Abstract:
A vehicle air suspension installation includes pneumatics configured for operation in conjunction with a compressed air supply installation, and comprises: a pneumatic line having a port connection to the air supply installation, air bellows, each serving as a pressure chamber for an air spring, one air bellows being connected to the pneumatic line via a directional solenoid valve, and the air bellows being fillable/bleedable depending on a switching state of the valve, and first and second directional valves forming a solenoid valve system, which has a pneumatic part that is actuatable by a magnetic part. The first directional valve forms a primary valve, and the second forms a secondary valve. First and second air bellows associated with the valve system are fillable/bleedable depending on the switching state of the primary and secondary valves. The primary and secondary valves are controllable by a controller of the magnetic part. The controller is common to and acts upon both valves.

Description:
FIELD OF THE INVENTION 
     The invention generally relates to an air suspension installation for a vehicle. 
     BACKGROUND OF THE INVENTION 
     A pneumatic system of the general type under consideration, e.g., disclosed in EP 1 165 333 B2, is used in vehicles of all types. A pneumatic system of this type is used for the suspension and potentially also for level control in a vehicle, so that the distance between the axle of the vehicle and body of the vehicle can be adjusted. An air suspension installation of such type comprises a plurality of bellows, each acting as a pressure chamber for a pneumatic spring, which raises the body of the vehicle with increasing filling and can lower it with decreasing pressurization. With increasing distance between the axle of the vehicle and the body of the vehicle or ground clearance, the spring travels can be longer and more uneven surfaces can also be overcome without this leading to contact with the body of the vehicle. Such pneumatic systems are used in off-road vehicles and sport utility vehicles (SUV). Especially with SUVs with very powerful engines, it is desirable that the vehicle is provided with relatively little ground clearance on the one hand for high speeds on the road and on the other hand with a relatively large ground clearance for off-road. It is further desirable to carry out a change of the ground clearance as rapidly as possible, which increases the requirement for speed, flexibility and reliability of a pneumatic system having an air suspension installation. 
     A compressed air supply system typically supplies the air suspension system in a pneumatic system with compressed air from a compressed air supply, for example in the region of a pressure level of 5 to 20 bar. The compressed air is supplied by an air compressor of a pressure source. For supplying the air suspension installation, the pressure source is connected to a connection of a pneumatic line of the air suspension installation, which can also be pneumatically connected to a vent. The compressed air supply installation and the air suspension installation can be vented by blowing off air via a venting valve apparatus. A pneumatic supply line of the compressed air supply installation usually comprises an air dryer in order to ensure long-term operation of the compressed air supply installation or to prevent the collection of moisture in the pneumatic system. 
     A control valve in the form of a solenoid valve—also referred to as a switching valve—is usually connected upstream of each air bellows of the air suspension installation. A control valve is connected to the pneumatic line of the air suspension installation such that the air bellows can be filled or emptied—depending on a switching state of the control valve. In other words, air masses can be adjusted in an air bellows in order to thereby regulate the suspension characteristics or the level for a vehicle. 
     It is desirable to provide an improved air suspension installation not only with respect to the increasing requirement for speed, flexibility and reliability, but in addition—without having to make restrictions of a functional nature—as simply as possible. 
     SUMMARY OF THE INVENTION 
     Generally speaking, it is an object of the present invention to provide an air suspension installation and a pneumatic system as well as a vehicle and a method, in which the pneumatics of the air suspension installation are simplified such that relatively simplified control of the air suspension installation is achieved without functional restriction of the air suspension installation or of the pneumatic system. In particular, an object is to increase the functionality of the air suspension installation with respect to speed, flexibility and reliability. 
     According to an embodiment of the present invention, the air suspension installation for a vehicle having a pneumatic installation provided for operation with a compressed air supply installation comprises: 
     1. a pneumatic line having a connection to the compressed air supply installation; 
     2. a plurality of air bellows, wherein an air bellows is connected to the pneumatic line by means of a control valve in the form of a solenoid valve, and wherein the air bellows can be filled or vented depending on a switching state of the control valve; 
     3. a first control valve and a second control valve, which form a control (solenoid) valve assembly, wherein the first control valve forms a primary valve and the second control valve forms a secondary valve of the solenoid valve assembly; in particular, the solenoid valve assembly comprises a pneumatic part that can be actuated by a magnetic part; 
     4. a first air bellows and a second air bellows associated with the solenoid valve assembly, which can be filled or vented depending on a switching state of the primary valve and of the secondary valve; and 
     5. a controller, in particular of the magnetic part of the solenoid valve assembly, for controlling the primary valve and the secondary valve, acting on both together. 
     Embodiments of the invention are based on the consideration that a switching valve is connected upstream of an air bellows of the air suspension installation that is used as a pressure chamber for a pneumatic spring, namely via a control valve in the form of a solenoid valve for controlling an air mass in the air bellows. In particular, each air bellows has an associated control valve for reasons of functional safety. Each of the control valves can, in principle, be connected by means of a separate control line to a controller and could be separately controlled or monitored. However, the inventors have recognized that it is possible, without restrictions on functional safety, to assign a control valve to each air bellows and yet to provide simplifications with respect to the controller. For this purpose, the invention provides for the formation of the first and second control valve as the control (solenoid) valve assembly, wherein the first control valve forms the primary valve and the second control valve forms the secondary valve of the solenoid valve assembly. The solenoid valve assembly advantageously comprises the pneumatic part that can be actuated by the magnetic part. Depending on the switching state of the primary valve and of the secondary valve, a first and a second air bellows associated with the solenoid valve assembly can be filled or vented. The combination of the first and second control valve in a solenoid valve assembly forms the basis for the simplification of the pneumatics and of the controller of the first and second control valves. Also, the primary valve and the secondary valve can be controlled by a controller of the solenoid valve assembly, especially of the magnetic part, common to both and acting on both. With the controller acting on both the primary valve and the secondary valve, the same can be electronically controlled simultaneously or sequentially. In particular, however, the number of control valves associated with the air bellows (e.g., four for a four-wheel vehicle) can be combined into groups of (e.g., two) solenoid valve assemblies, each having a primary valve and a secondary valve. 
     It will be appreciated that, with the inventive embodiments, costs and risks of failure of an air suspension installation can be reduced, primarily because the jointly acting controller of the magnetic part of the solenoid valve allows assembly interfaces, lines and final control stages, which are provided separately for each control valve in the prior art, to be omitted. Moreover, connections in the cabling and the electronics can be omitted; and the number of controllers can be reduced, because at least the primary valve and the secondary valve can be controlled by a controller of the magnetic part of the solenoid valve assembly common to both and acting on both. In this way, the power consumption is significantly reduced, which likewise increases reliability. The inventive embodiments also lead to a smaller structural volume and reduced weight of the air suspension installation. 
