Method of controlling thermally controlling a thermal printing head

A transistor causes a current to flow through a heat generating resistor in response to a picture signal and simultaneously, a capacitor coupled to the transistor is charged. When a charged voltage on the capacitor reaches a predetermined magnitude, the transistor stops the current to prevent the resistor from exceeding a predetermined magnitude and the capacitor begins to discharge. A plurality of resistors mentioned above are disposed in a row to form a thermally sensitive head which records a picture image on thermally sensitive paper as determined by the respective picture signals.

BACKGROUND OF THE INVENTION 
This invention relates to improvements in a method of driving a thermal 
printing head, and more particularly to a method of thermally controlling 
a thermal printing head so as to enable the head to effect high speed 
recording. 
Since thermal recording systems can reproduce picture images simply by 
means of thermal energy, they are useful as simple recording systems and 
the range of their use is rapidly increasing. For example, such recording 
systems are widely used in facsimile equipment and the like. 
The conventional thermal recording systems have comprised a thermal 
printing head including a plurality of heat generating resistors disposed 
in a row, and a transistor connected between each of the heat generating 
resistors and an associated one of a plurality of signal terminals to 
which picture signals are applied respectively. The transistors are 
responsive to associated picture signals for being put in their ON states 
thereby to permit a current from a common source terminal to flow into 
those heat generating resistors connected to the conducting transistors to 
generate Joule heat. The resistors thus heated from elemental picture 
images or dots on a section of thermally sensitive paper contacted by the 
resistors as determined by the picture signals. This process is repeated 
with successive rows of the section of thermally sensitive paper to form a 
received picture image on the section of paper. 
However, during repetition of the process as described above, the 
temperature of the heat generating resistors slowly rises due to the 
residual heat effect caused from the preceding flows of current through 
the resistors resulting in blurring of the recorded dots. Alternatively, 
the thermal printing head might be broken. Further the 
conduction-of-current cycle for each row on the thermally sensitive paper 
has been unable to be shortened beyond a particular limit because time 
intervals are required for heating and cooling the heat generating 
resistors. 
Accordingly it is an object of the present invention to provide a new and 
improved method of thermally controlling a thermal printing head so as to 
increase the recording speed required for facsimile equipment and the 
like. 
It is another object of the present invention to provide a new and improved 
method of thermally controlling a thermal printing head so as to improve 
the quality of pictures recorded by facsimile equipment and the like. 
It is still another object of the present invention to provide a new and 
improved method of thermally controlling a thermal printing head so as to 
decrease a driving power required for recording by facsimile technique and 
the like. 
SUMMARY OF THE INVENTION 
The present invention provides a method of thermally controlling thermal 
printing head including a plurality of heat generating resistors disposed 
in a predetermined pattern and driven by respective driving circuits so 
that currents flow through the heat generating resistors in accordance 
with associated picture signals to generate heat, thereby to record a 
picture image on a section of thermally sensitive paper as determined by 
the picture signals. The method comprises the steps of finding the 
relationship between the conduction-of-current cycles and the 
conduction-of-current time intervals for each of the heat generating 
resistors which will prevent the temperature of each of the heat 
generating resistors from exceeding a predetermined magnitude, and 
controlling a current flowing through each of the heat generating 
resistors in accordance with the thus found conduction-of-current cycle 
and the conduction-of-current time interval.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to FIG. 1 of the drawings, there is illustrated a 
conventional arrangement for thermally controlling a group of thermal 
printing heads. The arrangement illustrated comprises a thermal printing 
head generally designated by the reference numeral 10 including an 
electrically insulating substrate 12 formed, for example of a ceramic 
material and a plurality of heat generating resistors, in this case, 
either rectangular resistors 14a, 14b, 14c, 14d 14e, 14f, 14g and 14h 
disposed at predetermined equal intervals in a row on the surface of the 
substrate 12. The resistors 14a through 14h are connected at one end to 
respective terminals 16a, 16b, 16c, 16d, 16e, 16f, 16g and 16h and at the 
other ends to a common terminal 18 which is, in turn, connected to ground. 
The terminals 16a through 16h are connected to emitter electrodes of npn 
type transistors 20a, 20b, 20c, 20d, 20e, 20f, 20g including collector 
electrodes connected together to a source terminal 22. Each of the 
transistors 20a through 20h has a base electrode connected to a signal 
terminal 24 suffixed with the same reference character as the transistor. 
For example, the transistor 20b includes the base electrode connected to 
the signal terminal 24b. 
The transistors 20a through 20h form a switching circuit which is 
selectively responsive to picture signals supplied to the signal terminals 
24a through 24h to apply selectively a recording voltage from the source 
terminal 22 to the heat generating resistors 14a through 14h. 
