JPS63178061A - Middle tone thermal printer - Google Patents

Abstract

PURPOSE:To prevent the variation in recording density due to the change in circumferential temp., by employing a gradation level threshold value as the function of the circumferential temp. of a thermal head to compensate the circumferential temp. of the thermal head by a recording/auxiliary pulse generating circuit of simple constitution. CONSTITUTION:A gradation level threshold value calculation means 11 calculates a gradation level threshold value from an inputted sub-line value and the circumferential temp. value of a thermal head to output the same to a comparing means 12. The comparing means 12 compares the gradation level threshold value with gradation data and forms a signal having the recording pulse width (Wa-Wc) and auxiliary pulse width (Wa1-Wc1) corresponding to a gradation level to output the same to a polarity selection means 14. The polarity selection means 13 selects the polarities of the recording pulse and auxiliary pulse inputted by a recording/auxiliary pulse applying state signal to apply the same to a thermal head. ROM 61 as a means generating the threshold value for binarizing the gradation data uniquely, a comparator 62 as a means for comparing said threshold value with the gradation data, AND gates 1, 2 calculating AND and an OR gate 3 calculating OR are provided.

Description

[Detailed Description of the Invention] [Summary] The present invention provides a multi-value drive circuit for a thermal head in which the recording energy monotonically increases with respect to the gradation level, and the auxiliary heating energy for canceling the temperature history monotonically decreases. Focusing on this, a single circuit with a simple configuration using a comparator is designed to generate recording signal and auxiliary heating signal waveforms by following changes in ambient temperature.

[Industrial application field]

The present invention relates to a driving device for a thermal head, and more particularly to a multivalued driving device that can accurately control the amount of heat supplied by a thermal head to an object to be heated regardless of the ambient temperature of the thermal head.

Thermal printers that use heat to record information such as characters include a thermal recording method and a thermal transfer recording method. The former is a method that records by bringing the thermal head into direct contact with the recording paper, which changes color due to heat, and heating the heating element of the thermal head to change the color of the recording paper, while the latter heats the ink with the thermal head to melt or melt the ink. This is a method of sublimating and transferring it to recording paper for recording.

These thermal printers require correction means for correcting recording density fluctuations caused by changes in the ambient temperature of the thermal head.

[Conventional technology]

FIG. 7 shows an outline of a line recording type thermal transfer recording device, and FIG. 8 shows the structure of a thermal head.

The thermal head 71 has an ink sheet 72. Recording paper 73
It faces the platen 74 via. Ink sheet 7
2 is heat-fusible, and by heating this ink sheet 72 with the thermal heating door 1, the ink sheet 7
The ink No. 2 is melted and transferred onto the recording paper 73, and recording is performed. The thermal head 71 has heating elements for one line arranged along the direction perpendicular to the plane of the paper, and recording for one line is performed almost simultaneously. When one line of recording is completed, the recording paper 73 and the ink sheet 72 are simultaneously transported in the direction of the arrow.

The thermal head 71 has a multilayer structure as shown in FIG. That is, a glaze layer 84, a heating element 83, and an electrode 82 are provided in a layered structure on a substrate 85, and a protective layer 81 is provided on the surface that contacts the recording paper.

Note that 86 is a heat sink.

Such thermal heads accumulate heat within them.

That is, as shown in FIG. 9+8), when a drive signal Pa with a pulse width -a is applied, the temperature of the heating element at time ti becomes Ta, and as shown in FIG.
The temperature when the drive signal pb of b < Wa) is applied is Tb
(Tb < Wb), and as shown in (C), when a drive signal Pc with a pulse width Wc (Wc < Wb) is applied, the temperature becomes Tc (Tc < Tb). That is,
The temperature T of the heating element at time t1 varies greatly depending on the amount of heating (corresponding to the pulse width of the drive signal) applied to the heating element.

