JPS6228264A - Multi-value driving system for thermal head - Google Patents

Abstract

PURPOSE:To perform multi-value recording with high accuracy, by a method wherein heating elements of a thermal head are sequentially selected in a cooling period after impressing a recording signal, an electric current is supplied to the selected heating element for a short period of time, the temperature of the heating element is calculated from the current value, and the recording signal is corrected based on the temperature. CONSTITUTION:In the cooling period after impressing the recording signal on the heating elements, a driving part 2 supplies an electric current to the heating element 1 selected according to a controlling signal MC for supplying electric currents for temperature measurement to sequentially selected heating elements 1, and the current value is measured by a current-measuring part 4. A temperature-calculating part 5 obtains the temperature of the heating element 1 from the measured current value by a ROM or the like. With the temperature information on the heating element 1 applied to a controlling part 3, the part 3 corrects a recording-controlling signal PC corresponding to gradation data Dn according to the temperature information, whereby multi-value recording can be performed with high accuracy.

Description

[Detailed Description of the Invention] [Summary] In a thermal printer that performs multi-value recording, the heating elements of the thermal head are sequentially selected and supplied with short-term current during the cooling period after the application of a recording signal. The current is measured to calculate the temperature of the heating element, and based on the calculated temperature, the recording signal is corrected to perform highly accurate multi-value recording.

[Industrial application field]

The present invention relates to a multi-value drive method for a thermal head in a multi-value thermal printer, which calculates the temperature of a heating element of the thermal head and performs highly accurate multi-value recording.

There are two types of thermal printers: a thermal recording method and a thermal transfer recording method.The former uses a thermal head that comes into direct contact with recording paper that changes color due to heat, and heats the heating element of the thermal head to change the color of the recording paper. The latter is a method in which an ink sheet is heated by a thermal head to melt or sublimate the ink sheet, and then the ink sheet is transferred to recording paper for recording. The heating element scanning method is roughly divided into a line recording method and a serial recording method. A line recording method is used for high-speed recording, and a serial recording method is used for low-speed recording. Furthermore, multilevel recording is possible by controlling the peak value or application time of the recording signal applied to the thermal head.

[Prior Art] In a line recording type thermal transfer thermal printer, as shown in FIG.
and are arranged opposite to the platen 44 via the recording paper 43,
By heating the ink sheet 42 with the thermal head 41, the ink on the ink sheet 42 is melted and transferred onto the recording paper 43, thereby performing recording. The thermal head 41 has heating elements for one line arranged along the direction perpendicular to the paper surface, and records for one line are performed almost simultaneously, and when the recording for this one line is completed, the recording paper 43 and The ink sheet 42 is simultaneously transported in the direction of the arrow.

As shown in FIG. 7, the thermal head 41 has a multilayer structure, in which a glaze layer 54, a heating element 53, and an electrode 52 are formed on a substrate 55, and a protective layer 5 is formed on the surface facing the recording paper 43.
1 is formed. Also, there are heat dissipation fins 56 on the back side of the board.
is provided.

In the line recording method, the number of heating elements is large, for example, 1728 in the case of 8 dots/mm and A4 size, so a method using a diode matrix or 32 to 32 A method using a dedicated drive integrated circuit in which launch circuits and drive transistors for 64 heating elements are integrated has been put into practical use. The former is called a diode matrix (DM) type, and the latter is called a direct drive (DD) type.Recently, the DD type, which can simplify the peripheral circuitry, has become mainstream.

Due to its structure, this D-type thermal head has
Since it is difficult to vary the voltage applied to the heating elements for each heating element, in the case of multi-value driving, the pulse width of the recording signal is controlled as shown in FIG. 8.

