Abstract:
A method and apparatus for use with a web of material having a web length dimension and a web surface, the method for placing mark sequences on the web surface every X distance along the web length dimension identifying location along the web length dimension, the method comprising the steps of monitoring web location, every X distance, placing a sequence of N marks on the web surface along the web length wherein each two adjacent marks define a space length dimension and wherein the pattern of space length dimensions formed by the N marks in the sequence together specify a specific web length location. The invention also includes a marking and defect locating system including a high speed printer and a high resolution, high speed camera to facilitate the methods.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   Not applicable. 
   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   Not applicable. 
   BACKGROUND OF THE INVENTION 
   The field of the invention is web inspection systems and more specifically a system for encoding web (continuous) material with position information that can later be used to rapidly and precisely identify locations of interest on the web and performs some action based on those locations. 
   Hereinafter, while the present invention is applicable to various types of web materials and systems used to manufacture, process and repair such materials, unless indicated otherwise, the invention will be described in the context of paper manufacturing and systems used to locate and repair defects in large rolls of paper. 
   To meet the every increasing demand for paper products, the paper industry is constantly searching for ways to reduce costs and increase efficiencies of paper production. As in other industries, one way to reduce costs appreciably in the paper industry is to adopt mass production procedures. A paper manufacturing machine can rapidly produce extremely long (e.g., tens of miles) continuous ribbons of paper or webs. To store and handle these massive paper webs, a spindle (dowel) is mounted on a wind-up device, the leading end of the web is attached to the spindle and the spindle is wound at a high rate to take up slack in the web as the sheet emerges from an outlet end of the paper manufacturing machine. A paper web and associated spindle are referred to hereinafter as a reel. 
   After a complete reel (i.e., a full spindle) has been wound, the following end of the web is cut, the reel is removed from the wind-up device, a second spindle is mounted to the wind-up device and the process is repeated. Full reels are taken for further processing, by the paper manufacturer, or by end users (e.g., large printing companies, newspaper publishers, etc.). For this further processing, the reels are mounted on an unwind device that feeds a finishing machine, including paper coaters, patch and splice un-reelers, slitters, sheeters, folders, printing presses, etc. 
   Great effort has been exerted to minimize the number of manufacturing defects in paper webs like those described above. Nevertheless, a reel typically includes at least a small number of paper defects (e.g., 5-30 per roll) including holes, foreign objects, discolorations, edge tears, cracks, etc. At first blush, the small number of defects per reel may not seem troublesome, however, upon a more detailed perusal of paper processing and usage, it becomes apparent that even a small number of defects cannot be tolerated in many applications. 
   Perhaps the most important reason defects cannot be tolerated is that defects can cause finishing machines to malfunction. In this regard, as well known in the industry, even a small defect can, when subjected to the strains associated with high speed unwinding and other finishing activities, lead to an enlargement of the defect, the production of other defects, and even complete sheet breaks (a break in the web). When a sheet breaks, a large amount of sheet material is typically damaged as a large spinning unwind device typically takes a long time (e.g., minutes) to stop rotating and web material shoots off the reel during the slowing process. In addition, such breaks usually result in finishing machine downtime as the unwind device has to be stopped, scrap material has to be removed and discarded, the finishing machine line must be re-threaded and the finishing process has to be restarted. 
   One common method for dealing with paper defects is to identify and repair defects before they might impact further processing stages or compromise end-user quality standards. For example, an exemplary inspection system may include an inspection camera or cameras positioned between the output end of a paper manufacturing machine its wind-up device. The camera produces images of the complete area of paper manufactured and provides those images to an inspection processor. The inspection processor is programmed to identify defects in the received images. When a defect is identified, the inspection processor stores an indication of the defect type correlated with defect location along the web (hereinafter a “defect/location pairing”) in a database. 
   After a reel is full, the reel is removed from the wind-up device and mounted on the unwind device of another machine, a repair machine, with the free end (i.e., the following end) of the web linked to another wind-up device. The unwind and wind-up devices of the repair machine and the are controlled to unwind and rewind the paper web. The paper web can be stopped periodically to expose a defect or defect region for repair (e.g., a defect may be spliced out and a patch melded in its place). After a defect is repaired, the unwind and wind-up devices are again advanced until the next defect or region is exposed and can be repaired. After all defects are repaired, the remainder of the web is wound on the machine&#39;s wind-up spindle to form a complete reel and then the reel is removed and sent for further processing. 
   While inspection and repair systems like the one described above can yield reels free of critical defect, several difficulties have to be overcome in order to run such a system efficiently. In particular, to avoid a manufacturing bottleneck effect at a facility, the process of advancing between defects has to be rapid and stopping at defects must be precise. Overshooting or undershooting by a even a small amount can hide a defect and cause consternation as a system operator is forced to manually drive the system forward and backward to locate the defect. Advancing slowly between defects increases the time required for repair appreciably. 
   High speeds are not a problem as large motor systems are capable of rapidly accelerating even full reels to speeds of 8,000 feet per minute or more. Precisely stopping to expose a defect for repair, however, has proven to be more difficult. When a large paper reel is rotated at high speed, rotating momentum has to be overcome to stop the reel and the time required to stop the reel may be several minutes and may require several thousand reel rotations. Also, the paper must be accelerated and decelerated slowly (gently) to avoid ripping or deformation. Thus, the location of a defect has to be known well in advance of the defect being exposed in order to stop a rotating reel precisely for repair (e.g., so that the defect is exposed). 
   One solution for providing advanced warning that a defect is coming is to apply marks on the paper web that, upon unwinding, appear prior to the defect and that can be sensed to indicate that deceleration should commence. One particularly useful type of marking machine is positioned between a paper manufacturing machine&#39;s output and its wind-up device (e.g., proximate the inspection camera or cameras). The marking machine is linked to the inspection processor and provides “absolute position” mark sequences on the surface of a web and proximate a web edge as the web passes by. As the label implies, each absolute position mark sequence identifies a specific location along the length of the web. A typical mark sequence may include a series of marks that together form a code that can be readable at high speeds. 
