Abstract:
Weighing device with at least one weighing cell and with a receiving structure serving to hold the at least one weighing cell, wherein the at least one weighing cell includes a first fastener device serving to fasten the weighing cell in the receiving structure, and the receiving structure includes a second fastener device which is a complementary counterpart of said first fastener device. The fastener devices include a detent engagement mechanism and are designed in such a way that they hold as well as release the weighing cell by means of a form-locking engagement which can be locked and released, respectively, by a simple action in the form of pushing in the direction of the load and pulling against the direction of the load.

Description:
TECHNICAL FIELD 
     The invention concerns an arrangement for fastening a weighing cell or a plurality of weighing cells in a weighing device. 
     BACKGROUND OF THE ART 
     A weighing device of the generic type addressed by the invention, for example a device for weighing objects of a uniform kind, finds application primarily in automated production—and test systems, where scales of a modular design, so-called weighing modules are particularly well suited for incorporation in such systems. Frequently, the weighing modules employed here, which are also often referred to as weighing cells, are of a type where the indicator unit is arranged in separation from the weighing module or weighing cell, as is the case for example in a system with a central indicator for a plurality of such weighing modules or weighing cells. Areas of application for integrated weighing modules of this kind are installations for the production and/or testing of small, relatively expensive parts, for example in filling—and packaging machines for tablets, capsules ampoules, etc., in the pharmaceutical industry, or for the checking of ball bearings. The weighing of uniform objects and also the so-called batch weighing are processes where multiple loads have to be weighed individually in a confined space for a diversity of purposes such as checking, dispensing measured doses, and/or fill-weighing. 
     Systems of this kind which serve to weigh uniform objects belong to the known state of the art. The prevalent types are linear arrays of weighing cells. Other arrangements are based on a concept of spreading the weighing cells out in a two-dimensional layout like satellites associated with a linear array of load receivers, wherein the latter have to match the spacing of the conveyor elements of an existing conveyor system, as the individual weighing modules or weighing cells are in many cases too large to be lined up at the required small intervals. 
     In a weighing cell which operates according to the principle of electromagnetic force compensation, the force generated by a load on the weighing pan is compensated by a force-compensating member consisting of a permanent magnet and a coil, wherein the current is measured which flows through the coil and generates the compensating force. This measurement value is in proportion to the weighing load. However, this measurement value is also dependent on the position of the coil in the magnetic field of the permanent magnet, which is why the coil always has to be in the same position relative to the magnet at the moment when the measurement value is captured. The position of the coil after applying a load is determined by a position sensor, and the current through the coil is increased until the displacement of the coil which was caused by the load is compensated. Now the coil current is measured which is representative of the weight of the applied load. A weighing cell of this kind is disclosed in U.S. Pat. No. 4,280,577, wherein the weighing cell includes between the load receiver and the force-compensating member a force-transmitting mechanism whereby the force generated by the load on the load receiver is transmitted to the force-compensating member, either reduced or amplified in magnitude depending on the load range. 
     A weighing cell operating according to the same principle is disclosed in U.S. Pat. No. 4,099,587. In this patent, the load receiver is coupled directly to the force-compensating member through a force-transmitting rod. Attached to the force-transmitting rod is the movable portion of the position sensor, while the stationary part of the position sensor is rigidly connected to the housing-mounted part of the weighing cell, specifically to the part of the force-compensating member that has a fixed position relative to the housing. This arrangement, which is referred to as the direct-measurement principle, is used with preference in the range of small weighing loads. 
     A weighing cell that is designed for installation in a receiving holder arrangement is described in WO 2007/041979 A1. In order to establish a releasable mechanical and electrical connection with an associated receiving holder arrangement of an appropriate complementary design, the weighing cell has alignment elements which are designed to cooperate with alignment elements of the receiving holder arrangement. The receiving holder arrangement is intended for permanent installation in a machine that is designed to perform weighing functions. 
