Abstract:
A sensing method and system to protect printer print heads from substrate media contact comprising a trip wire sensor to detect and signal a printer control system that substrate media carried by transport media is positioned to strike or contact the print head. The trip wire sensor is located upstream of the print heads, a controlled distance above the transport media in the direction normal to the plane of the media, and comprises a trip wire operatively connected to at least one transducer. Substrate media exceeding the height requirements associated with the print heads contact the trip wire sensor causing the generation of an electrical signal which is received by the printer control system which then takes corrective action to prevent damage to the print heads, for example, by raising the print heads further away from the transport media and allowing the non-conforming substrate media to pass through and be purged. Also disclosed is a protective guide positioned to constrain substrate media from going over the top of the trip wire which can result in the substrate media becoming jammed in the trip wire sensor.

Description:
BACKGROUND 
       [0001]    1. Field of the Disclosure 
         [0002]    The present disclosure relates to methods of document creation. More specifically, the present disclosure is directed to an apparatus and method for printing in which the printer&#39;s print heads are protected from substrate media contact. 
         [0003]    2. Brief Discussion of Related Art 
         [0004]    In certain printers using ink jet direct marking technology, it is expected that marking inks, e.g., solid inks, UV gel inks, aqueous inks and others, will be jetted directly onto cut sheet substrate media. A critical parameter in this printing process is the size of the print head to media gap. In certain current technology, the gap is set as small as 0.5 mm in order to minimize the pixel placement errors due to misdirected jets. For other print heads, for example those having high drop velocity, it is possible that the gap can be opened to 0.75-1.0 mm. Nevertheless, these tight print head to media gaps pose a challenge for any cut sheet media substrate media printer, since the sheet lead edge (LE) and trail edge (TE), and to a lesser extent the sheet body are generally not perfectly flat. 
         [0005]    For accurate pixel placement and color registration, it is desired to keep the print head to substrate media gap within a +/−0.1 mm range about the nominal. To avoid print head front face damage, the substrate media should not be allowed to “close the gap”, i.e., to contact the print head(s). Both vacuum escort belt and/or electrostatic tack escort belt technology are technologies which may be employed to hold cut sheets of substrate media sufficiently flat. However, neither technology is completely robust against LE and TE upcurl defects. 
         [0006]    One method of addressing the problem of upcurl defects is to provide the cut sheet printer with a pre-curler subsystem which biases sheets into a flat or down-curl configuration. However, certain initial sheet substrate media non-uniform conditions, such as corner curl, edge wave, and cockle, can be difficult to detect and fully compensate for within the precurler. Hence, sheets may not be held sufficiently flat in the print zone, to the extent that the print head(s) may be damaged. 
       SUMMARY 
       [0007]    The present disclosures are directed to printers having a first media transport operative to receive a substrate media, to convey the substrate media towards, into, through, out of or away from print head(s) in the marking zone, and methods and apparatus to detect and take corrective action to avoid non-conforming substrate from damaging the prints head(s). 
         [0008]    A sensing method and system is disclosed comprising a trip wire sensor to detect and signal a printer control system that non-conforming substrate media carried by the transport media is positioned to strike or contact the print heads. The trip wire sensor is located upstream of the print heads, a controlled distance above the transport media in the direction normal to the plane of the media, and comprises a trip wire operatively connected to at least one transducer. Trip wire height adjusters may be provided to raise and lower the sensor as a function of expected media thickness. Substrate media exceeding the height requirements associated with the print heads contact the trip wire sensor causing the generation of an electrical signal which is received by the printer control system which then takes corrective action to prevent damage to the print heads, for example, by raising the print heads further away from the transport media and allowing the non-conforming substrate media to pass through and be purged. A sufficiently sized reaction zone is provided between the trip wire sensor and the print heads to afford the printer control system enough time to effectuate corrective action as the non-conforming substrate media moves from the trip wire sensor toward the print heads. 