     A pneumatic part of a solenoid valve assembly includes the pneumatically operating parts, such as the valve with a valve body, valve seat, valve housing or the like. A magnetic part of a solenoid valve assembly includes the electrically and/or magnetically working parts, such as an actuator and/or controller for the valve and coil, with a winding, armature, yoke and core as well as a control line or the like and the connections of the control line to the controller and coil. 
     The jointly acting controller is preferably a magnetic part jointly acting on the primary and secondary valves, in particular a coil body, e.g., forming a dual armature solenoid valve or a common control line. 
     The primary valve and secondary valve of the solenoid valve assembly can be connected either in parallel or in series. The primary valve and secondary valve should be normally closed. In principle, however, a normally open variant is not excluded. All four variants can be combined with each other in any way. A series arrangement of the primary and secondary valves is suitable for a particularly flexibly configurable, time-sequential switching sequence of primary and secondary valves. A parallel arrangement of the primary and secondary valves is particularly suitable for the simultaneous switching of primary and secondary valves. Each primary and/or secondary valve is preferably in the form of a 2/2-way valve. A series arrangement of the primary and secondary valves is particularly suitable for the sequential switching of primary and secondary valves. The primary valve is preferably in the form of a 2/2-way valve or the secondary valve is preferably in the form of a 3/2-way valve. 
     According to an embodiment of the invention, a vehicle is equipped with an inventive air suspension installation as well as a compressed air supply installation. The compressed air supply installation preferably comprises a pressure source, a vent and a pneumatic supply line connected to the connection of the pneumatic line, into which an air dryer is connected. 
     In a preferred embodiment of the invention, the common magnetic part acting on the primary valve and the secondary valve comprises a common coil body acting on the primary and secondary valves as a common controller. This has the advantage that a separate coil body for each of the control valves can be omitted. For each solenoid valve assembly, only a single coil body is provided as a common controller jointly acting on the primary and secondary valves. Advantageously, practically half of all otherwise necessary coil bodies are omitted, which means a significant reduction in weight and control costs. In particular, the solenoid valve assembly is in the form of a dual armature solenoid valve. A dual armature solenoid valve preferably comprises a primary armature carrying a first seal element of the primary valve and secondary armature carrying a second seal element of the secondary valve, which are disposed in a common coil body as a common controller. The primary armature has an associated primary valve seat and the secondary armature has an associated secondary valve seat here. The basic functionality of a conventional dual armature solenoid valve is described in the applicant&#39;s DE 35 01 708 A1, which is hereby incorporated herein by reference in its entirety. 
     According to an embodiment of the present invention, the pneumatic installation comprises a gallery line, which is connected to the pneumatic line. A gallery line is a pneumatic distribution line, from which at least two branch lines emanate. In particular, an air bellows is connected to a branch line that is connected to the gallery line. The design of the pneumatic installation with a pneumatic line for connection to the compressed air supply installation as well as a gallery line and branch lines connected thereto, each with at least one air bellows and at least one control valve—i.e., primary valve and/or secondary valve—is salutary with regard to dynamics and the manner of operation. 
     Advantageously, the pneumatic installation comprises at least one other pressure chamber, e.g., in the form of a compressed air reservoir, a pressure sensor or the like. In particular, such a pressure chamber can be connected to the pneumatic line via a further control valve in the form of a solenoid valve such that the pressure chamber—depending on a switching state of the control valve—can be filled or vented. Preferably, the other control valve part is also part of a solenoid valve assembly in the form of a primary valve or a secondary valve. 
     In a pneumatic installation with a pneumatic line, gallery line and branch lines, the primary valve and/or secondary valve is preferably connected to at least one branch line to the pneumatic line, advantageously via the gallery line. In particular, it is advantageously provided that a bellows-side pneumatic connection of the primary valve is connected to a first branch line and a bellows-side pneumatic connection of the secondary valve is connected to a second branch line. The first and second branch lines are advantageously separate branch lines, each comprising an air bellows. This ensures that at least one control valve, namely the primary valve and/or the secondary valve, is disposed upstream of each air bellows. A pressure sensor can also be connected to the gallery line directly or via a branch line. 
     A pressure limiter advantageously holds a pneumatic connection sealed up to a relatively high limiting pressure and only opens it at a relatively high limiting pressure. 
     The invention encompasses a vehicle having at least two axles, wherein the first and second air bellows are disposed on one of the two axles to form a pneumatic spring in each case. Such a vehicle can, e.g., be a private vehicle, in particular an SUV (Sport Utility Vehicle) as already mentioned. The invention also has application in commercial vehicles. 
     In one embodiment, a vehicle can comprise an inventive single-axle air suspension. That is, for a single axle of the vehicle, a solenoid valve assembly is provided with a primary valve and a secondary valve. These can be controlled by a controller of the magnetic part of the solenoid valve assembly common to both and acting on both. 
     It is also possible to provide another axle of the vehicle with an inventive solenoid valve assembly. In particular, in addition to the first axle, at least one second axle of the vehicle also comprises a further solenoid valve assembly with a third and a fourth air bellows, with which a primary valve and a secondary valve according to the further solenoid valve assembly are associated. 
     So-called average value control leads to a pressure being set for the first and second air bellows associated with an axle of a vehicle according to an average value of the pressure in the first and second air bellows. With so-called left-right control, a separate pressure setting results in a pressure in the first and second air bellows being separately adjustable. 
     According to an embodiment of the invention, average value control can be provided for only one or both or a plurality of, possibly all, axles. Also, left-right control can be provided for one or both or a plurality of, possibly all, axles. Further, average value control can be provided for one of the axles and left-right control can be provided for another axle. Depending on requirements, combinations of the foregoing can be implemented for specified operating states of a vehicle as well as for setting to work processes or special applications with more complex control technology. 
     Advantageously, the primary valve and the secondary valve can be disposed in a parallel circuit. Also, it is desirable to provide a gallery-side pneumatic connection of the primary valve to a first branch line and a gallery-side pneumatic connection of the secondary valve to a second—separate—branch line. The design of the solenoid valve assembly as a parallel circuit of control valves is particularly advantageous in the context of average value control, involving control to an average value of the pressure of the first and second air bellows. It is also advantageous, and relatively simple, to control the primary and secondary valve with the same switching states. A corresponding controller configured to regulate the first and second air bellows pressure only according to an average value of the pressure of the first and second air bellows by specifying simultaneously identical switching states for the primary valve and secondary valve at all times, can be used. Such a controller is comparatively simple because it does not involve current control. For the case in which the primary valve and the secondary valve are associated with the two wheels of a single axle, in the context of average value control, the axles are each controlled in relation to the average value of a pressure at the air bellows of the two wheels. As a result, the axle is thus controlled equally on the left and right. 
     A control arrangement can be particularly simply implemented with which only one axle of a vehicle is provided with air springs, which are controlled according to average value control. This is preferably the front axle of a vehicle. The rear axle of a vehicle can, e.g., be provided with conventional control valves. 
     According to another embodiment, the controller can be configured such that a different pressure is controlled in the first and second air bellows. Such a controller can include a current control module with which different switching states can be specified for the primary and secondary valves. Also, this variation of a controller can be implemented with a solenoid valve assembly forming a parallel circuit. 
     By specifying different switching states of the primary and secondary valves, so-called right-left control can be achieved, with which a control valve assigned to the left wheel—e.g., the primary valve—and a control valve assigned to the right wheel—e.g., the secondary valve—are switched with different switching states, so that the associated air bellows can be subjected to different pressures. 
     Such left-right control can be achieved particularly simply based on a second variant of a solenoid valve assembly in a series circuit. That is, the primary valve and secondary valve can be disposed in a series circuit, wherein a gallery-side pneumatic connection of the primary and secondary valves of the solenoid valve assembly is connected to the same branch line. 
     In particular, at least one pneumatic line of the pneumatic installation comprises a residual pressure holding valve. Advantageously, at least one pneumatic line of the pneumatic installation—in particular the pneumatic line, the gallery line or a branch line—can comprise a residual pressure holding valve and/or a pressure limiter. It is advantageous for certain requirements to hold a residual pressure in the system. This can prevent a collapse of air bellows in an air suspension installation. A pressure limiter has the advantage that the pneumatic installation is protected against an excessive pressure loading. 
     In a method according to one embodiment of the present invention, the primary valve and the secondary valve of the solenoid valve assembly are designed to be activated time-sequentially in a first operating mode with a control current rising above a current amplitude. It can also be provided that the control current at first only exceeds a current amplitude for activation of the primary valve and thereafter remains in a range that only keeps the primary valve activated, while the secondary valve is not activated. At a later point in time, the control current can exceed a second current amplitude, which is sufficient to also activate the secondary valve. Thereafter, the control current can have a current amplitude that is sufficient to keep both the primary valve and also the secondary valve in the activated state. For deactivation of the secondary valve and the primary valve, a control current can be reduced below the foregoing current amplitudes, possibly with a known hysteresis. In a second operating mode the control current can rise comparatively rapidly to a current amplitude that lies above a current amplitude that is sufficient to activate the primary valve and the secondary valve simultaneously. For this, the current amplitude should lie above the highest current amplitude for activation of the primary valve and the secondary valve. 
     Activation of the primary valve and/or the secondary valve can take place for blowing off air or for delivering air to an air bellows. The air can be passed as an air flow via the pneumatic line. Activation of a primary or secondary valve can, e.g., take place according to a ride height control of the vehicle. For instance, air mass control connected to ride height control can also be provided, which controls a volumetric flow through the pneumatic line. Activation of the primary valve and/or the secondary valve can take place under time control. Time control is recommended, e.g., if a ride height display is too noisy. Time-controlled activation of the primary valve and the secondary valve can, e.g., also take place in the context of a regeneration cycle for the compressed air supply installation. 
     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, arrangement of parts, and the various steps and the relation of one or more of such steps with respect to each of the others, all as exemplified in the constructions herein set forth, and the scope of the invention will be indicated in the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is explained in greater detail below using exemplary embodiments on the basis of the accompanying drawings, in which: 
         FIG. 1  is a schematic illustration of a pneumatic system with an air suspension installation and a compressed air supply in the form of a reservoir, according to an embodiment of the present invention; 
         FIG. 2  shows in views (a) through (d) different switching states of a solenoid valve assembly consisting of a primary valve and a secondary valve in a pneumatic parallel circuit—i.e., in relation to two bellows, which are used to form two air springs of an axle for a vehicle—in accordance with an embodiment of the present invention; 
         FIG. 3  shows in views (a) through (c) different switching states of a solenoid valve assembly consisting of a primary valve and a secondary valve in a pneumatic series circuit—i.e., in relation to two air bellows, which are used to form two air springs of an axle for a vehicle—in accordance with an embodiment of the present invention; 
         FIG. 4  is a graph of an exemplary current amplitude of a control signal as a time variation illustrating switching currents for a solenoid valve assembly in the form of a dual armature solenoid valve, by which the primary valve and the secondary valve are controlled via a common control line, in accordance with an embodiment of the present invention. 
     
    
    
     LIST OF REFERENCE CHARACTERS 
     
         
         
           
               1  control unit 
               2  compressed air supply installation 
               3  pneumatic line 
               4 ,  5 ,  6 ,  10 ,  15  control line 
               7  coil body 
               8 ,  9 ,  16 ,  17  connection 
               11  control valve 
               12  gallery line 
               13  control valve 
               14  coil body 
               18  control valve 
               19  control valve 
               20  compressed air line 
               21  coil body 
               22  control valve 
               23 ,  24 ,  25 ,  27 ,  28 ,  29 ,  30  control line 
               26  pressure sensor 
               31 ,  32 ,  33 ,  34  sensor 
               35 ,  36 ,  37 ,  38  branch line 
               39 ,  40 ,  41 ,  42  residual pressure holding valve 
               43 ,  45 ,  47 ,  49  air bellows 
               44 ,  46 ,  48 ,  50  damping valve 
               51  compressed air reservoir 
               52 ,  53 ,  54 ,  55  control line 
               60  coil body 
             I, II primary valve, secondary valve 
               62 ,  63  bellows line 
               61 ,  64  control valve 
               65 ,  66  air bellows 
               100  vehicle 
               110  air suspension installation 
               120 ,  120 ′, solenoid valve assembly 
               130 ,  130 ′ solenoid valve assembly 
               200  pneumatic installation 
               210  pneumatic system 
               350 ,  360 ,  370 ,  380  branch line 
               600  common branch line 
             SA 1 , SA 2 , SA 3 , SA 4  current amplitude 
             A 1 , A 2  first axle, second axle 
             R 1 , R 2 , R 3 , R 4  wheel 
             X 1 , X 2  gallery-side connection of the primary or secondary valve 
             Y 1 , Y 2 , Y 2   a , Y 2   b  bellows-side connection of the primary or secondary valve 
           
         
       
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  shows an example embodiment of an air suspension installation  110  for a schematically illustrated vehicle  100 , which comprises a pneumatic installation  200  electronically controlled by means of a control unit  1  as part of one or a plurality of controllers. The pneumatic installation  200  forms a pneumatic system  210  together with a compressed air supply installation  2  symbolically illustrated here as a reservoir. A compressed air supply installation  2  usually comprises a compressed air source not illustrated here in detail, a vent and a pneumatic supply line with an air dryer connected to the connection of the pneumatic line  3 . 
