Patent Publication Number: US-2006012618-A1

Title: Method and apparatus for adjusting the alignment of printing

Description:
PRIORITY  
      This application claims the benefit under 35 U.S.C. §119(a) of Korean Patent Application Nos. 10-2004-0055477, filed on Jul. 16, 2004, and 10-2004-0081085, filed on Oct. 11, 2004, respectively, in the Korean Intellectual Property Office, the entire disclosures of which are hereby incorporated by reference.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a printing device using a thermal head, and more particularly, to a method and apparatus for adjusting the alignment between a first media surface and a second media surface in a printing device that prints by heating the first and second media surfaces using a single thermal head.  
      2. Description of the Related Art  
      A thermal printing device transfers ink to a medium by heating an ink ribbon contacting the medium with a thermal head or heating a medium having an ink layer revealing a predetermined color in response to the heat from the thermal head, thereby forming an image.  
       FIG. 1  illustrates a sectional view of a conventional thermal-reactive medium. The conventional thermal-reactive medium has ink layers with predetermined color on both surfaces, a first surface  10   a  and a second surface  10   b , respectively, of a base sheet  11 . The ink layers include different color layers. For example, a yellow (Y) layer and a magenta (M) layer are sequentially stacked on the first surface  10   a , and a cyan (C) layer is formed on the second surface  10   b . The base sheet  11  may be transparent. A reflective layer  13  reflects light to show a color image on the first surface  10   a.    
       FIG. 2  illustrates a sectional view of a conventional printing device using a thermal head. The conventional printing device includes a thermal-reactive medium  200 , a driving roller  210 , a driven roller  220 , a platen roller  230 , and a thermal head  240 .  
      The driving roller  210  engages with a driving source, typically a motor (not shown) and rotates, thereby feeding the thermal-reactive medium  200 . The driven roller  220  engages with the driving roller  210  and rotates such that the medium  200  passes therebetween.  
      The thermal head  240  heats the thermal-reactive medium  200  to print yellow, magenta and cyan data. The platen roller  230  faces the thermal head  240  such that the medium  200  is supported therebetween, thereby allowing ink to be fused to the medium  200 . The platen roller  230  is rotated by the feeding of the medium  200 .  
      In order to print an image by heating the first and second surfaces of the medium  200  using the thermal head  240 , the thermal head  240  is moved to face the second surface of the medium after heating the first surface of the medium  200 , or the medium  200  is fed so that the second surface of the medium  200  faces the thermal head  240  after the first surface thereof is heated.  
      As described above, when an image is printed by heating the opposing first and second surfaces of a medium using a single thermal head, the printing position of the first surface and the printing position of the second surface may not be exactly the same due to differences between the feed path of the first surface and the feed path of the second surface or due to a deviation in the mechanics of the printing device. As a result, the color desired for an image may not be printed on the medium.  
     SUMMARY OF THE INVENTION  
      The present invention provides a method and apparatus for conveniently and accurately adjusting the alignment between the opposing first and second surfaces of a medium by detecting a distance deviation between the printing position of the first surface and the printing position of the second surface based on patterns respectively printed on the first and second surfaces and by adjusting either of the printing positions based on the detected distance deviation.  
      According to an aspect of the present invention, there is provided a method of adjusting the alignment of a printing device which prints by heating a first surface and a second surface of a medium using a thermal head, including the operations of (a) printing a first pattern on the first surface of the medium by heating the first surface at first intervals using the thermal head, (b) printing a second pattern on the second surface of the medium by heating the second surface at second intervals using the thermal head, (c) detecting an overlapping print position where the first pattern completely overlaps the second pattern; (d) calculating a distance deviation between a print position of the first surface and a print position of the second surface using the overlapping print position; and (e) adjusting the print position of either of the first and second surfaces of the medium based on the calculated distance deviation.  
      Operation (a) may comprise the operations of (a1) feeding the medium until an edge detection sensor detects an edge of the medium, (a2) further feeding the medium by a predetermined distance and heating the first surface of the medium using the thermal head to print a predetermined image, and (a3) feeding the medium and heating the first surface of the medium at increasing distance intervals a predetermined number of times, thereby printing the first pattern.  
      Operation (b) may comprise the operations of (b1) rotating the thermal head so that the thermal head faces the second surface of the medium, (b2) feeding the medium until an edge detection sensor detects an edge of the medium, (b3) further feeding the medium by a predetermined distance and heating the second surface of the medium using the thermal head to print a predetermined image, and (b4) feeding the medium and heating the second surface of the medium at increasing distance intervals a predetermined number of times, thereby printing the second pattern.  
