Abstract:
A valve unit for an electro-pneumatic brake control is connected to an input of an air-quantity-boosting valve for the aeration/venting thereof. A double-armature solenoid valve includes primary and secondary armatures each spring-loaded and actuated by a common magnet coil. The primary armature is a switch for a vent valve; the secondary armature is a switch for an intake valve. When the coil is not drawing current, the armatures are in spring-loaded position, the intake valve blocking intake and the vent valve venting. When a first current flows through the coil, the primary armature enters switching position, with the secondary armature in spring-loaded position; the intake valve blocking intake and the vent valve blocking venting. When a second current greater than the first flows through the coil, both primary and secondary armatures are moved into switching positions, so that the intake valve admits air and the vent valve blocks venting.

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to a valve unit for an electro-pneumatic brake control device for controlling a parking brake. The present invention also relates to the electro-pneumatic brake control device and an electrically controlled pneumatic vehicle brake system equipped therewith. 
     BACKGROUND OF THE INVENTION 
     Valve devices for electro-pneumatic brake control devices for controlling vehicle parking brakes are known, for example, from DE 103 36 611 A1 or EP 1 571 061 A1. These known brake control devices are used in brake systems provided not only with a service brake, which can be actuated by means of a brake pedal, but also with a parking brake (often also 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 brake cylinders designed as spring-actuated brake cylinders. In order to release the parking brake, the spring-actuator part of the spring-actuated brake cylinders is pressurized with compressed air. For this purpose, air is 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 may even be shut off. Furthermore, the pressure in the spring-actuated brake cylinder may also be lowered, and so the spring-actuator part may be vented. 
     To control the pressure in the spring-actuated brake cylinder, conventionally, a relay valve is provided by means of which 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 electro-pneumatic valve devices, especially, by means of electrically actuatable solenoid valves, that regulate a control pressure supplied to the relay valve. 
     In the known brake system described in DE 103 36 611 A1, a bistable valve, or, in other words, a valve that can occupy two stable conditions and that, in the event of a power failure, maintains the condition set at that instant, is used for this purpose. 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 can be 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, in the event of failure of the electrical power supply, cannot be parked such that the spring-actuator part of the spring-actuated brake cylinder is vented and, as a result, the parking brake is applied. 
     Electromagnetically actuatable multi-way valves of less complex construction have been proposed in DE 35 01 708 A1. In particular, it was proposed that two valves disposed opposite one another be provided in a multi-way valve. Armatures that cooperate with a coil disposed between the two valves are 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, in the de-energized state of this known valve, the input of the valve unit is in communication with the output leading to the consuming load. If such a valve unit were to be used instead of the bistable valve and holding valve discussed above, the full reservoir pressure might be applied to the control input of the relay valve in the event of failure of the electrical power supply, thus, potentially causing the parking brake to be released by the resulting admission of air to the spring-actuator part of the spring-actuated brake cylinders. These known valves might not, therefore, be best suited for controlling the pressure in a spring-actuated brake cylinder of a parking brake. 
     SUMMARY OF THE INVENTION 
     Generally speaking, in accordance with embodiments of the present invention, a simple, suitable valve unit for an electro-pneumatic brake control device for controlling a parking brake is provided that permits the vehicle to be parked safely even in the event of failure of the electrical voltage supply. 
     The inventive valve unit is constructed and arranged such that, in the de-energized condition, no air is admitted to an air-flow-boosting valve device, but, instead, the valve device is vented. Thus, the control input of the air-flow-boosting valve device is permanently vented even in the event of failure of the electrical power supply. A parking brake system having spring-actuated brake cylinders can be connected to the output of the air-flow-boosting valve device. Such spring-actuated brake cylinders are constructed and arranged such that they apply the parking brake in the vented condition, and only when air is being admitted to the spring-actuated brake cylinders do they release the parking brake. In this way, venting of the spring-actuated brake cylinders of the parking brake is ensured and, thus, the parking brake is applied even in the event of failure of the electrical power supply. 
