JP2761915B2 - Thermal printer - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a thermal printer of a density gradation control type in which the density gradation of each pixel is controlled by energizing each heating resistor for an energization time that is an integral multiple of the unit energization time. About. 2. Description of the Related Art A thermal line head in which a large number of heating resistance elements corresponding to pixels on one printing line in a one-to-one manner are arranged in a line is used in this type of density gradation control type printer. During printing, these heating resistance elements are pressed directly onto the recording paper or indirectly via an ink ribbon. Then, by selectively energizing the individual heating resistance elements in accordance with the image signal, the recording paper is colored in a dot shape, or is transferred from the ink ribbon to the recording paper in a dot shape, and one printing line at a time. Is recorded. By repeating such a printing operation in the paper feeding direction, for example, for 600 printing lines, one image is recorded. By the way, in such a thermal printer of the density gradation control type, there is one in which the printing speed can be switched from a normal low-speed mode to a high-speed mode. Conventionally, a method often used is a method in which the power supplied to the heating resistor element during energization in the high-speed printing mode is increased, thereby increasing the heating energy per unit time of the heating resistor element. According to this conventional method, for example, in order to obtain a constant density gradation D, in the high-speed mode, 32 times the unit energizing time is required, whereas in the high-speed mode, the energizing time is 16 times the unit energizing time. Will be done. Therefore, when the high-speed mode is set, the printing time for one printing line is reduced, and the recording time for one image is reduced. [Problems to be Solved by the Invention] In the above conventional method, the number of density gradations in the low-speed mode is, for example,

[1] ~

If there are levels, the levels are [1] to [32] in the high-speed mode, and

[2]
The [D] level in the high-speed mode corresponds to the D level. That is, in the high-speed mode, the number of density gradations becomes half of that in the low-speed mode, and the number of times of energization of the unit energization time is repeated, and thus the cumulative time of the unit energization time is reduced by half.
There is a problem that the density gradation of the pixel becomes coarse and the image quality remarkably deteriorates. Japanese Patent Application Laid-Open No. 60-9271 discloses a "halftone recording method for a thermal recording apparatus", which compensates for density changes due to external conditions such as environmental temperature and heat storage phenomena, so that the density of each gradation level becomes halftone. In order to maintain this, the number of pulse signals to be applied to the heating element was set in advance in accordance with the gradation level, and the number of pulse signals was controlled to change the energization time and divide the density equally. A desired temperature is obtained corresponding to the gradation level, and the temperature of the thermosensitive element or the vicinity thereof is detected, and the pulse width is controlled according to the temperature detection output, so that the density of the same gradation level can be obtained. There is disclosed a thermal printer which always keeps it constant. However, this thermal printer corrects gradation in accordance with the temperature of the thermal element or its vicinity, does not vary the unit energization cycle in accordance with the printing speed mode, and has an arrangement structure of the thermal element of 32. N thermal heads formed by linearly arranging the thermal elements are arranged and the thermal elements are arranged on one straight line. The n thermal heads U1 to Un are divided into eight blocks, and each thermal head is divided into eight blocks. A printing method in which time-division driving is performed in block order by a strobe signal output in time series corresponding to a block is adopted. For this reason, the seven blocks excluding the block being driven are kept in the non-conducting state during the energizing time of the block being driven, and the ratio of the driving period to the driving suspension period is 1: 7 for each block. Is the ratio of Therefore, the heat-sensing element can be cooled down to almost the steady-state temperature by the heat-dissipation cooling during the drive suspension period for each block until the next drive period arrives. When compared with the driving method, 1
