JP2014210637A - Sheet conveyance device and image forming device - Google Patents

Abstract

To provide a sheet conveying apparatus capable of obtaining a sheet conveying distance with high accuracy even when a disk is eccentric.
SOLUTION: Detection means for detecting a conveyed sheet, conveying means for nipping and conveying the sheet between a driving roller and the driving roller or a driven roller that rotates following the sheet, and a plurality of slits A disk provided around the disk that rotates with the driven roller; and two sensors that are provided at substantially opposite positions around the rotation axis of the disk and that detect the plurality of slits and output detection signals, respectively. And a conveyance distance calculation unit that calculates a conveyance distance of the sheet based on a detection result of the detection unit and detection signals output from the two sensors.
[Selection] Figure 3

Description

  The present invention relates to a sheet conveying apparatus and an image forming apparatus.

  In the commercial printing industry, small-lot, multi-variety, variable data printing, etc. is moving from a conventional offset printing machine to POD (Print On Demand) using an image forming apparatus using an electrophotographic method. For example, in order to meet such needs, an electrophotographic image forming apparatus is required to have front and back registration accuracy comparable to an offset printing machine, image uniformity, and the like.

  The causes of misregistration in the image forming apparatus can be broadly classified into vertical and horizontal registration errors, skew errors between the recording medium and the printed image, and image length expansion and contraction during toner image transfer. Further, in an image forming apparatus having a fixing device, a front / back misregistration occurs due to an image magnification error due to expansion / contraction of a recording medium caused by heating by the fixing device.

  Such front and back misregistration can be prevented, for example, by measuring the length of the sheet in the conveyance direction before and after fixing the toner image and correcting the image printed on the back side of the sheet based on the expansion / contraction ratio of the sheet.

  Therefore, the rotation length of the length measuring roll that rotates in contact with the conveyed paper is detected by a rotary encoder, and the length of the paper in the conveyance direction is calculated based on the detected amount of rotation of the length measuring roll. A measuring device is disclosed (for example, refer to Patent Document 1 or 2).

  However, in the sheet length measuring apparatus according to Patent Document 1 or 2, if the encoder disk of the rotary encoder is eccentric with respect to the length measuring roll axis, there is a possibility that the sheet length measurement result varies.

  For example, FIG. 9 shows a case where the encoder disk is eccentric with respect to the length measuring roll axis, and counts the pulses output from the encoder sensor provided on the encoder disk when the length measuring roll rotates at a constant speed. It is a graph which illustrates a number.

  As shown in FIG. 9, when the encoder disk is eccentric with respect to the length measuring roll axis, the cycle of pulses output from the encoder sensor varies even if the length measuring roll rotates at a constant speed. Therefore, the pulse count number varies periodically. Therefore, for example, even when the time interval T is the same as the time t1 to t2 and the time t3 to t4, the pulse count numbers during the time vary as n1 and n2, respectively. Therefore, the measurement result varies depending on the measurement timing, and the sheet length measurement accuracy may be reduced.

  The present invention has been made in view of the above, and an object of the present invention is to provide a sheet conveying apparatus capable of obtaining the sheet conveying distance with high accuracy even when the disk is eccentric. .

  According to the sheet conveying apparatus of one aspect of the present invention, the sheet is nipped and conveyed between the detection unit that detects the conveyed sheet, and the driving roller and the driven roller or the driven roller that rotates following the sheet. A plurality of slits are provided around the disk, the disk is rotated with the driven roller, and is provided at a position substantially opposed to the rotation axis of the disk, and each of the plurality of slits is detected. Two sensors for outputting detection signals, and conveyance distance calculation means for calculating the conveyance distance of the sheet based on the detection results of the detection means and the detection signals output from the two sensors.

  According to the embodiment of the present invention, it is possible to provide a sheet conveying apparatus capable of obtaining the sheet conveying distance with high accuracy even when the disk is eccentric.

1 is a diagram illustrating a schematic configuration of an image forming apparatus according to an embodiment. 1 is a diagram illustrating a schematic configuration of a main part of an image forming apparatus according to an embodiment. 1 is a schematic cross-sectional view illustrating the configuration of a sheet conveying apparatus according to an embodiment. 2 is a schematic top view illustrating the configuration of the sheet conveying apparatus according to the embodiment. FIG. It is a figure which illustrates the pulse count number based on each sensor output in an embodiment. It is a figure which illustrates the output from each sensor in an embodiment. 1 is a diagram illustrating a functional configuration of an image forming apparatus according to an embodiment. It is a figure which illustrates the output of the start trigger sensor, stop trigger sensor, and each sensor in an embodiment. It is a figure which illustrates the pulse count number based on the sensor output in case the encoder disk is eccentric.

