Resistive ribbon thermal transfer printing apparatus

A resistive ribbon thermal transfer printing apparatus produces two kinds of electric pulses for selectively energizing recording electrodes of a printing head according to a data to be printed. A first electric pulse (normal pulse) is applied to a recording electrode which is to be energized and is disposed between two recording electrodes which are to be energized. A second electric pulse (specific pulse) is smaller in energy than the normal pulse and is applied to a recording electrode which is to be energized and is not disposed between two recording electrodes which are to be energized. This selective application of the two kinds of pulses allows the size of the printed dots to become uniform.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a resistive ribbon thermal transfer printing 
apparatus which uses a resistive ribbon comprising a resistive material 
layer and a thermally molten ink layer and a plurality of selectively 
energized electrodes for causing a current to pass through the resistive 
material layer to cause the ink layer to be selectively molten and 
transferred to a receiving material such as a paper. 
2. Description of the Prior Art 
As a thermal transfer printing technology, which is known as a low-cost and 
high-quality printing technology, resistive ribbon thermal transfer 
printing technology is known as shown in "Resistive ribbon thermal 
transfer printing: A historical review and introduction to a new printing 
technology" by K. S. Pennington, IBM J. RES. DEVELOP. VOL. 29 NO. 5 
SEPTEMBER 1985. 
The basic method for energizing the plurality of electrodes is to apply 
voltage pulses of a same pulse width to the electrodes for printing dots 
at the same time as shown in FIG. 17(a). In FIG. 17(a), the printing data 
"W" denotes "white" where the corresponding electrode is not energized, 
and the printing data "B" denotes "block" where the corresponding 
electrode is energized to print a dot. In this method, however, the flow 
of the current passed through the part of the resistive material layer 
under an energized electrode between two adjacent energized electrodes is 
different from the flow of the current passed through the part under an 
energized electrode adjacent to an unenergized electrode. The currents 
caused to flow by two adjacent energized electrodes interfere with each 
other to be reduced by each other. But the current caused to flow by an 
energized electrode adjacent to an unenergized electrode passes through a 
larger area than the area through which the current caused to flow by an 
energized electrode between two adjacent energized electrode. This causes 
the printed dots to be not-uniform as shown in FIG. 17(b), which shows a 
printed image by the pulses shown in FIG. 17(a). In FIG. 17(b), the dots 
formed by the electrode Nos. 2 and 5 are larger than those formed by the 
electrode Nos. 3 and 4. 
To solve the above problem, a time-divisional energizing method was 
introduced as shown, for example, in Japanese Laid-Open patent application 
No. 59-167279. In this method, a plurality of electrodes are divided into 
blocks and the electrodes in each block are energized time-divisionally by 
time-divisional pulses as shown in FIG. 18. This method can solve the 
above problem of not uniform printed image, but has some new problems. One 
problem is that the printing speed becomes low due to the time-divisional 
driving. Another problem is that the linearity of the printed image 
becomes worse because the different electrodes are energized at different 
timings. Still another problem is that the resistive ribbon would be 
damaged due to a shock of a large pulse current flown through a small area 
in a short period. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a resistive ribbon thermal 
transfer printing apparatus which is capable of printing an image of 
uniform dot size and of good linearity at high speed. 
To achieve this object, the resistive ribbon thermal transfer printing 
apparatus according to the present invention uses a resistive ribbon 
comprising a resistive material layer and an ink layer being in contact 
with a surface of a receiving member on which an image is to be printed, 
and comprises: 
a printing head having a plurality of recording electrodes and a common 
electrode disposed in a spaced relationship to the recording electrodes, 
the recording and common electrodes being made in contact with the 
resistive material layer of the resistive ribbon; 
a driving unit for moving at least one of the resistive ribbon and the 
printing head relatively to each other; 
an energizing circuit for selectively energizing the plurality of recording 
electrodes at substantially the same time by electric pulses; and 
a control unit for controlling the energizing circuit according a data to 
be printed, the control unit causing the energizing circuit to apply a 
normal electric pulse having a predetermined energy to a recording 
electrode which is to be energized and disposed between two recording 
electrodes which are to be energized, and causing the energizing circuit 
to apply a specific electric pulse which is smaller in energy than the 
normal electric pulses to a recording electrode which is to be energized 
but is not disposed between two recording electrodes which are to be 
energized. 
