JP2006088658A - Thermal printer and thermal printing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To suppress the occurrence of density unevenness occurring due to transfer load fluctuations of recording sheet caused by a thermal head. <P>SOLUTION: Printing energy at the time of recording one line by the thermal head 6 is calculated. A transfer load fluctuation amount is calculated on the basis of the printing energy. The relationship between the transfer load fluctuation amount and a transfer speed of a platen roller 7 is calculated beforehand and stored in an LUT memory 26. The transfer speed Vp of the platen roller 7 is calculated on the basis of the transfer load fluctuation amount for each line from the LUT memory 26. The speed of the platen roller 7 is rotationally controlled so as to become the transfer speed Vp. The transfer speed Vp of the platen roller 7 is changed in the direction of canceling the transfer load fluctuation amount. Tension of the recording sheet becomes almost constant. The fluctuations of pixel density caused by the transfer load fluctuations are reduced. The occurrence of the density unevenness can be suppressed. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a thermal printer and a thermal printing method for thermally recording an image on a recording paper using a thermal head.

  2. Description of the Related Art There is known a thermal printer that heats a thermal recording paper provided with a thermal coloring layer with a thermal head to color and record an image. In the thermal head, a large number of heating elements are arranged in a line along the main scanning direction. The thermal printer records an image line by line while relatively moving the thermal head and the thermal recording paper in the sub-scanning direction. Each heating element is driven based on image data for one line, and gives printing energy to the thermal recording paper.

  The coefficient of friction between the thermal recording paper and the thermal head varies depending on the printing energy. That is, when the printing energy is large, the temperature of the heating element increases and the friction coefficient decreases, while when the printing energy is small, the temperature of the heating element decreases and the friction coefficient increases. Therefore, in the portion where the density of the original image changes suddenly, the fluctuation of the printing energy is large, so that the fluctuation of the friction coefficient also becomes large. When the friction coefficient varies, the conveyance load of the thermal recording paper varies, so the conveyance pitch of the thermal recording paper changes. As a result, there is a problem that the printing energy given per unit area changes and density unevenness occurs.

For example, as shown in Patent Document 1, there is described a thermal printer that does not generate density unevenness even when a load fluctuation caused by the density change occurs. In this thermal printer, the load amount of the thermal head is obtained for each line, the load fluctuation amount is obtained from the difference between the load amount of the line to be recorded and the load amount of the adjacent line, and recording is performed based on the load fluctuation amount. The printing energy of the line to be corrected is corrected. For this reason, even when the conveyance pitch of the thermal recording paper is changed due to fluctuations in the conveyance load, the printing energy is also corrected in accordance with the change, thereby preventing density unevenness.
JP 2002-67370 A

  By the way, even if the printing energy is corrected based on the conveyance load variation as described above, it does not act directly on the recording paper, and the conveyance load variation is suppressed in the form of correction of the printing energy. There is a problem that correction cannot be performed.

  SUMMARY OF THE INVENTION An object of the present invention is to provide a thermal printing method and a thermal printer capable of suppressing the occurrence of density unevenness due to a change in conveyance load based on a change in printing energy when recording each line with a simple configuration.

  In order to achieve the above object, according to the present invention, a recording head supported by a platen roller is configured to heat each heating element in accordance with image data using a thermal head in which a plurality of heating elements are arranged along the main scanning direction. In a thermal printer that feeds paper in a sub-scanning direction intersecting the main scanning direction and records an image line by line, image data of each pixel for one line is converted into printing energy to be applied to the recording paper, and this printing A conveyance load fluctuation amount calculation unit for obtaining a conveyance load fluctuation amount at the time of recording each line based on energy, and the conveyance load fluctuation amount from the conveyance load fluctuation amount calculation unit based on the conveyance load fluctuation amount, A platen roller control means for obtaining a control amount of the platen roller and controlling the platen roller in accordance with the control amount is provided.

