Printing head drive apparatus and method for driving printing head

A printing head drive apparatus comprising: a plurality of printing elements which operate upon receipt of electric power; switching elements which are provided for the respective printing elements and which supply power to the corresponding printing elements; a control signal output section which outputs a control signal for actuating the respective switching elements in response to print data; and a timing control section which arbitrarily sets output timing of the control signal for each printing element.

BACKGROUND OF INVENTION 
The present invention relates to a printing head drive apparatus for 
printing an image through use dots arranged in a staggered pattern by a 
thermal head or the like, and a method for driving a printing head. 
For instance, in a thermal printer, an image is formed through use of a 
thermal head having heating elements placed in a line thereon. The thermal 
head forms an image on paper by controlling heating of each heating 
element corresponding to each dot. The outline of a printer equipped with 
a thermal head will be described by reference to FIG. 5. 
A print data signal is input to the printer in a serial manner in 
accordance with a data transfer clock signal. The printer stores the thus 
input print data into a shift register 114. The instant when print data 
for one line are completely received, a latch circuit 115 latches the 
print data. 
The latch circuit 115 actuates in unison switching elements 112 provided 
for respective heating elements 111, by outputting a strobe pulse signal 
in synchronism with a strobe synchronization signal. Consequently, the 
heating elements 111 whose switching elements 112 are brought into 
conduction receive power from a common electrode 125, thus becoming 
heated. By virtue of the heat, dots are formed on paper. 
The conducting state (i.e., an ON/OFF state) of each switching element 112 
changes in accordance with print data. A period of time during which the 
switching element remains in a conducting state also changes in accordance 
with print data for the dots (an area ratio of dots). 
Recently, there has been employed a control technique whose principal 
purpose is to improve image quality and which involves forming pixel dots 
in a staggered pattern on paper by shifting timing at which a voltage is 
applied to the heating elements (i.e., heating timing). such a technique 
is described in, e.g., Japanese Patent Application Laid-open Hei. 
7-312677. 
Because of circuit configuration, heating is commenced in a synchronized 
manner for all the heating elements 111 of the existing printer (see FIG. 
5) used for forming one line, and the timing at which a voltage is applied 
to the heating element cannot be uniquely controlled an 
each-heating-element basis (nor on a group-by-group basis, provided that 
the heating elements for one line are divided into a plurality of groups). 
Accordingly, to commence application of a voltage to a certain group of 
heating elements, application of voltage to another group of heating 
elements must be completed. 
In order to divide the heating elements for one line into a plurality of 
groups and to apply a voltage to the groups at different timing, a time 
period capable of being assigned to one group (i.e., the length of time 
during which a voltage can be applied to the group) must be reduced to a 
time period shorter than that used for a printer which does not apply a 
voltage to the heating element at shifted timing. 
Further, because of circuit configuration, print data must be input to the 
printer immediately before application of a voltage to the heating 
elements. Since solely the timing of heating of an element cannot be 
shifted without reference to transfer of print data, there arises dead 
time during which print data are transferred. Accordingly, in effect, the 
length of time during which a voltage can be applied (i.e., the width of a 
strobe pulse) is reduced to a greater extent. 
FIG. 6 shows strobe pulse signals for black (K) and cyan (C) in a case 
where the timing of heating is controlled by dividing the heating elements 
into an odd-numbered group and an even-numbered group. As shown in FIG. 6, 
the length of time capable of being assigned to each group is reduced to a 
value (e.g., 3.5 ms in the example) less than one-half that used for the 
known printer (i.e., a printer in which all the heating elements used for 
forming one line are heated in a synchronized manner). 
Such being the case, if dots are output at an area ratio of 100% (i.e., 
what is called a solidly shaded image is formed), clearance is created 
among the dots, resulting in a failure to reach an area ratio of 100%. For 
this reason, in a case where dots having a large area ratio are formed, 
the voltage applied to the heating elements by way of the common electrode 
is increased to a value greater than an ordinary voltage to thereby supply 
greater power (or energy) to the heating elements within a shorter period 
of time, thus compensating for creation of clearance. 
However, an increase in voltage results in an excessive increase in the 
peak temperature of the head, and anomalies stemming from overheat 
sometimes arise in printing material. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a printing head drive 
apparatus capable of reliably printing tone in accordance with the tone in 
the area (particularly, dots having an area ratio of 100%) without 
application of an excessive voltage, and a method for driving the printing 
head. 
