Abstract:
A pneumatic vacuum generator includes at least one venturi nozzle having a flow cross section which deviates for a circularity. The venturi nozzle may thus have a substantial rectangular or non-circular flow cross section like for example an oval flow cross section or an elliptical flow cross section. At least two plates are disposed in parallel relationship and joined in sandwich construction, with one of the plates constructed to accommodate the venturi nozzle.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0001]    This application claims the priority of German Patent Application, Serial No. 10 2009 047 085.9, filed Nov. 24, 2009, pursuant to 35 U.S.C. 119(a)-(d), the content of which is incorporated herein by reference in its entirety as if fully set forth herein. 
       BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to a pneumatic vacuum generator. 
         [0003]    The following discussion of related art is provided to assist the reader in understanding the advantages of the invention, and is not to be construed as an admission that this related art is prior art to this invention. 
         [0004]    Different kinds of vacuum generators are used to produce a negative pressure. In the field of automation, vacuum generators are used which generate a negative pressure using the Venturi principle. These vacuum generators are also called ejectors and require compressed air for building up the negative pressure. Prior art ejectors with cylindrical venturi nozzles or multistage ejectors with cylindrical venturi nozzles have been in use for some time. Also known are cylindrical transport ejectors that operate according to the Coanda principle and the planar Coanda principle. 
         [0005]    U.S. Pat. No. 6,394,760 describes a multistage ejector, shown in more detail in  FIG. 1   a - 1   d  and designated by reference numeral  10 . The multistage ejector  10  has four suction stages  12 ,  14 ,  16 ,  18  with cylindrical venturi nozzles  20  to  26 .  FIGS. 1   a - 1   d  show schematically, in four cross-sectional views, the ejector  10  at gradually increased vacuum levels in a vacuum chamber  28  and overall decreasing vacuum flow. In  FIG. 1   a  the ejector  10  is shown in a mode of operation in which compressed air is introduced in a direction of arrow  30  into the first venturi nozzle  20  so that air is drawn from the vacuum chamber  28  in a direction of arrow  32 . Compressed air flows also through the venturi nozzle  22  so that air is drawn in a direction of arrow  34 . The same happens also with respect to the venturi nozzles  24 ,  26  so that air is drawn in a direction of arrows  36 ,  38 , respectively. Compressed air exits the multistage ejector  10  together with the aspirated air in a direction of arrow  40  through port  42 . The total amount of suction air (arrows  44 ) enters the multistage ejector  10  via port  46 . Flap valves  48 ,  50 ,  52  in the suction stages  14 ,  16 ,  18  are all open. As a result, the vacuum flow is high.  FIG. 1   b  shows the multistage ejector  10  in an operating position in which the flap valve  52  is closed. When a particular negative pressure has been reached in the vacuum chamber  28 , the flap valve  52  closes spontaneously so that suction air is drawn only via the suction stages  12 ,  14 ,  16  in the direction of arrows  32 ,  34 ,  36 , respectively. As a result, the vacuum flow decreases while the negative pressure in the vacuum chamber increases.  FIG. 1   c  shows the multistage ejector  10  in an operating position in which the flap valve  50  is closed as a result of the still higher negative pressure has been reached in the vacuum chamber  28 . Thus, air is drawn only via the suction stages  12 ,  14  in the direction of arrows  32 ,  34 , respectively. In  FIG. 1   d , also flap valve  48  closes as a result of a still higher negative pressure in the vacuum chamber  28 , i.e. all flap valves  48 ,  50 ,  52  are now closed. Air is now drawn solely via the suction stage  12  in the direction of arrow  32 . The vacuum flow is thus further decreased, indicated by the lesser number of arrows. On the other hand, a maximum negative pressure is generated in the vacuum chamber  28 . 
         [0006]      FIG. 2  shows a conventional multistage ejector  10   a  with three suction stages  12 ,  14 ,  16  and two flap valves  48 ,  50  which assume their closed positions. Parts corresponding with those in  FIG. 1  are denoted by identical reference numerals and not explained again. Compressed air is introduced via two ports  54 , whereas outgoing air exits through two ports  42  and one port  56 , as indicated by the arrows. The mode of operation corresponds to the multistage ejector  10 , as described with reference to  FIGS. 1   a - 1   d.    