   A processor at the repair machine, the repair processor, is programmed with an algorithm for de-coding the mark sequences to identify absolute web locations and is linked to access the defect/location pairings (i.e., the pairings stored by the inspection processor). The repair processor is also programmed with a stopping algorithm that can determine the typical slowing and stopping requirements of the machine, based on the current condition of the rotating reel (e.g. speed and acceleration). Specifically, the stopping algorithm attempts to predict the distance of web that would pass (be unwound) from the time a normal (non-emergency) slowing and stopping process commences until the web on the repair machine comes to a complete stop. For instance, the algorithm might calculate that slowing and stopping from a constant speed of 6,000 feet per minute should result in 7,000 feet of web being transported. In addition, the repair processor can commence processes to slow and stop the repair machine. 
   In addition to the components described above, the repair system also includes a camera or cameras linked to the repair processor and positioned between the unwind and wind-up devices of the repair machine to examine the mark sequences as the web is unwound for repair. The camera(s) provides images of the web near the edges to the repair processor which in turn identifies the mark sequences, decodes the sequences and thereby identifies location along the web length. 
   Having access to the defect/location pairings, being able to determine current web location and being programmed with the stopping algorithm, the repair processor should be able to precisely and efficiently stop the unwinding process to expose defects for repair. For example, assuming 7,000 feet are required to stop a high speed reel, the processor can be programmed to commence a stopping process when a mark 7,000 feet before a defect/location pairing is identified. 
   Unfortunately, while the system described above works well in theory, in reality there are several shortcomings. First, because of perceived hardware constraints, the industry has generally used relatively long marks to form mark sequences. For instance, an exemplary shortest possible mark in many cases is one or more yards long and may be ½ inch or more wide. While readily identifiable as a mark by many different types of camera systems, one or more yard long and relatively wide marks require excessive amounts of ink to produce and hence are costly. In addition, wide and long marks take up a relatively large amount of paper surface area and hence reduce the amount of paper useable by end users. 
   Second, marking hardware has some shortcomings that limit the characteristics of marks that can be uniquely distinguished with an acceptable degree of certainty. As known in the industry, mark making machines typically apply marks by spraying ink onto a surface of the paper web. While the beginning of a mark application is relatively precisely controllable (e.g., the turn on instant of a mark is controllable), the process of stopping mark application or turning off the mark sprayer is not very precise and, as a result, the ends of marks often “dribble” on past the points intended. This dribbling results in an unintended tailing effect. Hereinafter the unintended section of a mark will be referred to as a mark tail. 
   Because of the tailing effect, the differential between lengths of marks intended to indicate different information must be greater than the longest expected mark tail and the spaces between marks must also be greater than the longest expected mark tail. Thus, for instance, where an expected longest dribble or mark tail is 0.90 yards, the differential between marks intended to have different lengths that can be sensed has to be greater than 0.90 yards. Here, where a minimum mark length is 1 yard and numbers from 0 to 9 are to be represented by different length marks, the longest mark will have to be approximately 10 yards. Where value 9 is represented by a 10 yard mark, the value 999 may be encoded using a first ten yard mark (e.g., the first 9), a one yard space, a second ten yard mark (e.g., the second 9), a one yard space and a third ten yard mark (e.g., the third 9) so that the total length of the exemplary mark sequence associated with value 999 requires 32 yards. 
   Third, because of the length required to generate even a simple uniquely distinguishable mark sequence and the ink costs associated with such marks, most marking systems are limited to a small number of different mark sequences (e.g., 1000) and the space between marks equi-spaced along a multi-kilometer long web may be 1000 or more yards. Here, locations between mark sequences (i.e., intra-mark locations) have to be determined in some other fashion. 
   One way to identify intra-mark locations has been to provide an encoder on a roller proximate the repair machine unwind device that generates signals indicating roller velocity usable to determine web length travel from the most recent mark sequence. The web length travel is added to the location associated with the most recent mark sequence to determine instantaneous intra-mark location. Unfortunately, an encoder may not accurately measure the actual web movement, for a variety of reasons. For example, the web material may slip relative to the encoder&#39;s roll, the encoder may not be properly calibrated, and/or the material may shrink or stretch. Over a distance of 1000 yards or more, small errors may accumulate, resulting in a positional inaccuracy of several yards, yards causing the repair controller to stop the repair machine prior to or subsequent to desired defect exposure. 
   Fourth, when a reel of paper is first mounted on a repair machine, the material location is unknown until the first mark sequence is read. It is difficult to estimate the material location for numerous reasons: operators cut wraps of paper off the reel (e.g. to take samples for testing, to clean up the tail of the reel, or to prepare for threading), an unpredictable amount of material is transported during threading, etc. Thus, in cases where there is a great distance between mark sequences, it can potentially take a significant movement of web material before the first mark sequence is detected and decoded. This in turn can cause inaccuracies and failures when attempting to stop at a defect in the first portion of a reel that is being unwound. 
   Fifth, there are instances where a web breaks during the unwinding repair process and has to be repaired. Often to repair a web break, some paper material has to be discarded and hence the distance along a repaired web from the most recent location mark sequence is imprecise. To re-synchronize, the repair processor has to wait until the next location mark sequence is encountered and successfully read. In this case, the locations of defects prior to the next mark sequence (e.g., prior to re-synchronization), if any, cannot be precisely determined from the mark sequences and, in some cases, may not be repaired. 
