This application is a 35 U.S.C. § 371 National Stage Application of PCT/DE2018/000282, filed on Oct. 3, 2018, which claims the benefit of priority to Serial No. DE 10 2017 009 374.1, filed on Oct. 10, 2017 in Germany, the disclosures of which are incorporated herein by reference in their entirety.

TECHNICAL FIELD

The disclosure relates to a device and to a method for controlling the throughflow of blow-molding fluid during the blow molding of containers.

BACKGROUND

The use of pneumatically pilot-controlled valves in controllers for pneumatic drives is, in certain fields of application, subject to requirements in respect of operational safety, which requirements are specified, for example, in provisions of Directive 2006/42/EG (Machine Directive) or the safety-related standard EN ISO 13849. For example, in door controllers in machine tools, an unexpected movement of the working cylinder has to be reliably prevented during manual intervention by the operator. In accordance with ISO 13849-2:2012, this results in a requirement for the fault exclusion “automatic change in the initial switching position without an input signal” for valves, amongst others.

For the purpose of controlling pneumatic drives, for example a double-acting pneumatic working cylinder, the prior art discloses the use of electropneumatically pilot-controlled valves with an electrically directly operated pilot stage (pilot-control valve, pilot valve) and a main stage (main valve) which is indirectly pneumatically operated by means of the pilot stage. Pneumatically pilot-controlled valves of this kind are also called multistage valves and comprise, for example, an electrically operated 3/2-way pilot-control solenoid valve of seat-type valve construction (also called a “pilot solenoid valve”) with mechanical spring resetting as the pilot stage and a 5/2-way spool valve of longitudinal or piston spool construction, which is pneumatically operated likewise against a mechanical spring, as the main stage. The design of electropneumatically pilot-controlled valves of this kind is evident, for example, from the prior art disclosed in documents EP 0 846 873 A2 and EP 0 463 394 B1. Here, the electrically operated 3/2-way pilot-control solenoid valve, as a pilot stage, passes the control air which is applied to its input to the likewise spring-loaded longitudinal or piston spool of the main stage. The control air can be drawn from the pilot-control valve either internally by means of the compressed air connection of the multistage valve (that is to say the compressed air supply, which is switched by the main stage, to the working connections of the drive) or externally via a separate control air connection. External supply of the control air is used, for example, if the main stage is intended to switch only very low pressures which themselves are inadequate for operating the drive piston. When used in safety-related applications, pilot-controlled valves of this kind have the disadvantage that possible fracture of the spring of the pilot-control valve reduces its closing force and the applied air pressure can push against the pilot-control valve and thereby control air can reach the drive spool or piston of the main stage. As a result, the main stage can switch and unexpectedly automatically influence the pneumatic drive.

