Abstract:
A scan axis assembly for a printer comprises first and second tracks, the tracks being rigidly located relative to one another by one or more track support members, and each track being arranged to support a print carriage such that the print carriage may move along the track to traverse a print zone.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS 
     GB Priority Application 0209699.8, filed Apr. 27, 2002 including the specification, drawings, claims and abstract, is incorporated herein by reference in its entirety. 
     FIELD OF THE INVENTION 
     This invention relates to printers having printing elements which are scanned along a scan axis relative to a print medium. The invention has particular application to printers such as inkjet printers in which a print carriage carrying one or more printheads is scanned along a scan axis to print an image in swaths between advances of the print medium relative to the scan axis. 
     BACKGROUND OF THE INVENTION 
     Increased printing speeds are continually sought by users of printers, and there is therefore an ongoing desire by printer manufacturers to provide increased printing speeds. This is addressed in many ways, including improvements in software and firmware, improvements to the printing elements themselves (such as the speeds attainable by inkjet printheads) and improvements in the speed at which the moving parts of the printer can reliably move while maintaining the necessary accuracy of the printheads relative to the print medium. 
     The Hewlett Packard DesignJet 5000 is an example of a typical high quality inkjet printer. Inside the print body a scan axis comprising a track is mounted, and a print carriage scans along the track between incremental paper (or other print medium) advances. The print carriage has three bushings mounted on rails of the track, and a drive mechanism causes the carriage to be traversed back and forth while firmware within the printer converts received image files to swaths of halftone pixels in six colours. The firmware converts these halftone swaths to firing instructions for individual nozzles in a set of six printheads each provided on a print cartridge held on the carriage. 
     In high quality print modes, the swath is laid down in overlapping multiple passes. One of the reasons for employing higher numbers of passes is that in an eight pass print mode, for example, each pass lays down approximately ⅛th of the ink in a given area and thus each droplet laid down has an opportunity to dry before the next adjoining or overlapping droplet reaches the print medium. There is however a trade-off in that the printing speed of an eight-pass print mode is approximately eight times less than a single pass mode to achieve better print quality. 
     A further problem which arises in printing high quality textiles for example, is that a conventional CcMmYK set of six inks (comprising cyan, light cyan, magenta, light magenta, yellow and black) may not give sufficient colour fidelity. While one solution is to add additional ink colour cartridges to the print carriage, this results in a carriage which is larger than before. The scan axis or chassis along which the carriage travels will not be truly straight, and as a carriage travels along a non-linear scan axis, the increased length of the carriage will lead to greater errors in positioning droplets from different pens on the carriage. 
     It would therefore be desirable to provide a scan axis assembly which enables increased printing speeds to be attained. It also aims to provide improved printers and methods of manufacture of printers and their constituent parts. 
     SUMMARY OF THE INVENTION 
     The invention provides a scan axis assembly for a printer comprising first and second tracks, the tracks being rigidly located relative to one another by one or more track support members, and each track comprising a rail support member and at least one rail mounted on the rail support member to support a print carriage such that the print carriage may move along the rail to traverse a print zone. 
     The invention enables the track support members to fix the positions of the tracks relative to one another in a general sense, while a carriage-supporting rail on one track can be very accurately positioned relative to a carriage-supporting rail on the other track. This is of particular importance where the two print carriages are for printing on the same medium, so that the printing actions performed by the two carriages are very accurately aligned with respect to one another. 
     Preferably, the one or more track support members comprise a pair of frame members spaced apart from one another along a scan axis direction, said first and second tracks spanning said frame members and lying along said scan axis direction parallel to one another, whereby said frame members maintain the spacing between said parallel tracks when assembled in a printer. 
     By providing two parallel axes the printing throughput can surprisingly be more than doubled. The twin tracks provide a pair of scan axes for two print carriages and the carriages thus have parallel and offset print areas. The printer therefore prints two swaths simultaneously and in multiple pass print modes, each print carriage will print half of the total number of passes specified by the print mask. 
     Thus if each print carriage prints its swath (containing half of the droplets required in the full image) in two passes (so each pass lays down one quarter of the droplets), the printer will operate at the same speed as a conventional printer in two-pass mode (the processing power is increased to allow the image to be processed for the two scan axes), but will in fact utilise what is effectively a four pass print mask. However the print quality will be better than conventional four pass print mode output. 
