U.S. patent application number 13/572832 was filed with the patent office on 2013-02-28 for sheet conveying apparatus and image forming apparatus.
This patent application is currently assigned to RICOH COMPANY, LTD.. The applicant listed for this patent is Akira Kobashi, Koichi Kudo, Makoto Nakura, Shingo Takai, Naoto UEDA, Satoshi Ueda. Invention is credited to Akira Kobashi, Koichi Kudo, Makoto Nakura, Shingo Takai, Naoto UEDA, Satoshi Ueda.
Application Number | 20130049287 13/572832 |
Document ID | / |
Family ID | 47742515 |
Filed Date | 2013-02-28 |
United States Patent
Application |
20130049287 |
Kind Code |
A1 |
UEDA; Naoto ; et
al. |
February 28, 2013 |
SHEET CONVEYING APPARATUS AND IMAGE FORMING APPARATUS
Abstract
A sheet conveying apparatus includes a sheet conveying unit
including a drive roller and a driven roller; a downstream
detection unit and an upstream detection unit; a conveying amount
measuring unit that measures a conveying amount of the sheet; and a
conveying distance calculation unit that calculates a conveying
distance of the sheet, wherein a distance between the downstream
detection unit and the upstream detection unit or a perimeter of
one of the drive roller and the driven roller is set such that an
expected conveying distance calculated based on a set sheet length
of an expected value of the sheet becomes a substantially integer
multiple of a perimeter of the one of the drive roller and the
driven roller.
Inventors: |
UEDA; Naoto; (Ibaraki,
JP) ; Nakura; Makoto; (Ibaraki, JP) ; Takai;
Shingo; (Ibaraki, JP) ; Kudo; Koichi;
(Kanagawa, JP) ; Ueda; Satoshi; (Ibaraki, JP)
; Kobashi; Akira; (Ibaraki, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
UEDA; Naoto
Nakura; Makoto
Takai; Shingo
Kudo; Koichi
Ueda; Satoshi
Kobashi; Akira |
Ibaraki
Ibaraki
Ibaraki
Kanagawa
Ibaraki
Ibaraki |
|
JP
JP
JP
JP
JP
JP |
|
|
Assignee: |
RICOH COMPANY, LTD.
Tokyo
JP
|
Family ID: |
47742515 |
Appl. No.: |
13/572832 |
Filed: |
August 13, 2012 |
Current U.S.
Class: |
271/110 |
Current CPC
Class: |
B65H 9/002 20130101;
B65H 85/00 20130101; G03G 15/5029 20130101; B65H 2553/51 20130101;
B65H 2511/11 20130101; B65H 9/20 20130101; G03G 15/6561 20130101;
B65H 7/06 20130101; G03G 2215/00734 20130101; B65H 2701/1311
20130101; B65H 2511/414 20130101; G03G 2215/0059 20130101; B65H
2701/1313 20130101; B65H 5/068 20130101; G03G 2215/00586 20130101;
B65H 2701/1313 20130101; B65H 2511/11 20130101; B65H 2511/414
20130101; B65H 2553/81 20130101; G03G 15/231 20130101; G03G 15/6567
20130101; B65H 7/14 20130101; B65H 5/062 20130101; B65H 2701/1311
20130101; B65H 2220/01 20130101; B65H 2220/02 20130101; B65H
2511/172 20130101; B65H 2511/172 20130101; B65H 2553/82 20130101;
B65H 2220/03 20130101; B65H 2220/03 20130101; B65H 2220/01
20130101 |
Class at
Publication: |
271/110 |
International
Class: |
B65H 7/02 20060101
B65H007/02; B65H 5/06 20060101 B65H005/06 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 25, 2011 |
JP |
2011-183771 |
May 30, 2012 |
JP |
2012-123112 |
Claims
1. A sheet conveying apparatus comprising: a sheet conveying unit
that conveys a sheet including a drive roller which is driven to be
rotated by a driving unit, and a driven roller which is rotated in
accordance with the drive roller while the sheet is interposed
between the drive roller and the driven roller; a downstream
detection unit that detects the sheet downstream of the sheet
conveying unit in a conveying direction of the sheet; an upstream
detection unit that detects the sheet upstream of the sheet
conveying unit in the conveying direction of the sheet; a conveying
amount measuring unit that measures a conveying amount of the sheet
conveyed by the sheet conveying unit; and a conveying distance
calculation unit that calculates a conveying distance of the sheet
conveyed by the sheet conveying unit based on the conveying amount
measured by the conveying amount measuring unit within a period
determined by detections made by the first detection unit and the
second detection unit, wherein an expected value of the conveying
distance calculated based on a set sheet length of an expected
sheet for which the conveying distance is to be calculated becomes
a substantially integer multiple of a perimeter of one of the drive
roller and the driven roller.
2. The sheet conveying apparatus according to claim 1, further
comprising a sensor provided upstream or downstream of the sheet
conveying unit to function as the upstream detection unit or the
downstream detection unit, respectively.
3. The sheet conveying apparatus according to claim 2, wherein the
conveying distance calculation unit selects a combination of the
upstream detection unit and the downstream detection unit such that
the expected conveying distance calculated based on the set sheet
length of the expected sheet and the distance between the selected
downstream detection unit and the upstream detection unit becomes a
substantially integer multiple of a perimeter of the one of the
drive roller and the driven roller.
4. The sheet conveying apparatus according to claim 1, further
comprising a sensor position adjusting unit that adjusts the
position of at least one of the upstream detection unit and the
downstream detection unit in the conveying direction of the
sheet.
5. The sheet conveying apparatus according to claim 4, wherein the
sensor position adjusting unit adjusts the position of the at least
one of the upstream detection unit and the downstream detection
unit in the conveying direction of the sheet such that the expected
conveying distance calculated based on the set sheet length of the
expected sheet and the distance between the selected downstream
detection unit and the upstream detection unit becomes a
substantially integer multiple of a perimeter of one of the drive
roller and the driven roller.
6. The sheet conveying apparatus according to claim 1, wherein the
conveying distance calculation unit calculates a length of the
sheet in the conveying direction of the sheet by adding a distance
between the first detection unit and the second detection unit to
the calculated conveying distance of the sheet.
7. The sheet conveying apparatus according to claim 1, wherein a
conveying amount measuring unit measures the conveying amount of
the sheet conveyed by the sheet conveying unit based on a rotation
amount of one of the drive roller and the driven roller.
8. The sheet conveying apparatus according to claim 7, wherein a
distance between the downstream detection unit and the upstream
detection unit or a perimeter of the one of the drive roller and
the driven roller is set such that an expected conveying distance
calculated based on a set sheet length of an expected sheet for
which the conveying distance is to be calculated becomes a
substantially integer multiple of a perimeter of one of the drive
roller and the driven roller.
9. An image forming apparatus comprising: a transfer unit that
transfers a toner image onto a sheet; and the sheet conveying
apparatus according to claim 1.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a sheet conveying apparatus
and an image forming apparatus.
[0003] 2. Description of the Related Art
[0004] In a commercial printing business, Print on Demand (POD) by
an image forming apparatus using electrophotography instead of
using an offset printing machine has been provided for printing
small lots of data, various types of data or variable data has been
increasing. In order to meet this kind of need, registration on
both surfaces is required for the image forming apparatus using
electrophotography comparable to that of the offset printing
machine.
[0005] There are two main reasons for causing a registration error
occurring in both-sides printing, including registration error in
the lateral and the vertical directions, and a skew error between a
sheet and an image. Further, for an image forming apparatus
including a heat fixing device, an image size error caused by
expansion and contraction of the sheet is also a reason for
registration error occurring in both-sides printing.
[0006] In order to automatically correct the registration error in
both-sides printing caused by the image size error, it is required
to use a technique to automatically and accurately measure the size
of a sheet, the conveying distance of the sheet or the like. Thus,
a technique to measure the length of the sheet by detecting passing
of a front end and a rear end of the sheet and calculating the
length of the sheet based on the period between the passing of the
front end and the rear end of the sheet, or the like is known.
[0007] For example, according to Patent Documents 1 to 3, a sheet
length measurement means is disclosed. The sheet length measurement
means includes a rotation amount measurement means that measures a
rotation amount of a length measuring roller which is rotated in
accordance with a movement of a sheet or the like, and edge sensors
provided before and after the length measuring roller to detect
passing of the sheet. The sheet length measurement means measures
the length of the sheet or the like in the conveying direction of
the sheet based on the rotation amount of the length measuring
roller and detections by the edge sensors.
[0008] However, when there is an eccentric amount of the length
measuring roller, if the phases of the length measuring roller at a
start timing and an end timing are different, an error may be
caused in the measured sheet length.
[0009] Thus, according to Patent Document 4, a length measuring
apparatus including a length measuring roller, a first upstream
edge sensor, a second upstream edge sensor and a downstream edge
sensor is disclosed. In the length measuring apparatus, a length of
the sheet in the conveying direction is calculated by selecting a
length among a first length of a sheet measured within a first
detection period by the first upstream edge sensor and the
downstream edge sensor, and a second length of a sheet measured
within a second detection period by the second upstream edge sensor
and the downstream edge sensor, which becomes closer to an integer
multiple of the perimeter of the length measuring roller.
[0010] According to Patent Document 4, it is described that a
measurement error in the measured sheet length obtained by using
the length measuring roller caused by the eccentric amount of the
length measuring roller can be reduced.
[0011] However, according to the length measuring apparatus
disclosed in Patent Document 4, there may be a case when both the
first length of the sheet measured within the first detection
period by the first upstream edge sensor and the downstream edge
sensor, and the second length of the sheet measured within the
second detection period by the second upstream edge sensor and the
downstream edge sensor do not become an integer multiple of the
perimeter of the length measuring roller. In such a case, the
measurement error in the measured sheet length obtained by using
the length measuring roller caused by the eccentric amount of the
length measuring roller cannot be reduced.
PATENT DOCUMENT
[0012] [Patent Document 1] Japanese Laid-open Patent Publication
No. 2010-241600 [0013] [Patent Document 2] Japanese Laid-open
Patent Publication No. 2011-006202 [0014] [Patent Document 3]
Japanese Laid-open Patent Publication No. 2011-020842 [0015]
[Patent Document 4] Japanese Laid-open Patent Publication No.
