Abstract:
A method and apparatus are provided for monitoring the wire-stitching on print products in a wire-stitching machine. The wire-stitching includes wire staples having ends to be closed. The wire-stitching machine includes a measuring device operative to measure a density on the ends of passing wire staples to test the quality of the passing wire staples.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application claims the priority of European Patent Application No. 04405256.1-2304, filed on Apr. 26, 2004, the subject matter of which is incorporated herein by reference. The disclosure of all U.S. and foreign patents and patent applications mentioned below are also incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   The invention relates to a method and device for monitoring the wire-stitching on print products in a wire-stitching machine, wherein measuring devices are provided for testing the wire staple quality. 
   The technique of stapling together print products by means of wire staples in a wire-stitching apparatus is known. Wire-stitching machines typically comprise a stitching head and a wire-bending device for realizing the stitching operation. The operation involves supplying a wire, cutting the blank, forming the staple, pushing the staple through the product to be stapled, and bending the two staple legs. 
   Methods and devices are known which can be used to test for the existence of a wire staple on a product, thus making it possible to remove a product that is missing a wire staple. The testing can be realized, for example, by means of a metal detector which is arranged downstream from the wire-stitching apparatus. Each passing wire staple triggers an impulse. A missing impulse therefore indicates a product with a missing wire staple. Furthermore, European Patent Document EP 0 205 144 teaches an apparatus wherein a missing wire staple is detected by means of a sensor arranged on a stitching machine, wherein the sensor comprises one of an approximation switch or an optical sensor. 
   However, the above-mentioned methods and devices can only be used to detect the presence, and not the quality, of the wire-stitching. Thus, wire staples which are defective, for example those that have an outward-projecting leg, are nonetheless indicated as being present. Wire-stitching defects of this type are highly undesirable because they can result in injuries to the users and/or readers of such a print product. For that reason, numerous measures have already been proposed for detecting such defective stitching operations and for removing the corresponding print products. Thus, a device for monitoring the stitching of products is known from European patent document EP 1 029 643 A, which is co-owned by the assignee of the present application, wherein the wire-stitching machine is provided with measuring devices for detecting changes in the condition of the bending device or stitching head. For example, these devices use wire strain gauges to detect the force generated at the bending device during the forming of the wire staple. If this force deviates from a predetermined value, it is assumed that the wire-stitching is defective and the product is accordingly removed via the machine control. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to make available further suitable measures for testing wire-staple quality. 
   The above and other objects are achieved according to the invention by the provision of a method for monitoring wire staples on print products applied by a wire-stitching machine, the wire staples having ends to be closed by the wire stitching machine, the method comprising: arranging a measuring device to measure a density of the ends of the wire staples passing by the measuring device; and evaluating a curve obtained from the measured density to test a quality of the passing wire staples. 
   The invention is based on the finding that with defective wire staples, e.g. staples where a leg is projecting or missing, the density curve deviates considerably from that of a non-defective wire staple. With a non-defective wire staple both legs are present and are bent in the intended manner to rest against the print product, such that the staple normally does not pose a risk of injury. With the method according to the invention, however, other defects in a stitching operation can also be determined. For example, the method can also be used to detect defects in so-called eyelet wire staples, such as bent eyelets. 
   According to another exemplary embodiment of the invention, the measuring device is positioned downstream of a stitching head of the wire-stitching machine. The passing wire staples are tested. A method of this type is particularly suitable for a gathering and wire-stitching machine on which print products are conveyed on a transport chain. 
   According to another exemplary embodiment of the invention, the measuring operation is particularly reliable and operationally safe if the measuring device is positioned on the inside of the opened product during the measuring operation. In this embodiment of the invention, the measuring device can be moved comparatively close to the wire staples to be tested. 
   In yet another exemplary embodiment of the invention, the measuring device includes a sensor that generates a magnetic field. The wire staples to be tested pass through the magnetic field, thus permitting a particularly precise testing of the density curve of each wire staple. More particularly, the measuring device includes an electric resonating circuit having a coil, the inductance of which is change by the density of the metal staple as it passes by the measuring device. 
   The invention furthermore relates to an apparatus to monitor wire staples applied by a wire-stitching machine on print products, the apparatus comprising: a measuring device operative to measure a density of wire staples inserted into print products passing by the measuring device; and an evaluating device to evaluate a density curve obtained from the measured density to test the quality of the wire staples. 
   In a further exemplary embodiment of a gathering and wire-stitching machine according to the invention, the measuring device is arranged near the transport chain, below the opened print products, such that the staples can be closely measured at the ends to be closed. 
   Further advantageous features will become apparent from the following description, drawings and examples. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An exemplary embodiment of the invention is explained in further detail with the aid of the accompanying drawings. 
       FIG. 1  schematically depicts a partial section through a wire-stitching machine provided with a device according to the invention. 
