JP2001335192A - Sheet carrying device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the occurrence of a malfunction caused by irregularity of the sheet feeding timing. SOLUTION: In this sheet carrying device having a pair of carrying members 1 arranged in pressure contact so as to be rotatable and carrying a sheet S while sandwiching the sheet, a driving means 2 for driving the carrying members 1, a detecting means 3 arranged on the upstream side or the downstream side of the carrying members 1 and detecting the existence of the carried sheet S and a control means 4 for sending out a signal for stopping driving of the carrying members 1 to the driving means 2 after specified time passes after detecting the tip of the sheet S by the detecting means 3 and thereafter sending out a signal for resuming the driving of the carrying members 1 to the driving means 2 according to the prescribed timing, the control means 4 changes the length of the specified time to the next sheet S according to the length of refeeding time up to detecting the rear end of the sheet S by the detecting means 3 after sending out the signal for resuming the driving of the carrying members 1 to the driving means 2.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a sheet conveying apparatus used in, for example, an image forming apparatus, and more particularly, to a sheet conveying apparatus effective for feeding a sheet to an image forming section or an image reading section at a predetermined timing. Regarding improvement.

[0002]

2. Description of the Related Art Conventionally, as a sheet feeding device used in an image forming apparatus such as an electrophotographic copying machine, a sheet loaded in a sheet cassette is taken out by a pickup roll.
There is a type in which the taken out sheets are separated and sent out one by one by a separating mechanism composed of, for example, a feed roll and a retard roll, and the fed out sheets are transported to an image forming unit by a pair of transport rolls.

In such a paper feeding apparatus, since the slip amount of the paper in the separation mechanism differs for each paper, the paper feeding timing of the paper sent from the separation mechanism may vary. Therefore, a paper detection sensor is provided downstream of the separation mechanism, and based on the detection result, the transport roll pair is stopped and the paper is temporarily stopped while the fed paper is nipped by the transport roll pair. In addition, there is known an apparatus in which the driving of the pair of transport rolls is restarted at a predetermined timing and the sheet is re-fed to correct the variation in the feeding timing (Japanese Patent Laid-Open No. 5-3).
No. 19630).

[0004]

However, in this type of paper feeding apparatus, there is a time lag between sending a drive stop signal to the transport roll pair and actually stopping the transport roll pair. This time lag varies depending on the variation of the drive system such as the response speed of the motor and the electromagnetic clutch to the on / off signal, and the variation of the load system such as the rotational resistance of the pair of transport rolls. The paper may stop at a position different from the target position, or the rotation restart timing of the transport roll pair may be shifted, and the paper may not start moving at the target timing. There was a technical problem that paper jams and misregistration occurred without being able to do so.

In order to avoid such technical problems, it is conceivable to design a sheet feeding apparatus in consideration of a large variation in sheet feeding timing in advance. Inevitably increases the number of sheets that can be sent out per unit time,
There is a technical problem that productivity is reduced.

Japanese Patent Application Laid-Open No. 9-167820 discloses a technique in which the timing for increasing the speed of a sheet is changed in accordance with the timing at which the sheet reaches a sheet detection sensor. As a result of having to mount the variable mechanism, there is a technical problem that the apparatus configuration becomes complicated and the cost is increased.

Incidentally, such a technical problem is not limited to the above-described paper feeding apparatus, and a document feeding apparatus used in a document reading apparatus of an image forming apparatus, or a single-sided image forming apparatus in a double-sided image forming apparatus. This also occurs in a sheet conveying device such as a sheet reversing device used for reversing the sheet after forming the sheet.

SUMMARY OF THE INVENTION The present invention has been made to solve the above technical problems, and can prevent the occurrence of sheet jams and misregistration due to variations in sheet feeding timing with a simple configuration. A sheet conveying device is provided.

