Optimal and robust scheduling for networked control systems

1. Introduction

In recent years, networked control systems (NCSs) have gradually become an active research area due to their advantages in many aspects such as low-cost, reduced weight and power requirements, simple installation and maintenance, as well as high reliability. Generally speaking, NCSs are spatially distributed systems in which the communication between sensors, actuators, and controllers occurs through a shared bandlimited digital communication network (Hespanha, Naghshtabrizi, & Xu, 2007). Indeed, an NCS is a limited communication system and hence some network-induced issues have inevitably emerged that may deteriorate the performance of the whole NCSs such as the time delays, packet dropouts, and network congestions. The past few years have witnessed constant research interests on various analysis and synthesis problems of NCSs with such networked-induced phenomena, and a rich body of research results have been reported in the literature (Bemporad, Heemels, & Vejdemo-Johansson, 2010; Xia, Fu, & Liu, 2011; Yüksel & Başar, 2013). Note that most existing results have been established under some assumptions which are imposed for the purpose of convenience of the theoretical analysis. These assumptions would largely limit the application of these results in the real networked industrial process, especially when several systems or control loops need to use the same communication mechanism at the same time.

On the other hand, with the rapid development of computer science, the capability of both the individual control units and the transport layers has been improved significantly, which gives rise to high data transfer rates and makes the implementation of the NCSs possible. However, for a real-time networked system, there is always a gap between control theory and computer science (Törngren, 1998) since the software engineers usually implement a controller on hardware only after the controller is designed by the control engineers. This may neglect many important properties of NCSs and lead to only an overall suboptimal design. Therefore, in order to improve the control performance, it seems to be of both theoretical importance and practical significance to take into account the issues related to the practical implementation (e.g. the bandwidth constraints of the communication medium) at the controller design stage. However, it should be pointed out that there seems to be a lack of books dealing with the above codesign problems of a controller and communication sequence and, therefore, there is an urgent need to provide a textbook for graduate courses with extensive computer examples and case studies. The present book by Longo, Su, Herrmann, and Barber (2013) is coming with such a purpose.

2. The book

The book by Longo et al. (2013) is self-contained, which covers the topic of NCSs at different levels, from technical results to practical applications. It is unique in that it includes all background materials needed to understand the traditional networked control theory and also introduces up-to-date advances in the scheduling techniques for NCSs with successful application examples.

The book focuses mainly on subjects of optimal and robust scheduling techniques for NCSs and consists of the following 10 chapters.

• Chapter 1: Introduction.

• Chapter 2: Control of plants with limited communication.

• Chapter 3: A general framework for NCS modelling.

• Chapter 4: Controllability and observability.

• Chapter 5: Communication sequence optimization.

• Chapter 6: Optimal controller and schedule codesign.

• Chapter 7: Optimal schedule design.

• Chapter 8: Robust schedule design.

• Chapter 9: Application to an automotive control system.

• Chapter 10: Schedule design for nonlinear NCSs.

A brief summary of these chapters is given as follows. Chapter 1 gives an overview of the NCSs as well as the motivation of writing this book. Chapter 2 reviews the techniques related to the NCSs in details. In Chapter 3, a unified framework is described for the modelling of NCSs with time-triggered communication. Chapter 4 provides technical results that is useful to prove the preservation of the structural properties of NCSs. In Chapter 5, the communication sequence optimization problem is addressed and the corresponding optimization methods are also introduced. In Chapter 6, the codesign problem of controllers and communication sequences is addressed and some techniques are presented that well solve the addressed codesign problem. Chapter 7 presents a technique to design the communication sequence when the controller is given. In Chapter 8, the idea of the previous chapter is extended and the design problem of robust communication sequences is discussed. In Chapter 9, the design techniques developed in the previous chapters are applied to hardware in the loop automotive control systems. Chapter 10 is an extension of the nonlinear case.

3. Comments from this reviewer

Some comments from this reviewer are given as follows.

• (1) The materials presented in this book are new. The book not only reviews the existing modelling techniques and theoretical results on the NCSs but also introduces the up-to-date advances in the implementation techniques of NCSs. In particular, the codesign of controllers and communication sequences is now a frontier in the field of NCSs.

• (2) The techniques introduced in this book are timely. The existing results on the NCSs are theoretical that largely limit their applications in practice. The authors propose a unifying framework for modelling NCSs with time-triggered communication and present corresponding design techniques. Therefore, the book is timely since the proposed techniques well achieve the implementation issue of NCSs.

• (3) The contents of this book are rich. The book includes all background materials needed to understand the traditional networked control theory and is also a timely reflection of the rapidly developing research area of NCSs. It is comprehensive to capture recent advances of theory, techniques, and applications of optimal and robust scheduling techniques for NCSs.

• (4) The organization of this book is designed well. The book is composed of 10 chapters which are arranged logistically from modelling, approach, and application. It is worth mentioning that the book has not been designed to be read from first to last chapter. Some chapters could be omitted by readers according to their individual interests. The nice organization would definitely attract more audiences.

• (5) The references listed in this book are both comprehensive and up-to-date. All these references are the representatives in the latest development of the NCSs. The references justify again the novelty and timeliness of the book and would play an indicative role for further study of the readers.

• (6) The prospective audience of this book may include both students and researchers. This book would be an ideal textbook for students since it covers the evolving technologies in the area of current NCSs. Researchers may also benefit from the art of scientific innovation by blending the concepts, methodologies and principles contained in the book. The book would definitely meet the needs of the prospective audiences well.

4. Conclusion

In summary, this book presents the most recent results in NCSs as well as new developments related to scheduling techniques of NCSs. It contains details of case studies, experimental, simulation and/ or other application-related work showing how the theories put forward can be implemented in real systems. This book can serve as an essential reference for both researchers and graduate students who have been engaging in the area related to NCSs.