Adaptive PI-Control of Superheat in a Refrigeration System

Abstract

This paper describes adaptive control of the superheat in a refrigeration system. Traditionally, the superheat is PI controlled using constant gain and integration time, which often will have a conservative tuning. On-line update of the controller parameters in self-tuning scheme by identification in open loop or by using external signals in closed loop is cumbersome. In this work a method using PI controllers with shifting parameters in a bumpless set-up has been used for finding better parameters. Each controller is used in a certain time interval and ISE (Integrated Squared Error) is calculated as a measure for the quality. This is used in a gradient method to increase the performance. The improved PI controller gives the possibility to optimise the superheat reference via an additional control loop. The controllers are designed and tested in a laboratory set-up. The performance of the shifting adaptive controller turns out to have a better performance than a fixed parameter PI controller.