     The vehicle  100  can be, e.g., a private vehicle such as an SUV or the like, or a commercial vehicle. A first axle A 1  with two wheels R 1  and R 2  and a second axle A 2  with two wheels R 3  and R 4  of the vehicle  100  are symbolically illustrated. The air suspension installation  110  of  FIG. 1  is configured in this respect as a two-axle air suspension installation for a two-axle vehicle  100 . In a variation, one or a plurality of similar air suspension installations can also be realized for only one axle or more than two axles of a vehicle as required—the number of control valves and solenoid valve assemblies can be adapted accordingly. In general, not only compressed air but also a different gaseous or hydraulic pressure fluid can be used. The invention enables a simple implementation of an air suspension or general fluid suspension, with which a plurality of air springs or bellows can be controlled with a reduced number of solenoid valves. 
     In one embodiment, the air suspension installation  110  comprises a first and a second solenoid valve assembly  120 ,  130 . The air suspension installation  110  for a two-axle vehicle  100 , i.e., with a first axle A 1  and a second axle A 2 , is provided with four control valves  11 ,  13 ,  18 ,  19 , each of which is disposed upstream of a respective air bellows  43 ,  45 ,  46 ,  47 , thereby forming a pneumatic spring. Each of the control valves  11 ,  13 ,  18 ,  19  is formed as a controllable 2/2-way valve, namely as a solenoid valve. Using the control valves  11 ,  13 ,  18 ,  19 , air masses in the four air bellows  43 ,  45 ,  46 ,  47  can be regulated. As can be seen symbolically in  FIG. 1  by means of dashed lines, the four wheels R 1 , R 2 , R 3 , R 4  mounted on the vehicle axles A 1 , A 2  each have an associated pneumatic spring. With the example embodiment shown in  FIG. 1 , the two air bellows  43 ,  45  are associated with the first axle A 1  of the vehicle  100  forming the front axle, while the air bellows  47 ,  49  are associated with the second axle A 2  of the vehicle  100  forming the rear axle. 
     The first control valve  11  forms a primary valve and the second control valve  13  forms a secondary valve of the solenoid valve assembly  120 . The other first control valve  18  forms a primary valve and the other second control valve  19  forms a secondary valve of the other solenoid valve assembly  130 . A primary valve of a solenoid valve assembly is referred to throughout below by “I”. A secondary valve of a solenoid valve assembly is referred to throughout below by “II”. In the illustrations of the switching states of  FIG. 2  and  FIG. 3 , the control valves are characterized accordingly as a primary valve I or a secondary valve II. The control valves  11 ,  13 ,  18 ,  19  can optionally be combined in a valve block. 
     The control unit  1 , e.g., an electronic control unit ECU, is connected via control lines  5 ,  6  to a coil body  14 ,  7  forming the magnetic part of a solenoid valve assembly  120 ,  130  in each case. That is, the control unit  1  is designed to carry out an actuation of the first control valves  11 ,  18  and second control valves  13 ,  19  in each case such that a primary valve I and a secondary valve II—denoted in more detail in  FIG. 2 —of one of the solenoid valve assemblies  120 ,  130  are controllable via the control lines  5 ,  6  and coil bodies  14 ,  7 . Here, the control lines  5 ,  6  and coil bodies  14 ,  7  each form the controller common to both and acting on both the primary valve I and the secondary valve II. The coil body  7 —as shown above the connections  8 ,  9 —is an essential component of the magnetic part of the control valves  11 ,  13  and is coupled thereto. The coil body  14 —as shown above the connections  16 ,  17 , which are connected to each other—is an essential part of the magnetic part of the control valves  18 ,  19  and is coupled thereto. Each solenoid valve assembly  120 ,  130  is also in the form of a conventional dual-armature solenoid valve. A dual-armature solenoid valve also comprises a primary armature supporting a first seal element of the primary valve I (one of the first control valves  11 ,  18 ) and a secondary armature supporting a second seal element of the secondary valve II (one of the second control valves  13 ,  19 ), which are disposed in a common coil body  14 ,  7 . Here, the primary armature is associated with a primary valve seat and the secondary armature is associated with a secondary valve seat. 
     Moreover, another coil body  21  of another control valve  22  in the form of a solenoid valve can be controlled via a control line  4  by the control unit  1  in order to selectively connect a compressed air reservoir  51  via the gallery line  12  to the compressed air supply installation  2  or to decouple it. 
     The other connections of the coil body  7 ,  14 ,  21  are each connected via individual control lines, e.g.,  10 ,  15 , to a common control line  23  and via the same to the control unit  1 , wherein the common control line  23  can be set by the control unit  1  to a common potential, e.g., reference potential, null potential or supply voltage potential. 
     A compressed air supply installation  2  can, e.g., be a simple compressed air container or a compressed air supply installation  2  comprising a pressure source, a vent and a pneumatic supply line having an air dryer and connected to the connection of the pneumatic line  3 . A gallery line  12  forming a compressed air distribution line is connected to the pneumatic line  3 . A respective gallery-side compressed air connection of the control valves  11 ,  13 ,  18 ,  19  is connected to the gallery line  12  via branch lines  350 ,  360 ,  370 ,  380 . 
