SHEET FEEDING APPARATUS AND IMAGE FORMING APPARATUS

TECHNICAL FIELD

The present invention relates to a sheet feeding apparatus and an image forming apparatus, and more specifically, to a configuration to feed a sheet by using an electrostatic attraction force.

BACKGROUND ART

In an image forming apparatus such as a copying machine and a printer, many apparatuses adopting a friction separation-type system as a system for conveying sheets from a cassette in which the sheets are stacked. In the friction separation system, a sheet feed roller formed of a rubber material is rotated while being pushed against the sheet stacked in the cassette, so as to convey an uppermost sheet from the sheets stacked on an intermediate plate. In order to prevent multiple feeding where a lower sheet in contact with the uppermost sheet is conveyed with the uppermost sheet, known configurations such as pressing the sheet against a separation pad during conveyance and applying a conveying force in a direction opposite to the conveyance direction with the sheets other than the uppermost sheet by a retard roller are applied in the friction separation system. In such friction separation configurations, noise caused during the feeding operation becomes a problem, since sheets are conveyed while applying a large vertical drag to the sheets.

One example for solving this problem is disclosed in Japanese Patent Application Laid-Open Publication No. 2012-140224, providing an apparatus having a configuration adopting an electrostatic attraction and separation system. In the disclosed apparatus, when attracting the sheet, an endless belt is sagged to increase the attraction area in order to separate the sheet, and after the sheet is attracted, tension is applied to the belt to make the belt planar when carrying the sheet, so that the noise generated in the sheet feeding portion can be reduced significantly.

Further, Japanese Patent Application Laid-Open Publication Nos. 116-255823 and 2001-48370 propose apparatuses adopting a configuration where power is fed to an endless electrostatic attraction belt having electrodes formed thereon. According to the apparatus disclosed in Japanese Patent Application Laid-Open Publication No. 116-255823, positive and negative voltages are respectively fed from two rollers on which the electrostatic attraction belt is stretched to an integral electrode disposed on the endless electrostatic attraction belt. Further, the apparatus disclosed in Japanese Patent Application Laid-Open Publication No. 2001-48370 adopts a configuration where electrodes divided in the circumferential direction are disposed on the endless electrostatic attraction belt and a power feed brush configured to come into contact only with an electrode that is located in a adhesion range among the electrodes.

In the apparatus disclosed in Japanese Patent Application Laid-Open Publication No. 116-255823, the positive and negative voltages are fed at one location, so that the voltages are applied to the whole area of the electrostatic attraction belt during power feed. Thereby, the sheet will be separated from the electrostatic attraction belt in the state where power is fed while conveying the sheet, so that electric charges will reside in the electrostatic attraction belt by separation discharge. This residual electric charge will deteriorate the electrostatic attraction force of the electrostatic attraction belt, so that it is required to discharge the residual electric charge.

According to the above-mentioned configuration, the voltages are applied to the whole area of the electrostatic attraction belt during the operation of attracting the sheet, so that sufficient discharging effect cannot be achieved by having the discharge portion contact the electrostatic attraction belt in this state. Therefore, after conveyance of a preceding sheet is completed, the endless electrostatic attraction belt is rotated once while power feed is stopped to have the whole area contact the discharge portion to perform discharge, and then power is fed again to carry out conveying operation of a subsequent sheet. Therefore, the productivity may be deteriorated significantly.

In an attempt to apply the power feed configuration disclosed in Japanese Patent Application Laid-Open Publication No. 2001-48370 to the apparatus disclosed in Japanese Patent Application Laid-Open Publication No. 2012-140224, since the attraction range is moved up and down, it is difficult to have the power feed brush come into secure contact with electrodes disposed on the electrostatic attraction belt to feed power.

SUMMARY OF INVENTION

The present invention provides a sheet feeding apparatus and image forming apparatus that do not require switching control of power feed and discharge, so that generation of electrostatic attraction force and discharging can be realized at the same time, and deterioration of productivity can be prevented.

According to a first aspect of the present invention, a sheet feeding apparatus includes a support portion supporting a sheet, a first rotator disposed above the support portion, a second rotator disposed downstream, in a sheet feeding direction, of the first rotator, an endless attraction member, whose inner surface is supported at least by the first and second rotators, rotating in a circumferential direction, the attraction member configured to feed the sheet by attracting the sheet by its outer surface opposing to the sheet supported by the support portion, a power feed portion feeding positive and negative voltages to the attraction member such that electrostatic attraction force is generated when the attraction member contacts the sheet on the support portion, and a discharge portion discharging an electric charge on the attraction member by being in contact with the attraction member. The attraction member includes a plurality of electrode pattern areas extending along the circumferential direction and each of the electrode pattern areas includes first and second electrodes supplied the voltages from the power feed portion respectively. The electrode pattern areas are arrayed in the circumferential direction in such a manner that the electrode pattern areas are electrically insulated each other, and each circumferential direction length of the electrode pattern areas is set shorter than a circumferential direction length, between the power feed portion and the discharge portion, of the attraction member.

According to a second aspect of the present invention, a sheet feeding apparatus includes an endless attraction member configured to have its outer surface attract the sheet and rotate, a power feed portion configured to feed voltage to the attraction member at a power feed position, and a discharge portion configured to discharge the attraction member at a discharging position. The attraction member includes a first electrode pattern area including an electrode, to which the voltage is fed from the power feed portion, extending along a circumference direction of the attraction member and a second electrode pattern area arrayed in the circumference direction with the first electrode pattern area. The second electrode pattern area includes an electrode, to which the voltage is fed from the power feed portion, extending along the circumference direction of the attraction member. Each circumferential direction length of the first and second electrode pattern areas is set shorter than a circumferential direction length, between the power feed position and the discharging position, of the attraction member.

Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic cross-sectional view illustrating an image forming apparatus having a sheet feeding apparatus according to a first embodiment.

FIG. 2 is a cross-sectional view illustrating the sheet feeding apparatus according to the first embodiment.

FIG. 3A is a plan view illustrating an attraction member according to the first embodiment. FIG. 3B is a cross-sectional view taken at line IIIB-IIIB of the attraction member of FIG. 3A. FIG. 3C is a cross-sectional view taken at line IIIC-IIIC of the attraction member of FIG. 3A.

FIG. 4A is a perspective view illustrating a configuration of a power feed portion according to the first embodiment. FIG. 4B is a perspective view illustrating a configuration of a discharge portion according to the first embodiment.

FIG. 5A is a view illustrating a state where voltage is applied to the electrostatic attraction belt according to the first embodiment. FIG. 5B is a view illustrating an attraction principle of an electrostatic attraction belt according to the first embodiment.

FIG. 6A illustrates a principle of how a peeling charge is generated in the electrostatic attraction belt. FIG. 6B illustrates a configuration for discharging the peeling charge generated in the electrostatic attraction belt. FIG. 6C illustrates a configuration where peeling charge generated in the electrostatic attraction belt cannot be discharged.

FIG. 7 is an expansion plan illustrating a configuration of an attraction member according to the first embodiment.

FIG. 8A illustrates a positional relationship between electrode pattern area, power feed, and discharge of the attraction member according to the first embodiment. FIG. 8B illustrates the attraction member in a state having moved from the state of FIG. 8A to a direction of arrow D. FIG. 8C illustrates the attraction member in a state having moved from the state of FIG. 8B to the direction of arrow D.

FIG. 9A illustrates an initial state of sheet separating operation of the sheet feeding apparatus according to the first embodiment. FIG. 9B illustrates an approximating operation of the sheet separating operation. FIG. 9C illustrates an increasing operation of the sheet separating operation.

FIG. 10A illustrates an attracting operation of the sheet separating operation. FIG. 10B illustrates a separating operation of the sheet separating operation. FIG. 10C illustrates a conveyance operation of the sheet separating operation.

FIG. 11 is a control block diagram of the sheet feeding apparatus according to the first embodiment.

FIG. 12 is a cross-sectional view illustrating the sheet feeding apparatus according to a second embodiment of the present invention.

FIG. 13 is a perspective view illustrating a configuration of a power feed portion according to the second embodiment.

FIG. 14A illustrates an electrode pattern area of the attraction member and a positional relationship of power feed and discharge according to the second embodiment. FIG. 14B illustrates the attraction member in a state being migrated from the state of FIG. 14A to a circumferential direction. FIG. 14C illustrates the attraction member in a state being migrated from the state of FIG. 14B to the circumferential direction.

FIG. 15A illustrates an initial state of a sheet separating operation of a sheet feeding apparatus according to the second embodiment. FIG. 15B illustrates an approximating operation of the sheet separating operation. FIG. 15C illustrates an increasing operation in the sheet separating operation.

FIG. 16A illustrates an attracting operation of the sheet separating operation. FIG. 16B illustrates a separating operation of the sheet separating operation. FIG. 16C illustrates a conveying operation of the sheet separating operation.

FIG. 17 is an expansion plan of an attraction member according to the second embodiment.

DESCRIPTION OF EMBODIMENTS
First Embodiment

Now, a preferred embodiment of the present invention will be described in detail with reference to the drawings. In the description, the present invention is implemented in an electro-photographic image forming apparatus, but the size, material, shape, relative configuration and the like disclosed in the present embodiments are not intended to restrict the scope of the present invention in any way.

FIG. 1 is a schematic view illustrating the image forming apparatus having a sheet feeding apparatus according to the present embodiment. In FIG. 1, 100 denotes the image forming apparatus, and 100A denotes an image forming apparatus body (hereinafter referred to as apparatus body). On the upper portion of the apparatus body 100A is arranged an image reader 41, which irradiates light onto a document placed on a platen glass as a document positioning plate and has an image sensor and the like that converts reflected light into digital signals. The apparatus body 100A includes a controller 70 as a control portion having a CPU, a ROM and a RAM for controlling the respective portions of the image forming apparatus.

A document from which image is to be read is conveyed by an automatic document feeder 41a onto the platen glass. The apparatus body 100A further includes an image forming portion 55, sheet feeding apparatuses 51 and 52 for feeding sheets S to the image forming portion 55, and a sheet reversing portion 59 for reversing the sheet S and conveying the sheet to the image forming portion 55.

The image forming portion 55 includes an exposure unit 42, and four process cartridges 43y, 43m, 43c and 43k for forming toner images of four colors, which are yellow (Y), magenta (M), cyan (C) and black (Bk). Further, the image forming portion 55 configured to form the image on the sheet includes an intermediate transfer unit 44 disposed above the process cartridges 43y, 43m, 43c and 43k, a secondary transfer portion 56 and a fixing portion 57. The sheet feeding apparatuses 51 and 52 feed sheets S to the image forming portion 55.

The process cartridges 43y, 43m, 43c and 43k include photosensitive drums 21y, 21m, 21c and 21k, charging rollers 22y, 22m, 22c and 22k, and developing rollers 23y, 23m, 23c and 23k, respectively. The process cartridges 43y, 43m, 43c and 43k further include drum cleaning blades 24y, 24m, 24c and 24k.

The intermediate transfer unit 44 includes a belt driving roller 26, an intermediate transfer belt 25 stretched by a secondary transfer inner roller 56a and so on, and primary transfer rollers 27y, 27m, 27c and 27k abutted against the intermediate transfer belt 25 at positions opposing to the photosensitive drums 21y, 21m, 21c and 21k. As mentioned later, by applying a transfer bias of positive polarity from the primary transfer rollers 27y, 27m, 27c and 27k to the intermediate transfer belt 25, toner images having negative polarity on the photosensitive drums 21y, 21m, 21c and 21k are sequentially superimposed and transferred onto the intermediate transfer belt 25. Thereby, a full color image is formed on the intermediate transfer belt 25.

The secondary transfer portion 56 is composed of a secondary transfer inner roller 56a, and a secondary transfer outer roller 56b that is in contact with the secondary transfer inner roller 56a via the intermediate transfer belt 25. As described later, by applying a secondary transfer bias of positive polarity to the secondary transfer outer roller 56b, a four-colored full color image formed on the intermediate transfer belt 25 is transferred onto the sheet S.

The fixing portion 57 includes a fixing roller 57a and a fixing backup roller 57b. By having a sheet S nipped and conveyed between the fixing roller 57a and the fixing backup roller 57b, the toner image on the sheet S is pressed and heated, and fixed onto the sheet S. The sheet feeding apparatuses 51 and 52 respectively includes cassettes 51a and 52a for storing sheets S, and sheet attraction, separation and feeding portions 51b and 52b having a function of attracting sheets S stored in the cassettes 51a and 52a by static electricity and feeding the sheets one by one.