A printing head drive apparatus according to the present invention, 
comprises: a plurality of printing elements which operate upon receipt of 
electric power; switching elements which are provided for the respective 
printing elements and which supply power to the corresponding printing 
elements; a control signal output section which outputs a control signal 
(a strobe signal) for actuating the respective switching elements in 
response to print data; and a timing control section which arbitrarily 
sets output timing of the control signal for each printing element. 
The printing elements may include heating elements which are heated upon 
receipt of the electric power. 
Further, the timing control section is capable of arbitrarily setting the 
output timing of the control signal for each of groups into which the 
printing elements are classified according to the sequence in which the 
elements are placed. 
Still further, the timing control section is capable of arbitrarily setting 
the output timing of the control signal according to an external signal. 
A thermal head drive apparatus according to the present invention, 
comprises: a plurality of thermal elements which are heated upon receipt 
of electric power; switching elements which are provided for the 
respective heating elements and which supply power to the corresponding 
heating elements; a control signal output section which outputs a control 
signal for actuating the respective switching elements in response to 
print data; and a timing control section which arbitrarily sets output 
timing of the control signal for each heating element. 
A method, according to the present invention, for driving a printing head 
which has a plurality of printing elements which operate upon receipt of 
electric power, the method comprising the steps of: generating a control 
signal for driving switching elements to control electric power supply to 
the printing elements in response to print data; and setting arbitrarily 
output timing of the control signal for each printing element. 
Further, in the setting step, the output timing of the control signal is 
arbitrarily set for each of groups into which the printing elements are 
classified in accordance with a sequence in which they are placed. 
Still further, the setting step, the output timing of the control signal is 
set according to an external signal.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
With reference the accompanying drawings, a preferred embodiment of the 
present invention will be described hereinbelow. 
A printing head drive apparatus according to the present embodiment is 
designed so as to be able to set the start timing of heating for each of 
groups into which heating elements of the printing head, for instance a 
thermal head, are divided. Particularly, according to the present 
embodiment, independent control of start timing of heating is effected 
without an accompanying reduction in a time during which a voltage can be 
applied to each of the heating elements (i.e., the maximum width of a 
strobe pulse signal), by adoption of a drive apparatus capable of shifting 
the actuation timing of a switching element of each heating element. 
FIG. 1 shows the printing head drive apparatus according to the present 
embodiment. In this embodiment, the thermal head is provided as a printing 
head. As shown in FIG. 1, the thermal head drive apparatus comprises 
heating elements 11, switching elements 12, a shift register circuit 14 
having a control signal output section, latch circuits 15a, 15b, 15c, and 
15d, a timing control circuit 13 which controls actuation timing of the 
switching element 12, a power supply circuit 20, and a control circuit 30, 
which produces a strobe synchronization signal. Further, the thermal head 
drive apparatus includes wiring for connecting the above-described 
elements together (e.g., a common electrode 25 which connects a power 
circuit 20 to the heating elements 11). 
The heating elements 11 generate heat required to form dots on paper, from 
the power supplied from the power supply circuit 20. In the present 
embodiment, the heating elements 11 are placed in a line and are divided 
into four groups, thus enabling the start timing of heating to be 
controlled for each group by means of individual sections, which will be 
described later. 
The heating elements 11 are classified into groups according to the 
sequence in which they are placed (or according to the system of residues 
of four of their positions in the line in the present embodiment). More 
specifically, the heating elements are classified into a group of heating 
elements 1, 5, . . . (4N-3) (hereinafter referred to as a "first group"), 
a group of heating elements 2, 6, . . . , (4N-2) (hereinafter referred to 
as a "second group"), a group of heating elements 3, 7, . . . , (4N-1) 
(hereinafter referred to as a "third group"), and a group of heating 
elements 4, 8, . . . , (4N) (hereinafter referred to as a "fourth group"). 
In the drawing, the start timing of heating for each of the first through 
fourth groups of colors (K, C, M, Y) is provided in the form of a table. 
The heating elements 11 are placed in the widthwise direction of printing 
paper (or the primary scanning direction). A thermal head is positioned in 
such a way that the line of the heating elements 11 becomes orthogonal to 
the direction in which paper is fed (or the secondary scanning direction). 
