Jose L. Agraz, PhD

Piezoelectric Materials Background

The piezoelectric effect is a property that exists in many materials. The name combines the word “piezo” or Greek for pressure, and “electric”, or Greek for amber. Piezoelectric material converts electrical to mechanical energy and vice versa. As a result of an electric field E, piezoelectric material produces three different deformation modes; d 33 , d 31 , and d 15.
The d 33 and d 31 modes take place when the applied electric field is in the same direction as the poling direction. Poling is achieved by subjecting the material to a DC voltage across the material, causing permanent dipole alignment in the material. However, the quality of domains is proportional to voltage amplitude, exposure time, and temperature. The applied field is parallel to the poling direction. By convention, the 3 direction is parallel to the poling direction and the 1 and 2 directions are perpendicular to it.
When an electric field is perpendicular to the poling direction, shear strain occurs, the d 15 mode. The shear is in the plane created by the poling and applied electric field vectors (1 and 3). In most piezoelectric materials, d 15 is the largest piezoelectric coefficient.

Piezoelectric Motors

Ultrasonic motors are defined as devices that utilize vibration in the ultrasonic range to generate motion. This motion is the result of the contraction and expansion of the PZT ceramic elements. However, the contraction and expansion, or displacement is very small (about 1μm), but by supplying power at resonance frequencies, the displacement may be considerably larger, thus, making the PZT elements capable of thrust.
Piezoelectric motors are categorized in two types: Rotary and linear type. Each of these two types of ultrasonic motors benefits from characteristic electromagnetic motors may lack, such as; high torque densities at low speeds, fast response, self locking, flexibility, and flexibility in structural design. These characteristics make ultrasonic motors a genuine contender for high resolution and long travel intermittence motion generating purposes.

Linear Piezoelectric Definition

Linear motors or traveling wave motors generate motion by combining standing waves on two or more pieces of PZT material. These standing waves share the same frequency, but are 1⁄4 wave length out of phase. The generated traveling wave in turn produces an elliptical displacement which generates motion by friction between PZT pieces.

Kurosawa Motor

The Kurosawa motor is the closest design that resembles this project’s objective. The motor consists of two sandwiched type vibrators, arranged as a bolt-clamped Langevin transducer setup. The vibrators cross at right angles and operate on two vibration modes; symmetrical (normal motion) and anti-symmetrical (lateral motion) mode, both are driven a resonance and with a 90o phase shift with respect to each other. However, the design requires heat treated high strength chromium manganese steel bolts binding the PZT material, and a Zirconia ceramics plate at the tip as a frictional material. Although, the motor produces high speeds and thrusts, the motor is large, complex, and difficult to miniaturize.

Kyongjai Motor

The design consists of the shaking of a beam by two actuators resembling the letter “π”, producing an elliptical displacement at the tip located at the mid point of the beam.
The displacement is produced by actuators generating longitudinal and flexural motion being powered at resonance and 90o out of phase with respect to each other. The amplitude of the displacement is dependent of the power applied, frequency, and load pressure. In addition, the speed of the load is dependent on friction between the load and the driving, and the displacement amplitude at the motor’s tip. Although, the motor is easy to miniaturize, the motor is complex and requires a dual source of power.

Mozair Linear Motor

This motor was developed by the Ecole Polytechnique Federale De Lausanne (EPFL) in Switzerland and little literature other than a picture is available. The motor is composed of two layered PZT actuators at a right angle driven π/2 out of phase. The tip is composed of a brass 90o junction with square opening fitting the PZT actuators. Although, the motor is simple, the motor is difficult to miniaturize and requires a dual source of power.

PI Linear Motor

This design is the closest to the Λ shape motor. However, the main difference is that the PZT actuators are actually blocks transmitting a traveling wave to a hardy tip of metal located between the two actuators.
Displacement is produced by two rectangular pieces of PZT material, each alternating to change thrust direction. Although, only one PZT element is excited at a time while the other remains passive, two standing waves are produced, resulting in a traveling wave. Thrust is generated at an alumina tip placed at the junction of the two PZT blocks. Thrust is amplified by preloading a friction bar mounted on the moving portion of the translation stage. Although, the motor produces large thrust, the motor is difficult to miniaturize and lacks two dimensional capabilities