Patent Publication Number: US-8973919-B2

Title: Double-feed detection apparatus and image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 12/412,173, filed Mar. 26, 2009, which claims the benefit of Japanese Application No. 2008-091068, filed Mar. 31, 2008, both of which are hereby incorporated by reference herein in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a double-feed detection apparatus and an image forming apparatus. More particularly, the present invention relates to detection of double feed of sheet-like members in an apparatus for conveying the sheet-like members, and also relates to an image forming apparatus for forming images on the sheet-like members. 
     The term “image forming apparatus” implies an apparatus of the type forming an image on a sheet-like member by employing, e.g., electrophotography. Examples of the image forming apparatus include an electrophotographic copying machine, an electrophotographic printer (such as a laser beam printer or an LED printer), a facsimile, and a word processor. 
     2. Description of the Related Art 
     In an image forming apparatus such as a copying machine or a laser beam printer, an image has hitherto been formed on a transfer material (sheet-like member) as follows. First, transfer materials set in a sheet-like member stacking unit, which serves as a paper feed unit, are separated one by one. The separated transfer material is conveyed to an image forming section including various image forming units. In the image forming section, a formed image is transferred onto the transfer material by a transfer unit. Thereafter, the image is fused to the transfer material by a fusing unit, and the transfer material is ejected out of the apparatus. The image is formed on the transfer material through the above-described steps. When the transfer materials set in the paper feed unit are separated and conveyed, there may occur such a state that two or more transfer materials are conveyed to the transfer unit or the fusing unit in an overlapped relation (hereinafter referred to as “double feed”). If the transfer materials in the double-feed state are conveyed, as they are, up to the transfer unit or the fusing unit, the fusing unit being located downstream of the transfer unit, in the image forming section for forming the image, the transfer and the fusing are often not performed under appropriate conditions. In other words, the transfer materials need to be prevented from being conveyed in the double-feed state to the transfer unit or the fusing unit, which is the image forming unit for forming the image on the transfer material. In the following description, the transfer unit or the fusing unit is also called a double-feed prohibition unit. 
     For that reason, a technique has hitherto been widely practiced in which a double-feed detection mechanism for detecting double feed is disposed on a conveying path extending from a position where the transfer material is separated in the paper feed unit to the transfer unit and, upon detection of the double feed, the transfer materials in the double-feed state are stopped before reaching the double-feed prohibition unit, i.e., the transfer unit or the fusing unit in the image forming section. 
     In the above-described double-feed detection mechanism, if the transfer materials are double-fed with a mutual deviation in the feed direction (hereinafter referred to as “dragged-in double feed”), there is a possibility of generating erroneous detection when the double-feed detection is performed with respect to only the leading end of the transfer material. For example, the dragged-in double feed is not correctly detected (an erroneous detection indicating no double-feed is resulted) in some cases when the double-feed detection is performed with respect to only the leading end of the transfer material. 
     In view of such a problem, Japanese Patent Laid-Open No. 2001-063872 proposes a double-feed detection mechanism. More specifically, the double-feed detection mechanism includes a light-emitting unit, e.g., an LED, arranged near a conveying path along which a transfer material is conveyed, and a light-receiving unit, e.g., a photo-transistor. Further, a light quantity received by the light-receiving unit is sampled within a sample range on the transfer material in a predetermined number of samples, and double-feed detection is performed based on each sampled light quantity data. Double feed of the transfer materials is determined based on the results of plural double-feed detections obtained from the sample range. 
     However, the above-described known double-feed detection mechanism has the following problems. 
     In the above-cited Japanese Patent Laid-Open No. 2001-063872, when the double feed is detected, the determination as to the double feed is made after the completion of all samplings within the sample range. Therefore, when the sample range is larger than the distance from the double-feed detection mechanism to a particular member, such as a roller or a belt, (in the double-feed prohibition unit), the double feed cannot be determined before the transfer material reaches the particular member, such as the roller or the belt. The particular member, such as the roller or the belt, is a member for forming an image on the transfer material in the image forming apparatus. In other words, such a member is included in, e.g., the transfer unit or the fusing unit in which the transfer materials are apt to jam (namely, a paper jam tends to occur) if the transfer materials are conveyed in the double-feed state. Particularly, the fusing unit is a unit in which a difficulty arises in a process of removing the jammed transfer materials after the occurrence of a jam. Thus, it is often difficult to perform a process for coping with a jam occurred when the transfer materials are conveyed in the double-feed state. If a jam occurs, the transfer materials are wastefully consumed. Further, if the transfer materials are conveyed up to the fusing unit in the double or more feed state, the fusing unit may fail. 
