Title:
SHEET CONVEYING APPARATUS AND IMAGE FORMING APPARATUS
Kind Code:
A1


Abstract:
A sheet conveying apparatus including: a skew feed amount detecting portion for detecting a skew feed amount of a sheet; a pair of skew feed correcting portions for correcting the skew feed by conveying the sheet by their independent rotations; a conveying amount detecting portion for detecting the sheet conveying amounts by the correcting portions; and a controller for controlling rotations of the correcting portions based on detection results obtained by the skew feed amount detecting portion and the conveying amount detecting portion, wherein, after a skew feed correcting operation-is started, the controller detects a sheet correction amount by the correcting portions based on a detection result obtained by the conveying amount detecting portion, and controls the rotations of the correcting portions so that the sheet correction amount equals to the skew feed amount of the sheet detected by the skew feed amount detecting portion.



Inventors:
Katayama, Hiromasa (Toride-shi, JP)
Application Number:
11/459045
Publication Date:
04/05/2007
Filing Date:
07/21/2006
Assignee:
CANON KABUSHIKI KAISHA (Tokyo, JP)
Primary Class:
International Classes:
B65H9/04
View Patent Images:



Primary Examiner:
SUAREZ, ERNESTO A
Attorney, Agent or Firm:
Venable LLP (New York, NY, US)
Claims:
What is claimed is:

1. A sheet conveying apparatus, comprising: a skew feed amount detecting portion configured to detect a skew feed amount of a sheet; a pair of skew feed correcting portions arranged in a direction perpendicular to a sheet conveying direction, configured to correct a skew feed of the sheet by conveying the sheet through independent rotations of said pair of skew feed correcting portions; a conveying amount detecting portion configured to detect respective sheet conveying amounts of said pair of skew feed correcting portions; and a control portion configured to control rotations of said pair of skew feed correcting portions according to detection results obtained by said skew feed amount detecting portion and said conveying amount detecting portion, wherein, after a skew feed correcting operation by said pair of skew feed correcting portions is started, said control portion calculates a sheet correction amount by said pair of skew feed correcting portions based on a detection result obtained by said conveying amount detecting portion, and controls the rotations of said pair of skew feed correcting portions so that the sheet correction amount becomes equal to the skew feed amount of the sheet detected by said skew feed amount detecting portion.

2. A sheet conveying apparatus according to claim 1, wherein said pair of skew feed correcting portions effect a skew feed correction by a difference between sheet conveying speeds, and wherein said control portion controls the sheet conveying speeds of said pair of skew feed correcting portions so that a difference between the sheet conveying amounts of said pair of skew feed correcting portions when the skew feed correction is performed becomes equal to a difference between sheet conveying amounts of said pair of skew feed correcting portions set based on the skew feed amount of the sheet detected by said skew feed amount detecting portion.

3. A sheet conveying apparatus according to claim 2, wherein, when correcting the skew feed of -the sheet, said control portion controls the sheet conveying speeds of said pair of skew feed correcting portions so as to change the difference between the sheet conveying speeds of said pair of skew feed correcting portions according to the skew feed amount of the sheet.

4. A sheet conveying apparatus according to claim 1, wherein said pair of skew feed correcting portions effect a skew feed correction by a difference between sheet conveying speeds, and wherein said control portion controls the sheet conveying speeds of said pair of skew feed correcting portions so that the sheet correction amount based on a difference between the sheet conveying amounts of said pair of skew feed correcting portions when the skew feed correction of the sheet is effected becomes equal to the skew feed amount of the sheet detected by said skew feed amount detecting portion.

5. A sheet conveying apparatus according -to claim 1, wherein said conveying amount detecting portion is provided with rotary members respectively provided in said pair of skew feed correcting portions so as to be opposed to each other, and a rotation amount detecting portion for detecting rotation amounts of said rotary members, wherein said rotation amount detecting portion detects the rotation amounts when the rotary members rotate in association with a movement of the sheet conveyed by said pair of skew feed correcting portions, and wherein said control portion detects the sheet conveying amount based on detection results obtained by said rotation amount detecting portions.

6. A sheet conveying apparatus according to claim 5, wherein said rotary members are arranged along a curved path and on an inner side of the curved path for guiding the sheet, and wherein said pair of skew feed correcting portions are arranged along the curved path and on outer peripheral surfaces of said rotary members, the sheet being conveyed while undergoing a skew feed correction by said pair of skew feed correcting portions and said rotary members.

7. A sheet conveying apparatus according to claim 6, wherein said pair of skew feed correcting portions are provided with driving rollers and driven rollers arranged along the outer peripheral surfaces of said rotary members, and conveyor belts stretched around said driving rollers and said driven rollers, and conveys the sheet while effecting the skew feed correction on the sheet between said conveyor belts and said rotary members.

