U.S. patent application number 13/395015 was filed with the patent office on 2012-07-12 for vehicle motion control device.
This patent application is currently assigned to Hitachi Automotive Systems, Ltd.. Invention is credited to Shinjiro Saito, Junya Takahashi, Makoto Yamakado, Atsushi Yokoyama, Tatsuya Yoshida.
Application Number | 20120179349 13/395015 |
Document ID | / |
Family ID | 43825949 |
Filed Date | 2012-07-12 |
United States Patent
Application |
20120179349 |
Kind Code |
A1 |
Yamakado; Makoto ; et
al. |
July 12, 2012 |
Vehicle Motion Control Device
Abstract
There is provided a vehicle drive control system that feels less
unnatural and that enables an improvement in safety performance. A
vehicle motion control system capable of independently controlling
a driving force and a braking force of four wheels comprises: a
first mode (G-Vectoring control) in which substantially the same
driving force and braking force are generated with respect to left
and right wheels among the four wheels based on a longitudinal
acceleration/deceleration control command that is coordinated with
the vehicle's lateral motion; and a second mode (sideslip
prevention control) in which different driving forces and braking
forces are generated with respect to the left and right wheels
among the four wheels based on a target yaw moment derived from the
vehicle's sideslip information, wherein the first mode is selected
when the target yaw moment is equal to or less than a pre-defined
threshold, and the second mode is selected when the target yaw
moment is greater than the threshold (FIG. 11).
Inventors: |
Yamakado; Makoto;
(Tsuchiura, JP) ; Saito; Shinjiro; (Kasumigaura,
JP) ; Yokoyama; Atsushi; (Tokyo, JP) ;
Takahashi; Junya; (Hitachinaka, JP) ; Yoshida;
Tatsuya; (Naka, JP) |
Assignee: |
Hitachi Automotive Systems,
Ltd.
Hitachinaka-shi
JP
|
Family ID: |
43825949 |
Appl. No.: |
13/395015 |
Filed: |
August 2, 2010 |
PCT Filed: |
August 2, 2010 |
PCT NO: |
PCT/JP2010/062996 |
371 Date: |
March 8, 2012 |
Current U.S.
Class: |
701/89 |
Current CPC
Class: |
B60T 2270/302 20130101;
B60W 10/184 20130101; B60W 2720/30 20130101; B60T 8/1755 20130101;
B60W 10/08 20130101; B60W 30/18145 20130101; B60W 10/119 20130101;
B60W 10/04 20130101; B60W 2720/14 20130101; B60W 10/18 20130101;
B60W 30/045 20130101 |
Class at
Publication: |
701/89 |
International
Class: |
B60W 30/045 20120101
B60W030/045; B60W 10/119 20120101 B60W010/119; B60W 10/184 20120101
B60W010/184; B60W 10/04 20060101 B60W010/04 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 30, 2009 |
JP |
2009-225938 |
Claims
1. A vehicle motion control system capable of independently
controlling a driving force and braking force of four wheels, the
vehicle motion control system comprising: a first mode in which
substantially the same driving force and braking force are
generated with respect to left and right wheels among the four
wheels based on a target longitudinal acceleration/deceleration
control command that is coordinated with a lateral motion of the
vehicle; and a second mode in which different driving forces and
braking forces are generated with respect to the left and right
wheels among the four wheels based on a target yaw moment derived
from sideslip information of the vehicle, wherein the first mode is
selected when the target yaw moment is equal to or less than a
pre-defined threshold, and the second mode is selected when the
target yaw moment is greater than the threshold.
2. The vehicle motion control system according to claim 1, further
comprising: a vehicle lateral motion model that estimates estimated
lateral acceleration, a target yaw rate, and a target sideslip
angle based on inputted steering angle .delta. and vehicle speed V;
a first processing unit that calculates the target longitudinal
acceleration/deceleration control command based on lateral
acceleration and lateral jerk that are calculated based on the
estimated lateral acceleration and actual lateral acceleration; a
second processing unit that calculates the target yaw moment based
on a deviation between the target yaw rate and an inputted actual
yaw rate, and on a deviation between the target sideslip angle and
an inputted actual sideslip angle; and a braking force/driving
force distribution unit that calculates a braking force/driving
force of each wheel of the vehicle based on the target longitudinal
acceleration/deceleration control command or on the target yaw
moment.
3. The vehicle motion control system according to claim 2, wherein
it is determined whether or not the target longitudinal
acceleration/deceleration control command is zero, and if the
target longitudinal acceleration/deceleration control command is
not zero and the target yaw moment is equal to or less than the
pre-defined threshold, a braking force/driving force of each wheel
of the vehicle is calculated at the braking force/driving force
distribution unit based on the target longitudinal
acceleration/deceleration control command so as to distribute the
braking forces/driving forces of the left and right wheels
substantially evenly.
4. The vehicle motion control system according to claim 2, wherein
it is determined whether or not the target longitudinal
acceleration/deceleration control command is zero, and if the
target longitudinal acceleration/deceleration control command is
zero, or if the target longitudinal acceleration/deceleration
control command is not zero and the target yaw moment is greater
than the pre-defined threshold, a braking force/driving force of
each wheel of the vehicle is calculated at the braking
force/driving force distribution unit based on the target yaw
moment so as to distribute the braking forces/driving forces of the
left and right wheels individually.
5. The vehicle motion control system according to claim 1, wherein
actual longitudinal acceleration during control in the second mode
is correctively controlled in such a manner as to apply
substantially the same braking force and/or driving force to the
left and right wheels among the four wheels so as to reduce the
difference relative to the target longitudinal
acceleration/deceleration control command.
6. The vehicle motion control system according to claim 1, wherein
the target longitudinal acceleration/deceleration control command
is so determined as to transition in a curved manner in a diagram
with the passage of time, the diagram being defined in such a
manner that its horizontal axis represents the vehicle's
longitudinal acceleration and the vertical axis the vehicle's
lateral acceleration.
7. The vehicle motion control system according to claim 1, wherein
the target longitudinal acceleration/deceleration control command
is determined in such a manner that the vehicle decelerates as the
vehicle's lateral acceleration increases, and that the vehicle
accelerates as the vehicle's lateral acceleration decreases.
8. The vehicle motion control system according to claim 1, wherein
the target longitudinal acceleration/deceleration control command
is determined in such a manner that the vehicle decelerates as the
vehicle's steering angle increases, and that the vehicle
accelerates as the vehicle's steering angle decreases.
9. The vehicle motion control system according to claim 1, wherein
the target longitudinal acceleration/deceleration control command,
G.sub.xc, is determined by [ Eq . 1 ] G xc = - sgn ( G y G . y ) C
xy 1 + Ts G . y + G x_DC ( G . y = G y _ dot ) ( Eq . 1 )
##EQU00013## (where G.sub.y is vehicle lateral acceleration,
G.sub.y.sub.--dot is vehicle lateral jerk, C.sub.xy is gain, T is a
first-order lag time constant, s is a Laplace operator, and
G.sub.x.sub.--.sub.DC is an acceleration/deceleration command by a
driver or that is automatically inputted based on external
information).
10. The vehicle motion control system according to claim 1,
wherein, where the vehicle's measured longitudinal acceleration and
lateral acceleration are indicated in a diagram whose horizontal
axis represents the vehicle's acceleration in the positive
direction and deceleration in the negative direction, and whose
vertical axis represents the vehicle's leftward lateral
acceleration in the positive direction and rightward acceleration
in the negative direction, if the target yaw moment is a clockwise
value as viewed from above the vehicle, a greater deceleration
force is imparted to the left wheels than the right wheels or a
greater driving force is imparted to the right wheels than the left
wheels, and if the target yaw moment is an anti-clockwise value as
viewed from above the vehicle, a greater deceleration force is
imparted to the right wheels than the left wheels or a greater
driving force is imparted to the left wheels than the right
wheels.
11. The vehicle motion control system according to claim 6,
wherein, where the vehicle's measured longitudinal acceleration and
lateral acceleration are indicated in a diagram whose horizontal
axis represents the vehicle's acceleration in the positive
direction and deceleration in the negative direction, and whose
vertical axis represents the vehicle's leftward lateral
acceleration in the positive direction and rightward acceleration
in the negative direction, longitudinal acceleration/deceleration
is determined in accordance with lateral motion in such a manner as
to exhibit a clockwise curved transition with the passage of time
if an anti-clockwise motion is started as viewed from above the
vehicle, and to exhibit an anti-clockwise curved transition with
the passage of time if a clockwise motion is started as viewed from
above the vehicle.
12. The vehicle motion control system according to claim 1, wherein
the sideslip information of the vehicle comprises a steering angle,
a vehicle speed, a yaw rate, and a sideslip angle.
13. The vehicle motion control system according to claim 2, wherein
the braking force/driving force distribution unit calculates the
braking force and/or driving force of each wheel of the vehicle
based on a deviation between the target longitudinal
acceleration/deceleration control command and a value obtained by
multiplying measured actual longitudinal acceleration by a
pre-defined gain or by differentiating the measured actual
longitudinal acceleration.
14. The vehicle motion control system according to claim 2, wherein
the target longitudinal acceleration/deceleration control command
is determined using one of lateral jerk calculated using the
estimated lateral acceleration estimated with the vehicle lateral
motion model and actual lateral jerk obtained by differentiating
the vehicle's actually measured lateral acceleration.
15. The vehicle motion control system according to claim 2, further
comprising: a plurality of modes with varying calculation methods
for the target longitudinal acceleration/deceleration control
command; and switching means for switching between the plurality of
modes.
16. The vehicle motion control system according to claim 15,
wherein the plurality of modes are modes of calculation methods for
the target longitudinal acceleration/deceleration control command
that vary in accordance with a traveled road surface.
17. The vehicle motion control system according to claim 15,
wherein the switching means comprises a control selector that is
provided within the vehicle and that is switchable through driver
operation.
Description
TECHNICAL FIELD
[0001] The present invention relates to a vehicle motion control
system capable of controlling the driving forces and braking forces
of four wheels.
BACKGROUND ART
[0002] A command value for automatically performing
acceleration/deceleration that is coordinated with steering
operations is disclosed, for example, in Non-Patent Document 1
((Eq. 1)).
[ Eq . 1 ] G xc = - sgn ( G y y ) C xy 1 + Ts G . y + G x_DC ( Eq .
1 ) ( G . y = G y _dot ) ##EQU00001##
[0003] This is basically a simple control rule where lateral jerk
G.sub.y.sub.--dot is multiplied by gain C.sub.xy, and a value to
which a first-order lag is imparted is taken to be longitudinal
acceleration/deceleration control command G.sub.xc (equivalent to
target longitudinal acceleration/deceleration control command
(G.sub.xt)). It is confirmed in Non-Patent Document 2 that an
expert driver's coordinated control strategy for lateral and
longitudinal motions may thus be partially simulated.
G.sub.x.sub.--.sub.DC in the equation above is a deceleration
component that is not coordinated with lateral motion. It is a term
that is required in cases where there is anticipatory deceleration
when a corner is ahead or where there is a zone speed command.
Further, the sgn (signum) term is a term provided so that the
operation above may be attained with respect to both right corners
and left corners. Specifically, an operation may be attained where
deceleration is performed at turn-in upon starting steering,
deceleration is suspended once at steady turn (since lateral jerk
becomes zero), and acceleration is performed upon starting to ease
steering when exiting the corner.
