Conveyance device, image reading device, and image forming apparatus

A conveyance device includes a conveyance roller pair to convey a recording medium to an image reading position of an image reading unit. The conveyance roller pair includes a drive roller and a driven roller. The driven roller contacts the drive roller and rotates following the drive roller. The drive roller has a diameter satisfying a relation in which a detection mark on the recording medium is on a position away from a leading end of the recording medium by an integral multiple of a circumference of the drive roller in a conveyance direction of the recording medium.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application is based on and claims priority pursuant to 35 U.S.C. § 119(a) to Japanese Patent Application Nos. 2019-217111, filed on Nov. 29, 2019 and 2020-185932, filed on Nov. 6, 2020, in the Japan Patent Office, the entire disclosure of each of which is hereby incorporated by reference herein.

BACKGROUND

Technical Field

Embodiments of the present disclosure relate to a conveyance device, an image reading device, and an image forming apparatus.

Description of the Related Art

There is known a conveyance device that includes a first conveyance roller pair to convey a recording medium to an image reading position of an image reading unit. The first conveyance roller pair includes a first drive roller and a first driven roller that contacts the first drive roller and rotates following the first drive roller.

SUMMARY

Embodiments of the present disclosure describe an improved conveyance device that includes a conveyance roller pair to convey a recording medium to an image reading position of an image reading unit. The conveyance roller pair includes a drive roller and a driven roller. The driven roller contacts the drive roller and rotates following the drive roller. The drive roller has a diameter satisfying a relation in which a detection mark on the recording medium is on a position away from a leading end of the recording medium by an integral multiple of a circumference of the drive roller in a conveyance direction of the recording medium.

The accompanying drawings are intended to depict embodiments of the present disclosure and should not be interpreted to limit the scope thereof. The accompanying drawings are not to be considered as drawn to scale unless explicitly noted. In addition, identical or similar reference numerals designate identical or similar components throughout the several views.

DETAILED DESCRIPTION

It is to be noted that the suffixes Y, M, C, and Bk attached to each reference numeral indicate only that components indicated thereby are used for forming yellow, magenta, cyan, and black images, respectively, and hereinafter may be omitted when color discrimination is not necessary.

In a comparative example, an image forming apparatus includes a conveyance device to convey a recording medium to an image reading position of an image reading unit. The image forming apparatus performs image correction of images formed on a recording medium, such as skew correction, positional deviation correction, and the like. The image forming apparatus forms a cross-shaped detection marks near four corners of the recording medium, and the image reading unit reads the recording medium on which the detection marks are formed. Next, the position of each detection mark (e.g., the distance from the leading end of the recording medium in the conveyance direction to the detection mark and the distance from one end of the recording medium in the width direction to the detection mark) is pinpointed based on the scanned image. Then, based on the position of each pinpointed detection mark, the amount of skew of the images and the like are determined, and a predetermined image correction, such as the skew correction and the positional deviation correction, is performed.

However, due to the influence of fluctuation of the conveyance speed of the recording medium by the conveyance device, a positional deviation may occur between the position of the detection mark of the scanned image and the actual position of the detection mark on the actual recording medium. Accordingly, the image correction may not be performed with high accuracy.

According to the present disclosure, the image correction can be performed with high accuracy. A description is given below of a printer that is a full-color electrophotographic image forming apparatus according to an embodiment of the present disclosure.

The configuration of an image forming apparatus (printer)300according to the present embodiment are schematically described.FIG. 1is a schematic view illustrating the configuration of the image forming apparatus300according to the present embodiment. The image forming apparatus300according to the present embodiment can function as a copier by adding an optional scanner to the upper portion of an apparatus body100thereof, and further as a multifunction peripheral having a facsimile function by adding an optional facsimile board inside the apparatus body100.

As illustrated inFIG. 1, the image forming apparatus300according to the present embodiment includes a control panel200disposed on the apparatus body100. The control panel200displays the operation state of the image forming apparatus300, and a user can set the operation condition of the image forming apparatus300with the control panel200. In the image forming apparatus300, an image is formed by the electrophotographic method on a sheet P which is a sheet-shaped recording medium based on image data received from an external device such as a personal computer and the operation condition set by the control panel200.

A description is given below of the configuration and operation of the apparatus body100that performs image formation in the image forming apparatus300.

As illustrated inFIG. 1, the apparatus body100of the image forming apparatus300includes four process units1Y,1C,1M, and1Bk as image forming units and a transfer unit7including an intermediate transfer belt10as an intermediate transferor. The process units1Y,1C,1M, and1Bk are arranged in parallel on the stretched surface of the intermediate transfer belt10and constructs a tandem type image forming device together with the transfer unit7. The process units1Y,1C,1M, and1Bk are removably installable in the apparatus body100and have the same configuration except for containing different color toners, i.e., yellow (Y), magenta (M), cyan (C), or black (Bk) toners, respectively, corresponding to decomposed color components of full-color images.

Specifically, the process unit1includes a drum-shaped photoconductor2as an electrostatic latent image bearer, a charging device3to charge the surface of the photoconductor2, a developing device4to form a toner image on the surface of the photoconductor2. The process unit1further includes a cleaning blade5as a cleaning device to clean the surface of the photoconductor2. InFIG. 1, reference numerals of the photoconductor2, the charging device3, the developing device4, and the cleaning blade5are indicated in the process unit1Bk but are omitted in the process units1Y,1C, and1M for simplicity.

As illustrated inFIG. 1, an exposure device6to expose the surface of the photoconductor2is disposed above the process units1Y,1C,1M, and1Bk. The exposure device6includes a light source, a polygon mirror, an f-O lens, and reflection mirrors to irradiate the surfaces of the photoconductors2with laser beams according to the image data.

The transfer unit7is disposed below the process units1Y,1C,1M, and1Bk. As described above, the transfer unit7includes the intermediate transfer belt10that is an endless belt as the intermediate transferor. The inner circumferential surface of the intermediate transfer belt10is stretched around a first stretch roller21, a second stretch roller22, and a third stretch roller23as supports, and a tension roller24presses the intermediate transfer belt from the outer circumferential surface toward the inner circumferential surface, thereby applying tension to the intermediate transfer belt10. As a drive roller rotates, which is one of the first stretch roller21, the second stretch roller22, and the third stretch roller23, the intermediate transfer belt10rotates in the clockwise direction indicated by arrow A1inFIG. 1.

Four primary transfer rollers11are disposed opposite the respective four photoconductors2via the intermediate transfer belt10. At the position opposite the corresponding photoconductor2, each of the primary transfer rollers11presses the inner circumferential surface of the intermediate transfer belt10against the corresponding photoconductor2to form a primary transfer nip where a pressed portion of the intermediate transfer belt10contacts the photoconductor2. The primary transfer rollers11are electrically connected to a power source, and a predetermined voltage that is either direct current (DC) voltage, alternating current (AC) voltage, or including both is applied to the primary transfer rollers11.

A secondary transfer roller12is disposed opposite the third stretch roller23that stretches the intermediate transfer belt10. The secondary transfer roller12is pressed against the outer circumferential surface of the intermediate transfer belt10to form a secondary transfer nip where the secondary transfer roller12contacts the intermediate transfer belt10. Similarly to the primary transfer rollers11, the secondary transfer roller12is electrically connected to a power source, and a predetermined voltage that is either DC voltage, AC voltage, or including both is applied to the secondary transfer roller12.

A plurality of sheet feeding trays13is disposed at the lower portion of the apparatus body100to accommodate sheets P as sheet-shaped recording media, such as paper sheets, overhead projector (OHP) transparencies, and the like. A sheet feeding roller14is provided in the sheet feeding tray13to feeds the sheets P accommodated in the sheet feeding tray13. A sheet ejection tray20is disposed on the left outer surface of the side plate of the apparatus body100inFIG. 1. The sheets P ejected from the apparatus body100are stacked on the sheet ejection tray20.

A conveyance path25is formed inside the apparatus body100, and the sheet P is conveyed from the sheet feeding tray13to the sheet ejection tray20via the secondary transfer nip along the conveyance path25. Along the conveyance path25, a registration roller pair15is disposed upstream from the secondary transfer roller12in a direction of conveyance of the sheet P (hereinafter referred to as a conveyance direction). A fixing device8, a cooling device9, an image reading device50, and an output roller pair16are disposed downstream from the secondary transfer roller12in the conveyance direction in order. The fixing device8includes, for example, a fixing roller17including a heater therein and a pressure roller18that presses the fixing roller17. The portion where the fixing roller17and the pressure roller18contact each other is referred to as a fixing nip.

