| 1. |
- Barbosa, Fernando S., 1992-, et al.
(författare)
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Provably safe control of Lagrangian systems in obstacle-scattered environments
- 2020
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Ingår i: 2020 59th IEEE Conference on Decision and Control (CDC). - : Institute of Electrical and Electronics Engineers (IEEE).
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Konferensbidrag (refereegranskat)abstract
- We propose a hybrid feedback control law that guarantees both safety and asymptotic stability for a class of Lagrangian systems in environments with obstacles. Rather than performing trajectory planning and implementing a trajectory-tracking feedback control law, our approach requires a sequence of locations in the environment (a path plan) and an abstraction of the obstacle-free space. The problem of following a path plan is then interpreted as a sequence of reach-avoid problems: the system is required to consecutively reach each location of the path plan while staying within safe regions. Obstacle-free ellipsoids are used as a way of defining such safe regions, each of which encloses two consecutive locations. Feasible Control Barrier Functions (CBFs) are created directly from geometric constraints, the ellipsoids, ensuring forward-invariance, and therefore safety. Reachability to each location is guaranteed by asymptotically stabilizing Control Lyapunov Functions (CLFs). Both CBFs and CLFs are then encoded into quadratic programs (QPs) without the need of relaxation variables. Furthermore, we also propose a switching mechanism that guarantees the control law is correct and well-defined even when transitioning between QPs. Simulations show the effectiveness of the proposed approach in two complex scenarios.
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| 2. |
- Groen, Solveig Skovlund, et al.
(författare)
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A serological type II collagen neoepitope biomarker reflects cartilage breakdown in patients with osteoarthritis
- 2021
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Ingår i: Osteoarthritis and Cartilage Open. - : Elsevier BV. - 2665-9131. ; 3:4
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Tidskriftsartikel (refereegranskat)abstract
- Objectives: There is an unmet medical need for biomarkers in OA which can be applied in clinical drug development trials. The present study describes the development of a specific and robust assay measuring type II collagen degradation (T2CM) and discusses its potential as a noninvasive translational biomarker. Methods: A type II collagen specific neoepitope (T2CM) was identified by mass spectrometry and monoclonal antibodies were raised towards the epitope, employed in a chemiluminescence immunoassay. T2CM was assessed in bovine cartilage explants with or without MMP-13 inhibitor, and explant supernatants were analyzed by Western blot. T2CM was measured in plasma samples from one study (n = 48 patients) where OA patients were referred to total knee replacement (TKR). Additionally, T2CM was quantified in serum from OA patients receiving salmon calcitonin treatment (sCT) (n = 50) compared to placebo (n = 57). Results: The T2CM assay was technically robust (13/4 % inter/intra-variation) and specific for the type II collagen fragment cleaved by MMP-1 and -13. The MMP-13 inhibitor reduced the T2CM release from bovine cartilage explants receiving catabolic treatment. These results were confirmed by Western blot. In human end-stage OA patients (scheduled for TKR), the T2CM levels were elevated compared to moderate OA (p<0.004). The OA patients receiving sCT had lower levels of T2CM compared to placebo group after 1, 6, and 24 months of treatment (p = 0.0285, p = 0.0484, p = 0.0035). Conclusions: To our knowledge, T2CM is the first technically robust serological biomarker assay which has shown biological relevance in ex vivo models and OA cohorts. This suggests that T2CM may have potential as a translational biomarker for cartilage degradation.
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| 3. |
- Lindemann, Lars, et al.
(författare)
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Barrier Function-based Collaborative Control ofMultiple Robots under Signal Temporal Logic Tasks
- 2020
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Ingår i: IEEE Transactions on Control of Network Systems. - : Institute of Electrical and Electronics Engineers (IEEE). - 2325-5870. ; 7:4, s. 1916-1928
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Tidskriftsartikel (refereegranskat)abstract
- Motivated by the recent interest in cyber-physicaland autonomous robotic systems, we study the problem ofdynamically coupled multi-agent systems under a set of signaltemporal logic tasks. In particular, the satisfaction of each ofthese signal temporal logic tasks depends on the behavior of adistinct set of agents. Instead of abstracting the agent dynamicsand the temporal logic tasks into a discrete domain and solvingthe problem therein or using optimization-based methods, wederive collaborative feedback control laws. These control laws arebased on a decentralized control barrier function condition thatresults in discontinuous control laws, as opposed to a centralizedcondition resembling the single-agent case. The benefits of ourapproach are inherent robustness properties typically present infeedback control as well as satisfaction guarantees for continuous-time multi-agent systems. More specifically, time-varying controlbarrier functions are used that account for the semantics of thesignal temporal logic tasks at hand. For a certain fragment ofsignal temporal logic tasks, we further propose a systematic wayto construct such control barrier functions. Finally, we showthe efficacy and robustness of our framework in an experimentincluding a group of three omnidirectional robots
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| 4. |
- Lindemann, Lars, et al.