2011-079662
SUMMARY OF THE INVENTION
[0016] The present invention is made in light of the above
problems, and provides a sheet conveying apparatus capable of
reducing a measurement error in a sheet conveying distance caused
by an eccentric amount of a roller whose rotation amount is counted
to obtain the sheet conveying distance.
[0017] According to an embodiment, there is provided a sheet
conveying apparatus including a sheet conveying unit that conveys a
sheet including a drive roller which is driven to be rotated by a
driving unit, and a driven roller which is rotated in accordance
with the drive roller while the sheet is interposed between the
drive roller and the driven roller; a downstream detection unit
that detects the sheet downstream of the sheet conveying unit in a
conveying direction of the sheet; an upstream detection unit that
detects the sheet upstream of the sheet conveying unit in the
conveying direction of the sheet; a conveying amount measuring unit
that measures a conveying amount of the sheet conveyed by the sheet
conveying unit based on a rotation amount of one of the drive
roller and the driven roller; and a conveying distance calculation
unit that calculates a conveying distance of the sheet conveyed by
the sheet conveying unit based on the conveying amount measured by
the conveying amount measuring unit within a period determined by
detections made by the first detection unit and the second
detection unit, wherein a distance between the downstream detection
unit and the upstream detection unit or a perimeter of the one of
the drive roller and the driven roller is set such that an expected
value of the conveying distance calculated based on a set sheet
length of an expected sheet for which the conveying distance is to
be calculated becomes a substantially integer multiple of a
perimeter of one of the drive roller and the driven roller.
[0018] According to another embodiment, there is provided an image
forming apparatus including a transfer unit that transfers a toner
image onto a sheet; and the sheet conveying apparatus.
[0019] Note that also arbitrary combinations of the above-described
constituents, and any exchanges of expressions in the present
invention, made among method, device, system, recording medium,
computer program and so forth, are valid as embodiments of the
present invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] Other objects, features and advantages of the present
invention will become more apparent from the following detailed
description when read in conjunction with the accompanying
drawings.
[0021] FIG. 1 a plan view schematically showing an example of a
structure of a sheet conveying apparatus of an embodiment;
[0022] FIG. 2 is a cross-sectional view schematically showing an
example of a structure of a sheet conveying apparatus of an
embodiment;
[0023] FIG. 3 is a block diagram showing an example of a functional
structure of a sheet conveying apparatus of an embodiment;
[0024] FIG. 4 is a view showing output signals output by a start
trigger sensor, a stop trigger sensor and a rotary encoder;
[0025] FIG. 5A and FIG. 5B are views for explaining a conveying
distance "P" of a sheet of an embodiment;
[0026] FIG. 6A and FIG. 6B are views for explaining a relationship
between the eccentric amount of a driven roller and a measurement
error of an embodiment;
[0027] FIG. 7 is a view showing an example of a relationship
between a set length "Ls" of an expected sheet, an expected
conveying distance "Pe", and the perimeter of a driven roller of an
embodiment;
[0028] FIG. 8 is a graph showing a relationship between measurement
error "C" and phase ".theta.s" of a driven roller of an
embodiment;
[0029] FIG. 9 is a schematic diagram showing an example of a sheet
conveying apparatus of an embodiment;
[0030] FIG. 10 is a plan view schematically showing an example of a
structure of a sheet conveying apparatus of an embodiment;
[0031] FIG. 11 is a schematic diagram showing an example of a sheet
conveying apparatus of an embodiment;
[0032] FIG. 12 is a schematic diagram showing an example of a sheet
conveying apparatus of an embodiment;
[0033] FIG. 13 is a schematic diagram showing an example of an
image forming apparatus of an embodiment;
[0034] FIG. 14 is a schematic diagram showing an example of an
image forming apparatus of an embodiment;
[0035] FIG. 15 is a schematic diagram showing an example of an
image forming apparatus of an embodiment;
[0036] FIG. 16 is a flow chart showing an example of operations of
determining a distance "a" or a perimeter "2.pi.r"; and
[0037] FIG. 17 is a block diagram showing an example of functional
components of a conveying distance calculation unit.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0038] The invention will be described herein with reference to
illustrative embodiments. Those skilled in the art will recognize
that many alternative embodiments can be accomplished using the
teachings of the present invention and that the invention is not
limited to the embodiments illustrated for explanatory
purposes.
[0039] It is to be noted that, in the explanation of the drawings,
the same components are given the same reference numerals, and
explanations are not repeated.
First Embodiment
Structure of Sheet Conveying Apparatus
[0040] FIG. 1 and FIG. 2 are views showing an outline constitution
of a sheet conveying apparatus 100 of the embodiment. FIG. 1 is a
plan view schematically showing an example of a structure of the
sheet conveying apparatus 100 and FIG. 2 is a cross-sectional view
schematically showing an example of a structure of the sheet
conveying apparatus 100.
[0041] The sheet conveying apparatus 100 includes a sheet conveying
unit 110 provided on a conveying path of a sheet S, a start trigger
sensor 11, a stop trigger sensor 12, and a rotary encoder 15. The
sheet S may be a paper, an OHP or the like. The sheet conveying
unit 110 includes a drive roller 14 and a driven roller 13. The
drive roller 14 is driven to be rotated by a driving unit 20 (see
FIG. 2) such as a motor or the like and a driving force
transmitting unit 22 (see FIG. 2) such as a gear, a belt or the
like. The driven roller 13 is rotated in accordance with the
rotation of the drive roller 14 while a sheet S is interposed
between the drive roller 14 and the driven roller 13.
[0042] FIG. 3 is a block diagram showing an example of a functional
structure of the sheet conveying apparatus 100 of the
embodiment.
[0043] As shown in FIG. 3, the sheet conveying apparatus 100
includes the sheet conveying unit 110 (the driven roller 13 and the
drive roller 14), the rotary encoder 15, the start trigger sensor
11, the stop trigger sensor 12, a pulse measuring unit 116 and a
conveying distance calculation unit 117. The structure of the sheet
conveying apparatus 100 is explained with reference to FIG. 1 to
FIG. 3.
[0044] The drive roller 14 includes an elastic layer at a surface
in order to generate a sufficient friction force with the sheet S
so that the sheet S becomes intervened between the drive roller 14
and the driven roller 13.
[0045] The driven roller 13 is provided to be pushed by a pushing
member (not shown in the drawings) such as a spring or the like to
be in contact with the drive roller 14. With this structure, when
the drive roller 14 is rotated to convey the sheet S, the driven
roller 13 is also rotated by the friction force generated with the
sheet S.
[0046] The rotary encoder 15 is provided at a rotational axle of
the driven roller 13 in this embodiment. The rotary encoder 15
includes an encoder disk 15a mounted on the rotational axle and an
encoder sensor 15b. The encoder sensor 15b generates a pulse signal
when the encoder disk 15a is being rotated with the driven roller
13.
[0047] The pulse measuring unit 116, which is an example of a
conveying amount measuring unit, measures a rotation amount of the
driven roller 13 as a conveying amount of the sheet S based on
counting the pulse signal generated by the encoder sensor 15b in
accordance with the rotation of the encoder disk 15a.
[0048] Alternatively, the rotary encoder 15 may be provided at a
rotational axle of the drive roller 14, it means that the encoder
disk 15a is mounted on the rotational axle. Further alternatively,
the driven roller 13 and the drive roller 14 may be oppositely
positioned.
[0049] The diameter of a roller (the driven roller 13 or the drive
roller 14) to which the rotary encoder 15 is provided may be as
small as possible so that the number of rotations of the roller in
accordance with the conveying amount of the sheet S becomes larger
to accurately measure the conveying distance of the sheet S.
[0050] The driven roller 13 or the drive roller 14 to which the
rotary encoder 15 is provided may be made of metal in order to
reduce deflection of the rotational axle. By reducing the
deflection of the rotational axle, the conveying distance of the
sheet S, which will be explained later, can be accurately
measured.
[0051] As shown in FIG. 1, the width "Wr" of the driven roller 13
is set to be smaller than the minimum width "Ws" of an expected
sheet S adaptable to the sheet, in a direction perpendicular to a
conveying direction of the sheet S. Thus, when conveying the sheet
S, the driven roller 13 does not directly contact the drive roller
14 so that the driven roller 13 can be rotated by the friction
force generated with the sheet S. Therefore, the conveying distance
of the sheet S can be accurately measured without being influenced
by the drive roller 14.
[0052] The start trigger sensor 11 and the stop trigger sensor 12
are provided downstream and upstream, respectively, of the driven
roller 13 and the drive roller 14 on a conveying path of the sheet
S. The start trigger sensor 11 and the stop trigger sensor 12 are
configured to detect passing of a front end portion (front edge) of
the sheet S and passing of a rear end portion (rear edge) of the
sheet, respectively. Each of the start trigger sensor 11 and the
stop trigger sensor 12 may be a transmission or reflection optical
sensor capable of detecting an end portion of the sheet S with high
accuracy. In this embodiment, the start trigger sensor 11 and the
stop trigger sensor 12 are reflection optical sensors.
[0053] The start trigger sensor 11 is an example of a downstream
detection unit that detects passing of the front end portion of the
sheet S. The stop trigger sensor 12 is an example of an upstream
detection unit that detects passing of the rear end portion of the
sheet S.
[0054] The start trigger sensor 11 and the stop trigger sensor 12
are positioned to be substantially at the same position in a
direction perpendicular to the conveying direction of the sheet S.
With this structure, it becomes possible to more precisely measure
the conveying distance of the sheet S by minimizing the influence
of the attitude of the sheet S (skew with respect to the conveyance
direction).
[0055] Furthermore, the start trigger sensor 11 and the stop
trigger sensor 12 are not necessarily positioned in the middle but
may be positioned at an outer portion in the direction
perpendicular to the conveying direction of the sheet S provided
that they are positioned within the path of the sheet S.
[0056] In this embodiment, it is assumed that the distance between
the start trigger sensor 11 and the driven roller 13 (or the drive
roller 14) is "A", and the distance between the stop trigger sensor
12 and the driven roller 13 (or the drive roller 14) is "B", in the
conveying direction of the sheet S. The distances "A" and "B" will
be further explained later.