The power is supplied to the heating elements from the power supply 
circuit 20 by way of the common electrode 25. 
The switching elements 12 act to selectively set the power supply to the 
heating elements 11 (or application of a voltage to the heating elements) 
to a conducting/nonconducting state and are provided for the respective 
heating elements 11. The conducting state (or ON/OFF state) of each 
switching element 12 is controlled by a corresponding latch circuit 15. 
In the present embodiment, when the strobe pulse signal output from the 
latch circuit 15 is low, the switching element is brought into conduction 
(or a voltage is applied to the switching element). In contrast, when the 
strobe signal is high, the switching element is brought out of conduction. 
Further, the time during which each switching element is in conduction (or 
the width of the strobe pulse signal) is changed according to the input 
print data so as to correspond to the switching element 12 (i.e., the area 
ratio of dots to be formed by the heating element 11). 
The timing control circuit 13 produces four types of strobe synchronization 
signals, which differ in timing from one another, by shifting the strobe 
synchronization signal received from the control circuit 30 (hereinafter 
referred to as an "original strobe synchronization signal") by merely a 
given amount. The timing control circuit 13 has a variable delay line 
capable of being delayed every line. 
As shown in FIG. 1, with regard to black (K) according to the present 
embodiment, the first and third strobe synchronization signals have the 
same timing as that of the original strobe synchronization signal (i.e., 
delay time=0). In contrast, the second and fourth strobe signals are 
delayed from the original strobe synchronization signal by 3.5 ms. 
Likewise, with regard to the other colors (i.e., cyan (C), magenta (M), 
and yellow (Y)), the amount of shift in the strobe synchronization signals 
is set. For cyan (C) and yellow (Y) colors, the first and second strobe 
synchronization signals have the same timing, and the third and fourth 
strobe signals have the same timing. In short, the timing of voltage 
application is set so as to change every two heating elements. For magenta 
(M) color, the timing of voltage application is set so as to become 
alternate between two values (i.e., odd-numbered strobe synchronization 
signals have one timing and even-numbered strobe synchronization signals 
have another timing), as in the case of black (K) color. 
The shift register circuit 14 stores print data for one line input in a 
serial manner in accordance with a data transfer clock signal. 
The latch circuits 15a, 15b, 15c, and 15d latch the print data stored in 
the shift register circuit 14. The latch circuits 15a, 15b, 15c, and 15d 
control the conducting state (or ON/OFF state) of the switching element 12 
by outputting a strobe pulse signal in accordance with the timing of the 
strobe synchronization signal received from the timing control circuit 13. 
As mentioned previously, when the strobe pulse signal from the latch 
circuit 15 is low, the switching element 12 is brought into conduction. In 
contrast, when the strobe pulse signal is high, the corresponding 
switching element 12 is brought out of conduction. In such a case, the 
timing at which the switching element 12 is brought into conduction (i.e., 
the start timing of heating) changes according to the strobe pulse signal 
input to each of the latch circuits 15a, 15b, 15c, and 15d. The duration 
of the strobe pulse signal (i.e., a pulse width) also changes depending on 
the input print data so as to correspond to the respective hating element 
11 and the respective switching element 12. 
The latch circuit 15a corresponds to the heating element 11 and the 
switching element 12 of the first group. The latch circuit 15b corresponds 
to the heating element 11 and the switching element 12 of the second 
group. The latch circuit 15c corresponds to the heating element 11 and the 
switching element 12 of the third group. The latch circuit 15d corresponds 
to the heating element 11 and the switching element 12 of the fourth 
group. 
The power supply circuit 20 supplies power to each of the heating elements 
11 by way of the common electrode 25. 
The control circuit 30 produces the original strobe synchronization signal, 
controlling the overall printer operations. More specifically, the control 
circuit comprises memory which stores control data or programs, and a 
processor which executes control programs. 
The delay circuit comprises the control circuit 30, the timing control 
circuit 13, and the latch circuit 15a, 15b, 15c, and 15d. 
A method for driving the printing head of an embodiment according to the 
present invention will now be described by reference to FIG. 2. 
FIG. 2 shows a timing chart of strobe pulse signals output to switching 
elements 12(1), 12(2), 12(3), and 12(4) divided into four groups. 