The time required for printing a line is eight times as long. For this reason, while it is possible to maintain the printing quality while securing a sufficient cooling period for each block, there is a problem that printing takes too much time. In addition, if an attempt is made to shorten the printing time by high-speed printing, the unit energization cycle is naturally shortened. Therefore, for example, the non-energization time during the unit energization cycle is kept constant according to the same method as that corresponding to environmental changes. If the energization time is variably controlled while keeping the current, the duty ratio of the energization time in the unit energization cycle decreases as the energization time is shortened in accordance with high-speed printing. As a result, the gradation density changes to the light color density side. Therefore, there is a problem that the gradation density cannot be kept constant regardless of the printing speed. Japanese Patent Application Laid-Open No. 59-5772 discloses a thermal printer having a structure in which a unit energization cycle and an energization duty are changed according to different printing speed modes. In this apparatus, the unit energizing cycle is shortened and the energizing duty is increased at the time of high-speed printing. However, since one line printing cycle is not divided into the energizing interval and the cooling interval, temporarily When performing high-speed printing in which the energization duty needs to be increased to about 100%, the thermal element continues to be energized without cooling time, and as a result, it is inevitable that the gradation control will be disturbed due to the temperature rise. Considering that such disturbance of gradation control decisively degrades print quality, it is clear that the actual allowable energization duty is only about 70%, and there is a limit to high-speed printing. It was something to have. Furthermore, Japanese Patent Application Laid-Open No. 62-28265 discloses a thermal printer in which the thermal energy to be applied to a thermal head is varied according to the scanning linear velocity. However, this method also does not divide a one-line printing cycle into an energizing interval and a cooling interval, so that it is impossible to reduce the non-energizing time to zero in a unit energizing cycle, and it is not possible to reduce the level due to a rise in temperature. From the viewpoint of preventing disturbance of the tone control, there is a problem that high-speed printing by increasing the energizing duty has a certain limit. SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and has as its object to provide a thermal printer in which a difference in density gradation or image quality due to a change in printing speed is reduced. Means for Solving the Problems In order to achieve the above object, according to the present invention, a plurality of heating resistance elements provided in one-to-one correspondence with a plurality of pixels on a printing line and one printing line are printed. The line printing cycle is divided into an energizing interval and a cooling interval. In the energizing interval, a unit energizing cycle in which the occupation ratio of the energizing time and the non-energizing time differs depending on the printing speed is determined according to the density gradation for each heating resistor element. The printing control means for simultaneously executing the same number of times for all the heating resistance elements and stopping the energization in the cooling interval, and receiving a designation of a printing speed mode indicating the level of the printing speed. In addition to shortening the power-on cycle, the duty ratio of the power-on time in the unit power-on cycle is increased, and a substantially constant density gradation is given to each pixel regardless of the printing speed. Power supply mode switching means. Further, the energization mode switching means receives a mode selection switch for selecting a printing speed mode indicating the level of the printing speed, and a designation of the printing speed mode by the mode selection switch, and shortens the unit energization cycle as the printing speed increases. In addition, the CPU includes a CPU that increases a duty ratio of an energizing time in a unit energizing cycle and gives a substantially constant density gradation to each pixel regardless of a printing speed. [Operation] According to the present invention, a line printing cycle for printing one printing line is divided into an energizing interval and a cooling interval, and in this energizing interval, the occupation ratio of the energizing time and the non-energizing time differs depending on the printing speed. The energization cycle is executed by the number corresponding to the density gradation for each heating resistor element, energization is stopped in the cooling interval, and energization is performed in response to designation of the printing speed mode indicating the printing speed. The mode control means shortens the unit energizing cycle for high-speed printing, increases the duty ratio of the energizing time to the unit energizing cycle, and gives each pixel a substantially constant density gradation regardless of the printing speed. Example An example of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing a configuration of a main part of an embodiment of the thermal printer of the present invention. FIG. 2 is a timing chart of a line printing cycle in a low-speed mode by the thermal printer shown in FIG. FIG. 4 is a timing diagram of a unit energizing cycle in the low-speed mode by the thermal printer shown in FIG. 1, FIG. 4 is a timing diagram of a unit energizing cycle in the medium speed mode by the thermal printer shown in FIG. FIG. 6 is a timing chart of a line printing cycle in the high-speed mode by the thermal printer shown in FIG.
