Improved PI control for a surge tank satisfying level constraints

Abstract

This paper considers the case of averaging level control, where the main objective is to reduce flow variations by using varying liquid levels. However, to avoid overfilling or emptying the tank, the liquid level needs to satisfy safety-related constraints. In the simplest case, a P-controller can be used, but may not give acceptable averaging of the flow, especially if the surge tank is relatively small. In addition, the P-controller does not allow the level setpoint to be adjusted. We propose a simple scheme with a PI-controller for normal operation and two high-gain P-controllers to avoid the liquid level constraints, which is compared with a benchmark MPC strategy. We demonstrate that the proposed method has similar performance, but with less modeling effort, less computational time and simpler tuning.