Abstract:
An apparatus for measuring web planarity includes a horizontal reference surface and a light source for projecting a light pattern onto the reference surface at an angle. A web is supported above the reference surface. An imaging device detects (1) positions on the reference plane of interception of discrete regions of the projected light pattern and (2) respective positions on the web of interception of the same discrete regions of the projected light pattern. The imaging device determines the vertical offset of the respective positions on the web as a function of differences in the detected positions on the reference plane and the respective detected positions on the web. A measure of non-planarity of the web is calculated based upon a comparison of a plurality of such vertical offsets from a plurality of detected positions of the web.

Description:
FIELD OF THE INVENTION 
   The present invention relates to measuring or quantifying the bagginess of webs. 
   BACKGROUND OF THE INVENTION 
   Paper materials, plastic film materials, and other materials commonly provided in sheet and strip form are often initially produced as continuous lengths of material, usually referred to as “webs,” which are wound onto cores. As used herein, the term “web” is intended to refer both to such continuous lengths of material and to shorter and narrower, so called “sheets” of material. During manufacture, the web can stretch or shrink unevenly across the cross-web (CD) direction and in the machine direction (MD) of the web. When the web returns to its steady-state condition, the stretched or shrunken areas will often become baggy, which can develop into creases or wrinkles in the web. These changes in the condition of the web can cause problems in later web handling operations, including conveying through nips, slitting and winding. For example, it is more difficult to properly slit a baggy web into strips than to slit a web without bagginess. 
   According to David R. Roisum, in his 2002 PFFC Peer-Reviewed Paper entitled “BAGGY WEBS: MAKING, MEASUREMENT AND MITIGATION THEREOF,” bagginess can be defined as a deviation of flatness. That is, web that refuses to lay or run flat and straight is said to be baggy. Depending on the particular industry, bagginess is also related to terms such as “baggy lanes”, “camber”, “frogbellies”, “layflat”, “puckers”, and “non-planarity”. 
   Bagginess in a web poses several difficulties including poor visual appearance, defects during coating or lamination, impediments to floating over rollers, differences in winding tightness, and problems in web guiding, tracking and path control. A web having excessive bagginess may refuse to go through nips. 
   Bagginess is very difficult to measure in a quantifiable way. Common instruments and measurements do not correlate well to bagginess because they do not measure anything closely related to it. While there are ways to measure bagginess more directly, most are tedious or fraught with uncertainty or both. Thus, culling and rejection is typically done by subjective visual inspection. 
   One test for bagginess involves laying a web on an inspection table whereby the web is under no external stresses that can disturb flatness. A baggy web will not lay flat and/or the edges will not be straight. Roisum suggests that the most important or effective tool for bagginess detection is the eye of the operator. Baggy edges will appear as ruffles. A baggy lane will appear as stitches or “tractor tire” marks oriented in the machine direction. However, Roisum recognizes that a web can appear to be baggy when it is not because hard wrinkles and curl can cause the web to not lay dead flat even if it is not inherently baggy. Thus, a more rigorous test for inherent bagginess is needed. 
   Another method of monitoring bagginess of a web is disclosed in U.S. Pat. No. 5,778,724, which issued to T. Clapp et al. on Jul. 14, 1998. In the Clapp et al. method, a first reference light is projected onto a front face of the web transverse to the web, and a first measurement light is projected onto the front face of the web non-perpendicular to the front face and transverse to the web. The longitudinal distance on the front face of the web between a point along the first reference light and a corresponding point along the first measurement light are compared to determine bagginess of the web. In the Clapp et al. process, two laser line sources are required: one that is normal to the web in the cross-direction and another that grazes the web in the cross-direction. Relative distance between the lines indicates extent of bagginess. This is primarily an “on-line” tool. Only a single line of cross-direction web bagginess information is collected at a time. The method does not attempt to reconstruct the three-dimensional surface, as very simple two-dimensional geometry math is used. Clapp et al. make no reference to a need to control web tension. For such an on-line system, the minimum amount of tension that is needed to convey the web may indeed be too high to expose problematic bagginess (i.e., non-planarity). Even minor amounts of non-planarity can cause severe problems in subsequent finishing operations requiring conveyance of adjacent web slits through nip rollers and crosscut sheeters. 
