Sheet stacking apparatus

A sheet stacking apparatus includes an alignment member to align a sheet stacked on a stacking tray in a width direction perpendicular to a direction in which the sheet is discharged. When stacking a plurality of sheets on the stacking tray, the sheet stacking apparatus lowers the stacking tray, so that a predetermined distance is maintained between the top surface sheet and a sheet discharge port. If the stacking tray is lowered to a predetermined position while the alignment unit is executing an alignment operation at a first position, the sheet stacking apparatus lowers the alignment member to a second position lower than the first position and causes the alignment member to execute the alignment operation at the second position.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present disclosure relates to a sheet stacking apparatus having a function of aligning discharged sheets.

2. Description of Related Art

Conventionally, a sheet post-processing apparatus (finisher) is discussed. After receiving a sheet from an image forming apparatus, the finisher discharges the sheet onto a discharge tray and causes alignment members arranged on the discharge tray to align the sheet in a direction perpendicular to the discharging direction. Since the alignment members arranged on the discharge tray align such discharged sheets, the sheets can be stacked neatly on the discharge tray. Such alignment processing is referred to as “neat stack.”

An image forming apparatus uses heat to fix a toner image formed on a sheet onto the sheet. Thus, because of the heat added for fixing, a sheet transferred from the image forming apparatus to the finisher may be curled on a discharge tray.

FIGS. 14A and 14Billustrate states of sheets P stacked on a stacking tray731. InFIG. 14A, the stack of sheets P on the discharge tray731is not curled. The discharge tray731is controlled to be positioned so that a sheet surface detection sensor732does not detect a sheet. Thus, the position of the top surface sheet of the stacked sheets P is maintained at approximately the height of the sheet surface detection sensor732.

In contrast, inFIG. 14B, the stack of sheets P on the discharge tray731is curled. Based on a signal from the sheet surface detection sensor732, the discharge tray731is lowered to a position where no sheet trailing edge blocks the discharge port. Thus, as illustrated inFIG. 14B, the position of the discharge tray731on which the curled sheets Pare stacked is lower than that of the discharge tray731on which the curled sheets P are not stacked. Therefore, depending on curling of the sheets P, even if an alignment member730moves in a width direction perpendicular to the sheet conveyance direction, the alignment member730does not come into contact with the side of the stacked sheets P. As a result, since the alignment member730executes a missed swing, an appropriate alignment operation may not be executed.

In response to this problem, U.S. Pat. No. 7,681,881 discusses arranging, other than the sheet surface detection sensor732, a detection mechanism for detecting the position of the top surface of the stacked sheets at a position where the alignment member730comes into contact with the sheets in a sheet discharging direction. Based on the position of the sheet detected by the detection mechanism, the position of the alignment member730is adjusted vertically. Thus, if strongly curled sheets are stacked, the alignment member703is controlled to be lowered further, compared with when no curled sheets are stacked. Thus, when an alignment operation is executed, the alignment member730is prevented from executing a missed swing.

However, U.S. Pat. No. 7,681,881 requires a dedicated detection mechanism for detecting the position of the sheet surface at the position where the alignment member730comes into contact with the sheets, requiring a more complex configuration and increasing the cost.

SUMMARY OF THE INVENTION

The present disclosure is directed to a sheet stacking apparatus that is capable of preventing alignment members from executing a missed swing even when the alignment members align a curled sheet stack, without a cost increase.

According to an aspect disclosed herein, a sheet stacking apparatus includes a discharging unit configured to discharge a conveyed sheet, a stacking tray on which a sheet discharged by the discharging unit is stacked, an alignment unit configured to align a sheet stacked on the stacking tray in a width direction perpendicular to a direction in which the sheet is discharged, the alignment unit including an alignment member configured to be movable in the width direction, and a first moving unit configured to move the alignment member in the width direction, the alignment member aligning the sheet by coming into contact with a side of the sheet stacked on the stacking tray in the width direction, a detection unit configured to detect a position of a top surface of a plurality of sheets stacked on the stacking tray, a second moving unit configured to vertically move the stacking tray while a plurality of sheets are being stacked on the stacking tray, so that a predetermined distance is maintained between a position detected by the detection unit and a discharge port of the discharging unit, a third moving unit configured to vertically move the alignment member, and a control unit configured to cause, if the second moving unit lowers the stacking tray to a predetermined position while the alignment unit is executing an alignment operation at a first position, the third moving unit to lower the alignment member to a second position lower than the first position and cause the alignment member to execute the alignment operation at the second position.

Further features and aspects of the present disclosure will become apparent from the following detailed description of exemplary embodiments with reference to the attached drawings.

DESCRIPTION OF THE EMBODIMENTS

Various exemplary embodiments, features, and aspects of the disclosure will be described in detail below with reference to the drawings.

FIG. 1is across section illustrating configurations of main sections of an image forming system according to an exemplary embodiment. The image forming system includes an image forming apparatus10and a finisher500used as a sheet stacking apparatus. The image forming apparatus10includes an image reader200reading an image from a document and a printer350forming the read image on a sheet.

A document feeding apparatus100sequentially feeds documents, which are set face-up on a document tray101, one by one from the first page. After conveying a document to a predetermined reading position on a platen glass102, the document feeding apparatus100discharges the document onto a discharge tray112. In a read operation, a scanner unit104is fixed at a predetermined reading position. When a document travels through the reading position, the scanner unit104reads a document image. More specifically, when a document travels through the reading position, the document is illuminated with light emitted from a lamp103of the scanner unit104. The reflected light from the document is guided to a lens108via mirrors105to107. Next, by using the light transmitted through the lens108, an image sensor109forms an image on an imaging plane, converts the image into image data, and outputs the image data as a video signal to an exposure unit110of the printer350.

The exposure unit110of the printer350modulates and outputs a laser beam, based on the video signal supplied from the image reader200. A polygonal mirror110ascans and illuminates a photosensitive drum111with the laser beam. Based on the scanning laser beam, an electrostatic latent image is formed on the photosensitive drum111. Next, a developing device113supplies developer, to make the electrostatic latent image on the photosensitive drum111visible as a developed image.

In the printer350, pick-up rollers127and128feed a sheet from upper and lower cassettes114and115to registration rollers126via feeding rollers129and130, respectively. When the leading edge of the sheet reaches the registration rollers126, the registration rollers126are driven at a predetermined timing. Consequently, the registration rollers126feed the sheet to the path between the photosensitive drum111and a transfer unit116. The transfer unit116transfers the developed image, which is formed on the photosensitive drum111, onto the fed sheet. After the developed image is transferred onto the sheet, the sheet is conveyed to a fixing unit117, which adds heat and pressure to the sheet to fix the developed image on the sheet. After travelling through the fixing unit117, the sheet is discharged from the printer350to the outside of the image forming apparatus10(to the finisher500) via a flapper (diverter)121and discharging rollers118. If the image forming apparatus10executes two-sided printing on the sheet, the sheet is conveyed to a two-sided conveyance path124via a reversal path122. As a result, the sheet is conveyed to the registration rollers126, again.

Next, a configuration of the finisher500will be described with reference toFIGS. 2A and 2B.FIGS. 2A and 2Billustrate a configuration of the finisher500inFIG. 1.FIG. 2Aillustrates the finisher500as viewed from the front side, andFIG. 2Billustrates stacking trays700and701of the finisher500as viewed from the sheet discharging direction.

The finisher500sequentially receives the sheets discharged from the image forming apparatus10and executes sheet post-processing. For example, the finisher500aligns and stacks a plurality of supplied sheets together or staples a trailing end of a sheet stack. In the finisher500, a conveyance roller pair511receives and conveys the sheets one by one discharged from the image forming apparatus10along the conveyance path520. A sheet conveyed to the inside of the finisher500by the conveyance roller pair511is further conveyed by conveyance roller pairs512to514. Conveyance sensors570to573, each of which detects passage of the sheet, are arranged along the conveyance path520. The conveyance roller pairs512and the conveyance path sensor571are included in a shift unit580. Driven by a shift motor M11, the shift unit580can move the sheet in the sheet width direction, which is perpendicular to the conveyance direction. While the conveyance roller pairs512are pinching a sheet, if the shift motor M11is driven, the sheet can be offset in the width direction while being conveyed. In a shift-sort mode, the sheet stack position is shifted in the width direction on a copy-by-copy basis. The sheet can be offset toward the front or rear side by 15 mm from the center position of the paper in the width direction (shift to the front side or shift to the rear side). If no shift amount is specified, the sheet is discharged to the front side (15 mm from the center position in the width direction). When the finisher500receives an input from the conveyance sensor571and detects that the sheet has traveled through the shift unit580, the finisher500drives the shift motor M11and returns the shift unit580to the center position.

A switching flapper540for guiding a sheet reversely conveyed by the conveyance roller pair514to a buffer path523is arranged between the conveyance roller pair513and the conveyance roller pair514. The switching flapper540is driven by a solenoid (not illustrated), and a buffer path roller pair519is arranged along the buffer path523. In addition, a switching flapper541for switching upper and lower discharge paths521and522is arranged between the conveyance roller pair514and an upper discharge roller pair515. If the switching flapper541is controlled so that the sheet travels along the upper discharge path521, the conveyance roller pair514driven by a conveyance motor M1guides the sheet to the upper discharge path521. Next, the upper discharge roller pair515driven by a discharge motor M2discharges the sheet onto the upper discharge tray701. An upper-tray discharge sensor574for detecting passage of the sheet is arranged along the upper discharge path521. If the switching flapper541is controlled so that the sheet travels along the lower discharge path522, the conveyance roller pair514driven by the conveyance motor M1guides the sheet to the lower discharge path522. Next, a first lower conveyance roller pair516, a second lower conveyance roller pair517, and a processing tray conveyance roller pair518driven by the conveyance motor M1guide the sheet to a processing tray630. First and second conveyance sensors575and576for detecting passage of the sheet are arranged along the lower discharge path522.

A stack discharge roller pair680driven by a stack discharge motor (not illustrated) discharges the sheet onto the processing tray630or the lower discharge tray700, depending on the post-processing mode. A lower tray discharge sensor577for detecting passage of the sheet is arranged on the processing tray630. In addition, a stapler unit601is arranged on the processing tray630. The stapler unit601staples a sheet stack aligned on the processing tray630.

In addition, alignment plates711aand711bare arranged on the stacking tray701, as illustrated inFIG. 2B. These alignment plates711aand711bare used as alignment members for aligning the sheets discharged on the stacking tray701in the sheet width direction. Hereinafter, the alignment plates711aand711bmay collectively be referred to as the alignment plate711, as needed. Likewise, alignment plates710aand710bare arranged on the stacking tray700, as illustrated inFIG. 2B. These alignment plates710aand710balign the sheets discharged onto the stacking tray700in the sheet width direction. Hereinafter, the alignment plates710aand710bmay collectively be referred to as the alignment plate710, as needed. Lower-tray alignment motors (front-side) M6and (rear-side) M7can move the alignment plates710aand710bin the sheet width direction, respectively. The alignment plates710aand710bare arranged on the front and rear sides, respectively. Upper-tray alignment motors (front side) M3and (rear side) M4can move the alignment plates711aand711bin the sheet width direction, respectively. The alignment plates711aand711bare arranged on the front and rear side, respectively. In addition, an upper-tray alignment plate elevating motor M5vertically drives the alignment plates710aand710b, and a lower-tray alignment plate elevating motor M8vertically drives the alignment plates711aand711b.

In addition, upper- and lower-tray elevating motors M9and M10can vertically drive the stacking trays700and701. Upper- and lower-tray sheet surface detection sensors720and721detect the trays700and701or the top surface of the sheets on the trays700and701, respectively. Based on a detection result obtained by the sheet surface detection sensor720/721, the finisher500drives and controls the tray elevating motor M9/M10so that a certain distance is always maintained between the tray700/701and the corresponding discharge port or between the top surface of the sheets on the tray700/701and the corresponding discharge port. In addition, the sheet surface detection sensors720and721detect whether any sheet is present on the stacking trays700and701.

Next, movement of the alignment plates will be described with reference toFIG. 3. The alignment plates711aand711barranged on the stacking tray701move in the same way as the alignment plates710aand710barranged on the stacking tray700. Thus, only the alignment plates711aand711barranged on the stacking tray701will hereinafter be described.

The alignment plate711ais formed by two members coupled by a rotating shaft714and is connected to an elevating shaft712. In addition, the elevating shaft712is connected to the upper-tray alignment plate elevating motor M5via a driving belt713, and the alignment plate711arotates around the elevating shaft712in the directions of the arrow illustrated inFIG. 3. The alignment plate711bis formed in the same way as the alignment plate711a. Namely, since connected to the elevating shaft712, the alignment plate711brotates in synchronization with the alignment plate711a. If the upper-tray alignment plate elevating motor M5is driven, since the alignment plates711aand711bvertically move, the position relationship between the stacking tray701and the pair of the alignment plates711aand711bchanges, as illustrated inFIG. 12. In addition, the alignment plates711aand711bare formed to extend toward the stacking tray701from a position above the discharge port near the upper discharge roller pair515used as a discharge means. In addition, the stacking tray701has a concave upper surface, so that the alignment plate711does not come into contact with the stacking tray701when moving in the width direction. In this way, even when only a single sheet is stacked on the stacking tray701, the alignment plate711can come into contact with the sides of the sheet without fail.

Next, a configuration of a controller comprehensively controlling the image forming system and a block diagram of the entire system will be described with reference toFIG. 4.FIG. 4is a block diagram illustrating a configuration of a controller comprehensively controlling the image forming system inFIG. 1.

As illustrated inFIG. 4, the controller includes a central processing unit (CPU) circuit unit150including a CPU153, a read-only memory (ROM)151, and a random access memory (RAM)152. The CPU153executes a basic control operation for the entire image forming system and is connected to the ROM151storing control programs and the RAM152executing processing via address and data buses. Based on the control programs stored in the ROM151, the CPU153comprehensively controls an image reader control unit201, an image signal control unit202, a document feeding apparatus control unit211, a printer control unit301, an operation display apparatus control unit401, and a finisher control unit501. The RAM152temporarily stores control data and is used as a work area for calculation processing executed for a control operation.

The document feeding apparatus control unit211drives and controls the document feeding apparatus100, based on instructions from the CPU circuit unit150. The image reader control unit201drives and controls the above scanner unit104, the image sensor109, and the like and transfers an analog image signal supplied from the image sensor109to the image signal control unit202. After converting the analog image signal supplied from the image sensor109into a digital signal, the image signal control unit202executes various processing on the digital signal, converts the processed digital signal into a video signal, and outputs the video signal to the printer control unit301. In addition, the image signal control unit202executes various processing on a digital image signal supplied from a computer210via an external interface (I/F)209, converts this processed digital image signal into a video signal, and outputs the video signal to the printer control unit301. The CPU circuit unit150controls processing executed by the image signal control unit202. Based on the supplied video signal, the printer control unit301controls the exposure unit110and the printer350, to execute image formation and sheet conveyance. The finisher control unit501is included in the finisher500. By exchanging information with the CPU circuit unit150, the finisher control unit501comprehensively drives and controls the finisher500. An operation of controlling the finisher500will be described in detail below. The operation display apparatus control unit401exchanges information between an operation display apparatus400and the CPU circuit unit150. For example, the operation display apparatus400includes: a plurality of keys for setting various functions relating to image formation; and a display unit for displaying information indicating setting statuses. When detecting a key operation, the operation display apparatus control unit401outputs a corresponding key signal to the CPU circuit unit150. When detecting a signal from the CPU circuit unit150, the operation display apparatus control unit401causes the operation display apparatus400to display corresponding information. The finisher control unit501may be included in the image forming apparatus10.

Next, a configuration of the finisher control unit501will be described with reference to a block diagram inFIG. 5. The finisher control unit501includes a CPU412, a RAM414, a ROM415, an input/output unit (I/O)411, and a serial communication interface (SCI)413, for example. The finisher control unit501communicates with the CPU circuit unit150, exchanges data such as commands, job information, and sheet transfer notifications, and executes various programs stored in the ROM415, to drive and control the finisher500. The RAM414temporarily stores control data and is used as a work area for calculation processing executed for a control operation. The SCI413executes serial communication with the CPU circuit unit150of the image forming apparatus10, to transfer operation instructions and control data. The I/O unit411transmits an on/off signal supplied from the CPU412to an output device such as a motor. In addition, the I/O unit411transmits a signal from an input device such as a sensor to the CPU412. The I/O unit411is connected to the conveyance motor M1, the discharge motor M2, the upper-tray alignment motor (front side) M3, the upper-tray alignment motor (rear side) M4, and the upper-tray alignment plate elevating motor M5. In addition, the I/O unit411is connected to the lower-tray alignment motor (front side) M6, the lower-tray alignment motor (rear side) M7, the lower-tray alignment plate elevating motor M8, the upper-tray elevating motor M9, the lower-tray elevating motor M10, and the shift motor M11. In addition, the I/O unit411is connected to the upper-tray sheet surface detection sensor721, the lower-tray sheet surface detection sensor720, the upper-tray discharge sensor574, and the lower-tray discharge sensor577.

In addition, the I/O unit411is connected to upper- and lower-tray driving encoders578and579, respectively. Each of the upper- and lower-tray driving encoders578and579outputs a pulse signal depending on the movement of the stacking trays701and700, respectively, which is lifted or lowered based on the detection of the top surface of the sheets on the stacking trays701and700. By counting the pulse signals output from the upper- and lower-tray driving encoders578and579, the CPU412can determine the movement amounts of the stacking trays701and700.

Next, an alignment plate control operation executed by the finisher control unit501will be described.FIG. 6is a flow chart illustrating an alignment control operation executed by the CPU412.

First, in step S601, the CPU412determines whether the CPU412has received a job start instruction from the CPU circuit unit150. Along with a job start instruction, for each sheet, sheet information including a sheet output destination and a sheet size is transmitted from the CPU circuit unit150to the CPU412. If the CPU412determines reception of a job start instruction from the CPU circuit unit150(YES in step S601), the operation proceeds to step S602. In step S602, the CPU412sets a flag in the RAM414, to start a program for controlling the alignment plates of a discharge destination stacking tray in the width direction. This width-direction control program will be described below.

Next, in step S603, the CPU412sets a flag in the RAM414, to start a program for controlling the alignment plates of the discharge destination stacking tray in the vertical direction. This vertical-direction control program will be described below.

Next, in step S604, the CPU412sets a flag, to start a program for controlling detection of the sheet surface on the discharge destination stacking tray. This sheet surface detection control program will be described below. If the above flags are set, the CPU412repeatedly executes the width- and vertical-direction control programs and the sheet surface detection control program in a time-division manner. Next, in step S605, the CPU412determines whether the CPU412has received a job end instruction from the CPU circuit unit150. If the CPU412determines reception of a job end instruction (YES in step S605), the operation proceeds to steps S606to S608in which the CPU412resets the above flags. Thus, the width- and vertical-direction control programs and the sheet surface detection control program are ended.

Next, the width-direction control program will be described with reference toFIG. 7. Since the alignment plate711is controlled in the same way as the alignment plate710, only an operation of controlling the alignment plate711will be described.FIG. 7is a flow chart illustrating an operation of controlling movement of the alignment plate711in the width direction, which is executed by the CPU412in a time-division manner.

First, in step S701, based on the sheet information about a sheet N (N is a natural number) transmitted from the CPU circuit unit150, the CPU412determines whether the sheet N needs to be shifted to the rear side. Assuming thatFIG. 2Aillustrates the front side of the finisher500, this rear side corresponds to the depth direction of the finisher500. In other words, this rear side corresponds to the left side of the finisher500inFIG. 2B. In addition to the shift direction, the sheet information includes information about the sheet size, material, surface property, and grammage. Before the finisher500receives a sheet from the image forming apparatus10, the CPU412is notified of the sheet information.

In step S701, if the CPU412determines that the sheet needs to be shifted to the rear side (YES in step S701), the operation proceeds to step S702. If not (NO in step S701), the operation proceeds to step S703. In step S702, by driving the upper-tray alignment motors M3and M4, the CPU412moves the alignment plate711to a rear-side shift position. In step S703, by driving the upper-tray alignment motors M3and M4, the CPU412moves the upper-tray alignment plate711to a front-side shift position. Namely, the upper-tray alignment motors M3and M4function as a third moving unit. After the CPU412moves the alignment plate711in step S702or S703, the operation proceeds to step S704.

In step S704, the CPU412determines whether a sheet has been discharged onto the stacking tray701. More specifically, if a predetermined time period elapses after the upper-tray discharge sensor574detects passage of the trailing edge of the sheet N, the CPU412determines that the sheet N has been discharged. If the CPU412determines discharge of the sheet N (YES in step S704), the operation proceeds to step S705.

In step S705, the CPU412moves the alignment plate711in the width direction to align the sheet N. This alignment control operation will be described in detail below.

Next, in step S706, the CPU412determines whether the job has been completed. The CPU412determines that the job has been completed, if the flag is reset in step S606inFIG. 6. If the CPU412determines that the flag has been reset (YES in step in S706), the CPU412ends the operation. If not (NO in step in S706), the operation proceeds to step S707.

In step S707, the CPU412determines whether the discharged sheet N is the last sheet of a copy set. More specifically, the CPU412determines whether the discharged sheet N is the last sheet of a copy set of a plurality of copy sets or the last sheet of a copy set of a single print job. Namely, after the last sheet of a copy set, the sheet stacking position on the tray is changed. For example, if the sheet N is set to be stacked on the rear side and the next sheet N+1 is set to be stacked on the front side, the sheet stacking position is changed between the sheet N and the sheet N+1. Thus, the CPU412determines that the sheet N is the last sheet of a copy set. There are cases where booklets as print products are alternately stacked on the front and rear sides on a stacking tray and where booklets are alternately stacked every ten booklets. The CPU circuit unit150notifies the CPU412of the timing of switching the stacking position.

In step S707, if the CPU412does not determine that the discharged sheet N is the last sheet of a copy set (NO in step S707), the operation returns to step S704, and the CPU412waits for a sheet to be discharged. In step S707, if the CPU412determines that the discharged sheet N is the last sheet of a copy set (YES in step S707), the operation proceeds to step S708.

In step S708, to move the alignment plate711in the width direction, the CPU412waits until the alignment plate711is completely retracted by a vertical direction control operation inFIG. 10. More specifically, the CPU412waits until the alignment plate711is completely moved to a retracted position in step S909inFIG. 10. If the alignment plate711is completely retracted (YES in step S708), the operation proceeds to step S709.

In step S709, the CPU412determines whether the next sheet needs to be shifted to the rear side. If so (YES in step S709), the operation returns to step S702. If not (NO in step S709), the operation returns to step S703.

Next, the operation of controlling the alignment plate711in the width direction will be described in detail with reference toFIGS. 8A,8B,8C,8D,8E, and8F andFIGS. 9A,9B,9C,9D,9E, and9F.

FIG. 8Aillustrates an initial state of the stacking tray701and the alignment plate711. A job is started in this state. The dashed line inFIGS. 8A to 8Eindicates the center position of the stacking tray701in the width direction. This operation will be described, assuming that, at the start of the job, the CPU circuit unit150notifies the CPU412that the sheet needs to be shifted to the rear side. In this case, the operation proceeds from step S701to step S702in the flowchart inFIG. 7, and the alignment plates711aand711bare positioned as illustrated inFIG. 8B. Namely, while the alignment plate711bis not moved, the alignment plate711ais moved toward the center position. Next, as illustrated inFIG. 8C, a sheet is discharged onto the stacking tray701. Next, as illustrated inFIG. 8D, the alignment plate711bis moved by the upper-tray alignment motor M4in the direction indicated by the arrows inFIG. 8D. Accordingly, the alignment plate711acomes into contact with the sheet, and the sheet is then aligned. Next, as illustrated inFIG. 8E, the alignment plate711bis moved in the direction of the arrow inFIG. 8E. This state is maintained until the next sheet is discharged. Each time a sheet is stacked on the tray701, the alignment plate711bis repeatedly moved as illustrated inFIGS. 8D and 8E. As a result, the sheets are stacked as illustrated inFIG. 8F. As the number of the sheets stacked on the stacking tray701is increased, the stacking tray701is lowered, and the position of the top sheet surface is maintained at a certain height.

Next, as illustrated by the arrow inFIG. 9A, the alignment plate711is first lifted above the sheet stack and is next moved to the front side (to the right side inFIG. 9A) to change the alignment position. Next, the alignment plate711is lowered to an alignment standby position as illustrated inFIG. 9B. In this state, if a sheet is discharged onto the stacking tray701, the alignment plate711ais moved toward the center position, as illustrated inFIG. 9C. Accordingly, the alignment plate711bcomes into contact with the sheet, and the sheet is then aligned, as illustrated inFIG. 9D. Next, as illustrated inFIG. 9E, the alignment plate711areturns to the alignment standby position and waits until the next sheet is stacked. Each time a sheet is stacked on the tray701, the alignment plate711ais repeatedly moved as illustrated inFIGS. 9C to 9E. As a result, the sheets are stacked as illustrated inFIG. 9F.

Next, the operation of controlling the alignment plate711in the vertical direction will be described. Since the alignment plate711is controlled in the same way as the alignment plate710, only the operation of controlling the alignment plate711will be described.

FIG. 10is a flow chart illustrating an operation of controlling movement of the alignment plate711in the vertical direction, executed by the CPU412in a time-division manner. Prior to this control operation, the alignment plate711is maintained at an initial position as illustrated inFIG. 12A. At this initial position, the alignment plate711is positioned above the stacking tray701.

First, in step S901, the CPU412drives the upper-tray alignment plate elevating motor M5to move the alignment plate711to a first position as illustrated inFIG. 12B.

Next, in step S902, the CPU412waits for a sheet to be discharged. If the CPU412determines that a sheet has been discharged onto the stacking tray701(YES in step S902), the operation proceeds to step S903. In step S903, the CPU412checks the position of the stacking tray701. A procedure for checking the position of the stacking tray701will be described in detail below. As illustrated inFIG. 12C, the stacking tray701is lowered, as the number of the stacked sheets is increased. Next, in step S904, the CPU412determines whether the stacking tray701has been lowered to a predetermined position. If so (YES in step S904), the operation proceeds to step S905. In step S905, the CPU412drives the upper-tray alignment plate elevating motor M5to move the alignment plate711to a second position. Namely, the upper-tray alignment plate elevating motor M5functions as a second moving unit. More specifically, in step S904, the CPU412determines whether the stacking tray701is approximately 10 mm below an adjusted position. This adjusted position is set when a sheet surface detection control operation is executed on the upper discharge tray701having no sheet. In addition, as illustrated inFIG. 12D, the alignment plate711at the second position is lower in the moving direction of the stacking tray701than that at the first position illustrated inFIGS. 12B and 12C.

Next, in step S906, the CPU412determines whether the job has been completed. If so (YES in step S906), the operation proceeds to step S907. If not (NO in step S906), the operation proceeds to step S908.

In step S907, the CPU412drives the upper-tray alignment plate elevating motor M5to move the alignment plate711to the initial position as illustrated inFIG. 12G.

In step S908, the CPU412determines whether the discharged sheet is the last sheet of a copy set. If so (YES in step S908), the operation proceeds to step S909. If not (NO in step S908), the operation returns to step S902and waits for the next sheet to be discharged.

In step S909, to change the sheet alignment direction, the CPU412retracts the alignment plate711to the retracted position where the alignment plate711does not come into contact with the stacked sheets. The retracted position is the same as the above initial position. Namely, the alignment plate711is retracted as illustrated inFIG. 12G.

Next, in step S910, the CPU412determines whether the alignment plate711has completely been moved in the width direction. More specifically, the CPU412determines whether movement of the alignment plate711in step S702or S703in the flow chart inFIG. 7has been completed. If the movement of the alignment plate711in the width direction is completed (YES in step S910), the operation returns to step S901.

As described above, when the alignment plate control operation in the vertical direction is started, the CPU412moves the alignment plate711to the first position. When the position of the discharge tray701is lowered by a predetermined amount by the stacked sheets, the CPU412moves the alignment plate711to the second position. Thus, even if curled sheets are stacked, the alignment plate711does not execute a missed swing. For example, as illustrated inFIG. 12E, if the alignment plate711attempts to align the sheets at the first position, the alignment plate711does not come into contact with the sides of the sheets, possibly resulting in a missed swing. However, as illustrated inFIG. 12F, as the stacking tray701is lowered to the predetermined position, since the alignment plate711is moved to the second position, the alignment plate711comes into contact with the sides of the sheets. Thus, the alignment plate711does not execute a missed swing. When stacked sheets are not curled, even if the alignment plate711is moved to the second position, the alignment plate711simply comes into contact with the sides of the sheets at a different position. Thus, the alignment operation is not affected.