The operation of the arrangement shown in FIG. 1 will now be described with 
reference to the waveforms in FIGS. 2a-2h. In FIGS. 2a-2h the waveforms 
are applied to those signal terminals 24 suffixed with the same reference 
characters as those identifying the waveforms and plotted on the same time 
base as shown in FIG. 2i. For example, the waveform of FIG. 2a is applied 
to the signal terminal 24a to put the associated transistor 20a in its ON 
state between time points O and T.sub.H. Similarly the transistors 20e and 
20h are put in their ON state between time points O and T.sub.H as will 
readily be understood from the waveforms in FIGS. 2e and 2h. 
As a result, a recording current through the source terminal 22 flows via 
the now conducting transistors 20a, 20e and 20h to the associated heat 
generating resistors 14a, 14e and 14h to cause the latter to generate 
Joule heat. 
The thermal printing recording method utilizes that heat to form visible 
images in a first row on a section of thermally sensitive paper put under 
a pressure by the thermal printing head 10. 
At time point T.sub.H the flows of currents into the selected heat 
generating resistors are ended whereupon, the thermally sensitive paper is 
displaced to a position where a second row is recorded. 
Then the recording starts for the second row on the section of thermally 
sensitive paper at time point T.sub.H +T.sub.C. This recording is ended at 
time point 2T.sub.H +T.sub.C as will be seen from the time base FIG. 2i. 
Thereafter the process as described above is repeated for the succeeding 
rows to complete the thermal recording. 
While the arrangement of FIG. 1 has been illustrated and described in 
conjunction with the eight heat generating resistors, the same may include 
any desired number other than eight of the heat generating resistors. 
The thermal printing heads 10 presently in existence include the heat 
generating resistors disposed at equal intervals of 165 .mu.m to form an 
array having a length of 200 mm. 
The arrangement of FIG. 1 used in a conventional driving method has been 
disadvantageous in that high speed recording can not be effected for the 
following reasons: 
The waveform shown in FIG. 2j depicts the relationship between time and the 
surface temperature of the heat generating resistor 14a due to the 
application of current with the waveform of FIG. 2a to the signal terminal 
24a as observed through an infrared microscope. More specifically, the 
current from the source terminal 22 starts to flow through the heat 
generating resistor 14a at time point O while the transistor 20a is 
maintained in its ON state between time points O and T.sub.H. Accordingly 
the surface temperature of the resistor 14a rises at some time constant 
until it reaches a magnitude t.sub.f. Thereafter the surface temperature 
descends at another time constant. The temperature t.sub.f is the 
temperature required for thermally sensitive paper to be colored with a 
satisfactory density and normally is about 400.degree. C. A 
conduction-of-current time interval T.sub.H required for the heat 
generating resistor to rise to the temperature t.sub.f and a cooling time 
interval T.sub.C required for the heat generating resistor 14a raised to 
that temperature once to cool to a temperature t.sub.i before the current 
is initiated to flow through the resistor 14a depend upon the type of 
material of the thermal printing head, the structure thereof, and the 
thermal resistance and a heat capacity of the entire recording system 
including the thermally sensitive paper and platen therefor. 
In the conventional driving method, therefore, the conduction-of-current 
cycle has been required to be set to a time no shorter than T.sub.H 
+T.sub.C resulting in being unable to increase the recording speed beyond 
a certain limit. If the conduction-of-current cycle is set so as to be no 
longer than T.sub.H +T.sub.C, for example, to T.sub.H +T.sub.B where 
T.sub.B is shorter than T.sub.C, then the surface temperature of the heat 
generating resistor 14a changes as shown by the waveform in FIG. 2k. As 
shown by the waveform in FIG. 2k, the heat generating resistor 14a has a 
maximum surface temperature t.sub.m exceeding the predetermined 
temperature t.sub.f. This is because heat is accumulated by the thermal 
printing head or the heat generating resistor due to the residual heat 
from the preceding conduction of the current. This results in a slow 
increase in the temperature of the thermal printing head as the conduction 
is repeated. As a result, there have been disadvantages in that recorded 
dots are blurred and the thermal printing head may be broken due to an 
accummulation of heat. 
Even though a thermal printing head having a short thermal time constant 
was operatively associated with thermally sensitive paper having a high 
sensitivity, the conduction-of-current or heating time interval T.sub.H 
and the cooling time interval T.sub.C are, at present, required to be no 
shorter than 2 ms and no shorter than 8 ms required in order to provide a 
recording density exceeding a reflective optical density of 1.0. In 
conventional methods for driving the thermal printing head by an 
arrangement such as shown in FIG. 1, therefore, the conduction-of-current 
cycle for each row has been unable to be reduced to less than 10 ms by any 
means. 
