U.S. patent application number 09/893483 was filed with the patent office on 2003-01-02 for anti-lock brake control method having adaptive exit criteria.
Invention is credited to Leppek, Kevin Gerard, Walenty, Allen John.
Application Number | 20030004632 09/893483 |
Document ID | / |
Family ID | 25401642 |
Filed Date | 2003-01-02 |
United States Patent
Application |
20030004632 |
Kind Code |
A1 |
Walenty, Allen John ; et
al. |
January 2, 2003 |
ANTI-LOCK BRAKE CONTROL METHOD HAVING ADAPTIVE EXIT CRITERIA
Abstract
An improved anti-lock brake control method adaptively determines
exit criteria for terminating anti-lock brake control based on rate
of brake pedal release and estimates of the brake torque and road
surface coefficient of friction. The brake torque and road surface
coefficient of friction are estimated based on a periodically
updated characterization of the relationship between brake pedal
position and vehicle deceleration.
Inventors: |
Walenty, Allen John;
(Macomb, MI) ; Leppek, Kevin Gerard; (Rochester
Hills, MI) |
Correspondence
Address: |
CHRISTOPHER DEVRIES
General Motors Corporation
Legal Staff, Mail Code 482- C23- B21
P.O. Box 300
Detroit
MI
48265-3000
US
|
Family ID: |
25401642 |
Appl. No.: |
09/893483 |
Filed: |
June 29, 2001 |
Current U.S.
Class: |
701/71 ; 303/150;
701/80 |
Current CPC
Class: |
B60T 8/17636 20130101;
B60T 2220/04 20130101; B60T 2270/602 20130101; B60T 2210/12
20130101 |
Class at
Publication: |
701/71 ; 303/150;
701/80 |
International
Class: |
B60T 008/32 |
Claims
1. A method of operation for a vehicle braking system including a
driver manipulated brake pedal and an ABS system for modulating
vehicle braking when activated based on an ABS command, the method
comprising the steps of: detecting a rate of change in brake pedal
movement when the driver is releasing the brake pedal; biasing the
ABS command toward deactivation of the ABS system in response to
the detected rate of change in brake pedal movement when the ABS
system is activated; deactivating the ABS system if the ABS command
indicates that the ABS system should be deactivated for at least an
exit time interval.
2. The method of claim 1, including the steps of: estimating a road
surface coefficient of friction; and adaptively adjusting said exit
time interval based on the estimated road surface coefficient of
friction.
3. The method of claim 2, wherein the exit time interval is
adjusted in inverse relation to the estimated road surface
coefficient of friction.
4. The method of claim 2, including the steps of: periodically
measuring vehicle deceleration and a brake pedal position during
activation of the braking system; constructing and periodically
updating a brake system characterization table representing a
relationship between the measured vehicle deceleration and measured
brake pedal position; estimating said road surface coefficient of
friction based on the characterization table and changes in the
changes in the characterization table.
5. The method of claim 1, including the steps of: periodically
measuring vehicle deceleration and a brake pedal position during
activation of the braking system; determining a desired vehicle
deceleration based on the measured brake pedal position and the
detected rate of change in brake pedal movement; and deactivating
the ABS system if the ABS command indicates that the ABS system
should be deactivated and the measured deceleration exceeds the
desired vehicle deceleration.
6. The method of claim 5, including the steps of: constructing and
periodically updating a brake system characterization table
representing a relationship between the measured vehicle
deceleration and measured brake pedal position; and determining an
anticipated brake pedal position by decreasing the measured brake
pedal position in relation to the detected rate of change in brake
pedal movement; and determining the desired vehicle deceleration
retrieving a vehicle deceleration from the characterization table
corresponding to the anticipated brake pedal position.
7. The method of claim 1, including the steps of: measuring a brake
pedal position; estimating a brake torque corresponding to the
measured brake pedal position; estimating a road surface
coefficient of friction; determining a maximum braking torque
corresponding to the estimated road surface coefficient of
friction; and deactivating the ABS system if the ABS command
indicates that the ABS system should be deactivated and the
estimated brake torque is less than said maximum braking
torque.
