Microstepping bipolar stepping motor controller for document positioning

A controller for operating a stepping motor having a plurality of motor coils and defining rotor and stator poles, the controller comprising:

• at least one processor having control inputs for receiving operational commands, data and program memory means including random access memory for storage of information defining at least one stepping motor operation;
• a digital memory addressable by the processor and a digital to analog converter coupled to an output of the digital memory, the digital memory storing data values representing sinusoidally varying current levels in the motor coils over at least 90.degree. of a passage of rotor and stator poles, and the digital to analog converter being operable to convert data values selected in the digital memory by the processor to analog levels simultaneously for a plurality of outputs of the digital to analog converter at least equal in number to the motor coils;
• a plurality of power stages coupled to said outputs of the digital to analog converter and operable to produce currents in the motor coils corresponding to the analog levels, the power stages each comprising a comparator and a driver responsive to the comparator, a drive current level produced in a respective one of the motor coils being sensed and fed back to one input of the comparator, and an analog level from the RAMDAC being coupled to another input of the comparator, such that the comparator energizes the driver until the drive current level reaches the analog level from the RAMDAC;
• means coupled to an output of the comparator and to the driver, operable to disable the driver for a predetermined period following a time at which the drive current level reaches the analog level;
• and,
• wherein the driver comprises a bridge driver having phase control inputs coupled to outputs of the processor, and operable for reversing an output polarity of the bridge driver.