Abstract:
Scanning sensor for measuring properties of continuous flat sheet, that is moving in the machine direction, employs an IR radiation source for directing a beam of incident IR radiation that impinges the sheet. The IR source has elongated lamp filament that generates IR radiation and the corresponding spot size formed on the sheet has elongated dimensions with its long axis being aligned with the machine direction. Aligned with the MD maximizes sensor spatial resolution in the cross direction. The sensor can employ a receiver having rectangular geometry with its long axis being aligned also in the MD. Scanning sensor can operate in the reflective, transmissive, or offset transmission mode to monitor characteristics of flat sheets, particularly of paper or plastic products.

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to optical sensors and more particularly to optimized cross direction spatial resolution for spectroscopic sensors used for measuring characteristics of flat sheet products including paper and plastics. 
     BACKGROUND OF THE INVENTION 
     During manufacture or flat paper and plastics products, various sheet properties of multi-layered and single layer sheets can be detected with visible and infrared radiation while the sheet-making machine is operating. Characteristics of the sheet including composition, basis weight, coating weight, moisture content, opacity and layer thicknesses can be measured by sensors which detect the amount of radiation that the sheets absorb, transmit or reflect from a beam of infrared light or other radiation. A typical sensor includes an infrared (IR) radiation source that directs a beam of IR radiation towards a sample and the beam is transmitted through beam conditioning optics, such as collimating lenses and/or focusing lenses. These lenses condition the optical radiation for optimal sensor efficiency. The optics in front of the detectors typically comprises focusing lenses and those adjacent to the sample are typically collimating or focusing lenses. IR radiation is partly absorbed, reflected and transmitted by the sample depending on its various properties. A beam splitter splits the IR radiation into two separate beams with each beam being directed to separate band pass filters that are positioned and aligned immediately before detectors. The hand pass filters are configured to pass IR radiation at selected regions of the infrared spectrum. IR radiation, which is not within the selected region of the spectrum, is reflected by the filters back to the beam splitter. Adsorption-type filters can be used although they are less efficiency that the band pass filters which are interference-type filters. Instead of employing a beam splitter which requires a multiplexing arrangement, the sensor can use a rotating filter-wheel assembly. For example, a circular array of filters rotating around a shall or pivot is positioned to the side of the optical path defined by IR radiation reflected from the sample such that a circle drawn through the centers of the filters passes through the center of the optical path. As the filter-wheel rotates, different filters are introduced into and removed from the optical path. 
     Depending on the intensity of the radiation detected, the detector generates an analog electrical signal that may be converted to a digital signal for observation. The described sensor arrangement can measure different properties of the sample under observation. For instance, in the thickness measurement of thin plastic films, one of the two infrared band pass filters only passes infrared radiation having wavelengths in a selected region of the infrared spectrum. This first region of the spectrum is called the “reference” region, and the associated detector is called the “reference” detector. The reference channel spectral range is located in a specific region of the IR spectrum, which is not associated with a signature absorption band of the material or materials, which the film is composed of. This reference channel however should be indicative of all other optical loss mechanisms in the sensor system and sheet that are not indicative of the optical absorption of the material being sensed. These other properties may include such things as scattering loss from the sheet or the insertion losses of the optical components used. 
     Similarly, in papermaking, it is well known to continuously measure certain properties of the paper material in order to monitor the quality of the finished product. These on-line measurements often include basis weight, moisture content, gloss, and sheet caliper. The measurements can be used for controlling process variables with the goal of maintaining output quality and minimizing the quantity of product that must be rejected due to disturbances in the manufacturing process. 
     Generally, on-line measurements of sheet properties are made by scanning sensors that travel back and forth across the width of the sheet in the cross-machine direction (CD). In the manufacturing of a flat sheet of paper, the cross-machine direction uniformity is a critical issue. The scanning sensors are located downstream of actuators that are controlled to adjust the sheet properties. The scanning sensors collect information about the sheet properties to develop a property profile across the sheet and provide control signals to the appropriate actuators to adjust the profile toward a desired target profile in a feedback loop. In practice, the actuators provide generally independent adjustment at adjacent cross-directional locations of the sheet, normally referred to as slices or profile zones. 
