Abstract:
Systems and methods for imaging an imprinted substrate on a printing press is provided. One method comprises sensing light reflected by the substrate using a contact image sensor to produce data representative of the imprinted substrate. The substrate has been imprinted with different colors at a plurality of printing units of the printing press. Each printing unit comprises a plate cylinder. The data representative of the imprinted substrate is output by the contact image sensor as analog voltage signals. The method further comprises receiving the analog voltage signals from the contact image sensor at a sensor interface circuit and converting the analog voltage signals to digital signals using an analog-to-digital converter of the sensor interface circuit. The method further comprises processing the digital signals using the sensor interface circuit to produce corrected digital signals and storing data based on the corrected digital signals in a memory.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. application Ser. No. 13/172,522 filed Jun. 29, 2011 now U.S. Pat. No. 8,183,550, which is a continuation of U.S. application Ser. No. 12/765,723 filed Apr. 22, 2010 now U.S. Pat. No. 8,039,826, which is a continuation of U.S. application Ser. No. 12/174,481 filed Jul. 16, 2008 now U.S. Pat. No. 7,732,796, which is a continuation of U.S. application Ser. No. 10/914,372 filed Aug. 9, 2004 now U.S. Pat. No. 7,423,280, all of which are herein incorporated by reference in their entireties. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to a web inspection module for a printing press, and more particularly, to a web inspection module including a plurality of contact image sensors for obtaining image data from an imprinted web moving at a high rate of speed. 
     BACKGROUND OF THE INVENTION 
     In an exemplary printing press such as a web offset press, a web of material, typically paper, is fed from a storage mechanism, such as a reel stand, to one or more printing units that repetitively imprint the web with images. The imprinted web is typically driven through a number of processing units such as a dryer unit, a chill stand, and possibly a coating machine. The web is then typically fed to a former/folder to be slit, folded, and cut into multi-page signatures. 
     It is desirable to monitor the quality of the imprinted web, to ensure that the amount of applied ink is appropriate and produces the desired optical characteristics, and to ensure that the different ink colors are properly aligned (registered) with respect to one another. Further, monitoring the web is important to ensure that the imprinted web does not include defects such as ink blots, lack of ink in areas where ink should be, smears, streaks, or the like, and to insure that various print processes occur at a correct location with respect to the ink on the web. For example, ink color control systems, color registration systems, and defect detection systems are known systems used in connection with monitoring the quality of the imprinted web. Various other types of control systems are also known for controlling the position of the web with respect to a processing unit of the printing press. For example, a cutoff control system operates to control the longitudinal position of the web so that the cutting of the web into signatures occurs at a desired location. 
     Such systems generally include an imaging assembly for obtaining image data from a portion of the moving imprinted web. Typically, the acquired image data is compared to reference image data. The resultant information is used, for example, to control the amount of ink applied to the web, the alignment of the printing plates with respect to each other, to mark or track the whereabouts of resultant defective printed product, or to control the location of the imprinted web with respect to a processing unit. 
     More specifically, in a typical ink color control system for controlling the amount of ink applied on a printing press, the camera collects image data representative of color patches printed on the web. These patches generally extend across the width of the web. Pixels of the color patch image data are then processed, and assigned a color value that is compared against a desired color value. If the absolute difference between the desired color value and the determined color value for a number of pixels in an ink key zone is outside a predetermined tolerance, an associated ink key is then controllably adjusted to effect a change in the ink flow rate. Markless color control systems are also known that do not require the use of separate color patches but instead measure color values in the desired graphical/textual printed work itself. Examples of ink color control systems are described in U.S. Pat. Nos. 5,967,049 and 6,318,260. 
     A typical defect detection system also acquires an image of the imprinted web. The acquired image is subsequently compared to a stored digital template image. Any discrepancy between the acquired image and the template image beyond some tolerance is considered to be a defect. The defects are then logged in a data file, and can be categorized as isolated defects or non-isolated defects. Non-isolated defects occur when the system detects a change in color due to a change in inking level over a large portion of the web. When non-isolated defects are reported, an alarm will subsequently be set off to alert an operator to take appropriate corrective action. Isolated defects can be tracked such that the associated printed products are marked as defective, or are otherwise separated from the acceptable printed products. 
     Typically, color registration systems also compare acquired image data to reference image data and adjust the registration or alignment of each ink color with respect to the others by adjusting the positions of the printing plates with respect to each other. Color registration systems using marks or patches are known, as are markless systems. Examples of such systems are described in U.S. Pat. Nos. 5,412,577 and 5,689,425. 
     These control systems all require image data to be acquired from the printed work on the web, and vary in the amount and resolution of data required. For example, to detect defects in the entire printed work, it is desirable to acquire image data for the entire width of the web, as well as the entire length of the web. An ink key control system, because it controls ink keys across the lateral extent of the web, would preferably obtain image data from patches (or the desired printed work itself) across the entire width of the web, but only once per image repeat. Similarly, a color registration system using color marks would obtain image data only once per image repeat. Additionally, marks for color registration or cutoff control generally do not extend across the web. 
     Typical imaging assemblies include lighting elements for illuminating the web, and a camera having sensors for sensing light and optical elements for focusing light reflected from the imprinted web to the sensors. Known sensors include area array sensors having two-dimensional arrays of sensing elements, and line scan sensors, which include a single line of sensing elements aligned across the web. With line scan sensors, two dimensional image data is obtained by acquiring successive lines of data as the imprinted web moves with respect to the line sensors. 
     Typical optical elements are lenses that reduce the image on the web in order to obtain a desired resolution for the image data. This typically results in a field of view for the camera that is several inches in width. With such prior art imaging assemblies, the distance between the web and the camera generally needs to be comparable to the width of the web being imaged. Thus, prior art imaging assemblies for printing presses generally require a distance on the order of approximately four feet between the web and the camera. Further, because the cameras themselves were often expensive, prior art systems typically minimized costs by using a single camera with a positioning unit to move the imaging assembly across the width of the web. 
     SUMMARY 
     According to one exemplary embodiment, a method of imaging an imprinted substrate on a printing press comprises sensing light reflected by the substrate using a contact image sensor to produce data representative of the imprinted substrate. The substrate has been imprinted with different colors at a plurality of printing units of the printing press. Each printing unit comprises a plate cylinder. The data representative of the imprinted substrate is output by the contact image sensor as analog voltage signals. The method further comprises receiving the analog voltage signals from the contact image sensor at a sensor interface circuit and converting the analog voltage signals to digital signals using an analog-to-digital converter of the sensor interface circuit. The method further comprises processing the digital signals using the sensor interface circuit to produce corrected digital signals and storing data based on the corrected digital signals in a memory. 
     According to another exemplary embodiment, a system for imaging an imprinted substrate on a printing press comprises a light source configured to illuminate a portion of the substrate which has been imprinted with different colors at a plurality of printing units of the printing press. Each printing unit comprises a plate cylinder. The system further comprises a contact image sensor configured to sense light reflected by the substrate, to produce data representative of the imprinted substrate based on the sensed light, and to output analog voltage signals based on the data representative of the imprinted substrate. The system further comprises a sensor interface circuit configured to receive the analog voltage signals. The sensor interface circuit comprises an analog-to-digital conversion circuit configured to convert the analog voltage signals to digital signals and a digital processing circuit configured to process the digital signals to produce corrected digital signals. The system further comprises a memory configured to store data based on the corrected digital signals. 
     According to another exemplary embodiment, a system comprises a plurality of elements. Each element senses light reflected by a corresponding region on an imprinted substrate on a printing press to produce data representative of the corresponding region printed on the substrate. A dimension of each element is substantially equal to a dimension of the corresponding region printed on the substrate. The substrate has been imprinted with an image at a printing unit of the printing press. The printing unit comprises a plate cylinder. Each element is configured to output analog voltage signals based on the data representative of the imprinted substrate. The system further comprises a sensor interface circuit configured to receive the analog voltage signals. The sensor interface circuit comprises an analog-to-digital conversion circuit configured to convert the analog voltage signals to digital signals and a digital processing circuit configured to process the digital signals to produce corrected digital signals. The system further comprises a memory configured to store data based on the corrected digital signals. 
     Other features and advantages of the invention will become apparent by consideration of the detailed description and accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a typical printing press; 
         FIG. 2  is a block diagram of a web inspection module; 
         FIGS. 3(   a )- 3 ( b ) are perspective views of a web inspection module according to one embodiment; 
         FIGS. 4(   a )- 4 ( e ) are exploded views of a web inspection module illustrating the various components and their arrangement according to one embodiment; 
         FIG. 5(   a ) is a perspective view of a web inspection system according to one embodiment; 
         FIG. 5(   b ) is a perspective view of a web inspection system and further illustrating light sources for two of the web inspection modules; 
         FIG. 5(   c ) is a front view of the web inspection system illustrated in  FIG. 5(   b ) and showing the components within the light source housing; 
         FIG. 5(   d ) is a top view of the web inspection system illustrated in  FIG. 5(   b ); 
         FIG. 6  is a side view of the web inspection system illustrated in  FIG. 5(   a ) including the web inspection modules; 
         FIG. 7  is a schematic of a contact image sensor in the form of a sensor board; and 
         FIG. 8  is a schematic of a contact image sensor and GRIN lens array. 
     
    
    
     Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  illustrates a representative printing press  10  for repetitively printing desired images upon a substrate such as a paper web. The printing press  10  illustrated is a web offset press and includes a reel stand  14  that supports a reel  16  of the web  12 . It should be noted that the invention is equally applicable to sheet fed presses and other non-offset presses such as gravure presses and newspaper presses for example. 
     The printing press  10  includes printing units  18 ,  20 ,  22 , and  24 , each of which prints using a different color ink. For example, in the illustrated printing press  10 , the first printing unit  18  encountered by the web  12  prints with black ink and the other printing units  20 ,  22  and  24  respectively print with magenta ink, cyan ink, and yellow ink. It should be understood, however, that the invention is capable of being carried out with printing units that print in different colors, and/or with fewer or additional printing units. The printing press  10  includes a drive system  26 , including drive rollers  28  that move the web  12  from the reel  16  through each of the printing units  18 ,  20 ,  22 , and  24 . 
     Each printing unit  18 ,  20 ,  22 , and  24  includes a pair of parallel rotatable blanket cylinders  30  and  32  that nip the web  12 . Each printing unit  18 ,  20 ,  22 , and  24  further includes a plate cylinder  34  which has a printing plate thereon, and which applies an ink image to the blanket cylinder  30 . The images printed by each of the printing units  18 ,  20 ,  22  and  24  overlap to create composite multi-color images on the traveling web  12 . Optionally, if it is desired to print on both sides of the web  12 , each printing unit  18 ,  20 ,  22 , and  24  will also include a plate cylinder  36  having a printing plate thereon for applying an ink image to the blanket cylinder  32 . The blanket cylinders  30  and  32  transfer the ink images, received from the plate cylinders  34  and  36 , to the web  12 . 
     After exiting the printing stations  18 ,  20 ,  22 , and  24 , the now imprinted web  12  is guided through various processing units, such as a tensioner  38 , a dryer  40 , and a chill stand  42 . The imprinted web is then fed to a former/folder  44 . 
     As shown in  FIGS. 5(   a )- 5 ( d ), a web inspection system  48  includes a plurality of web inspection modules  50  for scanning the web  12  to produce image data representative of the imprinted web. In particular,  FIG. 5(   a ) is a perspective view of a web inspection system according to one embodiment. A longitudinal direction  46  is defined as the direction of web travel, with a lateral direction  47  substantially perpendicular to the longitudinal direction  46 .  FIG. 6  is a side view of the web inspection system shown in  FIG. 5(   a ). 
