Patent Publication Number: US-6714758-B2

Title: Printing system that positions web at accurate waiting position

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a printing system that prints images on both surfaces of a web. 
     2. Related Art 
     There has been proposed a printing system that prints images on both surfaces of a web, which is in a continuous belt shape. The printing system includes a pair of print devices arranged in a row, wherein a web is formed with an image on its front surface by a first print device, discharged from the first print device, turned upside down by a reversing device, fed into a second print device, and then formed with an image on its rear surface. 
     FIG. 1 shows components of an electrophotographic print device that can be used in such a printing system. In this device, a photosensitive drum  101  is formed with an electrostatic latent image on its surface at a position EP. Then, toner is selectively supplied onto the surface of the photosensitive drum  101 , so that a visible toner image corresponding to the electrostatic latent image is formed on photosensitive drum  101 . When the toner image comes into contact with a web W at a transfer point TP, the toner image is transferred onto the web W. Afterwards, the toner image is thermally fused onto the web. 
     This thermal fusion shrinks the web, so that the web has a length shorter than its original length. Accordingly, when an electrophotographic print device is used as the first print device, then there is a danger of positional deviation between the front surface image and the rear surface image on the web W. 
     However, when n-number of sprockets are formed in each page of the web, then it is possible to avoid such a positional deviation by transporting the web while counting the number of the sprockets in the second print device. That is, transporting the web by n-sprocket-worth of distance means transporting the web by a single-page worth of distance regardless of whether or not the web has shrunk. Because a location of the sprocket in each page head of the web never changes, it is possible to transfer a single-page worth of toner image from the photosensitive drum  101  onto a corresponding page of the web by transporting the web by a distance equivalent to n-number of sprockets. Subsequent pages can also be formed with corresponding single-page worth of images in the same manner. 
     In other words, if each page head of the web W meets a corresponding page-head position of the photosensitive drum  101  at the transfer point TP, the rear surface image is formed in a positional alignment with a corresponding front surface image, and there is no danger that positional deviation accumulates to greatly deviate the positional relationship between downstream-side front and rear surface images. 
     Here, when a printing process is started, a web transport speed is accelerated to a predetermined speed. Since it is necessary to reach the predetermined speed before the page head of the first page of the web reaches the transfer point TP, the web is positioned, after a previous printing operation has completed, such that the page head of a first page for a subsequent printing operation locates at a predetermined waiting position WP. The waiting position WP is downstream from the transfer point TP with respect to the web transport direction by a distance α, which is required to reach the predetermined speed. 
     However, when the last-page images are completely transferred onto the web W at the transfer point TP, the page head of the first page for the subsequent printing process has already passed the waiting position WP. Therefore, after the printing has completed, the web W is transported back to the waiting position in the following manner. 
     When an electrostatic latent image for the last page is completely formed on the photosensitive drum  101 , a CPF-OFF signal is generated. When a predetermined time T1 elapses after the CPF-OFF signal was generated, then a PF position clear signal is generated. Here, the time duration T1 is a time between when the CPF-OFF signal is generated and when the page head of the subsequent first page reaches the waiting position WP, and is expressed by the formula: 
     
       
           T= ( L 1−α)/ VP    (1)  
       