         [0009]    Also disclosed is a protective guide positioned to constrain substrate media from going over the top of the trip wire which can result in the substrate media becoming jammed in the trip wire sensor. Embodiments of the protective guide provide that its lower surface is positioned flush or slightly above the bottom of the trip wire. The protective guide or constraining baffle gives stiffness to the substrate media sheet which is also constrained by the height of the protective guide to contact the underside of the trip wire, all to avoid jamming. 
         [0010]    The marking zone is an area associated with the printer in which the substrate media is marked with an image. Generally, though not exclusively, the first media transport comprises a belt routed over a plurality of rollers, the belt being moved and moveable under the influence of a motive force applied to at least one roller among the plurality of rollers. 
         [0011]    A hold-down system of the printer and/or first media transport, optionally a vacuum powered hold-down system, an electrostatic hold-down system, or a combination of the two, generates a hold-down pressure applied to the substrate media in the direction of the first media transport. A pre-curler unit is operative to apply a predetermined degree of curl to the substrate media. A trip wire sensor signals the control system if the substrate media height above the first media transport exceeds a predetermined height. Generally, the predetermined height is such that the substrate media will not touch the print head(s) or otherwise approach the print head(s) too closely. If the trip wire sensor signals the printer control system that the substrate media has hit it, the printer control system in turn can take corrective action to prevent print head damage. The corrective action could comprise the printer control system signaling for the stopping or slowing the media transport while increasing the gap between the media transport and the print head(s) and allowing the non-conforming sheet substrate media to pass through without touching the print head(s) and, if necessary, be purged. The corrective action could also comprise the printer control system signaling for the stopping and reversing the travel direction of the first media transport in order to re-route the hitting substrate media back to an upstream purge or other location. 
         [0012]    A print head array, comprising at least one print head, marks the substrate media with an image in the marking zone, and an actuator adjusts the relative spacing between the print head array and the first media transport. The actuator may include at least one of a linear and rotary actuator, powered by at least one of a fluid or electric motive power, and be configured to move at least one or both of the print head array and the first media transport to alter the distance between the two. 
         [0013]    The actuator is operative to set the gap between the media transport and the print head array at a first relative spacing at which marking of the substrate media may selectively occur, based in part on the expected substrate media thickness and hence height above the first media transport, and at least one second relative spacing greater than the first spacing to permit substrate media exhibiting a greater than expected height above the first media transport. The printer control system can signal the actuator to set the gap in accordance with the trip wire sensor signals. 
         [0014]    The printer may optionally include a media feeding unit for supplying substrate media to the printer, the media feeding unit having a plurality of selectable trays from which substrate media is selectably sourced. A precurl may apply a selectable degree of precurl to the substrate media. 
         [0015]    The method optionally includes tacking the substrate media to the first media transport, as an illustrative example only by pressing the substrate media against the first media transport with a roller. 
         [0016]    These and other purposes, goals and advantages of the present disclosure will become apparent from the following detailed description of example embodiments read in connection with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    Some embodiments are illustrated by way of example and not limitation in the figures of the accompanying drawings, in which like reference numerals refer to like structures across the several views, and wherein: 
           [0018]      FIG. 1  illustrates schematically a printer with print head array for marking an image on substrate media. 
           [0019]      FIG. 2  illustrates schematically a print head array and marking zone transport. 
           [0020]      FIG. 3  illustrates the plan view schematic of the substrate media marking zone transport of the printer. 
           [0021]      FIG. 4  illustrates a partial sectional view of the substrate media marking zone transport of the printer through section line  4  of  FIG. 3 . 
           [0022]      FIG. 5  illustrates the transducer output of one embodiment using copper wire for the trip wire. 
           [0023]      FIG. 6  illustrates the transducer output using of one embodiment using fiberglass line for the trip wire. 
           [0024]      FIG. 7  illustrates schematically an embodiment of a trip wire protective guide. 
           [0025]      FIG. 8  illustrates schematically another embodiment of a trip wire protective guide. 
