Abstract:
Dual mounting head scanners measure the thickness of flexible moving porous webs and employ an air clamp on the operative surface of the lower head to maintain the web in physical contact with a measurement surface. As the web is held firmly by the clamp, the vacuum level that is established is indicative of the porosity of the membrane. As compressed air is supplied to a vacuum generator at a given operational pressure, the rate of airflow through the web can be interred from the vacuum pressure measurements. The rate of airflow through the membrane and therefore the porosity of the membrane are related to the vacuum level. It is not necessary to measure the airflow through the membrane. From the vacuum pressure measurements, the membrane&#39;s permeability can also be determined by correlation to empirical data. Thickness measurements are effected by optical triangulation and inductive proximity measurements.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates generally to a dual mounting head scanner system for simultaneously measuring the thickness and porosity 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. Typically, an optical triangulation sensor measures the distance between the upper head and the upper surface of the web while a displacement sensor such as an inductive proximity sensor measures the distance between the upper head and measurement surface. The airflow through the web and thus the porosity of the membrane can be inferred from the vacuum pressure that is established by the air clamp. 
       BACKGROUND OF THE INVENTION 
       [0002]    The thickness and porosity or permeability of web products such as paper and plastics are important product parameters. For example, lithium-ion battery separators, which are made of porous polymeric membranes, must have uniform thickness (typically to less than one micron) and meet specific electrolyte permeability standards. The an is desirous of developing reliable on-line techniques for measuring the thickness and calculating the permeability of web materials during production. 
       SUMMARY OF THE INVENTION 
       [0003]    In the manufacture of web or sheet products such as plastic membranes used as separators in lithium-ion batteries, sensors that measure web properties are housed in enclosures that are scanned across the web as the membrane is produced. These opposite. facing enclosures are positioned on either side of the web, which is approximately centered in the gap formed between them. As the scanner moves laterally from one edge of the traveling web to the other, mechanical and thermal variations cause the distance between the two enclosures to change. 
         [0004]    The precise positions of both sides of the web must he identified in order to make accurate dynamic web thickness measurements. With the inventive technique, the lower side of the moving web 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 from the upper enclosure to the measurement surface that is in contact with the sheet is measured, preferably with an electromagnetic induction sensor. The thickness of the web is the difference between the two distance measurements with a constant offset. 
         [0005]    An air clamp or stabilizer and vacuum assembly subjects the moving flexible web, which is traveling in the machine direction, to forces sufficient to support and pull the web toward the measurement surface that is formed on an operative surface in the lower enclosure, In particular, suction forces generated by vacuum channels, which are in gaseous communication with a vacuum generator and 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. 
         [0006]    The present invention is based, in part, on the recognition that when the web is held firmly on the measurement surface by the vacuum of the air clamp, the vacuum level that is established is indicative of the porosity of the membrane. In particular, as compressed air is supplied to a vacuum generator at a given operational pressure, the rate of airflow through the web as it is being held can be inferred from the vacuum pressure measurements. With the inventive detector device, a good seal is established around a selected vacuum channel so that air entering the vacuum channel must travel through the web. In this fashion, the rate of airflow through the membrane is controlled, in part, by the vacuum level within the vacuum channel. The porosity of the membrane in turn is correlated to rate of airflow through the membrane. With the inventive technique, it is not necessary to measure the airflow through the membrane to ascertain its porosity. From the vacuum pressure measurements, the membrane&#39;s permeability to various substances can also be determined by correlation to empirical data using calibration techniques. 
         [0007]    In one aspect, the invention is directed to a detector device, for measuring the porosity and thickness of a flexible continuous web that is moving in a downstream machine direction, which includes: 
         [0008]    (a) a first mounting head disposed on a first side of the moving web and which has a reference surface; 
         [0009]    (b) a second mounting head disposed on a second side of the moving web and which has 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; 
         [0010]    (c) a displacement sensor that determines a distance from the reference surface to the measurement surface 
         [0011]    (d) one or more channels that are disposed on the operative surface and that are in gaseous communication with a vacuum source so that the second side of the moving web is in vacuum engagement with the operative surface and the second side of the web comes into contact with the measurement surface as the web passes over the measurement surface; 
         [0012]    (e) means for measuring the pressure in the one or more of the channels when the second side of the moving web is vacuum engaged to the operative surface; 
         [0013]    (g) means for calculating, the porosity oldie moving web; and 
         [0014]    (f) means for calculating the thickness of the moving web. 
         [0015]    In a preferred embodiment, the operative surface has a first inner channel that at least partially encircles the measurement surface and a second outer channel that is located up stream of the first inner channel. Each channel being in communication with a vacuum source. The suction in the second outer channel removes or strips off entrained gas (e.g., air) between the moving web and the operative surface. This ensures that as the web approaches in the machine direction toward the first inner channel that the web covers the outer perimeter of the first inner channel so as to establish a good seal so that the inner vacuum can measure the air flowing through the web. 
