Method for driving thermal print head to maintain more constant print density

A method of driving a thermal head enabling recording of multiple gradations and having a plurality of heating resistors, comprising: a first step of calculating an amount of density correction, corresponding to an amount of decrease of printed density of the specific ones of the heating resistors due to reduction of quantity of heat generated by the specific ones of the heating resistors, which reduction is caused by simultaneous heating of the specific ones of the heating resistors and the remaining heating resistors; and a second step of applying to the specific ones of the heating resistors, a print signal corrected on the basis of the amount of density correction so as to drive the specific ones of the heating resistors such that a desired printed density is obtained; in which in the first step, the amount of density correction is calculated on the basis of a weight factor, an imaginary number of the heating resistors and a maximum amount of density decrease.

BACKGROUND OF THE INVENTION 
The present invention relates to a method of controlling heating resistors 
in a sublimation thermal transfer recording apparatus for performing 
recording having density of multiple gradations by using a thermal head. 
In a known sublimation thermal transfer recording apparatus, a plurality of 
heating resistors are arranged in a line on a thermal head and are 
energized to be heated such that printing is performed on a recording 
medium. FIG. 1 shows an electric circuit of the known thermal head. The 
known thermal head includes heating resistors R.sub.i (i=0-1279 
typically), a common resistor r.sub.1 and an FPC electrode resistor 
r.sub.2. From FIG. 1, a voltage V applied to the heating resistors R.sub.i 
is given by the following equation (a): 
EQU V=V.sub.H .times.R/(R+n.multidot.r) (a) 
where V.sub.H denotes a voltage applied to the thermal head, R denotes a 
resistance value of each heating resistor, n denotes the number of the 
heating resistors R.sub.i driven simultaneously and r denotes a sum of a 
resistance value of the common resistor r.sub.1 and a resistance value of 
the FPC electrode resistor r.sub.2. It will be seen from the equation (a) 
that the voltage V is a function of the number n, the resistance value of 
the common resistor r.sub.1 and the resistance value of the FPC electrode 
resistor r.sub.2. 
It is understood from the equation (a) as follows. Namely, in the case 
where the number n of the heating resistors R.sub.i driven simultaneously 
is increased, a so-called voltage drop phenomenon takes place in which the 
voltage V applied to the heating resistors is reduced unless the sum r of 
the resistance value of the common resistor r.sub.1 and the resistance 
value of the FPC electrode resistor r.sub.2 is minimized. As a result, the 
driven heating resistors Ri do not generate a desired quantity of heat, 
thereby resulting in the decrease of printed density. 
In this known thermal head, while printing at a fixed density is being 
performed by using specific ones of the heating resistors Ri, printing at 
a density identical with that of the specific heating resistors is 
performed by the remaining heating resistors by gradually increasing the 
number of the remaining heating resistors subjected to heating. At this 
time, the solid lines in FIG. 2 show the relation before density 
correction between actual density of the specific heating resistors 
(ordinate) and the number of the remaining heating resistors subjected to 
heating (abscissa). In FIG. 2, assuming that the known sublimation thermal 
transfer recording apparatus enables recording of 128 graduations of print 
density, the indication 20", for example, represents the 20th gradation 
counted from the lightest gradation. In FIG. 2, the left ordinate 
represents optical density of the specific heating resistors, while the 
right ordinate represents gradation of the specific heating resistors. It 
will be seen from FIG. 2 that even if printing at a fixed density is 
performed by the specific heating resistors, printed density of the 
specific heating resistors linearly decreases from desired printed density 
as the number of the remaining heating resistors subjected to heating is 
increased gradually. 
A method of correcting the decrease of printed density due to voltage drop 
of the heating resistors caused at the time of drive of the thermal head 
is disclosed in Chapter 4 of a book entitled "Sublimation dye transfer 
process" (1988). In order to implement the method, the sum r of the 
resistance value of the common resistor r.sub.1 and the resistance value 
of the FPC electrode resistor r.sub.2 is required to be reduced. To this 
end, a ceramic substrate of the thermal head is made larger in size, 
thereby resulting in rise of its production cost. 
Therefore, so long as the sum r of the resistance value of the common 
resistor r.sub.1 and the resistance value of the FPC electrode resistor 
r.sub.2 is not reduced, decrease of printed density due to the above 
mentioned voltage drop should occur as the number of the heating resistors 
driven simultaneously is increased, so that the thermal head is incapable 
of outputting accurate printed density. 
SUMMARY OF THE INVENTION 
Accordingly, an essential object of the present invention is to provide, 
with a view to eliminating the above mentioned disadvantages inherent in 
the prior art, a method of driving a thermal head, which enables recording 
of desired density accurately even if the number of the heating resistors 
subjected to heating is increased. 
