Abstract:
The present invention provides a sheet conveying apparatus comprising a sheet feeding device for separating and feeding stacked sheets one by one, a convey guide device for guiding the sheet fed out by the sheet feeding device, a spacing device provided on the guide device and adapted to deviate positions of a trailing end of a preceding sheet and a leading end of a succeeding sheet which is fed continuously to the preceding sheet, a sheet detecting device for detecting the leading end of the succeeding sheet deviated from the trailing end of the preceding sheet by the spacing device.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a sheet conveying apparatus for conveying a cut sheet in an image forming apparatus such as a printer, a facsimile, a copying machine, a printing device and the like. 
     2. Related Background Art 
     In image forming apparatuses such as copying machines, printers, facsimiles and the like, there has widely been used an apparatus for fixing a non-fixed toner image formed on a material to be recorded (for example, a paper sheet such as transfer material, photosensitive paper, electrostatic recording paper and printing paper) in correspondence to image information with a transfer system (indirect system) or a direct system by using an appropriate imaging process mechanism of electrophotographic type, electrostatic type or magnetic recording type. 
     Further, there has been used an apparatus (of ink jet type, for example) for directly forming an image on a material to be recorded, by using liquid including dye or/and pigment. 
     As a sheet conveying apparatus for conveying the material to be recorded (referred to as “sheet” hereinafter) in the above-mentioned apparatuses, there is a sheet conveying apparatus of automatic feeding type in which one cassette corresponding to desired sheet kind or sheet size is selected among a plurality of sheet feeding cassettes each containing sheets and a sheet is fed out one by one from such a sheet feeding cassette in synchronous with an image forming operation. 
     In such a sheet conveying apparatus, the following sheet detecting means have widely been used as a means for detecting sheet jam or detecting a leading end or a trailing end of the sheet. 
     A. Lever Type 
     As shown in FIG. 22A, a leading end of a sheet P conveyed along between guides G falls a lever L down so that an photo-interrupter I is blocked by a flag F opposite end of the lever, thereby detecting the leading end of the sheet. The lever L is biased in a clockwise direction by a spring or a gravity force to the extent that the sheet is not buckled. 
     B. Sensor of Reflection Type 
     An infrared ray is emitted toward a sheet, and reflected light or scattered light from the sheet is detected by a light receiving element. On the basis of a light amount received, presence/absence of the sheet is judged. 
     C. Sensor of Permeable Type 
     An infrared ray is emitted toward a sheet, and light passed through the sheet is received by a light receiving element. On the basis of a light amount passed, presence/absence of the sheet is detected. 
     In recent years, digitization has been progressed. For example, the more the number of revolutions of a polygon mirror is increased and drive frequencies of an LED head and an ink jet head are increased the more technical problems are increased, thereby increasing solution cost. Thus, in apparatuses having the same image forming speed, it has been requested that a distance between the sheets continuously conveyed be decreased to improve productivity. Further, regarding energy aspect and endurance aspect of all of parts moved or rotated, it is desirable that the image forming speed be reduced as less as possible. 
     However, if the distance between the sheets continuously conveyed is reduced to zero ultimately, since the leading and trailing ends of the sheet cannot be detected, control for sheet conveyance cannot be effected. For example, alignment between the sheet and an image position and detection of a jammed sheet cannot be effected. 
     In the lever type (A), as shown in FIG. 2B, if the trailing end of the sheet does not pass through the distal end of the fallen lever L, since the lever cannot be returned to its original position, at least a sheet-to-sheet distance (La) corresponding to a length of the lever L is required, and a time period of several tends of milliseconds is required for returning the lever L. Regarding this time period, as the sheet conveying speed is increased, loss of the sheet-to-sheet distance becomes greater; for example, when the time period for returning the lever L is 30 milliseconds and the sheet conveying speed is 500 mm/sec, the sheet-to-sheet distance of 15 mm is further required. 
     In the sensor of reflection type (B) and the sensor of permeable type (C), although the loss of the sheet-to-sheet distance is smaller than in the lever type, the sheet-to-sheet distance of about 5 mm is required for preventing erroneous detection. 
     SUMMARY OF THE INVENTION 
     The present invention aims to eliminate the above-mentioned conventional drawbacks, and an object of the present invention is to provide a sheet conveying apparatus in which, even when a sheet-to-sheet distance is approached to zero, the sheet can be detected. 
