Adaptive variable effort power steering system

• US 6,591,937 B2
• Filed: 12/05/2001
• Issued: 07/15/2003
• Est. Priority Date: 12/05/2001
• Status: Expired due to Fees

• Current Assignee: BWI Company Limited SA (Shougang Corporation)
• Original Assignee: Delphi Technologies, Inc. (BorgWarner Incorporated)
• Inventors: Belvo, Todd A., Schubert, Raymond Kurt, Badenoch, Scott Wilson
• Primary Examiner(s): Morris, Lesley D.
• Assistant Examiner(s): WINNER, TONY H
• Application Number:
  US10/007,647
• Publication Number:
  US 20030102180A1
• Time in Patent Office: 587 Days
• Field of Search: 180/446, 180/421, 180/444, 180/443, 701/41, 701/42
• US Class Current: 180/446
• CPC Class Codes: B60G 2400/104 lateral or transversal with regard to vehicle
• CPC Class Codes: B60G 2400/822 Road friction coefficient determination affecting wheel traction
• CPC Class Codes: B60G 2800/212 Transversal; Side-slip during cornering
• CPC Class Codes: B60G 2800/24 Steering, cornering
• CPC Class Codes: B60G 2800/702 Improving accuracy of a sensor signal
• CPC Class Codes: B60G 2800/95 Automatic Traction or Slip Control [ATC]
• CPC Class Codes: B60G 2800/96 ASC - Assisted or power Steering control
• CPC Class Codes: B60W 2510/105 Output torque
• CPC Class Codes: B60W 2510/20 Steering systems
• CPC Class Codes: B60W 2510/202 Steering torque
• CPC Class Codes: B60W 2520/125 Lateral acceleration
• CPC Class Codes: B60W 2540/18 Steering angle
• CPC Class Codes: B60W 2710/20 Steering systems
• CPC Class Codes: B60W 2720/125 Lateral acceleration
• CPC Class Codes: B60W 30/18172 Preventing, or responsive to skidding of wheels
• CPC Class Codes: B60W 40/064 Degree of grip
• CPC Class Codes: B62D 6/006 using a measured or estimated road friction coefficient

Abstract

A variable power steering system for a motor vehicle is responsive to an activity signal from a vehicle stability enhancement (VSE) system and a lateral surface coefficient of friction estimator to modify steering assist in an amount dependent on the estimated surface coefficient when the vehicle stability enhancement system is active. The modification is phased in and out with slew limiting in steps also determined by the estimated lateral surface coefficient, with both the maximum modification and the step size varying inversely with lateral surface coefficient. The system preferably determines offset current values for the VSE system and estimated lateral acceleration, as well as for any other vehicle traction limit handling systems such as anti-lock braking (ABS) or traction control (TCS) and chooses the greatest in magnitude as a road surface adaptation offset current for addition to a vehicle speed offset current to provide a total current signal for the modification.

Claims

1. An adaptive variable power steering system for a motor vehicle comprising steerable wheels for operating on roads having variable road surface friction, the vehicle having a vehicle stability enhancement control automatically activated in a vehicle traction limit handling situation to maintain vehicle directional stability, the power steering system comprising, in combination:

means for generating an assist torque in response to sensed handwheel input torque;

a vehicle lateral surface coefficient estimator;

means responsive to a lateral surface coefficient signal received from the vehicle lateral surface coefficient estimator for generating a vehicle stability offset current value having a magnitude determined from the lateral surface coefficient signal, the means generating the vehicle stability offset current value when the vehicle stability enhancement control signals its activation;

means for generating a lateral acceleration offset signal value indicative of vehicle lateral acceleration;

a maximum value selector applied to the vehicle stability offset current value and at least the lateral acceleration offset current value, whereby the largest one of the offset current values is selected; and

means for modifying the assist torque in response to the selected largest one of the offset values, whereby modifications to the assist torque responsive to activation of the vehicle stability enhancement control have a magnitude dependent on the estimated surface coefficient.

2. An adaptive variable power steering system according to claim 1 wherein the magnitude of the vehicle stability offset current value varies inversely with the magnitude of the estimated surface coefficient.

3. A method for controlling a variable power steering system for a motor vehicle having steerable wheels for operating on roads with variable road surface friction and a vehicle stability enhancement control automatically activated in a vehicle traction limit handling situation to maintain vehicle directional stability, the method comprising the steps:

generating an assist torque in response to sensed handwheel input torque;

estimating a vehicle lateral surface coefficient;

when the vehicle stability enhancement control signals its activation, generating a vehicle stability offset current value having a magnitude greater at a first predetermined value of the lateral surface coefficient signal than at a second, larger predetermined value of the lateral surface coefficient signal; and

selectively decreasing the assist torque in response to the vehicle stability offset current value.

4. The method of claim 3 further comprising the step of slew limiting the decrease of the assist torque in response to the vehicle stability offset current at a rate responsive to the lateral surface coefficient.

5. The method of claim 3 further comprising the steps of:

generating a lateral acceleration offset signal value indicative of vehicle lateral acceleration; and

modifying the assist torque in response to the larger one of the vehicle stability offset current value and the lateral acceleration offset current value.