Abstract:
A trailer device for driving stabilization of vehicle combinations is provided. In order to design a device for driving stabilization of passenger vehicle trailers in a cost-effective and simple manner, a brake system for braking the trailer by an overrun brake and for stability regulation is designed so as to have separating valves, switching valves, inlet and outlet valves and a pressure build-up pump. A stabilizing coupling is also connected by means of a second pressure medium pump to actuators which can vary the clamping force of the stabilizing coupling between the trailer and tractor vehicle if the second pressure medium pump conveys pressure medium from the storage tank into or out of the actuators.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is the U.S. national phase application of PCT International Application No. PCT/EP2007/054970, filed May 22, 2007, which claims priority to German Patent Application No. DE102006024160.6, filed May 22, 2006 and German Patent Application No. DE102006030924.3, filed Jul. 3, 2006, the contents of such applications being incorporated by reference herein. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The invention relates to a hydraulic trailer brake system. 
         [0004]    2. Description of the Related Art 
         [0005]    EP 1 375 279 discloses a generic trailer brake system for braking the wheels of an inertia-braked trailer in order to reduce rolling movements. The brake system has an electrohydraulic stabilizing device EHS which is combined with an antilock system and which can brake both sides of the trailer individually in the event of rolling movements. For this purpose, special additional pressure-oil reservoirs, special ABS piston/cylinder components and special EHS cylinder/piston components are provided, by means of which the ABS function and the EHS function can be carried out. 
         [0006]    A device for stabilizing a road vehicle, in particular a passenger car, having a trailer drawn by the road vehicle is known from DE 19964048 A1, the road vehicle being monitored with regard to rolling movements, and, when a rolling movement is detected, an essentially periodic yawing moment is imparted automatically to the road vehicle and is essentially in phase opposition to the rolling movement. 
         [0007]    A method for stabilizing a vehicle is known from DE 100 48 418 A1, a periodically varying yawing moment being applied to the vehicle by means of alternating brake action. 
       SUMMARY OF THE INVENTION 
       [0008]    The object on which the invention is based is to provide a simple and cost-effective trailer brake system of the type initially mentioned, by means of which rolling movements of trailers can be prevented or reduced in a simple way, while high braking stability is maintained. 
         [0009]    This object is achieved by means of a hydraulic trailer brake system including a device for braking the wheels of the trailer and with a controllable pressure generator for the wheel-individual or axle-individual activation of the wheel brakes, the device being assigned a hydraulic brake force transmission device, with a reservoir for the pressure medium, and a brake master cylinder, and the brake force transmission device being connected to the wheel brakes via brake lines, and also with an electronic control or regulation unit which receives energy via a supply unit, is distinguished in that at least two wheel brakes are connected to the brake master cylinder via a first isolating valve arranged in the brake lines and via a first changeover valve, in that the wheel brakes are assigned at least one first inlet valve and one first outlet valve, in that there is provided in a bypass line, downstream of the first changeover valve and of the first isolating valve, a first electrically activatable pressure medium pump, the suction side of which is connected to the first changeover valve, to at least one first outlet valve and to a first low-pressure accumulator and the delivery side of which is connected to the first isolating valve and to at least one first inlet valve, and in that a stabilizing coupling is connected via a second pressure medium pump to actuators which can vary the tension force of the stabilizing coupling between the trailer and traction vehicle, when the pressure medium pump conveys pressure medium out of the reservoir into or out of the actuators. 
         [0010]    The hydraulic trailer brake system for the driving stabilization of vehicle trailers is advantageously based on a modified ESP system used in vehicles and having preferably two brake circuits. To stabilize an at least single-axle passenger car trailer, according to advantageous exemplary embodiments ( FIGS. 1 to 8 ) the wheel brakes are connected to one brake circuit and the ball-shaped stabilizing coupling is connected to the other pressure circuit. Further components or subassemblies, such as, for example, high-pressure accumulators and/or hydraulic cylinders, may also be connected to the brake or pressure circuits. The wheel brakes may in this case be activated individually or in pairs (the left and the right wheel of an axle). 
         [0011]    According to a further advantageous exemplary embodiment ( FIG. 9 ), the wheels of one axle are connected to one brake circuit, the ball-shaped stabilizing coupling and the wheels of a further axle being connected to the other brake circuit. 
         [0012]    It has proved to be particularly advantageous if the exemplary embodiment according to  FIG. 9 , or design variants of this, is modified such that the delivery sides of the pressure medium pumps arranged in both circuits are connected to one another via a connecting line, as illustrated in  FIGS. 11 to 14 . The two pumps can thereby generate the brake pressure in the wheel brakes of one axle. By means of the short-circuited pumps, the wheels of one axle can be braked in a directed manner according to predetermined criteria, while the wheels of the other axle can be braked at a later time point according to the same or other stipulations. In this case, the low-pressure accumulators of the two circuits are likewise also connected to one another via a connecting line. 
         [0013]    By means of a modified switching circuit of an ESP system, an active pressure build-up in the wheel brakes of the trailer can advantageously take place. 
         [0014]    Further technical benefits are the possible axle-related or side-related activation of two wheels in each case (2-channel). As a result, a higher control performance due to the directed introduction of a yawing moment in order to reduce the rolling movements can be achieved. 
         [0015]    The stabilizing coupling may likewise be activated via 2 channels. 
         [0016]    A further technical benefit, as compared with existing systems, is that leaks in the ball-shaped stabilizing coupling circuit are detected, and this circuit has a follow-up connection. 
         [0017]    It is advantageous that the master cylinder is connected to a brake line which is connected downstream of the first isolating valve via a brake pressure subline having a first inlet valve to the first wheel brake and via a brake pressure subline having the first inlet valve to the second wheel brake of a first trailer axle, and that the brake line is connected downstream of the isolating valve via a brake pressure subline having a first inlet valve to the wheel brakes of a second trailer axle ( FIG. 1 ). 
         [0018]    To reduce the brake pressure, it is advantageous that the wheel brakes are connected to return lines in each of which a first outlet valve is arranged, the return lines being connected to the first low-pressure accumulator and, via a line, to the suction side of the first pump and to the first changeover valve which is connected to the brake line ( FIG. 1 ). 
         [0019]    Two wheels and one pair of wheels can be regulated by means of this pressure force or brake force transmission circuit. 
         [0020]    In normal braking, pressure is generated in all 4 wheels by the brake master cylinder. 
         [0021]    In drives in reverse, the inlet valves are closed and the volume in the wheel saddles is conducted to the low-pressure accumulator by means of outlet valves. 
         [0022]    In hazardous situations, the isolating valve is closed and the electrical changeover valve is open. The pump then conveys pressure medium out of the low-pressure accumulator or through the electrical changeover valve out of the master cylinder via the inlet valve or inlet valves to the corresponding wheels. In order to achieve a rapid response of the trailer, the inlet valve for the pair of wheels is opened with a time delay (only when the desired pressure prevails in the two regulated wheels). The reduction in the brake pressure takes place by means of a pressure or volume reduction (in the wheel saddles) via the outlet valve or outlet valves, from which the pressure medium flows to the low-pressure accumulator and (if necessary) is conveyed into the master cylinder again by means of the pump. 
         [0023]    ABS: When a wheel locks, the corresponding inlet valve is closed and the corresponding outlet valve is opened. The pressure medium flows via the outlet valve from the corresponding wheel or pair of wheels to the low-pressure accumulator, from which the pressure medium is conveyed back to the master cylinder by means of the pump via the isolating valve. 
         [0024]    Furthermore, the hydraulic trailer brake system is expediently designed such that the reservoir is connected to a second pressure line which is connected downstream of the second isolating valve via a pressure subline having a second inlet valve to actuators of a ball-shaped stabilizing coupling, and the actuators are connected to a return line in which a second outlet valve is arranged, the return line being connected to the second low-pressure accumulator and, via a line, to the suction side of the second pump and to the second changeover valve which is connected to the line. 
         [0025]    In a hazardous situation, the pressure medium supply, normally regulated as a function of the speed, into the actuators of the stabilizing coupling may additionally be used for stabilizing the connection between the traction vehicle and trailer. For this purpose, the isolating valve is closed and the electrical changeover valve, via which the pump conveys the pressure medium to two pistons of the actuators at the ball-shaped coupling, is opened. The friction acting on the coupling can thereby be set, as desired. If the moment of friction is to be lowered, the inlet valve is closed and the outlet valve is opened. The fluid flows via the outlet valve from the piston to the low-pressure accumulator, from which the fluid is conveyed back to the master cylinder by means of the pump via the isolating valve. 
