JP3017881B2 - Thermal printer - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thermal printer for recording a halftone image by an area gradation method, and more particularly to a thermal printer in which a dot pattern is changed according to the temperature of a thermal head. is there.

[0002]

2. Description of the Related Art In a thermal printer, a thermal head in which a plurality of heating elements are arranged in a line in a main scanning direction is used. Are recorded with ink dots. Further, one pixel is constituted by a plurality of sub-lines arranged in the sub-scanning direction, and the thermal head operates in the sub-scanning direction by one pixel.
Each time it moves by the number of sub-lines, it drives an elongated heating element in the main scanning direction, changes the number of sub-lines on which ink is recorded according to image data, and thereby, the area of ink dots recorded in one pixel A printing method for expressing a halftone is known from Japanese Patent Application Laid-Open No. HEI 4-19163.
issue). This area gradation method is particularly suitable for a printer in which the density of the ink dot itself cannot be adjusted, for example, a fusion-type thermal transfer printer that transfers the melted or softened ink to the recording paper by heating the back of the ink film. is there.

[0003]

In the conventional thermal head, it was difficult to accurately control the area of the ink dot recorded in the pixel according to the image data due to the influence of the heat storage characteristic and the heat radiation characteristic. For example, when printing a low density data portion between continuous high density data portions, etc.,
Due to the local heat storage effect, printing is performed at a higher density than a predetermined density.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a thermal printer which corrects the thermal influence of a thermal head, controls the area of ink dots accurately, and as a result, can print at an accurate density. Is what you do.

[0005]

In order to achieve the above object, a thermal printer according to the present invention comprises a temperature sensor for detecting the temperature of a heating element and at least two sensors corresponding to a temperature range.
A dot pattern LUT storing comparison data for the position of the sub-line, a table selecting means for selecting one of the two types of tables according to the temperature detected by the temperature sensor; A counter for detecting the position of the sub line facing the element, comparison data read from the dot pattern LUT based on the output signal of the counter and image data are compared, and ink is to be recorded on the sub line based on the comparison result. And a driver for controlling the energization of the heating element in accordance with the drive signal.

[0006]

In FIG. 1, a time-series image signal is sent to an A / D converter 11 via an input terminal 10 and synchronized with a sampling signal from a timing signal generation circuit 12,
It is converted into 8-bit image data corresponding to "1" to "256" in decimal system. The 8-bit image data is written to the frame memory 14 controlled by the memory controller 13. At the time of printing, image data of, for example, M pixels for one row arranged in the main scanning direction is sequentially read from the frame memory 14 and the drive signal conversion circuit 15
Send to At the time of monitoring, the image data read from the frame memory 14 is converted into an analog signal by the D / A converter 16, and then sent to a monitor (not shown) via the output terminal 17.

The drive signal conversion circuit 15 receives a clock and a dot clear signal from the timing signal generation circuit 12, a platen synchronization signal from a platen synchronization signal generation circuit 20, and a table switching signal from a system controller 21. The image data is converted into a drive signal in consideration of the temperature of the thermal head 22. That is, a dot pattern corresponding to the image data and the temperature of the thermal head 22 is selected, and the number of sub-lines on which ink is to be printed is determined among the n sub-lines constituting one pixel. In this embodiment, as shown in FIG.
Although the pixel is composed of 16 sub-lines, it is preferable to use "32" to "128" as the number n of sub-lines in order to express almost the same halftone as a photograph.

In order to detect the temperature of the thermal head 22, a temperature sensor, for example, a thermistor 23, is attached to the thermal head 22. This thermistor 23
Since the resistance value changes according to the temperature, the current flowing therethrough changes. This current is converted into a voltage by the voltage conversion circuit 24 and then converted into temperature data by the A / D converter 25. This temperature data is stored in the system controller 2
1 to determine the temperature range.

The M pieces of drive data are sent to the thermal head 22 via the thermal head driver 26. The thermal head driver 26 includes an energization time control circuit 2
7, the energizing time of the heating element is controlled when recording each sub-line.
As shown in FIG. 4, heating elements 22A to 22M are arranged in the main scanning direction in the thermal head 22, and each heating element has a length A in the main scanning direction and a length B in the sub-scanning direction. It has an elongated shape. The length in the sub-scanning direction is 20
Because of its small size of μm, the amount of heat generated by continuous energization becomes too high and burns out in a short time. Then, each heating element 22A ~
Burnout is prevented by pulse driving 22M.

