![]() ![]() Piezo motors can be fast, can reach into the multi-kHz range - impossible with electromagnetic motors - and are precise, repeatable, and controllable. Piezo-based motion is used in infusion pumps, microscope stages, optical positioning, instrumentation, inkjet nozzles, and more inexpensive, lower-quality piezo devices are used for audio sounders, alarms, and even small loudspeakers, but those uses have relaxed performance requirements. With one end fixed in place, the piezo motor becomes a precise, highly controllable piston. An array of multiple piezo motors can also be arranged in a circle to provide rotary motion, although their primary use is linear motion. Alternatively, one end of the crystal can be clamped in place while allowing the other end to move back and forth as the voltage is applied and removed, resulting in a piston or slip-stick motion (Figure 2). In one arrangement, the piezo material can alternately be held and then released by a set of tiny piezo-based clamps, thus allowing the crystal to inch forward (appropriately called inchworm mode), as shown in Figure 1. This physical elongation can be employed in two ways. (Source: Laurensva Lieshour/CC BY-SA 3.0) ![]() With appropriate timing of clamping and unclamping with respect to piezo-motor actuation, the motor can move ahead in tiny increments similar to an inchworm (1-housing, 2-moving crystal, 3-locking crystal, 4-rotary part). Larger elongations with more force can be achieved by stacking and driving multiple piezoelectric crystals as a single unit. These motors are small (one of their many virtues) with a representative device being about 10 mm in each dimension (larger ones are also in use), and the resultant motion on the order of microns, yet with Newtons of force. In the piezoelectric motor, an electric field is applied to the crystal material (via a voltage across the material) and the material elongates very slightly, on the order of 0.01 to 0.1 percent for typically applied voltages. This pair of piezoelectric properties has been exploited with great success in the classic crystal-based oscillator, which has formed the basis of a clock/frequency source for nearly 100 years (although MEMS-based oscillators are coming on strong as an alternative in recent years). Under this effect, when a crystal material is subject to mechanical stress (squeezed), it generates a voltage when a voltage is applied to the same crystal, the material expands by a very small amount. This unconventional motor is based on the well-known and widely used piezoelectric effect, which is a symmetrical electrical-mechanical relationship. Fortunately, there is a very viable alternative: the piezoelectric motor, which is used extensively in a wide variety of applications that need precise control of tiny ranges of linear motion. ![]() However, the conventional electromagnetic motor - whether rotary or linear, or large or small - is often not the best choice for precise, minute linear motion because of challenges in control, mechanical tolerances, backlash, and other electrical and mechanical issues. Engineers who need linear rather than rotary motion consider adding a mechanical linkage of some sort, or perhaps a linear-induction electromagnetic motor. For most electrical engineers, the simple term “motor” means one thing: an electromagnetic rotary-motion unit. ![]()
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