Abstract:
A printing plate mounted on an external drum platesetter is verified to be securely fastened to the drum using an autofocus system prior to spinning the drum to maximum operating speed. An autofocus mode is employed to set a reference signal using a light beam and light sensor. After establishing the reference signal at one end of the plate, the system switches to a second mode of operation where the autofocus system is fixed, and the reference signal is monitored as the drum and plate are rotated. If the reference signal deviates from the initial value, the rotation of the drum is stopped and an operator is alerted.

Description:
BACKGROUND OF THE INVENTION 
     The invention herein generally relates to the field of imaging systems for use in the graphic arts industry. More particularly, the invention relates to detection of improperly installed printing plates on an external drum platesetter used to image printing plates. 
     Printing plates, often made from aluminum sheets or polymer based materials, are manufactured in various sizes. Platesetters, machines used to transfer an electronic image onto a printing plate, must be capable of accommodating different size printing plates. 
     One technique of mounting a printing plate is to provide a clamp (or clamps) positioned on the outside surface of an external drum platesetter in a fixed location. This fixed position clamp is used to hold one end of the plate, often referred to as the leading edge, onto the external drum. A second clamp, also positioned on the outside surface of the external drum platesetter is moveable, and may be positioned most anywhere on the outside circumference of the drum. The moveable, or “variable” clamp is preset a predetermined distance from the fixed clamp corresponding to the longitudinal length of a particular printing plate to be mounted. The variable clamp is used to hold the end of the printing plate that is opposite from the end being held by the fixed clamp. The end of the plate held by the variable clamp (or clamps) is often referred to as the trailing edge. The variable clamp may be either manually positioned, or automatically positioned via a motor and a controller. 
     A common failure mode of a platesetter is displacement of one end of the printing plate from under one of the clamps during spin up of the drum. This can occur because of a failure of one of the clamps, or because the variable clamp was set an incorrect distance from the fixed clamp for the size plate being used. A spinning drum having a printing plate clamped at only one end is considered a dangerous condition. In addition to destroying the plate and possibly the platesetter itself, the loose end of the plate can injure an operator or even fly off the drum. 
     Adding components to the existing system to detect a loose plate increases product cost, decreases reliability, and adds complexity to the system, and hence is undesirable. 
     SUMMARY OF THE INVENTION 
     The invention herein solves at least the problem of spinning up a drum to full speed, having a plate, or other substrate either partially or completely unsecured onto the drum. The invention herein prevents injury to an operator caused by: 
     1) a loose printing plate when an operator attempts to stop the drum or otherwise intervene, and 
     2) precluding a printing plate from flying off the drum. 
     The invention herein automatically detects a printing plate or other recordable substrate that is not securely fastened to a rotating drum machine, and subsequently stopping rotation of the drum. 
     The invention herein uses an existing autofocus sub-system in an imaging system to detect an unsecured printing plate mounted to an external drum imaging machine while rotating the drum at a fraction of normal operating speed. The drum is slowly rotated to position the leading edge clamp proximate the incident light beam used for autofocusing. This maneuver places one end of the printing plate such that an autofocus light beam is incident upon (or impinges on) one end of the plate. The autofocus system positions a focusing lens such that optimum focus is obtained. The focus lens is then fixed in place. The drum is slowly rotated at only a fraction of the normal operating speed keeping centrifugal forces low (compared to full speed) thereby preventing the plate from being a danger to an operator and/or the machine itself. 
     The drum is slowly rotated so at least a portion of the length of the plate is exposed to the autofocus light beam. Preferably, at least one half of the length of the plate is exposed to the autofocusing beam. As the drum is rotated, an autofocus signal is monitored. If the autofocus signal does not deviate from a predetermined value, the plate is determined to be securely mounted, and the drum is spun up to normal operating speed. If the autofocus error signal exceeds a predetermined value, the plate is determined to be unsecurely mounted and the rotation of the drum is immediately stopped. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The following description may be further understood with reference to the accompanying drawings in which: 
     FIG. 1 shows one embodiment of an autofocus system in accordance with the invention. 
     FIGS. 2 a - 2   d  show light patterns illuminating a quadcell light sensor representative of various focus conditions of a light beam incident upon a printing plate. 
     FIG. 3 shows a portion of the autofocus system of FIG. 1 with a printing plate securely mounted to an imaging machine. 
     FIG. 4 shows a portion of the autofocus system of FIG. 1 with a printing plate insecurely mounted to an imaging machine. 
     FIG. 5 shows a portion of the autofocus system of FIG. 1 with Hall mode apparatus in accordance with the invention. 
     The drawings are shown for illustrative purposes only, and are not to scale. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Though the following description of the invention herein is described in the context of an external drum platesetter, the application of the invention should not be limited to such. For example, the invention herein may also be employed on external drum imagesetters and/or printing presses. Printing plate  34  may alternatively be a piece of film in lieu of a printing plate without deviating from the spirit of the invention. 