     If the sample range is widened, a light emission time of the light-emitting unit made of, e.g., an LED is prolonged correspondingly and the light-emitting unit is degraded to a larger extent. Large degradation of the light-emitting unit causes such a risk that the quantity of emitted light is reduced and accuracy of the double-feed detection is reduced. Conversely, when an LED capable of stably emitting light for a long time is used, the apparatus cost is increased because of the necessity of using an expensive LED. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in view of the above-described state of the art. One exemplary embodiment of the present invention provides an apparatus which can accurately detect double feed of sheet-like members to prevent the sheet-like members from being conveyed in a double-feed state up to a member where a jam is apt to occur or where a jam is difficult to cope with, which can improve efficiency in coping with the jam, and which can prevent a failure of the apparatus. 
     According to an aspect of the invention, a double-feed detection apparatus is provided that detects double feed of transfer materials. The double-feed detection apparatus includes a conveying path along which the transfer materials are each conveyed, a sensor configured to detect the double feed of the transfer materials conveyed along the conveying path, and a control unit configured to execute detection of the transfer material under conveyance plural times with the sensor, and to determine the double feed of the transfer materials based on detection results, wherein the control unit determines a detection timing of the sensor based on both a distance along the conveying path from the sensor to an image forming unit which forms an image on the transfer material and a length of the transfer material in the feed direction. 
     According to another aspect of the present invention, an image forming apparatus including a stacking unit in which transfer materials are stacked, a conveying path along which the transfer materials supplied from the stacking unit are each conveyed, a sensor configured to detect the double feed of the transfer materials conveyed along the conveying path, an image forming unit configured to form an image on the transfer material, and a control unit configured to execute detection of the transfer material under conveyance plural times with the sensor, and to determine double feed of the transfer materials based on detection results, wherein the control unit determines a detection timing of the sensor based on both a distance along the conveying path from the sensor to the image forming unit and a length of the transfer material in the feed direction. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an overall construction of a laser printer according to a first exemplary embodiment. 
         FIG. 2  illustrates an example of a double-feed detection sensor used in the first to third exemplary embodiments. 
         FIG. 3  is a block diagram illustrating a configuration of a double-feed detection control unit in the first exemplary embodiment. 
         FIG. 4  is a flowchart illustrating a double-feed detection process in the first exemplary embodiment. 
         FIG. 5  illustrates a second detection position when a double-feed detection distance is larger than a length of a transfer material in the feed direction in the first exemplary embodiment. 
         FIG. 6  illustrates a second detection position when the double-feed detection distance is not larger than the length of the transfer material in the feed direction in the first exemplary embodiment. 
         FIG. 7  illustrates a third detection position in the first exemplary embodiment. 
         FIG. 8  is a block diagram illustrating a configuration of a double-feed detection control unit in a second exemplary embodiment. 
         FIG. 9  is a flowchart illustrating a double-feed detection process in the second exemplary embodiment. 
         FIG. 10  illustrates an overall construction of a laser printer according to a third exemplary embodiment. 
         FIG. 11  is a block diagram illustrating a configuration of a double-feed detection control unit in the third exemplary embodiment. 
         FIG. 12  is a flowchart illustrating a double-feed detection process in the third exemplary embodiment. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Exemplary embodiments of the present invention will be described in detail for only illustrative purposes with reference to the drawings. It is to be noted that components in the following exemplary embodiments are described for only illustrative purposes, and that the technical scope of the present invention is defined by claims and is not limited by the following exemplary embodiments. 
     The best mode for carrying out the present invention will be described in detail in connection with the exemplary embodiments. 
     In a first exemplary embodiment of the present invention, a timing of double-feed detection is determined depending on a length of a transfer material (sheet-like member) in the feed direction, which has a minimum size among the sheet-like members capable of being stacked in a paper feed unit, i.e., a sheet-like member stacking unit. 
     &lt;Construction of Laser Printer&gt; 
     An overall construction of a laser printer as an example of an image forming apparatus will be described with reference to  FIG. 1 . 
