ULTRASONIC

MOTORS

SONAL MAHESHWARIB110986EE

CONTENTS

INTRODUCTION

PRINCIPLE OF OPERATION

CONSTRUCTION

WORKING

FRICTIONAL COUPLING

CLASSIFICATION

CONTROL TECHNIQUE

ADVANTAGES

DISADVANTAGES

APPLICATIONS

CONCLUSION

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INTRODUCTION

Motor is a device that converts electrical energy to mechanical energy.

Almost all the motors work on the principle of Faraday’s Law of Electromagnetic Induction.

The energy conversion in such motors involves two stages, electrical energy to magnetic

energy and further magnetic to mechanical energy.

Because of two-stage energy conversion, electromagnetic motor suffer from several losses

that lead to a rigorous energy wastage.

The electromagnetic motors that are widely used in home, industrial and other sectors suffer

from several disadvantages –

1. Noisy operations

2. Surge currents and spikes

3. Magnetic losses

3

Contd..

4. High power consumption

5. Low power factor

6. Comparatively lesser efficiency

Therefore, a new class of motors using high power ultrasonic energy, ultrasonic motors were introduced.

The ultrasonic motors belong to the class of piezoelectric motors, which directly convert electric energy to mechanical energy.

USM plays an important role in few niche markets, where the size, torque, speed or other requirements couldn’t be satisfied by electromagnetic motors.

USMs replaced not only the EM motors, but also the servomotors, stepper motors and synchronous motors.

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PRINCIPLE OF OPERATION

Before learning about USMs it is required to know about piezoelectric effect.

Piezoelectric effect was discovered by Jacques Curie & Pierre Curie [France-

1880].

It was found that in certain types of crystals when a pressure is applied across a

pair of opposite faces, an equivalent potential difference is developed across the

other pair of opposite faces.

Further, if we reverse the direction of application of force, the polarity of the

potential difference developed also reverses.

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In figure given below, a compressive force is applied across the crystal and we obtain a

potential difference as shown but if the compressive force is replaced by an elongation

force, the polarity reverses.

It was later discovered that the converse of this phenomenon is also possible.

Piezoelectric Effect

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Converse Piezoelectric Effect

When a potential difference is applied across the pair of opposite faces, compressions or

elongations are obtained across the other pair of opposite faces depending upon the polarity

of the applied PD.

Further, the application of AC voltage across these faces resulted in alternating

compressions and elongations (mechanical vibrations) across the other pair of faces. This is

the driving force behind the USMs.

Crystals that exhibit the above phenomenon are called piezoelectric materials.

In short, the Ultrasonic Motors works on the principle of converse piezoelectric effect.

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CONSTRUCTION

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.

8

Contd..

The Ultrasonic Motors constitutes mainly four parts, viz.

1. Actuator

2. Stator

3. Rotor

4. Casing

1. ACTUATOR

• It is a driving unit of USM.

• It is made up of piezoelectric material such as Quartz, Barium Titanate, Tourmaline, Rochelle salt, etc.

• It is fixed on the stator using thin metal sheets and bearings.

• It is directly connected to the supply mains.

9

Contd..

2. STATOR

• Stator is the stationary but vibrating part.

• It is constructed using a malleable material, usually steel.

• It can be of ring, cylindrical or rod shaped.

3. ROTOR

• It is the rotating part, which acquires the energy conversion and produces the desired

torque on the shaft.

• It is made of the same material as that of the stator and does have the same shape.

10

Contd..

4. CASING

• To provide protection against abrasive forces, external interferences and extreme

environmental conditions.

• They are made of non- corrosive alloys or fiber.

• Cylindrical, disc or box shaped.

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Comb tooth grooves are created on the stator surface that adheres to the rotor.

These are devised to make the amplitude of elliptic motion large and to reduce abrasions.

Further feature of comb tooth is to amplify vibrations.

The grooves also allow the dust created by friction to escape and thus keep the contact surface dust free.

So the USMs are well known for their low speed operation.

Torque ratings averaging 10 times greater than a comparably sized electromagnetic motor can be achieved.

Voltage inputs vary piezo-crystal assembly. That is, if the piezo-ring assembly is of a thinner design, the voltage requirements will be less than that of a thicker type piezo-ring assembly.

Power requirements for the USM usually rate in the low range.

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FUNDAMENTAL CONSTRUCTION

OF USM

► Figure shows the basic construction of

USMs, which consist of a high-

frequency power supply, a vibrator

and a slider.

► Further, the vibrator is composed of a

piezoelectric driving component and

an elastic vibratory part.

► The slider is composed of an elastic

moving part and a friction coat.

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ULTRASONIC MOTOR BY

BARTH

The practical ultrasonic motor was proposed

firstly by H V Barth of IBM in 1973.

As shown in figure, the rotor was pressed

against two horns placed at different locations.

