U.S. patent application number 13/529488 was filed with the patent office on 2012-10-18 for vehicle systems control for improving stability.
This patent application is currently assigned to HONDA MOTOR CO., LTD.. Invention is credited to Xiaodi Kang, William Monsma, James W. Post, II.
Application Number | 20120265402 13/529488 |
Document ID | / |
Family ID | 39645161 |
Filed Date | 2012-10-18 |
United States Patent
Application |
20120265402 |
Kind Code |
A1 |
Post, II; James W. ; et
al. |
October 18, 2012 |
VEHICLE SYSTEMS CONTROL FOR IMPROVING STABILITY
Abstract
Improved methods of controlling the stability of a vehicle are
provided via the cooperative operation of vehicle stability control
systems such as an Active Yaw Control system, Antilock Braking
System, and Traction Control System. These methods use recognition
of road surface information including the road friction coefficient
(mu), wheel slippage, and yaw deviations. The methods then modify
the settings of the active damping system and/or the distribution
of drive torque, as necessary, to increase/reduce damping in the
suspension and shift torque application at the wheels, thus
preventing a significant shift of load in the vehicle and/or
improving vehicle drivability and comfort. The adjustments of the
active damping system or torque distribution temporarily override
any characteristics that were pre-selected by the driver.
Inventors: |
Post, II; James W.; (Dublin,
OH) ; Kang; Xiaodi; (Dublin, OH) ; Monsma;
William; (Marysville, OH) |
Assignee: |
HONDA MOTOR CO., LTD.
Tokyo
JP
|
Family ID: |
39645161 |
Appl. No.: |
13/529488 |
Filed: |
June 21, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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12019341 |
Jan 24, 2008 |
8229642 |
|
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13529488 |
|
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|
60886536 |
Jan 25, 2007 |
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Current U.S.
Class: |
701/38 |
Current CPC
Class: |
B60G 2800/213 20130101;
B60W 40/064 20130101; B60T 2270/302 20130101; B60T 8/1769 20130101;
B60G 2800/212 20130101; B60G 2800/016 20130101; B60W 2720/403
20130101; B60T 2260/06 20130101; B60G 2800/214 20130101; B60G
2800/912 20130101; B60G 2800/215 20130101; B60T 2201/14 20130101;
B60G 17/0165 20130101; B60W 40/114 20130101; B60G 2800/94 20130101;
B60W 2720/14 20130101; B60W 10/18 20130101; B60W 10/22 20130101;
B60W 10/184 20130101; B60G 17/0195 20130101; B60W 30/18172
20130101; B60T 2210/12 20130101; B60W 40/112 20130101; B60W 2710/22
20130101; B60W 2720/30 20130101; B60T 8/17555 20130101; B60G 17/06
20130101; B60W 40/11 20130101; B60W 30/02 20130101 |
Class at
Publication: |
701/38 |
International
Class: |
B60G 17/016 20060101
B60G017/016; B60G 17/0195 20060101 B60G017/0195 |
Claims
1. A cooperative Active Damping System (ADS) method for providing
vehicle stability comprising the steps of: providing a vehicle
stability assist (VSA) system that determines yaw deviation from a
desired condition, the system including a vehicle stability
assist--Active Yaw Control electronic control unit (VSA-AYC ECU);
providing an Active Damping System (ADS) for adjusting the
suspension stiffness on the vehicle, at least independently between
the front and the rear of the vehicle; shifting the front to back
damping distribution between using the ADS in order to correct the
yaw deviation.
2. The method according to claim 1, further comprising the step of
reducing yaw deviation via braking if sufficient yaw rate error
reduction cannot be obtained via the shifting of suspension
stiffness from front to back.
3. The method according to claim 1, wherein the ADS includes
electronically controlled, fast-acting Magneto-Rheological fluid
dampers.
4. The method according to claim 1, wherein the ADS includes rotary
valve dampers or controllable disc dampers.
5. The method of according to claim 1, wherein the ADS includes and
ADS electronic control unit ECU that constantly monitory the
VSA-AYC ECU and when the VSA-AYC ECU becomes activated the ADS ECU
adjust the front/rear damping force distribution to generate a
corrective yaw moment.
Description
[0001] This application is a divisional of U.S. patent application
Ser. No. 12/019,341 filed on Jan. 24, 2008, currently pending, and
which in turn claims priority to U.S. Provisional Application Ser.
No. 60/886,536 filed on Jan. 25, 2007, the entire disclosure of
which is incorporated by reference herein.
BACKGROUND OF THE INVENTION
[0002] Modern vehicles have been increasingly equipped with various
chassis control systems, such as an Anti-lock Braking System (ABS),
advanced Four Wheel Drive (4WD) systems, Vehicle Stability Assist
(VSA) systems, and active suspension systems, such as for example,
an Active Damping System (ADS), as ways to further improve vehicle
handling quality, drive comfort and stability. However, these
chassis control systems usually have been designed and implemented
to work independently of one another with only minimal information
sharing, although they have been arranged on the same vehicle and
may facilitate one another's functions. It is expected that the
vehicle overall performance can be further enhanced if the existing
chassis control systems are able to share or exchange some
operational information.
[0003] As an example, in the current state of the art, a vehicle
Active Yaw Control system (AYC) is usually designed to control
vehicle yaw rate to follow a certain target or desired yaw rate
based on some yaw rate reference model. During vehicle operations,
AYC constantly monitors the vehicle actual yaw rate and calculates
the difference between the actual yaw rate and the target yaw rate
(i.e. yaw rate error). When the vehicle yaw rate difference is
larger than some preset threshold limit, AYC initiates to regulate
the yaw rate by applying a corrective yaw moment through
differential braking, for example, applying braking to the outside
wheels of the vehicle to mitigate oversteering (OS) or applying
braking to the inner-side wheels of the vehicle to reduce
understeer (US).
[0004] These braking applications are effective in reducing the
vehicle yaw rate error and, thus, maintain driver intended line
trace while ensuring vehicle stability, but at the same time,
because they are braking operations, they slow down the vehicle and
are also obtrusive to the driver. In addition, in the case of a
vehicle that is also equipped with an active drive torque
modulation system, such as Front Wheel Drive (FWD), Rear Wheel
Drive (RWD) or 4WD control devices, there is a possibility that
while the AYC is applying braking to an individual wheel of the
vehicle, the drive torque control system may be still delivering
some drive torque to the same wheel, causing conflicting torque
control and power wastage.
[0005] In another vehicle control situation, the braking efficiency
and stability of a vehicle is dependent upon many factors, such as
initial speed, surface conditions, wheel load distribution, braking
pressure, etc. During normal vehicle braking (where wheels are not
significantly slipping), braking pressure is directly related to
the driver braking pedal force, while during hard braking with ABS
activation, the braking pressure is modulated to regulate wheel
slip around some preset optimal region to maximize braking force
while maintaining vehicle stability. Since the braking pressure
modulation logic does not have any prior knowledge about the wheel
loads, whose fluctuations cause considerable variation in
achievable braking force and thus compromise braking efficiency, it
is desirable that the wheel load variation be kept as small as
possible during braking operation.
[0006] In yet another situation, tire-road friction and road
profile roughness considerably affect ADS performance. For example,
on high-mu flat surfaces, such as dry concrete and asphalt roads,
ADS is primarily calibrated to control body motion so as to enhance
vehicle handling characteristics, while on rough or low-mu
surfaces, such as a bumpy road, or snow and ice roads, ADS is
primarily set to facilitate driving comfort and drivability. In the
current state of art, ADS calibration is usually a trade-off
amongst vehicle handling performance on flat high-mu surfaces, ride
comfort on bumpy roads, and drivability and stability on low-mu
roads.
[0007] Different vehicle operation conditions require different ADS
settings to achieve optimal overall vehicle performance in terms of
handling and body motion control, ride comfort, drivability and
stability. For example, for low coefficient of friction operations,
a soft ADS setting provides the best drivability and stability, for
rough road operation, a moderate ADS setting offers very good road
isolation, ride comfort and body motion control, while on high
coefficient of friction operations, a firm ADS setting provides
best body motion control and handling stability.
[0008] Ideally, the ADS setting should be automatically adjusted
according to the prevailing operation conditions to enhance vehicle
overall performance. However, the current ADS systems only stay on
one predetermined setting, often pre-selected by the driver, and do
not change setting automatically based directly on sensed road
conditions.
[0009] In another vehicle control situation within the current
state of the art, Traction Control Systems (TCS) are designed to
regulate wheel slip around some preset optimal region to maximize
wheel traction. During vehicle operation, TCS constantly monitors
the slip ratio of each wheel of the vehicle. The slip ratio
typically is the difference between wheel speed and vehicle speed,
divided by the vehicle's speed or another comparison of wheel and
vehicle speed. Whenever excessive wheel slip occurs, TCS brings
down the wheel slip to the optimal region through either throttle
intervention, braking application or a combination of the two.