However, with such a multi-value driving method, it is difficult to obtain sufficient gray level accuracy. This is because heat accumulates within the thermal head, so even if the same recording signal is applied, the heat generation temperature of the thermal head may differ. That is, when the recording signal Pa with the pulse width Wa shown in (al) in FIG. 8 is applied, the temperature of the heating element at time t1 becomes Ta, and the pulse width Wb (<
The temperature at time t1 when the recording signal pb of Wa) is applied is Tb. In addition, (pulse width WC shown in C1 (<W
The temperature at time t1 when the recording signal Pc of b) is applied is Tc. In this way, the temperature T at time M1
a, Tb, and Tc vary depending on the pulse width of the recording signal, that is, the heating amount of the heating element. This is due to heat storage fluctuations in the heating element due to changes in the amount of heating.

Also, when the recording signal Pc of pulse width Wc is applied, time 1
The temperature at is Tc, and further time has elapsed at time t
2, Tel (<Tc). In other words, the temperature is different from the temperature at the start of recording. This is a change in heat storage in the heat generating element due to a change in the recording cycle. In this way, since the initial temperatures of the heating elements are different, when we consider the time when a recording signal with a predetermined pulse width is applied in the next recording cycle, the temperature characteristics of the heating elements are different depending on the past recording signals. , it becomes difficult to control the temperature of the heating element to a desired temperature.

To deal with this problem, a method can be considered in which the temperature of the thermal head is detected, the detected temperature is feedhacked, and conditions such as the peak value and pulse width of the recording signal applied to the heating element are changed. . Hereinafter, this method will be referred to as the feed pack method.

The feed hack method is effective against slow heat accumulation in the substrate of the thermal head. For example, the temperature of the entire thermal head may rise due to heat accumulation in the substrate of the thermal head, or the temperature may locally rise in areas where high-density recording is concentrated, so when recording for a long time, , the recording density increases, causing fine patterns to collapse, blank areas to darken, or density differences to occur in the longitudinal direction of the thermal head. Since this heat accumulation phenomenon is slow, it can generally be corrected by providing a substrate temperature detection means. That is, as the substrate temperature rises, the driving power for the thermal head can be reduced. In addition, in order to compensate for the temperature distribution along the longitudinal direction of the thermal head, temperature detection is performed at multiple locations.The thermal head drive circuit is divided into multiple blocks in response to this, and the drive conditions are controlled for each block. I will do it.

FIG. 9 is a block diagram of the main part of the conventional example, showing the main part of the control section for the DD type thermal head.

In the figure, 11 is a counter, 12 is a random access memory RAM, 13 is a read-only memory ROM, 1
4 is a comparison circuit, 15 is an AND circuit, 16 is a blocking circuit, SBL is a supply line value, CL is a scan clock signal, Dn is gradation data, RW is a continuous write signal, PE
is an enable signal, DVI to DV54 are drive signals for the drive integrated circuit, and the number of heating elements is 1728 (A4 size,
8 mm/dot) and 3 per drive integrated circuit.
This is for the case of driving two heating elements, and is 54
drive integrated circuits will be provided.

The recording cycle of one line is usually divided into 100 to 1000 parts, and one divided period is called a supply line, and the supply line value SEL at the start of the line recording cycle is set to 0, and thereafter, the supply line value SEL is set to 1,
2, 3. ...and... In addition, in order to scan all the heating elements within one line, this is divided equally by the number of heating elements. Each of these equally divided periods is called a scan, and the scan value at the start of the supply line is set to 0, and the following 1.2, 3. ...
shall be.

The counter 11 counts the scan clock signal CL and adds the scan value as an address signal of the RAM 12.

One line before the start of the line recording cycle, the gradation data Dn for all the heat generating elements is prepared, and when the successive write signal RW indicating writing is applied, the gradation data Dn for all the heat generating elements is written to the RAM 12. It will be done. After writing this,
The successive write signal RW becomes "1" to indicate reading, and the scan value becomes 0 to 1727 in one supply line, so the scan value from the counter 11 is repeatedly sent to the RAM 12 as an address signal. Reading of gradation data Dn is performed.

