Industrial design of a centimetric “TWILA” ultrasonic motor

Thanks to their qualities, ultrasonic motors could well challenge electromagnetic motors in the small actuators market (mechanical power of few watts). As typical of industrial applications, we can quote watch movements (SEIKO) or auto-focus lenses drive (CANON USM lenses serie, NIKON AFS lenses serie). But at present time, the industrialization cost of this kind of motor is still standing in the way of an important industrial development. This paper describes design phases for an ultrasonic motor industrialization. Several motor generations have been developed to obtain the actual Traveling Wave induced by Longitudinal Actuators (TWILA) structure. The reduction of production cost and the reliability of manufacture process have always been preference shares while preserving workable mechanical characteristics. By doing so, cost reductions of 50% and a three-fold increase of the mechanical power have been obtained. All these results essentially arise from the use of a specific stator geometry (the “three quarter of wave length” disposition) connected with conical couplers. It is also shown that the electrical control of the motor is rather difficult due to the presence of a mechanical coupling between the two motor phases.