     In the embodiment of  FIG. 1 —as will be explained in detail using  FIG. 2  and  FIG. 3 —another separate branch line  35 ,  36 ,  37 ,  38  continuing a respective branch line  350 ,  360 ,  370 ,  380  is connected to a bellows-side compressed air connection of each of the control valves  11 ,  13 ,  18 ,  19 . An air bellows  43 ,  45 ,  46 ,  47  is connected to the respective branch lines  35 ,  36 ,  37 ,  38 . The air bellows  43 ,  45 ,  47 ,  49  respectively associated with the vehicle wheels R 1 , R 2 , R 3 , R 4  are also connected via residual pressure holding valves  39 ,  40 ,  41 ,  42  in the branch lines  35 ,  36 ,  37 ,  38  to the bellows-side compressed air connections of the first and second control valves  11 ,  18  and  13 ,  19  in the manner shown in  FIG. 1 . Furthermore, respective damping valves  44 ,  46 ,  48 ,  50  are associated with the respective air bellows  43 ,  45 ,  47 ,  49 , which can be controlled via control lines  52 ,  53 ,  54 ,  55  by the control unit  1 . 
     For measurement of the pressure in the supply line  3  and the gallery line  12 , a pressure sensor  26  is provided, which is connected via a compressed air line  20  to the gallery line  12  for pressure measurement and via electrical control lines  24 ,  25  to the control unit  1 . The voltage supply for the pressure sensor  26  from the control unit  1  and the signal return to the control unit  1  can take place via the electrical control lines  24 ,  25 . Alternatively, a separate third electrical control line for the signal return to the control unit  1  can also be provided. The pressure sensor  26  converts the measured air pressure into an electrical signal that can be delivered via the control lines  24 ,  25  to the control unit  1  and is evaluated by the same for system control purposes. 
     With the example embodiment shown in  FIG. 1 , there are sensors  31 ,  32 ,  33 ,  34  that apply their sensor signals via control lines  27  through  30 , as shown, to the control unit  1  and that can be supplied with the required energy via the control lines. The sensors  31 ,  32 ,  33 ,  34  can be designed as sensors operating with pulse width modulation (PWM), which, e.g., can detect the distance between the wheels of the vehicle and the body of the vehicle. The control of the air bellows can take place depending on the sensor signals in order to achieve a constant, load-independent distance between a wheel R 1 , R 2 , R 3 , R 4  and a vehicle body, or else in order to vary the distance between a wheel R 1 , R 2 , R 3 , R 4  and the vehicle body in a controlled manner. There is, e.g., also the possibility of venting the system, e.g., by controlled connection of the air bellows  43 ,  45 ,  47 ,  49  to atmosphere via venting valves controllable by the control unit  1 . This enables, e.g., a level correction by controlling the respective air masses in the four air bellows  43 ,  45 ,  47 ,  49  via the control valves  11 ,  13 ,  18 ,  19  and the venting valves, so that, e.g., lowering of the vehicle body can be achieved. 
     With the example embodiment shown in  FIG. 1 , an air mass in the four air springs or air bellows  43 ,  45 ,  47 ,  49  is controlled by means of the control valves  11 ,  13 ,  18 ,  19 . One primary valve I (control valves  11 ,  18 ) in each case is controlled together with a secondary valve II (control valves  13 ,  19 ) by one of the two coil bodies  7 ,  14 . Specifically, as can be seen from  FIG. 1 , the first and second control valves  11 ,  13  and the other first and second control valves  18 ,  19  are each equipped with a common electrically controllable coil body  7  or  14 , which are respectively part of a solenoid coil of the magnetic part of the solenoid valve assembly  120  and the other solenoid valve assembly  130 . Therefore only two coil bodies  7 ,  14  are necessary for operating the four control valves  11 ,  13 ,  18 ,  19 , which simplifies the structure and control complexity. 
     According to the embodiment of  FIG. 1 , a balanced wheel load distribution can be achieved with a primary valve I and a secondary valve II of a solenoid valve assembly  120 ,  130 . In a solenoid valve assembly  120 ,  130  the first and second control valves  11 ,  13  and the other first and second control valves  18 ,  19  are respectively connected pneumatically in parallel. In addition, a gallery-side pneumatic connection of a primary valve (first control valves  11 ,  18 ) is connected to a first branch line  350 ,  370  and a gallery-side pneumatic connection of a secondary valve (second control valves  13 ,  19 ) is connected to a second branch line  360 ,  380 . 
     This enables average value control to be achieved—preferably in the context of a switching state shown in  FIG. 2( d )  of a solenoid valve assembly  120 ,  130  having a series circuit of a primary valve and a secondary valve I, II. With the exemplary embodiment shown in  FIG. 1 , e.g., an average value control established for one of the axles A 1 , A 2  can contribute to the transverse stabilization of the vehicle  100 , e.g., when travelling round a curve. Two control valves  11 ,  13  or  18 ,  19  can be used here. Despite the use of two control valves, however, only one coil body is necessary for their control, wherein both control valves may be controlled in parallel. This design enables both control valves to be controlled and monitored while ensuring functional reliability, without each requiring its own control valve cable and output stage of the electronic control unit. The reliability can thereby be increased and the costs reduced. 
     The 2/2-way valves  11 ,  13 ,  18 ,  19  in  FIG. 1  thus form primary and secondary valves I, II of the solenoid valve assemblies  120 ,  130  as air bellows valves. Also representative of the solenoid valve assembly  130  are the solenoid valve assemblies  120  with switching states shown in  FIG. 2 . The coil bodies  7 ,  14  of the magnetic parts of the solenoid valve assemblies  120 ,  130  are used to control the pneumatic part thereof—the latter is formed by means of the pneumatically parallel connected primary and secondary valves I, II. 
     In another embodiment, at least one of the first or second axles A 1 , A 2  can be implemented with an individual control building on the embodiment according to  FIG. 1 . Also only a single coil body  7 ,  14  per axle is necessary here as an actuator, which can be separately controlled and monitored by the electronics. The control valves of the axle can optionally be controlled separately, resulting in individual control. This allows straightening of the vehicle  100  about the longitudinal axis, whereby, e.g., an asymmetric loading can be compensated for. 
     With the exemplary embodiment shown in further detail in  FIG. 2 , only the solenoid valve assembly  120  of  FIG. 1  is shown in different switch positions for average value control of a single axle. Here, the same reference characters are used for identical or similar parts or parts having identical or similar function. The discussion that follows can also apply to the solenoid valve assembly  130  of  FIG. 1 . The coil bodies  7 ,  14  enable, by means of current control, the independent control of two respective control valves  11  (primary valve I),  13  (secondary valve II) or  18  (primary valve I),  19  (secondary valve II) using only one coil body  7  or  14 . 
