Abstract:
A printhead carriage for an ink jet printer having a plurality of printhead stalls therein and a printhead holddown cover having spring biased printhead holddown plungers on the underside of the cover and equal volume fluid passageways in the cover for conducting fluid from fluid inlets on said cover exposed to atmosphere to printheads in the stalls. An arcuately moveable air pump is engageable with one of a number of arcuately positioned inlets to the passageways in the carriage cover to enable a single pump to selectively apply positive pressure to prime each of the printheads in a desired sequence.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS, IF ANY 
     None. 
     BACKGROUND OF THE INVENTION AND PRIOR ART 
     The present invention relates to the art of computer driven printers, particularly, color inkjet printers. Printers of this type have a printhead carriage which is mounted for reciprocal movement on the printer in a direction orthogonal to the direction of movement of the paper or other medium on which printing is to take place through the printer. The printer carriage of a color printer typically has four or more removable thermal ink jet printheads mounted thereon. Each of the printheads contains or is attached to a supply of ink and occasionally it is necessary to prime one or more printheads by creating a pressure differential to force ink to flow through the ink delivery orifices. 
     Printhead priming has previously been done by positioning a compliant seal around the nozzle plate of the printhead after the printhead carriage has been parked at a service station. In these systems, ink is drawn through the printhead nozzles by applying a negative pressure to the outside of the nozzle plates of the printheads to suck ink through the orifices. The source of the negative air pressure differential has been, among others, a collapsing air bellows or a remote pump connected by a fluid conduit. In these systems, the pressure is maintained by pressing a compliant cap against the surface surrounding the nozzles to create a chamber closed to the atmosphere but connected to the pressure source. The use of negative pressure to prime a printhead can have several disadvantages such as ink foaming, excessive waste ink and lack of precise control over the priming operation. Accordingly a system for printhead priming is required which does not rely upon negative pressure priming and by which a printhead can be primed in a controlled manner with minimal risk of system damage. 
     SUMMARY OF THE INVENTION 
     In its broadest aspects, the present invention provides a printhead carriage for an ink jet printer which includes a frame having at least one printhead stall therein, the improvement comprising: a printhead holddown cover connected to said frame for fastening said cover in a closed printhead holddown position relative to the frame, said cover having at least one fluid passageway therein for conducting fluid between at least one fluid port on said cover exposed to atmosphere and a fluid port in said printhead stall, said printhead stall fluid port being engageable with a fluid port on a printhead positioned in said stall to exchange fluid with said printhead. 
     The present invention further provides a printer which includes the moveable carriage of the preceding paragraph and at least one inkjet printhead in operative position in a stall on said carriage and further comprising: 
     a) a fluid pump having a fluid outlet for delivering fluid to said inkjet printhead, said pump outlet comprising a moveable outlet proximate an end of the path of carriage travel for engagement by said carriage to actuate said pump to deliver a controlled volume of said fluid to said printhead, and 
     b) a pump position actuator for moving said pump outlet to a selected position to connect said pump outlet to supply fluid under pressure to a selected fluid passageway in said cover and an associated printhead. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a large format printer in which the present invention is useful. 
     FIG. 2 is a top plan view of the printer with its cover removed to show the automatic priming pump and service station at the right end of the path of travel of the printhead carriage. 
     FIG. 3 is a front elevation view of the service station and priming pump. 
     FIG. 4 is a right side elevation view of the service station and priming pump. 
     FIG. 5 is a cross-sectional elevation view taken at line  5 — 5  in FIG. 3, of the mechanism for moving the pump to selected positions to prime selected printheads. 
     FIG. 6 is a cross-sectional elevation view through the pump. 
     FIG. 7 is a right side elevation view of the printhead carriage with cover in the closed position. 
     FIG. 8 is a front elevation view of the carriage showing the printhead cover in the raised position. 
     FIG. 9 is a top plan view of the carriage with printheads installed in two stalls and the cover in raised position. 
     FIG. 10 is a plan view of the carriage cover partly broken away showing air passageways therein. 
     FIG. 11 is a graph plotting air pressure profiles delivered by the pump. 
     FIG. 12 is a graph of a velocity servo soft bump algorithm implementation. 
