Method and apparatus for determining faults in an electric assist steering system

1. A method for determining a fault in a system including an electric motor, said method comprising the steps of:

determining a calculated star voltage of the electric motor comprising the steps of:

determining phase voltages to apply to the phases of the electric motor; and

determining the sum of said phase voltages, said calculated star voltage being equal to one-third of said sum of said phase voltages;

determining a measured star voltage of the electric motor;

comparing said calculated star voltage to said measured star voltage of the electric motor to determine a star voltage error; and

determining the fault in the system when said star voltage error exceeds a predetermined maximum star voltage error.

2. The method as recited in claim 1 , wherein the system is an electric assist steering system and the electric motor is an electric assist motor for providing steering assist torque for helping to steer steerable wheels of a vehicle, said step of determining phase voltages comprising the steps of:

determining a required steering assist torque as a function of a sensed applied steering torque and a sensed vehicle speed; and

determining said phase voltages as a function of said required steering assist torque and a sensed rotor position of the electric motor.

3. A method for determining a fault in a system including an electric motor, said method comprising the steps of:

determining a calculated star voltage of the electric motor;

determining a measured star voltage of the electric motor;

comparing said calculated star voltage to said measured star voltage of the electric motor to determine a star voltage error by determining an absolute difference between said calculated star voltage and said measured star voltage; and

determining the fault in the system when said star voltage error exceeds a predetermined maximum star voltage error.

4. Apparatus for determining a fault in a system including an electric motor, said apparatus comprising:

means for determining a calculated star voltage of the electric motor comprising:

means for determining phase voltages to apply to the phases of the electric motor; and

means for determining the sum of said phase voltages, said calculated star voltage being equal to one-third of said sum of said phase voltages;

means for determining a measured star voltage of the electric motor;

means for comparing said calculated star voltage to said measured star voltage of the electric motor to determine a star voltage error; and

means for determining the fault in the system when said star voltage error exceeds a predetermined maximum star voltage error.

5. The apparatus as recited in claim 4 , wherein the system is an electric assist steering system and the electric motor is an electric assist motor for providing steering assist torque for helping to steer steerable wheels of a vehicle, said means for determining phase voltages comprising:

means for determining a required steering assist torque as a function of a sensed applied steering torque and a sensed vehicle speed; and

means for determining said phase voltages as a function of said required steering assist torque and a sensed rotor position of the electric motor.

6. Apparatus for determining a fault in a system including an electric motor, said apparatus comprising:

means for determining a calculated star voltage of the electric motor comprising means for determining an absolute difference between said calculated star voltage and said measured star voltage;

means for determining a measured star voltage of the electric motor;

means for comparing said calculated star voltage to said measured star voltage of the electric motor to determine a star voltage error; and

means for determining the fault in the system when said star voltage error exceeds a predetermined maximum star voltage error.