Abstract: To provide a controller of an electric power-steering system, capable of improving the steering performance by presuming a motor angular speed .omega. in a range in which the angular speed of a motor is small and moreover very accurately presuming the motor angular speed and thereby, completely showing the functions for compensating the inertia of the motor and controlling the astringency of a vehicle. Because the presumed error of a back electromotive force between a model motor and a motor to be actually mounted is proportional to a motor current, a dead zone of a motor angular speed having a width proportional to a motor current is set to the presumed value of the motor angular speed. When the motor current is small, the width of the dead zone also decreases. Therefore, it is possible to presume an angular speed .omega. even in a region where the motor angular speed is small.

Abstract: A control apparatus for an electric power steering system assures stability of the feedback control and control generation of noise from the control circuit when the steering wheel automatically returns to the neutral position by means of the self-aligning torque in order to improve the feeling of the steering operation. In the motor driving circuit connecting four semiconductor elements in the shape of H bridge, the semiconductors of the first arm are driven by the duty ratio D1 while the semiconductors of the second arm are driven by the duty radio D2 defined by the function of the duty ratio D1 (D2 is longer in time than D1).