       FIG. 2  schematically depicts a spatial view of a wire-stitching print product and a device according to the invention. 
       FIG. 3  depicts a block diagram of a resonating circuit for implementing a sensor according to an embodiment of the invention. 
       FIGS. 4   a  and  4   b  schematically depict representations of non-defective wire staples. 
       FIGS. 5   a  and  5   b  schematically depict representations of defective wire staples. 
       FIG. 6  is a functional block diagram of components for the apparatus according to the invention. 
       FIG. 7  depicts the signal curve for a print product, provided with two spaced apart non-defective wire staples, wherein the horizontal axis represents time and the vertical axis represents density. 
       FIG. 8  depicts the signal curve for a wire staple with one cut-off leg, wherein the horizontal axis represents time and the vertical axis represents density. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   According to an exemplary embodiment of the invention,  FIG. 1  shows a print product  1  which is located on a gathering and wire-stitching machine S that is known per se in the print-processing industry. The print product  1 , for example, is a booklet consisting of several pages  2 , for example held together along a spine  3  with two wire staples  10 , as shown in  FIG. 2 . The product  1  can also be held together by a single wire staple  10  or by more than two wire staples  10 . The wire staple  10  is a standard wire staple as shown in  FIG. 4   a , having a substantially straight wire staple back  10   b  and two wire staple legs  10   a  that are bent by 180 degrees. The wire staple legs  10   a  form the ends of the wire staples  10  and, as can be seen, are bent toward the inside so that they rest against the inside  4  of the print product  1  pointing toward one another. The wire staples  10  can also be designed as shown in  FIG. 4   b  to comprise an eyelet  10   c  in the center which projects upward from the spine  3  and/or the outside  5  of the print product  1 . The wire staple  10 ′ also has wire-staple legs  10   a  which are bent toward the inside. Other wire staple configurations are also conceivable. 
   The gathering and wire-stitching machine S comprises a saddle  8  with saddle ridge  9  which is rigidly attached to a frame of the gathering and wire-stitching machine S, not shown herein. The print products  1  are transported by a transport chain  6  which is an endless link chain provided at specified intervals with wing-type carriers  7  that carry along the print products  1 . The gathering and wire-stitching machine S and the transport chain  6  in this case are only examples for transporting and/or gathering devices for assembling print products  1 , e.g. booklets. Thus, other transporting means can also be used for transporting the print products  1 . 
   The wire staples  10  are formed in a wire-stitching machine S having a stitching head  11 , which is arranged so that the print products  1  are stapled from above, as shown in  FIGS. 1 and 2 . During the wire-stitching operation, the print product  1  is arranged between the stitching head  11  and a bending device, not shown herein. Once the staple is formed, it is then punched through the print product  1  and the two staple legs are bent in the manner known per se with the aid of leg benders which are also not shown herein. For example, if two wire staples  10  are formed as shown in  FIG. 2 , the print product  1  is transported on the transport chain  6  in the direction of arrow  22  ( FIG. 2 ) for further processing. For example, the print product  1  is supplied to a cutter. 
   If the above-described operation for forming the wire staple  10  is faulty, the print product  1  may contain defective wire staples  10 ″ or  10 ′″ shown in  FIGS. 5   a  and  5   b , respectively. For example, one staple leg  10   a ′ of the wire staple  10 ″ shown in  FIG. 5   a  is not bent toward the inside, as intended, but instead projects outward from the wire staple back  10   b  at about 90 degrees. The wire staple leg  10   a ′ accordingly projects on the inside  4  of the print product  1 . Likewise, one staple leg  10   a ′″ of the wire staple  10 ′″ is also not bent correctly. The two wire staples  10 ′″ and  10 ′″ carry the risk of injury to the user of the print product  1 , this danger being particularly high for children. 
   According to an exemplary embodiment of the invention shown in  FIGS. 1 and 2 , at least one measuring device  12  is provided for detecting such defective wire staples  10 ″ and  10 ′″, as well as other defective forms, and for removing the respective print products  1 . In a further exemplary embodiment, the measuring device  12  is arranged such that the print products  1  are transported across the measuring device  12 , as shown in  FIGS. 1 and 2 . The measuring device  12  is located on the inside  4  of the opened print product  1  and directly below a spine  3  of a print product  1 . 
   According to  FIG. 6 , each measuring device  12  is provided with a sensor  13  comprising a sensor head  17  having a coil  18  arranged therein. The sensor  17  is connected via a signal line  14  to an oscillator  19  in an evaluation unit  20  for signal processing. The evaluation unit  20  also includes a sensor card  21  with microprocessor. As shown in  FIG. 3 , the sensor  13  of the measuring device  12  includes a resonating circuit K. The resonating circuit K comprises the above-referenced coil  18  which is connected in parallel with a capacitor  24  and a resistor  25 . Also provided are an ASIC (application-specific integrated circuit)  26 , a rectifier  27 , a low pass  28 , and a microcontroller  29 . The aforementioned components and the mode of operation of such a resonating circuit are known to the person skilled in the art. 
