Technical Support
Feedback
What Is a Piezoelectric Element?
Piezoelectric elements are ceramics that change shape when an electric charge is applied, or (when subjected to a mechanical load) output an electrical charge.
Manufacturers process and press together lead, zircon, and titanate (PZT) powders, then fire them into quartz. They then integrate the ceramic with ferroelectric materials to create electrodes.
Finally, manufacturers apply an electromagnetic field to the piezoelectric element to align and polarize the material.
Discovered by Pierre and Jacques Curie in the late 1800s, piezoelectric elements are named after the Greek word for press — piezein. The Curies found that compressing crystals of potassium sodium tartrate (also known as Rochelle salt, after its original formulation) produces a voltage... and that a charged piezoelectric crystal deforms. Today, other moisture-resistant piezoelectric crystals are used in engineering systems... but the ability to change shape is what all piezoelectric technologies use today.
What Types of Piezo Motors Are There?
Piezoelectric elements go into countless transducers and sensors, and this use is perhaps the most widely known. However, certain arrangements of piezoelectric elements act as solid-state piezo motors to move loads over short distances.
The most basic piezo motors generate motion by expanding proportionally to the applied voltage. Stacked, shear, and tube piezo motors are three common options here. The former produce high force but only about 20 μm of travel. The latter (common in medical and dispensing applications) can move several millimeters but with less force.
Slightly more complex are flexure-guided piezo motors with motion amplifiers, used for long, straight-line motion. Motion is proportional to the drive voltage; multi-axis stages move items up several millimeters or more.
Ultrasonic friction piezo motors have oscillating plates that serve as stators, propelling a slider or rotor through friction. These are fast, with unlimited output motion, sometimes within 0.8 ms of a distance input command — although resolution is only about 60 nm.
Note that some sources call ultrasonic piezo motors standing-wave piezo motors because of the way they output motion. The only issue with ultrasonic piezo motors is that they rely on friction to propel the slider or rotor, which essentially limits resolution to about 38 nm.
For stepper piezo motors, manufacturers combine multiple piezoelectric elements to obtain actuator setups that can move more than 100 lb. The motors have longer travel than most other options and are fast (within 0.8 ms in some cases). Piezo stepper motors can also achieve picometer resolution under direct piezo drive in what is called dithering.
"Moving forward... control devices are getting increasingly stringent performance from piezo flexure stages, which output high-bandwidth actuation across multiple degrees of freedom. These actuators... advanced microscopy that breaks the Rayleigh limit; semiconductor lithography that tracks Moore's Law down to the atomic scale; silicon photonics manufacturing; and genomics analyzers," said Stefan Vorndran of Physik Instrumente in a statement recently published for Design World.
As mentioned earlier, in ultrasonic piezo motors, the ceramic material vibrates a few nanometers at a time, outputting linear or rotary motion up to 550 mm/s.
Current passing through the electrodes excites the piezoelectric ceramic plate (acting as the motor stator). This causes oscillation at a frequency of several thousand hertz at the material's natural frequency. The plate has friction pushers that move it along an inclined path at the characteristic frequency. With each oscillation cycle, the motor advances. The two main advantages of ultrasonic piezo motors are:
1) They are an effective alternative to miniature motorized spindles
2) They move with 50 nm resolution
The leading piezo motor option for long travel is the stepper. They have a series of individual piezoelectric elements that perform a sequence of extensions and contractions. Each cycle moves the attached load only a few micrometers... but at an average of 500 to 3,000 Hz, the steps bring the motor to 12 mm/s. Stepper piezo motors can be used in scanning applications and can also withstand external magnetic fields.
Rotary stepper piezo motors cycle a frame with crystals attached through states. One crystal group locks the rotor while another crystal group moves to advance a third crystal. The control then releases the first crystal group and retracts the third crystal. Both locking devices then return home.
By contrast, linear stepper piezo motors cycle a frame holding crystals through locking and moving setups. Typically one group is moving while the other two are locking. This brings the output stage plate to as much as 1,100 mm/s.
Jectronic Instruments Co., Ltd. can provide professional piezoelectric device and motor testing research, production, and application organizations in China with high-frequency high-voltage excitation sources, frequency response analysis, signal integrity checks, and customized test solution integration. For details, please call: 18665876229, Engineer Wu.