   Sixth, despite efforts to apply clear and precise marks, sometimes marks are misapplied or misread. For instance, assume a sequencing system where a one yard mark corresponds to a zero, a two yard mark corresponds to a one and so on up to a ten yard mark corresponding to a 9. If the mark sprayer machine fails to apply ink (e.g., sputters) during a central section of a ten yard mark corresponding to a 9 value, the resulting two marks may be read as a 3 and a 4 (e.g., where the first mark is four yards long, the second mark is five yards long and the sputter accounts for a one yard space). Many other errors due to a sputtering mark sprayer machine are contemplated. Where errors of this type occur, in some cases, the repair processor may not be programmed to recognize the errors and incorrect control may result. In the alternative, some processors have been programmed to recognize mark sequences that are not supported by the system and to disregard the data related thereto. Where mark sequences are disregarded, the processor simply waits for the next mark sequence to relocate and, in the interim, may use encoder information to roughly determine location for control purposes. Here until a valid mark sequence is again read, imprecision is compounded. 
   Seventh, upon commencing reel advancement from one defect to another, prior known systems have tied commencement of the stopping process to velocity and distance (e.g., web length) to next defect. To this end, prior systems have recognized that, just as a reel spinning at a top speed or velocity requires a specific web length of unwinding to stop, intermediate speeds also require specific web lengths of unwinding to stop. For instance, if a reel spinning at a top speed of 6,000 ft./min. requires 7,000 feet of unwinding to stop, a reel spinning at 5,000 ft./min. may require 5,500 feet of unwinding to stop, reels spinning at 4,000 ft./min. may require 4,700 feet of unwinding to stop and so on. In some systems these velocity-required web length pairings are stored for use during unwinding. Here, upon commencing advancing movement toward a next defect, the top speed command signal is used to drive the reels and mark sequences and encoder data are used to identify current web length location and the web length to the next defect. In addition reel velocity is tracked and, when a web length paired with a stored reel velocity is equal to or less than the web length to next defect, the stopping process is started. For instance, consistent with the example above, where the reels are spinning at 4,000 ft./min and the web length to next defect is 4,700 feet the stopping process is commenced. 
   While systems like the one described above work well in theory, in reality such systems have important shortcomings. In particular, the assumption that there is a one to one pairing between reel velocities and web lengths required to stop is not completely accurate. Here, it should be sufficient to note that stopping length may depend on current acceleration as well as velocity. Thus, for instance, a reel rotating at a steady state 5,000 feet per minute may require less stopping length than a similar reel at 5,000 feet per minute that is accelerating at a high rate. Similarly, a full reel will typically require more stopping length than a half reel. 
   One way to deal with the shortcomings described above has been to adopt stopping algorithms that include some leeway for errors (i.e., a “fudge factor”) and that drives the reels at two different velocities. For instance, in the example above, where a high speed reel (e.g., 6,000 ft./min.) requires 7,000 feet of unwinding to stop, the stopping algorithm may be set to commence stopping at 7,500 feet. After a reduced velocity (e.g., at 500 ft./min.) is achieved (e.g., at 800 feet prior to the defect) the algorithm may maintain the reduced velocity until the defect is exposed. 
   Here, because of the location uncertainties described above, prior systems have required a relatively large fudge factor and hence a relatively long time to decelerate and stop proximate a defect. In addition, despite providing an encoder, these systems have been known to miss their target (i.e., the defect) by several yards thereby requiring a system operator to manually hunt for defects and thus further increasing the overall time required to find each defect. 
   While each individual process of locating and exposing a defect may not appear too burdensome, when a reel includes many (e.g., 30) defects, the cumulative additional time required to expose the defects is appreciable. Moreover, the enhanced stopping algorithms do not address the re-synchronization problems associated with web breaks described above. 
   BRIEF SUMMARY OF THE INVENTION 
   It has been recognized that a new type of mark sequence can be used to provide absolute web length location sequences at relatively close web length locations. For instance, instead of providing a sequence every 1,000 yards, now absolute web length location sequences can be provided every 10 yards. Many advantages are derived from having relatively close mark sequences. For instance, stopping on defects (i.e., to expose defects) is much more accurate as the inaccuracies associated with web elasticity are negligible along web lengths on the order of tens of yards. As another instance, the time to start a stopping process can be identified with much more certainty and hence the fudge factor can be reduced appreciably. As one other instance, the web length require to re-synchronize web location is reduced to the length between consecutive mark sequences (e.g., 10 yards or less) which, in almost all cases, will mean that resynchronization can be achieved prior to the next defect location. Even if a next defect occurs prior to a next mark sequence, the reverse hunting process to expose the defect is simple and would require only a short amount of time. 
   According to at least some embodiments of the invention, instead of relying on mark lengths in sequences to encode absolute position information, spaces formed by marks are employed to encode position information. More specifically, it has been recognized that the inaccuracies associated with mark tails can be completely avoided by using the spaces defined by the starting points of consecutive marks to encode data. Here, because starting points of marks can be precisely controlled (as compared to the ends of marks which tend to tail off as described above), spaces defined by the points can likewise be precise. 
   In addition, it has been recognized that an inkjet printer (e.g., continuous, impulse, drop-on-demand and variations such as wax-based impulse, gravity-feed, etc.) or laser marking printer (i.e., burning) can be employed to make relatively short and thin marks that are individually distinguishable via a suitably configured high speed and high resolution camera. Here, for instance, each mark may be on the order of one inch long and spaces between marks may be on the order of one to six inches long and marks may be on the order of 1 mm wide. 
   Moreover, it has been recognized that the number of independently markable web length locations can be increased appreciably by coding with spaces that indicate binary information. For instance, in some embodiments, each space defined by consecutive mark starting points may be one of four distinct lengths, correspondingly representing four values (0, 1, 2, or 3) Hence, each space corresponds to two bits of data (00, 01, 10, or 11). Thus, where consecutive mark starting points define twelve separate spaces (i.e., the sequence includes 13 marks), a total of 24 bits of data can be encoded by each sequence and literally hundreds of thousands of distinct sequences can be generated. 