The motor driver 30 is controlled by the system controller 21 to drive the pulse motor 31 to rotate the platen drum 33 around which the recording paper 32 is wound. The pulse motor 31 rotates the platen drum 33 by one sub-step by one pulse. For example, the platen drum 33 is rotated by one step by three sub-steps. In this embodiment, in order to record an image of 16 gradations, the recording paper 32 is moved by the length L of one pixel 37 in the sub-scanning direction in 16 steps of the pulse motor 31, and a fixed width is set for each step. Is supplied to the heating element. The ink film 36 hung on the rollers 34 and 35 comes into close contact with the recording paper 32 and moves together with the recording paper 32 in the direction of the arrow. When the ink film 36 is heated from the back by the thermal head 22, the melted or softened ink is transferred to the recording paper 32.

A rotary encoder 38 is connected to the platen drum 33.
Generates an encoder pulse every time a sub-step is sent,
This is sent to the timing signal generation circuit 12 and the platen synchronization signal generation circuit 20. This timing signal generation circuit 1
2 creates a dot clear signal from the encoder pulse and sends it to the drive signal conversion circuit 15. Further, the platen synchronization signal generation circuit 20 generates one platen synchronization signal every time one encoder pulse is counted, and sends it to the drive signal conversion circuit 15. The system controller 21 performs sequence control of each unit based on the timing signal from the timing signal generation circuit 12.

FIG. 2 shows the configuration of the drive signal conversion circuit 15 and the thermal head driver. The drive signal conversion circuit 15 includes a shift register 40, a latch array 41,
It comprises a comparator array 42. The shift register 40 includes M 8-bit latch circuits 40A to 40M connected in series. These 8-bit latch circuits 40A to 40M have eight input terminals and eight output terminals, and input / output 8-bit image data in parallel. The shift register 40 stores 1 in the main scanning direction.
M image data constituting a row are sequentially input and shifted in synchronization with a clock from the timing signal generation circuit 12.

A latch array 41 is connected to the shift register 40. The latch array 41 includes M 8-bit latch circuits 41A to 41M for latching each image data. Shift register 4
The M image data converted into a parallel state at 0 are:
The data is latched in the latch array 41 by the dot clear signal from the timing signal generation circuit 20. This dot clear signal is one for every 16 falling edges of the platen synchronization signal.
The time becomes "H". The comparator array 42 is connected to the latch array 41. This comparator array 42 is composed of M 8-bit magnitude comparators 42A to 42M, and has a dot pattern LUT
(Look-up table memory) Only when the image data is larger than the comparison data in 43, an "L" drive signal is output.

The thermal head driver 26 is connected to the comparator array 42. The thermal head driver 26 includes M NAND gate circuits 26A.
To 26M, and each NAND gate circuit 26A to 26M
To M, the strobe signal of the conduction time control circuit 27 and the output signal of the comparator are separately input. When the output signal of the comparator is “L” and a strobe signal of “L” is input, the output signal of the NAND gate circuit becomes “L”. This strobe signal regulates the energization time of the heating element, and is generated only once in synchronization with the platen synchronization signal. In this embodiment, since there are 16 sub-lines, 16 sub-lines are generated for one pixel recording.

A heating element array 45 is connected to the thermal head driver 26. The heating element array 45 includes M heating elements 2 arranged in a line in the main scanning direction.
2A to 22M. Each of the heating elements 22A to 22M is
When the corresponding NAND gate circuits 26A to 26M are at "L", power is supplied to heat the back of the ink film 36 and transfer the ink to the recording paper 32.

The counter 44 counts a platen synchronizing signal generated every time the recording paper 32 is moved by one sub-line, and detects a sub-line position within one pixel 37. The counter 44 is reset to zero every 16 falling edges of the platen synchronization signal by the dot clear signal. The 8-bit output signal output from the counter 44 is a dot pattern LU as an address signal.
Sent to T43. The dot pattern LUT 43 stores three types of tables, a low-temperature table shown in Table 1, a medium-temperature table shown in Table 2, and a high-temperature table shown in Table 3. The types of tables may be two, four, and the like.

[0017]

[Table 1]

[0018]

[Table 2]

[0019]

[Table 3]

The dot pattern LUT 43 converts the image data into a dot pattern and compresses the image data having 256 gradations into 16 gradations. The system controller 21 responds to the temperature of the thermal head 22 by
A table switching signal is generated, and one of three types of tables shown in Tables 1 to 3 is selected. Using this selected table, the comparison data corresponding to the output signal of the counter 44 is read and sent to the comparator array 42.

