Abstract:
A continuous-sheet printing tandem electrophotography system for printing a continuous sheet includes first and second electrophotography units. A first size of the continuous sheet is measured before an image printed by the first electrophotography unit with a first parameter value is fused on the continuous sheet. A second size of the continuous sheet is measured after the image printed by the first electrophotography unit is fused on the continuous sheet. The second electrophotography unit then prints the continuous sheet with a second parameter value that is determined by a size difference between the first and the second sizes. The first and the second sizes include a page length and a page width of the continuous sheet. The parameter values include a print speed, a polygon mirror rotating speed, a video clock frequency, and a laser power.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a continuous-sheet printing tandem electrophotography system having a plurality of electrophotography apparatuses coupled to one another for printing a continuous sheet of a recording medium, and a method of printing a continuous sheet. In particular, the present invention relates to the correction of a print position error between the both sides of a printed continuous sheet. 
     2. Description of the Related Art 
       FIG. 1  schematically illustrates the principle of operation of a conventional electrophotography apparatus. A laser light source  301  is turned on or off in accordance with image data transmitted in synchronization with a video clock. The laser light source  301  emits a laser beam  302  that is reflected by a polygon mirror  303  as it rotates at a certain angular velocity, thereby scanning the surface of a photosensitive drum  304  rotating at a predetermined velocity with the laser beam  302 . As a result, a latent image is formed on the surface of the photosensitive drum  304 . 
     A beam detector  305  is disposed along the scanning line of the laser beam  302 . Upon detection of the laser beam  302 , the beam detector  305  outputs a horizontal synchronization signal, in accordance with which the output timing of the image data is determined so that an accurate write start position can be obtained. 
     The latent image on the photosensitive drum  304  is then developed using a magnetic brush of a two-component developer consisting of a mixture of a toner  306  and a carrier at a certain ratio. Specifically, the toner  306  is caused to attach to the surface of the photosensitive drum  304 , thereby making the latent image visible as a toner image. 
     A continuous sheet  308  is transported by tractors or rollers  307  at a speed corresponding to the circumferential speed of the photosensitive drum  304 , and the toner image on the photosensitive drum  304  is transferred onto the continuous sheet  308  by a transfer unit  309 . The toner image on the continuous sheet  308  is then fused thereon by pressing and heating by a fusing unit including rollers  310 , thus completing the print process. 
     In this case, it is necessary to synchronize the rotating speed of the polygon mirror  303  as it reflects the laser beam, the rotating speed of the photosensitive drum  304 , and the sheet transport speed. For this purpose, a single oscillator is generally used. Specifically, the individual devices are driven in accordance with a control clock, so that their relative synchronization can be ensured as long as the control clock is generated by the same oscillator. If the devices are controlled by different oscillators, the difference in the clock signals accumulates in the continuous-sheet electrophotography apparatus and the devices lose synchronization, rendering the realization of normal apparatus performance impossible. 
     The frequency of the control clock is uniquely determined by the optical specifications of the apparatus, a sheet transport speed which is equivalent to the print speed, and the photosensitive drum rotation speed. Another condition is that there should be only one oscillator, as mentioned above. Thus, the oscillating frequency is calculated from the least common multiple of the clock frequencies required by the individual devices, and an appropriate crystal oscillator is selected from the viewpoint of accuracy. 
     A continuous-sheet printing tandem electrophotography system is known in which a couple of continuous-sheet electrophotography apparatuses of the aforementioned type are disposed upstream and downstream along the transport of a continuous sheet, for printing both sides of the sheet, for example. Such a system has a market under the category of electrophotography equipment as a relatively simple commercial printing machine capable of high-speed, high-availability, and low-cost operations. Although there are also special-purpose offset printing machines, such as rotary presses, these are designed to compensate for the time-consuming setup process with the number of printed pages and are therefore not suitable for low-volume production. Thus, a small-volume, small-lot commercial printer market is being developed in which electrophotography systems and offset printing machines are competing against each other. 
     There has recently been a growing demand for coupling a plurality of continuous sheet electrophotography apparatuses for printing.  FIG. 2  schematically shows a continuous-sheet printing tandem electrophotography system. In this system, two continuous-sheet electrophotography apparatuses of the type shown in  FIG. 1  may be coupled and used in various combinations. For example, an upstream device  401  to the right in  FIG. 2  prints an upper surface of a continuous sheet, followed by the printing of a lower surface by a downstream device  402  to the left, thus forming a double-side printing system. Alternatively, the upstream device  401  may use black toner while the downstream device  402  may use a color toner, thereby forming a spot color printing system. In the illustrated example, a sheet inverting unit  403  is provided between the upstream device  401  and the downstream device  402 , forming a double-side printing system. 
