Configuration for ETR print head triggering

An ETR printing unit has electrodes and an ETR print head. A configuration for triggering the ETR print head includes a controllable energy source supplying energy for various pixels of a printed image to the electrodes of the ETR printing unit. A switching unit is provided through which the controllable energy source acts upon the electrodes temporarily connected to the energy source with a voltage or with a constant current, having a magnitude with a dependency on a temporarily different number of electrodes for supplying a larger number of electrodes with a higher voltage or a higher constant current than a lesser number would be. A microprocessor control unit for the ETR printing unit supplies the controllable energy source with a control signal corresponding to a dependency on the number of the triggered electrodes, for specifying the number of electrodes temporarily connected to the controllable energy source. A memory is connected to the microprocessor control unit.

BACKGROUND OF THE INVENTION 
Field of the Invention 
The invention relates to a configuration for ETR (electrothermal) print 
head triggering, with memory means and a control for an ETR printing unit, 
wherein energy from an energy source for various pixels of a printed image 
are furnished to the electrodes of the ETR printing unit. An ETR printer 
can be used in a postage meter, for instance, for franking mail. 
An ETR printer includes not only mechanics but also an electronic head 
control, an ETR print head with a number of electrodes, and a current 
collector electrode, which are connected to an energy supply. The printing 
energy is fed as a constant current into each current path belonging to 
each electrode, to assure uniform print quality. 
The ETR print head acts upon the recording medium, preferably paper, 
through a resistance ink ribbon moved along with the recording medium. The 
resistance ink ribbon has an upper resistor layer which is in contact with 
the ETR print head, a middle current return layer, and a lower ink layer 
that is in contact with the recording medium. 
The ETR print head includes a number of electrodes that are disposed in 
such a way that they are insulated from one another, and each of which can 
generate one pixel of the printed image. The energy delivered through the 
electrodes is converted, in the region of the resistor layer assigned to 
each pixel, into electrical heat that leads to melting of the ink of the 
ink layer located in that region. 
Published European Application No. 0 301 891 A1, corresponding to U.S. Pat. 
No. 5,005,993, discloses such an ETR printer with return electrodes. The 
energy to be delivered is dependent on the resistance of each current path 
assigned to a pixel, on the melting temperature of the ink, on the 
intended contrast of the printed image, and on the speed of the moving 
resistor ink ribbon, and rises non-linearly with the roughness of the 
surface of the paper. 
German Published, Non-Prosecuted Application 38 33 746 A1 has already 
disclosed a switch unit being acted upon by a trigger unit, for a print 
head, which unlike the ETR print head, already contains the resistor 
elements themselves (thermotransfer printing) and has selective triggering 
with preheating of the resistor element to reduce the heating output in 
printing. 
A serial/parallel shift register acted upon by the serial printing data 
passes the printing data in a first triggering phase to the latches of a 
buffer memory or store. In a second triggering phase, during a strobe 
pulse, each gate triggered by the associated outputs of the latches is 
switched open, and a trigger pulse is output to the applicable resistor 
element. The resistor heating elements are preheated directly, by means of 
a clock frequency that is adapted in both pulse height and pulse width to 
the necessary heating energy. 
In an ETR printer, such preheating by energy from a voltage source is 
impossible in principle, because the resistor elements are located in the 
resistor layer of the resistor ink ribbon. 
Since a very great number of parasitic serial resistances of variable value 
(junction resistance between the electrode and the ribbon, track 
resistance of the layer of aluminum in the ribbon, junction resistance 
between the ribbon and the return electrode) occur in the overall system 
including the ETR head with the electrodes, the ETR ribbon and the return 
electrode, which lead to a variation in the total resistance during 
operation, an energy supply by means of a voltage source is not suitable, 
since the varying partial voltage through the heating (printing) resistor 
would lead to varying printing energies. The result would be fluctuating 
print quality. 
Energy supply to the various electrodes of an ETR head is best done, from a 
technical standpoint, by means of a constant current source, because a 
very uniform printing output can be guaranteed as a result of the accuracy 
of the constant current and of the specific ribbon resistance. 
