Motor-driven power steering system for front and rear road wheels

A motor-driven power steering system for front and rear road wheels of a vehicle adapted to provide a fail-safe steering operation, said power steering system comprising:

• a motor electrically connected to receive current from an electric power source and mechanically connected to transmit a force to steer the rear road wheels;
• a steering condition sensor electrically connected to detect at least one of a plurality of steering conditions of a steering wheel connected to the front wheels, such as direction, steering angle, steering torque, and steering speed, and generating an output signal representative thereof;
• a vehicle speed sensor electrically connected to detect travelling speed of the vehicle and generating an output signal representative thereof;
• a motor rotation sensor electrically connected to detect rotational direction and rotational speed of said motor and generating an output signal representative thereof;
• a rear-wheel steering angle sensor electrically connected to detect steering angle of the rear road wheels and generating an output signal representative thereof;
• an electromagnetic brake electrically connected to the power source and operable independently of said motor to lock steering motion of said rear road wheels when the electrical power is disconnected from said electromagnetic brake;
• and
• a control unit connected to receive the output signals of said sensors and operable to disconnect the power supply to said motor and said electromagnetic brake, responsive to failure of any one of said sensors and electrical connections of said system, so as to lock the rear wheels against steering while allowing the vehicle to be steered by the steering wheel through the front wheels.