  Hereinafter, embodiments for carrying out the invention will be described with reference to the drawings. In the drawings, the same components are denoted by the same reference numerals, and redundant description may be omitted.

<Configuration of image forming apparatus>
FIG. 1 is a diagram illustrating a schematic configuration of an image forming apparatus 101 according to the present embodiment.

  The image forming apparatus 101 forms an image on a sheet S as a recording medium such as paper or OHP by an image forming unit having a tandem image forming apparatus 54, an intermediate transfer belt 15, a secondary transfer apparatus 77, and a fixing apparatus 50. .

  The tandem image forming apparatus 54 includes a plurality of developing devices 53y, 53m, 53c, and 53k (hereinafter, a description of y, m, c, and k is omitted) arranged along the intermediate transfer belt 15. . An exposure device 55 is provided above the tandem image forming apparatus 54. Each developing device 53 of the tandem image forming apparatus 54 includes a photosensitive drum 71 as an image carrier that carries a toner image of each color.

  A primary transfer roller 81 is provided at a primary transfer position where the toner image is transferred from the photosensitive drum 71 to the intermediate transfer belt 15 so as to face each photosensitive drum 71 with the intermediate transfer belt 15 interposed therebetween.

  The secondary transfer device 77 is provided on the opposite side of the intermediate transfer belt 15 from the tandem image forming device 54 (on the downstream side in the transport direction of the intermediate transfer belt 15). The secondary transfer device 77 transfers the image on the intermediate transfer belt 15 to the sheet S by pressing the secondary transfer roller 14 against a roller 62 as a secondary transfer counter roller and applying a transfer electric field. The secondary transfer device 77 changes the transfer current of the secondary transfer roller 14, which is a transfer condition parameter, according to the type of the sheet S and the like.

  Further, the image forming apparatus 101 includes a sheet conveying apparatus 100 that can calculate the conveyance distance and the length in the conveyance direction (hereinafter referred to as “sheet length”) of the sheet S, and conveys the sheet S by a configuration and a method described later. In addition, the conveyance distance or the sheet length of the sheet S is calculated.

  The fixing device 50 includes a halogen lamp 57 as a heat source, and a pressure roller 52 is pressed against a fixing belt 56 that is an endless belt. The fixing device 50 changes the temperature of the fixing belt 56 and the pressure roller 52, the nip width between the fixing belt 56 and the pressure roller 52, and the speed of the pressure roller 52, which are parameters of the fixing conditions, according to the sheet S. From the secondary transfer device 77 to the fixing device 50, the conveyance belt 41 conveys the sheet S after the image transfer.

  When the image forming apparatus 101 receives image data and receives an image formation start signal, the image forming apparatus 101 rotates the intermediate transfer belt 15 by rotating a roller 61 by a drive motor (not shown). At the same time, each developing device 53 forms a single color image on each photosensitive drum 71. Then, the single color image formed by the developing device 53 is sequentially superimposed and transferred onto the rotationally driven intermediate transfer belt 15 to form a composite color image.

  Further, the sheet S is selectively rotated by one of the sheet feeding rollers 72 of the sheet feeding table 76, fed out from any one of the sheet feeding cassettes 73, conveyed by the conveying roller 74, and abutted against the registration roller 75. Stop. The registration roller 75 corrects the conveyance posture of the sheet S and conveys the sheet S in accordance with the timing when the composite color image on the intermediate transfer belt 15 reaches the secondary transfer device 77. The composite color image formed on the intermediate transfer belt 15 is transferred onto the surface of the sheet S conveyed to the secondary transfer device 77.

  The sheet S after the image transfer is transported by the transport belt 41 and sent to the fixing device 50, and heat and pressure are applied to melt and fix the transferred image. After the image is fixed on the front side, the sheet S is conveyed to the sheet reversing path 93 by the branching claw 91 and the flip roller 92 in the case of double-sided printing. Thereafter, the sheet S is switched back by a branching claw / roller pair (not shown) and conveyed to the double-sided conveyance path 94 to form a composite color image on the back surface side.

