Note: Descriptions are shown in the official language in which they were submitted.
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ALFRED TEVES GMBH :
Frankfurt am Main
Method and Device for Controlling the
Distribution of Brake Force
The present invention relates to a method for control-
ling the distribution of brake force onto the front axle and
the rear axle of an automotive vehicle in dependence upon
the static and dynamic axle load:distribution as well as
other measured variables derived from the braking behaviour,
wherein the measured variables utilized for the control are
ascertained, electronically combined and processed by pick
ups for measuring data or sensors, respectively, and wherein
the braking pressure applied to the rear wheels i5 governed
in relation to the braking pressure applied to the front
wheels~ In addition, the present invention relatas to a
brake force distributor for implementing the method, which
i5 equipped with sensors for sensing measured vaeiables uti-
lizable for the control of the brake force distribution,
with electronic circuits for the combina:tion, processing and
evaluation of the sensor signals as well as for the genera-
tion of actuating signals for one or for several braking .
pressure modulators~
A:brake force distributor of this type is already known,
wherein the sensors serve to measure the static axle load
distribution with ~he vehicle at:standstill and to feed the
data into a microcomputer which latter,~while taking into
account these measured variables, controls the brake force
distribution pursuant a memorized mathematical expression
and, in addition, under consideration of the pressure
measured in the front-axle and in ~he rear-axle circuit
(European patent application ÆP-Al 062246)~ It is a
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disadvantage in this arrangement that the actual value of
adherence between road and vehicle at the rear axle and the
front axle cannot take influence on the brake force distribu-
tion, for what reason the rear axle will contribute little to
the braking action in most cases, with a view to avoidin0 the
dangerous overbraking of the rear axle.
The brake force distributors commonly used nowadays
are limited to an invariably adjusted, pressure-responsive con-
trol. Load-responsive or deceleration-responsive brake force
regulators are likewise known in a great number of variants.
Even when making great assembling and adjusting efforts, a
satisfactory adaption will be accomplished at most in one of
the two extreme conditions "unloaded/loaded", so that under
many operating condi-tions the exploitation of the adherence
value at -the front and the rear axle that is theoretically
possible will not be achieved.
It was therefore an object of the present invention to
improve the brake force distribution in automotive vehicles
such that, in both extreme conditions unloaded/loaded and with
every braking, the brake force distribution wlll be in close
approximation adapted to the actual static and dynamic axle load
distribution. It has likewise been attached importance to meet
the requirements of manufacturing a corresponding brake force
distributor with comparatively little effort.
The invention provides a brake force distributor for
automotive vehicles, for controlling the distribution of brake
force onto the front axle and the rear axle of the vehicle with
sensors for sensing rotating wheel and an axle load character-
istics utilizable for the control of the brake force distribu-
tion, with electronic circuits for the combination, processingand evaluation of the sensor signals and for the generation of
control signals with modulator means for the control of the brak-
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ing fluid pressure at the rear axle as a function of the
braking fluid pressure at the front axle and the sensor signals,
wherein said sensor are provided for the measurement of rota-
tional behavior of the wheels at the front axle and at the rear
axle, and said modulator means including a modulator for con-
trolling the braking pressure at the rear wheels so that the
coefficient of friction at the unlocked rear wheels is a pre-
determined fixed value in the range of 80 to 99~ of the coeffi-
cient of friction at the front wheels; said modulator adapted to
control a sequential connectlon of said rear wheel circuits.
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Hence, this invention is based on the knowledge that the
brake slip at the front and at the rPar axle of an automo-
tive vehicle is particularly suitable for the control of the
brake force distribution and for the design of an effective
brake force distributor which affords to be realized rela-
tively easily~ The requisite measured variables can be
ascertained by means of conventional velocity sensors, the
signals whereof permit to likewise derive the deceleration
and a reference velocity, as well as~ if necessary, by means
of an additional sensor for measuring the translational
velocity of the vehicle. After having processed and log-
ically combined these sensor signals in a conventional
fashion by electronic circuits, there will result control
signals for solenoid valves which controI directly the share
of braking pressure acting on the rear axle. On the one
hand, it will be accomplished thereby that front and rear
wheels contribute approximately evenly to the braking of the
vehicle; on the other hand, it will be ensured that the rear
wheels are allowed to lock but after the front wheels, what
i5 of great significance for the directional stability of
the vehicle.
Further advantages and applicabilities of the invention
will become obvious from the following description of de-
tails as well as of embodiments of this invention.
