EP0320435B1 - Thermal printer - Google Patents

Thermal printer Download PDF

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Publication number
EP0320435B1
EP0320435B1 EP88730271A EP88730271A EP0320435B1 EP 0320435 B1 EP0320435 B1 EP 0320435B1 EP 88730271 A EP88730271 A EP 88730271A EP 88730271 A EP88730271 A EP 88730271A EP 0320435 B1 EP0320435 B1 EP 0320435B1
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EP
European Patent Office
Prior art keywords
heating elements
temperature
measured
thermal printing
determined
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EP88730271A
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German (de)
French (fr)
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EP0320435A1 (en
Inventor
Ralf Dipl.-Ing. Kaiser
Hartmut Dipl.-Ing. Kirchhoff
Wolfgang Dr.-Ing. Pekruhn
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Siemens AG
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Siemens AG
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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B41—PRINTING; LINING MACHINES; TYPEWRITERS; STAMPS
    • B41J—TYPEWRITERS; SELECTIVE PRINTING MECHANISMS, i.e. MECHANISMS PRINTING OTHERWISE THAN FROM A FORME; CORRECTION OF TYPOGRAPHICAL ERRORS
    • B41J2/00—Typewriters or selective printing mechanisms characterised by the printing or marking process for which they are designed
    • B41J2/315—Typewriters or selective printing mechanisms characterised by the printing or marking process for which they are designed characterised by selective application of heat to a heat sensitive printing or impression-transfer material
    • B41J2/32—Typewriters or selective printing mechanisms characterised by the printing or marking process for which they are designed characterised by selective application of heat to a heat sensitive printing or impression-transfer material using thermal heads
    • B41J2/35—Typewriters or selective printing mechanisms characterised by the printing or marking process for which they are designed characterised by selective application of heat to a heat sensitive printing or impression-transfer material using thermal heads providing current or voltage to the thermal head
    • B41J2/355—Control circuits for heating-element selection
    • B41J2/36—Print density control
    • B41J2/365—Print density control by compensation for variation in temperature

