Controlling method for heat of thermal head
This application is a continuation of application Ser. No. 07/938,788, now abandoned filed Sep. 1, 1992. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a method for controlling heat of a thermal head in a thermal printer for carrying out a printing on a recording medium such as form, film or the like, and is particularly concerned with a method for controlling heat of a thermal head for which a heat generated state is properly controlled to produce a better quality printing. 
     2. Description of the Prior Art 
     Generally, a thermal head is formed to have a plurality of heat-generating elements arrayed in a single or plural row on a metallic or ceramic substrate, and a printing operation is performed by actuating each heat-generating element by applying a current selectively thereto according to printing information. 
     The thermal head brings about a temperature rise on each heat-generating element and substrate at the time of printing, therefore in consideration of the temperature rise, an arrangement is such that a power-on time to each heat-generating element is adjusted to prevent an uneven printing from resulting. 
     For the correction, a past record correcting method and an area correcting method were employed. 
     The past record correcting method comprises correcting a power-on time in accordance with a past power-on state of the heat-generating element to which to carry a current and its neighboring heat-generating elements. 
     On the other hand, temperature of the substrate gradually rises from carrying a current to the heat-generating element, therefore if correction is not applied there may arise a difference in print density between the beginning and the end of a line, and hence the power-on time will be corrected therefor in the area correcting method. 
     A power-on time T on  for the area correction is controlled by a drive circuit wherein, as shown in FIG. 2, a control command is written by turns in a plurality of latches connected to a data bus, thereby outputting a power to each heat-generating element. For obtaining output to 48 dots (48 heat-generating elements) by means, for example, of an 8-bit latch, six latches will be used. Further, a control of the power-on time by the drive circuit is effected according to the control command written in each latch from accessing by turns to the plurality of latches through the data bus, therefore the power-on time to each heat-generating element cannot be controlled by a resolution finer than a minimum power-on time T res  (resolution) expressed by the product TN of a time T for accessing one latch and a number N of the latches, and hence the power-on time to each heat-generating element is controlled by means of a value given integral times as high as the minimum power-on time (T res  ×n). 
     However, in the prior art controlling method for heat of a thermal head described as above, since a power-on time to each heat-generating element is controlled by means of the minimum power-on time multiplied integrally, a problem inherent therein is such that a fine control of the power-on time for securing a quality print is no more realizable for the recent high-speed requirement of printing, a multiplicity of the heat-generating elements (characters more than 48 dots being used generally on the latest thermal printer) and so forth. 
     Additionally, a size of each heat-generating element is 0.1 mm or below, and hence when the power-on time to each heat-generating element is to be controlled minutely, a drive circuit complicate in construction must be used, a big-sized system is entailed all the more and a cost increases inevitably as well. 
     SUMMARY OF THE INVENTION 
     This invention has been done in view of the problems mentioned above, and its object is to provide a method for controlling heat of a thermal head wherein the aforementioned problems inherent in the prior art system are overcome, a power-on time to each heat-generating element can be controlled more minutely without complicating a construction of the drive circuit as ever before, a uniform and quality print is obtainable despite a high-speed requirement of printing and multiplicity of the heat-generating elements. 
     Another object of this invention refers to a controlling method for, controlling the quantity of heat generated on the thermal head by correcting a power-on time to each heat-generating element of the thermal head with a plurality of heat-generating elements arrayed on a substrate, thereby printing characters on a recording medium, which is characterized in that the power-on time to each heat-generating element is calculated on a time smaller than a minimum power-on time, and where the calculated power-on time to each heat-generating element is integral times as high as the minimum power-on time, a current is carried to each heat-generating element exactly for the calculated power-on time, but where the calculated power-on time to each heat-generating element is different from the value integral times as high as the minimum power-on time, whether a printing place of the heat-generating element in the column direction is an odd-number place or an even-number place is decided, the power-on time in each place is given by an integral value obtained from carrying up fractions below a decimal point of the calculated power-on time in either one of the places and by an integral value obtained from rounding down fractions below a decimal point of the calculated power-on time in the other place. 
