JPS61202858A - Thermal printer control system - Google Patents

Abstract

PURPOSE:To accelerate recording speed and enable power consumption to be reduced by controlling a line/dot recording cycle to a required minimum value according to image data to be recorded. CONSTITUTION:A thermal head drive circuit 33 output data D2 from a memory 32, creates a drive signal after D/A conversion, and applies it a thermal head heating element. That is, data D2 output from a memory 32 at 1 point of time is 5-bit data for the current-recording and data D1 output from a memory 31 likewise is 5-bit dot data for the next recording line. therefore, a required cooling time is sought by reading ROM 34 where said time is written beforehand using said data as addresses. Further, the maximum value of each output T in a single line is sought by a maximum value detection circuit 35, and the next recording timing signal LT is output at a maximum value in a single line to be output by the circuit 35 using a timer 36.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a control method for a thermal printer, and relates to a control method for a thermal printer, and is capable of recording gradation images without sacrificing accuracy in gradation levels in monochrome/color thermal printers that record multi-valued gradation images. The present invention aims to provide a print control method that can increase speed and reduce power consumption.

[Conventional technology]

Thermal printers that use heat to record information such as characters include a thermal recording method and a thermal transfer recording method. The former is a method that records by bringing the thermal head into direct contact with the recording paper, which changes color due to heat, and heating the heating element of the thermal head to change the color of the recording paper, while the latter heats the ink with the thermal head to melt or melt the ink. This is a method of sublimating and transferring it to recording paper for recording. Furthermore, based on the heating element scanning method, there are two main types: line recording method and serial recording method. A line recording method is used for high speed recording, and a serial recording method is used for low speed recording.

FIG. 4 shows an outline of a line recording type thermal transfer recording apparatus, and FIG. 5 shows the structure of its thermal head. A thermal head 11 faces a platen 14 with an ink sheet 12 and a recording paper 13 in between. The ink sheet 12 is heat-meltable, and by heating the ink sheet 12 with the thermal head 11, the ink on the ink sheet 12 is melted and transferred onto the recording paper 13 for recording. The thermal heater 11 has heating elements for one line arranged in a direction perpendicular to the plane of the paper, and recording for one line is performed almost simultaneously. When recording for one line is completed, the recording paper 13 and the ink sheet 12 are simultaneously transported in the direction of the arrow, and the next line is printed.

The thermal helmet 11 has a multilayer structure as shown in FIG. That is, a glaze layer 24, a heating element 23, and an electrode 22 are provided in a layered structure on a substrate 25, and a protective layer 21 is provided on a surface that contacts the recording paper. Note that 26 is a heat sink.

Such thermal heads accumulate heat within them.

That is, the temperature of the heating element at time t1 when the drive signal Pa with the pulse width Wa is applied as shown in FIG. 6 (al) is Ta, and as shown in FIG.
When the driving signal pb of (W b < W a ) is applied, the temperature becomes Tb (Tb < Ta), and as shown in (e), the driving signal Pc of the pulse width W c (W c < W b ) is applied. The temperature at that time becomes Tc (Tc<Tb). That is, the temperature T of the heating element at time t1 varies greatly depending on the amount of heating (corresponding to the pulse width of the drive signal) applied to the heating element. This is the M heat fluctuation of the heating element due to the change in the amount of heating. Furthermore, when the drive signal Pc with the pulse width Wc is applied, the temperature at time t1 is Tc, but at time tll the temperature is Tc 1 (Tc 1<T
c), that is, the temperature of the heating element varies depending on the elapsed time from the start of recording. This fact causes a change in the heat storage of the heating element due to a change in the recording cycle, and a change in the initial temperature when the heating element is driven. If the initial temperatures of the heating elements are different, the temperature of the heating elements will not reach the expected temperature even if a drive signal with a predetermined pulse width is applied in the next recording cycle, and therefore the expected density will not be obtained. Since the temperature of the heating element changes depending on the initial temperature at the time of application of the drive signal and the past history, the temperature of the heating element cannot be controlled to a desired temperature only by the pulse width of the drive signal applied this time.

