Sensorless control of a three-degree-of-freedom ultrasonic vibration tool holder

Abstract

Piezoelectric actuators are commonly used to deliver vibrations with an amplitude range of (5 μm–25 μm) at ultrasonic frequencies (≥15 kHz) in machining applications. The natural frequencies of ultrasonic vibration actuators vary under the varying cutting force disturbances during machining. Consequently, the amplitudes of delivered vibrations deviate from the desired reference values. As opposed to using vibration sensors, this paper presents a sensorless method to track and control the frequency and amplitude of ultrasonic vibrations during machining. The dynamic model of the actuator is first modeled to obtain a transfer function between the supply voltage and driving current. An observer with Kalman filters per actuator direction is designed to estimate the vibration velocity amplitude and the phase between the supply voltage and current. The estimated vibration amplitude is used as a feedback in a proportional integral (PI) controller to maintain at the desired level during machining. The estimated phase is also kept at the desired reference level by pushing the excitation frequency towards resonance with another PI controller. The proposed control method has been digitally implemented in controlling a three degree of freedom rotary ultrasonic vibration actuator, and the crosstalks between the axes have been compensated when generating elliptical vibrations. The system has been experimentally tested in unidirectional turning and drilling, as well as two directional milling with elliptical vibrations.