Abstract:
A method and apparatus for positioning a fluid ejection device such as an inkjet pen, or a plurality of aligned fluid ejection devices, on a supporting frame known as a printbar for page wide array printing involves provision of datum surfaces on each ejection device extending in three orthogonally related planes. The datum surfaces on the fluid ejection devices mate with positioning surfaces on the printbar to hold each fluid ejection device in correct position. A datum surface on a laterally outwardly extending key on each device is engaged by a biasing spring mounted on the printbar to position the device lengthwise of the printbar and also to prevent incorrect installation of the device.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to the art of fluid ejection devices and, more particularly, to inkjet printers which use a plurality of pens supported in a page wide array in a frame referred to herein as a printbar which extends across a path of movement of media on which printing is to take place. The individual pens used in this field usually comprise generally rectangular configured fluid containing housings of molded plastic or other suitable material so that a number of pens can be aligned end-to-end in the printbar. Maximization of the size of the pens for ease of manufacture and close positioning of the pens in the printbar without pen to pen contact is important for accurate ink drop placement and in keeping size of the printer footprint to a minimum. for these reasons it is considered particularly advantageous to utilize pens which have protruding fore and aft lower orifice array portions on opposite sides of the lower portion of the pen so that the pens can be aligned with the fore and aft portions of adjacent pens in a side-to-side or nested relationship. These pens are rotationally symmetrical about a vertical axis. Electrical interconnects comprising conductive contacts arranged in a pattern are provided on opposite sides of the pen and mating electrical contacts are mounted on the frame for engagement with the interconnects on the pens for transmitting power to control the ejection of fluid from the pens. The pens must be closely and accurately positioned without contacting each other and they must each be stabilized against linear and rotational movement in the spaces on the printbar in which they are mounted. 
     Microscopic fluid droplets are ejected usually downwardly from thermal or piezoelectric printheads onto media on which a print pattern is to be produced from orifice arrays or orifice plates mounted on the lower surfaces of the pens. The need for precise alignment of the pens relative to each other and to the frame or printbar on which they are supported is quite apparent to those skilled in the art. 
     SUMMARY OF THE INVENTION 
     The present invention therefore provides a method of positioning a fluid ejection device in a support to stabilize the device against movement in at least one direction relative to said support comprising urging said device into engagement with said support by biasing a surface on said device which protrudes from one side of said device to urge said device in said direction. 
     The present invention further provides a fluid ejection assembly comprising a frame and at least one fluid ejection device positioned in a mounting location on said frame, said device including a housing for containing fluid and having a positioning member laterally protruding from one side of said housing, said frame having a guide at said mounting location for receiving said member to guide said device into said mounting location in said frame. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a printbar which holds a plurality of inkjet pens for a page wide array printer. 
     FIG. 2 is a top plan view of an inkjet pen. 
     FIGS. 3A and 3B respectively comprise left and right side elevation views of the pen of FIG.  2 . 
     FIG. 4 is a bottom plan view of the pen of FIG.  2 . 
     FIGS. 5A and 5B respectively comprise front and rear elevation views of the pen of FIG.  2 . 
     FIG. 6 is a top plan view of the printbar of FIG. 1 with one pen installed in a pen pocket therein and a broken away portion shown to an enlarged scale as FIG. 6A to show a Y-direction biasing spring. 
     FIG. 7 is a bottom plan view of the printbar and pen seen in FIG. 6, FIG. 7A showing a portion thereof to an enlarged scale. 
     FIG. 8 is a vertical cross-section view taken at lines  8 — 8  on FIG.  6 . 