A switching pawl26is disposed between the image reading device50and the output roller pair16. A reverse path27is formed between the sheet feeding trays13, and fixing device8and the cooling device9. When the duplex printing, in which images are formed on both sides of the sheet P, is selected among printing modes as image formation modes, the switching pawl26swings to guide the sheet P from the conveyance path25to the reverse path27. The sheet P guided to the reverse path27switchbacks in the reverse path27to reverse the front and back surfaces of the sheet P. Then the sheet P enters the conveyance path25upstream from the registration roller pair15to form an image on the back surface of the sheet P.

The cooling device9includes a front side belt97aand a back side belt97b. The front side belt97ais an endless cooling belt that removes heat from the front surface of the sheet P, while conveying the sheet P. The back side belt97bis an endless cooling belt that removes heat from the back surface of the sheet P, while conveying the sheet P. The sheet P is conveyed, while being sandwiched between the stretched surfaces of the front side belt97aand the back side belt97b. The cooling device9further includes a front side cooling plate91aand a back side cooling plate91b. The front side cooling plate91ais disposed inside the stretched surface of the front side belt97a. The back side cooling plate91bis disposed inside the stretched surface of the back side belt97b. Further, the cooling device9includes a pump92, a tank93, a radiator94, and a cooling fan95. The front side cooling plate91aand the back side cooling plate91bare heat receivers that receive the heat from the sheet P. The tank93stores a coolant. Pipes96are coupled to the inlet and outlet provided in each of the front side cooling plate91aand the back side cooling plate91b, and the coolant is circulated between the front side cooling plate91a, the back side cooling plate91b, the radiator94, the tank93, and the pump92via the pipes96, thereby forming a circulation path. The pump92transports the coolant stored in the tank93through the pipes96. The front side cooling plate91aand the back side cooling plate91btransfer the heat from the sheet P to the coolant. The radiator94dissipates the heat removed by the coolant to the outside of the image forming apparatus300. The cooling fan95is attached to the radiator94and generates an airflow around the radiator94to cool the radiator94.

As indicated by arrow A2, in the circulation path, the coolant is cooled by the radiator94and supplied to the front side cooling plate91aand the back side cooling plate91bthrough the circulation path. Then, the coolant is discharged from the back side cooling plate91bthrough the front side cooling plate91a. After that, the coolant is transported to the pump92and the tank93, and then returned to the radiator94again. The coolant is circulated by the pump92, and the radiator94dissipates heat to cool the coolant, thereby cooling the front side cooling plate91aand the back side cooling plate91b. The liquid transport capacity of the pump92and the size of the radiator94are based on the flow rate, pressure, cooling efficiency, and the like determined by thermal design conditions (e.g., conditions of the amount of heat removed by the front side cooling plate91aand the back side cooling plate91band the temperature of the front side cooling plate91aand the back side cooling plate91b).

The cooling fan95and the radiator94are disposed in a duct28. The duct28is arranged inside the side plate of the apparatus body100on which the sheet ejection tray20is disposed. When the cooling fan95is driven (rotated), low temperature air is suck into the duct28through an intake port28a. Then the air passes through the cooling fan95and the radiator94, thereby becoming high temperature. The high temperature air is exhausted from an exhaust port28b. The intake port28ais disposed in the lower portion of the duct28, and the exhaust port28bis disposed in the upper portion of the duct28inFIG. 1.

Next, a description is given of the basic operation of the image forming apparatus300when the single-sided printing is selected among the printing modes. As the image forming apparatus300receives image data from an external device such as a personal computer and starts the image forming operation, the photoconductor2of each of the process units1Y,1C,1M, and1Bk rotates counterclockwise inFIG. 1, and the charging device3uniformly charges the surface of the photoconductor2in a predetermined polarity. Then, the exposure device6irradiates the charged surfaces of the respective photoconductors2with laser beams based on the image data received from the external device and processed by an image processor. Thus, electrostatic latent images are formed on the surfaces of the respective photoconductors2. At this time, the image data for exposing the photoconductor2is single-color image data obtained by decomposing a desired full-color image into individual color components, that is, yellow, cyan, magenta, and black components. The electrostatic latent image thus formed on the photoconductor2is developed into a toner image (visible image) with toner deposited by the developing device4.

The intermediate transfer belt10rotates in the direction indicated by arrow A1inFIG. 1as the drive roller rotates, which is one of the stretch rollers21to23around which the intermediate transfer belt10is stretched. The power source applies a constant voltage or a voltage controlled at a constant current, which has a polarity opposite a polarity of the charged toner, to the primary transfer rollers11. As a result, primary transfer electric fields are generated at the respective primary transfer nips between the primary transfer rollers11and the photoconductors2. The primary transfer electric fields generated at the primary transfer nips sequentially transfer and superimpose the toner images of respective colors from the photoconductors2onto the intermediate transfer belt10. Thus, a full-color toner image, which is the superimposed toner images, is formed on the surface of the intermediate transfer belt10. Residual toner remaining on the photoconductor2failed to be transferred onto the intermediate transfer belt10is removed by the cleaning blade5in preparation for subsequent image formation.

Meanwhile, as the sheet feeding roller14rotates, the sheet P is fed out from the sheet feeding tray13. The registration roller pair15forwards the sheet P fed from the sheet feeding tray13to the secondary transfer nip between the secondary transfer roller12and the intermediate transfer belt10at appropriate timing to synchronize with the arrival of the toner images carried on the intermediate transfer belt10. At that time, a secondary transfer voltage opposite in polarity to the toner images on the intermediate transfer belt10is applied to the secondary transfer roller12, and a secondary transfer electric field is generated in the secondary transfer nip. The secondary transfer electric field generated in the secondary transfer nip collectively transfers the toner images (full-color toner image) from the intermediate transfer belt10onto the sheet P.

The sheet P bearing the full-color toner image is then conveyed to the fixing device8. The fixing roller17and the pressure roller18apply heat and pressure to the sheet P to fix the full-color toner image on the sheet P. The cooling device9cools the sheet P, and the output roller pair16ejects the sheet P onto the sheet ejection tray20. By cooling the sheet P by the cooling device9, the toner on the sheet P can be reliably cured at the time when the sheet P is stacked on the sheet ejection tray20.

Described above is the image forming operation to form a full-color toner image on the sheet P. Alternatively, the image forming apparatus300may form a monochrome toner image by using any one of the four process units1Y,1C,1M, and1Bk, or may form a bicolor toner image or a tricolor toner image by using two or three of the process units1Y,1C,1M, and1Bk.

FIG. 2is a schematic view illustrating the configuration of a sheet conveyor including the fixing device8, the cooling device9, and the image reading device50. The sheet P after the cooling process by the cooling device9is then conveyed to the image reading device50. The image reading device50includes a reader51, an illumination unit52, a background member54, a first conveyance roller pair55including a first drive conveyance roller55aand a first driven conveyance roller55b, and a second conveyance roller pair56including a second drive conveyance roller56aand a second driven conveyance roller56b. The first conveyance roller pair55and the second conveyance roller pair56construct a conveyance device to convey the sheet P in the image reading device50. The reader51, the illumination unit52, and the background member54construct an image reading unit to read an image on the sheet P being conveyed. The reader51includes an image sensor51a, a lens51b, mirrors51c,51d, and51e, and the like to read an image on the sheet P illuminated by the illumination unit52.

The first drive conveyance roller55aand the second drive conveyance roller56aare elastic rollers provided with an elastic layers, and the first driven conveyance roller55band the second driven conveyance roller56bare hard rollers such as metal rollers. The first and second driven conveyance rollers55band56bare movably supported in the direction to contact and separate from the first and second drive conveyance rollers55aand56a, and pressed against the first and second drive conveyance rollers55aand56aby biasing members such as springs, respectively, to form conveyance nips. Note that the first and second driven conveyance rollers55band56bmay be elastic rollers provided with elastic layers, and the first and second drive conveyance rollers55aand56amay be hard rollers such as metal rollers.