In order to overcome this disadvantage for safety-related functions, it is generally known in the prior art to employ the option of external supply of the control air for redundant switching. A redundant valve arrangement of this kind known in the prior art comprising the two valves101and102is illustrated in the circuit diagram ofFIG. 10in its unoperated (unenergized) initial position with the valves101and102in their inoperative position. The two valves101and102are designed with mechanical spring resetting in their pilot stage and main stage in each case. The valve101, which is configured as an electropneumatically pilot-controlled 5/2-way valve, is arranged immediately upstream of the working connections103and104which it alternately oppositely connects to the compressed air connection105and one of the compressed air outputs106or107(also called exhaust air connections) in an inoperative position and a switching position. The two chambers of a double-acting pneumatic drive which is connected to the working connections103and104are alternately oppositely influenced and vented by means of the valve101, that is to say in its two switching states (inoperative position on the one hand and switching position on the other hand). The valve102, which is configured as an electropneumatically pilot-controlled 3/2-way valve102comprising the pilot-control valve110, is connected upstream of the control air supply of the pilot-control valve108, which pilot-control valve provides the control air which is required for influencing the main stage109of the valve101. The pilot-control valve110draws the control air, which is switched by it, internally by means of the compressed air connection111. The valve102releases the compressed air supply for the pilot-control valve108of the valve101in its switching state and blocks said compressed air supply in its inoperative state. Therefore, it is always necessary to switch both valves101and102at the same time in order to be able to cause a change in state at the working connections103and104. A circuit of this kind has the advantage that a spring fracture in one of the two pilot-control valves108or110without an input signal cannot alone lead to a change in state at the working connections103and104. Therefore, a spring fracture in the pilot-control valve110may possibly lead to switching of the main stage112of the valve102, which would lead to a control pressure being provided at the pilot-control valve108, but since said pilot-control valve does not switch owing to the lack of an electrical control signal, the valve101does not change its switching state. Conversely, a spring fracture in the pilot-control valve108cannot alone lead to switching of the valve101either because no control pressure is applied to the pilot-control valve108since the valve102does not switch on account of the lack of an electrical control signal. However, one disadvantage of a circuit of this kind is that a spring fracture in only one of the two pilot-control valves108or110during operation cannot be respectively identified. Therefore, in these two cases, the two valves101and102would switch when an electrical control signal is applied because the pilot-control valves108and110are electromagnetically directly switching and each change their position even without an opposing force. Furthermore, in these two cases, the valve101would also always switch back again when the control signals are removed because either no control pressure, which counteracts the spring-loaded main stage109, is applied (=fracture of the spring of the pilot-control valve108) owing to the valve102(with an undamaged return spring) returning to its inoperative position on the pilot-control valve108or, in the other case, the undamaged valve101returning to its inoperative position in any case (=fracture of the spring of the pilot-control valve110). Since the valve101therefore returns to its inoperative position in both fault situations, the working connections103and104each also change state again (opposite venting/influencing), and therefore a pneumatic drive which is connected to the working connections103and104would also change state again. The operation of a double-acting pneumatic drive which is connected to the working connections103and104would therefore continue in an unaffected manner—inasmuch as is perceivable from the outside—in both fault situations, and therefore individual faults of this kind during operation remain unidentified without additional devices. With the aid of considerations which are routine in the art, this could be achieved, for example, by the integration of appropriate electronic monitoring measures, for example the use of position sensors for checking the switching position of the valves and delayed circuit signals and also appropriate evaluation within a superordinate controller, but this is associated with corresponding expenditure on design and implementation which significantly increases the costs of the actual valve function in practice.

WO 03/004194 A1 discloses a valve arrangement with two automatically resetting main valves which are connected in series and pilot-control valves which are respectively associated with said main valves and can serve, for example, to drive a double-acting pneumatic cylinder. The main valves are arranged in an inoperative position and a switching position for the purpose of alternately oppositely influencing and venting the two chambers of the working cylinder, wherein in each case both main valves have to switch to assume the switching position. For the purpose of implementing a safety function, the main valves are each designed for operating switches which allow electrical operation of the electrically operated pilot-control valves by means of external relays. Electrical operation takes place only when both main valves are in their inoperative position and the switches are closed. If, when the main valves are reset, a fault occurs by way of one of the main valves not being returned, the electrical circuit remains interrupted, as a result of which renewed operation is not possible. This requires the integration of an appropriate electrical circuit with switches and relays, and this generates corresponding expenditure on design and costs.

DE 10 2007 041 583 A1 discloses a valve arrangement comprising a first main valve, which is driven by a first pilot-control valve, and a second main valve, which is driven by means of a second pilot-control valve, which first main valve and second main valve are interconnected in such a way that, when the two main valves are driven at the same time by means of the pilot-control valves, a switching process from a default position to a working position takes place, as a result of which two working connections are respectively alternately oppositely influenced and vented in the default position and in the working position. In order to realize a safety function for preventing a change in load at the working connections when only one of the two main valves is driven, a pneumatic circuit comprising two changeover valves, in each case arranged upstream of the pilot-control valves and having three connections in each case, is provided, said pneumatic circuit being relatively complex and requiring comparatively complicated pneumatic duct guidance and interconnection together with a correspondingly large installation space.

DE 10 2009 037 120 A1 discloses a pneumatic safety valve device comprising two bistable main valves which can each be operated by a pilot-control valve in order to be able to be switched over to a working position in which they have the effect that a pneumatic pressure is applied to two working connections. The structural design of the safety valve device causes the main valves to switch over to the working position only when the two pilot-control valves are operated substantially synchronously. If, in the event of only asynchronous operation, only one of the main valves is switched over to the working position, this fault state continues to be stored until resetting is performed by means of a separate resetting valve device. The pneumatic interconnection of the safety valve device is, on account of its intended purpose for providing pneumatic fault storage, relatively complex and requires comparatively complicated pneumatic duct guidance and interconnection together with a correspondingly large installation space.