Definitions

  • the invention relates to a thermal printing method, in which the temperature of each individual heating element is first measured in each control cycle for the cyclical activation of heating elements of a thermal printhead, before a current pulse is applied to the heating element in question, the energy of which is controlled as a function of the measured temperature becomes.
  • the heating elements of a thermal printing head are controlled in such a way that the instantaneous temperature of each individual heating element intended for printing is measured individually and in succession by means of a thermocouple individually assigned to it and immediately afterwards the relevant one
  • a current pulse is applied to the heating element, the current strength of which is controlled by an amplifier device arranged downstream of the temperature sensor as a function of the measured temperature. This is intended to reduce the influence of the ambient temperature and the print head temperature, which is dependent on the activation of the heating elements, on the print quality. Since the heating elements intended for printing are controlled individually in succession, the printing time required for printing a dot matrix corresponding to the arrangement of the heating elements is relatively long.
  • EP-A 0174751 it is known from EP-A 0174751 to measure defective heating elements of a thermal print head by measuring their electrical resistances individually and in succession with a measuring current; the measured resistance values are compared with corresponding reference values stored in a memory device, whereupon the heating element in question is recognized as defective in the event of a deviation exceeding a predetermined dimension.
  • the thermal printing method specified at the outset that at the beginning of each scanning cycle there is a cooling pause for all heating elements following the respective previous scanning cycle, during which the temperatures of the heating elements are measured in succession, that from the temperature measurement values held in succession and one
  • the characteristic cooling curve of the heating elements stored in a storage device for all heating elements is determined in relation to a common point in time, and that all heating elements intended for printing are simultaneously subjected to the current pulses after the cooling pause, the respective pulse duration of which is individual for each heating element depending on the cooling temperature value determined for this heating element.
  • the main advantage of the thermal printing process according to the invention lies in the high achievable printing speed because, in contrast to the thermal printing process known from DE-OS 1964389, all heating elements intended for printing are simultaneously subjected to the current pulses, the respective pulse duration for each printing heating element in accordance with the temperature measured there is individually dimensioned.
  • the temperature measurements take place in the cooling pause common to all the heating elements, which is required anyway between successive time intervals of the current application to the heating elements.
  • the temperatures of the heating elements their electrical resistances are measured and compared with resistance reference values stored in a further memory device, which were determined once for the heating elements at a reference temperature.
  • the main advantage of the resistance measurement on the heating elements is the fact that the actual temperature conditions on the heating elements are detected directly since the electrical resistance of each heating element is directly dependent on its current temperature.
  • the resistance reference values are preferably determined before the respective thermal printing device, in which the method according to the invention is used, by measuring the electrical resistances of all the heating elements in succession at room temperature and storing them as resistance reference values in the further memory device. This measuring cycle can advantageously be carried out automatically before each renewed start-up of the thermal printing device, so that age-related changes in the electrical resistance values of the heating elements have no influence on the accuracy of the Have temperature measurements in the method according to the invention.
  • the heating elements each intended for printing as a function of cooling temperature values standardized to a common point in time, which are derived from the temperature measurement values measured in succession.
  • the measured temperature measured value of each heating element is reduced by a discount value which is determined from the course of the stored cooling curve of the heating elements and the respective time difference between the temperature measurement and the subsequent current pulse exposure to all heating elements becomes.
  • the discount values by which the temperature measurement values are reduced correspond in each case to the amount of cooling of the heating elements in question from the respective time of the temperature measurement to the point in time at which they are acted upon by the current pulses for all heating elements.
  • the discount values and thus the cooling temperature values of the heating elements at the time of their current application can be determined on the basis of the respective measured temperature values using the stored cooling curve for each individual heating element .
  • the temperature of the thermal print head in the area of the heating elements is measured with a temperature sensor device and that, depending on this measured temperature, the course of the stored Cooling curve of the heating elements is adapted to the temperature of the thermal print head.
  • the temperature of the thermal printhead can be determined in a computer program as a function of the temperature measurement values measured in previous control cycles and the course of the stored cooling curve can be adapted to the temperature of the thermal printhead determined in this way.
  • the power to be provided for controlling the heating elements of a thermal print head can be reduced in that the heating elements are activated in groups in succession.
  • the temperatures of the heating elements of the group to be subsequently actuated are measured simultaneously with the currently activated group.
  • FIG. 1 shows a schematic representation of the circuitry structure of an embodiment of a thermal printing device for performing the method according to the invention.
  • a large number of individual heating elements are shown on a thermal print head 1, which is indicated here only by a dash-dotted border R1 ... Rn arranged close to each other along a line.
  • Each individual heating element R1 ... Rn is connected in series with a controllable switch S1 ... Sn assigned to it to a common line 2 for all heating elements R1 ... Rn.
  • the controllable switches S1 ... Sn each consist of NAND elements, each of which is connected via a first input together with an output-side control clock signal line STROBE of a control device 3;
  • Sn are each connected via a second input to the outputs of a holding circuit 4 which are assigned to them and which have a number corresponding to the number of heating elements R1 ... Rn or the controllable switches S1 ... Sn Has storage spaces.
  • the holding circuit 4 is connected via a control input 5 to a data transfer signal line LATCH of the control device 3 on the output side.
  • the holding circuit 4 is connected to parallel data outputs of a series / parallel shift register 6, which has the same number of memory locations as the holding circuit 4.
  • the series / parallel shift register 6 is connected to an output-side data signal line DATA of the control device 3.
  • the control device 3 has a data input 8 for accepting print data, according to which the heating elements R1 ...
  • Rn are to be individually heated, in order to record the information of the print data on a heating element R1 ... Rn (not shown here) and on them to pass by heat-sensitive recording media.
  • the heating elements R1 ... Rn are connected in series with a measuring resistor 9 and a switch 10 which can be controlled in parallel to a voltage source V + via the common conductor 2.
  • a first analog / digital converter 11 is connected on the input side to the measuring resistor 9 and is connected on the output side to a corresponding input 12 of the control device 3.
  • the controllable switch 10 is connected on the control input side to a control line 13 coming from the control device 3.
  • On the Thermal print head 1 is also a temperature sensor device 14 consisting of a temperature-dependent resistor, which is connected to the input of a second analog / digital converter 15, which in turn is connected on the output side to a corresponding input 16 of the control device 3.
  • the control device 3 contains a storage device 17, which is only indicated here, in which the characteristic cooling curve of the heating elements R1 ... Rn is stored, and a further storage device 18, in which for the heating elements R1 ... Rn at one predetermined reference temperature, once determined resistance reference values are stored.
  • the information supplied to it via the inputs 8, 12 and 16 is processed by program execution, the contents of the memory devices 17 and 18 being used and output signals being generated on the lines STROBE, LATCH, DATA and 13.
  • the thermal printing method when the thermal printing device is started up, the electrical resistances of the individual heating elements R1 ... Rn are measured at a reference temperature, which can be measured, for example, by means of the temperature sensor device 14 and fed to the control device 3 via the analog / digital converter 15.
  • the controllable switch 10 is opened by a switching command issued by the control device 3 via the control line 13, so that the heating elements R1 ... Rn are connected to the voltage source V + via the measuring resistor 9.
  • a single data bit is supplied from the control device 3 to the serial / parallel shift register 6 via the data signal line DATA and is shifted step by step through the series / parallel shift register 6 from memory location to memory location.
  • the data bit is reloaded into the hold circuit 4 by a data transfer signal on the data transfer signal line LATCH, so that the data bit in the series / parallel shift register thereafter 6 can be moved by one space.
  • a control clock signal generated by the control device 3 on the control clock signal line STROBE the individual heating elements R1 ... Rn are successively connected to the voltage source V + via the measuring resistor 9 in accordance with the current position of the data bits temporarily stored in the holding circuit 4.
  • the measuring resistor 9 is dimensioned with regard to the electrical resistances of the heating elements R1 ... Rn such that only a small measuring current flows through the heating elements R1 ... Rn and the temperature increase thereby caused in the heating elements R1 ... Rn is negligible .
  • the voltage drop across the measuring resistor 9 is converted as a measure of the electrical resistance of each of the successively controlled heating elements R1 ... Rn by the analog / digital converter 11 into a digital resistance reference value for the heating element R1 ... Rn in question and stored in the memory device 18.
  • This measuring process can be repeated several times during the useful life of the thermal printing device, so that age-related changes in resistance of the heating elements R1 ... Rn are taken into account.
  • FIG. 2 shows the measuring currents M1 ... Mn fed to the individual heating elements R1 ... Rn for temperature measurement and the current pulses I1 ... In subsequently fed to the heating elements R1 ... Rn during a drive cycle t A ;
  • FIG. 2 also shows the associated temperature profiles T1 ... Tn in the individual heating elements R1 ... Rn.
  • Each control cycle t A consists of a measurement phase t M , which corresponds to a cooling pause for all heating elements R1... Rn following the respective previous control cycle, and a control phase t I that follows.
  • the controllable switch 10 is open during the measurement phase t M ; Via the data line DATA, a single data bit is shifted step by step through the series / parallel shift register 6 and at each step via the The holding circuit 4 is fed to another of the NAND elements S1 ... Sn on the input side, so that the heating elements R1 ... Rn are connected in series with the measuring resistor 9 to the voltage source V + in dependence on the drive clock signal on the drive clock signal line STROBE.
  • the respective voltage drop across the measuring resistor 9 is converted as a measure of the current electrical resistance values of the individual heating elements R1 ... Rn in the analog / digital converter 11 into a corresponding digital value and fed to the control device 3.
  • the respectively measured resistance value is compared in the control device 3 with the associated resistance reference value from the storage device 18 and the instantaneous temperature TM1 ... TMn of the heating element R1 ... Rn in question is determined from the comparison result.
  • the temperature measured value TM2 measured at the time t2 is reduced by a discount value A2 which is derived from the course of the stored cooling curve 19 of the heating elements R1 ... Rn and the time difference between the measurement time t2 and the beginning of Control phase t I results.
  • the cooling curve 19 is stored in the form of measured value data which can be obtained by a large number of individual resistance measurements on one or more selected heating elements R1 ... Rn during a cooling phase following heating.
  • the controllable switch 10 is closed, so that the heating elements R1... Rn are directly connected to the voltage source V + via the bus 2.
  • the print data which in the relevant control cycle TA designate the heating elements R1 ... Rn which are intended for printing, are fed to the control device 3 via the data input 8 and are read by the control device 3 as serial data into the series / parallel shift register 6 via the data signal line DATA , whose parallel outputs they are in the form of parallel data.
  • a data transfer signal generated by the control device 3 on the data transfer signal line LATCH reloads the parallel data from the series / parallel shift register 6 into the hold circuit 4, so that the individual heating elements R1...
  • Rn can be supplied with a current from the voltage source 2 in accordance with the data temporarily stored in the holding circuit 4.
  • the times of current application are measured differently for each individual heating element R1 ... Rn depending on the cooling temperature value TA1 ... TAn determined for this heating element R1 ... Rn; the lower this determined cooling temperature value TA1 ... TAn, the longer the duration of the current pulse application of the relevant heating element R1 ... Rn, so that all printing heating elements R1 ... Rn each have approximately the same printing temperature T D at the end of their respective current pulse application to reach.
  • the thermal print head 1 heats up as a result of heat accumulation, the cooling behavior changes of heating elements R1 ... Rn during the printing period. Therefore, the temperature of the thermal printhead 1 is measured at regular intervals by means of the temperature sensor device 14 and the course of the stored cooling curve 19 of the heating elements R1 ... Rn is adapted to the temperature of the thermal printhead.

