Patent Publication Number: US-6222575-B1

Title: High precision dye donor web positioning in thermal color printer

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an apparatus and method for positioning a dye donor web in a thermal color printer for minimal waste and cost. 
     In a color thermal printing process of a thermal color printer, the finished print is made by successively transferring color dyes from respective dye patches of a dye donor web onto a dye receiver medium with a thermal printer head. In general, a dye donor web contains a repeating series of frames of different colored, heat transferable dyes. The color series of a dye donor web can be YMC (yellow, magenta, cyan), YMCK (YMC, black), YMCO (YMC, overlay) or YMCKO. During the color printing process, it is necessary to have the dye donor web properly positioned relative to the dye receiver medium to ensure full coverage of the image area by successive color frames. Since the donor web has a repeating series of different colored dye frames, it is necessary to identify the leading edge of each different frame of each color series. One way to do this is to provide sensor marks on the donor, such as disclosed in U.S. Pat. No. 5,466,075 which inserted different black bars at the leading edges of the colored frames (the yellow frame) of each series as the sensor marks. Another way to identify the frames is to provide color discriminating optical sensors directly located in the donor web path just past the print line of the thermal print head in the direction of travel of the donor web, such as disclosed in U.S. Pat. Nos. 4,710,781 and 5,266,967. These sensors detect the presence of different colored patches on the donor as they move forward. The particular dye frame can be identified by analyzing the light intensity of transmission or reflection of a light beam or a plurality of light beams with different colors. It is noted that the sensor marks and the physical configuration of the print head and surrounding mechanisms indeed limit the minimum size of the color frames. Among the consequences of having unused donor are: a higher cost of material for making prints, reduced donor web capacity in the printer, and a great amount of material requiring environmentally safe disposal after use. 
     As shown in FIG. 1, the thermal color printer of the prior art comprises a dye donor web  10  having a plurality of dye frames in a repeating series of different colors; a web drive  20  adapted to move and receive the dye donor web  10 , which includes a ribbon supply spool  21 , a received spool  22 , a motor  23  used to rotate the received spool  22 , two idle rollers  24 ,  25  and a platen roller  26 ; a sensor device  30  adapted between the idle roller  25  and the platen roller  26 , which consists of a light source  31  and an optical sensor  32  for identifying the dye frames; and a controller  40  used to position the dye donor web  10  with high precision by properly driving the web drive  20 . During the color thermal printing process, the web drive  20  is driven by the controller  40  to deliver the dye donor web  10  adapted on the ribbon supply spool  21  to the received spool  22  through the idle roller  24 , the platen roller  26  and the idle roller  25  for taking up the used donor web. At the opposite position of the platen roller  26 , there is a thermal print head  50 , and a print line  60  is consequently formed between the thermal print head  50  and the platen roller  26 . The thermal print head  50  is energized to transfer dye from the donor web at the print line  60  to a receiver medium. In the system shown in FIG. 1, it is desirable to position the sensor device  30  as close as possible to the print line  60  because the amount of the donor web  10  after positioning is not used in printing, and is therefore wasted. Unfortunately, the physical configuration of the print head and surrounding mechanisms indeed limit the minimum distance that can be achieved. This, in sum, limits the minimum size of the color frames. Among the consequences of having unused donor are: a higher cost of material for making prints, reduced printing capacity in the printer, and increased volume of material requiring environmentally safe disposal after use. 
     In order to minimize the waste of the donor web, an improved configuration of the thermal color printer was disclosed in U.S. Pat. No. 4,710,781 and shown as FIG. 2. A motor  27  linked with the ribbon supply spool  21  is programmed to rotate the ribbon supply spool  21  through a predetermined arc length in the reverse-feed direction to draw the web  10  backward for minimizing the distance between the leading edge of every color frame and the print line  60 . However, since the distance of web moving is not only a function of the amount of supply spool rotation, but also a function of supply roll diameter, the amount of rotation of the ribbon supply spool  21  must be determined for a full supply roll. Thus, operation with anything other than a full supply roll still results in additional dye donor web waste. In U.S. Pat. No. 5,549,400, an encoder wheel  28  and an encoder sensor  29  are disposed in the ribbon supply spool  21  for positioning the dye donor web  10  with a high precision control to the rotation angle of the ribbon supply spool  21 . This configuration indeed reduces the waste of the web, however increases the cost and assembly complexity of the thermal color printer. 