   Accordingly, there exists a need to control web tension at a level much lower than the typical paper manufacturing process in order to expose and measure small degrees of non-planarity that are not visible at higher tensions. There is also a need to use multiple lines, in order to gather significantly more information about true non-planarity, and analyze and reconstruct the entire surface topology of a 2D section of web. 
   SUMMARY OF THE INVENTION 
   According to a feature of the present invention, an apparatus for measuring web planarity includes a reflective reference surface lying in a horizontal plane and a light source adapted to project a light pattern onto the reference surface along a non-vertical axis. The light pattern has discrete regions that intercept the reference surface. A support is adapted to receive a web and to bear the received web vertically offset from the reference surface such that the received web intercepts the discrete regions of the projected light pattern. An imaging device is adapted to detect (1) positions on the reference plane of interception of the discrete regions of the projected light pattern and (2) respective positions on the received web of interception of the same discrete regions of the projected light pattern. The imaging device determines the vertical offset of the respective positions on the received web as a function of differences in the detected lateral positions on the reference plane and the respective detected lateral positions on the received web. A measure of non-planarity of the received web is calculated based upon a comparison of a plurality of such vertical offsets from a plurality of detected positions of the received web. 
   In a preferred embodiment, the support is a pair of parallel rollers on opposed sides of the reference surface. The rollers may have guide flanges at one end. The support may further comprise a tensioner for applying a predetermined tension to the web. 
   According to another feature of the present invention, a method of measuring web planarity includes the steps of providing a reflective reference surface and adjusting the reference surface to be level in a horizontal plane; projecting a light pattern of discrete regions onto the reference surface along a non-vertical axis so as to intercept the reference surface; supporting a web spaced vertically above the reference surface such that the received web intercepts the discrete regions of the projected light pattern; detecting positions on the reference plane of interception of the discrete regions of the projected light pattern; detecting respective positions on the received web of interception of the same discrete regions of the projected light pattern; determining the vertical offset of the respective positions on the received web as a function of differences in the detected lateral positions on the reference plane and the respective detected lateral positions on the received web; and calculating a measure of non-planarity of the received web based upon a comparison of a plurality of such vertical offsets from a plurality of detected positions of the received web. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view of a portion of a planarity gauge according to the present invention; 
       FIG. 2  is a side elevation view of a portion of the planarity gauge of  FIG. 1 ; 
       FIG. 3  is a front elevation view of a portion of the planarity gauge of  FIG. 1 ; 
       FIG. 3A  is a detail view of a portion of the planarity gauge within circle A of  FIG. 3 ; 
       FIG. 3B  is a detail view of a portion of the planarity gauge within circle B of  FIG. 3 ; 
       FIG. 4  is a perspective view of a baggy sample with core set before weights are attached to its corners; 
       FIG. 5  is a perspective view, similar to  FIG. 4 , after weights are attached to its corners; 
       FIG. 6  is a perspective view of a misaligned sample; 
       FIG. 7  is a perspective view, similar to  FIG. 6 , but of a properly aligned sample; 
       FIG. 8  is a perspective view showing a baggy web before tension is applied; 
       FIG. 9  is a perspective view similar to  FIG. 8  showing the effect of too much applied tension; 
       FIG. 10  is a perspective view similar to  FIGS. 8 and 9  showing the correct amount of applied tension; 
       FIGS. 11A and 11B  are perspective views showing a web, baggy on one side only, before tension is applied; 
       FIGS. 12A and 12B  are perspective views showing the same web as in  FIGS. 11A and 11B  after tension has been incorrectly applied; 
       FIGS. 13A and 13B  are perspective views showing the same web as in  FIGS. 11A and 11B  after tension has been correctly applied; 
       FIG. 14  is a perspective view of the planarity gauge of  FIG. 1  further showing upper and lower frame, light sources, and a digital array camera; 
       FIG. 15  is a front elevational view showing the position of the light sources and digital camera of  FIG. 14 , along with the area viewed by the camera, and the areas illuminated by the light sources; 