         [0026]    The supply of energy to the pump drive motors assigned to the pumps takes place via a separate supply unit. For this purpose, there is provision for the supply unit to contain accumulators, preferably an accumulator pack, which are connected to the battery of the vehicle via a connecting element between the vehicle and trailer, the accumulators being connected to the control and regulation unit. 
         [0027]    In an embodiment according to aspects of the invention, the master cylinder is connected to a brake line which is connected downstream of the first isolating valve via a brake pressure subline having a first inlet valve to the first wheel brake and via a brake pressure subline having the first inlet valve to the second wheel brake of a first trailer axle, and the brake line is connected upstream of the isolating valve via a brake pressure subline having a first inlet valve to the wheel brakes of a second trailer axle ( FIG. 2 ). 
         [0028]    By means of this pressure force or brake force transmission circuit, an individual regulation of the pressure build-up for two wheels on one axle and for a pair of wheels on another axle can be implemented. 
         [0029]    Pressure reduction regulation is likewise provided for two wheels and a pair of wheels. 
         [0030]    In normal braking, pressure is generated in all four wheels by the brake master cylinder. 
         [0031]    In drives in reverse, the inlet valves are closed and the volume in the wheel saddles is conducted to the low-pressure accumulator by means of outlet valves. 
         [0032]    In hazardous situations, the isolating valve is closed and the electrical changeover valve is open. The pump then conveys pressure medium out of the low-pressure accumulator or through the electrical changeover valve out of the master cylinder via the inlet valve or inlet valves to the corresponding wheels. The reduction in the brake pressure takes place by means of a pressure or volume reduction (in the wheel saddles) via the outlet valve or outlet valves, from which the fluid flows to the low-pressure accumulator and, if necessary, is conveyed into the master cylinder again by means of the pump. 
         [0033]    ABS: When a wheel locks, the corresponding inlet valve is closed and the corresponding outlet valve is opened. The fluid flows via the outlet valve from the corresponding wheel or pair of wheels to the low-pressure accumulator, from which the brake fluid is conveyed back to the master cylinder by means of the pump via the isolating valve. 
         [0034]    In a hazardous situation, in this embodiment, too, the connection between the traction vehicle and trailer is stabilized. For this purpose, the isolating valve is closed and the electrical changeover valve, via which the pump conveys the brake fluid to two pistons at the ball-shaped coupling, is opened. The friction acting on the coupling can thereby be set, as desired. If the moment of friction is to be lowered, the inlet valve is closed and the outlet valve is opened. The brake fluid flows via the outlet valve from the piston to the low-pressure accumulator, from which the brake fluid is conveyed back to the master cylinder by means of the pump via the isolating valve. 
         [0035]    A further hydraulic trailer brake system for regulating two pairs of wheels is advantageously designed such that the master cylinder is connected to a brake line which is connected downstream of the first isolating valve via a brake pressure subline having a first inlet valve to first wheel brakes of a first trailer axle and via a brake pressure subline having the first inlet valve to the second wheel brakes of a second trailer axle. It is advantageous that the wheel brakes are connected to return lines, in each of which a first outlet valve is arranged, the return lines being connected to the first low-pressure accumulator and, via a line, to the suction side of the first pump and to the first changeover valve which is connected to the brake line ( 12 ). ( FIG. 3 ). 
         [0036]    By means of this pressure force or brake force transmission circuit, a regulation of the pressure build-up for two pairs of wheels can be implemented. In normal braking, pressure is generated in all four wheels by the brake master cylinder. 
         [0037]    In drives in reverse, the inlet valves are closed and the volume in the wheel saddles is conducted to a low-pressure accumulator by means of outlet valves. In hazardous situations, to increase the stability, the isolating valve is closed and the electrical changeover valve is opened. The pump then conveys pressure medium out of the low-pressure accumulator or through the electrical changeover valve out of the master cylinder via the inlet valve or inlet valves to the corresponding pairs of wheels. The reduction in the brake pressure takes place by means of a pressure or volume reduction (in the wheel saddles) via the outlet valve or outlet valves, from which the pressure medium flows to the low-pressure accumulator and, if necessary, is conveyed into the master cylinder again by means of the pump. 
         [0038]    ABS: If a wheel locks, the corresponding inlet valve is closed and the corresponding outlet valve is opened. The pressure medium flows via the outlet valve from the corresponding pair of wheels to the low-pressure accumulator, from which the brake fluid is conveyed back to the master cylinder by means of the pump via the isolating valve. 
         [0039]    In this embodiment, too, the connection between the traction vehicle and trailer is stabilized. For this purpose, the isolating valve is closed and the electrical changeover valve, via which the pump conveys the pressure medium to two pistons at the ball-shaped stabilizing coupling, is opened. As a result, the friction acting on the coupling can be set, as desired. If the moment of friction is to be lowered, the inlet valve is closed and the outlet valve is opened. The pressure medium flows via the outlet valve from the piston to the low-pressure accumulator, from which the brake fluid is conveyed back to the master cylinder by means of the pump via the isolating valve. 
         [0040]    According to a fourth embodiment, there is provision for the brake line to be connected downstream of the first isolating valve to a high-pressure accumulator. In this case, it is expedient that preferably an inlet valve serving as a filter is arranged between the isolating valve and the outlet valve in the brake pressure subline ( FIG. 4 ). 
         [0041]    By means of this pressure force or brake force transmission circuit, a regulation of the pressure build-up for two pairs of wheels can be implemented. As soon as the pressure in the high-pressure accumulator falls, the pump conveys pressure medium out of the low-pressure accumulator or through the electrical changeover valve out of the master cylinder via the inlet valve and the switched outlet valve to the high-pressure accumulator. The inlet valve serves merely for filtering the pressure medium. By a second filter being integrated into the outlet valve or by a filter being integrated into the corresponding line, the inlet valve could be saved ( FIG. 4 ). 
         [0042]    In normal braking, pressure is generated in all four wheels by the brake master cylinder. 
         [0043]    In drives in reverse, the inlet valves are closed and the volume in the wheel saddles is conducted to the low-pressure accumulator by means of outlet valves. 
         [0044]    In hazardous situations, the isolating valve is closed and the electrical changeover valve is opened. The pump then conveys brake fluid out of the low-pressure accumulator or through the electrical changeover valve out of the master cylinder via the inlet valve or inlet valves to the corresponding pairs of wheels. To shorten the response time of the trailer, moreover, the outlet valve of the high-pressure accumulator opens so that the latter can assist the pump in supplying the wheels. The reduction in the brake pressure takes place by means of a pressure or volume reduction (in the wheel saddles) via the outlet valve or outlet valves, from which the brake fluid flows to the low-pressure accumulator and (if necessary) is conveyed into the master cylinder or into the high-pressure accumulator again by means of the pump. 
         [0045]    ABS: If a wheel locks, the corresponding inlet valve is closed and the corresponding outlet valve is opened. The pressure medium flows via the outlet valve from the corresponding pair of wheels to the low-pressure accumulator, from which the brake fluid is conveyed back to the master cylinder by means of the pump via the isolating valve. 
         [0046]    Furthermore, advantageously, a hydraulic trailer brake system is designed, with regard to the circuit II for the ball-shaped stabilizing coupling, such that the reservoir is connected to a second pressure line in which a second isolating valve is connected via a pressure subline and a pressure line having second inlet valves to the actuators of the ball-shaped stabilizing coupling, and the actuators are connected to return lines in which the second outlet valve is arranged, the return lines being connected to the second low-pressure accumulator and, via a line, to the suction side of the second pump and to the second changeover valve which is connected to the line. In this case, each of the pressure sublines connected in each case to an actuator of the ball-shaped stabilizing coupling is assigned in each case an inlet and an outlet valve and a separate pressure sensor. The circuit I and the circuit II are set up identically. The pressure force or brake force transmission circuit I in this case corresponds in its design to the pressure force or brake force transmission circuit in  FIG. 3 , and therefore there is no need for a more detailed description ( FIG. 5 ). 
         [0047]    Furthermore, a hydraulic trailer brake system is designed such that the reservoir is connected to a second pressure line in which a second isolating valve is connected via a pressure subline having a second inlet valve to the actuators of the ball-shaped stabilizing coupling and a pressure line having a second inlet valve is connected to a high-pressure accumulator, and the actuators are connected to a return line and the high-pressure accumulator is connected to a return line, in which return lines second outlet valves are arranged, the return lines being connected to the second low-pressure accumulator and, via a line, to the suction side of the second pump and to the second changeover valve which is connected to the line. In this case, the pressure subline connected to the two actuators of the ball-shaped stabilizing coupling and the pressure subline connected to a high-pressure accumulator are assigned in each case an inlet and an outlet valve and a separate pressure sensor. The circuit I and the circuit II are set up identically. The pressure force or brake force transmission circuit I in this case corresponds in its design to the pressure force or brake force transmission circuit in  FIG. 3 , and therefore there is no need for a more detailed description ( FIG. 6 ). 