     According to the present invention, there is provided a sheet conveying apparatus comprising a sheet feeding means for separating and feeding sheets one by one, a convey guide means for guiding the sheet fed out by the sheet feeding means, a spacing means provided on the guide means and adapted to deviate positions of a trailing end of a preceding sheet and a leading end of a succeeding sheet which is fed continuously to the preceding sheet, and a sheet detecting mean for detecting the leading end of the succeeding sheet deviated from the trailing end of the preceding sheet by the spacing means. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional view of an image forming apparatus according to a first embodiment of the present invention; 
     FIGS. 2A,  2 B,  2 C,  2 D and  2 E are sectional views showing a sheet detecting portion according to a first embodiment of the present invention; 
     FIGS. 3A,  3 B and  3 C are sectional views of a sheet detecting portion according to a first embodiment of the present invention; 
     FIG. 4 is a sectional view showing another example of a sheet detecting portion; 
     FIG. 5 is a sectional view showing a further example of a sheet detecting portion; 
     FIG. 6 is a sectional view showing a still further example of a sheet detecting portion; 
     FIG. 7 is a sectional view showing an example of an auxiliary means; 
     FIG. 8 is a sectional view showing another example of an auxiliary means; 
     FIG. 9 is a sectional view showing a further example of an auxiliary means; 
     FIG. 10 is a view showing a construction of an image forming apparatus according to a second embodiment of the present invention; 
     FIG. 11 is a schematic enlarged view of a convey path for conveying a sheet to an image forming portion of the image forming apparatus; 
     FIG. 12 is a control block diagram of the image forming apparatus; 
     FIG. 13 is a view for explaining a non-detection area of a leading end detection sensor of the image forming apparatus; 
     FIG. 14 is a view for explaining a leading end detection condition of the leading end detection sensor; 
     FIGS. 15A and 15B are views for explaining a leading end detecting operation of the image forming apparatus; 
     FIGS. 16A and 16B are views for explaining a leading end detecting operation of the image forming apparatus; 
     FIG. 17 is a view showing another construction of the image forming apparatus; 
     FIG. 18 is a view showing a construction of an image forming apparatus according to a third embodiment of the present invention; 
     FIG. 19 is a schematic enlarged view of a longitudinal path portion of the image forming apparatus; 
     FIGS. 20A and 20B are views for explaining a leading end detecting operation of the image forming apparatus; 
     FIGS. 21A and 21B are views for explaining a leading end detecting operation of the image forming apparatus; and 
     FIGS. 22A and 22B are sectional view of a conventional sheet detecting means. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Now, an example that a sheet conveying apparatus according to the present invention is applied to a copying machine of electrophotographic type will be explained. FIG. 1 is a sectional view showing a schematic construction of the copying machine according to a first embodiment of the present invention. 
     First of all, the schematic construction of the copying machine will be described with reference to FIG.  1 . In the copying machine, image information read in a reader portion  1  having a scanning optical system is photo-electrically converted, and the converted data is sent to an image forming portion (image forming means)  2 , an image is formed on a sheet P fed by a sheet feeding portion (sheet feeding means)  3  in the image forming portion  2 . After the image formation, the sheet P is conveyed to a fixing device  4 , where a transferred image is a fixed to the sheet by heat and pressure. Since a series of electrophotographic processes are well-known, detailed explanation thereof will be omitted. 
     Next, a construction of the reader portion  1  will be described. An original D rested on an original glass plate  1   a  is illuminated by light from a scanning optical system  1   b  having a light source and a group of mirrors, and reflected light is focused on a CCD  1   d  through a reducing glass  1   c . Then, after the light is photo-electrically converted and A/D-converted, image information is transferred to a memory. A maximum original size is LTR size or A 3  size. 
     A sheet feeding cassette  3   a  containing sheets P is detachably mounted to a lower part of the copying machine. In the apparatus according to the illustrated embodiment, the sheets are conveyed with a sheet-to-sheet distance of “zero”. Now, an operation will be described with reference to FIGS. 2A to  2 E. 
     Before the sheet is fed, as shown in FIG. 2A, a solenoid (not shown) connected to a pick-up roller  3   c  is turned ON to space the pick-up roller  3   c  apart from a surface of a sheet stack. 
     When the sheet is fed, as shown in FIG. 2B, the solenoid is turned OFF to abut the pick-up roller  3   c  against the sheet stack. 
     A first (preceding) sheet starts to be fed by the rotating pick-up roller  3   c . A driving force is transmitted from a conveying roller  3 e through a timing belt  3   b . 
     After the first sheet was fed, when a trailing end of the sheet reaches the pick-up roller  3   c , as shown in FIG. 2C, since the pick-up roller  3   c  still abuts against the sheet stack, a second (succeeding) sheet is also fed in an overlapped condition by a distance X. 
     The sheet is pinched between the conveying roller  3   e  and a retard roller  3   f  and is conveyed by these rollers. The conveying roller  3   e  is rotated in a sheet conveying direction, and the retard roller  3   f  is rotatingly driven in a direction opposite to the sheet conveying direction via a torque limiter (not shown). Thus, since only a leading end portion of the first sheet exists between the rollers  3   e ,  3   f , a friction force between the sheet and the rollers overcomes the torque limiter, with the result that the retard roller  3   f  is rotated in the sheet conveying direction. 
     As shown in FIG. 2D, an X portion of the trailing end of the first sheet (where two sheet are overlapped) reaches the nip between the rollers  3   e ,  3   f , since the friction force between the first and second sheets is smaller than load (limit value) for interrupting the driving of the torque limiter, the driving force is transmitted from the torque limiter to the retard roller  3   f  to rotate the retard roller  3   f  in the direction opposite to the conveying direction, with the result that only the uppermost sheet (first sheet) is separated and fed. Even if a plurality of sheets are picked up, only the uppermost sheet is separated and fed in the similar manner. 
