| 1. |
- Adib Yaghmaie, Farnaz, et al.
(författare)
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Differential graphical games for H-infinity control of linear heterogeneous multiagent systems
- 2019
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Ingår i: International Journal of Robust and Nonlinear Control. - : WILEY. - 1049-8923 .- 1099-1239. ; 29:10, s. 2995-3013
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Tidskriftsartikel (refereegranskat)abstract
- Differential graphical games have been introduced in the literature to solve state synchronization problem for linear homogeneous agents. When the agents are heterogeneous, the previous notion of graphical games cannot be used anymore and a new definition is required. In this paper, we define a novel concept of differential graphical games for linear heterogeneous agents subject to external unmodeled disturbances, which contain the previously introduced graphical game for homogeneous agents as a special case. Using our new formulation, we can solve both the output regulation and H-infinity output regulation problems. Our graphical game framework yields coupled Hamilton-Jacobi-Bellman equations, which are, in general, impossible to solve analytically. Therefore, we propose a new actor-critic algorithm to solve these coupled equations numerically in real time. Moreover, we find an explicit upper bound for the overall L2-gain of the output synchronization error with respect to disturbance. We demonstrate our developments by a simulation example.
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| 2. |
- Boyd, Stephen, et al.
(författare)
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MIMO PID Tuning via Iterated LMI Restriction
- 2015
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Ingår i: International Journal of Robust and Nonlinear Control. - : Wiley. - 1099-1239 .- 1049-8923. ; 26:8, s. 1718-1731
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Tidskriftsartikel (refereegranskat)abstract
- We formulate multi-input multi-output (MIMO) proportional-integral-derivative (PID) controller design as an optimization problem that involves nonconvex quadratic matrix inequalities. We propose a simple method that replaces the nonconvex matrix inequalities with a linear matrix inequality (LMI) restriction, and iterates to convergence. This method can be interpreted as a matrix extension of the convex-concave procedure, or as a particular majorization-minimization (MM) method. Convergence to a local minimum can be guaranteed. While we do not know that the resulting controller is globally optimal, the method works well in practice, and provides a simple automated method for tuning MIMO PID controllers. The method is readily extended in many ways, for example to the design of more complex, structured controllers.
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| 3. |
- Dai, L., et al.
(författare)
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Distributed stochastic MPC for systems with parameter uncertainty and disturbances
- 2018
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Ingår i: International Journal of Robust and Nonlinear Control. - : John Wiley and Sons Ltd. - 1049-8923 .- 1099-1239. ; 28:6, s. 2424-2441
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Tidskriftsartikel (refereegranskat)abstract
- A distributed stochastic model predictive control algorithm is proposed for multiple linear subsystems with both parameter uncertainty and stochastic disturbances, which are coupled via probabilistic constraints. To handle the probabilistic constraints, the system dynamics is first decomposed into a nominal part and an uncertain part. The uncertain part is further divided into 2 parts: the first one is constrained to lie in probabilistic tubes that are calculated offline through the use of the probabilistic information on disturbances, whereas the second one is constrained to lie in polytopic tubes whose volumes are optimized online and whose facets' orientations are determined offline. By permitting a single subsystem to optimize at each time step, the probabilistic constraints are then reduced into a set of linear deterministic constraints, and the online optimization problem is transformed into a convex optimization problem that can be performed efficiently. Furthermore, compared to a centralized control scheme, the distributed stochastic model predictive control algorithm only requires message transmissions when a subsystem is optimized, thereby offering greater flexibility in communication. By designing a tailored invariant terminal set for each subsystem, the proposed algorithm can achieve recursive feasibility, which, in turn, ensures closed-loop stability of the entire system. A numerical example is given to illustrate the efficacy of the algorithm. Copyright
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| 4. |
- Dong, Yi, et al.
(författare)
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Coordinated control with multiple dynamic leaders for uncertain Lagrangian systems via self-tuning adaptive distributed observer
- 2017
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Ingår i: International Journal of Robust and Nonlinear Control. - : John Wiley & Sons. - 1049-8923 .- 1099-1239. ; 27:16, s. 2708-2721
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Tidskriftsartikel (refereegranskat)abstract
- This paper studies coordinated control of multiple Lagrangian systems with parametric uncertainties subject to external disturbances by proposing a fully distributed continuous control law based on the improved self-tuning adaptive observer inspired by non-identifier-based high-gain adaptive control technique. Under this distributed continuous control law, a group of Lagrangian systems are driven to the convex hull spanned by multiple heterogenous dynamic leaders, which can be any combination of step signals of arbitrary unknown magnitudes, ramp signals of arbitrary unknown slopes, and sinusoidal signals of arbitrary unknown amplitudes, initial phases, and any unknown frequencies. It is also worth to mention that this control law we propose, depending neither on any information of leader systems for uninformed followers, nor on external disturbances, even independent of neighbors' velocity, can achieve asymptotic tracking of multiple leaders without any additional condition instead of ensuring the ultimate boundedness of the containment error as in the literature.