       FIG. 16  is a top plan view showing the light patterns from the light sources of  FIGS. 14 and 15 ; 
       FIG. 17  is a top plan view showing how one such line from one of the light sources shifts laterally when projected onto a perfectly flat but tensioned web; 
       FIG. 18  is a side sectional view taken along line A-A of  FIG. 17 ; 
       FIG. 19  is a view similar to  FIG. 17  showing how one such line from one of the light sources shifts laterally when projected onto a baggy tensioned web; 
       FIG. 20  is an illustration of how the projected light patterns might appear in the sample illustrated in  FIG. 11A , where the web illustrates bagginess on the left side but not on the right; 
       FIG. 21  is a side sectional view taken through the baggy web on the left side of  FIG. 20  showing the fundamental parameters used to define bagginess, or non-planarity; 
       FIG. 22  is a graph of a cross-web planarity profile of a web having one area of non-planarity or one baggy lane; 
       FIG. 23  is a graph of a cross-web planarity profile of a web having three areas of non-planarity or three baggy lanes, and how the non-planarity is distributed across multiple slit lanes during converting; and 
       FIG. 24  sets forth the recommended steps to be taken practicing the preferred embodiment of the present invention 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to  FIGS. 1 ,  2 ,  3 ,  3 A and  3 B, a near roller  10  is aligned with a second, far roller  14 . The top surface of table  12  is preferably painted white for maximum reflectivity and lies in a horizontal plane. Terms of horizontal orientation such as “near”, “far”, “left”, “right”, etc. are used as an aid to understanding the apparatus being described herein, and are not intended to imply any particular orientation of the apparatus during manufacture or use. However, terms of vertical orientation such as “top”, “bottom”, “above”, “below”, up “down”, etc. do have significance in the description and the appended claims. Also, the phrases “machine direction” (MD) and “cross-web direction” (CD) are intended to respectively connote the direction of web movement from a supply roll and the direction normal to that of web movement from a supply roll, respectively. Both MD and CD directions lie in the plane of the web. 
   Measurements taken by the apparatus according to the present invention are intended to be extremely accurate and reliable to quantify flatness of a web material. Flatness is measured and displayed in basic engineering units of differential length per unit length (dL/L) in the machine direction as a function of cross-web direction location. Typical units are microns per meter, but other units may be used. To attain precision results, it is important that care be taken to assure that table  12  be as flat as possible. If the table sags in the middle, for example, and the web were perfectly flat, a three-dimensional plot of web height above the table would appear as the inverse of the tabletop sag. This is because the analysis program assumes that the table is perfectly flat. During experimental measurements of a 54″ wide web, the inventors first used a simple 0.12″ thick aluminum sheet as the table and noted this problem. They then converted to a 0.50″ thick aluminum jig plate that was ground flat on both sides to resolve this issue. 
   Rollers  10  and  14  have guide flanges  16 - 19  at their ends. Near roller  10  rotatably sits in bearing blocks  20  and  21 , while far roller  14  is carried in opposed load cells  22 , of which one is shown in  FIG. 1 . The horizontal distance between the rollers must be precisely set to be uniform. During development, uniformity to within 0.002″ appeared to give satisfactory results for a 54″ wide web. The rollers and tabletop are also aligned in the vertical plane using a precision level. 
   After machine calibration, to be discussed later in this writing, and in preparation for a planarity measurement, a web sample  24  is draped over rollers  10  and  14  inside of flanges  16 - 19 . The near end of web  24  is placed between opposed halves of an open stationary clamp  26 , while the far end of web  24  is placed between opposed halves of an open movable clamp  28 . The ends of the web must extend beyond the clamps and be accessible. Clamps are not closed yet. 
   Small preload weights  32  are clipped to the ends of web  24 , two per end, near each edge of the web. This preloading is desirable to assure an accurate measurement by preventing web sample web  24  from shifting as the clamps close. The operator now makes sure that all four preload weights and both web ends are hanging freely and not touching anything, including the frame, floor, pneumatic tubing, etc. Preloading also removes some of the waviness (e.g. core set) in the web (shown in  FIG. 4 ) that is not due to bagginess.  FIG. 5  illustrates the web after the preload weights are attached and while the clamps are open. 