FIG. 4 is a timing chart of a unit energizing cycle in the high-speed mode by the thermal printer shown in FIG. In FIG. 1, a thermal line head 10 includes, for example, a heating resistor 12 in which 512 heating resistor elements R1 to R512 are arranged in a line, and a shift having the same number (512) of bit capacities as the heating resistor elements. A register 14 and a latch circuit 16 are provided. Further, a thermistor 18 for temperature compensation is provided close to the heating resistor 12, and its output signal is
The data is converted into digital temperature detection data DT by the A / D exchanger 44 and supplied to the CPU 40. During the printing time of each printing line, 51
The 512-bit serial gradation data [CkP1j to CkP512] for each of the two heating resistance elements R1 to R512 are continuously and repeatedly supplied to the shift register 14 a plurality of times, for example, 64 times (K = 1 to 64). . Here, the n-th bit CkPnj
Has information on whether or not the nth heating resistor Rn should be energized during the unit energization cycle ΔT.
That is, if "1", energization is instructed, and if "0", non-energization is instructed. When the grayscale data of each time is loaded into the shift register 14 in synchronization with the clock signal CK from the clock circuit 34, the latch signal LA from the latch signal generation circuit 36
CKP1j to CKP512j are latched by the latch circuit 16
Are supplied to the heating resistor 12 as electric pulses through the heating resistors 12, and the heating resistor elements R1 to R512 are selectively energized during the unit energizing cycle ΔT in accordance with the information content of the corresponding bits to generate heat. This unit energization cycle ΔT is actually equal to the heating resistance element R1
512R512 consists of a conduction time tE during which a current flows through R512 and a time tC during which no current flows. The length of the energization time tE is defined by the “L” level duration (pulse width) of the strobe signal ST bar supplied from the strobe signal generation circuit 38 in synchronization with the latch signal LA. In this embodiment, as will be described later, the “L” level duration of the strobe signal ST par is changed according to the printing speed mode, so that the ratio of the energization time tE to the unit energization cycle ΔT, that is, the energization duty ratio is changed. Can be The energizing operation of the unit energizing cycle as described above is repeated 64 times (K = 1 to 64) during the energizing interval TE of one printing line in accordance with the gradation data of 64 times (K = 1 to 64). The energization is performed for an energization time T64 corresponding to 64 times the time tE, and each of the pixels on one print line is given one of 64 density gradation levels. That is, the number of times of energization of the corresponding heating resistance element Rn is determined by the number of times of the grayscale data in which the information content of each grayscale bit CKPnj is “1”, and thereby the grayscale level of the density of the corresponding pixel is determined. For example, the gradation bit CKP1j
Assuming that "1" is continued until the gradation data of FIG. 10, in this case, CKPnj to C10Pij are each "1", C11Pij to C64Pij are each "0", and the heating resistance element R1 is energized for the unit energization cycle ΔT. The energization is performed for an energization time equivalent to 10 times tE, and the density gradation level of the corresponding pixel is

[10] The gradation data [CKP1j output from the data comparison circuit 28
CKCKP512] (K = 1 to 64) is formed as follows. First, the digital video signal DV is input to the frame memory 20 as pixel data. Each row of the frame memory 20 corresponds to a horizontal scanning line of a television image, and pixel data is written in an order corresponding to raster scanning. Next, starting from the first column of the frame memory 20, pixel data a1j, a2j,... A512j for one line are read out for each column (j) and supplied to the color processing circuit 22, where the data is inverted. After undergoing image processing such as gamma correction, they are converted into 8-bit density data b1j, b2j,... B512j. Each of these density data bij has a value (gradation level) corresponding to the density of the corresponding pixel within the range of << 0 >> (minimum density) to << 64 >> (maximum density). The density data b1j, b2j,... B512j for one printing line output from the color process circuit 22 are converted into, for example, 8-bit conduction time data B1j, B2 by the density-conduction time conversion circuit 24.
j,..., B512j. At the time of this conversion, the energization time data undergoes temperature correction according to the temperature correction value δb from the CPU 40. The energization time data B1j, B2j,... B512j for one printing line generated by the density-energization time conversion circuit 24 are once taken into the line buffer 26, and thereafter, the data comparison circuit 28
To one of the input terminals. The other input terminal of the data comparison circuit 28 is supplied with an 8-bit comparison reference value DN that is incremented by one at regular intervals from the gradation counter 32 during the energization interval TE. The data comparison circuit 28 compares this comparison reference value DN with each density data, and sets a bit of “1” when the latter is equal to or larger than the former, and sets a bit of “0” when the latter is not (smaller). The bits are generated as gradation bits. For example, assume that the values of the energized alicyclic data B1j, B2j,..., B512j are << 10 >>, << 2 >>,.