     As shown in FIG. 2E, the overlapped sheets are separated, and, in a condition that the sheet-to-sheet distance is zero, the sheet are fed repeatedly for a predetermined number of sheets. After the predetermined number of sheets were fed, the driving is interrupted and the pick-up roller  3   c  is released. In this way, the sheets can be fed with the sheet-to-sheet distance of “zero”. 
     Incidentally, other than the above-mentioned arrangement, there are many means for maintaining the sheet-to-sheet distance to zero, and, alternatively, the trailing end of the preceding sheet and the leading end of the succeeding sheet may slightly be overlapped. 
     Then, the leading end of the sheet fed by the sheet feeding portion  3  is temporarily stopped by a pair of registration rollers  22 . Thereafter, the sheet is conveyed again in synchronous with the image formed by the image forming portion  2  to a transfer portion, where the image is transferred onto the sheet. The driving of the pair of registration rollers  22  is effected by connection of a clutch (not shown) which is controlled by a controller C in a main body of the copying machine. 
     In response to the image information read by the reader portion  1 , a laser light emitting portion  2   a  emits a laser beam via a laser driver. The laser beam is scanned along the generatrix of a photosensitive drum  2   c  by rotation of a polygon mirror  2   b , thereby forming a latent image on a surface of the photosensitive drum which was previously charged by a charger  2   d . The latent image is developed by a developing unit  2   e  disposed around the photosensitive drum  2   c  to form a toner image which is in turn transferred onto the sheet P conveyed by the pair of registration rollers  22  by means of a transfer charger  2   g . After the image transferring, residual toner remaining on the photosensitive drum is removed by a cleaning device  2   h.    
     The timing for forming the latent image is controlled by the controller C in the main body on the basis of a detecting timing of a leading end detection sensor  23  for detecting the leading end of the sheet in consideration of a time period during which the sheet reaches from the sensor to the transfer point. Thus, in the illustrated embodiment, it is selected so that a distance L 1  from the exposure point to the transfer point becomes smaller than a distance L 2  from a sensor lever  24  of the leading end detection sensor  23  (described later) to the transfer point (L 1 &lt;L 2 ). 
     The sheet P to which the toner image was transferred at the image forming portion  2  is sent, by a convey belt  8 , to the fixing device  4 , where, while the sheet is being passed between a pair of fixing rollers  4   a ,  4   b , the toner image is fused and fixed to the sheet P by heat and pressure. 
     The sheet P to which the image was fixed by the fixing device  4  is discharged out of the copying machine by a pair of discharge rollers  5   a ,  5   b.    
     Next, a detecting mechanism for detecting the sheet conveyed with zero sheet-to-sheet distance, which is a characteristic portion of the present invention will be fully explained. 
     Since the sheets fed from the sheet feeding portion  3  have zero sheet-to-sheet distance or are partially overlapped by a predetermined amount, if the leading end of the sheet cannot be detected, alignment between the sheet and the image formed in the image forming portion  2  cannot be achieved. Further, in a case where the sheet is jammed, if the sheet jam cannot be detected, the main body of the copying machine may be damaged. 
     To avoid this, in the illustrated embodiment, a stepped portion (spacing means) is provided on a guide between the conveying roller  3   e  and the pair of registration rollers  22  so that the leading end of the sheet can be detected. 
     Such an arrangement will be described with reference to FIGS. 3A to  3 C. 
     The guide between the conveying roller  3   e  and the pair of registration rollers  22  is constituted by an upper guide  25  and a lower guide  26  opposed to the upper guide. 
     The upper guide  25  has an inclined portion  25   a  inclined downwardly toward the downstream pair of registration rollers  22  so that the sheet can smoothly be guided in a nip of the pair of registration rollers  22 . The lower guide  26  is provided with a stepped portion  26   a  bent at a right angle at a position upstream of he inclined portion  25   a , so that a space is formed between the inclined portion  25   a  of the upper guide  25  and the stepped portion  26   a  of the lower guide  26 . 
     The sensor lever  24  of the leading end detection sensor  23  is inserted into the space through an opening portion  25   b  formed in the upper guide  25 . The lever can be rocked to be fallen (laid) down by the sheet P being conveyed. The leading end detection sensor  23  has the sensor lever  24 , a flag  27  provided at the end of the sensor lever  24 , and a photo-interrupter  28  ON/OFF-controlled by the flag  27 . The leading end detection sensor  23  is disposed at a center in a width-wise direction of the sheet P. 
     The photo-interrupter  28  has a light emitting portion and an opposed light receiving portion. A light amount in the light receiving portion is photo-electrically converted to obtain a voltage value, and, on the basis of the voltage value, it is judged whether the flag  27  exists in a space  28   a  between the light emitting portion and the light receiving portion. Incidentally, the sensor lever  24  is biased toward a clockwise direction by a spring (not shown) to be kept in a position shown in FIG.  3 A. 
     Next, the sheet detecting operation of the leading end detection sensor  23  will be described. 
     As shown in FIG. 3B, the leading end of the conveyed first sheet is directed to the pair of registration rollers  22  along the inclined portion  25   a  of the upper guide  25  by resiliency (rigidity) of the sheet itself. Meanwhile, the leading end of the sheet pushes the sensor lever  24  to rotate the lever in a clockwise direction. When the flag  27  of the sensor lever  24  enters into the space  28   a  between the light emitting portion and the light receiving portion of the photo-interrupter  28 , the leading end of the sheet P is detected. 
