Research On The Performance Of Synchronous Control Of Networked Control System

NCS (Networked Control System) is one of the focused research branches inautomatic control field. There are design requirements for synchronous control inmany practical control systems. When introducing NCS to those systems, it canobtain all the advantages of NCS but with lower synchronism because of networkdelay. And the uncertainty of network delay also makes it much more difficult tocontrol synchronously. The improvement of synchronous control performance ofNCS is very important to ensure the consistency of distributed network nodes, theaccuracy of controlde cision-making, as well as the overall performance of NCS.So the research on performance of synchronous control of NCS has important valuein both theory and application.In this paper, NCS synchronous control is studied according to two types ofessential synchronization control requirements in applications. One is timesynchronization requirement, which requests different network nodessynchronously response the same control command. The other is valuesynchronization requirement, which their outputs response satisfy with certainfunctional relationship at the same time in NCS with several plants. Based on someanalysis and synthesises to the status of research and development on performanceof synchronous control of NCS at home and abroad, performance evaluationindexes for synchronous control of NCS are set up firstly. And then, informationtransmission characteristics of NCS are analyzed. On base of the analysis to timecharacteristics, several of synchronous control strategies are put forward in view ofworking mode of network node, design of intelligent controller, etc. And finally thesimulation is done for further studies on the performance of synchronous control ofthe NCS.There are no definitive evaluation indexes for synchronous controlperformance of NCS other than those for network performances. In order toevaluate control performances under different types of synchronous control inapplications, some performance evaluation indexes for synchronous control of NCSare established herein, such as synchronous input, synchronous output andsynchronous output response of plants. Some performance evaluation indexes ofcontrol system and network are discussed. Based on full consideration to thesynchronization, stability, dynamic characteristic，steady state characteristic aswell as network performance of NCS, a kind of integrated evaluation method isproposed in this paper. Other influence factors, such as network clock, network information transmission time delay, network node working modes, networkscheduling and so on, are studied for analysis.Network information transmission time is one important factor to influencesynchronous performances of NCS. In order to study the time characteristics ofnetwork information transmission, some time analysis models are set up in thispaper. These models include analysis model of transmission time of NCS in thegeneral sense, time model of synchronous control with compensation, time modelof isochronous control. In view of PROFIBUS NCS, based on study on the accessmechanism of its token ring, its time analysis model is set up, an experimentalplatform of PROFIBUS-DP NCS is built, an experimental method is proposed tomeasure the transmission time, and some experiments are done to study the timecharacteristics of network information transmission. They could provide referencedata to study the transmission performance, node synchronization error calculation,as well as time delay prediction and compensation synchronous control.In order to realize synchronous control of input or output for NCS nodes,several control strategies are put forward including grouping synchronous control,improved combined node-driven mode, time compensation control by data buffer ofnodes, setting up global state observer for nodes, etc. Studies have shown thatgrouping synchronous control can more effectively improve the performance ofsynchronous control nodes; the proposed improved combined drive mode for nodesis not only with minimum synchronization error but also with minimum networkdelay than other node-driven approaches; the proposed time compensation controlby data buffer of nodes can reduce network synchronization error of node causedby the difference in transmission time; the global state observer established incontroller node of NCS is used to collect state information of every network node,to calculate the global state variable of the synchronous control, and to determinethe synchronous behaviour of NCS. This kind of control strategy provides afeasible and with practical design ideas in application programs to achievesynchronous control of network nodes.Based on analysis of information transmission characteristics of NCS, in orderof task series, a kind of modular simulation model of NCS is set up byMatlab/Simulink, and some simulation studies on the performance of synchronouscontrol of NCS are done. An improved combined control strategy of Fuzzy andSmith is proposed, and simulation results show that the strategy can reduce theadverse effects of the network delay to the performance of synchronous control.Aiming at those NCS with several controlled objects with requirements ofsynchronous control to their response output, an integrated synchronous control strategy is proposed including small step given signal, synchronization errorcompensation control and Fuzzy-PI dual-mode control. Simulation results showthat the synchronization error of output response of the controlled object can beeffectively reduced by synchronization error compensation control. The problem ofoutput response synchronization error bigger of controlled object in dynamic statecan be resolved by small step given signal control and Fuzzy-PI dual-mode control.