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Description  |
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FIELD OF THE INVENTION
This invention relates to a wheel lock control method and system
particularly for use on split coefficient surfaces.
BACKGROUND OF THE INVENTION
When the brakes of a vehicle are applied, a braking force between the wheel
and the road surface is generated that is dependent upon various
parameters including the road surface conditions and the amount of slip
between the wheel and the road surface. The braking force increases as
slip increases, until a critical value of slip is surpassed. Beyond this
critical slip value, the braking force decreases and the wheel rapidly
approaches lockup. If the wheel is allowed to lock, unstable braking
occurs, and vehicle stopping distance on uniform nondeformable surfaces
increases. Thus, stable vehicle braking occurs when wheel slip does not
exceed this critical slip value. An antilock control system achieves
stable braking and minimizes stopping distance by detecting incipient
wheel lock. Criteria used to sense incipient wheel lock are excessive
wheel deceleration and/or excessive wheel slip. Once an incipient wheel
lock has been detected, pressure is relieved at the wheel brake. Upon
releasing the brake pressure, the wheel will begin to recover from the
incipient wheel lock condition. When the wheel has substantially
recovered, brake pressure is reapplied. One criterion that is typically
used to indicate wheel recovery is a positive wheel acceleration.
Reapplication of brake pressure results in the wheel again approaching
lockup and the wheel cycle process is repeated. Brake force and vehicle
braking efficiency are maximized during braking by cycling the brake
pressure around an optimum pressure for the particular road surface. The
optimum pressure corresponds to the brake force generated while at the
critical wheel slip value. Since the brake force is a function of wheel
brake pressure and road surface conditions, the optimum brake force and
the corresponding optimum brake pressure will change as road surface
conditions vary. To optimize vehicle braking during a stop on a changing
or non-uniform road surface, the antilock control system must be able to
respond to each road surface and seek a new optimal pressure quickly to
insure maximum braking efficiency. The U.S. Pat. No. 4,881,784 issued to
Leppek discloses an example of such a system and is incorporated herein by
reference.
When vehicle braking occurs on a road surface which has one coefficient of
friction on one side of the vehicle and a markedly different coefficient
on the other side, the surface is said to have a split coefficient. An
example of this is a road which is covered with ice or snow along one side
and is clear or dry near the center so that the right side wheels engage a
low coefficient of friction and the left side wheels engage a high
coefficient. The result of braking on such a surface is that the vehicle
tends to yaw toward the high coefficient side. Most wheel lock control
systems are designed to deal with the split coefficient surface in either
of two ways if left and right wheels can be separately controlled. One
approach is to independently control the brakes according to the optimum
operation on each side. The result is that yaw occurs but stopping
distance is minimized. The other approach is to control the brakes
according to the optimum operation on the low coefficient side for a
programmed period of time and then gradually resume independent brake
control. This forces both brakes to have the same low pressure initially
and increases vehicle stability but also increases the stopping distance.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to provide a method of brake
control for split coefficient surfaces which initializes the system to
limit yaw rate while retaining a short stopping distance. Another object
is to provide a wheel lock control system for carrying out such a method.
The invention is carried out in an antilock brake control having
independently controlled left and right wheels subject to causing vehicle
yaw rate on a split coefficient surface, and having for each wheel a power
mode for full brake application, a regulation mode for modulated
application and a dump mode for brake release; by the method of
initializing the control for split coefficient surfaces comprising the
steps of: sensing the presence of split coefficient surface during braking
by determining when one wheel is in dump mode and the opposite wheel is in
power mode; then assessing the tendency for yaw rate to exceed an
undesirable amount for setting a yaw rate indication, setting the said
opposite wheel to dump mode in response to a yaw rate indication, and
subsequently terminating the dump mode for the said opposite wheel upon
recovery thereof, whereby undesirable yaw is avoided.
The invention is further carried out by apparatus for executing such a
method.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and other advantages of the invention will become more apparent
from the following description taken in conjunction with the accompanying
drawings wherein like references refer to like parts and
FIG. 1 is a schematic diagram of an antilock braking system according to
the invention;
FIG. 2 is a diagram of the electronic controller;
FIGS. 3 through 6 and 8 through 10 are flowcharts detailing the operation
of the electronic controller, according to the invention; and
FIG. 7 is a graphical illustration of wheel brake pressures and vehicle yaw
occurring during brake operation.