[0004] When thus controlled, with respect to a diagram in which the
horizontal axis represents the longitudinal acceleration of a
vehicle and the vertical axis the lateral acceleration of the
vehicle, the combined acceleration (denoted by G) of longitudinal
acceleration and lateral acceleration is so oriented (vectored) as
to transition in a curved manner with the passage of time. It is
therefore called "G-Vectoring control."
[0005] In addition, with respect to a sideslip prevention system
for improving safety performance at the critical driving region, it
is reported in Non-Patent Document 3 that because it becomes
unstable (divergent) when vehicle behavior transitions to a region
in a phase plane for vehicle sideslip angle .beta. and vehicle
sideslip angular speed (.beta._dot) that is apart from the origin
and where the signs of .beta. and .beta._dot are the same (the
first and third quadrants), it is effective when used in the
determination for activating the sideslip prevention system. It is
disclosed that the vehicle is stabilized by applying different
brake hydraulic pressures on the left and right wheels based on
sideslip information, generating decelerating forces that differ
between the left and the right, and generating a yaw moment in a
direction in which the sideslip angle becomes smaller. [0006]
Non-Patent Document 1: M. Yamakado, M. Abe: Improvement of Vehicle
Agility and Stability by G-Vectoring Control, Proc. of
AVEC2008-080420. [0007] Non-Patent Document 2: M. Yamakado, M. Abe:
Proposal of the longitudinal driver model in coordination with
vehicle lateral motion based upon jerk information, Review of
Automotive Engineering, Vol. 29. No. 4. October 2008,
P.533.about.541. [0008] Non-Patent Document 3: S. Inagaki, I.
Kushiro, M. Yamamoto: Analysis on Vehicle Stability in Critical
Cornering Using Phase-Plane Method, Proc. of AVEC1994-9438411
DISCLOSURE OF THE INVENTION
Problems to be Solved by the Invention
[0009] It is indicated in Non-Patent Documents 1 and 2 that this
control method is extracted from brake and accelerator operations
corresponding to steer operations that an expert driver performs
voluntarily, and that there is a possibility that it would not feel
unnatural even if there is automatic intervention from a normal
region, and improvements in the mechanical rationality,
maneuverability, and stability of this control method are presented
as simulation results. This means that because
acceleration/deceleration is controlled in a coordinated manner so
that the behavior of the vehicle would respond appropriately to the
driver's steering operation, it is consequently possible to prevent
the sideslip angle of the vehicle from becoming large. In
particular, it is effective in reducing so-called "understeer"
where the turning radius becomes too large relative to
steering.
[0010] On the other hand, this control does not guarantee that,
should the sideslip angle inadvertently become large for some
reason, it will be reduced for certain. By way of example, if the
vehicle lateral motion stabilizes while in a drifting state where
the sideslip angle has become large, lateral acceleration becomes
constant, and lateral jerk becomes zero. As a result, the
acceleration/deceleration control command represented by (Eq. 1)
becomes zero, and a stable state is entered while the vehicle is
still drifting. Although stable mechanically, there is no guarantee
that driving that does not feel unnatural to any driver is
attained.
[0011] In addition, although the sideslip prevention system
disclosed in Non-Patent Document 3 operates based on sideslip
information, no guidance is provided with respect to operating from
the normal region where there is little or no sideslip. Further,
from the perspective of "understeer" prevention, which is a forte
of "G-Vectoring control," it would mean that the "sideslip
prevention system" is such that a moment is introduced only after
sideslip has occurred to some significant extent. Thus, control
tends to be after the fact, requiring a large moment to reduce
understeer. As a result, there are concerns that the understeer
reducing effect would become smaller, while causing an unnatural
feel due to excessive deceleration.
[0012] In addition, no consideration is given to the deceleration
that occurs when the sideslip prevention system generates a yaw
moment. Thus, the moment to be generated is determined first, and
the vehicle's acceleration/deceleration is determined by the
combined force of the left and right braking forces. Given the
above, it cannot be said that acceleration/deceleration is
coordinated with lateral motion.
[0013] An object of the present invention is to provide a vehicle
drive control system that reliably reduces sideslip in the critical
driving region, feels less unnatural, and enables an improvement in
safety performance.
Means for Solving the Problems
[0014] With a view to attaining the object above, the present
invention is a vehicle motion control system capable of
independently controlling driving forces and braking forces of four
wheels, comprising: a first mode in which substantially the same
driving force and braking force are generated with respect to left
and right wheels among the four wheels based on a longitudinal
acceleration/deceleration control command that is coordinated with
a lateral motion of the vehicle; and a second mode in which
different driving forces and braking forces are generated with
respect to the left and right wheels among the four wheels based on
a target yaw moment derived from sideslip information of the
vehicle, wherein the first mode is selected when the target yaw
moment is equal to or less than a predefined threshold, and the
second mode is selected when the target yaw moment is greater than
the threshold.
Effects of the Invention
[0015] A vehicle drive control system that feels less unnatural and
enables an improvement in safety performance may be provided.
[0016] The present specification incorporates the contents of the
specification and/or drawings of JP Patent Application No.
2009-225938 from which the present application claims priority.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 is a diagram showing the overall configuration of a
vehicle motion control system according to the present
invention.
[0018] FIG. 2 is a diagram showing lateral acceleration and lateral
jerk estimation using a vehicle model of the present invention.
[0019] FIG. 3 is a diagram showing lateral acceleration and lateral
jerk estimation using an acceleration sensor of the present
invention.
[0020] FIG. 4 is a diagram showing a concept of the present
invention where estimated signals and measured signals complement
each other.
[0021] FIG. 5 is a diagram showing a process from entry to exit for
a left corner with respect to a G-vectoring controlled vehicle of
the present invention.
[0022] FIG. 6 shows charts indicating time series data from when
the travel path in FIG. 5 is traveled.
[0023] FIG. 7 is a diagram showing the application of a positive
yawing moment by left and right differential braking forces/driving
forces of the present invention.
[0024] FIG. 8 is a diagram showing the application of a negative
yawing moment by left and right differential braking forces/driving
forces of the present invention.
[0025] FIG. 9 is a diagram showing a process from entry to exit for
a left corner with respect to a sideslip prevention controlled
vehicle of the present invention.
[0026] FIG. 10 shows charts indicating time series data from when
the travel path in FIG. 9 is traveled.
[0027] FIG. 11 is a diagram showing a control block of a vehicle
motion control system according to the present invention.
[0028] FIG. 12 is a diagram showing forces, accelerations and a
yawing motion exerted on a vehicle.
[0029] FIG. 13 is a diagram showing yaw moments resulting from load
shifts by G-Vectoring control of the present invention.
[0030] FIG. 14 is a diagram showing a flowchart for G-Vectoring
control and sideslip prevention control of the present
invention.
[0031] FIG. 15 shows charts indicating time series data during
fused control of G-Vectoring and sideslip prevention of the present
invention.
[0032] FIG. 16 shows charts indicating time series data during
fused control of G-Vectoring and sideslip prevention of the present
invention.
[0033] FIG. 17 is a diagram showing control effects of the present
invention observed in a "g-g" diagram.
[0034] FIG. 18 is a diagram showing a situation where a mountainous
area in a snowy region is being traveled.
[0035] FIG. 19 is a diagram showing a situation where a slope is
being descended.
[0036] FIG. 20 is a diagram showing a longitudinal acceleration
feedback loop of the present invention.
[0037] FIG. 21 is a diagram showing a situation where a bumpy road
is traveled.
[0038] FIG. 22 shows charts indicating changes in steer response
due to changes in road surface characteristics.
[0039] FIG. 23 is a diagram showing a control selector and
multi-information display of the present invention.
LIST OF REFERENCE NUMERALS
[0040] 0 Vehicle [0041] 1 Motor [0042] 2 Driving force distribution
mechanism [0043] 7 Power steering [0044] 10 Accelerator pedal
[0045] 11 Brake pedal [0046] 16 Steering [0047] 21 Lateral
acceleration sensor [0048] 22 Longitudinal acceleration sensor
[0049] 23, 24, 25 Differentiating circuit [0050] 31 Accelerator
sensor [0051] 32 Brake sensor [0052] 33 Steering angle sensor
[0053] 38 Yaw rate sensor [0054] 40 Central controller [0055] 44
Steering controller [0056] 46 Power train controller [0057] 48
Pedal controller [0058] 51 Accelerator reaction motor [0059] 52
Brake reaction motor [0060] 53 Steering reaction motor [0061] 61
Left front wheel [0062] 62 Right front wheel [0063] 63 Left rear
wheel [0064] 64 Right rear wheel [0065] 70 Millimeter wave vehicle
ground speed sensor [0066] 81 Control selector [0067] 82
Multi-information display [0068] 121 Left front wheel motor [0069]
122 Right front wheel motor [0070] 451, 452 Brake controller
BEST MODES FOR CARRYING OUT THE INVENTION
[0071] The overall configuration of an embodiment of a vehicle
motion control system of the present invention is shown in FIG.
1.
[0072] In the present embodiment, a vehicle 0 is of a so-called
by-wire system, and there is no mechanical link between the driver
and the steering mechanism, acceleration mechanism and deceleration
mechanism.
<Driving>
[0073] The vehicle 0 is a four-wheel-drive vehicle (All Wheel
Drive: AWD vehicle) that drives a left rear wheel 63 and a right
rear wheel 64 with a motor 1, while driving a left front wheel 61
with a left front wheel motor 121, and a right front wheel 62 with
a right front wheel motor 122. A driving force distribution
mechanism 2 capable of freely distributing the torque of the motor
across the left and right wheels is so mounted as to be connected
with the motor 1. Differences in power source, e.g., electric
motors, internal combustion engines, etc., are not particularly
relevant to the present invention. As a most suitable example
representing the present invention, and by being combined with the
later-discussed four-wheel independent brake, the configuration is
such that the driving forces and braking forces of the four wheels
are freely controllable. The configuration is presented in detail
below.
[0074] The left front wheel 61, the right front wheel 62, the left
rear wheel 63, and the right rear wheel 64 are each equipped with a
brake rotor, a wheel speed sensing rotor, and, on the vehicle-side,
a wheel speed pickup, thereby providing a configuration that allows
the wheel speed of each wheel to be sensed. Then, the amount by
which the driver steps on an accelerator pedal 10 is sensed by an
accelerator position sensor 31, and is processed at a central
controller 40, which is a vehicle motion control system, via a
pedal controller 48. This processing includes therein torque
distribution information that is in accordance with "sideslip
prevention control" as an object of the present invention. Then, in
accordance with this amount, a power train controller 46 controls
the outputs of the motor 1, the left front wheel motor 121, and the
right front wheel motor 122. In addition, the output of the motor 1
is distributed across the left rear wheel 63 and the right rear
wheel 64 at the optimal ratio via the driving force distribution
mechanism 2, which is controlled by the power train controller
46.
[0075] An accelerator reaction motor 51 is also connected to the
accelerator pedal 10, and reactions are controlled by the pedal
controller 48 based on a computed command of the central controller
40.
[0076] It is noted that the central controller 40, which is a
vehicle motion control system, is a vehicle motion control system
capable of independently controlling the driving forces and braking
forces of the four wheels.