     The solenoid valve assembly  120  is connected to the pneumatic line  3  via the gallery line  12  and via at least one branch line  350 ,  360 , wherein a bellows-side pneumatic connection Y 1  of the primary valve I is connected to a first branch line  35  and a bellows-side pneumatic connection Y 2  of the secondary valve II is connected to a second branch line  36 , wherein the first branch line  35  and the second branch line  36  are separate branch lines. The primary valve I and the secondary valve II are disposed in a parallel circuit, wherein a gallery-side pneumatic connection X 1  of the primary valve I is connected to a first branch line  350  and a gallery-side pneumatic connection X 2  of the secondary valve II is connected to a second branch line  360 —in the present case, the first branch line  350  and the second branch line  360  are separately connected to the gallery line  12 .  10 . A control unit  1  is formed to regulate the pressure in the first and second air bellows  43 ,  45  according to an average value of the pressure of the first and second air bellows—also identical switching states can be specified for the primary and secondary valves I, II of the pneumatic parallel circuit in the solenoid valve assembly  120 . 
     A first state is shown in  FIG. 2 ( a ) , in which the coil body  7  is not energized and both air bellows  43 ,  45  are shut off. The control valves  11 ,  13  are in their blocking position here. 
     In the second state shown in  FIG. 2 ( b ) , only the primary valve I is connected, i.e., the control valve  11 , so that the air bellows  43 , e.g., on wheel R 1  left, can be pressurized or vented—this is also in accordance with the switch position of the control valve  22  and the pressure supply provided by the compressed air supply installation  2 . 
     In the third state shown in  FIG. 2 ( c ) , only the secondary valve II is connected, i.e., the control valve  13 , while the control valve  11  is blocking. This allows the pressurizing or venting of the other air bellows  45 , e.g., of the air bellows on wheel R 2  right, in the third state. 
     In the fourth state shown in  FIG. 2 ( d ) , both the primary valve I and also the secondary valve II, i.e., the control valves  43 ,  45 , are connected, so that pressurizing or bleeding of both air bellows  43 ,  45  can take place simultaneously. 
     With the embodiment according to  FIG. 2 , switching back and forth between the state according to  FIG. 2( a )  and the state according to  FIG. 2( d )  can take place without current control. This presents the concept of “two-stage magnet in pneumatic parallel circuit” for average value control. Using control with current control, the state according to  FIG. 2( b )  can be switched additively, e.g., by regulating the current delivered from the control unit  1  via the control line  6  so that the coil body  7  can now switch the control valve  11  into the through position, but the generated magnetic force is not yet sufficient to switch the control valve  13  from the blocking state to the passing state as well. 
     In order to achieve the state according to  FIG. 2( c ) , structural measures can also be taken in addition to current control, e.g., by ensuring a specified switching hysteresis by structural means. This can be used for a switching behavior such as is explained using  FIG. 4 . For example, by current control and the switching hysteresis, the control valve  13  remains switched to the open position, while the control valve  11  returns to the blocking position—as shown in  FIG. 2 ( c ) . It can also be held in the blocking position continuously independently of the switching operation of the control valve  13 . 
     By means of the states shown in  FIGS. 2 ( b )  and  2  ( c ), right-left control can also be achieved, so that the air bellows on the left and right can be controlled independently of each other and can be pressurized or vented, whereby, e.g., a tilted position of the vehicle caused by asymmetric loading can be balanced out. 
     Another exemplary embodiment is shown in  FIG. 3 —again, the control valves  61 ,  64  shown there, also referred to as a primary valve  61  and a secondary valve  64 , each form a solenoid valve assembly  120 ′ or  130 ′. These could be used instead of the solenoid valve assembly  120  or  130  of the embodiment of  FIG. 1 . 
     In the present case, a coil body  60 —as with the embodiment of  FIG. 1 —of primary valve I (control valve  61 ) and secondary valve II (control valve  64 ) is also used as a common coil body  60  acting on both for controlling the switching states thereof. The coil body  60  of  FIG. 3  is part of a coil of the solenoid valve assembly  120 ′,  130 ′ forming a magnetic part, which is designed for selectively controllable switching of a primary valve I and a secondary valve II. The primary valve I is designed as a 2/2-way valve  61 ; the secondary valve II is designed as a 3/2-way valve  64 . The latter primary valve I and secondary valve II are, unlike the embodiment of  FIG. 1  in the context of the dual-armature solenoid valve, pneumatically connected in series. The 2/2-way valve  61  and the 3/2-way valve  64  are pneumatically connected in a series circuit and both use a common gallery-side pneumatic connection of the solenoid valve assembly  120 ′,  130 ′ to the same branch line  600  for the primary and secondary valve I, II. The branch line  600  in turn is connected to the gallery line  12 . The common branch line  600  thus leads—as in  FIG. 1 —to a gallery line  12  that is shown there. 
     The solenoid valve assembly  120 ′,  130 ′ is connected to the pneumatic line  3  via the gallery line  12  by at least one branch line  600 , wherein a pneumatic bellows-side connection Y 1  of the primary valve I is connected into a first branch line  600  and a pneumatic bellows-side connection Y 2   a , Y 2   b  of the secondary valve II is connected into a second branch line  62 ,  63 , wherein the first branch line  600  and the second branch line  62 ,  63  are separate branch lines. The primary valve I and the secondary valve II are disposed in a series circuit, wherein a gallery-side pneumatic connection X 1 , X 2  of the primary valve I and secondary valve II of the solenoid valve assembly  120 ′,  130 ′ is connected to the same branch line  600 . In the present case, the branch line  600  is connected as a single branch line  600  to the gallery line  12 . A control unit  1  is designed to control a different pressure in the first and second air bellows  65 ,  66 . In particular, different switching states can be specified for the primary and secondary valves I, II of a pneumatic series circuit in a solenoid valve assembly  120 ′,  130 ′. 
     The embodiment shown in  FIG. 3  is suitable for an axle A 1 , preferably having left-right control—but, if required, also having average value control. A primary valve I formed by the control valve  61  is connected directly into the branch line  600  by a gallery-side connection X 1  and a bellows-side connection Y 1  for conducting compressed air. The secondary valve II formed by the control valve  64  is in turn connected directly into the same branch line  600  by a gallery-side connection X 2  for conducting compressed air. Here, the gallery-side connection X 2  of the secondary valve II is facing the bellows-side connection Y 1  of the primary valve I—i.e., the same branch line  600  connects the bellows-side and gallery-side connections Y 1 , X 2 . Moreover, the secondary valve II formed by the control valve  64  comprises bellows-side connections Y 2   a , Y 2   b  to bellows lines  62 ,  63 , each leading to an air bellows  65 ,  66 . The common branch line  600  is thus used at the same time as an input connection for the control valve  64 , which is connected to the air bellows  65 ,  66  via two bellows-side connections Y 2   a , Y 2   b  disposed at the output and the bellows lines  62 ,  63 . Instead of a bellows line, in principle a line, e.g., as shown in  FIG. 1 , can be connected to a compressed air reservoir  51 . 