Further, in the present embodiment, the first and second driven conveyance rollers55band56bare arranged on the background member54side with respect to the conveyance path25of the sheet P. Alternatively, the first and second drive conveyance rollers55aand56amay be arranged on the background member54side and the first and second driven conveyance rollers55band56bmay be arranged on the reader51side.

The background member54is disposed at an image reading position by the reader51where the sheet P is illuminated by the illumination unit52. The first conveyance roller pair55and the second conveyance roller pair56convey the sheet P at the image reading position. Illumination light from the illumination unit52is reflected by the sheet P and enters the reader51. The reader51starts reading an image with the image sensor51aimmediately before the leading end of the sheet P enters the image reading position, and finishes reading the image with the image sensor51aimmediately after the trailing end of the sheet P exits the image reading position. As a result, the reader51can read the image on the sheet P and the outline of the sheet P for each sheet P.

The background member54of the image reading device50according to the present embodiment includes a large-diameter black roller54ahaving a black outer circumference, a small-diameter black roller54bhaving a black outer circumference, a large-diameter white roller54chaving a white outer circumference, and a small-diameter white roller54dhaving a white outer circumference (hereinafter, simply referred to as “rollers54a,54b,54c, and54d”). These four rollers54a,54b,54c, and54dare rotatably supported by a rotary support54e. As the rotary support54erotates, one of the rollers54a,54b,54c, and54dis located at the image reading position. The background member54positions the corresponding one of the rollers54a,54b,54c, and54dat the image reading position depending on data of the sheet P that identifies the thickness, the color, and the like of the sheet P, and the operation mode of the image forming system (e.g., difference in conveyance speed).

The gap between the illumination unit52and the one of the rollers54a,54b,54c, and54dof the background member54at the image reading position is preferably narrow enough to reliably convey the sheet P. Further, the second conveyance roller pair56is preferably driven with high accuracy and controlled so that the sheet P does not bend directly under the illumination unit52. In particular, two types of transport paths, i.e., the reverse path27and a sheet ejection path, are disposed, and a curl correction mechanism may be disposed downstream from the second conveyance roller pair56. Thus, there may be many error factors that deteriorate the conveyance performance downstream from the second conveyance roller pair56. Therefore, preferably, the conveyance force of the second conveyance roller pair56is increased and the rotation unevenness of the second conveyance roller pair56is reduced in order to maintain the reading performance.

A rotary encoder59is disposed on one end of the rotation shaft of the first driven conveyance roller55b. The rotary encoder59includes an encoder disc and an encoder sensor. The encoder disc is secured onto the rotation shaft of the first driven conveyance roller55band rotates together with the first driven conveyance roller55b. The encoder sensor detects a slit formed in the encoder disc.

Although the rotary encoder59is disposed on the rotation shaft of the first driven conveyance roller55bin the present embodiment, the rotary encoder59may be disposed on the rotation shaft of the first drive conveyance roller55a. A driven conveyance roller to which the rotary encoder59is attached is preferably a metal roller in order to secure the accuracy of runout of the rotation shaft.

As the first driven conveyance roller55brotates, a pulse is generated from the rotary encoder59on the rotation shaft. A pulse measuring instrument is coupled to the rotary encoder59, and the number of pulses from the rotary encoder59is measured by the pulse measuring instrument.

A stop trigger sensor57ais disposed on the upstream side of the first conveyance roller pair55in the conveyance direction, and a start trigger sensor57bis disposed on the downstream side of the first conveyance roller pair55in the conveyance direction. The stop trigger sensor57aand the start trigger sensor57bdetect the end of the sheet P passing through in the conveyance direction. For example, a transmissive photosensor or reflective photosensor having high detection accuracy of the end of the sheet P is available for the stop trigger sensor57aand the start trigger sensor57b. In the present embodiment, the reflective photosensor is used.

The start trigger sensor57bdetects the leading end of the sheet P in the conveyance direction. The stop trigger sensor57adetects the trailing end of the sheet P and the rear end of a detection image.

In the present embodiment, the length of the sheet P in the conveyance direction is measured by the stop trigger sensor57a, the start trigger sensor57b, and the rotary encoder59. Specifically, the length of the sheet P in the conveyance direction is measured as follows.

As described above, as the first driven conveyance roller55brotates, a pulse signal is generated from the rotary encoder59. When the start trigger sensor57bdetects the passage of the leading end of the sheet P, the rotary encoder59starts measuring the number of pulses, and when the stop trigger sensor57adetects the passage of the trailing end of the sheet P, the rotary encoder59finishes measuring the number of pulses.

The length Lt of the sheet P in the conveyance direction is expressed by the following expression.
Lt=A+B+(nx/N)×π×D1bExpression 1,

where D1brepresents the diameter of the first driven conveyance roller55bonto which the rotary encoder59is attached, N represents the number of pulses of the rotary encoder59during one rotation of the first driven conveyance roller55b, and nx represents the number of pulses after the start trigger sensor57bdetects the passage of the leading end of the sheet P until the stop trigger sensor57adetects the passage of the trailing end of the sheet P. Further, A represents the conveyance distance from the stop trigger sensor57ato the first conveyance roller pair55, and B represents the conveyance distance from the first conveyance roller pair55to the start trigger sensor57b.

Generally, the conveyance speed of the sheet P fluctuates depending on mechanical tolerances such as external dimensional tolerances of the roller (in particular, the drive roller) that conveys the sheet P and the runout of the shaft. Accordingly, the pulse cycle and the pulse width of the rotary encoder59constantly fluctuate, but the number of pulses does not change. Therefore, the length Lt of the sheet P in the conveyance direction can be obtained without depending on the conveyance speed of the sheet P by Expression 1.

FIG. 3is a schematic view illustrating an example of a detection pattern formed on the sheet P for image alignment. The image forming apparatus300has an adjustment mode to align an image. The image forming apparatus300forms L-shaped detection marks a, b, c, and d near the four corners on the sheet P when the adjustment mode is automatically (for example, when the image forming apparatus300is turned on) or manually selected. The sheet P on which the detection marks a, b, c, and d have been formed is conveyed to the image reading device50via the fixing process by the fixing device8and the cooling process by the cooling device9.

The first conveyance roller pair55and the second conveyance roller pair56conveys the sheet P in the image reading device50. The reader51of the image reading unit optically reads the end of the sheet P and the detection marks a, b, c, and d. Then, a controller110(seeFIG. 2) calculates the coordinates (e.g., H0, V0) of the center position of each of the detection marks a, b, c, and d on the sheet P based on the length Lt of the sheet P in the conveyance direction calculated by Expression 1. Specifically, a scale for the scanned image is defined based on the length Lt of the sheet P in the conveyance direction calculated by Expression 1, and the coordinates (e.g., H0, V0) of the center position of each of the detection marks a, b, c, and d are calculated based on the scale. Note that, instead of the L-shaped detection marks a, b, c, and d illustrated inFIG. 3, detection marks having a shape such as a cross, a rectangle, or a straight line may be used.

For example, the coordinate V0of the front detection mark a in the conveyance direction (i.e., a distance V0from the leading end of the sheet P to the center position of the front detection mark a) are obtained as follows. First, the position of the leading end of the sheet P, which is the origin in the conveyance direction, is pinpointed. In the present embodiment, in the case of a white sheet P, the black roller (i.e., the large-diameter black roller54aor the small-diameter black roller54b) of the background member54is positioned at the image reading position, and the image reading is started before the leading end of the sheet P passes through the image reading position. Therefore, the front side of the scanned image is black. The controller110detects a position P1of the edge portion at which the scanned image turns from black to white first from the front side of the scanned image in the conveyance direction. The position P1detected by the controller110corresponds to the leading end of the sheet P, that is, the origin in the conveyance direction. In the present embodiment, the detection marks a, b, c, and d are painted, for example, in solid black as illustrated inFIG. 6Abut outlined inFIG. 3for understanding the coordinates of the center positions of the detection marks a, b, c, and d. The controller110detects a position P2of the edge portion at which the scanned image turns from white to black, and further, a position P3of the edge portion at which the scanned image turns from black to white at the lateral bar portion of the front detection mark a. The position P2corresponds to the front end of the front detection mark a. The position P3corresponds to the rear end of the lateral bar portion of the front detection mark a. InFIG. 3, the origin is the upper left corner of the sheet P, and the coordinate V0in the conveyance direction of the center position of the front detection mark a is obtained by the expression of (P3+P2−2×P1)/2. The front detection mark b is disposed on the other side in the width direction and on the front side of the sheet P, and the coordinate of the front detection mark b is similarly obtained.

