U.S. patent application number 13/602368 was filed with the patent office on 2013-11-07 for system and method for controlling a brake system in a vehicle.
This patent application is currently assigned to FORD GLOBAL TECHNOLOGIES, LLC. The applicant listed for this patent is Dale Scott Crombez, Andy Chuan Hsia, Scott J. Lauffer. Invention is credited to Dale Scott Crombez, Andy Chuan Hsia, Scott J. Lauffer.
Application Number | 20130297164 13/602368 |
Document ID | / |
Family ID | 49513226 |
Filed Date | 2013-11-07 |
United States Patent
Application |
20130297164 |
Kind Code |
A1 |
Lauffer; Scott J. ; et
al. |
November 7, 2013 |
SYSTEM AND METHOD FOR CONTROLLING A BRAKE SYSTEM IN A VEHICLE
Abstract
A method for controlling a brake system in a vehicle includes
using a first brake pedal map when the vehicle has a first load;
the first brake pedal map allows a first predetermined non-friction
braking torque to be reached. The method further includes using a
second brake pedal map allowing a second predetermined non-friction
braking torque, lower than the first predetermined non-friction
braking torque, to be reached. The second brake pedal map is used
when the vehicle has a second load lower than the first load.
Inventors: |
Lauffer; Scott J.;
(Northville, MI) ; Crombez; Dale Scott; (Livonia,
MI) ; Hsia; Andy Chuan; (Ann Arbor, MI) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Lauffer; Scott J.
Crombez; Dale Scott
Hsia; Andy Chuan |
Northville
Livonia
Ann Arbor |
MI
MI
MI |
US
US
US |
|
|
Assignee: |
FORD GLOBAL TECHNOLOGIES,
LLC
Dearborn
MI
|
Family ID: |
49513226 |
Appl. No.: |
13/602368 |
Filed: |
September 4, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
61643669 |
May 7, 2012 |
|
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|
Current U.S.
Class: |
701/70 |
Current CPC
Class: |
B60W 10/18 20130101;
B60Y 2300/89 20130101; B60T 7/042 20130101; B60T 13/585 20130101;
B60T 2270/604 20130101; B60W 2540/12 20130101; B60T 13/662
20130101; B60W 30/18127 20130101; B60T 1/10 20130101 |
Class at
Publication: |
701/70 |
International
Class: |
B60T 8/18 20060101
B60T008/18 |
Claims
1. A method for controlling a brake system in a vehicle comprising:
using a first brake pedal map allowing a first predetermined
non-friction braking torque to be reached when the vehicle has a
first load; and using a second brake pedal map allowing a second
predetermined non-friction braking torque lower than the first
predetermined non-friction braking torque to be reached when the
vehicle has a second load lower than the first load.
2. The method of claim 1, wherein the first and second brake pedal
maps are defined by vehicle braking torque versus brake pedal
input.
3. The method of claim 2, wherein the second brake pedal map
corresponds to a lower set of vehicle braking torques than the
first brake pedal map over a range of brake pedal inputs.
4. The method of claim 1, wherein the vehicle includes a rear-axle
regenerative brake system, the first predetermined non-friction
braking torque corresponding to a maximum desired regenerative
braking torque when the vehicle has the first load, and the second
predetermined non-friction braking torque corresponding to a
maximum desired regenerative braking torque when the vehicle has
the second load.
5. The method of claim 1, further comprising providing the first
predetermined non-friction braking torque at a first position of a
brake pedal, and providing the second predetermined non-friction
braking torque at a second position of the brake pedal equal to or
less than the first position of the brake pedal.
6. The method of claim 1, wherein the first brake pedal map is
represented by a first curve, and the second brake pedal map is
represented by a second curve, the first and second curves being
nonparallel for at least a range of brake pedal positions.
7. The method of claim 1, further comprising choosing an initial
value for the first predetermined non-friction braking torque, and
modifying the initial value of the first predetermined non-friction
braking torque based on a front-to-back distribution of the first
load.
8. The method of claim 7, further comprising choosing an initial
value for the second predetermined non-friction braking torque, and
modifying the initial value of the second predetermined
non-friction braking torque based on a front-to-back distribution
of the second load.
