Abstract:
For a valve arrangement of a pneumatically actuated, spring-loaded control cylinder, a non-return valve in pneumatic communication with the control cylinder piston chamber. At very low backpressures in the control cylinder piston chamber caused by leaks of the valve responsible for pressurization of the piston chamber, the chamber is vented by passage of a very small air stream via the non-return valve. In this way, leaks cannot cause a very slow pressure buildup in the control cylinder piston chamber, followed at some time by undesired shifting of the control cylinder. In contrast, in the case of normal pressure buildup, i.e., during switching of the pressurizing valve, the non-return valve is promptly closed and the pressure in the control cylinder piston chamber is not influenced.

Description:
BACKGROUND OF THE INVENTION 
   The present invention is directed generally to a valve device for a control cylinder, which is preferably of the type used for electronically controlled pneumatic actuation of the clutch of a motor vehicle. 
   In conventional single-acting pneumatic control cylinders in which the pneumatic piston is shifted into its initial position (zero position of the piston rod fixed to the piston) via a return spring, a specified actuation position is established by virtue of the pneumatic pressure prevailing in the piston chamber of the control cylinder. This means that the piston rod of the control cylinder is extended by a specified travel distance compared with its zero position. The air pressure in the control cylinder piston chamber determines the position of the control cylinder piston rod; the air pressure is lowered for retraction of the piston rod toward its zero position and raised for extension toward its maximum position allowed by the cylinder length. 
   The air pressure in the control cylinder piston chamber is varied by means of valves. In the simplest case, a switching pressurizing valve raises the air pressure and a switching venting valve lowers the air pressure. 
   For application of the control cylinder as an electronically controlled, pneumatically actuated clutch control cylinder as mentioned above, the pressurizing and venting valves are designed as electrically switched valves; the air pressure in the control cylinder piston chamber being varied as desired by the switching of these valves. For precision adjustment of a specified pressure or for establishing a specified pressure gradient, such as in the clutch-engagement process, the valves are actuated in a pulsed mode. 
   The control cylinder is connected to the motor vehicle clutch (which can be a push-type or pull-type clutch) in such a way that the motor vehicle clutch is completely disengaged in the piston rod zero position corresponding to a piston chamber pressure of zero. During an increase in the piston chamber pressure, the piston rod becomes extended, engagement begins at a specified piston rod position (clutch engagement point) and, beginning with a further specified position, the clutch is then completely engaged. 
   In the zero position of the control cylinder piston rod, in which, as explained, the piston chamber of the cylinder is depressurized and, also, the two valves are in unactuated position, it is possible that small leaks in the pressurizing valve in communication with the supply pressure can cause a gradual pressure buildup in the control cylinder piston chamber. This pressure buildup could be accompanied by undesired extension of the control cylinder piston rod from zero position, which could potentially lead to undesired engagement of the motor vehicle clutch, with the result that the vehicle might experience undesired movement under certain circumstances. 
   Undesired pressure buildup can be prevented by occasional actuation of the venting valve. For this purpose, however, the control electronics would require additional programming which may not be consistent with the program that controls the desired switching processes of the valve. Furthermore, additional functions may be required, for example, pressure sensing, that may not be needed for other purposes. Moreover, the system would then have to be continuously energized (current consumption, battery discharge). Such a solution is therefore not particularly advantageous. 
   SUMMARY OF THE INVENTION 
   Generally speaking, in accordance with the present invention, a valve device is provided which avoids the foregoing disadvantages associated with prior art devices and arrangements. 
   A valve device for pressurizing or venting the piston chamber of a control cylinder (such as is used, for example, to actuate the clutch of a motor vehicle) according to a preferred embodiment of the present invention includes at least one solenoid-actuated multi-way pressurizing valve and at least one solenoid-actuated multi-way venting valve. Preferably, the pressurizing and venting valves include at least 2/2 ways. The pressurizing valve has a port in communication with a supply pressure and another port in communication with the control cylinder piston chamber; the venting valve has a port in communication with a vent and another port in communication with the control cylinder piston chamber. Both the pressurizing valve and the venting valve are in closed position when deenergized. 
   The valve device further includes at least one non-return valve. The non-return valve has a pneumatic inlet in communication with the control cylinder piston chamber. The non-return valve is constructed and arranged to assume an unactuated position when the pneumatic inlet is depressurized, and an actuated position when the pneumatic inlet is pressurized. In unactuated position, the non-return valve has a defined pressure leak. The pressure leak is established as a nominal width which corresponds to a preselected proportion of the nominal width of the pressurizing valve. 