Since TCS regulates wheel slip on a feedback basis without any
prior knowledge about the factors that affect the wheel slip,
especially the wheel load (and ground surface friction), whose
fluctuations cause considerable wheel slip variation and thus may
compromise TCS control efficiency and smoothness, especially during
TCS braking operation, it is desirable that the wheel load
variation be kept as small as possible during TCS operation.
[0010] Therefore, there exists a need in the art for control of a
plurality of vehicle subsystems that have not worked together
synergistically in the past.
BRIEF SUMMARY OF THE INVENTION
[0011] COOPERATIVE-AYC: Considering the above drawbacks associated
with the braking applications for vehicle yaw rate correction, it
is desirable that the vehicle front to rear drive torque
distribution be controlled by a drive torque control actuator to
achieve a yaw moment change that is similar to or equivalent to the
effect of AYC activated differential braking.
[0012] In a first embodiment of the invention, directed to a
cooperative Active Yaw Control (AYC) system, a method includes the
steps of 1) providing a vehicle stability assist system that
determines when the actual vehicle yaw rate deviates from a target
yaw rate, wherein the assist system includes an Vehicle Stability
Assist-Active Yaw Control (VSA-AYC) ECU, 2) providing a system for
delivering/distributing torque, including utilizing a Direct Yaw
Control (DYC) Drive Torque Control ECU, to both of the front wheels
and both of the rear wheels of the vehicle and shifting torque
between the front and rear wheels. Torque is shifted from rear
wheels to front wheels either by placing an upper limit on the
amount of torque applied at each rear wheel or by reducing the
amount of torque applied at each rear wheel by an equivalent
amount.
[0013] Additionally the system allows for shifting the torque
distribution side to side between wheels on the front axle and
wheels on the rear axle in order to correct the yaw deviation.
[0014] COOPERATIVE-ADS: Considering the above drawbacks associated
with the braking efficiency and wheel load variation that affects
vehicle stability, a second embodiment of the invention provides a
control concept that adjusts ADS damping force distribution during
braking operation to enhance braking smoothness and stability and
thus is considered to be a cooperative ADS system.
[0015] The ADS system continuously operates independently from the
ABS system (normal operation) except in the event that ABS becomes
active. According to this embodiment, an ADS Electronic Control
Unit (ECU) constantly monitors the vehicle's braking status. When
the braking system (ABS) is activated by the driver, the ADS ECU
determines that the vehicle is in a slip-controlled braking
operation or in a state where differential braking is applied to
affect vehicle yaw stability (electronic brake distribution, EBD,
control which may be just below the point of tire slip) and
temporarily overrides the normal ADS control by switching to a
braking-event based control setting with appropriate firm damping
calibration to reduce vehicle body motion and wheel load variations
so as to facilitate braking efficiency and vehicle stability. After
a fixed period of time has expired, representing a critical braking
period, the ADS reverts back to the setting prior to application of
the brakes.
[0016] In a third embodiment of the invention, which like the
second embodiment is directed to cooperative ADS, available road
surface information is inferred by the VSA system and is used to
eliminate the trade-off of ADS capabilities under different road
conditions and thus maximize ADS potential benefits. This
embodiment includes a preemptive ADS control concept and
implementation, which makes use of available VSA determined road
surface friction and roughness information to adjust ADS
calibration based on prevailing road conditions provided by the VSA
system, and to enhance vehicle performance under all surface
conditions.
[0017] The third embodiment uses existing surface information
inferred by a VSA system equipped in the same vehicle, thus
improving ADS performance with minimum cost. This invention
eliminates the trade-off of ADS performances under different road
conditions and, thus, maximizes the ADS potential benefits.
[0018] The method for providing enhanced vehicle overall
maneuverability, ride comfort, and stability in this embodiment
includes the steps of: providing an ADS for adjusting the
suspension characteristics on the vehicle, providing a VSA system
on the vehicle, using the VSA system to infer the coefficient of
friction of the road upon which the vehicle is traveling by
checking the operational status of an ABS, TCS and AYC control
systems and a vector representation of longitudinal and lateral
acceleration of the vehicle, and upon inferring the coefficient of
friction of the road, changing the damping state of the ADS as
follows: If the coefficient of friction is high, the damping state
of the ADS is adjusted to a firm setting. If the coefficient of
friction is low, the damping state of the ADS is adjusted to a soft
setting.
[0019] Considering the above drawbacks associated with TCS
operation, in a fourth embodiment of the invention, also considered
cooperative ADS, a method for providing enhanced vehicle stability
is provided that includes providing an Active Damping System (ADS)
for adjusting the suspension characteristics on the vehicle,
providing a Traction Control System (TCS) on the vehicle, and using
the TCS to determine the slip ratio at each wheel of the vehicle so
that if the slip ratio is high at a wheel, the suspension in an
area adjacent to that wheel is stiffened.
[0020] Further, regarding the above drawbacks associated with the
braking applications for vehicle yaw rate correction, in a fifth
embodiment of the invention, also considered cooperative-ADS, a
method for providing vehicle stability is provided that includes
the steps of providing a vehicle stability assist (VSA) system that
determines yaw offset from a desired position, the system including
a vehicle stability assist--Active Yaw Control electronic control
unit (VSA-AYC ECU), providing an Active Damping System (ADS) for
adjusting the suspension stiffness on the vehicle, at least
independently between the front and the rear of the vehicle, and
shifting the front to back damping distribution using the ADS in
order to correct the yaw deviation.
[0021] COOPERATIVE-TCS: In a sixth embodiment of the invention,
involving cooperative TCS operation, a method is provided which
includes providing a Traction Control System (TCS) on the vehicle,
using the TCS to determine the slip ratio at each wheel of the
vehicle, providing a system including a DYC drive torque control
ECU for selectively delivering torque to each of the front wheels
and rear wheels of the vehicle in different amounts if desired, and
shifting the front to back torque distribution between wheels on
the front axle and wheels on the rear axle in order to correct the
detected wheel slippage.
[0022] Also directed to cooperative TCS operation, in a seventh
embodiment of the invention, a method of stabilizing a vehicle is
shown including the steps of providing a Traction Control System
(TCS) on the vehicle, using the TCS to determine the slip ratio at
each wheel of the vehicle, providing a system including a DYC drive
torque control ECU for selectively delivering torque to each of the
front wheels and rear wheels of the vehicle in different amounts,
and shifting the side to side distribution of torque between wheels
on the front axle and wheels on the rear axle in order to correct
the detected wheel slip.
[0023] As previously stated, tire-road friction and road profile
roughness considerably affect torque control system calibration and
performance. For example, on high-mu flat surfaces, such as dry
concrete and asphalt roads, drive torque control is primarily set
to improve turning capability (cornering performance) and driving
pleasure, while on rough or low-mu surfaces, such as a bumpy road,
or snow and ice roads, drive torque control is primarily calibrated
to improve vehicle traction, drivability and stability. In the
current state of art, because the drive torque distributions do not
change regularly, drive torque control system calibration is
usually a trade-off of vehicle handling performance on flat high-mu
surfaces, and drivability and stability on low-mu surfaces, which
unavoidably limits drive torque control system potential
capability. However, the drive torque distribution does not
currently change settings based directly on sensed road
conditions.
[0024] In an eighth embodiment of the invention, a method for
providing vehicle stability is shown that includes the steps of
providing a vehicle with all wheel drive, providing a Vehicle
Stability Assist (VSA) system on the vehicle, using the VSA system
to infer the coefficient of friction of the road upon which the
vehicle is traveling, providing a DYC Torque Control Electronic
Control Unit capable of adjusting the drive torque at each vehicle
wheel, and upon determining the coefficient of friction of the
road, changing the distribution of torque amongst the vehicle
wheels as follows: if the coefficient of friction is inferred to be
high, implementing a strong side to side and front to rear torque
bias; and if the coefficient of friction is inferred to be low,
implementing a moderate side to side and front to rear torque
bias.
[0025] These and other aspects of the invention are described
herein with further reference to the figures.
BRIEF DESCRIPTION OF THE DRAWINGS
[0026] FIG. 1 is a schematic showing interaction between vehicle
components;
[0027] FIG. 2 is a schematic showing where specific vehicle control
systems operate;
[0028] FIG. 3 is a schematic showing the basis of a method for
inferring road surface conditions;
[0029] FIG. 4 is a graph showing the operating range of a damper
within an Active Damping System;
[0030] FIG. 5A is a flowchart showing steps within a first
embodiment of the claimed invention;
[0031] FIG. 5B is a schematic showing torque shifting caused by
Cooperative AYC on a vehicle when cornering;
[0032] FIG. 5C is a flowchart showing firewall operation;
[0033] FIG. 6 is a flowchart showing steps within a second
embodiment of the claimed invention;
[0034] FIG. 7 is a flowchart showing steps within a third
embodiment of the claimed invention;
[0035] FIG. 8 is a flowchart showing steps within a fourth
embodiment of the claimed invention;
[0036] FIG. 9 is a flowchart showing steps within a fifth
embodiment of the claimed invention;
[0037] FIG. 10A is a flowchart showing steps within a sixth
embodiment of the claimed invention;
[0038] FIG. 10B is a schematic showing shifting of torque within a
cooperative TCS;
[0039] FIG. 10C is a graph showing different thresholds for
triggering different solutions for controlling the vehicle.