8. A sheet conveying apparatus according to claim 1, wherein said conveying amount detecting portion is provided with a sensor for detecting a speed fluctuation of the sheet in a non-contact fashion, and detects the sheet conveyed by said pair of skew feed correcting portions by said sensor in a non-contact fashion to detect the sheet conveying amount.

9. A sheet conveying apparatus according to claim 8, wherein said sensor also serves as a skew feed amount detecting means for detecting a leading edge of the sheet conveyed to said sensor.

10. An image forming apparatus which has an image forming portion to form an image to the sheet conveyed by a sheet conveying apparatus, comprising: a skew feed amount detecting portion configured to detect a skew feed amount of a sheet; a pair of skew feed correcting portions arranged in a direction perpendicular to a sheet conveying direction, configured to correct a skew feed of the sheet by conveying the sheet through independent rotations of said pair of skew feed correcting portions; a conveying amount detecting portion configured to detect respective sheet conveying amounts of said pair of skew feed correcting portions; and a control portion configured to control rotations of said pair of skew feed correcting portions according to detection results obtained by said skew feed amount detecting portion and said conveying amount detecting portion, wherein, after a skew feed correcting operation by said pair of skew feed correcting portions is started, said control portion detects a sheet correction amount by said pair of skew feed correcting portions based on a detection result obtained by said conveying amount detecting portion, and controls the rotations of said pair of skew feed correcting portions so that the sheet correction amount becomes equal to the skew feed amount of the sheet detected by said skew feed amount detecting portion.

Description:

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a sheet conveying apparatus, and more particularly, to a sheet conveying apparatus for correcting skew feed of a sheet such as a recording sheet or an original conveyed to an image forming portion or an image reading portion.

2. Related Background Art

Conventionally, an image forming apparatus or an image reading apparatus, such as a copying machine, a printer, or a facsimile apparatus, is equipped with a sheet conveying apparatus for conveying a sheet such as a recording sheet or an original to an image forming portion or an image reading portion. Some of such sheet conveying apparatuses are equipped with a skew feed correcting unit for effecting sheet skew feed correction to correctly adjust the attitude and position of the sheet until the sheet is conveyed to the image forming portion or the image reading portion.

In a system of such the skew feed correcting unit, a leading edge of a sheet is caused to abut a nip of a roller pair at rest to generate bending in the sheet, and the leading edge of the sheet is caused to extend along the roller nip by utilizing an elasticity of the sheet, thereby correcting skew feed. In another system, a shutter member is retractably provided in a sheet conveying path, and the leading edge of a sheet conveyed is caused to abut the shutter member -to thereby effect skew feed correction. In the system using the shutter member, there is no need to stop the sheet being conveyed, so it is possible to reduce an interval between the sheets, making it possible to enhance throughput in image formation, etc.

Further, as disclosed in Japanese Patent Application Laid-open No. H04-277151, there has been proposed an active skew feed correction system, in which skew feed is corrected while conveying a sheet without stopping it, making it possible to effect skew feed correction more reliably and with high accuracy.

FIG. 8 shows an example of a conventional skew feed correcting unit for correcting skew feed of a sheet by such the active skew feed correction system.

Two sheet leading edge detecting sensors 201a and 201b are provided in a sheet conveying path. The two sheet leading edge detecting sensors 201a and 201b are arranged in a direction (i.e., width direction of the sheet being conveyed) which is perpendicular to the sheet conveying direction indicated by an arrow A.

Further, there are provided skew feed correcting rollers 222a and 222b. The skew feed correcting rollers 222a and 222b are arranged in the direction perpendicular to the sheet conveying direction and coaxially at a predetermined interval. The skew feed correcting rollers 222a and 222b are driven independently by separate drive sources 221a and 221b, respectively.

In this skew feed correcting unit, constructed as described above, when a leading edge of a sheet S conveyed from a downstream side traverses the sheet leading edge sensors 201a and 201b, signals are output from the sheet leading edge detecting sensors 201a and 201b, respectively. Based on those leading edge detecting signals, the skew feed amount (i.e., skew angle) of the sheet leading edge is calculated, and, based on the calculated value, rotating speeds of the skew feed correcting rollers 222a and 222b are respectively controlled to thereby correct the skew feed of the sheet S. That is, by producing a difference in the speed at which the sheet is conveyed by the skew feed correcting rollers 222a and 222b, the sheet is turned by an amount corresponding to the skew feed amount to thereby correct the skew feed.

FIG. 9 is a control block diagram showing the skew feed correcting unit. A skew feed amount detecting portion 215 is formed by the two sheet leading edge detecting sensors 201a and 201b and a skew feed detecting portion 200. The sheet leading edge detection signals from the sheet leading edge detecting sensors 201a and 201b are input to the skew feed detecting portion 200, which outputs a skew feed detection signal to a control portion 210.

