BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention concerns a method for inspecting a weld seam. The weld seam between the two materials is produced by a laser beam One of the plastic materials of which the workpiece is made is essentially transparent to this laser radiation, whereas the other plastic material absorbs the laser radiation. 
     2. Description of the Related Art 
     In the previously known method of this type, the inspection was performed with a pyrometer, which responds to the thermal radiation emitted by the weld seam that has been produced. For the pyrometer to reach the highest possible temperature, the measurement must be made during welding. In addition, the material of which one of the plastic parts is made must be capable of transmitting thermal radiation. 
     Laser radiation has proven to be very effective for welding plastics. There are three welding methods for laser radiation, namely, “track welding”, “simultaneous welding”, and “quasi-simultaneous welding”. Pyrometric inspection of the weld seam is possible only with track welding and cannot be used in simultaneous welding or quasi-simultaneous welding, although the latter provides a time advantage over track welding. In the case of a welded product produced by the simultaneous welding method, inspection of the welded materials could be made only indirectly, by dimensional changes in the workpiece and could only be used with certain workpiece geometries. The quality of a simultaneous weld or quasi-simultaneous weld generally could not be inspected. 
     The pyrometric weld inspection possible in the case of track welding is also subject to error and can be used only if the material of one of the plastic parts transmits not only laser radiation but also thermal radiation. Therefore, in many cases, weld inspection of the finished workpiece is not possible at all. 
     In one well-known method of a different type (DE 196 03 675 A1), the weld joint is produced by contact welding of two superimposed plastic sheets from which a bag is to be produced. Each of the two plastic sheets itself consists of two layers, namely, a transparent, infusible outer support layer and a colored inner sealing layer. The contacting sealing layers of the two sheets are welded together by two heated sealing jaws pressing against each other. As a result of the weld joint on the colored sealing layers, the transparency of the weld seams changes relative to places that were not welded or were not adequately welded. These transparency differences are determined by a light transmission method and used to inspect the quality of the weld joints. The two sheets are transilluminated in the region of the weld seam by a light source, and the light emerging at the other side of the sheet is detected by a sensor and analyzed. This well-known method cannot be applied to weld seams produced by laser radiation, because one of the plastic parts of this welded product is absorbent and therefore opaque. 
     In another method for inspecting weld seams in bags produced from two sheets, which are then to be immediately filled with some substance (U.S. Pat. No. 5,260,766 A), laser light is projected into a transparent heated seal bar through a large number of glass fibers. In this way, the light reaches the contact area between this heated seal bar and an opposing bar, between which the two sheets are positioned and sealed. The light reflected from this contact area must pass back through the transparent material of the heated seal bar to produce an image, which is picked up by a camera and analyzed to determine whether particles of the material used to fill the bag are enclosed in the weld seam. This makes it possible to determine the quality of the weld seam. This method can be used only with thin flat sheets in which linear weld seams are formed and requires a transparent welding tool. It cannot be applied to the laser welding of three-dimensional plastic parts with two-dimensional or three-dimensional weld seams, especially if two plastics with different optical and/or mechanical properties are to be welded together. 
     Furthermore, it is also well known (DE 298 16 401 U1) that a transillumination technique can be used to detect cracks in a welded lap joint produced by the lap welding of sheets. In this method, the weld seam is placed between an optical transmitter and an optical receiver. To increase the accuracy of the measurement, this transillumination technique is carried out in a liquid with an extremely low viscosity. This method is not suitable for the inspection of weld seams produced by laser radiation between two plastic parts, one of which is absorbent. 
     Finally, it is well known (JP 10[1998]-100,259 A, Patent Abstracts of Japan, Vol. 1998, No. 09, Jul. 31, 1998) that two similar polyethylene materials can be irradiated with broadband infrared radiation. As long as the resulting weld seam is in the liquid state, the infrared radiation reflected by the liquid or passing through the liquid is optically detected and analyzed. A laser beam is not used. Inspection radiation is not used in addition to the infrared radiation, so it does not matter whether one of the polyethylene materials is transparent to inspection radiation. 
