JPH023348A - Control system of gradation of color thermal printer - Google Patents

Abstract

PURPOSE:To obtain constant printing density regardless of the magnitude of load even at any gradation level by determining the quantity of the lengthening of the conduction time at the time of the drop of the supply voltage of a head on the basis of a correction data. CONSTITUTION:Fundamental conduction data are set so that specified printing density is acquired under the state in which a voltage drop DELTAV1 can be ignored. Simultaneous conduction heating-element number Pi is increased successively so that an output from a voltage drop detector is brought to di=1, 2, 3 at a gradation level i=1 in ink A, and conduction pulse width coefficients kA11, kA12, kA13 are set so that printing density in each case is made to the same fixed density as the voltage drop DELTAV1 can be ignored. Even when load is changed while the gradation level is brought to i=2, conduction pulse width coefficients kA21, kA22, kA23 are set so that printing density is brought to specified density at all times. Likewise, a coefficient data table is acquired even regarding ink B. Printing is conducted while conduction pulse width data are determined on the basis of correction data obtained.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a gradation control method for a color thermal printer, and in particular to a method for controlling print density due to a drop in head power supply voltage that occurs when multiple heating elements of a thermal head are simultaneously energized. This invention relates to a gradation control method for color thermal printers designed to eliminate unevenness as much as possible.

[Prior Art] FIG. 5 is a schematic explanatory diagram of a thermal head. In this figure, R1-R2 are heating elements for one line, 81-S
2 is a transistor switch that controls energization to each heating element, Q1 and Q2 are print signals for each heating element, and STB is an energization pulse (pulse width t,
, i indicates the gradation level), VT)I indicates the head power supply voltage.

The head voltage V is applied to each heating element R only during the period (1+) when the print signal Q is °゛L°' and the energization pulse STB is L°゛.
7H is applied to generate heat, and the print density increases in accordance with the sum of the products of this heat generation amount. Therefore, if the head power supply voltage vH is constant, the gradation control of print density can be performed simply by the energization time (
This can be done by changing the length of T, =Σ1+).

However, in reality, the value of the current supplied to the thermal head at one time varies depending on the size of the load, that is, the number of heating elements (p+) that are energized simultaneously, and the voltage drop due to wiring resistance and contact resistance changes. Furthermore, the voltage applied to the heating element also fluctuates. Therefore, the amount of heat generated is determined by the head power supply voltage V7 in addition to the energization time tl.
It becomes a function of H. Further, the relationship between the amount of heat generated and the print density differs depending on the type of ink. Therefore, in order to perform gradation control, it is necessary to change the energization time depending on the type of ink, head power supply voltage, and gradation level.

Figure 6 is a diagram showing the voltage drop in VTH due to energization of the thermal head. (a) shows the energizing pulse to the heating element when printing with gradation control, and (b) shows the voltage when energizing. It is a figure showing a descent. When the heating element is intermittently energized by the energizing pulse shown in FIG. 6(a), a voltage drop Δ■1 occurs as shown in FIG. 6(b). The degree of this voltage drop ΔV1 increases as the load increases, that is, as the number p1 of simultaneously energized heating elements increases. Therefore, when the number of simultaneously energized heating elements p+ is large, unless the energization time t is lengthened, the printing density will decrease and printing with a well-balanced color tone cannot be performed.

On the other hand, in conventional color thermal printers, the width t1 of the energizing pulse is determined so as to obtain the desired density ODl for each gradation level i with negligible voltage drop, and the width t1 of the energizing pulse is determined to Density unevenness due to load fluctuations has been corrected by extending the energization pulse width uniformly at all gradations and linearly with respect to load fluctuations so that the maximum density under load or a desired density is achieved.

This correction method will be explained below using FIGS. 7 to 10. This correction method can be divided into the following three steps.

(2) Creation of basic energization data First, a curve (hereinafter referred to as ideal 0D-i curve) showing the relationship between ideal print density (ODI) and gradation level i is set [FIG. 7(a)]. This ideal 0D-i curve is
It is selected depending on the desired color tone, and may be a straight line as well as a curve.