SUMMARY

The disclosure is based on the object of avoiding the disadvantages outlined. In particular, the intention is to create a structurally simple valve arrangement for reliably controlling pneumatic drives which provides protection against a sudden automatic change in the initial switching position without an input signal in the event of a fault in a resetting device of a pilot stage and, for this situation, enables effective fault identification by purely pneumatic means.

According to the disclosure, the object is achieved by a valve arrangement as claimed in claim1, advantageous embodiments being described in the dependent claims.

The core of the disclosure is formed by a valve arrangement, comprising a first and a second working connection, which can be connected to a pneumatic drive, and a first and a second, in each case electropneumatically pilot-controlled directional valve, in which valve arrangement one or both directional valves is or are arranged upstream of the working connections for the purpose of influencing and venting said working connections, wherein the pilot stages of both directional valves are of automatically resetting design and the second directional valve is designed for alternately assuming an inoperative position and a switching position and the pilot stage of the first directional valve has an external control connection which can be influenced by means of the second directional valve in its switching position and can be vented by means of said second directional valve in its inoperative position, wherein the second directional valve has, as a resetting device for the main stage, an air spring which can be influenced and can be vented externally by means of the first directional valve, and a change in state between influencing or venting of the air spring of the second directional valve after the first directional valve assumes a switching position takes place only depending on the change in the switching state of the first directional valve, and a change in state between influencing or venting at one of the two working connections after previous influencing or venting which took place with the second directional valve assuming the switching position takes place only depending on the second directional valve assuming the inoperative position. The valve arrangement provides a structurally simple controller for a double-acting pneumatic drive, which controller provides effective protection against a sudden automatic change in the initial switching position without an input signal in the event of a fault in a resetting device of a pilot stage and which, for this situation, also at the same time renders possible effective fault identification by purely pneumatic means. During operation, the valve arrangement causes alternating influencing and venting of the working connections and therefore control of a double-acting pneumatic drive, which is connected to the working connections, in both of its directions of movement. On account of the redundant arrangement of the two directional valves, initially the fundamental fault exclusion that a fault in the resetting device of one the two pilot stages (for example a spring fracture in a pilot-control valve) does not lead to an unintended change in state at the working connections is ensured. It is always necessary to switch both directional valves in order to be able to cause a change in state (opposite venting/influencing) at the working connections. A fault in the pilot stage of the second directional valve in the unoperated (unenergized) inoperative position can lead to switching of its main stage, which would lead to a control pressure being provided at the control connection of the first directional valve, but since said first directional valve does not switch owing to the lack of an electrical control signal, the first directional valve does not change its switching state. Conversely, a fault in the pilot stage of the first directional control valve in the unoperated inoperative position also cannot lead to switching of its main stage because no control pressure is applied to its pilot stage since the second directional valve does not switch without an electrical control signal. However, furthermore, the valve arrangement has the further advantage that a fault in one of the two pilot stages of the two directional valves during operation is reliably identified from the outside. For example, the two directional valves initially switch normally when electrical control signals are applied in the event of a fault in the resetting device in one of the two pilot-control valves because the pilot stages which are operated electrically (for example by switching magnets) each also change their position even without an opposing force of an automatic resetting device (for example a mechanical spring). However, in these fault situations, a pneumatic drive which is connected to the drive connections would not return again when the electrical control signals are removed once again. This is because, on account of the crosswise interconnection, both directional valves must leave their previously assumed switching state again (switch back) so that a renewed change in state (reversed opposite influencing/venting of the working connections) can occur at a connected pneumatic drive. If only the first directional valve changes its switching position when the electrical input signal is removed (=fault in the resetting device of the pilot stage of the second directional valve), no renewed change in state occurs at the connected cylinder because no renewed change in state between influencing or venting takes place at a working connection which took place and was influenced or vented previously with the second directional valve assuming the switching position. In this case, the main stage of the second directional valve cannot return to its inoperative position when the first directional valve influences the air spring either because, on account of the defective pilot-control valve which cannot switch back, a control pressure which counteracts the return movement of the main stage continues to be applied (which control pressure is not externally controlled in contrast to the manner of operation of the first directional valve). Consequently, the state of a chamber of the pneumatic drive, which chamber is connected to the drive connection in question, also cannot change from influencing to venting, or vice versa; the movement of the pneumatic drive is blocked. The pneumatic drive cannot reverse; the fault is identified. In the converse case (=fault in the resetting device of the pilot stage of the first directional valve), neither the first nor the second directional valve return to their inoperative position because they block each other. The main stage of the first directional valve cannot switch back as long as the second directional valve has not switched back because a control pressure, which counteracts the return movement of its main stage, is still applied by means of the control connection and the defective pilot stage of the first directional valve. In turn, the main stage of the second directional valve cannot switch back as long as the main stage of the first directional valve has not switched back because the air spring which is influenced externally by means of the first directional valve in its inoperative position does not build up any pressure. Since there is no renewed change in state at the working connections and therefore no renewed change in state (reversed opposite influencing/venting) occurs at a connected pneumatic drive either, a pneumatic drive which is connected to the working connections cannot reverse and the fault is identified. Therefore, a pneumatic drive which is connected to the working connections cannot change its state in either of the fault situations. Consequently, both fault situations can be identified from the outside on the basis of the unchanged position of the drive after switching.