To deal with this, detect the temperature of the thermal head,
It is conceivable to feed back the detected temperature and change the drive conditions (pulse width, wave height, etc.) of the drive signal applied to the heating element (hereinafter referred to as a feedback method).

The feedback method is effective against heat accumulation at low speeds in the substrate of the thermal head. The temperature of the entire thermal head increases due to heat accumulation in the substrate of the thermal head, or the temperature locally increases in areas where high-density recording is concentrated.As a result, recording density increases when recording for a long time. The fine pattern may be crushed or
Blank paper parts become dark, or recording density differences occur in the length direction of the thermal head. Since this heat accumulation phenomenon is slow, it can generally be corrected using a substrate temperature detection system. That is, the drive power may be reduced as the substrate temperature rises. In order to compensate for the temperature distribution in the length direction of the thermal head, it is recommended to detect the temperature at multiple locations, divide the thermal head drive circuit into multiple blocks, and control the drive conditions for each block. It's normal.

However, with such a feedback method, it is not possible to completely correct changes in recording density due to heat accumulation unless the recording speed is very slow.

In other words, heat storage can be roughly divided into those caused by the substrate of the thermal head, and those caused by the glaze layer.
Since heat changes at a slow rate with respect to the recording cycle, it can be corrected by the feedback method described above. However, the heat storage by the glaze layer changes rapidly with respect to the recording period. Therefore, during high-speed recording, it is virtually impossible to correct heat accumulation by the glaze layer using the feedback method.

Conventionally, for high-speed recording, attempts have been made to control the power applied to the next recording pulse signal based on the temperature history of the heating element used for recording in the past. Taking binary recording as an example, if the previous record was black, the pulse width of the drive signal applied to the heating element would be shorter or the pulse height would be shorter than if it was white. This method reduces the supplied power by reducing the

As described above, by adopting the temperature history method, it is possible to reduce changes in recording density due to heat accumulation in the thermal head, but it has the following drawbacks. In the temperature history method, the temperature of the heating element at the start of the next recording is determined by applying past drive conditions to the theoretical formula for heating and cooling characteristics, and from this temperature the conditions for the drive signal corresponding to the amount of heat to be supplied are determined. This is essential. This calculation requires complicated calculations such as exponential functions, and all heating elements installed in the thermal head (for example, if the recording paper is A4
It is necessary to perform the above calculation for 1,680 heating elements (assuming 8 dots per crane), and a circuit that can perform high-speed calculations is required.

Furthermore, the larger the number of traced histories, the more the error in recording density due to heat accumulation can be reduced, but the greater the number of traced histories, the longer the time required for the above calculation.

Furthermore, in order to increase the recording speed and to limit instantaneous power consumption, the recording cycle of the heating element is not constant, and in order to calculate the drive signal conditions from the past drive conditions, it is necessary to take into account changes in this recording cycle. It is also necessary to take these factors into consideration, which complicates calculations and results in a huge variety of combinations of driving conditions.

These drawbacks are inconvenient for improving recording speed. Since the next drive signal cannot be applied until the above calculation is completed, the recording cycle is substantially reduced. It also requires a high-speed and high-precision arithmetic circuit.

In the initial temperature stabilization method proposed earlier, such calculations are unnecessary and high-speed recording is possible.

This method prevents the influence of heat storage fluctuations from affecting the next recording period by canceling the heat storage fluctuations in each recording period within that recording period. In other words, in response to changes in the amount of heating, by controlling the temperature of the heating element to a constant value at a certain time from the start of recording, the temperature history of the heating element is erased, and the subsequent temperature characteristics of the heating element are determined by heat storage. can be unrelated. In addition, in response to changes in the recording cycle, the heating element temperature at the next recording start point can be kept at a constant value by controlling the driving conditions of the drive signal according to the time from the above-mentioned fixed time to the next recording start point. , so that the change in the heat storage of the heating element due to the change in the recording period can be made irrelevant to the subsequent temperature characteristics of the heating element.

In this way, temperature changes due to differences in the amount of heating to the heating element and temperature changes due to changes in the recording cycle are canceled out within the recording period, thereby making the temperature of the heating element at the start and end of recording the same, thereby making it possible to There is no need to refer to the driving conditions of .