   The non-return valve according to the present invention can be disposed separate from or in the control cylinder. If in the control cylinder, the non-return valve can be disposed in the piston or, alternatively, in the housing (including in the end wall thereof). The non-return valve can also be disposed in the pressurizing or venting valves. 
   Accordingly, it is an object of the present invention to provide a valve device which is constructed and arranged such that slight leaks in the pressurizing valve do not lead to undesired extension of the control cylinder piston rod, and which does not require additional programming to accomplish such purpose. 
   It is also an object of the present invention to provide a valve device which can be readily integrated as a component in devices that are present in any case, whereby additional assembly and connecting-line costs can be avoided. 
   Still other objects and advantages of the present invention will in part be obvious and will in part be apparent from the specification. 
   The present invention accordingly comprises the features of construction, combination of elements, and arrangement of parts which will be exemplified in the constructions hereinafter set forth, and the scope of the invention will be indicated in the claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a fuller understanding of the invention, reference is had to the following description taken in connection with the accompanying drawings in which: 
       FIG. 1   a  is schematic diagram depicting the pneumatic valve connections for admission of compressed air to a spring-loaded control cylinder containing a valve device according to a preferred embodiment of the present invention; 
       FIG. 1   b  is an enlarged view of the inventive valve device depicted in  FIG. 1   a;    
       FIG. 1   c  is a cross-sectional view of the inventive valve device taken along lines  1   c — 1   c  in  FIG. 1   b;    
       FIG. 2  is a sectional view depicting the valve device according to a preferred embodiment of the present invention disposed in the piston of a spring-loaded control cylinder; 
       FIG. 3  is a sectional view depicting the valve device according to a preferred embodiment of the present invention alternatively disposed in the housing of a spring-loaded control cylinder; 
       FIG. 4  is a sectional view depicting the valve device according to a preferred embodiment of the present invention alternatively disposed in the venting valve of a spring-loaded control cylinder; and 
       FIG. 5   a  is an enlarged sectional view depicting a valve device in accordance with an alternative embodiment of the present invention; and 
       FIG. 5   b  is a cross-sectional view of the valve device depicted in  FIG. 5   a  taken along lines  5   b - 5   b  in  FIG. 5   a.   
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring to the drawing figures where like reference numerals are used for corresponding parts,  FIG. 1  shows a pressurizing (solenoid) valve  1  having a first port  2  in communication with a supply pressure  10  and a second port  3  in communication with a piston chamber  9  of a spring-loaded control cylinder  8 . A venting (solenoid) valve  4  is also provided having a first port  5  in communication with a vent  11  and a second port  6  also in communication with control cylinder piston chamber  9 . 
   Solenoid valves  1  and  4  are preferably provided with 2/2 ways, which represent the smallest possible number of ways for such directional multi-way control valves. It should be understood, however, that more than 2/2 ways can also be provided for these valves. 
   Solenoid-actuated valves  1  and  4  can be switched via an electronic control device (not shown in the drawings). In order to adjust pressure exactly in control cylinder piston chamber  9 , for example, during the process of clutch engagement in the application of control cylinder  8  as a clutch control cylinder, the valves can be switched in pulsed mode. 
   For such an application, it may also be advantageous to provide a further pressurizing valve and a further venting valve, each with larger nominal widths, for example, for the purpose of both rapid pressurizing and rapid venting. Except for the changed nominal width, such valves can have similar designs to those of valves  1  and  4  and can be connected in parallel therewith. In such a paired arrangement, therefore, faster or slower pressure buildup or pressure reduction can be achieved as desired by appropriate valve actuation. 
   In the event of a leak in pressurizing valve  1  (in this regard, an explanation of how a sealing seat  28  of venting valve  4 —having the same design as that of pressurizing valve  1 —can be achieved via a magnet armature sealing element  30  is set forth hereinafter in connection with FIG.  4 ), air passes from supply  10  to control cylinder piston chamber  9 . Even though the air flow is relatively small, pressure that can lead to shifting of control cylinder  8  will eventually build up in piston chamber  9 . 
   To prevent such a pressure buildup, pneumatic communication can be provided between the control cylinder piston chamber and an inlet  12  of a non-return valve  7  constructed and arranged in accordance with the present invention. 
   Non-return valve  7  preferably has two switched positions. A first, unactuated position is occupied when pneumatic inlet  12  is depressurized. In this situation, as shown in  FIG. 1   b , a sealing ball  13  is pressed by the force of a spring  19  against a first sealing seat  17  at pneumatic inlet  12 . In this position, non-return valve  7  is designed to allow a defined leak. 