     FIG. 9 is a rear side elevation view of the printbar partly broken away and showing the Y-direction biasing spring in FIG.  9 A. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The invention will be described with reference to a rectangular coordinate system in which the X, Y and Z directions extend as seen in FIG. 1 A frame or printbar  100 , elongated in the Y direction, is shown for supporting an aligned plurality of fluid ejection devices such as inkjet pens  200  includes spaced end pieces  102 ,  104  each having supporting feet  106  with apertures  108  for attaching of the printbar  100  to printer chassis structure. Each pen  200  must be accurately and repeatably positioned in the print bar  100  with respect to six degrees of freedom—three translational and three rotational. The exemplary printbar shown in the drawings includes a pair of spaced side frames each having elongated lower beams  112 ,  122  and upper beams  114 ,  124  connected by spaced columns  130  which extend in the Z direction which is generally vertical as shown. The spaced columns  130  define windows  132  therebetween for reception of electrical interconnects (not shown) assembled onto the printbar which mate with electrical interconnects on the inkjet pens when installed in the respective pen locations in the printbar  100 . As seen when comparing the top plan view of FIG. 6 with the bottom plan view of FIG. 7, and in FIG. 9, the columns  130  and windows  132  are not directly aligned with each other on opposite sides of the printbar. A rectangular cover  140  may be provided and affixed to the upper beams  114 ,  124  and to the end pieces  102 ,  104  by a plurality of fasteners  142  or any other suitable means. 
     Spaced generally horizontally extending Z-datum supports  150  protrude inwardly of the frame in the X direction from the lower ends of the columns  130  for supporting the fluid ejection devices. Spring support posts  134  are affixed to or integrally formed with the columns  130  on one side of the printbar near the upper ends of the columns as seen in FIGS. 6,  8  and  9 . Aligned cutouts  136  below the support posts  134  are provided in the upper portions of the columns  130  and upper beam  114  and cover  140  to afford clearance space for reception of pen positioning springs  160 . The support posts  134  in one ( 124 ) of the upper beams  114 ,  124  each have a Y direction bias torsion spring  160  positioned thereon so that one free end of the spring  160  extends downwardly in the cutout  136  and engages a transversely extending surface of the upper beam  114  in the cutout  136  and the other free end of each of the springs  160  extends downwardly to engage a surface on each of the pens  200  as will be explained below to bias the individual pens lengthwise of the printbar  100  in the Y direction. Other functionally equivalent types of springs and spring mounts can of course be substituted for the torsion springs and mounting posts shown in the drawings. 
     Spaced vertically extending guides in the form of keyways  180  best seen in FIG. 9A are also provided in the cover  140  (keyway portion  180   a ) and in the same upper beam (keyway portion  180   b ) in which the springs  160  are mounted in locations alongside the springs as shown to receive pen positioning members to be described. The keyways  180  open downwardly into the cutouts  136 . 
     One or more fluid ejection devices such as inkjet pens  200  are received in individual mounting locations or pockets (four are shown in the illustrated example) in the printbar  100 . Each pen  200  is comprised of a generally rectangular plastic housing  210  having protruding fore and aft portions  212 ,  214  at the lower portion of each pen so that the lower portion of each pen can be said to define a generally offset or S configuration as best seen in FIGS. 2 and 4. The bottom plan view of the pen seen in FIG. 4 shows four elongated overlapping (in the Y direction) nozzle arrays  220 ,  222 ,  224 ,  226  which each contain inkjet orifices  228  through which ink may be fired by electrical activation of fluid vaporizing resistors or by piezoelectric firing as is conventional. Each pen housing  210  may contain a supply of ink and may also be provided with a fill port  230  for replenishment of ink in the housing from remote sources of ink, not shown. 
     Generally rectangular recesses  240 ,  250  are formed in the offset lower portions  212 ,  214  of the housing for reception of electrical interconnects  242 ,  252  which may be resilient and engageable with similarly configured resilient electrical interconnects (not shown) which are positioned in the windows  132  of the printbar  100  during assembly of the printer so that fluid droplet firing energy can be supplied to each pen  200 . The electrical interconnects  242 ,  252  are not directly aligned in the X direction since they are on the oppositely facing sides of the fore and aft lower portions  212 ,  214  of the housing  210 . 
     As seen in FIGS. 3 and 5, the pen housing  210  is configured with shoulders which define horizontally extending Z datum surfaces  260 ,  262 ,  264 ,  266  in the locations shown on the housing which engage the spaced Z datum supports  150  on the printbar  100  to provide accurate Z direction positioning and support for the pens  200  when properly positioned in the printbar  100 . Three of the Z datum surfaces are operational and one is redundant and rotationally position each pen about the X and Y axes when the pen is held in place by a latch (not shown) which urges the pen in the Z direction to engage the Z datum surfaces with the supports  150 . 
