Abstract:
A sheet feeding apparatus includes first and second sheet sensors provided in order from upstream to downstream sides in feeding direction on a paper feed path, sheet conveyor roller sets arranged along the path, and a roller controller individually controlling groups of rollers on the upstream and downstream sides of the first sheet sensor, in response to a change in values of output signals from first and second sheet sensors. The roller controller includes an overlapped feeding determining device outputting a determination signal assuming a first value when overlapped feeding occurs and a second value otherwise, in response to the change in values of the output signals from the sensors, an overlapped feeding eliminating device eliminating overlapped feeding by controlling the roller controller upon receiving the determination signal of the first value, and a conveyor roller stopping device stopping the sheet conveyor rollers if the overlapped feeding eliminating process should fail.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This nonprovisional application claims priority under 35 U.S.C. §119(a) on Patent Application No. 2008-015830 filed in Japan on Jan. 28, 2008, the entire contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a sheet feeding apparatus and, more specifically, to a technique for detecting state of sheet feeding on a paper feed path. 
     2. Description of the Background Art 
     As is well known, an image forming apparatus such as a printer or a copying machine is provided with a paper feeder for feeding sheets of paper one by one. If a mechanism for separating sheets of paper in the paper feeder does not work well, overlapped feeding occurs. 
     The term “overlapped feeding” means that a plurality of sheets are fed together, with a part of one sheet overlapped by a part of another sheet. 
     Japanese Patent Laying-Open No. 9-124173 (hereinafter referred to as “&#39;173 application”) proposes a technique of mechanically preventing overlapped feeding by providing a peeling means that freely moves to be attached to/detached from one surface of a sheet, between a sheet feeding function and an image forming function. 
     Recently, however, various types of sheets having different colors, thicknesses and materials are used. Depending on the sheet type, sheet separation is impossible by the peeling function described in &#39;173 application, and overlapped feeding may occur. 
     On the other hand, in an image forming apparatus, sometimes a feed-failure jam occurs, that is, a sheet of paper is not well fed but stopped on the feeding path. If such a feed-failure jam is left unaddressed, it possibly leads to a complicated jam as a paper path obstruction. Therefore, if a feed-failure jam is detected, paper feed should be stopped immediately. 
     As a typical method of detecting a feed-failure jam, a time from detection of a leading edge to detection of a trailing edge of a sheet at a certain position is measured, and if it exceeds a time corresponding to the sheet length, it is determined that a feed-failure jam has occurred. 
     According to this method of detecting a feed-failure jam, however, overlapped feeding would be mistaken as a feed-failure jam. If such erroneous detection of feed-failure jam occurs, paper feeding stops. Therefore, it becomes necessary for the user to manually remove the sheets fed in overlapped manner, which is rather troublesome. 
     Generally, when overlapped feeding occurs, it is considered easier for the user to have the sheets discharged by continuing paper feeding, than to stop feeding. 
     In view of the foregoing, Japanese Patent Laying-Open No. 2007-206572 (hereinafter referred to as “&#39;572 application”) proposes a technique that alleviates burden of removing a sheet of paper, by reducing erroneous detection of feed-failure jam caused by overlapped feeding. 
     Specifically, on a paper feed path, a sheet passage sensor and a thickness sensor are arranged in this order from the upstream side to the downstream side in the direction of paper feeding. Time from detection of a leading edge to detection of a trailing edge of a sheet by the sheet passage sensor is detected, and whether or not feed-failure jam occurred is determined thereby. Specifically, if the detected time is longer than a normal time used for detecting feed-failure jam, it is determined that a feed-failure jam has occurred. Based on information from thickness sensor, whether or not overlapped feeding has occurred is determined. For instance, if the detected thickness is larger than the thickness of one sheet, it means that sheets are fed in an overlapped manner. If overlapped feeding is detected, paper feeding is continued. If feed-failure jam is detected but overlapped feeding is not detected, operation is controlled such that paper feeding is stopped. 
     The technique disclosed in &#39;572 application uses a thickness sensor in addition to the sheet passage sensor, in order to reduce erroneous detection of feed-failure jam caused by occurrence of overlapped feeding. The technique, however, has the following problems. 
     The thickness sensor described in &#39;572 application includes a light source for emitting light to the fed sheet of paper, and a function of measuring the intensity of light transmitted through the sheet that differs depending on the thickness of the sheet, and the sensor is adopted to detect sheet thickness based on the measured light intensity. 
     The types of paper, however, increase year after year as mentioned above and in order to attain detection accuracy of thickness sensor accordingly, necessary cost would be very high. This makes it difficult to adopt a thickness sensor for detecting overlapped feeding. 
     Meanwhile, it is among major problems recently in designing paper feeding system, to enable reliable detection of overlapped feeding and feed-failure jam by a combination of ON/OFF of sensors of one same type, that is, sheet passage sensors. 
     SUMMARY OF THE INVENTION 
     The present invention was made in view of the technical problems described above, and its object is to provide a sheet feeding apparatus capable of automatically detecting and eliminating feed-failure jam caused by overlapped feeding of sheets based on a combination of ON/OFF of sheet passage sensors, without using a thickness sensor for detecting overlapped feeding, and thereby alleviating burden of removing a sheet by the user, as well as to provide an image forming apparatus with the sheet feeding apparatus. 
     According to a first aspect, the present invention provides a sheet feeding apparatus, including: first and second sheet sensors provided in order from upstream to downstream sides in a sheet feeding direction on a paper feed path through which sheets of paper are fed one by one by a prescribed paper feeder, each outputting a signal that is on when a sheet is detected and off otherwise; and a plurality of sets of sheet conveyor rollers arranged at a plurality of positions along the paper feed path. Two or more sheets may possibly be fed partially overlapped from the paper feeder to the paper feed path. The plurality of sets of sheet conveyor rollers include a group of upstream rollers on the upstream side than the first sheet sensor, and a group of downstream rollers on the downstream side than the first sheet sensor. The sheet feeding apparatus further includes a roller controller connected to receive output signals of the first and second sheet sensors, and individually controlling the group of upstream rollers and the group of downstream rollers in accordance with a change in value of the output signals. The roller controller includes an overlapped feeding determining device activated in response to start of paper feeding from the paper feeder to the paper feed path, and outputting a determination signal assuming a first value when overlapped feeding of sheets has occurred and a second value otherwise, in response to a change in values of output signals of the first and second sheet sensors, an overlapped feeding eliminating device performing a process for eliminating overlapped feeding by controlling the roller controller, in response to reception of the determination signal of the first value, and a conveyor roller stopping device stopping the sheet conveyor rollers in response to failure of the process for eliminating overlapped feeding by the overlapped feeding eliminating device. 