     One drawback of this system is that when a double-side printing is performed, thermal contraction of the sheet occurs in the fusing unit of the upstream device  401 , so that a print position error is caused when the lower surface is printed by the downstream device  402 . Solution of the problem is earnestly desired because the above system enables the small-volume, small-lot production of printed matter for commercial printing purposes by a simple operation. 
     Various methods for correcting the contraction of the sheet have been proposed, such as Japanese Laid-Open Patent Application Nos. 2004-347842 and 2005-186614 teaching controlling the operating frequency of a laser clock, the speed of a polygon mirror motor, or the PWM output of laser power. However, these methods are all directed to electrophotography apparatuses using cut-sheets, where the upper and lower surfaces of a cut-sheet are printed in a single printing system along separate time axes by switching control values and by inverting the cut-sheet. Although the time for transition between the control values is ensured during the time of no printing between pages, the conventional methods do not take into consideration the decrease in throughput, which is a serious concern from the viewpoint of commercial printing. Further, the aforementioned related art does not provide any quantitative definition concerning main and sub scan operations and laser power correction. 
     In a continuous-sheet tandem printing system using a continuous sheet, the operation of one printing unit may need to be temporarily stopped when the individual printing units are allocated different numbers of pages to process, thus resulting in a decrease in throughput. If a sheet stays between the upper- and lower-surface print units, problems other than a print quality problem may be caused. Therefore, it is necessary for the upper- and lower-surface print units to process the same number of pages along the same time axis, and to achieve print position alignment between the lower and upper surfaces when a sheet contraction develops. 
     SUMMARY OF THE INVENTION 
     It is a general object of the present invention to provide a continuous-sheet printing tandem electrophotography system and a method of printing a continuous sheet by which one or more of the aforementioned problems of the related art are eliminated. 
     A more specific object of the present invention is to provide a continuous-sheet printing tandem electrophotography system by which a high-quality printed output having no print position error can be obtained. 
     According to one aspect of the present invention, a continuous-sheet printing tandem electrophotography system for printing a continuous sheet includes a first electrophotography unit disposed upstream of a direction of transport of the continuous sheet and configured to print a first image on the continuous sheet with a first parameter value; a second electrophotography unit disposed downstream of the direction of transport of the continuous sheet and configured to print a second image on the continuous sheet with a second parameter value; a size measuring unit configured to measure a first size of the continuous sheet before the first image is printed on the continuous sheet by the first electrophotography unit, and configured to measure a second size of the continuous sheet after the first image is printed on the continuous sheet by the first electrophotography unit; a control unit configured to compare the first size and the second size of the continuous sheet in order to obtain a difference value indicating a size difference between the first and the second sizes. The second parameter value is determined by the difference value obtained by the control unit. 
     According to another aspect of the present invention, a method of printing a continuous sheet by an electrophotographic process includes the steps of measuring a first size of the continuous sheet before the continuous sheet is printed; printing a first image on the continuous sheet with a first parameter value; measuring a second size of the continuous sheet after the first image is printed on the continuous sheet; comparing the first size and the second size of the continuous sheet in order to obtain a value indicating a size difference between the first and the second sizes; and printing a second image on the continuous sheet after the first image is printed thereon, with a second parameter value that is determined by the size difference between the first and the second sizes of the continuous sheet. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other objects, features and advantages of the present invention will become apparent upon consideration of the specification and the appendant drawings, in which: 
         FIG. 1  schematically shows an electrophotography apparatus according to the related art; 
         FIG. 2  shows a continuous-sheet printing tandem electrophotography system according to the related art; 
         FIG. 3  shows a block diagram of a continuous-sheet printing tandem electrophotography system according to an embodiment of the present invention; 
         FIGS. 4(   a ) and  4 ( b ) illustrate a contraction of a sheet after a fusing process in an upstream device of the system shown in  FIG. 3 ; 
         FIG. 5  shows a table indicating the relationships between an upstream device and a downstream device in terms of print speed, the rotating speed of the polygon mirror, video clock frequency, and laser power; and 
         FIG. 6  shows an arrangement of sensors relative to a printed sheet according to an embodiment of the present invention for measuring a page length and a sheet width of the sheet simultaneously. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, embodiments of the present invention are described.  FIG. 3  shows a block diagram of a continuous-sheet printing tandem electrophotography system including an upstream device  401  and a downstream device  402  according to an embodiment of the present embodiment. In this system, both sides of a continuous sheet are printed, as in the case of  FIG. 2 . 
     In  FIG. 3 , the designation of each of the units of the upstream device  401  and the downstream device  402  is suffixed with “A” or “B”, indicating that it belongs to the upstream device  401 (A) or the downstream device  402 (B). The suffixes “A” and “B”, however, are omitted in the following description of the embodiments whenever appropriate. 
