Abstract:
A method of controlling a moving web in relation to a selected transverse position comprising positioning a first positioning guide proximate a second positioning guide wherein the second positioning guide has a mechanism for positioning the web having minimal backlash. The web is passed through the first positioning guide and the second positioning guide. A sensor detects the transverse position of the moving web at the second positioning guide. The sensor transmits the transverse location of the web at the second positioning guide to a controller. The controller manipulates a zero-backlash actuator wherein the zero-backlash actuator is coupled to the second positioning guide such that the transverse position of the web is controllable to within a preselected dimension of the selected transverse position.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention generally relates to a method and an apparatus for controlling a moving web. More specifically, the present invention relates to a web guide apparatus having minimal mechanical backlash cooperating with a high speed control system which allows for precise control of a transverse location of the moving web. The present invention further includes a method of controlling the transverse location of the web.  
         [0002]     Generally, there are two types of guide systems for controlling a transverse position of a moving web. A first type of guide system for controlling a transverse position of a moving web is a passive system.  
         [0003]     An example of a passive system is a crowned roller, also called a convex roller, having a greater radius in the center than at the edges. Crowned rollers are effective at controlling webs that are relatively thick in relation to the width of the web such as sanding belts and conveyor belts.  
         [0004]     Another passive type of guide system is a tapered roller with a flange. The taper on the roller directs the web towards the flange. The web edge contacts the flange and thereby controls the transverse position of the web. A tapered roller with a flange is commonly used to control the lateral position of a narrow web, such as a videotape.  
         [0005]     However, a passive guide system cannot guide wide, thin webs because, depending on the type of passive guide system, either the edge of the web tends to buckle or the web tends to develop wrinkles. To effectively control a wide, thin web an active guide system is required.  
         [0006]     A typical active guide system includes a sensing device for locating the position of the web, a mechanical positioning device, a control system for determining an error from a desired transverse location and an actuator that receives a signal from the control system and manipulates the mechanical positioning device. A typical control system used for actively guiding a thin, wide web is a closed loop feedback control system.  
         [0007]     Typically, a web to be processed has been previously wound onto a spool. During the winding process, the web is not perfectly wound and typically has transverse positioning errors in the form of a zigzag or a weave. When the web is unwound, the zigzag or weave errors recur causing transverse web positioning problems.  
         [0008]     In precision web applications such as webs used in optics and electronics, the transverse location of the web must be precisely controlled. Most commercially available active web guide systems are not capable of controlling the transverse location to the level of precision required for these web applications. Commercial web guides typically employ rod ends, belts, sheaves, slides and threaded nuts and bolts, each of which has some mechanical play. Often, in a commercially available guide, the total mechanical play is in range of 125-375 microns (0.005-0.015 inches). A control system cannot guide a web to within a range of the guide&#39;s backlash or mechanical play.  
         [0009]     While the control system of a commercially available web guide has some error, often the error caused by the control system is insignificant when compared to the error caused by the mechanical backlash or play in the guide. The mechanical backlash, without accounting for any other error can preclude many commercially available web guides from being used for precisely locating a transverse location of a moving web.  
       BRIEF SUMMARY OF THE INVENTION  
       [0010]     The present invention includes a method of controlling a moving web in relation to a selected transverse position comprising positioning a first positioning guide proximate a second positioning guide wherein the second positioning guide includes a mechanism for positioning the web having minimal backlash. The web is passed through the first positioning guide and the second positioning guide. A sensor detects the transverse position of the moving web at the second positioning guide. The sensor transmits the transverse location of the web at the second positioning guide to a controller. The controller manipulates a zero-backlash actuator where the zero-backlash actuator is coupled to the second positioning guide such that the transverse position of the web is controllable to within a preselected dimension of the selected transverse position. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  is a schematic view of the precision web guide assembly of the present invention.  
         [0012]      FIG. 2  is a perspective view of a precision web guide of the present invention.  
         [0013]      FIG. 3  is an additional perspective view of the precision web guide of the present invention.  
         [0014]      FIG. 4  is an additional perspective view of the precision web guide of the present invention.  
