Patent Abstract:
The spacing between the printhead of an inkjet printer cartridge and the print medium is dynamically controlled by an apparatus that slightly rotates the carriage that holds the cartridge. In one embodiment, the rotation is effected by actuation of a cam that resides between the rotatable carriage and a fixed part of the printer. The printhead-to-paper spacing is thus optimized for highest print quality.

Full Description:
TECHNICAL FIELD 
     This invention relates to inkjet printers, and particularly to a technique for adjusting the printhead-to-paper spacing of an inkjet printer. 
     BACKGROUND AND SUMMARY OF THE INVENTION 
     An inkjet printer includes one or more ink-filled cartridges that are mounted to a carriage in the printer body. The carriage is reciprocated across the width of the printer as paper or other print media is advanced through the printer. Each ink-filled cartridge includes a printhead that is driven to expel droplets of ink though nozzles in the printhead toward the paper in the printer. The timing and nominal trajectory of the droplets are controlled to generate the desired text or image output and its associated quality. 
     An important design consideration in connection with such printers involves the spacing between the printhead nozzles and the paper, which can be called the pen-to-paper or printhead-to-paper (PTP) spacing. Generally, the resultant print quality is highest when the PTP spacing is minimized. In this regard, minimizing the PTP spacing reduces print quality degradation from “spray,” which is the presence of small droplets having a trajectory that strays from that of the primary droplet. Moreover, minimizing PTP spacing is useful for minimizing the effects of errors that may be present in the trajectory of the primary droplet. 
     It is important, however, that this PTP spacing is sufficient to ensure that the printhead does not contact the paper, which could damage the printhead and/or smear the printed image. The possibility of contact may arise in instances where the amount and absorption of the liquid ink in the paper is such that the paper buckles upwardly, towards the nozzle. This buckling effect is known as “cockle.” Even in the absence of cockle, other factors, such as the tolerances between the parts that support the paper and printer cartridge, dictate the minimum allowable PTP spacing. 
     Versatile inkjet printers allow one to print onto print media having different thicknesses. A simple example of this is a printer that allows printing on conventional paper stock and a relatively thicker envelope. Best print quality is achieved, therefore, where the PTP spacing is adjusted to account for variations in paper thickness. 
     Color inkjet printers commonly employ either one colored-ink cartridge, which may be interchangeable with a black-ink cartridge, or two to four cartridges mounted in the printer carriage to produce black print or a full spectrum of colors. In a printer with four cartridges, each print cartridge contains a different color ink, with the commonly used base colors of black, cyan, magenta, and yellow. 
     Generally, the print quality of black-ink images or text is more sensitive to PTP spacing than is color image quality. Also, color-ink printing is likely to involve a number of passes over substantially the same area on the paper. Thus, the possibility of smearing the ink may be greater when colored ink is printed. Accordingly, it would be desirable to vary the PTP spacing during a print job that calls for both colored and black ink sections, thereby to optimize the PTP spacing for color and black printing. 
     Instantaneous PTP spacing control, whether undertaken during a particular print job or between sheets of paper having different thicknesses, can be characterized as dynamic control, as opposed to, for example, mechanical methods developed with impact printers or the like, which halt printing until some mechanical intervention (such as lever movement) takes place to adjust the PTP spacing. 
     The present invention is directed to an apparatus and method for dynamically controlling PTP spacing. To this end, the relative position of the carriage is selectively and dynamically changed to vary the PTP spacing. As one aspect of this invention, this control is provided by electronic actuation that requires no manual intervention by the printer user. 
     As another aspect of the invention, the control is provided via the printer control firmware, thereby employing the primary components of the printer control system that exist for controlling the printhead operation and carriage movement normally required for printing. 
     Other advantages and features of the present invention will become clear upon study of the following portion of this specification and the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram showing the side view of an irkjet printer carriage that incorporates the dynamic PTP spacing components of the present invention. 
     FIG. 2 is a diagram showing a view taken along line  2 — 2  of FIG.  1 . 
     FIG. 3 is a diagram of a view taken along line  3 — 3  of FIG.  1  and enlarged to show an assembly that includes a cam member that is movable for changing the position of the carriage that carries the inkjet printer cartridges. 
     FIG. 4 is a diagram like FIG. 3, but showing the cam member moved into another position for placing the carriage in an alternative position. 
     FIG. 5 is a view like FIG. 3, but illustrating one of the alternative approaches to actuating the cam member. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     With reference to FIGS. 1 and 2, a preferred embodiment of the present invention includes a carriage  20  that is slidable along a support rod  22  that is housed within an inkjet printer. The rod  22  extends across the printer, oriented perpendicularly to the direction the paper  24  (or any other printing medium) is advanced through the printer. Bushings  26  may be fit into the carriage  20  to facilitate sliding. 
