Abstract:
Toner is conserved in an image forming apparatus, such as an electrophotographic printer (e.g., laser printer), by varying the interpage gap when approaching an exhaustion of the recording media supply (“paper out”). A supply level sensor is used to determine whether the supply of discrete recording media (e.g., paper sheets) is above or below a predetermined level. A smaller value interpage gap is used when the supply level is above the predetermined level. The interpage gap is automatically lengthened to a second larger value when the sensed media supply level drops below the predetermined level. The longer second interpage gap allows for “paper out condition” that may result from printing of sheet N−1 to be determined prior to image formation beginning for sheet N. The interpage gap may be reset to the smaller value when the supply has been refilled to above the pre-determined level.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates generally to the field of image forming, and more particularly to an image forming apparatus that varies the spacing between successive recording media based on the recording media supply level.  
           [0002]    One of the carefully controlled functions in an image forming apparatus, such as a laser printer, is the control of the relative timing of image formation and the feeding of recording media. For laser printers using an intermediate transfer module in particular, coordination of the toner image formation on the photoconductors and/or intermediate transfer module and the feeding of paper to be printed so that both arrive at the appropriate transfer point at the same time is an important function. This task may be complicated somewhat, particularly in prior art multi-color printers, by the physical path lengths involved. In particular, it is common for the image path length between the farthest upstream toner cartridge and the relevant transfer point to be longer than the paper feed path length between the paper supply and the relevant transfer point. As such, it is common for toner image formation to start for a particular sheet before that sheet is “picked” from the paper supply. That is, the image formation at that toner cartridge for sheet N begins substantially before the picking of the sheet N from the paper supply is initiated. Indeed, due to the geometries involved, the image formation for sheet N is typically initiated while the sheet N−1 is still being fed from the paper supply. As such, prior art devices often begin forming an image before it is possible to determine if the paper supply is empty. If this occurs, then toner is unnecessarily wasted.  
         SUMMARY OF THE INVENTION  
         [0003]    The present invention provides a method and apparatus for conserving toner in an image forming apparatus, such as a laser printer, by varying the interpage gap when approaching an exhaustion of the recording media supply (e.g., a paper out condition). A supply level sensor is used to determine whether the supply of discrete recording media (e.g., paper sheets) is above or below a predetermined level, such as 5% or 10%. When the supply level is above the predetermined level the printer operates normally. That is, the interpage gap is at a first value, say two inches. With this arrangement, it is necessary for image formation for sheet N to begin prior to when a “paper out condition” would result from printing sheet N−1. When the sensed media supply level drops below the predetermined level, the interpage gap is automatically lengthened to a second value, such as approximately thirteen inches. This longer interpage gap allows for the exhaustion of the recording media supply that may result from printing of sheet N−1 to be determined prior to image formation beginning for sheet N. The interpage gap may be increased by allowing the ITM belt to continue moving, but without adding the image for sheet N+1 until after the “paper out” would normally be sensed. For instance, the relative timing between the start of image formation and the picking of the corresponding sheet may be maintained (to maintain proper alignment at the relevant transfer point), but the start of image formation may be delayed until the “paper out” would be detected if it exists after printing sheet N−1. If a “paper out” condition is detected, the printer responds as usual (e.g., alerts user), but toner waste is reduced. This longer interpage gap/slower throughput process should be used when the media supply level is low, so that higher throughput is available for the majority of the time. The interpage gap may be reset to the normal value when it has been detected that the supply has been refilled to above the pre-determined level. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0004]    [0004]FIG. 1 shows an image forming apparatus.  
         [0005]    [0005]FIG. 2 shows one example of a media supply tray with one possible arrangement of a media supply level sensor useful with the present invention.  
         [0006]    [0006]FIG. 3 shows one process flow according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0007]    As the present invention relates to the process flow of various materials in an electrophotographic image forming apparatus, an understanding of the basic elements of an electrophotographic image forming apparatus may aid in understanding the present invention. For purposes of illustration, a four cartridge color laser printer will be described; however one skilled in the art will understand that the present invention is applicable to other types of electrophotographic image forming apparatuses that use one or more toner colors for printing. Further, for simplicity, the discussion below will use the terms “sheet” and/or “paper” to refer to a discrete unit of recording media  5 ; this term is not limited to paper sheets, and any form of discrete recording media is intended to be encompassed therein, including without limitation, envelopes, transparencies, postcards, and the like.  
