JPH0377067B2 - - Google Patents

Description

Detailed Description of the Invention Technical Field The present invention relates to a thermal recording head drive control device, and more specifically, to a thermal recording head that thermally records information on a recording medium by applying energy to a heating element. This invention relates to a drive control device.

BACKGROUND OF THE INVENTION For example, in thermal transfer recording or thermal recording using a thermal recording medium, heat remains in the structure of the thermal recording head, that is, the structure of the thermal head, depending on the previous driving state of the heating element. This heat storage effect decays exponentially over time. In thermal recording, good recording can basically be obtained by appropriately controlling the thermal state of the thermal head depending on the image information to be recorded. However, if the heat storage effect from the previous drive remains in the thermal head, it will appear as noise in the next new recording. For example, in a recorded image, in a portion with a high black pixel density, the pixels are mutually blacked out, resulting in a decrease in resolution. Therefore, if the amount of heat generated is lowered, the black part with low pixel density will be missing. Therefore,
High-speed recording cannot be performed in which driving is performed at such short intervals that the influence of the previous driving is not sufficiently eliminated.

In order to improve this drawback, a method was developed in which the width of the driving pulse of the heating element was controlled according to the interval between the previous driving during recording (Japanese Patent Application Laid-Open No.
142675), or a method for performing additional printing when the pre-recorded signal is a space signal (Japanese Patent Application Laid-open No.
55831) have been proposed. There is also a method (Japanese Patent Publication No. 55-47980) in which the next driving time is controlled in proportion to the length of the driving stop period of the heating element.

However, in either method, the energy to be applied to the heating element is not controlled according to the nature of the image information to be recorded or the state of heat storage in the head, so it is difficult to obtain recorded images of uniformly good quality. Can not. This tendency is particularly noticeable in Japanese printers, which use a mixture of various types of fonts and characters of various sizes, and in facsimile machines, which transmit various images, and are therefore not suitable for high-speed recording.

OBJECTIVES Accordingly, it is an object of the present invention to provide a thermal recording head drive control device that eliminates the drawbacks of the prior art and is capable of recording good and uniform image quality at high speed.

Configuration The configuration of the present invention will be described below based on one embodiment.

In thermal recording using a thermal recording head (thermal head), that is, recording that uses a thermal recording medium, or thermal transfer recording, that is, recording that transfers pigments such as ink to a normal storage medium such as plain paper using heat, the recording speed is As the speed increases, the image quality, that is, the quality of recorded images such as characters and images, is greatly influenced by the image information and the heat storage state of the head.

For example, in thermal transfer recording, as shown in FIG. 1, a thermal recording head, that is, a thermal head structure or substrate 10 is disposed facing a plantain roller 12, and a recording medium 14 such as plain paper and an ink sheet 16 are placed between them. Intervening.

In this example, the thermal head assembly 10 is a line head, and has one line of heating elements 18 in the direction perpendicular to the plane of the drawing, and a heating element array 180.
(Fig. 5). Therefore, as the platen roller 12 rotates in the direction of the arrow A, the recording medium 14 and the ink sheet 16 are fed in the sub-scanning direction B, and ink is ejected from the ink sheet 16 for recording according to the heat generated by the heating element 18. By transferring the image to the medium 14, line recording is performed.

FIGS. 2A and 2B show examples of images recorded using the conventional recording method, that is, a method in which the energy applied to the heating element 18 of the thermal head assembly 10 is not controlled in accordance with the image information to be recorded. In these examples, each character is formed of a number of dots or pixels, and an energy of 0.8 millijoules/dot in FIG. 2A and 0.4 millijoules/dot in FIG. 2B is applied to the heating element 18.

Figure 2A shows the heating element 18 at a location where the image is densely packed.
This is an example in which the heat accumulation state is large and the temperature becomes too high, causing part of the image to collapse. If the applied energy is lowered regardless of the image information in order to avoid overheating of the heating element 18, part of the image will be missing, as shown in the example shown in FIG. 2B.