[0009]

That is, as shown in FIG. 1, the present invention relates to a pair of transport members 1 which are rotatably pressed against each other and transport a sheet S while sandwiching the same. A driving unit 2 for driving, a detecting unit 3 arranged upstream or downstream of the conveying member 1 for detecting the presence or absence of a sheet S to be conveyed, and a detecting unit 3 for detecting the leading end of the sheet S After a lapse of a predetermined time from the above, a signal for stopping the driving of the conveying member 1 is sent to the driving means 2 and, thereafter, a signal for restarting the driving of the conveying member 1 is sent to the driving means 2 at a predetermined timing. In the sheet conveying apparatus including the control unit 4, the control unit 4 sends a signal for restarting the driving of the conveying member 1 to the driving unit 2 and then detects the rear end of the sheet S by the detection unit 3. Depending on the length of the sheet re-feeding time until the detected, and changes the length of the prescribed time for the next sheet S.

In such technical means, the conveying member 1 is rotatably pressed and arranged so long as it can convey the sheet S while pinching it, such as a roll-shaped member, a belt-shaped member or a combination thereof. You can choose it.

The driving means 2 may have any function to drive and stop the conveying member 1. For example, a driving means for turning on / off a driving force from a driving source by a clutch or a switch. A device for turning on / off the drive source may be appropriately selected.

Further, the detecting means 3 detects the conveyed sheet S
If it is possible to detect the presence or absence of the above, the disposition position may be on the upstream side of the transport member 1 or on the downstream side.

The control means 4 sends a signal to the drive means 2 to stop driving the transport member 1 after a lapse of a predetermined time from the detection of the leading end of the sheet S by the detection means 3, and thereafter a predetermined signal is outputted. A signal for restarting the driving of the transport member 1 is sent to the driving means 2 in accordance with the timing of, and further, a signal for restarting the driving of the transport member 1 is sent to the driving means 2 The length of the specified time for the next sheet S is changed in accordance with the length of the sheet re-feeding time until the trailing end of the sheet S is detected by the detection means 3.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail based on an embodiment shown in the accompanying drawings. First Embodiment FIG. 2 shows a first embodiment of an electrophotographic copying machine to which a sheet conveying device according to the present invention is applied. In the figure, reference numeral 11 denotes a photosensitive drum rotating in the direction of an arrow,
Reference numeral 12 denotes a charger such as a corotron for charging the photosensitive drum 11 in advance, and reference numeral 13 denotes an image writing device (ROS) such as a laser scanning device, which is a laser modulated by a laser drive signal generated for each color component. The surface of the photosensitive drum 11 is irradiated with the beam. This R
The OS 13 writes a digital image input from the image reading device 80 via the image processing unit (IPS) 15 on the photosensitive drum 11 as an electrostatic latent image in accordance with an image output instruction input from a host device (not shown). Also, reference numeral 1
Reference numeral 4 denotes a rotary developing device equipped with developing units corresponding to the respective colors of Y, M, C, and K, and develops an electrostatic latent image formed on the photosensitive drum 11 to form a toner image of each color component. It has become. Reference numeral 16 denotes the photosensitive drum 1
1 is a drum cleaner for removing residual toner,
Reference numeral denotes an intermediate transfer belt arranged so as to be in contact with the surface of the photosensitive drum 11, and is stretched around a plurality of rolls so as to rotate in the arrow direction. Further, a transfer roll 18 is provided at a portion (primary transfer position) of the intermediate transfer belt 17 facing the photoconductor drum 11, and a voltage having a polarity opposite to the charge polarity of the toner is applied to the transfer roll 18. Thus, the toner image on the photosensitive drum 11 is electrostatically attracted to the intermediate transfer belt 17.

Further, a secondary transfer device 19 composed of a secondary transfer roll and an opposing roll is provided at a secondary transfer position of the intermediate transfer belt 17 facing the transport path of the sheet P as a sheet, and is transported. The electrostatic latent image is transferred onto the paper P. Thereafter, the sheet P to which the unfixed image has been transferred is thermally fixed by the fixing device 20. Reference numeral 70 denotes a control unit that controls the supply and conveyance of the paper P in addition to various controls during image formation.

Next, the paper transport system of the image forming apparatus according to the present embodiment will be described. The paper transport system has a plurality of paper trays 31 to 34 on which paper P is stacked, and a manual feed tray 35 for supplying paper P from outside the apparatus. Also,
On the upper part of each of the paper trays 31 to 34, the stacked paper P
And a pick-up mechanism 36 composed of a feed roll and a retard roll for picking up the sheets P one by one at the downstream side, and temporarily stopping the sheets P at the downstream side. A take-away roll 38 (hereinafter, referred to as a T / A roll) is provided as a transport member for re-sending the sheet at a predetermined timing after the transfer. These T / A rolls 38 also have a function as transport rolls for simply transporting the paper P when the paper P is supplied from the lower paper trays 32 to 34.

