Conventionally electric power steering apparatuses are in widespread use as a steering apparatus, which are configured to give a steering assist force to a steering mechanism by driving an electric motor in accordance with the steering torque of a steering wheel steered by a driver.
Patent Literature 1, for example, describes a technique for such an electric power steering apparatus. This technique is configured to, if abnormality is detected in the output value of a torque sensor, control an electric motor using an alternative value calculated based on the past output value of the torque sensor. This can ease a sudden change in assisting when abnormality occurs at the torque sensor.
Recent electric power steering apparatuses, however, are required to have a double safety configuration from demanding for a redundant system of the steering function. Patent Literature 2, for example, describes a technique for such an electric power steering apparatus. This technique is configured to provide an abnormality detection function at a sub-microcomputer, and to permit the operation of power-steering only when a main microcomputer drives a motor in the driving permissive range, and to prohibit the operation of power-steering in the prohibition range. This can prevent serious abnormality where the generated assist torque value is different from the steering direction.