The coordinate H0in the width direction of the center position of the front detection mark a can also be obtained in the same manner. That is, in the width direction, the controller110detects a position Pa of the edge portion (i.e., one side end of the sheet P) as the origin in the width direction at which the scanned image turns from black to white first from one side of the scanned image. Then, the controller110detects a position Pb of the edge portion at which the scanned image turns from white to black, and further, a position Pc of the edge portion at which the scanned image turns from black to white at the longitudinal bar portion of the front detection mark a. The position Pb corresponds to the one side end of the longitudinal bar portion of the front detection mark a. The position Pc corresponds to the other side end of the longitudinal bar portion of the front detection mark a. When the origin is the upper left corner of the sheet P inFIG. 3as described above, the coordinate H0in the width direction of the center position of the front detection mark a is obtained by the expression of (Pc+Pb−2×Pa)/2. The rear detection mark c is disposed on the one side in the width direction and on the rear side of the sheet P, and the coordinate in the width direction of the rear detection mark c is similarly obtained.

The coordinates in the conveyance direction of the rear detection marks c and d disposed on the rear side of the sheet P in the conveyance direction are obtained as follows. In the conveyance direction, the controller110detects a position P4at which the scanned image turns to white from the rear side of the scanned image as the trailing end of the sheet P. Then, the controller110detects a position P5at which the scanned image turns from white to black at the lateral bar portion of the rear detection mark c (or d), and further, a position P6at which the scanned image turns from black to white at the lateral bar portion. Accordingly, a distance V1from the trailing end of the sheet P to the center position of the rear detection marks c and d is calculated by the expression of (P6+P5−2×P4)/2. The coordinate (Lt−V1) in the conveyance direction of the center position of the rear detection marks c and d is obtained by subtracting the distance V1from the length Lt of the sheet P.

The coordinates in the width direction of the detection marks b and d disposed on the other side of the sheet P in the width direction are obtained as follows. That is, in the width direction, the controller110detects a position Pd at which the scanned image turns to white from the other side of the scanned image as the other side end of the sheet P. Then, the controller110detects a position Pe at which the scanned image turns from white to black, and further, a position Pf at which the scanned image turns from black to white at the longitudinal bar portion of the detection marks b and d. Accordingly, a distance H1from the other side end of the sheet P to the center position of the detection marks b and d on the other side in the width direction is calculated by the expression of (Pf+Pe−2×Pd)/2. The coordinate (Ly−H1) in the width direction of the center position of the detection marks b and d is obtained by subtracting the distance H1from the length Ly of the sheet P in the width direction.

FIG. 4is a schematic diagram illustrating types of image corrections. The controller110calculates the amount of deviation (i.e., correction value) of the calculated center position of each of the detection marks a, b, c, and d from the target position, and corrects the writing timing or position of the exposure device6or image data for forming images so that each of the detection marks a, b, c, and d is formed at the target position. As illustrated inFIG. 4, the image forming apparatus according to the present embodiment performs various corrections to correct the image position, such as registration correction (that is, correction for translating the image position in the width direction or the conveyance direction of the sheet P), magnification correction, skew correction, trapezoidal correction, and other corrections. The type of correction is not limited to the above examples. These corrections can be performed by any known methods, and detailed description thereof is omitted.

Further, in the present embodiment, the output image on the sheet P read by the reader51is compared with the master image that is the original data of the output image, thereby inspecting the output image. Specifically, the controller110generates a difference image indicating the difference between the master image and the output image read by the reader51(i.e., the scanned image). Defects (defective pixels) that are not found in the master image remain in the generated difference image. If the number of the defects (defective pixels) is equal to or greater than the threshold, the controller110determines that the output image is a defective image. The inspection of the output image can be performed by any known methods, and detailed description thereof is omitted.

Further, in the present embodiment, the controller110corrects a gradation reproduction curve based on the full-color output image on the sheet P read by the reader51and the master image which is the original data of the output image to prevent the color output on the sheet P from fluctuating. Specifically, the controller110calculates the difference between the color of the master image and the color of the output image read by the reader51(i.e., the scanned image). Next, the controller110determines the amount of correction for correcting the current set value indicating the gradation reproduction curve of the image processing parameter based on the calculated difference. The control to prevent the fluctuation of the output color on the sheet P can be performed by any known methods, and detailed description thereof is omitted.

In the image forming apparatus300described above, the sheet P may be expanded or contracted, or deformed by the fixing process, and so-called front-back misregistration may occur in which the images formed on the front surface and the back surface of the sheet P are misaligned with each other.

In addition, due to cutting tolerances of the bundle of sheets P, one end of the sheet P or the other end of the sheet P may be tilted with respect to the conveyance direction. Here, the one end is the leading end of the sheet P and the other end is the trailing end of the sheet P in the conveyance direction when an image is formed on the front surface of the sheet P. When an image is formed on the back surface of the sheet P, the sheet P is reversed in switchback manner and conveyed to the secondary transfer nip again. Therefore, the other end of the sheet P, which is the trailing end of the sheet P in the conveyance direction when an image is formed on the front surface, becomes the leading end of the sheet P in conveyance direction when an image is formed on the back surface.

The leading end of the sheet P in the conveyance direction contacts the registration roller pair15before the sheet P is conveyed to the secondary transfer nip. If there are cutting tolerances of the bundle of sheets P, the posture of the sheet P when one end of the sheet P contacts the registration roller pair15is different from the posture of the sheet P when the other end of the sheet P contacts the registration roller pair15. The one end is the leading end in the conveyance direction when an image is formed on the front surface of the sheet P, and the other end is the leading end in the conveyance direction when an image is formed on the back surface of the sheet P. As a result, the posture of the sheet P being conveyed when an image is transferred to the front surface of the sheet P and the posture of the sheet P being conveyed when an image is transferred to the back surface of the sheet P are different from each other. Accordingly, the front and back misregistration may occur due to the cutting tolerances of the bundle of sheet P.

Therefore, the image on the front surface is preferably aligned with the image on the back surface of the sheet P by the above-described corrections. When the images on the front and back surfaces are aligned with each other, the controller110causes the image forming apparatus300to transfer a detection pattern onto the front surface, fix the detection pattern, cool the sheet P, and read the detection marks on the front surface. In the same order, the controller110causes the image forming apparatus300to transfer a detection pattern onto the back surface, fix the detection pattern, cool the sheet P, and read the detection marks on the back surface. Then, based on the result of reading the detection patterns on the front and back surfaces, the controller110corrects the writing timing and position by the exposure device6and/or the image magnification of the image data so that the positions of the images on the front and back surfaces coincide with each other. This configuration can prevent the images on the front and back surfaces from being misaligned with each other.

FIG. 5is a graph illustrating conveyance of the sheet P passing through the image reading device50.FIGS. 6A and 6Bare schematic views illustrating a dimensional relation of the image reading device50. InFIG. 5, t1represents the time when the leading end of the sheet P passes through the image reading position, and t2represents the time when the front detection marks a and b of the sheet P pass through the image reading position. In addition, inFIG. 5, t3represents the time when the leading end of the sheet P passes through the second conveyance roller pair56, and t4represents the time when the trailing end of the sheet P passes through the first conveyance roller pair55. Further, inFIG. 5, t5represents the time when the rear detection marks c and d of the sheet P pass through the image reading position, and t6represents the time when the trailing end of the sheet P passes through the image reading position. Furthermore, inFIG. 5, t7represents the time when the trailing end of the sheet P passes through the second conveyance roller pair56.

As illustrated inFIG. 5, due to the eccentricity of the first drive conveyance roller55aof the first conveyance roller pair55, the conveyance speed of the sheet P conveyed to the image reading position E fluctuates with the rotation cycle of the first drive conveyance roller55athat applies conveyance force to the sheet P. Further, due to the eccentricity of the second drive conveyance roller56aof the second conveyance roller pair56, the conveyance speed of the sheet P passing through the image reading position E fluctuates with the rotation cycle of the second drive conveyance roller56athat applies conveyance force to the sheet P.