In this case, in the first comparison, the comparison reference value DN is << 1 >>
Then, the first tone data [CKP1j, C1
P2j,... C1P512j] becomes [1,1,. Second
In the second comparison, the comparison reference value DN << 2 >> is used, and the second gradation data [C2P1j, C2P2j,.
j] is [1,1,... 0]. In the third comparison, the comparison reference value DN << 3 >> is used and the third gradation data [C3
P1j, C3P2j,... C3P512j] is [1, 0,. In this way, each time the density gradation comparison reference value DN is incremented by one step during the energization interval TE, it is compared with each of the density data B1j, B2j,... B512j. Gradation data [C1P1j, C1P2j,...
C1P512j] [C2P1j, C2P2j,... C2P512j],... Are sequentially and serially transmitted to the shift register 14 of the thermal head 10 at a constant cycle. Next, with reference to the timing charts of FIGS. 2 to 6, differences in operation between the printing speed modes according to this embodiment will be described. 2 and 5 show the line printing cycle in the low speed mode and the high speed mode, respectively. The printing time TPL and the energizing interval TEL in the high-speed mode are much shorter than the printing time TPL and the energizing interval TEL in the low-speed mode.
Note that TCL and TCH are cooling intervals. 3 and 6 show the unit energization cycle in the low-speed mode and the high-speed mode, respectively. The unit energization cycle ΔTH in the high-speed mode is much shorter than the unit energization cycle ΔTL in the low-speed mode. However, the unit energization cycle TL
The ratio of the energization time tEL (tEH) to (ΔTH), that is, the energization duty ratio is much larger in the high-speed mode (100% in the illustrated example). As a result, the heat energy generated by the heating resistance element during both the unit energization cycles TL and ΔTH is maintained substantially the same. Therefore, even if the unit energization cycle, energization interval, and printing time are shortened by switching from the low-speed mode to the high-speed mode, the same number of unit energization cycle repetitions as in the low-speed mode, that is, the number of density gradations (up to 64) is obtained. And substantially the same heat generation energy can be maintained. As a result, even when the mode is switched to the high-speed mode, it is possible to obtain a density gradation property and a good image quality that are less different from those in the low-speed mode. On the other hand, in the low-speed mode, the heating duty element is sufficiently cooled in each unit energization cycle due to a small energization duty ratio. Control can be performed. FIG. 4 shows a unit energization cycle in the middle speed mode. Compared to the low-speed mode (Fig. 3), in the medium-speed mode, instead of the unit energization cycle ΔTM being somewhat shorter,
It can be seen that the energization duty ratio (t / ΔTM) increases somewhat. Generally, two modes of the low speed mode and the high speed mode will be sufficient, so that the medium speed mode may not be provided. In this embodiment, a mode selection switch 42 is provided to switch the printing speed mode as described above.
When this switch 42 is operated to the high-speed mode position, the CPU 4
0 controls each part as follows. First, the read cycle of the frame memory 20 is shortened so that the print time for each print line is set to a short value (TPH).
Through 0, the writing cycle of the line buffer 26 is shortened. Further, the read cycle of the line buffer 26 and the progressive cycle of the comparison reference value of the gradation counter 32 are shortened through the DMA controller 30 so that the unit energization cycle ΔT becomes a short value (ΔTH). Each cycle of the CK and latch signal generation circuit 36 is shortened. Then, increase the energization duty ratio to a large value (100%).
