Patent ID: 7616023

Claim:
A method of detecting a malfunction of an encoder used in a vehicle drive system comprising: determining an error of a motor speed (ω Δ ) based on an estimated motor speed (ω x ), the estimated motor speed being a function of measured current over a predetermined interval of time; determining a maximum allowable error of the motor speed (ω E ) at the measured current; comparing the error of the motor speed (ω Δ ) with the maximum allowable error of the motor speed (ω E ), thereby detecting the malfunction of the encoder, wherein the encoder is considered to be malfunctioning if the error of the motor speed is substantially greater than the maximum allowable error; and determining the estimated motor speed by calculating the estimated motor speed using the equation: ω x = ( V PS ⁢ CDC X 100 - I x ⁢ R x ) × 9.55 K BackEMF x wherein: ω x is the estimated motor speed in rpm; V PS is the applied voltage from the power source in V; CDC x is a command duty cycle of the motor; I x , is the current of the motor in A; R x is the resistance of the motor in Ω; K BackEMFx is the back EMF constant of the motor in V/(rad/s); and 9.55 is a conversion factor having the units of rpm/(rad/s).