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| 5. |
- Jafarian, Matin
(författare)
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Robust consensus of unicycles using ternary and hybrid controllers
- 2017
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Ingår i: International Journal of Robust and Nonlinear Control. - : Wiley-Blackwell. - 1049-8923 .- 1099-1239. ; 27:17, s. 4013-4034
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents consensus of the orientations and average positions for a group of unicycles using ternary and hybrid controllers. The decentralized controllers designed to reach consensus of the average positions take only values in the set {-1; 0; +1}. In addition, a hybrid controller is introduced to control the orientations. Finite-time practical consensus of the average positions is proven despite the simple ternary control laws together with asymptotic consensus of the orientations. Furthermore, the consensus problem is studied in the presence of matched input disturbances that are locally rejected using an internal-model-based controller. The analysis is performed in a hybrid framework. Simulation results illustrate the effectiveness of the design.
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| 6. |
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| 7. |
- Kouzoupis, D., et al.
(författare)
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A dual Newton strategy for scenario decomposition in robust multistage MPC
- 2018
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Ingår i: International Journal of Robust and Nonlinear Control. - : Wiley. - 1099-1239 .- 1049-8923. ; 28:6, s. 2340-2355
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Tidskriftsartikel (refereegranskat)abstract
- This paper considers the solution of tree-structured quadratic programs as they may arise in multistage model predictive control. In this context, sampling the uncertainty on prescribed decision points gives rise to different scenarios that are linked to each other via the so-called nonanticipativity constraints. Previous work suggests to dualize these constraints and apply Newton's method on the dual problem to achieve a parallelizable scheme. However, it has been observed that the globalization strategy in such an approach can be expensive. To alleviate this pro blem, we propose to dualize both the nonanticipativity constraints and the dynamics to obtain a computationally cheap globalization. The dual Newton system is then reformulated into small highly structured linear systems that can be solved in parallel to a large extent. The algorithm is complemented by an open-source software implementation that targets embedded optimal control applications.
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| 8. |
- Kouzoupis, D., et al.
(författare)
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A dual Newton strategy for tree-sparse quadratic programs and its implementation in the open-source software treeQP
- 2019
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Ingår i: International Journal of Robust and Nonlinear Control. - : Wiley. - 1099-1239 .- 1049-8923. ; 29:8, s. 2438-2457
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Tidskriftsartikel (refereegranskat)abstract
- This paper presents a dual Newton scheme for tree-sparse quadratic programs as they may arise in the field of stochastic programming. Previous work suggests to introduce auxiliary variables to decompose the tree into scenarios and use Newton's method to solve a dual problem formulation. Following a different approach, we apply the same principle directly on the tree-sparse problem, avoiding the increase in dimensionality. In combination with a tailored algorithm for the calculation of the step direction, which is typically the most expensive operation per iteration, the proposed algorithm achieves a linear complexity in the number of nodes and supports parallel processing of the tree branches in a stage-wise fashion. An open-source implementation of the presented dual Newton strategy is publicly available in treeQP, a toolbox of open-source solvers for tree-sparse quadratic programs.
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| 9. |
- Köroglu, Hakan, 1971, et al.
