Patent Publication Number: US-6334723-B1

Title: Print medium feed system using pre-existing printing apparatus

Description:
CROSS REFERENCE TO RELATED APPLICATION(S) 
     This is a divisional of application Ser. No. 09/205,863 filed on Dec. 4, 1998 now U.S. Pat. No. 5,997,197. Which is a DIVISIONAL of application Ser. No. 08/979,391 filed Nov. 28, 1997, now U.S. Pat. No. 5,913,625 issued Jun. 22, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to feeding a print medium into a printer and to guiding a print medium through a path in a printer. 
     2. Related Art 
     Many printers provide multiple operational modes in which a print medium (e.g., paper, plastic transparency) is fed into a print drive mechanism used to advance the print medium through a printing zone of the printer during printing. For example, the standard mode of operation of many desktop printers is an automatic multi-sheet feeding mode in which individual sheets from a stack of print media (held in, for example, a tray) are, upon appropriate electronic control (e.g., computer control), automatically picked from the stack by a pick mechanism and fed into the print drive mechanism. Many of those printers also include a secondary mode of operation, usually intended for occasional and/or specialized use. The secondary operational mode is often a single sheet mode in which a single sheet of a print medium is fed (often manually) into the print drive mechanism. The secondary operational mode is sometimes a mode in which a small number of sheets of print media are placed in a feed tray, and automatically picked and fed to a print drive mechanism. 
     Generally, in printers that include a secondary mode of operation in addition to a standard mode of operation, a print medium is fed into the print drive mechanism during the secondary operational mode in the same direction as that in which a print medium is fed during the standard operational mode. For example, in some printers, single sheet mode consists of nothing more than placing a single sheet of a print medium on a stack of print media used in an automatic multi-sheet feeding mode so that the added single sheet is the next sheet picked and fed into the print zone. Even in printers in which a print medium is initially fed into a physically separate loading slot during the secondary operational mode, the print medium is usually eventually directed into the print drive mechanism through the same feed path (i.e., in the same direction) as used during the standard operational mode. 
     Physical constraints may preclude location of a loading slot for a secondary operational mode so that the feed path for the standard operational mode can be used. For example, a printer designed to be operated within a small area or volume (i.e., a printer having a small form factor) may be restricted so that a user can only access the front of the printer, necessitating that a loading slot for manually feeding a print medium be placed at the front of the printer. If the printer is also designed so that, in the standard operational mode, sheets of print media are fed in a direction from the back of the printer to the front, it becomes, at best, impractical to feed a sheet from the manual feed loading slot into the print drive mechanism in the same direction as that in which sheets are fed during the standard operational mode. 
     In an inkjet printer, feeding a sheet of a print medium into a print drive mechanism of the printer in a direction opposite that in which sheets are fed into the print drive mechanism during standard operation can be problematic because, just prior to being fed into the print drive mechanism, the sheet must traverse the area (swath gap) through which the print cartridge(s) pass during printing. Since the sheet, which is typically thin and flexible, is not supported in this area, it can be difficult to ensure that the sheet is properly fed into the print drive mechanism. Similar problems (e.g., the necessity for a sheet of a print medium to traverse a gap in the feed path) can exist in other types of printers when a sheet of a print medium is fed into the print drive mechanism in other than the normal feed direction. 
     One inkjet printer enables a sheet of a print medium to be fed into a print drive mechanism of the printer in a direction opposite that in which sheets are fed into the print drive mechanism during a standard operational mode by providing a sheet feed drive mechanism for such sheet. The sheet feed drive mechanism is positioned opposite the print cartridge swath gap from the print drive mechanism. In a secondary operational mode, a sheet of a print medium is fed into the sheet feed drive mechanism which acts to guide and support the sheet as the sheet traverses the print cartridge swath gap. However, the addition of another drive mechanism significantly increases the cost and complexity of the printer. The additional drive mechanism also necessitates additional space so that the printer is larger than desirable. Additionally, in this printer, a user must effect a special control instruction to cause the sheet feed drive mechanism to operate to feed a sheet of a print medium; the user cannot simply insert the sheet in the feed path. Further, it may not be possible or desirable to feed a sheet of a print medium back through the additional drive mechanism immediately after printing on the sheet, since the ink on the sheet may not be dry. (This problem can be overcome by, for example, pausing during printing to let the ink dry before feeding the sheet through the additional drive roller, or by guiding the sheet through a different exit path using additional structure provided in the printer; neither solution is desirable.) Thus, a better solution is needed for situations in which it is necessary or desirable to provide a second, distinct feed path to a print drive mechanism in addition a first feed path, and, in particular, situations in which such a second feed path is positioned so that a print medium fed therethrough is fed into the print drive mechanism in a direction opposite that in which print media are fed into the print drive mechanism through the first feed path. 
     In a mode of operation in which a sheet of a print medium is manually fed into a printer, it is often necessary to detect the insertion of a sheet into the print medium feed path associated with that mode of operation. This is typically done by adding an electronic sensor, either an interrupter-type sensor or a reflective-type sensor, that is dedicated to the task of print medium insertion detection. The additional electronics add to the cost and complexity of the printer. An alternative solution to such print medium insertion detection that is more inexpensive and simple is desirable. 
     SUMMARY OF THE INVENTION 
     The invention accomplishes print medium feed functions, at least in part, by making use of pre-existing printer apparatus that is used to accomplish other functions of the printer. For example, the invention can enable the use of a single print drive mechanism to accommodate multiple print medium feed paths within a printer and, in particular, feed paths through which print media are fed into the print drive mechanism in different (e.g., opposite) directions. Additionally, the invention can enable a rotatable media guide that can be positioned in one position to guide a sheet of a print medium during a print operation and in another position to release the sheet of the print medium after the printing operation, to be positioned in still another position to facilitate guiding a sheet of a print medium into a print drive mechanism after the sheet has traversed a gap in the feed path. The invention can also enable sensing of print medium insertion into a print medium feed path using pre-existing sensing apparatus within a printer that is used for other sensing purposes. 
