JPH0234363A - Drive method of thermal head - Google Patents

Abstract

PURPOSE:To prevent the generation of density unevenness, contour fading, the deterioration of gradation, etc., by conducting intermediate recording while fluctuating voltage applied to the common electrode of at least a heating element on the basis of the temperature data of a thermal head. CONSTITUTION:Applied voltage VH where specified density is obtained at a temperature detected by a head-temperature detector is determined, and applied to a common electrode 18. A printing data is stored in a shift register 17 while being synchronized with a clock signal, output to an AND gate 15 in parallel at the same time as a latch signal, and held to a latch circuit 16. When a strobe signal is input from a terminal 19, a heating resistance element 13 is driven selectively through a switching element 14 both by the strobe signal and the data of the latch circuit 16, and thermal transfer is performed by voltage VH from the common electrode 18. Accordingly, the thermal efficiency of the heating element is improved, no density unevenness, contour fading, gradation deterioration, etc., is generated, and intermediate tone recording at high speed can be performed.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for driving a thermal head, and more particularly to a method for driving a thermal head that performs halftone recording while performing temperature compensation based on environmental temperature.

[Conventional technology]

Conventionally, thermal printing methods have been widely used as output printers for office automation equipment and video equipment due to their superiority in maintainability, running cost, image quality, and the like. In particular, in order to simultaneously improve image quality and printing speed, thermal printers that use direct-drive line-type thermal heads capable of recording halftones have become mainstream.

The drive circuit for the thermal head has a numerical configuration as shown in FIG. 2, and is driven according to the head drive timing chart shown in FIG. 3.

In this conventional thermal head driving method, one line of data divided for each gradation level is loaded into a shift register, and at the same time as latching, each heating resistor element is energized for a time corresponding to the strobe pulse width, and this is repeated a number of times. This is a method of recording halftones by controlling the heat generation energy. In this case, temperature compensation is usually performed by changing and controlling the strobe pulse width depending on the temperature of the thermal head. Furthermore, the gradation characteristics are also usually controlled by changing the strobe pulse width or by inputting data to which γ characteristics are added.

[Problem to be solved by the invention]

However, in such conventional methods, the strobe pulse width is changed for each environmental temperature (head temperature) and for each gradation level, so the variable range of the strobe pulse width becomes too large, resulting in the following problems. was occurring.

First, if the strobe pulse width is shorter than the data transfer time for the next gradation level, there will be a period of no electricity until the end of the data transfer, resulting in a cooling period, so the temperature of the thermal head will decrease as shown in Figure 4. As shown in the figure, the temperature rose in a sawtooth pattern, and the current application time tl until reaching the predetermined temperature TI became extremely long. This was very uneconomical due to poor heat storage efficiency in the glaze layer on the thermal head.

Furthermore, due to the effect of heat release during the cooling period, adjacent heating elements are heated, causing color development of the thermal paper or transfer of ink from the transfer sheet to the receiving paper, resulting in blurred outlines and low density color development, resulting in image The quality was significantly degraded.

On the other hand, if the strobe pulse width is too long, it will take a very long time to energize one line, which will shorten the pause time for head cooling within the prescribed one line period, resulting in thermal The next line is energized before the head is sufficiently cooled. This causes density unevenness in which density increases line by line within the same screen, which can be followed by temperature compensation control using strobe pulse width control.

Furthermore, in order to prevent the sawtooth temperature rise characteristic shown in FIG. 4, there is an example in which printing is performed by continuously applying a constant strobe width that is longer than the data transfer time. However, this method has the disadvantage that sufficient gradation reproduction cannot be obtained because the gradation characteristics (γ curve) must be added to the data in advance. Because this is done by controlling the width, as mentioned above, there are cases where the head does not have enough time to cool down after printing one line, and residual heat remains when printing the next line, causing uneven density and blurred outlines. It became.

SUMMARY OF THE INVENTION An object of the present invention is to solve the problems of the conventional method and to provide a thermal head driving method that has good thermal efficiency and can perform high-speed printing without blurring the outline or lowering the gradation.

