Abstract:
A parking brake module for a pressure medium-operated brake system of a vehicle having a parking brake function enabled by at least one spring brake cylinder includes a first valve device that increases the amount of pressure medium and has an inlet that can be connected to a pressure medium reservoir, an outlet that can be connected to at least one spring-loaded part of a spring brake cylinder, and a control input connected to a control line. A second valve device encompasses at least one first connection used as a trailer control valve connection and can be connected to a trailer control valve to control a trailer brake. To provide a valve concept for different vehicle configurations, particularly including an anti jackknifing function, pressure prevailing at the first connection can be adjusted regardless of pressure prevailing in the pressure medium reservoir and at the outlet of the first valve device.

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to a parking brake module for a pressure medium operated brake system of a vehicle having a parking brake function. 
     BACKGROUND OF THE INVENTION 
     A parking brake module of the general type under consideration is described in DE 103 36 611 A1, in particular FIG. 8. This module has a plurality of valves or valve devices that can mainly be activated electrically. On the one hand, the parking brake module is connected to a compressed air reservoir vessel. On the other hand, a first compressed air line leads to a spring-loaded component of a spring-type brake cylinder embodied as a combined spring-type/diaphragm brake cylinder. Such combined brake cylinders have both the function of diaphragm brake cylinders and a spring-loading function. The brake cylinders each include a diaphragm component that is pneumatically connected to the service brake system and to which the actual brake pressure can be applied. In addition, such brake cylinders each include a spring-loaded component that is pneumatically disconnected from the diaphragm component and to which compressed air can be applied via separate compressed air lines. 
     The spring-loaded component implements the spring-loading function by virtue of the fact that when compressed air is applied to the spring-loaded component a pre-loaded spring is pre-stressed and, in the process, a braking effect of the spring energy stores is reduced. When the spring-loaded component is vented, the pre-loaded spring is relaxed so that, in the scope of the spring-loading function, a braking effect is applied to the brake that is connected to the respective brake cylinder. Brake cylinders of this type are referred to hereinafter as spring-type brake cylinders. Such spring-type brake cylinders enable a parking brake function that permits the vehicle to be braked or arrested even when there is a lack of compressed air. 
     The above-mentioned parking brake module has a further connection to a trailer control valve. The trailer control valve is used to control the brake pressure of an attached trailer vehicle; and, in this context, this should also be understood to apply to a semitrailer. The trailer control valve has a port for a control line of the trailer service brake that leads to the trailer vehicle. The trailer control valve inverts the pressure applied to its inlet. That is, if a high pressure coming from the service brake module is present at its input, a low service brake pressure is modulated, and vice-versa. 
     The parking brake module has a first valve device, specifically a relay valve, and a second valve device, a pressure-holding valve and a bistable valve. The relay valve includes an inlet that can be connected to a compressed air reservoir vessel. The relay valve further includes an outlet that can be connected to at least one spring-loaded component of a spring-type brake cylinder. The relay valve also includes a control inlet. The pressure applied to the control inlet is modulated at the outlet of the relay valve, but with a significantly larger quantity of air. In this way, the necessary volumes for activating the spring-type brake cylinders can be made available. 
     The control pressure applied to the control inlet of the relay valve can be set using a pressure-holding valve and a bistable valve. In this way, the pressure at the outlet of the relay valve can also be set. 
     The second valve device includes a 3/2 way solenoid valve. A first port of this 3/2 way solenoid valve leads to the line that leads to the trailer control valve. A second port of this valve is connected to the outlet of the relay valve. A third port can be connected to the compressed air reservoir vessel. In a de-energized state, this 3/2 way solenoid valve connects the first port to the second port, while the first port is shut off from the third port. In contrast, in an energized state, the first port is shut off from the second port and the first port is connected to the third port. 
     In an energized state of this 3/2 way solenoid valve, the trailer control valve is therefore vented, and, in this way, the trailer brake is released. Venting the trailer control valve at this port leads, specifically, to a reduction in the service brake pressure at the outlet of the trailer control valve, with the result that the trailer brake pressure decreases and the trailer brake is, thus, released. 
     However, in a de-energized state, the pressure in the trailer control valve is the same as the pressure at the outlet of the relay valve, and, therefore, identical to the pressure in the spring-loaded component of a spring-type brake cylinder. 