     A laser printer  101  is constructed as follows. The laser printer  101  includes photosensitive drums  5 Y,  5 M,  5 C and  5 K, chargers  7 Y,  7 M,  7 C and  7 K, and laser scanners  10 Y,  10 M,  10 C and  10 K in a one-to-one relation to stations which are arranged side by side corresponding to the number of development colors. Further, the laser printer  101  includes developing units  8 Y,  8 M,  8 C and  8 K, and toner cartridges  11 Y,  11 M,  11 C and  11 K in the respective stations. Moreover, the laser printer  101  includes an intermediate transfer belt  12 , primary transfer rollers  6 Y,  6 M,  6 C and  6 K, a secondary transfer roller  9  (transfer unit), a paper feed unit, a fuser  13  (fusing unit), etc. The photosensitive drums  5 Y,  5 M,  5 C and  5 K are each formed of an aluminum cylinder coated with an organic photoconductive layer on an outer circumference of the aluminum cylinder, and are rotated counterclockwise, as viewed in  FIG. 1 , by a driving motor (not shown) when a printing operation is started. 
     The chargers  7 Y,  7 M,  7 C and  7 K include charging sleeves  7 YS,  7 MS,  7 CS and  7 KS to perform primary charging of the photosensitive drums  5 Y,  5 M,  5 C and  5 K, respectively. 
     Surfaces of the photosensitive drums  5 Y,  5 M,  5 C and  5 K are selectively exposed by the laser scanners  10 Y,  10 M,  10 C and  10 K in accordance with signals of input image data (i.e., image signals), thus successively forming electrostatic latent images. The developing units  8 Y,  8 M,  8 C and  8 K include respectively developing sleeves  8 YS,  8 MS,  8 CS and  8 KS to visualize the electrostatic latent image. 
     The intermediate transfer belt  12  is an endless belt running over a driving roller  18   a  and driven rollers  18   b  and  18   c  under tension. The intermediate transfer belt  12  is rotated clockwise while contacting the photosensitive drums  5 Y,  5 M,  5 C and  5 K. During the rotation, toner images are successively primary-transferred onto the surface of the intermediate transfer belt  12  with primary transfer processes performed by the primary transfer rollers  6 Y,  6 M,  6 C and  6 K. 
     Transfer materials P are stacked in a paper feed cassette  2  or a paper feed tray  3 , each of which serves as a paper feed unit. The transfer materials P are each fed (supplied) onto a conveying path  25  by a paper feed roller  4 . The transfer material P is conveyed along the conveying path  25  which is constructed of conveying rollers  24 , etc., and then reaches the position of a pre-registration sensor  19 . The transfer material P is further conveyed through a certain distance. Upon reaching a registration roller  23 , the transfer material P forms a loop and comes into a standby state. When the transfer material P in the standby state is conveyed again, the transfer material P is advanced in a sandwiched relation between the intermediate transfer belt  12  and the secondary transfer roller  9  in a state of the secondary transfer roller  9  contacting the intermediate transfer belt  12 . Accordingly, color visible images having been multi-transferred onto the intermediate transfer belt  12  are secondary-transferred onto the transfer material P together. The secondary transfer roller  9  comes into contact with the intermediate transfer belt  12  during the secondary transfer, as indicated by a solid line. After the secondary transfer, the secondary transfer roller  9  is moved away from the intermediate transfer belt  12  to a position indicated by a dotted line. 
     A cleaner container  21  cleans the intermediate transfer belt  12  by a cleaning blade mounted in the cleaner container  21 , and holds therein, as waste toner, toner that remains on the intermediate transfer belt  12  after the secondary transfer without being transferred. 
     The fuser  13  fuses the toner image (developer image) on the transfer material P while the transfer material P is transferred. The fuser  13  includes a fusing roller  14  to heat the toner, and a pressing roller  15  to bring the transfer material P into pressure contact with the fusing roller  14 . The fusing roller  14  and the pressing roller  15  are formed to be hollow and include respectively heaters  16  and  17  mounted in their inner spaces. After the toner image on the transfer material P has been fused by the fuser  13 , the transfer material P passes through a conveying path  26  and is ejected out of the laser printer  101 . 
     The cleaner container (cleaning unit)  21  removes the toner remaining on the photosensitive drums  5 Y,  5 M,  5 C and  5 K and the intermediate transfer belt  12 . Waste toner left after transferring the color visible images, which are formed on the intermediate transfer belt  12 , onto the transfer material P is held in the cleaner container  21 . 
     Reference numeral  27  denotes a double-feed detection sensor (detection unit) for detecting double feed of the transfer materials P. In the laser printer  101  illustrated in  FIG. 1 , the double-feed detection sensor  27  is arranged upstream of the registration roller  23  in the transfer-material conveying path to detect the double feed of the transfer materials P passing the sensor. 