[0057] In this embodiment, it is assumed that the drive roller 14
is rotated in a direction shown by an arrow in FIG. 2. The driven
roller 13 is rotated with respect to the drive roller 14 by the
drive roller 14 when the sheet S is not conveyed (at an idling
time) and by the sheet S when the sheet S is conveyed. When the
driven roller 13 is rotated, the pulse signal is generated from the
rotary encoder 15 provided at the rotational axle of the driven
roller 13.
[0058] The pulse measuring unit 116 starts counting the number of
pulses of the rotary encoder 15 based on the pulse signal when the
start trigger sensor 11 detects passing of the front end portion of
the sheet S, and stops counting the number of pulses of the rotary
encoder 15 when the stop trigger sensor 12 detects passing of the
rear end portion of the sheet S while the sheet S is being conveyed
in a direction shown by an arrow X.
[0059] The conveying distance calculation unit 117 calculates the
conveying distance of the sheet S by the sheet conveying unit 110
based on the detection of the sheet S by the start trigger sensor
11 and the stop trigger sensor 12, and the rotation amount of the
driven roller 13 measured by the pulse measuring unit 116.
(Calculation of Conveying Distance of Sheet)
[0060] FIG. 4 is a view showing output signals output by the start
trigger sensor 11, the stop trigger sensor 12 and the rotary
encoder 15.
[0061] As described above, when the driven roller 13 is rotated,
the pulse signal is generated from the rotary encoder 15 which is
provided at the rotational axle of the driven roller 13.
[0062] It is assumed that the stop trigger sensor 12 detects
passing of a front end portion of the sheet S at time "t1" and
after that, the start trigger sensor 11 detects passing of the
front end portion of the sheet S at time "t2" while the sheet S is
being conveyed.
[0063] Subsequently, it is assumed that the stop trigger sensor 12
detects passing of a rear end portion of the sheet S at time "t3"
and after that, the start trigger sensor 11 detects passing of the
rear end portion of the sheet S at time "t4".
[0064] The pulse measuring unit 116 counts the number of pulses of
the rotary encoder 15 at a pulse counting period "Tp", which is
from time "t2" at which the start trigger sensor 11 detects that
the front end portion of the sheet S passes to time "t3" at which
the stop trigger sensor 12 detects that the rear end portion of the
sheet S passes.
[0065] Here, it is assumed that a radius of the driven roller 13 to
which the rotary encoder 15 is provided is "r", the number of
pulses of the rotary encoder 15 while the driven roller 13 is
rotated 360 degrees is "N", and the number of pulses counted by the
pulse measuring unit 116 during the pulse counting period "Tp" is
"n". Under this condition, the sheet conveying distance "P" (see
FIG. 1) of the sheet S during the pulse counting period "Tp" (from
time "t2" to time "t3") is expressed by the following equation
(1).
P=(n/N).times.2.pi.r (1)
[0066] n: the counted number of pulses
[0067] N: the number of pulses of the rotary encoder 15 while the
driven roller 13 is rotated 360 degrees
[0068] r: radius [mm] of the driven roller 13
[0069] Generally, a sheet conveying speed is easily varied based on
mechanical accuracy such as structural accuracy of the rollers
(especially the drive roller 14) which convey the sheet S,
deflection of rotational axle or the like, rotational accuracy of
the motor or the like, or accuracy of the driving force
transmitting unit such as a gear, a belt or the like. Further, the
sheet conveying speed is varied based on a slipping phenomenon
between the drive roller 14 and the sheet S, looseness generated by
the difference in conveying force or conveying speed of conveying
units provided upstream or downstream of the sheet conveying unit
110 or the like. Thus, a pulse period or pulse width of the rotary
encoder 15 may always vary. However, the number of pulses does not
easily vary.
[0070] Thus, the conveying distance calculation unit 117 can
accurately obtain the sheet conveying distance "P" of the sheet S
conveyed by the driven roller 13 and the drive roller 14 in
accordance with the above equation (1), without depending on the
sheet conveying speed.
[0071] The conveying distance calculation unit 117 can further
obtain a relative ratio of the conveying distances of a previous
sheet S and a next sheet S, a relative ratio of the conveying
distances of a front surface of the sheet S and a back surface of
the sheet or the like.
[0072] The conveying distance calculation unit 117 may obtain a
ratio of expansion and contraction "R" based on a relative ratio of
the conveying distances before and after the heat fixing by
electrophotography in accordance with the following equation
(2).
R=[(n2/N).times.2.pi.r]/[(n1/N).times.2.pi.r] (2)
n1: the number of pulses measured when the sheet S before the heat
fixing is conveyed
[0073] n2: the number of pulses measured when the sheet S after the
heat fixing is conveyed
[0074] Examples are explained in the following.
[0075] In this embodiment, when the measured number of pulses is
n1=18816 under a condition that N=2800, r=9 mm and the sheet S of
A3 size is conveyed in the longitudinal direction, the conveying
distance "P1" of the sheet S becomes,
P1=(18816/2800).times.2.pi..times.9=380.00 mm
[0076] Further, when the measured number of pulses is n2=18759
after the heat fixing of the sheet S, the conveying distance "P2"
of the sheet S becomes,
P2=(18759/2800).times.2.pi..times.9=378.86 mm
[0077] Thus, the difference between before and after the heat
fixing .DELTA.P of the conveying distances "P1" and "P2" of the
sheet S becomes as follows.
.DELTA.P=380.00-378.86=1.14 mm
[0078] Thus, the ratio of expansion and contraction "R" (the
relative ratio between before and after the heat fixing (front side
surface and back side surface of the sheet S, respectively)) of the
sheet S may be obtained as follows.
R=378.86/380.00=99.70%
[0079] Thus, in this case, the length of the sheet S in the
conveying direction of the sheet S is shrunken about 1 mm by the
heat fixing. Therefore, if the lengths of the images to be formed
on the front surface and the back surface of the sheet S are the
same, registration error between two surfaces of about 1 mm is
generated. Thus, by correcting the length of the image printed on
the back surface of the sheet S based on the calculated ratio of
expansion and contraction "R", the registration in two-sided
printing can be improved.
[0080] Here, for the above described example, the ratio of
expansion and contraction "R" is obtained by calculating the
conveying distances "P1" and "P2" of the sheet S before and after
the heat fixing. Alternatively, the ratio of expansion and
contraction "R" may be calculated based on the numbers of pulses
"n1" and "n2" which are counted by the pulse measuring unit 116
such as R=n2/n1.
[0081] For the above example, when the number of pulses n.sub.1,
which is measured when the sheet S is conveyed before the heat
fixing, is n1=18816, and the number of pulses n2, which is measured
when the sheet S is conveyed after the heat fixing, is
n.sub.2=18759, the ratio of expansion and contraction "R" may be
obtained as follows.
R=n2/n1=18759/18816=99.70%
[0082] Here, by adding a distance "a" between the start trigger
sensor 11 and the stop trigger sensor 12 shown in FIG. 2 to the
sheet conveying distance "P" obtained by the above equation (1),
the length "L" of the sheet S in the conveying direction becomes as
follows.
L=(n/N).times.2.pi.r+a (1')
[0083] a: the distance between the start trigger sensor 11 and the
stop trigger sensor 12
[0084] The conveying distance calculation unit 117 of the sheet
conveying apparatus 100 can obtain the length "L" of the sheet S in
the conveying direction based on the equation (1') in which the
distance "a" between the start trigger sensor 11 and the stop
trigger sensor 12 is added to the conveying distance "P" of the
sheet S obtained based on the above equation (1).
[0085] Further, the conveying distance calculation unit 117 can
obtain the ratio of expansion and contraction "R" from the relative
ratio of the length "L" of the sheet S in the conveying direction
before and after the heat fixing by the electrophotography in
accordance with the following equation (2').
R=[(n2/N).times.2.pi.r+a]/[(n1/N).times.2.pi.r+a] (2')
[0086] As described above, the conveying distance calculation unit
117 of the sheet conveying apparatus 100 can accurately obtain the
length "L" of the sheet S in the conveying direction and the ratio
of expansion and contraction "R".
(Relationship Between Perimeter of Driven Roller and Measuring
Length of Sheet)
[0087] FIG. 5A and FIG. 5B are views for explaining the conveying
distance "P" of the sheet S, at which the pulses are counted, in
the sheet conveying apparatus 100 of the first embodiment.
[0088] As shown in FIG. 5A, the rotary encoder 15, which is
provided at the driven roller 13 although not shown in FIG. 5A,
starts counting pulses when a front end portion of the sheet S is
detected by the start trigger sensor 11.
[0089] When the sheet S is conveyed by the drive roller 14 and the
driven roller 13 and a rear end portion of the sheet S is detected
by the stop trigger sensor 12 at a position as shown in FIG. 5B,
the rotary encoder (although not shown in FIG. 5) stops counting
the pulses.
[0090] The conveying distance "P" is a conveying amount of the
sheet S conveyed by the drive roller 14 and the driven roller 13
within a pulse counting range, which is between a start timing when
the sheet S is detected by the start trigger sensor 11 and counting
of the pulses is started, and an end timing when the sheet S is
detected by the stop trigger sensor 12 and counting of the pulses
is stopped.
[0091] Specifically, the conveying distance "P" becomes a length
obtained by subtracting a distance "A" between the start trigger
sensor 11 and the driven roller 13 and a distance "B" between the
driven roller 13 and the stop trigger sensor 12 from the length "L"
of the sheet S in the conveying direction (P=L-(A+B)). In other
words, the conveying distance "P" becomes P=L-a which is obtained
by subtracting a distance "a" between the start trigger sensor 11
and the stop trigger sensor 12 from the length "L" of the sheet
S.
[0092] FIG. 6A and FIG. 6B are views for explaining a relationship
between an eccentric amount of the driven roller 13 of the first
embodiment and a measurement error in the conveying distance "P" of
the sheet S.
[0093] For example, as shown in FIG. 6A, it is assumed that the
driven roller 13, to which the rotary encoder 15 is provided, is
rotated around an eccentric center O' which is decentered for "z"
from the center O of its circumferential circle. At this time, a
measurement error "C" in the conveying distance "P" of the sheet S
can be calculated as follows.
