| 1. |
- Johansson, Alexander
(författare)
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Coordination of cross-carrier truck platooning
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The need for sustainable transportation solutions is urgent as the demand for mobility of goods and people is expected to multiply in the upcoming decades. One promising solution is truck platooning, which shows great potential in reducing the energy consumption and operational costs of trucks. To utilize the benefits of truck platooning to the fullest, trucks with different schedules and routes in a road network need coordination to form platoons. This thesis addresses platoon coordination when trucks can wait at hubs to form platoons. We assume there is a reward for driving in a platoon and a cost for waiting at a hub, and the objective is to maximize the overall profit. We focus on coordinating trucks from different carriers, which is important considering that many platoon opportunities are lost if only trucks from the same carrier form platoons.In the first contributions of the thesis, we propose coordination solutions where carriers aim to maximize their own profits through cross-carrier platoon cooperation. We propose an architecture of a platoon-hailing service that stores reported platooning plans of carriers and, based on these, informs carriers about the platoons their trucks can join when they make platooning decisions. A realistic simulation study shows that the cross-carrier platooning system can achieve energy savings of 3.0% and 5.4% when 20% and 100% of the trucks are coordinated, respectively. A non-cooperative game is then formulated to model the strategic interaction among trucks with individual objectives when they coordinate for platooning and make decisions at the beginning of their journeys. The existence of at least one Nash equilibrium is shown. In the case of stochastic travel times, feedback-based solutions are developed wherein trucks repeatedly update their equilibrium decisions. A simulation study with stochastic travel times shows that the feedback-based solutions achieve platooning rates only $5\%$ lower than a solution where the travel times are known. We also explore Pareto-improving coordination guaranteeing each carrier is better off coopering with others, and models for distributing the profit within platoons.In the last contributions of the thesis, we study the problem of optimally releasing trucks at hubs when arriving according to a stochastic process, and a priori information about truck arrivals is inaccessible; this may be sensitive information to share with others. First, we study the release problem at hubs in a hub-corridor where the objective is to maximize the profit over time. The optimality of threshold-based release policies is shown under the assumption that arrivals are independent or that arrivals are dependent due to the releasing behavior at the preceding hub in the corridor. Then, we study the release problem at a single hub where the aim is to maximize the profit of trucks currently at the hub. This is realistic if trucks are only willing to wait at the hub if they can increase their own profits. Stopping time theory is used to show the optimality of a threshold-based release policy when arrivals are independent and identically distributed. These contributions show that simple coordination approaches can achieve high profits from platooning, even under limited information.
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| 2. |
- Alam, Assad, 1982-
(författare)
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Fuel-Efficient Distributed Control for Heavy Duty Vehicle Platooning
- 2011
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Freight transport demand has escalated and will continue to do so as economiesgrow. As the traffic intensity increases, the drivers are faced with increasinglycomplex tasks and traffic safety is a growing issue. Simultaneously, fossil fuel usageis escalating. Heavy duty vehicle (HDV) platooning is a plausible solution to theseissues. Even though there has been a need for introducing automated HDV platooningsystems for several years, they have only recently become possible to implement.Advancements in on-board and external technology have ushered in new possibilitiesto aid the driver and enhance the system performance. Each vehicle is able to serveas an information node through wireless communication; enabling a cooperativenetworked transportation system. Thereby, vehicles can semi-autonomously travel atshort intermediate spacings, effectively reducing congestion, relieving driver tension,improving fuel consumption and emissions without compromising safety. This thesis presents contributions to a framework for the design and implementation of HDV platooning. The focus lies mainly on establishing and validating realconstraints for fuel optimal control for platooning vehicles. Nonlinear and linearvehicle models are presented together with a system architecture, which dividesthe complex problem into manageable subsystems. The fuel reduction potentialis investigated through simulation models and experimental results derived fromstandard vehicles traveling on a Swedish highway. It is shown through analyticaland experimental results that it is favorable with respect to the fuel consumption tooperate the vehicles at a much shorter intermediate spacing than what is currentlydone in commercially available systems. The results show that a maximum fuelreduction of 4.7–7.7 % depending on the inter-vehicle time gap, at a set speedof 70 km/h, can be obtained without compromising safety. A systematic designmethodology for inter-vehicle distance control is presented based on linear quadraticregulators (LQRs). The structure of the controller feedback matrix can be tailoredto the locally available state information. The results show that a decentralizedcontroller gives good tracking performance, a robust system and lowers the controleffort downstream in the platoon. It is also shown that the design methodologyproduces a string stable system for an arbitrary number of vehicles in the platoon,if the vehicle configurations and the LQR weighting parameters are identical for theconsidered subsystems. With the results obtained in this thesis, it is argued that a vast fuel reductionpotential exists for HDV platooning. Present commercial systems can be enhancedsignificantly through the introduction of wireless communication and decentralizedoptimal control.
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| 3. |
- Alisic, Rijad
(författare)
-
Defense of Cyber-Physical Systems Against Learning-based Attackers
- 2023
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Cyberattacks against critical infrastructures pose a serious threat to society, as they can have devastating consequences on the economy, security, or public health. These infrastructures rely on a large network of cyber components, such as sensors, controllers, computers, and communication devices, to monitor and control their physical processes. An adversary can exploit the vulnerabilities in these cyber components to gain access to the system and manipulate its behavior or functionality.This thesis proposes methods that can be employed as a first line of defense against such attacks for Cyber-Physical Systems. In the first part of the thesis, we consider how uninformed attackers can learn to attack a Cyber-Physical System by eavesdropping through the cyber component. By learning to manipulate the plant, the attacker could figure out how to destroy the physical system before it is too late or completely take it over without raising any alarms. Stopping the attacker at the learning stage would force the attacker to act obliviously, increasing the chances of detecting them.We analyze how homomorphic encryption, a technique that allows computation on encrypted data, hinders an attacker's learning process and reduces its capabilities to attack the system. Specifically, we show that an attacker must solve challenging lattice problems to find attacks that are difficult to detect. Additionally, we show how the detection probability is affected by the attacker's solution to the problems and what parameters of the encryption scheme can be tweaked to increase the detection probability. We also develop a novel method that enables anomaly detection over homomorphically encrypted data without revealing the actual signals to the detector, thereby discouraging attackers from launching attacks on the detector. The detection can be performed using a hypothesis test. However, special care must be taken to ensure that fresh samples are used to detect changes from nominal behavior. We also explore how the adversary can try to evade detection using the same test and how the system can be designed to make detection easier for the defender and more challenging for the attacker.In the second part of the thesis, we study how information leakage about changes in the system depends on the system's dynamics. We use a mathematical tool called the Hammersley-Chapman-Robbins lower bound to measure how much information is leaked and how to minimize it. Specifically, we study how structured input sequences, which we call events, can be obtained through the output of a dynamical system and how this information can be hidden by adding noise or changing the inputs. The system’s speed and sensor locations affect how much information is leaked. We also consider balancing the system’s performance and privacy when using optimal control. Finally, we show how to estimate when the adversary’s knowledge of the event becomes accurate enough to launch an attack and how to change the system before that happens. These results are then used to aid the operator in detecting privacy vulnerabilities when designing a Cyber-Physical System, which increases the overall security when removed.
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| 4. |
- Björk, Joakim, 1989-
(författare)
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Fundamental Control Performance Limitations for Interarea Oscillation Damping and Frequency Stability
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- With the transition towards renewable energy and the deregulation of the electricity markets, the power system is changing. Growing electricity demand and more intermittent power production increase the need for transfer capacity. Lower inertia levels due to a higher share of renewables increase the need for fast frequency reserves (FFR). In this thesis, we study fundamental control limitations for improving the damping of interarea oscillations and frequency stability.The first part of the thesis considers the damping of oscillatory interarea modes. These system-wide modes involve power oscillating between groups of generators and are sometimes hard to control due to their scale and complexity. We consider limitations of decentralized control schemes based on local measurements, as well as centralized control schemes with limitations associated to actuator dynamics and network topology. It is shown that the stability of asynchronous grids can be improved by modulating the active power of a single interconnecting high-voltage direct current (HVDC) link. One challenge with modulating HVDC active power is that the interaction between interarea modes of the two grids may have a negative impact on system stability. By studying the controllability Gramian, we show that it is possible to improve the damping in both grids as long as the frequencies of their interarea modes are not too close. It is demonstrated how the controllability, and therefore the achievable damping, deteriorates as the frequency difference becomes small. With a modal frequency difference of 5%, the damping can be improved by around 2 percentage points whereas a modal frequency difference of 20% allows for around 8 percentage points damping improvement. The results are validated by simulating two HVDC-interconnected 32-bus power system models. We also consider the coordinated control of two and more HVDC links. For some network configurations, it is shown that the interaction between troublesome interarea modes can be avoided. The second part considers the coordination of frequency containment reserves (FCR) in low-inertia power systems. A case study is performed in a 5-machine model of the Nordic synchronous grid. We consider a low-inertia test case where FCR are provided by hydro power. The non-minimum phase characteristic of the waterways limits the achievable bandwidth of the FCR control. It is shown that a consequence of this is that hydro-FCR fails at keeping the frequency nadir above the 49.0 Hz safety limit following the loss of a HVDC link that imports 1400 MW. To improve the dynamic frequency stability, FFR from wind power is considered. For this, a new wind turbine model is developed. The turbine is controlled at variable-speed, enabling FFR by temporarily borrowing energy from the rotating turbine. The nonlinear wind turbine dynamics are linearized to facilitate a control design that coordinate FFR from the wind with slow FCR from hydropower. Complementary wind resources with a total rating of 2000 MW, operating at 70–90% rated wind speeds, is shown to be more than enough to fulfill the frequency stability requirements. The nadir is kept above 49.0 Hz without the need to install battery storage or to waste wind energy by curtailing the wind turbines.
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| 5. |
- Gao, Yulong, 1990-
(författare)
-
Safe Autonomy under Uncertainty: Computation, Control, and Application
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Safety is a primary requirement for many autonomous systems, such as automated vehicles and mobile robots. An open problem is how to assure safety, in the sense of avoiding unsafe subsets of the state space, for uncertain systems under complex tasks. In this thesis, we solve this problem for certain system classes and uncertainty descriptions by developing computational tools, designing verification and control synthesis algorithms, and evaluating them on two applications.As our first contribution, we consider how to compute probabilistic controlled invariant sets, which are sets the controller is able to keep the system state within with a certain probability. By using stochastic backward reachability, we design algorithms to compute these sets. We prove that the algorithms are computationally tractable and converge in a finite number of iterations. We further consider how to compute invariant covers, which are covers of sets that can be enforced to be invariant by a finite number of control inputs despite disturbances.A necessary and sufficient condition on the existence of an invariant cover is derived. Based on this result, an efficient computational algorithm is designed.The second contribution is to develop algorithms for model checking and control synthesis. We consider discrete-time uncertain systems under linear temporal logic (LTL) specifications. We propose the new notion of temporal logic trees (TLT) and show how to construct TLT from LTL formulae via reachability analysis for both autonomous and controlled transition systems. We prove approximation relations between TLT and LTL formulae. Two sufficient conditions are given to verify whether a transition system satisfies an LTL formula. An online control synthesis algorithm, under which a set of feasible control inputs can be generated at each time step, is designed, and it is proven to be recursively feasible.As our third contribution, we study two important vehicular applications on shared-autonomy systems, which are systems with a mix of human and automated decisions. For the first application, we consider a car parking problem, where a remote human operator is guided to drive a vehicle to an empty parking spot. An automated controller is designed to guarantee safety and mission completion despite unpredictable human actions. For the second application, we consider a car overtaking problem, where an automated vehicle overtakes a human-driven vehicle with uncertain motion. We design a risk-aware optimal overtaking algorithm with guaranteed levels of safety.