The shift register circuit 14 stores print data for one line, and the latch 
circuit 15 latches the thus-stored print data. In accordance with the 
strobe synchronization signal controlled by the timing control circuit 13, 
the print data are supplied to the switching element 12 in the form of a 
strobe pulse signal. 
As shown in FIG. 2, first, the control circuit 13 outputs the first and 
third strobe signals. In response to these signals, the latch circuits 15a 
and 15c output strobe pulse signals at time (t1). These strobe pulse 
signals bring into conduction the switching elements 12(1), 12(3), 12(5), 
12(7), . . . 12(4N-3), 12(4N-1) of the first and third groups. As a 
result, heating of the heating elements 11(1), 11(3), 11(5), 11(7), . . . 
, 11(4N-3), 11(4N-1) corresponding to the switching elements is commenced. 
After a given period of time has elapsed after commencement of output of 
the first and third strobe synchronization signals, the timing control 
circuit 13 outputs the second and fourth strobe synchronization signals. 
In response to the output of the synchronization signals, the latch 
circuits 15b and 15d outputs strobe pulse signals at timing (t2). The 
strobe pulse signals bring into conduction the switching elements 12(2), 
12(4), 12(6), 12(8), . . . , 12(4N-2), 12(4N) of the second and fourth 
groups. Heating of the heating elements 11(2), 11(4), 11(6), 11(8), . . . 
, 11(4N-2), 11(4N) corresponding to the switching elements is commenced. 
Even after application of a voltage to the heating elements 11 of the 
second and fourth groups has been commenced, the voltage is continually 
applied to the heating elements 11(1), 11(3), 11(5), 11(7), . . . , 
11(4N-3), 11(4N-1), as is. When the strobe pulse signal becomes off at 
timing (t3) after elapse of a given period of time, the switching elements 
12(1), 12(3), 12(5), 12(7), . . . , 12(4N-3), 12(4N-1) are correspondingly 
brought out of conduction, completing application of a voltage to the 
heating elements 11 of the first and third groups. 
In FIG. 2, for brevity, the strobe pulse width is set to the maximum width 
corresponding to the dots having an area ratio of 100%. As a matter of 
course, the timing at which the strobe pulse signal is completed (i.e., 
the width of the strobe pulse signal) changes according to the input print 
data so as to correspond to the respective heating element 11. 
Subsequently, the shift register circuit 14 waits for another new print 
data set, and voltage application is repeated after timing (t4) in a 
similar manner described previously. 
FIG. 3(a) shows a dot pattern of black (K) thus formed. Numerals (1 though 
4) provided in the drawing represent element numbers corresponding to 
respective dots. In the drawing, the primary scanning direction designates 
the direction in which the heating elements are positioned, and the 
secondary scanning direction designates the direction in which paper is 
fed. FIGS. 3(b), 3(c), and 3(d) show dot patterns of the other colors (C, 
M, Y) when the start timing of heating of each group is controlled in 
accordance with the combinations provided in FIG. 1. 
According to the foregoing embodiment, as is evident from a comparison 
between the present invention shown in FIG. 2 and the conventional one 
shown in FIG. 6, the strobe pulse width (i.e., the time during which a 
voltage is applied to the switching element) is not reduced even when the 
start timing of heating of the heating element 11 is controlled on a 
per-group basis. Accordingly, even when dots having a high area ratio are 
formed it is not necessary to increase the voltage applied to the heating 
elements 11. Therefore, anomalies are prevented which would otherwise 
arise in printing material as a result of overheating of a head, hence 
increasing the life of a head. 
Since dots do not become thick in the primary scanning direction, the 
gradient of a concentration-to-strobe curve (or .gamma. characteristic) 
becomes gentle (see FIG. 4). This in turn contributes to easy tone 
control, and various types of inconsistencies stemming from resistance or 
head temperature are reduced in severity and become easy to correct. 
Although the foregoing embodiment has been described with reference to an 
area-tone printer, the present invention can also be applied to a 
concentration-tone printer such as a sublimation-type printer. In this 
case, inconsistency, such as a sticking phenomenon, arising in the 
direction in which paper is fed can be made less visible. 
Although in the previous embodiment the heating elements are divided into 
four groups according to the sequence in which they are positioned, the 
number of groups or the way in which the heating elements are divided is 
not limited to that described for the previous embodiment. 
