Abstract:
Proposed is a technique for operating a hydraulic motor vehicle brake system in an operating situation which requires the formation of a hydraulic pressure difference at opposite wheel brakes of a vehicle axle. Here, each wheel brake is assigned a first slip regulating valve device for decoupling the respective wheel brake from a hydraulic pressure generator, and a second slip regulating valve device for dissipating hydraulic pressure at the respective wheel brake. A method implementation of this technique comprises the steps of building up a hydraulic pressure at the opposite wheel brakes during the course of a braking process, detecting a requirement for forming a hydraulic pressure difference at the opposite wheel brakes, actuating one or more of the slip regulating valve devices assigned to the opposite wheel brakes so as to form the hydraulic pressure difference by virtue of different hydraulic pressures being set at the opposite wheel brakes, and, in reaction to a driver demand, increasing the hydraulic pressure at the opposite wheel brakes, including the wheel brake at which a relatively low hydraulic pressure is to be set, while maintaining the hydraulic pressure difference by transferring hydraulic fluid from the hydraulic pressure generator via the first slip regulating valve devices to the opposite wheel brakes.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a national stage of International Application No. PCT/EP2011/003336 filed Jul. 5, 2011, the disclosures of which are incorporated herein by reference in entirety, and which claimed priority to German Patent Application No. DE 10 2010 033 496.0 filed Aug. 5, 2010, the disclosures of which are incorporated herein by reference in entirety. 
     BACKGROUND OF THE INVENTION 
     The invention relates generally to the field of brake systems. More precisely, the invention relates to the operation of a motor-vehicle brake system in a braking situation in which differing coefficients of friction of the road prevail on opposite sides of the vehicle (μ-split situation), or in comparable braking situations. 
     It is generally known that when the brakes of a motor vehicle are applied in a μ-split braking situation the vehicle has a tendency to rotate about the vertical axis of the vehicle (also called yawing).  FIG. 1  illustrates this yawing in connection with a motor vehicle  10 , the left wheels  12 ,  14  of which are running on ice, and the right wheels  16 ,  18  of which are running on dry asphalt. 
     The coefficient of static friction of ice amounts to approximately μ s =0.1, whereas the corresponding value of dry asphalt is approximately μ s =0.8. By reason of these greatly differing coefficients of friction, the wheels  12 ,  14  on the low-friction-coefficient side (on the left in  FIG. 1 ) attain a state that requires a slip regulation by an anti-lock system (ABS) more quickly than the wheels  16 ,  18  on the high-friction-coefficient side (on the right in  FIG. 1 ). By reason of this asymmetrical slip regulation, when the brakes of the motor vehicle  10  are applied differing braking forces act on the left wheels  12 ,  14  and on the right wheels  16 ,  18 , the difference of which at the front wheels  12 ,  16  may be particularly strongly pronounced by reason of the dynamic axle-load shift. These differing braking forces lead, in turn, to a torque about the vertical axis  20  of the vehicle (the so-called yawing moment), and hence, under certain circumstances, to a yawing of the motor vehicle  10 . 
     In the case of heavy motor vehicles the yawing illustrated in  FIG. 1  occurs so slowly that, with slip regulation activated, it can be compensated by a driver sufficiently quickly by steering in the opposite direction. Above all in the case of lighter motor vehicles, however, measures have to be taken additionally in order to assist the driver in the course of braking in a μ-split situation. 
     One possibility to counteract the build-up of a yawing moment in a μ-split situation is the implementation of a so-called select-low regulation in the ABS control software. With such a regulation, in the case of a detected μ-split situation the braking force at the wheels of the rear axle is set in accordance with the ABS-regulated braking force on the low-friction-coefficient side. Whereas in the case of the select-low regulation a yawing can be very largely avoided and the controllability of the vehicle is therefore preserved well, a considerable underbraking of the wheels on the high-friction-coefficient side arises. This underbraking results in an unacceptable lengthening of the braking distance. 
     For this reason, in L. M. Ho et al., The Electronic Wedge Brake—EWB, XXVIth International μ Symposium 2006, pages 248f, a description is given of allowing a small difference in braking force at the opposite wheels of each axle (that is to say, between the wheels on the high-friction-coefficient side and the wheels on the low-friction-coefficient side). The braking-force difference is then gradually increased, in axle-specific manner, up to a defined value. The gradual axle-specific increase of the braking-force difference results only in a slow build-up of yawing moment. In each case the build-up of yawing moment is distinctly delayed in comparison with a ‘pure’ ABS regulation. The driver therefore has sufficient time to compensate a possibly resulting yawing of the vehicle by means of steering movements. 
     In  FIG. 2  the ramp-like increase of the braking-force differences in combination with a select-low regulation is illustrated in a braking-force/time diagram according to L. M. Ho et al. In  FIG. 2  it is assumed that (as represented in  FIG. 1 ) the left side of the vehicle is the low-friction-coefficient side and the right side of the vehicle is the high-friction-coefficient side. Accordingly, at the left front and rear wheels (FL/RL in  FIG. 2  and reference symbols  12  and  14  in  FIG. 1 ) only low braking forces can be generated, whereas at the right front and rear wheels (FR/RR in  FIG. 2  and reference symbols  16  and  18  in  FIG. 1 ) distinctly higher braking forces can be built up. Overall, the braking distance in this case can be distinctly reduced in comparison with a ‘pure’ select-low regulation. Simultaneously, the driver is still given enough time to react to a possibly incipient yawing by steering in the opposite direction. 
     Now, it has turned out that in the case of the adapted select-low regulation illustrated in  FIG. 2  an underbraking of the wheels on the high-friction-coefficient side still occurs. In other words, in μ-split situations the braking distance is frequently still unnecessarily long. With a view to shortening the braking distance, it is proposed in DE 10 2008 027 093 A1 to carry out a braking-force regulation in a μ-split situation with the proviso of the setting of a side-slip angle different from zero. This means that a slight yawing of the vehicle is permitted selectively. The braking-force regulation at the individual wheels is effected in this connection on the basis of a desired side-slip angle to be set within the range between 0.5° and 8°. 
     Conventional regulating strategies of the ABS control software in μ-split situations uncouple the driver (i.e. the brake pedal) from the braking-force regulation. For this purpose, ABS slip-regulating valves, which have been arranged between a hydraulic-pressure generator (for example, the master cylinder), on the one hand, and the wheel brakes covered by the braking force regulation, on the other hand, are closed. For the driver, the closing of the slip-regulating valves becomes noticeable in a ‘hard’ pedal feedback, since no more hydraulic fluid can be displaced out of the master cylinder to the wheel brakes. 
     In order nevertheless in the case of closed slip-regulating valves to be able to register a wish of the driver with respect to an increase in braking force, a pressure sensor for registering the master-cylinder pressure may be provided. An increase of the master-cylinder pressure registered by this pressure sensor with slip-regulating valves closed indicates a further depression of the brake pedal by the driver and can be taken into consideration in suitable manner within the scope of the ABS regulating operation. 
     BRIEF SUMMARY OF THE INVENTION 
     A feature underlying the invention is to improve the operation of a hydraulic motor-vehicle brake system in a braking situation that requires the creation of a hydraulic-pressure difference at opposite wheel brakes of a vehicle axle (that is to say, for example, in a μ-split situation). 
     According to a first aspect, a method is provided for operating a hydraulic motor-vehicle brake system in a braking situation that requires the creation of a hydraulic-pressure difference at opposite wheel brakes of a vehicle axle, wherein a first slip-regulating valve device for a decoupling of the respective wheel brake from a hydraulic-pressure generator and a second slip-regulating valve device for a reduction of hydraulic pressure at the respective wheel brake have been assigned to each wheel brake. The method comprises the steps of building up a hydraulic pressure at the opposite wheel brakes within the scope of a braking procedure, of registering a requirement to create a hydraulic-pressure difference at the opposite wheel brakes, of driving one or more of the slip-regulating valve devices assigned to the opposite wheel brakes for the purpose of creating the hydraulic-pressure difference by differing hydraulic pressures being set at the opposite wheel brakes, and, as a reaction to a driver request, of increasing the hydraulic pressure at the opposite wheel brakes, inclusive of the wheel brake at which a lower hydraulic pressure is to be set, while maintaining the hydraulic-pressure difference by transfer of hydraulic fluid from the hydraulic-pressure generator via the first slip-regulating valve devices to the opposite wheel brakes. 
     Merely the wheel brakes of one vehicle axle or alternatively the wheel brakes of several (where appropriate, of all) vehicle axles may have been covered by the increase of hydraulic pressure. Furthermore, the increase of hydraulic pressure may occur after the initial build-up of hydraulic pressure, or may be part of the initial build-up of hydraulic pressure. 
     At least one of the operations of the creation and maintenance of the hydraulic-pressure difference may include the step that at that one of the opposite wheel brakes at which a lower hydraulic pressure is to be set an adjustable pressure difference between the hydraulic-pressure generator and the wheel brake is brought about by means of the first slip-regulating valve device assigned to this wheel brake. Accordingly, the hydraulic pressure in a hydraulic line from the hydraulic-pressure generator to an inlet of the first slip-regulating valve device may be higher by this pressure difference than the hydraulic pressure in a hydraulic-pressure line between an outlet of the first slip-regulating valve device and the assigned wheel brake. The pressure difference at the first slip-regulating valve device may correspond to the hydraulic-pressure difference at the opposite wheel brakes or alternatively may influence it (i.e. establish it). 
     Upon increasing the hydraulic pressure on the inlet side of the first slip-regulating valve device an overflow of this valve device may occur while maintaining the set pressure difference. This overflow, in turn, enables an increase of hydraulic pressure also on the outlet side of the first slip-regulating valve device and hence at the wheel brake assigned to this valve device. The pressure difference can be maintained in the course of the increase of hydraulic pressure (with, where appropriate, altered magnitude). 
