Electric power steering system

• US 6,364,051 B1
• Filed: 10/05/2000
• Issued: 04/02/2002
• Est. Priority Date: 10/07/1999
• Status: Active Grant

• Current Assignee: Koyo Seiko Co., Ltd. (Toyota Motor Corporation)
• Original Assignee: Koyo Seiko Co., Ltd. (Toyota Motor Corporation)
• Inventors: Segawa, Masaya, Hayama, Ryouhei, Nakano, Shiro, Kada, Tomoyasu, Takamatsu, Takanobu, Nishizaki, Katsutoshi
• Primary Examiner(s): Hurley, Kevin
• Application Number:
  US09/679,633
• Time in Patent Office: 544 Days
• Field of Search: 180/443, 180/446, 701/41, 701/42
• US Class Current: 180/446
• CPC Class Codes: B62D 5/0463 calculating assisting torque from the motor based on driver input

Abstract

An electric power steering system which assists a steering operation by controlling an electric motor on the basis of a target electric current determined in accordance with a steering torque applied to an operation member and applying a driving force to a steering mechanism from the electric motor. The system includes steering torque detecting section; target electric current determining circuit for determining a target electric current in accordance with the steering torque on the basis of an assist characteristic; and motor controlling circuit for controlling the electric motor on the basis of the target electric current. The assist characteristic is offset from an initial characteristic when a motor vehicle incorporating the system is in a predetermined turning state.

Claims

1. An electric power steering system which assists a steering operation by controlling an electric motor on the basis of a target electric current determined in accordance with a steering torque applied to an operation member and applying a driving force to a steering mechanism from the electric motor, the electric power steering system comprising:

steering torque detecting section for detecting a steering torque;

target electric current determining circuit for determining a target electric current in accordance with the steering torque detected by the steering torque detecting section on the basis of an assist characteristic indicative of a relationship between the steering torque and the target electric current;

motor controlling circuit for controlling the electric motor on the basis of the target electric current determined by the target electric current determining circuit for driving the electric motor; and

assist characteristic modifying circuit for modifying the assist characteristic by offsetting the assist characteristic from an initial characteristic when a motor vehicle incorporating the electric power steering system is in a predetermined turning state.

2. An electric power steering system as set forth in claim 1, wherein the assist characteristic modifying circuit modifies the assist characteristic so that the electric motor generates a greater driving force when the motor vehicle is in the predetermined turning state than when the initial characteristic is employed as the assist characteristic.

3. An electric power steering system as set forth in claim 1, wherein the assist characteristic modifying circuit comprises a circuit for determining that the motor vehicle is in the predetermined turning state on condition that the steering torque detecting section keeps detecting a torque not smaller than a predetermined level for not less than a predetermined period.

4. An electric power steering system as set forth in claim 1, further comprising steering angle detecting circuit for detecting a steering angle of the operation member, wherein the assist characteristic modifying circuit comprises a circuit for determining that the motor vehicle is in the predetermined turning state on condition that the steering angle detecting circuit keeps detecting a steering angle not smaller than a predetermined level for not less than a predetermined period.

5. An electric power steering system as set forth in claim 1, further comprising vehicle speed detecting section for detecting a speed of the motor vehicle, wherein the assist characteristic modifying circuit offsets the assist characteristic from the initial characteristic on condition that the vehicle speed detected by the vehicle speed detecting section is not higher than a first vehicle speed.

6. An electric power steering system as set forth in claim 1, further comprising vehicle speed detecting section for detecting a speed of the motor vehicle, wherein the assist characteristic modifying circuit offsets the assist characteristic from the initial characteristic on condition that the vehicle speed detected by the vehicle speed detecting section is not lower than a second vehicle speed.

7. An electric power steering system as set forth in claim 6, wherein the second vehicle speed is higher than the first vehicle speed.

8. An electric power steering system as set forth in claim 1, further comprising vehicle speed detecting section for detecting a speed of the motor vehicle, wherein the assist characteristic modifying circuit comprises:

a circuit for offsetting the assist characteristic from the initial characteristic on condition that the vehicle speed detected by the vehicle speed detecting section is not higher than a first vehicle speed; and

a circuit for offsetting the assist characteristic from the initial characteristic on condition that the vehicle speed detected by the vehicle speed detecting section is not lower than a second vehicle speed which is higher than the first vehicle speed,

wherein the assist characteristic is gradually changed from the initial characteristic to the offset characteristic so that the rate of a change in the assist characteristic is lower when the vehicle speed is not lower than the second vehicle speed than when the vehicle speed is not higher than the first vehicle speed.

9. An electric power steering system as set forth in claim 5, wherein the assist characteristic modifying circuit offsets the assist characteristic from the initial characteristic on condition that the motor vehicle is in a turning state with a steering angle not smaller than a first steering angle when the vehicle speed detected by the vehicle speed detecting section is not higher than the first vehicle speed.

10. An electric power steering system as set forth in claim 5, wherein the assist characteristic modifying circuit offsets the assist characteristic from the initial characteristic on condition that the motor vehicle is in a turning state with a steering angle not smaller than a second steering angle when the vehicle speed detected by the vehicle speed detecting section is not lower than the second vehicle speed.

11. An electric power steering system as set forth in claim 10, wherein the second steering angle is smaller than the first steering angle.

12. An electric power steering system as set forth in claim 1, wherein the assist characteristic modifying circuit comprises assist characteristic transition circuit for gradually effecting transition of the assist characteristic to an initial offset characteristic which is offset by a predetermined value from the initial characteristic when the motor vehicle comes into the predetermined turning state.

13. An electric power steering system as set forth in claim 12, wherein the assist characteristic modifying circuit further comprises torque adapting circuit for further modifying the assist characteristic in accordance with the change in the steering torque detected by the steering torque detecting circuit after the transition of the assist characteristic to the initial offset characteristic.

14. An electric power steering system as set forth in claim 12, wherein the assist characteristic modifying circuit further comprises initial offset determining circuit for determining the initial offset characteristic in accordance with a magnitude of a steering assist force applied during a predetermined period before the assist characteristic is changed to the initial offset characteristic.