     In cases where it is important to quickly measure the individual masses of weighing objects, it suggests itself for example to use a setup of several weighing cells to allow the individual masses such as weighing objects of a uniform kind to be weighed in parallel. A need exists in particular for weighing systems that are suitable for use in the area of mass determination in the range of milligrams to grams. In the case of weighing cells whose dimensions are large in comparison to the spacing between the conveyor elements of a conveyor system, and with the use of a complex conveyor system, in particular of a kind where the conveyor elements are set up in a two-dimensional arrangement, even a star-shaped arrangement as disclosed in the prior art will not meet the requirements. As the dimensions of the weighing cells, length as well as width, are often larger than the required spacing between the central longitudinal axes of their load receivers, only a limited number of weighing cells can be placed around an area provided for the arrangement of the load receivers which is determined by the conveyor elements of the conveyor system. 
     SUMMARY 
     It is therefore the object of the invention to provide a weighing cell which can be arranged in a system in a way that requires little space. The aim is for example to provide an arrangement of load receivers, and thus an arrangement of weighing cells that are connected to the load receivers, which is adapted to a complex conveyor system for weighing objects in which the conveyor elements are spaced apart at narrow intervals. It is a requirement that each weighing cell can be installed in this arrangement and removed from it in a simple manner. 
     This task is solved by a weighing device with at least one weighing cell and with a receiving structure that serves to hold at least one weighing cell. The at least one weighing cell has a first fastener device which serves to secure the weighing cell in the receiving structure, while the receiving structure has a second fastener device which is the complementary counterpart of the first fastener device. 
     To allow a simple installation of a weighing cell in the receiving structure and a correspondingly simple separation of the weighing cell from the receiving structure, the fastener devices include a detent engagement mechanism and are configured in such a way that they hold and release the weighing cell by means of a form-locking engagement which can be locked and unlocked by simple operator action directed at the weighing cell. In particular, the action is applied to the weighing cell directly and without intermediary means, specifically in the form of pushing in the direction of the load and pulling against the direction of the load. 
     In a weighing device with a receiving structure serving to hold a plurality of weighing cells, the at least one weighing cell is arranged in the receiving structure in a design space whose dimensions in a plane running perpendicular to the load direction match the largest dimension of the weighing cell in the same plane, and which is delimited by the design spaces reserved for neighboring weighing cells that are to be arranged in the receiving structure. The weighing device can thus be adapted to a conveyor device for weighing objects in which the conveyor elements are spaced at narrow intervals. 
     A weighing cell, as the term is used in the present context, means a device that serves to measure the force of a load, in particular a load that is placed on a load receiver connected to the weighing cell, wherein the presence of electronic components for the evaluation of the electrical measurement signal is however not necessarily required. 
     As the weighing cells are preferably of a uniform design, they are individually interchangeable. This makes a weighing device of this type more service-friendly. 
     The weighing device is thus distinguished by a high level of compactness, and by making it possible to weigh for example small, relatively expensive products in parallel, it ensures a fast weighing process and thus a high throughput of weighing objects. 
     The receiving structure is configured in a special way so that it can accommodate a plurality of weighing cells in a two-dimensional area or matrix structure. A design space in the aforementioned plane thus matches the largest dimension of the weighing cell in said plane. This has the consequence that each of the load receivers can be arranged in the proximity of the weighing cell to which it belongs. 
     The dimension of the design space in the direction of the load is delimited above and below the weighing cell only by adjacent systems, for example by the operating space of a conveyor system or for example by a housing wall of the weighing device. In one embodiment where the weighing cells are arranged on several levels, a neighboring level of the weighing device can delimit the design spaces for the weighing cells below it. 
     The receiving structure can for example have a base plate on which the second fastener device is arranged. The second fastener device can work together with a first fastener device that is arranged at the underside of the weighing cell. In this case, the weighing cell is fastened to and separated from the receiving structure preferably from above. 
     As a further embodiment, the receiving structure has a fastening plate in which the at least one weighing cell can be installed, and the second fastener device is incorporated in the fastening plate. In this case, the weighing cell can be fastened to and separated from the receiving structure either from above or below, depending on the specific design of the fastener device. 
