DE4300048A1 - Method for determining the adhesion/slip characteristics of the tyres of a road vehicle - Google Patents

Abstract

For determining the adhesion/slip characteristics of the tyres of a road vehicle, which is equipped with an anti-lock system configured for individual wheel control, during an initial phase of a target braking, only the wheel brake of a single vehicle wheel is subjected to brake pressure and this is dimensioned in such a way that the initial deceleration corresponding to the desire of the driver results. During this initial test phase of braking, the absolute brake slip (B) and the utilisation (B) of adhesion coupled with this are continuously determined; as soon as the vehicle deceleration z no longer increases because of the sole braking of the test wheel and/or the braked vehicle wheel decelerates more severely than corresponds to a predetermined threshold value, the test braking is terminated and the braking is continued by applying pressure to the further vehicle wheels; corresponding test braking phases are carried out in cyclic sequence for all the reference wheels. <IMAGE>

Description

The invention relates to a method for determining the Traction / slip characteristics of the tires of a street vehicle with a single wheel control designed anti-lock braking system, after which in driving mode of the vehicle from measured value pairs hatching and given hatching used adhesion coefficient on the course of each tire characteristic curve in the entire µ / λ field is closed.

One such method is through the German patent registration P 41 02 301.3 known.

According to this known method, traction drive the vehicle's tire characteristics determined vehicle wheels and because of the equivalence of traction and brake slip that mitit Tire characteristics also for the determination of Brake slip thresholds are used when they are exceeded the anti-lock braking system of the vehicle should respond. On the maximum exploitable coefficient of adhesion - that  µ maximum of the respective tire characteristic curve is from the Spin behavior of the driven vehicle wheels ge closed, the speed of which exceeds a Maximum of the characteristic value corresponding to the slip value grows dramatically. A precise determination of the tires Characteristic curves in braking operation is known after this Procedure not possible.

The object of the invention is therefore a method of Specify the type mentioned in the braking mode of the vehicle an exact determination of the tire characteristics enables knowledge of which is a prerequisite for a the dynamic stability of a vehicle in one slip re-maintaining as far as possible is, as well as a slip control system with which the procedure is feasible.

This task is accomplished with regard to the procedure the characteristic features a) to d) of the patent Proverb 1 and in a further embodiment of this procedure rens by the features of claims 2 to 10 and back visually of the slip control system by the kenn Drawing features of claim 11 and the to these related claims 12 to 20 solved.

After that, in an initial phase, one with moderate Vehicle deceleration linked target braking the wheel brake only a single vehicle wheel with brake pressure acted upon and this brake pressure increased so quickly gert that a Ent corresponds to the driver's request  winding deceleration results. In this one The initial test phase is continuously the absolute Brake slip measured, which is very possible is that at least the non-braked vehicle wheels roll freely and therefore very precise information can be achieved via the vehicle speed, the exact knowledge essential prerequisite for the exact te slip determination is. The vehicle deceleration can also very accurate based on the wheel speeds of the decelerated vehicle wheels can be determined. That the Braking under the vehicle wheel can be very close to the "blocking limit" will be slowed down because the free rolling vehicle wheels the vehicle enough sides convey leadership to the required Stabili to ensure activity. The test braking is canceled as soon as by braking the test bike alone the vehicle deceleration z no longer increases and / or the braked vehicle wheel is decelerated more than it corresponds to a predetermined threshold. By performing these measures cyclically, all vehicle wheels have the valid tire characteristic averages, depending on the duration of the trip or number of Braking carried out several times during a journey can be updated so that changes in the Tire characteristics are detectable, which are then si tuation-appropriate setting of response thresholds z. B. can be used for an anti-lock braking system.

Because there are fewer vehicle decelerations via the rear wheels are reachable than via the front wheels, it is in front  partial if the test braking phases in the by the Features of claims 2 to 4 specified "order ge "can be carried out.

Furthermore, it is advantageous if in a through Management of the test braking phase on a driven vehicle test wheel this abge from the drive train of the vehicle is coupled so that a reaction of the drive train is excluded on the braked vehicle wheel.

The driver's request "target braking" can alternatively or in Combination can be recognized by the fact that the driver Brake pedal of the brake system operated with a force which is less than a predetermined threshold or that the vehicle deceleration is kept smaller is set as an adjustable threshold very precisely because the brake pressure that the Fah rer by actuating the braking device in the braking system is less than a threshold.

Instead of a tabular input of measured force final slip value pairs in a memory of an elec tronic control unit, it is advantageous to follow the tire characteristics determined by the method according to the invention Generate lines continuously, as per the requirements Chen 9 and 10 provided.

With regard to the implementation and application of the Suitable slip rain method according to the invention is by the features of claim 11  and in a further embodiment by those of the An Proverbs 12 and 13 a structurally simple and space Economical, especially reliable design ei provided in the vehicle's braking system specified braking device, with the brake closed circles are exploitable, what from safety-related Reasons is particularly advantageous.

By means of a provided between claim the front axle brake circuit of the driver's braking system witness assigned output pressure space of the tandem main cylinders and continue to the front brakes de Main brake line of the front axle brake circuit ge switched, acting as a pressure converter secondary cylinder the one who in turn has a control pressure room in the a controllable outlet pressure of the control pressure source can be coupled in to this in the output pressure space Secondary cylinder by simply actuating the master cylinder Lindner's output pressure can be additively superimposed is additional variability in terms of Braking force distribution control is achieved with it before is partial if the secondary cylinder has its own Control pressure output assigned to the control pressure source is.

By means of a pressure provided according to claim 16 sors, the one for printing in the tandem master cylinder characteristic electrical output signal generated, is the one in terms of the desired Existing driver request recognizable.  

Pressure sensor provided according to claims 17 and 18 ren, for the at the respective pressure output of the tax Pressure source provided control pressure characteristics electrical output signals or for those in the brake pressure coupled to individual wheel brakes generate teristic electrical output signals that the electronic control unit as an information input are supplied, allow a precisely dosed Pressure metering to the individual wheel brakes under wah optimal dynamic stability of the vehicle.

Both for detecting cornering situations as well also to control the braking force distribution on the individual vehicle wheels in the sense of e.g. B. a uniform use of traction on all vehicle wheels, it is advantageous if the vehicle with a Querbe acceleration sensor and / or a yaw angle sensor is equipped with the electronic control unit of the slip control system processable generated electrical output signals.

To detect the driver's request "target braking" or "Full braking", it is also advantageous if a power Sensor is provided, one for the force with which the driver operates the brake pedal of the brake system, characteristic electrical output signal generated, by that when the signal level is a threshold exceeds the execution of a test braking phase canceled or their initiation ver is prevented.  

The method according to the invention and one to its through suitable slip control system are implemented following with reference to a vehicle brake system the drawing described and explained in more detail. It shows gene:

Fig. 1 equipped with a brake slip control system according to the invention hydraulic two-circuit brake system of a vehicle in simplified, schematic block diagram representation;

Fig. 2 constructive details of a usable in the brake system of FIG 1 Bremsge device;

Fig. 3 shows a detail of the tandem master cylinder of the braking device according to FIG. 2 and

FIG. 4 shows a friction / brake slip tire characteristic curve to explain the function of the slip control system according to FIG. 1.

