Recording control apparatus

A recording head having actuators driven by voltage supplied from a drive circuit constitutes a part of paths of a compensating circuit for compensating the drive voltage from the drive circuit. The paths are selectively cut off to compensate the drive voltage supplied to the actuators.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to recording control apparatus having 
actuators as recording elements on a recording head and capable of forming 
characters and symbols on a recording medium by controlling the voltage 
supplied to the actuators. 
2. Description of the Prior Art 
The recording head employed in thermal printers or ink jet printers is 
generally provided with an array, in a matrix or line form, of actuators 
as recording elements to which voltage is selectively supplied to form 
characters and symbols on a recording medium. However such actuators are 
inevitably associated with fluctuations in the characteristics thereof 
among themselves and between different heads. For example resistors used 
in the thermal heads are prepared by semiconductor technology such as 
diffusion, thin film formation or thick film formation but the fluctuation 
in resistance is unavoidable in the technology. 
Such fluctuation in resistance leads to a density fluctuation in printing 
on recording paper due to the difference in heat generation among the 
resistors, thus resulting in a deteriorated print quality. 
It is empirically confirmed that the difference in density of characters 
printed on recording paper is visually not noticeable if the fluctuation 
in resistance is within a range of .+-.5% with respect to the mean value 
thereof. In the case of thermal heads having resistors arranged in a 
5.times.7 or 1.times.7 dot matrix, it is technically possible to maintain 
the fluctuation in resistance within the range of .+-.5% with respect to 
the mean value within the same head. 
Furthermore it is generally considered economically advantageous to 
maintain the tolerance in the fluctuation of resistance between different 
heads within a range of .+-.15% with respect to the means resistance. 
Consequently, the conventional method of avoiding the fluctuation in 
density of printed characters among different heads consists of 
classifying the heads according to the average resistance into the 
following three groups: 
group A: 15%-5% of the mean resistance, 
group B: 5%--5% of the mean resistance, and 
group C: -5%--15% of the mean resistance, 
and changing the supplied voltage according to the groups thereby achieving 
constant heat generation. 
FIG. 1 shows an example of circuitry for such a case, in which a slide 
switch 1 is connected at a terminal A directly, at a terminal B through a 
diode D1, and at a terminal C through diodes D1 and D2, to a power supply 
+V. Also a common terminal D of the slide switch 1 is connected to the 
common electrode for resistors R1, R2, . . . , Rn of a thermal head 2. 
Those resistors are connected at the other terminals thereof to drive 
transistors Tr1, . . . , Trn. 
In the above-mentioned circuit the heat generation by the resistors is 
achieved by selective switching of drive transistors Tr1-Trn. 
In the above-explained circuit, three different voltages can be supplied to 
the common terminal, depending upon the voltage drop across the diodes D1, 
D2 according to three different positions A-D, B-D and C-D of the slide 
switch. Positions A-D, B-D and C-D are selected respectively when the 
average resistance of the thermal head 2 belongs to the group A, B or C. 
In this manner a constant heat generation can be obtained through 
appropriate selection of the diodes D1, D2, and it is rendered possible to 
avoid fluctuation in the print density on a thermographic paper sheet. 
Such a conventional method is however defective in that the slide switch 1 
has to be suitably shifted each time the thermal head 2 is exchanged, and 
in that the switch position A-D providing a high voltage, if erroneously 
selected for a low-resistance head of group C, will not only provide an 
abnormally high print density but also may undesirably affect the 
reliability of the recording head. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide recording control 
apparatus having a recording head capable of achieving compensation for 
the drive signals applied thereto. 
Another object of the present invention is to provide recording control 
apparatus having a thermal head capable of achieving compensation for the 
drive signals supplied thereto. 
Still another object of the present invention is to provide an inexpensive 
and compact thermal head. 
