Analysis And Solution For Network-induced Delays In Networked Tuning Based Control Systems

With the advent and rapid development of information technologies, communication networks are being used as popular transmission medium in control systems. In traditional networked control systems (NCSs), control loop is closed through special control networks with different incarnations: DeviceNet, ControlNet, Profibus, Modbus, etc. The emergency of NCSs provides a novel and effective way to implement device level communication and control.In the meantime, plenty of advanced control strategies with identification or tuning algorithms are presented to realize the control of complex objects. Therefore, the controller can automatically adapt itself to retain control performance as the controlled plant is under changes. This technique has been well-accepted in academic area. But in real applications, the complicated tuning algorithms can not be carried out with limited computation resources in device level controller. This problem is the major obstacle for the feasible implementation of advanced control strategies in real applications.In this paper, a new control system framework, defined as Networked Tuning based Control Systems (NTCS), is first presented to remove above obstacle. Its defining feature is a tuning loop consisting local controller and remote computational and tuning device, and this loop is closed through communication networks. Obviously, the insertion of network can offer modularity and flexibility in practical applications. More importantly, the controller is developed with the ability to communicate to remote tuning device through networks. Thus, the remote tuning device can handle very complex tasks such as online parameter estimation for the controller while the local controller can be simple and cheap enough to only do limited work.Local controller and remote computational and tuning device are connected through communication network in NTCSs. Hence, the non-deterministic phenomena such as network-induced delays are inevitable during data transmission, and will deteriorate the real-time transmission of tuning result from tuning device to controller and even destabilize the entire system. This paper aims to study network-induced delays' influence on control systems and what we can do to relieve this influence, guarantee system stability and improve control quality.First of all, network-induced delay, which is the most important non-deterministic phenomena, is studied. The sources of delays including communication protocols and control devices are discussed and the possibility of reducing all delays into an equivalent delay is proposed. Moreover, the criteria to choose suitable communication protocol and control device in NTCSs are presented. In general cases, clock-driven local controller and event-driven remote tuning device connected by Ethernet is a good option...