Abstract:
A multi-purpose, transmissive paper sensor includes a light beam projector and light detector having an analog output signal. Changes in the output signal from an open loop condition indicate the presences of at least one print medium being in the field-of-view of the sensor. Output signals indicative of print media leading edge, trailing edge, and number of sheets interrupting the light beam provide improved print media transport control for hard copy apparatus.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to hard copy apparatus, more particularly to print media sensing and, more specifically, to a transmissive optical sensing method and apparatus for print media sheet recognition. 
     2. Description of the Related Art 
     In designing paper path for hard copy apparatus, a designer must address the problem of print media (hereinafter referred to generically as “paper,” regardless of form, e.g., plain paper, special media, envelopes, and the like as would be common to the state of the art) recognition and positioning, multiple sheet picks, and transport to and through the printing zone. It is rare that multiple sheet picks present perfectly registered sheets of paper where a printing error—generally print registration on the page—will not occur. Therefore, when multiple sheets are overlapped, state of the art paper length sensing devices generally indicate a single sheet that is longer than the actual media in the input supply because the commonly used optical detectors or opto-mechanical interrupters inherently do not have the capability to sense the presence of two overlapped sheets of paper since the overlapped region between the two sheets generates the same signal as a single sheet. Exemplary optical media sensing methods and apparatus are shown in U.S. Pat. No. 5,135,321 (Olsen et al.), U.S. Pat. No. 5,466,079 (Quintana) and its divisional, U.S. Pat. No. 5,564,848 (each assigned to the common assignee herein and incorporated by reference in their entireties). 
     Multiple sheet picks effect a throughput loss as any print is likely to be mis-registered, requiring a reprinting. 
     There is a need for a method and apparatus which has the capability to view overlapped print media as being different than a single sheet. Moreover, such an apparatus is useful in the detection of top-of-form and bottom-of-form in the feeding and transport of multi-page printed form sheets are being fed and small overlaps are intentional. Furthermore, such an apparatus is useful in detecting the gap length between tail-gating sheets of paper. 
     SUMMARY OF THE INVENTION 
     In a basic aspect, the present invention provides a print media sensor device, including: mounted for bracketing a print media transport path, at least one emitter for directing a light beam across the transport path, the light beam having predetermined intensity and wavelength for penetrating print media, and aligned with the emitter, receptor mechanisms for receiving the light beam, wherein the receptor mechanisms provides a first output signal indicative of no paper breaking the beam, a second output signal indicative of a single sheet of print media interrupting the beam, and at least one other signal level indicative of multiple sheets of print media interrupting the beam. 
     Another basic aspect of the invention is a method of detecting print media in a print media path, including the steps of: positioning a transmissive light sensor along the print media path; calibrating the sensor for providing a first signal indicative of no print media within the field-of-view of the sensor, a second signal indicative of one sheet of print media being within the field-of-view of the sensor, and at least a third signal indicative of multiple sheets of print media being with the field-of-view of the sensor, wherein a change from the first signal is also indicative of a leading edge position of the print media in the print media path and a change from the second signal to the first sign is indicative of a trailing edge position of the print media in the print media path. 
     Another basic aspect of the present invention is a hard copy apparatus having a predefined paper path including a region upstream of a printing zone of the apparatus, associated with the printing zone, mechanisms for printing on paper, mechanisms for transporting paper from an upstream side of printing zone to an input of the printing zone, and paper position detector and indicator mechanisms, the mechanisms including: aligned with respect to associated with the predefined paper path region, a sensor having at least one light transmitter and at least one light receptor respectively positioned bracketing a predetermined position of the region, wherein the receptor provides signals indicative of conditions within the paper path including a first signal indicative of no print media within the field-of-view of the sensor, a second signal indicative of one sheet of print media being within the field-of-view of the sensor, and at least a third signal indicative of multiple sheets of print media being with the field-of-view of the sensor, and wherein a change from the first signal is also indicative of a leading edge position of the print media in the print media path in the field-of-view and a change from the second signal to the first signal is indicative of a trailing edge position of the print media in the print media path within the field-of-view. 
     Some of the advantages of the present invention are: 
     it provides an substantially immediate detection of a multiple print media sheet pick; 
     it provides leading edge and trailing edge detection; 
     it provides a means for top-of-form and bottom-of-form detection; 
     it provides improvement in media edge detection for full-bleed printing; and 
     it provides for improvements to hard copy apparatus throughput. 