      Operation (c) may comprise the operations of (c1) extracting image data from an image printed on the medium using a sensor, and (c2) comparing values of the image data and detecting the overlapping print position where the first pattern completely overlaps the second pattern. Operation (c2) may further comprise the operation of detecting a print position of a black image.  
      Operation (d) may comprise the operations of (d1) calculating a distance between the overlapping print position and a print start position of the first surface of the medium, (d2) calculating a distance between the overlapping print position and a print start position of the second surface of the medium, and (d3) calculating a difference between the distance calculated in operation (d1) and the distance calculated in operation (d2).  
      According to another aspect of the present invention, there is provided an apparatus for adjusting the alignment of a printing device which prints by heating a first surface and a second surface of a medium using a thermal head, comprising a pattern printing unit for respectively printing a first pattern and a second pattern on the first and second surfaces of the medium, a position detection unit for detecting an overlapping print position where the first pattern completely overlaps the second pattern, a deviation calculation unit for calculating a distance deviation between a print position of the first surface and a print position of the second surface using the overlapping print position, and an adjustor for adjusting a print position of either of the first and second surfaces of the medium based on the distance deviation.  
      The pattern printing unit may comprise a feeder for feeding the medium, the thermal head for printing an image by heating the first and second surfaces of the medium, an edge detection sensor for detecting an edge of the medium fed by the feeder, and a print controller for controlling the feeder and the thermal head to feed the medium by a predetermined distance from a position where the edge of the medium is detected by the edge detection sensor, to heat the medium to print a predetermined image, and to feed the medium and heat the medium at increasing distance intervals a predetermined number of times, thereby printing the first pattern on the first surface and the second pattern on the second surface. The apparatus may further comprise a position adjustor for rotating the thermal head so that the thermal head facing either of the first and second surfaces of the medium faces the other surface of the medium.  
      The position detection unit may comprise a sensor for detecting an image printed on the medium and outputting image data, an analog-to-digital converter for converting the image data from an analog form into a digital form, and a data comparator for comparing values of the digital image data and detecting an overlapping print position where the first pattern completely overlaps the second pattern. The data comparator may detect a print position of a black image. The sensor may be a reflective sensor.  
      The deviation calculation unit may comprise a memory section for storing print positions of the patterns printed on the first and second surfaces, respectively, of the medium, a memory controller for storing the print positions of the patterns printed on the first and second surfaces, respectively, of the medium in the memory section, a distance calculator for calculating a distance between the overlapping print position where the first and second patterns completely overlap and a print start position of the first surface that is stored in the memory and a distance between the overlapping print position where the first and second patterns completely overlap and a print start position of the second surface that is stored in the memory, and a difference calculator for calculating and outputting a difference between the two distances calculated by the distance calculator.  
      According to still another aspect of the present invention, there is provided a method for adjusting the alignment of a printing device, which uses a thermal head for heating a first surface and a second surface of a medium, in a direction perpendicular to a feed direction, the method comprising the operations of (a) printing a first pattern on the first surface of the medium by heating the first surface at predetermined intervals using a predetermined heating element from among a plurality of heating elements included in the thermal head while feeding the medium; (b) printing a second pattern on the second surface of the medium by heating the second surface at the predetermined intervals using heating elements at different positions from among the plurality of heating elements included in the thermal head while feeding the medium; (c) detecting an overlapping print position where the first pattern completely overlaps the second pattern; (d) calculating a distance deviation between a print position of the first surface and a print position of the second surface using the overlapping print position; and (e) adjusting the print position of either of the first and second surfaces of the medium based on the calculated distance deviation.  
      Operation (c) may comprise the operations of (c1) extracting image data from an image printed on the medium using a sensor, and (c2) comparing values of the image data and detecting the overlapping print position where the first pattern completely overlaps the second pattern. Operation (c2) may further comprise the operation of detecting a print position of a black image.  
      Operation (d) may comprise the operation of calculating a distance between a position of the predetermined heating element heating the first surface of the medium and a position of a heating element heating the second surface of the medium at the overlapping print position.  
      According to yet another aspect of the present invention, there is provided an apparatus for adjusting the alignment of a printing device, which uses a thermal head for heating a first surface and a second surface of a medium, in a direction perpendicular to a feed direction, the apparatus comprises a feeder for feeding the medium, the thermal head printing an image by heating the first and second surfaces of the medium, print controller for controlling the feeder and the thermal head to heat at predetermined intervals the first surface of the medium using a predetermined heating element from among a plurality of heating elements included in the thermal head and to heat at the predetermined intervals the second surface of the medium using heating elements at different positions among the plurality of heating elements included in the thermal head, thereby printing a first pattern on the first surface and a second pattern on the second surface, a position detection unit for detecting an overlapping print position where the first pattern completely overlaps the second pattern, a deviation calculation unit for calculating a distance deviation between a print position of the first surface and a print position of the second surface using the overlapping print position, and an adjustor for adjusting the print position of either of the first and second surfaces of the medium based on the distance deviation.  