After this, the control portion 210 calculates the skew feed amount by a skew feed amount calculating portion 210a based on the skew feed detection signal. Further, after that, based on the calculated skew feed amount, the respective rotating speeds of two skew feed correcting portions 220 formed by the skew feed correcting rollers 222a and 222b and the drive sources 221a and 221b are determined by a control amount calculating portion 210b. By driving the skew feed correcting rollers 222a and 222b at the respective determined rotating speeds, skew feed correction is effected on the sheet.

However, this control involves the following problem: the respective rotating speeds of the skew feed correcting rollers 222a and 222b are determined based on the detection signals from the skew feed amount detecting portion 215 formed by the sheet leading edge detecting sensors 201a and 201b and the skew feed detecting portion 200, and the skew feed is corrected according to a difference in the speed at which the sheet is conveyed by the skew feed correcting rollers 222a and 222b. Thus, when, for example, there has been generated a change in conveyance efficiency due to wear of the rollers, etc., even if the rotating speeds of the skew feed correcting rollers 222a and 222b are set, the sheet conveying speeds may differ from those when the rollers were in initial states, resulting in a deterioration in accuracy in skew feed correction.

In view of this, as disclosed, for example, in Japanese Patent Application Laid-open NO. H06-191684, there has been proposed a skew feed correcting unit, which uses, instead of the sheet leading edge detecting sensors 201a and 201b shown in FIG. 8, line sensors 301a and 301b as shown in FIG. 10. FIG. 12 is a side view of a skew feed correcting portion.

By using such the line sensors 301a and 301b, it is possible to continuously detect the leading edge of the sheet S, whereby it is possible to perform control so as to reduce the sheet skew feed amount (i.e., skew angle) to 0 (zero).

FIG. 11 is a block diagram showing such the skew feed correcting unit. A skew feed amount detecting portion 315 is formed by two line sensors 301a and 301b and a skew feed detecting portion 300. Sheet leading edge detection signals from the line sensors 301a and 301b are input to the skew feed detecting portion 300, and the skew feed detecting portion 300 outputs a skew feed detection signal to a control portion 310.

After this, the control portion 310 calculates the skew feed amount based on the skew feed detection signal by a skew feed amount calculating portion 310a. Further, after that, based on the skew feed amount calculated, the respective rotating speeds of two skew feed correcting portions 320, formed by skew feed correcting rollers 322a and 322b and drive sources 321a and 321b, are determined by a control amount calculating portion 310b. Then, the skew feed correcting rollers 322a and 322b are driven at the determined rotating speeds, and the sheet is turned according to a difference in sheet conveying speed between the skew feed correcting rollers 322a and 322b to thereby effect skew feed correction.

Here, due to the use of the line sensors 301a and 301b, after the start of the skew feed correcting operation, signals are continuously input to the skew feed detecting portion 300 from the line sensors 301a and 301b. Then, a result of the skew feed correction by a skew feed correcting portion 320 is input to the control portion 310 from the skew feed detecting portion 300. As a result, it is possible to make a judgment as to whether the skew feed amount of the sheet leading edge has been reduced to 0 or not, thus making it possible to effect the skew feed correction with high accuracy.

However, the conventional sheet conveying apparatus equipped with a skew feed correcting unit using the line sensors 301a and 301b has the following problems.

In order to arrange the line sensors 301a and 301b within a requisite correcting section for correcting the skew feed of the sheet, it is necessary to provide a linear sheet conveying path. That is, to always detect the leading edge of the sheet, it is necessary to provide, as shown in FIG. 12, the line sensors 301a and 301b that are of the same length as the correcting section X, which is of the requisite length for skew feed correction. To arrange the line sensors 301a and 301b, it is necessary to provide a sheet conveying path whose correcting section X is linear. This correcting section X must be made longer in proportion to the level of accuracy for sheet skew feed correction to be attained.

Further, when the line sensors 301a and 301b are used, deterioration in detection accuracy is involved due to curling of the leading edge of the sheet, etc. Thus, to detect the Leading edge of the sheet with high accuracy, it is necessary to press the sheet S against the line sensors 301a and 301b. This requires a device for pressing the sheet. Further, all the while the sheet is conveyed through the correcting section X, the sheet S is rubbed against the line sensors 301a and 301b, with the result that marks, etc. are undesirably left on the sheet surface.

Further, it is impossible for the line sensors 301a and 301b -to extend to an image transfer position, which locates between a photosensitive drum 21 and a transfer charger 22 shown in FIG. 12, with the result that there is generated a conveying section Y where the skew feed correction is impossible. In this case, the skew feed correcting rollers 321a and 321b may involve a difference in conveying amount as a result of long-term use. When such the difference in conveying amount is generated, there is a possibility of skew feed being newly generated in the conveying section Y.

Thus, while allowing feedback of the skew feed correction results, the method using the line sensors 301a and 301b involves limitations in terms of arrangement, and cannot be easily applied to an image forming apparatus, such as a copying machine or a printer, or an image reading apparatus, such as a scanner, which is nowadays being increasingly reduced in size.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above-mentioned problems in the conventional art. It is therefore an object of the present invention to provide a sheet conveying apparatus, an image forming apparatus, and an image reading apparatus capable of realizing a reduction in size and performing sheet skew feed correction with high accuracy.

