An optical printer has a camera and several projectors driven by stepping motors. Provision is made to control these motors so that they may be driven in synchronization or the projector motors may be driven out of synchronization and independently of the camera drive. Provision is also made to insure that the motors always stop in a predetermined home position, get up to full speed after rotating through a predetermined arc and begin to slow down with a predetermined arc of the home position.
The present invention comprises a method and apparatus for driving and homing a stepper motor. In a preferred embodiment, 24 micro step pulse width modulated (PWM) voltage signals are used to drive the coils of the stepper motor. The phases between the driving signals are 90.degree. out of phase. Thus, a sine/cosine methodology is used to drive the motor. Using micro-programmable pulse width modulation (PWM) levels involves the microprocessor reading stored voltage levels from a table stored in memory which corresponds to the amount of angular displacement desired by the motor. These voltage levels are then applied to the motor's coils. The microprocessor performs these operations by executing software instructions stored in memory. The software can be stored in memory located in the controller or in a separate logic block or logic chip. The magnitude of the bounce of a pointer attached to the output shaft of the motor and the magnitude of the noise generated as the pointer contacts a mechanical stop, are both directly related to the applied voltage and speed or frequency of the homing strategy. To reduce the bounce of the pointer and the generated noise to barely discernable levels, the applied voltage is reduced to between 15% and 30% of the normal driving voltage (approximately 1 volt for a 5 volt system). Additionally, the speed of homing is set to a value below the new start-stop frequency of the motor.
