Discontinuous ‘PD’ Control of Thermal Processes

Abstract

In thermal processes (boilers, furnaces, torches etc.) that are often discontinuously controlled (to reduce the cost of control) a great, deal of confusion has existed as to the effect of derivative signals in the feedback loop. This has resulted in empirical design rules for the derivative feedback settings of their controllers. This paper presents an analysis of the effect of derivative feedback upon the performance indices of such processes. Methods of evaluating these indices, for various combinations of command and derivative feedback, are given. The boundaries are clearly defined for the process instability that results from excessive derivative feedback. It is shown that the well-known cycling characteristic display is inadequate for conveying to the practising engineer the effect of derivative feedback. Two new displays have been derived that clearly show the effect of command and derivative feedback upon the process performance. Finally, the optimum derivative feedback conditions are given for the typical process. The presentation is intended for the professional engineer concerned with the control of industrial processes.