Title:
Sheet feeding unit
Kind Code:
A1


Abstract:
A sheet feeding unit including a feeding system, a controlling system to control the feeding system, and a detecting system, which is provided in between the target position and the feeding system and is adapted to detect a predetermined position of the recording sheet at a detecting position, is provided. The controlling system controls the feeding system to carry the recording sheet at a second feeding speed, at a third feeding speed, and at a first feeding speed, so that a period in which the predetermined position of the recording sheet being carried at the first feeding speed changed from the third feeding speed is carried from the detecting position to the target position is equivalent to a period in which the predetermined position of the recording sheet being carried constantly at the first feeding speed is carried from the detecting position to the target position.



Inventors:
Takano, Masatoshi (Saitama-ken, JP)
Application Number:
11/430998
Publication Date:
11/16/2006
Filing Date:
05/10/2006
Assignee:
PENTAX Corporation (Tokyo, JP)
Primary Class:
International Classes:
B65H5/34
View Patent Images:
Related US Applications:



Primary Examiner:
GOKHALE, PRASAD V
Attorney, Agent or Firm:
GREENBLUM & BERNSTEIN, P.L.C. (RESTON, VA, US)
Claims:
What is claimed is:

1. A sheet feeding unit, comprising; a feeding system, which is adapted to carry a recording sheet to a target position on a feeding path of the recording sheet, a controlling system, which is adapted to control the feeding system so that the recording sheet is carried at a first feeding speed, at a second feeding speed being slower than the first feeding speed, and at a third feeding speed being faster than the first feeding speed, and a detecting system, which is provided in between the target position and the feeding system and is adapted to detect a predetermined position of the recording sheet at a detecting position, wherein the controlling system controls the feeding system to carry the recording sheet at the second feeding speed until the detecting system detects the predetermined position of the recording sheet, at the third feeding speed changed from the second feeding speed, and at the first feeding speed changed from the third feeding speed, so that a period in which the predetermined position of the recording sheet being carried at the first feeding speed changed from the third feeding speed is carried from the detecting position to the target position is equivalent to a period in which the predetermined position of the recording sheet being carried constantly at the first feeding speed is carried from the detecting position to the target position.

2. The sheet feeding unit according to claim 1, wherein the feeding system is adapted to carry the recording sheet by a force provided from a motor.

3. The sheet feeding unit according to claim 2, wherein the motor is a stepping motor.

4. The sheet feeding unit according to claim 3, wherein the second feeding speed is caused by torque within a pull-in torque range of the stepping motor.

5. The sheet feeding unit according to claim 1, wherein the predetermined position of the recording sheet includes a front end of the recording sheet.

6. The sheet feeding unit according to claim 1, wherein a reflectance of the predetermined position of the recording sheet is different from a reflectance of the other area of the recording sheet excluding the predetermined position.

7. The sheet feeding unit according to claim 1, wherein a transmittance of the predetermined position of the recording sheet is different from a transmittance of the other area of the recording sheet excluding the predetermined position.

8. The sheet feeding unit according to claim 1, wherein the recording sheet is carried at the first feeding speed when the recording sheet is at the target position on the feeding path.

9. An image forming apparatus, comprising; a sheet feeding unit, which is adapted to carry a recording sheet and to guide the recording sheet in a target position on a feeding path in the image forming apparatus, and an image forming unit, which is adapted to form an image on the recording sheet at the target position on the feeding path, wherein the sheet feeding unit includes a feeding system, which is adapted to carry the recording sheet to the target position on the feeding path of the recording sheet, a controlling system, which is adapted to control the feeding system so that the recording sheet is carried at a first feeding speed, at a second feeding speed being slower than the first feeding speed, and at a third feeding speed being faster than the first feeding speed, and a detecting system, which is provided in between the target position and the feeding system and is adapted to detect a predetermined position of the recording sheet at a detecting position, wherein the controlling system controls the feeding system to carry the recording sheet at the second feeding speed until the detecting system detects the predetermined position of the recording sheet, at the third feeding speed changed from the second feeding speed, and at the first feeding speed changed from the third feeding speed, so that a period in which the predetermined position of the recording sheet being carried at the first feeding speed changed from the third feeding speed is carried from the detecting position to the target position is equivalent to a period in which the predetermined position of the recording sheet being carried constantly at the first feeding speed is carried from the detecting position to the target position.

10. A sheet feeding unit comprising; a feed roller, which is adapted to carry a recording sheet to a predetermined location on a feeding path, a stepping motor, which is adapted to rotate the feed roller, a controlling system, which is adapted to control a feeding speed of the recording sheet being carried by the feed roller, a detecting system, which is adapted to detect a predetermined position of the recording sheet, a retainer system, which is adapted to retain the recording sheet at an upstream side of the recording sheet from the feed roller with respect to a normal feeding direction of the recording sheet, and a load adjusting system, which is provided on the feeding path in between the feed roller and the retainer system so that a load applied on the feed roller to carry the recording sheet is adjusted, wherein the controlling system is adapted to switch the feeding speed of the recording sheet from a first feeding speed to a second feeding speed before the predetermined position of the recording sheet is detected by the detecting system, the first feeding speed corresponding to a rotation frequency that is within a pull-in torque range of the stepping motor, the second feeding speed being faster than the first feeding speed and corresponding to a rotation speed that is out of the pull-in torque range of the stepping motor, and wherein the load adjusting system is adapted to reduce the load applied on the feed roller to carry the recording sheet when the feeding speed of the recording sheet is switched from the first feeding speed to the second feeding speed.

11. The sheet feeding unit according to claim 10, wherein a part of the feeding path between the feed roller and the retainer system is shortened so that the load applied on the feed roller to carry the recording sheet is reduced.

