Sheet stacking apparatus

Abstract: The present invention provides a sheet stacking apparatus for stacking discharged sheets on a sheet stacking plate (23,24 or 25), comprising a distance sensor (60) disposed above the sheet stacking plate and having a light illumination portion for illuminating light toward the sheet stacking plate and a light receiving portion for receiving reflected light, and a control device for determining a distance between the distance sensor and a sheet stack rested on the sheet stacking plate and for judging a sheet stacking condition of the sheet stacking plate on the basis of the determined distance.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a sheet stacking apparatus in which sheets discharged from an image forming apparatus such as a copying machine, a laser beam printer and the like are stacking on a sheet stacking plate, and an image forming apparatus having such a sheet stacking apparatus, and more particularly, it relates to control of a position of such a sheet stacking plate. Related Background Art Among conventional image forming apparatuses such as copying machines, laser beam printers and the like, there is an image forming apparatus having a post-treatment apparatus for effecting a post-treatment such as sheet stapling. In such a post-treatment apparatus, as shown in Fig. 42, a tray (sheet stacking plate) 103 on which sheets are stacked is provided on a lift/lower tray shifting stage 102. On the other hand, an upper rockable guide 88 is provided with a sheet level detection sensor 105 for detecting the fact that a height of a sheet stack S comprised of sheets discharged on the tray 103 reaches a predetermined level. The sheet level detection sensor 105 includes a pivotable sensor lever 106 contacted with the sheet stack S rested on the tray 103 and having an upper end pivotablly supported, and a photo-sensor 107 for emitting a signal representing the fact that the height of the sheet stack S reaches the predetermined level when the sensor lever 106 is rotated by a predetermined angle. Incidentally, the sensor lever 106 is gradually rocked upwardly as the sheets S are successively stacked on the tray 103, and, when the height of the sheet stack S reaches the predetermined level, the sensor lever reaches the predetermined angle. With this arrangement, the sheet level detection sensor 105 can detect the fact that the height of the sheet stack S reaches the predetermined level. When the fact that the height of the sheet stack S reaches the predetermined level is detected by the sheet level detection sensor 105 in this way, a tray lift/lower motor provided on the tray shifting stage 102 is driven, so that the tray 103 is lowered to substantially a position where a first sheet is discharged on the tray. As a result, it is possible to prevent any sheet from jamming at a sheet discharge opening 50 and to increase a maximum stackable amount of sheets on the tray 103. By the way, recently, there have been proposed sheet stacking apparatuses each having a plurality of trays. In such sheet stacking apparatuses, for example, when the sheets are successively received one by one in trays from an uppermost one to a lowermost one, after a maximum allowable sheets are stacked on a certain tray, it is necessary that this tray is shifted above the discharge opening and a next (lower) tray is shifted below the discharge opening. However, when the tray on which the sheets were fully stacked is shifted, as is in the conventional case, if the conventional sheet level detection sensor is provided, since the sensor lever interferes with the movement of the tray, there must be provided for retarding the sensor lever to a position where the sensor lever does not interfere with the movement of the tray whenever the tray is lifted, thereby making the apparatus complicated. Further, since the conventional sheet level detection sensor cannot detect whether the sheets are stacked on the tray or not, for example, there arises a problem that a different kind of sheets are stacked on the tray on which a certain kind of sheets were stacked. Moreover, document EP-A-0 562 387 shows a sheet stacking apparatus according to the preamble portion of claim 1. This apparatus is intended for the use of films or any other moving sheets which are to be fed to a receiving magazine like a sheet stacking plate. An LED and a photosensor form a distance sensor means disposed above the sheet stacking plate. The LED emits a light beam oriented toward the last arrived film and is partially reflected back to the phototransistor which emits a detecting signal. A control means determines a distance between the distance sensor and a sheet stack of films rested on the receiving magazine in order to judge a sheet stacking condition on the basis of the determined distance. Furthermore, "ELEKTRO-TECHNIK" magazine; No. 33;12/10/1966; pages 772,773; describes a distance sensor arranged in a vertical manner above stacked sheets on a sheet stack plate, the distance sensor using focused light which generates a signal only if the uppermost sheet is in the focus point. Finally, US-A-5,017,972 discloses optical switches functioning as upper and lower stack toprange sensors, wherein a damper arm rotatably supported on the apparatus and resting on the sheet stack rotates and the position thereof is detected by the optical switches. SUMMARY OF THE INVENTION It is the object of the present invention, to provide a sheet stacking apparatus in which, even when a plurality of sheet stacking plates are provided, the positions of the sheets can easily be controlled with a simple construction. This object is achieved with a sheet stacking apparatus having the features of claim 1. The present invention can also be applied to an image forming apparatus comprising an image forming portion and a sheet stacking apparatus for containing sheets on which images were formed in the image forming portion. The present invention can also be applied to an image forming apparatus comprising an image forming portion and a sheet stacking apparatus for effecting a post-treatment regarding sheets on which images were formed in the image forming portion. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a side sectional view of a sheet stacking apparatus and an image forming apparatus according to the present invention; Fig. 2 is a side sectional view of the sheet stacking apparatus; Fig. 3 is a plan view of a staple tray portion of the sheet stacking apparatus; Fig. 4 is a side sectional view of the staple tray portion; Fig. 5 is a side view of a main portion of a tray unit of the sheet stacking apparatus; Fig. 6 is an enlarged sectional view of main portion of the sheet stacking apparatus; Fig. 7 is an explanatory view showing a condition that a rockable guide of the sheet stacking apparatus is rocked; Fig. 8 is an explanatory view showing a condition that a discharge opening is closed by a stopper of the sheet stacking apparatus; Fig. 9 is an explanatory view showing a condition that the rockable guide is rocked upwardly; Fig. 10 is an explanatory view showing a position where an escape portion is formed by a roller guide of the sheet stacking apparatus; Fig. 11 is a block diagram of a distance measurement sensor of the sheet stacking apparatus; Fig. 12 is an explanatory view showing a portion of a block diagram of a CPU of the sheet stacking apparatus; Fig. 13 is an explanatory view showing the other portion of the block diagram of the CPU of the sheet stacking apparatus; Fig. 14 is an explanatory view showing a distance measuring principle of the sheet stacking apparatus; Fig. 15 is an explanatory view showing a signal outputted from the CPU to the distance measurement sensor and a signal inputted form the distance measurement sensor to the CPU; Fig. 16 is a flow chart showing a portion of a control operation of the CPU; Fig. 17 is a flow chart showing another portion of the control operation of the CPU; Fig. 18 is a flow chart showing a further portion of the control operation of the CPU; Fig. 19 is an explanatory view for staple positions of a staple unit of the sheet stacking apparatus; Fig. 20 is a partial side sectional view of the staple unit; Fig. 21 is a schematic plan view showing moving routes of the staple unit; Fig. 22 is a partial right side sectional view of the staple unit; Fig. 23 is an explanatory view showing an operation of a retard means of the staple unit; Fig. 24 is an explanatory view showing operations of the staple unit and an abutment member; Fig. 25 is an explanatory view showing a structure of a stapler of the staple unit; Fig. 26 is a plan view of the stapler; Fig. 27 is a view showing a wave form applied to a stapler motor in a stapling operation of the stapler; Fig. 28 is an explanatory view showing a condition that a central portion of a first staple needle is held by a holding groove of a needle bending block; Fig. 29 is an explanatory view showing a stapling step of a forming portion of the stapler; Fig. 30 is an explanatory view showing a condition that a sheet is discharged on a second tray of the sheet stacking apparatus; Fig. 31 is an explanatory view showing a condition that the sheets were discharged on the second tray of the sheet stacking apparatus; Fig. 32 is an explanatory view showing a condition of the second tray in a staple sort mode; Fig. 33 is an explanatory view showing a condition a number of sheets set by an operator are aligned on a staple tray; Fig. 34 is an explanatory view showing a condition that a stapled sheets bundle is discharged; Fig. 35 is an explanatory view showing a condition that a stapled sheets bundle was discharged; Fig. 36 is an explanatory view showing a condition that the sheet starts to enter into the sheet stacking apparatus; Fig. 37 is an explanatory view showing a condition that a first sheet is wound around a buffer roller; Fig. 38 is an explanatory view showing a condition that first and second sheets are conveyed in an overlapped condition; Fig. 39 is an explanatory view showing a condition that two sheets are discharged in an overlapped condition; Fig. 40 is a side view showing an alteration of the tray unit of Fig. 5; Fig. 41 is an explanatory view showing an alteration of the principle of Fig. 14; and Fig. 42 is a side view of a main portion of a conventional sheet stacking apparatus. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will now be explained in connection with embodiments thereof with reference to the accompanying drawings. Fig. 1 shows an internal construction of a copying machine as an example of an image forming apparatus to which the present invention can be applied. In Fig. 1, the reference numeral 1 denotes a sheet stacking apparatus according to the present invention; 100 denotes a copying machine; 200 denotes a cassettes containing sheets having different sizes; and 300 denotes an original supplying apparatus or automatic document feeder (referred to as "ADF" hereinafter). The copying machine 100 comprises an original platen glass 101, scan reflection mirrors (scan mirrors) 103, 104 for changing light path for light reflected by an original, a lens 105 having a focusing function and a magnification change function, and a first scan mirror portion having an illumination lamp for reading the original supplied from the ADF 300 and a mirror. The copying machine further comprises a pair of regist rollers 107, a photosensitive drum 108, a pressure roller 110, a convey belt 111 for conveying a recording sheet on which an image was recorded to a fixing device 112 for fixing the image to the recording sheet, convey rollers 113, 117 for conveying the recording sheets, a flapper 114 for switching or changing a conveying direction of the recording sheet, a pair of convey rollers 115 for conveying the recording sheet toward the sheet stacking apparatus 1, a reverse rotation path 116 for turning up the recording sheet, a pair of convey rollers 118 for conveying the recording sheet supplied from any one of cassettes 200 to a photosensitive drum unit portion, and a portion including a roller 119, a tray 120 and a separation pad 121 which elements 119-121 serve to convey a recording sheet supplied from a manual sheet insertion unit. The copying machine further includes a laser 122 for forming an image on the photosensitive drum, a polygon mirror 123, a mirror 125 for changing a light path, and a motor 124 for driving the polygon mirror 123. In association with each cassette 200, there are provided a pair of convey rollers 201 for picking up the recording sheet from the corresponding cassette 200, and a pair of intermediate rollers 202 for feeding the recording sheet picked-up from the cassette 200. A surface of the photosensitive drum 108 is formed from a seamless photosensitive body using photoconductive material and electrically-conductive material, and the photosensitive drum 108 is rotatably supported so that, when a copy start key is depressed, the drum is rotated in a direction shown by the arrow in Fig. 1 by means of a main motor (not shown). Incidentally, when predetermined rotation control and potential control for the drum 108 are completed, the original rested on the original platen glass 101 is illuminated by the light from the illumination lamp of the first scan mirror portion 106, and the light reflected from the original is passed through the scan mirrors 103, 104 and the lens 105 and is focused on a light receiving element of the lens unit. In the light receiving element, the light reflected from the original is converted into an electric signal which is in turn sent to an image process portion (not shown). On the other hand, in the image process portion, after predetermined data treatment inputted by the operator is performed, the signal is sent to the laser 112. The data-treated electric signal is converted into light in the laser portion 112. Then, the light is incident on the photosensitive drum 108 through the polygon mirror 123 and the mirror 125 to form an electrostatic latent image on the drum. The latent image is then visualized by toner as a toner image, and the toner image is transferred onto the recording sheet (transfer sheet), as will be described later. On the other hand, the recording sheet (transfer sheet) supplied from cassette 200 or the manual insertion tray 120 is sent toward the interior of the copying machine by the rollers 118, 201, 202 or the roller 119 and then is sent to the photosensitive drum 108 with a predetermined timing controlled by the pair of regist rollers 107 so that an image tip end of toner image is aligned with a tip end of the transfer sheet. Thereafter, while the transfer sheet is being passed between the photosensitive drum 108 and the roller 110, the toner image on the drum 108 is transferred onto the transfer sheet. Then, the transfer sheet is separated from the drum 108, and the separated transfer sheet is sent, through the convey belt 111, to the fixing device 112, where the image is fixed to the transfer sheet with heat and pressure. The transfer sheet to which the image was fixed is introduced into the path 116 through the flapper 114. When a trail end of the transfer sheet leaves the flapper 114, the pair of convey rollers 117 are rotated in a direction opposite to a direction shown by the arrow. As a result, the sheet is moved along the path 116 in the opposite direction so that the trail end of the sheet is directed toward the discharge rollers 115 by the flapper 114. In this way, the sheet is outputted to the sheet stacking apparatus with the imaged surface facing downwardly. On the other hand, the ADF 300 comprises a stacking tray 301 on which the originals 302 are stacked with images surfaces facing downwardly, a pickup roller 304 for feeding out the originals one by one from a lowermost one, a separation means 305 for separating the originals one by one if several originals are supplied simultaneously, and a pair of regist rollers 306 for effecting registration of a tip end of the original. Incidentally, the original passed through the regist rollers 306 is sent to a reading portion 307, where the original is red in a condition that the first scan mirror unit 106 is fixed (so-called "flow-reading"). Then, the original is discharged onto a discharge tray 309 through a pair of discharge rollers 308. A stopper member 2 is provided on an upper portion of the sheet stacking apparatus 1, so that, when the sheet stacking apparatus is connected to the copying machine 100, the stopper member 2 is engaged by a hold portion 2A formed on a side surface of the copying machine 100, thereby positioning and holding the sheet stacking apparatus. Further, a folding unit or an attachment base 70 having casters 80 is provided below the sheet stacking apparatus. With the arrangement as mentioned above, when a jam treatment is performed in the vicinity of the discharge portion of the copying machine or at a transit portion between the sheet stacking apparatus 1 and the copying machine 100, first of all, the stopper member 2 is released and then the sheet stacking apparatus 1 is shifted to be separated from the copying machine 110. As a result, the jam treatment can easily be performed. On the other hand, when the sheets discharged from the discharge portion of the copying machine 100 are treated in the sheet stacking apparatus 1, in Fig. 2, an upstream end of a flapper 3 is shifted downwardly and an upstream end of a flapper 4 is shifted upwardly. As a result, the sheet is introduced into a first convey path 6 via a pair of rollers 5. Incidentally, when the sheet is sent to the folding unit 70, the upstream end of the flapper 3 is shifted upwardly, with the result that the sheet is sent toward a direction shown by the broken line arrow through a third convey path 7. Incidentally, in Fig. 2, the reference numeral 8 denotes a second convey path; 9 denotes a buffer roller; 14, 15 and 16 denote buffer rollers; 10, 11, 12 and 13 denotes sheet detection sensors for detecting the sheet passing therethrough or trapped therein; 17 denotes a first discharge roller; 18 denotes a hold-down roller; 19 denotes a discharge align belt pinched and rotated between the first discharge roller 17 and the hold-down roller 18 and having an endless rib (not shown) at its central portion on its inner surface to prevent the dismounting of the belt. The belt is driven by rotation of the first discharge roller 17. Further, when the stapling operation (described later) is performed, the trail end of the sheet abuts against an abutment member 20 so that the sheet is aligned in a longitudinal direction. The abutment member 20 can be shifted between a home position where the trail end of the sheet is aligned and a retard position where the abutment member does not interfere with a movement of a stapler 400. That is to say, when the stapler 400 is shifted, the abutment member is rotated to the retard position shown by the broken line so that the shifting movement of the stapler is not obstructed by the abutment member. On the other hand, as shown in Figs. 3 and 4, the alignment of the sheet in a width-wise direction is effected by a side align guide 21. Further, the stapler 400 is shifted within a range shown by the arrows in Fig. 3 to effect two-points staple at intermediate positions and front and rear one staple at both end positions. Incidentally, in Fig. 3, the