       FIG. 2  illustrates an example of construction of the double-feed detection sensor  27 . The double-feed detection sensor  27  includes an LED  201  serving as a light-emitting unit, and a detector  202  serving as a light-receiving unit. The double-feed detection sensor  27  detects the occurrence of double feed based on a light quantity received by the detector  202 . In other words, the double-feed detection sensor  27  is a sensor for detecting a quantity of light passing through the transfer material and obtaining information regarding the thickness of the transfer material. The detecting operation of the double-feed detection sensor  27  in this first exemplary embodiment is performed by turning on the LED  201  to emit light for a predetermined time while the transfer material P is conveyed, and detecting the occurrence of double feed when the quantity of light received by the detector  202  is larger than a preset threshold. The light emission time during which the LED  201  is turned on to emit the light is set to, e.g., several tens milliseconds. The threshold is set to a value that is previously obtained with experiments depending on the type of paper (such as ordinary paper, thick paper, or thin paper). Another sensor for detecting, e.g., information with respect to the basis weight of the transfer material can also be used instead of the sensor for detecting information with respect to the thickness of the transfer material. 
     &lt;Configuration of Double-Feed Detection Control Unit&gt; 
       FIG. 3  is a block diagram illustrating a configuration of the double-feed detection control unit in the first exemplary embodiment when the double-feed detection control unit is applied to the laser printer  101 . Note that various components of the laser printer other than the double-feed detection control unit are not described here. 
     A laser printer control unit  301  for operating the laser printer  101  includes a memory  302  (storage unit) and a double-feed detection control unit  303  (detection control unit). The laser printer control unit  301  displays, on a display panel  304 , information such as an error occurred in the laser printer  101 . 
     Reference numeral  305  denotes a driving motor that drives the driving roller  18   a ,  306  denotes a driving motor that drives the registration roller  23 , and  307  denotes a driving motor that drives the conveying rollers  24 . 
     The laser printer control unit  301  executes a double-feed detection process by obtaining a value held in the memory  302  and the detection result of the pre-registration sensor  19 , and by controlling the double-feed detection sensor  27  and the driving motors  305 ,  306  and  307 . A double-feed detection apparatus employed in the laser printer  101  in this first exemplary embodiment is made up of the double-feed detection sensor  27 , the memory  302 , and the double-feed detection control unit  303 . 
     &lt;Manner of Obtaining Double-Feed Detection Position on Transfer Material in Double-Feed Detection Process&gt; 
       FIG. 4  is a flowchart illustrating the double-feed detection process in the first exemplary embodiment, and each of  FIGS. 5 ,  6  and  7  illustrates the double-feed detection position in the laser printer  101 . 
     In this first exemplary embodiment, it is assumed that the fuser  13  as one of image forming units is a double-feed prohibition unit, and the distance from the double-feed detection sensor  27  to the fuser  13  along the conveying path  25  is a double-feed detection distance L (see  FIG. 1 ). In other words, the occurrence of double feed is detected before the transfer materials conveyed in the double-feed state reach the fuser  13 . Further, it is assumed that l represents the length of a transfer material in the feed direction, which has a minimum size among the transfer materials capable of being stacked in the paper feed tray  3 , and α represents the distance through which the transfer material is conveyed until an image forming process is interrupted after executing the double-feed detection. Stated another way, the length l of the transfer material in the feed direction is a value that is preset on the basis of the transfer material which has a minimum size among the transfer materials capable of being stacked at each paper feed inlet (i.e., in the paper feed cassette  2  or the paper feed tray  3 ). A value (&gt;0) of α represents a distance through which the transfer material is conveyed until, after the double-feed detection control unit  303  detects the occurrence of double feed and instructs the driving motor for conveying the transfer material to stop, the driving motor is stopped and the conveyance of the transfer material is actually stopped. The α is a distance transported in an inertia of the driving motor. Thus, the value of α is preset depending on parameters such as the conveyance speed of the transfer material and the type of the transfer material. The values of l and α are stored in the memory  302  (or a not-shown ROM). In addition, it is assumed that the double-feed detection process is started at a time when the laser printer  101  starts the printing operation and the leading end of the transfer material arrives at the pre-registration sensor  19 . Namely, the double-feed detection process is assumed to be started with respect to a leading end portion of the transfer material. 
     When the double-feed detection process is started, the double-feed detection is performed on the leading end portion of the transfer material (i.e., the leading end portion thereof in the feed direction) corresponding to a first detection position (step  401  (hereinafter referred to simply as “S 401 ”)), thus determining whether double feed occurs (S 402 ). 
     If the double feed is determined in S 402 , a double-feed treating process is executed (S 403 ), following which the double-feed detection process is brought to an end. 