C=sin .theta.s.times.z (3)
[0094] Here, ".theta.s" is phase of the driven roller 13 when a
point S1 at which the measuring of the conveying amount is started
is defined as .theta.s=0.
[0095] FIG. 6B shows the measurement error "C" when z=-0.1 mm. It
means that if the phases of the driven roller 13 at the start
timing and the end timing are different from each other, and the
eccentric amount z=-0.1 mm, the measurement error "C" becomes
.+-.0.1 mm at the maximum.
[0096] Thus, in this embodiment, the radius "r" of the driven
roller 13 to which the rotary encoder 15 is provided and the
distance "a" between the start trigger sensor 11 and the stop
trigger sensor 12 are determined to satisfy the following equation
(4). In the following equation (4), "Ls" is a set length of an
expected sheet (which will be referred to as the expected sheet Se
hereinafter) for which the actual length "L" is to be measured by
the sheet conveying apparatus 100, and "Pe" is an expected value of
the conveying distance (simply referred to as the "expected
conveying distance" hereinafter) of the expected sheet Se.
Pe(=Ls-a)=2.pi.r.times.k (4)
[0097] k: positive integer
[0098] It means that according to the embodiment, the radius "r" of
the driven roller 13 or the distance "a" between the start trigger
sensor 11 and the stop trigger sensor 12 is determined such that
the expected conveying distance "Pe" becomes an integer multiple of
a perimeter of the driven roller 13. With this structure, the
phases of the driven roller 13 at the start timing and the end
timing are expected to become substantially the same to reduce the
measurement error "C".
[0099] FIG. 7 is a view showing an example of a relationship
between the set length "Ls" of the expected sheet Se in the
conveying direction, the expected conveying distance "Pe", and the
perimeter "2.pi.r" of the driven roller 13 in the embodiment.
[0100] Thus, for a case when the radius "r" of the driven roller 13
is previously fixed, the distance "a" between the start trigger
sensor 11 and the stop trigger sensor 12 is determined to satisfy
the following equation (4-1).
a=Ls-(2.pi.r.times.k) (4-1)
[0101] Further, for a case when the distance "a" between the start
trigger sensor 11 and the stop trigger sensor 12 is previously
fixed, the perimeter "2.pi.r" (or the radius "r") of the driven
roller 13 is determined to satisfy the following equation
(4-2).
2.pi.r=(Ls-a)/k (4-2)
[0102] For example, it is assumed that the distance "a" between,
the start trigger sensor 11 and the stop trigger sensor 12 is
previously fixed as 70 mm. Further, it is assumed that two kinds of
sheets are expected to be used in the sheet conveying apparatus
100, whose set lengths "Ls" are 210 mm (a case when A4 sheet is
conveyed in the lateral direction is assumed) and 420 mm (a case
when A3 sheet is conveyed in the longitudinal direction is
assumed), which are most commonly used in Japan. In this case, the
expected conveying distances "Pe" for these expected sheets Se
become 140 mm and 350 mm, respectively.
[0103] Thus, the driven roller 13 is determined to have the
perimeter "2.pi.r" selected from among 2 mm, 4 mm, 5 mm, 7 mm, 10
mm, 14 mm, 20 mm, 28 mm, 35 mm and 70 mm, which are common divisors
of the expected conveying distances "Pe" for both the expected
sheets Se based on the necessity. For example, when the perimeter
2.pi.r=70 mm is selected, the radius "r" becomes about 11.14
mm.
[0104] With this structure, for the both expected sheets Se having
the set lengths "Ls" 210 mm and 420 mm, the expected conveying
distances "Pe" become an integer multiple of the perimeter of the
driven roller 13. Thus, the measurement error "C" caused by the
eccentric amount of the driven roller 13 can be reduced.
[0105] As described above, it is desirable to have the expected
conveying distance "Pe" become an integer multiple of the perimeter
of the driven roller 13. However, a predetermined margin may be
provided based on an allowable measurement error "C.sub.a".
[0106] Thus, in this embodiment, the radius "r" of the driven
roller 13 or the distance "a" between the start trigger sensor 11
and the stop trigger sensor 12 may be determined such that the
expected conveying distance "Pe" becomes a substantially integer
multiple of a perimeter of the driven roller 13 as follows.
Pe(=Ls-a)=2.pi.r.times.k' (4')
[0107] Here, "k'" is a substantially positive integer determined
based on an allowable measurement error "C.sub.a" as follows.
[0108] An example when the radius "r" of the driven roller 13 is
previously fixed will be explained.
[0109] It is assumed that the eccentric amount "z" of the driven
roller 13 is 0.1 mm. Further, if an allowable measurement error
"C.sub.a" in the conveying distance "P" of the sheet S is .+-.0.02
mm, an allowable phase ".theta.s.sub.a" of the driven roller 13 is
calculated as follows based on the above equation (3).
.+-.C.sub.a=sin .theta.s.sub.a.times.z
sin .theta.s.sub.a=.+-.C.sub.a/z=.+-.0.02/0.1
.theta.s.sub.a=.+-.11.54
[0110] FIG. 8 shows a relationship between the measurement error
"C" and the phase ".theta.s" of the driven roller 13 of the
embodiment. It means that the measurement error "C" becomes within
.+-.0.02 mm when the phases ".theta.s" of the driven roller 13 at
the start timing and the end timing are within .+-.11.54.degree..
Thus, the allowable phase ".theta.s.sub.a" becomes
.+-.11.54.degree. when the allowable measurement error "C.sub.a" is
.+-.0.02 mm.
[0111] Provided that the perimeter of the driven roller 13 is 70
mm, when the driven roller 13 is rotated .+-.11.54.degree., which
is the allowable phase ".theta.s.sub.a", the conveying amount of
the sheet S becomes .+-.2.244 mm as follows.
2 .PI. r .times. ( .theta. s / 360 ) = 70 .times. ( .+-. 11.54 /
360 ) = .+-. 2.244 mm ##EQU00001##
[0112] The above means that an allowable margin in the distance
"a.sub.a" becomes .+-.2.244 mm. Thus, an allowable distance
"a.sub.a" between the start trigger sensor 11 and the stop trigger
sensor 12 can be obtained as follows by adding the above distance
".+-.2.244 mm" to the distance "a" obtained based on the above
equation (4-1). Here, it is assumed that the set length "Ls" of the
expected sheet Se is 210 mm (a case when A4 sheet is conveyed in
the lateral direction is supposed), and k=2.
a a = Ls - ( 2 .PI. r .times. k ) .+-. 2 .PI. r ( .theta.s / 360 )
= 210 - ( 70 .times. 2 ) .+-. 2.244` = 70 .+-. 2.244
##EQU00002##
[0113] Thus, when the perimeter of the driven roller 13 is 70 mm,
the eccentric amount "z" of the driven roller 13 is less than or
equal to 0.1 mm, and k=2, the allowable distance "a.sub.a" becomes
70.+-.2.244 mm in order to meet the allowable measurement error
"C.sub.a" to be .+-.0.02 mm.
[0114] Thus, in the equation (4'), "k'" can be expressed as
follows.
k'=k.+-.(.theta.s.sub.a/360)
[0115] The sheet conveying unit 110 may further include a relation
information storing unit as will be explained later that stores the
relationship between the measurement error "C" and the phase
".theta.s" of the driven roller 13 as shown in FIG. 8. In such a
case, the value of "k'" may be obtained using the relationship
stored in the relation information storing unit based on the
allowable measurement error "C.sub.a". Further, the value of "k'"
may be calculated as follows based on the allowable measurement
error "C.sub.a". The allowable measurement error "C.sub.a" may be
determined based on the set length "Ls", the kind of the conveying
apparatus 100, an expected value of the ratio of expansion and
contraction "R", or the like, but may be .+-.0.05 mm as an
example.
[0116] Based on the above described equation (3), .theta.s can be
expressed as follows.
.theta.s=sin.sup.-1(C/z)
[0117] Thus, "k'" can be expressed as follows.
k'=k.+-.2.pi.r(sin.sup.-1(C.sub.a/z)/360)
[0118] As described above, in this embodiment, the distance "a"
between the start trigger sensor 11 and the stop trigger sensor 12
or the perimeter "2.pi.r" of the driven roller 13 is determined as
follows.
[0119] FIG. 16 is a flowchart showing an example of operations of
determining the distance "a" or the perimeter "2.pi.r". This
operation may be performed by the conveying distance calculation
unit 117.
[0120] First, sheet information including set lengths "Ls1", "Ls2",
and "Lsn" of expected sheets Se1, Se2, and Sen in the conveying
direction are obtained (step S100).
[0121] Subsequently, allowable error information is obtained to
determine the value "k'" (step S101).
[0122] Then, if the perimeter "2.pi.r" (or the radius "r") of the
driven roller 13 is previously fixed (YES in step S102), the
distance "a" is determined based on the equation (4') (step S104).
Subsequently, calculated result is output from the conveying
distance calculation unit 117 (step S108).
[0123] At step S102, if the perimeter "2.pi.r" (or the radius "r")
of the driven roller 13 is not previously fixed (NO in step S102),
and the distance "a" is previously fixed (YES in step S110), the
perimeter "2.pi.r" (or the radius "r") of the driven roller 13 is
determined based on the equation (4') (step S112). Then, calculated
result is output from the conveying distance calculation unit 117
(step S108).
[0124] As described above, by determining the distance "a" between
the start trigger sensor 11 and the stop trigger sensor 12 or the
perimeter "2.pi.r" of the driven roller 13 such that the expected
conveying distance "Pe" becomes a substantially integer multiple of
a perimeter of the driven roller 13, influence of the eccentric
amount of the driven roller 13 is reduced so that the conveying
distance "P" of the sheet S in the conveying distance can be
accurately measured.
[0125] When the distance "a" is to be determined is previously
known, step S102 can be omitted and only steps S100 and S101, and
steps S104 and S108 are performed. Similarly, when the perimeter
"2.pi.r" is to be determined is previously known, steps S102 and
S110 can be omitted and only steps S100 and S101, and steps S112
and S108 are performed.