  When the sheet S is reversed and discharged, the branching claw 91 guides the sheet S to the sheet reversing path 93, and the sheet S is reversed from the front surface to the back surface and discharged. In the case of single-sided printing and no sheet reversal, the sheet S is conveyed to the paper discharge roller 95 by the branching claw 91.

  Thereafter, the sheet S is conveyed to the decurler unit 96 by the paper discharge roller 95, and the decurler unit 96 changes the decurler amount according to the sheet S. The decurler amount is adjusted by changing the pressure of the decurler roller 97, and the sheet S is discharged by the decurler roller 97. The purge tray 40 is disposed below the reverse paper discharge unit.

  As a registration mechanism for correcting the position in the conveyance direction of the sheet S and the position in the width direction orthogonal to the conveyance direction, for example, a registration gate and a skew correction mechanism may be provided instead of the registration roller 75. In this case, the sheet conveying apparatus 100 controls the conveyance timing of the sheet S to the secondary transfer unit between the roller 62 and the secondary transfer roller 14. Specifically, the sheet conveying apparatus 100 includes a registration mechanism and a sheet so that the timing at which the toner image on the intermediate transfer belt 15 reaches the secondary transfer unit matches the timing at which the sheet S reaches the secondary transfer unit. The conveyance speed of the sheet S is controlled based on the detection result of the sheet detection sensor provided between the conveyance apparatus 100 and the conveyance apparatus 100.

  The image forming apparatus 101 according to the present embodiment is configured to transfer the color toner image formed on the intermediate transfer belt 15 to the sheet S. However, the single color toner images formed on the plurality of photosensitive drums 71 are transferred to the sheet S. Alternatively, the image may be transferred directly on the screen. The image forming apparatus 101 may be, for example, a monochrome image forming apparatus that forms a monochrome image. Further, the image forming method is not limited to the electrophotographic method, and may be an ink jet method, for example.

  FIG. 2 is a diagram illustrating a schematic configuration of a main part of the image forming apparatus 101 according to the embodiment.

  As shown in FIG. 2, the image forming apparatus 101 is provided with a sheet conveying apparatus 100 in the sheet S conveying path.

  The sheet conveying device 100 conveys the sheet S to the secondary transfer device 77 and measures the conveying distance or the sheet length of the sheet S.

  The sheet conveying apparatus 100 measures the sheet length before image formation on one surface (front surface) and the sheet length after image formation on one surface (front surface) during duplex printing, and the sheet length before and after image formation. Seek changes. The image forming apparatus 101 corrects the front and back registration accuracy by correcting the magnification of image data to be printed on the other side (back side) of the sheet S based on the change in sheet length during double-sided printing required in the sheet conveying apparatus 100. Can be improved.

  Here, at the time of double-sided printing of the sheet S, the sheet S is stretched and deformed by being heated and pressurized when passing through the fixing device 50 in surface printing, and the temperature decreases after passing through the fixing device 50. Continue to deform. Therefore, in order to measure the sheet length and perform magnification correction of an image printed on the back side with high accuracy, it is desirable to measure the sheet length immediately before transferring the image to the sheet S. It is preferably provided immediately upstream of the secondary transfer device 77.

<Configuration of sheet conveying device>
A configuration of the sheet conveying apparatus 100 according to the present embodiment will be described. FIG. 3 is a schematic cross-sectional view of the sheet conveying apparatus 100, and FIG. 4 is a schematic top view of the sheet conveying apparatus 100.

  As an example of a conveying unit, the sheet conveying apparatus 100 receives a driving force of a driving unit (not shown) (for example, a motor) and rotates and drives the sheet S between the driving roller 12 and the driven roller 12 to rotate and follow the sheet S. The driven roller 11 is provided.

  As shown in FIG. 4, the length Wr of the driven roller 11 in the width direction orthogonal to the conveyance direction of the sheet S is configured to be smaller than the minimum width Ws of the sheet S to which the sheet conveyance device 100 corresponds. Accordingly, the driven roller 11 does not come into contact with the driving roller 12 when the sheet S is conveyed, and thus the driven roller 11 is driven to rotate only by friction generated between the driven roller 11 and the sheet S. Therefore, when the sheet S is conveyed, the driven roller 11 can obtain the conveyance distance or the sheet length of the sheet S with higher accuracy by a method described later without being affected by the driving roller 12.

