JPH0574467B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a printing control device for a thermal printer having a thermal head equipped with a plurality of heat generating elements.

[Conventional technology]

In general, thermal printers suffer from density differences in printed dots due to the effects of ambient temperature and self-heating of the thermal head itself, and dot shapes are distorted due to trailing phenomenon.

For this reason, there have conventionally been methods for converting temperature changes into a power supply voltage for a heating element, as disclosed in JP-A-54-36737, and head temperature detection means as disclosed in JP-A-59-7068. A method has also been proposed in which the value of the head is read by an AD converter, the read value is discriminated by the CPU, and the voltage applied to the head is determined by the DA converter.

[Problem that the invention seeks to solve]

These conventional examples had the following problems.

In general, heating resistors formed by sputtering such as tantalum nitride are extremely sensitive to overvoltage, and when using a method that varies the applied voltage in response to temperature changes, extreme care must be taken to prevent damage to the heating resistor. I had to pay.

Furthermore, A-D converters and D-A converters are large-scale devices and are expensive, so they have not been used in serial printers and the like.

An object of the present invention is to provide a printing control device for a thermal printer for temperature compensation, which can supply appropriate applied energy in a wide temperature range in response to changes in the temperature of the thermal head and its surroundings. The aim is to realize these in a simple and low cost manner.

Another object of the present invention is to provide a printing control device that is safe against damage to the thermal head.

[Means for explaining the problem]

The print control device for a thermal printer according to the present invention has the following means.

(a) A thermal head or a heat-sensitive resistance element that detects the ambient temperature; (b) A resistance network that includes the heat-sensitive resistance element; (c) An amplifier circuit that amplifies the potential at a predetermined voltage dividing point of the resistance network. (d) a voltage comparator circuit that uses the output potential of the amplifier circuit as a reference potential and converts the charging/discharging time of the capacitor into a pulse width; (e) the output time of the voltage comparator circuit as a reference for the heating element on the thermal head; Power supply control circuit that determines the energization time.

[Effect]

The output of this operational amplifier corrects the temperature characteristics of the thermistor using a resistance network that includes a thermistor, which is a kind of heat-sensitive resistance element that detects the temperature of the thermal head or its surroundings, and amplifies it using an operational amplifier that is an amplifier circuit. By using the potential as a reference potential and outputting the charging time of the capacitor to this reference potential as a pulse width by a voltage comparator circuit, and using this pulse width as the reference energization time for the heat generating element on the thermal head, temperature fluctuations can be constantly avoided. The optimal pulse width is applied to

〔Example〕

FIG. 1 is a schematic diagram of a circuit diagram showing an embodiment of a printing control device according to the present invention.

1 is a resistance network including a thermistor 7 which is a type of heat-sensitive resistance element for detecting a thermal head or ambient temperature; 11 and 12 are resistors;
13 is a capacitor for noise prevention. The capacitor 13 has no relation to the characteristics at all.

2 is an amplifier circuit having an operational amplifier 8; as an input to the operational amplifier, a predetermined voltage dividing point of the resistor network and a voltage dividing circuit 14 having a variable resistor 16;
It has a predetermined partial pressure point. The operational amplifier 8 also includes a feedback resistor 19, a diode 20, and an input resistor 18. The voltage dividing circuit 14 includes resistors 15 and 17 and a variable resistor 16, and forms a reference input section of the operational amplifier 8.

3 is a voltage comparison section, which includes a voltage comparison circuit 9, a capacitor 10, resistors 23, 24, a transistor 21,
22 as basic components.

4 is a power supply control circuit, which includes a buffer 25, a head driver 26, and a switching transistor 27.
It consists of

5 is a thermal head having a heating resistor 30 which is a heating element.

6 is a CPU that centrally controls the thermal printer.

〔motion〕

The resistance network 1 is a curve correction section that corrects the temperature characteristics of the thermistor.

The resistance value Rt of the thermistor is generally expressed by the following formula using the B constant, where R(20) is the value at 20°C, and T is the absolute temperature.

Rt=R(20)expB(1/Tt-1/T20)...Equation 1 Tt=Absolute temperature at any temperature T20=Absolute temperature at 20℃ Looking at Equation 1, the term exp is included. Therefore, the reciprocal of temperature is log linear. That is, the resistance value decreases as the temperature rises, and the rate of change is smaller on the high temperature side.