In an preferred embodiment, each normal electric pulse is a single voltage 
pulse of a predetermined pulse width, and the specific electric pulse is a 
single voltage pulse having a smaller pulse width than that of the normal 
voltage pulse and occurring during the duration of the normal voltage 
pulse. 
In another preferred embodiment, each normal voltage pulse is divided into 
at least two sequentially occurring sub-pulses, and the specific voltage 
pulse is produced by removing at least one sub-pulse, preferably the 
earlier occurring one, from the normal voltage pulse. 
In still another preferred embodiment, each normal electric pulse is a 
single current pulse, and the specific electric pulse is produced by 
delaying the normal current pulse by a predetermined time so that a part 
of the specific electric pulse overlaps a part of the normal current 
pulse. 
In a further preferred embodiment, the normal and specific electric pulses 
are produced according to both a data to be printed and a data which has 
been printed previously. 
The above and other objects and features of the invention will be apparent 
from the following description taken in connection with the accompanying 
drawings in which:

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows an outline of an embodiment of resistive ribbon thermal 
transfer printing apparatus (RRTT printer, hereafter) according to the 
present invention. A printing head 1 and a ribbon cartridge 4 in which a 
resistive ribbon 5 is stored are mounted on a carriage 2 which is driven 
by a motor 7 via a belt 6 to move reciprocally along a guide bar 3. A 
sheet of paper 8 is fed between a platen 9 and the resistive ribbon 5. 
During printing, the printing head 1 is pressed onto the resistive ribbon 
5 so that the printing head 1 is kept in contact with the resistive ribbon 
5 and the resistive ribbon 5 is kept in contact with the paper 8. The 
resistive ribbon 5 is moved in one direction in synchronization with the 
printing operation by a known mechanism such as the one used in 
conventional typewriters. 
FIGS. 2 and 3 show principal portions of the RRTT printer shown in FIG. 1. 
The printing head 1 has a common electrode 11 and a plurality of recording 
electrodes 10 each being spaced from the common electrode 11 at a fixed 
distance. In the illustrated example, eight recording electrodes 10a 
through 10h are arranged in a straight line parallel to the common 
electrode 11. The resistive ribbon 5 has two layers--a resistive material 
layer 12 made of a resin such as a polycarbonate containing carbon, and an 
ink layer 13 made of a thermally meltable ink. The common and recording 
electrodes are in contact with the resistive material layer side surface 
of the resistive ribbon 5. The ink layer side surface of the resistive 
ribbon 5 is in contact with the paper 8 shown in FIG. 1 but not shown in 
FIGS. 2 and 3. 
In the embodiment shown in FIG. 1, the resistive ribbon 5 moves either in a 
direction shown by an arrow 40 in FIG. 2 so that the relative position of 
the printing head to the resistive ribbon moves in a direction from the 
recording electrode side to the common electrode side or in a direction 
shown by an arrow 41 in FIG. 2 so that the relative position of the 
printing head to the resistive ribbon moves in a direction from the common 
electrode side to the recording electrode side. The direction of the 
relative movement of the printing head to the resistive ribbon is in a 
direction perpendicular to the line along which the recording electrodes 
are arranged. Alternatively, the resistive ribbon may be fixed and the 
printing head may be moved in either the direction 41 or the direction 40. 
Between the respective recording electrodes 10a10h and the common electrode 
11 of the printing head 1 are selectively supplied with voltage pulses for 
energizing the recording electrodes from a head driving circuit 14 under 
control of a control unit 15. 
An exemplary configuration of the head driving circuit 14 and the control 
unit 15 is shown in FIG. 4. The head driving circuit 14 comprises a 
plurality of switching transistors 16 which are connected at their 
respective collector terminals to the recording electrodes 10a-10h, 
respectively, and at their respective emitter terminals in common to a 
power source 18 which is connected at its ground terminal to the common 
electrode 11. Each of the switching transistors turns on in response to a 
negative logic voltage pulse applied from the control unit 15 to its base 
terminal to energize the corresponding recording electrode connected to 
its collector terminal. 