     In the double-feed treating process in S 403 , the driving motors  305 ,  306  and  307  are stopped to interrupt the conveyance of the transfer material (i.e., to stop the conveying operation). Thereafter, an error is notified to the display panel  304 . The error is notified, for example, as a message indicating that the double feed has occurred, or that an abnormality has occurred in the conveyance of the transfer material. 
     On the other hand, if the double feed is not determined in S 402 , the double-feed detection distance L and the length l of the transfer material in the feed direction, which are stored in the memory  302 , are obtained (S 404 ). 
     Next, the double-feed detection distance L and the length l of the transfer material in the feed direction obtained in S 404  are compared with each other (S 405 ). 
     If the double-feed detection distance L is larger than the length l of the transfer material in the feed direction, a next double-feed detection position corresponding to a second detection position is set, as illustrated in  FIG. 5 , to a position away from the current detection position through (l−α) (i.e., a position resulting from adding a value (l−α), which is smaller than the length l of the transfer material in the feed direction, to the first detection position) (S 406 ). 
     On the other hand, if the double-feed detection distance L is not larger than the length l of the transfer material in the feed direction, the next double-feed detection position corresponding to the second detection position is set, as illustrated in  FIG. 6 , to a position away from the current detection position through (L−α) (i.e., a position resulting from adding a value (L−α), which is not larger than the double-feed detection distance L, to the first detection position) (S 407 ). 
     After waiting for that the next double-feed detection position on the transfer material reaches the double-feed detection sensor  27  (S 408 ), the double-feed detection is executed again (S 409 ), thus determining whether double feed occurs (S 410 ). 
     If the double feed is determined in S 410 , the double-feed treating process in S 403  is executed, following which the double-feed detection process is brought to an end. On the other hand, if the double feed is not determined in S 410 , the double-feed detection position is calculated in order to continuously perform the double-feed detection. More specifically, a next double-feed detection position corresponding to a third or subsequent detection position is set, as illustrated in  FIG. 7 , to a position away from the current detection position (at that time) through (L−α) (i.e., a third or subsequent position resulting from adding the value (L−α), which is smaller than the double-feed detection distance L, to the second detection position) (S 411 ). 
     The above-described processing of S 408  to S 411  is executed until the tailing end of the transfer material is detected by the pre-registration sensor  19 . At a time when the tailing end of the transfer material is detected, the double-feed detection process is brought to an end (S 412 ). 
     According to the first exemplary embodiment, as described above, the double-feed detection can be performed plural times at proper timings depending on the double-feed detection distance L and the length l of the transfer material in the feed direction, which has a minimum size among the transfer materials capable of being stacked at the paper feed inlet. Therefore, even if dragged-in double feed occurs, the transfer material is prevented from being conveyed up to the fuser  13  which is the double-feed prohibition unit. Hence, efficiency in coping with a jam can be improved and a failure of the fuser  13  can be prevented. Moreover, wasteful consumption of the transfer materials can be avoided. 
     Further, since the LED  201  serving as the light-emitting unit is not required to continuously emit light over a wide range to perform samplings, degradation of the LED  201  can be suppressed and accuracy of the detection using the LED  201  can be maintained. In addition, the detection accuracy can be maintained by using an inexpensive LED without using an expensive LED. 
     It is be noted that the above-described first exemplary embodiment can be variously modified in conformity with the gist of the present invention and is not intended to exclude those various modifications from the scope of the invention. 
     The paper feed inlet in the first exemplary embodiment can be constituted by using a known paper feed cassette as the paper feed tray  3  that can hold plural sizes of paper. For example, when a cassette capable of holding B5-size paper and A4-size paper is used as the paper feed cassette, the length of the B5-size paper in the feed direction is stored as l in the memory. 
     In the case of an image forming apparatus, such as the laser printer  101 , in which a plurality of paper feed units are employed and the length of a minimum-size transfer material in the conveying direction, which is capable of being stacked in each paper feed unit, differs between or among the paper feed units, the length l of the transfer material in the feed direction for each paper feed unit can be stored in the memory  302  and the setting length l of the transfer material in the feed direction can be changed depending on the paper feed unit selected in use. 
     In a second exemplary embodiment of the present invention, the timing of the double-feed detection is determined based on a detection result of a tray size sensor for detecting the length of the transfer material in the feed direction. 
     &lt;Configuration of Double-Feed Detection Control Unit&gt; 
       FIG. 8  is a block diagram illustrating a configuration of a double-feed detection control unit in the second exemplary embodiment when the double-feed detection control unit is applied to the laser printer  101 . Note that the components described above in the first exemplary embodiment are denoted by the same reference numerals and a description of those components is not repeated here. 
