Vehicle yaw rate control with yaw rate command limiting

Details Vehicle yaw rate control with yaw rate command limiting Vehicle yaw rate control with yaw rate command limiting

1998-08-24

2001-01-16

Cuchlinski, Jr., William A. (Department: 3661)

Data processing: vehicles, navigation, and relative location

Vehicle control, guidance, operation, or indication

Vehicle subsystem or accessory control

C303S146000, C701S072000

Reexamination Certificate

active

06175790

ABSTRACT:

TECHNICAL FIELD
This invention relates to a vehicle yaw rate control, and more particularly to a method for limiting the yaw rate command consistent with the lateral adhesion capability of the road surface.
BACKGROUND OF THE INVENTION
In general, vehicle yaw rate control systems determine a desired yaw rate based on driver steering angle and other parameters, develop a yaw rate control command, and selectively brake one or more vehicle wheels in accordance with the yaw rate control command so that the vehicle achieves the desired yaw rate. In open-loop systems, the yaw rate control command is determined primarily as a function of the desired yaw rate, whereas in closed-loop systems, the yaw rate control command is determined primarily as a function of the difference, or error, between the desired yaw rate and a measure or estimate of the actual yaw rate. In either type of system, the determination of the desired yaw rate is based on an assumption that the vehicle is operating on dry pavement—that is, a surface having a high lateral coefficient of adhesion. Thus if the surface adhesion capability is not taken into account, the desired yaw rate can easily exceed the surface adhesion capability when the vehicle is operating on a slippery surface such as snow. In such circumstances, the yaw rate control will not be optimal.
Two different approaches for addressing reduction in surface adhesion capability have been considered. The simplest approach is to reduce the control gains, but this also reduces the overall aggressiveness of the control, and is often not favored. The second approach is to estimate the surface adhesion capability during a steering maneuver based on a measure of instantaneous lateral acceleration, and to dynamically limit the desired yaw rate accordingly. While this approach works well when the steering input is substantially constant, it does not work well during transient steering because the desired yaw rate and the lateral acceleration (and therefore, the surface adhesion estimate) are not in phase with each other. This phenomenon can be seen in the graph of
FIG. 4A
, which depicts a yaw rate limit based on lateral acceleration (solid trace) with the desired yaw rate (broken trace) during a transient steering maneuver on a low adhesion surface. As soon as the steering begins to change significantly, the desired yaw rate begins to lead the yaw rate limit, to the point of being completely out of phase. Accordingly, the robustness of the second approach is limited.
SUMMARY OF THE INVENTION
The present invention is directed to an improved closed-loop vehicle yaw control in which a yaw rate limit based on measured lateral acceleration is used during transient steering maneuvers to dynamically limit a desired yaw rate derived from driver steering input. According to the invention, a preliminary yaw rate limit is computed based on the measured lateral acceleration, and a dynamic yaw rate limit having a proper phase relationship with the desired yaw rate is developed based on the relative magnitudes of the desired yaw rate and the preliminary yaw rate limit. In the preferred embodiment, a two-stage process is used to develop the dynamic yaw rate limit. A first stage yaw rate limit is determined according the lower in magnitude of the desired yaw rate and the preliminary yaw rate limit, and a second stage yaw rate limit (i.e., the dynamic yaw rate limit) is determined according to the relative magnitudes of (1) the desired yaw rate and the second stage yaw rate limit, and (2) the first stage yaw rate limit and the second stage yaw rate limit. The desired yaw rate, as limited by the dynamic yaw rate limit, is then combined with the actual or estimated yaw rate to form a yaw rate error, which in turn, is used to develop a yaw rate command for the vehicle.


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