JPS62234955A - Thermal head driving system - Google Patents

Abstract

PURPOSE:To prevent thermal accumulation of a thermal head and enable recording with stable density to be performed by subdividing recording data of each line into plural data per single dot, supplying each of subdivided data to a shift register while updating it and controlling the drive pulse width of a thermal element through variation of a high-level signal count. CONSTITUTION:Recording data transmitted serially from a circuit is converted to a parallel data by a series/parallel conversion part 6 of CPU5 to constitute a gradation matrix 7. Further it is converted to a serial data by a parallel/series conversion part 8 and a subdivision data is created referring to a table in ROM10. The data is transmitted to a thermal head drive circuit 11, thus subdividing recording data of a single dot to be given to a single thermal element into plural data segments. After this, these segments are updated and the number of high-level signals in the segments is made variable in accordance with the output of a temperature sensor. Then the drive pulse width of the corresponding thermal element is controlled to prevent heat accumulation of the thermal head.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for driving a thermal head in a thermal recording apparatus.

In general, in a thermal type recording device, a thermal head in which heating elements are arranged in dot units in one line in the main scanning direction is driven to generate heat according to the recording data for each line. Each line is sequentially recorded on the thermal recording paper that is fed in the sub-scanning direction.

FIG. 2 shows an example of the configuration of a thermal head drive circuit. Here, the thermal head 1 in which heating elements H1 to H2O48 for one line are arranged is divided into eight blocks 81 to B.
The blocks 81 to B8 are time-divisionally driven for each line of recording data DATA.

That is, one line of binary recording data DATA sent serially is taken into the shift register 2 with a capacity of 2048 bits, and the register contents are held in the latch circuit 3 according to the latch signal LATCH. and each of the first to eighth strobe signals STB1
5TB8 are sequentially activated to drive each block 81 to B8 in the thermal head 1 in a time-division manner via the driver 4.

In the thermal head 1, as each heating element is repeatedly driven, the substrate gradually accumulates heat, and as a result, the recording s'
There is a problem of thermal effects, such as the temperature becoming darker and the recording dots being crushed.

Therefore, in order to always record with stable density without being affected by heat accumulation in the thermal head, it is necessary to detect the temperature of the substrate of the thermal head with a sensor and respond to the detected temperature as shown in Figure 3. It is necessary to adjust the energy applied to each heating element.

Furthermore, the area gradation method, which is a means of expressing halftones, is also applied to thermal recording.

One of the drawbacks of the area gradation method is the deterioration of the resolution of recorded images.

This problem becomes more serious as the matrix is made larger to increase the number of gradations.

Therefore, one countermeasure is what is called the three-level method.

As shown in Fig. 4, the color density of the thermal recording paper is determined by the energy El applied to each heating element in the thermal head,
It takes advantage of the fact that E2 causes the color to change from black to gray.

By adding gray to the binary values of white and black, it becomes possible to express nine gradations in a 2x2 matrix configuration, as shown in Figure 5, and the apparent resolution improves. Become.

In the figure, W indicates a white part, G indicates a bottom part, and B indicates a black part.

Conventionally, as a means of adjusting the energy applied to the heating element in a thermal head, the pulse width of the strobe signal STB is controlled to make the energization time of the heating element variable while keeping the applied voltage constant. If such a method is used, each strobe signal S will be
All pulse widths of TB1 to TB8 must be made variable, and the circuit configuration for this purpose has become complicated.

Furthermore, it is possible to control the number of pulses while keeping the pulse width of the strobe constant; for example, it is possible to set the number of pulses to 1 for gray and 2 for black. When each heating element is driven, a time interval between pulses occurs, during which time the heating element cools down, resulting in poor thermal efficiency.

1 blood The present invention has been made in consideration of the above points.

The present invention provides a thermal head driving method that allows easy adjustment of the driving pulse width of the thermal head, that is, the energization time of each heating element.

42 In order to achieve the object of the present invention, each line of recording data is given to a shift register, the contents of the register are held in parallel in a latch circuit, and the data is transferred via a driver according to the contents of the latch. When driving each heating element in the thermal head, each line of recording data is divided into multiple pieces of data per dot, and each divided data is updated and fed to the shift register. The number of high-level signals is made variable to control the driving pulse width of the heating element.

Hereinafter, one embodiment of the present invention will be described in detail with reference to the accompanying drawings.

For convenience of explanation, one block of the thermal head has three dots Di, D2, . For example, dot D1 is black and dot D2 is gray.

Assume that dot D3 is to be recorded in white.

Further, the black drive pulse width is 1 ms, the gray drive pulse width is 0.5 ms, and the latch period is 0.1 m5 (latch clock IOK ball).

In this case, the number of divisions of recording data is L (ms) ÷ 0.1 (
ms)+1=11, and the divided data is as shown in the table below.

The control timing by the thermal head drive method according to the present invention in such a device will be explained below with reference to FIG.

First, initial data 1, 1. O is Di, D2. Each bit position in block 1 in the shift register corresponding to D3 is transferred. Thereafter, the first latch signal LATCH- is output at a predetermined timing, and as a result, S1="1", S2="1", and S3="OII".Here, Sl.

S2. Each number g of S3 corresponds to a dot DI, D2 . This is the drive signal for each heating element for D3, and 1'' drives the heating element.

II becomes non-driven at O11. The Sl, S2. The state of each signal in S3 is held until the second latch signal LATCH-■ is output after 7:1721 (0, 1m5).