   Consistent with the comments above, the invention includes a method for use with a web of material having a web length dimension and a web surface, the method for placing mark sequences on the web surface every X distance along the web length dimension identifying location along the web length dimension, the method comprising the steps of monitoring web location and, every X distance, placing a sequence of N marks on the web surface along the web length wherein each two adjacent marks define a space length dimension and wherein the pattern of space length dimensions formed by the N marks in the sequence together specify a specific web length location. 
   According to one aspect, each mark may begin at a mark start point and wherein the space length dimension defined by each two adjacent marks includes the dimension between the start point of a first of the two adjacent marks and the start point of the second of the two adjacent marks. In some cases N may be at least two but in other cases N may be greater (e.g., 6, 13, etc.). In some embodiments the ratio of the longest to the shortest space length dimensions is less than two. In some embodiments each space length dimension is a space dimension range and wherein the space dimension ranges are completely distinct. In some cases the space dimension ranges include four separate space dimension ranges and each space dimension range indicates four values or two bits of information. Here, the longest acceptable length in the longest space dimension range may be less than twice the shortest acceptable length in the shortest space dimension range. For example, the space dimension ranges may include first, second, third and fourth ranges between approximately 3.75 and 4.25 inches, 4.75 and 5.25 inches, 5.75 and 6.25 inches and 6.75 and 7.25 inches, respectively. In some cases each mark is between 1/50 th  inch and five inches and each space length dimension is greater than the longest mark length dimension. In some embodiments each mark is between one inch and 2 inches and each space length dimension is between 3 inches and 8 inches and each (variable-length) mark sequence is less than 5 yards. X is sometimes less than 20 yards. In some cases mark lengths may be as short as 1/50 th  of an inch. 
   In some embodiments the method is also for, subsequent to placing mark sequences on the web surface, identifying locations along the web surface via the mark sequences, the method further including the steps of while unwinding the web, examining the mark sequences and, for each sequence identifying the space length dimension pattern corresponding to the sequence and using the space length dimension pattern to determine location along the web length. 
   In some cases the step of placing the marks includes using a high speed printer to place the marks and the step of examining the mark sequences includes providing a high speed, high resolution camera to sense the marks. Here, the high speed printer may be one of an ink jet printer and a laser marking (i.e., burning) printer. Other high speed non-contact printers are contemplated that are capable of placing a distinct mark of less than one foot in length and typically one or two inches in length on a web that is moving at speeds in excess of 500 feet per minute and sometimes as high as 8,000 or more feet per minute. Here, an important limitation is some embodiments is that, whatever type of printer or marker is employed, the printer be capable of turning on and off at an extremely high rate to provide at least 1000 separate and distinct marks per minute. In some cases at least 1500 marks per minute may be required while in other cases 3000 marks per minute may be required. 
   Consistent with the above comments, some of the embodiments of the invention also includes a method for use with a web of material having a web length dimension and a web surface, the method for placing marks on the web surface identifying location along the web length dimension, the method comprising the steps of providing a printer capable of generating at least 1000 separate marks per minute moving the web material past a marking station at a rate of at least 500 feet per minute, monitoring web length location, using the printer to place marks on the web surface along the web length wherein the marks indicate an associated web location. Here, the step of using the printer may include using the printer to place mark sequences on the surface along the length wherein the sequences indicate web location. 
   The step of using the printer may include placing a sequence every X distance where each sequence includes N marks and each sequence is less than twenty yards long. Each mark sequence may be less than five yards long and X is less than 20 yards. In some embodiments the printer may generate at least 1500 separate marks per minute and in others the printer may generate at least 3000 separate marks per minute. 
   Moreover, the invention also includes an apparatus for use with a web of material having a web length dimension and a web surface, the apparatus for placing mark sequences on the web surface every X distance along the web length dimension identifying location along the web length dimension, the apparatus comprising a processor for monitoring web location and a high speed printer for, every X distance, placing a sequence of N marks on the web surface along the web length wherein each two adjacent marks define a space length dimension and wherein the pattern of space length dimensions formed by the N marks in the sequence together specify a specific web length location. 
   These and other objects, advantages and aspects of the invention will become apparent from the following description. In the description, reference is made to the accompanying drawings which form a part hereof, and in which there is shown a preferred embodiment of the invention. Such embodiment does not necessarily represent the full scope of the invention and reference is made therefore, to the claims herein for interpreting the scope of the invention. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       FIG. 1  is a schematic diagram illustrating a paper manufacturing machine, inspection system, web marking assembly, and wind-up device according to the present invention; 
       FIG. 2  is a schematic illustrating a repair machine assembly according to the present invention; 
       FIG. 3  is a schematic top plan view illustrating a segment of the web of  FIG. 1  and showing a 13-mark sequence according to one aspect of the present invention; 
       FIG. 4  is similar to  FIG. 3 , albeit illustrating a smaller segment of the web and a sub-set of marks from a sequence; 
       FIG. 5  is a flow chart illustrating one method according to the present invention; 
       FIG. 6  is a flow chart illustrating a second method according to the present invention; and 
       FIG. 7  is a schematic illustrating another configuration according to the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   A. Hardware 
   Referring now to the drawings wherein like reference numerals correspond to similar elements throughout the views and, more specifically, referring to  FIGS. 1 and 2 , the present invention will be described in the context of a paper manufacturing system including first and second assemblies  10  and  60 , respectively. In general, first assembly  10  produces a large reel of paper  15  including absolute web location mark sequences (two exemplary marks identified by numeral  19 ) where reel  15  has a width dimension W and a thickness dimension T. Assembly  60  is provided to unwind a full reel  15  that includes mark sequences, identify locations of web defects and stop the unwinding process at each defect thereby exposing the defect for repair. 
   Referring to  FIG. 1 , to produce web  40  with the mark sequences thereon, assembly  10  includes a paper machine  12 , a wind-up device spindle  14  for receiving the paper web  40 , a processor  16 , a database  18 , an encoder  20 , encoder rollers  26  and  28 , an inspection camera  22  and a marker device  24 . 
