JPS62120166A - Multivalue driving device for thermal head - Google Patents

Abstract

PURPOSE:To perform gradation recording with high picture quality by dividing the recording cycle of one line into sublines with quite short cycles, and generating a binary recording data string sampled with each of the sublines. CONSTITUTION:Assuming that a cycle shorter than the minimum value DELTAPmin out of driving pulse widths corresponding to gradation levels is set as a subline cycle Ts, one line of the recording cycle is divided into K-number of cycles, and the driving pulse of each dot is generated with the K-number of binary data strings. A ROM1 outputs a subline number which generates data 0 in the first place out of generated binary data strings setting a gradation data as an address. A CNT2 generates one line of subline number for one dot of gradation data, and a CMP4 compares the subline number outputted from the ROM1 with the subline number outputted from the CNT2, and generates the binary recording data string.

Description

[Detailed Description of the Invention] [Summary] The present invention is based on the fact that the recording characteristics of a thermal printer are non-linear, and that the pulse width of the driving pulse necessary to obtain the density of the same gradation level depends on the environmental temperature, M Focusing on the fact that it changes slightly due to heat, etc., we have developed 1.
The recording cycle of a line is divided into sub-lines with extremely short cycles, and a binary recording data string sampled at each sub-line is generated, thereby making it possible to perform high-quality gradation recording.

[Industrial application field]

The present invention relates to a thermal head driving device, and more specifically, the present invention relates to a thermal head driving device that uses a direct drive method (hereinafter referred to as DD) for high-speed binary recording or multi-gradation recording using pulse width modulation.
The present invention relates to a thermal head multi-value drive device that can be realized using the following methods.

2. Description of the Related Art A thermal recording method and a thermal transfer recording method are widely used as methods for recording characters, figures, etc. using heat. There are two methods for scanning the heating element: a line recording method and a serial recording method. Generally, the serial method is used for low-speed and inexpensive devices, and the line method is used for high-speed devices. However, since there is no difference in the method of controlling the heating element for each method, a description will be given below using a line method thermal transfer recording apparatus as an example.

The configuration of the thermal transfer recording apparatus is shown in FIG. 5. A thermal heater 11, which is a heating element, is heated to heat a heat-melting or heat-sublimating ink sheet 12 according to the dot pattern to be recorded. As a result, the ink is transferred to the recording paper 13. Note that a constant pressing force is applied to the recording paper 13 in the direction of the ink sheet 12 by the platen 14 . A dot pattern is recorded on the recording paper 13 by repeatedly heating the thermal head 1 while moving the ink sheet 12 and the recording paper 13 by a fixed distance.

A line type thermal head has heating resistors of the same number as all recording dots within a recording width formed in a line on a substrate. As is clear from the recording principle, thermal transfer recording requires extremely precise control of the heating characteristics of the thermal head. However, since a considerable amount of heat accumulates in the thermal head at -i (hereinafter referred to as heat accumulation), the heating characteristics become uneven in the length direction of the thermal head and in the direction perpendicular to this, that is, in the paper feeding direction. Recording quality deteriorates significantly.

The structure of the thermal head is shown in FIG. For heat storage in the thermal head, the glaze layer 24 has a small time constant (on the order of milliseconds), the glaze layer 24, the substrate 25,
Furthermore, there is a type with a large time constant (on the order of seconds to minutes) that includes up to the heat sink 26. (a) Due to heat accumulation in the glaze layer, the driving conditions corresponding to the recording density at a certain dot position largely depend on the immediately previous recording data at that dot position. That is, taking the case of black/white binary recording as an example, when the immediately preceding data is black, the initial temperature of the heating resistor is higher than when it is white, so the recording density becomes higher. Since this heat accumulation phenomenon occurs at a high speed, it is generally impossible to apply feedback using a temperature detection system or the like, and the driving conditions are corrected based on a theoretical formula for heating and cooling characteristics. That is, in the previous example, if the immediately preceding data is black, the driving power is reduced compared to when it is white. That is, the drive pulse width is shortened or the drive pulse height is reduced.

When using a thermal head with a large time constant, when performing high-speed recording at a cycle shorter than the time constant, or when performing multi-tone recording, the recorded data is not only recorded immediately before, but also traced back several lines. must be investigated and corrective calculations must be made for the driving conditions. In addition, (o) heat accumulation in the substrate or the like causes the temperature of the entire thermal head to rise, or the temperature to rise locally in areas where high-density recording is concentrated. For this reason, when recording is performed for a long time, the recording density increases, causing fine patterns to become blurred, blank areas to become dark, or differences in recording density to 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.

The time-temperature characteristics of the heating resistor can be approximated by the following equation.

However, t: time, T(t) where t=Q when the driving pulse is applied? Heating resistor temperature Ta at time t; Ambient temperature tW: Driving pulse width W: Applied power R: Thermal resistance Kyou Co., Ltd. W+7 This is illustrated in FIG. 7.

Further, the relationship between the heating resistor temperature and the recording density Dc is given by the following equation.