By exciting one of the horns, the rotor was

driven in one direction, and by exciting the

other horn, the rotation direction was reversed.

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WORKING OF USM

When the supply is switched ON, the actuator starts vibrating owing to

converse piezoelectric effect.

The particles of the stator receive energy from the actuator and starts

vibrating in the plane. This results in the formation of a surface wave.

The stator and rotor are placed so close to each other that their surfaces

almost grazes upon each other.

The surface waves so produced have a frequency in the ultrasonic range and

are not visible by our bare eyes. These waves cause the stator to slide against

the rotor.

15

Contd..

As the wave propagates, the rotor is pulled

back in the opposite direction of movement

of the wave.

In the figure, the surface wave produced is

propagating in the anti-clockwise direction,

whereby, the rotor is pulled to rotate in the

clockwise direction.

The shaft, upon which the rotor is mounted,

now rotates and the output torque is thus

obtained.

From the figure, it is evident that the stator

and rotor always possess the same shape.

The figure depicts a rotational USM.

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The above figure is the schematic diagram of a linear USM driven by dual actuators.

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FRICTIONAL COUPLING

As we have discussed in the working of USM that a surface wave is generated in the

stator, by the converse piezoelectric effect occurring in the actuator.

As the wave propagates through the stator, the rotor is pulled back in the opposite

direction.

Now, there is a chance that the rotor may get slipped off from the stator. To avoid this

hitch, the surface of contact between rotor and stator is made rough or rather uneven.

This method of coupling of rotor and stator is known as frictional coupling.

The grooves are designed in such a shape as to toss away the dust produced by this

process.

During the design of frictional grooves, care is to be taken that during slow speeds,

interlocking of rotor and stator doesn’t occur.

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CLASSIFICATION OF USMs

There are several classifications for ultrasonic motors depending upon their shape, size,

type of motion, application etc.

The USMs are also classified according to the type of wave that is being generated in the

stator.

If the wave produced in the stator has its particles vibrating only in the vertical direction,

(standing wave), such type of motors are called Standing Wave type USMs.

And, if the wave produced in the stator has its particles vibrating both in the vertical

direction as well as horizontal direction (travelling wave), such type of motors are called

Travelling Wave type USMs.

Even though the type of wave produced is different, both these types of USMs has got the

same construction and working principles.

However, the Travelling Wave-type USMs are found to have better operating

characteristics.

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The other methods of classifications of ultrasonic motors are,

1. Mode of operation:

a. Static

b. Resonant

2. Type of motion:

a. Rotary

b. Linear

c. Spherical

3. Shape of implementation:

a. Beam

b. Rod

c. Disk

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► If we categorize them from the

vibrator shape, there are

• Rod type

• π-shaped

• Ring (square)

• Cylinder types.

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TYPES OF ULTRASONIC

MOTORS

ULTRASONIC

MOTOR

STANDING

WAVE TYPE

LINEAR

MOTOR

ROTARY

MOTOR

TRAVELLING

WAVE TYPE

LINEAR

MOTOR

ROTARYMOTOR

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STANDING WAVE TYPE USM

The first ultrasonic standing wave motor was proposed by H.V. Barth in 1973.

The standing-wave type is sometimes referred to as a vibratory-coupler type or a

woodpecker type.

A vibratory piece is connected to a piezoelectric driver and the tip portion generates

flat elliptical movement.

Fig: Vibratory coupler type motor (a) and its tip locus (b).

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► When a vibration displacement:

u(x) = u0 sin(ωt +α)

is excited at the piezoelectric vibrator, the vibratory piece generates bending because

of restriction by the rotor,

► So that the tip moves along the rotor face between A → B, and freely between B →

A.

► If the vibratory piece and the piezo-vibrator are tuned properly, they form a

resonating structure.

► Therefore, only the duration A → B provides a unidirectional force to the rotor

through friction, i.e. intermittent rotational torque or thrust.

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LINEAR TYPE STANDING WAVE

USM

K Uchino et al invented a π-shaped linear motor.

This linear motor is equipped with a multilayer piezoelectric

actuator and fork-shaped metallic legs.

Since there is a slight difference in the mechanical resonance

frequency between the two legs, the phase difference between

the bending vibrations of both legs can be controlled by

changing the drive frequency.

A test motor 20×20×5 mm 3in dimension exhibited a maximum

speed of 20 cm/s and a maximum thrust of 0.2 kgf with a

maximum efficiency of 20%, when driven at 98 kHz at 6 V

(actual power = 0.7 W).

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The figure given below shows the motor characteristics of the linear motor.

Applications of linear standing wave type motors are paper or card senders.

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ROTATING TYPE STANDING

WAVE USM

The torsional coupler consisting of two legs which transform

longitudinal vibration.

