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:
FIELD OF THE INVENTION 
     The present invention relates to printers, and especially to inkjet printers. 
     BACKGROUND OF THE INVENTION 
     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. 
     The present invention seeks to overcome or reduce one or more of the above problems. 
     SUMMARY OF THE INVENTION 
     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 in a plurality of successive print medium positions, characterised in that at least one component of the drive system is configured to exert different mechanical forces on the print medium in successive print medium positions. 
     The component may comprise one or more rollers or wheels with a non-circular periphery. 
     The component is preferably an overdrive roller arrangement. 
     According to a second aspect of the present invention, there is provided a print medium roller arrangement characterised in that it comprises one or more rollers having a drive surface with a non-circular periphery. 
     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. 
     According to a third aspect of the present invention, there is provided a printer platen comprising a plurality of suction holes and means for controlling the suction pressure applied by said holes, characterised in that said suction control means are arranged to apply different pressures to different ones of said holes at successive times. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     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. 4 a  and  b  are diagrams respectively illustrating the PPS along the line of the overdrive rollers and along a line spaced 2 cm 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. 5 a  shows an overdrive arrangement for use in a printer according to another 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. 7 a  and  b  are diagrams corresponding to FIGS. 4 a  and  b , but relating to the overdrive arrangement of FIG. 5; 
     FIG. 8 a  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. 8 b  is a partial top view of the platen of FIG. 8 a.   
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     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 
                 3 
                 mm 
               
               
                   
                 length of roller 
                 15 
                 mm 
               
               
                   
                 diameter of roller 
                 18.1 
                 mm 
               
               
                   
                 centre-to-centre spacing of rollers 
                 76 
                 mm. 
               
               
                   
                   
               
             
          
         
       
     
     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. 4 a  illustrates the PPS measured at the location of the rollers  11  themselves, with black portions representing essentially zero spacing and white portions representing approximately 1 mm spacing. Since the overdrive rollers are under the paper, the paper is sucked against the rollers, i.e. towards platen. FIG. 4 b  illustrates the PPS measured along a line located in the region of suction aperture  17 , located 2 cm away from the shaft  12 . The position of the line is indicated by “x” in FIG.  3 . In FIGS. 4 a  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 9.05 mm 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 3.5 mm 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. 7 a  and  7   b  which correspond to FIGS. 4 a  and  4   b . 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. 7 a.    
     As can be seen indicated in white in FIG. 7 b , the PPS crests still exist, but they are interrupted and much less evident than in FIG. 4 b.    
     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. As shown in Figure 5 a , another overdrive arrangement  20   a  may include example rollers  23   a  and  24   a , where 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. 8 a  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. 8 b , 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.