  In addition, the present invention provides a thermal head in which a plurality of heating elements are arranged along the main scanning direction to cause each heating element to generate heat in accordance with the image data, and the recording paper supported by the platen roller is placed in the main scanning direction. In the thermal printing method in which an image is recorded line by line in the sub-scanning direction crossing the line, image data of each pixel for one line is converted into printing energy to be given to the recording paper, and each line is based on this printing energy. Obtaining a conveyance load fluctuation amount at the time of recording, obtaining a control amount of the platen roller for suppressing the conveyance load fluctuation of the recording paper based on the obtained conveyance load fluctuation amount, and controlling the platen roller according to the control amount. It is characterized by. The control amount of the platen roller is preferably the rotational speed of the platen roller or the pressing force of the platen roller against the heating element.

  According to the present invention, the image data of each pixel for one line is converted into printing energy to be applied to the recording paper, the amount of conveyance load fluctuation at the time of recording each line is obtained based on this printing energy, and the obtained conveyance load variation is obtained. The control amount of the platen roller for suppressing the change in the conveyance load of the recording paper is obtained based on the amount, and the platen roller is controlled in accordance with the control amount. Variations can be predicted. Then, by controlling the rotation speed of the platen roller or the pressing force of the platen roller against the heating element so as to suppress the fluctuation of the conveyance load, the fluctuation of the conveyance load can be suppressed with a simple configuration. The occurrence of uneven color can be suppressed.

  FIG. 1 is a schematic view showing a main part of a color thermal printer embodying the present invention. In this color thermal printer, a long color thermal recording paper (hereinafter simply referred to as recording paper) 2 fed from a recording paper roll (not shown) is used. The recording paper 2 is fed by the conveying roller pair 3 in a paper feeding direction and a printing direction opposite to the paper feeding direction. The conveyance roller pair 3 is composed of a capstan roller 3a driven by a conveyance motor 4 composed of a pulse motor, and a pinch roller 3b that is movable between a position where it is in pressure contact with the capstan roller 3a and a position where it is separated. Has been. The transport motor 4 is controlled by the controller 10 via the driver 4a.

  As is well known, the recording paper 2 has a cyan thermosensitive coloring layer, a magenta thermosensitive coloring layer, a yellow thermosensitive coloring layer, and a protective layer sequentially formed on a support. The yellow thermosensitive coloring layer, which is the uppermost layer as the thermosensitive layer, has the highest thermal sensitivity and develops yellow with a small amount of heat energy. The cyan thermosensitive coloring layer, which is the lowermost layer, has the lowest thermal sensitivity and develops cyan with large heat energy. The yellow thermosensitive coloring layer loses its coloring ability when irradiated with 420 nm blue-violet light. The magenta thermosensitive coloring layer develops magenta with a thermal energy intermediate between the yellow thermosensitive coloring layer and the cyan thermosensitive coloring layer, and loses the coloring ability when irradiated with near ultraviolet rays of 365 nm.

  A thermal head 6 is disposed on the downstream side in the paper feeding direction of the pair of conveying rollers 3. The thermal head 6 includes a heating element array 6a in which a large number of heating elements are arranged in a line, and is fixed in the printer.

  A platen roller 7 is arranged at a position facing the thermal head 6 so as to sandwich the recording paper 2. The platen roller 7 is movable in the vertical direction, and is biased in a direction in which it is pressed against the thermal head 6 by a spring (not shown). A platen motor 8 made up of a pulse motor is connected to the platen roller 7. The platen motor 8 is rotationally controlled by the controller 10 via a driver 8a. Then, the rotation of the platen roller 7 is controlled separately from the conveyance motor 4, and the platen roller 7 is controlled in such a direction as to cancel the additional conveyance fluctuation caused by the fluctuation of the printing energy for each line.

  When the recording paper 2 is conveyed in the printing direction by the conveying roller pair 3, the thermal head 6 generates heat at each heating element of the heating element array 6 a to a predetermined temperature, and each thermal coloring in the recording area of the recording paper 2. Color the layer. The platen roller 7 is lowered by a shift mechanism (not shown) using a cam, a solenoid or the like at the time of paper supply and paper discharge, thereby forming a clearance for passing the recording paper 2 between the thermal head 6.

  On the downstream side of the thermal head 6 in the paper feeding direction, an optical fixing device 11 is arranged facing the recording paper 2. The optical fixing device 11 includes a yellow fixing lamp 12, a magenta fixing lamp 13, and a reflector 14. The yellow fixing lamp 12 emits blue-violet light having an emission peak of 420 nm, and the magenta fixing lamp 13 emits ultraviolet light having an emission peak of 365 nm. By irradiating the fixing light from these fixing lamps 12 and 13, the yellow thermosensitive coloring layer and the magenta thermosensitive coloring layer of the recording paper 2 are fixed so as not to develop color even when heated.