In FIG. 1, 103 denotes a pre-secondary-transfer conveyance path for guiding the sheet S fed from the cassettes 51a and 52a to the secondary transfer portion 56. Reference number 104 denotes a pre-fixture conveyance path for guiding the sheet S having been conveyed to the secondary transfer portion 56 from the secondary transfer portion 56 to the fixing portion 57. Reference number 105 denotes a post-fixture conveyance path for guiding the sheet S having been conveyed to the fixing portion 57 from the fixing portion 57 to a switching member 61. Reference number 106 denotes a discharge path for guiding the sheet S having been conveyed to the switching member 61 from the switching member 61 to a discharge portion 58. Reference number 107 denotes a re-conveyance path for guiding a sheet S having been reversed by the sheet reversing portion 59 to the image forming portion 55 again, so as to have an image formed on a rear surface of the sheet S having an image already formed on one side thereof by the image forming portion 55.

Next, we will describe an image forming operation of the image forming apparatus 100 having the above configuration. When the image forming operation is started, at first, based on an image information from a personal computer and the like not shown, the controller 70 control the exposure unit 42 to irradiate laser beams to surfaces of the photosensitive drums 21y, 21m, 21c and 21k. At this time, the surfaces of the photosensitive drums 21y, 21m, 21c and 21k are charged evenly to a predetermined polarity/potential by the charging rollers 22y, 22m, 22c and 22k, and when laser beams are irradiated, electrical charges of the portions to which the laser beams have been irradiated are attenuated, and electrostatic latent images are formed on the surfaces of photosensitive drums.

Thereafter, the controller 70 causes the formed electrostatic latent images to be developed by the yellow (Y), magenta (M), cyan (C) and black (Bk) toners supplied from developing rollers 23y, 23m, 23c and 23k, respectively, so that the electrostatic latent images are developed as toner images. Then, the toner images of the respective colors are sequentially transferred to the intermediate transfer belt 25 by a primary transfer bias respectively applied to the primary transfer rollers 27y, 27m, 27c and 27k, so that a full-color toner image is formed on the intermediate transfer belt 25.

On the other hand, simultaneously as the toner image forming operation described above, the controller 70 activates the sheet feeding apparatuses 51 and 52 and causes the sheet attraction, separation and feeding portions 51b and 52b to separate and feed only one sheet S at a time from the cassettes 51a and 52a. The sheet S is detected by a sheet detection sensor 51c, and reaches a drawing roller pair 71 composed of drawing rollers 51d and 51e. Further, the sheet S is detected by a sheet detection sensor 52c, and reaches a drawing roller pair 72 composed of drawing rollers 52d and 52e. The sheet S nipped by the drawing roller pair 71 or 72 is sent into a conveyance path 103, and abuts against a nip of a registration roller pair 62 composed of registration rollers 62a and 62b which are stopped, by which a position of a front end of the sheet is adjusted (skew correction). The drawing roller pairs 71 and 72 constitute a drawing rotator pair for nipping the sheet S attracted and fed from intermediate plate 51f/52f, i.e., a stacking portion, by an attraction member 200 and conveying the sheet downstream. It is noted that each of the intermediate plates 51f and 52f as the stacking portion on which the sheet is stacked is also referred to a supporting portion supporting the sheet.

Next, in the secondary transfer portion 56, the controller 70 drives the registration roller pair 62 at a timing matching the position of the sheet S with the full color toner image on the intermediate transfer belt. Thereby, the sheet S is conveyed to the secondary transfer portion 56, and in the secondary transfer portion 56, the full color toner image is transferred collectively on the sheet S by a secondary transfer bias applied on the secondary transfer outer roller 56b.

The controller 70 causes the sheet S having the full color toner image transferred thereon to be conveyed to the fixing portion 57, where the sheet is heated and pressed in the fixing portion 57 so that the respective-colored toners are melted and mixed, and a full color image is fixed onto the sheet S. Thereafter, the controller 70 discharges the sheet S onto which the image has been fixed through the discharge portion 58 disposed downstream of the fixing portion 57. When forming images on both sides of the sheet S, the conveyance direction of the sheet S is reversed in the sheet reversing portion 59, and then the sheet S is conveyed again to the image forming portion 55 through a re-conveyance path 107.

Sheet Feeding Apparatus

Now, the sheet attraction, separation and feeding portions 51b and 52b in the sheet feeding apparatuses 51 and 52 will be described in detail. In the following description, the configuration of the sheet attraction, separation and feeding portion 51b in the sheet feeding apparatus 51 will mainly be described, and since the sheet attraction, separation and feeding portion 52b in the sheet feeding apparatus 52 has a similar configuration, the description thereof will be omitted.

As already described, the sheet feeding apparatus 51 includes the cassette 51a, and the sheet attraction, separation and feeding portion 51b attracting the sheet S stored in the cassette 51a by static electricity and feeding the sheet one by one. The sheet feeding apparatus 51 includes a lifting portion 301 arranged liftably to the cassette 51a for lifting the intermediate plate 51f on which the sheet S is stacked, and the sheet detection sensor 51c for detecting passing of the sheet S fed from the sheet attraction, separation and feeding portion 51b.

The lifting portion 301 changes the position of the uppermost sheet Sa stacked on the intermediate plate 51f by changing the position of the intermediate plate 51f according to a pivot angle of a lifter (not shown) disposed pivotably at a lower portion of the intermediate plate 51f. Further, the sheet detection sensor 51c is disposed within a sheet conveyance path between the sheet attraction, separation and feeding portion 51b and the drawing roller pair 71 (refer to FIG. 1). Whether sheet feed has succeeded is determined based on whether the sheet detection sensor 51c detects the sheet S at a predetermined timing. In the present embodiment, the sheet detection sensor 51c is a noncontact reflection-type photosensor that irradiates a spot light to a detection target, measures the quantity of reflected light and detects the presence or absence of the detection target.

The sheet attraction, separation and feeding portion 51b includes a second nipping-conveying roller pair 201, a first nipping-conveying roller pair 202, a discharge roller pair 250, and an endless attraction member 200 having flexibility. The attraction member 200 is nipped and conveyed by an extension roller 260, the first nipping-conveying roller pair 202 and the second nipping-conveying roller pair 201.

In FIG. 2, 302 denotes a sheet level detecting portion (refer to FIG. 11) for detecting the upper surface position of the sheets S stacked on the intermediate plate 51f of the cassette 51a. The sheet level detecting portion 302 is disposed above the intermediate plate 51f, and composed of a sensor flag 302a and a photosensor 302b. The sensor flag 302a is supported pivotably by a support portion not shown and configured so that one end can be in contact with an upper surface of the uppermost the sheet Sa, and the other end can shade the photosensor 302b.

When the upper surface of the sheet Sa is positioned at a predetermined height, the sheet level detecting portion 302 detects the upper surface position of the sheet Sa by having the sensor flag 302a pivot and shade the photosensor 302b. The controller 70 controls the movement of the lifting portion 301 so that the upper surface of the sheet Sa is constantly detected by the sheet level detecting portion 302, and maintains the position of the intermediate plate 51f so that the upper surface height of the uppermost sheet Sa is substantially fixed. As a result, a clearance Lr between the first nipping-conveying roller pair 202 and the upper surface of the sheet Sa can be maintained substantially constant.

The first nipping-conveying roller pair 202 is composed of a first nipping-conveying inner roller 202a and a first nipping-conveying outer roller 202b. Similar to the second nipping-conveying inner roller 201a, the first nipping-conveying inner roller 202a is disposed on the inner side of the attraction member 200, and a rotation shaft 202d is rotatably supported by a shaft support member (not shown) whose position is fixed. A driving force from a first driving source 204 is transmitted to the first nipping-conveying inner roller 202a via a drive transmission portion not shown. The first driving source 204 and a second driving source 203 illustrated in FIG. 2 are composed of stepping motors, for example, designed to rotate for a predetermined number of steps before moving onto a next operation step. The first nipping-conveying outer roller 202b constitutes a first nipping member nipping the attraction member 200 together with the first nipping-conveying inner roller (rotator) 202a.

Similar to a second nipping-conveying outer roller 201b, the first nipping-conveying outer roller 202b as a following rotator is disposed at a position opposing to the roller 202a in a state nipping the attraction member 200 together with the first nipping-conveying inner roller 202a, and supported rotatably by a shaft support member not shown. The first nipping-conveying outer roller 202b is driven to rotate by the attraction member 200 rotated by the first nipping-conveying inner roller 202a that rotates in a counterclockwise direction of FIG. 2 in the same direction. A first pressing spring 202c is coupled to a shaft support member not shown, and the first nipping-conveying inner roller 202a is biased by the first pressing spring 202c toward the axial center direction of the first nipping-conveying inner roller 202a, and nips the attraction member 200 together with the roller 202a.

The second nipping-conveying roller pair 201 is disposed downstream in a sheet feeding direction with respect to the first nipping-conveying roller pair 202, and is composed of the second nipping-conveying inner roller 201a and the second nipping-conveying outer roller 201b. The second nipping-conveying inner roller 201a is disposed on the inner side of the attraction member 200, and supports a rotation shaft 201d rotatably via a shaft support member (not shown) whose position is fixed. The second nipping-conveying inner roller 201a receives the drive from the second driving source 203 via a drive transmission portion not shown.

The second nipping-conveying outer roller 201b as a following rotator is configured to nip the endless belt-like attraction member (belt ember) 200 together with the second nipping-conveying inner roller 201a at a position opposing to the roller 201a, and supported rotatably by a shaft support member not shown. The second nipping-conveying outer roller 201b is co-rotated (driven to rotate) by the attraction member 200 rotated by the second nipping-conveying inner roller 201a that rotates in a counterclockwise direction of FIG. 2 in the same direction. A second pressing spring 201c is coupled to a shaft support member not shown, and the second nipping-conveying inner roller 201a is biased by the second pressing spring 201c toward the axial center direction of the second nipping-conveying inner roller 201a, and nips the attraction member 200 together with the roller 201a. The second nipping-conveying outer roller 201b constitutes a second nipping member nipping the attraction member 200 together with the second nipping-conveying inner roller (second rotator) 201a.

The discharge roller pair 250 is composed of a discharge inner roller 250a and a discharge outer roller 250b. The discharge inner roller 250a is disposed on the inner side of the attraction member 200, similar to the first nipping-conveying inner roller 202a and the second nipping-conveying inner roller 201a, and supported rotatably by a shaft support member not shown whose arrangement position is fixed. The discharge roller pair 250 constitutes a discharge portion coming in contact with the attraction member 200 at an upper section (second section) 200b distant from a lower sagged section (first section) 200a in the attraction member 200 and discharging residual electric charges. That is to say, the discharge roller pair 250 contacts the attraction member 200 and discharges the residual electric charge at a second position separated by a predetermined distance in a circumferential direction from a first position where the attraction member 200 is sagged downward to attract the sheet.

The discharge outer roller 250b as a following rotator is disposed on an outer side of the discharge inner roller 250a with the attraction member 200 intervened, and supported rotatably by a shaft support member not shown. A third pressing spring 250c is connected to a shaft support member not shown, and the discharge outer roller 250b is biased toward an axial center direction of the discharge inner roller 250a by the third pressing spring 250c, nipping the attraction member 200 together with the discharge outer roller 250b.

The extension roller 260 is disposed between the discharge roller pair 250 and the first nipping-conveying roller pair 202, and configured to rotate either at a same speed with or slightly slower than the first nipping-conveying inner roller 202a. Thereby, a tension occurs to the attraction member 200 between the extension roller 260 and the first nipping-conveying roller pair 202. The extension roller 260 constitutes a first rotator disposed above the intermediate plates 51f and 52f as stacking portion.

The extension roller 260 is composed of an insulator at least having a portion in contact with the attraction member 200 formed of insulating material. The extension roller 260 can be driven by the first driving source 204 which also drives the first nipping-conveying inner roller 202a, or driven by a driving force transmitted from a supporting shaft of the first nipping-conveying inner roller 202a via a gear or a belt.