DESCRIPTION OF THE INVENTION
The ensuing description is directed to a method and system for
independently controlling the left and right wheels on a given axle to
achieve the desired yaw control on split coefficient surfaces. The
illustrated example depicts a vehicle with independent brake control for
each wheel. The system requires at least a pair of right and left wheels
be independently controlled while the other pair may have a common
control. While a separate modulator is provided for each of the pair of
wheels so that different pressures can be applied to each wheel, an
algorithm is used to initially coordinate the modulator pressures thereby
limiting the independence in a manner to manage the yaw rate of the
vehicle. Two embodiments are described: one requires a yaw rate sensor to
provide information used in initially coordinating the modulator pressures
and the other requires no such sensor.
FIG. 1 illustrates the vehicle brake system. A hydraulic boost unit 2
couples master-cylinder pressure to brake modulators 4 which SUPPLY brake
pressure via brake lines 6 to each wheel brake 8. The construction and
operation of the modulators is more clearly set forth in the
above-mentioned U.S. Pat. No. 4,881,784. Each modulator has an electric
motor which controls a piston for regulating the brake pressure during
antilock operation. To assure no piston movement and thus a constant
pressure during hold mode, an electromagnetic brake on the motor may be
provided as shown in U.S. application Ser. No. 352,971 filed May 17, 1989
and assigned to the assignee of this invention, and which is incorporated
herein by reference. A wheel speed sensor 10 on each brake adjacent a tone
wheel 12 is connected by conductors 14 to an electronic controller 16
which is programmed to supply a modulator control signal via conductors 18
to each of the modulators 4. A discrete brake application sensor 20 such
as a switch responsive to brake pedal movement and a vehicle accelerometer
22 also provide input signals to the controller 16. The modulators 4,
under control of the controller 16, have four operational modes: a "power"
mode in which 100% of the master cylinder pressure is transferred to brake
pressure, a "dump" mode in which brake pressure is relieved to reduce
braking effort, a "regulation" mode which controls brake pressure at a
modulated rate, and a "hold" mode which maintains existing brake pressure
at a constant value but not to exceed the master-cylinder pressure.
The controller 16 monitors the wheel speed and calculates wheel slip and
wheel deceleration for each wheel. Upon detecting high wheel slip or
deceleration indicative of an incipient wheel lock condition, the
controller 16 initiates antilock activity for that wheel by commanding the
corresponding modulator to dump mode. As the brake pressure for that wheel
decreases the wheel is allowed to recover as indicated by wheel
acceleration. Then the regulation mode is commanded and the pressure is
gradually increased toward the optimal pressure for that road surface.
When such optimal pressure is exceeded the incipient wheel lock is again
detected and the wheel cycle is repeated.
As shown in FIG. 2, the electronic controller 16 consists of a common
digital computer composed of a read-only memory (ROM) 25, a random access
memory (RAM) 26, an analog-to-digital conversion port (A/D) 27, a power
supply device 28, an instruction processing architecture embodied in a
central processing unit (CPU) 29, and input/output (I/O) ports 30 which
interface to a modulator driver circuit 31 and a wheel speed sensor buffer
circuit 32. The modulator driver circuit 31 receives control commands and
also feeds back brake pressure information to the ports 30. The A/D 27
accepts input signals from the accelerometer 22 and an optional yaw rate
sensor 110.
The ROM 25 contains the instructions necessary to implement the algorithm
diagrammed in FIGS. 3-6 and 8-10. In describing the functions of the
algorithm, references to tasks which have been detailed in flow diagram
function blocks are designated by <nn>, where nn is the block reference
number.
When the antilock system is powered up, via the vehicle ignition circuit or
other means, the controller 16 will begin executing the instructions coded
in ROM 25. As shown in FIG. 3, the controller 16 will first perform system
initialization <35>, which entails clearing registers, initializing
specified RAm variables to calibrated values, stabilizing voltage levels
at the A/D, and other basic functions of the digital computer. The system
initialization process also includes insuring the modulators are in the
power mode to facilitate brake pressure control directly by the booster
unit 2 until antilock functions are invoked.