     Although the web inspection system  48  can be mounted at any convenient location on the printing press  10 , in one embodiment, the web inspection modules  50  are mounted to a mounting bar  52  that is mounted to side plates  54  of an idler roller  56  such as at the chill stand  42 . In this manner, the web  12  is stabilized on the surface of the idler roller  56  when the imprinted web is scanned and the system  48  is readily incorporated on an existing printing press. The web inspection system  48  also includes a distribution box  58  having, for example, an Ethernet hub for coupling signals to and from each web inspection module  50  to a central processing unit of the press (not shown). The web inspection system  48  is low profile and is located in close proximity to the web  12 . 
     In the preferred embodiment, a single web inspection module  50  is designed to include a contact image sensor  66  (one embodiment shown in  FIG. 7 ) to acquire image signals corresponding to approximately 12.4 inches across the web, i.e., in the lateral direction. Thus, four web inspection modules  50  can be used to acquire data across the entire width of a 48 inch web, with the web inspection modules being aligned such that their contact image sensors  66  slightly overlap in the lateral direction. In one embodiment, this overlap is on the order of 0.1 inch. The web inspection system  48  can also be designed in order to take into account web weave, i.e., the lateral movement of the web itself, which in some presses can be on the order of two inches or so. In such a case, the web inspection system  48  can include contact image sensors  66  that image an area having a width that is greater than the width of the web by the amount of expected lateral web weave. Each module  50  essentially provides image signals for a longitudinally extending slice of the imprinted web. Using multiple modules  50  allow image signals corresponding to the entire width of the web to be obtained. 
       FIG. 2  schematically illustrates in block diagram form one embodiment of a web inspection module  50  in accordance with the invention. The web inspection module  50  includes components such as a light source  62 , a lens array  64 , a contact image sensor  66 , a sensor interface circuit  68 , a power/interface circuit  70 , an image processor  72 , and cooling devices  74 . The web inspection module  50  is operable to scan at least a portion of an imprinted web moving in the longitudinal direction  46  in a printing press. Each web inspection module  50  receives from the distribution box  58  a plurality of signals including an encoder signal (as is known in the art), power and ground signals, and optionally, a light control signal. In particular, the power/interface circuit  70  receives these signals, buffers them as necessary, and supplies appropriate signals to several of the other components. As more fully explained below, the light source  62  provides light to illuminate a portion of the web. Reflected light from the web passes through the lens array  64  and is measured by a contact image sensor  66  having a plurality of sensing elements  67  (one embodiment shown in  FIG. 7 ) to generate image signals. The sensor interface circuit  68  receives the image signals from the sensing elements  67 , performs analog to digital conversion of the signals, and processes the digital image signals to produce image data that is then sent to the image processor  72 . The image data is representative of the imprinted web and may represent color information or monochromatic information, as explained below. The cooling devices  74  operate to cool the contact image sensor  66  and several other circuit components in order to allow the contact image sensors to operate at an appropriate clock rate to provide image signals at a desired longitudinal resolution. The image processor  72  performs calculations and operations using the image data according to a desired application, such as a defect detection application, color registration application, or the like. Output data from the image processor  72  is then transmitted to the distribution box  58  to be transferred to a central processing unit of the press. 
       FIGS. 3(   a ) and  3 ( b ) illustrate perspective views of a web inspection module  50  according to one embodiment. This web inspection module  50  includes a compact housing  76 , having dimensions on the order of sixteen inches wide, ten inches high, and a depth of five inches. The housing  76  provides protection for several of the module components.  FIG. 3(   a ) also illustrates the input ports  78  for chilled water for the cooling devices  74 , and also an access panel  80  for easy access to the components inside the housing  76 , and in particular to the power/interface circuit  70 .  FIG. 3(   b ) illustrates one embodiment of an input light port  82  and light distributor  84  for receiving light from the light source and distributing light to a portion of the web. 