8. The method of claim 7, including the steps of: periodically
measuring vehicle deceleration and a brake pedal position during
activation of the braking system; constructing and periodically
updating a brake system characterization table representing a
relationship between the measured vehicle deceleration and measured
brake pedal position; estimating the brake torque based on the
measured brake pedal position and said characterization table; and
estimating said road surface coefficient of friction based on the
characterization table and changes in the changes in the
characterization table.
9. A method of operation for a vehicle braking system including a
driver-manipulated brake pedal and an ABS system for modulating
vehicle braking when activated based on an ABS command, the method
comprising the steps of: estimating a road surface coefficient of
friction; and adaptively adjusting an exit time interval based on
the estimated road surface coefficient of friction; and
deactivating the ABS system if the ABS command indicates that the
ABS system should be deactivated for at least said exit time
interval.
10. The method of claim 9, including the steps of: periodically
measuring vehicle deceleration and a brake pedal position during
activation of the braking system; detecting a rate of change in
brake pedal movement when the driver is releasing the brake pedal;
determining a desired vehicle deceleration based on the measured
brake pedal position and the detected rate of change in brake pedal
movement; and deactivating the ABS system if the ABS command
indicates that the ABS system should be deactivated and the
measured deceleration exceeds the desired vehicle deceleration.
11. The method of claim 10, wherein including the steps of:
constructing and periodically updating a brake system
characterization table representing a relationship between the
measured vehicle deceleration and measured brake pedal position;
and determining an anticipated brake pedal position by decreasing
the measured brake pedal position in relation to the detected rate
of change in brake pedal movement; and determining the desired
vehicle deceleration retrieving a vehicle deceleration from the
characterization table corresponding to the anticipated brake pedal
position.
12. The method of claim 9, including the steps of: measuring a
brake pedal position; estimating a brake torque corresponding to
the measured brake pedal position; estimating a road surface
coefficient of friction; determining a maximum braking torque
corresponding to the estimated road surface coefficient of
friction; and deactivating the ABS system if the ABS command
indicates that the ABS system should be deactivated and the
estimated brake torque is less than said maximum braking
torque.
13. The method of claim 12, including the steps of: periodically
measuring vehicle deceleration and a brake pedal position during
activation of the braking system; constructing and periodically
updating a brake system characterization table representing a
relationship between the measured vehicle deceleration and measured
brake pedal position; estimating the brake torque based on the
measured brake pedal position and said characterization table; and
estimating said road surface coefficient of friction based on the
characterization table and changes in the changes in the
characterization table.
14. A method of operation for a vehicle braking system including a
driver-manipulated brake pedal and an ABS system for modulating
vehicle braking when activated based on an ABS command, the method
comprising the steps of: periodically measuring vehicle
deceleration and a brake pedal position during activation of the
braking system; detecting a rate of change in brake pedal movement
when the driver is releasing the brake pedal; determining a desired
vehicle deceleration based on the measured brake pedal position and
the detected rate of change in brake pedal movement; and
deactivating the ABS system if the ABS command indicates that the
ABS system should be deactivated and the measured deceleration
exceeds the desired vehicle deceleration.
15. The method of claim 14, including the steps of: estimating a
brake torque corresponding to the measured brake pedal position;
estimating a road surface coefficient of friction; determining a
maximum braking torque corresponding to the estimated road surface
coefficient of friction; and deactivating the ABS system if the ABS
command indicates that the ABS system should be deactivated and the
estimated brake torque is less than said maximum braking
torque.
16. A method of operation for a vehicle braking system including a
driver-manipulated brake pedal and an ABS system for modulating
vehicle braking when activated based on an ABS command, the method
comprising the steps of: measuring a brake pedal position;
estimating a brake torque corresponding to the measured brake pedal
position; estimating a road surface coefficient of friction;
determining a maximum braking torque corresponding to the estimated
road surface coefficient of friction; and deactivating the ABS
system if the ABS command indicates that the ABS system should be
deactivated and the estimated brake torque is less than said
maximum braking torque.