     According to the controlling method constructed as above, a power-on time to each heat-generating element is calculated on a time smaller than a minimum power-on time, and if the calculated power-on time to each heat-generating element is integral times as high as the minimum power-on time, a current is carried to each heat-generating element exactly for the calculated power-on time, but if the calculated power-on time to each heat-generating element is different from the value integral times as high as the minimum power-on time, whether a printing place of the heat-generating element in the column direction is an odd-number place or an even-number place is decided, the power-on time in each place is given by an integral value obtained from carrying up fractions below a decimal point of the calculated power-on time in either one of the places and by an integral value obtained from rounding down fractions below a decimal point of the calculated power-on time in the other place, thereby obtaining apparently a print quality by a power-on time more minutely than the minimum power-on time. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1. is a block diagram representing a hardware configuration which realizes a method for controlling heat of a thermal head according to this invention. 
     FIG. 2 is a block diagram representing a circuit configuration to which the method of this invention is applied. 
     FIG. 3 is a flowchart for determining a power-on time to heat-generating elements of a thermal head given in a first embodiment of this invention. 
     FIG. 4 is a flowchart representing a second embodiment to which the method according to this invention is applied. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of this invention will now be described with reference to FIG. 1 to FIG. 4. In this connection, a thermal head with 48 heat-generating elements disposed in one row is subjected to control in the embodiments. 
     FIG. 1 is a block diagram representing a hardware configuration of a thermal head to which a controlling method for heat of a thermal head relating to this invention is applied, FIG. 2 is a block diagram representing a circuit configuration, and FIG. 3 is a flowchart for determining a power-on time to heat-generating elements of the thermal head. 
     As illustrated in FIG. 1, a hardware configuration of the thermal head to which the method according to this invention is applied comes in a circuit A for calculating a power-on time T on  at every heat-generating element, a latch control circuit B for outputting the power-on time at every heat-generating element, an 8-bit latch in 6 pieces, and a thermal head with 48 heat-generating elements disposed in one row. 
     Further, as shown in FIG. 2, a circuit configuration of the hardware is such that control commands based on a calculation result of the circuit A operating for calculation of the power-on time at every heat-generating element are accessed by turns to each latch through a data bus to writing therein, and an electric power is outputted in latch units to each heat-generating element for a desired power-on time from each latch controlled by the latch control circuit. Then, as mentioned hereinbefore, a control of the power-on time according to such circuit configuration is effected on control commands written in each latch by accessing by turns to the plurality of latches through the data bus, therefore the power-on time to each heat-generating element is controlled with a minimum power-on time T res  (resolution) expressed by the product TN of a time T for accessing one latch and a number N of the latches as a minimum unit. 
     Described next is a method for determining the power-on time to each heat-generating element of the thermal head according to this embodiment with reference to FIG. 3. 
     First, in step ST11, an area correction time for all the heat-generating elements is calculated, and a calculated power-on time T ona  is obtained in a precision of 1/2 of the minimum power-on time (resolution) T res . 
     Next, in step ST12, whether or not the calculated power-on time T ona  is integral times as high as the minimum power-on T res  is decided, and if YES (T ona  =T res  ×n), the calculated power-on time T ona  is chosen as the power-on time T on , but if NO {T ona  =T res  ×(n+1/2)}, the flow proceeds to next step ST14. 
     In step ST14, whether a printing place of the print position is odd-number place or even-number place is decided according to printing information with reference to the print position in the column direction included in the printing information, and if YES (odd-number place), the flow proceeds to step ST15, where the power-on time T on  is obtained from adding 1/2 of the minimum power-on time T res  to the calculated power-on time T ona , but if NO (even-number place), then the flow proceeds to step ST16, where the power-on time T on  is obtained from subtracting 1/2 of the minimum power-on time T res  from the calculated power-on time T ona . 
     An operation of the embodiment constructed as above will be described next. 
     In this embodiment, the arrangement is such that the power-on time T ona  is calculated in the unit of 1/2 of the minimum power-on time T res , and when the calculated power-on time T ona  results in indicating a value integral times as high as the minimum power-on time T res , the calculated power-on time T ona  is decided to be the power-on time T on , but when the calculated power-on time T ona  is different from the value integral times as high as the minimum power-on time T res , whether the printing place of a print position is odd-number place or even-number place is decided, and if the odd-number place, the power-on time T on  is obtained from adding 1/2 of the minimum power-on time T res  to the calculated power-on time T ona , but if the even-number place, then the power-on time T on  is obtained from subtracting 1/2 of the minimum power-on time T res  from the calculated power-on time T ona , thereby varying the power-on time T on  according to the printing place despite the calculated power-on time T ona  being identical. 