According to the present invention, there is provided a sheet conveying apparatus including:

a skew feed amount detecting portion configured to detect a skew feed amount of a sheet;

a pair of skew feed correcting portions arranged in a direction perpendicular to a sheet conveying direction, configured to correct skew feed of the sheet by conveying the sheet through independent rotations;

a conveying amount detecting portion configured to detect the respective sheet conveying amounts of the skew feed correcting portions; and

a control portion configured to control rotations of the skew feed correcting portions according to detection results obtained by the skew feed amount detecting portion and the conveying amount detecting portion, and

in the sheet conveying apparatus, after a skew feed correcting operation by the skew feed correcting portions is started, the control portion calculates a sheet correction amount by which the skew feed of the sheet is to be corrected by the pair of skew feed correcting portions based on a detection result obtained by the conveying amount detecting portion, and controls the rotations of the skew feed correcting portions such that the sheet correction amount becomes equal to the skew feed amount of the sheet detected by the skew feed amount detecting portion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a printer constituting an example of an image forming apparatus equipped with a sheet conveying apparatus according to a first embodiment of the present invention;

FIG. 2 is a perspective view showing the construction of a skew feed correcting unit provided in the sheet conveying apparatus;

FIG. 3 is a block diagram showing the skew feed correcting unit;

FIG. 4 is a side view of the skew feed correcting unit;

FIG. 5 is a perspective view of a skew feed correcting unit provided in a sheet conveying apparatus according to a second embodiment of the present invention;

FIG. 6 is a block diagram of the skew feed correcting unit;

FIG. 7 is a side view of the skew feed correcting unit;

FIG. 8 is a perspective view showing the construction of a conventional skew feed correcting unit;

FIG. 9 is a block diagram showing the conventional skew feed correcting unit;

FIG. 10 is a perspective view showing the construction of another conventional skew feed correcting unit;

FIG. 11 is a block diagram of still another conventional skew feed correcting -unit; and

FIG. 12 is a side view of still another conventional skew feed correcting unit.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following, most preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

FIG. 1 is a sectional view of a printer, which constitutes an example of an image forming apparatus equipped with a sheet conveying apparatus according to the first embodiment of the present invention.

In FIG. 1, a printer 1, which is an image forming apparatus, is equipped with a printer main body 2 provided with an image forming portion 3, a scanner 11 arranged on the upper surface of the printer main body 2, and a feeding deck 12 which is arranged on one side of the printer main body 2 and which accommodates a large amount of sheets S in a stacked state.

Here, the printer main body 2 is equipped with the image forming portion 3 provided with a photosensitive drum 21 serving as the image bearing member, and retard separation type sheet feeding apparatuses 1.6 and 17 for feeding the sheets S. Further, the printer main body 2 is equipped with a sheet conveying apparatus 4 for conveying the sheets S.,which are fed by the sheet feeding apparatuses 16 and 17, to the image forming portion 3.

Here, the sheet feeding apparatuses 16 and 17 are equipped with cassettes 13 and 14 containing a plurality of sheets S, feeding rollers 16a and 17a for sending out the sheets S from the cassettes 13 and 14, and the like. Further, the sheet conveying apparatus 4 is equipped with a conveying roller 41 and a skew feed correcting unit 18, and the sheets S sent out from the sheet feeding apparatuses 16 and 17 are sent to the skew feed correcting unit 18 by the conveying roller 41. The sheets contained in the feeding deck 12 are sent out by a feeding roller 10, and sent to the skew feed correcting unit 18 by a retard separation type sheet feeding apparatus 15 and a conveying roller 12a.

Then, after undergoing skew feed correction by the skew feed correcting unit 18, the sheet S is transmitted to a transfer portion of the image forming portion 3, which is formed by the photosensitive drum 21 and the transfer charger 22. Then, at the transfer portion, a toner image previously formed on the photosensitive drum 21 is transferred. After that, the sheet S to which the toner image has been transferred is sent to a fixing device 24 by a conveyor belt 23, and the transferred toner image is fixed to the sheet S by the fixing device 24.

Here, the printer 1 is provided with a two-sided copying mode in which copying is performed on both sides of the sheet S. In the case of the normal copying mode (i.e., one-side copying mode), the sheet S that has undergone the fixing process is delivered onto a delivery tray 27 outside the printer 1 by a delivery roller pair 26.

In the case of the two-sided copying mode, the sheet S is temporarily stacked on an intermediate tray 31 by an in-delivery roller pair 25 or a switchback roller pair 29 through a re-feed path 28 and a duplex transport path 30. After that, the sheet S placed on the intermediate tray 31 is conveyed to the skew feed correcting unit 18 again by the re-feeding apparatus 32 for image formation, and from this onward, the sheet S undergoes the same process as in the one-side copying mode before being delivered to the outside.