     After the leading end of the first sheet P is detected by the leading end detection sensor  23 , the leading end of the sheet reaches the pair of registration rollers  22  which are now stopped. The conveying roller  3   e  is rotated to further convey the sheet until a predetermined loop is formed in the sheet to correct skew-feed. Rotation of the pair of registration rollers  22  is started at a predetermined timing after a waiting condition that the sheet abuts against the nip of the pair of registration rollers, thereby sending the sheet to the image forming portion. 
     Then, when the trailing end of the first sheet P passes through the stepped portion  26   a  of the lower guide  26 , due to the resiliency of the sheet and a gravity force, the sheet is dropped along the lower guide  26 . At the same time, the sensor lever  24  is returned to its initial position by the spring and the flag  27  leaves the space  28   a  between the light emitting portion and the light receiving portion of the photo-interrupter  28 , thereby preparing for detection of a leading end of a next sheet. 
     If the flag  27  of the sensor lever  24  is not detected by the photo-interrupter  28  after a predetermined time period is elapsed or if the flag continues to be detected, the controller C of the main body judges that the sheet jam occurs, and the copying machine is stopped. 
     The leading end of the second sheet P with zero sheet-to-sheet distance with respect to the first sheet is similarly detected by the leading end detection sensor  23 . In this way, the sheets continuously fed with zero sheet-to-sheet distance can surely be detected. 
     Incidentally, rather than zero sheet-to-sheet distance, if the sheets are partially overlapped, the leading end of the sheet can be detected. That is to say, if the sheets are overlapped by an amount corresponding to a length between the stepped portion  26   a  of the lower guide  26  and the lever  24  of the leading end detection sensor  23  or less, since the first sheet is dropped after passing the stepped portion  26   a , so long as the leading end of the second sheet is not detected by the leading end detection sensor  23  when the first sheet is dropped, the second sheet can be detected by the leading end detection sensor  23 . 
     FIG. 4 shows a sensor  200  of permeable type used in place of the leading end detection sensor  23  of lever type according to the illustrated embodiment. A light emitting portion  201  and a light receiving portion  202  are opposed to each other with the interposition of the upper guide  25  and the lower guide  26 , and opening portions  25   b ,  26   b  for passing light are formed in the guides. 
     In the sensor  200  of permeable type, a light amount in the light receiving portion  202  is photo-electrically converted to determine a voltage value, and, on the basis of the voltage value, it is judged whether the sheet P exists between the light emitting portion  201  and the light receiving portion  202 . Incidentally, since the elements designated by the same reference numerals as those in FIG. 3 have the same functions, explanation thereof will be omitted. 
     FIG. 5 shows an example that a sensor  203  of reflection type is used in place of the leading end detection sensor  23 . The sensor  203  of reflection type is disposed above the upper guide  25 . In this sensor, light from a light emitting portion is illuminated on the sheet P and, by detecting an amount of reflected light from the sheet by a light receiving portion, presence/absence of the sheet is judged. An opening portion  26   c  for passing the light is formed in the upper guide  25 . A detection distance from the sensor  203  of reflection type is finite, and thus, if a distance from the sensor  203  of reflection type is too long, detection becomes impossible. In the illustrated embodiment, a detection possible range is shown by “Y”, so that, when the trailing end of the sheet is positioned along the lower guide  26 , it is judged that the sheet is absent. 
     FIG. 6 shows an example that a sensor  204  of reflection type is disposed below the lower guide  26 . Since the other arrangements are the same as those shown in FIG. 5, explanation thereof will be omitted. 
     FIGS. 7 to  9  show various auxiliary means for positively dropping the trailing end of the sheet along the lower guide  26  when the trailing end of the sheet passes through the stepped portion  26   a  of the lower guide  26 . 
     In the auxiliary means shown in FIG. 7, in order to positively drop the trailing end of the sheet along the lower guide  26  even if the sheet is curled, air blow is applied from a fan  210  disposed above the upper guide  25 . An opening portion (not shown) for passing the air blow is formed in the upper guide  25 . Incidentally, a suction fan  210  may be arranged below the lower guide  26  to such the sheet, thereby positively drawing the trailing end of the sheet toward the lower guide  26 . Since the other arrangements are the same as those described above, explanation thereof will be omitted. 
     In the auxiliary means shown in FIG. 8, voltage is applied from a power supply  220  to the lower guide  26  to charge the lower guide, thereby positively drawing the sheet toward the lower guide  26 . Since the other arrangements are the same as those described above, explanation thereof will be omitted. 
     In the auxiliary means shown in FIG. 9, by inclining a tangential angle of the nip of the pair of registration rollers  22  by an angle a with respect to the horizontal direction, the trailing end of the sheet P is positively dropped toward the lower guide  26  by the resiliency of the sheet itself. According to this arrangement, since installation of the pair of registration rollers  22  may merely be adjusted, the construction can be simplified not to make the machine expensive. Since the other arrangements are the same as those described above, explanation thereof will be omitted. 
