Frequency behaviour and speed control of piezomotors

The rotational speed of a piezomotor is directly linked to the vibration ve locity of its stator. Because some drifts of the stator characteristics lea d to-an unwanted variation of the motor speed, a control is necessary. This paper discusses the origins of drifts in travelling wave ultrasonic motors (TWUM). The typical behaviour of this kind of motor and its consequences o n speed control are presented. While control is easily achieved when the ap plied torque is constant, variations of this quantity make more difficult t o obtain an efficient method. Filially, the basic idea of this paper is to derive a general control method by using the absorbed current, which is the only always detectable quantity. (C) 2000 Elsevier Science S.A. All rights reserved.