Next, the skew feed correcting unit 18 will be described.

FIG. 2 is a perspective view of the skew feed correcting unit 18. In FIG. 2, two optical sheet leading edge detecting sensors 101a and 101b are arranged at a fixed interval in a direction (i.e., width direction of the sheet being conveyed) which is perpendicular to the sheet conveying direction indicated by an arrow A. Driving rollers 122a and 122b are arranged coaxially in a direction perpendicular to the sheet conveying direction indicated by the arrow A and independently driven by pulse motors 121a and 121b serving as drive sources.

Conveyor belts 123a and 123b are respectively stretched between the driving rollers 122a and 122b, and driven rollers 124a and 124b arranged on the upstream side of the driving rollers 122a and 122b. Driven rotary members 131a and 13!b are held in press contact with the conveyor belts 123a and 123b by a pressurizing device (not shown). The driven rotary members 131a and 131b are associated with the rotation of the conveyor belts 123a and 123b to rotate respectively. The driving rollers 122a and 122b, the driven rollers 124a and 124b, the conveyor belts 123a and 123b, and the driven rotary members 131a and 131b constitute a pair of right and left skew feed correcting portions 18a and 18b.

A curved path is formed by the conveyor belts 123a and 123b and the driven rotary members 131a and 131b arranged on the inner side, making it possible to correct skew feed while conveying the sheet in a curved state.

Then, the sheet S sent out and conveyed by the sheet feeding apparatuses 15, 16, and 17 is conveyed while being nipped by the conveyor belts 123a and 123b, and the driven rotary members 131a and 131b.

Rotary encoders 132a and 132b are rotation amount detecting portions connected to the driven rotary members 131a and 131b. The respective rotation amounts of the driven rotary members 131a and 131b are detected by the rotary encoders 132a and 132b.

FIG. 3 is a control block diagram showing the skew feed correcting unit 18, constructed as described above. A skew feed amount detecting portion 105 for detecting the skew feed amount (i.e., skew angle) of the sheet is formed by the two sheet leading edge detecting sensors 101a and 101b serving as sheet detecting devices and a skew feed detecting portion 100 to which signals from the sheet leading edge detecting sensors 101a and 101b are input. A control portion 110 calculates the skew feed amount of the sheet based on a signal from the skew feed detecting portion 100, and which determines the respective control amounts of the driving rollers 122a and 122b for the skew feed correcting portions 18a and 18b to correct the skew feed of the sheet based on the calculated skew feed amount.

Here, the control amounts of the driving rollers 122a and 122b are the respective rotating speeds of the driving rollers 122a and 122b for correcting the skew feed of the sheet S in a predetermined period of time. That is, in the control portion, there is calculated the amount by which the sheet is to be conveyed by the conveyor belts 123a and 123b to correct the skew feed of the sheet in a predetermined period of time. Then, the respective rotating speeds of the driving rollers 122a and 122b are set such that the sheet conveying amount of the conveyor belts 123a and 123b is the calculated sheet conveying amount. By driving the driving rollers 122a and 122b at the set rotating speeds, there is generated a difference in sheet conveying speed (i.e., peripheral speed), and the sheet S is turned for a predetermined period of time, whereby the skew feed of the sheet is corrected. Here, the predetermined period of time is set as appropriate to a period of time that allows sheet skew feed correction within the correction section X for correcting the skew feed of the sheet until the sheet from the detecting position P1 reaches the transfer portion formed by the photosensitive drum 21 and the transfer charger 22. The control portion 110 is equipped with a comparing portion 110c and a storage portion 110d described below.

Here, the operation of the skew feed correcting unit 18 will be described with reference to FIG. 4, which is a side view of the skew feed correcting unit 18.

The sheet S, which is conveyed from the downstream side by the sheet feeding apparatuses 15, 16, and 17, is conveyed along a curved path which is formed by arranging the conveyor belts 123a and 123b on the outer side and arranging the driven rotary members 131a and 131b on the inner side. Then, the leading edge of the sheet S traverses the sheet leading edge detecting sensors 101a and 101b. As a result, the sheet leading edge detecting sensors 101a and 101b output signals indicating the traversing of the sheet to the skew feed detecting portion 100.

Here, for example, when the sheet leading edge traverses the detecting position P1 of the two sheet leading edge detecting sensors 101a and 101b with different timings, the detection signals are output from the sheet leading edge detecting sensors 101a and 101b to the skew feed detecting portion 100 with different timings.