However, a technologically optimal construction with current regulation for 
each electrode path is often unsupportable in price, because of the 
(sometimes) very high numbers of the electrodes in an ETR head. 
Structures are already known with which the attempt has been made to 
achieve a technologically feasible construction at acceptable expense. 
They includes the method of integrating a dropping, protective or 
multiplier resistor into each electrode path, having a resistance which is 
dimensioned as approximately 3 to 4 times higher than the effective 
heating (printing) resistance of the ETR ribbon. 
Due to such an artificially increased total resistance of the system, the 
narrow relatively slight changes in the parasitic serial resistances in 
the system cannot cause any substantial change in the effective voltage 
across the heating resistor. In that way, the current of each electrode 
path has been "stabilized", and an improvement in print quality is 
attained as a function of a ratio between the dropping, protective or 
multiplier resistors and the effective heating resistance of the ETR 
ribbon. 
Although that structure is inexpensive and technologically simple on one 
hand, nevertheless on the other hand it has the considerable disadvantage 
of needing only a fraction of the energy fed into the complete system for 
the actual printing process. The great majority of the energy is converted 
into lost heat. Moreover, a fluctuation in the voltage across the 
applicable heating resistor is unavoidable, because in contrast to the 
principle of thermal transfer printing, in the ETR printing principle, 
during the motion of the ribbon, varying junction resistances at the 
contact points of the resistor layer of the resistor ink ribbon with the 
electrodes of the ETR print head and of the current collector electrode, 
as well as varying resistor heating elements in the ribbon, are operative 
during the motion of the ribbon. 
SUMMARY OF THE INVENTION 
It is accordingly an object of the invention to provide a configuration for 
ETR print head triggering, which overcomes the hereinafore-mentioned 
disadvantages of the heretoforeknown devices of this general type and 
which provides a technological way of triggering an arbitrary ETR print 
head that combines a simple and therefore economical technological 
construction with minimal power loss in the system and therefore only 
entails low operating costs, while at the same time the print quality is 
maximal. 
With the foregoing and other objects in view there is provided, in 
accordance with the invention, in an apparatus having an ETR printing unit 
with electrodes and an ETR print head, a configuration for triggering the 
ETR print head, comprising a controllable energy source supplying energy 
for various pixels of a printed image to the electrodes of the ETR 
printing unit; a switching unit through which the controllable energy 
source acts upon the electrodes temporarily connected to the energy source 
with a voltage or with a constant current, having a magnitude with a 
dependency on a temporarily different number of electrodes for supplying a 
larger number of electrodes with a higher voltage or a higher constant 
current than a lesser number would be; a microprocessor control unit for 
the ETR printing unit supplying the controllable energy source with a 
control signal corresponding to a dependency on the number of the 
triggered electrodes, for specifying the number of electrodes temporarily 
connected to the controllable energy source; and memory means connected to 
the microprocessor control unit. 
The invention assumes that the configuration for ETR print head triggering 
is equipped with memory means and with control means for the ETR print 
unit, and the triggering of an ETR print head within a printing system is 
carried out entirely with the aid of microprocessors, microcomputers or 
computers, and energy for the various pixels of the printed image is 
furnished to the electrodes of an ETR print unit from a voltage source. 
The number of electrodes temporarily connected to the controllable voltage 
source is specified by a microprocessor control that outputs a control 
signal corresponding to the dependency on the number of triggered 
electrodes, to the controllable voltage source. 
The invention is also based on the concept that with a microprocessor 
control unit, the relevant print information at any given time is loaded 
into the switching unit at the correspondingly correct moment. In the 
active state, the switching unit assures that the pixels to be printed 
have current supplied to them for a defined period of time, in order for 
the heat required for the printing process to be generated in the ETR 
ribbon. 
In accordance with another feature of the invention, the controllable 
energy source is a digitally triggerable voltage source connected directly 
with control outputs of the microprocessor control unit, the voltage 
source supplies a voltage, and there is provided a measuring resistor 
across which a total current flows. 