     However, the conventional pneumatic circuit arrangement described above leads to a situation in which it is not possible to brake the trailer while the towing vehicle is unbraked. With the conventional parking brake module, it is, therefore, not possible to implement what is referred to as a trailer safety brake function. 
     Such parking brake systems are used in different vehicle control systems such as, for example, electronic brake systems, anti-lock brake systems or systems without anti-lock brake protection. The parking brake module has to be adapted to these different vehicle control systems. It is, therefore, manufactured in a wide variety of designs. This reduces the respective number of a specific type of parking brake module. However, a reduced number increases the costs of manufacture, administration and stock keeping. 
     SUMMARY OF THE INVENTION 
     Generally speaking, it is an object of the present invention to provide a single valve design for a parking brake module suitable for various vehicle configurations, and, in particular, a valve design that also permits a trailer safety brake function. 
     In accordance with embodiments of the present invention, a pressure prevailing at the first port of the second valve device, i.e., the pressure conducted to the trailer control valve, can be set independently of the pressure in the pressure medium reservoir vessel as well as of the pressure at the outlet of the pressure medium-quantity-boosting valve device, i.e., independently of the pressure in the spring-loaded component of the spring-type brake cylinder. The spring energy store and the trailer control valve can therefore be decoupled. 
     The possibility of setting the pressure for the trailer control valve independently of the pressure for the spring-type brake cylinders allows the trailer to be braked while the towing vehicle is unbraked. This also permits a trailer safety brake function to be implemented. 
     Furthermore, the inventive construction also permits a trailer monitoring setting, in particular in ABS vehicles. In addition, the implementation of a basic parking brake function and a hill-holder function, i.e., a function in which a vehicle is held with the brake on even without activating the brake pedal, is permitted. With such a function, it is necessary to provide particular safety precautions so that the vehicle does not unintentionally roll away. 
     The parking brake module according to exemplary embodiments of the present invention can be used in virtually all vehicles with any vehicle control systems, such as, for example, vehicles with an electronic brake system or anti-lock brake system or without anti-lock brake protection, in which context a variety of functions of a parking brake can be carried out without the parking brake module being over-equipped for minimum requirements. 
     Overall, a universal valve design for a parking brake module is provided that can implement a variety of functions in vehicles with electronic brake systems or anti-lock brake systems or without vehicle control systems. 
     Still other objects and advantages of the present invention will in part be obvious and will in part be apparent from the specification. 
     The present invention accordingly comprises the features of construction, combination of elements, and arrangements of parts which will be exemplified in the constructions hereinafter set forth, and the scope of the invention will be indicated in the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further advantageous embodiments can be found in the claims and from the exemplary embodiments that are discussed in greater detail with reference to the accompanying drawings, in which 
         FIG. 1  is a schematic diagram of a parking brake module and further components of a brake system of a vehicle in a first exemplary embodiment according to the present invention 
         FIG. 2  is a schematic diagram of the parking brake module according to  FIG. 1  in a state for clarifying a parking brake; 
         FIG. 3  is a schematic diagram of the parking brake module according to  FIG. 1  in a state for clarifying a trailer monitoring setting; 
         FIG. 4  is a schematic diagram of the parking brake module in a state for clarifying a trailer safety brake function; 
         FIG. 5  is a schematic diagram of a parking brake module according to a second exemplary embodiment of the present invention; 
         FIG. 6  is a schematic diagram of the parking brake module according to  FIG. 5  in a state for clarifying the parking brake function; 
         FIG. 7  is a schematic diagram of the parking brake module according to  FIG. 5  in a state for clarifying the trailer monitoring setting; 
         FIG. 8  is a schematic diagram of the parking brake module according to  FIG. 5  in a state for clarifying the trailer safety brake function; 
         FIG. 9  is a schematic diagram of a parking brake module according to a third exemplary embodiment of the present invention; and 
         FIG. 10  is a schematic diagram of a parking brake module according to a fourth exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the drawing figures, identical reference symbols are used for corresponding components. The figures also show exemplary embodiments that are used in a compressed-air-operated brake system, i.e., a pneumatic brake system. In this respect, compressed air is used as the pressure medium. As an alternative, however, the exemplary embodiments of the present invention can also be used with hydraulic systems, in which case, for example, an hydraulic oil is used as the pressure medium. 
