Abstract:
An electropneumatic brake control device for controlling an air-quantity-boosting valve device which controls a parking brake of a vehicle. A valve unit is provided that has a vent valve for venting a control input of the air-quantity-boosting valve device. The vent valve has three states. In a first state, the control input of the air-quantity-boosting valve device can be vented in a throttled manner by using an aperture. In a second state, the control input of the air-quantity-boosting valve device cannot be vented. In a third state, the control input of the air-quantity-boosting valve device can finally be vented in an unthrottled manner.

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to a valve unit for an electropneumatic brake control device for controlling an air-flow-boosting valve device which controls a vehicle parking brake. 
     BACKGROUND OF THE INVENTION 
     Valve devices for electropneumatic brake control devices for control of vehicle parking brakes are known, for example, from DE 103 36 611 A1 and EP 1571061 A1. Such known brake control devices are used in brake systems that are provided not only with a service brake, which can be actuated by means of a brake pedal, but also with a parking brake (often referred to as a handbrake), which can be actuated by means of an electrical signal transducer. 
     In these known brake systems, the parking brake is regularly applied by means of spring-actuated brake cylinders. In order to release the parking brake, the spring-actuator part of the spring-actuated brake cylinders must be pressurized with compressed air. For this purpose, air must be admitted to the spring-actuator part. The necessary pressure for air admission is supplied from a compressed-air reservoir. However, this pressure supply is not permanently turned on, but can be shut off. Furthermore, the pressure in the spring-actuated brake cylinder can also be lowered, and, so, the spring-actuator part can be vented. The parking brake is applied by such venting. 
     To control the pressure in the spring-actuated brake cylinder, a relay valve is conventionally provided. With this relay valve, the pressure supply from the compressed-air reservoir tank to the spring-actuator part of the spring-actuated brake cylinders can be controlled. Control is exercised with the assistance of electropneumatic valve devices, especially, by means of electrically actuatable solenoid valves, which regulate a control pressure supplied to the relay valve. 
     In the brake system described in DE 103 36 611 A1, a bistable valve is used for this purpose. This valve can occupy two stable conditions. In the event of a power failure, it maintains the condition set at that instant. Furthermore, a holding valve is connected between the bistable valve and the control input of the relay valve in this brake system. By means of the bistable valve and of the holding valve, the pressure at the control input can be held, raised or lowered. The pressure at the output of the relay valve varies correspondingly. In this way, the parking brake can be released or applied by means of electrical signals to the bistable valve and holding valve. 
     Because of its construction, however, the bistable valve is complex and, therefore, expensive. Furthermore, as noted above, the bistable valve remains in its previous condition in the event of failure of the electrical power supply. Thus, a vehicle having such a brake system might not be able to be safely parked in the event of failure of the electrical power supply. In other words, it might not be able to be parked by automatic and permanent venting of the spring-actuator part of the spring-actuated brake cylinders—that is, by application of the parking brake. 
     Electromagnetically actuatable multi-way valves of less complex construction are proposed in DE 35 01 708 A1. In particular, it is proposed that two valves disposed opposite one another be provided in a multi-way valve. An armature that cooperates with a coil disposed between the two valves is associated with each of these two valves. The springs associated with these armatures are designed such that different magnetic forces are necessary for actuation of the two valves. These magnetic forces are generated by application of a current flowing through the coil. Because a separate armature is associated with each valve, mutually independent actuation of the two valves is possible by appropriate energization of the coil. 
     However, this known valve has the disadvantage that, in de-energized condition, its input is in communication with the output leading to the consuming load, whereas the outlet of the valve is shut off. If such a valve were to be used instead of the aforesaid bistable valve and holding valve, the full reservoir pressure would be injected into the control input of the relay valve in the event of failure of the electrical power supply, causing the parking brake to be released by the resulting admission of air to the spring-actuator part of the spring-actuated brake cylinders. This is undesirable, however, since the vehicle might no longer be safely parked in the event of failure of the electrical power supply. 
     SUMMARY OF THE INVENTION 
     Generally speaking, it is an object of the present invention to provide a valve unit for an electropneumatic brake control device for control of an air-flow-boosting valve device for actuating a parking brake of a vehicle, which valve unit permits the vehicle to be parked safely even in the event of failure of the electrical power supply. 
     In accordance with embodiments of the present invention, by using a vent valve with three operating state or conditions to vent a control input of an air-flow-boosting valve device, it is possible to achieve multi-stage venting. In a first condition of the vent valve, the control input of the air-flow-boosting valve device is throttled via an orifice and, in this way, can be vented slowly. This throttled venting corresponds to a first venting stage. In a further operating condition of the vent valve, the control input of the air-flow-boosting valve device is not throttled and, thus, can be vented suddenly. This unthrottled venting corresponds to a second venting stage. In yet a further operating condition of the vent valve, it is provided that the control input of the air-flow-boosting valve device is not vented. 