reference numeral 29 denotes an align reference plate. In Fig. 2, the reference numerals 23, 24 and 25 denotes first, second and third trays on which the sheets discharged from a discharge opening 50; and 26 denotes a tray unit (stacking plate unit) which can be shifted in an up-and-down direction while holding the first, second and third trays 23, 24, 25. The tray unit 26 has a rack gear 26a (Fig. 5) meshed with a lift/lower gear 601a of a shift motor (drive device) 601. Thus, the tray unit can be shifted in the up-and-down direction by driving the shift motor. Further, as shown in Fig. 2 and 6, a rockable guide 31 serves to rotatably hold a shiftable discharge roller 33. When the sheet is discharged, the rockable guide is rocked around a pivot pin 31a by rotating a cam 35 (Fig. 7) in a direction shown by the arrow (Fig. 7) by means of a discharge motor 35a, thereby urging the shiftable discharge roller 33 against the discharge roller 32. Incidentally, in a staple mode (described later), as shown in Fig. 9, the rockable guide 31 is rocked to an upper position where the shiftable discharge roller 33 is separated from the discharge roller 32. In this position, a pair of rollers comprised of the shiftable discharge roller 33 and the discharge roller 32 cannot discharge the sheet. On the other hand, when the tray unit is shifted, a stopper 30 (Fig. 6) is rocked around a pivot pin 30a to close the discharge opening 50, as shown by the solid line in Fig. 8. By closing the discharge opening 50 in this way, when the tray is being passed through the discharge opening 50, the sheets stacked on the tray can be prevented from re-entering into the discharge opening 50. Incidentally, in Fig. 8, the reference numeral 27 denotes an upper net guide. Further, when the sheet is discharged, the stopper 30 is rocked in a direction shown by the arrow Y in Fig. 6, thereby opening the discharge opening 50. And, in the staple mode (described later), as is in the rockable guide 31, the stopper opens the discharge opening 50, as shown in Fig. 9. Further, in Fig. 6, a roller guide 34 is pivotally supported between a lower net guide 27a and the discharge opening 50 and is provided at its upper end with an outwardly extending locking pawl 35 (Fig. 8). When the rockable guide 31 is rocked downwardly, the roller guide 34 is rocked while expanding a spring 37 via a link 36, with a result that the roller guide is retarded to a retard position where a tip end of the locking pawl 35 is retracted into the interior of the copying apparatus 1 than a forward end of the discharge roller 32. Consequently, when the sheet is discharged, the sheet S can be prevented from being caught between the roller guide 34 and the discharge roller 32, thereby surely discharging the sheet S. Further, as shown in Fig. 10, an escape surface shown by an hatched area I can be formed between the lower net guide 27a and the roller guide, thereby smoothly guiding the sheet S to the tray 24. As shown in Fig. 6, the roller guide 34 is biased toward a direction shown by the arrow A by means of the spring 37, so that, in the staple mode, the roller guide is maintained in flush with the lower net guide 27a by the spring 37 as shown in Fig. 9. As a result, in the staple mode, even if an inclined end of the sheet stack Sa rested on the tray is flexed upwardly, the inclined end can be prevented from being caught between the lower net guide 27a and the discharge roller 32. Further, in the staple mode, as shown in Fig. 9, the locking pawl 35 is protruded above the tray 24. Consequently, even if the inclined end of the sheet stack Sa is flexed upwardly, a free end of the inclined end portion of the sheet stack is prevented from exceeding a point G, so that a next sheet can be prevented from being caught by the inclined end to prevent the sheet jam, and, the burden to the operation of the side align guide 21 can be eliminated, thereby preventing the aligning ability of the side align guide from being worsened. On the other hand, in Fig. 2, there is provided a distance sensor 60 of non-contact type having an illumination portion for emitting light toward the tray 23, 24 or 25 and a light receiving portion for receiving reflected light. For example, in the staple mode, whenever a stapling operation is performed, a CPU (control device) (described later) drives the distance sensor 60 to emit the light toward the trays 23, 24 and 25 and determines distances between the distance sensor 60 and the sheet stack rested on the trays 23, 24 or 25 on the basis of a position of received light on the light receiving portion. In the staple mode, after the sheet bundle (stack) is stapled by staple needles, the sheet bundle is discharged onto the tray 23, 24 or 25. Fig. 11 schematically shows a block diagram of the distance sensor 60. In Fig. 11, the reference numeral 61 denotes a light emitting element (LED); and 62 denotes a burst wave generate circuit for generating a signal for activating the light emitting element 61. The light emitting element 61 and the burst wave generate circuit 62 constitute the illumination portion. Further, the reference numeral 63 denotes a PSD (position sensitive detector) light receiving element for receiving the light reflected from the first, second or third tray 23, 24 or 25. The PSD light receiving element 63 comprises an amplifier 63a, a limiter 63b, a band pass filter (BPF) 63c, a demodulator 63d, an integrator 63e and a comparator 63f and serves to generate current having different value in accordance with a light receiving position for the light reflected from the sheet. A signal process circuit 64 serves to output a trigger signal to the burst wave generate circuit 62 and to convert the current into voltage information. As mentioned above, the distance sensor 60 is disposed within the sheet stacking apparatus 1 and is connected to the CPU 600 having block construction as shown in Figs. 12 and 13 so that, when the signal is inputted from the CPU 600, it causes the burst wave generate circuit 62 to output a trigger signal to thereby illuminate the light emitting element 61 and it outputs the voltage information corresponding to the light receiving position for the light in the PSD light receiving element 63 to the CPU 600. As shown in Fig. 14, the distance sensor 60 is disposed above the tray 23 and is inclined with respect to the tray by a predetermined angle a (30 degrees in the illustrated embodiment) so that the light is illuminated on the tray 23 (sheet S). On the other hand, the CPU 600 firstly determines a distance A between the distance sensor 60 and the upper surface of the sheet stack S on the tray 23 on the basis of magnitude of the voltage signal from the distance sensor 60. By determining the distance A or A' (A' is a distance when only a single sheet is rested on the tray) in this way, it is possible to determine a vertical distance L2 or L2' between the distance sensor 60 and the surface of the sheet stack on the basis of the following equations: L2 = A × cos 30° L2' = A' × cos 30° Incidentally, the distance L2' indicates the vertical distance when the tray 23 is positioned at a position where the first sheet is discharged onto the tray. Since a distance L1 between the distance sensor 60 and the discharge opening 50 already known, a distance (L3') between the tray 23 and the discharge opening 50 and a distance (L3) between the surface of the sheet stack and the discharge opening 50 can be determined on the basis of the following equations: L3 = L2 - L1 L3' = L2' - L1 By the way, such distance measurement is effected whenever the sheet discharge (single sheet discharge) or post-treatment (sheet bundle discharge) such as stapling is performed by the CPU 600, by intermittently inputting a signal as shown in Fig. 15 to the burst wave generate circuit 62 through the signal process circuit 64. In Fig. 15, a signal Vin is a signal for illuminating the light, for example, every stapling operation. When a low level L of the signal Vin continues longer than 70 msec, the light emitting element 61 starts to emit the light to start the measurement, and, thereafter, eight clock signals shorter than 0.2 msec are inputted to the burst wave generate circuit 62, for example, for 1 msec or more, thereby measuring the distance. The measurement is finished by providing a H (high level) signal longer than 1.5 msec after the eight clock signals have been inputted. Regarding the signals at the light emitting side, in the PSD light receiving element 63, the received reflected light is outputted to the CPU 600 as 8-bit voltage information. On the other hand, in the CPU 600, 8-bit distance data which was previously sought by tests is stored in a table of a ROM (read only memory) 610 (Fig. 13) in which control procedure to be executed by the CPU 600 is stored. On the basis of the table, the distance A between the distance sensor 60 and the sheet stack is determined in accordance with the data sent from the distance sensor 60. For example, as shown in Fig. 14, the sheet stacking apparatus 1 is provided with a tray HP sensor (stacking plate position detection means) S180. By this sensor S180 and an actuator 23b of the tray 23 for activating the sensor S180, it is possible to detect a home position of the tray 23 and to detect a home position of the next tray 24 or 25 when the tray is shifted. By providing the sensor S180 in this way and by providing the actuators on the respective trays 23, 24 and 25, it is possible to detect the surfaces of the sheet stacks on the trays 23 to 25. With this arrangement, a plurality of trays can be positioned at any level. The CPU 600 determines the height H of the sheet stack on the tray 23, 24 or 25 on the