System and method for attenuating noise associated with a back electromotive force signal in a motor
A method for attenuating switching noise from back electromotive force (EMF) observations in a motor is described. The motor is controlled by pulse width modulation (PWM) signals provided at a PWM frequency. The method comprises sampling back EMF observed in the motor at a sampling frequency at least two times greater than the PWM frequency to obtain over-sampled back EMF readings; and filtering the over-sampled back EMF readings to remove switching noise and obtain cleaned back EMF observations. A system implementing the method is also described.
1. A method for attenuating switching noise from back electromotive force (EMF) values in a motor controlled by pulse width modulation (PWM) signals provided at a PWM frequency, comprising:
sampling back EMF values on poles in the motor at a sampling frequency that is at least two times greater than the PWM frequency;
selecting from said back EMF values any two consecutive readings that have a change in sign of the rate of change in current on the poles between said any two consecutive readings;
filtering the two consecutive readings to remove switching noise to obtain cleaned back EMF values; and
utilizing said cleaned back EMF values to commutate said motor,
wherein the back EMF values are sampled from a sensing coil in said motor that is not being driven by the PWM signals when two poles of said motor are being driven by the PWM signals.
2. The method of claim 1 , wherein the filtering is performed by a digital filter.
3. The method of claim 2 , wherein the digital filter has a cutoff frequency approximately half of the frequency of the PWM signals.
4. The method of claim 3 , wherein the digital filter attenuates signals below the cutoff frequency by at least 12 decibels.
5. The method of claim 2 , wherein the sampling is done at regular intervals.
6. The method of claim 5 , wherein the motor is a three-phase motor.
7. The method of claim 6 , wherein the poles of the motor are provided in a delta configuration.
8. The method of claim 6 , wherein the poles of the motor are provided in a Y configuration.
9. A system for attenuating switching noise from back electromotive force (EMF) values in a motor controlled by pulse width modulation (PWM) signals provided at a PWM frequency, comprising:
a sense circuit for sampling back EMF values on poles in the motor from a sensing coil in said motor that is not being driven by the PWM signals when two poles of said motor are being driven by the PWM signals, said back EMF values being at a sampling frequency that is at least two times greater than the PWM frequency;
a selection module for identifying from said back EMF values any two consecutive readings that have a change in sign of the rate of change in current on the poles between said any two consecutive readings;
a filtering module for filtering the two consecutive readings to remove switching noise to provide cleaned back EMF values; and
a motor controller for using the cleaned back EMF values to commutate said motor.
10. The system of claim 9 , wherein the sense circuit samples said back EMF values at regular intervals.
11. The system of claim 10 , wherein the motor is a three-phase motor.
12. The system of claim 11 , wherein the filtering module utilizes a digital filter.
13. The system of claim 12 , wherein the digital filter has a cutoff frequency approximately half of the frequency of the PWM signals.
14. The system of claim 13 , wherein the digital filter attenuates signals below the cutoff frequency by at least 12 decibels.
15. The system of claim 11 , wherein the motor controller includes a digital signal processor.
16. The system of claim 11 , wherein the poles of the motor are provided in a delta configuration.
17. The system of claim 11 , wherein the poles of the motor are provided in a Y configuration.