JPS63199660A - Drive controller for thermal head - Google Patents

Abstract

PURPOSE:To inhibit a rush current at the start of printing and to print a sharp image, by a method wherein the drive of a resistance element is made to complete at a fixed point in a printing cycle when dots from the first step of gradation to a predetermined step of gradation are obtained, and it is started from the front end of the printing cycle when dots from the predetermined step of gradation to the n-th step of gradation are obtained. CONSTITUTION:Data SD for one line are supplied to an input terminal 11 and further supplied to a line memory 12 having a capacity for one line, thereafter being transferred to a line memory 13 at a predetermined timing. The inputted data for 512 dots are successively read dot by dot by a 512-notation address counter 14 and added to a comparison circuit 15. In the comparison circuit 15, the data for one line including 512 dots are compared with reference gradation data correspondingly one by one. PWM data each having a different pulse width are serially outputted from the comparison circuit 15 according to each gradation for 512 dots and supplied to a shift register 19. Head elements 41-4512 are made conductive only for a time of period according to the pulse widths of the respective PWM data.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a device for controlling the drive of a thermal head used in a color printing device or the like.

[Summary of the invention]

In the case of expressing dots in n gradations, the present invention drives the resistance element of the thermal head at a predetermined point in the print cycle when obtaining the dots from the first gradation to a predetermined gradation. At the same time, when obtaining the dots from the predetermined gradation level to the nth gradation level, the driving of the resistance element is controlled to start from the front end of the printing period, thereby controlling the rush. This is designed to reduce the current and the shift of the printing center of each color, and to prevent printing time from being eaten into the data transfer period.

[Conventional technology]

FIG. 4 schematically shows a printing device using a thermal head, in which an ink ribbon 2 and printing paper 3 are wrapped around a rotating drum 1 in an overlapping manner, and a thermal head device 4 is brought into contact with the ink ribbon 2. It is configured to allow In this state, the ink on the ink ribbon 2 is thermally transferred to the print paper 3 by rotating the rotating drum 1 in the direction of arrow a and driving the thermal head device 4 while continuously feeding the ink ribbon 2 and print paper 3. Then, the desired image or the like is printed.

Ink ribbon 2 is Y (yellow) and M (magenta).
, C (cyan) are used, and color printing is performed by sequentially overlapping and printing these three colors.

FIG. 5 shows the relationship between the print screen 6 and the thermal head device 4 when a print screen 6 consisting of pixels 5 of m dots in the vertical direction and n dots in the horizontal direction is printed on the print paper 3. is pixel 5 of m dots in the vertical direction
It consists of thermal head elements 4+, 4g-4, which are composed of m resistive elements, respectively, and these head elements 4
．． ~4. The image is printed on the printing screen 6 by selectively driving the image forming apparatus according to the image to be printed. Therefore, when printing the screen 6, one line consisting of m dots of pixels 5 in the vertical direction is simultaneously printed while the printing paper 3 is continuously fed in the direction of the arrow a. 6 is formed.

Each pixel 5 expresses the color density of each color using gradations, for example, 256 gradations. In order to express these 256 gradations, each head element 4. ~45 energization time (heating time)
is controlled in 256 steps. For this purpose, PWM (Pulse Width Modulation) signals are supplied to each of the head elements 4I to 4° in accordance with the gradation.

Each head element4. ~4. As a method of supplying the above-mentioned PWM signal to the motor, the energization method shown in FIG. 6 or FIG. 7 is known.

In the method shown in FIG. 6, printing of pixels 5 in one line in the vertical direction is started at the same time, and in the example shown, m=512
In the case of dots, the first dot of one vertical line has a gradation level of 1.2 dot has a gradation level of 3.3rd dot has a gradation level of 3.51.
A case where the second dot represents floor 115 is shown as an example. A PWM signal having a pulse width corresponding to these gradations is sent to the head elements 41 to 4. supplied to

The method shown in FIG.
This is a method of simultaneously ending energization by the WM signal.

Further, in the methods shown in FIGS. 6 and 7, when printing of one line is completed, a data transfer period is provided in which PWM data of the next line is transferred to the memory.

Furthermore, when printing one line, the energization time is actually corrected in accordance with the number of head elements to be energized as follows.

Head element 4. ~4. The m resistance elements for heating are connected to a common heater power source through m transistors, and each transistor is connected to a PWM signal.
Each resistance element is energized by control. Therefore, a voltage drop occurs due to the wiring resistance, the characteristics of the heater power supply, etc., and the magnitude of this voltage drop changes depending on the number of head elements to which electricity is applied when printing one line. For this reason, the current supplied from the heater power source changes, and as a result, the color density of the pixel may change even if the pixel is energized with a PWM signal of the same pulse width.

In order to solve this problem, the number of head elements to be energized when printing one line is determined, and the energization time is corrected according to the number.

This correction is performed by extending the energization time for each gradation. Regarding the correction of the energization time, the present applicant has proposed an invention in Japanese Patent Application No. 101394/1983.