[0126] FIG. 17 is a block diagram showing an example of the
functional components of the conveying distance calculation unit
117. Functional components of the conveying distance calculation
unit 117 for calculating the conveying distance of the sheet S by
the sheet conveying unit 110 based on the detection of the sheet S
by the start trigger sensor 11 and the stop trigger sensor 12, and
the rotation amount of the driven roller 13 measured by the pulse
measuring unit 116 are not shown in FIG. 17. The conveying distance
calculation unit 117 includes an information input unit 152, a
sensor distance calculation unit 150 and a relation information
storing unit 156.
[0127] The information input unit 152 inputs information input by a
user 200 or the like. The information input unit 152 may input the
sheet information explained above with reference to step S100 in
FIG. 16 input by the user 200 or the like. Further, the information
input unit 152 may input the allowable error information explained
above with reference to step S101 in FIG. 16 input by the user 200
or the like. Further, if the perimeter "2.pi.r" (or the radius "r")
of the driven roller 13 is previously fixed, the information input
unit 152 may input the value "2.pi.r" (or the radius "r") input by
the user 200 or the like. On the contrary, if the distance "a" is
previously fixed, the information input unit 152 may input the
value "a" input by the user 200 or the like.
[0128] If the perimeter "2.pi.r" (or the radius "r") of the driven
roller 13 is previously fixed, the sensor distance calculation unit
150 may calculate the distance "a" as explained above with
reference to step S104 in FIG. 16. The relation information storing
unit 156 stores the relationship between the measurement error "C"
and the phase ".theta.s" of the driven roller 13 as shown in FIG.
8. At this time, the sensor distance calculation unit 150 may refer
to the relation information storing unit 156 for obtaining the
allowable phase ".theta.s.sub.a" based on the allowable measurement
error "C.sub.a". Further, if the distance "a" is previously fixed,
the sensor distance calculation unit 150 may calculate the
perimeter "2.pi.r" (or the radius "r") of the driven roller 13 as
explained above with reference to step S112 in FIG. 16.
[0129] Although in the above embodiment, an example where the
rotary encoder 15 is attached to the driven roller 13 is explained,
the rotary encoder 15 may be attached to the drive roller 14. In
this case, the radius of the drive roller 14 or the distance "a"
between the start trigger sensor 11 and the stop trigger sensor 12
is determined such that the expected conveying distance "Pe"
becomes a substantial integer multiple of a perimeter of the drive
roller 14. With this structure, a measurement error caused by
eccentric of the drive roller 14 can be reduced.
[0130] Further, the distance "a" between the start trigger sensor
11 and the stop trigger sensor 12 may be arbitrary determined based
on the radius "r" of the driven roller 13, sizes of the start
trigger sensor 11 and the stop trigger sensor 12, or a space in the
sheet conveying apparatus 100 or the like.
[0131] Further, the driven roller 13 (or the drive roller 14) may
be configured to be capable of changing the perimeter. In this
case, the driven roller 13 (or the drive roller 14) may be
configured to have plural perimeters which are varied stepwise. In
this case, the driven roller 13 (or the drive roller 14) is
positioned to face the drive roller 14 (or the driven roller 13) at
the edge side in the width direction of the sheet and hold a sheet
there between. In this case, the driven roller 13 (or the drive
roller 14) may be configured to be capable of moving toward and
away from the drive roller 14 (or the driven roller 13) as well as
in the width direction of the sheet.
[0132] Thus, in this embodiment, by determining the distance "a"
between the start trigger sensor 11 and the stop trigger sensor 12
or the perimeter "2.pi.r" of the driven roller 13 to be within a
predetermined range, the measurement error "C" can be reduced to be
a predetermined value.
Second Embodiment
[0133] In this embodiment, a case when the perimeter "2.pi.r" (or
the radius "r") of the driven roller 13 is previously fixed, in
other words, the driven roller 13 is previously fixed, is
explained.
[0134] The sheet conveying apparatus 100 may be configured to
include plural sensors for at least one of the start trigger sensor
and the stop trigger sensor.
[0135] As described above, when the set lengths "Ls" of the
expected sheets Se are 210 mm (a case when A4 sheet is conveyed in
the lateral direction is supposed) or 420 mm (a case when A3 sheet
is conveyed in the longitudinal direction is supposed), the
distance "a" or the perimeter of the driven roller 13 can be
obtained based on the common divisor of the expected conveying
distances "Pe" as shown in FIG. 7. Thus, in such a case, the
conveying distance "P" of the sheet S can be accurately measured by
setting the distance "a" and the perimeter of the driven roller 13
to satisfy the above described equation (4), (4') or the like.
[0136] However, there may be a case when there are no common
divisors of the expected conveying distances "Pe" for the expected
sheets Se. Thus, in this embodiment, plural sensors for at least
one of the start trigger sensor and the stop trigger sensor are
provided.
[0137] FIG. 9 is a schematic diagram showing an example of a sheet
conveying apparatus 101 of the embodiment. In this embodiment, the
sheet conveying apparatus 101 includes plural stop trigger
sensors.
[0138] The sheet conveying apparatus 101 of the embodiment further
includes a stop trigger sensor 22 in addition to the components of
the sheet conveying apparatus 100 of the first embodiment explained
above with reference to FIG. 1 to FIG. 3.
[0139] The sheet conveying apparatus 101 is configured to be
adaptable for a LETTER size sheet as the expected sheet Se, which
is commonly used in North America or the like and whose set length
"Ls" is 216 mm (a case when it is conveyed in the lateral direction
is supposed) in addition to 210 mm (a case when A4 sheet is
conveyed in the lateral direction is supposed) or 420 mm (a case
when A3 sheet is conveyed in the longitudinal direction is
supposed).
[0140] Thus, in this embodiment, similar to the first embodiment,
the start trigger sensor 11 and the stop trigger sensor 12 are
provided such that the expected conveying distances "Pe" obtained
by subtracting the distance "a" between the start trigger sensor 11
and the stop trigger sensor 12 from the set lengths 210 mm and 420
mm, respectively becomes a substantially integer multiple of the
perimeter of the driven roller 13.
[0141] Further, in this embodiment, the start trigger sensor 11 and
the stop trigger sensor 22 are provided such that the expected
conveying distances "Pe" obtained by subtracting the distance "a'"
between the start trigger sensor 11 and the stop trigger sensor 22
from the set length 216 mm becomes a substantially integer multiple
of the perimeter of the driven roller 13.
[0142] Thus, according to the embodiment, even when there are no
common divisors of the expected conveying distances "Pe" for the
expected sheets Se, by providing plural combinations of the start
trigger sensor and the stop trigger sensor, the distances of which
are different from each other, the conveying distance "P" of
various kinds of sheets can be accurately calculated.
[0143] Alternatively, the sheet conveying apparatus 100 may be
configured to include plural start trigger sensors, or plural start
trigger sensors and plural stop trigger sensors. Thus, in this
embodiment, plural sensors for at least one of the start trigger
sensor and the stop trigger sensor are provided.
[0144] In this embodiment, the conveying distance calculation unit
117 selects a combination of the start trigger sensor 11 and the
stop trigger sensor 12 or a combination of the start trigger sensor
11 and the stop trigger sensor 22 based on the set lengths "Ls" of
the expected sheets "Se". Then, the conveying distance calculation
unit 117 calculates the conveying distance "P" based on the
selected combination of the start trigger sensor and the stop
trigger sensor.
[0145] With this structure, the sheet conveying distance "P" or the
length "L" of the sheet S can be accurately measured. Thus, the
measurement error "C" caused by the eccentric amount of the driven
roller 13 is reduced and for the distances which are different from
each other, the conveying distance "P" of various kinds of sheets
can be accurately calculated.
[0146] The start trigger sensor 11 and the stop trigger sensors 12
and 22 may be positioned on a line extending in the conveying
direction of the sheet S, in other words, the start trigger sensor
11 and the stop trigger sensors 12 and 22 may be positioned to be
substantially at the same position in a direction perpendicular to
the conveying direction of the sheet S. Alternatively, the start
trigger sensor 11 and the stop trigger sensors 12 and 22 may be
positioned at different positions in the direction perpendicular to
the conveying direction of the sheet S as shown in FIG. 10.
[0147] FIG. 10 is a plan view schematically showing an example of a
structure of the sheet conveying apparatus 101 of the embodiment.
In this example, the stop trigger sensors 12 and 22 are positioned
at different positions in the direction perpendicular to the
conveying direction of the sheet S. With this structure,
interference between the stop trigger sensors 12 and 22 can be
avoided.
Third Embodiment
[0148] In this embodiment as well, a case when the perimeter
"2.pi.r" (or the radius "r") of the driven roller 13 is previously
fixed, in other words, the driven roller 13 is previously fixed, is
explained.
[0149] In this embodiment, at least one of the start trigger sensor
11 and the stop trigger sensor 12 may be provided to be movable in
the conveying direction of the sheet S to correspond to various
sizes of the sheets.
[0150] FIG. 11 is a schematic diagram showing an example of a sheet
conveying apparatus 102 of the embodiment.
[0151] The sheet conveying apparatus 102 of the embodiment further
includes a sensor position adjusting unit 130 that adjusts the
position of the stop trigger sensor 12 in the conveying direction
of the sheet S.
[0152] The sensor position adjusting unit 130 includes a sensor
support member 30 provided with plural locating holes 34 and plural
long holes 35, a bracket 31 provided with two protruding portions
32, and a screw 33 with knob.
[0153] The stop trigger sensor 12 is attached to the bracket 31 to
be supported by the sensor support member 30.
[0154] When the protruding portions 32 of the bracket 31 engage one
of the locating holes 34 and one of the long holes 35,
respectively, and fixed by the screw 33, the bracket 31 is fixed to
the sensor support member 30.
[0155] The plural locating holes 34 and the long holes 35 are
provided such that the expected conveying distances "Pe" obtained
by subtracting the distance "a'" between the start trigger sensor
11 and the stop trigger sensor 12 from set lengths "Ls" for plural
expected sheets Se become a substantially integer multiple of the
perimeter of the driven roller 13.
