WO2016185723A1

WO2016185723A1 - Sheet feeding apparatus and image forming apparatus

Info

Publication number: WO2016185723A1
Application number: PCT/JP2016/002451
Authority: WO
Inventors: Takeshi Aoyama; Yasumi Yoshida; Takaaki Aoyagi; Takashi Hiratsuka; Isao Hayashi; Hisae Shimizu; Masakazu Mori
Original assignee: Canon Inc
Current assignee: Canon Inc
Priority date: 2015-05-20
Filing date: 2016-05-19
Publication date: 2016-11-24
Anticipated expiration: 2017-11-20
Also published as: JP6478803B2; JP2016216190A

Abstract

An attraction member (200) is moved from a standby position separated from sheets (S) stored in a cassette (51a) to come into surface contact with the sheet (S) while being elastically deformed to attract the sheet to the attraction member (200), and thereafter, the attraction member is moved upward to separate the sheet from lower sheets. When conveying the sheet, rotation of a first nip-conveyance roller pair (201) and a second nip-conveyance roller pair (202) are controlled so as to move the attraction member (200) to a conveyance position conveying the uppermost sheet in a state closer to a subsequent sheet compared to when the attraction member (200) is positioned at the separation position.

Description

SHEET FEEDING APPARATUS AND IMAGE FORMING APPARATUS

The present invention relates to a sheet feeding apparatus and an image forming apparatus, and specifically, relates to an apparatus that feeds sheets using electrostatic attraction force.

Hitherto, image forming apparatuses such as copying machines and printers include sheet feeding apparatuses that feed sheets, and there is a sheet feeding apparatus adopting a friction sheet feeding system that separates and feeds an uppermost sheet from a cassette supporting a sheet bundle by using a friction force of a rubber roller and the like. In the sheet feeding apparatus adopting such friction sheet feeding system, a rubber roller is pressed against the sheet bundle and rotated, thereby conveying the uppermost sheet from the sheet bundle. During conveyance of the sheet, a so-called multiple sheet feeding may occur, where a plurality of sheets are conveyed due to the friction between the sheets. Therefore, only the uppermost sheet is made to be fed to the image forming unit by applying a conveyance resistance to the sheets excluding the uppermost sheet, through use of a separation pad or a retard roller.

Incidentally, in the sheet feeding apparatus adopting such friction separation method, a large pressure is applied to the sheets from a rubber roller during sheet feed, so that noise is generated when a sheet is rubbed against another sheet or when a sheet is slid between rubber rollers, causing a problem. Further, a large sliding noise of the sheets is generated when the separation pad or the retard roller is adopted to prevent multiple feeding of sheets. Further, the separation pad or the retard roller applies conveyance resistance to the uppermost sheet when multiple feeding of sheets has not occurred, so that noise caused by stick-slip may be generated between the separation pad or the retard roller and the sheets.

For example, Japanese Unexamined Patent Application Publication No. 2012-140224 and Japanese Unexamined Patent Application Publication No. 2012-193010 solve this problem by providing a sheet feeding apparatus where electrostatic attraction force is used to perform separation and feeding by attracting a sheet through an electric field formed at a surface of a belt, i.e., attraction member. In such sheet feeding apparatus adopting an electrostatic attraction and separation system, the sheet can be conveyed by drawing an uppermost sheet away from the sheet bundle, so that noise generated at the feeding portion can be reduced significantly.

According to the conventional sheet feeding apparatus feeding sheets using electrostatic attraction force, as taught in Japanese Unexamined Patent Application Publication No. 2012-140224, sufficient electrostatic attraction force can be generated by expanding the contact area of the belt with the sheet by loosening a sheet-attracting side of the belt. However, during conveyance of the attracted sheet, the sheet is conveyed in a state being in contact with the lower sheet, so that the sliding noise of sheets still occurs. Further, during conveyance of sheets, force acts on the lower sheet in a direction of moving together with the uppermost sheet. Therefore, a force in a disadvantageous direction from the force separating the lower sheet from the uppermost sheet (hereinafter referred to as separation force) acts on the lower sheet, so that the configuration of Japanese Unexamined Patent Application Publication No. 2012-140224 is not preferable from the viewpoint of preventing multiple feeding.

Meanwhile, in the configuration of Japanese Unexamined Patent Application Publication No. 2012-193010, the whole sheet attracting and separating unit is swung, and the cassette unit supporting the sheets is lowered, by which the uppermost sheet is separated from the sheet bundle. In the case of this sheet feeding apparatus, the amount of swinging of the sheet attraction and separating unit or the amount of lowering of the cassette portion can be adjusted in accordance with the stiffness of the sheet, so that peeling of the attracted sheet caused by the stiffness of the sheet can be prevented.

However, in a configuration where the sheet attracting and separating unit is swung, collision noise occurs between the sheet attracting and separating unit and the sheets. Further, in a configuration where the cassette unit is elevated, the cassette unit having sheets supported thereon has a large mass, and when such cassette unit is elevated each time a sheet feeding operation is performed, a large mechanism operation noise is generated. Further, during conveyance of the attracted sheet, an end portion of the sheet upstream of the portion attracted to the attraction member is pressed against the lower sheet, and sliding noise occurs.

According to one aspect of the present invention, a sheet feeding apparatus including a supporting unit configured to support sheets, a first rotary member disposed above the supporting unit, a second rotary member disposed downstream in a sheet feed direction of the first rotary member, an attraction member having an inner surface supported by the first and second rotary members, and attracting a sheet supported on the supporting unit, and a control unit controlling the first and second rotary members such that the attraction member is moved to an attraction position where the attraction member is abutted against an uppermost sheet supported on the supporting unit and electrically attracting the uppermost sheet, a separation position where an attracted uppermost sheet is deflected and separated from a subsequent sheet, and a conveyance position where the separated uppermost sheet is conveyed in a state approaching the subsequent sheet than in a state where the attraction member is positioned at the separation position.

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

Fig. 1 is a view of a schematic configuration of an image forming apparatus including a sheet feeding apparatus according to a first embodiment of the present invention.

Fig. 2 is a view illustrating a configuration of the sheet feeding apparatus.

Fig. 3A is a view of a surface of an attraction member disposed in a sheet attraction, separation and feeding unit of the sheet feeding apparatus.

Fig. 3B is a perspective view of the attraction member disposed in the sheet attraction, separation and feeding unit.

Fig. 3C is a cross-sectional view of a power feed unit.

Fig. 3D is a view of a concept of an electrostatic attraction force operating between the attraction member and the sheet.

Fig. 4 is a view illustrating the configuration of a sheet thickness detection unit disposed in the sheet feeding apparatus, and a detection principle.

Fig. 5 is a control block diagram of the sheet feeding apparatus.

Fig. 6A is an explanatory view of an initial operation of the sheet attraction, separation and feeding unit.

Fig. 6B is an explanatory view of an approaching operation of the sheet attraction, separation and feeding unit.

Fig. 6C is an explanatory view of a contact area increasing operation of the sheet attraction, separation and feeding unit.

Fig. 6D is an explanatory view of an attracting operation of the sheet attraction, separation and feeding unit.

Fig. 6E is an explanatory view of a separating operation of the sheet attraction, separation and feeding unit.

Fig. 6F is an explanatory view of a re-approaching operation of the sheet attraction, separation and feeding unit.

Fig. 6G is an explanatory view of a conveying operation of the sheet attraction, separation and feeding unit.

Fig 6H is an explanatory view of a standby operation of the sheet attraction, separation and feeding unit.

Fig. 7 is a timing chart during separation and feeding of sheets of the sheet attraction, separation and feeding unit.

Fig. 8 is an explanatory view of a mechanism of separating a sheet fed by the sheet attraction, separation and feeding unit.

Fig. 9 is a control flowchart of the sheet separation control unit disposed in the sheet feeding apparatus.

Fig. 10 is a timing chart of a driving unit during separating operation of the sheet attraction, separation and feeding unit.

Fig. 11 is an explanatory view of change of sagged amount when the separation position of the attraction member is changed.

Fig. 12 is an explanatory view of a data table stored in the sheet separation control unit.

Fig. 13 is a flowchart illustrating a separation position change operation of the attraction member by the sheet separation control unit.

Fig. 14 is a timing chart during separation and feeding of the sheet in a sheet attraction, separation and feeding unit disposed in a sheet feeding apparatus according to a second embodiment of the present invention.

Fig. 15 is an explanatory view of a configuration of a sheet feeding apparatus according to a third embodiment.

Fig. 16 is a control block diagram of the sheet feeding apparatus.

Fig. 17A is an explanatory view of an initial operation of the sheet attraction, separation and feeding unit.

Fig. 17B is an explanatory view of an approaching operation of the sheet attraction, separation and feeding unit.

Fig. 17C is an explanatory view of an attracting operation of the sheet attraction, separation and feeding unit.

Fig. 17D is an explanatory view of a separating operation of the sheet attraction, separation and feeding unit.

Fig. 17E is an explanatory view of a re-approaching operation of the sheet attraction, separation and feeding unit.

Fig. 17F is an explanatory view of a conveying operation of the sheet attraction, separation and feeding unit.

Fig. 17G is an explanatory view of a standby operation of the sheet attraction, separation and feeding unit.

Fig. 18 is a timing chart during separation and feeding of the sheet in the sheet attraction, separation and feeding unit.

Now, embodiments for carrying out the present invention will be described in detail with reference to the drawings. Fig. 1 is a view of a general arrangement of an image forming apparatus having a sheet feeding apparatus according to a first embodiment of the present invention.

As illustrated in Fig. 1, an image reading unit 41 having an image sensor and the like that irradiates light onto a document placed on a platen glass and converts reflected light into digital signals is disposed at an upper portion of an image forming apparatus body (hereinafter referred to as apparatus body) 100A of an image forming apparatus 100. The document from which image is to be read is conveyed onto the platen glass by an automatic document feeder 41a. The apparatus body 100A further includes an image forming unit 55,

sheet feeding apparatuses

51 and 52 that feed sheets S to the image forming unit 55, and a sheet reverse unit 59 that reverses the sheet S and conveys the sheet S to the image forming unit 55.