As shown in FIG. 2, the endless belt-shaped attraction member 200 is supported by the first nipping-conveying inner roller 202a, the second nipping-conveying inner roller 201a, the discharge inner roller 250a and the extension roller 260 disposed along the sheet feeding direction (direction of arrow D). The attraction member 200 has a length longer than a minimum length in which a belt can be wound around the respective rollers. The attraction member 200 formed to have such length can be sagged downward while being rotated (moved) along with the rotation of the second nipping-conveying inner roller 201a and the first nipping-conveying inner roller 202a. Thereby, although a clearance Lr exists between a tangent line connecting outer surfaces of the second nipping-conveying roller pair 201 and first nipping-conveying roller pair 202 and the uppermost sheet Sa of the stacked sheets Sm, the attraction member 200 can contact the sheet Sa.

The endless attraction member 200 has its inner side supported by at least the extension roller 260 and the second nipping-conveying inner roller 201a, and rotates in a circumferential direction. The attraction member 200 attracts the sheet S at the lower sagged section (first section) 200a on an attraction side opposing to the sheet S stacked on the intermediate plate (stacking portion) 51f, and feeds the sheet S in the sheet feeding direction (direction of arrow D). That is, the attraction member 200 is configured to feed the sheet by attracting the sheet by its outer surface opposing to the sheet supported by the support portion at a first position. Further, the second nipping-conveying inner roller 201a constitutes a second rotator disposed downstream, in the sheet feeding direction, of the extension roller (first rotator) 260.

The first nipping-conveying inner roller 202a applies voltage from a positive voltage supply portion 265 and a negative voltage supply portion 266 as power source (high voltage power supply) to the attraction member 200. The first nipping-conveying inner roller 202a and the second nipping-conveying inner roller 201a constitute power feed portion for respectively supplying positive and negative voltages to the attraction member 200 so as to generate electrostatic attraction force when the lower sagged section (first section) 200a contacts the sheet S on the intermediate plate 51f. The details of the method for feeding power will be described later.

The discharge outer roller 250b and the discharge inner roller 250a of the discharge roller pair 250 are ground-connected, and are configured to discharge the residual electric charges on the front and rear surfaces of the attraction member 200. The details of this configuration will be descried later.

The discharge inner roller 250a is provided with a load torque applying portion 251, and the load torque applying portion 251 provides the discharge inner roller 250a with a resistance to a direction in which the attraction member 200 is fed. The discharge roller pair 250 is driven to rotate by the conveyance of the attraction member 200. Thus, the first nipping-conveying roller pair 202 continues to pull the discharge roller pair 250 driven to rotate through the attraction member 200. Thereby, a tension corresponding to the load torque of the load torque applying portion 251 is applied to the attraction member 200 between the first nipping-conveying roller pair 202 and the discharge roller pair 250. Therefore, the first nipping-conveying inner roller 202a connecting with the voltage supplying portion can be made to contact the attraction member 200 infallibly. The discharge roller pair 250 constitutes a discharge portion that contacts the attraction member 200 at the upper section (second section) 200b distant from the lower sagged section (first section) 200a in the attraction member 200 and discharges the residual electric charge (refer to FIG. 8B).

FIG. 11 is a common control block diagram of the sheet feeding apparatuses 51 and 52 according to the present embodiment. As shown in FIG. 11, the sheet level detecting portion 302, the sheet detection sensors 51c and 52c and so on are coupled to an input port of the controller 70. The lifting portion 301 (refer to FIG. 2), the first driving source 204, the second driving source 203, the positive voltage supply portion 265, the negative voltage supply portion 266 and so on are connected to an output port of the controller 70.

The controller 70 as a control portion controls the above-mentioned driving sources 204 and 203 respectively so that the rotational speeds of the first nipping-conveying inner roller 202a and the second nipping-conveying inner roller 201a differ. Thus, the sheet S on the intermediate plate 51f (52f) (on the stacking portion) is attracted to the attraction member 200 by increasing the sagging quantity of the attraction member 200 downward, and then the sheet S attracted to the attraction member 200 is conveyed while reducing the downward sagging quantity of the attraction member 200. The intermediate plate 51f (52f) (refer to FIG. 1) constitutes a stacking portion on which the sheets S are stacked.

The sheet level detecting portion 302 detects the upper surface position of the sheets S stacked on the intermediate plates 51f and 52f disposed respectively within the cassettes 51a and 52a. The sheet level detecting portion 302 is disposed above the intermediate plates, and is composed of a sensor flag and a photosensor not shown.

The sheet detection sensors 51c and 52c (refer to FIG. 1) detect the passing of a sheet S fed by the sheet attraction, separation and feeding portions 51b and 52b, respectively. The sheet detection sensor 51c is disposed on the sheet conveyance path between the sheet attraction, separation and feeding portion 51b and the drawing roller pair 71. Further, the sheet detection sensor 52c is disposed on the sheet conveyance path between the sheet attraction, separation and feeding portion 52b and the drawing roller pair 72. Then, whether the feeding of a sheet has succeeded is detected by whether the sheet detection sensor 51c or 52c detects a sheet S at a predetermined timing. In the present embodiment, each of the sheet detection sensors 51c and 52c is a noncontact reflection-type photosensor, which irradiates spot light to the detection target, measures the quantity of reflected light and detects the presence of the detection target.

The lifting portion 301 lifts the intermediate plates 51f and 52f configured liftably in the cassettes 51a and 52a on which sheets S are stacked, based on the control of the controller 70. The lifting portion 301 is equipped with a lifter (not shown) disposed pivotably below the intermediate plates 51f and 52f, and according to the pivot angle of the lifter, the lifting portion changes the position of the uppermost sheet of the sheets S stacked on the intermediate plates 51f and 52f and on the intermediate plate.

The first driving source 204 includes a pulse motor and the like, and drives the first nipping-conveying inner roller 202a to rotate based on the control of the controller 70. Further, the second driving source 203 includes a pulse motor, and drives the second nipping-conveying inner roller 201a to rotate based on the control of the controller 70.

Next, we will describe the attraction member 200, as shown in FIGS. 3A to 3C. FIG. 3B is a cross-sectional view taken at line IIIB-IIIB of FIG. 3A and viewing from the arrow direction, and FIG. 3C is a cross-sectional view taken at line IIIC-IIIC and viewing from the arrow direction. The attraction member 200 is an endless electrostatic attraction belt as described earlier, and a portion of the inner side of the member is illustrated in FIG. 3A.

As illustrated in FIG. 3A, the attraction member 200 has comb teeth-shaped electrode patterns 502 provided thereon. The comb teeth-shaped electrode patterns 502 are respectively alternately coupled to power feed electrodes 510 and 511 disposed on both ends of the attraction member 200. As illustrated in FIG. 3B, the power feed electrodes 510 and 511 are exposed on the rear surface of the attraction member 200, and high voltages are applied from the exposed portions. The rear surface refers to the inner surface that contacts the first nipping-conveying inner roller 202a, the second nipping-conveying inner roller 201a, the discharge inner roller 250a and the extension roller 260 when the attraction member 200 is stretched.

Now, an example of the material and dimension of the attraction member 200 will be described. In FIG. 3C, a base material 500 is formed of a PI (polyimide:volume resistivity=10¹⁵Ωcm) having a thickness of 50 μm, for example. The power feed electrodes 510 and 511 and the electrode pattern 502 are formed of copper having a layer thickness of 10 μm, for example, as a conductor having a volume resistivity of 10⁶Ωcm or smaller. A cover member 501 is formed of a PVDF (polyvinylidene fluoride:volume resistivity=10¹¹Ωcm) having a thickness of 50 μm, for example. Further according to FIG. 3A, the width of the comb teeth electrode of the electrode pattern 502 is set for example to 1 mm, the interval between one comb teeth electrode and an adjacent comb teeth electrode is set for example to 0.5 mm, and the width of the power feed electrodes 510 and 511 is set to 5 mm. The materials and dimensions described above are merely an example, and the materials and dimensions are not restricted to the above description.

Further according to the present embodiment, as mentioned later, the material and thickness of the attraction member 200 is adjusted to provide appropriate elasticity to the attraction member 200, so that the attraction member 200 is sagged downward when the attraction member 200 approximates the sheet S.

FIGS. 4A and 4B are perspective views respectively illustrating the first nipping-conveying roller pair 202 and the discharge roller pair 250, and the vicinity of the roller pairs.

In FIG. 4A, the first nipping-conveying inner roller 202a is formed of a cylindrical insulating material, and has ring-like conductive materials (hereinafter referred to as roller power feed portions 230 and 231) at two areas denoted by numbers 230 and 231. The roller power feed portions 230 and 231 are formed at positions capable of being in contact with the power feed electrodes 510 and 511 of the attraction member 200 illustrated in FIG. 3B. The roller power feed portions 230 and 231 is provided with conductive contact terminals 270 and 271 coupled to the positive voltage supply portion 265 and the negative voltage supply portion 266 (such as +1200 V and −1200 V) as positive and negative high voltage power supplies, and disposed in a slidable manner with the roller power feed portions 230 and 231. The first nipping-conveying inner roller 202a abutted against the first nipping-conveying inner roller 202a with the attraction member 200 intervened has a contact portion formed of an insulating material such as rubber that contacts the attraction member 200.

According to the above configuration, positive and negative high voltages are applied from the positive voltage supply portion 265 and the negative voltage supply portion 266 via the contact terminals 270 and 271 and the roller power feed portions 230 and 231 to the power feed electrodes 510 and 511 of the attraction member 200.

In FIG. 4B, the discharge inner roller 250a is formed of a cylindrical insulating material, and has ring-like conductive materials as roller power feed portions 232 and 233 disposed at two areas denoted by reference numbers 232 and 233. The roller power feed portions 232 and 233 are formed at positions capable of being in contact with the power feed electrodes 510 and 511 of the attraction member 200 illustrated in FIG. 3B. At positions respectively opposing to the roller power feed portions 232 and 233 are provided with conductive contact terminals 275 and 276 coupled to a ground potential, hereinafter referred to a GND potential, supported on the apparatus body 100A side so that the terminals can respectively slide against the roller power feed portions 232 and 233.

According to the above configuration, since the power feed electrodes 510 and 511 of the attraction member 200 becomes conductive with the GND potential through the contact terminals 275 and 276, and the roller power feed portions 232 and 233, the power feed electrodes 510 and 511 become the GND potential. The discharge outer roller 250b has a configuration where a roller 250bR formed of a conductive (or low-resistance) rubber material is fixed to a metal shaft 250bM, and the metal shaft 250bM is coupled to GND potential.

The discharge outer roller 250b is disposed at a position opposing to the discharge inner roller 250a and in contact with the surface (rear surface in the drawing) of the attraction member 200.

Next, the principles of how the attraction member performs electrostatic attraction of the sheet will be described with reference to FIGS. 5A and 5B. FIGS. 5A and 5B are cross-sectional views of the attraction members 200 which are the same as FIGS. 3B and 3C.

As shown in FIG. 5A, a positive high voltage (such as +1200 V) is applied to a power feed electrode 510 of the attraction member 200 from the positive voltage supply portion 265 as a high voltage power supply, and a negative high voltage (such as −1200 V) is applied to a power feed electrode 511 from the negative voltage supply portion 266 as a high voltage power supply. Then, as illustrated in FIG. 5B, positive and negative high voltages are applied alternately to each of the electrode patterns 502. Actually, negative voltage is applied to electrode pattern 5020, positive voltage is applied to electrode pattern 5021, negative voltage is applied to electrode pattern 5022, positive voltage is applied to electrode pattern 5023, and negative voltage is applied to electrode pattern 5024.

In FIG. 5B, when positive and negative high voltages are applied to the respective electrode patterns 502, dielectric polarization occurs within the cover member 501, and a potential of the same polarity as the electrode pattern 502 is generated at the surface of the attraction member 200. In this state, if a sheet S approximates the attraction member 200, dielectric polarization occurs within the sheet S (accompanying transfer of electrons), and the surface of the sheet S is charged with opposite polarity as the potential on the surface of the attraction member 200. As described, by having the surface of the attraction member 200 and the surface of the sheet S charged with opposite polarities, they are attracted to each other by Coulomb force, and the sheet S is thereby attracted to the attraction member 200.

Now, FIGS. 6A to 6C illustrates configurations capable of performing peeling charge, capable of performing discharge, and not capable of performing discharge, respectively.

When the attracted sheet S in the state where high voltage is still applied to the electrode pattern 502 of the attraction member 200 is peeled, as illustrated in FIG. 6A, a peeling charge 600 is generated at the surface of the attraction member 200. By experiment, it has been confirmed that by the peeling charge 600, the surface of the attraction member 200 is charged with opposite polarity as the applied voltage (for example, the surface of an electrode applied with +1200 V is charged with −600 V), and the electrostatic attraction force to the sheet S is deteriorated. Therefore, it is necessary to discharge the peeling charge 600 and to recover the electrostatic attraction force to the original state.