In order to accomplish this object of the present invention, a method of 
driving a thermal head enabling recording of multiple gradations and 
having a plurality of heating resistors is disclosed in which while 
printing at a fixed density is being performed by specific ones of the 
heating resistors, printing at the fixed density is performed by the 
remaining heating resistors by gradually increasing the number of the 
remaining heating resistors subjected to heating. The method comprises a 
first step of calculating an amount of density correction, corresponding 
to an amount of decrease of printed density of the specific ones of the 
heating resistors due to reduction of quantity of heat generated by the 
specific ones of the heating resistors, which reduction is caused by 
simultaneous heating of the specific ones of the heating resistors and the 
remaining heating resistors. The second step of applying to the specific 
ones of the heating resistors, a print signal corresponding to the amount 
of density correction so as to drive the specific ones of the heating 
resistors such that a desired printed density is obtained; wherein in the 
first step, the amount of density correction is calculated on the basis of 
a weight factor, an imaginary number of the heating resistors and a 
maximum amount of density decreases. The weight factor, assuming that the 
amount of decrease of printed density of the specific ones of the heating 
resistors through heating of the specific ones of the heating resistors 
and a predetermined number of the remaining heating resistors at a fixed 
density, is identical with an amount of decrease of printed density of the 
specific ones of the heating resistors through heating of the specific 
ones of the heating resistors at the fixed density and the remaining 
heating resistors at a different density not more than the original fixed 
density, being a ratio of the predetermined number of the remaining 
heating resistors to the original number of the remaining heating 
resistors. The imaginary number of the heating resistors is a sum of the 
weight factors calculated for all the heating resistors with respect to an 
arbitrary one of the heating resistors. The maximum amount of density 
decrease is a difference between a maximum printed density and a minimum 
printed density in the specific ones of the heating resistors at the fixed 
density. 
In the case where printing is performed on the recording medium by heating 
the specific heating resistors, voltage applied to the specific heating 
resistors drops in response to increase of the number of the remaining 
heating resistors subjected to heating for their drive and thus, printed 
density of the specific heating resistors decreases due to reduction of 
the quantity of heat generated by the specific heating resistors. 
In order to prevent this decrease of printed density of the specific 
heating resistors caused by the voltage drop, the method of the present 
invention calculates the amount of density correction, corresponding to 
the amount of decrease of printed density and applies to the specific 
heating resistors, the print signal corresponding to the amount of density 
correction so as to drive the specific heating resistors such that the 
desired printed density is obtained.

DETAILED DESCRIPTION OF THE INVENTION 
Referring now to the drawings, there is shown in FIG. 3, a sublimation 
thermal transfer recording apparatus in which a method of driving a 
thermal head is performed. The sublimation thermal transfer recording 
apparatus enables recording of multiple gradations, for example, 128 
gradations and includes a thermal head 1 having a plurality of heating 
resistors arranged in a line, an image memory 2 for storing data of one 
image, a line buffer 3 for storing data of one line and a circuit 10 for 
calculating decrease of printed density from an imaginary number of the 
heating resistors, etc. It should be noted that the method of the present 
invention is mainly concerned with operation of the circuit 10. 
The sublimation thermal transfer recording apparatus further includes a 
temperature sensor 4, a density table ROM 5 for storing a density table 
showing the relation between gradation and applied pulse width at a 
predetermined temperature in a state of accumulation of no heat in the 
heating resistors, a pulse control circuit 6 for generating heating pulse 
signals (strobe signals) which are determined in accordance with the 
density table so as to be applied to the heating resistors, a driver 
control circuit 7 for transmitting control signals to the line buffer 3 
and the pulse control circuit 6 and a central processing unit (CPU) 8 for 
controlling the sublimation thermal transfer recording apparatus as a 
whole. 
Hereinbelow, the method of the present invention, which is mainly based on 
operation of the circuit 10, is described. As shown in FIG. 8, binary data 
of one line used in the present invention is formed by a matrix of 1280 
rows and 127 columns. 
In the case where while printing at a fixed density is being performed by, 
for example, 32 specific heating resistors, printing is performed by the 
remaining 1248 heating resistors by gradually increasing printed density 
of the remaining heating resistors, FIG. 4 shows relation between actual 
density of the specific heating resistors (ordinate) and density of the 
remaining heating resistors (abscissa). In FIG. 4, the left and right 
ordinates represent optical density and gradation of the specific heating 
resistors, respectively. FIG. 4 reveals that actual density of the 
specific heating resistors decreases sharply in the vicinity of an initial 
density of the remaining heating resistors, then, linearly decreases to 
such a point as to be identical with density of the remaining heating 
resistors and thereafter, assumes a substantially fixed value. 
By using FIGS. 2 and 4, weight factor X(n(i)), the amount of density 
correction in the method of the present invention is based, is obtained. 