     As shown in  FIG. 3 , each of the upstream device  401  and the downstream device  402  includes a main control unit  118 , an oscillator  101 , a selector  103 , a image data output unit  106 , an exposure control unit  111 , a laser light source  112 , a polygon mirror  114 , a drive motor  119  for rotating a photosensitive drum (not shown in  FIG. 3 ), and a drive motor  120  for rotating rollers of a sheet transport unit (not shown in  FIG. 3 ). 
     The oscillator  101  includes plural oscillators  101   1  to  101   n  generating different frequencies. The selector  103  selects one of the oscillators  101   1  to  101   n  in accordance with a clock select signal from the main control unit  118 , and outputs a video clock F (F′). 
     Input image data is fed to the image data output unit  106 , which processes the image data into image data that is outputted to the exposure control unit  111  in synchronism with the video clock F (F′). The main control unit  118  also outputs a laser power setting signal to the exposure control unit  111 . 
     The exposure control unit  111 , to which the image data and the laser power setting signal are fed, then outputs a laser on/off signal and a laser power signal P(P′) to a laser light source  112 . 
     In accordance with the input laser on/off signal, the laser light source  112  controls the emission of a laser beam. When the laser light source  112  emits the laser beam, the laser power is controlled in accordance with the laser power signal P(P′). The laser beam emitted by the laser light source  112  is reflected by the polygon mirror  114  rotating at a certain angular velocity, thus scanning the surface of the photosensitive drum with the laser beam. The angular velocity of the polygon mirror  114  is determined by a rotation drive clock that is outputted by a variable frequency output unit  116 . The rotation drive clock is switched by a print speed signal V(V′) from the main control unit  118 . 
     The rotation drive clock is also fed to the drive motor  119  for driving the photosensitive drum and to the drive motor  120  for driving the sheet transport rollers. Thus, the rotation speed of the photosensitive drum and the sheet transport speed, i.e., print speed, are controlled by the rotation drive clock. 
     A latent image formed on the surface of the photosensitive drum by exposure to the laser beam is developed and then transferred onto a sheet (not shown in  FIG. 3 ) as a toner image. The toner image is then fused onto the sheet by the application of heat and pressure by a fusing unit (not shown in  FIG. 3 ). 
     With reference to  FIG. 4 , contraction of the sheet due to the application of heat by the fusing unit is described, by referring to the upstream device  401  of the continuous-sheet printing tandem electrophotography system. 
       FIG. 4(   a ) shows a sheet  201   a  before fusing in the upstream device  401  for upper surface print. The sheet  201   a  has a nominal page length L and a nominal page width W.  FIG. 4(   b ) shows a sheet  201   b  that has been fused by the fusing unit of the upstream device  401 . The sheet  201   b  has a page length L′ and a page width W′, indicating a print position error due to thermal contraction. 
     With reference to  FIG. 5 , a method of correcting the print position error in the contracted sheet by adjusting the print speed, the rotating speed of the mirror, the video clock frequency, and the laser power in the downstream device is described.  FIG. 5  shows a table indicating the relationships between the upstream and downstream devices in terms of the aforementioned parameters. 
     For example, the upstream device  401  has a print speed V and a page length L, and the downstream device  402  has a print speed V′ and a page length L′. Because a condition “L/V=L′/V′=constant” must be satisfied in order for the upstream and downstream devices to have the same page print time, the print speed of the downstream device  402  is V′=(L′/L)×V. 
     The page length L may be measured by printing a mark at the head of each page and then optically measuring the mark intervals after the transfer step in the upstream device  401 , using a reflective optical sensor. After the sheet has passed through the fusing unit of the upstream device  401 , the mark intervals may be measured again in the downstream device  402  before the transfer step, thus determining the page length L′. 
     If the rotating speed (angular velocity) of the polygon mirror is changed from R to R′ by changing the print speed from V to V′, the number of scans, i.e., the rotating speed of the mirror, per unit print speed is constant. Because R/V=R′/V′=constant, when the rotating speed of the polygon mirror of the upstream device  401  is R, the rotating speed of the polygon mirror of the downstream device  402  is R′=(V′/V)×R=(L′/L)×R. 
     The video clock frequency F′ is related to the correction for the change in the rotating speed (angular velocity) of the polygon mirror, and to the correction for the contraction of the sheet in its width direction. When print speed is changed from V to V′, the rotating speed of the mirror is changed from R to R′. When video clock time T=1/F, and the number of items of image data per scan is n, where the distance per scan is constant, F′=1/T′=(R′/R)×F=(L′/L)×F since R×T×n=R′×T′×n=constant. 