In a structurally simple refinement with commercial and cost effectively available pneumatic components, the main stages of the two directional valves are configured with a spool construction and/or the pilot stages of the two directional valves are configured with a seat-type construction.

Proceeding from the basic configuration of the valve arrangement, different valve functions are realized in different detailed embodiments:

In a structurally simple embodiment for alternately oppositely influencing and venting the chambers of a double-acting pneumatic drive (for example of a double-acting cylinder), the first directional valve is designed for alternately assuming an inoperative position and a switching position with an automatically resetting main stage, wherein the second directional valve connects the external control connection of the first directional valve, in its switching position, to a compressed air source via a control line and, in its inoperative position, to a compressed air output and its air spring is influenced by means of the first directional valve in its inoperative position and is vented by means of said first directional valve in its switching position, and wherein the first directional valve is arranged upstream of the two working connections and, in the switching position, connects the first working connection to a compressed air source and connects the second working connection to a compressed air output and, in the inoperative position, connects the second working connection to a compressed air source and connects the first working connection to the control line via a connecting line, wherein a check valve which provides blocking in the opposite direction is arranged in the connecting line and/or a throttle device is or are arranged upstream of the compressed air connection of the second directional valve. This embodiment of the valve arrangement provides a structurally simple controller for a double-acting pneumatic drive, which controller provides effective protection against a sudden automatic change in the initial switching position without an input signal in the event of a fault in a resetting device of a pilot stage and, for this situation, also renders possible effective fault identification by purely pneumatic means. During operation, the valve arrangement causes, in the parallel inoperative positions and switching positions of the two directional valves, alternating opposite influencing and venting of the working connections and therefore control of a double-acting pneumatic drive, which is connected to the working connections, in both of its directions of movement. On account of the redundant arrangement of the two directional valves, initially the fundamental fault exclusion that a fault in the resetting device of one the two pilot stages (for example a spring fracture in a pilot-control valve) does not lead to an unintended change in state at the working connections is ensured. It is always necessary to switch both directional valves together in order to be able to cause a change in state (opposite influencing/venting) at the working connections. A fault in the resetting device of the pilot stage of the second directional valve in its unoperated (unenergized) inoperative position can lead to switching of its main stage, which would lead to a control pressure being provided at the control connection of the first directional valve, but since said first directional valve does not switch owing to the lack of an electrical control signal, the first directional valve does not change its switching state. At the same time, in this fault situation, the check valve or the throttle device prevent or delay influencing of the first working connection via the control line, the connecting line and the first directional valve, which is in the inoperative position, either entirely or in any case in such a way that this cannot lead to a dangerous—sudden—movement of a pneumatic drive which is connected to the working connection. If the valve arrangement is designed only with a throttle device which is arranged upstream of the compressed air connection of the second directional valve, instead of with a check valve, a change in state (influencing) of the first working connection is not completely suppressed in each case in the event of a fault in the resetting device of the pilot stage of the second directional valve in the unoperated inoperative position—depending on the condition of a pneumatic drive which is connected to the working connections. However, since the second working connection is influenced by means of the first directional valve and the compressed air source at the same time in this fault situation, there is, in principle, a counterpressure which counteracts the unintended change in position of a pneumatic drive which is connected to the working connections. However, in the event of any force differences which are produced depending on the condition of a pneumatic drive which is connected to the working connections, it is ensured, on account of the throttle device, that a change in position can occur at most at a considerably reduced speed, which generally satisfies existing practical stipulations for operational safety and at the same time likewise ensures the ability to identify the fault. Conversely, a fault in