     The cooperating fastener devices establish a mechanical and preferably also an electrical connection between the at least one weighing cell and the receiving structure, so that the weighing cell can be anchored safely in the receiving structure and released again in a simple manner, while the weighing cell is at the same time supplied with power and can send and/or receive electrical signals through this connection. 
     The weighing cell in a preferred embodiment has a housing on which the first fastener device is arranged. 
     In order to make the process of connecting and/or separating the weighing cell and the receiving structure safe and simple, the first and/or the second fastener device has a guide feature which indicates how the weighing cell is to be connected to the receiving structure. The guide feature can also be designed so as to make it impossible to connect the fastener elements the wrong way. A guide feature is also possible which positions and guides the elements that cooperate in the fastening attachment, thus facilitating the connecting process. The guide features can have the form of a simple guide mark and/or of a pin cooperating with a groove and/or a seating recess. 
     A detent mechanism or snap mechanism can be realized through a concept where the first fastener device has at least one groove or recess and the second fastener device has at least two, and in particular three, balls which are seated against spring elements in such a way that when the weighing cell is set into the receiving structure, the balls will snap into the at least one groove or recess. 
     In a further embodiment, the first fastener device has at least two passage holes and the second fastener device has at least two spreader pins arranged diagonally across from each other, wherein when the weighing cell is set into the receiving structure, the spreader pins snap into engagement with the holes and thereby ensure a safe connection between the weighing cell and the receiving structure. 
     In a preferred embodiment, the mechanical attachment and the electrical connection are spatially separated from each other in the receiving structure. This separation is very advantageous, because the electrical connecting lines can thus be screened in a simple way against electromagnetic interference. As a further consequence of the separation, and with an appropriate design configuration, mechanical stress on the electrical connections is avoided or minimized. 
     To identify each individual weighing cell in a weighing device and also to determine the reproducibility of the weighing results, it may be advisable to provide each weighing cell with a unique identification code. This code can then be picked up by a suitable counterpart which is arranged on the receiving structure. Among other possibilities, the code can consist of a bar code, a matrix code or a passive element of an RFID system. In particular, it is also conceivable that there is a memory chip on which relevant data of the weighing cell are stored. 
     For the weighing of uniform weighing objects, it suggests itself in particular to configure a weighing device with a specified number of weighing cells and an equal number of load receivers. This number may be based primarily on the number of conveyor elements of a conveyor system. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the following, the invention is described through examples which are illustrated schematically in the drawings, wherein: 
         FIG. 1  represents a perspective view of a weighing device with six weighing cells which are arranged in a two-dimensional matrix and are equipped with a first embodiment of fastener devices for securing each weighing cell in a receiving structure; 
         FIG. 2  represents a perspective view of a weighing device with four weighing cells which are arranged in a two-dimensional matrix and are equipped with a second embodiment of fastener devices for securing each weighing cell in a receiving structure; 
         FIG. 3  represents a perspective view of a weighing device with a plurality of weighing cells which are set up in two-dimensional matrix arrangements, wherein the receiving structure is designed to accommodate weighing cells simultaneously on several levels; 
         FIG. 4  represents a perspective view of a detail of a weighing cell with a further embodiment of fastener devices, wherein a fastener device includes an element for a coupling connection to a receiving structure; 
         FIG. 5  represents the weighing cell of  FIG. 4  in a different perspective view, wherein the element for the coupling connection is shown in its connected state with the receiving structure; 
         FIG. 6  represents a perspective view of a weighing cell with a second embodiment of fastener devices for securing the weighing cell in a receiving structure, wherein the electrical connection and the mechanical connection lie in different planes; and 
         FIG. 7  represents a side view of a detail of a weighing cell in the area of the fastener devices. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows in a perspective view a weighing device  1  with six weighing cells  2  which are arranged in a two-dimensional matrix. Each of the weighing cells  2  contains in its interior a built-in arrangement of a coil and a magnet (not shown here) that is connected to the load-receiving area  3 . From each of the load-receiving areas  3 , a force-transmitting rod  4  extends in the opposite direction of the load direction, carrying at its end a load receiver  5 . Above the load-receivers  5  is normally the operating space of a conveyor system which is not illustrated here. Each of the load-receiving areas  3  of a weighing cell  2  is part of a parallel-guiding mechanism which, when a load is applied, is movable in the direction of the latter. Parts of the parallel-guiding mechanism that are shown for each weighing cell  2  in the drawing are the load-receiving area  3 , the upper parallel guide  6 , and the stationary part  7  of the weighing cell  2 . 