     In an embodiment of the inventive valve unit provided as a double-armature solenoid valve, two valves, namely, an air-admission valve and a vent valve, can be actuated with only one coil. This reduces the complexity of the construction of the valve unit. Also, the complexity of contacting and of electrical activation of the valve unit is reduced, since only two ports are necessary for the solenoid. Likewise, the number of output stages needed for energization of this solenoid valve is reduced, including the components belonging to the output stages. Furthermore, the entire current consumption is reduced by the use of only one coil for two valves. As a result, more favorable heating behavior of the brake control device is achieved. 
     Furthermore, the construction of the inventive valve unit is compact compared with a conventional valve unit comprising a bistable valve and a holding valve. By virtue of the more compact construction and of the smaller number of components, it is also possible to lower the manufacturing costs significantly. 
     The valve unit according to embodiments of the present invention can be constructed and arranged either as a stand-alone device or as a non-independent, integral part of a brake control device. 
     When the solenoid is de-energized, both the primary armature and the secondary armature of the double-armature solenoid valve are located in a home position established by corresponding springs, in which position the vent valve of the double-armature solenoid valve activates venting of the control input of the air-flow-boosting device, especially, by placing a connecting member, referred to as the third connecting member, leading to the control input of the air-flow-boosting valve device in communication with a connecting member, referred to as the second connecting member, leading to a venting device. In this way, the control input can be vented. At the same time, the air-admission valve is located in its home position, specifically, such that admission of air to the control input of the air-flow-boosting valve device is shut off, especially, because a first connecting member of the valve unit leading to the compressed-air reservoir is shut off from the third connecting member leading to the control input of the air-flow-boosting valve device. No further compressed air is supplied to the control input. In the de-energized condition, therefore, permanent, throttled, venting of the control input of the air-flow-boosting valve device is achieved and, thus, via the air-flow-boosting valve device, the spring-actuated brake cylinders are vented. In this way, it is ensured that the parking brake is applied. 
     Upon injection of a first low current into the solenoid, only the primary armature and, thus, the vent valve of the valve unit is energized at first, and, in this way, is displaced to its switched position. At this low current, however, the secondary armature remains in its spring-loaded home position. In the switched position of the primary armature, venting of the control input of the air-flow-boosting valve device is shut off by means of the vent valve, especially, since the second connecting member leading to the venting device is shut off from the third connecting member leading to the control input. In this way, the pressure at the control input can be held at its existing value. By injection of a higher current into the solenoid, the secondary armature is then also displaced to its switched position, and, so, the air-admission valve activates admission of air to the control input of the air-flow-boosting valve device. In particular, this air-admission valve places the first connecting member leading to the compressed-air reservoir in communication with the third connecting member leading to the control input. Because of such communication, air is admitted to the air-flow-boosting valve device and, consequently, to the spring-actuator part of the spring-actuated brake cylinder. In this condition, the parking brake is released, and, so, the vehicle is now in a drivable condition. 
     When the solenoid is de-energized, the vent valve preferably places the second connecting member leading to the venting device in communication with the third connecting member leading to the air-flow-boosting valve device via an orifice or throttle. In this way, the control input of the air-flow-boosting valve device can be vented in throttled manner. This has the advantage that the vehicle can be braked slowly and parked safely even in the event of failure of the electrical power supply, especially, of the brake control device. This is achieved by an orifice acting as a throttle on the vent valve of the valve unit, which is active in the de-energized condition of the solenoid. That is, the pressure at the control input of an air-flow-boosting valve device is slowly lowered via this orifice, and, so, the pressure in the spring-actuator part of the spring-actuated brake cylinder is also lowered slowly and, thus, the parking brake is applied slowly. 
     Preferably, the solenoid is supplied with an alternating solenoid current, especially, a pulsed solenoid current, this alternating current being of such magnitude that the primary armature is actuated in pulsed manner, whereas the secondary armature remains in its home position, without being actuated. In this way, rapid venting of the control chamber of the air-flow-boosting valve device can be achieved. In particular, by rapidly switching the vent valve to and fro between its home position and its switched position, compressed air from the control chamber of the air-flow-boosting valve device passes first to a first outlet of the vent valve, which is in communication with the throttle of the vent valve. Therefore, the compressed air actually cannot escape rapidly via this path. By virtue of the immediately following changeover of the vent valve to its switched position, the compressed air now present upstream from the throttle is discharged directly to a venting device via a corresponding path or duct in the vent valve. The amount of air escaping in a single switching operation is not very large during a single switching operation of the vent valve, and it depends on an available volume. Nevertheless, rapid venting of the control chamber of the air-flow-boosting valve device can be achieved by switching to and fro frequently and rapidly. For this purpose, a compressed-air accumulator is advantageously provided between the orifice and an outlet of the vent valve, in order to increase the amount of air to be discharged. 
