Controlling a nonlinear process with variable dead-time: A computer-based laboratory experiment

Abstract

In process control, nonlinearities and dead-time often play a significant role in the unsuccessful implementation of control systems. This is due to variations in operating conditions and process behaviour affecting controller performance within the stability limitations of the feedback loop. In a laboratory experiment, where a liquid is heated by means of submerged electrical heating elements, temperature control of liquid in a tank by means of a feedback control loop can be investigated. The relationship between the manipulated variable and the tank temperature is non-linear. The loop was designed to contain significant dead-time, dependent on the variable flowrate of liquid through the system. By means of experimentally obtained open loop process reaction curves and by continously measuring the volumetric flowrate, enough information about the process can be obtained to develop a process model compensating for the presence of dead-time and to take the nonlinear behaviour of the process into account. Using a digital measurement and control system, an algorithm was developed which can be used for accurate control of the process. In this paper emphasis is placed on the current ability of computing equipment to apply theoretical concepts which had been known for a long time.