     The reason for this unexpected increase in quality is that because the two print zones are spaced apart from one another on the print medium the ink will have an opportunity to dry between the print zones. Thus, the final swath will be of equivalent quality to a hypothetical four pass print mode image in which the print carriage and paper advance mechanism is paused between the printing of the first two passes and the second two passes (this hypothetical pause being equivalent to the time taken for a point on the page to advance from the first print zone to the second print zone). 
     The invention is not of course limited to two parallel scan axes; three, four or even more parallel tracks, each for supporting a print carriage can be employed by extension. 
     Preferably, two or more rails are mounted on the rail support member of each track. 
     Further, preferably, each frame member comprises a locating point for accurately locating one rail of one track to that frame member. 
     As will be explained further below, the accurate location of one particular rail with respect to each frame member allows that rail to be used as a datum point for the location of the other elements of the assembly. 
     Thus, in a preferred embodiment each frame member comprises means for adjustably affixing the tracks to the frame member, whereby when said one rail of one track is located at the locating point the distance between the tracks can be adjusted before the other track is affixed to the frame member. 
     Preferably, the first track is accurately located on each frame member, while the second track is affixed to both frame members when it has been positioned a predetermined distance from the first track. In this way the tracks are kept accurately parallel and are held parallel by the frame members. 
     Suitably the locating point may comprise a “V”-shaped recess which receives said rail. The rail can be typically a round bar which is uniquely located when it sits in the internal angle of the V. 
     Preferably, each frame member comprises a support structure for supporting each of the rails not located at said locating point. Thus, in a two track assembly, with two rails per track, each frame member will preferably have one locating point (for one rail of one track) and three support structures for supporting but not for accurately locating the other three rails. 
     Preferably, each frame member comprises a plate having a central aperture for receiving said tracks, said locating point and said support structures extending into said aperture for receiving the rails, and means for affixing the rail support members of the tracks to the opposed internal faces of the plates when the rails are located on the locating point and the support structures. 
     Further, preferably, said plate is generally “C”-shaped such that the aperture is open to the external periphery of the plate. 
     In the preferred embodiment of assembly, one end of each track extends beyond one of the frame members by a distance at least equal to the width of the carriage, whereby the carriage may be traversed along the track past the frame member to allow access to or maintenance of the carriage. 
     Preferably, the first and second tracks extend beyond the frame members as aforesaid at opposite ends. 
     In this way there are two maintenance stations, one adjacent to each frame member for accessing one of the two print carriages. Such maintenance stations may be used for e.g. removing the carriage from the track, cleaning the printheads, or replacing or adjusting the print cartridges on the print carriage. 
     Preferably, the first and second tracks are provided with stops against which the frame members are mounted. Where the tracks comprise an extrusion, part of the extrusion can be stripped away to leave a stop surface against which the frame members abut and to which the frame members can be affixed. 
     Preferably, the rail support members of the tracks are manufactured to a first tolerance below that required for accurate positioning of a print carriage in use, but comprise a rail mounting feature manufactured to a second tolerance at least equal to that required for accurate positioning of a print carriage in use, the rails being affixed to the rail mounting feature and the rails being adapted to receive the print carriage. 
     This is advantageous in view of the fact that the rail support members of the tracks, being extended generally linear structures, are most conveniently manufactured by extrusion processes. However, the tolerances achievable by conventional extrusion for e.g. an aluminium extrusion, are of the order of 1 mm per meter. If the print area has a width of e.g. 1.6 m the tolerances may give rise to lateral variations of 1.6 mm along the length of the scan axis. 
     However, by making the rail support member according to this first tolerance (e.g. by extrusion) and then accurately machining a rail mounting feature into the extrusion, the tolerances of the rail mounting feature can be improved tenfold using conventional machining techniques at a reasonable cost. 
     Preferably, the rail mounting feature comprises a pair of parallel accurately machined grooves on each rail support member, each groove being adapted to receive a respective rail of the track. 
     In another aspect the invention provides a printer comprising a scan axis assembly according to the invention. 
     Preferably, the printer will also comprise a print medium advance mechanism for advancing a print medium past the scan axis assembly, and a print carriage traverse mechanism for causing a pair of print carriages mounted on the tracks of the scan axis assembly to traverse the tracks and thereby enable the printing of swaths on the print medium. 
