Gain-adaptive control applied to a heat exchange process using a first order plus deadtime compensator

Abstract

The Smith deadtime compensator with and without model gain-adaptation is applied via a digital computer to a heat exchange process. A first order model with deadtime forms the basis of the well-known compensation scheme which is used in combination with a main process controller of the proportional plus integral (PI) form. Here, adaptation of the static model gain is added in an attempt to stabilize the control system in the face of certain types of process changes. This gain-adaptor also uses PI control of the static model gain to force the undelayed model response into agreement with the process response. Further, as the static model gain changes, the main process controller gain is changed in an attempt to maintain stability. Test runs were made on an actual double-pipe heat exchanger of an industrial size using steam to heat water. Control of the outlet water temperature (process response) was tried using the deadtime compensation schemes and regular PI control by itself. The responses for step changes in set-point water temperature show the superior tracking behavior of the deadtime compensation schemes. For decreases in the water flow rate, it is shown that regular PI control and Smith's method can give oscillatory responses. With proper tuning the gain-adaptive procedure is shown to maintain stability for this type of process change.