A piezoelectric micromotor with a stator of φ=1.6 mm and l=4 mm using bulk PZT

The smallest discrete piezoelectric ultrasonic motor using bulk ceramics was developed. We are proposing basically a two-part motor: stator and rotor. The stator of the present motor consists of a hollow metal brass tube with outer diameter of 1.6 mm, inner diameter of 0.8 mm and length of only 4 mm with 2 PZT plates bonded onto it. Owing to the asymmetrical stator surface, two degenerated orthogonal bending modes were slightly split, resulting in a wobbling motion. Thus, the motor can be driven by a single driving source. The rotor is a spring, which is basically different from previous designs, pressed at both ends to the stator by a pair of ferrules. Consequently, the length of the whole motor assembly was reduced significantly; a final motor length cf only 5 mm was obtained. The working frequency under zero load was approximately 227-233 kHz. Although the size is small, relatively high power was obtained under an optimized load condition: torque of 0.06mNm, maximum power of 3.2 mW with a speed of 118 rad/s, and maximum efficiency of 11% under 48 Vrms at 221 kHz.