A paper path 40 downstream of the T / A roll 38 has a registration roll (registration roll) 39 for transporting the paper P to the secondary transfer device 19 at the timing of image formation. After the positioning is stopped by the registration roll 39, the unfixed image is transferred onto the sheet P. Further, a transport belt 41 is provided on the downstream side, and the paper P is transported to the fixing device 20 by the transport belt 41.

Further, in this embodiment, when the duplex mode is selected, there is provided a sheet return mechanism for reversing the sheet P fixed on one side by the fixing device 20 and returning it to the secondary transfer position again. The paper return mechanism is provided with a paper branch path 44 that branches downward with respect to the paper discharge path 43 from the fixing device 20, and a paper reversing path 45 is further extended downward in the paper branch path. At the same time, a paper return path 46 returning from the paper reversing path 45 to the paper path 40 just before the secondary transfer position is connected. Then, the sheet branch path 44, the sheet reversing path 45, and the sheet return path 46
Has a plurality of transport rolls 47 (specifically, 47a to 47d).
And the like). In particular, the transport rolls 47 (specifically, 47a and 47b) provided in the sheet reversing path 45 rotate forward and backward at appropriate timing. Further, between the sheet discharge path 43 and the sheet branch path 44, the sheet branch path 44, the sheet reversing path 45, and the sheet return path 46.
A switching gate (not shown) for switching the transport path is provided between the printer and the printer, so that the paper path is appropriately switched according to the selected mode.

FIG. 3 is a schematic configuration diagram for explaining the sheet supply mechanism in the present embodiment. Each of the paper trays 31 to 34 loaded with the paper P includes a tray sensor 51 for detecting insertion / removal of the paper tray and a tray size sensor 52 for detecting a tray size corresponding to the size of the paper loaded on the paper tray. Further, it has a lifter 60 that operates when the paper tray is mounted, and raises the loaded paper P to a position where it can be supplied. Here, the nudger roll 36 is configured to move up and down via a bracket 59 with the axis of the feed roll 57 in the separation mechanism 37 as a fulcrum, and falls on the paper surface when the nudger solenoid 55 is turned on (power supply). It has a function of sending out the paper P. Reference numeral 53 is
This is a no-paper sensor that detects the presence of the paper P in the paper tray.
When the so-called READY state in which the sheet P can be conveyed, if the no-paper sensor 53 is ON (light receiving), it is detected that the sheet P exists in the sheet tray. Furthermore,
Reference numeral 54 denotes a tray level sensor for detecting that the height of the sheet P is at the reference value, and a part of the bracket 59 turns on the tray level sensor 54 following the vertical movement of the nadger roll 36. By turning on / off, the sheet height is detected.

The separating mechanism 37 includes a feed roll 57 that rotates in the transport direction of the paper P and a retard roll 58 that stops or rotates in the opposite direction to the transport direction.
The paper P sent from the nager roll 36 is separated and
It is configured to convey only the sheets downstream. Reference numeral 56 denotes a feed motor as a drive source of the feed roll 57 and the nudger roll 36. The rotation of the feed motor is performed by the operation of the feed clutch 68. Further, a pre-feed sensor 62 for detecting misfeed in the separating mechanism 37 is disposed near the downstream side of the separating mechanism 37, and further downstream, the presence or absence of the supplied sheet P is detected and the sheet P is detected. A feed-out sensor 63 (detection means) for temporarily stopping is provided. The feed-out sensor 63 is used as a sensor for detecting, for example, a tear of the sheet when the sheet tray is pulled out, and for controlling the sheet P not to stop at this position.

The T / A roll 38 provided further downstream of the separating mechanism 37 comprises a drive roll 65 made of EPDM and a driven roll 66 made of polyacetal (POM). Here, reference numeral 64 denotes a T / A feed clutch (electromagnetic clutch) for turning on / off the drive transmission from the main motor 67. When the T / A feed clutch 64 is turned on, the main motor 6 is turned on.
7 is transmitted to the drive roll 65. The main motor 67 is provided in each of the paper trays 31 to
34 are provided separately. And the separating mechanism 3
Between the T / A roll 7 and the T / A roll 38, a paper transport path is formed by an outer guide and an inner guide (not shown), and the paper P supplied from the separation mechanism 37 is reliably transferred to the T / A roll.
It leads to the roll 38.