<Braking>
[0077] The left front wheel 61, the right front wheel 62, the left
rear wheel 63, and the right rear wheel 64 are each equipped with a
brake rotor, and, on the vehicle-side, a caliper that decelerates
the wheel by pinching the brake rotor with pads (not shown). The
caliper is hydraulic, or electric with an electric motor for each
caliper.
[0078] Each caliper is controlled by a brake controller 451 (for
the front wheels) or 452 (for the rear wheels) based generally on a
computed command of the central controller 40.
[0079] A brake reaction motor 52 is also connected to the brake
pedal 11, and reactions are controlled by the pedal controller 48
based on a computed command of the central controller 40.
<Integrated Control of Braking and Driving>
[0080] With the present invention, braking forces and driving
forces that differ between the left and right wheels would be
generated based on sideslip angle information, however, what
contributes as a yaw moment is the difference between the left and
right braking forces or driving forces. Accordingly, in order to
create this difference, there may be unordinary operations, such as
driving one side while braking the other. An integrated control
command under such circumstances is such that a command is
determined in an integrated manner by the central controller 40 and
appropriately controlled via the brake controllers 451 (for the
front wheels) and 452 (for the rear wheels), the power train
controller 46, the motor 1, and the driving force distribution
mechanism 2.
<Steering>
[0081] The steering system of the vehicle 0 is of a steer-by-wire
structure where there is no mechanical link between the driver's
steering angle and the tire turning angle. It comprises power
steering 7, which includes therein a steering angle sensor (not
shown), steering 16, a driver steering angle sensor 33, and a
steering controller 44. The amount by which the driver steers the
steering 16 is sensed by the driver steering angle sensor 33, and
is processed at the central controller 40 via the steering
controller 44. In accordance with this amount, the steering
controller 44 then controls the power steering 7.
[0082] A steer reaction motor 53 is also connected to the steering
16, and reactions are controlled by the steering controller 44
based on a computed command of the central controller 40.
[0083] The amount by which the driver steps on the brake pedal 11
is sensed by a brake pedal position sensor 32, and is processed at
the central controller 40 via the pedal controller 48.
<Sensors>
[0084] A group of motion sensors of the present invention are next
discussed. With respect to sensors that measure the motion of the
vehicle in the present embodiment, there are provided an absolute
vehicle speed meter, a yaw rate sensor, an acceleration sensor,
etc. In addition to the above and at the same time, vehicle speed
and yaw rate are estimated with a wheel speed sensor, yaw rate and
lateral acceleration are estimated using vehicle speed, steering
angle, and vehicle motion models, and so forth.
[0085] The vehicle 0 is equipped with a millimeter wave vehicle
ground speed sensor 70, and longitudinal speed V.sub.x and lateral
speed V.sub.y may be sensed independently. In addition, the wheel
speed of each wheel is inputted to the brake controller 451 or 452
as mentioned above. Based on the wheel speeds of the four wheels,
the absolute vehicle speed may be estimated by averaging the wheel
speeds of the front wheels (non-driven wheels). With respect to the
present invention, the configuration is such that the absolute
vehicle speed (V.sub.x) is measured accurately, even in cases where
all four wheels decrease in wheel speed at the same time, by using
the method disclosed in JP Patent Application Publication (Kokai)
No. 5-16789 A (1993) and adding signals of the wheel speeds and of
an acceleration sensor that senses the vehicle's longitudinal
acceleration. In addition, it also incorporates a feature that
estimates the yaw rate of the vehicle body by obtaining the
difference between the left and right wheel speeds of the front
wheels (non-driven wheels), thereby improving the robustness of
sensing signals.
[0086] These signals are then constantly monitored within the
central controller 40 as shared information. The configuration is
such that the estimated absolute vehicle speed is compared and
referenced against the signal of the millimeter wave vehicle ground
speed sensor 70, and each complements the other if some anomaly
were to occur in either of the signals.
[0087] As shown in FIG. 1, the lateral acceleration sensor 21, the
longitudinal acceleration sensor 22, and the yaw rate sensor 38 are
disposed near the center of gravity. In addition, there are
provided differentiating circuits 23 and 24 which obtain jerk
information by differentiating the outputs of their respective
acceleration sensors. There is further provided a differentiating
circuit 25 for obtaining a yaw angular acceleration signal by
differentiating the sensor output of the yaw rate sensor 38.
[0088] In the present embodiment, in order to make the presence of
the differentiating circuits clear, each sensor is shown to be
provided with one. However, in reality, acceleration signals may be
inputted directly to the central controller 40, and differentiation
processes may be performed after various processes have been
performed. Thus, using the yaw rate estimated based on the
above-mentioned wheel speed sensors, the yaw angular acceleration
of the vehicle body may be obtained by performing a differentiation
process within the central controller 40.
[0089] In addition, although acceleration sensors and
differentiating circuits are used in order to obtain jerk, the jerk
sensor disclosed in JP Patent Application No. 2002-39435 may be
used instead.
[0090] In addition, the present embodiment also employs a method of
estimating lateral acceleration and lateral jerk. A method of
estimating lateral acceleration estimated value G.sub.ye and
lateral jerk estimated value G.sub.ye.sub.--dot based on steering
angle .delta. is discussed using FIG. 2.
[0091] First, with respect to a vehicle lateral motion model, with
steering angle .delta. [deg] and vehicle speed V [m/s] as input,
yaw rate r during a steady circular turn disregarding dynamic
characteristics is calculated as follows (Eq. 2).
[ Eq . 2 ] r = 1 1 + AV 2 V I .delta. ( Eq . 2 ) ##EQU00002##
[0092] With respect to the equation above, stability factor A and
wheel base 1 are parameters unique to the vehicle, and are
empirically derived constant values. In addition, lateral
acceleration G.sub.y of the vehicle may be represented through the
following equation, (Eq. 3), where V is the vehicle speed,
.beta._dot the rate of change in the vehicle's sideslip angle, and
r the yaw rate.
[Eq. 3]
G.sub.y=V({dot over (.beta.)}+r).apprxeq.Vr( {dot over
(.beta.)}=.beta._dot) (Eq. 3)
[0093] .beta._dot is a motion within the linear range of tire
force, and is a quantity that may be omitted as being negligible.
Here, lateral acceleration G.sub.ye-wod is calculated by
multiplying yaw rate r, for which dynamic characteristics have been
disregarded as mentioned earlier, by vehicle speed V. This lateral
acceleration does not take into account the dynamic characteristics
of the vehicle having response lag characteristics in the low
frequency region. This is for the following reason. In order to
obtain lateral jerk information G.sub.y.sub.--dot of the vehicle,
time-discrete differentiation needs to be performed on lateral
acceleration G.sub.y.
[0094] In so doing, the noise component of the signal is
reinforced. In order to use this signal for control, it has to be
passed through a low-pass filter (LPF), which would, however, cause
a phase lag. As such, a decision was made to derive jerk by
employing a method where acceleration with an earlier phase than
the actual acceleration, and for which dynamic characteristics are
disregarded, is calculated, and passed through an LPF with time
constant T.sub.lpfe after undergoing discrete differentiation.
Another way of looking at the above is that the lag caused by the
LPF represents the dynamic characteristics of lateral acceleration,
and the acceleration thus derived is simply differentiated. Lateral
acceleration G.sub.y is also passed through an LPF with the same
time constant T.sub.lpf. This would be equivalent to having dynamic
characteristics imparted to acceleration as well, and, although not
shown in the drawings, it has been confirmed that in the linear
rage, actual acceleration response is favorably represented.
[0095] A method in which lateral acceleration and lateral jerk are
thus calculated using the steering angle is advantageous in that
the influence of noise is suppressed, while reducing the response
lag of lateral acceleration and lateral jerk.
[0096] However, since this estimation method omits sideslip
information of the vehicle and ignores non-linear characteristics
of tires, should the sideslip angle become significant, it would be
necessary to measure and utilize the actual lateral acceleration of
the vehicle.
[0097] FIG. 3 shows a method of obtaining lateral acceleration
G.sub.ys and lateral jerk information G.sub.ys.sub.--dot for
control using sensed signal G.sub.yso of the lateral acceleration
sensor 21. Since it contains noise components, e.g., bumps on the
road surface, etc., the sensor signal also needs to be passed
through a low-pass filter (time constant T.sub.lpfs) (not for
dynamics compensation).
[0098] In order to balance the above-discussed respective merits of
estimating and of measuring lateral acceleration and jerk, in the
present embodiment, a method is employed where both signals are
used in a complementary fashion as shown in FIG. 4. An estimated
signal (indicated with the index "e" for "estimated") and a sensed
signal (indicated with the index "s" for "sensed") are to be
multiplied by a gain, which varies based on sideslip information
(sideslip angle .beta., yaw rate r, etc.), and added.
[0099] This variable gain K.sub.je (where K.sub.je<1) with
respect to lateral jerk estimated signal G.sub.ye.sub.--dot is so
varied as to assume a greater value in a region where the sideslip
angle is small, and to assume a smaller value as sideslip
increases.
[0100] In addition, variable gain K.sub.js (where K.sub.js<1)
with respect to lateral jerk sensed signal G.sub.ys.sub.--dot is so
varied as to assume a smaller value in a region where the sideslip
angle is small, and to assume a greater value as sideslip
increases.
[0101] Similarly, variable gain K.sub.ge (where K.sub.ge<1) with
respect to lateral acceleration estimated signal G.sub.ye is so
varied as to assume a greater value in a region where the sideslip
angle is small, and to assume a smaller value as sideslip
increases. In addition, variable gain K.sub.gs (where
K.sub.gs<1) with respect to lateral acceleration sensed signal
G.sub.ys is so varied as to assume a smaller value in a region
where the sideslip angle is small, and to assume a greater value as
sideslip increases.
[0102] By adopting such a configuration, there is provided a
configuration in which noise is low in regions ranging from the
normal region where the sideslip angle is small and up to the
critical region where sideslip has become significant, and in which
acceleration and jerk signals suitable for control may be obtained.
It is noted that these gains are determined through a sideslip
information function or map.
[0103] A system configuration and a method of estimating lateral
acceleration and lateral jerk according to the first embodiment of
the present invention have thus far been discussed (and these are
incorporated as logic within the central controller 40).
Hereinbelow, a "longitudinal acceleration/deceleration control
command coordinated with lateral motion" and a "yaw moment control
command calculated derived from sideslip information of the
vehicle" are discussed.
<Longitudinal Acceleration/Deceleration Control Command
Coordinated with Lateral Motion: G-Vectoring>
[0104] An outline of acceleration/deceleration control coordinated
with lateral motion is, for example, presented in Non-Patent
Document 1.
[0105] It is basically a simple control rule where lateral jerk
G.sub.y.sub.--dot is multiplied by gain C.sub.xy, and a value to
which a first-order lag is imparted is taken to be a longitudinal
acceleration/deceleration control command. It is confirmed in
Non-Patent Document 2 that a lateral and longitudinal motion
coordinated control strategy of an expert driver may thus be
partially simulated.
[0106] G.sub.x.sub.DC in (Eq. 1) is a deceleration component that
is not coordinated with lateral motion (an
acceleration/deceleration command that is inputted by the driver or
automatically based on external information). It is a term that is
required in cases where there is anticipatory deceleration when a
corner is ahead or where there is a zone speed command. It is noted
that longitudinal acceleration/deceleration control command
G.sub.xc is equivalent to target longitudinal
acceleration/deceleration control command G.sub.xt.