Next, the voltage drop △vI is an almost negligible number (
For example, by energizing only the heating elements with a diameter of less than /10 of all heating elements, the energizing time T + = t is applied so that the printing density becomes oDl when i = 1, as shown in Fig. 7(a). Decide on I. Similarly, the energization time T, =Σtl is determined so that the print density when i=i is OD [Figure 7(b)]
. As a result, the energization pulse width tI required to advance each gradation level i by one step is determined. Hereinafter, the energization pulse widths t+, t2. ...tn is called basic energization data.

■Determining the extension factor Next, the load is the largest and the voltage drop △Vl is the largest (△V
MAX), all heating elements are used and energized according to the basic energization data obtained above to obtain the 0D-i curve [FIG. 8(a)]. At this time, the 0D-i curve (-・
-1111.-), the print density is lower than the ideal 0D-i curve (-m---'') due to the influence of the voltage drop ΔVMAX.

Therefore, the energization time tl' at each gradation is set at a rate (T,'=
Σt,'=Σ(1+K)t, ), and find the extension coefficient K at which the print density ODA at the maximum gradation level IMAX ("n) becomes the predetermined density ODn.The energization data at this time ( 1+K)f;,, (1+K)t2
...(1+K)t, after correction obtained using 0D-
The i-curve is shown as a solid line in FIG. 8(a). As mentioned above, the extension coefficient K is determined from the density of the maximum gradation level i MAX, and this K is uniformly used for all energization data, so the obtained 0D-1 curve is similar to the ideal 0D-i curve. On the other hand, in the intermediate range of gradation level i, over-correction occurs,
The correction may be insufficient.

■If the load is medium, the voltage drop △■ at gradation level i is expressed as A of N bits.
/D converter detects the output d of the A/D converter.
l is d+=φ at no load, d,=2' − at maximum load
1=m.

Therefore, when the load is medium, that is, when φ<di<m, the energization time T1 is determined as shown in the following equation using the basic energization data 1+ obtained in ① and the extension coefficient K obtained in ③.

T+=Σ(++(at/m)K)++(I
) In other words, the energization time at no load is tl, and the time at maximum load is (
++K)++ is closed, and the energization time is extended at equal intervals (linearly) in response to the voltage drop during that time.

FIG. 10 shows a block diagram for realizing equation (I). 11 is a memory that stores basic energization data tl determined in advance by ■ for each ink type V; 12 is (I
) of the equation tl (D (d 17m) K) to the extension coefficient determined in advance by ■ and the voltage drop detector output d
A coefficient generator (memory) calculates and outputs from 1, and 13 indicates a multiplier. In other words, the basic energization data 1+ at gradation level i is sent from the memory 11 to the multiplier 13, and the coefficient (D(d+
/m) is determined, and the result of this multiplication is output as energization pulse width data.

The energization pulse and 0D-i curve at this time are shown in FIG.

As can be seen from this figure, conventionally, the extension coefficient obtained from the maximum density matching condition is used uniformly at all gradation levels;
Since the energization time is extended linearly with respect to the voltage drop, appropriate correction is not made for load fluctuations at each gradation.

[Problem to be solved by the invention]

The density characteristics of thermal transfer ink with respect to the supplied power differ depending on the type of ink (V) and also vary depending on the gradation level i, so unlike the conventional method described above, the head If the voltage drop △vl of the power supply is the same, the energization time is fixed at a constant rate, and the energization time is linearly corrected for the amount of voltage drop. This means that print density unevenness can be corrected as effectively as possible.
As shown in FIG. 9A and FIG. 9, in other condition ranges, the amount of correction will be too much or too little.

SUMMARY OF THE INVENTION An object of the present invention is to provide a gradation control method for a color thermal printer that can eliminate as much as possible print density unevenness caused by head power supply voltage drop ΔV1.

