By Bill Schweber for Mouser Electronics

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 fascinating, 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 10mm 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.

Figure 1: 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). (Source: Laurensva Lieshour / CC BY-SA 3.0)

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.

Figure 2: With one end fixed in place, the piezo motor becomes a precise, highly-controllable piston. (Source: Inductiveload / CC BY 2.5)

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 a neon lights, special vacuum tubes, electrometers, and optical equipment, to cite a few.

Driver Design Offers Options

Developing and delivering the relatively-high voltages needed for piezo drivers is a challenge in many cases because most amplifier ICs are low-voltage devices, while the higher-voltage ones are usually optimized for the current drive needed by MOSFETs/IGBTs, rather than for voltage drive. There are some specialized operational amplifiers (op amps) that are designed for piezo drive at the higher currents and voltages using high-voltage IC processes or for hybrid devices that combine lower-voltage op amps with voltage-boosting transistors on their output.

In principle, it is possible to build a basic high-voltage driver using just a transistor with adequate voltage rating (Figure 3). However, this design lacks the precision, controllability, and stability that a higher-performance piezo driver needs, and it also lacks protection features for failure modes. Further, it is not capable of delivering bipolar output, while its base-circuit drive requires suitable circuitry. Therefore, this type of basic design is better suited for less technically-challenging applications such as piezo alarms and sounders.

Figure 3: While a piezo element can be controlled by a basic transistor, the configuration is suitable only for modestly-challenging applications such as piezo-based speakers. (Source: Murata)

Fortunately, vendors have developed ICs that are specifically designed for piezo drive and simplify the task while adding needed features and functions, including control of the high-voltage waveform slew. These ICs also offer thermal, overload, and short-circuit protection, which are must haves in practical designs.

From Analog Devices

For example, Analog Devices offers the ADA4700-1, a high-voltage precision amplifier with a wide operating voltage range of (±5 to ±50V). Although this eight-lead SOIC device looks like a standard op amp, it is optimized for providing high-slew rate output into capacitive loads while remaining stable, Figure 4. It is fully characterized for a wide variety of operating conditions (at various voltages, loads, temperatures, distortion levels, and overshoot, for example) and is supported by a data sheet that features over 60 graphs of performance across many scenarios.

Figure 4: The ADA4700-1 high-voltage precision amplifier has carefully defined slew rate performance, along with many other attributes that are fully detailed in the data sheet. (Source: Analog Devices)

The ADA4700-1 is stable with minimal overshoot when driving capacitive loads, but extra compensation can enhance the response when driving larger capacitances. This requires a small snubber network, Figure 5; for unity-gain applications and capacitive loads up to 1nF (1,000pF), a combination of a 150a?| resistor and 10nF capacitor are all that is needed. For larger loads up to 10nF and higher gains such as tenfold, the resistor is decreased to 22a?| while the capacitor increases to 100nF. Finally, the drive-current level can be boosted by adding an external pair of complementary (PNP/NPN) transistors (Figure 6).

Figure 5: For driving highly capacitive loads, a simple external RC snubber circuit is added to the ADA4700-1. (Source: Analog Devices)

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