In FIG. 4, assuming that the sublimation thermal transfer recording 
apparatus enables recording of 128 gradations, the indication "20", for 
example, represents the 20th gradation counted from the lightest 
gradation. For example, when printed density of the specific heating 
resistors is set to the 80th gradation and printed density of the 
remaining heating resistor is set to the 1st gradation, actual density of 
the specific heating resistors descends sharply through an optical density 
of about 0.18 as shown by a portion A in FIG. 4. This descent of optical 
density of 0.18 in FIG. 4 corresponds to heating of 750 heating resistors 
in the case of printing at the density of the 80th gradation in FIG. 2. 
Thus, an imaginary number of the heating resistors subjected to heating in 
FIG. 4 becomes identical with that of FIG. 2. Hence, the following 
equation (1) is established: 
EQU 1248.times.X(80)=750.times.X(0) (1) 
where X(n(i)) denotes weight factor at the time when difference in density 
between the specific heating resistors and the remaining heating resistors 
is n(i) and X(0)=1.0 is set. 
As shown in FIG. 7, the difference n(i) in density between the specific 
heating resistors and the remaining resistors is initially obtained at 
step S1 in the method of the present invention. By solving the equation 
(1), X(80)=0.601 is obtained. When the weight factor X(n(i)) is obtained 
for each density of the specific heating resistors, FIG. 5 shows relation 
between the weight factor X(n(i)) and the difference n(i). FIG. 5 
illustrates that the weight factor X(n(i)) linearly decreases in response 
to increase of the difference n(i). 
In the foregoing, the number of the specific heating resistors is set to 
32, while the number of the remaining heating resistors is set to 1248. 
However, since density correction is performed for each of the heating 
resistors, calculation of density correction is performed for each of the 
heating resistors, hereinbelow. By obtaining the weight factors X(n(i)) 
corresponding to the differences n(i) for all the heating resistors and 
taking a sum of the weight factors X(n(i)), an imaginary number S of the 
heating resistors is obtained. 
Therefore, assuming that the difference n(i) on the abscissa of FIG. 5 
represents difference in density between a specific heating resistor (an 
arbitrary one of the 1280 heating resistors) and the remaining heating 
resistors, the imaginary number S of the heating resistors is given by the 
following equation (2). 
##EQU1## 
In the above equation (2), X(n(i)) denotes the linear function of FIG. 5. 
Thus, at step S2 in FIG. 7, the imaginary number S of the heating 
resistors is calculated from the equation (2) by obtaining the weight 
factor X(n(i)) from FIG. 5. 
FIG. 6 shows, at the time of printing at a fixed density by the specific 
heating resistors as shown by the solid lines in FIG. 2, the relation 
between maximum amount of density decrease i.e. difference between maximum 
and minimum printed densities of the specific heating resistors and 
desired printed density. FIG. 6 reveals that the maximum amount of density 
decrease is increased as the desired printed density is increased 
gradually and reaches its peak when the desired printed density ranges 
from the 80th gradation to the 100th gradation. After the peak, the 
maximum amount of density descent decreases. 
Namely, assuming, that character M denotes the maximum amount of density 
decrease, the maximum amount M of density decrease for an inputted density 
(inputted gradation) is read from a ROM in the circuit of the sublimation 
thermal transfer recording apparatus at step S3 in FIG. 7. Then, at step 
S4 in FIG. 7, amount H of density correction for each specific heating 
resistor is obtained from the following equation (3). 
EQU H=M.times.S/1280 (3) 
Finally, at step S5 in FIG. 7, the amount H of density correction is added 
to the inputted density (inputted gradation) so as to obtain a corrected 
print signal and the corrected print signal is applied to the specific 
heating resistors. 
One-dot chain lines in FIG. 2 show the relation between actual printed 
density of the specific heating resistors and the number of the remaining 
heating resistors at the time of printing at a fixed density by the 
specific heating resistors after printed density for each specific heating 
resistor has been corrected. By comparing the solid lines with the one-dot 
chain lines in FIG. 2, printed density of the specific heating resistors 
before density correction, namely the solid lines linearly decrease as the 
number of the remaining heating resistors is increased, so that desired 
printed density cannot be obtained. On the other hand, printed density of 
the specific heating resistors after density correction, namely the 
one-dot chain lines run substantially horizontally without any noticeable 
decrease and thus, desired printed density can be obtained. 
In the method of the present invention, printed density of the specific 
heating resistors is corrected on the basis of the imaginary number of the 
heating resistors representing a sum of the weight factors of the 
respective remaining heating resistors, etc. in order to prevent decrease 
of printed density due to voltage drop of the specific heating resistors 
caused by heating of the remaining heating resistors, whereby desired 
printed density can be obtained. 
Although the present invention has been fully described by way of example 
with reference to the accompanying drawings, it is to be noted here that 
various changes and modifications will be apparent to those skilled in the 
art. Therefore, unless otherwise such changes and modifications depart 
from the scope of the present invention, they should be construed as being 
included therein.