| 摘要 |
A permanent magnet DC variable length driven electric motor (not shown) is typically used to actuate the mechanism of a vehicle rear view mirror (not shown). When the rotating portion of such a motor is rotating as a result of the application of pulses of current, the motor also generates a voltage that is proportional to its angular velocity, known as its back EMF. Thus during the time the current pulse is not being applied, a back EMF is generated by the motor which is proportional to the angular velocity of the rotating portion of the motor. The back EMF of the motor can be measured and using a calibration table the motor's angular velocity can be determined and compared with a desired angular velocity of the motor. If for example, measured back EMF corresponds to an angular velocity higher than that desired, the on time of the current pulse driving the motor can be reduced the next time the motor is energised. Correspondingly, if the measured back EMF is indicative of a lower than desired angular velocity of the motor, the on time of the current pulse driving the motor is increased. This closed loop measurement and control technique is not affected by the load on the motor. The magnitude of back EMF is generated merely by the angular velocity of the motor and not the amount of work it is doing. A method and means for control of the speed and the positioning of the rotating portion of the motor is possible.
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