This is the M heat fluctuation of the heating element due to the change in the amount of heating. Furthermore, even when a drive signal Pc with a pulse width of -C is applied, the temperature at time t1 is Tc, but at time tll the temperature becomes Tc1 (Td < Tc), and the temperature of the heating element changes depending on the elapsed time from the start of recording. It's different. This is a heat storage fluctuation in the heating element due to a change in the recording cycle.

In this way, since the initial temperature of the heating element is different, when we consider when applying a drive signal with a predetermined pulse width in the next recording cycle, the temperature characteristics of the heating element will differ depending on the driving conditions of the past drive signal, and the temperature characteristics of the heating element will differ depending on the driving conditions of the past drive signal. The temperature cannot be controlled to the desired temperature.

In order to cope with this problem, an application has been filed for a method of keeping the initial temperature of the heating element constant (see Japanese Patent Application No. 1988-41188).

According to this method, heat storage fluctuations in each recording period are canceled within that recording period, so that the influence of heat storage fluctuations does not affect the next recording period.

This method will be explained with reference to the time-temperature characteristics and drive signal waveform diagram of FIG.

In FIG. 10, when a recording signal Pa having a as shown in (a) is applied to the heating element, the temperature of the heating element changes as shown by A and becomes equal to the target temperature Ti at time t1. When a recording signal pb having a pulse width as shown in Fig. 1 is applied, the temperature of the heating element changes as shown in Fig. B, and the temperature becomes less than the target temperature 71 at time t1, so at time tb. Temperature compensation signal Pbl of pulse width 1
Apply. This allows the temperature of the heating element at time t1 to be Ti even when the recording signal pb is applied.

Also, a recording signal Pc with a pulse width -C as shown in tc+
When applying , a pulse @ is generated at time tc as shown in C.
By applying the temperature compensation signal Pcl having Wcl, the temperature of the heating element can be set to Ti at time t1.

Here, the pulse widths of the recording signals Pa, Pb, and Pc are defined by the gradation level of the information to be recorded, and the temperature compensation signals Pbl and Pcl are determined by changes in the gradation level, that is, the change in the heating element due to the recording signal. This compensates for the heat storage capacity of the heating element due to changes in the amount of heating.

The pulse width of the auxiliary signals Pbl and PCI can be defined by the gradation level.

As is clear from the above explanation, the following relational expression holds between the pulse width of the recording signal and the pulse width of the auxiliary signal.

Wa > Wb > Wc (1) W
al <Wbl <Wcl...(2) However, (a) gradation level>(b) gradation level> (C)
gradation level.

+11. +2) The recording pulse width (WaJ6.T) monotonically increases with respect to the gradation level, and the auxiliary pulse width (WaJ6.T) increases monotonically with respect to the gradation level.
i, Wbl.

Wc1) is monotonically decreasing.

Next, the drive signal for the direct drive type thermal head will be explained. In a direct drive type thermal head, driving transistors corresponding to all heating elements are mounted, and the driving signal is a binary logic signal (true T or false F).

Therefore, in order to perform the drive shown in Figure 10, the recording period is subdivided as shown in Figure 11, and the recording signal and auxiliary signal pulses are divided into a series of binary signals with a constant minute pulse width. It needs to be translated (converted).

Each minute section obtained by subdividing the recording cycle is hereinafter referred to as a supply line.The recording paper is A4. If the dot density is 8 dots/fin, the number of heating elements is 1728, and gradation data for 1728 dots must be translated into a binary signal within the time of each line. Set the recording cycle to 20...S,
If the number of suplines is 1000, the translation time per dot is 20 x 10 + 1000 ÷ 1728 = 11
．． 6×10 (3) That is, 12 ns or less. In many thermal printers, the recording speed is faster than this, there is a strong demand for recording paper of B4 or A3, and the dot density is also moving to 12 to 16 dots/H. Therefore, the translation time becomes even shorter.

As a result, real-time translation in each supply line, i.e. RO
Since direct conversion using the M reference becomes impossible, a thermal head multi-value drive circuit shown in FIG. (see issue).