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Description

Die Erfindung betrifft ein Thermodruckverfahren, bei dem zur zyklischen Ansteuerung von Heizelementen eines Thermodruckkopfes in jedem Ansteuerzyklus zunächst die Temperatur jedes einzelnen Heizelementes gemessen wird, bevor das betreffende und zum Drucken bestimmte Heizelement mit einem Stromimpuls beaufschlagt wird, dessen Energie in Abhängigkeit von der gemessenen Temperatur gesteuert wird.The invention relates to a thermal printing method, in which the temperature of each individual heating element is first measured in each control cycle for the cyclical activation of heating elements of a thermal printhead, before a current pulse is applied to the heating element in question, the energy of which is controlled as a function of the measured temperature becomes.

Bei einem derartigen, aus der DE-OS 1964389 bekannten Thermodruckverfahren erfolgt eine Ansteuerung der Heizelemente eines Thermodruckkopfes in der Weise, daß einzeln und nacheinander die momentane Temperatur jedes einzelnen, zum Drucken bestimmten Heizelementes mittels eines diesem individuell zugeordneten Thermofühlers gemessen wird und unmittelbar darauf das betreffende Heizelement mit einem Stromimpuls beaufschlagt wird, dessen Stromstärke durch eine dem Temperaturfühler nachgeordnete Verstärkereinrichtung in Abhängigkeit von der gemessenen Temperatur gesteuert wird. Hierdurch sollen Einflüsse der Umgebungstemperatur und der von der jeweils zuvor erfolgten Ansteuerung der Heizelemente abhängigen Druckkopftemperatur auf die Druckqualität verringert werden. Da die jeweils zum Drucken bestimmten Heizelemente einzeln nacheinander angesteuert werden, ist die zum Drucken eines der Anordnung der Heizelemente entsprechenden Punkterasters benötigte Druckzeit relativ groß.In such a thermal printing method known from DE-OS 1964389, the heating elements of a thermal printing head are controlled in such a way that the instantaneous temperature of each individual heating element intended for printing is measured individually and in succession by means of a thermocouple individually assigned to it and immediately afterwards the relevant one A current pulse is applied to the heating element, the current strength of which is controlled by an amplifier device arranged downstream of the temperature sensor as a function of the measured temperature. This is intended to reduce the influence of the ambient temperature and the print head temperature, which is dependent on the activation of the heating elements, on the print quality. Since the heating elements intended for printing are controlled individually in succession, the printing time required for printing a dot matrix corresponding to the arrangement of the heating elements is relatively long.

Bei einem aus der EP-A 0112474 bekannten Thermodruckverfahren ist eine gleichzeitige temperaturabhängige Strombeaufschlagung von Heizelementen eines Thermodruckkopfes vorgesehen; hierbei bleiben die momentanen tatsächlichen Temperaturverhältnisse an den einzelnen Heizelementen unberücksichtigt, weil die Temperaturmessung und damit die Strombeaufschlagung für alle Heizelemente global erfolgt.In a thermal printing method known from EP-A 0112474, a simultaneous temperature-dependent application of current to heating elements of a thermal printing head is provided; the current actual temperature conditions at the individual heating elements are not taken into account here, because the temperature measurement and thus the current applied to all heating elements are global.

Aus der EP-A 0174751 ist es schließlich bekannt, zum Feststellen defekter Heizelemente eines Thermodruckkopfes deren elektrische Widerstände durch Beaufschlagung der Heizelemente einzeln und nacheinander mit einem Meßstrom zu messen; die gemessenen Widerstandswerte werden mit entsprechenden, in einer Speichereinrichtung abgelegten Referenzwerten verglichen, woraufhin bei einer ein vorgegebenes Maß überschreitenden Abweichung das betreffende Heizelement als defekt erkannt wird.Finally, it is known from EP-A 0174751 to measure defective heating elements of a thermal print head by measuring their electrical resistances individually and in succession with a measuring current; the measured resistance values are compared with corresponding reference values stored in a memory device, whereupon the heating element in question is recognized as defective in the event of a deviation exceeding a predetermined dimension.

Gemäß der Erfindung ist bei dem eingangs angegebenen Thermodruckverfahren vorgesehen, daß zu Beginn eines jeden Abtastzyklus eine auf den jeweils vorangegangenen Abtastzyklus folgende Abkühlungspause für alle Heizelemente vorgesehen ist, während der die Temperaturen der Heizelemente nacheinander gemessen werden, daß aus den so nacheinander haltenden Temperaturmeßwerten und einer in einer Speichereinrichtung abgelegten charakteristischen Abkühlungskurve der Heizelemente für alle Heizelemente jeweils auf einen gemeinsamen Zeitpunkt bezogene Abkühlungstemperaturwerte ermittelt werden und daß alle jeweils zum Drucken bestimmten Heizelemente im Anschluß an die Abkühlungspause gleichzeitig mit den Stromimpulsen beaufschlagt werden, deren jeweilige Impulsdauer individuell für jedes Heizelement in Abhängigkeit von dem für dieses Heizelement jeweils ermittelten Abkühlungstemperaturwert bemessen ist. Der wesentliche Vorteil des erfindungsgemäßen Thermodruckverfahrens liegt in der hohen erreichbaren Druckgeschwindigkeit, weil im Unterschied zu dem aus der DE-OS 1964389 bekannten Thermodruckverfahren alle zum Drucken bestimmten Heizelemente gleichzeitig mit den Stromimpulsen beaufschlagt werden, deren jeweilige Impulsdauer für jedes druckende Heizelement entsprechend der dort gemessenen Temperatur individuell bemessen ist. Die Temperaturmessungen erfolgen in der allen Heizelementen gemeinsamen Abkühlungspause, die ohnehin zwischen jeweils aufeinanderfolgenden Zeitintervallen der Strombeaufschlagung der Heizelemente erforderlich ist. Dadurch, daß die für die einzelnen Heizelemente nacheinander ermittelten Temperaturmeßwerte unter Zugrundelegung der abgespeicherten charakteristischen Abkühlungskurve der Heizelemente auf zeitgleiche Abkühlungstemperaturwerte zum Zeitpunkt der Beaufschlagung der Heizelemente mit den Stromimpulsen extrapoliert werden, wird erreicht, daß die Steuerung der Stromimpulsdauern der den Heizelementen gleichzeitig zugeführten Stromimpulse exakt in Abhängigkeit von den gleichzeitigen momentanen Temperaturverhältnissen an den einzelnen Heizelementen erfolgt. In diesem Zusammenhang wäre es zwar denkbar, die Temperaturen an den einzelnen Heizelementen gleichzeitig zu messen, jedoch wäre hierzu eine der Zahl der Heizelemente entsprechende Anzahl von Meßeinrichtungen erforderlich, was mit einem sehr hohen Aufwand verbunden wäre.According to the invention, it is provided in the thermal printing method specified at the outset that at the beginning of each scanning cycle there is a cooling pause for all heating elements following the respective previous scanning cycle, during which the temperatures of the heating elements are measured in succession, that from the temperature measurement values held in succession and one The characteristic cooling curve of the heating elements stored in a storage device for all heating elements is determined in relation to a common point in time, and that all heating elements intended for printing are simultaneously subjected to the current pulses after the cooling pause, the respective pulse duration of which is individual for each heating element depending on the cooling temperature value determined for this heating element. The main advantage of the thermal printing process according to the invention lies in the high achievable printing speed because, in contrast to the thermal printing process known from DE-OS 1964389, all heating elements intended for printing are simultaneously subjected to the current pulses, the respective pulse duration for each printing heating element in accordance with the temperature measured there is individually dimensioned. The temperature measurements take place in the cooling pause common to all the heating elements, which is required anyway between successive time intervals of the current application to the heating elements. Thereby, That the temperature measured values determined for the individual heating elements in succession are extrapolated on the basis of the stored characteristic cooling curve of the heating elements to simultaneous cooling temperature values at the time when the heating elements are acted upon by the current pulses the simultaneous instantaneous temperature conditions on the individual heating elements. In this context, it would be conceivable to measure the temperatures on the individual heating elements simultaneously, but this would require a number of measuring devices corresponding to the number of heating elements, which would be very expensive.