     The throttled venting of the control input is advantageous when the parking brake of the vehicle is to be applied slowly. This is desirable, in particular, in the event of failure of the electrical power supply, since the vehicle can then be braked slowly and parked safely. Sudden complete venting of the control input of the air-flow-boosting valve device and, thus, of the spring-actuator part of the spring-actuated brake cylinders, is practical when the driver has already brought the vehicle to a stop, for example, by means of the service brake, and intends to apply the parking brake. This can take place suddenly without the expectation that it will cause a traffic hazard. 
     Preferably, the vent valve is designed as a double-armature solenoid valve having two magnet armatures disposed in a common armature-guide housing and loaded with one spring each, namely, a primary armature and a secondary armature, which can be actuated respectively by a solenoid provided for both magnet armatures. By means of a vent valve provided as a double-armature solenoid valve, two armatures and, thus, two sub-valves of the double-armature solenoid valve can be actuated with only one coil. First, this reduces the complexity of the construction of the multi-stage vent valve. Second, the complexity of contacting and of electrical activation of the valve unit is reduced, since only two ports are necessary for the solenoid, even in the case of the multi-stage vent valve. Likewise, the number of output stages needed for energization of the solenoid valve is reduced, including the components belonging to such output stages. Furthermore, the entire current consumption is reduced by the use of only one coil for two sub-valves. As a result, favorable heating behavior of the brake control device is achieved. 
     Furthermore, the construction of the valve unit is compact compared with a valve unit that would be provided for multi-stage venting of a plurality of independent valves. By virtue of the more compact construction and of the smaller number of components, it is also possible to lower the manufacturing costs. 
     Preferably, the double-armature solenoid valve has an inlet that can be placed in communication with the control input of the air-flow-boosting valve device and is associated with the secondary armature. Advantageously, it further has a first outlet that can be placed in communication with a venting device and is associated with the primary armature as well as a second outlet that can be placed in communication with the venting device and is associated with the secondary armature. In this case, the first and second outlets can be placed in communication with the venting device in such a way that the first is throttled via the orifice while the second is unthrottled. When the solenoid is de-energized, the primary armature and the secondary armature are in their home positions established by spring loading, wherein the vent valve occupies its first condition and the control input of the air-flow-boosting device can be vented in throttled manner via an orifice. For this purpose, the inlet of the vent valve is in communication with its first outlet and the inlet is shut off from the second outlet. In this de-energized condition, permanent throttled venting of the control input of the air-flow-boosting valve device is achieved and thus, throttled venting of the spring-actuated brake cylinders. In this way, the parking brake is applied slowly. Thus, a moving vehicle is braked slowly and can be parked safely because of the permanent venting of the spring-actuated brake cylinders. 
     Upon injection of a first low current into the solenoid, only the primary armature is energized at first and, in this way, is displaced to its switching position. At this low current, however, the secondary armature remains in its spring-loaded home position. In these switching positions, the inlet of the vent valve is shut off from both the first and second outlets. The vent valve therefore occupies its second condition, in which the control input of the air-flow-boosting valve device cannot be vented but the pressure at the control input can be maintained at its existing value. 
     By injection of a higher current into the solenoid, the secondary armature is also displaced to its switching position, and, so, the inlet of the vent valve is in communication with its second outlet, which leads directly to venting. Preferably, this inlet of the vent valve is shut off from its outlet leading to the throttled first outlet. The vent valve then occupies its third condition, in which the control inlet of the air-flow-boosting valve device can be vented in unthrottled manner. In this condition, the control input of the air-flow-boosting valve device and, thus, also of the spring-actuator part of the spring-actuated brake cylinders can be vented suddenly. Consequently, the parking brake can be applied suddenly by establishing this condition. 
     In an alternative embodiment, the third condition for sudden venting can already be attained during injection of the lower, first solenoid current, whereas the second condition, in which the inlet of the vent valve is shut off from both of its outlets, is occupied only during injection of the second, higher solenoid current. 
     In a further advantageous embodiment, the spring force exerted on the primary armature by an associated spring is smaller than the spring force exerted on the secondary armature by a further spring associated with the secondary armature. Advantageously, springs of different strengths are used for this purpose. By this measure, the switching behavior of the solenoid-valve system can be improved. 
