Nonlinear feedback control of dual-stage actuator system

Abstract

The wafer stage of lithography is a dual-stage actuator (DSA) system. An nm-level positioning precision is required by using macro movement of long-stroke linear motor and high-precision micro movement of short-stroke voice coil motor, while the platform is moving in high-speed. This brief presents a nonlinear control method for dual-stage actuator systems to track a step command input fast and accurately. To further reduce the settling time, we design the long-stroke actuator controller with the control law of proximate time optimal control (PTOC) to yield a closed-loop system with a small damping ratio for a fast rise time and certain allowable overshoot. Moreover, for the purpose of reducing the overshoot caused by the long-stroke actuator as the system output approaches the target location, a composite nonlinear feedback (CNF) control law is designed for the short-stroke actuator to yield a closed-loop system with a large damping ratio. The linear extended state observer (LESO) was designed to estimate the unknown velocity and compensate the disturbance of servo systems, thus static error could be effectively decreased. We applied this proposed control method to an actual DSA positioning system, which consists of a linear motor and a voice coil motor. Experimental results show that our approach can improve the dynamic performance and the anti-jamming capability of the system, enhance the control precision.