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Description  |
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FIELD OF THE INVENTION
The present invention relates to a circuit arrangement which is provided
for an automotive vehicle brake system with electronic anti-lock control
to enhance the driving stability in critical driving situations and
includes circuits for the individual control of the braking pressure in
the wheel brakes of the front wheels in response to the rotational
behavior of the wheels and for the limitation of yawing torques which
occur due to braking maneuvers on road surfaces having a different
friction coefficient on the right/left sides. The circuit may include a
hard wired circuit, a programmed circuit, or mixed forms thereof.
BACKGROUND OF THE INVENTION
German patent application No. 39 25 828 discloses a circuit arrangement of
this type. An anti-lock control system with individual control of the
braking pressure on both wheels of an axle is described. The braking
pressure difference on the two wheels of an axle is restricted to limit
the yawing torque on split road surfaces, and the allowable braking
pressure difference is determined as a function of the difference in
friction coefficients and the magnitude of the lower coefficient of
friction. In the event of exceeding of the allowable braking pressure
difference, in consideration of the motional condition of the LM-wheel,
that is the wheel on the lower coefficient side, the braking pressure on
the HM-wheel, i.e. the other wheel of the same axle, is reduced. To
determine the allowable pressure difference in this known anti-lock
control system, the pressure introduced by the driver is measured on the
right and the left wheel, and the nominal pressure is compared to the
actual pressure in each case. The coefficient of friction is assessed on
the basis of the braking pressure.
Further, German patent application No. 41 14 734 discloses a circuit
arrangement for anti-lock control with an individual braking pressure
control and yawing torque limitation which is based on the fact that a
value, representative of the pressure difference on the two wheels of an
axle, is constantly determined from the pressure reduction signals and,
under .mu.-split conditions, the medium pressure increase gradient on the
HM-wheel is varied as a function of the pressure difference and the
vehicle deceleration and that the braking pressure on the HM-wheel is
reduced by a value, which is responsive to the vehicle deceleration and
the pressure difference, at the time of the occurrence of the so-called
yawing torque peak.
The above mentioned, known provisions do not permit preventing the
generation of yawing torques of a dangerous amount in defined, especially
critical situations. For example, different wheel lock pressure levels in
the individual wheel brakes occur on road surfaces having varying
coefficients of friction, so-called .mu.-patch road surfaces, due to a
changing dynamic axle load distribution and the related variation of the
wheel vertical forces. When these variations occur on the right and the
left road surface side, they cause yawing torques which may jeopardize the
driving stability. Especially in vehicles having a short wheelbase and
front-wheel drive, the rear axle alone cannot ensure that driving
stability is maintained.
In principle, the previous solutions involve slowing down the occurrence of
yawing torques during braking on .mu.-split road surfaces by a decelerated
braking pressure rise on the front HM wheel to give the driver time for a
reaction, i.e., for countersteering. However, when the coefficient of
friction changes, in particular from a high value to a low value, the
described effects will be encountered.
OBJECT OF THE INVENTION
An object of the present invention is to prevent the occurrence of
dangerous yawing torques even in defined, especially critical driving
situations, above all when passing from a high, homogeneous coefficient of
friction to a .mu.-split road surface or during braking maneuvers on
.mu.-patch road surfaces, and to virtually enhance the driving stability
also in such driving situations.
SUMMARY OF THE INVENTION
It has been found that this object can be achieved by a circuit arrangement
which in defined, critical driving situations, in particular when passing
into a .mu.-split road surface or during braking maneuvers on .mu.-patch
road surfaces, and upon identifying predetermined criteria for such a
critical driving situation, initiates a special control which causes
braking pressure reduction in the wheel brake of the front wheel having
the higher coefficient of friction, the so-called "HM-wheel", for a
predetermined period of time which is responsive to the vehicle speed or
vehicle reference speed.
Thus, the present invention initially determines whether there is a
defined, especially critical driving situation, and switch over is made to
a special control which is appropriate for this situation only and is used
for a one-time reduction (such a reduction may also be effected several
times on .mu.-patch road surfaces) of the braking pressure on the HM-wheel
in favor of driving stability. In some cases and with some vehicle types,
it is helpful to additionally reduce the braking pressure on the rear
wheels.