The gradation data Dn read from the RAM12 is stored in the ROM1.
3, the drive signal is converted into a supply line value that should change from "1" to "0". That is, the gradation data Dn is converted into a supply line number corresponding to the pulse width. This ROM
The supply line value outputted from 13 and the input supply line value SBL are compared in the comparator circuit 14, and this supply line value SBI.

is smaller than the output subline value of the ROM 13, the output of the comparison circuit 14 becomes "1", and the output of the comparison circuit 14 becomes "1" until the supply line value SBL equal to the supply line value of the output of the ROM 13 is reached. Therefore, the gradation data D
n will be converted to pulse width.

Also, the enable signal P is supplied for a predetermined time from the start of the line recording cycle.
B becomes 1", thereby opening the AND circuit 15 and adding the output of the comparison circuit 14 to the blocking circuit 16. The blocking circuit 16 receives the output signal of the AND circuit 15 in synchronization with the scan clock signal CL. and outputs drive signals DVI to DV54 to the 54 drive integrated circuits mounted on the thermal head.Therefore, the supply signal DVI to DV54 is outputted to the heating element connected to each drive integrated circuit in accordance with the gradation data Dn. A recording signal with a pulse width of which unit length is applied is applied, and multilevel recording is performed.

The aforementioned blocking circuit 16 will be briefly explained.

This blocking circuit 16 sequentially inputs binary drive data BDi (i=1°2...1728) for the 1st to 1728th heating elements in each supply period. On the other hand, the 54 drive integrated circuits mounted on the thermal head are 1 to 32.
33rd to 64th...1697th to 1728th, total 172
Each receiver has a drive, and the drive signal DVI of the drive integrated circuit is binary drive data BDI to BD32, D
V2 is BD33-BD64. ...DV54 is BD
1697 to BD1728 in order, and the possible transfer speed is determined by the scan clock signal C.
It is usually one order of magnitude slower than the frequency of L.

Therefore, the blocking circuit 16 has 54 32×1 bit FIFOs (first in, first out).
The binary drive data BDi is serially written at high speed, and the drive signals DVI to DV54 are read out in parallel at low speed. When writing, 54 FIFs
By connecting all O in series, 54X32X1
It is equivalent to a 1728x1 bit FIFO, and binary drive data BDI to BD1728 are written.

After that, the FIFO enters the read mode, and each FIFO is separated to enable parallel reading.

Therefore, the first continuous output is BDI, BD33°...
BD1697, and the next successive output is BD2. BD3
4. ...BD1698. The final sequential output, ie, the 32nd output, is BD32, BD64.・・・
- BD1728, and the output of each FIFO matches the required data format of the drive signals DVI to DV54.

FIG. 10 is an explanatory diagram of the operation, in which (a) shows a low gradation level recording signal, (bl shows a high gradation level recording signal, fcl shows a continuous write signal RW, and "0" indicates writing. ,“
1” is a read instruction, and the successive write signal RW
is “0”, the gradation data Dn for all heating elements
will be written to the RAM 12. Furthermore, by applying a recording signal of a low gradation level shown in fal to the heating element, the temperature of the heating element changes as shown in curve a, and (b
By applying a recording signal of a high gradation level shown at 1 to the heating element, the temperature of the heating element changes as shown by the curve 1. Therefore, since the applied energy is different, the recording density is correspondingly different, and multilevel recording is performed.

[Problem that the invention seeks to solve]

When the feedback method described above is applied, it is not possible to completely correct changes in recording density due to heat accumulation. That is, heat storage can be roughly divided into heat storage due to the thermal head substrate, etc., and heat storage due to the glaze layer of the heating element.
Heat accumulation due to the substrate etc. changes slowly with respect to the recording cycle, so it can be corrected using the feedback method described above, but heat accumulation due to the glaze layer changes quickly with respect to the recording cycle, so it can be corrected by the feedback method. Correction by method is virtually impossible.

Therefore, conventionally, it has been proposed to control the power applied to the next recording signal based on the temperature history of the heating element at which recording operation was performed in the past. This is called the temperature history method. To explain this method using binary recording as an example, if the previous recording is black, the pulse width of the recording signal applied to the heating element is made shorter or the peak value is made smaller than when it is white. This method reduces applied power.