         [0048]    Furthermore, in contrast to the version according to  FIG. 1 , it is expedient, in the pressure transmission circuit for activating the ball-shaped stabilizing coupling, that the reservoir is connected to a second pressure line in which a second isolating valve is connected via a pressure subline to the actuators of the ball-shaped stabilizing coupling, the pressure line being connected via a line to the suction side of the second pump. The pressure force or brake force transmission circuit I in this case corresponds in its design to the pressure force or brake force transmission circuit in  FIG. 1 , and therefore there is no need for a more detailed description ( FIG. 7 ). 
         [0049]    Moreover, in contrast to the version according to  FIG. 7 , it is expedient, in the pressure transmission circuit for activating the ball-shaped stabilizing coupling, that an outlet valve is arranged upstream of the actuators of the ball-shaped stabilizing coupling in the pressure subline ( FIG. 8 ). 
         [0050]    A hydraulic trailer brake system according to aspects of the invention in which a tandem brake master cylinder is provided is also advantageous. By means of the tandem master cylinder, the two circuits function independently of one another. One circuit supplies the ball-shaped stabilizing coupling and one pair of wheel brakes and the other circuit supplies the other pair of wheel brakes with fluid. The trailer brake system in this case therefore provides for one pair of wheel brakes to be arranged in the circuit of the ball-shaped stabilizing coupling and for another pair of wheel brakes to be arranged in a brake circuit. Of course, only one wheel brake per circuit may also be provided if the trailer has only one axle. The embodiment is characterized in that the brake master cylinder has two pressure chambers with two outlets, the brake line being connected to the one outlet and the pressure line to the other outlet, and the brake line being connected downstream of the first isolating valve via a brake pressure subline having a first inlet valve to the wheel brakes of a first trailer axle, and the wheel brakes being connected to a return line in which a first outlet valve is arranged, the return line being connected to the first low-pressure accumulator and, via a line, to the suction side of the first pump and to the first changeover valve which is connected to the brake line. In this case, the pressure line is connected via a second isolating valve in a pressure subline having a second currentlessly closed inlet valve to the actuators of the ball-shaped stabilizing coupling and via a further pressure subline and a second inlet valve to the wheel brakes of a second trailer axle, the actuators and the wheel brakes being connected in each case to a return line in each of which a second outlet valve is arranged. The return lines are connected in each case to the second low-pressure accumulator and, via a line, to the suction side of the second pump and to the second changeover valve which is connected to the line. In contrast to the embodiment according to  FIG. 1 , the inlet valve of the ball-shaped stabilizing coupling circuit is designed as a currentlessly closed valve without a nonreturn valve. By means of the currentlessly closed inlet valve in the ball-shaped stabilizing coupling circuit, during normal braking via the actuation of the master cylinder, for example by means of the inertia brake, a pressure drop in the ball-shaped stabilizing coupling is prevented. ( FIG. 9 ). 
         [0051]    According to a further embodiment of the hydraulic trailer brake system, it is expedient that the brake master cylinder is connected to a second pressure line in which a second isolating valve is connected in a pressure subline having a second currentlessly closed inlet valve to the actuators of the ball-shaped stabilizing coupling and via a pressure subline and a second currentlessly open inlet valve to the wheel brakes of a second trailer axle, the actuators and the wheel brakes being connected in each case to a return line in each of which a second outlet valve is arranged, the return lines being connected to the second low-pressure accumulator and, via lines, to the suction side of the second pump and to the second changeover valve which is connected to the second pressure line. Consequently, in contrast to the design variant according to  FIG. 9 , the trailer brake system provides only one brake master cylinder having a pressure chamber, the second pressure line then being connected to the brake line. It is in this case advantageous that two wheel brakes are connected to the brake master cylinder via a first isolating valve arranged in the brake line and via a first changeover valve, that, furthermore, the wheel brakes of the first axle are assigned a first inlet and a first outlet valve, that, moreover, there is provided in a bypass line, downstream of the first changeover valve and of the first isolating valve, the first electrically activatable pressure medium pump, the suction side of which is connected to the first changeover valve, to the first outlet valve and to the first low-pressure accumulator and the delivery side of which is connected to the first isolating valve and to the first inlet valve. In the hydraulic trailer brake system in the exemplary embodiment according to  FIG. 10 , the wheel brakes of the first axle are connected to the first low-pressure accumulator via the return line having the outlet valve. In contrast to the embodiment according to  FIG. 1 , in this embodiment, too, the inlet valve of the ball-shaped stabilizing coupling circuit is designed as a currentlessly closed valve without a nonreturn valve. By means of the currentlessly closed inlet valve in the ball-shaped stabilizing coupling circuit, during normal braking via the actuation of the master cylinder, for example by means of the inertia brake, a pressure drop in the ball-shaped stabilizing coupling is prevented. The trailer brake system therefore in this case provides for one pair of wheel brakes to be arranged in the circuit of the ball-shaped stabilizing coupling and for another pair of wheel brakes to be arranged in a separate brake circuit. Of course, only one wheel brake per circuit may also be provided if the trailer has only one axle ( FIG. 10 ). 
         [0052]    The hydraulic trailer brake system illustrated diagrammatically in  FIG. 11  advantageously provides, as compared with the system illustrated, for example, in  FIG. 10 , for the return lines with the first outlet valve and with the second outlet valve to be connected to the first low-pressure accumulator and to the second low-pressure accumulator via a connecting line. Furthermore, this embodiment also provides for the delivery sides of the first pressure medium pump and of the second pressure medium pump to be connected to one another via a connecting line. In this case, the connecting line is connected to the brake line between the first isolating valve and the bypass line having the first pressure medium pump and to the pressure subline between the second isolating valve and the bypass line having the second pressure medium pump. This embodiment makes it possible that fluid can be conveyed to a pair of wheel brakes of a trailer axle by means of the first and the second pump. By means of this “short-circuited” variant, in which the low-pressure accumulator and the two pumps of the two circuits are connected to one another, a trailer axle can be braked in a directed manner according to specific criteria by means of both pumps and therefore with high power, while the other axle is braked according to other or identical criteria, for example according to criteria which take into account a braking time point. The criteria are stipulated by the electronic control unit on the basis of measured and/or calculated information on the driving behavior of the trailer and/or of the vehicle ( FIG. 11 ). 
         [0053]    The hydraulic trailer brake system is expediently designed such that, in contrast to the design variant according to  FIG. 11 , the second isolating valve is dispensed with. For this purpose, the brake system is designed such that the brake master cylinder is connected to the suction side of the second pressure medium pump by means of a second pressure line via a second changeover valve, and that the delivery side of the pump is connected via the pressure subline having the inlet valve to the actuators of the ball-shaped stabilizing coupling and via a pressure subline and the inlet valve to the wheel brakes of a second trailer axle, the actuators and the wheel brakes being connected in each case to a return line in each of which a second outlet valve is arranged, the return lines being connected via the connecting line to the first and the second low-pressure accumulator and, via a line, to the suction side of the second pump and to the second changeover valve which is connected to the line. Furthermore, for this purpose, there is provision for two wheel brakes to be connected to the brake master cylinder via a first isolating valve arranged in the brake line and via a first changeover valve, for the wheel brakes to be assigned a first inlet and a first outlet valve, for there to be provided in a bypass line, downstream of the first changeover valve and of the first isolating valve, the first electrically activatable pressure medium pump, the suction side of which is connected to the first changeover valve, to the first outlet valve and to the first low-pressure accumulator and the delivery side of which is connected to the first isolating valve and to the first inlet valve, and for the delivery sides of the first pressure medium pump and of the second pressure medium pump to be connected to one another via a connecting line. In this design variant, the ball-shaped stabilizing coupling is acted upon by the hydraulic medium via the two currentlessly closed valves, to be precise the changeover valve and the inlet valve, when the electronic control unit outputs the control signals for opening the changeover valve and the inlet valve as a function of the driving behavior of the trailer and/or of the vehicle ( FIG. 12 ). 
