![Insides of a piezoelectric motor](https://upload.wikimedia.org/wikipedia/commons/4/42/Insides_of_a_piezoelectric_motor.JPG)
A piezoelectric motor or piezo motor is a type of
electric motor
An electric motor is an Electric machine, electrical machine that converts electrical energy into mechanical energy. Most electric motors operate through the interaction between the motor's magnetic field and electric current in a Electromagneti ...
based on the change in shape of a
piezoelectric material
Piezoelectricity (, ) is the electric charge that accumulates in certain solid materials—such as crystals, certain ceramics, and biological matter such as bone, DNA, and various proteins—in response to applied mechanical stress. The word ''p ...
when an
electric field
An electric field (sometimes E-field) is the physical field that surrounds electrically charged particles and exerts force on all other charged particles in the field, either attracting or repelling them. It also refers to the physical field fo ...
is applied, as a consequence of the converse piezoelectric effect. An electrical circuit makes acoustic or
ultrasonic
Ultrasound is sound waves with frequencies higher than the upper audible limit of human hearing. Ultrasound is not different from "normal" (audible) sound in its physical properties, except that humans cannot hear it. This limit varies fr ...
vibrations in the piezoelectric material, most often
lead zirconate titanate
Lead zirconate titanate is an inorganic compound with the chemical formula (0≤''x''≤1), commonly abbreviated as PZT. Also called lead zirconium titanate, it is a ceramic perovskite material that shows a marked piezoelectric effect, meaning t ...
and occasionally
lithium niobate
Lithium niobate () is a non-naturally-occurring salt consisting of niobium, lithium, and oxygen. Its single crystals are an important material for optical waveguides, mobile phones, piezoelectric sensors, optical modulators and various other linea ...
or other
single-crystal
In materials science, a single crystal (or single-crystal solid 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.RIWD. "Re ...
materials, which can produce
linear
Linearity is the property of a mathematical relationship (''function'') that can be graphically represented as a straight line. Linearity is closely related to '' proportionality''. Examples in physics include rectilinear motion, the linear r ...
or
rotary motion depending on their mechanism.
Examples of types piezoelectric motors include
inchworm motors, stepper and slip-stick motors as well as
ultrasonic motors which can further be further categorized into standing wave and travelling wave motors. Piezoelectric motors typically use a cyclic stepping motion, which allows the oscillation of the crystals to produce an arbitrarily large motion, as opposed to most other
piezoelectric actuators where the range of motion is limited by the static
strain
Strain may refer to:
Science and technology
* Strain (biology), variants of plants, viruses or bacteria; or an inbred animal used for experimental purposes
* Strain (chemistry), a chemical stress of a molecule
* Strain (injury), an injury to a mu ...
that may be induced in the piezoelectric element.
The growth and forming of piezoelectric crystals is a well-developed
industry
Industry may refer to:
Economics
* Industry (economics), a generally categorized branch of economic activity
* Industry (manufacturing), a specific branch of economic activity, typically in factories with machinery
* The wider industrial sector ...
, yielding very uniform and consistent distortion for a given applied
potential difference
Voltage, also known as electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge to m ...
. This, combined with the minute scale of the distortions, gives the piezoelectric motor the ability to make very fine steps. Manufacturers claim precision to the
nanometer
330px, Different lengths as in respect to the molecular scale.
The nanometre (international spelling as used by the International Bureau of Weights and Measures; SI symbol: nm) or nanometer (American and British English spelling differences#-re ...
scale. High response rate and fast distortion of the crystals also let the steps happen at very high frequencies—upwards of 5
MHz
The hertz (symbol: Hz) is the unit of frequency in the International System of Units (SI), equivalent to one event (or cycle) per second. The hertz is an SI derived unit whose expression in terms of SI base units is s−1, meaning that one he ...
. This provides a maximum linear speed of approximately 800 mm per second, or nearly 2.9 km/h.
A unique capability of piezoelectric motors is their ability to operate in strong magnetic fields. This extends their usefulness to applications that cannot use traditional electromagnetic motors—such as inside
nuclear magnetic resonance
Nuclear magnetic resonance (NMR) is a physical phenomenon in which nuclei in a strong constant magnetic field are perturbed by a weak oscillating magnetic field (in the near field) and respond by producing an electromagnetic signal with a ...
antennas. The maximum operating temperature is limited by the
Curie temperature
In physics and materials science, the Curie temperature (''T''C), or Curie point, is the temperature above which certain materials lose their permanent magnetic properties, which can (in most cases) be replaced by induced magnetism. The Cur ...
of the used piezoelectric ceramic and can exceed +250°C.
The main benefits of piezoelectric motors are the high positioning precision, stability of position while unpowered, and the ability to be fabricated at very small sizes or in unusual shapes such as thin rings. Common applications of piezoelectric motors include focusing systems in camera lenses as well as precision motion control in specialised applications such as microscopy.
Resonant motor types
Ultrasonic motor
Ultrasonic motors differ from other piezoelectric motors in several ways, though both typically use some form of piezoelectric material, The most obvious difference is the use of
resonance
Resonance describes the phenomenon of increased amplitude that occurs when the frequency of an applied periodic force (or a Fourier component of it) is equal or close to a natural frequency of the system on which it acts. When an oscillatin ...
to amplify the vibration of the stator in contact with the rotor in ultrasonic motors.
