Abstract:
A method for carrying out a stud joining process by a tool performing a working step on a workpiece and the working step is carried out while taking into account at least one parameter value which is selected from a set of values, and the working step is to be carried out at a certain position on the workpiece, and the position determines the parameter value. The method comprises the steps of: storing the parameter value for the position in an RFID transponder; locating the RFID transponder on the workpiece in the region of the position before the working step is carried out, reading the parameter value out from the RFID transponder with an RFID communication device associated with the tool.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of PCT Application No. PCT/EP2012/050587, filed on Jan. 17, 2012 which claims priority from German Patent Application No. DE 102011016132.5, filed on Mar. 29, 2011, the disclosure of which is incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to a workpiece processing method for carrying out a working step on a workpiece by means of a tool, in particular for carrying out a stud joining process, wherein the working step is carried out while taking into account at least one parameter value which is selected from a set of values, and wherein the working step is to be carried out at a certain position on the workpiece, said position determining the parameter value. 
         [0003]    Furthermore, the present invention relates to a tool for carrying out such a method, in particular a stud joining tool, and to a positioning device for carrying out this method. 
         [0004]    In workpiece processing methods of the type described above, working steps of the same type can be carried out in succession by means of the tool, for example successive stud joining processes. On account of the boundary conditions, however, it may be necessary to set the tool and/or other parameters differently for each of these processes. In stud joining processes, for example, the stud materials and/or the workpiece materials or workpiece thicknesses can vary from process to process. As a rule, this is taken into account by other joining parameters being preselected, depending on process or position. These joining parameters (for example welding voltage or current, frequency, joining stroke, joining time, etc.) are as a rule stored beforehand in a control device for each process or for each type of process. Before each joining operation, the respective joining parameters are then selected manually according to the position and the process thus to be carried out. This applies in particular to the case where the joining tool is a hand-operated joining tool, for example a joining gun. 
         [0005]    Errors can occur during the manual selection, and therefore the incorrect joining parameters are selected for the respective joining process. This may lead to the joint not having the adequate strength, to the workpiece being damaged, etc. 
         [0006]    Document DE 20 2009 012 370 U1 discloses a welding apparatus in which a hand unit has at least one signal means for optically displaying operating information of the welding apparatus. This is intended to make it possible to render information about the operating state of the welding apparatus discernible for the user at any time. 
       BRIEF SUMMARY OF THE INVENTION 
       [0007]    Against this background, one object of the invention is to specify an improved workpiece processing method and improved tools for this purpose. 
         [0008]    This object is achieved firstly by a workpiece processing method for carrying out a working step on a workpiece by means of a tool, in particular for carrying out a stud joining process, wherein the working step is carried out while taking into account at least one parameter value which is selected from a set of values, and wherein the working step is to be carried out at a certain position on the workpiece, said position determining the parameter value, wherein an RFID transponder has been arranged in the region of the position before the working step is carried out, in which RFID transponder the parameter value for this position is stored, and wherein the tool has an RFID communication device, by means of which the parameter value is read out from the RFID transponder. 
         [0009]    Furthermore, the above object is achieved by a tool which is suitable for carrying out this method, in particular in the form of a stud joining tool, wherein the tool has an RFID communication device for reading out a parameter value from an RFID transponder and/or for writing a parameter value into the RFID transponder. 
         [0010]    Furthermore, the above object is achieved by a positioning device for carrying out the method, wherein the positioning device has a plurality of positioners for defining a plurality of positions on a workpiece and wherein an RFID transponder is secured to at least one of the positioners. 
         [0011]    Finally, the above object is achieved by the use of RFID identification for determining a parameter value, with reference to which a working step is carried out on a workpiece by means of a tool, in particular with a method of the above-mentioned type. 
         [0012]    As a result of the measure of arranging an RFID transponder in the region of the position of the operating step to be carried out, in which RFID transponder the parameter value or a number of a set of joining parameters or the like for this position is stored, this parameter value can be read out by means of the RFID communication device when the tool is placed in the region of the position. As a result, it is possible to automatically set the tool or other devices with reference to the parameter value, such that a manual selection of the parameter value is dispensed with. This can ensure that the correct parameter value can be used for the respective working step to be carried out. Incorrect working steps on account of the selection of an incorrect parameter value can be avoided in this way. 