     Incidentally, in the above-mentioned embodiment, while an example that the leading end of the sheet conveyed with zero sheet-to-sheet distance is detected was explained, even when the trailing end of the preceding sheet and the leading end of the succeeding sheet is slightly overlapped, the leading end of the sheet being conveyed can be detected by such an embodiment. In this case, the sheet are conveyed in such a manner that the preceding sheet is positioned under the succeeding sheet, and the overlapped amount may be selected to become smaller than a distance between a start point of the stepped portion  26   a  of the lower guide and the lever  24  of the leading end detection sensor  23 . 
     Further, in the above-mentioned embodiment, while an example that the present invention is applied to a position where the sheet is guided by the upper and lower guides in order to convey the sheet substantially along the horizontal direction was explained, the present invention may be applied to guides for conveying the sheet along a vertical direction. 
     Next, a second embodiment of the present invention will be explained with reference to the accompanying drawings. 
     FIG. 10 shows a construction of an image forming apparatus according to a second embodiment of the present invention. In FIG. 10, the reference numeral  200  denotes an image forming apparatus, and  201  denotes to a main body of the image forming apparatus (referred to merely as “main body” hereinafter). 
     In the image forming apparatus  200 , after a sheet P on which an image is to be formed is fed from a sheet feeding cassette (sheet containing means)  101 , the sheet is passed through a convey path  140  by a pair of longitudinal path rollers (first conveying means)  102  and a pair of post-registration rollers (second conveying mean)  105 , and, thereafter, the sheet is rested on a convey belt  106  to be conveyed to an image forming portion  150 . 
     Further, according to this image forming apparatus  200 , in the image forming portion  150 , toner images formed on electrophotographic photosensitive drums (four image forming means)  107 ,  108 ,  109 ,  110  in response to image information signals sent from a reader scanner or a personal computer (not shown) are successively transferred onto the sheet P, and then the toner images are permanently fixed to the sheet by a fixing device  115 . Thereafter, the sheet on which the monochromatic image or full-color image was formed is discharged onto a discharge tray  16 . 
     In FIG. 10, LEDs (light emitting diodes)  111 ,  112 ,  113 ,  114  acts as light source devices for forming the toner images on the photosensitive drums  107 ,  108 ,  109 ,  110  and are controlled independently. The LEDs  111 ,  112 ,  113 ,  114  are opposed to surfaces of the respective photosensitive drums  107 ,  108 ,  109 ,  110 . 
     On the other hand, the image forming apparatus  200  has an automatic both-face function. Thus, when the images are formed on both surfaces of the sheet P, after the sheet P having one surface on which the image was formed is turned over (surface reversal) by a both-face path portion  130  of the main body  201 , the sheet P is again conveyed to the image forming portion  150 , where the image is formed on the other surface of the sheet. 
     By the way, in this image forming apparatus  200 , in order to utilize a process speed of the image forming portion  150  to the maximum extent, in a continuous recording mode, the sheets P are rested on the convey belt  106  in such a manner that the sheet-to-sheet distance becomes zero, and more preferably, in such a manner that the sheets are slightly overlapped. 
     Next, detection of a leading end of the sheet when the sheets are conveyed in the overlapped condition will be explained with reference to FIG. 11 which is an schematic enlarged view showing the convey path  140 . 
     In FIG. 11, the reference numeral  117  denotes a lower guide; and  118  denotes an upper guide cooperating with the lower guide  117  to form the convey path  140  therebetween. Incidentally, the upper and lower guides  118 ,  117 , the pair of longitudinal path rollers  102  and the pair of post-registration rollers  105  constitutes a spacing means for temporarily spacing the leading end of the succeeding sheet P overlapped with the preceding sheet P apart from the trailing end of the preceding sheet P in the convey path. 
     A registration member (skew-feeding correction member)  104  can be entered into or retarded from the convey path  140 . When the sheet P is conveyed, the registration member  104  enters into the convey path  140 , and, by abutting the leading end of the sheet P sent from the pair of longitudinal path rollers  102  against the registration member  104 , registration of the sheet P is effected. 
     Such registration is effected by forming a loop in the sheet P after the leading end of the sheet P abuts against the registration member  104 . To this end, the upper guide  118  are swollen upwardly to be separated from the lower guide  117  thereby to permit formation of the loop in the sheet P, thereby providing a loop forming space S. 
     On the other hand, after the registration is effected, when the registration member  104  is retarded from the convey path  140  at a predetermined timing, the sheet P elastically enters into a nip of the pair of post-registration rollers  105 . When the pair of post-registration rollers  105  are rotated, the sheet is conveyed to the image forming portion  150  in the condition that the skew-feeding is corrected. 
     Incidentally, a length of the sheet P in the conveying direction is longer than a distance between the pair of longitudinal path rollers  102  and the pair of post-registration rollers  105 , so that the sheet P is conveyed by the pair of longitudinal path rollers  102  and the pair of post-registration rollers  105  until the trailing end of the sheet P leaves the pair of longitudinal path rollers  102 . 
