Reference Modulation for Performance Enhancement in Motion Control Systems

In control engineering, there are many situations, where a system designed with a fixed feedback controller is not customizable, and yet its closed-loop performance (e.g., disturbance attenuation) is not satisfactory. To further enhance the closed-loop performance for such systems, we propose a reference modulation method that is compatible with any prefixed controller. By introducing an additional modulated function and a direct feedforward channel, our method allows more flexibilities in customizing the closed-loop sensitivity function, so that both the disturbance attenuation and the tracking performance can be improved. We also provide robust stability analysis of the control systems designed by the proposed method. Simulations are performed on a wafer scanner system with a pre-designed feedback controller. The simulation results show that via reference modulation, both the tracking and disturbance rejection of the closed-loop system are enhanced.