Head controller of thermal printer and thermal printer...

Details Head controller of thermal printer and thermal printer... Head controller of thermal printer and thermal printer...

2000-09-06

2002-05-14

Tran, Huan (Department: 2861)

Incremental printing of symbolic information

Thermal marking apparatus or processes

Gradational recording

C347S211000

Reexamination Certificate

active

06388691

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a printer printing an image with a line head and particularly relates to a printer characterized by having a line head controller for controlling the line head.
2. Description of the Related Art
A thermosensitive printer (thermal printer) is a widely used printer type. The thermal printer applies current according to gradations (image tones from white to black) to the head elements of a head, thereby transferring a thermal-melting or thermal-sublimating material coated on a ribbon to a medium by the heat of the head elements or coloring a thermosensitive material coated on a medium by the heat of the head elements.
FIG. 1
shows one example of the constitution of a line head controller which is a circuit provided to control current applied to the respective head elements of the line head in a conventional thermal printer for printing an image by the line head. In this line head controller (to be referred to simply as head controller hereinafter)
51
, N pieces of image data for one line among the image data represent gradations at the positions of dots on the respective lines of a medium width, for example, eight bits (i.e., with 256 density levels), respectively. If the number of dots per line is, for example, 2056, then 2056 image data are written to a line memory
52
.
A gradation counter
53
is a counter which counts up gradation data from a minimum value 0 to a maximum value 255 of 256 gradations each represented by eight bits, with a clock signal CK used as an operating clock, after the counter
53
is reset at an initial value 0 by a reset signal RS.
First, when the value of the gradation counter
53
is 0, image data are read out one by one from the line memory
52
and a comparator
54
compares each of the image data with the value of the gradation data. When the value of the image data is higher than the value of the gradation data, a H (high) signal CP is outputted from the comparator
54
and when lower, an L (low) signal CP is outputted from the comparator
54
.
The signals CP each indicating a comparison result as well as shift pulses (not shown) are sequentially fed from the head controller
51
to a sift register
62
in the line head (to be referred to simply as head hereinafter)
61
. As a result, by the time the comparator
54
completes comparison for all the image data for one line read from the line memory
52
, signals indicating whether the density levels of the image at the respective dot positions on this one line are higher than 0 are stored in the shift register
62
in the head
61
.
When the comparator
54
completes comparison with respect to all the image data corresponding to one line and read from the line memory
52
, the head controller
51
allows the signals stored in the shift register
62
in the head
61
to be outputted simultaneously and to be latched by a latch circuit
63
in the head
61
. The respective signals latched by the latch circuit
63
are supplied to head elements in the head
61
.
FIG. 2
shows one example of the structure of the head element. Transistors
65
are arranged on a substrate (not shown in
FIG. 2
) so as to correspond to the respective positions of dots on one line (therefore, the number of transistors
65
is N, equal to the number of dots per line), and each of the transistors
65
functions as one piece of the head element. The collector of each transistor
65
is connected in parallel to a semiconductor
66
through a resistor
64
. Both ends of the semiconductor
66
are connected to a power terminal
68
using a copper wire
67
. The emitter of each transistor
65
is connected to an earth terminal
70
using a copper wire
69
.
Each of the signals latched by the latch circuit
63
shown in
FIG. 1
is supplied to the base of each transistor
65
at the position of a corresponding dot. Thus, current is applied only to transistors
65
to the bases of which H signals are supplied (which transistors
65
are located at positions corresponding to the positions of dots having density levels higher than 0) and only these transistors
65
are heated.
Next, in the head controller
51
, the gradation counter
53
counts up gradation data by one. Image data are read again from the line memory
52
one by one and each of the image data is compared with the value 1 of the gradation data by the comparator
54
. When the comparison is completed, the signals stored in the shift register
62
in the head
61
are latched anew by the latch circuit
63
in the head
61
. By doing so, current is applied only to transistors to which bases H signals are supplied (which transistors
65
in this case are located at positions corresponding to the positions of dots having density levels higher than 1) and only these transistors
65
are therefore heated.
Thereafter, the head controller
51
continues the same processings repeatedly, while the gradation counter
53
counts up gradation data one at a time up to a maximum value 255. As a result, H signals (i.e., PWM signals which density levels are modulated to pulse widths) are supplied to the bases of the respective transistors
65
for a period of time according to gradations at their corresponding dot positions. Therefore, current flows for a period of time according to the gradations. A thermal-melting or thermal-sublimating material coated on a ribbon is transferred to a medium by the heat of the respective transistors
65
(or a thermosensitive material coated on the medium is colored as a result of heat generated at the transistors
65
due to the application of current to the transistors
65
), whereby an image corresponding to one line is written on the medium.
After the image corresponding to one line is written, the gradation counter
53
is reset at the initial value 0 in the head controller
51
and image data corresponding to the next line are written to the line memory
52
. The above-stated steps are conducted repeatedly for these image data.
In the meantime, according to the conventional head controller
51
as stated above, while the value of the gradation data at the gradation counter
53
is close to a minimum value 0, H signals CP are outputted from the comparator
54
for almost all image data read from the line memory
52
. During this period, therefore, current is simultaneously applied to almost all transistors
65
of the head
61
.
As stated above, if current is simultaneously applied to almost all transistors
65
, high current flows through the semiconductor
66
and the copper wire
67
(a resistor common to the transistors
65
) correspondingly. If the number of transistors
65
is 2056, current flowing through the common resistor (to be referred to as “common resistance” hereinafter) is, for example, 8 to 10 amperes. If such high current is applied to the common resistance, the following disadvantages occur.
(1) Generally, in a thermal printer, the length of time for allowing the latch circuit
63
of the head
61
to latch signals (i.e., the length of time for applying current to the transistors
65
) is corrected within a range of a cycle in which the gradation counter
53
counts up gradation data, in accordance with the number of image data for which H signals CP are outputted from the comparator
54
of the head controller
51
, thereby suppressing the occurrence of uneven printing derived from a voltage drop at the common resistance (particularly the voltage drop at the semiconductor
66
), which correction will be referred to as “number correction”. If current as high as 8 to 10 amperes is applied to the common resistance, however, the voltage drop becomes larger at the common resistance. Thus, even if this number correction is made, it becomes impossible to suppress the occurrence of uneven printing.
(2) The period of time for which current flows through almost all transistors
65
is a period for which the value of the gradation data at the gradation counter
53
is close to a minimum value 0. For that reason, although the period is only part of th

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