     The slip-regulating valve devices may have been designed in varying ways. Accordingly, each slip-regulating valve device may include one or more hydraulic valves and, if required, further fluid-controlling components (such as an adjustable check valve connected in parallel with the hydraulic valve). According to one realisation, at least each of the first (and—as an option—also each of the second) slip-regulating valve devices has been designed to enable a fluid connection between the hydraulic-pressure generator and the assigned wheel brake if a first pressure difference between an inlet and an outlet of the first slip-regulating valve device exceeds a predetermined maximal value, and to interrupt the fluid connection again if the excess pressure between the inlet and the outlet exceeding the predetermined maximal value has been reduced. The predetermined maximal value of the pressure difference between the inlet and the outlet of the first slip-regulating valve device may establish the hydraulic-pressure difference at the opposite wheel brakes. In other words, this maximal value may correspond to the hydraulic-pressure difference or alternatively may at least bring influence to bear thereon. 
     At least one of the operations of the creation and maintenance of the hydraulic-pressure difference may include the step that at that one of the opposite wheel brakes at which a higher hydraulic pressure is to be set the first slip-regulating valve device assigned to this wheel brake is kept completely open. In other words, no (additional) drop in pressure is generated at this valve device. For this purpose, for example, the predetermined maximal value for the pressure difference between the inlet and the outlet of the first slip-regulating valve device may be set to zero. 
     The slip-regulating valves may take the form of adjustable valve devices. The driving of the first slip-regulating valve devices and, if required, also of the second slip-regulating valve devices may happen in varying ways. Accordingly, drive signals can be generated, for example, on the basis of a pulse-width modulation or on the basis of a current regulation. 
     At least one of the operations of the creation and maintenance of the hydraulic-pressure difference may further include the step of the at least partial opening of the second slip-regulating valve device that has been assigned to that one of the opposite wheel brakes at which a lower hydraulic pressure is to be set. In this way the hydraulic pressure at the wheel brake assigned to this second slip-regulating valve device is accordingly reduced in order to create the hydraulic-pressure difference at the opposite wheel brakes. 
     The hydraulic-pressure difference may remain the same during the entire braking situation or may alternatively be altered (e.g. in accordance with the possibly changing requirements during the braking situation). Accordingly, the hydraulic-pressure difference may remain approximately the same during the increasing of the hydraulic pressure or alternatively may be maintained at an increased level (in each instance with respect to the hydraulic-pressure difference prevailing prior to the increasing of the hydraulic pressure). 
     At least one of the operations of the build-up of hydraulic pressure and the increase of hydraulic pressure by means of the hydraulic-pressure generator can be carried out as a reaction to an actuation of the brake pedal by the driver. The hydraulic-pressure generator may be a master cylinder coupled with the brake pedal. Alternatively or additionally to this, the hydraulic-pressure generator may also be an electrically operated hydraulic-pressure transducer unit (for example, a hydraulic pump). Consequently, the method of operation presented here may also come into operation in an electrohydraulic brake system or in a regenerative brake system (hybrid brake system). 
     The requirement to create a hydraulic-pressure difference at the opposite wheel brakes can be registered in various ways. Accordingly, the hydraulic-pressure difference may be created when registering a wheel-speed difference between the wheels assigned to opposite wheel brakes. Additionally or alternatively to this, an incipient yawing motion of the motor vehicle may also be registered and recognised as a requirement for creating the hydraulic-pressure difference. 
     The motor vehicle may comprise two or more axles. If the motor vehicle comprises a front axle and a rear axle, the increasing of the hydraulic pressure at the opposite wheel brakes can be carried out exclusively at the wheel brakes of one of the two axles (usually of the front axle). A distribution of braking force between the axles may also take place. In this case, differing hydraulic pressures may be set at the wheel brakes of the same side of the vehicle within the scope of an axle-specific braking-force distribution. 
     Furthermore, a computer-program product with program-code means is provided for implementing the method of operation presented here when the computer-program product is executed by a processor. The computer-program product may have been recorded on a computer-readable recording medium (for example, on a memory chip). The recording medium with the computer-program product stored thereon may be part of a control unit (electronic control unit, ECU) which also includes the processor for executing the computer-program product. 
     Furthermore, a hydraulic brake system of a vehicle is specified that is capable of being operated in a braking situation that requires the creation of a hydraulic-pressure difference at opposite wheel brakes of a vehicle axle. The brake system includes a hydraulic-pressure generator for the build-up of a hydraulic pressure at the opposite wheel brakes within the scope of a braking procedure, a first slip-regulating valve device for each wheel brake for the purpose of decoupling the respective wheel brake from the hydraulic-pressure generator, a second slip-regulating valve device for each wheel brake for the purpose of reducing the hydraulic pressure at the respective wheel brake, a registration device for registering a requirement to create a hydraulic-pressure difference at the opposite wheel brakes, and also a drive device for driving one or more of the slip-regulating valve devices assigned to the opposite wheel brakes for the purpose of creating the hydraulic-pressure difference by differing hydraulic pressures being set at the opposite wheel brakes. The drive device has been designed to increase, as a reaction to a driver request, the hydraulic pressure at the opposite wheel brakes, inclusive of the wheel brake at which a lower hydraulic pressure is to be set, while maintaining the hydraulic-pressure difference by transfer of hydraulic fluid from the hydraulic-pressure generator via the first slip-regulating valve devices to the opposite wheel brakes. 