         [0008]    A four color laser printer, generally designated  10  in FIG. 1, typically includes a plurality of optionally removable toner cartridges  20  that have different toner color contained therein, an intermediate transfer module  40 , a fuser  50 , and one or more recording media supplies  100 . For instance, the printer  10  may include a black (k) cartridge  20 , a magenta (m) cartridge  20 , a cyan (c) cartridge  20 , and a yellow (y) cartridge  20 . Typically, each different color toner forms an individual image of a single color that is combined in a layered fashion to create the final multi-colored image, as is well understood in the art. Each of the toner cartridges  20  may be substantially identical; for simplicity only the operation of the cartridge  20  for forming yellow images will be described, it being understood that the other cartridges  20  may work in a similar fashion.  
         [0009]    The toner cartridge  20  typically includes a photoconductor  22 , a charger  24 , a developer section  26 , a cleaning section  28 , and a toner supply bin  30 . The photoconductor  22  is generally cylindrically-shaped with a smooth surface for receiving an electrostatic charge over the surface as the photoconductor  22  rotates past charger  24 . The photoconductor  22  rotates past a scanning laser  70  directed onto a selective portion of the photoconductor surface forming an electrostatically latent image representative of the image to be printed. Drive gears (not shown) may rotate the photoconductor  22  continuously so as to advance the photoconductor  22  some uniform amount, such as {fraction (1/600)}th or {fraction (1/1200)}th of an inch, between laser scans. This process continues as the entire image pattern is formed on the surface of the photoconductor  22 .  
         [0010]    After receiving the latent image, the photoconductor  22  rotates to the developer section  26  which has a toner bin  30  for housing the toner and a developer roller  27  for uniformly transferring toner to the photoconductor  22 . The toner is typically transferred from the toner bin  30  to the photoconductor  22  through a doctor blade nip formed between the developer roller  27  and the doctor blade  29 . The toner is typically a fine powder constructed of plastic granules that are attracted and cling to the areas of the photoconductor  22  that have been discharged by the scanning laser  70 . To prevent toner escape around the ends of the developer roller  27 , end seals may be employed, such as those described in U.S. patent application Ser. 09/833,888, filed 12 Apr. 2001, entitled “Dynamic End-Seal for Toner Development Unit,” which is incorporated herein by reference.  
         [0011]    The photoconductor  22  next rotates past an adjacently-positioned intermediate transfer module (“ITM”), such as belt  40 , to which the toner is transferred from the photoconductor  22 . The location of this transfer from the photoconductor  22  to the ITM belt  40  is called the first transfer point (denoted X in FIG. 1). After depositing the toner on the ITM belt  40 , the photoconductor  22  rotates through the cleaning section  28  where residual toner is removed from the surface of the photoconductor  22 , such as via a scraper well known in the art. The residual toner may be moved along the length of the photoconductor  22  to a waste toner reservoir (not shown) where it is stored until the cartridge  20  is removed from the printer  10  for disposal. The photoconductor  22  may further pass through a discharge area (not shown) having a lamp or other light source for exposing the entire photoconductor surface to light to remove any residual charge and image pattern formed by the laser  70 .  
         [0012]    As illustrated in FIG. 1, the ITM belt  40  is endless and extends around a series of rollers adjacent to the photoconductors  22  of the various cartridges  20 . The ITM belt  40  and each photoconductor  22  are synchronized, via gears and the like well known in the art, so as to allow the toner from each cartridge  20  to precisely align on the ITM belt  40  during a single pass. By way of example as viewed in FIG. 1, the yellow toner will be placed on the ITM belt  40 , followed by cyan, magenta, and black. The purpose of the ITM belt  40  is to gather the image from the cartridges  20  and transport it to the sheet  5  to be printed on.  