In accordance with a further feature of the invention, there is provided a 
D/A converter for analog-triggering of the controllable voltage source, 
the D/A converter having digital inputs connected to outputs of the 
microprocessor control unit, and a control element having means for at 
least one of adjusting a basic amplification and adapting a printing 
intensity to a set printing speed. 
In accordance with an added feature of the invention, the switching unit 
has outputs each having a current source character for the electrodes of 
the ETR printing unit or dropping, protective or multiplier resistors for 
the electrodes. 
In accordance with an additional feature of the invention, there is 
provided a resistor in each current path for adjusting each current source 
or current distribution. 
In accordance with yet another feature of the invention, there is provided 
a one dropping, protective or multiplier resistor being located in each 
current path and assigned to the ETR electrodes, preferably having 
one-half to one-eighth the resistance of an effective resistor heating 
element. 
In accordance with yet a further feature of the invention, the controllable 
voltage source has a triggering input for a control voltage and a 
connection for additional regulation of a print quality by means of a 
measuring voltage, and including an inverting amplifier having a node 
point, a first resistor applying the measuring voltage to the node point, 
and a second resistor applying the inverted control voltage to the node 
point, or including a subtracting amplifier having inverting and 
non-inverting inputs, a first resistor applying the measuring voltage to 
the inverting input, and a second resistor applying the non-inverted 
control voltage to the non-inverting input or the non-inverted control 
voltage being applied directly to the non-inverting input. 
In accordance with yet an added feature of the invention, the switching 
unit receives relevant printing information for a given time at a 
correspondingly correct time in a first trigger phase, and the 
microprocessor control unit controls the switching unit in such a way that 
in an activated state of gates on an output side of a driver, during a 
second trigger phase, resistor heating elements in an ETR ribbon being 
assigned to the pixels to be printed are supplied with current for a 
defined period of time corresponding to a selected printing speed, so that 
requisite heat for a printing process is generated in the ETR ribbon. 
In accordance with yet an additional feature of the invention, the 
switching unit has a decoder with an input side being acted upon with at 
least one of data, commands and signals by the microprocessor control 
unit. 
In accordance with again another feature of the invention, there is 
provided another unit having components, such as microprocessors having 
memories, microcomputers or computers, the triggering of the ETR print 
head within the printing unit being carried out entirely with the 
components. 
In accordance with a concomitant feature of the invention, the ETR print 
head to be triggered is part of a postage meter. 
Other features which are considered as characteristic for the invention are 
set forth in the appended claims. 
Although the invention is illustrated and described herein as embodied in a 
configuration for ETR print head triggering, it is nevertheless not 
intended to be limited to the details shown, since various modifications 
and structural changes may be made therein without departing from the 
spirit of the invention and within the scope and range of equivalents of 
the claims. 
The construction and method of operation of the invention, however, 
together with additional objects and advantages thereof will be best 
understood from the following description of specific embodiments when 
read in connection with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to the figures of the drawing in detail and first, 
particularly, to FIG. 1 thereof, there is seen a configuration for ETR 
print head triggering which has a controllable energy source 1, a 
switching unit 2, an ETR printing unit 3, a microprocessor unit 5, a 
current collector electrode 6, and memory means 7, which are connected to 
the microprocessor control unit 5 for triggering the ETR printing unit 3. 
The memory means 7 contain at least graphic data for one printed image. 
Energy for electrodes of the ETR printing unit 3 is furnished from the 
single controllable energy source 1. A number n of electrodes 31, 32, 33, 
. . . that are temporarily connected to the controllable energy source 1 
is specified by the microprocessor control unit 5, which additionally 
outputs a control signal to the controllable energy source 1, wherein the 
control signal corresponds to the dependency on the number of triggered 
electrodes. 
The switching unit 2, that is acted upon through the microprocessor control 
unit 5, passes the energy on to an ETR print head 30 of the ETR printing 
unit 3 that is in contact with an ETR resistor ink ribbon 10 through the 
electrodes 31, 32, 33, . . . The relevant printing information at any 
given time is loaded at a correspondingly correct moment t.sub.1 into the 
switching unit 2, which in an activated state from a time t.sub.2 assures 
that the pixels to be printed are supplied with current for a defined 
period of time t.sub.j, so that the heat required for the printing process 
is generated in triggered, briefly electrically contacted regions 101, 
102, . . . of a resistor layer 100 of the resistor ink ribbon 10. 
