An ultrasonic motor uses vibrations too fast for human hearing to create useful mechanical motion. In a traveling-wave ultrasonic motor, a piezoelectric stator ring flexes at ultrasonic frequency and drives a rotor through friction. This design matters in robotics because it can provide precise motion, compact size, and strong holding torque without gears.
It is especially useful where quiet operation and accurate positioning are important.
Understanding Robotics: Ultrasonic Motor
The important physics is resonance. A piezoelectric ceramic changes shape only by a very small amount when voltage is applied. By itself, that motion would not turn a useful load.
The stator is shaped so that the electrical drive excites one of its natural bending patterns. Near that resonant pattern, each tiny push adds to the previous motion. The ring therefore develops a much larger surface vibration than a single electrical pulse could produce.
Many motors use separate electrode groups to create two bending patterns around the ring. Their timing is shifted so the combined pattern appears to travel around the stator. Reversing that timing reverses the rotor direction.
A point on the stator surface does not simply move up and down. It follows a small oval path. During one part of the cycle, the point presses into the rotor.
During another part, it moves sideways and drags the rotor a tiny distance. The contact then relaxes before the next cycle. Millions of these tiny slips build continuous rotation.
The pressure between stator and rotor is called preload. Too little preload causes slipping and weak torque.
Too much preload increases rubbing losses, heat, and wear. Surface material matters because the contact must provide enough grip while surviving repeated microscopic impacts.
The driver circuit has a harder job than supplying alternating voltage. Resonance changes when the motor warms up, when the rotor load changes, or when parts age. If the drive frequency drifts away from the best resonant value, vibration falls and the motor becomes inefficient.
Some systems monitor electrical current and voltage to estimate this condition, then adjust the drive frequency. This is a form of feedback control. Speed is often controlled by changing drive frequency, voltage, or the time spent driving.
Holding position can be very strong because preload creates friction even when motion stops. However, a robot designer must remember that friction can vary with contamination, humidity, and surface wear.
Students may meet ultrasonic motors in camera lens focusing systems, small positioning stages, medical equipment, and robots that need compact actuators near sensors. They are useful near strong magnetic fields because their motion does not depend on magnetic coils in the same way as many common motors. Their limits are important too.
They usually work best over a narrower speed range than a standard motor. They can lose performance in cold conditions or after long use if contact surfaces change.
When studying them, pay attention to the chain from electrical signal to ceramic strain, resonant bending, surface motion, frictional contact, and final rotor motion. Each link affects accuracy, torque, efficiency, and reliability.
Key Facts
- Ultrasonic frequency means f > 20,000 Hz.
- Piezoelectric strain is approximately S = dE, where S is strain, d is the piezoelectric coefficient, and E is electric field.
- Wave speed, frequency, and wavelength are related by v = fλ.
- The stator surface moves in tiny elliptical paths that push the rotor by friction.
- Friction force is limited by Ff ≤ μN, where μ is the coefficient of friction and N is normal force.
- Motor output power can be estimated by P = τω, where τ is torque and ω is angular speed.
Vocabulary
- Ultrasonic motor
- A motor that converts high-frequency mechanical vibration into continuous or stepwise motion.
- Piezoelectric effect
- The ability of certain materials to change shape when an electric field is applied, or to produce voltage when stressed.
- Stator
- The stationary vibrating part of the motor that creates the traveling wave.
- Rotor
- The moving part of the motor that is pushed by frictional contact with the stator.
- Traveling wave
- A wave pattern that moves around the stator and carries vibration energy along its surface.
Common Mistakes to Avoid
- Treating the motor like a normal electromagnetic motor is wrong because ultrasonic motors use piezoelectric vibration and friction instead of magnetic forces on coils.
- Assuming the rotor is glued to the stator is wrong because the rotor slips microscopically and is driven by controlled frictional contact.
- Ignoring preload is wrong because the normal force between rotor and stator strongly affects friction force, torque, wear, and efficiency.
- Confusing vibration frequency with rotor speed is wrong because the stator may vibrate tens of thousands of times per second while the rotor turns much more slowly.
Practice Questions
- 1 An ultrasonic motor stator vibrates at 40 kHz. What is the period of one vibration cycle in seconds?
- 2 A rotor experiences a torque of 0.18 N·m while spinning at 30 rad/s. What is the mechanical output power?
- 3 Explain why an ultrasonic motor can hold its position without continuous large current, and connect your answer to friction and preload.