However, by adopting the temperature history method, it is possible to correct the change of 4 degrees in recording due to heat accumulation in the thermal head, but it has the following drawbacks.

In other words, in the temperature history method, the temperature of the heating element at the start of the next recording is estimated by applying past driving conditions to a theoretical formula for heating and cooling characteristics, and from this estimated temperature, the recording corresponding to the amount of heating is Signal conditions are determined, and this calculation involves complex calculations such as exponential functions. Then, all the heating elements (172 in the above example) installed in the thermal head are
Since it is necessary to perform the above-mentioned calculation for 8 pieces of data, a high-speed arithmetic circuit is required, resulting in a complicated and expensive configuration. Furthermore, as the number of histories going back to the past increases, errors in recording density due to heat accumulation can be reduced, but the amount of calculation increases rapidly, and the time required for calculation becomes longer. Furthermore,
In order to improve recording speed and limit instantaneous power consumption, the recording cycle of the heating element is made variable, and changes in this recording cycle are also taken into consideration when calculating the recording signal conditions from past drive conditions. Therefore, the number of combinations of driving conditions becomes enormous, making calculations even more complicated.

In order to improve the drawbacks of the temperature history method described above, a method has been proposed in which the resistance value of the heating element is detected and the temperature is calculated from this resistance value. However, as mentioned above, in a thermal printer that records A4 size, 8 mm/dot, the thermal head has as many as 1728 heating elements, and it is not easy to detect the resistance value of the heating elements in practice. Even if implemented, the disadvantage is that the circuit scale will be large and the cost will be very high.

An object of the present invention is to calculate temperature from the resistance value of a heating element with a simple circuit configuration, and to enable highly accurate multi-value recording.

[Means for solving problems]

The multi-value driving method of the thermal head of the present invention will be explained with reference to FIG. In addition to applying the recording control signal pc corresponding to the data Dn, during the cooling period after applying the recording signal to the heating element 1,
A control unit 3 that sequentially selects the heating elements 1 and applies a control signal MC to supply a current for temperature measurement for a short time; and a current measurement unit 4 that measures the temperature measurement current flowing through the selected heating elements 1. and a temperature calculation unit 5 that calculates the temperature of the heating element 1 from the current value measured by the current measurement unit 4, and the temperature information of the heating element 1 calculated by the temperature calculation unit 5 is transmitted to the control unit 3. In addition, the recording control signal PC applied from the control section 3 to the driving section 2 is corrected, and the recording signal applied from the driving section 2 to the heating element 1 is corrected in accordance with the calculated temperature of the heating element 1.

[Effect]

The control unit 3 sends a recording control signal P to the drive unit 2 in accordance with the gradation data Dn and the temperature information obtained by the temperature calculation unit 5.
The drive unit 2 outputs a control signal Me for sequentially selecting the heating elements 1 and supplying current for temperature measurement during the cooling period after applying the recording signal to the heating elements 1. , a current for temperature measurement is supplied to the selected heating element 1 according to the control signal MC, and the current at that timing is measured by the current measuring section 4. Therefore, the configuration is simple because it is sufficient to measure the current for temperature measurement flowing through the heat generating element selected during the cooling period.

Furthermore, since the relationship between the temperature and resistance value of the heating element is determined in advance, and the measured current value and resistance value are uniquely determined, the temperature calculation section 5 is configured to calculate the temperature from the measured current value using a read-only memory or the like. Therefore, the temperature of the heating element 1 can be easily determined. Since this temperature information of the heating element 1 is added to the control section 3, the control section 3 receives the gradation data D.
By correcting the recording control signal pc corresponding to n in accordance with the temperature information, it is possible to perform highly accurate multi-value recording.

〔Example〕

Embodiments of the present invention will be described in detail below with reference to the drawings.

