Printers

Abstract: To avoid the effects of unwanted variations in the pen-to-paper spacing of a printer, deliberate and effectively randomised variations are introduced by using paper drive rollers (20) with an irregular cross-section. In one embodiment, an overdrive roller comprises rollers (23-26) the surfaces of which incorporate flattened portions (27, 28) of differing widths and spacings. The number, size and/or spacing of the portions (27, 28) differ from roller to roller. Instead or in addition, variations may be incorporated into a pinch wheel (101), platen (15) or other component of the paper drive system.

Description: The present invention relates to printers, and especially to inkjet printers. A problem with such printers is that, when an absorbent print medium such as paper is being used, the medium has a tendency to move out of its plane. This movement up from the printing zone is caused by local expansion of the medium as it absorbs the printing ink and creates ruffles or cockle in the medium. Such movements cause variation of the pen-to-paper spacing (PPS) and have an adverse impact on image quality. Another problem with such printers, which also affects PPS, is the creation of so-called "worms" due to the interaction of the print medium with paper-motion mechanisms. Print medium drive members, such as pinch wheels and overdrive wheels, act on the print medium at specific locations spaced along the scan axis of the printer. Thus they determine different boundary conditions at different locations across the print medium and this effect causes the medium to have a tendency to create ruffles at preferred scan-axis locations. The boundary conditions do not change with time and the resulting variations in PPS affect dot placement so that, in graphics applications, clearer areas or tracks (known as "worms") are observed extending along the paper axis. Such an impact on image quality is not acceptable. DE-A-4212900 discloses a printer with a roller which has a slightly greater diameter towards one end. EP-A-0395887 discloses a roller with two portions of maximum diameter spaced from its ends which tapers towards its ends and towards its centre. Thus these rollers exert a constant force at each particular location along the scan axis. US-A-5813782 discloses an arrangement in which paper passes between four circular rolls and four respective "flat-spot" rolls with flattened portions which have a fixed mutual stagger of 90°. In this arrangement a similar pattern (though shifted in time) of mechanical forces is applied to the paper at the location of each "flat-spot" roll. The present invention seeks to further break the symmetry of the mechanical forces applied to the print medium and thus to further reduce the possibility of "worms". According to a first aspect of the present invention, there is provided a printer comprising a printhead and a print medium drive system for advancing a print medium past the printhead, the printhead being arranged to print on the medium along a scan axis in a plurality of successive print medium positions, in each of which the print medium is stationary, at least one component of the drive system being configured to engage the print medium at a plurality of locations along the scan axis, and to exert different mechanical forces on the print medium in successive ones of said print medium positions, characterised in that the configurations of the component are different at different ones of the locations along the scan axis whereby to substantially reduce or eliminate printing artefacts. The component may comprise one or more rollers or wheels with a non-circular periphery. The component is preferably an overdrive roller arrangement. Alternatively the component may comprise a platen extending along the print axis and incorporating a plurality of suction holes, the platen incorporating a mechanism that selectively closes, at least partially, some of the holes. Preferred embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Fig. 1 represents a conventional overdrive roller arrangement for a printer; Fig. 2 shows the printing region of an actual printer; Fig. 3 is an enlarged sectional view of part of Fig. 2 showing the printhead and the various rollers; Figs. 4a and b are diagrams respectively illustrating the PPS along the line of the overdrive rollers and along a line spaced 2cm from the line of these rollers; Fig. 5 shows an overdrive arrangement for use in a printer according to a first embodiment of the present invention; Fig. 6 shows the cross-section on an enlarged scale of a roller of the arrangement of Fig. 5; Figs. 7a and b are diagrams corresponding to Figs. 4a and b, but relating to the overdrive arrangement of Fig. 5; Fig. 8a is a schematic perspective view of a vacuum platen arrangement for use in a printer according to a second embodiment of the present invention; and Fig. 8b is a partial top view of the platen of Fig. 8a. Referring to the drawings, Fig. 1 represents a conventional overdrive roller arrangement 10 (not to scale) comprising a plurality of spaced cylindrical rollers 11 attached to a rotatable shaft 12. A practical arrangement 10 is shown in the printer 100 of Figs. 2 and 3 which also comprises printhead 50, pinch wheel 101, a central platen 15 with a suction aperture 17, and a drive roller 102 all defining the path for print medium 104. Typical