Sheet Feed Device for Image Forming Equipment Using a Dielectric, Reciprocating Plate

The present invention relates to a laser beam printer, copier, facsimile transceiver or similar image forming equipment and, more particularly, to a device incorporated in such equipment for sequentially separating and feeding cut sheets one by one from a stack.

A predominant type of sheet feed device for the above application uses a pick-up member in the form of a roller or a belt made of a material having a high coefficient of friction, e.g., rubber. The pick-up member feeds sheets on the basis of friction. Another conventional sheet feed device feeds sheets by sucking them, i.e., under vacuum.

The friction type sheet feed device is simple in construction. However, this type of device cannot exert a great frictional force unless a spring or similar resilient member presses the pick-up member against the top of the stack. i.e., uppermost sheet. Another drawback with such a device is that rubber or similar material having a high coefficient of friction is apt to have the coefficient noticeably changed due to the varying ambient conditions, degrading the stability of operation. In addition, since the pressing contact of the pick-up member with the sheet stack is not delicately adjusted, two or more sheets are often fed out together. To eliminate this occurrence, an extra mechanism for surely separating the uppermost sheet from the others is indispensable.

The suction type sheet feed device is advantageous over the friction type device in respect of stable sheet feed. However, the suction scheme generates great noise in the event of suction of air and increases the overall dimensions of the device.

Japanese Utility Model Laid-Open Publication (Kokai) No. 85543/1985 discloses a sheet feed device capable of separating sheets by electrostatic adhesion. Specifically, this sheet feed device has a flat sheet feed member capable of exerting an electrostatic adhering force, a means for moving the flat member substantially in a linear reciprocating motion, and a means for selectively applying or interrupting a voltage for generating or cancelling the adhering force. With this kind of approach, however, it is difficult to separate one sheet from the others due to the leak of electric field, resulting in the simultaneous feed of two or more sheets. In the light of this, Japanese Patent Laid-Open Publication (Kokai) No. 176472/1985 proposes an improved electrode pattern.

It is therefore an object of the present invention to provide a sheet feed device for image forming equipment which can separate and feed sheets stably with no regard to aging or varying ambient conditions and which is simple in construction.

In accordance with the present invention, a sheet feed device for image forming equipment and capable of feeding sheets one by one from a stack by separating them comprises a plate having a dielectric portion capable of making surface-to-surface contact with the top of the stack, a reciprocating mechanism for causing the plate to move in a reciprocating motion, a moving mechanism for selectively moving the plate into or out of contact with the top of the stack, and a voltage applying circuit for applying a voltage to the plate to cause it to exert an electrostatic adhering force.

The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description taken with the accompanying drawings in which:

FIG. 1 is a perspective view showing a first embodiment of the sheet feed device in accordance with the present invention;

FIG. 2 is a fragmentary sectional side elevation of first embodiment;

FIG. 3 shows an electric arrangement included in the first embodiment;

FIGS. 4A-4F are views demonstrating the operation of the first embodiment;

FIG. 5 is a side elevation showing a second embodiment of the present invention;

FIG. 6 is a perspective view of a charge roller included in the second embodiment;

FIG. 7 shows an electric arrangement included in the second embodiment;

FIG. 8 is a perspective view of a third embodiment of the present invention;

FIG. 9 is a fragmentary side elevation of the third embodiment;

FIG. 10 shows an electric arrangement included in the third embodiment;

FIGS. 11A-11E are views representative of the operation of the third embodiment;

FIG. 12 is a section of a charge roller included in the third embodiment;

FIG. 13 is a perspective view of a fourth embodiment of the present invention;

FIG. 14 is a fragmentary sectional side elevation of the fourth embodiment;

FIG. 15 shows an electric arrangement included in the fourth embodiment; and

FIGS. 16A-16E are views demonstrating the operation of the fourth embodiment.

Preferred embodiments of the sheet feed device for image forming equipment in accordance with the present invention will be described.

