Abstract:
A method of correcting a conveyance error of a continuous sheet having a train of segments each including a respective segment mark and being conveyed for printing in accordance with the number of steps is disclosed. The method counts steps between consecutive segment marks during conveyance of the sheet to thereby output an actual number of steps, compares the actual number of steps with a preselected number of steps determined by a distance between the consecutive segment marks, and corrects, based on the result of comparison, the start of a new line at even positions of a single segment during subsequent conveyance. An apparatus for practicing the method is also disclosed.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a printer for printing an image on a sheet while conveying the sheet and more particularly to a method of correcting a sheet conveyance error and an apparatus therefor. 
     Printers extensively used today include a printer of the type using a continuous sheet made up of a train of segments. Each segment of this kind of sheet is provided with a respective segment mark beforehand. The problem with this type of printer is that by simply sensing the segment mark of the first segment to cause it to start at a preselected position, it is impossible to prevent an image from being shifted on the last segment. Because the shift of an image is ascribable even to the varying environment and the deterioration of a platen, it cannot be obviated by mechanical adjustment. Moreover, a conveyance error is apt to occur due to, e.g., an error in the diameter of the platen. Such errors would accumulate and also bring about the shift of an image. 
     In light of the above, it has been proposed to correct a conveyance error at the time of starting a new line. Japanese Patent Laid-Open Publication Nos. 63-1352277 and 63-137876, for example, each teach a system that provides a printing medium with marks and correct the positional error of a new line at the time of printing without regard to the kind of a new line pitch or the amount of a new line. However, the new line width corrected by the systems taught in the above documents is constant, so that an odd conveyance error of, e.g., ½ step and ⅓ step cannot be corrected. The systems therefore cannot obviate the shift of an image and make it difficult to print an image at the preselected position of a preselected format. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a method capable of correcting the conveyance error of a sheet to thereby insure constant, accurate conveyance, and an apparatus therefor. 
     It is another object of the present invention to provide a printer capable of conveying a sheet constantly and accurately and easily printing an image at the preselected position of a preselected format. 
     In accordance with the present invention, a method of correcting the conveyance error of a continuous sheet having a train of segments each including the respective segment mark and being conveyed for printing in accordance with the number of steps begins with a step of counting the steps between consecutive segment marks during conveyance of the sheet to thereby output an actual number of steps. The actual number of steps is compared with a preselected number of steps determined by a distance between the consecutive segment marks to thereby output a result of comparison. The start of a new line is corrected on the basis of the result of comparison at even positions of a single segment during subsequent conveyance. 
     Also, in accordance with the present invention, an apparatus for correcting the conveyance error of a continuous sheet having a train of segments each including the respective segment mark and being conveyed for printing in accordance with a number of steps includes a counting section for counting the steps between consecutive segment marks during conveyance of the sheet to thereby output an actual number of steps. A comparing section compares the actual number of steps with a preselected number of steps determined by a distance between the consecutive segment marks to thereby output a result of comparison. A correcting section corrects, based on the result of comparison, the start of a new line at even positions of a single segment during subsequent conveyance. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description taken with the accompanying drawings in which: 
     FIG. 1 is a fragmentary plan view showing a continuous sheet having a series of segments on which segment marks are printed beforehand; 
     FIG. 2 is a block diagram schematically showing a thermal printer operable with the continuous sheet; 
     FIG. 3 is a view showing the start of a new line in accordance with a conveyance error correcting method embodying the present invention; and 
     FIG. 4 is a flowchart demonstrating a specific operation of the illustrative embodiment. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1 of the drawings, a continuous sheet  1  apapplicable to a thermal line printer, which is a specific form of a printer, is shown. As shown, the sheet  1  has a train of segments divided by dotted lines and is conveyed in a direction indicated by an arrow. Segment marks  2  each are printed on the right edge of the respective segment in the vicinity of the leading edge. 
     A specific configuration of the thermal line printer is shown in FIG.  2 . As shown, the printer includes a thermal head  3  for printing images on the sheet  1  with heat. A platen  4  faces the thermal head  3  and conveys the sheet  1  by being driven by a feed motor  5 . A gearbox  6  accommodates a gear mechanism, not shown, for transferring the output torque of the feed motor  5  to the platen  4 . A mark sensor  7  senses the segment mark  2  of each segment of the sheet  1  being conveyed. A storage  8  stores a preselected number of steps determined by the distance between nearby segment marks  2 . A controller  9  controllably drives the feed motor  5  in accordance with a signal output from the mark sensor  7  and the preselected number of steps, as will be described more specifically later. The feed motor  5  is implemented by a stepping motor whose rotation angle is controlled in accordance with the number of steps. 
     When the printer having the configuration of FIG. 2 sequentially prints images on the consecutive segments of the sheet  1  of FIG. 1, a range between the position where the mark sensor  7 senses the first segment mark  2  and the position where it senses the second segment mark  2  is a single segment. So long as the conveyance of the sheet  1  is free from an error, the next segment mark  2  is surely sensed if the feed motor  5  is driven by a number of steps corresponding to the preselected number of steps. However, when an error occurs in the conveyance, the mark sensor  2  senses the second segment mark  2  before the feed motor  5  reaches the preselected number of steps or does not sense it even after the feed motor  5  has reached the preselected number of steps. 
     To obviate the above occurrence, the illustrative embodiment counts the steps of the feed motor  5  during the interval between the sensing of the first segment mark  2  and the sensing of the next segment mark following the printing of a single segment. The number of steps counted (actual number of steps hereinafter) is compared with the preselected number of steps stored in the storage  8 . 
     Referring also to FIG. 3, if the actual number of steps is smaller than the preselected number of steps, a difference between the former and the latter is evenly omitted from the new line portions of the next segment when an image is printed on the segment. If the actual number of steps is greater than the preselected number of steps, a difference between the former and the latter is evenly added to the new line port ions of the above segment. In this manner, a conveyance error is adequately corrected. 
     More specifically, assume that the thermal line printer has the following specifications: 
     