      The position detection unit may comprise a sensor for detecting an image printed on the medium and outputting image data, and a data comparator for comparing values of the image data and detecting an overlapping print position where the first pattern completely overlaps the second pattern.  
      The data comparator may detect a print position of a black image.  
      The deviation calculation unit may comprise a memory section for storing positions of the heating elements used to print the first and second patterns on the medium, a memory controller for storing the positions of the heating elements used to print the first and second patterns on the medium in the memory section, and a distance calculator for reading from the memory section a position of the predetermined heating element for heating the first surface of the medium and a position of a heating element for heating the second surface of the medium at the overlapping print position, and calculating a distance between the two positions.  
      The method for adjusting the alignment in a printing device may be implemented by a computer-readable recording medium storing a program that is executed on a computer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The above and other features and advantages of the embodiments of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which:  
       FIG. 1  illustrates a sectional view of a conventional thermal-reactive medium;  
       FIG. 2  illustrates a sectional view of a conventional printing device using a thermal head;  
       FIG. 3  illustrates a sectional view of a printing device having a single thermal head adapted to operate according to an embodiment of the present invention;  
       FIG. 4  illustrates a sectional view of a printing device using a method for adjusting the alignment according to an embodiment of the present invention;  
       FIG. 5  is a block diagram of an apparatus for adjusting the alignment between media surfaces according to an embodiment of the present invention;  
       FIG. 6  is a detailed block diagram of an exemplary pattern printing unit shown in  FIG. 5 ;  
       FIG. 7  is a detailed block diagram of an exemplary position detection unit shown in  FIG. 5 ;  
       FIG. 8  is a detailed block diagram of an exemplary deviation calculation unit shown in  FIG. 5 ;  
       FIG. 9  is a flowchart of a method for adjusting the alignment according to an embodiment of the present invention;  
       FIG. 10  is a detailed flowchart of operations for printing patterns on the first and second surfaces, respectively, of a medium in the embodiment illustrated in  FIG. 9 ;  
       FIG. 11  is a detailed flowchart of an operation for detecting an overlapping printing position in the embodiment illustrated in  FIG. 9 ;  
       FIG. 12  is a detailed flowchart of an operation for calculating a distance deviation in the embodiment illustrated in  FIG. 9 ;  
       FIGS. 13A through 13E  are diagrams for explaining the operations included in the method illustrated in  FIG. 9 ;  
       FIG. 14  is a block diagram of an apparatus for adjusting the alignment according to another embodiment of the present invention;  
       FIG. 15  is a detailed block diagram of an exemplary position detection unit shown in  FIG. 14 ;  
       FIG. 16  is a detailed block diagram of an exemplary deviation calculation unit shown in  FIG. 14 ;  
       FIG. 17  is a flowchart of a method for adjusting the alignment according to another embodiment of the present invention;  
       FIG. 18  is a detailed flowchart of operations for printing a first pattern on a first surface of a medium according to the embodiment illustrated in  FIG. 17 ;  
       FIG. 19  is a detailed flowchart of operations for printing a second pattern on a second surface of the medium according to the embodiment illustrated in  FIG. 17 ;  
       FIG. 20  is a detailed flowchart of an operation for detecting a printing position where the first pattern and the second pattern meet each other in the embodiment illustrated in  FIG. 17 ;  
       FIG. 21  is a detailed flowchart of an operation for calculating a distance deviation in the embodiment illustrated in  FIG. 17 ; and  
       FIGS. 22A through 22E  are diagrams for explaining the operations of an embodiment of the present invention illustrated in  FIG. 17 . 
    
    
      It should be understood that throughout the figures like reference numbers refer to like features, structures and elements.  
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS  
      Reference will now be made in detail to the exemplary embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like reference numerals refer to the like elements throughout. The embodiments are described below in detail in order to explain the embodiments of the present invention by referring to the figures.  
       FIG. 3  illustrates a sectional view of a printing device to explain the operation of the printing device using a single thermal head according to an embodiment of the present invention. The printing device includes a platen roller  305 , a thermal head  310 , a driving roller  335 , a driven roller  340 , an edge detection sensor  345 , a media guide  350 , a discharge driven roller  365 , a discharge roller  370 , a pickup roller  380 , and a media reservoir  390 .  
