1. Field of the Invention
The present invention relates to a method for initializing an indicating instrument for a vehicle.
2. Description of Related Art
Conventionally, an indicating instrument for a vehicle is known. The indicating instrument rotates a pointer through application of a drive signal, which alternates in accordance with an electrical angle, to a field winding of a step motor, and indicates a vehicle state value which accords with a rotational position of the pointer. In such an indicating instrument, the pointer returns to a zero position, which indicates a zero value of the vehicle state value, as a result of rotation of the pointer in a zero-reset direction, and the pointer is stopped at a stopper position, which is located within a predetermined range from the zero position in the zero-reset direction, through a stopper mechanism. The control of the drive signal is performed based on the electrical angle corresponding to this stopper position.
For example, an indicating instrument for a vehicle described in Patent No. JP3770095B2 (corresponding to US2002/0117988A1) detects an induced voltage generated in a field winding, controlling a drive signal applied to the field winding of a step motor so as to rotate the pointer in the zero-reset direction. Accordingly, the induced voltage is generated in the field winding during the rotation of the pointer, whereas the induced voltage generated in the field winding is reduced when the pointer stops. For this reason, if a detected voltage of the induced voltage generated in the field winding is equal to or smaller than a set value, the electrical angle corresponding to the stopper position is updated on the assumption that the pointer has stopped at the stopper position. By such a series of processings, even though the step motor has lost synchronization due to disturbance such as vibration, so that the rotational position of the pointer is displaced before the indicating instrument is started, the drive signal is accurately controlled based on the updated electrical angle.
An indicating instrument for a vehicle described in Patent No. JP3389727B2 (corresponding to U.S. Pat. No. 5,877,694A) continues application of the drive signal to the field winding of a step motor, to rotate the pointer in the zero-reset direction thereby pressing against a stopper mechanism. Accordingly, the pointer is reliably stopped at the stopper position, and the electrical angle corresponding to the stopper position is correctly updated. Therefore, even if the rotational position of the pointer is shifted as a result of the loss of synchronization of the step motor before the indicating instrument is started, the drive signal is accurately controlled based on the updated electrical angle.
In the above-described indicating instruments in JP3770095B2 and JP3389727B2, the subsequent control of the drive signal is carried out based on the electrical angle, which is initialized corresponding to the stopper position before product shipment. In consequence, if the initialization of the electrical angle that corresponds to the stopper position is inaccurate, the drive signal control contains an error from the beginning, i.e., before product shipment. Because of this, there is concern in the worst case that unintended synchronization loss of the step motor is caused after product shipment and erroneous indication of the vehicle state value by the pointer may be thereby brought about.
Accordingly, in JP3770095B2, the electrical angle of the drive signal is gradually changed so as to rotate the pointer, which has been rotated to the stopper position, in an indication value increasing direction that is opposite from the zero-reset direction. Then, the electrical angle when the pointer is separated from the stopper position or immediately before this separation, is initialized as the electrical angle corresponding to the stopper position. However, by such a method, the shift of the rotational position of the pointer in the indication value increasing direction in accordance with the gradual change of the electrical angle is small. Thus, the separation of the pointer from the stopper position is difficult to detect, so that accuracy of the initialization of the electrical angle corresponding to the stopper position may be reduced.