Sheet conveying apparatus and image forming apparatus

A sheet conveying apparatus has an upstream roller and a registration roller configured so that the sheet conveyed by the upstream roller abuts the registration roller to form a loop in the sheet to correct for skew. The skew of the sheet is corrected for by the leading edge of the sheet abutting the registration roller to bend the sheet so as to form the loop in the sheet. A resist sensor detects a time difference of passing time between the advanced leading edge and the retarded leading edge in the downstream leading edge of the sheet conveyed by the upstream roller, detects the skew amount of the sheet based on the time difference, and changes the backward rotation time of the registration roller depending on the skew amount.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a sheet conveying apparatus and an image forming apparatus such as a printer or photocopier including the same.

2. Description of the Related Art

In general, it is known that an image forming apparatus such as a laser beam printer or color photocopier includes a sheet conveying apparatus which conveys a sheet fed from a sheet feeding cassette or a manual feeding tray into the image forming apparatus.

In a sheet conveying apparatus, it is also known that the sheet is conveyed along a conveying path with the leading edge of the sheet in a skew feeding state slightly rotated with respect to the conveying direction due to such as a difference of the conveying speed by conveying rollers or a misalignment of the image transferring portion of the conveying rollers. If an image is formed with the sheet conveyed in a skew feeding state like this, it becomes a cause for forming a bad image such as an image formed on the sheet bent with respect to the sheet.

Therefore, conventionally, a sheet conveying apparatus has a skew correction apparatus which corrects for the skew feeding of a sheet to prevent the sheet from being conveyed in a skew feeding state. The skew correction apparatus includes a registration roller (downstream roller) for conveying the sheet to the transferring portion and an upstream roller for conveying the sheet to the registration roller. In addition, the sheet conveyed by the upstream roller abuts the registration roller and bends the sheet itself to form a loop therein, thereby correcting for the skew feeding of the sheet.

In the conventional skew correction apparatus, a loop is formed in the sheet by the leading edge of the sheet abutting a nip portion of the registration roller in a stop state (non-rotating state). Conveying the sheet to the registration roller by the upstream roller continues until the whole of the sheet leading edge abuts the nip portion of the registration roller, and the sheet itself is rotated to correct for the skew feeding of the sheet (See Japanese Patent Laid-Open Nos. 6-336353 and 6-345294).

However, in the conventional skew correction apparatus, if the rigidity of the sheet conveyed from the upstream roller is high and the leading edge of the sheet abuts the nip portion of the registration roller in a skew feeding state, the leading edge of the sheet encroaches on the nip portion of the registration roller. If the leading edge of the sheet encroaches on the nip portion of the registration roller like this (the state in which the sheet is squeezed into the registration roller), the sheet is made immovable by the registration roller.

Even if a loop is formed by bending the sheet in such a state, the sheet itself cannot rotate because it is made immovable by the registration roller, and the skew feeding of the sheet cannot be corrected for.

Therefore, conventionally, a sheet conveying apparatus, which is provided with a skew correction apparatus for preventing the sheet from encroaching on the nip portion of the registration roller by rotating the registration roller backward in a direction reverse to the rotation for conveying the sheet, has been devised (See Japanese Patent No. 4016621).

By rotating the registration roller backward, it is clearly possible to resolve the sheet-encroaching state even if the sheet has encroached on the nip portion of the registration roller, and it is also possible to prevent the sheet from encroaching on the registration roller.

However, in the sheet conveying apparatus described in the Japanese Patent No. 4016621, the registration roller is rotated backward a given amount regardless of the extent of the amount of encroachment of the sheet on the registration roller. Thus, if the registration roller is rotated backward a given amount even when the amount of encroachment of the sheet on the registration roller is small, there is a concern that damage such as “leading edge curling” or “leading edge folding” may occur in the sheet due to the backward rotation of the registration roller as illustrated inFIGS. 13A and 13B.

