Disturbance Compensation by Reference Profile Alteration with Application to Tray Indexing

In precision motion control systems, the two-degree-of-freedom (2-DOF) control structure is widely utilized, but its performance is often deteriorated due to various forms of disturbances. In prior works, it is common to compensate the disturbances by directly modifying the control input with the disturbance observer. However, in many industrial applications, such approaches may not be suitable, as the commercial controllers are often proprietary with a closed architecture. To address this practical difficulty, in this paper, we propose a disturbance compensation scheme which alters the reference profile instead of the control input. The non-causality problem incurred by the altered reference profile is then solved by using a predictive feedforward approach. In addition, a general offsetting mechanism is developed to eliminate the effect of the prediction error within finite sampling intervals. To further enhance the performance, a data-based fine-tuning procedure is implemented. Simulation and experiments are conducted to further illustrate the proposed method and show its practical appeals in industrial applications.