JPH0825296B2 - Thermal head drive - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a thermal head driving device in a thermal transfer recording device or a thermal recording device, and more particularly to control of recording density of a thermal head.

2. Description of the Related Art Thermal transfer recording devices and thermal recording devices are widely used as printing (recording) means for printers, copiers, facsimiles, etc. because of their relatively simple configurations.

There is a method using a sublimation type ink sheet, for example, in order to record a halftone in a thermal transfer recording apparatus. In this method, the dye ink is sublimated according to the amount of heating of the heating resistor that constitutes the thermal head, and the recorded image is transferred and recorded on the recording paper. The number of pulses applied to the heating resistor or The heating amount is controlled by the pulse width.

In the thermal transfer recording apparatus using the sublimation type ink sheet, good halftone recording is performed by the simple control method as described above, however, the main factor that determines the recording density at each gradation level is the heating resistor of the thermal head. Since the temperature is the same as above, there is a problem in that the fluctuation of the temperature caused by the heat accumulation or the change of the environmental temperature greatly affects the recording density, and the recording density of each gradation level is not faithfully recorded. Therefore, many correction methods have been conventionally proposed.

FIG. 3 and FIG. 4 are waveform diagrams showing waveforms of pulse signals applied to the thermal head in the conventional thermal head driving device disclosed in, for example, Japanese Patent Laid-Open No. 60-9271.
FIG. 6 is a characteristic diagram showing a relationship between temperature and pulse width of a pulse signal. In FIG. 3, Tw is the pulse width of the pulse signal applied to the thermal head (hereinafter referred to as "strobe signal"), Tp is the repetition period of the strobe signal, and N is the number of pulses. In FIG. 4, the horizontal axis represents the temperature and the vertical axis represents the pulse width Tw of the strobe signal.

Next, the operation of the above-mentioned conventional example will be described with reference to FIGS. 3 and 4.

First, the pulse number N is set in advance corresponding to the recording density of each gradation level. For example, the strobe signal shown in FIG. 3 is a case where three pulses (N = 3) are set corresponding to the recording density of a predetermined gradation level.

However, in order to obtain an arbitrary gradation level, the number of pulses N
Even if the value is constant, the recording density changes due to the influence of temperature and the like, so that the recording density at the same gradation level also changes.

Therefore, with respect to fluctuations in temperature and the like, a temperature detection element such as a thermistor is used to refer to the temperature line by line, and the pulse width Tw of the strobe signal is controlled with the characteristics shown in FIG. It was corrected so that the same recording density can be obtained with the number N of pulses.

[Problems to be Solved by the Invention] In the conventional thermal head drive device as described above,
The time constant of the thermistor is about several seconds, and it is from several tens of μS to several
Since it is impossible to accurately control the heat generation temperature of the thermal head which changes with the time constant of mS, even if the temperature is detected by a thermistor and the strobe signal is corrected, a sufficient effect cannot be obtained.

In particular, when there are many pixels with a high gradation level in one recorded image, the heat accumulation effect of the heating resistors that make up the thermal head has a large effect, and at the start and end of printing, the gradation level is tens of gradation levels. Also changes, and there is a problem that the recording density for each gradation level cannot be faithfully reproduced.

The present invention has been made in order to solve the above-mentioned problems, and it is possible to prevent variations in the recording density at the same gradation level due to the thermal storage phenomenon of the thermal head, and to obtain a uniform recording density. An object is to obtain a thermal head drive device.

[Means for Solving the Problems] A thermal head drive device according to the present invention divides a gradation level into a plurality of groups each having a gradation level or less, has a heat storage index assigned to each group, and is inputted. The gradation level of the gradation level signal is judged, and the gradation level judgment means for outputting the heat storage index of the group into which the gradation level is divided and the gradation level judgment means for a predetermined printing period are outputted. A heat storage index counter that counts the heat storage index and outputs the count value, a reference level generator that outputs the reference value of the heat storage amount of the thermal head, and outputs a judgment signal by comparing the count value with the reference value. Based on the heat storage index determination means and the determination signal output by the heat storage index determination means, the count value output by the heat storage index counter is added or subtracted up to a predetermined number of lines before the printing line. The heat storage index calculating means for outputting this cumulative heat storage index, the cumulative heat storage index is divided into a plurality of groups, and has a correction coefficient assigned to each group, the cumulative heat storage index output by the heat storage index calculating means is Pulse generating means for correcting and outputting the strobe signal of the thermal head according to the inputted gradation level signal based on the correction coefficient of the divided group.