     In a printer having two completely distinct feed paths to a single print drive mechanism, the invention can enable a sheet of a print medium to be guided to the print drive mechanism via at least one of the feed paths without using a drive mechanism, such as the sheet feed drive mechanism of the printer described above. For example, one of the feed paths of the printer can be one in which a print medium is manually introduced into the feed path, while the other feed path can be one in which a print medium is automatically introduced into the feed path. In particular, the feed paths can enable print media to be fed to the print drive mechanism in opposite directions. The invention can enable, if necessary, a sheet of a print medium fed through a feed path that does not contain a drive mechanism to traverse a gap in the feed path in which the sheet is unsupported. In particular, the invention can enable the sheet to traverse such a gap that is proximate to the print drive mechanism, while maintaining adequate guidance of the sheet to ensure that the sheet is directed into a nip region of the print drive mechanism. As explained further below, the invention can enable such sheet feeding to be accomplished with one or more simple and inexpensive mechanisms that necessitate the addition of little or no additional space to a printer in which the invention is used. 
     To enable a print medium to traverse a gap in a path (which can be, in general, any path) in a printer, the invention can make use of a guiding mechanism positioned on a first side of the gap past which the print medium travels to begin traversing the gap, as well as a print medium receiving mechanism positioned on a second side of the gap to which the print medium travels to finish traversing the gap. The former can be a guide surface for bending the print medium prior to crossing the gap. The latter can be a media guide having a media contact surface which slopes downward, relative to the path, from a first location to a second location that is near the first side of the gap than the first location. The latter can also be a movable media guide that can be placed in a specified position to facilitate directing the print medium along the path. The invention can enable gaps of greater than 0.25 inches to be traversed. 
     As indicated above, the invention can enable a movable (e.g., rotatable) media guide proximate to a print drive mechanism to be moved to a first position when the print drive mechanism is operated to advance a print medium over the media guide through the print zone during a printing operation, and to a second position when a print medium is being fed over the media guide into the print drive mechanism. The first position is specified to maintain a desired spacing within the print zone between the print medium and an apparatus for printing on the print medium, while the second position is specified to facilitate directing the leading edge of a print medium into the print drive mechanism. Typically the second position is a position at which the media guide has been moved such that a media contact surface of the media guide is lower, relative to a feed path into the print drive mechanism, than at the first position. For example, the second position can be a position at which the media guide has been rotated through an angle of about 18° to the lower position. 
     As mentioned above, the invention can also enable sensing of the insertion of a print medium into a feed path by making use of a sensor that is adapted to perform a sensing function other than sensing insertion of a print medium into the feed path. The invention can accomplish this by activating a sensor that senses the presence of an object in a space outside of the print medium path, and causing the introduction of an object into that space in response to insertion of a medium into the feed path. The latter function can be accomplished by disposing an object in the feed path so that a print medium inserted into the feed path will contact the object, then transmitting a signal, either mechanically or electrically, in response to contact in the feed path to cause the object to be moved into the space being monitored by the sensor. The sensor can be, for example, a sensor that is positioned on the print carriage of the printer to enable detection of the leading edge of a print medium during a printing operation. Since an existing sensor is used, print medium insertion detection is accomplished in a more inexpensive and a simpler fashion. 
     When print medium insertion is detected, as described above, by disposing an object in the feed path so that a print medium inserted into the feed path will contact the object, the invention can further provide compliant support of the object so that when a print medium is removed from the feed path after having been inserted, the object can move in response to a force exerted by the print medium, thereby preventing buildup of force between the object and the print medium that may result in marking or scarring of the print medium. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of apparatus including a print drive mechanism, structure defining print medium feed paths, and a print carriage assembly of an inkjet printer, viewed from above the apparatus, illustrating structure according to the invention. 
     FIG. 2 is a cross-sectional view of the apparatus shown in FIG. 1, viewed in the direction of the section line A—A of FIG. 1, together with other printer apparatus not shown in FIG. 1, illustrating two modes of operation of the printer with which the apparatus is used. 
     FIG. 3 is a detailed view of a portion of the cross-sectional view of FIG. 2, illustrating the operation of structure according to the invention during feeding of a sheet of a print medium into a print drive mechanism via a secondary feed path. 
     FIG. 4 is a detailed view of a portion of the cross-sectional view of FIG. 2, illustrating the operation of structure according to the invention during printing. 
     FIG. 5 is a perspective view of the apparatus shown in FIG. 1, viewed in the same direction as in FIG. 1, without some of the structure shown in FIG. 1, illustrating the presence of “frown” features according to the invention. 
     FIG. 6A is a perspective view of the apparatus shown in FIG. 5, viewed from underneath the apparatus, illustrating ribs formed on the chassis that reduce friction between the chassis and a sheet of a print medium passing through a secondary feed path. 
     FIG. 6B is a detailed view of a portion of the perspective view of FIG. 6A, better illustrating the ribs formed on the chassis. 
     FIGS. 7A and 7B are each perspective views of a portion of the apparatus shown in FIG. 1, viewed from underneath the apparatus and in the direction of the section line B—B of FIG. 1, illustrating a print medium insertion sensing system according to the invention. FIG. 7A illustrates the sensing system when insertion of a sheet of a print medium has not been sensed, while FIG. 7B illustrates the sensing system when insertion of a sheet of a print medium has been sensed. 
     FIGS. 8A and 8B are detailed views of a portion of the perspective views of FIGS. 7A and 7B, respectively, illustrating the trigger flag of the print medium insertion sensing system in more detail. 
     FIGS. 9A and 9B are cross-sectional views, viewed in the direction of the section line C—C of FIGS. 7A and 7B, of a trigger structure of a print medium insertion sensing system according to the invention. FIG. 9A illustrates the trigger structure when insertion of a sheet of a print medium has not been sensed. FIG. 9B illustrates the operation of a mechanism for avoiding self-locking of the trigger structure when a sheet of a print medium is removed from the secondary feed path. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 is a perspective view of apparatus including a print drive mechanism, structure defining print medium feed paths, and a print carriage assembly of an inkjet printer, viewed from above the apparatus, illustrating structure according to the invention. It is to be understood that, generally, the invention can be used with printing devices other than dedicated printers (e.g., facsimile machines or multifunction machines that include the capacity to print, transmit facsimiles and/or produce copies) and that printing devices with which the invention can be used may accomplish printing using other than inkjet technology (e.g., bubblejet or laser technology). It is also to be understood that the invention can be implemented with and by structures other than the particular structures shown in FIG. 1 (and the other drawings of the application). 