     
     wherein L1 is a moving distance of the photosensitive drum  101  from the position EP to the transfer point TP; 
     α is a distance from the waiting position WP to the transfer point TP; and 
     VP is a process speed of the second print device, which equals to the rotational speed of the photosensitive drum  101 . 
     Upon reception of the PF position clear signal, a measuring unit starts measuring a web transport distance. After the toner image for the last page is completely transferred onto the web W, the web transport is stopped. Then, the web W is transported in a reverse direction by the amount of the web transport distance measured by the measuring unit. In this manner, the web is transported back to the waiting position WP. 
     There is also provided a printing system of a type that uses a web formed with no sprockets. In this type of printing system, positioning marks are used instead of the sprockets for achieving the positional alignment between the front-surface images and the rear-surface images. More specifically, the first print device prints positioning marks on a predetermined position in each page in addition to the images. A detection unit of the second print device detects the positioning marks and outputs output signals accordingly. Then, the second print device controls the web transport speed such that the output timings of the output signals have the constant phase with respect to CPF-N signals which are generated periodically. The control of the web transport speed is necessary since the web W has a different length between when the front surface printing and when the rear surface printing as mentioned above. 
     SUMMARY OF THE INVENTION 
     As described above, when the web with no sprockets is used, the web transport speed is controlled to change during the printing in the above described manner. Therefore, if the PT position clear signal is output when the predetermined time T1 elapses after the CPF-OFF signal was generated, then the web W may not be positioned at the waiting position WP. 
     Such a problem does not occur in the conventional printing system of a type that uses a web W with sprockets, since the web transport speed can be maintained constant in this case. 
     In the view of foregoing, it is an object of the present invention to overcome the above problems, and also to provide a method for accurately positioning a web with no sprockets at a predetermined waiting position in a print device. 
     In order to achieve the above and other objects, according to the present invention, there is provided a printing system including a first printing means for printing images on a first surface of a web, a second printing means for printing images on a second surface of the web, and a control means for controlling both the first and second printing means. The second printing means includes a photosensitive member, an irradiating unit, a developing means, a calculating means, a transport means, and a measuring means. The irradiating unit irradiates a laser light onto the photosensitive member for forming latent images thereon. The control means generates a reference signal when the irradiating unit completes irradiating a laser light for a last page image. The developing means develops the latent images into toner images. The calculating means calculates a time duration required for the web to reach a predetermined waiting position after the reference signal was generated. The transport means stops transporting the web in a forward direction after a last-page toner image is completely transferred from the photosensitive member onto the web. The measuring means measures a web transport distance between when the time duration elapses from when the reference signal was generated and when the transport means stops transporting the web in the forward direction. The transport means transports the web in a reverse direction by the web transport distance measured by the measuring means so as to transport the web back to the web waiting position. The calculating means calculates the time duration based on a web transport speed at the time of when the reference signal was generated. 
     There is also provided a control method for controlling a second printing means of a printing system that includes a first printing means for printing images on a first surface of a web and the second printing means for printing images on a second surface of the web, the second printing means including a photosensitive member, an irradiating unit that irradiates a laser light onto the photosensitive member, and a web transport means for transporting the web. The control method includes the steps of a) generating a reference signal at a time of when the irradiating unit completes irradiating a laser light for a last page image, b) calculating a time duration required for the web to reach a predetermined waiting position after the reference signal was generated, c) controlling the web transport means to stop transporting the web in a forward direction after a last-page image is completely transferred from the photosensitive member onto the web, d) measuring a web transport distance by which the web has been transported between when the time duration has elapsed from when the reference signal was generated and when the web transport was stopped in the step c), and e) controlling the web transport means to transport the web back to the waiting position, by transporting the web in a reverse direction by the web transport distance measured in the step d). The time duration is calculated in the step b) based on a web transport speed at the time of when the reference signal was generated. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
     FIG. 1 is an explanatory plan view of components of a conventional print device; 
     FIG. 2 is a perspective phantom view of a printing system according to an embodiment of the present invention; 
     FIG. 3 is a plan view showing an internal configuration of a print device of the printing system; 
     FIG. 4 is a plan view of a web printed with positioning marks; 
     FIG. 5 is an explanatory view of web transport control of the embodiment of the present invention; and 
     FIG. 6 is a block diagram of a controller of a second print device of the printing system. 
    
