Time Delay Compensation in Networked Control System

Abstract

Networked control system is a special type of distributed control system where control loop is enclosed by communication medium. Networked Control System (NCS) suffers from the networked induced delay which may be induced in the forward path as well as in the feedback path. This delay is variable in nature. So if a controller is designed without considering the delays or considering the fixed delay then system performance will be degraded and in the worst case the system become unstable. To compensate the network induced variable transporting delay,a number of methods have been proposed in the literature such as robust control, Smith Predictor and Inteligent control theory. But there are few works reported in literature that employ Linear Quadratic Regulator (LQR) to compensate the networked induced delays.Firstly an LQR controller is designed to compensate the networked induced variable delay which is varied up to a maximum value. Then an LQG controller is designed to compensate the networked induced delay in noisy environment. Here the controller is the same controller used in LQR technique. Only difference between the standard LQR and the LQG controller said now is that it uses Kalman Filter to estimate the plant output using noisy measurement. Then an Model Predictive Controller (MPC) controller is designed using Laguerre network considering the constraints on control input and on the rate of control input. An integrator plant is considered for simulation where the above three controllers are applied. From the simulation result, it is observed that LQR gives a better step response but MPC has better disturbance rejection capacity. To validate the controllers in real-time, an experiment has been conducted in the Labrotory. In the experimental setup using one PC is considered as controller and other one is considered as plant. They are connected through an Ethernet network. From the real time experiment results it is seen that LQR exibits superior delay compensation performance.