         [0054]    A further advantageous design variant according to aspects of the invention is designed in such a way that the second changeover valve is dispensed with, as compared with the design according to  FIG. 12 . Consequently, the costs of the trailer brake system can be reduced. The second ball-shaped stabilizing coupling circuit with the ball-shaped stabilizing coupling and with the second pair of wheel brakes is supplied with hydraulic medium via the opening or closing of the currentlessly closed changeover valve of the first circuit and the currentlessly closed inlet valve or the currentlessly open inlet valve having the nonreturn valve according to corresponding activation via the electronic control unit. For this purpose, the design variant according to aspects of the invention is characterized in that two first wheel brakes are connected to the brake master cylinder via an isolating valve arranged in the brake line and via a changeover valve, in that the first wheel brakes are assigned a first inlet and a first outlet valve, in that there is provided in a bypass line, downstream of the changeover valve and of the isolating valve, the first electrically activatable pressure medium pump, the suction side of which is connected to the changeover valve, to the first outlet valve and to the first low-pressure accumulator and the delivery side of which is connected to the first isolating valve and to the first inlet valve, and the first changeover valve is connected via a connecting line to the suction side of the second pressure medium pump, to the second low-pressure accumulator and to the second outlet valve, the second low-pressure accumulator being connected to the first low-pressure accumulator via the connecting line, and the delivery side of the second pressure medium pump being connected via the connecting line to the delivery side of the first pressure medium pump and being connected via the pressure subline having the inlet valve to the actuators of the ball-shaped stabilizing coupling and via a pressure subline and the inlet valve to the wheel brakes of a second trailer axle, the wheel brakes being connected to the connecting line via the return line and the outlet valve ( FIG. 13 ). 
         [0055]    Furthermore, a further advantageous version is provided, in which the hydraulic trailer brake system is designed in such a way that the master cylinder is connected to a brake line which is connected downstream of the first isolating valve via a brake pressure subline having a first inlet valve to first wheel brakes of a first trailer axis and via a brake pressure subline having the first inlet valve to the second wheel brakes of a second trailer axle. All the wheel brakes are connected to return lines in each of which a first outlet valve is arranged, the return lines being connected to the first low-pressure accumulator and, via a line, to the suction side of the first pump and to the first changeover valve which is connected to the brake line. The brake master cylinder is in this case connected to a second pressure line in which a second isolating valve is connected via a pressure subline having a second inlet valve to the actuators of the ball-shaped stabilizing coupling. The actuators are connected to a return line in which a second outlet valve is arranged, the return line being connected to the second low-pressure accumulator and, via a line, to the suction side of the second pump and to the second changeover valve. The changeover valve is connected to the line. Furthermore, the return lines having the first outlet valves are connected via a connecting line to the first low-pressure accumulator and to the second low-pressure accumulator. Moreover, the delivery sides of the first pressure medium pump and of the second pressure medium pump are connected to one another via a connecting line. A nonreturn valve opening to the stabilizing coupling is advantageously arranged between the inlet valve and the connection point of the return line having the outlet valve to the brake subline ( FIG. 14 ). 
         [0056]    An exemplary embodiment of the invention is illustrated in the drawing and is described in more detail below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0057]    The invention is best understood from the following detailed description when read in connection with the accompanying drawing. Included in the drawing are the following figures: 
           [0058]      FIG. 1  shows a diagrammatic illustration of the circuit of the hydraulic trailer brake system according to aspects of the invention, 
           [0059]      FIG. 2  shows an embodiment of the hydraulic circuit diagram illustrated in  FIG. 1 , 
           [0060]      FIG. 3  shows a further embodiment of the hydraulic circuit diagram illustrated in  FIG. 1 , 
           [0061]      FIG. 4  shows a further embodiment of the hydraulic circuit diagram illustrated in  FIG. 1 , 
           [0062]      FIG. 5  shows a further embodiment of the hydraulic circuit diagram illustrated in  FIG. 1  with a separate activation of the actuators of the ball-shaped stabilizing coupling, 
           [0063]      FIG. 6  shows a further embodiment of the hydraulic circuit diagram illustrated in  FIG. 1  with a high-pressure accumulator, 
           [0064]      FIG. 7  shows an embodiment of the hydraulic circuit diagram illustrated in  FIG. 1  with a simplified circuit for activating the ball-shaped stabilizing coupling, 
           [0065]      FIG. 8  shows an embodiment of the hydraulic circuit diagram illustrated in  FIG. 1  with a modified ball-shaped coupling circuit, 
           [0066]      FIG. 9  shows an embodiment of the hydraulic circuit diagram illustrated in  FIG. 1  with a brake master cylinder having two pressure chambers and with a brake circuit, provided in the activation circuit of the ball-shaped stabilizing coupling, for a pair of wheel brakes, 
           [0067]      FIG. 10  shows an embodiment of the hydraulic circuit diagram illustrated in  FIG. 9  with a brake master cylinder having only one pressure chamber, 
           [0068]      FIG. 11  shows an embodiment of the hydraulic circuit diagram illustrated in  FIG. 10  with “short-circuited” pumps and low-pressure accumulators of the two circuits, 
           [0069]      FIG. 12  shows an embodiment of the hydraulic circuit diagram illustrated in  FIG. 11  with “short-circuited” pumps and low-pressure accumulators of the two circuits, without the isolating valve of the second ball-shaped stabilizing coupling circuit, 
           [0070]      FIG. 13  shows an embodiment of the hydraulic circuit diagram illustrated in  FIG. 12  with “short-circuited” pumps and low-pressure accumulators of the two circuits, without the isolating valve and the changeover valve of the second ball-shaped stabilizing coupling circuit, 
           [0071]      FIG. 14  shows an embodiment of the hydraulic circuit diagram illustrated in  FIG. 3 , but with “short-circuited” pumps and low-pressure accumulators of the two circuits, and with a special return circuit of the ball-shaped stabilizing coupling with an outlet valve and nonreturn valve. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0072]    The hydraulic trailer brake system for drive stabilization, illustrated in  FIG. 1 , has pressure or force transmission circuits which are designated as a whole by  9 . One brake pressure transmission circuit I consists of a brake cylinder  10  of an inertia brake  80  which is actuated by means of a mechanical unit  81  of the inertia brake  80 . Arranged on the brake cylinder  10  is a reservoir  27  which contains a pressure medium and which is connected in the brake release position to the working chamber of the brake cylinder  10 . The brake pressure transmission circuit illustrated has a brake line  12  connected to at least one working chamber of the brake cylinder  10  and having a first isolating valve  14  which in its rest position forms an open passage for the brake line  12 . Connected in parallel to the isolating valve  14  is a nonreturn valve  15  which opens from the direction of the brake cylinder  10 . The isolating valve  14  is actuated electromagnetically in the conventional way. However, variations in which hydraulic actuation takes place may also be envisaged. 
         [0073]    The brake line  12  branches into three brake pressure sublines  12   a ,  12   b ,  12   c  which lead to wheel brakes  18   a  to  18   d . The brake pressure sublines  12   a ,  12   b  lead here in each case to a wheel brake  18   a  or  18   b  of a trailer axle, while the brake pressure subline  12   c  leads to the pair of wheel brakes  18   c  and  18   d  of a trailer axle. The brake pressure sublines contain in each case an electromagnetically actuable inlet valve  16   a ,  16   b ,  16   c  which are open in their rest position and can be switched into a blocking position by the actuating magnet being excited. A nonreturn valve  26   a  to  26   c  is connected in parallel to each inlet valve  16   a  to  16   c  and opens from the direction of the wheel brakes  18   a ,  18   b  or  18   c  and  18   d . Connected in parallel to these wheel brake circuits  12 ,  12   a  to  12   c  is what is known as a recirculating circuit which consists of return lines  20   a  to  20   c  with a pressure medium pump  25 . The wheel brakes  18   a  and  18   b , in each case via an outlet valve  22   a  and  22   b , and the wheel brakes  18   c  and  18   d , via an outlet valve  22   c , are connected via the return lines  20   a  to  20   c  to the line  24  and consequently to the suction side of the pressure medium pump  25 , the delivery side of which is connected to the brake pressure sublines  12   a  to  12   c  at a point of issue between the isolating valve  14  and the inlets valves  16   a  to  16   c.    
         [0074]    The conveying pump  25  is designed as a reciprocating pump with a delivery valve and a suction valve which are not illustrated in any more detail. Located on the suction side of the pressure medium pump  25  is a low-pressure accumulator  23  which is connected to the return lines  20   a  to  20   c.    
         [0075]    In the connection between the low-pressure accumulator  23  and the pressure medium pump  25  is inserted a pretensioned nonreturn valve  29  opening to the pump. 
         [0076]    Furthermore, the suction side of the pump  25  is connected via the additional line  24  having a first changeover valve  28  to the brake cylinder  10 . 
