GB2438649A

GB2438649A - Improving print quality affected by malfunctioning printing element

Info

Publication number: GB2438649A
Application number: GB0610830A
Authority: GB
Inventors: Alan Beech
Original assignee: Markem Technologies Ltd
Current assignee: Markem Imaje Industries Ltd
Priority date: 2006-06-01
Filing date: 2006-06-01
Publication date: 2007-12-05
Also published as: GB0610830D0

Abstract

A print head (14) comprising a plurality of print elements (P1-P8), e.g. inkjet nozzles or thermal elements, is tested prior to use and data relating to any defective elements stored. In use, when a print controller processes an instruction to activate a defective element (P3), an adjacent element (e.g. P4 at R6, C4; P2 at R10, C2) is activated instead. The data relating to defective elements may be stored in the print head (14), and may be read by a print controller prior to printing, or processed by the print head (14) during printing. In this way, print quality may be improved by pixels on an image appearing slightly displaced (R6, C4; R10, C2), as opposed to being missing (R6, C3; R10, C3), and resolution thereby increased. The elements (P1-P8) may be arranged as a linear or two-dimensional array. The test may be conducted prior to installation of the print head (14), or at manually-requested or scheduled frequency post installation.

Description

Title: Method of Operating a Printing Apparatus

Description of Invention

This invention relates to a method of operating a printing apparatus and more particularly to a method of operating a pixel based printing apparatus such as a thermal or ink jet printer for examples only.

Thermal printers are known which have an array of printing elements, each printing element of the array being individually selectable for energisation in order to effect the printing of a pixel on a substrate. Printing elements are selected and energised at each of a plurality of printing positions along a substrate.

To print high resolution images, a high density of printing elements in the array is required. As print head design has improved, particularly high printing element densities, for example of at least 6 printing elements per linear millimetre have become common. However with such.a high density of printing elements, in manufacture, inevitably there are some printing element failures or other malfunctions which results in images being printed with missing pixels.

Because the density of printing elements is high, images with many missing pixels may still be read, but if too many pixels are missing, the print head may be inadequate for use.

According to a first aspect of the invention we provide a method of operating a printing apparatus which has a print head including a plurality of printing elements arranged in an array, the printing elements being individually and selectably operable to print pixels on a substrate, the method including performing a printing operation by providing relative movement between the print head and the substrate and in each of a plurality of positions along the substrate selecting and operating printing elements to print the image, the method including prior to performing a printing operation, testing the performance of each of the printing elements to identify any malfunctioning printing element, and in a subsequent printing operation, at each printing position where such identified malfunctioning printing element would otherwise be selected for operation, selecting and operating an adjacent printing element.

Thus instead of the image simply having one, or more than one, missing pixel, in the image printed, in at least some of the of the printing positions where the rows of pixels are printed, the or each missing pixel is masked by the pixel printed by the adjacent printing element. Where the image being printed is of high resolution i.e. the density of pixels in the image to be printed is large, even though a pixel which is printed by the adjacent printing element at at least some of the printing positions, instead of the identified malfunctioning printing element, will be displaced from their ideal positions in which pixels would be printed if the malfunctioning printing element was operable, such pixel displacement is less noticeable in reading the image, than missing pixels Preferably the adjacent printing element which is selected and operated at each printing position along the substrate, is a printing element which is immediately adjacent the identified malfunctioning printing element of the array. If desired the same adjacent printing element is selected for operation at each printing position along the substrate where the identified malfunctioning printing element would otherwise be selectable and operated, but if desired different adjacent printing elements are selectable and operated at some or each printing position where otherwise the identified malfunctioning printing element would be selected and operated.

For example at one printing position or at each of a plurality of printing positions along the substrate where the malfunctioning printing element would otherwise be selected and operated, the adjacent printing element selected and operated may be at one side of the malfunctioning printing element, whereas at another printing position or at each of another plurality of printing positions along the substrate where the malfunctioning printing element would otherwise be selected and operated, the adjacent printing element selected and operated may be at the other side of the malfunctioning printing element.

In this way, pixels in the image displaced from their natural positions, may be even less noticeable.