  A cutter 16 is provided on the downstream side of the optical fixing unit 11 in the paper feeding direction. The cutter 16 separates the full color image from the unrecorded area. As a result, the sheet-like recording paper is discharged from the discharge port (not shown) to the outside of the printer.

  The controller 10 controls each unit to record a full color image on the recording paper 2 by three-color surface sequential recording of yellow, cyan, and magenta in the same recording area. Further, as shown in FIG. 2, in order to suppress the occurrence of density unevenness based on the fluctuation in the conveyance load caused by the fluctuation amount of the printing energy at the time of recording each line, the conveyance load fluctuation suppression process is performed. In this transport load suppression process, the platen motor 8 is rotationally controlled so that the transport speed Vp of the platen roller 7 is different from the transport speed Vo of the pair of transport rollers 3, thereby canceling the transport load fluctuations during printing of each line.

  For this reason, the controller 10 includes a frame memory 21, a line memory 22, an energization time calculation control unit 23, a conveyance load variation amount calculation unit 25 for obtaining a conveyance load variation amount based on the printing energy of each line, and a platen from the conveyance load variation amount. A platen roller transport speed conversion LUT 26 for determining the transport speed of the roller 7, a transport load fluctuation / transport speed calculation unit 27, and an LUT writing unit 28 are provided.

  The image data written in the frame memory 21 is sent to the line memory 22 line by line. The line memory 22 stores image data for a plurality of lines, for example, 5 lines. The image data input to the line memory 22 is sent to the energization time calculation control unit 23 and the transport load fluctuation amount calculation unit 25 line by line. The energization time calculation control unit 23 obtains energization time control data for each heating element based on one line of image data, and sends this to the thermal head 6. The head drive unit of the thermal head 6 causes each heating element to generate heat with the energization time control data. In synchronization with the driving of the heat generating elements, the rotation of the conveying roller pair 3 is controlled via the driver 4a and the conveying motor 4, and an image is recorded on the recording paper 2 line by line.

  The transport load fluctuation amount calculation unit 25 includes an energy conversion LUT memory 25a, a dynamic friction coefficient LUT memory 25b, and a work memory 25c. In the energy conversion LUT memory 25a, a table is written in which the gradation level of the image data is used as an address and the printing energy given to the recording paper 2 from the heating element of the thermal head 6 is used as data. In the dynamic friction coefficient LUT memory 25b, a table indicating the relationship between the printing energy of each heating element and the dynamic friction coefficient of the surface of the recording paper 2 and the heating element is written. In this table, the dynamic friction coefficient decreases as the printing energy of the heating element increases. The table is obtained in advance for each type of recording paper by experiments or the like, and is written in the dynamic friction coefficient LUT memory 25b. Then, a corresponding table is selected according to the type of recording paper 2 to be used. The work memory 25c temporarily stores the calculation result and the like in the transport load fluctuation amount calculation unit 25.

The transport load fluctuation amount calculation unit 25 refers to the energy conversion LUT memory 25a to convert the image data of the i-th line and the j-th dot captured from the line memory 22 into the printing energy E (i, j), which is converted into the work energy. Store in the memory 25c. Then, the dynamic friction coefficient μ (E (i, j)) corresponding to the printing energy E (i, j) is read from the dynamic friction coefficient LUT memory 25b, and this is multiplied by the pressing force P with which the thermal head 6 presses the recording paper 2. Thus, for the image data of the i-th line and the j-th dot, the transport load amount that the corresponding heating element applies to the recording paper 2 is obtained. The transport load amount is obtained for all the heating elements by the following equation, and the transport load amount F (i) received by the recording paper when the thermal head records the i-th line image on the recording paper is calculated.
F (i) = ∫P · μ (E (i, j)) dj

  Then, by calculating a difference from the transport load amount F (i−1) of the (i−1) -th line as the previous line, the transport load fluctuation amount ΔF (i) (= F (i) of the i-th line. -F (i-1)).