     The first slip-regulating valve devices may have been designed to bring about an adjustable pressure difference between the hydraulic-pressure generator and the respective wheel brake—that is to say, between an inlet side and an outlet side of each valve device. Furthermore, the brake system may include an anti-lock system (ABS) to which the slip-regulating valve devices pertain. The drive device of the brake system may have been implemented as a control unit. 
     Other advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiments, when read in light of the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic illustration of the yawing of a motor vehicle in a μ-split situation; 
         FIG. 2  is a schematic diagram of a select-low regulation with ramp-like increase of pressure difference according to the state of the art; 
         FIG. 3  is a schematic illustration of an embodiment of a motor-vehicle brake system; 
         FIG. 4  is a schematic representation of a regulating device of the motor-vehicle braking system according to  FIG. 3 ; 
         FIG. 5  is a schematic flow chart which illustrates an embodiment of a method of operation for the brake system according to  FIG. 3  in a μ-split situation; and 
         FIGS. 6 and 7  are schematic hydraulic-pressure curves for differing pressure-increase scenarios. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following, various embodiments for the operation of a hydraulic motor-vehicle brake system in a braking situation that requires the creation of a hydraulic-pressure difference at opposite wheel brakes of a vehicle axle will be elucidated. These embodiments relate, merely in exemplary manner, to the μ-split braking situation illustrated in  FIG. 1 . 
       FIG. 3  illustrates schematically an embodiment of a motor-vehicle brake system  100  that implements a so-called electronic stability control (ESC) and is capable of being operated in order to create a hydraulic-pressure difference at opposite wheel brakes  102 ,  104  and  106 ,  108  of a front axle and rear axle, respectively. The brake system  100  comprises two brake circuits  110 ,  112 , the wheel brakes  102 ,  104 ,  106 ,  108  having been fluidically coupled with the brake circuits  110 ,  112  in accordance with a diagonal distribution. This means that a first brake circuit  110  supplies the wheel brake  104  at the left front wheel (FL) and also the wheel brake  106  at the right rear wheel (RR) with hydraulic fluid. A second brake circuit  112 , on the other hand, supplies the wheel brake  102  at the right front wheel (FR) and also the wheel brake  108  at the left rear wheel (RL) with hydraulic fluid. Deviating from the diagonal distribution illustrated in  FIG. 3 , the wheel brakes  102 ,  104  on the front axle could be assigned to the one brake circuit  110 , and the wheel brakes  106 ,  108  on the rear axle to the other brake circuit  112  (‘black/white distribution’). 
     The brake system  100  includes a master cylinder  114  with two hydraulic chambers which have each been assigned to one of the two brake circuits  110 ,  112 . The master cylinder  114  has to be actuated by the driver by means of a (brake) pedal  116 , whereby the actuating force introduced into the pedal  116  by the driver is ordinarily boosted—pneumatically, hydraulically or electromechanically—by a power brake unit (not represented). In the event of an actuation of the master cylinder  114 , hydraulic fluid is conveyed out of a pressureless reservoir  118  via the hydraulic chambers into the two brake circuits  110 ,  112 . The master cylinder  114  therefore constitutes a driver-actuated hydraulic-pressure generator. 
     The brake system  100  further includes, for each brake circuit  110 ,  112 , a hydraulic-pressure generator that is capable of being actuated independently of the driver, in the form of, respectively, a hydraulic pump  120 ,  122  operated by an electric motor. Accordingly, hydraulic fluid for generating hydraulic pressure can be conveyed to the wheel brakes  102 ,  104 ,  106 ,  108  both by means of the master cylinder  114  and by means of the hydraulic pumps  120 ,  122 . 
     In the following, the components of the first brake circuit  110  will be elucidated in more detail. It will be understood, however, that the components of the second brake circuit  112  have the same structure and the same functionality. 
     In the first brake circuit  110  a plurality of valve devices  130 ,  132 ,  134 ,  136 ,  138 ,  140  and also a pressure accumulator  142  for hydraulic fluid are arranged. The valve devices  130  to  140  are represented in  FIG. 3  in their initial position, in which hydraulic fluid can be conveyed out of the master cylinder  114  to the wheel brakes  104 ,  106  of the left front wheel and of the right rear wheel. 