         [0077]    The pressure or force transmission circuit  9  has a second control circuit II with a line  30  which is connected to the reservoir  27  and which branches into a pressure subline  30   a  and a line  46 . The pressure subline  30   a  leads via a second isolating valve  32  and a parallel-connected nonreturn valve  33  to actuators  36   a ,  36   b  of a stabilizing coupling  38  of the trailer. Via branches of the pressure subline  30   a , the two actuators  36   a ,  36   b  can be acted upon by the pressure generated in the lines  30 ,  30   a . Alternatively, it is also possible to act upon the actuators individually with the hydraulic medium contained in the reservoir  27 , in that two identical pressure transmission circuits are provided. Furthermore, the pressure line  30   a  contains an electromagnetically actuable inlet valve  34  which is open in its rest position and which can be switched into a blocking position by the actuating magnet being excited. A nonreturn valve  35  which opens in the direction of the reservoir  27  is connected in parallel to the inlet valve  34 . Connected in parallel to this pressure circuit  30 ,  30   a  is what is known as a recirculating circuit which consists of return lines  40 ,  46  with a pressure medium pump  48 . The actuators  36   a ,  36   b  are connected via an outlet valve  42  arranged in the line  40  to the line  46  and consequently to the suction side of the pressure medium pump  48 , the delivery side of which is connected to the brake pressure subline  30   a  at a point of issue between the isolating valve  32  and the inlet valve  34 . 
         [0078]    The conveying pump  48  is designed as a reciprocating pump with a delivery valve and a suction valve which are not illustrated in any more detail. Located on the suction side of the pressure medium pump  48  is a low-pressure accumulator  44  which is connected to the return lines  40 ,  46 . 
         [0079]    In the connection between the low-pressure accumulator  44  and the pressure medium pump  48  is inserted a pretensioned nonreturn valve  49  opening to the pump. 
         [0080]    Furthermore, the suction side of the pump  48  is connected to the reservoir  27  via the line  46  and a second changeover valve  50 . 
         [0081]    As can be seen from  FIG. 1 , the pressure circuit for the actuators  36   a ,  36   b  of the stabilizing coupling  38  and the brake pressure circuit for the wheel brakes  18   a  to  18   d  are advantageously set up identically. While in the brake pressure circuit, three wheel brakes  18   a  to  18   d , to be precise in each case the wheel brake  18   a  and the wheel brake  18   b  and a pair of wheel brakes  18   c  and  18   d , are activated separately, in the pressure circuit of the stabilizing coupling one “actuator” only, to be precise the two cylinder/piston units, is activated. 
         [0082]    Moreover, the pressure or force transmission circuit  9  has an electronic control unit  62 . Control or regulating signals for the valves and the pumps  25 ,  48  are generated in the electronic control unit on the basis of the wheel rotational speed sensors  94  to  97 , assigned to the wheels  90  to  93  illustrated merely diagrammatically, and of the pressure sensors  98 ,  99 ,  100 ,  101  assigned to the pressure or brake circuits or the further sensors  64 , for example the sensors of a sensor cluster, such as, for example, transverse-acceleration sensors, yaw-rate sensors, longitudinal-acceleration sensors. The energy supply to the electronic control unit  62  takes place in this case via accumulators, that is to say an accumulator pack  60 , which is connected to the vehicle battery via a supply line between the vehicle and the trailer. 
         [0000]    The device operates as follows: 
       Braking: 
       [0083]    The braking of the trailer by means of the inertia brake  80  takes place whenever the trailer speed is higher than the driving speed of the traction vehicle. In this case, by means of the mechanical unit  81  of the inertia brake  80 , pressure medium is displaced from the master cylinder  10  via the brake line  12 , open isolating valve  14  and open inlet valves  16   a  to  16   c  to the wheel brakes  18   a  to  18   d . The trailer is braked via all four wheels  90  to  93 . 
       ABS Control: 
       [0084]    By means of rotational speed signals from the wheel rotational speed sensors  90  to  93  which are made available to the control unit  62 , the circumferential speeds of each individual wheel are determined and are evaluated logically in terms of brake slip by means of the driving speed calculated in the control unit  62 . When, as a rule, the brake slip rises above 8 to 30 percent or in this case a wheel of the wheel brakes  18   a  to  18   d  locks (100 percent brake slip), the respective inlet valve  16   a  to  16   c  is closed, so that the pressure medium conveyed from the master cylinder  10  into the brake line  12  no longer arrives at the corresponding wheel. By the respective outlet valve  22   a  to  22   c  being opened, a pressure drop occurs, in that pressure medium is discharged from the respective wheel brake  18   a  to  18   d  into the low-pressure accumulator  23 . The pressure medium of the low-pressure accumulator  23  can be recirculated into the reservoir  27  at a later time point by means of the pump. 
       Stability Braking: 
       [0085]    The build-up of pressure in the wheel brake circuits of the wheel brakes  18   a ,  18   b ,  18   c  and  18   d  takes place, with the changeover valve  28  open and the isolating valve  14  closed, via the activation signals of the control unit  62  when the isolating valve  14  is open currentlessly in the initial position and the changeover valve  28  is currentlessly closed. In this case, by means of the pressure medium pump  25 , pressure medium is conveyed via the master cylinder  10  out of the reservoir  27  or the low-pressure accumulator  23  into the wheel brake circuits in which pressure medium is thus introduced according to the calculated brake pressure requirement. Activation in this case takes place as a function of the determined rolling movement of the trailer. The rolling movement is determined on the basis of the transverse-acceleration or yaw-rate or wheel rotational speed signals from the corresponding sensors  94  to  97 ,  64  which are evaluated logically in the control unit according to a program and assessed. Then, as a function of the evaluation result, the wheel brakes  18   a  or  18   b  are activated individually, specifically according to a program, essentially in phase opposition to the rolling movement, in order to damp the rolling movement. Another possibility is to activate the wheel brakes simultaneously. The two wheel brakes  18   c  and  18   d  can be activated simultaneously or even by being switched before or after via the common brake circuit in order to reduce the trailer speed. 
         [0086]    When the trailer is damped, brake pressure is discharged into the low-pressure accumulator  23  via the opening of the respective outlet valves  22   a  to  22   c  and of the return lines  20   a  to  20   c , the inlet valves  16   a  to  16   c  being closed. 
         [0087]    When the rolling movement of the trailer is to be reduced by means of the ball-shaped coupling  38 , the isolating valve  32  is closed and the changeover valve  50  is opened, and the pump  48  conveys pressure medium out of the reservoir  27  to the actuators  36   a ,  36   b . The ball-shaped coupling is tensioned with increased frictional force. 
         [0088]    The following design variants of  FIGS. 2 to 8  all have the two separate circuits described in connection with  FIG. 1 , to be precise the activation circuit for the wheel brakes  18   a  to  18   d  and the activation circuit for the stabilizing coupling  38  or further components. The hydraulic trailer brake systems operate essentially correspondingly according to the functioning described previously in connection with  FIG. 1 . Deviations in functioning which arise from the modification of the circuit are in each case described individually. The same reference symbols are used for identical components, not all the reference symbols being included in the alternative design variants for the sake of clarity. It is possible in a simple way for a person skilled in the art to transfer the reference symbols from  FIG. 1  to the respective components or structural parts of the corresponding circuit. Of course, individual circuits of  FIGS. 1 to 8  may also be replaced by circuits of the other figures in each case, thus one axle with two wheels or more than one axle with more wheels can be activated by means of the trailer brake systems. For this purpose, merely, identical activation circuits have to be provided. Thus, two pairs of wheel brakes of two axles or else wheel brakes individually can be activated. 
         [0089]      FIG. 2  shows an alternative, according to aspects of the invention, for the pressure or force transmission device  9  of  FIG. 1 . In contrast to the version in  FIG. 1 , in  FIG. 2  the brake pressure subline  122   c  leading to the wheel brakes  18   c  and  18   d  and having the inlet valve  16   c  and outlet valve  22   c  branches upstream of the isolating valve  14  and the changeover valve  28 . It is connected directed to the brake pressure line  12 . By means of this design, pressure medium cannot be made available by the pump  25  to the wheel brakes  18   c  and  18   d , that is to say to the pair of wheel brakes connected to the line  122   c . By means of the inertia brake  80 , however, brake pressure can be introduced into the brake circuit by means of the brake pressure subline  122   c  during stability control, that is to say during the side-related activation of the wheel brakes  18   a  or  18   b , by means of the pump  25 . The pressure reduction in the wheel brakes  18   a  to  18   d  takes place according to the way described in  FIG. 1 . The second activation circuit for the stabilizing coupling  38  corresponds to the version described in more detail in  FIG. 1 . 