   Furthermore, as shown in  FIGS. 1 and 2 , the stitching head  11  of the wire-stitching machine S includes a locally fixed brush  23  for pressing the spine  3  of the passing print product  1  downward against the ridge  9 . The measuring device  12  is arranged below and downstream from the stitching head  11 . As a result, the distance between the spine  3  and the sensor  13  is essentially always the same. The formed wire staples  10  thus pass across the sensor  13  with uniform spacing. Alternatively, the sensor  13  could move back and forth in the transport direction. Thus, the relative movement between sensor  13  and print product  1  is important. When a wire staple  10  is positioned above the sensor  13 , the wire staple  10  influences the inductance of the resonating circuit K and causes the frequency to change. This frequency change signal is detected by the evaluation unit  20  with oscillator  19  and sensor card with microprocessor  21 . The inductance of the resonating circuit K depends on the metal density of the wire staple  10 . The metal density for each wire staple  10  is the amount of metal per unit of length. As a result, the signal curve substantially corresponds to the shape of the wire staple  10 . Since the shape of wire staples  10 ′′ and  10 ′″ differs substantially from that of wire staple  10 , the signal curve differs in the same way, wherein this difference is illustrated in the following with the aid of  FIGS. 7 and 8 . 
     FIG. 7  shows the signal curve during the testing of a print product  1  with two non-defective wire staples  10  which are arranged at a distance from each other, as shown in  FIG. 2 , wherein the spacing between the two wire staples  10  is 27 mm. The staples are formed from a copper wire or steel wire having a diameter of 0.6 mm. The two staples  10  generate two pulses P 1  and P 2 , as shown in  FIG. 7 . From these peaks, digital signals D 1  and D 2  are generated with the aid of an algorithm. This algorithm is explained in further detail in the following. 
   An idle signal indicates the normal, uninfluenced state of the sensor  13  and forms the basis of the algorithm. This idle signal is temperature-dependent and can be influenced by surrounding metal parts. Consistent operation of the sensor  13  is ensured by a reference signal generated by machine control unit  16  to continuously adjust the idle signal. This reference signal is generated during the start-up of the gathering and wire-stitching machine S. 
   The digital signals D 1  and D 2  are generated by the above-mentioned algorithm if a wire staple  10  is located above the sensor  13 . A threshold  30  that is below the idle signal is additionally computed. When an analog signal  100  drops below the threshold  30 , the digital “wire staple detected” signal D 1  is emitted and a hysteresis value is added to the threshold value  30 , thus preventing a bouncing at the switching point  110 . The aforementioned threshold  30  follows the actual analog signal  100  until a minimum  120  is reached. Once the analog signal  100  reaches the minimum  120 , the threshold  30  remains constant. When the analog signal  100  subsequently exceeds the threshold  30 , the digital “wire staple detected” signal D 1  is reset and a new threshold  30  is computed on the basis of the analog signal  100 . The threshold  30  again follows the analog signal  100  until a maximum value  130  is reached. Following this, the threshold  30  remains constant, awaiting a new drop below the threshold  30  due to a new wire staple  10 . 
   The degree of adaptation of the threshold  30  can be adjusted via two parameters, wherein one parameter adjusts the strength of the adaptation in the OFF state and the other parameter adjusts the adaptation of the threshold  30  in the ON state of the digital “wire staple detected” signal. In the normal, uninfluenced state, a specified offset to the idle signal is subtracted to compute the threshold  30 . 
   As shown in  FIG. 7 , a passing print product  1  having two non-defective wire staples  10  or  10 ′ generates two digital signals D 1  and D 2 . In contrast, a passing print product  1  having one defective wire staple  10 ″ or  10 ′″ therein generates two digital signals D 1 ′ and D 2 ′, as shown in  FIG. 8 . Owing to the density curve for a defective wire staple  10 ′′ or  10 ′′′, the corresponding peak P′ is irregular. As a result, two digital signals D 1 ′ and D 2 ′ are generated for one defective wire staple  10 ″ or  10 ′″ instead of just one digital signal per wire staple as in the case of a passing non-defective wire staple  10 ′. The control recognizes that two digital signals D 1 ′ and D 2 ′ are generated for the defective wire staple  10 ″ or  10 ′″ and triggers the removal of the defective print product  1 . 
   The invention has been described in detail with respect to preferred embodiments, and it will now be apparent from the foregoing to those skilled in the art, that changes and modifications may be made without departing from the invention in its broader aspects, and the invention, therefore, as defined in the appended claims, is intended to cover all such changes and modifications that fall within the true spirit of the invention.