(författare)
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Control Barrier Functions for Nonholonomic Systems under Risk Signal Temporal Logic Specifications
- 2020
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Ingår i: Proceedings of the IEEE Conference on Decision and Control. - : Institute of Electrical and Electronics Engineers Inc.. ; , s. 1422-1428
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Konferensbidrag (refereegranskat)abstract
- Temporal logics provide a formalism for expressing complex system specifications. A large body of literature has addressed the verification and the control synthesis problem for deterministic systems under such specifications. For stochastic systems or systems operating in unknown environments, however, only the probability of satisfying a specification has been considered so far, neglecting the risk of not satisfying the specification. Towards addressing this shortcoming, we consider, for the first time, risk metrics, such as (but not limited to) the Conditional Value-at-Risk, and propose risk signal temporal logic. Specifically, we compose risk metrics with stochastic predicates to consider the risk of violating certain spatial specifications. As a particular instance of such stochasticity, we consider control systems in unknown environments and present a determinization of the risk signal temporal logic specification to transform the stochastic control problem into a deterministic one. For unicycle-like dynamics, we then extend our previous work on deterministic time-varying control barrier functions.
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| 5. |
- Lindemann, Lars, et al.
(författare)
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Coupled Multi-Robot Systems Under Linear Temporal Logic and Signal Temporal Logic Tasks
- 2021
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Ingår i: IEEE Transactions on Control Systems Technology. - : Institute of Electrical and Electronics Engineers (IEEE). - 1063-6536 .- 1558-0865. ; 29:2, s. 858-865
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Tidskriftsartikel (refereegranskat)abstract
- This brief presents the implementation and experimental results of two frameworks for multi-agent systems under temporal logic tasks, which we have recently proposed. Each agent is subject to either a local linear temporal logic (LTL) or a local signal temporal logic (STL) task where each task may further be coupled, i.e., the satisfaction of a task may depend on more than one agent. The agents are represented by mobile robots with different sensing and actuation capabilities. We propose to combine the two aforementioned frameworks to use the strengths of both LTL and STL. For the implementation, we take into account practical issues, such as collision avoidance, and, in particular, for the STL framework, input saturation, the digital implementation of continuous-time feedback control laws, and a controllability assumption that was made in the original work. The experimental results contain three scenarios that show a wide variety of tasks.
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| 6. |
- Lindemann, Lars, et al.
(författare)
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Efficient Automata-based Planning and Control under Spatio-Temporal Logic Specifications
- 2020
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Ingår i: Proceedings 2020 American Control Conference, ACC 2020, Denver, CO, USA, July 1-3, 2020. - : IEEE Institute of Electrical and Electronics Engineers Inc..
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Konferensbidrag (refereegranskat)abstract
- The use of spatio-temporal logics in control is motivated by the need to impose complex spatial and temporal behavior on dynamical systems, and to control these systems accordingly. Synthesizing correct-by-design control laws is a challenging task resulting in computationally demanding methods. We consider efficient automata-based planning for continuous-time systems under signal interval temporal logic specifications, an expressive fragment of signal temporal logic. The planning is based on recent results for automata-based verification of metric interval temporal logic. A timed signal transducer is obtained accepting all Boolean signals that satisfy a metric interval temporal logic specification, which is abstracted from the signal interval temporal logic specification at hand. This transducer is modified to account for the spatial properties of the signal interval temporal logic specification, characterizing all real-valued signals that satisfy this specification. Using logic-based feedback control laws, such as the ones we have presented in earlier works, we then provide an abstraction of the system that, in a suitable way, aligns with the modified timed signal transducer. This allows to avoid the state space explosion that is typically induced by forming a product automaton between an abstraction of the system and the specification.
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| 7. |
- Lindemann, Lars, et al.
