The technical field of this invention is vehicle power steering systems.
Basic power steering systems for vehicles provide a fixed steering assist offset curve regardless of driving conditions. More sophisticated power steering systems, however, are available that are capable of changing power steering effort in different driving situations. For example, one such system, marketed by Delphi Automotive Systems, provides a decrease in steering assist with vehicle speed, so that greater assist will be provided at very low speed vehicle maneuvers such as parking, while significantly less assist is provided at high speed highway driving for a tighter, xe2x80x9cmanual steering feelxe2x80x9d and greater stability. In a recent version of this system, a variation responsive to estimated lateral acceleration has been added to improve steering linearity.
Another such system is described in U.S. Pat. No. 6,062,336, issued May 16, 2000 and entitled Adaptive Variable Effort Steering System. The power steering system described therein takes advantage of the advent of vehicle low friction road surface handling controls for vehicle brakes and/or suspension components to provide special control of power steering assist near or at the limit of vehicle handling. Such controls include anti-lock braking controls (ABS), traction controls (TCS) and vehicle stability enhancement (VSE) controls. Each of these controls responds to sensors indicating when a vehicle approaches or reaches a vehicle handling limit, where traction is decreased or lost, to override normal vehicle control while the normal handling is not optimal. The power steering system of U.S. Pat. No. 6,062,336 is responsive to signals from one or more such controls that indicate when the control has become active in modifying handling, and thus indicate the existence of a near limit or at limit vehicle handling situation. When such a handling limit signal is received in the system of the patent, the power steering system responds by decreasing steering assist, and thus increasing steering effort, to provide a more xe2x80x9cmanualxe2x80x9d steering feel as long as the handling limit situation is indicated.
But vehicles with an adaptive power steering system as described do not experience loss of traction only on low coefficient road surfaces. They may also lose traction if driven in a xe2x80x9cperformancexe2x80x9d mode (in aggressive or evasive maneuvers) on high coefficient surfaces. But on a high coefficient surface, high lateral forces can be generated on the tires and transmitted through the steering system to produce an inherent increase in steering effort; and an additional increase in steering effort can produce an unpleasant sensation to a driver, particularly at the initiation of a counter-steering input. Thus, the increase in steering effort beneficial in improving driver comfort near or at the handling limit on low coefficient of friction surfaces has proven to be unnecessary and undesirable in similar circumstances on high coefficient surfaces. To modify the added steering downward for the system""s behavior on high coefficient surfaces can reduce its desired effect on low coefficient surfaces.
The system described in U.S. Pat. No. 6,062,336 is described as having embodiments responsive to variable vehicle dynamic parameters such as yaw rate error, in the case of a vehicle stability enhancement system, to provide a variable magnitude signal of approaching vehicle handling limit; but this signal does not distinguish between loss of traction on a low coefficient surface and loss of traction on a high coefficient surface.
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 to provide a greater steering assist when the VSE system is active on a low coefficient surface than when the VSE system is active on a high coefficient surface. Thus, the power steering assist is automatically varied, when the vehicle stability enhancement system is active, to provide the optimal handling feel on both low coefficient and high coefficient surfaces.
Preferably the modification is phased in and out in slew limited steps that also decrease in size as lateral surface coefficient increases. In a preferred embodiment, a power steering assist correction comprises the sum of a vehicle speed offset value varying inversely with vehicle speed and a road surface adaptation value that is the greatest in magnitude of a plurality of offset values comprising a vehicle stability enhancement offset value determined as described above, a lateral acceleration offset value varying inversely with estimated lateral acceleration and offset values used during the activity of any other vehicle traction limit handling controls such as anti-lock braking (ABS) or traction control (TCS).