A Research On Bandwidth Management And Priority Scheduling Of Networked Control Systems

Networked control systems refer to closed-loop control systems formed by exchanging data in controllers,actuators,and sensors located in different geographic locations through communication networks.Networked control systems,with its intelligence,network and distribution,can meet the requirements of complex structures,large scale and remote control by large-scale industrial control systems,so it attracts attention from researchers,engineers and technicians at home and abroad.However,networked control systems also face such problems as bandwidth constraints,network scheduling,packet loss,node-driven and network induced delay because of the introducing of shared network,which makes the design and analysis of them more complicated and challenging compared to traditional control systems.Now,this issue has become a hot point in the studies of networked control systems.Several control loops' sharing the limited network bandwidth resources in the network control systems is bound to produce competition.When bandwidth occupancy rate for a certain control loop is too high,it will affect other control loops or even the entire systems.The guarantee of good control performance and stability need not only superior control algorithm but also proper allocation of limited bandwidth resources.Therefore dynamical allocation and adjustment of the control loop bandwidth are critical to weigh the network bandwidth resource competition and control performance.In this paper,the bandwidth management and priority scheduling in the networked control systems are studied from the point of view of network bandwidth limitation and two strategies are proposed correspondingly to improve the system's control performance and bandwidth resource utilization.The first one is bandwidth management strategy based on bandwidth constraints and the second one is a two-parameter priority scheduling strategy based on bandwidth management.The main task is as follows:Firstly,the bandwidth management strategies based on bandwidth constraints are proposed to timely return extra bandwidth resources step by step and optimize allocation of overall expectation bandwidth overrun.The management strategies are to establish a global network scheduler,predict the loop control error,determine the expected bandwidth of the control loop periodically,and make the design of bandwidth allocation for bandwidth within and out of constraints.The experiment results show that the scheduling strategy can effectively improve the bandwidth utilization and system control performance.Secondly,a two-parameter priority scheduling strategy based on bandwidth management strategy is proposed to solve the poor stability of single parameters with priority scheduling and the difficulty that the bandwidth management fully combines with priority scheduling strategy.Based on the centralized bandwidth scheduling of the bandwidth management strategy,the distributed scheduling of the priority is carried out at each sensor node.The sampling period of the global network scheduler output in the bandwidth management strategy is taken as the network demand parameter and the idle time is taken as the network emergence parameter,and then combine these two parameters to determine priority parameter.Besides,the principles responsible for the adjustment of priority parameter are: the priority of the control loop with larger load is determined mainly by the network demand,then the network emergency.In addition,the identifier subsection method of the CAN extension frame is proposed not only to reflect the dynamic characteristics and fixed uniqueness of the strategy,but also increase the capacity of system node.Finally,the experimental results show that the scheduling strategy can dynamically adjust the priority of each communication node and further improve the control performance of the system.