Referring to FIGS. 1-4F, a sheet feed device embodying the present invention is shown. As shown, the device has a tray 2 including a bottom plate 2a. Cut sheets 1 are stacked on the bottom plate 2a and surrounded by a front fence 2b, side fences 2c, and a rear fence 2d. The front fence 2b is notched in the perpendicular direction over a width greater than the width of a flat plate 101 which will be described. An elevation sensing means 5 is disposed above the downstream side of the sheet stack 1 with respect to an intended direction of sheet feed. It is to be noted that the word "downstream", as well as the word "upstream", to frequently appear in the following description is used in the above-mentioned sense. The sensing means 5 is responsive to the elevation of the sheet stack 1 and implemented by a reflection type photoelectric sensor. Located at the downstream side of the sheet stack 1 is a pick-up unit 100 made up of the flat plate 101, a drive roller 102, a charge roller 103, FIG. 2, a plate sensing means 104 responsive to the flat plate 101 and implemented by a reflection type photosensor, guide members 105, and support plates 106 supporting the guide members 105 as well as other members.

As shown in FIG. 3, the flat plate 101 has a dielectric layer 101a whose volume resistivity is 10.sup.12 .OMEGA.cm, and a conductive layer 101b whose volume resistivity is 10.sup.4 .OMEGA.cm or less. The plate 101 is positioned such that the dielectric layer 101a, for example, faces the top of the stack 1. As shown in FIG. 1, the drive roller 102 has a gear-toothed portion 102a meshing with teeth 101c provided on the plate 101. As shown in FIG. 2, the plate 101 is held between the drive roller 102 and the charge roller 103 and is moved in a reciprocating motion above the stack 1 in the sheet feed direction as the drive roller 102 is rotated. The plate 101 has a metallic gloss except for two extremely small non-reflective portions 101d. The plate sensing means 104 disposed above the plate 101 determines the position of the plate 101 by sensing either of the non-reflective portions 101d. A guide roller 101e is provided on the downstream end of the plate 101 and is guided by the drive roller 102 and the guide member 105 located downstream of the drive roller 102.

The drive roller 102 and charge roller 103 both are conductive, and each is constituted by a metallic shaft and conductive rubber wound around the shaft and having a volume resistivity of 10.sup.4 .OMEGA.cm or less. The charge roller 103 extends over substantially the entire width of the flat plate 101 while the drive roller 102 has a small length. As shown in FIG. 3, the drive roller 102 is connected to ground via the support plate 106 and other metallic parts. The charge roller 103 is connected to a power source 110 via a feed section 111, FIG. 1, and insulated from the other constituent parts. Made up of a waveform generator 110a and a voltage amplifier 110b, the power source 110 applies a sinusoidal wave of .+-.2 kV to the plate 101 by using the drive roller 102 and charge roller 103 as electrodes.

As shown in FIG. 1, an upper and a lower transport guide 3 are disposed below the pick-up unit 100 and extend obliquely downward from a sheet outlet provided in the unit 100. A pair of transport rollers 4 are positioned at substantially the intermediate positions between opposite ends of the transport guides 3. Particularly, the distance between the front end of the sheet stack 1 in the sheet feed direction and the transport roller pair 4 is selected to be shorter than the length which the plate 101 would overly the stack 1 when protruded most onto the stack 1.

The operation of the embodiment will be described with reference also made to FIGS. 4A-4F. Before a sheet feeding operation begins, the bottom plate 2a of the tray 2 loaded with the sheet stack 1 is raised by an elevating mechanism, not shown. When the top of the stack 1 reaches an adequate position as determined by the elevation sensing means 5, the elevating mechanism stops operating. Such a procedure is repeated every time the stack 1 decreases in height due to the sequential sheet feed, so that the top of the stack 1 remains at a constant level.

As a control means, not shown, causes the device to start feeding a sheet, the drive roller 102 is rotated in a direction indicated by an arrow in FIG. 4A. Then, the flat plate 101 is moved to above the sheet stack 1 due to the gear portion 102a and teeth 101c meshing with each other, as shown in FIG. 4A. At the same time, the waveform generator 10a of the power source 110 generates a sinusoidal wave of predetermined frequency. As a result, an alternating voltage of .+-.2 kV is generated via the voltage amplifier 110b and is applied to the plate 101 via the drive roller 102 and charge roller 103. This voltage forms an electrostatic pattern alternating in a stripe configuration on the surface of the dielectric layer 101a of the plate 101. The pitch of such a stripe pattern is selected to be 5 mm to 20 mm on the surface of the plate 101 on the basis of the feed speed of the plate 101 and the frequency of the alternating voltage of the power source 110.

