Development of high-precision short-stroke actuator enabling tunable motor constant

Abstract

High-speed high-precision actuators are crucial in scanning systems with nanometer resolution, such as wafer scanners. To improve the through put and production quality, actuators have to realize a high motor constant (force to current ratio) and a high motion resolution. In comparison with state-of-the-art Lorentz actuators, which have a relatively low motor constant, reluctance actuators possess a higher motor constant. However, the high motor constant amplifies the noise in current, impairing the motion resolution, which results in a motor-constant dilemma.This thesis proposes a reluctance actuator capable of tuning its motor constant to overcome the dilemma. A typical scanning trajectory can be divided into acceleration/deceleration phase and constant-velocity phase. By maximizing the motor constant at the acceleration/deceleration phase, the mover can be brought to its desired velocity in a short time, and the minimum motor constant can be used at the constant-velocity phase to achieve a high motion resolution. Thus, both high motor constant and high resolution demands can be satisfied.The proposed tunable reluctance actuator, which consists of a stator, a mover and two identical coils, is designed based on analytical analysis method. Flexures are designed to guide the mover for a one-degree-of-freedom motion. The coils are driven by a combination of a constant bias current and a control current. For validation of the tunable motor constant, frequency responses of the actuator with different bias currents are measured. The measurement results show that by tuning the bias current, the motor constant of the actuator can be successfully tuned by a factor of 5.13. To investigate the influence of motor constant on motion resolution, experiments are conducted. Experimental results show that with a lower motor constant, a higher motion resolution can be realized. When the motor constant is varied from its maximum to the minimum, the motion resolution of the actuator is improved by a factor of 5.3. The experiments confirm the existence of the motor-constant dilemma and demonstrate that the proposed actuator can overcome it by tuning its motor constant.