   Preferably, non-return valve  7  is provided with a groove comprising a radial portion  36  and a longitudinal portion  16  (shown in cross section in  FIG. 1   c  taken along line  1   c — 1   c  in  FIG. 1   b  through the center of sealing ball  13 ) by which a small air opening to vent  11  is formed. Sealing ball  13  is housed in a cylindrical guide  14 , and groove  16  in the cylindrical guide therefore extends as far as a bore  15  at pneumatic inlet  12 . 
   A second or actuated position of non-return valve  7  is established when pneumatic inlet  12  is pressurized. Because pressure is present at pneumatic inlet  12 , and also because the leak is relatively small, a backpressure sufficient to press sealing ball  13  sealingly against a second sealing seat  18  can build up, and so inlet  12  is pneumatically shut off from vent  11 . 
   The defined leak of non-return valve  7  in its first, unactuated position is determined by the cross section of groove  36 ,  16  ( FIG. 1   c ). A leak nominal width, such as, for example, 0.3 mm can be established. 
   In designing the individual valves, the leak nominal width is preferably matched to the nominal widths of the other valves, especially that of the pressurizing valve. Preferably, the leak nominal width is small compared to the nominal width of the pressurizing valve (in the case of two pressurizing valves, the nominal width of the pressurizing valve having larger cross section is the determining factor for the leak, since a larger sealing seat also exhibits larger leaks). When the pressurizing valve is opened, a backpressure that presses sealing ball  13  against the force of spring  19  onto second sealing seat  18 , thus closing it, builds up promptly at inlet  12  of non-return valve  7 . 
   For comparison with inventive non-return valve  7 , the construction of a conventional non-return valve is now described. A conventional non-return valve is used to shut off the air stream in one flow direction and to allow it to pass in the other flow direction. In a conventional ball type non-return valve, the sealing ball is pressed via a spring against a sealing seat at the inlet. In the case of air flow directed from the inlet outward, this sealing seat is opened because air flowing in this direction lifts the sealing ball from the valve seat. On the other hand, the sealing seat remains closed in the case of air flow directed toward the inlet. 
   In comparison with the conventional non-return valve, the non-return valve  7  according to the present invention exhibits a very different functional principle. At very low backpressures at inlet  12 , the valve is opened with very small nominal width, to allow a small air stream to pass through. Even at a “normal” small backpressure, however, second sealing seat  18  is promptly occupied, and so the valve is closed. Spring  19  is therefore preferably designed with a relatively compliant spring rate, in such a way that the second sealing seat position is already occupied at a sufficiently low backpressure desired for this purpose, such as, for example, 0.2 bar. On the other hand, it should be appreciated that the ability of a non-return valve of conventional construction to shut off an air stream directed toward the inlet has no bearing for inventive non-return valve  7 . 
   It should be understood that, besides application in a spring-loaded control cylinder designed as an actuating cylinder for a motor vehicle clutch, the valve arrangement depicted in  FIG. 1  is suitable for all applications in which a gradual pressure buildup in the piston chamber of the spring-loaded control cylinder as a result of valve leaks can lead to undesired actuation of the control cylinder. 
   Considering the connections illustrated in  FIG. 1 , since inlet  12  of non-return valve  7  is in pneumatic communication with a compressed air port  21  of control cylinder piston chamber  9 , with second port  3  of pressurizing valve  1  and with second port  6  of venting valve  4 , it should be understood that non-return valve  7  can also be installed at a position other than that shown in  FIG. 1   a , in which case it would not be designed as a separate valve unit, as is the case in  FIG. 1   a . For example, non-return valve  7  can also be disposed in spring-loaded control cylinder  8 , which is advantageous because separate installation of a non-return valve is obviated and also because separate connecting lines are not required. 
   Referring to  FIG. 2 , non-return valve  7  is shown installed inside a piston  23  of spring-loaded control cylinder  8 . A piston rod  24  is also depicted. The cylinder return spring is not shown; it is disposed in the inside chamber of piston  23 . 
   It is possible to dispense with a return spring in control cylinder  8 , namely in applications in which a return spring is installed in the very device that is actuated by control cylinder  8  (for example, this is the case in certain embodiments of vehicle clutches described above). Moreover, in contrast to the embodiment of a single-acting control cylinder  8 , a control cylinder can also be provided as a double-acting control cylinder, in which a further piston chamber for retraction of piston rod  24  is provided in addition to piston chamber  9  for extension of piston rod  24 . Valves for raising and lowering the pressure are also provided for this further piston chamber. 
   Referring now to  FIG. 3 , as an alternative to the arrangement depicted in  FIG. 2 , non-return valve  7  can also be installed in a housing  22  of spring-loaded control cylinder  8 , specifically, in the end wall thereof. 
