GB2435952A

GB2435952A - A method of operating a print head and a print head with associated memory for storing the value of the electrical resistance of the print head

Info

Publication number: GB2435952A
Application number: GB0604716A
Authority: GB
Inventors: Alan Beech
Original assignee: Markem Technologies Ltd
Current assignee: Markem Imaje Industries Ltd
Priority date: 2006-03-09
Filing date: 2006-03-09
Publication date: 2007-09-12
Also published as: GB0604716D0

Abstract

A method of operating a printing apparatus 10 (eg a thermal printer) of the kind including a print head 12 having an array 14 of printing elements which in use are selectively energised for an "on" period in each of a plurality of row positions of an image to be printed, to effect printing, and a power supply 15 to provide electrical power to the print head 12 in each row position, includes setting the duration of the "on" period for which the printing elements are selectively energised in each row position, depending upon a characteristic of the print head 12. The value of the electrical resistance of the print head can be stored on a memory in the print head. The printer apparatus can measure the resistance of the print head.

Description

Title: Method of Operating a Printing Apparatus

Description of Invention

This invention relates to a method of operating a printing apparatus of the kind including a print head having an array of printing elements which in use are selectively energised for an "on" period in each of a plurality of row positions of an image to be printed, to effect printing.

In one arrangement, the printing apparatus includes an inked ribbon or other carrier which is interposed between the print head and a substrate onto which an image is to be printed, the printing elements when energised, removing pixels of ink from the ribbon which are deposited onto the substrate. In another embodiment, where no carrier is provided, the substrate may be heat sensitive whereby the energisation of the printing elements effect changes in the substrate thereby to print or record an image.

To ensure that when each of the printing elements is energised, an appropriate amount of energy is dissipated successfully to print a pixel of the image, i.e. for a pixel of a desired size to be printed, the printing apparatus of which the print head is a part, when the print head is installed, requires careful calibration. Because the electrical resistance of each print head is, due to manufacturing tolerances, a fixed and unique parameter of each print head, upon installation of the print head, it is usual practice for an engineer to have to set the voltage to be applied to the print head in use, for example by adjusting a potentiometer of a control board. Even though the electrical resistance of a print head may be factory determined and for example marked on the print head, it is obviously inconvenient for a user to have to engage an engineer's services when it is necessary to replace a print head.

According to a first aspect of the invention we provide a method of operating a printing apparatus of the kind including a print head having an array of printing elements which in use are selectively energised for an "on" period in each of a plurality of row positions of an image to be printed, to effect printing, and a power supply to provide electrical power to the print head in each row position, the method including setting the duration of the "on" period for which the printing elements are selectively energised in each row position, depending upon a characteristic of the print head.

Thus utilising the present invention, it is unnecessary for an engineer to have to set the voltage to be applied to the print head, as a nominal print head voltage may be applied to print heads which may have differing electrical resistance. By adjusting the duration of the "on" period for which each selected printing element of a print head is selectively energised at each row position, the desired energy may be dissipated by each printing element successfully to print a pixel of the image.

Desirably, the printing apparatus includes a controller which determines the durations of the "on" periods depending upon the electrical resistance of the print head. Whereas it is within the scope of this invention for the controller to determine the resistance of the print head by measurement during a pre-printing calibration period, e.g. upon installation of the print head, in a preferred embodiment, the print head carries previously determined data relating to the print head resistance, and the controller reads the data.

For example, the print head may include a memory into which the data is stored, the controller reading the data, and in response, determining the duration of the "on" period of each selected printing element for the desired energy to be dissipated by the printing elements successfully to print pixels.

The data may have been written into the memory during a factory calibration/test.

According to a second aspect of the invention we provide a method of operating a printing apparatus of the kind including a print head having an array of printing elements which in use are selectively energised for an "on" period in each of a plurality of row positions of an image to be printed, to effect printing, and a power supply to provide electrical power to the print head in each row position, the method including determining the electrical resistance of the print head, storing data relating to the determined electrical resistance of the print head in or on the print head, reading the stored data and setting the durations of the "on" periods for which the printing elements are selectively energised in each row position, depending upon the stored data.

