TITLE OF INVENTION
[0001] Stability-Enhanced Traction Control with Electronically Controlled
Center Coupler.
CROSS-REFERENCE TO RELATED APPLICATIONS
[0002] The present application claims priority to U.S. Provisional Patent
Application Serial No. 60/773,493, which was filed on February 15, 2006 and is
incorporated herein by reference in its entirety.
BACKGROUND OF THE DISCLOSURE
[0003] The present invention relates to vehicle control systems that enhance
vehicle stability.
[0004] Vehicle stability-control systems are being increasingly used in the
automotive industry and are becoming standard equipment in many vehicles. A
majority of the vehicle stability-control systems in the market are brake-based.
Brake-based stability-control systems use Anti-Lock Braking System (ABS)
hardware to apply individual wheel braking forces to correct vehicle yaw
dynamics. While brake-based systems are acceptable in many situations, they
tend to deteriorate longitudinal performance of the vehicle, especially during
vehicle acceleration.
BRIEF SUMMARY OF THE INVENTION
[0005] A control system for a vehicle having first and second axles is
provided that includes a coupling apparatus adapted to distribute torque
between the first and second axles, and a traction controller for controlling
operation of the coupling apparatus from vehicle launch up to a predetermined
vehicle speed. The traction controller is configured to engage the coupling
apparatus in a first vehicle operating state according to at least one vehicle
operating parameter indicative of a low traction operating condition and to

further control engagement of the coupling apparatus in a second vehicle
operating state during the low traction operating condition according to a
difference between an actual vehicle yaw rate and a predetermined target
vehicle yaw rate.
[0006] An embodiment of the present invention includes an active stability
control method using a coupling apparatus to enhance the vehicle lateral
dynamics while preserving longitudinal motion. Another embodiment of the
present invention includes a control system that provides stability enhancement
of the traction control. The stability-enhanced traction control was evaluated
under the condition of an on-throttle, T-junction vehicle launch. The experimental
data shows a significant stability improvement in the traction control operating
mode.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] FIG. 1 is a schematic illustration of an exemplary all-wheel-drive
vehicle drive-train configuration.
[0008] FIG. 2 is an exemplary electronically controlled coupling apparatus
for use in a control system according to an embodiment of the present
invention.
[0009] FIG. 3 illustrates the dynamics of an exemplary vehicle employing an
electronically controlled center coupling apparatus and the effect on vehicle
yaw control when disengaging and engaging the electronically controlled center
coupling apparatus.
[0010] FIG. 4 is a schematic illustration of a control system according to an
embodiment of the present invention.
[0011] FIG. 5 is a schematic illustration of an electronic control unit
according to an embodiment of the present invention for use in the control
system of FIG. 4.
[0012] FIGS. 6 and 7 graphically illustrate a performance comparison for an
on-throttle vehicle turning maneuver on a low friction surface for a vehicle
employing a center coupler.

[0013] FIGS. 8 and 9 graphically illustrate a performance comparison for an
on-throttle turning maneuver on a low friction surface using a vehicle employing
a center coupler.
[0014] FIGS. 10 and 11 graphically illustrate a performance comparison for
an on-throttle T-junction vehicle launch using a vehicle employing a control
system according to an embodiment of the present invention.
DETAILED DESCRIPTION
[0015] Referring now to the drawings, which are not intended to limit the
invention, FIG. 1 schematically illustrates an exemplary all-wheel-drive vehicle
drive-train configuration 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 (PTU) 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. While, the drive-train
configuration illustrated in FIG. 1 is a normally front-drive configuration in which
torque is transmitted to the rear axle 30 on-demand, the invention is not
intended to be limited thereto and may include a normally rear-drive
configuration.
[0016] The center coupler 32, such as an electronically controlled center
coupler manufactured by Eaton Corporation under the name HTC-I™ and
illustrated in FIG. 2, is connected to the prop-shaft and is adapted to transmit
torque from the front transaxle 26 to the rear axle 30. The exemplary locking
center coupler 32 shown in FIG. 2 is an electronically controlled all-wheel-drive
coupler designed as an integrated component of the vehicle's rear axle drive
module. The center coupler 32 transfers power smoothly and rapidly from the
vehicle prop-shaft to a hypoid pinion gear of the rear drive module in response