         [0013]    The expression “parameter value” should in this case be understood in general terms. This can be a number of a program, by means of which, for example, the tool is programmed. Since a plurality of parameters for the respective working step are to be set as a rule, it is preferred if the parameter value is an individual numerical value which refers to such a set of working step parameters. In general, however, it is also conceivable for the parameters to be stored directly in the RFID transponder. 
         [0014]    In the simplest case, the RFID communication device can be a reader device which is designed to read out a parameter value from the RFID transponder. Alternatively or additionally, however, the RFID communication device can be designed to write a parameter value into the RFID transponder. 
         [0015]    The processing method is preferably a stud joining process, such as a stud welding process or a stud adhesive-bonding process, in which a stud or another part is joined to a workpiece along a joining axis from one side. In a stud joining process, stud and workpiece are generally made of a metallic material. In a stud adhesive-bonding process, both stud and workpiece can also be made of other materials, such as, for example, plastic, etc. 
         [0016]    In a stud welding process, the welding parameters for carrying out this process are, for example, the welding current, the welding stroke, the welding time, etc. 
         [0017]    Although the present method is preferably used in the field of stud joining processes, the method can also be used, for example, in drilling processes or screwing processes. Here, the working step parameters can be, for example, the rotational speed, clockwise/anticlockwise rotation, feed, torque, angle of rotation, etc. Furthermore, a drilling parameter can be, for example, whether a drilling coolant is fed or not. 
         [0018]    In the simplest case, the parameter value can also be a position recognition value. In this case, the method according to the invention can also be used for locating the respective position without different process parameters being set in the tool from process to process. In this type of method, the coupling between the transponder and the reader device is used in order to determine the position at which the working step or process is to be carried out. 
         [0019]    The expression “RFID” is in this case to be understood in very general terms. The expression “RFID identification” refers both to RFID techniques in which the coupling between transponder and reader device is effected inductively and to those techniques in which the coupling is effected electromagnetically (by radio). It is preferred if the RFID transponder is a passive transponder, which has no independent energy supply. In this case, the transponder can have, for example, a chip in which there is an electronic memory for storing a parameter value. Furthermore, such a passive transponder can have a coil, via which energy is fed in by means of the reader unit in order to cause the chip to transmit the parameter value. This method is used in particular in an inductive coupling. The coil serves in this case as a type of aerial, via which a communication with an RFID communication device can be set up. In an active transponder, an aerial of another type can be used as communication means. 
         [0020]    In general, however, the RFID transponder can also be a simple tag which can be read by means of a reader device. The RFID transponder could in this case be a barcode, and the RFID communication device could in this case be a barcode scanner. 
         [0021]    The object is therefore completely achieved. 
         [0022]    It is especially preferred if the RFID transponder, for carrying out the working step, is arranged temporarily on the workpiece. 
         [0023]    As a result of this measure, it is possible to further process or deliver the processed workpiece without an RFID transponder arranged thereon. 
         [0024]    The temporary attachment can be effected, for example, via an adhesive. 
         [0025]    In a stud joining process, the transponder can be attached in the region next to a joining position on the surface of the workpiece, or also on the rear side thereof. 
         [0026]    However, it is especially preferred if the RFID transponder is secured to a holding device which is attached to the workpiece before carrying out the working step. 
         [0027]    Such a holding device can also easily be removed again after the working step has been carried out. For example, the holding device can be temporarily connected to the workpiece, using conventional fastening means such as clamps, screws, etc. 
         [0028]    It is especially advantageous in this case if the holding device is designed as a positioning device which defines the position of the working step. 
         [0029]    In this embodiment, the holding device serves as a type of template in order to mechanically define the position of the working step. 