According to a third aspect of the invention we provide a print head for use in a printing apparatus, the print head having an array of printing elements which in use are selectively energised for an "on" period in each of a plurality of row positions of an image to be printed, to effect printing, the print head including a memory in which is stored data relating to the electrical resistance of the print head.

According to a fourth aspect of the invention we provide a printing apparatus including a print head according to the third aspect of the invention, and a power supply to provide electrical power to the print head in each row position of an image to be printed, a controller to read the data stored in the memory of the print head and to adjust the durations of the "on" periods for which the printing elements are selectively energised in each row position, depending upon the electrical resistance of the print head.

Embodiments of the invention will now be described with reference to the accompanying drawings in which:-FIGURE 1 is an illustrative diagram of a printing apparatus for use in performing the method of the present invention; FIGURE 2 is a graphical representation illustrating the conventional operation of a print head in each of a plurality of row positions of an image to be printed; FIGURE 3 is an illustration similar to figure 2 but illustrating the operation of the print head in accordance with the present invention.

Referring to figure 1 a printing apparatus 10 includes a print head 12 which has a linear array 14 of thermal printing elements, the array 14 having for example, 600 printing elements per linear inch (236 per centimetre). The printing elements of the array 14 are selectively electrically energisable under computer control, and when energised, produce heat.

In figure 1 a computer controller is illustrated at 15, the controller 15 receiving data input 17 and controlling the print head 12, as well as a ribbon drive mechanism described below, to effect printing of images on each of a plurality of substrates, which in this example are peelable labels 16 provided on a continuous elongate backing web 18. The controller 15 in this example thus supplies power to the printing elements of the print head 12.

Inked ribbon 20 is provided on a supply spool 21 from which the ribbon 18 is drawn and fed past the print head 12 during printing, and is taken up onto a take-up spool 22.

In this example, each of the storage 21 and take-up spools 22 is driven by a respective drive motor 21a, 22a to effect ribbon 20 movement, but in another example only the take-up spool 22 may be driven, and/or ribbon 20 movement past the print head 12 may be effected by a drive capstan roller or otherwise, in a ribbon feed path.

In each case, in this example, during printing, both substrate 16 and inked ribbon 20 move relative to a stationary print head 12 in the direction indicated in the figure at A. A backing roller 24 is provided in alignment with the print head 12, and the inked ribbon 20 is interposed between the print head 12 and the substrate 16, with the substrate 16, or rather the backing web 1 8 in this example, in contact with the backing roller 24.

In another example, the print head 12 may move during printing relative to a stationary or moving ribbon 20 and substrate 16, but in each case, there will be relative movement between the print head 12, and the ribbon 20 and the substrate 16 whilst printing is effected. In each case, as the printing elements of the array 14 are heated in use, pixels of ink on the ribbon 20 are melted, and are removed from the ribbon 20 and are deposited onto the substrate 16.

In another example, no inked ribbon 20 may be required, where for example the substrate 16 is of the kind which is heat sensitive, whereby the printing elements of the array 14 as they heat up, cause a change in colour of pixels of the substrate 16.

Referring now to figure 2, as the substrate 16 and ribbon 20 move past the array 14 of printing elements of the print head 12, for each row position of an image to be printed, selected printing elements of the array 14 are electrically energised for an "on" period, to print a column of pixels of the image to be printed. In the figure, three alternative scenarios are depicted for each of the row positions at I, II, and Ill.

For the first scenario I, the selected printing elements of the array are energised for an "on" period, and during "off" periods at either side of the "on" period, the substrate 16 and ribbon 20 will be moved to and from a position at the array 14 of the print head 12. When the substrate 16 and ribbon 20 are stationary relative to the print head 12, the selected printing elements are energised during an "on" period t.

It will be appreciated that the print head 12 has an internal resistance, and that the magnitude of the internal resistance may vary from print head to print head 12 due to manufacturing tolerances. Thus conventionally, each print head 12 is factory calibrated to determine its internal resistance R. Successfully to print pixels of image, i.e. successfully to remove pixels of ink of a desired size from the ribbon 20 for application to the substrate 16, requires that a certain energy J is dissipated by the printing elements. Figure 2 shows that conventionally, whatever the internal resistance of the print head 12, the durations of the "on" periods for which selected printing elements are energised at each row position, is arranged to be the same i.e. t. Accordingly to ensure that the desired energy J is dissipated by each selected energised print head during the "on" period of duration t, the voltage of the electrical power supplied to the print head 12 by the controller 15 is set above or below a nominal voltage.