The image forming unit 55 includes an exposing unit 42, four process cartridges 43 (43y, 43m, 43c and 43k) forming toner images of four colors, which are yellow (Y), magenta (M), cyan (C) and black (Bk). Further, the image forming unit 55 includes an intermediate transfer unit 44 disposed above the process cartridges 43, a secondary transfer unit 56, and a fixing unit 57.

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

The intermediate transfer unit 44 includes a belt driving roller 26, an intermediate transfer belt 25 stretched between a secondary transfer inner roller 56a and the like, and primary transfer rollers 27 (27y, 27m, 27c and 27k) abutted against the intermediate transfer belt 25 at positions opposing to the photosensitive drums 21. As described later, toner images having negative polarity formed on the photosensitive drums 21 can be superimposed and transferred onto the intermediate transfer belt 25 through application of a transfer bias of positive polarity to the intermediate transfer belt 25 via the primary transfer roller 27. Thereby, a full-color image is formed on the intermediate transfer belt 25.

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

The fixing unit 57 includes a fixing roller 57a and a fixing backup roller 57b. The toner image on the sheet S is pressed, heated, and fixed onto the sheet S by having the sheet S nipped and conveyed between the fixing roller 57a and the fixing backup roller 57b.

Sheet feeding apparatuses

51 and 52 respectively include

cassettes

51a and 52a, i.e., storage portions storing sheets S, and sheet attraction, separation and feeding

units

51b and 52b attracting one sheet S at a time using static electricity from the sheets S stored in the

cassettes

51a and 52a.

The apparatus body 100A has a pre-secondary-transfer conveyance path 103 that conveys a sheet S fed from the

cassettes

51a and 52a to the secondary transfer unit 56, and a pre-fixing conveyance path 104 that conveys the sheet S having been conveyed to the secondary transfer unit 56 to the fixing unit 57. Further, the apparatus body 100A has a post-fixing conveyance path 105 that conveys the sheet S having been conveyed to the fixing unit 57 further to a switching member 61, and a sheet discharge path 106 that conveys the sheet S having been conveyed to the switching member 61 further to a sheet discharge portion 58. Further, the apparatus body 100A has a re-conveyance path 107 that conveys the sheet S having been reversed by the sheet reverse unit 59 to the image forming unit 55 again, so as to form an image on a rear surface of the sheet S having an image already formed on one side via the image forming unit 55.

Next, we will describe an image forming operation of the image forming apparatus 100 having the above-described configuration. When the image forming operation is started, at first, the exposing unit 42 irradiates laser beams to the surface of the photosensitive drums 21 based on image information from a personal computer and the like not shown. At this time, the surfaces of the photosensitive drums 21 are charged evenly to predetermined polarity and potential by the charging rollers 22, and when laser beams are irradiated to the drums, charges in areas irradiated with laser beams are attenuated, and electrostatic latent images are formed on the surface of the photosensitive drums.

Thereafter, the electrostatic latent images are developed using yellow (Y), magenta (M), cyan (C) and black (Bk) toners supplied from the respective developing rollers 23, and the electrostatic latent images are developed as toner images. Then, the toner images of respective colors are transferred sequentially to the intermediate transfer belt 25 by primary transfer biases applied to the respective primary transfer rollers 27, by which a full-color toner image is formed on the intermediate transfer belt 25.

Meanwhile, in parallel with the above-mentioned toner image forming operation, the

sheet feeding apparatuses

51 and 52 separate and feed only one sheet S at a time from the

cassettes

51a and 52a via the sheet attraction, separation and feeding

units

51b and 52b. Thereafter, the sheet S reaches the drawing

roller pairs

51c, 51d, 52c and 52d. Further, the sheet S nipped by the drawing

roller pairs

51c, 51d, 52c and 52d passes through sheet thickness detection via a sheet thickness detection unit 53 described later, and is sent into the pre-secondary-transfer conveyance path 103. Then, the sheet is abutted against a stopped

registration roller pair

62a and 62b, and the

registration roller pair

62a and 62b adjust the front end position of the sheet at a downstream end in the sheet feeding direction.

Next, in the secondary transfer unit 56, the

registration roller pair

62a and 62b is driven at a timing of matching the full-color toner image on the intermediate transfer belt and the position of the sheet S. Thereby, the sheet S is conveyed to the secondary transfer unit 56, and the full-color toner image is collectively transferred onto the sheet S via the secondary transfer bias applied to the secondary transfer outer roller 56b in the secondary transfer unit 56.

The sheet S on which the full-color toner image has been transferred is conveyed to the fixing unit 57, and heat and pressure is applied to the sheet in the fixing unit 57, by which the various colored toners are melted, mixed, and fixed onto the sheet S as a full-color image. Thereafter, the sheet S on which the image has been fixed is discharged via the sheet discharge portion 58 disposed downstream of the fixing unit 57. In order to form images on both sides of the sheet, the conveyance direction of the sheet S is switched by the switching member 61 toward the sheet reverse unit 59 where the sheet is reversed, and the sheet S is conveyed to the image forming unit 55 again.

Next, the configuration of a sheet feeding apparatus 51 according to the present embodiment will be described with reference to Fig. 2. Fig. 2 is a schematic diagram illustrating a cross-sectional view of a vicinity of the sheet feeding apparatus 51. As described earlier, the sheet feeding apparatus 51 includes a sheet attraction, separation and feeding unit 51b attracting, separating and conveying one sheet S at a time from the sheets stored in the cassette 51a using static electricity, and a

drawing roller pair

51c and 51d. Further, the sheet feeding apparatus 51 includes a conveyance guide 51e guiding the sheet S separated one by one and conveyed by the sheet attraction, separation and feeding unit 51b to the

drawing roller pair

51c and 51d positioned obliquely upward of the sheet attraction, separation and feeding unit 51b. The sheet thickness detection unit 53 is arranged on the pre-secondary-transfer conveyance path 103 between the sheet feeding apparatus 51 and the secondary transfer unit 56.

The cassette 51a, i.e., a supporting unit on which sheets are supported, includes an elevating unit 301 that elevates an elevating intermediate plate 301a on which sheets S are supported, and a sheet surface height detection unit 302 that detects an upper surface position of sheets S supported on the intermediate plate 301a. The elevating unit 301 includes a lifter 301b disposed pivotally below the intermediate plate 301a, and the position of the intermediate plate 301a and an uppermost sheet Sa supported on the intermediate plates 301a is changed by the pivot angle of the lifter 301b.

The sheet surface height detection unit 302 is arranged above the intermediate plate 301a, and includes a sensor flag 302a and a photosensor 302b. The sensor flag 302a is rotatably supported by a support unit not shown, and has one end positioned to contact an upper surface of the uppermost sheet Sa, and the other end positioned to shield the photosensor 302b.

When the upper surface of the uppermost sheet Sa is positioned at a predetermined height, the sensor flag 302a is rotated, and the photosensor 302b is shielded. A control unit 70 illustrated in Fig. 5 described later detects the upper surface position of the uppermost sheet Sa by detecting the shielded state of the photosensor 302b. Then, the control unit 70 controls the operation of the elevating unit 301 so that the upper surface of the uppermost sheet Sa is always detected by the sheet surface height detection unit 302, and maintains the position of the intermediate plate 301a so that the upper surface height of the uppermost sheet Sa is positioned substantially in a fixed position. As a result, clearances Lr1 and Lr2 between a first nip-conveyance roller pair 201 or a second nip-conveyance roller pair 202 and the upper surface of the uppermost sheet Sa are maintained substantially constantly. In the present embodiment, Lr1 is greater than Lr2.

The sheet attraction, separation and feeding unit 51b includes the first nip-conveyance roller pair 201, the second nip-conveyance roller pair 202, and a feed unit frame 206 supporting the first nip-conveyance roller pair 201 and the second nip-conveyance roller pair 202. Further, the sheet attraction, separation and feeding unit 51b includes an endless attraction member 200 having flexibility nipped and conveyed by the first nip-conveyance roller pair 201 and the second nip-conveyance roller pair 202. Further, it includes a first driving unit (second driving unit) 203 that generates driving force in the first nip-conveyance roller pair 201, and a second driving unit (first driving unit) 204 that generates driving force in the second nip-conveyance roller pair 202. The sheet attraction, separation and feeding unit 52b disposed on the sheet feeding apparatus 52 has a similar configuration as the sheet attraction, separation and feeding unit 51b in the sheet feeding apparatus 51, so that the description will be omitted.

The first nip-conveyance roller pair 201 is arranged downstream in the sheet feeding direction of the second nip-conveyance roller pair 202, and composed of a first nip-conveyance inner roller 201a and a first nip-conveyance outer roller 201b. The first nip-conveyance inner roller 201a is arranged on an inner side of an attraction member 200, supported rotatably by axial support members not shown having fixed positions, and a drive from a first driving unit 203 is transmitted to the first nip-conveyance inner roller 201a via a drive transmission unit not shown. Further, the first nip-conveyance inner roller 201a is arranged with a clearance Lr1 with respect to the upper surface of the uppermost sheet Sa.

The first nip-conveyance outer roller 201b, i.e., driven rotary member, is arranged on an outer side of the first nip-conveyance inner roller 201a and nipping the endless belt-shaped attraction member 200. A first pressing spring 201c is connected to an axial support portion not shown, and the first nip-conveyance outer roller 201b is biased toward an axial center direction of the first nip-conveyance inner roller 201a by the first pressing spring 201c and nips the attraction member 200 together with the first nip-conveyance inner roller 201a.