However, DO = saturation concentration Ci: ink transfer constant Q: barrier potential for ink transfer: Boltzmann constant CH2 constant related to heat transfer from heating resistor to ink sheet [Prior art] As a conventional thermal head multi-value drive system: , (1) one in which each dot is driven by a set of counters, (2) one in which redundant recording is performed according to the gradation level without performing a feed-through, (
3) Depending on the gradation level, it can be roughly divided into K types, which sample the number of gradation levels and decompose it into binary data strings.

[Problem that the invention seeks to solve]

Method (1) is mainly applied to diode matrix (hereinafter abbreviated as DM) type thermal heads, but each dot has one
Since two sets of counters are required, the circuit scale becomes enormous, which is impractical. One way to reduce the circuit size of this system is to drive one line by dividing it into m (m≧2). In this case, if the recording period of one dot is Td, the recording period T of one line is T. =mXTd, and there is a drawback that the recording speed is significantly reduced.

As a method to improve method (1), methods (2) and (3) using a DD type thermal head have been adopted.

Method (2) performs overlapping recording without paper feeding a number of times according to the gradation level Lk, and repeats one line cycle (heating + cooling) of normal binary recording once. . Method (2) further includes (2-1) a method in which the applied pulse width is not controlled, and (2-2) a method in which each gradation level Lk (k=1.2, . . . ) is adjusted according to the recording characteristic curve.

Those that control the applied pulse width in i) are divided into K.

However, since the thermal recording characteristic curves are generally not in a proportional relationship as shown in Figure 8, method (2-i > impractical). T, the recording period T of one line is T=ixTL, which has the disadvantage that the recording speed is significantly reduced.

Method (3) is to repeat the heating period of normal binary recording a number of times according to the gradation level Lk.

However, in method (3), like method (2-1), the heating time is proportional to the input gradation level, so the number of gradations in the recorded image is small, and the gradation quality in the intermediate area cannot be obtained. There is.

The present invention was created in view of these points, and a period sufficiently shorter than the minimum time among the differences in the pulse widths of drive pulses necessary to obtain each gradation level is defined as a subline, and one By sampling the drive pulses of each dot on the line in sub-lines to generate a binary data string, the density interval of each gradation level can be made equal, and the thermal head multi-4IFI drive system can easily deal with heat accumulation compensation. is intended to provide.

[Means for solving problems]

For this reason, the present invention includes m (m, 12) n-bit latches and n-bit shift registers (n-2) that hold one line of binary recording data, and m x n heating resistors. In a thermal printer using a line-type thermal head, the gradation level to be recorded is Li, i=1.2...m), the drive pulse width corresponding to the gradation level Li is Pi, Let the recording period of one line be T, and the corresponding recording surface! When each period obtained by dividing tIIT into subline periods Ts, the drive pulse for each dot is j
T 5 (j=0.1..., K-1) has a means for sampling at each point in time and converting it into binary data strings, and also has a means for sampling at each time point of T 5 (j=0.1..., K-1) and converting it into binary data strings at two adjacent gradation levels L i 5Li
The difference in pulse width corresponding to −+ is ΔPi (= Pi − Pi,
, 1), the subline period Ts is set to be smaller than ΔPmin, which is the smallest among ΔPi.

[For production]

Subline cycle is shorter than ΔPmin! iJJ'r
By dividing the recording period T for one line into equal parts and using the drive pulse for each dot as s, the multi-value drive using pulse width modulation in the DD thermal head can be achieved with high precision. can be controlled.

〔Example〕

Methods for realizing multi-gradation recording include a method of controlling drive voltage and a method of controlling pulse width. However, from the viewpoint of ease of control and control accuracy, a pulse width modulation method is generally used in which pulse width is controlled to perform multi-gradation recording.

FIG. 2 shows the principle of multi-gradation recording using pulse width modulation. Pulse width (to-to), (tt-to),...
, (tt-to) is the gradation level (Lo, Ll,...
, L7), and no recording pulse is applied at the lowest level (ie, "white"). A common method for realizing multi-gradation recording by pulse width modulation is to generate recording pulses based on pulse width data using a counting circuit. However, in this case, a counting circuit is required for each heating resistor of the thermal head, and the circuit scale becomes enormous. Further, the usable thermal head is also limited to the DM type.

Therefore, as shown in Figure 2 (waveform (B)) and Figure 3, the drive pulse width corresponding to the maximum gradation level is equally divided by (the number of gradation levels - 1), and normal binary recording is performed. A method is proposed in which multi-gradation recording is performed by pulse width modulation by repeatedly driving (floor level - 1) times. In this case, a DD type thermal head can be used. However, as shown in Fig. 2, the pulse width (to'-to') + (t+'-to
')+..., (t7'-to'), the resulting recording density is LQ
'+LI' + . . ., as shown in L7', the number of executed gradations decreases, and the gradation properties in the intermediate area are impaired. Furthermore, if the ambient temperature fluctuates, even if the thermal head is driven at the same gradation level, the resulting recording density will not be constant.

