Abstract:
A system for characterizing a set of properties for a moving substance are disclosed. The system includes: a first near-infrared linear array; a second near-infrared linear array; a first filter transparent to a first absorption wavelength emitted by the moving substance and juxtaposed between the substance and the first array; a second filter blocking the first absorption wavelength emitted by the moving substance and juxtaposed between the substance and the second array; and a computational device for characterizing data from the arrays into information on a property of the substance. The method includes the steps of: filtering out a first absorption wavelength emitted by a substance; monitoring the first absorption wavelength with a first near-infrared linear array; blocking the first wavelength from reaching a second near-infrared linear array; and characterizing data from the arrays into information on a property of the substance.

Description:
REFERENCE TO PROVISIONAL APPLICATIONS TO CLAIM PRIORITY 
     This application claims priority in three provisional applications, all entitled “A Dual-Band Linescan Camera System for Measuring Water Content on a Paper Web.” The first provisional application was filed on Oct. 2, 1998 and was assigned serial number 60/102,859, the second provisional application was filed on Oct. 12, 1998 and was assigned serial number 60/103,863, the third provisional application was filed on Mar. 15, 1999 and was assigned serial number 60/124,452. 
    
    
     STATEMENT OF GOVERNMENT INTEREST 
     The United States Government has rights in this invention pursuant to Contract No. W-7405-ENG-48 between the United States Department of Energy and the University of California for the operation of Lawrence Livermore Laboratory. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to systems and methods for paper moisture and basis weight measurement, and more particularly for continuous, wide coverage measurement of paper moisture, and basis weight using a camera assembly. 
     2. Discussion of Background Art 
     Paper manufacture is a very energy and waste-intensive industry. A typical paper machine makes paper that is three-hundred inches wide at speeds of up to five-thousand feet per minute. In order to produce a high quality paper product manufacturers would prefer to monitor as many of the properties of a paper web as possible during manufacture to ensure uniformity and high quality throughout the entire web. 
     During manufacture there are about a dozen properties that are preferably measured, including moisture, thickness, weight, formation, fiber orientation, color, and printability. In an attempt to meet this need, some current on-line paper web measurement systems use a slow-moving scanning platform containing a suite of sensors for measuring many of these properties, including moisture. Moisture is a critical parameter for paper. For instance, since paper is very hydroscopic and too much moisture affects printability, manufactures tend to take a conservative approach and over-dry their paper products, greatly increasing energy costs. 
     Methods for making moisture measurements based on near-infrared absorption techniques where originally developed in the sixties, e.g., U.S. Pat. No. 3,405,268. Other patents further refined these method by using specific wavelength combinations and mathematical formulations, e.g. U.S. Pat. Nos. 3,551,678, 3,793,524, 3,851,175, 4,052,615, and 4,823,008. 
     Current practices for online paper measurement of moisture and cellulose content (which is proportional to basis weight) via absorption techniques, incorporate several lead sulfide sensors for measuring wavelengths between 1.8 μm and to 2.2 μm. These sensors are mounted on a scanning platform in close proximity to the paper web, and take point measurements. Point measurements are those which only look at one “point” of about an inch square at any one time. This small sensing area, along with the fact that it takes about a minute or more for the scanning platform to move across the width of a typical moving paper web, can result in the production of thousands of feet of paper before a point sensor can cross the full width of the web even once. In such point measurement based systems, typically less than two percent of a total paper web area is actually measured. 
     In another approach, Charged Couple Device (CCD) linescan camera technology has been used on paper webs for inspection of visible defect flaws and non-uniformity detectable in a visible range, e.g. U.S. Pat. Nos. 4,950,911 and 5,563,809. However, CCD sensor technology, which is based on Silicon, is not capable of detecting those wavelengths in the near-infrared range needed for accurately measuring paper moisture or cellulose content. 
     If manufacturers had a method for conducting measurements for moisture and cellulose content over one hundred percent of a paper web, paper-drying time could be substantially reduced, resulting in significant energy savings, and improved paper quality. 
     In response to the concerns discussed above, what is needed is a system and method for 100% paper moisture and cellulose measurement that overcomes the problems of the prior art. 