     Each weighing cell  2  is fastened to a receiving structure in the form of a base plate  8 . For the weighing cell  2   a  which is shown to the right in the foreground of  FIG. 1 , the break-away portion of the base plate  8  opens up the view into a first embodiment of fastener devices for securing the weighing cell  2   a  on the base plate  8 . The fastener devices include a second fastener device  13   a  which is incorporated directly in the base plate. The second fastener device has at least two—but preferably three or four—balls  12   a , each of which is seated on a spring  11   a . When the weighing cell  2   a  is in its connected state with the base plate  8 , the balls  12   a  engage or snap into a perimeter groove  14   a  of a first fastener device  13   a  which is connected to the weighing cell  2   a . As a result, the weighing cell  2   a  is releasably connected to the base plate  8  and can be exchanged against another weighing cell of the same kind, if necessary, for example in a service case or if the weighing device  1  needs to be adapted for another load range. 
     For interchangeability, all of the other weighing cells  2  that are shown in  FIG. 1  are of course equipped with identical fastener devices. To ensure a secure engagement, the respective axes of the balls  12   a  and of the groove  14   a  are slightly offset against each other in the vertical direction, so that when the weighing cell is set in place, there is a contact force acting on the balls which pulls them into the groove. 
     Likewise in a perspective representation,  FIG. 2  shows a further embodiment of a weighing device  201  with four weighing cells  202  which are arranged in a two-dimensional matrix. Each weighing cell  202  is arranged within a design space  220  which is indicated in the drawing by dash-dotted lines and whose dimensions measured in a plane orthogonal to the load direction are delimited by the design spaces  220  of neighboring weighing cells  202  that are arranged in the receiving structure. The receiving structure in this example essentially comprises a base plate  208 . Working together with the base plate  208 , the weighing cells  202  include a second embodiment of fastener devices serving to secure each weighing cell  202  in the receiving structure, i.e. in the base plate  208  in the present example. The fastener devices are made up of a second fastener device  213  which has two spreader pins  215  that are arranged diagonally on the base plate  208  and engaged in a first fastener device  213 , the latter being incorporated in the weighing cell  202  which has holes  216  arranged diagonally opposite each other. After passing through the holes  216 , the spreader pins lock the weighing cell  202  against separating itself from the base plate  208  and only release their hold on the weighing cell  202  if a large enough pulling force is applied to the latter. Also shown in  FIG. 2  are guide bolts  217  which are likewise fastened to the base plate  208  and are engaged in further holes  219  of the weighing cell  202  to exactly position the latter on the base plate  208 . A weighing cell  202  is preferably equipped with two posts  218  providing a grip for pulling the weighing cell out of the receiving structure, for example with a tool. 