     Referring to FIG. 1, an autofocus system  1  is employed with an external drum imaging system generally shown at  26 . A Printing plate  34  is mounted to a rotatable external drum (or cylinder)  28  using clamps  30  and  32 . 
     Light source  2  creates a beam of light  3  that is collimated using lens  4 . Light beam  3  is directed through magnifying lens  18  and focusing lens  20  using fold mirrors  6  and  16 . Light source  2  is preferably a semiconductor laser, but any type of laser source may be employed. Further, other light sources may be suitable as well. Light beam  3  is a different wavelength than a laser beam (or beams) used to transfer an electronic image to plate  34 . However, light beam  3  and a “writing” laser beam (not shown) may use a portion of the same optical path in the invention herein. 
     Light beam  3  is focused into a small spot onto printing plate  34  using focusing lens  20  and hence may be referred to as an incident light beam  3 . Light beam  3  is directed onto plate  34  at a location above (or below) the spin axis  35  of drum  28 . This is to create diffuse light being reflected back toward focusing lens  20 , and avoid a direct reflected beam that would be present if light beam  3  were to be applied to plate  34  perpendicularly. 
     A portion of incident light beam  3  is reflected from printing plate  34  forming reflected light beam  5 . Light beam  5  passes back through focusing lens  20 , and magnifying lens  18 . Reflected light beam  5  is directed to beamsplitter  8  by fold mirror  16 . Beamsplitter  8  prevents reflected light beam  5  from passing to light source  2  by directing light beam  5  toward aspherical lens  10 . 
     Aspherical lens  10 , may be a cylindrical plano-concave lens that provides magnification (or in other words, image compression) in one axis only. Reflected light beam  5  illuminates a light sensor  12  after passing through aspherical lens  10 . 
     Light sensor  12  is preferably a quadcell light sensor that is well known in the art of imaging optics, but is not limited to such. Light sensor  12  creates an electrical signal representative of a focus condition of incident light beam  3  on plate  34  and is coupled to a controller  14 . 
     Controller  14  provides a drive signal to motor  22  that is mechanically coupled to focusing lens  20 . Controller  14  adjusts the spatial position of focusing lens  20  relative to printing plate  34  using motor  22 , to achieve optimum focus of incident light beam  3  upon plate  34 . An algorithm stored in controller  14  analyses the output from light sensor  12  to determine the position of focusing lens  20  by moving focusing lens  20  along a direction shown by arrow  24 . 
     Controller  14 , light sensor  12 , and motor  22  from a feedback loop for positioning focusing lens  20  using light reflected from plate  34 . This is the autofocus mode of operation. 
     FIGS. 2 a - 2   d  will be used to describe the operation of light sensor  12 . Light sensor  12  is composed of four quadrants  12 A- 12 D, each quadrant exposes an active semiconductor PN junction to incident light. When light illuminates any or all of the PN junctions (not shown) an electrical current is generated in each quadrant (note the light that actually illuminates light sensor  12  is reflected light beam  5 ). If light illuminates only one quadrant, electrical current is generated in only that particular quadrant. This provides an ability to discriminate between different shapes of light spots that illuminate light sensor  12 . FIG. 2 b  shows a small round light spot  36 A illuminating light sensor  12 . This case yields equal electrical currents generated in each quadrant and may be considered an in focus condition representing incident light beam  3  is in focus on printing plate  34 . 
     However, if light beam  3  illuminating printing plate  34 , and hence reflected light beam  5 , is out of focus, the size of light spot  36 A is much larger. Since the out of focus reflected light beam  5  passes through aspherical lens  10 , the beam is compressed in only one direction yielding a non-round light spot as shown in FIG. 2 c  as light spot  36 B. The amount of compression, and hence how elliptical light spot  36 B becomes, depends on how far out of focus reflected light beam  5  is. Depending if the out of focus condition is the result of focusing lens  20  being either too far or too close to printing plate  34 , light spot  36  appears angled left or right as shown in FIGS. 2 c  and  2   d  as  36 B and  36 C respectively. Proper rotational alignment between aspherical lens  10  and light sensor  12  is required in order to obtain focal discrimination as is well known in the art. If light spot  36 B or  36 C is perfectly vertical (or perfectly horizontal) and centered on light sensor  12 , equal currents would be generated in each quadrant preventing focal discrimination. Preferably, a 45 degree angle is introduced between aspherical lens  10  and light sensor  12 . 
     Referring to FIG. 2 c , quadrants  12 A and  12 C have unequal areas,  38  and  40 , illuminated by light spot  36 B. Hence, two different electrical currents are generated in the two quadrants  12 A and  12 C (and quadrants  12 B and  12 D). Many different combinations of using quadrants  12 A- 12 D are possible to determine a focus condition and are well known in the art and will not be elaborated upon. It is recognized that a focal condition may be determined using only two of the quadrants of light sensor  12 . 
