In the context of motion control, the term “motor” generally refers to a single electromechanical device that produces rotary or linear motion to drive a system, while the term “actuator” typically refers to an integrated mechanical system that is driven by a motor. But in the context of piezo drives, the term “actuator” refers to a group of piezo elements that produce motion solely through the inverse piezoelectric effect, with no mechanical components. And the term “motor” refers to a piezo actuator that incorporates mechanical elements to produce motion – typically with longer strokes than piezo actuators alone.
Confused? Here’s another way to look at it…
Piezo actuators are simply layers or stacks of piezo elements that expand and contract in proportion to an applied voltage. The arrangement of the elements determines the manner in which the piezo actuator moves – parallel or orthogonal to the electrical field. Piezo actuators produce small movements – typically in the range of several microns, up to a maximum of a few millimeters – and can generate very high forces, although at a tradeoff with displacement. There are four general categories of piezo actuators: longitudinal, shear, tube, and contracting, which are described in detail here.
A piezo motor incorporates a piezo actuator (very often a longitudinal, or “stack,” actuator) and other mechanical elements to produce longer strokes – up to several hundred millimeters. Common piezo motor types are ultrasonic piezo motors, piezo inertia motors, and piezo stepper motors.
In an ultrasonic piezo motor, the piezo actuator is electrically excited to produce high-frequency oscillations. The actuator is preloaded against a runner via a coupling element. When the actuator oscillates, the coupling moves along an inclined path at the same frequency, making contact with the runner and causing it to move linearly.
Piezo inertia motors (also referred to as “stick-slip” piezo motors) also use a coupling element and runner to produce motion. But in this design, the piezo actuator expands slowly and contracts very rapidly. During expansion, the runner is able to move along with the actuator, but during the rapid contraction, the inertia of the runner prevents it from following the actuator, so the runner effectively stays in place.
A piezo inertia motor can also be used to drive a lead screw. The piezo actuator is mounted perpendicular to the axis of the lead screw, with two “jaws” that extend to the top and bottom of the screw. As the actuator slowly expands, these jaws engage with the screw and cause it to turn. Like the linear version, as the actuator rapidly contracts, the jaws slip due to the inertia of the screw, so the screw remains in place until the next cycle re-engages the jaws and turns the screw again.
Piezo stepper motors use multiple actuators that both expand (or contract) and bend sideways as voltage is applied. These actuators act in pairs to grip a longitudinal runner and move it forward. The first pair of actuators then releases the runner, and the next pair of actuators takes over. Because of the walking-type motion they produce, the actuators of a piezo stepper motor are often referred to as “legs.” Although each pair of actuators moves just a few microns per cycle, piezo stepper motors operate at very high frequencies and make thousands of “steps” per second, achieving very long travel lengths at high speeds.
Due to their operating principle, linear stepper piezo motors are often described as having “legs” or as “walking.” This video from MICROMO demonstrates why those descriptions are so fitting.
Feature image credit: Physik Instrumente GmbH
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