DIAGNOSTIC METHOD AND APPARATUS FOR ELECTRIC POWER STEERING SYSTEM

A method and apparatus diagnoses a correct operation of a phase or star point isolation relay in an electric power steering system of the kind including a motor drive circuit and at least one phase isolation relay which connects a part of the drive circuit to a phase of the motor. The relay is opened during shutdown to isolate the motor phase from the part of the drive circuit and is normally closed during operation of the motor to connect the phase to the part of the drive circuit. The method includes the steps of: closing the relay; demanding a current through the motor phase associated with the relay that is equal to a preset value; measuring at least one characteristic of the phase current in that phase after a predetermined period of time has elapsed, the elapsed time being less than a time needed for the phase current to reach the demanded current value; comparing the measured characteristic of the phase current with a reference characteristic of the phase current; and, in the event that the measured characteristic does not correspond with the reference characteristic, opening the relay and subsequently repeating the steps.