Abstract:
A web edge sensor is disclosed. The web edge sensor has a light source directing light incident to an edge of a web. The edge of the web scatters the light and a sensor array that detects a first portion of the light scattered from the edge of the web and rejects a second portion to determine a position of the web edge.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of prior filed U.S. Provisional Patent Application No. 60/865,000 entitled “FIBER OPTIC WEB EDGE SENSOR,” filed Nov. 9, 2006, the contents of which are hereby incorporated by reference. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    This invention was made with government support under contract CMS 0428397 awarded by the National Science Foundation. The government has certain rights in the invention. 
     
    
     FIELD OF THE INVENTION 
       [0003]    The present invention is related to object position sensors in general and, more particularly, to position sensors for locating flexible web materials. 
       BACKGROUND 
       [0004]    The term web is used to describe materials which are manufactured and processed in a continuous, flexible strip form. Web materials cover a broad spectrum from extremely thin plastics to paper, textiles, metals, and composites. Web handling refers to the physical processes related to the transport and control of web materials through processing machinery. Web handling systems facilitate transport of the web while it is being processed in processing machinery, which is typically an operation specific to the particular web product. For example, in the case of webs used for consumer products, the web is coated, printed, or laminated. A typical operation involves transporting a web in rolled, unfinished form from an unwind roll to a rewind roll through processing machinery where the required processing operations are performed. The web is transported by rollers driven by motors in a process line. The quality of the finished web depends on how it is handled on the rollers during transport. Lateral and longitudinal motion of the web, and web tension, are controlled during transport. 
         [0005]    Accurate web guiding is becoming increasingly demanding due to variation in web materials, both in the gauge and opacity of the materials, being processed in a single process line. Since web lateral position is used as feedback for controlling a web guide, accurate web edge detection becomes increasingly important for proper guiding of the web. Web lateral position is defined generally to mean the position of the center and edges of the web relative to a fixed position on the axis of rotation of the roller along the axis of rotation of the roller. This axis of rotation of the roller is generally perpendicular to the direction of travel of the web. 
         [0006]    What is needed is a system and method for addressing the above, and related, issues. 
       SUMMARY OF THE INVENTION 
       [0007]    The present invention disclosed and claimed herein, in one aspect thereof, comprises a web edge sensor. The web edge sensor has a light source directing light incident to an edge of a web. The edge of the web scatters the light and a sensor array that detects a first portion of the light scattered from the edge of the web and rejects a second portion to determine a position of the web edge. 
         [0008]    In various embodiments the sensor array comprise a plurality of fiber optic strands that may be jacketed. The sensor array may be a linearly arranged plurality of fiber optic strands or may comprise a two dimensional array. Photodiodes may be connected to the fiber optic strands. The sensor array may be positioned to detect web flutter and/or lateral movement of the web 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a schematic illustration of one embodiment a web edge sensor according to aspects of the present disclosure. 
           [0010]      FIG. 2  is a schematic illustration of the cone of acceptance principle for an optical fiber. 
           [0011]      FIG. 3  is an end view of a linear array of optical fibers. 
           [0012]      FIG. 4  is an illustration of the cone of acceptance principle for an exemplary fiber optic array. 
           [0013]      FIG. 5  is an end view of a two dimensional array of optical fibers 
           [0014]      FIG. 6  illustrates the illumination of a single optical fiber due to scattered light from a web edge. 
           [0015]      FIG. 7A  is an end view of a combined fiber optic light source and fiber optic sensor array. 
           [0016]      FIG. 7B  is a side view of the operation of the sensor array of  FIG. 7A . 
           [0017]      FIG. 7C  is a perspective view of the operation of the sensor array of  FIG. 7A . 
           [0018]      FIG. 8  is a top down view of a center pivot guide roller. 
           [0019]      FIG. 9  is a top down view of an end pivot guide roller. 
           [0020]      FIG. 10A  is a top down view of an offset pivot guide roller. 
           [0021]      FIG. 10B  is a side view of an offset pivot guide roller. 
           [0022]      FIG. 11A  is a top down view of a remotely pivoted steering guide. 
           [0023]      FIG. 11B  is a side view of a remotely pivoted steering guide. 
           [0024]      FIG. 12  is a perspective view of a web edge sensor for detecting a web position in two dimensions. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0025]    Referring now to  FIG. 1 , a schematic illustration of one embodiment of a web edge sensor according to aspects of the present disclosure is shown. In the present embodiment, the web edge sensor is a fiber optic web edge sensor comprised of a laser light source  103 , a one-dimensional or two-dimensional array of optical fibers  108 , and a means for detecting the illumination of the optical fiber or fibers at the distal end of the optical fibers from the sensor end of the optical fibers. 