9. The method of claim 8, wherein the steps of modifying the
initial values of the first and second predetermined non-friction
braking torques include reducing the first and second predetermined
non-friction braking torques when the first and second loads are
distributed toward a front of the vehicle.
10. A method for controlling a brake system in a vehicle
comprising: braking the vehicle with at least some non-friction
braking until a first non-friction braking torque is reached when
the vehicle has a first load; and braking the vehicle with at least
some non-friction braking until a second non-friction braking
torque, lower than the first non-friction braking torque, is
reached when the vehicle has a second load lower than the first
load.
11. The method of claim 10, further comprising providing the first
non-friction braking torque at a first position of a brake pedal,
and providing the second non-friction braking torque at a second
position of the brake pedal equal to or less than the first
position of the brake pedal.
12. The method of claim 10, further comprising defining vehicle
braking torque as a first function of brake pedal position when the
vehicle has the first load, and defining the vehicle braking torque
as a second function of the brake pedal position different from the
first function when the vehicle has the second load.
13. The method of claim 12, wherein the first function is
represented by a first curve, and the second function is
represented by a second curve, the first and second curves being
nonparallel for at least a range of brake pedal positions.
14. The method of claim 10, wherein the vehicle includes a
rear-axle regenerative brake system, the first non-friction braking
torque corresponding to a first desired regenerative braking torque
when the vehicle has the first load, and the second non-friction
braking torque corresponding to a second desired regenerative
braking torque when the vehicle has the second load.
15. The method of claim 14, further comprising: choosing respective
initial values for the first and second desired regenerative
braking torques; reducing the initial value of the first desired
regenerative braking torque from its initial value when the first
load is distributed toward a front of the vehicle; and reducing the
initial value of the second desired regenerative braking torque
from its initial value when the second load is distributed toward a
front of the vehicle.
16. A control system for controlling a brake system in a vehicle
comprising: a controller configured to brake the vehicle with at
least some non-friction braking until a first predetermined
non-friction braking torque is reached when the vehicle has a first
load, and to brake the vehicle until a second predetermined
non-friction braking torque, lower than the first predetermined
non-friction braking torque, is reached when the vehicle has a
second load lower than the first load.
17. The control system of claim 16, wherein the vehicle includes a
rear-axle regenerative brake system, the first predetermined
non-friction braking torque corresponding to a maximum desired
regenerative braking torque when the vehicle has the first load,
and the second predetermined non-friction braking torque
corresponding to a maximum desired regenerative braking torque when
the vehicle has the second load.
18. The control system of claim 17, wherein the controller is
further configured to receive inputs related to a front-to-back
distribution of vehicle load, and to reduce the first and second
predetermined non-friction braking torques when the first and
second loads are distributed toward a front of the vehicle.
19. The control system of claim 16, wherein the control system is
further configured to receive inputs corresponding to brake pedal
position, and to control the brake system to: reach the first
predetermined non-friction braking torque at a first position of
the brake pedal, and reach the second predetermined non-friction
braking torque at a second position of the brake pedal equal to or
less than the first position of the brake pedal.
20. The control system of claim 16, wherein the control system is
further configured to output vehicle braking torque: as a first
function of brake pedal position when the vehicle has the first
load, and as a second function of brake pedal position when the
vehicle has the second load, the first function yielding higher
vehicle braking torques than the second function over a range of
pedal positions.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. provisional
patent application No. 61/643,669 filed 7 May 2012, which is hereby
incorporated herein by reference.
TECHNICAL FIELD
[0002] The present invention relates to a system and method for
controlling a brake system in a vehicle.
BACKGROUND
[0003] Vehicles today are increasingly equipped with electric drive
motors, which, in addition to propelling the vehicle, can capture
braking energy to charge a battery. Depending on how the vehicle
powertrain is configured, this process, known as "regenerative
braking", can occur at the front axle, the rear axle, or both.
There are other kinds of non-friction braking, for example, engine
braking, that occur when the compression of the engine provides a
negative torque to the vehicle drive wheels. Where the engine is
only connected to one axle, as in a two-wheel-drive vehicle, or
where the regenerative braking is only available at one axle, there
may be competing interests between trying to brake in such a way as
to maximize non-friction braking, for example, to maximize energy
capture in a regenerative brake system, and more evenly
distributing braking torque between the front and rear wheels to
provide better vehicle handling.