     In yet a further advantageous embodiment, the primary armature and the secondary armature have different diameters. In particular, the secondary armature has a smaller diameter than the primary armature. This results in the advantage that the structure of the armature-guide arrangement can be configured such that it can be mounted in the coil from one side, together with the two magnet armatures; also, this results in the advantage that, by virtue of the different sizes, especially diameters of the armatures, different magnetic forces act on the armatures. As a result, the switching behavior of the valve unit can be favorably influenced. More specifically, it is intended that the primary armature will be pulled in first by the solenoid at a first low current flowing through the solenoid and that the secondary armature will also be pulled in only when a higher current is flowing through the solenoid. 
     In another advantageous embodiment, the primary armature and the secondary armature are of identical design. This has the advantage that the manufacturing costs of these armatures can be reduced by virtue of larger production runs. 
     In a still further advantageous embodiment, the primary armature and the secondary armature are pulled to different depths into the solenoid. In particular, the primary armature is pulled more deeply into the solenoid than is the secondary armature. This has the advantage that the magnetic force exerted by the solenoid on the primary armature is greater than the magnetic force exerted by the solenoid on the secondary armature. Also, as a result, the switching behavior of the valve unit is favorably influenced. 
     Preferably, both the vent valve and an air-admission valve are provided in the valve unit. In this case, the inlet of the air-admission valve is in communication with a first connecting member that can be placed in communication with a compressed-air reservoir. Furthermore, the first outlet of the vent valve is in communication via the orifice and the second outlet of the vent valve with a second connecting member that can be placed in communication with the venting device. Also, the outlet of the air-admission valve is in communication with the inlet of the vent valve and with a third connecting member that can be placed in communication with the control input of the air-flow-boosting valve device. 
     In the present context, the term “connecting member” is to be understood to comprise any kind of connecting means, especially, for example, pneumatic connecting lines, connecting ducts, connecting bores or other passages as well as connecting ports, especially ports for pneumatic connecting lines, ducts and bores. 
     The valve unit can be designed either as a stand-alone device or as a non-independent, integral part of a brake control device. 
     In an exemplary embodiment of the valve unit as a stand-alone device, a very compact construction is achieved, in which both the vent valve and the air-admission value are mounted in a common unit, advantageously in a common housing. This unit functions with only three pneumatic and three electrical ports. This compact construction reduces the number of necessary components and, thus, also the manufacturing costs. 
     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 arrangement of parts that 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 
       For a fuller understanding of the invention, reference is had to the following description taken in connection with the accompanying drawings, in which: 
         FIG. 1  is a simplified schematic diagram of an air-brake system having an electropneumatic brake control device for control of a parking brake, including two valve units according to an exemplary embodiment of the present invention; 
         FIG. 2  shows a valve unit according to an exemplary embodiment of the present invention for the brake control device according to  FIG. 1 ; 
         FIG. 3  is a simplified schematic diagram of a brake control device for a parking brake, including a valve unit according to a further exemplary embodiment of the present invention; and 
         FIG. 4  is a simplified schematic diagram of a brake control device for a parking brake, including a valve unit according to a further exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawing figures,  FIG. 1  schematically shows part of an air-brake system  10  for a vehicle, especially an electropneumatic brake control device for control of a parking brake of the vehicle. Such air-brake systems are used, for example, in commercial vehicles, heavy motor trucks or buses. Such brake systems are used in particular in vehicle trains comprising a tractor and a trailer. 
       FIG. 1  shows only some components of brake system  10  that are helpful to understanding the present invention. Brake system  10  is electrically controlled, meaning that the metering of brake pressure to brake cylinders for actuation of wheel brakes provided on the vehicle wheels is controlled by electrical or electronic control elements. The brake cylinders are designed partly or completely as combination service and spring-actuated brake cylinders  12  (for simplicity, only one such brake cylinder is illustrated in  FIG. 1 ). The spring-actuator part is controlled by an electropneumatic brake control device designed as parking-brake module  14  for control of the parking brake. 
     Brake system  10  is provided with a brake-force transducer  16 , which senses braking intent of the vehicle operator. Brake-force transducer  16  comprises an electrical part and a pneumatic or hydraulic part (only the pneumatic part is illustrated in  FIG. 1 ). Via compressed-air lines (not illustrated), the pneumatic part is supplied with compressed air by a first compressed-air reservoir tank  18  and a second compressed-air reservoir tank  20 . Compressed-air reservoir tanks  18 ,  20  are used to supply compressed air to the brake cylinders of the service brakes. As illustrated in  FIG. 1 , however, they can also be used to supply compressed air to the parking brake. Alternatively, the compressed air for the parking brake can be supplied by a separate compressed-air reservoir tank. 
     By actuation of a brake pedal  22 , brake-force transducer  16  generates a pneumatic manipulated variable either by electrical activation of electropneumatic devices or directly, the variable being passed via a compressed-air line  24 ,  26  to combination service and spring-actuated brake cylinder  12 . Combination service and spring-actuated brake cylinder  12  is designed as a combination spring-actuator/diaphragm cylinder. Besides the function of a diaphragm cylinder, it additionally has a spring-actuator function. Brake cylinder  12  is therefore provided with a diaphragm part, which is in communication pneumatically with the service-brake system and can be pressurized with the actual brake pressure, and with a spring-actuator part  30 , which is pneumatically separated from diaphragm part  28  and can be pressurized with compressed air via separate compressed-air lines  32 ,  34 . Spring-actuator part  30  forms part of the parking brake. Spring-actuator part  30  includes the spring-actuator function, which preloads an actuator spring upon pressurization of spring-actuator part  30  and, thus, prevents or diminishes braking action of the spring-actuator function, whereas the actuator spring relaxes upon venting of spring-actuator part  30  and, thus, in connection with the spring-actuator function, exerts a braking action on the brake in communication with the respective brake cylinder. In the present context, brake cylinders of this type will be referred to as “spring-actuated brake cylinders”. 
     To prevent mechanical overloading of the brake mechanism, an overload-protection valve  35  is provided, which is designed as a changeover valve or select-high valve and is connected between spring-actuator part  30 , a pneumatic output  102  of the parking-brake module and the output of brake-force transducer  16  having the modulated pressure. Overload-protection valve  35  selects the higher of two pressures present at its two inputs  35   a ,  35   b , namely, the higher of the modulated brake pressure at the output of brake-force transducer  16  and the pressure made available by air-flow-boosting valve device  64 . It supplies this selection via its output  35   c  to spring-actuator part  30  of spring-actuated brake cylinder  12 . Overload-protection valve  35  prevents addition of the brake force supplied by the service brake, or, in other words, via the pneumatic part of diaphragm part  28 , to the brake force supplied by the parking brake, or, in other words, spring-actuator part  30 , so that, in this way, it prevents mechanical overloading of the brake mechanism of the wheel brake associated with this brake cylinder. By virtue of the illustrated structure, the brake force supplied to the brake cylinder via diaphragm part  28  is not increased by the brake force exerted by spring-actuator part  30 , since, in the case of actuation of the service brake, the brake force exerted by the actuator spring is reduced by a force corresponding to actuation of the service brake. In this way, critical overloading of the corresponding wheel brake can be avoided. 
     By means of the spring-actuated brake cylinder, a parking-brake function is achieved that also permits the vehicle to be braked or immobilized even in the absence of compressed air. The parking-brake function is active when the respective spring-actuator part  30  of spring-actuated brake cylinder  12  is vented below a minimum pressure value. Via compressed-air lines  32 ,  34 , spring-actuator part  30  of brake cylinder  12  is pneumatically in communication with parking-brake module  14 . This parking-brake module  14  permits pressure control by way of electronic control means. 
     A manually actuatable parking-brake signal transducer  36  is electrically connected via a multi-conductor electrical line  38  to an electrical control unit  40  of parking-brake module  14 . Via appropriate electrical lines, the electrical devices in the vehicle are supplied with energy by an electrical power supply device (not illustrated), such as, for example, a vehicle battery. 
     The vehicle is suitable for coupling a trailer having a further parking brake equipped with spring-actuated brake cylinders. Brake system  10  is therefore provided with what is known as a tractor-truck protection valve  42 , which is used for brake-pressure control, especially of the parking brake of the trailer. Via compressed-air lines  44 ,  46 , tractor-truck protection valve  42  is supplied with the reservoir pressure of compressed-air reservoir tanks  18 ,  20 . Furthermore, a pressure modulated by means of an air-flow-boosting valve device, namely, a relay valve  48 , and intended for the parking brake of the trailer is supplied to tractor-truck protection valve  42 . 
     Relay valve  48  is provided with a control input  50 , a vent port  52  that can be placed indirectly or directly in communication with the atmosphere and an inlet  56  that, via a compressed-air line  54 , can be placed in communication with the reservoir pressure of compressed-air reservoir tanks  18 ,  20  as well as an outlet  60  that, via a compressed-air line  58 , can be placed in communication with tractor-truck protection valve  42 . Via a compressed-air line  62 , control input  50  is in communication with parking-brake module  14 . 
     At its outlet  60 , relay valve  48  delivers to compressed-air line  58  an output pressure that corresponds to the pressure injected via compressed-air line  62  at control input  50  and, thus, to the pressure in a control chamber of relay valve  48 . Relay valve  48  draws the compressed air needed for this purpose from compressed-air supply line  54 , which is in communication with inlet  56  of relay valve  48  and, via further compressed-air lines, is in communication with compressed-air reservoir tanks  18 ,  20 . 
