Networked control system: stability, robustness and controllability with respect to packet dropouts and scheduling

© 2015 Dr. Merid LjesnjaninControl systems which use a communication network as a communication medium are the focus of this thesis. These systems are widely recognized as Networked Control Systems (NCSs). In particular, we consider NCSs in which the corresponding network induces two communication issues. One of them is a packet dropout(s) while the other is scheduling. To mitigate the undesirable effects of packet dropouts and scheduling, such as instability or deteriorated performance, we use a protocol and controller co-design. More precisely, we use a Model Predictive Control (MPC) framework and the flexible nature of NCS architecture which allows for distributed computation. Considering a specific NCS architecture affected with packet dropouts and/or scheduling we focus on stability, robustness and controllability properties of the corresponding NCS. In particular, we begin by considering stability property of a NCS in which the corresponding network is located between the controller output and the plant input. The network is prone to packet dropouts and it induces scheduling of its communication resource. To address these communication constraints we employ a protocol and controller co-design and show stability, in particular, Uniform Global Asymptotic Stability (UGAS) of the corresponding NCS state. We use two approaches to establish this result. One approach consists of finding an appropriate Lyapunov function while the other approach uses a cascade idea. Following this is an investigation of the same NCS architecture with the difference that it is governed not with a standard MPC controller but an Economic MPC controller. Here we combine several recently established results for an Economic MPC in a way so that our result can be applied off the shelf to establish UGAS of the corresponding NCS state. We then proceed by considering robustness properties of the same NCS architecture. In particular, we consider the case where, additionally, the plant is affected with exogenous disturbances. Here we exploit a concept of nonlinear gains to establish the corresponding result, namely, partial nonlinear gain l 2 stability. We also establish partial linear gain l 2 stability, recover and strengthen a result from the literature for the case when there is no disturbance, provide an alternative robustness characterization for the case when there is no scheduling and finally, by using stronger assumptions, we establish Input-to-State Stability (ISS) of the corresponding NCS state. The last theoretical contribution is controllability of a NCS where the network only induces the scheduling of its communication resource. Namely, we first provide an interesting and novel model which is followed by splitting attention to a NCS with a nonlinear and a NCS with a linear plant. In the former case, we establish general results while in the latter case we extend a result from the corresponding literature and use it to establish our controllability result. Finally, we finish with the implementation of the obtained results within a Hardware-in-the-loop (HIL) simulation. We verify the expectations from theoretical stability and robustness results. Finally, we encounter several issues while carrying out the implementation which will be used as a motivation for further research

Controllability of discrete-time networked control systems with try once discard protocol

This paper investigates controllability of discrete-time Networked Control Systems. The distinguishing feature is that the network imposes scheduling. The network is characterized by a dynamic protocol and different types of additional processing capabilities, as determined by available technology. For NCS with general nonlinear plants we present general controllability results. Finally, for NCS with linear plants we extend ideas motivated by NCS architectures with static protocols to state corresponding controllability results

Uniform Global Asymptotic Stability of Networked Control Systems affected with packet dropouts and scheduling issues

We focus on Networked Control Systems where the network induces two communication issues: one of them is packet dropouts while the other is scheduling. To mitigate the corresponding undesirable effects, such as instability or deteriorated performance, we use a protocol and controller co- design method. In particular, we adopt a deterministic Model Predictive Control (MPC) framework. We establish Uniform Global Asymptotic Stability (UGAS) by assuming a finite bound on the number of consecutive packet dropouts and appropriate modifications to standard MPC stability-related assumptions. We show UGAS through finding an appropriate Lyapunov candidate function.<br/

Spatial iterative learning control: systems with input saturation

This paper proposes a novel Iterative Learning Control (ILC) framework for spatial tracking. Spatial tracking means that the temporal component is not fixed which violates the standing assumption on time intervals in traditional ILC. The proposed framework allows for the length of the time interval to change with each iteration. To relate the spatial information from the past to the present iteration, the concept of spatial projection is proposed. A class of nonlinear uncertain systems with input saturation is chosen for demonstration. An a appropriate ILC control law, exploiting the spatial projection idea, is proposed and the corresponding convergence analysis, based on the Composite Energy Function, is carried out. It is shown that spatial tracking is achieved under appropriate assumptions related to spatial projection and provided that the desired trajectory is realizable within the saturation bound. Finally, simulation results illustrate the predicted convergence