[0004] Adding complexity to the braking control system is
consideration of the vehicle carrying load. This may be of
particular concern with commercial vehicles where the difference
between the loaded weight and unloaded weight is significant. If,
for example, a brake system is configured to maximize non-friction
braking at the rear axle for the fully loaded vehicle, the brake
system may over brake at the rear wheels when the vehicle is
unloaded. In addition, if the brake pedal travel is mapped the same
for the loaded and unloaded conditions, the brake pedal may be "too
sensitive" when the vehicle is in the unloaded condition--i.e., a
very hard braking may occur for a very small amount of pedal
travel. Conversely, if the brake system is configured to maximize
non-friction braking at the rear axle for the unloaded vehicle, the
brake system may not utilize all of the available non-friction
braking--e.g., it may not capture all of the possible regenerative
braking--when the vehicle is loaded. This may be due, in part, to
the lack of sensitivity of the brake pedal, which now may be
depressed so far as to engage the vehicle's friction brakes before
all of the available non-friction braking energy is utilized.
SUMMARY
[0005] At least some embodiments of the invention include a method
for controlling a brake system in a vehicle. The method includes
using a first brake pedal map allowing a first predetermined
non-friction braking torque to be reached when the vehicle has a
first load, and using a second brake pedal map allowing a second
predetermined non-friction braking torque lower than the first
predetermined non-friction braking torque to be reached when the
vehicle has a second load lower than the first load.
[0006] At least some embodiments of the invention include a method
for controlling a brake system in a vehicle. The method includes
braking the vehicle with at least some non-friction braking until a
first non-friction braking torque is reached when the vehicle has a
first load, and braking the vehicle with at least some non-friction
braking until a second non-friction braking torque, lower than the
first non-friction braking torque, is reached when the vehicle has
a second load lower than the first load.
[0007] At least some embodiments of the invention include a control
system for controlling a brake system in a vehicle. The control
system includes a controller configured to brake the vehicle with
at least some non-friction braking until a first predetermined
non-friction braking torque is reached when the vehicle has a first
load, and to brake the vehicle until a second predetermined
non-friction braking torque, lower than the first predetermined
non-friction braking torque, is reached when the vehicle has a
second load lower than the first load.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] FIG. 1 shows a simplified schematic diagram of a vehicle
having a control system in accordance with embodiments of the
present invention;
[0009] FIG. 2 shows a brake distribution chart for a vehicle in a
fully loaded condition;
[0010] FIG. 3 shows a brake distribution chart for the vehicle in
an unloaded condition using the same braking torque control as
shown in FIG. 2;
[0011] FIG. 4 shows a brake distribution chart for the vehicle in
the unloaded condition using a different braking torque
control;
[0012] FIG. 5 shows a brake distribution chart for the vehicle in
the loaded condition using the same braking torque control as shown
in FIG. 4;
[0013] FIG. 6 shows a flowchart illustrating a method in accordance
with embodiments of the present invention;
[0014] FIG. 7 shows a chart illustrating the relationship between
vehicle braking torque and pedal input in accordance with
embodiments of the present invention; and
[0015] FIG. 8 shows additional details of the brake system shown in
FIG. 1.
DETAILED DESCRIPTION
[0016] As required, detailed embodiments of the present invention
are disclosed herein; however, it is to be understood that the
disclosed embodiments are merely exemplary of the invention that
may be embodied in various and alternative forms. The figures are
not necessarily to scale; some features may be exaggerated or
minimized to show details of particular components. Therefore,
specific structural and functional details disclosed herein are not
to be interpreted as limiting, but merely as a representative basis
for teaching one skilled in the art to variously employ the present
invention.
[0017] FIG. 1 shows a simplified schematic diagram of a portion of
a vehicle 10. The vehicle 10 includes a friction brake system 12,
controlled by a brake controller 14, and a non-friction,
regenerative brake system 16, which is part of the vehicle
powertrain. The regenerative brake system 16 includes one or more
electric machines, such as electric motors, which are operable to
provide regenerative braking for the vehicle 10. The regenerative
brake system 16 is controlled by a control system, or vehicle
system controller (VSC) 18, which communicates with the brake
controller 14, for example, through a controller area network
(CAN). The VSC 18 may include other controllers, such as a
powertrain control module (PCM), and in some embodiments, the brake
controller 14 may be integrated into the VSC 18. Thus, a control
system in accordance with embodiments of the present invention may
control various systems within the vehicle 10 by using a single
controller, separate software controllers within a single hardware
device, or a combination of separate software and hardware
controllers.