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| 6. |
- Guo, Meng, 1988-
(författare)
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Hybrid Control of Multi-robot Systems under Complex Temporal Tasks
- 2015
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Autonomous robots like household service robots, self-driving cars and dronesare emerging as important parts of our daily lives in the near future. They need tocomprehend and fulfill complex tasks specified by the users with minimal humanintervention. Also they should be able to handle un-modeled changes and contingentevents in the workspace. More importantly, they shall communicate and collaboratewith each other in an efficient and correct manner. In this thesis, we address theseissues by focusing on the distributed and hybrid control of multi-robot systemsunder complex individual tasks.We start from the nominal case where a single dynamical robot is deployed in astatic and fully-known workspace. Its local tasks are specified as Linear TemporalLogic (LTL) formulas containing the desired motion. We provide an automatedframework as the nominal solution to construct the hybrid controller that drives therobot such that its resulting trajectory satisfies the given task. Then we expand theproblem by considering a team of networked dynamical robots, where each robot hasa locally-specified individual task also as LTL formulas. In particular, we analyzefour different aspects as described below.When the workspace is only partially known to each robot, the nominal solutionmight be inadequate. Thus we first propose an algorithm for initial plan synthesis tohandle partially infeasible tasks that contain hard and soft constraints. We designan on-line scheme for each robot to verify and improve its local plan during runtime, utilizing its sensory measurements and communications with other robots. Itis ensured that the hard constraints for safety are always fulfilled while the softconstraints for performance are improved gradually.Secondly, we introduce a new approach to construct a full model of both robotmotion and actions. Based on this model, we can specify much broader robotic tasksand it is used to model inter-robot collaborative actions, which are essential for manymulti-robot applications to improve system capability, efficiency and robustness.Accordingly, we devise a distributed strategy where the robots coordinate theirmotion and action plans to fulfill the desired collaboration by their local tasks.Thirdly, continuous relative-motion constraints among the robots, such as collision avoidance and connectivity maintenance, are closely related to the stability,safety and integrity of multi-robot systems. We propose two different hybrid controlapproaches to guarantee the satisfaction of all local tasks and the relative-motionconstraints at all time: the first one is based on potential fields and nonlinear controltechnique; the second uses Embedded Graph Grammars (EGGs) as the main tool.At last, we take into account two common cooperative robotic tasks, namelyservice and formation tasks. These tasks are requested and exchanged among therobots during run time. The proposed hybrid control scheme ensures that the real-time plan execution incorporates not only local tasks of each robot but also thecontingent service and formation tasks it receives.Some of the theoretical results of the thesis have been implemented and demonstrated on various robotic platforms.
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| 7. |
- Johansson, Anette
(författare)
-
Ways forward : Effectual and causal approaches to innovation in the Swedish magazine industry
- 2014
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This dissertation builds on a study of key decision makers in the Swedish magazine publishing industry with a particular focus on how they think and act in their work to innovate their industry. This industry, much like the rest of the media industry, is facing increased unpredictability regarding for example the impact of new technology on the business and future demand. Traditional planning (causal) approaches can be greatly questioned in times of uncertainty, when the task at hand include creating products and services that do not yet exist.In this study I examine an alternative logic forward which takes its starting point in the interests, experiences, knowledge and networks of decision makers – instead of given goals. This logic is known as an effectual logic, and offers an alternative approach to move forward with innovation work. I argue, that the effectual logic can enable organizations to capitalize better from people’s experiences, networks and “gut-feeling” and create the innovations we now know nothing about. The effectual process involves developing ideas that coevolve together with others who want to commit, in a spirit that welcomes surprise, flexibility and experimentation. In this process products and services are created which could not have been predicted and planned – innovations. This logic of entrepreneurial action stems from research on expert entrepreneurs, however limited research has been done on its use and relevance in an established firm context. In this dissertation I examine this in the setting of magazine publishing firms to better understand the underlying mechanisms of effectuation in the firm setting, and its consequences for learning, innovation and performance. Alongside the effectual logic, I examine the traditional, causal logic dominating management education and practice.Effectuation is generally used among the decision makers in the study, inparticular among the most senior managers. Effectuation is positively linked to innovation, in particular radical innovation, and a combination of both logics is positively linked to innovation in general. Combining causal and effectual logics also gives rise to more knowledge useful for future innovation projects. One challenge for organizations is how to transfer this logic from the most senior managers to the whole organization, and thus make the most of the experiences and networks of their members.
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| 8. |
- Li, Yuchao
(författare)
-
Approximate Methods of Optimal Control via Dynamic Programming Models
- 2023
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Optimal control theory has a long history and broad applications. Motivated by the goal of obtaining insights through unification and taking advantage of the abundant capability to generate data and perform online simulation, this thesis studies the discrete-time infinite horizon optimal control problems and introduces some approximate solution methods via abstract dynamic programming (DP) models. The proposed methods involve approximation in value space through the use of data and simulator, apply to a broad class of problems, and strike a good balance between satisfactory performance and computational expenditure.First, we consider deterministic problems with nonnegative stage costs. We derive sufficient conditions under which a local controllability condition holds for the constrained nonlinear systems, and apply the results to establish the convergence of the classical algorithms, including value iteration, policy iteration (PI), and optimistic PI. These results provide a starting point for the design of suboptimal schemes. Then we propose algorithms that take advantage of system trajectory or the presence of parallel computing units to approximate the optimal costs. These algorithms can be viewed as variants of model predictive control (MPC) or rollout, and can be applied to deterministic problems with arbitrary state and control spaces, and arbitrary dynamics. It admits extensions to problems with trajectory constraints, and a multiagent structure. Via the viewpoint provided by the abstract DP models, we also derive the performance bounds of MPC applied to unconstrained and constrained linear quadratic problems, as well as their nonlinear counterparts. These insights suggest new designs of MPC, which likely lead to larger feasible regions of the scheme while costing hardly any loss of performance measured by the costs accumulated over infinite stages. Moreover, we derive algorithms to address problems with a fixed discount factor on future costs. We apply abstract DP models to analyze $\lambda$-PI with randomization algorithms for problems with infinite policies. We show that a contraction property induced by the discount factor is sufficient for the well-posedness of the algorithm. Moreover, we identify the conditions under which the algorithm is convergent with probability one. Guided by the analysis, we exemplify a data-driven approximate implementation of the algorithm for the approximation of the optimal costs of constrained linear and nonlinear control problems. The obtained optimal cost approximations are applied in a related suboptimal scheme. Then we consider discounted problems with discrete state and control spaces and a multiagent structure. When applying rollout to address the problem, the main challenge is to perform minimization over a large control space. To this end, we propose a rollout variant that involves reshuffling the order of the agents. The approximation of the costs of base policies is through the use of on-line simulation. The proposed approach is applied to address multiagent path planning problems within a warehouse context, where through on-line replanning, the robots can adapt to a changing environment while avoiding collision with each other.
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| 9. |
- Li, Yuchao
(författare)
-
Approximate Solution Methods to Optimal Control Problems via Dynamic Programming Models
- 2021
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Optimal control theory has a long history and broad applications. Motivated by the goal of obtaining insights through unification and taking advantage of the abundant capability to generate data, this thesis introduces some suboptimal schemes via abstract dynamic programming models.As our first contribution, we consider deterministic infinite horizon optimal control problems with nonnegative stage costs. We draw inspiration from the learning model predictive control scheme designed for continuous dynamics and iterative tasks, and propose a rollout algorithm that relies on sampled data generated by some base policy. The proposed algorithm is based on value and policy iteration ideas. It applies to deterministic problems with arbitrary state and control spaces, and arbitrary dynamics. It admits extensions to problems with trajectory constraints, and a multiagent structure.In addition, abstract dynamic programming models are used to analyze $\lambda$-policy iteration with randomization algorithms. In particular, we consider contractive models with infinite policies. We show that well-posedness of the $\lambda$-operator plays a central role in the algorithm. The operator is known to be well-posed for problems with finite states, but our analysis shows that it is also well-defined for the contractive models with infinite states. Similarly, the algorithm we analyze is known to converge for problems with finite policies, but we identify the conditions required to guarantee convergence with probability one when the policy space is infinite regardless of the number of states. Guided by the analysis, we exemplify a data-driven approximated implementation of the algorithm for estimation of optimal costs of constrained linear and nonlinear control problems. Numerical results indicate the potentials of this method in practice.
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| 10. |
- Liang, Kuo-Yun
(författare)
-
Fuel-Efficient Heavy-Duty Vehicle Platoon Formation
- 2016
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- There is a need for intelligent freight transport solutions as the demand for road freight transport is continuously increasing while carbon footprint needs to be significantly reduced. Heavy-duty vehicle (HDV) platooning is one potential solution to partially mitigate the environmental impacts as well as to reduce fuel consumption, improve traffic safety, and increase traffic throughput on congested highways. However, as each goods transport has different origin, destination, and time restriction, it is not evident how the HDVs, carrying the goods, can fully utilize the platooning benefits during individual transport missions. Thus, there is a need to systematically coordinate scattered vehicles on the road to form platoons in order to maximize the benefits of platooning. This thesis addresses the problem of merging scattered HDVs to form platoons in traffic. The first contribution of the thesis is the investigation of how and when a pair of HDVs should form platoons given their positions, speeds, and destinations. We formulate the problem as an optimization problem and we derive a break-even ratio that describes how far a vehicle should check for possible vehicles to platoon with. The second contribution is to consider traffic during the merging maneuver when forming a platoon. Traffic may disturb and delay when the two HDVs will form a platoon and such delay leads to less fuel saved than initially planned. Based on shockwave and moving bottleneck theories, we derive a merge distance predictor that calculates where the HDVs will merge depending on the traffic condition. We first validate this in a microscopic traffic simulation tool. Then, we also conduct an experimental study during one month on a public highway between Stockholm and Södertälje to evaluate the merging maneuver with different traffic densities. Lastly, we use vehicle probe data obtained from a fleet management system to investigate the potential fuel savings from coordination in a larger road network. The number of vehicles platooning can be increased significantly through coordination compared to today. The main result of this thesis indicates that merging HDVs to form platoons leads to great fuel savings and that there are significant potentials to do so in reality. Traffic needs to be considered in order to guarantee that the HDVs save fuel and deliver the goods in time. Furthermore, the earlier the transport assignment is planned ahead of time, the more opportunities there are to collaborate with other fleet owners to reduce the fuel consumption.
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| 11. |
- Liu, Hanxiao, 1995-
(författare)
-
Analysis, Detection, and Mitigation of Attacks in Cyber-physical Systems
- 2021
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Cyber-Physical Systems (CPS) offer close integration among computational elements, communication networks, and physical processes. Such systems play an increasingly important role in a large variety of fields, such as manufacturing, health care, environment, transportation, defence, and so on. Due to the wide applications and critical functions of CPS, increasing importance has been attached to their security. In this thesis, we focus on the security of CPS by investigating vulnerability under cyber-attacks, providing detection mechanisms, and developing feasible countermeasures against cyber-attacks.The first contribution of this thesis is to analyze the performance of remote state estimation under linear attacks. A linear time-invariant system equipped with a smart sensor is studied. The adversary aims to maximize the state estimation error covariance while staying stealthy. The maximal performance degradation that an adversary can achieve with any linear first-order false data injection attack under strict stealthiness for vector systems and $\epsilon$-stealthiness for scalar systems is characterized. We also provide an explicit attack strategy that achieves this bound and compare it with strategies previously proposed in the literature. The second problem of this thesis is about the detection of replay attacks. We aim to design physical watermark signals and corresponding detector to protect a control system against replay attacks. For a scenario where the system parameters are available to the operator, a physical watermarking scheme to detect the replay attack is introduced. The optimal watermark signal design problem is formulated as an optimization problem, and the optimal watermark signal and detector are derived. Subsequently, for systems with unknown parameters, we provide an on-line learning mechanism to asymptotically derive the optimal watermarking signal and corresponding detector.The third problem under investigation is about the detection of false-data injection attacks when the attacker injects malicious data to flip the distribution of the manipulated sensor measurements. The detector decides to continue taking observations or to stop based on the received signals, and the goal is to have the flip attack detected as fast as possible while trying to avoid terminating the measurements when no attack is present. The detection problem is modeled as a partially observable Markov decision process (POMDP) by assuming an attack probability, with the dynamics of the hidden states of the POMDP characterized by a stochastic shortest path (SSP) problem. The optimal policy of the SSP solely depends on the transition costs and is independent of the assumed attack probability. By using a fixed-length window and suitable feature function of the measurements, a Markov decision process (MDP) is used to approximate the POMDP. The optimal solution of the MDP is obtained by reinforcement learning. The fourth contribution of this thesis is to develop a sensor scheduler for remote state estimation under integrity attacks. We seek a trade-off between the energy consumption of communications and accuracy of state estimation when the acknowledgment (ACK) information, sent by the remote estimator to the local sensor, is compromised. The sensor scheduling problem is formulated as an infinite horizon discounted optimal control problem with infinite states. We first analyze the underlying MDP and show that the optimal schedule without ACK attack is of threshold type. Thus, we can simplify the problem by replacing the original state space with a finite state space. For the simplified MDP, when ACK is under attack, the problem is modelled as a POMDP. We analyze the induced MDP that uses a belief vector as its state for the POMDP. The properties of the exact optimal solution are studied via contractive models and it is shown that the threshold solution for the POMDP cannot be readily obtained. A suboptimal solution is provided instead via a rollout approach based on reinforcement learning. We present two variants of rollout and provide corresponding performance bounds.