[0156] With this structure, when the conveying distance "P" of the
sheet S or the length "L" of the sheet S in the conveying direction
is measured, the position of the stop trigger sensor 12 is manually
adjusted using the sensor position adjusting unit 130 such that the
expected conveying distance "Pe" obtained by subtracting the
distance "a'" between the start trigger sensor 11 and the stop
trigger sensor 12 from a set length "Ls" of a current expected
sheet Se becomes a substantially integer multiple of the perimeter
of the driven roller 13.
[0157] Thus, by providing the stop trigger sensor 12 or the start
trigger sensor 11 movable with respect to the start trigger sensor
11 or the stop trigger sensor 12, respectively, the distance "a'"
between the start trigger sensor 11 and the stop trigger sensor 12
can be variable. Therefore, the conveying distance "P" of various
kinds of sheets can be accurately calculated.
[0158] FIG. 12 is a schematic diagram showing another example of
the sheet conveying apparatus 102 of the embodiment.
[0159] In this example, the structure of the sensor position
adjusting unit 130 is different from that shown in FIG. 11.
[0160] The sensor position adjusting unit 130 includes a carriage
41, a guide-rail 42, plural belt pulleys 46, an endless belt 45, a
carriage position sensor 44 and a protruding portion for sensor
43.
[0161] The stop trigger sensor 12 is attached to the carriage 41.
The carriage 41 is fixed to the endless belt 45 which is suspended
around the plural belt pulleys 46. When the belt 45 is rotated in
accordance with the rotations of the belt pulley 46, the carriage
41 is moved along the guide-rail 42 in the conveying direction of
the sheet S.
[0162] The protruding portion for sensor 43 is attached to the
carriage 41 to be positioned upstream of the carriage 41 in the
conveying direction of the sheet S. The carriage position sensor 44
detects the position of the carriage 41 when the protruding portion
for sensor 43 reaches the carriage position sensor 44. When the
protruding portion for sensor 43 reaches the carriage position
sensor 44 and is detected by the carriage position sensor 44, the
movement of the carriage 41 is stopped and the position of the
carriage 41 is controlled while having the stopped position as an
initial position.
[0163] The position of the carriage 41 from the initial position
can be accurately determined by driving and rotating the belt
pulley 46 using a stepping motor or the like that controls a phase
of the belt pulley 46, for example, so that the position of the
stop trigger sensor 12 can be controlled.
[0164] Thus, by controlling the position of the stop trigger sensor
12 based on the set length "Ls" of the expected sheet Se such that
the expected conveying distance "Pe" obtained by subtracting the
distance "a'" between the start trigger sensor 11 and the stop
trigger sensor 12 becomes a substantially integer multiple of the
perimeter of the driven roller 13, the measurement error "C" in the
measured conveying distance "P" caused by the eccentric amount of
the driven roller 13 can be reduced to accurately measure the
conveying distance "P" or the length of the sheet S "L" in the
conveying distance.
[0165] Although the sensor position adjusting unit 130 is provided
to adjust the position of the stop trigger sensor 12 in the
conveying direction of the sheet S in this embodiment,
alternatively, the sensor position adjusting unit 130 may be
provided to adjust the position of the start trigger sensor 11.
Further, the sensor position adjusting units 130 for both the start
trigger sensor 11 and the stop trigger sensor 12 may be
provided.
Fourth Embodiment
[0166] FIG. 13 and FIG. 14 are views schematically showing an
example of an image forming apparatus including the sheet conveying
apparatus 100. FIG. 13 shows an example of a monochrome image
forming apparatus 103, and FIG. 14 shows an example of a tandem
color image forming apparatus 104.
[0167] In the monochrome image forming apparatus 103 shown in FIG.
13, an image is printed on the conveyed sheet S as follows. First,
a whole surface of a photoconductor drum 1 is charged while the
photoconductor drum 1 is rotated. Then, an electrostatic latent
image is formed on the surface of the photoconductor drum 1 by a
light writing unit, not shown in the drawings. Then, the
electrostatic latent image is developed to form a toner image by a
developing unit, not shown in the drawings.
[0168] Subsequently, when the sheet S passes between the
photoconductor drum 1 and a transfer unit 5, the toner image formed
on the surface of the photoconductor drum 1 is transferred onto the
sheet S. Thereafter, when the sheet S passes between a heat roller
2 and a pressure roller 3, the toner image is melted and fixed on
the sheet S so that a printed image is formed on the sheet S.
[0169] In the tandem color image forming apparatus 104 shown in
FIG. 14, an image is printed on the conveyed sheet S as follows.
First, similar to the photoconductor drum 1 of the monochrome image
forming apparatus 103, toner images formed on surfaces of
photoconductor drums 1K, 1C, 1Y and 1M respectively provided for
black (K), cyan (C), yellow (Y) and magenta (M) are primary
transferred onto an intermediate transfer belt 4 in a superposed
manner. Then, the superposed color toner image on the intermediate
transfer belt 4 is secondary transferred onto the sheet S when the
sheet S passes between the intermediate transfer belt 4 and the
transfer unit 5.
[0170] The sheet S on which the color toner image is formed is
further conveyed to pass between the heat roller 2 and the pressure
roller 3 so that a printed image is formed on the sheet S.
[0171] For the image forming apparatuses 103 and 104 shown in FIG.
13 and FIG. 14, the sheet conveying apparatus 100 is placed right
before (upstream of) the transfer unit 5 on the conveying path of
the sheet S. Even for another image forming apparatus having a
different structure, by placing the sheet conveying apparatus 100
right before (upstream of) a transfer unit, the conveying distance
of the sheet S or the length of the sheet S in the conveying
direction before transferring can be measured.
[0172] In the image forming apparatuses 103 and 104, first, the
conveying distance of the sheet S is calculated by the sheet
conveying apparatus 100. Then, a toner image is transferred on the
sheet S by the transfer unit 5. Subsequently, when the sheet S is
conveyed between the heat roller 2 and the pressure roller 3, a
printed image is formed on one surface of the sheet S.
[0173] When printing images on both surfaces, the sheet S is
reversed by a reverse mechanism, not shown in the drawings, and is
conveyed again in a direction shown by an arrow X in FIG. 13 and
FIG. 14. At this time, the sheet S is generally contracted by the
heat so that the sheet S is conveyed under a condition that the
size of the sheet S is changed. Then, the conveying distance is
calculated by the sheet conveying apparatus 100 again, and a toner
image is transferred and fixed on the back surface.
[0174] In this embodiment, the length of the toner image to be
transferred on the back surface is corrected (image size correction
is performed) based on the calculated relative ratio of the
conveying distances before and after the heat fixing. Then, the
corrected toner image is transferred on the back surface of the
sheet S. Thus, the length of the images formed on the front surface
and the back surface of the sheet S become the same to improve the
registration in two-sided printing.
[0175] The contraction of the sheet S caused by the heat fixing
recovers in accordance with time, thus, by measuring the conveying
distance "P" right before the transfer unit 5, the length of the
sheet S-after the heat fixing can be accurately measured to improve
the registration in two-sided printing.
[0176] By correcting the size of the image data or the timing of
transferring the toner image on the sheet S based on the thus
obtained conveying distance "P" of the sheet S or the length of the
sheet S in the conveying direction, the registration error in
two-sides printing caused by the expansion and contraction of the
sheet S can be corrected to improve the registration in two-sided
printing.
[0177] As described above, according to the sheet conveying
apparatus 100, by setting the distance "a" between the start
trigger sensor 11 and the stop trigger sensor 12 and the perimeter
"2.pi.r" of the driven roller 13 to satisfy the above equation (4)
or (4'), the phases of the driven roller 13 at the start timing and
the end timing are expected to become substantially the same within
an allowable error range. Thus, the measurement error "C" caused by
the eccentric amount of the driven roller 13 is reduced so that the
conveying distance "P" or the length "L" of the sheet S in the
conveying distance of the sheet S can be accurately measured.
[0178] According to the image forming apparatus 103 or 104
including the sheet conveying apparatus 100, as the conveying
distance "P" or the length "L" of the sheet S can be accurately
measured so that images can be printed on the sheet S with a higher
registration in two-sided printing.
[0179] FIG. 15 is a view schematically showing an example of an
image forming apparatus 105 including the sheet conveying apparatus
100.
[0180] The image forming apparatus 105 includes an intermediate
transfer belt 52, a tandem image forming device 54, an exposure
device 55, first transfer rollers 57, a second transfer device 59,
the sheet conveying apparatus 100, a fixing device 32, a resist
roller 75, a conveying belt 62, a feeding table 71, a de-curl unit
26 and a purge tray 40.
[0181] The intermediate transfer belt 52 is an endless belt and is
provided at almost the center of the image forming apparatus 105.
The intermediate transfer belt 52 is supported by plural support
rollers 58 to be rotated in a clockwise direction in FIG. 15.
[0182] The tandem image forming device 54 includes plural image
forming units 53 which are laterally aligned above the intermediate
transfer belt 52 along the conveying direction of the transfer belt
52. The exposure device 55 is provided above the tandem image
forming device 54.
[0183] Each of the image forming units 53 of the tandem image
forming device 54 includes a photoconductor drum 56 as an image
retaining member which retains a toner image of a respective
color.
[0184] The first transfer rollers 57 are positioned to face the
photoconductor drums 56 with the intermediate transfer belt 52
interposed therebetween at first transferring positions at which
toner images are transferred to the intermediate transfer belt 52,
respectively. The support rollers 58 function as drive rollers that
rotate the intermediate transfer belt 52.
[0185] The second transfer device 59 is provided at an opposite
side (downstream of the conveying direction of the intermediate
transfer belt 52) of the tandem image forming device 54 while
contacting the intermediate transfer belt 52. The second transfer
device 59 includes a second transfer roller 61 and a second
transfer opposing roller 60 which is facing the second transfer
roller 61. The second transfer device 59 transfers a toner image
formed on the intermediate transfer belt 52 onto the sheet S by
pushing the second transfer roller 61 toward the second transfer
opposing roller 60 while applying a transferring electric field.
The second transfer device 59 varies the transferring current of
the second transfer roller 61, which is a parameter for
transferring, in accordance with the sheet S.
[0186] The sheet conveying apparatus 100 is provided upstream of
the second transfer device 59 in the conveying direction of the
sheet S. The fixing device 32 is provided downstream of the second
transfer device 59 in the conveying direction of the sheet S. The
fixing device 32 melts and fixes a toner image on the sheet S.