     The foregoing summary and list of advantages is not intended by the inventor to be an inclusive list of all the aspects, objects, advantages and features of the present invention nor should any limitation on the scope of the invention be implied therefrom. This Summary is provided in accordance with the mandate of 37 C.F.R. 1.73 and M.P.E.P. 608.01(d) merely to apprize the public, and more especially those interested in the particular art to which the invention relates, of the nature of the invention in order to be of assistance in aiding ready understanding of the patent in future searches. Other objects, features and advantages of the present invention will become apparent upon consideration of the following explanation and the accompanying drawings, in which like reference 
    
    
     Other objects, features and advantages of the present invention will become apparent upon consideration of the following detailed description and the accompanying drawings, in which like reference designations represent like features throughout the figures. 
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view drawing of a paper transport mechanism for a hard copy apparatus in accordance with the present invention. 
     FIG. 2 is a close-up detail from FIG. 1 highlighting the detector. 
     FIG. 3 is an close-up detail from FIG. 1 in an elevation view, showing a case of two sheets of media and a gap between the trailing edge of one and the leading edge of the other. 
     FIG. 4 is a close-up detail as shown in FIG. 3, showing a case of two sheets of media and an overlap between the trailing edge of one and the leading edge of the other. 
     FIG. 5 is an alternative embodiment of the present invention as shown in FIGS. 1 through 4. 
     FIG. 6 is a graph showing exemplary measurements in accordance with the present invention. 
     FIG. 7 is a flow chart of the operation of the present invention in accordance with FIGS. 1-4. 
     The drawings referred to in this description should be understood as not being drawn to scale except if specifically noted. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference is made now in detail to a specific embodiment of the present invention, which illustrates the best mode presently contemplated by the inventors for practicing the invention. Alternative embodiments are also briefly described as applicable. 
     FIG. 1 depicts an exemplary embodiment paper transport mechanism  101  for a hard copy apparatus. Many such mechanisms are well-known in the art; the implementation shown is for convenience in explaining the present invention which is associated with the paper transport mechanism and no limitation on the scope of the invention is intended by the inventors nor should any be implied from the specifics of this example. The transport mechanism  101  is suitably mounted within the hard copy apparatus adjacent a supply of paper (not shown), such as would be placed by the end user in an input tray. In this mechanism, at least one pick roller  103  proximate the input tray is designed for appropriately picking a sheet and transporting it along a paper path initially determined by a rear paper guide  105 . Assuming the input tray is subjacent the pick roller  103 , rotating the pick roller counter-clockwise transports a picked sheet—or in a double pick, both sheets—such that a first leading edge is driven along the curvilinear surface  107  in the direction indicated by arrow  109 , used hereinafter to generally represent the paper path through the transport mechanism  101 . 
     Following the curvilinear surface  107 , the picked sheet will make a one-hundred and eighty degree turn about the pick roller  103  and then the leading edge will next encounter an upper paper guide  111 . The paper path then assumes a more linear transport mechanism region as the sheet is directed between the upper paper guide  111  and a lower paper guide  113 . As the leading edge of the sheet exits the linear transport region of the paper path, it will be engaged by axle  119  driven feed rollers  115 ,  116 ,  117 ,  118  which will deliver the sheet across a pivot  121  to the printing zone of the hard copy apparatus. A paper sensor  123  is positioned in the linear transport region of the paper path. 
     Turning now to FIG. 2, the sensor  123  device in accordance with the present invention is illustrated in further detail. A paper path bracket  201  has upper arm  203  and lower arm  205  members that extend laterally from an upright  206  with the upper arm  203  extending over the paper path (again indicated by arrows  109 ) and the lower arm  205  extending under the paper path such that at least one side edge of a sheet of paper will pass through the bracket  201  as a sheet is transported along the paper path linear region. In other words, the bracket  201  generally forms a “C” in which the open cavity  209  formed by the arms  203 ,  205  is in the paper path. 
     An optical emitter  207 , such as a light emitting diode (“LED”) is mounted on the upper arm  203  to project a light beam across the paper path. A photo-receptor  211 , having an analog output, is mounted on the lower arm  205  in visual alignment with the emitter  207  for detecting the light beam projected across the paper path. Commercially available LED and photo-receptor elements can be employed in the present invention. These elements can be selected or tailored to any specific implementation. Selection or design is based on the wavelength and intensity of light needed for the various forms of paper used with the printer. The LED has to have an output beam capable of penetrating at least two sheets of the densest media used in the hard copy apparatus. Standard electrical connections  213  are provided for “POWER,” “GROUND,” and “SIGNAL OUT.” 