Description
TECHNICAL FIELD
[0001] This invention relates to motor vehicle anti-lock brake
systems, and more particularly to a control method that adaptively
determines exit criteria for terminating anti-lock brake pressure
modulation.
BACKGROUND OF THE INVENTION
[0002] A motor vehicle anti-lock braking system (referred to herein
as an ABS system) repeatedly releases and re-applies hydraulic
brake pressures during conditions of wheel lock to maximize the
tractive force between the vehicle tires and the road surface.
Typically, the control is initiated in response to insipient wheel
lock detection, and is exited when the driver fully releases the
brake pedal or the vehicle deceleration vs. wheel slip operating
point is in an exit region for at least a predetermined exit time
interval. This strategy can unnecessarily delay the termination of
ABS control when the driver only partially releases of the brake
pedal, which is undesirable, particularly when the road surface
coefficient of friction is relatively high. Accordingly, what is
needed is method of exiting ABS control that reduces the exit
delay, consistent with the road surface coefficient of friction,
when the brake pedal is only partially released.
SUMMARY OF THE INVENTION
[0003] The present invention is directed to an improved ABS control
method in which exit criteria for terminating ABS control are
adaptively determined based on rate of brake pedal release and
estimates of the brake torque and road surface coefficient of
friction. In the preferred embodiment, the brake torque and road
surface coefficient of friction are estimated based on a
periodically updated characterization of the relationship between
brake pedal position and vehicle deceleration. In one aspect of the
invention, an exit time interval is adaptively adjusted based on
the estimated road surface coefficient of friction, and a control
value used to determine if ABS control should be exited is
adaptively biased toward exiting ABS control based on the brake
pedal release rate and the estimated brake torque. In another
aspect of the invention, ABS control is exited independent of the
timer when the control value indicates that ABS control should be
exited and the brake pedal release rate exceeds a threshold,
provided the vehicle deceleration is higher than expected, based on
the estimated road surface coefficient of friction. In another
aspect of the invention, the ABS control is exited independent of
the timer if the control value indicates that ABS control should be
exited and the brake torque corresponding to the pedal position is
significantly less than the maximum possible brake torque, given
the estimated road surface coefficient of friction.
BRIEF DESCRIPTION OF THE DRAWINGS
[0004] FIG. 1 is a schematic diagram of a vehicle ABS system,
including a brake pedal position sensor, and a microprocessor-based
control unit programmed to carry out the method of this
invention.
[0005] FIG. 2 is a graph depicting a prior art technique for
defining ABS control regions in terms of vehicle deceleration and
wheel slip.
[0006] FIG. 3 is a graph depicting vehicle deceleration as a
function of brake pedal position for the braking system of FIG.
1.
[0007] FIG. 4 is a graph depicting an exemplary brake system
characterization according to this invention.
[0008] FIGS. 5-7 depict a computer software routine executed by the
control unit of FIG. 1 for carrying out the control method of this
invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0009] Referring to FIG. 1, the present invention is disclosed in
the context of an ABS system 10 for a vehicle 12 having
hydraulically-activated friction brakes 14, 16, 18, 20 at each of
four vehicle wheels 22, 24, 26, 28. A driver-manipulated brake
pedal 30 is mechanically coupled to a master cylinder (MC) 32 for
producing hydraulic pressure in proportion to the force applied to
pedal 30. Master cylinder 32, which may include a pneumatic booster
(not shown), proportions the hydraulic pressure between front and
rear brake supply lines 34 and 36 in a conventional manner. Front
supply line 34 is coupled to the left front service brake 14 via
left-front ABS modulator (M) 38, and to the right front service
brake 16 via right-front ABS modulator (M) 40. Rear supply line 36
is coupled to the left and right rear wheel brakes 18, 20 via rear
ABS modulator (M) 42.