FIG. 2 shows a circuit of the switching unit 2. A serial/parallel shift 
register 21 of the switching unit 2, which is acted upon by the serial 
printing data directly or through a decoder 20 as shown in FIG. 9 sends 
the printing data in a first triggering phase, beginning at the time 
t.sub.1, to latches of a buffer memory or store 22. Accordingly, the 
current printing information is available in the control unit 2 for a 
sufficiently long time prior to the actual printing process. 
In a second trigger phase beginning at the time t.sub.2, each gate G.sub.1, 
G.sub.2, . . . triggered by the associated outputs of the latches of a 
driver 23 on the output side is switched open during one strobe pulse, and 
a trigger pulse is output to the appropriate current path at the 
associated resistance R.sub.p. An SN 75518 triggering circuit with a 
32-bit shift register, 32 latches and 32 AND gates can advantageously be 
used as the switching unit 2. Once a predetermined period of time has 
elapsed, the new printing data are furnished by the microprocessor control 
unit 5 and stored in the latches of the buffer memory 22. 
In order to provide a constant print quality, the printer driver is set in 
such a way that for each ribbon speed V.sub.bj, where j=1, 2, . . . , m, 
the following equation applies: 
EQU t.sub.j *V.sub.bj =c, where c equals a constant (1) 
FIG. 3a shows an electrical substitute circuit diagram for ETR printers 
with a current path selected, having the associated resistance R.sub.p and 
a single constant current source I.sub.s. The resistance R.sub.p is a sum 
of resistances as follows: 
EQU R.sub.p =R.sub.v +R.sub.k +R.sub.h +R.sub.r +R.sub.b +R.sub.u +R.sub.1(2) 
where the symbols have the following meanings: 
R.sub.v : heating resistor 
R.sub.k : contact resistor of an electrode 
R.sub.h : resistance heating element 
R.sub.r : current return resistance 
R.sub.b : ribbon resistance 
R.sub.u : junction resistance between the ribbon and the return electrode 
R.sub.l : line resistance 
The contact resistance R.sub.k of an electrode having the upper resistor 
layer 100 of the resistance ink ribbon 10 is dependent on the size of the 
effective electrode surface area and on the contact pressure against the 
ribbon. The current return resistance R.sub.r of a middle layer 8 of the 
resistance ink ribbon preferably is formed of aluminum and depends on the 
total current and on the distance from the return electrode. The aluminum 
layer 8 is approximately 0.8 .mu.m thick, as compared with the resistor 
layer 100 which is approximately 15 .mu.m and as compared with an ink 
layer 9 having a thickness which is approximately 6 .mu.m. If the current 
collector electrode 6 is disposed near the electrodes of the ETR print 
head 30, the current return resistance R.sub.r is negligibly low. The 
ribbon resistance R.sub.b of the resistor layer 100 of the resistance ink 
ribbon 10 is determined by a contact angle .beta. of the surface of the 
return electrode 6. The junction resistance R.sub.u between the ribbon 10 
and the current collector electrode 6 depends on the pressure and on the 
return electrode surface area. 
The resistance heating elements R.sub.h are triggered by a clock frequency 
which is adapted in its pulse height and pulse width to the required 
heating energy. An energy W.sub.p, in each resistance heating element 
R.sub.h, that determines the print quality, thus becomes as follows: 
EQU W.sub.p =(I.sub.p2 *R.sub.h)*t.sub.j =(U.sub.h2 /R.sub.h)*t.sub.j(3) 
The requisite pulse height is furnished by the triggered energy source 1, 
which acts upon the electrodes 31, 32, 33, . . . that are temporarily 
connected to it through the switching unit 2, with a current I.sub.s or a 
voltage U.sub.s, having a magnitude which has a dependency on the 
temporarily different number n of triggered electrodes in such a way that 
a larger number of electrodes is supplied with a higher current or with a 
higher voltage than a lesser number would be. 