Extruder is aligned with a certain cant angle to the legs, which

transforms the bending to a torsion vibration.

This transverse moment coupled with the bending up–down

motion leads to an elliptical rotation on the tip portion.

The optimum pressing force to obtain the maximum thrust is

obtained, when the ellipse locus is deformed roughly by half.

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However, this type provides only

unidirectional rotation.

Even though the drive of the motor is

intermittent, the output rotation becomes very

smooth because of the inertia of the rotor.

Provides high speed than linear motors

because of high frequency (160kHz)&

amplified vibration.

Provides speed of 1500 rpm, torque of 0.08

Nm and efficiency of 80%.

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The standing-wave type is

• Low in cost (one vibration source) and

• High efficiency (up to 98% theoretically)

• But lack of control in both clockwise and counterclockwise directions.

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TRAVELLING WAVE USM

The propagating-wave type combines two standing waves with a 90

degree phase difference both in time and in space.

Representation of travelling wave

U(x ,t)= A cos(k x) cos(wt) + A cos(k x - 90) cos (wt-90)

A surface particle of the elastic body draws an elliptical locus due to the

coupling of longitudinal and transverse waves.

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LINEAR TYPE TRAVELLING WAVE

USM

Linear motor using bending vibration.

The two piezoelectric vibrators installed at

both ends of a transmittance steel rod

excite and receive the traveling transverse

wave.

Adjusting a load resistance in the receiving

vibrator leads to a perfect traveling wave.

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Exchanging the role of exciting and receiving piezo-components provided a

reverse moving direction.

The bending vibration transmitting via the rail rod.

Low efficiency around 3% because the whole rod must be excited even when

only a small portion is utilized for the output.

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ROTARY TYPE TRAVELLING

USM

Two voltage sources are used to produce

travelling wave.

Vibrations of the piezoelectric material is

amplified by the stator teeth.

Due to frictional forces rotor rotates.

Resonant frequency- 46kHz.

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The travelling-wave type-

• Requires two vibrating source.

• Controllable in both direction.

• Silent operation, so suitable to video cameras with

microphone.

• Thinner design, leading to space saving.

• Low efficiency(not more than 50%)

• Energy saving.

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Comparison of the motor characteristics of the vibration coupler standing

wave type, surface propagating wave type and a compromised ‘teeth’

vibrator type.

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CONTROL TECHNIQUE OF

USM

PWM control is used.

Motor is operated at

resonance frequency(low

impedance) to reduce losses &

to reduce pressure on

piezoelectric material.

Various control methods are :

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Block diagram of the total ultrasonic drive system

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ESTIMATIONS FOR SMALLER

USMs

The output force is proportional to the force factor A.

The relationship between the force factor and stator dimensions-

Force factor is expressed as-

where ƛ is a constant equivalent to 4.730, R0 is outer radius of piezoelectric

material, Ri is inner radius of piezoelectric material, l is length and e is

piezoelectric coefficient.

Maximum output force, Fmax is expressed as AV and the maximum output torque

is proportional to FmaxRo.

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Calculation results for micro ultrasonic motors with various

diameters

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ADVANTAGES

They have got high output torque & efficiency.

They exhibit high torque to weight ratio.

Good positioning accuracy

Capable of working in extreme environmental conditions.

No magnetic interference.

Simple construction

Compact size and Low cost

Energy saving

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DISADVANTAGES

Requires high frequency power supply(kHz range).

Piezoelectric material is expensive.

Ultrasonic noise occurs.

Drooping torque-speed characteristics.

Suppression of heat is required.

Less constancy.

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APPLICATIONS

Auto focusing & optical zooming in digital cameras &

surveillance cameras.

In the automobile industry, the USMs are used for the operation

of power steering, power windows, tilt steering, ORVMs, car

seat adjustment, etc.

The disk heads of hard disks, floppies & CD drives can be

controlled by the USM.

Wrist watches & Clocks

Robotics

Aerospace

Medicine

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CONCLUSION

The above prose has gone through almost all the fundamental details of this

modern technology machine. Hence the ultrasonic motors, which is a new

step in the miniaturized electrical technology has got many applications in

small appliances because of its high torque at low density. Though USMs are

not widely used for heavy motoring activities because of the requirement of a

larger & expensive piezoelectric material that causes the production cost to

shoot up double fold. However, researches are in progress to improve the

technology to meet the heavy industrial requirements and also to minimize

the problems caused by ultrasonic noise. A world might be expected in the

near future that replaces the futile electromagnetic motors by the proficient

ultrasonic motors.

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REFERENCES

http://www.noliac.com/Files/Billeder/Pdf/Pdf%20%20external/

Piezoelectric_ultrasonic_motors.pdf

http://www.americanpiezo.com/piezo_theory /piezo_theory.pdf

http://www.slideshare.net

www.pdf-searchengine.com

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Questions?