Abstract:
Nuclear-based basis weight sensors are passline-sensitive. Error in measurement is induced when the sheet moves up and down in the gap between the radiation source and detector. A passline-insensitive basis weight sensor includes a triangulation sensor to measure the position of the sheet within the gap. The sensor and gap is characterized in the laboratory for its passline behavior over a range of basis weights. The curves are either parameterized or a lookup table is created for each weight and passline position and the data added to the sensor&#39;s processor. The basis weight measured can be automatically corrected to account for deviations from the passline or nominal path through the sensor.

Description:
REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Application 61/990,537 that was filed on May 8, 2014 and which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention generally relates to basis weight sensors and more particularly to techniques for automatically correcting for measurement errors introduced by changes in the position of the moving web within the measurement gap. 
     BACKGROUND OF THE INVENTION 
     In the forming sheets of material, such as plastics, paper or other web-based products, the sheet is generally formed in a continuous process as a moving film or web. In order to most effectively monitor and control the process, one or more properties of the web must be determined while the web is in motion. The properties of interest include web basis weight, which is the weight per unit area of the web. 
     To measure the desired web property, one or more sheet sensors are employed in fixed or web traversing structures. The moving web can be constrained to move over a fixed reference surface, but more commonly the moving web is unsupported in the measurement region and is subject to deviations from the nominal path through the measurement region. The nominal path through the measurement region is usually referred to as a “pass-line”, and deviations from that pass-line such as “flutter” or other lower frequency deviations alter the pass-line of the web through the measurement region. 
     A desirable method of forming a measurement region across the width of the web is to utilize nuclear radiation in a traveling sensor arrangement that traverses the moving web to measure the desired web property. A radioactive thickness/density gauge is based upon the principle that a mass of material will absorb the products of radioactive emission in a known and repeatable manner. An industrial web-gauging instrument, known as a beta-gauge, typically utilizes a radioactive isotope that decays through beta particle emission. The radioactive isotope is mounted in an enclosed head or source, which projects the radiation through the web to a second head that includes a radiation detector. The amount of radiation sensed by the detector is directly related to the amount of radiation absorbed by the web material being measured. 
     The relative position of the moving web in the measurement region space or gap between the source and detector heads is called the web pass-line. Due to the nature of beta particle interaction with the web material, specifically due to the angular dispersion of the beta particles as they pass through the web, the amount of radiation sensed by the detector varies with pass-line deviations. Since the fluctuating web pass-line can vary significantly and at a high frequency during a measurement of the moving web, the accuracy of the gauge or sensor in part depends on the ability to compensate for pass-line variations. In particular, basis weight sensors (especially nuclear-based ones) are passline-sensitive. Error in measurement is induced when the sheet moves up and down in the gap. 
     SUMMARY OF THE INVENTION 
     The present invention is based in part on the recognition that a basis weight sensor especially a nuclear-based sensor can be rendered passline-insensitive by characterizing the sensor&#39;s passline behavior as a function of sheet weight (not composition) and position within the gap in the sensor. 
     In one aspect, the invention is directed to a method of measuring the basis weight of a moving sheet of material under test with a sensor that includes a basis weight gauge that includes a radiation source and radiation detector with a gap between the source and detector that defines a radiation path towards a sheet substrate and for detecting the amount of first radiation transmitted through the sheet substrate wherein the transmitted radiation that is detected being an initial measurement of the basis weight of the sheet substrate, said method includes the steps of: 
     establishing a calibration for the material under test over a range of basis weights and positions within the gap; 
     moving the sheet of material under test through the gap to obtain an initial basis weight measurement; 
     measuring the position of the sheet of material within the gap; and 
     determining a final basis weight measurement by correcting the initial basis weight measurement with the calibration. 
     In another aspect, the invention is directed to a sensor device for measuring the basis weight of a sheet of material that includes: 
     a basis weight gauge that includes a radiation source and radiation detector with as gap therebetween for directing radiation along a path towards a sheet of material and for detecting the amount of radiation transmitted through the sheet and generating a first signal that corresponds to the radiation transmitted, wherein the first signal corresponds to an initial basis weight measurement of the sheet; 
     means for measuring the position of the sheet of material within the gap and deriving a correction factor corresponding to the position of the sheet; and 
     means for adjusting the initial basis weight measurement by applying the correction factor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1 and 1A  show basis weight sensors with automatic z-correction; 
         FIG. 2  shows a sheet making system implementing a caliper sensor in a dual head scanner; and 
         FIG. 3  is a diagram of a system employing process measurements to calculate the final basis weight of the web 
     
    
    
     DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a sensor apparatus  10  for measuring the basis weight of paper and other web product  20 . The apparatus includes scanner head  2  that houses a radiation source  6  that emits radiation that is directed along a radiation path toward web or sheet  20 , which moves in the machine direction (MD). The radiation can be of the beta type. A radiation detector  8  that is housed in scanner head  4  senses the intensity of radiation that is transmitted through web  20 . Sheet  20  is advanced through the gap configured between scanner heads  2  and  4  and the sheet is not supported in the gap and therefore is susceptible to flutter within the gap; the distance z of the gap is preferably fixed. Suitable beta sensors include the 4203 model series of nuclear sensors from Honeywell International, Inc. 
     The radiation detector senses the amount of radiation absorbed by the web material, which gives an indication of the basis weight (that is, weight per unit area) of the web material. Specifically, the basis weight being inversely proportional to the level of absorption by the material. 
     With the present invention the passline behavior of the apparatus is determined in the laboratory; particularly for paper products, this behavior is characterized and is a function of sheet mass per unit area (not composition) and position in the gap. The sensor head is outfitted with a single triangulation sensor and in practice the distance to the sheet of paper can be measured to an accuracy of a few microns. With the knowledge of this distance as well as the sheet mass per unit area (from the sensor), a basis weight corrector can be generated based on the characteristic passline curve. With the present invention, it is not necessary to measure the thickness of web  20 . 
     In particular, the apparatus is equipped with a laser triangulation source  12  and detector  14 , collectively being referred to as an interrogation laser  18 . The source/detector arrangement is referred to generally as a distance determining means. From the measured path length from the source to the detector, values for the distance between each distance determining means and a measurement or interrogation spot on an upper web surface may be determined. The heads  2  and  4  are typically fixed in the position so that the interrogations spots do not move in the machine direction even as the heads are scanned in the cross direction, which is transverse to the machine direction. 
     Referring to  FIG. 1 , in the laboratory during the behavior characterization process, standard means can be employed instead of web  20 . The standard means has a range of predetermined and stable basis weights that are selectively interposed in the radiation path between scanner heads  2  and  4 . The standard means can be polyester (MYLAR from DuPont) disc  22  of a predetermined and stable basis weight which is secured to a frame that is pivoted for rotation on a shaft which is driven through a universal joint by a rotary solenoid unit. For each disc, the standard means is positioned at different vertical positions within the gap. In this fashion, the desired sensor model (and gap) is characterized in the laboratory for its passline behavior over a range of weights. 
     As shown in  FIG. 1 , disc  22  is positioned at an initial lower position within the gap adjacent radiation detector  8 . Once in position, a conventional gauge measures the distance from the upper surface of disc  22  to the radiation source  6 . Finally, as radiation source  6  emits a measurement level of radiation towards disc  22 , the radiation detector  8  measures the intensity of the radiation passing through disc  22 . All data being is recorded and stored. Thereafter disc  22  is raised to a second position and its distance to the radiation source  6  and the radiation intensity are measured as when disc  22  was at the initial lower position. This behavior characterization process continues with disc  22  being is raised incrementally and measured until a final upper position adjacent radiation source  6  is reached. The initial lower position and the final upper position for disc  22  represent the lowest and highest positions, respectively, in the gap where a web  20  traveling through sensor device  10  might travel. The behavior characterization process next employs a plurality of different sample discs with known basis weights. The laboratory measurements thus establish a library of basis weight measurements using different disc standards that are positioned at different locations within the gap of the sensor device  10 . The curves generated by the measurements are either parameterized using standard curve-fitting techniques, or a lookup table is created for each weight and passline position. The inventive method is suited for measuring the basis weight of any web material in which nuclear sensors are applicable. The materials include, for instance, paper, plastic, sandpaper, thin metals and especially non-planar sheets and heavily coated sheet products that exhibit edge curl. In the case of measuring paper, the typical thickness ranges from 20 to 200 microns. 
       FIG. 2  illustrates a scanning sensor system  30  whereby a basis weight sensor is incorporated into a dual head scanner  38  that measures the basis of sheet  36  during continuous production. Scanner  30  is supported by two transverse beams  32 ,  34  on which are mounted upper and lower scanning heads  2 ,  4 . The operative faces of scanner heads define a measurement gap that accommodates sheet  36 . 
     The movement of the dual scanner heads  2 ,  4  is synchronized with respect to speed and direction so that the are aligned with each other. The radiation source  6  ( FIG. 1 ) emits radiation onto an illumination (spot) on sheet  36  as the sensor moves repeatedly back and forth in the cross direction (CD) across the width of the moving sheet  36 , which moves in the machine direction (MD), so that the basis weight of the entire sheet can be monitored. 
     Referring to  FIG. 1 , during operations, basis weight sensor  10  continuously measures the position of sheet  20  within the gap using interrogation laser  18  while radiation source  6  directs a beam of radiation the sheet and radiation detector  8  measures the intensity of the radiation transmitted through the sheet. The intensity of the radiation front the source  6  should be the same as that used during the laboratory behavior characterization phase. By knowing the sheet&#39;s position and radiation intensity, the library generated during the above-described characterization phase can be employed to automatically generate the corrected basis weight. 
     As shown in  FIG. 1 , the interrogation spot for interrogation laser  18  is upstream of the spot on the sheet where radiation from source  6  passes through the sheet. In the case where sheet  20  remains substantially planar as it passes through gap in sensor  10 , the position of sheet  20  as it passes under radiation source  6  is actually measured by the interrogation laser  18 . However, in the event sheet  20  does not remain planar, an optical translation technique such as that described in U.S. Pat. No. 7,528,400 to Duck et al., which is incorporated herein, can be employed to move the interrogation spot of the interrogation laser  18  to coincide with basis weight spot where radiation from radiation source  6  is incident on sheet  20 . In this fashion, interrogation spot and basis weight spot coincide and both z distance and the transmitted radiation can be measured at the same spot simultaneously.  FIG. 1A  shows an alternative embodiment of the sensor apparatus  11  that includes (i) scanner head  3  that houses radiation source  7  and laser triangulation source  13  and detector  15  and (ii) scanner head  5  that houses radiation detector  9 . A first transparent optical element  24  can be positioned along the path of incident light from laser source  13  so as to refract the light to interrogation spot  29  on the surface of sheet  21 . In addition, a second transparent optical element  26  is positioned so that reflected light is refracted into detector  15 . In this fashion, the interrogation spot of the triangulation laser coincides with the basis weight spot. 
       FIG. 3  depicts a process for controlling the manufacture of paper or other web product by continuously measuring the basis weight of the web. Digitized signals representing the intensity of the measured radiation transmitted through the web as the position of the web generated by the signal conditioning and digitizing stage  40 . A basis weight analyzer  46  includes a microprocessor  42  and memory  44 , that contains tables and/or parametric equations, calculates the basis weight signals  48  which can be employed to control actuators upstream and/or downstream of the scanner system  30  ( FIG. 2 ) to regulate production mechanisms in response to the basis weight measurements. 
     With the present invention, one the sensor apparatus is initially calibrated for a particular material, such as paper, it is not necessary to recalibrate the basis weight sensor each time the web material changes, in thickness or composition such as grade changes. For example, in the production of paper, it is not necessary to recalibrate the sensor when there is a paper grade change as the sensor continues to automatically calculate the basis weight of the sheet product. However, recalibration is necessary when measuring a different type of material such as when the sensor apparatus is switch from measuring paper to plastic or metal. As is apparent, a different calibration library is needed. 
     The present invention can be implemented by reconfiguring existing nuclear gauges for measuring the basis weight per unit area of the sheet material such as that disclosed in U.S. Pat. No. 4,692,616 to Hegland et al. that is incorporated herein by reference. Once the passline behavior is characterized and the laboratory data stored in a computer, measurements from the gauge can be automatically corrected for variations in the passline to yield accurate basis weight measurements. 
     The foregoing has described the principles, preferred embodiments and modes of operation of the present invention. However, the invention should not be construed as being limited to the particular embodiments discussed. Thus, the above-described embodiments should be regarded as illustrative rather than restrictive, and it should be appreciated that variations may be made in those embodiments by workers skilled in the art without departing from the scope of the present invention as defined by the following claims.