Next, the paper supply operation of the paper supply mechanism according to the present embodiment will be described with reference to the flowchart shown in FIG. 4 and the timing diagram shown in FIG. 5, in addition to FIGS. In FIG. 5, reference symbol Ps indicates the leading edge of the sheet P, and symbol Pe indicates the trailing edge of the sheet P.

Paper trays 31 to 34 loaded with paper P
Is inserted into the apparatus main body, the control unit 70 receives the output signal of the tray sensor 51, drives the lifter 60, and pushes up the paper P.

When a paper feed signal is sent from the control unit 70 (S1), the nudger solenoid 55 is turned on, and the nudger roll 36 is lowered toward the surface of the paper P, and
When the paper P is pressed against the upper surface and the feed clutch 68 is engaged, the driving force is transmitted from the feed motor 56 and the paper P is drawn. At the same time, the T / A feed clutch 64 is engaged, and the driving force is transmitted from the main motor 67 to the T / A roll 38 to start rotation (S2). The paper P drawn in by the nudger roll 36 is separated into one sheet by the feed roll 57 and the retard roll 58 of the separation mechanism 37, and is transported to the T / A roll 38 mainly by the transport force of the feed roll 57.

When the leading edge Ps of the sheet P reaches the feed-out sensor 63, the feed-out sensor 63 is turned on (S3). Upon receiving the output of the feed-out sensor 63, the control unit 70 measures the time by a built-in timer. Is started (S4), and after the timer value reaches a predetermined time T1 (S5), a release signal for disconnecting the feed clutch 68 and the T / A feed clutch 64 is sent to feed rolls 57 and T / A. A roll 3
8 is stopped (S6) to form a so-called pre-feed state. The specified time T1 is equal to the length of the feed clutch 68.
At the time when the T / A feed clutch 64 is disengaged, the time is set to a time necessary for the leading end portion of the paper P to be already sandwiched by the T / A roll 38.

Next, when there is an instruction to restart paper feeding (S7),
At the same time that the paper feed restart signal is issued from the control unit 70, the measurement is started by a timer built in the control unit 70 (S8), and the T / A feed clutch 64 is engaged and the T / A feed clutch 64 is engaged.
/ A roll 38 is restarted (S9), and the sheet P is
The paper is conveyed toward the paper path 40 as the / A roll 38 rotates.

Thereafter, when the rear end Pe of the sheet P reaches the feed-out sensor 63, the feed-out sensor 63 is turned off (S10). Then, in the control unit 70, after the timer measurement is started in step S8, step S1 is executed.
The refeeding time T2 until the feed-out sensor 63 is turned off at 0 is compared with the standard refeeding time Ts (S
11, S12). Here, the standard re-feeding time Ts is a time required when the paper P is temporarily stopped at a target position and is re-feeded at a target timing. As a result, if T2 = Ts, there is no problem and there is no problem. However, the temporary stop position of the paper P is downstream from the target position or the re-feeding starts up quickly. In other words, when T2 <Ts (see T2 'in FIG. 5), the timer value T1 for the specified time is set to T1 = Ts.
1− (Ts−T2) so that the paper stop position is shifted to the upstream side more than before (S13, T in FIG. 5).
1 '). Further, when the pause position of the sheet P is on the upstream side of the target or when the re-feeding is delayed, that is, when T2> Ts (T2 in FIG. 5).
2 "), the timer value T1 of the specified time T1 is set to T1 = T1.
+ (T2-Ts) so that the paper stop position is shifted downstream from the target position (S14, see T1 "in FIG. 5), and it is determined whether or not the next paper P is present. Is performed (S15), and if there is the next sheet P, the above-described process is repeated. If not, a release signal for disconnecting the T / A feed clutch 64 is sent to stop the T / A roll 38 ( S16), a series of processes ends.