     Accurate positioning of the pens  200  in the X direction is provided by two vertically extending datum surfaces  270 ,  272  in the locations shown on the diagonally opposite lower corners of the main housing portion  210 . These X datum surfaces  270 ,  272  engage X datum surfaces  274 ,  276  which slightly protrude inwardly from the inner faces of the columns  130  near the supports  150 . The pens are held in the correct position by engagement of the X datum surfaces on the pen with the X datum surfaces on the columns due to the bias provided by the contact of the resilient electrical interconnects  242 ,  252  on the pens with mating resilient electrical interconnects which have been installed in the windows of the printbar. This engagement of the pen X datum surfaces  270 ,  272  with the column X datum surfaces  274 ,  276  provides both accurate linear positioning of each pen in the X direction and prevents rotation of the pens  200  around a line parallel to the Z axis (which as shown is generally vertical) which ordinarily would occur due to pen biasing when the electrical interconnects  242 ,  252  on the pens resiliently engage the electrical interconnects on the printbar  100  during installation of a pen into its mounting location. FIG. 8 shows one pen having its X datum surface  272  properly positioned in contact with an X datum surface  276  on the inside of the column  130  seen to the left of the pen. 
     Each pen  200  also has a laterally extending positioning member in the form of a guide rail or key  280  preferably integrally formed with the pen housing and protruding from one side thereof. The key  280  extends horizontally in the X direction and vertically in the Z direction as shown and is received in an associated guide track or keyway  180  in the printbar to guide the pen into its mounting location in the printbar and provide accurate longitudinal and rotational positioning of the pens. The non-symmetrical positioning of a key  280  protruding laterally from one side of each pen  200  and associated keyways in the printbar thus prevents incorrect installation of the pens in the printbar. The ends  162  of the torsion springs  160  bias each pen  200  to urge the pen keys  280  firmly against the sides of the keyways  180  to provide accurate positioning in the Y direction. The keys thus provide datum surfaces for Y directional positioning and the firm engagement of the keys  280  with the keyways  180  and the engagement of the Z datum surfaces  260 ,  262 ,  264 ,  266  with the associated Z axis supports  150  on the printbar  100  prevents rotation of the pens  200  around an axis which extends parallel to the X direction. 
     The electrical interconnects  242 ,  252  are provided on both sides of the S-shaped pens  200  because there is not enough room on one side surface for all of the electrical connections which need to be made. It is desirable that the interconnects on the opposite sides of the pen be identical but, as previously mentioned due to the S configuration of the lower portion of the pen, the interconnects are not aligned with each other in the X direction. Accordingly, the contact forces generated by deflection of resilient electrical interconnects during installation of the pens  200  in the printbar  100  tends to twist the pens around axes extending in the Z direction. The X datum surfaces  270 ,  272  are positioned to engage like X direction datum surfaces  274 ,  276  on the columns  130  and thus resist the twisting moment without the necessity for special bias springs as is typical in the prior art. Accordingly, the cost of providing reliable biasing springs to oppose X direction pen motion and the increased pen-to-pen spacing required thereby is avoided leading to closer pen spacing and reduced costs. 
     The rotational symmetry of S-shaped pens can and does occasionally result in incorrect pen installation by the user. Good ergonomic design dictates that prevention of incorrect pen installation be prevented as early as possible in the process of installing the pen. Also, since the space between adjacent stacked pens should be reduced to the minimum necessary while preventing pen to pen contact, the Y directional bias springs  160  are provided at locations such, as shown, to the side of the pens but within the confines of the printbar  100  rather than between the end faces of the pens and engage the laterally extending keys  280  to provide the appropriate Y directional bias and resultant positioning. In the event of attempted incorrect installation of a pen  200  in the printbar  100 , interference between the key  280  and the cover  140  occurs well before any electrical contact between the electrical interconnects on the pen  200  and printbar  100  is made. 
     Persons skilled in the art will also appreciate that various additional modifications can be made in the preferred embodiments shown and described above and that the scope of protection is limited only by the wording of the claims which follow. Parenthetical directional references in the claims are provided for convenience and are not intended as limitations on the scope of protection.