     SUMMARY OF THE INVENTION 
     The objective of the invention is to develop a reliable inspection method of the type specified above, which avoids the aforementioned disadvantages of the state of the art. In accordance with the invention, this objective is achieved by the measures specified below. 
     In the interior of the welded product, reflections of the inspection radiation occur at all material interfaces between the two plastic parts of the workpiece, and, in accordance with the invention, the reflections of the inspection radiation from the already solidified, finished weld seam are analyzed. If the welding seam should have an unintended gap where imperfect welding or no welding has taken place, the reflections emanating from this location are of course also detected and evaluated in the same manner. If the weld seam that has been produced is imperfect, the radiation emerging from the workpiece is significantly changed. The quality of the weld seam can be clearly determined in this way. An evaluation unit detects the inspection radiation emerging from the welded workpiece and triggers suitable reactions in a monitoring device in the event of problems with the measured inspection radiation due to an imperfect weld. 
     The invention proposes two different measures as inspection radiation, each of which has independent inventive significance. One possibility, in accordance with Claim  2 , consists in using additional radiation, that is completely independent of the laser radiation, for the inspection. It is only necessary to select as the inspection radiation an electromagnetic frequency at which at least one of the two plastic parts is transparent to this inspection radiation. These measures can then be used not only during the welding operation itself, but also later on the finished welded product. This control method could also be used if the welding seam has not been produced by laser radiation, but in some other manner. 
     However it is especially advantageous to use the laser radiation used to produce the weld seam as the inspection radiation. The measurement then detects the radiation emanating from an already solidified area of the resulting weld seam. This is possible, because the laser radiation reaching the interior of the workpiece is scattered inside the workpiece, and by suitably offsetting the detector, the area that is detected is some distance from the focus of the laser beam. The laser radiation that has already been repeatedly scattered in the laser-welded workpiece is used for the inspection. In this way, one obtains results based on reflections after solidification of the weld seam. When an unsatisfactory result is obtained, the defective workpiece can be removed immediately. 
     Additional measures and advantages of the invention are apparent from the dependent claims, the following description, and the drawings. Several embodiments of the invention are illustrated in the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic perspective view of an example of a welded product, namely, a transponder integrated in a two-layer workpiece. 
         FIG. 2  shows a first, photometric method for inspecting the quality of the weld seam. 
         FIG. 3  shows the operating principle of the method used in  FIG. 2 , when there is no weld seam between the two plastic parts of the workpiece. 
         FIG. 4  shows, in a representation analogous to  FIG. 3 , the conditions that exist, when a weld seam has formed between the two plastic parts. 
         FIG. 5  shows another method of inspection in accordance with the invention for determining the quality of the weld seam. 
         FIG. 6  shows a schematic longitudinal cross section through a device for another embodiment of the inspection method of the invention. 
         FIG. 7  shows an enlarged section of the workpiece indicated in  FIG. 6 , on the basis of which the special manner of operation of this method will be explained. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The workpiece  10  shown in  FIG. 2  consists of two plate-like plastic parts  11 ,  12 , which, at least in certain places between them, have a contact surface  13 , on which a weld seam  15  is to be formed. An example of a finished welded product of this type is illustrated in  FIG. 1 . 
     For better recognition of the parts, the upper plastic part  11  is shown as transparent in  FIG. 1  to allow a view of its contact surface  13  with the other plastic part  12  beneath it. The second plastic part  12  has a seat  14  for holding a plate-shaped transponder  16 , which is able to receive, store, and transmit electronic data. A transponder of this type is temperature-sensitive and must be protected from environmental influences, such as moisture. The inserted transponder  16  is enclosed here by an annular weld seam  15 , which joins the two layers of plastic  11 ,  12  in the area of the contact surface  13 . This weld seam  15  provides media-tight enclosure of the transponder inside the workpiece  10 . 