When the detection signals are thus output with different timings, the skew feed detecting portion 100 outputs an instantaneous skew feed detection signal to the control portion 110, and the control portion 110 calculates the skew feed amount (i.e., skew angle) by a skew feed amount calculating portion 110a based on the skew feed detection signal. Based on the skew feed amount thus calculated, the respective control amounts for the skew feed correcting portions 18a and 18b of the skew feed correcting unit 18 are determined by a control amount calculating portion 110b. That is, the amount by which the sheet is to be conveyed by the conveyor belts 123a and 123b in the predetermined period of time to correct the skew feed amount of the sheet S is calculated, and the respective rotating speeds of the driving rollers 122a and 122b for achieving this are determined.

Based on this control amount, the respective rotating speeds of the pulse motors 121a and 121b are controlled, and the conveyor belt 123a of the front-side skew feed correcting portion 18a and the conveyor belt 123b of the depth-side skew feed correcting portion 18b, shown in FIG. 2, are driven for a predetermined period of time with a difference in sheet conveying speed (i.e., peripheral speed). By thus producing a difference in the conveying amount of the sheet S (obtained by sheet conveying speed×time) between the front-side skew feed correcting portion 18a and the depth-side skew feed correcting portion 18b, the sheet is turned, and the skew feed is corrected.

This difference in sheet conveying speed (i.e., peripheral speed) may be controlled so as to be always at a fixed value, or may be controlled such that the difference in sheet conveying speed is increased according to the skew feed amount. Further, skew feed correction can also be effected by performing control to delay the advancing end portion side of the sheet S, control to promote the delayed end portion side thereof, or both controls. Further, control is possible if the pulse motors 121a and 121b serving as the drive sources are DC motors or AC motors.

When skew feed correction is thus effected on the sheet S by the skew feed correcting unit 18, there occurs a difference between the rotation amount of the front-side driven rotary member 131a and the rotation amount of the depth-side driven rotary member 131b, both of which rotate with the sheet conveyed by the conveyor belts 123a and 123b. Accordingly, the respective detection values of the rotary encoders 132a and 132b vary accordingly.

Here, the difference in detection value between the rotary encoders 132a and 132b corresponds to a difference in the amount by which the sheet is conveyed by the conveyor belts 123a and 123b, so this difference constitutes the correction amount to be obtained through actual correction on the sheet S. Then, the detection value detected by the conveying amount detecting portion 130 including the driven rotary members 131a and 131b and the rotary encoders 132a and 132b is output to the control portion 110, where the actual difference in sheet conveying amount is calculated as the correction amount result.

Further, according to the detection results obtained by the skew feed amount detecting portion 105 and the conveying amount detecting portion 130, the comparing portion 110c of the control portion 110 compares the actual correction amount result with the skew feed amount which is detected by the skew feed amount detecting portion 105 and then calculated by the skew feed amount calculating portion 110a. Then, a judgment is made as to whether the skew feed correction has been effected to a sufficient degree or as to whether overshoot (i.e., getting beyond the proper position) in skew feed correction has been generated or not. That is, when determining the control amount by the control amount calculating portion 110b, the difference in the amount by which the sheet is conveyed by the conveyor belts 123a and 123b is calculated from the skew feed amount of the sheet S. Thus, the difference between this calculated sheet conveying amount and the actual sheet conveying amount input from the conveying amount detecting portion 130 are compared with each other to make a judgment as to whether the skew feed correction has been performed properly or not.

When the comparison result shows that the skew feed amount is insufficient, the difference in the sheet conveying speed (i.e., peripheral speed) at which the sheet is conveyed by the conveyor belts 123a and 123b is maintained. When overshoot has been generated, the pulse motors 121a and 121b are controlled such that the relationship of the difference in the sheet conveying speed is reversed. This operation is continued until the calculated sheet conveying amount and the actual sheet conveying amount become equal to each other; even after they are judged to be equal, correction control is conducted again when any skew feed of the sheet is detected.

It is also possible for the control portion 110 to calculate the actual sheet skew feed correction amount (i.e., correction angle) from the detection value detected by the conveying amount detecting portion 130, and to compare the correction amount with the skew feed amount (i.e., skew angle) calculated by the skew feed amount calculating portion 110a based on the skew feed detection signal from the skew feed detecting portion 100. Based on this comparison, a judgment is made as to whether the sheet skew feed correction is proper or not. In this case also, when the comparison result shows that the skew feed amount is insufficient, the difference in conveying speed (i.e., peripheral speed) at which the sheet is conveyed by the conveyor belts 123a and 123b is maintained. When overshoot has been generated, the pulse motors 121a and 121b are controlled such that the relationship of the difference in sheet conveying speed is reversed. Then, this operation is continued until the skew angle based on the detection by the skew feed detecting portion 100 and the actually corrected correction angle become equal to each other. Even after those angles are judged to be equal, correction control is conducted again if any skew feed of the sheet is detected.

This operation can be continued through the correction section X from the moment that skew feed of the leading edge of the sheet is detected at the detecting position P1 shown in FIG. 4 to the moment that the leading edge of the sheet reaches the transfer portion formed by the photosensitive drum 21 and the transfer charger 22.

