The Study About Time Delay Compensation Method For A Kind Of Network Cascade Control System

Cascade control systems, wherein the outer lop and inner loop are inserted through a real-time communication network or control network, are called Networked Cascade Control Systems (NCCS for short). It has the advantages of single-loop network control system (NCS for short), such as resources sharing of remote node, telemetry control, fault diagnosis, system maintenance, and can improve reliability, maintainability and extensible ability of the system. At the same time, it has the merits of conventional cascade control system, which can overcome the interference of sub loop and improve the performance of major loop.At the present, the study about NCCS at home and abroad keeps the initial stage, and the research fruit about type I NCCS which the network delay existing the forward path of inner-outer closed loops is few, the research is limited to that the network delay is constant(the conventional PID parameter is adjusted),or network delay is assumed to smaller than one period, or network delay is random but not more than three sampling periods and it is used by advanced control to realize, or the network delay of various nodes is measured by using the time stamp method (the hypothesis is keeping the clock synchronization),or the medium value of network delay is obtained through adopting location of probability and prediction estimation,so it is not the true value of network.In addition,the study is limited to structure classification, building models, designing main controller and adjusting the parameter of conventional PID)To aim at the present studying condition, we proposed the method of new Smith predictor from angle of structure for type I NCCS, it is suitable for the situation that the network delay is random, time variant and uncertain, and it is also adequate for the occasion that the network delay is long and is more than tens of simple period. The key of this method is that true network data transmission replace the predictor compensation models among nodes, no matter what the system satisfy all conditions of Smith predictor, but make sure that it satisfy the network time delay conditions, it can avoid the request for the clock synchronous, reduce the influence of stability caused by network time delay. The implementation of presented method has nothing to do with the size and nature of network time delay, and it is also irrespective with the network protocols and control scheme of inner-outer loop, the arithmetic is simple and it is easy to realized, so it has better engineering application prospect. Some detailed researchers are done about the type I NCCS which are choosing by wired multi-bop, heterogeneous and hybrid network protocols respectively based on new Smith predictor in Truetime1.5. The simulation results demonstrate that the robustness is strong when the control plants model parameters are changed more and anti-interference ability of the system is better when network time delay changed or existing interference.