In the first variant shown in FIG. 1, an analog-triggerable energy source 1 
is provided, which is triggerable by the analog output of a digital/analog 
converter 4, that is connected by its digital inputs to outputs of the 
microprocessor control unit 5. 
In the case of each current print column, the number n of printing points 
to be activated is output, in binary coded form, to the D/A converter 4 of 
the microprocessor control unit 5, prior to the outputting of the printing 
information to the switching unit 2 in accordance with that number. Even 
with a simple eight-bit D/A converter, 256 different analog levels can be 
generated in this way, which correspond directly to the applicable number 
of pixels to be printed. These analog levels serve to trigger a 
triggerable and adjustable energy source 1. Accordingly, a defined energy, 
corresponding exactly to the number of pixels to be printed in each 
printing column, is fed into the system. 
This has the advantage firstly of enabling a single controllable and 
adjustable constant current source I.sub.s, for instance, to be sufficient 
for the entire system having arbitrarily many ETR electrodes, rather than 
one such current source having to be made available for each current path. 
Secondly, only a very small heating resistor R.sub.v is then necessary in 
each current path I, II, III, . . . for adjusting the current 
distribution. At the same time, however, because of the controllable 
constant current source for each printed column, provision is made for the 
precise predetermined printing energy to always be available for melting 
the lower ink layer 9. The following equation is approximately valid for 
the controllable constant current: 
EQU I.sub.s =(I.sub.p1 +I.sub.p2 +. . . +I.sub.pi) (4) 
The resistance of the heating resistor R.sub.v is one-half to one-eighth 
that of the effective heating resistor and preferably one-third to 
one-fourth of it, which minimizes the energy loss of the system as 
compared with the aforementioned prior art, with a very much larger 
resistor R.sub.v. If R.sub.h +R.sub.v &gt;R.sub.r +R.sub.b +R.sub.u +R.sub.l, 
the losses are minimal 
Another advantage of this invention is based on the fact that the printing 
intensity of the entire ETR head can be very easily achieved by varying a 
single control element S, and current source I.sub.s or constant voltage 
U.sub.s of the controllable namely by varying a factor y of the 
controllable constant energy source 1. If a further factor z is varied 
with the same control element S, the print speed or ribbon speed V.sub.b 
can additionally be taken into account. 
If the print speed V.sub.b and print intensity (contrast) increase, then 
the factors y and z increase as well. Since the partial currents in the 
current paths are equal, and the relationship I.sub.p =I.sub.p1 =I.sub.p2 
=. . . =I.sub.pi is established by means of the dropping, protective or 
multiplier resistors R.sub.v, the following equation applies: 
EQU I.sub.s =y*z*n*I.sub.p (5) 
FIG. 3b shows an electrical substitute circuit diagram for ETR printers 
with a single constant voltage source U.sub.s. When the voltage source 
U.sub.s is used as the energy source 1, provision is made, by 
incorporating a serial measuring resistor R.sub.m into the current 
circuit, for linearizing the voltage drop across the residual resistor 
R.sub.rest =R.sub.r +R.sub.b +R.sub.u +R.sub.m. If R.sub.m &gt;&gt;R.sub.r 
+R.sub.b +R.sub.u, then the following equation approximately applies: 
EQU R.sub.rest =R.sub.m (6) 
Since the current I.sub.p of each current path flows across the measuring 
resistor R.sub.m (which includes the line resistor R.sub.l), the total 
current I.sub.g =n*I.sub.p can be measured through U.sub.m. The following 
equation applies: 
EQU U.sub.m =n*I.sub.p *R.sub.m (7) 
If only one current path is included, in other words the smallest unit that 
corresponds to the value of one ETR electrode, then the factor n=1. 