       FIG. 3  shows in a perspective view a weighing device  301  with several weighing cells  302 ,  302   a  which are arranged in a two-dimensional matrix layout. For a description of the structure of the weighing cells  302 ,  302   a , the reader is referred to the description of  FIGS. 1 and 2 . The weighing device  301  has two base plates  308 ,  337  arranged parallel to each other, spaced apart from each other in the direction of the load, and rigidly connected to each other through at least one vertically oriented connecting element  321 . The connecting element  321  can for example be configured as an essentially closed frame which covers the gap between the two base plates  308 ,  337  against the outside, or it could also be configured as a box in which the base plates  308 ,  337  are installed. The lower base plate  308 , as part of a receiving structure, essentially corresponds to one of the base plates  8 ,  208  illustrated in  FIG. 1  or  2 . The upper base plate  337 , as a further part of the receiving structure, additionally includes passages  322 . Continuations of these passages  322  are formed along the meeting corners of the design spaces through cutbacks  323  in the vertical edges of the weighing cells  302 . The passages  322  as well as the cutbacks  323  along the edges of the stationary parts  307  of the weighing cells  302  serve as passage ways for the force-transmitting rods  304  of the weighing cells  302   a  which are arranged on the lower base plate  308 . The lower base plate  308  can accommodate weighing cells without cutbacks as well as the weighing cells  302   a  with cutbacks that are shown here. By arranging the weighing cells  302 ,  302   a  on two different base plates  308 ,  337  aligned parallel to each other, it is possible to accommodate essentially twice the number of weighing cells  302 ,  302   a  on the same footprint area. The weighing mechanism of the weighing cells  302 ,  302   a  is the same as in the weighing cells which have already been described in the context of  FIGS. 1 and 2 . It is considered self-evident that for example the load receivers  305  as well as the upper base plate  337  with the weighing cells  302  arranged on it can easily be removed as a whole in order to allow individual weighing cells  302   a  on the base plate  308  to be exchanged if necessary. 
     As is evident from  FIG. 3  and consistent with the force-transmitting rods  304  reaching through the passages  322  and the cutbacks  323 , the weighing cells  302   a  on the lower base plate  308  are offset from the weighing cells  302  of the upper base plate  337  by half the length and half the width of their respective design spaces. 
       FIGS. 4 and 5  each show a detail of a first embodiment of fastener devices for a weighing cell seen from two different perspectives, wherein identical elements are shown with the same reference symbols. On the weighing cell  402  a first fastener device  410  is arranged which extends perpendicular to the load direction. The first fastener device  410  is configured as a cylindrical sleeve  428  with a circumferential groove  414 . Arranged inside the sleeve  428  is an electrical connector plug  424  through which the weighing cell can exchange electrical signals and/or receive its power supply. The second fastener device  413 , which works together with the first fastener device  410  so that the weighing cell can snap into a receiving structure, is shown here in the form of a socket  429  with a circular-shaped seating recess  430  oriented parallel to the direction of the load. The internal diameter of the circular-shaped seating recess  430  essentially matches the external diameter of the cylindrical sleeve  428 . The socket  429  further has at least two holes  426  directed parallel to the direction of the load, which serve to couple the second fastener device  413  to a base plate (not shown in  FIG. 4 ) represented in  FIG. 5  as base plate  408 . The holes  426  are arranged opposite each other at an angle of 180° in relation to the circular-shaped seating recess  430 . As shown in  FIG. 4 , the socket  429  has a setback  427  above each of the vertical holes  426 . Relative to the circular-shaped seating recess  430 , two passage holes  425  which are oriented perpendicular to the load direction are arranged at locations between the holes  426 . These passage holes  425  serve for the arrangement of the balls  412  and springs illustrated in  FIG. 1 , which in the installed state of the weighing cell are pushed against the groove  414 , so that the weighing cells  402  snap into the socket  429 . 
       FIG. 5  shows the second fastener device  413  arranged on a base plate  408 . Also visible inside the circular-shaped seating recess  430  is the connector socket  431  for the electrical connector plug  424 . 