     Reference numeral  50  denotes a tray size sensor (i.e., a detection unit configured to detect the length of the sheet-like member in the feed direction), which is disposed within the paper feed cassette  2  and which is connected to the laser printer control unit  301 . The laser printer control unit  301  calculates the length of the transfer material in the feed direction, which is present within the paper feed cassette  2 , based on information obtained with the tray size sensor  50 , and determines the timing of the double-feed detection based on the calculation result. The tray size sensor  50  is, e.g., a sensor configured to detect the position of a member for restricting the transfer materials set in the paper feed cassette  2 . More specifically, when the paper feed cassette  2  includes a plate which is movable to slide in conformity with the sheet size (size in each of the feed direction and a direction perpendicular to the feed direction) at the time of stacking the transfer materials in the paper feed cassette  2 , the tray size sensor  50  is a sensor configured to detect the sheet size from the position of the plate after the sliding. Instead of such a sensor, other type of sensor can also be used as the tray size sensor  50  so long as the sensor is able to detect the length of the transfer material in the feed direction, which is set in the paper feed cassette  2 . 
     &lt;Manner of Obtaining Double-Feed Detection Position on Transfer Material in Double-Feed Detection Process&gt; 
       FIG. 9  is a flowchart illustrating a double-feed detection process in the second exemplary embodiment. 
     In this second exemplary embodiment, as in the first exemplary embodiment, it is assumed that the fuser  13  is the double-feed prohibition unit, and the distance from the double-feed detection sensor  27  to the fuser  13  is a double-feed detection distance L. Further, it is assumed that Cst_l represents the length of the transfer material in the feed direction, which is obtained from the tray size sensor  50 , and α represents the distance through which the transfer material is conveyed until an image forming process is interrupted after executing the double-feed detection. In addition, it is assumed that the double-feed detection process is started at a time when the laser printer  101  starts the printing operation and the leading end of the transfer material arrives at the pre-registration sensor  19 . 
     When the double-feed detection process is started, the double-feed detection is performed on a leading end portion of the transfer material corresponding to a first detection position (step  421 ), thus determining whether double feed occurs (S 422 ). 
     If the double feed is determined in S 422 , a double-feed treating process is executed (S 423 ), following which the double-feed detection process is brought to an end. 
     In the double-feed treating process in S 423 , the driving motors  305 ,  306  and  307  are stopped to interrupt the conveyance of the transfer material. Thereafter, an error is notified to the display panel  304 . The error is notified, for example, as a message indicating that the double feed has occurred, or that an abnormality has occurred in the conveyance of the transfer material. 
     On the other hand, if the double feed is not determined in S 422 , the double-feed detection distance L is obtained from the memory  302  and the length Cst_l of the transfer material in the feed direction is obtained from the tray size sensor  50  (S 424 ). 
     Next, the double-feed detection distance L and the length Cst_l of the transfer material in the feed direction both obtained in S 424  are compared with each other (S 425 ). 
     If the double-feed detection distance L is larger than the length Cst_l of the transfer material in the feed direction, a next double-feed detection position corresponding to a second detection position is set to a position away from the current detection position through (Cst_l−α) (S 426 ). On the other hand, if the double-feed detection distance L is not larger than the length Cst_l of the transfer material in the feed direction, the next double-feed detection position corresponding to the second detection position is set to a position away from the current detection position through (L−α) (S 427 ). 
     After waiting for that the next double-feed detection position on the transfer material reaches the double-feed detection sensor  27  (S 428 ), the double-feed detection is executed again (S 429 ), thus determining whether double feed occurs (S 430 ). 
     If the double feed is determined in S 430 , the double-feed treating process in S 423  is executed, following which the double-feed detection process is brought to an end. On the other hand, if the double feed is not determined in S 430 , a next double-feed detection position corresponding to a third or subsequent detection position is set, for continuing the double-feed detection, to a position away from the current detection position (at that time) through (L−α) (S 431 ). 
     The above-described processing of S 428  to S 431  is executed until the tailing end of the transfer material is detected by the pre-registration sensor  19 . At a time when the tailing end of the transfer material is detected, the double-feed detection process is brought to an end (S 432 ). 
     According to the second exemplary embodiment, as described above, the double-feed detection can be performed plural times at proper timings depending on the double-feed detection distance and the length of the transfer material in the feed direction, which is set at the paper feed inlet. Therefore, even if dragged-in double feed occurs, the transfer material is prevented from being conveyed up to the fuser  13  which is the double-feed prohibition unit. Hence, efficiency in coping with a jam can be improved and a failure of the fuser  13  can be prevented. Moreover, wasteful consumption of the transfer materials can be avoided. 
