Wear compensation device of a label printer

Abstract

The invention relates to a wear compensation device (1) of a label printer (2) which prints labels (3) by means of thermal printing, having a thermal head (4) which has a thermal strip (5) with a plurality of heating resistors (6), having a label feeding device (8) which feeds the respective label (3) to the active region (9) of the heating resistors (6), and having a control device (10) which actuates the thermal head (4) for printing the respective label (3). It is proposed that the control device (10) is configured to monitor the electric resistance (R) of one or more of the heating resistors (6) and, if a predefined threshold value (R1) for the electric resistance (R) is exceeded, to increase the energization duration of the respective heating resistor (6) during a printing operation.

Claims

1. A wear compensation device for a label printer that includes a thermal head having a thermal strip provided with a plurality of heating resistors and a label feed device for feeding respective labels to an active region of the plurality of heating resistors and that prints labels by means of thermal printing in a printing operation, the wear compensation device comprising: a control device for activating the thermal head for printing the respective labels; wherein the control device is configured to monitor electric resistance of one or more of the plurality of heating resistors and, when it is determined that a predetermined threshold value for the electric resistance for a respective one or more of the plurality of heating resistors is exceeded, to increase energization duration for the respective one or more of the plurality of heating resistors during the printing operation.

2. The wear compensation device as claimed in claim 1, wherein the control device is configured to monitor the electric resistance of all the heating resistors of the thermal strip.

3. The wear compensation device as claimed in claim 1, wherein the control device is configured to monitor the electrical resistance of heating resistors of at least one predefined group of heating resistors of the thermal strip.

4. The wear compensation device as claimed in claim 1, wherein the control device is configured such that voltage and/or current intensity during the printing operation with the increased energization duration is not changed in comparison with at least a last preceding printing operation in which the predetermined threshold value was not determined to be exceeded.

5. The wear compensation device as claimed in claim 1, wherein the control device is configured to determine the electric resistance of the respective one or more of the plurality of heating resistors continuously.

6. The wear compensation device as claimed in claim 1, wherein the control device is configured to determine the electric resistance of the respective one or more of the plurality of heating resistors at time intervals.

7. The wear compensation device as claimed in claim 1, wherein the control device comprises a current measuring device for determining the electric resistance of the respective one or more of the plurality of heating resistors.

8. The wear compensation device as claimed in claim 1, wherein the control device comprises a voltage measuring device for determining the electric resistance of the respective one or more of the plurality of heating resistors.

9. The wear compensation device as claimed in claim 1, wherein the control device comprises a comparator for comparing the electric resistance determined for the respective one or more of the plurality of heating resistors with the predefined threshold value.

10. The wear compensation device as claimed in claim 1, wherein the control device is configured such that the control device adjusts the energization duration depending on the electric resistance determined for the respective one or more of the plurality of heating resistors such that the energization duration rises with increasing electric resistance.

11. The wear compensation device as claimed in claim 1, wherein the control device comprises a memory for storing the predetermined threshold value.

12. The wear compensation device as claimed in claim 11, wherein the memory is configured to store an individual predetermined threshold value for each of the one or more of the plurality of heating resistors being monitored.

13. The wear compensation device as claimed in claim 1, wherein the control device is configured such that the predetermined threshold value does not change as the thermal strip ages through use.

14. The wear compensation device as claimed in claim 1, wherein the control device is configured such that the predetermined threshold value is raised from an initial value to a subsequent value continuously as the thermal strip ages through use.

15. The wear compensation device as claimed in claim 1, wherein the control device is configured such that the predetermined threshold value is raised from an initial value to a subsequent value at time intervals as the thermal strip ages through use.

16. The wear compensation device as claimed in claim 1, wherein the control device is configured to fix the predetermined threshold value for the respective one or more of the plurality of heating resistors at an initial value when a new thermal strip is installed.

17. The wear compensation device as claimed in claim 1, wherein the initial value is 1 to 20% higher than an initial value for the electric resistance of the respective one or more of the plurality of heating resistors.

18. The wear compensation device as claimed in claim 1, wherein the initial value is 1 to 10% higher than an initial value for the electric resistance of the respective one or more of the plurality of heating resistors.

19. The wear compensation device as claimed in claim 1, wherein the initial value is 1 to 5% higher than an initial value for the electric resistance of the respective one or more of the plurality of heating resistors.