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
Canon or Canons may refer to:
Arts and entertainment
* Canon (fiction), the conceptual material accepted as official in a fictional universe by its fan base
* Literary canon, an accepted body of works considered as high culture
** Western can ...
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.
Non-resonant motor types
Inchworm motor
The inchworm motor uses piezoelectric ceramics to push a
stator
The stator is the stationary part of a rotary system, found in electric generators, electric motors, sirens, mud motors or biological rotors. Energy flows through a stator to or from the rotating component of the system. In an electric mot ...
using a walking-type motion. These piezoelectric motors use three groups of crystals—two 'locking', and one 'motive' that permanently connects to either the motor's casing or stator (not both). The motive group, sandwiched between the other two, provides the motion.
The non-powered behaviour of this piezoelectric motor is one of two options: 'normally locked' or 'normally free'. A normally free type allows free movement when unpowered but can still be locked by applying a voltage.
Inchworm motors can achieve nanometre-scale positioning by varying the voltage applied to the motive crystal while one set of locking crystals is engaged.
Stepping actions
The actuation process of the inchworm motor is a multistep cyclical process:
# First, one group of 'locking' crystals is activated to lock one side and unlock other side of the 'sandwich' of piezo crystals.
# Next, the 'motive' crystal group is triggered and held. The expansion of this group moves the unlocked 'locking' group along the motor path. This is the only stage where the motor moves.
# Then the 'locking' group triggered in stage one releases (in 'normally locking' motors, in the other it triggers).
# Then the 'motive' group releases, retracting the 'trailing locking' group.
# Finally, both 'locking' groups return to their default states.
Stepper or walk-drive motor
![Piezomotor type bimorph](https://upload.wikimedia.org/wikipedia/commons/2/29/Piezomotor_type_bimorph.gif)
Not to be confused with the similarly named electromagnetic
stepper motor
A stepper motor, also known as step motor or stepping motor, is a brushless DC electric motor that divides a full rotation into a number of equal steps. The motor's position can be commanded to move and hold at one of these steps without any pos ...
, these motors are similar to the inchworm motor, however, the piezoelectric elements can be
bimorph actuators which bend to feed the slider rather than using a separate expanding and contracting element.
Slip-stick motor
![Slip-stick actuator operation](https://upload.wikimedia.org/wikipedia/commons/a/a8/Slip-stick_actuator_operation.svg)
The mechanism of slip-stick motors rely on the inertia in combination with the difference between static and dynamic friction. The stepping action consists of a slow extension phase where static friction is not overcome, followed by a rapid contraction phase where static friction is overcome and the point of contact between the motor and moving part is changed.
Direct drive motors
The direct drive piezoelectric motor creates movement through continuous ultrasonic vibration. Its control circuit applies a two-channel sinusoidal or square wave to the piezoelectric elements that matches the
bending resonant frequency
In applied mechanics, bending (also known as flexure) characterizes the behavior of a slender structural element subjected to an external load applied perpendicularly to a longitudinal axis of the element.
The structural element is assumed to ...
of the threaded tube—typically an ultrasonic frequency of 40 kHz to 200 kHz. This creates orbital motion that drives the screw.
A second drive type, the squiggle motor, uses piezoelectric elements bonded orthogonally to a nut. Their ultrasonic vibrations rotate a central lead screw.
Single action
Very simple single-action stepping motors can be made with piezoelectric crystals. For example, with a hard and rigid rotor-spindle coated with a thin layer of a softer material (like a
polyurethane
Polyurethane (; often abbreviated PUR and PU) refers to a class of polymers composed of organic chemistry, organic units joined by carbamate (urethane) links. In contrast to other common polymers such as polyethylene and polystyrene, polyurethan ...
rubber), a series of angled piezoelectric
transducer
A transducer is a device that converts energy from one form to another. Usually a transducer converts a signal in one form of energy to a signal in another.
Transducers are often employed at the boundaries of automation, measurement, and contr ...
s can be arranged. (see Fig. 2). When the control circuit triggers one group of transducers, they push the rotor one step. This design cannot make steps as small or precise as more complex designs, but can reach higher speeds and is cheaper to manufacture.
Patents
The first U.S. patent to disclose a vibrationally-driven motor may b
"Method and Apparatus for Delivering Vibratory Energy"(U.S. Pat. No. 3,184,842, Maropis, 1965). The Maropis patent describes a "vibratory apparatus wherein longitudinal vibrations in a resonant coupling element are converted to torsional vibrations in a toroid type resonant terminal element." The first practical piezomotors were designed and produced by V. Lavrinenko in Piezoelectronic Laboratory, starting 1964, Kiev Polytechnic Institute, USSR. Other important patents in the early development of this technology include:
* "Electrical motor", V. Lavrinenko, M. Nekrasov, Patent USSR # 217509, priority May 10, 1965.
(U.S. Pat. No. 4,019,073, Vishnevsky, et al., 1977)
(U.S. Pat. No. 4,210,837, Vasiliev, et al., 1980)
See also
*
Ultrasonic motor An ultrasonic motor is a type of piezoelectric 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). Ultraso ...
*
Ultrasonic Motor Drive as used in the Canon EF Mount
*
Ultrasonic homogenizer
A sonicator at the Weizmann Institute of Science during sonicationSonication is the act of applying sound energy to agitate particles in a sample, for various purposes such as the extraction of multiple compounds from plants, microalgae and seawe ...
References
{{Electric motor
Electric motors