Ultrasonic motor
Encyclopedia
An ultrasonic motor is a type of electric motor
powered by the ultrasonic vibration
of a component, the stator
, placed
against another component, the rotor
or slider depending on the scheme of operation (rotation or linear translation). Ultrasonic motors
differ from piezoelectric actuator
s in several ways, though both typically use some form of piezoelectric material, most often lead zirconate titanate
and occasionally lithium niobate
or other single-crystal
materials. The most obvious difference is the use of resonance
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 static strain
that may be induced in the piezoelectric element.
One common application of ultrasonic motors is in camera lenses where it, as part of the autofocus system, is used to move lens elements. It's replacing the noisier and often slower conventional micro-motor.
Two different ways are generally available to control the friction along the stator-rotor contact interface, traveling-wave vibration and standing-wave
vibration. Some of the earliest versions of practical motors in the 1970s, by Sashida, 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 at Matsushita, ALPS, and
Canon
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 constant vibration velocity regardless of frequency. The vibration velocity is simply the time derivative
of the vibration displacement in a structure, and is not (directly) related to the speed of the wave 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 a woofer
would give displacements of about 10 mm, which is visible to the eye. 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 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 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 Dr. Hu Junhui. Research in this area
continues, particularly in near-field acoustic levitation for
this sort of application. (This is different from far-field acoustic levitation,
which suspends the object at half to several wavelengths away from
the vibrating object.)
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. Canon has not only included an ultrasonic motor (USM) in their DSLRs but also in the Bridge camera Canon PowerShot
SX1 IS. 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:
Electric motor
An electric motor converts electrical energy into mechanical energy.Most electric motors operate through the interaction of magnetic fields and current-carrying conductors to generate force...
powered by the ultrasonic vibration
Vibration
Vibration refers to mechanical oscillations about an equilibrium point. The oscillations may be periodic such as the motion of a pendulum or random such as the movement of a tire on a gravel road.Vibration is occasionally "desirable"...
of a component, the stator
Stator
The stator is the stationary part of a rotor system, found in an electric generator, electric motor and biological rotors.Depending on the configuration of a spinning electromotive device the stator may act as the field magnet, interacting with the armature to create motion, or it may act as the...
, placed
against another component, the rotor
Rotor (electric)
The rotor is the non-stationary part of a rotary electric motor, electric generator or alternator, which rotates because the wires and magnetic field of the motor are arranged so that a torque is developed about the rotor's axis. In some designs, the rotor can act to serve as the motor's armature,...
or slider depending on the scheme of operation (rotation or linear translation). Ultrasonic motors
differ from piezoelectric actuator
Actuator
An actuator is a type of motor for moving or controlling a mechanism or system. It is operated by a source of energy, usually in the form of an electric current, hydraulic fluid pressure or pneumatic pressure, and converts that energy into some kind of motion. An actuator is the mechanism by which...
s in several ways, though both typically use some form of piezoelectric material, most often lead zirconate titanate
Lead zirconate titanate
Lead zirconate titanate , also called PZT, is a ceramic perovskite material that shows a marked piezoelectric effect. PZT-based compounds are composed of the chemical elements lead and zirconium and the chemical compound titanate which are combined under extremely high temperatures. A filter is...
and occasionally lithium niobate
Lithium niobate
Lithium niobate is a compound of niobium, lithium, and oxygen. Its single crystals are an important material for optical waveguides, mobile phones, optical modulators and various other linear and non-linear optical applications.-Properties:...
or other single-crystal
Single crystal
A single crystal or monocrystalline solid is a material in which the crystal lattice of the entire sample is continuous and unbroken to the edges of the sample, with no grain boundaries...
materials. The most obvious difference is the use of resonance
Resonance
In physics, resonance is the tendency of a system to oscillate at a greater amplitude at some frequencies than at others. These are known as the system's resonant frequencies...
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 static strain
Strain (materials science)
In continuum mechanics, the infinitesimal strain theory, sometimes called small deformation theory, small displacement theory, or small displacement-gradient theory, deals with infinitesimal deformations of a continuum body...
that may be induced in the piezoelectric element.
One common application of ultrasonic motors is in camera lenses where it, as part of the autofocus system, is used to move lens elements. It's replacing the noisier and often slower conventional micro-motor.
Mechanism
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 and standing-wave
Standing wave
In physics, a standing wave – also known as a stationary wave – is a wave that remains in a constant position.This phenomenon can occur because the medium is moving in the opposite direction to the wave, or it can arise in a stationary medium as a result of interference between two waves traveling...
vibration. Some of the earliest versions of practical motors in the 1970s, by Sashida, 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 at Matsushita, ALPS, and
Canon
Canon Inc.
is a Japanese multinational corporation that specialises in the manufacture of imaging and optical products, including cameras, camcorders, photocopiers, steppers and computer printers. Its headquarters are located in Ōta, Tokyo, Japan.-Origins:...
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 constant vibration velocity regardless of frequency. The vibration velocity is simply the time derivative
Time derivative
A time derivative is a derivative of a function with respect to time, usually interpreted as the rate of change of the value of the function. The variable denoting time is usually written as t\,.-Notation:...
of the vibration displacement in a structure, and is not (directly) related to the speed of the wave propagation
Wave propagation
Wave propagation is any of the ways in which waves travel.With respect to the direction of the oscillation relative to the propagation direction, we can distinguish between longitudinal wave and transverse waves....