12. The sheet feeding unit according to claim 10, wherein the load adjusting system is provided with a guide roller to guide the recording sheet in the feeding path, and wherein the guide roller is adapted to be shifted so that the part of the feeding path between the feed roller and the retainer system is shortened when the feeding speed of the recording sheet is switched from the first feeding speed to the second feeding speed.

13. The sheet feeding unit according to claim 12, wherein the load adjusting system includes a supporting member, which rotatably supports the guide roller and swingably rotates about an axis, and wherein the supporting member is attracted in a direction so that the part of the feeding path between the feed roller and the retainer system is elongated by the attraction.

14. The sheet feeding unit according to claim 9, wherein the recording sheet is in a continuous form, and wherein the retainer system retains the recording sheet by rolling the recording sheet around the retainer system.

15. An image forming apparatus, comprising; a sheet feeding unit, which is adapted to carry a recording sheet and to guide the recording sheet to a predetermined location on a feeding path in the image forming apparatus, and an image forming unit, which is adapted to form an image on the recording sheet at the predetermined location on the feeding path, wherein the sheet feeding unit includes a feed roller, which is adapted to carry the recording sheet to the predetermined location on the feeding path, a stepping motor, which is adapted to rotate the feed roller, a controlling system, which is adapted to control a feeding speed of the recording sheet being carried by the feed roller, a detecting system, which is adapted to detect a predetermined position of the recording sheet, a retainer system, which is adapted to retain the recording sheet at an upstream side of the recording sheet from the feed roller with respect to a normal feeding direction of the recording sheet, and a load adjusting system, which is provided on the feeding path in between the feed roller and the retainer system so that a load applied on the feed roller to carry the recording sheet is adjusted, wherein the controlling system is adapted to switch the feeding speed of the recording sheet from a first feeding speed to a second feeding speed before the predetermined position of the recording sheet is detected by the detecting system, the first feeding speed corresponding to a rotation frequency that is within a pull-in torque range of the stepping motor, the second feeding speed being faster than the first feeding speed and corresponding to a rotation speed that is out of the pull-in torque range of the stepping motor, and wherein the load adjusting system is adapted to reduce the load applied on the feed roller to carry the recording sheet when the feeding speed of the recording sheet is switched from the first feeding speed to the second feeding speed.

Description:

BACKGROUND OF THE INVENTION

The present invention relates to an image forming apparatus capable of forming an image on a recording sheet in an electrophotographic method, and particularly to a sheet feeding unit of the image forming apparatus capable of controlling feeding speed of a recording sheet.

Conventionally, an image forming apparatus employing an electrophotographic technology, such as a copier and a laser beam printer, is known. Such an image forming apparatus is adapted to form a latent image corresponding to image data on a surface of a photoconductive drum by laser beam or the like. When toner is adhered to the latent image, a toner image is formed on the surface of the photoconductive drum. The toner image is then in a developing unit transferred to a surface of a recording sheet that has been introduced via a sheet feed roller of a sheet feeding unit, and is permanently fixed on the recording sheet in a fixing unit. This type of image forming apparatus is often equipped with a feeding unit including feed rollers and a sensor to detect the recording sheet, and the recording sheet is carried from a sheet inserting portion to the developing unit via the feed roller, and the sensor.

The feed rollers are generally rotated by a stepping motor, and with the rotating force carry the recording sheet nipped therebetween. The sensor detects a predetermined position of the recording sheet being carried, so that the image forming apparatus can synchronize emission of the laser beam on the photoconductive drum based on the detected position of the recording sheet.

In Japanese Patent Provisional Publication No. HEI11-265096, a scanning device with an original sheet feeding unit to scan an image on an original whilst the original is being carried to the scanning unit is disclosed. In the scanning device, the original being carried is paused when a front end of the original reaches a predetermined position at an upstream side of the scanning unit. Thereafter, according to timing of the scanning unit to scan the original, a pulse signal to accelerate rotation of the stepping motor, starting from a pull-in torque range, in a substantially low rate to avoid stepping out of the motor is inputted to the stepping motor, so that the stepping motor can be rotated at a speed that is beyond the pull-in torque range. In this configuration, the original can be carried to the scanning unit, which corresponds to the developing unit of the image forming apparatus, at a feeding speed corresponding to the scanning speed that may otherwise be limited within the pull-in torque range of the stepping motor. Thus, a faster feeding speed of the original is attained.

A faster speed for feeding the recording sheet in the image forming apparatus as described above is preferable as well. The mechanism of the original sheet feeding unit of the scanning device in the above-referenced publication, for example, may be employed in the image forming apparatus so that the recording sheet can be carried to the developing unit at a faster speed without the limitation of the pull-in torque range. In the image forming apparatus, however, a feeding speed at the sensor and a feeding speed at the developing unit are required to be equivalent, as detection of the predetermined position of the recording sheet and the emission of the laser beam to the photoconductive drum must be synchronized. When the recording sheet is carried at a faster feeding speed that requires torque beyond the pull-in torque range, the stepping motor requires to be controlled to accelerate in a substantially moderate rate to avoid stepping out. In order for the motor to be properly accelerated, a substantial length to carry the recording sheet is required before the recording sheet is carried at the faster speed. Consequently, the image forming apparatus requires substantial space corresponding to the length to carry the recording sheet between the feeding rollers and the sensor, and thus a size of the image forming apparatus may be increased.

SUMMARY OF THE INVENTION

In view of the foregoing shortcomings, the present invention is advantageous in that a sheet feeding unit capable of feeding the recording sheet at a faster speed is provided, whilst an entire configuration of the image forming apparatus can be downsized.