(författare)
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Funnel control for fully actuated systems under a fragment of signal temporal logic specifications
- 2021
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Ingår i: Nonlinear Analysis. - : Elsevier Ltd. - 1751-570X .- 1878-7460. ; 39
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Tidskriftsartikel (refereegranskat)abstract
- Temporal logics have lately proven to be a valuable tool for various control applications by providing a rich specification language. Existing temporal logic-based control strategies discretize the underlying dynamical system in space and/or time. We will not use such an abstraction and consider continuous-time systems under a fragment of signal temporal logic specifications by using the associated robust semantics. In particular, this paper provides computationally-efficient funnel-based feedback control laws for a class of systems that are, in a sense, feedback equivalent to single integrator systems, but where the dynamics are partially unknown for the control design so that some degree of robustness is obtained. We first leverage the transient properties of a funnel-based feedback control strategy to maximize the robust semantics of some atomic temporal logic formulas. We then guarantee the satisfaction for specifications consisting of conjunctions of such atomic temporal logic formulas with overlapping time intervals by a suitable switched control system. The result is a framework that satisfies temporal logic specifications with a user-defined robustness when the specification is satisfiable. When the specification is not satisfiable, a least violating solution can be found. The theoretical findings are demonstrated in simulations of the nonlinear Lotka–Volterra equations for predator–prey models.
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| 8. |
- Lindemann, Lars, et al.
(författare)
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Learning Hybrid Control Barrier Functions from Data
- 2020
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Ingår i: Proceedings of the 2020 Conference on Robot Learning, CoRL 2020. - : ML Research Press. ; , s. 1351-1370
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Konferensbidrag (refereegranskat)abstract
- Motivated by the lack of systematic tools to obtain safe control laws for hybrid systems, we propose an optimization-based framework for learning certifiably safe control laws from data. In particular, we assume a setting in which the system dynamics are known and in which data exhibiting safe system behavior is available. We propose hybrid control barrier functions for hybrid systems as a means to synthesize safe control inputs. Based on this notion, we present an optimization-based framework to learn such hybrid control barrier functions from data. Importantly, we identify sufficient conditions on the data such that feasibility of the optimization problem ensures correctness of the learned hybrid control barrier functions, and hence the safety of the system. We illustrate our findings in two simulations studies, including a compass gait walker.
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| 9. |
- Lindemann, Lars
(författare)
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Planning and Control of Multi-Agent Systems under Signal Temporal Logic Specifications
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Engineered systems are becoming more connected due to the availability of affordable and mobile communication and computation devices and form, already today, interacting and dependent complex networks. Examples of such systems range from multi-robot systems, e.g., fleets of drones or autonomous cars, over smart grids and smart home technologies. Besides these systems being connected and dependent, they have to satisfy complex individual and global specifications. These specifications may in particular include nested combinations of temporal and spatial requirements, e.g., a fleet of drones may be required to repeatedly inspect particular areas of interest within certain time intervals and change its formation pattern over time, while ensuring safety at all times. The challenge in such networks under complex specifications is to formally verify and ensure the correct and safe behavior of the system.This thesis proposes planning and feedback control algorithms towards achieving this goal. In particular, we consider coupled multi-agent systems under signal temporal logic specifications. In the first part of the thesis, we propose timed automata-based planning algorithms that decompose the complex specification into a sequence of simpler specifications that can be realized by feedback control laws, such as those proposed in the second part of the thesis. These planning algorithms take uncontrollable events such as failures of agents/sensors into account and address perception issues, e.g., when the environment is not perfectly known. The second part proposes feedback control laws for multi-agent systems under fragments of individual and global signal temporal logic specifications. We propose time-varying feedback control laws that induce a desired temporal behavior, according to the specification at hand, into the closed-loop of the system. The combination of timed automata-based planning and feedback control is efficient in the sense that the computationally expensive planning algorithms can be performed offline, while only the efficient low-level feedback control laws are used during runtime of the system. The proposed algorithms additionally ensure robustness of the system. All theoretical results are illustrated in simulations or experiments involving mobile robots.
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| 10. |
- Mehdifar, Farhad, 1992-, et al.
(författare)
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Control of Nonlinear Systems Under Multiple Time-Varying Output Constraints : A Single Funnel Approach
- 2023
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- This paper proposes a novel control framework for handling (potentially coupled) multiple time-varying output constraints for uncertain nonlinear systems. First, it is shown that the satisfaction of multiple output constraints boils down to ensuring the positiveness of a scalar variable (the signed distance from the time-varying output-constrained set’s boundary). Next, a single funnel constraint is designed properly, whose satisfaction ensures convergence to and invariance of the time-varying output-constrained set. Then a robust and low-complexity funnel-based feedback controller is designed employing the prescribed performance control method. Finally, a simulation example clarifies and verifies the proposed approach
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