         [0030]    The positioning device can in this case have a positioner which is designed, for example, as a guide bushing. A mouthpiece or another head part of a joining tool can be inserted into such a bushing. In this case, the positioning device is secured to the workpiece in such a way that the positioner or the guide bushing lies exactly over the position. Here, the RFID transponder can be integrated, for example, into the bushing. 
         [0031]    Such a positioning device is used, for example, for carrying out stud joining processes in the prototype construction of motor vehicles, partly also in series production and at repair and rework stations. In this case, the template is secured to the workpiece (vehicle body). 
         [0032]    It is preferred in this case if the positioning device has a plurality of positioners for defining a plurality of positions on the workpiece, wherein an RFID transponder is secured to at least one of the positioners. 
         [0033]    Positioning devices of this type have, for example, a plurality of positioners in the form of bushings in order to define different joining positions on the workpiece. In the different joining positions, the joining parameters may be different, and so the securing of an RFID transponder to one or more of the positioners can serve to automatically set the joining parameters by means of the method according to the invention. 
         [0034]    On the whole, therefore, it is preferred if the read-out parameter value is transmitted to a control device which sets the tool on the basis of the parameter value. 
         [0035]    As a result of this measure, it is possible to automatically set the tool for the respective working step to be carried out as soon as the tool has been arranged in the region of the position and the RFID transponder arranged in this region has been read out. Incorrect settings of the tool when carrying out the working step can be avoided in the process. 
         [0036]    Furthermore, it is possible to count in the control device the number of working steps to be carried out. For example, if a plurality of working steps are to be carried out one after the other, an internal counter of the control device can be incremented at the same time during the reading-out of each RFID transponder. This enables monitoring to be carried out to determine whether a working step has also actually been carried out at each position or whether possibly a working step has been forgotten at one of the positions. 
         [0037]    Furthermore, it is preferred on the whole if the RFID transponder has a coil which is arranged concentrically around a processing axis. 
         [0038]    In this embodiment, it is possible to arrange the tool in any desired rotary position with respect to the processing axis, and therefore an exact rotary orientation of the tool at the position is not necessary. 
         [0039]    Furthermore, it is preferred in the tool according to the invention if the RFID communication device is mounted on the tool so as to be movable in a direction parallel to a processing axis. In particular, it is preferred if the RFID communication device is mounted so as to be movable in this direction relative to a mouthpiece and/or relative to a holding device for a component to be joined. 
         [0040]    This is in particular advantageous if the tool is provided for carrying out working steps during which an RFID transponder is secured to a holding device. It may be the case here that the holding device or the RFID transponder is arranged at a different height with respect to the workpiece from working step to working step. The mobility of the RFID communication device can ensure that said RFID communication device can always be positioned optimally with respect to the RFID transponder, in particular as close to the RFID transponder as possible, irrespective of the relative position of the RFID transponder with respect to the workpiece. 
         [0041]    It is especially preferred in this case if the RFID communication device is elastically preloaded in a basic position, for example by means of a spring device. This can ensure that the RFID communication device as far as possible always bears against an intended bearing surface of the holding device and/or of the RFID transponder. 
         [0042]    According to a further preferred embodiment of the tool according to the invention, the RFID communication device has a coil or aerial which is arranged concentrically about a processing axis in such a way that a communication with the RFID transponder can be effected substantially independently of a rotary position of the tool about the processing axis. 
         [0043]    This embodiment is in particular preferred when the RFID transponder also has a coil which is arranged concentrically to the processing axis. In this case, it is especially preferred if the coils are matched to one another in their arrangement and size in such a way that said coils are substantially in alignment with one another in a direction parallel to the processing axis. As a result, the communication reliability can be markedly increased. 
         [0044]    In the positioning device according to the invention, it is preferred if at least one of the positioners is designed as a bushing. 
         [0045]    Furthermore, it is preferred in this case if the bushing has an outer sleeve and an inner ring connected thereto, wherein the RFID transponder is integrated into the inner ring. The outer sleeve of the bushing is preferably made of a metallic material and can therefore be secured to the positioning device (template) by conventional joining methods. The inner ring preferably has a non-conductive and non-magnetic material, such as, for example, plastic or synthetic resin. Owing to the fact that the outer sleeve surrounds the inner ring, interference between bushings adjacent to the transponder can be avoided. Furthermore, the outer sleeve can remain on the template, even if the RFID transponder is exchanged. 