The controllable constant voltage, taking n current paths into account, 
then becomes as follows: 
EQU U.sub.s =y*z*(U.sub.1 +[n*U.sub.2 ]) (8) 
In this case, the following equation applies: 
EQU U.sub.1 =U.sub.v +U.sub.k +U.sub.h, and U.sub.2 =R.sub.rest *I.sub.p(9) 
The adjustment times of the controllable energy source 1 are not critical, 
with a view to the maximum printing speeds in the range of approximately 
500 mm/s that can be attained with ETR technology. The engineering effort 
and expense is comparatively low, for optimal printing results. 
FIG. 4 presents a variant of a controllable voltage source, having a linear 
regulator 11, which is supplied with an unregulated input voltage U.sub.g 
and a command value amplified through a non-inverting operational 
amplifier 12, and which outputs a voltage U.sub.s on its output side. The 
command value voltage is obtained from the analog control voltage as 
follows: 
EQU U.sub.soll =(1+R.sub.s /R.sub.e)*U.sub.stell (10) 
A resistance ratio R.sub.s /R.sub.e of a control element S permits the 
adjustment of the basic amplification and/or a switchover of a resistor 
chain corresponding to the required factors y and z, which switchover is 
controlled by the microprocessor control unit 5 and is shown in FIG. 5. 
FIG. 6 shows a further variant for a controllable voltage source, which is 
equipped with a connection for additionally regulating the print quality 
by means of the measuring voltage U.sub.m. The measuring voltage drops at 
the measuring resistor R.sub.m, which is smaller by orders of magnitude 
than the dropping, protective or multiplier resistors R.sub.v or the 
heating resistors R.sub.h and is less than the current return resistance 
R.sub.r. The measuring voltage U.sub.m, through a first resistor R.sub.d, 
and an inverted control voltage U.sub.stell, through a second resistor 
R.sub.t, is located at a node point of an inverting amplifier 13. As the 
total resistance rises, the total current decreases, and accordingly 
U.sub.m also decreases, which leads to an increase in a command voltage 
U.sub.soll. 
FIG. 7 shows a further variant for a controllable voltage source with 
additional regulation. The amplifier 12 is constructed as a subtracting 
amplifier. Unlike the variant of FIG. 6, positive voltages U.sub.stell and 
U.sub.m can be applied on the input side, while the mode of operation is 
otherwise the same. 
A further variant for a controllable voltage source with digital control 
inputs, for correspondingly adjusting the selected printing speed, for 
adjusting the contrast per se, and with additional regulation of the print 
quality by means of the measuring voltage U.sub.m, is illustrated by FIG. 
5 in combination with an expansion of the block circuit diagram, that is 
not shown in FIG. 1, but which will be discussed below. The microprocessor 
unit 5 is additionally equipped on the input said with an analog/digital 
converter 14 as shown in FIG. 8, at an input of which the measuring 
voltage U.sub.m is applied. The digital data corresponding to the 
measuring voltage U.sub.m are input into the microprocessor unit 5 and 
form a correction variable, which additionally enters into the 
aforementioned equation (8). The following equation thus results for the 
control voltage: 
EQU U.sub.stell =(U.sub.1 -U.sub.3 +[n*U.sub.2 ]) (11) 
In a further variant that is not shown in FIG. 1, a digitally triggerable 
energy source 1 (current source I.sub.s or voltage source U.sub.s) is 
connected directly to the outputs of the microprocessor control unit 5. 
For instance, the number n of the electrodes temporarily connected to the 
controllable current source I.sub.s is specified directly by the 
microprocessor control unit 5, which outputs a control signal to the 
controllable energy source 1 that corresponds to the dependency on the 
number n of triggered electrodes, so that each resistance heating element 
R.sub.p brings to bear the requisite uniform heating output in printing. 
If the ETR printer is used for a postage meter, then the memory and the 
microprocessor control unit of the postage meter can be jointly used for 
triggering purposes. A postage meter of this kind includes memory means 
and receiving means connected to it, for data which can be transmitted 
through transmission means, input means, a control module, and the ETR 
printer. 
The invention is not limited in its structure to the preferred exemplary 
embodiment described above. On the contrary, there are a number of 
conceivable variants, which make use of the provisions described and 
illustrated herein, even in embodiments that are fundamentally different.