     Further, since the LED  201  serving as the light-emitting unit is not required to continuously emit light over a wide range to perform samplings, degradation of the LED  201  can be suppressed and accuracy of the detection using the LED  201  can be maintained. In addition, the detection accuracy can be maintained by using an inexpensive LED without using an expensive LED. 
     It is be noted that the above-described second exemplary embodiment can be variously modified in conformity with the gist of the present invention and is not intended to exclude those various modifications from the scope of the invention. 
     In the case of an image forming apparatus, such as the laser printer  101 , in which a plurality of paper feed units are employed and the length of a minimum-size transfer material in the conveying direction, which is capable of being stacked in each paper feed unit, differs between or among the paper feed units, the length Cst_l of the transfer material in the feed direction for each paper feed unit can be stored and the setting length Cst_l of the transfer material in the feed direction can be changed depending on the paper feed unit selected in use. 
     In a third exemplary embodiment of the present invention, the timing of the double-feed detection is determined based on a result of detecting an actual length of the transfer material in the feed direction during conveyance of the transfer material and a detection result of the tray size sensor. In this third exemplary embodiment, it is assumed that the distance from the double-feed detection sensor  27  to the fuser  13  is a double-feed detection distance L, and the distance from a feed sensor  51  (described later) to the double-feed detection sensor  27  is L 2 . The third exemplary embodiment will be described below in connection with the case of L≦L 2  (see  FIG. 10 ). 
     &lt;Construction of Laser Printer&gt; 
       FIG. 10  illustrates an overall construction of a laser printer as an example of an image forming apparatus, which is similar to that described in the first and second exemplary embodiments. In this third exemplary embodiment, however, the feed sensor  51  is disposed near the paper feed cassette  2 . 
     &lt;Configuration of Double-Feed Detection Control Unit&gt; 
       FIG. 11  is a block diagram illustrating a configuration of a double-feed detection control unit in the third exemplary embodiment when the double-feed detection control unit is applied to the laser printer  101 . Note that the components described above in the first exemplary embodiment are denoted by the same reference numerals and a description of those components is not repeated here. 
     The laser printer control unit  301  determines the timing of the double-feed detection based on information from the tray size sensor  50  as a first unit for detecting the length of the sheet-like member (transfer material) in the feed direction, and from the feed sensor  51  as a second unit for detecting the length of the sheet-like member (transfer material) in the feed direction. The reason why the double feed is determined based on the information from both the tray size sensor  50  and the feed sensor  51  in this third exemplary embodiment is as follows. 
     For example, in a state that the paper feed cassette  2  is set as a cassette for stacking A4-size sheets (transfer materials), a user may erroneously set B5-size sheets (transfer materials) in the paper feed cassette  2 . In that case, if the double-feed detection is performed based on the information from the tray size sensor  50 , the double-feed detection cannot be correctly performed. Even in such a situation, this third exemplary embodiment can ensure reliable double-feed detection. 
     &lt;Manner of Obtaining Double-Feed Detection Position on Transfer Material in Double-Feed Detection Process&gt; 
       FIG. 12  is a flowchart illustrating a double-feed detection process in the third exemplary embodiment. 
     In this third exemplary embodiment, as in the first and second exemplary embodiments, it is assumed that the fuser  13  is the double-feed prohibition unit, the distance from the double-feed detection sensor  27  to the fuser  13  is a double-feed detection distance L, and the distance from the feed sensor  51  to the double-feed detection sensor  27  is L 2 . Further, it is assumed that Real_l represents the length of the transfer material in the feed direction, which is obtained from the feed sensor  51 , and α represents the distance through which the transfer material is conveyed until an image forming process is interrupted after executing the double-feed detection. In addition, it is assumed that the double-feed detection process is started at a time when the laser printer  101  starts the printing operation and the leading end of the transfer material arrives at the pre-registration sensor  19 . 
     When the double-feed detection process is started, the double-feed detection is performed on a leading end portion of the transfer material corresponding to a first detection position (step  461 ), thus determining whether double feed occurs (S 462 ). 
     If the double feed is determined in S 462 , a double-feed treating process is executed (S 463 ), following which the double-feed detection process is brought to an end. 
     In the double-feed treating process in S 463 , the driving motors  305 ,  306  and  307  are stopped to interrupt the conveyance of the transfer material. Thereafter, an error is notified to the display panel  304 . The error is notified, for example, as a message indicating that the double feed has occurred, or that an abnormality has occurred in the conveyance of the transfer material. 