     FIG. 13 is a graph of a velocity servo hard bump algorithm implementation. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 shows a large format printer  10  of the type which includes a transversely movable printhead carriage enclosed by a cover  12  which extends over a generally horizontally extending platen  14  over which printed media is discharged into a catcher basket. At the left side of the platen are four removable ink reservoirs  20 ,  22 ,  24 ,  26  which, through a removable flexible tube arrangement to be described, supply ink to four inkjet printheads mounted on the moveable carriage. 
     In the plan view of FIG. 2 in which the carriage cover  12  has been removed, it is seen that the printhead carriage  30  is mounted on a pair of transversely extending slider rods or guides  32 ,  34  which in turn are affixed to the frame of the printer. Also affixed to the frame of the printer are a pair of tube guide support bridges  40 ,  42  from which front and rear tube guides  44 ,  46  are suspended. The printhead carriage  30  has a pivotal printhead hold down cover  36  connected to the carriage  30  by a hinge  31  (FIG. 7) and fastened by a latch  38  at the front side of the printer which securely holds four inkjet printheads, two of which is shown in FIG. 9 in place in stalls C, M, Y, K on the carriage. The front tube guide  44  is angled near the left bridge support  40  to provide clearance for opening the printhead cover  36  when the carriage is slid to a position proximate the left side of the platen  14  so that the printhead hold down cover  36  can be easily opened for changing the printheads. 
     A flexible ink delivery tube system conveys ink from the four separate ink reservoirs  20 ,  22 ,  24 ,  26  at the left side of the printer through four flexible ink tubes  50 ,  52 ,  54 ,  56  which extend from the ink reservoirs through the rear and front tube guides  44 ,  46  to convey ink to printheads on the carriage  30 . The ink tube system may be a replaceable system as described and claimed in co-pending application Ser. No. 09/240/039 filed Jan. 29, 1999 owned by the assignee of the present invention, the disclosure of which is hereby incorporated herein by reference. 
     At the right side of the printer is a printhead service station  48  at which the printhead carriage  30  may be parked for cleaning and priming the printheads. The printhead service station  48  is comprised of a plastic frame mounted on the printer adjacent the right end of the transversely extending path of travel of the printhead carriage  30 . The printhead carriage  30  (FIGS. 8 and 9) includes four stalls C, M, Y, K which respectively receive four separate printheads containing colored ink such as cyan, magenta, yellow and black. The service station  48  also includes four separate servicing stalls C, M, Y, K which may be provided on a drawer which is moveable forwardly and rearwardly of the printer. The servicing stalls each include a spittoon to capture ink discharged by the printheads during priming. The moveable drawer construction of the servicing station forms no part of the present invention. 
     A printhead servicing pump  50  is mounted on the upper end of a pump positioning arm  80 . A gear enclosure frame  60  is affixed to the right sidewall of the frame of the service station  48  and is spaced therefrom to provide a pocket containing a speed reduction gear mechanism which positions the arm  80  and thus the pump  50  with respect to the printhead carriage  30 . The positioning arm  80  is mounted for movement on a pivot axis  82  extending between the right sidewall of the service station frame and the gear enclosure frame  60 . An arm positioning electric step motor  90  rotates a drive gear  92  thereon which is engaged with the teeth of a large driven gear  94  connected on a common shaft to a small driven gear  96  having teeth which mesh with an arcuate arm positioning gear  98  formed on the pump positioning arm  80  to move the arm through an angle of slightly less than 90°. Movement of the arm  80  positions the pump at various locations along an arc centered on the pivot axis  82  of the arm to align a pump outlet  520  with the inlet end of one of four air conduits  100 ,  102 ,  104 ,  106  arcuately positioned on the side of a pivotally mounted printhead holddown cover  36  on the printhead carriage  30 . 