     As can now be recognized, by using an emitter on one side of a sheet of paper in the paper path paired with a photo-receptor capable of an analog output on the opposite side of the paper, the output of the photodiode will give an indication of the total light that is being transmitted across the paper path. Two sheets of paper in the path simultaneously will theoretically transmit less light than a single sheet of paper which in turn transmits significantly less light than when no sheet is present. In general, the actual levels will be a function of wavelength and intensity of the particular LED subsystem employed in a specific implementation. Thus, at least three distinct output signal levels are detectable, corresponding to the state of 0, 1 or 2 sheets of paper at a given point in the paper path. This output signals are thus indicative of a multiple-pick or, if appropriate to the current print job to measure the relationship of the top-of-form and bottom-of-form positions of intentionally overlapped pages. 
     This is illustrated by the graph of FIG. 6; this data of this graph was empirically generated as an example of operation of the present invention using the invention with HP™ bright white, twenty four pound paper; the implementation shown is for convenience in explaining the present invention which is associated with the paper transport mechanism and no limitation on the scope of the invention is intended by the inventors nor should any be implied from this example. Looking also to FIGS. 3 (showing two separated paper sheets, appropriately picked and traversing the paper path)and  4  (a multi-pick condition), when no paper is in the light beam  301  the full intensity of the light hits the photo-receptor of the sensor  123  device when the gap,“G,” between a first sheet  303  and a second sheet  305  in the paper path  109  passes the sensor. An output voltage—or other state indicator signal, “Signal Out,” FIG. 2, as would be known in the art—is generated by the photo-receptor  211  of the sensor  123 , shown in FIG. 6 as an exemplary, calibrated, two-volt signal, “No Paper.” It has been found that even when the gap, G, is reduced to zero, where the trailing edge  303 ′ of the leading sheet  303  appears to be touched by the leading edge  305 ′ of a trailing sheet  305 , a spike signal Signal Out (FIG. 2) Output Voltage (FIG. 6) generally will be sent by the photo-receptor. Thus the sensor  123  acts as a leading edge-trailing edge detector. 
     As a single sheet passes through the sensor  123 , less light is received at the photo-receptor  211 . In the example, where the open loop condition Signal Out is two volts, the intensity falls into a first range (“1 Sheet of Paper”) of approximately 0.8 to 0.9 volts, or approximately half the open loop, “No Paper,” condition. 
     Turning also to FIG. 4, a multi-pick condition is illustrated. In the exemplary measurement chart of FIG. 6, when two (or more) sheets overlap in the sensor  123  region of the paper path, the Output Voltage drops to a range of approximately 0.2 volts to 0.5 volts. A triple pick would result in a lower Signal Out. 
     Thus the sensor acts as a multi-pick detector. Rather than printing downstream of the transport  101 , an eject cycle—or other action as will be explained with respect to FIG. 7 —can be initiated, simultaneously including the next pick if appropriate. 
     The operation is illustrated by the flow chart of FIG. 7, with cross-references to the hardware of the other FIGS. The operational cycle begins with a print job (or during calibration phase in a power-on or a hard copy apparatus device driver boot-up routine as would be known in the art), step  701 . The Signal Out from the photo-receptor  211  should be at its calibrated maximum, No Paper, level, step  703 . If not, the eject cycle, step  705 , can be implemented to clear the paper path. If no paper sheet was in the paper path, a recalibration of the No Paper level can be instituted, re-setting the sensor to its appropriate design range or trouble-shooting routine can be implemented as needed and as would be known in the art of device driver software; further discussion here is not essential to an understanding of the present invention. If the paper path is clear, step  703 , YES-path, the sensor device is set to the appropriate, calibrated, detecting range level, step  704 , and monitoring of the Signal Out is instituted, step  707 . When a Signal Out change occurs, it is assumed that a next sheet of paper is being transported along the paper path  109  and that the leading edge is breaking the light beam  301  and a position indicator signal is sent to the device driver, step  711 , and used in transporting the sheet through the printing zone. If the substantially immediate Signal Out is not indicative of a single sheet (FIG. 6, 1 Sheet of Paper range), step  713 , an error has occurred, assumed to be a multi-pick, and the sheets are ejected, step  705 . 