     By the way, in the illustrated embodiment, when it is assumed that sheet conveying speed of the pair of longitudinal path rollers  102  is V 1  and a sheet conveying speed of the pair of post-registration rollers  105  is V 2 , the conveying speeds V 1 , V 2  are selected to satisfy the following relationship: 
     
       
           V   1 ≦ V   2   (1) 
       
     
     By selecting the conveying speeds V 1 , V 2  to satisfy the above relationship, after the registration is effected, while the sheet is being conveyed by the pair of longitudinal path rollers  102  and the pair of post-registration rollers  105 , slack is not formed in the sheet P within the loop forming space S. In other words, the loop formed in the sheet P for registration can be removed during the conveyance after registration. 
     On the other hand, in FIG. 11, a leading end detection sensor (leading end detecting means)  103  comprises a sensor arm  103   a  rotatably provided within the loop forming space S, and a photo-interrupter  103   b . The sensor arm  103   a  is pushed by the leading end of the sheet P conveyed by the pair of longitudinal path rollers  102  to be rotated. When the sensor arm  103   a  is rotated in this way to reach a position shown by the two dot and chain line in FIG. 11, the photo-interrupter  103   b  is turned ON. 
     FIG. 12 shows a control block diagram of the image forming apparatus. On the basis of an ON signal of the photo-interrupter  103   b  of the leading end detection sensor  103 , a controller  135  shown in FIG. 12 detects the fact that the leading end of the sheet P has passed. Further, in response to the leading end detection, the controller  135  retards the registration member  104  from the convey path  140  at a predetermined timing after registration and drives the pair of longitudinal path rollers  102  and the pair of post-registration rollers  105  at the conveying speeds satisfying the above relationship (1). 
     Incidentally, when the sensor arm  103   a  is pushed by the leading end of the sheet P, the sensor arm can be retarded sufficiently not to obstruct the conveyance of the sheet P, and, when the pressure of the sheet P is released, the sensor arm can returned to an original condition by itself. 
     By the way, when the sheet P is conveyed in a condition that the sheet is pinched between the sensor arm  103   a  and the lower guide  117 , the leading end detection sensor  103  does not detect the sheet P. That is to say, as shown in FIG. 13, the sheet P from which the loop is removed is shifted along the lower guide  117 , and, when a distance L between the sensor arm  103   a  and the lower guide  117  becomes smaller than &lt;(thickness of single sheet)+(predetermined small amount)&gt;, the photo-interrupter  103   b  is turned OFF. 
     Namely, in this leading end detection sensor  103 , when the sensor arm  103   a  is temporarily pushed by the leading end of the sheet P to be shifted to the position shown by the two dot and chain line in FIG. 11, a leading end detection signal for the sheet P can be outputted, and, when the sheet P from which the loop is removed is shifted along the lower guide  117  and the sensor arm  103   a  is rocked downwardly to a position shown by the broken line, output of the leading end detection signal is stopped. 
     Incidentally, an area where the output of the leading end detection signal is stopped by the leading end detection sensor  103 , i.e., an area where the sheet P is not detected by the leading end detection sensor  103  is sown by “H” in FIG.  13  and is referred to as “non-detection area” hereinafter. 
     With this arrangement, on the basis of the leading end detection signal from the leading end detection sensor  103 , the controller  135  can detect only passing of the leading end of the sheet P. Incidentally, the controller  135  controls the image forming portion  150  to effect the image forming operations successively on the basis of an OFF signal after the ON signal from the photo-interrupter  103   b.    
     On the other hand, when the trailing end of the sheet P conveyed by the pair of post-registration rollers  105  reaches a curved portion  141  of the convey path  140 , the leading end of the succeeding sheet Pa (referred to as “next sheet P” hereinafter) entered into the conveying path  140  in the condition that the next sheet P is overlapped with the preceding sheet P is separated from the trailing end of the sheet P, and, ultimately urges the sensor arm  103   a  abutting against the sheet P. 
     As a result, the sensor arm  103   a  is rocked upwardly again. As the sensor arm  103   a  is rocked upwardly, the photo-interrupter  103   b  is turned ON, and the controller  135  detects the leading end of the next sheet Pa accordingly. 
     Next, the leading end detecting operation of the image forming apparatus having the above-mentioned leading end detection sensor  103  will be explained. 
     When the continuous recording mode (image formation) is selected and start command is emitted, the controller  135  feeds the sheet P from the sheet feeding cassette  101 . When the sheet is fed, the next sheet P 2  is picked up before the first sheet P 1  is completely removed from the sheet feeding cassette  101  so that two sheets P 1 , P 2  are partially overlapped. 
     Incidentally, regarding all of the sheets to be fed, the same overlapping is performed. Preferably, the overlapped among is selected so that, after the registration is effected by the pair of post-registration rollers  105  in front of the image forming portion  150 , the sheets are rested on the convey belt  106  of the image forming portion  150  in such a manner that the sheet-to-sheet distance becomes zero or the sheets are overlapped only at their margins (non-image formed areas). 
     Then, the leading end of the first sheet P 1  fed in the overlapped condition enters into the convey path  140  and, as shown in FIG. 15A, passes along the upper guide  118 . Namely, the leading end of the first sheet P 1  passes above the non-detection area H (FIG. 13) of the leading end detection sensor  103 . 
