Abstract:
Dual mounting head scanner system measures the thickness of a flexible continuous moving web such as paper by employing an optical senor positioned in the upper head to determine the distance between the optical sensor and the upper surface of the paper while a displacement sensor positioned in the lower head determines the distance between the displacement sensor, which includes an RF coil, and a reference surface on the upper head. An air clamp and vacuum source assembly on the operative surface of the lower head maintains the moving web in physical contact with a measurement surface that is incorporated in the operative surface. The optical sensor directs incident radiation onto the web at the measurement surface. Thermal isolation of the two sensors eliminates thermal interactions.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to a dual mounting head scanner system for measuring the thickness of a flexible continuous moving web. An air clamp and vacuum source assembly maneuvers the moving web into physical contact with a measurement surface that is incorporated in the operative surface of the lower head. An optical senor located in the upper head measures the distance between the optical sensor and the upper surface of the web while a displacement sensor located in the lower head measures the distance between the displacement sensor and a reference surface on the upper head. The optical sensor directs incident radiation onto the web at the measurement surface. 
     BACKGROUND OF THE INVENTION 
     In modern papermaking very high standards exist for many of the physical properties of a manufactured sheet. These properties are manipulated by complex control systems that require very accurate, robust measurements for control. Of these properties, thickness is a commonly required characteristic that poses significant problems for the measurement system. Many grades of paper are delicate and cannot be measured using conventional contacting measurement methods, which can either mark the sheet or tear it altogether. While non-contacting optical measurements of web thickness have been used, these techniques are extremely susceptible to errors that are caused by sheet motion. Sheet stabilization is the main technical hurdle to obtaining consistent measurements. The sheet must be held essentially flat within a narrow Measurement range even as the sheet travels at speeds that reach up to 120 km/h. The industry is in need of a non-marking, sub-micron accurate, thickness measurement apparatus. 
     SUMMARY OF THE INVENTION 
     In papermaking machines, sensors that are employed to measure paper properties are housed in enclosures that are scanned across the sheet as the paper is produced. These opposite-facing enclosures are positioned on either side of the sheet, which is approximately centered in the gap formed between them. Typically, a sensor includes two halves, each in its respective enclosure, on either side of the sheet. As the scanner moves laterally from one edge of the traveling sheet to the other, mechanical and thermal variations cause the distance between the two enclosures to change. 
     The precise positions of both sides of a sheet must be identified in order to make accurate dynamic sheet thickness measurements. With the inventive technique, the lower side of the moving sheet is held against a flat measurement surface. Subsequently, the distance from an opposing side on the upper enclosure to the exposed, visible upper side of the moving sheet is measured with an optical sensor, such as a laser triangulation device, while, simultaneously, the position of the optical sensor from the lower side holding the sheet is measured, preferably with an electromagnetic induction sensor. The thickness of the sheet will be the difference between the two distance measurements with a constant offset. 
     The present invention is based in part on the development of an air clamp or stabilizer and vacuum assembly that subjects a moving flexible web, which is traveling in the machine direction, to forces sufficient to support and pull the web toward a measurement surface that is formed on an operative surface. In particular, suction forces generated by vacuum channels that are configured adjacent the measurement surface flatten the contour of the web and holds the web in physical contact against the measurement surface as the web passes over the measurement surface. The above-described two distance measurements are conducted as the moving web is held on the measurement surface thereby yielding accurate continuous web thickness measurements. 
     In one aspect, the invention is directed to a detector device for contact support of a flexible continuous web being monitored and that is moving in a downstream machine direction, that includes:
         (a) a first mounting head disposed on a first side of the moving web;   (b) a second mounting head disposed on a second side of the moving web and which comprises a body having an operative surface facing the second side of the web wherein the operative surface defines a measurement surface and has a web entry end and a web exit end that is downstream from the web entry end;   (c) a displacement sensor that is positioned in the second mounting head to determine a distance from the displacement sensor to a reference surface on the first mounting head; and   (d) means for positioning the moving web such that the second side of the web comes into contact with the measurement surface as the web passes over the measurement surface.       