     SUMMARY OF THE INVENTION 
     The present invention is a system and method for 100% measurement of moisture and basis weight on moving paper. Within the system of the present invention, a camera assembly monitors a water absorption wavelength radiated by a substance, such as a paper web, through a first array of photo detectors. The camera assembly monitors a cellulose absorption wavelength radiated by the substance through a second array of photo detectors. The camera assembly monitors a reference wavelength radiated by the substance through a third array of photo detectors. A single optical axis system is used to image the web from a distance and provide the same field of view to each array in the camera assembly. Optical filters, which could include dichroic mirrors or a prism, are used to filter the specific wavelengths to be measured with each of the three arrays. The filters could also be coated directly on the array photo detectors. A computational device or lookup table is used to calculate the moisture content and basis weight of the substance from the water absorption, cellulose absorption, and reference wavelength data received from the camera assembly. Supporting electronics interface signals from the three arrays with the computational device or lookup table. In other aspects of the invention, the first array has a band-pass filter in its optical path transparent to a water absorption wavelength such as 1.45 μm or 1.94 μm wavelength; the second array has a band-pass filter in its optical path transparent to a cellulose absorption wavelength such as 1.57 μm or 2.1 μm; the third array has a band-pass filter in its optical path transparent to other wavelengths in the near-infrared range which do not include water or cellulose absorption wavelengths, such as 1.3 μm, or 1.8 μm. 
     In other aspects of the invention the first, second, and third linear arrays consist of Indium Gallium Arsenide (InGaAs) linear arrays. These arrays consist of a plurality of photodiodes, sensitive to the near-infrared spectrum. Unlike CCD arrays, InGaAs arrays are sensitive to wavelength between 0.9 μm and 2.2 μm. Therefore, these arrays are very suitable for measuring all the wavelength of interest for this application. Furthermore, the array nature of this sensor technology makes it practical to build a high resolution linescan camera system with the use of optics and supporting electronics for 100% inspection of high-speed web processes such as paper. 
     In other aspects of the invention a light source, which radiates energy within at least an infrared spectral band, is used to illuminate a paper web. The current invention can be used in a transmission mode where the light source is on one side of the web and the camera assembly is on an opposite side, or in a reflection mode where the light source is on a same side of the web as the camera assembly. 
     In other aspects of the invention, depending on a width of the web, the size of the arrays, a desired resolution of measurement, a distance between the camera assembly and the web, and a field of view of the camera assembly, several camera assemblies might be needed to monitor a full width of the paper web. 
     The system and method of the present invention are particularly advantageous over the prior art because they permit a full width paper web to be continuously monitored for moisture and basis weight during manufacture. Such monitoring enables faster detection of moisture and basis weight irregularities so that manufacturers can correct problems before miles of off-quality paper are produced. Such monitoring also helps reduce energy costs and waste because the paper web would not be over-dried as now often happens. Integrating the present invention with existing paper web visible defect inspection systems is straightforward, since the same light source and camera enclosures can be shared between the two systems. It might also be practical in some cases to integrate the present invention with other array sensors such as CCD arrays through a single optical axis to measure other properties of the web in parallel with those described here. 
     These and other aspects of the invention will be recognized by those skilled in the art upon review of the detailed description, drawings, and claims set forth below. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a system for moisture and basis weight measurement according to the present invention; 
     FIG. 2 is a pictorial diagram of a first embodiment of a camera assembly within the system; 
     FIG. 3 is a block diagram of a second embodiment of the camera assembly within the system; 
     FIG. 4 is an exemplary graph of optical transmittance verses optical wavelength for water; 
     FIG. 5 is an exemplary graph of optical transmittance verses optical wavelength for cellulose; and 
     FIG. 6 is a graph illustrating several possible optical filtering configurations for moisture and basis weight measurement. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 is a block diagram of a system  100  for moisture and basis weight measurement according to the present invention. The system  100  includes a light source  102 , a linescan camera assembly  104 , data acquisition electronics  106 , and a computational device  108 . The system  100  can measure moisture content and basis weight of paper produced by a paper manufacturing machine (not shown) at either a dry end of the machine, or further upstream in the process toward a wet end. 