Still another object of the present invention is to provide recording 
control apparatus having a drive control circuit capable of providing 
drive signals suitable for a thermal head.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
With reference to FIG. 2 showing a thermal head 2 in accordance with the 
present invention in an external view, there are formed seven resistors Rn 
on an insulating substrate 3. A common electrode of the resistors is 
divided on substrate 3 into four patterns constituting common terminals 
C1, C2 and voltage compensating patterns is followed by voltage 
compensating terminals C(b), C(c) in an area 4. The other terminals of the 
resistors are also patterned on substrate 3 to form seven signal terminals 
DTn in area 4. These eleven terminals are connected at area 4 to an 
flexible cable (not shown) either by contact or by soldering and thus 
further to a drive control circuit. 
FIG. 3 shows a drive control circuit for the thermal head of the present 
invention, in which there are shown switching transistors Tr1, Tr2, . . . 
, Tr7, bias resistors r1, r2, r3, r4 for an amplifying transistor Tr8, and 
a stabilizing resistor r5 for stabilizing the potential at the point E. 
The potential of point E connected to an emitter electrode of the 
transistor Tr8 is determined by the ratio of total resistance of r2, r3 
and r4 to the resistance of r1. The bias resistors r3, r4 are respectively 
connected to the voltage compensating terminals C(b), C(c) of the thermal 
head 2 to constitute a compensating circuit for adjusting the voltage 
supplied to the head from a thermal head drive circuit composed of 
transistors, resistors, etc. The resistance of the thermal head 2 is 
measured beforehand, and the voltage compensating terminal C(c) or C(b) is 
cut at the broken line L(c) or L(b) in FIG. 2 respectively when the head 
belongs to the group C or B. In case the head belongs to the group A, both 
voltage compensating terminals are cut at the broken lines L(b) and L(c). 
Thus the voltage at the point E is determined by the ratio of r1 to 
r2r3/(r2+r3) when a head of group C is mounted, by the ratio of r1 to 
r3r4/(r3+r4) when a head of group B is mounted, and by the ratio of r1 to 
r2 when a head of group A is mounted. 
In this manner it is possible to adjust the supplied voltages so as to 
obtain a constant print density from the heads of the groups A, B and C 
through suitable selection of the resistances of the bias resistors r1, 
r2, r3 and r4. 
When the heads are classified into the three groups A, B and C at the last 
step of manufacturing the head, the voltage compensating terminals C(b) 
and C(c) are selectively cut off as shown in FIGS. 4(A)-4(B) according to 
the classification. In this manner the heads can receive appropriate 
voltage to provide a constant print density, thus achieving constant print 
quality and reliability without any adjustment in the drive control 
circuit at the mounting or replacement of the head. 
FIG. 5 shows another embodiment of the thermal head and the drive control 
circuit therefor in which bias resistors r3, r4 connected in parallel on 
the substrate 3 are connected, through a voltage compensating terminal 
C(a), which units terminals C(b) and C(c), to a drive control circuit 5'. 
Such an embodiment is advantageous in reducing the cost and the size of 
the thermal head as the number of compensating terminals is reduced to 
one. 
It is furthermore possible to eliminate the voltage compensating terminal 
for the resistor Rn, thus enabling further cost reduction and 
miniatuarization, by inserting resistors in the lines L1, L2 connected to 
common terminals C1, C2 and cutting off those resistors in determined 
combinations thereby adjusting the resistance in the paths L1, L2 to 
adjust the voltage supplied to the resistor Rn. 
FIG. 6 shows still another embodiment in which a dummy resistor R0 is 
prepared with the resistors Rn under the same conditions in the 
manufacture of the head in such a manner that the resistance of the dummy 
resistor is maintained within a tolerance of .+-.5% with respect to the 
resistances of heating resistors in the same head. In a drive control 
circuit 5", a print signal PT from printer control circuit (not shown) is 
supplied through a capacitor C to the base electrode a of a transistor 
Tr1, of which the emitter electrode is grounded and also connected through 
a resistor r7 to the base electrode b of a transistor Tr2. Also a power 
supply +V is connected through a resistor r8 to the base electrode b of 
the transistor Tr2 and connected directly to the emitter electrode of the 
transistor Tr2, of which collector electrode d is connected to the emitter 
electrodes of switching transistors Tr1-Tr7 for the resistors Rn, wherein 
the heat generation is achieved by current supply to the resistors R1-R7 
through selective switching of transistors Tr1-Tr7. 
