Patent Publication Number: US-6701837-B2

Title: Widthwise paper drift correction device for elongated web-like print paper

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a printing device for printing on an elongated web-like print paper. More particularly, the invention relates to a paper drift correction device for correcting the position of a print paper drifted in the widthwise direction during travel of the print paper within the printing device. 
     2. Description of the Related Art 
     In a printing device of the type in which an elongated web-like print paper travels along a predetermined meandering path to reach a printing position, the print paper is liable to drift or shift in the widthwise direction. To correct the widthwise drift of the print paper, conventional printing devices employ a correction device having a sensor that detects the side edge of the print paper. With the correction device, the print paper is moved back to the right position when the output from the sensor indicates that the side edge of the print paper is drifted from a reference position. 
     However, the cut condition in the side edges of the print paper differs in different manufacturers and in different slots of paper produced even by the same manufacturer and also differs depending upon a side edge cutting machine used. For the print papers with side edges that are not cut to the same condition, the sensor outputs a signal to correct the widthwise position of the print paper. Moving the print paper in accordance with the output of the sensor may result in incorrect positioning of the print paper. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing, it is an object of the invention to provide a printing device that is capable of stably transporting an elongated web-like print paper while not allowing the print paper to drift in the widthwise direction. 
     To achieve the above and other objects, there is provided an improved printing device for printing on an elongated web-like print paper. The print paper has a width and side edges substantially perpendicular to a widthwise direction of the print paper. A printing device includes a paper conveying mechanism for conveying the print paper along a predetermined path, the print paper being conveyed so that side edges are in coincidence with a paper conveying direction; a first sensor disposed at a first position in the predetermined path, for sensing a position of a side edge of the print paper and outputting a first signal; a second sensor disposed at a second position in the predetermined path, the second position being apart a predetermined distance from the first position and downstream of the first position with respect to the paper conveying direction, the second sensor sensing a position of the side edge of the print paper and outputting a second signal; delaying means for delaying outputting the first signal by a predetermined period of time corresponding to a paper conveying time for conveying the print paper from the first position to the second position; computing means for computing a difference between the first signal output from the delaying means and the second signal and outputting a difference signal; a widthwise moving mechanism for moving the print paper in the widthwise direction; and a controller for generating a drive signal based on the difference signal and outputting the drive signal to the widthwise moving mechanism so that the widthwise moving mechanism is driven in response to the drive signal and corrects the position of the print paper in the widthwise direction. 
     Averaging means may further be provided for computing an average of difference signals output from the computing means over a predetermined period of time. In this case, the controller generates the drive signal based on the averaged difference signal. 
     A low-pass filter may be connected to the output of the computing means for removing a high frequency component contained in the drive signal. In this case, the controller generates the drive signal based on the filtered drive signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The particular features and advantages of the invention as well as other objects will become apparent from the following description taken in connection with the accompanying drawings, in which: 
     FIG. 1 is a schematic diagram illustrating a printing device according to one embodiment of the invention; 
     FIG.  2 ( a ) is a perspective view showing a paper guide; 
     FIG.  2 ( b ) is a cross-sectional view showing the paper guide and a print paper passing therethrough; 
     FIG.  3 ( a ) is a side view showing a base and swing rollers; 
     FIG.  3 ( b ) is a top view showing the base and swing rollers; 
     FIG.  4 ( a ) is a cross-sectional side view showing a sensor and the print paper to be sensed by the sensor; 
     FIG.  4 ( b ) is a top view showing the sensor and the print paper to be sensed by the sensor; 
     FIG. 5 is a sensor output processing system illustrated in a block form; and 
     FIG. 6 is a flowchart illustrating operation of the system shown in FIG.  5 . 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     A printing device according to the embodiment of the invention will be described with reference to the accompanying drawings. 
     As shown in FIG. 1, the printing device  1  includes a sheet feed section (not shown), an in-feed section  100 , a printing section  200 , a fixing section  300 , and an out-feed section  400 . The sheet feed section feeds an elongated web-like print paper  2  into the in-feed section  100 . The in-feed section  100  is made up of three sections including a buffer lead-in section, a tension imparting section and a paper drift correction section. 