   Manufacturing machine  12  includes a paper outlet port  32  and, as its label implies, outputs web  40  through port  32 . Encoder rollers  26  and  28  are spaced from outlet port  32  thereby forming an inspection and marking space  37 . Rollers  26  and  28  are mounted to be parallel to web width W and, in the illustrated embodiment, so that roller  26  is below roller  28  with a narrow space therebetween. Winding spindle  14  is positioned proximate roller  26  and is also parallel to web width W. A leading end of web  40  is fed from port  32  around encoder roller  28 , between roller  28  and roller  26  and to spindle  14  where the leading end is attached. In operation, as web  40  emerges from port  32 , web  40  travels in the directions indicated by arrows  34 ,  36  and  38  and is wound on spindle  14  to form reel  15 . 
   Web  40  is taut between port  32  and spindle  14  and there is very little slippage between roller  28  and web  40 . Thus, as roller  28  rotates, at least over short distances, there is generally to one-to-one relationship between roller rotation and web length advance (e.g., when roller  28  rotates once, web  40  advances a distance essentially equal to the circumference of the roller  28 ). Encoder  20  senses roller  28  rotation and provides an encoder signal to processor  16 . Processor  16  uses the encoder signal to, at least over short web distances, precisely determine the web length passing by roller  28 . 
   Inspection camera  22  is generally a high speed, high resolution elongated line camera which extends across web width W and is positioned proximate output port  32  of machine  12  and on a top side of web  40  in inspection and marking space  37 . As well known in the art, camera  22  generates images of web  40  as web  40  exits port  32  and provides the image data to processor  16 . 
   Marker device  24  is positioned next to camera  22  within inspection and marking space  37  and so that a marker spray head (not illustrated) is slightly offset from a lateral web edge  17 . In at least some embodiments marker device  24  is a high speed printer capable of producing relatively short distinct marks on the web surface while the web is moving at high speeds. For instance, while the web is moving at 6,000 feet per minute, marker device  24  may have to be capable of applying marks of approximately one inch length to the web surface. Minimal marker device capabilities may require a device that can apply at least 1000 separate marks per minute. In some cases 1500 or 3000 marks per minute may be required. Known conventional web marking devices are not capable of such functions. Exemplary high speed printers may include various types of ink jet printers (e.g., continuous, impulse, drop-on demand, etc.), laser markers or any other printers having the required fast mark applying capabilities that are contemplated. Marks generated by device  24  are, in at least some embodiments, relatively thin (e.g., on the order of 1/50 th  of an inch in width). In some embodiments, device  24  must be capable of generating marks with clearly defined starting points so that, when turned on to provide a mark, device  24  goes on essentially instantaneously. In embodiments where space lengths defined by consecutive marks as opposed to mark lengths are used to code web length locations, marker device  24  need not be able to precisely define the tailing ends of marks and instead, may generate marks having mark tails within a range up to a known maximum length. 
   Referring still to  FIG. 1 , in addition to being linked to camera  22  and encoder  20 , processor  16  is also linked to marker device  24  and database  18 . In addition to other functions, processor  16  performs three functions that are important to the present invention. First, processor  16  uses encoder signals from encoder  20  to continually track the web length location at which roller  28  resides. For example, if 3,000 yards of web material have already accumulated on reel  15  and the distance between roller  28  and reel  15  is 4.5 yards, processor  16  generates an instantaneous web length location signal indicating 3,004.5 yards 
   Second, processor  16  is programmed to examine the image data generated by camera  22  and identify any defects that occur in web  40 . To this end, when properly illuminated, virtually all web defects show up as either a light spot or a dark spot in image data generated by camera  22  and therefore can be distinguished form non-defective web material. Many web inspection algorithms have been developed within the industry and any of the those algorithms may be used here. Typical defects include holes, dirt, water drops, oil drop, streaks, scratches, edge cracks, coating skips or voids, wrinkles, caliper tears, calendar cuts, scabs and edge tears. 
   When processor  16  identifies a defect, processor  16  correlates a web length location with the defect and stores the correlated location and defect information in database  18 . For instance, if a defect is identified at the instant when a web length location corresponding to data from encoder  20  is 3,000 yards and there are 6 yards between roller  28  and the position of camera  22 , processor  16  correlates the web length location 3,006 yards with the defect and stores the correlated information in database  18 . In addition to storing an indication that a defect occurred along with a location, processor  16  may also store other information such as the type of defect, the position along web width W at which the defect occurred, etc. 
   Third, processor  16  is programmed to control marker device  24  to apply mark sequences in a precise fashion to identify equispaced absolute web length locations. Thus, as encoder  20  generates web length location data, processor  16  causes marker device  24  to apply corresponding mark sequences to the surface of web  40 . In a similar fashion, processor  16  may cause marker  24  to apply a “defect mark” along edge  17  at a location corresponding to the web length at which a defect occurs. A defect mark will typically have an appearance similar to that of a mark in one of the absolute position sequences but may be offset a different distance from the web edge  17 . In this regard,  FIG. 3  illustrates an exemplary mark sequence including 13 separate marks M 1 , M 2  . . . M 12  and M 13  proximate edge  17  and an exemplary defect mark DM on a side of the mark sequence opposite edge  17  and at a web length location corresponding to a defect  44 . Defect marks (e.g. DM) maybe used to provide visual feedback to an operator or to assist in automatic stopping. 
   Referring now to  FIG. 2 , to rapidly and precisely unwind reel  15  and stop web advance to expose defects, assembly  60  includes a “downwind” reel  15  to be unwound, an exemplary receiving or “upwind” reel  21  which is wound while downwind reel  15  is unwound, first and second encoder rollers  44  and  46 , a processor  48 , database  18 , a controller  62 , first and second motors  64  and  66 , an encoder  68 , a code reading camera  70  and a start button  63 . As illustrated, web  40  is fed from downwind reel  15  around roller  44 , between rollers  44  and  46  and then to upwind reel  21  so that, as reel  15  is unwound, the web  40  travels along the directions indicated by arrows  80 ,  78  and  76 . 
