TITLE OF INVENTION
[0001] Method of Identifying Predictive Lateral Load Transfer Ratio for Vehicle
Rollover Prevention and Warning Systems
BACKGROUND OF THE DISCLOSURE
[0002] Vehicle rollover has the highest fatality rate among non-collision vehicle
accidents. To prevent vehicle rollover, a rollover index called Lateral Load Transfer
Ratio (LTR) has been used to detect vehicle rollover propensity. Typically, LTR is
estimated from vehicle information measured at a fixed point in time. In analogy, it is
like taking a snap-shot of a dynamic system and using this information (frozen in time)
to determine the vehicle rollover threat. If the threshold of the LTR is set to be too low,
it will give a warning or prematurely activate the vehicle rollover prevention system
during normal driving. If the threshold is set to be too high, it may be too late to
prevent the vehicle from rollover. Determining the LTR threshold is difficult due to
dynamic changes in vehicle operation or unexpected disturbances, which cannot be
captured using only static LTR.
BRIEF SUMMARY OF THE INVENTION
[0003] A method for controlling stability of a vehicle is provided that includes the
steps of determining a predictive lateral load transfer ratio of the vehicle by evaluating
vehicle performance factors over a period of time, and
controlling operation of the vehicle based on the predictive lateral load transfer
ratio. In an embodiment of the invention, the predictive lateral load transfer ratio
may be used to detect the rollover propensity of a vehicle prior to the vehicle
operating in a condition that induces vehicle rollover. With this prediction
capability, operation of a vehicle rollover warning system may be improved to
provide a vehicle operator with advanced warning of an impending rollover.
Moreover, a rollover prevention system, including torque-biasing devices such as
electronic limited-slip differentials, may be operated to prevent vehicle rollover.

BRIEF DESCRIPTION OF THE DRAWINGS
[0004] FIG. 1 is a schematic illustration of an exemplary all-wheel-drive vehicle
employing a vehicle stability control system according to an embodiment of the present
invention;
[0005] FIG. 2 is a schematic illustration of a vehicle stability control system
according to an embodiment of the present invention;
[0006] FIG. 3 is a model of vehicle dynamics during lateral operation; and
[0007] FIG. 4 is a model of vehicle dynamics during vehicle roll.
DETAILED DESCRIPTION
[0008] Referring now to the drawings, which are not intended to limit the invention,
FIG. 1 schematically illustrates an exemplary all-wheel-drive vehicle 20 including a
laterally-positioned engine 22. The engine 22 is linked to a pair of front wheels 24a,
24b through a front axle or transaxle 26 and to a pair of rear wheels 28a, 28b through
a rear axle 30. The front axle 26 is primarily and directly driven by the engine 22. The
rear axle 30 is indirectly driven via a power transfer unit (not shown) and a center
coupling apparatus or coupler 32. The rear axle 30 is mechanically linked to the front
transaxle 26 through one or more drive- or prop-shafts. An optional electronically
controlled limited slip differential (ELSD) 34 is used to bias the rear prop-shaft torque
to the rear wheels 28a, 28b. Coupler 32 and ELSD 34 may be well known devices in
the art, and may be controlled by a vehicle control system 58 (shown in FIG. 2), such
as a vehicle electronic control unit (ECU) or other controller. It will be appreciated that
vehicle 20 is not limited to the configuration shown in the drawings and may include
other configurations, including, without limitation, two-wheel drive configurations.
[0009] As shown in FIG. 2, the control system 58 may include a control unit 60,
such as a microprocessor-based ECU including a memory device having stored
therein, for example, one or more maps containing vehicle operating parameter
information, and at least one vehicle sensor 62 for measuring a vehicle performance
factor(s), such as, without limitation, a yaw rate sensor, wheel speed sensor, lateral
acceleration sensor and/or a steering angle sensor. The control unit 60 provides an
input signal to the center coupler 32 and/or the ELSD 34 to control engagement and

disengagement of the devices to distribute torque between the wheels or axles. The
control unit 60 may also control operation of a roll warning device 64, such as, for
example, and audible warning device or visual indicator in the vehicle dash, to warn a
vehicle operator of an impending vehicle rollover.
[0010] Referring now to FIGS. 3 and 4, the vehicle LTR may be determined from
vehicle nonlinear models as:
(D
[0011] Taking into account the lateral dynamics of the vehicle:

or

• wherein m Is the vehicle mass, v is the vehicle lateral velocity, r is the vehicle yaw
rate, u is the vehicle longitudinal velocity, g is the acceleration of gravity, Ay is the
vehicle lateral acceleration, and h is the height of the center of gravity relative to the
vehicle rotational point as shown in FIG. 4.
[0012] Taking into account the roll dynamics of the vehicle:

[0013] Taking into account the vertical dynamics of the vehicle:
(4)
wherein z is the vehicle vertical acceleration.
[0014] Accordingly, taking into account equations (1 )-(4), the LTR may be
expressed as follows:


wherein, Ay _meas is the lateral acceleration, which may be obtained using an
accelerometer, and hCG is the total vehicle center of gravity (CG) height.
[0017] In accordance with an embodiment of the present invention, a method for
determining LTR of a vehicle is provided that includes a predictive LTR (PLTR), which
evaluates vehicle performance factors over a period of time, rather than a fixed point in
time. The method is designed to accurately "count-down" toward rollover or evaluate
vehicle performance prior to rollover under a wide range of vehicle operating
conditions. Derivation of PLTR is shown as follows:


wherein Ay_ meas is the measured vehicle lateral acceleration.
[0018] Equation (10) expresses PLTR at time t0 predicted for a future time horizon
At. The effect of the sign of the measured lateral acceleration may be neglected in
this derivation for proof-of-concept purposes. The measured lateral acceleration
Ay_ meas in equation (10) is typically noisy and, therefore, it is difficult to obtain a smooth
value after a derivation. The following filtering technique is used to reduce noise:

wherein  is a time constant.
[0019] The measured lateral acceleration can be further estimated from a
relationship with the steering angle using a linear model,

wherein TFModel(s) is a linear transfer function of the steering angle and the lateral
acceleration based on the linear model, and ip is an actual average steered wheel


angle. By using this model-based filter, the noise from the derivation of the lateral
acceleration can be filtered out using a low-pass filter. Moreover, driver's steering input
information plays an important role in predicting the rollover index due to the delay of
the steering system.
[0020] Accordingly, the PLTR is provided as follows:


selected At needs to be long enough to cover the rollover prevention system response
time.
[0022] Unlike more conventional methods of determining LTR, the new method of
determining PLTR may be used by control system 58 to control a vehicle torque
biasing device (e.g., center coupling 32 and ELSD 34) to improve vehicle stability and
inhibit vehicle rollover. In the embodiment shown in FIGS. 1 and 2 for example, the
PLTR may be used by controller 60 to determine when and to what extent to engage
the torque biasing devices to increase yaw damping of the vehicle and prevent vehicle
rollover. For example, and without limitation, control unit 60 may be configured such
that center coupler 32 and/or ELSD 34 are operated once the PLTR exceeds a
threshold value(s) or the rate of change of PLTR exceeds a predetermined threshold
rate. The PLTR may also be used to control operation of roll warning device 64 to
warn a vehicle operator of an impending vehicle rollover. It will also be appreciated
that use of the PLTR to control operation of a vehicle torque biasing device or roll
warning device is not limited thereto, and that the PLTR may be used to control other
vehicle systems, such as the vehicle brake system or power steering system, to inhibit
vehicle rollover.
[0023] The invention has been described in great detail in the foregoing
specification, and it is believed that various alterations and modifications of the
invention will become apparent to those skilled in the art from a reading and
understanding of the specification. It is intended that all such alterations and
modifications are included in the invention, insofar as they come within the scope of
the appended claims.

What is claimed is:
1. A method for controlling stability of a vehicle comprising the steps of:
determining a predictive lateral load transfer ratio of the vehicle by
evaluating vehicle performance factors over a period of time; and
controlling operation of the vehicle based on the predictive lateral load
transfer ratio.
2. The method of claim 1, wherein the vehicle includes a torque biasing
device for controlling the distribution of torque between a pair of vehicle
wheels or between a pair of vehicle axles, and the step of controlling
operation of the vehicle includes engaging the torque biasing device to
control torque distribution between the pair of vehicle wheels or the pair of
vehicle axles.
3. The method of claim 1, wherein vehicle includes a roll warning device,
and the step of controlling operation of the vehicle includes operating the
roll warning device to warn a vehicle operator of an impending vehicle
rollover.
4. The method of claim 1, wherein the step of determining predictive lateral
load transfer ratio includes estimating vehicle lateral acceleration from a
relationship with a vehicle steering angle using a linear model:

wherein TFModel (s) is a linear transfer function of the steering angle and the lateral
acceleration based on the linear model, and 8„ is an actual, average steered
wheel angle.
5. The method of claim 1, wherein the determining step includes determining
the predictive lateral load transfer ratio as follows:


7. The method of claim 5, wherein the vehicle includes a control system and the
determining step includes selecting the future time horizon At to be at least as
long as the vehicle rollover prevention system response time.

A method for controlling stability of a vehicle
includes the steps of determining predictive lateral
load transfer ratio of the vehicle by evaluating
vehicle performance factors over a period of time, and
controlling operation of the vehicle based on the
predictive lateral load transfer ratio.