Entsprechend einer bevorzugten Ausbildung des erfindungsgemäßen Verfahrens werden zur Messung der Temperaturen der Heizelemente deren elektrische Widerstände gemessen und mit in einer weiteren Speichereinrichtung abgelegten Widerstandsreferenzwerten verglichen, die bei einer Referenztemperatur einmalig für die Heizelemente ermittelt worden sind. Der wesentliche Vorteil der Widerstandsmessung an den Heizelementen ist darin zu sehen, daß hierdurch die tatsächlichen Temperaturverhältnisse an den Heizelementen unmittelbar erfaßt werden, da der elektrische Widerstand jedes Heizelementes in direkter Abhängigkeit zu dessen momentaner Temperatur steht. Die Ermittlung der Widerstandsreferenzwerte erfolgt vorzugsweise vor Inbetriebnahme der jeweiligen Thermodruckvorrichtung, bei der das erfindungsgemäße Verfahren zur Anwendung kommt, indem bei Zimmertemperatur in einem Meßzyklus die elektrischen Widerstände aller Heizelemente nacheinander gemessen und als Widerstandsreferenzwerte in der weiteren Speichereinrichtung abgelegt werden. Dieser Meßzyklus kann in vorteilhafter Weise vor jeder erneuten Inbetriebnahme der Thermodruckvorrichtung automatisch ausgeführt werden, so daß alterungsbedingte Änderungen der elektrischen Widerstandswerte der Heizelemente keinen Einfluß auf die Genauigkeit der Temperaturmessungen bei dem erfindungsgemäßen Verfahren haben.According to a preferred embodiment of the method according to the invention, to measure the temperatures of the heating elements, their electrical resistances are measured and compared with resistance reference values stored in a further memory device, which were determined once for the heating elements at a reference temperature. The main advantage of the resistance measurement on the heating elements is the fact that the actual temperature conditions on the heating elements are detected directly since the electrical resistance of each heating element is directly dependent on its current temperature. The resistance reference values are preferably determined before the respective thermal printing device, in which the method according to the invention is used, by measuring the electrical resistances of all the heating elements in succession at room temperature and storing them as resistance reference values in the further memory device. This measuring cycle can advantageously be carried out automatically before each renewed start-up of the thermal printing device, so that age-related changes in the electrical resistance values of the heating elements have no influence on the accuracy of the Have temperature measurements in the method according to the invention.

Wie oben bereits erläutert, erfolgt die Strombeaufschlagung der jeweils zum Drucken bestimmten Heizelemente in Abhängigkeit von auf einen gemeinsamen Zeitpunkt normierten Abkühlungstemperaturwerten, die aus den nacheinander gemessenen Temperaturmeßwerten abgeleitet werden. In diesem Zusammenhang ist im Rahmen der Erfindung vorgesehen, daß zur Ermittlung der Abkühlungstemperaturwerte der jeweils gemessene Temperaturmeßwert jedes Heizelementes um einen Abschlagswert verringert wird, der aus dem Verlauf der abgespeicherten Abkühlungskurve der Heizelemente und der jeweiligen Zeitdifferenz zwischen der Temperaturmessung und der nachfolgenden Stromimpulsbeaufschlagung aller Heizelemente bestimmt wird. Die Abschlagswerte, um die die Temperaturmeßwerte verringert werden, entsprechen dabei jeweils dem Betrag der Abkühlung der betreffenden Heizelemente vom jeweiligen Zeitpunkt der Temperaturmessung an bis zu dem für alle Heizelemente gemeinsamen Zeitpunkt ihrer Beaufschlagung mit den Stromimpulsen. Da es sich bei den jeweiligen Zeitdifferenzen zwischen den Temperaturmessungen und der Stromimpulsbeaufschlagung um für alle Heizelemente individuell vorgegebene Werte handelt, lassen sich die Abschlagswerte und damit die Abkühlungstemperaturwerte der Heizelemente zum Zeitpunkt ihrer Strombeaufschlagung ausgehend von den jeweiligen Temperaturmeßwerten anhand der abgespeicherten Abkühlungskurve für jedes einzelne Heizelement ermitteln.As already explained above, current is applied to the heating elements each intended for printing as a function of cooling temperature values standardized to a common point in time, which are derived from the temperature measurement values measured in succession. In this context, it is provided within the scope of the invention that, in order to determine the cooling temperature values, the measured temperature measured value of each heating element is reduced by a discount value which is determined from the course of the stored cooling curve of the heating elements and the respective time difference between the temperature measurement and the subsequent current pulse exposure to all heating elements becomes. The discount values by which the temperature measurement values are reduced correspond in each case to the amount of cooling of the heating elements in question from the respective time of the temperature measurement to the point in time at which they are acted upon by the current pulses for all heating elements. Since the respective time differences between the temperature measurements and the current impulse are individually preset values for all heating elements, the discount values and thus the cooling temperature values of the heating elements at the time of their current application can be determined on the basis of the respective measured temperature values using the stored cooling curve for each individual heating element .

Zur Berücksichtigung des Einflusses der Umgebungstemperatur bzw. des Temperaturniveaus des Thermodruckkopfes auf das Abkühlverhalten der Heizelemente ist gemäß einer vorteilhaften Weiterbildung des erfindungsgemäßen Thermodruckverfahrens im Zusammenhang mit der Bestimmung der zeitgleichen Abkühlungstemperaturwerte vorgesehen, daß die Temperatur des Thermodruckkopfes im Bereich der Heizelemente mit einer Temperaturfühlereinrichtung gemessen wird und daß in Abhängigkeit von dieser gemessenen Temperatur der Verlauf der abgespeicherten Abkühlungskurve der Heizelemente an die Temperatur des Thermodruckkopfes angepaßt wird. Alternativ oder ergänzend hierzu läßt sich im Rahmen eines Rechenprogramms die Temperatur des Thermodruckkopfes in Abhängigkeit von den in vorangegangenen Ansteuerzyklen gemessenen Temperaturmeßwerten ermitteln und der Verlauf der abgespeicherten Abkühlungskurve an die so ermittelte Temperatur des Thermodruckkopfes anpassen.To take into account the influence of the ambient temperature or the temperature level of the thermal print head on the cooling behavior of the heating elements, according to an advantageous development of the thermal printing method according to the invention in connection with the determination of the simultaneous cooling temperature values, it is provided that the temperature of the thermal print head in the area of the heating elements is measured with a temperature sensor device and that, depending on this measured temperature, the course of the stored Cooling curve of the heating elements is adapted to the temperature of the thermal print head. As an alternative or in addition to this, the temperature of the thermal printhead can be determined in a computer program as a function of the temperature measurement values measured in previous control cycles and the course of the stored cooling curve can be adapted to the temperature of the thermal printhead determined in this way.