Especially when the surface of the registration roller is made of rubber material, in case of the leading edge of the sheet being curled or the like, the leading edge of the sheet is caught by the surface of the roller because the friction coefficient of the rubber surface is high, whereby the above-mentioned “leading edge curling” or “leading edge folding” is apt to occur.

Thus, it is desirable to provide a sheet conveying apparatus in which the above-mentioned problem is solved by changing the backward rotation time for rotating the downstream roller backward based on the skew amount of the conveyed sheet.

SUMMARY OF THE INVENTION

The sheet conveying apparatus of the present invention, in which a loop is formed in a sheet between an upstream roller and a registration roller by the registration roller abutting the sheet conveyed by the upstream roller to correct for the skew feeding of the sheet, includes a driving apparatus which rotates the registration roller forward or backward with respect to the sheet conveying direction, a detection portion which detects a skew amount of the sheet before the sheet abuts the registration roller, and a controller which determines the backward rotation amount for the downstream roller to rotate backward based on the detection results of the detection portion and controls the driving apparatus to rotate the downstream roller backward based on the backward rotation amount determined by the controller with the sheet conveyed by the upstream roller abutting the downstream roller.

According to the present invention, the backward rotation time of the registration roller which the leading edge of the sheet abuts is changed to correct for the skew feeding of the sheet, based on the detected skew amount. By this, the registration roller makes a minimum required backward rotation at the time of skew correction of the sheet, so that it is possible to resolve the state in which the leading edge of the sheet encroaches on the nip portion of the registration roller and reduce the occurrence of “leading edge curling” or “leading edge folding” of the leading edge of the sheet.

DESCRIPTION OF THE EMBODIMENTS

A digital color printer1(hereinafter referred to in brief as a “printer”), which is an example of an image forming apparatus according to an embodiment of the present invention, is formed in rectangular shape when seen from a side, as illustrated inFIG. 1. In the upper side of the printer, sheet discharging trays140and141are installed in two upper and lower stages respectively for discharging a sheet P with an image formed thereon, and in the lower side thereof, an image forming portion2for forming an image on the sheet P is installed. In the lower side of the image forming portion2, sheet feeding cassettes111and112are installed in two upper and lower stages for stacking the sheet P on which the image is to be formed. The sheets P of these sheet feeding cassettes111and112are conveyed to the image forming portion2and sheet discharging trays140and141by a sheet conveying apparatus3.

First, the image forming portion2will be described in detail. The image forming portion2installed in the printer1has laser beam scanners103ato103dof a four-drum full color type, as illustrated inFIG. 1. The image forming portion2also includes four image forming units including photosensitive drums101ato101d, charging rollers102ato102d, development devices104ato104dand cleaners107ato107d. These four image forming units form toner images of four colors of yellow (Y), magenta (M), cyan (C), and black (Bk). The photosensitive drums101ato101dare configured to rotate in an arrow direction illustrated inFIG. 1by the driving force of a driving apparatus not illustrated.

As illustrated inFIG. 1, primary transferring rollers105ato105dare arranged facing the respective photosensitive drums101ato101d. An intermediate transfer member belt106, which is supported by the respective photosensitive drums101ato101dand the primary transferring rollers105ato105d, passes between the respective photosensitive drums101ato101dand the primary transferring rollers105ato105d.

The intermediate transfer member belt106is wound on a driving roller5, a tension roller6and a secondary transferring opposite roller109b, the driving roller5rotates in the arrow direction as illustrated in the drawing, and the intermediate transfer member belt106also rotates in the same direction as the driving roller5. A fixing portion110is installed near a secondary transferring portion118.

Next, the image forming operation of the image forming portion2of the printer1configured as described above will be described. When the image forming operation starts and image signals of four colors of yellow (Y), magenta (M), cyan (C), and black (Bk) are input into the laser beam scanners103ato103d, the laser beam scanners103ato103demit laser beams.