     Referring now to the drawing figures,  FIG. 1  shows a parking brake module  10  of a pneumatic brake system for a vehicle, in particular, a utility vehicle or truck, that can be operated with a trailer vehicle including a brake system that is also pneumatic. The illustrated parking brake module  10  is located in the towing vehicle. The towing vehicle has both a service brake and a parking brake, which brakes are embodied by means of spring-type brake cylinders  12  of the type discussed hereinabove. That is, the spring-type brake cylinder  12  has a spring-loaded component  14  and a diaphragm component  16  for the service brake. In order to release the spring-loaded brake or parking brake, the spring-loaded component  14  is ventilated. This ventilation is carried out via a compressed air line  18  connected to the parking brake module  10 . 
     Just one spring-type brake cylinder  12  is illustrated in each of the drawing figures. Such spring-type brake cylinders are, however, preferably used on at least the wheels of one axle or a plurality of axles of the towing vehicle (and, if appropriate, also of the trailer vehicle). The compressed air line  18  therefore preferably leads not only to one spring-type brake cylinder, but rather to a plurality, i.e., at least two, of such spring-type brake cylinders, but only one thereof is illustrated here. However, in another exemplary embodiment, just one spring-type brake cylinder that acts on, for example, a Cardan shaft, is provided. 
     The parking brake module  10  also has a connection, via a further compressed air line  20 , to a trailer control valve  22  provided for controlling the trailer brakes. The trailer control valve  22  is arranged in the towing vehicle. The trailer control valve  22  operates in the way discussed hereinabove. Reference is made to the above discussion, in particular with respect to spring-loaded brakes and trailer control valves. 
     The trailer control valve  10  is also connected to a compressed air reservoir vessel  24  via a further compressed air line  26 . This compressed air reservoir vessel  24  provides the parking brake module  10  with compressed air that the parking brake module  10  can feed to the spring-type brake cylinder  12  via the compressed air line  18 , or to the trailer control valve  22  via the compressed air line  20 . 
     The way in which compressed air can be fed to, or carried away from, the spring-type brake cylinder  12  or the trailer control valve  22  is determined by means of an electronic control unit  28  arranged in the parking brake module  10  or in a separate unit. This electronic control unit  28  is designed to receive electrical signals from external and internal components. For example, the electronic control unit  28  receives an electrical activation signal for the trailer brake from an electrical activation unit  30  for the trailer brake. This electrical activation unit  30  is preferably arranged in the driver&#39;s cab of the towing vehicle and is designed, for example, as an operator control element that can be activated manually. 
     An actuation signal for the trailer brake can be generated by means of this electrical activation unit. The actuation signal can represent, in particular, either a desired brake pressure or else a vehicle deceleration. In order to provide a means of control that corresponds to this actuation signal, the pressure applied to the trailer control valve  22  is measured by means of a pressure sensor  32  arranged in or outside the parking brake module  10  that can determine the pressure in the compressed air line  20 , and, therefore, the pressure that is fed to the trailer control valve  22 . 
     If a deceleration request can be preselected by means of the activation unit  30 , the electronic control unit  28  must be able to access a vehicle deceleration value that is derived, for example, from a vehicle velocity value obtained by means of wheel speed sensors, for example. 
     The electrical activation unit  30  is connected to the electronic control unit  28  via electrical lines  34  in order to exchange corresponding signals. 
     The electronic control unit  28  is also connected via electrical lines  36  to an operator control element  38  for activating a trailer monitoring function. This operator control element  38  is also accommodated in the driver&#39;s cab of the towing vehicle. 
     Although  FIG. 1  shows a plurality of electrical lines  34  and  36  that are each connected to the electronic control unit  28 , an individual signal line connected to the electronic control unit is sufficient. The bodywork of the vehicle can also be used as a ground line. 
     Instead of transmitting electrical signals from the activation unit  30  for the trailer brake or from the operator control element  38  for the trailer monitoring function, it is also possible to provide for signals to be correspondingly transmitted to the electronic control unit  28  by radio. 
     The electronic control unit  28  is also connected to further electrical lines  40  for supplying electrical power and for connecting to other systems. 
