Comparison of industrial controllers : disturbance rejection in an up-and-running industrial flotation process

Abstract

Linear quadratic (LQ) control optimizes a quadratic cost function while following a linear model. It is commercially available in the process industry but often not labeled as such and infrequently used. Froth flotation is a process in the minerals industry that extracts precious metals from a slurry of finely ground rock in consecutive tanks called cells. Flotation cells are often arranged in two parallel streams and the main control task is to regulate the cell levels. In this work, a commercial solution is used to assess the performance of LQ control and compare it to existing PI/PID controllers. Comparing control solutions in the process industry in general and the minerals industry in particular is fraught with difficulty because operating conditions change frequently, especially when studying the capability to reject disturbances. In the flotation process studied here, the disturbances act on two parallel lines, one controlled by an LQ algorithm and the other with the existing PI/PID controllers. This work develops data assessment strategies to isolate events of interest and analyses them both with classical metrics such as integrated absolute error, and with metrics relevant for the operators, such as maximum level deviations. Both these metrics clearly show that the LQ controller performs significantly and consistently better than the PI/PID controllers.