  A rotary encoder 21 is provided on the rotation shaft of the driven roller 11 of the sheet conveying apparatus 100. The configuration of the rotary encoder 21 will be described later.

  Sensors 3 and 4 as detection means are provided in the vicinity of the upstream side and the downstream side in the conveyance direction of the sheet S of the driven roller 11 and the driving roller 12. The sensors 3 and 4 detect the passage of the edge of the sheet S being conveyed. For the sensors 3 and 4, for example, a transmissive or reflective optical sensor with high sheet edge detection accuracy can be used. In this embodiment, a reflective optical sensor is used.

  A sensor 3 on the downstream side in the conveyance direction of the sheet S of the driven roller 11 and the driving roller 12 is a start trigger sensor 3 as a downstream detection unit that detects passage of the front end of the sheet S. Further, the upstream side sensor 4 in the conveyance direction of the sheet S of the driven roller 11 and the driving roller 12 is a stop trigger sensor 4 as upstream side detection means for detecting passage of the rear end portion of the sheet S.

  As shown in FIG. 4, the start trigger sensor 3 and the stop trigger sensor 4 have substantially the same width direction position orthogonal to the sheet S conveyance direction. By providing in this way, it is possible to minimize the influence of the conveying posture of the sheet S (skew with respect to the conveying direction) and more accurately measure the conveying distance of the sheet S.

  In the present embodiment, the two sensors 3 and 4 are arranged at the center position in the width direction orthogonal to the conveyance direction of the sheet S. It may be arranged to be shifted in either direction.

  A distance A illustrated in FIGS. 3 and 4 is a distance between the start trigger sensor 3, the driven roller 11, and the driving roller 12 in the conveyance path of the sheet S. The distance B is a distance between the stop trigger sensor 4 and the driven roller 11 and the driving roller 12. The distances A and B are preferably made as small as possible because the pulse count range described later becomes large.

  The driving roller 12 is rotated in the direction of the arrow shown in FIG. 3 by driving means (not shown). The driven roller 11 is driven and rotated by the driving roller 12 when the sheet S is not conveyed, and is rotated by the sheet S when the sheet S is conveyed. When the driven roller 11 rotates, a pulse signal is output from the rotary encoder 21 provided on the rotating shaft.

  A pulse counting unit (not shown) connected to the rotary encoder 21 counts pulse signals output from the rotary encoder 21, and the counted result is used to calculate the transport distance or sheet length of the sheet S.

(Configuration of rotary encoder)
Next, the configuration of the rotary encoder 21 will be described based on the drawings.

  As shown in FIGS. 3 and 4, the rotary encoder 21 includes an encoder disk 18 that is provided around the slit 19 and rotates with the driven roller, and detects the slit 19 of the encoder disk 18 to detect a pulse signal as a detection signal. Sensor A20a and sensor B20b. The pulse signals output from the sensors A20a and B20b are counted by a pulse counting unit (not shown), and the counting result is used for calculating the transport distance or the sheet length of the sheet S.

  As shown in FIG. 3, the encoder disk 18 is provided with a plurality of slits 19 at substantially equal angular intervals in the circumferential direction. In addition, the sensor A 20 a and the sensor B 20 b that detect the slit 19 and output a pulse signal are provided at positions that oppose each other about the rotation axis of the encoder disk 18.

  FIG. 5 is a diagram illustrating the pulse count numbers of the pulse signals output from the sensors A20a and B20b of the rotary encoder 21 when the driven roller 11 rotates at a constant speed.

  When the encoder disk 18 is eccentric with respect to the rotation shaft of the driven roller 11, the pulse count numbers based on the outputs from the sensor A 20a and the sensor B 20b change periodically as shown in FIG. . Here, since the sensor A 20a and the sensor B 20b are provided at positions that are substantially opposed to each other with the rotation axis of the encoder disk 18 as a center, the pulse count numbers based on the outputs from the sensors are shifted by a half cycle with respect to the other. Will fluctuate periodically.