As the plate 101 is sequentially moved by the drive roller 102 as stated above, it rotates about the point thereof held between the drive roller 102 and charge roller 103 with the guide roller 101e moving along the guide surface of the guide member 105. Consequently, as shown in FIG. 4B, the plate 101 lies flat on the uppermost sheet 1a when protruded most. The plate sensing means 104 determines that the plate 101 has been protruded most by sensing one of the non-reflective portions 101d of the plate 101.

On receiving the resulting output of the plate sensing means 104, the control means reverses the rotation of the drive roller 102 which in turn starts moving the plate 101 in the opposite direction, as shown in FIG. 4C. At this instant, the plate 101 lifts the uppermost sheet 1a due to the electrostatic pattern formed thereon. The adhesion due to the electrostatic pattern is so adjusted as not to act on the second sheet 1b underlying the uppermost sheet 1a, so that the sheet 1a may be successfully lifted away from the sheet 1b.

As shown in FIG. 4D, the sheet 1a is moved to the left together with the plate 101. However, since the upper transport guide 3 is protruded above the plate 101 which is comparatively narrow, the sheet 1a is sequentially separated from the plate 101 from the leading edge to the trailing edge and is guided into the path between the upper and lower transport guides 3.

As shown in FIG. 4D, before the sheet 1a is entirely separated from the plate 101 by the upper guide plate 3, the leading edge of the sheet 1a is nipped by the transport roller pair 4. As a result, the sheet 1a is transported by the roller pair 4 thereafter.

When the fiat plate 101 is pulled back or retracted most, the plate sensing means 104 senses the other non-reflective portion 101d of the plate 101. Then, the control means stops driving the drive roller 102, as shown in FIG. 4F.

To feed the sheets 1 continuously, as soon as the trailing edge of the first sheet 1a enters the path between the transport guides 3, the drive roller 102 is again rotated to repeat the above-described procedure.

Referring to FIGS. 5-7, a second embodiment of the present invention will be described. Since this embodiment is essentially similar to the first embodiment, the same constituent parts will be designated by the same reference numerals, and a detailed description thereof will not be made to avoid redundancy.

As shown in FIG. 6, the charge roller 103 is implemented as an assembly of circular roller elements and also formed by winding conductive rubber having a volume resistivity of 10.sup.4 .OMEGA.cm or less around a metallic shaft. The roller elements may each be about 3 mm wide and spaced apart about 3 mm from nearby elements. FIG. 7 shows the power source 110 which is a DC power source capable of generating a voltage of 4 kV. As shown in FIG. 5, a balancer 101f is mounted on the downstream end of the flat plate 101. The balancer 101f is selected such that when the plate 101 protruded above the sheet stack 1 most, the upstream side of the plate 101 falls onto the sheet stack 1.

The operation of this embodiment will be described hereinafter, except for the steps identical with the steps of the first embodiment. As the drive roller 102 driven to move the flat plate 101 to above the sheet stack 1, an electrostatic stripe pattern parallel to the sheet feed direction is formed on the surface of the dielectric layer 101a of the plate 101 by the voltage applied via the charge roller 103. The plate 101 usually held in a position where the side thereof where the balancer 101f is located is lower than the other side. When the plate 101 is protruded above the stack 1 most, it rotates about the point thereof held between the drive roller 102 and charge roller 103. As a result, the side of the plate 101 remote from the balancer 101f falls onto the uppermost sheet 1a. Thereafter, as the drive roller 102 is reversed, the plate 101 is returned to the downstream side. As soon as the weight of the side of the plate 101 where the balancer 101f is located overcomes the weight of the other side, the plate 101 is moved away from the sheet stack 1 while lifting the uppermost sheet 1a therewith. As a result, the sheet 1a is separated from the underlying sheet 1b.

A reference will be made to FIGS. 8-12 for describing a third embodiment of the present invention. As shown in FIG. 8, the device has the tray 2 including the bottom plate 2a. The cut sheets 1 are stacked on the bottom plate 2a and surrounded by the front fence 2b , side fences 2c, and rear fence 2d. The front fence 2b is notched in the perpendicular direction over a width greater than the width of a flat plate 201 which will be described. The downstream side of the bottom plate 2a is elevated by an elevating mechanism, not shown, as needed. The elevation sensing means 5 is disposed above the downstream side of the stack 1. The sensing means 5 is responsive to the elevation of the sheet stack 1 and implemented by a reflection type photoelectric sensor. The upper and lower transport guides 3 are located downstream of the stack 1. Also located downstream of the stack 1 are the flat plate 201, a drive roller 202, a charge roller 203, FIG. 9, a plate sensing means 204 implemented by a reflection type photosensor, support brackets 205, and biasing means 206 and 207.