[Means to solve the problem]

The present invention provides a color thermal printer that performs printing while correcting print density unevenness by detecting the amount of voltage drop in the head power supply of the thermal printer and extending the width of the energizing pulse to the heating element by a predetermined amount in accordance with the amount of the drop. In the gradation control method, the amount of extension of the energizing pulse width is set for each thermal head used, and for each head power supply voltage drop, ink type, and gradation level to achieve a predetermined print density. This is a gradation control method for a color thermal printer characterized in that the gradation is determined based on data (coefficient data table).

[Operation] The correction data according to the present invention has a function of determining the optimum extension amount for the energization pulse width t1 based on the measured value ΔvI of the head power supply voltage drop amount of the thermal printer.

〔Example〕

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a flowchart for determining the energization pulse width in the method of the present invention, in which 21 is a memory storing basic energization data prepared for each ink type (■) and gradation level 1, 22 is an ink type (V), and each In the memory that stores the voltage drop Δvl and the energization pulse width coefficient kvld at each gradation level 1, the corrected energization pulse width is kvz·1. Determined by.

Next, the present invention has eight gradation levels (i=1 to 8), a total number of heating elements of four (R, to R4), and an output of an A/D converter for detecting voltage drop Δvl that has four levels. (d, = 0-3). Figure 2 ~ takes the case where there are two types of ink (A and B) as an example.
This will be explained using FIG.

First, in the same way as the conventional method, the voltage drop △vI is negligible (dl = 0. In this example, the number of simultaneously energized heating elements pI
The basic energization data (tAl"tAg, ta
+~tBa) (see Fig. 7).

Next, for ink A, the gradation level i=1, the number of simultaneously energized heating elements is 11+, and the output of the voltage drop detector is d.
The energization pulse width coefficient kAIlk Al1 k Al1 is increased sequentially so that i = 1.2.3, and the printing density in each case becomes the same predetermined density OD+ as when the voltage drop △Vl can be ignored. Set. Next, assuming that the gradation level is i=2, the energizing pulse width coefficients k A2+, k A□2. k Set A23. Figure 3 shows (a) voltage drop △vl and (b)
An example of an energization pulse is shown. Below, the gradation levels are sequentially i=
Repeat the same operation until the number is 8, and all (i, di
) is obtained (hereinafter referred to as a coefficient data table) of the energization pulse coefficient kA+a for the combination. Similarly, a coefficient data table is obtained for ink B as well.

FIG. 2 is an explanatory diagram of the voltage drop ΔVI of the head power supply as the number p1 of simultaneously energized heating elements increases and the output d1 of the voltage drop detector at this time. In the present example, since the voltage drop detector is 2 bits, the detection sensitivity of the voltage drop ΔV1 is low, but it goes without saying that in reality, a detector with even higher sensitivity is used.

In general, the print density in the low density range (for example, 1-1) does not change much with respect to changes in the applied voltage, so even if the number of simultaneously energized heating elements pI increases (as a result, d1 increases), the energization pulse width The change in the coefficient k vld is small. On the other hand,
In the medium density range, the print density changes greatly with respect to the applied voltage, so the difference in the coefficient kvld is large. Furthermore, in a high density region, printing characteristics vary greatly depending on the type of thermal head or ink used, so the coefficient kvld needs to have a different value accordingly.

The method of the present invention is used in the following manner using the correction data obtained as described above. As with the conventional method, gradation level 1, ink type ■, voltage drop detector output d1
Printing is performed while determining the energization pulse width data based on the . In the method of the present invention, the width of one energization pulse is (v, i
, 'd+) is determined by the product of the energization pulse width coefficient k vld and the basic energization time tvI determined by (v, i). The total energization time TI of each heating element is given as the sum of each energization pulse width kv+a·jv+.