  In such a configuration, the average value of the pulse count numbers based on the outputs from the sensors A20a and B20b is offset by the influence of the eccentricity of the encoder disk 18, and becomes a straight line proportional to the time t as shown in FIG. . Similarly, the total value (not shown) of the pulse count number based on the outputs from the sensor A 20a and the sensor B 20b is also a straight line proportional to the time t because the influence of the eccentricity of the encoder disk 18 is offset.

  Therefore, by using the average value or the total value of the pulse count numbers based on the outputs from the sensor A 20a and the sensor B 20b, it is possible to eliminate variations in the pulse count numbers due to the measurement timing. In the sheet conveying apparatus 100, it is possible to obtain the sheet conveying distance or the sheet length with high accuracy by eliminating the variation in the pulse count number with such a configuration.

  In addition, as shown in FIG. 6, it is preferable that a plurality of slits 19 are formed in the encoder disk 18 so that the phases of the pulse signals output from the sensor A 20a and the sensor B 20b are different. In this embodiment, the slit 19 is provided in the encoder disk 18 so that the phases of the pulse signals output from the sensor A 20a and the sensor B 20b during the constant speed rotation of the driven roller 11 are shifted by 90 °.

  With such a configuration, as shown in FIG. 6, when the encoder disk 18 rotates with the driven roller 11, the pulse signal output from the sensor B 20 b is less than the pulse signal output from the sensor A 20 a. Output is shifted by two cycles. The phase difference between the pulse signals of the sensor A 20a and the sensor B 20b is not limited to the example shown in the present embodiment, and can be set as appropriate.

  When counting a pulse signal output from one sensor, a delay of a period t of the pulse signal occurs at the maximum from the count start timing until the pulse is detected. However, by using the sensor A20a and the sensor B20b having the above-described configuration, the interval from the count start timing to the actual detection of the pulse can be suppressed to a half of the period t at the maximum. Further, since the pulse signal interval between the sensor A 20a and the sensor B 20b is shortened, many pulse signals can be counted at the count end timing. Therefore, since the pulse count can be started with high sensitivity from the start to the end of the count, the sheet conveyance distance or the sheet length can be obtained with higher accuracy.

  Although the same effect can be obtained by increasing the number of slits 19 provided in the encoder disk 18, there is a limit in manufacturing technology for narrowing the slit interval, which may be limited by the resolution on the sensor side. In addition, when the outer diameter of the encoder disk 18 is increased to increase the number of slits 19, an increase in the size of the apparatus is inevitable. However, according to the present embodiment, it is possible to improve the resolution of the sensor output without increasing the number of slits 19 or increasing the size of the apparatus, and to obtain the sheet conveyance distance or the sheet length with high accuracy.

  In addition, although illustrated about the structure which provides sensor A20a and sensor B20b, the number of the sensors provided in the rotary encoder 21 is not restricted to two. For example, a plurality of combinations of two sensors provided facing the rotation axis of the encoder disk 18 may be provided.

  Further, the smaller the roller diameter of the driven roller 11 to which the rotary encoder 21 is attached, the more the number of pulses to be counted by increasing the number of rotations associated with the sheet conveyance, and the sheet S conveyance distance or the sheet length is highly accurate. This is preferable because it can be obtained.

  The driven roller 11 to which the rotary encoder 21 is attached is preferably composed of a metal roller in order to ensure shaft deflection accuracy. By suppressing the deflection of the rotating shaft, it becomes possible to measure the transport distance or the sheet length of the sheet S described later with high accuracy.

<Functional configuration of image forming apparatus>
FIG. 7 is a block diagram illustrating a functional configuration of the image forming apparatus 101 according to the embodiment.

  As shown in FIG. 7, the image forming apparatus 101 includes a start trigger sensor 3, a stop trigger sensor 4, a rotary encoder 21, a pulse counting unit 23, a conveyance distance calculating unit 25, and an image data correcting unit 27. The functions of the pulse counting unit 23, the conveyance distance calculating unit 25, the image data correcting unit 27, and the like included in the image forming apparatus 101 are realized by, for example, cooperation between a program stored in the ROM and hardware such as a CPU and a RAM. The

  The pulse counting means 23 counts pulse signals output from the sensor A 20a and the sensor B 20b as the encoder disk 18 of the rotary encoder 21 provided on the driven roller 11 rotates.

  The conveyance distance calculation unit 25 calculates the conveyance distance or the sheet length of the sheet S based on the detection result of the sheet S by the start trigger sensor 3 and the stop trigger sensor 4 and the pulse count number by the pulse counting unit 23.