The surfaces of the photosensitive drums101ato101dare charged with uniform electric charge beforehand by charging rollers102ato102dand the charged surfaces are irradiated with laser beams emitted by the laser beam scanners103ato103d. By the irradiation with laser beams by the laser beam scanners103ato103d, electrostatic latent images of yellow, magenta, cyan, and black are formed on the photosensitive drums101ato101d.

The electrostatic latent images formed on the photosensitive drums101ato101dare developed by yellow, magenta, cyan, and black toners to be visualized by the development devices104ato104d. The toners developed on the respective photosensitive drums101ato101dare sequentially transferred to the intermediate transfer member belt106as transfer bias is applied to the intermediate transfer member belt106from the primary transferring rollers105ato105d, so as to form a full color image on the intermediate transfer member belt106. After being transferred, the toners remaining on the photosensitive drums101ato101dare removed by the cleaners107ato107dto be prepared for the next image forming.

Meanwhile, the sheet P to which the full color image formed on the intermediate transfer member belt106is to be transferred is fed from the sheet feeding cassettes111and112or a manual feeding portion113. The uppermost sheet P is separated from the sheet bundle stacked in the sheet feeding cassettes111and112by a pickup roller150and is fed to the conveying apparatus3by conveying rollers114. Further, by feeding from the manual feeding portion113, the same uppermost sheet P is separated to be fed to the sheet conveying apparatus3.

The skew of the sheet P fed to the sheet conveying apparatus3from the sheet feeding cassettes111and112or the manual feeding portion113is corrected for by the sheet conveying apparatus3before it is synchronized with the leading edge of the image on the intermediate transfer member belt106and is conveyed to the secondary transferring portion118. The image formed on the intermediate transfer member belt106is transferred to the sheet P conveyed to the secondary transferring portion118by the secondary transfer bias applied to the secondary transferring roller109a, and the sheet P is conveyed to the fixing portion110. The sheet P is heated and pressed in the fixing portion110, so that the toner image on the sheet P is melted and mixed for the image to be fixed on the sheet.

The sheet P with the image fixed in the fixing portion110passes the conveying path25to be discharged from the discharging portion119aor119bto the sheet discharging trays140and141.

Next, the sheet conveying apparatus3will be described in detail. The sheet conveying apparatus3includes a skew correction apparatus30which corrects for the skew of the conveyed sheet P. The skew correction apparatus30includes a registration roller120as a downstream roller which conveys the sheet P to the secondary transferring portion118and an upstream roller115which conveys the sheet P to the registration roller120, as illustrated inFIG. 2. The skew correction apparatus30includes a resist motor61as a driving apparatus which rotates the registration roller120forward or backward with respect to the sheet conveying direction. Further, the sheet conveying apparatus3includes resist sensors117ato117dwhich are a detection portion arranged across the sheet conveying direction in a position more upstream than the registration roller120to detect the leading edge of the sheet P and the skew amount S of the sheet. The resist sensors117ato117d(hereinafter referred to in brief as numeral117to indicate all of the resist sensors) include optical sensors, for instance CCD (Charge Coupled Device) image sensors.

Here, the registration roller120will be described in detail. The registration roller120includes, as illustrated inFIG. 2, a lower resist roller10, both ends of which are journaled, and which can rotate forward or backward with respect to the sheet conveying direction, and an upper resist roller20, both ends of which are also journaled, and which can rotate forward or backward with respect to the sheet conveying direction.

The lower resist roller10is formed as one body with a plurality of rubber rollers10battached to a metal shaft10a. A gear71is mounted at one end of the metal shaft10a, and is connected to an output shaft61aof the resist motor through a gear72. The upper resist roller20is also formed as one body with a plurality of rubber rollers20battached to a metal shaft20a. The outer shape of the rubber roller10bitself is formed to 20Ø, and the outer shape of the roller20bitself is also formed to 20Ø like the rubber roller10b. Further, the roller20bis made of polyacetal (POM).

