EP0575668A2 - Drive circuit for an electrothermal recorder with resistive ribbon - Google Patents

Abstract

A drive circuit for an electrothermal recorder with resistive ribbon (10) has a printing unit (3), a current collector electrode (6), a storage means (7) and a pressure control unit (5). A resistive ink ribbon (10) which is subject to relative movement transmits the ink particles from the ink layer (9) when the associated heating resistor in the resistive layer (100) is heated in areas (101, 102, ...,) on the recording medium. By means of at least one measurement electrode (29) which is arranged near to the print head, the voltage drop Um caused by the total current Lg and by the variation of the resistances is measured over the unselective current path (feedback layer (8)) in the resistive ink ribbon and the constant voltage source (1) which has a reference voltage input causes the electrodes (31, 32, 33, ...) which are temporarily connected to the said source (1) via the switching unit (2) to be fed with a feed voltage Us = alpha Up + UB. <IMAGE>

Description

The invention relates to a control circuit for an electrothermal printing device with a resistance band of the type specified in the preamble of claim 1. Such printing devices, which print printing patterns on a recording medium that is relatively moved, are to be printed, whereby a likewise relatively moving ink carrier with defined electrical resistance transfers the color particles For example, suitable for franking mail with franking machines.

Franking machines have input, storage and display means and a print control unit for a printing device. The pressure control unit contains a microprocessor control and acts on a switching unit.

It is already known from DE 38 33 746 A1 a switching unit for a print head which is acted on via a control unit (ASE) and which, in contrast to an ETR print head, contains the resistance elements themselves (thermal transfer printing method) and a selective control with preheating of the resistance elements to reduce the heating power when printing, known.

A series / parallel shift register loaded with the serial print data transfers the print data to the latches of an intermediate store in a first control phase. In a second control phase, each gate controlled by the associated outputs of the latches is switched to continuity during a strobe pulse and a control pulse is emitted to the respective resistance element. The resistance heating elements are preheated directly by a clock frequency which is adapted to the required heating energy in terms of its pulse height and pulse width. Such preheating via energy from a voltage source is in principle not possible in a printer with an electrothermal resistance ribbon (ETR) because the resistance elements are located in the resistance layer of the resistance ribbon and because the resistance ribbon is moved relative to the print head and also to the recording medium to be printed.

Such a (ETR) printer, which has an electrothermal resistance ribbon, is already known from DE 21 00 611, the pin electrodes of which are encased by a counter electrode. The electrodes are supplied by applying a voltage potential from a constant voltage source. This type of control has the advantage of a simple, cheap power supply, but it is Print resolution too low due to the small number of electrodes. If the casing is omitted, the number of electrodes in the pressure bar can be increased. However, this has the disadvantage that a common current collecting electrode is then used as the counterelectrode, that the individual currents fed simultaneously via n electrodes add up in one point in a returning metal layer in the resistance ribbon and that the voltage drop between this point and the current collecting electrode, that is to say via the unselective one Part of the current path leading through the resistance ink ribbon, through which the currently activated number of printing electrodes is determined, which leads to undetectable printing performance fluctuations and thus to different printing qualities.

In addition to the mechanics, a modern ETR printer includes an electronic head control, an ETR print head with a large number of electrodes and a current collecting electrode which are connected to a power supply unit. The expansion of the application area of thermal printing technology, in particular to include label and bar code applications, has increased the need for print heads with a larger print width (1 inch and more) and a higher geometric resolution (200 dots per inch and more). This can only be achieved with print heads with a large number of selectively controllable electrodes. While 25 to 50 electrodes were originally sufficient for the conventional line printer, the number of electrodes increases to 150 to 250 in the above-mentioned applications. Since under certain operating conditions (pressure of a continuous pressure column) all electrodes have to be supplied with current at the same time, considerable effort has to be expended for the potential provision of this electrical power.

Additional circuit measures have already attempted to keep the power conversions per electrode approximately constant despite the influences mentioned. For example, the pressure energy is fed as a constant current in each current path associated with each electrode in order to ensure a uniform print quality. This type of control is the optimal solution from a technical point of view, but has the disadvantage of very high costs for the power supply if the ETR print head has a large number of electrodes.

In simple known cases, the control circuit for an ETR printhead control has a common voltage source and series resistors for the electrodes in each partial current path. The ETR print head contains a multiplicity of electrodes which are arranged insulated from one another, each of which can generate a pixel of the printed image. The energy supplied via these electrodes is converted into current heat in the area of the resistance layer assigned to each pixel, which leads to the melting of the color of the color layer located in the area and thus to the printing of a dot.

The ETR printhead acts on the recording medium, preferably paper, via a resistance ink ribbon that is moved with the recording medium. The resistance ink ribbon has an upper resistance layer in contact with the ETR print head, a middle current return layer and a lower ink layer in contact with the recording medium (EP 88 156 B1).

It is known to install such a series resistor in each control circuit of an electrode of the head, the resistance value of which is constant in each case and is considerably above the sum of the resistances of the resistance band in the current path.

In this respect, these fixed resistors dominate the variable resistors that are on the way of the print head-ribbon-back electrode and relatively reduce the influence of these variances on the overall resistance. The series resistors used have the task of keeping the current for the electrodes as constant as possible. This happens the better, the relatively larger these resistances are to the sum of all resistances of the actual pressure current path (tape resistance, resistance of the returning metal layer, contact resistances). These series resistors are currently selected to be approx. 3 to 4 times larger, i.e. of course also that only about a quarter of the energy used is used for printing, the rest is converted into heat loss.

Such a solution is used, for example, in the Hermes printer 820 equipped with an ETR printing unit. The additional loss of electrical energy in the series resistors is disadvantageous.