In a favorable embodiment of the present invention, a driving situation is
assessed as critical or identified as critical if one or a plurality of
the following criteria simultaneously are satisfied:
The braking pressure difference or the difference in braking pressure
reduction times on the front wheels exceeds a predetermined braking
pressure difference limit value.
The vehicle speed or vehicle reference speed ranges above a predetermined
speed limit value.
The slip of the low coefficient side front wheel, the so-called "LM-wheel",
ranges above a slip limit value which is responsive to the vehicle
(reference) speed.
The duration of the instability of the LM-wheel exceeds a predetermined
time.
The HM-wheel is in the control mode, or the vehicle deceleration is above a
predetermined deceleration limit value, and special control has not yet
taken place during the instantaneous control action.
The commencement of the special control is only permitted if a defined
critical situation is undoubtedly identified. It is favorable in some
cases to assess the driving situation as critical only if all listed
criteria are satisfied.
In another favorable aspect of the present invention, the circuit
arrangement is so designed that the special control during a braking
maneuver is repeated when a critical driving situation is identified once
more or the critical driving situation continues if, priorly, the braking
pressure difference or the difference in braking pressure reduction times
has exceeded the predetermined limit value with reversed signs or if there
occurred a change in the coefficient of friction conditions right/left.
This embodiment relates to the so-called .mu.-patch road surfaces.
The present invention further discloses that the special control is not
initiated, or is prevented, when re-acceleration of the LM-wheel is
identified during a braking maneuver as being above a predetermined first
acceleration limit value. The re-acceleration limit value is suitably set
to a value between 5 and 10 g. Such a high amount of re-acceleration is an
indication of a high coefficient of friction. Hence, the wheel concerned
is not the LM-wheel.
Appropriately, the special control is interrupted at once when one or a
plurality of predetermined criteria for the identification of a critical
driving situation is not satisfied any more. The special control will be
terminated when re-acceleration of the LM-wheel in excess of roughly 10 to
30 g occurs during the special control interval.
In another aspect of the present invention, the duration of braking
pressure reduction due to the special control at a vehicle speed ranging
between 30 and 50 km/h is limited to a value between 20 and 40 msec, and
this duration is increased in steps or continuously to values up to
roughly 50 to 70 msec at higher speeds.
Further, it has proved suitable to set the slip limit value to a value
ranging between 60 and 70% at a vehicle speed of about 40 km/h and to
reduce this value at higher speeds linearly or in steps until a limit
value of about 40%.
The circuit arrangement of the present invention, according to latest
findings, is favorably realized by an electronic controller which
comprises programmed circuits such as microprocessors, microcontrollers,
etc. The controller is part of an anti-lock control system which includes
a hydraulic brake system with electrically operable hydraulic valves and
sensors to determine the rotational behavior of the vehicle wheels. The
programmed circuits are used to evaluate the sensor signals and to
generate braking pressure control signals. The circuits are programmed so
that the braking pressure in the wheel brakes of the front wheels is
controlled individually as a function of the rotational behavior of the
wheels and, in doing so, yawing torques caused by driving maneuvers on
.mu.-split road surfaces are limited. A corresponding programming will
achieve that the special control is started by way of the above described
criteria when the so-called critical driving situations are identified.
The special control causes braking pressure reduction on the HM-wheel and,
if necessary, also on the rear wheels for a predetermined duration which
depends on the vehicle speed. All other above mentioned preferred aspects
of the present invention may also be realized by the programmed circuits.
Further details of the present invention can be seen in the following
description of embodiments, making reference to the accompanying drawings
and diagrams.
BRIEF DESCRIPTION OF THE DRAWINGS
In the drawings,
FIG. 1 is a schematically simplified view of the most important components
or function blocks of a circuit arrangement of the present invention.
FIG. 2 is a diagram showing the braking pressure variation on the two front
wheels in the represented driving situation (abrupt change in
.mu.-conditions) according to an embodiment of the present invention.
