JPS61167591A - Thermal head driving device - Google Patents

Abstract

PURPOSE:To prevent deterioration of printing quality attributable to the generation of uneven accumulation phenomenon in a substrate by estimating the temperature distribution of a substrate with a thermal head and cooling each prearranged division of the substrate using a mutually independent cooling power. CONSTITUTION:A thermal head 1 is mounted in straight line on the upper surface of a ceramic substrate 2, and thermometers (thermister, etc.) 111, 112, 113 are inserted into apertures on the sides of the substrate 2. Fans 41, 42, 43 and an airflow control circuit 12 are provided right under the substrate 2 together with an AC power supply 14, a switching circuit (SCR) 15 and a phase control circuit 16 incorporated in the device.

Description

DETAILED DESCRIPTION OF THE INVENTION "Field of Industrial Application" The present invention relates to a thermal head drive device used in a device that performs thermal recording or display.

"Prior Art" Recording devices that perform thermal recording using thermal recording paper or thermal transfer recording media are widely used in facsimile machines, printers, and the like. Usually, such a recording device uses a unit heating element (
A thermal head in which heating elements (heat generating elements) are arranged in a row is used as a recording head.

Similar thermal heads are also used in display devices that perform recording and display using magnetized latent images.

Now, when recording or displaying using a thermal head,
Electric pulses are selectively applied to a large number of unit heating elements provided in the thermal head according to print data. This electric pulse generates heat in each unit heating element, and at that temperature, for example, a specific portion of the thermosensitive recording paper develops color. If this temperature is not high enough, color development will be insufficient and the recorded image will be faint and faded.

Furthermore, if this temperature is too high, the color may develop more intensely than necessary, or the area where the color develops will become large, causing the details of the recorded image to become blurred. In addition, even if good printing is performed at the beginning of recording, in the latter half of a long recording operation, the average temperature of the unit heating element rises due to heat accumulation in the entire thermal head, which may cause density unevenness in the recorded image as a whole. .

FIG. 6 shows an example of the temperature distribution (vertical axis) in the longitudinal direction (horizontal axis) of the substrate of the thermal head.

In the figure, characteristic I is an example when a heat accumulation phenomenon with a certain pattern occurs in the longitudinal direction of the thermal head, and characteristic ■ is an example when a wavy heat accumulation phenomenon occurs unevenly in the longitudinal direction of the thermal head. shows.

Phenomena such as characteristic I are often caused by problems with the structure of the board, and as shown in FIG. This can be suppressed by making a difference in the length of the fins.

In addition, the phenomenon shown in characteristic (■) occurs during continuous recording of special images, and usually recovers in a relatively short time due to thermal diffusion.
This is suppressed by installing a fan 4 as shown in FIG. 9 to forcibly cool the board.

"Problems to be Solved by the Invention" However, even with the above methods, problems may arise due to the installation environment of the recording device, the peculiarity of the recorded image, and other various factors.
It is not possible to completely prevent uneven heat storage from occurring on the substrate of the thermal head.

In particular, when recording is performed using a line-type thermal head, if uneven heat accumulation occurs in the longitudinal direction, striped shading will occur in the recorded image, resulting in a significant deterioration in image quality. When a heat accumulation phenomenon with a certain scent as shown in characteristic I in FIG. 6 occurs, the recorded image becomes darker toward the left, which also deteriorates the image quality.

The present invention has been made to solve the above problems, and an object of the present invention is to provide a thermal head drive device that can prevent deterioration in print quality due to uneven heat accumulation on the substrate of the thermal head. It is something to do.

"Means for Solving the Problems" The thermal head driving device of the present invention includes a thermal head, a substrate on which the thermal head is mounted, a detection means for predicting the temperature distribution of the substrate, and a forced cooling device for cooling the substrate. This forced cooling device is characterized in that it cools the substrate in each predetermined section with mutually independent cooling power based on the detection result by the detection means.

