The Analysis, Modeling And Stablity For Networked Control Systems

Networked Control Systems (NCS) are the combinations of sensors, controllers, actuators in some area and the communication networks, providing the data transmission among the equipments and making different users in different places realize resource share and coordinated operations. Networked control systems are the entirely distributed, networked, real-time feedback control systems. Because of the stronger diagnosis capability, the simple installation and maintenance, reducing the weight and the volume effectively, increasing the agility and the reliability of systems, and so on, networked control systems are becoming the new hot research points and new trends of industrial control systems. When networks are introduced into the control systems, networked-induced delays are produced inevitably by waiting, collision and congestion, making data transmission be in disorder and resulting in data dropout, because of bandwidth constraint, time-sharing multiplexing, uncertainty in transmission path, and so on. The network-induced delays, which are affected by network protocols, driven modes of nodes, data sampling methods, and some other factors, whether it is constant, time-varying, bounded or unbounded, can influence the performances of control systems. Sometimes they can cause system's instability, which brings difficulties to the analysis and designs of NCS. Although the researches have obtained many results in the analysis, modeling, and stability of delay systems, because of the own characteristics and complexities of NCS, the existed control theory and methodology can only be used to NCS after being reanalyzed and redesigned. Specifically, the case coupling of control and communication must be taken into account during the analysis and designs of NCS. It is necessary to establish the mathematic models so as to make NCS be run steadily and have well dynamic performances. At the same time, it is necessary to develop the methodologies of analysis and designs corresponding to NCS. So this dissertation has very important theoretical significance and practical application worth.The main purpose of this dissertation is to study the influences of network-induced delay upon the analysis, modeling and stability of NCS, and to explore effective solution methods, so as to provide meaningful use for reference. The following outlines the research results.Firstly, some typical research methods of NCS in recent years are analyzed. Some of them are listed in this dissertation. Also they can be used for future research work of NCS. Secondly, some basic issues of NCS are analyzed in detail. Some interesting research aspects are dug and can become the future study trends. At the same time, hoping that these issues can arouse the recognition of researchers who can continue to study the theory and practical applications of NCS, the theories and methodologies corresponding to the characteristics of NCS can be made perfectly.Thirdly, network-induced delay of NCS is studied. This is a basic and important issue of NCS. The analysis and research of the delay's composition, the influences to the stability and performance indexes of NCS, the characteristics of data transmission in networks and the factors of influencing on delay are investigated. The choice standards of network protocols, driven modes of nodes and sampling period are discussed. Based on the above discussions, the analysis and designs of stability of NCS with deterministic and constant delay which is less than one sampling period are studied.Fourthly, the modeling of NCS is presented. Based on the continuous models of single-input-single-output NCS, the general continuous models of multi-input-multi-output (MIMO) NCS with system noise and observation noise are built. The discrete models of short delays, single-packet transmission and multiple-packet transmission with no packets dropout are established. The discrete models of MIMO NCS with any bounded delays based on the augmented states are constructed. The open-loop mathematic models and unified mathematic model of NCS with long time delay are developed when the system is driven by anyone of the four different driven modes. The random time-varying delay is divided into two parts which are constant delay and time-varying delay, and then, discrete uncertain model of NCS is built.Fifthly, based on the theory of interval algebra, modeling and stability of NCS with time-varying sampling period and time-varying delay are studied. This method simplifies the stability analysis of original time-varying closed-loop system model which is changed into a time-independent one. The deduced conclusion is simple and effective although it is conservative.Sixthly, modeling and stability of NCS with data-packet dropout are studied. The asymptotic stability criteria of NCS are derived theoretically. The achieved conclusions need to be verified in the engineering applications. First of all, based on the discrete model of single-packet transmission with data-packet dropout of NCS, the discrete models of associated plant and closed-loop system considering multiple-packet transmission with data-packet dropout are set up. And then, considering the network-induced delay and data-packet dropout synthetically, asymptotic stability conditions of NCS with unbounded delays and bounded delays based on single-packet transmission are derived. Based on multiple-packet transmission, asymptotic stability conditions with time-delay variate rate which is less than one or less than zero are deduced.Finally, the method which can change the random delay into the deterministic delay by means of installing queue buffer at the sending node or receiving node is studied. Three kinds of state observers of NCS whose states are unmeasured are designed. The fact that observer errors are asymptotically convergent is proved theoretically only if original system is observable.