[Means to solve the problem]

In order to achieve the above object, in a first embodiment of the present invention, parallel data corresponding to an image signal is arbitrarily input to each heating element of a thermal head in which a plurality of heating elements are arranged in a row, and the A thermal head driving method for performing halftone recording is characterized in that halftone recording is performed while changing at least the voltage applied to the common electrode of the heating element based on temperature data of the thermal head.

Further, in a second embodiment of the present invention, parallel data corresponding to an image signal is arbitrarily applied to each heating element of a thermal head in which a plurality of heating elements are arranged in a row, and halftone recording is performed line sequentially. The thermal head driving method is characterized in that the applied voltage applied to the common electrode of the heating element is varied based on the gradation data and the temperature data of the thermal head, and halftone recording is performed in synchronization with the thermal head driving. shall be.

(Function) According to the first aspect of the present invention, at least the applied voltage applied to the common electrode of the heating element is changed based on the temperature data of the thermal head, and halftone recording is performed while temperature compensation is being performed. . Therefore, the load on strobe pulse width control can be reduced and its variable range can be narrowed.

Furthermore, since there is no gradation drop, it is possible to output gradation levels equivalent to human input signals.

According to the second aspect of the present invention, the voltage applied to the common electrode of the heating element is changed based on the gradation data and the temperature data of the thermal head, and halftone recording is performed in synchronization with the driving of the thermal head. . Therefore, since the strobe pulse width is constant and does not need to be controlled, density unevenness and outline blurring do not occur. Furthermore, since there is no gradation drop, gradation output equivalent to that of human input signals can be obtained at high speed.

〔Example〕

The present invention will be described in detail below with reference to the drawings.

FIG. 1 is a block diagram showing a control circuit of a thermal printer for implementing the present invention, in which 1 is a color image input terminal for R, G, and B, 2 is a color conversion circuit, 3 is a data line memory, and 4 is a control circuit for a thermal printer. is a halftone control circuit that controls the energy applied to the thermal head to produce halftones of color images;
5 is a gradation ROM that stores gradation data suitable for the characteristics of consumables such as thermal paper and ink sheets, and 6 is a D/ROM that stores digital data of the applied voltage determined based on the head temperature.
^ Conversion and amplification circuit, 7 is a control unit that controls the entire system of the thermal printer, 8 is a thermal head,
9 is a temperature detection element for detecting head temperature; IO is an ink sheet (not used in the case of thermal paper); 11 is a receiving paper or thermal paper that receives ink separated from the ink sheet by melting or sublimation; and 12 is thermal paper. It is a platen roller.

FIG. 2 is a schematic diagram of the thermal head 8 and its drive circuit, which are used in the conventional method and also used in this embodiment. 13 is a heat generating resistor element, 14 is a switching transistor, 15 is an AND gate between a data signal and a strobe signal, 16 is a latch circuit, 17 is a shift register that converts a serial data signal into a parallel data signal, and 18 is a heat generating resistor element 13. a common electrode for applying an applied voltage, 19
2 is a strobe signal input terminal, and 20 is a data signal input terminal.

Hereinafter, the operation of the first embodiment of the first form of the present invention will be described first.

From an image input terminal 1, B, G, and R signals corresponding to Y, M, and C prints are sequentially input from a color image memory (not shown), and a color conversion circuit 2 inputs Y, M signals, respectively.
, is converted into a C signal.

From the color conversion circuit 2, the image data is stored in a data line memory 3 for each print line, and is transferred line by line to the halftone control unit 4. The image data for this one line is 1
The data is divided into gradation levels within the printing time of a line, and for each gradation level, data for one line is transferred to the data input terminal 20 of the thermal head 8 as many times as the number of gradations. On the other hand, in order to obtain gradation characteristics corresponding to the different consumable materials for each color of Y, M, and C, the amount of energy given to the heating resistor element 13 is stored in advance for each gradation level from the gradation ROM 5. The signals are sequentially read out and transferred to the strobe signal input terminal 19 as a strobe signal. Furthermore, the thermal head temperature at that point is detected by the temperature detection element 9, and the control unit 7 sets the applied voltage to obtain a predetermined concentration at the temperature.
After determining this, the data is converted into analog data by the D/A converter 6, amplified, and then applied to the common electrode 18 as an applied voltage.