  Next, the platen roller conveyance speed conversion LUT memory 26 calculates the conveyance speed Vp of the platen roller 7 when recording each line with each heating element based on the obtained conveyance load fluctuation amount ΔF (i) of the i-th line. The platen roller 7 is driven at this transport speed Vp. FIG. 3 shows an example of the relationship between the conveyance load fluctuation amount ΔF (i) of each line and the conveyance speed Vp of the platen roller 7 for canceling the fluctuation of the recording paper conveyance speed due to the conveyance load fluctuation. This relationship is obtained in the form of a look-up table (LUT), and a table using the transport load fluctuation amount ΔF (i) as an address and the transport speed of the platen roller at this time as data is written by the LUT writing unit 28. It is.

  Therefore, during the thermal recording of each line, when the conveyance load fluctuation becomes + side (the conveyance load of the current line (i-th line) becomes larger than the conveyance load of the previous line (i-1th line)), the corresponding amount. As a result, the transport speed Vp of the platen roller 7 is increased, the transport speed of the platen roller 7 is slightly higher than the transport speed Vo of the transport roller pair 3, and the transport load fluctuation is canceled. As a result, the amount of recording paper transported per line is slightly reduced, and the occurrence of density unevenness is suppressed accordingly. Further, when the conveyance load fluctuation becomes negative in the thermal recording of each line, the conveyance speed Vp of the platen roller 7 is lowered by that amount, and the conveyance speed of the platen roller 7 is slightly lower than the conveyance speed Vo by the conveyance roller pair 3. The transfer load fluctuation is canceled out. As a result, the amount of recording paper transported per line is slightly increased, thereby suppressing the occurrence of density unevenness. Further, when there is no change in the conveyance load or when it is slight, the platen roller 7 is controlled to rotate at the same speed Vp = Vo as the conveyance roller pair 3.

  3 is shifted from “0” to a position on the “+” side (indicated by a two-dot chain line in FIG. 3), the transport speed Vp of the platen roller 7 is always set to the transport roller pair. The control is to decelerate and apply braking to the transport speed Vo of 3, and the behavior is always in only one direction, so that density unevenness correction based on transport load fluctuations can be easily performed. Conversely, the control of constantly increasing the speed of the platen roller 7 may be performed by shifting the reference position of the fluctuation amount of the transport load from the position “0” to the position “−”.

  The relationship between the transport load fluctuation amount ΔF (i) and the transport speed Vp of the platen roller 7 for canceling this is obtained in advance by the transport load fluctuation / transport speed calculator 27 as follows. First, the pulse generator 17 is connected to the drive shaft 7 a of the platen roller 7 in order to detect fluctuations in the conveyance speed Vp of the platen roller 7. Then, the printing energy of each line is added so as to change the conveyance load fluctuation amount ΔF (i) at the time of each line recording step by step, and the fluctuation amount of the conveyance speed at the platen roller 7 at the time of line recording at this time is expressed by a pulse generator 17. From the output of. Next, the transport speed Vp of the platen roller 7 is obtained so as to cancel out this transport speed fluctuation amount. That is, when the conveyance speed of the current line recording is slightly higher than the previous line recording due to the fluctuation of the conveyance load due to the fluctuation of the printing energy, the conveyance speed Vp of the platen roller 7 is made higher than the conveyance speed Vo of the conveyance roller pair 3. And the platen roller 7 is braked. Further, when the conveyance speed of the current line is slightly lower than the previous line recording due to fluctuations in printing energy, the conveyance speed Vp of the platen roller 7 is increased more than the conveyance speed Vo of the conveyance roller pair 3. As a result, even if the printing energy fluctuates during recording on each line and the conveying load fluctuates, the recording paper tension between the conveying roller pair 3 and the platen roller 7 can be made substantially the same, and density unevenness occurs. Is suppressed.

  The relationship between the transport load fluctuation amount ΔF (i) and the transport speed Vp of the platen roller 7 is obtained for each type of the recording paper 2 according to the difference in thickness, size, etc. of the recording paper 2, and the recording paper type The most suitable one is used according to. In addition to obtaining the above relationship for each type of recording paper, one of the above relationships is obtained for a typical or average recording paper, and a recording coefficient to be used is applied by applying a correction coefficient for each recording paper type. You may obtain | require optimal conveyance speed of the platen roller for every paper type.

