(Source: Jaanus.kalde, CC BY-SA 3.0, via Wikimedia Commons)
There’s a world of motors beyond the well-known electromagnetic energy-based ones that most engineers are familiar with. One standout example is the piezoelectric motor and actuator, which uses an entirely different physics and materials-science principle to provide precise, swift, and controllable linear motion over distances from nanometers to millimeters.
Applications for piezoelectric motors comprise a surprisingly long list, including medical infusion pumps, ultrasound diagnostic transducers, microscope-positioning stages, disk-drive arm assemblies, ink-jet printers, haptics systems, and mirror/lens micro-positioning. This type of motor is a “direct drive” unit, with no gears or linkages needed. As with most other motors, effective use of the piezoelectric motor is really a two-part process: the motor itself and its drive circuitry.
In this blog, we will examine how the widely used piezoelectric-based motor and actuator, unlike electromagnetic motors, provides precise and repeatable linear motion over short distances.
Developers can apply the piezoelectric principle to create a motor using two approaches. In one approach, an end of the piezoelectric crystal element is permanently “clamped” or fixed in position (Figure 1).
Figure 1: This diagram shows the operation of a slip-stick piezoelectric actuator, providing coarse and fine positioning modes. With one end fixed in place, the piezoelectric motor becomes a precise, highly controllable piston. (Source: Inductiveload, CC BY 2.5, via Wikimedia Commons)
As a voltage is applied and removed, the crystal elongates and then returns to its original dimensions. In this way, the actuator moves back and forth from a known zero-point reference. The motion is like that of a tightly controlled piston, as the amount of elongation is closely proportional to the applied voltage.
In the other approach, often referred to as the “inchworm” mode, the piezo material is alternately held and then released by a set of tiny piezo-based clamps, thus allowing the crystal to “inch” forward (Figure 2).
Figure 2: 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: LaurensvanLieshout, CC BY-SA 3.0, via Wikimedia Commons)
The motion of a piston-like motor is quite small, which is well suited to typical applications for piezoelectric motors. The crystal material elongates very slightly, on the order of 0.01 to 0.1 percent. Using larger crystals or stacking multiple ones makes it possible to have motion in the order of tens of millimeters and even centimeters, with forces ranging into the tens of newtons.
Despite the minute dimensions and motion, these piezo actuators can develop a significant force—on the order of newtons. It’s somewhat analogous to liquid water turning into solid ice: the volume of the water mass expands by about 4 percent, but the force it develops as it does can break pipes!
Piezo actuators are available as standard devices from many vendors in sizes ranging from small to very large multilayered units, as well as custom units often used at modest cost.
In fact, due to their small size, low mass, and the inherent physics of the situation, piezoelectric motors can be operated at a high rate, with motion “action” reaching into the multi-kilohertz range—a rate which is difficult to nearly impossible with conventional electromagnetic motors—while their motion is precise, repeatable, and controllable. As an added advantage, they are “clean” with no bearings and require no lubrication (avoiding potential contamination), and their non-metallic nature is not only an advantage in many situations but may even be a necessity.
The motor transducer is only half of the piezoelectric motor story, as the electronics which drive the piezo motor are also critical. Specifically, a piezo motor requires a voltage drive ranging from 30V to hundreds and even thousands of volts depending on the crystal size, desired elongation, and other factors.
The need for voltage drive differs from conventional electromagnetic motors that are driven by current flowing through their coils. The piezoelectric crystal needs an applied voltage, and it is this voltage which determines and drives its action.
Of course, a current accompanies a voltage any time real work is done (in the physics sense, as is the case here), but voltage is the controlling parameter. The associated current can be in the range of a few milliamps to several amps.
In contrast to the inductive load of an electromagnetic motor and its coils, the piezoelectric motor element looks like a capacitive load, which may range as high as 1000 nanofarads (nF). Therefore, its voltage driver must be designed for supporting capacitive loads without concern for oscillation or stability issues.
The high-voltage driver itself needs a high-voltage DC rail as its supply. Providing this high-voltage supply can be a challenge in some circuits, as most basic AC/DC or DC/DC converters produce a much lower-voltage rail at up to several tens of volts. Thus, the drive problem has two parts: providing a DC supply of high enough voltage and then developing a high-voltage amplifier that operates from this supply to drive the element.
Also note that the piezo element is “floating” (not ground referenced), so the driver must have a differential output with no reference to system ground. This complicates the driver design and topology but is a solvable problem.
There are also potential safety issues. Depending on application, region, and regulatory standards, once the drive voltage gets above around 50–60 volts, there are issues of user safety, physical isolation, and protection from the voltages. Additionally, designers must follow mandates defining minimum creepage and clearance dimensions, which are a function of the voltage level.
At lower voltages in the tens of volts, standard op amps may be suitable for piezoelectric drive circuits if their process supports these higher voltages. Providing the drive signal at those voltages often requires devices and process technology which differ from those for low-voltage regimes.
Another option is to use a standard lower-voltage op amp with voltage-boosting transistors (usually bipolar, sometimes FETs) on their output. For example, the Analog Devices ADA4700-1 is a high-voltage precision amplifier operating to ±50V. With a suitable “snubber” network, this 8-lead small outline integrated circuit (SOIC) device is optimized for providing high slew-rate output into capacitive loads as high as 100nF while remaining stable (Figure 3).
Figure 3: The ADA4700-1 op amp is designed to drive large capacitive loads, such as a piezoelectric motor element, while the added RC components function as a snubber circuit to ensure proper behavior with this load class. (Source: Analog Devices)
To meet the drive needs of the piezoelectric element with higher voltages, specialized op amps in monolithic and hybrid forms are available to deliver the voltage and current needed for highly capacitive loads. Vendors such as Analog Devices, Texas Instruments, and Microchip Technology offer these specialized components. For higher voltage levels and capacitive-load handling, vendors like Apex Microtechnology offer optimized, application-specific drivers.
Designing, building, and certifying these higher-voltage amplifiers is not an easy task, and so it is often a “buy” rather than “make” decision. For designers who prefer or need to make their own drive circuit, vendors of ICs and modules help complete reference designs.
The piezoelectric-based motor and actuator, while not an obvious solution, is an innovative answer to a class of motion problems that would otherwise be difficult to solve. It provides high-speed, precise, accurate motion over a limited range and with significant force. Unlike the better-known current-driven electromagnetic motors, piezoelectric motors are voltage-drive devices with unique driver needs in order to deliver higher voltages into capacitive loads.
Bill Schweber is a contributing writer for Mouser Electronics and an electronics engineer who has written three textbooks on electronic communications systems, as well as hundreds of technical articles, opinion columns, and product features. In past roles, he worked as a technical web-site manager for multiple topic-specific sites for EE Times, as well as both the Executive Editor and Analog Editor at EDN.
At Analog Devices, Inc. (a leading vendor of analog and mixed-signal ICs), Bill was in marketing communications (public relations); as a result, he has been on both sides of the technical PR function, presenting company products, stories, and messages to the media and also as the recipient of these.
Prior to the MarCom role at Analog, Bill was associate editor of their respected technical journal, and also worked in their product marketing and applications engineering groups. Before those roles, Bill was at Instron Corp., doing hands-on analog- and power-circuit design and systems integration for materials-testing machine controls.
He has an MSEE (Univ. of Mass) and BSEE (Columbia Univ.), is a Registered Professional Engineer, and holds an Advanced Class amateur radio license. Bill has also planned, written, and presented on-line courses on a variety of engineering topics, including MOSFET basics, ADC selection, and driving LEDs.