Anultrasonic motor is a type ofpiezoelectric motor powered by the ultrasonicvibration of a component, thestator, placed against another component, therotor or slider depending on the scheme of operation (rotation or linear translation). Ultrasonic motors differ from otherpiezoelectricmotors in several ways, though both typically use some form of piezoelectric material, most oftenlead zirconate titanate and occasionallylithium niobate or othersingle-crystal materials. The most obvious difference is the use ofresonance to amplify the vibration of the stator in contact with the rotor in ultrasonic motors. Ultrasonic motors also offer arbitrarily large rotation or sliding distances, while piezoelectric actuators are limited by the staticstrain that may be induced in the piezoelectric element.
One common application of ultrasonic motors is in camera lenses where they are used to move lens elements as part of the auto-focus system. Ultrasonic motors replace the noisier and often slowermicro-motor in this application.
Dry friction is often used in contact, and the ultrasonic vibration induced in the stator is used both to impart motion to the rotor and to modulate the frictional forces present at the interface. The friction modulation allows bulk motion of the rotor (i.e., for farther than one vibration cycle); without this modulation, ultrasonic motors would fail to operate.
Two different ways are generally available to control the friction along the stator-rotor contact interface,traveling-wave vibration andstanding-wave vibration. Some of the earliest versions of practical motors in the 1970s, by Sashida,[1] for example, used standing-wave vibration in combination with fins placed at an angle to the contact surface to form a motor, albeit one that rotated in a single direction. Later designs by Sashida and researchers atMatsushita, ALPS, andCanon made use of traveling-wave vibration to obtain bi-directional motion, and found that this arrangement offered better efficiency and less contact interface wear. An exceptionally high-torque 'hybrid transducer' ultrasonic motor uses circumferentially-poled and axially-poled piezoelectric elements together to combine axial and torsional vibration along the contact interface, representing a driving technique that lies somewhere between the standing and traveling-wave driving methods.
A key observation in the study of ultrasonic motors is that the peak vibration that may be induced in structures occurs at a relatively constantvibration velocity regardless of frequency. The vibration velocity is simply thetime derivative of the vibration displacement in a structure, and is not (directly) related to the speed of thewave propagation within a structure. Many engineering materials suitable for vibration permit a peak vibration velocity of around 1 m/s. At low frequencies — 50 Hz, say — a vibration velocity of 1 m/s in awoofer would give displacements of about 10 mm, which is visible. As the frequency is increased, the displacement decreases, and the acceleration increases. As the vibration becomes inaudible at 20 kHz or so, the vibration displacements are in the tens of micrometers, and motors have been built[2] that operate using 50 MHzsurface acoustic wave(SAW) that have vibrations of only a few nanometers in magnitude. Such devices require care in construction to meet the necessary precision to make use of these motions within the stator.
More generally, there are two types of motors, contact and non-contact, the latter of which is rare and requires a working fluid to transmit the ultrasonic vibrations of the stator toward the rotor. Most versions use air, such as some of the earliest versions by Hu Junhui.[3][4] Research in this areacontinues, particularly innear-field acoustic levitation forthis sort of application.[5] (This is different fromfar-field acoustic levitation,which suspends the object at half to several wavelengths away fromthe vibrating object.)
Canon was one of the pioneers of the ultrasonic motor, and made the "USM" famous in the late 1980s by incorporating it into itsautofocus lenses for theCanon EF lens mount. Numerous patents on ultrasonic motors have been filed by Canon, its chief lensmaking rivalNikon, and other industrial concerns since the early 1980s. Canon has not only included an ultrasonic motor (USM) in their DSLR lenses, but also in theCanon PowerShot SX1 ISbridge camera.[6] The ultrasonic motor is now used in many consumer and office electronics requiring precision rotations over long periods of time.
The technology has been applied to photographic lenses by a variety of companies under different names.