Preferably the array of printing elements is a linear array although the invention may be utilised for two dimensional arrays.

The method of the invention may include, upon testing the performance of the individual print heads, creating data and storing the data for use in controlling and operating the print head.

In one embodiment, the testing method step may be carried out with the print head at a printing station of the printing apparatus where in use the print head will perform printing operations.

For example, the testing method step may be automatically carried out upon installing the print head at the printing station, and/or may be carried out manually or at scheduled frequencies, or upon the occurrence of another trigger.

The data created upon testing the performances of the printing elements may be provided for use by a controller of the printing apparatus which selects and effects operation of the printing elements in response to command signals.

Such a controller typically would also co-ordinate selecting and operating printing elements with the relative movement between the print head and the substrate.

Thus where the data identifies a malfunctioning printing element, the controller of the printing apparatus may select the adjacent printing element to operate at each printing position where the command signal indicates that a malfunctioning printing element should be selected and operated.

Whether or not the testing method step is carried out with the print head at the printing station, if desired the data created upon testing the performances of the printing elements may be stored in a memory of the print head. In a preferred method, the data is provided to a controller of the printing apparatus which selects and effects operating of the printing elements in response to command signals. In another embodiment though, the print head may process an instruction from the controller to select and operate, at a printing position, a printing element which has been identified as a malfunctioning printing element, by selecting the adjacent printing element instead.

An advantage of storing the data in a memory of the print head is that the print head may be used in different printing apparatus without the print head having to be tested in the printing apparatus to create the data for the controller.

The invention has been devised particularly but not exclusively for use where the print head is of the thermal kind in which the printing elements when operated, are heated. The heated printing elements may soften and remove pixels of marking medium from a carrier such as an inked ribbon, for depositing on the substrate, or the printing elements when operated may effect printing of pixels by actuating pixels of a heat sensitive material of the substrate.

Typically in a thermal printer, the array of printing elements would contain at least six printing elements per mm in a direction transverse to the direction of relative movement between the print head and the substrate, and more particularly, more than six printing elements per mm, e.g. eight printing elements per mm.

However the invention may be applied to other pixel based printing apparatus, such as for examples only, ink jet type printers.

Embodiments of the invention will now be described with reference to the accompanying drawings in which: FIGURE 1 is an illustrative view of a printing apparatus with which the method of the present invention may be used; FIGURE 2 is an illustrative diagram showing where pixels are printed in each printing position, according to the method of the invention.

Referring to figure 1 a printing apparatus 10 includes a print head 11 provided at a printing station 12 thereof. The apparatus 10 is a thermal printer, that is the print head 11 has an array 14 of printing elements each of which can be individually addressed and energised to produce heat. The heat is used to soften and remove pixels of ink from a carrier ribbon 15 which is fed between the print head 11 and a substrate 18 to be printed upon.

The array 14 is in this example linear, and during a printing operation, relative movement is effected.between the print head 11 and the substrate 18 in a direction transverse to the linear extent of the array 14, and the printing elements are selected and selected printing elements energised at each of a plurality of printing positions along the substrate 18, thereby to build up an image of ink pixels.

In this example, the print head 11 is held stationary at the printing station 12 and during printing, relative movement is effected between the substrate 18 and the print head 11 by moving the substrate 18 relative to the print head 11.

In this case, usually the carrier ribbon 15 is moved with the substrate 18 past the print head 11.

In another example, the substrate 11 may be held stationary at the printing station 12 while the print head 11 traverses the substrate 18, and the carrier ribbon 15 typically is held stationary with the substrate 18. In another example, the print head 11 and the substrate 18 and the carrier ribbon 15 may move at the printing station 12 relative to a base structure which mounts the components.

The carrier ribbon 15 is moved either during printing and/or before of after printing so that fresh carrier ribbon 15 is located at the print station 12 before or during printing. The carrier ribbon 15 may be stored on a storage spool C and taken up onto a take-up spool D, passing around a carrier ribbon feed path between the spools C, D. In this example, each of the storage and take-up spools C, D are simultaneously driven to effect carrier ribbon 15 movement. Thus as indicated in the figure, the storage spool C has an associated drive motor Ml and the take-up spool D has an associated drive motor M2.

