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INTRODUCTION• An Ultrasonic motor is a type of electric motor formed from the ultrasonic vibration of a component, the stator being placed against another, the rotor depending on the scheme of operation.• These motors offer high torque density at low speed, high holding torque, simple construction and have a quick response.
• To obtain the levels of torque speed characteristics of USM using conventional motors we require to add a gear system to reduce the speed.• USM’s are fundamentally designed to have high holding force,providing zero backlash.• These characteristics of USM makes them attractive for robotic applications where small motions are required.
WORKING OF USM• A travelling wave is established over the stator surface,which behaves as an elastic ring and produces elliptical motion at the inter face with the rotor.• This elliptical motion of the contact surface propels the rotor and the drive shaft connected to it.• The teeth which are attached to the stator are intended to increase the moment arm to amplify the speed.
Principle of operation• The general principle of operation of the ultrasonic motors is to generate gross mechanical motion through the amplification and repetition of micro deformations of the active material.• The active material, induces an orbital motion of the stator of the stator at the rotor contact points to produce macro motion of the stator.
• The active material which is a peizo electric material excites a traveling flexural wave within the stator that leads to the elliptical motion of the stator.• Teeth are used to enhance the speed that is associated with the propelling effectof these particles.• The frictional force between the stator and rotor causes the rotor to spin.
Mechanism• Dry friction is often used in contact and the the ultra sonic vibration induced in the stator is used both to impart motion to the rotor and to modulate the frictional force present at the interface.• An exceptionally high torque ‘hybrid transducer’ ultrasonic motor uses circumferentially poled and axially poled piezoelectric elements to combine axial and torsional vibration.
Applications• Canon was one of the pioneers of the ultrasonic motors and made it famous for the manufacture of the autofocus lenses.• USM’s are also used in watch motors and compact paper handling,in spacecraft instruments and subsystems.