[0107] Further, the sgn (signum) term is a term provided so that
the operation above may be attained with respect to both right
corners and left corners. Specifically, an operation may be
attained where deceleration is performed at turn-in upon starting
steering, deceleration is suspended once at steady turn (since
lateral jerk becomes zero), and acceleration is performed upon
starting to ease steering when exiting the corner.
Accelerating/decelerating in accordance with lateral jerk may be
construed as decelerating when lateral acceleration increases and
accelerating when lateral acceleration decreases.
[0108] Further, drawing on (Eq. 2) and (Eq. 3), it may also be
construed to mean that the vehicle decelerates when the steering
angle increases, and that the vehicle accelerates when the steering
angle decreases.
[0109] When thus controlled, with respect to a diagram whose
horizontal axis represents the longitudinal acceleration of the
vehicle and the vertical axis the lateral acceleration of the
vehicle, the combined acceleration (denoted by G) of longitudinal
acceleration and lateral acceleration is so oriented (vectored) as
to transition in a curved manner with the passage of time. It is
therefore called "G-Vectoring control."
[0110] Vehicle motion with respect to a case where the control of
(Eq. 1) is applied is described assuming a specific case of
traveling. FIG. 5 assumes a common travel scene where a corner is
entered and exited, namely, straight road A, transition zone B,
steady turn zone C, transition zone D, and straight zone E. In this
case, it is assumed that no acceleration/deceleration operation is
performed by the driver.
[0111] In addition, FIG. 6 shows charts where steering angle,
lateral acceleration, lateral jerk, longitudinal
acceleration/deceleration control command as calculated through
(Eq. 1), and the braking forces/driving forces of the four wheels
(61, 62, 63, 64) are represented as time history waveforms. With
respect to the front outer wheel (62 in the case of a left turn)
and the front inner wheel (61), as well as the rear outer wheel
(64) and rear inner wheel (63), braking forces and driving forces
are so distributed as to assume the same value between the left and
the right (the inner side and the outer side). This will be
discussed in detail later.
[0112] The term braking force/driving force is used herein to
collectively refer to forces of the respective wheels that are
generated in the vehicle longitudinal direction, where braking
force is defined as a force in a direction that decelerates the
vehicle, and driving force as a force in a direction that
accelerates the vehicle.
[0113] First, the vehicle enters the corner from straight road zone
A. In transition zone B (point 1 to point 3), as the driver
gradually increases steering, the vehicle's lateral acceleration
G.sub.y increases. Lateral jerk G.sub.ydot assumes a positive value
while lateral acceleration is increasing near point 2 (and returns
to zero at 3, at which point lateral acceleration ceases to
increase).
[0114] In this case, according to (Eq. 1), as lateral acceleration
G.sub.y increases, a deceleration (G.sub.xc is negative) command is
generated with respect to the controlled vehicle. In accordance
therewith, braking forces (with a minus sign) of generally the same
magnitude would be applied to the front outer, front inner, rear
outer, and rear inner wheels.
[0115] Then, as the vehicle enters steady turn zone C (point 3 to
point 5), the driver stops increasing steering, thereby maintaining
a constant steering angle. In so doing, since lateral jerk
G.sub.y.sub.--dot becomes 0, longitudinal acceleration/deceleration
control command G.sub.xc becomes 0. Accordingly, the braking
forces/driving forces on the wheels also become zero.
[0116] Next, in transition zone D (points 5 to 7), due to the
driver's easing of steering, the vehicle's lateral acceleration
G.sub.y decreases. At this point, the vehicle's lateral jerk
G.sub.y.sub.--dot is negative, and, according to (Eq. 1),
longitudinal acceleration/deceleration control command G.sub.xc is
generated with respect to the controlled vehicle. In accordance
therewith, driving forces (with a plus sign) of generally the same
magnitude would be applied to the front outer, front inner, rear
outer, and rear inner wheels.
[0117] Further, in straight zone E, lateral jerk G.sub.y becomes 0,
and lateral jerk G.sub.y.sub.--dot also becomes zero. Consequently,
no acceleration/deceleration control is performed. Thus,
deceleration takes place from turn-in (point 1) upon starting
steering up to the clipping point (point 3), deceleration is
suspended during the steady circular turn (point 3 to point 5), and
acceleration takes place from when the easing of steering starts
(point 5) up to when the corner is exited (point 7). Thus, by
applying G-Vectoring control to the vehicle, the driver would be
able to attain an acceleration/deceleration motion that is
coordinated with lateral motion by simply performing steering to
make a turn.
[0118] In addition, when this motion is represented in a "g-g"
diagram depicting the acceleration mode occurring with respect to
the vehicle, where the horizontal axis represents longitudinal
acceleration and the vertical axis lateral acceleration, a
characteristic motion that transitions in a smooth and curved
fashion is observed. This signifies that the longitudinal
acceleration/deceleration control command is so determined as to
transition in a curved fashion in the diagram with the passage of
time. With respect to left corners, this curved transition would be
a clockwise transition as shown in the diagram. With respect to
right corners, the transition path is inverted about the G.sub.x
axis, and its transition direction becomes anti-clockwise. When a
transition occurs in this manner, the pitching motion that is
generated with respect to the vehicle due to longitudinal
acceleration is favorably coordinated with the rolling motion that
is generated due to lateral acceleration, and peak values for the
roll rate and pitch rate are reduced.
<Yaw Moment Control Command>
[0119] Next, yaw moment control based on left/right wheel driving
force/braking force distribution is briefly presented using the
drawings. FIG. 7 shows schematic diagrams depicting a situation
where a yaw moment in an in turning direction (positive) is
inputted with respect to an anti-clockwise turn steady state (A) of
the vehicle 0. First, a lateral motion equation and yawing
(rotating) motion equation for the vehicle 0 in the steady state
are provided below as (Eq. 4) and (Eq. 5).
[Eq. 4]
mG.sub.y=F.sub.yf+F.sub.yr (Eq. 4).
[Eq. 5]
M=I.sub.z{dot over (r)}=0=F.sub.yfl.sub.f-F.sub.yrl.sub.r( {dot
over (r)}_dot) (Eq. 5).
where m is the mass of the vehicle 0, G.sub.y the lateral
acceleration exerted on the vehicle 0, F.sub.yf the lateral force
of the two front wheels, F.sub.yr the lateral force of the two rear
wheels, M the yaw moment, I.sub.Z the yawing moment of inertia of
the vehicle 0, r_dot the yaw angular acceleration of the vehicle 0
(r being the yaw rate), l.sub.f the distance between the center of
gravity of the vehicle 0 and the front axle, and l.sub.r the
distance between the center of gravity of the vehicle 0 and the
rear axle. During a steady circular turn, the yawing motion
balances out (the yaw moment is zero), and angular acceleration
becomes zero.
[0120] From this state, (B) is an example where a brake is applied
only to the inner rear wheel (the left rear wheel 63) thereby
imparting braking force (F.sub.xrl), (C) is an example where, in
addition to the above, a brake is also applied to the inner front
wheel thereby imparting braking force (F.sub.xfl), and (D) is an
example where, in addition to (C), driving forces (F.sub.fxr,
F.sub.xrr) are imparted to the outer front and rear wheels. In this
case, the yawing moment of (Eq. 6) below would act on the vehicle
0.
[ Eq . 6 ] M d = d 2 { ( F xfr + F xrr ) - ( F xfl + F xrl ) } = d
2 ( F xr - F xl ) > 0 ( Eq . 6 ) ##EQU00003##
[0121] In the equation above, forces in the forward direction,
i.e., the driving direction, are defined as being positive, and
forces in the braking direction negative, where d represents the
distance (tread) between the left and right wheels. Further, the
combined braking force/driving force of the left front and rear
wheels is denoted by F.sub.xl, and the combined braking
force/driving force of the right front and rear wheels by
F.sub.xr.
[0122] Similarly, FIG. 8 shows a distribution of braking
forces/driving forces that generates a negative direction, i.e.,
clockwise (restoring side), yaw moment when there is a negative
moment, that is, when a left turn is being made. In this case, too,
the equation for the yawing motion would be (Eq. 6).
[0123] With respect to the vehicle 0, since it is possible to
freely generate braking and driving forces for each of the four
wheels via commands by the central controller 40, both positive and
negative yaw moments may be generated.
[0124] With the present invention, it is assumed that, when the
vehicle's measured longitudinal acceleration G.sub.x and lateral
acceleration G.sub.y are indicated in a diagram where the positive
direction and negative direction of the horizontal axis
respectively represent the vehicle's acceleration and deceleration,
and where the positive direction and negative direction of the
vertical axis respectively represent the vehicle's leftward lateral
acceleration and rightward acceleration, if target yaw moment
M.sub.t is a clockwise value as viewed from above the vehicle,
either a greater decelerating force is imparted to the left wheels
relative to the right wheels or a greater driving force is imparted
to the right wheels relative to the left wheels, whereas if target
yaw moment M.sub.t is an anti-clockwise value as viewed from above
the vehicle, either a greater decelerating force is imparted to the
right wheels relative to the left wheels or a greater driving force
is imparted to the left wheels relative to the right wheels.
[0125] In addition, it is assumed that, when the vehicle's measured
longitudinal acceleration G.sub.x and lateral acceleration G.sub.y
are indicated in a diagram where the positive direction and
negative direction of the horizontal axis respectively represent
the vehicle's acceleration and deceleration, and where the positive
direction and negative direction of the vertical axis respectively
represent the vehicle's leftward lateral acceleration and rightward
acceleration, the longitudinal acceleration/deceleration is
determined in accordance with the lateral motion in such a manner
that a clockwise curved transition would be observed with the
passage of time when starting an anti-clockwise motion as viewed
from above the vehicle, whereas an anti-clockwise curved transition
would be observed with the passage of time when starting a
clockwise motion as viewed from above the vehicle.
[0126] Next, with respect to a specific case of traveling, the
application of such yaw moment control to "sideslip prevention" is
described including an overview of the operating conditions
thereof. With respect to a travel scene where a corner is entered
and exited, namely, straight road A, transition zone B, steady turn
zone C, transition zone D, and straight zone E, FIG. 9 shows the
results of performing "sideslip prevention control" in a situation
where, as provided below, "understeer" and "oversteer" occur to
cause the vehicle to sideslip and fall off course.
[0127] Using the three yaw rates and the sideslip angle in FIG. 10,
a brief description is provided with respect to the judgment of
"understeer" and "oversteer." FIG. 10 shows charts where the
steering angle, the yaw rates including estimated values and the
estimated vehicle sideslip angle, which are to be used for
"sideslip prevention control" intervention conditions, the yaw
moment command derived from the above, the braking forces/driving
forces of the four wheels (61, 62, 63, 64), and the vehicle
longitudinal acceleration and lateral acceleration in this case are
represented as time history waveforms.
[0128] First, yaw rate r.sub..delta. derived from steering is
calculated through (Eq. 2) using stability factor A, wheel base 1,
vehicle speed V, and steering angle .delta.. Since it takes the
driver's steering angle as input, it may be said that it best
reflects the driver's intention.
[0129] Next, yaw rate r.sub.Gy derived from lateral acceleration is
calculated by omitting, as in (Eq. 3), sideslip angle change
.beta._dot to obtain (Eq. 7), and dividing lateral acceleration by
vehicle speed.