This loss is no longer acceptable, in particular, and would lead to oversized power supply units if high electrical pressure output and a large number of electrodes to be controlled in parallel are used. With a print width of 1 inch and a resolution of 250 dpi, which, for example, meets the requirements for high quality label printing, 250 electrodes are to be controlled. In this case, the power loss would increase at R = 300 ohms and I = 50 mA to P = 250 (I² * R) ≈ 187 W. Another problem with 250 activated electrodes, with a total current of 12.5 A flowing in the unselective part of the resistance ribbon, is its return from the resistance ribbon via a current collecting electrode.

An ETR printer with two back electrodes is known from EP 0 301 891 A1. Although this leads to a current distribution when the total current is returned, it does not improve the overall power balance. When feeding the electrodes, it should also be noted that the energy to be supplied depends on the resistance of each current path assigned to a pixel, on the melting temperature of the color, the intended contrast of the printed image and on the speed of the resistive ink ribbon being moved, and is non-linear with the surface roughness of the recording medium (Paper type) increases.

The print quality in the ETR process depends crucially on the fact that the electrical power, which is converted into thermal energy per electrode in the resistance band, is the same for all electrodes and all times.

A too low electrical power leads to an insufficient heating of the corresponding pixel area in the ink layer of the resistance band. This then results in a smaller volume of melted-out color and ultimately an insufficient contrast of the corresponding pixel on the substrate to be printed. On the other hand, an excessive electrical power leads to a strong heating of the ETR band, which also affects the support layer of the band and reduces its strength. In addition, excessive electrical power continues to overload the power supply assembly. In any case, differences in the contrast of the imprint would become visible with changing electrical power.

• a) Der Übergangswiderstand R_k zwischen einer Elektrode des Druckkopfes und der Widerstandsschicht des ETR-Bandes, der vor allem vom gerade herrschenden Anpreßdruck abhängig ist. Letzterer ist durch die Oberflächenbeschaffenheit des Aufzeichnungsträgers aber auch durch den Verschleißzustand des Druckkopfes beeinflußt.
• b) Der Heizwiderstand R_h der Widerstandsschicht des Widerstandsfarbbandes, der von der Dickentole-ranz und Homogenität der Widerstandsschicht abhän-gig ist.
• c) Der Widerstand R_r der rückleitenden Metallschicht des Widerstandsfarbbandes, der von der Homogenität und Dickentoleranz der Metallschicht des Bandes sowie von der Entfernung der Stromsammelelektrode zu den Druckkopfelektroden abhängig ist.
• d) Der integrale Widerstand der Widerstandsschicht des Bandes bei der Rückleitung des Stromes (Bandwiderstand) R_b, der von deren Dickentoleranz und Homogenität der Widerstandsschicht sowie der Kontaktoberfläche mit der Stromsammelelektrode abhängig ist.
• e) Der integrale Übergangswiderstand R_ü der Widerstandsschicht gegenüber der Stromsammelelektrode, der vor allem vom gerade herrschenden Anpreßdruck abhängig ist. Dieser ist durch den Umschlingungswinkel des Bandes mit der Stromsammelelektrode und die herrschenden Bandzugkräfte beeinflußt.

The following are essential for a variance in the electrical pressure output:

• a) The contact resistance R _k between an electrode of the print head and the resistance layer of the ETR tape, which is primarily dependent on the contact pressure currently prevailing. The latter is influenced by the surface condition of the record carrier but also by the state of wear of the print head.
• b) The heating resistance R _{h of} the resistance layer of the resistance ribbon, which is dependent on the thickness tolerance and homogeneity of the resistance layer.
• c) The resistance R _{r of} the returning metal layer of the resistance ribbon, which is dependent on the homogeneity and thickness tolerance of the metal layer of the ribbon and on the distance of the current collecting electrode to the printhead electrodes.
• d) The integral resistance of the resistance layer of the tape when the current is returned (tape resistance) R _b , which is dependent on its thickness tolerance and homogeneity of the resistance layer and the contact surface with the current collecting electrode.
• e) The integral contact resistance R _{ü of} the resistance layer with respect to the current collecting electrode, which is primarily dependent on the prevailing contact pressure. This is influenced by the wrap angle of the tape with the current collecting electrode and the prevailing tape tensile forces.

Since in the overall system consisting of the ETR head with the electrodes, the ETR ribbon and the back electrode, there are very many parasitic series resistances of variable value (electrode / band transition resistances, return resistance of the Aluminum layer in the strip, contact resistance between strip and back electrode), which lead to a variation of the total resistance during operation, it is not possible to dispense with the series resistances while maintaining the principle of a constant voltage source, since the then also varying partial voltage across the heater (= pressure) resistance would lead to different printing energies. This would result in fluctuating print quality.

The main influence on the fluctuation of the voltage drop arises in addition to the above-mentioned factors but by printing variable data, a number between 0 and the number n of the electrodes present being generally controlled per printing column. The voltage drop across the resistors c) to e) in the unselective (return) current path depends on the current flowing through. This in turn is equal to the sum of the individual currents in the selective part of the current path with the resistors a) + b) and thus depends on the number of activated electrodes of the print head.

In order to improve the print quality while reducing the power loss in the application P 42 14 545.7, an arrangement for an ETR printhead control, with storage means, with a microprocessor control for an ETR printing unit has been proposed, energy being used for the electrodes of the ETR printing unit a controllable energy source is provided.

The number of electrodes temporarily connected to the controllable energy source is predefined by the microprocessor control, which outputs a control signal corresponding to the dependency on the number of activated electrodes delivers the controllable energy source. The latter applies a current or a voltage to the electrodes which are temporarily connected via a switching unit, the height of which has such a dependence on the temporarily different number of controlled electrodes that a larger number of electrodes are supplied with a higher current or voltage, than a smaller number. A control voltage, which is preferably generated via a D / A converter, is passed to an amplifier input of an amplifier, which outputs the required target voltage for the controllable voltage source. The total current flowing in the resistance ribbon is dissipated to ground using a current collecting electrode.