The second nip-conveyance roller pair 202 is composed of a second nip-conveyance inner roller 202a and a second nip-conveyance outer roller 202b. The second nip-conveyance inner roller 202a is arranged on an inner side of the attraction member 200, similar to the first nip-conveyance inner roller 201a, and axially supported in a rotatable manner by an axial support member whose position of arrangement is fixed. Further, a driving force is transmitted through a drive transmission unit not shown to the second nip-conveyance inner roller 202a from a second driving unit 204. Further, the second nip-conveyance inner roller 202a is arranged with a clearance Lr2 with respect to the upper surface of the uppermost sheet Sa.

The second nip-conveyance outer roller 202b, i.e., driven rotary member, is arranged on an outer side of the second nip-conveyance inner roller 202a and nipping the attraction member 200, similar to the first nip-conveyance outer roller 201b, and axially supported in a rotatable manner by an axial support member not shown. A second pressing spring 202c is connected to an axial support portion not shown. The second nip-conveyance outer roller 202b is biased toward an axial center direction of the second nip-conveyance inner roller 202a by the second pressing spring 202c and nips the attraction member 200 together with the second nip-conveyance inner roller 202a. As described, the attraction member 200 has its inner surface supported by the second nip-conveyance inner roller 202a, i.e., first rotary member, and the first nip-conveyance inner roller 201a, i.e., second rotary member.

In the present embodiment, the clearance Lr1 between the first nip-conveyance inner roller 201a and the upper surface of the uppermost sheet Sa and the clearance Lr2 between the second nip-conveyance inner roller 202a and the upper surface of the uppermost sheet Sa are set so that Lr1 > Lr2, as described earlier. Therefore, the sheet attraction, separation and feeding unit 51b is arranged at an arrangement angle θu with respect to the sheet S. The arrangement of the sheet attraction, separation and feeding unit 51b as described above enables to attract and separate the sheet S by lifting, so that separation force to the sheet S described later can be generated effectively. The relationship between Lr1 and Lr2 is not restricted to Lr1 > Lr2, as long as the sheet S can be attracted and separated by lifting.

In the present embodiment, the endless attraction member 200 has its inner side supported by the first nip-conveyance inner roller 201a and the second nip-conveyance inner roller 202a. Further, a circumference of the attraction member 200 is longer than a length of [twice a distance between rotation centers of the first and second nip-conveyance

inner rollers

201a and 202a + half a length of circumferential surfaces of each of the

rollers

201a and 202a]. In other words, the circumference of the attraction member 200 is set to have a margin (be loosened) compared to a minimum winding circumference of the sheet attraction, separation and feeding unit 51b, so that a sheet contact area Mn as illustrated in Fig. 6D for realizing a sheet attraction force necessary for attraction and separation can be ensured.

Since the attraction member 200 has the above-described length, the member 200 can be sagged downward while rotating (moving) along with the rotation of the first and second nip-conveyance

inner rollers

201a and 202a. Thus, even though there are clearances Lr1 and Lr2 existing between the first nip-conveyance inner roller 201a or the second nip-conveyance inner roller 202a and the uppermost sheet Sa of the sheets S supported on the intermediate plate 301a, the attraction member 200 can be in contact with the uppermost sheet Sa.

Now, in the present embodiment, during attraction and conveyance of a sheet by the attraction member 200, the sheet is attracted to the attraction member 200 by static electricity, and the attraction member 200 is elastically deformed and drawn upward, so that the sheets are not rubbed against one another. In this manner, the attraction member 200 is elastically deformed and drawn upward, by which a sheet is separated from other sheets.

A positive voltage supplying unit 205a to which positive voltage is supplied and a negative voltage supplying unit 205b to which negative voltage is supplied are electrically connected to the attraction member 200. Then, an electrostatic attraction force attracting the sheet S is generated in the attraction member 200 by the positive and negative voltages supplied from the positive voltage supplying unit 205a, i.e., first power supply, and the negative voltage supplying unit 205b, i.e., second power supply.

A sheet separation control unit 210 included in the control unit 70 illustrated in Fig. 5 described later is connected to each of the first driving unit 203 and the second driving unit 204. Then, the first and

second driving units

203 and 204 transmit driving forces to the first and second nip-conveyance

inner rollers

201a and 202a. At this time, the first driving unit 203 and the second driving unit 204 respectively receive individual speed command pulse strings transmitted from the sheet separation control unit 210 and perform cooperative operations, by which an optimum separating operation of sheet S in accordance with the classification of the sheet S being fed is performed. The details of this operation will be described later.

Next, a detailed configuration of the attraction member 200 and a generation principle of attraction force by which the attraction member 200 attracts the sheet S will be described with reference to Fig. 3. FIG. 3A is a view of a surface of the attraction member 200, Fig. 3B is a perspective view of the attraction member 200, Fig. 3C is a cross-sectional view of a power feed unit of the attraction member 200, and Fig. 3D is a view showing a concept of the electrostatic attraction force acting between the attraction member 200 and the sheet S.

As illustrated in Figs. 3A through 3D, the attraction member 200 includes a base layer 200c, a positive electrode 200a, i.e., first electrode, and a negative electrode 200b, i.e., second electrode. The positive electrode 200a and the negative electrode 200b are comb-tooth shaped, and arranged alternately within the base layer 200c. In the present embodiment, the base layer 200c is a polyimide, i.e., dielectric with a volume resistivity of 10⁸ Ω cm or greater, having a layer thickness of approximately 100 μm. The positive electrode 200a and the negative electrode 200b are conductors with a volume resistivity of 10⁶ Ω cm or smaller, and use copper with a layer thickness of approximately 10 μm.

According to the present embodiment, the material, the thickness and the like of the attraction member 200 is controlled to realize a moderate elasticity, so that the attraction member 200 is sagged downward and realizes a barrel shape when the attraction member 200 approaches the sheet S, as described later. On an inner circumferential surface of the attraction member 200 facing the first nip-conveyance inner roller 201a and the second nip-conveyance inner roller 202a are provided exposed

areas

200d and 200e where the positive and

negative electrodes

200a and 200b are exposed. A positive contact 206a connected to the positive voltage supplying unit 205a is in contact with an exposed area 200d of the positive electrode 200a, and a negative contact 206b connected to the negative voltage supplying unit 205b is in contact with an exposed area 200e of the negative electrode 200b.

In the present embodiment, a positive voltage of approximately +1 kV is applied to the positive electrode 200a, and a negative voltage of approximately -1 kV is applied to the negative electrode 200b. Further, the positive and

negative contacts

206a and 206b respectively adopt a structure where a carbon brush is crimped at the tip of a metal plate material having elasticity, and the carbon brush is in contact with either the exposed

area

200d or 200e of the positive electrode 200a or the negative electrode 200b. Since the positive contact 206a and the negative contact 206b have elasticity, they can follow the movement of the attraction member 200 whose cross-sectional shape is changed moment by moment, maintaining contact, so that stable power supply is enabled.

When positive and negative voltages are applied to the positive and

negative electrodes

200a and 200b, as illustrated in Fig. 3D, a non-uniform electric field is formed at a vicinity of the surface of the attraction member 200 by the positive and

negative electrodes

200a and 200b to which voltage is applied. When the attraction member 200 with such non-uniform electric field approaches a sheet S, dielectric polarization occurs to the surface layer of the sheet, i.e., a dielectric, and electrostatic attraction force occurs between the attraction member 200 and the sheet S by Maxwell's stress.

As described earlier, the present embodiment is configured to generate electrostatic attraction force between the attraction member 200 and the sheet S by disposing electrodes within the attraction member 200 and applying voltages to the electrodes. However, the present invention is not restricted to such configuration, and other units can be used as long as electrostatic attraction force is generated. For example, the attraction force of the sheet S can be achieved by abutting a charging roller against a surface layer of an insulation attraction member at a position upstream in a conveyance direction of an attraction portion of the sheet S, and applying voltage to the charging roller to charge the surface layer of the attraction member from the outside. At this time, the power supply connected to the charging roller can be an AC power supply or a DC power supply.

Next, a configuration and a principle of sheet thickness detection of the sheet thickness detection unit 53 will be described with reference to Fig. 4. In the present embodiment, the sheet thickness detection unit 53 uses ultrasonic waves to detect sheet thickness and determine a stiffness of the sheet. The sheet thickness detection unit 53, i.e., stiffness detector, includes an ultrasonic element unit 53a for wave transmission, an ultrasonic element unit 53b for wave reception, and a phase difference operation circuit 53c connected to the

units

53a and 53b.

Ultrasonic waves USW1 are transmitted from the ultrasonic element unit 53a for wave transmission toward the surface of the sheet S. When the ultrasonic waves USW1 reach the surface of the sheet S, the waves are divided into an ultrasonic wave component USW2 penetrating the sheet S and a component USW3 reflected by the sheet S, and the reflected ultrasonic waves USW3 are received by the ultrasonic element unit 53b for wave reception. On the other hand, the ultrasonic wave component USW2 having penetrated the sheet is reflected by the rear surface of the sheet S, and the reflected ultrasonic waves are received as USW4 by the ultrasonic element unit 53b for wave reception.

Since the propagation path lengths from the ultrasonic element unit 53a for wave transmission to the ultrasonic element unit 53b for wave reception differ between ultrasonic waves USW3 and ultrasonic waves USW4, a phase difference Δt occurs between the timings at which the waves are received by the ultrasonic element unit 53b for wave reception. Actually, since the path lengths differ for approximately double the distance of thickness St of the sheet S, through measurement of the phase difference Δt at the phase difference operation circuit 53c, the thickness St of the sheet S can be detected (St = c × Δt/2, when sonic speed is represented by c).