By in FIG. 1, its basic structure according represented generally by 10 denoted hydrau metallic dual-circuit brake system, a road vehicle is represented, which is lung system equipped with a complex slip Rege that the vehicle, not the possibility of damaging an advantage of high driving Tool accelerations and decelerations in statistically significant driving situations enable a high degree of dynamic stability and thus safety in driving. This slip control system mediates the functions of both an anti-lock braking system (ABS) and that of a traction control system (ASR), as well as that of an - electronic - brake force distribution control (EBKV), which is suitable for the vehicle desired braking force distribution to stamp on the z. B. in a partial braking of the Ausnut tion increased rear axle braking force shares can correspond and with full braking an ideal braking force distribution, the circuit utilization essentially the same force on the - braked - front and rear wheels of the vehicle corresponds.

In addition, the slip control system is gone designed that the wheel brakes in any Combination and regardless of the driving mode, be it that the vehicle is accelerating or decelerating or in a ride at a uniform speed speed, can be activated automatically, the individual contributions of the unfolded via the wheel brakes Braking forces are individually adjustable.

In order to achieve these functions, the brake system 10 is constructed in more detail as explained below, reference being made to FIG. 2 for the explanation of constructive and functional details of the brake system 10 .

For the vehicle, advance for the purpose of explanation set that it has a rear axle drive in which the possibility of an automatic "interruption" of the Drive train is provided, be it that the vehicle has an automatic transmission that works rend while driving, especially when braking the driver stuff, automatically switched to idle can, or in that if the vehicle with a Manual gearbox is equipped with an electric control The clutch can be released and reinserted over which the drive motor and the gearbox with are detachable.

The one to release the clutch and turn on the empty Automatic gearbox position required Chen facilities, the simplicity of the presentation sation, not shown in the drawing figures.

For the brake system 10 , for the purpose of explanation, it is further assumed that the left front wheel brake (VL) 11 and the right front wheel brake (VR) 12 form a front axle brake circuit I and the left rear wheel brake (HL) 13 and the right rear wheel brake (HR ) 14 are combined to form a rear axle brake circuit II.

Provided for the brake pressure supply of the two brake circuits I and II, designated overall by 16 brake device of the brake system 10 , which has a pressure output 19 assigned to the front axle brake circuit I, to which the main brake line 22 of the front axle branching to the front wheel brakes 11 and 12 -Brake circuit I is connected and a pressure output 21 assigned to the rear axle brake circuit II, to which the main brake line 23 branching to the rear wheel brakes 13 and 14 is connected to the rear axle brake circuit II, comprises a stepped tandem master cylinder 16 'of a special type, whose structural details can be seen in FIG. 2, which can be actuated by means of a brake pedal 17 via a brake booster 18 , which can be designed as a hydraulic or as a pneumatic brake booster and has a pressure output 19 'assigned to the front axle brake circuit I, to which a essentially functioning as a pressure transducer Secondary cylinder 16 '' is connected, the sen pressure output 19 '' is directly connected to the front axle brake circuit I assigned pressure output 19 of the braking device 16 , and a rear axle brake circuit II assigned pressure output 21 ', which is directly connected to the rear axle -Brake circuit II assigned pressure output 21 of the braking device 16 is connected or forms this directly.

The brake calipers 24 and 25 of the front brakes 11 and 12 and the rear brakes 13 and 14 , which are each designed as disc brakes, are required as two-piston calipers, which are each designed identically on the front brakes and also brake on the rear wheel have an identical, but different from that of the front wheel brakes, which in the sense of a fixed adjustment of the front / rear axle brake force distribution to dynamic-stable deceleration behavior of the vehicle in the entire braking range - partial braking to full braking - is taken.

It goes without saying, however, that instead of two-piston calipers 24 and 25 , four-piston brake calipers, each with two pairs of pistons, could be used, where - for each brake - only one piston pair of brake calipers 24 and 25 for a "normal", ie braking without slip control can be used and the other piston pairs could be used for slip control.

The vehicle is equipped with an anti-lock braking system (ABS), which is represented by its electro-hydraulic control unit, designated 20 in FIG. 1.

For this anti-lock braking system, it is assumed in the special exemplary embodiment shown that, in order to be able to keep the brake circuits I and II closed, it works according to the recirculation principle, after which, in brake pressure reduction phases of the anti-lock control, from the front wheel brakes 11 and / or 12 or the rear wheel brakes 13 and / or 14 drained brake fluid back into the main brake line 22 and / or 23 of the front axle brake circuit I or the rear axle brake circuit II and is pumped back to the brake device 16 via this.

This anti-lock braking system is - in a known manner of implementation - designed in such a way that on the wheel brakes 11 and 12 or 13 and 14 of the respective brake circuit I or II "opposite phase" brake pressure changes are possible, that is, on one Wheel brake 11 or 12 or 13 or 14 of the respective brake circuit I or II brake pressure can be reduced or built up, while on the other wheel brake of the respective brake circuit I or II brake pressure is built up again.

Accordingly, within the scope of the electrohydraulic control unit 20 of the anti-lock braking system, the front axle brake circuit I and the rear axle brake circuit II are individually assigned subunits 20 'and 20 ''hen, each of which, seen in isolation, has an opposite-phase brake pressure reduction or Enable brake pressure build-up on the left front wheel brake 11 and the right front wheel brake 12 or the left rear wheel brake 13 and the right rear wheel brake 14 of the vehicle.

The front axle brake circuit I assigned subunit 20 'of the electrohydraulic control unit 20 of the anti-lock braking system comprises, in a known circuit arrangement 2 of the left front brake 11 and the right front brake 12, the individually assigned inlet valves 26 and 27 , by means of which the branching point of the United States 28 of the main brake line 22 of the front axle brake circuit I outgoing, to the individual wheel brakes 11 and 12 leading brake line branches 22 'and 22 ''are individually releasable and lockable. These A lassventile 26 and 27 are - in the example shown Ausfüh approximately - designed as 2/2-way solenoid valves, the basic position 0 is their flow position, in which the front wheel brakes 11 and 12 with the front axle brake circuit I assigned pressure output 19 of the braking device 16 are communicatively connected, and their energized position I is their blocking position, in which the front wheel brakes 11 and 12 against the branching point 28 of the main brake line 22 of the front axle brake circuit I and thus also against the pressure output 19 of the brake device 16 assigned to this are blocked.

Next, the sub-unit 20 'of the electrohydraulic control unit 20 of the anti-lock braking system the derradbrems 11 and 12 each individually assigned outlet valves 29 and 30 , by means of which the two front brakes 11 and 12 - individually or collectively - to a return line 31 of the front axle brake circuit I can be closed, from the return pump 32 assigned to the front axle brake circuit I in brake pressure reduction phases of the brake pressure control to at least one of the front wheel brakes 11 and / or 12 let off brake fluid into the main brake line 22 of the front axle brake circuit I and can be pumped back into the braking device 16 .

The exhaust valves 29 and 30 of the front axle brake circuit I are, as shown in FIG. 1, designed as 2/2-way solenoid valves, the basic position 0 is their blocking position, in which the wheel brake (s) 11 and / or 12 against the return line 31 is / are shut off, and the excited position I is their Durchflußstel development in which the wheel brake (s) 11 and / or 12 of the front axle brake circuit I is / are connected to the return line 31 .