In a variant with a controllable voltage source, the total current also flows through an external measuring resistor, from which a measuring voltage is tapped and fed to a second input of the amplifier. This combination of control and regulation is circuit-intensive. With a higher (lower) measuring voltage, the target voltage and thus the supply voltage of the print head is reduced (increased). However, this means that only the fluctuations in the total resistance caused by the strip quality can be compensated for, but no errors can be detected. The measuring voltage drops with a higher total resistance, in particular to compensate for contact problems of the electrodes, the supply voltage is increased. However, the failure of an electrode cannot be detected. The measuring voltage then drops and the remaining electrodes are supplied with a somewhat too high supply voltage, which leads to a somewhat higher contrast in the printed image. On the other hand, an increase in the total current caused by an error in the print head drive circuit would only lead to an insignificant reduction in the supply voltage and thus in the contrast, and would initially go unnoticed stay. However, this can lead to serious damage to the device during continuous operation.

The invention is based on the fact that, with a larger number n of existing electrodes to be controlled simultaneously, supplying the individual electrodes with the previous control circuits is too expensive and too complex.

It is the object of the invention to propose a circuit arrangement for an ETR printhead control which allows the deficiencies of the prior art to be remedied with a cheaper supply. The circuit arrangement should be usable for ETR high-performance printers with a large number of electrodes, with a drastic reduction in power loss and consistently good print quality. Protection of the printing device against destruction should also be ensured.

The object is achieved with the characterizing features of claim 1.

The invention is based on the consideration, taking into account the total resistance, with a regulation of the supply voltage in accordance with the constantly changing power requirement to create an inexpensive alternative to the solution with a control of the supply voltage, as was proposed in the application P 42 14 545.7.

For the common supply of the electrodes, an adjustable constant voltage source is used which, compared to ground potential, supplies a supply voltage consisting of a constant, adjustable print voltage, which is increased by a variable reference voltage.

The reference voltage in relation to ground potential can be changed in accordance with the number n of electrodes activated simultaneously and in accordance with the variance of certain resistances in the resistance band. The invention is based on the fact that a compensation of the occurring variance of the voltage drop across the heating resistors can be carried out in the resistance ribbon.

According to the invention, the voltage drop caused by the total current is measured via the unselective (return) current path in the resistance ink ribbon by means of one or more additional or existing electrodes which are arranged on the print head. This measured value forms the reference voltage, preferably at the same level. It is added to the set print voltage. Then the supply voltage of the activated electrodes of the print head results in such a way that an increase in the measured value leads to an increase and a decrease leads to a decrease in the supply voltage, the print voltage remaining constant.

With a reduced reference voltage compared to the measuring voltage, the level of the supply voltage has such a dependency on the temporarily different number n of activated electrodes that a larger number of activated electrodes are supplied with a higher supply voltage but per dot with a lower pressure energy , than a smaller number of activated electrodes, which are supplied with a higher pressure energy with a lower supply voltage per dot.

In addition, the variance of the resistances in the non-selective (return) current path in the resistance ribbon is also taken into account.

The measuring electrode is a separately arranged and / or just not activated normal printhead electrode. For this purpose, the ETR print head can advantageously be equipped with edge electrodes which are located at the ends of the electrodes of the print head arranged in line in the print bar, but which are not used for the franking imprint.

Figur 1,

Blockschaltbild der erfindungsgemäßen elektrothermischen Druckvorrichtung

Figur 2,

elektrisches Ersatzschaltbild der Ansteuerschaltung mit einer einzigen Konstantleistungsquelle

Figur 3,

Ausführungsvariante der Ansteuerschaltung der elektrothermischen Druckvorrichtung

Figur 4,

Variante der Druckvorrichtung mit einer gesondert angeordneten Meßelektrode

Figur 5,

Variante der Druckvorrichtung mit Meßelektrode in der Druckleiste und mit großflächiger Stomsammelelektrode

Figur 6,

erste Variante der Anpaßschaltung

Figur 7,

zweite Variante der Anpaßschaltung

Die Figur 1 zeigt ein Blockschaltbild der erfindungsgemäßen elektrothermischen Druckvorrichtung mit einer Ansteuerschaltung, bestehend aus einer Konstantspannungsquelle 1, einer Schalteinheit 2, einer ETR-Druckeinheit 3, einer Drucksteuereinheit 5, einer Stromsammelelektrode 6 und mit einem Speichermittel 7, das mit der Drucksteuereinheit 5 für die Ansteuerung der ETR-Druckeinheit 3 verbunden ist. Das Speichermittel 7 enthält mindestens die Grafikdaten für ein Druckbild.Advantageous developments of the invention are characterized in the subclaims or are shown below together with the description of the preferred embodiment of the invention with reference to the figures. Show it:

Figure 1,

Block diagram of the electrothermal printing device according to the invention

Figure 2,

Electrical equivalent circuit diagram of the control circuit with a single constant power source

Figure 3,

Design variant of the control circuit of the electrothermal pressure device

Figure 4,

Variant of the printing device with a separately arranged measuring electrode

Figure 5,

Variant of the printing device with measuring electrode in the pressure bar and with a large current collecting electrode

Figure 6,

first variant of the matching circuit

Figure 7,

second variant of the matching circuit

1 shows a block diagram of the electrothermal printing device according to the invention with a control circuit, consisting of a constant voltage source 1, a switching unit 2, an ETR printing unit 3, a pressure control unit 5, a current collecting electrode 6 and with a storage means 7 which is connected to the pressure control unit 5 for controlling the ETR printing unit 3. The storage means 7 contains at least the graphic data for a print image.

The print control unit (DS) 5 of the control circuit acts on the switching unit 2, with the electrodes being supplied with energy from a controllable constant voltage source 1 for the individual pixels of the print image in order to control a print head 30, and a print pattern being printed on a record carrier which is relatively moved and is to be printed , in that the likewise relatively moved resistance ink ribbon 10 transfers the color particles from the ink layer 9 when the associated heating resistor is heated in the resistance layer 100 in areas 101, 102, 103, ....

