A control apparatus of an electric power steering system has first to
fourth calculators and an abnormality detector. The first calculator
calculates an assist current command value, which corresponds to assist
force for reducing steering force of the steering wheel, based on the
steering torque of the steering wheel. The second to fourth calculators
calculate a compensation current command value, which corresponds to
compensation force for compensating for movement of the steering wheel,
based on the angular velocity of the steering wheel that has been
estimated based on the terminal-to-terminal voltage of the motor. The
abnormality detector detects an abnormality of the terminal voltages of
the motor. When the abnormality detector detects an abnormality, the
second to fourth calculators substitute zero for the steering wheel
angular velocity and then calculates the compensation current command
value.