     The four air conduits each  100 ,  102 ,  104 ,  106  are each sized to have a substantially equal volume and extend from the inlet ends at the side of the hold down cover  36  internally of the cover and terminate in downwardly directed (when the cover is closed) fluid outlets  110 ,  112 ,  114 ,  116  on the underside of the printhead holddown cover. The air outlets each have a compliant seal  111 ,  113 ,  115 ,  117  therearound which mates with corresponding air inlet ports on the top surfaces of the four printheads when positioned in their respective stalls in the printhead carriage. Also shown on the underside of the printhead holddown cover  36  are spring loaded printhead positioners  120 ,  122 ,  124 ,  126 . It will be seen that the printhead holddown cover is pivotally connected to the carriage and fastened in its closed or printhead holddown position by a finger latch  38  and retainer  39 . 
     The air pump  50 , which may be removably affixed to the upper end of the positioning arm  80  or permanently attached thereto as desired, comprises an open ended cylinder  510  in which an elongated piston  522  having a pair of spaced piston alignment discs  523 ,  524  or collars slideably engageable with the inner wall of the cylinder is received. The piston  522  is biased outwardly of the cylinder by a compression spring  525  which is seated at one end against a spring seat  526  in the pump cylinder and which is seated at its other end against a collar  57  surrounding the inner end of a hollow piston stem  58  having an elongated axial passageway  59  therethrough. A compliant seal  61  is seated against the inner piston alignment disc  54  and slideably engages the inner wall of the cylinder to provide an air seal therebetween. The walls of the seal  61  engage the cylinder  510  at an angle so that the seal  61  unidirectionally holds a positive pressure within the air chamber  68  when the piston  522  moves to the right, but does not hold a vacuum when piston  522  moves to the left. The cylinder is closed by a cover  63  attached to the outer wall of the cylinder by one or more fasteners  65 , the construction of which is not relevant to the present invention. Alternatively, the cover may be threadedly affixed to the cylinder. The piston  522  has an enlarged collar  67  at its outer end on which a compliant gasket  69  is affixed for engaging the side wall of the printhead holddown cover  36  and providing an air seal between the outlet  52  of the piston and the side wall of the printhead holddown cover  36  during positioning of the carriage against the piston at the service station. 
     Servicing of the printheads on the printhead carriage is accomplished in part by positioning the pump  50  for alignment with the air passageway  102 ,  104 ,  106 ,  108  in the printhead holddown cover which conveys air to the printhead to be serviced. Movement of the carriage  30  into the service station  48  with the pump so positioned causes the carriage to engage the compliant gasket  69  at the outlet of the pump with continued movement of the carriage moving the pump piston  52  to the right into the cylinder to discharge air from the air chamber  68  in the cylinder through the central passageway  59  in the piston to thus provide a source of positive air pressure to the printhead which causes ink to be forced through the printhead orifices at the bottom of the printhead into the appropriate spittoon in the service station  48 . The nozzles of the printheads C, M, Y, K may thus be primed with ink flow caused by a positive air pressure supplied by the pump  50 . It will be appreciated by persons skilled in the art that the air pressure supplied by the pump need not contact the ink in the printheads and in fact should not do so to avoid introducing air which must be warehoused in the pen body. Accordingly, a printhead configuration in which ink in the printhead is contained in a chamber having a volume which can be reduced by application of air pressure to another chamber in the printhead is preferred. Travel of the printhead carriage away from the pump  50  as it leaves the service station  48  extracts the air which has been previously forced into the printhead cover. If some of the air introduced under pressure to the printhead has escaped during the process, the pump may apply an undesired amount of vacuum to the printhead. The pump design allows the pressure to be clipped at a small negative pressure of approximately −5.0 inches of water to avoid creating a vacuum before damage is done to the printhead. The seal between the pump outlet and the passageway in the printhead holddown cover is broken after the pump piston has travelled under the bias of the spring  55  to the end of its stroke. Thus any backpressure within the printhead necessary for its correct functioning should remain unaffected by the priming operation. 
     The pump  50  is arcuately postionable as best seen in FIG. 5 anywhere between a rest position  0  and a reference position R which are defined by stops  84 ,  86  on the gear enclosure frame  60  which are engaged by the sides of the positioning arm  80 . Positions of the arm for delivery of air by the pump to the cyan, magenta, yellow and black ink printhead conduits  100 ,  102 ,  104 ,  106  on the printhead carriage holddown cover  36  are shown in FIG. 5 at positions preferably spaced by approximately 6° degrees from each other. 