In the example, each print head 12 is intended when manufactured to operate at a nominal 24 volts. So in the event that the internal resistance R is determined to be above the designed norm, during factory calibration, the voltage necessary to ensure that the desired energy J is dissipated by the selectively energised printing elements, is derived.

For illustrative purposes, in scenario I, it can be seen that the voltage required to be applied for "on" period t is slightly above the nominal 24 volts. For illustrative purposes an alternative scenario is depicted at II in which the print head 12 has an internal resistance at or towards an upper end of an allowable tolerance range, and thus the actual applied voltage is significantly greater that 24 volts. For illustrative purposes in scenatio Ill, the voltage required to be applied for "on" period t for the printing elements to dissipate the required energy J, is slightly below the nominal 24 volts as the internal resistance of the print head 12 is slightly less than the norm.

In each of the three illustrated scenarios, the durations of the "on" periods during which voltage is applied to the print head 12, is constant, and by varying the magnitude of the applied voltage with respect to a nominal voltage, the same energy J can be dissipated by the printing elements of the array 14.

Currently, during factory calibration, each print head 12 is marked with an indication of the voltage it is required to be applied for that particular print head 12. Thus during installation of the print head 12 into the printing apparatus 10, an engineer is required to set the print head voltage, for example by adjusting a potentiometer on a control/power supply board.

Thus using the conventional method of operating the printing apparatus 10 described and ignoring losses, the instantaneous power P dissipated by the selected energised printing elements during the duration t of each "on" period is given by:-p= V2/R where V 24 volts + or -the voltage adjustment from 24 volts to be effected due to the variance of the internal resistance of the print head from the norm R is the internal resistance of the particular print head 12.

Thus the total energy dissipated by the selected energised printing elements during each of the "on" period durations, is Pt.

Referring now to figure 3 in accordance with the present invention for each of the same scenarios I, II and Ill, the same voltage is applied to the print head, namely the nominal voltage 24 volts.

To vary the amount energy dissipated by the printing elements of the print head 12, the durations of the "on" periods is adjusted and set. Thus for scenario I, where the internal resistance of the print head 12 is slightly above the norm, the selected printing elements of the array 14 are energised for an enhanced "on" period ti. In the example, the durations of the "off" periods to either side of the "on" periods at each row position are equalised but in another example, the durations of the "on" periods may be enhanced by extending the "on" periods from the same switching on position S as in figure 2, or by advancing the switching on positions or delaying the switching off of the electrical energy after each "on" period.

In each case, it will be appreciated that comparing scenario I in figure 3 to the same scenario in figure 2, the print head 12 is energised at a lesser voltage, i.e. the nominal 24 volts so that the instantaneous power applied to the print head 12 is less than the instantaneous power in the comparable scenario I of figure 1, but power is supplied for longer durations of "on" period, tl such that the same energy is dissipated by the selected printing elements during the duration of each "on" period, as is achived in figure 1.

In the second scenario II in which the internal resistance of the print head 12 is at or towards an upper end of an allowable tolerance range, the durations of the "on" periods t2 are substantially greater that the constant duration "on" periods t depicted in figure 2. However the same energy is dissipated as still, only the nominal 24 volts is applied.

In the third scenario Ill in which the internal resistance of the print head 12 is lower than the norm, it can be seen that the duration of the "on" period t3 is significantly less than the constant "on" period duration t of figure 2, but the energy dissipated by the print head 12 is the same as again, 24 volts is applied to the print head 12.

Because in figure 3 the same voltage is applied irrespective of the internal resistance of the print head 12, it is unnecessary for a user to engage an engineer to set the print head voltage when installing the print head 12.

However the controller 15 needs to have data relating to the internal resistance of the print head 12 in order that the controller 15 can set the durations of the "on" periods for a particular print head 12.