to input signals from an electronic control unit (ECU).
[0017] Referring to FIG. 2, power transfer is provided by an actively
controlled wet multi-plate clutch 40 disposed between a coupling shaft 42, to
which the vehicle's prop-shaft is attached, and a hypoid pinion gear 44. Clutch
engagement limits the slip between the vehicle's prop-shaft and the hypoid
pinion gear 44 and, in doing so, torque is transferred from the prop-shaft to the
hypoid pinion gear 44, the magnitude of which will be less then or equal to the
clutch torque. A gerotor-type on-board hydraulic oil pump 46 provides hydraulic
pressure to actuate a clutch piston 48 when coupling shaft 42 is rotating. A
stationary hydraulic manifold 50 includes an oil inlet 52 through which pump 46
draws oil from a sump for discharge into a passage in direct communication
with both piston 48 and at least one solenoid operated pressure regulation
valve 54. When the valve 54 are de-energized, oil flows freely through the
valve and back to the sump, resulting in little or no hydraulic pressure against
the clutch actuation piston 48. When valve 54 is energized, oil flow is restricted
by the valve creating hydraulic pressure against the actuation piston 48 to
engage the clutch 40 according to a level proportional to that of the hydraulic
pressure. Further details regarding the structure and operation of center
coupler 32 are described in pending U.S. Patent Application Serial No.
11/167,474, which is owned by the Assignee of the present invention and
incorporated herein by reference in its entirety.
[0018] The locking center coupler 32 provides fast coupling torque
application and removal as is desired for both driveline torque control-based
vehicle dynamic operations (as is the focus of the present invention), and also
for compatibility with many of the current brake-based vehicle dynamic
intervention systems. To support this operation, the center coupler 32 exhibits
engagement and disengagement times of less than about 50 milliseconds. In a
mode of operation, torque is transmitted from the front transaxle 26 to the rear
axle 30 if the front axle wheel speed is greater than the rear axle wheel speed
and positive engine torque is being delivered to the drive-train (e.g., engine is
being throttled).

[0019] During vehicle operation, the center coupler 32 is periodically
engaged or locked to transfer torque from the front axle 26 to the rear axle 30.
This operation is performed to maintain vehicle traction. For vehicle traction
control, the extent to which the center coupler 32 is engaged may be based on,
without limitation, the vehicle throttle position and the degree of front wheel slip
(e.g., the greater the slip, the greater the engagement). However, operating the
center coupler 32 during a T-junction (junction between two road surfaces that
intersect at a right angle) launch or during a severe turn with the engine under
heavy throttle enforces oversteer behavior in the vehicle or introduces large slip
angles at the vehicle's rear wheels. FIG. 3 illustrates the dynamics of an
exemplary vehicle employing a center coupler and the effect on vehicle yaw
control when disengaging and engaging the center coupler.
[0020] In accordance with an embodiment of the present invention, a control
system 58 and method for controlling engagement of a center coupler is
provided to maintain stability in a vehicle while preserving traction. The control
system 58 and method of the present invention modulates center coupler
engagement based on a difference between a desired yaw rate and the actual
vehicle yaw rate. Actively controlling engagement of the center coupler, and
accordingly the amount of torque transmitted by the center coupler, mitigates
vehicle oversteer behavior that induces undesirable vehicle yaw motion. The
present invention controls the center coupler during launch and acceleration to
preserve both driyeline power and yaw stability.
[0021] As shown in FIG. 4, the control system includes a control unit 60,
such as a microprocessor-based electronic control unit (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, such as, without limitation, a yaw rate sensor, wheel speed sensor,
lateral acceleration sensor and/or a steering angle sensor. The control unit
provides an input signal to the center coupler 32 and the ELSD 34 to control
engagement and disengagement of the devices. A method of controlling the
ELSD 34 is disclosed in a co-pending U.S. Patent Application entitled "Stability-

Enhanced Traction and Yaw Control using Electronically Controlled Limited-Slip
Differential," which is owned by the Assignee of the present invention and
incorporated herein by reference in its entirety.
[0022] In an embodiment of the invention shown in FIG. 5, the control unit 60
includes a traction controller 64 for controlling operation of the center coupler
from vehicle launch up to a predetermined vehicle speed. The traction
controller 64 is configured to engage the coupling apparatus in a first vehicle
operating state according to at least one vehicle operating parameter, such as
wheel speed, indicative of a low traction operating condition and to further
control engagement of the coupling apparatus in a second vehicle operating
state during the low traction operating condition according to a difference
between the actual vehicle yaw rate and a predetermined target or desired
vehicle yaw rate. The electronic control unit 60 may also include a stability
controller 66 for controlling engagement of the coupling apparatus at or above
the predetermined vehicle speed. Traction controller 64 and stability controller
66 may be hardware provided in communication with control unit 60, made
integral within control unit 60, or form a non-hardware component (e.g.,
software) of control unit 60 or other vehicle controller.
[0023] During vehicle operation, the actual yaw rate is periodically or
continuously compared to the desired yaw rate. The actual yaw rate may be
measured using a yaw rate sensor or may be calculated based on vehicle
operating parameter information received from various vehicle sensors, as is
known in the art. The desired yaw rate may be stored in a map in the control
system memory or may also be calculated based on the vehicle operating
parameter information received from various vehicle sensors.
[0024] The center coupler locking torque required to maintain vehicle
stability and traction may be determined according to the following equation:


wherein des is the desired center coupler locking torque, nom is the normal
center coupler locking torque, rmax a predetermined maximum yaw rate
difference, r is the actual yaw rate, and rdes is the predetermined target or
desired yaw rate, and deadband is a threshold function for the yaw rate
difference allowed, rthreshold. The control unit 60 then converts the desired
locking torque value into an input signal, which is communicated to the center
coupler 32 for control. During vehicle operation, if the vehicle is experiencing
excessive oversteer (see, e.g., FIG. 3), the center coupler locking torque is
reduced in proportion to the degree of vehicle oversteer.
[0025] To simulate performance of the proposed control system with the
exemplary drive-train configuration shown in FIG. 1, a full vehicle model
employing a center coupler and an ELSD was developed using CarSim
software. A simulated turning maneuver was first performed to validate the
effect of engaging the center coupler 32 and ELSD 34 on vehicle yaw
dynamics. FIGS. 6 and 7 graphically illustrate a performance comparison for an
on-throttle turning maneuver on a low friction surface (e.g.,  = 0.2). As shown
in FIG. 6, locking the center coupler 32 during an on-throttle turning maneuver
induces less understeer. By contrast, locking the rear axle ELSD has little or no
effect on the vehicle dynamics on a low friction surface.
[0026] FIGS. 8 and 9 graphically illustrate a performance comparison for an
on-throttle turning maneuver on a low friction surface (e.g.,  = 0.2) using a test
vehicle employing a center coupler according to the present invention. As
shown in FIG. 8, engaging the center coupler 32 during an on-throttle turning
maneuver again induces less understeer compared to a similar maneuver with
the center coupler disengaged.
[0027] FIGS. 10 and 11 graphically illustrate a performance comparison for
an on-throttle T-junction vehicle launch using a test vehicle employing a control
system according to the present invention and a test vehicle without stability-
enhanced traction control. As shown in FIG. 10, actively controlling
engagement of the center coupler during vehicle launch using the stability-

enhanced traction control (SE TC) according to an embodiment of the invention
induces significantly less understeer compared to a similar maneuver in which
the center coupler is not actively controlled. It will also be appreciated with
reference to FIGS. 10 and 11 that a vehicle without stability-enhanced traction
control may exhibit too large a reduction in understeer. In both stability-
controlled and non-stability controlled modes of operation, some degree of
torque is transferred from the front axle 26 to the rear axle 30 by the center
coupler 32. Unlike ABS-based stability-control systems that dissipate vehicle
energy, thereby degrading the vehicle performance and efficiency, the control
system and method according to the present invention maintains vehicle
stability while preserving vehicle traction.
[0028] 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 control system for a vehicle having first and second axles
comprising:
a coupling apparatus adapted to distribute torque between the first
and second axles;
a traction controller for controlling operation of the coupling apparatus
from vehicle launch up to a predetermined vehicle speed, the traction
controller configured to engage the coupling apparatus in a first vehicle
operating state according to at least one vehicle operating parameter
indicative of a low traction operating condition and to further control
engagement of the coupling apparatus in a second vehicle operating state
during the low traction operating condition according to a difference between
an actual vehicle yaw rate and a predetermined target vehicle yaw rate.
2. The control system of claim 1, wherein the traction controller is
configured to modulate engagement of the coupling apparatus during the
low traction operating condition according to a difference between the
actual vehicle yaw rate and the predetermined target vehicle yaw rate.
3. The control system of claim 1, wherein the traction controller 64 is
configured to engage the coupling apparatus according to a desired
coupling applied torque signal that is based on a modified normal
coupling applied torque signal.
4. The control system of claim 3, wherein the desired coupling
applied torque signal is equal to the normal coupling applied torque signal
multiplied by a modifier, the modifier including in its numerator the
difference between a predetermined maximum yaw rate difference and
the multiplication of a deadband and the difference between the actual
vehicle yaw rate and the predetermined target vehicle yaw rate, and the
modifier including in its denominator the predetermined maximum yaw

rate difference.
5. The control system of claim 1, wherein in the first vehicle operating
state, the actual vehicle yaw rate is less than or substantially equal to the
predetermined target vehicle yaw rate and, in the second vehicle
operating state, the actual vehicle yaw rate is greater than the
predetermined target vehicle yaw rate.
6. The control system of claim 1, further including a stability controller 66 for controlling engagement of the coupling apparatus at or above the predetermined vehicle speed.

A control system for a vehicle having first and
second axles in provided that includes a coupling
apparatus adapted to distribute torque between the first and second axles and a traction controller for controlling operation of the differential apparatus from vehicle launch up to a predetermined vehicle speed. The traction controller is configured to engage the coupling
apparatus in a first operating state according to at least one vehicle operating parameter indicative of a low traction operating condition and to further control engagement of the
coupling apparatus in a second vehicle operating
state during the low traction operating condition according to a difference between an actual vehicle yaw rate and a predetermined
target vehicle yaw rate.