The present embodiments adopts a sheet thickness detection method utilizing ultrasonic waves as the sheet thickness detection unit 53, but the present embodiment is not restricted to this example. For example, the embodiment can adopt a method that detects the sheet thickness St by irradiating light to the sheet S and detecting the amount of penetrated light. Further, though the object differs according to the conventional configuration, there is a case where a unit for detecting or setting a surface property or thickness of the sheet S is provided to change a secondary transfer condition or image fixing condition in accordance with the classification of the sheet S. In that case, the information from such detection or setup unit can be used in substitution. For example, it is possible to utilize the sheet classification information set up by the user when a print job is entered, without disposing sensors.

Next, a control unit configuration of the sheet feeding apparatus 51 will be described with reference to Fig. 5. Fig. 5 is a control block diagram of the sheet feeding apparatus 51 according to the present embodiment. As illustrated in Fig. 5, in addition to the elevating unit 301, the positive voltage supplying unit 205a, the negative voltage supplying unit 205b, the sheet surface height detection unit 302 and the sheet thickness detection unit 53 described earlier, a timer 71, a cassette opening/closing detection sensor 74 and so on are connected to the control unit.

Further, a sheet separation control unit 210 is included as a subsystem in the control unit 70, and the first and

second driving units

203 and 204 are connected to the sheet separation control unit 210. In the example, the sheet separation control unit 210 includes a built-in storage area, i.e., storage unit, 210A not shown. The built-in storage area 210A stores a data table Mt1 determining a sheet separation condition described later and a data table Mt2 determining a conveyance position, based on a sheet thickness St detected by the sheet thickness detection unit 53.

Then, the sheet separation control unit 210 determines the sheet separation condition and the conveyance condition based on the detected sheet thickness St and the data tables Mt1 and Mt2, and transmits speed command pulse strings to the first and

second driving units

203 and 204 in accordance with the determined sheet separation condition and conveyance condition. The details of the method for controlling the sheet separation control unit 210 will be described later.

Next, a sheet separation and feeding operation of the sheet attraction, separation and feeding unit 51b will be described with reference to Figs. 6A through 6H. Figs. 6A through 6H are schematic diagrams showing the operation in which the sheet S is fed by the sheet attraction, separation and feeding unit 51b in time series. The separation and feeding operation of the sheet S is composed, in time series order, of eight steps illustrated in Figs. 6A through 6H, which are an initial operation, an approaching operation, a contact area increasing operation, an attracting operation, a separating operation, a re-approaching operation, a conveying operation, and a standby operation. These steps will now be described in order. In the present embodiment, the positive and negative

voltage supplying units

205a and 205b are connected to the attraction member 200 in all the operation steps, and attraction force is constantly generated.

An initial operation illustrated in Fig. 6A is an operation of having the attraction member 200 stand-by at an initial operation of feed operation, i.e., standby position. In the present embodiment, during the initial operation, the sheet separation control unit 210 separates the attraction member 200 from the uppermost sheet Sa by a predetermined clearance Ln.

An approaching operation illustrated in Fig. 6B is an operation of sagging the attraction member 200 downward, that is, moving the sagged portion downward, to thereby deform the attraction member into a barrel shape, and cause an attraction surface side of the attraction member 200 to approach the uppermost sheet Sa. During this operation, the sheet separation control unit 210 rotates the second nip-conveyance roller pair 202 in an arrow direction at a rotational speed U by the second driving unit 204. At the same time, the control unit 210 either stops the first nip-conveyance roller pair 201 or rotates the first nip-conveyance roller pair 201 by the first driving unit 203 at a speed slower than the rotational speed U, by which the attraction member 200 is conveyed in a direction of arrow Ad, and the attraction member 200 is deformed into a barrel shape. This deformation of the attraction member 200 into a barrel shape causes the surface of the attraction member 200 to contact the uppermost sheet Sa.

A contact area increasing operation illustrated in Fig. 6C is an operation of increasing a contact area Mc between a surface of the attraction member 200 having been moved (displaced) to a position for attracting sheets (attraction position) and the uppermost sheet Sa by continuing the above-described approaching operation. During this operation, the sheet separation control unit 210 rotates the second nip-conveyance roller pair 202 by the second driving unit 204 in the direction of the arrow at rotational speed U, similar to the approaching operation. At the same time, the control unit 210 either stops the first nip-conveyance roller pair 201 or rotates the first nip-conveyance roller pair 201 by the first driving unit 203 at a speed slower than the rotational speed U to convey the attraction member 200 in the direction of arrow Ad, and increases the contact area Mc.

The sheet separation control unit 210 continues this contact area increasing operation until the contact area Mc becomes equivalent to a predetermined sheet contact area Mn, and stops the first and

second driving units

203 and 204. It is possible to provide a detection unit for directly detecting the size of the sheet contact area Mc, but in the present embodiment, the size of the sheet contact area Mc is alternately detected by a difference in the conveyance amount between first and second nip-conveyance roller pairs 201 and 202 based on measurement of time by the timer 71.

An attracting operation illustrated in Fig. 6D is an operation of attracting the uppermost sheet Sa to the attraction member 200 at a position, i.e., attraction position, where the upper surface of the uppermost sheet Sa and the surface of the attraction member 200 are in surface contact with a predetermined sheet contact area Mn. When the uppermost sheet Sa and the attraction member 200 contact each other, an electrostatic attraction force acts between the attraction member 200 and the sheet S, since voltage is applied to the attraction member 200 through positive and negative

voltage supplying units

205a and 205b, as mentioned earlier. Then, in the state where the attraction member 200 is in surface contact with the uppermost sheet Sa via a predetermined sheet contact area Mn, the uppermost sheet Sa is attracted to the attraction member 200.

A separating operation illustrated in Fig. 6E is an operation of deflecting the uppermost sheet Sa attracted to the attraction member 200 upward by the attraction member 200 and thereby separating the sheet Sa from a lower sheet Sb, i.e., subsequent sheet. During this operation, the sheet separation control unit 210 rotates the first nip-conveyance roller pair 201 in the direction of the arrow at rotational speed U by the first driving unit 203. At the same time, the control unit 210 either stops the second nip-conveyance roller pair 202 or rotates the second nip-conveyance roller pair 202 slower than the rotational speed U by the second driving unit 204, to reduce the sagging and convey the attraction member 200 in the arrow Au direction.

In other words, based on the separating operation, the attraction member 200 deforms the uppermost sheet Sa for a distance Lb from a front end of the sheet Sa to a base of deflection Pb illustrated in Fig. 8 descried later, and for a distance Lh from a front end of the supported sheet Sa to a front end of the sheet having been drawn upward. Thereby, the uppermost sheet Sa is moved to a position, i.e., separation position, separated from a lower sheet Sb. In the present embodiment, the sheet separation control unit 210 changes the separation position in accordance with a sheet thickness detected by the sheet thickness detection unit 53. The details will be described later.

A re-approaching operation illustrated in Fig. 6F is an operation of deforming the attraction member 200 into a barrel shape by sagging the member downward prior to the conveying operation, and reducing a deflection angle of the uppermost sheet Sa having been deflected upward by the separating operation. During this operation, the sheet separation control unit 210 rotates the first nip-conveyance roller pair 201 in the arrow direction at rotational speed U by the first driving unit 203. At the same time, the control unit 210 rotates the second nip-conveyance roller pair 202 in the arrow direction at a rotational speed 2U faster than the rotational speed U by the second driving unit 204, to convey the attraction member 200 in the direction of arrow Ad, and deform the attraction member 200 into a barrel shape. In the present embodiment, the sheet separation control unit 210 changes the amount of deformation, i.e., moving amount, of the attraction member 200 according to the sheet thickness detected by the sheet thickness detection unit 53. The details will be described later.

A conveying operation illustrated in Fig. 6G is an operation of having the uppermost sheet Sa attracted to the attraction member 200 sagged by the re-approaching operation attracted and fed to the

drawing roller pair

51c and 51d, i.e., sheet conveying unit, downstream in sheet feeding direction. During this conveying operation, the attraction member 200 is deformed into a barrel shape, so that a deflection angle θs of the uppermost sheet Sa having been deflected by the attraction member 200 is in a reduced state than during the separating operation. It is most preferable that the state of the uppermost sheet Sa at this position (conveyance position) has the deflection angle θs of 0, and that the uppermost sheet Sa is not in contact with the lower sheet Sb in a range where the sheet Sa is attracted to the attraction member 200.

During this operation, the sheet separation control unit 210 respectively rotates the first nip-conveyance roller pair 201 and the second nip-conveyance roller pair 202 at a rotational speed U to the direction of the arrow. Thereby, the attraction member 200 attracting the sheet Sa is conveyed while maintaining the shape of the attraction surface side. As a result, the uppermost sheet Sa attracted to the attraction member 200 is conveyed to the arrow A direction while maintaining a deflection angle θ of approximately 0 and a state where the sheet Sa does not contact the lower sheet Sb in a range attracted by the attraction member 200. Of course, the effect of the present invention of reducing the noise generated during conveyance of a sheet described later can be achieved even if the deflection angle θs is not 0 and the uppermost sheet Sa is in contact with the lower sheet Sb in a range not attracted by the attraction member 200.

Thereafter, when a front end of the uppermost sheet Sa approaches a vicinity of a curvature portion of the attraction member 200 formed by the first nip-conveyance inner roller 201a, the front end of the uppermost sheet Sa detaches from the attraction member 200. This detachment occurs by a reaction force generated in the sheet Sa by the curvature portion of the attraction member 200 becoming greater than the electrostatic attraction force generated in the attraction member 200. In other words, according to the present embodiment, the reaction force generated in the sheet Sa by the curvature portion of the attraction member 200 is set to be greater than the electrostatic attraction force generated in the attraction member 200. That is, the attraction member moves to a position where the uppermost sheet Sa is alienated (alienation position) by the conveying operation.

After the front end is detached from the attraction member 200, the detached area of the uppermost sheet Sa is increased from the front end, but the sheet Sa is attracted to the attraction member 200 at a trailing edge area, i.e., upstream area, in the sheet feeding direction of the sheet Sa. Thereby, the sheet Sa is continuously conveyed by the attraction member 200, abutted against the conveyance guide 51e, and thereafter, deflected and moved along the conveyance guide 51e, and transferred to the

drawing roller pair

51c and 51d.