Once the system has been initialized, the controller 16 will enable the
control cycle interrupt <36>. The control cycle interrupt provides a means
for accurately calculating the critical vehicle parameters of wheel slip
and acceleration by insuring that the time between calculations is fixed
at a value such as 8 msec. Once a control cycle interrupt has occurred,
the controller 16 proceeds through the major loop referred to as the
"control cycle". During the control cycle, the controller performs brake
control processing tasks <38> for each wheel and then background tasks
<39>. The brake control tasks include: reading and processing the wheel
speed and pressure feedback information, determining whether antilock
control is necessary, and performing antilock control functions as
necessary. The background tasks consist of diagnostic self-check
activities and communication with other vehicle controllers or service
tools. All of these control cycle tasks are performed once every control
cycle interrupt.
FIG. 4 is a flow diagram of the antilock brake control algorithm of block
38. It comprises reading input information <50> such as wheel speed sensor
output, the brake apply signal and the vehicle accelerometer output, then
calculating from the input information vehicle speed and wheel slip <52>,
determining the brake operation mode <54>, executing the initial
left-and-right coordination that strikes the balance between the stopping
distance and initial yaw rate when braking on split-coefficient surfaces
<56>, repeating the mode determination and initial coordination for all
wheels <57>, and finally executing the pressure control for each wheel
according to the respective operating mode <58>.
FIG. 5 sets forth the details of block 54 which determines the brake
operation mode. Power mode is available for power assisted braking, dump
mode for brake pressure relief during antilock braking, regulation mode
for regulating wheel slips to desired targets, and hold mode to hold the
brake pressure not to exceed the master-cylinder pressure generated by the
brake pedal force and the hydraulic boost. The four modes are mutually
exclusive and a flag is set to indicate the current mode for each wheel.
If the brake is not applied <60> the power mode is set <62> for full
braking pressure. If the brake is applied <60>and the regulation mode is
not set <64> and the power mode is still set <66>, a test is made for
impending lock <68> based on wheel deceleration or wheel slip. If there is
impending wheel lock the dump mode is set <70> and, in either case, the
program exits. If the power mode is not set <66> and the dump mode is not
set <76>, the control passes to block 72. Where the vehicle speed is below
some low threshold, say 5 mph <72>, the hold mode is set <74> to sustain
the existing brake pressure. If the dump mode is set <76> and wheel speed
recovery is detected <78>, regulation mode is set <80> to effect a
controlled pressure increase and the control passes to block 72. When the
control is in regulation mode <64> so that the pressure is increasing,
impending wheel lock is tested <82> and if it is affirmed the dump mode is
set <84> to reduce the brake pressure to induce wheel recovery. Again the
control goes to block 72 when dump mode is set or if there is no impending
wheel lock <82>. The program 54 is exited when the flags have been set for
each of the wheels <63>, and then the left-right coordination program 56
is entered. Thus, in general the power mode is set when the brakes are not
applied, the dump mode is entered whenever impending wheel lock is
detected, the regulation mode is entered when wheel recovery is detected,
and, overriding dump and regulation modes, the hold mode is set when
vehicle speed is at a low value during antilock activity.
The left-right coordination program 56 shown in FIG. 6, is executed for all
axles where left and right brakes have independent control. The wheel that
is first set at dump mode is identified as the wheel of low-.mu. side and
the opposite wheel is assumed to be at the high-.mu. side. This program is
executed by first testing for the dump mode <90>. If the control is not in
dump mode the routine is bypassed. If it is in dump mode <90> and the
opposite wheel is in power mode <94>, the slip S of the opposite wheel is
obtained <96>, a wheel slip threshold S.sub.th is determined <98> and
compared to the slip S <100>. When the slip is greater than the threshold
the control for the opposite wheel is set to dump mode <102> before
passing control to block 92. The effect of this algorithm is to force the
opposite wheel into antilock operation, initially arrest any increase in
the opposite pressure and likely to decease the opposite pressure for one
or a few control loops until wheel recovery is detected in block 78 to
cause entry into regulation mode. Thus the system is prepared for
split-coefficient surface operation upon initial antilock operation and
the left-right coordination function does not come into play again until a
brake reapplication puts the brakes into power mode and impending slip is
sensed.
The graphs of FIG. 7 illustrate the effect of the coordination program 56.
Graph A tracks the brake pressure on the high-.mu. wheel and graph B shows
the brake pressure of the low-.mu. wheel. The first spike of pressure in
both graphs occurs when brakes are first applied. The dump mode is
triggered almost immediately in the low-.mu. side to cause a pressure drop
followed by the beginning of the regulation mode which maintains the
pressure at a very low value on the order of 100 psi. The coordination
program 56 causes the pressure on the high-.mu. side to dump shortly after
the low side dumps and prevent the initial pressure from becoming large.