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| 12. |
- Olguín Muñoz, Manuel Osvaldo Jesús, 1992-
(författare)
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An Emulation-Based Performance Evaluation Methodology for Edge Computing and Latency Sensitive Applications
- 2023
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Cloud Computing, with its globally-accessible nature and virtually unlimited scalability, has revolutionized our daily lives since its widespread adoption in the early 2000s. It allows us to access our documents anywhere, keep in touch with friends, back up photos, and even remotely control our appliances. Despite this, Cloud Computing has limitations when it comes to novel appli- cations requiring real-time processing or low-latencies. Applications such as Cyber-Physical Systems (CPSs) or mobile eXtended Reality (XR), which in turn also have great transformative potential, are unable to run on the Cloud. Edge Computing is emerging as a potential solution to these limitations by bringing computation closer to the edge of the network, thereby reducing latency and enabling real-time decision making. The combination of Edge Computing and modern mobile network technologies such as 5G offers potential for massive deployments of latency-sensitive applications. However, scaling and understanding these deployments poses important challenges such the optimization of latency through multiple processing steps and trade-offs in wireless system choice, protocols, hardware, and algorithms. Existing approaches have so far been unsuccessful in capturing the complex effects arising from the interplay between network and compute in these systems. This dissertation addresses the challenge of performance evaluation of Edge Computing and the applications enabled by this paradigm with two key contributions to literature. First, a methodological approach to experimentally studying the trade-offs between system responsiveness and resource consumption in latency-sensitive applications such as CPSs and XR is introduced. These applications and systems feature characteristics, such as tight interaction with the physical world and the involvement of humans, that make them challenging to study through simulated approaches or analytical modeling. The approach presented in this thesis involves therefore the emulation of the client-side workload while maintaining the real server-side process and network stack to retain the realism of network and compute effects. Next, an exploration of the requirements for accuracy in the emulation is presented. This work discusses the extent to which accuracy in the emulation can open new avenues for optimization of these systems. To this end, the first-ever realistic model of human timings for a particular class of Mobile Augmented Reality (MAR) applications is provided. The model is combined with a mathematical framework to study the potential for optimization in Edge Computing applications. Results indicate that the methodology introduced in this work offers advantages over existing methods by improving efficiency, repeatability, and replicability. By fully integrating workload components into the emulated software domain, this methodology reduces complexity while still capturing complex effects of network and compute factors that are challenging to model. This approach represents thus an important contribution to literature, as it consists of a comprehensive method for the performance evaluation of Edge environments, encompassing both the application and the infrastructure. Furthermore, results from the exploration into the implications of realism in the emulation suggest that incorporating enhanced realism in client-side emulation can enable the implementation of optimization approaches that would otherwise be infeasible. In particular, this work highlights the significance of considering human behavior and reactions in addition to system-related metrics and performance optimizations in the context of MAR.
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| 13. |
- Ramesh, Chithrupa, 1982-
(författare)
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State-based Channel Access for a Network of Control Systems
- 2014
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Wireless networked control systems use shared wireless links to communicate between sensors and controllers, and require a channel access policy to arbitrate access to the links. Existing multiple access protocols perform this role in an agnostic manner, by remaining insular to the applications that run over the network. This approach does not give satisfactory control performance guarantees. To enable the use of wireless networks in emerging industrial applications, we must be able to systematically design wireless networked control systems that provide guaranteed performances in resource-constrained networks.In this thesis, we advocate the use of state-based channel access policies. A state-based policy uses the state of the controlled plant to influence access to the network. The state contains information about not only the plant, but also the network, due to the feedback in the system. Thus, by using the state to decide when and how frequently to transmit, a control system can adapt its contribution to the network traffic, and enable the network to adapt access to the plant state. We show that such an approach can provide better performance than existing methods. We examine two different state-based approaches that are distributed and easy to implement on wireless devices: event-based scheduling and adaptive prioritization.Our first approach uses events to reduce the traffic in the network. We use a state-based scheduler in every plant sensor to generate non-coordinated channel access requests by selecting a few critical data packets, or events, for transmission. The network uses a contention resolution mechanism to deal with simultaneous channel access requests. We present three main contributions for this formulation. The first contribution is a structural analysis of stochastic event-based systems, where we identify a dual predictor architecture that results in separation in design of the state-based scheduler, observer and controller. The second contribution is a Markov model that describes the interactions in a network of event-based systems. The third contribution is an analysis of the stability of event-based systems, leading to a stabilizing design of event-based policies.Our second approach uses state-based priorities to determine access to the network. We use a dominance protocol to evaluate priorities in a contention-based setting, and characterize the resulting control performance. An implementation and evaluation of this channel access mechanism on sensor nodes is also presented.The thesis finally examines the general networked control problem of jointly optimizing measurement and control policies, when a nonlinear measurement policy is used to perform quantization, event-triggering or companding. This contribution focuses on some of the fundamental aspects of analyzing and synthesizing control systems with state-based measurement policies in a more generalized setting. We comment on the dual effect, certainty equivalence and separation properties for this problem. In particular, we show that it is optimal to apply separation and certainty equivalence to a design problem that permits a dynamic choice of the measurement and control policies.
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| 14. |
- Stefansson, Elis
(författare)
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Complexity-aware Decision-making with Applications to Large-scale and Human-in-the-loop Systems
- 2023
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis considers control systems governed by autonomous decision-makers and humans. We formalise and compute low-complex control policies with applications to large-scale systems, and propose human interaction models for controllers to compute interaction-aware decisions.In the first part of the thesis, we consider complexity-aware decision-making, formalising the complexity of control policies and constructing algorithms that compute low-complexity control policies. More precisely, first, we consider large-scale control systems given by hierarchical finite state machines (HFSMs) and present a planning algorithm for such systems that exploits the hierarchy to compute optimal policies efficiently. The algorithm can also handle changes in the system with ease. We prove these properties and conduct simulations on HFSMs with up to 2 million states, including a robot application, where our algorithm outperforms both Dijkstra's algorithm and Contraction Hierarchies. Second, we present a planning objective for control systems modelled as finite state machines yielding an explicit trade-off between a policy's performance and complexity. We consider Kolmogorov complexity since it captures the ultimate compression of an object on a universal Turing machine. We prove that this trade-off is hard to optimise in the sense that dynamic programming is infeasible. Nonetheless, we present two heuristic algorithms obtaining low-complexity policies and evaluate the algorithms on a simple navigation task for a mobile robot, where we obtain low-complexity policies that concur with intuition. In the second part of the thesis, we consider human-in-the-loop systems and predict human decision-making in such systems. First, we look at how the interaction between a robot and a human in a control system can be predicted using game theory, focusing on an autonomous truck platoon interacting with a human-driven car. The interaction is modelled as a hierarchical dynamic game, where the hierarchical decomposition is temporal with a high-fidelity tactical horizon predicting immediate interactions and a low-fidelity strategic horizon estimating long-term behaviour. The game enables feasible computations validated through simulations yielding situation-aware behaviour with natural and safe interactions. Second, we seek models to explain human decision-making, focusing on driver overtaking scenarios. The overtaking problem is formalised as a decision problem with perceptual uncertainty. We propose and numerically analyse risk-agnostic and risk-aware decision models, judging if an overtaking is desirable. We show how a driver's decision time and confidence level can be characterised through two model parameters, which collectively represent human risk-taking behaviour. We detail an experimental testbed for evaluating the decision-making process in the overtaking scenario and present some preliminary experimental results from two human drivers.
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| 15. |
- Teixeira, André
(författare)
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Toward Cyber-Secure and Resilient Networked Control Systems
- 2014
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Resilience is the ability to maintain acceptable levels of operation in the presence of abnormal conditions. It is an essential property in industrial control systems, which are the backbone of several critical infrastructures. The trend towards using pervasive information technology systems, such as the Internet, results in control systems becoming increasingly vulnerable to cyber threats. Traditional cyber security does not consider the interdependencies between the physical components and the cyber systems. On the other hand, control-theoretic approaches typically deal with independent disturbances and faults, thus they are not tailored to handle cyber threats. Theory and tools to analyze and build control system resilience are, therefore, lacking and in need to be developed. This thesis contributes towards a framework for analyzing and building resilient control systems.First, a conceptual model for networked control systems with malicious adversaries is introduced. In this model, the adversary aims at disrupting the system behavior while remaining undetected by an anomaly detector The adversary is constrained in terms of the available model knowledge, disclosure resources, and disruption capabilities. These resources may correspond to the anomaly detector’s algorithm, sniffers of private data, and spoofers of control commands, respectively.Second, we address security and resilience under the perspective of risk management, where the notion of risk is defined in terms of a threat’s scenario, impact, and likelihood. Quantitative tools to analyze risk are proposed. They take into account both the likelihood and impact of threats. Attack scenarios with high impact are identified using the proposed tools, e.g., zero-dynamics attacks are analyzed in detail. The problem of revealing attacks is also addressed. Their stealthiness is characterized, and how to detect them by modifying the system’s structure is also described.As our third contribution, we propose distributed fault detection and isolation schemes to detect physical and cyber threats on interconnected second-order linear systems. A distributed scheme based on unknown input observers is designed to jointly detect and isolate threats that may occur on the network edges or nodes. Additionally, we propose a distributed scheme based on local models and measurements that is resilient to changes outside the local subsystem. The complexity of the proposed methods is decreased by reducing the number of monitoring nodes and by characterizing the minimum amount of model information and measurements needed to achieve fault detection and isolation.Finally, we tackle the problem of distributed reconfiguration under sensor and actuator faults. In particular, we consider a control system with redundant sensors and actuators cooperating to recover from the removal of individual nodes. The proposed scheme minimizes a quadratic cost while satisfying a model-matching condition, which maintains the nominal closed-loop behavior after faults. Stability of the closed-loop system under the proposed scheme is analyzed.
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| 16. |
- Terelius, Håkan, 1987-
(författare)
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Optimization and Control in Dynamical Network Systems
- 2016
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Dynamical network systems are complex interconnected systems useful to describe many real world problems. The advances in information technology has led the current trend towards connecting more and more systems, creating "intelligent" systems, where the intelligence originates in the scale and complexity of the network. With the growing scale of networked systems comes also higher demands on performance and continuous availability and this creates the need for optimization and control of network systems. This thesis makes four important contributions in this area.In the first contribution, we consider a collaborative road freight transportation system. An efficiency measure for the road utilization in collaborative transportation scenarios is introduced, which evaluates the performance of collaboration strategies in comparison to an optimal central planner. The efficiency measure is used to study a freight transport simulation in Germany and taxi trips using real data from New York City. This is followed by a study of the optimal idling locations for trucks, and the optimal locations for distribution centers. These locations are then exploited in a simulation of a realistic collaborative freight transport system.The second contribution studies the important problem of gathering data that are distributed among the nodes in an anonymous network, i.e., a network where the nodes are not endowed with unique identifies. Two specific tasks are considered: to estimate the size of the network, and to aggregate the distribution of local measurements generated by the nodes. We consider a framework where the nodes require anonymity and have restricted computational resources. We propose probabilistic algorithms with low resource requirements, that quickly generate arbitrarily accurate estimates. For dynamical networks, we improve the accuracy through a regularization term which captures the trade-off between the reliability of the gathered data and a-priori assumptions for the dynamics.In the third contribution, a peer-to-peer network is utilized to improve a live-streaming media application. In particular, we study how an overlay network, constructed from simple preference functions, can be used to build efficient topologies that reduce both network latency and interruptions. We present necessary and sufficient convergence conditions, as well as convergence rate estimates, and demonstrate the improvements for a real peer-to-peer video streaming application.The final contribution is a distributed optimization algorithm. We consider a distributed multi-agent optimization problem of minimizing the sum of convex objective functions. A decentralized optimization algorithm is introduced, based on dual decomposition, together with the subgradient method for finding the optimal solution. The convergence rate is analyzed for different step size rules, constant and time-varying communication delays, and noisy communication channels.
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| 17. |
- Alam, Assad, 1982-
(författare)
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Fuel-Efficient Heavy-Duty Vehicle Platooning
- 2014
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The freight transport industry faces big challenges as the demand for transport and fuel prices are steadily increasing, whereas the environmental impact needs to be significantly reduced. Heavy-duty vehicle (HDV) platooning is a promising technology for a sustainable transportation system. By semi-autonomously governing each platooning vehicle at small inter-vehicle spacing, we can effectively reduce fuel consumption, emissions, and congestion, and relieve driver tension. Yet, it is not evident how to synthesise such a platoon control system and how constraints imposed by the road topography affect the safety or fuel-saving potential in practice.This thesis presents contributions to a framework for the design, implementation, and evaluation of HDV platooning. The focus lies mainly on establishing fuel-efficient platooning control and evaluating the fuel-saving potential in practice. A vehicle platoon model is developed together with a system architecture that divides the control problem into manageable subsystems. Presented results show that a significant fuel reduction potential exists for HDV platooning and it is favorable to operate the vehicles at a small inter-vehicle spacing. We address the problem of finding the minimum distance between HDVs in a platoon without compromising safety, by setting up the problem in a game theoretical framework. Thereby, we determine criteria for which collisions can be avoided in a worst-case scenario and establish the minimum safe distance to a vehicle ahead. A systematic design methodology for decentralized inter-vehicle distance control based on linear quadratic regulators is presented. It takes dynamic coupling and engine response delays into consideration, and the structure of the controller feedback matrix can be tailored to the locally available state information. The results show that a decentralized controller gives good tracking performance and attenuates disturbances downstream in the platoon for dynamic scenarios that commonly occur on highways. We also consider the problem of finding a fuel-efficient controller for HDV platooning based on road grade preview information under road and vehicle parameter uncertainties. We present two model predictive control policies and derive their fuel-saving potential. The thesis finally evaluates the fuel savings in practice. Experimental results show that a fuel reduction of 3.9–6.5 % can be obtained on average for a heterogenous platoon of HDVs on a Swedish highway. It is demonstrated how the savings depend on the vehicle position in the platoon, the behavior of the preceding vehicles, and the road topography. With the results obtained in this thesis, it is argued that a significant fuel reduction potential exists for HDV platooning.
