1. Field of the Invention
The present invention relates to an apparatus for driving a stepping (stepper) motor, in particularly incorporated in a digital still camera.
2. Description of the Related Art
Recently cameras have been employing a motor for driving the auto-focus system. In the case of a DC motor, a relative position detecting device which detects a relative position with respect to a reference position using an encoder or the like is used as a means for detecting a position of a focusing lens. It is necessary to provide a space in which the encoder is accommodated in the camera. Also, a means for detecting the output of the encoder must be incorporated in the camera.
To this end, it is possible to use a stepping motor (stepper motor) in which the angular displacement can be controlled stepwise in the auto-focusing system for a photographing lens. In a 1-2 phase excitation system stepping motor (half-step drive system) which has a large rotational torque and a simple control circuit, if the power supply is stopped at a 1-phase excitation position, i.e., if no excitation occurs, the rotor is stopped at the 1-phase excitation position (detent position). However, if the power supply is stopped at a 2-phase excitation position (equilibrium position), the rotor is in a position between the detent positions and hence, the rotor tends to rotate to one of the detent positions. Consequently, if the excitation is ceased at the 2-phase excitation position, the rotor rotates to and stops at a detent position adjacent to the 2-phase excitation position, thus resulting in an occurrence of an error in the stop position. In order to prevent this, it is necessary to hold the 2-phase excitation to maintain a stable position when the power supply is ceased at the 2-phase excitation position. This causes an increase in the power consumption.
It is an object of the present invention to provide a drive apparatus for a stepping motor of a camera, in which the power consumption of a 1-2 phase excitation stepping motor can be reduced.