         [0013]    The paper  5  may be stored in paper supply tray  100  and supplied, via a suitable series of rollers, belts, and the like, to the location where the sheet  5  contacts the ITM belt  40 . At this location, called the second transfer point (denoted Z in FIG. 1), the toner image on the ITM belt  40  is transferred to the sheet  5 . If desired, the sheet  5  may receive an electrostatic charge prior to contact with the ITM belt  40  to assist in attracting the toner from the ITM belt  40 . The sheet  5  and attached toner next travel through a fuser  50 , typically a pair of rollers with an associated heating element, that heats and fuses the toner to the sheet  5 . The paper  5  with the fused image is then transported out of the printer  10  for receipt by a user. After rotating past the second transfer point Z, the ITM belt  40  is cleaned of residual toner in any conventional fashion, so that the ITM belt  40  is clean again when it next approaches the first transfer point X.  
         [0014]    A recording media supply level sensor  110  (or “paper level sensor”) may be used to monitor the supply of sheets  5  in the paper supply tray  100 . The paper level sensor  110  should preferably be able to distinguish between three states—above a threshold, at or below the threshold but more than zero, and zero (paper out). Any one of numerous paper supply level sensors  110  known in the art may be used for such purpose, whether mechanical, optical, capacitive, or any other known type. One suitable arrangement, shown in FIG. 2, is for the paper supply level sensor  110  to include a moveable flag arm  112  that pivotally rests on the top of the paper stack in the paper supply  100 , and a pair of optical emitter/detector pairs  114 ,  116  communicating with the controller  60  of the printer  10 . One optical emitter/detector pair  114 ,  116  would be at a level corresponding to the threshold. As the paper supply level falls to the threshold, the flag arm  112  would interrupt the light from the emitter  114  to the detector  116 , thereby telling the controller  60  that the threshold had been reached. Eventually, the last sheet is reached; as this sheet  5  is fed to the printing operation, the flag arm  112  should move (e.g., fall down through a hole normally blocked by the sheets  5 ) so as to interrupt the light from the second emitter  114  to the second detector  116 , thereby telling the controller  60  that the paper out condition has been reached. Of course, the paper supply level sensor  110  may also be used to detect additional thresholds without departing from the present invention. Further, by employing the teachings of the present invention, the paper supply level sensor  110  may be positioned close to the exit point of the paper  5  from the paper supply tray  100 , but this is not required. For example, FIG. 2 shows the flag arm  112  disposed farther upstream than the pick mechanism; however, the flag arm  112  may instead be disposed downstream of the pick mechanism. Indeed, in some arrangements, the flag arm  112  may be disposed outside the footprint of the sheets  5  in the paper supply tray  100 , but connected to the pick mechanism so as to move appropriately in conjunction with the paper supply level in the paper supply tray  100 .  
         [0015]    The present invention addresses the problem of wasted toner due to image formation prior to detecting a paper out condition by increasing the gap between successive sheets  5  (the “interpage gap”) when the paper supply level is low so that a paper out condition can be detected prior to forming the corresponding image at the toner cartridges  20 .  
         [0016]    The overall process flow may be seen in FIG. 3. The printer  10  prints using its normal interpage gap (box  210 ), such as two inches, while the paper supply level is above a first threshold level, such as above 10% full. The paper supply level is determined (box  220 ) by reference to the paper supply level sensor  110 . If the supply level is above the threshold (box  230 ), the process returns to box  210 . If the supply level is zero (box  240 ), corresponding to a paper out condition causing the printer  10  to alert the user in any fashion known in the art (box  250 ). If the supply level is more than zero (box  240 ), and necessarily below the threshold, this means that a low supply level has been detected. In response to the detection of the low supply level, the printer controller  60  increases the interpage gap to a second larger size, such as approximately thirteen inches (box  260 ). It should be understood that the actual value of the larger interpage gap will necessarily depend on the particular geometries of a given printer, especially the location of the paper sensor  110  relative to the rest of the paper supply tray  100  and any speed variations in the paper supply path. However, the second interpage gap is sized such that the previous sheet  5 , (sheet N−1) will fully pass the relevant paper sensor  110  before the controller  60  initiates the image formation on the photoconductor  22  of the farthest upstream toner cartridge  20 . Thus, if sheet N−1 is the last available sheet  5  in the paper supply  100 , the paper sensor  110  will have time to note that there are no remaining sheets  5 , meaning there is no sheet N available, and warn the controller  60 , before the image starts being formed for the now-missing sheet N. The printer  10  is then ready to print the next sheet  5  using the larger interpage gap (box  270 ). The process then loops back to box  220 .  