Next, the operation of the above embodiment will be described with reference to FIG. At the time of image input, a scanner or a video player is connected to the input terminal 10, and an analog image signal is input to the A / D converter 11. This A / D converter 1
The image data of 256 tones converted into a digital signal at 1 is written to the frame memory 14.

At the time of printing, the system controller 2
1 drives the pulse motor 31 via the motor driver 30 to rotate the platen drum 33 holding the recording paper 32 stepwise. The rotation of the platen drum 33 is detected by a rotary encoder 38, and an encoder pulse output from the rotary encoder 38 is supplied to the timing signal generation circuit 12
And the platen synchronization signal generating circuit 20. The timing signal generation circuit 12 outputs the timing signal generated based on the encoder pulse to the system controller 21.
Send to The system controller 21 performs sequence control on the basis of the timing signal so that each unit is synchronized.

The timing signal generation circuit 12 generates a dot clear signal based on the encoder pulse,
This is sent to the drive signal conversion circuit 15. The platen synchronizing signal generation circuit 20 also generates one platen synchronizing signal from the encoder pulse every time the platen drum 33 moves by one subline, and sends this to the drive signal conversion circuit 15.

The temperature of the thermal head 22 is detected by the thermistor 23, and the temperature data is taken into the system controller 21. The system controller 21 constantly monitors the temperature of the thermal head 22, and determines whether the temperature of the thermal head 22 is high, medium, or low. For example, in the case of medium temperature,
A switch signal is sent to the dot pattern LUT 43, and a table 2
The table is switched to the medium temperature table shown in (1).

The system controller 21 sends a read signal to the memory controller 13 immediately before the recording paper 32 reaches the print start position. The memory controller 13 sequentially reads out image data of M pixels constituting the first row extending in the main scanning direction from the frame memory 14. The M image data are supplied to the drive signal conversion circuit 15.
To the shift register 40. The shift register 40 shifts and latches M image data in response to a clock from the timing signal generation circuit 12. The latched M image data are latched by the latch array 41 by the dot clear signal output from the timing signal generation circuit 12 when the recording start position of the recording paper 32 faces the heating element array 45.

The counter 44 is reset by the dot clear signal, and its content becomes zero in decimal system. When the value of the counter 44 becomes zero, the comparison data of "15" is outputted from the dot pattern LUT 43 in the decimal system as shown in the table of Table 2. As shown in FIG. 3, when the image data is "30" in decimal notation, a signal of "L" is output from the magnitude comparator 42A. In this state, when the “L” strobe signal is output from the conduction time control circuit 27, the NAND gate circuit 2
The output of 6A becomes "L". The voltage V _P is applied to the heating element 22A.
Is applied, a current flows toward the NAND gate circuit 26A. Since the time during which this current flows is regulated by the strobe signal, a pulse current having a constant width is supplied to the heating element 22A.

The heating element 22A generates heat by the pulse current, heats the back of the ink film 36, and transfers the ink to the recording paper 32. Thus, as shown in FIG. 4, ink dots are recorded on the first sub-line. After this transfer, the platen drum 33 rotates by one sub-line, so that the second sub-line faces the heating element 22A. The content of the counter 44 is incremented by the platen synchronization signal to "1". In this case, the comparison data of “31” is output from the dot pattern LUT 43. Since this comparison data is larger than the image data, the output signal of the 8-bit magnitude comparator 42A becomes "H". As a result, the output signal of the NAND gate circuit 26A becomes "H", so that the heating element 22A is not energized even when the second strobe signal is input. Therefore, the heating element 22A is connected to the first of the pixels 37.
Transfer ink only to sub-line.

Similarly, when the image data is "40", the output of the magnitude comparator 42A is "L" until the content of the counter 44 becomes "2".
The ink is transferred to both the first sub-line and the second sub-line. When the image data is "256", which is the highest value, the output of the magnitude comparator 42A is kept at "L" during the recording of one pixel, so that ink is transferred to all of the first to sixteenth sublines. . If the image data is smaller than “15”, no ink is recorded on any of the sublines.

As described above, the platen drum 33 is
In one step rotation, one pixel 37 is moved by the length L in the sub-scanning direction, and a pulse current is supplied to the heating element during each step to record ink dots of the number of sub-lines according to the image data. . During the recording of the M pixels belonging to the first row, the image data of the M pixels belonging to the second row is input to the 8-bit cyst register 40. Recording continues.