Wie beispielsweise aus der DE-OS 3302388 bekannt ist, läßt sich die zum Ansteuern der Heizelemente eines Thermodruckkopfes bereitzustellende Leistung dadurch verringern, daß die Heizelemente gruppenweise nacheinander angesteuert werden. In diesem Zusammenhang werden im Rahmen der Erfindung bei einer gruppenweise nacheinander erfolgenden Ansteuerung der Heizelemente mit der Stromimpulsbeaufschlagung der Heizelemente zeitgleich der jeweils aktuell angesteuerten Gruppe die Temperaturen der Heizelemente der jeweils nachfolgend anzusteuernden Gruppe gemessen.As is known, for example, from DE-OS 3302388, the power to be provided for controlling the heating elements of a thermal print head can be reduced in that the heating elements are activated in groups in succession. In this context, within the scope of the invention, when the heating elements are actuated in groups one after the other with the current impulses applied to the heating elements, the temperatures of the heating elements of the group to be subsequently actuated are measured simultaneously with the currently activated group.

Zur Erläuterung der Erfindung wird im folgenden auf die Figuren der Zeichnung Bezug genommen; im einzelnen zeigen

Figur 1
ein Ausführungsbeispiel einer Thermodruckvorrichtung zur Durchführung des erfindungsgemäßen Verfahrens,
Figur 2
ein Diagramm mit der zeitlichen Aufeinanderfolge der Temperaturmessungen und der nachfolgenden Stromimpulsbeaufschlagung der einzelnen Heizelemente sowie deren Temperaturverläufe während eines Ansteuerzyklus und
Figur 3
in einem Diagramm den Verlauf der abgespeicherten Abkühlungskurve der Heizelemente.

To explain the invention, reference is made below to the figures of the drawing; show in detail
Figure 1
an embodiment of a thermal printing device for performing the method according to the invention,
Figure 2
a diagram with the chronological sequence of the temperature measurements and the subsequent current pulse exposure of the individual heating elements and their temperature profiles during a control cycle and
Figure 3
in a diagram the course of the saved cooling curve of the heating elements.

Figur 1 zeigt in schematischer Darstellung den schaltungstechnischen Aufbau eines Ausführungsbeispiels einer Thermodruckvorrichtung zur Durchführung des erfindungsgemäßen Verfahrens. Auf einem hier lediglich durch eine strichpunktierte Umrandung angedeuteten Thermodruckkopf 1 sind eine Vielzahl einzelner Heizelemente R1... Rn entlang einer Zeile dicht nebeneinanderliegend angeordnet. Jedes einzelne Heizelement R1... Rn ist in Reihe mit einem ihm jeweils zugeordneten steuerbaren Schalter S1... Sn an einer für alle Heizelemente R1... Rn gemeinsamen Sammelleitung 2 angeschlossen. Die steuerbaren Schalter S1... Sn bestehen jeweils aus NAND-Gliedern, die über jeweils einen ersten Eingang gemeinsam mit einer ausgangsseitigen Ansteuertaktsignalleitung STROBE einer Steuereinrichtung 3 verbunden sind; darüber hinaus sind die NAND-Glieder S1... Sn über jeweils einen zweiten Eingang an ihnen zugeordneten Ausgängen einer Halteschaltung 4 angeschlossen, die eine der Anzahl der Heizelemente R1... Rn bzw. der steuerbaren Schalter S1... Sn entsprechende Anzahl von Speicherplätzen aufweist. Die Halteschaltung 4 ist über einen Steuereingang 5 mit einer ausgangsseitigen Datenübernahmesignalleitung LATCH der Steuereinrichtung 3 verbunden. Eingangsseitig ist die Halteschaltung 4 mit Paralleldatenausgängen eines Serien/Parallel-Schieberegisters 6 verbunden, das eine gleiche Anzahl von Speicherplätzen wie die Halteschaltung 4 aufweist. An ihrem seriellen Dateneingang 7 ist das Serien/Parallel-Schieberegister 6 an einer ausgangsseitigen Datensignalleitung DATA der Steuereinrichtung 3 angeschlossen. Die Steuereinrichtung 3 weist einen Dateneingang 8 zur Übernahme von Druckdaten auf, entsprechend denen die Heizelemente R1... Rn einzeln aufgeheizt werden sollen, um eine Informationsaufzeichnung der Druckdaten auf einem hier nicht dargestellten, an den Heizelementen R1... Rn anliegenden und an ihnen vorbeigeführten, wärmeempfindlichen Aufzeichnungsträger zu erreichen. Die Heizelemente R1... Rn sind über den ihnen gemeinsamen Sammelleiter 2 in Reihe mit einem Meßwiderstand 9 und einem zu diesem parallelen steuerbaren Schalter 10 an einer Spannungsquelle V + angeschlossen. An dem Meßwiderstand 9 ist ein erster Analog/Digital-Umsetzer 11 eingangsseitig angeschlossen, der ausgangsseitig mit einem entsprechenden Eingang 12 der Steuereinrichtung 3 verbunden ist. Der steuerbare Schalter 10 ist steuereingangsseitig mit einer von der Steuereinrichtung 3 kommenden Steuerleitung 13 verbunden. Auf dem Thermodruckkopf 1 ist weiterhin eine Temperaturfühlereinrichtung 14 bestehend aus einem temperaturabhängigen Widerstand angeordnet, der an dem Eingang eines zweiten Analog/Digital-Umsetzers 15 angeschlossen ist, der seinerseits ausgangsseitig mit einem entsprechenden Eingang 16 der Steuereinrichtung 3 verbunden ist.Figure 1 shows a schematic representation of the circuitry structure of an embodiment of a thermal printing device for performing the method according to the invention. A large number of individual heating elements are shown on a thermal print head 1, which is indicated here only by a dash-dotted border R1 ... Rn arranged close to each other along a line. Each individual heating element R1 ... Rn is connected in series with a controllable switch S1 ... Sn assigned to it to a common line 2 for all heating elements R1 ... Rn. The controllable switches S1 ... Sn each consist of NAND elements, each of which is connected via a first input together with an output-side control clock signal line STROBE of a control device 3; In addition, the NAND elements S1 ... Sn are each connected via a second input to the outputs of a holding circuit 4 which are assigned to them and which have a number corresponding to the number of heating elements R1 ... Rn or the controllable switches S1 ... Sn Has storage spaces. The holding circuit 4 is connected via a control input 5 to a data transfer signal line LATCH of the control device 3 on the output side. On the input side, the holding circuit 4 is connected to parallel data outputs of a series / parallel shift register 6, which has the same number of memory locations as the holding circuit 4. At its serial data input 7, the series / parallel shift register 6 is connected to an output-side data signal line DATA of the control device 3. The control device 3 has a data input 8 for accepting print data, according to which the heating elements R1 ... Rn are to be individually heated, in order to record the information of the print data on a heating element R1 ... Rn (not shown here) and on them to pass by heat-sensitive recording media. The heating elements R1 ... Rn are connected in series with a measuring resistor 9 and a switch 10 which can be controlled in parallel to a voltage source V + via the common conductor 2. A first analog / digital converter 11 is connected on the input side to the measuring resistor 9 and is connected on the output side to a corresponding input 12 of the control device 3. The controllable switch 10 is connected on the control input side to a control line 13 coming from the control device 3. On the Thermal print head 1 is also a temperature sensor device 14 consisting of a temperature-dependent resistor, which is connected to the input of a second analog / digital converter 15, which in turn is connected on the output side to a corresponding input 16 of the control device 3.