         [0015]      FIG. 5  is an additional perspective view of the precision web guide of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0016]     The present invention generally relates to an assembly for controlling a transverse location of a moving web. The assembly includes a first web guide in series with a second web guide. The first web guide is manipulated by a first control system and the second web guide is manipulated by a second control system. The first and second control systems control the first and second web guides independent of each other to provide precision control of the transverse position of the moving web.  
         [0017]     The assembly provides precise control of the transverse position of the moving web because of a number of design features including, but not limited to, positioning the first web guide, having a short exit span, and upstream and proximate the second web guide. The first web guide reduces the input angle error, the transverse position error, and the error rate of the moving web entering the second web guide.  
         [0018]     With the input angle error, the transverse position error, and the error rate reduced by the first web guide, the second web guide precisely controls the transverse position of the moving web. The second web guide is designed to be lightweight and stiff while minimizing backlash caused by mechanical play. The lightweight, stiff second web guide with minimal backlash allows the second control system, having a fast, high resolution sensor communicating with a fast control system, to precisely control the transverse location of the moving web with a high bandwidth, zero backlash actuator connected to the second web guide with a zero backlash connection.  
         [0019]     The second web guide also includes a relatively long guide span and a relatively short exit span. The long guide span reduces an angle needed to produce a correction to the transverse position of the moving web and reduces a twist angle of the moving web in the entrance and exit spans. The short exit span reduces the transverse position error caused by the input angle error.  
         [0020]     As used herein, the terms “precision control” or “precise control” means controlling a transverse position of the web to within less than about 0.004 inches (0.0102 mm) of a desired location.  
         [0021]     As used herein, the term “backlash” corresponds to the amount of mechanical play or lost motion found in the web guide. Backlash adversely affects the ability of a control system to precisely control the transverse position of the moving web.  
         [0022]     As used herein, the term “zero-backlash” means tolerances or mechanical play of less than about 0.0001 inch (0.0025 mm).  
         [0023]     As used herein, the term “exit span” means the distance between the last frame roller and the second base roller of the web guide that is preferably expressed in terms of a factor of a width of the web.  
         [0024]     As used herein, the term “entrance span” means the distance between the first base roller and the first frame roller of the web guide that is preferably expressed in terms of a factor of a width of the web.  
         [0025]     As used herein, the term “guide span” means the distance between the entrance span and the exit span. The guide span is preferably expressed in terms of a factor of a width of the web.  
         [0026]     As used herein, the term “input angle error” is the error in the angular position of the web from the desired angle of the web as the web is detected by the sensor. Typically, the input angle error of the moving web is undetectable by a single web position sensor. Since a web position sensor detects the position of the web at only one point, the sensor detects the position of the web, but not the input angle of the web. Therefore, a single sensor may detect no positional error while there may be a significant amount of input angle error that is undetected. The input angle error, although undetected by a single position sensor, may result in a significant downstream position error.  
         [0027]     The present invention generally includes an assembly  10  and method for precisely controlling a transverse position of a moving web  12  as illustrated in  FIG. 1 . The moving web  12  is passed through a first web guide  14  followed by a second web guide  16 . While an exact distance between the first web guide  14  and second web guide  16  is not critical to practice the invention, it is preferred that first web guide  14  and second web guide  16  be disposed in close proximity with minimal or no intermediate processing of the web  12 . In an exemplary embodiment, an idler roller  18  is disposed within the path of the moving web  12  between the first web guide  14  and the second web guide  16 .  
         [0028]     The first web guide  14  can include any conventional commercially available web guide. It is preferred that an exit span  20  between the last roller  21  and the second to the last roller  19  of the first web guide  14  be relatively short compared to an exit span of a conventional web guide. A short exit span  20  on the first web guide  14  significantly reduces the transverse angular error of the moving web  12 , reduces the input angle error, and minimizes output error. The exit span  20  of the first web guide  14  is preferably less than about one-half of the width of the moving web  12 . Upon reading this specification, one skilled in the art will appreciate that the shortest exit span possible is preferred that does not result in the wrinkling of the moving web  12 . An exemplary commercially available web guide that can be used as the first web guide is a DF Rotating Frame Guide “P-Model” manufactured by BST Pro Mark of Elmhurst, Ill.  