     The first and second sensors output a signal that is on if a sheet is detected and off otherwise. The roller controller controls operations of a plurality of sets of sheet conveying rollers in the feed path, in accordance with signals output from the first and second sheet sensors. The overlapped feeding determining device outputs a determination signal of a first value if overlapped feeding of sheets is determined to have occurred, and outputs the determination signal of a second value otherwise. Receiving the determination signal of the first value, the overlapped feeding eliminating device performs an overlapped feeding eliminating process for eliminating overlapped feeding. If the overlapped feeding eliminating process fails, the conveyor roller stopping device stops the sheet conveying rollers. As a result, a sheet feeding apparatus can be provided that is capable of automatically detecting and eliminating feed-failure jam caused by overlapped feeding of sheets based on a combination of ON/OFF of sheet passage sensors, without using a thickness sensor for detecting overlapped feeding, and thereby alleviating burden of removing a sheet by the user. 
     Preferably, the overlapped feeding eliminating device includes an upstream roller stopping device stopping the paper feeder and the upstream rollers, in response to reception of the determination signal of the first value, and a roller activating device activating the paper feeder and the upstream rollers, in response to a change from on to off of the signal output by the first sheet sensor within a prescribed time period from turning on of the second sheet sensor. 
     Receiving the determination signal of the first value, the overlapped feeding eliminating device stops the upstream rollers. Therefore, it is unlikely that a new sheet is fed and causes a jam. On the other hand, the sheet at the position of overlapped feeding would be conveyed by the downstream rollers, eliminating overlapped feeding. If the output signal from the first sheet sensor turns from on to off within a prescribed time period, it is supposed that the sheet is fed, and hence, the upstream rollers, which have been stopped, may be activated to resume paper feeding. As a result, it becomes possible to grasp the state of paper feed path and to control feeding simply based on the output signals of sheet sensors. 
     More preferably, the conveyor roller stopping device includes a downstream roller group stopping device stopping the group of downstream rollers when the prescribed time period passed with the signal output from the first sheet sensor kept on, after the upstream rollers were stopped by the upstream roller stopping device. 
     When the upstream rollers were stopped by the upstream roller stopping device and thereafter a prescribed time period passed with the signal output from first sheet sensor kept on, it is determined that an attempt to eliminate overlapped feeding has failed and caused a feed-failure jam in the paper paper feed path. In that case, downstream roller stopping device stops the group of downstream rollers, to stop sheet feeding. As a result, paper feed stops completely, and the sheet left on the feed path can be removed manually. 
     Preferably, the overlapped feeding determining device includes an overlapped feeding occurrence determining device outputting the first determination signal of the first value, when a prescribed time period passed with the output signal from the first sheet sensor kept on, after the output signal from the first sheet sensor changed from off to on. 
     If the output signal of first sheet sensor turns from off to on, it is highly likely that the sheet is at the position of first sheet sensor. If a prescribed time period passes with the signal output kept on, it is considered that overlapped feeding has occurred and it takes time for the sheets to pass through the first sheet sensor. Therefore, occurrence of the overlapped feeding can be recognized without using a thickness sensor. 
     Preferably, the roller controller further includes a conveyor roller reactivating device activating both the upstream and downstream rollers for feeding a sheet from the prescribed paper feeder to the paper feed path, in response to turning off of both output signals from the first and second sheet sensors after the downstream roller group was stopped by the downstream roller group stopping device. 
     When the downstream roller group is stopped by the downstream roller group stopping device, it is possible for the user to remove the sheet left staying on the feed path of the sheet feeding apparatus. When the sheet is removed, outputs of the first and second sheet sensors turn off. Therefore, here, the paper feeding operation can be resumed, and hence, the conveyor roller resetting device activates both the upstream and downstream roller groups. As a result, after the process of removing the sheet left on the path, the sheet feeding operation can be resumed smooth. 
     Preferably, the overlapped feeding determining device includes a timer activated in response to a change from off to on of the output signal of the first sheet sensor, and a circuit outputting the determination signal that assumes a first value if the output signal of the first sheet sensor is on and a second value if it is off, when a prescribed time is counted by the timer. 
     Preferably, the downstream roller group stopping device includes a timer activated when the second sheet sensor is turned on, and a device inspecting an output of the first sheet sensor when a prescribed time is counted by the timer and stopping, if the output is on, the downstream roller group. 
     Preferably, the roller activating device includes a timer activated when the second sheet sensor is turned on, and a device inspecting an output of the first sheet sensor when a prescribed time is counted by the timer and activating, if the output is off, the paper feeder and the upstream rollers. 
     Preferably, the apparatus includes only a pair of the conveyor rollers between the first and second sheet sensors. 
     According to a second aspect, the present invention provides an image forming apparatus, including: a sheet feeding apparatus feeding sheets of paper on a paper feed path on which sheets of paper are fed one by one by a prescribed paper feeder; and an image forming unit printing image data on the sheet fed through the paper feed path. The sheet feeding apparatus includes first and second sheet sensors provided in order from upstream to downstream sides in a sheet feeding direction on the paper feed path, each outputting a signal that is on when a sheet is detected and off otherwise; and a plurality of sets of sheet conveyor rollers arranged at a plurality of positions along the paper feed path. Two or more sheets may possibly be fed partially overlapped from the paper feeder to the paper feed path. The plurality of sets of sheet conveyor rollers include a group of upstream rollers on the upstream side than the first sheet sensor, and a group of downstream rollers on the downstream side than the first sheet sensor. The sheet feeding apparatus further includes a roller controller connected to receive output signals of the first and second sheet sensors, and individually controlling the group of upstream rollers and the group of downstream rollers in accordance with a change in value of the output signals. The roller controller includes an overlapped feeding determining device activated in response to start of paper feeding from the paper feeder to the paper feed path, and outputting a determination signal assuming a first value when overlapped feeding of sheets has occurred and a second value otherwise, in response to a change in values of output signals of the first and second sheet sensors, an overlapped feeding eliminating device performing a process for eliminating overlapped feeding by controlling the roller controller, in response to reception of the determination signal of the first value, and a conveyor roller stopping device stopping the sheet conveyor rollers in response to failure of the process for eliminating overlapped feeding by the overlapped feeding eliminating device. 
