Study On Hybrid Control Of NCS And AQM Algorithms

Compared with the traditional point-to-point control systems, Networked Control System (NCS) has enormous advantages such as the shared resources of information, reduced system wiring, easy extending and maintenance of system, higher reliability and flexibility, and so on. However, the problems of network delays, packet dropout, and multiple-packet transmission exist. So it has practical significance to study the modeling and control of NCS. Network brings a great deal of convenience to our daily life, while the problems of network emerge. Presently, network congestion is a thorny problem in network research, and Active Queue Management (AQM) is the research hotspot of congestion control. The works about NCS and AQM that are studied in the thesis are as follows:Firstly, some existing problems of NCS are introduced, and research progresses of hybrid control of NCS and AQM in congestion control are expounded briefly.Then, a class mathematical model of NCS is proposed. The double tank system with continuous flow rate regulation is taken as the controlled object, Mixed Logical Dynamic (MLD) is applied to model the NCS with delay and packet dropout based on the mathematical model, and predictive controller is designed. The double tank system with hierarchical flow rate regulation is taken as the controlled object, hierarchical structure model is used to model the NCS with delay and packet dropout based on the mathematical model, and limited lookahead controller is designed. The simulation model of NCS is performed by TrueTime. In the case of delay and packet dropout, the designed controllers obtain better control effect by means of simulation.Finally, the TCP/AQM model is studied. According to the requirement of Smith predictor for high precision of the controlled object model structure, an AQM algorithm is proposed on the basis of a modified Smith predictor that has lower requirement for precision of the controlled object model structure and better robustness. NS2 simulation results show that the proposed algorithm is better than Smith-PI algorithm in control effect. Considering the better fast response characteristic of time optimal controller, and when control system closes to stable, PID controller has a better control action. An AQM algorithm is designed using controllers switching technique between optimal controller and PID controller combined the advantage of two controllers. Through NS2 simulation, it can achieve the expected queue length more quickly, abbreviate adjusting time and reduce the probability of buffer overflow effectively.