Abstract:
An apparatus and method for on line detection and measurement of cold emulsion adhesives, including a sensor of a presence of an adhesive on a substrate, the sensor being arranged to sense an amount of absorption of two specific wavelengths of near infrared energy by the adhesive, the two specific wavelengths including a first wavelength within an absorption band for water and a second wavelength outside an absorption band for water. The two sensed amounts are converted into two signals, a difference between the signals being indicative of the moisture content of the adhesive. If the two signals are the same, then the moisture content approximates zero. Otherwise, the difference is proportional to the moisture content.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to an apparatus for on-line detection and measurement of cold emulsion adhesives applied to a substrate based on determining near infrared light energy absorption at two specific wavelengths, one being within an absorption band of water and the other being outside the absorption band of water. 
     2. Discussion of Related Art 
     On-line measurement of key glue-line characteristics of corrugated board is known from U.S. Pat. No. 5,663,565 (the &#39;565 patent), entitled SYSTEM AND METHOD FOR THE ON-LINE MEASUREMENT OF KEY GLUE-LINE CHARACTERISTICS ON CORRUGATED BOARD and issued Sep. 2, 1997 and whose contents are incorporated herein by reference. According to its disclosure, the measurement signal is typically generated by either an infrared absorption sensor, which is used to measure the mass of moisture and/or starch in the glue-line, or an infrared pyrometer, which is used to measure the temperature of the glue-line. The chosen sensor is installed so as to measure the exposed glue-line prior to bonding of the medium with a liner. In addition, the disclosure reveals sensing isolated glue-based variations found in the sensor output signal that oscillate at a known fluting frequency of the corrugated board. 
     Variation in the sensor output signal that are not oscillating at the fluting frequency are canceled out, while those that are oscillating at the fluting frequency are isolated and amplified. An incremental amount of infrared radiation that is absorbed by starch and/or water in the glue-lines is isolated from the predominant, more random background absorption due to cellulose and water in the paper substrate. The amplitude of the extracted signal component, which reflects only starch and/or water in the glue, is then converted using empirically derived calibration constants into a final starch mass value. The output signal of an infrared absorption sensor, therefore, provides an improved on-line starch measurement for corrugators. 
     While the &#39; 565  patent mentions that to date no on-line glue measurement techniques have been successfully commercialized, the present inventors believe that have found a solution. The conventional problem with glue application on substrates lies with detection and correction of irregularities, which the &#39; 565  patent identifies and elaborates upon in some detail. However, the present inventors have devised a technique to discriminate the adhesive from its substrate and to quantify the moisture content of the substrate and adhesive to promote a more uniform glue application. 
     SUMMARY OF THE INVENTION 
     One aspect of the invention relates to an apparatus and method for on line detection and measurement of cold emulsion adhesives, including a sensor of a presence of an adhesive on a substrate, the sensor being arranged to sense an amount of absorption of two specific wavelengths of near infrared energy by the adhesive, the two specific wavelengths including a first wavelength within an absorption band for water a second wavelength outside an absorption band for water. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a better understanding of the present invention, reference is made to the following description and accompanying drawings, while the scope of the invention is set forth in the appended claims. 
     FIG. 1 is a functional block diagram for a glue detection apparatus in accordance with the invention. 
     FIG. 2 is a schematic diagram of a glue detection apparatus that incorporates the functional block diagrams of FIG.  1  and of a carton substrate with glue applied. 
     FIG. 3 is a schematic diagram of amplifier electronics for use in the glue detection apparatus of FIG.  2 . 
     FIG. 4 is a schematic diagram of a power supply shown in box form in FIG. 1 for use to power the glue detection apparatus of FIG.  2 . 
     FIG. 5 is a schematic diagram of a near infrared light source shown in box form in FIG. 1 for use in generating near infrared light energy. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Turning the drawing, FIG. 1 shows in block form an optical module  2  having collection optics  4  and fiber optics  6 , an optical module  8  having applicable passband filters  10 , a detector assembly  12  having detector substrates  14 , a preamp module  16 , signal processing electronics  18  having a differential amplifier  20 , voltage comparator  22 , zero offset  24  and an output driver  26 . Also shown is a near infrared light source  28  and a power supply  30 . 
     FIG. 2 is the same glue detection apparatus as in FIG.  1 . The optical module  2  is in the form of a lens assembly. The optical module  8  is in the form of a housing  32  containing the near infrared light source  28  in the form of the halogen light source, two filters and detectors  34 ,  36  of the passband filters  10  and detector substrates  14 , the preamp module  16 , the signal processing electronics  18 , a zero adjustment control  38 , a glue threshold adjustment control  40 , an output indicator  42  and an input/output cable  44 . Between the optic module  2  and the optic module  8  is trifurcated fiber optics  6 . 
     The arrangement of FIG. 2 is used to detect water based adhesives  46  on a substrate  48 , but the detection technique may be applied to non-aqueous adhesives on a substrate as well. When aqueous based adhesives are used, the invention detects and measures the presence of the adhesive on a substrate by measuring the absorption of near infrared energy at specific wavelengths. The specific wavelengths of interest are those where there is an absorption band for water and a second wavelength where there is no absorption band for water. 
     The substrate is monitored by the filtered detectors  34 ,  36 , which are used to measure the absorption of near infrared energy on the substrate. When adhesive is present upon the substrate, the detector that is filtered to the absorption band of water detects a decrease in signal strength. The second detector, however, sees no change in signal. A differential measurement of absorption of near infrared energy is used to discriminate the adhesive from its substrate. The differential signal between the two detectors may be used to quantify the moisture content of the substrate and adhesive by offsetting one signal from the other. 
     The near infrared light source  28  is guided to collection optics  4  of the optical module  2 . The collection optics  4  is exemplified by the lens assembly that uses the fiber optics  6  such as the trifurcated fiber optic bundle. Both the lens assembly and the trifurcated fiber optic bundle are transmissive in the near infrared region. 
     The trifurcated fiber optic bundle consists of fibers coming from the two filtered detectors  34 ,  36  and the near infrared light source  28 . The fibers are combed so as to evenly distribute the light source and detectors within the trifurcated fiber bundle. 
     The lens assembly focuses the near infrared light at the focal length of the lens and collects reflected light energy, which is returned to the detectors. The detectors  34 ,  36  used are exemplified by high quality extended wavelength indium gallium arsenide photodiodes for use in the wavelength range from 800 nm to 2600 nm, but may instead be detectors such as lead sulfide, lead selenide, indium antimonide or indium arsenide. The indium gallium arsenide (InGaAs) is preferred due to its stability at room temperature and speed of response. 
     The detector that is used to inspect for presence of moisture is passband filtered at 1900 nm and is exemplified by the J18 Series InGaAs photodiodes (extended wavelength) manufactured by EG&amp;G Judson of Montgomeryville, Penn. Other water absorption bands in the near infrared region could be used as well. The background reference detector may be a passband filtered at 1720 nm. Other non aqueous absorption bands in the near infrared region may be used as well. 
     The signal processing electronics  18  of FIGS. 1 and 2 is exemplified by the amplifier electronics of FIG.  3 . Transimpedence amplifier (current mode) preamplifiers  50 ,  52  are used to convert the current output of the InGaAs detectors into a voltage output. They amplify the signal for subsequent use by an offset device such as a differential amplifier  54 . 
     When no moisture is present, the sensors generate outputs that pass through preamplifiers  50 ,  52  and are nearly equal. Thus, when such outputs are fed into the differential amplifier  54 , the ensuing differential output of the differential amplifier  54  is nearly 0 volts, indicative of the condition where no moisture is present. 
     When moisture is present, the filtered detector  34  produces an output of smaller magnitude than that of the filtered detector  36  due to the adsorption of energy (within the absorption band of water) at its filtered wavelength. The differential amplifier  54  amplifies the difference in detector voltages, which is proportional to moisture content. The analog output is a negative voltage whose magnitude is proportional to detected moisture. 
     A voltage comparator  56  is used to digitize the analog glue/moisture signal and whose switching threshold is set by a resistor  58  to the moisture content of the adhesive to be detected and is higher than background levels of moisture content. By relying on this threshold as a cut-off level, there is no need to further compensate for moisture levels in the background, unlike conventional techniques. 
     The output of the voltage comparator  56  is buffered by a transistor  60 , which serves as a driver to external devices. These external devices may then be driven by the transistor  60  to take corrective measures or other action with respect to the deposition of adhesive on the substrate. The output indicator  42  may be in the form of a light emitting diode. 
     If no adhesive is detected where the adhesive is supposed to be, appropriate external devices that control the deposition of adhesive may be triggered by the transistor  60  to deposit the adhesive in the area where the adhesive was supposed to be but was detected as missing, thereby correcting irregularities of adhesive deposition. Such external devices are conventional. 
     For those applications where sensing the presence or absence of the adhesive from a given area on a substrate does not go far enough to ensure uniformity of the adhesive in all areas or meet desired amounts, the detected moisture content may be used instead. Since the moisture content detected is indicative of the amount of adhesive present in a sensed region of the substrate, irregularities in the evenness of the adhesive present may be corrected by appropriate external devices triggered by the transistor  60 , based on signals indicative of the sensed moisture content. 
     FIG. 3 shows various electronic components such as resistors, capacitors, voltage sources, diodes and grounds arranged in the circuitry to provide an operative circuit in accordance with conventional techniques. FIGS. 4 and 5 show circuits for the power supply and light in accordance with conventional techniques. The circuit components and their arrangement in FIGS. 3-5 that are ancillary to the detectors  34 ,  36 , amplifiers  50 ,  52 , differential amplifier  54 , voltage comparator  56 , output indicator  42 , power supply  30 , and near infrared light source  28  are conventional or readily apparent in accordance with conventional techniques to render operative the invention so that further description of such components would be superfluous information to what is known conventionally. 
     The glue sensor of FIG. 2 of the present application is commercially available from the VeriTec., Technologies, Inc., a wholly owned subsidiary of Nordson Corporation (Westlake, Ohio), model identification VeriTec GD 100 Glue Sensor and may be used for carton folder gluers, converting machinery, envelope machines, corrugated converting, mailers and folding machines, and any application requiring adhesive detection. Preferably, the present invention uses 1 mm glue spot resolution. 
     While the foregoing description and drawings represent the preferred embodiments of the present invention, it will be understood that various changes and modifications may be made without departing from the spirit and scope of the present invention.