When the thermal printer is used in a high-temperature environment or in a state in which the head has accumulated heat after continuous printing, the heat storage element causes the heating element to exceed the ink transfer temperature even immediately after recording a predetermined number of sub-lines. In many cases, the number of sub-lines is increased due to the characteristics of the ink. Therefore, when the temperature of the thermal head 22 is higher than the predetermined temperature, the system controller 21 sends a table switching signal to the dot pattern LUT 43 to switch to the high temperature table shown in Table 3. Since the high-temperature table has a dot pattern in which the number of times of heat generation of the heating element is smaller by “1”,
As a result, ink dots having the same area as when the medium temperature table is used can be recorded.

When the thermal printer is used in a low-temperature environment or when performing the first printing, the temperature of the ink may not reach the ink transfer temperature even when the power is started because the heating elements are cooled too much. is there. In this case, the number of sub-lines on which ink is recorded decreases.
If the temperature of the thermal head 22 is lower than the predetermined temperature, the system controller 21 sends a table switching signal to the dot pattern LUT 43 to switch to the low temperature table shown in Table 1. This low-temperature table has a dot pattern in which the number of times of heat generation of the heat-generating element is larger by “1”.
In the present embodiment, the increase / decrease of the dot pattern is set to “1”, but this value changes as appropriate depending on the characteristics of the ink.

Although the above embodiment has been described with reference to monochrome printing for convenience, the present invention can also be used for color printing. In this color printing, a decoder is provided between the input terminal 10 and the A / D converter 11 to separate a color video signal into three color signals. Then, each color signal is converted into digital data, and then written into three types of frame memories provided for each color. At the time of printing, the image data is read by designating the frame memory of the color to be printed. Then, the platen drum is rotated three times to record an image in three color planes sequentially. In this color printing, a color ink film in which a cyan ink area, a magenta ink area, and a yellow ink area are formed in a predetermined order is used.

Also, the line printer which moves the recording paper and the ink film one-dimensionally relative to each other has been described.
The present invention can also be used for a serial printer whose relative movement is two-dimensional. In this serial printer, a movable thermal head is used, and a plurality of heating elements are arranged in a line in the recording paper feed direction. Therefore, in a serial printer, the feed direction of the recording paper and the arrangement direction of the heating elements are the main scanning direction,
The moving direction of the thermal head is the sub-scanning direction.

In the above embodiment, the heating element and the sub-line have the same length in the sub-scanning direction. However, the heating element may be longer in consideration of heat resistance. In this case,
The width of the first sub-line is wider than that of the second to sixteenth sub-lines. One sub-line is recorded by one pulse current. By outputting a plurality of strobe signals, a plurality of pulse currents are supplied to the heating element and one sub-line is recorded by a plurality of driving operations. May be. Further, the present invention can be applied to a sublimation type thermal transfer printer, a thermal printer and the like as long as a halftone image is printed by the area gradation method.

[0035]

As described in detail above, according to the present invention, the temperature of the thermal head is monitored and the dot pattern is changed according to the temperature, so that even if the environmental temperature is different, For the same image data, the area of the ink dot can be kept the same. As a result, the same concentration can be maintained.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of a fusion type thermal transfer printer.

FIG. 2 is a block diagram illustrating a configuration of a drive signal conversion circuit and a thermal head driver.

FIG. 3 is a timing chart of the circuit shown in FIG. 2;

FIG. 4 is an explanatory diagram showing a recording state of a heating element.

[Explanation of symbols]

 22 Thermal Head 22A-22M Heating Element 23 Thermistor 37 Pixel 43 Dot Pattern LUT 44 Counter 45 Heating Element Array

Claims (1)

(57) [Claims] 1. A thermal head in which elongated heating elements are arranged in a main scanning direction, and one of the heating elements and one of recording paper are moved in a sub-scanning direction while each heating element constitutes one pixel. And a temperature sensor for detecting the temperature of the heating element, and at least two in accordance with the temperature range.
A dot pattern LUT storing comparison data for the position of the sub-line, a table selecting means for selecting one of the two types of tables according to the temperature detected by the temperature sensor; A counter for detecting the position of the sub line facing the element, comparison data read from the dot pattern LUT based on the output signal of the counter and image data are compared, and ink is to be recorded on the sub line based on the comparison result. A thermal printer, comprising: a comparator for generating a drive signal indicating the fact; and a driver for controlling energization of the heating element according to the drive signal.