[0040] FIG. 11A is a schematic showing the firewall function of the
4WD Control System used to regulate the available control authority
being requested by the VSA;
[0041] FIG. 11B is a graph showing how priority is determined
between two operating control systems;
[0042] FIG. 12 is a flowchart showing steps within a seventh
embodiment of the claimed invention;
[0043] FIG. 13 is a flowchart showing steps within an eighth
embodiment of the claimed invention; and
[0044] FIG. 14 is a flowchart showing steps within a variation of
the eighth embodiment of the claimed invention.
DETAILED DESCRIPTION OF THE INVENTION
[0045] The invention is directed to a method of the hierarchical
control of stability systems within a vehicle. Referring to FIG. 1,
the vehicle, in general, includes four wheels 22, 24, 26, 28; each
wheel includes an associated brake 32 and speed sensor 34. The
vehicle also includes a controllable suspension, typically
including at least four (rear left 36, rear right 38, front left
40, front right 42) zones that are individually controllable
(spring rate, damper rate).
[0046] The vehicle also includes at least the following sensors:
lateral vehicle acceleration sensor 44, longitudinal vehicle
acceleration sensor 46, yaw rate sensor 48, vehicle speed sensor
50, and steering sensor 52.
[0047] The vehicle includes at least the following performance
enhancement systems as further described below: Active Damping
System (ADS) 54, Controllable Suspension System (CSS) 56, Direct
Yaw Control (DYC) Torque Adjustment/Distribution System (such as a
continuously controllable 4WD, FWD, or RWD) 58, each having an
associated electronic control unit (ECU).
[0048] The vehicle includes the following stability systems, as
further described below: Active Yaw Control System (AYC) 60,
Anti-lock Braking System (ABS) 61, and a Traction Control System
(TCS) 62 each having an associated electronic control unit (ECU) or
each system incorporated into a common ECU in the VSA.
[0049] Referring to FIGS. 1 and 2, a Vehicle Stability Assist (VSA)
system 63 is illustrated and comprises one or more of the vehicle
stability systems AYC 60, ABS 61, and TCS 62. As shown, depending
on the measured longitudinal and lateral traction of the vehicle as
well as the slip conditions of individual wheels, one or more of
these stability systems activates to control the vehicle stability
providing improved vehicle yaw response and wheel traction. These
systems are designed to be reactive feedback control systems and
therefore activate in a boundary that is near the limit of adhesion
of the road surface. In general, TCS works most strongly during
acceleration in a straight line, AYC while turning and ABS during
limit braking events. Also, more than one stability control system
may operate at the same time. In a range that is well within the
road adhesion limits (shown in the middle of FIG. 2), the stability
control systems are generally not active.
[0050] Performance enhancement systems ADS 54, CSS 56, and DYC
Torque Distribution Control 58 work at all levels of lateral and
longitudinal traction and are not necessarily restricted by the
proximity to the road adhesion limits. In the same way that
stability control systems (ABS, AYC and TCS) may operate
simultaneously, there is an opportunity for performance enhancement
systems (ADS, CSS, DYC Torque Distribution Control) to also operate
simultaneously in areas of overlap in FIG. 2.
[0051] Communications between the ADS ECU 54, CSS ECU 56, ABS ECU
61, Torque Distribution ECU 58, AYC ECU 60, TCS ECU 62 and sensors
occur within a Controller Area Network (CAN) 64. The CAN 64 is
preferably wiring running throughout the vehicle.
[0052] ACTIVE DAMPING SYSTEM (ADS): The vehicle's controllable
suspension includes the Active Damping System (ADS) 54 which is
active at all times. The Active Damping System 54, in normal
operation, gives the vehicle an elevated level of handling
precision, while maintaining a smooth and controlled ride. The ADS
54 includes, preferably, electronically controlled, fast-acting
Magneto-Rheological fluid dampers and achieves outstanding levels
of handling response while maintaining refined levels of ride
comfort. The quick response of the dampers in combination with
computer algorithms produce fast-acting vertical force modulation
at each corner of the vehicle to result in smooth vehicle body
movements and provide the driver intuitive vehicle control in a
relaxed environment--free from neck straining head toss and other
sudden vehicle motions.
[0053] The dampers of the ADS 54 are fast-acting "semi active shock
absorbers". These dampers offer the ability to individually adjust
from minimum to maximum damping force very quickly. Each damper
carries a field coil that generates an electromagnetic field when
current is passed through it. The fluid within the dampers contains
microscopic (on the order of 10 microns in diameter) ferric spheres
that align when surrounded by the electromagnetic field,
dramatically and instantly changing the effective fluid viscosity
within the damper. The overall range of damping force available is
significantly more than that of a conventional damper as it is
dependent on the control current being applied to the damper.
Additionally, there is a continuum of damping force steps within
the range of the lower and upper capacity of the damper.
[0054] The current that passes through the field coils is
controlled by the ADS ECU 54 that uses special algorithms to
determine the best control for the road conditions. This, combined
with nearly instantaneous reaction time of the dampers, allows
damping control to occur before the vehicle's tires or body are
allowed to have extraneous motion. Active dampers can run with low
damping when the road is smooth and level and the vehicle is being
driven at a constant speed. This further reduces the amount of
vibration and harshness that passes from the suspension to the
body, quieting and improving the vehicle's ride quality.
[0055] Using the dampers' fast response time, the ADS 54 reacts to
sudden changes in driver or road inputs. The high damping force
achieved in this short time allows the dampers to aid the springs
and stabilizer bars in roll, pitch and heave (vertical body motion)
control, greatly improving the handling of the vehicle.
[0056] In alternative variations, the ADS 54 comprises rotary valve
dampers or controllable disc dampers instead of a
Magneto-Rheological fluid dampers.
[0057] A result is the improved transient handling, road isolation,
and body control of the vehicle with the Active Damping System 54,
which noticeably reduces the driver's workload. The ADS 54 is able
to achieve the benefits of a sporty suspension without the
traditional ride comfort tradeoffs.
[0058] FIG. 4 shows the adjustable range of a damper in a Active
Damping System 54 as compared to a conventional damper. Because the
ADS 54 system has this large range of capability, it is feasible to
employ a number of driver-selectable control gain settings. The ADS
54 preferably provides at least the driver-selectable
settings--Sport (firm), and Comfort (soft). Sport mode prioritizes
handling response, vehicle body control and tire adhesion to allow
for spirited driving with high levels of precision and composure.
Sport mode keeps the vehicle body as flat as possible in motion.
Comfort mode allows for a more relaxed driving experience by
prioritizing road isolation and reducing passenger fatigue caused
by road inputs while still providing sufficient damping for overall
vehicle motion control. Comfort mode allows for greater vehicle
body movement when the vehicle is in motion. Either of the two
modes can be used in any type of operating conditions as selected
by the driver. Alternatively, the mode may be selected directly by
the ADS ECU 54. Other settings are possible that are located
between Sport mode and Comfort mode.
[0059] The vehicle also includes a Controllable Suspension System
(CSS) 56 that controls the spring rate in each of the four
suspension zones. The Active Suspension System 56 includes a CSS
ECU 56.
[0060] DYC TORQUE DISTRIBUTION SYSTEM: The driving torque that is
applied to each axle or wheel is individually adjustable. The
amount of torque generated by the engine is controllable by
throttle adjustment or other known means. The engine torque is then
distributed between the two axles and/or four wheels, the amount
directed to each wheel being controllable using slip clutches or
other means known in the art. The vehicle distribution system may
be a 4 wheel drive system (4WD) where addition/reduction can be
made at any of the four wheels, and on an axle by axle basis (front
to rear axle) or a system where the torque is varied between the
left and right wheels on a common axle (FWD, RWD).
[0061] The variations of vehicle drive torque distribution (also
referred to as calibration) depend upon the torque control
components in a vehicle. If the vehicle is a DYC four wheel drive
(DYC all wheel drive) vehicle, torque is distributed between the
front axle and the rear axle as well as between the two wheels on
the rear axle. In this DYC 4WD system, torque control components
comprise a conventional front wheel drive system (engine,
transmission, driveshafts) with an attached transfer case that
distributes drive torque to a propeller shaft running to the rear
axle of the vehicle. At the rear axle, the system includes a rear
drive unit that contains multi-plate controllable slip clutches at
either side of the unit which direct power to output driveshafts
that transmit torque to each of the rear wheels. By varying the
pressure symmetrically on each of the two clutches hydraulically or
electronically, torque distribution between the front and rear
axles is continuously shifted. By varying the pressure in different
amounts between the right and left clutches, torque may be
continuously distributed to either side of the axle, more or less
to each side depending on the control. The two clutches, being
activated either symmetrically or asymmetrically can deliver a
pre-described front/rear torque distribution as well as a rear
side-to-side distribution simultaneously. This can be performed in
a continuous manner with infinitely adjustable levels of front/rear
and rear side/side distribution. Hence it is termed a direct yaw
control (DYC) system.