On the other hand, when it is desired to equally divide the recording density obtained for a predetermined number of gradation levels into , Ll , ..., Lヮ, the drive pulse width (t
o-to), (tt-to), ...+ (ty-t
@) difference (b-tt-1) (t=t.

2,...,7) are not constant, and the smallest part is much smaller than the simple average value (tt-to)/7. In addition, in the case of a color thermal transfer printer, a second
Considering that the color development characteristics shown in the figure are different and further consideration is given to heat accumulation compensation, etc., the drive pulse width corresponding to each gradation level is determined by the above (ti
tf-1) (hereinafter abbreviated as ΔPmin).

Therefore, in the present invention, the recording surface MT for one line is divided into equal parts, with a cycle shorter than ΔPmin as the subline cycle Ts, and the drive pulse for each dot is generated by two binary data strings. Here, the recording characteristics shown in FIG. 2 will be explained as an example. From FIG. 2, the maximum pulse width (tt-to) is 4.
9 ms and ΔPmin is 0.3 ms, so the subline period Ts is now set to 0.1 ms. Then, t
The time from 0 to t7 is represented by a subline number as shown in FIG. Basically, as shown in FIG. 3, the generated binary recording data string is "1" after time 1° if the gradation level is Lt, and "1" after time ti.
0" data is generated. Since the method of the present invention uses subline numbers instead of time data as described above,
For example, if i=4, as shown in FIG. 4, "1" data is generated up to subline number 31, and "0" data is generated from subline number 32 onwards.

As an example of a circuit that implements this method, a circuit block diagram is shown in FIG. In FIG. 1, a ROM 1 outputs a subline number NSL that generates "0" data first in a binary data string to be generated, using gradation data as an address. CNT(1)2 corresponds to one line of subline numbers 0 to (K-
This is a counting circuit that generates 1). CHF2 is a comparator that compares the subline number Nst output from the ROMI with the subline number A31 output from the CNT(1)2 to generate a binary recording data string. RAM5 is CHF2
This is a memory that stores one line of the binary recording data string generated in . CNT(■) 3 is a counting circuit that generates a dot address Ad in one line. When writing a binary record data string to the RAM 5, one clock 2 is input each time the clock 1 is input, and the writing is completed when the clock 2 is input for the number of dots Nd of one line. Conversely, when reading a binary recorded data string from the RAM 5, one clock 1 is input every time Nd clocks 2 are input, and when one clock is input, one line of gradation recording ends. do. Actually CNT(1)2. CNT(If)
3 and I? Two AM5s and I1 are prepared, and writing/sequential output of R^fat is carried out exclusively by switching between the two sets for each line. The capacity of RAM5 is 2 in the example shown in Figure 4.
56KX1 focus is sufficient. (MXNd bit) Colorization or heat storage compensation can be easily handled by inputting color type or temperature data to 170M1.

The present invention is suitable not only for multi-gradation recording but also as a driving method for high-speed binary recording. In binary recording, the line recording period is becoming shorter than the main time constant of the thermal head. In such a case, it is effective to apply an auxiliary pulse to compensate for heat accumulation in the glaze layer, and if the present invention is applied as a means for generating the auxiliary pulse, it will be effective in increasing precision and downsizing the circuit. .

〔Effect of the invention〕

As described above, according to the present invention, it is possible to control multi-value driving using pulse width modulation in a DD thermal head with high precision, thereby significantly improving the quality of gradation recording, and improving color recording, heat storage compensation, etc. This can be easily dealt with and is extremely useful in practice.

[Brief explanation of drawings]

Figure 1 is a circuit block diagram for realizing the present invention, Figure 2 is a diagram showing recording characteristics and drive waveforms to explain pulse width modulation, and Figure 3 is a diagram showing recording characteristics and drive waveforms for explaining pulse width modulation. A diagram showing a binary recording data string in the value driving method, FIG. 4 is a diagram for explaining the recording density linearization method in the present invention, and FIG. 5 shows an outline of the thermal transfer recording bag 10) of the conventional line recording method. Figure 6 shows the structure of a conventional thermal head, Figure 7 shows the time-temperature characteristics of a heating resistor, and Figure 8 shows an example of the recording characteristic curve of a thermal printer. be. In Figure 1, 1 is a ROM, 2.3 is a counting circuit, 4 is a comparator,
5 is a RAM.

Claims (1)

[Claims] m pieces (m≧
2) n-bit latch and n-bit shift register (
In a thermal printer using a line-type thermal head equipped with m x n heat generating resistors (n≧2), the gradation level to be recorded is Li (i = 1, 2...m).
), the driving pulse width corresponding to the gradation level Li is Pi, the recording period of one line is T, and each period obtained by dividing the recording period T by K is a subline period Ts, then the driving pulse for each dot is j× Ts (j=0, 1,..., K-1)
It has means for sampling at each point in time and converting it into K binary data strings, and also calculates the difference in pulse widths corresponding to two adjacent gradation levels Li, Li_-_1 by ΔPi(
=Pi-Pi_-_1), the sub-line period Ts is set smaller than the smallest ΔPmin among ΔPi.