       FIG. 2  shows a first method for both producing and inspecting a weld seam  15  of this type. This is accomplished with a combined device  40  for producing and directing a laser beam  20 , which is shown as a shaded arrow. The laser beam is produced in a semiconductor laser module  21 . The laser beam  20  strikes a deflecting mirror  22 , which is transparent to another, especially electromagnetic, beam  30 , whose formation will be described in greater detail below. As the beam path in  FIG. 2  shows, the laser beam  20  strikes two movable beam-deflecting mirrors  23 ,  24 . These two mirrors  23 ,  24  are moved in a well-defined way to direct the laser beam  20  to the workpiece  10  through a theta objective  35 . With the two mirrors  23 ,  24 , the weld seam  15  can be produced by the simultaneous welding method mentioned at the beginning, which can be carried out especially fast and inexpensively. The two plastic parts  11 ,  12  of the workpiece have the following properties in this case: 
     The material  18  of the first plastic part  11  is essentially transparent to the laser beam  20 , but the material  19  of the second plastic part  12  absorbs the laser beam  20 . The transparent plastic may consist of an amorphous material and thus cause little scattering. However, the plastic  18  may also be semicrystalline, i.e., it may have a large scattering effect. This causes liquefaction of the two plastic materials  18 ,  19  in some areas of the contact surface  13 . The enlargement in  FIG. 4  shows what happens in the workpiece  10 .  FIG. 4  shows the cross section of the resulting weld seam  15 , which consists of a mixture of the two starting materials  18 ,  19 . As  FIG. 4  shows, compared to the original contact surface  13 , another interface  25  forms relative to the two plastic materials  18 ,  19 , which remain unmixed and enclose the interface  25 . 
     Next to the workpiece  10 , there is a source  31  for electromagnetic radiation  30 , which is independent of the laser beam  20  and hereinafter will be referred to simply as “inspection radiation” for reasons that will become apparent. Used as the inspection radiation  30  may also be a laser radiation, but also another electromagnetic radiation, such as ultraviolet or infrared radiation, or also visible light. The selection is also in this case dependent on the materials used. As  FIG. 2  illustrates, this inspection radiation  30  is introduced into the workpiece  10  at a suitable angle  26  to the laser beam  20  entering from the combination device  40 . In the present case, the plastic material  18  of the upper layer  11  should also be transparent to the inspection radiation  30 . What then happens is explained in greater detail below with reference to  FIG. 3 , on the one hand, and to  FIG. 4 , on the other hand. 
       FIG. 3  shows the conditions in the workpiece  10 , when, in the extreme case, no welding of the plastic parts  11 ,  12  occurs at their contact surface  13 . In the interior  27  of the workpiece  10 , the inspection radiation  30  experiences multiple reflections  32  between the contact surface  13  and the outer surface  17 . A portion of the reflected radiation  32  striking the outer surface  17  exits, as the arrows  33  in  FIG. 3  show, and is gathered by the theta objective  35  of the device  40  of  FIG. 2 . As  FIG. 2  makes clear, this exiting inspection radiation  33  travels along the segments  28 ,  29  of the optical path in the device  40  that the laser beam  20  follows. However, due to the transparency of the deflecting mirror  22  to the inspection radiation, this radiation passes through the deflecting mirror  22  and a lens  34  and reaches a sensor  36 , which is connected to an evaluation unit  37 . The evaluation unit  37  detects the measured inspection radiation  33  and triggers the desired reactions in devices  38  connected at its output end as a function of the detected radiation  33 . In the present case, the device  38  is a monitor, whose screen displays the quality of the weld seam  15  that has been produced in the workpiece  10 . 
     If a weld seam  15  is present in the area of the workpiece  10  that is being covered, then, depending on the condition of the weld seam, the special conditions shown in  FIG. 4  result. In front of the weld seam  15 , the reflected radiation  32  and the exit radiation  33  described above remain the same relative to the angle of incidence of the inspection radiation  30 , but after the weld seam  15 , the reflected radiation  32 ′ and exit radiation  33 ′ deviate significantly. The rough interface  25  in the region of the weld seam  15 , where diffuse scattering  32 ″ occurs, contributes to this deviation. 