     Preferably, the image forming apparatus further includes a discharge device discharging, when two or more sheets are fed in partially overlapped manner to the paper feed path and a preceding sheet is for printing last page of the image data, a sheet following the preceding sheet without performing printing process. 
     If the preceding one of the sheets fed in the overlapped manner is the last page of image forming process, the succeeding one of the sheets fed in the overlapped manner must be discharged without performing any image forming process. Therefore, if the preceding one of the sheets fed in the overlapped manner is the last page of image forming process, the discharge device discharges the sheet following the preceding sheet without performing image processing. 
     According to a third aspect, the present invention provides a method of sheet feeding, in a sheet feeding apparatus including first and second sheet sensors provided in order from upstream to downstream sides in a sheet feeding direction on a paper feed path through which sheets of paper are fed one by one by a prescribed paper feeder, each outputting a signal that is on when a sheet is detected and off otherwise, and a plurality of sets of sheet conveyor rollers arranged at a plurality of positions along the paper feed path. Two or more sheets may possibly be fed partially overlapped from the paper feeder to the paper feed path. The plurality of sets of sheet conveyor rollers include a group of upstream rollers on the upstream side than the first sheet sensor, and a group of downstream rollers on the downstream side than the first sheet sensor. The sheet feeding apparatus further includes a roller controller connected to receive output signals of the first and second sheet sensors, and individually controlling the group of upstream rollers and the group of downstream rollers in accordance with a change in value of the output signals. The sheet feeding method includes: the step, started in response to start of paper feeding from the paper feeder to the paper feed path, of outputting a determination signal assuming a first value when overlapped feeding of sheets has occurred and a second value otherwise, in response to a change in values of output signals of the first and second sheet sensors; the step of performing a process for eliminating overlapped feeding by controlling the roller controller, in response to reception of the determination signal of the first value; and the step of stopping the sheet conveyor rollers in response to failure of the process for eliminating overlapped feeding at the step of performing the process for eliminating overlapped feeding. 
     According to the present invention, it becomes possible to automatically detect an overlapped feeding of sheets based on a combination of ON/OFF of sheet passage sensors and to eliminate the overlapped feeding without using a thickness sensor for detecting the overlapped feeding. Therefore, the burden on the user to remove the sheet can be alleviated. 
     The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an overall configuration of an image forming apparatus  20  in accordance with an embodiment of the present invention. 
         FIG. 2  shows a structure of an upper portion of image forming apparatus  20 . 
         FIG. 3  schematically shows structures of main portions of a second sheet feeding system related to a second paper feed cassette and a third sheet feeding system related to a third paper feed cassette. 
         FIG. 4  is a block diagram representing hardware configuration of image forming apparatus  20 . 
         FIG. 5  is a block diagram showing an electric structure related to a second sheet feeding system related to a second paper feed cassette  36  of a sheet feeding unit  40 . 
         FIG. 6  is a flowchart of a program structure of a main routine realizing the sheet feeding function of image forming apparatus  20 . 
         FIG. 7  is a flowchart of a program structure of a main routine realizing the sheet feeding function of image forming apparatus  20 , showing processes following step S 128  of  FIG. 6 . 
         FIG. 8  is a flowchart of a program structure of a main routine realizing the sheet feeding function of image forming apparatus  20 , showing processes following step S 136  of  FIG. 6 . 
         FIG. 9  is a time chart showing a program structure of a subroutine realizing a normal sheet feeding function. 
         FIG. 10  is a time chart showing a program structure of a subroutine realizing overlapped feeding eliminating function. 
         FIG. 11  schematically shows a state when overlapped feeding occurred. 
         FIG. 12  schematically shows a state when the overlapped feeding occurred and sheet feeding is continued. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following description and in the appended drawings, corresponding components are denoted by the same reference characters. Their names and functions are also the same. 
     &lt;Overall Structure&gt; 
     Referring to  FIGS. 1 and 2 , the image forming apparatus  20  in accordance with the present embodiment is, by way of example, a multifunctional apparatus including copy, printer and facsimile functions, and it outputs image data read by a scanner or the like or image data transmitted from an external device such as a client personal computer (hereinafter referred to as a “client PC”)  320  shown in  FIG. 4  or the like, by electrophotographic technique. Specifically, an electrostatic latent image is formed on a surface of a photoreceptor drum  22 , which is driven to rotate, the formed electrostatic latent image is visualized as a toner image by a two-component developer charged by mixing electric toner with magnetic carrier, and the image is transferred onto a sheet of paper  24  and fixed as a monochrome (single color) image. Therefore, image forming apparatus  20  includes an image forming unit  26  having the photoreceptor drum  22 , a transfer mechanism  28  for directly or indirectly transferring the toner image formed on photoreceptor drum  22  to the sheet of paper, and a fixing unit  30  for melting and fixing the not-yet-fixed toner image transferred to the sheet of paper on the sheet of paper. 
     Further, image forming apparatus  20  includes multi-stage paper feed cassettes  34 ,  36  and  38  functioning as a sheet containing unit  32  and capable of storing sheets of paper  24 , a sheet feeding unit  40  feeding sheet  24  fed from multi-stage paper feed cassette  34 ,  36  or  38  to image forming unit  26 , and a sheet feeder  42  feeding the sheet  24  on which printing has been done at image forming unit  26 , to fixing unit  30  for fixing. 
     As shown in  FIG. 1 , image forming apparatus  20  mainly includes an apparatus body  58  including an exposure unit  44 , a developer  46 , a toner supply device  48 , photoreceptor drum  22 , a charger  50 , a neutralizer  52 , a photoreceptor drum cleaning unit  54 , fixing unit  30 , sheet feeder  42 , multi-stage paper feed cassettes  34 ,  36 ,  38 , a discharge tray  56  and a transfer mechanism  28 , and an automatic document feeder  60 . 
     On an upper surface of apparatus body  58 , a platen  62  of transparent glass for receiving a document is provided. Automatic document feeder  60  is attached to apparatus body  58  to open/close platen  62 . 
     Below the platen  62 , a scanner unit  64  is provided for reading image information of the document. Below scanner unit  64 , exposure unit  44 , developer  46 , photoreceptor drum  22 , charger  50 , neutralizer  52 , photoreceptor drum cleaning unit  54 , fixing unit  30 , sheet feeder  42 , discharge tray  56  and transfer mechanism  28  are arranged. 