[ Eq . 7 ] r G y = G y V ( Eq . 7 ) ##EQU00004##
[0130] This value may be thought of as an indication of the
vehicle's orbital speed, and may be thought of as a quantity
indicating a vehicle turn limit.
[0131] Further, yaw rate r.sub.s sensed by the yaw rate sensor 38
indicates the actual rotating speed of the vehicle.
[0132] While sideslip angle .beta. is by definition obtained by
calculating arctan(v/u) using the vehicle's longitudinal speed u
and the vehicle's lateral speed v, it may be thought of as the
angle formed between the vehicle and the travel direction. By way
of example, the arrows passing through the vehicle's center of
gravity in FIG. 7 and FIG. 8 indicate the vehicle's travel
direction, and the angle formed between that and the vehicle's
longitudinal direction is the sideslip angle, where the
anti-clockwise direction with respect to the vehicle fixed
coordinate system is taken to be positive. FIG. 7 shows a state
where the sideslip angle is negative and significant, and where
oversteer.fwdarw.spin is induced. Conversely, FIG. 8 shows a state
where the sideslip angle is positive and significant, and where
understeer.fwdarw.k path departure is induced.
[0133] Sideslip angle .beta..sub..delta. derived from steering may
be calculated as in (Eq. 8) below using a vehicle motion model.
[ Eq . 8 ] .delta. = 1 - m 2 l l f l r K V 2 1 + AV 2 l r l .delta.
( Eq . 8 ) ##EQU00005##
[0134] where m is the vehicle mass, and Kr the cornering stiffness
representing the lateral force gain per unit sideslip angle of the
rear wheels.
[0135] The sideslip angle may be calculated through (Eq. 9) below
by independently sensing longitudinal speed V.sub.x and lateral
speed V.sub.y with the millimeter wave vehicle ground speed sensor
70, or an integration method such as (Eq. 10) may be used.
[ Eq . 9 ] .beta. = arctan ( V y V x ) ( Eq . 9 ) [ Eq . 10 ]
.intg. .beta. . t = .intg. ( G y V x - r ) t ( Eq . 10 )
##EQU00006##
[0136] Using yaw rate r.sub..delta. derived from steering, yaw rate
r.sub.Gy derived from lateral acceleration, yaw rate r.sub.s sensed
with the yaw rate sensor 38, sideslip angle .beta..sub..delta.
derived from steering, and sideslip angle .beta. derived from
sensed or estimated values, (1) "sideslip prevention control"
intervention conditions and (2) yaw moment control amount are
determined using a method similar to that disclosed in JP Patent
Application Publication (Kokai) No. 09-315277 A (1997).
(1) Intervention Conditions
[0137] The yaw rate derived from lateral acceleration is compared
with the actual yaw rate, and it is determined to be understeer
when the actual yaw rate is smaller, and oversteer when greater,
and, further, oversteer when the sideslip angle is negative and
large. The threshold, dead zone, etc., for the above are adjusted
through sensory tests on test drivers, etc.
(2) Yaw Moment Control Amount
[0138] A yaw moment is generally applied in such a manner that the
yaw rate and sideslip angle derived from steering would be close to
the actual values. Further, the sideslip angle derivative value,
etc., are multiplied by a gain that has been so adjusted as to feel
natural, and corrections are made using their sum.
[0139] The occurrence of understeer and oversteer in the present
embodiment, and "sideslip prevention control" with respect thereto
will now be presented using FIG. 10. First, at positions 2 and 3 in
transition zone B upon entering the corner, there is a possibility
that understeer may occur and that the vehicle may deviate from the
course. This may be sensed from the fact that actual yaw rate
r.sub.s is less than yaw rate r.sub.GY derived from lateral
acceleration. As such, a yaw moment command in the in turning
direction (positive) is calculated. Then, in the present
embodiment, a braking force is generated with respect to the left
(inner) rear wheel, thereby applying a moment in the in turning
direction (positive). Due to this braking force, as indicated by
the longitudinal acceleration in FIG. 10 (second from the bottom),
deceleration with a profile similar to that of the rear inner wheel
braking force would be at work.
[0140] In addition, in steady turn zone C, in a maximum lateral
acceleration state, the equivalent cornering stiffness of the rear
wheels drops relatively, and oversteer occurs, thereby creating a
situation likely to trigger spinning. This may be sensed from the
fact that actual yaw rate r.sub.s is greater than yaw rate r.sub.Gy
derived from lateral acceleration, and it further may be sensed
from the fact that the sideslip angle has exceeded .beta..sub.th,
which is the threshold. In order to restore the excess yawing
motion, in the present embodiment, a braking force is generated
with respect to both the right (outer) front wheel and rear wheel,
thereby applying a clockwise moment. Due to this braking force, as
indicated by the longitudinal acceleration in FIG. 10 (second from
the bottom), deceleration with a profile similar to that of the sum
of the braking forces for the front outer wheel and the rear outer
wheel would be at work.
[0141] Braking forces are distributed among the front outer wheel
(62 in the case of a left turn), the front inner wheel (61), the
rear outer wheel (64), and the rear inner wheel (63) so as to
assume different values between the left and the right (inside and
outside) only when there exists a yaw moment command.
[0142] By thus controlling braking forces (driving forces) so as to
assume different values between the left and the right, it is
possible to attain yaw moment control for preventing vehicle
sideslip, thereby ensuring vehicle maneuverability (tunability) and
stability. However, as shown in FIG. 10, in this case, deceleration
would occur depending on the occurrence of sideslip. Naturally,
since a change in speed, etc., would also occur, fluctuation would
occur in the lateral acceleration even if the handle is steered
smoothly as in FIG. 10.
[0143] When this motion is depicted in a "g-g" diagram indicating
the acceleration mode occurring with respect to the vehicle, where
the horizontal axis represents longitudinal acceleration and the
vertical axis lateral acceleration, anti-clockwise loops would
occur at two places between 1 and 5 as shown in the lower part of
FIG. 9. As such, the pitching motion and the rolling motion would
be asynchronous, resulting in a jerky motion as compared to the
motion under G-Vectoring control in FIG. 5. It would be, so to
speak, an acceleration/deceleration motion that is not coordinated
with the lateral motion caused by driver input.
[0144] This is why a sense of speed loss and an unnatural feel
would be caused. With respect to such problems, the present
invention automatically performs acceleration/deceleration in
coordination with steering operations and which operates from the
normal driving region (G-Vectoring), and seeks to fuse control in
which sideslip is reliably reduced in the critical driving region
(sideslip prevention control), thereby causing less of an unnatural
feel and enabling an improvement in safety performance. A specific
control system configuration and method are disclosed below.
<Fusion of G-Vectoring Control and "Sideslip Prevention
Control">
[0145] FIG. 11 schematically shows the relationship between a
processing control logic of the central controller 40 and an
observer that estimates the sideslip angle based on the vehicle 0,
a group of sensors and signals from the sensors (although processed
within the central controller 40). The logic as a whole generally
comprises a vehicle lateral motion model 401, a G-Vectoring
controller unit 402, a yaw moment controller unit 403, and a
braking force/driving force distribution unit 404.
[0146] Using (Eq. 2), (Eq. 3) or (Eq. 8), the vehicle lateral
motion model 401 estimates the estimated lateral acceleration
(G.sub.ye), target yaw rate r.sub.t, and target sideslip angle
.beta..sub.t based on steering angle .delta. that is inputted from
the driver steering angle sensor 33 and on vehicle speed V. In the
present embodiment, the settings are such that target yaw rate
r.sub.t would be equal to yaw rate r.sub..delta. mentioned above
which is derived from steering.
[0147] With respect to the lateral acceleration and lateral jerk to
be inputted to the G-Vectoring controller 402, which is the first
processing unit, a logic 410 that uses both signals in a
complementary fashion as shown in FIG. 4 is adopted. The logic 410
is a logic that calculates lateral acceleration and lateral jerk
based on the estimated lateral acceleration (G.sub.ye) that has
been estimated, and the actual lateral acceleration that has
actually been measured.
[0148] Using the lateral acceleration and lateral jerk mentioned
above and in accordance with (Eq. 1), the G-Vectoring controller
402 determines, of target longitudinal acceleration/deceleration
control command G.sub.Xt, the component that is coordinated with
the present vehicle lateral motion. Further, G.sub.x.sub.--.sub.DC,
which is the deceleration component that is not coordinated with
the present vehicle lateral motion, is added to calculate target
longitudinal acceleration/deceleration control command G.sub.Xt,
which is then outputted to the braking force/driving force
distribution unit 404.
[0149] In this case, G.sub.x.sub.--.sub.DC is a term that is
required in cases where there is anticipatory deceleration when a
corner is ahead or where there is a zone speed command. The zone
speed command is information that is determined by the coordinates
at which the host vehicle lies. It may therefore be determined by
matching coordinate data obtained with a GPS, etc., against map
information in which zone speed commands are listed. Next, as for
anticipatory deceleration with respect to a corner ahead, although
details of the sensing will be omitted in the present embodiment,
it may be attained by a method in which, by way of example,
information on what lies ahead of the host vehicle, e.g., monocular
or stereo cameras, laser or millimeter wave ranging radars, GPS
information, etc., is taken in, and in which
acceleration/deceleration is performed in accordance with future
lateral motion (lateral jerk) that has not yet become apparent.
Using a path with respect to forward gaze distance and time, and
deviation information with respect to anticipated host vehicle
arrival position, a future steering angle is estimated in a manner
similar to a so-called "driver model" that determines steering
angles. Then, by performing G-Vectoring as in (Eq. 1) in accordance
with future lateral jerk that is likely to be caused with respect
to the vehicle due to this steering operation (Preview
G-Vectoring), it becomes possible to perform anticipatory
deceleration with respect to a corner ahead.
[0150] Next, with respect to the yaw moment controller 403, which
is the second processing unit, in accordance with a logic such as
that mentioned earlier, target yaw moment M.sub.t is calculated
based on respective deviations .DELTA.r and .DELTA..beta. between
target yaw rate r.sub.t (r.sub..delta.) and the actual yaw rate,
and between target sideslip angle .beta..sub.t and the actual (or
estimated) sideslip angle, which is then outputted to the braking
force/driving force distribution unit 404.
[0151] The braking force/driving force distribution unit 404
determines the braking forces/driving forces (F.sub.xfl, F.sub.xfr,
F.sub.xrl, F.sub.xrr) for the four wheels of the vehicle 0 based on
target longitudinal acceleration/deceleration control command
G.sub.xt or on target yaw moment M.sub.t. In the following, a basic
distribution rule will first be presented. In addition to the
above, the effects of indirect yaw moment control (IYC), which is
characteristic of the "G-Vectoring" control of the present
invention will be described generally. Characteristic points to be
careful of with respect to braking force/driving force distribution
will be discussed.
[0152] First, using FIG. 12, motion equations for longitudinal
motion, lateral motion, and yawing motion are considered. In order
to improve the clarity of the equations, with respect to braking
force/driving force and tire lateral force, the force for two
wheels are redefined as in (Eq. 11), (Eq. 12), (Eq. 13), and (Eq.
14) below.