dimensions of the overdrive roller arrangement 10 are: diameter of shaft 3mm length of roller 15mm diameter of roller 18.1mm centre-to-centre spacing of rollers 76mm. As the rollers 11 advance the print medium (not shown), they exert thereon mechanical forces, the pattern of which does not change from one print medium position to the next. Accordingly unwanted variations in PPS are produced and do not change over time. Figs. 2 and 3 show the relationship of the overdrive rollers 11 to central platen 15, the rollers protruding from below the substantially flat platen. Fig. 4a illustrates the PPS measured at the location of the rollers 11 themselves, with black portions representing essentially zero spacing and white portions representing approximately 1mm spacing. Since the overdrive rollers are under the paper, the paper is sucked against the rollers, i.e. towards platen. Fig. 4b illustrates the PPS measured along a line located in the region of suction aperture 17, located 2cm away from the shaft 12. The position of the line is indicated by "x" in Fig. 3. In Figs. 4a and b arrow 18 represents the scan (or printing) axis and arrow 19 represents the paper advance axis. The unwanted PPS crests can be seen (indicated in white) corresponding to the locations of the overdrive rollers. Stronger crests are produced near the cut edge of the paper. As soon as the paper advances enough as to curl down, crests diminish, though they remain troublesome. There are, therefore, a transient situation and a steady state situation. The present invention is based on the realisation that it is not necessary to eliminate variations in PPS to reduce the impact on image quality. Rather, by breaking the symmetry which was previously maintained along the paper axis, it is possible to effectively randomise the variations in PPS and thus to substantially reduce or eliminate "worms" and other such printing artefacts. Referring now to Figs. 5 and 6, there is shown an overdrive roller arrangement 20 in accordance with the present invention comprising a plurality of spaced rollers 23, 24, 25, 26 attached to a rotatable shaft 22. Each roller has a surface 30 of generally circular cross-section of radius 9.05mm but has one or more flat surfaces 27, 28 extending along its entire length and spaced around its circumference or periphery. The maximum cut depth "d" of the flat surfaces is typically 1.5 to 3.5mm and the angle therebetween may be selected as desired. The combination of the number, width and spacing of the flat surfaces are selected differently for each roller 23-26 to introduce a further randomising factor into the PPS across the print medium. As the print medium advances, a different profile of mechanical forces is exerted thereon from one print medium position to the next position after an advance movement. Thus the PPS is different for every position of the print medium. With the conventional arrangement of Fig. 1 there is always constructive interference which produces defects in the image quality. With the arrangement of Figs. 5 and 6 there is random interference, occasionally constructive but predominantly destructive, and this produces a different visual effect, which is more pleasing to the eye. This can be seen in Figs. 7a and 7b which correspond to Figs. 4a and 4b. The effect of the interruptions caused by flat surfaces 27 and 28 in the circumferences of the rollers 11 can be seen in the differing patterns shown in Fig. 7a. As can be seen indicated in white in Fig. 7b, the PPS crests still exist, but they are interrupted and much less evident than in Fig. 4b. Another advantage of the arrangement of Figs. 5 and 6 is that it is possible to compensate for errors or defects, the size of which is comparable with the dimensions of the entire printing zone. With other methods, e.g. the use of printing masks, there can be corrected only errors, the size of which is comparable to the dimensions of the printhead. Various modifications may be made to the above-described arrangement. For example, the rollers 23 to 26 may have one or three or a greater number of flat surfaces. Some of the flat surfaces 27, 28 may directly adjoin each other so that part of the roller has a polygonal cross-section. One or more of the flat surfaces 27, 28 may be replaced by a curved surface having a radius of curvature greater than that of the rest of the respective roller. Any convenient number of rollers may be provided on shaft 22. Although the arrangement of Fig. 5 has different rollers 23-26, some of them may be identical since this will still have the effect of reducing "worms" extending along the paper axis. It is preferable that the circumference or peripheral dimension of the rollers does not equal the amount of a print medium advance and is not a multiple or factor thereof. This avoids the introduction of unwanted artefacts and provides a further randomising effect. One or more of the rollers may have a conventional circular cross-section, but such an arrangement is not as effective. Although described in connection with an overdrive roller arrangement, the changes in configuration may be introduced, instead or in addition, into other paper-drive components such as pinch-wheels 101. The component may be an elongate roller which has variations in cross-section along its length in addition to around its circumference. If desired, an additional set of wheels