For the strobe signal generation circuit 38, the "L" level maintaining time of the strobe signal ST bar is increased. When switch 42 is operated to the low speed mode position, CPU 4
A value of 0 controls each unit to the opposite side so that the line printing time and the unit energization cycle are set to long values (TPL, ΔTL) and the energization duty ratio is set to a short value (for example, 60%). The strobe signal generation circuit 38 includes, for example, a plurality of mono-multi circuits that output “L” levels having different durations in response to the timing signals from the latch signal generation circuit 36,
And a select circuit for selecting an appropriate one of the mono-multis in accordance with the printing speed mode. As described above, the thermal printer executes the energization interval TE for all the heating resistor elements R1 to R512 at the same time, so that the heating resistor elements are divided into several blocks and all blocks are time-divisionally driven. It is possible to reduce the printing time comparable to the simultaneous driving. Also, since the cooling interval TC is always reserved after the energizing interval TE in one line printing cycle, it is sufficient for the heating resistance element that has performed the printing of the maximum gradation before starting the printing of the next printing line. Cooling time can be given. In addition, the unit energization cycle ΔT is shortened at the time of high-speed printing, and the duty ratio of the energization time tE in the unit energization cycle ΔT is increased, so that the cumulative time of the unit energization cycle ΔT of the number corresponding to the density gradation is increased. On the other hand, the energization duty ratio within the unit energization cycle tE is increased, while the energization duty ratio within the unit energization cycle tE is shortened to shorten the energization time occupying the energization interval TE, thereby reducing the heat generation energy generated by the heat generation resistance elements R1 to R512 seen in one line printing cycle. It can be kept almost constant, whereby a substantially constant density gradation can be given to each pixel regardless of the printing speed. Even if the energizing time occupying the unit energizing cycle at high-speed printing becomes 100% and the cooling period becomes 0%, the necessary cooling is performed at the cooling interval TC following the energizing interval TE, and the cooling time is 1%.
The heat generation energy generated in the line printing cycle can be maintained at substantially the same amount as that at the time of low-speed printing, and high-speed printing can be performed while achieving substantially constant density gradation control regardless of the printing speed. Also, the energization mode switching means is provided with a mode selection switch 42 for selecting a printing speed mode indicating the printing speed.
In response to the designation of the printing element mode by the mode selection switch 42, the unit energization cycle is shortened at the time of high-speed printing, the duty ratio of the energization time in the unit energization cycle is increased, and the printing speed is increased for each pixel. Because it is composed of CPU40 which gives almost constant density gradation regardless of
By operating the mode selection switch 42, a mode such as a high-speed printing mode or a low-speed printing mode can be designated, and the CPU 40 sets the unit energization cycle ΔT and the unit energization cycle in accordance with the printing speed mode selected by the mode selection switch 42. Since the duty ratio of the energizing time tE occupied is variably controlled, density gradation management can be performed with extremely high precision and strictness for each printing speed mode, and as a result, printing quality is somewhat inferior, but high-speed printing requires a shorter printing time. Choose or
Alternatively, it is possible to arbitrarily and arbitrarily select a low-speed printing which requires a relatively long printing time but has excellent printing quality. In the above embodiment, the CPU 40, which is a component of the energization mode switching means, is also a component of the printing control means. That is, the CPU 40 sets the energization interval TE
The unit energizing cycle in which the occupation ratio of the energizing time and the common energizing time differs depending on the printing speed
All heat-generating resistor elements R1 to R512
Executes all at once for R512, and also functions as a printing control center that stops energization during the cooling interval TC. In addition, both functions of the energization mode switching means and the printing control means can be classified and defined only by software that governs the operation of the CPU 40, and the same circuit elements share common hardware, so that Classification cannot be defined by visual inspection. [Effects of the Invention] As described above, according to the present invention, a line printing cycle for printing one printing line is divided into an energizing interval and a cooling interval. A unit energization cycle having a different energization ratio of energization time is simultaneously executed for all the heating resistance elements by the number corresponding to the density gradation for each heating resistance element,
In the cooling interval, the energization mode control means, which stops the energization and operates in response to the designation of the printing speed mode indicating the level of the printing speed, shortens the unit energizing cycle as the printing speed increases, and reduces the unit energization. Since the duty ratio of the energizing time in the cycle is set to be large and a substantially constant density gradation is applied to each pixel regardless of the printing speed, the energizing interval is simultaneously performed for all the heating resistance elements. In comparison with the method in which the heating resistor element is divided into several blocks and time-division driving is performed, the printing time can be shortened, which is equivalent to simultaneous driving of all blocks. In addition, the cooling interval always follows the energization interval in one line printing cycle. Before starting printing on the next printing line, even for the heating resistance element that has performed printing at the maximum gradation, It can give a minute cooling time,
Also, the higher the printing speed, the shorter the unit energizing cycle and the duty ratio of the energizing time in the unit energizing cycle is increased, so that the cumulative time of the unit energizing cycle of the number corresponding to the density gradation is shortened, and the energizing interval is reduced. On the other hand, the energizing duty ratio within a unit energizing cycle is increased, while the energizing duty ratio within a unit energizing cycle is increased, so that the heat generating energy generated by the heat generating resistive element seen in one line printing cycle can be kept almost constant, thereby An almost constant density gradation can be given to each pixel regardless of the printing speed.