EXAMPLE 
The following are printing results which were produced by a printer 
equipped with a thermal head based on the present invention under the 
following conditions. 
1. Printing Conditions 
(1) Specifications Concerning Thermal Head 
Heating Element Pitch: 300 dpi 
Heating Element Size: 70 .mu.m in the primary scanning direction, and 80 
.mu.m in the secondary scanning direction 
Number of Heating Elements: 3,648 
Resistance: Mean Resistance of 3,550 .OMEGA. 
(2) Printing Material 
(Ribbon: Proof Ribbon J for First Proof of Digital Color Proof 
(manufactured by Fuji Photo Film Co., Ltd.) 
(Receiver: Receiver Sheet for First Proof of Digital Color Proof 
(manufactured by Fuji Photo Film Co., Ltd.) 
(3) Printing Conditions 
Line Speed: 7 msec/line (on the basis of 300-dpi resolution) 
Array of Dots: See FIGS. 3(a) to 3(d) 
Dot Pitch in the Secondary Scanning Direction 
K: 300 dpi, C: 250 dpi 
M: 200 dpi, Y: 150 dpi 
Dot Pitch in the Primary Scanning Direction 
K: 150 dpi, C: 75 dpi 
M: 150 dpi, Y: 75 dpi 
Applied Voltage: 14.6 V 
2. Conclusion 
The print head controlled by the conventional method requires application 
of a voltage of 16.3V in order to prevent clearance from being created 
among dots in a solidly shaded yellow (Y) image. In contrast, according to 
the present invention, it was acknowledged that there was prevented 
creation of clearance among the dots at an applied voltage of 14.6 V. 
Microscopic observation of the dots in the solidly shaded yellow (Y) image 
formed by the existing method clearly shows that a reduction in 
concentration, stemming from deformation at the center of the dots, and 
that according to the present invention there is no such reduction in 
concentration, but concentration is uniform. 
In the above described embodiment, the printing head drive apparatus drives 
the thermal head, for printing an image though use of dots arranged in a 
staggered pattern. Furthermore, the present invention may apply to other 
types of a printing head having plural printing elements, and achieve 
similar advantages. 
For instance, the present invention may apply to an LED (Light Emitting 
Diode) head. The LED head has plural LEDs aligned thereon, and exposes the 
light on a medium in response to the driving signals of the respective 
LEDs. Since a driving circuit of the LEDs can use a switching circuit such 
as transistors, the delay circuits being similar to the embodiment 
described above may connect with respective transistors. Therefore, in 
this case, the same advantages are achieved. 
Furthermore, the present invention may apply to a liquid crystal line head. 
The liquid crystal line head has plural elements, which are individually 
driven, arranged on a board. The liquid crystal line head is put between a 
linear light source and a medium so that the light in response to the 
driving signals of the respective elements is exposed on the medium. A 
driving circuit, for instance in the TFT system, has transistors, each 
connecting to the respective element. Accordingly, the delay circuits 
being similar to the embodiment described above may be provided with the 
respective transistors so that the same advantages are achieved. 
Moreover, the similar delay circuits can be applied to a head in which 
plural printing elements are individually driven, such as an EL 
(electro-luminescence) head, a PLZT (lead lanthanum zirconate titanate) 
head, a LD (laser diode) array head, an electro thermo-recording head, and 
an electrostatic printing head. 
According to the present invention, the printing tone can be reliably 
expressed in accordance with an area ratio of dots without an accompanying 
increase in a voltage applied to the printing elements. Further, since the 
peak temperature of a head using the heating elements can be reduced to a 
lower temperature, soil is less apt to stick to the head. 
The load exerted on an apparatus (particularly a head) is reduced, and 
hence an apparatus can be expected to have a life longer than that of an 
existing apparatus. Conversely, if the apparatus is required to have only 
substantially the same life as that of the existing apparatus, the cost of 
the apparatus can be reduced. 
Since dots do not become thick in the primary scanning direction (or the 
direction in which elements are placed), the gradient of a 
concentration-to-strobe curve (or .gamma. characteristic) becomes gentle. 
This in turn contributes to easy tone control, and various types of 
inconsistencies stemming from resistance or head temperature are reduced 
in severity and become easy to correct.