JPH0326139B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a thermal head printing temperature control device for maintaining uniform printing density of a thermal head in a recording apparatus that drives a thermal head to perform printing.

In a recording apparatus that employs a thermal recording method such as a thermosensitive color recording method or a thermal transfer recording method, a thermal head is used as a printing head.

One type of thermal head is a matrix drive type (MD) head. The matrix drive type head is a head developed to be smaller and more dense than the large single head type head. In the matrix drive type head, all n heating elements are grouped together to form m blocks, and the blocks are driven using n+m lead terminals using diodes and matrix wiring. However, since this method requires m scanning energizations, there is a limit to its high speed performance.

For example, direct drive type (DD) heads have been developed that can perform high-speed printing.
There is a head of shape). A direct drive head is a head in which one end of the heating resistor is connected to a common electrode, and the other ends are directly connected to each transistor driver, so that any number of currents can be energized at the same time. Since the transistor driver can be opened and closed simultaneously by the recording signal stored in the shift register, the head has a very small number of input terminals and is fast and easy to use.

In recording devices using these thermal heads, a large number of heating elements arranged on the substrate of the thermal head selectively generate heat, and the heat is used to color thermal paper or transfer ink to the paper. , is recording image information. The temperature of the heating element during printing is desirably kept within a predetermined temperature range in order to make the printing density uniform. For this reason, in conventional recording devices, the temperature of the substrate of the thermal head is detected by a heat-sensitive element such as a thermistor, and the substrate is heated accordingly, or the voltage and pulse width of the drive pulse applied to the heat-generating element are changed. The print density was being adjusted.

However, although such conventional devices were sufficiently effective for matrix drive type heads since high-speed printing was not possible, they were not suitable for heads that performed high-speed printing. In other words, there is a time delay before the temperature of the heating element is reflected in the substrate temperature, so even if the temperature of the thermal head substrate is detected by a heat-sensitive element, there is a limit to the response speed, and when high-speed printing is performed, The substrate temperature corresponding to the temperature of the heat generating element from a long time ago was detected, and the temperature of the heat generating element was corrected according to that temperature. For this reason, in recording devices that use heads that perform particularly high-speed printing, such as direct-drive heads, if you try to adjust the print density by detecting the temperature of the thermal head line by line, the error will increase. Put it away. Furthermore, since the thermal element only detects the overall temperature (average temperature) of the substrate of the thermal head, regardless of whether it is a high-speed or low-speed head, it is not possible to compensate for local temperature fluctuations. As described above, in the conventional recording apparatus, the print density becomes uneven, and sufficient image quality cannot be obtained.

Therefore, several proposals have been made to increase the speed of recording by a recording apparatus by taking into account the current supply state of the thermal head itself. Among these, in JP-A-57-69074 and JP-A-56-137978, power supply increase/decrease control is performed by detecting the recording state of the previous line and calculating the logic with the current recording line on a pixel-by-pixel basis. I'm going. Also, in Japanese Patent Application Laid-open No. 57-4783,
The printed dot information is stored and the energy to the thermal head is controlled accordingly.

However, in the former two proposals, the thermal energy is only adjusted according to the previous driving situation of the same pixel, and no consideration is given to the transfer of thermal energy to surrounding pixels. In the latter proposal, in order to keep the surface temperature of the thermal head constant, thermal energy is only controlled according to the driving status of the thermal head immediately before printing, and no consideration is given to the past driving status of the pixels to be printed next. I haven't been there. In addition, from these proposals, it is possible to assume thermal energy control that takes into consideration the driving status of the thermal head immediately before printing and the past driving status of the pixel to be printed next, but the No consideration is given to heat transfer due to past driving conditions. Therefore, in a recording device using such a proposal, as the recording speed becomes faster and the amount of thermal energy applied per unit time to each pixel increases overall, the effect of Heat transfer can no longer be ignored, and sufficient temperature compensation for the thermal head cannot be performed, leading to problems similar to those in the case of using a heat-sensitive element.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a thermal head printing temperature control device that can sufficiently increase the recording speed without deteriorating printing quality.