Whereas the method of the invention could provide for the measurement of the internal resistance of the print head 12 during installation or at least during a pre-printing calibtation period, desirably, each print head 12 has a memory, 30 such as an EEPROM, in which data relating to the internal resistance of the print head 12 may be stored. Thus data relating to the internal resistance of the print head 12 obtained during calibration of the print head 12 e.g. in the factory, is written to the memory 30.

Upon installation of the print head 12 and/or subsequently, the controller 15 reads the data in the memory 30 relating to the internal resistance of the print head 12, an as a result, sets the durations of the "on" periods of the print head 12 at each of the pixel row printing positions, for the required amount of energy to be dissipated by the selectively energised printing elements for pixels of ink successfully to be printed, or where the substrate 16 is heat sensitive, for pixels successfully to be recorded by the heating effect of the printing elements of the array 14.

Various modifications may be made without departing from the scope of the invention.

For example whereas in the examples described, the voltage to be applied to the print head is 24 volts, this is for illustrative purposes only and the invention my be used for print heads 12 having any desired operating voltage.

Whereas in the examples, the substrate is a label 16 carried on a backing web 18, the invention may be applied to a thermal printer for another application, for example only for printing onto packages passing the print head 12 in a packaging environment. Thus a backing roller 24 need not be provided, or where some kind of backing is required, this could alternatively be provided by a platen or otherwise.

Whereas ion the examples described, the data relating to the internal resistance of the print head 12 has been stored in an electronic memory of the print head 12, it is within the scope of the invention for such data to be stored otherwise, for example visually so that it may be optically read for use by the controller 15, or magnetically.

The features disclosed in the foregoing description, or the following claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

Claims

CLAIMS

1. A method of operating a printing apparatus of the kind including a print head having an array of printing elements which in use are selectively energised for an "on" period in each of a plurality of row positions of an image to be printed, to effect printing, and a power supply to provide electrical power to the print head in each row position, the method including setting the duration of the "on" period for which the printing elements are selectively energised in each row position, depending upon a characteristic of the print head.

2. A method according to claim 1 wherein the printing apparatus includes a controller which determines the durations of the "on" periods depending upon the electrical resistance of the print head.

3. A method according to claim 1 or claim 2 wherein the controller determines the resistance of the print head by measurement during a pre-printing calibration period.

4. A method according to claim 1 or claim 2 wherein the print head carries previously determined data relating to the print head resistance, and the controller reads the data.

5. A method according to claim 4 wherein the print head includes a memory in which the data is stored, the controller reading the data, and in response, determining the duration of the "on" period of each selected printing element for the desired energy to be dissipated by the printing elements successfully to print pixels.

6. A method according to claim 5 wherein the data is written into the memory during a factory calibration/test.

7. A method of operating a printing apparatus of the kind including a print head having an array of printing elements which in use are selectively energised for an "on" period in each of a plurality of row positions of an image to be printed, to effect printing, and a power supply to provide electrical power to the print head in each row position, the method including determining the electrical resistance of the print head, storing data relating to the determined electrical resistance of the print head in or on the print head, reading the stored data and setting the durations of the "on" periods for which the printing elements are selectively energised in each row position, depending upon the stored data.

8. A method of operating a printing apparatus substantially as hereinbefore described with reference to figure 1 and figure 3 of the accompanying drawings.

9. A print head for use in a printing apparatus, the print head having an array of printing elements which in use are selectively energised for an "on" period in each of a plurality of row positions of an image to be printed, to effect printing, the print head including a memory in which is stored data relating to the electrical resistance of the print head.

10. A print head substantially as hereinbefore described with reference to and/or as shown in figure 1 and figure 3 of the accompanying drawings.

11. A printing apparatus including a print head according to claim 9 or claim 10, and a power supply to provide electrical power to the print head in each row position of an image to be printed, a controller to read the data stored in the memory of the print head and to adjust the durations of the "on" periods for which the printing elements are selectively energised in each row position, depending upon the electrical resistance of the print head.

12. A printing apparatus substantially as hereinbefore described with reference and/or as shown in figure 1 and figure 3 of the accompanying drawings.

13. Any novel feature or novel combination of features described herein and/or as shown in figure 1 or figure 3 of the accompanying drawings.