Pseudo half-step motor drive method and apparatus

A positioning device comprising:

• (a) a stepper motor having a rotor and a plurality of windings, said plurality of windings being selectively energized to rotate the rotor in a stepped motion from a current stepped position to an adjacent stepped position, said stepper motor further having a characteristic detent torque that tends to maintain the rotor in a stepped position;
• (b) a driver circuit coupled to the windings, said driver circuit producing current to drive selected windings among the plurality of windings in the stepper motor based on control signals that are input to the driver circuit;
• and
• (c) a driver control device, which produces the control signals that cause the driver circuit to step the stepper motor through a single step by:
• (i) energizing selected windings among said plurality of windings for a predetermined step time at current levels selected to produce a magnetic field vector that is not aligned with a stepped position such that the rotor completes the single step by rotating to the adjacent stepped position without an overshoot, and with substantially a zero velocity when the predetermined step time has expired;
• and
• (ii) de-energizing the windings that were energized in step (i) after the predetermined step time has expired, the rotor being then held in the adjacent stepped position by the stepper motor detent torque.

A method for driving a stepper motor having a detent torque and defining a plurality of stepped positions, the stepper motor being operatively coupled to a load and comprising a magnetized rotor and a plurality of windings, said plurality of windings being selectively energized to rotate the rotor from a current stepped position to an adjacent stepped position, rotation of the rotor between the current stepped position and the adjacent stepped position corresponding to a fundamental electrical step angle, the method comprising the steps of:

• (a) determining an attenuated electrical step-command angle corresponding to a magnetic field vector that produces a torque on the magnetized rotor such that the magnetized rotor is caused to rotate with a damped oscillation having a first angular peak equal to the fundamental electrical step angle, said first peak being reached after an elapsed time;
• (b) energizing a pair of selected windings among said plurality of windings at different predetermined current levels based on the attenuated electrical step-command angle determined in step (a) for a step time equal to the elapsed time, thereby causing the magnetic rotor to rotate from the current stepped position to the adjacent stepped position such that the rotor has substantially a zero velocity upon reaching the adjacent stepped position;
• and
• (c) de-energizing the windings that were selectively energized in step (b) after the step time, whereby the magnetic rotor is held in the adjacent stepped position by the stepper motor detent torque.