Until the leading end of the sheet P reaches the second conveyance roller pair56(i.e., the time t3inFIG. 5), the sheet P is conveyed by the first conveyance roller pair55, and the conveyance speed of the sheet P fluctuates with the rotation cycle of the first drive conveyance roller55a. Before the leading end of the sheet P reaches the second conveyance roller pair56, the leading end of the sheet P passes through the image reading position E at the time t1inFIG. 5, and the front detection marks a and b passes through the image reading position E at the time t2inFIG. 5. Accordingly, A portion of the scanned image between the leading end and the position away from the leading end by a distance X2(seeFIGS. 6A and 6B) is affected by the fluctuation of the conveyance speed of the first drive conveyance roller55a. Specifically, due to the fluctuation of the conveyance speed of the first drive conveyance roller55a, the scanned image expands and contracts with the rotation cycle of the first drive conveyance roller55a. When the scanned image expands and contracts, the actual positions of the front detection marks a and b on the sheet P in the conveyance direction (i.e., the distance from the leading end of the sheet P to the front detection marks a and b) may deviate from the positions of the front detection marks a and b of the scanned image in the conveyance direction. As a result, accurate image correction may not be performed based on the scanned image.

Further, after the trailing end of the sheet P passes through the first conveyance roller pair55(i.e., the time t4inFIG. 5) until the trailing end of the sheet P reaches the image reading position E (i.e., the time t6inFIG. 5), the sheet P is conveyed by the second conveyance roller pair56, and the conveyance speed of the sheet P fluctuates with the rotation cycle of the second drive conveyance roller56a. Therefore, due to the fluctuation of the conveyance speed of the second drive conveyance roller56a, a portion of the scanned image between the trailing end of the sheet P and the position indicated by the coordinate (Lt−X1), i.e., the position away from the trailing end by a distance X1(seeFIGS. 6A and 6B), expands and contracts with the rotation cycle of the second drive conveyance roller56a. Since the rear detection marks c and d are disposed from the position indicated by the coordinate (Lt−X1) of the scanned image to the trailing end of the sheet P, the actual distance from the trailing end of the sheet P to the rear detection marks c and d may deviate from the distance from the trailing end of the sheet P to the rear detection marks c and d of the scanned image in the conveyance direction. As a result, accurate image correction may not be performed based on the scanned image.

Therefore, in the present embodiment, the distance V0from the leading end of the sheet P to the center position of the front detection marks a and b in the conveyance direction satisfies the following relation expressed by Expression 2.
V0≈n1×π×D1aExpression 2,

where D1arepresents the diameter of the first drive conveyance roller55a, and n1is an integer.

Due to the expansion and contraction of the scanned image with the rotation cycle of the first drive conveyance roller55a, the actual positions of the front detection marks a and b on the sheet P in the conveyance direction (i.e., the distance from the leading end of the sheet P to the front detection marks a and b) may deviate from the positions of the front detection marks a and b of the scanned image in the conveyance direction. The deviation between the actual positions on the sheet P and the positions on the scanned image becomes 0 at the position away from the leading end of the sheet P by an integral multiple of the circumference of the first drive conveyance roller55a. Therefore, by satisfying Expression 2 described above, the distance from the leading end of sheet P to the front detection marks a and b of the scanned image can substantially coincide with the actual distance from the leading end of sheet P to the front detection marks a and b on the sheet P. As a result, the registration correction can be performed with high accuracy based on the scanned image. In addition, the position of the image on the front surface of the sheet P can be accurately aligned with the position of the image on the back surface of the sheet P.

To satisfy Expression 2, the reference position for forming the front detection marks a and b is located at the position away from the leading end of the sheet P by an integral multiple of the circumference (π×D1a) of the first drive conveyance roller55a. Thus, the detection marks a and b can be formed at the reference position. Alternatively, to satisfy Expression 2, the first drive conveyance roller55ahaving a diameter satisfying the relation in which the distance from the leading end of the sheet P to the reference position is an integral multiple of the circumference (π×D1a) of the first drive conveyance roller55ais used.

Further, in the present embodiment, the distance V1from the center position of the rear detection marks c and d to the trailing end of the sheet P in the conveyance direction satisfies the following relation expressed by Expression 3.
V1≈n2×π×D2aExpression 3,

where D2arepresents the diameter of the second drive conveyance roller56a, and n2is an integer.

The actual distance from the trailing end of the sheet P to the rear detection marks c and d on the sheet P may deviate from the distance from the trailing end of the sheet P to the rear detection marks c and d of the scanned image. The deviation between the actual distance on the sheet P and the distance on the scanned image becomes 0 at the position away from the trailing end of the sheet P by an integral multiple of the circumference of the second drive conveyance roller56a. Therefore, by satisfying Expression 3, the distance from the center position of the rear detection marks c and d of the scanned image to the trailing end of the sheet P in the conveyance direction can substantially coincide with the actual distance from the rear detection marks c and d on the sheet P to the trailing end of the sheet P. As a result, the magnification correction in the conveyance direction can be performed with high accuracy. In addition, the size of the image on the front surface of the sheet P can accurately coincide with the size of the image on the back surface of the sheet P.

To satisfy Expression 3, the reference position for forming the rear detection marks c and d is located at the position away from the trailing end of the sheet P by an integral multiple of the circumference (π×D2a) of the second drive conveyance roller56a. Thus, the rear detection marks c and d can be formed at the reference position. Alternatively, to satisfy Expression 3, the second drive conveyance roller56ahaving a diameter satisfying the relation in which the distance from the trailing end of the sheet P to the reference position is an integral multiple of the circumference (π×D2a) of the second drive conveyance roller56ais used.

FIG. 7is a graph illustrating an amount of deviation of the detection marks. The horizontal axis indicates the positional difference between the position of the front detection mark and the position away from the leading end of the sheet P by an integral multiple of the circumference (π×D1a) of the first drive conveyance roller55a, and the vertical axis indicates the amount of deviation between the position of the front detection mark of the scanned image and the actual position of the front detection mark on the sheet P.

As illustrated inFIG. 7, when there is not much positional difference between the position (distance V0) of the front detection mark formed on the sheet P and the position which is away from the leading end of the sheet P by an integral multiple of the circumference (π×D1a) of the first drive conveyance roller55a, that is, when the front detection mark is formed in the vicinity of the position away from the leading end of the sheet P by an integral multiple of the circumference of the first drive conveyance roller55a, there is not much deviation between the actual position of the front detection mark formed on the sheet P and the position of the front detection mark of the scanned image. On the other hand, the positional difference between the position (distance V0) of the front detection mark and the position away from the leading end of the sheet P by an integral multiple of the circumference (π×D1a) of the first drive conveyance roller55abecomes larger (i.e., the front detection mark is separated from the position away from the leading end of the sheet P by an integral multiple of the circumference (π×D1a) of the first drive conveyance roller55a), the amount of deviation between the actual position of the front detection mark formed on the sheet P and the position of the front detection mark of the scanned image becomes larger.

Therefore, even if the position (distance V0) of the front detection mark on the sheet P is slightly separated from the position away from the leading end of the sheet P by an integral multiple of the circumference (π×D1a) of the first drive conveyance roller55a, the amount of deviation between the actual position of the front detection mark formed on the sheet P and the position of the front detection mark of the scanned image is small. As a result, the good image correction can be performed based on the scanned image. Therefore, the center position of the front detection mark in the conveyance direction does not need to completely coincide with the position away from the leading end of the sheet P by an integral multiple of the circumference (π×D1a) of the first drive conveyance roller55a. When the position away from the leading end of the sheet P by an integral multiple of the circumference (π×D1a) of the first drive conveyance roller55afalls within the range of the front detection mark in the conveyance direction, that is, when the front detection mark is on the position away from the leading end of the sheet P by an integral multiple of the circumference of the first drive conveyance roller55ain the conveyance direction, the good image correction can be performed based on the scanned image.

