Networked Control Systems And Its Application In Hydropower Plant

Networked Control System (NCS) is a feedback control system where controller, sensor and actuator nodes are connected through real-time network. NCS is an outcome of development and integration of control science, computer technology, network communication and other technologies. It has the advantages of resources share, reduced system cost, increased system agility and ease of system maintenances, so NCS has been used widely in the fields of industrial automatics, navigation technology and aerospace science in the past few years. Because network is introduced in the feedback control system, delay is induced by waiting, collision and congestion for bandwidth constraint, time sharing multiplexing, uncertainty in transmission path, etc. The delay can influence control performance, it can even cause system instability, which brings difficulties to analysis and design of NCS.Aiming at the problem of time delay in NCS, the main research works are as follows:(1) With typical NCS consisted of PLC and PROFIBUS-DP, which is used commonly in automation engineering, as a study object, the inside work in master station and the characteristics of PROFIBUS-DP communication protocol are studied., Based on the characters of mono-master network, the "1-n" processing model of high priority messages is set up, subsequently, the maximal transmitting delay is formulated.(2) To resolve the problem of time delay in NCS, a predictive control strategy is presented to compensate for the delays based on analysis generation mechanism the characters of network-induced delay. The controller predication generator and networked delay compensator are designed. The difficulties of the clock synchronization in the present network makes it not easy to measure the individual forward and feedback network delays, So predication control system is designed based on the round trip time (RTT) delay which is easy to measure.(3) Aiming at the requirement of the project-the Design of Supervisory Control System (SCS) of Hydropower Station, NCS is built up on the base of PROFIBUS-DP. The predication controller strategy is presented to compensate the network-induced delays in NCS. TrueTime toolbox is obtained to simulate experiments for the Hydropower Station NCS. The results of the simulations show that excellent network performance is ensured by this strategy not only on compensating for the constant delay but also on compensating for the random delay.