As shown in FIG. 10, the flat plate 201 is made up of a dielectric layer 201a whose volume resistivity is 10.sup.12 .OMEGA.cm, and a conductive layer 201b whose volume resistivity is 10.sup.4 .OMEGA.cm or less. The dielectric layer 201a, for example, faces the top of the sheet stack 1. As shown in FIG. 8, the plate 201 is in the form of a rectangle having a smaller width than the stack 1 and extending in the sheet reed direction. Further, the plate 201 includes a curved portion where the surface of the dielectric layer 201a is convex. As shown in FIG. 8, the drive roller 202 has a gear-toothed portion 202a meshing with teeth 201c provided on the flat plate 201. As shown in FIG. 9, the flat plate 201 is held between the drive roller 202 and the charge roller 203 and is moved in a reciprocating motion above the sheet stack 1 in the sheet feed direction as the drive roller 202 is rotated. The plate 201 has a metallic gloss except for two extremely small non-reflective portions 201d. The plate sensing means 204 disposed above the plate 201 determines the position of the plate 201 by sensing either of the non-reflective portions 201d.

The brackets 205 of the charge roller 203 are rotatable coaxially with the shaft of the drive roller 202. The biasing means 206 constantly biases the charge roller 203 toward the drive roller 202. The other biasing means 207 constantly biases the brackets 205 to the upstream side. As shown in FIG. 12, the charge roller 203 is made up of a roller portion 203a, a shaft 203b, and a torque limiter 203c intervening between the roller portion 203a and the shaft 203b. The torque limiter 203c generates a torque when the roller portion 203a is rotated at least in the direction for moving the fiat plate 201 to the downstream side, and it is reversible. The drive roller 202 and charge roller 203 both are conductive, and each is constituted by winding conductive rubber having a volume resistivity of 10.sup.4 .OMEGA.cm or less around a metallic shaft. The drive roller 202 is connected to ground via the transport guides 3 and other metallic parts. The charge roller 203 extends over substantially the entire width of the plate 201. The roller portion 203a, FIG. 12, is connected to the power source 210, FIG. 10, via a leaf spring 203d, the shaft 203b, and a feed section 211 and insulated from the other constituent parts.

As shown in FIG. 10, the power source 210 has a charge voltage generating section 210a made up of a waveform generator 210b and a voltage amplifier 210c, a discharge voltage generating section 210d made up of a waveform generator 210e and a voltage amplifier 210f, and a switching means 210g for selecting either of the voltage generating sections 210a and 210d. The power source 210 applies an alternating voltage to the plate 210 by using the drive roller 202 and charge roller 203 as electrodes. An arrangement may be so made as to discharge only a part of a charging area by any conventional scheme.

The upper transport guide 3 adjoining the drive roller 202 has a removed portion or notch 3c in a part thereof which would otherwise interfere with the plate 201 if the plate 201 were returned to the downstream side. The distance between the front end of the stack 1 and the transport roller pair 4 is selected to be shorter than the length which the plate 101 would overly the stack 1 when protruded onto the stack 1 most.

A reference will also be made to FIGS. 11A-11E for describing the operation of the third embodiment. Before a sheet feeding operation begins, the bottom plate 2a of the tray 2 loaded with the stack 1 has the downstream side thereof angularly raised by an elevating mechanism, not shown. When the top of the stack 1 reaches an adequate position as determined by the elevation sensing means 5, FIG. 8, the elevating mechanism stops operating. Such a procedure is repeated every time the sheet stack 1 decreases in height due to the successive sheet feed, so that the top of the stack 1 remains at a constant level.