      A printing device using a single thermal head usually has at least three paths to feed a medium. The pickup roller  380  picks up a medium  320  from the media reservoir  390  and feeds the medium  320  through a first path. Through the second path, the medium  320  is fed in a reverse print direction (the B direction) to be positioned in a printing area and in a forward print direction (the F direction) to be printed by heat of the thermal head  310 . The medium  320  is fed through a third path after a first surface of the medium  320  is heated for printing and before the medium  320  is fed in the B direction and returned to the second path. The medium  320  is fed along the third path to be finally discharged when it is fed in the F direction after printing is completed on both of the first and second surfaces thereof.  
      The media guide  350  may be provided between the first and third paths. The media guide  350  controls the medium  320  fed through the first path to advance to the second path and the medium  320  fed through the second path to advance to the third path.  
      In the second path, an image printing unit  300  performs the image printing. The image printing may be performed two times or more than two times when necessary. In the embodiments of the present invention, the image printing is preferably performed two times, once on the first surface and once on the second surface, of the medium  320 .  
      Before image printing is performed on either of the first and second surfaces of the medium  320 , the thermal head  310  is positioned for image printing. For example, when image printing is performed on the first surface of the medium  320 , the thermal head  310  is positioned at a position D. When image printing is performed on the second surface of the medium  320 , the thermal head  310  is positioned at a position C. The position of the thermal head  310  may be changed by rotating the platen roller  305  and the thermal head  310  around a rotational axis of the platen roller  305 . The position of the thermal head  310  is changed when interference with the medium  320  does not occur, for example, before the medium  320  is fed from the first path or after the medium  320  whose first surface has been printed is fed to the third path before the medium  320  is returned to the second path.  
      After image printing is performed on the first surface of the medium  320 , and when the medium  320  is fed from the third path back to the second path in the reverse print direction, image printing is performed on the second surface of the medium  320  using the thermal head  310  that has been rotated. During image printing, the medium  320  is gradually fed by a feeding unit  330  in the forward print direction (F). After, image printing is completed on the second surface of the medium  320 , the medium  320  that is fed to the third path is discharged by a media discharging unit  360 .  
      The edge detection sensor  345  detects an edge of the medium  320  fed by the feeding unit  330  and may be implemented by an optical sensor.  
       FIG. 4  illustrates a sectional view of a printing device using a method for adjusting the alignment according to an embodiment of the present invention. The printing device includes a thermal head  400 , a platen roller  410 , a motor  420 , a driving roller  430 , a driven roller  440 , an encoder  450 , and an edge detection sensor  460 .  
      The thermal head  400  heats a medium  470  to print an image. The platen roller  410  fuses ink onto the medium  470  using heat from the thermal head  400 . The motor  420  drives the driving roller  430  that feeds the medium  470  in a forward print direction and backward print direction. The edge detection sensor  460  detects an edge of the medium  470  and senses a position of the medium  470 . The encoder  450  converts the operation of the driving roller  430  into an electrical signal to measure the feed distance or the feed speed of the medium  470 . The encoder  450  may also be attached to the driven roller  440  or the motor  420  besides the driving roller  430  and may convert an operation of the driven roller  440  or the motor  420  into electrical signal to measure the feed distance or the feed speed of the medium  470 .  
       FIG. 5  is a block diagram of an apparatus for adjusting the alignment according to an embodiment of the present invention. The apparatus includes a pattern printing unit  510 , a position detection unit  520 , a deviation calculation unit  530 , and an adjustor  540 . The apparatus illustrated in  FIG. 5  will be described with reference to a flowchart of a method for adjusting the alignment according to an embodiment of the present invention illustrated in  FIG. 9 .  
      In operation  900 , the pattern printing unit  510  prints a first pattern on a first surface of the medium  470  by heating the first surface at regular or irregular intervals using the thermal head  400  while feeding the medium  470 .  
      In operation  910 , the pattern printing unit  510  prints a second pattern on a second surface of the medium  470  by heating the second surface at regular or irregular intervals using the thermal head  400  while feeding the medium  470 .  
      In operation  920 , the position detection unit  520  detects an overlapping print position where the first pattern printed on the first surface of the medium  470  completely overlaps the second pattern printed on the second surface thereof. In operation  930 , the deviation calculation unit  530  calculates the distance deviations between the detected overlapping position and other print positions on the respective first and second surfaces of the medium  470 .  
      In operation  940 , the adjustor  540  adjusts the print position of either of the first and second surfaces of the medium  470  based on the distance deviations received from the deviation calculation unit  530 . For example, when the print position of the first surface is ahead of the print position of the second surface by 0.1 mm, a print start position of the first surface may be moved back 0.1 mm or a print start position of the second surface may be moved forward 0.1 mm to adjust the alignment between the first and second surfaces of the medium  470  in a feed direction.  