   Encoder  68  and associated rollers  44  and  46  are similar to the encoder and rollers described above with respect to  FIG. 1  and therefore will not again be described in detail. Here, it should suffice to say that encoder signals are provided to processor  48  indicating roller  46  rotational velocity. As illustrated, rollers  44  and  46  are spaced from upwind reel  21  so as to form an exposure space  35 . As its label implies, space  35  is the area in which assembly  60  exposes web defects for repair (i.e., web  40  is stopped when a defect is exposed in space  35 ). 
   Motors  64  and  66  are mechanically coupled to reels  15  and  21 , respectively, to control rotational velocity. Controller  62  is linked to each of motors  64  and  66  to control those motors in unison to unwind the paper web from reel  15  and to wind the paper web on reel  21  while keeping the web section between reels  15  and  21  taut. Start button  63  is linked to controller  62  and indicates when assembly  60  should advance web  40  to the next defect that occurs along the web length. Thus, after one defect is repaired, a system operator activates button  63  and assembly  60  advances web  40 . 
   In addition, other controller operating parameters are selectable by a system operator. One selectable parameter is a normal operating speed or normal command speed value S* which indicates a normal unwinding speed in feet per minute. The normal speed S* is selected as a function of various assembly  60  parameters and typically will be in the range of 4,000 to 8,000 feet per minute. Hereinafter it will be assumed the normal or top speed is 6,000 feet per minute. 
   Code reading camera  70  is a high speed, high resolution line or area camera including a lens (e.g., 50 mm) positioned within exposure space  35  proximate (e.g., within 1 meter) the path traveled by the mark sequences and the defect marks adjacent lateral web edge  17 . Camera  70  reads mark sequences and provides data associated with the sequences to processor  48 . Importantly, camera  70  must be capable of sensing the short marks generated by the high speed marker device  24  while the web is moving past camera  70  at an extremely high rate. At a minimum camera  70  should be able to sense a five inch mark passing by at 4,000 feet per minute. In some cases camera  70  should be able to sense a one inch mark passing by at 8,000 feet per minute. In some cases camera  70  need only be able to identify the starting points of marks that are within a minimum distance (e.g., 2-3 inches). Known conventional cameras used with web mark systems are not capable of accurately sensing marks and spaces of the magnitudes described herein. 
   In  FIG. 2 , database  18  is labeled with the same number as the data base in  FIG. 1  to emphasize that the data used by processor  48  is the same data stored by processor  16  regarding defect locations along web  40 . 
   Referring still to  FIG. 2 , in addition to other processes that may be performed by processor  48 , processor  48  performs two processes that are important in the context of the present invention. First, processor  48  decodes the mark sequences from camera  70  to determine the absolute web length locations of each mark sequence. In addition, to determine the precise location of points between mark sequences, processor  48  uses, in at least some embodiments, signals from encoder  68  to identify absolute locations between marks. Thus, for instance, if a mark sequence indicates that the beginning of the mark sequence (i.e., the starting point of the first mark in the sequence) is at 3,000 yards and processor  48  uses the encoder signals to determine that the web has moved 8 yards past the beginning of the first mark in the sequence, processor  48  identifies an absolute instantaneous web length location of 3,008 yards. 
   Second, processor  48  uses the information in database  18  to determine the web length locations of defects on web  40  and then overrides the normal command speed value S* to controller  62  to reduce reel unwinding speed and to eventually stop the unwinding process such that the next defect to occur along the web length is exposed within exposure space  35  for repair. To this end, processor  48  performs an algorithm which, based on system characteristics and instantaneous operating parameters, identifies substitute command speed signals to provide to controller  62 . In at least one embodiment, the substitute command signals include a slow command signal, a crawl command signal, and a stop command signal. 
   The algorithms performed by processor  48  to determine at which web length location to override the normal command speed signal may take any of several different forms and therefore no specific form is described here in detail. Here, it should suffice to say that whatever algorithm is employed, because the mark sequences applied via the high speed printer are much closer together, accuracy is increased appreciably and the fudge factor required to facilitate an accurate stop on a next defect can be minimized thereby speeding up the overall web advancing process. 
   B. Exemplary Code 
   Referring again to  FIG. 3 , an exemplary web length section  40  is illustrated which includes an exemplary  13  mark sequence, the first and second of the 13 marks identified by labels M 1  and M 2  and the 12 th  and 13 th  marks identified by labels M 12  and M 13 , respectively. In  FIG. 3 , an exemplary defect  44  corresponding to a dark spot is illustrated along with a corresponding exemplary defect mark DM. In addition, an inspection camera window  70 ′ corresponding to exemplary camera  70  in  FIG. 2  is illustrated to show that camera  70  is generally aligned along lateral web edge  17  and the path traveled by the mark sequences. 
     FIG. 4  is similar to  FIG. 3 , albeit illustrating a smaller set of marks (i.e., an incomplete mark sequence). In  FIGS. 3 and 4 , it is assumed web  40  travels in direction  55  from right to left during unwinding so that the left end of each mark (e.g., M 1 ) is sensed first within window  70 ′ followed by the right end. Note this order of sensing during unwinding is opposite the order in which each mark was generated. In other words, the starting points (i.e., where marker unit  24  in  FIG. 1  was turned on to start the mark) of each mark (e.g., M 1 , M 2 , etc.) are at the right ends of each mark and the mark tails are at the left ends as illustrated in  FIGS. 3 and 4 . For instance, mark M 1  starts at starting point SP 1 , mark M 2  starts at starting point SP 2  and so on. 