[Eq. 11]
F.sub.xr=F.sub.xfr+F.sub.xrr (Eq. 11)
[Eq. 12]
F.sub.xl=F.sub.xfl+F.sub.xrl (Eq. 12)
[Eq. 13]
F.sub.yf=F.sub.yfl+F.sub.yfr (Eq. 13)
[Eq. 14]
F.sub.yr=F.sub.yrl+F.sub.yrr (Eq. 14)
[0153] which result in (Eq. 15), (Eq. 16), and (Eq. 17) below.
<Longitudinal Motion>
[0154] [Eq. 15]
mG.sub.xt=F.sub.xl+F.sub.xr (Eq. 15)
<Lateral Motion>
[0155] [Eq. 16]
mG.sub.y=F.sub.yf+F.sub.yr (Eq. 16)
<Yawing Motion>
[0156] [ Eq . 17 ] I z r . = ( l f F yf - l r F y r ) + d 2 ( F xr
- F xl ) ( Eq . 17 ) ##EQU00007##
[0157] Further, a description regarding the target yaw moment and
braking forces/driving forces for the respective wheels would be as
in (Eq. 18) below.
[ Eq . 18 ] M t = d 2 ( F xr - F xl ) ( Eq . 18 ) ##EQU00008##
[0158] In this case, by linking the longitudinal motion (Eq. 15)
and the yawing moment (Eq. 18), they may be analytically solved as
in (Eq. 19) and (Eq. 20) below with two unknowns and two
equations.
[ Eq . 19 ] F xr = m 2 G xt + M t d ( Eq . 19 ) [ Eq . 20 ] F xl =
m 2 G xt - M t d ( Eq . 20 ) ##EQU00009##
[0159] Thus, it was possible to attain a distribution for the
braking force/driving force of the two right front and rear wheels
and for the braking force/driving force of the two left front and
rear wheels where the longitudinal acceleration/deceleration
control command based on "G-Vectoring control" and the moment
command based on "sideslip prevention control" are simultaneously
attained. Next, these are distributed across the front and rear
wheels in accordance with the front and rear wheel vertical load
ratio. Assuming now that h is the height of the sprung center of
gravity of the vehicle 0 relative to the ground, and that the
vehicle 0 is accelerating/decelerating due to target longitudinal
acceleration/deceleration control command G.sub.xt, then the loads
(W.sub.f, W.sub.r) for the respective two wheels at the front and
the rear would respectively be given by (Eq. 21) and (Eq. 22)
below.
[ Eq . 21 ] W f = mgl r - mhG xt l ( Eq . 21 ) [ Eq . 22 ] W r =
mgl f + mhG xt l ( Eq . 22 ) ##EQU00010##
[0160] Thus, the braking forces/driving forces for the four wheels
distributed in accordance with the load ratio would be given by
(Eq. 23), (Eq. 24), (Eq. 25), and (Eq. 26) below.
[ Eq . 23 ] F xfr = gl r - hG xt gl ( m 2 G xt + M t d ) ( Eq . 23
) [ Eq . 24 ] F xfl = gl r - hG xt gl ( m 2 G xt - M t d ) ( Eq .
24 ) [ Eq . 25 ] F xrr = gl f + hG xt gl ( m 2 G xt + M t d ) ( Eq
. 25 ) [ Eq . 26 ] F xrl = gl f + hG xt gl ( m 2 G xt - M t d ) (
Eq . 26 ) ##EQU00011##
[0161] However, (Eq. 27) and (Eq. 28) below hold true
[ Eq . 27 ] G xt = - sgn ( G y G . y ) C xy 1 + Ts G . y + G x_DC (
Eq . 27 ) [ Eq . 28 ] M t = M ( r .delta. , r G y , r s , .beta. t
, .beta. s ) ( Eq . 28 ) ##EQU00012##
[0162] The details of (Eq. 28) are calculated using a method
similar to that disclosed in JP Patent Application Publication
(Kokai) No. 09-315277 A (1997).
[0163] The above is a basic distribution rule of the present
invention. Looking at (Eq. 23) through (Eq. 26), it may be
construed that when the "G-Vectoring" control command value (target
longitudinal acceleration/deceleration control command G.sub.xt) is
zero, the yaw moment command based on "sideslip prevention control"
is distributed in accordance with the static loads on the front and
rear wheels, whereas when "G-Vectoring" control command value
G.sub.xt is not zero, the braking forces and driving forces for
attaining that longitudinal acceleration are distributed across the
front and the rear at the load distribution ratio with the left and
right wheels being identical in value so as not to generate any
excess moment.
[0164] With the central controller 40, which is a vehicle motion
control system of the present invention, fusion and decoupling of
"G-Vectoring control," which works from the normal region, and a
"sideslip prevention system," which works in the critical region,
become necessary.
[0165] When vehicle motion is considered as motion in a plane, it
may be described in terms of (1) longitudinal motion, (2) lateral
motion, and rotation about the center of gravity, that is, (3)
yawing motion. "G-Vectoring control," which attains
acceleration/deceleration that is coordinated with lateral motion,
controls (1) longitudinal acceleration/deceleration, and does not
directly control (3) the yawing moment. In other words, the yawing
moment is "arbitrary" and has some degree of freedom.
[0166] In addition, the "sideslip prevention system" directly
controls the (3) yaw moment, and does not control (1)
acceleration/deceleration. In other words, longitudinal
acceleration/deceleration is "arbitrary" and has some degree of
freedom.
[0167] Accordingly, in order to attain fusion of these controls,
one may control (1) longitudinal acceleration in accordance with an
acceleration/deceleration control command coordinated with lateral
motion that is determined by "G-Vectoring control" and control (3)
yawing moment in accordance with a yaw moment command determined by
the "sideslip prevention control system."
[0168] Specifically, a system is configured so as to have the
following two modes.
[0169] (1) In the normal region where sideslip is not pronounced,
braking forces/driving forces that are generally the same are
generated with respect to the left and right wheels based on a
"G-Vectoring control" command (first mode).
[0170] (2) As sideslip increases, braking forces/driving forces
that differ between the left and the right are generated based on a
yaw moment command determined through "sideslip prevention control"
(second mode).
[0171] Then, when a state of the second mode is entered, if, for
example, the longitudinal acceleration caused by the braking
forces/driving forces of the four wheels differs from the
longitudinal acceleration command determined through "G-Vectoring
control," the braking forces/driving forces to be applied to the
vehicle in order to generate that difference acceleration are
calculated, and values obtained by evenly distributing them may be
added to the left and right wheels. Thus, it is possible to attain
the commanded acceleration/deceleration while maintaining the
commanded yawing moment (attaining fusion and decoupling of the two
controls).
[0172] In other words, the present invention is able to provide a
vehicle drive control system comprising: a first mode (G-Vectoring
control), in which, based on longitudinal acceleration/deceleration
control command G.sub.xc that is coordinated with the lateral
motion of the vehicle, driving forces and braking forces that are
generally the same are generated with respect to the left and right
wheels among the four wheels thereof; and a second mode (sideslip
prevention control), in which different driving forces and braking
forces are generated with respect to the left and right wheels
among the four wheels based on target yaw moment M.sub.t derived
from the vehicle's sideslip information (steering angle .delta.,
vehicle speed V, yaw rate r, and sideslip angle .beta.), wherein
the vehicle drive control system causes less of an unnatural feel
and enables an improvement in safety performance by being of a
configuration where the first mode is selected when target yaw
moment M.sub.t is equal to or less than pre-defined threshold
M.sub.th, and where the second mode is selected when the target yaw
moment is greater than the threshold.
[0173] In addition, for example, in the case of two-wheel drive, or
if the yaw moment is to be controlled through brake control only,
there may be cases where the desired driving force cannot be
generated. In such cases, the configuration is made to be such that
safety is ensured by prioritizing "sideslip prevention control,"
and reliably generating a moment.
[0174] Regarding the fusion of "G-Vectoring control" and "sideslip
prevention control" with respect to the present invention, there is
one more point that should be considered, and that is the indirect
yaw moment control (IYC) effect that stems from the load dependence
of tire lateral force. This effect will be described generally
using FIG. 13. It is assumed, for purposes of brevity, that l.sub.f
(the distance from the center of gravity to the front axle) and
l.sub.r (the distance from the center of gravity to the rear axle)
are equal. In other words, it is assumed that the front and rear
wheel loads of the front wheels and rear wheels at rest are
equal.
[0175] As shown in FIG. 13, tire lateral force is proportional to
tire sideslip angle when the sideslip angle is small, and has
saturation properties when the sideslip angle is large. Since it is
assumed that the loads on the front and rear wheels are equal, the
same lateral force would be generated for the same sideslip angle.
Assuming now that the vehicle 0 decelerates based on "G-Vectoring"
control value G.sub.xt, the front wheel load increases as indicated
in (Eq. 21), and the rear wheel load decreases as indicated in (Eq.
22). As a result, if deceleration occurs while turning, lateral
force F.sub.yf of the front wheels increases, while lateral force
F.sub.yr of the rear wheels decreases. Considering this phenomenon
based on the yawing motion equation of (Eq. 17), an in turning
moment would be at work. In addition, if acceleration occurs while
turning, a yaw moment on the restoring side would be at work as
shown in the lower part of FIG. 13.
[0176] With respect to "G-Vectoring" control that is coordinated
with lateral motion, as lateral acceleration increases, that is, as
turning is started, deceleration occurs, thereby causing a yaw
moment in the direction for in turning. In addition, as lateral
acceleration decreases, that is, as turning is finished,
acceleration occurs, thereby causing a yaw moment in the direction
for restoring turning and heading straight ahead. The above
indicate that they both have potential for improving
maneuverability and stability.
[0177] If a yaw moment for "sideslip prevention control" were to be
applied to such "G-Vectoring control," there is a possibility that
failure may be caused due to excess control amounts. By way of
example, this may occur when a yaw moment for understeer prevention
control is inputted from the perspective of "sideslip prevention
control" upon entering a corner, and "G-Vectoring" control is
further applied thereto, and so forth. Another concern is that the
control amount for understeer prevention may become too large,
thereby going beyond neutral steer to become oversteer. A method of
avoiding such situations will be described generally using the
flowchart in FIG. 14.
[0178] First, vehicle speed V, yaw rate r, lateral acceleration
G.sub.y, lateral jerk G.sub.y.sub.--dot, sideslip angle .beta., and
sideslip angular speed .beta._dot are sensed or estimated (step
(1)), and target longitudinal acceleration/deceleration control
command G.sub.xt that is based on G-Vectoring control rules and
that is coordinated with lateral motion is calculated (step (2)).
Further, (1) intervention conditions and (2) a control amount, that
is, target yaw moment M.sub.t, are calculated in such a manner as
to reduce the vehicle's sideslip (step (3)). A short description
will now be added with respect to target yaw moment M.sub.t. When
sideslip occurs with respect to the vehicle, sideslip also occurs
with respect to the front wheels and the rear wheels. As is
well-known, under such circumstances, there is generated a
cornering force that is substantially proportional to cornering
stiffness (unit: N/rad), which represents a tire's lateral
stiffness. A combined moment of a yaw moment on the in turning
side, which may be expressed as the product of the cornering force
generated by the front wheels and the distance from the vehicle's
center of gravity to the front axle, and of a yaw moment on the
turn stopping side, which may be expressed as the product of the
cornering force generated by the rear wheels and the distance from
the vehicle's center of gravity to the rear axle, is a restoring
yaw moment that naturally occurs with respect to the vehicle when
sideslip occurs. Accordingly, if the target yaw moment command is
equal to or less than the restoring yaw moment, it would naturally
converge to a state with little sideslip without having to apply
any yaw moment control. If control were to be applied under such
conditions, it would create a subjective impression of overcontrol
for the driver. In order to avoid such a phenomenon, a method is
adopted where yaw moment control is not performed at or below a
threshold, which is the restoring yaw moment unique to the vehicle.