(författare)
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Robust static output feedback synthesis for platoons under leader and predecessor feedback
- 2017
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Ingår i: International Journal of Robust and Nonlinear Control. - : Wiley. - 1099-1239 .- 1049-8923. ; 27:10, s. 1726-1756
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Tidskriftsartikel (refereegranskat)abstract
- A multi-objective static output feedback synthesis problem is considered for the control of vehicle platoonsunder leader and predecessor feedback. Sufficient linear matrix inequality conditions are derived for thesolvability of the problem in a way to facilitate static feed-forward as well. A novel velocity-dependent spacingpolicy is integrated into the control scheme together with a platoon model in which the emphasis on thepredecessor information can be adjusted by a normalized scalar weight. It is shown that the string stability ofthe spacing errors and the acceleration signals can always be guaranteed by choosing this weight sufficientlysmall. Moreover, provided that the time headway is chosen sufficiently large, the synthesis can be performedin a way to avoid the amplification of acceleration energies if compared with the leader. As a particularlyconvenient feature, the target spacing between the vehicles becomes smaller when moving backward alongthe platoon. The total increase in the platoon length caused by the introduction of the velocity-dependentscheme is shown to be bounded and decreasing with decreasing predecessor weight. It is also establishedthat the predecessor weight can be adjusted smoothly over time without endangering the formation stability.In addition to the optimization of the parameters of common fixed-structure controllers for general vehicle models, the proposed synthesis procedure provides various tools for improving robustness against measurement noise, communication delay, and model uncertainty.
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| 10. |
- Long, Yushen, et al.
(författare)
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Distributed nonlinear model predictive control based on contraction theory
- 2018
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Ingår i: International Journal of Robust and Nonlinear Control. - : WILEY. - 1049-8923 .- 1099-1239. ; 28:2, s. 492-503
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Tidskriftsartikel (refereegranskat)abstract
- A novel distributed model predictive control algorithm for continuous-time nonlinear systems is proposed in this paper. Contraction theory is used to estimate the prediction error in the algorithm, leading to new feasibility and stability conditions. Compared to existing analysis based on Lipschitz continuity, the proposed approach gives a distributed model predictive control algorithm under less conservative conditions, allowing stronger couplings between subsystems and a larger sampling interval when the subsystems satisfy the specified contraction conditions. A numerical example is given to illustrate the effectiveness and advantage of the proposed approach.
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| 11. |
- Luspay, T., et al.
(författare)
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Set-theoretic analysis of the isolated ramp metering problem
- 2016
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Ingår i: International Journal of Robust and Nonlinear Control. - : Wiley. - 1099-1239 .- 1049-8923. ; 26:6, s. 1246-1266
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Tidskriftsartikel (refereegranskat)abstract
- Analysis of ramp metering, as an effective freeway traffic control solution, is focused in the paper. Thestudied traffic control problem is discussed in a set-theoretic context in order to quantitatively characterize its effectiveness. The notions of maximal robust controlled invariant set, as well as t-step robust controllable set are defined and used for analyzing the ramp metering problem independently of the control policy applied. Algorithms are developed to compute these sets, with special attention to their practical interpretations.Numerical examples with graphical representation of the proposed methodology are given to examine localramp metering and conclude implications to control strategies.
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| 12. |
- Nikou, Alexandros, et al.
(författare)
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Decentralized tube-based model predictive control of uncertain nonlinear multiagent systems
- 2019
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Ingår i: International Journal of Robust and Nonlinear Control. - : WILEY. - 1049-8923 .- 1099-1239. ; 29:10, s. 2799-2818
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Tidskriftsartikel (refereegranskat)abstract
- This paper addresses the problem of decentralized tube-based nonlinear model predictive control (NMPC) for a general class of uncertain nonlinear continuous-time multiagent systems with additive and bounded disturbance. In particular, the problem of robust navigation of a multiagent system to predefined states of the workspace while using only local information is addressed under certain distance and control input constraints. We propose a decentralized feedback control protocol that consists of two terms: a nominal control input, which is computed online and is the outcome of a decentralized finite horizon optimal control problem that each agent solves at every sampling time, for its nominal system dynamics; and an additive state-feedback law which is computed offline and guarantees that the real trajectories of each agent will belong to a hypertube centered along the nominal trajectory, for all times. The volume of the hypertube depends on the upper bound of the disturbances as well as the bounds of the derivatives of the dynamics. In addition, by introducing certain distance constraints, the proposed scheme guarantees that the initially connected agents remain connected for all times. Under standard assumptions that arise in nominal NMPC schemes, controllability assumptions, communication capabilities between the agents, it is guaranteed that the multiagent system is input-to-state stable with respect to the disturbances, for all initial conditions satisfying the state constraints. Simulation results verify the correctness of the proposed framework.
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| 13. |
- Ren, Wei, et al.
