Finite element models for the piezoelectric actuation in ultrasonic traveling wave motors

By appropriate superposition of two standing waves, a traveling wave can be excited in the ring-shaped stator of a traveling wave motor. Thus, the material points on the surface of the stator perform an elliptical motion. A second disk, pressed against the stator, is driven by frictional forces, generated in the interface between stator and rotor. Traveling wave motors produce a very high torque at low rotational speed. Their simple mechanical design and good controllability make these motors a very promising alternative to small electro-magnetic motors. The mechanical oscillations of high frequency are excited by piezoelectric ceramic actuators, bonded to the surface of the stator. The dynamic behavior of the stator is strongly influenced by these actuators. Since it is difficult to produce big ceramic rings and bond them to the stators, single piezoelectric elements have to be used for the excitation of large stators. The geometry of the ceramic actuators as well as the stiffness of the bonding layer do have marked influence on the efficiency of the excitation, because the actuation mechanism is of induced strain type and the stator is excited by tangential stresses in the bonding layer. In this paper, the influence of the shape of the ceramics on the vibration of the stator is investigated using the finite element method. A detailed model taking into account the piezoelectric effect in the ceramic actuators as well as the exact geometry of the stator is used to calculate the transfer function between the electrical excitation and the mechanical vibrations of the stator. The results allow to compare the efficiency of different actuator configurations.