FIG. 3 shows, in the same illustration as FIG. 2, the braking pressure
variation on the two front wheels in another driving situation which is
also represented (.mu.-patch road surfaces).
FIG. 4 is a diagram showing the dependence of the pressure reduction time
on the vehicle (reference) speed.
FIG. 5 is a diagram showing the variation of the predetermined slip limit
value as a function of the vehicle (reference) speed.
FIG. 6 is a flow chart to illustrate the operation of the circuit
arrangement of FIG. 1.
DETAILED DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates the basic design and effect of a circuit arrangement of
the present invention. The schematically simplified block diagram shows
the electronic part of an antilock control system of the present
invention. Block 1 represents wheel sensors S.sub.1 to S.sub.4 by which
the rotational behavior of the individual vehicle wheels is determined in
a known fashion. The output signals of the sensors S.sub.1 to S.sub.4
represent the speeds of the individual wheels. The signals are conditioned
and wheel speed signals v.sub.1 to v.sub.4 are produced in a circuit block
2. Further, the vehicle reference speed v.sub.Ref, wheel acceleration
signals b.sub.1 to b.sub.41 slip values .lambda..sub.1 to .lambda..sub.4
and, if necessary, still other quantities are determined or calculated in
the circuit block 2.
A signal E.sub.2 responsive to the instantaneous vehicle (reference) speed
v.sub.Ref is derived in a circuit 3.
In the present embodiment of the invention, a vehicle deceleration signal
b.sub.FZ is produced in another circuit 4 by logically combining the
deceleration signals b.sub.1 to b.sub.4 of the individual vehicle wheels
in consideration of the wheel speeds v.sub.1 to v.sub.4. The output signal
E.sub.3 of circuit 4, exactly as E.sub.2 and the signals
E.sub.1,E.sub.4,E.sub.5,E.sub.6 explained hereinbelow, is conducted to an
identification logic 5. The input signal E.sub.5 informs the
identification logic 5 about the instantaneous magnitude of the slip
.lambda..sub.1, .lambda..sub.2 of the front wheels.
In the circuit arrangement of FIG. 1, a circuit block 6 accommodates the
actual control logic which produces braking pressure control signals or
brake valve control signals pursuant to complex computing principles which
embody the control philosophy that is essential for a control system. Of
course, a control logic of this type, exactly as the other circuit blocks
and circuit components of the circuit arrangement of FIG. 1, may also be
realized by a corresponding program structure of a microprocessor or
microcontroller.
The control logic 6 typically includes several function blocks or program
parts. In the present case, there is, among others, a so-called yawing
torque limitation unit GMB which decelerates the braking pressure rise on
the higher coefficient side in the presence of different coefficients
right/left.
Valve actuation units 9,10 are connected to outputs A1,A2 of the control
logic 6 by way of commutators 7,8 in the circuit arrangement of FIG. 1.
The valve actuation units 9,10 include final stages having output signals
which finally actuate hydraulic valves 11,12 in the hydraulic brake
circuits of an anti-lock brake system (ABS). In the embodiment of FIG. 1,
the components and signal lines which lead to the wheel brakes of the
front wheels 7,9,11 and the wheel brakes of the rear wheels 8,10,12 are
shown separately because the special control (MFO) of the present
invention depends in first place on the rotational behavior of the front
wheels.
Another output A3 of the control logic 6 leads to the identification logic
5. Above all, circuit 5 is used for the AND-linking of the signals
introduced through inputs E.sub.1 to E.sub.6. When the stipulated
conditions are satisfied, the special control MFO
(Mu-Flecken-Option-Mu-patch option) is initiated or prepared by a circuit
13. However, circuit 13 cannot cause actuation of the hydraulic valves 11
in the front-wheel brake circuits and, if necessary, also the hydraulic
valves 12 in the rear-wheel brake circuits until switches 7,8 are changed
over.
