There are several types of piezo motor that provide linear motion, with one of the most widely used being the linear stepper piezo motor. These devices harness the coordinated motion of multiple piezo elements to produce virtually unlimited travel with high resolution and very high stiffness.
Construction and operation
In the linear stepper design, multiple piezo elements are mounted in a row and act as pairs of “legs.” When voltage is applied, the piezo elements experience two motions: expansion (or contraction) in length, and sideways bending. Linear stepper piezo motors synchronize the movements of the piezo elements to produce a walking motion. This is translated into useful linear travel by mounting the elements against a longitudinal rod or runner. When the assembly is activated, one pair of legs grips the rod through friction, moves it forward as the legs expand and bend, and then releases it as the next pair of legs takes over.
Each piezo element moves just a few microns per cycle, but linear stepper piezo motors operate at thousands of hertz, making several thousand “steps” per second, which allows them to produce long travel lengths and achieve 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.
Performance characteristics
Because linear stepper piezo motors use friction to move the drive rod or runner, no mechanical elements are needed and there is no backlash, so resolution in the nanometer-level (and in some cases, picometer-level) range can be achieved. The use of multiple piezo elements gives linear steppers very good stiffness and high holding forces, making them self-locking when powered down.
Linear stepper designs can be operated at either very slow or very fast speeds, with maximum speeds up to 15 mm/s. While unlimited motion is possible, the length of the drive rod or runner determines the stroke, which is typically in the range of several centimeters, although maximum strokes of up to 10 cm are are available in some designs.
Like other piezo motors, linear steppers are non-magnetic, so they can be used in applications with magnetic fields, and they are inherently vacuum compatible.
Linear stepper piezo motor applications
The walking motion of linear stepper designs makes them the best choice for piezo applications that require long strokes. This, combined with their backlash-free motion give them ideal characteristics for driving linear stages in scanning and microscopy applications and in semiconductor lithography equipment. Because of their extremely high resolution, linear stepper piezo motors are useful for nanoliter dispensing and for positioning of medical equipment or samples.
To see how linear stepper piezo motors are used to drive stages, check out the video below from PI (Physik Instrumente).
Feature image credit: Physik Instrumente GmbH
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