     In another aspect, the invention is directed to a system for dynamic thickness measurements of a flexible continuous web, that has a first surface and a second surface, and which is moving in a downstream machine direction (MD) that includes:
         (a) a first mounting head disposed adjacent to the first side of the web, the first mounting head including:
           (i) a first operative surface facing the first side of the web; and   (ii) an optical sensor for measuring the distance from the optical sensor to the first side of the web; and   
           (b) a second mounting head disposed adjacent to the second side of the web, the second mounting head including:
           (i) a body having a second operative surface facing the second side of the web wherein the second operative surface defines a measurement surface wherein the first operative surface and the second operative surface define a measurement gap, that has a web entry end and a web exit end that is downstream from the web entry end, through which the continuous web travels;   (ii) an air stabilizer that supports the flexible continuous web as the web travels through the measurement gap;   (iii) a first vacuum channel formed on the second operative surface that applies a suction force on the web to maintain the web in contact with the measurement surface as the web passes over the measurement surface; and   (iv) a displacement sensor that determines a distance from the displacement sensor to a reference surface on the first mounting head.   
               

     In yet another aspect, the invention is directed to a method of measuring the thickness of a flexible continuous web that is moving in a downstream machine direction (MD) along a path that includes the steps of:
         (a) maneuvering the continuous web through a dual scanner head that includes:
           (i) first mounting head disposed adjacent to a first side of the web, the first mounting head including:   (A) a first operative surface facing the first side of the web; and   (B) an optical sensor for measuring the distance from the optical sensor to the first side of the web; and   (ii) a second mounting head disposed adjacent to the second side of the web, the second mounting head including:   (A) a body having a second operative surface facing the second side of the web wherein the second operative surface defines a measurement surface wherein the first operative surface and the second operative surface define a measurement gap, that has a web entry end and a web exit end that is downstream from the web entry end, through which the continuous web travels;   (B) an air stabilizer that supports the flexible continuous web as the web travels through the measurement gap;   (C) a first vacuum channel formed on the second, operative surface that applies a suction force on the web to maintain the web in contact with the measurement surface as the web passes over the measurement surface; and   (D) a displacement sensor that determines a distance from the displacement sensor to a reference surface on the first mounting head;   
           (b) measuring the distance between the displacement sensor and the reference surface;   (c) measuring the distance between the optical sensor and the first side of the web; and   (d) calculating the thickness of the moving web.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross sectional schematic view of a dual scanner head system employing the caliper measurement device; 
         FIG. 2A  is a cross sectional schematic view of the lower scanner head illustrating the measurement surface and vacuum channels on the operative surface of the an air stabilizer and vacuum system; 
         FIGS. 2B and 2C  are enlarged cross sectional views of Coanda nozzles; 
         FIG. 3  shows a perspective cross sectional view of the web thickness measurement device as part of a sensor head; and 
         FIG. 4  shows a perspective view of the operative surface of the measurement device as part of the sensor head. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  illustrates an embodiment of the non-contacting caliper sensor system that includes upper and lower sensing scanner heads  62  and  64 , which are positioned on opposite sides of web or sheet  22 . The two heads define a measurement gap and, if the caliper measurement is to be performed in a scanning manner across the web in the cross direction, the heads are aligned to travel directly across from each other as they traverse the moving web which is moving in the machine direction. 
     In a preferred embodiment, upper head  62  includes a laser triangulation device  66  that gauges the perpendicular distance between a base surface  80  of device  66  to the top of moving web  22  as the web is in contact with measurement surface  36 . This operation being referred to as the laser measurement. Laser triangulation device  66  includes radiation source  68  and detector  70 ; incident radiation from source  68  passes through an aperture  78  in upper head  62  and detector  70  captures reflection radiation. From the measured path length from the source to the detector, values for the distance between base surface  80  and a measurement or interrogation spot on upper surface of the web  22  can be determined. The heads  62  and  64  are typically fixed in positions so that the interrogations spots do not move in the machine direction even as the heads are scanned in the cross direction. Laser triangulation devices are further described, for example, in U.S. Pat. No. 6,281,679 to King et al., and U.S. Pat. No. 7,528,400 to Duck and Hughes, which are incorporated herein by reference. 
     As further illustrated in  FIG. 1 , lower head  64  incorporates an air clamp and vacuum assembly  10  which supports moving web  22  and which initially flattens the contour of the moving web as it approaches measurement surface  36  and then holds web  22  in contact with measurement surface  36  as web  22  passes over it. In addition, lower head  64  includes a displacement or distance measurement apparatus that measures the distance from the apparatus to a reference surface that is positioned above web  22 . A preferred apparatus is an inductive-type sensor that has an RF or z-coil  74 , which is positioned directly below measurement surface  36  and measures the distance from z-coil  74  to lower surface  82  of upper head  62 . Suitable z-coils can be made of aluminum nitride. This latter operation being referred to as the inductive measurement. Lower surface  82  thus which serves as the reference surface or plate. Z coil sensors are described in U.S. Pat. No. 6,281,679 to King et al. and U.S. Pat. No. 4,160,204 Holmgren et al, which are incorporated herein by reference. 