The control unit 15 comprises a memory 21 having stored therein data to be 
printed, a microprocessing unit (MPU) 20 which reads the printing data 
from the memory 21 and produces driving data and control signals, and a 
drive control circuit 19 which is controlled by the control signals for 
producing negative logic driving pulses for driving the switching 
transistors 16 from the driving data. 
FIG. 5 shows an example of driving data produced by the MPU 20 in the case 
that the relative position of the printing head to the resistive ribbon 
moves in the direction from the recording electrode side to the common 
electrode side. The positive logic pulses shown in FIG. 5 are inverted in 
polarity to become the negative logic driving pulses by the drive control 
circuit 19. The driving data (a) through (h) are for energizing the 
recording electrodes 10a through 10h, respectively. The printing data "W" 
denotes "white" (corresponding to logic "0") where no dot is printed, and 
the printing data "B" denotes "black" (corresponding to logic "1") where a 
dot is printed. Pulses each having a pulse width T1 are generated at the 
same timing and are called "normal pulses". Pulses each having a pulse 
width T2 are generated at the timing delayed by T3 from the leading edge 
of the normal pulse and are called "specific pulses". The normal and 
specific pulses are terminated at the same timing, i.e., T1=T3+T2. 
The normal pulse is used for energizing a recording electrode disposed 
between two recording electrodes which are to be also energized. The 
specific pulse is used for energizing a recording electrode which is not 
disposed between two recording electrodes which are to be also energized. 
This way of selection of pulses may be easily understood from FIG. 5. 
The MPU 20 produces the driving data shown in FIG. 5 according to a program 
shown by a flow chart in FIG. 6. In step 101, a present printing data is 
read from the memory 21 as a data A, which is "01111100" in the case of 
FIG. 5. In step 102, data A is shifted to right for 1 bit, the result 
being a data B, "00111110". In step 103, data A is shifted to left for 1 
bit, the result being a data C, "11111000". In step 104, a logical AND 
operation of A.multidot.B.multidot.C is executed to obtain a data D, 
"00111000". Thus, the step comprising steps 101 through 104 is a data D 
calculating step 100. In step 200, the MPU 20 outputs data D for the 
period T3 to the drive control circuit 19. In step 300, the MPU 20 outputs 
data A for the period T2 to the drive control circuit 19. As a result, the 
pulses shown in FIG. 5 are produced and inverted in polarity in the drive 
control circuit 19 to be the negative logic pulses, which are respectively 
applied to the respective base terminals of the switching transistors 16. 
In response to the negative logic pulse having the same logic as the pulse 
shown in FIG. 5, the switching transistors 16 apply voltage pulses 
corresponding to the pulses shown in FIG. 5 to the recording electrodes. 
Each of the voltage pulses applied to the recording electrodes 10(b) and 
10(f), which correspond to the specific pulses in FIG. 5, has a smaller 
energy than that of each of the voltage pulses applied to the recording 
electrodes 10(c) through 10(e), which correspond to the normal pulses in 
FIG. 5. The current caused to flow through the resistive material layer of 
the resistive ribbon by each of the recording electrodes 10(c) through 
10(e) energized by the normal voltage pulses is interacted by the currents 
caused to flow by the adjacent two energized recording electrodes to be 
reduced in the flowing area. The current caused to flow by each of the 
recording electrodes 10(b) and 10(f) energized by the specific voltage 
pulses is interacted only by the current caused to flow by one adjacent 
energized recording electrode, so that its flowing area is less reduced. 
But, since the energy given by the specific voltage pulse is smaller in 
amount than and different in timing from that given by the normal voltage 
pulse, the current flowing area under the recording electrode energized by 
the specific voltage pulse becomes almost equal to the reduced current 
flowing area under the recording electrode energized by the normal voltage 
pulse. In other words, the specific voltage pulses and the normal voltage 
pulses are selectively applied to the recording electrodes so that the 
currents caused to flow by the respective energized electrodes become 
uniform, which allows the printed dots to be equal in size. Accordingly, a 
high quality image can be printed. 