For example, a forced cooling device is composed of a fan and an air volume control circuit placed opposite each section of the board, and each fan increases the air volume as the board temperature of the section it faces is higher. controlled to become

In this way, for example, if the temperature of each part of the board is measured, the temperature distribution is predicted, and the board is cooled with independent cooling power for each predetermined section according to the results, localized heat accumulation will occur on the board. In this case, the temperature of the entire substrate can be made uniform by intensively cooling that region.

Embodiment FIG. 1 is a schematic diagram showing an embodiment of a thermal head driving device of the present invention.

A thermal head 1 made of a heat-generating resistance wire or the like is placed linearly on the upper surface of a substrate 2 made of ceramic, for example. As is well known, lead wires and peripheral circuits for supplying printing pulses are also arranged on this substrate, but illustration thereof is omitted. A hole is provided in the side surface of the substrate 2 at one longitudinal end, the other end, and the center, and a temperature measuring device such as a thermistor LIT, 11. ．． 113 is inserted.

Directly below the board 2, a forced cooling device 13 consisting of three fans 4I, 42, 43 and an air volume control circuit 12 is arranged. The air volume control circuit 12 is connected to an alternating current power source (AC) 1
A switching circuit (SCR) 15 consisting of a SIRISK, a TRIAC, etc., inserted in a circuit that transmits the power supplied from 4 to each fan 4I, 42.43, and a phase control circuit (SCR) 15 that determines the phase angle of each switching circuit 15. FC)
It consists of 16. 3 thermistors 111.
11. 11. The temperature detection output of this phase control circuit 16
is man-powered.

This thermal head driving device operates as follows.

Suppose that some external factor causes a heat accumulation phenomenon in the substrate 2, which has a certain characteristic as shown in characteristic (2) in FIG. At this time, the thermistor 11° detects the maximum temperature,
Thermistor 11. ．． Detect lower temperatures in the order of 11°.

Utilizing the change in electrical resistance of each thermistor at this time,
The phase angle of the switching circuit 15 is made to correspond to the detected temperature by a phase control method that combines a capacitor and a resistor. Second
The figure shows a specific example of wiring of this forced cooling device 13.

Fan (F) 4 using AC power supply 14 and single-phase AC motor
．． A switching circuit 15 using a triac 18 is connected in series to the circuit connecting the two. Furthermore, a phase circuit 16 using a trigger diode 19 is connected to the gate of the triac 18 . In the phase circuit 16, a thermistor (T) 1 is fixed to a substrate on which a resistor 20, a capacitor 21, and a thermal head are mounted.
1° are connected in series. Thermistor 11. For this purpose, use one whose resistance value decreases as the temperature rises. A resistor 22 and a capacitor 23 connected in parallel with the triac 18 are for surge absorption.

Here, the thermistor 11. As the fixed substrate temperature increases, the resistance value of the thermistor 111 decreases. As the resistance value of the thermistor 111 decreases, the charging speed of the capacitor 21 becomes faster, and the voltage across the capacitor 21 reaches the breakover voltage of the trigger diode 19 faster. Therefore, the gate current is supplied at a faster timing, the conduction angle of the triac becomes larger, and the air volume of the fan increases.

Conversely, when the board temperature decreases, the conduction angle of the triac becomes smaller and the fan air volume decreases.

Figure 3 shows each fan 4. ．． 4. ．． 4. represents the power supply voltage waveform applied to Thermistor 111.112.113
Based on the substrate temperature measurement data, the conduction angle α11α2,
α3 is controlled differently. That is,
If the board 2 is divided into three sections 21122.23, the fan 4 facing the section 21 with the highest temperature will be supplied with the most power to increase the air volume, and the fans facing the other sections will be supplied in order. Control to reduce air volume. Each section of the board 2. ．． 22.2.0 The correlation between temperature and air volume is
This largely depends on the heat dissipation of the board itself.