In the present invention, the thermal history at each position of the line to be currently printed is grasped based on the image data of the past line already printed and the degree of influence (weight) of these existing in a predetermined range that is symmetrical in the line direction with the printing position as the center. In addition to reflecting this in the amount of power supplied to each heating element for printing,
The degree of influence, or weight, should be symmetrical in the line direction, that is, the main scanning direction, with the same position as the current printing position on the past line, that is, the same sub-scanning position, and also more The above object is achieved by making the farther the line the smaller it is.

The present invention will be explained in detail with reference to Examples below.

FIG. 1 shows the main parts of the thermal head printing temperature control device of this embodiment. This device
RAM (Random Access Memory) 1 has
Image data 2 output from an image sensor (not shown) and binarized by a binarization circuit (also not shown) is input. The RAM 1 has a memory area for two lines, and the image data 2 is written alternately one line at a time.

Now, as shown in Figure 2, the Nth recording line l
It is assumed that the power for printing is determined for the M-th bit B[N,M] of [N] (N, M are arbitrary positive integers). In this device, bit B
M-3 in recording lines l[1] to l[N-1] before recording line l[N] to which [N,M] belongs
This determination is made with reference to the M+3rd bit from . The area for reference is the internal area 3 surrounded by the dashed line in the figure.

To make such a determination, the address counter 4 outputs address information 5 specifying the address of the M-3rd to M+3rd bits.
When the address information 5 is supplied to the RAM 1, the corresponding bits B[N-1, M-3] to B[N-1, M
+3] image data 9 (white image signal or black image signal) is read out serially. The image data 9 is supplied to a shift register 11 and subjected to serial-parallel conversion. Each bit B[N-1, M-3]~
The parallel image data 12 for B[N-1, M+3] is input to the front line input terminal of the arithmetic circuit 13.
Supplied from A ₀ to A ₆ .

As can be easily understood from FIG. 3, the thing that has the strongest influence on the print density of bit B[N,M] in recording line l[N-1] is the bit B[N- -1,M]. Also, the one with the weakest influence on the recording line l[N-1] in the internal area 3 is the farthest bit B[N-1].
-1, M-3] and B[N-1, M+3].
The weighting of these influences changes depending on various factors such as the recording method and recording speed. In this embodiment, each bit is set to 5 from "1" to "5" as follows.
Weighting is done in stages.

B[N-1,M-3] and B[N-1,M+3]
...Weight "1" B[N-1, M-2] and B[N-1, M+2]
...Weight "2" B[N-1, M-1] and B[N-1, M+1]
...Weight "3" B[N-1,M] ...Weight "5" In this case, these bits are bit B
The sum of influences S[N-1] on [N, M] can be expressed by the following formula.

S[N-1]=b÷a...(1) However, b={B[N-1, M-3]+B[N-1,
M+3]}+2{B[N-1, M-2]+B[N-1,
M+2]}+3{B[N-1, M-1]+B[N-1,
M+1]}+5B[N-1,M] The arithmetic circuit 13 converts the black data into a signal “1”,
In addition, the white data is made to correspond to the signal "0", and the above equation (1) is first calculated. Then, the value 0.8×ΣS is obtained by weighting the above calculation value of the line before the previous line supplied to the input terminals A ₇ to _{A 10} of the line before the previous line by “0.8”.
[M] Add ^* . The weighting of "0.8" represents that as the recording line moves away by one line, the influence on bit B[N,M] decreases by a factor of 0.8 in this case.

ΣS[M] ^* is the value shown below.

ΣS[M] ^* =S[N-2]+0.8× S[N-3]+(0.8) ² × S[N-4]+(0.8) ³ × S[N-5]+…… Calculation The value S finally calculated by the circuit 13 is as follows.

S=S[N-1]+0.8×ΣS[M] ^* ...(2) Now, the influence on bit B[N,M] is expressed as
If it is expressed as a step), the value S needs to be in the following range.

O≦S≦15 (3) That is, the value S needs to converge to the numerical value 15 when all the bits in the internal area 3 are “1” (black image data). Therefore, the following equation is established using equations (2) and (3).

15=S[N-1]+0.8×15...(4) The numerical value b in equation (1) becomes the numerical value 17 in this case. Therefore, by substituting equation (1) in this case into equation (4), the numerical value a can be found.

