Patent Publication Number: US-6222565-B1

Title: Scanning device for scanning an image carrier

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a scanning device for scanning an image carrier, comprising a linear array of scanning elements, a holder for the array, which extends in the longitudinal direction of the array, and positioning means to maintain the scanning elements at a predetermined position from the image carrier. A scanning device of this kind is known from European Patent 0 401 316, which describes a scanning device for forming an image on a photoconductive image carrier by means of a linear array of co-operating LEDs and lenses. In this device the lens array focuses the light emitted by the array of LEDs into the plane in which the image carrier moves. In order to keep the linear array in the required position with respect to the imaging plane occupied by the image carrier, the arrays of LEDs and lenses in this known scanning device are fixed on a holder made from relatively thick metal and provided with stiffening ribs in order to keep the linear array in a predetermined focusing position throughout with respect to the image carrier. 
     Since in order to form an uninterrupted image the linear array holder can bear against the image carrier only outside its operative zone, the array is sensitive to sagging or deflection, and particularly in the case of a long linear array in large-format scanning devices in which the linear array is disposed beneath or above the imaging plane. This manifests itself in the form of locally non-sharp imaging. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide a scanning device of the type referred to in the preamble, wherein a linear imaging array can be maintained in a predetermined position with respect to the image carrier without the array having to be made in an extremely stiff construction. 
     To this end, according to the present invention, positioning means are formed by projections on the holder for the array, which projections extend substantially perpendicular to the array and parallel to one another, and an adjustment means which acts on the projections is provided to adjust the distance between the projections. As a result, the imaging array can, with simple means, be so adjusted that optimum focusing can be set at every part of the linear array, thus compensating for any array sag. 
     In one advantageous embodiment of the scanning device according to the present invention, the adjustment means comprises a pressure element which exerts on two spaced-apart projections, forces directed away from one another and operative in the longitudinal direction of the array. As a result, a linear array with the projections directed upwardly, which array sags in the middle due to its weight, is pressed straight. 
     Preferably, the pressure element comprises a first compression spring which presses against one side of a projection which is directed towards the other projection in order to press the two projections apart. As a result, a construction is obtained in which the spring force of the compression spring determines the force with which the linear array is pressed straight. 
     Further, preferably, the first compression spring is provided with first tensioning means which give the first compression spring an adjustable prestressing. As a result a very sensitive control is obtained for straightening the linear array, for example to compensate for other initial deviations in the straightness of the linear array. 
     In another embodiment of the scanning device according to the present invention, the adjustment means comprises a tension element which exerts on two spaced-apart projections, forces which are directed towards one another and which act in the longitudinal direction of the array. As a result, the sagging of a linear array disposed beneath the image carrier can be readily compensated. Another effect is that in the case of a linear array disposed above the image carrier and initially having an upwardly deflected form, straightening can be obtained in a simple manner. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description given hereinbelow and the accompanying drawings which are given by way of illustration only, and thus are not limitative of the present invention, and wherein: 
     FIG. 1 is a cross-section of a printing apparatus in which a scanning device according to the present invention is disposed in the form of a printhead; 
     FIG. 2 is a side elevation in detail of the printhead of FIG. 1; 
     FIG. 3 is a section taken along line III—III of FIG. 2; 
     FIG. 4 is a side elevation of another printhead according to the present invention; and 
     FIG. 5 is a cross-section taken along line V—V of FIG.  4 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The printing apparatus shown in FIG. 1 is provided with an image carrier in the form of a photoconductive drum  1  which is rotated at a uniform speed by drive means (not shown) in the direction of the arrow. 
     The photoconductive surface of the drum  1  is electrostatically charged by means of a charging device  2  disposed above the photoconductive drum  1 . A scanning device  3  in the form of an LED exposure array disposed next to the charging device  2  above the photoconductive drum  1  discharges the charged drum  1  image-wise in zones corresponding to the image to be formed and printed. 
     A scanning device  3  of this type which exposes in accordance with (black) image portions is generally referred to as a black writer. The LED exposure array  3  will be described in detail hereinafter. 
     A developing device  4  disposed next to the photoconductive drum  1  functions as a reverse developing device for covering the exposed areas of the photoconductor with a toner. A corona transfer device  5  disposed beneath the photoconductive drum  1  transfers the formed toner image to a receiving sheet fed along a sheet transport path  7  by a pair of transport rollers  6 . The toner image printed on the receiving sheet is then fused on the receiving sheet in a fixing device (not shown). 
     After the transfer of the toner image any remaining toner is removed from the photoconductive drum  1  in a cleaning device  8 , whereafter the photoconductive drum can be re-charged for the printing of a subsequent image. 
     The scanning device  3  shown in detail in FIGS. 2 to  5  comprises a linear LED array  10  and a Selfoc lens array  11  disposed at a specific distance from the LED array to focus the light emitted by the LEDs on to a narrow strip of the surface  12  of the photoconductive drum  1 . The LED array  10  is mounted on a baseplate  13  fixed on an extruded aluminium profile  14  provided with fins  15  which provide some rigidity for the profile and can also provide cooling for the scanning device. The Selfoc lens array  11  is contained in a slot  16  formed in an extruded profile  17  fixed on the baseplate  13 . Thus the extruded profiles  14  to  17  together with the baseplate  13  form a rigid unit for the LED array therebetween. 