The switching unit 2 acted upon by the pressure control unit 5 transmits the power to an ETR printhead 30 of the ETR printing unit 3, which is in contact with an ETR resistance ink ribbon 10 via electrodes 31, 32, 33,.. Relevant print information at the correspondingly correct time t 1 is loaded into the switching unit 2, which in the activated state from t 2 ensures that the pixels to be printed are energized for a defined time t _j , so that the heat required for the printing process in the briefly activated contacted areas 101 , 102, ..., 105, ..., the resistance layer 100 of the resistance ribbon 10 is produced.

The energy for the electrodes of the ETR printing unit 3 is provided by an adjustable constant voltage source 1, those being temporarily connected to the controllable voltage source 1 standing electrodes 31, 32, 33, ..., are specified by the pressure control unit 5. In FIG. 1, electrodes 31, 32, 33, 34 and 35 are connected via the switching unit 2 to the positive pole + U _{s of} the constant voltage source 1, each partial current causes heating in the respectively contacted areas of the resistance layer 100.

The current collects in the return layer 8, which is preferably made of aluminum, and which has a current return resistance R _r — not shown in FIG. 1. The current flows through the resistance layer 100 to the current collecting electrode 6 connected to ground (or to the negative pole -U _s ) and thereby generates a voltage drop. This can be tapped with a measuring electrode 29.

The voltage drop caused by the total current I _g and the variance of the resistances across the unselective (return) current path in the resistance ink ribbon is measured by means of at least one electrode 29 arranged close to the print head, and the constant voltage source 1 is caused, which is temporarily used with it via the switching unit 2 connected electrodes 31, 32, 33, ... to be supplied with a supply voltage U _s , the level of the supply voltage of the activated electrodes of the printhead being controlled in such a way that an increase in the measured value leads to an increase in the supply voltage of the electrodes leads and a drop to lower the supply voltage. This compensates for the existing variance in the voltage drop across the heating resistors in the resistance ribbon.

The constant voltage source 1 has a reference voltage input for the measuring voltage emitted by at least one measuring electrode, that of the Number n of the controlled electrodes and the residual resistance R _{r is} dependent. For the measuring electrode 29, at least one additional printhead electrode, which is present due to the manufacturing process but is not used during printing, can advantageously be used.

FIG. 2 shows an electrical equivalent circuit diagram with a constant voltage source having an input for the reference voltage U _B and with the switching unit 2. From the switching unit 2, only the gates G₁ to G₄ are shown as switches with associated series resistors R _v in FIG. 2 for the sake of simplicity. The switches are shown in the closed state during the energization time t _j , ie when a strobe pulse is applied to the switching unit.

Das elektrische Ersatzschaltbild für ETR-Drucker zeigt vier eingeschaltete Strompfade mit den zugehörigen Widerständen R_p1, R_p2, R_p3 und R_p4 und mit einem Restwiderstand R_rest, mit einem Meßstrompfad und mit einer Konstantspannungsquelle U_s angegeben. Jeder Widerstand R_pi ergibt sich als Widerstandssumme zu:

${\text{R}}_{\text{pi}}{\text{= R}}_{\text{vi}}{\text{+ R}}_{\text{ki}}{\text{+ R}}_{\text{hi}}\text{ (2)}$

mit i = 1, 2, 3, 4 für die einzelnen Strompfade. Der gemeinsame Restwiderstand ist gleich:

${\text{R}}_{\text{rest}}{\text{= R}}_{\text{r}}{\text{+ R}}_{\text{b}}{\text{+ R}}_{\text{ü}}{\text{+ R}}_{\text{l}}\text{ (3)}$

mit    R_v - Vorwiderstand
   R_k - Kontaktwiderstand einer Elektrode
   R_h - Widerstandsheizelement
   R_r - Stromrückleitwiderstand
   R_b - Bandwiderstand
   R_ü - Übergangswiderstand Band/Rückelektrode
   R_l - Leitungswiderstand
Der Wert der Vorwiderstände R_v und R_k ist wesentlich kleiner als der Wert der Heizwiderstände R_h. Die Widerstandsheizelemente R_h ≈ R_p werden durch eine in ihrer Impulshöhe und Impulsbreite an die benötigte Heizenergie angepaßte Taktfrequenz angesteuert. Damit ergibt sich die die Druckqualität bestimmende Energie W_p in jedem Widerstandsheizelement R_h zu:

${\text{W}}_{\text{r}}{\text{= (U}}_{\text{r}}{\text{²/R}}_{\text{h}}{\text{) * t}}_{\text{j}}{\text{, bei R}}_{\text{h}}{\text{» R}}_{\text{v}}{\text{+ R}}_{\text{k}}\text{ (4)}$

Die erforderliche Impulshöhe U_p wird von der einstellbaren Konstantspannungsquelle 1 bereitgestellt, welche zu diesem Zweck die mit dieser über die Schalteinheit 2 temporär in Verbindung stehenden Elektroden 31, 32, 33,..., mit einer Spannung U_s beaufschlagt, deren Höhe eine derartige Abhängigkeit von der temporär verschiedenen Anzahl n an angesteuerten Elektroden aufweist, daß eine größere Anzahl an Elektroden mit einem höheren Strom oder mit einer höheren Spannung versorgt werden, als eine geringere Anzahl.For a constant print quality, the printer drive is set so that for each belt speed V _bj with j = 1, 2, ..., m:

${\text{t}}_{\text{j}}{\text{* V}}_{\text{bj}}\text{= c with c = constant (1)}$

The electrical equivalent circuit diagram for ETR printers shows four switched current paths with the associated resistors R _p1 , R _p2 , R _p3 and R _p4 and with a residual resistance R _rest , with a measuring current path and with a constant _{voltage source} U _s . Each resistance R _pi is the _{sum of the} resistances:

${\text{R}}_{\text{pi}}{\text{= R}}_{\text{vi}}{\text{+ R}}_{\text{ki}}{\text{+ R}}_{\text{Hi}}\text{(2)}$

with i = 1, 2, 3, 4 for the individual current paths. The common residual resistance is the same:

${\text{R}}_{\text{rest}}{\text{= R}}_{\text{r}}{\text{+ R}}_{\text{b}}{\text{+ R}}_{\text{ü}}{\text{+ R}}_{\text{l}}\text{(3)}$

with R _v - series resistor
R _k - contact resistance of an electrode
R _h - resistance heating element
R _r - current return resistance
R _b - band resistance
R _ü - contact resistance band / back electrode
R _l - line resistance
The value of the series resistors R _v and R _k is significantly smaller than the value of the heating resistors R _h . The resistance heating elements R _h ≈ R _p are controlled by a clock frequency which is adapted to the required heating energy in terms of its pulse height and pulse width. This results in the energy W _p determining the print quality in each resistance heating element R _h :

${\text{W}}_{\text{r}}{\text{= (<figure-callout id="2" label="U r" filenames="imgf0001.png,imgf0002.png" state="{{state}}">U </figure-callout>}}_{\text{r}}{\text{² / R}}_{\text{H}}{\text{) * t}}_{\text{j}}{\text{, at R}}_{\text{H}}{\text{»R}}_{\text{v}}{\text{+ R}}_{\text{k}}\text{(4)}$

The required pulse height U _p is provided by the adjustable constant voltage source 1, which for this purpose applies a voltage U _{s to} the electrodes 31, 32, 33, ... which are temporarily connected to it via the switching unit 2, the level of which is such Depending on the temporarily different number n of activated electrodes, a larger number of electrodes are supplied with a higher current or with a higher voltage than a smaller number.

Der Gesamtwiderstand R_g ergibt sich zu:

${\text{R}}_{\text{g}}{\text{= (R}}_{\text{p1}}{\text{∥R}}_{\text{p2}}{\text{∥R}}_{\text{p3}}{\text{∥ ... ∥R}}_{\text{pi}}{\text{) + R}}_{\text{rest}}\text{ (6)}$

vereinfacht gilt bei R_p1 = R_p2 = R_p3 = ... = R_pi und i = n

${\text{R}}_{\text{g}}{\text{= (R}}_{\text{p}}{\text{/n) + R}}_{\text{rest}}\text{ (7)}$

Der Wert des Vorwiderstandes R_v beträgt 1/10 bis 1/100 vom Wert des effektiven Heizwiderstandes R_h. Das minimiert gegenüber dem o.g. Stand der Technik die Verluste des Systems noch weiter. Im Vorwiderstand gehen bei R_v = 1,2 Ohm (R_v = 15 Ohm) ca. 3 mW (37,5 mW) als Wärme verlohren, da I_p = 50 mA bei nur n = 1 Elektrode fließen. Bei n = 192 gleichzeitig aktivierten Elektroden wird eine ganze Druckspalte gedruckt und zur Kompensation der sich ergebenden zusätzlichen Kontrasterhöhung sollen pro Elektrode nur noch 40 mA fließen. In den Vorwiderständen werden somit insgesamt ca. 0,6 W (4,6 W) in Wärme umgesetzt. Der Restwiderstand R_rest ≈ 1 Ohm ist demgegenüber bei einer hohen Anzahl gleichzeitig angesteuerter Elektroden verlustleistungsintensiv (bei n = 192, ca. 90 bis 100 W). Für R_rest « R_p und nur einer einzigen angesteuerten Elektrode sind die Verluste minimal (bei n = 1, ca. 50 mW).The relationship approximately applies to the total current:

${\text{I.}}_{\text{G}}{\text{= (I}}_{\text{p1}}{\text{+ I}}_{\text{p2}}{\text{+ ... + I}}_{\text{pi}}{\text{) = n * I}}_{\text{p}}\text{(5)}$

The total resistance R _g results in:

${\text{R}}_{\text{G}}{\text{= (R}}_{\text{p1}}{\text{∥R}}_{\text{p2}}{\text{∥R}}_{\text{p3}}{\text{∥ ... ∥R}}_{\text{pi}}{\text{) + R}}_{\text{rest}}\text{(6)}$

R _p1 = R _p2 = R _p3 = ... = R _pi and i = n

${\text{R}}_{\text{G}}{\text{= (R}}_{\text{p}}{\text{/ n) + R}}_{\text{rest}}\text{(7)}$

The value of the series resistor R _v is 1/10 to 1/100 of the value of the effective heating resistor R _h . This further minimizes the losses of the system compared to the prior art mentioned above. At R _v = 1.2 Ohm (R _v = 15 Ohm) approx. 3 mW (37.5 mW) dissipate as heat, since I _p = 50 mA flow with only n = 1 electrode. If n = 192 electrodes are activated at the same time, an entire print column is printed and only 40 mA should flow per electrode to compensate for the additional contrast increase. A total of approximately 0.6 W (4.6 W) is converted into heat in the series resistors. The residual resistance R _rest ≈ 1 ohm, on the other hand, is power-intensive if there are a large number of electrodes that are driven simultaneously (at n = 192, approx. 90 to 100 W). For R _rest «R _p and only one controlled electrode, the losses are minimal (at n = 1, approx. 50 mW).

Es wurde ermittelt, daß die Meßspannung U_m wegen der nicht zu vermeidenden Widerstände R_km = 5 Ohm und R_hm = 115 Ohm im Meßstromkreis nur um 4,8 mV bei einem Meßstrom von 40 µA verfälscht wird.The following applies to the measuring voltage U _m when the current flow in the measuring circuit is negligible:

${\text{U}}_{\text{m}}{\text{= n * I}}_{\text{p}}{\text{* (R}}_{\text{r}}{\text{+ R}}_{\text{ü}}{\text{+ R}}_{\text{l}}\text{) (8th)}$

It was determined that the measuring voltage U _{m is} only falsified by 4.8 mV at a measuring current of 40 µA due to the unavoidable resistances R _km = 5 ohms and R _hm = 115 ohms in the measuring circuit.