As a control means, not shown, causes the device to start feeding a sheet, the drive roller 202 is rotated in a direction indicated by an arrow in FIG. 11A. Then, the flat plate 201 is moved to above the stack 1 due to the gear portion 202a and teeth 201c meshing with each other, as shown in FIG. 11A. At the same time, the waveform generator 210a of the power source 210 generates a sinusoidal wave of predetermined frequency. As a result, an alternating voltage of .+-.2 kV is generated via the voltage amplifier 210b. At this time, the switching means 210g has connected the charge voltage generating section 210a to the charge roller 203. Hence, the alternating voltage is applied to the plate 201 via the drive roller 202 and charge roller 203. This voltage forms an electrostatic pattern alternating in a stripe configuration on the surface of the dielectric layer 201a of the plate 201. The pitch of such a stripe pattern is selected to be 5 mm to 20 mm on the surface of the plate 201 on the basis of the feed speed of the plate 201 and the frequency of the alternating voltage of the power source 210. Further, the charge roller 203 is sequentially moved toward the upstream side about the shaft of the drive roller 202 together with the brackets 205 due to the pulling force of the plate 201 and the bias of the biasing means 207.

As shown in FIG. 11B, when the plate 201 is moved to above the stack 1 most, the plate sensing means 204 senses one of the non-reflective portion 201d of the plate 201. On receiving the resulting output of the sensing means 204, a control means, not shown, reverses the drive roller 202 to thereby move the plate 201 in the opposite direction. The plate 201 in turn urges the charge roller 203 to the downstream side. At this instant, since the force of the biasing means 207 is selected to be smaller than the force of the plate 201 acting on the charge roller 203, the charge roller 203 is moved to the downstream side about the shaft of the drive roller 202 together with the brackets 205. As a result, the plate 201 held between the rollers 202 and 203 is further rotated about the shaft of the roller 202 until it abuts against the uppermost sheet 1a.

The uppermost sheet 1a is electrostatically adhered to the plate 201 due to the electrostatic pattern formed on the plate 201. At this instant, despite that the angle at which the plate 201 contacts the uppermost sheet 1a depends on the amount of the stack 1, the plate 201 can surely cause the uppermost sheet 1a to adhere thereto due to the previously mentioned convex portion. Further, as shown in FIG. 11 C, since the uppermost sheet 1a adheres to the plate 201 in a curved configuration, it is positively separated from the underlying sheet 1b. The control means reverses the drive roller 202 and, at the same time, causes the switching means 210g of the power source 210 to select the discharge voltage generating section 210d. The waveform generator 210e and voltage amplifier 210f of the voltage generating section 210d generate an alternating voltage of about .+-.800 V and having a frequency which is one-tenth or less of the output voltage of the other voltage generating section 210a. Consequently, an alternating voltage is applied to the surface of the plate 201 for erasing the electrostatic pattern ascribable to the previous charging.

The sheet 1a electrostatically adhered to the plate 201 is moved to the left, as viewed in the figures, by being nipped by the drive roller 202 and charge roller 203 together with the plate 201. The part of the sheet 1a moved away from the rollers 202 and 203 is separated from the plate 201 by the discharge voltage applied thereto from the charge roller 203.

When the sheet 1a is adhered to the plate 201, it may occur that the underlying sheet 1b is moved toward the nipping portion of the rollers 202 and 203 due to the irregularity in the cut edges of the sheets, friction between the sheets, etc. In the illustrative embodiment, when only the plate 201 or only the plate 201 and sheet 1a are moved to the downstream side via the nipping portion, the friction between the plate 201 or sheet 1a and the charge roller 203 overcomes the torque of the torque limiter 203c. As a result, the charge roller 203 is rotated by the plate 201 which is in movement. However, when the second sheet 1b is moved to the nipping portion together with the sheets 1a, the torque of the torque limiter 203c overcomes the friction between the sheets 1a and 1b and prevents the sheet 1b from reaching the nipping portion.

As shown in FIG. 11D, before the sheet 1a is entirely separated from the plate 201, the leading edge thereof arrives at the nipping portion of the transport roller pair 4. Thereafter, the sheet 1a is transported by the roller pair 4. When the plate 201 is returned or pulled back most, the plate sensing means 204 senses the other non-reflective portion 201d with the result that the control means stops driving the drive roller 202 and power source 210. The charge roller 203 is returned to the upstream side by the biasing means 207 together with the brackets 205, as shown in FIG. 11E.

When the sheet 1b should be fed just after the sheet 1a, the drive roller 202 is gain driven just after the trailing edge of the sheet 1a has moved away from the nipping portion of the drive roller 202 and charge roller 203. This is followed by the procedure described above.

While the shaft 203b of the charge roller 203 has been shown and described as being fixed in place, it may be driven in a direction for returning the sheet 1 to the upstream side. Then, even when two or more sheets arrive at the nipping portion of the drive roller 202 and charge roller 203, only the uppermost sheet 1a will be more surely separated from the others.