Similarly, regarding the rear detection mark, even if the position (distance V1) of the rear detection mark on the sheet P is slightly separated from the position away from the trailing end of the sheet P by an integral multiple of the circumference (π×D2a) of the second drive conveyance roller56a, the amount of deviation between the actual distance from of the rear detection mark formed on the sheet P to the trailing end of the sheet P and the distance from the rear detection mark of the scanned image to the trailing end of the sheet P is small. As a result, the good image correction can be performed based on the scanned image. Therefore, the center position of the rear detection mark in the conveyance direction does not need to completely coincide with the position away from the trailing end of the sheet P by an integral multiple of the circumference (π×D2a) of the second drive conveyance roller56a. When the position away from the trailing end of the sheet P by an integral multiple of the circumference (π×D2a) of the second drive conveyance roller56afalls within the range of the rear detection mark in the conveyance direction, that is, when the rear detection mark is on the position away from the trailing end of the sheet P by an integral multiple of the circumference of the second drive conveyance roller56ain the conveyance direction, the good image correction can be performed based on the scanned image.

Further, preferably, the diameter D1bof the first driven conveyance roller55bis the same as the diameter D1aof the first drive conveyance roller55aor an integral multiple of the diameter D1aof the first drive conveyance roller55a. In the present embodiment, the nip pressure of the first conveyance roller pair55is increased so that the cooling device9disposed upstream from the first conveyance roller pair55in the conveyance direction does not affect the conveyance of the sheet P passing through the image reading position. The first drive conveyance roller55ahas the elastic layer, and the first driven conveyance roller55bis the metal roller. Therefore, in the nip, the outer circumferential surface of the first drive conveyance roller55ais deformed according to the curvature of the first driven conveyance roller55b. Due to the eccentricity of the first driven conveyance roller55b, the radius of curvature at the nip may change, and the conveyance speed of the sheet P may fluctuate with the rotation cycle of the first driven conveyance roller55b.

As described above, the diameter D1bof the first driven conveyance roller55bis preferably an integral multiple of the diameter D1aof the first drive conveyance roller55a. With this configuration, the positional deviation between the position of the front detection marks a and b of the scanned image and the actual position of the front detection marks a and b on the sheet P due to the fluctuation of the conveyance speed with the rotation cycle of the first driven conveyance roller55bcan be prevented.

Further, preferably, the diameter D2bof the second driven conveyance roller56bis an integral multiple of the diameter D2aof the second drive conveyance roller56a. Also in the second conveyance roller pair56, the second drive conveyance roller56ahas the elastic layer, and the second driven conveyance roller56bis the metal roller. Further, as described above, the nip pressure of the second conveyance roller pair56is increased so that the mechanisms disposed downstream from the second conveyance roller pair56in the conveyance direction does not affect the conveyance of the sheet P passing through the image reading position, thereby enhancing the conveyance force. As a result, the outer circumferential surface of the second drive conveyance roller56ais deformed according to the curvature of the second driven conveyance roller56bin the nip of the second conveyance roller pair56. Therefore, due to the eccentricity of the second driven conveyance roller56b, the radius of curvature at the nip may change, and the conveyance speed of the sheet P may fluctuate with the rotation cycle of the second driven conveyance roller56b.

Therefore, preferably, the diameter D2bof the second driven conveyance roller56bis the same as the diameter D2aof the second drive conveyance roller56aor an integral multiple of the diameter D2aof the second drive conveyance roller56a. With this configuration, the deviation between the distance from the rear detection mark of the scanned image to the trailing end of the sheet P and the actual distance from the rear detection mark on the sheet P to the trailing end of the sheet P due to the fluctuation of the conveyance speed with the rotation cycle of the second driven conveyance roller56bcan be prevented.

Further, preferably, the diameter D2aof the second drive conveyance roller56ais the same as the diameter D1aof the first drive conveyance roller55a, and the cyclic fluctuation of the first drive conveyance roller55ais in phase with the cyclic fluctuation of the second drive conveyance roller56awhen the leading end of the sheet P reaches the second conveyance roller pair56. This is because the sheet P may be greatly bent or excessively stretched between the first conveyance roller pair55and the second conveyance roller pair56if the speed difference between the fluctuation of the conveyance speed with the rotation cycle of the first drive conveyance roller55aand the fluctuation of the conveyance speed with the rotation cycle of the second drive conveyance roller56ais large when the sheet P is conveyed by both the first conveyance roller pair55and the second conveyance roller pair56. Therefore, the output image may not be read accurately.

In the present embodiment, as described above, the output image on the sheet P read by the reader51is compared with the master image that is the original data of the output image, thereby inspecting the output image and correcting the graduation reproduction curve of the image processing parameter. Accordingly, if the sheet P is greatly bent or excessively stretched between the first conveyance roller pair55and the second conveyance roller pair56and the reading accuracy deteriorates, the inspection of the output image or the correction of the graduation reproduction curve of the image processing parameter may not be performed with high accuracy.

In the present embodiment, the fluctuation of the conveyance speed with the rotation cycle of the first drive conveyance roller55ais in phase with the fluctuation of the conveyance speed with the rotation cycle of the second drive conveyance roller56awhen the leading end of the sheet P reaches the second conveyance roller pair56. Accordingly, when the sheet P is conveyed by the first conveyance roller pair55and the second conveyance roller pair56, the difference between the conveyance speed by the first drive conveyance roller55aand the conveyance speed by the second drive conveyance roller56acan be reduced. Therefore, the sheet P is not greatly bent or excessively stretched between the first conveyance roller pair55and the second conveyance roller pair56, thereby preventing the reading accuracy from deteriorating. As a result, the inspection of the output image and the correction of the graduation reproduction curve of the image processing parameter can be performed with high accuracy.

Further, preferably, the distance X1from the first conveyance roller pair55to the image reading position E is an integral multiple of the circumference of the first drive conveyance roller55a(i.e., X1=na×π×D1a, where na is an integer), and the distance X2from the image reading position E to the second conveyance roller pair56is an integral multiple of the circumference of the first drive conveyance roller55a(i.e., X2=nb×π×D1a, where nb is an integer), resulting in the distance from the first conveyance roller pair55to the second conveyance roller pair56(i.e., X1+X2) being an integral multiple of the circumference of the first drive conveyance roller55a.

By setting the distance from the first conveyance roller pair55to the second conveyance roller pair56to an integral multiple of the circumference of the first drive conveyance roller55a, the cyclic fluctuation of the first drive conveyance roller55acan be easily in phase with the cyclic fluctuation of the second drive conveyance roller56awhen the leading end of the sheet P reaches the second conveyance roller pair56. That is, the image reading device50is assembled so that the cyclic fluctuation of the first drive conveyance roller55ais in phase with the cyclic fluctuation of the second drive conveyance roller56a. After that, by just matching the drive start and drive stop timing between of the first drive conveyance roller55aand the second drive conveyance roller56a, the cyclic fluctuation of the first drive conveyance roller55acan be in phase with the cyclic fluctuation of the second drive conveyance roller56awhen the leading end of the sheet P reaches the second conveyance roller pair56.

Further, by setting the distance X1from the first conveyance roller pair55to the image reading position E to an integral multiple of the circumference of the first drive conveyance roller55a(i.e., X1=na×π×D1a, where na is an integer), the timing at which the leading end of the sheet P passes through the image reading position E can be stabilized, and the reading accuracy can be stabilized.

FIG. 8is a schematic view illustrating an example of the detection pattern in which detection marks are formed outside an output image area of the sheet P. InFIG. 8, cutting marks Sa to Sd are reference marks for cutting the sheet P after the image formation, and the inside enclosed by the broken line connecting between the cutting marks Sa to Sd is the output image area.

Generally, the cutting marks Sa to Sd also serves as the detection marks for adjusting the image position. The distance from the cutting marks Sa and Sb on the front side to the leading end of the sheet P and the distance from the cutting marks Sc and Sd on the rear side to the trailing end of the sheet P are short. There is a limit to reducing the diameters of the first drive conveyance roller55aand the second drive conveyance roller56adue to the effect of the conveyance force on the sheet P. Therefore, when the cutting marks Sa to Sd are used as the detection marks for adjusting the image position, the distance from the leading end of the sheet P to the front detection marks is too short to be an integral multiple of the circumference of the first drive conveyance roller55a. Therefore, as illustrated inFIG. 8, front detection marks al and b1are provided outside the output image area. As a result, the distance from the leading end of the sheet P to the front detection marks al and b1can be an integral multiple of the circumference of the first drive conveyance roller55a.