     The electronic control unit  28  is connected by means of a data bus to data lines  42  for exchanging data with further components. 
     The electronic control unit  28  serves to electrically activate valves or valve devices in the parking brake module  10 . The parking brake module  10  has, in particular, a first air-quantity-boosting valve device in the form of a relay valve  44 . The relay valve  44  includes an inlet  46  that is connected to a port  48  for the compressed air reservoir vessel  24 . The relay valve  44  also has an outlet  50  that is connected to a port  52  of the parking brake module  10 . The port  52  can be connected to the compressed air line  18  that leads to the spring-type brake cylinder  12 . In addition, the parking brake module  10  has a further port  54  that can be connected to the trailer control valve  22  by the compressed air line  20 . 
     The relay valve  44  also includes a schematically illustrated venting means  56  via which compressed air can escape. The venting means  56  and further venting means of further valves are preferably connected to a common venting device via which the compressed air can then escape centrally to atmosphere. 
     The relay valve  44  also has a control inlet  58  via which a control pressure can be fed to the relay valve  44  via a control line  60 . The relay valve  44  modulates precisely this control pressure at its port  50 , but the relay valve boosts the quantity of air, i.e., the volume made available at the outlet  50  of the relay valve  44  is significantly greater than the volume transported in the control line  60 . 
     The pressure at the outlet  50  of the relay valve  44 , and, therefore, the pressure that can be conducted to the spring-type brake cylinder  12 , is measured by means of a pressure sensor  62 . This pressure sensor  62  supplies, as does the pressure sensor  32 , an electrical signal to the electronic control unit  28 . 
     The control pressure at the control inlet  58  of the relay valve  44  is determined by means of a pressure-holding valve  64  and bistable valve  66 . For this purpose, the pressure-holding valve  64  is connected upstream of the control inlet  58  of the relay valve  44 , with the result that the control line  60  can be shut off or opened by means of this pressure-holding valve  64 . 
     The pressure-holding valve  64  is embodied as an electrically activated 2/2 way solenoid valve. 
     The bistable valve  66  has an inlet  68  connected to the port  48  for the compressed air reservoir vessel  24 . In addition, the bistable valve  66  has an outlet  70  that is connected to the pressure-holding valve via a pressure medium duct  72 . Finally, the bistable valve  66  has a venting means  74  via which compressed air can escape. 
     The bistable valve  66  includes a bistable switching function with two switched states. That is, the bistable valve retains its state even if it is no longer energized. 
     In contrast, the pressure-holding valve  64  is a spring-loaded valve, and in a de-energized state, this spring-loaded valve assumes the first switched state illustrated in  FIG. 1 , i.e., it connects through, and, in so doing, connects the control line  60  to the pressure medium duct  72 . In an energized, second switched state it closes so that the pressure in the control line  60 , and, therefore, the pressure at the control inlet  58 , and also at the outlet  50  of the relay valve  44 , is maintained. 
     The bistable valve  66  has a first switched state in which compressed air can be conducted away from the pressure-holding valve  64  via the venting means  74 . This switched state is illustrated in  FIG. 1 . In a second switched state of the bistable valve  66 , compressed air can be fed to the pressure-holding valve  64  from the compressed air reservoir vessel  24 . 
     In a first switched state of the bistable valve  66 , the outlet  70  is therefore connected to the venting means  74 , while the outlet  70  is shut off from the inlet  68 . In contrast, in the second switched state the outlet  70  is connected to the inlet  68 , while the outlet  70  is shut off from the venting means  74 . 
     The parking brake module  10  also has a second valve device  76 . This second valve device  76  includes, for its part, at least a first port  78  connected to the port  54  for the trailer control valve  22 . In addition, this second valve device  76  has a second port  80  that is connected to the pressure medium duct  72 , and, therefore, any pressure that can be set in the pressure medium duct  72  between the atmospheric pressure and the pressure in the compressed air reservoir vessel  24  can be applied to second port  80 . 
     The second valve device  76  has at least two switched states. In the first switched state (illustrated in  FIG. 1 ), the first port  78  is connected to the second port; in the second switched state, the first port  78  is shut off from the second port  80 . The compressed air line  20  that leads to the trailer control valve  22  can therefore be shut off by means of second valve device  76 . 