     Referring to FIGS. 3-5, operation of the inventive system and method herein will be described. 
     The end  31  of printing plate  34  is referred to as the leading edge as it is the end first fed onto drum  28  during the plate loading process. Leading edge  31  is clamped to drum (or cylinder)  28  with leading edge clamp (or clamps)  30 . Trailing edge  33  of printing plate  34  is clamped to drum  28  using trailing edge clamp (or clamps)  32 . 
     Drum  28  is rotated about spin axis  35  to position leading edge clamp  30  (and by default leading edge  31  of plate  34 ) such that incident light beam  3  illuminates a portion of leading edge  31  in the middle of the plate. 
     Autofocus system  1  then focuses incident light beam  3  onto printing plate  34  such that light spot  36 A becomes small and generally round. 
     After achieving desired focus, via monitoring electrical signal(s) from light sensor  12  using controller  14 , focusing lens  20  is fixed in place, at the spatial position that yields the desired focus of incident light beam  3  (and hence reflected light beam  5 ). 
     Drum  28  is slowly rotated in the direction that causes incident light beam  3  to travel toward trailing edge clamp  32 , without causing incident light beam  3  to pass over leading edge clamp  30 . The focus condition of incident light beam  3  impinging upon plate  34  is monitored using light sensor  12  and controller  14  as drum  28  is rotated at least half the length of plate  34  (or alternatively half the distance between leading edge clamp  30  and trailing edge clamp  32 ). 
     If plate  34  is not securely clamped at both ends  31  and  33  by clamps  30  and  32  respectively, a gap  42  is introduced between plate  34  and drum  28  as shown in FIG.  4 . Gap  42  causes incident light beam  3  to be out of focus as plate  34  is now closer to lens  20  which has been spatially fixed. Light spot  36 A becomes elliptically shaped as shown in FIG. 2 c  or  2   d , and may even become line shaped if severely out of focus. Light sensor  12  provides an electrical signal representative of elliptically shaped light spot  36 B or  36 C to controller  14 . Controller  14  recognizes the electrical signal(s) representative of elliptically shaped light spot  36 B or  36 C to be different than the electrical signal(s) representative of small generally round light spot  36 A previously measured. If the difference between the electrical signals representative of light spots  36 A and  36 B(or  36 C) exceeds a predetermined value, plate  34  is determined to be unsecured on drum  28  by controller  14 . Controller  14  causes drum  28  to stop rotating and optionally provides an alarm  37 , visually and/or audibly, to an operator. A reset circuit  39  may be employed operative to prevent drum  28  from rotating until an operator resets the circuit. 
     If plate  34  is properly secured at both ends  31  and  33  by clamps  30  and  32  respectively, incident light beam  3  remains in focus, and light spot  36  remains generally round shaped. If drum  28  is rotated a predetermined distance without light beam  3  going out of focus, controller  14  determines plate  34  to be properly secured to drum  28 , and allows drum  28  to reach normal operating speed of rotation. 
     FIG. 5 shows one embodiment of providing a second mode of operation of autofocus system  1  for use with the invention herein. 
     A magnet  44 , preferably a permanent magnet, is fixedly coupled to focusing lens  20 . A Hall effect sensor  46 , operatively coupled to controller  14 , is fixedly positioned proximate to magnet  44 . An electrical signal is created by Hall effect sensor  46  representative of a spatial position of magnet  44 , and hence the spatial position of focusing lens  20 . The manner in which Hall effect sensor  46  and magnet  44  operates is well known and will not be described herein. In Hall mode, controller  14 , Hall effect sensor  46 , magnet  44 , and motor  22  form a second feedback loop for controlling a spatial position of focusing lens  20  independent of a focus condition of light beam  3 . 
     Once focusing lens  20  is fixed in place during autofocus mode of operation, autofocus system  1  then switches to Hall mode of operation. In Hall mode, controller  14  determines the value of a reference signal provided by Hall effect sensor  46  that corresponds to the in focus condition determined by the autofocus mode. 
     Controller  14  maintains focusing lens  20  in a fixed position by monitoring Hall effect sensor  46 , and adjusting the position of focusing lens  20  using motor  22 , to keep the reference signal provided by Hall effect sensor constant. 
     Though the invention herein has been described for verifying a printing plate (or other recordable substrate) is securely mounted to a surface, other mounting anomalies may also be detected with the invention. 
     For example, the invention herein may be used to detect and locate debris trapped between a printing plate and a support surface. Such a piece of debris could cause a bulge to appear in the plate thereby introducing artifacts while imaging the plate. Further, if the printing plate is accidentally buckled, kinked or bent during handling or loading of the plate, the invention herein may be used to detect and locate such an anomaly. The afore mentioned examples of mounting anomalies are cases where the media is also not considered securely mounted to a support surface. 
     Those skilled in the art will appreciate that numerous modifications and variations may be made to the above disclosed embodiments without departing from the spirit and scope of the present invention.