[0187] The sheet conveying apparatus 100 measures the conveying
distance "P" of the sheet S or a length "L" of the sheet in the
conveying direction of the sheet S before and after the sheet S
passes the fixing device 32 in duplex printing. The image forming
apparatus 105 corrects the size of the image to be formed on the
back surface of the sheet S based on the ratio of expansion and
contraction "R" which is calculated from the measured conveying
distance "P" or the length "L" of the sheet S. Further, in this
embodiment, the sheet conveying apparatus 100 is placed right
before (upstream of) the second transfer device 59 and after
(downstream of) the resist roller 75.
[0188] The fixing device 32 includes a pressure roller 29, a
halogen lamp 30 as a heat source, and a fixing belt 31 which is an
endless belt. The pressure roller 29 is pushed toward the fixing
belt 31. The fixing device 32 changes a parameter for fixing such
as temperatures of the fixing belt 31 and the pressure roller 29, a
nip width between the fixing belt 31 and the pressure roller 29,
and the speed of the pressure roller 29 in accordance with the
sheet S. The sheet S on which the toner image is formed is conveyed
to the fixing device 32 by the conveying belt 62.
[0189] When image data is sent to the image forming apparatus 105,
and the image forming apparatus 105 receives a signal to start
image formation, one of the support rollers 58 is rotated by a
driving motor, not shown in the drawings, so that other support
rollers 58 are also driven by the rotated support roller 58 to
rotate and convey the intermediate transfer belt 52. At the same
time, monochromatic images are formed on the respective
photoconductor drums 56 of the image forming units 53. Then, the
monochromatic images are transferred onto the intermediate transfer
belt 52 by the first transfer rollers 57 while the intermediate
transfer belt 52 is being conveyed so that a combined superposed
color toner image is formed on the intermediate transfer belt
52.
[0190] One of feeding rollers 72 of the feeding table 71 is
selected to be rotated so that a sheet S is sent from one of
feeding cassettes 73 and is conveyed by conveying rollers 74 to the
resist roller 75. Then, when the sheet S reaches the resist roller
75, there is a pause in the conveying of the sheet S. Then, the
resist roller 75 is rotated at a timing of the combined color toner
image on the intermediate transfer belt 52 so that the combined
color toner image is transferred onto the sheet S at the second
transfer device 59. The sheet S on which the combined color toner
image is formed is further conveyed from the second transfer device
59 to the fixing device 32 where heat and pressure are applied to
melt and fix the transferred combined color toner image on the
sheet S.
[0191] Then, when forming images on both surfaces of the sheet S,
the sheet S is conveyed on a sheet reversing path 23 and a two-way
path 24 by a changeover claw 21 and a flip roller 22. Then, a
combined color image toner is formed on the back surface of the
sheet S by repeating the above described method.
[0192] When reversing and ejecting the sheet S, the sheet S is
conveyed to the sheet reversing path 23 by the changeover claw 21,
and then the sheet S is further conveyed to an ejecting roller 25
side by the flip roller 22 to reverse the front surface and the
back surface of the sheet S.
[0193] When an image is formed only on one surface and reversing of
the sheet S is not necessary, the sheet S is conveyed to the
ejecting roller 25 by the changeover claw 21.
[0194] Subsequently, the ejecting roller 25 conveys the sheet S to
the de-curl unit 26. The de-curl unit 26 includes a de-curl roller
27 and removes curling of the sheet S. The de-curl unit 26 changes
the de-curl amount in accordance with the sheet S. The de-curl
amount is adjusted by changing the pressure of the de-curl roller
27. Then, the sheet S is ejected from the de-curl roller 27. The
purge tray 40 is provided below a sheet reversing unit such as the
changeover claw 21, the flip roller 22 and the sheet reversing path
23.
(Correction of Image Size Based on Conveying Distance of the Sheet
S)
[0195] The sheet conveying apparatus 100 measures the conveying
distance "P" of the sheet S or the length "L" of the sheet S in the
conveying direction of the sheet S by the above described method.
Further, the sheet conveying apparatus 100 can measure the width of
the sheet S in the direction (width direction) perpendicular to the
conveying direction of the sheet S by contact image sensors (CISs),
not shown in the drawings, positioned at edges of the sheet S,
respectively.
[0196] After the conveying distance "P" of the sheet S or the sizes
of the sheet S in the conveying direction and in the width
direction are measured by the sheet conveying apparatus 100, the
CISs or the like, a toner image is transferred onto the sheet S at
the second transfer device 59. The sheet S on which the toner image
is transferred is conveyed to the fixing device 32 where the toner
image is fixed. There is a case where the sheet S is contracted by
heat when passing through the fixing device 32.
[0197] Thereafter, the sheet S is reversed in the sheet reversing
path 23 to be conveyed again to the sheet conveying apparatus 100.
Then, the conveying distance "P" of the sheet S or the sizes of the
sheet S in the conveying direction and in the width direction are
measured again. Subsequently, a toner image is transferred and
fixed on the back surface of the sheet S.
[0198] For a subsequent sheet S, the size or position of the toner
image to be transferred on the back surface of the sheet S is
corrected based on the ratio of expansion and contraction "R" of
the measured sheet S. As a result, the size of the images to be
formed on a front surface and a back surface of the sheet S are
matched to improve the registration in two-sided printing.
[0199] The contraction of the sheet S after fixing recovers in
accordance with time. Thus, by providing the sheet conveying
apparatus 100 right before the second transfer device 59, the
conveying distance "P" of the sheet S or the length "L" of the
sheet S in the conveying direction is measured right before the
toner image is transferred. With this structure, the ratio of
expansion and contraction "R" can be accurately measured so that
the registration in two-sided printing can be improved.
[0200] Correction of size of image based on the sheet size measured
by the sheet conveying apparatus 100 is explained. As described
above, in this embodiment, the sheet conveying apparatus 100 is
provided right before the second transfer device 59; thus, the
correction of the exposing data size or exposing timing based on
the measured sheet size is not reflected on the sheet S for which
the sheet size is measured, but reflected on a subsequent sheet
S.
[0201] The exposure device 55 includes a data buffer unit that
buffers input image data, an image data generating unit that
generates image data for forming an image, an image size correction
unit that corrects the size of the image data in the conveying
direction of the sheet S based on the sheet size, a clock
generating unit that generates a writing clock, and a light
emitting device that forms an image by emitting a light on the
photoconductor drum 56.
[0202] The data buffer unit is composed of a memory or the like.
The data buffer unit stores the input image data sent from a host
apparatus such as a controller or the like, not shown in the
drawings, at a transferring clock.
[0203] The image data generating unit generates the image data
based on the writing clock sent from the clock generating unit and
size correction data sent from the image size correction unit.
Then, the light emitting device is controlled to be ON/OFF by drive
data output from the image data generating unit while having a
length corresponding to one cycle of a writing clock as one
pixel.
[0204] The image size correction unit generates the size correction
data based on the sheet size measured by the sheet conveying
apparatus 100.
[0205] The clock generating unit is operated at high frequency
which is a few times of the writing clock in order to change clock
period, and performs an image correction with such as a known
technique called pulse width modulation. The clock generating unit
generates the writing clock at a frequency basically corresponding
to the speed of the image forming apparatus 103.
[0206] The light emitting device is composed of one or a
combination of a diode laser, a diode laser array, a vertical
cavity surface emitting laser and the like. The light emitting
device irradiates light on the photoconductor drum 56 in accordance
with the drive data to form the electrostatic latent image on the
photoconductor drum 56.
[0207] A pre-fixed image, which is a toner image, formed on the
sheet S is fixed on the sheet S at the fixing device 32 by being
heated and pressed. The sheet S may be deformed by the heat or the
pressure so that the length of the sheet S in the conveying
direction of the sheet S may be changed by expansion and
contraction. As a result, there may be caused a difference in
position between an image forming region on the back surface and
that of the front surface of the sheet S to have influence on
quality of output images, and registration in two-sided printing
(as the image on the front surface is deformed so as to be shifted
from the image on the back surface). The fixing device 32 may
separately perform heating and pressing, or may be a flash fixing
type.
[0208] Thus, according to the image forming apparatus 105, size of
image and the image forming region are changed in accordance with
the measured sheet size to compensate for the deformation of the
sheet S caused by the fixing device 32. With this structure, even
when the sheet S is deformed, registration in two-sided printing of
the sheet S can be improved.
[0209] The sheet size, including the deformation of the sheet S, is
obtained from the sheet conveying apparatus 100. Further, the image
forming apparatus 105 can perform only expanding, only reducing, or
a combination of expanding and reducing based on the deformation of
the sheet S.
[0210] In duplex printing, the sheet S is deformed when fixing the
toner image formed on a front surface of the sheet S while the
sheet S is conveyed with a first end of the sheet S in front.
Thereafter, the sheet S is reversed in the sheet reversing path 23
of the image forming apparatus 105. Then, the sheet S is conveyed
with a second end, opposite end of the first end, of the sheet S in
front to be inserted into the fixing device 32. At this time, if
the image forming region is not corrected, a back end of an image
formed on the back surface of the sheet S is shifted from a back
end of an image formed on the front surface of the sheet S to
reduce registration in two-sided printing.
[0211] However, according to the image forming apparatus 105, as
the size of image and the image forming region are corrected when
forming an image on the back surface of the sheet S, the
registration in two-sided printing of the sheet S can be
improved.
(Peripheral Speeds of Rollers of the Second Transfer Apparatus and
the Sheet Conveying Apparatus)
[0212] The relationship of the peripheral speeds of the second
transfer opposing roller 60 and the second transfer roller 61 of
the second transfer device 59, and the driven roller 13 and the
drive roller 14 of the sheet conveying apparatus 100 is
explained.
[0213] The sheet conveying apparatus 100 includes the driven roller
13, the drive roller 14, a motor (an example of the driving unit
20) and a one-way clutch (an example of the driving force
transmitting unit 22) provided between the drive roller 14 and the
motor.
[0214] As described above, the drive roller 14 is rotated by the
driving force by the motor via the driving force transmitting unit.