       FIG. 6  is a detailed block diagram of the pattern printing unit  510  shown in  FIG. 5 . The pattern printing unit  510  comprises an edge detection sensor  600 , a print controller  610 , a feeder  620 , and the thermal head  400 . The pattern printing unit  510  illustrated in  FIG. 6  will be described with reference to  FIG. 10 , which is a detailed flowchart of the operations of printing the first and second patterns on the first and second surfaces, respectively, of the medium  470  according to an embodiment of the present invention.  
      In operation  1000 , the feeder  620  feeds the medium  470  in the backward print direction under the control of the print controller  610  until the edge detection sensor  600  detects an edge of the medium  470  fed by the feeder  620 . The edge detection sensor  600  may be an optical sensor.  
      In operation  1010 , under the control of the print controller  610 , the feeder  620  feeds the medium  470  in the forward print direction by a feed distance L (not shown) from a position where the edge of the medium  470  has been detected. In operation  1020 , the thermal head  400  heats the medium  470 , thereby printing a pattern image.  
      In operation  1030 , the print controller  610  increases the feed distance L by a predetermined value “d”. In operation  1040 , under the control of the print controller  610 , the feeder  620  feeds the medium  470  in the forward print direction by the increased feed distance L. In operation  1050 , the thermal head  400  heats the medium, thereby printing a pattern image.  
      In operation  1060 , the print controller  610  determines whether pattern printing has been completed and, if not, operations  1030  through  1050  are repeated until the pattern printing is completed.  
      When the first and second patterns are respectively printed on the first and second surfaces of the medium  470 , the feed distance L by which the medium  470  is initially fed in the forward print direction in operation  1010  may be the same for both of the first and second surfaces, but the predetermined value “d” by which the feed distance L is increased in operation  1030  may be different for the first and second surfaces.  
      To print the second pattern on the second surface of the medium  470  after printing the first pattern on the first surface of the medium  470  using the thermal head  400 , a position adjustor may be further provided to rotate the thermal head  400  to face either the first surface or second surface of the medium  470  to face the other surface.  
       FIG. 7  is a detailed block diagram of the position detection unit  520  shown in  FIG. 5 . The position detection unit  520  comprises a sensor  700 , an analog-to-digital (A/D) converter  710 , and a data comparator  720 . The operation of the position detection unit  520  illustrated in  FIG. 7  will be described with reference to the flowchart illustrated in  FIG. 11 .  
      In operation  1100 , the sensor  700  senses an image printed on the medium  470  and outputs image data. Here, the sensor  700  may be a reflective sensor. A single sensor may be commonly used as the edge detection sensor  600  and the sensor  700  detecting the image data.  
      In operation  1110 , the A/D converter  710  converts the image data from an analog form into a digital form to generate digital data. In operation  1120 , the data comparator  720  compares values of the digital data with each other and detects an overlapping print position where the first pattern on the first surface of the medium  470  completely overlaps the second pattern on the second surface thereof.  
      When the first and second patterns are respectively printed on the first and second surfaces of the medium  470  under the condition that the color depth of data printed on the first surface is the same as that of data printed on the second surface, a black image is formed at an overlapping print position where the first pattern on the first surface completely overlaps the second pattern on the second surface. Accordingly, the data comparator  720  may detect a data value corresponding to the color black from among the digital data values detected from the overlapping print position where the first pattern and the second pattern completely overlap. For example, when a yellow (Y) layer and a magenta (M) layer are sequentially formed on the first surface of the medium  470  and a cyan layer (C) is formed on the second surface of the medium  470 , the image data may be set such that a black image is formed on the medium  470  when Y, M, and C are printed at the same position by the thermal head  400  and may be printed in a predetermined pattern on the medium  470 .  
       FIG. 8  is a detailed block diagram of the deviation calculation unit  530  shown in  FIG. 5 . The deviation calculation unit  530  comprises a memory controller  800 , a memory section  810 , a distance calculator  820 , and a difference calculator  830 . The operation of the deviation calculation unit  530  illustrated in  FIG. 8  will be described with reference to a flowchart illustrated in  FIG. 12 . The memory controller  800  stores the print positions of the first and second patterns printed on the first and second surfaces, respectively, of the medium  470  in the memory section  810  in advance.  
      In operation  1200 , the distance calculator  820  receives the overlapping print position, where the first and second patterns on the respective first and second surfaces of the medium  470  completely overlap, from the position detection unit  520  and a print start position of the first pattern on the first surface of the medium  470  from the memory section  810  and calculates a distance d 1  between the two positions.  