     Exposure unit  44  has a function of irradiating a surface of photoreceptor drum  22  charged uniformly by charger  50  with a laser beam in accordance with the image data output from image processing unit  312  shown in  FIG. 4  for exposure, and thereby forming an electrostatic latent image in accordance with the image data on the surface of photoreceptor drum  22 . Exposure unit  44  is arranged immediately below scanner unit  64  and above photoreceptor drum  22 . Exposure unit  44  includes laser scanning units (hereinafter referred to as “LSUs”)  72  and  74  with laser emitting units  66 A,  66 B, and reflection mirrors  70 A,  70 B. In the present embodiment, for high-speed printing, two-beam technique is adopted, in which a plurality of laser beams are utilized to attain moderate emission timing. Though LSUs  72  and  74  are used in exposure unit  44  in the present embodiment, an array of light emitting elements such as EL (Electro Luminescence) or LED (Light Emitting Diode) write heads, for example, may be used. 
     Photoreceptor drum  22  is arranged below exposure unit  44  and controlled such that it rotates in a prescribed direction (direction of arrow  76  in the figure) by a driving device, not shown, and controller  300  shown in  FIG. 4 . 
     Around photoreceptor drum  22 , a sheet separating pawl  78 , photoreceptor drum cleaning unit  54 , charger  50  as electric field generator, developer  46  and neutralizer  52  are arranged in this order along the direction of rotation of photoreceptor drum  22 , with the position at the end of image transfer being a reference, as shown in  FIG. 2 . 
     Sheet separating pawl  78  is arranged to be brought into contact with/separated from the outer circumferential surface of photoreceptor drum  22 , by a solenoid  80 . Sheet separating pawl  78  separates, in a state in contact with the outer circumferential surface of photoreceptor drum  22 , the sheet of paper  24  adhering to the surface of photoreceptor drum  22  when the toner image formed on the surface of photoreceptor drum  22  is transferred to the sheet of paper  24 . In place of solenoid  80 , a driving motor or the like may be used as the device for driving sheet separating pawl  78 , or other driving method may be used. 
     Developer  46  visualizes the electrostatic latent image formed on the surface of photoreceptor drum  22  by black toner. Below developer  46  and upstream side in the paper feeding direction, a register roller  82  is arranged. 
     A toner supply device  48  is arranged adjacent to developer  46 , and it stores toner, discharged from a toner container  84  filled with toner, temporarily in a hopper  86  and then supplies the toner to developer  46 . 
     Charger  50  is arranged above photoreceptor drum  22 , near the outer circumferential surface of photoreceptor drum  22 , and it uniformly charges the surface of photoreceptor drum  22  to a prescribed potential. Though a non-contact type charger is used as charger  50  in the present embodiment, a contact type charger, such as a roller charger or a brush charger, may be used. 
     Neutralizer  52  lowers the surface potential of photoreceptor drum  22  so as to facilitate transfer of the toner image formed on the surface of photoreceptor drum  22  to the sheet of paper  24 . Though neutralizer having neutralizing electrode is used as neutralizer  52  in the present embodiment, one that utilizes a neutralizing lamp, or other type of neutralizer may be used. 
     Photoreceptor drum cleaning unit  54  removes and recovers the toner left on the surface of photoreceptor drum  22 , after development and transfer of toner image. 
     As described above, the electrostatic latent image that has been visualized on the surface of photoreceptor drum  22  is transferred to the sheet of paper  24  by applying, from transfer mechanism  28  to the fed sheet of paper  24 , electric field of opposite polarity to the charges of the electrostatic latent image. For instance, if the electrostatic latent image has charges of (−) polarity, the polarity applied by transfer mechanism  28  is of (+) polarity. 
     Transfer mechanism  28  includes a driving roller  88 , a driven roller  90 , and a transfer belt  92  wound around driving and driven rollers  88  and  90  and other rollers, and formed as a unit including these components. Transfer belt  92  has a prescribed resistance value (in the present embodiment, in the range of 1×10 9  Ω·cm to 1×10 13  Ω·cm), and it is arranged below photoreceptor drum  22  such that the surface of transfer belt  92  is in contact with a part of outer circumferential surface of photoreceptor drum  22 . In transfer mechanism  28 , the sheet of paper  24  is fed pressed against photoreceptor drum  22  by means of transfer belt  92 . 
     Near the portion  94  at which photoreceptor drum  22  and transfer belt  92  are in contact with each other, an elastic conductive roller  96 , which has conductivity type different from that of driving and driven rollers  88  and  90  and is capable of applying transfer electric field, is arranged. 
     Elastic conductive roller  96  is formed of a soft material such as elastic rubber or foamable resin. With elastic conductive roller  96  being elastic, the contact between photoreceptor drum  22  and transfer belt  92  becomes a plane contact of a prescribed width, which is referred to as a transfer nip, rather than a line contact. This improves transfer efficiency of the toner image to the fed sheet of paper  24 . 
     Further, on the downstream side of paper feeding direction than the transfer area of transfer belt  92 , a neutralizing roller  98  is arranged, on the back side of transfer belt  92 , for neutralizing the electric field applied to the fed sheet of paper  24  at the transfer area, to enable smooth feeding of the sheet of paper  24  to the next step. 
     As shown in  FIG. 2 , in transfer mechanism  28 , a transfer belt cleaning unit  100  for removing smudge of toner left on the surface of transfer belt  92 , and a plurality of neutralizing mechanisms  102  for neutralizing transfer belt  92  are arranged. The neutralizing method applied to neutralizing mechanism  102  may be a method of grounding through the apparatus, or a method of positively applying an electric field having opposite polarity to the transfer electric field. 
     The electrostatic latent image transferred to the sheet at transfer mechanism  28  is fed to fixing unit  30  and pressurized and heated, whereby the not-yet-fixed toner is melt and fixed on the sheet of paper  24 . 
     In fixing unit  30 , the not-yet-fixed toner on the fed sheet of paper  24  is heated and melt by a heating roller  104  at a pressure-contact portion  108  generally referred to as a fixing nip, where heating roller  104  and a pressurizing roller  106  are in pressure contact with each other, and by the function of pressure contact between heating roller  104  and pressurizing roller  106 , the melt toner is fixed on the sheet of paper  24 . 