         [0016]    In yet another aspect, the invention is directed to a method of measuring the porosity and thickness of a flexible continuous web that is moving in a downstream machine direction (MD) along as path that includes the steps of: 
         [0017]    (a) maneuvering the continuous web through a dual scanner bead 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; and   (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;   
           (iii) a displacement sensor that determines a displacement distance from the first operative surface to the measurement surface;       
 
         [0026]    (b) measuring the displacement distance; 
         [0027]    (c) measuring the distance between the first operative surface and the first side of the web; 
         [0028]    (d) calculating the thickness of the moving web; 
         [0029]    (e) measuring the pressure in the first vacuum as the web moves over the first vacuum; and 
         [0030]    (f) calculating the porosity of the moving web based on the pressure measured in step (e) wherein the calculation of the porosity does not require measurement of airflow through the moving web. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0031]      FIG. 1  is a cross sectional schematic view of a dual scanner head system employing the co-located caliper and porosity measurement sensors; 
           [0032]      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 an air stabilizer and vacuum system to measure porosity; 
           [0033]      FIGS. 2B and 2C  are enlarged cross sectional views of Coanda nozzles; 
           [0034]      FIGS. 2D and 2E  illustrate two vacuum generators; 
           [0035]      FIG. 3  shows a perspective cross sectional view of the web thickness measurement device as part of a sensor head; 
           [0036]      FIG. 4  shows a perspective view of the operative surface of the measurement device as part of the sensor head; and 
           [0037]      FIG. 5  is a diagram of a system employing process measurements to calculate the caliper and porosity of the web. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0038]      FIG. 1  illustrates a caliper and porosity 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 measurements are to be performed in a scanning manner across the web in the cross direction, the heads are aliened to travel directly across from each other as they traverse the web which is moving in the machine direction (MD). The sensor system can be employed to continuously measure porous webs made of paper, plastics and the like, as further described herein. 
         [0039]    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 that is preferably configured as a zircornia disk or button  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. The distance between base surface  80  and a measurement or interrogation spot on upper surface of the web  22  can be inferred by measuring the spot on the detector where the image is. Since the distance between detector  70  and source  68  is known, the distance between base surface  80  and the spot of the web can be calculated. In addition, upper head  62  includes a displacement measurement apparatus that measures the distance from the apparatus at base surface  80  to measurement surface  36  that is in contact with web  22  below. A preferred apparatus is an inductive-type sensor that has an RF or z-coil  74 , which measures the distance from z-coil  74  to measurement surface  36  of lower head  64 . Suitable z-coils consist of a metal wire that is preferably made of aluminum nitride. The laser triangulation sensor projects a beam onto the center of the zirconia button  36 . The center of z-coil  74  preferably matches with the center of the zirconia button  36 . Aside from employing triangulation, another method to measure the distance between the upper head and lower and the web uses chromatic confocal sensors which are described for example in U.S. Pat. No. 7,738,113 to Marx et at and U.S. Pat. No. 8,212,997 to Xie, which are incorporated herein by reference. 
         [0040]    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. 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. 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. 
         [0041]    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. The maintain accuracy the temperatures of z-coil  74  and laser triangulation device  66  can be independently controlled. 
         [0042]    A feature of the present invention is that moving web  22  remains in contact with measurement surface  36  to ensure 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. 
         [0043]    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. 
         [0044]      FIG. 2A  illustrates an air clamp and vacuum assembly  10  that incorporates opposite-facing nozzles that are configured with backsteps to generate suction farces that are applied to a moving web  22 . The assembly  10  includes a stainless steel body that is segmented into a central region  12 , lateral region  14 A and 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. 
         [0045]    Chamber  18 A is connected to plenum chamber  46 A which in turn is connected to a source of vas  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  30 B. 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. 
         [0046]    Central region  12  includes a lower compartment  4  that houses hard ceramic disk  2  that is secured with epoxy in a depression  6  that has been milled out from the stainless body. A preferred material for the disk is zirconium dioxide. The planar, upper surface of disk  2  serves as the measurement surface  36 . Encircling at least a portion of the outer perimeter of depression  6  is inner vacuum channel  58  and upstream from inner vacuum channel  58  is second vacuum channel  8 . Vacuum channel  58  is connected to an air valve  38 A, a pressure sensor  39  and a vacuum generator  38  via conduit  48 . The pressure sensor  39  measures the pressure in vacuum channel  58  when the air valve  38 A is open. Vacuum channel  8  is connected to a vacuum generator  41  which is usually set to a generate a higher suction force relative to that of vacuum generator  38  in order to remove any entrained air under the moving web  22 . 