[0010] A microprocessor-based control unit 50 receives various
inputs, including wheel speed signals on lines 52, 54, 56, 58 from
respective wheel speed sensors 60, 62, 64, 66 and a brake pedal
position signal PP on line 68 from pedal position sensor 70. The
sensors 60, 62, 64 66 and 70 may be implemented with conventional
devices in a manner known to those skilled in the art. In response
to the various inputs, the control unit 50 outputs modulator
control signals on lines 72, 74, 76 during wheel lock-up
conditions, and diagnostic information signals on line 80 for
display on a driver information device 82.
[0011] In general, the control unit 50 monitors the measured wheel
speeds to detect a condition of insipient wheel lock, and then
controls modulators 38, 40, 42 to modulate the respective hydraulic
brake pressures so as to maximize the tractive force between the
vehicle tires and the road surface. In a conventional system, ABS
control is terminated when the brake pedal 30 is fully released, or
when the vehicle deceleration and wheel slip are such that ABS
control is no longer needed. FIG. 2 illustrates a prior art
approach in which a vehicle deceleration vs. wheel slip table is
used to delineate two regions: Region I above the trace 84 for
which ABS control is not needed, and Region II below the trace 84
for which ABS control is needed. As a practical matter, Region II
of the table may contain other information, such as brake pressure
apply and release rates. According to the conventional approach,
ABS control is terminated if the vehicle remains in Region I for at
least a predetermined exit time interval. As indicated above,
however, this exit strategy tends to unnecessarily delay
termination of ABS control when the driver only partially releases
the brake pedal 30. This issue is addressed by the present
invention, which adaptively determines exit criteria based on the
rate of brake pedal release and estimates of the brake torque and
the road surface coefficient of friction. As described below, the
brake torque and road surface coefficient of friction are estimated
based on the brake pedal position and a periodically updated
characterization of the relationship between brake pedal position
and vehicle deceleration.
[0012] FIG. 3 graphically depicts a representative relationship
between vehicle deceleration and brake pedal position for defined
braking of the vehicle 12, assuming that there is no lock-up
condition and the modulators 38, 40, 42 are inactive. Typically,
the "knee" portion of the relationship varies considerably from
stop to stop, whereas the portion of the relationship above the
knee tends to be linear and repeatable from stop to stop. For this
reason, the knee portion of the relationship is ignored for
purposes of this invention, and the brake pedal position vs.
vehicle deceleration relationship is characterized only for pedal
positions and vehicle decelerations in the linear portion above the
knee. In the illustrated embodiment, depicted in FIG. 4, the
characterization data is collected by identifying the pedal
position values PPCUR1, PPCUR2 and PPCUR3 corresponding to three
different vehicle deceleration values D1, D2 and D3. Of course, any
number of data points may be used, and the data points may be
defined in terms of pedal position, if desired. In any case, the
braking data is only collected during braking operation when the
pedal 30 is depressed at a "normal" rate or held at an essentially
static position; data is not collected upon release of the pedal 30
or during panic braking. This eliminates the need to compensate for
the effects of suspension and powertrain dynamics, tire and sensor
dynamics, and so on. The vehicle acceleration at the onset of
braking is saved and subtracted from the deceleration during
braking operation in order to compensate for the effects of engine
braking and road grade. Of course, the road grade and other factors
such as vehicle weight and the effects of brake heating may be
estimated and used to compensate the collected braking data; see
for example, the U.S. Pat. No. 6,212,458 to Walenty et al., issued
on Apr. 3, 2001, and incorporated herein by reference.
[0013] The characterization table is periodically updated to
reflect a current condition of the braking effectiveness, and the
brake torque BRAKE_TQ for any brake pedal position PP greater than
or equal to PPCUR1 can be determined according to the
expression:
BRAKE.sub.--TQ=[((PP-PPCUR1)*(PPCUR3-PPCUR1)/(D3-D1))*Kbt]+(UPDATE_BRAKE_H-
EAT-BRAKE_HEAT)*Kheat (1)
[0014] where Kbt is a brake torque constant, UPDATE_BRAKE_HEAT is
the estimated brake temperature when the characterization table was
last updated, BRAKE_HEAT is a current estimate of the brake
temperature, and Kheat is a constant for converting the quantity
(UPDATE_BRAKE_HEAT-BRAKE_- HEAT) to a corresponding difference in
brake torque. Thus, the brake torque obtained from the
characterization table is compensated for differences in brake
temperature; for example, BRAKE_TQ is reduced if BRAKE_HEAT is
higher than UPDATE_BRAKE_HEAT, and vice-versa.