At this time, the attraction member 200 is sagged downward than the separation position and deformed in a barrel shape, so that the deflection angle θg of the sheet Sa by the attraction member 200 and the conveyance guide 51e is reduced. A small deflection angle θg is effective in preventing the trailing edge area of the sheet Sa from being detached from the attraction member 200. However, in a configuration where the rear end area of the sheet Sa is detached from the attraction member 200 even in the conveyance position, the attraction member 200 can be loosened further downward from the conveyance position before the sheet Sa is deflected by the attraction member 200 and the conveyance guide 51e.

A standby operation illustrated in Fig. 6H is an operation of returning the attraction member 200 to the initial operation position. During this operation, the sheet separation control unit 210 rotates the first nip-conveyance roller pair 201 in the arrow direction at rotational speed U by the first driving unit 203. At the same time, the control unit 210 either stops the second nip-conveyance roller pair 202 or rotates the second nip-conveyance roller pair 202 at a speed slower than the rotational speed U by the second driving unit 204, to thereby reduce the sagging and convey the attraction member 200 in the direction of the arrow Au. Only one uppermost sheet Sa is fed at a time from the multiple sheets S supported on the cassette 51a by carrying out the eight steps mentioned above. The sheets S can be fed one at a time and continuously by repeating these eight steps.

Fig. 7 is a timing chart of the initial operation, the approaching operation, the contact area increasing operation, the attracting operation, the separating operation, the re-approaching operation, the conveying operation and the standby operation illustrated in Figs. 6A through 6H. In Fig. 7, b represents a loosening amount of the attraction member 200, u1 represents a conveyance speed of the first nip-conveyance roller pair 201, and u2 represents a conveyance speed of the second nip-conveyance roller pair 202. Further, vp represents a positive voltage supplied from the positive voltage supplying unit 205a, and vn represents a negative voltage supplied from the negative voltage supplying unit 205b.

In Fig. 7, a section from time T0 to T1 denoted by (a) is an initial operation section, and at this time, conveyance speed u1 and conveyance speed u2 are set to 0, the supply voltage vp is set to +V, and the supply voltage vn is set to -V. Further, a loosening amount b of the attraction member 200 is set to a smallest value B1. In the present embodiment, the supply voltage vp and the supply voltage vn are set to +V and -V throughout all feeding operations of the sheet S, and they are not changed.

Further, a section from time T1 to T2 denoted by (b) is an approaching operation section. At this time, the conveyance speed u1 is set to 0 and the conveyance speed u2 is set to speed U. Based on the difference between conveyance speeds, the loosening amount b is increased. The speed U is a speed determined based on productivity and the like of the image forming apparatus, and in the present embodiment, U equals 200 mm/s. A section from time T2 to T3 denoted by (c) is a contact area increasing operation section, and continuously from time T1, the conveyance speed u1 is set to 0 and the conveyance speed u2 is set to speed U. Based on the difference between the conveyance speeds, the loosening amount b is increased, and reaches a maximum value B3. A section from time T3 to T4 denoted by (d) is an attracting operation section, where the conveyance speeds u1 and u2 are both set to 0. In that case, no difference occurs between conveyance speeds, so that the loosening amount b is fixed to B3.

A section from time T4 to T5 denoted by (e) is a separating operation section, where the conveyance speed u1 is set to U and the conveyance speed u2 is set to 0. Based on the difference between conveyance speeds, the loosening amount b is reduced to B1. A section from time T5 to T6 denoted by (f) is a re-approaching operation section, where the conveyance speed u1 is set to speed U and the conveyance speed u2 is set to speed 2U. Based on the difference between the conveyance speeds, the loosening amount b is increased again, and reaches B2 (B1 < B2 < B3). The speed 2U is a speed determined based on productivity and the like of the image forming apparatus similar to speed U, and the present embodiment, 2U equals 400 mm/s.

A section from time T6 to T7 denoted by (g) is a conveying operation section, where the conveyance speed u1 and the conveyance speed u2 are set to U, and the loosening amount b is fixed as it is to B2. A section from T7 to T8 denoted by (h) is a standby operation section, where the conveyance speed u1 is set to speed U, and the conveyance speed u2 is set to 0. Based on the difference between the conveyance speeds, the loosening amount b is reduced, and becomes B1. A section from time T8 to T9 denoted by (a) refers to the initial operation section again, where the operation prepares for the feeding of the subsequent sheet S. Continuous feeding of sheets is performed by repeating the above operations.

According to the present embodiment, the first and

second driving units

203 and 204 are stopped in the initial operation, but it is also possible to drive the two units at a same speed, and alienate the attraction member 200 with a predetermined clearance from the sheet S. Further, in the approaching operation and the contact area increasing operation, the attraction member 200 approaches the sheet S and the contact area is increased by the difference in conveyance speeds between the second nip-conveyance roller pair 202 and the first nip-conveyance roller pair 201. However, it is also possible to approximate the attraction member 200 to the sheet S and increase the contact area by driving the first driving unit 203 in reverse rotation and stopping the second driving unit 204.

During the attracting operation, the first and

second driving units

203 and 204 are stopped, but as long as the uppermost sheet and the attraction member 200 are in surface contact with a predetermined sheet contact area Mn, the first and

second driving units

203 and 204 can be operated. In the present embodiment, the positive and negative

voltage supplying units

205a and 205b are connected to the attraction member 200 in the respective operation steps described above to generate attraction force constantly, but the present embodiment is not restricted to such example. For example, it is possible to connect the positive and negative

voltage supplying units

205a and 205b only during three steps, which are the attracting operation, the separating operation and the conveying operation, to generate attraction force.

Next, a mechanism of separation of the sheet S according to the present invention will be described with reference to Fig. 8. Fig. 8 is a schematic diagram illustrating forces acting on sheets Sa and Sb in the separation position of the sheet feeding apparatus 51. As have been described, in the sheet feeding apparatus 51 of the present embodiment, during separation of sheets, the sheet Sa is attracted to the attraction member 200 by static electricity, and thereafter, the attraction member 200 is drawn upward while being elastically deformed.

Now, the uppermost sheet Sa is drawn upward by an electrostatic attraction force Fe, but an attraction force Fa with the uppermost sheet Sa caused, for example, by edge burr or charging of sheets, and separation force Fd caused by stiffness (rigidity) of a sheet Sb itself acts on a subsequent supported sheet Sb, i.e., target of separation. In the present embodiment, a condition in which the subsequent supported sheet Sb is not drawn upward together with the uppermost supported sheet Sa, that is, a condition in which the separation of sheets is realized, is represented by a following expression (1).
Fd > Fa (1)

Further, a separation force Fd of the sheet Sb can be expressed by a following expression (2), through approximation of a simple beam model. E represents a Young's modulus of the sheet Sb, and I represents a geometrical moment of inertia.
Fd = 3 EI/Lb³ × Lh (2)

In order to realize expression (1), that is, in order to separate the uppermost sheet Sa from the subsequent sheet Sb, the Lb (deflection length) and the Lh (deflection height) should be set appropriately, and the separation force Fd of the sheet Sb should be controlled to exceed an assumed attraction force Fa. However, when the uppermost sheet Sa is attracted to the attraction member 200, an equivalent force as the separation force Fd of the sheet Sb also acts on the uppermost sheet Sa. Since the separation force Fd is a force resisting against the electrostatic attraction force Fe, attraction peeling may occur where the uppermost sheet Sa is detached from the attraction member 200 if the separation force Fd is controlled excessively, which may lead to occurrence of misfeed.

According to studies performed by the present inventors, if the sheet S as feed target is a paper, it has been discovered that the stiffness corresponding to a product of Young's modulus E and geometrical moment of inertia I is approximately proportional to a cube of the paper thickness or basis weight. Therefore, according to expression (2), the separation force Fd of the sheet Sb is also approximately proportional to the cube of the paper thickness or basis weight. The present embodiment assumes a paper with a basis weight of 60 to 160 g/m² as the feed target sheet S. When such paper is fed, a ratio of maximum and minimum values of the separation force Fd of the sheet Sb will be as large as approximately 20 times, by considering only the basis weight of paper.

Therefore, in the present invention, Lb or Lh is changed in accordance with the thickness of the sheet S so as to exceed the assumed attraction force Fa for sheets S having various basis weight, i.e., stiffness, while preventing attraction peeling, so that the separation force Fd is maintained to a substantially fixed level capable of realizing sheet separation. Actually, after detecting the thickness of the sheet S by the sheet thickness detection unit 53, the sheet separation control unit 210 changes the separation position for separating the sheet by the attraction member 200 to a desirable separation position corresponding to the thickness of the sheet S.

In some cases, the electrostatic attraction force Fe may be sufficiently high, that is, the electrostatic attraction force Fe may be set to exceed the assumed attraction force Fa, and set to such a high value exceeding the separation force Fd that changes within the range of the basis weight of the feed target sheet S. The attraction peeling of the sheet S can be prevented even in such case, but generally, it is not easy from the viewpoint of safety and apparatus size to increase the electrostatic attraction force Fe, and the configuration of the present invention is superior in that point.

Next, a method for changing a separation position of the attraction member 200 by the sheet separation control unit 210 according to the present invention will be described with reference to Figs. 9 through 11. Fig. 9 is a flowchart of the sheet separation control unit 210, Fig. 10 is a timing chart of a driving unit during separating operation, and Fig. 11 is a schematic diagram of a case where the separation position of the attraction member 200 is changed.

