Patent ID: 6449567
Filing Date: 2002-09-10
Classification: G01R

Abstract:
A method of determining a shaft speed of a motor by using an electrical signature of the motor, comprising the steps of:(a) sensing an electrical current supplied to the motor to generate a current sensor output signal for at least one electrical phase of the motor; (b) demodulating the current sensor output signal for a predetermined time interval to obtain an instantaneous amplitude of the current sensor output signal; (c) generating a frequency spectrum of the instantaneous amplitude of the current sensor output signal; (d) finding at least one spectral peak of the instantaneous current amplitude frequency spectrum within a predetermined shaft frequency sideband range, wherein the predetermined shaft frequency sideband range extends between a shaft frequency sideband upper bound and a shaft frequency sideband lower bound, and wherein the predetermined shaft frequency sideband range is determined by the steps of: (d)(1) collecting over a measurement interval a set of data points representative of the relationship between: (i) one of the motor electrical current sensor output signal and a function of the motor electrical current sensor output signal and (ii) the motor shaft speed and a function of the motor shaft speed; (d)(2) determining a regression line from the collected set of data points; (d)(3) determining a shaft speed upper bound and a shaft speed lower bound based on the dispersion of the collected data points above and below the regression line; (d)(4) converting the shaft speed upper bound to the shaft frequency sideband upper bound by the expression: ShaftFrequencyUpper=lesser of (RPMUpper/60,fsynchâˆ’&agr;Â·&Dgr;f) â€ƒwhere: ShaftFrequencyUpper=the shaft frequency sideband upper bound in revolutions per second; RPMUpper=the motor shaft speed upper bound in revolutions per minute; fsynch=a synchronous frequency of the motor in revolutions per second; &Dgr;=a constant; and &Dgr;f=a reciprocal of the measurement interval in secondsâˆ’1; (e) converting the shaft speed lower bound to the shaft frequency sideband lower bound by the expression: ShaftFrequencyLower=RPMLower/60 â€ƒwhere: ShaftFrequencyLower=the shaft frequency sideband lower bound in revolutions per second, and RPMLower=the motor shaft speed lower bound in revolutions per minute; (f) estimating a shaft frequency from the at least one spectral peak; and (g) converting the shaft frequency to shaft speed by the expression: RPM=ShaftFrequencyÂ·60 where RPM is the shaft speed in revolutions per minute and ShaftFrequency is the estimated shaft frequency in revolutions per second.