[0018] The brake controller 14 receives vehicle operator inputs
from a brake pedal 20, and the VSC 18 receives operator inputs from
an accelerator pedal 22. A brake sensor 24 (which can be more than
one sensor), is configured to detect the position of the brake
pedal 20, and send one or more signals to the brake controller 14.
Similarly, an accelerator pedal sensor 26 (which can also be more
than one sensor), is configured to detect the position of the
accelerator pedal 22, and send one or more signals to the VSC 18.
The VSC 18 and the brake controller 14 use various inputs,
including the inputs from the sensors 24, 26, to decide how to
control the friction brake system 12 and the regenerative brake
system 16. The friction brake system 12 operates to slow the speed
of rear vehicle wheels 28 and the front wheels (not shown) through
the application of one or more friction elements in accordance with
methods known in the art. The regenerative brake system 16 is also
operable to reduce the speed of the rear vehicle wheels 28 by
having at least one electric motor produce a negative torque which
is transferred through the powertrain to the rear vehicle wheels
28.
[0019] The friction brake system 12 includes one or more sensors,
represented in FIG. 1 by a single sensor 30. The sensor 30 is
configured to send signals to the brake controller 14 related to
various conditions within the friction brake system 12. For
example, if the friction brake system 12 should experience reduced
braking capability, perhaps due to a loss of boost or the loss of a
hydraulic circuit, the sensor 30 can communicate this condition to
the brake controller 14, which in turn communicates with the VSC
18. Similarly, the regenerative brake system 16 has one or more
sensors, represented in FIG. 1 by the sensor 32. The sensor 32 may
detect such conditions as motor speed, motor torque, power, etc.
The sensor 32 communicates directly with the VSC 18, which can use
these inputs in combination with the other inputs to control the
brake systems 12, 16.
[0020] The vehicle 10 also includes a body/chassis system 34. The
body/chassis system 34 includes structural elements of the vehicle
10, including such things as a vehicle suspension system. The
vehicle wheels 28, shown separately in FIG. 1, may be considered a
part of the larger body/chassis system 34. One or more sensors,
shown in FIG. 1 as a single sensor 36, are configured to detect
various conditions of the body/chassis system 34, and to
communicate with the VSC 18. The sensor 36 may detect such
conditions as the deflection of, or the load on, various elements
of the body/chassis system 34, as well as load distribution.
Similarly, a sensor 38, which represents one or more sensors, is
configured to detect conditions of the vehicle wheels 28, including
the wheel speed. The sensor 38 is shown in FIG. 1 communicating
with the larger body/chassis system 34, which in turn communicates
with the VSC 18. Alternatively, the sensor 38 can be directly
connected to the VSC 18.
[0021] In the embodiment shown in FIG. 1, the regenerative brake
system 16 is a rear-axle regenerative brake system, configured to
capture braking energy from the rear wheels 28 only. Although
embodiments of the invention are described and illustrated in
conjunction with a rear axle, regenerative brake system, other
embodiments may include other types of non-friction braking, such
as engine braking, and may also include front axle or four-wheel
non-friction brake systems. With a regenerative brake system, it is
often desirable to capture as much braking energy as possible,
while not allowing too great a difference in braking distribution
between the front and rear brakes so as to affect vehicle handling.
Toward that end, a controller, such as the VSC 18, can be
programmed to perform a number of steps in accordance with
embodiments of the present invention. Initially, a first
predetermined, non-friction braking torque, which in this
embodiment is a maximum desired regenerative braking torque, for
the vehicle 10 can be provided when the vehicle 10 is carrying a
first load, which, for example, may be a maximum capacity load
conveniently identified by the vehicle's "gross vehicle weight"
(GVW). This value of the braking torque can be "provided" to the
VSC 18 by direct programming, or information can be provided to the
VSC 18 and it can perform an internal calculation.