     This has the result that the area covered by the device  40  in  FIG. 2  produces cumulative exit radiation  33 ′ that differs significantly in  FIG. 4  from that which is produced in the corresponding area without a weld seam in the workpiece  10  in  FIG. 3 . Multiple reflections occur in the transparent plastic  11  and possibly also in the area of the weld seam  15 . The region of the exit radiation of interest  33 ′ can be detected by adjusting the optics. This is accomplished by a sensor  36 , whose output is connected to an evaluation unit  37 . The measurement result is displayed on a display device  38 . The use of the method of the invention showed that even small deviations from the reference value in the formation of the weld seam  15  can be clearly detected. Therefore, workpieces with weld seams  15  that are within tolerance can be clearly distinguished from those to be regarded as rejects. 
     The method described above can be used not only for workpieces  10  in which the weld seam is formed by laser radiation, but also in workpieces in which the weld seams are produced by any other desired method, e.g., friction welding or ultrasonic welding. Moreover, the method of the invention does not have to be used at the same time as the welding operation, because, in contrast to the state of the art, the thermal radiation emitted by the weld seam is not used for the measurement. As was explained above, this method uses inspection radiation  30  that is completely independent of the thermal radiation and can be used at any time. The inspection radiation  30  can also act from several sides on the workpiece  10 . Consequently, it is also possible to use several radiation sources  31 . This can be done, for example, with the additional method of the invention shown in  FIG. 5 . 
     In  FIG. 5 , analogous parts are indicated by the same reference numbers as in the preceding embodiment, and to this extent, the preceding description also applies here. It will only be necessary to describe the differences. 
     In  FIG. 5 , the workpiece  10  previously described in connection with  FIG. 1  is being inspected with respect to the quality of the weld seam  15  produced in it. The inspection radiation  30  can act on the workpiece  10  from several sides. Therefore, as  FIG. 5  illustrates, several radiation sources  31  can be used. In  FIG. 5 , the inspection radiation  30  is directed at the workpiece  10  from all sides. Depending on the quality of the weld seam  15 , there are differences in the radiation  33  exiting the workpiece  10 . In  FIG. 5 , this radiation  33  is detected by a CCD camera  39 , which receives an image of the weld seam  15 . The image is analyzed in the associated evaluation unit  37  by image-processing software. A suitably intensified and enlarged image  41  of the weld seam  15  previously produced in the workpiece  10  then appears on the display device  38 . Depending on the result of the image  41 , suitable reactions can then be carried out by monitoring personnel or by an automatic monitoring unit. 
     However, it is also possible to use as the inspection radiation  30  the laser radiation  20  itself, however, offset with respect to time from the welding process. In that case, the laser radiation  20  is once again moved along the path of the welding seam  15  after the welding procedure has been concluded. It is advantageous if the laser radiation  20  is raised to a lower power in order not to damage the workpiece  10  or the welding seam  15 . Depending on the quality of the welding seam  15 , the radiation  33  emanating from the workpiece  10  differs. Depending on the determined quality, appropriate reactions can be carried out by a control person or by an automated monitoring device. 
       FIG. 6  shows a processing head  50 , which can be moved relative to a workpiece  10  in the directions indicated by the two arrows  42 . A semiconductor laser module not shown in  FIG. 6  produces a laser beam  20 , which enters the processing head  50  by an entrance  61 , specifically, a beam waveguide, and is indicated by solid arrows in  FIG. 6 . The laser beam  20  is collimated by a lens  45 , passes through two mirrors  43 ,  44  and is bundled by a collimator  46  and focused on a well-defined area at  47 . As the enlargement in  FIG. 7  shows, the focus  47  is then located on the contact surface  13  between the two plastic parts  11 ,  12  already described in connection with  FIGS. 3 and 4 , of which the upper part  11  is made of a plastic material  18  that is transparent or either slightly or strongly scattering. The critical factor is that the plastic part  12  consists of a plastic material  19  that absorbs the laser radiation  20 . The wavelength of this laser radiation  20  can be about 800-940 nm. 
