Piezoelectric motor
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A piezoelectric motor or piezo motor is a type of electric motor based upon the change in shape of a piezoelectric material when an electric field is applied. Piezoelectric motors make use of the converse piezoelectric effect whereby the material produces acoustic or ultrasonic vibrations in order to produce a linear or rotary motion. In one mechanism, the elongation in a single plane is used to make a series stretches and position holds, similar to the way a caterpillar moves.
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[edit] Current designs
Piezoelectric motors are made in both linear and rotary types.
Commonly known under the trademark names of Inchworm or PiezoWalk motors, the most common type of piezoelectric motor uses three groups of crystals: two which are Locking and one Motive, permanently connected to either the motor's casing or stator (not both) and sandwiched between the other two, which provides the motion.
Current piezoelectric motors are fundamentally stepping motors, with each step comprising either two or three actions, based on the locking type.
Another mechanism employs the use of surface acoustic waves (SAW) to generate linear or rotational motion.
[edit] Locking mechanisms
The non-powered behaviour of a piezoelectric motor is one of two options: Normally Locked or Normally Free. When no power is being applied to a Normally Locked motor, the spindle or carriage (for rotary or linear types, repectively) will not move under external force. For a Normally Free motor, the spindle or carriage will move freely under external force; However, if both locking groups are powered at rest, a Normally Free motor will resist external force without providing any motive force.
A combination of mechanical latches and crystals could be used, but this would restrict the maximum stepping rate of the motor.
[edit] Stepping actions
Regardless of locking type, piezoelectric motors — both linear and rotary — use the same mechanism to provide movement.
First, one group of locking crystals is activated — this gives one locked side and one unlocked side of the 'sandwich'.
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 motor movement takes place.
Then the locking group triggered in stage one is released (in Normally Locking motors, the other is triggered). Then the motive group is released, retracting the 'trailing' locking group. Finally, both locking groups are returned to their default states.
[edit] Speed and precision
The growth and forming of piezoelectric crystals is a well developed industry, yielding very uniform and consistent distortion for a given applied potential difference. 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 scale.
The high response rate and fast distortion of the crystals also allows the steps to be made at very high frequencies — upwards of 5 MHz. This gives a maximum linear speed of approximately 800 mm per second, or nearly 1.8 miles per hour.
[edit] Other designs
[edit] 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 rubber), a series of angled piezoelectric transducers can be arranged. (see Fig. 2). When one group of transducers is triggered, the rotor will be pushed around one step. This design is not capable of such small or precise steps as more complex designs, but can reach higher speeds and are cheaper to manufacture.
[edit] Patents
The first U.S. patent to disclose a vibrationally-driven motor may be "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." Other important patents in the early development of this technology include:
- "Piezoelectric motor structures" (U.S. Pat. No. 4,019,073, Vishnevsky, et al., 1977)
- "Piezoelectrically driven torsional vibration motor" (U.S. Pat. No. 4,210,837, Vasiliev, et al., 1980)
