For most electrical engineers, the simple term “motor” means one thing: an electromagnetic rotary-motion unit. Engineers who need linear rather than rotary motion consider adding a mechanical linkage of some sort, or perhaps a linear-induction electromagnetic motor. 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. 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.
Piezoelectric Motor Overview
This unconventional motor is based on the well-known and widely used piezoelectric effect, which is a symmetrical electrical-mechanical relationship. 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 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).
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. 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. Larger elongations with more force can be achieved by stacking and driving multiple piezoelectric crystals as a single unit.
This physical elongation can be employed in two ways. 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. 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). An array of multiple piezo motors can also be arranged in a circle to provide rotary motion, although their primary use is linear motion.
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. Piezo motors can be fast, can reach into the multi-kHz range — impossible with electromagnetic motors — and are precise, repeatable, and controllable. Further, they are clean, with no bearings needing lubricant that may cause contamination, while their non-metallic nature is also an advantage in many situations (and may even be a necessity, as in MRI machines).
Drivers Make the Difference
As with electromagnetic motors, a complete and useable piezo motor assembly consists of three parts: the electronic drive, the electrical-mechanical transducer (motor) itself, and the output linkage. We’ll be focusing on the drive electronics.
For electromagnetic motors, the drive function requires sourcing and sinking current into the electromagnetic coils, which is usually done using power semiconductors (MOSFETs or IGBTs). These power devices are controlled by drivers that turn them on and off at the correct times with appropriate slew rates, and they must source/sink the required current into their highly inductive loads. The voltage that is applied to the MOSFET or IGBT output stage is needed to force the current into the coils, but it is the current that provides the electromagnetic force for the motor coils.
For piezo motors, the situation is very different. Instead of driving current, the driver must supply a relatively high voltage to create the electric field, and current is the secondary factor accompanying this applied voltage. Thus, the piezo drive scenario is the complement of the electromagnetic drive, where current drive is needed and voltage is a consequence; here, voltage is what is needed and current is the consequence. The piezo driver must supply the needed voltage (not current) into a capacitive (not inductive) load, and it must control and modulate this voltage to force the desired crystal elongation. In other words, current is the independent parameter and voltage is the dependent parameter for conventional motors, but for piezo motors, the situation is the opposite.
The piezo motor’s needed voltage (and therefore current) levels depend on the size of the piezoelectric element, the intended elongation, and the rate of motion. At the low end, voltage and current values may be 20–30V and 10–30mA, respectively, but most higher-performance piezo units need at least 10V and 10 to several hundred milliamps, and there are even piezo motors using 1,000V and above at several amps.
It’s this need to provide high voltage at moderate currents that is the electrical design challenge. In addition, the piezo driver must remain stable despite the highly capacitive nature of the load, which can read 1,000pF (1nF) and more. Also, as the piezo device is a floating, differential device, most applications require a differential, bipolar driver output.
One major design caution: Since these motors operate at higher voltages, there are issues of user safety, physical isolation and protection from the voltages, and regulatory mandates defining minimum creepage and clearance dimensions, which are a function of the voltage level. Therefore, any driver circuit for a piezo motor must keep these layout and placement conditions in mind, in addition to what works for the circuit’s electrical performance. Also note that these high voltages and modest current pairings are not unique to piezo devices, as many scientific and even commercial products need this combination, such as neon lights, special vacuum tubes, electrometers, and optical equipment, to cite a few.