By experiment, as illustrated in FIG. 6B, it has been confirmed that by coupling the electrode pattern 502 of the attraction member 200 to GND potential and brushing the surface of the attraction member 200 with an anti-static brush 430 coupled to the GND potential, the peeling charge 600 can be discharged. According to this discharging method, it is important to stop applying high voltage to the electrode pattern 502 and have the pattern coupled to GND potential.

By experiment, as illustrated in FIG. 6C, it has been confirmed that by brushing the surface of the attraction member 200 with the anti-static brush 430 in a state where high voltage is applied to the electrode pattern 502, the charge cannot be discharged and electrostatic attraction force remains deteriorated. In a state where high voltage is applied, electrons flow in through the anti-static brush 430, and the surface of the attraction member 200 will be charged so as to be drawn toward the polarity of the potential at the surface of the attraction member 200. In other words, the surface applied with positive voltage is charged to negative voltage. Therefore, it is considered that the applied voltage and the surface of the charged voltage will have opposite polarities and will cancel out the other voltage, causing deterioration of the electrostatic attraction force. In order to perform continuous sheet conveyance operation, it is necessary to have the sheet S attracted while satisfying the above-mentioned discharge conditions.

Next, the details of the attraction member 200 according to the present embodiment will be described with reference to FIG. 7. FIG. 7 is a view illustrating a state where the endless attraction member 200 is expanded.

The plane illustrated in FIG. 7 is the plane on the side of the attraction member 200 in contact with the second nipping-conveying inner roller 201a, the first nipping-conveying inner roller 202a, the discharge inner roller 250a and the extension roller 260 when the attraction member 200 is stretched. In the present embodiment, electrode pattern areas composed of the power feed electrodes 510 and 511 and the comb teeth-shaped electrode patterns 502 are divided into three areas within the attraction member 200. Electrode pattern areas 520a, 520b and 520c composed of wirings are disposed in each of the areas 515a, 515b and 515c shown by dashed lines, and the three electrode pattern areas 520a, 520b and 520c are not electrically connected with one another. The wirings of the electrode pattern areas 520b and 520c are omitted, except for some portions.

The wirings within the respective electrode pattern areas are configured similarly as FIGS. 5A and 5B, and the wiring within the electrode pattern area 520a is described as an example in the following description. That is to say, n number of electrode patterns, which are 5020a, 5021a . . . 502na from the end portion in the named order, are wired alternately from the power feed electrodes 510a and 511a receiving power feed from the positive voltage supply portion 265 and the negative voltage supply portion 266, respectively. The respective dimensions and pitches of the power feed electrodes 510a, 510b, 510c, 511a, 511b and 511c of the electrode pattern areas 520a, 520b and 520c and the electrode patterns 502a, 502b and 502c are the same as those described in FIGS. 5A and 5B. In the present embodiment, sheet feeding direction lengths 530a, 530b and 530c of the electrode pattern areas 520a, 520b and 520c are all set the same.

As described, the electrode pattern area 520a has n number of electrode patterns 5020a, 5021a . . . 502na as the comb teeth-shaped electrodes. The n number of electrode patterns are protruded alternately in comb teeth-shapes from each of the power feed electrode (first electrode) 510a and the power feed electrode (second electrode) 511a in a width direction orthogonal to the circumferential direction of the attraction member 200. Further, the electrode pattern area 520b has n number of electrode patterns 5020b . . . 502nb as comb teeth-shaped electrodes protruded in comb teeth-shapes from each of the power feed electrode 510a and the power feed electrode 511a in the width direction orthogonal to the circumferential direction. Further, the electrode pattern area 520c has n number of electrode patterns 5020c, 5021c . . . 502nc as the comb teeth-shaped electrodes protruded alternately in comb teeth-shapes from each of the power feed electrode 510a and the power feed electrode 511a in a width direction orthogonal to the circumferential direction of the attraction member 200. That is to say, in the electrode pattern areas 520a to 520c, the comb teeth-shaped electrodes 5020b to 502n are configured so that positive and negative polarities are disposed alternately in the circumferential direction.

According to this configuration, the respective electrode pattern areas 520a to 520c can stably generate a good electrostatic attraction force by the comb teeth-shaped electrode patterns in a state where power is supplied from the power feed electrodes 510a and 511a. In the present embodiment, the distance between adjacent areas of the electrode pattern areas 520a to 520c (refer to FIG. 7) is set greater than the electrode pitches of the respective electrode patterns within the electrode pattern area.

Now, the positional relationship between electrode pattern areas and power feed electrodes will be described with reference to FIGS. 8A to 8C. FIGS. 8A to 8C are views illustrating the position of electrode patterns of the electrode pattern areas 520a, 520b and 520c (refer to FIG. 7) and the positional relationship between the first nipping-conveying roller pair 202 as the power feed portion and the discharge roller pair 250 as the discharge portion.

The attraction member 200 illustrated in FIGS. 8A to 8C is in a most sagged state between the second nipping-conveying roller pair 201 and the first nipping-conveying roller pair 202, and it is extended by a minimum distance in a stretched manner with tension applied thereto between the second nipping-conveying roller pair 201 and the discharge roller pair 250.

In FIGS. 8A to 8C, the area shown by the dashed line in the inner side of the attraction member 200 is the position of each electrode pattern area. Reference numbers 5020a, 5020b and 5020c denote electrode patterns as heads in the respective electrode pattern areas in the sheet feeding direction, and 502na, 502nb and 502nc denote electrode patterns at ends.

Reference number 201N denotes a nip position (conveyance nip) where the attraction member 200 is nipped by the second nipping-conveying roller pair 201. Reference number 202N denotes a nip position (power-feed nip) where the attraction member 200 is nipped by the first nipping-conveying roller pair 202, and through this portion, power is fed to the power feed electrodes 510 and 511 of the attraction member 200. Reference number 250N denotes a nip position (discharge nip) where the attraction member 200 is nipped by the discharge roller pair 250, and the residual electric charge caused by the peeling charge of the sheet S described in FIG. 6A will be removed at this portion.

In the state illustrated in FIG. 8A, a distance from a point on an upstream side (right side in the drawing) of the discharge nip 250N to a point on a downstream side (lower side in the drawing) in the power-feed nip 202N is referred to as 205a. A distance from a point on an upstream side of the power-feed nip 202N to a point on a downstream side of the discharge nip 250N is referred to as 205b. In the present embodiment, the respective portions are configured as follows.

(1) The distance 205a>the sheet feeding direction lengths 530a, 530b and 530c of the respective electrode pattern areas 520a, 520b and 520c illustrated in FIG. 7.

(2) The distance 205b>the sheet feeding direction lengths 530a, 530b and 530c of the respective electrode pattern areas 520a, 520b and 520c illustrated in FIG. 7. The sheet feeding direction lengths 530a, 530b and 530c do not necessarily have to be the same lengths, as long as the above conditions are satisfied, and they can be set to different lengths.

As described, the attraction member 200 has a plurality of electrode pattern areas 520a to 520c configured to extend along the circumferential direction and mutually insulated in the circumferential direction. Each of the electrode pattern areas 520a to 520c includes the power feed electrode (511a to 511c) and the power feed electrode (510a to 510c) as first and second electrodes to which voltages are respectively fed from the first nipping-conveying inner rollers (power feed portion) 202a. The first nipping-conveying inner rollers (power feed portion) 202a is configured to feed voltage to the attraction member 200 at the power feed position and the discharge roller pair (discharge portion) 250 configured to discharge the attraction member 200 at the discharging position. Each circumferential direction length of the electrode pattern areas is set shorter than a circumferential direction length, between the power feed portion and the discharge portion, of the attraction member 200. More specifically, the circumferential direction length of the respective electrode pattern areas is set to be shorter than the circumferential direction length of the attraction member 200 set to the minimum length between adjacent first nipping-conveying inner roller (power feed portion) 202a and discharge roller pair (discharge portion) 250. In other words, the attraction member 200 includes a first electrode pattern area (the electrode pattern area 520a and/or 511b) including an electrode (the power feed electrode 510a), to which the voltage is fed from the power feed portion 202a, extending along the circumference direction of the attraction member 200 and a second electrode pattern area (the electrode pattern area 520b) arrayed in the circumference direction with the first electrode pattern area, the second electrode pattern area including an electrode (the power feed electrode 510b and/or 511b), to which the voltage is fed from the power feed portion (the first nipping-conveying inner roller 202a), extending along the circumference direction of the attraction member, and each circumferential direction length of the first and second electrode pattern areas is set shorter than a circumferential direction length, between the power feed position and the discharging position, of the attraction member 200.

That is to say, the power feed portion according to the present embodiment is composed of only one first nipping-conveying inner roller (power feed rotator) 202a disposed at one location in the circumferential direction of the attraction member 200. The circumferential direction lengths of the respective electrode pattern areas are set shorter than the circumferential direction length between the first nipping-conveying inner roller 202a and the discharge roller pair (discharge portion) 250, of the attraction member 200 in a case where the attraction member 200 set to the minimum length.

Since rollers are pressed against the power-feed nip 202N and the discharge nip 250N, the nips have certain widths in the sheet feeding direction, and in FIG. 8A, the widths of the nips are illustrated in an emphasized manner. The distances 205a and 205b are distances that do not include the widths of the nip.

FIG. 8A illustrates a state where the electrode pattern 5020a being the head pattern of the electrode pattern area 520a (FIG. 7) is at a position having passed through a conveyance nip 201N, and the electrode pattern 502na being the tail pattern is positioned before the power-feed nip 202N. At this time, in the vicinity of the first nipping-conveying inner roller 202a, as illustrated in FIG. 4A, power is fed from the positive voltage supply portion 265 and the negative voltage supply portion 266 via the contact terminals 270 and 271 and the roller power feed portions 230 and 231 to the power feed electrodes 510 and 511.

Therefore, all the electrode patterns from the electrode pattern 5020a to the electrode pattern 502na receive power so as to have positive and negative polarities alternately, and the surface of the attraction member 200 from the conveyance nip 201N to the power-feed nip 202N opposing to the sheet is in a state capable of attracting the sheet. At this time, the electrode pattern 5020b being the head pattern of the electrode pattern area 520b (FIG. 7) is positioned downstream of 502na, and the electrode pattern 502nb being the tail pattern is at a position having passed through the discharge nip 250N. Therefore, the surface (the sheet attraction surface) of the electrode pattern area 520b is in a state where discharge has been completed (the details of which will be described later).

In this case, the electrode pattern 5020c being the head pattern of the electrode pattern area 520c (FIG. 7) passes through the discharge nip 250N, and the electrode pattern 502nc being the tail pattern is placed before the discharge nip 250N. Therefore, the surface (sheet attraction surface) of the electrode pattern having passed through the discharge nip 250N is in a state where discharge has been completed (the details of which will be described later).

FIG. 8B illustrates a state where the electrode pattern 5020a being the head pattern of the electrode pattern area 520a has passed through the discharge nip 250N, and the electrode pattern 502na being the tail pattern is placed before the discharge nip 250N. In the vicinity of the discharge inner roller 250a, as illustrated in FIG. 4B, power is conducted from the GND potential to the contact terminals 275 and 276, the roller power feed portions 232 and 233, and the power feed electrodes 510 and 511 of the attraction member 200, and the power feed electrodes 510 and 511 of the attraction member 200 are set to GND potential.

When the electrode pattern 5020a reaches the discharge nip 250N, the electrode pattern 502na is in a positional relationship having passed through the power-feed nip 202N. Therefore, all the electrode patterns from 5020 to 502n are in a no-power-feed state, that is, in the dischargeable state similar to FIG. 6B. The electrode pattern area 520c in FIG. 8A is in the same state as the electrode pattern area 520a of FIG. 8B.

In the electrode pattern area 520b (refer to FIG. 7) of FIG. 8B, all electrodes are in a no-power-feed state that is not in contact with any of the power-feed nips 202N. At this time, the electrode pattern area 520c is positioned so that the electrode pattern 5020c being the head pattern has passed the conveyance nip 201N, and the electrode pattern 502nc being the tail pattern is placed before the power-feed nip 202N. Therefore, similar to the electrode pattern area 520a illustrated in FIG. 8A, power is fed to all the electrode patterns.

The sheet S starts to separate from the attraction member 200 in the vicinity of a contact start point between the attraction member 200 and the second nipping-conveying inner roller 201a, and on the downstream side from that position, the surface of the attraction member 200 is in a peeling-charged state, as shown in FIG. 6A.

