JPH0584960A - Video printer - Google Patents

Abstract

PURPOSE:To improve the density irregularity due to the of nonuiformity the resistance values of the heating elements of the thermal head of a video printer printing the stationary image due to the thermal head having heating elements arranged thereto in a row. CONSTITUTION:The resistance values of the heating elements of a thermal head 5 are measured by a resistance measuring circuit 9 under the control of a system controller 2 and the current energy to the thermal head 5 is controlled by the system controller 2 so as to compensate the irregularity of the resistance values. Therefore, the irregularity of printing density caused by the irregularity of the resistance values of the heating elements of a video printer performing multigradation contrast medium control is improved and effect for enhancing printing image quality is obtained and, further, the thermal head at an abnormal time can be protected from damage.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a video printer device for printing a still image, and more particularly to a video printer device suitable for improving density unevenness due to variations in resistance values of thermal head heating elements.

[0002]

2. Description of the Related Art Conventionally, various systems have been proposed as a system for obtaining a hard copy from a still image of a television or movie. Among them, a thermal transfer type video printer apparatus has been put into practical use, which uses an ink paper coated with a heat-melting or heat-subliming ink on its surface, selectively heats the ink with a thermal head, and transfers the ink onto a recording paper.

In this system, a thermal head in which heating elements are linearly arranged is used to perform multi-gradation printing line by line.

Recently, the number of gradations has been increased, but with this, it is difficult to obtain the same color density at the same gradation. Most of them are caused by non-uniformity and variation in resistance value of the thermal head heating element.

Conventionally, when print data has the same density,
To obtain the same color density, for example, Japanese Patent Application No. 63-504.
As described in Japanese Patent No. 931, a technique is known in which a heating element is divided and heat is uniformly distributed during the time when image pixels are printed.

[0006]

In the above-mentioned prior art, when performing multi-tone halftone control in which the coloring density is more meticulous, the variation in the resistance value of the heating element is not taken into consideration. There has been a problem that uneven color density is detected and print quality is impaired.

An object of the present invention is to compensate for the density unevenness caused by the variation in the head resistance value and improve the print image quality.

Further, it is also possible to prevent the thermal head from being damaged by being over-energized due to a malfunction of the printer.

[0009]

The above-mentioned object is achieved by automatically measuring the resistance value of the heating element of the thermal head and compensating the dispersion of the resistance value so that the color density is uniform.

Further, the protection of the thermal head can be achieved by prohibiting the energizing energy to the heating element to be a certain value or less.

[0011]

The circuit for measuring the resistance value of the thermal head is controlled by the system controller, and a signal necessary for measuring the resistance value is input to the thermal head. Then, the resistance value is detected as a voltage at least one dot at a time and transmitted as resistance information to the system controller. The system controller controls the energizing energy applied to the thermal head during printing by increasing or decreasing energizing time information such as data so as to correct variations in resistance. As a result, it is possible to improve density unevenness due to variations in resistance value.

Further, the protection of the thermal head due to a malfunction of the system controller or the like is achieved by providing a circuit for generating a fixed time and providing a gate circuit so as not to energize the thermal head for more than this time.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to the drawings.

FIG. 1 shows an embodiment of the present invention. The image source 1 stores digital data for one screen,
Under the control of the system controller 2 which controls the entire system of the printer operation, the data for one line and the control signal described later are sequentially transmitted to the energization time control circuit 3 which controls the energization time to the thermal head 5.

The energization time control circuit 3 includes a thermal head 5
Based on the head temperature information detected by the temperature sensor 6 and the temperature detection circuit 8 attached to the head, and the head resistance information obtained by measuring the resistance value of the head heating element by the resistance measurement circuit 9, the head is read from the gradation ROM 7 corresponding to the gradation number. Read the energization time information,
Data for energizing the head is calculated. Then, the thermal head 5 is energized via the halftone control circuit 4 which controls the color development of multi-tone density.

Next, the main part of FIG. 1 will be described.