The upper resist roller20and the lower resist roller10are arranged facing each other so that the roller20battached to the upper resist roller20and the rubber roller10battached to the lower resist roller10contact each other. The upper resist roller20and the lower resist roller10are pressed by springs13mounted respectively on a plurality of unillustrated bearing portions that bear the upper resist roller20. Therefore, as illustrated inFIG. 2, the nip portion15is formed at the position at which the rubber roller10band the roller20bcome into contact.

Next, a controller of the printer1will be described.FIG. 3is a block diagram of the controller50which is a control portion of the sheet conveying apparatus3. An operation portion200of the printer1, the resist motor61, a preresist motor60, a resist sensor117and a sheet feeding motor54are each connected to the controller50connected to an external computer201through a network.

The controller50outputs a signal to the sheet feeding motor54when a signal is output from the operation portion200or the connected external computer201. The controller50sets a backward rotation start timing for the registration roller120to start backward rotation, and the backward rotation time for the registration roller120to rotate backward, based on the detection results by the resist sensor117. Based on these, the controller50outputs signals to the resist motor61or the like to rotate forward or backward and stop the resist motor61with respect to the sheet conveying direction, and further controls backward start timing and backward rotation time (backward rotation amount).

Next, the operation of the sheet conveying apparatus3configured as described above will be described in detail. When the start of a print job is executed by the external computer201connected by the network to the operation portion200of the printer1or the printer1, supplying (feeding) of the sheet P starts (step101ofFIG. 4, hereinafter referred to in brief as “SXXX”). When the feeding is started, the uppermost sheet is separated by the pickup roller150from the sheet bundle stacked in the sheet feeding cassettes111and112or the manual feeding portion113to be fed to the sheet conveying apparatus3(S102).

The sheet P fed to the sheet conveying apparatus3is conveyed at a given sheet conveying speed (so-called process speed), and after being conveyed to the upstream roller115, is conveyed to the registration roller120by the upstream roller115(S103).

The leading edge downstream in the sheet conveying direction of the sheet P (hereinafter referred to in brief as a “downstream leading edge”) conveyed by the upstream roller115is detected by the resist sensor117. As the leading edge of the sheet P is detected by each of the resist sensors117ato117d, the skew amount S of the conveyed sheet is detected (S104).

Here, the skew amount S refers, as illustrated inFIG. 7, to a tilt angle (amount) of a corner portion Pb of the sheet leading edge on the side being retarded (hereinafter, referred to in brief as a “retarded corner portion”) with respect to a corner portion Pa of the sheet leading edge on the side being advanced (hereinafter, referred to in brief as an “advanced corner portion”), in the conveying direction in the downstream leading edge of the sheet P.

Next, the phenomenon of encroachment of the sheet P on the registration roller120will be briefly described. The encroachment phenomenon refers, as illustrated inFIG. 5A, to the sheet P being conveyed toward the registration roller120by the upstream roller115entering the nip portion15of the registration roller120. X2is a position of a center line of the registration roller120, X3is a position at which the sheet has not encroached on the registration roller120but has stopped when the leading edge of the sheet P abuts, and X1is a position at which the leading edge of the sheet P has encroached on the registration roller120and stopped. The distance from the X1to X3becomes the encroachment amount Lb.

FIG. 5Bis a view showing the state in which the leading edge of the sheet P has encroached on the nip portion15of the registration roller120. The greater the skew amount of the sheet P being conveyed is, the more easily the encroachment phenomenon occurs; the encroachment amount Lb depends greatly on the skew amount S. Like the graph illustrated inFIG. 6, the encroachment amount Lb of the sheet P into the nip portion15of the registration roller is increased almost in proportion to the skew amount S.

Next, detection of the skew amount S of the conveyed sheet P will be described in detail. Each of the resist sensors117ato117ddetects the leading edge of the sheet P conveyed in the direction of arrow A from the upstream roller115in a skew feeding state. In the downstream leading edge of the sheet P, a time difference of passing time between both leading edges is obtained from a passing time of the leading edge P1on the side advanced in the conveying direction detected by the resist sensor117dand a passing time of the leading edge P2on the side retarded in the conveying direction detected by the resist sensor117a. A delay amount s1is calculated by multiplying the time difference by the conveying speed of the sheet P conveyed.