In another example though, the ribbon 15 may be driven by a single motor driving the take-up spool D, and/or by a driven capstan roller, and/or by a shuttle or otherwise.

The printing apparatus 10 is controlled by a controller 20. The controller 20 selects and effects operation of the printing elements of the array 14 in response to command signals by issuing instructions to the printer head 11.

Such a controller 20 typically would therefore co-ordinate the selecting and operating of printing elements of the array 14 at each printing position along the substrate 18, and control the motors Ml and M2 and the print head 11 so that printing is properly co-ordinated and effected.

Information relating to the nature of an image to be printed typically is provided to the controller 20 as a control signal Al from an input device or a command bus to which the controller 20 may be connected with other kinds of apparatus, for controlling the printing apparatus 10 in synchronism with other apparatus e.g. in a factory environment.

For completeness, the printing apparatus 10 further includes guide rollers 22, 23 for guiding the carrier ribbon 15 around the carrier ribbon path, and a reaction roller 25 which is provided on the opposite side of the substrate 18 to the print head 11, and against which the print head may effect pressure to facilitate the transfer of ink pixels to the substrate during printing. Also, practically, the print head 11 would be able to move towards and away from the substrate 18, for example only, by pivoting about an axis B as shown by arrow E, so that when printing is not being effected, substrate 18 and/or carrier ribbon 18 movement between the print head 11 and roller 25 can be more easily achieved.

Finally, it can be seen in the figure that the substrate 18 passes between a pair of rollers 26, 27 at least one of which may include an encoder so that the speed of the substrate 18 through the printing station 12 can be determined.

The encoder provides an input A2 to the controller 20 which uses the input to co-ordinate operation of. the printing apparatus 10 with substrate 18 movement.

Referring now to figure 2 there is shown how an image may be printed by the method according to the invention. It can be seen that the image is of the letter "A", and is made up in the very diagrammatic representation, of relatively few pixels of ink, derived from an array 14 having only eight printing elements P1 to P8. Practically, the image "A" would be made up of a much greater number of pixels, where the density of the printing elements 30 may be at least six, preferably at least eight, and possibly many more printing elements per linear millimetre of the array 14.

Along the substrate 18, practically there are a large number of printing positions, although only fifteen printing positions are shown at RI to R15 to facilitate explanation of the invention.

At each printing position Rl to R15 along the substrate 18, the controller 20 selects a set of the printing elements P1 to P8 to energise to print the row of pixels, depending upon the command signal Al to the controller 30.

In the example, in the first printing position Ri, only printing element P8 is selected and energised, and a pixel of ink is printed at a column grid position (the grid being representative of each row and column pixel position of the image) indicated at C8. At the second printing position R2, only printing element P7 is selected and a pixel is printed at C7, and so on for the whole image.

However, in the example, it has previously been determined as described below, that printing element P3 of the array 14 is malfunctioning. For example, the printing element P3 may not be working at all, or may be under or over heating to some defect. In any event, as described below, the printing element P3 is disabled and is not used for printing.

Thus if operated conventionally, the "A" image would have missing pixels at the grid row positions R6 and Rl0 where otherwise printing element P3 would be selected for energisation. Whereas in a practical embodiment where there is a sufficiently high density of printing elements, missing ink pixels due to a single or a few malfunctioning printing elements P1 to P8 may be tolerable, as these may not affect the readability of the image, it is desirable to mask such omissions to improve image quality.

In accordance with the present invention, at each of the row positions R6 and RiO where the malfunctioning printing element P3 would be selectable according to the command signal A to the controller 30 in order to print a pixel of the "A" image, an adjacent printing element is selected and energised by the controller 30. In the example, at row printing position R6, the adjacent printing element P4 to one side of the malfunctioning printing element P3 is selected and energised, whilst in row RiO, the adjacent printing element P2 to the other side of the malfunctioning printing element P3, is selected and energised.