According to an aspect of the invention, there is provided a sheet feeding unit, including a feeding system, which is adapted to carry a recording sheet to a target position on a feeding path of the recording sheet, a controlling system, which is adapted to control the feeding system so that the recording sheet is carried at a first feeding speed, at a second feeding speed being slower than the first feeding speed, and at a third feeding speed being faster than the first feeding speed, and a detecting system, which is provided in between the target position and the feeding system and is adapted to detect a predetermined position of the recording sheet at a detecting position, is provided. The controlling system controls the feeding system to carry the recording sheet at the second feeding speed until the detecting system detects the predetermined position of the recording sheet, at the third feeding speed changed from the second feeding speed, and at the first feeding speed changed from the third feeding speed, so that a period in which the predetermined position of the recording sheet being carried at the first feeding speed changed from the third feeding speed is carried from the detecting position to the target position is equivalent to a period in which the predetermined position of the recording sheet being carried constantly at the first feeding speed is carried from the detecting position to the target position.

Optionally, the feeding system may be adapted to carry the recording sheet by a force provided from a motor.

Optionally, the motor may be a stepping motor.

Optionally, the second feeding speed may be caused by torque within a pull-in torque range of the stepping motor.

Optionally, the predetermined position of the recording sheet may include a front end of the recording sheet.

Optionally, a reflectance of the predetermined position of the recording sheet may be different from a reflectance of the other area of the recording sheet excluding the predetermined position.

Optionally, a transmittance of the predetermined position of the recording sheet may be different from a transmittance of the other area of the recording sheet excluding the predetermined position.

Optionally, the recording sheet may be carried at the first feeding speed when the recording sheet is at the target position on the feeding path.

According to an aspect of the invention, there is provided an image forming apparatus, including a sheet feeding unit, which is adapted to carry a recording sheet and to guide the recording sheet in a target position on a feeding path in the image forming apparatus, and an image forming unit, which is adapted to form an image on the recording sheet at the target position on the feeding path, is provided. The sheet feeding unit includes a feeding system, which is adapted to carry the recording sheet to the target position on the feeding path of the recording sheet, a controlling system, which is adapted to control the feeding system so that the recording sheet is carried at a first feeding speed, at a second feeding speed being slower than the first feeding speed, and at a third feeding speed being faster than the first feeding speed, and a detecting system, which is provided in between the target position and the feeding system and is adapted to detect a predetermined position of the recording sheet at a detecting position. The controlling system controls the feeding system to carry the recording sheet at the second feeding speed until the detecting system detects the predetermined position of the recording sheet, at the third feeding speed changed from the second feeding speed, and at the first feeding speed changed from the third feeding speed, so that a period in which the predetermined position of the recording sheet being carried at the first feeding speed changed from the third feeding speed is carried from the detecting position to the target position is equivalent to a period in which the predetermined position of the recording sheet being carried constantly at the first feeding speed is carried from the detecting position to the target position.

According to an aspect of the invention, there is provided a sheet feeding unit including a feed roller, which is adapted to carry a recording sheet to a predetermined location on a feeding path, a stepping motor, which is adapted to rotate the feed roller, a controlling system, which is adapted to control a feeding speed of the recording sheet being carried by the feed roller, a detecting system, which is adapted to detect a predetermined position of the recording sheet, a retainer system, which is adapted to retain the recording sheet at an upstream side of the recording sheet from the feed roller with respect to a normal feeding direction of the recording sheet, and a load adjusting system, which is provided on the feeding path in between the feed roller and the retainer system so that a load applied on the feed roller to carry the recording sheet is adjusted, is provided. The controlling system is adapted to switch the feeding speed of the recording sheet from a first feeding speed to a second feeding speed before the predetermined position of the recording sheet is detected by the detecting system. The first feeding speed corresponds to a rotation frequency that is within a pull-in torque range of the stepping motor. The second feeding speed is faster than the first feeding speed and corresponds to a rotation speed that is out of the pull-in torque range of the stepping motor. The load adjusting system is adapted to reduce the load applied on the feed roller to carry the recording sheet when the feeding speed of the recording sheet is switched from the first feeding speed to the second feeding speed.

Optionally, a part of the feeding path between the feed roller and the retainer system may be shortened so that the load applied on the feed roller to carry the recording sheet is reduced.

Optionally, the load adjusting system may be provided with a guide roller to guide the recording sheet in the feeding path. The guide roller may be adapted to be shifted so that the part of the feeding path between the feed roller and the retainer system is shortened when the feeding speed of the recording sheet is switched from the first feeding speed to the second feeding speed.

Optionally, the load adjusting system may include a supporting member, which rotatably supports the guide roller and swingably rotates about an axis. The supporting member may be attracted in a direction so that the part of the feeding path between the feed roller and the retainer system is elongated by the attraction.

Optionally, the recording sheet may be in a continuous form, and the retainer system may retain the recording sheet by rolling the recording sheet around the retainer system.

According to an aspect of the invention, there is provided an image forming apparatus, including a sheet feeding unit, which is adapted to carry a recording sheet and to guide the recording sheet to a predetermined location on a feeding path in the image forming apparatus, and an image forming unit, which is adapted to form an image on the recording sheet at the predetermined location on the feeding path, is provided. The sheet feeding unit may include a feed roller, which is adapted to carry the recording sheet to the predetermined location on the feeding path, a stepping motor, which is adapted to rotate the feed roller, a controlling system, which is adapted to control a feeding speed of the recording sheet being carried by the feed roller, a detecting system, which is adapted to detect a predetermined position of the recording sheet, a retainer system, which is adapted to retain the recording sheet at an upstream side of the recording sheet from the feed roller with respect to a normal feeding direction of the recording sheet, and a load adjusting system, which is provided on the feeding path in between the feed roller and the retainer system so that a load applied on the feed roller to carry the recording sheet is adjusted, is provided. The controlling system may be adapted to switch the feeding speed of the recording sheet from a first feeding speed to a second feeding speed before the predetermined position of the recording sheet is detected by the detecting system. The first feeding speed corresponds to a rotation frequency that is within a pull-in torque range of the stepping motor. The second feeding speed is faster than the first feeding speed and corresponding to a rotation speed that is out of the pull-in torque range of the stepping motor. The load adjusting system is adapted to reduce the load applied on the feed roller to carry the recording sheet when the feeding speed of the recording sheet is switched from the first feeding speed to the second feeding speed.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING

FIG. 1 is a diagram to illustrate a general configuration of an image forming apparatus according to a first embodiment of the present invention.