15=17÷a+0.8×15 ∴a=17÷3=5.667...(5) That is, equation (1) can be rewritten as follows.

S[N-3]=b÷5.667... Now, each image data supplied to the previous line input terminals _A0 to _A6 is calculated according to equation (6), and then as shown in equation (2). A weighted number of 0.8 is added to the previous calculated value ΣS[M] ^* supplied to the input terminals A ₇ to _{A 10} on the line before the previous line. The value S thus obtained is the total influence of the internal region 3 on the bit B[N,M]. This value (O≦S=15) is
Output terminal O ₁ as 4-bit power control information 15
~ Output from _O4 .

The pulse width correction circuit 16 inputs the power control information 15 and outputs a pulse width correction signal 17. That is, if the value S exceeds the numerical value 10, for example, the pulse width of the print pulse that determines the time to print bits B[N,M] is reduced by 10%. If the value S is less than 5, the pulse width is increased by 10%.

Such a pulse width correction signal 17 is created using an output signal of a comparator that compares digital values. Of course, it is also possible to perform pulse width correction using an analog signal using a DA converter and an operational amplifier. Since the pulse width correction signal 17 adjusts the power supplied to each heating element during printing, it is possible to eliminate uneven print density.

On the other hand, the power control information 15 is also supplied to the RAM 18. In the RAM 18, the supplied value S is set as the calculated value S[M] for the M-th bit.
Write to the th address. When the correction of the power supply for each bit of the recording line l[N] is completed in this way, the calculated values for one line have been written in the RAM 18 in correspondence with the bits. When the power is corrected for the next recording line l[N+1], the address counter 4 selects the corresponding bit B.
M-th bit information 1 corresponding to [N+1,M]
8, and the calculated value S[M] of the Mth address
read out. The calculated value S[M] is supplied to the latch circuit 21 as the line-before-before information 19, and in the latched state is supplied to the line-before-before input terminals A ₇ to A ₁₀ of the RAM 13. In this way, the information from the previous line is added to the information from the previous line, and the optimum power supply is determined.

In the above description, temperature control at the end of each line was not explained, but it is no different from control at other locations. In other words, at these ends, the number of bits constituting the internal region 3 decreases and the value S decreases, but in reality the amount supplied from other heating elements tends to decrease at these locations. . Therefore, in the present invention, there is no problem in that the power supplied to these parts tends to increase. Similarly, when recording the first line, since the value S is small, the supplied power tends to increase, and in this case as well, uniformity of print density is ensured.

Further, in the embodiment, the pulse width of the printing pulse was changed to increase or decrease the supplied power, but it goes without saying that the applied voltage may also be varied. Furthermore, it goes without saying that the environmental temperature may be measured using a heat sensitive element and the supplied power may be further corrected based on this.

As explained above, according to the present invention, especially in a recording device that performs high-speed printing, the power supply for current printing is adjusted by weighting past image data in relation to the current printing position. ,
Not only does it not cause uneven print density, it also allows you to set the print density itself very precisely.
For example, it has the effect of improving color stability and reproducibility in multicolor recording.

[Brief explanation of drawings]

The drawings are for explaining the embodiments of the present invention; Fig. 1 is a block diagram showing the main parts of the thermal head printing temperature control device, and Fig. 2 shows the internal area used for reference when determining the power supply. Explanatory diagram,
FIG. 3 is an explanatory diagram showing the weighting of each bit in the recording line one line before. 2... Image data, 13... Arithmetic circuit, 16...
Pulse width correction circuit (print density correction means).

Claims (1)

[Scope of Claims] 1. In a recording device that records image data line by line using a thermal head, for each printing position of a line to be printed from among a predetermined number of lines of image data that have already been printed, , select image data having equal distance ranges symmetrically in the main scanning direction centering on the same sub-scanning position as the printing area, and weights of these image data decrease symmetrically as they move away from the center. weighting setting means for weighting the thermal influence on the printing location so that the weight decreases as the line is farther from the printing line; and the weighting set by the weighting setting means. a thermal history calculation means for calculating the state of heat accumulation due to past printing at the printing location from the signal state of each image data of the line that has already been printed; A thermal head printing temperature control device comprising a print density correction means for adjusting the amount of power supplied for printing to the printing location.