When the peeling-charged portion of the attraction member 200 contacts the discharge outer roller 250b of the discharge nip 250N, the electric charge at the surface of the attraction member 200 flows through the roller 250bR as conductive rubber portion described in FIG. 4B and the metal shaft 250bM to the GND potential portion. Thereby, the surface of the attraction member 200 is discharged.

FIG. 8C illustrates a state where the electrode pattern 5020a of the electrode pattern area 520a is placed before the power-feed nip 202N, and the electrode pattern 502na being the tail pattern has passed through the discharge nip 250N. Before the electrode pattern 502na passes through the discharge nip 250N, the electrode pattern 5020a is in a no-power-feed position before the power-feed nip 202N, so that the dischargeable state of FIG. 6B is maintained. Therefore, when all the electrode patterns from the electrode pattern 5020a to the electrode pattern 502na passes through the discharge nip 250N (discharge outer roller 250b), discharging of the surface of the attraction member 200 of this electrode pattern area is completed.

Thereafter, the electrode pattern 5020a reaches the power-feed nip 202N after discharge is completed, and power feed from the first nipping-conveying inner roller 202a to the power feed electrodes 510 and 511 is started, by which the surface of the attraction member 200 is set to a state capable of attracting the sheet S. At this time, in the electrode pattern area 520b, the electrode pattern 5020b has passed through the discharge nip 250N, and the electrode pattern 502nb being the tail pattern is positioned before the discharge nip 250N. In the electrode pattern area 520b, the surface of the electrode pattern (sheet attraction surface) having passed through the discharge nip 250N is in a discharge-completed state. At this time, in the electrode pattern area 520c, the electrode pattern 502nc being the tail pattern is positioned downstream from the power-feed nip 202N, and similar to FIG. 8B, a state is maintained where power is fed to all electrode patterns.

Next, with reference to FIGS. 9A to 9C and FIGS. 10A to 10C, the sheet separation and feeding operation by the sheet attraction, separation and feeding portion 51b according to the present embodiment will be described. FIGS. 9A to 9C and FIGS. 10A to 10C illustrate the operations where the sheet S is fed by the sheet attraction, separation and feeding portion 51b in time series. The feeding operation of sheet S is composed of six steps in time-series order as illustrated in FIGS. 9A to 9C and FIGS. 10A to 10C, which are an initial operation, an approximating operation, a contact length increasing operation, an attracting operation, a separating operation, and a conveying operation. Now, these operations will be described in this order.

In the present embodiment, the controller 70 controls the first driving source 204 and the second driving source 203, respectively, so that there is a difference in rotational speeds of the first nipping-conveying inner roller 202a and the second nipping-conveying inner roller (second rotator) 201a, which are rotators that are driven to rotate. Thus, by increasing the downward sagging quantity of the attraction member 200 in the lower sagged section (first section) 200a, the sheet S on the intermediate plate 51f can be attracted to the attraction member 200. Then, the sheet attracted on the attraction member 200 can be fed while reducing the downward sagging quantity of the attraction member 200.

Now, the first and second driving source 204 and 203 composed for example of stepping motors as illustrated in FIG. 2 is controlled to move onto the next operation step after rotating for a predetermined number of steps. At first, the initial operation illustrated in FIG. 9A is an operation for arranging the attraction member 200 at the initial position of the feeding operation, and in the present embodiment, the sagging of the attraction member 200 is gathered between the second nipping-conveying roller pair 201 and the discharge roller pair 250. In order to realize the initial state, the controller 70 rotates the second nipping-conveying roller pair 201 toward the direction of arrow R at a velocity (peripheral velocity or conveyance velocity) faster than the first nipping-conveying roller pair 202, and sends the sagging of the attraction member 200 to the downstream side than the second nipping-conveying roller pair 201.

As mentioned earlier, the load torque applying portion 251 as illustrated in FIG. 2 is disposed on the discharge inner roller 250a, which acts as a conveyance resistance with respect to the direction in which the attraction member 200 is conveyed. Since the sagging of the attraction member 200 will not be transmitted downstream by the discharge roller pair 250, it can be gathered between the second nipping-conveying roller pair 201 and the discharge roller pair 250. At this time, tension is applied between the second nipping-conveying roller pair 201 and the first nipping-conveying roller pair 202 on the attraction member 200, and the member is extended by the minimum distance.

In this case, the first nipping-conveying roller pair 202 can either be stopped or rotated. When the initial operation is completed, the distance between the uppermost sheet Sa and the attraction member 200 is in a state separated by only the clearance Lr between the sheet Sa and the first nipping-conveying inner roller 202a. The second nipping-conveying roller pair 201 and the first nipping-conveying roller pair 202 can either be transited to the next operation while being continuously rotated from the initial position, or transited to the next operation after stopping the rotation temporarily.

In the configuration of the present embodiment, it is necessary to maintain a fixed positional relationship between the electrode pattern area and the sheet, since the power feed to the electrode pattern area contributing to attracting the sheet Sa must be continued until the drawing roller pair 71 nips the sheet Sa (the reason of which will be described later).

In the present embodiment, the state where the electrode pattern 5020a is in the position illustrated in FIG. 9A is referred to as a home position (HP) of the attraction member 200. A home position refers to a state where not only the electrode pattern area 520a but also any one of the head patterns of the three electrode pattern areas 520a, 520b and 520c (FIG. 7) is at a position of the electrode pattern 5020a of FIG. 9A. The feeding of the sheet is started from this state.

In the present invention, an configuration is adopted where the position of the attraction member 200 can be detected so as to stop the attraction member 200 at the home position. For example, a surface roughness or a hole is formed at an arbitrary position outside the attraction range of the attraction member 200, and the controller 70 detects the position of the attraction member 200 using a transmission type or reflection type sensor or micro switches, for example, so as to control the position of the electrode pattern 5020 to be at the position illustrated in FIG. 9A.

When a signal to feed the sheet S is received, the controller 70 performs control to have the first and second nipping-conveying roller pairs 201 and 202 drive the attraction member 200 and search the home position, or stop the driving of the attraction member 200 when the home position is detected after completing conveyance of the sheet S.

The approximating operation illustrated in FIG. 9B is an operation of deforming the attraction member 200 to be sagged downward, and to move the attraction surface side of the attraction member 200 close to the sheet Sa. At first, the controller 70 drives the first driving source 204 to rotate the first nipping-conveying inner roller (power feed portion) 202a in a direction of arrow R, and at the same time, drives the second driving source 203 to rotate the second nipping-conveying inner roller 201a in the same direction to convey the attraction member 200.

At this time, the first nipping-conveying inner roller 202a (first nipping-conveying roller pair 202) is rotated faster than the second nipping-conveying inner roller 201a (second nipping-conveying roller pair 201). Thus, the lower side of the attraction member 200 between the first nipping-conveying roller pair 202 and the second nipping-conveying roller pair 201 is deformed to be sagged downward. At this time, the second nipping-conveying roller pair 201 may either be stopped or rotated. Then, by deforming the attraction member 200, the lower surface of the attraction member 200 can approximate the sheet Sa.

An increasing operation of the contact length illustrated in FIG. 9C is an operation where the above-described approximating operation is continued to thereby have the lower side surface of the attraction member 200 contact the sheet Sa, and to increase a contact length Mc of the lower sagged section (first section) 200a of the attraction member 200. In the present embodiment, the contact length Mc is increased by rotating the first nipping-conveying inner roller 202a in the direction of arrow R faster than the second nipping-conveying inner roller 201a, similar to the approximating operation.

Voltages are applied respectively from the positive voltage supply portion 265 and the negative voltage supply portion 266 to the attraction member 200, and at least the area of the contact length Mc is within a range where power is fed from the first nipping-conveying inner roller 202a. Thereby, a state is realized where electrostatic attraction force is applied between the attraction member 200 and the sheet Sa. However, if the contact length Mc is shorter than a predetermined contact length Mn (refer to FIG. 10A), the force in which the attraction member 200 attracts the sheet Sa is small, so that it cannot overcome the sheet feed resistance acting on the sheet Sa. Therefore, the increasing operation of the contact length can be continued while the sheet Sa is stored within the cassette 51a.

The attracting operation illustrated in FIG. 10A is an operation where after the upper surface of the sheet Sa is in surface contact with the surface of the attraction member 200 by a predetermined contact length Mn, the sheet Sa is started to be conveyed by the attraction member 200. After the above-described contact length increasing operation is continued, when the contact length reaches Mn and the conveyance power to the sheet Sa is increased, the sheet Sa is started to be conveyed, overcoming the conveyance resistance.

In order to convey the sheet S infallibly to the drawing roller pair 71 composed of the drawing rollers 51d and 51e disposed downstream of the second nipping-conveying roller pair 201, it is desirable that the attraction member 200 attract the front end of the sheet Sa infallibly in the state illustrated in FIG. 10A. In the present embodiment, the driving of the first and second nipping-conveying roller pairs 202 and 201 is controlled so that the electrode pattern 5020 of the electrode pattern area (dashed line) contributing to the attraction of the sheet Sa reaches a position 903 separated by a distance 902 downstream than a leading end position 901 of the sheet Sa. However, even if the front end position 901 of the sheet Sa is positioned downstream than the electrode pattern 5020, the sheet Sa can be conveyed to the drawing roller pair 71 as long as the front end of the sheet Sa is within the range where the front end of the sheet Sa constitutes a substantially straight line with the attraction member 200 by the stiffness of the sheet.

In the attraction member 200, when the attraction area of the lower sagged section 200a with respect to the sheet Sa becomes maximum and before the uppermost sheet Sa on the intermediate plate 51f is separated from the next sheet Sb (FIG. 10A), the front end of the electrode pattern area can be positioned adjacent to the front end of the sheet Sa. Thereby, when the uppermost sheet Sa is separated from the subsequent sheet Sb, it becomes possible for the front end of the corresponding electrode pattern area to start attracting the area approximate the front end of the sheet Sa and convey the sheet Sa infallibly when the attraction area of the attraction member 200 becomes maximum. Feeding of sheets can be started not only when the front end portion of the electrode pattern area is positioned downstream from the uppermost the sheet Sa, but also when it is positioned upstream therefrom.

The separating operation illustrated in FIG. 10B is an operation of lifting the sheet Sa attracted by the attraction member 200 to the upper direction, and separating the sheet Sa from the sheet Sb below. In the separating operation, the second nipping-conveying roller pair 201 is rotated in the direction of arrow R at a velocity (peripheral velocity and conveyance velocity) faster than the first nipping-conveying roller pair 202. Thereby, the sagging of the surface of the attraction member 200 opposed to the sheet Sa can be moved downstream than the second nipping-conveying roller pair 201.

Further, since the load torque applying portion 251 on the rotation shaft of the discharge inner roller 250a acts as a resistance against the conveyance direction of the attraction member 200, the first nipping-conveying roller pair 202 will continue pulling the discharge roller pair 250 driven to rotate by the attraction member 200. That is to say, tension will constantly be applied to the attraction member 200 between the first nipping-conveying roller pair 202 and the discharge roller pair 250, and the sagging of the attraction member 200 will be gathered between the second nipping-conveying roller pair 201 and the discharge roller pair 250. As a result, the sagging of the surface of the attraction member 200 opposing to the sheet Sa will be eliminated and elastically deformed to a substantially linear shape, according to which the sheet Sa attracted to the attraction member 200 is lifted upward and separated from the sheet below (subsequent sheet) Sb.

During this time, the electrode pattern 502n being the pattern at the very end of the electrode pattern area contributing to the attraction of the sheet Sa is positioned upstream of the power-feed nip 202N, so that the attraction member 200 attracts the area from the front end of the sheet Sa to position 904.

The conveying operation illustrated in FIG. 10C is an operation of conveying the attraction member 200 having the attraction surface of the sheet Sa deformed to a substantially linear shape, to thereby attract and convey the attracted sheet Sa to the drawing roller pair 71 disposed downstream in the sheet feeding direction. FIG. 10C illustrates a state just before the front end of the sheet Sa is nipped by the nip of the drawing rollers 51d and 51e.

