Piezoelectric motors are precise and silent, but their high cost has limited their applications to specialty niches in chipmaking and optics.
A tiny new piezomotor from Elliptecmotor, Dortmund, Germany (www.elliptec.com), costs much less than current piezomotors. It applies a pulsing voltage to a piezoceramic, which contracts and expands in response. A patented stator converts these motions to oscillations, causing the tip of the stator to trace out an ellipse. At a frequency of about 85 kHz, this motion is clockwise; at 95 kHz it's counterclockwise. When the stator tip is pressed against a movable object, the object moves forward with each rotation of the stator tip.
The tip exerts about one Newton of force. Its velocity is a function of the number of pulses sent to the motor and can be tightly controlled from 0 to 300 mm/sec, so it needs no gearbox. It has no minimum velocity like some dc motors. Input voltages can range from 2.8 to 48 V, given the proper conditioning electronics, and it can be battery operated. Total weight is 1.5 gm, length is under an inch, and because force is transferred using friction (the tip on the moving object), it effectively has a sliding clutch with overload protection built in.