the resetting device of the pilot stage of the first directional valve in the unoperated (unenergized) inoperative position also cannot lead to switching of its main stage because no control pressure is applied to its pilot stage since the second directional valve does not switch without an electrical control signal. However, furthermore, the valve arrangement has the further advantage that a fault in the resetting device of one of the two pilot stages (for example a spring fracture in one pilot-control valve) of the two directional valves during operation is reliably identified. In both of these cases, the two directional valves initially switch normally when electrical control signals are applied because the electrically operated pilot stages each also change their position without an opposing force of the resetting devices (for example mechanical springs). The first directional valve connects, in the switching position, the first working connection to the compressed air source and the second working connection to a compressed air output; a pneumatic drive which is connected to the working connections changes its position. However, in these fault situations, a pneumatic drive which is connected to the drive connections would not reverse again when the electrical control signals are removed once again. This is because, on account of the crosswise interconnection, both directional valves always have to switch back so that a renewed change in state (reversed opposite influencing/venting of the working connections) can occur at the connected cylinder. If only the first directional valve returns the inoperative position without an electrical input signal (=fault in the resetting device of the pilot stage of the second directional valve), no renewed change in state occurs at the connected cylinder because either the check valve (if present) or the first working connection (if only a throttle device is present) is still influenced by means of the second directional valve and the first working connection and the associated chamber of the pneumatic drive are not vented. In this fault situation, the main stage of the second directional valve cannot return to its inoperative position in spite of influencing of the air spring because a control pressure which counteracts the return movement of the main stage of said second directional valve is still applied by means of the defective pilot stage (which control pressure is not externally controlled in contrast to the manner of operation of the first directional valve). If the valve arrangement is designed only with a throttle device which is arranged upstream of the compressed air connection of the second directional valve, instead of with a check valve, likewise no renewed change in state occurs at the connected cylinder because the first working connection and the associated chamber of the pneumatic drive are still influenced by means of the second directional valve and are not vented. There is, in principle, a counterpressure which counteracts the change in position of a pneumatic drive which is connected to the working connections. In this fault situation, a movement of a pneumatic drive which is connected to the working connections is not completely suppressed in every case—depending on the condition of the pneumatic drive. However, in the event of any force differences which are produced depending on the condition of a pneumatic drive which is connected to the working connections, it is ensured, on account of the throttle device, that a change in position can occur at most at a considerably reduced speed, which generally satisfies existing practical stipulations for operational safety and at the same time likewise ensures the ability to identify a fault. In the converse case (=fault in the resetting device of the pilot stage of the first directional valve), neither the first nor the second directional valve return to their inoperative position because they block each other. The main stage of the first directional valve cannot switch back as long as the second directional valve has not switched back because a control pressure, which counteracts the return movement of its main stage, is still applied by means of the external control connection. In turn, the main stage of the second directional valve cannot switch back as long as the main stage of the first directional valve has not switched back because the air spring which is influenced externally by means of the first directional valve only in its inoperative position does not build up any pressure. Since no renewed change in state occurs at the working connections, a pneumatic drive which is connected to the working connections cannot reverse and the fault is identified.

In a structurally simple refinement of the above embodiments, the second directional valve is configured as a 3/2-way valve and is designed, as a resetting device for the main stage, with an air spring which can be influenced and can be vented externally by means of the first directional valve.

In a structurally simple refinement of the above embodiments, the second directional valve is configured as a 4/2-way valve and is designed, as a resetting device for the main stage, with an air spring which can be influenced and can be vented externally by means of the first directional valve.