     The sensors include a radiation source that typically comprises a broadband infrared source and a receiver with one or more detectors with the wavelength of interest being selected by narrow-band filters such as, for example, an interference type filter. The sensor gauges used fall into two main types: the transmissive type in which the source and detector are on opposite sides of the web and, in a scanning gauge, are scanned in synchronism across it, and the scatter type (typically called “reflective” type) in which the source and detector are in a single head on one side of the web, the detector responding to the amount of source radiation scattered from the web. 
     SUMMARY OF THE INVENTION 
     The present invention is based in part on the recognition that the spatial resolution of infrared spectroscopic sensors is dictated by the radiation source and receiver dimensions and by their fields of view. The present invention optimizes the source&#39;s design by aligning the long dimensions of the source lamp filament in the machine direction of a scanning sensor system and at the same time preferably using a rectangular geometry on the receiver with the long axis thereof being aligned also in the MD. In this fashion, the areas of the source and receiver are designed to maximize the cross direction resolution. The incident radiation beam from the infrared radiation source illuminates a small spot size (the area measured on the sheet) on the flat product surface and analysis thereof yields more precise and detail information of the sheet properties of interest. 
     In one aspect, the invention is directed to a sensor for measuring at least one selected component in a continuous sheet composition having a length and width and that is moving in a machine direction (MD) which is parallel to the sheet length wherein the sheet has a first surface and a second surface that is opposite to the first side that includes: 
     an infrared (IR) radiation source for directing a beam of incident infrared radiation to the first surface of the sheet wherein the source has a filament that emits IR radiation having an elongated beam profile that impinges on the first surface with an impinging elongated profile; and 
     a detector operable to receive IR radiation that emerges from the second surface of the sheet wherein (i) the impinging elongated profile has a length that is aligned parallel with the MD to maximize detector spatial resolution in the cross direction (CD) or (ii) the detector comprises at least one detector element having a length that is aligned parallel in the MD. In a preferred embodiment, both the impinging elongated profile has a length that is aligned parallel with the MD and the detector comprises at least one detector elements having a length that is aligned parallel in the MD. That is, both the IR radiation source and the IR detector have rectangular fields of view to obtain maximum CD resolution. 
     In another aspect, the invention is directed to a system for continuous online measurement of a characteristic of a moving sheet that is traveling lengthwise in the machine direction (MD) includes: 
     an infrared (IR) radiation source which emits radiation from an elongated filament such that an incident elongated beam of IR radiation is directed on a first side of the sheet wherein the IR radiation source travels over the cross direction (CD) of the moving sheet; 
     a receiver operable to detect radiation emerging from the moving sheet and provides electrical detection signals and wherein the receiver travels over the CD of the moving sheet wherein (i) the long axis of the elongated beam is aligned parallel with the MD or (ii) the receiver comprises at least one detector element having a length that is aligned parallel with the MD; and 
     a processor that receives the electrical detection signals and that is operable to determine at least one property of the sheet 
     In a further aspect, the invention is directed to a method of measuring at least one property of a sheet that is traveling lengthwise in the machine direction (MD) that includes the steps of: 
     (a) directing a beam of infrared (IR) radiation from an IR radiation source having an elongated filament that emits IR radiation having an elongated beam profile at the moving sheet such that impinging IR radiation has an impinging elongated profile; 
     (b) measuring radiation emerging from the sample and generating electrical signals therefrom wherein (i) the impinging elongated profile has a length that is aligned parallel with the MD such that the orientation enhances the spatial resolution or (ii) step (b) employs a receiver that has at least one detector element having a length that is aligned parallel with the MD; and 
     (c) determining at least one property of the sample from the electrical signals. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A and 1B  are perspective and side views of an infrared sensor emitting a beam of radiation onto the surface of a moving sheet; 
         FIG. 2  shows an imaging optics arrangement to image a beam from of an infrared radiation source to a material being measured; 
         FIG. 3  shows a linear array of detectors; 
         FIG. 4  shows a linear array of optical fibers to transport light that is reflected from or transmittal through material being measured; 
         FIGS. 5 and 6  show sensor devices operating in the reflective mode; 
         FIG. 7  shows a top view of spectral wheel; 
         FIG. 8  shows a sensor device operating in the transmissive mode; 
         FIG. 9  shows sensor device operating in the offset mode; and 
         FIG. 10  shows a scanning sensor system for measuring properties a moving sheet. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention relates to a sensor device for detecting properties of a composition, especially material that is in the form of a film, web or sheet. While the sensor will be illustrated in measuring properties of paper, it is understood that the sensor can be employed to detect a variety of components in a number of different flat materials including, for example, coated materials, plastics, fabrics, and the like. 