As illustrated inFIG. 7, provided that the distance (interval) from the position at which the resist sensor117ddetects the advanced leading edge P1of the sheet P to the position at which the resist sensor117adetects the retarded leading edge P2of the sheet P is L1, the skew amount S can be calculated (defined) from the skew amount s1by the equation below.
S=atan(s1/L1)  (Equation 1)

When a sheet with the width narrower than the distance (interval) L1between the detection position of the resist sensor117aand the detection position of the resist sensor117dis conveyed, the resist sensor117cand the resist sensor117bdetect the leading edge of the sheet.

In the downstream leading edge of the sheet P, a time difference of passing time between both leading edges is obtained from a passing time of the advanced leading edge P3in the conveying direction detected by the resist sensor117cand a passing time of the retarded leading edge P4in the conveying direction detected by the resist sensor117b. A delay amount s2is calculated by multiplying the time difference by the conveying speed of the conveyed sheet P.

As illustrated inFIG. 7, if the distance (interval) from the position at which the resist sensor117cdetects the advanced leading edge P3of the sheet P to the position at which the resist sensor117bdetects the retarded leading edge P4of the sheet P is L2, the skew amount S can be calculated (defined) from the skew amount s2by the following equation.
S=atan(s2/L2)  (Equation 2)

The controller50rotates the registration roller120backward to prevent the sheet P from encroaching on the nip portion15of the registration roller. The controller50sets the backward rotation time for the registration roller120to rotate backward from the skew amount S of the sheet P detected by the resist sensor117(S105).

Next, the changing of the backward rotation time of the registration roller120will be described in detail. As described above, the encroachment amount Lb tends to be proportional to the skew amount S of the sheet P; the greater the skew amount S of the sheet P is, the more the encroachment amount of the sheet P on the nip portion15of the registration roller becomes. Therefore, the greater the skew amount S of the sheet P is, the longer the backward rotation time of the registration roller120(the greater the backward rotation amount) is set, with the leading edge of the sheet contacting the registration roller120. On the other hand, the smaller the skew amount S of the sheet P is, the shorter the backward rotation time of the registration roller120(the smaller the backward rotation amount) is set. The encroachment amount G of the sheet for calculating the backward rotation time (backward rotation amount) of the registration roller120can be calculated (defined) by the following equation using the skew amount S and a proportional constant a (S105).
G=a·S(Equation 3)

The backward rotation time of the registration roller120is set by the controller50to correspond to the encroachment amount G. In addition, the backward rotation timing for starting the backward rotation of the registration roller120is set to correspond to the backward rotation time of the registration roller120(S105).

Next, the backward rotation operation of the registration roller120will be described in detail. After the leading edge of the conveyed sheet P is detected by the resist sensor117and the leading edge of the sheet has passed the resist sensor117, the registration roller120starts rotation in the direction opposite to the conveying direction of the sheet P by the backward rotation of the resist motor61.

At this time, if the registration roller120stops backward rotation before skew correction of the sheet P is completed, there is a concern that the sheet P will encroach on the nip portion15of the registration roller again, and the loop R will need to be formed to correct for the skew of the sheet P. In such a case, a sufficient effect of skew correction cannot be obtained. Therefore, setting is performed such that the backward rotation of the registration roller120continues until the retarded corner portion Pb of the downstream leading edge of the skewed sheet P abuts the nip portion15of the registration roller (S105). That is, with the whole region of the downstream leading edge of the sheet abutting the nip portion of the registration roller, the controller50stops the backward rotation of the registration roller120once the skew of the sheet is corrected for.

The start time of the backward rotation, which is the start timing of the backward rotation of the registration roller120, is set based on the time at which the retarded corner portion Pb of the downstream leading edge of the sheet P abuts the nip portion15of the registration roller, when the skew correction of the sheet P is completed. For this, the time t_G2when the retarded corner portion Pb of the downstream leading edge of the sheet P abuts the nip portion15of the registration roller is calculated.