   Non-return valve  7  can also be disposed in at least one of the two solenoid-actuated multi-way valves  1  or  4 ; this is depicted in  FIG. 4  for the example of venting valve  4 . Indeed, instead of one non-return valve  7 , two non-return valves can be mounted in an arrangement of connections as depicted in FIG.  1 . 
   Referring now to  FIG. 4 , venting valve  4  preferably includes a first housing part  26  and a second housing part  27  joined to the first housing part. An armature guide tube  32  together with a magnet coil  31  is preferably disposed in first housing part  26 , and an O-ring seal  33  ensures that second port  6  of the venting valve, in pneumatic communication with control cylinder port  21 , is also sealed relative to armature guide tube  32  and, thus, relative to first housing part  26 . 
   A magnet armature  29  is preferably mounted displaceably inside armature guide tube  32 . While magnet coil  31  is not energized, the armature is pushed toward second housing part  27  by a spring (not shown) disposed in a valve pressure volume  35 , as explained in greater detail hereinafter, in such a way that a magnet armature sealing element  30  bears compliantly on a sealing seat  28  of the second housing part, thus closing the sealing seat when the magnet is not actuated (in pressurizing valve  1  of similar design, a slight leakage in the air stream to control cylinder piston chamber  9  can develop due to leaks from supply  10  via the sealing seat). 
   When magnet coil  31  is energized, magnet armature  29  is lifted toward a stationary core  37 , magnet armature sealing element  30  lifts up from sealing seat  28 , and pneumatic communication is established between first port  5  of venting valve  4  in communication with vent  11  and control cylinder piston chamber  9 , thus permitting venting of the piston chamber. Preferably, non-return valve  7  is disposed in stationary core  37  such that valve pressure volume  35  is formed between the valve and magnet armature  29 . 
   Valve pressure volume  35  is in communication with second port  6  of venting valve  4  via a pressure channel  34  in magnet armature  29 . Valve pressure volume  35  is therefore always in pneumatic communication with control cylinder piston chamber  9 , regardless of the switched position of venting valve  4 . Venting of valve pressure volume  35  is therefore synonymous with venting of control cylinder piston chamber  9  itself. 
   As indicated above, the design of pressurizing valve  1  is desirably similar to that of venting valve  4 , with the difference that first port  2  is in communication not with vent  11  but with supply pressure  10 . Just as for venting valve  4 , however, second port  3  is in communication with control cylinder piston chamber  9 , and so non-return valve  7  for venting control cylinder piston chamber  9  can also be disposed above valve pressure volume  35  of pressurizing valve  1 . 
   In the embodiment of non-return valve  7  according to  FIGS. 1   b  and  1   c , sealing ball  13  at first sealing seat  17  forms a circumferential sealing edge which forms an airtight sealing edge. As explained above, the defined leak is achieved by the cross section of radial portion  36  of the groove in the circumferential sealing edge. 
   As shown in  FIGS. 5   a  and  5   b , the defined leak can also be established by a leak built into the structure of a circumferential sealing edge  38  of first valve seat  17 . Notches  41  are provided in the valve housing around the circumference at first valve seat  17  in order to establish the defined leak (for example, eight notches  41  are provided in the preferred embodiment depicted). 
   Non-return valve  7  according to  FIG. 5   a  and  5   b  also exhibits further differences in configuration compared with non-return valve  7  according to  FIGS. 1   b  and  1   c . For example, instead of a ball there is provided a rotationally symmetric sealing member  39 , which is mounted in cylindrical guide  14  with a certain play (exaggerated in  FIG. 5   a ), which ensures the leakage function of longitudinal portion  16  of the groove depicted in  FIGS. 1   b  and  1   c.    
   Furthermore, second sealing seat  18  is desirably formed not as a seat sealing by metal-to-metal contact but as an elastomeric sealing seat. For this purpose, an elastomeric sealing cone  40  can be provided on sealing member  39 . This embodiment is particularly advantageous because of its airtight and “compliant” sealing effect. It should be understood that the sealing member can also have a different shape, such as a rotationally symmetric shape (e.g., like a torpedo or an egg). 
   Accordingly, a valve device is provided which is constructed and arranged to prevent undesired extension of the control cylinder piston rod caused by slight leaks in the pressurizing valve, and which does not require additional programming to accomplish this function. The valve device according to the present invention can be readily integrated as a component in devices that are present in any case, such that additional assembly and connecting-line costs can be avoided. 
   It will thus be seen that the objects set forth above, among those made apparent from the preceding description, are efficiently attained, and since certain changes may be made in the above constructions without departing from the spirit and scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense. 
   It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described and all statements of the scope of the invention which, as a matter of language, might be said to fall therebetween.