         [0046]    It goes without saying that the abovementioned features and the features still to be explained below can be used not only in the respectively specified combination but rather also in other combinations or on their own without departing from the scope of the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         [0047]    Exemplary embodiments of the invention are shown in the drawing and are described in more detail below. In the drawing: 
           [0048]      FIG. 1  shows a schematic cross-sectional view of a joining arrangement having a stud joining tool according to an embodiment of the present invention; 
           [0049]      FIG. 2  shows a schematic illustration of a positioning device; 
           [0050]      FIG. 3  shows a perspective sectional view of a bushing for a positioning device of the type shown in  FIG. 2 ; 
           [0051]      FIG. 4  shows a sectional view through a further embodiment of a bushing for a positioning device of the type shown in  FIG. 2 ; and 
           [0052]      FIG. 5  shows a cross-sectional view of a joining arrangement with a further embodiment of a stud joining tool according to the invention and of a positioning device according to the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0053]    A joining arrangement in the form of a stud welding arrangement is designated generally by  10  in  FIG. 1 . The joining arrangement  10  serves to join a stud  12  to a workpiece  14  in the form of a sheet or the like in one working step. The joining operation is effected in this case preferably in such a way that an integral connection, such as a welded or adhesively bonded connection, is set up between the stud  12  and the workpiece  14 . The joining operation is carried out only from one side of the workpiece  14 . 
         [0054]    The joining arrangement  10  has a joining tool  20  in the form of a stud welding gun. The joining tool  20  has a handle  22  and is connected via a schematically indicated supply line  24  to a power source and possibly to a control device. 
         [0055]    In the region of a head, the joining tool  20  has a holding device  26  for holding a stud  12 . The holding device  26  is surrounded by a mouthpiece  28 , which can be connected, for example, rigidly to the housing of the joining tool  20 . The holding device  26  can be displaceable in the axial direction relative to the housing. Furthermore, the holding device  26  is preferably driven in the axial direction by means of a mechanical or electromechanical device. 
         [0056]    To carry out a stud welding operation, as a rule the stud  12  is first of all put onto the workpiece  14  and then the mouthpiece  28  comes down on the workpiece  14 . A pilot current, which flows via the workpiece  14  and the stud  12 , is then switched on. Subsequently, the stud  12  is lifted from the workpiece  14 , such that an arc is produced between the stud  12  and the workpiece  14 . The current is then increased to a welding current, such that those surfaces of stud  12  and workpiece  14  which are opposite one another are fused. In a last step, the stud  12  is lowered again onto the workpiece  14 , such that the molten pools intermix and the arc is short-circuited. The welding current is switched off. The complete molten pool solidifies, such that the stud  12  is integrally connected to the workpiece  14 . In a stud adhesive-bonding process, it is normal practice to heat a layer of adhesive on that side of the stud  12  which faces the workpiece  14  before the stud  12  is lowered onto the workpiece  14 . 
         [0057]    To carry out such a joining process, the joining tool  20  is positioned with respect to an intended position  30  at the workpiece  14 . 
         [0058]    To carry out a plurality of such joining processes one after the other, during which different studs  12  are joined one after the other to the same workpiece  14  or different workpieces  14 , it may be necessary to set the respective joining parameters differently for the individual processes. As a rule, the joining parameters are stored for the individual welding processes in a control device which is connected to the joining tool  20 . 
         [0059]    In order to be able to select the suitable set of joining parameters automatically, an RFID transponder  32  has been arranged in the region of the position  30 . In the illustration of  FIG. 1 , the transponder  32  is arranged on that side of the workpiece  14  which is opposite the joining location. As a rule, this embodiment is only relevant when the workpiece  14  is not made of a metallic material. As an alternative, the transponder  32  can also be arranged next to the position  30  or the joining location on the top side of the workpiece  14 . The transponder  32  is in this case preferably arranged temporarily, such that the workpiece  14  can then be further processed or delivered without the transponder  32 . Stored in the RFID transponder  32  is a parameter value which is selected from a range of values and contains the joining parameters or a reference to the joining parameters in the control device which are to be set for the joining process to be carried out at this position  30 . 
