Piezoelectric Ultrasonic Motor Technology Working and Applications

Ultrasonic motors were invented in 1965 by V.V Lavrinko. In general we are aware of the fact that the motive force is given by the electromagnetic field in the conventional motors. But, here to provide a motive force, these motors utilize the piezoelectric effect in the ultrasonic frequency range, which is from 20 kHz to 10 MHz and is not audible to normal human beings. Hence, it is termed as piezoelectric USM technology. Ultrasonic technology is used by the USMs which utilize the ultrasonic vibration power from a component for their operation.

Before discussing in detail about this technology we need to know about the information regarding the ultrasonic sensors, piezoelectric sensors and piezoelectric actuators.

Piezoelectric Sensor

The changes in the physical quantities like strain, force, stress and acceleration can be measured by converting these into electrical energy. The devices or sensors that are used for this process are called as the piezoelectric sensors. And this process is termed as the piezoelectric effect. If a voltage is applied across a crystal, then the pressure will be posed on the atoms of crystal causing deformation of atoms which is of 0.1% only.

Ultrasonic Sensor

The transducers that generate high frequency – a frequency of around 20 kHz to 10 MHz sound waves – and, attribute the target by reading the time interval between receiving the echo after sending the signal are called as Ultrasonic sensors. Hence, Ultrasonic sensors can be used for obstacle detection and to avoid collision.

Piezoelectric Actuator

For the fine adjustment of the lenses of a camera, mirror, machining tools and other similar equipment a precise movement control is required; this precise movement control can be achieved by the piezoelectric actuators. The electrical signal can be converted into precisely controlled physical displacement by using a piezoelectric actuator. These are used for controlling the hydraulic valves and special purpose motors.

Piezoelectric Ultrasonic Motor Technology

Simply we can call the ultrasonic technology as inverse of the piezoelectric effect because, in this case, the electric energy is converted into motion. Hence, we can call it as a Piezoelectric USM Technology.

The piezoelectric material named Lead zirconate titanate and quartz are used very often for USMs and also for piezoelectric actuators even though the piezoelectric actuators are different from the USMs. The materials like lithium niobate and some other single crystal materials are also used for USMs and piezoelectric technology.
The major difference between the piezoelectric actuators and USMs is stated as the vibration of the stator in contact with the rotor, which can be amplified by using the resonance. The amplitude of the actuator motion is in between 20 to 200nm.

Types of Ultrasonic Motors

The USMs are classified into different types based on different criteria, which are as follows:

Classification of USMs based on the type of motor rotation operation

• Rotary type motors
• Linear type motors

Classification of USMs based on the shape of the vibrator

• Rod type
• П shaped
• Cylindrical shaped
• Ring (square) type

Classification based on the type of vibration wave

• Standing wave type – it is further classified into two types:

1. Unidirectional
2. Bidirectional

• Propagating wave type or travelling wave type

Working of the Ultrasonic Motors

The vibration is induced into the stator of the motor, and it is used for conveying the motion to the rotor and also to modulate the frictional forces. The amplification and (micro) deformations of active material are utilized for generation of the mechanical motion. The macro-motion of the rotor can be achieved by the rectification of the micro-motion using the frictional interface between the stator and the rotor.

The ultrasonic motor consists of stator and rotor. The operation of the USM changes the rotor or linear translator. The stator of the USM consists of piezoelectric ceramics for generating vibration, a metal of the stator for amplifying the generated vibration and a friction material for making contact with the rotor.

Whenever voltage is applied, a travelling wave is generated on the surface of the stator metal which causes the rotor to rotate. As the rotor is in contact with the stator metal, as mentioned above – but only at each peak of the travelling wave – which causes the elliptical movement – and, with this elliptical movement, the rotor rotates in the direction conversely to the direction of the travelling wave.

Features & Merits of Ultrasonic Motors

• These are small in size and are excellent in response.
• These have low speed of ten to several hundred rpm and high torque, and hence reduction gears are not required.
• These consist of high-holding power, and even if the power is turned off, they don’t need brake and clutch.
• They are small, thin and have less weight compared to other electromagnetic motors.
• These motors don’t contain any electromagnetic material and they do not generate electromagnetic waves. So, these can be used even in high magnetic field areas as these are unaffected by the magnetic field.
• These motors don’t have any gears, and an inaudible frequency vibration is used for driving these motors. So, they do not generate any noise and their operation is very quiet.
• Accurate speed and position control are possible with these motors.
• The mechanical time constant for these motors is less than 1ms and the speed control for these motors is step less.
• These motors have very high efficiency, and their efficiency is insensitive to their size.

Demerits of Ultrasonic Motors

• A high-frequency power supply is required.
• As these motors operate on friction, durability is very less.
• These motors have drooping speed-torque characteristics.

Applications of Ultrasonic Motors

• Used for the autofocus of camera lens.
• Used in compact paper handling devices and watches.
• Used in conveying machine parts.
• Used for drying and ultrasonic cleaning.
• Used to inject oil into the burners.
• Used as the best motors known to offer high potential for miniaturization of equipments.
• Used in MRI magnetic resonance imaging scanning in medicine.
• Used to control the disk heads of computer like floppies, hard disk and CD drives.
• Used in many applications in the fields of medicine, aerospace and robotics.
• Used to automatically control Rolling screen.
• In future, these motors may find applications in the fields like automobile industry, nano-positioning, microelectronics, Micro Electro Mechanical System technology and consumer goods.

This article discusses about the piezoelectric ultrasonic motors, ultrasonic sensors, piezoelectric sensors, piezoelectric actuators, working of USMs, merits, demerits and applications of USMs in brief. For more information regarding the above topics, please post your queries by commenting below.

Photo Credits:

• Ultrasonic Motor by seminarsonly
• Piezoelectric Sensor by imagesco
• Ultrasonic Sensor by iseerobot
• Piezoelectric Actuator by robotplatform
• Working of UltrasonicMotors by elektroda