     With this embodiment, the components  60  through  66  shown in  FIG. 3  can replace the components  7 ,  11 ,  13 ,  43 ,  45  shown in  FIG. 1 . Alternatively or additionally, the components  60  through  66  shown in  FIG. 3  can also be used in place of the components  14 ,  18 ,  19 ,  47 ,  49  shown in  FIG. 1 . The air bellows  65 ,  66  can be provided with damping valves and/or residual pressure holding valves, as shown in  FIG. 1 . 
       FIG. 3 ( a )  specifically shows a solenoid valve assembly  120 ′,  130 ′ in the form of a dual-armature solenoid valve having a primary valve I (control valve  61 ) and a secondary valve II (control valve  64 ) connected in series—and these are shown in a state in which the coil body  60  is not energized and the solenoid valve assembly  120 ′,  130 ′ is blocking. While the control valve  64  is in the open position, it is, however, blocked by the upstream control valve  61 . Both air bellows  65 ,  66  are therefore neither pressurized nor vented. 
       FIG. 3 ( b )  shows the solenoid valve assembly  120 ′,  130 ′ in a second state, in which only the control valve  61  is switched to the forward direction. Because the control valve  64  remains in its position, the air bellows  66 , i.e., for example the air bellows on the right, is pressurized and vented. The air bellows  65 , for example the air bellows on the left, remains blocked. 
       FIG. 3 ( c )  shows the solenoid valve assembly  120 ′  130 ′ in a third state, in which both the control valve  61  and also the control valve  64  are connected. In this switch position, the control valve  61  is in the conducting position, as in  FIG. 3 ( b ) , but the control valve  64  is also switched over so that the air bellows  66  is now blocked, but the air bellows  65  is pressurized and vented via the control valve  64  connecting the air bellows  65  to the bellows line  63 . With the embodiment shown in  FIG. 3 , the three states are switched by current control. Simultaneous pressurization or venting of the two air bellows  65 ,  66  is not provided. A series circuit of primary and secondary valves I, II in a solenoid valve assembly  120 ′,  130 ′ particularly provides left-right control, however. 
     In another embodiment of the invention, it is also possible to combine a solenoid valve assembly as described above as a dual-armature solenoid valve for an axle having a pneumatic parallel circuit of primary and secondary valves with optional average value control—for example in the embodiment of  FIG. 1  or  FIG. 2 —or as a dual-armature solenoid valve for the axle having a pneumatic series circuit of primary and secondary valves—for example in the embodiment of  FIG. 3 —with two conventional solenoid valves for another further axle. Here, too, the corresponding switching and cabling complexity as well as the monitoring complexity are advantageously reduced with a dual-armature solenoid valve for at least one axle. 
     By the use of a dual-armature solenoid valve or a different solenoid valve assembly having a primary valve I and a secondary valve II, e.g., in a parallel circuit or a series circuit, in principle, a coil body can be saved, whereby the costs of the vehicle cabling and the electronic control unit are reduced. 
     The solenoid valve assembly  120 ,  13 ,  120 ′ 130 ′ can, but does not have to, be provided in each case as a dual-armature valve with a coil body  7 ,  14 ,  60 , by whose energization the respective armature is operated in a controlled manner and hence controlled switch positions of the two control valves can be set. Such a dual armature solenoid valve can, e.g., comprise two magnet armatures (primary armature and secondary armature) disposed in a common or separate armature guide assembly and optionally each loaded by a spring, which can be selectively or jointly operated by a common coil body. With the coil body not energized, the two armatures can each be in their basic positions, e.g., determined by the spring loading, while, when supplied with suitable currents in the coil body, the first and/or second armatures can be activated and changed into the other control valve switch position. Using current control, these switch positions can be specified and achieved in a controlled manner. 
       FIG. 4  is a symbolic illustration of a control current amplitude for the switching of a dual armature solenoid valve—in general, for a solenoid valve assembly  120 ,  120 ′, 130 ,  130 ′—in different switching states, as shown, e.g., in  FIG. 2  and  FIG. 3 . The current amplitude SA is illustrated here by a time profile t. The time profile can be used to determine an exact switching point of a primary valve I and/or of a secondary valve II. This can take place in the context of a diagnostic task for a vehicle controller ECU. 
     The graph of  FIG. 4  shows a controlled current amplitude SA. In an initial state of a solenoid valve assembly, the primary valve I is closed and the secondary valve II is closed (e.g.,  FIG. 2 ( a ) —air bellows  43 ,  45  isolated from gallery line  12 ). 
     On exceeding a first current amplitude SA 1 , the primary valve I changes from a normally closed state into an opened switching state because of the setting of a control valve spring of the primary valve I (e.g., first control valves  11 ,  18  of  FIG. 1  and  FIG. 2  or control valve  61  of  FIG. 3 ), i.e., it is switched over. In the switching state of a solenoid valve assembly, the primary valve I is open and the secondary valve II is closed (e.g.,  FIG. 2 ( b ) —air bellows  43  connected to gallery line  12 , air bellows  45  isolated from gallery line  12 ). 
     On exceeding a second, in the present case higher, current amplitude SA 2 , the secondary valve II also switches (e.g., second control valves  13 ,  19  of  FIG. 1  and  FIG. 2 ), i.e., it changes in the present example from a normal closed state into a switching state that corresponds to the open state of the secondary valve II. In the switching state of a solenoid valve assembly the primary valve I is open and the secondary valve II is open (e.g.,  FIG. 2 ( d ) —air bellows  43 ,  45  connected to gallery line  12 ). 
     In principle, in a modified embodiment, the de-energized and energized switching states can also be selected oppositely; i.e. the primary and/or the secondary valve can be designed for a normally open and closed switching state when energized. 
     In the present case—with the primary and secondary valves I, II designed to be normally closed—when reducing the control current amplitude, the primary valve closes again, taking into account a certain hysteresis, when the current falls below a third current amplitude SA 3 , i.e., the primary valve now changes from the energized open state into the closed switching state occurring at lower current. In this switching state of a solenoid valve assembly, the primary valve I is closed and the secondary valve II is open (e.g.,  FIG. 2 ( c ) —air bellows  45  connected to gallery line  12 , air bellows  43  isolated from gallery line  12 ). 