     Near the outer circumferential surface of heating roller  104 , a paper separation pawl  110 , a thermister  112  and a fixing roller cleaning unit  114  for cleaning the outer circumferential surface of heating roller  104  are arranged. On the inner circumferential portion, a heat source  116  is provided, to heat the surface of heating roller  104  to a prescribed temperature (set fixing temperature: approximately 160° C. to approximately 200° C.). 
     On pressurizing roller  106 , pressurizing members  118  are arranged to enable pressure contact of roller  106  to heating roller  104  with a prescribed pressure, and near the outer circumferential surface of pressurizing roller  106 , a paper separation pawl  120  and a pressurizing roller surface cleaning member  122  are arranged, as in the vicinity of heating roller  104 . 
     Near the fixing unit  30 , a conveyor roller  124  is provided, for feeding the sheet of paper  24  through fixing unit  30 . On the downstream side in the paper feeding direction of conveyor roller  124 , a discharge roller  126  is provided for discharging the sheet of paper  24  to discharge tray  56 . 
     Multi-stage paper feed cassettes  34 ,  36  and  38  are to store a plurality of sheets of paper  24  of mutually different sizes, as shown in  FIG. 1 , and arranged below image forming unit  26 . 
     Sheet feeding unit  40  includes three paper feeding systems for feeding the sheet of paper  24  from cassettes  34 ,  36  and  38  to a paper feed path leading to image forming unit  26 , and three sheet feeding systems for feeding sheet of paper  24  from each of the paper feeding systems through the paper feed path to image forming unit  26 . 
     The first paper feeding system includes a pick-up roller  128  for feeding the sheets of paper  24  in the uppermost, first paper feed cassette  34  one by one to the paper feed path, and a paper feed roller  130  and a separation roller  132  forming a vertical pair and serving as retard rollers. The sheets of paper  24  stacked in cassette  34  are picked-up one by one from the uppermost layer, and fed to the first sheet feeding system on the paper feed path, by the rotations of rollers  128 ,  130  and  132 . Pick-up roller  128 , paper feed roller  130  and separation roller  132  are arranged at an end portion of paper discharging side of first cassette  34 . The sheet of paper  24  fed from the inside of first cassette  34  to the first sheet feeding system by the operations of rollers  128 ,  130  and  132  is transmitted to a register roller  82  positioned at the terminal end in the feeding direction of paper feed path, by the rotations of two sets of conveyor roller pairs  134  and  136  of the first sheet feeding system. 
     The second paper feeding system includes a pick-up roller  138  for feeding the sheets of paper  24  in the middle, second paper feed cassette  36  one by one to the paper feed path, and a paper feed roller  140  and a separation roller  142  forming a vertical pair and serving as retard rollers. The sheets of paper  24  stacked in cassette  36  is picked-up one by one from the uppermost layer, and fed to the second sheet feeding system on the paper feed path, by the rotations of rollers  138 ,  140  and  142 . Pick-up roller  138 , paper feed roller  140  and separation roller  142  are arranged at an end portion of paper discharging side of second cassette  36 . The sheet of paper  24  fed from the inside of second cassette  36  to the second sheet feeding system by the operations of rollers  138 ,  140  and  142  is transmitted to a register roller  82  positioned at the terminal end in the feeding direction of paper feed path, by the rotations of five sets of conveyor rollers  144 ,  146 ,  148 ,  134  and  136  of the second sheet feeding system. 
     The third paper feeding system includes a pick-up roller  150  for feeding the sheets of paper  24  in the lowermost, third paper feed cassette  38  one by one to the paper feed path, and a paper feed roller  152  and a separation roller  154  forming a vertical pair and serving as retard rollers. The sheets of paper  24  stacked in cassette  38  is picked-up one by one from the uppermost layer, and fed to the third sheet feeding system on the paper feed path, by the rotations of rollers  150 ,  152  and  154 . Pick-up roller  150 , paper feed roller  152  and separation roller  154  are arranged at an end portion of paper discharging side of third cassette  38 . The sheet of paper  24  fed from the inside of third cassette  38  to the third sheet feeding system by the operations of rollers  150 ,  152  and  154  is transmitted to a register roller  82  positioned at the terminal end in the feeding direction of paper feed path, by the rotations of six sets of conveyor rollers  156 ,  158 ,  146 ,  148 ,  134  and  136  of the third sheet feeding system. 
     Referring to  FIG. 3 , on the paper feed path shared by the second sheet feeding system related to the second paper feed cassette  36  and the third sheet feeding system related to the third paper feed cassette  38 , a first sheet passage sensor  200  and a second sheet passage sensor  202  are arranged in this order from the upstream side to the downstream side in the sheet feeding direction, with conveyor rollers  146  positioned therebetween. As sheet passage sensors  200  and  202 , a sensor that can detect passage of sheet of paper  24  in a non-contact manner, such as an optical sensor or an ultrasonic sensor, may be used. 
     Again referring to  FIGS. 1 and 2 , register roller  82  has its operation controlled by a driving device, not shown, and controller  300  shown in  FIG. 4 , such that a sheet of paper  24  fed from each of cassettes  34 ,  36  and  38  is fed between photoreceptor drum  22  and transfer belt  92  with tip end of the sheet aligned with the toner image formed on the surface of photoreceptor drum  22 . 
     To one side surface of apparatus body  58  (on the right side surface in  FIG. 1 ), an automatic paper feeding cassette  160  is connected, which is capable of storing sheets of paper of different types in large volume. Above the automatic paper feeding cassette  160 , a manual feed tray  162  is provided mainly for handling sheets of paper of irregular size. From automatic paper feeding cassette  160  and manual feed tray  162  also, sheet of paper  24  is fed to image forming unit  26  through the paper feed path. 
     Paper discharge tray  56  is arranged opposite to the side where the manual feed tray  162  is provided. Image forming apparatus  20  may have a post processing device for stapling or punching of the discharged sheets, a multi-stage discharge tray or the like arranged as an optional component, in place of paper discharge tray  56 . 
     Sheet feeder  42  is arranged between photoreceptor drum  22  and multi-stage paper feed cassettes  34 ,  36  and  38  described above. Sheet feeder  42  is provided with the paper feed path, a branching pawl and the like. Sheet feeder  42  has functions of feeding sheets of paper  24  supplied from cassettes  34 ,  36  and  38  one by one to transfer mechanism  28 , feeding the sheet of paper  24  on which the toner image is transferred from photoreceptor drum  22  by transfer mechanism  28  to fixing unit  30 , and after the toner image is transferred by fixing unit  30 , feeding the sheet of paper  24  in accordance with a designated paper discharge mode. 