Eine Ausführungsvariante der Ansteuerschaltung wird anhand der Figur 3 erläutert.From this measuring voltage, the reference potential for the constant voltage source 1 is preferably formed by impedance conversion. The electrodes with a supply voltage U _{s are} equal to the sum from reference voltage U _B and a voltage U _p adjustable with the defined factor α:

${\text{U}}_{\text{s}}{\text{= αU}}_{\text{p}}{\text{+ U}}_{\text{B}}\text{(9)}$

An embodiment variant of the control circuit is explained with reference to FIG. 3.

For the switching unit 2, six pieces of the control circuits SN 75518, each with 32 bit shift registers, 32 latches of the buffer memory and 32 AND gates, can advantageously be used for the control of 192 electrodes in a pressure bar. The "data out" output of the first control circuit is connected to the "data in" input of the second control circuit. In the following, the inputs / outputs are also interconnected in order to load all print data for one print column. After a defined time has elapsed, the new print data are provided by the print control unit 5 and can be stored in the latches of the intermediate store.

Each with the serial print data directly at the input "data in" series / parallel shift register of the switching unit 2 transfers the print data in a first control phase from t 1 to the latches of an associated buffer, which has a control input "latch enable". The current printing information is therefore available in the switching unit 2 sufficiently long before the actual printing process. In a second drive phase from t₂, each gate driven by the associated outputs of the latches gate G₁, G₂, ..., an output-side driver is switched to pass during a strobe pulse and a drive pulse of pulse width t _j to the current path with the associated resistances R _p and R _rest delivered.

In the control circuit on which the exemplary embodiment is based, the best printing results can be achieved with an electrode current of approximately 45 to 50 mA, which corresponds to the preferred number of electrodes of n = 192 electrodes and the band type used with a heating resistor R _h of approximately 120 ohms and a power of approx. 300 mW converted into heat in each heating resistor.

If the 192 electrodes are activated at the same time and the residual resistance R _{Rest is} approx. 1 ohm, a measuring voltage U _m of max. 10 V measured and thus a supply voltage U _s of approx. 19 V is required. Only a voltage of approximately 1 V then drops across the series resistors R _v between the driver output of the switching unit 2 and the electrodes, which have a value between one eighth and one hundredth of the value of the heating resistor R _h in the resistance layer 100 of the resistance ribbon 10 .

In Figure 3, a voltage supply unit SVE with an adjustable constant voltage source 11 and a power supply unit 14 is also presented that a first DC voltage U _g of a maximum of 30 V and a second DC voltage U _c = + 5 V for supplying the rest of the circuit, in particular the Switch unit 2 outputs. The adjustable constant voltage source is in particular a linear regulator 11, which contains, for example, a parallel connection of the circuit type LM 317, to which the first DC voltage U _{g is} supplied and which outputs a regulated voltage U _s on the output side for supplying the drivers in the switching unit 2. The reference voltage U _B at the control input of the Linear regulator 11 results directly from the analog measuring voltage U _m or from the amplified measuring voltage via a matching circuit 12. The matching circuit 12 contains for impedance conversion at least one non-inverting amplifier 13 connected as a voltage follower and a protective circuit 17 against an excessively high output. It contains a Zener diode, which limits the reference voltage to U _B ≦ +10 V.

FIG. 4 relates to a further variant with a flat measuring electrode 29 arranged on one side of the pressure bar and the current collecting electrode 6 arranged on the other side.

In a preferred variant - shown in FIG. 5 - the measuring electrodes are each arranged at the two ends of the print bar of the print head 30 at a distance from the printing electrodes. The edge electrodes are also in contact with the resistance ribbon, but are not acted upon by drive pulses from the printhead drive electronics. The current collecting electrode 6 flatly surrounds the print bar and preferably consists of a piece of sheet metal with a central opening as a cutout for the print head 30.

Das Widerstandsverhältnis gestattet das Einstellen der Grundverstärkung. Die Verstärkung liegt theoretisch bei 1, kann aber durch die äußere Beschaltung des Verstärkers auch andere Werte annehmen, falls das für eine verbesserte Druckqualität erforderlich ist. Aufgrund der kühleren Druckpunktumgebung wird beim Abdruck nur eines einzigen Dots mehr Energie benötigt, als beim Abdruck einer ganzen Druckspalte. Bei n = 192 gleichzeitig aktivierten Elektroden wird zur Kompensation der sich ergebenden Kontrasterhöhung, die durch gegenseitige Aufheizung der benachbarten Elektroden bedingt ist, pro Elektrode nur noch ein Strom von ca. I_p = 40 mA benötigt.The measuring voltage is tapped virtually without power by integrating a non-inverting amplifier 13 - shown in FIG. 6 - into the measuring branch:

${\text{U}}_{\text{B}}{\text{= (R}}_{\text{n}}{\text{/ R}}_{\text{d}}{\text{) * [(R}}_{\text{d}}{\text{+ R}}_{\text{s}}{\text{) / (R}}_{\text{t}}{\text{+ R}}_{\text{n}}{\text{)] * U}}_{\text{m}}\text{(10)}$

The resistance ratio allows the basic gain to be set. The gain is theoretically 1, but can also assume other values due to the external wiring of the amplifier if this is necessary for an improved print quality is. Due to the cooler pressure point environment, more energy is required to print a single dot than to print an entire print column. With n = 192 electrodes activated at the same time, only a current of approx. I _p = 40 mA is required per electrode to compensate for the resulting increase in contrast, which is caused by mutual heating of the neighboring electrodes.

It was found that the total energy required when printing a column in which all printing electrodes are applied simultaneously is approximately 80% of the printing energy per dot. When the gain V _u <1 is defined, the supply voltage U _s , which is divided between the print voltage U _p and the measurement voltage U _m , is automatically reduced in accordance with the number n of electrodes which is controlled at the same time. For example, when n = 192, U _m is reduced from 10 V to a smaller value, as a result of which less total current I _{g flows} through the total resistance R _g and U _m drops further until a stable state is reached. The voltage across the heating resistor then reaches a lower limit.