     On reducing below a fourth, in the present case lower, current amplitude SA 4 , the secondary valve II changes from the energized open switching state back into the de-energized closed switching state. A corresponding control current module can be provided in a control unit  1  in order to define switching states SA 1  through SA 4 . 
     The foregoing switching sequence results independently from a series or parallel circuit of a solenoid valve assembly  120 ,  130  or  120 ′,  130 ′. 
     However, in contrast to the foregoing examples of a parallel circuit of primary and secondary valves I, II in a solenoid valve assembly  120 ,  130  of  FIG. 2( a )-( d ) , a through-flow pneumatic connection of the gallery line  12  to the bellows is assigned differently for a series circuit of primary and secondary valves I, II in a solenoid valve assembly  120 ′,  130 ′. 
     With a series circuit of the primary and secondary valves I, II in a solenoid valve assembly  120 ′,  130 ′, with the primary valve closed (prior to exceeding the current amplitude SA 1  and after reducing below the current amplitude SA 3 ) none of the air bellows is connected to the gallery line  12  ( FIG. 3 ( a ) ). Blocking of the left to right air bellows (or vice-versa) is ensured with no control, e.g., with a series circuit of the above-mentioned type, whereby transverse stabilization is ensured by the suspension springs when travelling round a curve. This effect is also given by the embodiment according to  FIG. 3 . 
     With a series circuit of primary and secondary valves I, II in a solenoid valve assembly  120 ′,  130 ′, with the primary valve open (after exceeding the current amplitude SA 1  and prior to reducing below the current amplitude SA 3 ) either the air bellows  65  or the air bellows  66  is connected to the gallery line. Prior to exceeding the current amplitude SA 2 , the air bellows  66  is connected to the gallery line ( FIG. 3 ( b ) ). After exceeding the current amplitude SA 2 , the air bellows  65  is connected to the gallery line ( FIG. 3 ( c ) ). Left-right control is optionally achieved with the series circuit shown in  FIG. 3  by current control and a suitable design. 
     The switching currents of the individual switching stages can be adjusted in individual learning steps. Learning the current amplitudes SA 1 , SA 2 , SA 3 , SA 4  as relevant switching points for an on/off switching state transition of the primary valve I (e.g., first control valves  11 ,  18  of  FIG. 1  and  FIG. 2  or primary valve  61  of  FIG. 3 ) or of secondary valve II (e.g. second control valves  13 ,  19  of  FIG. 1  and  FIG. 2  or secondary valve  64  of  FIG. 3 ) can take place during vehicle work—even only following a component replacement. An exact determination of a point in time of the movement of a control valve armature, namely a primary armature or secondary armature, for a primary or secondary valve cannot be readily detected. Learning the switching points, however, enables exact switching of the primary valve and of the secondary valve in a dual armature solenoid valve assembly  120 ,  130 ,  120 ′,  130 ′. A learning procedure can advantageously be provided as a diagnostic task in the vehicle controller ECU in a module for current control, i.e., it can be stored in the ECU. A possible sequence can be: 
     1. For a current amplitude greater than SA 2 : switch on primary valve I and secondary valve II and pressurize air bellows to a pressure of, e.g., 6 bar. 
     2. For a current amplitude less than SA 4 : switch off primary valve I and secondary valve II and change to the de-energized state. 
     3. Vent gallery line  12  once and measure the pressure with a pressure sensor  26 . 
     4. Slowly increase the current amplitude SA for primary valve I and secondary valve II and continuously measure the pressure using pressure sensor  26  to determine when, for an initial pressure rise, the current amplitude SA 1  of the primary valve is reached, i.e., the primary valve is opened and connects the corresponding branch line to the corresponding air bellows, which can be recorded from the initial pressure increase. 
     5. Pressurize the gallery line  12  with the primary valve open, e.g., to 10 bar. 
     6. Vent the gallery line with the primary valve open and continuously measure the pressure using the pressure sensor  26 . 
     7. Reduce the current amplitude SA and continuously measure the pressure using the pressure sensor  26 . As a result, the current amplitude SA 3  of the primary valve I is reached if the pressure does not reduce further. A pressure that does not reduce further is the result of the primary valve I changing from the open switching state into the closed switching state. The current amplitudes SA 1 , SA 3  of the switching currents of the primary valve are determined by steps  4  and  7 . 
     8. With the switched current amplitude SA 1  and the gallery line  12  and the air bellows thereby connected via the primary valve I, vent once to, e.g., 1 bar. 
     9. Increase the current amplitude SA further and continuously measure the pressure using pressure sensor  26 . If a pressure increase is observed, the current amplitude SA 2  is reached to switch on the secondary valve II. Because the secondary valve II opens, this is noticeable through the increase. The pressure increase is established by the air bellows that are connected to each other via the open primary valve and secondary valve I, II. 
     The current amplitude SA 4  does not have to be explicitly determined, because the closure of the primary and secondary valves I, II can be achieved simply by turning off the current amplitude SA. The current amplitudes SA 1  through SA 4  for switching points exactly determinable in this way can be used for the further optimized functional operation of the primary valve and secondary valve. 
     Accordingly, the present invention provides an air suspension installation  110  for a vehicle  100  having a pneumatic installation  200  designed for operation with a compressed air supply installation  2 , which comprises a pneumatic line  3  having a connection to the compressed air supply installation  2 , a plurality of air bellows  43 ,  45 ,  47 ,  49 , each used as a pressure chamber for a pneumatic spring, wherein an air bellows is connected via a control valve in the form of a solenoid valve to the pneumatic line  3 , and wherein the air bellows can be pressurized or vented depending on a switching state of the control valve. And, a first control valve  11 ,  18  and a second control valve  13 ,  19  form a controllable solenoid valve assembly  120 ,  130 , which comprises a pneumatic part that can be operated by a magnetic part, wherein the first control valve  11 ,  18  forms a primary valve and the second control valve  13 ,  19  forms a secondary valve of the solenoid valve assembly  120 ,  130 , a first air bellows  43 ,  47  and a second air bellows  45 ,  49  of the solenoid valve assembly  120 ,  130  is each assigned to and can be pressurized or vented depending on a switching state of the primary and secondary valve, and wherein the primary valve and the secondary valve can be controlled by a controller of the magnetic part of the solenoid valve assembly  120 ,  130  common to both and acting jointly on both. 
     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 there-between.