     In image forming apparatus  20 , a one-sided printing mode and a two-sided printing mode are set in advance as the discharge modes. In the image forming apparatus  20 , in the one-sided printing mode, it is possible to selectively set either a face-up discharging in which the sheet of paper is discharged with the printed side facing upward or a face-down discharging in which the sheet of paper is discharged with the printed side facing downward. 
     &lt;Hardware Configuration&gt; 
     Referring to  FIG. 4 , image forming apparatus  20  includes a controller  300  for overall control of image forming apparatus  20 . 
     Controller  300  is substantially a computer, including a main CPU (Central Processing Unit)  302 , an ROM (Read Only Memory)  304 , an RAM (Random Access Memory)  306 , an HDD (Hard Disk Drive)  308 , an image memory  310  and an image processing unit  312 . 
     A common bus line  314  is connected to main CPU  302  and, to the common bus line  314 , ROM  304 , RAM  306 , HDD  308 , image memory  310  and image processing unit  312  are connected. 
     Main CPU  302  realizes the function of sheet feeding unit  40  shown in  FIG. 5 , by executing a computer program for realizing the sheet feeding process in accordance with the present embodiment. The program to be executed by main CPU  302  is stored in ROM  304  or HDD  308 . 
     The program stored in ROM  304  or HDD  308  is read from ROM  304  or HDD  308  at the time of execution and stored in RAM  306 . The program is read from an address in RAM  306  indicated by a register functioning as a program counter in main CPU  302 , and interpreted and executed by main CPU  302 . Data necessary for execution are read from a register in main CPU  302 , RAM  306  or HDD  308  at an address designated by the instruction. Similarly, the result of execution is stored in the register in main CPU  302 , RAM  306  or HDD  308  at an address designated by the instruction. 
     To common bus line  314 , connected are image forming unit  26 , transfer mechanism  28 , fixing unit  30 , sheet feeding unit  40 , sheet feeder  42 , automatic document feeder  60 , scanner unit  64 , automatic paper feeding cassette  160 , operating unit  316  of image forming apparatus  20 , and an NIC (Network Interface Card)  322  serving as an interface to client PC  320  or the like as an external device through a LAN (Local Area Network) line  318 . Main CPU  302  controls image forming unit  26 , transfer mechanism  28 , fixing unit  30 , sheet feeding unit  40 , sheet feeder  42 , automatic document feeder  60 , scanner unit  64 , automatic paper feeding cassette  160  and NIC  322 , causing these components to execute desired operations such as document reading, document output, feeding and discharge of sheets and communication with an external device such as client PC  320 , and stores data in or reads data from RAM  306 , HDD  308  and image memory  310 . 
     Operating unit  316  is provided on a front surface of apparatus body  58 . Operating unit  316  is provided with a start key  324 , a display panel  326  and the like. 
     The paper feed program in accordance with the present embodiment is transmitted from other device to controller  300  substantially functioning as a computer, through LAN line  318  and NIC  322 , and stored in ROM  304  or HDD  308 . 
     &lt;Electrical Structure of Sheet Feeding Unit  40 &gt; 
     Referring to  FIG. 5 , sheet feeding unit  40  includes a sub-CPU  400  as a control nerve center of control of the sheet feeding unit  40 . 
     Sub-CPU  400  is connected to common bus line  314  and transmits/receives various data and the like to/from main CPU  302  shown in  FIG. 4 , through common bus line  314 . Sub-CPU  400  is connected to motors  402 ,  404 ,  406 ,  408  and  410  as driving sources of five sets of conveyor rollers  144 ,  146 ,  148 ,  134  and  136  forming the second sheet feeding system, sheet passage sensors  200  and  202 , and first, second and third timers  412 ,  414  and  416 . 
     To sub-CPU  400 , sensing outputs of sheet passage sensors  200  and  202 , and timer outputs (time count values) of timers  412 ,  414  and  416  are applied. Based on the input sensing outputs from sheet passage sensors  202  and  204  and timer outputs from timers  412 ,  414  and  416 , sub-CPU  400  controls driving of motors  402 ,  404 ,  406 ,  408  and  410 . 
     In the present embodiment, sheet feeding unit  40  and controller  300  function as the sheet feeding apparatus of the present invention. 
     &lt;Software Configuration&gt; 
     In the image forming apparatus  20 , even when a plurality of sheets of paper  24  are fed continuously to the paper feed path and the first sheet passage sensor  200  is turned ON by the preceding and succeeding sheets, the first process for continuing sheet feeding based on a determination of overlapped feeding is executed, if a leading edge portion of the preceding sheet has turned ON the second sheet passage sensor  202 . If the states of first and second sheet passage sensors  201  and  202  are not changed even after the execution of the first process, it is determined that a feed-failure jam has occurred, and the program executes a second process for stopping sheet feeding. More specifically, the program executes the first process while driving of the conveyor rollers on the upstream side in the sheet feeding direction than the first sheet passage sensor  200  is stopped. 
     Such a program is stored in RAM  306  or HDD  308  of controller  300 , and it realizes various functions of image forming apparatus  20  as will be described in the following. These functions are realized by main CPU  302  in controller  300  described above, which is substantially a computer, and sub-CPU  400  in sheet feeding unit  40 , executing the program. 
     In the flowchart representing the program structure of the main routine for realizing the sheet feeding function of image forming apparatus  20 , a control flow is shown assuming that sheets of paper  24  are continuously fed to the paper feed path from the second paper feed cassette  36  and a print job remains to be done on last two sheets. Therefore, in the following, of the two sheets on which print job is to be done, the sheet that is fed first will be referred to as the “first sheet” and the sheet that is fed later will be referred to as the “second sheet.” 
     In the present image forming apparatus  20 , receiving an ON operation signal of a start key  324  or receiving a print start signal from an external device such as client PC  320 , main CPU  302  controls driving of image forming unit  26 , transfer mechanism  28 , fixing unit  30 , sheet feeder  42 , scanner unit  64  and sheet feeding unit  40 , based on the signal. 
     Referring to  FIG. 6 , main CPU  302  transmits a sheet feeding command to sub-CPU  400 , and through sub-CPU  400 , causes operations of pick-up roller  138 , paper feed roller  140  and separation roller  142 , whereby the operation of feeding the first sheet from the second paper feed cassette  36  starts (step S 100 ). 