     According to the embodiment of the present invention illustrated in  FIG. 1 , the second valve device  76  is an electrically activated solenoid valve, specifically, a 2/2 way solenoid valve. This electrically activated solenoid valve is spring-loaded; and, in a de-energized state, it assumes the first switched state, and, in an energized state, it assumes the second switched state. 
     The illustrated parking brake module  10  permits implementation of a basic parking brake function as described below with reference to  FIG. 2 , of a trailer monitoring function as described below with reference to  FIG. 3 , and of a trailer safety brake function as described below with reference to  FIG. 4 .  FIGS. 1 to 4  therefore show the same design but in different switched states of the valves. 
     In addition, the parking brake module  10  implements protection of what is referred to as the hill-holder function that can be implemented in particular in conjunction with an electronic brake system. This hill-holder function is a starting aid that facilitates starting on a slope by maintaining the pressure in the service brake even if the vehicle driver does not activate (or no longer activates) the service brake. 
       FIG. 2  shows the parking brake module  10  according to  FIG. 1 , and serves to illustrate the basic parking brake function. In the state illustrated in  FIG. 2 , the reservoir pressure is fed to the control inlet  58  of the relay valve  44  via the bistable valve  66  and the pressure-holding valve  64 , and, therefore, a pressure that corresponds to the reservoir pressure is modulated at the outlet  50  of the relay valve  44  and is then fed to the spring-type brake cylinder  12  via the compressed air line  18 . As a result, the spring-loaded component  14  is ventilated and the parking brake is therefore released. 
     At the same time, in the illustrated position of the valves of the parking brake module  10 , the trailer control valve  22  is also ventilated and supplied with the pressure of the compressed air from the compressed air reservoir vessel  24 . However, since the trailer control valve  22  inverts, at its outlet, the pressure that is present at its inlet, the pressure at the outlet of the trailer control valve  22  is at a minimum in the state illustrated in  FIG. 2 , with the result that a trailer that is connected to the trailer control valve  22  would be unbraked. 
     By means of the pressure-holding valve  64  and the second valve device  76 , it is possible to limit the pressure in the pressure medium duct  72  in so far as this is intended by the driver or by the brake system. 
     In order to apply the parking brake, the bistable valve  66  is switched over to its first switched position so that the control pressure is vented at the control inlet  58  of the relay valve  44  and, therefore, also at the outlet  50  of the relay valve  44 , and, therefore, in total, the spring-loaded component  14  of the spring-type brake cylinder  12  is also vented. The spring energy store then generates a pressure on the wheel brakes with the result that the towing vehicle is braked. At the same time, the pressure in the line  20  to the trailer control valve  22  is also reduced with the result that the service brake in the trailer increases a pressure in the brake cylinders of the trailer and, therefore, a braking force owing to the inverting effect of the trailer control valve  22 . That is, the trailer is also braked when the bistable valve  66  is switched over to venting. 
     If the hill-holder function is activated, the control unit  28  energizes the pressure-holding valve  64  and the second valve device  76 , with the result that both valves  64 ,  76  close. In addition, the bistable valve  66  is switched over to its first switched state, with the result that it vents. If, when the hill-holder function is activated, the electrical power supply fails, the spring-loaded valves, specifically the pressure-holding valve  64  and the second valve device  76 , switch back into the basic position illustrated in  FIG. 2  (the first switched state in each case), with the result that these valves are opened, and the spring-loaded component  14  of the spring-type brake cylinder  12  and the trailer control valve  22  are, therefore, vented. In this way, the vehicle is securely held by means of the parking brake of the towing vehicle and the brake of the trailer vehicle. 
       FIG. 3  shows the parking brake module  10  from  FIGS. 1 and 2 , with the trailer monitoring position being illustrated with reference to  FIG. 3 . The trailer monitoring position is intended to enable the driver to check whether the parking brake, i.e., the spring energy store of the towing vehicle, is alone sufficient to hold the entire vehicle combination including the towing vehicle and trailer vehicle. To do this, the vehicle combination has to be first stopped and the parking brake applied. For this purpose, both the spring-type brake cylinder  12  and the trailer control valve  22  are therefore vented. However, as a result of the inverting function of the trailer control valve, the trailer vehicle is also braked by means of the trailer brake. In order to determine whether the towing vehicle also has to hold an unbraked trailer, the driver must first release the brake of the trailer vehicle. To do this, it is necessary to ventilate the trailer control valve  22 . This is illustrated in  FIG. 3 . 