Next, calculation of the time t_G2will be described in detail. Time t_a is the time at which the leading edge123of the sheet P reaches the sheet leading edge detection position of the resist sensor117a, as illustrated inFIG. 8. As for the conveying direction of the sheet, the distance from the sheet leading edge detection position of the resist sensor117to the nip portion15of the registration roller is referred to as L3. As for the width direction of the sheet conveying direction, the distance from the leading edge123of the sheet P to the leading edge122of the retarded corner portion Pb of the sheet P that abuts the position121of the widthwise outer end portion of the roller20barranged outermost is referred to as L4. Further, the conveying speed of the sheet v and t_G2from the skew amount S is calculated by the following equation.
t—G2=t—a+{(L3+L4)tan(S)}/v(Equation 4)

The time t_G2at which the leading edge122of the retarded corner portion Pb of the sheet P abuts the nip portion15of the registration roller, and the start time of the backward rotation of the registration roller120from the backward rotation time that is changed based on the skew amount S are determined. Based on these, the backward rotation start time is set such that the smaller the detected skew amount S is, the shorter the backward rotation time becomes (the smaller the backward rotation amount becomes), and the backward rotation stop of the registration roller120becomes later than t_G2(S105).

FIG. 9is an example of the timing chart for the operation of the registration roller that is set such that the rotation drive of the registration roller stops later than t_G2, the time at which the leading edge122of the retarded corner portion Pb of the sheet P abuts the nip portion15of the registration roller. InFIG. 9, t_G1is the time at which the advanced corner portion Pa of the sheet P abuts the nip portion15of the registration roller120. As illustrated inFIG. 9, the backward rotation of the registration roller120is started after time t_G1has passed. That is, the registration roller120rotates backward in a state in which the leading edge of the sheet abuts the registration roller120. In addition, the backward rotation of the registration roller120is stopped after time t_G2has passed. Unlike the example inFIG. 9, the stop of the backward rotation of the registration roller120becomes later than the time t_G2, even when the backward rotation of the registration roller120starts earlier than the time t_G1from the time t_G2and the backward rotation time of the registration roller120.

When the backward rotation time and the start time of the backward rotation for the registration roller120are set by the controller50, the skew correction of the sheet P starts (S106), and the registration roller120starts backward rotation at the set start time of the backward rotation (S107).

As illustrated inFIGS. 10A and 11A, the sheet P conveyed to the registration roller120continues to be conveyed in the arrow A direction by the upstream roller115even after the leading edge of the sheet P abuts the nip portion15of the registration roller. By the force pushing out the sheet P ahead by the conveying of the upstream roller115, the sheet P is bent so as to form a loop, as illustrated inFIGS. 10B and 11B.

By the force pushing out the leading edge of the sheet P by the loop R formed like this, the retarded corner portion Pb of the sheet P is pushed out toward the registration roller120. As the retarded corner portion Pb of the sheet P is pushed out toward the registration roller120, the whole of the leading edge downstream of the sheet conveying direction rotates to correct for the skew of the sheet.

After the skew of the sheet P is corrected, the registration roller120is rotated forward again by the resist motor61to convey the sheet P to the secondary transferring portion118, as illustrated inFIGS. 10C and 11C. The sheet P that has the image transferred on the sheet P in the secondary transferring portion118is discharged out of the printer1after the image is fixed in the fixing portion110on the sheet (S109), and printing by the printer1is completed (S110).

As described above, the skew amount S of the sheet is detected, and based on the detected skew amount S, the backward rotation time of the registration roller120is changed to rotate backward until the retarded corner portion Pb of the downstream leading edge of the sheet P abuts the nip portion15of the registration roller. As the registration roller120is thus rotated backward at the backward rotation time reduced depending on the skew amount S of the sheet to suppress the damage such as “leading edge curling” or “leading edge folding” being generated on the sheet P, it is possible to resolve the state in which the leading edge of the sheet P encroaches on the nip portion15of the registration roller. By rotating the registration roller backward, it is also possible to prevent the leading edge of the sheet P from encroaching on the nip portion15of the registration roller.

Further, it is also possible to shorten the total skew correction time of the sheet P since the interval between the forward rotation and the backward rotation is shortened by setting the backward rotation time of the registration roller120to the minimum. It is also possible to increase the printing speed of the printer1, because the conveying efficiency of the sheet conveying apparatus is improved.

In the present embodiment, four resist sensors117which detect the sheet leading edge downstream of the sheet conveying direction were arranged across the sheet conveying direction, but only two resist sensors117band117cthat can detect the skew amount S of the sheet P of minimum width may be included.

In addition, the controller50included in the sheet conveying apparatus may be configured so as to have the backward rotation time changed such that the higher the rigidity Z of the sheet P conveyed to the registration roller120is, the longer the backward rotation time of the registration roller120becomes.

Here, the proportional constant a of Equation 3 is calculated from the following equation by using a given proportional constant b in a function of the rigidity Z of the sheet P.
a=b·Z(Equation 5)

By using the calculated a, the backward rotation time is changed depending on the rigidity Z of the sheet P from the skew amount S.

As described above, change is performed such that the higher the rigidity Z of the conveyed sheet P is, the longer the backward rotation time of the registration roller120becomes. Thereby, if a sheet P of high rigidity is conveyed to the registration roller120as the rotary driving force of the resist motor61is transmitted to the gears71and72, as illustrated inFIG. 2, the encroachment amount becomes greater as the registration roller120is rotated by the backlash of the gears71and72. Like this, even if the sheet P of high rigidity, which makes the encroachment amount to the registration roller120greater, is conveyed, it is possible to resolve the encroachment of the sheet P on the registration roller with a high degree of precision.

The resist sensor that detects the leading edge of the conveyed sheet P may also be configured to be installed across the CIS (Contact Image Sensor), an optical line sensor, at right angles with the sheet conveying direction, as illustrated inFIGS. 12A and 12B.

Here, the CIS will be described briefly. The CIS is one of optical sensors of light and compact construction, in which modules including a light source, a light system, and light amount detection system are made as one body.

The resist sensor116including the CIS detects the leading edge plane downstream in the conveying direction of the sheet P being conveyed to determine the backward rotation start timing of the registration roller120and also continually detects the change of the lateral cross section along the conveying direction to detect the skew amount S.

As for the sheet skew amount S, as illustrated inFIG. 12, the CIS detects the retarded corner portion Pb of the downstream leading edge of the sheet P and also detects the side edge along the conveying direction of the sheet P. When the advanced corner portion Pa of the sheet P abuts the nip portion15of the registration roller, the side edge P5along the conveying direction of the sheet P is detected. From the side edge P5along the conveying direction of the sheet P, a travel amount d that the side edge has moved along the conveying direction of the sheet P in the sheet width direction crossing the sheet conveying direction at right angles from the position of the detected retarded corner portion Pb is detected.

As illustrated inFIG. 12, if the travel amount is d and the distance (interval) from the detection position of the sheet P of the resist sensor116to the nip portion15of the registration roller is Lc, the skew amount S is calculated by the following equation.
S=atan(d/Lc)  (Equation 6)

As described above, the skew amount S of the sheet P can be detected properly by detecting the lengthwise side edge of the sheet P by the CIS. By using a relatively low-priced CIS for the resist sensor116that detects the leading edge of the sheet P, the manufacturing cost can be reduced.

Note that, the embodiment of the sheet conveying apparatus according to the present invention is described as a color digital printer, which is one of image forming apparatuses, as an example, but the present invention is not limited thereto, and can obviously be applied to a printer of an ink jet type as well.

This application claims the benefit of Japanese Patent Application No. 2011-195286, filed Sep. 7, 2011, which is hereby incorporated by reference herein in its entirety.