     The drawbacks of the thermal color printers of the prior art are listed as follows: 
     1. In order to ensure full coverage of the image area by successive color frames, the optical sensors directly located in the donor web path just past the print line of the thermal print head in the direction of travel of the donor web. The amount of dye donor web between the sensors and the print line of the thermal print head is unused. 
     2. In order to identify the color frame, sensor marks are inserted on the donor web. The total length of the donor web is consequently reduced and may result in reducing the capacity of printing. 
     3. In the configuration of disposing a motor in the supply spool, since the rotation angle of the motor is a function of the spool roll diameter, the operation with anything other than a full supply roll still results in additional dye donor web waste. 
     4. The cost and assembly complexity of the thermal color printer with additional encode wheel and sensors are obviously increased. 
     SUMMARY OF THE INVENTION 
     The objective of the invention is to overcome the problems of dye donor waste, high cost and assembly complexity as set forth above. 
     According to the present invention, the donor web positioning apparatus of a thermal color printer of the invention includes a dye donor web having a plurality of dye frames in a repeating series of different colors; a web drive adapted to move the dye donor web, which includes: a ribbon supply spool; and a sensing device positioned between the ribbon supply spool and the print line of the printer, which includes at least two colored light sources with the same color and optical sensors. The optical sensors detect the light intensity of transmission of the light beams for identifying the color of the frame through the sensing device, and generates a signal to the web drive for moving and positioning the dye donor web. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects, features, and advantages of the invention will become apparent from the following detailed description of the preferred but non-limiting embodiments. The description is made with reference to the accompanying drawings in which: 
     FIG. 1 shows the first embodiment of the donor web positioning apparatus of a thermal color printer according to the prior art; 
     FIG. 2 shows the second embodiment of the donor web positioning apparatus of the prior art; 
     FIG. 3 shows the third embodiment of the donor web positioning apparatus of the prior art; 
     FIG. 4 is a diagrammatic side view of a donor web positioning apparatus of the present invention; 
     FIG. 5 is a diagram of the donor web positioning apparatus shown in FIG. 4; 
     FIG. 6 is a diagrammatic top view of the donor web positioning apparatus shown in FIG. 4; 
     FIG. 7 is the first embodiment of a sensor mark on a dye donor web according to the present invention; 
     FIG. 8 is the second embodiment of a sensor mark of the present invention; 
     FIG. 9 is the third embodiment of a sensor mark of the present invention; 
     FIG. 10 is the forth embodiment of a sensor mark of the present invention; 
     FIG. 11 is a table of the truth values of the colors: Y, M, C, K,  0 ; and 
     FIG. 12 is a flow chart of a donor web positioning procedure according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A donor web positioning apparatus of the present invention shown in FIGS. 4,  5  and  6  for a thermal color printer comprises a web drive  20  adapted to move a dye donor web  10 , which includes a ribbon supply spool  21 , a received spool  22 , a power generator  23   a  (for example a DC motor) provided to rotate the received spool  22 , two idle rollers  24 ,  25  and a platen roller  26 ; a sensing device  30   a  disposed between the ribbon supply spool  21  and the platen roller  26 , which consists of at least two optical sensors  32   a  and light generators  31   a  providing light beams  311  of the same color but different light intensity; and a controller  40  provided to judge the color of the frame through the sensing device  30   a  and position the leading edge of a frame of the dye donor web  10  related to the print line  60  with high precision according to the signal delivered from the optical sensors  32   a . In addition, the dye donor web  10  disposed on the ribbon supply spool  21  consists of a plurality of frames  11  in a repeating series of different colors such as: yellow (Y), magenta (M), cyan (C), black (K) and overlay (O). At the opposite position of the platen roller  26 , there is a thermal print head  50 . A print line  60  is consequently formed between the thermal print head  50  and the platen roller  26 . The light generators  31   a  can be light emission diodes (LEDs). 