     One or more inkjet cartridges  28  are removably connected to the carriage. In the illustrated embodiment, four cartridges  28  are depicted to represent a printer that is adapted for color printing and carries cartridges of black, cyan, yellow, and magenta inks for this purpose. The cartridges  28  include plastic bodies that comprise liquid ink reservoirs  30  shaped to have a downwardly depending snout  32 . A printhead  34  (the size of which is greatly enlarged in the drawing for clarity) is attached to the end of the snout. The printhead is formed with minute nozzles from which are ejected ink droplets onto the paper  24 . 
     Each ink cartridge  28  has a circuit mounted on a wall  38 . The circuit includes exposed contacts that mate with contacts of a circuit carried inside the carriage  20 . The carriage is connected, as by a flexible, ribbon-type multi-conductor to the printer microprocessor, which provides to the cartridges control signals for precisely timed ejection of ink droplets. The droplets render text or images on the advancing paper as the carriage is reciprocated across the printer (i.e., into and out of the plane of FIG.  1 ). 
     FIG. 1 illustrates in somewhat simplified fashion a small portion of the path of the paper  24  through the printer. Each cartridge  28  is supported above the paper  24  by the carriage  20  such that printhead  34  is maintained at a desired PTP spacing “D.” The paper  24  is picked from an input tray and driven into the paper path in the direction of arrow  40 . The leading edge of the paper is fed into the nip between a drive roller  42  and an idler or pinch roller  44  and is driven in a controlled manner into the zone underlying the printhead  34 , from where it encounters an output roller  46 , and then advances into an output tray. Although an output roller  46  is shown in FIG. 1, a stationary surface may be placed in that vicinity for supporting the advancing paper. 
     The carriage  20  is mounted for limited rotational movement about the central axis of support rod  22 , which axis is depicted as point  23  in the cross section portion of FIG.  1 . Moreover, the center of gravity of the carriage  20  is located on one side of the rod  22  (FIG. 1, the left side) so that the carriage is normally urged by its weight toward a counterclockwise rotation. As a result, the upper portion of the carriage bears against a stationary part of the printer designated as a guide rail  48 . 
     The guide rail  48  may be in the shape of an inverted “U” or channel and extends substantially across the width of the printer, parallel to the support rod  22 . In a preferred embodiment, the rail  48  includes a downwardly depending outer leg  50  and inner leg  52 . 
     The upper portion of the carriage  20  bears against the surface of the rail outer leg  50  to limit the counterclockwise rotation of the carriage with respect to the rod  22 . Here, in the region where there is contact between the carriage  20  and rail leg  50  one finds the primary mechanisms of the present invention as now explained. 
     These control mechanisms are used to make the carriage  20  move toward and away from the rail leg  50 , thus rotating, slightly, the carriage and its cartridges  28  about the rod  22 . This carriage rotation changes the PTP spacing “D.” In a preferred embodiment, the PTP spacing may vary from about 0.5 mm to slightly more than 1.5 mm, and can be dynamically changed to suit changes in print media type (envelopes, plastic film, coated paper, etc.) or thickness, colors employed in printing (i.e., color or black ink), or amount of ink. 
     More particularly, the apparatus and method of the present invention includes a cam  60  that is rotatably mounted at the top of the carriage  20 . The cam includes at least two planar contact faces  62 ,  64 , which may be alternately moved, as a result of cam rotation, into contact with a bendable but substantially incompressible reference slider  66  that is also mounted to the carriage. The amount of bending of the slider  66  is shown greatly exaggerated in the figures for the sake of illustration. 
     The reference slider  66  is a plastic member that is mounted to the top of the carriage  60  by attachment of a base portion  68  of the slider to the carriage. Away from the base, the slider  66  presents a bendable beam-like member that extends to be pinched between cam surfaces  62 ,  64  and the surface of the rail leg  50 . Preferably, the spacer has a low coefficient of friction to facilitate sliding along the rail. 
     The pivot axis  70  of the cam  60  is fixed relative to the carriage. In one preferred embodiment, this axis is defined by the shaft of a small, reversible motor  72  that underlies the cam and is fastened to the carriage. The motor  72  is actuated via drive signals to rotate the cam as described more below. The drive signals are provided by leads (not shown) that extend between the motor  72  and small printed circuit (PC) board  74  that is mounted to one side of the carriage. This PC board  74  conveys the drive signal to the motor from a printer controller  76  via a flexible multi-conductor, such as shown at  78 . 
     The printer controller  76  may be a conventional microprocessor based unit carried on board the printer and includes suitable signal conditioning, drivers, and interfaces for providing the motor control signals at selected times for actuating the cam  60 . 