FIG. 2 shows a configuration diagram of the thermal head 5, and FIG. 3 shows a waveform diagram of signals. 2 and 3, serial data (b) is transferred to the shift register 51 by the clock (c) based on the one-line start pulse (a). The transferred data for one gradation of one line is input to the latch circuit 52 in parallel at the timing of the latch (d). Then, the heating element 54 is energized by the driver 53 only during the strobe (e). While energizing a certain gradation, that is, during printing, the data of the next gradation (b)
Are transferred into the shift register 51 by the clock (c). Data transfer and head energization are continuously performed by these series of operations.

As for the structure of the thermal head 5, the shift register 51, the latch circuit 52, and the driver 53 shown in FIG. 2 are mounted on a printed circuit board by IC, and are mounted on a heat radiating plate (not shown) together with the heating element 54. ing.

Although not shown, the recording paper and the ink paper are superposed on the rotating drum, which is the main mechanism, and pressed by the summer head 5. Then, the amount of heat generated by the heating element 54 is controlled in accordance with the energization time of the strobe, and the ink is transferred to the recording paper to perform halftone recording.

Ink paper is used for printing in a frame-sequential system in three colors of yellow, cyan and magenta having density characteristics as shown in FIG.

As shown in FIG. 1, the temperature detection circuit 8 outputs the temperature of the thermal head 5 by the temperature sensor 6, converts it into a digital signal, and transmits it to the system controller 2. The operation of the printer device according to the detected temperature is shown in FIG. 5. The system controller 2 energizes the head to preheat when the head temperature is lower than T1, and performs the printing operation when the head temperature is between T1 and T2. If the head temperature rises to reach temperature 2 due to excessive continuous printing or runaway or damage of the system controller 2 or the like, the printing operation is stopped to protect the head.

Next, the resistance value measuring circuit 9 will be described. Figure 6
The signal waveform of the energization timing to the thermal head 5 for measuring the resistance value is shown in FIG. In the figure, the heating element is sequentially energized dot by dot, and the resistance value of the heating element is measured dot by dot. This operation is performed by the system controller 2. That is, in the waveform diagram shown in FIG. 3, data is set to 1 and a clock is input, then data is set to 0 and a clock is input one clock at a time. Immediately after the one clock, a latch and a strobe are generated one pulse each. By doing so, the heating element can be energized dot by dot as shown in FIG.

A concrete circuit of the resistance value measuring circuit 9 is shown in FIG. The common terminal of the heating element 54 is connected with a resistor 91 having a resistance value that is sufficiently small compared to the resistance value of the heating element, and the other end is connected to a power source. Since the heating element is energized dot by dot, a current during energization flows to one dot of the heating element selected as the resistor 91, causing a voltage drop in the resistor 91. This is amplified and detected by the voltage detection circuit 92, converted into a digital signal of several bits by the A / D conversion circuit 93, and transmitted to the system controller 2 for correcting the resistance value. For example, before correcting the resistance value, as described above, the resistance value is measured dot by dot, and those values and the average value are calculated by the system controller 2
Calculated and stored in the built-in RAM. When the resistance value is smaller than the resistance value of a certain heating element, the heat generation energy depends on the electric power value obtained by dividing the square of the voltage applied to the heating element by the resistance value. On the contrary, when the resistance value of the heating element is larger than the average value, light printing is performed. Since the resistance value of the heating element is usually several hundred to several thousand ohms, the resistance 9
A value of 1 is suitably about 1 ohm.

Another embodiment of the resistance measuring circuit 9 is shown in FIG. A constant current is supplied from the constant current circuit 94 to the heating element 54. As described above, since the heating element 54 is energized dot by dot, a voltage proportional to the resistance value is generated in each dot at the common terminal of the heating element 54, and the voltage detection circuit 92, A / It is input to the system controller 2 via the D conversion circuit 93.

When the measurement of the resistance measuring circuit 9 described with reference to FIGS. 7 and 8 is completed, although not shown, the common terminal of the heating element is automatically disconnected from the resistor 91 and the constant current circuit 94 by the system controller. , Connected to the thermal head power supply.

The resistance value of the heating element is automatically measured and corrected, for example, by operating a dedicated switch provided on the panel surface of the video printer before printing a print image.

When a command for automatic resistance value measurement is input to the system controller 2, the system controller 2 automatically measures each thermal head heating element and its average resistance value as described above. This resistance information is input to the RAM in the system controller 2.