The current that flows to ground via the measuring electrodes and the current feedback circuit is set by the dimensioning of the amplifier circuit far below the threshold value above which this measuring current would cause an additional impression pixel (dot). A protective circuit 17 contains a Zener diode, which limits the reference voltage to U _B ≦ +10 V and is preferably connected in parallel with the negative feedback resistor R _s . The protective circuit 17 is intended to prevent the destruction of the print head in the event of a fault and, for this purpose, interacts with the print control unit (DS) and with a switching element S.

One or more measuring means 18, 19 and / or 20 can be used. A measuring device consists of at least one Schmitt trigger, comparator or threshold switch, which can be queried by the pressure control unit 5 in order to interrupt the printing operation if necessary and emit an error message. The switching voltage S is then used to set the reference voltage to U _B = 0 V.

The linear regulator 11 shown in FIG. 3 has a means 16 for setting the print voltage U _p . It is provided that the means 16 is an adjusting resistor.

In a further variant, the means 16 for setting the print voltage U _{p is} an actuator which can be controlled electronically via line D _{α of} the pressure control unit 5 and with which a control value α is set for a specific tape speed V _bj depending on the material of the recording medium used, in particular the type of paper becomes.

In addition, it is provided that the energization time t _j assigned for a defined belt speed V _bj is preset by the print control unit 5 via the strobe pulse duration t _j in accordance with the desired contrast in the print image.

In the event of a fault, if none of the measuring electrodes is in contact with the resistance ink ribbon 10 or the reference voltage U _B is too high in comparison with the number n of electrodes driven at the same time, the actuator 16 is controlled by the pressure control unit 5 to a lower actuating value α, so that the print voltage is set to a harmless value of αU _p ≈ 1 V.

The other fault case, when the reference voltage U _B is too low, is evaluated by a second one measuring means 19 which can also be queried by the pressure control unit 5. The measuring means 19 also has at least one threshold switch, comparator or Schmitt trigger. The threshold value of each measuring means 18, 19, 20 is preferably set in accordance with a defined number n of electrodes to be activated simultaneously.

An error message is then issued by the print control unit 5 when a suitable location in the print image is printed and the correspondingly set threshold value is not reached or exceeded.

It is further provided that the protective circuit 17 has a Zener diode ZD and a window comparator 20 which can be scanned by the pressure control unit 5 and whose output is present at the D input of a buffer store 21. The measurement is performed at the end of the transient process, because the measurement triggering signal D _st via a delay circuit 22 is connected to the strobe pulse to the clock input of latch 21, the supernatant D _l is acted upon by a reset pulse (latch enable), and a to Pressure control unit 5 has leading data output D _d .

The advantageous variant of the adapter circuit - shown in FIG. 6 - has at least one window comparator, which can be queried by the pressure control unit 5 and whose output is present at the D input of a D flip-flop 21, that has a strobe pulse at a delay circuit 22 the corresponding signal D _st is present and the output is connected to the clock input of the D flip-flop 21, which can be acted upon with a reset pulse by means of a signal enable corresponding signal D _l and has a data output D _d .

The pressure control unit 5 evaluates the signal on the data output D _d and outputs control signals to the control circuit. In the event of a signal D _u for interrupting the printing operation, the measuring voltage U _m and thus the reference voltage U _{B can be set} to U _B = 0 V with a switching element S. In addition, the print voltage U _{p is} reduced.

In a further variant of the solution according to the invention, shown in FIG. 7, the electrodes of the printhead 30 that are not currently being driven are used as measuring electrodes together with the measuring electrode 29 for the measurement. At the outputs Q₁ to Q _{x of} the switching unit 2, all or part of the voltages U₁ to U₄ are tapped and each connected to the inputs e₁ to e₄ and the voltage U _m tapped at the measuring electrode 29 at the input e₉ of the adapter circuit 12. The matching circuit 12 has a circuit for evaluating a plurality of DC voltages with regard to the lowest DC voltage, consisting of a corresponding number of non-inverting operational amplifiers 15, each with a diode D connected on the output side. Each diode D is connected with its n-region at the amplifier output and with its p-region at the inverting input (-) of the amplifier 15 directly (voltage follower) or via a voltage divider, not shown in FIG. 7, in order to obtain the reference voltage U _B = V _u * To form _m .

At the output there is also a protection circuit 17 (not shown in FIG. 7). The protection circuit 17 also contains a Zener diode, measuring means 18, 19 or 20, buffer store 21 and a pulse delay circuit 22, as has already been explained with reference to FIG.

This type of control of the print head with the help of an adjustable constant voltage source 11 has the Advantage that with the help of at least one non-activated printhead electrode a voltage drop U _m in the resistance ribbon is measured during the ETR printing or franking process, that the compensation of the variance of the voltage drop U _p existing in the resistance ribbon 10 due to the above-mentioned influences by means of the for the activated Pressure electrodes are supplied by the constant voltage source 11 supply voltage U _s and that an evaluation and appropriate control can be carried out by the pressure control unit 5 to ensure the functionality and for a high print quality.

The invention is not limited to the present embodiment. Rather, a number of variants are conceivable which make use of the solution shown, even in the case of fundamentally different types.