           [0026]      FIG. 9  depicts a flowchart showing an exemplary mode of operation according to the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
     Introduction 
       [0027]    As used herein, a “printer” refers to any device, machine, apparatus, and the like, for forming images on substrate media using ink, toner, and the like. A “printer” can encompass any apparatus, such as a copier, bookmaking machine, facsimile machine, multi-function machine, etc., which performs a print outputting function for any purpose. Where a monochrome printer is described, it will be appreciated that the disclosure can encompass a printing system that uses more than one color (e.g., red, blue, green, black, cyan, magenta, yellow, clear, etc.) ink or toner to form a multiple-color image on a substrate media. 
         [0028]    As used herein, “substrate media” refers to a tangible medium, such as paper, transparencies, parchment, film, fabric, plastic, vellum, paperboard or other substrates on which an image can be printed or disposed. 
         [0029]    As used herein “process path” refers to a path traversed by substrate media through a printer to be printed upon by the printer on one or both sides of the substrate media. Substrate media moving along the process path away from its beginning and towards its end will be said to be moving in the “process direction”. 
         [0030]    As used herein, “transport” when used as a noun, “media transport” or “transport apparatus”, each and all refer to a mechanical device operative to convey a substrate media through a printer. 
         [0031]    As used herein, a “trip wire sensor” refers to a tensioned wire or other body operatively connected to one or more transducers. Transducers, as used herein, are preferably based upon piezo-electric, micro-electromechanical system (MEMS), strain gauge, or similar technology capable of converting a mechanical force or displacement into an electric signal. 
         [0032]    As used herein, “upcurl”, is substrate media curvature towards the print head, in other words curl around a radius centered on the side of a cut sheet substrate media in the same direction as the print head. 
         [0033]    As used herein, “downcurl” is curvature in the substrate media around a radius centered on the side of the cut sheet away from the print head. 
         [0034]    As used herein, “print head array” refers to at least one print head or multiple print heads for printing or disposing images on substrate media. 
         [0035]    As used herein, an “actuator” refers to a device operative to move at least one or both of the print head array and the first media transport so as to adjust the relative spacing between the print head array and the first media transport. 
       Description 
       [0036]    Referring now to  FIG. 1 , illustrated is a printer, generally  10 , according to a first embodiment of the present disclosure. The printer  10  may include a media feeding unit  12  in which one or more types of substrate media  15  may be stored and from which the substrate media  15  may be fed, for example sheet-by-sheet feeding of a cut sheet medium, to be marked with an image. The media feeding unit  12  delivers substrate media  15 , for example from one or more media trays  13 , to a marking unit  14  to be marked with a document image. The marking unit delivers marked substrate media  15  to an interface module (not shown) which may, for example, prepare the substrate for a finishing operation. Optionally the printer  10  may include a finishing unit (not shown), which receives printed documents from the interface module. The finishing unit, for example, finishes the documents by stacking, sorting, collating, stapling, hole-punching, or the like. 
         [0037]    Marking unit  14  includes a marking zone, generally  20 , within the marking unit  14 . A marking zone  20  encompasses a marking engine, in this example an ink jet marking engine, having one or more print heads  22   a,    22   b,  etc., collectively print head array  22 , any of which are operative to directly mark the substrate media  15  and thereby form an image on the substrate media  15 . Ink jet print head configuration is not the exclusive marking engine, and is offered as an example only. The ink jet print heads  22   a,    22   b,  etc. may draw ink from respective reservoirs  24   a,    24   b,  etc., or in some instances a collective reservoir (not shown). A marking zone transport  26  is operative to hold a substrate media  15  to itself securely, for example by electrostatic means or vacuum means, without limitation. In other embodiments, the marking engine may comprise any technology for printing, marking images or document creation in which a controlled gap must be maintained between the marking member and the surface of the substrate media  15 . 
         [0038]    The marking zone transport  26  is further operative to receive a substrate media  15  delivered towards the marking zone  20  and to convey the substrate media  15  towards, into, through, out of, and/or away from the marking zone  20 , with positive control of the motion of the substrate media  15 . The marking zone transport  26  maintains the substrate media  15  within the marking zone  20  in sufficient proximity to the print head array  22  to permit print heads  22   a,    22   b,  etc. to mark the substrate media  15 , but is designed and operated to avoid any contact between the substrate media  15  and the print head array  22 . Contact between the substrate media  15  and the print head array  22  is to be avoided to negate the possibility of damage to the precise size and shape of the ink jet openings in the print head array, or to any coatings applied thereto, for example those which may facilitate precise ink particle/droplet formation. Such damage may be caused by impact or abrasion due to contact with the substrate media  15 . Contact between the substrate media  15  and the print head array  22  may also be the cause of media jams leading to unscheduled stoppage of printing, wasting media and ink, requiring attention to service the error, and generally leading to customer dissatisfaction. 