     On the assumption that the distance per scan should be corrected from W to W′ by the video clock frequency when the sheet width has changed from W to W′, the frequency is switched to F′=(W/W′)×F because W/(T×n)=W′/(T′×n)=constant. Thus, a correction is made so that F′=(L′/L)×(W/W′)×F. When the ratio of change in sheet width (W′/W) is equal to the ratio of change in sheet length (L′/L), F′=F; namely, the video clock frequency F′ of the downstream device  402  is the same as the video clock frequency F of the upstream device  401 , and therefore no correction is required. 
     As to the laser power P′, when the energy per unit scan is constant, since P/(R×T×n)=P′/(R′×T′×n)=constant, P′=(P×R′)/(R×T′)/T=(L′/L)×(W′/W)×P. 
     For measuring the sheet widths W and W′, marks may be printed at the side edges of the sheet in its width direction (perpendicular to the sheet transport direction), and then the mark intervals may be optically measured after the transfer step in the upstream device  401  to determine the sheet width W. Thereafter, after the sheet has passed the fusing unit of the upstream device  401 , the mark intervals may be optically measured in the downstream device  402  prior to the transfer step in order to determine the sheet width W′. 
     Thus, referring to  FIG. 5 , when the upstream device  401  has print speed V, the print speed of the downstream device  402  is set so that V′=(L′/L)×V. When the rotating speed of the polygon mirror in the upstream device  401  is R, the rotating speed of the polygon mirror in the downstream device  402  is set so that R′=(V′/V)×R=(L′/L)×R. When the video clock frequency of the upstream device  401  is F, the video clock frequency of the downstream device  402  is set so that F′=(L′/L)×(W/W′)×F. When the upstream device  401  has a laser power P, the laser power of the downstream device  402  is set so that P′=(L′/L)×(W′/W)×P. 
     The aforementioned print speed may be set by adjusting the control clock supplied to the drive motor  119  for the photosensitive drum and the drive motor  120  for the sheet transport unit. The rotating speed of the polygon mirror  114  may be set by adjusting the control clock for the corresponding drive motor (not shown). The video clock frequency may be adjusted by selecting the oscillator  101  appropriately. The laser power may be adjusted by adjusting the current supplied to the laser light source  112 . 
     With reference to  FIG. 6 , a method of measuring the page length L(L′) and the sheet width W(W′) of the sheet  201  simultaneously is described. As shown in  FIG. 6 , marks M 1  and M 2  are printed at a front edge of the page of the sheet  201 , one on either side in the width direction. Marks M 3  and M 4  are also printed at the front edge of the next page, one on either side in the width direction. While in accordance with the present embodiment these marks M 1  to M 4  are lines inclined at an angle (45°) with respect to the transport direction of the sheet  201 , they may be triangular in shape in another embodiment. 
     On a line extending through the marks M 1  and M 3 , a reflective optical sensor S 1  is disposed. A reflective optical sensor S 2  is disposed on a line extending through the marks M 2  and M 4 . In the upstream device  401 , the optical sensors S 1  and S 2  are disposed upstream of the fusing device in the sheet transport direction. In the downstream device  402 , similar optical sensors S 1  and S 2  are disposed upstream of the fusing device in the sheet transport direction. Based on the timing of detection of the interval between the marks M 1  and M 3  (M 2  and M 4 ), and the interval between the marks M 1  and M 2  (M 3  and M 4 ) with the optical sensors S 1  and S 2  in the upstream and downstream devices  401  and  402 , the page length L(L′) and the sheet width W(W′) of the sheet  201  are simultaneously measured. 
     Detection signals (sheet information) from the optical sensors S 1  and S 2  in the upstream device  401  are fed to the main control unit  118 A of the upstream device  401  and the main control unit  118 B of the downstream device  402 . Detection signals (sheet information) from the optical sensors S 1  and S 2  in the downstream device  402  are supplied to the main control unit  118 B of the downstream device  402 . 
     In accordance with the present embodiment, both sides of a continuous sheet are printed by the upstream device  401  and the downstream device  402 . However, the present invention is not limited to such an embodiment. In another embodiment, the upstream device may print with a black toner and the downstream device may print with a color toner in a spot color print system. 
     The sensors for measuring the page length L′ and the page width W′ of the continuous sheet may be disposed at any location between the downstream of the fusing unit of the upstream device  401  and the upstream of the fusing unit of the downstream device  402 . 
     In accordance with another embodiment of the present invention, processing of a lower surface of a sheet medium may be adjusted depending on any change in the shape of the sheet that may be caused by the processing of an upper surface of the sheet medium. 
     Although this invention has been described in detail with reference to certain embodiments, variations and modifications exist within the scope and spirit of the invention as described and defined in the following claims. 
     The present application is based on the Japanese Priority Application No. 2008-216645 filed Aug. 26, 2008, the entire contents of which are hereby incorporated by reference.