(författare)
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Razumikhin stability theorems for a general class of stochastic impulsive switched time-delay systems
- 2019
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Ingår i: International Journal of Robust and Nonlinear Control. - : Wiley. - 1049-8923 .- 1099-1239. ; 29:12, s. 3988-4001
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Tidskriftsartikel (refereegranskat)abstract
- This paper studies stability of a general class of impulsive switched systems under time delays and random disturbances using multiple Lyapunov functions and fixed dwell-time. In the studied system model, the impulses and switches are allowed to occur asynchronously. As a result, the switching may occur in the impulsive intervals and the impulses can occur in the switching intervals, which have great effects on system stability. Since the switches do not bring about the change of the system state, we study two cases in terms of the impulses, ie, the stable continuous dynamics case and the stable impulsive dynamics case. According to multiple Lyapunov-Razumikhin functions and the fixed dwell-time, Razumikhin-type stability conditions are established. Finally, the obtained results are illustrated via a numerical example from the synchronization problem of chaotic systems.
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| 14. |
- Sayyaddelshad, Saleh, et al.
(författare)
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H∞ observer design for uncertain nonlinear discrete-time systems with time-delay: LMI optimization approach
- 2015
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Ingår i: International Journal of Robust and Nonlinear Control. - : Wiley. - 1049-8923 .- 1099-1239. ; 25:10, s. 1514-1527
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Tidskriftsartikel (refereegranskat)abstract
- We present a robust H∞ observer for a class of nonlinear discrete-time systems. The class under study includes an unknown time-varying delay limited by upper and lower bounds, as well as time-varying para- metric uncertainties. We design a nonlinear H∞ observer, by using the upper and lower bounds of the delay, that guarantees asymptotic stability of the estimation error dynamics and is also robust against time-varying parametric uncertainties. The described problem is converted to a standard optimization problem, which can be solved in terms of linear matrix inequalities (LMIs). Then, we expand the problem to a multi-objective optimization problem in which the maximum admissible Lipschitz constant and the minimum disturbance attenuation level are the problem objectives. Finally, the proposed observer is illustrated with two examples.
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| 15. |
- Soltesz, Kristian, et al.
(författare)
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An automatic tuner with short experiment and probabilistic plant parameterization
- 2017
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Ingår i: International Journal of Robust and Nonlinear Control. - : Wiley. - 1099-1239 .- 1049-8923. ; 27:11, s. 1857-1873
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Tidskriftsartikel (refereegranskat)abstract
- A novel automatic tuning strategy is proposed. It is based on an experiment of very short duration, followed by simultaneous identification of LTI model parameters and an estimate of their error covariance. The parametric uncertainty model is subsequently exploited to design linear controllers with magnitude bounds on some closed-loop transfer function of interest, such as the sensitivity function. The method is demonstrated through industrially relevant examples. Robustness is enforced through probabilistic constraints on the H∞ norms of the sensitivity function, while minimizing load disturbance integral error (IE) to ensure performance. To demonstrate the strength of the proposed method, identification for the mentioned examples is carried out under a high level of measurement noise.
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| 16. |
- Song, Qiang, et al.
(författare)
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Semi-global and global containment control of multi-agent systems with second-order dynamics and input saturation
- 2016
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Ingår i: International Journal of Robust and Nonlinear Control. - : Wiley. - 1049-8923 .- 1099-1239. ; 36:16, s. 3460-3480
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Tidskriftsartikel (refereegranskat)abstract
- This paper considers both semi-global and global containment control for a second-order multi-agent system that is composed by a network of identical harmonic oscillators or double integrators with multiple leaders and input saturation. A distributed low gain feedback algorithm is proposed to solve the semi-global containment control problem for the network whose topology is directed and initial condition is taken from any a priori given bounded set. In particular, by using a parametric Lyapunov equation approach, M-matrix properties and algebraic graph theory, an upper bound of the low gain parameter is estimated such that the low gain feedback matrix can be analytically determined without involving numerical computation. Furthermore, under the assumption that the induced subgraph formed by the followers is strongly connected and detail balanced, two linear feedback protocols are designed for coupled harmonic oscillators and coupled double integrators, respectively, to asymptotically achieve the global containment control of the network with any initial condition. Finally, numerical examples are given to illustrate the effectiveness of the theoretical results
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| 17. |
- Song, Wenjun, et al.
