JP2000071505A - Thermal printer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To correct a density unevenness brought about when recording papers of different breadths are used and eliminate the density unevenness by detecting a density of the printed result by a plurality of density-detecting means, comparing with original image data, obtaining a correction value for correcting the density unevenness and correcting the original image data. SOLUTION: When the printer drives to print, data at the same position as a position where a sensor group 21 for detecting densities of printed images is arranged are extracted along a print direction at the time of printing from original image data stored in an image buffer memory 16 and the data are integrated. Printing is started after the density on data at the sensor position is obtained from the original image data. Every time printed recording papers are gradually delivered, densities of printed images are detected by the sensor group 21 and integrated. A correction value is calculated on the basis of each integrated value and the original image data are corrected. The printing is conducted according to the corrected image data, so that beautiful print results without a density unevenness are obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thermal printer, and more particularly to a thermal printer that can use a plurality of recording papers having different widths.

[0002]

2. Description of the Related Art Thermal printers include heat-sensitive papers (including color-formed heat-sensitive papers that are not limited to monochrome ones) that generate heat by means of heat generated by heating elements provided on a thermal head.
Alternatively, the printer is a printer that conveys an ink film together with a sheet (image receiving paper) and transfers the ink of the ink film to the sheet by the heat applied by the heating element to perform printing.

As one form of such a thermal printer, a thermal head in which a plurality of heating elements are arranged for printing a plurality of dots for one line at a time is used. There is a type capable of performing printing on recording paper (collectively referred to as recording paper).

In such a thermal printer,
When printing on narrower recording paper is performed more often, only the part of the thermal head where the narrower recording paper comes into contact wears out, and the printing result becomes uneven when using wider recording paper. There is a problem.

More specifically, in a thermal printer capable of printing on two types of recording paper, for example, an A4 size and a post card (postcard) size, it is assumed that a post card size paper is used. After printing about several hundred sheets, if you print on A4 size recording paper,
Since the thermal head in the portion where the post card size recording paper, which is narrower than the A4 size recording paper, has passed is worn, when printing is performed on the A4 size recording paper, the A4 size recording paper is printed. The part of the size of the post card on the printing surface of the image becomes darker than the surroundings, and density unevenness appears. This problem is a problem particularly when a graphic image such as a photograph is printed with high definition, and becomes a problem, for example, in a sublimation printer that mainly prints a photographic image.

[0006]

Conventionally, density unevenness caused by such a difference in recording paper width is printed every time density unevenness occurs, for example, by printing a gray scale image and visually observing the printing source. The problem was solved by performing density correction of the image data.

[0007] Therefore, the method of solving such density unevenness is very complicated, and in some cases, it is said that printing is repeated many times and corrected. It is troublesome.

Accordingly, an object of the present invention is to provide a thermal printer capable of automatically correcting density unevenness due to such a difference in recording paper width and performing printing without density unevenness.

[0009]

The object of the present invention is achieved by the following means.

(1) In a thermal printer capable of printing on recording paper of different widths using a thermal head having a plurality of heating elements, the density of a plurality of portions of an image printed on the recording paper A plurality of density detecting means for detecting the density, and comparing the print density information from the density detecting means with the original image data to be printed to obtain a correction value for correcting density unevenness at the time of printing. A thermal printer comprising: a value calculating unit; and an image data correcting unit that corrects image data of a printing source based on a correction value obtained by the correction value calculating unit.

(2) A thermal printer, wherein the correction value obtained by the correction value calculating means is a value sloping at a boundary between recording papers having different widths.

[0012]

BRIEF DESCRIPTION OF THE DRAWINGS FIG.
An embodiment of the present invention will be described.

FIG. 1 is a block diagram showing the internal configuration of a thermal printer to which the present invention is applied, FIG. 2 is a schematic diagram showing the configuration of a normal thermal head printing unit, and FIG. 2A is a side view. FIG. 2B is a diagram viewed from the direction a in FIG. 2A.