         [0047]    As shown in  FIG. 2D , a vacuum generator preferably comprises a venturi vacuum pump  138  that has an air-inlet port  140 , an air-discharge port  142 , and a vacuum port  144 . Channel  146  connects inlet port  140  to discharge port  142 . As operational compressed air from air supply  152  is introduced through inlet port  140 , the gas flows through channel  146  and passes through nozzle  150  whereupon channel  146  narrows. The narrowing of the channel results in the gas&#39;s speed increases, and because of the Venturi effect, its pressure decreases. A vacuum at vacuum port is established through channel  148 . By measuring the vacuum in inner vacuum channel  58  ( FIG. 2A ) at a given operational pressure for the compressed air, air porosity of membrane  22  ( FIG. 2A ) can be inferred, as described herein. An alternate method of generating a vacuum in addition to the venturi vacuum pump is illustrated in  FIG. 2E  in which an air amplifier operates by using the Coanda effect to create a low pressure volume from a high pressure supply. In this configuration, an air amplifier directs air from the inlet port  171  to a circumferential plenum  172 . A circumferential nozzle,  174  throttles the flow and directs it towards a curved surface  173  where it is directed towards the outlet port by the Coanda effect. This generates to low pressure region drawing flow from the inlet port  175 . Entrained air  176  flows along the perimeter. 
         [0048]    As illustrated in  FIG. 23 , 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  configured so that gas has room to expand to create the large, low pressure area. The combination of the Coanda slot and backstep generates an amplified suction force and an extensive air bearing. 
         [0049]    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 . 
         [0050]    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 farming Coanda nozzle  16 B, including backstep  30  and lower surface  34 B, can be the same as those for Coanda nozzle  16 A. 
         [0051]    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 MD. 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. 
         [0052]    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 channel  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. 
         [0053]      FIGS. 3 and 4  show an air clamp and vacuum assembly that is incorporated into a recess compartment within 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. Inner vacuum channels  58  form a ring structure that partially encircles measurement surface  36  and outer vacuum channel  8  and that has a curved, arch-shaped configuration that partially encircles inner vacuum channels  67 ,  59 . 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, 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 
         [0054]    When employed for measuring porous membranes, 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 straight Coanda nozzles  16 A and  16 B which are located at the web entrance and exit ends of measurement surface  32 , respectively, have linear slots that are perpendicular to the machine direction. 
         [0055]    The present invention is particularly suited for calculating the caliper and permeability of plastic membranes used as separators in lithium-ion batteries, which consists of anode, cathode, separator there-in-between, and lithium electrolyte that includes lithium salt dissolved in an organic solvent. The separator is a barrier through which the lithium ions migrate back and forth during charging and discharging of the battery. The separator must exhibit sufficient structural integrity and permeability to lithium ions in the electrolyte. A suitable material is NAFION, which is a synthetic copolymer membrane available from Du Pont (Wilmington Del.). Co-located porosity and caliper measurements can also be made for other plastic membrane materials that include, for instance, polyethylene, polypropylene, polyethylene terephthalate, polytetrafluoroethylene and polyvinyl chloride. 
         [0056]    The vacuum pressure measurements P can be used to estimate the porosity and permeability of the web. To achieve this, normalized pressure Pnorm measurements are computed. Referring to  FIG. 2A , normalization of the pressure takes into account the maximum vacuum pressure that can be generated by the vacuum pump  38 . Normalization is also used to limit the effect of dirt accumulation in the vacuum channel  58  in the porosity and permeability measurements. A maximum vacuum pressure Pmax is measured periodically, for example when the scanner head is in an offsheet position, by closing air valve  38 A. A measurement of the amount of dirt in vacuum channel  58  is performed by measuring the pressure Pdirt using the pressure sensor  39  when the scanner head is offsheet and no web is present near operative surface  32  and measurement surface  36 . The normalized pressure Pnorm is defined as: Pnorm=(P−Pmax)/(Pdirt−Pmax), The normalized pressure depends on the porosity and thickness of web  22 . The more porous the material forming the web, the lower the vacuum level (the higher the Pnorm). A method of correlating pressure measurements to porosity is to create a library of calibration standards by producing membranes over a range of porosities that are then measured and assigned porosity values in the laboratory. Another method is to develop models based on the data. The membranes with their known porosities are then subject to thickness and porosity measurements with the device of the present invention to establish the required correlations. Each membrane material of interest will require a different set of calibration standards. These correlations are subsequently used to relate thickness and porosity measurements to actual porosities. 
         [0057]    An analogous procedure can be used to create calibration standards to relate measured thickness and porosity measurements to a material&#39;s permeability to lithium-ions in electrolyte. A web material&#39;s permeability to substance(s) in general can be so established. 
         [0058]      FIG. 5  depicts a process for controlling the manufacture of porous membranes or similar webs by continuously measuring the caliper and the porosity of the web. Digitized signals  160 ,  162  representing the caliper of the membrane  22  and pressure within inner vacuum channel  58  ( FIG. 2A ), respectively, are fed to a computer  164 . Memory  166  includes data from calibration measurements that correlate the vacuum pressure to the air porosity of the membrane. In the case where the membrane is to be used as a separator in lithium ion batteries, the calibration data can further correlate the pressure to its permeability to lithium ions in an electrolyte. 
         [0059]    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.