     The buffer lead-in section includes an upstream lead-in portion disposed in a position adjacent to a print paper entrance port from which the print paper  2  is introduced, and a downstream lead-in portion. The upstream lead-in portion includes a sliding roller  4 , a booster motor  5 , and a timing belt  6 . The sliding roller  4  is rotatably coupled via the timing belt  6  to the booster motor  5  and rotates at a peripheral speed higher than a paper traveling speed. The downstream lead-in portion includes a motor  8 , a buffer roller  9  driven by the motor  8 , and a driven roller  10   a  urged against the buffer roller  9 . 
     An air buffer  7  is provided downstream of the buffer lead-in section for slackening the print paper  2 . The print paper  2  between a tension roller  11  and the buffer roller  9  is slackened. The tension roller  11  is driven by a torque motor  12  that stably generates a predetermined level of torque and can control the level of the torque as desired. A load roller  10   b  is movable toward and away from the tension roller  11 . When the load roller  10   b  is moved toward the tension roller  11  and urged thereagainst, the load roller  10   b  is driven by the tension roller  11 , thereby conveying the print paper  2  nipped between the rollers  10   b  and  11 . 
     A paper guide  13  is disposed upstream of the tension roller  11  to prevent the slackened print paper  2  from being moved in the widthwise direction. As shown in FIG.  2 ( a ), the paper guide  13  is configured by a pair of guide blocks  13   a  and  13   b  and a pair of rods  13   c  and  13   d  extending in parallel to each other and passing through the guide blocks  13   a  and  13   b . The guide block  13   a  is fixed to the two rods  13   c  and  13   d  whereas the guide block  13   b  is movably supported by the two rods  13   c  and  13   d  so as to be movable toward and away from the guide block  13   a . The position of the guide block  13   b  is adjusted depending upon the size of the print paper  2 . As shown in FIG.  2 ( b ), the print paper  2  passes through a space between the two rods while contacting the peripheral surfaces of the two rods  13   c ,  13   d . The print paper  2  that has passed through the paper guide  13  is shifted 1 mm or so in the widthwise direction. 
     The tension of the print paper  2  is primarily determined by the torque generated by the tension roller  11  and the winding angle of the print paper  2  wound around the periphery of a fixed roller  14  disposed downstream of the tension roller  11 . With the paper guide  13  and the fixed roller  14 , the distance of the print paper  2  drifted in the widthwise direction of the print paper  2  can be restricted to some extent. 
     A dancing arm  17  is disposed near the corner diagonally opposite the corner where the paper entrance port is formed. One end of the dancing arm  17  is fixedly secured to a housing with a spring  18  and another end of the arm  17  rotatably supports a dancing roller  15 . The dancing roller  15  rotates following the transportation of the print paper  2 . The arm  17  is pivotally movable about the pin  16  fixed to the arm  17  at a position between the two ends but closer to the end supporting the dancing roller  15 . By the pivotal movement of the arm  17 , the dancing roller  15  moves toward and away from an in-feed roller  19 . 
     A position sensor (not shown) is disposed near the dancing arm  17  to sense the position of the dancing arm  17 . The rotations of the in-feed roller  19  are controlled in accordance with the output from the position sensor. A feed-in motor  20  drives the in-feed roller  19  via a gear  21   a . A nip roller  22  is urged against the in-feed roller  19  to nip the print paper  2  therebetween. 
     The tension imparting section is configured by the tension roller  11 , fixed roller  14 , dancer roller  15 , pin  16 , dancer arm  17 , spring  18 , in-feed roller  19 , feed-in motor  20 , gear  21   a , and nip roller  22 . 
     A base  25  is disposed downstream of the arm  17 . As shown in FIG.  3 ( a ), the base  25  has an L-shaped cross-section and is configured by a pair of opposing side frames. A pair of swing rollers  27   a ,  27   b  is rotatably supported by a pair of shafts that is bridged between the side frames  25   a ,  25   b . A pin  26  is downwardly protruded from the base  25  and is positioned just beneath the swing roller  27   a  and between the side frames  25   a ,  25   b . The base  25  is swingably movable about the pin  26 , so that the swing rollers  27   a ,  27   b  conveying the print paper  2  can move the print paper  2  in the widthwise direction, i.e., a direction perpendicular to the paper traveling direction, depending upon the swung position of the base  25 . 