     The stepper motor  90  preferably steps the gear  92  at 3.75°/half-step and the gear train preferably provides a 30:1 reduction between the stepper motor  90  and the gear  98  on the pump positioning arm  80 . 
     The hard stops  84 ,  86  which define the limits of travel of the pump positioning arm are preferably placed at 84° from one another. For each printhead servicing cycle, the pump  50  is moved from the parking or rest position  0  in which the arm  80  engages the parking hard stop  84  to the reference position R in which the positioning arm engages the reference stop  86 . The reference stop  86  is positioned closer than the parking or rest stop  84  to the functional angular positions K, Y, M, C in which the pump  50  engages the cyan, magenta, yellow and black printhead conduits  100 ,  102 ,  104 ,  106  on the carriage holddown cover. After movement of the pump positioning arm from the rest position  0  to the reference position R, the arm is then moved in a reverse (clockwise as seen in FIG. 3) direction to the preliminary position P. The stepper motor  90  then moves the pump positioning arm  80  in the original direction (counterclockwise in FIG. 3) to position the pump  50  in alignment with the desired functional location C, M, Y or K for connection to the related conduit  100 ,  102 ,  104 ,  106 . This movement is performed to assure that, due to backlash, the same gear tooth face set that is used to move the pump positioning arm against the reference hard stop  86  is used to complete the accurate positioning of the pump  50  in the selected functional position. 
     The hard stops  84 ,  86  are integrally formed with the gear enclosure frame  60 . This design sacrifices a small amount of positional accuracy in the nominal position of the pump  50  but decouples the hard stop function from the vertical adjustment of the gear enclosure frame  60 . An over-stepping algorithm is used to ensure that the pump positioning arm  80  has contacted the reference hard stop  86 . The over-stepping algorithm includes margin for both backlash and possible lost steps. 
     All functional angles are placed at even multiples of the nominal angular resolution. This is done to ensure that there are no pump positioning errors because an odd step total for a half-stepping algorithm is, by definition, less stable than an even step total. 
     The inlets on the printhead holddown cover to the conduits  100 ,  102 ,  104 ,  106  are placed at angles of 6° from one another and are centered around a vertical line which extends through the axis  82  of rotation of the pump positioning arm  80  and are located at the same radius as the outlet of the pump  50 . The axis  82  of rotation of the positioning arm  80  is placed at a maximum reasonably feasible radius from the inlets to the conduits  100 ,  102 ,  104 ,  106  to minimize the vertical distance (FIG. 4) between the inlets to facilitate the design of the holddown cover  36 . 
     The radial margin around each air inlet is preferably about 2.5 mm to the inner diameter of the pump discharge gasket and 3.5 mm to the outside diameter. In the case that the vertical and horizontal alignment error of the axis of rotation  82  of the positioning arm  80  is  0 , this translates to a stepping error of about 16 half-steps before the interface fails. 
     The stroke length or axial displacement of the pump  50  may be easily selected or adjusted to discharge a controlled volume of air to each of the printheads on the carriage. Design control of the length and cross-sectional area of each of the air passageways  100 ,  102 ,  104 ,  106  in the printhead holddown cover  36  to insure that the total volume of each passageway is substantially the same insures that, for a given pump stroke, the pump delivers the same volume and pressure of air to each printhead regardless of which printhead is being serviced. Each printhead priming process may be tuned individually by adjusting the pump stroke appropriately. 
     The pressure profile delivered by the pump is shown in FIG.  11  and is dependent upon the volume of the air passageways  102 ,  104 ,  106 ,  108  in the printhead holddown cover, the resting volume of the air chamber  68  in the pump itself and the rest position of the printhead carriage prior to priming. The curves shown in FIG. 11 are based upon an air passageway volume of 1.8 cc and a resting pump chamber volume of 3.2 cc. Three curves are shown. The 3.5 mm COMP curve shows the pressure profile at 3.5 mm axial displacement of the pump while the 7.0 mm COMP curve shows the pressure profile at 7.0 mm axial displacement of the pump. The third curve demonstrates the curve form when an air leak in the system is present. In this case, the priming pressure delivered to the printheads is slightly diminished but is still adequate to perform the priming function. 