To the return line 31 of the front axle brake circuit I, a low-pressure accumulator 33 is connected, which is configured in a conventional design as a piston-spring accumulator and can quickly absorb drained brake fluid speed in brake pressure reduction phases that take place on the front axle brake circuit I, to achieve the necessary quick le pressure relief of the respective front brake 11 and 12 , respectively, the return pump 32 , the der der pressure accumulator 31 via an input check valve and an output check valve to the main brake line 22 of the front axle brake circuit I is connected, the drained brake fluid, if necessary, pumps back from the low-pressure accumulator 33 into the braking device 16 .

With this analog circuit combination and function, the rear axle brake circuit II assigned subunit 20 'of the electro-hydraulic control unit 20 of the anti-lock braking system, the rear wheel brakes 13 and 14 individually assigned inlet valves 34 and 35 , by means of which the branch brake 36 of the main brake line 23 of the rear axle Brake circuit II to the sen wheel brakes 13 and 14 continuing brake line branches 23 'and 23 ''are each individually releasable or lockable, and the wheel brakes 13 and 14 of the rear axle brake circuit II individually assigned exhaust valves 37 and 38 , by means of which the wheel brakes 13 and 14 of the rear axle braking circuit individually II or 'of the rear axle brake circuit II are lockable connectable or against these, as well as of the pump an the rear axle brake circuit II associated Rückför 32' common to a return line 31 and a to the return line 31 'of the Rear axle brake circuit II connected n Low pressure memory 33 ', from / in the brake pressure reduction phases that are required on the rear axle brake circuit II, sas brake fluid discharged into the drain line 23 of the rear axle brake circuit II or the associated functional part of the braking device 16 of the brake system 10 can be pumped back .

The return pumps 22 and 22 'of the front axle brake circuit I and the rear axle brake circuit II can, as not specifically shown, be designed as piston pumps, in particular as free-piston pumps, which have a common eccentric drive.

In the context of the slip control system of the brake system 10 , a total of 40 control pressure source is provided, which has a front axle brake circuit I assigned control pressure output 41 and a rear axle brake circuit II assigned control output 42 , to which defined, if necessary ver different pressures p _s1 and / or p _{s2 can be} provided, these pressures p _s1 and p _s2 behaving according to a desired time can be raised and lowered accordingly.

A pump 44 , which can be driven by means of an electric drive motor 43, is provided as the pressure generating unit, the delivery rate of which is assumed to be proportional to the speed, the electric motor 43 being controllable with respect to the speed, at least depending on the frequency of a pulsed operating voltage or the level of the supply voltage voltage with which the electric motor 43 is operated.

The control pressure source 40 expediently works with brake fluid as the pressure medium, for which a separate reservoir 45 of the control pressure source 40 is provided.

The control pressure source 40 is provided with a pressure limiting valve 46 which limits the outlet pressure P _s provided at the high pressure outlet 47 of the pump 44 to a value of approximately 80 bar maximum.

Between the high pressure outlet 47 of the pump 44 and the control pressure outputs 41 and 42 of the control pressure source 40 , an outlet control valve 48 or 49 is connected, by means of which the pressures p _s1 and p _{s2 are} controllable, the pressure source at the control pressure outputs 41 and 42 40 can be provided. This outlet control valves 48 and 49 are formed as 2/2-solenoid valves, which are pulsed by electrical output signals from an electronic control unit 50 , which is provided as the central control unit of the slip control system and also for the control the inlet valves 26 , 27 and 34 , 35 and the outlet valves 21 , 30 and 37 , 38 of the electrohydraulic brake pressure control unit 20 of the antiblocking system stems necessary electrical output signals, which testifies the vehicle wheels individually from a comparative and differentiating processing of output signals Wheel speed sensors 51 ge won, the output signals according to level and / or frequency contain the information about the dynamic behavior - wheel circumferential speed, acceleration and deceleration - of the vehicle wheels.

Between the control pressure outputs 41 and 42 of the control pressure source 40 and the reservoir 45 each a pressure reduction control valve 52 or 53 is switched by its z. B. pulsed control, with output signals from the electronic control unit 50 which at the control pressure outputs 41 and 42 of the control pressure source 40 prevailing output pressures can be reduced. The pressure reduction control valves 52 and 53 are also designed as 2/2-way solenoid valves.

The basic positions 0 of both the outlet control valves 48 and 49 and the pressure reduction control valves 52 and 53 of the control pressure source 40 are their blocking positions, in which, on the one hand, the high pressure outlet 47 of the pump 44 is shut off against the pressure outputs 41 and 42 of the control pressure source 40 and others, the control pressure outlets 41 and 42 of the control pressure source 40 are blocked off from their reservoir 55 . Your alternative flow positions I are he excited positions in which the control pressure outputs 41 and 42 - alternatively or together - are connected to the high pressure outlet 47 of the pump, alternatively connected to the reservoir 45 of the control pressure source 40 .

To detect the output pressures p _s1 and p _s2 provided at the control pressure outputs 41 and 42 of the control pressure source 40 , pressure sensors 54 and 55 are provided, which generate characteristic electrical output signals for these pressures p _s1 and p _s2 , which are generated by the electronic control unit 50 forwarded and by which these can be processed.

As further information inputs to the electronic control unit's 50, the output signals of the wheel brakes 11 to 14 individually assigned brake pressure sensors 60 are supplied, which are coupled for the brakes 11 and 12 coupled into the front wheel brake pressures p _vl and p _vr and for the in the rear wheel brakes 13 and 14, coupled brake pressures p _hl and p _hr characteristic, processable by the electronic control unit 50, produce electrical output signals.

To explain constructive details of the braking device 16 , which is shown only schematically in FIG. 1, reference is now made to the relevant detailed illustration of FIG. 2: The tandem master cylinder 16 'is designed as a so-called stepped master cylinder, in which the one axially movable limit its primary outlet pressure chamber 56 forming primary piston 58 and the axially movable boundary of a secondary outlet pressure chamber 57 forming secondary piston 59 un have different cross-sectional areas A ₁ and A ₂ , the cross-sectional area A _{1 of} the primary piston 58 being the larger.

Between the bore step 61 , in which the primary piston 58 is guided in a pressure-tight manner, on which the actuating force 18, which is increased by means of the brake force amplifier 18 , engages via a push rod 62 , and the bore step 63 of the master cylinder housing, denoted overall by 64 , in which the axially movable union Limitation of the secondary outlet pressure chamber 57 forming secondary piston 59 is pressure-tightly displaceable leads, extends a further, central drilling stage 66 , which closes via an annular shoulder 67 to the housing-fixed, radial boundary of the primary outlet pressure chamber 56 forming bore stage 61 and opposite that bore step 63 , in which the secondary piston 59 is displaceably guided in a pressure-tight manner, is delimited by an intermediate wall 68 of the cylinder housing 64 .

In this central bore step 66 , an actuating piston 69 forming the inner axially movable boundary of the primary outlet pressure chamber 56 is displaceably guided in a pressure-tight manner, which is displaceably displaceable by means of a slim, axial plunger 71 which passes through a central bore 72 of the intermediate wall 68 of the master cylinder 64 , centrally of the secondary piston is at an inner piston flange 73 59 supportable, the secondary output pressure chamber 57 directly limiting, "outer" plunger flange 74 is connected to the hold ren plunger flange 73 through a slotted piston rod 76, radially passes through the slot a housing-fixed stop tubes 77, that with a leading to the brake fluid reservoir 78 Ge housing channel 79 is in communicating connection and is provided with openings 81 , on the brake fluid speed in the extending between the two piston flanges 73 and 74 of the secondary piston 59 wake rough m 82 can overflow.