The high side wheel quickly recovers and enters the regulation mode which
holds the maximum pressure to about 1000 psi. The high side alternates
between the regulation and dump modes to cause wheel cycles having periods
of about one second. Graph C depicts the vehicle yaw rate during the
braking action. Upon initial braking, the yaw rate approaches -30
degrees/second. The rapid release of the high-.mu. wheel prevents a
greater yaw rate. Thereafter, the yaw rate oscillates about a zero value
and has peaks which are generally somewhat less than plus or minus 20
degrees/second. These yaw rate values are reasonably manageable by the
vehicle operator.
The process of selecting the wheel-slip threshold S.sub.th required by
block 98 is illustrated in FIG. 8. The wheel-slip threshold is a function
of both vehicle speed v and deceleration. The speed component S.sub.V is a
monotonically decreasing function of vehicle speed as shown in block 104.
The function decreases with speed since a high yaw rate may be acceptable
at a low vehicle speed and a lower yaw rate is preferred at higher speeds.
The function may be stored as a formula or, preferably, as a table giving
the S.sub.V value for several discrete speeds and using interpolation for
intermediate speeds. The deceleration dependent component S.sub.D is a
simple increasing curve as shown in block 106. This too, may be stored as
a formula or as a table. It has a zero value when vehicle deceleration is
below a certain value, and then increases with deceleration increase. This
makes the threshold higher for high deceleration which would occur only on
relatively high coefficient surfaces but the threshold is lower and the
left-right coordination more sensitive to slip at low vehicle
deceleration. As shown in block 108, the wheel-slip threshold S.sub.th is
the sum of the components S.sub.V and S.sub.D. The selection of the
threshold S.sub.th and comparison with the wheel-slip of the high-.mu.
wheel is a way of estimating the tendency of the vehicle to exceed a
certain yaw rate and the dump mode command given in block 102 is in effect
an indication of excess yaw rate. By proper selection of the threshold
components the yaw rate can be limited to a desired value. The particular
functions of the threshold components are determined empirically for a
given vehicle to achieve a balance of comfort and drivability with
stopping capability.
A specific example of lookup tables for the functions S.sub.V and S.sub.D
are given in Tables 1 and 2.
TABLE 1
______________________________________
speed (mph)
S.sub.V
______________________________________
0 0.8
10 0.54
15 0.28
20+ 0.02
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TABLE 2
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decel (g)
S.sub.D
______________________________________
0 0
0.125 0.005
0.250 0.015
0.375 0.030
0.500 0.045
0.625+
0.060
______________________________________
Thus the respective values of S.sub.D and S.sub.V change step-wise as the
deceleration or speed increase from zero or if a higher resolution
function is desired interpolation can be used to determine intermediate
values.
As an alternative to the coordination program 56, if the vehicle is
equipped with an optional yaw rate sensor 110, as shown in FIG. 1, the
measured yaw rate can be compared with a threshold, which may be set at,
say, 20 or 30 degrees/second, to directly assess the tendency to exceed a
desirable yaw rate. The alternate coordination program 56' is shown in
FIG. 9 wherein a split coefficient surface is detected by testing for dump
mode <90'> and the opposite side in power mode <94'>. If the split
coefficient is confirmed <94'> the yaw rate is read <96'> and compared
with a threshold <100'>. If the yaw rate exceeds the threshold the
opposite side is set to dump mode <102'>.
After the left-right coordination program 56 or 56' has been executed, the
pressure control is executed for each wheel by the routine 58 as shown in
FIG. 10. If the control for a given wheel is in power mode <120>, the
command is that the master-cylinder pressure be fully applied to that
wheel <122>. If the control is in regulation mode <124> a pulse width
modulated pressure command is issued <126>. If the control is in the dump
mode <128> a full dump command is issued <130>. If none of the above modes
is set, the hold mode applies and the brake pressure is held at its
present value, subject to master-cylinder pressure <132>.
It will thus be seen that the method and system described herein are
effective to detect when a vehicle is being braked on a split coefficient
surface and to control the antilock function in a way to avoid undesirable
vehicle yaw rates.
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