Through the above operations, a predetermined applied voltage and strobe pulse width are determined and sent to the heating resistor element 13. Thermal energy of the thermal head 8 sublimates or melts the sublimable ink or meltable ink applied to the ink sheet 10, and the sublimable ink or meltable ink is applied to the receiving paper 11, thereby obtaining a desired multi-tone image.

FIG. 5 shows a timing chart for the thermal head driving method implemented in this example. DATA.

CLOCK, LATCH, 5TRB, and VH represent a data signal, a clock signal, a latch signal, a strobe signal, and an applied voltage, respectively.

The data signal DATA is 4 MHz in synchronization with the clock signal CLOCK, and one gradation level of one line (512 pixels) is transferred to the shift register 17 in 128 μsec.

The strobe signal 5TRB is read out in advance from the ROM 5, which is stored as energy amount data required for each gradation level, as many as the number of gradations (64 gradations) so that the minimum pulse width is 128 μsec.

Furthermore, the latch signal LATII:H is the strobe signal 5T.
It is set to be synchronized with RB and latched in synchronization with the falling edge of strobe signal 5TRB.

The applied voltage VH increases the head temperature by 10°C from 15°C to 55°C.
It is set to be divided into four stages at intervals of 0.degree. C., and sent as 2-bit data to the D/A conversion circuit 6 to apply a desired voltage. This head temperature data is detected at the start of printing one screen and is set to be fixed during printing one screen.

Next, the sequence of this driving method will be explained.

First, an applied voltage that provides a predetermined concentration at the temperature detected by the head temperature detection element 9 is determined.
is applied to the common electrode 18.

Next, the data signal DA is synchronized with the clock signal CLOC.
TA is stored serially in the shift register 17, and simultaneously with the latch signal LATCH, it is stored in the AND gate 15 in parallel.
is output to. Here, a predetermined voltage is applied to a predetermined heat generating resistor element 13 by a logical sum with a strobe signal which is the data of the gradation ROM 5, causing heat generation.

This operation is repeated 64 times for each gradation, forming one line of a 64-gradation image.

The energization time for this one line is about 12 m5 ec, including the preheating time for 0th gradation printing, and if the 1 line printing cycle is a typical 33.3 m5 ec,
The cooling time of each heating resistor element 13 can be as long as 21.3 m5ec.

FIG. 6 schematically shows a temperature characteristic diagram in this case.

In the thermal head driving method according to the first aspect of the present invention, the strobe signal can be applied in a nearly continuous manner, so the temperature rise characteristic does not show a microscopic sawtooth temperature rise, but shows a continuous curve. and rise. This shows good thermal efficiency.

In addition, since there is sufficient cooling time after printing one line,
There is almost no thermal effect on the next line printing.

Furthermore, the strobe pulse width does not change depending on the head temperature, and sufficient cooling time can be taken regardless of the environmental temperature, so that the influence of temperature on image density is extremely small. Furthermore, it is possible to adapt to other consumable materials by simply changing the applied voltage, and for example, it is easy to share thermal paper and sublimation transfer in one printer.

In this embodiment, a head driving method using a sublimation transfer type full color printer has been described, but it goes without saying that similar effects can be seen in a thermal melting type full color printer or a multi-gradation printer using thermal paper.

Next, a second embodiment of the first embodiment of the present invention will be described.

The head is driven using the same device as in the previous example and with the same timing, but the voltage V)l applied to the common electrode for temperature compensation is changed from 15°C to 55°C.
Divided into 20 steps at intervals of ℃, D/
The voltage was sent to the A conversion circuit 6, and settings were made to apply a desired voltage. Furthermore, temperature detection was performed for each line printed, and printing was performed with settings such that the applied voltage changed to a predetermined value depending on the temperature data even within one screen. As a result, 1
There was no difference in density at the switching point of the applied voltage within the screen, and no change in density was observed even after continuous printing of 10 sheets. In addition, the printed matter was a high-quality image with no density unevenness, no blurring of outlines, and no deterioration of gradation.

Next, the operation of the first example in the second form of the present invention will be described. However, in this example, the halftone control section 4 and the D/^ converter 6 are also connected as shown by broken lines in FIG.

Similar to the embodiment of the first form of the present invention described above,
From the image input terminal 1, B is output corresponding to prints Y, M, and C from a color image memory (not shown).