     A pair of sensors  23   a ,  23   b  is disposed downstream of the swing rollers  27   a ,  27   b . Each sensor includes a light emitting section  231 , such as an LED (light emitting diode), and a light receiving section  232 , such as a photodiode. The side edge portion of the print paper  2  is positioned between the light emitting section  231  and the light receiving section  232 . As shown in FIGS.  4 ( a ) and  4 ( b ), the sensor  23   a  ( 23   b ) detects the paper side edge. Specifically, the position of the paper side edge can be determined based upon an amount of light received at the light receiving section  232 , which amount will reduce when the print paper  2  shifts outwardly in the widthwise direction whereas increase when the print paper  2  shifts inwardly in the widthwise direction. As will be described later, the position control motor corrects the widthwise position of the print paper  2  based on the outputs from the sensors  23   a ,  23   b  by swingingly moving the base  25 . The sensors  23   a ,  23   b , the position control motor, and swing rollers  27   a ,  27   b  configure the paper drift correction section. 
     The print section  28  is disposed downstream of the in-feed section  3  and includes a light source, such as LEDs or a source of laser, a photosensitive member, a developing unit, a transfer unit  31  for transferring toner images formed on the photosensitive member onto the print paper  2 . The print paper  2  carrying the toner images thereon is fed into the fixing section  300  for thermally fixing the toner images on the print paper  2 . 
     The fixing section  300  includes a hear source and a plurality of heat plates for supplying thermal energy to the print paper  2 . The fixing section  300  heats the print paper  2  so as to melt toner and adhere the melted toner to the print paper  2 . When the temperature of the print paper  2  is cooled down, the toner image is fixed to the print paper  2 . The print paper  2  with the toner image fixed thereon is discharged by an out-feed section  400  out to the printer  1 . The out-feed section  400  includes an out-feed roller  35 , an out-feed motor  36  for driving the out-feed roller  35 , a gear  21   b  for transmitting the driving power of the motor  36  to the roller  35 , a nip roller  37  urged against the out-feed roller  35  and nipping the print paper  2  therebetween, and a puller  38 . The tension of the print paper  2  located between the in-feed roller  19  and the out-feed roller  35  is determined by the dancing roller  15 . Generally, the tension imparted upon a sheet of paper is set to a range between 30 to 200N and is adjusted depending upon the length and width of the paper. 
     FIG. 5 shows two sensors  23   a ,  23   b  and their associated processing system. FIG. 6 is a flowchart illustrating the operation of the processing system shown in FIG.  5 . 
     As shown in FIG. 5, two sensors  23   a ,  23   b  are disposed in spaced-apart relation along the paper traveling path to detect the edge of the print paper  2 . A voltage signal is output from the sensor  23   a  and is subjected to analog-to-digital conversion by an A/D converter  40   a . The output of the A/D converter  40   a  is temporarily stored in a data storage  42  and is supplied to a subtracter  44 . Likewise, a voltage signal is output from the sensor  23   b  and is subjected to analog-to-digital conversion by an A/D converter  40   b . However, the output of the A/D converter  40   b  is directly supplied to the subtracter  44 . 
     The subtracter  44  computes a difference of the two input signals S 1  and S 2  supplied from the data storage  42  and the A/D converter  40   b  and outputs a difference signal Sc to a main controller  46 . The main controller  46  produced a drive signal based on the difference signal Sc, a drive gain and other factors. The drive signal is supplied to a driving mechanism (not shown) for moving the base  25 . 
     A distance between the two sensors  23   a  and  23   b  will be represented by L(m), and a print paper traveling speed by Vp(m/sec). The date storage  42  is provided for delaying the output of the A/D converter  40   a  by a duration of time L/Vp (sec). The portion of the paper side edge detected by the sensor  23   a  is moved to the position of the sensor  23   b  during a time L/Vp. Accordingly, the two sensors  23   a ,  23   b  detect the same portion of the print paper  2  and so the output from the subtracter  44  is not influenced by the change in paper edge condition. 
