Research On The Delay And Packet Loss Issues In Networked Control Systems

Compared with conventional point-to-point control systems, the advantages of net-worked control systems (NCSs) are less wiring, lower install costs as well as greater agilityin maintenance, interaction and diagnosis. Due to these distinctive benefits, NCSs havefound applications in a variety of areas, and it has now been one of the hot research topicsin the control society.However, new problems and difficulties in the analysis and synthesis of control sys-tems are introduced by using communication networks of finite capacity to transmit largeamounts of real-time data involved in control tasks. Network-induced delay and packetloss are two of these newly introduced problems, and are commonly recognized as majorcauses of deterioration of system performance and potential of system instability. Thoughthere have been some useful results on the two issues, lots of challenging problems remainto be unsolved, especially, investigations of the following issues are of great significance,i.e., figure out fundamental difficulties that the delay and packet loss bring upon to the syn-thesis of the NCSs, and then propose some systematic modeling, analysis and synthesisapproaches for the NCSs; establish relations among system parameters and performances,and parameters characterizing the delays and packet losses; reduce the conservatism of thedesign. These are the problems to be solved in this thesis.For NCSs with time-varying delays, the following problems should be concerned.The first is the exponential time-varying term induced by the variation nature of the delays.The second is the timing disorder problem caused by the long delays, furthermore, for theNCSs with large bursty delays, how to reduce the conservatism of the design by solelyconsidering the delay upper bound is a problem needs full investigation. Finally, the mostimportant problem is how to establish relations among system performances and parameterscharacterizing the delays. Robust control methods and switched system approaches are proposed in this thesis to solve the aforementioned problems, and two main results areobtained as follows.Firstly, an uncertain system approach and a switched system approach are proposedto deal with the exponential time-varying term for NCSs with short time-varying delays.In the uncertain system approach, the delay is separated into a nominal part and a time-varying uncertain part, and the NCSs is modeled as an uncertain system with norm-boundeduncertainties. In the switched system approach, the delay is quantized and is allowed totake values in a finite set, and the variation nature of the delay is thus transformed into theswitching property of the system models. Based on the proposed two approaches, statefeedback stabilizing controllers are designed, and relations among system performances,delay upper bound, delay variation range and delay variation rate are established for theNCSs.Secondly, a switched system approach and a robust control method are proposed tosolve the modeling, analysis and synthesis problems for NCSs with long time-varying de-lays. In the switched system approach, relations among system performances, delay upperbound, and large delay occurrence rate are established. The relations reveal that there exista state feedback stabilizing controller for the NCSs with large bursty delays if the largedelay occurrence rate is small enough. In the robust control approach, perturbations oflarge delays on system dynamics are described as system uncertainties, and state feedbackcontrollers are designed by using small gain theorem.The key problem in NCSs with packet losses is how to establish relations among sys-tem performances and parameters characterizing the packet losses. Several effective ap-proaches, such as the stochastic system approach, the switched system approach and thetime-delay system approach, are proposed to solve the aforementioned problem accordingto different descriptions of the packet loss processes, and the main achievements are asfollows.The control problem is investigated for NCSs with random packet losses. Firstly, forthe NCSs with integrated structures (where all the system measured outputs/control inputsare transmitted/received via a single node), a design method for state feedback stabilizingcontrollers is presented, and relations among system performances, packet loss probabilityand maximal consecutive packet losses are established. Secondly, for the NCSs with dis-tributed structures(the system measured outputs/control inputs are transmitted/received via multiple nodes), a co-design method for state feedback stabilizing controllers and schedul-ing protocol is proposed, and relations are established among system performances, packetloss probability and network accessing probabilities of various nodes.The signal estimation problem is investigated for networked systems with randompacket losses and distributed structures. Firstly, an H∞filter design method is presentedfor a class of networked discrete-time linear systems by considering the case that all thenodes can complete communications with the remote filter within one sampling period, andrelations among the H∞filtering performance and packet loss probabilities of the variousnodes are established. Secondly, optimal minimum mean-square estimators, including fil-ters, predictors and smoothers, are designed for a class of networked discrete-time linearsystems by considering the case that not all the nodes can complete communications withthe remote filter within one sampling period, and the relations are established among the op-timal estimation performances, packet loss probability and network accessing probabilitiesof the nodes.The quantitative relation between the performance of NCSs and the packet loss rateis investigated. State feedback and output feedback stabilizing controllers are designed forNCSs with packet losses by using switched system approach, and the maximal allowablepacket loss rate is presented as a linear function of the exponential decay rate of the NCSs.The relation between the sampling period and the performance of NCSs with packetlosses is investigated by using switched time-delay system approach, and a quantitativerelation between the sampling period and the exponential decay rate of the NCSs is estab-lished.A number of illustrative examples and simulations, which are carried out by the Mat-lab and Truetime softwares, are given to show the effectiveness of the proposed designmethods. Finally, the conclusion and future work are presented.