Method and System for Determining Motor Shaft Position

1 . A method for determining a rotational position of a rotor of a brushless DC motor having at least a first phase and a second phase, the first phase and second phase being driven by a first pulse-width modulated (“PWM”) drive signal and a second PWM drive signal, the rotational position of the rotor being determined relative to a stator of the motor, the method comprising the steps of:

shifting a phase of the first PWM drive signal relative to a phase of the second PWM drive signal to produce a current ripple;

obtaining a plurality of measurements, over a sampling period, of a current of the first phase of the motor and a current of the second phase of the motor;

determining a current ripple based on the plurality of measurements;

determining an inductance of the stator over time based on the current ripple and the first and second PWM drive signals; and

determining the rotational position of the rotor based on the determined inductance.

2 . The method of claim 1 , wherein the brushless DC motor includes a third phase driven by a third PWM drive signal, the method further comprising the steps of:

shifting a phase of the third PWM drive signal relative to a phase of the first and second PWM drive signals; and

obtaining a plurality of measurements, over the sampling period, of a current of the third phase; and

wherein the step of determining an inductance of the stator over time is further based on the third PWM drive signal.

3 . The method of claim 2 , wherein the phase of each of the first, second, and third PWM drive signals is shifted 120° from each other.

4 . The method of claim 1 , wherein the number of measurements of the plurality of measurements during the sampling period (the “sampling rate”) is higher than a frequency of the PWM drive signals.

5 . The method of claim 4 , wherein the sampling rate is greater than 10 times more than the frequency of the PWM drive signals.

6 . A controller for a three-phase BLDC motor having a saliency, the controller comprising:

a first PWM generator adapted to be in electrical communication with a first phase of the motor and configured to generate a first PWM drive signal, the first PWM drive signal having a drive frequency and a first signal phase;

a second PWM generator adapted to be in electrical communication with a second phase of the motor and configured to generate a second PWM drive signal, the second PWM drive signal having the drive frequency and a second signal phase which is different than the first signal phase;

a third PWM generator adapted to be in electrical communication with a third phase of the motor and configured to generate a third PWM drive signal, the third PWM drive signal having the drive frequency and a third signal phase which is different than the first and second signal phases; and

a ripple analyzer configured to sample a voltage of each of the PWM drive signals and a current of each phase of the motor at a sampling rate to determine a position of a rotor of the motor.

7 . The controller of claim 6 , wherein the ripple analyzer further comprises:

a current sensor configured to sample a current of each phase of the motor at a sampling rate and determine a ripple current;

a ripple flux generator configured to sample a voltage of each of the PWM drive signals at the sampling rate and determine a ripple flux; and

a position sensor in electrical communication with the current sensor and the ripple flux generator and configured to determine a position of a rotor of the motor based on the ripple current and the ripple flux.

8 . A processor-based controller for a BLDC motor having a saliency, the controller comprising:

a processor;

interface circuitry configured to operably couple the processor to the motor; and

wherein the processor is programmed to:

provide a first PWM drive signal having a first voltage waveform to a first phase of the motor and a second PWM drive signal having a second voltage waveform to a second phase of the motor, the first and second PWM drive signals being phase-shifted relative to each other;

obtaining a plurality of measurements of a current of each phase of the motor over time; and

determine a position of a rotor of the motor based on the first and second voltage waveforms and the measured currents of the motor.

9 . A method for fault detection of a brushless DC motor having at least a first phase and a second phase, the first phase and second phase being driven by a first pulse-width modulated (“PWM”) drive signal and a second PWM drive signal, the method comprising the steps of:

shifting a phase of the first PWM drive signal relative to a phase of the second PWM drive signal;

obtaining a plurality of measurements, over a sampling period, of a current of the first phase of the motor and a current of the second phase of the motor;

determining whether or not a current ripple exists based on the plurality of measurements; and

when the current ripple does not exist, providing a fault indication.

10 . The method of claim 9 , wherein determining whether or not a current ripple exists comprises the sub-steps of:

calculating, using the plurality of measurements; an average current over a PWM cycle;

calculating, using the plurality of measurements, a previous average current over a previous PWM cycle;

extrapolating a line through the calculated average current and the previous average current; and

subtracting the extrapolated line from the measured currents to determine if a current ripple remains.