     The light source  102  illuminates a paper web  110 . Paper web is an industry term for a moving stream of paper, and while the present invention is discussed with respect to the paper web  110 , those skilled in the art will know that the system  100  can also measure moisture in other substances such as textiles, sand, salts, aluminum oxide, gypsum, wood chips, fertilizer, plastics, and saw dust for wood products. The light source  102  illuminates an under side of the paper web  110 ; however, those skilled in the art will know that illumination could also come from a top or at various angles to the paper web  110 . The light source  102  radiates energy within the infrared spectral band. 
     The linescan camera assembly  104  has a set of optics and sensor arrays for acquiring data on various wavelengths of interest re-radiated from the paper web  110  in response to energy absorbed from the light source  102 . The wavelengths monitored are chosen so that moisture content and basis weight can subsequently be calculated. Linescan cameras are ideally suited for high speed paper web imaging applications due to their fast scanning rates and compact size. Details on the linescan camera assembly  104  are provided below with reference to FIGS. 2 and 3. 
     The data acquisition electronics  106  includes electronics for interfacing the camera assembly  104  to the computational device  108 . The data acquisition electronics  106  include: a web speed encoder for synchronizing data acquisition with a speed of the paper web; supporting electronics for routing acquired data over cables; and a high speed data capturing system to transfer the data, in either analog or digital form, to the computational device  108 . 
     The computational device  108  incorporates either a computational algorithm for calculating, or a lookup table for mapping the data acquired on the various wavelengths to moisture and basis weight values across the entire paper web. Standard computational algorithms and/or lookup tables are used. 
     FIG. 2 is a pictorial diagram of a first embodiment  200  of the camera assembly  104  within the system  100 . The first camera assembly  202  is positioned above a paper web  204  moving within a paper manufacturing machine  206 . An below-red light source  208  is positioned underneath the web  204  in this embodiment. 
     The first camera assembly  202  includes two near-infrared Indium Gallium Arsenide (InGaAs) linescan cameras, such as those manufactured by Sensors Unlimited located in Princeton, N.J. The first camera assembly  202  can be mounted directly on the paper machine and continuously monitor moisture content or basis weight of the paper web during production. Wider paper webs may require additional camera assemblies. 
     FIG. 3 is a block diagram  300  of a second embodiment  302  of the camera assembly  104  within the system  100 . The second camera assembly  302  receives the near-infrared energy radiated from the paper web  110  from a single optical axis  302   1 . The radiated energy then passes through a first set of optics  304  which focus and control aperture of the second camera assembly  302 . A beam splitter  306  divides the radiated energy three ways. The beam splitter  306  can include a prism and/or dichroic mirrors. A second set of optics  308 ,  310 , and  312  focus radiated energy from the beam splitter  306  to a set of near-infrared InGaAs linear arrays  314 ,  316 , and  318 . 
     Three linear arrays  314 ,  316 , and  318  enable continuous real-time monitoring of three sets of near-infrared wavelengths. By monitoring three wavelengths, both moisture content and cellulose measurements can be made. Those skilled in the art however will recognize that the second camera assembly  302  can also function with only two linear arrays  314  and  316  should only one paper web measurement be required. Since the arrays  314 ,  316 , and  318  are configured about the single optical axis  302 , each array is at a same distance from and has a same field of view of the paper web, enabling accurate measurements. 
     The second camera assembly  302  also includes a set of filters (not shown) placed somewhere from the first set of optics  304  to the linear arrays  314 ,  316 , and  318 . For instance, the filters could either be incorporated into the beam splitter  306 , into the second set of optics  308 ,  310 , and  312 , or directly coated onto the linear arrays  314 ,  316 , and  318 . The set of filters are selected so that the linear arrays  314 ,  316 , and  318  can most effectively monitor selected near-infrared wavelengths radiated from the paper web  110 . Possible filter selections and combinations will be suggested below with respect to FIGS. 4,  5 , and  6 . 
     Supporting electronics  320  collect and interface output signals from the linear arrays  314 ,  316 , and  318  with the computational device  108 . The supporting electronics  320  can include a multiplexer for each array, digitizers for digitizing the data of each array, amplifiers, and other signal conditioning electronics. 