     In the apparatus shown in FIG. 1, a print carriage  101  is mounted on a slider rod  102  so that the print carriage  101  can move along the slider rod  102  and does not rotate about the slider rod  102 . As illustrated in FIG. 1, the print carriage  101  is constructed with two stalls  103   a  and  103   b  into which corresponding print cartridges (not shown in FIG. 1, but partially visible in FIGS. 2,  3  and  4 , described below) are inserted. (Generally, the print carriage  101  can be constructed to accommodate any appropriate number of print cartridges). An ink reservoir is formed in each print cartridge. Each print cartridge also includes a printhead having a plurality of nozzles through which ink can be ejected. During printing, the print carriage  101  is moved laterally back and forth along the slider rod  102 , and a print medium (such as paper, plastic transparency, white plastic with inkjet coating, or laminate) is advanced past the print carriage  101  (in particular, proximate to the printheads of the print cartridges in the space indicated generally by the bi-directional arrow  104 ) by a print drive mechanism to enable printing of a desired image or images on the print medium. The above-described structure and corresponding operation is conventional; the details of implementation of such structure and operation are well-known to those skilled in the art. 
     FIG. 2 is a cross-sectional view of the apparatus shown in FIG. 1, viewed in the direction of the section line A—A of FIG. 1, together with other printer apparatus not shown in FIG. 1, illustrating two modes of operation of the printer with which the apparatus is used. In one mode of operation, a sheet (not shown) of a print medium is fed from a primary feed tray (which, typically, is a multi-sheet feed tray) to a print drive mechanism of the printer. (In the drawings, with the exception of a primary feed tray cover  204 , for clarity, the primary feed tray and the structure for picking a sheet of a print medium from the tray and advancing the sheet to a drive roller  201  are not shown; the construction and operation of such structure is known to those skilled in the art.) Herein, “print drive mechanism” refers to the structure used to advance a print medium into a print zone during a printing operation to enable printing on the print medium to occur. In FIG. 2, the print drive mechanism includes the drive roller  201  and four pinch rollers (best shown in FIG. 1, and in FIGS. 3 and 4, described below); however, generally, any appropriate number of drive rollers and pinch rollers can be used. A print drive feed guide  202  helps to guide the sheet of the print medium to a nip region between the drive roller  201  and the pinch rollers. The “nip region” is a region of the print drive mechanism within which a normal force is applied to the print medium to ensure that sufficient friction exists between the print drive mechanism and the print medium to ensure that the print drive mechanism can advance the print medium in a controlled fashion. The drive roller  201  is rotated (clockwise, as shown in FIG. 2) to advance the sheet through the nip region and into a print zone (indicated by a bidirectional arrow  203 ), located underneath the printhead(s) of the print cartridge(s) (not shown in the drawings), within which printing on the sheet of the print medium occurs. For convenience, the above-described mode of operation is referred to hereinafter as the “standard” mode of operation and the path  205  that a sheet of a print medium follows from the primary feed tray to the drive roller  201  of the print drive mechanism is referred to hereinafter as the “standard feed path.” Typically, the standard mode of operation encompasses automatic feeding of sheets of print media through the print zone  203  of the printer and/or feeding of sheets from a stack of multiple sheets of print media. 
     In another mode of operation of the printer, described in greater detail below, a sheet (not shown) of a print medium is fed from a secondary feed tray  206  to the print drive mechanism and advanced through the print zone  203 . For convenience, this mode of operation is referred to hereinafter as the “secondary” mode of operation and the path  207  that the sheet follows from the secondary feed tray  206  to the drive roller  201  of the print drive mechanism is referred to hereinafter as the “secondary feed path.” In the secondary mode of operation, often a single sheet of a print medium is positioned in the secondary feed tray  206  for feeding into the print drive mechanism, though, strictly speaking, this need not necessarily be the case. Additionally, in the secondary mode of operation, the sheet or sheets of a print medium are typically manually fed through the secondary feed path  207  to the print drive mechanism. Again, however, this need not necessarily be the case; in some embodiments of the invention, the printer can be constructed so that sheets are automatically picked from the secondary feed tray  206  and fed through the secondary feed path  207 . 
     As can be appreciated from FIG. 2, the position of the secondary feed tray  206  relative to the drive roller  201  makes it preferable during the secondary mode of operation to feed sheets into the drive roller  201  in a direction opposite that in which sheets are fed during the standard mode of operation, i.e., the direction of motion along the secondary feed path  207  into the print drive mechanism is preferably opposite that of the direction of motion along the standard feed path  205  into the print drive mechanism. As can also be appreciated from FIG. 2, this necessitates that, in the secondary mode of operation, sheets be fed into the drive roller  201  after traversing the print zone  203 . A permanent support cannot be provided for a sheet in the region of the print zone  203 , since the upper portion of the print zone  203  must be open during printing to allow movement of the print carriage, and the lower portion of the print zone  203  must be open after printing to allow the sheet of the print medium to be released from the printer (as described further below). Because most print media are relatively flexible, this lack of support can be problematic, since, without support, the sheet may bend during feeding through the secondary feed path  207  such that the leading edge of the sheet does not properly enter the nip region between the drive roller  201  and pinch rollers. Further, the leading edge of the sheet is typically curved, warped and/or curled (viewed in the plane of the sheet in the direction of the feed path), i.e., has a “wavy” shape, thus exacerbating any difficulty in ensuring that the sheet is properly fed into the nip region. 
     As will be better appreciated from the description below, the invention can enable a sheet of a print medium to be fed along a secondary feed path in a manner that ensures accurate introduction of the sheet into the nip region of a print drive mechanism without addition of expensive, complex or bulky structure to the printer. In particular, the invention can enable this to be accomplished for a secondary feed path in which the sheet is fed into the print drive mechanism in a direction opposite that in which sheets of print media are fed using a standard feed path and/or for a secondary feed path in which the sheet must traverse a relatively lengthy unsupported area (e.g., a print zone of the printer) just prior to entering the nip region. Advantageously, the invention can accomplish the above-described feeding of a print medium along a secondary feed path without necessity to add a drive mechanism to the printer to facilitate such feeding. 