     The hydraulic fluid displaced out of the master cylinder  114  passes firstly through a first valve device  130 . The valve device  130  is an adjustable 2/2-way valve. In the direction of build-up of hydraulic pressure downstream of the valve device  130  there has been assigned to each wheel brake  104 ,  106  a first slip-regulating valve device  134 ,  138 , respectively. Each of the first slip-regulating valve devices  134 ,  138  corresponds in structure to valve device  130  and includes an adjustable 2/2-way valve. This 2/2-way valve has been designed to enable a fluid connection between the master cylinder  114  or the hydraulic pump  120  (inlet side) and the assigned wheel brake  104 ,  106  (outlet side) if a first pressure difference between the inlet side and the outlet side exceeds a predetermined maximal value, and to interrupt the fluid connection again if the excess pressure between the inlet side and the outlet side exceeding the predetermined maximal value has been reduced again. A possible realisation of the first slip-regulating valve device  134 ,  138  is described in DE 102 47 651 A1. The disclosure content of this document with respect to the first slip-regulating valve devices  134 ,  138  is included here by reference. 
     Each of the two first slip-regulating valve devices  134 ,  138  enables a decoupling of the respective wheel brake  104 ,  106  from the master cylinder  114  and also from the hydraulic pump  120 . Such a decoupling is required, for example, during pressure-maintaining phases within the scope of an ABS regulating operation. 
     For the reduction of pressure at wheel brakes  104 ,  106  in ABS regulating operation, to each of these two wheel brakes  104 ,  106  a second slip-regulating valve arrangement  136 ,  140  in a return line leading to the pressure accumulator  142  and to the pressureless reservoir  118  has furthermore been assigned. Second slip-regulating valve devices  136 ,  140  include a non-adjustable 2/2-way valve (non-return valve with two switching states), which in the basic position represented in  FIG. 3  is closed. In the open state of second slip-regulating valve devices  136 ,  140 , pressurised hydraulic fluid is able to flow back from wheel brakes  104 ,  106  into the pressure accumulator  142 . In this way, a reduction of pressure at these wheel brakes  104 ,  106  is effected. 
     The further valve device  132  provided in the return line leading to the reservoir  118  likewise includes a non-adjustable 2/2-way valve (non-return valve with two switching states) and enables a decoupling of the input side of the hydraulic pump  120  from the master cylinder  114  and from the pressureless reservoir  118 . In its blocking position the hydraulic pump  120  in ABS regulating operation therefore conveys hydraulic fluid out of the pressure accumulator  142  back into brake circuit  110  or into wheel brakes  104 ,  106  for future pressure-build-up phases. 
     For automatic brake engagements, which, as a rule, occur independently of an actuation of the pedal  116  by the driver and require a decoupling of the master cylinder  114  from wheel brakes  104 ,  106 , valve device  130  is closed and the further valve device  132  is opened. By virtue of the opening of the further valve device  132 , the hydraulic pump  120  is able to withdraw hydraulic fluid from the master cylinder  114  or from the reservoir  118 , so that a build-up of hydraulic pressure or an increase of hydraulic pressure at wheel brakes  104 ,  106  is effected by means of the hydraulic pump  120 . By virtue of the closing of valve device  130 , hydraulic fluid conveyed by the hydraulic pump  120  is prevented from being conveyed to the master cylinder  114  instead of to wheel brakes  104 ,  106 . 
     Such automatic brake engagements include, for example, an anti-slip regulation (ASR), which prevents a spinning of individual wheels in the course of a start-up procedure by selective deceleration of a wheel involved, an electronic stability program (ESP), which adapts the behaviour of the vehicle within the limiting range to the wish of the driver and to the road conditions by selective deceleration of individual wheels, an adaptive cruise control (ACC), which by (inter alia) automatic braking maintains a spacing of the driver&#39;s own vehicle from a vehicle in front, and/or a hill descent control (HDC), which monitors and keeps constant the speed and directional stability of a vehicle when travelling downhill on loose ground or on a road with a low coefficient of friction, such as snow, inter alia by means of brake engagements. 
     As already elucidated above, in the second brake circuit  112  corresponding valve devices  150 ,  152 ,  154 ,  156 ,  158 ,  160  and a corresponding pressure accumulator  162  are located. With a view to avoiding repetition, these components will not be elucidated here in any detail. 
     According to  FIG. 3 , the brake system  100  further includes a regulating device  170 , which within the scope of an ABS regulating operation and, in particular, in a μ-split braking situation automatically intervenes in a braking procedure. The regulating device  170  is, inter alia, provided for the purpose of driving all the valve arrangements  130  to  140  and  150  to  160  and also the hydraulic pumps  120 ,  122 . 
     As represented in  FIG. 4 , the regulating device  170  includes a registration device  172  for registering a requirement to create a predetermined hydraulic-pressure difference at opposite wheel brakes  102 ,  104 , and  106 ,  108 . The registration device  172  may, for example, include wheel-speed sensors. The regulating device  170  further possesses a device  174  for μ-split detection. Device  174  receives wheel-speed signals from the registration device  172  and detects a μ-split situation if a wheel-speed difference between the wheels that have been assigned to the wheel brakes  102 ,  104  of the front axle and to the wheel brakes  106 ,  108  of the rear axle exceeds a predetermined threshold value. If device  174  establishes an exceeding of the threshold value, this is signalled to a drive device  176  of the regulating device  170 . The drive device  176  has been designed to drive the slip-regulating valve devices  134 ,  136 ,  138 ,  140 ,  154 ,  156 ,  158 ,  160  in suitable manner in order to counteract the creation of a yawing moment in the detected μ-split situation, or at least to limit the build-up of the yawing moment. 