Of the secondary output pressure chamber 57 movably begren collapsing flange 74 of the secondary piston 59 is provided with a, generally designated 83 central valve, which in the illustrated, the non-actuated state of the brake system corresponding basic position of the seconding därkolbens 59 by detent action between a pusher of its valve body and the Stop tube 77 is held in its open position, so that pressure compensation is possible between the secondary outlet pressure chamber 57 and the brake fluid reservoir 78 via the trailing chamber 82 , the central valve 83 after a small initial section of the brake pressure build-up stroke of the secondary piston 59 reaches its closed position, in which the pressure build-up in the secondary outlet pressure chamber 57 is possible. For corresponding pressure equalization in the front axle brake circuit I assigned primary output pressure chamber 56 is a communicating with the brake fluid reservoir 78 in communicating connection bore 84 hen, the pressure chamber-side opening in the non-actuated state of the brake system 10 corre sponding basic position of the Primary piston 58 is released and after a small initial section of the brake pressure build-up stroke of the primary piston 58 is blocked against the primary output pressure chamber 56 , after which brake pressure build-up is possible with further piston displacement.

The penetrated by the axial plunger 71 of the actuating piston 69 , axially fixed to the housing through the intermediate wall 68 of the housing 64 and axially movable by the actuating piston 69 limited annular space 86 , the ring-shaped cross-sectional area by the cross-sectional area A _{4 of} the axial plunger 71 is smaller than the total Cross-sectional area A _{3 of} the actuating piston 69 , on which this can be acted upon by the pressure generated in the primary outlet pressure chamber 56 , is permanently connected via a further housing channel 87 to the brake fluid reservoir 78 and is therefore kept depressurized.

The inner piston flange 73 of the secondary piston 59 is, as can best be seen from the detailed illustration in FIG. 2a, provided with a blind bore 88 open to the intermediate wall 68 of the housing 64 , in which the free end section 71 'of the axial plunger 71 of the Actuating piston 69 is guided so that it can be displaced in a pressure-tight manner, a seal in this regard between the plunger 71 , 71 'and the secondary piston 59 providing de ring seal 89 piston-fixed on the side of the secondary piston flange 73 facing the intermediate wall 68 of the housing.

In the illustrated basic position of the secondary piston 59 and the actuating piston 69 , its plunger 71 is axially supported with the end face of its end section 71 'at the bottom 91 of the blind bore 88 .

In the blind bore 88 of the inner piston flange 73 of the secondary piston 59 , a central compensating bore 92 with a smaller diameter opens out, via which, in the case of relative movements between the secondary piston 59 and the actuating piston 69, brake fluid flows from the follow-up chamber 82 into the blind bore 88 or from the water into the trailing space 82 can be displaced.

The radially against the supply chamber 82 and by means of the fixed to the housing, within the centra len bore 72 of the intermediate wall 68 arranged ring seal 94 against the pressure-less annular space 68 special sealed by means of the piston-fixed ring seal 89 radially in NEN and using a sealing collar 93 of the inner piston flange 73 outwardly, from the free end portion 71 'of the plunger 71 of the actuating piston 69 axially penetrated annular space 96 is connected via a control connection 97 to the control pressure output 42 of the control pressure source 40 assigned to the rear axle brake circuit II and thus forms a control pressure space, by acting on it with the output pressure p _{s2 of} the control pressure source 40 of the pressure output 21 'of the secondary output pressure chamber 57 of the tandem master cylinder 16 ' in the rear wheel brakes 13 and 14, the brake pressure which can be coupled in can be changed in a targeted manner, as a result of which the front axle / rear axle braking force distribution can be changed accordingly.

The primary piston 58 , the secondary piston 59 and thus also the actuating piston 69 are urged by Rückstellfe 98 and 99 in their basic positions marked by stop action, the return spring 98 acting on the primary piston 58 engaging the actuating piston, the basic position of which is thus supported by its axial support on the base 91 of the blind bore 88 of the secondary piston is marked, the basic position of which is in turn marked by rearward contact of its outer piston flange 74 on the stop tube 77 , the return spring 99 of the secondary piston 59 having to be somewhat stronger than that of the primary piston 58 .

The further belonging to the braking device 16 secondary cylinder 16 '', to the pressure output 19 of the main brake line 22 of the front axle brake circuit I is, according to its structure, to the tandem master cylinder 16 'largely analog and mediates in "normal", ie one Slip-control operation not subjected to the braking process, the function of a 1/1 pressure converter, the 'in the primary output pressure chamber 56 when operating the tandem master cylinder 16 ' built up brake pressure on the pressure output 19 - essentially unabated - transmits.

As far as the structural and functional elements of the secondary cylinder 16 '' are assigned reference numerals with respect to the construction and functional elements of the tandem Hauptzylin DERS 16 'are elevated reference numerals used by 100, this should logy reporting the identical construction or -ana the thus designated elements mean and at the same time include the reference to the given by the tandem master cylinder 16 'given description of these functional elements to avoid repetitions ver. The description of the secondary cylinder 16 'is therefore - essentially - limited to the existing over the tandem master cylinder 16 ' of the braking device 16 under different.

The functionally the primary output pressure chamber 56 of the tandem master cylinder 16 'corresponding pressure chamber 156 , which is axially movable by the functionally the actuation piston 69 corresponding actuating piston 169 be limited, has the function of an input pressure chamber here, which is connected via a connecting line 101 to the primary -Ausdruckdruckraum 56 of the tandem master cylinder 16 'is in constantly communicating connection. The water inlet pressure chamber 156 is completed by an axially movable primary piston 58 through a housing-fixed, pressure-tightly attached to the housing 164 from cylinder-end part 102 . In the non-actuated state of the brake system 10 corresponding basic position of the secondary piston 159 of the secondary cylinder 16 'and the actuating piston 169 is the latter via an axially only slightly extended stop extension 103 directly on the housing-from the closing part 102 - axial - supported.

The bore step 161 , in which the actuating piston 169 is displaceably guided in a pressure-tight manner, has the same cross-sectional area A ₁ as the bore stage 61 of the tandem master cylinder 16 'in which the primary piston 58 is displaceably guided in a pressure-tight manner.

Also compared to the actuating piston 169 containing pressure tightly displaceable bore step 161 through the partition 168 delimited bore step 163 , within which by the outer piston flange 174 of the secondary piston 159 of the secondary outlet pressure chamber 157 of the secondary cylinder 16 '' is axially movably limited and by its interior Piston stage 173 , which is firmly connected by the slotted piston rod 176 to the outer piston stage 174 , which is axially movably bounded by the axial plunger 171 of the actuating piston 169 through the annular space 196 , has the same cross-sectional area A ₁ as the inlet pressure chamber 156 of the secondary cylinder 16 '' Or the primary output pressure chamber 56 of the tandem master cylinder 16 'of the braking device 16th This annulus 196 is via its control connection 197 and an existing connection line 104 if necessary with the front axle brake circuit I assigned control pressure output 41 of the control pressure source 40 in connection.