     In a preferred embodiment, the alternating solenoid current jumps to and fro between the value zero and a value corresponding to the intensity of the first solenoid current. This embodiment has the advantage that this first solenoid current merely has to be pulsed, or, in other words, turned on and off. Alternatively, the alternating solenoid current can also be generated between two closely spaced current values; as a result, the switched positions of the primary armature can be changed over more rapidly. 
     In a further embodiment, the primary armature and the secondary armature have different diameters. In particular, the secondary armature has a smaller diameter than the primary armature. As a result, the structure of the armature-guide arrangement can advantageously be configured such that the armature-guide tube, together with the two magnet armatures, can be mounted in the coil from one side. Also, advantageously, 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. As discussed above, it is intended specifically that the primary armature will be pulled in first by the solenoid and that the secondary armature will also be pulled in only at a higher current. 
     Advantageously, 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. As a result, the switching behavior of the valve unit is favorably influenced. 
     In a further 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. The switching behavior of the solenoid-valve system is also improved by this measure. 
     In a still further 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. 
     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 
       The present invention will be described in greater detail hereinafter on the basis of the accompanying drawings, wherein: 
         FIG. 1  is a simplified schematic diagram of an air-brake system having an electro-pneumatic brake control device for controlling 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 a brake control device according to  FIG. 1 ; 
         FIG. 3  shows a valve unit according to a further exemplary embodiment of the present invention for a brake control device according to  FIG. 1 ; and 
         FIG. 4  is a 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 electro-pneumatic brake control device for controlling a parking brake of the vehicle. Such air-brake systems are used, for example, in commercial vehicles, heavy motor trucks or buses, and, in particular, in vehicle trains comprising a tractor and a trailer. 
       FIG. 1  shows the components of brake system  10  that are helpful for understanding the present invention. Brake system  10  is electrically controlled, meaning that the metering of brake pressure to the 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 clarity, only one such brake cylinder is illustrated in  FIG. 1 ), the spring-actuator part being controlled by an electro-pneumatic brake control device constructed and arranged as parking brake module  14  for controlling the parking brake. 
     Brake system  10  is provided with a brake force transducer  16 , which senses a braking intent of the driver. Brake force transducer  16  comprises an electrical part and a pneumatic or hydraulic part, only the pneumatic part being 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 electro-pneumatic 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. Aside from its function as a diaphragm cylinder, it additionally has a spring-actuator function. Brake cylinder  12  is, therefore, provided with a diaphragm part  28 , 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 in the form of a changeover valve or select-high valve connected between spring-actuator part  30 , a pneumatic output  102  of parking brake module  14  and the output of brake force transducer  16  having the modulated pressure. This overload protection valve  35  selects the higher of the two pressures, namely, the modulated brake pressure at the output of brake force transducer  16  and the pressure made available by air-flow-boosting valve device  67 , and it supplies this 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 or diaphragm part  28 , to the brake force supplied by the parking brake, or, in other words, spring-actuator part  30 , to prevent 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 , which 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 electronic 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 suited 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 , 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 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  80  in communication via compressed-air lines  78 ,  34 ,  32  with spring-actuator part  30  of brake cylinder  12 . Relay valve  64  has a control input  82 , which is in communication via a compressed-air line  84  with a valve unit  86  for controlling 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  34  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 loss from occurring in parking brake module  14 . Such a pressure drop or pressure loss is undesirable, since it might lead 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 controlling 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  108 ,  44  and  46 . 
     Electronic 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 . 
     Valve units  86  and  110  are of identical design. Accordingly, hereinafter, the discussion will focus on valve unit  86 . 