       FIGS. 1A and 1B  show a continuous sheet or web  2  that is moving lengthwise along the machine direction. An infrared radiation source  4  such as a tungsten halogen lamp (THL) includes an elongated filament  6  that is located at the foci of the elliptical reflector  8 . The filament is typically configured as a coil. In this fashion, elliptical reflector  8  re-directs IR rays  12  to converge to form an elongated incident beam  10  with an elongated profile on the top surface of sheet  2 . Filament  6  is preferably 5 to 30 mm in length; the elongated beam  10  impinging on sheet  2  preferably has a length of 5 to 30 mm. As shown in  FIG. 1B , an IR energy modulator  14  that is positioned before the second foci of elliptical reflector  8  can be employed to provide a high level of IR energy modulation. Modulator  14  preferably comprises a mechanical chopper with one or more elongated or rectangular slots  16  that are aligned parallel to the long axis of elongated beam  10 . An advantage of having the long axis of elongated beam  10  aligned parallel with chopper slots  16  is that maximum chopping frequency can be achieved for the mechanical constraints such as motor speed and slot width. Suitable chopper include tuning forks, shutters, and chopper wheels that are equipped with a plurality of radial slots, and which is described in U.S. Pat. No. 4,770,538 to Orkosalo, and that is incorporated herein by reference. 
       FIG. 2  illustrates an optical technique for imaging an elongated beam  20  to the surface of moving sheet  42  so that beam  20  is aligned with the MD of moving sheet  42 . The image transmission optics includes an IR source optical head  26  with coupler  30  and a projection optical head  36  with coupler  34 . Couplers  30  and  34  are connected to a linear array of optical fibers  31 , 32 . IR rays  42  that are generated by filament  22  of a THL are focused by elliptical reflector  24  to form an elongated beam that is by imaged by lens  28  into linear array of optical fibers  31 , 32 . Thereafter, the transmitted radiation is imaged by  38  lens as an elongated beam  20  and projected onto sheet  42 . The long axis of beam  20  is parallel to the MD of sheet  42 . Instead of using a linear array of optical fibers, other radiation transmitting channels such as waveguide or light pipe can be employed. 
     The arrangement illustrated in  FIG. 2  can be incorporated into a sensor system employed to monitor paper quality by scanning the apparatus over a moving sheet of paper during production. Projection optical head  36  would move continuously back-and-forth along the CD relative to moving sheet  42 . The light source  22 , 24  and other devices of the sensor system such as the signal processing components can be located remote from the hostile environment that is usually associated with the sheet making process. The linear array of optical fibers  31 , 32  can be part of a cable take-up mechanism that manage the fiber while projection optical head  36  is being moved as well as to preserve the overall bend length and radius. 
     The sensor device of the present invention includes a receiver that detects radiation that emerges, that is, reflected from or transmitted through the product being measured.  FIG. 3  depicts a detector device  50  with a linear array of IR sensors for detecting radiation emerging from a sheet. The array includes a plurality of sensors  52  that are mounted and bonded within a detector module. The rectangular-shaped array is scaled to image the entire illuminating profile of the emerging light. Each sensor  52  can have an associated band-pass filter  54  that can be selected so that it passes IR in a separate region of the IR band. Suitable IR sensors include InGaAs photovoltaic sensors from Hamamatsu Photonics K.K. (Japan) or Teledyne Judson Technologies (Montgomeryville, Pa.). Generally, any suitable photo-detector such as, for example, photoconductive, photovoltaic, pyroelectric type IR sensors can be employed. 
       FIG. 4  depicts a radiation transmission device with an array  70  of optical fibers or optical fiber bundles  76  that is used to transport emerging light from sheet  62  to individual detectors. The radiation transmission includes rectangle-shaped, light receiving module  66  that is equipped with an array of radiation directing optics  68 , such as mirrors and/or lenses, which captures radiation. In operation, an elongated beam  64  that emerges from sheet  62  is captured by light receiving module  66 , transported through an array  70  of optical fibers  76  and directed by a plurality of directing optics  78 , which are housed in module  74 , into a radiation receiver  72  where the intensities of the beam at different wavelengths are measured. The long axis of beam  64  is parallel to the MD of sheet  62 . Alternatively, each of the optical fibers  76  in the array  70  can be coupled directly to corresponding detectors without using coupling optics. 