In this thermal head multivalue drive circuit, ROMB
2 is referenced only once per line, and the above translation is performed by the comparator 62, thereby making it possible to perform real-time processing in a circuit using inexpensive elements.

C Problems to be Solved by the Invention] In the thermal head multi-value drive method shown in FIG. 6, temperature changes around the thermal head are not taken into account, so there is a problem that recording density fluctuates due to changes in ambient temperature. There is.

The present invention was created in view of these points, and an object of the present invention is to provide a halftone thermal printer that can compensate for temperature fluctuations around a thermal head and obtain accurate recording density.

[Means for solving problems]

FIG. 1 is a principle block diagram of a thermal head multi-value drive pulse generation circuit according to the present invention, which includes a gradation level threshold calculating means 11 for calculating a gradation level threshold corresponding to the ambient temperature of the thermal head, and the threshold and each recording. The configuration includes a comparison means 12 for comparing the gradation data of the dots, and a polarity selection means 13 for selecting the polarity of the comparison means 12.

[Effect]

The gradation level threshold calculating means 11 calculates a gradation level threshold from the input supply line value and the thermal head ambient temperature value and outputs it to the comparing means 12. The comparing means 12 compares the gradation level threshold value with the gradation data and generates a recording pulse @(Wa) corresponding to the gradation level.

Wb, Wc) and auxiliary pulse 'I'l (Wal, W
bl, Wc1) and outputs it to the polarity selection means. The polarity selection means 13 selects the polarity of the input recording pulse and auxiliary pulse according to the recording/auxiliary pulse application state signal and applies the polarity to the thermal head.

The present invention has a method of translating gradation data into a binary data string by comparing gradation data with a gradation level threshold in each supply line, and by making the gradation level threshold a function of the ambient temperature of the thermal head, the configuration is simple. The recording/auxiliary pulse generation circuit makes it possible to compensate for the ambient temperature of the thermal head.

〔Example〕

FIG. 2 is a diagram of a thermal head multivalue drive pulse generation circuit according to an embodiment of the present invention, and FIG. 3 is a time chart thereof.

As shown in FIG. 2, this embodiment uses a ROMB2 as a means for generating a threshold value for uniquely binarizing tone data corresponding to a recorded tone, and as a means for comparing this threshold value with the tone data. A comparator 62, an AND gate 1 for calculating the AND of the signal from the comparator 62 (BkA) and the recording pulse mask signal, and an AND gate 1 for calculating the AND of the signal from the comparator 62 (B<A) and the auxiliary pulse mask signal. AND gate 2 and the two AND gates 1
and an OR gate 3 for calculating the logical sum of the outputs from 2 and 2.

The operation will be explained with reference to the time chart of FIG.

(al, (b), fc) in Figure 3 are (a) in Figure 10.
Drive signal waveform corresponding to l, ('b), (C), (a''
), (b'), (c') are (a), (
(B≧A) of the comparator corresponding to g)), fc)
The outputs, (a”), (b”), and (c”) are respectively (a
), middle school 1. (B<A) output corresponding to (C), (d)
is a recording pulse mask signal q, and (e) is an auxiliary pulse mask signal. Moreover, the threshold value is shown in (f). However, Ga.

Gb and Gc are (al + (bl, (c
) is gradation day 9.

The output of AND gate 1 is (al, (b'), (
The logical product of c') and (d), the output of AND gate 2 is (
The logical products of (a')), (b), (c'') and (fl) are respectively (al, (ill), (equal to the CJ recording pulse and auxiliary pulse, so the OR gate 3 output which is their logical sum is (al, (bl, (C)), and the drive signal for the direct drive type thermal head is obtained.

The circuit shown in FIG. 2 cannot deal with changes in the ambient temperature of the thermal head, that is, the temperature distribution in the longitudinal direction of the thermal head and the throat. For many applications, this can provide sufficient accuracy, but in cases where very high accuracy is required, such as full-color thermal transfer recording, the temperature distribution in the longitudinal direction of the thermal head cannot be ignored. In such a case, the ROMB
This can be handled by puffing the output of No. 2 and delaying the gradation data by the read time of the ROM 61. This embodiment is shown in FIG. 4, and its time chart is shown in FIG. In FIG. 4, a latch 4L42, a comparator 43, and delay circuits 44 and 45 are added to the circuit in FIG. 2.