         [0060]    An RFID communication device  34  is provided on the joining tool  20 . As shown in  FIG. 1 , the RFID communication device  34  is preferably arranged in the region of the mouthpiece  28  of the joining tool  20 . The RFID communication device  34  is connected to a control device  36  of the joining tool  20 . Alternatively, the RFID communication device  34  can be connected to a control device which is connected to the tool  20  via the supply line  24 . 
         [0061]    When the mouthpiece  28  is put onto the workpiece  14 , before the joining process is actually carried out, the RFID communication device  34  reads the parameter value from the transponder  32  and transmits this parameter value to the control device  36 . With reference to the parameter value, the control device  36  selects the joining parameters relevant to this position and accordingly sets the joining tool  20 . Furthermore, an energy supply source can also be set with reference to this parameter value, said energy supply source being connected to the joining tool  20  via the supply line  24 . Finally, the type of stud  12  which is to be joined in the working step and is possibly fed automatically to the tool  20  can be alternatively or additionally selected via the parameter value. 
         [0062]    The joining process is then carried out with reference to the joining parameters selected automatically in this way. Manual selection errors of incorrect joining parameters can be avoided as a result. 
         [0063]    For the correct positioning of the joining tool  20  with respect to the joining position  30 , it is known to mount a positioning device  38  at the workpiece  14  beforehand. The positioning device  38  is attached to the workpiece  14  at schematically indicated attachment points  40 ,  41 , for example in the form of screwed connections or by clamping connections or the like. The connection should at any rate also be easily releasable again, since the positioning device  38  is secured only temporarily to the workpiece  14 . 
         [0064]    Furthermore, the positioning device  38  is preferably mounted at a distance  42  from the surface of the workpiece  14 . 
         [0065]    The positioning device  38  has a positioner in the form of a bushing  44 . The bushing  44  defines a joining axis  46  which, when the positioning device  38  is attached, is disposed exactly and perpendicularly at the position  30  on the surface of the workpiece  14 . 
         [0066]    To carry out the joining process, the mouthpiece  28  is inserted into the bushing  44  and then put onto the top side of the workpiece  14 . As a result, it can be ensured that the stud  12  is joined to the workpiece  14  at the correct position  30 . 
         [0067]    When such a positioning device  38  is used, it is preferred to secure the RFID transponder  32  to the positioning device  38 . It is especially preferred to secure a transponder to the positioning device  38  in the region of the bushing  44 , as schematically shown in  FIG. 1  at  32 ′. In this case, on account of the close proximity between the mouthpiece  28  and the bushing  44  during the joining process, reliable recognition and reading-out of the transponder  32  by means of the RFID communication device  34  can be ensured. 
         [0068]    Shown in  FIG. 2  is a further embodiment of a positioning device  38 ′. The positioning device  38 ′ has a plurality of bushings  44   a ,  44   b ,  44   c ,  44   d  which are rigidly connected to one another by struts. Furthermore, the positioning device  38 ′ has a plurality of stirrups or clamping devices  48 ,  50  for securing the positioning device  38 ′ to the workpiece  14 . 
         [0069]    The bushings  44   a - 44   d  define respective joining axes  46   a - 46   d , which define a plurality of different positions  30  on the workpiece  14 . 
         [0070]    Although not shown in  FIG. 2  for reasons of clarity, a transponder  32  is preferably arranged on at least one of the bushings  44   a - 44   d , as shown at  32 ′ in  FIG. 1 . Such a transponder  32 ′ is preferably arranged on each bushing  44   a - 44   d . The transponders  32 ′ of the bushings  44   a - 44   d  define, via the parameter value stored therein, in each case the joining parameter or the set of joining parameters which are to be used when carrying out the joining processes in the region of the respective bushings  44   a - 44   d.    