      In operation  1210 , the distance calculator  820  calculates a distance d 2  between the overlapping print position received from the position detection unit  520  and a print start position of the second pattern on the second surface of the medium  470 , which is received from the memory section  810 .  
      In operation  1220 , the difference calculator  830  calculates the difference between the calculated distances d, and d 2  and outputs information regarding a distance deviation between the print positions of the respective first and second surfaces of the medium  470 . For example, when the distance d, is 31.4 mm and the distance d 2  is 31.1 mm, the difference calculator  830  outputs information indicating that the print position of the first surface of the medium  470  is ahead of the print position of the second surface thereof by the difference between d, and d 2 , which is 0.3 mm.  
       FIGS. 13A through 13E  are diagrams for explaining the operations of the method illustrated in  FIG. 9 . Referring to  FIG. 13A , after the feeder  620  (not shown) feeds the medium  470  in the backward print direction until the sensor  1300  detects an edge of the medium  470 , the thermal head  400  applies heat at regular or irregular intervals to the first surface of the medium  470  that is being fed in the forward print direction (F) by the feeder  620 , thereby printing the first pattern shown in  FIG. 13B  on the first surface of the medium  470 .  
      After pattern printing is completed on the first surface of the medium  470 , the thermal head  400  and the platen roller  410  are rotated so that the thermal head  400  faces the second surface of the medium  470 .  
      Referring to  FIG. 13C , after the feeder  620  feeds the medium  470  in the backward print direction until the sensor  1300  detects the edge of the medium  470 , the thermal head  400  applies heat at regular or irregular intervals to the second surface of the medium  470  that is being fed in the forward print direction (F) by the feeder  620 , thereby printing the second pattern shown in  FIG. 13D  on the second surface of the medium  470 . Here, a print start position  1320  of the second pattern is set to be the same as a print start position  1310  of the first pattern.  
       FIG. 13E  illustrates an image formed on the medium  470  after pattern printing on both of the first and second surface of the medium  470  is completed. The position detection unit  520  (not shown) detects an overlapping print position  1300  where the first pattern completely overlaps the second pattern. Then, the deviation calculation unit  530  calculates the distance d 1  between the overlapping print position  1300  and the print start position  1310  of the first surface and the distance d 2  between the overlapping print position  1300  and the print start position  1320  of the second surface and outputs a distance deviation (x) between the distances d 1  and d 2 , which represented by the equation x=d 2 −d 1 .  
       FIG. 14  is a block diagram of an apparatus for adjusting the alignment according to another embodiment of the present invention. The apparatus shown in  FIG. 14  comprises an edge detection sensor  1400 , a print controller  1410 , a feeder  1420 , a thermal head  1430 , a position detection unit  1450 , a deviation calculation unit  1460 , and an adjustor  1470 . The operations of the apparatus illustrated in  FIG. 14  will be described with reference to a flowchart of a method for adjusting the alignment according to another embodiment of the present invention illustrated in  FIG. 17   
      In operation  1700 , the thermal head  1430  is controlled by the print controller  1410  to print a first pattern on a first surface of the medium  1440  fed by the feeder  1420  by heating the first surface at predetermined intervals using a single particular heating element from among a plurality of heating elements included therein.  
      In operation  1710 , thermal head  1430  is controlled by the print controller  1410  to print a second pattern on a second surface of the medium  1440  fed by the feeder  1420  by heating the second surface at predetermined intervals using heating elements at different positions. Alternatively, the heating elements at different positions may be used to print the first pattern while a single particular heating element is used to print the second pattern. As another alternative, both of the first and second patterns may be printed using heating elements at different positions.  
      To print the second pattern on the second surface of the medium  1440  after printing the first pattern on the first surface of the medium  1440  using the thermal head  1430 , a position adjustor (not shown) may be further provided to rotate the thermal head  1430  to face one of the first and second surfaces of the medium  1440  to face the other surface.  
      In operation  1720 , the position detection unit  1450  detects a position where the first pattern printed on the first surface of the medium  1440  completely overlaps the second pattern printed on the second surface thereof. In operation  1730 , the deviation calculation unit  1460  calculates a distance deviation between the print positions of the first and second surfaces of the medium  1440  using the overlapping print position.  
      In operation  1740 , the adjustor  1470  adjusts the print position of either of the first and second surfaces of the medium  1440  based on the distance deviations received from the deviation calculation unit  1460 . For example, when the print position of the first surface is deviated 0.1 mm to the left from the print position of the second surface, a print start position of the first surface may be moved 0.1 mm to the right or a print start position of the second surface may be moved 0.1 mm to the left to adjust alignment between the first and second surfaces of the medium  1440  in a direction perpendicular to the feed direction.  