     In one embodiment, valve units  86  and  110  are stand-alone subassemblies. Alternatively, however, valve units  86  and  110  are 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 explained in greater detail hereinafter. However, if the valve units are integrated in the parking brake module, such ports are omitted in favor of appropriate connecting lines. This application, therefore, uses the generic term “connecting member”, which is to be understood to comprise any kind of connecting means, including both ports and other types of connections, such as, for example, pneumatic connecting lines, connecting ducts or bores or other passages and other like elements. 
     Also, the term “port” as used in connection with valve units  86  and  110  is to be understood to refer to a connecting member, so that, in this way, the embodiment in which valve units  86  and  110  are integrated in parking brake module  14  can also be discussed. 
     Valve unit  86  is provided with a first port  122 , which, via compressed-air lines  126 ,  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 designed as a double-armature solenoid valve. The structural design can be seen in  FIG. 2 , which shows double-armature solenoid valve  134 . 
     Double-armature solenoid valve  134  is provided with two magnet armatures  138 ,  140  disposed in a common armature-guide arrangement  136 . It is constructed and arranged 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 vent valve  148  ( FIG. 1 ), and secondary armature  140  is provided as an operating element for an air-admission valve  150 . Solenoid  146  is provided with two electrical ports  152 , which are connected by means of electrical lines  118  to electronic control unit  40 . 
     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 to 3 . Air-admission valve  150 , when in its home position, shuts off first port  122  from third port  132 , and vent valve  148 , when in its home position, places second port  128  in communication with third port  132  via an interposed orifice  154  acting as a throttle. A compressed-air accumulator  158  that can be switched by means of primary armature  138  is provided between orifice  154  and an outlet  160  of vent valve  148 . Compressed-air accumulator  158  is designed as a chamber within valve unit  86 . 
     Outlet  160  of vent valve  148  is in communication with second port  128  of valve unit  86 . Furthermore, vent valve  148  has an inlet  162 , which, via corresponding connecting ducts within valve unit  86 , is in communication with third port  132 . 
     In the home position of primary armature  138 , inlet  162  is pneumatically in communication, via compressed-air accumulator  158  and orifice  154 , with second port  128 . Furthermore, second outlet  160  is shut off in the home position of primary armature  138 . In a switched position of primary armature  138 , or, in other words, when the primary armature is pulled inward in the direction of the interior of solenoid  146  by injection of a first solenoid current of predetermined magnitude, compressed-air accumulator  158  is pneumatically in communication with second outlet  160  and inlet  162  is shut off. 
     Elastomeric inserts  164 ,  166  are disposed at the respective two ends of primary armature  138 . Elastomeric inserts  164 ,  166  can also be formed in one piece, by providing primary armature  138  with a through bore, through which there extends elastomeric inserts  164 ,  166 . Elastomeric inserts  164 ,  166  each form a valve seat together with a corresponding shaped projection  168  on armature-guide arrangement  136  and, respectively, a shaped projection  170  on a vent-valve head  172 . 
     Vent valve  150  has an inlet  174  in communication with first port  122  of valve unit  86  and an outlet  176  in communication with third port  132 . Via corresponding ducts in valve unit  86 , outlet  176  is also in pneumatic communication with inlet  162  of the vent valve. 
     In its home position, secondary armature  140  of air-admission valve  150  shuts off inlet  174  of air-admission valve  150  from its outlet  176 . In its switched position, secondary armature  140  places inlet  174  in communication with outlet  176 . 
     Secondary armature  140  is provided with an elastomeric insert  178 ,  180  at each of its two ends. These elastomeric inserts can also be formed as separate pieces or, as illustrated in  FIG. 2 , in one piece. In the case of one-piece design, the elastomeric insert passes through a conduit extending through the secondary armature. Elastomeric inserts  178 ,  180  protruding at the two ends of secondary armature  140  are able to come into contact with corresponding shaped projections  182 ,  184  on an air-admission valve head  186  or on armature-guide arrangement  136 . A valve seat is formed by elastomeric insert  178  and shaped projection  182 , by which inlet  174  of air-admission valve  150  can be shut off. By virtue of a recess  187  in the region of shaped projection  184  on armature-guide arrangement  136 , the stop between the elastomeric insert and shaped projection  184  always remains open. Furthermore, this stop is pneumatically in communication via a duct-like conduit  188  with inlet  162  of the vent valve. 