FIG. 2 is a diagram to illustrate a feeding unit according to the first embodiment of the present invention.

FIG. 3 is a flowchart to illustrate a controlling process of a feeding speed of the recording sheet according to the first embodiment of the present invention.

FIG. 4 is a diagram to illustrate the feeding speed of the recording sheet in the controlling process according to the first embodiment of the present invention.

FIG. 5 is a diagram to illustrate a general configuration of an image forming apparatus according to a second embodiment of the present invention.

FIG. 6 is a flowchart to illustrate a controlling process of feeding speed of the recording sheet according to the second embodiment of the present invention.

FIG. 7 is a diagram to illustrate a relation between timing and the feeding speed of the recording sheet in the controlling process according to the second embodiment of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Referring to the accompanying drawings, an image forming apparatus with a feeding unit of a recording medium according to an embodiment of the present invention will be described in detail. FIG. 1 is a diagram to illustrate a general configuration of an image forming apparatus 100 according to a first embodiment of the present invention. The image forming apparatus 100, which is often used as an output device for a computer, is adapted to form an image on a continuous form recording sheet (hereinafter referred to as a recording sheet) 10 in an electrophotographic method by exposing a surface of a photoconductive drum 4 to a laser beam modulated according to information (image data) inputted by a user.

As shown in FIG. 1, the image forming apparatus 100 is provided with a feeding unit 50 including a core roll 1, around which the recording sheet 10 is rolled, feed rollers 2a, 2b, and a fed sheet sensor 3 (the feed rollers 2a, 2b and the fed sheet sensor 3 as a feeding unit). The image forming apparatus 100 is further provided with a developing unit (an image forming unit) 60 including a photoconductive drum 4, an intermediate transfer roller 5, a secondary transfer roller 6, and an optical unit 9a for outputting laser beam 9, and with a discharge portion 70 including fixing rollers 7a, 7b, and a discharged sheet sensor 8. It should be noted that in the present embodiment the recording sheet 10 is a rolled recording paper, however, the recording medium may not necessarily be in a continuous form, but may be a sheet of cut paper, for example.

The recording sheet 10 is forwarded in the image forming apparatus 100 from the feeding unit 50 to the discharge portion 70 via the developing unit 60. When image forming (i.e., printing) is completed, a front end portion of the recording sheet 10 on which the image is formed is discharged from the image forming apparatus 100 and is cut off with a cutter and the like automatically or manually. In this configuration, a newly created front end of the remaining recording sheet 10 is positioned in the discharge portion 70. When a new printing operation is conducted with the front end of the recording sheet in the discharge portion 70 (a normal print operation), a front end portion of the recording sheet 10 between the developing unit 60 and the discharge portion 70 is not provided with a new image, thus the front end portion is wasted. In consideration of this drawback, the image forming apparatus 100 may be configured to rewind the recording sheet 10 so that the front end is brought back to the feeding unit 50 before a new printing operation is started (a reverse-and-print operation).

The recording sheet 10 is rolled around a core roll 1 that is rotatably supported by a predetermined supporting member (not shown). A front end portion of the recording sheet 10 is unwound and directed to the feed rollers 2a, 2b by a guide 1a that extends in an axial direction of the core roll 1. The core roll 1 is adapted to rotate in a clockwise direction in FIG. 1 (hereinafter referred to as a normal direction) when the feed rollers 2a, 2b rotate to forward the recording sheet 10 and thereby pull the same. When a reverse-and-print operation is conducted, the feed rollers 2a, 2b rotate in respective reverse directions to rewind the recording sheet 10 after a previous printing operation is completed or before a new printing operation is started. The core roll 1 includes a tensile member (not shown) that rotates the core roll 1 in a counterclockwise direction in FIG. 1 (hereinafter referred to as a reverse direction) so that the recording sheet 10 is tensioned between the feed rollers 2a, 2b and the core roll 1.

The feed roller 2a is rotated by rotating force provided from a drive motor (a stepping motor) M. When the recording sheet 10 is fed in a direction from the feeding unit 50 toward the discharge portion 70 (i.e., a normal sheet feeding direction), the feed roller 2a rotates in a normal feeding direction (i.e., the counterclockwise direction in FIG. 1), and when the recording sheet 10 is reversed (i.e., in a reverse sheet feeding direction), the feed roller 2a rotates in a reverse feeding direction (i.e., the clockwise direction in FIG. 1). In a reverse-and-print operation, the front end of the recording sheet 10, which is positioned in a vicinity to the discharge portion 70, is brought back to a position between the feed roller 2a and the fed sheet sensor 3. A number of pulses of the drive motor M to be activated for the reverse-and-print operation may be fixed in advance so that a length of the recording sheet 10 to be reversed can be fixed. The feed roller 2b is adapted to be rotated according to the rotation of the feed roller 2a, in an opposite direction from the rotating direction of the feed roller 2a. It should be noted that the feed roller 2b is mutually abut at a predetermined nip pressure with the feed roller 2a, so that the recording sheet 10 is effectively carried between the two feed rollers 2a, 2b.

The fed sheet sensor 3 is adapted to detect a predetermined position of the recording sheet 10. The predetermined position may be, for example, a front end of the recording sheet 10, or may be a position of a mark provided on a surface of the recording sheet 10. When a label paper is used as the recording sheet 10, for example, the recording sheet 10 includes a base sheet, on which labels are arranged, and the labels, on which printing is provided. In such a case, a front end of each label or a marked position on the base sheet may be the predetermined position.