[Operation] According to the present invention, the gradation level is divided into a plurality of groups of the gradation level or less, and the gradation level means having the heat storage index assigned to each group causes the level of the inputted gradation level signal to be divided. The tonal level is determined, and the heat storage index of the group into which the tone level is divided is output.
The heat storage index counter counts the heat storage index output by the gradation level determination means for a predetermined printing period and outputs the counted value. Further, the reference level generator outputs the reference value of the heat storage amount of the thermal head. Further, the heat storage index calculating means adds or subtracts the count value output by the heat storage index counter based on the determination signal output by the heat storage index determining means up to a predetermined plural lines before the printing line, and outputs the accumulated heat storage index. To be done. Further, the cumulative heat storage index is divided into a plurality of groups, and the cumulative heat storage index output by the heat storage index calculation means is based on the correction coefficient of the divided group by the pulse generating means having the correction coefficient assigned to each group. Then, the strobe signal of the thermal head corresponding to the input gradation level signal is corrected and output, and the variation of the recording density of the same gradation level is prevented and uniform recording density is achieved.

[Embodiment] FIG. 1 is a block diagram showing an embodiment of the present invention. In FIG. 1, (1) is a gradation level determining means connected to the input terminal T, (2) is a heat storage index counter connected to the preceding gradation level determining means (1), and (3) is a preceding step. The heat storage index determination means connected to the heat storage index counter (2), (4) to the reference level generator connected to this heat storage index determination means (3), and (5) to the heat storage index determination means (3) in the preceding stage. Connected heat storage index calculation means, (6) is connected to the heat storage index calculation means (5) and the input terminal T, and pulse generation means, and (7) is connected to the pulse generation means (6), for example, 1024 It is a thermal head composed of a heating resistor.

FIG. 2 is a characteristic diagram showing the relationship between the number of print lines and the recording density. In FIG. 2, the horizontal axis represents the number of print lines, the vertical axis represents the recording density, and the characteristic curves of A, B, C and D are
The gradation levels are "8", "16", "32" and "64" respectively.
Shows the case of full printing.

First, the principle of the present invention will be described with reference to FIG.

As shown by the characteristic curve A in FIG. 2, when the entire surface is printed at the gradation level “8”, the recording density hardly changes even if the number of printing lines increases, because the heat storage hardly occurs. However, as shown by characteristic curves B, C, and D in FIG. 2, when the entire surface is printed at the gradation levels “16”, “32”, and “64”, the higher the gradation level, the larger the heat accumulation. The recording density fluctuates significantly.

In this way, when the characteristics of the number of print lines and the recording density are obtained using the gradation level as a parameter, strictly speaking, a characteristic curve corresponding to the number of gradation levels is obtained, but characteristic curves that may be regarded as the same characteristics are obtained. Yes, it can be divided into several groups.

In the present invention, the gradation level signal L is classified into a plurality of groups each having a gradation level or less, and the heat storage index S1 is assigned to each group. Here, the heat storage index S1 is a value representing an unnecessary heat storage amount remaining in the heating resistor when one dot is printed, and the heat storage amount of the thermal head (7) can be expressed by calculating the heat storage index S1. . That is, when the heat storage index S1 is a predetermined value or more, the thermal head (7) is printed every time printing is performed.
Since the amount of stored heat increases, the heat storage index S1 is added and accumulated. On the other hand, when the heat storage index S1 is less than the predetermined value, the heat storage index S1 is subtracted because the heat storage amount of the thermal head (7) decreases due to heat radiation.

In the present invention, as described above, the heat storage index S1 is calculated according to the print progress, and the strobe signal S6 is corrected based on the calculation result. The heat storage index S1 described above
The value of can be easily obtained by thermal analysis or experiment.

Next, the operation of the above-described embodiment will be described with reference to Tables 1 and 2. Table 1 shows gradation levels and heat storage index
The relationship of S1 and Table 2 show the relationship between the cumulative heat storage index S5 and the correction coefficient K.

For example, a gradation level signal L having 6-bit gradation having 1 to 64 levels is input from an input terminal T and converted into a heat storage index S1 by a gradation level determination means (1). The gradation level determination means (1) determines the amount of heat storage when one dot is printed.

That is, the gradation level signal L is classified into four groups, and the heat storage index S1 (index indicating the heat storage amount) set for each group is output. As shown in Table 1, for example, the gradation level of the gradation level signal L is "1" to "8" or "3".
If it is 3 "to" 64 ", the heat storage index S1 of" 0 "or" 4 "is output to the heat storage index counter (2) of the next stage.

Subsequently, the heat storage index S1 is counted line by line by the heat storage index counter (2), and the count value S2 is supplied to the heat storage index determining means (3) and the heat storage index calculating means (5) in the next stage.

The count value S2 is compared with the reference value S3 supplied from the reference level generator (4) by the heat storage index determination means (3), and if it is equal to or greater than or less than the reference value S3, a determination signal of "1" or "0" S4 is output to the heat storage index calculation means (5) in the next stage.