20. A method for wear compensation of a label printer that prints labels by means of thermal printing in a printing operation and includes a thermal head having a thermal strip provided with a plurality of heating resistors, a label feed device for feeding respective labels to an active region of the plurality of heating resistors and a control device for activating the thermal head for printing the respective labels, the method comprising monitoring electric resistance of one or more of the plurality of heating resistors using a wear compensation device as claimed in claim 13, and, when it is determined that a predefined threshold value for the electric resistance for a respective one or more of the plurality of heating resistors is exceeded, increasing energization duration for the respective one or more of the plurality of heating resistors during the printing operation.

Description

(1) The invention will be explained in more detail below by using a drawing, merely illustrating an exemplary embodiment. In the drawing:

(2) FIG. 1 shows a schematic view of a wear compensation device as proposed from the side and from the front,

(3) FIG. 2 shows an example of the curve of the electric resistance of a heating resistor over its service life and

(4) FIG. 3 shows the energization of the heating resistor a) until a threshold value for the electric resistance is reached, and b) after the threshold value for the electric resistance has been exceeded.

(5) The wear compensation device 1 shown in FIG. 1 in the two views is a constituent part of a label printer 2, which prints labels 3 by means of thermal printing, for example by means of direct thermal printing. The labels 3 here are self-adhesive labels 3, for example, which are detachably arranged on a carrier strip (not illustrated) and are printed individually following the detachment.

(6) For the printing, a thermal head 4 is provided, which has a thermal strip 5 with a multiplicity of heating resistors (dots) 6, via which a printed image of a specific imprint quality is produced on the surface of the respective label 3, which is led past the thermal strip 5.

(7) On the side opposite the thermal strip 5, i.e. vertically underneath the label 3 which is just being printed here, there is arranged a mating printing element 7, which here and preferably is designed as a pressure-felt-coated strip. In principle, the mating printing element 7 can also be an impression cylinder.

(8) Furthermore, a label feed device 8, here and preferably in the form of a transport belt, is provided, which feeds the respective label 3 to the active region 9 of the heating resistors 6. Here and preferably, the labels 3 are fed to the active region after they have been separated or detached from a carrier strip. However, it is also conceivable to feed the labels 3 while they are still located on the carrier strip. The active region means the portion underneath the thermal strip 5, in which the heating resistors 6 can introduce thermal energy point by point into the thermosensitive medium of the label 3 and, as a result, can effect a color change, in particular blackening, of the label 3 at this point.

(9) The wear compensation device 1 as proposed further has a control device 10, which activates the thermal head 4 for printing the respective label 3.

(10) The activation of the thermal head 4 comprises the energization of the respective heating resistors 6 for predefined energization duration.

(11) It is then essential that the control device 10 as proposed monitors the electric resistance R of one or more of the heating resistors 6, here all of the heating resistors 6 of the thermal strip 5. The monitoring comprises the repeated determination of the electric resistance R of the respective heating resistor 6. When a predefined threshold value R.sub.1 for the electric resistance R is exceeded, the control device 10 then increases the energization duration of the respective heating resistor 6. The respective heating resistor 6 is also energized for longer as compared with the initially provided energization duration and is thus activated for longer. Therefore, the respective heating resistor 6 can act for longer on the thermosensitive medium of the label 3 and bring about a more intense color change or blackening. Ageing-induced retrogression of the level of color change or level of blackening can therefore be compensated.

(12) The control device 10 is here and preferably configured such that the voltage U and/or current intensity I are/is not changed during the printing operation carried out with the increased energization duration. In other words, the voltage U and/or current intensity I remain unchanged in comparison with at least the last preceding printing operation in which the threshold value R.sub.1 had not yet been exceeded or in comparison with all the preceding printing operations in which the threshold value R.sub.1 had not yet been exceeded, here according to FIG. 3 at a value U.sub.1 or I.sub.1. In principle, however, as an additional compensation measure in order to compensate for a decreasing level of color change or level of blackening, the voltage U and/or current intensity I of the respective heating resistor 6 can be increased.

(13) Here and preferably, the electric resistance R of the respective heating resistor 6 is determined continuously, i.e. each time it is switched on. Here, the electric resistance R is determined via a current measuring device 11 and/or a voltage measuring device 12.