  The image data correction unit 27 corrects image data formed on the back surface of the sheet S in the image forming apparatus 101 based on the sheet conveyance distance or the sheet length calculated by the conveyance distance calculation unit 25.

  In the image forming apparatus 101, the image data correction unit 27 corrects the image data based on the sheet conveyance distance or the sheet length calculated by the conveyance distance calculation unit 25, so that the back and front registration accuracy is high in double-sided printing of the sheet S. Images can be printed.

<Calculation of sheet conveyance distance and sheet length>
Next, a method for calculating the sheet conveyance distance and the sheet length in the image forming apparatus 101 will be described.

  FIG. 8 shows output examples of the start trigger sensor 3, the stop trigger sensor 4, the sensor A 20a, and the sensor B 20b in the present embodiment.

  As described above, when the driven roller 11 rotates, the sensors A20a and B20b of the rotary encoder 21 provided on the rotation shaft of the driven roller 11 output pulse signals.

  In the example shown in FIG. 8, after the start of conveyance of the sheet S, the stop trigger sensor 4 detects passage of the leading edge of the sheet S at time t <b> 1, and the start trigger sensor 3 detects passage of the leading edge of the sheet S at time t <b> 2. doing.

  Subsequently, the stop trigger sensor 4 detects passage of the rear end portion of the sheet S at time t3, and the start trigger sensor 3 detects passage of the rear end portion of the sheet S at time t4.

  At this time, after the start trigger sensor 3 detects that the leading edge of the sheet S has passed at time t2, until the stop trigger sensor 4 detects that the trailing edge of the sheet S has passed at time t3. The pulse counting means 23 counts the pulse signals output from the sensor A 20a and the sensor B 20b during the pulse count time.

  The radius of the driven roller 11 provided with the rotary encoder 21 is r, and the number of encoder pulses for one rotation of the driven roller 11 (the total number of pulses output from the sensor A 20a and the sensor B 20b) is counted as N and the pulse count time. The total number of pulses of the sensor A20a and the sensor B20b that has been performed is n. At this time, the conveyance distance Ld of the sheet S between the time t2 and the time t3 can be obtained by the following equation (1).

Ld = (n / N) × 2πr (1)
n: Total number of pulses output from sensor A 20a and sensor B 20b N: Number of encoder pulses for one revolution of driven roller 11 [/ r]
r: radius of the driven roller 11 [mm]
In general, the sheet conveyance speed depends on the mechanical accuracy such as the external accuracy of the roller (particularly the driving roller 12) that conveys the sheet S, the core flutter accuracy, the rotation accuracy of the motor, and the accuracy of the power transmission mechanism such as the gear and belt. fluctuate. Further, it fluctuates due to a slip phenomenon between the driving roller 12 and the sheet S, a slack phenomenon due to a difference in sheet conveying force or sheet conveying speed of the conveying means on the upstream side and the downstream side, and the like. However, the pulse count numbers output from the sensors A20a and B20b of the rotary encoder 21 increase in proportion to the sheet conveyance distance regardless of the sheet conveyance speed.

  Therefore, the conveyance distance calculation unit 25 provided in the sheet conveyance apparatus 100 can convey the sheet S by the driven roller 11 and the driving roller 12 as the sheet conveyance unit without depending on the sheet conveyance speed or the like according to the equation (1). Ld can be obtained with high accuracy.

  The average value of the pulse counts from the sensors A20a and B20b may be used. In this case, in the above equation (1), n is an average value of the pulse counts and N is provided on the encoder disk 18. By setting the number of slits 19 to be present, the conveyance distance Ld of the sheet S can be obtained.

  Further, the transport distance calculating unit 25 can obtain a relative ratio such as a ratio between pages of the sheet S or a front / back ratio.

  The conveyance distance calculation unit 25 can obtain the expansion / contraction rate R by the following equation (2) from the relative ratio of the sheet conveyance distance before and after image formation in the image forming apparatus 101, for example.

R = [(n2 / N) × 2πr] / [(n1 / N) × 2πr] (2)
n1: Total number of pulses counted during conveyance of the sheet S before thermal fixing n2: Total number of pulses counted during conveyance of the sheet S after thermal fixing Here, an example calculated in the present embodiment will be described below. To do.