     In the apparatus illustrated in FIG. 2, in the secondary mode of operation, a sheet of a print medium is moved from the secondary feed tray  206  into a slot (the “secondary feed path slot”) formed by a surface  208   b  of the chassis  208  and a planar portion  209   a  of a shim  209 . As discussed in more detail below, a sensing system detects when a sheet of a print medium has been inserted into the secondary feed path slot; when such insertion has been detected, a printer control system (e.g., an electronic control system) begins rotating the drive roller  201 . A bend  208   a  is formed in the chassis  208 , so that, just after the sheet leaves the secondary feed path slot, the leading edge of the sheet is forced away from the surface  208   b  of the chassis  208  by the bend  208   a  in the chassis  208 . The sheet traverses the print zone  203 , the leading edge of the sheet eventually contacting a media guide  210  (which is part of a rotatable structure that enables the media guide  210  to be rotated into a desired position, as explained in more detail below) that guides the sheet into the nip region between the drive roller  201  and the pinch rollers. The drive roller  201  is rotated (in a direction opposite that in which the drive roller  201  is rotated to push a sheet of a print medium into the print zone  203  for printing, i.e., counterclockwise, as shown in FIG. 2) to pull the sheet through the nip region. The primary media tray is positioned so that the portion of the sheet first pulled through the nip region is supported by the primary feed tray cover  204 . The sheet is not pulled completely through the nip region; the drive roller  201  stops rotating when the sheet has been pulled through the nip region to a printing position at which the sheet is positioned so that printing can begin. The drive roller  201  is then rotated in the opposite direction (i.e., clockwise, as shown in FIG. 2) to push the sheet back into the print zone  203  to enable printing to occur. 
     FIG. 3 is a detailed view of a portion of the cross-sectional view of FIG. 2, illustrating the operation of structure according to the invention during feeding of a sheet of a print medium into a print drive mechanism via a secondary feed path. 
     The media guide  210  is formed as part of a structure  301  that is mounted on a rotatable shaft  302 . Generally, the media guide  210  can be formed on, or attached to, any suitable rotatable structure that enables the media guide  210  to be positioned in predetermined positions as described below. The construction and operation of a rotatable structure including the media guide  210  according to the embodiment of the invention illustrated in FIGS. 3 and 4 is described in detail in commonly owned, co-pending U.S. patent application Ser. No. 08/814,564 now U.S. Pat. No. 5,797,687, entitled “Paper Stacker Activation for Printer Input/Output”, by David M. Petersen et al., filed on Mar. 11, 1997, the disclosure of which is incorporated by reference herein. 
     In the embodiment of a rotatable structure described in Petersen et al., the structure  301  and shaft  302  are constructed and mounted with respect to each other in a manner that enables the structure  301  to be engaged or disengaged from the shaft  302  to enable rotation and/or translation (along a translational axis parallel to the axis of rotation of the shaft  302 ) of the structure  301 . The structure  301  is spring-loaded so that the structure  301  is biased along the translational axis toward a print position in which the structure  301  can be engaged by a stop (formed as part of, or fixedly attached to, the chassis  208 ) that holds the structure  301  in a predetermined rotational position for printing (see FIG. 4 below). The structure  301  and shaft  302  are constructed such that, in the print position, the structure  301  is disengaged from the shaft  302  so that the shaft  302  can be operated to rotate the drive roller  201  (to enable printing) without rotating the structure  301 . 
     Movement of the print carriage  101  is used to effect translational movement of the structure  301 . A print carriage contact mechanism is mounted on the shaft  302  such that rotation of the shaft  302  can position the print carriage contact mechanism so that the print carriage contact mechanism either contacts or does not contact the print carriage  101  as the print carriage  101  moves. The print carriage contact mechanism is further positioned with respect to the structure  301  such that translational movement of the print carriage contact mechanism (in response to movement of the print carriage  101 ) results in contact with the structure  301  that moves the structure  301  in a direction opposite the spring bias force. This movement releases the structure  301  from the stop and engages the structure  301  with the shaft  302 , so that the shaft  302  can be rotated to change the rotational position of the structure  301 . As described in more detail below, in general, the shaft  302  can be rotated to position the structure  301  in any desired rotational position. Once the structure  301  has been rotated to a now rotational position, the print carriage  101  can be moved so that the print carriage contact mechanism (which is biased by a spring in the same direction as the structure  301 ) and the structure  301  are forced back to a translational position at which the structure  301  is no longer engaged with the shaft  302 . Further, at such translational position, for any particular rotational position the structure  301  can be held in place rotationally by a stop (formed as part of, or fixedly attached to, the chassis  208 ), similar to the stop described above for holding the structure  301  in the print position. (Such a stop may not be used when the structure  301  is positioned in a media release position, described below.) 
     In general, the rotatable structure can be constructed and/or operated so that the shaft  302  can be rotated to an appropriate angular position to enable the media guide  210  to be positioned at any desired position. Typically, the positions of the media guide  210  are confined to a range of positions, the limits of which are imposed by the presence of other structure of the printer. For example, in one embodiment of the invention, the rotatable structure including the media guide  210  can be rotated between a position (a “print position,” shown in FIG. 4, discussed below) in which a portion of a media contact surface  210   a  of the media guide  210  that is closest to the printhead(s) of the print cartridge(s) supports a print medium so as to maintain a desired spacing within the print zone  203  between the print medium and the printhead(s) (hereinafter defined as a position in which the rotatable structure has an angular orientation of 0°) and a position (a “media release position”) in which the rotatable structure has been rotated about 90°-95° clockwise (as viewed in FIGS. 3 and 4) from the print position (hereinafter defined as a position in which the rotatable structure has an angular orientation of about 90°-95°). 