   While each mark sequence includes 13 separate marks in the illustrated example, the lengths of the marks may be different. For instance, in  FIG. 4 , mark M 1  is characterized by a mark length MS 1  that is clearly shorter than mark length MS 3  corresponding to mark M 3 . Similarly, the spaces between marks in the inventive mark sequence have different lengths. For instance, space S 4  between the starting points SP 4  and SP 5  of marks M 4  and M 5  is greater than space S 1  between the starting points SP 1  and SP 2  of marks M 1  and M 2 . The differences in mark lengths illustrated are not intended to show desired differences but rather to illustrate the occurrence of differently sized marks due to inability to precisely control mark lengths. Thus, in  FIGS. 3 and 4  the intent may have been to apply marks having identical lengths but inability to precisely turn off the marker device  24  may have resulted in the disparate lengths. 
   It has been recognized that, while the length of a mark is difficult to control due to run-on problems with maker devices like device  24  in  FIG. 1 , the starting point of each mark (i.e., the instant at which a mark starts) is relatively precisely controllable. Thus, according to at least one aspect of the present invention, instead of using mark lengths to encode web length location data, the spaces between the starting points of marks are used to encode web length location. For example, in  FIG. 4 , mark M 1  begins at starting point SP 1  while the following mark M 2  begins at starting point SP 2 . A space SL 1  which is precisely controllable is formed between starting points SP 1  and SP 2 . Thus, precise space length SL 1  between starting points SP 1  and SP 2  can be used to code specific location data. Similarly, referring still to  FIG. 4 , mark M 3  begins at starting point SP 3  and starting points SP 2  and SP 3  can be precisely controlled to provide second space length SL 2  and hence to code information, starting points SP 3  and SP 4  can be controlled to define space length SL 3 , starting points SP 4  and SP 5  can be controlled to define space length SL 4 , and so on. Although not labeled in the figures, the 13 separate marks M 1  through M 13  that define a single mark sequence can be used to precisely define 12 separate space lengths between consecutive starting points (see 12 spaces defined by marks in  FIG. 3 ). 
   It has also been recognized that each space can be used to code different binary values such that, when a mark sequence is considered as a whole, the number of distinctly codable web locations is on the order of several hundred thousand. For instance, referring still to  FIG. 4 , in the illustrated embodiment of the present invention, four distinct space lengths are definable by adjacent mark pairs including 4, 5, 6 and 7 unit lengths SL 1 , SL 2 , SL 3  and SL 4 , respectively. Hereinafter it will be assumed the units are inches. Because any of the spaces defined by adjacent mark pairs may be found to have any of four different values, each space within a mark sequence corresponds to two bits of information. Therefore, the 12 spaces in an exemplary mark sequence, when combined, can be used to generate 24 bits of information and well over 200,000 distinct web length location codes. This large number of distinct markings reduces reliance on encoder data to determine web length location and hence increases speed and precision of the stopping process. 
   In addition to providing a huge number of specific and distinct location codes, other features of the inventive mark sequence concept are contemplated that add additional value. First, unlike other web coding algorithms, with the present code, a small number (e.g., 4 space lengths) of space lengths may be used as an error checking or correcting code (e.g., a cyclic redundancy check (CRC)) to make sure that a location determination by processor  48  is correct. Where the CRC indicates data that is inconsistent with a corresponding location, the location determination can be corrected or discarded. 
   Second, by limiting the different space lengths (e.g., SL 1 , SL 2 , etc.) by marker device  24  to within certain ranges, even where marking errors occur, processor  48  can identify the mark sequence errors. For instance, consistent with the example above where maximum and minimum space lengths SL 1  and SL 4  are 4 and 7 inches, respectively, if marker device  24  fails to generate one of the 11 marks M 2  through M 12  in a 13 mark sequence, a corresponding space between starting points will be at least 8 inches and hence easily recognizable as a space in which a mark was missed. Similarly, if a sequence includes all 13 intended marks but one mark is not sensed by processor  48 , processor  48  may recognize a space that is 8 or more inches long as a missed mark. Here, the important limitation is that the longest recognizable space length (e.g., SL 4  above) should be less than twice the shortest space length (e.g., SL 1  above). Where space lengths are specified in ranges, the upper limit for the longest space length should be less than twice the lower limit for the shortest space length. For instance, if length SL 1  may be between 3.75 and 4.25 inches, the upper limit for the longest space length SL 4  range should be less than 7.5 inches (e.g., less than 2×3.75 inches). 
   Third, by limiting the number of distinct space lengths used, many mismarking or misreading errors can be corrected by processor  48 . For instance, again assuming mark lengths of 4, 5, 6 and 7 inches, where a second mark that was to be provided between first and third marks was either not applied by device  24  or was not sensed by camera  70 , the space defined by the first and third marks is often useable to identify the location of the missing second mark. For instance, if the first and third marks define an 8 inch space, processor  48  can determine that the first and second marks and the second and third marks were to have defined two consecutive four inch space lengths as there is no other combination of 4, 5, 6 and 7 inch marks that make up an 8 inch mark. Similarly, if the first and third marks define a fourteen inch space, processor  48  can determine that the first through third marks were to have defined two consecutive 7 inch marks. Thus, here, to reduce the number of errors that may occur and result in similar erroneous space lengths, the number of distinct lengths employed should be small and the shortest employed length should not be extremely small. For instance, while some embodiments may include distinct space lengths on the order of one or two inches, 4 units or inches has been determined to be a good balance between competing considerations. 
   The mark length may be any dimension that fits certain criteria. Specifically, the mark length plus a maximum mark tail (MMT) (e.g., the maximum distance that a mark is expected to dribble on after marker device  24  is commanded to stop applying a mark) must be less than the minimum space length SL 1 . For instance, where the minimum space length SL 1  is four inches, the mark and MMT combined may be up to a length less than 4 inches. It has been determined that a one inch long mark is easily readable via a typical code reading camera  70 . Thus, where space length SL 1  is four inches, the MMT must be just less than 3 inches. 