         [0090]      FIG. 3  shows a further alternative, according to aspects of the invention, for the pressure or force transmission device  9  of  FIG. 1 . In contrast to  FIG. 1 , the wheel brakes  18   a  and  18   b  are connected correspondingly to the wheel brakes  18   c  and  18   d  described in  FIG. 1 . That is to say, the wheel brakes  18   a  and  18   b  or the pair of wheel brakes  18   a ,  18   b  can be activated only jointly via an inlet valve  16   b  and an outlet valve  22   a . The brake pressure subline  12   c  of  FIG. 1  having the inlet and outlet valve  16   c  and  22   c  is dispensed with. Here, too, the second activation circuit for the stabilizing coupling  38  corresponds to the version described in more detail in  FIG. 1 . 
         [0091]      FIG. 4  shows an alternative, according to aspects of the invention, for the pressure or force transmission device  9  of  FIG. 1 . Here, the wheel brakes  18   a  to  18   d  are activated according to the circuit diagram of the alternative according to  FIG. 3 . However, the alternative has, furthermore, the pressure subline  123   c  corresponding to the brake pressure subline  12   c  of  FIG. 1 , in which a high-pressure accumulator  70  is connected via an outlet valve  22   c . A filter  166   c  is provided between the isolating valve  14  and the outlet valve  22   c . The filter  166   c  is designed as a currentlessly open inlet valve without a nonreturn valve. Between the outlet valve  22   c  and the high-pressure accumulator  70  is arranged a further pressure sensor  102 , by means of which the pressure of the high-pressure accumulator  70  can be determined. 
         [0092]      FIG. 5  shows the pressure or force transmission device  9  illustrated in  FIG. 1 , with a modified activation of the ball-shaped stabilizing coupling  38 , as compared with  FIG. 1 . As can be seen from  FIG. 5 , the actuators  36   a ,  36   b  of the stabilizing coupling  38  are connected to the reservoir  27  via the line  30 . The line  30  branches downstream of the isolating valve  32  into pressure sublines  30   a ,  30   b  and downstream of the changeover valve  50  into a line  46 . The pressure subline  30   a  leads via the inlet valve  34   a  with the nonreturn valve  35   a  to one of the actuators  36   a  or  36   b , while the pressure subline  30   b  leads via the inlet valve  34   b  and nonreturn valve  35   b  in each case to the other actuator  36   a  or  36   b  of the stabilizing coupling  38 . The pump  48  conveys pressure medium out of the reservoir to the actuators  36   a ,  36   b  in order to stabilize the trailer. Via the switching intervals (open/shut) of the isolating valve  32 , then, a stipulated pressure determined by means of the pressure sensors  101 ,  102  for each actuator  36   a  and  36   b  can be introduced into the actuators  36   a  or  36   b . For this purpose, the inlet valves  34   a  or  34   b  of the respective actuator can be closed or opened by means of the electronic control unit. The two pressure sublines  30   a ,  30   b  have the return lines  40   a ,  40   b , described in connection with  FIG. 1 , in which an outlet valve  42   a ,  42   b  is arranged in each case. The return lines are connected to the low-pressure accumulator  44  and to the line  46 . 
         [0093]      FIG. 6  shows a modification of the stabilizing coupling circuit. In contrast to  FIG. 5 , hydraulic medium is introduced into the two actuators  36   a ,  36   b  of the stabilizing coupling  38  via the pressure subline  30   a  with the inlet valve  34   a  and with the nonreturn valve  35   a . A high-pressure accumulator  70  is connected to the pressure subline  30   b  with the inlet valve  34   b  and the nonreturn valve  35   b . The two pressure sublines  30   a ,  30   b  likewise have the return lines  40   a ,  40   b , described in connection with  FIG. 1 , in each of which an outlet valve  42   a ,  42   b  is arranged. The return lines are connected to the low-pressure accumulator  44  and to the line  46 . 
         [0094]      FIG. 7  shows the pressure or force transmission device  9  illustrated in  FIG. 1 , with a modified activation of the ball-shaped stabilizing coupling  38 , as compared with  FIG. 1 . The hydraulic circuit of the wheel brakes  18   a  to  18   d  connected to the brake line  12  corresponds in this case to the activation circuit illustrated and described in  FIG. 1  and therefore does not need to be described in any more detail. As illustrated diagrammatically in  FIG. 7 , the actuators  36   a ,  36   b  of the stabilizing coupling  38  are connected to the reservoir  27  via the line  30 . The line  30  branches into the pressure subline  30   a  and a line  46 . A currentlessly open isolating valve  32  with a nonreturn valve  33  is arranged in the pressure subline  30   a . Between the isolating valve  32  and the actuators  36   a ,  36   b , the delivery side of the second pump  48  is connected to the pressure subline  30   a , the suction side of which pump is connected to the line  46 . In the pressure subline  30   a , the pressure sensor  101  is arranged between the connection point of the suction side of the pump  48  and the actuators  36   a ,  36   b  of the stabilizing coupling  38 . After the activation of the pump  48  by means of the electronic control unit  62 , to stabilize the trailer, the pump  48  conveys pressure medium out of the reservoir  27  to the actuators  36   a ,  36   b . Via the switching intervals “open/shut” of the isolating valve  32 , then, a stipulated pressure determined by means of the pressure sensor  101  can be set in the actuators  36   a ,  36   b.    
         [0095]      FIG. 8  shows a modification of the stabilizing coupling circuit of  FIG. 7 . In contrast to the circuit variant of  FIG. 7 , a currentlessly closed outlet valve  42  is provided as a shutoff valve in the pressure subline  30   a  between the connection point of the suction side of the pump  48  and the connection point of the pressure sensor  101 . By means of the valve  42 , the pressure introduced into the actuators  36   a ,  36   b  of the stabilizing coupling can be blocked or held while the isolating valve  32  is open. 
         [0096]    The following design variants of  FIGS. 9 to 14  all have the two separate activation circuits described in connection with  FIG. 1 , but, in contrast to the preceding design variants, the two hydraulic activation circuits both comprise the activation of wheel brakes  18   a  to  18   d , that activation of the stabilizing coupling  38  or further components being additionally provided in the case of one activation circuit. The hydraulic trailer brake systems operate essentially correspondingly according to the functioning described previously in connection with  FIG. 1 . Deviations in functioning which arise from the modification of the circuit are in each case described individually. The same reference symbols are used for identical components, not all the reference symbols being included in the alternative design variants for the sake of clarity. It is possible in a simple way for a person skilled in the art to transfer the reference symbols from  FIG. 1  to the respective components or structural parts of the corresponding circuit. Of course, individual circuits of  FIGS. 1 to 14  may also be replaced by circuits of the other figures in each case, thus one axle with two wheels or more than one axle with more wheels can be activated by means of the trailer brake systems. For this purpose, merely, identical activation circuits have to be provided. Thus, two pairs of wheel brakes of two axles or else wheel brakes individually can be activated. By the activation of the wheel brakes  18   a ,  18   b  or  18   c ,  18   d  being allocated to two activation circuits, each with a pressure medium pump  25 ,  48 , pressure-dynamic benefits can be achieved. 
         [0097]      FIG. 9  shows a hydraulic trailer brake system with a tandem brake master cylinder  10  which has two pressure chambers with two separate outlets  200 ,  201 . The two control circuits I, II are supplied with hydraulic medium via the two pressure chambers of the brake master cylinder  10 . In the control circuit I supplying the wheel brakes  18   a ,  18   b  with a hydraulic medium, the brake line  12  is connected to the outlet  201  of the first pressure chamber, and, in the control circuit II, the pressure line  30  is connected to the outlet  200  in the second pressure chamber. The brake circuit I with the brake line  12  has, downstream of the first isolating valve  14  with the nonreturn valve  15 , a brake pressure subline  12   b  which is connected via the first inlet valve  16   a  arranged in it to the wheel brakes  18   a ,  18   b  of a first trailer axle. The wheel brakes  18   a ,  18   b  are connected to a return line  20   b  in which a first currentlessly closed outlet valve  22   b  is arranged. The return line  20   b  is connected to the first low-pressure accumulator  23  and, via a line  24 , to the suction side of the first pump  25 . In the line  24  is provided a nonreturn valve  29  which opens to the suction side of the pump and is also connected to the first changeover valve  28  via the line  24 . The changeover valve  28  is connected to the brake line  12 . The second hydraulic control circuit II provides, correspondingly to the version according to  FIG. 1 , a hydraulic circuit for the stabilizing coupling  38 , in such a way that the pressure line  30  is connected via a second isolating valve  32  with a nonreturn valve  33 , in a pressure subline  30   a  having a second inlet valve  34 , to the actuators  36   a ,  36   b  of the ball-shaped stabilizing coupling  38 . In contrast to the design in  FIG. 1 , the inlet valve  34  is designed as a currentlessly closed shutoff valve without a nonreturn valve. By means of the inlet valve  34  designed as a shutoff valve, the pressure in the actuators  36   a ,  36   b  can be blocked, while the isolating valve  32  is open. The pressure line  30  is connected to the wheel brakes  18   c ,  18   d  of a second trailer axle via a further pressure subline  30   b  and a second currentlessly open inlet valve  16   c  with a nonreturn valve  26   b . A pressure sensor  101  for determining the pressure in the actuators  36   a ,  36   b  is arranged between the actuators and the shutoff valve  34  in the pressure subline  30   a . The actuators  36   a ,  36   b  and the wheel brakes  18   c ,  18   d  of the second control circuit II are connected in each case to a return line  20   c ,  40  in which a second outlet valve  22   c ,  42  is arranged in each case. The two return lines  20   c ,  40  are connected in each case to the second low-pressure accumulator  44  and, via a line  46 , to the suction side of the second pump  48  and to the second changeover valve  50 . The changeover valve  50  is connected to the line  30 . 