     On the other hand, if the double feed is not determined in S 462 , it is determined whether the tailing end of the transfer material has passed the feed sensor  51  and the actual length of the transfer material in the feed direction has been confirmed (S 464 ). If the actual length of the transfer material in the feed direction is not confirmed, the double-feed detection distance L is obtained from the memory  302  and the length Cst_l of the transfer material in the feed direction is obtained from the tray size sensor  50  (S 465 ). Next, the double-feed detection distance L and the length Cst_l of the transfer material in the feed direction both obtained in S 465  are compared with each other (S 466 ). 
     If the double-feed detection distance L is larger than the length Cst_l of the transfer material in the feed direction, a next double-feed detection position corresponding to a second detection position is set to a position away from the current detection position through (Cst_l−α) (S 467 ). On the other hand, if the double-feed detection distance L is not larger than the length Cst_l of the transfer material in the feed direction, the next double-feed detection position corresponding to the second detection position is set to a position away from the current detection position through (L−α) (S 468 ). 
     If the actual length of the transfer material in the feed direction is confirmed in S 464 , the double-feed detection distance L is obtained from the memory  302  and the length Real_l of the transfer material in the feed direction is obtained from the feed sensor  51  (S 469 ). Next, the double-feed detection distance L and the length Real_l of the transfer material in the feed direction both obtained in S 469  are compared with each other (S 470 ). 
     If the double-feed detection distance L is larger than the length Real_l of the transfer material in the feed direction, a next double-feed detection position corresponding to a second detection position is set to a position away from the current detection position through (Real_l−α) (S 471 ). On the other hand, if the double-feed detection distance L is not larger than the length Real_l of the transfer material in the feed direction, the next double-feed detection position corresponding to the second detection position is set to a position away from the current detection position through (L−α) (S 472 ). 
     After waiting for that the next double-feed detection position on the transfer material reaches the double-feed detection sensor  27  (S 473 ), the double-feed detection is executed again (S 474 ), thus determining whether double feed occurs (S 475 ). 
     If the double feed is determined in S 475 , the double-feed treating process in S 463  is executed, following which the double-feed detection process is brought to an end. On the other hand, if the double feed is not determined in S 475 , a next double-feed detection position corresponding to a third or subsequent detection position is set, for continuing the double-feed detection, to a position away from the current detection position (at that time) through (L−α) (S 476 ). 
     The above-described processing of S 473  to S 476  is executed until the tailing end of the transfer material is detected by the pre-registration sensor  19 . At a time when the tailing end of the transfer material is detected, the double-feed detection process is brought to an end (S 477 ). 
     According to the third exemplary embodiment, as described above, the double-feed detection can be performed plural times at proper timings depending on the double-feed detection distance and the length of the transfer material in the feed direction, which is set at the paper feed inlet, or the actual length of the transfer material detected during the conveyance. Therefore, even if dragged-in double feed occurs, the transfer material is prevented from being conveyed up to the fuser  13  which is the double-feed prohibition unit. Hence, efficiency in coping with a jam can be improved and a failure of the fuser  13  can be prevented. Moreover, wasteful consumption of the transfer materials can be avoided. 
     Further, since the LED  201  serving as the light-emitting unit is not required to perform samplings over a wide range, degradation of the LED  201  can be suppressed and accuracy of the detection using the LED  201  can be maintained. In addition, the detection accuracy can be maintained by using an inexpensive LED without using an expensive LED. 
     It is be noted that the above-described third exemplary embodiment can be variously modified in conformity with the gist of the present invention and is not intended to exclude those various modifications from the scope of the invention. 
     For example, when the paper feed inlet is constituted by a free size cassette (not shown) or the like which allows a user to set transfer materials of desired sizes therein, the double feed can be detected by employing only the operation in which the double-feed detection process is executed using the length of the transfer material in the feed direction obtained from the feed sensor  51  and the double-feed detection distance L, as described above in the third exemplary embodiment. 
     In the case using the free size cassette, because the user can set transfer materials of desired sizes, the size of the transfer material is uncertain (i.e., the sheet size is not definite at a time when the transfer material starts to be fed). Therefore, the double-feed detection process can be performed in accordance with a flowchart obtained by excluding S 465 , S 466 , S 467  and S 468  from the flowchart of  FIG. 12  described above in the third exemplary embodiment. The use of the free size cassette can be designated by previously setting the paper feed inlet for the free size cassette, or designated in accordance with an instruction from the user or a print command. Such a modified exemplary embodiment can reliably detect the double feed even when the size of the transfer material is uncertain, while providing similar advantages to those of the first to third exemplary embodiments. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all modifications and equivalent structures and functions.