An Introduction to Piezoelectric Motors

Photo 1. The EDO piezoelectric motor is designed for precision micropositioning applications. This low-profile (<0.20 in. high), low-inertia motor uses solid-state piezoelectric crystals for accurate, repeatable motion.The 130 kHz drive frequency provides speeds of 250 mm/s, with a 6 ms response time and a dynamic resolution of <1.0 micron over several inches. The force output to weight ratio is 14:1.

The ultrasonic piezoelectric motor's faster response times, higher precision, hard brake with no backlash, high power-to-weight ratio, and smaller packaging envelope more than compensate for the lack of brute horsepower and speed associated with its electromagnetic motor counterparts. Its greater positioning accuracy, simplicity (fewer parts), lower profile (no iron cores required), and minimal EMI/RFI noise meet the stringent requirements of today's evolutionary product technology. A typical piezoelectric motor is shown in Photo 1.

Piezoelectric motors have been around for years, but acceptance has been slow because they were relatively expensive. However, recent advances have significantly reduced the channel cost of this technology for closed-loop systems that require high positioning accuracy. With the use of a wide range of PID controllers and/or position sensors, the list of piezoelectric motor product applications is growing daily (see "Piezoelectric Motor Applications?).

Piezoelectric Motor Applications • Micropositioning stages • Manufacturing process control • Fiber-optic positioning • Pick-and-place assembly • Camera autofocus • Medical catheter placement • Semiconductor test equipment • Computer disk drives • Robotic positioning • Pharmaceuticals handling

To understand the nature of piezoelectric materials we have to go back to 1881, when Pierre and Jacques Curie observed that quartz crystals generated an electric field when stressed along a primary axis. The term piezoelectric derives from the Greek word piezein, meaning to squeeze, and the electricity that results from pressure applied to the quartz crystal.

Today, modern polycrystalline piezoelectric ceramic is mass-produced for applications including underwater transducers, point level sensors, medical products, ultrasonic cleaners, actuators, fish finders, and motors. High-purity lead, zirconate, and titanate powders are processed, pressed to shape, fired, electroded, polarized, and tested. Polarization is achieved using high electric fields (2500 V/ mm) to align material domains along a primary axis (see Figure 1).

Figure 2. Piezoelectric material constants such as d33 are used to characterize crystal performance for a given applied voltage.

This process is functionally identical to the way material shapes containing iron are magnetized. Once poled, the material exhibits useful piezoelectric properties. Piezoelectric motors use this poled ceramic shape to create motion with the use of periodic (sinusoidal) electric fields.

In Figure 2, a 200 V charge is applied to a piezoelectric crystal with a displacement constant (d₃₃) equal to 500 pm/ V. This produces an axial displacement of 0.1 microns, a fairly small number that could be good or bad depending on the application. It is very repeatable and lasts a lifetime with a properly designed drive circuit, but the motor will not travel very far when 200 V are applied just once every second. This is the reason piezoelectric motors are driven at or near a resonant frequency mode of the crystal shape.

Figure 3. Standing-wave and propagating-wave piezoelectric motors are very flexible. They can provide rotary, linear, and hybrid motor motion to accommodate numerous user applications.

Piezoelectric motors can be classified as standing wave (single vibration driving source) or propagating wave (two standing waves with a 90° phase difference in both time and space) [1].

In the example shown in Figure 3, a simple standing wave is generated in a thickness-mode piezoelectric crystal to produce linear motion (U.S. Patent No. 4,622,483). Other piezoelectric motors may use shear, hoop, or planar vibration modes for motion, as called for by the application. Sophisticated designs that provide exceptional performance are possible, but at an increased cost. For now, simpler is better.

Piezoelectric crystals are positioned in a simple mechanical motor arrangement and driven in their thickness direction to produce useful work—linear motion—as shown in Figure 4.

Figure 4. In (A), the motor is at rest with pre-load. When a charge is applied, the element expands across its thickness (B). When the voltage goes negative, the element retracts, leaving a gap (C). The process is repeated 130,000 times/s (D).

Two piezoelectric crystals are preloaded against a flat wear surface, by way of the motor shoe, to produce a normal contact force. Friction is important here, since every piezoelectric motor needs friction to operate. As a "positive? sinusoidal voltage waveform:

(1)

drives the crystal to increase its thickness, the axial motion imparts a frictional force:

(2)

of ~ 2"3 N along the wear strip, moving it to the left.

When the sign of the drive voltage swings negative, the same crystal thickness contracts. This action creates a minuscule separation between the motor shoe and the wear strip, allowing the motor to return to its original position without dragging the wear strip backward. As the drive voltage swings positive again, the crystal stroke cycle repeats and the wear strip moves another incremental step to the left.

When the crystal driving process is repeated 130,000 times/s, with a resonant frequency amplification factor of 10, the resulting velocity is ~130 mm/s (5 ips) using mid-range control voltage. Faster or slower motor speeds are achieved by changing control (input) voltage (see Figure 5).

Photo 2. The Model LK-031 laser-based interferometer can be used to measure piezoelectric motor travel and position. (Photo courtesy of Keyence Corp. of America.)

Conventional displacement sensors may be used to measure motor travel and position. One example of this is the Model LK-031 (see Photo 2), a laser-based interferometer made by Keyence Corp. of America (Woodcliff Lake, NJ), that measures motor position over time. Nonlinear start and stop times are a function of system mass (inertia), while resolution of motor position is directly proportional to the voltage across the piezoelectric crystal.

Motor performance is enhanced with the strategic placement and use of additional piezoelectric crystals or structural elements. To meet additional force requirements, several motor elements may be driven in parallel. This process will cumulatively add force. For example, Figure 6B shows two crystals, working complementary to each other to provide twice the baseline motor force.

Figure 6. A single motor preloaded against a friction strip can produce ~1.3 N of thrust (A). Two symmetric motors, driven electrically in parallel, produce twice the thrust force, or ~2.7 N (B). Four motors can be arranged in a similar fashion for >5.0 N of thrust (C). Motor construction and packaging also facilitate rotary motion (D), with corresponding force and accuracy.

Photo 3. Very low profile EDO piezoelectric motors are capable of driving a small stage at a top speed of 300 mm/s and a position repeatability of 0.1 micron. The system shown here uses mature, commercially available hardware such as the Model 1412 motion controller made by Galil Motion Control Inc.

Motors can be packaged in series or parallel to increase speed and force, and linear motion is converted to circular motion by driving a round surface (wheel) vs. a flat wear strip.

Closed-loop systems can track and control linear motions to small fractions of a micron. Photo 3 shows very low profile EDO piezoelectric motors driving a small stage with a top speed of 300 mm/s and a position capability of 0.1 micron. This system uses mature, commercially available hardware such as Galil's Model 1412 motion controller (Galil Motion Control Inc., Rocklin, CA), with public domain software, and the Model RGH22 noncontact position encoder from Renishaw (Hoffman Estates, IL) shown in Photo 4.

Photo 4. The system in Photo 4 also incorporates a Model RGH22 noncontact position encoder made by Renishaw. (Photo courtesy of Renishaw, Inc.)

So what are you waiting for? The building blocks for precise, economical motion control in very small packages are available now, and that's both fact—and friction.

Reference
1. Kenji Uchino. 1997. Piezoelectric Actuators and Ultrasonic Motors, Kluwer Academic Publishers:269.