     Parking-brake module  14  is provided with an air-flow-boosting valve device in the form of a relay valve  64  for the tractor. Relay valve  64  comprises an inlet  76  in direct or indirect communication via compressed-air lines  66  to  75  with compressed-air reservoir tanks  18 ,  20 . Furthermore, relay valve  64  is provided with an outlet in communication via compressed-air lines  78 ,  34 ,  32  with spring-actuator part  30  of brake cylinder  12 , and has a control input  82 , which is in communication via a compressed-air line  84  with a valve unit  86  for control of the parking brake of the tractor. 
     At its outlet  80 , relay valve  64  delivers to a compressed-air line  78  an output pressure that corresponds to the pressure injected via compressed-air line  84  at control input  82  and, thus, to the pressure in a control chamber of relay valve  64 . Relay valve  64  draws the compressed air needed for this purpose from compressed-air supply line  66 , which is in communication with inlet  76  of relay valve  64 . Any venting of compressed-air line  78  that may be necessary takes place via a vent port  88  in indirect or direct communication with atmosphere. In the exemplary embodiment shown in  FIG. 1 , this vent port  88  is in communication via a compressed-air line  90  with a venting device  92 . 
     Parking-brake module  14  is further provided upstream from compressed-air reservoir tanks  18 ,  20  with check valves  94 ,  96 , respectively, which, in the event of a pressure drop or of detachment of or damage to compressed-air lines  71  and  75 , respectively, to compressed-air reservoir tanks  20  and  18 , respectively, prevent a pressure drop or pressure loss from occurring in parking-brake module  14 . Such a pressure drop or pressure loss is undesirable, since it could lead, in particular, to sudden application of the parking brake and, thus, to emergency braking of the tractor. Under certain circumstances, this might cause an uncontrollable driving situation. 
     Parking-brake module  14  is provided with a plurality of pneumatic ports  98 ,  100 ,  102 ,  104 ,  106 . Via port  98 , compressed-air line  74  is in communication with compressed-air line  75  for connection of first compressed-air reservoir tank  18 . Via port  100 , compressed-air line  70  is in communication with compressed-air line  71  for connection of second compressed-air reservoir tank  20 . Via port  102 , compressed-air line  78  is in communication with compressed-air line  34  for placing relay valve  64  in communication with brake cylinder  12 . Via port  104 , compressed-air line  44  to relay valve  48  is in communication via the trailer controller with a compressed-air line  108  and, thus, via compressed-air lines  67  to  75  with compressed-air reservoir tanks  18 ,  20 . Via port  106 , compressed-air line  62  to control input  50  of relay valve  48  for the trailer controller is in communication with a valve unit  110  disposed in parking-brake module  14  for control of the trailer parking brake. 
     Parking-brake module  14  is further provided with a pressure sensor  114 , which is mounted within cover  112  and is used for monitoring the reservoir pressure inside parking-brake module  14 . For this purpose, pressure sensor  114  is in communication via a pressure line  116  with pressure line  72  and, thus, is directly or indirectly in communication with pressure lines  66  to  71 ,  73  to  75  as well as lines  108 ,  44  and  46 . 
     Electrical control unit  40 , by means of which valve unit  86  as well as valve unit  110  can be switched electrically via electrical lines  118 ,  120 , is also disposed in the area of cover  112 . 
     The two valve units  86  and  110  disposed in parking-brake module  14 , just as relay valve  48  for the trailer and relay valve  64  for the tractor, are of identical design and are connected in the same way, as shown in  FIG. 1 . Hereinafter, the discussion will focus on valve unit  86 , although the discussion will apply equally to valve unit  110 . 
     According to an exemplary embodiment of the present invention, valve units  86  and  110  have the form of stand-alone subassemblies. Alternatively, however, valve units  86  and  110  can be implemented integrally in a single uniform parking-brake module  14 , either individually or together with relay valve  64 , and possibly also with relay valve  48 . When valve units  86  and  110  are stand-alone components, the valve units have pneumatic ports, as will be discussed in greater detail hereinafter. However, if the valve units are integrated into the parking-brake module, such ports are omitted in favor of appropriate connecting lines. The present application therefore uses the generic term “connecting member” to mean any kind of connecting means, or, in other words, both ports and other types of connections, such as, for example, connecting lines, connecting ducts or bores and other like structures. Hereinafter, the term “port” as used in connection with valve units  86  and  110  is to be understood as a connecting member, so that, in this way, the exemplary embodiment in which valve units  86  and  110  are integrated into parking-brake module  14  can also be explained. 