     In a further aspect, the invention provides a method of manufacturing a scan axis assembly for a printer, comprising the steps of:
         a) affixing a first linear track adapted to receive a print carriage to a first frame member;   b) affixing a second linear track adapted to receive a print carriage to said first frame member such that the points of attachment of the tracks to the first frame member are separated by a predetermined distance and the tracks are positioned generally parallel to one another;   c) affixing one of the first and second tracks to a second frame member such that the first and second frame members are spaced apart from one another along the direction of the tracks; and   d) affixing the other of the first and second tracks to the second frame member such that the points of attachment of the tracks to the second frame member are separated by said predetermined distance and the tracks are accurately positioned parallel to one another to define parallel scan axes for a pair of print carriages each mounted on a respective one of said tracks.       

     Preferably, step (a) comprises locating one rail of the first track at a locating point provided on the first frame member prior to affixing the first track to the first frame member. 
     Preferably, step (b) comprises employing a spacing template to separate the second track from the first track by said predetermined distance before affixing the second track to the first frame member. 
     Preferably, step (c) comprises locating one rail of said one of the tracks at a locating point provided on the second frame member prior to affixing said one of said tracks to the second frame member. 
     Preferably, step (d) comprises employing a spacing template to separate said other track from said one track by said predetermined distance before affixing said other track to the second frame member. 
     A relatively simple hardened steel template can be used at both ends to ensure that the tracks are equally spaced when affixed to the frame members, and in this way that the tracks are accurately parallel. 
     Preferably, the “one track” affixed in step (c) is the first track and the “other track” affixed in step (d) is the second track, so that each track is accurately located to the frame members. 
     Suitably, step (b) may involve resting the rails of the second track on a respective rail support structure provided on the first frame member, and step (d) may involve resting the rails of the other of said tracks on a respective rail support structure provided on the second frame member. 
     The invention further provides a method of manufacturing a printer comprising manufacturing a scan axis assembly in accordance with the invention, and mounting the assembly on a printer body. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be further illustrated by the following description of embodiments thereof, given by way of example only with reference to the accompanying drawings, in which: 
         FIG. 1  is a perspective view of an embodiment of a scan axis assembly according to the invention; 
         FIG. 2  is a sectional elevation of a rail support member of a track forming part of the assembly of  FIG. 1 , taken along the line II—II; 
         FIG. 3  is a sectional elevation of a rail support member of the track of  FIG. 2 , taken along the line III—III; 
         FIG. 4  is a sectional elevation of a rail support member of the track of  FIG. 2 , taken along the line IV—IV; 
         FIG. 5  is an elevation of a frame member forming part of the assembly of  FIG. 1 , shown before assembly; 
         FIG. 6  is an elevation of the frame member of  FIG. 5 , shown during assembly; 
         FIG. 7  is an elevation of the frame member of  FIG. 6 , shown after assembly. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In  FIG. 1  there is indicated, generally at  10 , an embodiment of a scan axis assembly according to the invention. The assembly comprises track support members  12  and  14 , in the form of a first frame member  12  and a second frame member  14 , spaced apart from one another along an axial direction defined by a first track  16  and a parallel second track  18 . 
     The first and second tracks  16 ,  18  span the distance between the frame members  12 , 14  and they each have a print carriage  20  mounted thereon. The print carriages are conventional print carriages each mounting a set of six inkjet cartridges, and are driven by a carriage drive mechanism (not shown) when the assembly is mounted within a printer to traverse the tracks and thereby deposit a swath of ink along a respective one of two print zones  22 , 24 , indicated in dotted outline. 
     When a print medium (e.g. a sheet of paper or a length of textile) is advanced under the assembly  10  by a print medium advance mechanism (not shown), it successively passes through the two print zones  22 , 24  and a swath of ink can be printed thereon by the print carriages. 
     Due to the separation between the print zones, ink laid down in the first print zone  22  will be relatively dry when the paper on which it has been printed reaches the second print zone  24  thus, the print mask of ink droplets can be divided between the print zones  22 , 24  to enable higher quality printing for a given number of passes as explained above. 
     Structural integrity of the assembly is ensured by a pair of longitudinally extending struts  26  affixed between the frame members  12 , 14 . 
     Each track comprises three basic components, namely a rail support member and a pair of rails. Referring to track  16 , the rail support member  28 , outer rail  30  and inner rail  32  can be identified. Similarly, track  18  comprises a rail support member  34 , inner rail  38  and outer rail  36 . The rails are used to mount and guide the carriages by means of bushings on the carriages which engage and run along the rails in conventional manner. 
     The rail support members  28 , 34  are manufactured as an aluminium extrusion.  FIG. 2  shows rail support member  28  of track  16  in sectional elevation (as taken along the line II—II in FIG.  1 ), with the rails  30 , 32  omitted. 