[0022] FIG. 2 shows a brake distribution chart 40 for the vehicle
10 at the vehicle's GVW, which, for example, may be 3000 kilograms
(kg). The chart 40 illustrates a rear deceleration for the vehicle
10 along the vertical axis, and a front deceleration along the
horizontal axis. The sum of these two decelerations is the total
deceleration for the vehicle 10, which can be easily converted into
a vehicle braking force or a vehicle braking torque because there
is a known relationship between each of these values. Therefore,
providing a vehicle with a desired non-friction braking torque as
discussed above can also be described and illustrated in terms of a
rear deceleration as shown in the chart 40 in FIG. 2.
[0023] The chart 40 shows a number of curves, including an ideal
brake distribution curve 42. The ideal brake distribution curve 42
illustrates a theoretical line along which the front and rear
brakes would lock-up simultaneously. An equal pressure curve 44 is
also illustrated in the chart 40, and represents a line along which
equal pressure is applied to both of the front and the rear brakes.
The ideal brake distribution curve 42 is not coincident with the
equal pressure curve 44, because in practice, a vehicle does not
have an equal weight distribution between the front and rear
wheels. As shown in FIG. 2, the lines 42, 44 cross at point
Z.sub.1, which may be different for different vehicles and
different loading conditions of the same vehicle. A number of equal
deceleration lines 46 are also illustrated in the chart 40, and
indicate lines along which the front and rear wheels of the vehicle
10 are decelerating equally.
[0024] As described above, it may be desirable to optimize
regenerative braking--i.e., to capture as much energy as
possible--while at the same time ensuring that there is not an
undesirable impact on vehicle handling. For any given vehicle, and
vehicle loading condition, the "optimum" amount of regenerative
braking that can be captured from the front, rear or both pairs of
wheels of a vehicle can be estimated. Using the vehicle 10 at GVW
as an example, a maximum desired regenerative braking torque (in
this example for the rear regenerative brake system) is shown in
the chart 40 by the line 48, which generally illustrates the rear
regenerative braking balance for the vehicle 10 at GVW. In the
chart 40, the maximum non-friction braking torque is shown as a
rear deceleration of -2 meters per second squared (m/s 2). For the
vehicle 10, this level of deceleration can be translated into a
deceleration torque of approximately 1700 Newton-meters (Nm). After
reaching this maximum value, the line 48 slopes downward and toward
the right of the chart 40, indicating a combination of front and
rear braking, until the equal pressure curve 44 is reached.
[0025] The slope of the line 48 is generally less than the slope of
the equal deceleration lines 46, and is brought below the ideal
brake distribution curve 42 somewhere at or before the intersection
point Z.sub.1. The specific way in which the maximum rear braking
torque (in this case -2 m/s 2) is chosen, and how the rest of the
brake balance line (or curve) is determined, can be based on any
number of factors a brake system designer wishes to consider. In
the examples of embodiments of the present invention described
herein, the optimum non-friction braking torque--which in this case
coincides with the optimum regenerative braking torque--is chosen
to provide a "maximum desired" amount of regenerative braking while
still providing a required level of vehicle handling. Although the
first part of the curve 48 is vertical, indicating exclusive use of
the regenerative (rear) brakes until a deceleration of 2 m/s 2 is
reached, the initial deceleration may also include some front
braking, as indicated by the line 48', which intersects the sloping
part of the line 48 and follows its path from there. As braking
occurs along the line 48', and along the sloped portion of line 48,
it may be a combination of friction and non-friction braking, or,
in the case where non-friction braking is available at both axles,
it may be exclusively non-friction braking even though both sets of
wheels are braking.
[0026] As discussed above, embodiments of a method of the present
invention may be executed, for example, by the VSC 18. Using
information, for example, from the chart 40 in FIG. 2, the method
may include braking the vehicle 10 with at least some non-friction
braking (such as regenerative braking) until the first
predetermined non-friction braking torque (in this case -2 m/s 2)
is reached. This does not mean that non-friction braking ceases
once the first non-friction braking torque is reached; rather, it
means that non-friction braking is controlled to not exceed this
level (as discussed above, the sloping portion of the line 48 may
include non-friction braking, but along this portion of the line
48, the amount of rear axle braking is being reduced). The first
predetermined non-friction braking torque is based on the vehicle
10 at a first load, which as described above, is its GVW. One of
the reasons that the chosen non-friction braking torque is load
dependent, is because braking conditions change with a vehicle when
it is loaded versus when it is unloaded.