     The spring-loaded component  14  of the spring-type brake cylinder  12  is first vented by virtue of the fact that the pressure-holding valve  64  is de-energized and the bistable valve  66  is placed in its first switched state. The pressure-holding valve  64  is then energized. It therefore shuts off the control inlet  58  of the relay valve  44 . This prevents venting of the spring-type brake cylinder  12 . The bistable valve  66  is then switched over to the position illustrated in  FIG. 3 , i.e., into the second switched position, while the second valve device  76  is de-energized, with the result that the pressure of the compressed air reservoir vessel  24  is conducted to the trailer control valve  22 . 
     In this state, the trailer control valve  22  is vented. The trailer brakes are therefore released without the parking brake of the towing vehicle being released. 
     In order to terminate the trailer monitoring position, the bistable valve  66  is first switched over again to ventilation, i.e., to the first switched state, and the pressure-holding valve  64  is then opened, i.e., placed in the de-energized state. 
       FIG. 4  illustrates the parking brake module  10  with the trailer safety brake function activated. With this function, the intention is that the trailer vehicle is braked while the towing vehicle is unbraked. A vehicle can therefore, to a certain extent, be straightened, which is advantageous, for example, on sections of roadway with negative gradients. In addition, such a trailer safety brake function can be appropriate on slippery underlying surfaces. It is also used to test the braking effect of the trailer. Finally, with the trailer safety brake function, it is also possible to check the connection of the towing vehicle to the trailer vehicle. 
     When the trailer safety brake function is activated, the parking brake is first released. For this purpose, the spring-type brake cylinder  12  is ventilated. If the driver then wishes to brake only the trailer, the trailer control valve  22  is vented. For this purpose, the pressure-holding valve  64  is energized and, therefore, shut off or closed, in order to keep the parking brake of the towing vehicle released—the parking brake being implemented by means of the spring-type brake cylinder  12 . If the bistable valve  66  is switched over to the position (first switched state) illustrated in  FIG. 4  and the trailer control valve  22  is vented, either the desired pressure or the desired deceleration of the vehicle is brought about. When the desired value is reached, the pressure in the trailer control valve can be held by means of the second valve device  76 . If the driver varies the deceleration request by means of the electrical activation unit  30 , the pressure can be modulated in accordance with the driver&#39;s request by means of the bistable valve  66  and the second valve device  76 . If the control is carried out by means of the pressure, the pressure sensor  32 , which can determine the pressure in the trailer control valve  22 , is used. The bistable valve  66  and the second valve device  76  are then correspondingly opened or closed as a function of the pressure measured by the pressure sensor  32 , until the desired pressure is reached. 
       FIGS. 5 to 8  show a further exemplary embodiment of a parking brake module  10  according to the present invention. This exemplary embodiment corresponds essentially to the exemplary embodiment shown in  FIGS. 1 to 4 , but the second valve device according to  FIGS. 5 to 8  is of a different design and is, therefore, denoted by  76 ′. 
     The second valve device  76 ′ has, in addition to a first port  78  and a second port  80 , a third port  82  that is connected to the outlet  50  of the relay valve  44 . The second valve device  76 ′ also has a third switched state, in which the first port  78  is connected to the third port  82 , and the second port  80  is shut off from the first port  78  and the third port  82 . This second valve device  76 ′ is embodied as an electrically activated double-armature solenoid valve  84  (that is additionally illustrated separately in a simplified sectional view in  FIGS. 5 to 8 ). In the de-energized state, the second valve device  76 ′ assumes a third switched state. In this third switched state, the first port  78  is connected to the third port  82 , and the second port  80  is shut off from the first port  78  and from the third port  82 . 
     The first switched state of the second valve device discussed above in conjunction with  FIGS. 1 to 4 , is assumed in a first energized state of the valve device  76 ′. In this state, the first port  78  is connected to the second port  80 , while the second port  80  is shut off from the first port  78  and the third port  82 . 