         [0039]    The marking zone transport  26  is configured and operative to pass the substrate media  15  to a downstream transport  30  for further handling. As example only, the downstream transport  30  includes a leveler transport, whose function is to bring all jetted ink to the same elevated temperature. The downstream transport  30  receives the substrate media  15  from the marking zone transport  26  and delivers the substrate media  15  to be subjected to a post-marking process  32 , including without limitation ultra-violet light curing, fusing, spreading, drying, etc., any or some combination of which may be included without departing from the scope of the instant disclosure. In certain embodiments, the post-marking process includes a spreader nip  32 , where the ink is spread under high pressure and elevated temperature to its final film thickness on the media. The post-marking process  32  may of course be omitted, if desired. 
         [0040]    Included in the marking unit  14  are a curl sensor  33  and pre-curler unit  34 , preferably upstream in the process path of the marking zone transport  26 . The pre-curler unit  34  is operative to apply a selectable degree of pre-curl to the substrate media  15 . In particular, a degree of curl in the substrate media  15  is detected by the curl sensor. The pre-curler unit  34  receives output from the curl sensor  33  in setting a desired degree of pre-curler. Also included in the marking unit  14  is a duplex path  36 , operative to selectively return printed cut sheet documents to the print zone, for example to be imaged in duplex, i.e., on a reverse side thereof. A document inverter  38 , operative to invert the orientation of the cut sheet substrate media  15  to facilitate printing on the reverse side thereof, may be located in the process path upstream of the diversion point for the duplex imaging path  36 . 
         [0041]    Referring now to  FIG. 2 , illustrated schematically is a print head array  22  and marking zone transport  26  in closer detail. Marking zone transport  26  includes an endless belt  40  in a path around rollers including  42 ,  44 ,  46  and  54   b.  In this case, roller  42  serves as a drive roller, roller  44  a tensioning roller, roller  46  a steering roller, and roller  54   b  as an idler roller. Other configurations will be seen as within the scope of the present disclosure to one skilled in the art. A marking zone transport drive unit  48  controls the motion of the drive roller  42  by commanding a motor (not shown) operatively connected with the drive roller  42 . The transport drive unit  48  can adjust the speed and/or direction of the first media transport. 
         [0042]    Substrate media hold-down systems are operative in the marking zone  20  and provide means for drawing substrate media  15  toward the endless belt  40 . Hold-down systems may comprise a vacuum powered hold-down system, an electrostatic hold-down system, or a combination of the two. 
         [0043]    The endless belt  40  in certain embodiments is air-permeable, and platen  50  may include a vacuum hold-down manifold  52  positioned beneath the endless belt  40 , including where the endless belt  40  passes beneath the print head array  22 . As described, the endless belt  40  lies at least in part between the vacuum hold-down manifold  52  and the print head array  22 . The vacuum hold-down manifold  52  introduces a negative atmospheric pressure at its top surface, which in turn draws air through the air-permeable endless belt  40 . Substrate media  15  lying on the endless belt  40  is therefore drawn against endless belt  40  by the air flow which passes through the endless belt  40  and the vacuum hold-down manifold  52 , and also by the air pressure differential between opposing sides of the substrate media  15  under the operation of the vacuum hold-down manifold  52 . The vacuum hold-down manifold  52  is in fluid communication with a source of negative vacuum air pressure via a vacuum line (not shown). Flow through the vacuum line may be optionally controlled or varied, for example by provision of a flow control valve, pressure regulator, or the like. Alternately, the vacuum source may itself be configured to provide variable vacuum pressure. 