If the board is provided with a heat sink 3 with a large cooling power as shown in FIG. 6, the cooling power can be varied by slightly increasing or decreasing the air volume.

Using the apparatus of this example, we first continuously recorded images in which the black ratio on the section 2I side of the board was significantly higher than on the other sections, while keeping the air volume of the three fans constant. After a certain period of time, thermistor 111 becomes 34.5°C1
Thermistor 113 read 30.3°C. Next, each thermistor 11, . When we continued recording by increasing or decreasing the fan air volume based on the measured temperature of the thermistor 11. is 31.1°C1 thermistor lla is 3
The temperature was 0.2°C, resulting in a temperature distribution that had almost no effect on recorded images.

FIG. 4 shows the relationship between the optical density difference (vertical axis) between the left and right sides of the recorded image obtained during this experiment and the number of recorded sheets (@ axis).

Data ■ is for the case where the fan air volume is kept constant, and it can be seen that the optical density difference increases significantly as the number of recorded sheets increases. On the other hand, it can be seen that when the air volume is controlled as in the thermal head driving device of the present invention, stable recorded image quality can be obtained with little change in the optical density difference even if the number of recorded sheets increases.

The thermal head driving device of the present invention is not limited to the above embodiment.

The temperature distribution of the substrate may be predicted from the printing pulses applied to each unit heating element or the printing data thereof.

If the device measures the substrate temperature to correct the print pulse width, all or part of the output of the temperature measuring device may be used. Forced cooling systems do not necessarily require the use of fans. For example, a heat pump may be used to control its cooling power.

In addition, air volume adjustment is performed not only by fan rotation control but also by
As shown in the figure, the inclination angle of the blade 25 for adjusting the wind direction arranged between the fan 4 and the board 2, that is, the direction of the wind blowing on the board may be changed. In this embodiment, a solenoid 27 moves a rod 26 inserted through the blade 25 back and forth in the direction of the arrow 28, thereby tilting the blade 25 left and right as shown by the arrow 29.

"Effects of the Invention" The thermal head driving device of the present invention described above has the following advantages:
Since the substrate is cooled by independent cooling power for each predetermined section, non-uniformity in substrate temperature is alleviated, and deterioration in printing quality can be prevented.

[Brief explanation of the drawing]

Fig. 1 is a schematic diagram showing an embodiment of the thermal head driving device of the present invention, Fig. 2 is a wiring diagram of its main parts, Fig. 3 is an explanatory diagram of its operation, and Fig. 4 is a diagram for explaining its effects. A graph showing the relationship between the number of recorded sheets and the optical density difference. Figure 5 is a sectional view of the main part of a modified example. Figure 6 is a temperature distribution diagram of the substrate of a conventional thermal head. Figures 7 and 8 are graphs of the conventional thermal head. FIG. 9 is a perspective view of a substrate of a thermal head, and FIG. 9 is a perspective view of a main part of a conventional thermal head driving device. 1... Thermal head, 2... Substrate, 2. 2. ．． 23...Category of board, 4...
...Fan, 11, ,11. ．． 11. ...Temperature measuring device, 1
2... Air volume control circuit, 13... Forced cooling device. Applicant: Fuji Xerox Co., Ltd. Representative
Patent Attorney Ume Yu Yamauchi 1st
Figure 4 Figure 2 Figure 3 Figure 5 Figure 7 Figure 8 Figure 9

Claims (1)

[Claims] 1. A thermal head, a substrate on which the thermal head is mounted, a detection means for predicting the temperature distribution of the substrate, and a forced cooling device for cooling the substrate, the forced cooling device The thermal head driving device is characterized in that the substrate is cooled in each predetermined section with mutually independent cooling powers based on the detection result by the detection means. 2. The forced cooling device is composed of a fan and an air volume control circuit arranged to face each board section, respectively, and the fan increases the air volume as the board temperature of the section facing each section increases The thermal head driving device according to claim 1, wherein the thermal head driving device is controlled such that the number of thermal heads increases.