       Braking: 
       [0098]    The braking of the trailer by means of the inertia brake  80  takes place whenever the trailer speed is higher than the driving speed of the traction vehicle. In this case, by means of the mechanical unit  81  of the inertia brake  80 , pressure medium is displaced from the first pressure chamber of the tandem brake master cylinder  10  via brake line  12 , open isolating valve  14  and open inlet valve  16   a  to the wheel brakes  18   a ,  18   b . The changeover valve  28  and the currentlessly closed outlet valve  22   b  are closed. Furthermore, hydraulic medium is introduced into the wheel brakes  18   c  and  18   d  from the second pressure chamber of the tandem brake master cylinder  10  via the pressure line  30 , the open isolating valve  32  and the open inlet valve  16   c . Via the switching intervals “open/shut” of the inlet valve  16   c , then, a stipulated pressure determined by means of the pressure sensor  99  can be set in the wheel brakes  18   c ,  18   d . The outlet valve  22   c  and the shutoff valve  34  of the stabilizing coupling are closed. The trailer can be braked via all four wheels  90  to  93 . 
       Stability Braking: 
       [0099]    The build-up of pressure in the wheel brake circuits, separate in this design variant, for the wheel brakes  18   a  and  18   b  or  18   c  and  18   d  takes place with the changeover valves  28  and  50  open and the isolating valves  14  and  32  closed via the activation signals of the control unit  62  when the isolating valves  14 ,  32  are currentlessly open in the initial position and the changeover valves  28 ,  50  are currentlessly closed. In this case, by means of the two pressure medium pumps  25 ,  48 , pressure medium is conveyed via the master cylinder  10  out of the reservoir  27  or the low-pressure accumulators  23 ,  44  into the control circuits I, II of the wheel brake circuits in which pressure medium is thus introduced according to the calculated brake pressure requirement. It is determined via the pressure sensors  98 ,  99  whether the brake pressure requirements are set in the wheel brakes  18   a  to  18   d . Activation in this case takes place as a function of the determined rolling movement of the trailer. The rolling movement is determined on the basis of transverse-acceleration or yaw-rate or wheel rotational speed signals from the corresponding sensors  94  to  97 ,  64  which are evaluated logically in the control unit according to a program and are assessed. As a function of the evaluation result, the wheel brakes  18   a  to  18   d  of a trailer axle are then activated according to a program in order to damp the rolling movement. 
         [0100]    Another possibility is to activate the wheel brakes individually. For example, the two wheel brakes  18   c  and  18   d  of the second control circuit II can be activated simultaneously or else by switching before or after, in order to reduce the trailer speed. In parallel with this, in the control circuit II, the shutoff valve  34  can be opened and a hydraulic medium can be introduced into the actuators  36   a ,  36   b . Via the shutoff valve  34 , then, a stipulated pressure determined by means of the pressure sensor  101  in the actuators  36   a ,  36   b  can be blocked. The rolling movement of the trailer can thereby be reduced by means of the ball-shaped stabilizing coupling  38 , in that the ball-shaped coupling is tensioned with increased frictional force. 
         [0101]    When the trailer is damped, brake pressure is discharged into the low-pressure accumulators  23 ,  44  via the opening of the respective outlet valves  22   b ,  22   c  of the return lines  20   b ,  20   c , the inlet valves  16   a ,  16   c  being closed. 
         [0102]      FIG. 10  shows a design variant of  FIG. 9  in which there is no tandem brake master cylinder with two pressure chambers provided, but, instead, a brake master cylinder  10  with only one pressure chamber as the essential difference. In this case, the brake master cylinder  10  is connected to the second pressure line  30  via the brake line  12 . Thus, in the pressure line  30 , the second isolating valve  32  is connected in the pressure subline  30   a  via the second inlet valve  34  to the actuators  36   a ,  36   b  of the ball-shaped stabilizing coupling  38  and via the pressure subline  30   b  and the second inlet valve  16   c  to the wheel brakes  18   c ,  18   d  of the second trailer axle. The actuators  36   a ,  36   b  and the wheel brakes  18   c ,  18   d  are in each case connected to a return line  20   c ,  40  in each of which a second outlet valve  22   c ,  42  is arranged, the return lines  20   c ,  40  being connected to the second low-pressure accumulator  44 . Furthermore, as already described in  FIG. 9 , the return lines  20   c ,  40  are connected via the line  46  to the suction side of the second pump  48  and to the second changeover valve  50 . The changeover valve  50  is connected to the second pressure line  30 . 
         [0103]    Since the control circuit I is set up identically to the control circuit I illustrated and described in  FIG. 9 , with the exception of the already mentioned connection of the pressure line  30  to the brake line  12 , in connection with  FIG. 10  reference may be made to the description of  FIG. 9 . The operation of the hydraulic trailer brake system also corresponds to that described in connection with  FIG. 9 , except that the two control circuits I, II are not supplied with a fluid by different pressure chambers, but by a common pressure chamber of the brake master cylinder  10 . 
         [0104]      FIG. 11  shows a hydraulic trailer brake system, which, as compared with the design variant according to  FIG. 10 , provides for the return lines  20   b ,  20   c  having the first outlet valve  22   b  and the second outlet valve  22   c  to be connected to the first low-pressure accumulator  23  and to the second low-pressure accumulator  44  via a connecting line  221 . In this case, the delivery sides of the first pressure medium pump  25  and of the second pressure medium pump  48  are also connected to one another via a connecting line  220 . In this case, the connecting line  220  is connected to the brake line  12   b  of the first control circuit I between the first isolating valve  14  having the nonreturn valve  15  and the connection point of the bypass line to the first pressure medium pump  25 . The other side of the connecting line  220  is connected to the pressure subline  30   a  of the second control circuit II between the second isolating valve  32  and the connection point of the bypass line to the second pressure medium pump  48 . By means of the hydraulically “short-circuited” pressure medium pumps  25 ,  48 , pressure medium can be conveyed into one of the two wheel brakes  18   a ,  18   b  or  18   c ,  18   d  by means of the two pumps, with the result that a faster build-up of pressure in the pair of wheel brakes and consequently an improved damping of the trailer during rolling are achieved. Correspondingly, the low-pressure accumulators  23 ,  44  are “short-circuited” in order to achieve a balanced pressure reduction with better volume distribution. By means of this design, the electronic control unit  62  can brake a pair of wheel brakes  18   a ,  18   b  or  18   c ,  18   d  of one axle according to specific criteria in a highly dynamic way by means of the two pressure medium pumps  25 ,  48 , while the other axle with the other pair of wheel brakes can be braked according to identical or other criteria at another time point. 
         [0105]      FIG. 12  shows a development of the hydraulic trailer brake system according to  FIG. 11 . In contrast to the design in  FIG. 11 , the second isolating valve  32  of the second control circuit II is dispensed with here. The brake master cylinder  10  is therefore connected to the suction side of the second pressure medium pump  48  by means of the second pressure line  30  via a second changeover valve  50 . The delivery side of the pump  48  is connected to the actuators  36   a ,  36   b  of the ball-shaped stabilizing coupling  38  via the pressure subline  30   a  having the currentlessly closed inlet valve  34  designed as a shutoff valve. The pressure subline  30   b  having the inlet valve  16   c  connects the wheel brakes  18   c ,  18   d  of a second trailer axle to the pressure subline  30   a . The actuators  36   a ,  36   b  and the wheel brakes  18   c ,  18   d  are in each case connected to a return line  20   c ,  40  in each of which a second outlet valve  22   c ,  42  is arranged. The return lines  20   c ,  40  are connected via the connecting line  221  to the first and the second low-pressure accumulator  23 ,  44  and via a line  46  to the suction side of the second pump  48  and to the second changeover valve  50 . The changeover valve  50  is connected to the line  30 . As described in  FIG. 11 , the control circuit I provides for the two wheel brakes  18   a ,  18   b  to be connected to the brake master cylinder  10  via a currentlessly open isolating valve  14 , arranged in the brake line  12 ,  12   b  and having the nonreturn valve  15 , and a changeover valve  28 , for the wheel brakes  18   a ,  18   b  to be assigned a first inlet and a first outlet valve  16   a ;  22   b , and for there to be provided in the bypass line, downstream of the changeover valve  28  and of the isolating valve  14 , the first electrically activatable pressure medium pump  25 , the suction side of which is connected to the changeover valve  28 , to the first outlet valve  22   b  and to the first low-pressure accumulator  23  and the delivery side of which is connected to the first isolating valve  14  and to the first inlet valve  16 . The delivery sides of the pumps  25 ,  48  are “short-circuited” via the connecting line  220  according to the design in  FIG. 11 . 
