Traveling wave motor

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Traveling wave motors are small motors that mostly use the piezoelectric effect to generate movement. Traveling wave motors have achieved greater popularity primarily through their use in camera lenses. The movement is generated by high-frequency vibrations. Since their frequency is mostly in the inaudible ultrasonic range, traveling wave motors are also known as ultrasonic motors . Other types of piezoelectric motors also use ultrasonic vibrations.


Traveling wave motors consist of a fixed part, the stator , and a moving part, the rotor . There are at least two piezoelectric transducers in the stator that convert the applied alternating voltage into mechanical vibrations. If the converters are operated with phase-shifted voltages, a traveling wave is created on the stator. This sets the latter in motion via the frictional contact between the stator and rotor. In order to achieve high oscillation amplitudes and thus speeds, the stator is usually operated in resonance .

Linear motors can use the same functional principle. However, it is significantly more complex to generate a traveling wave in a linear stator, which is why linear traveling wave motors are not yet commercially available.

advantages and disadvantages

The main advantages of the ultrasonic motors are the high dynamics , the exact positioning possibility and the lower noise pollution in the audible range of humans. Furthermore, there is no magnetic field in these types of motors .

The main disadvantage of ultrasonic motors is that they are somewhat more exposed to noise in the audible range of various animal species. This can have a negative effect on animal photography, for example.


Clock drives

Traveling wave motors can be used as clock drives (e.g. at Seiko ) due to their size .


Ultrasonic traveling wave motor from an autofocus lens

Traveling wave motors can be used with lenses for lens autofocusing . When using traveling wave motors in lenses, a distinction is made between ring-shaped ultrasonic motors and micro-ultrasonic motors.

Advantages of a ring-shaped ultrasonic motor

  • Ring-shaped traveling wave motors enable almost noiseless focusing, while operating noises can be heard with the micro ultrasonic motor due to the transmission.
  • Ring-shaped traveling wave motors are placed concentrically around the lens to save space, do not disturb the beam path and are exactly where the user would use his hand for manual focusing. Existing lens constructions require only minor modifications.
  • With ring-shaped traveling wave motors, you can intervene in the focusing process, which the micro-ultrasonic motor cannot do because of the reduction gear.
  • Focussing with a ring-shaped traveling wave motor is much faster than with a micro-ultrasonic motor, because "normal" motor constructions - especially with a gearbox - have more and faster moving rotating masses which first have to be accelerated and then decelerated again.

Because of these advantages, annular traveling wave motors are used in professional lenses.

Advantages of a micro ultrasonic motor

  • Micro ultrasonic motors are more compact than annular ultrasonic motors. Therefore this type can be found in smaller or narrower lenses.

Names of ultrasonic motors from lens manufacturers

The ultrasonic motors have different names depending on the lens manufacturer:

  • Canon: USM (Ultrasonic Motor)
  • Nikon: SWM (Silent Wave Motor)
  • Olympus: SWD (Supersonic Wave Drive)
  • Pentax: SDM (Supersonic Dynamic Motor)
  • Sigma: HSM (Hyper Sonic Motor)
  • SONY: SSM (Super Sonic Motor)
  • Tamron: PZD (PieZo Drive), USD (Ultrasonic Silent Drive)

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