With existing sideslip prevention systems, test drives are
repeatedly performed by test drivers, and this dead zone is set
based on feeling evaluations. In other words, target yaw moment
command M.sub.t calculated in step (3) represents a specific
control command value for a situation where control is required and
as a value in or above the dead zone (if a value simply obtained
from sideslip information is in or below the dead zone, M.sub.t is
made to be zero). This yaw moment command is a basic yaw moment
command for a case where no acceleration/deceleration is taking
place.
[0179] Next, in step (4), a determination is made as to whether or
not there is a longitudinal acceleration/deceleration control
command. First, a case where there is a longitudinal
acceleration/deceleration control command, that is, a case where a
transition to step (5) is made, will be discussed. In step (5),
control rules are changed based on the magnitude of target yaw
moment M.sub.t. First, a comparison between target yaw moment
M.sub.t and M.sub.th, which is a pre-defined threshold, is made,
and it is determined whether to perform yaw moment control where
the braking and driving forces of the left and right wheels are
distributed individually ((7) through (10)), or to perform only
G-Vectoring (5) where generally equal braking and driving forces
are distributed between the left and right wheels.
[0180] As discussed above, although the restoring yaw moment for
determining the dead zone may be set roughly based on tire
characteristics and vehicle specifications, tire characteristics
are dependent on load as was discussed in connection with FIG. 13.
Accordingly, taking into consideration the
acceleration/deceleration state by G-Vectoring control, which has
potential for improving maneuverability and stability, the
restoring yaw moment varies in an equivalent manner from moment to
moment, and the required yaw moment control amount becomes even
less. As such, in the present embodiment, threshold M.sub.th is
defined with G-Vectoring control effects taken into consideration
[see Non-Patent Document 1] based on the load dependence
coefficient of the tires. A comparison is then made with the
absolute value of basic yaw moment command M.sub.t determined in
step (3), and the configuration is such that if M.sub.t is equal to
or less than M.sub.th, acceleration/deceleration control is
performed at an even distribution between the left and right wheels
by G-Vectoring control (step (6)).
[0181] Thus, in the present embodiment, as shown in FIG. 14, the
configuration is such that it includes a logic where, when
G-Vectoring control is active, the left/right distribution of
braking forces and driving forces is not performed unless the yaw
moment control amount exceeds a given threshold. Consequently, when
the yaw moment command is small, it operates in the first mode
(G-Vectoring (step (6)), and when the yaw moment command is large,
it operates in the second mode (side slip prevention control (steps
(7) through (10)).
[0182] In addition, the vehicle longitudinal acceleration attained
in the second mode (sideslip prevention control) in which different
braking forces and/or driving forces are generated with respect to
the left and right wheels among the four wheels is correctively
controlled in such a manner that braking forces and/or driving
forces that are substantially equal are applied to the left and
right wheels among the four wheels so that the difference with
respect to the longitudinal acceleration/deceleration control
command of (G-Vectoring) becomes narrower (see also step (9) and
(Eq. 23) through (Eq. 26)).
[0183] However, when other embodiments where brake/drive
distribution is not at one's disposal, e.g., only brake control is
performed with respect to an ordinary two-wheel-drive vehicle (N in
step (8)), etc., are considered, the vehicle longitudinal
acceleration attained in the second mode (sideslip prevention
control) in which different braking forces and/or driving forces
are generated with respect to the left and right wheels among the
four wheels does not necessarily coincide with the longitudinal
acceleration/deceleration control command of (G-Vectoring).
[0184] By way of example, if brake control were to be performed
when the G-Vectoring command is zero, deceleration would inevitably
occur (step (10)). However, when the G-Vectoring control command is
greater than the deceleration caused by the sideslip prevention
control command, corrective control may be performed in such a
manner as to apply substantially equal braking forces and/or
driving forces to the left and right wheels among the four wheels
so that the difference with respect to the G-Vectoring control
command would be narrower. As such, there exists a scene where a
problem of the present invention is solved, and it therefore falls
within the scope of the present invention.
[0185] In sum, the present invention is such that it is determined
whether or not target longitudinal acceleration/deceleration
control command G.sub.xt is zero. If target longitudinal
acceleration/deceleration control command G.sub.xt is not zero and
target yaw moment M.sub.t is equal to or less than pre-defined
threshold M.sub.th, the braking forces/driving forces (F.sub.xfl,
F.sub.xfr, F.sub.xrl, F.sub.xrr) of the respective wheels of the
vehicle are calculated at the braking force/driving force
distribution unit 404 based on target longitudinal
acceleration/deceleration control command G.sub.xt in such a manner
that the braking forces/driving forces of the left and right wheels
would be distributed in a substantially even manner. In addition,
the configuration is such that, it is determined whether or not
target longitudinal acceleration/deceleration control command G, is
zero, and if target longitudinal acceleration/deceleration control
command G.sub.xt is zero, or if target longitudinal
acceleration/deceleration control command G.sub.xt is not zero and
target yaw moment M.sub.t is greater than pre-defined threshold
M.sub.th, the braking forces/driving forces (F.sub.xfl, F.sub.xfr,
F.sub.xrl, F.sub.xrr) of the respective wheels of the vehicle are
calculated at the braking force/driving force distribution unit 404
based on target yaw moment M.sub.t in such a manner that the
braking forces/driving forces of the left and right wheels would be
distributed individually.
[0186] Finally, effects of the present invention will be described
using FIG. 15, FIG. 16, and FIG. 17. FIG. 15, FIG. 16, and FIG. 17
are examples in which the present invention is applied to the scene
shown in FIG. 9 and FIG. 10 where only "sideslip prevention
control" is applied. In addition, although the locations at which
"understeer" and "oversteer" occur in FIG. 16 are the same as those
in FIG. 10 and FIG. 15, there is assumed a case in which there is
less fluctuation in the steer characteristics.
[0187] FIG. 15 shows a longitudinal acceleration/deceleration
control command, yaw moment control command, and brake/drive
distribution of the respective wheels that are determined in
accordance with the lateral motion that occurs in accordance with
the steering angle, and the vehicle yaw moment, vehicle
longitudinal acceleration, and vehicle lateral acceleration that
are brought about thereby. In this case, the yaw moment commands
for reducing the understeer at locations 2 to 3 and the oversteer
at locations 4 to 5 are values with greater absolute values than
control activating threshold M.sub.th in step (5) in FIG. 14
("sideslip prevention control" is active). In the charts
representing the braking forces and driving forces of the
respective wheels, the dotted line is the longitudinal
acceleration/deceleration control command of "G-Vectoring" control
only, and the dashed line is the deceleration amount based on the
yaw moment command of "sideslip prevention control." It can be seen
that, through braking force/driving force distribution to which
(Eq. 23) through (Eq. 26) of the present invention are applied,
braking forces are applied to the four wheels over the course of
locations 1 to 3, causing an in turning moment. It can further be
seen that, at location 2 and onward, a significant braking force is
applied only to the rear inner wheel, the braking forces of the
other wheels are reduced, and longitudinal acceleration follows the
"G-Vectoring" control command as net acceleration/deceleration,
while the yaw moment demanded by "sideslip prevention control" is
also attained. In addition, it can be seen that, at locations 4 to
5, the braking forces of the front outer wheel and rear outer wheel
are reduced, driving forces are imparted to the front inner wheel
and rear inner wheel, and vehicle longitudinal acceleration follows
the "G-Vectoring" control command while yaw moment follows the
"sideslip prevention control" command.
[0188] Likewise with respect to FIG. 16 for which a case is assumed
where there is less fluctuation in the steer characteristics, a yaw
moment command for reducing understeer occurs across locations 2 to
3. However, since there is a longitudinal acceleration/deceleration
control command while the yaw moment command is less than threshold
M.sub.th, left/right wheel independent braking control/driving
control is omitted (the same braking force for the left and right
wheels, step (6) in FIG. 14). In contrast, at locations 4 to 5,
there is shown an example where "sideslip prevention control" is
active because, although the yaw moment command is less than
threshold M.sub.th, there is no longitudinal
acceleration/deceleration control command by "G-Vectoring," and no
load shift occurs among the front and rear wheels (transition from
step (4) to step (7) in FIG. 14). It can be seen that because the
braking forces/driving forces of the respective wheels are
calculated using (Eq. 23) through (Eq. 26) of the present
invention, even in this state, vehicle longitudinal acceleration
follows the "G-Vectoring" control command, and yaw moment follows
the "sideslip prevention control" command.
[0189] By having the braking forces and driving forces of the four
wheels controlled as in FIG. 15 and FIG. 16, it is possible to
attain a characteristic motion that transitions in a smoothly
curved fashion in a "g-g" diagram like "G-Vectoring" control while
performing yaw moment control for "sideslip prevention" as shown in
FIG. 17. With respect to left corners, this curved transition would
be a clockwise transition as shown in the diagram, and for right
corners, the transition path becomes inverted about the G.sub.x
axis, and its transition direction becomes anti-clockwise. When
transitions occur in this manner, the pitching motion that occurs
with respect to the vehicle due to longitudinal acceleration
coordinates favorably with the rolling motion that occurs due to
lateral acceleration, and the peak values for the roll rate and
pitch rate are reduced. It can be seen that a technique and system
that cause less of an unnatural feel and enable an improvement in
safety performance are successfully realized where
acceleration/deceleration that is coordinated with steering
operations and that is advantageous in the normal driving region is
thus performed automatically, and where sideslip is reliably
reduced in the critical driving region.
[0190] It is of course necessary to consider situations in which
the system or the driver issues deceleration commands such as when
a vehicle ahead stops abruptly, or when information is received
that there is an obstacle on the road. In such situations, it is
necessary that these commands be reflected with utmost priority.
This may be done through system input at the part where
G.sub.x.sub.DC is added in the logic diagram in FIG. 11.
[0191] Up to this point, a situation in which the vehicle travels
along a plane without any bumps has been assumed, and a technique
and system that cause less of an unnatural feel and enable an
improvement in safety performance have been disclosed where
acceleration/deceleration that is coordinated with steering
operations and that is advantageous in the normal driving region is
performed automatically, and where sideslip is reliably reduced in
the critical driving region. Specifically, a method of controlling
the braking forces/driving forces of the respective wheels in such
a manner that the vehicle motion follows both the longitudinal
acceleration command and the yaw moment command has been disclosed
assuming a situation in which the vehicle travels along a plane
without any bumps.
[0192] Next, assuming a situation where a vehicle of the present
invention is traveling a mountainous area in a snowy region as
shown in FIG. 18, a more practical use situation of the present
system is presented, and the content of what has been devised to
solve practical control problems for obtaining control effects
similar to those in a situation where the vehicle travels along a
plane without any bumps will now be disclosed.
[0193] In situations like the one in FIG. 18, the following
practical control problems arise.
(1) Change in vehicle longitudinal acceleration due to gravity
component based on grade (2) Sense of jerkiness in
acceleration/deceleration control stemming from lateral
acceleration caused by road surface bump input (3) Change in steer
response due to change in road surface characteristics
[0194] With respect to each of the above, problems will be
clarified and solutions with regard to the present invention will
be disclosed.