     Valve unit  86  is provided with a first port  122 , which, via compressed-air lines  126 , and  68  to  75 , is in communication with compressed-air reservoir tanks  18 ,  20 . Valve unit  86  is further provided with a second port  128 , which, via compressed-air line  130 , is in communication with venting device  92 . Venting device  86  is further provided with a third port  132 , which, via compressed-air line  84 , is in communication with control input  82  of relay valve  64 . Valve unit  86  is provided with an air-admission valve  133  as well as a vent valve  134 . Air-admission valve  133  is designed as a 2/2-way solenoid valve. Vent valve  134  is designed as a double-armature solenoid valve with three switched conditions. 
     The structural design of valve unit  86  is shown in  FIG. 2 . Vent valve  134  is provided with two magnet armatures  138 ,  140  disposed in a common armature-guide arrangement  136 . Armature-guide arrangement  136  is designed as an armature-guide tube, wherein the inside tube diameter is constant at least over some portions and is matched to the outside diameter of magnet armatures  138 ,  140 . A first magnet armature, namely, primary armature  138 , is loaded by means of a spring  142  and is therefore compressed toward the right in the diagram according to  FIG. 2 . Analogously, a second magnet armature, namely, secondary armature  140 , is loaded with a spring  144 , which compresses magnet armature  140  toward the left in the diagram shown in  FIG. 2 . Armature-guide arrangement  136  is surrounded by a solenoid  146 . The outside diameter of armature-guide arrangement  136  is matched to the inside diameter of solenoid  146 . Upon injection of suitable solenoid currents into solenoid  146 , solenoid  146  pulls primary armature  138  and, possibly, secondary armature  140  in the direction of the interior of the coil. Primary armature  138  is provided as an operating element for a first sub-valve  148 , and secondary armature  140  is provided as an operating element for a second sub-valve  150 . Valve unit  86  is provided with three electrical ports  152 , which are connected by means of electrical lines  118  to electrical control unit  40 . Two of the three electrical ports  152  are connected to solenoid  146  of vent valve  134 . 
     When solenoid  146  is de-energized, both primary armature  138  and secondary armature  140  are located in their home positions, determined by springs  142 ,  144 , as illustrated in  FIGS. 1 and 2 . In its home position, sub-valve  148  associated with primary armature  138  places a first outlet  154  of vent valve  134  in communication with a conduit  156  extending along armature-guide arrangement  136 . In a switched position of sub-valve  148 , magnet armature  138  shuts off first outlet  154  from conduit  156 . In its home position, second sub-valve  150  shuts off an inlet  158  from a second outlet  160 . In this home position, conduit  156  is further in communication with inlet  158 , whereas conduit  156  is shut off from second outlet  160 . In its switched position, second sub-valve  150  places inlet  158  of vent valve  134  in communication with the second outlet of vent valve  160 . Via an orifice  162  and ducts  164 ,  166 , first outlet  154  of vent valve  134  is pneumatically in communication with second port  128 . Orifice  162  acts as a throttle and reduces the cross section of the pneumatic line at first outlet  154  of vent valve  134 . 
     When solenoid  146  is de-energized, both primary armature  138  and secondary armature  140  are in their illustrated home positions. If solenoid  146  is operated with a first, low current, which is supplied via ports  152  of first solenoid  146 , primary armature  138  shifts into its switched position, or, in other words, is pulled inward in the direction of the interior of solenoid  146 . If the current flowing through solenoid  146  is further increased, secondary armature  140  also shifts into its switched position, or, in other words, is pulled inward in the direction of the interior of the solenoid. 
     Elastomeric inserts  168 ,  170  are disposed at both ends of primary armature  138 , or at least at the end of primary armature  138  facing conduit  156 . Elastomeric inserts  168 ,  170  can also be designed in one piece, by providing primary armature  138  with a through bore, through which there extends an elastomeric of corresponding one-piece design. Elastomeric insert  168 , which faces conduit  156 , forms a valve seat together with a corresponding shaped projection  172  on armature-guide arrangement  136 . 
     Secondary armature  140  is provided with an elastomeric insert  174 ,  176  at each of its two ends. These elastomeric inserts may also be designed as separate pieces or, as illustrated in  FIG. 2 , as one piece. In the one-piece construction, the elastomeric insert passes through a conduit extending through secondary armature  140 . Elastomeric inserts  174 ,  176  protruding at both ends of secondary armature  140  form valve seats together with corresponding shaped projections  178 ,  180  on a valve head  182  or on armature-guide arrangement  136 . By means of elastomeric inserts  174 ,  176  and the shaped projections  178 ,  180 , it is possible to shut off second outlet  160  of vent valve  134  or conduit  156 . By virtue of a recess  184  in the region of shaped projection  180  on armature-guide arrangement  136 , the valve seat between the elastomeric insert and shaped projection  178  always remains open. However, recess  184  can also be omitted. In such a case, elastomeric insert  174  would completely close shaped projection  178  in the switched position of secondary armature  140 . Primary armature  138  and secondary armature  140  each have substantially rotationally symmetric design. However, they each have a slot-like recess  186 ,  188  extending along the respective armature. Recess  186  of the primary armature establishes communication between first outlet  154  of vent valve  134  and conduit  156  when primary armature  138  is in its home position. Recess  188  of secondary armature  140  establishes communication between recess  184  or conduit  156  and inlet  158  or second outlet  160  of the vent valve when secondary armature  140  is in its home position. 