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 a woofer
Woofer
Woofer is the term commonly used for a loudspeaker driver designed to produce low frequency sounds, typically from around 40 hertz up to about a kilohertz or higher. The name is from the onomatopoeic English word for a dog's bark, "woof"...
would give displacements of about 10 mm, which is visible to the eye. 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 that operate using 50 MHz surface acoustic wave
Surface acoustic wave
]A surface acoustic wave is an acoustic wave traveling along the surface of a material exhibiting elasticity, with an amplitude that typically decays exponentially with depth into the substrate.-Discovery:...
(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 Dr. Hu Junhui. Research in this area
continues, particularly in near-field acoustic levitation for
this sort of application. (This is different from far-field acoustic levitation,
which suspends the object at half to several wavelengths away from
the vibrating object.)
Applications
Canon was one of the pioneers of the ultrasonic motor, and made the "USM" famous in the late 1980s by incorporating it into its autofocusAutofocus
An autofocus optical system uses a sensor, a control system and a motor to focus fully automatic or on a manually selected point or area. An electronic rangefinder has a display instead of the motor; the adjustment of the optical system has to be done manually until indication...
lenses for the Canon EF lens mount
Canon EF lens mount
Introduced in 1987, the EF lens mount is the standard lens mount on the Canon EOS family of SLR film and digital cameras. EF stands for "Electro-Focus": automatic focusing on EF lenses is handled by a dedicated electric motor built into the lens...
. Numerous patents on ultrasonic motors have been filed by Canon, its chief lensmaking rival Nikon
Nikon
, also known as just Nikon, is a multinational corporation headquartered in Tokyo, Japan, specializing in optics and imaging. Its products include cameras, binoculars, microscopes, measurement instruments, and the steppers used in the photolithography steps of semiconductor fabrication, of which...
, and other industrial concerns since the early 1980s. Canon has not only included an ultrasonic motor (USM) in their DSLRs but also in the Bridge camera Canon PowerShot
Canon PowerShot
The PowerShot products are a line of consumer and prosumer grade digital cameras, launched by Canon in 1996. The PowerShot line has been successful for Canon, and is one of the best-selling digital camera lines worldwide....
SX1 IS. 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
- MinoltaMinolta AFThe Minolta Alpha camera system was a collection of photographic equipment from Minolta. The system used a lens mount called A-mount, with a flange focal distance 44.50 mm. The new mount was larger than the older SR-mount making old manual lenses incompatible with the new system...
, Konica MinoltaKonica Minoltais a Japanese manufacturer of office equipment, medical imaging, graphic imaging, optical devices, and measuring instruments. It is headquartered in the Marunouchi Center Building in Marunouchi, Chiyoda, Tokyo, with a Kansai office in Nishi-ku, Osaka, Osaka Prefecture...
, Sony – SSM, SuperSonic Motor - NikonNikon, also known as just Nikon, is a multinational corporation headquartered in Tokyo, Japan, specializing in optics and imaging. Its products include cameras, binoculars, microscopes, measurement instruments, and the steppers used in the photolithography steps of semiconductor fabrication, of which...
– SWM, Silent Wave Motor - OlympusOlympus Corporationis a Japan-based manufacturer of optics and reprography products. Olympus was established on 12 October 1919, initially specializing in microscope and thermometer businesses. Its global headquarters are in Shinjuku, Tokyo, Japan, while its USA operations are based in Center Valley, Pennsylvania,...
– SWD, Supersonic Wave Drive - PanasonicPanasonicPanasonic is an international brand name for Japanese electric products manufacturer Panasonic Corporation, which was formerly known as Matsushita Electric Industrial Co., Ltd...
– XSM, Extra Silent Motor - PentaxPentaxPentax is a brand name used by Hoya Corporation for its medical-related products & services and Pentax Ricoh Imaging Company for cameras, sport optics , etc. Hoya purchased and merged with the Japanese optics company on March 31, 2008. Hoya's Pentax imaging business was sold to Ricoh Company, Ltd...
– SDM, Silent Drive Motor - SigmaSigma Corporationis a Japanese company founded in 1961, manufacturing cameras, lenses, flashes and other photographic accessories. All Sigma products are produced in the company's own Aizu factory in Bandai, Fukushima, Japan...
– HSM, Hyper Sonic Motor - TamronTamronis a Japanese company manufacturing photographic lenses, optical components and commercial/industrial-use optics. Tamron Headquarters is located in Saitama City in the Saitama Prefecture of Japan....
- USD, Ultrasonic Silent Drive
External links
- Ultrasonic Actuators, Motors and Sensors page, from NASA JPL
- Design and performances of high torque ultrasonic motor for application of automobile
- Design of miniature ultrasonic motors
- Ultrasonic Lens Motor
- Micro/Nano Physics Research Laboratory, with research on ultrasonic piezoelectric actuators by Dr James Friend
- Institute of Piezomechanics, Kaunas University of Technology, Lithuania
- Disassembly of a Canon EF lens, revealing an ultrasonic motor
- Research Center for Microsystems and Nanotechnology, KTU, Lithuania