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| 18. |
- Alisic, Rijad, 1994-
(författare)
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Privacy of Sudden Events in Cyber-Physical Systems
- 2021
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Cyberattacks against critical infrastructures has been a growing problem for the past couple of years. These infrastructures are a particularly desirable target for adversaries, due to their vital importance in society. For instance, a stop in the operation of a critical infrastructure could result in a crippling effect on a nation's economy, security or public health. The reason behind this increase is that critical infrastructures have become more complex, often being integrated with a large network of various cyber components. It is through these cyber components that an adversary is able to access the system and conduct their attacks.In this thesis, we consider methods which can be used as a first line of defence against such attacks for Cyber-Physical Systems (CPS). Specifically, we start by studying how information leaks about a system's dynamics helps an adversary to generate attacks that are difficult to detect. In many cases, such attacks can be detrimental to a CPS since they can drive the system to a breaking point without being detected by the operator that is tasked to secure the system. We show that an adversary can use small amounts of data procured from information leaks to generate these undetectable attacks. In particular, we provide the minimal amount of information that is needed in order to keep the attack hidden even if the operator tries to probe the system for attacks. We design defence mechanisms against such information leaks using the Hammersley-Chapman-Robbins lower bound. With it, we study how information leakage could be mitigated through corruption of the data by injection of measurement noise. Specifically, we investigate how information about structured input sequences, which we call events, can be obtained through the output of a dynamical system and how this leakage depends on the system dynamics. For example, it is shown that a system with fast dynamical modes tends to disclose more information about an event compared to a system with slower modes. However, a slower system leaks information over a longer time horizon, which means that an adversary who starts to collect information long after the event has occured might still be able to estimate it. Additionally, we show how sensor placements can affect the information leak. These results are then used to aid the operator to detect privacy vulnerabilities in the design of a CPS.Based on the Hammersley-Chapman-Robbins lower bound, we provide additional defensive mechanisms that can be deployed by an operator online to minimize information leakage. For instance, we propose a method to modify the structured inputs in order to maximize the usage of the existing noise in the system. This mechanism allows us to explicitly deal with the privacy-utility trade-off, which is of interest when optimal control problems are considered. Finally, we show how the adversary's certainty of the event increases as a function of the number of samples they collect. For instance, we provide sufficient conditions for when their estimation variance starts to converge to its final value. This information can be used by an operator to estimate when possible attacks from an adversary could occur, and change the CPS before that, rendering the adversary's collected information useless.
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| 19. |
- Araújo, José, 1985-
(författare)
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Design, Implementation and Validation of Resource-Aware and Resilient Wireless Networked Control Systems
- 2014
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Networked control over wireless networks is of growing importance in many application domains such as industrial control, building automation and transportation systems. Wide deployment however, requires systematic design tools to enable efficient resource usage while guaranteeing close-loop control performance. The control system may be greatly affected by the inherent imperfections and limitations of the wireless medium and malfunction of system components. In this thesis, we make five important contributions that address these issues. In the first contribution, we consider event- and self-triggered control and investigate how to efficiently tune and execute these paradigms for appropriate control performance. Communication strategies for aperiodic control are devised, where we jointly address the selection of medium-access control and scheduling policies. Experimental results show that the best trade-off is obtained by a hybrid scheme, combining event- and self-triggered control together with contention-based and contention-free medium access control.The second contribution proposes an event-based method to select between fast and slow periodic sampling rates. The approach is based on linear quadratic control and the event condition is a quadratic function of the system state. Numerical and experimental results show that this hybrid controller is able to reduce the average sampling rate in comparison to a traditional periodic controller, while achieving the same closed-loop control performance.In the third contribution, we develop compensation methods for out-of-order communications and time-varying delays using a game-theoretic minimax control framework. We devise a linear temporal coding strategy where the sensor combines the current and previous measurements into a single packet to be transmitted. An experimental evaluation is performed in a multi-hop networked control scenario with a routing layer vulnerability exploited by a malicious application. The experimental and numerical results show the advantages of the proposed compensation schemes.The fourth contribution proposes a distributed reconfiguration method for sensor and actuator networks. We consider systems where sensors and actuators cooperate to recover from faults. Reconfiguration is performed to achieve model-matching, while minimizing the steady-state estimation error covariance and a linear quadratic control cost. The reconfiguration scheme is implemented in a room heating testbed, and experimental results demonstrate the method's ability to automatically reconfigure the faulty system in a distributed and fast manner.The final contribution is a co-simulator, which combines the control system simulator Simulink with the wireless network simulator COOJA. The co-simulator integrates physical plant dynamics with realistic wireless network models and the actual embedded software running on the networked devices. Hence, it allows for the validation of the complete wireless networked control system, including the study of the interactions between software and hardware components.
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| 20. |
- Baumann, Dominik
(författare)
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Learning and Control Strategies for Cyber-physical Systems: From Wireless Control over Deep Reinforcement Learning to Causal Identification
- 2020
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Cyber-physical systems (CPS) integrate physical processes with computing and communication to autonomously interact with the environment. This enables emerging applications such as autonomous driving or smart factories. However, current technology does not provide the dependability and adaptability to realize those applications. CPS are systems with complex dynamics that need to be adaptive, communicate with each other over wireless channels, and provide theoretical guarantees on proper functioning. In this thesis, we take on the challenges imposed by wireless CPS by developing appropriate learning and control strategies.In the first part of the thesis, we present a holistic approach that enables provably stable feedback control over wireless networks. At design time (i.e., prior to execution), we tame typical imperfections inherent in wireless networks, such as communication delays and message loss. The remaining imperfections are then accounted for through feedback control. At run time (i.e., during execution), we let systems reason about communication demands and allocate communication resources accordingly. We provide theoretical stability guarantees and evaluate the approach on a cyber-physical testbed, featuring a multi-hop wireless network supporting multiple cart-pole systems.In the second part, we enhance the flexibility of our designs through learning. We first propose a framework based on deep reinforcement learning to jointly learn control and communication strategies for wireless CPS by integrating both objectives, control performance and saving communication resources, in the reward function. This enables learning of resource-aware controllers for nonlinear and high-dimensional systems. Second, we propose an approach for evaluating the performance of models of wireless CPS through online statistical analysis. We trigger learning in case performance drops, that way limiting the number of learning experiments and reducing computational complexity. Third, we propose an algorithm for identifying the causal structure of control systems. We provide theoretical guarantees on learning the true causal structure and demonstrate enhanced generalization capabilities inherited through causal structure identification on a real robotic system.
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| 21. |
- Björk, Joakim, 1989-
(författare)
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Performance Quantification of Interarea Oscillation Damping Using HVDC
- 2019
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- With the transition towards renewable energy, and the deregulation of the electricity market, generation patterns and grid topology are changing. These changes increase the need for transfer capacity. One limiting factor, which sometimes leads to underutilization of the transmission grid, is interarea oscillations. These system-wide modes involve groups of generators oscillating relative to each other and are sometimes hard to control due to their scale and complexity. In this thesis we investigate how high-voltage direct current (HVDC) transmission can be used to attenuate interarea oscillations. The thesis has two main contributions.In the first contribution we show how the stability of two asynchronous grids can be improved by modulating the active power of a single interconnecting HVDC link. One concern with modulating HVDC active power is that the interaction between interarea modes of the two grids may have a negative impact on system stability. By studying the controllability Gramian, we show that it is always possible to improve the damping in both grids as long as the frequencies of their interarea modes are not too close. For simplified models, it is explicitly shown how the controllability, and therefore the achievable damping improvements, deteriorates as the frequency difference becomes small.The second contribution of the thesis is to show how coordinated control of two (or more) links can be used to avoid interaction between troublesome interarea modes. We investigate the performance of some multivariable control designs. In particular we look at input usage as well as robustness to measurement, communication, and actuator failures. Suitable controllers are thereby characterized.
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| 22. |
- Čičić, Mladen, 1991-
(författare)
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Modelling and Lagrangian control of mixed traffic: platoon coordination, congestion dissipation and state reconstruction
- 2021
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Traffic congestion is a constantly growing problem, with a wide array of negative effects on the society, from wasted time and productivity to elevated air pollution and reduction of safety. The introduction of connected, autonomous vehicles enables a new, Lagrangian paradigm for sensing andcontrolling the traffic, by directly using connected vehicles inside the traffic flow, as opposed to the classical, Eulerian paradigm, which relies on stationary equipment on the road. By using control methods specifically tailored to the Lagrangian paradigm, we are able to influence the traffic flow even if the penetration rate of connected vehicle is low. This allows us to answer one of the central impending questions of the traffic control using emerging technologies: How can we influence the overall traffic by using only a smallportion of vehicles that we can control directly?Traffic phenomena such as moving bottlenecks and stop-and-go waves are particularly pertinent to Lagrangian traffic control, and therefore need to be captured in traffic models. In this thesis we introduce the influence of these phenomena into the cell transmission model, multi-class cell transmission model, and tandem queueing model. We also propose a transition system model based on front tracking, which captures the relevant phenomena, and show under which conditions it corresponds to the Lighthill-Whitham-Richards model. Moving bottlenecks are introduced as a moving zone in which a reduced flux function describes the traffic flow, and their influence on the surrounding traffic is given by solving the Riemann problems at the flux function boundaries. Stop-and-go waves are introduced by constraining the wave speed of rarefaction, resulting in constant stop-and-go wave propagation speed and discharging flow lower than the road capacity, which is consistent with the empirical observations.We use the proposed traffic models to design control laws that address three problems: platoon merging coordination, congestion reduction, and traffic state reconstruction. We study the case when two trucks are closing the distance and merging into a platoon on a public road, and propose an optimal control algorithm which accounts for the mutual influence between the trucks and the surrounding traffic. The proposed control law minimizes the total fuel consumption of the trucks, and improves the reliability of platooning. Then, we consider two forms of the congestion reduction problem: stationary bottleneck decongestion, and stop-and-go wave dissipation. In both cases, connected vehicles are used as moving bottlenecks to restrict the traffic flow enough to let the congestion dissipate. By applying these control laws, the throughput of the road is increased and the total travel time of all vehiclesis reduced. Finally, we generalize the stop-and-go wave dissipation problem by dropping the assumption that the full traffic state is known, and instead propose traffic state reconstruction algorithms which use local measurements originating from the connected vehicles. We show that the proposed control laws can also be implemented using the reconstructed traffic state. In this case, as the number of available connected vehicles increases, the control performance approaches the full-information control case.
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| 23. |
- Di Marco, Piergiuseppe
(författare)
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Modeling and Design of Wireless Protocols for Networked Control Applications
- 2012
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Wireless networking offers great potentials for the development of new applications in real-time monitoring and control. However, current design processes do not simultaneously consider energy efficiency, system requirements, and standards compatibility. Modeling, optimization, and integration of communication and control protocols are essential to achieve efficient overall operations. We propose a holistic design framework, which includes physical channels, medium access control (MAC), multi-hop routing, and control applications. Accordingly, we provide the following contributions.First, we investigate the performance of the IEEE 802.15.4 MAC through an accurate Markov chain model and its simplified representation. The effects of traffic load, number of devices, and MAC parameters on reliability, delay, and energy consumption are determined analytically and experimentally. We show that the delay distribution is different with respect to commonly used models in networked control systems design. Moreover, we introduce an adaptive mechanism to minimize the energy consumption while fulfilling reliability and delay constraints.Second, we extend the analysis to multi-hop networks, including heterogeneous traffic distribution and limited carrier sensing range. Due to the contention-based channel access, routing decisions based on reliability or delay typically direct traffic toward nodes with high packet generation rates, leading to unbalanced performance and higher energy consumption. A load balancing metric is proposed for the IETF routing protocol for low-power and lossy networks. Furthermore, a mechanism to optimally select routes and MAC parameters is implemented.Third, we include a realistic channel model in the analysis. Multi-path and shadowing are modeled by a Nakagami-lognormal distribution. A moment matching approximation is used to derive the statistics of aggregate signals. The impact of fading on MAC and routing is determined for various traffic regimes, distances among devices, and signal-to-(interference plus noise)-ratio settings. The results show that a certain level of fading actually improves the network performance.Fourth, we propose TREnD, a cross-layer protocol that takes into account tunable application requirements. Duty cycling, data aggregation, and power control are employed to provide energy efficiency and an optimization problem is solved to select the protocol parameters adaptively. TREnD is implemented on a test-bed and it is compared to existing protocols. Experimental results show load balancing and adaptation for static and dynamic scenarios.Finally, the analytical models developed in the thesis are formalized into a contract-based design framework. We consider a building automation example with a feedback control system over a heterogeneous network. We include the effects of delays and losses in the controller synthesis and we compare various robust control strategies. The use of contracts allows for a compositional design that handles performance, heterogeneity, and reconfigurability in a systematic and efficient way.
