Closed-loop control of a three degree-of-freedom ultra accurate linear stepper motor

In this paper, we present a robust adaptive control system designed for three degree-of-freedom linear motion ultra accurate (Sawyer) stepper motors. Such motors are commonly used in the semiconductor industry for wafer-probing and in automated assembly. A detailed model of the motor is developed. A controller is designed for the motor that is robust to parametric uncertainties in the model and achieves good tracking performance in the presence of disturbances while compensating for the undesirable effects of yaw rotation of the motor in the platen plane. The proposed current level controller utilizes all the states for feedback. The stability of the closed-loop system is shown using Lyapunov techniques. The tracking errors are shown to be globally uniformly bounded. Simulation studies are carried out to show the efficacy of the proposed control design methodology.