When a predetermined position of the recording sheet 10 such as the front end thereof passes by the fed sheet sensor 3, the fed sheet sensor 3 detects the same, and a timing of the laser beam 9 to be emitted is calculated based on the detected predetermined position of the recording sheet 10. It should be noted that with this configuration the image can be formed on a predetermined position with respect to the recording sheet 10.

The fed sheet sensor 3 may be provided with, for example, a light emitting element and a light receiving element. In an embodiment of the fed sheet sensor 3, the light emitting element and the light receiving element are arranged to oppose to each other, so that the recording sheet 10 is carried therebetween. As the front end of the recording sheet 10 passes by the fed sheet sensor 3, the light receiving element stops receiving light from the light emitting element, and thereby the front end of the recording sheet 10 is detected. When the label paper as the recording sheet 10 is used, and transmittances of the base sheet and the labels are different, the fed sheet sensor 3 can detect the front end of the label by a difference in the transmittances. Further, when the transmittances of the marked position on the recording sheet 10 and of the other area are different, the fed sheet sensor 3 can similarly detect the marked position by the difference in the transmittances. In another embodiment of the fed sheet sensor 3, the light emitting element and the light receiving element may be arranged in adjacent to each other. In this configuration, when the front end of the recording sheet 10 passes by the fed sheet sensor 3, the light emitted from the light emitting element is reflected on the surface of the recording sheet 10 and is received by the light receiving element so that the front end of the recording sheet 10 can be detected. When the label paper as the recording sheet 10 is used, and the reflectance of the base sheet only and the reflectance of the labels with the base sheet are different, the front end of the label can be detected. Further, when the reflectance of the marked position on the recording sheet 10 and of the other area are different, the fed sheet sensor 3 can similarly detect the marked position by the difference in the reflectance.

The photoconductive drum 4 is adapted to rotate in the counterclockwise direction in FIG. 1. The photoconductive drum 4 is applied voltage and uniformly charged to a predetermined level, which is approximately +700 V, by a charger (not shown). The photoconductive drum 4 is thereafter rotated and exposed to the laser beam 9 that scans the surface of the photoconductive drum 4 in parallel with a rotation axis of the photoconductive drum 4 (i.e., a main scanning direction) according to the image data, and a latent image is formed on the surface of the photoconductive drum 4, as regions where the latent image is formed gains a lower potential, for example as low as +100 V, due to an effect of the laser beam 9.

The photoconductive drum 4 with the latent image on the surface is further rotated, and between the region excluding the latent image on the photoconductive drum 4, of which the electric potential is approximately +100 V, and a surface of a developing roller 11, of which the electric potential is approximately +500V, the toner remains closely stuck to the lower-potential region i.e. the surface of the developing roller 11, without being transferred to the region of which the electric potential is approximately +700 V and where no latent image exists. Consequently, the region excluding the latent image is not developed. By contrast, between the latent image region on the surface of the photoconductive drum 4 and the surface of the developing roller 11, the toner performs electrophoresis toward the lower-potential region. That is, the toner adheres to the latent image region on the surface of the photoconductive drum 4. That is how the latent image on the photoconductive drum 4 is developed, to turn into a toner image.

To an intermediate transfer roller 5 that rotates in the clockwise direction, a transfer bias of a reverse polarity to the toner is applied, which is approximately −100 V, so that the toner image developed on the surface of the photoconductive drum 4 is transferred as a primary step to the intermediate transfer roller 5, at the interface between the photoconductive drum 4 and the intermediate transfer roller 5.

The intermediate transfer roller 5 and a secondary transfer roller 6 are disposed so as to oppose to each other across the paper path of the recording sheet 10, and mutually abut at a predetermined nip pressure. The secondary transfer roller 6 rotates in the counterclockwise direction, and is applied voltage of approximately −1 kV. The toner image transferred to the surface of the intermediate transfer roller 5 is transferred to the recording sheet 10 being carried along the paper path at the interface with the secondary transfer roller 6, by the effect of a transfer electric field, the nip pressure and so on, and thus the image is formed on the recording sheet 10. It should be noted that the feeding speed at the developing unit 60 is configured to be faster than the feeding speed of at the feeding unit 50 caused by the feed rollers 2a, 2b.

The secondary transfer roller 6 can be retracted in a position indicated in the dotted line in FIG. 1. When the secondary transfer roller 6 is retracted, the recording sheet 10 is set apart from the intermediate transfer roller 5. With this configuration, when the recording sheet 10 is rewound in the reverse-and-print operation, as the feed roller 2a rotates in the reverse feeding direction, the surface of the intermediate transfer roller can be prevented from being damaged by friction that may otherwise be caused by the recording sheet 10.

The recording sheet 10 that has passed the developing unit 60 is forwarded to the fixing rollers 7a, 7b. The fixing roller 7a is adapted to apply heat to the recording sheet 10, whilst the fixing roller 7b is adapted to apply pressure toward the fixing roller 7a. With these fixing rollers 7a, 7b, the toner image is fixed onto the recording sheet 10. It should be noted that the fixing roller 7b can be retracted in a position indicated in a dashed line in FIG. 1 so that the surface of the fixing roller 7a may not be damaged by friction when the recording sheet 10 is rewound.

The discharged sheet sensor 8 is adapted to detect a predetermined position of the recording sheet 10 with the toner image fixed thereto that passes by the discharged sheet sensor 8 itself. With this configuration, the image forming apparatus 100 can detect an erroneous condition of the recording paper 10. For example, the image forming apparatus 100 can be configured to determine that paper jam has occurred at some point between the fed sheet sensor 3 and the discharged sheet sensor 8, when the recording sheet 10 is not detected by the discharged sheet sensor 8 after a predetermined period of time has passed since the predetermined position of the recording sheet 10 had passed the fed sheet sensor 3.