         [0044]    Alternately, or in addition, to the vacuum hold-down means described above, the marking zone transport may be provided with an electrostatic hold-down means. In one embodiment, an electrostatic charge is applied onto the upper surface of sheet  15  while an opposite polarity electrostatic charge is deposited onto the lower surface of belt  40 . The opposite charges are attracted to each other and a tack pressure is developed between sheet  15  and belt  40 . 
         [0045]    Further illustrated in  FIG. 2  is a tacking nip  54 , in this case a pair of tacking rolls  54   a,    54   b  with one roll of the pair each above and beneath, respectively, the endless belt  40 . In operation, substrate media  15  is delivered to the tacking roll  54  adjacent the endless belt  40 . The tacking roll  54  presses the substrate media  15  towards the endless belt  40  in tacking zone  55 , in order to initiate and/or assist the hold-down pressure applied by the marking zone transport  26  and to “tack” the substrate media  15  to the endless belt  40 . Tacking rolls  54   a  and  54   b  may be electrically biased so as to apply the electrostatic charges to the substrate media  15  and/or the surface of the endless belt  40  as previously described 
         [0046]    The trip wire sensor assembly  56  is operatively associated with the marking zone transport  26  and the printer control system (not shown). The printer control system is operatively connected to the actuator  60 . 
         [0047]    The marking zone transport  26  and/or the print head array  22  may each be mounted by, on or to a frame or chassis portion of the marking unit  14 . Furthermore, the print head array  22  may be mounted in order to permit it to adjust position with respect to the marking zone transport  26 . The adjustment can be controlled, for example, by an actuator  60 . The gap between a print head array  22  and the substrate media  15  is preferably variable between at least a nominal operating gap at which printing may occur and a second greater gap for a passing non-conforming substrate media  15  through the marking zone  20  without damaging the print head array  22 . 
         [0048]    Actuator  60  may be driven electrically, or by fluid power, and may be linear and/or vertical, as in the embodiment shown, or also rotary in nature (rack-and-pinion, rotary levers, etc.). The actuator  60  may also include an encoder (not shown) to provide feedback concerning the position of the print head array  22 . Alternately or additionally the marking zone transport, and/or at least the platen portion thereof that underlies the print head array, may be mounted for adjustable motion with respect to the print head array. Here again, the actuation may be driven by a variety of motive power sources, and/or in either a linear or rotary fashion, and optionally be associated with some form or positional feedback indication, e.g., an encoder. 
         [0049]      FIG. 3  illustrates a plan view schematic of the marking zone transport. Sheets  15  enter from the left and are acquired onto the marking zone transport belt at the tacking nip  54 . Each sheet then passes by the trip wire sensor assembly  56 , the reaction zone  100 , and then the print head array  22  at which point the print heads mark the substrate with an image. 
         [0050]      FIG. 4  illustrates a sectional view of the marking zone transport through section line  4  of  FIG. 3 , where the tacking roll has been removed for clarity. The trip wire sensor assembly  56  comprises two transducers  110  with a tensioned line, the trip wire  120  spanning between them. Transducers  110  are preferably based upon piezo-electric, micro-electromechanical system (MEMS), strain gauge, or similar technology capable of converting a mechanical force or displacement into an electric signal. Each transducer  110  may be mounted adjustably  130  so that the height of the tensioned line, the trip wire  56  above the transport belt  40 , that is, the sensing gap  180  can be varied as a function of expected media  15  thickness. The height adjustment mechanism  130  could be via cam or fine pitch screw. In general, the sensing gap  180  will be kept equal or less than the print head-to-transport belt gap, which may also be adjustable based on expected substrate media  15  thickness. 
         [0051]    One embodiment of a trip wire sensor assembly was separately tested using two standard piezo-electric buzzers as the transducers. Each consists of a plastic housing supporting a thin metal disk to which a thin piezo ceramic disk is bonded. Each disk assembly has a through hole to which one end of a tensioned line is anchored. Any significant deflection of the line thus causes a tension wave to propagate along the line to each disk. Two different line materials were tested: thirty-one gauge copper wire and twelve pound test fiberglass fishing line. Three different electrical configurations were tested. 
