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.
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.