As illustrated in Fig. 5 described earlier, the sheet thickness detection unit 53, the first driving unit 203 and the second driving unit 204 are electrically connected to the sheet separation control unit 210 within the control unit 70, as a subsystem of the control unit 70. When the sheet separation control unit 210 changes the separation position, at first as illustrated in Fig. 9, thickness St of the sheet S is acquired from the sheet thickness detection unit 53 (S101). Thereafter, a conveyance amount difference set value Uadiff between the first driving unit 203 and the second driving unit 204 in the separating operation section, which is an area of a shaded portion illustrated in Fig. 10 (e'), is calculated using the data table Mt1 stored in the storage area of the sheet separation control unit 210. Along with the change of separation position, a conveyance amount difference set value Ubdiff between the first driving unit 203 and the second driving unit 204 in the re-approaching operation section, which is an area of a shaded portion illustrated in Fig. 10 (f'), is calculated using the data table Mt2 (S102).

Next, the sheet separation control unit 210 determines a rotational speed Ua1 of the first driving unit 203, a rotational speed Ua2 of the second driving unit 204, and a separating operation section length ΔTa (= T5 - T4), based on the calculated value of the conveyance amount difference set value Uadiff. Further, the sheet separation control unit 210 determines a rotational speed Ub1 of the first driving unit 203, a rotational speed Ub2 of the second driving unit 204, and a re-approaching operation section length ΔTb (= T6 - T5), based on the calculated value of the conveyance amount difference set value Ubdiff (S103). Then, the sheet separation control unit 210 outputs a speed command pulse string to the first and

second driving units

203 and 204, based on the value determined in step S103 (S104).

Through the above-mentioned steps, a loosened shape of the attraction member 200 in the separating operation section, that is, the loosening amount of the attraction member 200, can be changed in accordance with the conveyance amount difference set value Uadiff, and the separation position is changed by the change in the loosened shape of the attraction member 200. Even when the separation position is changed in this manner, not only the separation position but also the conveyance position can be fixed by having the conveyance amount difference set value Ubdiff correspond to the changed separation position.

Now, the conveyance amount difference set value Uadiff is a value corresponding to a sag reduction quantity from an attraction position of the attraction member 200 illustrated in Fig. 6D to a separation position of the attraction member 200 illustrated in Fig. 6E described earlier. Then, when the conveyance amount difference set value Uadiff is set to a high value, the sag reduction quantity of the attraction member 200 is increased during separating operation, and along therewith, an attraction surface side of the attraction member 200 is deformed into an approximately linear shape as shown in P5-n of Fig. 11. The deflection height in this case is Lh.

When the conveyance amount difference set value Uadiff is set to a low value, the sag reduction quantity of the attraction member 200 becomes small, and the attraction surface side of the attraction member 200 is deformed in the Au direction, but the attraction member 200 maintains a barrel shape, as shown in P5-h. This deflection height will be lower Lh' than the previous deflection height Lh.

As described, when a set value of the conveyance amount difference set value Uadiff is varied to change the separation position of the attraction member 200, the deflection height Lh is changed. Then, the conveyance amount difference set value Uadiff should be set smaller as the thickness St of the sheet S increases, that is, as the stiffness of the sheet S increases, so as to maintain a low deflection height Lh, the separation force Fd illustrated in Fig. 8 described later can be maintained approximately constantly even when the separation position is changed.

On the other hand, the conveyance amount difference set value Ubdiff is a value corresponding to a sag increase quantity from a separation position of the attraction member 200 illustrated in Fig. 6E to a conveyance position of the attraction member 200 illustrated in Fig. 6G described earlier. The conveyance amount difference set value Ubdiff should be set so that the attraction member 200 at the separation position approximates the lower sheet Sb as illustrated in Fig. 6F, and that the portion of the uppermost sheet Sa attracted to the attraction member 200 does not contact the lower sheet Sb. Further, the conveyance amount difference set value Ubdiff should be set so that the conveyance position is fixed regardless of the separation position. Therefore, in the present embodiment, the conveyance amount difference set value Ubdiff is set so that Uadiff - Ubdiff is fixed within the range of Uadiff > Ubdiff > 0.

Next, a method for calculating the conveyance amount difference set value Uadiff of the driving unit based on the thickness St of the sheet S will be described with reference to Fig. 12. Fig. 12 is a schematic diagram illustrating a data table Mt1 stored in the storage area of the sheet separation control unit 210. In the present embodiment, a function expression expressing the conveyance amount difference set value Uadiff as a function of thickness St of the sheet S is stored as the data table Mt1. When calculating the conveyance amount difference set value Ubdiff of the driving unit from the thickness St of the sheet S, similar to calculating the conveyance amount difference set value Uadiff, the data table Mt2 storing the function expression expressing the set value as a function of thickness St of the sheet S is used.

In a process of calculating the function expression, at first, the thickness St of various sheets S is measured in advance. The conveyance amount difference set values Uadiff for realizing a desired separation force Fd when the various sheets S are used are also measured in advance. The relationship between the thickness St of the sheet S and the conveyance amount difference set value Uadiff is subjected to least squares approximation through polynomial expression, and converted into a function. The present inventors have discovered through studies that the conveyance amount difference set value Uadiff expressed using a function in inverse proportion to the cube of the thickness St of the sheet S approximately corresponds to the experimental value, so that such function is adopted in the present embodiment.

The present invention is not restricted to the illustrated example, and the effect of the present invention will not be inhibited by using a simplified function expression due to various restrictions. Further, the form of the data table Mt1 is not necessarily restricted to a function expression. For example, it is possible to store multiple sets of sheet thickness St and corresponding conveyance amount difference set value Uadiff in advance as a list. The set value can be substituted by checking the list for the detected sheet thickness St and reading the conveyance amount difference set value Uadiff corresponding to the closest sheet thickness St being stored.

Next, the timing at which the separation position of the attraction member 200 is changed by the sheet separation control unit 210 will be described with reference to Fig. 13. Fig. 13 is a flowchart starting from entry of a print job to the execution of operation of the sheet separation control unit 210.

At first, when a print job is entered (S201), the sheet separation control unit 210 detects whether the cassette 51a has been opened or closed after the previous print job, using a cassette opening/closing detection sensor (not shown) arranged near the cassette 51a (S202). If the cassette 51a has been opened or closed as a result of opening/closing detection (S202: Y), the control unit feeds a first sheet S in a state where the conveyance amount difference set value Uadiff is set to a factory shipment value, that is, where the separation position is set to a standard separation position (S203). In the present embodiment, a minimum conveyance amount difference set value Uadiff_min capable of feeding even a sheet S having a maximum corresponding thickness without causing peeling is set as the factory shipment value.

After step S203, when the fed sheet S reaches a detection area of the sheet thickness detection unit 53, the thickness St of the sheet S is detected by the sheet thickness detection unit 53 (S204). After detecting the thickness St of the sheet S, the separation position and the conveyance position of the attraction member 200 are changed in accordance with the thickness of the sheet by the sheet separation control unit 210, as described earlier. In this state, change parameters Ua1, Ub1, Ua2, Ub2, ΔTa and ΔTb are stored in a storage area not shown installed in the sheet separation control unit 210 (S205). The sheet feed is executed at the updated separation position and conveyance position based on the parameters stored in S205 when feeding second and subsequent sheets to carry out jobs, that is, when performing subsequent jobs (S206).

If the cassette 51a had not been opened and closed as a result of opening/closing detection of the cassette opening/closing detection sensor (S202: N), the sheet separation control unit 210 reads out the separation position information and the conveyance position information (Ua1, Ub1, Ua2, Ub2, ΔTa and ΔTb) (S207). Then, based on the separation position information and the conveyance position information having been read, the subsequent jobs are executed at the updated separation position and conveyance position (S208).

As have been described, the attraction member 200 is moved to a conveyance position where the uppermost sheet is conveyed in a state closer to the subsequent sheet than when the attraction member 200 is positioned at the separation position, when conveying the attracted sheet. According to this configuration, the attracted area of the attracted sheet will not be in contact with the subsequent sheet, and the deflection of the attracted sheet is reduced so that a contact pressure between the trailing edge of the sheet and the subsequent sheet is lowered. Thereby, the sliding noise generated during conveyance of the sheets can be reduced significantly, and the sheet can be separated and conveyed while generating only small noise.

In the present embodiment, an electrostatic attraction force is generated between the attraction member 200 and the sheet S by a configuration as described above, but the present embodiment is not restricted to such configuration. For example, the positive and

negative electrodes

200a and 200b do not have to be comb-tooth shaped, and can be a uniform electrode or other shapes capable of forming an electric field between the attraction member and the sheet S and subjecting the sheet S to dielectric polarization.

A case where initial operation is performed after completing conveyance of a sheet when performing continuous sheet feed has been described, but the present invention is not restricted thereto, and an approaching operation can be performed without performing initial operation after completing conveyance of a sheet.

Next, a second embodiment of the present invention will be described. Fig. 14 is a timing chart of separation and feeding performed by the sheet of the sheet attraction, separation and feeding unit disposed in the sheet feeding apparatus according to the present embodiment. In Fig. 14, sections of time T0 to T7 denoted by (a) through (g) are the same as the sections illustrated in Fig. 7 described earlier, and the operations are also the same. The section from time T7 to T8 denoted by (h) is an approaching operation and contact area increasing operation section, and at this time, the conveyance speed u1 is set to 0, and the conveyance speed u2 is set to U. At this time, the loosening amount b is increased to B3. The section from time T8 to T9 denoted by (d') is a re-attracting operation section, where the apparatus prepares for the feeding of a next sheet S. By repeating the above operations, continuous sheet separation and feeding operation is performed.

As described, the present embodiment performs the initial operation, the approaching operation, the contact area increasing operation, the attracting operation, the separating operation, the re-approaching operation and the conveying operation illustrated in (a) through (g) of Fig. 14, and thereafter, performs the approaching operation and contact area increasing operation illustrated in (h), before starting the separation and feeding operation of the subsequent sheet. In other words, in order to perform continuous sheet feed, the present embodiment performs the approaching operation and contact area increasing operation after completing conveyance of the sheet by the conveying operation, without performing the standby operation and the initial operation. The present embodiment enables to improve the productivity by omitting the two steps of initial operation and standby operation.