Further, by providing rear detection marks cl and dl outside the output image area, the rear detection marks cl and dl can be located at the positions away from the trailing end of the sheet P by an integral multiple of the circumference of the second drive conveyance roller56a.

InFIG. 8, the position in width direction of the sheet P is detected at the vertical bar portions of the cutting marks Sa to Sd extending in the conveyance direction. Alternatively, detection marks for detecting the position in the width direction of the sheet P may also be provided outside the output image area.

FIG. 9is a schematic view illustrating a variation of the sheet conveyor including the fixing device8, the cooling device9, and the image reading device50. In the sheet conveyor according to the variation, the cooling device9includes, for example, a cooling roller191including a heat pipe and a pressure roller192that presses the sheet P against the cooling roller191. Further, in the sheet conveyor according to the variation, the image reading device50includes an image reading unit that are an equal magnification optical system such as a contact image sensor (CIS)151. Other configurations are the same as in the above-described embodiment. Thus, the sheet conveyor according to the variation can be downsized as compared with the sheet conveyor including the image reading unit that are a reduced optical system such a charge-coupled device (CCD) as illustrated inFIG. 2.

In the present embodiment, as expressed by Expressions 2 and 3, the distances V0and V1are just an integral multiple of the circumference of the respective rollers (i.e., the first and second drive conveyance roller55aand56a). Such a configuration is most preferable. However, in consideration of the manufacturing tolerances of the respective rollers, the distances V0and V1allow ±5% error of the length obtained by an integral multiple of the circumference of the respective rollers. For example, an integer n1can be replaced with an integer n1′ in Expression 2, where 0.95×n1≤n1′≤1.05×n1. The same applies to an integer n2in Expression 3. In addition, the same applies to integers na and nb described above. The value of “an integral multiple” in the present embodiment is not limited to only just the value of an integral multiple and is defined so as to allow ±5% error of the value of an integral multiple.

The above descriptions concern about the electrophotographic image forming apparatus300, but the present disclosure can be applied to an inkjet image forming apparatus. Further, in the above embodiments, the image reading device50is arranged in the image forming apparatus300, but the image reading device50may be coupled to the image forming apparatus300and the sheet P may be conveyed from the image forming apparatus300to the image reading device50.

Further, the present disclosure is applicable to an apparatus including an image reading unit. The sheet P on which the detection marks a to d are formed is ejected from the image forming apparatus and set on an automatic document feeder (ADF). The ADF conveys the sheet P on which the detection marks a to d are formed, and the image reading unit reads the detection marks a to d on the sheet P.

The embodiments described above are examples and can provide, for example, the following effects, respectively.

A conveyance device includes a conveyance roller pair such as the first conveyance roller pair55to convey a recording medium to an image reading position E of an image reading unit. The conveyance roller pair includes a drive roller such as the first drive conveyance roller55aand a driven roller such as the first driven conveyance roller55b. The driven roller contacts the drive roller and rotates following the drive roller. The drive roller has a diameter satisfying a relation in which a detection mark on the recording medium is on a position away from a leading end of the recording medium by an integral multiple of a circumference of the drive roller in a conveyance direction of the recording medium.

The conveyance speed of the recording medium conveyed to the image reading position E fluctuates with the rotation cycle of the drive roller (first drive roller) due to the eccentricity of the drive roller of the conveyance roller pair (first conveyance roller pair). As the conveyance speed of the recording medium thus fluctuates with the rotation cycle of the drive roller, the scanned image expands and contracts with the rotation cycle of the drive roller. When the scanned image expands and contracts, the actual position of the detection mark on the sheet P in the conveyance direction (i.e., the distance from the leading end of the recording medium to the detection mark) may deviate from the position of the detection mark of the scanned image in the conveyance direction. As a result, the image correction may not be performed with high accuracy based on the scanned image.

Therefore, in Aspect 1, the drive roller has the diameter so that the detection mark is on the position away from the leading end of the recording medium by an integral multiple of the circumference of the drive roller in the conveyance direction of the recording medium. When the scanned image expands and contracts with the rotation cycle of the drive roller, the positional deviation between the actual position of the detection mark on the recording medium and the position of the detection mark of the scanned image in the conveyance direction becomes 0 at the position away from the leading end of recording medium by an integral multiple of the circumference of the drive roller. Further, as illustrated inFIG. 7, the positional deviation from the actual position in the conveyance direction is small in the vicinity of the position away from the leading end of the recording medium by an integral multiple of the circumference of the drive roller. Therefore, in Aspect 1, since the drive roller has the diameter so that the detection mark is on the position away from the leading end of the recording medium by an integral multiple of the circumference of the drive roller in the conveyance direction of the recording medium, the image correction such as the registration correction of the image formed on the recording medium and the skew correction of the image can be performed with high accuracy based on the position of the detection mark of the scanned image.

In Aspect 1, a diameter of the driven roller such as the first driven conveyance roller55bis an integral multiple of the diameter of the drive roller such as the first drive conveyance roller55a.

With this configuration, as described in the above embodiments, the detection mark is on the position away from the leading end of the recording medium such as the sheet P by an integral multiple of the circumference of the driven roller such as the first driven conveyance roller55bin the conveyance direction of the recording medium. As a result, even if the conveyance speed of the recording medium fluctuates with the rotation cycle of the driven roller, the positional deviation between the position of the detection mark of the scanned image and the actual position of the detection mark on the recording medium can be reduced.

In Aspect 1 or 2, the detection mark is a front detection mark such as the front detection marks a and b on a front side of the recording medium such as the sheet P.

With this configuration, as described in the above embodiments, the difference between the distance from the leading end of the recording medium to the front detection mark of the scanned image and the actual distance from the leading end of the recording medium to the front detection mark on the recording medium can be reduced.

In Aspect 3, the conveyance device further includes another conveyance roller pair such as the second conveyance roller pair56to convey the recording medium passing through the image reading position E. Said another conveyance roller pair includes another drive roller such as the second drive conveyance roller56aand another driven roller such as the second driven conveyance roller56b. Said another driven roller contacts said another drive roller and rotates following said another drive roller. Said another drive roller has a diameter satisfying a relation in which a rear detection mark on a rear side of the recording medium is on a position away from a trailing end of the recording medium by an integral multiple of a circumference of said another drive roller in the conveyance direction of the recording medium.

With this configuration, as described in the above embodiments, when the rear detection mark such as the rear detection marks c and d passes through the image reading position E, the trailing end of the recording medium such as the sheet P has passed through the first conveyance roller pair such as the first conveyance roller pair55, and the recording medium is conveyed by said another conveyance roller pair such as the second conveyance roller pair56. Therefore, the conveyance speed of the recording medium fluctuates with the rotation cycle of said another drive roller (second drive roller) such as the second drive conveyance roller56a. As a result, the portion of the scanned image from the rear detection mark to the trailing end of the recording medium expands and contracts with the rotation cycle of the second drive roller, and the distance from the rear detection mark of the scanned image to the trailing end of the recording medium may differ from the actual distance from the rear detection mark on the recording medium to the trailing end of the recording medium.

Therefore, in Aspect 4, the second drive roller has the diameter so that the rear detection mark on the rear side of the recording medium is on the position away from the trailing end of the recording medium by an integral multiple of the circumference of the second drive roller in the conveyance direction of the recording medium. When the scanned image expands and contracts with the rotation cycle of the second drive roller, the difference between the actual distance from the rear detection mark on the recording medium to the trailing end of the recording medium and the distance from the rear detection mark of the scanned image to the trailing end of the recording medium becomes 0 at the position away from the trailing end of the recording medium by an integral multiple of the circumference of the second drive roller. When the rear detection mark is located in the vicinity of the position away from the trailing end of the recording medium by an integral multiple of the circumference of the second drive roller, the difference between the actual distance from the rear detection mark on the recording medium to the trailing end of the recording medium and the distance from the rear detection mark of the scanned image to the trailing end of the recording medium is small. Therefore, since the second drive roller has the diameter so that the rear detection mark on the recording medium is on the position away from the trailing end of the recording medium by an integral multiple of the circumference of the second drive roller in the conveyance direction of the recording medium, the image correction such as the magnification correction of the image formed on the recording medium can be performed with high accuracy based on the position of the detection mark of the scanned image.

In Aspect 4, a diameter of said another driven roller such as the second driven conveyance roller56bis an integral multiple of the diameter of said another drive roller.