In the case of a thermal head, reducing the applied energy at higher temperatures provides better printing quality and is safer for the heat generating element, and it is necessary to correct the characteristics of this thermistor.

FIG. 2 is a graph showing the characteristics of the resistance value Rt of the thermistor, the output voltage at the voltage dividing point A of the resistor network 1, and the output voltage of the operational amplifier, where the horizontal axis is the absolute temperature;
The vertical axis indicates the resistance value Rt, the potential at the voltage dividing point A, and the output potential Vop of the operational amplifier.

41 is the temperature characteristic of the resistance value Rt of the thermistor,
42 is a characteristic curve showing the temperature characteristic of the potential at the voltage dividing point A, and 43 is a characteristic curve showing the temperature characteristic of the output potential Vop of the operational amplifier. The thermistor has negative characteristics with respect to temperature changes, as shown in the characteristic curve.

The following points can be cited as characteristics for performing such correction.

There are few peripheral elements, so there is little variation such as variation.

Linearity can be maintained over a relatively wide range.

Changes in output voltage with respect to temperature outside the operating temperature range, for example below 0°C, above 40°C, etc., can be made small, and the thermal head can be protected from destruction.

The amplifier circuit 2 uses the corrected temperature characteristics to partially expand the temperature characteristics, and has one input as the voltage dividing point A at which the characteristic curve 42 is obtained, and the other input as a voltage dividing circuit using a resistor.

The magnification of this circuit is expressed as follows.

The value of resistor 18 is R18, and the value of resistor 19 is R19.
Then, the magnification K becomes K=R19/R18.

The diode 20 built into the amplifier 2 is
This serves to place a certain limit on the expansion of the energization time at low temperatures. When the temperature is low, the voltage at the voltage dividing point A increases, and the voltage comparator circuit 9, which will be explained later, expands the energization time width.
The change in the potential at the voltage dividing point A cannot immediately follow the change in the heat generation temperature of the heat generating resistor 30 itself, and especially at low temperatures, the heat generating resistor 30 may generate abnormal heat and be thermally destroyed. The diode 20 is
This prevents thermal damage at low temperatures, and diode 2
0 suppresses the rise in the output voltage Vop of the operational amplifier 8 to a predetermined voltage, thereby placing a certain limit on the expansion of the current supply time. Note that the clamp voltage value of the output voltage Vop can be changed by changing the number of inserted diodes.

The variable resistor 16 in the voltage divider circuit 14 adjusts the output potential Vop without changing the rate of change of the output potential with respect to temperature, and sets the output pulse width of the voltage comparison section in the subsequent stage to a predetermined value. This is used to set the initial value of the energy applied to the head.

The transistor 21 in the voltage comparator 3 is a discharging transistor that momentarily discharges the capacitor 10.

FIG. 3 is an explanatory diagram showing input signals and output waveforms of the voltage comparison section, and 51 is a trigger signal sent from the CPU 6. 52 and 53 indicate output waveforms, and 52 indicates the pulse width switching transistor 22.
53 shows the output when it is off, and 53 shows the output when it is on. The pulse width TW1 of the output waveform 52 is the pulse width of the output waveform 53.
It is larger than TW2. This pulse width switching is performed when the energy applied to the thermal head is changed depending on the characteristics of the ink ribbon, the surface condition of the printing paper, etc. to obtain the optimum conditions.

This pulse width TW is determined by resistor 23 and resistor 24.
Let the values of R23 and R24 be respectively, and capacitor 10
When the value of is C10 and the output potential of the operational amplifier is Vop, when the transistor 22 is off, it is expressed as follows. At this time, the pulse width of the trigger input is extremely small, so it is ignored.

TW1=-C10×(R23+R24)ln(1-Vop/Vc) When transistor 22 is on, TW2=-C10×R24ln(1-Vop/Vc) ln=Natural logarithm Vc is the circuit voltage Since it is generally constant at 5V,
The pulse width TW is determined by the potential of Vop.

By selecting the values of C10, R23, and R24 as well as the value of the preceding stage amplifier circuit 2, it is possible to realize a characteristic in which the rate of change in pulse width is linear with temperature change within a limited temperature range. In addition, it is also possible to freely change the temperature characteristics.

The output of the voltage comparator circuit 9 is the inverter buffer 2
5 to the switching transistor 27.

