JP2788830B2 - Thermal head - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thermal head used in a printing section of a thermal transfer printer, and more particularly to a temperature control means for a heat generating element.

[0002]

2. Description of the Related Art A conventional thermal head used for a printing section of a thermal transfer printer has a low print density immediately after the printing operation is started, and if the printing operation is continuously performed, the printing is performed over time. There is a tendency for the density to increase. This is because the energy supplied to the heating element of the thermal head for printing is accumulated as heat on the base material near the heating element and the thermal head itself. Since the change in print density due to the heat accumulation lowers the print quality, various measures have been taken against the heat accumulation.

For example, there is a means for correcting the energy applied to a heating element based on print history information. This means, when the print rate is small and the change in the print rate is small, such as in the case of printing characters, it is possible to suppress a decrease in print quality by a correction operation in a relatively short time, In the case of a large change in the printing ratio in the arrangement direction of the heating elements of the thermal head and the direction of the passage of time, such as in the case of graphic printing, a very large amount of print history information is required. In order to
Requires large scale integrated circuits.

There is also a means in which a temperature sensor such as a thermistor is provided near the heating element, and the energy applied to the heating element is corrected based on the temperature information. In this case, since it is impossible to arrange the temperature sensor in contact with the heating element, a time delay occurs between the actual temperature of the heating element and the temperature detected by the temperature sensor. In addition, since the temperature sensor itself also has a thermal response characteristic, there is a time delay in the detected temperature due to the thermal response characteristic. Therefore, for these reasons, it is impossible to correct the applied energy in the unit of time required for printing.

On the other hand, color printing has become widespread in recent printers. However, in the case of color printing, printing with delicate gradations is required, and the demand for suppressing a change in print density is becoming strict. For this reason, it is necessary to improve the resolution as well as to correct the heat storage of the thermal head.

[0006]

As described above, in the conventional thermal head, the printing density is low immediately after the printing operation is started, and if the printing operation is continuously performed, the printing density is reduced with time. It has the disadvantage of darkening.

[0007]

According to the present invention, there is provided a thermal head including a heating element formed of a material having a high temperature dependence of electric resistance, and a change in a current value caused by a change in electric resistance of the heating element. A temperature detection circuit for detecting a temperature, and a microcomputer for controlling applied energy to the heating element based on the temperature detected by the temperature detection circuit, and further, the temperature of the heating element is set in advance. A microcomputer for stopping supply of energy to the heating element when the temperature reaches a certain upper limit temperature.

[0008]

Next, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a circuit diagram for explaining the operation principle of the present invention, FIG. 2 is a characteristic diagram showing the temperature dependence of the resistance value of the heating element of the circuit of FIG. 1, and FIG. 5 is a strobe in the circuit of FIG. FIG. 4 is a characteristic diagram illustrating a relationship between a signal and a temperature of a heating element.

First, the operation principle of the present invention will be described with reference to FIGS. 1, 2 and 5.

In FIG. 1, a strobe signal generator (F
F) 1 generates a strobe signal 10 for determining a voltage application time to the heating element 4 (resistance value R) every printing cycle. While the strobe signal 10 is at a high level ("H" level), the drive circuit 2 is driven, a constant voltage V (for example, 24 volts) is applied to the heating element 4, and a current I (I = 24 / R) is applied. (1)) flows. Therefore, the resistance R is determined from the current I. The heating element 4 is shown in FIG.
Has a negative temperature coefficient as shown by the curve 21 of FIG.
(For example, this can be obtained by a chromium alloy thin film containing 25 atomic% of aluminum.) The resistance value of the heating element 4 is 1.6 KΩ at room temperature at the start of printing, but the temperature rises as printing operation starts. The resistance value is
In accordance with the curve 21 of FIG.

A current detecting resistor 3 (resistance value r) is connected in series to the heating element 4, and the relationship between the resistance values is as follows.

[0013]

## EQU1 ##

On the other hand, the output voltage v of the current detector 5 is v = r × I = r × 24ｒR, and therefore, the resistor 8 connected to the current detector 5
The current flowing through (resistance value r) has the same value as the value of (1), and is a value inversely proportional to the resistance value R of the heating element 4.

When the upper limit temperature T0 of the heating element 4 is set to 350 ° C., the resistance value R0 at that time is 1.05 KΩ from the curve 21 in FIG. Therefore, the upper limit current value setting device 6
Calculates the current value I0 at that time as: I0 = 24 ÷ 1.05 = 22.86 mA, and inputs this value to the comparator 7.

When the current value I detected by the current detector 5 becomes larger than the current value I0, that is, when the temperature of the heating element 4 exceeds the upper limit temperature T0, the output signal of the comparator 7 becomes low. Level (“L” level) changes to a high level (“H” level), and the strobe signal generator (F
F) 1 is reset to set the strobe signal 10 to the “L” level. Therefore, the operation of the drive circuit 2 stops,
No current flows to the heating element 4.

The relationship between the strobe signal and the temperature of the heating element at this time is as shown in FIG. That is, during the period from time t0 to t1 (period S1), the strobe signal 10
Is an "H" level, a current flows through the heating element 4 to increase the temperature, and accordingly, the resistance value decreases and the current value increases. The temperature of the heating element 4 is the upper limit temperature T0 (resistance value R0)
, The strobe signal generator (FF) 1 is reset, so that the strobe signal 10 becomes “L” level, and the current to the heating element 4 is cut off. Therefore, the temperature curve 22 of the heating element 4 starts to decrease.