    
     PREFERRED EMBODIMENT OF THE PRESENT INVENTION 
     Next, a control method according to an embodiment of the present invention will be described with reference to the attached drawings. 
     As shown in FIG. 2, a printing system  100  according to the present embodiment includes a pair of print devices P 1  and P 2 , an inversion device T disposed between the print devices P 1  and P 2 , and a controller  17  for controlling both the print devices P 1  and P 2 . First, configuration of the print device P 1  will be described. Here, since the print devices P 1  and P 2  have basically the same configuration, only explanation for the print device P 1  will be provided. Also, since the inversion device T is well known in the art, explanation thereof will be omitted. 
     As shown in FIG. 3, the print device P 1  includes a pair of transport rollers  8 ,  9 , a printing unit  10 , a transport belt  11 , a buffer plate  12 , a fixing unit  13 , a discharge roller  14 , a swing fin  15 , and a mark sensor  16 . The transport roller  8  is a drive roller having its own driving source, and the transport roller  9  is a driven roller that is urged onto the transport roller  8  via a web W by an urging force of a spring  9   a . The transport belt  11  is wound around and extending between a driving roller  11   a  and a driven roller  11   b.    
     Rotation of the transport rollers  8 ,  9  transports the web W to the printing unit  10 , which is an electrophotographic print unit in this embodiment. The printing unit  10  includes a photosensitive drum  101 , a corona charging unit  102 , a light source  103 , a developing unit  104 , and a transfer unit  105 . When the photosensitive drum  101  starts rotating, the corona charging unit  102  is applied with a high voltage so as to uniformly charge the surface of the photosensitive drum  101 . The light source  103 , which is formed of a semiconductor laser or a light-emitting diode, irradiates a light beam on the photosensitive drum  101 , whereby an electrostatic latent image is formed on the photosensitive drum  101 . 
     When the electrostatic latent image comes into confrontation with the developing unit  104 , the electrostatic latent image is developed into a visible toner image on the photosensitive drum  101 . Thus formed toner image is transferred onto a front surface of the web W by the transfer unit  105  having an opposite polarity from that of the toner image. The web W with the toner image transferred thereon is supplied onto the transport belt  11 , and further transported along the buffer plate  12 . Although not shown in the drawings, there is provided a suction member that enables the transport belt  11  to transport the web W with its rear surface attached to the transport belt  11  by generating suctioning force. Then, the web W reaches the fixing unit  13 . 
     The fixing unit  13  includes a pre-heater  13   a , a heat roller  13   b , and a pressure roller  13   c  that presses against the heat roller  13   b , thereby defining a nip portion therebetween. The web W having reached the fixing unit  13  is preheated by the pre-heater  13   a , and then further transported through the nip portion between the pre-heater  13   a  and the heat roller  13   b . At this time, the toner image is thermally fused onto the web W. 
     The web W discharged from the fixing unit  13  is further transported to the discharge roller  14 ., and usually the web W is folded back and forth into an accordion fold by the swing movement of the swing fin  15  and stored in the print device P 1 . However, because the print device P 2  is disposed behind the print device P 1  in this printing system  100 , the web W discharged from the fixing unit  13  is discharged outside the print device P 1  via the discharge roller  14 . Thus discharged web W is inverted upside down by the inversion device T and then supplied into the print device P 2  where images are formed on a rear surface of the web W. 
     The mark sensor  16  is for detecting positioning marks (described later) formed in the web W and outputting mark detection signals. 
     Next, printing operation of the printing system  100  will be described. 
     First, as shown in FIG. 4, the first print device P 1  forms on the front surface of the web W an image Im based on print data and in addition the positioning mark (toner marks) Rm at the page head of each page. The same unit can be used to form both the positioning mark Rm and the image Im, or a separate unit can be provided for forming the positioning mark Rm. In the present embodiment, the same unit is used to form both the positioning mark Rm and the image Im, and the positioning mark Rm is formed at the same time as the image Im. 