         [0007]      FIGS. 3   a  and  3   b  show by way of example a conventional Coanda ejector as disclosed in International application WO 2009/054732 A1 and designated by reference numeral  58 . The Coanda ejector  58  is made in sandwich construction and includes a top plate  60 , a bottom plate  62 , and an intermediate plate  64 . In  FIG. 3   a , the Coanda ejector  58  is of single-stage configuration, whereas in  FIG. 3   b , the Coanda ejector  58  has several parallel stages. In  FIG. 3   a , compressed air enters through port  54  in a direction of arrow  30  into the Coanda ejector  58  and is introduced tangentially via a channel  65  into a chamber  66 . As a result, air is drawn in a direction of arrows  44  through a perforated inlet  46  in the bottom plate  62  and exits the chamber  66  together with compressed air via outlet channel  67 . In the variation of  FIG. 3   b , compressed air is dispersed via a manifold  68  to several channels  65 . Thus, compressed air is split over a total of six chambers  66 . The bottom plate  62  has thus six inlets  46  to enable a gripping of a workpiece  70  over a large area. 
         [0008]    A drawback common to all prior art vacuum generators or ejectors is their bulkiness. 
         [0009]    It would therefore be desirable and advantageous to address this problem and to obviate other prior art shortcomings. 
       SUMMARY OF THE INVENTION 
       [0010]    According to one aspect of the present invention, a pneumatic vacuum generator includes at least one venturi nozzle having a flow cross section which deviates from a circularity, and at least two plates disposed in parallel relationship and joined in sandwich construction, with one of the plates constructed to accommodate the venturi nozzle. 
         [0011]    According to another advantageous feature of the present invention, the venturi nozzle may have substantial rectangular flow cross section or substantial non-circular cross section, e.g. oval flow cross section or elliptical flow cross section. 
         [0012]    The present invention resolves prior art problems by providing a venturi nozzle with non-circular flow cross section. As a result, the planar venturi nozzle is compact and requires little installation space and may be constructed of multistage configuration. The flat structure of the vacuum generator allows the manufacture of the components from flat semifinished products so that production costs are reduced. The overall height is small so that the installation space is small as well. When combined with an area vacuum gripper, the vacuum generator can be best suited to the available space at hand. 
         [0013]    Currently preferred is the provision of a vacuum generator with planar venturi nozzle with or without vacuum control, with the vacuum control having a vacuum sensor and a flap valve. Multistage ejectors with several planar venturi nozzles placed in series behind one another can also be realized. The flap valves can hereby be arranged perpendicular to the gripping area or in the gripping area, i.e. the flap is oriented parallel to the gripping area. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0014]    Other features and advantages of the present invention will be more readily apparent upon reading the following description of currently preferred exemplified embodiments of the invention with reference to the accompanying drawing, in which: 
           [0015]      FIGS. 1   a - 1   d  show schematic cross sectional views of a prior art multistage ejector with cylindrical venturi nozzles and illustration of increased vacuum levels over four suction stages in a vacuum chamber and overall decreasing vacuum flow; 
           [0016]      FIG. 2  is a schematic illustration of a prior art multistage injector with cylindrical venturi nozzles, three suction stages and two flap valves, with both flap valves being closed; 
           [0017]      FIGS. 3   a - 3   b  show exploded views of a prior art Coanda ejector; 
           [0018]      FIG. 4  is an exploded view of an area vacuum gripper having embodied therein a multistage ejector according to the present invention; 
           [0019]      FIG. 5  is a schematic illustration of the area vacuum gripper of  FIG. 4  in assembled state; 
           [0020]      FIG. 6  is a schematic illustration of a multistage ejector with planar venturi nozzles and three suction stages; and 
           [0021]      FIG. 7  shows a sketch of the multistage ejector of  FIG. 6  with illustration of flow lines calculated by flow simulation. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0022]    Throughout all the figures, same or corresponding elements may generally be indicated by same reference numerals. These depicted embodiments are to be understood as illustrative of the invention and not as limiting in any way. It should also be understood that the figures are not necessarily to scale and that the embodiments are sometimes illustrated by graphic symbols, phantom lines, diagrammatic representations and fragmentary views. In certain instances, details which are not necessary for an understanding of the present invention or which render other details difficult to perceive may have been omitted. 