In the above description, after the last sheet of a copy set is stacked on the stacking tray701and the CPU412moves the alignment plate711to the retracted position, the CPU412first moves the alignment plate711to the first position, instead of immediately moving the alignment plate711to the second position. The reason for this operation will be hereinafter described.

FIGS. 13A and 13Billustrate the alignment plate711at the first and second positions, respectively. As illustrated inFIGS. 13A and 13B, when sheet stacks shifted to the front side and sheet stacks shifted to the rear side are stacked on each other, if the shift direction is changed and the alignment plate711is moved to the second position, the alignment plate711may press and damage a stacked sheet stack. Thus, in the present exemplary embodiment, if the shift direction is changed and if the position of the alignment plate711is changed in the width direction, the alignment plate711is returned to the first position. A one-way clutch may be arranged at the elevating shaft712of the alignment plate711(711aand711b). In this way, when the alignment plate711rotates in the direction in which the sheets are pressed, the sheets are not pressed more than necessary. If such configuration is used, the operation may proceed from S910to S905in the flow chart inFIG. 10.

Next, an operation of controlling detection of the sheet surface on the stacking tray will be described. Since the operation of controlling detection of the sheet surface on the stacking tray701is the same as that on the stacking tray700, only the operation of controlling detection of the sheet surface on the stacking tray701will be described.

FIG. 11is a flow chart illustrating an operation of controlling detection of the sheet surface on the stacking tray701, which is executed by the CPU412in a time-division manner.

First, in step S1001, the CPU412determines whether the upper-tray sheet surface detection sensor721is turned on. The sheet surface detection sensor721turns on when sheets are stacked on the stacking tray701up to a position near the discharge roller515. If the sheet surface detection sensor721is not turned on (NO in step S1001), the operation proceeds to step S1007. In step S1007, the CPU412determines whether the job has been completed. If not (NO in step S1007), the operation returns to step S1001. If so (YES in step S1007), the CPU412ends the operation.

If the CPU412determines that the sheet surface detection sensor721is turned on (YES in step S1001), the operation proceeds to step S1002. In step S1002, the CPU412drives the upper-tray elevating motor M9to start lowering the stacking tray701. Namely, the upper-tray elevating motor M9functions as a first moving unit. When the CPU412drives the upper-tray elevating motor M9, the encoder578attached to the rotation shaft of the motor M9outputs a pulse signal. The pulse signal is supplied to the CPU412via the I/O unit411. The CPU412counts the pulse signals to detect the movement amount of the stacking tray701.

Next, in step S1003, the CPU412waits until the sheet surface detection sensor721is turned off. The sheet surface detection sensor721is arranged below the sheet discharge port. If the CPU412detects that the sheet surface detection sensor721is turned off (YES in step S1003), the operation proceeds to step S1004. In step S1004, the CPU412stops the upper-tray elevating motor M9. Next, in step S1005, the CPU412detects the position of the stacking tray701, based on the counted pulses.

Next, in step S1006, the CPU412determines whether the job has been completed. If the flag is reset in step S608in the flow chart inFIG. 6, the CPU412determines that the job has been completed. If the job is completed (YES in step S1006), the CPU412ends the operation. If not (NO in step S1006), the operation returns to step S1001.

In the present exemplary embodiment, the CPU412counts the pulse signals from the encoder578to detect the position of the stacking tray701. However, the CPU412may count the stacked sheets to detect the position of the stacking tray701.

As described above, in the present exemplary embodiment, when sheets are stacked on a stacking tray, the position of the alignment plates is controlled in the vertical direction depending on the position of the stacking tray. More specifically, when stacking is started, the CPU412moves the alignment plates to the first position, where the alignment plates do not come into contact with the stacking tray, and causes the alignment plates to align the sheets. Subsequently, when sheets are stacked and the stacking tray is lowered, the CPU412moves the alignment plates to the second position and causes the alignment plates to align the sheets. In this way, curled sheets can be aligned, without requiring a dedicated sensor for detecting a sheet surface position at a position where the alignment plates come into contact with the sheets.

This application claims priority from Japanese Patent Application No. 2011-171996 filed Aug. 5, 2011, which is hereby incorporated by reference herein in its entirety.