Dynamic Preload Control for Rotary Ultrasonic Motors

Abstract

In typical rotary traveling-wave Ultrasonic motors, the preload force between the stator and the rotor is optimized to achieve the best compromise between output torque and speed to suit different applications. Due to this force, a high stationary torque is generated which is not suitable for many applications. This research investigates the effect of preload force on the motor performance. First, the preload force was statically adjusted using a helical spring. Then, the preload force was dynamically-controlled using a Piezo-linear actuator. Actively-controlling the preload enables free movement with a relatively low preload while allowing for higher output torque under fairly high preload. The results show a direct proportionality between preload and output torque, and an inverse proportionality with speed. Additionally, the transient characteristics of the motor could be adjusted resulting in an improved system response and a minimized system noise. Furthermore, dynamic preload control offers an efficient USM driving control scheme. By employing driving frequency, and preload force as control parameters, the driving efficiency could be enhanced. The efficiency was evaluated by measuring the input electrical power, and the output mechanical power (Torque and Speed).