     Then, the sheet P 1  advances along the upper guide  118  and then pushes the sensor arm  103   a . As a result, as shown in FIG. 15B, when the sensor arm  103   a  is rocked, the interrupter  103   b  is turned ON accordingly, and, on the basis of the ON signal from the photo-interrupter  103   b , the controller  135  detects the leading end of the sheet P 1  and effects the aforementioned registration on the basis of the detection timing. 
     On the other hand, after the registration is finished, the pair of longitudinal path rollers  102  and the pair of post-registration rollers  105  are driven. Incidentally, when the sheet P 1  is conveyed by these rollers  102 ,  105 , the sheet is conveyed at such a conveying speed that the slack in the first sheet P 1  is removed before the second sheet P 2  conveyed in the overlapped condition reaches the vicinity of the leading end detection sensor. Since this is selected on the basis of the above-mentioned relationship  1  detailed explanation thereof will be omitted. 
     When the slack is removed from the first sheet P 1  in this way, as shown in FIG. 16A, the first sheet P 1  is conveyed along the lower guide  117 , i.e., along the non-detection area, and, the sensor arm  103   a  is rocked downwardly, with the result that the leading end detection sensor  103  emits the OFF signal to the controller  135 . Incidentally, when the OFF signal is outputted from the leading end detection sensor  103 , the controller  135  start to drive the image forming portion  150  to form the image on the sheet P 1 . 
     Then, when the first sheet P 1  is further conveyed, the second sheet P 2  enters into the convey path  140 . The sheet P 2  is conveyed along the upper guide  118 . When the sensor arm  103   a  is pushed by the leading end of the sheet P 2 , the sensor arm  103   a  is rocked as shown in FIG.  16 B. In this way, the controller  135  detects the leading end of the second sheet P 2 . 
     Namely, the first sheet P 1  (except for the leading end thereof) is passed without being detected by the photo-interrupter  103   b , and the leading end detection sensor  103  detects the leading end of the second sheet P 2 . On the basis of the detection timing, the controller  135  effects the registration and image formation for the second sheet P 2 . Incidentally, regarding a third sheet and so on, a leading end is detected by repeating the above operation. 
     In this way, after the preceding sheet P 2  entered into the convey path  140  is conveyed along the upper guide  118 , the sheet is shifted along the lower guide  117 , and the leading end of the next sheet P 2  conveyed in the overlapped relationship to the preceding sheet P 1  is temporarily spaced apart from the trailing end of the preceding sheet P 1 . In this way, the leading end of the next sheet P 2  can be detected positively. 
     By positively detecting the leading end of the next sheet P 2  in this way, even when the sheets are conveyed in the manner that the sheet-to-sheet distance becomes zero or the sheets are slightly overlapped, the skew-feeding can be corrected and the leading margin can be maintained, thereby achieving the proper image formation. 
     In the above explanation, while an example that the leading end of the sheet P is detected within the loop forming space S of the convey path  140  for effecting the registration was explained, the present invention is not limited to such an example. For example, in an image forming apparatus  200  having an intermediate tray  131  as shown in FIG. 17, a sheet path portion  134  provided at a junction between a sheet re-feeding portion  132  and a sheet feeding longitudinal path  133  and adapted to change a conveying direction may be constituted by guides  134   a ,  134   b  having different curvatures, and a leading end detection sensor  120  having the same construction as the leading end detection sensor  1  may be arranged there to detect the leading end of the sheet P in the same manner as mentioned above. As well as the image forming apparatuses, the present invention can widely be applied to apparatuses having a means for conveying a sheet P such as an applicator. 
     By the way, according to the aforementioned first embodiment, in the convey path  140  for changing the conveying direction, by deviating the leading end of the next sheet P from the preceding sheet P toward the leading end detection sensor, the detection of the leading end of the sheet P is made possible. 
     However, in consideration of various image forming apparatuses, in some cases, it is required that the leading end of the sheet be detected within a straight convey path such as a horizontal sheet path portion or a vertical sheet path portion. 
     Now, an image forming apparatus according to a third embodiment of the present invention to cope with such cases will be explained. 
     FIG. 18 is a view of a construction of an image forming apparatus according to a third embodiment of the present invention. Incidentally, in FIG. 18, the same reference numerals as those in FIG. 10 designate the same elements. 
     In FIG. 18, a lower sheet feeding cassette  101 B is disposed below an upper sheet feeding cassette  101 A, a longitudinal path portion  142  serves to direct a sheet P from the lower sheet feeding cassette  101 B toward a convey path  140 . An upstream side conveying roller (first conveying means)  123  serves to convey the sheet P from the lower sheet feeding cassette  101 B to the longitudinal path portion  142 . A downstream side conveying roller (second conveying means)  125  serves to direct the sheet conveyed to the longitudinal path portion  142  toward an image forming portion  150  through the convey path  140 . 
     The longitudinal path portion  142  is substantially straight not to change the conveying direction of the sheet P; whereas the convey path  140  is curved to change the conveying direction of the sheet P. That is to say, as shown in FIG. 19, the longitudinal path portion  142  is constituted by an inner guide  126  extending substantially straightly toward the convey path  140 , and an outer guide  127  having a swollen portion swollen outwardly. 