It can be seen that whilst the resulting the "A" image is a little deformed compared to a perfect and regular image achievable if all the printing elements were operable, by printing using a printing element P2 or P4 adjacent to the malfunctioning printing element P3, the otherwise missing pixels are masked and readability ofihe "A" image is improved. Of course, where the printing element density is at least six per mm, the irregularity of the "A" image due to the occasional offset ink pixel, is largely unnoticeable.

When a print head 11 is manufactured, it is common practice to test the performance of the print head ii. This can be achieved conveniently ion a factory setting by selecting and energising each printing element P1 to P8 in turn. A known technique determines the electrical resistance of each printing element P1 to P8 and identifies any printing element where the determined resistance is outside of an allowable range. Usually such a technique is practised to establish the number of malfunctioning printing elements P1 to P8 in the array 14, and if this exceeds an acceptable number, the print head 11 is discarded.

In accordance with the present invention, provided that so many printing elements P1 to P8 are not found to be malfunctioning that it is determined that the print head II is unusable, the print head 11 may still be used, by selecting and energising printing elements P1 to P8 adjacent to any malfunctioning printing elements P1 to P8, and indeed a greater number of malfunctioning printing elements P1 to P8 may be tolerated than compared with a print head operated according to the prior art method, in which there is no compensation for malfunctioning print heads P1 to P8.

In a preferred method, a determination of which, if any, of the printing elements P1 to P8 is malfunctioning is made in a factory environment, in order to create data relating to the performance of the printing elements P1 to P8, which data is then stored in a memory 35 of the print head 11.

Thus when the print head 11 is installed at a printing station 12 of a printing apparatus 10, the stored data can be used to enable a printing method in accordance with the invention to be performed.

Such data in the memory 35 may upon installation of the print head 11, be uploaded or read by the controller 20 and used by the controller 20 when instructing the print head 11 to perform the method of the invention. Thus the controller 20 would need to be configured to read and use the data from memory 35 when sending instructions to the print head 11 at each printing position RI to R15 to energise selected printing elements P1 to P8.

Alternatively, the print head 11 may include a processor to process an instruction from the controller 20 to select and operate a malfunctioning printing element P3 at a printing position RI to R15, by selecting the adjacent printing element P2 or P4 instead.

Where the data relating to the identification of one or more malfunctioning printing elements P1 to P8 is stored in a memory 35 of the print head 11, no testing or calibration of the print head 11 in situ i.e. at the printing station 12 may be needed. However, this would mean that it would not be possible to identify any printing element 11 which fails in use, so that an adjacent printing element may be used instead of the malfunctioning printing element.

Accordingly if desired, the method of the invention may include carrying out testing on the print head 11 to identify any malfunctioning printing element P1 to P8 in situ, for example, upon installation of the print head 11 at the printing station 12, and/or periodically and/or as scheduled e.g. by user. In this case, the provision of a memory 35 in or on the print head 11 for the test data may not be required, but the controller 20 would need to be configured/programmed to perform the print head 11 testing, and then to use the data created when controlling the print head 11, to select and energise printing elements adjacent to any otherwise selectable malfunctioning printing element P3.

Various other modifications are possible without departing from the scope of the invention.

For example, whereas in the embodiment described, the immediately adjacent printing element P2 or P4 to the malfunctioning printing element P3 identified is selected instead of the malfunctioning printing element P3, in a practical embodiment there are a large density of printing elements along the linear array 14, a close adjacent, rather than immediately adjacent printing element may be selected for printing rather than the malfunctioning printing element P3. For example, where the immediately adjacent printing element is selected at a printing position in the course of the ordinary printing element selection process in response to command signals, another not necessarily adjacent printing element may be selected and operated.

In the example described, in printing position R6, a different adjacent printing element P4 is selected to the adjacent printing element P2 used in printing position RIO. The selection of which adjacent printing element to operate instead of a malfunctioning printing element may be made depending upon the nature of the image to be printed for example. In another example though, the same adjacent printing element may be selected and used for printing wherever the malfunctioning printing element would otherwise be selected and energised.

The printing apparatus 10 shown in figure 1 is purely exemplary and many other configurations are possible. For example, the printing apparatus 10 need not include an inked carrier ribbon 15, but such carrier may be provided as fed sheets. No carrier at all may be provided where for example the substrate 18 is of the heat sensitive kind, in which pixels of marking medium becomes visible when a material of the substrate 18 is subjected to heat from selected and energised printing elements P1 to P8.