   Web sample web  24  is gently slid up against guide flanges  16  and  18  as shown in  FIG. 3A . The web should touch only at very top of each roller (the so called 12 o&#39;clock position), being sure that web is right up against both flanges without being crumpled.  FIG. 3B  shows that the web has been pulled away from guide flanges  17  and  19  by this step. Failure to align sample properly will result in a misaligned sample as illustrated in  FIG. 6  and will result in large diagonal draw lines when tension is applied and will require unclamping and returning to this step. A properly aligned sample is shown in  FIG. 7 . 
   Stationary clamp  26  is now closed by suitable means readily selectable by one of ordinary skill in the art. For example, the illustrated embodiment uses a series of clamping air cylinders  30  for this purpose. A web tensioner draws web  24  by moving clamp  28  downward after clamps  26  and  28  are closed. The amount of tension in the web is predetermined to be sufficient to remove any core set in the web material that would introduce noise into the results of the subsequent planarity measurement. The predetermined tension should be less than the planned operating tension for the web from which the sample was cut, and should be low enough to still be able to see all non-planarity defects in the web.  FIGS. 8 ,  9  and  10  illustrate this procedure.  FIG. 8  shows web  24  unclamped. Note the existence of bagginess.  FIG. 9  shows the web when too much tension is applied. All core set and bagginess wrinkles have been removed, and will result in a false result. In  FIG. 10 , the proper amount of tension has been applied to remove the core set while retaining most of the true pattern of bagginess in the web. A skilled operator can easily determine the correct predetermined tension empirically, but it is very important that the test tension be standardized, since different test tensions will produce different values of dL/L. 
   Movable clamp  28  must be drawn downward in pure translation, that is, allowing for no rotation away from its original degree of levelness, during this tensioning step.  FIGS. 11A ,  11 B,  12 A,  12 B,  13 A and  13 B illustrate this requirement.  FIGS. 11A and 11B  show the web before tension is applied. In this example, web  24  exhibits bagginess on one side, represented by the wavy lines on the left side of the web as viewed in the figure. In  FIGS. 12A and 12B , movable claim  28  has be applied and drawn downward to tension the web, but the clamp has been allowed to rotate about point “A” of  FIG. 12B . Much of the bagginess of the non-tensioned web has been removed and the non-planarity has been masked. In  FIGS. 13A and 13B , movable claim  28  has be applied and drawn downward to tension the web. In this case, the clamp has not been allowed to rotate about point “A”. The original bagginess of the non-tensioned web has been retained and the non-planarity will show up in the subsequent measurement results. 
   Referring now to  FIG. 14 , the apparatus in accordance with a preferred embodiment of the present invention further includes a lower main frame  34  upon which sits bearing blocks  20  and  21  and load cells  20  of  FIG. 1 . The lower main frame also supports an upper main frame  36  that carries a digital camera  38  and a pair of opposed laser light sources of which one is visible in the figure. A third, florescent white light source  40  is positioned at the far end of the table  12  in order to create shadowing of any non-planarity when a white-light image is captured by the camera. This serves to document the visual appearance of the tensioned web during the test. 
   The two laser light sources mentioned immediately above are shown in  FIG. 15  and identified by reference numerals  42  and  44 . Laser light source  42  on the left side of upper main frame  36  projects a pattern  48  onto the right side of table  12  (see  FIG. 16 ). Laser light source  44  on the right side of upper main frame  36  projects a pattern  46  onto the left side of table  12 . 
     FIGS. 17 and 18  illustrate the result, for a single line in the pattern, of changing between illuminating table  12  with laser light source  42  and illuminating web sample web  24  with that light source.  FIG. 17  is the image viewed by digital camera  38 . When web  24  is not present, light source  42  projects a pattern  48  of lines, of which one  50  is illustrated. When web  24  is present, the light source projection moves to position  50 ′. The horizontal positional distance between projection line  50  and projection line  50 ′ as viewed by digital camera  38  is a function of the vertical height of sample web  24  above the reference plane of table  12 . 
