Motor-driven power steering system for a vehicle

Claim 1

A motor-driven power steering system for a vehicle having a steering wheel operatively connected to steer steerable road wheels in response to force exerted thereon by an operator, said motor-driven power steering system comprising: a torque sensor connected to detect operator-induced steering torque resulting from steering force exerted on a steering wheel by an operator and generating an output signal representative thereof; a vehicle-speed sensor detecting vehicle speed to generate an output signal representative of the detected vehicle speed; a motor operatively connected with steerable road wheels and adapted to be energized by a power source to transmit a torque to the steerable road wheels for power-assisting steering motion caused by the operator through the steering wheel; an electromagnetic hysteresis clutch interposed between said motor and the steerable road wheels, said clutch having drive and driven rotors free of mechanical contact and electromagnetically coupled to each other to operatively connect said motor with the steerable road wheels in such a manner that the power-assisting torque transmitted from said motor to the steerable road wheels is continuously and gradually changed in direct proportion to an exciting current supplied to said clutch; and a control unit connected to receive output signals from said torque sensor and said vehicle-speed sensor, said control unit including means for controlling respective running and coupling operations of said motor and said electromagnetic hysteresis clutch in such a manner that a current supplied to said motor is controlled in accordance with the detected steering torque and the detected vehicle speed whereas the exciting current supplied to said clutch is at a maximum when the detected vehicle speed is under a prescribed first level and is gradually decreased to zero after the detected vehicle speed increases to the predetermined level whereby said hysteresis clutch develops no slippage when the detected vehicle speed is under the prescribed first level, providing a manual steering mode, and develops a gradual change in the power-assisting torque after the detected vehicle speed exceeds the first level, providing a power-assisted steering mode and a transition between manual and power-assisted steering modes without generation of shock.