As shown in detail in FIG. 3A, the thermal transfer system is a heat-generating system that melts a portion 164 of the ink layer 162 on the ink sheet base 160 of the ink sheet 16 by heat generated by the heat generating element 18 and transfers it onto the recording medium 14. It is. However, a portion of the thermal energy applied to the heating element 18 flows into the ink layer 162, particularly a portion 164 thereof, through various thermal resistances, and also leaks into the head substrate 10 and the surrounding atmosphere.

This heat generation system is treated as a thermal model system and analyzed by replacing it with an electrical equivalent circuit model. This equivalent circuit is shown in FIG. 3B. Here, each element is as follows.

C ₁ : Heat capacity of heating element 18 C ₂ : Heat capacity of thermal head substrate 10 C ₃ : Heat capacity when raising 164 to be transferred to melting temperature C ₄ : Heat capacity of surrounding atmosphere R ₁ : Heat capacity from heating element 18 Thermal resistance to the head substrate (including the heat sink) 10 R ₂ : Thermal resistance from the thermal head substrate 10 to the atmosphere R ₃ : From the heating element 18 to the ink layer 162 via the air layer 30 and the ink sheet base 160 etc. Thermal resistance R ₄ : Thermal resistance when heat leaks from the heating element 18 to the substrate 10 or platen 12 via the air layer 30 and the ink sheet 16 etc. R ₅ : Heat capacity D when heat is absorbed by the ink 164 to be transferred ₁ : Zener diode indicating melting temperature T _p : Accumulation temperature of thermal head board 10 3rd
In the equivalent circuit shown in Figure B, the heat capacity C ₂ is much larger than C ₁ , and it can be considered that the temperature of the heat capacity C ₂ does not change T _p during the time the heating element 18 is driven. Furthermore, since the thermal resistance R ₃ is sufficiently larger than R ₁ , the temperature T of the heat capacity C ₁ after time t has elapsed after giving the temperature T ₀ to the heat capacity C ₁ can be approximated by equation (1). can.

However, e is the base of natural logarithm.

The melting energy of the ink 164 is several hundred times greater than the energy required to raise the temperature of a unit amount of ink by the equivalent of 1°C, so in a practical temperature range, the energy required to store heat capacity C ₃ to the melting temperature is necessary for transfer. It can be ignored from the point of view of energy.

The melting temperature T _K of the ink is sufficiently higher than T _p ,
Furthermore, considering that the ink sheet 16 moves according to the linear velocity during recording, the temperature of the heating element 18 that contributes to melting the ink is the temperature of the portion exceeding the melting temperature _TK . That is, as shown in Fig. 4, the horizontal axis represents the elapsed time t after the temperature _T0 is given to the heat capacity _C1 .
If we take the temperature T of the heat capacity C ₁ on the vertical axis, the curve 40 follows the above-mentioned equation (1), and the shaded portion 42 is the portion that contributes to the melting of the ink 164.

In order to avoid complication of calculation, the shaded part 42 is
is a rectangular wave 32 with a temperature T ₀ and a pulse width τ (Fig. 3B).
Approximating as, the heat flow i ₅ flowing through the thermal resistance R ₅ is (2)
The formula becomes

i ₅ =T ₀ R ₄ +T _p R ₃ /R ₃ R ₄ +R ₄ R ₅ +R ₃ R ₅ (2) Therefore, the energy Q absorbed by the ink 164 to be transferred is approximated by equation (3). .

Q≒∫〓 ₀ i ₅ dt=(T ₀ R ₄ +T _p R ₃ )τ/R ₃ R ₄ +R ₄ R ₅ +R ₃ R
₅ (3) The temperature T ₀ that makes Q constant is given by equation (4).

T ₀ ≒ Q / R ₄ 〓 (R ₃ R ₄ + R ₄ R ₅ + R ₃ R ₅ ) − R ₃ /R ₄ T _p (4) Regardless of the image information or the heat storage state of the thermal head 10, the ink to be transferred In order to give energy of a predetermined Q to 164, T _s obtained from equation (5)
It is necessary to provide heat to the heating element 18.

Note that in equation (5), equation (1) was substituted for T, and equation (4) was substituted for T ₀ .