Wie Figur 1 ferner zeigt, enthält die Steuereinrichtung 3 eine hier nur angedeutete Speichereinrichtung 17, in der die charakteristische Abkühlungskurve der Heizelemente R1... Rn abgelegt ist, sowie eine weitere Speichereinrichtung 18, in der für die Heizelemente R1... Rn bei einer vorgegebenen Referenztemperatur einmalig ermittelte Widerstandsreferenzwerte abgelegt sind. Innerhalb der Steuereinrichtung 3 werden durch Programmablauf die ihr über die Eingänge 8, 12 und 16 zugeführten Informationen verarbeitet, wobei auf die Inhalte der Speichereinrichtungen 17 und 18 zurückgegriffen wird und Ausgangssignale auf den Leitungen STROBE, LATCH, DATA und 13 erzeugt werden.As FIG. 1 also shows, the control device 3 contains a storage device 17, which is only indicated here, in which the characteristic cooling curve of the heating elements R1 ... Rn is stored, and a further storage device 18, in which for the heating elements R1 ... Rn at one predetermined reference temperature, once determined resistance reference values are stored. Within the control device 3, the information supplied to it via the inputs 8, 12 and 16 is processed by program execution, the contents of the memory devices 17 and 18 being used and output signals being generated on the lines STROBE, LATCH, DATA and 13.

Entsprechend dem erfindungsgemäßen Thermodruckverfahren werden bei Inbetriebnahme der Thermodruckvorrichtung bei einer Referenztemperatur, die beispielsweise mittels der Temperaturfühlereinrichtung 14 gemessen und über den Analog/Digital-Umsetzer 15 der Steuereinrichtung 3 zugeführt werden kann, die elektrischen Widerstände der einzelnen Heizelemente R1... Rn gemessen. Dazu wird durch einen von der Steuereinrichtung 3 über die Steuerleitung 13 abgegebenen Schaltbefehl der steuerbare Schalter 10 geöffnet, so daß die Heizelemente R1... Rn über den Meßwiderstand 9 an der Spannungsquelle V + liegen. Ein einzelnes Datenbit wird von der Steuereinrichtung 3 über die Datensignalleitung DATA dem Serien/Parallel-Schieberegister 6 zugeführt und schrittweise durch das Serien/Parallel-Schieberegister 6 von Speicherplatz zu Speicherplatz geschoben. Bei jedem Schritt wird das Datenbit durch ein Datenübernahmesignal auf der Datenübernahmesignalleitung LATCH in die Halteschaltung 4 umgeladen, so daß danach das Datenbit in dem Serien/Parallel-Schieberegister 6 um einen Speicherplatz weiterverschoben werden kann. Mit einem von der Steuereinrichtung 3 auf der Ansteuertaktsignalleitung STROBE erzeugten Ansteuertaktsignal werden die einzelnen Heizelemente R1... Rn entsprechend der momentanen Lage des in der Halteschaltung 4 zwischengespeicherten Datenbits nacheinander über den Meßwiderstand 9 mit der Spannungsquelle V + verbunden. Der Meßwiderstand 9 ist im Hinblick auf die elektrischen Widerstände der Heizelemente R1... Rn derart bemessen, daß durch die Heizelemente R1... Rn nur ein geringer Meßstrom fließt und die dadurch in den Heizelementen R1... Rn hervorgerufene Temperaturerhöhung vernachlässigbar gering ist. Die an dem Meßwiderstand 9 abfallende Spannung wird als Maß für den elektrischen Widerstand jedes einzelnen der nacheinander angesteuerten Heizelemente R1... Rn von dem Analog/Digital-Umsetzer 11 in einen digitalen Widerstands-Referenzwert für das betreffende Heizelement R1... Rn umgesetzt und in der Speichereinrichtung 18 abgespeichert. Dieser Meßvorgang läßt sich während der Nutzungsdauer der Thermodruckvorrichtung mehrfach wiederholen, so daß alterungsbedingte Widerstandsänderungen der Heizelemente R1... Rn ihre Berücksichtigung finden.According to the thermal printing method according to the invention, when the thermal printing device is started up, the electrical resistances of the individual heating elements R1 ... Rn are measured at a reference temperature, which can be measured, for example, by means of the temperature sensor device 14 and fed to the control device 3 via the analog / digital converter 15. For this purpose, the controllable switch 10 is opened by a switching command issued by the control device 3 via the control line 13, so that the heating elements R1 ... Rn are connected to the voltage source V + via the measuring resistor 9. A single data bit is supplied from the control device 3 to the serial / parallel shift register 6 via the data signal line DATA and is shifted step by step through the series / parallel shift register 6 from memory location to memory location. At each step, the data bit is reloaded into the hold circuit 4 by a data transfer signal on the data transfer signal line LATCH, so that the data bit in the series / parallel shift register thereafter 6 can be moved by one space. With a control clock signal generated by the control device 3 on the control clock signal line STROBE, the individual heating elements R1 ... Rn are successively connected to the voltage source V + via the measuring resistor 9 in accordance with the current position of the data bits temporarily stored in the holding circuit 4. The measuring resistor 9 is dimensioned with regard to the electrical resistances of the heating elements R1 ... Rn such that only a small measuring current flows through the heating elements R1 ... Rn and the temperature increase thereby caused in the heating elements R1 ... Rn is negligible . The voltage drop across the measuring resistor 9 is converted as a measure of the electrical resistance of each of the successively controlled heating elements R1 ... Rn by the analog / digital converter 11 into a digital resistance reference value for the heating element R1 ... Rn in question and stored in the memory device 18. This measuring process can be repeated several times during the useful life of the thermal printing device, so that age-related changes in resistance of the heating elements R1 ... Rn are taken into account.

