Iterative Feedforward Tuning With Multiplicative Nonlinearity: Application to Linear-Motor-Driven Wafer Stage With Multiplicative Force Ripple

Iterative feedforward tuning (IFFT) plays important roles in precision motion control systems to ensure high motion performance and high task flexibility simultaneously. In this article, IFFT subject to the unknown multiplicative nonlinearity is further investigated. The proposed method adopts the inverse compensation mechanism for the multiplicative nonlinearity. Thus, a rational feedforward controller is developed with the numerator compensating for the reference trajectory and the additive disturbance and with the denominator compensating for the multiplicative nonlinearity. After feedforward parametrization, the IFFT algorithm is proposed to realize the simultaneous tuning for the numerator and denominator polynomial parameters. To enhance the convergence speed, an inner-loop iteration procedure is further utilized in each trial to obtain a more accurate data-based model required by IFFT by more fully utilizing the servo error. The superiorities of the proposed method in achieving faster, more accurate and more flexible tuning compared to relevant methods are finally verified through experimental results on a linear-motor-driven wafer stage with multiplicative force ripple.