Design and Performance Analysis of Wireless Networked Control Systems

Abstract: Networked control systems (NCSs) are distributed systems that use shared communication networks to exchange information between system components such as sensors, controllers and actuators. The networked control system architecture promises advantages in terms of increased flexibility, reduced wiring and lower maintenance costs, and is finding its way into a wide variety of applications, ranging from automobiles and automated highway systems to process control, and power distribution systems. However, NCSs also pose many challenges in their analysis and design, since transmitting signals over wireless networks has several side effects, such as: (i) variable sampling intervals, (ii) variable communication delays, (iii) packet losses caused by the unreliability of the network. In this thesis, we aim at developing three different design frameworks, which take some of these side effects into account for improving the performance of the overall system.This thesis firstly presents the joint design of packet forwarding policies and controllers for wireless control loops where sensor measurements are sent to the controller over an unreliable and energy--constrained multi--hop wireless network. For fixed sampling rate of the sensor, the co--design problem separates into two well-defined and independent subproblems: transmission scheduling for maximizing the deadline--constrained reliability and optimal control under packet loss. We develop optimal and implementable solutions for these subproblems and show that the optimally co--designed system can be efficiently found. Numerical examples highlight the many trade-offs involved and demonstrate the power of our approach.Secondly, this thesis proposes a supervisory control structure for networked systems with time-varying delays. The control structure, in which a supervisor triggers the most appropriate controller from a multi-controller unit, aims at improving the closed-loop performance relative to what can be obtained using a single robust controller. Our analysis considers average dwell-time switching and is based on a novel multiple Lyapunov-Krasovskii functional. We develop stability conditions that can be verified by semi-definite programming, and show that the associated state feedback synthesis problem also can be solved using convex optimization tools. Extensions of the analysis and synthesis procedures to the case when the evolution of the delay mode is described by a Markov chain are also developed. Simulations on small- and large-scale networked control systems are used to illustrate the effectiveness of our approach.Lastly, we consider an event--triggered control framework for a linear time--invariant process. We introduce a range based event--triggering algorithm that is used to transmit information from the controller to the actuator. We also analytically characterize the control performance and communication rate for a given event threshold. Additionally, we provide a systematic way to analyze the trade--off between the communication rate and control performance by appropriately selecting an event threshold. Using numerical examples, we demonstrate the effectiveness of the proposed framework.