(14) The control device 10 further has a comparator 13, which compares the respectively determined electric resistance or the corresponding resistance value R of the respective heating resistor 6 with the predefined threshold value R.sub.1. Thus, FIG. 2 shows by way of example a curve of the electric resistance or resistance value R of one of the heating resistors 6 of the thermal strip 5. Thus, the heating resistor 6 has an electric resistance R.sub.0 at the start of its service life (time T.sub.0). The electric resistance R then initially falls as a result of a large number of printing operations in the course of the service life of the heating resistor 6 and then rises again. As FIG. 2 shows, the electric resistance R exceeds its initial value R.sub.0 at some point and continues to rise. As soon as a predefined threshold value R.sub.1 for the electric resistance R is exceeded at a specific time T.sub.1, the control device 10 changes the energization duration of this heating resistor 6.

(15) In the exemplary embodiment illustrated in FIG. 2, the threshold value R.sub.1 lies above the initial value R.sub.0 of the heating resistor 6, which has the advantage that the increase of the energization duration is performed only when the heating resistor 6 has actually reached a specific level of wear over the course of time. As a result, it is possible to rule out the energization duration also already being increased at the start when the thermal strip 5 or the heating resistor 6 is still new, but where it also has an electric resistance R with a value of R.sub.0 later in the critical state.

(16) In principle, however, it is also conceivable to fix the threshold value R.sub.1 at the initial value R.sub.0 of the heating resistor 6, wherein the control device 10 is able to distinguish the initial state of the heating resistor 6, in which the energization duration is not yet to be increased, from the critical state, starting from which the energization duration is to be increased, by using a series of resistance values stored over the course of time, wherein it is possible to draw conclusions about a rise in the electric resistance R by comparing at least two successive resistance values. If the value R.sub.0 is then reached, the control device 10 detects that the critical state which requires an increase in the energization duration has been reached.

(17) The energization duration is adjusted as proposed depending on the respectively determined electric resistance R of the respective heating resistor 6, the energization duration rising with increasing electric resistance R. In FIG. 3a), by way of example, the original energization duration of the heating resistor 6 is illustrated with the resistance curve according to FIG. 2. This energization duration is provided for the time period from T.sub.0 to T.sub.1 in FIG. 2. FIG. 3b) shows, for comparison and by way of example, an increased energization duration. The increased energization duration is provided after the time T.sub.1 in FIG. 2 has been reached, that is to say when the threshold value R.sub.1 has been exceeded. If the electric resistance R of the heating resistor 6 continues to rise, in particular the energization duration is also correspondingly increased further. As is likewise illustrated in FIG. 3, the voltage U and the current intensity I remain adjusted constantly to the value U.sub.1 and I.sub.1.

(18) The control device 10 also has a memory 14, in which the respective threshold value R.sub.1 is stored. An individual threshold value R.sub.1 is stored in the memory 14 here and preferably for each of the heating resistors 6. The threshold value R.sub.1 is in particular 1 to 20%, preferably 1 to 10%, particularly preferably 1 to 5%, higher than the initial value R.sub.0 for the electric resistance R of the heating resistor 6. In the present case, the threshold value R.sub.1 is 15% higher than the initial value R.sub.0, as illustrated by way of example in FIG. 2.

(19) In principle, the control device 10 can also be configured in such a way that the respective threshold value R.sub.1 or is adapted, in particular raised, continuously or at time intervals, in particular at regular time intervals, starting from its initial value R.sub.1, illustrated in FIG. 2 for the time T.sub.2. The adapted threshold value then replaces the respective previous threshold value R.sub.1 in the memory 14. However, here and preferably no adaptation of the threshold value R.sub.1 is provided, instead the latter remains constant or unchanged at R.sub.1, as FIG. 2 shows. Starting from the time T.sub.1, at which the constant threshold value R.sub.1 has been exceeded, the energization duration is adapted here and preferably always on the basis of the respectively determined electric resistance R of the heating resistor 6, i.e. the energization duration is changed with each newly determined value for the electric resistance.

(20) The present invention, finally, also relates to a method for wear compensation of a label printer 2 which prints labels 3 by means of thermal printing, which method can preferably be carried out by using the previously described wear compensation device 1.

(21) In the method as proposed, wear compensation is carried out via the control device 10 which activates the thermal head 4 for printing the respective label 3, by the electric resistance R of one or more heating resistors 6 of the thermal strip 5 of the thermal head 4 being monitored and, when a predefined threshold value R.sub.1 for the electric resistance R is exceeded, the energization duration of the respective heating resistor 6 being increased during a printing operation.