1-3-6. Ultrasonic Motor

Ultrasonic motor (USM) is an actuator that converts characteristic vibration (resonance) of several μm in amplitude generated in the metallic elastic body (transducer, stator) into rotation or translational movement of motive elements (rotor, slider) with friction. It was invented by Mr. Toshiiku Sashida in 1980. This motor is called an ultrasonic motor because the characteristic vibration number (resonance frequency) of the elastic body is in the ultrasonic range (20 kHz and over).

The characteristic vibration is generated by the PZT (piezoelectric ceramic) inside the transducer.

Compared with electromagnetic motors in general, the ultrasonic motor has the following characteristics:

  • <1> No speed reducing mechanism required due to low-speed and high-torque characteristics.
  • <2> Exhibits low noise characteristics as no reduction gear is needed.
  • <3> Retains holding torque when not energized.
  • <4> Unaffected by magnetism and does not emit any electromagnetic waves.
  • <5> Compact and lightweight

Shortcomings are as follows:

  • <1> Poor durability due to high wear
  • <2> Difficult to operate at high speed.
  • <3> Requires high-frequency power supply and complex drive circuits.

By optimizing the above-mentioned advantages <1> to <5>, the ultrasonic motor is applied to the auto-focusing mechanism of a single-lens reflex camera and precision positioning devices in a scanning electron microscope, semiconductor making equipment, micro machine manufacturing equipment, and so forth.

In addition, it is used to raise/lower the roll-screen curtain and position the headrest by taking advantage of the low-speed/high-torque features and the torque-holding characteristic during non-energized periods.

Moreover, as the motor is not affected by magnetic fields, it can be used on the main body and peripherals of MRI scanners, which use magnetic fields for medical diagnosis, etc.

<Short column> AC and DC

AC is an abbreviation of Alternating Current (Koryu in Japanese) and DC for Direct Current (Chokuryu in Japanese).

Incidentally, AC and DC are translated into German as:

  • AC: Wechselstrom
  • DC: Gleichstrom

<Short column> Overload

For example, if a mechanical load mounted on the shaft (output shaft) of a DC motor is small, the flow of current will be small. But if the load is heavy, the current increases if the applied voltage remains constant. An increasing current will cause the motor to overheat. Overload refers to a condition where a level of current greater than the appropriate load limit flows to the motor occasionally or for long periods.

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