JPS6342468B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a halftone recording device using a thermal head, and particularly proposes a halftone recording device in which the printing density of each heating element is corrected based on its heat generation characteristics depending on the energization conditions and the environmental temperature. It is something to do.

A thermal head is made up of a plurality of heating elements arranged in a single line array, and records a desired image on thermal recording paper by selectively energizing each heating element to generate heat. When recording halftones using a thermal head, the energization conditions such as voltage and current applied to the heating element are not proportional to the applied density, so it is necessary to make a certain γ correction to the energization conditions. Furthermore, since the print density changes depending on the environmental temperature, it is necessary to modify the energization conditions based on the environmental temperature, especially in order to record halftones.

As a conventional device for adjusting printing density, "a pulse signal with a constant frequency is applied as a drive signal to a thermal print element, and the driving time of the thermal print element is adjusted by changing the pulse width of the pulse signal. (Special Publication No. 51-3222), "The printing density is made variable by changing the number of recording signals applied to the heating resistor by the signal of the temperature detection element within the unit recording time. (Japanese Unexamined Patent Publication No. 52-143839) and "By using ROM to generate counter control signals, the application time can be controlled according to the color density characteristics according to gradation steps such as equal intervals or logarithmic characteristics. Yes, ROM
It is possible to easily change the color density characteristics by changing the contents (Japanese Patent Application Laid-Open No. 1983-69482)
etc. However, in these cases, the print density is adjusted by controlling the width or number of energizing pulses by considering only either γ correction or environmental temperature compensation, so in order to realize both corrections, it is necessary to There are disadvantages in that the circuit configuration becomes complicated and sufficient accuracy cannot be obtained.

The present invention has been made in view of the above points, and can realize both γ correction and temperature compensation simultaneously with a simple circuit configuration, and furthermore, can perform temperature compensation at each step of the gradation level. An object of the present invention is to provide a halftone recording device for a thermal head that can perform correction with high accuracy. A halftone recording device for a thermal head according to the present invention is a halftone recording device for a thermal head that adjusts printing density by adjusting a current pulse applied to a heating element.
There is a ROM table group in which a plurality of ROM tables are provided according to the environmental temperature, in which the relationship between the gradation level and the energizing pulse is set in advance, and which inputs the gradation level signal and outputs the corresponding pulse information. temperature detection means for detecting and converting the detection signal from analog to digital; and a temperature detection means corresponding to the temperature detected by the temperature detection means among the respective ROM tables.
The present invention is characterized by comprising a table selection circuit that selects and outputs a ROM table, and pulse output means that outputs a pulse signal corresponding to the output of the table selection circuit to a heating element of a thermal head. In this case, when the pulse information is a number of pulses, the pulse output means includes a pulse oscillator that oscillates a pulse of a predetermined width at a predetermined frequency, a counter that counts the number of pulses of the pulse oscillator, and a counter that counts the number of pulses of the pulse oscillator. and a comparator that opens and closes the gate circuit based on a comparison result between the number of pulses from the table selection circuit and the count value of the counter.

Hereinafter, specific embodiments of the present invention will be described with reference to the accompanying drawings. FIG. 1 is a circuit diagram showing an example of a direct drive type thermal head equipped with a shift register, and FIG. 2 is a timing diagram of each signal. A plurality of heating elements 1 of the thermal head are arranged in a single row array.
A transistor switch 2 is connected in series to each heating element 1. Each of these series connected bodies is a heating element 1
are connected in parallel between the power supply (V _HD ) and the ground (G _ND ), and voltage V _HD is applied to each heating element 1 when the switch 2 is turned on. An AND gate 3 is connected to the base of the switch 2, and one input terminal of each AND gate 3 is connected to a latch 4.
, and a strobe signal SB is input to the other input terminal. The latch 4 is connected to the shift register 5, and latches each output of the shift register 5 when the load signal LD is input. The pattern data DI is serially input to the shift register 5, and the pattern data DI is output in parallel in synchronization with the clock signal CK.

In this configuration, the shift register 5 outputs the pattern data DI to the latch 4 in parallel in synchronization with the clock signal CK, as shown in FIG. When the transfer of one line of pattern data DI from the shift register 5 to the latch 4 is completed, the load signal LD is input to the latch 4.
Latch 4 latches pattern data DI. The strobe signal SB with a pulse width t _pw is input to the AND gate 3, and the AND gate 3 corresponding to the bit in which the pattern data DI latched to the latch 4 is on outputs the strobe signal SB.
Switching transistor 2 is turned on for the time t _pw that the pulse is on. Therefore, the heating element 1 corresponding to the bit where the pattern data DI is on is energized by the pulse width t _pw of the strobe signal SB, and each heating element 1 to which the voltage V _HD is applied is selectively energized based on the pattern data DI. Electricity is applied for a time t _pw , which generates heat in the resistor and records an image on thermal paper.