         [0026]    Solid materials, when illuminated by light, scatter incident light in all directions. In the present embodiment, a laser source  103  generates a laser light beam or laser curtain  104  that is incident on the web edge  105  of the web  101 . The laser curtain  104  can be inclined at substantially any angle to the plane of the web  101 . Although the present embodiments discussed herein contemplate a laser light source, the light source  103  could be any monochromatic collimated light source. 
         [0027]    The web edge  105  intercepts the incident laser curtain  104  and scatters light in all directions. Regardless of the opacity of the web  101 , scattering of light takes place. Scattered light from the web edge  104  may be collected by an optical fiber array  108 . The fiber optic array  102  may be positioned such that the ends of the optical fibers are essentially parallel to the plane of the web  101 . 
         [0028]    Referring now to  FIG. 2 , a schematic illustration of the cone of acceptance principle for an optical fiber is shown. Optical fibers are directionally sensitive. This means that light must strike the end of the optical fiber within a specific range of angles (e.g., less than a critical angle) in order for the fiber to be illuminated. The critical angle depends on the numerical aperture (NA) of the fiber, which is given by NA=sin(θ c ), where θ c  is the critical angle of incidence above which no light is transmitted. 
         [0029]    Based on the core diameter and NA of an optical fiber, there is an invisible cone  203  which extends from the core  201 . The volume of the cone increases as distance from the core increases. An exemplary ray of light  204  traveling within the cone  203  that is incident on the fiber is transmitted into the fiber. An exemplary ray of light  205  outside of the cone  203  will not be transmitted by the fiber. This property of increasing the volume of the cone can be effectively used to make sure only one or a few fibers are illuminated by the scattered light from the web edge into the fibers. By moving the array  108  closer to the scattering material, the cone  203  can be made smaller and restrict the potential amount of light rays that will be transmitted. Conversely, moving the array  108  farther from the scattering source will increase the number of rays of scattered light transmitted. 
         [0030]    In the present embodiment, the optical fibers are made of a core  201  and a cladding  202 . Light may be transmitted through the core  201  while the cladding  202  protects the core  201  and serves to aid in the internal reflection of the light within the core  201 . Light rays  204  with angles of incidence less than the critical angle suffer total internal reflection, and hence, are transmitted inside the core  201  of the fiber. Light rays  205  with angle of incidence greater than the critical angle are not internally reflected and are not transmitted. 
         [0031]    Referring now back to  FIG. 1 , in the embodiment illustrated, a single fiber  107  is illuminated by the scattered light whose angle of incidence is less than the critical angle. This single illuminated fiber  107  can be any one of the fibers in the linear or two-dimensional array. All fibers receive some scattered light, but based on the angle of incidence only one or a few fibers are illuminated. Light entering the optical fiber  107  illuminated by the reflected light from the web may be detected by a photodiode or other detection means (not shown) on an end of the optical fiber opposite the web  101 . Because the sensors of the present disclosure utilize discrete fiber optic strands and photodiodes to detect position, calibration may not be necessary. 
         [0032]    Referring now to  FIG. 3 , an end view of a linear array of optical fibers is shown. This linear array of fibers  300  may be used to detect the scattered light as shown in  FIG. 1 . The spacing  301  between the individual fibers in the linear array  300  determine resolution of the array  300 . As the spacing  301  is decreased, the resolution is increased. 
         [0033]    Referring now to  FIG. 4  an illustration of the cone of acceptance principle for an exemplary fiber optic array is shown. The distance of the fiber ends from the scattering source at least partially determines how may scattered rays are captured. If scattered light is generated at a distance close to the fiber ends  401 , some scattered light rays may fall between the cones of acceptance from neighboring fibers. At distances far from the fiber ends  403 , the cones of acceptance overlap so multiple fibers may be illuminated. There is an optimal distance  402  at which the cones of acceptance intersect so that only a single cone is illuminated. 
         [0034]    It will be appreciated that the cone of acceptance principle also applies to two dimensional arrays of optical fibers. Referring now to  FIG. 5 , an end view of a two-dimensional array  500  of optical fibers is shown. The spacing between the fibers in a two-dimensional array  501  can be adjusted in concert with the distance from the web to provide the necessary resolution. 
         [0035]    Referring now to  FIG. 6 , the operation of an embodiment of a web edge sensor  600  using a laser in the plane of the web is shown.  FIG. 6  also illustrates one possible arrangement of the photodiodes  601  with respect to the fiber optic array  108 . The inset  610  illustrates the relation of a single fiber  109  to a single photodiode  601 . It can be seen that a single photodiode  601  may correspond to each fiber in the array  108 . Each photodiode may detect light from only one fiber such that the position of the web  101  and edge  105  may be ascertained. Thus, the optical fibers in the array  108  may serve to filter the photodiodes from being able to receive any scattered light rays that are not within the critical angle or cone of acceptance. 