     The precise location on the printer of the position of the compliant gasket at the pump outlet is determined by the use of a novel velocity servo bumping algorithm. The algorithm has general application to any two relatively moveable components but is more conveniently described in the context of an inkjet printer with reference to movement of the carriage  30  (a first component) with respect to the pump outlet  520  (a second component) to bump the components together preferably through a number of bumping cycles during which the current drawn by an electric motor used to move the carriage to cause the relative movement between the carriage and pump outlet is measured to establish a pulse width modification (PWM) threshold which is exceeded during the bumping. The deflection of one of the components (the pump outlet) has been characterized when the load power exceeds the threshold value. 
     Most bumping strategies require that the two contacting components have a minimum rigidity to function correctly. They typically assume that once the parts contact there will be no deformation or at least that the resulting deformation will be less than the precision required by the system. These algorithms, therefore, cannot be applied to systems having flexible components such as the compliant gasket  69  at the pump outlet  520 . FIG. 13 shows a plot of carriage drive motor load pulse width modification (PWM) against interruptions in milliseconds along the horizontal axis for printhead carriage measurements for a hard bump environment. 
     To recognize the contact of a flexible component, the algorithm must react to single impulses in the PWM profile. This is to say that the servo algorithm must respond if the threshold is exceeded for a single processor interruption (1/1000 sec.). Also, the servo parameters must have a very undamped response to velocity error. The algorithm depends on the PWM instability at the point of contact to recognize the flexible component. Because the impact can be somewhat unstable and because there is additional noise in the system due to other sources, several bumping samples must be taken to insure data consistency. This data must pass the following sanity checks to be considered valid: 
     1. The average reading must not exceed a maximum variation from the nominal value (taken as 4σ of the distribution across many previous printers); 
     2. The 3σ value of the measurement distribution must not exceed a critical value for mechanism function (reading Cp); and 
     3. No single reading can vary from each machine&#39;s own distribution average by more than a critical value (erroneous data point). 
     Because of the delay of the servo and the compressibility of the flexible components, an offset should be calculated when determining the bump position. As seen in the PWM evolution shown in FIG. 12 where the horizontal axis indicates interruptions in milliseconds, time B indicates when the PWM threshold (−28 as shown) was exceeded and time A indicates the point at which the true first contact occurred. The positional offset due to these effects has been characterized and shown to be repeatable. This occurs particularly in the case in which two flexible components are assembled in series (the gasket and the spring) with one of the two having a much higher stiffness and particularly preload. 
     FIG. 12 also demonstrates the transient noise which occurs due to both inertial and friction/stiction effects while accelerating the carriage and approaching the pump. To reduce the risk that the PWM threshold will be exceeded during this phase, carriage movement is started sufficiently far from the nominal position to ensure that discarding the first half of the PWM profile will both eliminate this noise and ensure the flexible component (the pump) is not touched during the initial movement. 
     The carriage is repeatedly positioned to deflect the pump outlet and during the bumping procedure. The currently preferred algorithm includes the following: 
     1. Number of bumping cycles:  12 . 
     2. Offset due to connect gasket compression: 6 encoder units (0.25 mm). 
     3. Maximum variation of average reading from nominal: 24 encoder units (1.0 mm). 
     4. Maximum 3σ value: 12 encoder units. 
     5. Maximum single point deviation from average: 6 encoder units. 
     It has been found that the position of the pump outlet can vary by up to 1.0 mm during construction of a printer. Use of the above positioning algorithm reduces the error between actual pump outlet position and optimum pump outlet position to a maximum of 0.25 of this amount. 
     It will be appreciated by those skilled in the art that, while the specific embodiment of the present invention described utilizes a carriage actuated pump to deliver air under pressure to a printhead, the invention also extends to the use of a carriage actuated pump to generate a vacuum within a printhead and to deliver a liquid, such as ink, to a printhead. 
     Persons skilled in the art will understand that the above disclosure of the preferred embodiment of the invention may be modified and that the scope of the invention is defined in its broadest sense only by the following claims.