Here, we will describe the equation representing the time-temperature characteristics of the heating resistor, which can be approximated by the following equation.

However, t: time, T (tl: heating resistor temperature at time t, Ta: ambient temperature, τ: thermal time constant of heat storage in the glaze layer, thermal time constant of heat storage in the substrate, etc.) Constant tW: Drive pulse width W: Applied power R: Thermal resistance Figure 6 shows the heating resistor temperature T (t).Heating resistor temperature T (tl and recording density [)c
The relationship is given by the following equation.

However, Do: Saturation concentration C1: Transfer constant of ink Q: Barrier potential of ink transfer: Portzmann constant CH: Constant related to heat transfer from heating resistor to ink sheet [Problem to be solved by the invention] As described above. In order to perform multi-value recording with a thermal printer, the amount of heating by the thermal head must be controlled with extremely high precision. However, because of the phenomenon in which heat accumulates within the thermal head, it is not easy to control the amount of heating, and the following control method has conventionally been used.

1) Recording is performed at such a low speed that high-speed heat accumulation can be ignored (usually line recording period > 100 m5 in line recording method) using only a feedback method that compensates for low-speed heat accumulation.

2) Use not only the feedback method but also a temperature history method that compensates for high-speed heat storage, and record at high speed within the range where temperature history calculation is possible.

3) In addition to the feedback method, an initial temperature-chemotaxis method is used to compensate for high-speed heat accumulation, and recording is performed at a high speed (usually line recording period ≦10 m S in a line recording method).

With the temperature history method, the recording speed is often limited by the calculation speed of the temperature history (there is little room for improvement. However, with the other two methods, there is much room for improvement. The line/dot recording cycle is constant except during printing, and its value corresponds to the required cooling time for thermal and dots under the worst-case conditions, that is, immediately after the highest gradation level (pure black) dot, the lowest gradation level (pure white) It is determined based on the conditions for recording dots.However, images with continuous worst conditions are extremely rare, and if it is a dark and dark image, the difference in gradation level is often small.In general, in a binary image, the black level is 10 to 30%, and the rest is at the white level.

Even in grayscale images, low gradation levels often occupy the majority of the image, and if such images are recorded at a cycle determined from the worst-case conditions above, high-speed recording would normally be possible.
This results in recording at an unnecessarily slow speed.

Also, high-speed recording saves power because the next recording is performed before it has cooled down, but if the cycle is set based on the worst-case conditions, the cooling time will be longer than necessary, resulting in excessive cooling.
Extra power is required to heat the thermal heating element.

The present invention provides a print control method for a multilevel thermal printer that increases the recording speed and reduces power consumption by controlling the line/dot recording cycle to the minimum necessary value according to the image data to be recorded. This is what we are trying to provide.

[Means for solving problems]

The present invention relates to a control method for a thermal printer that records an image by heating a plurality of heating elements of a thermal head using a driving signal created from multilevel gradation data. The method is characterized in that the required cooling time of each heating element is determined from the drive signal applied to each heating element at the next point in time, and the point in time of the method is determined according to the maximum value of the required cooling time. .

In the present invention, the gradation level to be recorded at a certain time point Ti and the gradation level to be recorded at the next time point T, + is checked for all the heating elements of the thermal head, and therefore for all dots on the line, and from these, Calculate the required cooling time of the heating element, and calculate the maximum value as the recording period at time Ti (=
T, +t Ti), thereby achieving the above objective. Thermal printers include a line recording method and a serial recording method, but both are basically the same in terms of driving (heating) and cooling of the heating element of the thermal head. Therefore, the line recording method will be explained below as an example.