Further, since all of the recording electrodes to be energized are 
energized at substantially the same time, the printing speed is higher 
than the time-divisional driving system. Moreover, since the pulse width 
of the energizing pulse can be made relatively larger than the 
time-divisional driving system, the resistive ribbon will not be damaged 
by the current pulse flown therethrough. 
The normal electric pulses and specific electric pulses for selectively 
energizing the recording electrodes to obtain the above-described effects 
can be produced in other ways as will be described below. 
FIG. 7 shows another example of driving data in the case that the position 
of the printing head relative to the resistive ribbon moves in the 
direction from the recording electrode side to the common electrode side. 
In FIG. 7, the normal pulse is divided into two sub-pulses--the first one 
having a pulse width T4, and the second one delayed by T5 from the 
trailing edge of the first sub-pulse and having a pulse width T2. The 
specific pulse is identical with the second sub-pulse of the normal pulse 
and occurring at the same timing as the second sub-pulse. 
FIG. 8 shows a flow chart of a program executed in the MPU 20 for producing 
the pulse shown in FIG. 7. In step 100, the same data D as that described 
with reference to FIG. 6 is produced. In step 210, the MPU 20 outputs data 
D for the period T4. In step 220, the MPU 20 outputs a data of all bits 
"0" for the period T5. In step 300, the MPU 20 outputs the present 
printing data A for the period T2. 
FIG. 9 shows an example of driving data in the case that the position of 
the printing head relative to the resistive ribbon moves in the direction 
from the common electrode side to the recording electrode side, i.e., the 
direction opposite to that in the case of FIGS. 5 and 7. In FIG. 9, the 
specific pulse is generated at the same timing as that of the normal 
pulse, but terminated at the timing prior by T3 to the trailing edge of 
the normal pulse. The pulses shown in FIG. 9 can be produced by exchanging 
the order of the steps 200 and 300 shown in the flow chart of FIG. 6 as 
shown in FIG. 10. 
When the printing speed is increased, the temperature rise of the printing 
head due to the heat transferred from the heated resistive ribbon is also 
increased. The excessive temperature rise of the printing head would cause 
a bad effect on printing quality. In this case, when turning the point of 
view, it can be understood that the area of the resistive ribbon to be 
heated for printing has been heated to a certain extent by the heat 
generated during the previous printing operation. This means that the 
recording electrodes to be energized next may be energized by less energy 
than that normally required. In view of the above, the pulses for 
energizing the recording electrodes may be produced according not only to 
the present printing data but also to the previous printing data. FIG. 11 
shows an example of driving data for satisfying such condition in the case 
that the relative position of the printing head to the resistive ribbon 
moves in the direction from the recording electrode side to the common 
electrode side. 
In FIG. 11, each period T in which one printing operation for printing one 
printing data is performed is divided into four periods--a first period T4 
for a first sub-pulse, a second period T5 in which no sub-pulse will 
occur, a third period T2 for a second sub-pulse, and a fourth period T6 
for a third sub-pulse. In the figure, although there are illustrated 
spaces between T2 and T6 and between T6 and the end of T, they are for the 
purpose of clearly showing the third sub-pulse distinguished from the 
second sub-pulse and the first sub-pulse in the next period T, and do not 
exist actually. The normal pulse for normal energization is composed of 
the first through third sub-pulses, and the specific pulse for normal 
energization is composed of the second and third sub-pulses. The first and 
second sub-pulses are produced according to the same rule between the 
normal pulse and the specific pulse as described before. That is, both of 
the first and second sub-pulses are produced for energizing a recording 
electrode which is to be energized and disposed between two adjacent 
recording electrodes which are to be energized, and only the second 
sub-pulse is produced for energizing a recording electrode which is to be 
energized and is not disposed between two adjacent recording electrodes 
which are to be energized. 
The third sub-pulse is produced only when a recording electrode which is to 
be energized by the present printing data was not energized by the 
previous printing data. The data for producing the third sub-pulse can be 
obtained from the present printing data and the previous printing data by 
a calculation described below. 
FIG. 12 shows a flow chart of a program executed in the MPU 20 for 
producing the pulses shown in FIG. 11. Here, suppose that the present 
printing data denoted by data A is the fifth one of the five printing data 
shown in FIG. 11. The previous printing data, the forth one in FIG. 11, is 
denoted by data E. Data A is "10111010" ("BWBBBWBW"), and data E is 
"10010010" ("BWWBWWBW"). 
