JP2014162599A - Paper transport device - Google Patents

Abstract

A sheet conveying device capable of detecting a bend or a crease occurring in the width direction of a sheet is provided.
In a sheet conveying device (20) including a sheet feeding unit and an image forming unit, at least two or more sheet height detection sensors for detecting the height of a sheet are provided with a pair of registration rollers. A bend / break determination unit which is arranged in parallel along the paper width direction in the vicinity of the central portion in the paper width direction that is orthogonal to the paper conveyance direction on the upstream side of 25 and at least one other than the vicinity of the central portion. 74, the maximum absolute difference value Dmax among the absolute difference values Dn obtained from the output values A, B, C of at least two or more of the paper height detection sensors 22A, 22B, 22C, and for bending / breaking determination. Is compared with the threshold value S of the sheet P to determine whether the sheet P is bent in the width direction T or bent O.
[Selection] Figure 5

Description

  The present invention relates to a sheet conveying apparatus capable of detecting bending or folding generated in a sheet width direction orthogonal to a sheet conveying direction while conveying a sheet along a sheet conveying path from a sheet feeding unit toward an image forming unit. .

  In general, a paper transport device that transports paper is applied to a printing device that prints images and characters on paper, a copying machine that copies images and characters on paper, and the like.

  This type of paper transport device includes a paper feed unit that feeds paper, an image forming unit that forms an image on the paper by printing or copying, and a paper transport formed between the paper feed unit and the image forming unit. And a control unit that controls the entire apparatus.

  In the above-described sheet conveying apparatus, a registration roller pair (also referred to as a timing roller pair) is located upstream of the image forming unit in the sheet conveying direction in the sheet conveying path formed between the sheet feeding unit and the image forming unit. is set up.

  While the paper fed by the paper feeding unit is being transported toward the registration roller pair, the leading end in the paper transport direction is abutted against the resist roller pair that is stopped rotating, and a loop ( By forming a slack), the skewed paper is corrected by the registration roller pair during paper feeding or paper conveyance, and then the skew-corrected paper is rotated in the image forming unit by rotating the registration roller pair. It is transported towards.

  As described above, when the sheet is conveyed from the sheet feeding unit to the image forming unit, the transmitted light amount measuring device, the medium identification device, and the medium conveyance that detect the type and double feed of the sheet upstream from the registration roller pair. Apparatus and image forming apparatus (for example, refer to Patent Document 1), an image forming apparatus (for example, refer to Patent Document 2) for detecting paper breakage upstream of the registration roller pair, and a pair of registration rollers There is an image forming apparatus (for example, see Patent Document 3) that detects an inclination angle due to a loop (sag) of a sheet.

JP 2009-137760 A JP 2007-331874 A JP 2010-269893 A

  By the way, when a sheet is conveyed from a sheet feeding unit to an image forming unit in a general sheet conveying apparatus, the sheet is abutted against a pair of registration rollers whose rotation is stopped and the skew of the sheet is corrected. The pair is rotated and the pair of registration rollers conveys the paper to the image forming unit. In particular, paper such as thin paper that is thin and weak, or plain paper that is thicker than thin paper and thinner than thick paper is used. In such a case, the rotating registration roller pair may bend or bend near the center in the paper width direction perpendicular to the paper (thin paper, plain paper) conveyance direction.

  The reason for this will be described. From the roller portions corresponding to both ends in the width direction of the sheet of the registration roller pair due to the twist of the rotation shaft of the registration roller pair and the variation in the roller shape of the registration roller pair. Each force toward the central portion in the width direction of the paper may be applied to both ends in the width direction of the paper, and as a result, the central portion in the width direction of the paper P as shown in FIGS. A bend T is generated in the vicinity, and this bend T may be formed across the registration roller pair across the front and rear in the sheet conveyance direction while having a substantially mountain-shaped bulge away from the upper surface of the lower sheet guide member.

  At this time, the deflection T generated in the vicinity of the central portion in the width direction of the paper P is such that the left and right inclined surfaces Ta and Tb inclined in a substantially mountain shape are directed toward the left and right ends Pa and Pb in the width direction of the paper P. Hereinafter, the bending T having such a shape will be referred to as “the bending T generated in the width direction of the paper P”.

  Further, as shown in FIGS. 9A and 9B, the deflection T generated in the vicinity of the central portion in the width direction of the paper P is crushed by the force niped by the registration roller pair, and the upper surface of the lower paper guide member. By overlapping the upper side, a fold O is generated in the vicinity of the central portion in the width direction of the paper P, and this fold O may also be formed across the registration roller pair in the front and rear directions in the paper transport direction.

  At this time, the fold O generated in the vicinity of the central portion in the width direction of the paper P is such that the left and right folds Oa and Ob bent substantially linearly are closer to the left and right ends Pa and Pb in the width direction than the central portion of the paper P. The fold O that is located and substantially parallel to the left and right ends Pa and Pb and having such a shape will be hereinafter referred to as “fold O generated in the width direction of the paper P”.

  In the case of paper P such as thin paper or plain paper that is relatively weak, there may be a fold O as described above from the beginning.

  If a bend T or a crease O occurs in the vicinity of the central portion in the width direction of the paper P, if an image is formed on the paper P by printing or copying in the image forming portion in this state, paper wrinkles occur. Image quality deteriorates and paper jam occurs.

  In addition, for example, when an ink jet system is employed as the image forming unit, there arises a problem that a portion where the paper sheet P is bent in the width direction T or folds O collides with the ink jet head and damages the ink jet head.

  On the other hand, in the transmitted light amount measuring device, the medium identifying device, the medium conveying device, and the image forming apparatus disclosed in Patent Document 1 (Japanese Patent Laid-Open No. 2009-137760), the paper conveying path on the upstream side of the registration roller pair. A plurality of transmitted light amount measuring means that measure the transmitted light amount in the thickness direction of the paper are provided, and the type and double feed of the paper are determined based on the detection result of each transmitted light amount measuring means. Since there is no means for determining whether or not a bend or fold has occurred near the center in the direction, the bend or fold that has occurred near the center in the width direction of the sheet as shown in FIG. It cannot be detected.

  Further, in the image forming apparatus disclosed in Patent Document 2 (Japanese Patent Application Laid-Open No. 2007-331874), along each sheet conveyance path upstream of the registration roller pair and corresponding to the right end and the left end of the sheet. A paper detection unit that includes a light emitting unit and a light receiving unit can be provided to detect paper breakage based on the amount of light input to each light receiving unit. Since it can only be detected, it is not possible to detect the bending or crease that occurs in the vicinity of the center in the width direction of the paper.

  Further, in the image forming apparatus disclosed in Patent Document 3 (Japanese Patent Application Laid-Open No. 2010-269893), an ultrasonic sensor in which a transmitting unit and a receiving unit are paired is provided on the upstream side of the registration roller pair. By detecting the inclination angle of the loop (slack) of the paper that has struck the roller pair, the paper can be deflected appropriately. However, since only one set of ultrasonic sensors is provided, the above paper is used. It is not possible to detect bending or bending that occurs in the vicinity of the central portion in the width direction.

  Accordingly, a paper transport device is provided that can detect a bend or a crease that occurs in the paper width direction orthogonal to the paper transport direction while transporting the paper along the paper transport path from the paper feed unit to the image forming unit. For the purpose.

The present invention has been made in view of the above problems, and the invention according to claim 1 includes a paper feeding unit that feeds paper, an image forming unit that forms an image on the fed paper, In a paper transport device provided with
A pair of registration rollers that rotate after correcting skew by correcting the skew of the sheet by forming a loop by abutting the leading end in the transport direction of the sheet,
One in the vicinity of the central portion in the paper width direction orthogonal to the paper conveyance direction on the upstream side of the pair of registration rollers, and at least one other than the vicinity of the central portion are juxtaposed along the paper width direction, At least two paper height detection sensors for detecting the height of the paper;
The width of the sheet is compared by comparing the maximum difference value among the difference values respectively obtained from the output values of at least two of the sheet height detection sensors with a preset threshold value for determination of bending / breaking. Bending / breaking determination means for determining the bending or bending of the direction;
A sheet transporting apparatus comprising:

  The invention according to claim 2 further includes a nip pressure adjusting means for adjusting a nip pressure of the pair of registration rollers, and the bending / folding determination means causes the sheet to bend or bend in the width direction. 2. The sheet conveying device according to claim 1, wherein the nip pressure adjusting unit adjusts the nip pressure of the registration roller pair to a value lower than an initial set value that is initially set. .

  According to the sheet conveying apparatus of the first aspect, in the sheet conveying apparatus including the sheet feeding unit and the image forming unit, at least two or more sheet height detection sensors for detecting the sheet height are provided as a pair of registration rollers. One side of the paper width direction perpendicular to the paper conveyance direction on the upstream side, and at least one other than the vicinity of the central portion are juxtaposed along the paper width direction, By comparing the maximum absolute difference value among the absolute difference values obtained from the respective output values of at least two paper height detection sensors with a preset threshold value for determination of bending / breaking, the width of the paper Since it is determined that the bending or folding occurred in the direction, the paper conveyance can be stopped immediately when it is determined that bending or folding occurs in the width direction of the paper. Can be suppressed Together we can form a good image of the image quality on a paper fold deflection or the width direction of the sheet does not occur.

  Furthermore, when an ink jet method is used as an example of the image forming unit installed in the paper conveyance device according to the present invention, at least two paper height detection sensors are provided on the upstream side of the pair of registration rollers. In order to be able to stop the paper immediately when the bending or folding that occurs in the width direction of the paper is detected by at least two or more paper height detection sensors, the paper can be stopped immediately. There is no fear of damaging one or more ink jet heads provided in the image forming unit, which can contribute to improving the reliability of the paper transport device.

  According to a second aspect of the present invention, the nip pressure adjusting means for adjusting the nip pressure of the registration roller pair is further provided, and the bending / folding determination means causes the sheet to bend or bend in the width direction. The nip pressure adjusting means adjusts the nip pressure of the pair of registration rollers to a value lower than the initial set value that is initially set, so that the user can bend or break the paper in the width direction. After removing the generated paper, the paper after feeding again does not bend or bend in the width direction of the paper, so that a reliable paper transport device can be provided.

(A), (b) is the top view and longitudinal cross-sectional view which showed the whole structure of the image forming apparatus to which the paper conveying apparatus based on this invention was applied. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an enlarged longitudinal sectional view illustrating a paper transport device according to a first embodiment of the present invention. (A), (b) is a longitudinal sectional view schematically showing a state in which a plurality of paper height detection sensors shown in FIGS. 1 and 2 are used to detect bending and crease occurring in the paper width direction. It is. FIG. 4 is a diagram showing detection results from a plurality of paper height detection sensors shown in FIGS. FIG. 5 is a first operation flow diagram for explaining an operation of detecting bending or folding generated in the width direction of a sheet by using a plurality of sheet height detection sensors in the sheet conveying apparatus according to the first exemplary embodiment. It is the longitudinal cross-sectional view which expanded and showed the paper conveying apparatus of Example 2 which concerns on this invention. FIG. 10 is a second operation flow diagram for explaining an operation of detecting a flexure / fold generated in the width direction of a sheet using a plurality of sheet height detection sensors in the sheet transport apparatus according to the second embodiment. (A), (b) is the perspective view and MM sectional drawing for demonstrating the bending produced in the center part vicinity of the width direction of a sheet | seat in a general paper conveying apparatus. (A), (b) is a perspective view and NN longitudinal cross-sectional view for demonstrating the folding produced in the vicinity of the center part of the paper width direction in a general paper conveying apparatus.

  Hereinafter, embodiments of a paper conveying device according to the present invention will be described in detail in the order of Example 1 and Example 2 with reference to FIGS.

  The paper transport device according to the present invention is applied to a printing device that prints images and characters on paper, a copying machine that copies images and characters on paper, and the like.

  The above-described paper transport device includes a paper feed unit that is installed upstream of the paper transport direction and feeds paper, and an image forming unit that is installed downstream of the paper feed unit and forms an image on the paper. The apparatus includes at least a sheet conveying unit that conveys a sheet between the sheet feeding unit and the image forming unit, and a control unit that incorporates a bending / breaking determination unit.

  Then, a registration roller pair is installed on the upstream side of the image forming unit in the paper conveyance path from the paper feeding unit to the image forming unit, and at least two or more paper height detection sensors are further upstream of the registration roller pair. A sheet that is orthogonal to the sheet conveyance direction is arranged side by side along the sheet width direction in the vicinity of the center in the sheet width direction perpendicular to the sheet conveyance direction and at least one other than the vicinity of the center. The present invention is characterized in that it can be detected by a bending / breaking determination unit provided in the control unit.

  In the following embodiments, an example of an ink jet printing apparatus using an ink jet system will be described as an example of an image forming apparatus to which the paper conveying apparatus according to the present invention is applied. However, as another example of the image forming apparatus, stencil printing is described. The present invention can also be applied to apparatuses, laser printing apparatuses, thermal transfer printing apparatuses, and copying machines.

  FIGS. 1A and 1B show the overall configuration of an image forming apparatus to which a paper conveying apparatus according to the present invention is applied. FIG. 2 is an enlarged view of the sheet conveying apparatus according to the first embodiment of the present invention.

  As shown in FIGS. 1A and 1B, the image forming apparatus 1 to which the paper conveying apparatus (20) according to the first embodiment of the present invention is applied is configured as an ink jet printing apparatus using an ink jet system.

  The image forming apparatus 1 described above includes a paper feed unit 10 that feeds the paper P, and a first paper transport unit (main book) that transports the fed paper P toward the belt platen unit 30 and the image forming unit 40 described below. (Corresponding to the sheet conveying apparatus of Example 1 according to the invention) 20 and the sheet P conveyed by the first sheet conveying unit 20 are directed to the downstream side of the image forming unit 40 and the image forming unit 40 described below. A belt platen unit 30 that conveys, an image forming unit 40 that is installed facing the upper side of the belt platen unit 30 to form an image on the paper P, and the paper P on which the image has been formed is directed to a paper discharge unit 60 described below A second paper transport unit 50 that transports the image forming paper P, a paper discharge unit 60 that discharges the image-formed paper P, and a control unit 70 that controls the image forming apparatus 1 as a whole.

  Here, each part in the image forming apparatus 1 will be described in order.

  First, in the paper supply unit 10, a plurality of sheets P of the same size are stacked on the paper supply table 11. A paper feed roller 12 is rotatably provided above the paper feed tray 11, and this paper feed roller 12 contacts the uppermost paper P among the paper P stacked on the paper feed tray 11. Thus, the uppermost sheet P is fed toward the downstream side.

  Further, the uppermost sheet P fed by the sheet feeding roller 12 is separated one by one while being nipped and conveyed between the sheeting roller 13 and the sheeting plate 14 provided on the downstream side of the sheet feeding roller 12. Double feed is prevented, and thereafter, the uppermost sheet P is conveyed toward the downstream side.

  At this time, the winding roller 13 is fixed to the rotary shaft 15, and the paper feed roller 12 is pivotally supported between a pair of arms 16, 16 that are swingably supported by the rotary shaft 15 with the rolling roller 13 interposed therebetween. Has been. The paper feed roller 12 and the rolling roller 13 are rotated in the counterclockwise direction by transmitting the rotation of the first pulse motor 17 serving as the same drive source via the speed reduction mechanism 18, so that the paper P is a predetermined amount. It is transported at a transport speed.

  Next, the first paper transport unit 20 is provided between the paper feeding unit 10, the belt platen unit 30, and the image forming unit 40. In the first sheet conveyance path for conveying the sheet P between the sheet feeding unit 10 and the belt platen unit 30 and the image forming unit 40, a plate-shaped first sheet that guides the lower surface of the sheet P along the conveyance direction. 1 A lower paper guide member 21 is fixedly installed. The first lower sheet guide member 21 has through holes formed at locations corresponding to a plurality of sheet height sensors 22A to 22C described below and a registration roller pair 25 described later.

  Further, on the upstream side of the first lower paper guide member 21, a plurality of paper height detection sensors 22A to 22C are arranged in a substantially straight line along the paper width direction orthogonal to the paper transport direction, in other words. Thus, the plurality of sheet height detection sensors 22A to 22C are installed further upstream than a registration roller pair 25 described later installed upstream from the belt platen unit 30 and the image forming unit 40.

  The plurality of paper height detection sensors 22A to 22C described above are members constituting a part of the main part of the present invention, and the vicinity of the central portion in the width direction of the paper P described above with reference to FIG. 8 or FIG. It is installed in order to detect the bending T or the fold O generated.

  At this time, it is essential that one paper height detection sensor 22B among the plurality of paper height detection sensors 22A to 22C is installed at a position corresponding to the vicinity of the central portion in the width direction of the paper P. At least one or more other sheet height detection sensors 22A and 22C other than the one sheet height detection sensor 22B are arranged in parallel along the sheet width direction with a predetermined interval from one sheet height detection sensor 22B. It is installed.

  In this embodiment, for example, using three sheet height detection sensors 22A to 22C, the sheet height detection sensor 22B is installed near the center in the width direction of the sheet P, and the sheet height detection sensor 22A, 22C is arranged substantially symmetrically with a predetermined interval left and right in the sheet width direction with respect to the sheet height detection sensor 22B.

  In this embodiment, as the plurality of paper height detection sensors 22A to 22C, for example, a transmissive optical sensor in which a light emitting element 22a and a light receiving element 22b are combined is used, and the light emitting element 22a is connected to the first lower sheet. The guide member 21 is installed above the through hole formed in the first lower sheet guide member 21 on the upper surface 21a side, and the light receiving element 22b is opposed to the light emitting element 22a and the first lower sheet guide member 21 On the lower surface 21 b side, it is installed below the through hole formed in the first lower paper guide member 21.

  Then, as will be described later, the plurality of paper height detection sensors 22A to 22C make the first reference with respect to the upper surface 21a of the first lower paper guide member 21, in other words, with the lower surface of the paper P as the height reference. The height on the upper surface side of the paper P conveyed on the lower paper guide member 21 is detected without contact.

  At this time, the plurality of paper height detection sensors 22A to 22C may be sensors of any form as long as they can detect the height on the upper surface side of the relatively weak paper P such as thin paper or plain paper in a non-contact manner. It is also possible to use an ultrasonic sensor that transmits and receives ultrasonic waves and an image sensor that reads and reflects light from a light emitting unit.

  Furthermore, each light receiving element 22b side of the plurality of paper height detection sensors 22A to 22C is connected to the signal amplifier 23, the AD converter 24, and the control unit 70 in the above order. The outputs from the light receiving elements 22b of the plurality of paper height detection sensors 22A to 22C are input to the signal amplifier 23 and amplified, and then converted into analog signals by the AD converter 24 to be described later. This is analyzed by a bending / breaking determination unit 74 provided in the control unit 70.

  The operation of analyzing the outputs of the plurality of paper height detection sensors 22A to 22C by the bending / breaking determination unit 74 provided in the control unit 70 will be described in detail later.

  A pair of registration rollers 25 for correcting the skew of the paper P is installed along the first lower paper guide member 21 and downstream of the plurality of paper height detection sensors 22A to 22C. In other words, the registration roller pair 25 is installed upstream of the belt platen unit 30 and the image forming unit 40.

  In the registration roller pair 25, the driving roller 25a that rotates counterclockwise by transmitting the rotation of the second pulse motor 26 via a speed reduction mechanism (not shown) is provided on the upper surface 21a side of the first lower paper guide member 21. The driven roller 25b that is installed above the through hole formed in the first lower paper guide member 21 and rotates in the clockwise direction following the drive roller 25a is a lower surface 21b of the first lower paper guide member 21. On the side, a through hole formed in the first lower sheet guide member 21 is installed facing the lower side, and the driving roller 25a and the driven roller 25b make a pair.

  At this time, the driving roller 25a and the driven roller 25b of the registration roller pair 25 are spaced apart from each other along upper and lower rotary shafts 25a1, (25b1... FIG. 2) in which a plurality of rubber rollers are formed to be long and perpendicular to the paper surface. The paper P is divided and fixed in the width direction.

  Here, the registration roller pair rotation driving means for rotating the registration roller pair 25 will be described with reference to FIG. 2 showing the paper conveying apparatus according to the first embodiment of the present invention in an enlarged manner.

  As shown in an enlarged view in FIG. 2, a plurality of paper heights for detecting a deflection T {FIG. 3 (a), FIG. 8} or fold O {FIG. 3 (b), FIG. A registration roller pair 25 in which a driving roller 25a and a driven roller 25b are paired is rotatably provided downstream of the detection sensors 22A to 22C.

  A second pulse motor 26 serving as a rotational drive source for the registration roller pair 25 is connected to the end of the rotation shaft 25a1 of the drive roller 25a via a speed reduction mechanism (not shown), and the second pulse motor 26 is driven. The driving roller 25a is rotated counterclockwise by the force.

  An intermediate pulley 27A is rotatably provided on the upper surface 21a side of the first lower sheet guide member 21 on the right side of the end of the rotation shaft 25a1 of the drive roller 25a.

  Further, on the right side of a gear portion (not shown) formed on a part of the end portion of the rotation shaft 25b1 of the driven roller 25b on the lower surface 21b side of the first lower paper guide member 21, this gear portion (not shown) is provided. The gear portion (not shown) of the pulley 27B with the reversing gear portion that is always meshed with the gear 27) is rotatably provided.

  A belt 28 is stretched endlessly between the end of the rotation shaft 25a1 of the drive roller 25a, the pulley 27B with a reverse gear portion, and the intermediate pulley 27A.

  Furthermore, one end of the compression spring 29 is hooked on the end of the rotation shaft 25b1 of the driven roller 25b, and the other end of the compression spring 29 is fixed. The compression spring 29 is configured to obtain a predetermined nip pressure between the driving roller 25a and the driven roller 25b of the registration roller pair 25 by pressing the driven roller 25b side toward the driving roller 25a.

  As described above, the pulley 27 </ b> B with the reverse gear and the intermediate pulley, whose rotation is transmitted via the belt 28 from the driving roller 25 a of the registration roller pair 25 that rotates counterclockwise by the driving force of the second pulse motor 26. Each of the rotation rollers 27A rotates counterclockwise, while the driven pulley 25b of the registration roller pair 25, which is always meshed with the pulley 27B with the reverse gear portion, reverses with respect to the pulley 27B with the reverse gear portion and rotates clockwise. By rotating, the driven roller 25b side can be rotated without slipping with respect to the driving roller 25a side via the sheet P, and the sheet nipped between the rollers 25a and 25b by a predetermined nip pressure by the compression spring 29. P can be reliably conveyed toward the image forming unit 40.

  Returning to FIG. 1, the registration roller pair 25 stops rotating when the leading end of the transport direction of the paper P transported here hits, and the transport of the paper P is continued while the rotation is stopped. Then, a loop (sag) L as shown by a dotted line is formed on the upstream side of the registration roller pair 25, and after the fixed amount of loop (sag) L is formed, the registration roller pair 25 is rotated at the sheet conveyance speed. Thus, the skew-corrected paper P is conveyed toward the belt platen unit 30 and the image forming unit 40.

  In the registration roller pair 25, during the rotation after the skew correction is completed, as described above with reference to FIG. 8 or FIG. Although it may occur, the operation for detecting the bending T or the break O will be described later.

  Next, the belt platen unit 30 is installed on the downstream side of the first sheet conveyance unit 20, and conveys the sheet P whose skew has been corrected by the registration roller pair 25 toward the image forming unit 40. The image forming unit 40 has a function of conveying to a second sheet conveying unit 50 provided on the downstream side of the image forming unit 40.

  In the belt platen section 30 described above, an endless belt 31 having a plurality of air suction holes (not shown) and formed in a belt shape is rotationally driven by a driven pulley 32 and a third pulse motor 35 at a constant conveyance speed. Air that is endlessly passed through two intermediate pulleys 34, 34 between the driven pulley 33 and the paper P conveyed on the endless belt 31 is installed inside the endless belt 31. While sucking air with the suction part 36, it is conveyed toward the downstream side.

  A paper holding roller 37 for holding the paper P toward the endless belt 31 is provided rotatably on the upstream side of the endless belt 31 above the endless belt 31.

  Next, the image forming unit 40 is provided on the downstream side of the first sheet conveying unit 20 and above the belt platen unit 30 with a slight distance therebetween, and is provided with a plurality of colors (cyan, cyan, etc.). It has a function of forming a color image on paper P by printing using black, magenta, yellow) ink.

  In the image forming unit 40 described above, a plurality of line-type ink jet heads (hereinafter referred to as ink jet heads) 41C, 41K, 41M, and 41Y corresponding to a plurality of colors of ink are moved from the upstream side to the downstream side in the conveyance direction of the paper P. On the other hand, they are fixedly installed in the order of C (cyan), K (black), M (magenta), and Y (yellow).

  In the first embodiment, an example in which inkjet heads 41C, 41K, 41M, and 41Y for four colors are installed corresponding to four colors (CKMY) of ink will be described. For example, when printing only characters. Since only one color (K) is required, at least one or more inkjet heads may be installed.

  Here, each of the inkjet heads 41C, 41K, 41M, and 41Y for each color has a head block in which a plurality of nozzles are arranged along a main scanning direction (paper width direction) orthogonal to the sub-scanning direction (paper conveyance direction). A plurality of lines are arranged along one line along the main scanning direction, and are divided into two lines in the sub-scanning direction orthogonal to the main scanning direction. The head blocks are arranged in a staggered pattern so that the head blocks partially overlap each other.

  The paper P conveyed on the endless belt 31 while air is sucked by the air suction unit 36 in the belt platen unit 30 is printed with a color image by the four-color ink jet heads 41C, 41K, 41M, and 41Y. Forming.

  Next, the second paper transport unit 50 is provided between the belt platen unit 30 and the image forming unit 40 and the paper discharge unit 60. In the second sheet conveyance path for conveying the sheet P between the belt platen unit 30 and the image forming unit 40 and the sheet discharge unit 60, a plate-shaped first sheet that guides the lower surface of the sheet P along the conveyance direction. 2 A lower paper guide member 51 is fixedly installed.

  A pair of paper transport rollers 52 is installed along the second lower paper guide member 51 so that the rotation of the fourth pulse motor 53 is transmitted through a speed reduction mechanism (not shown) so as to be rotatable. Then, the image-formed paper P formed in step 1 is conveyed toward the paper discharge unit 60.

  Next, the paper discharge unit 60 is installed on the downstream side of the second paper transport unit 50 and has a function of stacking the image-formed paper P on the paper discharge tray 61.

  Next, the control unit 70 is installed at an appropriate location in the image forming apparatus 1, and stores therein a CPU 71 that controls the entire image forming apparatus 1, an operation program of the image forming apparatus 1, and the like. ROM 72 and a RAM 73 that temporarily stores various information that can be changed in the image forming apparatus 1, and a flexure / break determination unit 74 that is a part of the main part of the present invention. I have.

  At this time, the RAM 73 provided in the control unit 70 includes a deflection T {FIG. 3 (a), FIG. 8} or a break O {FIG. 3 (b), FIG. } Is stored in advance in accordance with the type of the paper P (thin paper, plain paper, etc.) and the color of the paper P. Has been.

  When the state of bending / bending that occurs in the vicinity of the center of the sheet P in the width direction changes depending on environmental conditions such as temperature and humidity, the threshold value S for determining bending / bending according to the environmental conditions (FIG. 5). May be stored in the RAM 73 as a table.

  Further, the sheet thickness data (not shown) for one sheet corresponding to the type of sheet P (thin paper, plain paper, thick paper) is stored in the RAM 73 in advance, so that the registration roller pair 25 skews the sheet P. Before the correction, when the output value of any one of the plurality of paper height detection sensors 22A to 22C exceeds the paper thickness data for one sheet, the double feed of the paper P can be detected.

  The control unit 70 controls the entire image forming apparatus 1. Specifically, the control unit 70 specifically controls the paper feeding unit 10, the first paper transport unit 20, the belt platen unit 30, and the image forming unit 40. Each member provided in the second paper transport unit 50 is controlled.

  Here, the operation for detecting the deflection T or the fold O generated in the width direction of the paper P shown in FIG. 8 or 9 previously using the plurality of paper height detection sensors 22A to 22C has been described above. 1 will be described in combination with FIG. 3 and FIG.

  FIGS. 3A and 3B schematically show a state in which a plurality of paper height detection sensors shown in FIGS. 1 and 2 are used to detect bending and folding that occur in the width direction of the paper. Yes. FIG. 4 shows detection results from the plurality of paper height detection sensors shown in FIGS.

  As described above in the background art, after correcting the skew of the sheet P by the registration roller pair 25, the registration roller pair 25 is rotated to cause the sheet P to have a predetermined nip pressure by the compression spring 29 (FIG. 2). During conveyance to the belt platen unit 30 and the image forming unit 40 while nipping, due to torsion of the rotation shaft of the registration roller pair 25, variation in the roller shape of the registration roller pair 25, and the like, FIG. As shown in FIG. 3, the deflection T may occur in the vicinity of the central portion in the width direction of the paper P, or the folding O may occur in the vicinity of the central portion in the width direction of the paper P as shown in FIG. There is.

  In FIGS. 3A and 3B, in the registration roller pair 25, each force from each roller portion corresponding to both ends in the width direction of the paper P toward the central portion in the width direction of the paper P is substantially the same. In this case, the case where the bending T and the fold O occur substantially symmetrically about the central portion in the width direction of the paper P is illustrated, but it corresponds to both ends of the paper P in the width direction. When the respective forces from the respective roller portions are different on the left and right, the flexure T and the fold O are generated asymmetrically by shifting slightly to the left or right from the central portion in the width direction of the paper P. It is possible to detect a bend T or a fold O generated in the width direction of the paper P including such an asymmetric case.

  Here, when the bending T or the fold O generated in the width direction of the paper P is detected by using the plurality of paper height detection sensors 22A to 22C, as described above with reference to FIG. It is essential to install the paper height detection sensor 22B at a position corresponding to the vicinity of the central portion in the width direction of the paper P. In addition to the single paper height detection sensor 22B, at least one paper height detection sensor 22A. , 22C are juxtaposed along the paper width direction at a predetermined interval with respect to one paper height detection sensor 22B. In this embodiment, at least one of the paper height detection sensors 22 is sandwiched therebetween. The above-described sheet height detection sensors 22A and 22C are installed substantially symmetrically.

  If the output values from the plurality of sheet height detection sensors 22A to 22C are A, B, and C when the deflection T or the fold O is detected by the plurality of sheet height detection sensors 22A to 22C, each output The values A, B, and C are based on the upper surface 21a (or the lower surface of the paper P) of the plate-like first lower paper guide member 21 that guides the conveyance of the paper P, and the upper surface 21a (or the paper). A value corresponding to the height from the lower surface of P) to the upper surface of flexure T or fold O is output.

  At this time, when a transmission type optical sensor is used as the plurality of sheet height detection sensors 22A to 22C, the light transmittance of the sheet P decreases as the height of the flexure T or the fold O increases, so that the transmitted light amount level. FIG. 4 shows this state.

  FIG. 4 shows the detection of a plurality of paper heights when the paper P is transported in the transport direction and the bending T shown in FIG. 3A or the fold O shown in FIG. 3B is detected. The locus of each output value (each transmitted light level) A, B, C detected at the same timing by the sensors 22A, 22B, 22C is shown linearly. Thus, the locus of each output value (each transmitted light level) A, B, C is curved.

  As shown in FIG. 4, the transmitted light amount level B from the sheet height detection sensor 22B installed in the vicinity of the center in the width direction of the sheet P is the sheet height detection sensor 22B from the illustrations of FIGS. The values are lower than the transmitted light levels A and C from the paper height detection sensors 22A and 22C arranged side by side on both sides, and the transmitted light levels A and C are substantially the same value.

  Then, when determining whether or not the bending T or the folding O has occurred in the width direction of the paper P, the transmitted light amount levels A, B, and C from the plurality of paper height detection sensors 22A to 22C are shown in FIG. Is input to the bending / breaking determination unit 74 in the control unit 70 shown in FIG.

  Here, the bending / breaking determination unit 74 has an absolute difference value Dn (where n is a natural number, for example) for each of the transmitted light amount levels A, B, C from the plurality of sheet height detection sensors 22A, 22B, 22C. , D1 = | A−B |, D2 = | B−C |, D3 = | C−A |), and among the plurality of absolute difference values Dn, the maximum absolute difference value Dmax ( Figure 5).

  In FIG. 4, when the transmitted light amount level A and the transmitted light amount level C are substantially equal, the maximum absolute difference value Dmax (FIG. 5) among the plurality of absolute difference values Dn is | A−B | It is clear from the figure that | B−C |.

  On the other hand, the RAM 73 in the control unit 70 stores in advance a threshold value S (FIG. 5) for determination of bending / folding that is set according to the type of paper (thin paper, plain paper) and the color of the paper P.

  Then, the maximum absolute difference value Dmax obtained as described above in the bending / breaking determination unit 74 is compared with the threshold value S for bending / breaking determination, and the maximum absolute difference value Dmax is determined for bending / folding determination. Is greater than the threshold value S (maximum absolute difference value Dmax> threshold value S), it is determined that the deflection T or the breakage O has occurred.

  At this time, if it is determined that the bend T or the crease O has occurred, the conveyance state of the paper P becomes abnormal, so that it is possible to immediately stop the conveyance of the paper P and warn the user.

  Here, the operation of the sheet conveying apparatus (20) of the first embodiment configured as described above will be described with reference to FIG. 1 and FIG.

  FIG. 5 shows a first operation flow in the paper conveying apparatus of the first embodiment, in which a plurality of paper height detection sensors are used to detect bending / bending that occurs in the paper width direction.

  As shown in FIG. 5, when the first operation flow in the first embodiment is started, first, in step S11, the paper feed roller 12 and the separating roller 13 are rotated to move the paper P to the registration roller pair 25 provided on the downstream side. Transport toward.

  Next, in step S12, the leading end of the sheet P in the conveyance direction is abutted against the registration roller pair 25 that has stopped rotating, and the skew of the sheet P is corrected.

  Next, in step S13, after correcting the skew of the sheet P, the registration roller pair 25 is rotated to convey the sheet P toward the belt platen unit 30 and the image forming unit 40 provided on the downstream side.

  Next, in step S14, each output value (each transmitted light level) A, B, C from the plurality of sheet height detection sensors 22A, 22B, 22C is read, and the bending / breaking determination unit 74 in the control unit 70 is read. To enter.

  At this time, each output value A, B, C from the plurality of paper height detection sensors 22A, 22B, 22C is read using an average value of a predetermined number of samplings. Further, the reading timing of the output values A, B, C from the plurality of paper height detection sensors 22A, 22B, 22C is set in advance according to the paper conveyance speed after the skew correction of the paper P. Yes.

  Next, in step S15, the absolute difference value Dn (where n is a natural number) with respect to the output values A, B, and C from the plurality of paper height detection sensors 22A, 22B, and 22C by the bending / breaking determination unit 74. ) To obtain the maximum absolute difference value Dmax among the plurality of absolute difference values Dn.

  Next, in step S <b> 16, the deflection / fold determination unit 74 determines the maximum absolute difference value Dmax, the threshold value S for bending / break determination set in advance according to the type of paper P (thin paper, plain paper) and the color of the paper P. As to whether or not the maximum absolute difference value Dmax is larger than the threshold value S for determination of bending / breaking.

  Next, in step S17, when it is determined in step S16 that the maximum absolute difference value Dmax is smaller than the deflection / folding determination threshold value S (in the case of NO), the bending / folding determination unit 74 uses the width of the paper P. It is determined that no deflection T or fold O occurs near the center of the direction.

  Next, in step S18, since it is determined in step S17 that no bending T or fold O has occurred, there is no abnormality in the conveyance of the sheet P, and the conveyance of the sheet P is continued as it is, and the belt platen unit 30 and The ink is sent to the image forming unit 40, and an ink image is printed on the paper P by the image forming unit 40.

  On the other hand, in step S19, when it is determined in step S16 that the maximum absolute difference value Dmax is larger than the deflection / folding determination threshold value S (in the case of YES), the bending / folding determination unit 74 performs the width direction of the paper P. It is determined that a bend T or a fold O has occurred in the vicinity of the central portion.

  Next, in step S20A, since it is determined in step S19 that bending T or fold O has occurred, the conveyance state of the sheet P is abnormal, the rotation of the registration roller pair 25 is stopped, and the conveyance of the sheet P is stopped. Stop immediately.

  Next, in step S21A, a warning for stopping conveyance of the paper P is issued to the user, and the first operation flow is ended.

  As described above in detail, in the paper transport apparatus (20) according to the first embodiment of the present invention, the lower surface of the paper P is guided between the paper feeding unit 10 and the image forming unit 40 along the paper transport direction. A registration roller pair 25 is installed along the side paper guide member 21 and upstream of the image forming unit 40, and the upper surface 21a of the lower paper guide member 21 (or the lower surface of the paper P) is positioned below the height. At least two sheet height detection sensors 22A to 22C for detecting the height of the sheet P conveyed on the side sheet guide member 21 are further upstream than the registration roller pair 25, and the sheet width orthogonal to the sheet conveyance direction One in the vicinity of the central portion of the direction and at least one other than the vicinity of the central portion are juxtaposed along the paper width direction, and at least two paper height detection sensors 22A, 22B, The maximum absolute difference value Dmax among the absolute difference values Dn obtained from the output values A, B, and C of 2C is compared with a preset threshold value S for determination of bending / breaking, and the width of the paper P Since the bend T or fold O generated in the direction is determined, the conveyance of the sheet P can be stopped immediately when it is determined that the bend T or fold O occurs in the width direction of the sheet P. The generation of wrinkles and paper jam can be suppressed, and an image with good image quality can be formed on the paper P in which no bending T or bending O occurs in the width direction of the paper P.

  Further, when an ink jet system is adopted as an example of the image forming unit 40 installed in the paper transport device (20) according to the present invention, at least two or more paper height detection sensors 22A, 22B, and 22C are registered. When juxtaposed in the paper width direction on the upstream side of the roller pair 25 and at least two or more paper height detection sensors 22A, 22B, and 22C detect bending T or fold O generated in the width direction of the paper P Since the sheet P can be stopped immediately, there is no fear of damaging the plurality of ink jet heads 41C, 41K, 41M, 41Y provided in the image forming unit 40 due to the bending T or fold O in the width direction of the sheet P. It can contribute to the improvement of the reliability of (20).

  FIG. 6 shows an enlarged view of the sheet conveying apparatus according to the second embodiment of the present invention.

  The paper transport device (20) of the second embodiment shown in FIG. 6 has the same configuration except for the configuration of the paper transport device (20) of the first embodiment described above with reference to FIGS. Here, for convenience of explanation, the same reference numerals are given to the constituent members shown above, and the constituent members shown above will be described as necessary, and different from the first embodiment. This will be described with a new reference numeral.

  As shown in FIG. 6, the sheet conveying apparatus (20) according to the second embodiment of the present invention is applied to the image forming apparatus 1 shown in FIG. 1 instead of the sheet conveying apparatus (20) according to the first embodiment described above. Has been.

  The sheet conveying device (20) of the second embodiment described above is different from the first embodiment only in that a nip pressure adjusting means 80 for adjusting the nip pressure of the registration roller pair 25 is added to the first embodiment. is there.