   The patterns shown in  FIGS. 16 and 17  represent the projection pattern from the light sources directly onto the table with no web. If sample web  24  was perfectly planar, the patterns would look similar but both would be shifted equally toward the center of the table.  FIG. 19  is an illustration of what a projection line  50 ′ might actually resemble if the sample web was non-planar. Projection line  50 ′ appears wavy from above because the horizontal position of the line shifts as a function of the height of the sample web above table  12 . Positions X 1 , X 2  and X 3  are observed (by camera  38 ) of line  50  on table  12 . Positions Y 1 , Y 2  and Y 3  are observed positions on non-planar web  24 . Accordingly, the differences between the lengths Y 1 -X 1 , Y 2 -X 2 , and Y 3 -X 3  are indications of the amount of non-planarity of web  24  at specific positions of the web. 
     FIG. 20  is an illustration of how the projected light patterns might appear in the sample illustrated in  FIG. 11A , where the web illustrates bagginess on the left side but not on the right. Referring to  FIG. 21 , the pattern of projected lines allows for an analytical measurement of bagginess. First, for each right triangle, the differential length between the hypotenuse and the adjacent side is defined as dL. Pluralities of samples of these differential lengths are taken along a sample length at finite cross-web directional positions. A fundamental measure of non-planarity dL/L for each cross-web directional position is obtained by summing all of these differential lengths dL over the machine direction sample length (say, 36″ in a preferred embodiment), and then dividing by the sample length. Finally, a cross-web planarity profile is developed by plotting the measure of non-planarity dL/L for each cross-web directional location as a function of the cross-web directional location. The result of the sample of  FIG. 20  might look like the plot shown in  FIG. 22 , where there is only one large area of non-planarity or one wide baggy lane. The result of a different sample might look like the plot shown in  FIG. 23 , where there are three areas of non-planarity or three baggy lanes. 
     FIG. 24  sets forth the recommended steps to be taken practicing the preferred embodiment of the present invention. From time to time, the optical hardware should be calibrated, as represented in box  60 . One should determine the location of each laser light source  42 ,  44  and digital camera  38  relative to the rest of the structures; STEP  62 . The location of the light source and camera origins relative to the top of table  12  are important for the proper calculation of web height above the table; STEP  64 . Numerical constants describing these locations are easily determined by placing calibration fixture(s) of known height on top of the reference tabletop. A sufficient number of calibration positions are needed to reduce the light source and camera position errors to an acceptable level. A “Least-Squares” calibration method works well. Like the tabletop, the fixture plates are preferably painted white for maximum reflectivity. The image distortion, commonly called ‘barrel’ distortion, that would be introduced by the lens of camera  38  should be determined so that parameters can be developed to compensate for the distortion; STEP  66 . 
   Once calibration is complete, a reference image is captured (STEP  68 ) to determine the coordinates of various points along the laser lines on the top of table  12  without a sample web  24  present. The determined coordinates of each point are saved; STEP  70 . 
   Once a sample is properly aligned as described above, the laser lines are again sampled (STEP  72 ) and the coordinates of the points on the reference plane are determined for each sample laser line; STEP  74 . Comparing the sample data and the reference data, and using conventional three dimensional geometry equations, the spatial coordinates of the points along each sample laser line are calculated; STEP  76 . The results are smoothed and height above the table values are recalculated at evenly spaced points along x and y (STEP  78 ). Finally dL/L values are calculated along the MD sample length using x, y and z coordinates at finite intervals along the cross-web direction (STEP  80 ), and are displayed in graphical form at STEP  82   
   The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention. 
   PARTS LIST 
   
       
         10  near roller 
         12  table 
         14  far roller 
         16  guide flange 
         17  guide flange 
         18  guide flange 
         19  guide flange 
         20  bearing block 
         21  bearing block 
         22  load cells 
         24  web sample 
         26  stationary clamp 
         28  movable clamp 
         30  clamping air cylinders 
         32  preload weights 
         34  lower main frame 
         36  upper main frame 
         38  digital camera 
         40  florescent white light source 
         42  laser light source 
         44  laser light source 
         46  pattern 
         48  pattern 
         50  projection line on table 
         50 ′ projection line on web 
         60  calibration box 
         62  light source location determination step 
         64  camera location determination step 
         66  image distortion parameter determination step 
         68  reference image capture and processing step 
         70  reference line coordinates save step 
         72  sample image capture and processing step 
         74  coordinate determination step 
         76  coordinate calculation step 
         78  heights calculation step 
         80  calculation of dL/L step 
         82  display dL/L profile step