         [0023]    Turning now to the drawing, and in particular to  FIG. 4 , there is shown an exploded view of an area vacuum gripper generally designated by reference numeral  72  and having embodied therein a multistage ejector according to the present invention, generally designated by reference numeral  100 . The multistage ejector  100  includes a nozzle plate  174  having planar venturi nozzles  120 ,  122 ,  124 . Suction ports  146  and flap valves  148 ,  150 ,  152  are arranged in parallel relation to the venturi nozzles  120 ,  122 ,  124 . The multistage ejector  100  is configured in sandwich construction and includes a top plate  160 , the nozzle plate  174  disposed beneath the top plate  160 , a support plate  176  placed beneath the nozzle plate  174  and formed with oblong openings  178  for support of the flap valves  148 ,  150 ,  152  which are received in a plate  180 . The plate  180  can be made of any suitable material, e.g. elastomer and is provided with tongue-like or spoon-shaped valve tongues as a result of an omega-shaped (Ω-shaped) section line. Placed underneath the plate  180  is a plate  182  having suction ports, with a frame  184  abutting the underside of the plate  182  and configured to form a suction chamber  186  between the plate  182  and a perforated plate  188 . The plates  160 ,  174 ,  176 ,  180 ,  182 ,  188  can be made of any suitable material, e.g. metal, and the frame  184  can be made of metal or a sealing material of plastic. All plates may be punched or laser cut. The plates may also be cut by water jet application or by using coated EDM wires to prevent the formation or burrs. 
         [0024]    When the multistage ejector  10  with the planar venturi nozzles  120 ,  122 ,  124 , and with the suction ports  146  and flap valves  148 ,  150 ,  152  which are arranged in parallel relation to the plane of the venturi nozzles  120 ,  122 ,  124 , is assembled, the vacuum gripper  72  has a slender structure of slight height, as can be seen from  FIG. 5 . The rectangular cross section of the venturi nozzles  120 ,  122 ,  124  is rendered possible by covering the nozzle plate  74  with simple boards. 
         [0025]    The mode of operation of the vacuum gripper  72  is known to the artisan and follows essentially the mode of operation as described above with reference to  FIGS. 1   a - 1   d , so that further description is not necessary. For example, outgoing air flow from an outlet of the (upstream) venturi nozzle  120  constitutes a propellant air flow for an inlet of the (downstream) venturi nozzle  122 , whereas outgoing air flow from an outlet of the venturi nozzle  122  constitutes a propellant air flow for an inlet of the still further downstream venturi nozzle  124 . The flow cross section of the venturi nozzles  120 ,  122 ,  124  increases in flow direction of introduced compressed air. Currently preferred is a configuration of the venturi nozzles  120 ,  122 ,  124  with rectangular housing. 
         [0026]      FIG. 6  shows a schematic illustration of a multistage ejector, generally designated by reference numeral  200 . In the following description, parts corresponding with those in  FIG. 4  will be identified, where appropriate for the understanding of the invention, by corresponding reference numerals each increased by “100”. The ejector  200  includes planar venturi nozzles  220 ,  222 ,  224  and three suction stages  212 ,  214 ,  216 , with the suction ports  246  and the flap valves  248 ,  250  extending in a plane of nozzle plate  274  in which plane the venturi nozzles  220 ,  222 ,  224  are situated. The flap valves  248 ,  250  are shown here in a closed position. The flap valves  248 ,  250  may be provided on a separate plate  280  or integrated in the suction stages  214 ,  216 , e.g. in respective grooves  290 , as indicated in  FIG. 7 . 
         [0027]    In the illustration of  FIG. 6 , the suction ports  246  and the flap valves  248 ,  250  extend perpendicular to a plane of the venturi nozzles  220 ,  222 ,  224 , and the flap valves  248 ,  250  assume their closed position. 
         [0028]    Compressed air is introduced in a direction of arrow  30  to draw in suction air that enters the multistage ejector  200  via ports  246 , as indicated by arrows  244 . The suction air exits together with compressed air through outlet channel  267 . The flap valves  248 ,  250  open at a certain negative pressure and close the suction port  246  again when the vacuum flow falls below a threshold value. 
         [0029]      FIG. 7  illustrates the flow pattern of compressed air and suction air, when the flap valves  248 ,  250  are open. The flow lines have been determined through flow simulation. The nozzle plate  274  may also be punched or made by laser. The nozzle plate  274  may also be cut by water jet application or by using coated EDM wires to prevent the formation or burrs. The structure of the multistage ejector  200  is even flatter in this embodiment. As a result of the rectangular cross section of the venturi nozzles  220 ,  222 ,  224 , the nozzle plate  274  can be covered by simple boards. 
         [0030]    While the invention has been illustrated and described in connection with currently preferred embodiments shown and described in detail, it is not intended to be limited to the details shown since various modifications and structural changes may be made without departing in any way from the spirit and scope of the present invention. The embodiments were chosen and described in order to explain the principles of the invention and practical application to thereby enable a person skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated.