Cylinder-sphere 3-DOF ultrasonic motor and its control

A three degree-of-freedom (DOF) ultrasonic motor (USM) with cylinder-shaped stator and a spherical rotor was developed using one first order longitudinal and two second order bending nature vibration modes of the cylinder. Control strategies for two DOF trajectory following are studied and applied to the prototype USM. Vibration amplitude control is employed for speed regulation. Our first trajectory following strategy is the step-by-step interpolation. The second strategy is vector decomposition control. Three pulse width modulation (PWM) methods were investigated for the exciting source voltages regulation. These methods were compared and experimentally verified by several experiments. The key is to keep the phase differences of the three vibrations constant and small exciting voltage distortion while changes the exciting voltages for simplify the control process and obtain good control performance. The vector control method takes several advantages, such as small trajectory following error, smooth moving and hence low noise.