or rollers may be incorporated in a printer, specifically to introduce the controlled variations in PPS and thus dot placement. An advantage of introducing variations into two or more components of the print drive system is that a further randomising factor is introduced into the PPS. The radii of rotating components are preferably different and one is preferably not a multiple of the other. This avoids the introduction of unwanted artefacts and provides yet a further randomising effect. In a further arrangement, the paper path can include a centre platen with a plurality of suction/vacuum holes arranged to retain the paper against the platen and having a mechanism that moves to close different ones of said holes during successive printing passes. Thus, in a second embodiment of the invention, Figs, 8a and b, a member 30 of sheet material is provided beneath the central platen 15. Member 30 has a bearing element 35 at one end and is connected at the other end to the printer chassis by compression springs 31, 32 which use the bearing element 35 in the direction of arrow "y" against a cam wheel 36. Bearing element 35 engages a cam surface (not shown) of wheel 36. Wheel 36 is geared to the paper advance mechanism of the printer. Member 30 has a plurality of irregularly sized and spaced holes and slots 41, 42 arranged in a line which extends beneath the suction apertures 17 in the platen 15, so that suction is applied to the lower surface of the paper via both sets of openings 17 and 41, 42. In use, wheel 36 undergoes a certain rotation each time the paper (or other print medium) is advanced so that the cam surface causes member 30 to move to another position along the print axis. Holes and slots 41, 42 thus adopt another relative disposition to apertures 17 and thus, as will be appreciated from Fig. 8b, a different pattern of suction is exerted on the paper. This causes the vacuum pressure to vary over time at different locations. To increase the randomising effect, it is arranged that a single paper advance movement does not produce an integral number of rotations of the wheel 36, nor does a single rotation of wheel 36 correspond to an integral number of paper advance movements. In a modification the paper path includes a platen, the surface of which is flexible and has a profile which is altered over time by an underlying control mechanism.

Claims: A printer comprising a printhead and a print medium drive system for advancing a print medium past the printhead, the printhead being arranged to print on the medium along a scan axis in a plurality of successive print medium positions, in each of which the print medium is stationary, at least one component (20) of the drive system being configured to engage the print medium at a plurality of locations along the scan axis, and to exert different mechanical forces on the print medium in successive ones of said print medium positions, characterised in that the configurations of the component are different at different ones of the locations along the scan axis whereby to substantially reduce or eliminate printing artefacts. A printer according to claim 1, wherein said component (20) comprises one or more rollers (23-26) or wheels (101) with a non-circular periphery. A printer according to claim 2, wherein the or each roller or wheel has a generally circular periphery (30) interrupted by one or more non-circular portions (27, 28). A printer according to claim 3, wherein the non-circular surface portions (27, 28) are substantially flat. A printer according to claim 3 or 4, wherein the widths and/or spacings of the non-circular portions differ on the or each roller or wheel. A printer according to any of claims 3 to 5, wherein the component (20) comprises a plurality of rollers or wheels the number and/or width and/or mutual spacing of the non-circular portions (27, 28) differ from roller to roller, or wheel to wheel. A printer according to any preceding claim, wherein the component (20) is an overdrive roller arrangement. A printer according to any of claims 1 to 6, wherein the component (101) is a pinch wheel arrangement. A printer according to any of claims 2 to 8, wherein the print medium undergoes substantially equal advance movements between the successive print medium positions, and wherein the peripheral dimension of the roller(s) or wheel(s) does not equal the amount of said advance movement and is not a multiple or a factor thereof. A printer according to claim 1, wherein the component (15) is a platen of the print medium drive system, the platen incorporating a plurality of suction holes and means for varying the suction pressure applied through said holes. A printer according to claim 1, wherein the component (15) is a platen of the print medium drive system, the platen having a flexible surface with a variable profile. A printer according to any preceding claim, wherein two or more components (20, 101, 15) of the drive system are configured to exert different mechanical forces on the print medium in successive print medium positions. A printer according to claim 12 wherein said components (20, 10) include two rotating components the radii of which are different and not a multiple one of the other.

Inventors: GIL ANTONI, GIL, ANTONI
Applicants: HEWLETT PACKARD CO, HEWLETT-PACKARD COMPANY
Application year: 2000
Main classification: B41J13
IPC codes: B41J13, B41J11