basis of the height position L2' of the tray when detected by the sensor S180, and the distance L2 between the distance sensor 60 and the sheet stack (on the tray 23, 24 or 25) determined by the distance sensor 60. The value L2' may be measured as mentioned above, or, may previously be clarified in case of the sheet bundle. When the sheets are stacked by a predetermined amount, a shift motor 601 is driven through a driver D6 (Fig. 13) not to interfere with the discharge of the sheet, thereby shifting the tray unit 26 downwardly to lower the tray 23. In this way, a distance between the discharge opening 50 and the sheet stacking plate is maintained more than L3. As the sheets are successively stacked, when the sheet stacking height H exceeds a first predetermined height indicating the fact that a maximum amount of sheets are stacked on the tray 23, the tray unit 26 is shifted so that further sheets can be stacked on other tray. In this way, by measuring the height of the sheet S or the distance between the sheet stacking surface and the discharge opening 50, it is possible to calculate the proper shifting amount of the tray 23. Incidentally, the calculated result is stored in a RAM (random access memory) 620 for storing various data. The first, second and third trays 23, 24 and 25 are provided with through holes 23a, 24a and 25a (Figs. 2 and 14) at measuring points for the distance sensor 60, so that the presence/absence of the sheet on the trays 23 to 25 can be detected. That is to say, when the light is emitted toward the trays, if no sheet is rested on the trays 23 to 25, the illumination light passes through the through holes 23a, 24a, 25a to reach, for example, a lowermost tray and is reflected by the sheet on the lowermost tray. With this arrangement, the sheet stacking height determined in this case normally becomes greater than a second predetermined height indicating the fact that the tray is positioned at a position where the first sheet is discharged on the tray, with the result that the CPU 600 can judge that there is no sheet on the trays 23 to 25. Incidentally, after it was judged that there is no sheet on the trays 23, 24 and 25, the CPU 600 judges that the trays 23 to 25 are in a sheet stackable condition and discharges the first sheets on the trays 23 to 25. In addition to the distance sensor 60, as shown in Fig. 12, a buffer sensor (detection means) S10 for detecting the fact that the sheet is trapped in the sheet stacking apparatus, an inlet sensor S30 for detecting the fact that the sheet discharged from the copying machine 100 enters into the sheet stacking apparatus 1, an UP cover sensor S40 for detecting the fact that an upper cover of the sheet stacking apparatus 1 is opened, a discharge motor clock sensor S80 for sending information regarding speed control or abnormality of the discharge motor 35a when the sheets are discharged from the sheet stacking apparatus 1 onto the trays 23, 24 and 25, an align HP sensor S90 for detecting the abutment plate 20 in the stapling operation, and a staple tray sensor S100 for detecting presence/absence of the sheet on the staple tray 38 are electrically connected to the input side of the CPU 600. Further, first and second net sensors S130, S140 for detecting positions of the upper and lower net guides 27, 27a forming upper and lower walls of the discharge opening 50, a discharge sensor S150 for detecting the fact that the sheet is discharged from the sheet stacking apparatus 1 onto the tray, a staple shift HP sensor S170 for detecting the fact that the stapler 400 shiftable within the sheet stacking apparatus 1 is positioned at the home position, a shift clock sensor S190 for informing the CPU 600 of a shifted amount of the shiftable tray or abnormality of the shift motor as a drive source for the tray, an UP limit detection sensor S200 for detecting an upper limit of the shiftable tray, a door open/close detection sensor S210, and a joint SW sensor S220 for detecting the fact that the sheet stacking apparatus 1 is connected to the copying machine 100 are electrically connected to the input side of the CPU 600. On the other hand, in addition to the shift motor 601, as shown in Fig. 13, a convey motor M230 for conveying the sheet in the sheet stacking apparatus, a discharge motor 35a, an align motor M250 for aligning the sheet, a staple move motor (pulse motor) 452 for shifting the stapler 400, a staple motor 406 for causing the stapler 400 to effect the stapling operation to staple the sheet bundle, an inlet solenoid SL290 for switching or changing the convey path for the sheet discharged from the copying machine 100, a discharge opening solenoid SL300 for switching or changing the discharge opening for the sheet discharged from the copying machine 100, a switch solenoid S1310 for switching the sheet convey path in the sheet stacking apparatus 1, and a display means 650 for informing the operator of alarm when the over-stacking is detected in the sheet stacking surface distance measurement are electrically connected to the output side of the CPU 600. In the illustrated embodiment, the copying machine 100 is of digital type comprising a scanner portion for reading the image on the original and a printer portion for reproducing the image and can be operated independently. That is to say, in the scanner portion, the original is illuminated by a lamp, and the reflected light is decomposed to small spots (pixels) by a light receiving element, which small spots are converted into electric signals corresponding to light/shade of the image; whereas, in the printer portion, laser beams are illuminated onto the photosensitive drum in response to the electric signals, thereby forming an electrostatic latent image on the drum, and a copy image is formed on the sheet through developing, transferring and fixing processes. Accordingly, by connecting an interface 500 to the digital copying machine as shown in Fig. 1, the signals of the image read in the scanner portion can be transmitted to a facsimile 501, or, signals from the facsimile 501 can be transmitted to the printer portion through the interface 500, thereby copying the image on the transfer sheet. Similarly, signals from a computer equipment 502 such as a personal computer can be transmitted to the printer portion through the interface 500, thereby copying the image on the transfer sheet, or, the image read in the scanner portion can be transmitted to the personal computer through the interface 500. As mentioned above, in present digital copying machines, not only the original sent from the ADF 300 or the original rested on the original platen glass can be read to obtain the copy, but also the copying machine can be used as a facsimile or a printer of a personal computer by using the interface 500. Next, the sheet stacking control of the CPU 600 of the sheet stacking apparatus 1 connected to the digital copying machine will be explained with reference to flow charts shown in Figs. 16, 17 and 18. In Fig. 16 showing a flow chart for effecting initialization of the sheet stacking apparatus 1, when an electric power source is turned ON in a step S1001, in a step S1002, I/O ports and memory (RAM) are initialized. Then, in a step S1003, a communication mode (to FAX, printer, copying machine) is set, and, in a step S1004, it is judged whether the communication to the main body of the copying machine is established. If affirmative, in a step S1005, initializing communication data (sheet stacking apparatus stand-by signal) is emitted from the sheet stacking apparatus 1. On the other hand, after the initializing communication data is emitted, in Fig. 17 showing a flow chart for effecting the tray position control, when an operation start signal is sent to the sheet stacking apparatus 1, in a step S2001, it is judged whether an initial signal (operation start signal) indicating the fact that the sheet stacking apparatus 1 was initialized is turned ON. If ON, in a step S2002, it is judged whether a tray position is determined. If the tray position is not determined, in a step S2003, the tray is shifted to the home position. Then, in a step S2004, it is judged whether the tray has been shifted to the home position. If affirmative, the program goes to a step S2005. In the step S2002, if it is judged that the tray position is determined, the program also goes to the step S2005, where it is judged whether the present tray is positioned at a tray designated position. If affirmative (the designated position), the program goes to a step S2007; whereas, if negative, the program goes to a step S2006, where the tray is shifted to the designated position. After the tray was shifted to the designated position in the step S2006, in the step S2007, the sheet stacking height on the tray (referred to as "tray stacking height" hereinafter) is measured by the distance sensor 60. Then, the program goes to a step S2008, tray stacking height data D measured in the step S2007 is compared with sheet height reference data D1 corresponding to the second predetermined height. In this case, if there is no sheet on the tray, since the through hole is not closed by the sheet, the tray stacking height data D becomes greater than the reference data D1. In this case, it is judged that there is not sheet on the tray, and the program goes to a step S2012, where the first sheet is discharged on the tray maintaining the present tray height. On the other hand, if D1 > D, the program goes to a step S2009, where tray shift amount data B is determined on the basis of a difference between the tray stacking height data D and the sheet height reference data D2 corresponding to the predetermined height, and, in a step