In an alternative embodiment for alternately oppositely influencing and venting the chambers of a double-acting pneumatic drive (for example a double-acting cylinder) with modified duct guidance, the first directional valve is designed for alternately assuming an inoperative position and a switching position with an automatically resetting main stage, wherein the second directional valve connects the external control connection of the first directional valve, in its switching position, to a compressed air source via a control line and, in an inoperative position, to a compressed air output and its air spring is influenced by means of the first directional valve in its inoperative position and is vented by means of said first directional valve in its switching position, and wherein the first directional valve is arranged upstream of the first working connection and, in the switching position, connects said first working connection to a compressed air source and, in the inoperative position, connects said first working connection to a compressed air output, and wherein the second directional valve is arranged upstream of the second working connection and, in the inoperative position, connects said second working connection to a compressed air source and, in the switching position, connects said second working connection to a compressed air output. In this embodiment, it is possible to dispense with the arrangement of a check valve or a throttle device which is arranged upstream of the compressed air connection of the second directional valve, while maintaining the desired safety features, on account of the modified duct guidance. Owing to the redundant arrangement, both the pilot stage of the first directional valve, which pilot stage is designed with the external control connection, and also the pilot stage of the second directional valve have to switch in order to be able to cause a change in state (opposite venting/influencing) at the two working connections. In the event of a fault in the resetting device of the pilot stage of the second directional valve, no change in state (opposite venting/influencing) occurs at the two working connections because the pilot stage of the first directional valve does not switch without an electrical control signal. The fault in the resetting device of the pilot stage of the second directional valve can lead to switching of its main stage, but this only leads to additional venting of the second working connection as well. In this case, the first working connection continues to be vented by means of the first directional valve which remains in its inoperative position. A pneumatic drive which is connected to the two working connections stays in its position. No change in state (opposite venting/influencing) occurs at the two working connections in the event of a fault in the resetting device of the pilot stage of the first directional valve either because no control pressure is applied to its pilot stage since the second directional valve does not switch without an electrical control signal. The first working connection continues to be vented by means of the first directional valve; the second working connection is influenced by means of the second directional valve. A pneumatic drive which is connected to the two working connections stays in its position. In this embodiment, the valve arrangement furthermore also has the further advantage that a fault in the resetting device of one the two pilot stages of the two directional valves is reliably identified during operation in each case. In these fault situations, a pneumatic drive which is connected to the working connections would not reverse again when the electrical control signals are removed. This is because, on account of the crosswise interconnection, both directional valves always have to switch back so that a renewed change in state (reversed opposite influencing/venting of the working connections) can occur at the connected cylinder. If only the first directional valve returns the inoperative position, after previously assuming the switching position, without an electrical input signal (=fault in the resetting device of the pilot stage of the second directional valve), no renewed change in state occurs at the connected cylinder because the second working connection is still vented by means of the second directional valve. In this case, the first working connection is also vented by means of the first directional valve which is returned to its inoperative position. A pneumatic drive which is connected to the two working connections stays in the position it last assumed. In the converse case (=fault in the resetting device of the pilot stage of the first directional valve), neither the first nor the second directional valve return to their inoperative position because they block each other. The main stage of the first directional valve cannot switch back as long as the second directional valve has not switched back because a control pressure, which counteracts the return movement of its main stage, is still applied by means of the external control connection. In turn, the main stage of the second directional valve cannot switch back as long as the main stage of the first directional valve has not switched back because the air spring which is influenced externally by means of the first directional valve only in its inoperative position does not build up any pressure. The first working connection continues to be influenced by means of the first directional valve and the second working connection continues to be vented by means of the second directional valve. Since no renewed change in state (opposite venting/influencing) occurs at the working connections, a pneumatic drive which is connected to the working connections cannot reverse and the fault is identified.

In a structurally simple refinement of the above embodiments, the first directional valve is configured as a 5/2-way valve.