In this embodiment, N = 2800 [/ r], r = 9 [mm], and the sheet S when the total number of pulses counted when the A3 size sheet is conveyed vertically is n1 = 18816. The transport distance L1 of
L1 = (18816/2800) × 2π × 9 = 380.00 [mm]
It becomes.

Further, when the number of pulses counted again after the thermal fixing of the sheet S is n2 = 18759, the conveyance distance L2 of the sheet S is
L2 = (18759/2800) × 2π × 9 = 378.86 [mm]
The difference between the front and back of the transport distance of the sheet S is
ΔL = 380.00−378.86 = 1.14 [mm]
From the calculation result of the conveyance distance of the front and back of the sheet S, the expansion ratio R of the sheet S (the relative ratio of the length of the front and back of the sheet S),
R = 378.86 / 380.00 = 99.70 [%]
Can be obtained as

  Therefore, in this case, since the sheet length of the sheet S is contracted by about 1 mm due to thermal fixing, if the image lengths of the front and back surfaces of the sheet S are the same, a front-back registration deviation of about 1 mm occurs. Therefore, the image data correction unit 27 can improve the front / back registration accuracy by correcting the image length to be printed on the back surface of the sheet S based on the expansion / contraction ratio R.

  In the above-described example, the conveyance distances L1 and L2 of the sheet S by the conveyance unit before and after thermal fixing are calculated to obtain the expansion / contraction ratio R. For example, the number of pulses counted during conveyance of the sheet S before and after thermal fixing An expansion / contraction rate calculating means for obtaining the ratio of n1 and n2 as the expansion / contraction rate R may be provided.

  For example, in the above-described example, when the number of pulses n1 = 18816 counted during conveyance of the sheet S before heat fixing and the number of pulses n2 = 18759 counted during conveyance of the sheet S after heat fixing, the expansion / contraction ratio R is as follows: You can ask for it.

R = n2 / n1 = 18759/18816 = 99.70 [%]
Note that the sheet length Lp is obtained by adding the distance a between the start trigger sensor 3 and the stop trigger sensor 4 shown in FIG. 3 to the conveyance distance Ld obtained by Expression (1).

Lp = (n / N) × 2πr + a (3)
a: Distance between Start Trigger Sensor 3 and Stop Trigger Sensor 4 In this way, the conveyance distance calculation means 25 of the sheet conveyance apparatus 100 is the conveyance distance Ld of the sheet S by the sheet conveyance means obtained by the above equation (1). Further, the sheet length Lp can be obtained by the equation (3) in which the distance a between the sensors is added.

  Further, the transport distance calculation means 25 can obtain the expansion / contraction ratio R from the relative ratio of the sheet length Lp before and after the heat fixing by the electrophotographic method by the following expression (4).

R = [(n2 / N) × 2πr + a] / [(n1 / N) × 2πr + a] (4)
As described above, the conveyance distance calculation unit 25 of the image forming apparatus 101 can obtain the conveyance distance or the sheet length of the sheet S with high accuracy, and the expansion / contraction ratio R before and after the image formation is obtained as necessary. It is also possible to ask for it.

<Image data correction method>
Next, a processing procedure for image magnification correction based on the shape of the sheet S obtained by the sheet shape calculating unit 20 will be described. In this embodiment, the calculation of the shape of the sheet S by the sheet shape calculation unit 20 is performed immediately before the secondary transfer roller 14 (immediately upstream in the sheet S conveyance direction). Therefore, the reflection to the exposure data size and the exposure timing based on the calculated shape of the sheet S is performed not on the calculated sheet S itself but on the image data of the subsequent sheet S or the like.

  The exposure device 55 of the image forming apparatus 101 includes a data buffer unit configured to buffer input image data including a memory, an image data generation unit that generates image data for image formation, and a sheet conveyance direction based on sheet size information. An image magnification correction unit that performs the image magnification correction, a clock generation unit that generates a writing clock, and a light emitting device that irradiates light to the photosensitive drum 71 to form an image.

  The data buffer unit buffers input image data sent from a host device such as a controller, for example, with a transfer clock.

  The image data generation unit generates image data based on the writing clock from the clock generation unit and the pixel insertion / extraction information from the image magnification correction unit. The drive data output from the image data generation unit controls the light emitting device on / off with the length of one cycle of the write clock as one pixel for image formation.