FIG. 2 is a block diagram of an embodiment of the present invention, in which the same reference numerals as in FIG.
is an AND circuit, and 22 is an OR circuit. Further, DI and D2 are readout output signals of the ROM 18, SCL is a heating element count clock signal, and MC is a control signal for current measurement. FIG. 3 is an explanatory diagram of the operation, (al is a line start signal, (bl is an enable signal PE, (C1 is a heating element count clock signal SCR, (dl is a counter 1
7 count output, (el is the read output signal D I of the ROM 18, (fl is the successive output signal D2 of the ROM 18, (
g+ indicates the output signal of the AND circuit 21.

The scan clock signal CL is counted by a counter 11, and the count output is used as an address signal to write the gradation data Dn at the beginning of the line recording period according to the successive write signal RW, and then repeatedly read out the gradation data Dn. The length data of the supline value corresponding to the gradation data [)n is read from the ROM 13, and the supply line value S
BL is compared with the comparator circuit 14, and if the supply line value SBL is small, a signal of "1" is output to the AND circuit 15. OR circuit 2
2 to the blocking circuit 16, and the drive integrated circuits are controlled by drive signals DVI to DV54 in synchronization with the scan clock signal CL. Such operations are similar to those described with reference to FIG. 9, and redundant explanation will be omitted.

The supply line value is O at the time of the line start signal (al) in FIG. , the count output is added as an address signal to the ROM 18. Based on this address signal, the ROM 18 outputs a series of output signals D1 and (fl) of supply line values corresponding to the timing at which the resistance value of the heating element shown in (e) is to be detected. A successive output signal D2 of a heating element number indicating a heating element whose resistance value is to be measured.
are applied to comparator circuits 19 and 20, respectively.

For example, if the count output is O to 53, the ROM18
Readout output signal DI of supply line values from 500 to 553
were obtained, and 0, 32, 64. . . . A successive output signal D2 having a heating element number of 1696 is obtained. During the cooling period of the next line recording cycle, the count output is 54~
107, and as a result, from ROM18, 50
A continuous output signal DI with a supply line value of 0 to 553, and 1 more than in the case of the previous line recording cycle, 1,33°6
Successive output signal D2 of heating element numbers of 5, . . . 1697
is obtained.

The comparison circuit 19 receives a continuous output signal D1 of the supply line value, and
Compare the supply line value SBL and get “1” due to the comparison match.
The comparison circuit 20 compares the successive output signal D2 of the heating element number with the scan value of the count output of the counter 11, and outputs "1" if the comparison matches.

Since the output signals of the comparison circuits 19 and 20 are applied to the AND circuit 21, the output signal of the AND circuit 21 which becomes 1'' indicates that the heating element specified by the successive output signal D2 from the ROM 18 has a supply line value SBL. It is driven when the supply line value is indicated by the read output signal DI from the ROM 18.

That is, since the output signal of the AND circuit 21 and the output signal of the AND circuit 15 are applied to the blocking circuit 16 via the OR circuit 22, the drive to the selected drive integrated circuit is performed in the same way as a normal drive control signal. Signal DVI~D
V54 is output, and according to the output signal of the AND circuit 21, current is supplied for a short time. A signal at this timing becomes a control signal MC for current measurement.

FIG. 4 is a block diagram of main parts including a temperature calculation section, in which 1 is a heating element, 30 is a drive section, 31 is a current detection circuit, 32 is a hold circuit, 33 is a read-only memory ROM, 34
is the delay circuit, Q is the drive transistor, ■ is the power supply voltage, R
is a low resistance for current detection. Resistor R and current detection circuit 3
1 constitutes the current measuring section 4 in FIG. 1, and the ROM 33 includes:
This constitutes the temperature calculation section shown in FIG.

In the drive unit 30, the drive transistor Q is turned on in accordance with the drive control signal PC or the control signal MC for temperature measurement from the control unit, and a recording signal is applied to the heating element 1 with a constant power supply voltage V, and the current flowing therein is The voltage drop across the resistor R is detected by the current detection circuit 31. This resistance R
By connecting each heating element 1 to a power supply in common,
With one resistor R and current detection circuit 31, it is possible to detect the current flowing through successively selected heating elements 1.

