High Power Density Piezoelectric Motors

   A recent contract award from the Defense Advanced Research Projects Agency supports the efforts of Penn State CAV researchers to develop high power density piezo-electric motors for application to agile, high speed aerospace vehicles. This MEDIRRA program (MEchanical DIode Resonant Rectifying Actuator) is a spin-off from previous research performed for DARPA under the SPICES and SAMPSON programs. Partners in the new effort include the Virginia Tech Center for Power Electronics, Boeing Phantom Works (St. Louis), MPC Products, and Torrington.

   Drs. George Lesieutre and GaryKoopmann, the MEDIRRA PIs, explain that the key principle underlying motor operation is mechanical rectification of resonant oscillation of a piezoelectric drive element. Much recent research had addressed the use of smart (or active) materials such as piezoelectric ceramics for solid-state actuation. In many realistic applications having volume or weight constraints, however, the energy density available from such materials is inadequate. This motivates their use as active drive elements in non-solid state devices, or motors. In such devices, the active elements are driven in an oscillatory manner, and some means of rectification is used to develop one-way motion. Often, rectification involves a mechanical diode: pumps use check valves, while inchworms or ultrasonic wave motors use friction- or interference-based clamps.

   Output power is the product of force and rate. In inchworm motors, the maximum force is the lower of the pushing or holding force. The maximum rate, as the product of step and step size, is roughly independent of length. In practice, maximum specific power is obtained by using a drive element the is long enough to develop displacements that exceed the backlash, but otherwise as short as possible, to obtain a high resonance frequency.

The development of smart materials-based motors invites comparisons to competing technologies. Typical commercial AC motors under 1500 W (2hp) have specific powers of 50-100 W/kg. Nevertheless, the best DC brushless motors have continuous specific power of over 900 w/kg, and even higher intermittent output.

   The performance of piezoelectric motors developed under the SAMPSON program was recently measured. Torques of 0.5 N-m, speeds greater than 500 RPM, and peak power output of 5 W were developed. While performance does not yet exceed that of commercial motors, it is already comparable. The new MEDIRRA program promises performance and packaging that will substantially surpass those of available motors.