At the time of printing, the system controller 2 reads the image data from the image source 1 line by line,
The halftone control circuit 4 converts the image data of one line into serial data of the number of dots of the thermal head 5 for each gradation and transfers the data via the energization time control circuit 3. At this time, the lower data of the image data is increased or decreased according to the resistance value of the thermal head heating element.

For example, when the image data has 256 gradations of 8 bits, 16 gradations of the lower 3 bits are controlled to compensate for the print density due to variations in resistance value. That is, after adding / subtracting the function to the system controller 2 and detecting the resistance value and the average value of the thermal head heating element for each dot by the resistance measuring circuit 9, the resistance value of the image data output by the system controller 2 is When the average value is larger than the average value, the data is controlled so as to be increased by the lower 3 bits, when it is equal to the average value, it is not increased or decreased, and when it is smaller than the average value, the image data is controlled by the system controller 2. To do.

The compensation of the print density due to the variation in the resistance value of the thermal head heating element has been described above. However, when printing is performed with compensation, the schematic diagram of density compensation is shown in FIG. 9, but the density at both ends of the thermal head becomes slightly thin. In the figure, (a) shows the case where the preceding dots are printed at the same time, and (b) shows the case where the thermal head is divided and printed with a time difference.

The reason why both ends of the print line are thinned is that one side of both ends is not in contact with the heating element and is lower than the temperature of the other heating element even during preheating or printing. Therefore, in addition to the compensation of the resistance value described above, it is necessary to set the print data at both ends of these print lines by the system controller 2 in many gradations.

First, in FIG. 5, the thermal head has a constant temperature T.
When the number is 2 or more, the printing operation is performed by the system controller 2
It was explained that it would be stopped under the control of

However, in this operation, the detection time of the temperature sensor 6 which is mainly used in the thermistor in FIG. 1 is slow, and the thermal head 5 is over-energized for a short time due to runaway or damage of the system controller 2 and the circuit in the subsequent stage. Before the temperature sensor 6 detects the temperature T2 shown in FIG. 5, the performance of the thermal head heating element may deteriorate or be damaged. An embodiment in which this is prevented is shown in FIG. 11, and a waveform diagram of main signals is shown in FIG.

In FIG. 11, 10 is a head protection circuit, which is composed of a pulse generation circuit 11 and a gate circuit 12. The pulse generation circuit 11 is, for example, a monostable multivibrator, and when a conventional strobe is branched and input from the halftone control circuit 4 as shown in the figure, a pulse for a fixed time is generated in synchronization with the start of the input. That is, FIG.
A timing waveform diagram is shown in FIG. 4, where (a) shows a conventional strobe and (b) shows a pulse for a certain time. The fixed time pulse is set to a time slightly longer than the strobe.

When the strobe becomes longer than the pulse time of (b) due to a malfunction of the system controller 2 as shown by the broken line of FIG. 10 (a), the thermal head 5
The heating element may be damaged. Therefore, 12 in FIG.
The gate circuit of FIG. 10 causes the gate circuit to output the pulse when the strobe is longer than the pulse and the strobe when the strobe is shorter than the pulse as shown in FIG. Input to the thermal head as shown in.

By the above operation, the strobe for energizing the heating element of the thermal head does not exceed the constant time generated by the pulse generating circuit 11 and over-energization does not occur.

In FIG. 7, which shows an embodiment of the resistance measuring circuit 9 for the thermal head heating element described above, the driver 53 is usually composed of a transistor.

When the transistor is turned on, there is a saturation voltage between the collector and the emitter. That is, when the heating element generates heat, strictly speaking, a voltage obtained by subtracting the saturation voltage from the head voltage is applied to the heating element. Further, since the resistance 91 is inserted in the resistance measuring circuit of FIG. 7, it is necessary to subtract the voltage drop due to this. When measuring the resistance, the thermal head is not pressed against the ink paper or recording paper, so it is not printed on the recording paper.

The head voltage for measuring the resistance may be different from the voltage for printing.

The collector-emitter saturation voltage of the transistor depends on the collector current (almost equal to the emitter current), and is expressed by the sum of the collector current value multiplied by a constant and a constant voltage. Since the resistor 91 in FIG. 7 is known,
The voltage detection circuit 92 can obtain collector current information for each dot of the heating element. After that, the system controller 2 performs a digital operation via the A / D conversion circuit 93 to calculate the resistance value of the heating element for each dot.