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| 24. |
- Ekendahl, Karl
(författare)
-
The Good, the Bad, and the Dead : An Essay on Well-Being and Death
- 2019
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This book examines some central arguments in the debate about the value of death. The first main chapter, Chapter 2, begins with an introduction to the debate and a clarification of Epicureanism, i.e. the view that it is not bad to die. I then go on to evaluate several versions of a popular Epicurean line of argument, according to which death’s failure or inability to cause its victim any unpleasant experiences gives us reason to deny that death can be bad for the person who dies. I argue that none of these arguments succeeds. In Chapter 3, I turn to a more promising argument against the badness of death: the Timing Argument. Because there is no time at which death can be bad for its victim, the argument goes, it cannot be bad for her at all. To clarify the nature of this rather obscure argument, I offer two different interpretations, only one of which, I argue, should be considered a challenge to the anti-Epicurean. In Chapter 4, I review different attempts at refuting the Timing Argument, many of which fail to address the argument in its most challenging form. I also argue that there is no time at which death is bad for its victim, but that the conclusion to draw from this is that death can be bad for its victim without being bad for her at any time. The final chapter, Chapter 5, starts with the widespread worry that Epicureanism is hard to combine with certain normative commonsense ideas, e.g. the idea that we often prudentially ought to avoid death. As it turns out, however, the anti-Epicurean faces similar problems: in certain cases where, intuitively, a person has prudential reasons to avoid her death, the most prominent anti-Epicurean accounts fail to yield that her death is bad for her. This is a serious problem for anti-Epicureanism, and I end with a few remarks on its potential implications.
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| 25. |
- Farokhi, Farhad, 1987-
(författare)
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Decentralized Control Design with Limited Plant Model Information
- 2012
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Large-scale control systems are often composed of several smaller interconnected units. For these systems, it is common to employ local controllers, which observe and act locally. At the heart of common control design procedures for distributed systems lies the often implicit assumption that the designer has access to the global plant model information when designing a local controller. However, there are several reasons why such plant model information would not be globally known. One reason could be that the designer wants the parameters of each local controller to only depend on local model information, so that the controllers are not modified if the model parameters of a particular subsystem change. It might also be the case that the design of each local controller is done by individual designers with no access to the global plant model, for instance, due to the fact that the designers refuse to share their model information since they consider it private. This class of problems, which we refer to as limited model information control design, is the topic of the thesis. First, we investigate the achievable closed-loop performance of discrete-time linear time-invariant plants under a separable quadratic cost performance with structured static state-feedback controllers. To do so, we introduce control design strategies as mappings, which construct controllers by accessing the plant model information in a constrained way according to a given design graph. We compare control design strategies using the competitive ratio as a performance metric, that is, we compare the worst case control performance for a given design strategy normalized with the optimal control performance based on full model information. An explicit minimizer of the competitive ratio is sought. As this minimizer might not be unique, we further search for the ones that are undominated, that is, there is no other control design strategy in the set of limited model information design strategies with a better closed-loop performance for all possible plants while maintaining the same worst-case ratio. We study the trade-off between the amount of model information exploited by a control design strategy and the best possible closed-loop performance. We generalize this setup to structured dynamic state-feedback controllers for H_2-performance. Surprisingly, the optimal control design strategy with limited model information is still a static one. This is the case even though the optimal decentralized state-feedback controller with full model information is dynamic. Finally, we discuss the design of dynamic controllers for disturbance accommodation under limited model information. This problem is of special interest because the best limited model information control design in this case is a dynamic control design strategy. The optimal controller can be separated into a static feedback law and a dynamic disturbance observer. For constant disturbances, it is shown that this structure corresponds to proportional-integral control.
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| 26. |
- Flärdh, Oscar, 1980-
(författare)
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Modelling, analysis and experimentation of a simple feedback scheme for error correction control
- 2007
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Data networks are an important part in an increasing number of applications with real-time and reliability requirements. To meet these demands a variety of approaches have been proposed. Forward error correction, which adds redundancy to the communicated data, is one of them. However, the redundancy occupies communication bandwidth, so it is desirable to control the amount of redundancy in order to achieve high reliability without adding excessive communication delay. The main contribution of the thesis is to formulate the problem of adjusting the redundancy in a control framework, which enables the dynamic properties of error correction control to be analyzed using control theory. The trade-off between application quality and resource usage is captured by introducing an optimal control problem. Its dependence on the knowledge of the network state at the transmission side is discussed. An error correction controller that optimizes the amount of redundancy without relying on network state information is presented. This is achieved by utilizing an extremum seeking control algorithm to optimize the cost function. Models with varying complexity of the resulting feedback system are presented and analyzed. Conditions for convergence are given. Multiple-input describing function analysis is used to examine periodic solutions. The results are illustrated through computer simulations and experiments on a wireless sensor network.
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| 27. |
- Gouveia Fonseca, Joana Filipa, 1994-
(författare)
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Cooperative Multi-Vehicle Circumnavigation and Tracking of a Mobile Target
- 2020
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- A multi-vehicle system is composed of interconnected vehicles coordinated to complete a certain task. When controlling such systems, the aim is to obtain a coordinated behaviour through local interactions among vehicles and the surrounding environment.One motivating application is the monitoring of algal blooms; this phenomenon occurs frequently and has a substantial negative effect on the environment such as large-scale mortality of fish. In this thesis, we investigate control of multiple unmanned surface vehicles (USVs) for mobile target circumnavigation and tracking, where the target can be an algal bloom area.A protocol based on local measurements provided by the vehicles is developed to estimate the target's location and shape.Then a control strategy is derived that brings the vehicle system to the target while forming a regular polygon.More precisely, we first consider the problem of tracking a mobile target while circumnavigating it with multiple USVs. A satellite image indicates the initial location of the target, which is supposed to have an irregular dynamic shape well approximated by a circle with moving center and varying radius. Each USV is capable of measuring its distance to the boundary of the target and to its center. We design an adaptive protocol to estimate the circle's parameters based on the local measurements. A control protocol then brings the vehicles towards the target boundary as well as spreads them equidistantly along the boundary. The protocols are proved to converge to the desired state. Simulated examples illustrate the performance of the closed-loop system.Secondly, we assume that the vehicles can only measure the distance to the boundary of the target and not to its center. We propose a decentralised least-squares method for target estimation suitable for circular targets. Convergence proofs are given for also this case. An example using simulated algal bloom data shows that the method works well under realistic settings.Finally, we investigate how to extend our protocols to a quite general irregular mobile target. In this case, each vehicle communicates only with its two nearest neighbors and estimates the curvature of the target boundary using their collective measurements. We validate the performance of the protocol under various settings and target shapes through a numerical study.
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| 28. |
- Henriksson, Erik, 1982-
(författare)
-
Compensating for Unreliable Communication Links in Networked Control Systems
- 2009
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Control systems utilizing wireless sensor and actuator networks can be severely affectedby the properties of the communication links. Radio fading and interferencemay cause communication losses and outages in situations when the radio environmentis noisy and low transmission power is desirable. This thesis proposes amethod to compensate for such unpredictable losses of data in the feedback controlloop by introducing a predictive outage compensator (POC). The POC is a filter tobe implemented at the receiver sides of networked control systems where it generatesartificial samples when data are lost. If the receiver node does not receive thedata, the POC suggests a command based on the history of past data. It is shownhow to design, tune and implement a POC. Theoretical bounds and simulationresults show that a POC can improve the closed-loop control performance undercommunication losses considerably. We provide a deterministic and a stochasticmethod to synthesize POCs. Worst-case performance bounds are given that relatethe closed-loop performance with the complexity of the compensator. We also showthat it is possible to achieve good performance with a low-order implementationbased on Hankel norm approximation. Tradeoffs between achievable performance,communication loss length, and POC order are discussed. The results are illustratedon a simulated example of a multiple-tank process. The thesis is concludedby an experimental validation of wireless control of a physical lab process. Herethe controller and the physical system are separated geographically and interfacedthrough a wireless medium. For the remote control we use a hybrid model predictivecontroller. The results reflect the difficulties in wireless control as well as theyhighlight the flexibility and possibilities one obtains by using wireless instead of awired communication medium.
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| 29. |
- Henriksson, Erik, 1982-
(författare)
-
Predictive Control for Wireless Networked Systems in Process Industry
- 2014
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Wireless networks in industrial process control enable new system architectures and designs. However, wireless control systems can be severely affected by the imperfections of the communication links. This thesis proposes new methods to handle such imperfections by adding additional components in the control loop, or by adapting sampling intervals and control actions.First, the predictive outage compensator is proposed. It is a filter which is implemented at the receiver side of networked control systems. There it generates predicted samples when data are lost, based on past data. The implementation complexity of the predictive outage compensator is analyzed. Simulation and experimental results show that it can considerably improve the closed-loop control performance under communication losses.The thesis continues with presenting an algorithm for controlling multiple processes on a shared communication network, using adaptive sampling intervals. The methodology is based on model predictive control, where the controller jointly decides the optimal control signal to be applied as well as the optimal time to wait before taking the next sample. The approach guarantees conflict-free network transmissions for all controlled processes. Simulation results show that the presented control law reduces the required amount of communication, while maintaining control performance.The third contribution of the thesis is an event-triggered model predictive controller for use over a wireless link. The controller uses open-loop optimal control, re-computed and communicated only when the system behavior deviates enough from a prediction. Simulations underline the methods ability to significantly reduce computation and communication effort, while guaranteeing a desired level of system performance.The thesis is concluded by an experimental validation of wireless control for a physical lab process. A hybrid model predictive controller is used, connected to the physical system through a wireless medium. The results reflect the advantages and challenges in wireless control.
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| 30. |
- Johansson, Anders, 1975-
(författare)
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Context dependent adaptation of biting behavior in human
- 2014
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The focus of this thesis was to study an action that humans perform regularly, namely, to hold a morsel between the teeth and split it into smaller pieces. Three different issues related to this biting behavior were addressed: (1) the effect of reduced periodontal tissues on food holding and splitting behavior; (2) the behavioral consequences of performing different bite tasks with different functional requirements, i.e., to split a peanut half resting on a piece of chocolate or to split both the peanut and the chocolate; and (3) the reflex modulations resulting from such a change in the intended bite action. The main conclusions from the experimental studies were the following:First, periodontitis, an inflammatory disease that destroys the periodontal ligaments and the embedded periodontal mechanoreceptors, causes significant impairments in the masticatory ability: the manipulative bite forces when holding a morsel are elevated compared to a matched control population and the bite force development prior to food split is altered. These changes are likely due to a combination of reduced sensory information from the damaged ligaments and to changes in the bite strategy secondary to the unstable oral situation.Second, people exploit the anatomy of jaw-closing muscles to regulate the amount of bite force that dissipates following a sudden unloading of the jaw. Such control is necessary because without mechanisms that quickly halt jaw-closing movements after sudden unloading, the impact forces when the teeth collide could otherwise damage both the teeth and related soft tissues. Splitting a piece of chocolate, for instance, regularly requires >100N of bite force and the jaws collide within 5 ms of a split. On the other hand, when biting through heterogeneous food, the bite force needs to be kept high until the whole morsel is split. The required regulation is achieved by differentially engaging parts of the masseter muscles along the anteroposterior axis of the jaw to exploit differences between muscle portions in their bite force generating capacity and muscle shortening velocity.Finally, the reflex evoked by suddenly unloading the jaw—apparent only after the initial bite force dissipation—is modulated according to the bite intention. That is, when the intention is to bite through food items with multiple layers, the reflex response in the jaw opening muscles following a split is small, thus minimizing the bite force reduction. In contrast, when the intention is to rapidly decrease the bite force once a split has occurred, the reflex response is high. This pattern of reflex modulation is functionally beneficial when biting through heterogeneous food in a smooth manner.The presented studies show the significance of integrating cognitive, physiological and anatomical aspects when attempting to understand human masticatory control.