By the way, the heating element 1 of the thermal head board 10
8, that is, the thermal energy to be input into the heat capacity C ₁ ,
That is, the input energy E _io is given by equation (6).

E _io = C ₁ T _s (6) When the temperature T _p of the substrate 10 is maintained at an arbitrary reference temperature and the heat capacity C ₁ of the heating element 18 is the same temperature as this temperature T _p , in order to obtain a good record, Assuming that the required input energy is E ₀ , the input energy conditions for recording image information satisfactorily are expressed by equations (5) and (6) to equation (7).

However, A is a constant expressed by the following equation (8), ΔT _p
is the increase in the temperature of the substrate 10 from the reference temperature.

A=Qτ(R ₃ +R ₄ )/R ₃ R ₄ +R ₄ R ₅ +R ₃ R ₅ (8) Referring to FIG. 5, control is performed to control the thermal energy applied to the heating element 18 according to equation (7). circuit 100
It basically consists of two random access memories (RAM) 102 and 104, a shift register 106, an AND gate 108, a RAM 110, a latch 112, a counter 114, a read only memory (ROM) 116, and a latch multiplexer 118. ing.

In this embodiment, the thermal head structure 10 includes:
It forms a 2048-bit line head, has a 2048-bit shift register 182, and a latch 184 that holds the state of each bit. In other words, one recording line (row) consists of 2048 bits. In this embodiment, as shown in No. 6, the heating element 18 is driven by dividing one line period τ _L for recording one line into N sub-periods τ _B. For example,
τ _L is approximately 2.5 milliseconds, τ _B is approximately 300 microseconds, and N is 8
It is. Therefore, for example, when recording white on the recording medium 14, no pulse is applied to the stage of the shift register 182 corresponding to the white bit, as shown by reference numeral 322 in FIG. For example, the stage of the shift register 182 corresponding to the black bit has n (n
is a natural number from 1 to 8) pulses 320 are applied. This is the signal activated every sub-period τ _B
It is held for each sub-period τ _B by a latch 184 responsive to LOAD, during which the corresponding transistor 186 conducts. Therefore, for the entire one line period τ _L , a drive pulse 32 (FIG. F) equivalently having a pulse width of nτ _B is formed.

According to this embodiment, the driving period nτ _B of the heating element 18 in one line period τ _L , that is, the ratio n/N is (7)
It is controlled according to the formula, that is, depending on the length of the pause period from the previous drive and the temperature of the head substrate 10.

RAMs 102 and 104 are 2K-bit (2048-bit) random access memories, and the input data IDATA as image information to be recorded is alternately read from read 120 every 2K bits.
It is input to RAM 102 or 104 in a toggle manner. Its storage address is specified by IADD. When one of the RAMs 102 or 104 is full, the data is output bit-serial from the lead 122 and input to the shift register 106. Its speed is eight times the one line period τ _L ,
This is performed eight times during one line period τ _L . Its output address is specified by OADD.

FIG. 6 shows signal waveforms at various parts of the circuit of FIG. 5, but the period axes are the same in A, C to F, and the one line period τ _L is shown enlarged by B. That is, when the image information data IDATA to be recorded shown in FIG. C to E in the same figure are
Each left half shows a waveform focused on bit 0 of line 0, and each right half shows a waveform focused on bit 0 of line 1, and the other bits 1 to 2047 are omitted with waveform symbols.

RAMs 102 and 104 thus function as toggle buffers, which are controlled by toggle buffer control 130.

2-bit delay output Q _B of shift register 106
is input to one input 124 of the AND gate 108 (C in the same figure). The other input 128 of the AND gate 108 is supplied with a signal from the latch multiplexer 118 in synchronization with each bit, which determines the pulse width of the pulse 32 (FIG. F) that drives the heating element 18 for one bit. (Figure D). When this signal is significant, the data transferred to the thermal head 10 is valid, and when it is invalid, it is invalid. Therefore,
As described above, if it is significant n times during the transfer of one line of stored data eight times, the drive pulse 32 will have a pulse width of n/8 of the one line period τ _L (FIG. F).