When performing multi-level recording using a thermal head, the temperature of the heating element of the thermal head rises when one line is recorded, so the next line cannot be recorded until it has cooled down sufficiently, which requires the longest cooling period. Obviously, if the lowest gradation level is recorded immediately after the highest gradation level (
(worst case). This is shown in Figure 7(a), 2
In value recording, it is extremely common for any of the heating elements in one line to have such a recording pattern, so the recording cycle is adjusted to the worst case, and only consecutive blank lines are detected separately to achieve high-speed processing. If you do this, you will be recording at a speed close to the limit. On the other hand, in multi-level recording of gradation images, 1) intermediate gradation levels are overwhelmingly large among all pixel data, and 2) changes in gradation levels in images are generally gradual, and the highest gradation For reasons such as the fact that the frequency of the lowest gradation level appearing immediately after the level is extremely low, determining the recording period using the figure 7 (al) will result in recording at an unreasonably slow speed. However, if the frequency is low, However, there may be cases where the recording cycle shown in Figure 7(a) is necessary, so in the end, the required cooling time for all heating elements in one line is calculated, the maximum value is detected, and this is used as the recording cycle. If so, the average recording cycle is as shown in FIG. 7(b) without degrading the recording quality.

As shown in (C), it is significantly shortened. As a result, not only the recording speed increases, but also the wasteful dissipation of thermal energy is reduced, so power consumption can also be reduced.

A block diagram of an embodiment of the present invention is shown in FIG. 1, and a time chart is shown in FIG. 2.

〔Example〕

In Fig. 1, 31 and 32 are FIFO (first in, first out) memory with 1728 rows x 5 bits configuration, and each dot of 1728 dots in one line has 2 shades of gray.
Gradation data Do, D+ (D+ is the output of the memory 31 and is the same as Do) expressed in 5 bits (of which 1 bit is parity, so it is actually 4 bits) is input sequentially from the beginning of the memory, and the input data is Shift in memory and output sequentially from the end in the order in which it was input. 53 is a thermal head drive circuit which takes in the output data D2 of the memory 32 and converts it from D/A to the drive signal pa shown in FIG.

Pb or the like is prepared and applied to the thermal head heating element. The reason why the memories 31 and 32 are provided is to determine the density difference between the corresponding dots of the current recording line and the next recording line, and to determine the required cooling time from this. Data D2 outputted by the memory 32 at point 1 of the current recording line is 5.
Bit data, similarly data D1 output by the memory 31
is the 5-bit dot data of the next recording line, so
Using these (4+4=8 bits excluding parity) as addresses, write the required cooling time in advance in the ROM.
By reading the (read-only memory) 34, the required cooling time can be determined.

If the required cooling time is expressed in n bits, the address is 8 bits or 256 addresses, so the ROM 34
For example, one with n pints/word and 256 addresses may be used. To is the output data of the ROM 34. 35 is a maximum value detection circuit, which detects each output To in one line.
Find the maximum value of.

A timer 36 outputs a timing signal LT for the next recording based on the maximum value T+ in one line outputted by the circuit 35.

ST is a start signal, and DL is a gradation data latch signal.

The operation of this circuit will be explained below.

When the start signal ST rises, the output of the timer 36, that is, the line timing signal LT rises, and the gradation data Do is output from an image data control circuit (not shown). In FIG. 2, the gradation data corresponding to the j-th heating element of the first line is expressed as Dij, where i and j are natural numbers. The activation signal ST is also input to the thermal head drive circuit 33 at the same time, and initialization processing is performed.

The image data control circuit outputs the gradation data Do at regular intervals, and outputs the gradation data latch signal DL with a slight delay. When the data of the (i-3)th line is being recorded, the gradation data Do output from the image data control circuit is for the i-th line, and 1728 pieces of the gradation data are generated at the rising edge of the line timing signal LT. The data is sequentially taken into the first FIFO memory 31 by the latch signal DL that is output thereafter. These i-th line data are
-2) The second FI when the line data is being recorded.
It is recorded in the FO memory 32. These i-th line data are stored in the thermal head drive circuit 33 while the (i-1)th line data is being recorded, and are recorded at the next rising edge of the line timing signal LT.

The access address AoA+ of the ROM34 is the output D of the FIFo memory 1J32.31.2. D+Tearly, when the data of the (i-3)th line is recorded, ROM3
4 address A1 is the first (7) F I Fo memory 3
1 output D1, i.e. I)+-t, + ~D i-1,
1728, and address A2 is the second FIF
Output D2 of O memory 32, i.e. I) t-2+1~Di
-2.1728. The ROM 34 is accessed using these addresses, and each gradation data (
(Ih-t, + l Di-21,) , (Di-
1, 21D, -2, 2) 1... (DI-1,
1728, DI-2, 1728) ) corresponding to the minimum required recording cycle T o (Tl-21, Tl-22, .
...Tr-2, 1728) is output. ROM
The output Ta of 34 is input to the maximum value detection circuit 35, and when all the data for one line is input, the maximum value T+-2 (Max (Tl-2, t, Tr-
2,2+ "0--0Ti-2,17□8))" is output.

