Research On Real-time And Reliability Of CANopen Networked Control Systems

With more and more applications of the network technology in different fields, solving real-time and reliability problems given by the introduction of the network to control systems has become urgent and difficult. The researches of these relative problems have become the hottest topics of networked control systems (NCS). Real-time is an important characteristic of system performance of NCS, but the network protocols often cannot meet the requirements of higher control system design. In addition, with the expanding scale of control systems, bandwidth resource constraints in NCS are becoming obvious, which will lead to some problems--such as message information delay, deterioration of the system control performance, and less utilization of the network resource. As such, all performance parameters of control systems cannot be efficiently ensured. It is necessary that new analysis methods and scheduling strategies be presented to reevaluate the real-time of NCS and usable resources and achieve the global optimalization of NCS. Meanwhile, the reliability of the automatic control system is the most essential characteristics. With the automation control systems become more complex, more and more applications on the control system are also put forward higher requirements for reliability. Then higher reliability control system has become the most important design goal. Therefore, it is important to analyze and research on the the real-time and it’s scheduling network reliability of NCS. These works have important theory as well as the extensive and practice applications value.This work was supported in part by National Natural Science Foundation of China (NSFC)-GuangDong Province Union Foundation Project U0735003, and the Project of CAN in Water Electrolysis Hydrogen Generation Station(WEHGS) of Suzhou Etorq Co., Ltd. Using the theory analysis, actual industrial application designs and project experiments together, this paper mainly researches on the real-time and reliability of NCS based on Controller Area Network (CAN).Firstly, NCS, fieldbus, CAN and CANopen are summarized. Moreover, the researches about the real-time and reliability of CANopen network are emphatically analyzed and summarized. As regarding the real-time of CAN, this paper evaluates the timing of CAN existed in actual engineering applications platform using the online mensuration. Furthermore, according to the conflicts of CAN messages, the timing of systems can be improved using a flexible quantization scheduling. Reliability design is an important aspect of the NCS research, from the practical point of view, the specific CANopen protocol I/O modules are implemented, and verified by applied these modules to WEHGS system.In general, the main primary innovations and contents of this paper can be summarized as follows:1. Based on the timing characteristic of MAC layer protocol of CAN, the real-time and inherent limitation of the CAN bus have been analyzed. The corresponding optimized method is presented. In harsh real-time field, the TTCAN (Time Triggered CAN) protocol has been introduced, including the design and implementation. Using the online measurement of CAN message delay time in CANopen application platform, the differentiation between the actual message transmission time and the theoretic analysis is analyzed. Moreover, the conclusions can be summarized that error frames and network load lead to the longer network delay directly.2. To use the limited resources of the bus effectively, a uniform quantization scheduling strategy used to solve the problem of CAN-bus with bandwidth constraints is studied. By quantified the sampling signal, we can reduce the length of packet transmission, which can release part of the network bandwidth and improve network control performance. In this paper, the dynamic changes of uniform quantization step size are presented which can improve the control performance significantly. This quantization scheduling strategy can be extended to more general multi-loop network control system.3. Based on industrial practical applications experience, the method of how to improve the control system reliability of CANopen Network is analyzed. Especially, the fault of the fieldbus, in principle, comes from the physical layer, whose lay-out directly affects the fieldbus’s reliability. Furthermore, the paper presents the plan of using H-bridge type matching terminal, which aims at solving signal-reflection problems. This paper also studied through the node redundancy, the line redundancy and software redundancy to further increase system reliability.4. Based on the analysis of CANopen protocol, the CANopen I/O function module series are implemented, which include the CPU module, digital input module, digital output, analog input and analog output modules. Then how to improve the reliability of these I/O modules by the design of printed circuit boards is carefully presented.5. In terms of the network reliability and real-time, we used the CANopen function I/O module series in Water Electrolysis Hydrogen Generation Station, which was usually controlled by PLC modules. Moreover, the ideas and mothods of the softerware and hardware design are given. Experiment results show that these CANopen modules using WEHGS system has some advantage--such as real-time, reliability, extendibility, and lower price. This method has some reference value in the industrial promotion and application of the CANopen protocol.