         [0106]      FIG. 13  shows a development of the hydraulic trailer brake system according to  FIG. 12 . In contrast to the design of  FIG. 12 , in  FIG. 13  there is no changeover valve  50  provided in the second control circuit II. As a result, the first changeover valve  28  is connected directly via a connecting line  230  to the suction side of the second pressure medium pump  48 , to the second low-pressure accumulator  44  and to the second outlet valve  42 . The second low-pressure accumulator  44  is connected to the first low-pressure accumulator  23  via the connecting line  221 , and the delivery side of the second pressure medium pump  48  is connected to the delivery side of the first pressure medium pump  25  via the connecting line  220 . The pressure subline  30   a  having the currentlessly closed inlet valve  34  designed as a shutoff valve connects the actuators  36   a ,  36   b  of the ball-shaped stabilizing coupling  38  to the pumps  25 ,  48 , while the pressure subline  30   b  having the currentlessly open inlet valve  16   c  with the nonreturn valve  26   b  likewise connects the wheel brakes  18   c ,  18   d  of a second trailer axle to the pumps  25 ,  48 . The wheel brakes  18   c ,  18   d  are connected to the connecting line  221  via the return line  20   c  and the outlet valve  22   c.    
         [0107]      FIG. 14  shows a further design variant of the “short circuited” hydraulic trailer brake system. The control circuit I corresponds essentially to the design according to  FIG. 3 . The pressure or force transmission device  9  is designed such that the wheel brakes  18   a  and  18   b  and also  18   c  and  18   d  are connected correspondingly to the wheel brakes  18   c  and  18   d  described in  FIG. 1 . One brake pressure transmission circuit consists of a brake cylinder  10  of an inertia brake  80 , which is actuated by a mechanical unit  81  of the inertia brake  80 . On the brake cylinder  10 , a reservoir  27  is arranged, which contains a pressure medium and which is connected to the working chamber of the brake cylinder  10  in the brake release position. The brake pressure transmission circuit illustrated has the brake line  12  connected to at least one working chamber of the brake cylinder  10  and having a first isolating valve  14  which in its rest position forms an open passage for the brake line  12 . A nonreturn valve  15  is connected in parallel to the isolating valve  14  and opens from the direction of the brake cylinder  10 . The isolating valve  14  is actuated electromagnetically in the conventional way. However, variations in which hydraulic actuation takes place may also be envisaged. 
         [0108]    The brake line  12  branches into two brake pressure sublines  12   a ,  12   b  which lead in each case to two wheel brakes  18   a ,  18   b  and  18   c ,  18   d  of a trailer axle. The brake pressure sublines contain in each case an electromagnetically actuable inlet valve  16   a ,  16   b  which are open in their rest position and can be switched into a blocking position by the actuating magnet being excited. A nonreturn valve  26   a ,  26   b  is connected in parallel to each inlet valve  16   a ,  16   b  and opens from the direction of the wheel brakes  18   a ,  18   b  and  18   c ,  18   d . Connected in parallel to these wheel brake circuits  12 ,  12   a ,  12   b  is the recirculating circuit consisting of return lines  20   a ,  20   b  which are connected to the pressure medium pump  25 . The wheel brakes  18   a  and  18   b , via the outlet valve  22   a , and the wheel brakes  18   c  and  18   d , via the outlet valve  22   b , are connected via the return lines  20   a ,  20   b  to the line  24  and consequently to the suction side of the pressure medium pump  25 , the delivery side of which is connected to the brake pressure sublines  12   a ,  12   b  at a point of issue between the isolating valve  14  and the inlet valves  16   a ,  16   b.    
         [0109]    The conveying pump  25  is designed as a reciprocating pump with a delivery valve and a suction valve which are not illustrated in any more detail. Located on the suction side of the pressure medium pump  25  is a low-pressure accumulator  23  which is connected to the return lines  20   a ,  20   b.    
         [0110]    In the connection between the low-pressure accumulator  23  and the pressure medium pump  25  is inserted a pretensioned nonreturn valve  29  opening to the pump. 
         [0111]    Furthermore, the suction side of the pump  25  is connected to the brake cylinder  10  via the additional line  24  having the first changeover valve  28 . 
         [0112]    The second control circuit II of the pressure or force transmission circuit  9  has a line  30  which is connected to the brake master cylinder and which branches into a pressure subline  30   a  and a line  46 . The pressure subline  30   a  leads via a second currentlessly open isolating valve  32  and a parallel-connected nonreturn valve  33  to actuators  36   a ,  36   b  of the stabilizing coupling  38  of the trailer. The two actuators  36   a ,  36   b  can be acted upon via branches of the pressure subline  30   a  with the pressure generated in the lines  30 ,  30   a . Alternatively, it is also possible to act upon the actuators individually with the hydraulic medium contained in the reservoir  27  in that two identical pressure transmission circuits are provided. Furthermore, the pressure line  30   a  contains an electromagnetically actuable inlet valve  34  which is open in its rest position and can be switched into a blocking position by the actuating magnet being excited. Connected in parallel to the inlet valve  34  is a nonreturn valve  35  which opens in the direction of the reservoir  27 . Connected in parallel to this pressure circuit  30 ,  30   a  is the recirculating circuit which consists of return lines  40 ,  46  with the pressure medium pump  48  arranged in the line  46 . The actuators  36   a ,  36   b  are connected via a currentlessly closed outlet valve  42  arranged in the line  40  and via the second low-pressure accumulator  44  to the line  46  and consequently to the suction side of the pressure medium pump  48 , the delivery side of which is connected to the brake pressure subline  30   a  at a point of issue between the isolating valve  32  and the inlet valve  34 . Furthermore, a nonreturn valve  51  opening in the direction of the stabilizing coupling  38  is provided downstream of the connection point of the return line  40  and upstream of the inlet valve  34  in the pressure subline  30   a . As described previously in the design variants of  FIGS. 11 to 13 , the low-pressure accumulators  23 ,  44  and the delivery sides of the pumps  25 ,  48  are connected to one another via the connecting lines  220 ,  221 . 
         [0113]    In all the design variants, the stabilizing coupling may have a venting screw. 
         [0114]    The design according to aspects of the invention, as shown in  FIG. 14 , may, of course, be modified to the effect that it can also be used for a trailer with only one axle and consequently two wheel brakes. For this purpose, in contrast to  FIG. 14 , instead of the pair of wheel brakes  18   c ,  18   d  or  18   a ,  18   b , only one wheel brake, for example  18   c ,  18   a , of the one axle is connected to the brake master cylinder  10 . Furthermore, a design is possible which does not have the connecting lines  220 ,  221 . In this case, the low-pressure accumulators  23 ,  44  or the pumps  25 ,  48  are not short-circuited. 
         [0115]    Furthermore, the hydraulic circuit diagram according to  FIG. 14  may also, as illustrated, be implemented with two changeover valves  28 ,  50 , but with only one isolating valve  14  (see  FIG. 13 ) or with one changeover valve  50  and one isolating valve  14  (see  FIG. 12 ). 
         [0116]    Basically, all the control circuits I, II described can be combined with one another. 
         [0117]    Also, according to an advantageous design, instead of or in combination with two wheel brakes, a hydraulic cylinder for a cable brake can be connected, such as is used, for example, in an electronic parking brake. Thus, two wheels can be braked via the wheel brakes, the stabilizing coupling can be activated and a cable brake of the trailer can be actuated. 
         [0118]    While preferred embodiments of the invention have been described herein, it will be understood that such embodiments are provided by way of example only. Numerous variations, changes and substitutions will occur to those skilled in the art without departing from the spirit of the invention. It is intended that the appended claims cover all such variations as fall within the spirit and scope of the invention.