Ultrasonic Motor

Piezoelectric Effect

        Many polymers, ceramics and molecules are permanently polarized; that is some parts of the molecules are positively charged, while other parts are negatively charged. When an electric field is applied to these materials, these polarized molecules will align themselves with the electric field, resulting in induced dipoles within the molecular or crystal structure of the material. Further more a permanently polarized material such as Quartz(SiO2) or Barium Titanate(BaTiO3) will produce an electric field when the material changes dimensions as a result of an imposed mechanical force.

Absence Of Magnetic Motion

        As the ultrasonic motor does not use coil or magnets as its driving force, it does not generate magnetism. It can be operated without influence of magnetism even in strong magnetic fields.

Introduction

        All of us know that motor is a machine which produces or imparts motion, or in detail it is an arrangement of coils and magnets that converts electric energy into mechanical energy and ultrasonic motors are the next generation motors.  In 1980,the world’s first ultrasonic motor was invented which utilizes the piezoelectric effect in the ultrasonic frequency range to provide its motive force resulting in a motor with unusually good low speed, high torque and power to weight characteristics.

Working

        When a voltage having a resonance frequency of more than 20 KHz is applied to the piezoelectric element of an elastic body(a stator),the piezoelectric element expands and  expands and contracts. The piezoelectric ring is divided into two groups of alternated polarities, which are driven simultaneously by cyclic signals that are ninety degrees out of phase, to produce a traveling wave of flexural vibrations. The third input lead is ground and attached to the ring itself. It acts as a common return to both  the out of phase input leads.

Abstract

         Ultrasonic motors are characterized by the absence of noise during operation, High torque weight ratio, highly accurate speed and position control etc . The advantages of ultrasonic motors makes them open to a wide range of applications and technologies.

Piezoelectric Ceramic

        The piezoelectric ceramic used in the ultrasonic motor is an element that generates ultrasonic vibrations when a specified high frequency voltage is applied. The piezoelectric ceramic itself expands and contracts when high frequency voltage is applied. This phenomenon is called electrostriction.

Applications

1.     Lens actuator for automatic focusing of cameras.

2.     Watch movements

3.     Roll screen motor for automatic open close actions.

4.     Positioning in satellite reception devices.

5.     Actuators in strong magnetic field

Conclusion

        Utilising the large piezoelectric coefficients and high dielectric constants of ferroelectric thin films, it is possible to micro fabricate USMs on silicon substrates. By using ferroelectric thin films of lead zirconate titanate (PZT) a two order of magnitude improvement over bulk ceramic materials is obtained in break down strength.