     In the following, the modes of operation of the brake system  100  according to  FIG. 3  and, in particular, of the regulating device  170  according to  FIG. 4  will be elucidated in more detail with reference to the flow chart  500  according to  FIG. 5  and also the exemplary hydraulic-pressure curves according to  FIGS. 6 and 7 . 
     The method of operation of the brake system  100  in a μ-split situation, represented in  FIG. 5 , begins in step  502  with the initiating of a braking procedure by the driver. As a consequence of an actuation of the brake pedal in this case, a hydraulic pressure is built up by means of the master cylinder  114  in the two brake circuits  110 ,  112 . All the valve devices of the brake system  100  are located in their initial position illustrated in  FIG. 3 . For this reason, the pressurised hydraulic fluid displaced out of the master cylinder  114  is able to reach the wheel brakes  102 ,  104 ,  106 ,  108 . 
     The hydraulic-pressure curves in the master cylinder (MC)  114  and also at the wheel brakes  102 ,  104 ,  106  and  108  (FR, FL, RR, RL) are illustrated in  FIG. 6 . In this connection it becomes evident that at the start of the braking procedure (time t 0  until time t 1 ) a uniform build-up of hydraulic pressure occurs at all four wheel brakes  102 ,  104 ,  106 ,  108 . The build-up of hydraulic pressure at the wheel brakes  102 ,  104 ,  106 ,  108  in this case lags somewhat behind the build-up of hydraulic pressure in the master cylinder  114 , because the rise in pressure in the master cylinder  114 , as shown in  FIG. 6 , occurs very quickly. 
     In the present embodiment, upon a hydraulic-pressure threshold being attained at the wheel brakes  102 ,  104 ,  106 ,  108  at time t 1  a braking-force distribution occurs between the wheel brakes  106 ,  108  of the rear axle and the wheel brakes  102 ,  104  of the front axle. More precisely, at the wheel brakes  102 ,  104  of the front axle a higher hydraulic pressure is permitted than at the wheel brakes  106 ,  108  of the rear axle. This procedure is also designated as dynamic rear proportioning (DRP) or as dynamic braking-force distribution. 
     During the build-up of hydraulic pressure it is continuously ascertained by the regulating device  170  whether a braking situation has arisen that necessitates the creation of a hydraulic-pressure difference between the wheel brakes  102 ,  104  of the front axle or the wheel brakes  106 ,  108  of the rear axle by reason of a difference of coefficient of friction (μ-split situation; cf.  FIG. 1 ). In this connection, the wheel speeds of axle-specifically opposite wheels ascertained by the registration device  172  are continuously compared with one another by the device  174  for μ-split detection. If device  174  detects the exceeding of a threshold value, below which differences in wheel speed are still considered to be permissible, a μ-split situation—that is to say, a braking response with differing coefficient of friction on opposite sides of the vehicle—is detected by device  174 . This state of affairs corresponds to step  504  in  FIG. 5 . 
     In connection with the μ-split detection, for the rear axle and the front axle differing threshold values for the wheel-speed difference can be predetermined. In this way, the threshold value for the wheels of the rear axle can be set to be smaller than the threshold value for the wheels of the front axle, since—by reason of the dynamic axle-load shift from the rear axle to the front axle resulting in the course of deceleration of the vehicle—at the wheels of the front axle a higher braking force can be applied up until the onset of the ABS slip regulation. This has the result that a μ-split situation is detected earlier (time t 2 ) at the wheels of the rear axle than at the wheels of the front axle (time t 3 ). 
     After detection in step  504  of a safety-relevant μ-split situation, which requires an automatic intervention in the braking procedure, in step  506  a drive of the slip-regulating valve devices involved is effected by means of the drive device  176 . As illustrated in  FIG. 6 , this drive begins at time t 2  at the rear axle, and at time t 3  at the front axle. 
     As far as the rear axle is concerned, within the scope of the DRP at time t 1  the hydraulic pressure at both wheel brakes  106 ,  108  of the rear wheels is kept constant. This is done by closing the two assigned first slip-regulating valve devices  138 ,  154 . By reason of the μ-split situation, at time t 2  a build-up of hydraulic pressure at the wheel brake  106  of the right rear wheel begins, and also a simultaneous reduction of hydraulic pressure at the wheel brake  108  of the left rear wheel. The reduction of hydraulic pressure at wheel brake  108  is effected by brief opening and subsequent closing of second slip-regulating valve device  156 . The build-up of hydraulic pressure at wheel brake  106  may be effected by brief opening and subsequent closing of first slip-regulating valve device  138 , since the master-cylinder pressure (MC) is at a higher level than the hydraulic pressure (RR, RL) to be set at wheel brakes  106 ,  108 . As illustrated in  FIG. 6 , the pressure difference that has been set in this way can be retained continuously. 