[Problem that the invention seeks to solve]

The above-described energization method shown in FIG. 6 starts energization of each head element at the same time, and therefore has the drawback that a very large rush current flows at the start, as shown in FIG. In addition, this energization method can be used, for example, when the gradations of each color Y, M, and C for one pixel 5 are different, as shown in FIG.
Since the energizing time for each color is different, the printing center is shifted for each color, resulting in color bleeding. For this reason, the colors of the entire image may appear visually muddy.

In addition, in the energization method shown in Fig. 7, if the energization time is extended as shown by the dotted line in Fig. 7 in order to correct the energization time described above, this extended time may be inserted into the next data transfer period. there were. If the energization time intrudes into the data transfer period in this way, all the data will not be transferred and the next line will not be printed properly. If the extension of the current application time is sacrificed for this reason, correct color density cannot be obtained.

Particularly in the case of low gradations, omitting the extension time will have a large effect. It is also conceivable to lengthen the data transfer time or provide a buffer memory, but doing so would result in new problems such as longer printing time and more complicated circuitry. It has been confirmed that the energization method shown in FIG. 7 improves the color shift problem described in FIG. 8 compared to the energization method shown in FIG. 6.

[Means for solving problems]

In the present invention, when obtaining dots from the first gradation opening to a predetermined gradation level, there is provided a means for controlling the driving of the resistive element to end at a predetermined point in the printing cycle, and When obtaining the dots up to the n-th gradation, means is provided for controlling the driving of the resistive element so as to start from the front end of the printing period.

[Effect]

Since the number of head elements that start energizing at the same time is small, rush current can be reduced.

Furthermore, since the printing centers of each color substantially coincide, color misregistration is reduced. Furthermore, since there are few head elements whose energization ends at the same time, the energization time due to correction does not eat into the data transfer period.

〔Example〕

First, the energization method according to the present invention will be explained in principle with reference to FIG.

In Figure 3, the first half 1 to 128 of the total 256 gradations
For the gradation, use the method shown in Figure 7 above, and use the latter half 129 to 2.
For 56 gradations, the method shown in FIG. 6 described above is used.

According to this energizing method, when printing one line, the number of head elements that start energizing simultaneously is often smaller than in the case of FIG. 6, so that rush current can be suppressed. In addition, the printing centers for each gradation are substantially coincident, and the color shift as shown in FIG. 8 is reduced compared to the method shown in FIG. 6. Furthermore, the extended time of the energization time due to the above-mentioned correction of the energization time does not intrude into the data transfer period, as shown by the dotted line. For 256 gradations, the above-mentioned extension period will cut into the data transfer period, but in that case, priority should be given to the data transfer period and the extension period may be omitted. Since the 256th gradation has the highest color density, there is virtually no effect even if the extension time is omitted.

Next, an embodiment of the present invention based on the above-mentioned principle will be described with reference to FIG.

In FIG. 1, an input terminal 11 is supplied with one line of data SD obtained from a digital still image signal.

This data SD is data of 512 dots □ in which each dot is expressed in 8-bit gradation. After this data SD is supplied to a line memory 12 having a capacity for one line, it is transferred to the next stage line memory 13 at a predetermined timing.

This transfer period corresponds to the data transfer period in FIGS. 3, 6, and 7.

The 512 dots of data taken into the line memory 13 are converted into 1 dot (
8 bits) are sequentially read out and applied to the comparator circuit 15. The address counter 14 has a clock generator 1.
A data clock GK from 6 is supplied via a switch 17.

The comparison circuit 15 is composed of, for example, an exclusive OR circuit,
The data sent in 8-bit units from the line memory 13 and the 8-bit data sent sequentially from the 256-decimal gradation counter 17
Compare the bit reference gradation data. This reference gradation data is data of 1 to 256 gradations, and is supplied to the comparison circuit 15 through the reference gradation data control circuit 18 one gradation at a time.

In the comparison circuit 15, data for 512 dots of l line are compared one by one with respect to one reference gradation data. The gradation counter 17 is advanced by one gradation by a carry pulse outputted every time the address counter 14 reads one line of data.

Therefore, the comparator circuit 15 serially outputs PWM data having different pulse widths in accordance with each of the 512 dots. This PviM data is supplied to the shift register 19. This shift register 19 is connected to the clock C.
K, is used as a shift clock. The 512 PWM data stored in the shift register 19 are then transferred to the latch circuit 21 at the timing of the fall of the strobe pulse STP output from the 512-ary strobe counter 20 (to be described later), and then sent through the drive circuit 22 to the thermal sensitive Head elements 41 to 4 of the head device 4. I! added to. As a result, the head elements 41-4. l! is energized for a time corresponding to the pulse width of each PWM data.

The reference gradation data control circuit 18 controls the reference gradation data from the gradation address counter 17 in order to carry out the above-described energization method shown in FIG. It is configured as shown. Control of the reference gradation data by the control circuit 18 is performed as follows.

First, during a period in which the reference gradation data output from the gradation counter 17 is in the 1st to 128th gradations, the inverter 24 is operated and the buffer 23 is inoperative. For this purpose, the most significant bit (MSB) of the 8-bit data output from the gradation counter 17 is added to the buffer 23 to stop its operation, and the inverter 24 is operated via the inverter 25. This MSB is also applied to comparator circuit 15.