Also, even if the energizing time in the unit energizing cycle becomes 100% during high-speed printing and the cooling period becomes 0%,
By performing necessary cooling in the cooling interval following the energization interval, the heat generation energy generated in one line printing cycle can be maintained at substantially the same amount as in low-speed printing, and is substantially constant regardless of the printing speed. Excellent effects such as high-speed printing while realizing density gradation control are achieved. Further, the energization mode switching means receives a mode selection switch for selecting a printing speed mode indicating the level of the printing speed, and a designation of a printing element mode by the mode selection switch, and shortens the unit energization cycle at a high speed printing. At the same time, the CPU includes a CPU that increases the duty ratio of the energizing time in the unit energizing cycle and provides a substantially constant density gradation regardless of the printing speed for each pixel. A mode such as low-speed printing mode can be specified, and the CPU can be selected according to the printing speed mode selected by the mode selection switch.
Variably controls the duty ratio of the unit energizing cycle and the energizing time in the unit energizing cycle, so that density gradation management for each printing speed mode can be performed very accurately and strictly. It is possible to arbitrarily and arbitrarily select a high-speed printing which requires a short time or select a low-speed printing which requires a relatively long printing time but has excellent printing quality.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration of a main part of an embodiment of the thermal printer of the present invention. FIG. 2 is a timing chart of a line printing cycle in a low-speed mode by the thermal printer shown in FIG. FIG. 4 is a timing diagram of a unit energizing cycle in the low-speed mode by the thermal printer shown in FIG. 1, FIG. 4 is a timing diagram of a unit energizing cycle in the medium speed mode by the thermal printer shown in FIG. FIG. 6 is a timing chart of a high-speed mode line printing cycle by the thermal printer shown in FIG. 1;
FIG. 3 is a timing chart of a unit energization cycle in a high-speed mode by the thermal printer shown in FIG. 10: Thermal line head 12: Heating resistor 14: Shift register 16: Latch circuit 28: Data comparison circuit 30: DMA controller 32: Gray scale counter 34: Clock circuit 36: Latch signal generation Circuit 38: Strobe signal generation circuit 40: CPU 42: Mode selection switch R1, R2, to R512: Heating resistance element

Claims (2)

(57) [Claims] 1. A plurality of heating resistance elements provided in a one-to-one correspondence with a plurality of pixels on a printing line, and a line printing cycle for printing one printing line is divided into an energizing interval and a cooling interval. In the interval, the unit energization cycle in which the occupation ratio of the energization time and the non-energization time differs depending on the printing speed, is simultaneously performed for all the heating resistance elements by the number corresponding to the density gradation for each heating resistance element, In the cooling interval, the printing control means for stopping the energization and the designation of the printing speed mode indicating the level of the printing speed, and the higher the printing speed, the shorter the unit energizing cycle and the energizing time of the unit energizing cycle. Energizing mode switching means for increasing the duty ratio and providing a substantially constant density gradation to each pixel regardless of the printing speed. Thermal printer. An energizing mode switching means for selecting a printing speed mode indicating a level of the printing speed, and receiving a designation of the printing speed mode by the mode selecting switch; And a CPU that increases a duty ratio of an energizing time in a unit energizing cycle and gives a substantially constant density gradation to each pixel regardless of a printing speed. Thermal printer.