When, in the skew feed correcting portions 18a and 18b, there is no sheet S between the conveyor belts 123a, 123b and the driven rotary member 131a, 131b, the driven rotary members 131a and 131b follow the movement of the conveyor belts 123a and 123b. Thus, by detecting the signals from the rotary encoders 132a and 132b in this sate, it is possible to control the sheet conveying speed of the conveyor belts 123a and 123b. For example, when the surfaces of the conveyor belts 123a and 123b have been worn as a result of long-term use, and the sheet conveying speed is changed, it is possible to maintain the sheet conveying speed of the conveyor belts 123a and 123b at a desired value by the conveying amount detecting portion 130 and the control portion 110.

Further, the rotational accuracy of the driven rotary members 131a and 131b is strictly controlled, and variation in the rotation of the conveyor belts 123a and 123b is stored in the storage port-on 110d provided in the control portion 110, controlling the pulse motors 121a and 121b so as to cancel the variation in rotation. As a result, it is possible to equalize the rotating speeds of the two driving rollers 122a and 122b, and to obtain a desired stable sheet conveying speed free from variation in rotation. Further, in the case of this construction, by making the cycle of all the rotary members related to the skew feed correcting unit 18 and of all the rotary members related to the conveying amount detecting portion 130 an integral multiple, it is possible to shorten the cycle of the variation in rotation.

In this way, after the start of the skew feed correcting operation by the skew feed correcting unit 18, the difference in the sheet conveying amount between the skew feed correcting portions 18a and 18b of the skew feed correcting unit 18 is detected based on a signal from the conveying amount detecting portion 130. Then, the skew feed correcting portions 18a and 18b of the skew feed correcting unit 18 are controlled such that the difference in the detected sheet conveying amount becomes equal to the skew feed amount detected by the skew feed detecting portion 105. This makes it possible to achieve a reduction in size and to perform skew feed correction with which accuracy. Further, it is possible to perform skew feed correction until the sheet leading edge reaches the transfer position.

In this embodiment, as shown in FIG. 4, the rotation center of the driven rotary members 131a and 131b and of the rotary encoders 132a and 132b is provided at a center of a circle along which the curved sheet conveying path (i.e., curved path) L extends. By arranging the conveyor belts 123a and 123b and the driven rotary members 131a and 131b in the curved portion of the sheet conveying path L, the contact area between the conveyor belts 123a and 123b and the sheet S and the driven rotary members 131a and 131b is increased.

Due to this arrangement, it is possible to mitigate slippage between the sheet S and the driven rotary members 131a, 131b and between the conveyor belts 123a, 123b and the driven rotary members 131a, 131b. The same effect can be obtained even if the positional relationship of the skew feed correcting unit 18 and the conveying amount detecting portion 130 with respect to the sheet conveying path L is reversed.

In this case, the conveying amount detecting portion 130 is formed not by the rotary encoders 132a and 132b and the driven rotary members 131a and 131b but by the rotary encoders 132a and 132b and the conveyor belts 123a and 123b. In the case in which the skew feed correcting unit 18 and the conveying amount detecting portion 130 are provided in the substantially linear portion of the sheet conveying path, both may be provided as a belt structure.

Further, instead of using the conveyor belts 123a and 123b, it is also possible to convey the sheet, with a single driving roller 122a, 122b and a plurality of driven rotary members arranged along the peripheral surfaces of the driven rotary members 131a and 131b.

Next, the second embodiment of the present invention will be described.

FIG. 5 is a perspective view of a skew feed correcting unit provided in a sheet conveying apparatus according to an embodiment of the present invention, and FIG. 6 is a block diagram thereof. In FIGS. 5 and 6, the reference symbols that are the same as those of FIGS. 2 and 3 indicate the same or equivalent components.

In FIG. 5, skew feed correcting rollers 125a and 125b constitute a pair of skew feed correcting portions 20a and 20b of a skew feed correcting unit 20. The skew feed correcting rollers 125a and 125b are arranged coaxially in a direction perpendicular to the sheet conveying direction indicated by the arrow A, and are driven independently by pulse motors 121a and 121b serving as the drive sources.

Conveying rotary members 126a and 126b are held in press contact with the skew feed correcting rollers 125a and 125b by a pressurizing device (not shown). The sheet S conveyed by the sheet feeding apparatuses 15, 16, and 17 is conveyed while being sandwiched between the conveying rotary members 126a and 126b and the skew feed correcting rollers 125a and 125b.

In this embodiment, the axial width of the skew feed correcting rollers 125a and 125b is larger than the axial width of the conveying rotary members 126a and 126b. Due to this difference in width, there are generated, in the skew feed correcting rollers 125a and 125b, portions that are not in contact with the conveying rotary members 126a and 126b, and laser Doppler type sensors 133a and 133b are arranged so as to face these portions.