In an alternative embodiment for rendering possible both-way venting of the two working connections, the first directional valve is configured as a both-way electropneumatically pilot-controlled 5/3-way valve with a both-way automatically resetting main stage and is designed for assuming a vented central position, as the inoperative position, and also a first and a second switching position, wherein the first switching position is assumed in the event of operation and influencing of the pilot stage which is designed with the external control connection, and wherein the second directional valve connects the external control connection of the first directional valve, in its switching position, to a compressed air source via a control line and, in an inoperative position, to a compressed air output and its air spring is influenced by means of the first directional valve in its second switching position and is vented by means of said first directional valve in its first switching position and inoperative position, and wherein the first directional valve is arranged upstream of the first working connection and, in the first switching position, connects said first working connection to a compressed air source and, in the second switching position and the inoperative position, connects said first working connection to a compressed air output, and wherein the second directional valve is arranged upstream of the second working connection and, in the inoperative position, connects said second working connection to a compressed air source and, in the switching position, connects said second working connection to a compressed air output. In this embodiment, it is possible to dispense with the arrangement of a check valve or a throttle device which is arranged upstream of the compressed air connection of the second directional valve, while maintaining the desired safety features, on account of the modified duct guidance. Owing to the redundant arrangement, both the pilot stage of the first directional valve, which pilot stage is designed with the external control connection, and also the pilot stage of the second directional valve have to switch in order to be able to cause a change in state (opposite venting/influencing) at the two working connections. In the event of a fault in the resetting device of the pilot stage of the second directional valve, no change in state (opposite venting/influencing) occurs at the two working connections because the pilot stage of the first directional valve does not switch without an electrical control signal. The fault in the resetting device of the pilot stage of the second directional valve can lead to switching of its main stage, but this only leads to additional venting of the second working connection as well. In this case, the first working connection continues to be vented by means of the first directional valve which remains in its inoperative position. A pneumatic drive which is connected to the two working connections stays in its position. No change in state (opposite venting/influencing) occurs in the initial position at the two working connections in the event of a fault in the resetting device of the pilot stage of the first directional valve either because no control pressure is applied to its pilot stage since the second directional valve does not switch without an electrical control signal. The first working connection continues to be vented by means of the first directional valve; the second working connection is influenced by means of the second directional valve. A pneumatic drive which is connected to the two working connections stays in its position. In this embodiment, the valve arrangement furthermore also has the further advantage that a fault in the resetting device of one of the two pilot stages of the two directional valves is reliably identified during operation. In these fault situations, a pneumatic drive which is connected to the working connections would not reverse again when the electrical control signals are removed. This is because, on account of the crosswise interconnection, both directional valves always also have to change their previously assumed switching state again so that a renewed change in state (reversed opposite influencing/venting of the working connections) can take place at a connected pneumatic drive and the drive can reverse again. The renewed reversed opposite influencing/venting of the working connections after opposite influencing and venting which took place previously with the first directional valve assuming the first switching position and the second directional valve assuming the switching position takes place only with the first directional valve assuming the second switching state together with the second directional valve switching back.

In a structurally simple refinement of the above embodiments, the second directional valve is configured as a 5/2-way valve and is designed, as a resetting device for the main stage, with an air spring which can be influenced and can be vented externally by means of the first directional valve, provided that no refinement as a 3/2-way valve or 4/2-way valve is provided.

In order to increase the vibrational and operational stability, the resetting device of the main stage of the first directional valve is designed parallel with a mechanical spring and an air spring, wherein the air spring is influenced externally by means of the second directional valve in its inoperative position and is vented in its switching position, provided that the first directional valve is designed as a directional valve for assuming two switching states (an inoperative position and a switching position). Here, the second directional valve is configured as a 5/2-way valve.

An additional valve function is achieved by a particular control method, in which the pilot stages of the two directional valves can be both jointly and also individually electrically switched, provided that, in one of the above embodiments of the valve arrangement, the first directional valve is designed for alternately assuming an inoperative position and a switching position with an automatically resetting main stage and is configured as a 5/2-way valve, and is arranged upstream of the two working connections and, in the switching position, connects the first working connection to a compressed air source and connects the second working connection to a compressed air output and, in the inoperative position, connects the second working connection to a compressed air source and connects the first working connection to the control line via a connecting line, wherein a throttle device is arranged upstream of the compressed air connection of the second directional valve and there is no check valve arranged in the connecting line. As a result, with respect to the working connections, the valve arrangement can be controlled overall as a 5/3-way valve with an open central position (both working connections influenced). Here, the control position, which corresponds to a 5/3-way valve in its open central position, corresponds to the switching position only of the second directional valve (while the first directional valve is in the inoperative position).

An additional valve function is achieved by a particular control method, in which the pilot stages of the two directional valves can be both jointly and also individually electrically switched, provided that, in one of the above embodiments of the valve arrangement, the first directional valve is designed for alternately assuming an inoperative position and a switching position with an automatically resetting main stage and is configured as a 5/2-way valve, and is arranged upstream of the first working connection and, in the switching position, connects said first working connection to a compressed air source and, in the inoperative position, connects said first working connection to a compressed air output, while the second directional valve is arranged upstream of the second working connection and, in the inoperative position, connects said second working connection to a compressed air source and, in the switching position, connects said second working connection to a compressed air output. As a result, with respect to the working connections, the valve arrangement can be controlled overall as a 5/3-way valve with a vented central position (both working connections vented). Here, the control position, which corresponds to a 5/3-way valve in its vented central position, corresponds to the switching position only of the second directional valve (while the first directional valve is in the inoperative position).