     FIG. 3 depicts the cam  60  rotated by the motor  72  into one of two positions. It is noteworthy here that a cam shaped to provide more than two such positions is contemplated. In one such embodiment, the cam may have a continuously curved, eccentric contact face to thereby provided a very large or infinite number of positions. 
     In the position shown in FIG. 3, the face  62  of the cam bears against the slider  66 , which in turn bears against the rail leg  50 . The distance between the cam axis  70  and the cam face  62  (taken along a line normal to that face) is shown as W 1  in FIG.  3  and is greater than the distance W 2  (FIG. 4) between the cam axis  70  and the other cam face  64  (taken along a line normal to that face  64 ). 
     Movement of the cam into the position shown in FIG. 3 causes the cam to force the pivot axis  70  (hence, the carriage  20 ) to move away from the rail leg  50 , which movement occurs as a result of the bending or yielding of the beam portion of the slider  66 . This movement is rotational movement, in the clockwise sense with respect to FIG. 1, which has the effect of increasing or maximizing the PTP spacing “D.” As noted above, movement of the carriage into this position might be selected to accommodate, for example, a relatively thick printing medium  24 , such as an envelop. 
     It will be appreciated that the movement of the carriage may be a dynamic response to a user&#39;s selection of an “envelope” printing mode that may be available for selection from buttons on the printer, or from virtual buttons of the word processing software running on a computer that is associated with the printer. In this regard, the motor-drive signals generated by the printer controller in response to the user selection may be retained in the printer firmware and provided to the motor  72  on the carriage as required. 
     When it is desired to reduce the PTP spacing from that provided in the arrangement of FIG. 3, the cam  60  is moved to rotate in the direction of arrow  80  until its face  64  bears against the slider  66 . As noted earlier, the force for moving the carriage  20  in this direction (i.e., counterclockwise rotation in FIG. 1) is attributable to gravity and the moment that arises from the offset center of gravity of the carriage. 
     In the position shown in FIG. 4, the relatively small distance W 2  between the cam axis  70  and the slider results in a relatively closer PTP spacing “D,” which may be desired for paper of normal thickness. As before, the carriage movement into this position is dynamically controlled by the user. 
     In one embodiment, the motor  72  may be a stepper-type. Also, the printer controller  76  may verify the position of the cam  60  at any given time by the use of a microswitch  82 . To this end, the microswitch  82  may be mounted to an extension of the PC board  74  to reside near the cam  60  so that the switch is activated each time the cam moves into and out of the position shown in FIG.  3 . Another embodiment may directly verify or control PTP spacing “D” by placing an optical or other measuring device near the cartridge snout(s)  32 . If located at the top of the carriage, such a device could measure, for example, the changes in the distance between the cam axis  70  and the rail  48 . 
     Any of a variety of means (solenoid, etc.) may be used to actuate the cam movements described above. One such alternative actuation technique is depicted in FIG.  5 . There, a linear actuator  86  is pivotally mounted at each end between the PC board  74  (another extension of this board is shown in FIG. 5) and the cam  60 . In one embodiment, the linear actuator  86  may be formed of shape memory alloy and driven, via a lead  88 , with sufficient current to contract the link and move the cam from the position shown in FIG. 5 to the position shown in FIG.  4 . The cam can be returned to the FIG. 5 orientation by a suitable tensioned spring  90  that extends between the cam  60  and PC board  74 . Alternatively, another linear actuator may be employed for this purpose. 
     It is contemplated that the carriage may be moved to the extreme side of the printer away from engagement with the rail. In such an arrangement, a mechanical stop located there is used to limit the counterclockwise rotation of the carriage in lieu of the rail  48 . The cam may thus be rotated into a selected position without pinching the slider  66  between a cam face and rail leg. As a result, appreciably less energy is required for rotating the cam, as compared to rotating the cam while the slider beam is pinched against the rail (since in the latter case the entire mass of the carriage and cartridges is moved; in the former, only the beam of the slider  66  is bent). For such an arrangement, the slider  66  includes a beveled portion  92  for enabling the slider to fit between the legs  50 ,  52  of the rail  48  as the carriage  20  moves back from the extreme side region. The beveled portion  92  thus acts as a simple inclined plane for forcing the clockwise rotation of the carriage as the carriage returns to engagement with the rail. Alternatively or additionally, the rail may have an inclined plane feature. 
     It is contemplated that the cam may be mounted for translational movement relative to the carriage, thus acting like a wedge. Moreover, the cam may be mounted securely enough, and have sufficient low-friction characteristics to eliminate the need for a slider member, such that the cam would bear directly on the printer rail. 
     Thus, while the present invention has been described in terms of a preferred embodiment, it will be appreciated by one of ordinary skill that the spirit and scope of the invention is not limited to those embodiments, but extend to the various modifications and equivalents as defined in the appended claims.

Technology Classification (CPC): 1