     The web W discharged from the first print device P 1  is inverted upside down by the inverting unit T, and then supplied into the second print device P 2 . By inverting the web W upside down by the inverting unit T, the front surface of the web W formed with the images Im and the positioning marks Rm comes into confrontation with a detection surface of the mark sensor  16  in the print device P 2 , and the rear surface of the web W, which is still unprinted at this time, comes into confrontation with the surface of the photosensitive drum  101 . 
     In addition to the above configuration, the second print device P 2  further includes a controller  20  shown in FIG. 5 including a control unit  210 , a-memory  213 , a counter  214 , an encoder  220 , and a web-transport motor  230 . Various signals from the controller  17  and a mark detection signal from the mark sensor  16  are input to the control unit  210 . Details will be described later. 
     Each time the light source  103  of the second print device P 2  starts irradiating a laser light for a page, and the controller  17  generates a web-transport control signal (hereinafter referred to as “CPF-N signal”), which is input to the control unit  210 . 
     Next, basic principles behind the control for matching positions of images on the front and rear surfaces of the web W will be described. 
     FIG. 6 is a schematic view for explaining positioning control operations. During printing operations, the photosensitive drum  101  rotates at a predetermined process speed VP, and toner images formed on the photosensitive drum  101  are transferred onto the surface of the web W at a transfer point TP shown in FIG. 6 where the photosensitive drum  101  contacts the web W. The control unit  210  controls a web-transport speed through the web-transport motor  230  such that a positioning mark Rm on the web W and a corresponding position PP that is imaginary defined on the surface of the photosensitive drum  101  meet at the transfer point TP in order to achieve the positional alignment between the front-surface images and the rear-surface images. 
     In other words, the position PP indicates a position of a page head on the photosensitive drum  101 . As mentioned above, in the print device P 2 , each time the light source  103  starts irradiation for a page, the controller  17  produces the CPF-N signal. Because the photosensitive drum  101  rotates at the fixed process speed VP, the position PP reaches the transfer point TP at the cycle of the CPF-N signal, that is, each time the web W is transported by the length of CPF-N signal. Accordingly, by controlling the web-transport speed such that the difference between the generation timing of the CPF-N signal and the detection timing of the positioning mark Rm is fixed, the position PP on the photosensitive drum  101  and the corresponding positioning mark Rm at the page head of the web W can meet at the transfer point TP. 
     As shown in FIG. 6, there is a moving distance L1 of the photosensitive drum  101  from a position EP to the transfer point TP. The position EP is where the laser beam from the light source  103  is irradiated on the photosensitive drum  101 . Also, there is a moving distance L2 of the web W from a detection point DP where the mark sensor  16  detects the positioning mark Rm to the transfer point TP. 
     In order to make the position PP and the corresponding positioning mark Rm to reach the transfer point TP at the same time, the position PP should be located upstream from the transfer point TP by the distance L2 at the time of when the mark sensor  16  detects the corresponding positioning mark Rm at the detection point DP that is upstream from the transfer point TP by the distance L2. 
     In the present embodiment, “control timing” will be referred to a theoretical detection timing of the positioning mark Rm when the web W is being transported in an appropriate web-transport speed wherein the positioning mark Rm will meet a corresponding position PP at the transfer point TP. With this definition, appropriate positioning of a rear-surface image is achieved by controlling the web transport speed such that the actual detection timing constantly matches the control timing. 
     Next, the positioning control of the present embodiment will be described. 
     As described above, a CPF-N signal is generated each time the light source  103  of the second print device P 2  starts irradiating a laser light for a page. Accordingly, a first CPF-N signal is generated when the light source  103  starts irradiating a laser light for a first page. Then, a second CPF-N signal is generated when the light source  103  starts irradiating a laser light for a second page. Here, in order for a position PP indicating a page head position of the second page on the photosensitive drum  101  to meet the corresponding positioning mark Rm at the transfer point TP, it is necessary that the corresponding positioning mark Rm be detected at the time of when the position PP on the photosensitive drum  101  reaches a position FP which is upstream from the transfer point TP by a distance of L2. Accordingly, the following equation is obtained: 
     