Referring to FIG. 2, a feeding unit according to the first embodiment of the present embodiment will be described. FIG. 2 is a diagram to illustrate the feeding unit 50 with the developing unit 60 according to the first embodiment of the present invention. In the feeding unit 50, the recording sheet 10 is carried by the feed rollers 2a, 2b. The feed roller 2a is adapted to be rotated by the stepping motor M (see FIG. 1), and the feeding speed of the feeding unit 50 is controlled by controlling rotation speed of the stepping motor M.

A positional relation between the feeding unit 50 and the developing unit 60 will be described. In the present embodiment, a position in between the feed rollers 2a, 2b, whereat the recording sheet 10 is nipped, is represented as a feeding position A, whilst a position whereat the recording sheet 10 is detected by the fed sheet sensor 3 is represented as a detecting position C. A position whereat the toner image is transferred to the recording sheet 10 is represented as a transfer position D. A length between the feeding position A and the detecting position C is represented as a length L, and a length between the detecting position C and the transfer position D is represented as a length S. Further, a position on the surface of the photoconductive drum 4 whereat the laser beam is emitted is represented as an emission position E. A length including a circumferential length in the counterclockwise direction in FIG. 2 of the photoconductive drum 4 from the emission position E to a position whereat the photoconductive drum 4 is in contact with the intermediate transfer roller 5 (a position F) and a circumferential length in the clockwise direction in FIG. 2 of the intermediate transfer roller 5 from the position F to the transfer position D is represented as a path length (drawn in a thick line) N. At a ready position B, whereat the predetermined position of the recording sheet 10 is located when the image forming apparatus 100 is in a ready state and before the image forming apparatus 100 starts an printing operation.

In the present embodiment, the fed sheet sensor 3 is arranged at a predetermined position so that the length S is equivalent to the path length N, and the laser beam 9 is adapted to be emitted at the emission position E when the fed sheet sensor 3 detects the predetermined position of the recording sheet 10. Thus, a latent image formed by emitting the laser beam 9 at the emission position E is formed as a printed image on the predetermined position of the recording sheet 10, which was detected at the detecting position C. It should be noted that the fed sheet sensor 3 may be arranged at a position wherein the length S is longer than the path length N. In this configuration, the laser beam 9 is emitted when a predetermined period passed after the predetermined position of the recording sheet 10 had been detected at the detecting position C by the fed sheet sensor 3, so that the image can be formed on the predetermined position of the recording sheet 10.

FIG. 3 is a flowchart to illustrate a controlling process of the feeding speed of the recording sheet 10 according to the first embodiment of the present invention. As the process starts and the recording sheet is carried in the normal sheet feeding direction by the feed rollers 2a, 2b, the predetermined position of the recording sheet 10 is carried from the ready position B to the detecting position C. The predetermined position of the recording sheet 10 is carried to the detecting position C at a speed V1, which will be described in detail hereinbelow. In S10, the process examines whether the predetermined position of the recording sheet 10 is detected by the fed sheet sensor 3 at the detecting position C. When the predetermined position of the recording sheet 10 is detected (S10: YES), the process proceeds to S20. In S20, a feeding speed of the recording sheet 10 is accelerated in a predetermined rate until the feeding speed reaches V2(V2>V1), which will be described hereinbelow. When the feeding speed reaches V2, the feeding speed V2 remains constant until a predetermined period elapses.

In S30, the process examines whether the recording sheet 10 has been carried for the predetermined period at the feeding speed V2. The period to carry the recording sheet 10 is determined based on a calculation, which will be described in detail hereinbelow, so that dimensions of a shaded area a in FIG. 4 are equivalent to dimensions of a shaded area b. When the predetermined period elapses (S30: YES), the feeding speed of the recording sheet 10 is decelerated in a predetermined rate until the feeding speed reaches V0 (V1<V0<V2), which will be described in detail hereinbelow. When the feeding speed reaches V0, the feeding speed V0 remains constant so that the predetermined position of the recording sheet 10 is carried to the transfer position D at the feeding speed V0 (S40).

FIG. 4 is a diagram to illustrate the feeding speed of the recording sheet 10 in the controlling process executed in the feeding unit 50 according to the first embodiment of the present invention. When the image forming apparatus is in the ready state (i.e., before and after a printing operation,) the predetermined position of the recording sheet 10 is located at the ready position B. In the diagram, when t as time is 0, the predetermined position of the recording sheet 10 is detected at the detecting position C (see FIG. 2). The feeding speed V0, which is for example approximately 200 mm per second, is equivalent to a feeding speed of the recording sheet 10 at the developing unit 60 (i.e., the feeding speed V0 is equivalent to a speed of the image to proceed along the path length N). As an image forming operation is started, the stepping motor M (see FIG. 1) is activated to rotate the feed roller 2a. The feeding speed V1 of the recording sheet 10 (i.e., a rotation speed of the feed roller 2a), which can be reached instantaneously after the stepping motor M is activated, is determined based on a rotation frequency within the pull-in torque range of the stepping motor M. In the present embodiment, the feeding speed V1 (V1<V0) may be for example approximately 100 mm per second. The recording sheet 10 is thus carried at the feeding speed V1 until the predetermined position of the recording sheet 10 reaches the detecting position C. A length between the ready position B and the detecting position C is configured to be substantially long so that the feeding speed of the recording sheet 10 can be accelerated to V1 within the length.

When the predetermined position of the recording sheet 10 being carried at the feeding speed V1 is detected (t0), the stepping motor M is accelerated in a predetermined acceleration rate. Meanwhile, the feeding speed is increased from V1 to V0 when t is t1, and from V0 to V2 when t is t2 (V2>V0). From t2 to t3, the feeding speed is maintained constant at V2. At t3, the feeding speed is decelerated to V0 in a predetermined rate until t4. Thereafter, the feeding speed is maintained constant at V0, and the predetermined position of the recording sheet 10 reaches the transfer position D at the feeding speed V0.