     Primary armature  138  and secondary armature  140  each have a substantially rotationally symmetric design. However, they each have a slot-like recess  190  and  192 , respectively, extending along the respective armature. Recess  190  of primary armature  138  establishes communication between inlet  162  of the vent valve and compressed-air accumulator  158  when primary armature  138  is in its home position. 
     Recess  192  of secondary armature  140  establishes communication between recess  187  or conduit  188  and the outlet of air-admission valve  176 , regardless of the switched position of secondary armature  140 . 
     By virtue of this arrangement, vent valve  148  forms a 3/2-way solenoid valve. Air-admission valve  150  forms a 2/2-way solenoid valve. 
     The valve unit of  FIG. 3  corresponds largely to that illustrated in  FIG. 2  and therefore bears the reference numeral  86 ′. Hereinafter, therefore, only the differences with valve unit  86  will be discussed. All other elements are of identical design and/or identical function, as discussed in connection with  FIG. 2 . To this extent, the foregoing discussion is instructive. 
     Valve unit  86 ′ shown in  FIG. 3  differs from valve unit  86  shown in  FIG. 2 , on the one hand, by the design of armature-guide arrangement  136 ′ and, on the other hand, by the design of secondary armature  140 ′. 
     More particularly, secondary armature  140 ′ is designed with a smaller diameter than that of secondary armature  140  shown in  FIG. 2 . As a result, a particularly space-saving arrangement is achieved. Consequently, armature-guide arrangement  136 ′ can also be made more slender in the region of secondary armature  140 ′. In particular, the end of armature-guide arrangement  136 ′ facing secondary armature  140 ′ is designed with uniform cross section. Thus, armature-guide arrangement  136 ′ has an outside contour that corresponds to the inside contour of solenoid  146  substantially over the entire length of armature-guide arrangement  136 ′; only at its end associated with primary armature  138  does armature-guide arrangement  136 ′ have a projecting or thickened part. This construction of armature-guide arrangement  136 ′ makes it easy to assemble valve unit  86 ′, since armature-guide arrangement  136 ′ can be mounted from one side, namely, from the right side in the orientation shown in  FIG. 3 . Beyond this, the different diameters of the primary armature and secondary armature result in improved switching behavior of valve unit  86 ′. 
       FIG. 4  shows a further exemplary embodiment of a parking brake module  14 ′, which corresponds largely to parking brake module  14  shown in  FIG. 1 . However, parking brake module  14 ′ shown in  FIG. 4  reveals only valve unit  86  for the tractor. In  FIG. 4 , therefore, like reference numerals denote like parts as in  FIG. 1 , and, so, to this extent, the foregoing discussion should be consulted. 
     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  154 . 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  154 . Therefore, orifice  154  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 might no longer be assured. Nevertheless, in order to ensure slow venting with a sufficiently large cross section of orifice  154 , the control volume of the relay valve is seemingly or, in other words, 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  194 . This orifice  194  increases the amount of air to be vented in the control chamber 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  154  to reduce the danger of fouling by dirt or ice. 
     Orifice  194  of the relay piston of relay valve  64 ′ is advantageously provided with a larger cross-sectional area than the cross-sectional area of orifice  154  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  154  in sufficiently slow manner by virtue of the small opening of orifice  154 . 
     By the inventive double-armature valve having an orifice for slow venting, a valve unit of simple and therefore inexpensive design is provided that simultaneously ensures safe parking of the vehicle even in the event of failure of the electrical power supply. By injection of a high solenoid current, air can be admitted to the relay valve and, thus, to the spring-actuator part of the spring-actuated brake cylinders. By injection of a low current, the pressure can be held at the control input of the relay valve and, thus, also in the spring-actuator part of the spring-actuated brake cylinders. In the case of a pulsed low current at the primary solenoid valve, rapid venting is made possible by rapid to-and-fro movement of the primary armature. In the de-energized condition, on the other hand, only slow venting of the control chamber of the relay valve takes place via an orifice. 
     Accordingly, 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 so 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 in the above constructions 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 which, as a matter of language, might be said to fall therebetween.