The braking pressure in the front-wheel brakes is determined and assessed
by way of an integrator 14 having its output connected to a threshold
value switch 15. The difference of the braking pressures introduced into
the front-wheel brakes or the difference of the braking pressure reduction
times on the front wheels (this is a measured quantity which is especially
appropriate for assessment) is of interest for the initialization of the
special control MFO. Therefore, the actuation times of the braking
pressure reducing valves, associated with the two front-wheel brakes, on
each individual wheel are sensed to determine the control quantity. The
threshold value switch 15 signals that a predetermined braking pressure
limit value p.sub.0 is exceeded. The sign of the output signal of the
threshold value generator permits identifying which of the two front
wheels has the higher or lower braking pressure. It can be taken from this
fact which front wheel has the higher or the lower coefficient of
friction.
When the predetermined braking pressure difference limit value p.sub.0 is
exceeded, the output signal of the threshold value switch 15 causes
change-over of the switches 7,8 so that the special control MFO (provided
the other conditions are also satisfied) can now be initiated by way of
circuit 13. As will be explained hereinbelow, the special control MFO
effects the discharge of braking pressure from the wheel brake of the
front HM-wheel and also from the rear-wheel brakes.
In FIG. 1, the braking pressure reduction signals initiated by the special
control (MFO) 13 are counted as soon as a switch 16 is closed, or sensed
by an integrator 17. The result is transmitted to the identification logic
5 by way of input E.sub.6 because, among other criteria, the duration of
pressure reduction on the LM-wheel is a criterion for the identification
of a critical driving situation and the design of the special control.
For better understanding the present invention, FIG. 1 is shown in a
simplified fashion. Only the components, logic blocks, switches, etc.,
which are necessary for understanding the special control are represented.
Further, it should be noted that the described circuits, signal lines,
etc., even if not expressly indicated, sense the condition control
quantities on each individual wheel.
The diagrams in FIGS. 2 and 3 illustrate more clearly the circuit
arrangement of the present invention of FIG. 1 which is realized by
hard-wired circuits or by a program structure. On top of the diagrams, it
is represented that FIG. 2 refers to a driving siutation where a vehicle
FZ at point X passes from a road surface with a high coefficient of
friction to a .mu.-split surface. FIG. 3, however, relates to a so-called
.mu.-patch surface where, as soon as the vehicle passes point X1, the
coefficient conditions HM/LM from right to left vary greatly or are
reversed.
FIGS. 2 and 3 show the pressure variation on the right and left front wheel
of the vehicle FZ during a braking operation. It can be seen in FIG. 2
that, at time t.sub.1, approximately the braking pressure in the two front
wheels of the vehicle FZ, which is on a road surface with a high,
homogeneous coefficient of friction, i.e. a HM-road surface, reaches a
wheel lock pressure level. During such a controlled braking operation, the
vehicle reaches a .mu.-split surface and, thus, a particularly critical
driving situation, at time t.sub.x. At time t.sub.x, rapid pressure
reduction caused by anti-lock control starts at the left front wheel which
is now on a low coefficient surface and, hence, is the LM-wheel. The
controller identifies at time t.sub.s that there is a particularly
critical driving situation. The following criteria are satisfied at this
point of time:
The difference of braking pressure reduction times on both front wheels of
the vehicle FZ surpasses a limit value.
The vehicle (reference) speed v.sub.Ref is high and above a predetermined
limit value.
The slip of the LM-wheel surpasses a slip limit value.
The duration of instability of the LM-wheel exceeds a certain,
predetermined time.
The HM-wheel, i.e. the right wheel in FIG. 2, is in the control mode.
No special control has occurred during this control operation (which
commenced at time t.sub.1).
The special control MFO now initializes that, commencing time t.sub.s, the
pressure on the right (HM) wheel is reduced for a predetermined time of 40
msec, for example, (see FIG. 4) which depends on the instantaneous vehicle
(reference) speed. Upon expiry of this period, a new, decelerated braking
pressure increase starts on the HM-wheel so that the braking pressure on
the HM-wheel, with a predetermined gradient, reapproaches the locking
pressure level of this wheel, shown in dash-dotted lines in FIG. 2. In
addition (what is not shown in FIG. 2 for the sake of clarity), pressure
reduction on the rear axle can be effected simultaneously with the
pressure reduction on the HM-wheel in order to still further enhance the
lateral guidance and, thus, the stability of the vehicle. Because the
braking pressure in the rear-wheel brakes is generally controlled
according to the "select-low" principle, the braking pressure on both rear
wheels is simultaneously reduced.
