Patent ID: 8579072

Claim:
An electric power steering system comprising: a steering mechanism having (a) an input shaft which rotates according to a steering operation of a steering wheel; (b) an output shaft which is connected to the input shaft through a torsion bar, and to which the rotation of the input shaft is transmitted; and (c) a conversion mechanism which converts a rotation of the output shaft to a steering motion of steered road wheels; an input shaft side rotation angle sensor provided at an input shaft side and detecting a rotation angle of the input shaft; an output shaft side rotation angle sensor provided at an output shaft side and detecting or estimating a rotation angle of the output shaft; an electric motor providing a steering assist force to the steering mechanism; an operation circuit calculating, on the basis of a vehicle driving condition, a command current value that is sent to the electric motor; a rotation direction judging circuit provided in the operation circuit and judging whether or not a rotation direction of the input shaft and a rotation direction of the output shaft are identical; an advanced-phase judging circuit provided in the operation circuit and judging which phase of an input shaft side rotation angle signal that is an output signal of the input shaft side rotation angle sensor and an output shaft side rotation angle signal that is an output signal of the output shaft side rotation angle sensor is advanced; a normal/reverse input judging circuit provided in the operation circuit and judging that (i) when the rotation direction judging circuit judges that both rotation directions are identical and also the advanced-phase judging circuit judges that the phase of the input shaft side rotation angle signal is advanced, an input state is a normal input by which torsion of the torsion bar occurs by an input from the input shaft side, and (ii) when the rotation direction judging circuit judges that both rotation directions are identical and also the advanced-phase judging circuit judges that the phase of the output shaft side rotation angle signal is advanced, the input state is a reverse input by which the torsion of the torsion bar occurs by an input from the output shaft side; and a command current correcting circuit provided in the operation circuit and correcting the command current value for the electric motor so that when the normal input is judged by the normal/reverse input judging circuit, a generation torque in the same direction as the rotation direction of the input shaft by the electric motor is greater than that of a case where the reverse input is judged by the normal/reverse input judging circuit.