The transistor 27 is connected to the power supply terminal 26a of the head driver 26, and the head driver is enabled to operate for the pulse width TW time, and the reference energization time of the heat generating element 30 is determined.

Generally, in thermal history control and the like, when the heat generating element 30 is continuously energized, a method is adopted in which the time during which the heat generating element 30 is energized is reduced by a predetermined time from the pulse width TW.

In this case, when the head driver 26 is in an operable state, the data output signal 6a from the CPU side is used to reduce the power supply time.

Note that Vc indicates the circuit power supply, and Vh indicates the thermal head power supply.

Also, instead of using multiple diodes 20, one
Alternatively, one Zener diode may be used instead. Furthermore, it is also possible to use a heat-sensitive element whose resistance value increases as the temperature rises; in this case, the voltage at the voltage dividing point A is connected to the non-inverting input of the operational amplifier 8, and also to the variable resistor in the voltage dividing circuit 14. The divided voltage signal from 16 may be input to the inverting input of operational amplifier 8 to configure operational amplifier 8 as a non-inverting amplifier.

FIG. 4 shows another embodiment of the voltage comparison circuit of the printing control device according to the present invention.

60 is one shot timer IC555, 6
1 is a control terminal and is an input terminal of operational amplifier 8, 62 is a trigger input terminal, 63 is a resistor, 64
65 represents a capacitor, and 65 represents an output terminal.

This output pulse width is also exactly the same as the characteristics of the voltage comparator circuit shown in FIG.

FIG. 5 is a characteristic diagram showing the relationship between applied energy and temperature of the thermal head obtained by the printing control device according to the present invention.

The applied energy E changes the resistance value of the thermal head.
Rh is the saturation voltage of the head driver.
Assuming Vs, it is expressed by the following formula.

E=(Vh−Vs) ² /Rh×TW(j) The vertical axis in FIG.
2 represents the characteristic curve when it is off, and 72 represents the characteristic curve when the transistor 22 is on.

In this way, it is possible to provide linear characteristics with respect to temperature changes, and the rate of change of applied energy with respect to temperature is almost the same even when the transistor 22 is turned on or off. It is possible to stabilize the concentration against temperature changes.

〔Effect of the invention〕

According to the present invention, it is possible to provide various temperature characteristics of energization time in accordance with the temperature characteristics required by a thermal printer.

In addition, it is possible to suppress the sudden increase in pulse width at low temperatures, which has a great effect on preventing destruction of the heat generating element, and always provides the appropriate applied energy.
It is possible to achieve good print quality.

Further, there is no need for expensive circuits such as A-D converters, and various applications are possible with simple circuits.

In addition to the heat-generating resistor element, the heat-generating element includes an electrode that generates heat in the resistance layer on the back side of the thermal transfer film.
A similar effect is obtained even when an electrode type printing element is used.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram of an embodiment of a print control device for a thermal printer according to the present invention. FIG. 2 is an explanatory diagram showing the temperature characteristics of the resistance value of the thermistor, the voltage dividing point of the resistance network including the thermistor, and the potential of the output of the operational amplifier. FIG. 3 is an explanatory diagram showing input signals and output waveforms of the voltage comparison section. FIG. 4 is a diagram showing another embodiment of the voltage comparison circuit. FIG. 5 is a characteristic diagram showing the characteristics of the printing control device of the present invention. DESCRIPTION OF SYMBOLS 1... Resistance network, 2... Amplification circuit, 3... Voltage comparison section, 4... Power supply control circuit, 7... Thermistor, 8... Operational amplifier, 9... Voltage comparison circuit, 3
0...Heating element.

Claims (1)

[Claims] 1. A thermal printer that has a thermal head with a plurality of heat-generating elements and prints on recording paper via thermal paper or a transfer film, a thermal printer that detects the temperature of the thermal head or the ambient temperature, and a thermal printer that detects the temperature of the thermal head or the ambient temperature; an operational amplifier that connects a predetermined voltage dividing point of the resistive network to one input terminal and amplifies the potential of the voltage dividing point; and an output terminal of the operational amplifier and the input terminal. at least one diode or Zener diode connected between them; a voltage comparator circuit that uses the output potential of the operational amplifier as a reference potential and converts the charging/discharging time of the capacitor into a pulse width; 1. A printing control device for a thermal printer, comprising: a power supply control circuit that sets a standard energization time to the element, and controls the heating element to prevent the pulse width from expanding at low temperatures.