The thermal printer 1 includes a CPU 11 for controlling various parts of the thermal printer, a RAM 12 used by the CPU 11 for developing data, an EPROM 13 for storing an operation program of the thermal printer 1,
EEPR which is a rewritable nonvolatile memory for storing correction values of density unevenness described later and other various set values
An OM 14, an interface (I / F) 15 for transferring data (image data, control data, and the like) to and from an external computer, an image buffer memory 16 for storing image data to be printed, and a plurality of heating elements for printing. The arranged thermal heads 17, an applied energy control unit 18 for applying energy according to image data to the thermal heads 17, and used in the control unit 18 to obtain applied energy according to the density of image data. Look-up table (LTU) 19, motor 20 for driving each section of the transport system for transporting the recording paper, sensor group 21 as a plurality of density detecting means for detecting image density for correcting density unevenness described later , Motor 20
And I / O for transmitting / receiving signals to / from the sensor group 21
An operation panel 2 having an O port 22 and a display device for inputting various setting values and displaying the current status of the printer;
3. A power supply unit 24 for supplying necessary power to each unit in the thermal printer 1 (power supply lines to each unit are not shown), and a clock signal serving as a reference is supplied to a CPU.
It comprises a timer 25 to supply to 11. These units are interconnected by a signal line 26 for passing a control signal and a bus line 27 for passing data.

The mechanical configuration of such a thermal printer 1 is the same as that of a normal thermal printer, and therefore detailed description is omitted here. The schematic configuration of the printing unit will be described with reference to FIG. . 2A is a schematic side view of the printing unit, and FIG. 2B is a schematic diagram viewed from the direction of arrow a in FIG. 2A (from the paper discharge side). The printing section including the thermal head 17, as shown in FIG. 2, presses the recording paper against the thermal head 17 at a constant pressing force with the thermal head 17 for forming an image on the recording paper 50. Roller 31 for recording and recording paper 5
It is composed of a pair of transport rollers 32 that transport 0. The thermal head 17 is provided with a plurality of heating elements 17a arranged side by side, and the heating elements 17a are supplied with power from the applied energy control unit 18 in accordance with the density of image data to perform printing. In the thermal printer 1, a sensor group 21 (sensors 21a and 21b) for detecting the density of a printed image is provided near the thermal head 17 and downstream of the recording paper 50 in the paper discharging direction. .
In the illustrated case, one sheet of thermal paper is shown as the recording paper 50. However, the present invention is not limited to this, and there is also a case where an ink film is conveyed to the paper and the ink of the ink film is transferred to the paper for printing.

Here, in the present embodiment, the width of the recording paper usable in the above-described thermal printer 1 will be described. In the present embodiment, in order to make the description easy to understand,
It is assumed that two types of recording paper having different widths are used, and the narrow width is W1 and the wide width is W2.

FIG. 3 is a diagram for explaining a printing result on such recording papers having different widths.

Here, for example, it is assumed that there is image data as shown in FIG. 3A as image data to be printed. The image data is input to the thermal printer 1 from an external computer or the like via the I / F 15 and stored in the image buffer memory 16. Here, it is assumed that this image data is printed on a recording sheet having a wide width W2.

In FIG. 3B, after printing several hundred sheets on a recording sheet having a narrow width W1, the image data (FIG.
A shows the result of printing (image data indicated by A) on recording paper having a wide width W2. In such a case, when the density correction is not performed, as shown in the figure, the image density differs between the portion W1 and the portion Wg outside the portion, and density unevenness occurs.

In the present embodiment, in order to automatically correct the density unevenness caused by such a difference in the recording paper width, the image density printed by the sensor group 21 as the density detecting means is detected, and this is detected. The correction value is calculated based on this. When two types of recording papers are used as in the present embodiment, the necessary sensor group 21 is at least one outside the narrow width W1 and inside the W2 as shown in FIG. (Sensor 21a) and one inside W1 (sensor 21b)
It is. With these two sensors 21a and 21b,
A difference in density caused by a difference in recording paper width can be detected from the printed image. The sensor used is, for example, an optical sensor, which receives reflected light from a printed recording paper surface and detects the density from a change in the amount of light. In addition,
The number of sensors may be at least one in each area corresponding to the width of the recording paper as described above, but a plurality of sensors may be provided in each area.

Hereinafter, a method of correcting the density unevenness will be described.

FIG. 5 is a flowchart showing a procedure for correcting the density unevenness. This procedure is performed by the CPU 11 in the above-described thermal printer 1 executing a program created according to the procedure. Therefore, the CPU 11 functions as a correction value calculation unit and an image data correction unit in the present invention.