Dummy resistor R0 on the thermal head 2 is connected through a voltage 
compensating terminal C(a) to the base electrode of the transistor Tr1 to 
constitute a compensating circuit for determining the voltage supply time 
for resistors Rn in the following manner. 
Now referring to FIG. 7, upon receipt of the print signal PT, there is 
generated a signal P(a) of a logarithmic waveform at the point a by the 
dummy resistor R0, resistor r6 and capacitor C. Upon arrival of the signal 
P(a) at the threshold voltage V0 of the transistor Tr1, transistor Tr1 is 
closed to provide a pulse signal P(b) of a pulse width t0 at the point b. 
Consequently through the transistor Tr2 there is obtained a thermal head 
drive signal P(d) with a pulse width t0 and a voltage vp. A pulse width t0 
achieving the optimum print density is obtained by appropriate selection 
of the capacitor C and the resistor r6. If the resistances in a thermal 
head are higher than a standard value, the resistance of the dummy 
resistor R0 is accordingly higher to provide a milder slope in the signal 
P(a) thereby giving a shorter pulse width t0. On the other hand if the 
resistances in a thermal head are lower than the standard value, the 
resistance of the dummy resistor R0 is accordingly lower to provide a 
steeper slope in the signal P(a) thereby giving a longer pulse duration 
t0. 
As explained in the foregoing, the present embodiment allows provision of a 
longer pulse width t0 in the case of a head having a higher resistance and 
a shorter pulse width t0 in the case of a head with a lower resistance for 
a determined supply voltage. 
Since the print density is generally proportional to the product of the 
applied electric power W=V.sup.2 /R and the pulse width t0, and W is 
inversely proportional to the power W, then W.times.t0=V.sup.2 /t.sub.0 
.times.t.sub.0 becomes constant. Thus in this manner it is possible to 
obtain a constant print density regardless of the fluctuation in the 
average resistance of each thermal head 2 represented by the resistance of 
the dummy resistor R0. 
The dummy resistor R0, being only used for a small current for driving the 
transistor Tr1, does not generate sufficient heat to cause color 
development on the thermographic paper or generation of bubbles in the 
liquid to cause liquid droplet emission in the ink jet nozzle. 
As discussed in the foregoing, the present embodiment allows realization of 
a thermal printer maintaining a constant print density in a simple and 
inexpensive manner through the use of the dummy resistor R0 in the print 
density control circuit provided in the recording head. 
FIG. 8 shows still another embodiment of the present invention, in which a 
thermal head having a substrate and a flexible cable is connected to an 
ordinary circuit board 3c' not provided with voltage compensating 
terminals through soldering or detachable coupling of the flexible cable 
on which voltage compensating terminals C(b), C(c) are printed. Broken 
lines L(b), L(c) are printed on flexible cable 6 to facilitate cutting for 
example with scissors, thereby modifying the combinations of the bias 
resistors r1, r2, r3 and r4 for the transistor Tr8. 
The embodiments shown in FIGS. 2, 3, 4 and 5 may be employed in combination 
with the drive control circuit 5" as shown in the embodiments of FIGS. 6 
and 7 for modifying the voltage supply time in place of the drive control 
circuit 5 or 5' for compensating the voltage supplied to the resistors Rn, 
which in turn may be employed in combination with the embodiments shown in 
FIGS. 6 and 7. 
Although the foregoing explanation is made with respect to the recording 
control apparatus for a thermal head, the present invention is by no means 
limited to such applications but is also similarly applicable for example 
in a recording apparatus with an ink jet recording head provided with 
piezoelectric elements as recording elements.