   In addition to reducing reliance on encoder data to determine inter-mark location, providing marks every few yards facilitates rapid re-synchronization after a web break occurs. For instance, where mark sequences are 10 yards apart, when a break occurs, re-synchronization may be performed within 10 yards. Rapid re-synchronization within a short web length means essentially all defects can be exposed and repaired as quickly as possible. 
   Moreover, because the dimensions of the marks contemplated by the present invention are extremely small (e.g., 1/50 th ×½ inch), the sequence lengths are reduced appreciably. Thus an exemplary sequence may be 4 or less yards instead of 30 or more yards as in the prior art. Reduced sequence length means a reduction in required ink, system maintenance, etc. 
   C. Exemplary Methods 
   Referring now to  FIG. 5 , one method  100  according to the present invention is illustrated. Referring also again to  FIG. 1 , at block  102 , with web  40  output at port  32 , processor  16  monitors web location with signals from encoder  20 . At block  105 , processor  16  causes marker device  24  to apply equispaced (e.g., every 10 yards) mark sequences that indicate web locations as described above. At block  104  processor  16  identifies any defect in the web. Where no defect is identified, control passes to block  108 . At block  108 , if the reel is full the process is halted and if the reel is not full, control passes back up to block  102  where the process is repeated. Referring again to block  105 , where a defect is identified control passes to block  106  where processor  16  stores a correlated web location/defect pair. Next, control passes to block  107  where processor  15  controls marker device  24  to apply a defect mark laterally adjacent the defect after which control passes to block  108 . 
   Referring now to  FIG. 6 , a method  110  for unwinding a web on reel  15  where the web includes mark sequences is illustrated. Referring also and again to  FIG. 2 , at block  112 , processor  48  receives data from camera  70  corresponding to the web sequences and identifies the sequence spacings defined by the starting points of the marks. At block  114 , processor  48  converts a sensed sequence spacing pattern to a web location and at block  116  processor  48  controls the unwinding process as a function of the web location. After block  116  control passes back up to block  112  where the process is repeated. 
   It is contemplated that under certain circumstances marks placed on a web during a marking process may subsequently become obscured. For instance, some post-marking processes may print over existing marks, may trim web edge and mark sequences thereon or may otherwise obscure the mark sequences. In this case, where a process is known to obscure mark sequences, it is contemplated that a re-marking configuration may be employed wherein a second marker (e.g., 24 in  FIG. 1 ) may be used to remark a web within the same reference frame as the original mark sequences. To this end, referring to  FIG. 7 , an exemplary system including a component  170  that performs a process on a previously marked web is illustrated where a processor  48  is linked to a camera  70  and a marker unit  24 . In the illustrated configuration it is assumed that component  170  alters the web in some fashion that renders the previously applied mark sequences obstructed (e.g., trimming, printing over, etc.). As illustrated, camera  70  is placed upstream of component  170  and marker unit  24  is placed downstream. Here, processor  48  obtains mark sequences via camera  70  and identifies absolute position or web location. In addition, processor is programmed to know the distance between camera  70  and unit  24  and to re-apply the mark sequences obtained via camera  70  to the web downstream at the exact same locations via unit  24 . 
   From the foregoing, it will be observed that numerous modifications and variations can be effected without departing from the true spirit and scope of the novel concept of the present invention. It will be appreciated that the present disclosure is intended as an exemplification of the invention, and is not intended to limit the invention to the specific embodiment illustrated. The disclosure is intended to cover by the appended claims all such modifications as fall within the scope of the claims. For example, while a mark sequence protocol based on spaces formed by consecutive marks is described above, some embodiments may simply use a sequence protocol based on mark lengths as opposed to space length where novelty resided in the hardware configuration used to provide short and minimal ink marks (e.g., high speed cameras and marker devices). As another example, the protocol based on spaces as opposed to marks may be used with conventional marking devices, albeit a lot of the advantages associated with the high speed hardware may be minimized. As another example, while the invention is described above in the context of a system for identifying locations of defects via mark sequences, it should be appreciated that the invention has many other applications including any type of application where web location is to be determined or where a range of locations must be determined. In this regard the invention provides a common down-web coordinate system for recording event positions/ranges and, in at least some embodiments, subsequently taking actions based on those events. The events could include almost anything from optical detection of a defect, a process change, an operator-defined “region of interest”, detection of metal by a scanning device, a region where a solvent was added, shift change boundaries, etc. Any piece of information that can be associated (in a database) with a down-web location (or range of locations) can later be identified and acted on. The later-processing machines could be almost any type of machine including machines used to process, convert, enhance, transport, analyze, view or repair web material. Also, the actions taken could include almost any type of action including slowing, stopping, alerting an operator, controlling a shear (cutter), activating safety devices, triggering a camera, lifting coater blades, etc. 
   In addition, while the specification above describes absolute position marking and determination it should be appreciated that there are many different ways to mark absolute position. For instance, the mark sequences themselves may indicate a specific number associated with an absolute position such as the number 1010 for 1010 yards or, in the alternative, some type of look up table may be provided for correlating the content of a sequence with a specific location (e.g., the number 47 may be correlated with 1010 yards, the number 12 may be correlated with 1110 yards and so on). Thus, the phrase absolute position is used in a broad sense. 
   Moreover, while the inventive space based mark sequence described above is described as being useable to mark web locations, it should be appreciated that the inventive sequence may be used for other marking purposes. For example a space-oriented mark sequence may be used to encode a reel ID number, in addition to or instead of the position code. 
   Furthermore, the mark sequences do not have to be uniformly spaced. Sequences could be provided with almost any changing frequency (e.g., more frequently at the beginning of a reel) or even random frequency. 
   To apprise the public of the scope of this invention, the following claims are made: