Single-Crystal PMN-PT Stack-Driven High Torque Motor

Taking advantage of the special properties of single-crystal ferroelectrics, a unidirectional direct-drive rotary motor was developed. The goal was to demonstrate the benefit of using single crystal stacks as driving engines for a new generation of smart material motors - more compact, more reliable (fewer moving parts) and with higher torque and power density than existing designs.

Two prototype stack actuators (5x5x40 mm) were constructed from single-crystal PMN-PT layers. Driving a unidirectional roller clutch, the motor achieved a free speed of 330 RPM and a stall torque of 0.11 N-m. The high strain of a single-crystal stack (0.26% in this project, but projected as high as 0.5% to 1.0% with electric fields over 10 kV/mm) results in much more energy per cycle transferred to an external load.

If the optimal external load is applied through appropriate motion amplification, the single-crystal stack can deliver five to ten times as much work per cycle compared to a piezoceramic actuator. When the stack is driven at high frequency, an enormous amount of power can be generated relative to the stack weight. The power density for the piezocrystal stack (mechanical power divided by mass) can be well over 10,000 W/kg for a drive frequency in the kHz range.

Assuming a certain amount of unavoidable motor, housing and bearing mass, the power density for an actuator application can be well over 1000 W/kg. This compares favorably with even the highest-performing electromagnetic actuators.

Pre-compressive stack casing