During the cooling period of the heating element 1 after applying the recording signal to the heating element 1, a control signal MC for temperature measurement is applied from the control section.For example, as shown in FIG. After a time Td from the starting point, the control signal M
C is added. This control signal MC is corrected for the delay time in the drive section 30 etc. by the delay circuit 34 and is applied to the hold circuit 32, where the current value detected by the current detection circuit 31 is held.

The resistance value of the heating element 1 is either designed and manufactured in advance or measured after manufacturing, and the resistance temperature characteristics of the heating element 1 are also known in advance. The relationship with temperature is determined in advance, and this relationship is stored in the ROM 33. Temperature information is then read from the ROM 33 using the current value held in the hold circuit 32 as an address signal and applied to the control section. Therefore, the temperature calculation section 5 in FIG.
can be configured using a simple ROM 33.

In a DD type thermal head, if one power line is used for all heating elements, the current may become too large. In that case, divide it into 2 to 4 power lines and install a current detection circuit for each power line. 31 may also be provided. Further, the ROM 33 can be made common, and the timing of access from the hold circuit 32 can be made different depending on the current detection circuit. In this case, the time required to calculate the temperatures of all heating elements becomes shorter.

〔Effect of the invention〕

As explained above, in the present invention, during the cooling period after applying a recording signal to the heat generating elements 1, the heat generating elements 1 are sequentially selected.
The temperature of the heating element 1 can be calculated by passing a current for short-time temperature measurement through the heating element 1, and a recording signal corrected based on the calculated temperature information can be applied to the heating element 1, so high accuracy can be achieved. This has the advantage that multi-level recording is possible. Furthermore, since the configuration for this purpose is relatively simple, there is an advantage that the performance of the printer can be improved with an inexpensive configuration.

[Brief explanation of drawings]

Fig. 1 is a block diagram of the principle of the present invention, Fig. 2 is a block diagram of an embodiment of the invention, Fig. 3 is an explanatory diagram of operation, Fig. 4 is a block diagram of main parts including the temperature calculation section, and Fig. 5 is a block diagram of the main part including the temperature calculation section. An explanatory diagram of current measurement timing, Fig. 6 is an explanatory diagram of a thermal transfer thermal printer, Fig. 7 is a schematic cross-sectional view of a thermal head, Fig. 8 is an explanatory diagram of recording signal pulse width and temperature, and Fig. 9 is a conventional example. The main part block diagram and FIG. 1O are operation explanatory diagrams. 1 is a heating element, 2 is a drive section, 3 is a control section, 4 is a current measurement section, 5 is a temperature calculation section, Dn is gradation data, PC is a recording control signal, MC is a control signal for current measurement, 11. 17 is a counter, 12 is random access memory (RAM)
, 13.18 One-way only memory (ROM) , 1
4, 19.20 is a comparison circuit, 15.21 is an AND circuit,
16 is a blocking circuit, and 22 is an OR circuit.

Claims (1)

[Claims] A thermal head that applies a recording signal corresponding to gradation data to a heating element (1) of a thermal head and records on a recording medium at a recording density corresponding to the heat generation temperature of the heating element (1). In the multi-value drive system, a drive section (2) applies a recording signal to the heating element (1), and a recording control signal corresponding to the input tone data is applied to the drive section (2). In addition, a control unit (3) applies a control signal for sequentially selecting the heat generating elements (1) and supplying current for temperature measurement for a short time during a cooling period after applying the recording signal to the heat generating elements (1). ), a current measurement unit (4) that measures a temperature measurement current flowing through the selected heating element (1), and the current measurement unit (4).
and a temperature calculation unit (5) that calculates the temperature of the heating element (1) from the current value measured by the heating element (1) calculated by the temperature calculation unit (5).
temperature information is added to the control section (3).
) A multi-value drive system for a thermal head, characterized in that a recording control signal applied to the drive unit (2) is corrected.