As described above, in this embodiment, the re-feeding time of the sheet P is monitored by the feed-out sensor 63 and the variation in the sheet feeding timing of the T / A roll 38 is corrected. And the occurrence of misregistration can be prevented.

In this embodiment, the correction of the specified time T1 is performed every time one sheet of paper P is transported. However, the present invention is not limited to this. It may be performed when clogging or misregistration occurs, or immediately after the paper trays 31 to 34 are removed and inserted.

Further, in the present embodiment, the correction of the specified time T1 is performed based on the data of one sheet. However, the present invention is not limited to this. T1 may be corrected.

Embodiment 2 In this embodiment, the sheet conveying device according to the present invention is applied to the paper return mechanism of the electrophotographic copying machine shown in FIG. 2, and FIG. FIG. 2 is a schematic configuration diagram for explaining a sheet reversing mechanism of the photocopier. In the figure, the transport roll 47b has a drive roll 71 and a driven roll 72, and the transport roll 47c has a drive roll 73 and a driven roll 74. Reference numeral 75 denotes a transport motor as a drive source of the transport rolls 47b and 47c. Rotational driving and stop are executed by operation of the clutches 76 and 77. In the present embodiment, the transport rollers 47b and 47c can be rotated in the forward and reverse directions by a gear (not shown). Further, a paper detection sensor 78 for detecting the presence or absence of the transported paper P is provided below the transport roll 47b. Reference numeral 79 denotes a switching gate provided between the sheet branch path 44, the sheet reversing path 45, and the sheet return path 46.

Next, the sheet returning operation of the sheet returning mechanism according to the present embodiment will be described with reference to the flowchart shown in FIG. 7 and the timing diagram shown in FIG. 8 in addition to FIGS. In FIG. 8, reference symbol Ps 'indicates the leading edge of the inverted sheet P (the trailing edge Pe of the sheet P before the inversion), and symbol Pe' indicates the trailing edge of the inverted sheet P (the sheet P before the inversion).
At the tip Ps).

When the duplex mode is selected, the fixing device 20
The paper P having one side fixed thereon is carried in from the paper branch path 44 and is carried by the carrying roll 47a. When a paper feed signal is output from the control unit 70 (S21), the clutch 7
By inserting the rollers 6 and 77, the driving force is transmitted from the transport motor 75 to the transport rolls 47b and 47c to start rotation (S22). At this time, the rotation direction of the transport rolls 47b and 47c is a direction in which the paper P is transported downward in FIG.

When the leading edge Ps of the paper P reaches the paper detection sensor 78, the paper detection sensor 78 is turned on (S23), and the output of the paper detection sensor 78 is received.
The control unit 70 starts measurement by a built-in timer (S24). After the timer value reaches a predetermined time T1 (S25), the control unit 70 sends a release signal for disengaging the clutches 76 and 77 to send the transport roll. 47b and 47c are stopped (S26). Note that, during the specified time T1, when the clutches 76 and 77 are disengaged, the leading end portion of the sheet P has already been nipped by the transport roll 47c, and the trailing end Pe of the sheet P has exceeded the switching gate 79. Is set to the time required for

Next, when there is an instruction to restart paper feeding (S27),
At the same time that the paper feed restart signal is issued from the control unit 70, measurement is started by a timer built in the control unit 70 (S28), the clutches 76 and 77 are engaged, and the driving of the transport rolls 47b and 47c is restarted. (S29). At this time, the rotation direction of the transport rolls 47b and 47c is set to be opposite to the previous direction, whereby the paper P is
It is conveyed upward with the rotation of b and 47c.
At this time, the switching gate 79 simultaneously moves so as to close the sheet branch path 44, and the sheet P is transferred to the sheet return path 46.
It is conveyed toward.

Thereafter, when the rear end Pe 'of the paper P reaches the paper detection sensor 78, the paper detection sensor 78 is turned off (S30), and after a predetermined time TA has elapsed (S31), the clutches 76 and 77 are disengaged. A switching signal for switching is sent to rotate the transport rollers 47b and 47c in the forward direction again (S32), and the reversing process of the first sheet is completed.
The time TA is set to a time necessary for the rear end Pe 'of the sheet P to have already passed the transport roll 47b when the clutch 76 is switched.