         [0029]     Preferably, the first web guide  14  includes a first control system  22  that independently controls the first web guide  14 . The first control system  22  is preferably a closed loop feed back system, although a feed forward system, H infinity system, model based system, embedded model based system or any other control system which will effectively control the transverse position of the moving web  12  is also within the scope of the invention.  
         [0030]     The first control system  22  includes a first web position sensor  24  that preferably detects a position of an edge of the moving web  12 . One skilled in the art will recognize that other position detecting sensors besides edge position sensors are within the scope of the invention. The first web position sensor  24  communicates with a first controller  26 . The first controller  26  detects the error of the transverse position of the edge of the moving web  12  from a selected setpoint. The first controller  26  preferably employs a proportional-integral controller (PI) control scheme.  
         [0031]     The first controller  26  communicates the error to an actuator  28 . The actuator  28  adjusts the position of the first web guide  14  depending on the magnitude of error calculated by the first controller  26 .  
         [0032]     Referring to  FIG. 1 , after the moving web  12  exits the first web guide  14 , the moving web  12  preferably passes over the idler roller  18  prior to entering into the second web guide  16 . After passing through the first web guide  14 , the input error rate, the input angle error and the output transverse error of the moving web  12  have been significantly reduced as the moving web  12  enters the second web guide  16 . The second web guide  16 , as illustrated in  FIGS. 2-5 , is also referred to as a precision web guide. The precision web guide  16  manipulates the transverse position of the moving web  12  to within less than about 0.004 inches (0.102 mm) of a desired transverse location.  
         [0033]     The moving web  12  passes over a first base roller  32  disposed within a base  30  of the precision web guide  16 . The base  30  is fixed in a selected position, preferably with a plurality of bolts, however the base may be fixed into the selected position by a weld, a plurality of rivets or any other fastening means which fixedly retains the base in the selected position.  
         [0034]     The base  30  also includes a second base roller  34  disposed therein. Preferably, an axis  35  of the first base roller  32  is substantially parallel to an axis  37  of the second base roller  34 . Both the first and second base rollers  32 ,  34 , respectively, include laterally loaded or precision bearings. The laterally loaded or precision bearings are preferred to minimize or eliminate lateral backlash within the first and second base rollers  32 ,  34  respectively. An exemplary laterally loaded bearing can be purchased along with an Ultralight Aluminum Idler manufactured by Webex, Inc. of Neenah, Wis.  
         [0035]     After passing over the first base roller  32 , the moving web  12  contacts and passes over a first frame roller  38  that is disposed within a frame  36 . The frame  36  is connected to the base  30  but is also movable with respect to the base  30 . Preferably, the frame  36  is connected to the base  30  with a plurality of flexure plates  40 ,  42 ,  44 ,  46  as viewed in  FIGS. 1-5 . The plurality of flexure plates  40 ,  42 ,  44 ,  46  allows the frame  36  to move relative to the base  30  without any mechanical backlash or mechanical play. Although a plurality of flexure plates  40 ,  42 ,  44 ,  46  is preferred, one skilled in the art will recognize that other connecting mechanisms which allow the frame to move relative to the base with minimal or no mechanical backlash are within the scope of the invention. The alternative connecting mechanisms include, but are not limited to, linear ways, a precision pivot, and preloaded mechanical components.  
         [0036]     Referring to  FIGS. 2-5 , a length of each flexure plate  40 ,  42 ,  44 ,  46  is significantly longer when compared to a width of each flexure plate  40 ,  42 ,  44 ,  46 . The flexure plates  40 ,  42 ,  44 ,  46  are designed to flex along the width of the flexure plate while maintaining stiffness along the length of the plate. In the exemplary embodiment, the frame is connected to the base with four flexure plates  40 ,  42 ,  44 ,  46 .  
         [0037]     The four flexure plates  40 ,  42 ,  44 ,  46  connect the frame  36  to the base  30  such that the frame  36  rotates about a point  48  proximate the first frame roller  38 . Referring to  FIGS. 2 and 3 , an optional pivot pin  49  is disposed between the frame  36  and the base  30  where the pivot pin  49  is fixed to the frame  36  but rotatable with respect to the base  30 . The pivot pin  49  is disposed within a bracket  51  attached to the base  30  to retain the pivot pin  49  in the selected position while allowing the pivot pin  49  to rotate therein.  