     Rail support member  28  is formed as an aluminium extrusion with an inner “V”-shaped rail mounting feature  40  for mounting inner rail  32  ( FIG. 1 ) and an outer “V”-shaped rail mounting feature  42  for mounting outer rail  30 . Screw-receiving holes  44  are located at 180 mm intervals along the rail mounting features  40 , 42 , and the rails, which are formed as circular-section steel bars, are similarly provided with threaded holes at 180 mm intervals. This enables the rails to be securely mounted to the rail mounting features and to conform to their straightness (counteracting any non-linearity in the rails). 
     The aluminium extrusion of the rail support member  28  has a tolerance of 1 mm per meter. However, after extrusion, the “V”-shaped features  40 , 42  are accurately machined to a tolerance of just under 0.1 mm per meter, so that for a 1.6 m length of track between the frame members (as shown in FIG.  1 ), the variation from linearity is only 0.15 mm. Furthermore, since the print carriage shown has a length of 250 mm, the expected mean variation from linearity within one carriage length is only 0.025 mm, ensuring that all of the pens on the carriage are able to deposit droplets correctly with respect to the adjacent droplets deposited by the other pens on the carriage. The printer can then be calibrated in known manner to account for the variation from non-linearity along the axis in the secure knowledge that the carriage, when moving along its own length, is moving in a straight line. 
       FIG. 3  shows the rail support member  28  at the point where it meets the frame member  12 . At this location along the rail support member, the lower part of the extrusion is machined away to provide a bottom surface  46  for the remainder  48  (see  FIG. 1 ) of the length of the extrusion extending beyond the frame member  12 . The end surface  50  ( FIG. 3 ) of the lower part of the extrusion  28  thus provides a step against which the frame member  12  (a portion of which is indicated in dotted outline) is located. 
     The rail support member extrusion has a first slot  52  and a second slot  54  machined up through the rail mounting features  40 , 42  to receive a first tab  56  and second tab  58  respectively of the frame member  12 . First tab  56  has a “V”-shaped upper surface  60  which coincides with the “V”-shaped surface of the rail mounting feature  40 . This means that when the inner rail  32  is in place on rail mounting feature  40 , the rail  32  can be precisely positioned with respect to the frame member  12  by resting the rail  32  in the inner angle of the “V”-shaped upper surface  60 . 
     The upper surface  62  of the second tab  58  is flat and not “V”-shaped, so that while it provides a rail support structure (i.e. a surface) for resting the outer rail  30 , all of the location of track  16  is achieved by means of the location point provided by the surface  60  of tab  56 . 
     In assembling the track  16  to the frame member  12 , the tabs  56 , 58  are first fed through the slots  52 , 54 , and the rails  30 , 32  (not shown in  FIG. 3 ) are rested on the tabs. Due to the “V”-shaped profile of the upper surface  60 , this uniquely positions the inner rail  32  and thus the entire track  16  with respect to the first frame member  12 . A set of bolt receiving holes  64  in the frame member  12  are provided each of which allows a bolt to be fastened into a threaded section of a plurality of apertures  66  in the lower part of the extrusion, thereby fastening the first frame member  12  to the first track  16 . The bolt receiving holes are slightly larger than the diameter of the shanks of the bolts themselves (but smaller than the heads of the bolts) to provide a small amount of play and ensure that the relative positions of the frame member and track are determined by the interaction between the inner rail  32  and the location point provided by the surface  60  of first tab  56 . 
       FIG. 4  shows the remainder  48  of rail support member  28  (see  FIG. 1 ) in sectional elevation taken along the line IV—IV in FIG.  1 . In this view, it can be seen that the extrusion has been machined away below the bottom surface  46  of the remainder  48 . It can be seen that the frame member  12  (only a portion of which is shown) is affixed to the extrusion as described above by a set of bolts  68 , and that the tabs  56 , 58  extend into the upper part of the extrusion at the point where the frame member and extrusion are affixed together. 
     The second track  18  is affixed to the first frame member  12  in a mirror image to that described for track  16  with two important exceptions. 
     First, referring to  FIG. 5 , which shows frame member  12  in elevation, the tabs extending  70 ,  72  which extend up into slots created through rail support member  34  of track  18  are both provided with flat upper surfaces (i.e. of the four tabs  56 , 58 , 70 , 72 , only upper surface  60  of tab  56  is “V”-shaped as a location point, while the other tabs merely provide rail support structures on which the rails  30 , 36 , 38  rest. 