In the embodiment specifically described, the invention is applied to a printing apparatus 10 of the thermal kind, but it will be appreciated that the invention may be performed using any pixel based printing apparatus 10. Thus the printing elements P1 to P8 need not be thermal printing elements, but could for example be ink jets. In each case though, when a malfunctioning printing element is identified, the effect of otherwise missing pixels of ink on a printed image may be masked by performing the method of the invention.

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 which has a print

head including a plurality of printing elements arranged in an array, the printing elements being individually and selectably operable to print pixels on a substrate, the method including performing a printing operation by providing relative movement between the print head and the substrate and in each of a plurality of positions along the substrate selecting and operating printing elements to print the image, the method including prior to performing a printing operation, testing the performance of each of the printing elements to identify any malfunctioning printing element, and in a subsequent printing operation, at each printing position where such identified malfunctioning printing element would otherwise be selected for operation, selecting and operating an adjacent printing element.

2. A method according to claim I wherein the adjacent printing element which is selected and operated at each printing position along the substrate, is a printing element which is immediately adjacent the identified malfunctioning printing element of the array.

3. A method according to claim I or claim 2 wherein the same adjacent printing element is selected for operation at each printing position along the substrate where the identified malfunctioning printing element would otherwise be selectable and operated.

4. A method according to claim I or claim 2 wherein different adjacent printing elements are selectable and operated at some or each printing position where otherwise the identified malfunctioning printing element would be selected and operated.

5. A method according to claim 4 wherein at one printing position or at each of a plurality of printing positions along the substrate where the malfunctioning printing element would otherwise be selected and operated, the adjacent printing element selected and operated is at one side of the malfunctioning printing element, whereas at another printing position or at each of another plurality of printing positions along the substrate where the malfunctioning printing element would otherwise be selected and operated, the adjacent printing element selected and operated is at the other side of the malfunctioning printing element.

6. A method according to any one of the preceding claims wherein the array of printing elements is a linear array.

7. A method according to any one of the preceding claims wherein upon testing the performance of the individual print heads, data is created and stored for use in controlling and operating the print head.

8. A method according to any one of the preceding claims which includes carrying out the testing method step with the print,ead at a printing station of the printing apparatus where in use the print head will perform printing operations.

9. A method according to claim 8 wherein the testing method step is carried out automatically upon installing the print head at the printing station, and/or is carried out manually or at scheduled frequencies.

10. A method according to any one of claims 7 to 9 where dependant upon claim 7 wherein the data created upon testing the performances of the printing elements is provided for use by a controller of the printing apparatus which selects and effects operation of the printing elements in response to command signals.

11. A method according to claim 10 wherein the controller co-ordinates selecting and operating printing elements with the relative movement between the print head and the substrate.

12. A method according to claim 10 or claim 11 wherein where the data identifies a malfunctioning printing element, the controller of the printing apparatus selects the adjacent printing element to operate at each printing position where the command signal indicates that a malfunctioning printing element should be selected and operated.

13. A method according to any one of claims 7 to 12 where dependent upon claim 7 wherein the data created upon testing the performances of the printing elements is stored in a memory of the print head.

14. A method according to claim 13 wherein the data is provided to a controller of the printing apparatus which selects and effects operating of the printing elements in response to command signals.

15. A method according to claim 13 wherein the print head processes an instruction from the controller to select and operate a printing element at a printing position, which has been identified as a malfunctioning printing element, by selecting the adjacent printing element instead.

16. A method according to any one of the preceding claims wherein the print head is of the thermal kind in which the printing elements when operated, are heated.

17. A method according to any one of the preceding claims wherein there is an array of printing elements containing at least six printing elements per mm in a direction transverse to the direction of relative movement between the print head and the substrate.

18. A method according to claim 17 wherein there are more than eight printing elements per mm in the array.

19. A method of printing substantially as hereinbefore described with reference to the accompanying drawings.

20. Any novel feature or novel combination of features described herein and/or as shown in the accompanying drawings.