     Further, in the illustrated embodiment, a direction of a nip N of the downstream side conveying roller  125  is substantially in parallel with the inner guide  126  (parallel with the conveying direction; whereas, a nip N of the upstream side conveying roller  123  is directed toward the swollen portion of the outer guide  127 . With this arrangement, the sheet P from the lower sheet feeding cassette  101 B entered into the longitudinal path portion  142  by the upstream side conveying roller  123  is directed toward the downstream side conveying roller  125  along the outer guide  127 . 
     Further, the outer guide  127  is provided with a leading end detection sensor  124  comprising a sensor arm  124   a  and a photo-interrupter  124   b . Similar to the leading end detection sensor of the first embodiment, the leading end detection sensor  124  is constituted so that, when the sheet P passes through the non-detection area between the sensor arm  124   a  and the inner guide  126 , the sheet P is not detected. 
     Incidentally, in this embodiment, when it is assumed that a conveying speed of the upstream side conveying roller  123  is V 3  and a conveying speed of the downstream side conveying roller  125  is V 4 , the conveying speeds V 3 , V 4  are selected to satisfy the following relationship: 
     
       
           V   3 &lt; V   4   (2) 
       
     
     By selecting the conveying speeds V 3 , V 4  to satisfy such a relationship, even in the longitudinal path portion  142 , slack is not formed in the sheet P as less as possible. 
     Next, the leading end detecting operation of the image forming apparatus having the above-mentioned leading end detection sensor  124  will be explained. 
     When the continuous recording mode (image formation) is selected and start command is emitted, the controller  135  feeds the sheet P from the sheet feeding cassette  101 B. When the sheet is fed, the next sheet P 2  is picked up before the first sheet P 1  is completely removed from the sheet feeding cassette  101  so that two sheets P 1 , P 2  are partially overlapped. 
     Then, when the leading end of the first sheet P 1  fed in the overlapped condition passes through the longitudinal path portion  142 , as shown in FIG. 20A, the leading end passes along the outer guide  127  due to the direction of the nip N of the upstream side conveying roller  123  directed outwardly. Namely, the leading end of the sheet P 1  passes outwardly of the non-detection area of the leading end detection sensor  124 . 
     Then, the sheet P 1  advances along the outer guide  127  and then pushes the sensor arm  124   a . As a result, as shown in FIG. 20B, when the sensor arm  124   a  is rocked by an amount sufficient to detect the leading end of the sheet P 1  by means of the leading end detection sensor  124 , the interrupter  124   b  is turned ON accordingly, and, on the basis of the ON signal from the photo-interrupter  103   b , the controller  135  detects the leading end of the first sheet P 1  and effects the aforementioned registration in the conveying path  140  on the basis of the detection timing. 
     Incidentally, when the sensor arm  124   a  is pushed by the leading end of the sheet P 1 , it can be retarded sufficiently not to obstruct the further conveyance of the sheet P and can be returned to its original condition by itself. 
     On the other hand, after the leading end is detected, the controller  135  drives the upstream side conveying roller  123  and the downstream side conveying roller  125 . Incidentally, when the first sheet P 1  is conveyed by these rollers  123 ,  125 , the sheet is conveyed at such a conveying speed that the slack in the first sheet P 1  is removed before the second sheet P 2  conveyed in the overlapped condition reaches the vicinity of the leading end detection sensor. Since this is selected on the basis of the above-mentioned relationship (2), detailed explanation thereof will be omitted. 
     When the slack is removed from the first sheet P 1  in this way, i.e., when the first sheet P 1  is conveyed along the inner guide  126  of the longitudinal path portion  142 , as shown in FIG. 21A, the sensor arm  124   a  is rocked downwardly. 
     Then, when the first sheet P 1  is further conveyed, the second sheet P 2  enters into the longitudinal path portion  142 . The second sheet P 2  is conveyed along the outer guide  127 . When the sensor arm  124   a  is pushed by the leading end of the sheet P 2  to rock the sensor arm  124   a  as shown in FIG. 21B, the photo-interrupter  124  is turned ON. In this way, the controller  135  detects the leading end of the second sheet P 2 . Incidentally, regarding a third sheet and so on, a leading end is detected by repeating the above operation. 
     In this way, after the preceding sheet P 2  entered into the longitudinal path portion  142  is conveyed along the outer guide  127 , the sheet is shifted along the inner guide  126 , and the leading end of the next sheet P 2  conveyed in the overlapped relationship to the preceding sheet P 1  is temporarily spaced apart from the trailing end of the preceding sheet P 1  in the longitudinal path portion. In this way, the leading end of the next sheet P 2  can be detected positively. 
     The detection of the leading end detection sensor can be applied to various controls such as control for controlling the conveying amount to maintain a constant amount of the loop formed in front of the registration means to correct the skew-feeding or control for detecting sheet jam, as well as the timing adjustment between the image forming means and the sheet. 
     In the above-mentioned embodiments, while the optical sensors were explained, for example, a detecting means of pressure detecting type, a detecting means utilizing a Doppler effect, or a switch of electrical contact type may be used. 
     In the above-mentioned embodiments, while the conveying machine was explained as the image forming apparatus, the present invention can be applied to any apparatuses for conveying a cut sheet such as a printer, a facsimile or a printing device, as well as the image forming apparatus. Further, the present invention can also be applied to an auto document feeder for automatically feed an original in a copying machine.