1,720,952 research outputs found
Towards Traffic Bisimulation of Linear Periodic Event-Triggered Controllers
No full textWe provide a method to construct finite abstractions exactly bisimilar to linear systems under a modified periodic event-triggered control (PETC), when considering as output the inter-event times they generate. Assuming that the initial state lies on a known compact set, these finite-state models can exactly predict all sequences of sampling times until a specified Lyapunov sublevel set is reached. Based on these results, we provide a way to build tight models simulating the traffic of conventional PETC. These models allow computing tight bounds of the PETC average frequency and global exponential stability (GES) decay rate. Our results are demonstrated through a numerical case study.Green Open Access added to TU Delft Institutional Repository 'You share, we take care!' - Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Team Tamas Keviczk
Self-triggered output-feedback control of LTI systems subject to disturbances and noise
No full textSelf-triggered control (STC) and periodic event-triggered control (PETC) are aperiodic sampling techniques aiming at reducing control data communication when compared to periodic sampling. In both techniques, the effects of measurement noise in continuous-time systems with output feedback are unaddressed. In this work we prove that additive noise does not hinder stability of output-feedback PETC of linear time-invariant (LTI) systems. Then we build an STC strategy that estimates PETC's worst-case triggering times. To accomplish this, we use set-based methods, more specifically ellipsoidal sets, which describe uncertainties on state, disturbances and noise. Ellipsoidal reachability is then used to predict worst-case triggering condition violations, ultimately determining the next communication time. The ellipsoidal state estimate is recursively updated using guaranteed state estimation (GSE) methods. The proposed STC is designed to be computationally tractable at the expense of some added conservatism. It is expected to be a practical STC implementation for a broad range of applications.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Team Tamas Keviczk
Timing is everything: Analysis and synthesis of traffic patterns in event-triggered control
No full textEvent-triggered control (ETC) and self-triggered control (STC) are sample-and-hold control paradigms in which sensor data is only updated to the controller when necessary, often aperiodically, in contrast to the well-established periodic sampling paradigm. In ETC, a state-dependent event triggers a transmission, while in STC the controller decides when to request the next sample. The main objective of ETC and STC is to reduce sampling and transmissions when either sampling/transmitting is costly or network resources are scarce. However, despite years of development in the ETC/STC field, little is known about their sampling performances or how to accommodate the generated aperiodic traffic of multiple ETC systems in a shared communication medium. This dissertation presents methods to (i) schedule multiple ETC systems in a shared network, (ii) evaluate ETC systems' sampling performance, and (iii) creating STC strategies that improve ETC systems' sampling performance. In particular, we focus for the most part on ETC applied to linear time-invariant (LTI) systems. To solve these problems, we first model the timing behavior of ETC/STC systems, obtaining what we call traffic models. The states of a traffic model are the transmitted samples and its output is the elapsed time between consecutive transmissions, the inter-sample time (IST). These models are infinite-state systems that can exhibit very complex—even chaotic—behavior, as we demonstrate. To solve synthesis problems such as scheduling and optimal STC sampling strategies, we augment the models with early-sampling choices, which are guaranteed to preserve control stability and performance. The models are then abstracted into finite-state systems or timed automata, on which many of our problems can be computationally solved. Using these abstractions, the obtained schedulers are always valid for the real systems, and the obtained metrics are always formal bounds to the real system's performance. Our abstraction method is based on quotient and l-complete systems. That is, we partition the state-space into regions, each region comprising all states whose next IST, or next sequence of l ISTs, is the same. This is made possible by observing that periodic ETC (PETC)—a practical version of ETC where events are checked periodically—has a finite output set, and that each obtained region is described by an intersection of finitely many quadratic cones. The abstraction transitions, which enable predicting how samples and their corresponding ISTs evolve over time, can be computed exactly using nonlinear satisfiability-modulo-theories solvers, or approximately through convex semi-definite relaxations. Infinite periodic IST patterns arising from these abstractions can be verified to exist in the real traffic model via an eigenvector problem, which is central for solving problem (ii) exactly. Our methodology comprises a comprehensive framework for solving qualitative (scheduling) and quantitative (sampling performance) problems for ETC and STC, as well as a computational machinery that automates these processes, ultimately consolidated in the open-source tool ETCetera. With the developed methods, we can show cases where ETC significantly outperforms periodic sampling in terms of average inter-sample time, and how to increase this performance further using look-ahead. We also manage to solve the ETC scheduling problem efficiently, which is helped by an abstraction minimization algorithm that we propose. In summary, this dissertation provides new tools to understand and manipulate ETC traffic, and ultimately casts new light on the practical relevance of ETC and STC.Team Manuel Mazo J
Self-Triggered Control for Near-Maximal Average Inter-Sample Time
No full textSelf-triggered control (STC) is a sample-and-hold control method aimed at reducing communications in networked-control systems; however, existing STC mechanisms often maximize how late the next sample is, thus not optimizing sampling performance in the long-term. In this work, we devise a method to construct self-triggered policies that provide near-maximal average inter-sample time (AIST) while respecting given control performance constraints. To achieve this, we rely on finite-state abstractions of a reference event-triggered control, while also allowing earlier samples. These early triggers constitute controllable actions of the abstraction, for which an AIST-maximizing strategy can be obtained by solving a mean-payoff game. We provide optimality bounds, and how to further improve them through abstraction refinement techniques.Green Open Access added to TU Delft Institutional Repository 'You share, we take care!' - Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Team Manuel Mazo J
Scalable Traffic Models for Scheduling of Linear Periodic Event-Triggered Controllers