Noticing when the conduction-of-current cycle is short, the heat generating 
resistors slowly rise in due to the residual heat resulting from the flow 
of current therethrough during recording of the preceding row, because of 
the constant conduction-of-current time interval, the present invention 
seeks to provide a method for controlling thermal heat generating 
resistors or a thermal printing head comprising the step of controlling 
the conduction-of-current time interval for a row or recording line so as 
to prevent the maximum temperature of the thermal printing head from 
exceeding a predetermined constant magnitude whereby the high speed 
recording can be achieved. 
Referring now to FIG. 3, there is illustrated an arrangement for carrying 
out a method of thermally controlling a thermal printing head according to 
the present invention. Only for purposes of illustration FIG. 3 shows a 
single heat generating resistor, in this case, a resistor 14a similar to 
that shown in FIG. 1 and a driving circuit therefore, like reference 
numerals and characters designating the components identical to those 
illustrated in FIG. 1. As in the arrangement of FIG. 1, the heat 
generating resistor 14a and the emitter-to-collector circuit of the npn 
type transistor 20a are connected in series circuit relationship between 
ground and the source terminal 22. The emitter electrode of the transistor 
14a called a first transistor is also connected to ground through a series 
combination of a semiconductor diode 30, a resistor 36 and a capacitor 34 
forming a network for charging the capacitor 34. The junction 32 of the 
capacitor 34 and the resistor 32 is connected to ground through a resistor 
38 and a second npn transistor 40 including a collector electrode 
connected to the resistor 38 and an emitter electrode connected to ground. 
The resistor 38 and the second transistor 40 form a network for 
discharging the capacitor 34. Therefore, by alternatively placing the 
first and second transistors 20a and 40 in the ON state, the capacitor 34 
is alternately brought into its charged and discharged states. 
The junction 32 is further connected to a voltage comparator 42 through one 
input labelled -. The voltage comparator 42 has the other input labelled + 
and it is connected to a reference terminal 44 to which a reference 
voltage is applied. The voltage comparator 42 includes an output connected 
to an R-SS FLIP-FLOP 46. More specifically, the output of the comparator 
42 is connected to a reset input 48 labelled R to the R-S FLIP-FLOP 46 
including a set input 50 labelled S and connected to an output of a 
differentiating circuit 52. The differentiating circuit 52 is connected at 
the input to the signal terminal 24a. The R-S FLIP-FLOP 46 includes an 
output 56 labelled Q and connected to the base electrode of the first 
transistor 20a and an inverse output 58 labelled Q and connected to the 
second transistor 40 at the base electrode. 
While FIG. 3 shows the heat generating resistor 14a and the driving circuit 
therefor it is to be understood that the arrangement of the overall 
circuit comprises a plurality of heat generating resistors similar to and 
disposed in the same manner as those shown in FIG. 1 and driving circuits 
identical to the driving circuit as described above in conjunctio with 
FIG. 3, one for each of the heat generating resistors. In the latter case 
the components of the driving circuit are designated by the respective 
reference numerals as shown in FIG. 3 suffixed with the reference 
characters identifying that heat generating resistor operatively coupled 
thereto. For example, the second transistors operatively coupled to the 
heat generating resistors 14a and 14b respectively are designated by 40a 
and 40b respectively. 
The operation of the arrangement shown in FIG. 3 will now be described with 
reference to FIGS. 4a-4g wherein there are illustrated waveforms developed 
at various point in that arrangement on a common time base shown in FIG. 
4h. 
A picture signal having a waveform shown in FIG. 4a is applied via the 
signal terminal 24a to the differentiating circuit 52 where it is 
differentiated into the waveform shown in FIG. 4b. This waveform is 
supplied to the set input 50 of the R-S FLIP-FLOP 46. As a result, the R-S 
FLIP-FLOP 46 delivers an output changed from its low to its high level to 
the output 56 as shown by the waveform in FIG. 4c between time points O 
and T.sub.H (see FIG. 4h). Therefore the first transistor 20a is brought 
into its ON state in which a recording current from the source terminal 22 
flows through the heat generating resistor 14a to raise its surface 
temperature following a temperature rising portion t.sub.r of the waveform 
shown in FIG. 4d. 
Up to this point, the driving method of the present invention is 
substantially identical to that of the prior art practice such as shown in 
FIGS. 1 and 2. 
One of the characteristic features of the present invention is to charge 
the capacitor 30 through the diode 30 and the resistor 36 in 
synchronization with the flow of current through the heat generating 
resistor 14a. The waveform shown in FIG. 4e shows the change in voltage at 
the junction 32 of the resistor 36 and the capacitor 34 and includes a 
charging voltage portion v.sub.c and a discharging voltage portion v.sub.d 
for the capacitor 34. 