The first and second embodiments illustrate the sheet feeding apparatus performing attraction and separation of the sheet S through elastic deformation of the attraction member 200 itself, but the present invention is not restricted thereto. For example, the present invention can be applied to a sheet feeding apparatus designed to elevate the attraction member to attract sheets.

Next, a third embodiment of the present invention configured to elevate the attraction member to attract sheets will be described. Fig. 15 is an explanatory view of a configuration of a sheet feeding apparatus according to the present embodiment. In Fig. 15, the same reference numbers as Fig. 2 described earlier refer to the same or corresponding portions.

In the present embodiment, as illustrated in Fig. 15, an endless attraction member 221 is wound around a second conveyance roller 223, i.e., first rotary member, and a first conveyance roller 222, i.e., second rotary member. The circumference of the attraction member 221 is generally [double a distance between rotation centers of the first and

second conveyance rollers

222 and 223 + half a length of circumferential surfaces of each roller 222 and 223].

Here, the second conveyance roller 223 is driven by the second driving unit 204. Further, the second conveyance roller 223 is rotatably supported by a feed unit frame 224 axially and pivotally supported by a rotation shaft 223a of the second conveyance roller 223. A tension spring 227 is connected to an axial support member not shown of the first conveyance roller 222, and the tension spring 227 biases the first conveyance roller 222 toward a direction moving away from the second conveyance roller 223. Thereby, tension is applied by the tension spring 227 to the attraction member 221 wound around the first and

second conveyance rollers

222 and 223.

A gear portion 224a is disposed around the rotation shaft 223a of the second conveyance roller 223 on the upper surface of the feed unit frame 224, and the gear portion 224a is engaged with a gear 225 disposed on the sheet feeding apparatus body and rotated by a third driving unit 226. The third driving unit 226 is capable of performing normal/reverse rotation. When the gear 225 is rotated by the normal/reverse rotation of the third driving unit 226, the feed unit frame 224 is pivoted in the vertical direction around the rotation shaft 223a of the second conveyance roller 223. As described, according to the present embodiment, the attraction member 221 can approach or move away from the sheet S stored in the cassette 51a by pivoting the feed unit frame 224 in the vertical direction by the third driving unit 226, i.e., elevating unit.

An attraction position detecting unit 228 detecting that the attraction member 221 has reached an attraction position capable of attracting the uppermost sheet Sa from the sheets S stored in the cassette 51a when the feed unit frame 224 is pivoted downward is disposed on a side portion of the cassette 51a. In the present embodiment, the attraction position detecting unit 228 is a piezoelectric element having its electrical resistivity changed when pressure is applied, and the attraction position detecting unit 228 outputs a detection signal when the attraction member 221 is in contact with the attraction position detecting unit 228. In the present embodiment, the drawing

roller pair

51c and 51d are arranged downstream, in a sheet conveyance direction, of the sheet attraction, separation and feeding unit 51b.

Next, a configuration of a control unit of the sheet feeding apparatus 51 will be described with reference to Fig. 16. Fig. 16 is a control block diagram of the sheet feeding apparatus 51 according to the present embodiment. In Fig. 16, the same reference numbers as Fig. 5 described earlier refer to the same or corresponding portions. The elevating unit 301, the positive voltage supplying unit 205a, the negative voltage supplying unit 205b, the sheet surface height detection unit 302, the timer 71 and so on described earlier are connected to a control unit 75.

Further, the sheet separation control unit 215 is included as a subsystem in the control unit 75, and the second driving unit 204, the third driving unit 226 and the attraction position detecting unit 228 are connected to the sheet separation control unit 215. The sheet separation control unit 215 transmits drive command pulse strings to the second driving unit 204 and the third driving unit 226 based on the detection signal from the attraction position detecting unit 228. The control of the sheet separation control unit 215 will be described in detail later.

Next, a sheet separation and feeding operation of the sheet attraction, separation and feeding unit 51b will be described with reference to Figs. 17A through 17G. Figs. 17A through 17G are schematic diagrams showing the operation in which the sheet S is fed by the sheet attraction, separation and feeding unit 51b in time series. The feeding operation of the sheet S is composed, in time series order, of seven steps illustrated in Figs. 17A through 17G , which are an initial operation, an approaching operation, an attracting operation, a separating operation, a re-approaching operation, a conveying operation, and a standby operation. These steps will be described in order. In the present embodiment, the positive and negative

voltage supplying units

205a and 205b are connected to the attraction member 221 in all the operation steps, and attraction force is constantly generated.

An initial operation illustrated in Fig. 17A is an operation of having the attraction member 221 stand-by at an initial operation of feed operation, i.e., standby position. In the present embodiment, the sheet separation control unit 215 separates the first conveyance roller 222 from the uppermost sheet Sa during the initial operation, so that a height h thereof is set to H3 with respect to the uppermost sheet Sa, and stops the second and

third driving units

204 and 226.

An approaching operation illustrated in Fig. 17B is an operation of moving the attraction member 221 downward, to thereby have the attraction member 221 approach the uppermost sheet Sa. During this operation, the sheet separation control unit 215 rotates the gear 225 at an angular velocity Ω to a direction of arrow x by the third driving unit 226, with the second driving unit 204 stopped. Then, when the attraction member 221 contacts the uppermost sheet Sa, and the attraction position detecting unit 228 detects the attraction member 221, the control unit 215 stops the third driving unit 226.

An attracting operation illustrated in Fig. 17C is an operation of attracting the uppermost sheet Sa to the attraction member 221 in a state, i.e., attraction position, where the upper surface of the uppermost sheet Sa and the surface of the attraction member 221 are in surface contact. At this time, a height h of the first conveyance roller 222 is set to H1, lower than the uppermost sheet Sa. In this state, when the uppermost sheet Sa and the attraction member 221 contact each other, an electrostatic attraction force acts between the attraction member 221 and the sheet S, since voltage is applied to the attraction member 221 through positive and negative

voltage supplying units

205a and 205b, as mentioned earlier. Then, the uppermost sheet Sa is attracted to the attraction member 221.

A separating operation illustrated in Fig. 17D is an operation of deflecting the uppermost sheet Sa attracted to the attraction member 221 upward by the attraction member 221 and thereby separating the sheet Sa from a lower sheet Sb. During this operation, the sheet separation control unit 215 rotates the third driving unit 226 in the opposite direction while the second driving unit 204 is stopped, in a direction of arrow y at an angular velocity Ω. As a result, the first conveyance roller 222 is elevated, and the attraction member 221 is also elevated. By elevating the attraction member 221, the uppermost sheet Sa is deformed for distance Lb, i.e., deflection length, from a front end of the sheet Sa to the base of deflection Pb illustrated in Fig. 8 described earlier, and for distance Lh, i.e., deflection height, from a front end of the sheet Sa being supported to a front end of the sheet having been drawn upward. Thereby, the uppermost sheet Sa is moved to a position, i.e., separating position, separating from the lower sheet Sb. In this state, the height h of the first conveyance roller 222 is the same H3 as the height h in the standby position.

A re-approaching operation illustrated in Fig. 17E is an operation of moving the attraction member 221 downward, similar to the approaching operation, to reduce a deflection angle of the uppermost sheet Sa having been deflected upward by the separating operation. During this operation, the sheet separation control unit 215 rotates the third driving unit 226 in normal rotation to rotate the gear 225 in a direction of arrow x at an angular velocity Ω. By this re-approaching operation, the attraction member 221 moves to a position where the attracted uppermost sheet Sa can be conveyed without fail to the nip portion of the

drawing roller pair

51c and 51d.

A conveying operation illustrated in Fig. 17F is an operation of attracting and feeding the uppermost sheet Sa attracted to the attraction member 221 to the

drawing roller pair

51c and 51d. During this conveying operation, the deflection angle of the uppermost sheet Sa deflected by the attraction member 221 is reduced than during the separating operation. In this position, i.e., conveyance position, the height h of the first conveyance roller 222 is set to H2 (H1 < H2 < H3). At this time, it is most preferable that the deflection angle θs set to 0, and that the uppermost sheet Sa is not in contact with the lower sheet Sb in a range where the sheet Sa is attracted to the attraction member 200.

During this operation, the sheet separation control unit 215 drives the second driving unit 204 while the third driving unit 226 is stopped, to rotate the second conveyance roller 223 in the arrow direction at rotational speed U. Thereby, the uppermost sheet Sa attracted to the attraction member 221 is conveyed to the direction of arrow A while maintaining a deflection angle θs of approximately 0, and maintaining a state where the sheet Sa does not contact the lower sheet Sb in a range attracted to the attraction member 221. Of course, the effect of the present invention can be achieved even if the deflection angle θs is not 0 and the uppermost sheet Sa is in contact with the lower sheet in a range not attracted by the attraction member 221.

A standby operation illustrated in Fig. 17G is an operation of returning the attraction member 221 to the initial operation position. During this operation, the sheet separation control unit 215 rotates the third driving unit 226 in reverse rotation while maintaining the second driving unit 204 in a stopped state to rotate the gear 225 in the direction of arrow y at an angular velocity Ω. Only one uppermost sheet Sa is fed at a time from the multiple sheets S supported on the cassette 51a by carrying out the seven steps mentioned above. The sheets S can be fed one at a time and continuously by repeating these seven steps.

Fig. 18 is a timing chart of the initial operation, the approaching operation, the attracting operation, the separating operation, the re-approaching operation, the conveying operation and the standby operation illustrated in Figs. 17A through G. In Fig. 18, h represents a height of the first conveyance roller 222 with respect to the uppermost sheet Sa, u2 represents a conveyance speed of the second conveyance roller 223, and ω1 represents an angular velocity around a rotation axis of the second conveyance roller 223 of the feed unit frame 224. Further, vp represents a positive voltage supplied from the positive voltage supplying unit 205a, and vn represents a negative voltage supplied from the negative voltage supplying unit 205b.