Claims (20)

Control circuit for an electrothermal printing device with a resistance band, which transfers color particles when heated to a recording medium, with a current collecting electrode, with a storage means and with a pressure control unit, which acts on a switching unit for the ETR printing unit, the electrodes of the printhead providing energy from an energy source for the individual pixels of the print image are provided in a defined manner,
characterized in that
the energy source is a constant voltage source (1) with an input for a reference voltage U _B , with which the existing variance of the voltage drop across the heating resistors in the resistance ribbon (10) is compensated for by temporarily switching the supply voltage in via the switching unit (2) Contact electrodes (31, 32, 33, ...,) with a regulated supply voltage U _s equal to the sum of a defined adjustable print voltage αU _p corresponding to the voltage falling across the selective part of a current path and from the voltage across the unselective part of the current path measured in the resistance ribbon voltage drop U _m formed reference voltage U _B , the level of the supply voltage U _s for the activated electrodes of the printhead being regulated in such a way that the print voltage U _p remains constant while an increase (decrease) in the measurement voltage U _m to increase (decrease) the supply voltage f leads. Control circuit according to Claim 1, characterized in that the measurement voltage is a voltage drop across the unselective (return) current path in the resistance ink ribbon and caused by the total current I _g and the variance of the resistors and by means of one or more electrodes which are located on or near the print head are arranged, is measured, the measuring electrode being a non-activated electrode contacting the resistance ribbon, and that during the energization time t _j the electrodes (31, 32) temporarily connected to the constant voltage source (1) via the switching unit (2) and series resistors R _v , 33, ...) are subjected to a voltage, the height of which has such a dependence on the temporarily different number n of activated electrodes that a larger number of activated electrodes are supplied with a higher voltage than a smaller number. Control circuit, according to claims 1 and 2, characterized in that the constant voltage source (1) is part of a voltage supply unit (SVE) which contains a power supply unit (14) that a first DC voltage U _g and a second DC voltage U _c for the supply of the Switching unit (2) outputs that a linear regulator (11) is used as an adjustable constant voltage source, to which the first input voltage U _{g is} supplied and which outputs the voltage U _s for supplying the drivers in the switching unit (2) and that between the control input of the Linear regulator (11) and the measuring electrode a matching circuit (12) is connected, which forms the reference voltage U _B from the analog measurement voltage U _m . Control circuit according to claims 1 to 3, characterized in that with a reduced reference voltage U _B compared to the measuring voltage U _m , the level of the supply voltage U _s on the one hand has such a dependency on the temporarily different number n of activated electrodes that a a larger number of simultaneously activated electrodes with a higher supply voltage U _s but per dot are supplied with a lower pressure energy than a smaller number of simultaneously activated electrodes which are supplied with a higher pressure energy at a lower supply voltage per dot. Control circuit according to Claim 3, characterized in that the linear regulator (11) has means (16) for setting the print voltage U _p . Control circuit according to Claim 5, characterized in that the means (16) is an adjusting resistor. Control circuit according to Claim 6, characterized in that the means (16) for setting the print voltage U _{p is} an actuator which can be controlled electronically by the print control unit (5) and with which, via lines Da, an actuating value α, in particular depending on the material of the recording medium used the Paper type, for a certain belt speed V _{bj is} set. Control circuit according to one of the preceding claims 1 to 7, characterized in that the energization time t _j assigned for a specific belt speed V _bj is preset by the print control unit (5) in accordance with the desired contrast in the print image. Control circuit according to one of the preceding claims 1 to 8, characterized in that the printing device has a flat measuring electrode (29) arranged on one side of the pressure bar and one on the other side arranged with a current collecting electrode (6) connected to ground potential. Control circuit according to one of Claims 1 to 8, characterized in that the printing device has a single large-area current collecting electrode (6) with an opening for the printing head (30) and the measuring electrode (29). Control circuit according to one of the preceding claims 1 to 10 , characterized in that the print electrodes (30) which are just not activated are used as measuring electrodes together with the measuring electrode (29) for measurement, and in that the outputs Q₁ to Q _{x of} the switching unit ( 2) all or a part of the voltages U₁, U₂, U₃, U₄, ..., tapped and in each case to the inputs e₁, e₂, e₃, e₄, ..., and the voltage tapped at the measuring electrode (29) U _m to the input e₉ of the adapter circuit (12) that the adapter circuit (12) has a circuit for evaluating several DC voltages with regard to the lowest DC voltage. Control circuit according to Claims 3 and 4, characterized in that the matching circuit (12) has at least one non-inverting operational amplifier (13) with adjustable voltage amplification. Control circuit according to Claim 12, characterized in that the non-inverting operational amplifier (13) is connected as a voltage follower or has a voltage gain of V _u = 1. Control circuit according to one of the preceding claims 11 to 13, characterized in that the circuit for evaluating a plurality of DC voltages with regard to the lowest DC voltage in the matching circuit (12) consists of a corresponding number of non-inverting operational amplifiers (15), each with a diode D connected on the output side, wherein each diode D is connected to its n region at the amplifier output and to its p region at the inverting input (-) of the amplifier (15) directly or via a voltage divider. Control circuit according to one of the preceding claims 1 to 14, characterized in that the series resistors R _v between the driver output of the switching unit (2) and the electrodes have a value between one eighth and one hundredth of the value of the heating resistor R _h in the resistance layer of a resistance ribbon. Control circuit according to one of the preceding claims 1 to 15, characterized in that the matching circuit (12) contains a protective circuit (17) connected to the amplifier output with a Zener diode and with a measuring means (18, 19, 20) consisting of at least there is a Schmitt trigger, comparator, threshold switch and / or window comparator, which can be queried by the pressure control unit (5) in order to interrupt the printing operation if necessary and issue an error message. Control circuit according to Claim 16, characterized in that the protective circuit (17) is equipped with a Zener diode and with a measuring means (18, 19, 20) and with an intermediate store (22) which can be queried by the pressure control unit (5) to interrupt printing if necessary and issue an error message. Control circuit according to Claim 17, characterized in that the measuring means (20) has at least one window comparator (20) which can be queried by the pressure control unit (5) and whose output is present at the D input of the D flip-flop (21), that at a Delay circuit (22) a signal D _st corresponding to a strobe pulse is present and the output is connected to the clock input of the D flip-flop (21), the signal D _l corresponding to a latch enable with a Reset pulse can be applied and has a data output D _d . Control circuit according to Claim 18, characterized in that a switching element S connected to ground potential is connected to a non-inverting input of the amplifier (13, 15), the measuring voltage U _m and thus with a signal D _u to interrupt the printing operation with the switching element S U _{B can be set} to U _B = 0 V. Control circuit according to claim 7 and 19, characterized in that the control value α is changed when the printing operation is interrupted.

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