In this way, a current with the same waveform as the strobe signal SB flows through the heating element 1.
When recording halftones, there are cases in which the pulse width t _pw of the strobe signal SB is adjusted as shown in Fig. 3a, and a strobe signal having a plurality of narrow pulses with a constant width is used as shown in Fig. 3b. In some cases, the number of pulses N of the strobe signal SB is adjusted. In either case, when the applied voltage V _HD is constant, the relationship between the pulse width t _pw or the number of pulses N and the print density is as shown in Figure 4, and there is a difference between the two at least in part. There is no proportional relationship. Moreover, as shown in FIG. 5, the pulse width t _pw
Even when the pulse number N is constant, the print density varies depending on the environmental temperature, and there is no proportional relationship between the two. Therefore, when recording halftones, if the pulse width _tpw or the number of pulses N is changed in proportion to the tone level, the print density is not necessarily proportional to the tone level. Further, even if the pulse width t _pw or the number of pulses N is changed in proportion to the environmental temperature to compensate for the fluctuations in the environmental temperature, the print density will not necessarily become constant. Therefore, when recording halftones with a thermal head, the print density characteristics depending on the energization conditions (pulse width t _pw or number of pulses N) and the print density characteristics depending on the environmental temperature are shown in Figure 5. Based on this, it is necessary to adjust the pulse width t _pw or the number of pulses N of the voltage V _HD applied to the heating element 1 depending on the gradation level and the environmental temperature. An embodiment in which this correction is performed by controlling the number N of energizing pulses will be described below.

FIG. 6 is a schematic block diagram of the correction circuit. The environmental temperature is divided into n levels, and for each group
ROM tables 111, 112, ... 11n are provided, consisting of n ROM tables.
A ROM table group 11 is configured. each
In the ROM table 111, etc., the relationship between the gradation level and the number N of energizing pulses to be applied to the heating element 1 in order to obtain the print density corresponding to the gradation level is set at the environmental temperature level. There is. That is, a predetermined number of pulses is stored at an address provided for each gradation level. Then, the gradation level signal is input into each ROM table 111 etc. as an address, and each ROM table 111 etc.
The ROM table 111 etc. outputs the number of pulses No. required to obtain the print density of the gradation level specified by the gradation level signal. The temperature sensor 14 is
For example, it is a thermistor or the like, and measures the environmental temperature of the thermal head and outputs the measurement result as an analog signal. The output of the temperature sensor 14 is an analog/digital converter (hereinafter abbreviated as A/D converter).
13, and after being converted into a digital signal, it is input to the table selection circuit 12.
Based on the environmental temperature measured by the temperature detector 14, the table selection circuit 12 selects a ROM table corresponding to the environmental temperature level from the ROM table group 11, and transfers only the output of the selected ROM table to the comparator 17. This is a switching circuit. The oscillator 16, as shown in FIG.
Output to. The counter 15 integrates the number of oscillation pulses from the oscillator 16 and sends the integration result to the comparator 1.
7. The comparator 17 compares the set value No. of the number of energizing pulses to be applied to the heating element 1 inputted from the table selection circuit 12 and the integrated value of the counter 15, and when the two match, a match signal is sent to the gate circuit. Output to 18. The gate circuit 18 passes the pulse signal from the oscillator 16 and outputs it as a strobe signal SB, but when a match signal from the comparator 17 is input, the gate is closed and the output of the strobe signal SB is stopped.

When recording halftones using the apparatus of the present invention configured as described above, the temperature sensor 14 measures the environmental temperature of the thermal head, and the measurement result is sent to the table selection circuit 12 via the A/D converter 13.
You will be prompted to enter The table selection circuit 12 selects a ROM table corresponding to the environmental temperature level, and performs a switching operation to input the output of the selected ROM table to the comparator 17. Thus, the gray level of the intermediate tone is specified, and the gray level signal is sent to ROM table group 1.
When input to the ROM table 111 of No. 1, etc., the selected ROM table as described above outputs the number of energizing pulses required to obtain the print density of the gradation level. Therefore, the table selection circuit outputs to the comparator 17 the number of pulses No required to obtain the printing density of the gradation level designated by the gradation level signal at the ambient temperature. In other words, in order to obtain the desired print density corresponding to the specified gradation level, correction is made based on the relationship between the print density and the number of pulses N (see Figure 4), and also based on the relationship between the print density and the environmental temperature. The set value No. of the number of energizing pulses that has been temperature-compensated based on the relationship (see FIG. 5) is input to the comparator 17. Then, the pulse signal from the oscillator 16 is sent to the counter 15.
The gate circuit 18 is closed when the number of pulses matches the set value No.
The gate circuit 18 outputs No unit pulses of the pulse number setting value, and this pulse signal is inputted to the AND gate 3 (see FIG. 1) as the strobe signal SB. Then, as described above, the AND gate 3 selected based on the pattern data DI outputs a pulse signal with the number of pulses No to the base of the corresponding transistor switch 2. Then, the transistor switch 2 is turned on only while this pulse signal is on,
The heating element 1 has a voltage V _HD having the same frequency and pulse width as the pulses oscillated from the oscillator 16.
It is applied as pulses, generating resistance heat and recording halftones on thermal paper.