S2010, it is judged whether the tray shift amount data B is zero (i.e., the present sheet height is a maximum stacking amount). If affirmative, the program goes to the step S2012. On the other hand, if the present sheet height does not reach the maximum stacking amount, the program goes to a step S2011, where the tray is shifted by a distance corresponding to the tray shift amount data B. Then, the program is returned to the step S2007, where the tray stacking height data D is measured again, and, after the tray is shifted to the predetermined height, the program goes to the step S2012. In this way, by determining the sheet stacking height by the distance sensor 60 and the sensor S180 at a predetermined timing after the post-treatment, the position control of the trays can be properly effected without interference with the distance sensor 60. In Fig. 18 showing a flow chart for effecting the tray movement control, in a step S3002, it is judged whether the rockable guide is closed. If the rockable guide is opened, in a step S3003, the discharge motor 35a is rotated in a reverse direction until the rockable guide 31 is closed. When the rockable guide 31 is closed, or if the rockable guide 31 is initially closed, the program goes to a step S3004, where it is judged whether the stopper 30 is closed. If the stopper 30 is closed, the program goes to a step S3006, where the shift motor 601 is driven. In Fig. 2, a stapler unit 400A includes the stapler 400 for effecting the stapling operation regarding the sheet stack (sheet bundle) rested on the staple tray 38. As shown in Fig. 19, the staple unit is shifted in a direction shown by the arrow Y by means of a pulse motor (described later) so as to effect front one-point staple (staple position H 1 ), intermediate two-points staple (staple positions H 2 , H 3 ) and rear one-point staple (staple position H 4 ) regarding the sheet bundle rested on the staple tray 38. Incidentally, in Fig. 19, while an example that sizes of the sheets to be stapled are A3, A4, B4 and B5 is shown, the present invention is not limited to such sizes of the sheets. As shown in Fig. 20, the staple 400 is secured to a staple cover 430 and is supported for shifting movement in a direction shown by the arrow X by means of a support member 431 secured to a shift table 433. A spring member 439 is secured to the shift table 433 so that the staple cover 430 is biased upwardly by the spring member 439 and is positioned by a stopper 430a. Further, support shafts 441, 442, 443 are secured to the shift table 433. A pulley gear 440 is rotatably supported on the support shaft 441, and guide support members 434, 435, 436 are rotatably supported on the support shafts 441, 442, 443, respectively. Further, rollers 444 for permitting parallel movement of the shift table 433 are rotatably supported on the shift table 433, and a stopper regulating member 438 constituting a retard means (described later) for the abutment member 20 is secured to the shift table. On the other hand, as shown in Fig. 21, a stay 432 opposed to the staple tray 38 is provided with an elongated slot 447 for regulating the movement of the first guide support member 434, a rail 437 for regulating movements of the second and third guide support members 435, 436, and a fixed rack gear 445 meshed with the pulley gear 440. In Fig. 21, a photointerrupter 446 serves to detect whether the staple unit 400A is positioned at its home position (where the first guide support member 434 is positioned at a position shown by "A" in Fig. 21). By determining a rotational amount of the pulse motor (described later) from the home position on the basis of the number of pulses, the staple position of the staple unit 400A is controlled. Incidentally, the present invention is not limited to such an arrangement. Further, as shown in Fig. 22, a pulse motor 452 for shifting the staple unit 400A in a direction shown by the arrow Y is secured to the shift table 433, and a belt pulley 454 is secured to an output shaft of the pulse motor. The belt pulley 454 is connected to the pulley gear 440 via a timing belt 455, so that rotation of the motor 452 is transmitted to the pulley gear 440 through the belt pulley 454 and timing belt 455, with the result that the staple unit 400A is shifted in the direction Y. Incidentally, the reference numeral 453 denotes a cover for electric parts such as the pulse motor 452 and the like. When the staple unit 400A is shifted, the first guide support member 434 is shifted between positions A-E shown in Fig. 21 along the elongated slot 447 of the stay 432. On the other hand, the second guide support member 435 is shifted along the rail 437 while the first guide support member 434 is being shifted between the positions A-E, and the third guide support member 436 is shifted along the rail 437 while the first guide support member 434 is being shifted between positions E-G. For example, in Fig. 21, when the first guide support member 434 is positioned at the position A, the position of the second guide support member 435 is regulated by the rail 437 and the third guide support member 436 is in a free condition. In this case, corner staple at the staple position H 1 is permitted. Further, when the first guide support member 434 is shifted from the position A to the position C, the staple unit 400A inclined at a predetermined angle at the position A is rotated so that it is gradually brought to a horizontal position in the width-wise direction of the sheet by shifting the second guide support member 345 along the rail 437. And, while the first guide support member 434 is being shifted from the position C to a position D, the position of the staple unit 400A is controlled so that it is maintained in the horizontal condition. As a result, the parallel two-points staple (H 2 , H 3 ) can be performed in accordance with the size of the sheet. In this way, the staple unit 400A can be shifted in the direction Y while regulating the position and the inclination angle of the staple unit by means of two of three guide support members 434, 435 and 436. Thus, the front one-point staple and two-points staple can be effected regarding the sheets having various sizes. Incidentally, the shift amount of the first guide support member 434 is defined by the rotational amount of the pulse motor 452 as mentioned above. Further, in the illustrated embodiment, as shown in Fig. 3, by providing the sheet align reference plate 29 at only one side, while the front one-point staple position (H 1 ) was common to the sheets having various sizes, the present invention can be applied to a case where a sheet align reference is constituted by a sheet sensor and two-point staple positions (H 2 , H 3 ) are common to sheets having various sizes. When such stapling operation is performed, it is necessary to provide a regulating member against which trail ends of the sheets abut to align the sheet bundle. To this end, the abutment plate 20 is provided at the rear end of the staple tray 38. The abutment plate 20 is rotatably held by a shaft member 457 secured to the staple tray 38 and is biased toward an anti-clockwise direction by a spring member 448 mounted around the shaft member 457 so that a regulating portion 20a formed at one end of the abutment plate is protruded upwardly from a rear end of the staple tray 38. In this condition, when the sheets are stacked on the staple tray 38, the trail ends of the sheets abut against the abutment plate 20, thereby aligning the trail end of the sheet bundle. Since the abutment plate 20 is overlapped with the stapler 400, when the staple unit 400A is shifted or when the stapling operation is performed, the abutment plate 20 becomes an obstacle. In this regard, the abutment plate 20 is provided with a retard means 449 for retarding the abutment plate 20 to a retard position where the abutment plate does not interfere with the movement of the staple unit 400A when the latter is shifted. The retard means 449 comprises a gear portion 450 secured to the abutment plate 20 and attached to the shaft member 457, a rotatable sector gear 451 having a rotatably supported lower end and meshed with the gear portion 450 of the abutment plate 20, and a stopper regulating member 438 secured to the shift table 433 and adapted to abut against the sector gear 451 and rotate the latter around a shaft portion 456 when the staple unit 400A is shifted. Incidentally, the sector gear 451 is provided with an abutment portion 451a. When the staple unit 400A is shifted, the stopper regulating member 438 abuts against the abutment portion 451a, with the result that the sector gear 451 is pushed in a direction perpendicular to a shifting direction of the staple unit 400A, thereby rotating the sector gear to a position shown by the broken line. When the sector gear 451 is rotated in this way, the gear portion 450 meshed with the sector gear 451 is rotated, with the result that the abutment plate 20 is rotated downwardly around the shaft member 457 to the retard position (below the staple tray 38) where the abutment plate does not interfere with the movement of the staple unit 400A, while compressing the spring member 448. When the staple unit 400A is further shifted, since the stopper regulating member 438 is disengaged from the abutment portion 451a of the sector gear 451, under the action of the returning force of the spring member 448, the abutment plate 20 is returned, together with the sector gear 451, to a position where the trail end of the sheet bundle Sa is regulated (Fig. 23). As shown in Fig. 24, there are provided a plurality of abutment plates 20 in a width-wise direction of the sheets. Each of the abutment plates 20a, 20b, 