Torsional actuator and stepper motor based on piezoelectric d₁₅ shear response

Two novel devices, torsional actuator and torsional stepper motor, are described and their performance is investigated in detail. Both devices were designed to produce large angular displacement and large torque output from the piezoelectric d₁₅ shear response. In the torsional actuator, the large displacement can be achieved due to built-in amplification of piezoelectric strain using the aspect ratio of a piezoelectric tube. It is shown that the displacement can be controlled by changing the length of the tube, and that the blocking torque developed is independent of the length. It is also shown that the performance of the actuator does not change under external torque load, and that the prestress in radial direction improves the mechanical strength of the actuator without affecting the produced displacement. In the stepper motor, the unlimited angular displacement of the rotor is produced by accumulating the piezoelectric strain by using a direct coupling mechanism between the stator and the rotor, where a clutch drives the rotor by locking it. The direct coupling makes it possible to transmit the whole power generated in the piezoelectric tube to the rotor, and thus achieve the high efficiency of the motor. Also, the locking mechanism allows smooth motion either in a continuous or stepwise fashion within a 360° interval with a precise control over angular positioning. In addition, the advantage of both devices is their simplicity and a possibility to produce them in the compact package.