     The caliper of a moving sheet  22  that travels between two heads  62 ,  64  is determined by making the laser measurement, d (optical), and inductive measurement, d (inductive). Thereafter, the thickness (t) of sheet  22  is calculated as being the difference between the two measurements with a constant offset, that is: t=d (inductive)−d (optical)−C. The offset constant is determined by calibration that is preferably conducted by taking a zero measurement when the sensor is offsheet, that is, when there is no sheet between the heads. 
     Because laser triangular device  66  and the z-coil  74  are located in separate scanner heads,  62  and  64 , respectively, the two devices are effectively thermally isolated from each other. Given that both devices are susceptible to thermal drift, this arrangement eliminates thermal interaction between them. In this regard, the temperatures in the upper and lower heads can also be independently regulated with controller  90 , which actuates heat exchangers  94  and  98  in response to signals from temperature sensors  92  and  96 , respectively. Separating z-coil  74  from laser triangulation device  66  has the added benefit of allows the coil to operate with less “backloading” from conductive material located nearby. This allows for a cleaner inductive measurement. The laser and laser optomechanics can be made of conductive ‘target’ materials. Optomechanical components are preferably made of stiff materials with low coefficients of thermal expansion. Metallic materials are cheaper and easier to manufacture than complicated parts made of nonconductive materials such as ceramics. 
     A critical feature of the present invention is that moving web  22  remains in contact with measurement surface  36  to insure accurate and consistent thickness measurements. This is accomplished in part by employing an air clamp that supports and pulls the moving web toward measurement surface and one or more vacuum channels, which are disposed on the operative surface adjacent the measurement surface, which holds the moving web against the measurement surface. 
     Suitable air clamps or stabilizers include an operative surface and one or more nozzles that are disposed on the operative surface. As a moving web travels above the operative surface, gas jets from the nozzles establish pressure fields that support and maintain the moving web at a desired distance from the operative surface. Air clamps are described, for example, in U.S. Pat. No. 6,9356,137 to Moeller et al., U.S. Pub. Nos. 2009/0260771 to Alev et al., 2009/0260772 to Alev et al., and 2010/0078140 to Hughes, which are all incorporated herein by reference. 
       FIG. 2A  illustrates an air clamp and vacuum assembly  10  that incorporates opposite-facing nozzles that are configured with backsteps to generate suction forces that are applied to a moving web  22 . The assembly  10  includes a body that is segmented into a ceramic central region  12 , polymer lateral region  14 A and polymer lateral region  14 B. Central region  12  has an operative surface  32  that is situated between Coanda nozzles  16 A and  16 B, which are in gaseous communication with chambers  18 A and  18 B, respectively. Coanda nozzles  16 A and  16 B exhaust jets of gas in opposite directions toward surface  34 A and  34 B, respectively, which are downstream of the backstep features of nozzles. 
     Chamber  18 A is connected to plenum chamber  46 A which in turn is connected to a source of gas  24 A via conduit  30 A. The gas flow rate into plenum  46 A can be regulated by conventional means including pressure controller  28 A and flow regulator valve  26 A. Plenum  46 A essentially serves as a reservoir in which high pressure gas equilibrates before being evenly distributed along the length of Coanda nozzle  16 A via chamber  18 A. Similarly, chamber  18 B is in gaseous communication with plenum chamber  46 B, which is connected to a source of gas  24 B via conduit  3013 . Gas flowing into plenum  46 B is regulated by pressure controller  28 B and flow regulator valve  26 B. Any suitable gas can be employed in gas sources  24 A and  24 B including for example, air, helium, argon, carbon dioxide. 
     Central region  12  includes a lower compartment  4  that houses a z-coil (not shown) that serves as the z-direction source/detector of a z-sensor. Positioned immediately above compartment  4  is hard ceramic disk  2  that is partially housed in enclosure  6 . A preferred material for the disk is zirconium. The planar, upper surface of disk  2  serves as the measurement surface  36 . Encircling at least a portion of the outer perimeter of enclosure  6  is vacuum channel  58  and upstream from vacuum channel  58  is vacuum channel  8 . Both vacuum channels  58  and  8  are connected to a venturi vacuum pump  38  via conduit  48 . 