The rising edge of the next line timing signal LT, that is, the (
When recording the data of the i-2) line, the above values T and -2 are set in the timer 36, and when the equal time T i-2 has elapsed, the timing signal LT rises and the data of the (i-1)th line is recorded. recording will begin. That is, the (i-th
2) Line data recording cycle is T, from -211 to T
This is the maximum value up to i-2,1728, and multi-level recording can be performed as fast as possible.

Tl-3, T1-1, Ti is t4+t-2
+t-1 This is the recording timing signal for the next line obtained during line recording, and the line timing signal LT rises after these times have elapsed.

Here, the data to be written to the ROM 34, namely 2
A method for calculating the minimum required recording period corresponding to the gradation data value of 7 will be described. There are various possible calculation methods:
An example of a method that is easy to calculate and fully demonstrates the effects of the invention will be shown. In the feedback method, for the amount of high-speed heat storage that cannot be fed back, a permissible value is determined for each of the two gradation data, which will be described later, and the amount of heat storage that cannot be fed back is determined according to the data that is recorded first. The recording period is calculated from the allowable value using the above equations (11 to (3)).

Next, in the initial temperature-chemotaxis method, the initial temperature target value is set to the steady temperature of the heating element when the thermal head is repeatedly operated an infinite number of times at intermediate gradation level values that appear frequently.

The recording period can be calculated from the data recorded first and the initial temperature target value using the above equations (1) to (3).

That is, in the past, as shown in Fig. 3(a), the initial temperature target value ToQ was high and auxiliary heating was always required at the intermediate gradation level, but in the present invention, as shown in Fig. 3(a), 'l
'o3 can be set low, auxiliary heating is not required even at intermediate gradation levels, or the amount of heating can be very small (and the recording speed can be increased. In this example, it is necessary to refer to the data that will be recorded later. None.

If you refer to the data that will be recorded later and select one value of Tω from multiple reference values, the speed will be even higher.
Although the power consumption can be reduced, the calculation becomes quite complicated.

An arithmetic circuit may be used in place of the ROM 34, but since there are 16 types of gradations (D2) for each dot in the preceding recording line and 16 types for the gradation 11 (DI) of each dot in the subsequent recording line, the combination is 16×16. = 256, and it is easier and faster to calculate the required cooling time for each combination in advance, write it in the ROM 34, and read it out for use.

〔Effect of the invention〕

As explained above, in the present invention, the maximum density difference between corresponding dots between the preceding recording line and the succeeding recording line is determined, the recording interval necessary for the maximum difference is determined, and the recording interval for the subsequent recording line is determined from the recording interval. Since recording is performed, the recording interval of each line can always be set to the required value, and unlike the method of determining the recording interval according to the worst conditions or the most frequently occurring cases, there is no excess or deficiency. high speed,
Energy saving and high quality printing can be performed.

[Brief explanation of the drawing]

Fig. 1 is a block diagram showing an embodiment of the present invention, Fig. 2 is a time chart for explaining the operation, Fig. 3 is the same graph, Fig. 4 is an explanatory diagram of the thermal printer, and Fig. 5 is an explanation of the thermal head. 6 and 7 are explanatory diagrams of heating element temperature characteristics. In the drawing, Do indicates multilevel gradation data, 11 is a thermal head, 23 is a heating element, and 34 is a ROM that stores the required cooling time.
It is.

Claims (1)

[Claims] In a thermal printer control system that records an image by heating multiple heating elements of a thermal head using a drive signal created from multilevel gradation data, the drive signal applied to each heating element at one point in time and the next point in time are A control method for a thermal printer, characterized in that the required cooling time of each heating element is determined from the drive signal applied to each heating element, and the next point in time is determined according to the maximum value of the required cooling time.