     Air-admission valve  133  is formed within valve unit  86 . It is provided with a magnet armature  192  disposed in a further armature-guide arrangement  190 . Armature-guide arrangement  190  is provided with a tubular portion having an inside diameter matched to the outside diameter of magnet armature  192 . Magnet armature  192  is loaded by means of a spring  194  and is compressed toward the left in the diagram depicted in  FIG. 2 . Armature-guide arrangement  190  is surrounded by a solenoid  196 . The outside diameter of armature-guide arrangement  190  is matched to the inside diameter of solenoid  196 . Upon injection of a solenoid current of predefined magnitude into solenoid  196 , solenoid  196  pulls magnet armature  192  in the direction of the interior of the coil. Magnet armature  192  is provided as an operating element for air-admission valve  133 . Solenoid  196  is provided with two electrical ports  152 , one of the two ports coinciding with one of the two ports of solenoid  146 . Thus, three ports  152  in total are sufficient for valve unit  86  in order to electrically connect solenoid  146  of vent valve  134  and also solenoid  196  of air-admission valve  133 . All three ports  152  are connected by means of electrical lines  118  to electrical control unit  40 . 
     When solenoid  196  is de-energized, magnet armature  192  is located in its home position illustrated by springs  194  in  FIGS. 1 and 2 . In its home position, air-admission valve  133  shuts off its inlet  198  and, therefore, first outlet  122  of valve unit  86  in pneumatic communication with this inlet  198  from outlet  200  of air-admission valve  133 . On the one hand, outlet  200  is in communication with third port  132  of valve unit  86 , which leads to control input  82  of relay valve  64 . On the other hand, outlet  200  is also in communication with inlet  158  of vent valve  134 . 
     Upon injection of a current of predefined magnitude into solenoid  196 , magnet armature  192  is displaced into its switched position. In the process, inlet  198  and, thus, first port  122  of valve unit  86  is placed in communication with outlet  200  of air-admission valve  133 . In this way, compressed air can be supplied via compressed-air lines  68  to  75 ,  84 ,  126  from compressed-air reservoir tanks  18 ,  20  to control input  82  of relay valve  64 . If magnet armature  192  remains in its home position, the pressure at control input  82  of relay valve  64  can be maintained even if inlet  158  of vent valve  134  is shut off from its outlets  154 ,  160 . If, in contrast, the pressure at control input  82  of relay valve is to be lowered, air-admission valve  133  is de-energized, and, so, magnet armature  192  remains in its home position; in this way inlet  198  is shut off from outlet  200  of air-admission valve  133 . To vent control inlet  82  of relay valve  64 , vent valve  134  places its inlet  158  in communication with one of its two outlets  154  or  160 , depending on whether slow venting or sudden venting of control input  82  of relay valve  64  is to take place. 
     At both ends of magnet armature  192 , or at least at the end associated with inlet  198 , there is disposed an elastomeric insert  202 ,  204 . Elastomeric inserts  202 ,  204  can also be designed in one piece, by providing magnet armature  192  with a through bore, through which the elastomeric insert extends. Elastomeric insert  202  associated with inlet  198  of air-admission valve  133  forms a valve seat together with a shaped projection  206  on valve head  182 . 
       FIG. 3  shows a further exemplary embodiment of a parking-brake module  14 ′, which corresponds largely to parking-brake module  14  shown in  FIG. 1 . However, the parking-brake module  14 ′ shown in  FIG. 3  reveals only valve unit  86  for the tractor, whereas valve  110  for the trailer is not illustrated. In  FIG. 3 , therefore, like reference numerals denote like parts as in  FIG. 1  and  FIG. 2 . Accordingly, the foregoing discussion need not be repeated. 