The operation of the other embodiment shown in FIG. 4 will be explained with reference to FIG. 5.

Figure 5 1. Here, the ambient temperature of the thermal head is r'
= At the time of hanging, the gradation data is compared with the corresponding threshold value, that is, from Ga to Gf, and when the thermal head ambient temperature is Ti, the threshold value is updated, and the gradation data is compared with the corresponding threshold value, that is, from Gg to cp. indicates that are compared.

The threshold value is written in ROMB2, and reading it takes time. Therefore, a countermeasure is taken by delaying the inputs A and B of the comparator 62 by a time longer than the time required for reading.

The latch 42 and the comparator 43 are circuits that detect changes in the ambient temperature of the thermal head. When the thermal head ambient temperature and the latch 42 output do not match, the comparator 43 output becomes re 1 tt and the new thermal head ambient temperature is held by the latch 42 . Input A, l of comparator 43! :B matches, so the comparator 43 output returns to "OIT".The moment the thermal head ambient temperature changes, the comparator 43 output becomes "IT1".
tT, and after a certain amount of time has elapsed, the delay circuit 44
Since the output becomes 1'', the output of the latch 41 outputs the new thermal head ambient temperature after a period of time has elapsed since the thermal head ambient temperature changed.

On the other hand, since the gradation data is also delayed by a certain amount of time by the delay circuit 45, the comparator 62 always compares the thermal head ambient temperature with the corresponding gradation data.

This embodiment will not work properly if the thermal radar ambient temperature changes within a shorter time than the time interval. However, since the temperature fluctuation in the longitudinal direction of the thermal head is usually extremely gradual, there is no problem in practical use.

〔Effect of the invention〕

As explained above, according to the present invention, it is possible to realize a circuit that drives a thermal head in multiple values with high precision regardless of the ambient temperature using a circuit with a simple configuration. Not only can printers, full-color facsimiles, etc. be lower in price, but they can also be made more reliable by reducing the number of parts.

[Brief explanation of the drawing]

Fig. 1 is a principle block diagram of the present invention, Fig. 2 is a circuit diagram of an embodiment of the invention, Fig. 3 is a diagram showing the time chart of Fig. 2, and Fig. 4 is another embodiment of the invention. 5 is a time chart of another embodiment, FIG. 6 is a conventional thermal head multivalue drive circuit diagram, FIG. 7 is a schematic diagram of a line recording type thermal transfer recording device, and FIG. 8 is a thermal Figure 9 shows the structure of the head. Figure 9 shows the drive waveform of the thermal head and the time-temperature characteristics of the heat generating element. Figure 10 shows the drive waveform and heat generation in a method of keeping the temperature of the heat generating element of the thermal head constant. FIG. 11 is a diagram showing the time-temperature characteristics of the body, and is a diagram for explaining the data format and data conversion method for multi-value driving of the direct drive type Malhefd. In the figure, 1 and 2 are AND gates, 3 is an OR gate,
11 is a gradation level threshold calculation means, I2 is a comparison means, 13
is a polarity selection means, 41.42 is a launch, 43.62 is a comparator, 44.45 is a delay circuit, 61 is a ROM, 7
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Claims (1)

[Claims] Means (11) for subdividing the recording period of the thermal recording signal to be applied to the heating element (71) and generating a threshold value for uniquely binarizing the gradation data corresponding to the recording tone within each minute recording period. ), and means (12) for comparing the threshold value and the gradation data, in a thermal printer for controlling recording tone on a recording medium, the output threshold value of the means (11) for generating the threshold value is set to the output threshold value of the heating element. A halftone thermal printer characterized in that the temperature is a function of ambient temperature.