[0062] In a torque distribution control system including front
wheel driving capabilities, a clutch activated system can
distribute the available driving torque between the left and right
front wheels according to the same principles as the rear drive
unit in the 4WD system discussed above. This distribution can be
made in a vehicle having only front wheel drive or a vehicle with
four wheel drive.
[0063] In a torque distribution control system including rear wheel
driving capabilities, the operation would be the same as the
aforementioned FWD-based system except that the rear axle is the
drive axle. This distribution can be made in a vehicle having only
rear wheel drive or a vehicle with four wheel drive.
[0064] Torque distribution is controlled by a torque distribution
ECU 58.
[0065] VSA-ABS CONTROL SYSTEM: The ABS 61 is composed of a central
electronic control unit (ECU), four speed sensors 34 (one for each
wheel), and two or more hydraulic valves on each brake circuit. The
ABS ECU 61 constantly monitors the rotation speed of each wheel.
When it senses that any number of wheels are rotating considerably
slower than the others (a condition that will bring it to lock) it
adjusts the valves to decrease the pressure on the braking circuit,
effectively reducing the braking force on that wheel. Wheel(s) then
turn faster and when they turn too fast, force is reapplied. This
process is repeated continuously, and this causes the
characteristic pulsing feel through the brake pedal.
[0066] Application of the brakes is sensed by the ABS ECU 61 in any
one of a number of manners: by actual application of pressure to
the brake pedal, sensed pressure at the master cylinder, sensed
longitudinal acceleration "g", or sensed lateral acceleration
"g".
[0067] VSA-AYC CONTROL SYSTEM: Referring back to FIG. 1, The Active
Yaw Control (AYC) system includes an AYC ECU 60 that limits the
overall wheel slip and/or lateral skid and/or yaw motion through
braking and driveline torque modulation to achieve stable vehicle
response. The AYC ECU 60 is designed to operate in a slip-based
and/or lateral skid-based and/or yaw rate feedback manner and is
designed and implemented with high fidelity for this purpose. The
AYC controller 60 has the capability to: Calculate a reference
vehicle speed; Calculate each wheel's longitudinal slip condition
(either longitudinal slip rate or lateral skid); Compare to a
threshold speed and/or longitudinal slip and/or lateral skid; Judge
the proximity to a potentially unstable vehicle motion; and,
Calculate a required application torque (brake or otherwise) to
control wheel spin and/or wheel lateral skid and/or vehicle
motion.
[0068] The AYC ECU 60 receives signals from the yaw rate sensor 48,
wheel speed sensors 34 and from other vehicle parameter sensors in
the vehicle and determines, based upon predetermined control
algorithms, whether one or more of the wheels is either slipping
and/or skidding laterally (feedback-based control) or about to slip
and/or skid laterally (feed-forward based control) or if the
vehicle is progressing into a region of unstable motion. The AYC
ECU 60 calculates the necessary wheel brake torque (or actuation
force), the necessary throttle angle adjustment, and the necessary
drive torque reduction amount for the slipping and/or laterally
skidding wheel(s). The AYC 60 also monitors the braking status of
the vehicle.
[0069] VSA-TCS CONTROL SYSTEM: The TCS control system includes a
TCS ECU 62 that has sub-algorithms that receive information from
the wheel speed sensors 34 for each wheel, and provide torque
change commands associated with each of the wheels and an engine
ECU that is operable to control operation of the engine, namely the
throttle angle, to affect engine output torque.
[0070] The TCS ECU 62 is in communication with brake systems 32 for
each wheel, and with controls for individual wheel torques and the
overall engine torque. There are several ways to implement the
improved cooperative traction control made possible by
independently controlling the individual drive torques for each of
the wheels.
[0071] When wheel-slippage is detected at any wheel, the TCS ECU 62
pulses the brakes until traction is regained and all four wheels
are again traveling at the same speed. The TCS system 62 is very
effective at low speeds and when the vehicle is on a split traction
surface.
[0072] Referring to FIG. 11A, a firewall feature of the VSA is
shown. Specifically, the VSA is prevented from utilizing the full
capacity of any one of the performance enhancement systems (Torque
Distribution System 4WD is shown as an example). The performance
enhancement system maintains a certain degree of autonomy, and this
acts as a checking mechanism. The VSA can not force the performance
enhancement system to completely override the system's
independently generated control function.
[0073] Referring to FIG. 2 and FIG. 11B, it can be seen that at
certain conditions of longitudinal traction and lateral traction,
both the AYC and TCS control systems may be active concurrently.
Because each stability control system may be controlling the same
performance enhancement system(s), priority to one control system
must be given. FIG. 11B shows that priority is decided based on the
speed of the vehicle. Thus, for example, the torque distribution
performance enhancement system, such as 4WD, first receives
commands from a feedforward control based logic. These commands are
then superceded, if necessary, by the TCS stability control system
at a higher (>60 kph) vehicle speed. Note, that any commands
from the TCS control system are first subject to a firewall, as
previously described before being implemented by the 4WD. These
commands however are superceded, if necessary, by commands from the
AYC performance enhancement system. Note that any commands from the
AYC control system are also first subject to a firewall before
being implemented by the 4WD.
[0074] Distinct embodiments of the invention are described
below.
[0075] COOPERATIVE YAW STABILITY BY MODULATING INDIVIDUAL AXLE
DRIVE TORQUE: Referring to FIGS. 1 and 2, the first embodiment of
the invention is concerned with reducing AYC differential braking
intervention during stability events that have combined
acceleration and turning (refer to the upper portion of FIG. 2).
Through drive torque control cooperation, modulation of individual
wheel driving torque (as controlled through the DYC torque
distribution system) can reduce the yaw moment leading to an
unstable motion to improve VSA yaw rate control smoothness and
driver perception, especially during vehicle acceleration on low-mu
road surfaces. According to this embodiment, the DYC drive torque
distributor ECU 58 constantly monitors the AYC operation states
such as AYC activation flags, vehicle stability factor and
oversteer (OS)/understeer (US) indicators, which can be estimated
based on the vehicle yaw rate error, AYC activation modes, target
pressures, etc.
[0076] This embodiment is directed to controlling the yaw rate of a
vehicle in a manner that includes primarily shifting the
distribution of torque between the front and rear wheels. As
described in more detail below, a Yaw Stability Control Algorithm
receives information regarding the vehicle state from the sensors.
The AYC ECU 60 compares the measured yaw state to a desired yaw
operating state. The desired yaw operating state is set during
vehicle manufacture/testing by mapping yaw at different vehicle
speeds and steering angles. The AYC ECU 60 then determines if yaw
control is required to maintain a real yaw response to within a
specified error to the desired yaw rate. If yaw control is
required, the AYC ECU 60 calculates the amount of redistribution of
drive torque required and sends instructions to the DYC Drive
Torque Control ECU 58. If additional yaw correction is required,
the brakes 32 are subsequently applied accordingly.
[0077] Referring to FIGS. 1 and 5A, the AYC ECU 60 is shown
receiving, via the CAN 64, vehicle status information. This
information includes information from one or more of the following:
vehicle motion sensors including longitudinal and lateral
acceleration, wheel speed sensors, steering sensors. This
information is used to determine wheel speed changes and vehicle
motions including body lateral slippage. The wheel speed changes
and vehicle motions are compared to stability operational
boundaries in the AYC ECU 60 to determine whether one or more
wheels are slipping and/or skidding laterally, or about to slip
and/or skid laterally and/or whether the vehicle is progressing
into a region of unstable motion, and whether the actual or
anticipated wheel slip and/or lateral skid needs to be reduced. It
is noted that many algorithms are known in the art to qualify and
quantify wheel slip and/or lateral skid and/or stable vehicle
motion and that many of these known algorithms may be used by the
AYC ECU 60.
[0078] Referring to step 100 of FIG. 5A, based on the information
received, the AYC ECU 60 calculates a vehicle stability factor
and/or generates an oversteer (OS) or understeer (US) indication.
The vehicle stability factor and/or OS/US indicators are then
compared with the pre-mapped target values for vehicle stability
and OS/US. The target values represent the vehicle operating in a
stable condition.
[0079] Referring to step 110, depending on the difference between
the calculated vehicle stability and/or OS/US indicators and the
target values, yaw stability corrective action is or is not
required. Referring to step 120, if yaw stability correction is not
required, the AYC ECU 60 does not institute any changes. If yaw
stability is required, the AYC ECU 60 calculates the required
corrective yaw moment.