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| 31. |
- Minnema Lindhé, Magnus, 1978-
(författare)
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Communication-Aware Motion Planning for Mobile Robots
- 2012
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Mobile robots have found numerous applications in recent years, in areas such as consumer robotics, environmental monitoring, security and transportation. For information dissemination, multi-robot cooperation or operator intervention, reliable communications are important. The combination of communication constraints with other requirements in robotics, such as navigation and obstacle avoidance is called communication-aware motion planning. To facilitate integration, communication-aware methods should fit into traditional layered architectures of motion planning. This thesis contains two main contributions, applicable to such an architecture. The first contribution is to develop strategies for exploiting multipath fading while following a reference trajectory. By deviating from the reference, a robot can stop and communicate at positions with high signal strength, trading tracking performance for link quality. We formulate this problem in three different ways: First we maximize the link quality, subject to deterministic bounds on the tracking error. We control the velocity based on the position and channel quality. Second, we consider probabilistic tracking error bounds and develop a cascaded control architecture that performs time-triggered stopping while regulating the tracking error. Third, we formulate a hybrid optimal control problem, switching between standing still to communicate and driving to improve tracking. The resulting channel quality is analyzed and we perform extensive experiments to validate the communication model and compare the proposed methods to the nominal case of driving at constant velocity. The results show good agreement with the model and improvements of over 100% in the throughput when the channel quality is low. The second contribution is to plan velocities for a group of N robots, moving along pre-determined paths through an obstacle field. Robots can only communicate if they have an unobstructed line of sight, and the problem is to maintain connectivity while traversing the paths. This is mapped to motion planning in an N-dimensional configuration space. We propose and investigate two solutions, using a rapidly exploring random tree (RRT) and an exact method inspired by cell decomposition. The RRT method scales better with the problem size than the exact method, which has a worst-case time complexity that is exponential in the number of obstacles. But the randomization in the RRT method makes it difficult to set a timeout for the solver, which runs forever if a problem instance is unsolvable. The exact method, on the other hand, detects unsolvable problem instances in finite time. The thesis demonstrates, both in theory and experiments, that mobile robots can improve communications by planning trajectories that maintain visual connectivity, or by exploiting multipath fading when there is no line of sight. The proposed methods are well suited for integration in a layered motion planning architecture.
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| 32. |
- Palumbo, Alessandro
(författare)
-
Skriftsystem i förändring : en grafematisk och paleografisk studie av de svenska medeltida runinskrifterna
- 2018
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The aim of this dissertation is to study the development of Swedish runic writing during the Middle Ages in terms of runic shapes and orthography. It comprises three related investigations. A preparatory investigation surveys the runic shapes employed during the Middle Ages in Sweden. The second investigation aims to identify the graphematic distinctions used and developed during the Middle Ages, the chronological and geographical patterns evident in the use of the system of runic writing as well as the phonological conclusions which can be drawn from the graphematic analysis. The third and final section of the dissertation focuses on the use of certain long-branch and short-twig runes from a palaeographic perspective.The Swedish runic writing system changed during the course of the Middle Ages and varied in accordance with different factors, including chronology as well as local traditions. Västergötland, especially the area of Falbygden, appears the most prominent Swedish centre of innovation. As regards to the vowel system, the new grapheme <æ> was consistently used there in the 12th century, whereas not even in the 13th century was it completely established in Uppland. The grapheme <ø> instead was introduced only in the beginning of the 13th century and does not appear to have ever been consistently employed in any province. The analysis of the vowel system, in particular as regards to the unrounded front vowels, has also revealed several graphematic changes that can be explained phonologically and attest to ongoing dialectal developments.Even though no dotted graph type for consonants is used without exception and the double marking of long consonants is employed in less than a third of the attestations of long consonants, greater consistency in the use of both practices can be observed during the 13th century than during earlier periods. Also in this case, the development proceeded more rapidly in Västergötland than in Uppland. Among other provinces, several new features also occur in Småland. Also the inscriptions in Östergötland attest to an advanced use of writing, but the mediaeval innovations there seem to be younger than in Västergötland and western Småland, which indicates that they spread to more easterly areas at a later stage.Medieval innovations such as dotted runes with a consonantal value seem to have their origin and focus in East Scandinavia. Many of the earliest attestations occur on coins minted in Lund, and it is possible that they spread from there to Småland and Västergötland. In other cases, for example as regards the dotted f-rune, these Swedish areas may themselves have acted as centres of innovation. Here, closer contact between carvers and the Church and the introduction of the Roman script may have contributed to creating a more dynamic writing culture.As regards the alleged existence of a "completely dotted runic alphabet", the analyses in this dissertation have shown that there was no uniform use of those features which are often maintained to be characteristic of the completely dotted rune row. These results make it clear that the introduction of innovations could not have occurred via a reform and confirm that no completely dotted rune row was ever established.
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| 33. |
- Park, Pangun, 1980-
(författare)
-
Modeling, Analysis and Design of Wireless Sensor Network Protocols
- 2011
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Wireless sensor networks (WSNs) have a tremendous potential to improve the efficiencyof many systems, for instance, in building automation and process control.Unfortunately, the current technology does not offer guaranteed energy efficiencyand reliability for closed-loop stability. The main contribution of this thesis is toprovide a modeling, analysis, and design framework for WSN protocols used in controlapplications. The protocols are designed to minimize the energy consumption ofthe network, while meeting reliability and delay requirements from the applicationlayer. The design relies on the analytical modeling of the protocol behavior.First, modeling of the slotted random access scheme of the IEEE 802.15.4medium access control (MAC) is investigated. For this protocol, which is commonlyemployed in WSN applications, a Markov chain model is used to derive theanalytical expressions of reliability, delay, and energy consumption. By using thismodel, an adaptive IEEE 802.15.4 MAC protocol is proposed. The protocol designis based on a constrained optimization problem where the objective function is theenergy consumption of the network, subject to constraints on reliability and packetdelay. The protocol is implemented and experimentally evaluated on a test-bed. Experimentalresults show that the proposed algorithm satisfies reliability and delayrequirements while ensuring a longer lifetime of the network under both stationaryand transient network conditions.Second, modeling and analysis of a hybrid IEEE 802.15.4 MAC combining theadvantages of a random access with contention with a time division multiple access(TDMA) without contention are presented. A Markov chain is used to model thestochastic behavior of random access and the deterministic behavior of TDMA.The model is validated by both theoretical analysis and Monte Carlo simulations.Using this new model, the network performance in terms of reliability, averagepacket delay, average queueing delay, and throughput is evaluated. It is shown thatthe probability density function of the number of received packets per superframefollows a Poisson distribution. Furthermore, it is determined under which conditionsthe time slot allocation mechanism of the IEEE 802.15.4 MAC is stable.Third, a new protocol for control applications, denoted Breath, is proposedwhere sensor nodes transmit information via multi-hop routing to a sink node. Theprotocol is based on the modeling of randomized routing, MAC, and duty-cycling.Analytical and experimental results show that Breath meets reliability and delayrequirements while exhibiting a nearly uniform distribution of the work load. TheBreath protocol has been implemented and experimentally evaluated on a test-bed.Finally, it is shown how the proposed WSN protocols can be used in controlapplications. A co-design between communication and control application layers isstudied by considering a constrained optimization problem, for which the objectivefunction is the energy consumption of the network and the constraints are thereliability and delay derived from the control cost. It is shown that the optimaltraffic load when either the communication throughput or control cost are optimizedis similar.
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| 34. |
- Park, Pangun, 1980-
(författare)
-
Protocol Design for Control Applications using Wireless Sensor Networks
- 2009
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Given the potential benefits offered by wireless sensor networks(WSNs), they are becoming an appealing technology for process,manufacturing, and industrial control applications. In thisthesis, we propose a novel approach to WSN protocol design forcontrol applications. The protocols are designed to minimize theenergy consumption of the network, while meeting reliability andpacket delay requirements. The parameters of the protocol areselected by solving a constrained optimization problem, where theobjective is to minimize the energy consumption and theconstraints are the probability of successful packet reception andthe communication delay. The proposed design methodology allowsone to perform a systematic tradeoff between the controlrequirements of the application and the network energyconsumption. An important step in the design process is thedevelopment of analytical expressions of the performanceindicators. We apply the proposed approach to optimize the networkfor various communication protocols.In Paper A, we present an adaptive IEEE 802.15.4 for energyefficient, reliable, and low latency packet transmission. Thebackoff mechanisms and retry limits of the standard are adapted tothe estimated channel conditions. Numerical results show that theproposed protocol enhancement is efficient and ensures a longerlifetime of the network under different conditions. Furthermore,we investigate the robustness and sensitivity of the protocol topossible errors during the estimation process. In Paper B, we investigate the design and optimization ofduty-cycled WSNs with preamble sampling over IEEE 802.15.4. Theanalytical expressions of performance indicators are developed andused to optimize the duty-cycle of the nodes to minimize energyconsumption while ensuring low latency and reliable packettransmissions. The optimization results in a significant reductionof the energy consumption compared to existing solutions.The cross-layer protocol called Breath is proposed in Paper C. Theprotocol is suitable for control applications by using theconstrained optimization framework proposed in the thesis. It isbased on randomized routing, CSMA/CA MAC, and duty-cycling. Theprotocol is implemented and experimentally evaluated on a testbed,and it is compared with a standard IEEE 802.15.4 solution. Breathexhibits a good distribution of the work load among the networknodes, and ensures a long network lifetime.
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| 35. |
- Ramesh, Chithrupa, 1982-
(författare)
-
Contention-based Multiple Access Architectures for Networked Control Systems
- 2011
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Networked Control Systems (NCSs) use a wireless network for communication between sensors and controllers, and require a Medium Access Controller (MAC) to arbitrate access to the shared medium. Traditionally, a MAC for control systems is chosen primarily based on the delay it introduces in the closed loop. This thesis focuses on the design of a contention-based MAC, in a time-varying, resource-constrained network for closed loop systems. In this thesis, we advocate the use of a state-aware MAC, as opposed to an agnostic MAC, for NCSs. A state-aware MAC uses the state of the plant to influence access to the network. The state-aware policy is realized using two different approaches in the MAC: a regulatory formulation and an adaptive prioritization. Our first approach is a regulatory MAC, which serves to reduce the traffic in the network. We use a local state-based scheduler to select a few critical data packets to send to the MAC. We analyze the impact of such a scheduler on the closed loop system, and show that there is a dual effect for the control signal, which makes determining the optimal controller difficult. We also identify restrictions on the scheduling criterion that result in a separation of the scheduler, observer and controller designs. Our second approach is a prioritized MAC that uses state-based priorities called Attentions, to determine access to the network. We use a dominance protocol called tournaments, to evaluate priorities in a contention-based setting, and analyze the resulting performance of the MAC. We also consider a NCS that uses a wireless multihop mesh network for communication between the controller and actuator. We design an optimal controller, which uses packet delivery predictions from a recursive Bayesian network estimator.
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| 36. |
- Rosell, Erik, 1975-
(författare)
-
Entreprenörskap som kommunikativ handling : skapande av interaktion, uppmärksamhet och manifestationer
- 2013
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The purpose of the present thesis is to create an understanding of entrepreneurship interpreted as communicative action. This is done through reflections on an interactive study that was planned and conducted together with members of a civic network-organization called Societal Change in Practice (SIP). According to Habermas, civic organizations are ideally characterized by a communicative rationality that enables them to organize informal public spheres; that is, arenas in social life where individuals can come together to discuss and act upon societal problems or opportunities that they have experienced in their private life-spheres.I have actively participated in three ventures with members from SIP. The first venture revolves around my own and members from SIP’s respective practices as education coordinators. Based on our common interest in education and learning in relation to entrepreneurship, we planned and conducted a series of joint activities that also involved our respective student groups. The activities are interpreted based on my own personal experiences as a researcher participating in a project that requires commitment and responsibility. The second venture involves the creation of a local community magazine that highlights examples of civic initiatives in two municipalities. The production of the magazine is interpreted as an example of how SIP creates public opinion in the local community. The third event relates to the organization of a conference on the subject of youth and digital media. The main message of the conference is interpreted in terms of a manifestation of what the public sphere can accomplish, or as a reaction in defense of a well-functioning public sphere in society.The methodological contribution of the thesis is its definition of three interactive research roles based on my own interaction as a researcher in different kinds of ventures. Based on a theatrical metaphor, I argue that the researcher can participate as one of the directors of a venture, as a member of the ensemble that performs a venture, or as a member of the audience that observes an event.The theoretical contribution of the study is that it shows how Habermas’ theory of communicative action can be modified and made useful as a theoretical frame of reference for studying entrepreneurship in civil society. Entrepreneurship is understood as a way to vitalize the informal public sphere, thereby influencing society as a whole and not just its economy.