Then, in the control section 70, step S28
Then, the refeeding time (reverse feeding time) T2 from when the timer measurement is started until the paper detection sensor 78 is turned off in step S30 is compared with the standard refeeding time Ts (S
33, S34). Here, the standard re-feeding time Ts is a time required when the paper P is temporarily stopped at a target position and is re-feeded at a target timing. As a result, if T2 = Ts, there is no problem, so there is no problem. However, the temporary stop position of the paper P is lower than the target position or the rising of re-feeding is slow. In this case, that is, when T2> Ts (see T2 'in FIG. 8), the timer value T1 for the specified time is set to T1 = T1.
The paper stop position is shifted to the upper side than before by correcting to-(Ts-T2) (S35, see T1 'in FIG. 8). Further, when the pause position of the paper P is above the target or the re-feeding starts quickly,
That is, when T2 <Ts (see T2 "in FIG. 8), the timer value T1 of the specified time T1 is set to T1 = T1 +
(T2−Ts) so that the sheet stop position is shifted to a lower position than before (S36, see T1 "in FIG. 8), and it is determined whether or not there is the next sheet P. (S37) If the next sheet P is present, the above-described process is repeated. If not, a release signal for disconnecting the T / A feed clutch 64 is sent to stop the T / A roll 38 (S38). End a series of processes.

As described above, in the present embodiment, the paper re-feeding time of the paper P is monitored by the paper detection sensor 78 and the variation in the paper feeding timing of the transport rolls 47b and 47c is corrected. And the occurrence of misregistration can be prevented.

In the present embodiment, the correction of the specified time T1 is performed every time one sheet of paper P is conveyed. However, the present invention is not limited to this. It may be performed when clogging or misregistration occurs. Further, similarly to the first embodiment, the correction of the specified time T1 may be performed from the average value of a plurality of data.

Further, in addition to the paper feeding mechanism described in the first embodiment and the paper returning mechanism described in the second embodiment, the above-described control method is applied to the original feeding mechanism of the image reading device 60 shown in FIG. May be applied.

[0041]

As described above, according to the present invention,
Since the specified time is corrected in accordance with the refeeding time, it is possible to prevent the occurrence of sheet jams and misregistration due to variations in sheet feeding timing with a simple configuration.

[Brief description of the drawings]

FIG. 1 is a schematic explanatory view of a sheet conveying device according to the present invention.

FIG. 2 is an overall configuration diagram of the first embodiment of the electrophotographic copying machine to which the sheet feeding device according to the present invention is applied.

FIG. 3 is a schematic configuration diagram of a paper supply mechanism according to the first embodiment.

FIG. 4 is a control flow of the paper supply mechanism according to the first embodiment.

FIG. 5 is a timing diagram of the paper supply mechanism according to the first embodiment.

FIG. 6 is a schematic configuration diagram of a sheet supply mechanism according to a second embodiment.

FIG. 7 is a control flow of the paper supply mechanism according to the second embodiment.

FIG. 8 is a timing diagram of the paper supply mechanism according to the second embodiment.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Conveying member, 2 ... Driving means, 3 ... Detection means, 4 ... Control means, S ... Sheet

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 3F048 AA01 AB01 BA02 BA05 BA07 BA14 BA24 BA25 BA26 BA27 BB02 BB05 CA03 CB02 CB03 CC03 CC04 CC17 DA03 DA04 DA06 DB03 DB07 DB12 DC12 EA03 EA14 EB02 EB22 EB24 EB30 BA09 CA03 EA03 EC02 FA05 FA06 FA08

Claims (1)

[Claims] 1. A pair of conveying members rotatably pressed and arranged to convey a sheet while nipping the sheet; a driving unit for driving the conveying member; and an upstream or downstream side of the conveying member. Detecting means for detecting the presence or absence of a sheet to be conveyed, and sending a signal to stop driving of the conveying member to the driving means after a lapse of a predetermined time since the leading end of the sheet is detected by the detecting means, and Control means for sending a signal to the driving means to restart the driving of the conveying member in accordance with a timing.The control means outputs a signal to the driving means to restart the driving of the conveying member. The length of the specified time for the next sheet is changed according to the length of the sheet re-feeding time from when the sheet is sent to when the trailing end of the sheet is detected by the detecting means. Sheet conveying apparatus characterized by.