[0080] Referring to step 130, the AYC ECU 60 then calculates the
required shift in torque between the front wheels and rear wheels
of the vehicle to achieve the corrective yaw moment. The torque
change request is provided from the AYC ECU 60 to the DYC Drive
Torque Control ECU 58 via the Controller Area Network 64. Referring
to step 140, the DYC Drive Torque Control ECU 58 receives the
torque distribution request and implements the request accordingly
by shifting drive torque from the rear wheels to front wheels of
the vehicle. For the case of oversteer mitigation, this is done by
either enforcing an upper torque limit on each rear wheel torque or
reducing the torque applied to each rear wheel in an equivalent
manner. When an upper limit is applied, if either wheel is already
below the limit, no torque is shifted from that wheel. In a
reduction instance, torque is always reduced from both wheels. For
the case of understeer reduction, the same approach would be used
to reduce front axle torque by either imposing an upper limit value
or reducing the torque on both front wheels accordingly.
[0081] According to this proposed method, the DYC Drive Torque
Control ECU 58 constantly monitors the AYC operation states such as
AYC activation flags, vehicle stability factor and oversteer
(OS)/understeer (US) indicators generated by the AYC ECU 60, which
can be estimated based on the vehicle yaw rate error, AYC
activation modes, target pressures, etc.
[0082] Referring to step 150, the AYC ECU 60 determines if the
corrective yaw moment shift resulting from the drive torque
distribution carried out in step 140 was sufficient to provide a
desired yaw rate correction. If the shift was sufficient, the AYC
ECU 60 returns to monitoring the vehicle sensors for further
changes in vehicle conditions. Referring to step 160, if the shift
was not enough to reach the desired change in vehicle conditions,
the appropriate amount of braking torque required to obtain the yaw
rate change is calculated. Referring to step 170, the required
braking torque is then converted to line braking pressure and sent
to the brake actuators 32 for braking application.
[0083] FIG. 5B illustrates the shift of torque and other
compensation on a vehicle which is accelerating and turning in a
curve. The first schematic shows a representative drive torque
distribution resulting from the feed-forward control of a DYC
Torque Distribution System. In this example, the drive torque is
biased more towards the rear, outside wheel to take advantage of
the vertical load on the tires and to create an inward yaw moment
to reduce understeer. When operating near the limit, it is feasible
that a perturbation of the road mu (icy patch for example) can
initiate an oversteering movement to the point that a corrective
yaw moment must be introduced by the AYC controller to mitigate the
oversteer. In this case, the AYC controller would impose a
limitation on the rear axle torque leading to the corrected
distribution in the second schematic (center). The rear outside
wheel torque is reduced and this surplus is re-directed back to the
front axle via the DYC Torque Distribution System in the manner
explained above in the AYC discussion. Since the torque limiter
value is larger than the inside-rear wheel value prior to the AYC
intervention, it's value in the second schematic is unchanged. Once
the changes in yaw moment produced by the torque re-distribution in
the second schematic correct the yaw motion, the AYC generated
torque limiter is removed and the DYC Torque Distribution System
restores the distribution back to the original one prior to the
intervention (shown in the 3.sup.rd schematic at the right). In the
process, if the restoring moment created by the torque limitation
is sufficient to correct the yaw motion, then nearly no additional
brake application (shown as small) nor throttle reduction (shown as
small) is required to maintain stability and turning.
[0084] In an alternative of this embodiment of the invention, the
AYC ECU 60 does not calculate the correct amount of torque
distribution between the front and rear wheels of the vehicle to
achieve the desired corrective yaw moment, but instead the DYC
Drive Torque Distribution Control ECU 58 determines the correct
amount. Also, if sufficient torque reduction is not achieved via
torque shift and brake application, engine torque reduction may be
implemented as shown in the third vehicle schematic of FIG. 5B.
[0085] Additionally, if the torque distribution system is a four
wheel drive system, the AYC ECU 60 may request that the DYC torque
distribution system shift torque from the left wheel to the right
wheel (or vise-versa) on either the front axle, rear axle, or
both.
[0086] A section of FIG. 5C illustrates use of a firewall within
the present embodiment. A torque limiting request from the AYC 60
within the VSA passes through a VSA firewall, then to the DYC
torque distribution ECU (4WD) via the CAN, where a signal interface
within the torque distribution ECU interprets the request, as
described above, before it is implemented.
[0087] This embodiment is concerned about reducing VSA-AYC
differential braking intervention through a drive torque control
cooperation to improve VSA yaw rate control smoothness and driver
perception, especially during vehicle low-mu accelerations. The
concept is more efficient in correcting the yaw motion compared to
existing brake-only systems when considering the driver's intention
to continue acceleration through the turn. Braking forces generally
slow the vehicle speed while a re-distribution of the driving
torque away from critical wheels keeps the same overall drive
torque and does not inhibit forward momentum. This Cooperative
Direct Yaw Control can also occur earlier than conventional "brake
only" control because the DYC Torque Distribution System control
components do not create noise and vibration that is perceivable to
the driver or occupants. A more refined control entry condition can
be used without risk of excessive noise and vibration within the
sub-limit operational range of FIG. 2 which is a practical
development constraint of typical brake VSA systems. In feedback
systems that utilize brake and throttle control as a means to
stabilize yaw motion, control activation thresholds must be set
large enough such that perturbations in measured vehicle parameters
(yaw rate, lateral g, etc.) do not extraneously cause the VSA to
enter control when not needed (example: driving on a vertically
varying road combined with turning, or driving on a banked road
while turning). Without setting the thresholds high enough,
unnecessary activation can occur leading to undesirable brake
activation noise and/or vehicle hesitation through throttle
control. Since the DYC AWD system's operation shifts drive torque
to various wheels without any noise or vibration, then control
entry thresholds can be set low to initiate control sooner than in
typical brake and throttle only control systems.
[0088] COOPERATIVE ADS CONTROL: Referring to FIGS. 1 and 6, a
second embodiment of the invention including an improved method of
controlling the stability of a vehicle is provided. This embodiment
uses recognition of vehicle braking, determining the present state
of an active damping system in the suspension of the vehicle, and
then modifying the settings of the active damping system, if
necessary, to stiffen the suspension, thus, preventing a
significant shift of load in the vehicle and providing better
vehicle stability. The present embodiment's adjustment of the ADS
damping characteristics to minimize vehicle body motion and wheel
load variations results in improving vehicle braking smoothness and
stability, especially during combined braking in turning maneuvers
on high mu roads. In high mu braking situations, there is
considerable movement of the vehicle body which results in larger
load variations at the wheels. Therefore, a stiffer suspension
damping in these situations allows for better contact between the
tire and ground.
[0089] Known braking and ADS systems usually work independently of
each other with only minimum information sharing, such as braking
and ADS operational status, etc. The present embodiment of the
invention uses braking activation state information to adjust ADS
damping characteristics to minimize vehicle body motion and wheel
load variations for the purpose of improving vehicle braking
smoothness and stability.
[0090] With reference to FIG. 6, the improved method of vehicle
stability control is described. In step 200, readings of the
vehicle operating conditions 200 are made, including wheel speed,
longitudinal acceleration, lateral acceleration, etc. and the
decision to enter into ABS control is decided by the ABS controller
61. In step 210, information regarding the braking condition of the
vehicle (ABS active or not) is sent to the ADS ECU 54 via CAN. In
step 220, a limit braking condition is determined based on the
relation between the measured total G (refer to FIG. 3) and the
operational status of the ABS (either operating or not operating).
If the total G (vector sum of the lateral and longitudinal G) is
beyond a specified threshold (for example 0.5 g to correspond to
braking on a high mu surface while turning) and the ABS is active,
then the vehicle behavior inferred by the ADS ECU 54 is near limit
braking event on a high mu surface. Note, "g" is the gravitational
acceleration constant. As a second or complementary control entry
condition, if the total G is above the specified threshold and ABS
has entered electronic brake distribution (EBD) control (wheels not
yet slipping longitudinally but the ABS judges that a
redistribution of brake torque from the current one to one more
aligned with estimated wheel loads and/or a corrective yaw moment
is necessary to prevent a slipping situation), then the vehicle
behavior inferred by the ADS ECU is a condition just prior to near
limit braking on a high mu surface where a brake re-distribution is
helpful to avoid a skid or locking of any of the wheels.
[0091] ADS typically is preset for a vehicle type (comfort, sporty)
or alternatively is selectable by the driver. In step 230, the ADS
ECU 54 overrides the current pre-selected setting, when required
(based on the above determination of a limit braking event while
turning on high mu). For example, if the ADS is set at a "soft"
comfort setting, the ADS ECU 54 changes the setting to a more
"firm" sport setting. The braking trigger event results in
stiffening of the whole system, not just 1-2 wheels. If the driver
has already pre-selected a "firm" sport setting, the system may not
be required to do anything for override.
[0092] As a result of the shift to a firmer setting, there is
reduced body motion leading to steady load transfer amongst the
wheels which increases braking effectiveness. Referring to steps
240 and 250, the ADS ECU maintains the overridden setting for a
critical braking period. This period is preferably between 0.75 and
3 seconds. Referring to step 260, the system will revert back to
the original predetermined or driver-selected setting after the
critical braking time period expires.
