A survey of grinding circuit control methods:: from decentralized PID controllers to multivariable predictive controllers

A conventional grinding circuit consisting of one open-loop rod mill and one closed-loop ball mill is essentially a two-inputX two-output system, assuming that the classifier pump box level is controlled by a local loop. The inputs are the ore and water feed rates and the outputs are the product fineness and the circulating load. The design problem is to find a control algorithm and a tuning procedure which satisfy specified servo and regulatory robust performances. A first approach is to use decentralized PID controllers and systematic tuning methods which take into account loop interactions. Another technique consists of adding decouplers or pseudo-decouplers to the decentralized controllers. Finally, the design of a fully multivariable controller is a possible option. To face the problem of performance robustness related to change of process dynamics, two options are studied. A design criterion involving the minimization of a penalized quadratic function on a future trajectory can be used. A second alternative is to track process dynamics changes using adaptive process modelling. The paper will present a comparison of these various strategies, for a simulated grinding circuit. A benchmark test, involving a sequence of disturbances (grindability, feed size distribution, change of cyclone number...) and setpoint changes, is used to compare the performances of the controllers. (C) 2000 Elsevier Science S.A. All rights reserved.