FIG. 7 shows print density characteristics when the number N of energizing pulses to the heating element 1 is corrected based on the gradation level and temperature compensated based on the environmental temperature. This graph shows the environmental temperature at 7 levels (n=
7), and at each level, the print density increases slightly as the environmental temperature rises, but since temperature compensation is performed according to each environmental temperature level, the overall The print density remains constant despite changes in temperature. Further, when the gradation level changes at equal intervals, the print density also changes at equal intervals. In addition, in order to further improve the temperature compensation accuracy, it is necessary to increase the number of ROM tables and A/
Although the number of quantization steps of the D converter 13 increases,
The number n of environmental temperature levels to be classified into groups may be increased. In addition, the relationship between the number of pulses N to be corrected depending on the gradation level and the print density (γ correction characteristics) can be determined by changing the stored contents of the ROM table according to the thermal characteristics of the thermal paper used, and It is best to use the appropriate one.

By the way, when comparing both the signals in Figures 3a and 3b, if the duty ratio of the pulse signal in Figure 3b that controls the number of pulses is 50%, the energization time required to obtain the same concentration is 30%. In the case of FIG. 3B, the pulse width is twice as long as in the case of FIG. As shown in Figure 3a, even when recording halftones by controlling the pulse width, each heating element is usually arranged in multiple blocks because the power capacity of the thermal head is limited. Divide and drive in chronological order. Therefore, as shown in FIG. 8a, strobe signals SB1, SB2, . Therefore, even when recording halftones by controlling the number of pulses N as in this embodiment, the method of outputting the strobe signal is changed to the strobe signal SB1 for the first block as shown in FIG. 8b. By setting the strobe signal SB2 for the second block so that the pulses are turned on and off alternately, restrictions from the power supply capacity are satisfied and the recording time is the same as in the case of pulse width control. I can do it.

As explained above in detail, according to the present invention, both γ correction, which is based on the nonlinear relationship between pulse information (pulse number or pulse width) and print density, and temperature compensation, which is based on changes in environmental temperature, can be easily performed. Since the temperature compensation is performed based on a relationship appropriately set for each step of the gradation level, the printing density can be maintained constant with high precision. Note that the present invention should not be limited to the above-described specific embodiments, and various modifications can be made within the technical scope of the present invention. for example,
The correction of print density may be performed by controlling the pulse width instead of depending on the number of pulses as in the above embodiment.

[Brief explanation of drawings]

Figure 1 is a circuit diagram showing an example of a direct drive type thermal head, Figure 2 is a timing diagram of each signal, and Figures 3a and b are strobe signals SB.
The schematic diagram in Figure 4 shows the pulse width t _pw or the number of pulses N
Figure 5 is a graph showing the relationship between environmental temperature and print density; Figure 6 is a graph showing the relationship between environmental temperature and print density;
This figure is a schematic block diagram of the correction circuit, FIG. 7 is a graph diagram showing the effects of the present invention, and FIGS. 8a and 8b are schematic diagrams of strobe signals SB1 and SB2. Explanation of symbols, 1... Heat generating element, 2... Transistor switch, 3... AND gate, 11... ROM table group, 12... Table selection circuit, 14... Temperature detector, 15... Counter, 16... Oscillator, 17... Comparator, 18 ...Gate circuit, 111, 112
...11n...ROM table.

Claims (1)

[Scope of Claims] 1. In a halftone recording device for a thermal head that adjusts print density by adjusting a current pulse applied to a heating element, a relationship between a grayscale level and a current pulse is set in advance, and a grayscale level signal is generated. A ROM table group in which a plurality of ROM tables are provided according to the environmental temperature for inputting pulse information and outputting pulse information corresponding thereto; a temperature detecting means for detecting the environmental temperature and converting the detection signal from analog to digital;
A table selection circuit that selects and outputs a ROM table corresponding to the temperature detected by the temperature detection means from among the ROM tables, and a pulse output means that outputs a pulse signal corresponding to the output of the table selection circuit to the heating element of the thermal head. 1. A halftone recording device for a thermal head, comprising: 2. In the above item 1, the pulse information is the number of pulses, and the pulse output means includes a pulse oscillator that oscillates a pulse of a predetermined width at a predetermined frequency, and a counter that counts the number of pulses of the pulse oscillator.
A thermal head comprising: a gate circuit into which the output of the pulse oscillator is input; and a comparator that opens and closes the gate circuit based on a comparison result between the number of pulses from the table selection circuit and the count value of the counter. halftone recording device.