Referring to FIGS. 13-16E, a fourth embodiment of the present invention will be described. As shown in FIG. 13, the cut sheets 1 are also stacked on the tray 2 capable of accommodating about 200 sheets at a maximum. The tray 2 includes a front fence which is formed with a perpendicular notch greater in width than a flat plate 301 which will be described. An upper and a lower transport guide 3 are disposed downstream of the sheet stack 1. Also disposed downstream of the stack 1 are a plate sensing means 304 implemented by a reflection type photosensor, brackets 305, a biasing means 307, a solenoid 308, and guide grooves 309.

The flat plate 301 is flexible and constituted by about 1 mm thick PET. As shown in FIG. 15, the plate 301 is made up of a dielectric layer 301a whose volume resistivity is 10.sup.12 .OMEGA.cm, and a conductive layer 301b whose volume resistivity is 10.sup.4 .OMEGA.cm or less. The dielectric layer 301a, for example, faces the top of the stack 1. The plate 301 is in the form of a rectangle having a smaller width than the sheet stack 1 and extending in the sheet feed direction. As shown in FIG. 14, the plate 301 is held between a drive roller 302 and a charge roller 303 and is moved in a reciprocating motion above the stack 1 in the sheet feed direction as the drive roller 302 is rotated. The plate 301 has a metallic gloss except for two extremely small non-reflective portions 301d. The plate sensing means 304 disposed above the plate 301 determines the position of the plate 301 by sensing either of the non-reflective portions 301d.

The brackets 305 of the charge roller 303 are rotatable coaxially with the shaft of the drive roller 302. The biasing means 307 constantly biases the charge roller 303 toward the drive roller 302. The brackets 305 are rotated by the solenoid 308. The drive roller 302 and charge roller 303 both are conductive, and each is constituted by winding conductive rubber having a volume resistivity of 10.sup.4 .OMEGA.cm or less around a metallic shaft. The drive roller 302 is connected to ground via the transport guides 3 and other metallic parts, as shown in FIG. 15. The charge roller 303 is connected to a power source 310, FIG. 13, via a feed section 311 and is insulated from the other constituent parts.

As shown in FIG. 15, the power source 310 has a charge voltage generating section 310a made up of a waveform generator 310b and a voltage amplifier 310c, a discharge voltage generating section 310d made up of a waveform generator 310e and a voltage amplifier 310f, and a switching means 310g for selecting either of the voltage generating sections 310a and 310d. The power source 310 applies an alternating voltage to the plate 301 by using the drive roller 302 and charge roller 303 as electrodes. An arrangement may be so made as to discharge only a part of a charging area by any conventional scheme.

The transport guides 3 are each made up of a horizontal portion adjoining the drive roller 302 and charge roller 303, and a perpendicular portion extending from the downstream end of the horizontal portion. The upper transport guide 3 adjoining the drive roller 302 has a notch 3c for accommodating the plate 301 when the plate 301 is pulled back most. The guide grooves 309 are formed in the intermediate portion of the upper transport guide 3 at both sides of the notch 3c so as to guide the edges of the plate 301. In the perpendicular portion of the upper transport guide 3, the guide grooves 309 are shaped such that the downstream end of the plate 301 moves away from the guide 3, as shown in FIG. 13. The transport roller pair 4 is located just downstream of the curved portion of the transport guides 309 where the horizontal portion merges into the perpendicular portion, and at both sides of the guide grooves 309. Particularly, the nipping portion of the transport roller pair 4 is located substantially at the same position as the position where the guide grooves 309 are directed away from the guide plate 3.

The operation of this embodiment will be described with reference to FIGS. 16A-16E. As a control means, not shown, causes the sheet feed device to start operating, the drive roller 302 is rotated in a direction indicated by an arrow in FIG. 16A. Then, the flat plate 301 held between the drive roller 302 and charge roller 303 is driven to above the sheet stack 1. At the same time, in the power source section 310, the waveform generator 310b generates a sinusoidal wave of predetermined frequency. The charge voltage generating section 310a, therefore, generates an alternating voltage of .+-.2 kV via the voltage amplifier 310c. At this instant, the switching means 310g has connected the charge voltage generating section 310g to the charge roller 303. The alternating voltage is applied to the plate 301 via the drive roller 302 and charge roller 303. As a result, an electrostatic pattern alternating in a stripe configuration is formed on the surface of the dielectric layer 301a of the plate 301. The pitch of the stripe pattern is selected to be 5 mm to 20 mm on the surface of the plate 301 on the basis of the feed speed of the plate 301 and the frequency of the alternating voltage. The biasing means 307 causes the charge roller 202 to move to the upstream side about the shaft of the drive roller 302 together with the brackets 305.