         [0038]     Referring to  FIGS. 2-5 , the first and second flexure plates  40 ,  46 , respectively, attach the frame  36  to the base  30  proximate ends  39  of the first frame roller  38 . The first and second flexure plates  40 ,  46  are positioned such that the lengths of the flexure plates  40 ,  46  are substantially parallel to an axis of the first frame roller  38 .  
         [0039]     The third and fourth flexure plates  42 ,  44  connect the frame  36  to the base  30  between the first frame roller  38  and a second frame roller  50 . The third and fourth flexure plates  42 ,  44 , respectively are positioned at angles which are mirror images of each other as referenced from a plane perpendicularly intersecting a midpoint of the first frame roller  38 . While the first and second flexure plates  40 ,  46 , respectively, allow the frame  36  to move forward and backward relative to the path of the moving web  12 ; the third and fourth flexure plates  42 ,  44 , respectively, allow the frame  36  to twist or rotate relative to the path of the moving web  12 . The four flexure plates  40 ,  42 ,  44 ,  46  working in cooperation allow the frame  36  to pivot about the point  48  proximate the first frame roller  38 . An exemplary pivot point  48  is about at the midpoint of an entrance tangent line of the moving web  12  with the first frame roller  38 . In the context of this disclosure, what is meant by the entrance tangent line is the line defined by the first contact of the moving web with a roller.  
         [0040]     After passing over the first frame roller  38 , the moving web  12  passes over the second frame roller  50 . The first and second frame rollers  38 ,  50 , respectively, are also equipped with laterally loaded or precision bearings to minimize the amount of lateral backlash within the first and second frame rollers  38 ,  50 . An exemplary laterally loaded bearing can be purchased along with an Ultralight Aluminum Idler manufactured by Webex, Inc. of Neenah, Wis.  
         [0041]     One skilled in the art will recognize that one large roller may be substituted for the first and second frame rollers  38 ,  50 , respectively. Additionally, one skilled in the art will recognize that the moving web  12  may pass over more than two rollers within the frame  36  while precisely controlling the transverse location of the moving web  12 .  
         [0042]     An axis  51  of the second frame roller  50  is approximately parallel to an axis  41  of the first frame roller  38 . A distance from the first frame roller  38  to the second frame roller  50  defines a guide span  53  as best illustrated in  FIG. 1 . The guide span  53  is relatively long as compared to the width of the moving web  12 .  
         [0043]     One skilled in the art will recognize that a longer guide span reduces the amount of movement required by the flexure plates  40 ,  42 ,  44 ,  46  to produce a desired transverse position correction. The ability to control the transverse position of the moving web  12  with a minimal amount of movement allows for a more accurate web guide control because twist angles in an entrance span  55  and an exit span  57  are minimized.  
         [0044]     Additionally, minimizing the amount of movement while accurately controlling a transverse position of the moving web  12  allows use of the flexure plates  40 ,  42 ,  44 ,  46  that have no mechanical backlash, but also have a limited range of motion. If significant motion were required, the movement may exceed the flexibility of the flexure plates  40 ,  42 ,  44 ,  46 , thereby precluding the use of flexure plates in the present invention.  
         [0045]     After passing over the last frame roller  50 , the moving web  12  passes over the second base roller  34 . In an exemplary embodiment, the path of the moving web  12  in the entrance and exit spans  55 ,  57 , respectively is substantially perpendicular to a plane of rotation of the frame  36 . Applying the principles taught herein, one skilled in the art will appreciate that other web paths are within the scope of the invention, including but not limited to, the first base roller  32  being disposed above the first frame roller  38  and also at an angle not substantially perpendicular to the first frame roller  38 . Similarly, the second base roller  34  may be disposed in a position such that the path of the moving web  12  is not substantially perpendicular to the plane of rotation of the frame  36 .  
         [0046]     Referring to  FIG. 1 , a second control system  52  controls the precision web guide  16 . The second control system  52  is preferably a closed loop feed back system. However, a feed forward system, H infinity system, model based system, embedded model based system or any other control system which will effectively control the transverse position of the moving web  12  is also within the scope of the invention.  