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| 37. |
- Sahlholm, Per, 1979-
(författare)
-
Distributed Road Grade Estimation for Heavy Duty Vehicles
- 2011
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- An increasing need for goods and passenger transportation drives continued worldwide growth in traffic. As traffic increases environmental concerns, traffic safety, and cost efficiency become ever more important. Advancements in microelectronics open the possibility to address these issues through new advanced driver assistance systems. Applications such as predictive cruise control, automated gearbox control, predictive front lighting control, and hybrid vehicle state-of-charge control decrease the energy consumption of vehicles and increase the safety. These control systems can benefit significantly from preview road grade information. This information is currently obtained using specialized survey vehicles, and is not widely available. This thesis proposes new methods to obtain road grade information using on-board sensors. The task of creating road grade maps is addressed by the proposal of a framework where vehicles using a road network collect the necessary data for estimating the road grade. The estimation can then be carried out locally in the vehicle, or in the presence of a communication link to the infrastructure, centrally. In either case the accuracy of the map increases over time, and costly road surveys can be avoided. This thesis presents a new distributed method for creating accurate road grade maps for vehicle control applications. Standard heavy duty vehicles in normal operation are used to collect measurements. Estimates from multiple passes along a road segment are merged to form a road grade map, which improves each time a vehicle retraces a route. The design and implementation of the road grade estimator are described, and the performance is experimentally evaluated using real vehicles. Three different grade estimation methods, based on different assumption on the road grade signal, are proposed and compared. They all use data from sensors that are standard equipment in heavy duty vehicles. Measurements of the vehicle speed and the engine torque are combined with observations of the road altitude from a GPS receiver, using vehicle and road models. The operation of the estimators is adjusted during gearshifts, braking, and poor satellite coverage, to account for variations in sensor and model reliability. The estimated error covariances of the road grade estimates are used together with their absolute positions to update a stored road grade map. Highway driving trials show that the proposed estimators produce accurate road grade data. The estimation performance improves as the number of road segment traces increases. A vehicle equipped with the proposed system will rapidly develop a road grade map for its area of operation. Simulations show that collaborative generation of the third dimension for a pre-existing large area two-dimensional map is feasible. The experimental results indicate that road grade estimates from the proposed methods are accurate enough to be used in predictive vehicle control systems to enhance safety, efficiency, and driver comfort in heavy duty vehicles. The grade estimators may also be used for on-line validation of road grade information from other sources. This is important in on-board applications, since the envisioned control applications can degrade vehicle performance if inaccurate data are used.
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| 38. |
- Teixeira, André, 1986-
(författare)
-
Toward Secure and Reliable Networked Control Systems
- 2011
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Security and reliability are essential properties in Networked Control Systems (NCS), which are increasingly relevant in several important applications such as he process industry and electric power networks. The trend towards using non-proprietary and pervasive communication and information technology (IT) systems, such as the Internet and wireless communications, may result in NCS being vulnerable to cyber attacks. Traditional IT security does not consider the interdependencies between the physical components and the cyber realm of IT systems. Moreover, the control theoretic approach is not tailored to handle IT threats, focusing instead on nature-driven events. This thesis addresses the security and reliability of NCS, with a particular focus on power system control and supervision, contributing towards establishing a framework capable of analyzing and building NCS security. In our first contribution, the cyber security of the State Estimator (SE) in power networks is analyzed under malicious sensor data corruption attacks. The set of stealthy attacks bypassing current Bad Data Detector (BDD) schemes is characterized for the nonlinear least squares SE, assuming the attacker has accurate knowledge of a linearized model. This result is then extended to uncertain models using the geometric properties of the SE and BDD. Using the previous results, a security framework based on novel rational attack models is proposed, in which the minimum-effort attack policy is cast as a constrained optimization problem. The optimal attack cost is interpreted as a security metric, which can be used in the design of protective schemes to strengthen security. The features of the proposed framework are illustrated through simulation examples and experiments. As our second contribution, we analyze the behavior of the Optimal Power Flow (OPF) algorithmin the presence of stealthy sensor data corruption and the resulting consequences to the power network operation. In particular, we characterize the set of attacks that may lead the operator to apply the erroneous OPF recommendation and propose an analytical expression for the optimal solution of a simplified OPF problem with corrupted measurements. A novel impact-aware security metric is proposed based on these results, considering both the impact on the system and the attack cost. A small analytical example and numerical simulations are presented to illustrate and motivate our contributions. The third contribution considers the design of distributed schemes for fault detection and isolation in large-scale networks of second-order systems. The proposed approach is based on unknown input observers and exploits the networked structure of the system. Conditions are given on what local measurements should be available for the proposed scheme to be feasible. Infeasibility results with respect to available measurements and faults are also provided. In addition, methods to reduce the complexity of the proposed scheme are discussed, thus ensuring the scalability of the solution. Applications to power networks and robotic formations are presented through numerical examples.
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| 39. |
- Turri, Valerio, 1987-
(författare)
-
Fuel-efficient and safe heavy-duty vehicle platooning through look-ahead control
- 2015
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The operation of groups of heavy-duty vehicles at small inter-vehicular distances, known as platoons, lowers the overall aerodynamic drag and, therefore, reduces fuel consumption and greenhouse gas emissions. Experimental tests conducted on a flat road and without traffic have shown that platooning has the potential to reduce the fuel consumption up to 10%. However, platoons are expected to drive on public highways with varying topography and traffic. Due to the large mass and limited engine power of heavy-duty vehicles, road slopes can have a significant impact on feasible and optimal speed profiles. Therefore, maintaining a short inter-vehicular distance without coordination can result in inefficient or even infeasible speed trajectories. Furthermore, external traffic can interfere by affecting fuel-efficiency and threatening the safety of the platooning vehicles.This thesis addresses the problem of safe and fuel-efficient control for heavy-duty vehicle platooning. We propose a hierarchical control architecture that splits this complex control problem into two layers. The layers are responsible for the fuel-optimal control based on look-ahead information on road topography and the real-time vehicle control, respectively. The top layer, denoted the platoon coordinator, relies on a dynamic programming framework that computes the fuel-optimal speed profile for the entire platoon. The bottom layer, denoted the vehicle control layer, uses a distributed model predictive controller to track the optimal speed profile and the desired inter-vehicular spacing policy. Within this layer, constraints on the vehicles' states guarantee the safety of the platoon. The effectiveness of the proposed controller is analyzed by means of simulations of several realistic scenarios. They suggest a possible fuel saving of up to 12% for the follower vehicles compared to the use of existing platoon controllers. Analysis of the simulation results shows how the majority of the fuel saving comes from a reduced usage of vehicles brakes.A second problem addressed in the thesis is model predictive control for obstacle avoidance and lane keeping for a passenger car. We propose a control framework that allows to control the nonlinear vehicle dynamics with linear model predictive control. The controller decouples the longitudinal and lateral vehicle dynamics into two successive stages. First, plausible braking and throttle profiles are generated. Second, for each profile, linear time-varying models of the lateral dynamics are derived and used to formulate a collection of linear model predictive control problems. Their solution provides the optimal control input for the steering and braking actuators. The performance of the proposed controller has been evaluated by means of simulations and real experiments.
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| 40. |
- Yi, Xinlei, 1990-
(författare)
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Distributed Optimization and Control : Primal--Dual, Online, and Event-Triggered Algorithms
- 2020
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In distributed optimization and control, each network node performs local computation based on its own information and information received from its neighbors through a communication network to achieve a global objective. Although many distributed optimization and control algorithms have been proposed, core theoretical problems with important practical relevance remain. For example, what convergence properties can be obtained for nonconvex problems? How to tackle time-varying cost and constraint functions? Can these algorithms work under limited communication resources? This thesis contributes to answering these questions by providing new algorithms with better convergence rates under less information exchange than existing algorithms. It consists of three parts.In the first part, we consider distributed nonconvex optimization problems. It is hard to solve these problems and often only stationary points can be found. We propose distributed primal--dual optimization algorithms under different information feedback settings. Specifically, when full-information feedback or deterministic zeroth-order oracle feedback is available, we show that the proposed algorithms converge sublinearly to a stationary point if each local cost function is smooth. They converge linearly to a global optimum if the global cost function also satisfies the Polyak--{\L}ojasiewicz condition. This condition is weaker than strong convexity, which is a standard condition in the literature for proving linear convergence of distributed optimization algorithms. When stochastic gradient feedback or stochastic zeroth-order oracle feedback is available, we show that the proposed algorithms achieve linear speedup convergence rates, meaning that the convergence rates decrease linearly with the number of computing nodes.In the second part, distributed online convex optimization problems are considered. For such problems, the cost and constraint functions are revealed at the end of each time slot. We focus on time-varying coupled inequality constraints and time-varying directed communication networks. We propose one primal--dual dynamic mirror descent algorithm and two bandit primal--dual algorithms. It is shown that these distributed algorithms achieve the same sublinear regret and constraint violation bounds as existing centralized algorithms.In the third and final part, in order to achieve a common control objective for a networked system, we propose distributed event-triggered algorithms to reduce the amount of information exchanged. Specifically, we propose dynamic event-triggered control algorithms to solve the average consensus problem for first-order systems, the global consensus problem for systems with input saturation, and the formation control problem with connectivity preservation for first- and second-order systems. We show that these algorithms do not exhibit Zeno behavior and that they achieve exponential convergence rates.
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| 41. |
- Åberg, Pelle, 1977-
(författare)
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Translating Popular Education : Civil Society Cooperation between Sweden and Estonia
- 2008
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- By their very nature, attempts by civil society organizations to promote democracy abroad involve cooperation and contact across the borders of nation states. The dissemination of the ideas and practices of the promoters is often essential; in the case of Swedish democracy promotion, popular education or folkbildning has been important.This thesis investigates the dissemination of ideas and practices by civil society organizations in Sweden and Estonia in the field of popular education. More specifically, a number of projects run by member organizations of the Swedish study association ABF (Workers’ Educational Association) and member organizations of the Estonian AHL (Open Education Association) are studied. These projects are also part of democracy-promoting activities whose funding comes mainly from Swedish donor agencies. The thesis aims to understand not only what has been spread but also how ideas and practices have been translated to fit the Estonian context. The primary theoretical concept used is therefore translation, even though one aim of the thesis is to systematize the field of study concerning processes of dissemination and to enrich the concept of translation with the aid of previous work on diffusion and socialization.The main contribution of the thesis consists of the in-depth study of cooperation between Swedish and Estonian popular education organizations. It also contributes to the theoretical development of the concept of translation by identifying different phases in this kind of process and important elements of these phases. Popular education is an important part of Swedish democracy promotion and this study also contributes to this rather undertheorized subject, and also to the discussions of transnational civil society cooperation, by demonstrating an analytical framework that can be of use in future research into these issues.
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| 42. |
- Adaldo, Antonio
(författare)
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Event-triggered and cloud-support control of multi-robot systems
- 2018
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In control of multi-robot systems, the aim is to obtain a coordinated behavior through local interactions among the robots. A multi-agent system is an abstract model of a multi-robot system. In this thesis, we investigate multi-agent systems where inter-agent communication is modeled by discrete events triggered by conditions on the internal state of the agents. We consider two models of communication. In the first model, two agents exchange information directly with each other. In the second model, all information is exchanged asynchronously over a shared repository. Four contributions on control algorithms for multi-agent systems are offered in the thesis. The first contribution is an event-triggered pinning control algorithm for a network of agents with nonlinear dynamics and time-varying topology. Pinning control is a strategy to steer the behavior of the system in a desired manner by controlling only a small fraction of the agents. We express the controllability of the network in terms of an average value of the network connectivity over time, and we show that all the agents can be driven to a desired reference trajectory. The second contribution is a control algorithm for multi-agent systems where inter-agent communication is substituted with a shared remote repository hosted on a cloud. The communication between each agent and the cloud is modeled as a sequence of events scheduled recursively by the agent. We quantify the connectivity of the network and we show that it is possible to synchronize the multi-agent system to the same state trajectory, while guaranteeing that two consecutive cloud accesses by the same agent are separated by a lower-bounded time interval. The third contribution is a family of distributed controllers for coverage and surveillance tasks with a network of mobile agents with anisotropic sensing patterns. We develop an abstract model of the environment under inspection and define a measure of the coverage attained by the sensor network. We show that the network attains nondecreasing coverage, and we characterize the equilibrium configurations of the network. The fourth contribution is a distributed, cloud-supported control algorithm for inspection of 3D structures with a network of mobile sensing agents, similar to those considered in the third contribution. We develop an abstract model of the structure to inspect and quantify the degree of completion of the inspection. We demonstrate that, under the proposed algorithm, the network is guaranteed to complete the inspection in finite time. All results presented in the thesis are corroborated by numerical simulations and sometimes by experiments with aerial robotic platforms. The experiments show that the theory and methods developed in the thesis are of practical relevance.
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| 43. |
- Aguiar, Miguel
(författare)
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Learning flow functions : architectures, universal approximation and applications to spiking systems
- 2024
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Learning flow functions of continuous-time control systems is considered in this thesis. The flow function is the operator mapping initial states and control inputs to the state trajectories, and the problem is to find a suitable neural network architecture to learn this infinite-dimensional operator from measurements of state trajectories. The main motivation is the construction of continuous-time simulation models for such systems. The contribution is threefold.We first study the design of neural network architectures for this problem, when the control inputs have a certain discrete-time structure, inspired by the classes of control inputs commonly used in applications. We provide a mathematical formulation of the problem and show that, under the considered input class, the flow function can be represented exactly in discrete time. Based on this representation, we propose a discrete-time recurrent neural network architecture. We evaluate the architecture experimentally on data from models of two nonlinear oscillators, namely the Van der Pol oscillator and the FitzHugh-Nagumo oscillator. In both cases, we show that we can train models which closely reproduce the trajectories of the two systems.Secondly, we consider an application to spiking systems. Conductance-based models of biological neurons are the prototypical examples of this type of system. Because of their multi-timescale dynamics and high-frequency response, continuous-time representations which are efficient to simulate are desirable. We formulate a framework for surrogate modelling of spiking systems from trajectory data, based on learning the flow function of the system. The framework is demonstrated on data from models of a single biological neuron and of the interconnection of two neurons. The results show that we are able to accurately replicate the spiking behaviour.Finally, we prove an universal approximation theorem for the proposed recurrent neural network architecture. First, general conditions are given on the flow function and the control inputs which guarantee that the architecture is able to approximate the flow function of any control system with arbitrary accuracy. Then, we specialise to systems with dynamics given by a controlled ordinary differential equation, showing that the conditions are satisfied whenever the equation has a continuously differentiable right-hand side, for the control input classes of interest.
