CA1248160A

CA1248160A - Method and device for controlling the distribution of brake force

Info

Publication number: CA1248160A
Application number: CA000445780A
Authority: CA
Inventors: Hans-Christof Klein
Original assignee: Individual
Current assignee: Individual
Priority date: 1983-01-21
Filing date: 1984-01-20
Publication date: 1989-01-03
Also published as: GB2136519A; FR2539687B1; IT8419253A0; GB8400621D0; SE8400149D0; JPS59137245A; GB2136519B; SE8400149L; DE3301948A1; SE457785B; FR2539687A1; BR8400095A; IT1173090B

Abstract

Abstract S u m m a r y:
Method and Device for Controlling the Distribution of Brake Force With a view to controlling 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, sensors (2 through 4, 16 through 19) serve to ascertain the wheels' ro-tational behaviour at the front axle (VR) and at the rear axle (HR) as well as, if necessary, the translational decel-eration of the vehicle, and, by these data, there will be determined the brake slip ( .lambda. ) at the front and at the rear wheels or measured values that are proportional to the brake slip, respectively. After the electronic combination and processing of these signals, the brake force distribution will be controlled in dependence upon the brake slip such that the coefficient of friction (fHR) at the rear wheels amounts to about 80 through 100 %, preferably 85 through 97 %, of the coefficient of friction (fVR) at the front wheels.

The braking pressure build-up at the rear wheels is gov-erned preferably with electromagnetic switching valves (6, 13), the time-responsive pressure increment occurring step-wise.
Figure 1

Description

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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 lZ481~
~ ~ 2 -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-3B, lZ~ t;0 ~ 2a -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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o i 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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4 ~ -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 ~2~60 ~ 5 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.
.' 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 lZ~ 6V

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 iZ48~60 .

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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.

Z481~) 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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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. 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 characteristics utilizable for the control of the brake force distribution, with electronic circuits for the combination, processing and evaluation of the sensor signals and for the generation of control signals with modulator means for the control of the braking 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 rotational behavior of the wheels at the front axle and at the rear axle, and said modula-tor means including a modulator for controlling the braking pressure at the rear wheels so that the coefficient of friction at the unlocked rear wheels is a predetermined fixed value in the range of 80 to 99% of the coefficient of friction at the front wheels; said modulator adapted to control a sequential connection of said rear wheel circuits.

2. A brake force distributor as claimed in claim 1, wherein said sensors include a sensor for the measurement of the translational deceleration of the vehicle, whose output signal is combinable with the signals of the front axle and of the rear axle in the electronic circuit.

3. A brake force distributor as claimed in claim 1, where-in said modulator comprises a quick-acting electromagnetically operated two-way, two-position direction control valve which, when de-energized interconnects the hydraulic front wheel cir-cuit and the rear wheel circuit and which disconnects said cir-cuits when energized.

4. A brake force distributor as claimed in claim 1, wherein said modulator contains a quick-acting electromagnetical-ly operated two-way, two-position direction control valve which, when energized, interconnects the rear wheel circuits and dis-connects said circuits when de-energized.

5. A brake force distributor as claimed in claim 4, wherein the distributor controls a dual-circuit hydraulic brake system with diagonal brake force split-up, the distribution of the braking pressure in a first brake circuit being variable by means of said electromagnetically actuated two-way, two-position directional control valve governed as a function of the brake slip, while it is variable in the second brake circuit by means of a spherical separating valve of said modulator means which is hydraulically governed as a function of the braking fluid pres-sure in the rear wheel circuit of the first brake circuit.