| 摘要 |
PROBLEM TO BE SOLVED: To determine whether an indicating needle has reached a reset-to-zero position in a properly increased state of the rotating speed of a magnet rotor of a step motor, when an alternating-current reset-to-zero signal is inputted into the step motor, so as to cause the indicating needle to make turning motion toward the reset-to-zero position in the case of an indicating instrument for a vehicle, using the step motor as a driving power source. SOLUTION: A cosine wave-shaped reset-to-zero voltage is inputted to a field winding of the step moor M, when the indicating needle 20 is returned to the reset-to-zero position. When the reset-to-zero voltage has progressed to a phase angle, corresponding to zero level for the second time after the voltage is inputted into the field winding, an electrical angle appearing then is stored in an EEPROM 90. When the reset-to-zero voltage has progressed to the phase angle, corresponding to the electrical angle stored in the EEPROM 90 after the voltage has been inputted into the field winding, a microcomputer 50 cuts off the field winding from the reset-to-zero voltage by means of changeover switches 70b to 80c, and determines whether a voltage induced in the field winding at the time of cut-off is lower than a prescribed low voltage, thereby determining whether the indicating needle reaches the reset-to-zero position.
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