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Abstract

The rotation of a motor is converted into a swinging movement of an ultrasonic transducer by a conversion mechanism. The relationship between a rotation angle .theta. of the motor and a swinging angle .eta. of the ultrasonic transducer can be expressed by .eta.=tan.sup.-1 (sin.theta.). The rotation of the motor is detected by a rotation detector of a reluctance type having an eccentric rotor. A signal having a nonlinear relationship with respect to an actual rotation angle .theta. of the motor, which can be expressed by the following equation, is supplied to a motor driver from the rotation detector: .theta..sub.D =.theta.+K sin 2.theta.. A dummy rotation angular velocity .omega..sub.D which is a rate of change with respect to a time of a dummy rotation angle .theta..sub.D is obtained by the driver. The motor is servo-driven with reference to a deviation value between the dummy rotation angular velocity .omega..sub.D and a velocity setting value .omega..sub.M. Therefore, the motor is rotated so that the rotation angle .theta. has the nonlinear relationship with respect to time t, as expressed in the following equation: .theta.=.omega..sub.M t-K sin 2.omega..sub.M t. A scanning velocity V of an ultrasonic beam is substantially constant.

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