Study of Networked Control System Based on Switched Ethernet
|School||Huazhong University of Science and Technology|
|Course||Control Theory and Control Engineering|
|Keywords||Switched Ethernet Networked control system Delay Network calculus Network topology Genetic algorithm Scheduling algorithm|
Networked control system based on Ethernet breaks many restrictions of field-bus control systems and becomes the new development direction of the networked control system. Switching technology’s emergence can overcome the weakness of raditional Ethernet, raise the network performance and overcome the non-deterministic characteristic of delay in some degree, so switched Ethernet will push the Ethernet’s application and development in networked control system and will be the research emphasis and direction. Currently, systemic research on networked control system based on switched Ethernet is absent and many problemes need to be resolved. In this thesis, six problems are thoroughly studied and analyzed: delay’s making up, delay’s determinately analysising, network topology structure’s designing, switcher scheduling algorithm, random delay compensation method with considering the system noise and simulationtrue platform in NCS based on switched Ethernet.Firstly, the development of networked control system and the reported research works on networked control system are summarized.Delay is the most apparent disadvantage in NCS based on switched Ethernet. Delay composing is studied thoroughly and all parts of delay are made qualitative analysis, the main factor influcing system delay is derived. Based on above analysis, the worst-case delay in system with single switcher and system with multiple switchers are analyzed using timing diagram and the mathematics expression of the maximum delay is derived. Compared with former methodes and conclusions of delay analysis, it is the first time to be roundly considered that delay induced by switcher and conclusions are more integrated and full-scale in this thesis.In order to meet the real-time requirements of NCS, the real-time ability of switched Ethernet need to be determinately analysed and evaluated. This thesis proposes adopting the analytic tool of network calculus to determinately analyze the delay of NCS based on switched Ethernet, and makes the switcher with non-priority, buffers at output port and FIFO(first input first output)scheduling algorithm as study object for the first time. Firstly communication model is set. Then second-level and first-level switcher’service curves for some stream are calculated through the theory of service curve. Lastly, the analytic expressions of system service curves, delay, and buffer size are presented. The method and conclusions can determinately analyse and evaluate the real-time ability of switched Ethernet and can also analyze the network’s carrying capacity of load under the sistuation of satisfying stream performance.Reasonable network topology design is the first effective step to minish delay. But research work on the direction is seldom. This thesis proposes adopting Genetic Algorithms to optimize and design NCS’s topology based on switched Ethernet. Coding strategy, probability of crossover and probability of mutation are improved, which can conquer the shortcomings of premature convergence and contradiction between convergent rate and holistic convergence in simple Genetic Algorithms. Redundant topology structure and communication characteristics of switched Ethernet are considered in the thesis,which allows inter-node communication has different weights. Firstly, the problem of network partition is analyzed to be equivalent to a multi-objective optimization problem, the definition of network topology optimization is gived. Then improved Genetic Algorithms is used to optimize the network topology. The simulation experiment proves that the algorithm can rapidly depart from part optimization and evolve at the direction of holistic optimization, which can ensure the convergent rate at the same time. The simulation result shows that the algorithm is capable of realizing the system topology optimization and minishing delay effectively.In switched Ethernet, larger queue delay will be produced when multiple data packets are sent to only one destination simultaneously. So packets in buffer must be scheduled to assure the real-time ability. A innovative classed scheduling algorithm based switched Ethernet is proposed. The scheduling algorithm can be used in ordinary switcher and has some features such as not changing Ethernet’s MAC protocol, simple realization, small run spending and so on. The algorithm includes two levels scheduling. First scheduling is to apply IEEE802.1p non-preemptive priority scheduling policy in switcher; then second scheduling is to adopt different strategies to different priorities queue. For the real-time periodic queue, which has highest priority, Genetic Algorithm is adopted to create off-line scheduling table. Meterial policy of Genetic Algorithm is designed and the condition in which the queue can be scheduled is gived.A large amount of materials studied the delay of networked control system from the view of control, yet seldom considered system noise at present. To supply this gap, the thesis studies some kind of NCS with random delay and system noise and proposes a kind of state observer which can compensate random time-delay. Output feedback is used to construct the state observer, which can be realized by mathematics method in controller. It is proved that the compensator can assure the stability of the closed-loop system when the system meet some conditions in the thesis. The simulation result shows the state observer has the ability to effectively compensate random time-delay and filter system noise at the same time.A kind of networked control system simulation platform based on switched Ethernet is designed. The platform is hardware-in-the-loop simulation system and data can be transmitted in real network between sensor and controller, which can conquer the difficulty of not accurate network model when NCS is simulated by computer. The software is programmed by MATLAB and VC++.