In a gest your by operation of the brake pedal 17 th braking of the vehicle causes the group consisting of the primary output pressure chamber 156 of the tandem master cylinder 16 'in the inlet pressure chamber 156 of the secondary cylinder 16' 'coupled pressure P _V in that on the actuating piston 169 a in the direction of arrow 106, that in that the displacement direction of the actuating piston 169 and the motion-coupled thereto via the plunger 171 Se kundärkolbens 159 directed force is exerted whose magnitude is given p _v by the product of A ₁ *, whereby the secondary piston 159 ver in the direction of arrow 106 inserted is and after a small initial portion of this displacement stroke, the central valve 183 reaches its blocking position, so that now in the secondary output pressure chamber 157 of the secondary cylinder 16 '' a pressure p _{VA is} built up, the amount of which or almost that in the primary output pressure chamber 56 of Tandem Hauptzy linders 16 'built pressure P _v en speaks and is coupled as brake pressure in the front axle brake circuit I.

By coupling the on the rear axle brake circuit II - the brake circuit of the driven vehicle wheels - assigned control pressure output 42 defines adjustable pressure p _s2 in the annular space 96 of the tandem master cylinder 16 ', it is possible to brake the rear brakes 13 and 14 even then to act when the driver does not operate the brake pedal 17 of the tandem master cylinder 16 '. This makes it possible to implement a drive slip control on the - driven - rear wheels of the vehicle, which works on the principle of a vehicle wheel that tends to spin by - deliberately - activating its wheel brake again, thereby increasing torque the selection of the vehicle wheel to be braked, it follows that, while the control pressure source 40 is activated to generate control pressure at the control pressure outlet 42 , the inlet valve 34 or 35 , which is associated with the rear brake 13 or 14 to be activated is held in its basic position, while the inlet valve 35 or 34 of the wheel brake 14 or 13 of the rear wheel not tending to spin is switched into its blocking position 1 , the electronic control unit 15 providing the necessary control signals from a known criteria Processing the output signals of the Vehicle wheels individually assigned wheel speed sensors 60 he testifies.

Even during a braking process controlled by the driver by actuating the brake pedal 17, additional brake pressure can be controlled in the rear wheel brakes by pressurizing the annular space 96 of the tandem main cylinder 16 'in order to in the partial braking range, ie at relatively low vehicle decelerations, in which the maximum exploitable coefficient of adhesion between the road surface and the braked vehicle wheels is not fully exploited to achieve an increased rear axle braking force proportion in order not to unnecessarily load the front wheel brakes, so that their thermal stress is kept as low as possible and the brake load is more even on the front wheel and Rear brakes 11 and 12 and 13 and 14 is "distributed".

Likewise, it is also possible to activate the wheel brakes 11 and 12 of the - not driven - front wheels of the vehicle thereby or to apply a brake pressure which is higher than the pressure output 19 'of the tandem master cylinder 16 ' in one operation the brake system generated output pressure P _v that a control pressure p _{s1 is} coupled into the annular space 196 of the secondary cylinder 16 '', which is provided at a defined level or with a defined rate of increase at the control pressure outlet 41 of the control pressure source 40 assigned to the front brake circuit 1 .

Such coupling of control pressure p _s1 into the annular space 196 of the secondary cylinder 16 '' may be necessary in the course of a braking controlled by the driver by actuating the brake pedal 17 in order to achieve a desired - situation-appropriate - front / rear axle brake force distribution during a regardless of a driver-controlled braking successive activation of at least one of the wheel brakes 11 and / or 12 of the - not driven - front wheels of the vehicle z. B. may be necessary if, for example, in a neither accelerated nor decelerated cornering situation of the vehicle, an oversteering behavior of the vehicle and an associated risk of skidding can only be taken into account by the fact that a brake slip is built up on the front wheels of the vehicle , which at the same time has a reduction in the cornering force of the front wheels, so that the vehicle is somewhat "carried out of the curve", but still remains controllable.

An activation of the brake system 10 by means of the brake pedal 17 independent activation of both the front brakes 11 and 12 and the rear wheel brakes 13 and 14 is also required if the vehicle is equipped with a so-called "distance control" which, depending on the by means of a sensor system detectable distance to a preceding vehicle, an activation of the brake system 10 should trigger when this distance falls below a value considered critical.

The functional properties of the brake system 10 explained so far, essentially the possibility seen in this before, to be able to change the front axle / rear axle brake force distribution in a defined manner, in combination with the possibility of using the inlet valves 26 and 27 as well as 34 and 35 and the exhaust valves 29 and 30 as well as 37 and 38 of the electrohydraulic brake pressure control unit 20 of the anti-lock braking system in the individual wheel brakes 11 to 14 under different brake pressures and thus to be able to control different wheel brake forces, opens up the possibility of braking processes that are required during a journey and also of acceleration processes to be able to continuously determine the current characteristic curves for the individual vehicle wheels and to use the knowledge gained in this way for a z. B. from the point of view of optimizing the dynamic stability of the vehicle to take advantage of suitable slip control.

To determine the "tire characteristics", that is, the relationship between a frictional coefficient µ _B that can be used on the respective vehicle wheel and the absolute braking coefficient µ _B required for the utilization of this coefficient of friction µ _B , which is caused by the relationship

is given, according to a variant of a method suitable for this purpose, the sequence of which is automatically controlled by the electronic control unit 50, proceed as follows, provided that the braking takes place in the course of a straight travel of the vehicle and the driver wishes to carry out target braking , ie braking, in the course of which the driver increases the force K _p with which he actuates the brake pedal 17 , initially relatively slowly from the start of braking until a vehicle deceleration of approximately 0.2 g (g = 9.81 ms⁻ ² ) is reached and the driver then maintains the pedal actuation force K _p constant in order to maintain the vehicle deceleration mentioned for a desired period of time:

In such a situation, only one wheel brake, e.g. B. the left rear wheel brake 13 with brake pressure p _hl , which is dimensioned so that the driver's desired driving test delay z results. The selection of the rear wheel initially used for the brake solution is made by the fact that, controlled by output signals from the electronic control unit 50 , the inlet valves 26 and 27 of the front wheel brakes 11 and 12 and the inlet valve 35 of the right rear brake in their locking positions I are switched . In order to achieve the vehicle deceleration z corresponding to the driver's request with the alone braked left rear wheel, which is achieved by the relationship

z = F _Blh / Gg (2)

is given, with F _Bhl the braking force deployed by means of the left rear wheel brake 13 and G _g denoting the total weight of the vehicle, it is necessary in a customary design of a braking system of a passenger car corresponding cases that the coupled into the left rear wheel brake 13 Brake pressure speaks about 7 times that pressure ent that is built up in the primary output pressure chamber 56 of the tandem master cylinder 16 'of the braking device 16 of the brake system 10 when the driver presses the brake pedal 17 of this tandem master cylinder 16 ' with that actuating force K _p actuated with which he pre-tied the - as desired - vehicle deceleration z.

Accordingly, in the aforementioned, initial phase of braking - by activating the control pressure source 40 - via the control pressure outlet 42 assigned to the rear axle brake circuit II into the annular space 96 assigned to the rear axle brake circuit II of the tandem master cylinder 16 ', a control pressure p _{s2 is} coupled through whose effect on the exposed annular surface of the inner piston flange 73 of the secondary piston 59 of the tandem master cylinder 16 'on the secondary piston 59 is an additional, in the sense of a pressure increase in the secondary outlet pressure chamber 57 of the tandem master cylinder 16 ' we are exerting operating force, by the result that he sufficient to achieve the necessary vehicle deceleration brake pressure applied to the left rear brake 13 is achieved.