[0093] Alternatively, if the automobile is provided with a Traction
Control System (TCS) 62, this system may take the place of the ABS
ECU 61, and provide braking information to the ADS ECU 54.
[0094] The advantages of the ADS cooperating with the braking
system includes: Flat cornering feel--minimal roll or pitch even
during spirited driving and braking; Responsive, precise and secure
steering feel; Elimination of unwanted under-damped body heave,
roll and pitch motions; Improved tire adhesion, vehicle stability
and road isolation.
[0095] ENHANCING ADS CONTROL USING OPERATIONAL MODE INFORMATION
FROM VSA COMBINED WITH THE VEHICLE OPERATING STATE: Referring to
FIGS. 1, 3, and 7, a third embodiment of the invention, including
an improved method of controlling the overall maneuverability, ride
comfort, and stability of a vehicle is provided. This improved
method uses a technique to infer the road surface qualities
including the road friction coefficient (mu) and road roughness
index when combined with stability control intervention (TCS, AYC,
ABS) operational status. The method then modifies the settings of
the Active Damping System (ADS), if necessary, to stiffen the
suspension damping control, thus preventing a significant shift of
load in the vehicle, or to soften the suspension damping control
for improving vehicle drivability and comfort. The adjustment of
the ADS overrides any damping characteristics that were
pre-selected by the driver or preset by the vehicle.
[0096] The operational (ON/OFF) status of an AYC, ABS, and/or
Traction Control System (TCS) together with vehicle longitudinal
and lateral accelerations are used to formulate a trigger for
changing ADS calibration. As shown in FIG. 3, a range of vector G
between 0.5 g and 1.0 g is shown and is considered the high mu
operating area. The acceleration vector used to determine whether
the vehicle is operating in this range is calculated by taking the
square root of the sum of the squares of longitudinal and lateral
acceleration.
[0097] In practice, road roughness estimates by the TCS/VSA/ABS are
related to the perturbations in wheel speed sensor signals over a
period of time. There are several "levels" of roughness, each
corresponding to a given level of "noise" in the wheel speed
signals. Roughness for each of the wheels is estimated based on
this approach, then the roughness levels of each of the 4 wheels
are compared to determine if the overall "roughness" of the road
for the entire vehicle should be updated. When the roughness is
"high" or "low", the control gains for ABS/TCS control are modified
to take advantage of this information.
[0098] The coefficient of friction (mu) is a rating of the grip
between a road surface and a tire. The value of the coefficient of
friction is a fraction, which lies between zero and one. The lower
the value of the coefficient of friction of the roadway, the more
slippery the roadway will be. For example, an icy surface may have
a coefficient of friction in the range of 0.1, while a clean, dry
asphalt surface may have a coefficient of friction of approximately
0.9.
[0099] In step 310, and as illustrated by FIG. 3, if the AYC 60 or
ABS 61 or TCS 62 is activated (AYC, ABS or TCS status is ON
(activated); Operation bit=1) and, at the same time, the vehicle
total acceleration (G) level (vector combination of vehicle
longitudinal and lateral accelerations) is low (below 0.3 for
example), an operation condition at the limit of low coefficient of
friction is identified. If either AYC 60, ABS 61, or TCS 62 (AYC,
ABS, or TCS status is ON) is activated at the same time the vehicle
total acceleration level is high, an operation condition at the
limit of high coefficient of friction is identified.
[0100] In the prior art, the ADS setting is usually fixed and thus
is usually a trade-off of ride comfort, maneuverability and
stability under different operating conditions or is selected by
the driver through an external button located near the driver's
seat, and thus is not directly and automatically linked to
operating surface conditions.
[0101] Existing braking, AYC, and ADS systems usually work
independently of each other with only minimum information sharing.
The present embodiment of the invention uses road surface state
information to adjust ADS damping characteristics to either,
depending on the sensed road condition, minimize vehicle body
motion and wheel load variation or improve drivability.
[0102] Referring to FIG. 7, in step 320, the inferred conditions of
mu are sent to the ADS ECU 54 through the vehicle's CAN 64. After
receiving the information, the ADS ECU 54 determines appropriate
ADS settings based on the road conditions.
[0103] In step 330, the logic of determining the appropriate ADS
setting is shown. If the road has a high coefficient of friction
(mu), a strong damping force is instilled in the ADS to suppress
body motion so as to enhance handling control. If the road has low
coefficient of friction, a moderate or weak damping force is
instilled in the ADS and as a result improves vehicle drivability
and comfort.
[0104] ADS typically is preset for vehicle type (comfort or sporty)
or alternatively is selectable by the driver. Within step 330, the
ADS ECU overrides the current pre-selected setting, when required.
For example, if a high coefficient of friction operation is
identified and the ADS is set at a "soft" Comfort setting, the ADS
ECU changes the setting to a more "firm" setting. If the driver has
already pre-selected a "firm" setting, the system may not be
required to do anything for override. Likewise, if a low
coefficient of friction operation is identified and the ADS is set
at a "firm" setting, the ADS ECU changes the setting to a more
"soft" setting. If the driver has already pre-selected a "soft"
setting, the system may not be required to do anything for
override.
[0105] After the indicators (AYC and ABS and TCS activation status
is OFF (not activated) Operation bit=0) for the prevailing
operating surface are no longer true for some predetermined time,
approximately 1-3 seconds, the ADS system automatically shifts back
to the prior selected setting.
[0106] In one variation of this embodiment of the invention, any
one of the system (AYC, TCS, ABS) status indicators being activated
triggers use of the damping control, however in other variations,
two or three of the systems must be considered activated.
[0107] The present invention uses existing vehicle control systems
to infer road surface data without directly calculating the surface
mu. Previously, the surface information was usually obtained by a
separate estimator and was updated only when wheels began to slip.
Since the ABS, AYC or TCS activation occurs shortly after wheel
slip, there is no advantage to using the estimated surface mu as a
trigger to change damping force control The condition that any of
the feedback systems (AYC, ABS, TCS) has entered control, combined
with the total vehicle G at that instant is sufficient to infer the
road surface characteristics.
[0108] TCS ENHANCEMENT UTILIZING ADS: A fourth embodiment of the
invention is shown in FIGS. 1 and 8, wherein the TCS 62 and ADS 54
work in combination with each other. TCS 62 is usually designed to
regulate wheel slip around some preset optimal region to maximize
wheel traction. During vehicle operation, TCS 62 constantly
monitors the slip ratio of each wheel of the vehicle. Whenever
excessive wheel slip occurs, TCS 62 brings down the wheel slip to
the optimal region through either throttle intervention, braking
application or a combination of the two.
[0109] Since TCS 62 regulates wheel slip on a feedback basis
without any prior knowledge about the factors that affect the wheel
slip, especially the wheel load, whose fluctuation causes
considerable wheel slip variation and, thus, may compromise TCS
control efficiency and smoothness, especially during TCS braking
operation, it is desirable that the wheel load variation be kept as
small as possible during TCS 62 operation. The Active Damping
System (ADS) 54 on the same vehicle now provides the opportunity to
control wheel load variation by adjusting the damping force to
minimize the load transfer among the wheels of the vehicle. This
embodiment of the invention provides a control concept that uses
ADS devices to adjust suspension damping force distribution during
TCS 62 operation for the purpose of enhancing TCS 62 effectiveness
and vehicle acceleration smoothness.
[0110] Referring to FIG. 8, in step 510 the TCS ECU 64 monitors
vehicle operating conditions obtained from the vehicle sensors in
step 500. The sensed information includes, but is not limited to
vehicle speed 50 as well as individual wheel speeds 34. In step
520, if excessive wheel slip is determined, appropriate corrective
measures are taken by the TCS 62 and ADS 54. The TCS 62, in step
530 takes known corrective action, namely reducing the torque
supplied by the engine and/or application of the brakes 32 to the
slipping wheel(s). In addition, the TCS 62 sends the wheel slip
data and an indication that the TCS 62 is operative to the ADS ECU
54 via the vehicle CAN 64.
[0111] In step 550, the ADS ECU 54 determines the most appropriate
calibration of individual controllable suspension components in
order to minimize the load variations in the wheel which is
experiencing excessive slippage. For example, if slip is determined
to be excessive in the right rear wheel 26 of the vehicle, the
stiffness of the suspension 38 in an area adjacent to the right
rear wheel 26 is increased.
[0112] This embodiment of the invention adjusts ADS 54 damping
calibration to minimize wheel load variation, thus facilitating TCS
62 braking operation and improving vehicle acceleration smoothness
and quality feel.