       FIGS. 4(   a )- 4 ( e ) are exploded views that illustrate the physical arrangement of several of the module components within the housing  76 . In particular,  FIG. 4(   a ) shows the power/interface circuit  70 , and the image processor  72  coupled to a network board  86  providing connections, such as Ethernet connections, to the distribution box  58 .  FIG. 4(   a ) also illustrates the placement of a lens array  64  and lens array housing  94 , and various sealing elements  90 . The lens array  64  couples light reflected from the imprinted web to the contact image sensor  66 , in one embodiment, through a transparent protector  91 . 
       FIGS. 4(   c ) and  4 ( d ) illustrate the contact image sensor  66  and the sensor interface circuit  68  arranged substantially perpendicular to each other. A cooling device  74   a  in the form of tubes with chilled water operates to cool the sensor  66  and sensor interface circuit  68 .  FIG. 4(   b ) shows the placement of cooling device  74   b  for cooling the image processor  72 . In one embodiment, the cooling devices  74   a ,  74   b  are connected to the water supply of the chill unit  42 . Such chill units are typically part of a web offset printing press. The cooling devices  74   a ,  74   b  operate to keep the components within a specified operating temperature range, for example, at a temperature below 55 degrees centigrade. 
       FIG. 4(   e ) further illustrates the light distributor  84 , such as a fiber optic bundle, for transmission and distribution of the light from the light source  62  to a desired portion of the web. The desired web portion has a dimension measured in the lateral direction at least equal to the length of the sensing elements  67  (note that the length of the sensing elements  67  is also measured in the lateral direction). The light source  62  can be, for example, an AC or a DC light bulb. Using such an optical distributor, the AC or DC light bulb can be located on top of the housing and the light from the bulb transmitted to the desired portion of the web. Referring to  FIGS. 5(   b )- 5 ( d ), illustrated therein is a light source box  98  for housing the light source  62 , such as a light bulb  100 . Although only two boxes  98  are illustrated, in this embodiment, each web inspection module  50  would have its own light source box and bulb. Also illustrated is a light tube  102  for transmitting light from the light source box  98  to light distributor  84  via port  82  (both shown in  FIG. 3(   b )). Further illustrated are connections  104  between the web inspection modules  50  and the distribution box  58 , which are routed via the mounting bar  52 .  FIG. 5(   d ) is a top view of the web inspection system illustrated in  FIG. 5(   b ). 
     In the preferred embodiment, the AC or DC light sources are non-strobed such that light is continuously provided while the imprinted web is being scanned. Each web inspection module acquires a single line of data at a time, with the movement of the web providing additional lines over time. Thus, for each web inspection module  50 , image signals are obtained for the entire longitudinal extent of each repeat of the desired image on the web, for that portion of the web width scanned by that particular module  50 . Thus, the web inspection system can provide 100% coverage of the web  12 . 
     The lifespan and cost of the light source  62  are considerations in the design of the web inspection module  50 , with AC light bulbs typically being cheaper and lasting longer than DC light bulbs. Alternatively, a line array of LEDs can be used as the light source  62  for illuminating a portion of the imprinted web. In such a case, the LEDs can be arranged along the width of the web inspection module such that an optical distributor is not necessary. Preferably, LEDs emitting white light are employed, although other LEDs such as those emitting red, blue or green light can be used, depending upon the sensors used and the type of image data required for the application. The LEDs provide the option of pulsed operation. 
     Preferably, light is delivered to the web (directly or indirectly from a light source  62 ) at an angle of approximately 45 degrees from the reflected light travelling to the lens array  64 . The use of LEDs as a light source may require the use of reflectors to focus the emitted light in an advantageous manner. 
     The power/interface circuit  70  includes the necessary components to supply appropriate power and ground signals to the other components of the web inspection module. 