In this operation, by substantially matching the rotational speed of the first nipping-conveying roller pair 202 and the second nipping-conveying roller pair 201, the attraction member 200 attracting the sheet Sa is conveyed while maintaining the attraction surface side in a substantially linear shape. Thereby, the sheet Sa is conveyed while being attracted by the attraction member 200 with at least a front end portion of the sheet separated from the subsequent sheet Sb below. When the front end of the sheet Sa approaches a position 905 in the vicinity of a curved portion of the attraction member 200 formed by the second nipping-conveying inner roller 201a between FIGS. 10B and 10C, the front end of the sheet Sa is peeled off from the attraction member 200.

This peeling occurs by the reaction force of the sheet Sa becoming greater than the electrostatic attraction force generated in the attraction member 200. In other words, according to the present embodiment, the size of the electrostatic attraction force generated in the attraction member is set to the size for attracting the sheet with a power smaller than the reaction force of the sheet Sa (self stripping). That is to say, the attraction member 200 is moved to a position where the sheet Sa is separated (separation position) by the conveying operation. After the front end is peeled from the attraction member 200, the peeling is widened from the front end of the sheet Sa, but the rear end area of the sheet Sa is still attracted by the attraction member 200.

If the electrode pattern 502n being the end pattern is passed through the power-feed nip 202N before the front end of the sheet Sa is nipped by the drawing roller pair (drawing rotator pair) 71, the sheet Sa will be peeled off from the attraction member 200. Therefore, the sheet Sa cannot enter the nip of the drawing roller pair 71, and jamming (sheet jam) will occur. In order to prevent jamming, the electrode pattern 502n being the end pattern of the electrode pattern area contributing to attracting the sheet Sa is configured to be positioned upstream of the power-feed nip 202N until at least the front end of the sheet Sa is nipped by the drawing roller pair 71. Thereby, the attraction state of the sheet Sa can be maintained.

When the leading end of the sheet Sa is peeled off from the attraction member 200, the sheet Sa is nipped by the drawing roller pair 71, and when a sheet detection sensor not shown disposed on the downstream side detects the arrival of the sheet Sa, the controller 70 stops the positive voltage supply portion 265 and the negative voltage supply portion 266, and stops feeding power to the attraction member 200. When power feed stops, electrostatic attraction force will not occur to the electrode pattern area, and the sheet Sa is separated from the attraction member 200. Thereafter, the sheet Sa is conveyed downstream by the drawing roller pair 71. The controller 70 continues to drive the attraction member 200 even after the sheet Sa separates from the attraction member 200, and when the attraction member 200 detects that it has reached the home position (HP) 400, it stops the driving operation of the first and second nipping-conveying roller pairs 201 and 202. The stopped state is the state illustrated in FIG. 9A.

The configuration described above requires a relationship where the sheet feeding direction lengths 530a to 530c of the respective electrode pattern areas 520a to 520c illustrated in FIG. 7 are greater than the distance 206a plus distance 206b. Now, distance 206a is the distance from a point on the upstream side (upper side in the drawing) of the power-feed nip 202N to position 903 illustrated in FIG. 10A. This distance 206b is the distance in which the front end of the sheet Sa illustrated in FIG. 10C moves from the position 901 (refer to FIG. 10A) to the area being nipped by the drawing roller pair 71. As described, the respective circumferential direction lengths of the electrode pattern areas 520a to 520c is set so that the rear end side of the electrode pattern area is capable of being in contact with the first nipping-conveying inner roller 202a being the power feed portion even at the point of time when the sheet being attracted and conveyed by a single electrode pattern area, i.e., the sheet having a size capable of being attracted and fed by one electrode pattern area, is nipped by the drawing roller pair 71.

In the present embodiment, at the point of time when the sheet Sa is nipped by the drawing roller pair 71 (substantially in the state illustrated in FIG. 10C), the electrode pattern 5020b of the subsequent electrode pattern area approximates a home position 400. When power feed is continued in this state, when the electrode pattern 5020b arrives at the home position 400 and the attraction member 200 is stopped, the sheet Sa will remain attracted by the attraction member 200, and the conveyance load of the drawing roller pair 71 is increased, so that it is not preferable. Therefore, by stopping power feed before the sheet Sa is nipped by the drawing roller pair 71 and the subsequent head pattern of the electrode pattern area reaches the home position 400, the sheet Sa can be separated from the attraction member 200.

By setting the sheet feeding direction lengths 530a, 530b and 530c or the distances between respective electrode pattern areas to be longer than the setting of the present embodiment, it is also possible to realize a configuration where stopping of power feed is not necessary. The configuration allows power feed to be continued even after the sheet Sa is nipped by the drawing roller pair 71, and after the sheet Sa is separated from the attraction member 200, the head pattern of the subsequent electrode pattern area arrives at the home position. The total length of the attraction member 200 is elongated compared to the above-described embodiment, but control can be simplified since a fixed power feed should simply be continuously performed from the starting to the ending of sheet feed.

Further, during initial operation, voltage can either be applied constantly, or stopped at normal times and applied after the attraction member 200 and the sheet Sa come in contact. The present embodiment adopts a method of managing the operation steps by the first driving source 204 and the number of rotating steps of the first driving source 204 (refer to FIG. 11), but the present invention is not restricted thereto. For example, it is possible to adopt a method where the velocity (conveyance velocity) of nipping-conveying roller pairs 201 and 202 are controlled while detecting the shape of the attraction member 200 and the attraction timing of the sheet Sa.

Discharge cannot be performed thoroughly in the state where voltage is applied to the electrode pattern area to be discharged, as explained by FIG. 6C, so that the electrode pattern 5020 being the head pattern should not reach the discharge nip 250N before the electrode pattern 502n passes through the power-feed nip 202N. Further, the electrode pattern 5020 being the head pattern should not reach the power-feed nip 202N before the electrode pattern 502n passes through the discharge nip 250N.

In the present embodiment, as mentioned earlier, the distances 205a and 205b are set longer than the conveyance direction distances 530a to 530c of the electrode pattern area illustrated in FIG. 7, so that the electrode pattern areas will not contact the power-feed nip 202N and the discharge nip 250N at the same time. Therefore, the surface of the attraction member 200 can be discharged while conveying the sheets S.

Now, if the sheet Sa is not detected within a predetermined period of time by the sheet detection sensor 51c, the controller 70 (FIG. 11) determines that a failure has occurred during the feeding operation of the sheet Sa, and re-starts the feeding operation from the approximating operation (FIG. 9B) again. Based on the six steps described above, the sheet Sa can be fed one at a time from the plurality of sheets S stacked in the cassette 51a. By performing these six steps repeatedly, the sheets S can be fed one at a time and successively.

By adopting the above configuration, discharge can be performed while high voltage is not applied to the electrode pattern areas 520a to 520c, and there is no need to stop the sheet conveyance while performing discharge, so that the deterioration of throughput will no longer occur.

Power can be fed from the power-feed nip 202N nipped by the first nipping-conveying roller pair 202 to the respective electrode patterns of the electrode pattern areas 520a to 520c of the attraction member 200, and electrostatic attraction force can be generated at the lower sagged section 200a where the tension state of the attraction member 200 varies. Therefore, the sheet S can be fed infallibly to the drawing roller pair 71 disposed in the oblique upper direction by controlling the configuration of the first nipping-conveying roller pair 202 and the sagging of the attraction member 200, without having to swing the whole body of the sheet feeding apparatuses 51 and 52.

In the configuration of the present embodiment, the number of electrode pattern areas is three, but the number is not restricted thereto, and it can be two, or four or more. Regardless of the number of electrode pattern areas, the length of the attraction member 200, or the configuration of the second nipping-conveying roller pair 201 and the first nipping-conveying roller pair 202 (first nipping-conveying inner roller 202a) should be determined to allow discharge to be performed in the state where high voltage is not applied to the respective electrode pattern areas.

The present embodiment adopts an configuration where the electrode of the attraction member 200 is divided into multiple electrode pattern areas in the sheet feeding direction, and the circumferential direction lengths of the respective electrode pattern areas are set shorter than the minimum length of the attraction member 200 from the first nipping-conveying inner roller 202a to the discharge roller pair 250. Thereby, the electrode pattern areas 520a to 520c will not come in contact with the first nipping-conveying inner roller 202a and the discharge roller pair 250 at the same time. Further, the feeding of power to the attraction member 200 will be performed at the portion nipped by the power-feed nip 202N, and electrostatic attraction force can be generated at the lower sagged section 200a where the tension state varies in the attraction member 200.

According to this configuration, discharge can be performed without having to stop the feeding of sheets S, so that feeding of sheets can be performed infallibly while generating sufficient electrostatic attraction force without deteriorating throughput during image forming. A fixed voltage should simply be applied constantly to the first nipping-conveying roller pair 202 (first nipping-conveying inner roller 202a) without the need to perform switching control between power feed and discharge, so that a simple configuration without the need to perform intermittent driving or polarity control is made possible.

Second Embodiment

Now, a second embodiment of the present invention will be described with reference to FIGS. 12 to 17. In the present embodiment, the same components as the first embodiment are denoted with the same reference numbers, and the descriptions of components having the same configuration and function are omitted.

In the first embodiment described earlier, power feed to the attraction member 200 is performed from the positive voltage supply portion 265 and the negative voltage supply portion 266 through the first nipping-conveying inner roller 202a, but in the present embodiment, in addition to the above, power is also fed from the second nipping-conveying inner roller 201a.

FIG. 12 is a view illustrating an configuration of the sheet feeding apparatus 51 according to the present embodiment. In the present embodiment, the positive voltage supply portion 265 and the negative voltage supply portion 266 are connected to both the first nipping-conveying inner roller 202a and the second nipping-conveying inner roller 201a, so that power can also be fed from the second nipping-conveying inner roller 201a to the power feed electrodes 510 and 511 of the attraction member 200.

FIG. 13 is a perspective view illustrating in enlarged view a portion of the second nipping-conveying inner roller 201a and the second nipping-conveying outer roller 201b in the second nipping-conveying roller pair 201 illustrated in FIG. 12. As illustrated in FIG. 13, the second nipping-conveying inner roller 201a disposed on an inner circumference of the attraction member 200 is composed of a cylindrical insulator, and at two areas denoted by reference numbers 330 and 331, roller power feed portions 330 and 331 composed of ring-shaped conductive material are disposed.

The roller power feed portions 330 and 331 are formed at positions coming into contact with the power feed electrodes 510 and 511 of the attraction member 200. Conductive contact terminals 280 and 281 connected to the positive voltage supply portion 265 and the negative voltage supply portion 266 (such as +1200 V and −1200 V) are slidably disposed in areas opposing to the roller power feed portions 330 and 331.

In the above-described configuration, positive and negative high voltages are applied from the positive voltage supply portion 265 and the negative voltage supply portion 266 to the contact terminals 280 and 281, the roller power feed portions 330 and 331 and the power feed electrodes 510 and 511 of the attraction member 200.

FIG. 17 is a perspective view showing the attraction member 200 according to the present embodiment in the expanded state, having a similar configuration as FIG. 7 described in the first embodiment. Hereafter, the differences of the present configuration from the configuration illustrated in FIG. 7 will be described.

In the present embodiment, as illustrated in FIG. 17, the distance among the electrode pattern areas 520a to 520c, i.e., a distance between adjacent areas, is set to a same dimension as the electrode pitch of respective electrode patterns within the electrode pattern areas. That is, distances 540a, 540b and 540c between adjacent electrode pattern areas are set equal to the pitch between the respective electrode patterns within respective electrode pattern areas. In other words, the distances 540a, 540b and 540c between the respective electrode pattern areas 520a to 520c are set to the same dimension as the pitch of comb teeth-shaped electrodes in the respective electrode pattern areas.

By setting the pitches to be the same, starting of electrostatic attraction force that occurs between closest electrode patterns between the adjacent electrode pattern areas will be equivalent as the starting of electrostatic attraction force that occurs between closest electrode patterns within the same electrode pattern area. That is, the time required from when power feed is started to when the necessary electrostatic attraction force is achieved will be equivalent among adjacent electrode pattern areas and among the closest electrode patterns within a single electrode pattern area. Therefore, the electrostatic attraction force will not vary among different electrode pattern areas, and the power can be stabilized. This configuration can also be adopted in the first embodiment described earlier.

Further, it is possible to have electrode patterns in adjacent areas of the electrode pattern areas 520a to 520c couple to either one of the first nipping-conveying inner roller 202a and the second nipping-conveying inner roller 201a, which are different power feed portion, at the same timing. For example, there is a case where the electrode pattern 5020b of the electrode pattern area 520b receives power feed from the power feed electrode 510b at a timing where the electrode pattern 502na being the end pattern of the electrode pattern area 520a receives power feed from the power feed electrode 511a. Thereby, positive and negative high voltages can be applied appropriately in an alternative manner also among adjacent electrode pattern areas, so that uniform electrostatic attraction force can be generated throughout the whole length of the attraction member 200 without having the electrostatic attraction force varied, and sheets can be attracted by the same electrostatic attraction force at any position.