[0113] COOPERATIVE STABILITY CONTROL USING ADS FRONT TO REAR
SHIFTING: This fifth embodiment of the invention is concerned about
minimizing VSA differential braking intervention for yaw moment
control through an active suspension control system such as ADS 54
to improve VSA control smoothness and driver perception. According
to this embodiment, the ADS ECU 54 constantly monitors the AYC ECU
60 operation states such as AYC ECU activation flags, vehicle
stability factor and oversteer (OS)/understeer (US) indicators,
which can be estimated based on the vehicle yaw rate error, AYC
activation modes, target pressures, etc. When the AYC ECU 60
becomes activated, ADS ECU 54 determines that the AYC has judged
that the vehicle requires corrective yaw moment to compensate
either oversteer or understeer, and adjusts the front/rear damping
force distribution to generate the corrective yaw moment demanded
by AYC ECU 60 so as to minimize VSA-braking activation.
[0114] Referring to FIGS. 1 and 9, in step 710 the AYC ECU 60
monitors vehicle operating conditions via sensed parameters read by
the vehicle sensors. When the AYC ECU 60 determines that there is
an undesired understeer or oversteer condition as determined by
measured undesired sideslip or yaw rate (as determined by comparing
to threshold values in step 720), the AYC ECU 60 takes corrective
action in step 730.
[0115] As a corrective action, in step 740, the AYC ECU 60
calculates the required corrective yaw moment and sends it to the
ADS ECU 54 via the CAN 64. The ADS ECU 54 adjusts distribution, in
step 750, between the ADS system components in the front and rear
of the vehicle's suspension. The shift of suspension stiffness,
from front to rear or vice versa creates a counter-acting yaw
moment. At each of the 4 wheels, the ADS knows the damping force
(as an internal variable in its control loop). Depending on the
stroke rate of the damper (rate of change of the vertical wheel
travel), the ADS can determine if a stiffening or softening of the
damping force will either contribute or subtract from the desired
yaw moment change issued by the VSA. As the damping force stiffens
or softens, there will be a change in the vertical wheel load. When
the wheel load increases, so does the lateral tire force (generally
speaking). So for an oversteer event, the ADS would re-adjust the
front dampers such that the lateral forces are reduced (depending
on the wheel stroke, this could be either an increase or decrease
on either damper). At the rear, the ADS would adjust each damper to
attempt to raise the tire forces (depending on the prevailing wheel
stroke at the instant, this could be an increase or decrease in
damping force of a given wheel). In a simplified case, the front
dampers would likely be softened and the rear dampers stiffened to
mitigate oversteer.
[0116] In step 760, the AYC ECU 60 determines if additional yaw
corrective action is required beyond what was provided by the
shifting of stiffness within the controllable suspension. If
additional correction is required, appropriate braking torques are
determined by the AYC ECU 60 in step 770 and the appropriate brakes
34 actuated in step 780.
[0117] This embodiment, by making use of existing vehicle stability
state information from the AYC ECU 60 and adjusting the ADS ECU 54
control according to AYC corrective yaw moment requests to reduce
yaw rate error and stabilize the vehicle, minimizes VSA-AYC braking
activation and greatly improves vehicle control smoothness and
quality feel, especially for the vehicle during transient steering
operations.
[0118] WHEEL SLIP CONTROL UTILIZING A TCS SYSTEM WITH A FRONT/REAR
AXLE DRIVE TORQUE DISTRIBUTION SYSTEM: A sixth embodiment of the
invention is shown in FIGS. 1 and 10A, wherein the TCS 62 works to
distribute torque between the front and rear axles, as necessary.
TCS 62 is usually designed to regulate individual wheel or total
axle slip below some preset threshold limit to enhance traction
utilization. During vehicle operation, TCS 62 constantly monitors
the slip ratio of each wheel. Whenever excessive wheel slip or
wheel spinning occurs, TCS 62 brings down the wheel slip to the
allowable region through either throttle intervention to reduce
engine torque, braking or a combination of the two. TCS braking and
throttle intervention are both considered to be intrusive to the
driver, as they result in noise and vibration as well as hesitation
to the vehicle momentum. This is because the amount of engine
torque reduction required to reduce the wheel slips on the more
critical axle also reduces the amount of traction utilization on
the other axle that may have additional traction capability, as in
the cases of vehicle launch acceleration on a snow or ice surface,
on off-roads, or climbing slippery hills, as the wheel load on any
axle of the vehicle is typically not proportional to the drive
torque delivered to that axle.
[0119] In addition, in the case of a vehicle that is also equipped
with a DYC drive torque distribution control system 58, such as
4WD, there is a possibility that while the TCS 62 is applying
braking on a single wheel of an axle, the DYC drive torque control
system 58 may be still delivering some drive torque to the same
wheel, causing conflicting torque control and power wastage.
[0120] To overcome these drawbacks, wheel slips of each axle are
controlled independently through the utilization of an available
DYC drive torque control system 58 capable of Front to Rear drive
torque distribution so as to minimize the TCS throttle and brake
activity. The DYC drive torque control system 58, such as 4WD,
provides the opportunity to independently control the wheel slips
of each axle by drive torque redistribution between front and rear
axles. Here, the TCS ECU 62 issues demands for front/rear torque
distribution changes to the DYC Torque Distribution ECU 58 to
adjust vehicle Front/Rear drive torque distribution in response to
wheel slip conditions of each axle, as detected by TCS 62, for the
purpose of minimizing TCS throttle and brake activity and enhancing
vehicle drivability and smoothness, especially during low-mu launch
acceleration and off-road starting operations.
[0121] Referring to FIGS. 10A and 10B,and considering a driving
situation where the front wheels slip before the rear wheels due to
road condition variations, in step 610 the TCS ECU 62 monitors
vehicle operating conditions obtained from the vehicle sensors in
step 600. TCS estimates individual wheel slips in step 610
according to a variety of techniques known in typical TCS control
systems. In step 620, the amount of wheel slip occurring in wheels
on the front axle is compared to a control entry threshold to
determine if intervention is required to mitigate wheel spin. The
control entry threshold is a wheel slip of less than 15% or so,
preferably less than 5%.about.8% at higher speeds.
[0122] In step 630 a determination is made as to whether wheel slip
reduction is required. If wheel slip reduction is required, the
appropriate distribution of torque between the wheels on the front
axle and the wheels on the rear axle is determined by the TCS ECU
62 in step 640. Commands to implement the desired distribution of
torque are sent from the TCS ECU 62 to the DYC torque distribution
ECU 58. In step 650 the DYC drive torque distribution ECU 58 shifts
torque between wheels on the front axle and wheels on the rear
axle. FIG. 10B shows, schematically, a shift of torque from wheels
on the front axle to wheels on the rear axle, with the total drive
torque kept nearly constant (throttle reduction small).
[0123] Referring back to FIG. 10A, in step 660 the TCS ECU 62, in
the case where the output torque of the engine and transmission is
judged to be too large to sustain traction on all 4 wheels
simultaneously, determines if additional torque reduction is
required. If additional torque reduction is required, the TCS ECU
62, in steps 670-695 takes known corrective action, namely reducing
the torque supplied by the engine and/or application of the brakes
(in either order).
[0124] The process of steps 600-695 also applies to a driving
situation where the rear wheels slip first due to road conditions
and the cooperative relation between TCS and DYC Torque
Distribution System acts to shift torque from the rear axle to the
front axle to mitigate rear axle slippage.
[0125] FIG. 10C schematically shows the implementation of the three
stages of correction of slip through the TCS system 62. When the
line "A" that indicates the speed of the most dominant vehicle
wheel exceeds the body speed of the vehicle, slipping is occurring.
The first correction threshold that is crossed (vertically in the
figure) is the distribution of drive torque between the front and
rear wheels as described above. If this threshold is exceeded, the
TCS requests the DYC Torque Distribution system to shift torque
away from the axle of the slipping wheel according to the previous
discussion. If the slip rate continues to increase to where it
surpasses the throttle correction threshold, then throttle
variation control is implemented. Furthermore, if the slip
continues to grow and it surpasses the brake correction threshold,
then brake application is performed. The three thresholds can be
separated or combined depending on vehicle operating conditions to
ensure both smooth control as well as sufficient control authority
to manage a variety of slippery road conditions.
[0126] The method of the present embodiment significantly improves
vehicle performance during take-off from snow or icy surfaces, on
unpaved roads, or during the climbing of slippery hills. During
these conditions, there is a high possibility of traction loss
associated with non-ideal front to rear axle load distribution as
well as local perturbations in the road coefficient that can lead
to single wheel slippage, which can lead to a loss of vehicle
maneuverability. With this embodiment, wheel spin is reduced, wheel
control is smooth, and acceleration is improved.
[0127] A seventh embodiment of the invention is shown in FIG. 12,
wherein the TCS ECU 62 works to distribute torque between the left
and right side wheels on a given axle, as necessary. This action
helps to reduce differential braking control on a given axle (brake
control used to keep the left and right side wheels within a
specified speed difference) and leads to refinements in control
smoothness. Since throttle control itself can not easily mitigate
side-to-side differences in wheel speed (such as when one side of
the vehicle is on a low mu surface and the other on a high mu
surface), a side-to-side torque re-distribution to slow the faster
of the two spinning wheels provides enhanced control.