     The second valve device  76 ′ assumes the second switched state in a second energized state, all the ports  78 ,  80  and  82  being respectively shut off from one another in this switched state. 
     The double-armature solenoid valve  84  has a coil (not illustrated) that can be energized. Unless this coil is energized, the second valve device  76 ′ remains in the third switched state owing to the pretension of a spring. When the coil is energized with a first low current, the second valve device  76 ′ moves into the first switched state. When it is energized with a higher current, the second valve device  76 ′ goes into the second switched state. 
     The double-armature solenoid valve  84  has a primary armature  86  and a secondary armature  88 . At first, i.e., when there is a low current, the primary armature is attracted, while the secondary armature  88  remains essentially in a position of rest. However, with a relatively high current, the secondary armature  88  is also attracted, with the result that the second valve device  76 ′ then assumes its second switched state. 
     The exemplary embodiment shown in  FIG. 5  permits, when the second valve device  76 ′ is de-energized, pressure-actuation of the trailer control valve  22  according to the pressure-actuation of the spring-type brake cylinder  12 . That is, the spring-type brake cylinder  12  is coupled to the trailer control valve  22  with respect to the pressure. At the same time, energization of this second valve device  76 ′ permits the same states to be set as are possible in the exemplary embodiment shown in  FIGS. 1 to 4 . 
     The state according to  FIG. 6 , therefore, corresponds essentially to the state shown in  FIG. 2 , and, specifically, in the de-energized state of the pressure-holding valve  64  and of the second valve device  76 ′, the same pressure is present at the first port  78  of the second valve device, i.e., at the port leading to the trailer control valve  22 , as is present at the outlet  50  of the relay valve  44  (even if the first port  78  is shut off from the second port  80  of the second valve device  76 ′ in the exemplary embodiment shown in  FIG. 6 ). 
     In order to apply the parking brake, the bistable valve  66  is switched over to venting, i.e., into the first switched state, with the result that the vehicle is braked. 
       FIG. 7  corresponds to the state illustrated in  FIG. 3  for illustrating the trailer monitoring position. However, according to  FIG. 7 , for this purpose, the second valve device  76 ′ is placed in the first switched state in which the coil of the double-armature solenoid valve  84  is energized with a first, low current. However, for the rest, the mode of operation of the trailer monitoring position according to  FIG. 7  corresponds to the mode of operation described for  FIG. 3 , so that reference is made to the discussion hereinabove. 
       FIG. 8  corresponds, in turn, to the situation shown in  FIG. 4  for describing the trailer safety brake function; but, in turn, the second valve device  76 ′ is in a first energized state with a low current in order to connect the first port  78  of the second valve device  76 ′ to the second port  80  of this valve device. For the rest, the mode of operation corresponds, however, to the mode of operation discussed in conjunction with  FIG. 4 , so that reference is made to the discussion hereinabove with respect to  FIG. 4 . 
       FIG. 9  shows a further exemplary embodiment of a parking brake module  10 ″. This exemplary embodiment corresponds structurally essentially to the exemplary embodiment shown in  FIGS. 1 to 4 , and functionally essentially to the exemplary embodiment shown in  FIGS. 5 to 8 . In this exemplary embodiment, a further, third valve device  90  is arranged between the second valve device  76 , which is embodied as in  FIGS. 1 to 4 , and the pressure medium duct  72  or the bistable valve  66  and the relay valve  44 . This third valve device  90  has at least two switched states, and, in a first switched state, the second port  80  of the second valve device  76  is connected to the outlet  50  of the relay valve  44  and shut off from the pressure medium duct  72 . In a second switched state of this third valve device, the second port  80  of the second valve device is connected to the pressure medium duct  72  and shut off from the outlet of the relay valve  44 . 
     The third valve device is embodied as an electrically activated, spring-loaded 3/2 way solenoid valve. Owing to the spring loading, this third valve device  90  assumes the first switched state in a de-energized state, and the second switched state in an energized state. 
     The third valve device  90  has a first port  92 , a second port  94  and a third port  96 . In the first switched state, the first port  92  is connected to the third port  96 , while the second port  94  is shut off from the first port  92  and the third port  96 . In the second switched state, the first port  92  is connected to the second port  94 , while the third port  96  is shut off from the first port  92  and the second port  94 . 