  Each data of the platen roller conveyance speed conversion LUT 26 for canceling the conveyance load fluctuation is obtained for each printer before shipment after the assembly is completed, and is stored in a nonvolatile memory such as an EEPROM by the LUT writing unit 28. Instead of obtaining and storing before shipment, the data may be periodically obtained after printing a certain amount after shipment, or the data may be updated at the time of repair. Further, the pulse generator 17 or the like may be connected to the platen roller 7 only at the time of data acquisition at the factory and removed from the product printer. In addition to obtaining data individually for each printer, the data obtained by the representative machine may be used as it is or after being corrected for the same or similar products.

  Next, the operation of the above embodiment will be described. As shown in FIG. 1, at the time of printing, the recording paper 2 is fed in the printing direction by the conveying roller pair 3. When it is detected that the leading edge of the recording area of the recording paper 2 has reached the thermal head 6, the heating element array 6a generates heat, and one line of yellow image is recorded on the yellow thermosensitive coloring layer. Thereafter, the yellow image is recorded line by line in synchronization with the feeding of the recording paper 2.

  At the time of this line recording, the printing energy of each heating element for each line is obtained in advance based on the image data. Then, the conveyance load of each line is obtained based on the printing energy. Then, the conveyance load fluctuation amount ΔF (i) for each line is obtained from the conveyance load between the previous line and the current line. Next, the conveyance speed Vp of the platen roller 7 is obtained from the conveyance load fluctuation amount ΔF (i), and when the conveyance load fluctuation of the recording paper 2 occurs, the platen roller 7 is rotationally controlled in a direction to cancel the fluctuation. Therefore, the printing energy fluctuation occurs when each line is recorded, and even when the printing paper 2 conveyance load fluctuates due to this printing energy fluctuation, the conveyance speed of the recording paper 2 is controlled by the platen roller 7. The recording paper 2 is always conveyed at a substantially constant speed, and the occurrence of density unevenness is suppressed.

  In addition, the conveyance load fluctuation amount ΔF (i) is predicted based on the printing energy for each line, and the conveyance speed Vp of the platen roller 7 is controlled at the time of line recording in which the conveyance load fluctuation occurs, so that the platen roller is eliminated in the direction to eliminate the conveyance load fluctuation. Since the roller 7 is rotationally controlled, the occurrence of uneven density due to fluctuations in printing energy can be accurately suppressed.

  When the recording of all the lines of the yellow image is completed, the feeding of the recording paper 2 in the printing direction is stopped, and then the recording paper 2 is fed in the paper feeding direction. When the recording paper is fed in the paper feeding direction, the platen roller 7 is moved away from the thermal head 6 by a shift mechanism (not shown). Simultaneously with the conveyance in the paper feeding direction, the yellow fixing lamp 12 of the optical fixing device 11 is turned on to fix the yellow thermosensitive coloring layer in the recording area of the recording paper 2. When the optical fixing for the recording area is completed, the conveyance motor 4 stops. Thereafter, the recording paper 2 is fed in the printing direction, and magenta recording and fixing of the magenta thermosensitive coloring layer are performed in the same manner as yellow recording, and further cyan recording is performed. Also in this magenta recording and cyan recording, the conveyance speed Vp of the platen roller 7 is controlled based on the conveyance load fluctuation amount, and each color recording without density unevenness is performed.

  When the printed portion of the recording paper 2 is discharged out of the apparatus, the cutter 16 operates after the transport motor 4 is stopped. As a result, the recording area is separated from the unrecorded area, and a sheet-like color print is obtained. In the color print obtained by the color thermal printer according to the present embodiment, the recording position of each line is the same, so that density unevenness due to the variation in the transport load is eliminated. Further, since the recording position of each line is the same for each color, the occurrence of color unevenness is eliminated.