         [0036]    In one embodiment, the diodes  601  will connect to a common bus  602  to simplify wiring, but individually wiring each photodiode  601  is also possible. The bus  602  may interconnect with a control module  604  via interface  606 . The interface  606  may be an analog to digital (A/D) converter or other signal conditioning circuitry. The control module  604  may have an internal A/D converter and may also connect directly to the bus  602 . The control module  604  may be a digital controller and may be part of a larger system. The control module  604  may function alone or in combination with other componentry to direct and control web handling operations, as will be described in greater detail below. 
         [0037]    The fiber array  108  is spatially positioned such that the plane of fiber array is perpendicular to the plane of longitudinal motion of the web  101 , and is parallel to lateral motion of the web. In the present embodiment, the width of the beam  104  may only need to be as wide as the thickness of the web  101  since the beam  104  hit the edge  105  at substantially zero angle. The collimated light source  103  is oriented such that the fiber array receives maximum intensity of scattered light. When the web  101  is in sensing range, one (or a few fibers in a neighboring group) are illuminated. The fibers are illuminated sequentially as the web moves in either direction in the sensor range; the measurement is linear with actual web displacement. The sensors of the present disclosure are therefore able to directly measure displacement, rather than measuring by inference. 
         [0038]    By comparing the intensity of light received by each fiber, actual web edge position can be determined. The intensity of light received by each fiber may be measured using a photodiode detector  601  as described. The voltage developed by each photodiode may be proportional to the intensity of light received by the corresponding fiber. The photodiodes  601  may be scanned continuously to determine the photodiode with maximum voltage. The photodiode with the maximum voltage corresponds to a particular position of the web in the sensing window. In some cases, consecutive scans may be used to determine the displacement of the web over time. 
         [0039]    In one embodiment, the laser light can operate in the wavelength range of 100 to 1800 nm. However, the spectral range of the light source  103  is not limited as long as a photodiode of similar spectral range is used at the distal end of the fiber array. In some embodiments, the width of the collimated light source can be selected based on the location of the light source with respect to the plane of the web. 
         [0040]    Referring now to  FIGS. 7A-7C , the operation of another embodiment of a web edge sensor according to aspects of the present disclosure is shown. In  FIGS. 7A-7C , the light source is placed adjacent to the receiving fiber array as shown in  FIGS. 7A-C . The light source may be routed through fiber optics, thus allowing the light source and sensors to effectively operate in very close proximity. 
         [0041]      FIG. 7A  shows an end view of such a combination arrangement. Here, the top fiber array  108  is used for collecting the scattered light and may be connected on the distal end to photodiodes. The bottom fiber array  103  may be attached to a light or laser and used as the light source. 
         [0042]    It can be seen with reference to  FIG. 7B  that a beam of light emitted by the transmitting fibers  103  and incident on the web  101  scatters light in all directions, including the back scatter direction. The scattered light is picked by the receiving fiber array  108 . Hence, all the fibers which are directly below web surface will receive the scattered light, while the fibers which are not below the web surface will receive no scattered light. When the web is completely outside the sensing window, all the photodiodes will have the same voltage level. The photodiodes corresponding to the fibers which are directly below the web surface will have higher voltage levels, when compared to the rest of the photodiodes. 
         [0043]    With reference now to  FIG. 7C , it can be seen that as the web  101  displaces, the fibers in the sensing array  108  that are receiving light will change. The position of the web  101  can be determined by comparing the voltage levels of the photodiodes. In one embodiment, the transition from low voltage level to high voltage level on the photodiodes indicates the position of the web edge. If the web  101  is completely out of range of the sensing array  108 , all photodiodes will have a relatively low voltage level. If the web  101  completely covers the sensor array  108 , all the photo diodes will have a same voltage level, but the voltage level will be higher since all the photodiodes receive the scattered light. Hence, a clear distinction between different positions of the web is obtained using the configuration illustrated in  FIGS. 7A-7C . 
         [0044]    Referring now to  FIGS. 8-11 , exemplary web handling operations are shown. These are operations making use of the web edge sensors described herein. It will be appreciated that these are merely exemplary applications and the web edge sensors of the present disclosure are not limited to the enumerated exemplary applications. 
