Back-EMF detection for motor control

1. An apparatus, comprising:

a drive circuit configured to apply a plurality of excitation signals to respective inputs of a motor;

a signal inhibit circuit configured to convey a signal to inhibit application of the plurality of excitation signals for an interval, wherein the plurality of excitation signals include three excitation signals, and wherein the signal inhibit circuit is configured to inhibit all three of the excitation signals simultaneously for a same duration that is the interval; and

a measuring circuit configured to measure when a back-electromotive force (EMF) signal crosses a reference signal to determine a length of the interval, wherein the length of the interval is an amount of time for the back-electromotive force to decay to the reference voltage, and wherein the interval varies according to the back-electromotive force.

2. The apparatus of claim 1 , wherein the plurality of excitation signals are pulse-width-modulated excitation signals.

3. The apparatus of claim 2 , wherein at least one of the plurality of pulse-width-modulated excitation signals approximates a sinusoidal waveform.

4. The apparatus of claim 1 , wherein the drive circuit is configured to modify at least one of the plurality of excitation signals according to the determined length of the interval.

5. The apparatus of claim 4 , wherein the drive circuit is configured to modify the at least one of the plurality of excitation signals to align with an angular displacement within the motor.

6. The apparatus of claim 1 , wherein outputs of the drive circuit are configured to inhibit the plurality of excitation signals.

7. The apparatus of claim 1 , wherein the motor is configured to include three phases and wherein the reference signal corresponds to a voltage at a center tap of the motor.

8. The apparatus of claim 1 , wherein the interval occurs approximately one time during an electrical cycle of the plurality of excitation signals.

9. A method, comprising:

inhibiting a plurality of excitation signals output to a motor, wherein the plurality of excitation signals include three excitation signals, and wherein inhibiting the plurality of excitation signals includes inhibiting all three of the excitation signals simultaneously for a same time;

determining the time between the inhibiting of the plurality of excitation signals and a crossing of a back electro motive force (EMF) signal from the motor with a reference signal from the motor;

modifying the plurality of excitation signals as a result of the determined time; and

applying the modified plurality of excitation signals to the motor, wherein the time is an amount of time for the back-electromotive force to decay to the reference voltage, and wherein the time varies according to the back-electromotive force.

10. The method of claim 9 , wherein the inhibiting comprises:

increasing output impedance of a plurality of output amplifiers supplying the plurality of excitation signals, wherein the reference signal is a constant zero volts.

11. The method of claim 9 , wherein the reference signal corresponds to a signal present on a center tap of the motor.

12. The method of claim 9 , wherein the applying further comprises:

obtaining pulse widths from a lookup table, and aligning the plurality of excitation signals with an angular displacement of a rotor portion of the motor using the pulse widths.

13. The method of claim 9 , wherein the inhibiting occurs approximately once per electrical cycle of the plurality of the excitation signals.

14. A device comprising:

a circuit configured to couple a plurality of excitation signals to a motor, wherein the plurality of excitation signals include three excitation signals that are each associated with a different terminal of the motor;

a measuring circuit configured to receive a reference signal; and

logic configured to:

inhibit the coupling of the plurality of excitation signals to the motor, wherein the logic is configured to inhibit all three of the excitation signals simultaneously for a same time;

measure the time at which a back-electromotive force from the motor drops below a level of the reference signal while the plurality of excitation signals are inhibited; and

modify the plurality of excitation signals based, at least in part, on the time, wherein the measured time is an amount of time for the back-electromotive force to decay to the reference voltage, and wherein the measured time varies according to the back-electromotive force.

15. The device of claim 14 , wherein the logic is further configured to align the plurality of excitation signals with an angular displacement of a rotor of the motor according to the measured time.

16. The device of claim 15 , wherein the logic is further configured to determine pulse-width-modulation coefficients that align the plurality of excitation signals with the angular displacement of a rotor of the motor.

17. The device of claim 14 , wherein the plurality of excitation signals are coupled to a three-phase motor, and wherein the plurality of excitation signals include at least the three excitation signals.

18. The device of claim 14 , wherein outputs of the circuit configured to couple a plurality of excitation signals are tri-state amplifiers configured to be set to a high impedance state by the logic configured to inhibit the coupling of the plurality of excitation signals to the motor.

19. The device of claim 14 , wherein the reference signal corresponds to a center tap of the motor.

20. The device of claim 14 , further comprising:

restart the coupling of the plurality of excitation signals to the motor upon the back-electromotive force from the motor dropping below the level of the reference signal.