         [0017]    As described above, the printer  10  may operate in three conceptual modes: 1) “normal mode” with a small interpage gap, preferably as small as possible given the considerations known in the art, when the paper supply level is above the threshold (e.g., level &gt;10%); 2) “guard mode” with a larger interpage gap sufficient to allow the detection of a paper out condition prior to image formation, when the paper supply level is at or below the threshold but non-zero (e.g., 10%≦level &lt;0); and 3) “paper out mode” when the paper out condition is sensed.  
         [0018]    By increasing the interpage gap when the paper supply level is low, the present invention allows a paper out condition to be sensed before image formation for the next sheet begins, thereby conserving toner that would otherwise be wasted. While the approach of the present invention may have the effect of slightly decreasing throughput of the printer  10  when the paper supply is low, the toner savings may be significant. Indeed, provided that the threshold paper level is set relatively low, such as 10%, 6%, 5%, or less, the slower throughput should not be encountered very often. In addition, the larger the paper supply tray  100 , the lower the threshold level may be set, as a percentage of full.  
         [0019]    The discussion above has assumed that the response to a paper out condition will be a stoppage in printing. However, the present invention may also be applied in situations involving multiple paper supply trays  100 . In such a situation, it may be advantageous to switch to a different paper supply tray  100  in response to a paper out condition for the first paper supply tray  100 , rather than stopping printing entirely until the first paper tray  100  is refilled. Each paper supply tray  100  may advantageously have an associated recording media supply level sensor  110  and the controller  60  may be programmed to respond to the corresponding sensor  110  when feeding a given paper supply tray  100 .  
         [0020]    The discussion above has assumed that the recording media supply level sensor  10  performs the dual functions of determining the state of the media supply  100  with respect to the predetermined threshold level and detecting when the media supply is exhausted (commonly referred to as a “paper out” condition, or more generally as a “media-supply-out” condition). However, these conceptually distinct functions of the media supply level sensor  10  may optionally be divided amongst more than one sensor. For instance, a supply level sensor  10  may be used to determine whether the media supply  100  is in a normal state—corresponding to a supply somewhere between full and the threshold—or in a low supply level state—corresponding to a supply somewhere between the threshold and zero. A second sensor (not shown), distinct from the supply level sensor  110 , could then be used to detect when the supply is exhausted—corresponding to a paper out condition. Of course, these two functions can be combined into one sensor  10 , but this is not required.  
         [0021]    The “normal” interpage gap in the discussion above has been illustratively set to two inches and the longer interpage gap has been illustratively set to approximately thirteen inches. However, these values are meant for illustrative purposes only as being suitable for 8 ½×11 paper in a particular design of printer  10 , and not intended to be limiting. In general, it may be advantageous to apply to following logic to setting the respective interpage gap values:  
         [0022]    Normal gap (default for 8½×11 paper):=11 inches+2 inches (gap)−paper length, but not less than 2 inches.  
         [0023]    Longer gap:=11 inches+12 ⅞ inches (gap)−paper length, but not less than 12 ⅞ inches.  
         [0024]    This logic may be easily implemented in the controller  60 . Of course, the particular values for any given printer will depend on the geometry of that particular printer, particularly the path distance between the “paper-out” sensor  110  and the second transfer point Z, the path distance between the first transfer point X and the second transfer point Z, and the desired safety margin.  
         [0025]    As used herein, the term “image forming apparatus” should be broadly construed; specifically including, but not limited to, laser printers, facsimile machines, copiers, and the like that use an electrophotographic image forming process of any variety.  
         [0026]    Although the present invention has been described herein with respect to particular features, aspects and embodiments thereof, it will be apparent that numerous variations, modifications, and other embodiments are possible within the broad scope of the present invention, and accordingly, all variations, modifications and embodiments are to be regarded as being within the scope of the invention. The present embodiments are therefore to be construed in all aspects as illustrative and not restrictive and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.