     The calculated power-on time and the power-on time are exemplified in TABLE 1. 
     
                                           TABLE 1                                 
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Calculated Power-on Time and Power-on Time (Unit: Min. power-on time Tres 
= 1)                                                                      
       Printing place                                                     
       Print position in the column direction                             
       1      2      3      4      5      6      7      8                 
       Odd-number                                                         
              Even-number                                                 
                     Odd-number                                           
                            Even-number                                   
                                   Odd-number                             
                                          Even-number                     
                                                 Odd-number               
                                                        Even-number       
Item   place  place  place  place  place  place  place  place             
__________________________________________________________________________
Calculated                                                                
       2.5    2.5    2.0    2.5    2.5    2.0    2.5    2.5               
power-on                                                                  
time Tona                                                                 
Power-on                                                                  
       3      2      2      2      3      2      3      2                 
time Ton                                                                  
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     As will be apparent from TABLE 1, the calculated power-on time T ona  is 2.5 of the minimum power-on time T res , but this value is different from that integral times as high as the minimum power-on time T res  and the print position comes on an odd-number place, therefore the power-on time T on  on the first column in the direction of printing place takes a value 3 obtained from adding 1/2 of the minimum power-on time T res  to the calculated power-on time T ona  being 2.5. Further, the calculated power-on time T ona  is 2.5 of the minimum power-on time T res , but this value is different from that integral times as high as the minimum power-on time T res  and the print position comes on an even-number place, therefore the power-on time T on  on the second column in the direction of printing place takes a value 2 obtained from subtracting 1/2 of the minimum power-on time T res  from the calculated power-on time T ona  being 2.5. Still further, the calculated power-on time T ona  is 2.0 of the minimum power-on time T res , but this value coincides with that integral times as high as the minimum power-on time T res , therefore the power-on time T on  on the third column in the direction of printing place becomes 2 identical with that of the calculated power-on time T ona  irrespective of odd-number place and even-number place of the print position. 
     Thus, the print quality according to the controlling method for heat of a thermal head of this invention is ensured likewise apparently as in the case where printing is effected by controlling the power-on time T on  in the unit of 1/2 of the minimum power-on time T res  to each heat-generating element, and a precision (resolution) of the minimum power-on time T res  of the controllable power-on time T on  can be improved double. 
     Further, the number of heat-generating elements to which the method according to this invention is applied may be more than one, and hence it is not necessarily limited to the number specified in this embodiment, and thus, needless to say, the method is applicable to a multiplicity of heat-generating elements such as, for example, 64 pieces, 166 pieces, 1,024 pieces and so forth. 
     FIG. 4 is a flowchart for determining a power-on time to heat-generating elements of a thermal head given in a second embodiment to which the controlling method for heat of a thermal head relating to this invention is applied, and in this embodiment, the construction is such that where the calculated power-on time T ona  is different from that integral times as high as the minimum power-on time T res , the power-on time T on  is obtained from subtracting 1/2 of the minimum power-on time T res  from the calculated power-on time T ona  when the printing place of a print position is an odd-number place, and the power-on time T on  is obtained from adding 1/2 of the minimum power-on time T res  to the calculated power-on time T ona  when the printing place of a print position is an even-number place. Then, since the other construction is similar to the first embodiment described hereinabove, a further description will be omitted here. 
     According to such construction, an advantage similar to the first embodiment may be realized. 
     Further, this invention is not necessarily limited to the embodiments mentioned above, and hence may be modified as occasion demands. For example, the invention is applicable to a control of the power-on time in the direction of row of the heat-generating elements. 
     As described above, according to this invention, the power-on time can be changed according to printing places despite the calculated power-on time being identical. That is, a print quality according to such method for controlling heat of a thermal head is capable of enhancing and also substantially unifying apparently a precision (resolution) of the minimum power-on time of a controllable power-on time, therefore the power-on time to heat-generating elements can be controlled minutely for obtaining a quality printing to cope with a high-speed requirement of printing and a multiplicity of the heat-generating elements in number, and an advantage exceedingly prominent in economical need may be realized such that a large-sized construction of a system and a complication of a circuit configuration can securely be prevented.