     The first port  92  is connected to the second port  80  of the second valve device. The second port  94  is connected to the pressure medium duct  72 . The third port  96  is connected to the outlet  50  of the relay valve  44 . 
     By suitably actuating the second valve device  76  and the third valve device  90 , and also the other valves, specifically, the pressure-holding valve  64  and the bistable valve  66 , it is also possible to use this embodiment to carry out the functions discussed in conjunction with  FIGS. 1 to 4 . 
       FIG. 10  shows a further exemplary embodiment of a parking brake module  10 ′″. In this exemplary embodiment, the second valve device  76 ′″ has a third port  98  that serves as a further trailer control valve port. Port  98  is connected to a further port  100 , intended for a trailer control valve, on the parking brake module  10 ″. In the first switched state of the second valve unit  76 ″, the third port  98  is shut off from the other two ports  78 ,  80  of the second valve device  76 ′″. A line that can be connected to the third port  98  of the second valve device  76 ′″ can therefore be shut off with respect to a trailer control valve  22 ′. 
     In contrast, in the second switched state of the second valve device  76 ′″, the third port  98  is connected to the second port  80 . In this way, the pressure for a trailer control valve  22 ′ that can be connected to the third port  98  of the second valve device  76 ′″ can be open-loop or closed-loop controlled by means of the parking brake module  10 ′″. 
     The second valve device  76 ′″ is embodied as an electrically activated 3/2 way solenoid valve. 
     For the rest, the embodiment illustrated in  FIG. 10  corresponds to the embodiments shown in the other figures, so that reference is made to the discussions hereinabove. 
     By means of the embodiment of the present invention shown in  FIG. 10 , it is possible to implement further parking brake designs for the parking brake module. Providing two ports  54 ,  100  for a trailer control valve on the parking brake module  10 ′″ enables the trailer to be braked when the vehicle is parked by using the port  54  for a trailer control valve  22 . This is done by venting the line from the parking brake module  10 ′″ to the trailer control valve  22 . Since the trailer control valve carries out an inverting function, a pressure is therefore applied to the service brake of the trailer, with the result that the service brake brakes the trailer. However, such a braking effect can be partially problematic. Specifically, if the pressure in the trailer escapes, the entire vehicle combination can begin to move if the brakes of the towing vehicle do not provide a sufficient braking effect. 
     In some countries, there is therefore provision that the trailer vehicle must not be braked by means of the service brake when it is parked. In this way, when parking the vehicle combination, the driver immediately notices whether the braking effect of the brakes of the towing vehicle is sufficient. For these jurisdictions, the port  100  of the parking brake module  10 ′″ connected to the third port  98  of the second valve device  76 ′″ is used to connect the trailer control valve  22 ′. In this context, specifically the pressure in the line  20 ′ leading to the trailer control valve is held since, when the second valve device  76 ″/is de-energized or the vehicle is parked, the second valve device  76 ′″ shuts off the line  20 ′ from the trailer control valve  22 ′ when the port  100  is used. As a result, owing to the inverting effect of the trailer control valve  22 ′, the service brake of the trailer is released. When the driver parks the vehicle, the driver therefore realizes immediately whether the braking effect of the brakes of the towing vehicle are sufficient to hold the entire vehicle combination. 
     The exemplary embodiment of the present invention shown in  FIG. 10  can therefore be used for different parking brake designs, namely such designs in which either the trailer vehicle is braked or not braked when the vehicle combination is parked. Overall, the structure of parking brake modules can therefore be standardized, which considerably simplifies the administration for the components to be selected for the vehicle configuration and also the administration for spare parts. A saving is therefore obtained in the production of such parking brake modules and the administration of the corresponding parts numbers. 
     All the features mentioned in the above description and in the claims can also be combined individually with the parking brake module according to the present invention. The present invention is therefore not restricted to the described or claimed feature combinations. Instead, all combinations of individual features are to be considered as being disclosed. 
     It will thus be seen that the objects set forth above, among those made apparent from the preceding description, are efficiently attained, and since certain changes may be made without departing from the spirit and scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense. 
     It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described and all statements of the scope of the invention that, as a matter of language, might be said to fall therebetween.