     In particular, the rotatable structure can be rotated to position the media guide  210  into a position (a “sheet feed position”) that ensures that a sheet being fed through the secondary feed path  207  will be properly guided into the nip region between the drive roller  201  and the pinch rollers (one of the pinch rollers, designated by the numeral  303 , is visible in FIG. 3, as well as FIG. 4, described below). Generally, as illustrated in FIG. 3, this can be accomplished by positioning the rotatable structure at an angular orientation such that the media contact surface  210   a  of the media guide  210  is lower, relative to the secondary feed path  207 , than at the print position (though the media guide  210  can also be positioned in the print position during feeding of a sheet of a print medium). Preferably, in the sheet feed position, the rotatable structure has an angular orientation of greaser than about 0° measured in the clockwise direction in FIGS.  3  and  4 ), more preferably greater than about 5° and most preferably greater than about 10°. The magnitude of the angular orientation of the rotatable structure should not be so great that, if the leading edge of the sheet contacts the media guide  210 , the sheet binds up or is misdirected away from the nip region. Preferably, in the sheet feed position, the rotatable structure has an angular orientation of less than about 90°, more preferably less than about 45° and most preferably less than about 30°. In one particular embodiment of the invention, the rotatable structure has an angular orientation of about 18° (shown in FIG. 3) in the sheet feed position. 
     As indicated above, the bend  208   a  in the chassis  208  forces the leading edge of a sheet being fed through the secondary feed path  207  away from the surface  208   b  of the chassis  208 . The presence of the bend  208   a  to guide the sheet produces several advantageous effects. 
     First, since, absent the bend  208   a , the sheet would tend to continue moving along the plane defined by the secondary feed path slot between the surface  208   b  of the chassis  208  and the planar portion  209   a  of the shim  209 , the bend  208   a  applies a force to the sheet (deflecting the sheet) that is resisted by the sheet, thereby causing the sheet to stiffen locally near the end of the bend  208   a . In particular, this stiffening helps control the position of the leading edge of the sheet as the leading edge of the sheet traverses the print zone  203  so that proper entrance of the sheet into the nip region can be better ensured. 
     Second, the bend  208   a  is formed so that the sheet is deflected to direct the sheet to a position between the print drive feed guide  202  and the media guide  210 . The particular geometry of the bend  208   a  (e.g., in a structure as illustrated in FIG. 3, the angle between the substantially planar section of the bend  208   a  and the surface  208   b  of the chassis  208 ) can be established to accomplish this function, as can be appreciated by those skilled in the art, in view of other aspects of the construction and operation of the printer structure, such as, for example, the geometry and locations of the print drive feed guide  202  and the media guide  210  (e.g., the sheet feed position of the media guide  210 ) and a nominal specification of a location at which the leading edge of the sheet is to contact structure after traversing the print zone  203 . For example, in the structure shown in FIG. 3, the angle between the substantially planar section of the bend  208   a  and the surface  208   b  of the chassis  208  is about 15°. 
     FIG. 4 is a detailed view of a portion of the cross-sectional view of FIG. 2, illustrating the operation of structure according to the invention during printing. As indicated above, after the drive roller  201  has pulled a sheet through the nip region to a printing position at which the sheet is positioned so that printing can begin, the drive roller  201  stops rotating. The printer control system rotates the rotatable structure so that the media guide  210  is positioned in the print position (described in more detail above). The printer control system then begins rotating the drive roller  201  in the opposite direction to push the sheet back into the print zone  203  through a printing path  401  to enable printing to occur. After passing through the print zone  203 , the sheet moves into an output tray (not shown) positioned underneath the planar portion  209   a  of the shim  209 . Once printing has been completed (determined in any suitable manner, as known to those skilled in the art, such as by using, for example, a paper edge detector sensor positioned, for example, proximate to the printing path  401  on a side of the drive roller  201  opposite the print zone  203 ) and the sheet has been ejected from the nip region, the printer control system rotates the media guide  210  to the media release position, allowing the remainder of the sheet to fall into the output tray. 
     As discussed in more detail in the above-referenced U.S. patent application Ser. No. 08/814,564 U.S. Pat. No. 5,797,687 and indicated above, the rotatable structure of the embodiment of the invention shown in FIGS. 3 and 4 is a spring-loaded structure that can be locked into each of the print position, the sheet feed position and the media release position. Thus, in general, to place the media guide  210  in the sheet feed position from another position, the rotatable structure must be moved against the spring force to unlock the rotatable structure, then rotated into the sheet feed position. Thus, preferably, when the printer is not in use, the rotatable structure is rotated so that the “default” position of the media guide  210  is the sheet feed position, thus enabling the printer, when not in operation, to always be ready for use in the secondary mode of operation. However, other default positions of the media guide  210  can be used, particularly in embodiments of the invention in which a rotatable structure having another construction is used. 
     The media contact surface  210   a  of the media guide  210  can also be shaped in a particular manner to achieve particular functionality. For example, as discussed above, the media contact surface  210   a  should be shaped so that, when the media guide  210  is placed in the print position, the media contact surface  210   a  supports the print medium at a desired spacing from the printhead(s) of the print cartridge(s). The media contact surface  210   a  can also be shaped so that the media contact surface  210   a  slopes downward, relative to the secondary feed path  207 , from a first location to a second location that is nearer the secondary feed path slot than the first location, thus aiding in guiding a sheet of a print medium toward the print drive mechanism. It may also be desirable to shape the media contact surface  210   a  by providing a downward slope, relative to the secondary feed path  207 , from the location at which the print medium contacts the media contact surface  210   a  during printing to a location that is nearer the print drive mechanism than that contact location, such shaping causing the print medium to bend and locally stiffen just prior to entering the print drive mechanism, thereby enhancing the control of the leading edge of the print medium so that the likelihood of proper entry of the print medium to the nip region of the print drive mechanism is increased. In the embodiment of the invention shown in FIGS. 3 and 4, the media contact surface  210   a  of the media guide  210  has been shaped so as to accomplish these purposes. 