[0128] In the present variation, due to the inclusion of a torque
distribution system that can distribute torque between the four
wheels (side to side and front to back), such as a 4WD system, the
need to reduce engine torque or introduce braking can be lessened
or eliminated. The present variation combines the basic functions
of a feed-forward based drive torque control such as 4WD with the
control of a wheel slip-based feed-back brake and throttle control
system (TCS) 62 such that the TCS ECU 62 now has the capability to
request drive torque reduction at each rear wheel of the vehicle to
the drive torque control ECU 58.
[0129] The primary operational function of the proposed concept is
to redistribute the available drive torque between Rear Left and
Right wheels by use of an available drive torque actuator, such as
a 4WD system, in response to rear left and right wheel slip
conditions of the vehicle.
[0130] Referring to FIGS. 1 and 12, in step 1000 the TCS ECU 62
monitors vehicle operating conditions obtained from the vehicle
sensors. In step 1010 the amount of individual wheel slip is
calculated and in step 1020 the amount of wheel slip occurring in
wheels on the left and right sides on the front axle is compared.
The difference in slippage between the left and right sides is
compared to a target/threshold difference.
[0131] In step 1030 a determination is made as to whether wheel
slip reduction is required. If wheel slip reduction is required,
the appropriate distribution of torque between the left and right
wheels on the given axle is determined by the TCS ECU 62 in step
1040. Commands to implement the desired distribution of torque are
sent from the TCS ECU 62 to the DYC torque distribution ECU 58. In
step 1050 the DYC drive torque distribution system 58 shifts torque
between the left and right wheels on the given axle. Steps
1000-1050 can be applied to either the front or rear axle of a
vehicle (or both simultaneously) depending of what type of system
4WD, FWD, RWD is provided on the vehicle. This lateral control can
also be combined with the longitudinal control explained in the
previous embodiment of the invention.
[0132] In step 1060, the TCS ECU 62 determines if additional torque
reduction is required. If additional torque reduction is required,
the TCS ECU 62, in steps 1070-1095 takes known corrective action,
namely applying differential brake control (as described above), or
in extreme cases reducing the torque supplied by the engine and/or
application of the brakes.
[0133] The present embodiment is primarily concerned about
overcoming the drawbacks associated with existing TCS systems, and
to enhance vehicle acceleration control smoothness. The main
operational enhancement of this embodiment relates to, but is not
limited to, vehicle launch or acceleration on road surfaces where
individual tires do not have the same traction limit due to local
variations in road surface coefficient, split friction coefficient
(mu) surfaces or during cornering. During vehicle launch or
acceleration on split friction coefficient (mu) surfaces or during
cornering, there is a high possibility of traction loss on one side
of either the front or the rear axle, associated with asymmetrical
rear left and right tractions, which may lead to loss of vehicle
drivability and/or stability.
[0134] Referring to FIGS. 1, 3, 13 and 14, an eighth embodiment of
the invention including an improved method of controlling the
stability of a vehicle is provided. This improved method uses
recognition of road surface qualities including the road friction
coefficient (mu) as determined by a Vehicle Stability Assist system
(VSA). The method then modifies the settings of a drive torque
control device(s), if necessary, to redistribute drive torque
between 2 or 4 wheels, thus providing improved vehicle drivability
and stability or improved handling, depending upon the road
conditions. The adjustment of the drive torque distribution
overrides any torque distribution characteristics that were
pre-selected by the driver. In addition to, or as an alternative to
the AYC ECU 60 estimating road surface information, the operational
(ON/OFF) status of a AYC 60, and/or TCS 62 may be used as a trigger
for changing drive torque distribution calibration in combination
with a vector representation of vehicle acceleration (G).
[0135] Existing drive torque, and VSA systems usually work
independently of each other with only minimum information sharing.
The present invention uses calculated or inferred road surface
state information to adjust drive torque characteristics to either,
depending on the sensed road condition, provide moderate front to
rear and side to side torque biases to improve vehicle drivability
and stability, or provide strong front to rear and side to side
torque biases to improve vehicle handling driving pleasure.
[0136] DYC Torque Distribution Systems are capable of side to side
torque distributions of anywhere between moderate 50% right side
and 50% left side (equal distribution), to strong 100% right side
and 0% left side (or vise-versa) representing all the axle torque
to be on one wheel. DYC Torque Distribution Systems are capable of
front-to rear distribution ranging from moderate 50% front and 50%
rear (equal distribution), and strong 100% front and 0% rear,
representing all torque on a given axle.
[0137] With reference to FIG. 13, an improved method of vehicle
stability control is described. In step 1100, readings of vehicle
operating conditions are made, including wheel speed, longitudinal
acceleration, lateral acceleration, yaw rate, engine operational
parameters, and transmission operational parameters. In step 1110,
the AYC ECU 60 and/or TCS ECU 62 uses these measured operating
conditions to estimate road surface conditions, namely, the road
coefficient of friction (mu). Direct measurement of mu by known
methods may be performed or inference of mu may be done by the
method previously described.
[0138] In step 1120, the measured condition of mu is sent to the
DYC Drive Torque Control ECU 58 through the vehicle's CAN 64. After
receiving the information, the Drive Torque Control ECU 58
determines appropriate distribution of drive torque based on the
road conditions.
[0139] In step 1130, the logic of determining the appropriate
distribution of drive torque is shown. If the road has a high
coefficient of friction (mu), depending on the capability of the
systems on the vehicle, a strong front to rear and side to side
torque biases are set to improve vehicle handling and driving
pleasure. If the vehicle has all wheel drive, strong front to rear
torque bias is set and strong side to side torque bias is set. If
the vehicle has only a FWD system, strong side to side torque bias
is set only on the front wheels. If the vehicle has only a RWD
system, strong side to side torque bias is set only on the rear
wheels.
[0140] If the road has a low coefficient of friction (mu),
depending on the capability of the systems on the vehicle, moderate
front to rear and side to side torque biases are set to improve
vehicle drivability and stability. If the vehicle has all wheel
drive, moderate front to rear torque bias is set and moderate side
to side torque bias is set. If the vehicle has only a FWD system,
moderate side to side torque bias is set only on the front wheels.
If the vehicle has only a RWD system, moderate side to side torque
bias is set only on the rear wheels.
[0141] Drive Torque distribution typically is preset for vehicle
type (comfort, sporty) or alternatively is selectable by the
driver. As lateral acceleration increases during cornering, drive
torque is sent more strongly to the rear axle and to the outside
rear wheel. Nearly 70% of the total driveline torque can exist at
the outside, rear wheel at the limiting case. For a sporty setting
the progression from a nearly even distribution (25% at each wheel)
to this 70% outside-rear condition occurs at a lower lateral
acceleration than a more comfort oriented setting. The torque
transition gain is lower for a more comfort setting and higher for
a sporty setting. Within step 1130, the DYC Drive Torque Control
ECU overrides the current pre-selected setting, when required. For
example, if the distribution is set at a sporty setting with strong
front to rear and side to side bias, the Drive Torque Control ECU
changes the setting to a comfort setting with moderate front to
rear and side to side bias. If the driver has already pre-selected
a comfort setting, the system may not be required to do anything
for override.
[0142] The redistribution of drive torque occurs typically when the
vehicle is starting up from a stand still position. The
redistribution is maintained for a set period of time and then
reset back to the bias presetting selected by the driver.
Alternatively, as opposed to a set period of time, if longitudinal
acceleration reaches a threshold value, the system resets back to
the bias presetting selected by the driver.
[0143] Referring to FIG. 14, in an alternative variation of this
embodiment of the invention, instead of using sensed/measured road
conditions, in step 1210, the operational status of a vehicle
equipped with a AYC system, TCS system and ABS system is used in
combination with a vector representation of the vehicle's
acceleration G (as previous described) is used to infer road
conditions. The operational status of the AYC, TCS and ABS systems
is determined; Simply is the system operating or not operating? If
one of the systems is operating, it is an indication that the road
surface either has a low or high coefficient of friction.
[0144] After making a determination of road coefficient of friction
and roughness in step 1220, the remaining steps of the method are
unchanged.
[0145] Instead of relying solely on the VSA system to determine the
road coefficient of friction, the system may also sense
longitudinal acceleration, lateral acceleration, yaw rate, or sense
rain contacting the windshield and adjust the VSA calculated
coefficient of friction.
[0146] The present invention eliminates the trade-off of drive
torque control system capabilities under different road conditions
and thus maximizes the drive torque control system potential
benefits. The system also reduces unwanted VSA activation for
smooth vehicle handling and stability control.
[0147] Although the invention has been shown and described with
reference to certain preferred and alternate embodiments, the
invention is not limited to these specific embodiments. Minor
variations and insubstantial differences in the various
combinations of materials and methods of application may occur to
those of ordinary skill in the art while remaining within the scope
of the invention as claimed and equivalents.
* * * * *