Heat phenomena can be generally simulated by using an integrating circuit 
formed of a resistor and a capacitor, and the temperature can be in 
one-to-one correspondence with the output voltage from the integrating 
circuit. Accordingly, by selecting predetermined magnitudes of the values 
of the resistor and capacitor 36 and 34 respectively, the charging voltage 
portion v.sub.c of the waveform shown in FIG. 4e for the capacitor 34 can 
approximate the temperature rising portion t.sub.r of the waveform shown 
in FIG. 4d for the heat generating resistor 14a with a fairly high 
accuracy. 
On the other hand, the temperature t.sub.f required for thermally sensitive 
paper to be colored with the ncessary and sufficient density can be 
converted to a reference voltage v.sub.f (see the waveform in FIG. 4e). 
Thus the temperature of the heat generating resistor 14a can be controlled 
according to a method as will subsequently be described. 
Further, the magnitudes of the values of the resistor and capacitor 36 and 
34 respectively can be analytically found by using a mathematical method 
such as a finite element method or the like but rough approximate 
magnitudes can also be found from simple recording experiments. 
The voltage comparator 42 compares the voltage charged on the capacitor 34 
with the reference voltage applied thereto from the reference terminal 44. 
When both voltages are equal to each other as determined by the voltage 
comparator 42, the latter delivers to the reset input 48 of the R-S 
FLIP-FLOP 46 an equality sensed pulse as shown by the waveform in FIG. 4f. 
The R-S FLIP-FLOP 46 responds to that equality sensed pulse to stop the 
flow of current through the heat generating resistor 14a. Therefore the 
resistor 14a is controlled so that the maximum temperature thereof is 
prevented from exceeding the temperature t.sub.f. 
On the other hand, the output from the inverse output 58 of the R-S 
FLIP-FLOP 46 changes from its low to its high level by the and after time 
point T.sub.H as shown at waveform in FIG. 4g. This results in the second 
transistor 40 being put in its ON state. Accordingly, the electric charged 
on the capacitor 34 discharges through the resistor 38 and the now 
conducting transistor 40 with a time constant determined by the resistor 
38 and the transistor 40. By properly selecting a predetermined magnitude 
of the value of the resistor 38, the discharging voltage portion v.sub.d 
of the waveform 4 shown in FIG. 4e can approximate the temperature 
dropping portion t.sub.d of the waveform shown in FIG. 4d with sufficient 
accuracy for practical purposes. 
The process as described above is simultaneously effected for the remaining 
heat generating resistors in accordance with associated ones of the 
picture signals to complete the recording of a first row in a section of 
thermally sensitive paper. 
Then after the section of paper has been displaced widthwise thereof as 
described in conjunction with FIGS. 1 and 2, recording of a second row is 
carried out at and after time point T.sub.H +T.sub.B where T.sub.B is less 
than T.sub.C in the same manner as described above for the first row. That 
is, the temperature of the heat generating resistors is controlled in the 
manner as described above in conjunction with FIGS. 3 and 4. This results 
in a definite recording free from blurring. 
From the foregoing it is seen that, according to the present invention, 
each of the heat generating resistors have the maximum temperature thereof 
reach a predetermined constant magnitude by controlling the 
conduction-of-current time interval for each recording row. Therefore the 
present invention provides recorded pictures of high quality not only with 
high speed recording but also with a recording speed variable in 
accordance with the amount of information included in the material being 
transmitted. Also the present invention can effectivity utilize the 
residual heat due to the recording of the preceding row resulting in a 
sharp decrease in the burden on the associated electric source. 
While the present invention has been illustrated and described in 
conjunction with a single preferred embodiment thereof it is to be 
understood that numerous changes and modifications may be resorted to 
without departing from the spirit and scope of the present invention. For 
example, the present invention has been described in conjunction with the 
control of the conduction-of-current time interval by having the output 
voltage from the integrating circuit converted from the temperature of the 
heat generating resistor, but it is to be understood that the 
conduction-of-current time interval may be controlled by utilizing a 
non-volatile memory device such as a read only memory device. In the 
latter case the non-volatile memory device has preliminarily stored 
therein the relationship between the conduction-of-current cycle and the 
conduction-of-current time interval required for and determined by that 
cycle. Upon recording, the relationship between the conduction-of-current 
cycle and time interval may be read out from the memory device for each 
conduction-of-current cycle and the conduction-of-current time interval 
may be controlled by supplying the associated heat generating resistor 
with a current only for the conduction-of-current time interval readout 
from the memory device. This measure is advantageous over the embodiment 
of the present invention shown in FIG. 3 in that the number of circuit 
components such as the resistors and capacitors can be reduced when the 
number of the heat generating resistors increases. 
While the present invention has been illustrated in conjunction with a 
facsimile system including a plurality of heat generating resistors in the 
form of rectangles, it is to be understood that the same is equally 
applicable to a heat generating resistors having shapes other than a 
rectangular shape and also to printers, plotters etc.