FIGS. 14A to 14C respectively illustrate positions of the respective electrode patterns in the electrode pattern areas 520a to 520c and the positional relationship between the first nipping-conveying inner roller 202a and the second nipping-conveying inner roller 201a as power feed portion and the discharge roller pair 250 as discharge portion. FIGS. 14A to 14C illustrate a state where the lower sagged section (first section) 200a between conveyance roller pairs 202 and 201 of the attraction member 200 is in a most sagged state, and the upper section (second section) 200b between the upper roller 201a and the discharge roller pair 250 is stretched to a minimum distance.

In FIGS. 14A to 14C, the areas shown by the dashed line on the inner side of the attraction member 200 illustrate positions within the respective electrode pattern areas 520a to 520c. Reference numbers 5020a, 5020b and 5020c denote electrode patterns being a head of the respective electrode pattern areas in the sheet feeding direction, and reference numbers 502na, 502nb and 502nc denote tail positions of electrode patterns.

Reference numbers 201N and 202N respectively denote power-feed nips where the attraction member 200 is nipped by the first and second nipping-conveying roller pairs 201 and 202, and 201e denotes an end position of the area where the attraction member 200 and the second nipping-conveying inner roller 201a contact one another. The power feed end portion 201e is a point most downstream in the conveyance direction of the attraction member 200 within the area where the roller power feed portions 330 and 331 contact the power feed electrodes 510 and 511 of the attraction member 200.

In FIGS. 14A to 14C, the attraction member 200 is in a stretched state at a minimum distance between the conveyance roller pair 201 and the discharge roller pair 250, and the power feed end portion 201e is disposed downstream of the conveyance nip 201N, but the position thereof may vary depending on the state of the attraction member 200 (the details of which will be described later).

Power is fed from the power feed end portion 201e through the contact portion to the conveyance nip (power-feed nip) 201N and through the power-feed nip 202N to each of power feed electrodes 510 and 511 of the attraction member 200. Reference number 250N denotes a discharge nip where the attraction member 200 is nipped by the discharge roller pair 250, and the residual electric charge by the peeling charge of the sheet S described in FIG. 6A is removed by the discharge nip 250N.

In the state illustrated in FIG. 14A, a distance from the point on the upstream side (right side in the drawing) of the discharge nip 250N to the power feed end portion 201e is referred to as 205a. A distance from the point on the upstream side (right side in the drawing) of the power-feed nip 202N to a point on the downstream side of the discharge nip 250N is referred to as 205b. A distance from the power feed end portion 201e to a point on the upstream side of the power-feed nip 202N is referred to as 205c.

In the present embodiment, the respective portions are determined as follows.

(1) The distance 205a>the sheet feeding direction lengths 530a, 530b and 530c of the electrode pattern area illustrated in FIG. 17.

(2) The distance 205b>the sheet feeding direction lengths 530a, 530b and 530c of the electrode pattern area illustrated in FIG. 17.

(3) The distance 205c<the sheet feeding direction lengths 530a, 530b and 530c of the electrode pattern area illustrated in FIG. 17.

The sheet feeding direction lengths 530a, 530b and 530c can be set to different lengths, as long as the above conditions are satisfied.

As described, the power feed portion according to the present embodiment is composed of a first nipping-conveying inner roller (first power feed rotator) 202a and a second nipping-conveying inner roller (second rotator, second power feed rotator) 201a configured so that the lower sagged section 200a is positioned between the two rollers. Each circumferential direction length of the electrode pattern areas is set shorter than a circumferential direction length, between the roller 202a and the discharge roller pair 250, of the attraction member, and set shorter than a circumferential direction length, between the roller 201a and the discharge roller pair 250, of the attraction member 200. The circumferential direction lengths of the respective electrode pattern areas are set shorter than the circumferential direction length when the attraction member 200 is set to the minimum length between the above-mentioned roller 202a and the discharge roller pair 250. At the same time, the circumferential direction lengths of the respective electrode pattern areas are set shorter than the circumferential direction length when the attraction member 200 is set to the minimum length between the above-mentioned roller 201a and the discharge roller pair 250.

Since roller pairs are respectively pressed against the power-feed nip 202N and the discharge nip 250N, the nips have certain widths, and in FIG. 14A, the widths of the nips are illustrated in an emphasized manner. The distances 205a and 205b illustrate distances that do not include the widths of the nip. The distance 205c illustrates the distance including the width of the power-feed nip 202N and the width from the power feed end portion 201e to the point on the upstream side of the conveyance nip 201N.

In FIG. 14A, the electrode pattern 5020a of the electrode pattern area 520a is at a position having passed through the power feed end portion 201e, and the electrode pattern 502na is positioned before the power-feed nip 202N. At this time, in the vicinity of the second nipping-conveying inner roller 201a, as illustrated in FIG. 13, power is fed from the positive voltage supply portion 265 and the negative voltage supply portion 266 via the contact terminals 280 and 281 and the roller power feed portions 330 and 331 to the power feed electrodes 510 and 511.

On the other hand, in the vicinity of the first nipping-conveying inner roller 202a, as shown in FIG. 4A, power is fed from the positive voltage supply portion 265 and the negative voltage supply portion 266 via the contact terminals 270 and 271 and the roller power feed portions 230 and 231 to the power feed electrodes 510 and 511.

Accordingly, positive and negative voltages are applied alternately to all the electrode patterns in the electrode pattern area 520a from the electrode pattern 5020a to the electrode pattern 502na. Thereby, the lower sagged section (first section) 200a opposing to the sheet in the attraction member 200 from the power feed end portion 201e to the power-feed nip 202N generates an electrostatic attraction force.

Power is constantly fed to the electrode pattern area 520a from at least either the first nipping-conveying roller pair 202 or the second nipping-conveying roller pair 201 while moving on from the power-feed nip 202N to the power feed end portion 201e. At this time, the electrode pattern 5020b of the electrode pattern area 520b is positioned upstream from the power-feed nip 202N, and the electrode pattern 502nb being the end pattern is at a position having passed through the discharge nip 250N. In this case, the surface (sheet attraction surface) of the electrode pattern area 520b is in a state where discharge has been completed, similar to the description of the first embodiment.

At this time, the electrode pattern 5020c of the electrode pattern area 520c is positioned immediately before the discharge nip 250N. The electrode pattern 502nc being the end pattern is at a position having passed through the power feed end portion 201e, and the electrode pattern area 520c is in a state where neither discharge nor power feed is performed.

In the present embodiment, as illustrated in FIG. 17, the distances 540a, 540b and 540c between adjacent electrode pattern areas are configured similarly as the pitch within the electrode pattern area. Therefore, if the electrode pattern 502n being the end pattern of a certain electrode pattern area has passed through the power-feed nip 202N, the head pattern of the subsequent electrode pattern area will immediately reach the power-feed nip 202N and power feed to the attraction member 200 is continued. In FIGS. 14A to 14C, the distance between adjacent electrode pattern areas is emphasized and illustrated in a separated manner.

Also according to the present embodiment, the electrode pattern areas 520a to 520c have comb teeth-shaped electrodes that protrude in comb teeth shapes alternately from the power feed electrode (first electrode) 510a and the power feed electrode (second electrode) 511a in the width direction of the attraction member 200. There are n number of electrode patterns 5020a, 5021a to 502na, n number of electrode patterns 5020b to 502nb, and n number of electrode patterns 5020c to 502nc as the comb teeth-shaped electrodes. Further, comb teeth-shaped electrodes are configured such that positive and negative polarities are positioned alternately in the circumferential direction in the plurality of electrode pattern areas 520a to 520c. Thereby, an effect equivalent to the first embodiment can be achieved.

The distances 540a, 540b and 540c between adjacent electrode pattern areas should preferably be equal to the pitch between electrode patterns within each electrode pattern area, but it is also possible to adopt an configuration where the distances are set to a somewhat different dimension, as long as the distances are within the permissible range of the required electrostatic attraction force.

As mentioned earlier, since the positive and negative polarities of the adjacent electrode patterns between electrode pattern areas are disposed alternately, the attraction member 200 can generate a uniform electrostatic attraction force throughout the whole length.

Therefore, during the operation steps mentioned later, even if there are two electrode pattern areas positioned between the power feed end portion 201e and the power-feed nip 202N before and after the positions of the electrode pattern areas are in the state illustrated in FIG. 14A, the following configuration is enabled. That is, power can be fed from the first and second nipping-conveying roller pairs 201 and 202 to the respective electrode pattern areas, so as to generate a uniform electrostatic attraction force.

FIG. 14B illustrates a state where the electrode pattern 5020a of the electrode pattern area 520a has passed through the discharge nip 250N and the electrode pattern 502na is positioned before the discharge nip 250N.

In the vicinity of the discharge inner roller 250a, as shown in FIG. 4B, conduction is realized from the GND potential to the contact terminals 275 and 276, the roller power feed portions 232 and 233, and the power feed electrodes 510 and 511 of the attraction member 200, so that power feed electrodes 510 and 511 of the attraction member 200 are set to GND potential.

Since an configuration is adopted where the electrode pattern 502na passes through the power feed end portion 201e when the electrode pattern 5020a reaches the discharge nip 250N, all the electrode patterns from 5020a to 502na are set to a no-power-feed state, in other words, are set to a dischargeable state as illustrated in FIG. 6B.

In the vicinity of the contact start point of the attraction member 200 and the second nipping-conveying inner roller 201a, the sheet starts to separate from the attraction member 200, and downstream from that positon, the surface of the attraction member 200 is in a peeling-charged state, as illustrated in FIG. 6A.

When the peeling-charged portion of the attraction member 200 contacts the discharge outer roller 250b of the discharge nip 250N, the surface electric charge is discharged by flowing to the GND potential portion through the roller 250bR as the conductive rubber portion and the metal shaft 250bM, as described in FIG. 4B.

At this time, the electrode pattern areas 502b and 502c are positioned so that the respective electrode patterns 5020b and 5020c have passed through the power feed end portion 201e and the power-feed nip 202N, respectively, and in a charged state. Therefore, similar to FIG. 14A, electrostatic attraction force is generated in the lower sagged section (first section) 200a of the attraction member 200 from the power feed end portion 201e to the power-feed nip 202N opposing to the sheet.

In FIG. 14C, the electrode pattern 5020a of the electrode pattern area 520a is positioned before the power-feed nip 202N, and the electrode pattern 502na is in a state having passed through the discharge nip 250N. Before the electrode pattern 502na is passed through, the electrode pattern 5020a is in a no-power-feed position before the power-feed nip 202N, so that the dischargeable state of FIG. 6B is maintained. Accordingly, when all the electrode patterns from the electrode pattern 5020a to the electrode pattern 502na are passed through the discharge nip 250N, the surface of the attraction member 200 of the electrode pattern area is completely discharged.

After discharge is completed, the electrode pattern 5020a reaches the power-feed nip 202N, and power feed is started from the first nipping-conveying inner roller 202a to the power feed electrodes 510 and 511, so that the surface of the attraction member 200 will be in a state capable of attracting the sheet S. At this time, in the electrode pattern area 520b, the electrode pattern 5020b has passed through the discharge nip 250N, and the electrode pattern 502nb being the end pattern is positioned before the discharge nip 250N. In the electrode pattern area 520b, the surface of the electrode pattern having passed through the discharge nip 250N is in a state where discharge has been completed.

At this time, in the electrode pattern area 520c, the electrode pattern 5020c has passed through the power feed end portion 201e, and the electrode pattern 5020nc being the end pattern is positioned upstream of the power-feed nip 202N and in a power-fed state. Therefore, similar to FIGS. 14A and 14B, the lower sagged section 200a of the attraction member 200 from the power feed end portion 201e to the power-feed nip 202N opposing to the sheet generates electrostatic attraction force.

Next, with reference to FIGS. 15A to 15C and FIGS. 16A to 16C, the sheet separation and feeding operation of the sheet attraction, separation and feeding portion 51b according to the present embodiment will be descried. FIGS. 15A to 15C illustrate the feeding operation of sheets S by the sheet attraction, separation and feeding portion 51b in time series. FIGS. 16A to 16C illustrate the sheet S feeding operation by the sheet attraction, separation and feeding portion 51b in tine series.