     A melt  48  of both materials  18 ,  19  forms in the area of the focus  47 . During the movement  42  of the workpiece  10  relative to the processing head  50 , the focus  47  moves along the workpiece, and the melt gradually undergoes solidification  49 . The weld seam  15  forms in this way. At the same time, the laser light  20  in the interior of the transparent or slightly scattering material  18  of the first plastic part  11  is scattered in analogy to  FIGS. 2 and 4 . The scattered radiation is illustrated by solid arrows  52 . By repeated reflections, the scattered radiation  52  also reaches a specific inspection point  57 , which is located at a well-defined distance  51  from the focus  47 . After repeated scattering  52 , exit radiation  53  emerges, which is indicated by short-dash lines in  FIGS. 6 and 7 . The exit radiation  53  emerging from the inspection point  57  is detected by the optical component  46  and collimated. The exit radiation  53  passes through the lower mirror  44  but is reflected by the upper mirror  43  and finally reaches a detector  55 . The detector  55  is offset  54  relative to a central axis that defines the optical beam path, but is not shown in the drawing. This offset  54  takes into account the above-mentioned distance  51  of the observed inspection point  57  from the melting point  48 . The same process occurs at the detector that has already been described in connection with the first embodiment, following the sensor  36 . 
     The processing head  50  also has a radiation source  31  for electromagnetic inspection radiation  30  that is independent of the laser light, and has a wavelength of, for example, 750-800 nm. This processing head  50  thus makes it possible, as an alternative to or in addition to the above-described inspection based on the exit radiation  53  of the welding beam  20 , to perform an inspection that is independent of that method of inspection. This inspection can also be made by means of a detector  55  that detects the above-described inspection point  57  in the workpiece  10 . An optical component  56  focuses and collimates the exit radiation  53  and the inspection radiation  30 , respectively. 
     Finally, a pyrometer  58  is also integrated in the processing head  50 . The pyrometer  58  detects the thermal radiation, which is indicated in  FIG. 6  by dotted arrows  60 , after the thermal radiation passes through an optical component  59 . The thermal radiation  60  is emitted by the weld. This is used to regulate the melt temperature. At the same time, the welding result can be inspected by the inspection radiation  30 , whose exit radiation  33 ,  33 ′ from the inspection point  57  is detected by the detector  55 . The processing head  50  can thus cover all three of the methods described above for inspecting the resulting weld seam  15  or weld  47 . These measurement results are then analyzed either together or separately in connected devices. 
     The pyrometer  58  can be integrated with the source of the laser light  20  in a feedback control system. The thermal radiation emitted by the weld  47  is detected by the pyrometer  58  and analyzed in the connected devices. In the event of deviations from a desired reference value, the result of the analysis is used to regulate the intensity of the laser light  20 . 
     The above-described components integrated in the processing head  50  may also be housed in individual units. These individual units are then placed together to form groups of units.
The invention relates to an inspection method for inspecting the quality of a weld seam ( 15) during which a material, which is transparent to electromagnetic radiation ( 30) of a defined frequency, is used in a workpiece ( 10) consisting of two plastic parts ( 11, 12). In order to be able to reliably inspect the weld seam, the invention provides that an electromagnetic inspection radiation ( 30) is irradiated inside the workpiece ( 10). The resulting reflections between the boundary surfaces in the workpiece ( 10) and the portions of inspection radiation ( 30, 30 ') exiting from the workpiece ( 10) are affected by the quality of the produced weld seam ( 15). ( 15). By measuring the exiting radiation ( 33, 33 '), it can be clearly determined whether the workpiece ( 10) has a defective or a tolerable seam ( 15).