         [0071]      FIG. 3  shows a preferred embodiment of a bushing  44 ′, as can be used, for example, in the positioning device  38 ′ for each of the bushings  44   a - 44   d.    
         [0072]    The bushing  44 ′ of  FIG. 3  has an outer sleeve  54  made of a metallic material. The outer sleeve  54  has a shoulder  56  at a first longitudinal end, such that the outer sleeve  54  can be inserted into a preformed bore in a positioning device  38 . Furthermore, a notch  58  for fixing the bushing  44  to a positioning device is formed on the outer circumference of the outer sleeve  54 . 
         [0073]    Furthermore, the bushing  44 ′ has an inner ring  60 . The inner ring  60  is preferably made of a non-metallic and non-magnetic material, such as, for example, plastic or synthetic resin. For example, the inner ring  60  can be formed by casting a synthetic resin compound. 
         [0074]    A top side of the inner ring  60  is flush with the top side of the outer sleeve  54 . An underside of the inner ring  60  is in alignment with a radially inwardly projecting section of the outer sleeve  54 . An inside diameter of the radially projecting section of the outer sleeve  54  and an inside diameter of the inner ring  60  are coaxial to one another and jointly form a bore  66 . 
         [0075]    Integrated into the inner ring  60  is an electronic component in the form of a transponder chip  62 , which has a memory for storing the parameter value. In general, each transponder chip  62  has a unique ID worldwide, which can represent a reference to a parameter value. Furthermore, the chip  62  is connected to a coil  64  which forms an “aerial” of the transponder  32 ″. The transponder  32 ″ is preferably a passive transponder and does not have an independent energy supply source. The read-out operation is effected by the RFID communication device  34  supplying energy to the chip  62  by an inductive coupling with the coil  64 , said chip  62  reading out the memory by means of this energy and “transmitting” in turn the parameter value stored therein via an inductive coupling between the coil  64  and the RFID communication device  34 . As a result, the transponder  32 ″ can be read out. The transponder  32  can also be of the active type, which is fed from a battery. 
         [0076]    The coil  64  is preferably formed concentrically to the inner ring  60  and is jointly integrated with the chip  62  connected thereto into the inner ring  60 , preferably cast therein. As a result of the concentric arrangement of the coil  64 , it is possible to set up a communication with an RFID communication device which is substantially independent of a relative rotary position of the tool  20  about the joining axis  46 . 
         [0077]    On the top side of the inner ring  60 , an insertion taper  68  is provided in the region of the inside diameter in order to be able to insert the mouthpiece  28  more easily into the bore  66 . 
         [0078]    Shown in  FIG. 4  is a further preferred embodiment of a bushing  44 ″, which with regard to construction and functioning generally corresponds to the bushing  44 ′ of  FIG. 3 . The same elements are therefore provided with the same reference numerals. The differences are essentially explained below. 
         [0079]    It can thus be seen that the inner ring  60 ″ of the bushing  44 ″ is of multi-piece design, which can be advantageous from the production point of view. To be more precise, the inner ring  60 ″ has a first ring part  70 , which is arranged at the inner circumference of the outer sleeve  54 , and a second ring part  72 . The second ring part  72  forms, together with the first ring part  70 , a casting cavity and forms with its inner circumference the bore  66 . The casting cavity is filled with a casting compound  74  in which the RFID transponder is integrated, in particular cast. The coil  64  arranged rotationally symmetrically relative to the joining axis  46 ′ permits a communication with an RFID communication device independently of the relative rotary position. 
         [0080]    In the bushing  44 ″, the ring parts  70 ,  72  are arranged in such a way that they are jointly flush with a top side of the outer sleeve  54 . The casting compound  74  in this case bears against a shoulder (not designated in any more detail) of the outer sleeve  54 . 
         [0081]    The first and second ring parts  70 ,  72  are preferably electrically and magnetically non-conductive components and are preferably made of a plastic, for example by the injection moulding process. 