In FIG. 12, the same data D as that shown in FIG. 6 is produced in step 
100, here D="00010000". In step 401, the previous printing data E is 
inverted to be a data F (=E)="01101101". In step 402, a logical AND 
operation of A.multidot.F is executed to obtain a data G 
(=A.multidot.F)="00101000". This data G calculated in a step 400 composed 
of steps 401 and 402 is the data for producing the third sub-pulses in 
FIG. 11. The MPU 20 outputs data D for the period T4 in step 210, an all 
"0" data for T5 in step 220, and data A for T2 in step 300, in the same 
way as that shown in FIG. 8. Thereafter, in step 500, the MPU 20 outputs 
data G for the period T6. In this way, the pulses as shown in the last 
printing period T in FIG. 11 can be produced. 
The driving method described with reference to FIGS. 11 and 12 is effective 
to prevent the printing head from being excessively heated during a high 
speed printing operation. 
In the foregoing description, the recording electrodes are energized by 
voltage pulses applied thereto. Alternatively, the recording electrodes 
may be energized by current pulses supplied thereto. FIG. 13 shows an 
embodiment for energizing the recording electrodes by current pulses. The 
embodiment shown in FIG. 13 differs from the embodiment shown in FIG. 4 
only in the configuration of the head driving circuit 14 in which constant 
current generating circuits 22 are connected between the respective 
collector terminals of the switching transistors 16 and the recording 
electrodes 10a through 10h, respectively. Each of the constant current 
generating circuits 22 generates a constant current pulse corresponding to 
a voltage pulse applied thereto for energizing a recording electrode 
connected thereto. 
FIG. 14 shows an exemplary circuit arrangement of each of the constant 
current generating circuits 22. An input terminal 23 is connected to the 
collector terminal of corresponding one of the switching transistor 16. An 
output terminal 24 is connected to corresponding one of the recording 
electrodes 10a-10h. A resistor 25 is connected at one terminal to the 
input terminal 23 and at the other terminal to the emitter terminal of a 
transistor 26 and the inverting input terminal of an operational amplifier 
27. The non-inverting input terminal of the operational amplifier 27 is 
connected to a connection point of a zener diode 29 and a resistor 30 
which are connected in series between the input terminal 23 and the ground 
to keep constant the voltage at the connection point thereof. The output 
terminal of the operational amplifier 27 is connected via a resistor 28 to 
the base terminal of the transistor 26. The collector terminal of the 
transistor 26 is connected to the output terminal 24. The operational 
amplifier 27 operates to keep constant a voltage across the resistor 25 so 
that a constant current flows through the resistor 25 and the transistor 
26 to the output terminal 24 when a voltage pulse is applied to the input 
terminal 23. 
FIG. 15 shows an example of driving data for energizing the recording 
electrodes with the arrangement shown in FIG. 13. The normal pulses occur 
during the period T1. The specific pulses occur during the period T2 
delayed by T3 from the leading edge of the normal pulse so that the 
specific pulses overlap the normal pulses during the period T8. The length 
of T1 is equal to the length of T2. Thus, the timing difference T9 between 
the trailing edges of the normal and specific pulses is equal to the 
length of T3. In other words, the specific pulse may be regarded as such a 
pulse that is obtained by delaying the normal pulse by T3. The example 
shown in FIG. 15 is effective in the case that the relative position of 
the printing head to the resistive ribbon moves in the direction from the 
recording electrode side to the common electrode side. 
FIG. 16 shows a flow chart of a program executed in the MPU 20 for 
producing the driving pulses shown in FIG. 15. In step 100, the same data 
D as that shown in FIG. 6 is produced, i.e., data A="01111100" and data 
D="00111000". In step 601, data D is inverted to be a data H="11000111". 
In step 602, a logical AND operation of A.multidot.H is executed to obtain 
a data I=A.multidot.H="01000100". The MPU 20 outputs data D for the period 
T3 in step 700, data A for T8 in step 800, and data I for T9 in step 900.