     The operation for positioning the leading edge of a frame of the dye donor web is described as follows. During the color thermal printing process, the web drive  20  controlled by the controller  40  is driven by the power generator  23   a  to deliver the dye donor web  10  to the received spool  22  through the sensing device  30   a , the idle roller  24 , the platen roller  26  and the idle roller  25  for taking up the used donor web. The optical sensing device  30   a  is directly located in the donor web path just the length of a frame  11  preceding the print line  60  in the direction of travel of the donor web  10 . Between the optical sensors  32   a  and the light generators  31   a , there exists a gap which the donor web path goes through. The optical sensors  32   a  faced to the light generators  31   a  are used to detect the light intensity of transmission of light beams  311  through the donor web  10  for identifying the leading edge of a frame. Since the optical sensing device  30   a  is directly located in the donor web path just the length of a frame  11  preceding the print line  60 , the leading edge of a frame is located exactly at the print line  60  as the leading edge of a successive frame is located between the sensing device  30   a . Hence, as yellow dye of a yellow frame is required to be thermally transferred to a dye receiver medium, it is only needed to position the leading edge of the magenta frame successively after the yellow frame. The frame positioning is achieved by the cooperation of the sensing device  30   a , the controller  40  and the power generator  23   a . The method for identifying the leading edge of each frame is described below. 
     The operation for the identification of the color of a frame is described as follows. As the dye donor web  10  is delivered through the sensing device  30   a , the optical sensors  32   a  detect the light intensity of transmission of light beams  311  through the donor web  10  and generate analog signals for identifying frames colors. These signals are then transformed into digital signals by comparing them with a reference signal. The light beams  311  include a bright red light beam (R′)  311   a , a weak red light beam (R)  312   a  and a green light beam (G)  313   a . The digital signals for detecting the light intensity of transmission of each light beam through different color frames are listed in the FIG.  11 . The digital signals for detecting the transmission light intensity for a yellow frame are the same as those for an overlay frame, i.e. both codes are ( 111 ), and the digital signals for detecting the transmission light intensity through the other color frames are different. Hence, in order to distinguish the yellow and overlay frames, a sensor mark is inserted at the leading edge of each yellow or frame of the dye donor web  10 . Referring to FIGS. 7 to  10 , a sensor mark  106  added at the leading edge of a yellow frame  101  consists of three sections which are transparent or colored (e.g. black). The digital signals for detecting the transmission light intensity through the sensor mark  106  is different from those through the overlay and yellow frame. The digital signals for detecting the sensor marks  106  shown in FIGS. 7 to  10  are ( 010 ), ( 011 ), ( 001 ) and ( 101 ), respectively. They are not to be confused with the digital signals of the color frames  101 ,  102 ,  103 ,  104  and  105 . Hence, the frames color identification is achieved with the method of the present invention. For example, when the digital signals are changed from ( 111 ) to ( 110 ), the leading edge of a yellow frame is moved at the print line  60 . Similarly, when the digital signals are changed from ( 110 ) to ( 100 ), the leading edge of a magenta frame is moved at the print line  60 . As the digital signals are changed from ( 111 ) to any one of the codes ( 010 ), ( 011 ), ( 001 ) or ( 101 ), the leading edge of an overlay frame is located at the print line  60 . In addition, the leading edge positioning of each frame with high precision is easily achieved by identifying the variance of the digital signals. 
     Referring to FIG. 12, to sum up, the donor web positioning method of the present invention comprises the following steps: 
       101 . control the web drive  20  to move the dye donor web  10  with the controller  40 ; 
       102 . detect the light intensity of transmission of the light beams  311  going through the frame  11  into the sensing device  30   a  with the optical sensors  32   a;    
       103 . generate digital signals according to the detection result in the step  102 ; 
       104 . check if the digital signals are changed; if YES, go to the next step; otherwise, go to the step  101 ; 
       105 . check if the desired color of the frame  11  is arrived; if YES, go to the next step; otherwise, go to the step  101 ; 
       106 . stop moving the dye donor web  10  by controlling the web drive  20  with the controller  40 ; and 
       107 . thermally print. 
     With the donor web positioning apparatus and method of the invention, the rewinding motor, encode wheel and encode sensor used in the prior art are not needed without reducing the web positioning precision, and this indeed reduces the cost and assembly complexity of a thermal color printer. 
     It is noted that the donor web positioning apparatus and method for a thermal color printer described above are the preferred embodiments of the present invention for the purposes of illustration only, and are not intended as a definition of the limits and scope of the invention disclosed. Any modifications and variations that may be apparent to a person skilled in the art are intended to be included within the scope of the present invention.