It must be ensured in a special control of the type of the present
invention, which is based on the braking pressure reduction on the
HM-wheel, that a clear-cut distinction is made between a defined,
especially critical driving situation and similar events that are due to
special pavements, cobblestone pavement, rough roads, etc. In the
described embodiment of the present invention, initialization of the
special control is therefore prevented when a very high re-acceleration of
the LM-wheel of, for example, 8 g ("g"=9.81 m/s.sup.2) is detected during
a braking maneuver. When re-acceleration occurs during a special control
operation which is above a second, still higher limit value of e.g. 20 g,
the special control will be discontinued at once. The above criteria are
only a few of possible criteria for distinguishing between an actually
critical and a seemingly critical driving situation.
FIG. 3 shows the braking pressure variation on the front wheels on a
.mu.-patch surface. Braking commences at time t.sub.10, and the control on
the right front wheel which is the LM-wheel at this time commences at time
t.sub.11. For yawing torque limitation, further braking pressure rise on
the HM-wheel is prevented or decelerated by GMB at time t.sub.11. At time
t.sub.x1, the wheel which so far moved on the HM side passes to a low
coefficient surface (LM'). The coefficient of friction changes from LM to
HM' on the other road side. Because the difference of braking pressure
reduction times on both front wheels surpassed the predetermined limit
value already at time t.sub.x1, and all other criteria are satisfied in
addition, special control MFO commences. The braking pressure on the
(previous) HM-wheel is reduced rapidly by actuation of the braking
pressure reducing valves for a predetermined duration, which depends on
the vehicle (reference) speed. The braking pressure is increased with a
predetermined gradient or approached to the wheel lock pressure level on
the wheel brake of the right front wheel which moved from the low
coefficient of friction (LM) to the high coefficient road surface (HM').
In all other respects, the same criteria and measures of influencing
pressure as in the situation of FIG. 2 apply to the situation of FIG. 3.
FIG. 4 shows an example for the dependence of the predetermined duration of
pressure reduction on the vehicle reference speed. Pressure reduction by
the special control (MFO) will not occur below 40 km/h. Initially, i.e.
when the 40 km/h limit is exceeded, the duration of pressure reduction
covers five loops and is extended to a maximum value of seven loops. The
duration of a loop ranges between 5 to 10 msec, for example.
FIG. 5 shows the dependence of the slip threshold on the vehicle reference
speed. A criterion for the existence of a particularly critical driving
situation and for the initialization of the special control is satisfied
when the slip of the LM-wheel is in excess of the speed-responsive limit
value. For example, the slip in FIG. 5 must exceed 40% in the vehicle
speed range between 70 and 100 km/h.
FIG. 6 shows the linking of various criteria in a flow chart. A subroutine
for the start of the special control (MFO) according to the present
invention may be developed on the basis of such a type of linking the
individual functions when the invention is realized by programmed
circuits, such as microcomputers, microcontrollers, etc.
The subroutine is started in a step 20. A number of branching points 21,22
. . . 2n will follow in the program run where the program flow is
continued only if a defined condition or a defined criterion is satisfied
("yes"). If the criterion is not satisfied ("no"), the program run is
returned to start. For example, the decision in branch 21 depends on the
fact whether the difference in braking pressure reduction
(.DELTA.p-reduction) has surpassed a predetermined limit value p.sub.0.
Branch 22 interrogates whether the slip of the LM-wheel (.lambda..sub.LM)
has exceeded the slip limit value (.lambda..sub.0). If "no", the loop is
returned to the starting point in both cases, if "yes", the next step will
follow. Finally, special control (FMO) will be triggered in a step 30 when
all interrogated criteria are satisfied.
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