Dynamic Matrix Control Algorithm with Data Packet Dropout in Networked Control Systems
A new control strategy based on dynamic matrix control algorithm is proposed in the paper to deal with data packet dropout which universally exists in networked control systems. Proper output value of the system is selected by comparing time stamp of the data packet in controller and actuator, consequently the worsen system performance due to data packet dropout is overcome by the strategy. The simulation results show that the new strategy is effective. Introduction With the wide application of computer network and the continuous development of network technology, the structure of control system is being changed, and networked control systems (NCSs) have become a research hotspot of control field in recent years. NCSs are to introduce network into control system and are of distributed control system rather than independent control system, to connect main functional parts of sensor, executor, controller and other systems via network and transmit and exchange relevant signal and data through communication network. Although NCSs have many advantages over traditional control system, inevitably, they also have some frequently-seen problems of computer network due to the involvement of network, such as network delay, data packet dropout, messy code and disordering, leading to severe impacts on the performance of control system. Prediction control is a widely used new control algorithm in current industrial process control field. Due to the utilization of multistep prediction, rolling optimization and feedback compensation, it is of better control effect and applicable for the industrial processes less prone to establishing precise mathematic model. By the application of the design philosophy of prediction control to NCSs, a network forecast design method compensating network delay and data packet dropout to some extent is developed[1~4]. In the paper, dynamic matrix control algorithm (DMC) is used to compensate data packet dropout to a certain extent, and the simulation results show that the system performance is improved. PC-based Control Design of Networked Control Systems Figure 1. Networked Control System. The network based on TCP protocol is mainly used for ensuring the reliability of data transmission, and the data failing to arrive at the receiving end is usually sent in a repeated way. However, NCSs are of higher real-time demand of system data. Therefore, the repeated sending of old data is not applicable to NCSs. In actual NCSs, when new sampling data or control data arrives, the unsent old data will be deleted. In addition, network congestion or data destroy may cause misfit between the data finally arriving at the end and the data sent at the transmission end. Such phenomena are deemed as loss of network data. Figure 1 shows a typical NCS. NCSs are usually composed of 91 network module, controller, actuator, controlled object, sensor and other nodes. The switching on/off of the switch simulates data transmission and loss in network. From Figure 1, it is not difficult to find that two processes are vulnerable to encountering such data loss, i.e. data transmission from sensor to controller and data transmission from controller to actuator. Due to such two processes, the controller and actuator cannot timely update the measured output value of the controlled object and output value of actuator, thus seriously impacting the system performance. Dynamic Matrix Control Algorithm Dynamic matrix control is a prediction control algorithm based on object step response module. With this algorithm, the optimized control of system is realized through module prediction, rolling optimization and feedback compensation. [5] The amplitude k a of response curve at the sampling moment ⋅ ⋅ ⋅ = , 2 , 1 k can be measured through applying unit step signal to the system. The output of such an object at moment k is caused by all the previous input increments. The prediction module in DMC is of the said step response expression, i.e.