Methods and apparatus for robust and efficient stepper motor bemf measurement

A method for determining a back electromotive force (BEMF) voltage for a stepper motor, comprising:

• supplying a voltage to a coil in a stepper motor for an on time that corresponds to increasing a current in the coil to a predetermined maximum current;
• removing the voltage to the coil in the stepper motor for an off time that corresponds to allow the current in the coil to decrease to a predetermined minimum current;
• counting a number of pulses of a known frequency to form a count corresponding to a duration of the off time;
• and
• determining a voltage due to the BEMF using the change in the current between the predetermined minimum current and the predetermined maximum current, the off time, an inductance of the coil, a current in the coil, and a resistance of the coil.

An apparatus, comprising:

• an FET driver circuit configured to supply current to a coil in a stepper motor, the FET driver circuit configured to regulate the current to the coil using a fixed delta current ΔI;
• a current chopper pulse width modulated circuit coupled to the FET driver circuit configured to supply pulses corresponding to a step control signal and a direction control signal;
• a back electromotive force (BEMF) monitor coupled to the current chopper circuit configured to measure an off time pulse and to output a BEMF monitor signal;
• and
• a controller coupled to the current chopper pulse width modulated circuit to supply the step and direction control signals and coupled to receive the BEMF monitor signal.

An integrated circuit for driving a stepper motor, comprising:

• a current chopper pulse width modulation circuit for receiving a step control signal and a direction control signal and configured to output pulses to a FET driver circuit, the output pulses configured to control current to a stepper motor using a fixed delta current scheme;
• a FET driver circuit for supplying current to at least one coil in a stepper motor, the FET driver circuit coupled to receive the output pulses from the current chopper pulse width modulation circuit;
• and
• a back electromotive force monitor circuit coupled to the output pulses from the current chopper pulse width modulation circuit, the back electromotive force monitor circuit outputting a signal corresponding to a back electromotive force voltage of the stepper motor.