Reference Modulation for Performance Enhancement of Motion Control Systems With Nonlinear Parameter Variations

In general, it is not intuitive to quantify the performance of nonlinear systems; thus, designing controllers and/or estimators is not easy. Moreover, there are situations in which a system designed with a fixed feedback controller is not customizable, and yet its closed-loop performance (e.g., disturbance attenuation) is not satisfactory. In this study, we first present an equivalent linear model for motion control systems with nonlinear parameter variations. The linear model facilitates the quantitative analysis of the motion control system. Furthermore, we introduce a reference modulation technique that enhances the performance by boosting loop-gain over a desired range of frequencies. The proposed approach, by introducing an additional modulated function and a direct feedforward channel, facilitates the separation of the closed-loop sensitivity function from other issues of the closed-loop system design so that both disturbance attenuation and tracking performance can be improved. We also provide robust stability analysis of the motion control systems by reference modulation. The simulations and experiments are performed on a permanent magnet synchronous motor system with a predesigned feedback controller. The simulation and experimental results show that both the tracking and disturbance attenuation performance of the closed-loop system are enhanced via reference modulation.