       FIG. 6  shows a further embodiment of fastener devices in a perspective view, wherein a weighing cell  602  is coupled to a fastening plate  634 . Arranged on the fastening plate  634  is a second fastener device  613  with two spreader pins  615  arranged diagonally on the fastening plate  634 , which are inserted in a first fastener device  610 , the latter being incorporated in the weighing cell  602 , which has holes  616  arranged diagonally opposite each other. After passing through the holes  616 , the spreader pins  615  lock the weighing cell  602  against separating itself from the fastening plate  634  and only release their hold on the weighing cell  602  if a large enough pulling force is applied to the latter. Also shown in  FIG. 6  are guide bolts  617  which are likewise fastened to the fastening plate  634  and are engaged in further holes  619  of the weighing cell  602  to exactly position the latter on the fastening plate  634 . Visible between the guide bolts  617  and the spreader pins  615  is a circular-shaped recess  632  which surrounds a passage  633 . This passage  633  allows access for an electrical connector plug  624  of the weighing cell  602 . The connector plug  624  serves to establish an electrical connection to a connector socket  631  arranged on a base plate  608  which in this illustration is arranged below the fastening plate  634 , spaced apart from the latter. The electrical connection here has the form of a connector plug  624 , but it is also possible to configure the connection as a simple cable with an appropriate connection. The base plate  608  and the fastening plate  634  are connected to each other by spacer columns  635 . The border area of the recess  632  which surrounds the passage  633  serves as a seat for the structure that carries the plug  624 , which includes in particular a printed circuit board. In this illustration, the spreader pins  615  and the guide bolts  617  are connected directly to the fastening plate  634 . 
     It would likewise be conceivable to configure the fastener device  613  as an element which would be set onto and/or into a base plate. 
     Care should be taken when establishing the mechanical and electrical connection of the weighing cell  602  to the receiving structure that the system does not become over-determined. The problem is solved with a concept where the connector socket  631  is mounted in the base plate  608  with a floating play in the range of tenths of millimeters. 
     A further embodiment of fastener devices is shown in a sectional view in  FIG. 7 . The first fastener device  710  is largely analogous to the one shown in  FIG. 6 . The structure carrying the connector plug  724  is recognizable in  FIG. 7  as a printed circuit board  739 . On this circuit board  739 , there can be a memory chip  740  arranged on which specific data of the weighing cell  702  are stored. These data include for example the identification of the weighing cell or temperature compensation data or similar information. When a weighing cell is newly installed in the receiving structure, and immediately after contact has been established through the plug connection, the weighing cell is interrogated regarding its identity by a processor unit (not shown in the drawings) and the correction data for the weighing cell are transferred to the processor unit. 
     The second fastener device  713  consists of a socket  729  to which two spreader pins  715  as well as two guide bolts  717  are fastened. The spreader pins  715  and the guide bolts  717  are mounted, respectively, on diagonally opposite corners of the socket  729 . The socket  729  further has a recess  732  which surrounds a passage  733 . The recess  732  again serves to receive the structure  739  on which the electrical connector  724  of the weighing cell is arranged. The opening of the passage  733  that faces away from the weighing cell  702  is delimited by a further structure in which the connector socket  731  for the electrical connector plug  724  of the weighing cell  702  is located. 
     In some of the illustrated variants, the weighing cells are equipped with a plug which at the time of installation of a weighing cell into the weighing device is brought into contact with a matching connector socket. This separation is very advantageous, as the electrical connections are thus routed through a base plate, for example, as shown in  FIGS. 5 to 7 , in a recess of the base plate or the socket that is connected to the latter. This has the advantage that the base plate, being preferably made of metal, screens the connecting leads against electromagnetic interference. In addition, this concept allows mechanical stresses on the electrical leads to be avoided. 
     Besides the design configurations of the weighing cell as described and illustrated, it is also possible for the weighing cell to be equipped with an auxiliary device. Such an auxiliary device can for example be a grip hold on the outside of the weighing cell, which facilitates the removal of individual weighing cells from the receiving structure either manually and/or with a suitable tool. 
     The weighing cell can further be equipped with a marking and/or a safeguard against installing it the wrong way, for example by simple color markings, pins cooperating with grooves or holes, etc. 
     It is also conceivable to provide the weighing cell with a unique identifier code, be this in the form of a bar code, a matrix code, or as a passive element of an RFID-code setup. A corresponding counterpart is arranged for example at the receiving structure. Thus it is possible for example to realize a safety switch arrangement which would allow a weighing process to take place only after the required weighing cells are placed correctly in the receiving structure. However, identification data and further data could also be stored in the memory chip  740  which has been mentioned in context with the description of  FIG. 7 . 
     The weighing device according to the invention has been described and illustrated in a preferred embodiment. However, based on the teachings of the invention, other variants can also be realized within the scope of the pertinent art.