In the course of this first phase of braking, which does not necessarily have to be target braking, but can also be braking in which the driver wishes to achieve the most effective vehicle deceleration, the control pressure p _s2 , which is in the rear axle brake circuit II assigned annular space 96 of the tandem master cylinder 16 'must be coupled in, adjusted to approximately 6 times the value of the primary output pressure chamber 56 of the tandem master cylinder 16 ' - by pedal actuation - generated pressure. Of the primary output pressure chamber 56 of the tandem master cylinder 16 'generated pressure P _V is monitored by an electronic pressure sensor 107, which produces an electrical for in the primary output pressure space 56 of the tandem master cylinder pressure prevailing P _V characteristic output signal as Information input of the electronic control unit 50 is supplied.

From a comparison of the output signals of the Drucksen sensor 107 , which monitors the primary output pressure of the tandem master cylinder 16 'and the pressure sensor 55 , which generates a characteristic for the pressure at the control pressure output 42 of the control pressure pressure control 40 generated electrical output signal, produces the electronic Control unit 50 , the control signals for the activation of the motor 43 of the pump 47 and for the outlet control valve 49 and, if necessary, the pressure reduction control valve 53 of the control pressure source, by means of which the pressure output at the control outlet 42 is kept at the required value.

In this first - introductory - braking phase, an evaluation of the output signals of the wheel speed sensors 51 continuously determines the absolute brake slip according to the relationship (1), which can be determined very precisely, since the output signals in particular from the wheel speed sensors assigned to the non-braked front wheels provide signals for Are available, which are a very precise measure of the vehicle speed v _F.

From a temporally differentiated processing of the output signals of the wheel speed sensors 51 assigned to the non-driven front wheels, the vehicle deceleration z can also be determined very precisely, which is linked according to the relationship (2) with the wheel _{braking force} F _Bhl that can be _exerted by means of the braked left rear wheel, which is the calculation

F _Bhl = µBhlF _GHA / ² (3)

is enough so that the relationship

in which F _GHA / ² denotes the normal force acting on the braked rear wheel (wheel load), which corresponds to half the rear axle load and with µ _Bhl the one used in the respective deceleration and the associated brake _slip λ _Blh between the road and the braked vehicle wheel effective force are referred to.

Taking into account the dynamic axle or wheel load shifting that occurs during braking, for the total rear axle load F _GHA by the relationship

F _GHA = (Ψ - z · χ) · Gg (4)

given is,
in which Ψ denotes the rear axle load proportion based on the vehicle weight G _g and χ the wheelbase-related center of gravity of the vehicle (cf. Burkhardt, chassis technology: braking dynamics and car braking systems Vogelbuchverlag, 1st edition 1991, ISBN 3-8023-0184- 6, pages 74 to 80), follows directly from relations (2), (3) and (4):

By means of an evaluation of this relationship (5) carried out automatically by means of the electronic control unit 50 , the associated _frictional coefficient μ _Bhl can thus be determined in addition to the values of the brake slip λ _Blh determined from the output signals of the wheel speed sensors 51 and thus the overall interest is the so-called tire characteristic curve he indirect, for which a typical course is shown in FIG. 4, in which the frictional connection coefficient µ _{B is} plotted as the ordinate and the absolute brake slip λ _{B is} plotted as the abscissa.

The z. For a summer tire, for example, the course of the tire characteristic line 108 shown in FIG. 4 can be seen that with increasing brake slip λ _B initially, as represented by the increasing branch 109 , the exploitable coefficient of adhesion increases and with a brake slip λ _Bom reaches a maximum value of µ _Bmax , which, when used, also achieves a maximum vehicle _deceleration . From this maximum 111 of the tire characteristic curve 108 , the greater the values of the brake slip λ _B, the usable adhesion coefficient µ _B decreases again, which in the practical case of braking means that the brake slip λ _{B must} not be increased beyond the value λ _Bom , because otherwise - due to the decrease in the usable coefficient of adhesion - the braked wheel, which corresponds to the tire characteristic curve 108 , very quickly enters the locked state, which, when the braked wheel is stationary and the slip coefficient µ _BG corresponding to the brake slip 1 , is still reached ne quite effective braking of the vehicle enables light, but dynamic stability of the vehicle is no longer guaranteed and the vehicle is no longer steerable.

The exact knowledge of the tire characteristic curve 108 , at least the course thereof between the coordinate origin of the diagram of FIG. 4 and the maximum 111 of the tire characteristic curve 108 is therefore a prerequisite for the response thresholds of a slip control system, for. B. an anti-lock braking system can be chosen to be sufficiently high so that the highest possible adhesion values _{B can be} used, but on the other hand it is also reliably avoided that the maximum of the tire characteristic curve 108 is exceeded during braking and thereby the vehicle in a dynamically unstable state could get.

The "test phase" of the braking carried out to determine the tire characteristic curve 108 up to its maximum 111 on only one vehicle _wheel is terminated as soon as the brake _slip λ _{Bhl approaches} the value λ _Bom , which the electronic control unit 50 "recognizes" that the vehicle deceleration z no longer increases because the value d µ _B / d λ _B approaches the value 0 and / or because the braked vehicle wheel is increasingly decelerated.

After the end of this "test phase", the braking solution is "conventionally" continued by applying brake pressure to the previously unused wheel brakes 11 and 12 and 14 , the brake pressure p _{hl being} brought to the same pressure level on the left rear wheel which was braked first and lowered - Is, on which the now activated wheel brakes are applied, this pressure level p _hyd in the event that the test braking on a rear wheel of the vehicle has been started by the relationship

p _hyd = _{Φp hydT} / ² (6)

is given in which Φ the rear axle braking force is a share of a braking force distribution that is assumed to be fixed and p _{hydT denotes} the _braking pressure of the wheel brake used in the test phase at the time the test phase was aborted.

After a "first" tire characteristic curve 108 has been determined in the manner described, the tire characteristic curve of a second tire is determined during a next braking operation, e.g. B. that of the right rear wheel, or, if the driver, recognizable by the way in which he operates the brake system 10 , would like to brake with somewhat higher vehicle deceleration, the characteristic of a front wheel tire in an analogous manner, whereby now the relationships instead of the relationships (4), (5) and (6)

F _GVA = (1-Ψ + z · χ) · Gg (4 ′)

such as

and

p _hyd = p _hydT · (1 - Φ) / ² (6 ′)

be valid.

The indices "l, r" apply to "left, right".

The sequence of the braking characteristics from braking to braking can be cyclical, in such a way that after determining the tire characteristic curve applicable for a rear wheel, that of a front wheel, then that of the other rear wheel and then again the other front wheel is determined, etc. or also in such a way that whenever the type of actuation of the brake system 10 by the driver he knows that he wants to brake with relatively high vehicle deceleration, the tire characteristic curve of a front wheel is determined and that of a rear wheel when the driver is with wants to brake relatively low vehicle deceleration.