  Also in the second embodiment, a plurality of paper height detection sensors 22A for detecting the deflection T {FIG. 3 (a), FIG. 8} or the fold O {FIG. 3 (b), FIG. 9} in the width direction of the paper P. A registration roller pair 25 in which a driving roller 25a and a driven roller 25b are paired is provided on the downstream side of ˜22C to be rotatable.

  A second pulse motor 26 serving as a rotational drive source for the registration roller pair 25 is connected to the end of the rotation shaft 25a1 of the drive roller 25a via a speed reduction mechanism (not shown), and the second pulse motor 26 is driven. The driving roller 25a is rotated counterclockwise by the force.

  An intermediate pulley 27A is rotatably provided on the upper surface 21a side of the first lower sheet guide member 21 on the right side of the end of the rotation shaft 25a1 of the drive roller 25a.

  Further, on the right side of a gear portion (not shown) formed on a part of the end portion of the rotation shaft 25b1 of the driven roller 25b on the lower surface 21b side of the first lower paper guide member 21, this gear portion (not shown) is provided. The gear portion (not shown) of the pulley 27B with the reversing gear portion that is always meshed with the gear 27) is rotatably provided.

  A belt 28 is stretched endlessly between the end of the rotation shaft 25a1 of the drive roller 25a, the pulley 27B with a reverse gear portion, and the intermediate pulley 27A.

  As a result, the pulley 27B with the reversing gear portion and the intermediate pulley 27A, whose rotation is transmitted via the belt 28 from the driving roller 25a of the registration roller pair 25 that rotates counterclockwise by the driving force of the second pulse motor 26, respectively. On the other hand, the driven roller 25b of the registration roller pair 25 that is always meshed with the pulley 27B with the reverse gear portion rotates in the clockwise direction by rotating reversely with respect to the pulley 27B with the reverse gear portion. The configuration is the same as that of the first embodiment.

  Here, unlike the first embodiment, a nip pressure adjusting means 80 for adjusting the nip pressure of the registration roller pair 25 is additionally provided below the driven roller 25b of the registration roller pair 25 with respect to the first embodiment. ing.

  In the nip pressure adjusting means 80, an eccentric cam 83 is fixed to a rotary shaft 82 that is rotationally driven by a stepping motor 81, and the outer peripheral surface 83 a of the eccentric cam 83 is rotated by the driven roller 25 b of the registration roller pair 25. It is in contact with the end of the shaft 25b1.

  When the stepping motor 81 is operated to rotate the eccentric cam 83 integrally with the rotating shaft 82, the nip pressure of the registration roller pair 25 can be adjusted by the step position (0-1024) when the stepping motor 81 rotates. It has become.

At this time, the nip pressure adjustment value of the registration roller pair 25 depends on the type of the paper P, the deflection T in the width direction of the paper P {FIG. 3 (a), FIG. 8}, or the folding O {FIG. 3 (b), FIG. } Can be adjusted according to the sheet P on which the error occurs, and the nip pressure adjustment values described in Table 1 below are stored in advance in a table format in the RAM 73 in the controller 70 (FIG. 1).

  The nip pressure adjustment value shown in Table 1 will be specifically described. When the step position of the stepping motor 81 is 0, a nip pressure adjustment value of 36 [N] is obtained, and this nip pressure adjustment value 36 [ N] corresponds to paper P other than thick paper and thin paper such as plain paper.

  When the step position of the stepping motor 81 is 256, a nip pressure adjustment value of 39 [N] is obtained, and when the nip pressure adjustment value is 39 [N], it corresponds to thick paper.

  Further, when the step position of the stepping motor 81 is 512, a nip pressure adjustment value of 33 [N] is obtained. It corresponds to the plain paper in which the bending direction T or the fold O in the width direction has occurred.

  Further, when the step position of the stepping motor 81 is 768, a nip pressure adjustment value of 30 [N] is obtained, and when the nip pressure adjustment value is 30 [N], the deflection T or the folding O of the sheet P in the width direction is obtained. Corresponds to the thin paper that occurs.

  At this time, the shape of the outer peripheral surface 83 a of the eccentric cam 83 is formed into a curve that can obtain each nip pressure adjustment value shown in Table 1 according to each step position of the stepping motor 81.

  Therefore, when the image forming apparatus 1 and the sheet conveying device (20) provided in the apparatus 1 are in the initial state, the type of the sheets P stacked on the sheet feeding table 11 (FIG. 1) is set to an operation panel (not shown). Alternatively, the nip pressure adjustment in Table 1 stored in the RAM 73 in the control unit 70 (FIG. 1) according to the type of the paper P is set by a personal computer or detected using a paper type detection sensor (not shown). By referring to the value, the stepping motor 81 in the nip pressure adjusting means 80 is operated, and the nip pressure of the registration roller pair 25 can be adjusted to the initial setting value initially set according to the type of the paper P via the eccentric cam 83. It is like that.

  Further, after correcting the skew feeding of the fed paper P by the registration roller pair 25, a plurality of paper height detection sensors 22 </ b> A to 22 </ b> C for the paper P conveyed by the rotation of the registration roller pair 25 and the control unit 70. The nip pressure of the registration roller pair 25 is set to the initial setting value by the nip pressure adjusting means 80 when it is detected by the bend / break determination unit 74 provided inside that the occurrence of the bend T or fold O in the width direction of the paper P. Can be adjusted to a lower value.

  In the second embodiment, the nip pressure adjusting means 80 for adjusting the nip pressure of the registration roller pair 25 is configured using the eccentric cam 83 rotated by the stepping motor 81. However, the present invention is not limited to this. Although not shown in the drawings, as a nip pressure adjusting means of a modified example, for example, one end of the compression spring is hooked to the end of the rotation shaft of the driven roller of the registration roller pair, and the other end of the compression spring is displaced. The compression spring is expanded and contracted by hanging on a displacement member using a possible laminated piezoelectric element, etc., and displacing the displacement member in accordance with the type of paper and the paper in which the paper is bent or bent in the width direction. As a result, it is possible to adjust the nip pressure of the registration roller pair 25 according to the type of the paper P and the paper P in which the bending T or the folding O has occurred in the width direction of the paper P.

  Here, the operation of the sheet conveying apparatus (20) of the second embodiment configured as described above will be described with reference to Table 1 together with FIGS.

  FIG. 7 shows a second operation flow in the paper conveying apparatus of the second embodiment, in which a plurality of paper height detection sensors are used to detect bending / bending that occurs in the paper width direction.

  As shown in FIG. 7, when the second operation flow in the second embodiment is started, first, in step S10, at the initial stage, the nip pressure of the registration roller pair 25 is set by the nip pressure adjusting means 80 to the paper feed table 11 (FIG. 1). Initially set to a nip pressure adjustment value in accordance with the type of paper P stacked on top. The nip pressure adjustment value at this time is set as an initial set value.