[0015] The brake temperature term BRAKE_HEAT can be modeled
reasonably well, and is continuously updated regardless of whether
the brakes are activated. For example, BRAKE_HEAT may be estimated
as:
BRAKE_HEAT=BRAKE_HEAT-((VSPD+K1).sup.2*K2)*
(BRAKE_HEAT-(BRAKE_HEAT*Tamb)+-
(BRAKE.sub.--TQ*Kheat*VSPD)*(K3-BRAKE_HEAT)/K3 (2)
[0016] where K1, K2 and K3 are constants, Tamb is the ambient
temperature, and VSPD is the vehicle speed.
[0017] Since changes in the vehicle weight change the
characterization table data, the vehicle weight is updated each
time the braking characterization table is updated, according
to:
V.sub.--WT=V.sub.--WT(last)+WT_DELTA (3)
[0018] where WT_DELTA is a measure of the change in
characterization data, compensated for changes in brake
temperature. Specifically, WT_DELTA is given by the expression:
WT_DELTA=[(((PPCUR3old-PPCUR1old)-(PPCUR3-PPCUR1))/(D3-D1))*Kwt]+(UPDATE_B-
RAKE_HEAT-BRAKE_HEAT)*Kheat (4)
[0019] where Kwt is a weight constant. In turn, the coefficient of
friction between the vehicle tires and the road surface
(SURFACE_MU) is given by:
SURFACE.sub.--MU=(BRAKE.sub.--TQ/V.sub.--WT)*Kmu (5)
[0020] where Kmu is a constant.
[0021] According to the present invention, the above information is
used along with the brake pedal release rate to define adaptive
exit criteria that reduce the ABS exit delay, consistent with
SURFACE_MU, when the brake pedal 30 is only partially released. In
one aspect of the invention, the exit time interval is adaptively
adjusted based on SURFACE_MU, and a control value (ABS_COMMAND)
used to determine if ABS control should be exited is adaptively
biased toward exiting ABS control based on the brake pedal release
rate and BRAKE_TQ. In another aspect of the invention, ABS control
is exited independent of the timer when ABS_COMMAND indicates that
ABS control should be exited and the brake pedal release rate
exceeds a threshold, provided the vehicle deceleration is higher
than expected, based on SURFACE_MU. In another aspect of the
invention, the ABS control is exited independent of the timer when
ABS_COMMAND indicates that ABS control should be exited and BRAKE
TQ is significantly less than the maximum possible brake torque,
given SURFACE_MU.
[0022] The method of the invention is illustrated by the flow
diagram of FIGS. 5-7, which represents a software routine
periodically executed by control unit 50 of FIG. 1. The routine
serves to collect the braking system characterization data, to
update the estimated brake torque BRAKE TQ and road surface
coefficient of friction SURFACE_MU, and once ABS control has been
initiated, to determine if the adaptive exit criteria are met.
Referring to FIG. 5, the input processing blocks 90 and 92 are
first executed to read the brake pedal position PP and the wheel
speeds (WS1-WS4), to compute the vehicle acceleration ACCEL
(compensated for road grade and vehicle weight) and wheel slip, and
to update BRAKE_HEAT using equation (2). Also, the vehicle
acceleration when the brake pedal is initially depressed is saved
as the onset acceleration ACCEL_ONSET. Block 94 then checks the ABS
FLAG to determine if ABS control is active. Ordinarily, the ABS
FLAG is FALSE, and a portion of the routine comprising the blocks
96-124 is executed to collect braking system characterization data.