A length wherein the predetermined position of the recording sheet 10 is carried in a period between to (i.e., when the predetermined position is at the detecting position C) and t4 is equivalent to a integration value of the period from t0 to t4 and the feeding speed in the period (from t0 to t4). In the present embodiment, the feeding speed is controlled so that the dimensions of the shaded area a, which indicates a part of the integration value, become equivalent to the dimensions of the shaded area b, which indicates the other part of the integration value. When the dimensions of the shaded area a and the shaded area b are equivalent, the length wherein the recording sheet 10 is carried in the period from t0 to t4 is equivalent to a length wherein the recording sheet 10 is carried in a constant feeding speed V0 in the same period. That is, when the feeding speed at the detecting position C (i.e., t0) is V1, which is slower than V0, it is practically equivalent that the recording sheet 10 is carried at the constant feeding speed V0 in the same length. Therefore, emission of the laser beam 9 at the emission position E, which is based on the timing wherein the predetermined position of the recording sheet 10 is detected by the fed sheet sensor 3, can be controlled based on the timing wherein the predetermined position of the recording sheet 10 is carried from the detecting position C to the transfer position D at the constant feeding speed V0.

As shown in FIG. 4, a period t1 is required before the feeding speed V1 is accelerated to reach V0. Therefore, in order for the recording sheet 10 to be carried through the detecting position C at the feeding speed V0, a substantial length that corresponds to the period t1 is required between the ready position B and the detecting position C. However, when the recording sheet 10 is carried through the detecting position C at the feeding speed V1 with the configuration described above, a length between the ready position B and the detecting position C can be configured to be shorter than a corresponding length between the ready position B and the detecting position C, wherein the recording sheet 10 is carried through the detecting position C at the feeding speed V0. Consequently, the length L, which is between the feeding position A and the detecting position C can be configured to be shorter, and the feed rollers 2a, 2b can be arranged in positions closer to the fed sheet sensor 3.

In the present embodiment, as shown in FIG. 4, the feeding speed is accelerated from V1 to V2 and is decelerated from V2 to V0 in constant rates, although the rates may not necessarily be constant. The feeding speed in the period from t0 to t4 may be indicated in curved lines, as long as the dimensions of the shaded area a and the dimensions of the shaded area b are equivalent.

According to the present invention, the length between the feed rollers 2a, 2b and the fed sheet sensor 3 can be configured to be shorter, as the recording sheet 10 can be detected by the fed sheet sensor 3 before the feeding speed of the stepping motor M reaches a desired speed. Therefore, the feeding unit 50 can be provided with more options in design thereof and the image forming apparatus 100 can be downsized.

Next, a second embodiment according to the present invention will be described hereinbelow.

FIG. 5 is a diagram to illustrate a general configuration of an image forming apparatus 200 according to a second embodiment of the present invention. In the present embodiment, configurations corresponding to the configuration of the previous embodiment is referred to by the identical reference numerals, and description of those is omitted.

The recording sheet 10 is rolled around the core roll 1. The recording sheet 10 is lead in between the feed rollers 2a, 2b, and a buffering guide unit 300 is provided in a path of the recording sheet 10. The buffering guide unit 300 includes a guide roller 310, a pair of supporting members 320, and a pair of springs 340. The guide roller 310 is a roller to guide the recording sheet 10, and is provided with the supporting member 320 at each end. The guide roller 320 is configured to have a length in a longitudinal direction thereof being substantially equivalent to or greater than a width of the recording sheet 10, which is in parallel to the rotation axis thereof. Each of the supporting members 320 rotatably supports the guide roller 310 at one end, and is rotated about the other end. Each of the supporting members 320 is provided with a spring 340, which applies attraction force to the supporting members 340 respectively. Each supporting member 320 is an elongated plate member, which extends from a rotation axis thereof to a rotation axes of the guide roller 310. At an approximately midpoint of each supporting member 320 in a longitudinal direction thereof, one end of the spring 340 is coupled to the supporting member 320. The other end of the spring 340 is fixed to a predetermined position (not shown) of the housing of the image forming apparatus 200. As shown in FIG. 5, the springs 340 are adapted to attract the supporting members 320 from a lower side of the image forming apparatus 200 (i.e., a lower side in FIG. 5) so that the supporting members 320 can be rotated in a clockwise direction.

When the image forming apparatus 200 is in a ready state, i.e., when the stepping motor M is not activated, the buffering guide unit 300 is in an initial position A. In this position, a downstream portion (i.e., a front end portion) of the recording sheet 10 with respect to the normal sheet feeding direction is held in between the feed rollers 2a, 2b, and an upstream portion, which is closer to a portion rolled around the core roll 1, is tensioned toward an upstream end thereof by a rotation force provided by an attraction member (not shown). The attraction member is provided to the core roll 1, so that the rotation force of the attraction member in the counterclockwise direction is applied to the core roll 1. From this initial position A, as the recording sheet 10 is fixed to the feed roller 2a, 2b at the downstream end thereof, and the upstream end is tensioned by the core roll 1, the tension accordingly tends to lift the guide roller 310 upwardly. That is, the guide roller 310 is applied a rotation force to be rotated in the counterclockwise direction by the recording sheet 10. It should be noted that, in the initial position A, the rotation force to rotate the guide member 320 in the counterclockwise direction applied by the recording sheet 10 and the rotation force in the clockwise direction to rotate the guide member 340 applied by the springs 340 are substantially equivalent. It should be further noted that the springs 340 are adapted not to be expanded to an maximum extent thereof when the springs 340 are in the initial position A.