(1) Change in Vehicle Longitudinal Acceleration Due to Gravity
Component Based on Grade
[0195] Assuming that the vehicle weight is M, when a slope having
an angle of grade .theta. is descended as shown in FIG. 19, a
gravity component of M.sub.gsin .theta. would be applied to the
vehicle in the longitudinal direction. If, with respect to
longitudinal acceleration/deceleration control command G.sub.xc,
front wheel longitudinal force F.sub.xff and rear wheel
longitudinal force F.sub.xrr were to be controlled by performing
brake fluid pressure control or motor torque control, etc., with an
open-loop, the actual vehicle deceleration, as opposed to the
deceleration command value, would become
G.sub.x=G.sub.xc-M.sub.gsin .theta., making it impossible to
perform the intended control. In contrast to the above, as shown in
FIG. 20, actual longitudinal acceleration G.sub.x may be measured
with the longitudinal acceleration sensor 22 and be multiplied by
gain K1, or be differentiated to find the longitudinal jerk, and
the value obtained by multiplying it with gain K2 may be compared
with target longitudinal acceleration/deceleration control command
G.sub.xt, and braking forces/driving forces F.sub.xff and F.sub.xrr
may be determined based on deviation .DELTA.G.sub.x thereof. The
"s" in K2s is the Laplace operator, and feeds back a partial
derivative to improve response.
(Gain K2 in this Case is Intended to Improve Control Readiness, and
is not an Essential Feature)
[0196] Further, if contemplating a system without the longitudinal
acceleration sensor 22, the actual acceleration of the vehicle may
be measured using, for example, derivatives of the wheel speed, and
grade estimation may be performed.
[0197] By configuring such a feedback loop, it is possible to have
the actual longitudinal acceleration follow the target longitudinal
acceleration regardless of such disturbances as grade, etc., and
control degradation may be reduced.
[0198] Thus, even in a state where a road surface with a grade is
being traveled, a motion that is in accordance with the target
acceleration/deceleration control command may be attained, and
control effects similar to those in situations where the vehicle is
traveling along a plane without any bumps may be obtained.
(2) Sense of Jerkiness in Acceleration/Deceleration Control
Stemming from Lateral Acceleration Caused by Road Surface Bump
Input
[0199] In cases where, as shown in FIG. 21, the road surface is not
flat and the road shoulder portion is covered with frozen snow,
should the vehicle run onto a bumpy road, the tires on the road
shoulder side would constantly be vibrated by the road surface,
thereby generating small kickback torques in the driver's steering
angle, and steering angle change .DELTA..delta. would occur. In
addition, small rolls would occur to cause lateral acceleration
change .DELTA.G.sub.y at the center of gravity, and noise would
consequently occur in the lateral jerk. If, under such
circumstances, one were to adopt only a method such as that shown
in FIG. 4, a small high-frequency component would be generated in
the longitudinal acceleration as shown in the diagram. In order to
avoid such a situation, the present invention is so configured as
to reduce the sense of jerkiness by using a threshold and not
performing control with respect to longitudinal acceleration
commands that are equal to or less than the threshold. In addition,
although a threshold is defined with respect to the absolute value
of the longitudinal acceleration control command in the present
embodiment, as an idea in which a threshold is employed with
respect to frequency, actual actuator control may also be performed
based on vehicle longitudinal acceleration commands that have been
passed through a low-pass filter that only allows the frequency of
the vehicle's lateral motion (2 Hz at most) through. It would thus
be possible to reduce the sense of jerkiness in the longitudinal
direction even in cases where there are bumps in the road.
(3) Change in Steer Response Due to Change in Road Surface
Characteristics
[0200] When the road surface traction condition changes at, for
example, a snow surface as shown in FIG. 22, discrepancies in
amplitude and in phase occur respectively between the lateral
acceleration and lateral jerk, which are estimated using a vehicle
model with respect to steering angle such as that shown in FIG. 2,
and the lateral acceleration actually measured with the lateral
acceleration sensor 21 and lateral jerk obtained as the time
derivative thereof. In the low traction region, the measured value
is slightly delayed relative to the model estimated value. In the
previous embodiment, as shown in the drawings, a vehicle
longitudinal acceleration command that is coordinated with lateral
motion was formed using (Eq. 1) based on a linearly combined value
obtained by multiplying the lateral jerk that is based on a model
estimated value and the lateral jerk that is based on a measured
value by a gain and summing them up. If a compacted snow road were
to be traveled by a vehicle thus configured, the feel on the hand
at the moment when steering is begun and the turnability would be
compromised. Further, deceleration would drop while the actual
lateral acceleration is still increasing, and the continuity
between the rolling motion and the pitching motion would become
diluted. Under such circumstances, the feel on the hand as well as
a sense of continuity of motion may be simultaneously attained by
choosing the longitudinal acceleration using (Eq. 1) based on a
signal obtained by so-called select-high, where the one with the
greater amplitude is selected from the lateral jerk based on a
model estimated value, which has little response delay with respect
to steering operations, and the measured lateral jerk, which is
coordinated with the vehicle's actual lateral motion. In addition,
the sense of continuity of motion may be further improved by
smoothing the control command obtained by select-high by passing it
through a low-pass filter.
[0201] Thus, there are provided a plurality of modes with distinct
calculation methods for target longitudinal
acceleration/deceleration control command G.sub.xt depending on the
traveled road surface, and there is provided a switching means for
switching between these plurality of modes.
[0202] As shown in FIG. 23, such switching of control modes may be
done by the driver with a control selector 81 (switching means)
installed within the vehicle cabin. AUTOMATIC carries out mode
switching automatically, and is configured in such a manner that
linear combination, select-high, and the respective gains for the
lateral jerk based on a model estimated value and for the lateral
jerk based on a measured value are adjusted in accordance with
changes in road surface conditions, e.g., traction coefficient,
etc. The configuration is such that, by way of example, braking
force/driving force control is performed in accordance with a
longitudinal acceleration command and the actual longitudinal
acceleration is sensed, and if the actual longitudinal acceleration
is significantly less than the command value, it is determined as
being a case where the traction coefficient is small, and
select-high control is automatically chosen, or the gain for the
jerk on the model estimated value side is increased, etc., thereby
improving the feel on the hand. Such mode switching and gain
switching may also be of mapped formats in accordance with
estimated traction coefficients. Thus, in accordance with road
surface conditions, it is possible to automatically obtain good
longitudinal acceleration/deceleration control commands coordinated
with lateral motion. Further, although a detailed description is
omitted in the present embodiment, this AUTOMATIC mode may be
further divided into nimble mode in which the model estimated value
jerk gain is set slightly high so that it would move nimbly in
response to steering, and comfort mode in which the jerk gain based
on measured values is set slightly high to attain a laid-back
motion, and so forth. In addition, external information may be
incorporated to change the gain and mode as emergency avoidance
mode.
[0203] Further, modes other than AUTOMATIC will be briefly
described. These modes are modes that the driver may choose from as
desired.
[0204] TARMAC is intended mainly for use when traveling on dry
paved roads, and a jerk linear combination mode is used. Since it
has high responsiveness of vehicle motion with respect to steering,
the model estimated lateral jerk and the lateral jerk based on
measured values would be roughly the same value. In addition, it is
so configured that the gain of the model estimated lateral jerk and
the gain of the lateral jerk based on measured values would be
roughly the same.
[0205] Next, GRAVEL is intended mainly for use when traveling on
wet roads or dirt roads, and the control threshold indicated in
FIG. 21 is so set as to be slightly high. In addition, although a
linear combination mode is adopted, the configuration is such that
steerage is improved by having the gain of the model estimated
lateral jerk be slightly greater than the gain of the lateral jerk
based on measured values.
[0206] Further, SNOW is intended mainly for use when traveling on
snow roads, and the control threshold indicated in FIG. 21 is so
set as to be slightly high even in comparison to GRAVEL. In
addition, select-high control is adopted, and the gain of the model
estimated lateral jerk is greater than the gain of the lateral jerk
based on measured values. Thus, steerage and continuity of motion
with respect to lateral motion with a delayed response are
ensured.
[0207] Changes in vehicle response that accompany changes in road
surface conditions greatly affect the driver's driving operation,
and the vehicle motion itself also varies significantly as a
result. It becomes important to perform appropriate driving
operations with respect to vehicle response that varies from moment
to moment. With respect to a control system of the present
invention, appropriate driving operations by the driver are
assisted by displaying the control state and the vehicle motion
state on the multi-information display 82 within the vehicle cabin.
As for display modes, there are provided a plurality of modes for
indicating the present vehicle motion state and displaying
reference information to help the driver make driving operation
decisions, such as by indicating a "g-g" diagram where the
horizontal axis represents the vehicle's longitudinal acceleration
and the vertical axis the vehicle's lateral acceleration, or time
series data of acceleration.
[0208] Further, the tire braking forces/driving forces, or the
generated yaw moment, is/are displayed to make the control state
clear, thereby indicating whether the vehicle is currently in a
"G-Vectoring control state" or a "DYC state." The aim here is to
make the control effect with respect to the presently generated
vehicle motion clear, thereby having the vehicle driving operations
by the driver be performed more appropriately. In particular,
"G-Vectoring control" emulates "acceleration/deceleration
operations coordinated with lateral motion" performed by an expert
driver, and does not independently control the braking
forces/driving forces of the four wheels. Accordingly, if the
driver is able to perform comparable acceleration/deceleration
driving operations, a comparable motion may be attained without any
active involvement in the control by the system. It is speculated
that by physically feeling his own driving operations and the
vehicle motions that accompany them, and, further, seeing the
control state on the multi-information display 82, the driver will
more likely be able to perform "G-Vectoring control" on his
own.
[0209] Thus, a control configuration in which
acceleration/deceleration that is coordinated with steering
operations and that is active from the normal driving region is
automatically performed, and in which sideslip in the critical
driving region is reliably reduced has been addressed, as well as
solutions for its problems in practice. With the present invention,
it becomes possible to provide a technique and system that cause
less of an unnatural feel and enable an improvement in safety
performance.
[0210] Further, with respect to emergency avoidance, additional
notes are made below regarding the present invention's superiority
in performance over conventional sideslip prevention systems.
[0211] With conventional sideslip prevention systems, left and
right braking forces or driving forces would be controlled after
sideslip has occurred. With respect to emergency avoidance, if the
driver were to perform an abrupt steering operation in order to
avoid an obstacle ahead, thereby causing understeer, the occurrence
of understeer would first be awaited, and left and right braking
forces would then be applied so as to cause a moment that cancels
the understeer. In other words, between the occurrence of
understeer and its being sensed, there would be a no brake state,
and the vehicle would approach the obstacle. In contrast, with the
present invention, a braking force is generated from the moment
steering is started by the driver, as a result of which the speed
relative to the obstacle is clearly reduced, thereby enabling a
significant improvement in emergency avoidance performance.
[0212] Further, by virtue of an improvement in steer response, the
absolute value of the initial steering angle for performing
avoidance becomes smaller, and not as much easing of steering would
be necessary after avoidance. Thus, a stable avoidance operation
may be attained without voluntarily causing the vehicle response to
become jerky due to steer response delay (similar effects may be
attained when turning a sharp curve as well).
* * * * *