The driven roller 13 is rotated in accordance with the rotation of
the drive roller 14 with the sheet S interposed between the drive
roller 14 and the driven roller 13.
[0215] The one-way clutch provided between the drive roller 14 and
the motor transmits the driving force to the drive roller 14 in a
conveying direction in which the drive roller 14 conveys the sheet
S, and stops transmitting the driving force to the drive roller 14
in a direction which is opposite to the conveying direction by
slipping.
[0216] The sheet conveying apparatus 100 receives the sheet S from
the resist roller 75, and conveys the sheet S at a predetermined
speed such that a front end of the sheet S is inserted into the
second transfer device 59 at a predetermined timing. The speed of
conveying the sheet S by the sheet conveying apparatus 100 is
controlled by the speed of the drive roller 14.
[0217] The second transfer device 59 receives the sheet S from the
sheet conveying apparatus 100 and further conveys the sheet S. The
second transfer device 59 transfers the toner image onto a surface
of the sheet S.
[0218] The second transfer device 59 includes the intermediate
transfer belt 52, the second transfer roller 61, a motor that
independently drives the intermediate transfer belt 52 and the
second transfer roller 61 and a torque limiter provided between the
second transfer roller 61 and the motor.
[0219] The torque limiter provided between the second transfer
roller 61 and the motor transmits the driving force of the motor to
the second transfer roller 61 within a range of a limited load
torque and stops transmitting the driving force from the motor to
the second transfer roller 61 when the load torque exceeds a
predetermined value by slipping.
[0220] The sheet conveying apparatus 100 may include a contact
control mechanism that is configured to control the driven roller
13 or the drive roller 14 so that the driven roller 13 and the
drive roller 14 are apart from each other when the sheet S is not
being conveyed and the driven roller 13 and the drive roller 14 are
in contact with each other when the sheet S is being conveyed.
Further, the second transfer device 59 may also include a contact
control mechanism that is configured to control the second transfer
roller 61 or the second transfer opposing roller 60 so that the
second transfer roller 61 and the second transfer opposing roller
60 are apart from each other when the sheet S is not being conveyed
and the second transfer roller 61 and the second transfer opposing
roller 60 are in contact with each other when the sheet S is being
conveyed.
[0221] The sheet conveying apparatus 100 is configured to output a
driving force of the motor connected to and driving the drive
roller 14 at a peripheral (linear) speed "Va". When the sheet S is
conveyed only by the sheet conveying apparatus 100, the one-way
clutch transmits the driving force of the motor to the drive roller
14. At this time, as the drive roller 14 is being rotated at the
peripheral speed "Va", the sheet S is also conveyed at the speed
"Va".
[0222] In the second transfer device 59, the intermediate transfer
belt 52 is rotated at a peripheral (linear) speed "Vb" (Vb>=Va),
and the motor connected to the second transfer roller 61 outputs a
driving force that causes the second transfer roller 61 to be
rotated at a peripheral (linear) speed "Vc" (Vc>=Vb).
[0223] Here, slip torque "Ts" of the torque limiter provided
between the second transfer roller 61 and the motor is set between
load torque "To" when the intermediate transfer belt 52 and the
second transfer roller 61 are apart from each other, and load
torque "Tc" when the intermediate transfer belt 52 and the second
transfer roller 61 are in contact with each other
(To<Ts<Tc).
[0224] Thus, when the second transfer roller 61 is apart from the
intermediate transfer belt 52, the load torque "To" of the torque
limiter is less than the slip torque "Ts". Therefore, the torque
limiter transmits driving force of the motor to the second transfer
roller 61 so that the second transfer roller 61 is rotated at the
peripheral speed "Vc". When the second transfer roller 61 is in
contact with the intermediate transfer belt 52, the load torque
"Tc" of the torque limiter exceeds the slip torque "Ts". Thus, the
torque limiter stops transmitting the driving force from the motor
to the second transfer roller 61 so that the second transfer roller
61 is rotated in accordance with the intermediate transfer belt 52
at the peripheral speed "Vb".
[0225] Under this situation, when the sheet S is conveyed by both
the sheet conveying apparatus 100 and the second transfer device
59, the sheet S is conveyed at the peripheral speed "Vb" of the
intermediate transfer belt 52, where the one-way clutch of the
sheet conveying apparatus 100 slips to stop transmitting the
driving force from the motor to the drive roller 14. Thus, at this
time, the drive roller 14 is rotated in accordance with the sheet
S, which is conveyed at the linear speed "Vb" with the driven
roller 13.
[0226] With this structure, when the sheet S is passed from the
sheet conveying apparatus 100 to the second transfer device 59 and
the toner image is being transferred onto the sheet S, the sheet S
is conveyed at a constant linear speed "Vb", which is the
peripheral speed "Vb" of the intermediate transfer belt 52. By
maintaining the sheet conveying speed while the toner image is
being transferred, an abnormal image with such as banding or the
like can be prevented from being generated, and the image forming
apparatus 105 can form uniform images.
[0227] The peripheral speed "Va" of drive roller 14, the peripheral
speed "Vb" of the intermediate transfer belt 52 and the peripheral
speed "Vc" of the second transfer roller 61 may be defined as the
following equation (5). In this case, the above merit can be
obtained.
Va=<Vb=<Vc (5)
[0228] However, if the difference between the peripheral speed "Va"
and the peripheral speed "Vb" or between the peripheral speed "Vb"
and the peripheral speed "Vc" is large, a slipping amount of the
one-way clutch or the torque limiter when conveying the sheet S
becomes large and the service lifetime of the one-way clutch or the
torque limiter is lowered by heat, abrasion or the like. Thus, the
difference between these peripheral speeds may be preferably set
smaller and may be set equal to each other. However, if the
peripheral speeds of the drive roller 14, the intermediate transfer
belt 52 and the second transfer roller 61 change due to
environmental variation such as temperature and relative humidity
or the like and become not to meet the equation (5), the conveying
speed of the sheet S is varied when transferring the toner image
onto the sheet S to cause size change of the toner image formed on
the sheet S. Thus, predetermined margins may be provided between
the peripheral speed "Va" and the peripheral speed "Vb", and
between the peripheral speed "Vb" and the peripheral speed
"Vc".
[0229] The peripheral speeds "Va", "Vb" and "Vc" may be defined by
the following equations (6) and (7).
0.90Vb=<Va=<0.99Vb (6)
1.001Vb=<Vc=<1.05Vb (7)
[0230] Further, preferably, the peripheral speeds "Va", "Vb" and
"Vc" may be defined by the following equations (8) and (9) in order
to maintain the service lifetime of the one-way clutch or the
torque limiter, and obtain the above described merit considering
the environmental variation or the like.
0.95Vb=<Va=<0.99Vb (8)
1.001Vb=<Vc=<1.02Vb (9)
[0231] With the above structure, the sheet conveying speed of the
sheet S when transferring the toner image can be maintained at a
constant value so that an abnormal image with such as banding or
the like can be prevented from being generated, and the image
forming apparatus 105 can form uniform images on the sheet S.
[0232] Further, for an image forming apparatus in which a toner
image is directly transferred from the photoconductor drum to the
sheet S, the sheet conveying speed may be maintained at a constant
value when transferring the toner image by a similar method as
described above. In this case, the intermediate transfer belt 52
may correspond to the photoconductor drum, and the second transfer
roller 61 may correspond to a transfer roller that transfers an
image from the photoconductor drum to the sheet S.
[0233] Further, instead of the one-way clutch provided between the
drive roller 14 and the motor of the sheet conveying apparatus 100,
a torque limiter may be provided by which slip torque is set so
that the drive roller 14 is rotated in accordance with the sheet S
for both the sheet conveying apparatus 100 and the intermediate
transfer belt 52 when the sheet S is being conveyed.
[0234] The image forming apparatus 103, 104 or 105 may include the
sheet conveying apparatus 101 or 102 instead of the sheet conveying
apparatus 100. In such a case, the same merit can be obtained.
[0235] Further, after a predetermined period after the sheet S is
inserted between the driven roller 13 and the drive roller 14,
velocity turbulences of the driven roller 13 and the drive roller
14 are caused at the resonance frequencies of the driven roller 13
and the drive roller 14. This causes a measurement error. Thus, it
is necessary to set the distance "A" between the start trigger
sensor 11 and the driven roller 13 (and the drive roller 14) to be
larger than the distance necessary for the velocity turbulence of
the driven roller 13 to dissipate.
[0236] The individual constituents of the pulse measuring unit 116
and the conveying distance calculation unit 117 of the sheet
conveying apparatus 100 may be embodied by arbitrary combinations
of hardware and software, typified by a CPU of an arbitrary
computer, memory, a program loaded in the memory so as to embody
the constituents illustrated in the drawings, storage units for
storing the program such as a hard disk, and an interface for
network connection. It may be understood by those skilled in the
art that methods and devices for the embodiment allow various
modifications.
[0237] According to the embodiment, a sheet conveying apparatus
which is capable of reducing the measurement error "C" in the sheet
conveying distance "P" caused by the eccentric amount of the drive
roller or the driven roller as the phases of the drive roller or
the driven roller at the start timing and the end timing are
expected to become substantially the same within an allowable error
range. Further, according to the embodiment, a sheet conveying
apparatus is capable of improving the registration in two-sided
printing by accurately obtaining the conveying distance "P".
[0238] Further, in the above embodiments, in order to reduce
influence of velocity turbulence of a conveying unit other than
that of the sheet conveying apparatus 100 while the conveying
amount of the sheet S is being measured, the sheet S may be
conveyed only by the sheet conveying unit 110 when the conveying
amount of the sheet S is being measured.
[0239] Although a preferred embodiment of the sheet conveying
apparatus and the image forming apparatus has been specifically
illustrated and described, it is to be understood that minor
modifications may be made therein without departing from the sprit
and scope of the invention as defined by the claims.
[0240] The present invention is not limited to the specifically
disclosed embodiments, and variations and modifications may be made
without departing from the scope of the present invention.
[0241] The present application is based on Japanese Priority
Application No. 2011-183771 filed on Aug. 25, 2011, and Japanese
Priority Application No. 2012-123112 filed on May 30, 2012, the
entire contents of which are hereby incorporated by reference.
* * * * *