As a result, the lower 7 bits of data from the gradation counter 17 are inverted by the inverter 24 and applied to the comparison circuit 15 together with the MSB. As a result, from the comparison circuit 15,
PWM data in which pulses fall simultaneously during the 1st to 128th gradation period in FIG. 3 is obtained.

Next, during the period in which the gradation counter 17 indicates 129 to 256 gradations, the MSB is inverted from rOJ to "1", so
Inverter 24 is stopped and buffer 23 is activated. As a result, the lower 7 bits of data from the gradation counter 17 are applied to the comparison circuit 15 together with the MSB through the buffer 23. As a result, 129-2 in Figure 3
PWM in which pulses rise simultaneously in the period of 56 gradations
Data is obtained. Thus, the energization method according to the present invention shown in FIG. 3 is carried out.

Next, the operation of correcting the energization time described above will be explained with reference to FIGS. 1 and 2.

In FIGS. 1 and 2, the clock selection switch 27
selects two clocks CK, , CK, with different frequencies output from the clock generator 16 and supplies them to the correction counter 26. When inputting data to the shift register 19, the data clock GK is corrected. counter 2
6, and when correcting the energization time, use the correction clock CK.
t is supplied to the counter 26. This switch 27 and switch 17 are switched at the timing shown in FIG. 2 by the strobe pulse STP.

The strobe counter 20 counts the clock CK and outputs a strobe pulse STP that rises when the count value reaches 512. This pulse ST
The pulse width of P is determined by the carry pulse from the correction counter 26, as will be described later. This pulse STP
At the rising edge of , the latch circuit 7 latches the PWM data. Therefore, depending on the pulse width of the pulse STP, the transmission of one unit gradation to the head elements 41 to 4 s+z is determined by the width of the pulse STP.
, %+ '-' fl! I-stop data low data counter 26
For example, when "12" is input from the inverter 28, "10" is loaded into the counter 26 as correction data. The counter 26 counts only the H signal of data for 512 dots, and when this count is completed, the strobe pulse ST of the strobe counter 20 is
At the rising edge of P, the above count value changes to inverter 11 and R.
OM: is fed back to the correction counter 26 via OM:. This feedback correction counter counts up to 512 CKs for correction from the loaded value. When this count value reaches 124, a carry pulse is sent to the strobe counter 20 and the strobe pulse ST-
Therefore, if the loaded value is large, the counter value will decrease quickly < r512J, and if the loaded value is small, it will take time for the counter value to reach 2, and a carry pulse will be generated. The timing is 1L.

In this way, the strobe counter 20 receives the clock C.
A strobe pulse STP that rises when 512 K is counted and falls at the carry pulse from the correction counter 26 is formed. This pulse STP is supplied to the latch circuit 21, the correction counter 26, the switch 17.27, and the reset terminal R of the gradation counter 17. Note that during the above-mentioned correction period, the switch 17 is turned off and the clock CK1 is interrupted.

The latch circuit 21 launches the PWM data from the shift register 19 at the falling edge of the pulse STP, and updates the PWM data at the falling edge of the next pulse STP. Therefore, the energization time is extended by the pulse width of the pulse STP.

When the above steps are completed, the operation for the second gradation is performed in the same way, and when this is completed up to 256 gradations, printing of the next line is started, and this is repeated n times to finish printing, for example, Y.

〔Effect of the invention〕

According to the present invention, when printing one line, the number of head elements that start energizing at the same time is reduced, so that rush current at the start of printing can be suppressed.

Furthermore, since the printing center of the pixel is at approximately the same position for each color, color shift due to deviation of the printing center for each gradation is also reduced. Furthermore, the extension period of the energization time due to correction of the energization time does not intrude into the data transfer period. Therefore,
It is possible to print clear images without color turbidity.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an embodiment of the present invention, FIG. 2 is a timing chart showing the operation of FIG. 1, and FIG. 3 is a timing chart for explaining the principle of the embodiment of the present invention.
FIG. 4 is a perspective view of a printing device to which the present invention can be applied, FIG. 5 is a plan view showing the relationship between the thermal head device and the print screen, and FIGS. 6 and 7 are energization of the thermal head device by a PWM signal. FIG. 8 is a timing chart showing the method, and is a diagram showing color shift due to the energization method shown in FIG. 6. 4-------111------------- Thermal head devices 41 to 4, ------- Head element 13---------- −−・−−−−−1−−・Line memory 15・−・−−−−−−1−−−・・−−−11 Comparison circuit 17−・−・・−・−・−Gradation counter 18
・−・−・・−・−・−・・−Reference gradation data control circuit.

Claims (1)

[Claims] A thermal head that obtains dots expressed in n gradations by controlling the drive time of a resistive element within a predetermined printing cycle, from the first gradation to the predetermined gradation. When obtaining the dots from the predetermined gradation level to the nth gradation level, a means for controlling the driving of the resistor element to end at a predetermined point in the printing cycle; A drive control device for a thermal head, comprising means for controlling the drive of the resistor element so as to start from the front end of the print cycle.