[0027] This is illustrated in FIG. 3 where a braking distribution
chart 50 for the vehicle 10 is shown when it has a second load
lower than the first load; in this case the vehicle 10 is
unloaded--i.e., it is not carrying any payload. Thus, in FIG. 3,
the weight of the vehicle 10 is equal to its "curb weight".
Although the fully loaded GVW weight and the unloaded curb weight
are used in the examples described and illustrated herein, it is
understood that embodiments of the invention may be applied to any
or all of the various loading conditions that may exist between
these two extremes. In the chart 50, the equal deceleration lines
46 and equal pressure curve 44 are the same as in FIG. 2, while the
brake balance curve 52 and ideal brake distribution curve 54 are
different from their counterparts 48, 42 shown in FIG. 2.
[0028] If the same level of braking torque is applied to the
vehicle 10 at its curb weight as was applied at GVW (1700 Nm, see
above), the result is a greater rear deceleration as shown by the
brake balance curve 52 in the chart 50 in FIG. 3. In this example,
the rear deceleration has increased from -2 m/s 2 to --2.8 m/s 2,
as indicated by the label "Overbraking Rear Axle". As discussed
above, this level of rear braking may be undesirable; therefore,
embodiments of the present invention may utilize different
non-friction (in this embodiment, rear) braking torques for
different loading conditions of the same vehicle. This is
illustrated in FIG. 4, which shows a braking distribution chart 56
for the vehicle 10 at the second loading condition, which is its
curb weight. In this example, a different optimum rear braking
torque has been chosen so as to provide the desired vehicle
handling throughout the braking event; this is indicated by the
brake balance curve 58.
[0029] As shown in the chart 56 in FIG. 4, the maximum rear
deceleration is -1.3 m/s 2, which translates into a rear braking
torque of approximately 900 Nm. Therefore, a system and/or method
in accordance with embodiments of the present invention may provide
a second predetermined non-friction braking torque lower than the
first predetermined non-friction braking torque when the vehicle
has a second load lower than the first load. The vehicle 10 is then
braked exclusively with the rear brakes 28 using regenerative
braking until the first predetermined non-friction braking torque
of 1700 Nm is reached when the vehicle 10 is at GVW; however, when
the vehicle 10 is at its curb weight, it is braked exclusively with
the rear brakes 28 using regenerative braking only until a second
predetermined non-friction braking torque of 900 Nm is reached. As
described above, the first and second predetermined non-fiction
braking torques of 1700 Nm and 900 Nm represent first and second
desired regenerative braking torques for the vehicle 10 for the two
different loading conditions, each of which may be maximum desired
values for the respective loading conditions. Although the examples
above rely on exclusive use of the rear brakes until the desired
non-friction braking torque levels are reached, different
embodiments may use a combination of front and rear brakes, such as
described above in conjunction with the braking curve 48' shown in
FIG. 2. Moreover, some non-friction braking may still occur along
the sloping portion of the line 58, but non-friction braking is
controlled to not exceed the second predetermined non-friction
braking torque.
[0030] As described above, embodiments of the present invention
provide two different maximum desired regenerative braking torques
for two different loading conditions of a vehicle, such as the
vehicle 10. Using the maximum desired regenerative (in this case,
rear) braking torque from a fully loaded vehicle for the same
vehicle at a lower load resulted in the undesirable effect of
overbraking the rear axle, which was illustrated and described in
conjunction with FIG. 3. It is similarly undesirable to use the
maximum desired regenerative braking torque provided for the
unloaded condition--such as illustrated and described in FIG.
4--when the vehicle has a higher payload. This is illustrated in
FIG. 5, where a region of lost regenerative braking energy is
labeled "Lost Regen at GVW". This results from abandoning
regenerative braking too soon--i.e., at a braking torque level that
is below the maximum desired level.
[0031] FIG. 6 shows a flowchart 60 summarizing a method and system
in accordance with embodiments of the present invention. At step
62, the process is started, and at step 64 a determination is made
as to load estimation for a vehicle, such as the vehicle 10. This
load estimation comes from inputs 66, for example, to the VSC 18,
that may provide information on the load level and "load quality".