First, from the image data of the printing source stored in the image buffer memory 16, data at the same position as the position where the sensor 21a is arranged is extracted along the printing direction at the time of printing and integrated. (S1). Taking this extracted data as an example of the image data shown in FIG. 3A, FIG.
FIG. 6A shows data extracted along the dotted line indicated by a in FIG. 6A, and the density (data density) is as shown in FIG.

Next, the sensor 2 on the image data
The data at the same position as the position where 1b is arranged is extracted along the printing direction at the time of printing and integrated (S2). Similarly, taking the image data shown in FIG. 3A as an example, it is data along the dotted line indicated by b in FIG.
And the integral value D2 is 180.

In this manner, the density on the data at the sensor position is obtained from the image data of the printing source, and printing is started (or the figure containers D1 and D2 may be obtained simultaneously with the start of printing). Each time the printed recording paper is gradually discharged, the density of the printed image is detected from each of the sensors 21a and 21b, and this is detected by each of the sensors 21a and 21b.
Is integrated in the printing direction every time (S3). Taking this as an example of the printing result shown in FIG. 3B, the density on the dotted line indicated by a in FIG. 3B is detected by the sensor 21a, and the density (measured density) becomes as shown in FIG. 7A. , Its integral value P
1 becomes 50. On the other hand, the sensor 21b detects the density on the dotted line b in FIG. 3B, and the density is as shown in FIG. 6B, and the integrated value P2 is 200. In addition, for integration of the detected concentration, for example, a signal from the sensor is A / D
And may be added sequentially by the CPU 11,
Alternatively, a dedicated integration circuit may be provided.

Next, D1 and D2 obtained as described above
Then, a correction value is calculated from P1 and P2 (S4).

This correction value is calculated by calculating the density integrated values D1 and D2 on the image data and the density integrated values P1 and P2 on the print image.
From the above, a correction value that makes the density at the positions of the sensors 21a and 21b constant, that is, the density of the W1 portion and W1
The correction value is determined so that the density of the outside portion of the image becomes constant. Specifically, the correction value is calculated from the following equation (1). Correction value h = (P2 / P1 × D1-D2) / L (1) where L is the number of dots in the printing direction.

From equation (1), based on the above example, L = 1
000, and a specific correction value is calculated, h = (2
00/50 × 60-180) / 1000 = 60/100
It becomes 0.

The correction value thus obtained is stored in the EEPROM 14 for the next printing (S5).

Then, the image data is corrected by subtracting the correction value h obtained from the data corresponding to W1 in the image data (S6), and printing is performed in accordance with the corrected image data (S7). The correction of the image data may be performed on the image data itself stored in the image buffer memory 16 or the energy applied to the thermal head 17 during printing may be corrected.

The print result after the correction as described above will be described.

FIG. 8 shows the density (data density) of the image data before correction and the density (print density) when the image data is printed as it is (the direction along the dotted line c in FIGS. 3A and 3B). It was taken to. As shown in the drawing, a portion where the data density is constant inside and outside W1 has a step as the print density. This is as described with reference to FIG.

FIG. 9 shows the result of the correction.
The density of the image data after correction has a step in the density of the image data due to the correction. When the density is printed, the density as a printing result due to the correction is substantially constant between the inside of the width W1 and the outside thereof. It turns out that it becomes. Therefore,
This result indicates that the occurrence of density unevenness is suppressed by the correction.

It should be noted that the transport position of the recording paper may be slightly shifted depending on the thermal printer. This is, for many thermal printers,
In the conveyance of the recording paper, a regulating plate or the like for restricting the conveyance position of the recording paper is provided in the conveyance system to prevent the conveyance position from being shifted in the width direction, but a slight deviation may occur. In such a case, the wear amount of the thermal head is different from that of the central portion at the side end portion of the narrow recording paper, that is, at the boundary portion of the width W1, due to the shift of the transport position of the recording paper. When the data is corrected, the boundary may be emphasized instead. Therefore, in the present embodiment, the correction value at the boundary is inclined.

FIG. 10 is an enlarged view of a boundary portion (within a circle in FIG. 9). As shown in FIG. 10A, the boundary of the image data itself is inclined as shown by the dotted line as a correction value of the image data, so that the density change at the boundary of the image data itself is made gentle. As shown in FIG. 10B, the density of the printing result was slightly prominent at the boundary portion and the boundary was conspicuous (solid solid line in the drawing) when the inclination was not applied, but almost as shown by the dotted line in the drawing. , And the boundary portion is not visually noticeable, and a clearer print result can be obtained. The range of the inclination varies depending on the accuracy of the transport system of the thermal printer, but it is desirable to match the shift amount in the width direction when the recording paper is transported due to the accuracy of the transport system.

