Electric power steering system

• US 5,904,223 A
• Filed: 11/27/1996
• Issued: 05/18/1999
• Est. Priority Date: 12/01/1995
• Status: Expired due to Term

• Current Assignee: Honda Giken Kogyo Kabushiki Kaisha (Honda Motor Company)
• Original Assignee: Honda Giken Kogyo Kabushiki Kaisha (Honda Motor Company)
• Inventors: Shimizu, Yasuo, Tokunaga, Hiroyuki
• Primary Examiner(s): Oberleitner, Robert J.
• Assistant Examiner(s): Lipka, Pamela J.
• Application Number:
  US08/757,796
• Time in Patent Office: 902 Days
• Field of Search: 180/443, 180/446, 180/412, 180/413, 180/422, 303/150, 303/146, 303/148, 701/41, 364/424.05
• US Class Current: 180/443
• CPC Class Codes: B62D 5/0463 calculating assisting torque from the motor based on driver input
• CPC Class Codes: B62D 6/006 using a measured or estimated road friction coefficient
• CPC Class Codes: B62D 6/008 Control of feed-back to the steering input member, e.g. simulating road feel in steer-by-wire applications

Abstract

In an electric power steering system, a road surface reaction applied to a steering mechanism is estimated from a voltage and an electric current applied to an electric actuator, and a steering torque. Because the road surface reaction provides a useful data for assessing the condition of the road surface, it is possible to optimally control various control systems which depends on the condition of the road surface. In particular, a frictional coefficient between a road surface and a tire of the vehicle can be estimated according to a steering angle and the road surface reaction, for instance by comparing the estimated road surface reaction with a reference road surface reaction which is computed from a set of standard conditions. Thus, the road surface reaction and the road frictional coefficient can be obtained during a normal steering action by using functions which are normally provided in the electric power steering system. By knowing the current road surface condition, an optimum steering assist action can be executed according to each particular road surface condition, and can substantially reduce the effort required by the vehicle operator to steer the vehicle under all conditions.

Claims

1. 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

wherein said road frictional coefficient is estimated by comparing said estimated road surface reaction with a reference road surface reaction which is computed from a set of standard conditions.

2. 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.

3. 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.