Information such as this can come from, for example, a sensor or
sensors such as the sensor 36 shown in FIG. 1. A sensor that
detects deflection levels of a suspension system is one example of
a load detection sensor. The "load quality" factor may be provided
to give an indication of the accuracy of the sensor itself, or the
accuracy of the particular measurement as it relates to the vehicle
payload--i.e., a weight sensor may provide a higher quality
measurement than a deflection sensor, which must be used in a
calculation to estimate the actual load.
[0032] Next, at step 68, a front-to-back distribution of vehicle
load is determined based on inputs 70 providing a front-to-back
load distribution detection and distribution quality. When a
vehicle load is distributed toward a front of the vehicle, which
may be defined, for example, as in front of the rear axle, or in
front of a center of gravity for the vehicle, it may not be
possible to provide a desired level of braking torque at the rear
axle without having an impact on vehicle handling. Therefore, a
system and method in accordance with embodiments of the present
invention may choose an initial value for the first predetermined
non-friction braking torque, such as illustrated and described in
FIG. 2 for the rear wheels, and may also choose an initial value
for the second predetermined non-friction braking torque, such as
illustrated and described in FIG. 4 (also for the rear wheels).
Then, if it is determined that the first or second loads are
distributed toward a front of the vehicle, the first and second
predetermined non-friction braking torques can be modified such
that they are reduced to a somewhat lower level to account for the
low distribution. Although the "second load" illustrated and
described above was considered a zero payload for the vehicle 10,
the center of gravity of the vehicle at curb weight may be
distributed toward a front of the vehicle, and even in an unloaded
condition, the load distribution may be a factor to consider.
[0033] At step 72, a determination of brake torque level is made
based on brake pedal input, load, and load distribution. To make
such a determination, a controller, such as the VSC 18, may receive
a brake pedal input indicated at 74, for example, from a brake
pedal 20 and sensor 24 shown in FIG. 1. Other inputs are shown at
76 in FIG. 6 which are brake level versus pedal input maps,
described below in conjunction with FIG. 7. The process shown in
FIG. 6 is ended at 78.
[0034] FIG. 7 shows a graph 80, which shows brake level versus
pedal input maps for a vehicle, such as the vehicle 10 under two
different loading conditions: a GVW loading with good distribution
and a minimum loading with poor distribution. The first condition
is illustrated by a first brake pedal map, shown as a first curve
82, while the second condition is illustrated by a second brake
pedal map, shown as a second curve 84. Although the vertical axis
of the graph 80 is labeled as "vehicle braking torque" it is
understood that it could be labeled in terms of braking force or
deceleration as described above. Along the horizontal axis is
"pedal input" which relates to the travel of a brake pedal, such as
the brake pedal 20 shown in FIG. 1; thus, points on the curves 82,
84 at different positions along the pedal input axis represent
different brake pedal positions.
[0035] The curves 82, 84 respectively represent the vehicle braking
torque as first and second functions of the brake pedal position
for the vehicle having first and second loads, such as described
above. These functions can be programmed into the VSC 18 as
formulas, if they can be defined that way, or as data tables that
can be accessed, with certain values output based on certain inputs
received. Thus, the VSC 18 may output vehicle braking torque as
functions of brake pedal position. These functions, and the curves
that represent them, such as the curves 82, 84, can be chosen by a
brake system specialist so that different values of vehicle braking
torque are achieved for different pedal inputs. A curve
representing a maximum load with a poor distribution might appear
below the curve 82, while a curve representing a minimum load with
a good distribution might appear above the curve 84, but below the
second, lower maximum load curve, and these would represent
alternative pedal maps. As shown in FIG. 7, the second brake pedal
map 84 corresponds to a lower set of vehicle braking torques than
the first brake pedal map 82 over a range of brake pedal positions
or pedal inputs.
[0036] As discussed above in conjunction with FIG. 2, the total
vehicle braking torque can be obtained by adding the front and rear
decelerations and converting this sum to a braking torque, which is
shown on the vertical axis in the graph 80 in FIG. 7. Because the
level of pedal input for a particular vehicle braking torque or
deceleration has an impact on driver expectations, it may be
desirable to control the level of pedal input for any given vehicle
braking torque. In addition, as discussed below in conjunction with
FIG. 8, some vehicles may engage friction brakes when the pedal has
traveled a certain distance or angle; therefore, controlling pedal
input versus vehicle braking torque may be important to ensure that
the maximum desired non-friction braking is achieved before the
friction brake system engages or becomes the exclusive braking
mechanism.