20c, 20d and 20e has the retard means 449, and the abutment plates 20a to 20e can be rotated independently. Incidentally, in Fig. 24, in accordance with the position of the staple unit 400A, three abutment plates 20a, 20b, 20c are positioned at a position where the trail end of the sheet bundle is aligned by these abutment plates, and the other two abutment plates 20d, 20e is positioned at the retard position where these abutment plates do not interfere with the movement of the staple unit 400A. Next, a concrete construction and a fundamental operation of the stapler 400 will be explained. As shown in Fig. 25, the stapler 400 has a jaw structure and includes a needle stapling portion 400a comprised of an upper forming portion 401 and a lower staple table 402. A needle cartridge 403 is removably mounted on the forming portion 401, and about 5000 needles H connected to each other to form a plate shape are contained in the needle cartridge 403. The needle plate H contained in the needle cartridge 403 is biased downwardly by a spring 404 provided within an upper portion of the needle cartridge 403 to afford a conveying force to a lower feed roller 405. The needle H fed out by the feed roller 405 is formed as a U-shape by rocking the forming portion 401. When the staple motor 406 is driven, the forming portion 401 is rocked toward the staple table 402 as shown by the arrow to effect a clinching operation (stapling operation) under an eccentric cam integrally attached to an eccentric cam gear 408 by rotating the eccentric cam gear 408 through a gear train 407. Incidentally, the stapler 400 is provided with a sensor 409 of reflection type disposed below the needle cartridge 403 and adapted to detect absence of the staple needle H in the needle cartridge 403. The needle jam of the staple needle H fed out from the needle cartridge 403 can be detected by the sensor 409 of reflection type. Next, the staple needle jam detection will be explained. Fig. 26 is a plan view of the stapler 400. In Fig. 26, a cord 406a for feeding drive current is connected to the staple motor 406 and this cord 406a has a current sensor or load detection means (abnormality detection means) 406b for detecting a current value flowing in the cord. On the other hand, Fig. 27 shows a wave form of the current value flowing in the staple motor 406 (at one stapling process) detected by the current sensor 406b. In Fig. 27, W1 indicates a wave form when the needle H is normally penetrates through the sheet bundle S and is bent, and W2 indicates a wave form when idle stapling occurs (the needle does not penetrate through the sheet bundle even when the stapler 400 is operated). Upon the idle stapling, since there is no load when the needle H penetrates through the sheet bundle S or when the needle is bent, the current value is decreased. Incidentally, W3 indicates a wave form when the poor stapling or the needle jam occurs. In this case, generally, since overload is generate, the current value is increased remarkably. Accordingly, it can be judged that the normal stapling is effected when the current level is I 0 value (initial set value) or therearound, and it can be judged that the needle jam, poor stapling or abnormality of a mechanism of the stapler occurs when I > (I 0 + C) (C = dispersion), and further, it can be judged that the idle stapling occurs when I < (I 0 - C). Incidentally, the operator is informed of the needle absence condition or the needle jam condition of the stapler 400 through a display portion using an LED and the like. Next, the stapling operation of the stapler 400 will be explained. The plate-shaped staple needles H contained in the needle cartridge 403 are fed out one by one from the lowermost one by the feed roller 405, and the fed-out needle is sent to a needle bending block 415 as shown in Fig. 28, where a central portion of the first needle H1 is held in a holding groove 415a of the needle bending block 415. Thereafter, when the forming portion 401 is shifted to a lower operating position by rotating the eccentric cam gear 408, as shown in Fig. 29, a driver 416 is pushed downwardly by a drive mechanism (not shown), thereby shifting a plunger 416a downwardly. In this case, a U-shape bending block 417 is pushed by a push pawl 416b formed on the plunger 416a, thereby urging the block 417 against the needle bending block 415. As a result, as shown in Fig. 28, the staple needle H held in the holding groove 415a of the needle bending block 415 is bent to a U-shaped condition. Thereafter, the plunger 416a is further pushed downwardly, so that the push pawl 416b is disengaged from the U-shape bending block 417, with the result that only the plunger 416a is further pushed downwardly to reach a tapered portion of the needle bending block 415. The plunger 416a brings the first needle H1 alone to a needle cutting member 418 while shifting the needle bending block 415 to a position shown by the dot-and-chain line in Fig. 29. In the needle cutting member 418, the needle H1 is sheared, and the needle is stapled into the sheet bundle S, and then, the needle is pressed against the staple table 402, thereby stapling the sheet bundle S. Thereafter, when the forming portion 401 is shifted to an upper waiting position by further rotation of the eccentric cam gear 408, the driver 416 and the plunger 416a are shifted upwardly to reach their waiting positions. In this way, one process of the stapling operation is finished. Next, the sheet post-treatment of the sheet stacking apparatus having the staple unit 400A will be explained. When the sheet bundle is discharged without the stapling operation, the sheet bundle is directly discharged onto the first, second or third tray 23, 24 or 25. Fig. 30 shows a case where the sheet is discharged onto the second tray 24. When the non-staple mode is selected by the operator, the cam 35 shown in Fig. 7 is rotated in the direction shown by the arrow by means of the discharge motor 35a, with the result that, as shown in Fig. 6, the rockable guide 31 is rocked around the pivot shaft 31a so that the discharge rollers 32, 33 are urged against each other. Incidentally, in this case, the stopper 30 for closing the discharge opening 50 is rotated in the direction shown by the arrow with respect to the rockable guide 31 and is stopped there. In this condition, the sheet discharged from the copying machine 100 is passed through the convey path shown in Fig. 2 and is further conveyed toward a downstream side by the pairs of rollers 5, 17. Then, the sheet is directed toward the tray 24 by the rockable guide 31 and then is passed through the rollers 32, 33 to be discharged onto the tray through the discharge opening 50. In this way, the sheets are successively stacked on the tray 24. On the other hand, when a large number of normal copies are formed, first of all, it is ascertained that there is no sheet on the first tray 23 by the distance sensor 60. To this end, as mentioned above, the CPU 600 causes the distance sensor 60 to emit the light toward the first tray 23, thereby measuring the time for receiving the reflected light. In this case, since the measured time becomes longer than the second predetermined time, the CPU 600 judges that there is no sheet on the tray. After it is ascertained that there is no sheet on the tray 23, the tray 23 is shifted to the position where the first sheet is discharged, in order to stack the sheets from the present tray height. When a predetermined number of sheets are stacked, the tray unit 26 is lowered to a position where the upper surface of the sheet stack on the tray becomes substantially the same as the tray surface on which the first sheet was discharged. The above-mentioned operation is repeated. When it is detected that a maximum amount of sheets are stacked on the tray, a stop signal is sent to the copying machine, thereby stopping the discharging of sheets temporarily. Then, in order to stack the sheets on the second tray 24, the tray unit 24 is lowered to the position where the first sheet is discharged onto the tray 24. Thereafter, the copying operation of the copying machine 100 is re-started, thereby stacking the copied sheets on the tray. Thereafter, the above-mentioned operation is repeated until a maximum amount of sheets are stacked on the tray 24. Incidentally, when the sheets are stacked on the third tray 25, the similar operation is effected. In the illustrated embodiment, as mentioned above, the copying machine 100 is of digital type in which not only the original sent from the ADF 300 or the original rested on the original platen glass can be read to be copied but also the machine can be as a facsimile or a printer for a personal computer by using the interface 500. In order to use the copying machine in this way, it is necessary to sort and stack the sheets on the different trays or to stack the sheets on the tray which is designated by the operator. To this end, in the illustrated embodiment, for example, the output sheets from the facsimile are stacked on the first tray 23, the output sheets from the personal computer are stacked on the second tray 24, and the copied sheets are stacked on the third tray 25 in the copy mode. Now, the operations for discharging the sheets in this way will be explained. First of all, a case where the copied sheets are stacked in the copy mode (i.e., stacked on the third tray 25) from a condition that a certain number of output sheets were discharged from the personal computer onto the second tray 24 will be described. When the sheet stacking apparatus 1 is powered ON, the CPU 600 performs the initialization of the I/O ports and the memory (RAM) and then sets the communication mode to the