The data for determining this pulse width is provided by the circuit 11.
0, 112, 114, 116 and 118. The RAM 110 has a storage capacity of 2K x 4 bits, and is connected to the address line 130 in synchronization with the data output from the toggle buffer section in the upper half of FIG.
Addressed by Accumulation in RAM 110 is performed by activating signal WENA for the last one of the eight data transfers described above for each bit, and outputs Q _A , Q _B and Q of counter 114 are activated. The state of _C is written via data line 134 to the address corresponding to that bit.

The counter 114 is a counter that counts the elapsed time from the time when the heating element 18 corresponding to each bit was last driven by the number of line periods τ _L ,
In this example, the driving state of each bit, that is, the previous history, is monitored up to eight line periods ago. More specifically, RAM10 specified by output address OADD
One bit of image information data is read out eight times from address 2 or 104 and stored in shift register 10.
6 to the gate 108 (FIG. 6B, C), but in synchronization with the eighth and final transfer, the contents are read out from the memory location of the RAM 110 specified by the same address to the output 136, and the latch is 1
It is held at 12. This is applied to address input 138 of ROM 116 and counter 1
14 preset inputs A, B and C are also provided. Note that the signal ELCLK is a pixel clock given for each bit of image information data.

The counter 114 loads preset values A, B, and C in response to a signal CNTLOAD applied in synchronization with 1 bit of the image information data, and counts the counting clock CNTCLK, which is also synchronized with 1 bit. , B and C as 1
and outputs that value to outputs Q _A , Q _B and Q _C. This output passes through lead 134.
The data is transferred to the RAM 110 and written to the corresponding address. This count-up operation is performed in response to the signal CNTENA synchronized with the last of the eight transfers of 1-bit image information data.

By the way, one output of the shift register 106
Q _A has the same image information data as the other output Q _B (6th
Figure C) is output, which is NAND gate 1
40 is fed to one input 142 of 40. The 1-bit image information data to be recorded is bit 0 of line 0.
If it is black as shown by , the NAND gate 140 is activated in synchronization with the signal CNTENA, and the counter 114 is reset to "all 0s" by the reset terminal R. Therefore, “all 0s” of the outputs Q _A , Q _B and Q _C are stored at the corresponding address in the RAM 110 .

In this way, the RAM 110 and the counter 114 monitor the previous history of the driving state of each bit in one line up to eight lines before.

The ROM 116 stores the rest period t of the heating element 18.
Addresses A0 to A2 given from the leads 138 and the head board 1 in accordance with (formula (7) above)
In the storage locations specified by addresses A3 to A7 given by the signal THERM indicating a temperature of 0,
Drive pulse 32 corresponding to the appropriate input energy _Eio calculated in advance according to equation (7) (Fig. 6F)
The pulse width of is stored as the number n of pulses 320 (E in the figure). The signal THERM is the board 10
The temperature is detected by a temperature detector 22 (first
Analog/digital (A/D)
This is a converted signal and is related to ΔT _p in equation (7).

Data indicating this number of pulses n is read out to the outputs Q0 to Q7 of the ROM 116, and is read out to the outputs Q0 to Q7 of the ROM 116, and is divided into eight sub-periods τ _B
The signal MP×CK that specifies one of 0 to 7
Output Q2 to ROM116 each time
~ Q7 sequentially, multiplexer 118
is output to the lead 126 through the (FIG. 6D)
This signal is applied to the AND gate 108 as data for determining the pulse width described above. Note that such feedback of the temperature of the thermal head substrate 10 may be performed with respect to the power supply voltage that supplies the voltage V _HD applied to the heating element 18 from the sensor 22 on the reference 10.

In this way, a driving pulse 32 having a pulse width corresponding to the input energy _Eio determined according to equation (7) for each line is applied to the heating element 18.
In this embodiment, when black level recording is performed by dividing one line period τ _L into eight pulses 320, the first n pulses 320 are activated, but the present invention is not limited to this. , n pulses 320 including the last time may be activated, or discrete pulses 320 may be activated at random. Note that these numerical values are merely examples for explanation and do not limit the present invention in any way. Further, although the input energy to the heating element 18 is controlled by increasing or decreasing the number n of pulses 320, that is, the pulse width of the driving pulse 32 of the transistor 186, other methods such as increasing or decreasing the pulse width of the driving pulse 32 of the transistor 186 are performed. It will be appreciated that this may be done by increasing or decreasing the height, ie the magnitude of the energizing voltage or current, of a switching element such as transistor 186.