  In the above embodiment, a one-head three-pass color thermal printer has been described as an example. However, the present invention may be applied to a three-head one-pass color thermal printer as shown in FIG. In addition, the same code | symbol is attached | subjected to the same structural member as the said embodiment. Also in this case, the platen motors 33, 34, and 35 are provided on the platen rollers 30, 31, and 32 for the respective colors, and the rotational speeds of the platen rollers 30 to 32 are controlled separately from the conveyance roller pairs 36, 37, and 38. . Then, in the print stages 41 to 43 of each color, the conveyance load fluctuation amount of each line is obtained, and the platen rollers 30 to 32 are rotationally controlled so as to cancel the conveyance load fluctuation based on the fluctuation amount. Suppresses the occurrence of density unevenness based on load fluctuation. Reference numeral 44 denotes a yellow fixing device, and 45 denotes a magenta fixing device.

  Further, in the above embodiment, the density unevenness caused by the conveyance load fluctuation amount ΔF (i) based on the printing energy fluctuation is eliminated by controlling the conveyance speed Vp of the platen roller 7, but instead, As shown in FIG. 5, in addition to the conveyance speed Vp, the recording paper 2 between the conveyance roller pair 3 and the platen roller 7 is moved in a direction intersecting the conveyance direction by a free roller 50 and a roller shift mechanism 51, As shown in FIG. 6, by changing the pressing force of the platen roller 7 against the heating element array 6a of the thermal head 6 by the pressing force adjusting mechanism 52, the back tension of the recording paper 2 is changed, thereby causing fluctuations in the conveyance load. The occurrence of uneven density may be suppressed.

  In the above embodiment, the color thermal printer has been described. However, other printers in which the conveyance load fluctuates due to the fluctuation amount of the printing energy of each line, for example, a sublimation type thermal ink using a color ink sheet of yellow, magenta, and cyan. The present invention may be implemented in a melt type thermal transfer printer.

1 is a schematic view showing a color thermal printer of the present invention. It is a flowchart which shows the conveyance speed control process of a platen roller based on image data. It is a graph which shows an example of the relationship between conveyance load fluctuation amount and the conveyance speed of a platen roller. It is the schematic of another embodiment which shows the color thermal printer of a 1 pass 3 head system. FIG. 10 is a schematic view showing a main part of another embodiment in which the recording paper tension is made constant by changing the position of the free roller between the platen roller and the conveying roller pair. It is the schematic which shows the principal part of another embodiment which makes recording paper tension | tensile_strength constant by changing the pressing force with respect to the recording paper of a platen roller.

Explanation of symbols

2 Recording Paper 3 Conveying Roller Pair 4 Conveying Motor 6 Thermal Head 6a Heating Element Array 7 Platen Roller 8 Platen Motor 10 Controller 11 Optical Fixing Unit 17 Pulse Generator 1
30 to 32 Platen rollers 33 to 35 Platen motors 36 to 38 Conveying roller pair 50 Free roller 51 Roller shift mechanism 52 Pressing force adjustment mechanism

Claims (4)

Using a thermal head in which a plurality of heating elements are arranged along the main scanning direction, each heating element is heated according to the image data, and the recording paper supported by the platen roller intersects the main scanning direction. In a thermal printer that records images line by line,
A conveyance load fluctuation amount calculation unit for converting image data of each pixel for one line into printing energy to be applied to the recording paper, and obtaining a conveyance load fluctuation amount at the time of recording each line based on the printing energy;
A platen roller that obtains a control amount of the platen roller for suppressing the conveyance load variation of the recording paper based on the conveyance load variation amount from the conveyance load variation amount calculation unit and controls the platen roller according to the control amount. And a thermal printer.   2. The thermal printer according to claim 1, wherein the control amount of the platen roller is a rotation speed of the platen roller. Using a thermal head in which a plurality of heating elements are arranged along the main scanning direction, each heating element is heated according to the image data, and the recording paper supported by the platen roller intersects the main scanning direction. In the thermal printing method of recording images one line at a time,
The image data of each pixel for one line is converted into printing energy to be applied to the recording paper, and the conveyance load fluctuation amount at the time of recording each line is obtained based on the printing energy, and the recording paper is calculated based on the obtained conveyance load fluctuation amount. A thermal printing method characterized in that a control amount of the platen roller for suppressing a fluctuation in the transport load is obtained, and the platen roller is controlled in accordance with the control amount. 4. The thermal printing method according to claim 3, wherein the control amount of the platen roller is a rotational speed of the platen roller.

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