No full textThis paper addresses the problem of modeling and scheduling the transmissions generated by multiple event-triggered control (ETC) loops sharing a network. We present a method to build a finite-state similar model of the traffic generated by periodic ETC (PETC), which by construction mitigates the combinatorial explosion that is typical of symbolic models. The model is augmented with early triggering actions that can be used by a scheduler. The complete networked control system is then modeled as a network of timed game automata, for which existing tools can generate strategies that avoids communication conflicts, while keeping early triggers to a minimum. Our proposed model is relatively fast to build and is the first to constitute an exact simulation. Finally, we demonstrate modeling and scheduling for a numerical example.</p
Computing the average inter-sample time of event-triggered control using quantitative automata
No full textEvent-triggered control (ETC) is a major recent development in cyber–physical systems due to its capability of reducing resource utilization in networked devices. However, while most of the ETC literature reports simulations indicating massive reductions in the sampling required for control, no method so far has been capable of quantifying these results. In this work, we propose an approach through finite-state abstractions to do formal quantification of the traffic generated by ETC of linear systems, in particular aiming at computing its smallest average inter-sample time (SAIST). The method involves abstracting the traffic model through l-complete abstractions, finding the cycle of minimum average length in the graph associated to it, and verifying whether this cycle is an infinitely recurring traffic pattern. The method is proven to be robust to sufficiently small model uncertainties, which allows its application to compute the SAIST of ETC of nonlinear systems.</p
A Simpler Alternative: Minimizing Transition Systems Modulo Alternating Simulation Equivalence
No full textThis paper studies the reduction (abstraction) of finite-state transition systems for control synthesis problems. We revisit the notion of alternating simulation equivalence (ASE), a more relaxed condition than alternating bisimulations, to relate systems and their abstractions. As with alternating bisimulations, ASE preserves the property that the existence of a controller for the abstraction is necessary and sufficient for a controller to exist for the original system. Moreover, being a less stringent condition, ASE can reduce systems further to produce smaller abstractions. We provide an algorithm that produces minimal AS equivalent abstractions. The theoretical results are then applied to obtain (un)schedulability certificates of periodic event-triggered control systems sharing a communication channel. A numerical example illustrates the results. Team Tamas KeviczkyTeam Manuel Mazo J
Optimised State-Dependent Sampling Control for Heavy-Haul Trains
No full textThis thesis investigates the potential of state-dependent sampling strategies (SDSS) for the control of heavy-haul trains. Event-triggered control (ETC) is a control approach in which data is only sent when some state-dependent condition, the triggering condition, is satisfied. In this way, the number of communications required to stabilise a system can be drastically reduced. Periodic event-triggered control (PETC) is a variant of ETC in which the triggering condition is checked periodically. By sometimes sampling earlier than a PETC-generated deadline, long-term pay-offs in the average inter-sample times are possible. As searching for formal models of early-triggering controllers quickly becomes computationally infeasible as system dimensions grow, a sample-based approach using reinforcement learning was utilised to find the SDSS controller, which takes the form of a neural network mapping states to the time until the following sample, trained to hopefully yield long-term payoffs in sample efficiency. It was found that, by choosing suitable control parameters, the SDSS controller can outperform the PETC baseline in terms of inter-sample times (IST). Next, a hardware-in-the-loop (HIL) setup was made to evaluate the optimised controller’s performance in a real-time context controlling a non-linear train system subject to noise, disturbances, and tasked with attaining different speed setpoints while minimising the inter-wagon forces. It was found that the optimised controller did not perform well during these scenarios. Since the controller’s robustness was not considered during the training process, performance suffered under these conditions. To improve the robustness of the controller, the system must be trained on realistic (noisy) data. Simulations with more wagons must be performed to assess whether improvements can still be attained when using larger models. Finally, the accuracy of the HIL setup needs to be improved by using an appropriate network stack that would allow each wagon to send its own data and turn off the sensor node radios when otherwise idly listening for incoming data.Electrical Engineering | Embedded SystemsMechanical Engineering | Systems and Contro
Self-triggered output feedback control for perturbed linear systems
No full textIn this work we propose a Self-Triggered Control (STC) strategy for linear time-invariant (LTI) systems subject to bounded disturbances, using LTI discrete-time dynamic output-feedback. The STC logic computes worst-case triggering times from available information, based on a Periodic Event Triggered Control (PETC) triggering function. In the case of no perturbation and full state information, the discrete times can be determined exactly, defining unions of cones in the state space. When bounded disturbances are present, we compute worst-case triggering times and their associated state-space regions. If full state information is also not available, we use a special observer to compute the worst-case triggering times, yielding an STC logic that only needs the current system output and the controller state. For all cases, we provide sufficient conditions for stability and L2-gain from disturbance to output.Team Tamas Keviczk
Periodic event-triggered control with a relaxed triggering condition
No full textIn networked control systems (NCSs), extensive data exchange between plants and controllers leads to an unnecessary usage of communication and computational resources. Aperiodic sample-and-hold methods such as event-triggered control (ETC) can reduce the number of transmissions, allowing more applications to operate within the same network. However, most existing event-triggering mechanisms enforce a Lyapunov function of the continuous-time closed-loop system to be (almost) always decreasing. We propose a relaxed triggering condition for periodic event-triggered control (PETC) based on bounding the Lyapunov function with an exponentially decaying reference function, which reduces the communications while guaranteeing the same decay rate as competing strategies. We provide sufficient global exponential and input-to-state stability conditions for linear time-invariant (LTI) systems under our event-based state feedback, giving explicit performance guarantees in the presence of additive disturbances. Finally, some simulation results illustrate the performance of the proposed control strategy.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Team Tamas Keviczk
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