Industrial Electronics

A primer on piezoelectric ultrasonic motors

25 July 2023
The inside of a slip-stick piezoelectric motor. Two piezoelectric crystals are visible that provide the mechanical torque. Source: Jaanus.kalde/CC BY-SA3.0

For industrial applications and future space vehicles, improving functions and reducing weights and sizes are crucial challenges. One difficulty is the shrinking of motors; when scaled down to sizes of a few millimeters or less, the effectiveness of routinely used electromagnetic coil-based motors drops substantially. However, recent advances in piezoelectric ultrasonic motors (USMs) may help bridge this technological divide. When high speed and great accuracy are equally crucial in a motion application, piezoelectric USMs provide significant benefits. They are also ideal for applications where size and weight are critical factors, such as in drones or other unmanned aerial vehicles. Additionally, their high efficiency also results in lower energy consumption and reduced operating costs.

What are USMs?

In 1965, V.V Lavrinko created the first practical USM. In standard motors, the electromagnetic field provides the driving force, which is common knowledge. In USMs, however, the ultrasonic frequency range (20 kHz to 10 MHz) that is inaudible to human ears is used by the piezoelectric effect to power the motors. Therefore, we refer to it as piezoelectric USM technology. USMs are equipped with ultrasonic technology and run on the ultrasonic vibration power of a component. The theory behind piezoelectric USMs is the "converse piezoelectric effect," which states that applying an electrical field to piezoelectric materials causes them to vibrate.

How does a piezoelectric USM work?

The motor's vibration is induced into the stator and then used to transmit motion to the rotor and control the amount of friction between the two. The rotor is held in place by a friction liner, while the stator is coated in piezoelectric ceramic. A rotor with a friction liner attached to it makes axial contact with a stator. When current is passed through the stator, a traveling wave is produced on its surface, which in turn spins the rotor. Since the rotor only makes contact with the stator metal at the peaks of the traveling waves, the resulting motion is elliptical, and the rotor spins counterclockwise relative to the direction of the traveling waves. The elliptical motion of the rotor also allows for smooth acceleration and deceleration, making it ideal for use in conveyor systems or other automated machinery.

How do these differ from other motors?

While USMs include a stator and rotor like conventional electromagnetic motors, USMs replace the coil and magnet pairs in the stator with a piezoelectric device connected to the stator for convenience and portability. When a driving voltage is provided at a frequency that is a perfect match for the ultrasonic vibrations of the piezoelectric structure, the stator is subsequently vibrated. Therefore, the mechanical motion and torque result from the frictional contact force between the rotor and stator.

When compared to conventional electromagnetic motors, the USMs' small construction and adaptable design allow for a high torque density (torque/weight ratio). In some cases, they can bypass the need for a gear or gear train altogether by being able to directly drive the payloads. Importantly, thanks to the benefits of piezoelectric materials and the low inertia of rotors, USMs can respond to stimuli in a matter of microseconds. In addition, they can self-lock, have a high holding torque, and regulate motion precisely, making them useful in fields like medicine, manufacturing and quality assurance where a high degree of accuracy is required.

Moreover, USMs are very useful in magnetic resonance imaging since they produce no electromagnetic interference. USMs are the preferred choice for usage in aerospace applications such as space missions since they can be operated in extremely cold or hot environments. In addition to all the above qualities, their noiseless operation makes them ideal for places where noise reduction is a priority.

What are the different types of piezoelectric USMs?

Piezoelectric USM designs may be classified into linear and rotary subsets (type of motion), or standing and traveling wave vibration subsets. However, they are generally categorized into traveling wave and standing wave ultrasonic motors based on the type of wave used to generate motion. The delicate nature of the resonant high-voltage drive, minimal outgassing for space applications, frequency matching between the drive circuit and stator resonances, quality factor of resonance, and the material and geometrical qualities of an appropriate friction pair (to obtain high force and low wear) must be taken into account in all designs. Other factors such as the number of stators and rotors, driving frequency, and power source can also be used for classification.

Conclusion

USMs are equipped with ultrasonic technology and run on the ultrasonic vibration power of a component. The rotor is held in place by a friction liner, while the stator is coated in piezoelectric ceramic. When current is passed through the stator, a traveling wave is produced on its surface, which in turn spins the rotor. USMs can respond to commands in a matter of microseconds, can self-lock, have a high holding torque, and regulate motion precisely, making them useful in fields like medicine, manufacturing and quality assurance where a high degree of accuracy is required.

To contact the author of this article, email GlobalSpecEditors@globalspec.com


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