Electromagnetic motors work fine, but scale one down to a diameter below 10 mm with a 0.2-W input,and efficiency goes to just 2%, making them useless for most applications. To make a tiny motor withsome oomph, researchers at Penn State University use piezoelectrics and electromechancial motionrather than electromagnetics.

The motor, which measures 4-mm long with a 1.8-mm diameter, consistsof a hollow metal tube -- the stator -- with two flattened sides. A strip of PZT, a piezoelectricceramic of lead zirconate titanate, fastens inside the tube on each flat side. A rod held in placewith a spring, or just a spring, is the rotor. Applying ac current to one PZT strip causes it todeform. This creates a bending movement that works with the off-axis weight of the other PZT strip tocreate a wobbling, elliptical rotation. This rotation is then conveyed to the rod. Switching currentto the other plate continues the rotation of the rod. Current is switched from strip to strip atultrasonic frequencies, letting the motor spin at 1,800 rpm producing 1.0 mN-m. The output is fivetimes that of the smallest electromagnetic motor, which is 60 times larger in volume and uses 10 timesthe input power.

Researchers designed the motor to be inexpensive to manufacture and easy toassemble. They envision the motor being used in medical tools that work inside the human body to drivepropellers on miniature cameras exploring the heart, or break up stones in the kidneys And since themotor can be made without magnetic materials and uses no magnetic fields, it can be working inside thebody while the patient is in an MRI machine, a necessity in some microsurgeries. They also believe themotor could find applications in wristwatches.