     In the preferred embodiment, the lens array  64  is a gradient index (GRIN) lens array, such as a SELFOC brand lens array, available from NSG Europe, as illustrated in  FIG. 8 . This lens array has one or more rows of gradient index lenses, with each lens having a continuous change of refractive index inside a cylinder. The lenses couple light reflected from the imprinted web to a plurality of sensing elements of a contact image sensor  66 . The images from adjacent lenses overlap and form a continuous image adjacent the contact image sensor  66 . The array provides a one to one correspondence between the width of an image sensing region and the width W (illustrated in  FIG. 7 ) of a single sensing element  67 . In other words, each sensing element  67  measures light reflected by a corresponding image region on the web, wherein a width of each sensing element is substantially equal to a width of the corresponding image region measured in the lateral direction. If the bottom of lens array  64  is at a distance D 1  from the web  12 , then the distance between the top of the lens array and the contact image sensor  66  is substantially equal to distance D 1 . In a preferred embodiment, D 1  is approximately ¼ inch (a typical idler roller has a diameter of approximately four to six inches). The lens array has a height (measured radially outwardly from the idler roller) of approximately ½ to ¾ inches. 
     The contact image sensor  66  can include a plurality of sensing elements  67 , and one embodiment of the contact image sensor in the form of a sensor board with input/output (I/O) terminals is schematically illustrated in  FIG. 7 . In the preferred embodiment, the contact image sensor can include twenty identical image sensor chips  69  placed end to end, having a sensing length of 12.4 inches. Such sensors are known in the art and are commercially available. 
     Each sensor chip  69  can include four rows, denoted Mono, Red, Green and Blue, of sensing elements  67  for respectively sensing light having wavelengths within a particular range, such as white, red, blue and green light. Each row of the contact image sensor can include 7440 active sensing elements (i.e., 372 per sensor chip) and 120 dark sensing elements for reference purposes. For example, the sensing elements  67  are pn junction photodiodes fabricated using CMOS technology and have a width of 42.33 microns, which corresponds to 600 sensing elements per inch. Various other contact image sensors can be used utilizing other known sensing technologies such as CCD sensing elements. In the preferred embodiment, the contact image sensor  66  is externally configured to read out signals from the twenty sensing chips  69  in parallel. In one embodiment, the sensor chip is used in a monochromatic mode, while in another embodiment, the R, G, and B channels are used. 
     As stated, the image signals are acquired for one line at a time. The resolution in the longitudinal direction is determined by the web speed and a clock rate. For example, for a desired longitudinal resolution of 75 lines of image data per inch (75 pixels per inch), and a web speed of 3000 feet/min (600 inches/sec), the web will move 1/75 of an inch in 1/45,000 second. Thus, a line rate of 45 kHz is required to provide resolution of 75 pixels per inch. Each chip requires 372 clock cycles to output the image signals from each sensing element, so that a single line from all three channels requires a clock speed greater than 50.22 MHz (=45 kHz*372*3). In a preferred embodiment, a 60 MHz clock signal from the sensor interface board can be employed to clock out data from the R, G, B rows of each chip. 
     The sensor interface circuit  68  includes an analog front end and a digital processing circuit. In the preferred embodiment, the analog front end includes an A/D converter for converting the image signals from analog to digital. Further, the A/D converter includes a programmable gain amplifier, and the voltage value corresponding to an averaged output of two sensing elements is converted to an eight bit digital voltage signal. Thus, the lateral resolution at the output of the A/D converter corresponds to 300 pixels per inch. 
     The digital processing circuit  72  operates to further reduce the lateral resolution to around 75 pixels per inch. This can be accomplished by averaging every four values to produce a single value, or by simple deleting 75% of the values. The digital processing circuit also operates to adjust the digital values by an offset and gain amount. An appropriate offset and gain amount for the sensing elements can be determined by obtaining values for no light conditions, and full light conditions, as is known in the art. 
     The image processor processes the image data. The processing can include, for example, comparison with reference image data for ink color control, color registration, and/or defect detection purposes, or for other applications. 
     Various features and advantages of the invention are set forth in the following claims.