       FIG. 5  depicts an IR sensor  88  operating in the reflective mode that is used, for instance, to measure the amount of the coating material  90  applied to the base paper sheet  92 , which is moving in the MD. The sensor  88  includes a tungsten-halogen source  94  having a filament  96  that provides continuous wave radiation in the visible and infrared regions and a detector assembly of a plurality of IR detectors. The number of IR detectors employed depends on the number of properties being measured. The broadband infrared energy  94  is directed at the sheet  92  at an angle that minimizes sensitivity to sheet flutter and surface characteristics. Typically, when sensor  88  is employed to measure the concentrations of one or more components in a sheet material, a reference and associated measurement detector is configured to measure each component&#39;s concentration. Thus, IR sensor  88  with six channels can be employed, for instance, to monitor the concentration of three substances in the composition in coating material  90 . The first property is measured by first measure filter/detector  98 A and reference filter/detector  98 B. The second property is second measured by measure filter/detector  100 A and reference filter/detector  100 B. The third property measured by third measure filter/detector  102 A and reference filter/detector  102 B. The energy reflected from the sheet is wavelength-analyzed hr passing the beam through the beam splitters  104 ,  106 , and  108  and the appropriate filters to the individual detectors. This configuration of the optical analyzer comprising the beam splitters, filters, and detectors insures that all detector signals originate from the same location on the sheet, so that at any given time all of the information needed for accurate measurement is available. 
     Filament  96  is oriented so that its long axis is aligned with the machine direction of base paper sheet  92  so as to form an elongated illumination  124  on the surface of coating material  90  that is also aligned with the MD. In operation, radiation generated by filament  94  is modulated by an infrared enemy modulator  110 , which can be a rotating light chopper, for instance. Radiation that is reflected from coating material  90  is directed in the detectors of sensor  88 . A radiation transmission device  112  such as that shown in  FIG. 4  can be employed to capture the elongated beam reflected from coating material  90 . 
     The output of each of the detectors (both measure and reference) is transmitted to signal processing, circuitry in processor  120 . Demodulated and amplitude averaged detector signals are then measured by the signal processing circuitry, digitized and led to the process control computer  122 . The computer computes the properties of interested on the base sheet  92  utilizing the standard equations and techniques which are described for example in U.S. Pat. No. 7,494,567 to Haran, U.S. Pat. No. 7,382,456 to Tixier et al., and U.S. Pat. No. 7,868,296 to Haran et al, which are incorporated herein by reference. 
       FIG. 6  depicts an IR sensor  288  operating in the reflective mode which employs a spinning filter wheel and that is used, for instance, to measure the amount of the coating material  290  applied that is on base paper sheet  292 . Sensor  288  includes a tungsten-halogen source  294  having a filament  296  that provides continuous wave radiation and a single detector  298 . A spinning filter wheel  250  that is powered by a motor  249  spins the wheel about an axis and a synchronizing detecting device  252  tracks the position of the of the wheel and rotational speed.  FIG. 7  depicts a spinning filter wheel which contains a plurality of light filters  254 ,  266  about a central axis  268  with each filter designed to allow light of a specific wavelength or wavelength range to pass through it. Eight filters are illustrates, the number of filters being dependent on the number of characteristics being monitored. In this embodiment, four properties can be detected using 4 sets of reference and measurement filters. The wheel includes a synchronizing mark  270  that, when detected by synchronizing detector  252  ( FIG. 6 ). 
     Broadband infrared radiation  294  is directed at sheet  292  and the reflected energy from the sheet is wavelength-analyzed by passing the beam through a filter of spinning filter wheel  250  and into detector  298 . Filament  296  is oriented so that its long axis is aligned with the machine direction of base paper sheet  292  so as to form an elongated illumination  224  on the surface of coating material  290  that is also aligned with the MD. In operation, radiation generated by filament  294  is modulated by an infrared energy modulator  210 . Radiation that is reflected from coating material  290  is directed to detector  298 . A radiation transmission device  212  such as that shown in  FIG. 6  can be employed to capture the elongated beam reflected from coating material  290 . 
     The outputs from detector  298  (at the various measure and reference wavelengths) are transmitted to signal processing circuitry in processor  220 . Demodulated and amplitude averaged detector signals are then measured by the signal processing circuitry, digitized and fed to the process control computer  222 . 
       FIG. 8  illustrates a sensor that is configured to operate in the transmissive mode where the radiation source and radiation detector are direct opposite sides of web  238 . Optical sensor  230  includes an upper scanner head  232  housing light source  234  and a lower scanner head  250  housing detector  236 . Sensor  230  measures characteristics of a moving web  238  that comprises a layer of material  240  that is transmissive to radiation. The upper and lower scanner heads  232 , 250  are aligned and their movement is coordinated in the cross direction. Filament  280  is oriented so that its long axis is aligned with the machine direction of moving web  238  so as to form an elongated beam that is imaged by lens  282  onto material  240 . The foci of the elliptical reflector can also be used to image the beam. Specifically, incident light  262  from light source  234  passes through material  240  and enters receiver  236  through lens  284 . It is expected that the elongated shaped of the incident light remains somewhat aligned in the MD as the radiation is passes through material  240  so that exiting radiation that is directed into spectrometer  244 , for instance, retains its alignment to the MD. Instead of a spectrometer, a filter-beam splitter stack as shown in  FIG. 5  can be employed. 
       FIG. 9  illustrates a sensor that is configured to operate in the offset transmission geometry where the radiation source and radiation detector are laterally offset from one another with respect to the path of a flat product being monitored. Optical sensor  130  includes an upper scanner bead  132  that houses light source  134  and a lower scanner head  150  that houses detector  136 . Sensor  130  measures characteristics of a moving web  238  that comprises a layer of material  140  that is transmissive to radiation. A reflector  146  is secured to the lower surface of head  132 . The reflector  146  can be either specular or diffusive depending on the application. For measuring paper product, a diffusive reflector is preferred. Similarly, lower scanner head  150  has a reflective surface  152 , which can be either specular or diffusive, is positioned adjacent to the lower surface of the layer of material  140 . The upper and lower scanner heads  132 , 150  are aligned so that mirror  146  of the upper scanner bead  132  is parallel with and faces reflective surface  152 . In addition, the movement of the upper and lower scanner heads  132 , 150  is coordinated in the cross direction so that light is reflected between reflective surfaces  146  and  152  as radiation  164  propagates through layer of material  140 . Filament  170  is oriented so that its long axis is aligned with the machine direction of moving web  138  so as to form an elongated beam that is imaged by lens  172  onto material  140 . Specifically, incident light  162  from light source  134  is reflected by lower reflective surface  152  and upper mirror  146  multiple times (shown as reflected radiation  164 ) before the light enters receiver  136  through lens  175 . It is expected that the elongated shaped of the incident light remains aligned in the MD as the radiation is reflected through material  140  so that exiting radiation  166  that is directed into spectrometer  144 , for instance, retains its alignment to the MD. 
       FIG. 10  illustrates a scanning sensor system whereby the sensor is incorporated into a dual head scanner  198  of scanner system  180  that is employed to monitor one or more properties during continuous paper production. Scanner  198  is supported by two transverse beams  182 , 184 , on which are mounted upper and lower scanning heads  190 , 192 . The operative faces of the lower and upper scanner heads  190 , 192  define a measurement gap that accommodates sheet  186 . 
     When the sensor is operating in the reflective mode as illustrated in  FIG. 5 , both the radiation source and receiver are housed within upper scanner head  190 . When the sensor is operating in the transmission mode as illustrated in  FIG. 8  or  9 , the radiation source and receiver are housed within upper scanner head  190  and lower scanner head  192 , respectively. It should be noted that in alternative configurations of the offset transmission mode scanning sensor of  FIG. 9 , the source and receiver are housed in the same head. Finally, when operating in the standard transmissive mode, a radiation source is positioned in the upper scanning head  190  while the radiation receiver is positioned in the lower scanning head  192 . 
     The movement of the dual scanner heads  190 ,  192 , is synchronized with respect to speed and direction so that they are aligned with each other. The radiation source produces an elongated illumination (spot size) on the sheet  186  that is aligned with the MD as the sensor moves repeatedly back and forth in the CD across the width of the moving sheet  186 , so that the characteristics of the entire sheet can be monitored. 
     The foregoing has described the principles, preferred embodiment and modes of operation of the present invention. However, the invention should not be construed as limited to the particular embodiments discussed. Instead, 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 present invention as defined by the following claims.