  The image magnification correction unit generates an image magnification switching signal for switching the image magnification from the sheet shape calculated by the sheet shape calculation unit 20 of the sheet conveying apparatus 100.

  The clock generation means operates at a high frequency several times the write clock so as to change the clock cycle and to perform image correction such as pulse width modulation, which is a known technique, and basically has a device speed. A write clock is generated at a frequency in accordance with.

  The light emitting device includes any one or more of a semiconductor laser, a semiconductor laser array, a surface emitting laser, and the like, and forms an electrostatic latent image by irradiating light to the photosensitive drum 71 according to drive data.

  As described above, in the present embodiment, the sensor 20a and the sensor 20b provided to face each other around the rotation axis of the encoder disk 18 detect the slit 19 and output a pulse signal. Even when the encoder disk 18 is eccentric with respect to the rotation axis of the driven roller 11, the total value or average value of the pulse signals output from the sensor 20 a and the sensor 20 b is proportional to the rotation amount of the driven roller 11. To do. Accordingly, it is possible to obtain the sheet conveyance distance or the sheet length with high accuracy without causing variations in the measurement result due to the influence of the eccentricity based on the total value or the average value of the pulse signals output from the sensor 20a and the sensor 20b. become.

  Further, the image forming apparatus 101 according to the present embodiment is a magnification of image data to be printed on the other side (back side) of the sheet S based on a change in sheet length during double-sided printing that is required with high accuracy in the sheet conveying apparatus 100. By performing the correction, the front and back registration accuracy can be improved.

  The sheet conveying apparatus and the image forming apparatus according to the embodiments have been described above. However, the present invention is not limited to the above embodiments, and various modifications and improvements can be made within the scope of the present invention.

3 Start trigger sensor (downstream detection means)
4 Stop trigger sensor (upstream detection means)
11 driven roller (conveying means)
12 Driving roller (conveying means)
18 Encoder disk (disk)
19 Slit 20a, 20b Sensor 25 Conveyance distance calculation means 27 Image data correction means (correction means)
54 Tandem image forming apparatus (image forming means)
77 Secondary transfer device (transfer means)
DESCRIPTION OF SYMBOLS 100 Sheet conveying apparatus 101 Image forming apparatus S Sheet

JP 2011-6202 A JP 2011-20842 A

Claims (8)

Detecting means for detecting a conveyed sheet;
Conveying means for nipping and conveying the sheet between a driving roller and a driven roller that rotates following the driving roller or the sheet;
A plurality of slits are provided around the disk, and the disk rotates with the driven roller;
Two sensors provided at substantially opposing positions around the rotation axis of the disk, each detecting the plurality of slits and outputting a detection signal;
A conveyance distance calculation unit that calculates a conveyance distance of the sheet based on a detection result of the detection unit and detection signals output from the two sensors;
A sheet conveying apparatus comprising: The sheet conveying apparatus according to claim 1, wherein the plurality of slits are provided on the disk so that detection signals output from the two sensors have different phases. The detection means includes
An upstream side detection unit that is provided on the upstream side of the conveyance unit in the conveyance direction of the sheet and detects the sheet;
A downstream detection unit that is provided downstream of the conveyance unit in the conveyance direction of the sheet and detects the sheet;
The sheet conveying apparatus according to claim 1, wherein the sheet conveying apparatus includes: The transport distance calculation means includes
The conveyance distance of the sheet is calculated based on detection signals output from the two sensors after the downstream detection unit detects the sheet and before the upstream detection unit detects the sheet. The sheet conveying apparatus according to claim 3. The transport distance calculation means includes
The length in the conveyance direction of the sheet is calculated by adding a distance from the upstream detection unit to the downstream detection unit in the conveyance path of the sheet to the conveyance distance of the sheet. The sheet conveying apparatus according to 4.   An image forming apparatus comprising the sheet conveying device according to claim 1. Having transfer means for transferring an image to the sheet;
The image forming apparatus according to claim 6, wherein the sheet conveying device is provided immediately upstream of the transfer unit. Image forming means for forming an image to be transferred to the sheet based on image data;
Correction means for correcting the image data based on the conveyance distance of the sheet calculated by the conveyance distance calculation means;
The image forming apparatus according to claim 6, further comprising:

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