In the drawings,
Figure 1 is a schema~ically simplified block diagram of
an embodiment of the brake force distributor
according to the present invention,
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Figure 2 is the diagram of the coefficient of friction
in dependence upon brake slip and braking in a
brake force distribution of the known type
(Figure 2 a) and in a control according to the
inventive method (Figure 2 b),
Figure 3 is the diagram of the time-responsive curve of
i~ braking pressure at front and rear wheel when
utilizing the embodiment o~ Figure 1,
Figure 4 isj alike the illustration in Figure 3, the
braking pressure curve according to another
embodiment of the present invention.
Figure S is, alike the illustration in Figure ~,
another embodiment with pressure limitation at
. the commenc~ment of a locked condition, and
Figure 6 i5 a schematically simplified block diagram o~
an embodiment of the inventive brake force
distributor with diagonal brake-circuit
split-up.
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According to the embodiment vf the invention illustrated
in Figure 1, the inventive brake force distributor comprises
substantially a regulator 1, several sensors 2, 3, 4 and a
braking pressure modulator 5, which latter, in the embodi-
ment shown, is composed of a quick-acting electromagnetical-
ly actuated two-way~two-position direc~ional control valve 6
and a check valve 7 for the fast removal of the pressure
during release of the brake.
The sensors 2, 3 serve to ascertain the wheels' rota-
tional behaviour at the front axle ( C~ VR) and at the
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rear axle ( ~ HR). To this effect, for instance, each one
inductive speed sensor may be used in a known fashion, one
sensor per axle being assumed to suffice in most cases. For
measuring the deceleration of the vehicle, the sensor 4 may
be designed in the form of a known electromechanical trans-
lational sensor, for example. A corresponding reference V2-
locity is, however, permitted to be formed likewise with the
aid of wheel velocity sensors and ~y suitable combination of
the sensor signals or by forming the average value of the
measurements of several velocity sensors.
The electronic circuits for the processing and the com-
bination of the sensor signals and for the generation of the
control signals for the switching valve 6 are arranged in
the circuitry 1. For the formation of the control clock for
the switching valve 6, a microcomputer can be fit~ed into
the circuitry 1.
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The switchin~ valve 6 of Figure 1 connects, as long as
it is de-energized, the hydraulic front-wheel circuit VR
with the rear wheel circuit HR of the brake system ~ For
the reduction of the share oE braking force allotted to the
rear axle, the valve 6 will be closed pulsatingly, what will
be explained in more detail hereinbelow by way of Figures 3
and 4.
The brake characteristics which have so far been assumed
to be constant in the dimensioning of the brake force dis-
tribution are subject to considerable variations .in prac-
tical operation, for instance on account of manufacturing
tolerances of the friction value, due to aging, contamina-
tion, tempèrature changes etc. This has as a consequence
that the actual characteristic curve of the brake force dis-
tribution may differ considerably from the predetermined
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one. The inventive method, however, takes these variations
into account because the tire slip ~ that is proportional
to the actual adherence or the measured variables derived
from the tyre slip will be evaluated for the control of the
brake force distribution. Figures 2 a and 2 b depict this
mode of function and the progress achieved.
The diagram according to Figure 2 a applies for conven-
tional brake force distributors with invariably determined
brake force distribution between front axle and rear axle.
In the event of braking of, for instance, z = 0,3, as is il-
lustrated, there results for the front axle VR a c~efficient
of friction of FVR = 0,46, while compared thereto, there
results for the rear axle HR only a coefficient of friction
f fHR = 0,19~ The corresponding operating points on the
adherence~slip curve - that is the right-hand part of Figure
2 a - are therefore spaced considerably from one another.
Thus the rear axle contributes relatively little to the
braking action.
With a brake force distribution according to the
teachings of this invention or when using a brake force dis-
tributor of the inventive type, as is described by way of
Figure 1, both axles contribute almost evenly to the braking
because the share of braking pressure at the rear wheels
will be regulated such that the coefficient of friction a
the rear axle will be less than the coefficient o~ friction
at th~ front axle by merely the predetermined amount of a
few per cent, preferably 3 up to 15 ~. Therefore, with a
master cylinder pressure predefined, the stopping distance
wilL be considerably shorter, because the rear wheels will
take more effect, in particular when braking with lower
force.
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As has been indicated alxeady in he description of the
embodiment according to Figure 1, the variation of the share
in braking pressure applied to the rear axle is permitted to
be attained in a particularly simple fashion and yet very
precisely and sensitively by pulsating actuation of the
switching valve 6. As is shown in Figures 3 through 5,
there results a pulsewise increment of the pressure PHR at
the wheels of the rear axle. By variation of the pulse-to-
pulse ratio of the switching signals supplied from the cir-
cuitry 1 to the switching valve 6 and thus of the switching
position "open~interrupted", there will be achieved the de-
sired distribution of the braking pressure to the front axle
and the rear axle.