         [0052]    In the simplest (“Dual Passive”) mode, each transducer output was simply monitored for amplitude changes using an oscilloscope.  FIG. 5  shows the resulting transducer output for three different sheet corner ‘strike’ levels using the copper wire. Given the mirror symmetry of the mounted disk assemblies, it is evident that a differential signal can be extracted in this mode, which will provide some common mode noise immunity.  FIG. 6  shows a typical transducer response when using the fiberglass line. 
         [0053]    Two other electrical modes were also tested and the results provided support for their use: “Single Active” mode drives one transducer with a carrier frequency supplied by a signal generator and the output of the passive transducer is measured for amplitude changes; “Closed Loop” mode feeds back the output of the passive transducer to the base of a transistor that drives the active transducer, and excursions in the resonant frequency are then monitored. 
         [0054]    It is expected that the trip wire sensor assembly  56  could normally operate in Dual Passive mode but could periodically do a Single Active mode self-check, for example, to ensure the trip wire line  120  is intact and tensioned. It is also expected that the entire trip wire sensor assembly  56  could be spring loaded in place so that any significant force from a substrate media  15  collision can be limited well below the line&#39;s breaking strength. This action also limits the contact force acting on the substrate media  15  to prevent a lifted substrate media sheet corner from stubbing and jamming on the line. 
         [0055]    Generally the trip wire sensor assembly will produce an analog signal output where the output signal will be approximately proportional to the strike force. Thus, it is possible that a thin substrate media sheet  15  could ‘brush’ the trip wire line  120  and the resulting signal could fall within the normal noise range. However, the implication of a gentle brush also implies that any contact force of the sheet with the print heads will also be gentle and thus of less concern for potential print head damage. 
         [0056]      FIGS. 7 and 8  show schematic embodiments of a protective guide  140  which can be placed at least partially upstream of the sensor. The protective guide is expected to be much stiffer than either the tensioned line  120  or the substrate media  15 . The lower surface of the protective guide  140  is generally positioned flush or slightly above the bottom of the tensioned line  120  along the vertical axis and is placed close to the tensioned line along the horizontal axis as shown by example in  FIGS. 7 and 8 . This arrangement should eliminate or at least minimize the possibility, for example, that a sheet corner or other LE defect will go over the top of the tensioned line, which could result in a paper jam. Instead, the LE defect, for example, will generally be constrained in height by the protective guide  140  and will contact the underside of the tensioned wire  120 . 
         [0057]    The flowchart  200  in  FIG. 9  depicts an exemplary mode of operation according to an embodiment of the present disclosure. Beginning from a start condition  202 , the first media transport receives substrate media  204  and conveys the substrate media in the process path  206  for marking with an image. The trip wire sensor then determines whether or not it has been contacted by substrate media  208 . If the substrate media does not contact the trip wire sensor, the substrate media continues to the print head array  210  where it is marked with an image  212 . If the substrate media contacts the trip wire sensor, the trip wire sensor signals the printer control system  220  and the printer control system signals the actuator  222 . In response to the printer control system&#39;s signal, the actuator increases the gap between the first media transport and print head array  224 . Next, the substrate media is conveyed by the first transport media past the print head array in the increased gap provided by the actuator  226 . Finally, the substrate media may be purged  228 . 
         [0058]    In another embodiment of the present disclosure, the substrate media contacts the trip wire sensor, the trip wire sensor signals the printer control system  220  and the printer control system signals the actuator  222  and the transport drive unit  48 . In response to the printer control system&#39;s signal, the actuator increases the gap between the first media transport and print head array  224 , and the transport drive unit  48  may cause the first media transport to slow its speed in order to increase the available time for increasing the gap. Next, the substrate media is conveyed by the first media transport past the print head array in the increased gap provided by the actuator  226 . Finally, the substrate media may be purged  228 . Alternative system responses also include the transport drive unit  48  stopping and reversing the travel direction of the first media transport in order to re-route contacting substrate media back to an upstream purge or other location. 
         [0059]    Variants of the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Various presently unforeseen or unanticipated alternatives, modifications, variations, or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.