An experiment was conducted using an ink sheet 16 with a melting point of 63 DEG C. and a 2048-bit thin film type thermal head as the thermal head 10, and the results of determining the energy applied to the heating element 18 are shown in FIG. This data satisfies approximation formula (7). An image sample under this control condition is shown in FIG. 2C, and it can be seen that the drawbacks of the conventional example shown in FIGS. 2A and 2B have been improved.

Although the explanation so far has focused on thermal transfer recording, the above control conditions can also be applied to thermal recording. It can be considered that the ink melting energy in the equivalent circuit of FIG. 3B is replaced by the coloring energy, and the value of the thermal resistance _R3 is different.

The values of E ₀ , A, and C ₁ R ₁ are the melting temperature of the ink sheet,
Although it depends on various conditions such as the sensitivity of the thermal paper and the configuration of the thermal head, if the reference temperature of the substrate temperature _Tp is 20°C, the current thermal transfer recording system of, for example, 8 x 8 dots/mm ² generally has the following characteristics. Values in the range of are considered advantageous.

E ₀ = 0.5 to 1.0 millijoules/dot A = 1.0 x 10 ^-2 to 2.0 x 10 ^-2 C ₁ R ₁ = 2.0 x 10 ^-3 to 1.0 x 10 ^-2 The recording example in Figure 2C is based on the following condition values. This is what I got from.

E ₀ = 0.9 millijoules/dot A = 1.5 x 10 ^-2 C ₁ R ₁ = 5 x 10 ^-3 Reference temperature = 20°C Effect According to the present invention, the accumulation state of the thermal head heating element can be determined by its previous driving history and lead. Since the substrate temperature is monitored and the driving energy is controlled accordingly, good recording quality can always be obtained regardless of the image information to be recorded. Furthermore, since the driving interval of the heating element can be shortened, thermal recording can be achieved much faster than in the past. Therefore, the present invention is particularly advantageously applied to facsimiles and high-speed printers.

[Brief explanation of drawings]

Fig. 1 is a sectional view showing an example of a thermal transfer recording system for explaining the principle of the present invention, Fig. 2A and 2C are views showing recording samples according to the prior art and the present invention, respectively, and Fig. 3A is a cross-sectional view of a thermal transfer system. 3B is a circuit diagram showing an equivalent circuit of the thermal transfer system shown in FIG. 3A as an electrical model; FIG. 4 is a graph showing an example of temperature change of the thermal heating element; FIG. 5 is a block diagram showing an embodiment of a control circuit for a thermal recording head drive control device according to the present invention, FIG. 6 is a timing diagram showing waveforms appearing in the circuit of FIG. 5, and FIG. It is a graph which shows an example of the experimental result of a drive control device. Explanation of symbols of main parts, 10... thermal head structure, 18... heating element, 22... temperature detector,
100...control circuit, 102, 104, 110...
... Random access memory, 108 ... AND gate, 114 ... Counter, 116 ... Read-only memory, 130 ... Toggle buffer control, 182 ... Shift register.

Claims (1)

[Scope of Claims] 1. A thermal recording head drive control device for a recording apparatus that performs recording on a recording medium by applying input energy to a heating element of a thermal recording head to drive the thermal recording head, the drive control device comprising: a time counting means for counting the elapsed time t from the previous driving of the heat recording head; a temperature checking means for detecting the temperature of _the structure of the heat recording head; The thermal resistance to the head structure is R ₁ , the input energy required to obtain good recording quality when the structure is at the reference temperature is E ₀ , the constant is A, the base of the natural logarithm is e, and the above-mentioned time is When the temperature of the structure increases by ΔT _p after t has elapsed, the input energy E _io required to obtain good recording image quality is approximately expressed by the following formula: 1. A thermal recording head drive control device comprising drive control means for controlling said input energy in response to said time counting means and temperature detection means.