Research On Dynamic Scheduling Of Variable Sampling Period In Networked Control System

Networked control system(NCS) is a control system based on network. Compared with the traditional control system, data is transmitted from sensors to controllers and controllers to actuators through network. On the one hand, when communication network is used in NCS, it brings convenience to design of control system. Communication network in NCS is also widely used in metallurgy, chemical industry, mechanical manufacture, etc. On the other hand, it causes some questions such as data packet loss, data packet reordering and network delay, etc. Therefore, experts and scholars from domestic and overseas are concerned about how to reduce negative impact on NCS and increase overall performance of NCS. At present, the strategy of dynamic scheduling is used to enhance the performance of NCS. The study of the strategy of dynamic scheduling is an important research direction. It is an important theoretical value and practical significance to the development of control theory.In this dissertation, the strategy of dynamic scheduling of variable sampling period is studied in NCS. Performance of network and control system is considered simultaneously. The strategy of dynamic scheduling of variable sampling period is proposed in multi-loop NCS. It can improve performance of NCS. The main works are as follows:Sampling period on the influence of NCS is analyzed. The strategy of dynamic scheduling of variable sampling period is studied based on parameters of the model, scheduling algorithm and predictive control. The strategy of dynamic scheduling of variable sampling period is proposed in multi-loop NCS. The dynamic scheduler of variable sampling period is introduced to NCS. It is composed of network state monitor, scheduling algorithm, priority scheduling and data transmission. Network state monitor module is to monitor parameters of network delay and outputs of control system. Network delay is transmitted to scheduling algorithm based on OLE for Process Control(OPC) communication. Predictive algorithm is used to predict network delay of next period in scheduling algorithm module. Predictive sampling period of next period is obtained within fixed network utilization. According to error indexes of output values, scheduling algorithm makes decision whether or not sampling period of control loops need to be updated. Priority of control loops are obtained according to priority scheduling. Predictive sampling period of next period are allocated to each control loop in data transmission module.Because measurements of network delay is the basis and premise of dynamic scheduling of variable sampling period in multi-loop NCS. This paper discusses the approach of network delay measurement. Some factors that are the packet size and data transmission rate are studied for measurement of network delay. Network delay is obtained based on measurement method of Programmable Logic Controller(PLC) scan cycle. Obtained experimental data make preparation for the modeling and prediction of the network delay.According to samples of network delay, Auto Regressive Moving Average(ARMA) model of network delay is set up. Scheduling algorithm module adopts Robust Kalman filter to estimate parameters of ARMA model. Minimum variance is used to predict network delay of next period. In order to prove feasibility of the method, Matlab is applied to programme prediction algorithm. From simulation result, network delay converges towards real value.In order to prove feasibility of the strategy of dynamic scheduling of variable sampling period, experiment platform is set up based on PROFIBUS network. Some program is written to realize dynamic adjustment of sampling period. Network delay is obtained by using measurement method of PLC scan cycle. Data is transmitted between server and client based on OPC communication. According to the network delay, Matlab of client is applied to realize prediction algorithm. According to error indexes of outputs, priority of control loops which are about sampling period, need to be determined. Sampling period of next period is transmitted to some control loops based on OPC communication, which realizes allocation of sampling period for each control loop.