Loop priority based decentralized control for coal ball mill system

Coal ball mill pulverizing system is a multiple-input multiple-output (MIMO) system characterized with strong couplings and disturbances. In the coal ball mill control system, the three loops possess different dynamics with remarkable lag times. In this paper, we proposed a mixed control scheme based on loop priority (namely the outlet temperature loop, the inlet pressure loop, the pressure difference loop) considering the principle of the safe-stability-quality. The outlet temperature loop employees the feedforward compensation to suppress the interaction from the other two loops; the inlet pressure loop uses the disturbance observer based structure to eliminate internal and external disturbances because the characteristics of the fast response from recycling air flow rate to the inlet pressure is fit to cancel the estimated disturbances; The pressure difference loop controller is designed based on the dominant internal disturbance since it has the lowest priority. The three loop controllers are all in the form of two-degrees-of-freedom PID in order to get a better trade-off between set-point tracking and disturbance rejection. The proposed approach can maintain stable operation of the ball mill, and it is readily available to industrial practitioners and DCS software. Simulation results demonstrate the effectiveness of this control strategy.