If insipient wheel lock has been detected, and the modulators 38,
40, 42 have been activated to release brake pressure, the ABS FLAG
will be TRUE; in this case, the data collection portion of the
routine is skipped, and the block 134 is executed to determine the
value of ABS_COMMAND, as explained below. If block 94 is answered
in the negative, the data collection portion of the routine (blocks
96-102) is executed to detect the presence of braking activity that
is suitable for brake system characterization. Block 96 determines
if ACCEL exceeds a relatively high threshold acceleration Kaccel,
block 98 determines if the previous pedal position PPold is greater
than the current value PP, block 100 determines if the difference
(PP-PPold) exceeds a threshold rate Krate, and block 102 determines
if the difference (ACCEL-ACCEL_ONSET) is less than the minimum
deceleration table entry D1. Each of the blocks 96, 98, 100, 102
must be answered in the negative to proceed with data collection;
thus, a "normal" braking condition is defined as one in which (1)
ACCEL<Kaccel, (2) PPold<PP, (3) PP-PPold<Krate, and (4)
ACCEL-ACCEL_ONSET>D1. Stated oppositely, "normal" braking for
purposes of data collection does not include (1) panic braking, (2)
high rate brake pedal movement, (3) brake pedal releasing, or (4)
deceleration below the linear range of the deceleration vs. pedal
position relationship. If at least one of the blocks 96, 98, 100,
102 is answered in the affirmative, the blocks 104-124 are skipped,
and the control unit 50 proceeds to block 126, as indicated by the
circled letter B.
[0023] The data collection blocks 104-124 identify the brake pedal
positions P1, P2, P3 corresponding to the respective predefined
vehicle deceleration values D1, D2, D3, and periodically update a
brake system characterization table corresponding to the graph of
FIG. 4. The blocks 104, 110 and 116 respectively determine if the
pedal positions P1, P2 and P3 have been identified, based on the
status of the D1 FLAG, the D2 FLAG and the D3 FLAG. If block 104
determines that the D1 FLAG is not true, the block 106 determines
if the difference (ACCEL-ACCEL_ONSET) has reached the predefined
deceleration value identified in FIG. 4 as D1. If not, the data
collection portion of the routine is exited; if so, the block 108
sets the D1 FLAG to true, and uses the current value of PP to
update the pedal position variable P1 as shown. On the next
execution of the routine, block 106 will be answered in the
affirmative, and block 110 will determine if the D2 FLAG is true.
If not, the block 112 determines if the difference
(ACCEL-ACCEL_ONSET) has reached the predefined deceleration value
identified in FIG. 4 as D2. If not, the data collection portion of
the routine is exited; if so, the block 114 sets the D2 FLAG to
true, and uses the current value of PP to update the pedal position
variable P2 as shown. On the next execution of the routine, blocks
106 and 110 will both be answered in the affirmative, and block 116
will determine if the D3 FLAG is true. If not, the block 118
determines if the difference (ACCEL-ACCEL_ONSET) has reached the
predefined deceleration value identified in FIG. 4 as D3. If not,
the data collection portion of the routine is exited; if so, the
block 120 sets the D3 FLAG to true, uses the current value of PP to
update the pedal position variable P3, increments a brake event
counter BEC CTR, and sums the pedal position variables P1, P2 and
P3 with corresponding position summation values PS1, PS2 and PS3.
However, if the difference (ACCEL-ACCEL_ONSET) for the braking
event fails to reach the deceleration value D3, the block 120 is
not executed, and any pedal position data collected during the
respective brake application is discarded.
[0024] After each successful data collection, the block 122
compares the brake event counter BEC_CTR to a calibrated threshold
K_BEC indicative of the number of braking events needed to update
the braking system characterization table. Thus, when BEC_CTR
reaches K_BEC, the identified pedal position variables P1, P2, P3
for K_BEC (which may have a value of ten, for example) braking
events will have been accumulated in the respective position
summation values PS1, PS2, PS3. When this happens, the block 124
re-calculates the braking system characterization table values
PPCUR1, PPCUR2, PPCUR3 by dividing the respective pedal position
summation values PS1, PS2, PS3 by the brake event counter BEC_CTR.