Im folgenden wird der Druckbetrieb der Thermodruckvorrichtung anhand der Diagramme in Figur 2 und Figur 3 erläutert. Figur 2 zeigt die den einzelnen Heizelementen R1... Rn zugeführten Meßströme M1... Mn zur Temperaturmessung und die nachfolgend den Heizelementen R1... Rn zugeführten Stromimpulse I1... In während eines Ansteuerzyklus tA; ferner zeigt Figur 2 die dazugehörigen Temperaturverläufe T1... Tn in den einzelnen Heizelementen R1... Rn. Jeder Ansteuerzyklus tA besteht aus einer Meßphase tM, die einer auf den jeweils vorangegangenen Ansteuerzyklus folgenden Abkühlungspause für alle Heizelemente R1... Rn entspricht, und einer sich daran anschließenden Ansteuerphase tI. Während der Meßphase tM ist der steuerbare Schalter 10 geöffnet; über die Datenleitung DATA wird ein einzelnes Datenbit schrittweise durch das Serien/Parallel-Schieberegister 6 geschoben und bei jedem Schritt über die Halteschaltung 4 jeweils einem anderen der NAND-Glieder S1... Sn eingangsseitig zugeführt, so daß in Abhängigkeit von dem Ansteuertaktsignal auf der Ansteuertaktsignalleitung STROBE die Heizelemente R1... Rn nacheinander in Reihe mit dem Meßwiderstand 9 an die Spannungsquelle V + geschaltet werden. Der jeweilige Spannungsabfall an dem Meßwiderstand 9 wird als Maß für die aktuellen elektrischen Widerstandswerte der einzelnen Heizelemente R1... Rn in dem Analog/Digital-Umsetzer 11 in einen entsprechenden Digitalwert umgewandelt und der Steuereinrichtung 3 zugeführt. In der Steuereinrichtung 3 wird für jedes Heizelement R1... Rn der jeweils gemessene Widerstandswert mit dem zugehörigen Widerstands-Referenzwert aus der Speichereinrichtung 18 verglichen und aus dem Vergleichsergebnis die momentane Temperatur TM1... TMn des betreffenden Heizelementes R1... Rn bestimmt.The printing operation of the thermal printing device is explained below using the diagrams in FIG. 2 and FIG. 3. FIG. 2 shows the measuring currents M1 ... Mn fed to the individual heating elements R1 ... Rn for temperature measurement and the current pulses I1 ... In subsequently fed to the heating elements R1 ... Rn during a drive cycle t A ; FIG. 2 also shows the associated temperature profiles T1 ... Tn in the individual heating elements R1 ... Rn. Each control cycle t A consists of a measurement phase t M , which corresponds to a cooling pause for all heating elements R1... Rn following the respective previous control cycle, and a control phase t I that follows. The controllable switch 10 is open during the measurement phase t M ; Via the data line DATA, a single data bit is shifted step by step through the series / parallel shift register 6 and at each step via the The holding circuit 4 is fed to another of the NAND elements S1 ... Sn on the input side, so that the heating elements R1 ... Rn are connected in series with the measuring resistor 9 to the voltage source V + in dependence on the drive clock signal on the drive clock signal line STROBE. The respective voltage drop across the measuring resistor 9 is converted as a measure of the current electrical resistance values of the individual heating elements R1 ... Rn in the analog / digital converter 11 into a corresponding digital value and fed to the control device 3. For each heating element R1 ... Rn, the respectively measured resistance value is compared in the control device 3 with the associated resistance reference value from the storage device 18 and the instantaneous temperature TM1 ... TMn of the heating element R1 ... Rn in question is determined from the comparison result.

Da in der auf die Meßphase tM folgenden Ansteuerphase tI alle jeweils zum Drucken bestimmten Heizelemente R1... Rn gleichzeitig in Abhängigkeit von ihren momentanen Temperaturen angesteuert werden sollen, ist es erforderlich, der Steuerung der Stromimpulsbeaufschlagung der einzelnen Heizelemente R1... Rn jewils auf einem gemeinsamen Zeitpunkt bezogene Temperaturwerte der Heizelemente R1... Rn zugrunde zu legen. Aus diesem Grund werden aus den zeitlich nacheinander erhaltenden Temperaturmeßwerten TM1... TMn in Abhängigkeit von der in der Speichereinrichtung 17 abgelegten Abkühlungskurve der Heizelemente R1... Rn und der für jedes Heizelement R1... Rn jeweils anderen Zeitdifferenz zwischen dem Zeitpunkt der Temperaturmessung und dem Beginn der nachfolgenden Stromimpulsbeaufschlagung zeitgleiche Abkühlungstemperaturwerte TA1... TAn für alle Heizelemente R1... Rn ermittelt. Hierzu wird, wie Figur 3 beispielhaft für das Heizelement R2 schematisch zeigt, der zum Zeitpunkt t₂ gemessene Temperaturmeßwert TM2 um einen Abschlagswert A2 verringert, der sich aus dem Verlauf der abgespeicherten Abkühlungskurve 19 der Heizelemente R1... Rn und der Zeitdifferenz zwischen dem Meßzeitpunkt t₂ und dem Beginn der Ansteuerphase tI ergibt. Die Abkühlungskurve 19 ist in Form von Meßwertdaten abgespeichert, die durch eine Vielzahl einzelner Widerstandsmessungen an einem oder mehreren ausgesuchten Heizelementen R1... Rn während einer auf eine Aufheizung nachfolgenden Abkühlphase erhalten werden könnmen.Since in the control phase t M following the control phase t I all heating elements R1 ... Rn each intended for printing are to be controlled simultaneously depending on their current temperatures, it is necessary to control the current pulse application of the individual heating elements R1 ... Rn temperature values of the heating elements R1 ... Rn based on a common point in time. For this reason, the temperature measured values TM1 ... TMn obtained one after the other become dependent on the cooling curve of the heating elements R1 ... Rn stored in the storage device 17 and the different time difference between the time of the temperature measurement for each heating element R1 ... Rn and the start of the subsequent current pulse application, simultaneous cooling temperature values TA1 ... TAn were determined for all heating elements R1 ... Rn. For this purpose, as shown schematically by way of example in FIG. 3 for the heating element R2, the temperature measured value TM2 measured at the time t₂ is reduced by a discount value A2 which is derived from the course of the stored cooling curve 19 of the heating elements R1 ... Rn and the time difference between the measurement time t₂ and the beginning of Control phase t I results. The cooling curve 19 is stored in the form of measured value data which can be obtained by a large number of individual resistance measurements on one or more selected heating elements R1 ... Rn during a cooling phase following heating.