       
         
               
               
               
             
           
               
                   
                   
               
             
             
               
                   
                 line step 
                 one line = one step 
               
               
                   
                 print feed line 
                 one line = 0.125 mm 
               
               
                   
                 line 
                 one character = thirty-two lines 
               
               
                   
                   
                 one new line = eight lines 
               
               
                   
                   
               
             
          
         
       
     
     When the printer with the above specifications prints an image on a 100 mm long segment, the number of lines is twenty, the number of print feed lines is 800, and an adequate number of steps (preselected number of steps) is 800. 
     Assume that an error of +1% occurred in the conveyance of the sheet  1 . Then, the sheet  1  is conveyed by 101 mm when 800 lines are fully printed thereon. Stated another way, the sheet  1  is conveyed by 100 mm when only 792 lines are printed thereon, causing the mark sensor  7  to sense the next segment mark  2 . That is, 792 steps are counted between the sensing of the first segment mark  2  and the sensing of the next segment mark  2 . 792 steps is smaller than 800 steps, or preselected number of steps, by eight steps (1 mm). 
     In such a case, when an image is printed on the next segment of the sheet  1 , the number of steps between the lines is evenly reduced by one once for a distance of 12.5 mm over the entire segment so as to correct the error. The prerequisite is that one step be omitted at the time of a new line after the printing of characters; should one step be omitted during printing of characters, the characters would be smeared out. More specifically, because the third line is being printed at the position corresponding to 12.5 mm, the new line width between the third and fourth lines is reduced by one line, implementing seven lines. 
     On the other hand, when the conveyance error is −1%, the sheet  1  is conveyed only by 99 mm when 800 lines are fully printed on its first segment. Stated another way, the mark sensor  7  senses the next segment mark  2  when 808 lines are printed on the first segment; that is 808 steps are counted between the sensing of the first segment mark  2  and the sensing of the next segment mark  2 . 808 steps is greater than the preselected 800 steps by eight steps (1 mm). In this case, when an image is printed on the next segment of the sheet  1 , the number of steps between the lines is evenly increased by one step once for the distance of 12.5 mm so as to correct the error. This must also be done at the time of a new line after the printing of characters; otherwise, the characters would become discontinuous. Again, because the third line is being printed at the position corresponding to 12.5 mm, the new line width between the third and fourth lines is increased by one line, implementing nine lines. 
     Reference will be made to FIG. 4 for describing the operation of the illustrative embodiment, i.e., the controller  9  more specifically. As shown, after the start of the operation, the controller  9  drives the feed motor  5  in order to convey the sheet  1 . At the same time, the controller  9  causes the thermal head  3  to print an image on the first segment of the sheet  1  (step S 1 ). The controller  9  counts the number of steps of the feed motor  5  (step S 2 ) and, in this sense, plays the role of counting means. 
     Subsequently, the controller  9  determines whether or not the mark sensor  7  has sensed two consecutive segment marks  2  (step S 3 ). If the answer of the step S 3  is positive (Y), the controller  9  determines that the sheet  1  has been successfully conveyed by a single segment. The controller  9  then determines whether or not the actual number of steps counted is equal to the preselected number of steps (step S 4 ). If the answer of the step S 4  is Y, the controller  9  continues the conveyance of the sheet  1  and prints an image on the next segment (step S 10 ). If the answer of the step S 4  is negative (N), the controller  9  determines whether or not the actual number of steps is greater than the preselected number of steps (step S 5 ). In the steps S 4  and S 5 , the controller  9  serves as comparing means. 
     If the answer of the step S 5  is Y, the controller  9  produces a difference between the actual number of steps and the preselected number of steps (step S 6 ). At this instant, the controller  9  plays the role of difference calculating means. 
     After the step S 6 , the controller  9  evenly omits the difference between the actual number of steps and the preselected number of steps from the new line portions of the next segment of the sheet  1  (step S 7 ). The step S 7  is followed by the step S 10 . As for the step S 7 , the controller  9  serves as omitting means. 
     If the answer of the step S 5  is N, meaning that the actual number of steps is smaller than the preselected number of steps, the controller  9  calculates a difference between the former and the latter (step S 8 ). At this instant, too, the controller  9  serves as difference calculating means. Subsequently, the controller  9  the evenly adds the above difference to the new line portions of the next segment of the sheet  1  (step S 9 ) and, in this sense, plays the role of adding means. The omitting means and adding means constitute step correcting means in combination. Further, the difference calculating means and step correcting means constitute correcting means in combination. 
     While the illustrative embodiment has concentrated on a thermal line printer, the present invention is similarly applicable to a thermal serial printer. 
     In summary, it wiII be seen that the present invention provides a sheet conveyance error correcting method and an apparatus therefor capable of compensating for the influence of irregularity in the diameter of a platen and the expansion and contraction of a sheet ascribable to the varying environment, thereby insuring accurate sheet conveyance. This successfully obviates the shift of an image on the sheet, i.e., allows an image to be accurately printed at the preselected position of a preselected format without fail. 
     Various modifications will become possible for those skilled in the art after receiving the teachings of the present disclosure without departing from the scope thereof.