     If the print paper  2  does not move in the widthwise direction during transportation from the position of sensor  23   a  to the position of sensor  23   b , then paper side edge at position A in the sensor  23   a  is also detected by the sensor  23   b . The corresponding position at the sensor  23   b  is indicated by A′. In this case, the signals S 1  and S 2  are at the same level so that the difference signal Sc from the subtracter  44  is 0 (zero). However, if the paper slide edge is moved in the widthwise direction perpendicular to the paper conveying direction D, the paper side edge at position A in the sensor  23   a  will be detected at the sensor  23   b  to be positioned at A″. Therefore, the difference signal Sc output from the subtracter  44  is not zero and indicates the widthwise movement of the print paper  2 . 
     Referring to the flowchart of FIG. 6, when the operation of widthwise paper drift correction device is started, it is first checked in step (hereinafter abbreviated to “S”)  1  whether the printing device  1  is powered. If the printing device  1  has not yet been powered (“No” in S 1 ), then the operation of the correction device is ended. If the printing device  1  has been powered (“Yes” in S 1 ), then it is checked in S 2  whether the print paper  2  is being conveyed. If the conveyance of the print paper  2  has not yet been started (“No” in S 2 ), then the operation of the correction device is ended. If the print paper  2  is being conveyed (“Yes” in S 2 ), then the outputs of the sensors  23   a ,  23   b  are converted to digital signals by the A/D converter  40   a ,  40   b  (S 3  and S 4 ). In S 5 , the digital signal corresponding to the output of the sensor  23   a  is stored in the data storage  42  for a period of time L/Vp (sec) and then retrieved therefrom and supplied to the subtracter  44 . The digital signal corresponding to the output of the sensor  23   b  is directly supplied to the subtracter  44  with no time delay. 
     In S 6 , the subtracter  44  computes a difference between the two signals one supplied from the data storage  42  and the other from the A/D converter  40   b , and outputs the difference signal Sc. In S 7 , the main controller  46  converts the difference signal Sc to a drive signal while taking output gain and other factors into account. In S 8 , the drive signal is applied to the position control motor to thereby move the base  25  and to thus correct the paper position in the widthwise direction. In S 9 , it is checked if the print paper  2  is being conveyed. If affirmative (“Yes” in S 9 ), then the routine returns to S 1  and repeats the processes described above. If the print paper  2  is not being conveyed (“No” in S 9 ), then the control process will end. 
     With the above-described structure and control process, the widthwise paper drift can be accurately detected regardless of the paper side edge condition and the position error of the print paper can be corrected. 
     While the invention has been described in detail with reference to specific embodiments thereof, it would be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the spirit of the invention, the scope of which is defined by the attached claims. 
     For example, although the above-described embodiment employs only a pair of sensors, plural pairs of sensors may be provided. Further, a reflection type sensor is also usable instead of a transmissive type sensor as described in the embodiment. The above-described embodiment describes that the printing section  200  performs an electrophotographic printing, however, an ink jet printing section may be used in place of the electrophotographic printing section. In this case, the fixing section  300  needs to be replaced by a drying section for drying the ink on the print paper  2 . 
     It should be noted that an error contained in the difference signal Sc caused by the fluctuation of paper travel speed or measurement error of the sheet feed travel speed can be obviated by the following measure. Because the higher frequency components contained in the difference signal Sc indicates an error caused by the measurement error of averaged sheet feed travel speed, the error can be removed by passing the difference signal Sc through a low-pass filter. To this end, it is required that the low-pass filter be connected to the output or the subtracter  44 . The main controller  46  generates the drive signal based on the output of the low-pass filter. 
     In the case where the paper travel speed Vp fluctuates, the error caused thereby appears as the lowest frequency component in the difference signal Sc. Therefore, to eliminate the influence of the paper travel speed fluctuation, it is necessary to compute an average of the lowest frequency components over a predetermined period of time. To this end, the main controller  46  has to perform averaging operation for computing an average of difference signals output from the subtracter  44  over a predetermined period of time. In this case, the main controller  46  generates the drive signal based on the average difference signal.