       
           T 2=( L 1 −L 2)/ VP    (2)  
       
     
     wherein T2 is a time duration between when the second CPF-N signal was generated and the control timing; 
     L1 is a moving distance of the photosensitive drum from the exposure position EP to the transfer point TP; 
     L2 is a moving distance of the web W from the mark detection position DP to the transfer point TP; and 
     VP is a rotational speed of the photosensitive drum  101 . 
     Since the CPF-N signal is generated once for each page, the control timing is at the cycle of the CPF-N signal. In other words, the time interval of the successive control timings equals to a length of the CPF-N signal. 
     Then, a gap between the control timing and the actual mark detection timing is determined. If the mark detection timing is behind the corresponding control timing, then the control unit  210  controls the web-transport motor  230  to accelerate the web transport speed. Contrarily, if the mark detection timing is ahead of the corresponding control timing, then the control unit  210  controls the web-transport motor  230  to decelerate the web transport speed. In this manner, the controller  20  controls the web transport speed such that the mark detection timing matches the corresponding control timing. 
     Next, print-stop process according to the present embodiment will be described. 
     At the timing of when the light source  103  completes irradiation for the last page, the controller  20  receives a CPF-OFF signal at the control unit  210  from the controller  17 , whereupon the control unit  210  starts the print-stop process. 
     When the print-stop process starts, first the control unit  210  calculates a time T5 using the following equation: 
     
       
           T 5=( L 1−α)/ V 1  (3)  
       
     
     wherein T5 is a time that is required for a positioning mark Rm on the first page of a subsequent printing operation to reach a waiting position WP (FIG. 6) after the CPF-OFF signal was generated; 
     L1 is a moving distance of the photosensitive drum  101  from the position EP to the transfer point TP; 
     α is a moving distance of the web W from the waiting position WP to the transfer point TP; and 
     V1 is a web transport speed at the time of when the CPF-OFF signal was generated. 
     Here, the web transport speed V1 is detected by monitoring encoder pulses that encoder  220  outputs in synchronization with the driving movement of the web-transport motor  230 . 
     Then, the control unit  210  resets the counter  214  to zero after the time T5 elapses from when the CPF-OFF signal was received. Then, the counter  214  starts counting up the encoder pulse from the encoder  220 . After the images for the last page have completely been transferred from the photosensitive drum  101  onto the web W at the transfer point TP, the control unit  210  controls the web-transport motor  230  to stop transporting the web W. At the same time, the control unit  210  reads the counter value of the counter  214 . Then, the control unit  210  controls the web-transport motor  230  to transport the web W in a reverse direction by the distance corresponding to the counter value. In this manner, the web W is transported back to the waiting position WP by the distance of α. 
     In this manner, according to the above embodiment of the present invention, it is possible to precisely position the web W with no sprockets at the predetermined waiting position WP even if the web transport speed fluctuates, because the time T5 is determined based on the actual web-transport speed V1 rather than the process speed VP. 
     While some exemplary embodiments of this invention have been described in detail, those skilled in the art will recognize that there are many possible modifications and variations which may be made in these exemplary embodiments while yet retaining many of the novel features and advantages of the invention. 
     For example, the positioning control is not limited to what described in the above embodiment. For example, the control unit  210  can stores a mark detection time in the memory  213  each time the mark sensor  16  detects the positioning mark Rm. The mark detection time indicates a time interval between when the CPF-N signal is generated and when the corresponding positioning mark Rm is detected. Then, the control unit  210  calculates a time difference Δt between the mark detection time T4 which has been stored into the memory  213  this time and a mark detection time T3 which has been stored into the memory  213  last time, using the following equation: 
     
       
         Δ t=T 4 −T 3  (4)  
       
     
     Then, the control unit  210  controls the web-transport motor  230  to change the web-transport speed by an amount of Δv, which can be calculated using the equation: 
     
       
         Δ v= (Δ t/CPF  length)× v   (5)  
       
     
     wherein Δt is a time difference between a mark detection time T4 of this time and a mark detection time T3 of last time; 
     CPF length is a length of the CPF-N signal; and 
     v is a current web-transport speed. 
     By changing the current web-transport speed by the amount of Δv, the mark detection timing will match the corresponding control timing.