Ultrasonic Motor

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ULTRASONIC MOTOR

Abstract:
An ultrasonic motor is a type of electric motor formed from the ultrasonic vibration of a component, the stator, placed against another, the rotor or slider depending on the scheme of operation (rotation or linear translation). Piezoelectric ultrasonic motors are a new type of actuator. They are characterized by high torque at low rotational speed, simple mechanical design and good controllability. They also provide a high holding torque even if no power is applied. Compared to electromagnetic actuators the torque per volume ratio of piezoelectric ultrasonic motors can be higher by an order of magnitude. Many different types of ultrasonic motors have been proposed up to date.  Ultrasonic motors are of great interest due to the flexibility of miniaturization in comparison with conventional electromagnetic motors whose efficiency decreases significantly. Especially in information systems and medical industry, compact size of these motors makes them find wider applications. This paper describes the advantages of ultrasonic motors over the conventional electromagnetic motors, several types of piezoelectric ultrasonic motors. The basic working principle; the mechanism of the ultrasonic motor are also explained. Finally, its applications and the only disadvantage are discussed.
INTRODUCTION
An ultrasonic motor is a type of electric motor formed from the ultrasonic vibration of a component, the stator, placed against another, the rotor or slider depending on the scheme of operation (rotation or linear translation). Ultrasonic motors differ from piezoelectric actuators in several ways, though both typically use some form of piezoelectric material, most often lead zirconate titanate and occasionally lithium niobate or other single-crystal materials. The most obvious difference is the use of resonance to amplify the vibration of the stator in contact with the rotor in ultrasonic motors. Ultrasonic motors also offer arbitrarily large rotation or sliding distances, while piezoelectric actuators are limited by the static strain that may well be induced in the piezoelectric element.
Piezoelectric ultrasonic motors are a new type of actuator. They are characterized by high torque at low rotational speed, simple mechanical design and good controllability. They also provide a high holding torque even if no power is applied. Compared to electromagnetic actuators the torque per volume ratio of piezoelectric ultrasonic motors can be higher by an order of magnitude.
The ultrasonic motor is characterized by a ?low speed and high torque?, contrary to the ?high speed and low torque? of the electromagnetic motors. Two categories of ultrasonic motors are developed at our laboratory: the standing wave type and the traveling wave type.
The standing wave type is sometimes referred to as a vibratory-coupler type, where a vibratory piece is connected to a piezoelectric driver and the tip portion generates flat-elliptical movement. Attached to a rotor or a slider, the vibratory piece provides intermittent rotational torque or thrust. The travelling-wave type combines two standing waves with a 90?-phase difference both in time and space. By means of the traveling elastic wave induced by the thin piezoelectric ring, a ring-type slider in contact with the surface of the elastic body can be driven.

DRAWBACKS OF ELECTROMAGNETIC MOTORS

???????? Electromagnetic motors rely on the attraction and repulsion of magnetic fields for their operation. Without good noise suppression circuitry, their noisy electrical operation will affect the electronic components inside it. Surges? and spikes from these motors can cause disruption or even damage in nonmotor related items such as CRTs and various types of receiving? and? transmitting equipments. Also , electromagnetic motors are notorious for consuming high amount of power and creating high ambient motor temperatures. Both are undesirable from the efficiency point of view. Excessive heat energy is wasted as losses. Even the efficiently rated electromagnetic motor has high input to output energy loss ratios.

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Replacing these by ultrasonic motors would virtually eliminate these undesirable effects. The electromagnetic motors produce strong magnetic fields which cause interference. Ultrasonic motors use piezoelectric effect and hence no magnetic interference.????

Ultrasonic motor construction tends to be simpler than EM type motors. Fewer assembly parts mean fewer moving parts and consequently less wear. The number of components required to construct an USM is small thereby minimizing the number of potential failure points.

???????? As the ultrasonic motor uses ultrasonic vibrations as its driving force,it comprises a stator which is a piezoceramic material with an elastic body attached to it,and a rotor to generate ultrasonic vibrations. It therefore does not use magnets or coils. Therefore there is no problem of magnetic field and interference as in the case of electric motors. In ultrasonic motors, piezoelectric effect is used and therefore generates little or no magnetic interference.


???????? Constructionally,a stator and a rotor are coupled to form an ultrasonic motor. On one side of the metal stator, the piezoceramic is glued on.It is an element that generates ultrasonic vibrations when a high frequency voltage is applied. Ultrasonic motors make use of a ring shaped piezoelectric crystal with segments having sequentially reversed polarity.On the other side of the metal stator,comb tooth grooves are set up.The rotor (dynamic body) is pressed tightly against this side of the stator metal surface so that they are adhered together closely.

 

 

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