     The pressure differences that have been set at the wheel brakes  106 ,  108  of the rear axle have been chosen in this case in such a way that a desired hydraulic-pressure difference between these wheel brakes  106 ,  108  and an accompanying braking-force difference arise. This braking-force difference counteracts a rapid rise in the yawing moment and hence influences the stability of the vehicle positively. 
     As already mentioned, the creation of a hydraulic-pressure difference, enhancing the stability of the vehicle, at the wheel brakes  102 ,  104  of the front axle occurs only at a later time t 3 , since on the front axle a higher wheel-speed difference can be permitted. At time t 3  a reduction of hydraulic pressure then happens at the wheel brake  104  of the left front wheel to such an extent that the desired hydraulic-pressure difference in relation to the wheel brake  102  of the right front wheel is set. For this purpose, the second slip-regulating valve device  136  assigned to the left front wheel is briefly opened, in order that a reduction of hydraulic pressure can occur. Subsequently thereto or simultaneously therewith, the first slip-regulating valve device  134  assigned to the left front wheel has applied to it by the drive device  176  a drive signal, based on a pulse-width modulation and/or current regulation (closing current), that corresponds to a precisely defined valve-closing force and accordingly to a desired pressure difference between an inlet side and an outlet side of slip-regulating valve device  134 . This pressure difference between the inlet side and the outlet side of slip-regulating valve device  134  tallies with the hydraulic-pressure difference between the wheel brake  102  of the right front wheel and the wheel brake  104  of the left front wheel, since the master-cylinder pressure applied at the inlet of the first slip-regulating valve device  134  of wheel brake  104  is also applied directly at the wheel brake  102  of the right front wheel (cf.  FIG. 6 ). This is because the first slip-regulating valve device  158  assigned to the wheel brake  102  of the right front wheel is held by the drive device  176  in a maximally conducting state. 
     As long as the master-cylinder pressure remains constant (between times t 3  and t 4 ), the hydraulic-pressure difference at the opposite wheel brakes  102 ,  104  and  106 ,  108  of the front and rear axles, respectively, also remains constant. In this case it is assumed that the corresponding wheel-speed differences (and hence the coefficients of friction of the road) do not change significantly. 
     At time t 4 , in step  508  an increase of the hydraulic pressure is then requested by the driver. This request can be effected, for example, by further depression of the brake pedal, this becoming noticeable, starting from time t 4 , in a rise of the master-cylinder pressure (step  510 ) up until a time t 5 . Since the first slip-regulating valve device  158  assigned to the wheel brake  102  at the right front wheel continues to be held in the maximally open state by the drive device  176 , the hydraulic pressure at wheel brake  102  follows the master-cylinder pressure directly (cf.  FIG. 6 ). Since, furthermore, the drive device  176  continues to drive the first slip-regulating valve device  134  assigned to the wheel brake  104  at the left front wheel with regard to maintaining a predetermined pressure difference between inlet side and outlet side, the pressure at wheel brake  104  also follows the rise in hydraulic pressure in the master cylinder  114 , though reduced by the set pressure difference. 
     The rise in hydraulic pressure at wheel brake  104  between times t 4  and t 5  is connected with the fact that the first slip-regulating valve device  134  assigned to this wheel brake  104  enables the fluid connection between the master cylinder  114  and wheel brake  104  as soon as a pressure difference between an inlet side and an outlet side of this valve device  134  exceeds the pressure difference predetermined by the drive device  176  (by means of current regulation or pulse-width modulation). When the predetermined pressure difference is exceeded, a transfer of hydraulic fluid from the master cylinder  114  via first slip-regulating valve device  134  to wheel brake  104  occurs. Since the second slip-regulating valve device  136  assigned to wheel brake  104  remains closed, an increase of hydraulic pressure at wheel brake  104  therefore arises. First slip-regulating valve device  134  has been designed in such a manner that the fluid connection from the master cylinder to wheel brake  104  is interrupted again immediately if the excess pressure exceeding the predetermined differential pressure between its inlet side and outlet side has been reduced again. First slip-regulating valve device  134  therefore works like a pressure-relief valve with adjustable overflow pressure. 
     Starting from time t 5 , the further depression of the brake pedal comes to an end, and the rise in hydraulic pressure in the master cylinder  114  levels out correspondingly. Hence the increase of hydraulic pressure (step  510 ) ends with continued valve drive. At a time t 6  an end of the μ-split situation is finally detected by device  174  (for example, by wheel-speed differences on the front and rear axles falling). Thereupon a reduction of the hydraulic-pressure differences between the wheel brakes  102 ,  104  of the front axle is initiated by the drive device  176 . This reduction of hydraulic pressure is concluded at a time t 7 , so that, starting from this time, an automatic intervention in the braking procedure occurs merely by reason of the DRP (braking-force distribution). Not represented in any detail is the fact that for the purpose of reducing the hydraulic-pressure differences at the wheel brakes  106 ,  108  of the rear axle the hydraulic pressure in the wheel brake  108  of the left rear wheel can again be set to the pressure level in the wheel brake  106  of the right rear wheel (cf.  FIG. 7 ) by the drive device  176  by suitable pulse-width modulation and/or current regulation of first slip-regulating valve device  154 . 