         [0082]      FIG. 5  shows an alternative embodiment of a joining arrangement  10 ′″. With regard to construction and functioning, said joining arrangement  10 ′″ generally corresponds to the joining arrangement  10  of  FIG. 1 . The same elements are therefore provided with the same reference numerals. The differences are essentially explained below. 
         [0083]    Inserted into the positioning device  38 ′″ is a bushing  44 ′″ which, with regard to construction and functioning, generally corresponds to the bushing  44 ″ of  FIG. 4 . The same elements are therefore provided with the same reference numerals. In contrast to the bushing  44 ″ of  FIG. 4 , the bushing  44 ″ has yet a third ring part  76 , which is arranged at the inner circumference of the inner ring  60 ′″. This third ring part encloses with the outer sleeve  54  an annular space, into which the inner ring  60 ′″ is inserted. The third ring part  76  can in this case likewise be made of a plastic, for example by an injection moulding process, but can, if need be, also be made of a metallic material in order to increase the abrasion resistance. 
         [0084]    In the bushing  44 ′″, a casting cavity which is open towards the top side of the bushing  44 ′″ is formed by the first ring part  70  and the second ring part  72 . Consequently, the casting compound  74  forms part of the top side of the bushing  44 ′″. 
         [0085]    In the bushings  44 ″ and  44 ′″ of  FIGS. 4 and 5 , the inner ring  60 ″ or  60 ′″ can be produced separately and can then be connected to the outer sleeve  54 , for example by pressing, adhesive bonding or the like. 
         [0086]    As shown in  FIG. 5 , a distance  42  is provided between the positioning device  38 ′″ and a top side of the workpiece (sheet)  14 . In a positioning device having a plurality of bushings, this distance  42  can vary from bushing to bushing. 
         [0087]    In the joining tool  20  shown in  FIG. 5 , provision is therefore made for the RFID communication device  34 ′″ to be mounted on the joining tool  20 ′″ so as to be movable in a direction parallel to the joining axis  46 . To be more precise, the RFID communication device  34 ′″ is secured to a slide  80  which is axially movable relative to the mouthpiece  28  of the joining tool  20 ′″. The maximum stroke of the slide  80  is shown at  82  in  FIG. 5 . 
         [0088]    The RFID communication device  34 ′″ has a coil  84  as “aerial”, which is arranged concentrically around the mouthpiece  28  and preferably has a diameter which is equal to the diameter of the coil  64  of the RFID transponder. When the mouthpiece  28  is inserted into the bore  66  of the bushing  44 ′″, the RFID communication device  34 ′″ comes into contact with the top side of the bushing  44 ′″. Different distances  42  from bushing to bushing can be compensated for by the axial displaceability of the slide  80 . 
         [0089]    The coil  84  is preferably likewise cast in a casting compound  86  which has been accommodated in a cavity of a sleeve component  88 , which is connected to the slide  80 , or is formed by the slide  80 . During a movement relative to the mouthpiece  28  or the housing of the joining tool  20 ′″, the slide  80  can be guided on the mouthpiece  28  and/or on housing parts  91 ,  92  and/or on one or more guide rods  89 . The latter can also serve as anti-rotation locking means. 
         [0090]    Furthermore, the slide  80 , as shown, can be preloaded elastically in the joining direction by means of a spring device  90  (consisting of one or more springs). As a result, it can be ensured that the RFID communication device  34 ′″ is also pressed onto the top side of the bushing  44 ′″ irrespective of the distance  42  or other ambient conditions in order to minimize an air gap in between. As a result, the communication reliability can be increased. In the ideal case, the casting compounds  86 ,  74  bear directly against one another, such that no air gap at all is formed. Consequently, an inductive coupling between the coils  64 ,  84  can be realized with a lower magnetic resistance. It goes without saying that this equally applies if not only a casting compound but possibly also sections of other non-metallic or non-conductive elements, such as sections of the ring parts  70 ,  72  for example, are arranged between the coils  64 ,  84 . 
         [0091]    It will be appreciated by persons skilled in the art that the above embodiments have been described by way of example only, and not in any limitative sense, and that various alterations and modifications are possible without departure from the scope of the invention as defined by the appended claims.