Grinding circuit control: A hierarchical approach using extended 2-DOF decoupling and model approximation

During the operation of a grinding circuit (GC), the purpose of control is to ensure the final grinding production indices (GPIs) meet prescribed technical requirements. However, due to the complex dynamic characteristics between the GPIs and the control loops, such control objectives are difficult to achieve by using existing control methods. The complexity is reflected by the existence of strong process coupling, multiple time delays, and large time variations. In this paper, a hierarchical control approach which comprises a lower-level loop control system (LLCS) and a higher-level loop setting system (HLSS) has been presented to control the complex grinding process. The LLCS designed by the distributed PID control technique is responsible for forcing the key process variables that closely relate to the GPIs to track the given set-points. The HLSS, which mainly consists of a feedforward pre-setting controller and a feedback compensator, is used to auto-adjust the set-points of the LLCS in the presence of significant uncertainty about the plant behavior and disturbances so as to maintain good overall operation performance. To design the HLSS, an extended 2-degrees-of-freedom (2-DOF) decoupling control method is presented together with a model approximation method based on the multiple-point step response fitting technique. At last, several simulations have been performed to demonstrate the proposed control method for GC operation.