FAX, printer and copying machine. Thereafter, when the copied sheets are stacked on the third tray 25 from the condition that a certain number of output sheets were discharged from the personal computer onto the second tray 24, the tray unit 26 is lowered to a position where the first sheet is discharged onto the third tray 25. This operation is the same as that in the copy mode, except that the unit is lowered even when the maximum amount of sheets are not stacked on the tray. Next, a case where the output sheets from the facsimile are stacked (i.e., stacked on the first tray 23) from a condition that a certain number of output sheets were discharged from the personal computer onto the second tray 24 will be described. In this case, in the condition that the sheets were stacked on the second tray 24, the tray unit 26 is lifted to permit to stack the sheets on the first tray 23. In this case, in order to prevent the sheet from entering into a space F shown by the hatched area in Fig. 6, as shown in Fig. 8, the stopper 30 is rotated around the pivot pin 30a from the broken line position to the solid line position in Fig. 8, thereby closing the space F. Thus, the tray 24 on which the sheets were stacked can be shifted upwardly without any trouble. Consequently, since the tray on which the sheets were stacked can be moved across the discharge opening 50, the ability of the copying machine 100 having the interface can be realized adequately. Next, the stapling operation of the sheet stacking apparatus will be explained. First of all, in the staple sort mode, the sheets are not directly stacked on the trays 23, 24 and 25, but are stacked on the staple tray 38 (Fig. 2) temporarily. When the staple sort mode is selected by the operator, as shown in Fig. 9, the rockable guide 31 is rocked upwardly to release the discharge opening 50 and to separate the discharge rollers 32, 33 from each other. When the rockable guide 31 is rocked in this way, as mentioned above, roller guide 34 is held in flush with the lower net guide 27a by the spring 37, and the sheet stopper portion 35 is protruded above the sheet stack Sa on the tray 24. In this condition, the sheet discharged from the copying machine 100 is passed through the convey path 6 to reach the pair of rollers 17, 18. Then, the sheet is discharged by the pair of rollers 17, 18. In this case, since the rockable guide 31 was rocked upwardly, the sheet is not discharged but is rested on the staple tray 38. In this case, the tray 24 is positioned higher than that in the non-staple mode, so that, as shown in Fig. 32, the tray 24 supports the trail end of the sheet S, thereby helping the returning of the sheet to the upstream side of the discharging direction. On the other hand, as shown in Fig. 32, the sheet S discharged on the staple tray 38 is moved by own weight toward an upstream side of the discharging direction with the aid of the inclination of the staple tray 38 and by increasing the sheet dropping position from the tray 24. However, the sheet is further biased toward an upstream side of the staple tray 38 by a discharge align belt 19 rotated in synchronous with the discharge roller 17. As a result, the sheet S abut against the abutment plate 20, thereby aligning the sheet in the direction perpendicular to the discharging direction. Further, regarding the alignment of the sheet in the width-wise direction, the side align guide 21 (Figs. 3 and 4) starts to operate at a predetermined time when the sheet dropped on the staple tray 38 abuts against the abutment plate 20 and is shifted from the rear side to the front side by a predetermined distance smaller than a width of the sheet, thereby aligning the sheet S at the front side. This operation is repeated until all of the sheets are stacked on the staple tray. As shown in Fig. 33, when the predetermined number (set by the operator) of sheets are aligned with each other on the staple tray 38, the stapler is operated to effect the stapling operation so that the needles are stapled at positions set by the operator. When the stapling operation is finished, as shown in Fig. 34, the rockable guide 31 is lowered and the discharge roller 32 is rotated in the direction shown by the arrow, with the result that, as shown in Fig. 35, the stapled sheet bundle S is discharged. In the stapling operation, since the sheets are successively discharged from the copying machine 100, the first discharge sheet of the next job is trapped in the sheet stacking apparatus 1 and the second sheet is overlapped with the first sheet and then is discharged together with the first sheet. This operation will be explained with reference to Figs. 36 to 39 (Incidentally, Fig. 36 shows a condition that the sheet starts to enter into the sheet stacking apparatus). The first sheet S1 discharged from the copying machine 100 is sent to a buffer path 8 since the upstream ends of the flappers 3, 4 were shifted downwardly. Then, the sheet is conveyed to the direction shown by the arrow around a buffer roller 9. Now, a flapper 39 is rotated so that the sheet is sent to a roller 15. When the tip end of the sheet S1 is detected by a sensor 11, the sheet is stopped in a condition as shown in Fig. 37. As shown in Fig. 37, when the second sheet S2 enters into the sheet stacking apparatus, the buffer roller 9 starts to rotate to convey the first and second sheets S1, S2 in an overlapped condition, as shown in Fig. 38. When the trail end of the first sheet S1 leaves the flapper 39, as shown in Fig. 39, the flapper 39 is rotated so that the sheets are sent toward the discharge rollers 17, 18, with the result that two sheets are discharged on the staple tray 38 in the overlapped condition. By performing the above operation, during the stapling operation, the sheet is discharged from the discharge rollers 17, 18. Thus, the stapling operation can be carried out and the copying machine 100 is not stopped. Incidentally, in order to increase the stapling time, three or more sheets may be wound around the buffer roller 9. By repeating the above-mentioned operation, a plurality of stapled copy bundles Sa can be produced. As shown in Fig. 9, when the stapled copy bundle Sa was already rested on the tray 24, if an upper end of the copy bundle Sa upwardly exceeds the point G due to the flexion and/or expansion of the copy bundle Sa, the next sheet will be caught by the copy bundle to generate the sheet jam or the copy bundle will interfere with the side align guide to worsen the aligning ability. However, in this case, since the roller guide 34 is positioned in flush with the lower net guide 27a and the stopper member 35 is protruded above the tray 24 to hold down the upper surface of the copy bundle Sa on the tray 24, the upper end of the copy bundle Sa rested on the tray does not exceed the point G. Next, another embodiment of the present invention will be explained with reference to Figs. 40 and 41. Incidentally, the reference numerals which are not particularly explained designate the same elements shown in Figs. 5 and 14. In Fig. 40, a sensor S700 and a shift clock sensor S800 is connected to the input side of the CPU 600. For example, the sensor S700 serves to detect the fact that the tray unit 26 is positioned at the lowermost position, and the shift clock sensor S800 serves to count the number of clocks of the shift motor 601 to thereby measure the shift amount of the tray unit 26. The CPU 600 can detect how amount of the tray unit 26 is lifted from the lowermost position on the basis of signals from the sensors S700, S800 and can judge whether the tray is shifted to the home position on the basis of the detected amount. When the determined distance A becomes shorter than the first predetermined distance indicating the fact that the sheets are stacked in the tray 23 by a predetermined height (for example, height for preventing the discharging of the sheets), the shift motor 601 is driven through the driver D6 to shift the tray unit 26 downwardly to thereby lower the tray 23 so that the discharging of sheets is not prevented. Incidentally, after the tray 23 reaches the lowermost position by successively lowering the tray 23, when the determined distance A becomes shorter than the first predetermined distance, it is judged that the maximum amount of sheets are stacked on the tray 23, and the tray unit 26 is shifted so that further sheets can be stacked on another tray. In this way, by determining the height of the sheet stacking surface or the tray at the predetermined timing after the post-treatment by means of the distance sensor 60 of non-contact type, even when a plurality of trays are provided, the control of the positions of the trays can be properly effected without interference with the distance sensor 60. According to the illustrated embodiments, since the sheet stacking condition of the sheet stacking plate can be judged by the distance sensor of non-contact type and the sheet stacking plate can be shifted on the basis of the judged result, the control of the position of the sheet stacking plate can be effected without interference with the distance sensor. The present invention provides a sheet stacking apparatus for stacking discharged sheets on a sheet stacking plate, comprising a distance sensor disposed above the sheet stacking plate and having a light illumination portion for illuminating light toward the sheet stacking plate and a light receiving portion for receiving reflected light, and a control device for determining a distance between the distance sensor and a sheet stack rested on the sheet stacking plate and for judging a sheet stacking condition of the sheet stacking plate on the basis of the determined distance.