The time-responsive curve of the braking pressure PvR
at the front axle in relation to the braking pressure P~R
at the rear axle according to the illustration in E~igure 3
is preEerably applied ~or vehicles which are equipped with
disc brakes at the front axle and with drum brakes at the
rear axle. This is because it is advisable in such brake
systems, for the purpose of shortening the process of appli-
cation of the drum brake, to admit also a limited pressure
build-up at the rear axle at the commencement of braking for
a short interval tl without delay and to provide for a
pressure-retaining phase t2 only then, be~ore the further
pulsating pressure build-up at the rear axle takes place.
In contrast thereto, a control of the pressure rise ac-
cording to Figure 4 is to be preferred in vehicles with
similar brakes at front and r~ar axle. The front wheel
pressure PVR will rise again linearly proportional to the
pedal force. The pressure PHR at the rear wheels, which
is built up pulsatingly also in this case, follows, on the
other hand, the front wheel pressure staggeEed in time
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and will not reach its constant value, which latter is de
fined by the electronics according to the criteria chosen,
until the point of time tH.
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Furthermore, according to another embodiment of the
present invention which is shown in ~igure 5 t the electron~
ics can be dimensioned such as to prevent further increase
of the presssure PHR at the rear axle in excess of P~RO
upon recognition of an imminent locked condition at the
front wheels at the point of time tA; this is the point of
time the locked condition begins. A tendency to lock or an
imminent locked condition permits to be derived electroni-
cally, for instance, by the speed variation measured with
the sensors 2, 3 or by wheel deceleration, respectively; if
these measured values exceed an upper threshold val~e, this
may be interpreted as the beginning of wheel lock.
If the rise of the rear wheel pressure, as is illus-
trated in Figures 4 and 5, is kept below the front wheel
pressure, the limitation of the pressure increment at ~he
rear axle will serve in many cases to prevent locking of the
rear wheels upon recognition of the tendency to lack of the
front wheels. That is because the logic decides in a logi-
cal fashion to stop the further pressure build~up at the
rear axle. ~ince the pressure build-up at the rear axle, as
has been previously explained, succeeds the pressure at the
front axle with time lag, the rear wheel has not yet reached
the maximum of the adherence~slip curve at this psint of
time, that is to say, it will be safe not to lock. ~his
variant of the inventive brake slip regulator is of great
significance in respect of the track stability of the
vehicle.
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In the vehicle equipped with the inventive brake force
regulator illustrated symbolically in Figure 6, two hydrau-
lic brake circuits are provided which are split up diagonal-
ly to the wheels. At a tandem master cylinder 9 actuated by
the pedal 8, the hydraulic connection 10 leads directly to
the wheel brake cylinder of the right front wheel VreChts
and via a spherical separating valve 12 to the left rear
wheel HlinkS~ In a like manner, the second brake circuit
extends via the connection 11 directly to the left front
wheel Vlinks as well as via an electromagnetic separating
valve 13 to the right rear wheel hreCht5. C
parallel to the solenoid valve 13 is a check valve 14 for
quicker pressure removal during release of the brake.
~ The mode of operation of the device according to ~igure
6 corresponds to that one of Figure 1. Supplied to a cen-
tral electronic logic circuit 15 that contains a micropro-
I cessor in this arrangement will be signals which correspondi to the rotational behaviour of the rear wheels and which are
r~ceived by the sensors 16, 17, as well as a signal of a
sensor 18 corresponding to the average front wheel velocity,
and ~inally a deceler~tion signal produced with the aid o~ a
0 sensor 19. In this case, the sensor 18 is seated on a dif-
~ ferential gear 20 by which the front wheels are driven.
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Actuation of the brake via the pedal ~ will cause imme-
diate increase of the pressure at the two front wheels. As
; soon as the solenoid valve 13 is actuated or exci~ed and
switched over, pressure build-up is allowed to start in the
rechts Simultaneously, the~control piston of
this valve will be displaced via the hydraulic connection
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leading from the solenoid valve 13 to the spherical valve
12, as a result whereof the rear wheel ~links will be con-
nected to the second brake ~ircuit.
¦ The spherical valve 12 can be substituted by a second
s~lenoid valve which will then be likewise actuated by the
circuitry 15 and, suitably, operated in parallel to the
: solenoid valve 13.
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Besides, it is assumed to be favourable in many appli-
; cations to design both the solenoid valve 13 and the spher-
: ~ cal valve 12 as integral components of the tandem master
~ cylinder 9.
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