Speed control of elastic force motors by means of integrated piezoelectric sensors

The rotation speed of an ultrasonic flexure wave type micromotor has been regulated using piezoelectric detection of the rotating stress field induced by the contact force between the stator and the rotor. The regulation principle proposed is to insert the motor as a Voltage Controlled Oscillator in a Phase Lock Loop circuit. In normal operation, the rotation speed of the motor will be locked to an external control frequency. In addition, the number of rotor revolutions can be obtained on a counter for specific applications. With this configuration, quasi-synchronous or step-like operations are possible, increasing considerably the application field of such devices. This technique can be applied to a miniaturized version of the Elastic Force Motor (EFM) using a batch processed silicon micromachined stator with a patterned active piezoelectric layer. Based on this principle, future hybrid EFM associated with a low power, single chip control unit will provide a low cost actuator for mechanical microsystems.