In other words, PPCUR1=PS1/BEC_CTR, PPCUR2=PS2/BEC_CTR and
PPCUR3=PS3/BEC_CTR. Block 124 also saves the old table values for
updating V_WT, stores the current value of BRAKE_HEAT as
UPDATE_BRAKE_HEAT, and resets the brake event counter BEC_CTR and
the position summation values PS1, PS2, PS3 to zero.
[0025] Once the data collection portion of the routine has been
completed or exited, the block 126 is executed to determine if the
brake pedal position PP is at least as great as the lowest
characterization value PPCUR1. If not, the block 128 skipped; if
so, the block 128 is executed to update BRAKE_TQ and SURFACE_MU.
The brake torque BRAKE_TQ is determined using equation (1), the
vehicle weight V_WT is updated using equations (3) and (4), and
SURFACE_MU is estimated using equation (5). Block 130 then checks
for insipient wheel lock. If insipient wheel lock is not detected,
the routine is exited; if insipient wheel lock is detected, the
block 132 sets the ABS FLAG to TRUE, and the block 134 determines
the value of ABS_COMMAND.
[0026] In a conventional ABS system, ABS_COMMAND is determined by
table look-up as a function of vehicle deceleration and wheel slip
as described above in reference to FIG. 2. For operating points in
Region I, ABS_COMMAND indicates that ABS control should be exited;
for operating points in Region II, ABS_COMMAND indicates that the
respective brake pressure should be increased, decreased or
maintained at the current value. In most applications, ABS_COMMAND
values for different combinations of deceleration and wheel slip
are stored as digital numbers, with the magnitude of the numbers
corresponding to the commanded action. For example, if the
retrieved value of ABS_COMMAND is less than a relatively low
threshold, it indicates that ABS control should be exited; if
ABS_COMMAND is in successively higher ranges, it indicates a rate
at which the respective brake pressure should be decreased, that
the respective pressure should be maintained, or a rate at which
the respective pressure should be increased. According to this
invention, however, the ABS_COMMAND value retrieved from the table
is reduced as a function of the brake release rate BRR, and
BRAKE_TQ whenever BRR is greater than zero, as indicated at block
134. In the illustrated embodiment, this is achieved with the
expression:
ABS_COMMAND=ABS_COMMAND.sub.--R-(BRR*BRAKE.sub.--TQ) (6)
[0027] a where ABS_COMMAND_R is the ABS Command value retrieved
from the deceleration vs. wheel slip look-up table. In the above
expression, BRR cannot have a negative value, and ABS
COMMAND=ABS_COMMAND_R when brake pedal 30 is not being released.
However, when the brake pedal 30 is being released, ABS_COMMAND is
reduced as a function of both BRR and BRAKE_TQ, thereby biasing
ABS_COMMAND toward less aggressive ABS control.
[0028] Referring to FIG. 7, the control unit 50 then executes block
140 to determine if ABS_COMMAND indicates that ABS control should
be exited. If not, the blocks 142 and 144 are exited to determine
the exit time interval TIME_EXIT and to carry out a conventional
ABS control algorithm based on the ABS_COMMAND determined at block
134. The exit time interval TIME_EXIT is utilized when the
ABS_COMMAND indicates that ABS control should be exited--that is,
when block 140 is answered in the affirmative. As indicated at
block 142, TIME_EXIT is computed using the expression:
TIME_EXIT=Kexit*(1-SURFACE.sub.--MU) (7)
[0029] where Kexit is a relatively long time interval that would be
appropriate for a very low road surface coefficient of friction. As
the estimated road surface coefficient of friction SURFACE_MU
increases, however, the computed value of TIME_EXIT decreases.