     FIG. 5 is a perspective view of the apparatus shown in FIG. 1, viewed from the same direction as in FIG. 1, without some of the structure shown in FIG.  1 . FIG. 5 illustrates the presence of “frown” features  501   a ,  501   b  and  501   c  in structure according to the invention. (The frown features  501   a ,  501   b  and  501   c  need not necessarily be present in a structure according to the invention.) Each of the frown features  501   a ,  501   b  and  501   c  is a bend of a corner of the bend  208   a  of the chassis  208 , the bend being toward the secondary feed path (i.e., into the plane of FIG. 5, or downward). The downward bend of the frown features  501   a ,  501   b  and  501   c  locally deforms the print medium along the longitudinal edges of the print medium, helping to eliminate some of the “waviness” (as viewed in the plane of the print medium in the direction of the feed path) that is typically present in a print medium that may otherwise create a problem in accurately feeding the print medium along the secondary feed path  207  into the print drive mechanism. The frown features  501   a  and  501   c  can be used to provide such deformation in print media having a first particular width (e.g., letter size paper), while the frown features  501   b  and  501   c  can be used to provide such deformation in print media having a second particular width that is smaller than the first width. 
     FIG. 6A is a perspective view of the apparatus shown in FIG. 5, viewed from underneath the apparatus. As described above, the bend  208   a  (FIGS. 2,  3  and  4 ) of the chassis  208  imparts a force to the sheet as the sheet is fed through the secondary feed path  207 . In particular, this force generally produces an increase in the functional force resisting motion of the sheet through the secondary feed path  207 . Thus, ribs  601   a  through  601   f  can be formed on the bend  208   a  to reduce the area of contact between the chassis  208  and the sheet, thereby reducing the frictional force opposing motion of the sheet. More generally, the invention contemplates the formation of any other appropriate structure as part of, or attached to, the bend  208   a  that reduces the contact area between the chassis  208  and the sheet. FIG. 6B is a detailed view of a portion of the perspective view of FIG. 6A, better illustrating the ribs  601   a  and  601   b  formed on the bend  208   a.    
     The above-described structure according to the invention can enable a sheet of a print medium to traverse the relatively lengthy unsupported area of the print zone  203 . Returning to FIG. 3, the horizontal (i.e., parallel to the surface  208   b  of the chassis) distance  304  from the edge  202   a  of the print drive feed guide  202  nearest the print zone  203  to the end of the bend  208   a  adjacent the print zone  203  is about 0.9 inches. The horizontal distance  305  from the edge  202   a  of the print drive feed guide  202  to the end  209   h  of the planar portion  209   a  of the shim  209  nearest the bend  208   a  is about 1.5 inches. In the position shown in FIG. 3, the vertical (i.e., perpendicular to the surface  208   b  of the chassis) distance  306  from the end of the bend  208   a  adjacent the print zone  203  to the nearest part of the media contact surface  210   a  of the media guide  210  is about 0.5 inches. In general, similar structure according to the invention can be used to enable a print medium to traverse a gap in any path in a printer through which the print medium travels. The magnitude of such gaps can be, for example, greater than about 0.25 inches. With appropriate implementation of the principles of the invention, gaps greater than about 0.5 inches or greater than about 1.0 inches can be traversed. In particular implementations of the invention, the gaps that can be traversed can be up to, for example, about 1.5 inches or about 2.0 inches. 
     FIGS. 7A and 7B are each perspective views of a portion of the apparatus shown in FIG. 1, viewed from underneath the apparatus and in the direction of the section line B—B of FIG. 1, illustrating a print medium insertion sensing system according to the invention. FIG. 7A illustrates the sensing system when insertion of a sheet of a print medium has not been sensed, while FIG. 7B illustrates the sensing system when insertion of a sheet of a print medium has been sensed. FIGS. 8A and 8B are detailed views of a portion of the perspective views of FIGS. 7A and 7B, respectively, illustrating a trigger flag  703  of the print medium insertion sensing system in more detail. FIGS. 9A and 9B are cross-sectional views, viewed in the direction of the section line C—C of FIGS. 7A and 7B, of a trigger structure  702  of a print medium insertion sensing system according to the invention. FIG. 9A illustrates the trigger structure  702  when insertion of a sheet of a print medium has not been sensed. FIG. 9B illustrates the operation of a mechanism for avoiding self-locking of the trigger structure  702  when a sheet of a print medium is removed from the secondary feed path. 
     A print media edge detection sensor  701  (best shown in FIGS. 8A and 8B) is positioned on the print carriage  101 . The sensor  701  can be embodied by any appropriate sensing device, such as, for example, a reflective-type sensor that emits a beam and measures the intensity of any reflected beam. At the beginning of a printing operation (during either the standard or secondary mode of operation of the printer), the sensor  701  senses when the leading edge of a print medium passes underneath the sensor  701 . As is understood by those skilled in the art, this information is used to control movement of the print carriage  101  and advancement of the print medium so that printing is begun on the print medium at an appropriate location. As will be better appreciated from the further description below, the print medium insertion sensing system according to the invention makes use of the print media edge detection sensor  701 , which is already provided in the printer to enable printing operations, to enable sensing of insertion of a print medium into the secondary feed path  207  (FIG. 2) during the secondary mode of operation of the printer. 
     The trigger structure  702  (indicated generally in FIGS. 7A and 7B, and shown more completely in FIGS. 9A and 9B, described below) is rotatably supported in a trigger housing  209 b that is formed as part of the shim  209 , as described in more detail below with respect to FIG.  9 A. The trigger structure  702  includes a print media contact portion  702   a  that is moved in response to insertion of a sheet of a print medium into the secondary feed path slot (designated by the numeral  704  in FIGS. 7A,  7 B,  9 A and  9 B, and described above with respect to FIG.  2 ,), as described further below. The trigger structure  702  is, in turn, rotated as a result of movement of the print media contact portion  702   a  (as described further below with respect to FIG.  9 A). A trigger flag  703  is attached to the trigger structure  702  (as described in more detail below with respect to FIG. 9A) and supported by a trigger flag support portion  209   c  of a trigger flag housing  209   d  that is formed as part of the shim  209 . The trigger flag  703  moves over the trigger flag support portion  209   c  of the trigger flag housing  209   d  in response to rotation of the trigger structure  702 . The trigger flag  703  and the trigger structure  702  must have a mass low enough (in combination with the distribution of the mass about the axis of rotation of the trigger structure  702 ) to enable insertion of a sheet of a print medium into the secondary feed path slot  704  to move the trigger flag  703  and trigger structure  702 . Illustratively, the combined mass of the trigger flag  703  and the print media contact portion  702   a  can be between about 0.1 grams and 0.25 grams. 