         [0045]    Lateral control (also called web guiding) involves controlling web fluctuations in the plane of the web and perpendicular to web travel. Web guides are used to maintain the lateral position of the web on rollers during transport. Control of web guides to maintain the lateral position of the web on the rollers prior to coating, printing, winding and other web processes is critical for making a web which is suitable for commercial applications. For example, large lateral movements of the web on rollers can cause slackness of the web, which can result in wrinkles. Hence, monitoring and tight control of the lateral position of the web is essential for manufacturing a quality commercial web material. 
         [0046]    Referring now to  FIG. 8 , a top down view of a center pivot guide roller is shown. Generally, a web guide mechanism may comprise a roller sitting on a pivoted base, whose motion is controlled to change the axis of rotation. A center pivot web guide  802  is rotated around a center pivot  803 . In this operation, the web  101  travels across the center pivot web guide  802  and is transported to the next operation. A web edge sensor  800  monitors the position of the web in order to provide feedback to the control system of the web guide  802 . A web approaching a guide roller will tend to orient itself perpendicular to the axis of rotation of the roller. 
         [0047]    Referring now to  FIG. 9 , a top down view of an end pivot guide roller is shown. An end pivot web guide  901  is used to alter the direction of travel of the web. The end pivot roller  901  is rotated around an end pivot  902 . As in other arrangements of the web guides, a web edge sensor  800  is used to monitor the position of the web in order to provide feedback to the control system of the web guide  901 . 
         [0048]    Further illustrations of the use of the web sensor are seen in  FIGS. 10 and 11 .  FIG. 10A  is a top down view of an offset pivot guide roller, while  FIG. 10B  is a side view of the offset pivot guide roller.  FIGS. 10A-10B  illustrate the use of an offset pivot guide roller  1005  to change the direction of a web. In the offset roller guide operation  1005 , the web  101  is first wound around a fixed incoming roller  1002  and transported to the offset roller guide  1005 . The web is then transported to a fixed exiting roller  1006 . The inlet span  1003  is the distance between the incoming roller  1002  and the offset roller guide  1005 , and the exit span  1007  is the distance between the offset roller guide  1005  and the exit roller  1006 . A web edge sensor  800  provides the necessary measurement of web edge position required for feedback to the controller of the offset pivot guide roller  305 . 
         [0049]    Referring now to  FIG. 11A , a top down view of a remotely pivoted steering guide is shown while  FIG. 11B  is a side view of the same remotely pivoted steering guide. Here the remotely pivoted guide roller  1101  pivots about a remote point  1102  to move the web  101  over the distance R between the pivoted guide roller  1101  and a stationary roller  1104 . Again, a web edge sensor  800  provides the necessary measurement of web edge position required for feedback to the controller 
         [0050]    Referring now to  FIG. 12 , a web edge sensor for detecting a web position in two dimensions is shown. It will be appreciated that any of the previously described web edge sensors may be operable to detect movement of an edge  105  and/or a web  101  across multiple directions or dimensions. For example, a web  101  may displace laterally but may also displace perpendicularly to the plane of motion. This may be referred to as web flutter and it may be important to monitor this effect along with the lateral displacements or perturbations. By aligning the sensor array  108  perpendicularly to the plane of motion (and possibly adjusting the light source), web flutter may be monitored. 
         [0051]    In the embodiment of  FIG. 12 , flutter and lateral displacement are monitored simultaneously. Possible lateral displacements may be said to occur along the direction D, while web flutter may be said to occur along the direction F. In the present embodiment, there is a sensor array  108  for monitoring each of these perturbations. There may also be one or more light sources (not shown) if they are not incorporated into the sensor array as described with respect to  FIGS. 7A-7C . In  FIG. 12 , two sets of sensor arrays  108  are utilized for monitoring the edge  105  along one side of the web  101 . However, it is understood that additional sensors could be placed along the opposite edge of the web  101  or along a different location on the edge  105 . 
         [0052]    It is to be understood that the invention is not limited in this application to the details of construction and to the arrangements of the components set forth in the present disclosure or illustrated in the drawings. The invention is capable of other embodiments and of being practiced and carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein are for the purpose of description and should not be regarded as limiting. As such, those skilled in the art will appreciate that the conception of the invention, upon which this disclosure is based, may readily be utilized as a basis for the designing of other structures, methods and systems for carrying out the several purposes of the present invention. Additional benefits and advantages of the present invention will be apparent in those skilled in the art to which the present invention relates. It is important, therefore, that the claims be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the present invention. 
         [0053]    Further, the purpose of the abstract contained herein is to enable the U.S. Patent and Trademark Office and the public generally, and especially the scientist, engineers and practitioners in the art who are not familiar with patent or legal terms or phraseology, to determine quickly from a cursory inspection the nature and essence of the technical disclosure of the application. The abstract is neither intended to define the invention of the application which is measured by the claims, nor is it intended to be limiting as to the scope of the invention in any way.