     As clearly shown in FIG. 3 the Selfoc lens array  11  focuses the light emitted by the LEDs  10  onto the surface  12  of the photoconductive drum  1 . On imagewise selective triggering of the LEDs the Selfoc lens array projects a corresponding image on to the photoconductive drum  1 , which image delivers a print on to a receiving sheet in the manner indicated in the description of FIG.  1 . As clearly shown in FIG. 2, the extruded profile  14  projects on either side from the extruded profile  17 . U-shaped end blocks  18  and  19  are fixed on the projecting parts of the extruded profile  17 , e.g. by means of screws, at the locations indicated by references  20 . The limbs  21  and  22  of each U-shaped end block  18  and  19  and other the limbs  23  and  24  of each of the end blocks  18  and  19  extend parallel to the limbs  21  and  22  at a greater distance from the image plane  12  of the lens array. Thus the connecting members between the limbs  21 ,  23  and  22 ,  24  form projections  26  and  27  on the holder for the linear arrays of scanning elements  10  and  11 , which extend substantially perpendicularly to the array and parallel to one another. 
     The limbs  23  and  24  of the end blocks are provided with round holes  28  and  29 , the center-lines of which are situated in extension of one another and parallel to the linear LED array. The hole  29  in one of the end blocks ( 24 ) is provided with a screwthread. A pin  30  having the length of the linear arrays is provided at one end with a screwthread which fits in the screwthread in hole  29 . The other end of the pin  30  is smooth and fits slidingly in hole  28 . The head of the smooth end of the pin  30  is provided with a slot to enable pin  30  to be screwed further in or out of the screw hole  29  by means of a screwdriver. 
     The pin  30  is also provided with a collar  31  near the smooth end of the pin  30 . A compression spring  32  is fitted between this collar  31  and the end block  18 . On axial displacement of the pin  30  with respect to the end blocks  18  and  19  by means of a screwdriver the compression spring  32  is tensioned to a greater or lesser extent, so that it tends to press the limbs  23  and  24  apart by a variable force on the end blocks  18  and  19 , thus exerting a variable bending moment on the linear array  10  and  11 . The end blocks  18  and  19  are provided with adjustable supports  34  and  35  which bear against the photoconductive drum  1  in order to always hold the ends of the linear array at a distance from the photoconductive drum  1  such that a sharp image is obtained at the ends. 
     For the adjustment of the scanning device, the latter is placed in an optical measuring bench in the same position as the scanning device occupies in the printing apparatus. After adjustment of the scanning device by means of the adjustable supports, in order to obtain optimum image quality at the ends, the image quality in the middle of the linear array is measured. Any sagging of the linear array under the influence of gravity (indicated by a broken line in FIG. 2) results in a measurable unsharpness of the image. This deviation can be compensated by turning the pin  30 . With the increasing force of the compression spring  32 , the bending moment exerted on the linear array results in its displacement thereof in the upward direction. The turning of the pin  30  is stopped when a sharp imaging is also measured in the middle of the array. The pin  30  is locked in this position by means of a nut  33 . Of course, it is possible to dispense with a screwthread connection between the pin  30  and the hole  29  and to obtain axial displacement of the pin  30  simply by turning the nut  33 . 
     A sag of about 200 mm is not unusual, particularly in the case of a printing apparatus for wide formats in which the linear array may have a length of about 1 meter. With the construction according to the present invention this sag can be readily compensated for without excessively heavy construction being required. 
     FIGS. 4 and 5 show a scanning device according to the present invention for use in a printing apparatus in which the scanning device is disposed underneath the photoconductive drum. This scanning device differs from the scanning device  3  shown in FIGS. 2 and 3 in that instead of pin  30 , a longer pin  40  is used which projects beyond end block  18 . A collar  41  is fixed on this projecting end and a compression spring  42  is disposed between the collar  41  and the outside of the end block  18 . 
     Sagging of the holder  14  for the linear arrays (indicated by a broken line in FIG. 4) is compensated by turning the pin  40  in order to give the compression spring  42  a greater tensioning force so that the middle of the linear array can be moved upwards. As shown in broken lines in FIGS. 4 and 5, the collar  41  and compression spring  42  can also be disposed on pin  40 . As a result, a linear array can also be adjusted in order to compensate for other forms of non-linearity, such as a curvature of the extruded profiles  14  in opposition to and greater than the curvature that the holder for the linear array in the printing device experiences as a result of sagging due to its weight. 
     The use of compression springs between the holder and compression rod (pin  30  or  40 ) to compensate for sagging of the LED array holder has the advantage over a compression rod acting directly on the holder projections, e.g. by a right-hand and left-hand screwthread at the ends of the compression rod, that the compression rod must make a larger angular rotation for a specific sag correction, and can therefore be adjusted much more sensitively. 
     As shown in FIG. 1, the scanning device  3  is mounted in pivoting arms  36  which are adapted to hinge about axis  35 . Thus the scanning device  3  can be swung away for maintenance without losing the setting executed to compensate for the sag. 
     The scanning device according to the present invention can also be used for strip-wise scanning of an original with a lens array, for imaging on to a linear array of light-sensitive elements, e.g. CCDs, and the like. 
     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.