Electric power steering system

An electric power steering system for a vehicle, comprising:

• a steering system including a steering wheel and a steering mechanism for transmitting a steering torque applied to said steering wheel to steerable wheels;
• a steering torque sensor for detecting said manual steering torque applied to said steering wheel;
• an electric actuator for providing an assist steering torque to said steering mechanism;
• an electric current detecting means for detecting electric current supplied to said electric actuator;
• a control means for controlling an output of said electric actuator according to signals from said steering torque sensor and said electric current detecting means, wherein said control means is adapted to diminish said assist steering torque when said road frictional coefficient estimated by said road frictional coefficient estimating means is lower than a standard reference value;
• an electric voltage detecting means for detecting electric voltage applied to said electric actuator;
• a road surface reaction estimating means for estimating a road surface reaction applied to said steering mechanism according to signals from said electric voltage detecting means, said steering torque sensor, and said electric current detecting means;
• and
• a steering angle sensor for detecting a steering angle of said steering mechanism, and a road frictional coefficient estimating means for estimating a frictional coefficient between a road surface and a tire of the vehicle according to signals from said steering angle sensor and said road surface reaction estimating means.

An electric power steering system for a vehicle, comprising:

• a steering system including a steering wheel and a steering mechanism for transmitting a steering torque applied to said steering wheel to steerable wheels;
• a steering torque sensor for detecting said manual steering torque applied to said steering wheel;
• an electric actuator for providing an assist steering torque to said steering mechanism;
• an electric current detecting means for detecting electric current supplied to said electric actuator;
• a control means for controlling an output of said electric actuator according to signals from said steering torque sensor and said electric current detecting means;
• an electric voltage detecting means for detecting electric voltage applied to said electric actuator;
• a road surface reaction estimating means for estimating a road surface reaction applied to said steering mechanism according to signals from said electric voltage detecting means, said steering torque sensor, and said electric current detecting means;
• a steering angle sensor for detecting a steering angle of said steering mechanism, and a road frictional coefficient estimating means for estimating a frictional coefficient between a road surface and a tire of the vehicle according to signals from said steering angle sensor and said road surface reaction estimating means;
• and
• maximum steering angle determining unit for computing a maximum steering angle which can be taken without causing any excessive side slip angle according to said estimated road frictional coefficient, and damping means for increasing a resistance to a manual effort to turn said steering wheel when said steering angle sensor detects a steering angle greater than said maximum steering angle.