Due to the continuous determination of the tire characteristics, their changes can also be detected, which change z. B. from changes in tire temperature during a trip, he can give and can be significant. The tire characteristics determined in the course of the test phases of each braking operation, as it were "point by point" from continuously determined adhesion / slip values, can be stored in a memory in the electronic control unit 50 in tabular form and kept ready for a comparison with currently determined slip values which, as it were, the "stability reserve" can be determined, which is still available in a dynamic driving situation in order to, for. B. in the case of cornering, to be able to reliably determine whether with a given brake slip, the same applies analogously to be accelerated cornering for traction slip, sufficient cornering power reserve is still available, or whether brake or traction slip has to be reduced to the vehicle to be able to steer safely through a curve.

In a preferred design of the electronic control unit, this never generates the current tire map by adapting an exponential function corresponding to the characteristic curve 108 to a plurality of suitable support points, e.g. B. the maximum 111 of the tire characteristic curve according to FIG. 4 and a plurality of further support points which lie between the maximum 111 and the origin of the coordinate, this exponential function being represented by the relationship

is given, in which C ₁ , C ₂ and C ₃ - constant - mean tire values. These tire parameters are to be determined in such a way that the adapted curve according to the relationship (7) corresponds as closely as possible to the measured tire curve in the area accessible to the measurement. Suitable values for determination are e.g. B .:

The maximum adhesion coefficient µ _Bmax , the sliding coefficient µ _BG at λ _B = 1 and the optimal brake _slip λ _Bom , at which the maximum adhesion coefficient µ _{Bmax is} reached.

Based on the approximation according to the relationship (7) the following system of equations results

This system of equations is simple Iteration that quickly leads to a good approximation "solve", with who is assumed in a first approximation can:

(C ₁ ) ₁ = µ _Bmax (7.4)

and neglecting the size e ^-C₂ :

(C ₃ ) ₁ = (C ₁ ) ₁ -µ _G (7.5)

what follows

From this relationship, the characteristic value (c ₂ ) ₁ can be quickly determined by the electronic control unit according to an algorithm corresponding to the "Regula falsi", which gives the relationship in a first approximation:

If necessary, further approximation steps can be carried out by the electronic control unit 50 using simple algorithms.

By utilizing a characteristic 108 which essentially corresponds to the relationship (7) and can be generated on a case-by-case basis, rapid measurement of the measured µ _B and λ _B data is possible, as is necessary for effective slip control.

In an approximation that is usually sufficiently good for practical cases, a value between 0.2 and 0.3 can be set for the size C ₃ and for the term to be calculated

a value of about 7, from which the value C ₂ with measured λ _bom can be determined by simple iteration.

The electronic control unit 50 is also equipped with the ability to take into account lateral acceleration occurring in cornering situations and the resulting “lateral” wheel load shifts in the case of cornering braking when pressure is added to the individual wheel brakes 11 , 12 and 13 and 14 , that on all braked vehicle wheels a braking force control is given to the same force utilization, whereby an optimal braking from the vehicle can be achieved. The necessary feedback on the prevailing in the wheel brakes 11 , 12 and 13 and 14 brake pressures is achieved by these individually assigned pressure sensors 60 .

The information about the accelerations occurring in cornering can be obtained directly or indirectly from an evaluation of the different wheel speeds of the vehicle wheels by output signals from a lateral acceleration sensor 112 and / or by a steering angle sensor (not shown), possibly also by a - also not shown - yaw rate sensor.

With knowledge of the tire characteristic curves for straight-ahead driving, the entire tire characteristic curve field is known in principle, which is valid for a superimposition of longitudinal and transverse movements of the vehicle and can be used for the determination of "lateral force reserves" if a predetermined one - as explained measurable - longitudinal slip is given, whereby for a traction slip analogous relationships apply to the explanations given for the brake slip, which can be evaluated by means of the electronic control unit 50 and can be used for the slip regulation or control.

In order to enable reliable detection of the driver's request - target braking or full braking - the brake pedal 17 is also equipped with a z. B. with the help of strain gauges realized force sensor 113 equipped, the electrical output signal is also supplied to the electronic control unit 50 . Furthermore, an angular position transmitter 114 is provided, the electrical output signal of which represents a measure of the pedal position and thus also a measure of the vehicle deceleration desired by the driver. From a differentiating processing of the output signal of this angular position transmitter 114 , the electronic control unit 50 obtains the information about how quickly the driver actuates the brake pedal 17 . If the actuation speed is very high, the "electronic control unit 50 " concludes that the driver wants to trigger the most effective braking possible with the greatest possible braking deceleration and interrupts the execution of test cycles in this case.

Furthermore, it is useful to detect the driver's request if the secondary cylinder 16 '' at his pressure outlet 19 '' is verse hen with a pressure sensor 116 , by means of which the brake line actually in the main ( 22 ) of the front axle brake circuit I coupled pressure is detectable.

Claims (20)

1. A method for determining the adhesion / slip characteristic curves of the tires of a road vehicle which is equipped with an anti-lock braking system designed for a single-wheel control, according to which, when the vehicle is in operation, measured value pairs of the slip λ and the given coefficient of adhesion µ used for a given slip the course of the respective tire characteristic curve in the entire µ / λ field is inferred, characterized by the following features:

• a) in an initial phase of a target braking associated with moderate vehicle deceleration, only the wheel brake of a single vehicle wheel is subjected to brake pressure and this is dimensioned in the initial phase of increasing the brake pressure so that the driver's request - increasing - initial deceleration results at least approximately;
• b) during this initial test phase of the braking solution, the absolute braking slip λ _{B continues} according to the relationship and the slip with the respective value of the brake _B λ linked adhesion utilization μ _B determined in the case of the braking of a rear wheel by the relation and in the case of braking a front wheel by the relationship is given, in these relationships with v _F the vehicle speed, with v _R the speed of the braked vehicle wheel, with Ψ the rear axle load, with the wheelbase-related center of gravity of the vehicle and with z the measured braking deceleration of the vehicle, and where driving tool speed v _F a from the wheel circumferential speeds of the non-braked vehicle wheels the value obtained is applied;
• c) as soon as the sole braking of the Testra does not increase the vehicle deceleration z and / or the braked vehicle wheel decelerates more than it corresponds to a predefined threshold value, the test braking is aborted and the braking is continued by applying pressure to the other vehicle wheels;
• d) the test braking phases corresponding to the features a) to c) are carried out in a cyclical sequence for all vehicle wheels.