       FIG. 18  is a detailed flowchart of the operations for printing the first pattern on the first surface of the medium  1440  in the embodiment illustrated in  FIG. 17 . In operation  1800 , the edge detection sensor  1400  detects an edge of the medium  1440  fed by the feeder  1420  in a backward print direction. The edge detection sensor  1400  may be a reflective sensor.  
      In operation  1810 , the feeder  1420  feeds the medium  1440  in a forward print direction by a predetermined distance L from the position where the edge of the medium  1440  has been detected. In addition, in operation  1810 , “n”, which indicates the number of printed pattern images, is set to 1. In operation  1820 , the medium  1440  is fed by the feeder  1420  by a predetermined distance W, during which the thermal head  1430  heats the first surface of the medium  1440  using a particular heating element from among the plurality of heating elements, thereby printing a pattern image with a length W.  
      In operation  1830 , it is determined whether “n” is equal to “m”, which indicates the predetermined number of pattern images to be printed on the medium  1440 . When it is determined that “n” is not equal to “m”, the feeder  1420  feeds the medium  1440  in the forward print direction (F) by a predetermined distance R in operation  1840 . Thereafter, in operation  1850 , “n” is increased by 1, and operation  1820  is repeated. When it is determined that “n” is equal to “m” in operation  1830 , the operations for printing the first pattern ends. When the first pattern is printed using the operations illustrated in  FIG. 18 , the first pattern appears on the first surface of the medium  1440  as shown in  FIG. 22B .  
       FIG. 19  is a detailed flowchart of the operations for printing the second pattern on the second surface of the medium  1440  in the embodiment illustrated in  FIG. 17 . In operation  1900 , the edge detection sensor  1400  detects an edge of the medium  1440  fed by the feeder  1420  in the backward print direction. The edge detection sensor  1400  may be a reflective sensor.  
      In operation  1910 , the feeder  1420  feeds the medium  1440  in the forward print direction by the predetermined distance L from the position where the edge of the medium  1440  has been detected. In addition, in operation  1910 , “n”, which indicates the number of printed pattern images, is set to 1. In operation  1920 , the medium  1440  is fed by the feeder  1420  by the predetermined distance W, during which the thermal head  1430  heats the second surface of the medium  1440  using an i-th heating element from among the plurality of heating elements, thereby printing a pattern image with the length W.  
      In operation  1930 , it is determined whether “n” is equal to “m”, which indicates the predetermined number of pattern images to be printed on the medium  1440 . When it is determined that “n” is not equal to “m”, the feeder  1420  feeds the medium  1440  in the forward print direction by the predetermined distance R in operation  1940 . Thereafter, in operation  1950 , “n” is increased by 1 and “i” is increased by 1 to select another heating element at a position shifted by 1 dot, and then operation  1920  is repeated. For example, if an initial pattern image is printed using a 300th heating element of the thermal head  1430 , a subsequent pattern image may be printed using a subsequent 301st heating element so that print positions of adjacent pattern images are deviated 1 dot from each other in the second pattern.  
      When it is determined that “n” is equal to “m” in operation  1930 , the operations for printing the second pattern ends. When the first pattern is printed using the operations illustrated in  FIG. 19 , the second pattern appears on the second surface of the medium  1440  as shown in  FIG. 22D .  
       FIG. 15  is a detailed block diagram of the position detection unit  1450  shown in  FIG. 14 . The position detection unit  1450  comprises a sensor  1500 , an A/D converter  1510 , and a data comparator  1520 . The operation of the position detection unit  1450  illustrated in  FIG. 15  will be described with reference to the flowchart in  FIG. 20 .  
      In operation  2000 , the sensor  1500  senses an image printed on the medium  1440  and outputs image data. Here, the sensor  1500  may be a reflective sensor. A single sensor may be used in common as the edge detection sensor  1400  and the sensor  1500  detecting the image data.  
      In operation  2010 , the A/D converter  1510  converts the image data from an analog form into a digital form to generate digital data. In operation  2020 , the data comparator  1520  compares values of the digital data with each other and detects an overlapping print position where the first pattern on the first surface of the medium  1440  completely overlaps the second pattern on the second surface thereof.  
