Co-design Research Of Two-dimensional Scheduling Strategy In Networked Control Systems

Based on network transmission medium, Networked control systems(NCSs)integrate the spatial separation nodes of sensors, controllers and actors, and form a real-time close-loop control system, which are widely used in industrial control systems,electrical automation engineering and aerospace and other fields for its distributed,highly intelligent, sharing bandwidth resources, etc. Due to the introduction of network media, network control systems inevitably appear some problems, such as network congestion, packet loss, network information transmission delay, etc And these problems will reduce the control performance of the system, even makes the system unstable. So, how to reduce the impacts which are introduced by adverse factors in the network control systems is a hot problem of relative researches. As a reasonable allocation method of system resources, network scheduling is always the topic research in this field and has important theoretical significance and great value in industrial application which can adjust the information-sending orders of nodes and improve overall system performance effectively. The paper mainly focus on network scheduling and main research works and results are included as follows:Firstly,this paper introduces the background and current situation of the development of network control systems, and illustrates its main research focus; Then it lists several commonly used industrial control network communication medium access protocol; Then we focus on the study on the scheduling methods of network control system, analyses them briefly which in turn lead to the topic of this paper-- “the two-dimensional scheduling strategy and collaborative control research with the combination of variable sampling period scheduling priority scheduling in networked control systems”. Before stating the main research work in this paper, we introduce the CAN bus network communication protocol and the use of simulation tools- True Time based on which we carry the simulation on. Then we simply introduce network time delay,analyze how it affects the performance of the system problem, and introduceseveral related concepts involved in this paper.This paper proposes a design of information distribution in the communication message identifier domain for CAN bus, and adopts a modified fuzzy maximum priority scheduling strategy based on feedback(N-FFS) to calculate the message priority of communicating message. Identifier domain information is distributed into message ID and node number. Message ID is updated by N- FFS calculation, and the node number is set initially remaining unchanged. This can ensure the CAN data identifier domains do not conflict. N-FFS takes loop control error, error change and network latency as the input of fuzzy control algorithm and it dynamically updates priority of each loop in the network control system. By modeling and simulation, the simulation results show that the proposed scheduling strategy can balance the overall performance of the network control system, and it can effectively improve the performance of the system.Considering that the sampling period of sensor node varies by the changes which is caused by the controlled object itself or the over system performance difference, we should consider the impacts that the variable sampling period make on system performance. Under the condition when it guarantee the stability of system, satisfies the limitation of network load capacity and network scheduling constraints, this paper put forwards the collaborative design of variable sampling period scheduling and priority scheduling : it takes the variable sampling period as the first level of scheduling and priority scheduling as the second level and forms a closed loop of two-dimensional scheduling policy, which is applied in the network control system. First of all, according to the network running status and control performance, the two-dimensional scheduling policy adjusts the sampling period in corresponding circuit; Secondly, according to the performance of each circuit it adjusts their priorities dynamically. Then we make the preliminary research and simulation of the collaborative design of the two-dimensional scheduling. Through the simulation analysis, the proposed two-dimensional scheduling policies in this paper considers both variable sampling period scheduling and CAN control characteristic, and it can satisfy the requirement of system of network utilization,ensure good control loop in the control system performance. Lastly, we verify the superior performance of the co-design of variable sampling period scheduling algorithm and priority scheduling algorithm by simulation tools.