     Another condition may call for an immediate ejection of a sheet. The device driver software will know the expected length of the sheet and the time it takes to print that page. Thus, during monitoring, step  717 , after recognition of a leading edge, a change in Signal Out, step  719 , is expected at an approximate predetermined time. If that time is exceeded, step  721 , YES-path, an error is assumed and the eject cycle initiated, step  705 . 
     In a normal situation, the Signal Out is appropriate to one sheet in the paper path  109 . Monitoring of the Signal Out for a change continues, step  717 . When a signal change occurs such that Signal Out transitions to the No Paper maximum level, step  723 , YES-path, a trailing edge position indicator is set, step  725 , and the information sent to the device driver. The cycle continues by returning, step  727 , to monitoring, step  707 , for the next leading edge. 
     The operation is complicated by the use of multi-sheet forms which use an intentional overlap to continuously print from the bottom-of-form-l to the following top-of-form-2. The overlap will cause the Signal Out to drop to the lower “2 Sheets of Paper” (FIG. 6) range, but no error has occurred. Thus, the Signal Out change may transition from “1 Sheet of Paper” to “2 Sheets of Paper at a predetermined time significantly later than the indication of a leading edge, step  711 . Thus, if the Signal Out indicates at this change and the multi-sheet form flag is set, step  729 , YES-path, an indicator of the bottom-of-form-1 to the top-of-form-2, step  731 , is sent to the device driver. The process continues, step  733 , by monitoring, step  717 , for the next transition—possibly to a “3 Sheets of Paper” level, et seq.—until a trailing edge recognition, steps  719 ,  723 ,  725 ,  727 , sequence occurs. If at the test step  723  for the changed signal level is followed by a recognition that a multi-sheet form was not expected (step  729 , NO-path), an indication that a multi-sheet pick error occurred triggers the eject cycle, step  705 ′. 
     It should be noted by those skilled in the art that self-calibrating of the “No Paper,” FIG. 6, level can be implemented to account for the use of different types of media, such as by feeding a sheet of the next media to be used—e.g., a thicker photo-quality paper used in color ink-jet printing—and adjusting the multiple pick levels recognition accordingly. 
     An alternative and preferred embodiment of the present invention is shown in FIG.  5 . The sensor  123  is mounted an angle to the paper path  109 . The upper paper guide  111  and the lower paper guide  113  are provided with protrusions  501 ,  503  into the paper path  109 ; smooth transition bumps prevent binding of the leading edge  305 ′ of a paper sheet  305  through the field-of-view of the sensor  123 . The lower guide protrusion  501  is located just upstream of the sensor and the upper guide protrusion  503  is located just downstream of the sensor. Note that the protrusions can be a single construct proximate the sensor  123  or a series of bumps (or ramps or the like as is expedient for a particular design) or a continuous construct across the upper and lower guides  111 ,  113  as best suits a particular implementation. As demonstrated in FIG. 5, the upstream protrusion lifts the leading edge of a sheet  305 ; the downstream protrusion drops the trailing edge of a preceding sheet  303 . By doing so, the light beam  301  is ensured a clear transmissive gap between the emitter and receptor, yet still recognizes multiple picked sheet errors or intentionally overlapped sheets in the same operational manner as already described. 
     It will also be recognized by those skilled in the art that the transmissive sensor  123  device can be built into upper and lower guides  111 ,  113  or be mounted elsewhere upstream of the printing zone of the hard copy apparatus. 
     Furthermore, an implementation having a plurality of emitters and detectors may also be employed. 
     The foregoing description of the embodiments of the present invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in this art. Similarly, any process steps described might be interchangeable with other steps in order to achieve the same result. The embodiment was chosen and described in order to best explain the principles of the invention and its best mode practical application to thereby enable others skilled in the art to understand the invention for various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto and their equivalents. Reference to an element in the singular is not intended to mean “one and only one” unless explicitly so stated, but rather means “one or more.” Moreover, no element, component, nor method step in the present disclosure is intended to be dedicated to the public regardless of whether the element, component, or method step is explicitly recited in the following claims. No claim element herein is to be construed under the provisions of 35 U.S.C. Sec. 112, sixth paragraph, unless the element is expressly recited using the phrase: “means for . . . ”.