Bench Talk

When most engineers hear “motor,” they usually think “magnetics.” That makes sense because traditional motors use coils, windings, and magnetic materials to provide rotary or linear motion. Whether an AC or DC motor, brushed or brushless, stepper, or other configuration (see The Mystery and Magic of Motor Genealogy), the interaction between magnetic fields and materials converts electrical energy into mechanical motion.

So, when the time comes to implement small-scale and precise motion, designers often first consider a very small motor or even a small motor plus gearset. However, at these smaller scales, magnetic motors are difficult to control and still relatively large, while gearsets bring more issues of size, weight, cost, mechanical play, backlash, and wear. Even with advanced motion-control electronics and software, it can be a tricky and unsatisfactory solution.

But there exists an alternative to the magnetics-based motors currently in widespread use: The piezoelectric motor, also called a piezoelectric actuator. Its principles of operation leverage the well-known piezoelectric effect. Engineers are familiar with this two-way phenomenon and its many applications, such as converting vibration into electricity for energy harvesting, building pressure-sensing transducers, and implementing spark ignitors. In the complementary mode, the effect is used for transforming electrical energy into pressure and motion, in audio signaling devices/annunciators and, of course, as the core of ubiquitous crystal oscillators.

The piezo motor is built of a single ceramic crystal or stacked layers of these ceramic materials. When an electric field is applied across the assembly via a voltage, the material deforms, as seen in Figure 1. In the most common design, the elongation is restricted to a single plane of motion. The material is directed by on/off voltage pulsing and mechanical arrangement to make a series of stretches and position holds, and so moves like a caterpillar (sometimes also called an “inchworm” design).

Figure 1: The piezoelectric motor is usually made of stacked piezo elements and is stimulated by an applied voltage (left). Some units also include a strain gauge for closed-loop control (center) that enables even more-precise control of applied voltage versus displacement (right). (Source: Physik Instrumente GmbH & Co.)

The motion is both minute and precise. Piezo-based motors are used in nanoliter infusion pumps and optical-position mechanisms. These motors can provide positioning down to nanometer tolerances, with step rates into the MHz range—clearly an impossible specification for a magnetic-motor approach. Available force can range from nanonewtons to about one newton(1N), (though some motors can achieve hundreds of newtons, and motor weight for small ones is in the less-than-10g range.

These are generally not “high-power” motors, but they don’t need to be for the target applications. Also, the non-magnetic nature of piezo motors is a benefit (and even a necessity) in some situations. Piezo motors can be operated as open-loop transducers or used as a strain gauge in a feedback loop for the additional precision that closed-loop control offers.

Not only are the fundamental physics of the piezo motor very different from that of a magnetic motor, the drive requirements are also different. A magnetic-based motor is a current-driven device, as magnetic fields and strengths are a function of current through the windings (while there is voltage, of course, that is used to drive the current into the windings; the motor equations are all based on the interplay between current and magnetism).

The piezo-based motor is a voltage-driven scenario. The piezo material needs an electric field supplied by a voltage differential across the material. Depending on the size of the motor, this voltage can be as low as 50V or as high as as 1000V or more (simple piezo buzzers and vibrators typically require only about 25–30V).

This places several challenges on the design of the drive electronics:

• Because of the potentially lethal voltages, use of appropriate insulation and wire routing are critical, as well as attention to creep and clearance requirements as set by regulatory bodies.
• Unlike the MOSFET/IGBT switches used to control the flow of current in a magnetic motor, piezo motors are usually driven by high-voltage, low-current amplifiers (either standard op amps boosted by high-voltage transistors on their outputs or application-specific high-voltage op amps).
• The magnetic motor is a highly inductive load, so the drive circuitry must handle current inrush, inductive kicks, and other inductive-load attributes. In contrast, the piezo motor is highly capacitive, so the driver op amp must be capable of providing the needed voltage into loads of 1,000pF (picofarads) or more yet remain stable, which requires a special output-stage design even if the op amp can easily deliver the high voltage.

The widespread use of piezo-based, non-magnetic motors shows how clever engineers have adapted basic materials and physics principles to create innovative solutions to micro-motion applications. The next step in motion and motion control is developing practical MEMS-based motors too small to see with the unaided eye, for uses such as “pumping” individual cells through a micro-capillary path in a medical test instrument—lots of R&D work is already underway on those.