     In certain driving situations, such as, for example, during a failure of the electrical power supply, spring-actuator part  30  is to be vented slowly via orifice  162 . However, since the control volume of relay valve  64  or  64 ′ is very small, venting of control input  82  of relay valve  64 ,  64 ′ is permitted only through a very small orifice  162 . Therefore, orifice  162  would inherently have to be designed with a very small diameter. However, a very small diameter can become clogged by dirt or ice. In turn, throttled venting could be made inoperative by dirt or ice, and, thus, safe venting of spring-actuator part  30  of the spring-actuated brake cylinders could no longer be assured. Nevertheless, in order to ensure slow venting with a sufficiently large cross section of orifice  162 , the control volume of the relay valve is virtually enlarged by establishing communication between control input  82  and outlet  80  of relay valve  64 ′. Such communication has the form, for example, of a through bore in the relay piston, thus forming an orifice  208 . Orifice  208  increases the amount of air to be vented in the control area of relay valve  64 ′. In this way, the working volume at control input  82  of relay valve  64 ′ can be vented sufficiently slowly despite a sufficiently large opening of orifice  162 —to reduce the danger of fouling by dirt or ice. 
     Orifice  208  of the relay piston of relay valve  64 ′ is advantageously provided with a larger cross-sectional area than the cross-sectional area of orifice  162  of valve unit  86 . In this way, the pressure at control input  82  of relay valve  64 ′ corresponds substantially to the pressure at outlet  80  of relay valve  64 ′. Thus, venting of spring-actuator part  30  no longer takes place via vent port  88  of relay valve  64 ′ or does so to only an immaterial extent. Instead, substantially the entire volume of spring-actuator part  30  as well as the control volume at control input  82  of relay valve  64 ′ is vented via orifice  162  and in sufficiently slow manner by virtue of the small opening of orifice  162 . 
       FIG. 4  shows another exemplary embodiment of a parking-brake module  14 ″. However, the design of vent valve  134 ′ in the parking-brake module  14 ″ shown in  FIG. 4  differs from that of the previous exemplary embodiments. Otherwise, however, parking-brake module  14 ″ and, thus, also, valve  86 ′ corresponds to the parking-brake module  14 ′ and to the valve unit  86 , respectively, as shown in  FIG. 3 . Like reference numerals therefore denote like parts, and the foregoing discussion need not be repeated. 
     A special feature of valve unit  86 ′ includes the different design of vent valve  134 ′, in which second condition II and third condition III are interchanged. However, first condition I remains unchanged. This interchange of conditions means that, in the case of injection of a first low solenoid current into the solenoid of vent valve  134 ′, sudden venting of control input  82  of relay valve  64 ,  64 ′ takes place and, thus, the parking brake is applied suddenly. Only when vent valve  134 ′ is energized with a higher current does inlet  158  of vent valve  134 ′ become shut off from its outlets  154 ,  160 . 
     In the event of a power failure, this exemplary embodiment might seem to establish first condition I of vent valve  134 ′ only after third condition III has been briefly turned on. However, since vent valve  134 ′ is designed as a double-armature solenoid valve, both the primary armature and the secondary armature of vent valve  134 ′ are switched to their respective home positions at substantially the same time in the event that the coil of vent valve  134 ′ is switched to de-energized condition. To this extent, the duration for which third condition III is active, as shown in the exemplary embodiment in  FIG. 4 , is negligible, and it has no significant effect on the operation of venting of control input  82  of relay valve  64 ′. 
     A valve unit of simple and inexpensive design that simultaneously ensures safe parking of the vehicle even in the event of failure of the electrical power supply is provided by virtue of the inventive double-armature solenoid valve for the vent valve with an orifice for slow venting and an outlet for fast venting. 
     Because of the air-admission valve connected between the control input of the relay valve and the compressed-air reservoir, and also because of the vent valve connected between the control input of the relay valve and a venting device, almost any desired pressure value up to the level of the pressure in the reservoir tanks can be applied at the control input of the relay valve by means of appropriate opening times of the two valves. Since the spring-actuated brake cylinders do not have to be pressurized with the full reservoir pressure in order to release the parking brake, it is therefore also possible to specify a lower pressure at the control input of the relay valve, which pressure is then supplied correspondingly to the spring-actuator part of the spring-actuated brake cylinders. In this way, it is possible, by means of the two valves, namely, the air-admission valve and the vent valve, to also achieve a pressure-limiting function at the control input of the relay valve and, thus, also in the spring-actuated brake cylinders. Such pressure limitation is advantageous not only for energy consumption or for the consumption of compressed air but also for the noise generated during venting. 
     On the whole, the embodiments according to the present invention permits simple implementation of a parking brake that ensures a safe condition even in the event of failure of the electrical power supply and, moreover, that can be actuated by purely electrical means. In particular, the pneumatic tubing that has often been common heretofore in the operator&#39;s cab for the purpose of activating the parking brake can be omitted, and operator-control of the parking brake can be achieved completely via an electrical actuating means. 
     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.