Ultrasonic motor

An ultrasonic motor is a type of electric motor formedfrom the ultrasonic vibration of a component, the stator, placedagainst another, the rotor or slider depending on the schemeof operation (rotation or linear translation). Ultrasonic motorsdiffer from piezoelectric actuators in several ways, though bothtypically use some form of piezoelectric material, most often
lead zirconate titanate and occasionally
lithium niobate or other single-crystalmaterials. The most obvious difference is the use of resonanceto amplify the vibration of the stator in contact with the rotor inultrasonic motors. Ultrasonic motors also offer arbitrarily largerotation or sliding distances, while piezoelectric actuators arelimited by the static strain that maybe induced in the piezoelectric element.

Mechanism

Dry friction is often used in contact, and the ultrasonicvibration induced in the stator is used both to impart motion to therotor and to modulate the frictional forces present at theinterface. The friction modulation allows bulk motion of the rotor(i.e., for farther than one vibration cycle); without thismodulation, ultrasonic motors would fail to operate.

Two different ways are generally available to control the frictionalong the stator-rotor contact interface, traveling-wave vibration and standing-wavevibration. Some of the earliest versions of practical motors inthe 1970s, by Sashida, [Citation
last1 = Ueha | first1 = S.
last2 = Tomikawa | first2 = Y.
last3 = Kurosawa | first3 = M.
last4 = Nakamura | first4 = N.
title = Ultrasonic Motors: Theory and Applications
publisher = Clarendon Press
date = December 1993
year = 1993
isbn = 0-1985-9376-7 ] for example, used standing-wave vibration in combination with finsplaced at an angle to the contact surface to form a motor, albeitone that rotated in a single direction. Later designs by Sashidaand researchers at Matsushita, ALPS, and Canon made use oftraveling-wave vibration to obtain bi-directional motion, and foundthat this arrangement offered better efficiency and less contactinterface wear. An exceptionally high-torque 'hybrid transducer'ultrasonic motor uses circumferentially-poled and axially-poledpiezoelectric elements together to combine axial and torsionalvibration along the contact interface, representing a drivingtechnique that lies somewhere between the standing andtraveling-wave driving methods.

A key observation in the study of ultrasonic motors is that the peakvibration that may be induced in structures occurs at a relativelyconstant "vibration velocity" regardless of frequency. Thevibration velocity is simply the time derivative of thevibration displacement in a structure, and is not (directly) relatedto the speed of the wave propagation within a structure. Manyengineering materials suitable for vibration permit a peak vibrationvelocity of around 1 m/s. At low frequencies— 50 Hz, say — a vibration velocity of 1 m/s in a woofer wouldgive displacements of about 10 mm, which is visible to the eye. Asthe frequency is increased, the displacement decreases, and theacceleration increases. As the vibration becomes inaudible at 20 kHzor so, the vibration displacements are in the tens of micrometers,and motors have been built [Citation
last1 = Shigematsu | first1 = T.
last2 = Kurosawa | first2 = M.K.
last3 = Asai | first3 = K.
contribution = Nanometer stepping drives of surface acoustic wave motor
title = IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control
publisher = IEEE
pages = 376-385
year = 2003
date = April 2003
volume = 50
number = 4] that operate using 50 MHz surface acoustic wave(SAW) that have vibrations of only a few nanometers in magnitude.Such devices require care in construction to meet the necessaryprecision to make use of these motions within the stator.

More generally, there are two types of motors, contact andnon-contact, the latter of which is rare and requires a workingfluid to transmit the ultrasonic vibrations of the stator toward therotor. Most versions use air, such as some of the earliest versionsby Dr. Hu Junhui. [Citation
last1 = Hu | first1 = Junhui
last2 = Li | first2 = Guorong
last3 = Lai Wah Chan | first3 = Helen
last4 = Loong Choy | first4 = Chung
contribution = A standing wave-type noncontact linear ultrasonic motor
title = IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control
publisher = IEEE
pages = 699-708
year = 2001
date = May 2003
volume = 48, issue 3] [Citation
last1 = Hu | first1 = Junhui
last2 = Nakamura | first2 = Kentaro
last3 = Ueha | first3 = Sadauki
contribution = An analysis of a noncontact ultrasonic motor with an ultrasonically levitated rotor
title = Ultrasonics
publisher = Elsevier
pages = 459-467
year = 1997
date = May 1997
volume = 35
number = 6] Research in this areacontinues, particularly in near-field acoustic levitation forthis sort of application. [Citation
last1 = Koyama | first1 = D.
last2 = Takeshi | first2 = Ide
last3 = Friend | first3 = J.R.
last4 = Nakamura | first4 = K.
last5 = Ueha | first5 = S.
contribution = An ultrasonically levitated non-contact sliding table with the traveling vibrations on fine-ceramic beams
title = 2005 IEEE Ultrasonics Symposium
publisher = IEEE
pages = 1538-1541
year = 2005
date = September 2005
volume = 3] (This is different from far-field acoustic levitation,which suspends the object at half to several wavelengths away fromthe vibrating object.)

Commercial applications

Canon was one of the pioneers of the ultrasonic motor, and made the "USM" famous in the 1980s by incorporating it into its autofocus lenses for the Canon EF lens mount. Numerous patents on ultrasonic motors have been filed by Canon, its chief lensmaking rival Nikon, and other industrial concerns since the early 1980s. 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:

* Canon - USM, UltraSonic Motor
* Nikon - SWM, Silent Wave Motor
* Minolta, Sony - SSM, SuperSonic Motor
* Pentax - SDM, Supersonic Drive Motor
* Sigma HSM, Hyper Sonic Motor
* Olympus SWD, Supersonic Wave Drive
* Panasonic XSM, Extra Silent Motor

References

External links

* [http://ndeaa.jpl.nasa.gov/nasa-nde/usm/usm-hp.htm Ultrasonic Actuators, Motors and Sensors page, from NASA JPL]
* [http://www.eng.monash.edu.au/mnrl Micro/Nano Physics Research Laboratory, with research on ultrasonic piezoelectric actuators by Dr James Friend]
* [http://www.piezo.lt/ Institute of Piezomechanics, Kaunas University of Technology, Lithuania]
* [http://www.photoscene.com/sw/tour/inside.htm Disassembly of a Canon EF lens, revealing an ultrasonic motor]