     As illustrated in  FIG. 2B , Coanda nozzle  16 A has a Coanda slot  40  between upper surface  44  and operative surface  32  which are preferably coplanar. Coanda slot  40  has a curved convex surface  42  on its downstream side, with a radius of curvature (R) typically ranging from about 1.0 mm to about 10 mm. Airflow from the Coanda slot  40  follows the trajectory of the curved surface  42 . The term “backstep” is meant to encompass a depression on the stabilizer surface located a distance downstream from Coanda slot  40  preferably sufficient to create a vortex. The combination of the Coanda slot and backstep generates an amplified suction force and an extensive air bearing. 
     Backstep  20  is most preferably configured as a 90 degrees vertical wall. Preferably, Coanda slot  40  has a width (b) of about 3 mils (76 μm) to 5 about mils (127 μm). The distance (d) from the upper surface  44  to lower surface  34 A, which are preferably parallel to each other, is preferably between about 100 to 1000 μm. Preferably the backstep location (L) is about 1 mm to about 6 mm and preferably about 2 mm to 3 from Coanda slot  40 . 
     Similarly, as shown in  FIG. 2C , Coanda nozzle  16 B has a Coanda slot  50  between upper surface  54  and coplanar operative surface  32 . Coanda slot  50  has a curved surface  52  on its downstream side. The dimensions of structures forming Coanda nozzle  16 B, including backstep  30  and lower surface  34 B, can be the same as those for Coanda nozzle  16 A. 
     Referring to  FIG. 2A , the air clamp and vacuum assembly  10  is positioned underneath a web of material  22  which is moving from left to right relative to the assembly; this direction from the web entry end to the web exit end through the measurement gap being the downstream machine direction (MD) and the opposite direction being the upstream machine direction. The cross direction (CD) is transverse to the M.D. Operative surface  32  and measurement surface  36  are preferably not coplanar. The measurement surface is raised between 0.005 in. (0.127 mm) to 0.020 in. (0.508 mm) above the operative surface. The middle part of web  22  that is passing over operative surface  32  is not shown for clarity. 
     The contour of web  22  as it travels over operative surface  32  is manipulated with the air clamp and vacuum channels. In a preferred application, the profile of web  22  is substantially planar as in approaches measurement surface  36 . The sub-ambient pressure generated by vacuum channels  8  and  58  urges web  22  toward and into physical contact with measurement surface  36 . The higher the vacuum levels, the greater the suction force imparted on moving web  22 . 
     The thickness measurement devices of the present invention can be incorporated into on-line dual head scanning sensor systems for papermaking machines, which are disclosed in U.S. Pat. No. 4,879,471 to Dahlquist, U.S. Pat. No. 5,094,535 to Dahlquist et al., and U.S. Pat. No. 5,166,748 to Dahlquist, all of which are incorporated herein by reference. Besides the thickness of paper, other materials such as plastics, fabrics and the like can also be measured. The width of the paper in the papermaking machines generally ranges from 5 to 12 meters and typically is about 9 meters and travels at speeds of 200 m/min to 1800 m/min or higher. 
       FIGS. 3 and 4  show an air clamp and vacuum assembly that is incorporated into a recess compartment within polymer substrate  102  that is a part of lower head  100  of a dual scanning sensor. The upper surface of disk  2  serves as measurement surface  36 , which is located in the middle of operative surface  32  between Coanda nozzles  16 A and  16 B. Vacuum channel  58  has a ring structure that encircles measurement surface  36  and vacuum channel  8 , which has a curved, arch-shaped configuration that partially encircles vacuum channel  58 . As shown in  FIG. 3 , vacuum channel  8  has a proximal end  110  and distal ends  112  and  114  where gas vacuum ports  116  and  118 , respectively are located. Substrate  102  is positioned so that as a web product travels toward operative surface  32  in the machine direction (MD), the web after traveling over Coanda nozzle  16 A encounters the forces generated by vacuum channels  8  and  58 . The web&#39;s contour is flattened as it approaches measurement surface  36  and is held thereon as it passes over the surface. 
     When employed for measuring the caliper of paper, in one embodiment, the distance between nozzles  16 A and  16 B is about 50 mm and the length of each nozzle along the cross direction is about 75 mm. The zirconium disk  2  has a diameter of 0.375 inches (0.95 cm). 
     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.