G and R signals are sequentially inputted manually and converted into Y, M and C signals, respectively, in the color conversion circuit 2. From the color conversion circuit 2, the image data is stored in a data line memory 3 for each print line, and is transferred line by line to the halftone control unit 4. This one line of image data is divided into gradation levels within the printing time of one line, and one line is divided into gradation levels for each gradation level.
The thermal head 8 transfers the data for the line as many times as the number of gradations.
The data is transferred to the data input terminal 20 of.

On the other hand, a latch signal is sent in synchronization with the data transfer of each gradation level, and a strobe signal having a time equal to the data transfer time is transferred to the strobe signal input terminal 19 so as to always have a constant pulse width. Furthermore, the head temperature at that point is detected by the temperature detection element 9, and the halftone control section 4 detects the head temperature at that point.
Based on the temperature and color data, gradation data that provides a predetermined density is read out from the gradation ROM 5, and this gradation data is converted into analog data by the D/8 converter 6, amplified, and then applied as an applied voltage. It is applied to the common electrode 18 in synchronization with the latch signal.

Through the above operations, a predetermined applied voltage is determined and sent to the heating resistor element 13. Thermal energy of the thermal head 8 sublimates or melts the sublimable ink or meltable ink applied to the ink sheet 10 and is received by the receiving paper 11 to obtain a desired multi-tone image.

FIG. 7 shows a timing chart for the thermal head driving method implemented in this example. The data signal DATA is transferred to the shift register 17 at 4 MHz for one gradation level of one line (512 pixels) in 128 μsec in synchronization with the clock signal CLOCK.

Strobe signal 5TRB and latch signal LATCH are synchronized with data signal DATA and have a pulse width of 128 μs.
ec and the period is set to 130 μsec. Furthermore, the temperature data shows that the head temperature was increased from 15°C to 55°C.
It is divided into four stages at 0°C intervals and sent to the halftone control section 4 as 2-bit data, the II scale data corresponding to this temperature data is read out from the gradation table in the ROM 5, and the data corresponding to each gradation level is changed to the applied voltage. It is input to the common electrode V old 8 as analog data.

This head temperature data is detected at the start of one screen,
Set it to be fixed during one screen.

Next, the sequence of this driving method will be explained.

First, the temperature detected by the head temperature detection element 9 is recorded in the halftone control section 4. Next, clock signal CL
The data signal DA'TAh is stored in the shift register 17 serially in synchronization with OC, and the latch signal LATCH
At the same time, it is sent to the AND gate 15 in parallel. Here, the number of pulses corresponding to the data signal [IAT^ is determined by performing a logical sum with a strobe signal having a constant pulse width (128 μsec). Further, according to the gradation data read from the ROM 5 using temperature data and color data, the applied voltage corresponding to each gradation level is applied to each heating resistor element 1.
3 and generates heat. This operation is performed for each gradation by 64
This is repeated several times to form one line of a 64-gradation image.

The energization time of this - line is about 10 m5ec, including the preheating time for 0th gradation printing, and if the l line printing cycle is about 33.3m5ec, which is the general case,
The cooling time for each printing line of each heating resistor element 13 is 23 hours as in the embodiment of the first embodiment of the present invention described above.
．． It can be taken very long at 3m5ec. Therefore, the temperature rise characteristic diagram in this case is also schematically shown in FIG. 6. In the thermal head driving method according to the second embodiment of the present invention, the strobe signal is applied in a nearly continuous manner, so the temperature rise characteristic does not show a microscopic sawtooth temperature rise, but instead shows a continuous curve. It is clear that the thermal efficiency is good because it rises as it draws. Furthermore, since there is sufficient cooling time after printing one line, there is almost no thermal influence on the next line. Furthermore, as described above, it is possible to handle other consumable materials by simply changing the applied voltage, and for example, it is easy to share thermal paper and sublimation transfer in one printer.

In this example, we have explained the head driving method using a sublimation transfer type full color printer, but of course the same effect can be seen in a thermal melting type full color printer or a multi-gradation printer using thermal paper. Needless to say.

Next, a second example of the second form of the present invention will be described.