[0030] When block 140 determines that ABS_COMMAND indicates that
ABS control should be exited, the blocks 146-158 are executed to
determine how quickly ABS control should be exited. If the brake
release rate BRR is relatively high, as determined at block 146,
the blocks 148 and 150 are executed to determine if the actual
vehicle deceleration exceeds the vehicle deceleration ACCEL_DES
desired by the driver. Block 148 determines ACCEL_DES by using the
brake system characterization table to determine the desired brake
torque BRAKE_TQ_DES, compensated for vehicle weight V_WT, and then
converting BRAKE_TQ_DES to a corresponding desired vehicle
deceleration ACCEL_DES. In the illustrated embodiment, BRAKE_TQ_DES
is determined according to the expression
BRAKE.sub.--TQ.sub.--DES=[((PP-BRR-PPCUR1)*(PPCUR3-PPCUR1)/(D3-D1))*Kbt]+(-
UPDATE_BRAKE_HEAT-BRAKE_HEAT)*Kheat (8)
[0031] provided that the difference (PP-BRR) exceeds the lowest
pedal position table value PPCUR1. Reducing the pedal position PP
by the release rate BRR causes BRAKE_TQ_DES to lead or anticipate
BRAKE_TQ based on driver intent. The desired deceleration, in turn,
is given by:
ACCEL.sub.--DES=BRAKE.sub.--TQ.sub.--DES*Kd (9)
[0032] where Kd is a deceleration constant. If the actual
acceleration ACCEL exceeds ACCEL_DES by more than a reference
amount Kdd, the block 150 is answered in the affirmative, and the
block 158 is executed to exit ABS control by setting the ABS FLAG
to FALSE.
[0033] If the brake release rate BRR is less than Kbrr, or
ACCEL<=ACCEL_DES, the blocks 152 and 154 are exited to decrement
the exit time interval TIME_EXIT and to check if TIME_EXIT has
reached zero. Once TIME_EXIT reaches zero, block 154 is answered in
the affirmative, and block 158 is executed to exit ABS control by
setting the ABS FLAG to FALSE. Otherwise, the block 156 is executed
to determine if the BRAKE_TQ, compensated for the estimated vehicle
weight V_WT, is significantly less than the maximum possible brake
torque, given the estimated road surface coefficient of friction
SURFACE_MU. Thus, block 156 compares (BRAKE_TQ/V_WT) to
(SURFACE_MU*Kmbt), where Kmbt is the maximum brake torque
achievable on a high road surface coefficient of friction. If the
quantity (BRAKE TQ/V_WT) exceeds the product (SURFACE_MU*Kmbt) by
at least at release constant Krel, block 156 will be answered in
the affirmative, and block 158 will be executed to exit ABS control
by setting the ABS FLAG to FALSE.
[0034] In summary, the control of this invention provides a
reliable and cost-effective way of adaptively adjusting the exit
criteria for an ABS system based on the brake pedal release rate
and brake system characterization data to reduce the ABS exit delay
when the brake pedal 30 is only partially released. The ABS control
term ABS_COMMAND is adaptively adjusted based on BRAKE_TQ and BRR,
and TIME_EXIT is adaptively adjusted based on SURFACE_MU. The ABS
control is exited if ABS_COMMAND indicates that ABS control should
be exited for at least TIME_EXIT, or before the TIME_EXIT elapses
if (1) BRR exceeds a threshold and the vehicle deceleration is
higher than the desired deceleration, or (2) if BRAKE_TQ is
significantly less than the maximum possible brake torque, given
SURFACE_MU. While the brake system characterization table is
depicted as being developed for purposes of estimating BRAKE_TQ and
SURFACE_MU, for example, it may be also be advantageously used for
diagnosing brake system abnormalities. While described in reference
to the illustrated embodiment, it is expected that various
modifications in addition to those mentioned above will occur to
those skilled in the art. For example, the control is applicable to
other types of ABS systems, and other types of vehicles, including
electric or hybrid vehicles that utilize electric or regenerative
braking to decelerate the vehicle. Thus, it will be understood that
the scope of this invention is not limited to the illustrated
embodiment, and that control methods incorporating these and other
modifications may fall within the scope of this invention, which is
defined by the appended claims.
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