Theoretical foundations of process control

Abstract

The paper surveys the theoretical foundations which underlie the behaviour of automatic process control. The general nature of control systems is described and, for analytical purposes, the control action and the plant response are taken separately. Consideration is given to the plant characteristic as determining response to potential correction and mention is made of the analogies with electric circuits which can be usefully employed in this connection. Basic control actions are described and a fundamental distinction is drawn between action and control. The questions of stability and damping are examined with the help of vector diagrams and polar curves. Stress is laid on the limitations on damping which are imposed by the nature of the plant characteristic. Theoretical examples are given of the effect of various methods of control on a postulated plant in face of load and set-point changes. Attention is drawn to the approximations involved in the practical generation of integral and derivative control action, with particular reference to the control setting restrictions imposed by present methods of providing combined proportional, integral and derivative control. Finally, brief consideration is given to the possibilities of applying theoretical analysis to actual process control.