[0037] Thus, with regard to the examples described above, a
controller, such as the VSC 18, may provide the first predetermined
non-friction braking torque, or first maximum desired regenerative
braking torque, at a first position of a brake pedal, such as the
brake pedal 20 shown in FIG. 1. The first maximum desired
regenerative braking torque is indicated by point (a) on the curve
82, which corresponds to a pedal position of (d.sub.1). This point
may be chosen, for example, to ensure that the first maximum
desired regenerative braking torque is reached before the friction
brakes are engaged. This is illustrated in FIG. 8, which shows a
gap 86 between the brake pedal 20 and the point of engagement of
the friction brake system 12. Although FIG. 8 is a simplified
schematic drawing, it does illustrate the general relationship
between brake pedal travel and friction brake system engagement
that exists in some vehicles. Therefore, one factor to consider in
providing a brake pedal travel of a certain distance for a certain
vehicle loading condition is the distance allowed before the
friction brake system is engaged.
[0038] The VSC 18 may also provide the second maximum desired
regenerative braking torque, at a second position of the brake
pedal. If it is desired to have the sensitivity of the brake pedal
at the same level or perhaps slightly more sensitive when the
vehicle is in the unloaded condition, the second position of the
brake pedal may be equal to or less than the first position of the
brake pedal. This is illustrated in the graph 80 where the second
maximum desired regenerative braking torque is shown as point
(b.sub.1) on the curve 84, which also corresponds to the first
pedal position (d.sub.1). If, however, a somewhat more sensitive
brake pedal is desired for lower loading conditions, the curve 84
could be adjusted to include the point (b.sub.0) such that the
corresponding pedal position was (d.sub.0). If the first pedal
position (d.sub.1) is chosen to be the same or nearly equal to the
width of the gap 86, then it may not be possible to make the brake
pedal less sensitive for a lower loading condition, otherwise, the
brake pedal may engage the friction brake system before the maximum
desired regenerative braking is achieved.
[0039] As shown in FIG. 7, the curves 82, 84 are not parallel over
most of their range. They begin to approach parallelism at higher
pedal input positions. This is because when the brake pedal is at
or near a bottoming out position, front and rear wheels are braking
simultaneously and the difference between braking during a loaded
and unloaded condition is negligible. Although the curves 82, 84
could be made to be parallel over their entire range, having them
nonparallel as shown in FIG. 7 provides some advantages. For
example, at lower levels of pedal travel, the curve 82 rises more
steeply than the curve 84. This means that for the same change in
pedal travel, a greater vehicle braking torque is applied when the
brake system is controlled in accordance with curve 82. This may
result in similar deceleration regardless of the load; however,
providing substantially similar curves at the higher level may
provide tactile response (vehicle braking torque v. brake pedal
travel) that may be desirable as it provides feedback to the
operator of the load that is being carried.
[0040] As described above, braking control according to the curve
82 is applied when a vehicle has a large load with good
distribution. Therefore, it may be desirable to have a smaller
amount of pedal travel provide a greater increase in braking
torque, as this might be expected by a vehicle operator when it is
known that the vehicle has a large payload. The shapes of the
curves 82, 84, and their underlying functions, can be adjusted to
provide different vehicle braking torque outputs for different
pedal inputs as desired. In each case, however, the curves 82, 84
provide brake pedal maps that allow the maximum desired
regenerative braking torque to be achieved before the friction
brakes are engaged, or at least, before the friction brakes are
exclusively engaged, which could cause less than the maximum
desired regenerative braking to be achieved. Where the inputs are
based on actual measurements and accurate information, the curves
can be more aggressive; whereas, the use of estimates and the
absence of information may require a more conservative braking
control.
[0041] While exemplary embodiments are described above, it is not
intended that these embodiments describe all possible forms of the
invention. Rather, the words used in the specification are words of
description rather than limitation, and it is understood that
various changes may be made without departing from the spirit and
scope of the invention. Additionally, the features of various
implementing embodiments may be combined to form further
embodiments of the invention.
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