2. The method according to claim 1, characterized in that that the test braking on a rear wheel leads when the driver requests braking with moderate vehicle deceleration between 0.1 g and Corresponds to 0.2 g and on a front wheel if the Driver request a slightly higher vehicle deceleration tion between 0.2 g and 0.4 g. 3. The method according to claim 1 or claim 2, characterized characterized in that two driving in a test cycle Wheels are braked, the one diagonally are arranged opposite if the driver request a braking deceleration of more than 0.4 g corresponds. 4. The method according to claim 3, characterized in that the test phase first on the rear wheel and then is carried out on the front wheel. 5. The method according to any one of claims 1 to 4, characterized characterized in that when the Test braking phase on a driven vehicle wheel this uncoupled from the drive train of the vehicle pelt is. 6. The method according to any one of claims 1 to 5, characterized in that the operating situation "target braking" of the vehicle is recognized by the fact that the driver operates the brake pedal ( 17 ) of the brake system ( 10 ) with a force K _p that is less than is a predetermined, preferably adjustable, threshold value. 7. The method according to any one of claims 1 to 6, characterized in that the operating situation "target braking" of the vehicle can be recognized by the fact that the vehicle deceleration is less than a predetermined threshold z _min . 8. The method according to any one of claims 1 to 7, characterized in that the operating situation "target braking" of the vehicle is recognized by the fact that the brake pressure that the driver by actuating the braking device ( 16 ) in the brake system ( 10 ) einteu ert, is less than a preferably adjustable predetermined threshold value p _min . 9. The method according to any one of claims 1 to 8, characterized in that for the determined on the basis of slip and deceleration and possibly acceleration measurements, the coefficient of adhesion / slip, which applies to the respective vehicle wheel, by interpolation or adaptation of a means an electronic control unit ( 50 ) from a valuable approximate relationship to characteristic points of support of the measurement obtained by the stored µ _B / λ _B value pair stock, an algorithm is obtained which is used for the continuous processing of measured λ _B data in units of the used adhesion coefficient is exploitable. 10. The method according to claim 9, characterized in that the determination of the currently used final closing coefficient µ _B by evaluating a relationship of the shape in which C ₁ , C ₂ and C ₃ denote constants by adapting the relationship to
Measured λ _B and µ _B values are obtained and stored as parameters of the relationship used for the continuous evaluation and continuously updated. 11. Slip control system for performing the method according to one of claims 1 to 10, on a road vehicle that is equipped with both an anti-lock braking system that enables individual brake pressure control on the individual vehicle wheels, as well as with a device for one Electronically controlled distribution of the brake forces that can be exerted via the front wheel brakes and the rear wheel brakes, with a braking device comprising a tandem master cylinder and with a control pressure source with an electrically controllable outlet pressure that can be coupled into a control pressure chamber of the braking device and thereby by means of sole pedal actuation of the brake device generated brake pressure, which can be coupled into the rear brakes, is additively superimposed, the control pressure source comprising an electronic control unit, which processes at least wheel speed sensor output signals that individually assign the vehicle wheels t are, as well as pressure sensor output signals, by means of which characteristic electrical signals can be obtained for the producible brake pressures, which generate the control signals required for controlling the brake force distribution, characterized in that the front wheel brakes ( 11 , 12 ) to the primary output pressure space ( 56 ) and the rear wheel brakes ( 13 , 14 ) to the secondary output pressure chamber ( 57 ) of the tandem main cylinder ( 16 ') are connected so that the housing-fixed boundary of the secondary output pressure chamber ( 57 ) forming bore step ( 63 ) of the housing ( 64 ), within which the secondary outlet pressure chamber ( 57 ) is delimited by an - external piston flange ( 74 ) of the secondary piston ( 59 ) against an - unpressurized - trailing chamber ( 82 ) and this by an inner piston flange ( 73 ) is movably sealed against an inner portion of this bore step ( 63 ) by an intermediate wall ( 68 ) of the housing ( 64 ) g a central bore step ( 66 ) is delimited, which starts from the bore step ( 61 ) receiving the primary piston ( 58 ), the cross-sectional area A _{1 of which is} somewhat larger than the cross-sectional area A _{3 of} the central bore step ( 66 ), that in the central bore step ( 66 ) an actuating piston ( 69 ) is displaceable in a pressure-tight manner, which forms the axially movable - inner - loading boundary of the primary outlet pressure chamber ( 56 ) and this against an unpressurized, axially housing firmly delimited by the intermediate wall ( 68 ) after running space ( 86 ) delimits that this actuating piston ( 69 ) is provided with a plunger ( 71 ) which axially penetrates the central trailing chamber ( 86 ) and which can be moved in a pressure-tight manner through a central bore ( 72 ) in the intermediate wall ( 68 ) and axially on the inner piston flange ( 73 ) of the secondary piston ( 59 ) can be supported, and that through the inner piston flange ( 73 ) of the secondary piston ( 59 ) axially Movable and by the intermediate wall ( 68 ) axially fixed to the housing, which is also centrally penetrated by the plunger ( 71 ) through the annular space ( 96 ) to the pressure outlet ( 42 ) of the controllable pressure source ( 40 ). 12. skid control system according to claim 11, characterized in that the on the secondary piston (59) on cross-end portion (71 ') of the plunger (71) of the actuating piston (69) in a blind bore (88) of the secondary piston relative to (59) this pressure is arranged to be tightly displaceable and can be supported on the base ( 91 ) of this blind bore ( 88 ). 13. Slip control system according to claim 12, characterized in that the inner piston flange ( 73 ) of the secondary piston ( 59 ) with a central at the bottom ( 91 ) of the blind bore ( 88 ) opening into this and connecting this with the trailing space ( 82 ) from same bore ( 92 ) is provided. 14. Slip control system according to one of claims 11 to 13, characterized in that between the front axle brake circuit (I) of the brake system ( 10 ) of the vehicle associated output pressure chamber ( 56 ) of the tandem master cylinder ( 16 ') and the Before derradbremsen ( 11 , 12 ) advanced Hauptbremslei device ( 22 ) of the front axle brake circuit (I) a secondary cylinder acting as a pressure converter ( 16 '') is provided, which in turn has a control pressure chamber ( 196 ) in which a controllable output pressure Control pressure source ( 40 ) can be coupled, the output pressure in the output pressure chamber ( 157 ) of this secondary cylinder ( 16 '') can be generated additively by simply actuating the master cylinder ( 16 '). 15. Slip control system according to claim 14, characterized in that the secondary cylinder ( 16 '') has its own control pressure output ( 41 ) of the control pressure source ( 40 ). 16. Slip control system according to one of claims 11 to 15, characterized in that at least one of the pressure outlets ( 19 ', 21 '), preferably the pressure outlet ( 19 ') of the primary outlet pressure chamber ( 26 ) of the tandem master cylinder ( 16th ') A pressure sensor ( 107 ) is connected, which generates a characteristic for the pressure in the tandem master cylinder ( 16 ') electrical output signal, which is the electronic control unit ( 50 ) supplied as information input. 17. Slip control system according to one of claims 11 to 16, characterized in that the control pressure outputs ( 41 , 42 ) of the control pressure source ( 40 ) each have a pressure sensor ( 54 , 55 ) assigned to them for the respective pressure output ( 41 or 42 ) provided control pressure produces a characteristic electrical output signal which is fed to the electronic control unit ( 50 ). 18. Slip control system according to one of claims 11 to 17, characterized in that the front wheel brakes ( 11 , 12 ) and the rear wheel brakes ( 13 , 14 ) each individually assigned pressure sensors ( 60 ) are provided for the in the individual wheel brakes prevailing brake pressures generate characteristic electrical output signals which are sent to the electronic control unit as further information inputs. 19. Slip control system according to one of claims 11 to 18, characterized in that a Querbe acceleration sensor ( 112 ) and / or a yaw angle sensor is / are provided, which generate electrical output signals from the electronic control unit ( 50 ) in Units on the vehicle can be evaluated by gripping lateral forces. 20. Slip control system according to one of claims 11 to 19, characterized in that a force sensor is provided, which is a characteristic for the force with which the driver operates the brake pedal ( 70 ) of the brake system ( 10 ), electrical output generated signal that the electronic Steuerein unit ( 50 ) is supplied.