     If a sheet of a print medium has not been inserted into the secondary feed path slot  704 , the print media contact portion  702   a  of the trigger structure  702  is positioned as shown in FIG.  7 A. This position of the print media contact portion  702   a  of the trigger structure  702  results in a position of the trigger flag  703  in which the trigger flag  703  is withdrawn into the trigger flag housing  209   d , as shown in FIG.  7 A. This position is shown in more detail in FIG.  8 A. The trigger flag housing  209   d  helps to prevent contaminants (in particular, aerosol from the print cartridges) from collecting on the trigger flag  703 . This is particularly advantageous when the sensor  701  is a reflective-type sensor, since the collection of contaminants on the trigger flag  703  may eventually degrade the reflectivity of the trigger flag  703  so that the trigger flag  703  does not produce a reflection that is strong enough to be detected by the sensor  701 . 
     When a sheet of a print medium is inserted into the secondary feed path slot  704 , the sheet contacts the print media contact portion  702   a  of the trigger structure  702 . As the sheet moves further into the secondary feed path slot  704 , the sheet pushes the print media contact portion  702   a , causing the trigger structure  702  to rotate, which, in turn, moves the trigger flag  703  so that the trigger flag  703  extends outside of the trigger flag housing  209   d , as shown in FIG.  7 B. This position is shown in more detail in FIG.  8 B. At a time when a sheet of a print medium is inserted into the secondary feed path  207  (in accordance with correct operation of the printer), the print carriage  101  is positioned in a “park” position (i.e., a predetermined position of the print carriage when the print cartridges are not printing and are not being serviced) as shown in FIGS. 7A and 7B. The trigger structure  702 , trigger flag housing  209   d  and trigger flag  703  are positioned so that, when the trigger flag  703  extends out of the trigger flag housing  209   d , the trigger flag  703  passes underneath the sensor  701 . The sensor  701  is operated so that the sensor  701  is active (i.e., capable of sensing an object of the type that the sensor  701  is adapted to sense when such object is within the field of detection of the sensor  701 ) at times when a sheet of print medium can be (properly) inserted into the secondary feed path  207 . (This can be accomplished, for example, by making the sensor  701  always active, which is desirable, too, in view of the fact that the sensor  701  is also used to sense the leading edge of a print medium at the beginning of a printing operation). Thus, when the trigger flag  703  extends outside of the trigger housing  209   b , the sensor  701  senses this event, and transmits a signal to the printer control system indicating that a sheet of a print medium has been inserted into the secondary feed path  207 . It is important to note that this sensing signal from the sensor  701  indicating the presence of a sheet of a print medium in the secondary feed path  207  can be distinguished from a sensing signal produced by the detection of a sheet of a print medium during a printing operation because those two events occur (during proper operation of the printer) during distinct and non-overlapping periods of operation of the printer, e.g., a sheet of a print medium cannot permissibly be fed into the secondary feed path  207  during a printing operation (though such could, of course, occur if the printer is being used improperly). 
     FIG. 9A is a cross-sectional view, viewed in the direction of the section line C—C of FIGS. 7A and 7B, of the trigger structure  702 , illustrating the operation and construction of the trigger structure  702  in more detail. The trigger structure  702  includes a shaft portion  702   b  having multiple sections of different diameters, the aforementioned print media contact portion  702   a , and a trigger flag retention port on  702   c.    
     One end of the shaft portion  702   b  is rotatably held in place by opposed retention arms  209   e  formed on the trigger housing  209   b . The retention arms  209   e  are shaped and sized, and the diameter of the corresponding section of the shaft portion  702   b  is sized, so that the shaft portion  702   b  can be snapped into place by pushing the shaft portion  702   b  between the retention arms  209   e . The opposite end of the shaft portion  702   b  is fitted into a slot (not visible in FIG. 9A) formed in the trigger housing  209   b , the width of the slot and the diameter of the corresponding section of the shaft portion  702   b  being established so that the section of the shaft portion  702   b  is held in place along an axis perpendicular to the plane of FIG. 9A while allowing rotation of the shaft portion  702   b  within the slot. Normally, as explained further below, the end of the shaft portion  702   b  fits against one end of the slot so that the trigger structure  702  is positioned as shown in FIG.  9 A. 
     An opening is formed in the trigger housing  209   b  to allow the print media contact portion  702   a  to extend into the secondary feed path slot  704 . 
     A knob  702   f  is formed at the free end of the trigger flag retention portion  702   c . A flange  702   d  is formed on the trigger flag retention portion  702   c  a short distance from the knob  702   f . A hole (not readily visible in the drawings) is formed in the trigger flag  703 , the hole being made sufficiently large to enable the section  702   e  of the trigger flag retention portion  702   c  between the flange  702   d  and knob  702   f  to fit through the hole, but sufficiently small so that neither the flange  702   d  nor the knob  702   f  can fit through the hole. The flange  702   d  and knob  702   f  thus hold the trigger flag  703  in place on the trigger structure  702 . 
     A slit  703   a  is formed in the trigger flag  703  adjacent to the hole to enable an opening to be formed in the trigger flag  703  that allows the trigger flag  703  to be fitted over the knob  702   f  so that the trigger flag  703  can be attached to the trigger structure  702 . 
     The trigger flag retention portion  702   c  is formed with a bend that enables the trigger flag retention portion  702   c  to fit around an edge of the chassis  208  when insertion of a sheet of a print medium into the secondary feed path slot  704  causes the trigger structure  702  to rotate such that the trigger flag retention portion  702   c  moves in a direction out of the plane of FIG.  9 A. 