However, since the heating element 4 is provided on the glass substrate, heat is accumulated on the glass substrate, and when the next print signal is applied, the temperature of the heating element 4 decreases to room temperature. I haven't. The accumulation of the reduced temperature of the heating element 4 due to the accumulation of heat causes the deterioration of the print quality. In the present invention, when the strobe signal 10 becomes the next “H” level, (Period S2),
When the temperature of the heating element 4 reaches the upper limit temperature T0 (resistance value R0), the comparator 7 operates and the strobe signal generator (FF)
1 to reset strobe signal 10 at time t3
At "L" level. As described above, since the width of time during which the strobe signal 10 is at the “H” level changes depending on the temperature of the heating element 4, the temperature of the heating element 4 can always be kept within a certain range.

FIG. 3 is a circuit diagram showing one embodiment of the present invention, and FIG. 4 is a timing chart showing the operation of the embodiment of FIG.

The thermal head of the embodiment shown in FIG. 3 has 32 heating elements 46-1 to 46-32. The serial input signal (Sin) 52 is input to the thermal head at the timing shown in FIG. 4 together with the clock signal (Clock) 53, is converted from serial to parallel by the shift register 41, and is latched by the latch circuit 4 by the latch signal (Latch) 51.
2 is stored. At the same time, the strobe signal generators (FF) 47-1 to 47-32 are set, so that one of the input terminals of the AND gates 43-1 to 43-32 is at the "H" level. Accordingly, the AND gates 43-1 to 43-1
Only the output signal of the AND gate of which the data of the latch circuit 42 of the drive circuits 43-32 is at the "H" level is at the "H" level, and corresponds to those of the drive circuits 44-1 to 44-32. Things become operational. As a result, a current flows through the corresponding heating element among the heating elements 46-1 to 46-32 to generate heat. For example, when only the odd-numbered data of the latch circuit 42 is at the “H” level, a current flows through the odd-numbered heating elements to generate heat.

This current value is determined by the current detection resistor 45-1.
Analog-to-digital converter (A
/ D converter) 62. On the other hand, an upper limit current value is set in advance in an upper limit current value setting device (setting register) 63, and the current value of the setting register 63 and the output signal of the A / D converter 62 are compared with a comparator 64.
And the current values of the heating elements 46-1 to 46-32 indicated by the output signal of the A / D converter 62 are
3, the strobe signal generator (F
F) The corresponding strobe signal generator (FF) of 47-1 to 47-32 is reset. As a result, the corresponding one of the AND gates 43-1 to 43-32 is closed, and the current flowing to the heating element connected thereto is cut off.

The A / D converter 62 and the setting register 63 operate in a time-sharing manner according to the timing of the pulse signals TS1 to TS32 shown in FIG.

In the above embodiment, the temperature dependence of the electrical resistance of the material forming the heating element may be either positive or negative. Since it becomes remarkable, it is suitable for use in a printer that requires a fine gradation.

The microcomputer controls the energy applied to the heating element based on the temperature of the heating element detected by the temperature detection circuit. When the temperature of the heating element reaches a separately set upper limit temperature. At this time, the supply of energy to the heating element is suppressed, and when the temperature falls below a separately set lower limit temperature, the supply of energy to the heating element is increased to maintain the temperature of the heating element within a certain range. May have the ability to
When the temperature of the heating element reaches a separately set upper limit temperature, the supply of energy to the heating element is stopped to prevent the temperature of the heating element from exceeding a predetermined temperature. Good.

[0026]

As described above, the thermal head of the present invention uses a heating element formed of a material having a large temperature dependence of electric resistance, and the temperature of the heating element is changed by a change in current value due to a change in electric resistance. Is detected, and the energy applied to the heating element is controlled based on the detected temperature, whereby the temperature of the heating element can be always kept within a certain range. Therefore, it is possible to control the print density at a high speed, and it is possible to perform printing with a high gradation. Furthermore,
Since a special temperature sensor is required, the number of parts and the number of assembling steps can be reduced.

[Brief description of the drawings]

FIG. 1 is a circuit diagram for explaining the operation principle of the present invention.

FIG. 2 is a characteristic diagram showing a temperature dependence of a resistance value of a heating element of the circuit of FIG.

FIG. 3 is a circuit diagram showing one embodiment of the present invention.

FIG. 4 is a timing chart showing the operation of the embodiment of FIG.

FIG. 5 is a characteristic diagram showing a relationship between a strobe signal and a temperature of a heating element in the circuit of FIG.

[Explanation of symbols]

1.47-1 to 47-32 Strobe signal generator (FF) 2.44-1 to 44-32 Drive circuit 3.45-1 to 45-32 Current detection resistor 4.46-1 to 46-32 Heating element 5 Current detector 6 Upper limit current value setting device 7.64 Comparator 8 Resistor 10 Strobe signal 21 Curve 22 Temperature curve 41 Shift register 42 Latch circuit 43-1 to 43-32 AND gate 51 Latch signal 52 Serial input signal 53 clock signal 61 microcomputer 62 analog-to-digital converter (A / D converter) 63 setting register

Claims (2)

(57) [Claims] A heating element formed of a material having a large temperature dependence of electric resistance; a temperature detection circuit for detecting a temperature of the heating element by a change in current value due to a change in electric resistance of the heating element; A thermal head comprising: a microcomputer that controls energy applied to the heating element based on the temperature detected by the detection circuit. 2. The thermal head according to claim 1, further comprising: a microcomputer for stopping supply of energy to the heating element when the temperature of the heating element reaches a preset upper limit temperature.