         [0047]     The second control system  52  includes a second web position sensor  54  that detects a position of the edge of the moving web  12 . One skilled in the art will recognize that other position detecting sensors besides edge position sensors are within the scope of the invention. The second positioning sensor  54  preferably includes a fast, high-resolution means of sensing a transverse position of the moving web  12  at an edge of the moving web  12  such as, at a minimum, a fifty-hertz sensor with at least twelve-micron resolution. A preferred second sensor  54  is a high speed, high precision digital micrometer Model No. LS-7030M manufactured by Keyence Corporation of America of Woodcliff Lake, N.J.  
         [0048]     The second positioning sensor  54  preferable detects the transverse position of the moving web  12  at about or proximately below an exit tangent line  60  of the moving web  12  exiting the second frame roller  50 . In the context of this disclosure, what is meant by the exit tangent line is the line defined by the last contact of the moving web with a roller. By sensing the transverse position at about or proximately below the exit tangent line  60  of the second frame roller  50 , a transportation lag is minimized. What is meant by transportation lag is the transportation time from the last shifting roller, in this case the second frame roller  50 , to the second positioning sensor  54 .  
         [0049]     However, the transverse position of the moving web  12  can be measured at numerous other locations including lower on the exit span or at about an exit tangent line of the moving web  12  exiting the second base roller  34 . At these alternative transverse position sensing locations, the transportation lag will need to be accounted for in the control system.  
         [0050]     The detected transverse position of the moving web  12  by the second web position sensor  54  is transmitted to a second controller  56 . The second controller  56  compares the transverse position of the moving web  12  to a desired position or setpoint and calculates an error of the detected position from the desired position. The second controller  56  is typically a programmable logic controller using a proportional-integral (PI) controller with an update rate of at least about one millisecond. An exemplary controller is a TwinCAT PLC manufactured by Beckhoff Industrie Elektronik of Verl, Germany.  
         [0051]     The second controller  56  communicates the error to a second actuator  58 . The second actuator  58  is mounted to the base  30  or another stationary structure. Referring to  FIGS. 2-5 , the second actuator  58  is coupled to an extension  60  of the frame  36  that extends beyond the second frame roller  50  with a flexible bracket  62 . The flexible bracket  62  is preferred to provide a zero backlash coupling of the actuator  58  to the frame  36 . Further, the flexible bracket  62  allows the actuator  58  traveling in a linear motion to be coupled to the frame  36  that is traveling in an arcuate motion.  
         [0052]     The plurality of flexure plates  40 ,  42 ,  44 ,  46  are designed to allow the frame  36  to rotate in a plane about the point  48  proximate the first frame roller  38  at about a midpoint of the entrance tangent line. As the frame  36  pivots about the point  48 , an end  64  opposite the pivot point  48  moves in an arc. The flexible bracket  62  provides flexibility to allow the linear actuator  58  to cooperate with the frame  36  moving in an arcuate path.  
         [0053]     The second actuator  58  has zero-backlash allowing for precise movement without mechanical play. The second actuator  58  is capable of control frequencies in excess of five hertz. An exemplary actuator is Model No. SR31-0605-XFM-XX1-238-PF-19413 manufactured by EXLAR (www.exlar.com). One skilled in the art will recognize that a direct linear or rotary motor may be used to practice the invention in place of the zero-backlash actuator.  
         [0054]     The second actuator  58  does not require a significant amount of travel because the transverse position error is significantly reduced by the first web guide  14  and the first control system  22 . Referring to  FIGS. 4 and 5 , a member  66  extending from the frame  36  towards the base  30  cooperates with first and second limit switches,  68 ,  70 , respectively. If the member  66  contacts either of the limit switches  68 ,  70 , the moving web  12  is stopped so that the web  12  can be manually realigned within the assembly  10 .  
         [0055]     The frame  36  is designed to have excess material removed to decrease the mass of the frame  36  while maintaining the required stiffness. Removing the excess material results in the frame  36  having a high natural frequency. Further, the decrease in mass of the frame  36  allows for a high system gain on the precision guide  16 . The precision guide  16  of the present invention has a gain of greater than about thirty-three inverse seconds and a crossover frequency of greater than about five hertz.  
         [0056]     Although the present invention has been described with reference to preferred embodiments, one having ordinary skill in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.