As shown in FIG. 16B, when the plate 301 is fed out most, the plate sensing means 304 senses one of the non-reflective portions 301d of the plate 301. In response to the resulting output of the sensing means 304, the control means energizes the solenoid 308 for an extremely short period of time. Since the force of the biasing means 307 is selected to be smaller than the pulling force of the solenoid 308, the charge roller 303 is moved to the downstream side about the shaft of the drive roller 302 together with the brackets 305. As a result, the plate 301 held between the drive roller 302 and charge roller 303 is bent in the vicinity of the nipping portion of the rollers 302 and 303, as shown in FIG. 16C. Consequently, the plate 301 abuts against the uppermost sheet 1a of the stack 1. It is noteworthy that the plate 301 is flexible and, therefore, surely contacts the sheet 1a with no regard to the amount of the stack 1. The sheet 1a electrostatically adheres to the plate 301 due to the electrostatic pattern formed on the plate 301. Subsequently, as the control means deenergizes the solenoid 308, the sheet 1a is lifted together with the plate 301. Since the force of the electrostatic pattern is so adjusted as not to act on the sheet 1b underlying the sheet 1a, the sheet 1a is surely separated from the sheet 1b, as shown in FIG. 16D.

After the above procedure, the control means reverses the drive roller 302. Then, the plate 301 is moved to the left, as viewed in the figures, along the guide grooves 309 while carrying the sheet 1a therewith. When the leading edge of the sheet 1a moves, for example, about 30 mm away from the nipping portion of the drive roller 302 and charge roller 303, the plate sensing means 304 senses the other non-reflective portion 301d. On receiving the resulting output of the sensing means 304, the control means causes the switching means 310 of the power source to select the discharge voltage generating means generating means. The waveform generator 310e and voltage amplifier 310f generate an alternating voltage of about .+-.800 V and having a frequency which is one-tenth or less of the voltage of the charge voltage generating section 310a. As a result, an alternating voltage which erases the electrostatic pattern is applied to the surface of the plate 301. The sheet 1a passed the nipping portion of the drive roller 302 and charge roller 303 is separated, except for the leading edge portion thereof, from the plate 301 by the alternating voltage applied from the charge roller 303.

The sheet 1a is transported along the path between the transport guides 3 with the leading edge portion thereof continuously retained by the plate 301. The plate 301 is moved away from the guide plates 3 by having the leading edge thereof bent along the guide grooves 309. The leading edge portion of the sheet 1a arrived at the transport roller pair 4 is separated from the plate 301 due to the curvature of the plate 301. Thereafter, the sheet 1a is driven by the transport roller pair 4. This prevents the sheet 1a from jamming the path at the bent portion of the transport guides 3 and, in addition, eliminates the need for rollers exerting a great nipping force otherwise located at the bent portion. As shown in FIG. 16E, as soon as the plate sensing means 304 senses the other non-reflective portion 301d, the control means stops driving the drive roller and plate 301.

To feed the sheet 1b after the sheet 1a, the control means again starts driving the drive roller 302 just after the trailing edge of the sheet 1a has passed the nipping portion of the rollers 302 and 303. This is followed by the procedure described above.

While the embodiments have concentrated on a flat plate having a flat surface and movable along a linear path, they are similarly practicable with a bent plate movable in a rotary motion.

In summary, it will be seen that the present invention provides a sheet feed device which feeds a sheet by the simple reciprocating motion of a flat plate and electrostatic adhesion. The electrostatic adhesion effects more stable sheet feed against aging and environmental changes than friction. By adjusting the electrostatic adhesion, it is possible to reduce the probability of the simultaneous feed of two or more sheets. Since the flat plate is retracted except during sheet feed operation by the reciprocating motion thereof, the operator can supplement sheets with ease under safe conditions. In addition, the operator is prevented from inadvertently touching the flat plate; otherwise, the sheet feeding ability would be degraded.

Various modifications will become possible for those skilled in the art after receiving the teachings of the present disclosure without departing from the scope thereof.