Next, the printing operation using the method of the present invention will be specifically explained using FIG. 4. Now, the ink is A and 1
There are two heating elements with input data i=3 in the line (R+,
R2), there is one heating element (R3) with i=4, and one heating element (R4) with i=6. First, when gradation level 1 is 1, all input data is higher than this level, so all four heating elements (R, ~ R4) are energized, and the voltage drop is the maximum (a, = ] [Figure 4 (a)]. At this time, the energization pulse coefficient is k Al
1 is selected and the energization pulse width becomes kA+3·tAl. Thereafter, all four heating elements are energized up to gradation level i=3, so the energization time is T3''kA+3・t++kA23・tA2+kA33・as shown in FIG. 4(b).
tA3, and the heating element (R1, R2
), a predetermined print density OD3 is obtained. Next, when the gradation level becomes i=4, the number of heating elements that are energized at this time is two (R3, R4), so the voltage drop detector output is d4.
=1 [FIG. 4(a)], and kA41 is selected as the energization pulse coefficient at this time. Therefore, input data i=4
As shown in FIG. 4(b), the energization time to the heating element (R3) is T4=T3+kA41・tA4, and the predetermined print density OD for input data j=4.
, is obtained. Furthermore, when the gradation level is i=5.6, the number of energized heating elements is one (R4), so the voltage drop detector output is d5. d6=o [Fig. 4 (a
)], the coefficients at this time are both 1, so in the heating element (R4) with input data i = 6, electricity is applied for a time of T6 = T3 ÷ kA41 · tA4 + tA5 + tA6, and the predetermined print density (OD, ) is achieved. can get[
FIG. 4(b)].

In this embodiment, FIG. 1 uses two types of data tables and multipliers, but these calculation results are stored in one ROM (
It is possible to perform correction by storing this ROM in a read-only memory) and exchanging this ROM for each thermal head.

[Effects of the Invention] As described above in detail, according to the present invention, the amount of extension of the energization time when the head power supply voltage drops is determined based on the above-mentioned correction data. Even at different levels, a constant print density can be obtained regardless of the magnitude of the load.

[Brief explanation of the drawing]

Fig. 1 is a flowchart for determining the energization pulse width in the method of the present invention. Fig. 2 is a diagram showing the relationship between the number of simultaneously energized heating elements p1 and the voltage drop detector output d, and Fig. 3 is a diagram showing the relationship between the voltage drop △V1 and energization. A diagram showing an example of a pulse, (a) is a diagram showing an example of a pulse.
Figure 4 shows the head voltage V7H at +, Figure (b) shows the energizing pulse at each d, Figure 4 shows the head voltage and the energizing pulse, and Figure (a) shows the gradation level i, VTH, and voltage drop detection. Figure (b) shows the relationship with the output dI for each heating element R,
- Figure 5 is a diagram showing the energization pulse to R4, Figure 5 is a schematic explanatory diagram of the thermal head, Figure 6 is a diagram showing the voltage drop in vH due to energization of the thermal head, and (a) is the energization pulse to the heating element. , (b) is a diagram showing the voltage drop during energization, No. 7
Figure (a) is the ideal 0D-i curve, Figure 7 (b) is a diagram showing basic energization data, Figure 8 (a) is the 0D-i curve at high load, and Figure 8 (b) is after correction. Figure 9 shows the energizing pulse of
Figure (a) shows the 0D-i curve when the load is medium, and Figure 9 (
b) is a diagram showing the energization pulse after correction, and FIG. 10 is a flowchart for determining the energization pulse width in the conventional method. i...Gradation level, ■...Ink type, △V1・
... Voltage drop, d+... Voltage drop detector output, pl.
... Number of simultaneous energizing heating elements, k vld... Energizing pulse width coefficient, tvl... Basic energizing data. Patent applicant Kunio Kakiki O, representative of Victor Japan Co., Ltd.

Claims (1)

[Claims] A gradation control method for color thermal printers that corrects uneven print density while printing by detecting the amount of voltage drop in the thermal printer's head power supply and extending the width of the energizing pulse to the heating element by a predetermined amount according to this amount of drop. In this step, the amount of extension of the energizing pulse width is determined for each thermal head used, and based on correction data set for each head power supply voltage drop, ink type, and gradation level to achieve a predetermined print density. A gradation control method for a color thermal printer, characterized in that a gradation control method for a color thermal printer is determined.