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| 44. |
- Alcala, Karine, et al.
(författare)
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The relationship between blood pressure and risk of renal cell carcinoma
- 2022
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Ingår i: International Journal of Epidemiology. - : Oxford University Press (OUP). - 1464-3685 .- 0300-5771. ; 51:4, s. 1317-1327
-
Tidskriftsartikel (refereegranskat)abstract
- BACKGROUND: The relation between blood pressure and kidney cancer risk is well established but complex and different study designs have reported discrepant findings on the relative importance of diastolic blood pressure (DBP) and systolic blood pressure (SBP). In this study, we sought to describe the temporal relation between diastolic and SBP with renal cell carcinoma (RCC) risk in detail.METHODS: Our study involved two prospective cohorts: the European Prospective Investigation into Cancer and Nutrition study and UK Biobank, including >700 000 participants and 1692 incident RCC cases. Risk analyses were conducted using flexible parametric survival models for DBP and SBP both separately as well as with mutuality adjustment and then adjustment for extended risk factors. We also carried out univariable and multivariable Mendelian randomization (MR) analyses (DBP: ninstruments = 251, SBP: ninstruments = 213) to complement the analyses of measured DBP and SBP.RESULTS: In the univariable analysis, we observed clear positive associations with RCC risk for both diastolic and SBP when measured ≥5 years before diagnosis and suggestive evidence for a stronger risk association in the year leading up to diagnosis. In mutually adjusted analysis, the long-term risk association of DBP remained, with a hazard ratio (HR) per standard deviation increment 10 years before diagnosis (HR10y) of 1.20 (95% CI: 1.10-1.30), whereas the association of SBP was attenuated (HR10y: 1.00, 95% CI: 0.91-1.10). In the complementary multivariable MR analysis, we observed an odds ratio for a 1-SD increment (ORsd) of 1.34 (95% CI: 1.08-1.67) for genetically predicted DBP and 0.70 (95% CI: 0.56-0.88) for genetically predicted SBP.CONCLUSION: The results of this observational and MR study are consistent with an important role of DBP in RCC aetiology. The relation between SBP and RCC risk was less clear but does not appear to be independent of DBP.
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| 45. |
- Andersson, Therese, 1978-
(författare)
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En gemensam europeisk skogspolitik? : En integrationsteoretisk studie av ett politikområde på tillväxt
- 2007
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This dissertation examines efforts to integrate a “new” policy sector – forest and forestry – into the European Union (EU). There is currently no legal foundation for a common forest policy and some member states (not least Sweden), as well as parts of the forestry sector, have been strongly opposed to one. At the same time, administrative units and structures within the EU have been created and they and some member states have promoted a common policy. This raises the question how can we understand and explain this?The purpose of this dissertation is to problematise, map and analyse mainly Swedish actors’ attitudes to efforts to create a common forest policy within the EU. The study is based on neofunctionalism, which is a classic theory of integration, but it uses newer theorising (from intergovernmentalism and modern versions of neofunctionalism) to address some of the weaknesses of the approach.I investigate the role, preferences and strategies of the main actors. This includes EU institutions and member states. I also map European industry interests and other associations, interest groups and active networks and study their role in the process. In these multi-national settings, I pay particular (although not exclusive) attention to their Swedish members. Within Sweden, I examine how governmental and non-governmental forest actors behave vis-à-vis the EU.The empirical investigation shows that some of Swedish actors, for example the private forest owners’ organisation and forest industries associations, have change their preferences and strate gies over time. They have come to believe that whether they like it or not, other policy areas affect forest and forestry both directly and indirectly. Because of this, they now take the position that it is better to promote a limited European forest policy rather than remaining aloof and risk the creation of a much more comprehensive and centralised policy. At the same time (and for now at least), the Swedish government and most party politicians remain opposed to any attempt to formalise a forest policy within the EU.This study contributes new knowledge about how new policy areas become integrated within EU, including knowledge about the roles that different actors can have in such processes. The results are of interest to researchers, decision makers and the interested public. They can also influence thinking about Sweden’s influence in, and relation to, EU forest policy.Based on the empirical results, my theoretical conclusion is that organised interests have an important role in the integration process. The integration process of forest and forestry is not driven by one actor, but by many different actors, who operate on different levels and who have different interests.This study shows that forest and forestry-related questions have come to the EU, and they will remain there. The important question for the future is not if there will be some kind of European level policy on forest and forestry, but rather what form European policy will take.
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| 46. |
- Andreasson, Martin, 1987-
(författare)
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Control of Multi-Agent Systems with Applications to Distributed Frequency Control Power Systems
- 2013
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Multi-agent systems are interconnected control systems with many application domains. The first part of this thesis considers nonlinear multi-agent systems, where the control input can be decoupled into a product of a nonlinear gain function depending only on the agent's own state, and a nonlinear interaction function depending on the relative states of the agent's neighbors. We prove stability of the overall system, and explicitly characterize the equilibrium state for agents with both single- and double-integrator dynamics.Disturbances may seriously degrade the performance of multi-agent systems. Even constant disturbances will in general cause the agents to diverge, rather than to converge, for many control protocols. In the second part of this thesis we introduce distributed proportional-integral controllers to attenuate constant disturbances in multi-agent systems with first- and second-order dynamics. We derive explicit stability criteria based on the integral gain of the controllers.Lastly, this thesis presents both centralized and distributed frequency controllers for electrical power transmission systems. Based on the theory developed for multi-agent systems, a decentralized controller regulating the system frequencies under load changes is proposed. An optimal distributed frequency controller is also proposed, which in addition to regulating the frequencies to the nominal frequency, minimizes the cost of power generation.
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| 47. |
- Araujo, Jose, 1985-
(författare)
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Design and Implementation of Resource-Aware Wireless Networked Control Systems
- 2011
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Networked control over wireless sensor and actuator systems is of growing importancein many application domains. Energy and communication bandwidth are scarce resources in such systems. Despite that feedback control might only be needed occasionally, sensor and actuator communications are often periodic and with high frequency in today’s implementations. In this thesis, resource-constrained wireless networked control systems with an adaptive sampling period are considered. Our first contribution is a system architecture for aperiodic wireless networked control. As the underlying data transmission is performed over a shared wireless network, we identify scheduling policies and medium access controls that allow for an efficient implementation of sensor communication. We experimentally validate three proposed mechanisms and show that best performance is obtained by a hybrid scheme, combining the advantages of event- and self-triggered control as well as the possibilities provided by contention-based and contention-free medium accesscontrol. In the second contribution, we propose an event-triggered PI controller for wireless process control systems. A novel triggering mechanism which decides the transmission instants based on an estimate of the control signal is proposed. It addresses some side-effects that have been discovered in previous PI proposals, which trigger on the state of the process. Through simulations we demonstrate that the new PI controller provides setpoint tracking and disturbance rejection close to a periodic PI controller, while reducing the required network resources. The third contribution proposes a co-design of feedback controllers and wireless medium access. The co-design is formulated as a constrained optimization problem, whereby the objective function is the energy consumption of the network and the constraints are the packet loss probability and delay, which are derived from the performance requirements of the control systems. The design framework is illustrated in a numerical example.
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| 48. |
- Baumann, Dominik
(författare)
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Fast and Resource-Efficient Control of Wireless Cyber-Physical Systems
- 2019
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Cyber-physical systems (CPSs) tightly integrate physical processes with computing and communication to autonomously interact with the surrounding environment.This enables emerging applications such as autonomous driving, coordinated flightof swarms of drones, or smart factories. However, current technology does notprovide the reliability and flexibility to realize those applications. Challenges arisefrom wireless communication between the agents and from the complexity of thesystem dynamics. In this thesis, we take on these challenges and present three maincontributions.We first consider imperfections inherent in wireless networks, such as communication delays and message losses, through a tight co-design. We tame the imperfectionsto the extent possible and address the remaining uncertainties with a suitable controldesign. That way, we can guarantee stability of the overall system and demonstratefeedback control over a wireless multi-hop network at update rates of 20-50 ms.If multiple agents use the same wireless network in a wireless CPS, limitedbandwidth is a particular challenge. In our second contribution, we present aframework that allows agents to predict their future communication needs. Thisallows the network to schedule resources to agents that are in need of communication.In this way, the limited resource communication can be used in an efficient manner.As a third contribution, to increase the flexibility of designs, we introduce machinelearning techniques. We present two different approaches. In the first approach,we enable systems to automatically learn their system dynamics in case the truedynamics diverge from the available model. Thus, we get rid of the assumption ofhaving an accurate system model available for all agents. In the second approach, wepropose a framework to directly learn actuation strategies that respect bandwidthconstraints. Such approaches are completely independent of a system model andstraightforwardly extend to nonlinear settings. Therefore, they are also suitable forapplications with complex system dynamics.
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| 49. |
- Blomberg, Hans, 1963-
(författare)
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Influence of The Education and Training of Prehospital Medical Crews on Measures of Performance and Patient Outcomes
- 2013
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Prehospital care has developed dramatically the last decades with the implementation of new devices and educational concepts. Clinical decisions and treatments have moved out from the hospitals to the prehospital setting. In Sweden this has been accompanied by an increase in the level of competence, i.e. by introducing nurses in the ambulances. With some exceptions the scientific support for these changes is poor.This thesis deals with such changes in three different subsets of prehospital care: Cardiopulmonary resuscitation (CPR), the stroke chain of survival and trauma care.We assessed the performance of ambulance crews during CPR, using a mechanical compression device, as compared to CPR using manual compressions. There was a strikingly poor quality of compressions using the mechanical device compared to CPR with manual compressions. The result calls for caution when implementing a chest compression device in clinical practice and reinforce the importance of randomised controlled trials to evaluate new interventions. Careful attention should be given to the assurance of correct application of the device. Further implementation without evaluation of the quality of mechanical compressions in a clinical setting is discouraged.Among patients with a prehospital suspicion of stroke we analysed the ambulance nurses’ ability to select the correct patient subset eligible for a CT scan as a preparation for potential thrombolysis. The results do not support an implementation of a bypass of the emergency department, using ambulance nurse competence to select patients eligible and suitable for a CT scan without a preceding assessment by a physician.The association between the Prehospital Trauma Life Support (PHTLS) course and the outcome in victims of trauma was analysed in two observational studies. A study covering one county gave some support for a protective effect from PHTLS, but the estimate had a low precision. A nationwide study, covering all of Sweden, could not confirm those results. Although there was a reduction in mortality over time coinciding with the implementation of PHTLS, it did not appear to be associated with the implementation of PHTLS. Thus, we could not detect any clear beneficial impact of the PHTLS course on the outcome of trauma patients.
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| 50. |
- Champati, Jaya Prakash, et al.
(författare)
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Performance Characterization Using AoI in a Single-loop Networked Control System
- 2019
-
Ingår i: IEEE INFOCOM 2019 - IEEE Conference on Computer Communications Workshops, INFOCOM Workshops 2019. - : IEEE. - 9781728118789 ; , s. 197-203
-
Konferensbidrag (refereegranskat)abstract
- The joint design of control and communication scheduling in a Networked Control System (NCS) is known to be a hard problem. Several research works have successfully designed optimal sampling and/or control strategies under simplified communication models, where transmission delays/times are negligible or fixed. However, considering sophisticated communication models, with random transmission times, result in highly coupled and difficult-to-solve optimal design problems due to the parameter inter-dependencies between estimation/control and communication layers. To tackle this problem, in this work, we investigate the applicability of Age-of-Information (AoI) for solving control/estimation problems in an NCS under i.i.d. transmission times. Our motivation for this investigation stems from the following facts: 1) recent results indicate that AoI can be tackled under relatively sophisticated communication models, and 2) a lower AoI in an NCS may result in a lower estimation/control cost. We study a joint optimization of sampling and scheduling for a single-loop stochastic LTI networked system with the objective of minimizing the time-average squared norm of the estimation error. We first show that, under mild assumptions on information structure the optimal control policy can be designed independently from the sampling and scheduling policies. We then derive a key result that minimizing the estimation error is equivalent to minimizing a non-negative and non-decreasing function of AoI. The parameters of this function include the LTI matrix and the covariance of exogenous noise in the LTI system. Noting that the formulated problem is a stochastic combinatorial optimization problem and is hard to solve, we resort to heuristic algorithms by extending existing algorithms in the AoI literature. We also identify a class of LTI system dynamics for which minimizing the estimation error is equivalent to minimizing the expected AoI.
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