When a feeding operation is started, the stepping motor M starts rotating, and the feed roller 2a is rotated accordingly to carry the recording sheet 10 in the normal sheet feeding direction. In order for the recording sheet 10 to rotate the core roll 1 in accordance with the rotation of the feed roller 2a, torque against an impact load caused by inertia in the core roll 1 and recording sheet 10 is required. In the present embodiment, the guide roller 310 serves as a pulley until the core roll 1 starts rotating, and the supporting members 320 are rotated (uplifted) in the counterclockwise direction in FIG. 5 against the attraction force of the springs 340. Simultaneously, the springs 340 attract the supporting members 320, and thus the supporting members 340 are swayed. As the supporting members 320 are rotated, the feeding path of the recording sheet 10 becomes shorter, and the impact load on the stepping motor M is reduced. And as the supporting members 320 are swayed for a predetermined amount, the core roll 1 starts to rotate. It should be noted that the springs 340 are adapted not to be expanded to the maximum extent thereof when the core roll 1 starts rotating.

It should be noted that, as the core roll 1 is rotated at a constant rotation speed, and the rotation speed of the feed roller 2a is changed to be faster in an instant to carry the recording sheet 10 at a faster speed, the feed roller 2a also requires torque against the impact load. When the feed roller 2a is rotated at the faster speed, the supporting members 320 are swingably rotated in the counterclockwise direction. Accordingly, the impact load on the stepping motor M is similarly reduced by the effect of the buffering guide unit 300. Thus, a length of a path of the recording sheet 10 from the core roll 1 to the feed roller 2a is varied by the buffering guide unit 300, so that the impact load is reduced. Further, it should be noted that the buffering guide unit 300 prevents the recording sheet 10 from being loosened in the path so that the recording sheet 10 can be properly tensioned as the supporting members 320 are swingably rotated. With this configuration, the stepping motor M can be prevented from stepping out when the feeding speed of the recording sheet 10 (i.e., the rotation speed of the feed roller 2a) is accelerated instantly. That is, a maximum rotation frequency to be reached instantly with the stepping motor M maintained synchronized to the pulse signals, can be increased.

FIG. 6 is a flowchart to illustrate a controlling process of feeding speed of the recording sheet 10 according to the second embodiment of the present invention. The controlling process is executed before the predetermined position of the recording sheet 10 is detected by the fed sheet sensor 3. It should be noted that feeding speeds V0 and V1 referred to in the present embodiment are equivalent to the feeding speeds V0 and V1 in the first embodiment. As the process starts, in S110, the stepping motor M is controlled to start rotating the feeding roller 2a at a rotation speed corresponding to the feeding speed V1 of the recording sheet 10. Thereafter, in S120, the feeding speed of the recording sheet 10 is controlled to be V0, which corresponds to a feeding speed at the developing unit 60.

FIG. 7 is a diagram to illustrate a relation between timing and the feeding speed of the recording sheet 10 in the controlling process according to the second embodiment of the present invention. In the diagram, when t as time is 0, the predetermined position of the recording sheet 10 is detected at the ready position B (see FIG. 2).

When the stepping motor M is activated as the feeding operation starts (t0), the feeding speed of the recording sheet 10 is V1. Thereafter at t1(t1>0), the feeding speed is changed (switched) in an instant from V1 to V0. At t2 (t2≧t1), the predetermined position of the recording sheet 10 transits the detecting position C (see FIG. 2). It should be noted that t2 may be concurrent with t1. That is, the feeding speed V1 may be switched to V0 at the detecting position C.

With the buffering guide unit 300 described above, the impact load upon switching the feeding speeds of the recording sheet 10 is reduced by the effect of the springs 340, and the stepping motor M can be prevented from stepping out, so that the feeding speed of the recording sheet 10 can be instantly switched from V1, which corresponds to a rotation speed within the pull-in torque range of the stepping motor M, to V0, which corresponds to a rotation speed outside the pull-in torque range. Therefore, in the image forming apparatus 200, the length between the ready position B and the detecting position C can be configured to be shorten, as the recording sheet 10 can be carried to the detecting position C at the feeding speed V0, which is switched from the feeding speed V1 in a shorter period after the stepping motor M is activated. As the length between the ready position B and the detecting position C can be shorten, the feed rollers 2a, 2b can be arranged in closer positions to the fed sheet sensor 3 accordingly.

Although examples of carrying out the invention have been described, those skilled in the art will appreciate that there are numerous variations and permutations of the image forming apparatus and the drive direction switching system that fall within the spirit and scope of the invention as set forth in the appended claims. It is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or act described above. Rather, the specific features and acts described above are disclosed as example forms of implementing the claims.

For example, the recording sheet 10 to be carried in the image forming apparatus 200 may not necessarily be in a continuous form, but may be a cut sheet. When a cut sheet is used in the image forming appratus 200, the image forming apparatus 200 may be provided with a pair of rollers to hold the cut sheet at an upstream side of the feed rollers 2a, 2b with respect to the normal sheet feeding direction so that the cut sheet can be properly carried and/or the cut sheet may not be loosened. In such case, impact load on the stepping motor M by the pair of rollers is similarly generated when the feeding speed of the feed roller 2a is switched to carry the cut sheet at the faster speed. Thus, the present invention can be also applied to an image forming apparatus that uses a cut sheet of paper as a recording medium.

Further, it should be noted that a recoding sheet positioning member (not shown) is provided at the upstream side of the feed rollers 2a, 2b with respect to the normal sheet feeding direction in the image forming apparatuses 100, 200. The recording sheet positioning member is adapted to hold both sides of the recording sheet 10 so that the recording sheet 10 can be positioned properly in a width direction thereof. The recording sheet positioning member is linearly extended along the sheet feeding direction (i.e., in an approximately horizontal direction in FIGS. 1 and 5). As the feed rollers 2a, 2b are located in positions closer to the fed sheet sensor 3 (i.e., closer to a left-hand side in FIGS. 1 and 5), as described above, the recording sheet positioning member may also be located in a position closer to the left-hand side in FIGS. 1 and 5. Thus, the image forming apparatuses 100, 200 may be designed in a downsized configuration.

The present disclosure relates to the subject matter contained in Japanese Patent Application No. 2005-137997, filed on May 11, 2005, which is expressly incorporated herein by reference in its entirety.