     In the embodiment represented in  FIG. 6  it is assumed that the μ-split situation arises only after a master-cylinder target pressure predetermined by means of the brake-pedal position has been attained.  FIG. 7  illustrates another embodiment, in which the μ-split situation arises during a comparatively slow build-up of hydraulic pressure in the master cylinder. As a comparison of  FIGS. 6 and 7  shows, the build-up of hydraulic pressure in the master cylinder  114  in the embodiment according to  FIG. 7  occurs so slowly that the hydraulic pressures at the wheel brakes  102 ,  104 ,  106 ,  108  follow the hydraulic pressure in the master cylinder  114  practically instantaneously. 
     According to  FIG. 7 , at time t 1  a μ-split situation and also a DRP regulation occur simultaneously at the wheel brakes  106 ,  108  of the rear axle. Consequently, the build-up of a hydraulic-pressure difference between the wheel brake  108  of the left rear wheel and the wheel brake  106  of the right rear wheel begins at the same time as the creation of a hydraulic-pressure difference between the wheel brakes  102 ,  104  of the front axle and the wheel brakes  106 ,  108  of the rear axle. The drive of the first slip-regulating valve devices  138 ,  154  on the rear axle is effected in this connection substantially as elucidated in connection with the embodiment represented in  FIG. 6 . Deviating from this embodiment, however, no lowering of the hydraulic pressure is required. For this reason, second slip-regulating valve devices  140 ,  156  remain closed. 
     At a later time t 2  the μ-split situation is detected also with regard to the wheel brakes  102 ,  104  of the front axle. Accordingly, as already described above with reference to  FIG. 6 , the first slip-regulating valve device  134  assigned to the wheel brake  104  of the left front wheel has a drive signal applied to it by the drive device  176 , by means of pulse-width modulation or current regulation, that corresponds to the desired differential pressure between the wheel brake  104  of the left front wheel and the wheel brake  102  of the right front wheel. In contrast to the embodiment according to  FIG. 6 , a reduction of hydraulic pressure is once again not required. 
     Starting from time t 2 , the driver continues to demand an increase of hydraulic pressure. Accordingly, the hydraulic pressure at the wheel brake  102  of the right front wheel increases further, whereas the hydraulic pressure of the wheel brake  104  of the left front wheel is kept constant until, at time t 3 , the hydraulic-pressure difference between the two wheel brakes  102 ,  104  of the front axle has attained the hydraulic-pressure difference predetermined by the drive device  176 . Starting from this time t 3 , a further increase of the hydraulic pressure upon further depression of the brake pedal leads to a parallel rise in hydraulic pressure at the wheel brakes  102 ,  104  of the front axle while retaining the hydraulic-pressure difference predetermined by the drive device  176 . As illustrated in  FIG. 7 , this hydraulic-pressure difference is furthermore maintained even when, at time t 4 , no further increase of the hydraulic pressure in the master cylinder  114  arises—that is to say, the master-cylinder target pressure has been attained. 
     At a time t 5 , device  174  finally detects a cessation of the μ-split situation. Starting from this time t 5 , the hydraulic pressure in wheel brake  104  is therefore increased by the drive device  176  until a hydraulic-pressure difference between the wheel brakes  102 ,  104  of the front axle has been reduced again. Likewise, the hydraulic pressures in wheel brakes  106 ,  108  are reduced or increased by the drive device  176  until a hydraulic-pressure difference between the wheel brakes  106 ,  108  of the rear axle has been reduced again. 
     As follows from the embodiments, the method of operation described here permits a totally safe hydraulic-pressure regulation, in the course of which an increase of pressure by the driver is guaranteed at any time. The driver can therefore overstep a pressure difference that has been set at the first slip-regulating valve devices  134 ,  138  at any time. For this reason, the ‘hard’ pedal feedback which is familiar from the state of the art ceases to apply. The measurement of the wish of the driver by a (additional) pressure sensor for registering the master-cylinder pressure within the scope of a μ-split situation is also superfluous. The method of operation presented here can be employed in axle-specific manner and displays good effectiveness, above all, on the front axle by reason of the regularly higher wheel load there and the good brake parameters. For this reason, the method of operation proposed here can also be combined in outstanding manner with DRP mechanisms. 
     The method of operation proposed here is capable of being employed both in ESC or ESP brake systems and in brake systems that merely implement an ABS functionality. This applicability arises, above all, from the fact that the pressure-regulating strategies presented here are capable of being implemented, if required, exclusively by means of the ABS slip-regulating valve devices. Furthermore, the pressure-regulating strategies can be implemented in straightforward manner in the ABS regulating software and in a corresponding control unit. The regulating device  170  illustrated in  FIG. 4  can therefore be integrated at least partly into an ABS control unit. 
     In accordance with the provisions of the patent statutes, the principle and mode of operation of this invention have been explained and illustrated in its preferred embodiments. However, it must be understood that this invention may be practiced otherwise than as specifically explained and illustrated without departing from its spirit or scope.