     When the operation of feeding the first sheet starts, sub-CPU  400  turns ON the first timer  412 , and waits for the first sheet passage sensor  200  being turned ON by the first sheet (steps S 102  and S 104 ). 
     When the first sheet passage sensor  200  is turned ON by the first sheet, sub-CPU  400  turns ON the second timer  414 , and waits for the second sheet passage sensor  202  being turned ON by the first sheet (steps S 106  and S 108 ). 
     When the second sheet passage sensor  202  is turned ON by the first sheet, sub-CPU  400  turns ON the third timer  416 , and waits until a count value of first timer  412  attains to T 1  (steps S 110  and S 112 ). 
     When the count value of first timer  412  attains to T 1 , sub-CPU  400  turns OFF the first timer  412  (step S 114 ). At this time, sub-CPU  400  notifies main CPU  302  that the first and second sheet passage sensors  200  and  202  are both turned ON by the first sheet in a time period from when timer  412  was turned ON until the timer reached the time count value T 1 . 
     Then, main CPU  302  operates pick-up roller  138 , paper feed roller  140  and separation roller  142  through sub-CPU  400 , whereby the operation of feeding the second sheet from the second paper feed cassette  36  starts (step S 116 ). 
     When the operation of feeding the second sheet starts, sub-CPU  400  waits for the count value of second timer  414  attaining to T 2 , and when the time count value attains to T 2 , it turns OFF the second timer  414  (steps S 118  and S 120 ). At this time, sub-CPU  400  determines whether or not the first sheet passage sensor  200  made a transition from ON to OFF (step S 124 ). The control flow branches depending on the result of determination. 
     If the first sheet passage sensor  200  has made the transition from ON to OFF at step S 124 , sub-CPU  400  determines that the first and second sheets are fed not overlapped with each other, or the sheets are fed in a normal manner, and forces OFF the third timer  416  and performs a normal sheet feeding process (steps S 126  and S 128 ). When the normal sheet feeding process ends, the control proceeds to step S 200  shown in  FIG. 7 . 
     Now, the normal sheet feeding process will be described. 
     Referring to  FIG. 9 , for executing the normal sheet feeding process, until after the lapse of an idle time AT, sub-CPU  400  turns OFF motors  402 ,  404 ,  406 ,  408  and  410  and temporarily stops driving of all conveyor rollers  144 ,  146 ,  148 ,  134  and  136  of the second sheet feeding system related to the second paper feed cassette  36 , in order to effect print registration of sheets before register roller  82 . After the lapse of idle time AT, motors  402 ,  404 ,  406 ,  408  and  410  are turned ON and all conveyor rollers  144 ,  146 ,  148 ,  134  and  136  are driven again. 
     Again referring to  FIG. 6 , at step S 124 , if the first sheet passage sensor  200  has not made a transition from ON to OFF, sub-CPU  400  determines that the first and second sheets are fed in the overlapped manner, and performs a sheet feeding process to eliminate overlapped feeding (step S 130 ). At the end of sheet feeding process to eliminate overlapped feeding, control proceeds to step S 132 . 
     The sheet feeding process to eliminate overlapped feeding (overlapped feeding eliminating process) will be described. 
     Referring to  FIG. 10 , for executing the sheet feeding process to eliminate overlapped feeding until after the lapse of an idle time AT, sub-CPU  400  turns OFF motors  402 ,  404 ,  406 ,  408  and  410  and temporarily stops driving of all conveyor rollers  144 ,  146 ,  148 ,  134  and  136  of the second sheet feeding system related to the second paper feed cassette  36 , in order to effect print registration of sheets before register roller  82 . After the lapse of idle time AT, motors  404 ,  406 ,  408  and  410  are turned ON while the conveyor rollers  144  on the upstream side in the sheet feeding direction of first sheet passage sensor  200  (closer to the second paper feed cassette than the first sheet passage sensor  200 ) are kept stopped, and remaining conveyor rollers  146 ,  148 ,  134  and  136  are driven again. 
     Again referring to  FIG. 6 , when the sheet feeding process to eliminate overlapped feeding ends and control proceeds to step S 132 , sub-CPU  400  waits for the count value of third timer  416  to attain to T 3 , and when the time count value attains to T 3 , turns OFF the third timer  416  (step S 134 ). At this time, sub-CPU  400  determines whether or not the first sheet passage sensor  200  has made a transition from ON to OFF (step S 136 ). The control flow branches depending on the result of determination. 
     At step S 136 , if the first sheet passage sensor  200  has made a transition from ON to OFF, sub-CPU  400  determines that the first and second sheets have been separated and overlapped feeding is eliminated, and control proceeds to step S 300  shown in  FIG. 8 . On the contrary, if the first sheet passage sensor  200  has not made a transition from ON to OFF, sub-CPU  400  determines that a feed-failure jam has occurred and performs a feed-failure jam processing, in which the image forming apparatus  20  is stopped (step S 138 ). If the feed-failure jam is eliminated by a manual operation by the user, control resumes from step S 100 . 
     Referring to  FIG. 7 , when the normal sheet feeding process ends and control proceeds to step S 200 , sub-CPU  400  turns on the second timer  414  using turning ON of the first sheet passage sensor  200  by the second sheet as a trigger (step S 202 ). 
     Thereafter, when the second sheet passage sensor  202  is turned ON by the second sheet, using this as a trigger, sub-CPU  400  turns ON the third timer  416  (steps S 204  and S 206 ). 
     When the third timer  416  is turned ON, sub-CPU  400  determines whether the first sheet passage sensor  200  has made a transition from ON to OFF, on condition that the second timer count value has attained to T 2  (steps S 208  and S 210 ). The control flow branches depending on the result of determination. 
     At step S 210 , if the first sheet passage sensor  200  has made a transition from ON to OFF, sub-CPU  400  determines it is a normal sheet feeding state, forces OFF the third timer  416  and performs a normal sheet feeding process similar to step S 128  of  FIG. 6  described above (steps S 212  and S 214 ). Thereafter, when the print job ends at step S 216 , sub-CPU  400  ends the present program. 
     If the first sheet passage sensor  200  has not made a transition from ON to OFF at step S 210 , sub-CPU  400  determines that the second sheet and the following sheet are fed in the overlapped manner, and performs the sheet feeding process to eliminate overlapping similar to that of step S 130  of  FIG. 6  (step S 218 ). 