  At this time, the nip pressure adjustment value of the registration roller pair 25 according to the type of the paper P is referred to Table 1 stored in the RAM 73 in the control unit 70. If the fed paper P is thin paper, for example, Table 1 The nip pressure adjustment value 33 [N] is initially set as the initial setting value of the nip pressure of the registration roller pair 25.

  Next, the process from step S11 to step S19 in the second operation flow in the second embodiment is the process from step S11 to step S19 in the first operation flow in the first embodiment described above with reference to FIG. Since it is exactly the same, it will be briefly described here.

  That is, in the processes from step S11 to step S19 in the second operation flow, after correcting the skew of the sheet P fed by the registration roller pair 25, the registration roller pair 25 is rotated to rotate the registration roller pair. 25, the respective output values (each transmitted light level) A, B, C from the plurality of sheet height detection sensors 22A, 22B, 22C are read with respect to the sheet P being conveyed, and the respective output values A, B, C are read. The maximum absolute difference value Dmax among the obtained absolute difference values Dn is obtained, and the maximum absolute difference value Dmax is compared with a predetermined threshold value S for bending / folding determination by the bending / breaking determination unit 74. Thus, in step S17, it is determined that there is no bending T or fold O in the vicinity of the central portion in the width direction of the paper P. There is determined has occurred.

  Thereafter, when it is determined in step S19 that the bending T or the fold O is generated near the center in the width direction of the sheet P as described above, the conveyance state of the sheet P is abnormal, so in step S20B. Then, the rotation of the registration roller pair 25 is stopped, the conveyance of the paper P is immediately stopped, and the nip pressure of the registration roller pair 25 is adjusted to a value lower than the initial set value by the nip pressure adjusting means 80.

  At this time, as described above, when the initial setting value of the nip pressure of the registration roller pair 25 is initially set to 33 [N] for the thin paper in step S10, for example, the width T of the sheet P is bent or folded. From Table 1 in which the nip pressure adjustment value of the registration roller pair 25 is stored in the RAM 73 in the control unit 70 for the thin paper on which O is generated, the nip pressure adjustment value 30 [ N] is adjusted by operating the stepping motor 81.

  Next, in step S21B, a warning to stop conveyance of the paper P is issued to the user.

  Next, in step S22B, it is asked whether or not the paper P on which the flexure T or the fold O has occurred is removed. If the user does not remove the paper P on which the flexure T or the fold O has occurred (in the case of NO), After a predetermined time has elapsed (for example, about 10 minutes have elapsed) in step S23B, the second operation flow is terminated.

  On the other hand, when the user removes the sheet P on which the bending T or the fold O has occurred in step S22B (in the case of YES), the process returns to step S11 while the nip pressure of the registration roller pair 25 is lowered, and the sheet P again. Feed paper.

  Then, after confirming that the sheet P has been normally conveyed to the image forming unit 40 without causing any bending T or fold O in the width direction, the sheet P is fed again after that. The nip pressure adjustment value that has been adjusted to be lower than the initial setting value for the paper P is stored in the RAM 73 in the control unit 70 and the conveyance of the paper P is continued.

  When the paper feed setting for a print job from an operation panel (not shown) or a PC or the type of paper P is different from the previous print job, the nip pressure of the registration roller pair 25 is initialized by the nip pressure adjusting means 80. Return to the set value. Thereby, when the specification of the next print job is different from the previous print job, it is possible to prevent a trouble at the time of paper conveyance due to insufficient nip pressure of the registration roller pair 25.

  According to the sheet conveying device (20) of the second embodiment described in detail above, the nip pressure adjusting means 80 for adjusting the nip pressure of the registration roller pair 25 is further provided to the first embodiment, and the bending / breaking determination unit 74 is provided. When it is determined that a deflection T or a fold O in the width direction of the paper P has occurred, the nip pressure adjusting means 80 adjusts the nip pressure of the registration roller pair 25 to a value lower than the initial set value. Therefore, after the user removes the sheet P in which the paper sheet P has been bent or fold O in the width direction, the sheet P is fed again after the sheet P is fed again. Therefore, a highly reliable paper transport device (20) can be provided.

1 Image forming apparatus,
DESCRIPTION OF SYMBOLS 10 ... Paper feed part, 11 ... Paper feed stand, 12 ... Paper feed roller, 13 ... Separation roller, 14 ... Separation board,
20 ... 1st paper conveyance part (paper conveyance device),
21: First lower sheet guide member, 21a: Upper surface, 21b: Lower surface,
22A to 22C ... Plural paper height detection sensors, 22a ... Light emitting element, 22b ... Light receiving element,
23 ... Signal amplifier, 24 ... AD converter,
25 ... Registration roller pair, 25a ... Drive roller, 25a1 ... Rotating shaft,
25b ... driven roller, 25b1 ... rotating shaft, 26 ... second pulse motor,
27A ... intermediate pulley, 27B ... pulley with reversing gear, 28 ... belt,
29 ... compression spring,
30 ... belt platen, 31 ... endless belt,
40: image forming unit, 41C, 41K, 41M, 41Y, a plurality of inkjet heads,
50... Second paper transport section 51. Second lower paper guide member 52 52 Paper transport roller pair
60 ... paper discharge unit, 61 ... paper discharge stand,
70: Control unit, 71 ... CPU, 73 ... RAM, 74 ... Bending / breaking determination unit,
80 ... nip pressure adjusting means, 81 ... stepping motor, 82 ... rotary shaft,
83: Eccentric cam, 83a: Outer peripheral surface,
P ... paper, L ... loop (sag), T ... flexion, O ... fold,
Dn: absolute difference value, Dmax: maximum absolute difference value, S: threshold value.

Claims (2)

In a paper conveying apparatus comprising a paper feeding unit that feeds paper and an image forming unit that forms an image on the fed paper,
A pair of registration rollers that rotate after correcting skew by correcting the skew of the sheet by forming a loop by abutting the leading end in the transport direction of the sheet,
One in the vicinity of the central portion in the paper width direction orthogonal to the paper conveyance direction on the upstream side of the pair of registration rollers, and at least one other than the vicinity of the central portion are juxtaposed along the paper width direction, At least two paper height detection sensors for detecting the height of the paper;
The width of the sheet is compared by comparing the maximum difference value among the difference values respectively obtained from the output values of at least two of the sheet height detection sensors with a preset threshold value for determination of bending / breaking. Bending / breaking determination means for determining the bending or bending of the direction;
A sheet conveying apparatus comprising:   A nip pressure adjusting means for adjusting the nip pressure of the registration roller pair is further provided, and the nip pressure adjustment is performed when it is determined by the bending / folding determination means that the sheet is bent or folded in the width direction. The sheet conveying apparatus according to claim 1, wherein the nip pressure of the registration roller pair is adjusted to a value lower than an initial set value by the means.

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