As described above, in the thermal printer according to the present embodiment, when density unevenness occurs when printing is performed on a wide recording sheet, the above-described correction operation is performed so that the next time. As a result, a clear image without density unevenness can be obtained. In particular, it is suitable for a sublimation type printer which is required to print a graphic image such as a photograph with high definition, and can obtain a clear print result without density unevenness. This correction may be performed after printing hundreds of sheets, and need not be performed each time printing is performed. Therefore, a counter for counting the number of prints may be provided in the thermal printer, and this correction operation may be automatically performed every time the counter value reaches a predetermined value.

In the present embodiment described above, the usable width of the recording paper is two types. However, the present invention can be applied to a case where many different widths of the recording paper are used. . For example, as shown in FIG.
In the case where five types of recording papers up to 7 can be used, a sensor for detecting the density for each width is provided on the dotted lines shown in FIGS. In the same manner, it is possible to correct the density unevenness.

Note that the present invention is not limited to the above-described embodiment, and it goes without saying that various modifications can be made by those skilled in the art within the scope of the technical idea of the present invention.

[0039]

According to the present invention described above, the following effects can be obtained for each claim.

According to the first aspect of the present invention, the density of the printing result is detected by a plurality of density detecting means, and the detected density is compared with the image data of the printing source to determine a correction value for correcting the density unevenness. It was decided to correct the image data of the printing source according to this correction value, so that the uneven density caused by using recording paper with different widths was automatically corrected, and a clear print result without uneven density was obtained. Can be.

According to the second aspect of the present invention, the correction value has a slope at the boundary between recording papers having different widths, thereby preventing a sharp step from occurring on the data due to the correction. Even in the case where the recording paper is shifted in the width direction, the correction does not result in the boundary portion being emphasized, and a clear printing result without density unevenness can be obtained.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an internal configuration of a thermal printer to which the present invention has been applied.

2A and 2B are diagrams showing a schematic configuration of a printing unit including a thermal head of the thermal printer, wherein FIG. 2A is a side view and FIG. 2B is a view as seen from the direction of arrow a in FIG. 2A.

FIG. 3A is a diagram showing an example of image data, and FIG. 3B is a diagram showing a result of printing this image data.

FIG. 4 is a view showing an arrangement of sensors in the thermal printer.

FIG. 5 is a flowchart showing a procedure of correction in the thermal printer.

6A is a diagram showing the density of image data in a direction along a dotted line a in FIG. 3A, and FIG. 6B is a diagram showing the density of image data in a direction along a dotted line b in FIG. 3A.

7A is a drawing showing the density of a printed image in a direction along a dotted line a in FIG. 3B, and FIG. 7B is a drawing showing the density of a printed image in a direction along a dotted line b in FIG. 3B.

8 is a diagram showing the density of image data in a direction along a dotted line c in FIG. 3A before correction and the density of a print result in a direction along a dotted line c in FIG. 3B.

9 is a drawing showing the density of image data in the direction along the dotted line c in FIG. 3A after correction and the density of the print result in the direction along the dotted line c in FIG. 3B.

FIG. 10 is an enlarged view inside a circle in FIG. 9;

FIG. 11 is a diagram for explaining a sensor position when a plurality of recording papers having different widths are used.

[Explanation of symbols]

 11 CPU, 12 RAM, 13 EPROM, 14 EEPROM, 16 image buffer memory, 17 thermal head, 18 applied energy control unit, 21 sensor group, 21a, 21b sensor.

Claims (2)

[Claims] 1. A thermal printer capable of printing on recording paper of different widths using a thermal head having a plurality of heating elements, wherein the density of a plurality of portions of an image printed on the recording paper is determined. A plurality of density detecting means for detecting, and a correction value for obtaining a correction value for correcting density unevenness at the time of printing by comparing print density information from the density detecting means with original image data to be printed. Calculating means, based on the correction value obtained by the correction value calculating means,
Image data correction means for correcting the image data of the printing source;
A thermal printer comprising: 2. The thermal printer according to claim 1, wherein the correction value obtained by the correction value calculating means is a value that is inclined at a boundary portion between the recording papers having different widths.