     When the sheet feeding process to eliminate overlapped feeding ends, sub-CPU  400  turns OFF the third timer  416 , using count value of third timer  416  attaining to T 3  as a trigger (steps S 220  and S 222 ). At this time, sub-CPU  400  determines whether or not the first sheet passage sensor  200  has made a transition from ON to OFF (step S 224 ). The control flow branches depending on the result of determination. 
     At step S 224 , if the first sheet passage sensor  200  has made a transition from ON to OFF, sub-CPU  400  determines that the second sheet and the following sheet have been separated and overlapped feeding is eliminated, and performs a process of blank-feeding the next sheet (step S 226 ). At this time of blank-feeding, printing is not done on the following sheet, and the sheet is fed as a blank sheet. Thereafter, control proceeds to step S 216 , and waits for the end of job. On the contrary, if the first sheet passage sensor  200  has not made a transition from ON to OFF, sub-CPU  400  determines that a feed-failure jam has occurred, and performs a feed-failure jam processing, in which the image forming apparatus  20  is stopped (step S 228 ). If the feed-failure jam is eliminated, control resumes from step S 100 . 
     Referring to  FIG. 8 , if it is determined at step S 136  of  FIG. 6  that the first and second sheets are separated and the overlapped feeding is eliminated and control proceeds to step S 300 , sub-CPU  400  starts feeding of the next sheet, and waits for the second sheet passage sensor  202  to be turned ON by the second sheet (step S 302 ). 
     If the second sheet passage sensor  202  is turned ON by the second sheet, control proceeds to step S 210  of  FIG. 7 , and the process steps following step S 210  are executed. 
     &lt;Operation&gt; 
       FIGS. 11 and 12  assume a situation that a plurality of sheets are continuously fed to the paper feed path from the second paper feed cassette  36 . 
     As shown in  FIG. 11 , for feeding the sheets, all conveyor rollers  144 ,  146 ,  148 ,  134  and  136  of the sheet feeding system related to the second paper feed cassette  36  are driven. It is noted that  FIG. 11  only shows conveyor rollers  144 ,  146  and  148 . If the normal sheet separating operation by pick-up roller  138 , paper feed roller  140  and separation roller  142  on the second paper feed cassette  36  fails, overlapped feeding, in which two sheets are fed together with a trailing edge of a preceding sheet  500  and a leading edge of a succeeding sheet  502  overlapped, occurs between the first and second sheet passage sensors  200  and  202 . 
     In the overlapped feeding state, the first sheet passage sensor  200  is turned ON both by the preceding and succeeding sheets  500  and  502 . At this time, if the leading edge of preceding sheet  500  has turned ON the second sheet passage sensor  202 , it is determined to be an overlapped feeding and sheet feeding is continued as shown in  FIG. 12 . Specifically, sheet feeding is continued while conveyor rollers  144  on the upstream side in the sheet feeding direction than first sheet passage sensor  200  are stopped and remaining conveyor rollers  146 ,  148 ,  134  and  136  are driven. 
     The overlapped feeding may be eliminated as the preceding and succeeding sheets  500  and  502  are possibly separated by the continued feeding of sheets. 
     If the states of first and second sheet passage sensors  200  and  202  are unchanged even after the process of continuously feeding the sheets, it is determined to be a feed-failure jam, and sheet feeding is stopped. 
     The present embodiment attains the following functions and effects. 
     (1) Even when a plurality of sheets are fed continuously to the paper feed path and the first sheet passage sensor  200  is turned ON both by the preceding sheet  500  and succeeding sheet  502 , sheet feeding is continuously executed as it is determined to be overlapped feeding, if the leading edge of preceding sheet  500  has turned ON the second sheet passage sensor  202 . Thereafter, if the states of first and second sheet passage sensors  200  and  202  are unchanged, it is determined that a feed-failure jam has occurred, and sheet feeding is stopped. Therefore, it becomes possible to automatically detect and eliminate feed-failure jam caused by overlapped feeding of sheets simply by the combination of ON/OFF of sheet passage sensors  200  and  202  without using a thickness sensor to detect the overlapped feeding. As a result, the burden on the user to remove the sheet can be alleviated. 
     (2) In the process of continuously feeding the sheets, conveyor rollers  144  on the upstream side in the sheet feeding direction than first sheet passage sensor  200  are stopped and remaining conveyor rollers  146 ,  148 ,  134  and  136  are driven. Therefore, the preceding sheet  500  and succeeding sheet  502  can reliably be separated while sheet feeding is continued, to prevent new occurrence of overlapped feeding or feed-failure jam. Thus, smooth operation of continuous sheet feeding is realized. 
     (3) The leading edge of succeeding sheet  502  fed in the overlapped manner is positioned between the first and second sheet passage sensors  200  and  202 . Therefore, if the preceding sheet  500  is the last page of a print job, the succeeding sheet  502  must be discharged without performing any image forming process. In the present feeding control, if the preceding sheet  500  fed in the overlapped manner is the last page of a print job, the sheet following the preceding sheet  500  is discharged without performing any printing process thereon. Therefore, the present invention can easily and advantageously be applied even when a sheet is fed overlapped on a sheet of a last page. 
     The present invention is not limited to the embodiment above. In the embodiment above, an example has been described in which the present invention is applied to a sheet feeding system related to the second paper feed cassette. The present invention, however, is not limited to such an arrangement. By way of example, the present invention may be applied to the sheet feeding system related to the third paper feed cassette. In that case, for the continuous sheet feeding, driving of two sets of conveyor rollers  156  and  158  on the upstream side in the sheet feeding direction than the first sheet passage sensor  200  should be stopped and remaining conveyor rollers  146 ,  148 ,  134  and  136  should be driven. Further, in the embodiment above, an example has been described in which the sheet feeding program is transmitted from another device to the controller through LAN line and NIC and stored in ROM or HDD. The present invention, however, is not limited to such an embodiment. By way of example, a disc drive such as a DVD (Digital Versatile Disk) drive, CD-ROM (Compact Disk-Read Only Memory) drive or FD (Flexible Disk) drive, or a memory port may be provided in place of NIC, and the sheet feeding program recorded on an external recording medium may be introduced to the image forming apparatus thereby. It is naturally understood that various design changes and modifications may be made within the scope of claims as appended to the present specification. 
     The embodiments as have been described here are mere examples and should not be interpreted as restrictive. The scope of the present invention is determined by each of the claims with appropriate consideration of the written description of the embodiments and embraces modifications within the meaning of, and equivalent to, the languages in the claims.