In der auf die Meßphase tM folgenden Ansteuerphase tI wird der steuerbare Schalter 10 geschlossen, so daß die Heizelemente R1... Rn über die Sammelleitung 2 direkt an der Spannungsquelle V + liegen. Die Druckdaten, die in dem betreffenden Ansteuerzyklus TA die jeweils zum Drucken bestimmenden Heizelemente R1... Rn bezeichnen, werden über den Dateneingang 8 der Steuereinrichtung 3 zugeführt und von dieser als serielle Daten über die Datensignalleitung DATA in das Serien/Parallel-Schieberegister 6 eingelesen, an dessen Parallelausgängen sie in Form von parallelen Daten anliegen. Durch ein von der Steuereinrichtung 3 auf der Datenübernahmesignalleitung LATCH erzeugtes Datenübernahmesignal werden die parallelen Daten aus dem Serien/Parallel-Schieberegister 6 in die Halteschaltung 4 umgeladen, so daß mit einem von der Steuereinrichtung 3 auf der Ansteuertaktsignalleitung STROBE erzeugten Ansteuertaktsignal die einzelnen Heizelemente R1... Rn entsprechend dem in der Halteschaltung 4 zwischengespeicherten Daten mit einem Strom aus der Spannungsquelle 2 beaufschlagt werden. Die Zeiten der Strombeaufschlagung bemessen sich für jedes einzelne Heizelement R1... Rn unterschiedlich in Abhängigkeit von dem für dieses Heizelement R1... Rn jeweils ermittelten Abkühlungstemperaturwert TA1... TAn; je geringer dieser ermittelte Abkühlungstemperaturwert TA1... TAn ist, umso länger ist die Dauer der Stromimpulsbeaufschlagung des betreffenden Heizelementes R1... Rn, so daß alle druckenden Heizelemente R1... Rn zum Ende ihrer jeweiligen Stromimpulsbeaufschlagung jeweils annähernd die gleiche Drucktemperatur TD erreichen.In the control phase t I following the measurement phase t M , the controllable switch 10 is closed, so that the heating elements R1... Rn are directly connected to the voltage source V + via the bus 2. The print data, which in the relevant control cycle TA designate the heating elements R1 ... Rn which are intended for printing, are fed to the control device 3 via the data input 8 and are read by the control device 3 as serial data into the series / parallel shift register 6 via the data signal line DATA , whose parallel outputs they are in the form of parallel data. A data transfer signal generated by the control device 3 on the data transfer signal line LATCH reloads the parallel data from the series / parallel shift register 6 into the hold circuit 4, so that the individual heating elements R1... Are generated with a drive clock signal generated by the control device 3 on the drive clock signal line STROBE. Rn can be supplied with a current from the voltage source 2 in accordance with the data temporarily stored in the holding circuit 4. The times of current application are measured differently for each individual heating element R1 ... Rn depending on the cooling temperature value TA1 ... TAn determined for this heating element R1 ... Rn; the lower this determined cooling temperature value TA1 ... TAn, the longer the duration of the current pulse application of the relevant heating element R1 ... Rn, so that all printing heating elements R1 ... Rn each have approximately the same printing temperature T D at the end of their respective current pulse application to reach.

Da sich der Thermodruckkopf 1 während seiner Ansteuerung durch Wärmeakkumulation insgesamt aufheizt, ändert sich das Abkühlungsverhalten der Heizelemente R1... Rn während der Druckdauer. Daher wird in regelmäßigen Zeitabständen die Temperatur des Thermodruckkopfesl mittels der Temperaturfühlereinrichtung 14 gemessen und der Verlauf der abgespeicherten Abkühlungskurve 19 der Heizelemente R1... Rn an die Temperatur des Thermodruckkopfes angepaßt.Since the thermal print head 1 heats up as a result of heat accumulation, the cooling behavior changes of heating elements R1 ... Rn during the printing period. Therefore, the temperature of the thermal printhead 1 is measured at regular intervals by means of the temperature sensor device 14 and the course of the stored cooling curve 19 of the heating elements R1 ... Rn is adapted to the temperature of the thermal printhead.

Claims (6)

1. A thermal printing method in which for cyclic control of heating elements (R1... Rn) of a thermal printing head (1) in each control cycle (tA) firstly the temperature (T1... Tn) of each individual heating element (R1... Rn) is measured before the relevant heating element (R1... Rn), which is determined for printing, is acted upon by a current pulse (I1... In), the energy of which is controlled in dependence on the measured temperature (T1... Tn), characterised in that at the beginning of each control cycle (tA) there is provided a cooling pause (tM) for all heating elements (R1... Rn), following the respective preceding control cycle, during which the temperatures (T1... Tn) of the heating elements (R1... Rn) are measured one after the other, in that from the measured temperature values (TM1... TMn) received one after the other, and a characteristic cooling curve (19), of the heating elements (R1... Rn), deposited in a memory device (17), cooling temperature values (TA1.. TAn), relative in each case to a common point in time, are determined in each case for all heating elements (R1... Rn) and in that all heating elements (R1... Rn), determined for printing, are acted upon subsequent to the cooling pause (tM) at the same time by the current pulses (I1... In), the respective pulse durations of which are measured individually for each heating element (R1... Rn), in dependence on the cooling temperature value (TA1... TAn) respectively determined for this heating element (R1... Rn).
2. A thermal printing method according to claim 1, characterised in that for measuring the temperatures (T1... Tn) of the heating elements (R1... Rn), the electric resistances of the heating elements are measured and compared with reference resistance values deposited in a further memory device (18), which have been uniquely determined with one reference temperature for the heating elements (R1... Rn).
3. A thermal printing method according to one of the preceding claims, characterised in that for determining the cooling temperature values (TA1... TAn), the respective measured temperature value (TM1... TMn) of each heating element (R1... Rn) is reduced by an offset value (e.g. A2), which is determined from the shape of the stored cooling curve (19) of the heating elements (R1... Rn) and from the respective time difference between the temperature measurement and the following current pulse action of all heating elements (R1... Rn).
4. A thermal printing method according to one of the preceding claims, characterised in that the temperature of the thermal printing head (1) in the region of the heating elements (R1.... Rn) is measured with a temperature sensor device (14), and that in dependence on this measured temperature the shape of the stored cooling curve (19) of the heating elements (R1... Rn) is adapted to the temperature of the thermal printing head (1).
5. A thermal printing method according to one of the preceding claims, characterised in that the temperature of the thermal printing head is determined, in dependence on the temperature measured values measured in the preceding control cycles, and in that the shape of the stored cooling curve is adapted to the determined temperature of the thermal printing head.
6. A thermal printing method according to one of the preceding claims, characterised in that with a successive group control of the heating elements with the current pulse action of the heating elements of the respective current controlled group the temperatures of the heating elements of the respective following group to be controlled are measured at the same time.
EP88730271A 1987-12-07 1988-12-05 Thermal printer Expired - Lifetime EP0320435B1 (en)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
DE3741799 1987-12-07
DE19873741799 DE3741799A1 (en) 1987-12-07 1987-12-07 THERMAL PRINTING PROCESS

Publications (2)

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EP0320435A1 EP0320435A1 (en) 1989-06-14
EP0320435B1 true EP0320435B1 (en) 1991-11-21

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EP88730271A Expired - Lifetime EP0320435B1 (en) 1987-12-07 1988-12-05 Thermal printer

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DE (2) DE3741799A1 (en)

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US5132709A (en) * 1991-08-26 1992-07-21 Zebra Technologies Corporation Apparatus and method for closed-loop, thermal control of printing head
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US4887092A (en) 1989-12-12
EP0320435A1 (en) 1989-06-14
DE3741799A1 (en) 1989-06-15
DE3866338D1 (en) 1992-01-02

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