     FIG. 4 is an exemplary graph  400  of optical transmittance  402  verses optical wavelength  404  for water within the paper web  110 . As shown, optical transmittance  402  for water varies with the wavelength  404  and includes a first water absorption wavelength  406  at 1.45 μm and a second water absorption wavelength  408  at 1.94 μm. The camera assembly  104  can be designed to monitor either the first absorption wavelength  406  the second absorption wavelength  408  or both. “Standard” InGaAs near-infrared linear arrays can monitor the 1.45 μm wavelength, while “Stressed” InGaAs near-infrared linear arrays can monitor the 1.94 μm wavelength. Since there is very little water absorption around a wavelength of 1.3 μm  410 , that wavelength  410  is suitable for providing a reference measurement with which to compare the absorption wavelengths  406  and  408  and thus enable moisture content to be determined. 
     The camera assembly  104  can monitor either one or both of the water absorption wavelengths  406  and  408 . The camera assembly  104  also monitors either one or several predetermined reference wavelengths, such as 1.3 μm  410  and/or 1.8 μm. The predetermined reference wavelength need only be outside of the water absorption band regions of the paper web  110 . 
     In a preferred embodiment of the present invention, the camera assembly  104  includes standard InGaAs near-infrared linear arrays and monitors the 1.45 μm water absorption wavelength, and a predetermined reference wavelength located in a range from 1.1 μm to 1.35 μm. The standard InGaAs near-infrared linear arrays have better quantum efficiency and dark current characteristics than stressed InGaAs near-infrared linear arrays at these wavelengths. Selecting the 1.45 μm water absorption wavelength also permits measurement of higher moisture levels than is possible if the 1.94 μm water absorption wavelength was monitored. 
     FIG. 5 is an exemplary graph  500  of optical transmittance  502  verses optical wavelength  504  for cellulose within the paper web  110 . As shown, optical transmittance  502  for cellulose varies with the wavelength  504  and includes a first cellulose absorption wavelength  506  at 1.57 μm, a second cellulose absorption wavelength  508  at 1.73 μm, and a third cellulose absorption wavelength  510  at 2.1 μm. The third cellulose absorption wavelength  510  is strongest. Any of the absorption wavelengths  506 ,  508 ,  510  can be used with the current invention. “Standard” InGaAs near-infrared linear arrays can monitor the 1.57 μm wavelength, while “Stressed” InGaAs near-infrared linear arrays can monitor the 2.1 μm wavelength. Since there is very little cellulose absorption around a wavelength of 1.3 μm  512 , that wavelength  512  is suitable for providing a reference measurement with which to compare the absorption wavelengths  506 ,  508 ,  510  and thus enable basis weight to be determined. As an aside, while the graph  500  also shows strong absorption bands at 1.45 μm and 1.93 μm, these are probably due to cellulose&#39;s hydrophilic properties. 
     The camera assembly  104  monitors either one or several of the cellulose absorption wavelengths  506 ,  508  or  510 , and either one or several predetermined reference wavelengths, such as 1.3 μm  512 . The predetermined reference wavelengths need only be outside of the cellulose absorption band regions of the paper web  110 . 
     In a preferred embodiment of the present invention, the camera assembly  104  includes standard InGaAs near-infrared linear arrays and monitors the 1.57 μm cellulose absorption wavelength. The standard InGaAs near-infrared linear arrays have better quantum efficiency and dark current characteristics at 1.57 μm than stressed InGaAs near-infrared linear arrays do at 2.1 μm. 
     FIG. 6 is a graph  600  illustrating several possible optical filtering configurations for moisture and basis weight measurement. The graph  600  depicts filter spectral response verses wavelength for various filtering configurations. For instance, a first narrow-band filter  602  for monitoring a cellulose absorption wavelength can be centered about 1.57 μm. A second narrow-band filter  604  for monitoring a water absorption wavelength can be centered about 1.45 μm. A third narrow-band filter  606  for monitoring a reference wavelength can be centered about 1.3 μm. A fourth filter  608  can be a dichroic mirror for providing reference wavelengths for water absorption measurements. A fifth filter (not shown) can be a specialized prism for selectively separating various water, cellulose, and reference wavelengths over by a predetermined number of arc degrees. Using the prism, the arrays would need to be located at specific positions about the prism. In light of the teachings in this invention, those skilled in the art will know that additional filtering arrangements are possible. 
     While the present invention has been described with reference to a preferred embodiment, those skilled in the art will recognize that various modifications may be made. Variations upon and modifications to the preferred embodiment are provided by the present invention, which is limited only by the following claims.