     A spring retention block  702   g  having a small hole formed therethrough is formed as part of the trigger flag retention portion  702   c  near the shaft portion  702   b  of the trigger structure  702 . A similar spring retention block  209   f  having a small hole formed therethrough is formed as part of the trigger housing  209   b . A spring  901  is fitted over the shaft portion  702   b  of the trigger structure  702 , the spring  901  having extending portions at either end that fit through the holes in the spring retention blocks  702   g  and  209   f  of the trigger flag retention portion  702   c  and the trigger housing  209   b , respectively. The extending portions of the spring  901  are formed so that, when the trigger structure  702  is positioned as shown in FIG. 9A, the spring  901  is slightly deformed, thereby providing a force that tends to hold the trigger structure  702  in that position (contact between the trigger flag  703  and the trigger housing  209   b  prevents the trigger structure  702  from rotating further). This spring force increases as the trigger structure  702  is rotated-when a sheet of a print medium is inserted in the secondary feed path  207  (as shown in FIG.  7 B), so that, after the sheet has passed entirely beyond the print media contact portion  702   a , the trigger structure  702  rotates back to the position shown in FIG. 9A (and FIG.  7 B). The spring constant of the spring  901  and the geometry of the relevant parts of the structure that determine the amount of displacement of the spring  901  are established so that the spring  901  produces a nominal range of spring forces between about 0.25 grams force and about 0.5 grams force. 
     FIG. 9B is a cross-sectional view, viewed in the same direction as FIG. 9A, of the trigger structure  702 , illustrating the operation of a mechanism for avoiding self-locking of the trigger structure  702  when a sheet of a print medium is removed from the secondary feed path  207  (FIG.  2 ). A V-shaped spring  902  (“anti-lock spring”) is fitted around a spring retention feature  209   g  formed on a wall of the trigger housing  209   b . (The anti-lock spring  902  is not shown in FIGS. 7A and 7B.) One end of the anti-lock spring  902  is positioned within a recess formed by a lip that is part of the trigger housing  209   b , and is held in place by the lip. The other end of the anti-lock spring  902  is positioned against the shaft portion  702   b  of the trigger structure  702  and within the slot formed in the trigger housing  209   b . As will be more readily apparent from the description below, the anti-lock spring  902  provides a compliant support for the trigger structure  702  that allows the trigger structure  702  to move when a print medium is withdrawn from the secondary feed path  207  so that the print media contact portion  702   a  of the trigger structure  702  does not mark the print medium as the print medium is withdrawn. 
     When a sheet of a print medium is removed from the secondary feed path slot  704  after having been inserted in the slot  704 , contact between the sheet and the print media contact portion  702   a  can cause the trigger structure  702  to be rotated in a direction opposite that in which the trigger structure  702  is rotated when the sheet is inserted into the slot  704 . As the trigger structure  702  rotates in this direction, the print media contact portion  702   a  begins to dig into the sheet. The rotation is stopped by contact between the trigger flag  703  and the trigger housing  209   b  at a position at which there is a strong likelihood, absent the anti-locking mechanism according to the invention, of marking of the sheet as the sheet continues to be removed from the slot  704 . 
     In particular, the above-described rotation of the trigger structure  702  can occur when the print media contact portion  702   a  makes a relatively large angle (e.g., greater than about 35°) with the plane of a sheet of a print medium (0° being defined to be the position of the trigger structure  702  at which the print media contact portion  702   a is parallel to the plane of the sheet) when the trigger structure  702  has been rotated as a result of insertion of the sheet into the secondary feed path  207 . The magnitude of this angle is a function of the length of the print media contact portion  702   a : the shorter the length of the print media contact portion  702   a , the greater the magnitude of the angle. Thus, when the print media contact portion  702   a  must be made relatively short (perhaps because the print media contact portion  702   a  must fit within a small space, as is the case in the structure shown in FIGS. 7A and 7B, in which the print media contact portion  702   a  cannot extend beyond the shim  209 ), a sheet of a print medium is particularly susceptible to being marked by the print media contact portion  702   a  when being removed from the secondary feed path  207  after having been inserted. (In the structure shown in FIGS. 7A and 7B, the magnitude of the angle between the print media contact portion  702   a  and the plane of an inserted sheet is about 55°.) However, in accordance with this aspect of the invention, since the end of the trigger structure  702  near the print media contact portion  702   a  is free to move within the slot formed in the trigger housing  209   b , the trigger structure  702  can rotate, as shown in FIG. 9B, about an axis perpendicular to the plane of FIG. 9B, so that the print media contact portion  702   a  can move away from the sheet of the print medium as force begins to build up between the two, thus avoiding marking of the sheet. 
     The anti-lock spring  902  has an unstressed shape such that when the trigger structure  702  is positioned as shown in FIG. 9A, the anti-lock spring  902  is slightly deformed, thereby providing a force that tends to hold the trigger structure  702  in that position (contact between the ends of the shaft portion  702   b  of the trigger structure  702  and the retention arms  209   e  and slot wall, respectively, prevents the trigger structure  702  from rotating further). This spring force increases as the trigger structure  702  is rotated when a sheet of a print medium is withdrawn from the secondary feed path  207  and causes the trigger structure  702  to rotate as shown in FIG. 9B, so that, after the sheet has been entirely withdrawn from the secondary feed path  207 , the trigger structure  702  rotates back to the position shown in FIG.  9 A. The spring constant of the anti-lock spring  902  and the geometry of the relevant parts of the structure that determine the amount of displacement of the spring are established so that the anti-lock spring  902  produces a nominal range of spring forces between about 10 grams force and about 20 grams force. 
     The retention arms  209   e  are formed so that the opening formed between the retention arms  209   e  is sufficiently large to provide space to enable the end of the shaft portion  702   b  of the trigger structure  702  retained by the retention arms  209   e  to rotate as shown in FIG.  9 B. Similarly, the opening in the trigger housing  209   b  through which the print media contact portion  702   a  extends is made sufficiently large to enable the print media contact portion  702   a  to rotate as shown in FIG.  9 B. 
     Various embodiments of the invention have been described. The descriptions are intended to be illustrative, not limitative. Thus, it will be apparent to one skilled in the art that certain modifications may be made to the invention as described above without departing from the scope of the claims set out below.