With this configuration, as described in the above embodiments, the rear detection mark on the rear side of the recording medium is on the position away from the trailing end of the recording medium such as the sheet P by an integral multiple of the circumference of the second driven roller such as the second driven conveyance roller56bin the conveyance direction of the recording medium. As a result, even if the conveyance speed of the recording medium fluctuates with the rotation cycle of the second driven roller, the difference between the actual distance from the rear detection mark on the recording medium to the trailing end of the recording medium and the distance from the rear detection mark of the scanned image to the trailing end of the recording medium can be further reduced.

In any one of Aspects 1 to 5, the driven roller such as the first driven conveyance roller55bis configured to measure a length of the recording medium in the conveyance direction with a measuring instrument such as the rotary encoder59.

With this configuration, as described in the above embodiments, the length of the recording medium can be measured even if the conveyance speed of the recording medium fluctuates.

In any one of Aspects 1 to 6, a distance from the conveyance roller pair such as the first conveyance roller pair55to the image reading position E is an integral multiple of the circumference of the drive roller such as the first drive conveyance roller55a.

With this configuration, as described in the above embodiments, the leading end of the recording medium passes through the image reading position E at a predetermined timing, thereby stabilizing the reading accuracy.

In any one of Aspects 1 to 7, the conveyance device further includes another conveyance roller pair such as the second conveyance roller pair56to convey the recording medium passing through the image reading position E. Said another conveyance roller pair includes another drive roller such as the second drive conveyance roller56aand another driven roller such as the second driven conveyance roller56b. Said another driven roller contacts said another drive roller and rotates following said another drive roller. A fluctuation of a conveyance speed of the recording medium with a rotation cycle of the drive roller such as the first drive conveyance roller55ais in phase with a fluctuation of a conveyance speed of the recording medium with a rotation cycle of said another drive roller.

With this configuration, as described in the above embodiments, when the recording medium such as the sheet P is conveyed by the first conveyance roller pair and the second conveyance roller pair, the speed difference can be reduced between the fluctuation of the conveyance speed with the rotation cycle of the first drive roller and the fluctuation of the conveyance speed with the rotation cycle of the second drive roller. As a result, the sheet P is not bent or stretched between the first conveyance roller pair and the second conveyance roller pair, thereby reading an image on the recording medium with high accuracy.

In Aspect 8, a distance from the conveyance roller pair such as the first conveyance roller pair55to said another conveyance roller pair such as the second conveyance roller pair56is an integral multiple of the circumference of the drive roller.

As described in the above embodiments, the image reading device is assembled so that the cyclic fluctuation of the first drive roller such as the first drive conveyance roller55ais in phase with the cyclic fluctuation of the second drive roller such as the second drive conveyance roller56a. After that, by matching the drive start and drive stop timing between of the first drive roller and the second drive roller, the fluctuation of the conveyance speed of the recording medium with the rotation cycle of the first drive roller can be in phase with the fluctuation of the conveyance speed of the recording medium with the rotation cycle of the second drive roller.

A conveyance device includes a conveyance roller pair such as the second conveyance roller pair56to convey a recording medium passing through an image reading position E of an image reading unit. The conveyance roller pair includes a drive roller such as the second drive conveyance roller56aand a driven roller such as the second driven conveyance roller56b. The driven roller contacts the drive roller and rotates following the drive roller. The drive roller has a diameter satisfying a relation in which a rear detection mark on a rear side of the recording medium is on a position away from a trailing end of the recording medium by an integral multiple of a circumference of the drive roller in a conveyance direction of the recording medium.

With this configuration, as described in the above embodiments, when the rear detection mark such as the rear detection marks c and d passes through the image reading position E, the trailing end of the recording medium such as the sheet P has passed through the first conveyance roller pair such as the first conveyance roller pair55, and the recording medium is conveyed by the second conveyance roller pair such as the second conveyance roller pair56. Therefore, the conveyance speed of the recording medium fluctuates with the rotation cycle of the second drive roller such as the second drive conveyance roller56a. As a result, the portion of the scanned image from the rear detection mark to the trailing end of the recording medium expands and contracts with the rotation cycle of the second drive roller, and the distance from the rear detection mark of the scanned image to the trailing end of the recording medium may differ from the actual distance from the rear detection mark on the recording medium to the trailing end of the recording medium.

Therefore, in Aspect 10, the second drive roller has the diameter so that the rear detection mark on the rear side of the recording medium is on the position away from the trailing end of the recording medium by an integral multiple of the circumference of the second drive roller in the conveyance direction of the recording medium. When the scanned image expands and contracts with the rotation cycle of the second drive roller, the difference between the actual distance from the rear detection mark on the recording medium to the trailing end of the recording medium and the distance from the rear detection mark of the scanned image to the trailing end of the recording medium becomes 0 at the position away from the trailing end of the recording medium by an integral multiple of the circumference of the second drive roller. When the rear detection mark is located in the vicinity of the position away from the trailing end of the recording medium by an integral multiple of the circumference of the second drive roller, the difference is small between the actual distance from the rear detection mark on the recording medium to the trailing end of the recording medium and the distance from the rear detection mark of the scanned image to the trailing end of the recording medium. Therefore, the second drive roller has the diameter so that the rear detection mark on the rear side of the recording medium is on the position away from the trailing end of the recording medium by an integral multiple of the circumference of the second drive roller in the conveyance direction of the recording medium. Accordingly, the image correction such as the magnification correction of the image formed on the recording medium can be performed with high accuracy based on the position of the detection mark of the scanned image.

An image reading device includes an image reading unit to read an image on a recording medium and the conveyance device according to any one of Aspects 1 to 10.

In Aspect 11, the image reading unit includes one of an equal magnification optical system such as the CIS and a reduced optical system such as the CCD.

Aspect 13 An image forming apparatus includes an image forming device to form an image on a recording medium, an image reading unit to read the image on the recording medium, and the conveyance device according to any one of Aspects 1 to 10.

With this configuration, the image correction such as the registration correction of the image can be performed with high accuracy based on the scanned image.

An image forming apparatus includes an image forming device to form an image on a recording medium such as the sheet P, an image reading unit to read the image on the recording medium, and a conveyance device to convey the recording medium. The conveyance device includes a conveyance roller pair such as the first conveyance roller pair55to convey the recording medium to an image reading position E of the image reading unit. The conveyance roller pair includes a drive roller such as the first drive conveyance roller55aand a driven roller such as the first driven conveyance roller55b. The driven roller contacts the drive roller and rotates following the drive roller. The image forming device forms a detection mark on the recording medium, and the detection mark is on a position away from a leading end of the recording medium by an integral multiple of a circumference of the drive roller in a conveyance direction of the recording medium.

With this configuration, similarly to Aspect 1, the distance from the leading end of the recording medium to the detection mark of the scanned image read by the image reading unit can substantially coincide with the actual distance from the leading end of the recording medium to the detection mark on the recording medium. With this configuration, the image correction such as the registration correction and the skew correction of the image formed on the recording medium can be performed with high accuracy.

In Aspect 14, the detection mark is a front detection mark on a front side of the recording medium.

With this configuration, similarly to Aspect 3, the difference between the distance from the leading end of the recording medium to the front detection mark of the scanned image and the actual distance from the leading end of the recording medium to the front detection mark on the recording medium can be reduced.

In Aspect 15, the image forming apparatus further includes another conveyance roller pair such as the second conveyance roller pair56to convey the recording medium passing through the image reading position E. Said another conveyance roller pair includes another drive roller such as the second drive conveyance roller56aand another driven roller such as the second driven conveyance roller56b. Said another driven roller contacts said another drive roller and rotates following said another drive roller. The image forming device forms a rear detection mark on a rear side of the recording medium, and the rear detection mark is on a position away from a trailing end of the recording medium by an integral multiple of a circumference of said another drive roller in the conveyance direction of the recording medium.

With this configuration, similarly to Aspect 4, the distance from the rear detection mark such as the detection marks c and d of the scanned image read by the image reading unit to the trailing end of the recording medium can substantially coincide with the actual distance from the detection mark on the recording medium to the trailing end of the recording medium. As a result, the position of the rear end of the image formed on the recording medium can be detected with high accuracy, and the image correction such as the magnification correction and the registration correction can be performed with high accuracy.