The same device as in the first embodiment is used and the head is driven at the same timing, but the gradation table for each temperature is divided into 20 stages from 15°C to 55°C at 2°C intervals. It was stored in the gradation ROM5. Furthermore, temperature detection is performed for each line printed, and if there is a change in temperature data even within one screen, the gradation table corresponding to the temperature data is read out and applied voltage is applied to the thermal head 8.
was applied to. As a result, there was no difference in density at the switching point of the applied voltage within one screen, and no change in density was observed even after continuous printing of 10 sheets. The prints also had no density unevenness, resulting in high-quality images with no blurring of outlines or deterioration of gradation.

(Comparative example) As a comparative example, the head drive circuit shown in Fig. 2 was used.
Image output was also performed using the timing chart shown in the figure.

This is a method of driving by fixing the applied voltage at a constant value and changing the strobe pulse width for each gradation and for each head temperature. According to this method, the strobe width that changes for each gradation level must also be changed depending on the environmental temperature. For example, the range of the strobe pulse width becomes extremely large, such as the maximum pulse width of 240 .mu.sec at 15.degree. C. and the minimum pulse width of 110 .mu.sec at 55.degree. Furthermore, since the minimum pulse width was shorter than the data transfer time (128 μsec), as shown in Figure 4, there was a cooling time for each gradation print, resulting in a sawtooth temperature rise, resulting in extremely poor thermal efficiency. . Furthermore, when the maximum pulse width was long, the cooling time until the next line was printed was shortened, which appeared in the image as uneven density and blurred outlines.

〔Effect of the invention〕

As is clear from the above description, according to the first embodiment of the present invention, temperature compensation is performed by controlling at least the voltage applied to the common electrode of the heating element based on the temperature data of the thermal head. Therefore, the thermal efficiency of the heating element is good, and high-speed halftone recording can be performed without causing density unevenness, outline blurring, or gradation deterioration.

According to the second aspect of the present invention, the applied voltage applied to the common electrode of the heating element is varied based on the gradation data and the temperature data of the thermal head, and gradation control and temperature control are performed in synchronization with the thermal head drive. Since compensation is performed, the thermal efficiency of the heating element is also good, and density unevenness and
High-speed halftone recording can be performed without causing outline blurring or gradation deterioration.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing a control circuit of a thermal printer for implementing the present invention. FIG. 2 is a block diagram showing a head drive circuit for implementing the present invention and conventionally used numerically. , FIG. 3 is a timing chart of the conventional driving method, FIG. 4 is a temperature characteristic diagram of the conventional driving method, FIG. 5 is a timing chart of the thermal head driving method according to the first embodiment of the present invention, and FIG. 6 is a timing chart of the present invention. Temperature characteristic diagram of the driving method. FIG. 7 is a timing chart of the thermal head driving method in the second embodiment of the present invention. 19... Strobe signal input terminal, 20... Data signal input terminal. 1... Color image input terminal, 5... Gradation ROM storing gradation data 6... D/
A conversion circuit, 8... Thermal head, 9... Temperature detection element, 1O... Ink sheet, 11... Receiving paper, 12... Platen roller, 13... Heat generating resistor element, 15. ...And gate, ■8...Applied voltage common electrode, head drive circuit port Figure 2 Electrofishing block diagram with real part of Mudori Figure 1 Timing button for VH Ishibarai U17J method Figure 3 Thermal head drive timing chart ↑- Fig. 5 Humidity special 1st stage diagram of Seizokukan @ Nagisa Temperature characteristic diagram of the wooden receiver B111 drive vJ structure Fig. 6

Claims (1)

[Claims] 1) Parallel data corresponding to an image signal is arbitrarily input to each heating element of a thermal head in which a plurality of heating elements are arranged in a row, and halftone recording is performed line-sequentially. The thermal head driving method according to the present invention, wherein halftone recording is performed while changing at least the voltage applied to the common electrode of the heating element based on temperature data of the thermal head. 2) A thermal head driving method in which parallel data corresponding to an image signal is arbitrarily input to each heating element of a thermal head in which a plurality of heating elements are arranged in a row, and halftone recording is performed line-sequentially. A thermal head driving method characterized in that the applied voltage applied to the common electrode of the heating element is varied based on gradation data and temperature data of the thermal head, and halftone recording is performed in synchronization with thermal head driving. .