     Second, as can be seen with reference to  FIG. 1 , the second track  18  extends past the first frame member  12  to a far lesser extent than track  16 . Conversely, the mounting at the opposite end of the assembly  10  is a mirror image under point symmetry: Second track  18  extends past second frame member  14  to a far greater extent than first track  16  does. 
     Because of this central symmetry, the extended part  48  of first track  16  enables the carriage  20  mounted on that track to be traversed along the track and outside of first frame member  12 , while an identical extended portion  80  ( FIG. 1 ) of second track  18  allows the carriage  20  mounted on track  18  to be traversed past second frame member  14 . Since second track  18  only extends a short distance past first frame member  12 , there is a space provided adjacent the unextended end  82  of second track  18  for maintenance of the carriage on first track  16 , and there is a matching space provided adjacent the unextended end  84  of first track  16  for maintenance of the carriage on second track  18 , when the assembly  10  is mounted in a printer. In this space a printhead cleaning mechanism may be located to service the carriage when it moves alongside, and it also allows access to the carriage for maintenance or replacement of the carriage or of the print cartridges on the carriage. 
     As also seen in  FIG. 5 , the frame member  12  is in the form of a generally “C”-shaped plate (rotated through 90 degrees in the orientation shown), with a central aperture  74  which is open to the external periphery  76  of the plate.  FIG. 5  also shows two sets of mounting points, each indicated generally at  78  and having four mounting points which are used to mount the reinforcing struts  26  ( FIG. 1 ) after the frame members  12 , 14  and tracks  16 , 18  have been assembled together. 
       FIG. 6  shows first frame member  12  during the assembly of the tracks  16 , 18  to the frame member  12 . As previously described, the outer and inner rails  30 , 32  of track  16  are rested on the tabs  56 , 58  of the frame member  12 , causing the “V”-shaped surface  60  (see  FIG. 5 ) to locate the first track  16  precisely on the frame member. At this point the bolts  68  are inserted and tightened, affixing first track  16  in position. 
     The second track is then placed in approximate position with the outer and inner rails  36 , 38  of track  18  resting on the tabs  70 , 72  of the frame member  12 . Because of the flat upper surfaces of these tabs the second track can be moved from side to side as required. By placing the frame member flush with the surface revealed by machining away the lower part of rail support member  34  of track  18  (i.e. the surface which is equivalent to surface  50  ( FIG. 3 ) but at the opposite end), the two tracks are known to be generally parallel. The precise spacing between the inner rail  32  of first track  16  and the inner rail  38  of second track  18  is then adjusted with reference to a hardened steel template  86 . 
     Template  86  has a pair of co-planar surfaces  88  which are adapted to rest on the tops of the inner rails  32 , 38  and a pair of parallel, spaced-apart shoulders  90  which allow the inner rail  38  of second track  18  to be spaced a precisely predetermined distance from the inner rail  32  of first track  16 . 
     With the inner rails and template  86  touching in this way, the second track  18  can be affixed to the first frame member  12  with a set of bolts  68 ′. 
     The second frame member  14  ( FIG. 1 ) is then attached in identical manner to the first track  16  (which locates precisely by inner rail  32  resting in the angle of a “V”-shaped tab) and the second track  18  (which is spaced from the first track  16  at the location of second frame member  14  by means of the template  86 ). 
     Since the template  86  provides a precise spacing between the inner rails  32 , 38  at two points along their lengths, these inner rails are parallel to one another. Furthermore, since the inner and outer rails on each track are tightly mounted on the precisely machined rail mounting features  40 , 42 , all four rails can be assumed to be very close to truly parallel. This means that the carriage traversing first track  16  and the carriage traversing second track  18  will define a pair of parallel print zones  22 , 24 . Thus, a print mask in which the droplets for any given area are distributed between the two print zones can be reliably printed by both carriages to provide an accurate final image. 
     The template is used only in the assembly of the components.  FIG. 7  shows the scan axis assembly  10  as viewed from an end with first frame member shown in elevation and first and second tracks  16 , 18  affixed thereto. The scan axis assembly is finally mounted in the body of a printer and the various other printer components, including control circuitry for the two carriages, carriage drive mechanisms, paper advance mechanism, etc. also assembled in the printer body with respect to the assembly to provide a twin scan axis printer. 
     It will be appreciated that while the embodiment shown has two opposed print carriages located in mirror symmetry to one another on a pair of rails so that the respective print zones are closely spaced from one another, different mounting arrangements and/or spacings are possible. 
     The invention is not limited to the embodiments described herein which may be varied without departing from the spirit of the invention.