      When the first and second patterns are respectively printed on the first and second surfaces of the medium  1440  under the condition that color depth of data printed on the first surface is the same as that of data printed on the second surface, a black image is formed at an overlapping print position where the first pattern on the first surface completely overlaps the second pattern on the second surface. Accordingly, the data comparator  1520  may detect a data value corresponding to the color black from among the digital data values detected from the overlapping print position where the first pattern and the second pattern completely overlap. For example, when a yellow (Y) layer and a magenta (M) layer are sequentially formed on the first surface of the medium  1440  and a cyan layer (C) is formed on the second surface of the medium  1440 , the image data may be set such that a black image is formed on the medium  1440  when Y, M, and C are printed at the same position by the thermal head  1430  and may be printed in a predetermined pattern on the medium  1440 .  
       FIG. 16  is a detailed block diagram of the deviation calculation unit  1460  shown in  FIG. 14 . The deviation calculation unit  1460  comprises a memory controller  1600 , a memory section  1610 , and a distance calculator  1620 . The operation of the deviation calculation unit  1460  illustrated in  FIG. 16  will be described with reference to a flowchart in  FIG. 21 .  
      The memory controller  1600  stores the positions of the heating elements, which heat the medium  1440  to print patterns onto the first and second surfaces thereof, in the memory section  1610 .  
      In operation  2100 , the distance calculator  1620  receives from the position detection unit  1450  the overlapping print position, at which the first and second patterns on the respective first and second surfaces of the medium  1440  completely overlap, and reads from the memory section  1610  the position of the heating element heating the first surface of the medium  1440  at the overlapping print position.  
      In operation  2110 , the distance calculator  1620  receives from the position detection unit  1450  the overlapping print position, at which the first and second patterns on the respective first and second surfaces of the medium  1440  completely overlap, and reads from the memory section  1610  the position of the heating element heating the second surface of the medium  1440  at the overlapping print position.  
      In operation  2120 , the distance calculator  1620  calculates a distance between the positions of the two heating elements read from the memory section  1610  and outputs the distance. The distance between the positions of the two heating elements is the distance deviation between the print positions of the first and second surfaces of the medium  1440 .  
       FIGS. 22A through 22E  are diagrams for explaining the operations of the method according to an embodiment of the present invention illustrated in  FIG. 17 . Referring to  FIG. 22A , after the feeder  1420  feeds the medium  1440  in the backward print direction until a sensor  2230  detects an edge of the medium  1440 , the particular heating element of the thermal head  1430  heats at predetermined intervals the first surface of the medium  1440  that is being fed by the feeder  1420  in the forward print direction (F), thereby printing the first pattern, which is shown in  FIG. 22B , on the first surface of the medium  1440 .  
      After the printing of the first pattern on the first surface of the medium  1440  is completed, the thermal head  1430  and a platen roller  2200  are rotated so that the thermal head  1430  faces the second surface of the medium  1440 .  
      Referring to  FIG. 22C , after the feeder  1420  feeds the medium  1440  in the backward print direction until the sensor  2230  detects the edge of the medium  1440 , the thermal head  400  heats at the predetermined intervals the second surface of the medium  1440  that is being fed by the feeder  1420  in the F direction, sequentially using heating elements at different positions that are separated by 1 dot from each other, thereby printing the second pattern, which is shown in  FIG. 22D , on the second surface of the medium  1440 .  
       FIG. 22E  illustrates an image formed on the medium  1440  after pattern printing on both of the first and second surface of the medium  1440  is completed. The position detection unit  1450  detects the overlapping print position  2240  where the first pattern completely overlaps the second pattern. Then, the deviation calculation unit  1460  calculates a distance between the position of the heating element heating the first surface at the overlapping print position  2240  and the position of a heating element heating the second surface at the overlapping print position  2240  and outputs a distance deviation between the print positions of the first and second surfaces of the medium  1440 , which is the distance between the two heating elements.  
      The invention can also be embodied as computer readable codes on a computer readable recording medium. The computer readable recording medium is any data storage device that can store data which can be thereafter read and executed by a computer system. Examples of the computer readable recording medium include read-only memory (ROM), random-access memory (RAM), CD-ROMs, magnetic tapes, floppy disks, optical data storage devices, and carrier waves (such as data transmission through the Internet). The computer readable recording medium can also be distributed over network coupled computer systems so that the computer readable code is stored and executed in a distributed fashion. Also, functional programs, codes, and code segments for performing the functions of the embodiments of the present invention can be easily construed by programmers skilled in the art to which the present invention pertains.  
      As described above, according to an embodiment of the present invention, a distance deviation between a printing position of a first surface of a medium and a printing position of a second surface of the medium is detected based on patterns respectively printed on the first and second surfaces, and either of the printing positions is adjusted based on the detected distance deviation, thereby conveniently and accurately adjusting the alignment between the first and second surfaces of the medium While this invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.