Furthermore, the contents of the shift register are updated between the time when the first latch signal LATCH-■ is output and the time when the second latch signal LATCH-■ is output. 1.0).

After that, the latch signal LATCH-■ for cylinder 2 is output, and S
l=”1”.

S2="1" and S3="O". Similarly below,
Update of shift register contents and latch signal LATC
Each heating element is driven according to H.

Note that when initial data 1, 1°0 is transferred to block l of the shift register, update data ■ is transferred to block 2 next to it.
However, block 3 has update data ■, block 4 has update data ■, block 5 has update data ■, block 6 has update data ■, and block 7 has update data ■.
However, it is necessary to sequentially transfer the update data ■ to the block 8. When transfer data ■ is being transferred to block 1 of the shift register, update data ■ is coming to block 2, and update data ■ is coming to block 3. Furthermore, when the last updated data 0 is transferred to block 1 of the shift register, initial data for driving that block has arrived in block 2. Therefore 1
When recording of a block is completed, that is, when the strobe signal STB is switched, there is no need for the next data transfer, and it is only necessary to wait for a time or switch the latch signal LATCH. By allowing such recording data to be transferred, the data transfer can be performed efficiently and loss in transfer time can be reduced.

Note that while recording block 1, the corresponding strobe signal 5
Only TB-1 is active.

By sequentially transferring and latching each divided data in block 1 shown in the table above to the shift register, the heating drive signal S1 of dot D1 with a pulse width of 10T (1 ms) is generated as shown in FIG. But again 5T
A heat generation drive signal S2 of D2 having a pulse width of (0,5 m5) is obtained. Also, during the IOT period, the drive signal S3 corresponding to the dot D3 is connected to the non-driven II O7+
is maintained.

Therefore, according to the present invention, the driving pulse width of each heating element in the thermal head can be made variable in units of latch clock cycles in accordance with the number of divisions of recording data.

Therefore, not only three-value control such as the three-level method described above, but also general multi-value control is easily possible. Furthermore, according to the present invention, it is also possible to effectively control the adverse effects of heat accumulation on the substrate in the thermal head. Specifically, the I in the divided data is determined according to the output of the temperature sensor attached to the board of the thermal head.
The number of t I II is made variable. For example, for dot D1, normally it is 11 of 11111111110.
At high temperatures, the drive is performed using 1 divided data.
By driving with divided data of 11111111000 with a reduced number of ``, it is possible to adjust the driving pulse width for the dot D1 appropriately according to the substrate temperature.In that case, all bits of one block are O
When it reaches IT, the recording of that block is completed, so at that point the recording may be moved to the next block to shorten the recording time.

FIG. 1 shows an example of a configuration in which a thermal head driving method according to the present invention is specifically implemented.

Here, the recording data DATA serially sent from a line etc. is converted into parallel data in a serial-to-parallel converter 6 in the CPU 5, forms a gradation matrix 7, and is further converted into serial data by a parallel-to-serial converter 8. After converting to Data DATA' is transferred to the thermal head drive circuit 11. Needless to say, this processing is performed under the control of the CPU 5. For example, the number of heating elements in a thermal head is 2.
048, when performing the 8-division driving, if the clock CLK given to the shift register is 2 MHz, 1
It takes 0.128 m5 to transfer the recording data DATA for a block (256 bits). Therefore, the pulse width of black is 0.768 m5, and the pulse width of gray is 0.384 m5, which is the minimum latch clock period T.
If the power per dot is adjusted so that the divided data becomes 1111110 for black and 1110000 for gray (in this case, pulse width adjustment according to the temperature sensor output is not considered).

Note that the unit change time of the pulse width in the present invention can be shortened by increasing the number of divided drives of the thermal head (reducing the number of bits per block) or by increasing the clock frequency of the shift register.

Since the driving method of the present invention provides a considerable degree of freedom in setting the pulse width for each heating element, it can also be applied to adjust the variation in resistance value of each heating element.

More than a shame, in the thermal head driving method according to the present invention, the recording data for one dot applied to one heating element is divided into a plurality of parts and updated, and the number of II l 7j in the divided data is made variable. The drive pulse width of each heat generating element in the thermal head can be adjusted by a simple means, which is an excellent advantage.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing a configuration example for concretely implementing the thermal head drive method according to the present invention, FIG. 2 is an electric circuit diagram showing a general configuration example of a thermal head drive circuit, and FIG. A diagram showing the characteristics of applied energy with respect to substrate temperature in a thermal head, Figure 4 is a diagram showing characteristics of color density with respect to heating energy of thermal recording paper, and Figure 5 shows a state in which 9 gradations are expressed in a 2 x 2 matrix configuration. The diagram shown in FIG. 6 shows a latch signal when the present invention is applied. 5 is a time chart showing respective timings of divided data and drive pulse signals. 1... Thermal head 2... Shift register 3
...Latch circuit 4...Driver 5...CPU
6... Serial-parallel converter 7... Gradation matrix 8... Parallel-serial converter 9... Temperature sensor 10
...ROM 11...Thermal head drive circuit

Claims (1)

[Claims] Each line of recording data is given to a shift register, the register contents are held in parallel in a latch circuit, and each heating element in the thermal head is driven via a driver according to the latch contents. 1 each
The recorded data for a line is divided into multiple pieces of data per dot, each divided data is updated and fed to the shift register, and the number of high-level signals in the divided data is varied to drive the heating element. A thermal head drive system that controls the width.