In Fig. 18, a section from time T0 to T1 denoted by (a) is an initial operation section, and at this time, the conveyance speed u2 and the angular velocity ω1 are set to 0, the supply voltage vp is set to +V, and the supply voltage vn is set to -V. At this time, the height h is set to H3. In the present embodiment, the supply voltages vp and vn are set to +V and -V in all the feeding operations of the sheet S, and they are not changed.

Further, a section from time T1 to T2 denoted by (b) is an approaching operation section. At this time, the conveyance speed u2 is set to 0, the angular velocity ω1 is set to Ω, and thereby, the height h is reduced. The angular velocity Ω is a speed determined based on productivity and the like of the image forming apparatus, and in the present embodiment, Ω equals 2πrad/s. A section from time T2 to T3 denoted by (c) is an attracting operation section, where the conveyance speed u2 and the angular velocity ω1 are set to 0, and the height h is set to H1.

A section from time T3 to T4 denoted by (d) is a separating operation section, where the conveyance speed u2 is set to 0 and the angular velocity ω1 is set to -Ω, so that the height h is increased and reaches H3. A section from time T4 to T5 denoted by (e) is a re-approaching operation section, where the conveyance speed u2 is set to U and the angular velocity ω1 is set to Ω, by which the height h is reduced. A section from time T5 to T6 denoted by (f) is a conveying operation section, where the conveyance speed u2 is set to U, the angular velocity ω1 is set to 0, and the height h is set to H2. The speed U is a speed determined based on productivity and the like of the image forming apparatus, and in the present embodiment, U equals 200 mm/s.

A section from T6 to T7 denoted by (g) is a standby operation section, where the conveyance speed u2 is set to 0, and the angular velocity ω1 is set to -Ω, according to which the height h is increased and reaches H3. Continuous feeding of sheets is performed by repeating the above operations.

The above description has described a sheet feeding operation of a case where the drawing

roller pair

51c and 51d is arranged downstream, in the sheet conveyance direction, of the sheet attraction, separation and feeding unit 51b in the sheet feeding apparatus 51 according to the present embodiment. Incidentally, the sheet feeding apparatus 51 according to the present embodiment can adopt a configuration where the drawing

roller pair

51c and 51d is arranged obliquely upward of the sheet attraction, separation and feeding unit 51b, as illustrated by the dashed line in Fig. 15 described earlier. In this configuration, similar to the first and second embodiments already described, the separated sheet is abutted against the conveyance guide 51e, deflected along the conveyance guide 51e, and transferred to the

drawing roller pair

51c and 51d.

Prior to conveying the attracted sheet, as described earlier, the first conveyance roller 222 is lowered so that the attraction member 221 is moved to a conveyance position conveying the sheet so that the attracted area of the sheet is positioned closer to the subsequent sheet than during the separation position. In the re-approaching operation, the sheet separation control unit 215 determines the angular velocity Ω and the driving time based on the data table storing a lowering amount of the first conveyance roller 222 in accordance with the stiffness of the sheet and the sheet stiffness, and sets the lowering amount of the first conveyance roller 222.

Thereby, the attracted area of the attracted sheet will not contact the subsequent sheet, and the deflection of the attracted sheet is reduced so that the contact pressure between the trailing edge of the attracted sheet and the subsequent sheet is reduced. As a result, the sliding noise generated during conveyance of the sheet can be reduced significantly regardless of the stiffness of the sheet, and the sheet can be separated and conveyed with a low noise.

In the first and second embodiments, and in the third embodiment configured so that a drawing roller pair is disposed obliquely upward of the sheet attraction, separation and feeding unit, an example has been illustrated where sheets having a same stiffness are conveyed successively. However, in the sheet feeding apparatus according to these embodiments, there may be a case where sheets having different rigidities are stored in the cassette and the sheets having different rigidities are conveyed successively. In that case, sheets having different rigidities can be conveyed successively by conveying the sheet having a first stiffness at a first conveyance position, and conveying the sheet having a second stiffness at a second conveyance position. That is, sheets having different rigidities can be conveyed successively by moving the attraction member to a first conveyance position when conveying a sheet having a first stiffness and to a second conveyance position when conveying the sheet having a second stiffness.
Other Embodiments

Embodiments of the present invention can also be realized by a computer of a system or apparatus that reads out and executes computer executable instructions recorded on a storage medium (e.g., non-transitory computer-readable storage medium) to perform the functions of one or more of the above-described embodiment(s) of the present invention, and by a method performed by the computer of the system or apparatus by, for example, reading out and executing the computer executable instructions from the storage medium to perform the functions of one or more of the above-described embodiment(s). The computer may comprise one or more of a central processing unit (CPU), micro processing unit (MPU), or other circuitry, and may include a network of separate computers or separate computer processors. The computer executable instructions may be provided to the computer, for example, from a network or the storage medium. The storage medium may include, for example, one or more of a hard disk, a random-access memory (RAM), a read only memory (ROM), a storage of distributed computing systems, an optical disk (such as a compact disc (CD), digital versatile disc (DVD), or Blu-ray Disc (BD)^TM), a flash memory device, a memory card, and the like.

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. 2015-102862, filed May 20, 2015 which is hereby incorporated by reference herein in its entirety.

Claims

A sheet feeding apparatus comprising:
a supporting unit configured to support sheets;
a first rotary member disposed above the supporting unit;
a second rotary member disposed downstream, in a sheet feed direction, of the first rotary member;
an attraction member, whose inner surface is supported by the first and second rotary members, attracting a sheet supported on the supporting unit; and
a control unit controlling the first and second rotary members such that the attraction member is moved to an attraction position where the attraction member abuts against an uppermost sheet supported on the supporting unit and electrically attracts the uppermost sheet, a separation position where an attracted uppermost sheet is deflected and separated from a subsequent sheet, and a conveyance position where the separated uppermost sheet is conveyed in a state approaching the subsequent sheet more than a state where the attraction member is positioned at the separation position.
The sheet feeding apparatus according to claim 1, wherein the conveyance position is a position where a portion of the uppermost sheet attracted to the attraction member is not in contact with the subsequent sheet.
The sheet feeding apparatus according to claim 1, wherein the control unit performs control such that the attraction member is sagged and moved to the attraction position in a state where the uppermost sheet is attracted, the sagging of the attraction member is reduced and the attraction member is moved to the separation position in a state where the uppermost sheet is separated from the subsequent sheet, and the attraction member is sagged and moved to the conveyance position in a state where the uppermost sheet is conveyed.
The sheet feeding apparatus according to claim 3, further comprising
a first driving unit driving the first rotary member; and
a second driving unit driving the second rotary member,
wherein the control unit controls the first and second driving units to rotate the first rotary member at a speed faster than the second rotary member in a state where the attraction member is moved to the attraction position, rotate the secondary member at a speed faster than the first rotary member in a state where the attraction member is moved to the separation position, and rotate the first rotary member at a speed faster than the second rotary member in a state where the attraction member is moved to the conveyance position.
The sheet feeding apparatus according to claim 4, further comprising a storage unit storing a data table used to calculate rotational speeds of the first and second rotary members in accordance with a stiffness of the sheet in a state where the attraction member is moved to the conveyance position,
wherein the control unit sets up the rotational speeds of the first and second rotary members based on a stiffness of the sheet and the data table in a state where the attraction member is moved to the conveyance position.
The sheet feeding apparatus according to claim 5, further comprising a detection unit configured to detect a stiffness of a sheet,
wherein the control unit sets up the rotational speeds of the first and second rotary members based on a stiffness of the sheet detected by the detection unit and the data table in a state where the attraction member is moved to the conveyance position.
The sheet feeding apparatus according to claim 1, further comprising an elevating unit configured to elevate the second rotary member,
wherein the control unit controls the elevating unit to lower the second rotary member and move the attraction member to the attraction position in a state where the uppermost sheet is attracted, elevate the second rotary member and move the attraction member to the separation position in a state where the uppermost sheet is separated from the subsequent sheet and moving the attraction member to the separation position, and lower the second rotary member and move the attraction member to the conveyance position in a state where the uppermost sheet is conveyed.
The sheet feeding apparatus according to claim 7, further comprising a storage unit storing a data table used to calculate a lowering amount of the secondary rotary member in accordance with a stiffness of a sheet in a state where the attraction member is moved to the conveyance position,
wherein the control unit sets up the lowering amount of the second rotary member based on the stiffness of the sheet and the data table in a state where the attraction member is moved to the conveyance position.
The sheet feeding apparatus according to claim 8, further comprising a detection unit detecting a stiffness of a sheet,
wherein the control unit sets up the lowering amount of the second rotary member based on the stiffness of the sheet detected by the detection unit and the data table in a state where the attraction member is moved to the conveyance position.
The sheet feeding apparatus according to claim 1, wherein the control unit controls the first and second rotary members such that the attraction member is moved from a standby position separated from an uppermost sheet supported on the supporting unit to the attraction position, and after electrically attracting the sheet, the attraction member is moved to the standby position.
The sheet feeding apparatus according to claim 1, wherein the control unit controls the first and second rotary members such that the attraction member is moved to the attraction position after electrically attracting the sheet.
The sheet feeding apparatus according to claim 5, wherein the conveyance position is one of first and second conveyance positions and, the control unit controls the first and second rotary members such that the attraction member is moved to the first conveyance position in a state where a sheet having a first stiffness is conveyed, and the attraction member is moved to the second conveyance position in a state where a sheet having a second stiffness is conveyed, when the sheets having different stiffnesses are conveyed.
An image forming apparatus comprising:
an image forming unit configured to form an image on a sheet; and
a sheet feeding apparatus according to claim 1 feeding the sheet to the image forming unit.