Further, the heat storage index calculation means (5) calculates the count value S2 from the start of printing to the print line. That is,
When the determination signal S4 is "1", the heat storage amount becomes large, so the count value S2 supplied from the heat storage index counter (2) is added by the heat storage index calculation means (5), and as a result, the accumulated heat storage index S5. Is output to the pulse generating means (6) in the next stage. On the other hand, when the determination signal S4 is "0", the heat storage amount decreases due to heat dissipation, so the count value S2 is subtracted by the heat storage index calculation means (5), and as a result, the cumulative heat storage index S5 is similarly generated by the pulse generation means. It is output to (6).

Then, the cumulative heat storage index S5 is referred to and the gradation level signal L
, That is, the strobe signal S6 having the pulse number N such that the recording density of the same gradation level signal L is constant is generated by the pulse generating means (6) and applied to the thermal head (7).

As shown in Table 2, the cumulative heat storage index S5 is classified into, for example, 16 groups, and a correction coefficient K for correcting the heat storage amount is set for each group. Based on the correction coefficient K, the pulse number N of the strobe signal S6 corresponding to the gradation level signal L is corrected to optimize the heating amount of each heating resistor forming the thermal head (7), that is, the energization time. To be done.

In the above embodiment, the number of group classifications and the heat storage index S1 for the gradation level, and the number of group classifications and the correction coefficient K for the cumulative heat storage index S5 are set as shown in Table 1 and Table 2, respectively. The value of varies depending on the characteristics of the thermal head (7) and is not limited to the value set above.

Further, in the above embodiment, the number N of pulses of the strobe signal S6 is corrected, but it goes without saying that the intended purpose can be achieved even if the pulse width is corrected.

Further, in the above embodiment, when the heat storage index S1 is counted, it is counted in units of one line, but it may be counted in a plurality of lines, and one line is divided into a plurality of units and counted in each divided unit. Even if the same operation can be expected.

Further, in the above embodiment, the heat storage index S1 was accumulated from the start of printing to the print line, but it goes without saying that the desired purpose can be achieved even if accumulated up to a plurality of lines before the print line.

[Effects of the Invention] As described above, the present invention divides a gradation level into a plurality of groups of a gradation level or less, has a heat storage index assigned to each group, and receives an input gradation level signal. Of the heat storage index of the group into which the gradation level is divided, and the heat storage index output by the gradation level determination means for a predetermined printing period are counted. A heat storage index counter that outputs the count value, a reference level generator that outputs a reference value of the heat storage amount of the thermal head, and a heat storage index determination means that outputs a determination signal by comparing the count value with the reference value. , Based on the determination signal output by the heat storage index determination means, the count value output by the heat storage index counter is added or subtracted up to a predetermined number of lines before the printing line to output the cumulative heat storage index. Compensation of the group into which the heat storage index calculating means for inputting and the cumulative heat storage index are divided into a plurality of groups, and which has a correction coefficient assigned to each group, and the cumulative heat storage index output by the heat storage index calculating means is divided. Based on the coefficient
Since the pulse generation means for correcting and outputting the strobe signal of the thermal head according to the inputted gradation level signal is provided, the fluctuation of the recording density of the same gradation level due to the thermal storage phenomenon of the thermal head It is possible to prevent the occurrence and to obtain a uniform recording density.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention, FIG. 2 is a characteristic diagram showing the relationship between the number of print lines and recording density, FIG. 3 is a waveform diagram showing the waveform of a conventional strobe signal, and FIG. FIG. 4 is a characteristic diagram showing a relationship between a conventional temperature and a pulse width of a strobe signal. In the figure, (1) ... gradation level determination means, (2) ... heat storage index counter, (3) ... heat storage index determination means, (4) ... reference level generator, (5) ... heat storage index Computation means (6) ... Pulse generation means, (7) ... Thermal head. The same reference numerals in the drawings indicate the same or corresponding parts.

Claims (1)

[Claims] 1. A gradation level is divided into a plurality of groups of a gradation level or less, a heat storage index is assigned to each group, and the gradation level of an input gradation level signal is judged, A gradation level determination means for outputting the heat storage index of the group into which the gradation level is divided, and a heat storage index counter for counting the heat storage index output by the gradation level determination means for a predetermined printing period and outputting the count value. A reference level generator that outputs a reference value of the heat storage amount of the thermal head, a heat storage index determination means that outputs a determination signal by comparing the count value and the reference value, and a heat storage index determination means A heat storage index calculating means for adding and subtracting the count value output by the heat storage index counter up to a predetermined number of lines before the printing line based on the determination signal and outputting the accumulated heat storage index; The heat index is divided into a plurality of groups and has a correction coefficient assigned to each group, and the cumulative heat storage index output by the heat storage index calculation means is input based on the correction coefficient of the divided group. A thermal head driving device for correcting and outputting the strobe signal of the thermal head according to a gradation level signal.