FIG. 8 shows another embodiment of the resistance measuring circuit 9. In the figure, a constant current for each dot of the heating element, that is, the collector current of the driver transistor is passed from the constant current circuit 94, and the sum of the saturation voltage of the transistor and the voltage drop of the heating element is detected by the voltage detection circuit 92. The subsequent operation of measuring the resistance value of the heating element is as described above.

When there are multiple gradations such as 256 gradations and 512 gradations, it is necessary to control the energizing energy of the heating element for that number of gradations. Therefore, depending on the order of the resistance value of the heating element and the head voltage, the driver transistor The saturation voltage cannot be ignored.

It is difficult to control the head voltage as a means for controlling the energizing energy.
That is, it controls strobes and data. However, when the head voltage as one parameter of energizing energy is examined, for example, the number of dots is 480 and the head voltage is 20.
25 with a thermal head of V and head resistance of 1150 ohms
When the halftone control of 6 gradations is performed, the head voltage per gradation corresponds to 78 mV if the saturation voltage of the driver transistor is ignored. This value is larger than the actual saturation voltage of the driver transistor. Therefore, it is necessary to take into account the saturation voltage of the driver transistor described above for the head voltage.

As another parameter of other head energizing energy, focusing on the head resistance described above, that is, the resistance of the heating element, one gradation of 256,512 gradations is a ratio to the total number of gradations. 0.39 for each
The values are 1% and 0.195%, which are extremely smaller than dozens to several percent of the actual resistance value variation of the heating element.

Therefore, in multi-tone halftone control such as 256 and 512, variations in the resistance value of the heating element cause unevenness in the print density, and therefore it is necessary to compensate the resistance value according to the present invention.

[0046]

The present invention has the following effects.

In a video printer apparatus in which a still image output from a video device is hard-copied in color, the gradation of color density is increased and the thermal head density is increased.
At this time, a problem is color density unevenness due to variation in resistance value of the thermal head heating element. In the present invention, the energization energy to the heating element is controlled so that the resistance value of the heating element is automatically compensated, so that there is an effect that density unevenness is improved and print image quality performance is improved.

Further, there is an effect that the phenomenon that both ends of a line printed at one time becomes thin is improved.

Further, there is an effect of protecting the thermal head from damage due to over-energization of the thermal head.

[Brief description of drawings]

FIG. 1 is a block diagram of a video printer device according to an embodiment of the present invention,

FIG. 2 is an explanatory view of a thermal head in the video printer device shown in FIG.

FIG. 3 is a timing chart of the signals in FIG.

FIG. 4 is a density characteristic diagram for each color of the video printer device,

FIG. 5 is a temperature-printing operation diagram of the video printer device;

FIG. 6 is a timing chart of signals when measuring the resistance of a thermal head heating element,

7 is a block diagram of a resistance measuring circuit for a thermal head heating element in FIG.

FIG. 8 is another block diagram of the resistance measuring circuit of the thermal head heating element in FIG.

FIG. 9 is a print density characteristic diagram after resistance compensation of a thermal head heating element,

FIG. 10 is a timing chart of signals of a thermal head protection circuit,

FIG. 11 is a block diagram of a thermal head protection circuit.

[Explanation of symbols]

2 ... System controller, 5 ... Thermal head, 9 ...
Resistance measurement circuit.

Claims (3)

[Claims] 1. A video printer apparatus for controlling a printing operation by a system controller and printing by a thermal head in which heating elements are arranged in a line, wherein a resistance value of the heating element is measured at least dot by dot and measured. A function to control the energizing energy to each heating element is added to the system controller according to the resistance value,
A video printer device characterized by being configured to compensate for density unevenness due to variations in resistance value. 2. The video printer device according to claim 1, further comprising a function of increasing the energization energy compensation amount for the heating elements at both ends of the portion of the thermal head to be energized more than other heating elements. 3. The method according to claim 1, wherein in synchronization with the timing of energizing the thermal head,
A video printer device having a pulse generation circuit for generating a pulse of a predetermined fixed time and a gate circuit for inhibiting the head energization time from exceeding a fixed pulse time.