| 摘要 |
PROBLEM TO BE SOLVED: To make it possible to change rotary torque of an ultrasonic motor rapidly and smoothly. SOLUTION: As shown by Figure, rotary torqueτ generated in an ultrasonic motor changes with a change in a phase difference θof driving signals of two phase to be supplied to the ultrasonic motor. Therefore, a gradient α of a change in rotary torque against the change in a phase difference is measured and at the same time, a deviation in a phase difference β is also measured. The deviation in a phase difference β is a difference between a phase difference when the rotary torque reaches a specified value when the phase difference is so changed that the rotary torque in a certain direction may increase and a phase difference when a rotary torque reaches the specified value when the phase difference is so changed that the rotary torque in a certain direction may decrease. For controlling the rotary torque of the ultrasonic motor, a phase difference θ is found using a formula θ=1/α.τT+β/2 when a target value τT (a positive value in the clockwise direction and a negative value in the counter clockwise direction) of the rotary torque tends to increase, and using a formula θ=1/α.τT-β/2 when the target value tends to decrease, and thereby a phase difference between the driving signals of two phase to be supplied to the ultrasonic motor is agreed with the phase difference θ. |