Now, in the state illustrated in FIGS. 14A to 14C, the length of the attraction member 200 from the power feed end portion 201e to the upstream side of the power-feed nip 202N becomes longest. At that time, regardless of the positional relationship of the three electrode pattern areas, electrostatic attraction force is constantly generated at the lower sagged section 200a of the attraction member 200 from the power feed end portion 201e to the power-feed nip 202N opposing to the sheet. Therefore, in the six steps described below, electrostatic attraction force is constantly generate at the area from the power feed end portion 201e to the upstream side of the power-feed nip 202N of the attraction member 200, as long as power is fed.

In the state illustrated in FIGS. 14A to 14C, the length of the attraction member 200 from the upstream side of the discharge nip 250N to the power feed end portion 201e becomes shortest. The length of the electrode pattern area is set so that the end pattern passes through the power feed end portion 201e at the point of time when the head pattern of the electrode pattern area has reached the discharge nip 250N. Therefore, in the following six steps, the electrode pattern area whose head pattern has reached the discharge nip 250N is in a constantly dischargeable state.

There are no conditions for restricting the positional relationship between electrode pattern areas in the six steps described below, so that the description of the position of the electrode pattern areas in the respective steps will be omitted.

The feeding operation of sheet S is composed of six steps illustrated in FIGS. 15A to 15C and FIGS. 16A to 16C in the named time-series order, which are an initial operation, an approximating operation, a contact length increasing operation, an attracting operation, a separating operation, and a conveying operation.

The initial operation illustrated in FIG. 15A is an operation for arranging the attraction member 200 to the initial position of the feeding operation, and in the present embodiment, the sagging of the attraction member 200 is gathered between the second nipping-conveying roller pair 201 and the discharge roller pair 250. In order to realize the initial state, the second nipping-conveying roller pair 201 is rotated in the direction of arrow R at a velocity faster than the first nipping-conveying roller pair 202, and sends the sagging of the attraction member 200 to the downstream side than the second nipping-conveying roller pair 201. As mentioned earlier, the load torque applying portion 251 as illustrated in FIG. 12 is disposed on the discharge inner roller 250a, which acts as a conveyance resistance with respect to the direction in which the attraction member 200 is conveyed.

Since the sagging of the attraction member 200 will not be transmitted downstream by the discharge roller pair 250, it can be gathered between the second nipping-conveying roller pair 201 and the discharge roller pair 250. At this time, the first nipping-conveying roller pair 202 can either be stopped or rotated. When the initial operation is completed, the distance between the sheet Sa and the attraction member 200 is set to be separated only by a clearance Lr between the sheet Sa and the first nipping-conveying inner roller 202a. The rotation of the second nipping-conveying roller pair 201 and the first nipping-conveying roller pair 202 can be continued when moving on from the initial operation to the next operation, or can be stopped temporarily before moving on to the next operation.

The approximating operation illustrated in FIG. 15B is an operation of deforming the attraction member 200 to be sagged downward, and to approximate the attraction surface side of the attraction member 200 to the sheet Sa. The attraction member 200 is conveyed by rotating the second nipping-conveying roller pair 201 and the first nipping-conveying roller pair 202 respectively toward the arrow R direction. At this time, by rotating the first nipping-conveying roller pair 202 faster than the second nipping-conveying roller pair 201, the lower side of the attraction member 200 is deformed in a sagged manner. At this time, the second nipping-conveying roller pair 201 can either be stopped or rotated. By the attraction member 200 being deformed, the lower side surface of the attraction member 200 can approximate the sheet Sa.

The contact length increasing operation illustrated in FIG. 15C is an operation where the above-described approximating operation is continued to thereby have the surface of the attraction member 200 contact the sheet Sa, and to increase a contact length Mc of the attraction member 200. In the present embodiment, similar to the approximating operation, the contact length Mc is increased by rotating the first nipping-conveying inner roller pair 202 in the direction of arrow R faster than the second nipping-conveying roller pair 201.

Also during this state, electrostatic attraction force acts between the attraction member 200 and the sheet Sa. However, if the contact length Mc is smaller than the predetermined length, the force in which the attraction member 200 attracts the sheet Sa will also be small, so that the conveyance resistance applied on the sheet Sa cannot be overcome, and the sheet Sa remain stored in the cassette 51a while the contact length increasing operation is continued.

The attracting operation illustrated in FIG. 16A is an operation where after the upper surface of the sheet Sa is in surface contact with the surface of the attraction member 200 by a predetermined contact length Mn, the sheet Sa is started to be conveyed by the attraction member 200. After the above-described contact length increasing operation is continued, when the contact length reaches Mn and the conveyance power to the sheet Sa is increased, the sheet Sa is started to be conveyed, overcoming the conveyance resistance.

The distance from the power feed end portion 201e to the discharge nip 250N becomes minimum in this state. As described with reference to FIG. 14B, the circumferential direction length, between the first nipping-conveying inner roller 202a and a second nipping-conveying inner roller 201a, of the attraction member 200 is set shorter than each circumferential direction length of the electrode pattern areas. That is, the sheet feeding direction lengths 530a to 530c of the respective electrode pattern areas of FIG. 17 are set shorter than the minimum distance. Therefore, none of the electrode pattern areas will contact the second nipping-conveying inner roller 201a and the discharge roller pair 250 simultaneously.

Therefore, when the electrode pattern 5020 reaches the discharge nip 250N, all the electrode patterns including the electrode pattern 502n being the end pattern will be in a dischargeable state similar to FIG. 6B. Therefore, the portion of the surface of the attraction member 200 having passed through the discharge nip 250N will be discharged.

The distance from the discharge nip 250N to the power-feed nip 202N via the extension roller 260 is always fixed, and this distance is set longer than the sheet feeding direction lengths 530a, 530b and 530c of the electrode pattern area of FIG. 17. Therefore, none of the electrode pattern areas will contact the first nipping-conveying inner roller 202a and the discharge roller pair 250 simultaneously, so that the dischargeable state as illustrated in FIG. 6B will continue until the electrode pattern 502n passes through the discharge nip 250N.

The separating operation illustrated in FIG. 16B is an operation of lifting the sheet Sa attracted to the attraction member 200 to the upper direction and separating the sheet from the subsequent sheet Sb. In the separating operation, the second nipping-conveying roller pair 201 is rotated in the direction of arrow R at a velocity (peripheral velocity and conveyance velocity) faster than the first nipping-conveying roller pair 202. Thereby, the sagging of the surface of the attraction member 200 opposing to the sheet Sa can be sent downstream than the second nipping-conveying roller pair 201.

Further, since the load torque applying portion 251 on the rotation shaft of the discharge inner roller 250a (refer to FIG. 12) acts as a resistance with respect to the conveyance direction of the attraction member 200, the first nipping-conveying roller pair 202 will continue pulling the discharge roller pair 250 driven to rotate by the attraction member 200. That is to say, tension will constantly be applied to the attraction member 200 between the first nipping-conveying roller pair 202 and the discharge roller pair 250, and the sagging of the attraction member 200 will be gathered between the second nipping-conveying roller pair 201 and the discharge roller pair 250. As a result, the sagging of the surface of the attraction member 200 opposing to the sheet Sa will be eliminated and elastically deformed to a substantially linear shape, according to which the sheet Sa attracted to the attraction member 200 is lifted and separated from the sheet (subsequent sheet) Sb below.

The conveying operation illustrated in FIG. 16C is an operation of conveying the attraction member 200 having the attraction surface of the sheet Sa deformed to a substantially linear shape, to thereby attract and convey the attracted sheet Sa to the drawing roller pair 71 disposed downstream in the sheet feeding direction. In this operation, by substantially matching the velocity (peripheral velocity and conveyance velocity) of the second nipping-conveying roller pair 201 and the first nipping-conveying roller pair 202, the attraction member 200 attracting the sheet Sa can be conveyed while maintaining the attraction surface side in a substantially linear shape.

Thereby, the sheet Sa is conveyed while being attracted by the attraction member 200 with at least a front end portion of the sheet separated from the sheet Sb below. When the front end of the sheet Sa approaches a vicinity of a curved portion of the attraction member 200 formed by the second nipping-conveying inner roller 201a between FIGS. 16B and 16C, the front end of the sheet Sa is peeled off from the attraction member 200. The peeling occurs by the reaction force of the sheet Sa becoming greater than the electrostatic attraction force that has occurred in the attraction member 200.

That is to say, in the present embodiment, the size of the electrostatic attraction force occurring in the attraction member 200 is set to a level capable of attracting the sheet Sa with a force smaller than the reaction force of the sheet Sa. By the conveying operation, the attraction member 200 moved to a position (separation position) where the sheet Sa is separated.

Since power is constantly fed to the electrode pattern in the area from the power feed end portion 201e to the power-feed nip 202N while the peeling of the sheet Sa expands, this area is constantly in an attractable state. Therefore, the sheet Sa is attracted in the area from a peel start point 300 of FIG. 16C to a point 304 at the end of the contact point between the attraction member 200 and the sheet Sa, and the attracted state is maintained until the rear end of the sheet Sa reaches the peel start point 300.

In the present embodiment, the distances 205a and 205b are set longer than the sheet feeding direction lengths 530a, 530b and 530c of the electrode pattern area illustrated in FIG. 17, so that the electrode pattern area will not contact the power-feed nip 202N and the discharge nip 250N at the same time. Therefore, the surface of the attraction member 200 can be discharged while performing the feeding operation of the sheet S.

Now, when the sheet Sa is not detected within a predetermined period of time by the sheet detection sensor 51c, the controller 70 determines that a failure has occurred during the feeding operation of the sheet Sa, and performs control so as to re-start the feeding operation from the approximating operation (FIG. 15B) again. Based on the six steps described above, only the uppermost sheet Sa is fed one at a time from the plurality of sheet S stacked on the cassette 51a. By performing these six steps repeatedly, the sheets S can be fed one at a time and successively.

The power feed end portion 201e is a point at the most downstream area in the feeding direction of the attraction member 200 within the area where the roller power feed portions 330 and 331 and the power feed electrodes 510 and 511 of the attraction member 200 contact one another. The position of the power feed end portion 201e is varied between FIGS. 15A to 16C as shown in the drawing, and as the sagging of the lower sagged section (first section) 200a between the second nipping-conveying roller pair 201 and the first nipping-conveying roller pair 202 increases, the position is moved further downstream from the conveyance nip 201N. The power feed end portion 201e and the conveyance nip 201N are most separated in the state illustrated in FIG. 16A.

The present embodiment adopting the above configuration can be discharged in a state where high voltage is not applied to the electrode pattern areas 520a to 520c, similar to the first embodiment, so that the sheet feeding operation can be continued even during the discharge operation. Therefore, throughput will not be deteriorated, and productivity can be enhanced. Further, similar effects as the other effects described in the first embodiment can be achieved.

In the present embodiment, the distances 540a to 540c between adjacent electrode pattern areas are set equal to the pitch of the respective electrode patterns within each electrode pattern area. Thereby, a fixed electrostatic attraction force can be generated throughout the whole area from the conveyance nip 201N to the power-feed nip 202N as the power-feed nip, regardless of the position of the electrode pattern areas. Since the positions of the electrode patterns 5020a, 5020b and 5020c when starting feeding is not restricted, there is no need to set a home position to the attraction member 200, to detect the home position and perform control. Therefore, feeding operation can be started regardless of the position of the electrode patterns 5020a, 5020b and 5020c. Thereby, the control performed by the controller 70 can be simplified further.

According to the present embodiment, the circumferential direction length of the attraction member 200 when the length thereof is minimized between the first nipping-conveying inner roller 202a and the second nipping-conveying inner roller 201a is set shorter than the circumferential direction lengths of the respective electrode pattern areas 520a to 520c. Thereby, power can be fed to the electrode pattern areas 520a to 520c of the attraction member 200, respectively, in a state nipped by the first and second nipping-conveying roller pairs 202 and 201 disposed at two locations. Thus, by electrostatic attraction force can be generated with greater stability at the portion of the attraction member 200 where the extended state varies (lower sagged section 200a). Thereby, the controlling the configuration of the roller pairs and the sagging of the attraction member without swinging the whole body of the sheet feeding apparatus, the sheet can be fed further upward.

While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.

This application claims the benefit of Japanese Patent Application No. 2015-002702, filed Jan. 8, 2015, which is hereby incorporated by reference herein in its entirety.

SHEET FEEDING APPARATUS AND IMAGE FORMING APPARATUS

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