Here, the laser Doppler type sensors 133a and 133b are sensors capable of detecting fluctuation in the speed of an object in a non-contact fashion. By using the sensors 133a and 133b, it is possible to detect the conveying speed of the sheet S. Further, the sensors 133a and 133b are adapted to undergo a change in output when the sheet S is conveyed thereto. Due to this change in output, it is possible to detect any sheet S conveyed thereto. That is, in this embodiment, the skew feed amount detecting portion also serves as the conveying amount detecting portion, and the skew feed amount detecting portion 115 of the present invention is formed by the sensors 133a and 133b and the skew feed detecting portion 100.

In the skew feed correcting unit 20, constructed as described above, when the sheet S conveyed by the sheet feeding apparatuses 15, 16, and 17 is conveyed to the detecting point P2 of the sensors 133a and 133b as shown in FIG. 7, the values detected by the sensors 133a and 133b change.

Here, when the sheet S is conveyed in a skew feed state, the leading edge of the sheet S traverses the two sensors 133a and 133b with different timings, whereby detection signals are output from the sensors 133a and 133b with different timings.

As in the first embodiment described above, when the detection signals are thus output with different timings, the skew feed detection signals are input to the skew feed detecting portion 100 shown in FIG. 6 from the sensors 133a and 133b, and the skew feed detecting portion 100 outputs an instantaneous skew feed detection signal to the control portion 110. After this, the control portion 110 calculates the skew feed amount based on this skew feed detection signal. Then, based on the skew feed amounts thus calculated, there is determined the control amount with respect to the skew feed correcting portions 20a and 20b of the skew feed correcting unit 20, formed by the pulse motors 121a and 121b, the skew feed correcting rollers 125a and 125b, etc. Here, as in the first embodiment, the term “control amount” means a rotating speed set for each of the skew feed correcting rollers 125a and 125b in order to correct the skew feed of the sheet S in a predetermined period of time.

Based on this control amount, the rotating speeds of the pulse motors 121a and 121b are controlled to produce a difference in a rotating speed between the front-side skew feed correcting roller 125a and the depth-side skew feed correcting roller 125b, thereby producing a difference in the sheet conveying amount between the front side and the depth side. In this way, the sheet S is turned, and the skew feed thereof is corrected.

Here, the sheet conveying speeds (i.e., peripheral speeds) of the front-side skew feed correcting roller 125a and the depth-side skew feed correcting roller 125b are respectively detected by the sensors 133a and 133b. By calculating the sheet conveying amount based on the detection result, it is possible to detect the actual correction amount.

The comparing portion 110c of the control portion 110 compares the actual correction result input from the sensors 133a and 133b, also serving as the conveying amount detecting portion, with the skew feed amount detected by the skew feed amount detecting portion 115 and calculated by the skew feed amount calculating portion 110a. In this way, it is checked whether the skew feed correction has been effected to a sufficient degree, or whether any overshoot in a skew feed correction has been generated or not.

When the skew feed amount is insufficient, the difference in the sheet conveying speed (i.e., peripheral speed) is maintained; when an overshoot has been generated, the pulse motors 121a and 121b are controlled so as to reverse the relationship of the difference in the sheet conveying speed. This operation is continued until the skew feed amount becomes equal to the correction amount. Even after the skew feed amount and the correction amount are judged to be equal, correction control is conducted again if any skew feed of the sheet is detected.

Due to this construction, it is possible, as in the first embodiment described above, to provide a sheet conveying apparatus and a printer (i.e., image forming apparatus) allowing a reduction in size and capable of conducting sheet skew feed correction with high accuracy.

In the above-mentioned embodiments, after the start of the skew feed correcting operation, a difference in the sheet conveying amount of the skew feed correcting unit is detected based on the detection by the conveying amount detecting portion, and the skew feed correcting unit is controlled such that the difference in the sheet conveying amount detected becomes equal to the skew feed amount detected. As a result, it is possible to achieve a reduction in the size of a sheet conveying apparatus, and to,perform sheet skew feed correction with high accuracy.

When it is rather difficult to detect the sheet conveying speeds by the laser Doppler type sensors 133a and 133b due to the material of the skew feed correcting rollers 125a and 125b, the width of the conveying rotary members 126a and 126b is made larger than that of the skew feed correcting rollers 125a and 125b. In this case, it is also possible to arrange the laser Doppler type sensors 133a and 133b on the skew feed correcting rollers 125a and 125b side, and to detect the sheet conveying speeds of the conveying rotary members 126a and 126b to thereby control the pulse motors 121a and 121b.

While in the first and second embodiments described above, the sheet conveying apparatus of the present invention is provided in a printer (i.e., image forming apparatus). However, the apparatus according to the present invention should not be construed restrictively. The present invention is also applicable, for example, to an image reading apparatus, such as a scanner, which is equipped with an image reading portion for reading the image of an original.

This application claims priority from Japanese Patent Application No. 2005-218847 filed Jul. 28, 2005, which is hereby incorporated by reference herein.