Claims: A sheet stacking apparatus for stacking discharged sheets on a sheet stacking plate, comprising: a distance sensor (60) of a non-contact type disposed above said sheet stacking plate, and having a light illumination portion for illuminating a light toward said sheet stacking plate and a light receiving portion for receiving reflected light; and a control device (600) for determining a vertical distance (L2) between said distance sensor (60) and a sheet stack (S) rested on said sheet stacking plate, and judging a sheet stacking condition of said sheet stacking plate on the basis of the determined distance, characterized in that said distance sensor (60) is disposed in an inclined manner with respect to said sheet stacking plate by a predetermined angle 'a' so that a distance (A) between said distance sensor (60) and the upper surface of the sheet stack (S) is measured and that said control device (600) determines the vertical distance (L2) by the calculation L2 = A x cos 'a'. A sheet stacking apparatus according to claim 1, further comprising a drive device (601) for shifting said sheet stacking plate (23, 24, 25) in an up-and-down direction, and a control device (600) for controlling a driving of said drive device (601) to shift said sheet stacking plate on the basis of the determined distance. A sheet stacking apparatus according to claim 1 or 2, further comprising a stapling means (400), and wherein said light receiving portion of said distance sensor (60) includes a PSD light receiving element, and said illumination portion is designed to emit the light whenever the stapling operation is effected on the basis of a signal from said control device (600). A sheet stacking apparatus according to claim 2, wherein said control device (600) judges that a predetermined amount of sheets are stacked on said sheet stacking plate (23, 24, 25) when the distance becomes shorter than a first predetermined distance, thereby driving said drive device (601) to lower said sheet stacking plate. A sheet stacking apparatus according to claim 1, wherein said control device (600) judges that no sheet is stacked on said sheet stacking plate (23, 24, 25), when said distance is longer than a second predetermined distance. A sheet stacking apparatus according to claim 5, wherein a through opening (23a, 24a, 25a) for permitting passage of the illumination light from said distance sensor (60) is formed in said sheet stacking plate (23, 24, 25) so that, when no sheet is stacked on said sheet stacking plate, the illumination light is passed through said through opening (23a, 24a, 25a) to indicate the fact that said distance becomes longer than the second predetermined distance. A sheet stacking apparatus according to claim 2 or 3, further comprising a stacking plate position detection means (S180) for detecting a height position of said sheet stacking plate, and wherein said control device (600) controls said drive device (601) to determine a distance between said distance sensor (60) and a sheet stack rested on said sheet stacking plate (23, 24, 25), to judge a sheet stacking height of said sheet stacking plate (23, 24, 25) on the basis of the determined distance and the height position detected by said stacking plate position detection means (S180), and to shift said sheet stacking plate on the basis of a judged result. A sheet stacking apparatus according to claim 7, wherein said control device (600) judges that a maximum amount of sheets are stacked on said sheet stacking plate (23, 24, 25) when the sheet stacking height becomes greater than a first predetermined height. A sheet stacking apparatus according to claim 7, wherein said control device (600) judges that no sheet is stacked on said sheet stacking plate (23, 24, 25) when the sheet stacking height is smaller than a second predetermined height. A sheet stacking apparatus according to claim 9, wherein a through opening (23a, 24a, 25a) for permitting passage of the illumination light from said distance sensor (60) is formed in said sheet stacking plate (23, 24, 25) so that, when no sheet is stacked on said sheet stacking plate, the illumination light is passed through said through opening (23a, 24a, 25a) to indicate the fact that the sheet stacking height becomes smaller than the second predetermined height. A sheet stacking apparatus according to claim 7, wherein a plurality of sheet stacking plates (23, 24, 25) are provided along a vertical direction. A sheet stacking apparatus according to claim 11, wherein said plurality of sheet stacking plates (23, 24, 25) are integrally shiftable in the vertical direction. A sheet stacking apparatus according to claim 7, wherein said stapling device (400) is disposed at an upstream side of a discharge opening (50) to said sheet stacking plate (23, 24, 25) for stapling the sheets trapped as a sheet bundle and then discharging the stapled sheet bundle onto said sheet stacking plate. A sheet stacking apparatus according to claim 7, wherein, when a signal from a CPU (600) is inputted to said distance sensor (60), said distance sensor causes a burst wave generate circuit to emit a trigger signal to thereby illuminate a light emitting element, and outputs voltage information corresponding to a light receiving position of a light receiving element to said CPU (600), to thereby determine the distance from stored data on the basis of magnitude of the voltage information. An image forming apparatus comprising: an image forming portion; and a sheet stacking apparatus according to one of claims 1 to 14. A sheet stacking apparatus according to claim 11, wherein, when an electric power is turned ON, initialization for returning said sheet stacking plate (23, 24, 25) to a home position is effected, and, after the returning of said sheet stacking plate is completed, the following processes are performed; (a) the sheet stacking height is measured by said distance sensor (60), and the measured sheet stacking height data D is compared with sheet height reference data D1 corresponding to a second predetermined height; (b) when there is no sheet on said sheet stacking plate (23, 24, 25), the sheet stacking height data D becomes greater than the reference data D1 since a through hole (23a, 24a, 25a) is not closed by the sheet, and, in this case, it is judged that no sheet is stacked on said sheet stacking plate, and a first sheet is stacked at a present height of said sheet stacking plate; (c) when the reference data D1 is greater that the sheet stacking height data D, a sheet stacking plate shift amount data B is determined on the basis of a difference between the sheet stacking height data D and sheet height reference data D2 corresponding to a predetermined height, and it is judged whether the sheet stacking plate shift amount data B is zero, that is, the present sheet height is a maximum sheet stacking amount, and, if the present sheet height is the maximum sheet stacking amount, the sheet is stacked on a next sheet stacking plate; and (d) if the present sheet height is not the maximum sheet stacking amount, said sheet stacking tray is shifted by a distance corresponding to the sheet stacking plate shift amount data B, and, sheet stacking height data D is measured again and said sheet stacking plate is shifted to the predetermined height, and then, the sheet is stacked on a next sheet stacking plate.

Inventors: FURUKAWA HIDEAKI, NAKAZAWA NORIAKI, YONENUMA MASAHIRO, FURUKAWA, HIDEAKI, NAKAZAWA, NORIAKI, YONENUMA, MASAHIRO, Furukawa, Hideaki, c/o Canon Aptex K.K., Nakazawa, Noriaki, c/o Canon Aptex K.K., Yonenuma, Masahiro, c/o Canon Aptex K.K.
Applicants: CANON FINETECH INC, CANON FINETECH INC.
Application year: 1996
Main classification: B65H29
IPC codes: B65H29