(författare)
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Relative attitude formation control of multi-agent systems
- 2017
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Ingår i: International Journal of Robust and Nonlinear Control. - : John Wiley & Sons. - 1049-8923 .- 1099-1239. ; 27:18, s. 4457-4477
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Tidskriftsartikel (refereegranskat)abstract
- This paper investigates the relative attitude formation control problem for a group of rigid-body agents using relative attitude information on SO(3). On the basis of the gradient of a potential function, a family of distributed angular velocity control laws, which differ in the sense of a geodesic distance dependent function, is proposed. With directed and switching interaction topologies, the desired relative attitude formation is showed to be achieved asymptotically provided that the topology is jointly quasi-strongly connected. Moreover, several sufficient conditions for the desired formation to be achieved exponentially and almost globally are given. Additionally, numerical examples are provided to illustrate the effectiveness of the proposed distributed control laws.
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| 18. |
- Wang, Lin, et al.
(författare)
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Connectivity maintenance and distributed tracking for double-integrator agents with bounded potential functions
- 2015
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Ingår i: International Journal of Robust and Nonlinear Control. - : Wiley. - 1049-8923 .- 1099-1239. ; 25:4, s. 542-558
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Tidskriftsartikel (refereegranskat)abstract
- In this paper, we develop a set of decentralized control laws with bounded potential functions. The basic control law is a combination of attractive, repulsive, and alignment forces, which can keep connectivity, avoid collision, synchronize all agents, and further track a constant moving leader. Furthermore, we investigate the distributed tracking problem with a varying-velocity leader, where the acceleration of the leader can not be measured. Two cases are considered; the acceleration of the leader is bounded, and the acceleration function satisfies Lipschitz condition. In the first case, the relative velocities of neighbors are integrated and transmitted as a new variable to account for the uncertain time-varying acceleration. In the second case, two distributed estimators are added for the leader's position and velocity. Finally, some simulations are presented to show the effectiveness of the proposed algorithms.
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| 19. |
- Xu, Tao, et al.
(författare)
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Event-based distributed robust synchronization control for multiple Euler-Lagrange systems without relative velocity measurements
- 2019
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Ingår i: International Journal of Robust and Nonlinear Control. - : Wiley. - 1049-8923 .- 1099-1239. ; 29:11, s. 3684-3700
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Tidskriftsartikel (refereegranskat)abstract
- This paper focuses on the event-based distributed robust leaderless synchronization control for multiple Euler-Lagrange systems with directed communication topology that contains a directed spanning tree. Update frequency of the system is reduced by taking advantages of the event-triggered approach, which can help extend the service life of the controller. Robust control theory is employed to guarantee the synchronization stability of the networked Euler-Lagrange systems when unmodeled dynamics occur. The cost on the distributed synchronization protocol design can be saved due to the relaxation of the requirement on relative velocity measurements. Furthermore, our results are more practical because unknown disturbance is taken into consideration. In addition, it can be rigorously analyzed that each agent can exclude the undesired Zeno behavior. Some simulation examples are provided in the end to demonstrate the effectiveness of the proposed event-based distributed robust control algorithm.
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| 20. |
- Kokfelt, U., et al.
(författare)
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Diatom blooms and associated vegetation shifts in a subarctic peatland : responses to distant volcanic eruptions?
- 2016
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Ingår i: Journal of Quaternary Science. - : Wiley. - 0267-8179 .- 1099-1417. ; 31:7, s. 723-730
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Tidskriftsartikel (refereegranskat)abstract
- We test the hypothesis that rich occurrences of diatoms observed at transitions between major peat units representing different vegetation communities in a peat sequence from subarctic northern Sweden reflect responses to acid deposition from the Samalas AD 1257 and Laki AD 1783/1784 eruptions. We observe sudden changes in the mire ecosystem and thereby in the trophic status and biogeochemical cycling of the peatland. Both the eruptions are known to have been associated with significant acid deposition events and climatic anomalies, as recorded in polar ice cores. To test the hypothesis, new chronological analyses and age modelling were applied to existing biogeochemical and biological records from the peat sequence. This approach yielded modelled age ranges of AD 1239-1284 (1s)/AD 1210-1303 (2s) (median: AD 1260) and AD 1674-1795 (1s)/AD 1665-1875 (2s) (median AD 1743), respectively, for the stratigraphic transitions. Hence, the modelled age ranges bracket the ages of the eruptions in question and the hypothesis could therefore not be rejected. Impacts of acid deposition from the eruptions are assumed to have caused instant acidification, vegetation damage, increased nutrient cycling and blooms of opportunistic epiphytic diatoms. In addition, cooling may have contributed to vegetation changes through permafrost inception, frost heave and thereby altered hydrological conditions.
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