Patent ID: 8798844

Claim:
A method of diagnosing a correct operation of an isolation relay in an electric power steering system including a motor drive circuit and at least one phase isolation relay which connects a part of said motor drive circuit to a phase of said motor, said phase isolation relay being opened during shutdown to isolate said motor phase from said part of said drive circuit and being normally closed during operation of said motor to connect said phase to the part of said drive circuit, said method comprising the steps of: (a) closing said phase isolation relay; (b) demanding a phase current having a predetermined value through the motor phase associated with said phase isolation relay; (c) measuring at least one characteristic of an actual said phase current in said motor phase after a predetermined period of time has elapsed, said predetermined period of time being that is less than an amount of time needed for said measured characteristic of said actual phase current to reach said demanded predetermined current value of said demanded phase current; (d) comparing, using a processor, said measured characteristic of said actual phase current with a reference characteristic of said demanded phase current that is less than said predetermined value of said demanded phase current; and (e) in the event that said measured characteristic of said actual phase current does not correspond with said reference characteristic of said demanded phase current, opening said phase isolation relay and subsequently repeating steps (a) to (d).