JP2007004137A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To accurately detect the length of a sheet on which an image is formed. <P>SOLUTION: The image forming apparatus is configured so that a first detection position 3 and a second detection position 4 are set between adjacent conveyance rollers 5 and 6, and has two sensors SN1 and SN2 and two sensors SN3 and SN4 which are disposed in the first detection position 3 and in the second detection position at the interval of N. Four sensors SN1 to SN4 are arranged in such a way, and the actual length of transfer material can be detected with high accuracy by performing (1) cancellation of an error in detection caused by variation in a speed at which the transfer material is conveyed, (2) cancellation of an error in detection caused by a skew, and (3) cancellation of an error in detection caused by oblique passing according to a passing signal obtained from the sensors. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an image forming apparatus that forms an image on a transfer material, such as an electrophotographic printer, a copying machine, and a printing machine.

  The image forming apparatus includes a plurality of systems such as an electrophotographic system, an offset printing system, and an ink jet system. Here, a conventional image forming apparatus using an electrophotographic system will be described as an example.

  Color image forming apparatuses are mainly classified into a tandem method in which a plurality of image forming units are arranged side by side and a rotary method in which a plurality of image forming units are arranged in a cylindrical shape. Further, the transfer method is classified into a direct transfer method in which a toner image is directly transferred from a photosensitive member to a sheet material, and an intermediate transfer method in which the toner image is once transferred to an intermediate transfer member and then transferred to a sheet material.

  FIG. 9 is a cross-sectional view of an intermediate transfer tandem type image forming apparatus in which four color image forming units are arranged side by side on an intermediate transfer belt. The intermediate transfer method does not need to hold the transfer material on the transfer drum or the transfer belt as in the direct transfer method, and thus can cope with a wide variety of transfer materials such as ultra-thick paper and coated paper. Therefore, it is suitable for realizing high productivity due to the features of parallel processing in a plurality of image forming units and batch transfer of full-color images.

  The operation of the image forming apparatus will be described below with reference to FIG.

  The transfer material S is stored on the lift-up device 52 included in the paper feeding device 51 and is fed by the paper feeding unit 53 in accordance with the image forming timing of the image forming device 50. Here, the paper feeding means 53 includes a system using frictional separation by a paper feed roller or the like, a system using air separation adsorption, and the like. In FIG. The transfer sheet S sent out by the paper feeding means 53 passes through a transport path 54 a of the transport unit 54 and is transported to the registration unit 55. After the skew correction and timing correction are performed in the registration unit 55, they are sent to the secondary transfer section. The secondary transfer portion is a toner image transfer nip portion to the transfer material S formed by the substantially opposite secondary transfer inner roller 503 and secondary transfer outer roller 56, and has a predetermined pressure and electrostatic load bias. Is applied, the unfixed image is adsorbed on the transfer paper.

  With respect to the transfer process of the transfer material S to the secondary transfer portion described above, an image forming process that is sent to the secondary transfer portion at the same timing will be described.

  The image forming unit 513 mainly includes a photoconductor 508, an exposure device 511, a developing device 510, a primary transfer device 507, a photoconductor cleaner 509, and the like. An exposure device 511 emits light based on a signal of image information sent to the photosensitive member 508 whose surface is uniformly charged in advance by the charging unit and rotates in the direction of arrow A in the figure, and the diffraction unit 512 and the like. A latent image is formed through the above. In this way, the developing device 510 performs toner development on the electrostatic latent image formed on the photoconductor 508 to form a toner image on the photoconductor. Thereafter, a predetermined pressure and an electrostatic load bias are applied by the primary transfer device 507, and a toner image is transferred onto the intermediate transfer belt 506. Thereafter, the transfer residual toner slightly remaining on the photoconductor 508 is collected by the photoconductor cleaner 509 and prepared for the next image formation again. In the case of FIG. 9, the image forming unit 513 described above includes four sets of yellow (Y), magenta (M), cyan (C), and black (Bk).

  Next, the intermediate transfer belt 506 will be described. The intermediate transfer belt 506 is stretched by rollers such as a driving roller 504, a tension roller 505, and a secondary transfer inner roller 503, and is conveyed and driven in the direction of arrow B in the drawing. Therefore, the image forming process of each color processed in parallel by the above-described Y, M, C, and Bk image forming apparatuses 513 is performed at the timing of superimposing on the upstream toner image primarily transferred onto the intermediate transfer belt. As a result, a full-color toner image is finally formed on the intermediate transfer belt 506 and conveyed to the secondary transfer unit.

  As described above, the full color toner image is secondarily transferred onto the transfer material S in the secondary transfer portion by the transfer process and the image forming process of the transfer material S described above. Thereafter, the transfer material S is conveyed to the fixing device 58 by the pre-fixing conveyance unit 57. The fixing device 58 melts and fixes the toner on the transfer material S by applying a predetermined pressurizing force by a substantially opposing roller or belt and generally a heating effect by a heat source such as a heater. The transfer material S having the fixed image thus obtained is discharged as it is onto the paper discharge tray 500 by the branch conveyance device 59, or is conveyed to the reverse conveyance device 501 when double-sided image formation is required. Route selection is performed.

  A transport operation when double-sided image formation is required will be described. The transfer material S sent to the reversing and conveying apparatus 501 is switched back and then conveyed to the double-sided conveying apparatus 502 by performing a switchback (reversing) operation. Thereafter, the transfer unit 54 joins from the re-feed path 54b of the transport unit 54 at the same timing as the transfer material of the subsequent job transported from the paper feeder 51 and is similarly sent to the secondary transfer unit. The image forming process on the back surface (second surface) is the same as that on the front surface (first surface), and is therefore omitted.

  Here, in the image forming apparatus 50, a switchback method is used to invert the transfer material. However, since the method for inverting the transfer material is generally an easy configuration and advantageous in terms of space, In many cases, the switchback method is employed. However, when transferring the front and back images, there is a known defect that the reference of the transfer direction of the transfer material, that is, the front end and the rear end are switched. As described above, the image forming apparatus configured as shown in FIG. 9 has high productivity and media compatibility, and in recent years, for example, light printing as represented by print-on-demand has been considered. Often put. In such a case, the required image printing accuracy becomes strict, and the registration unit 55 also has a configuration excellent in skew feeding correction capability such as a skew feeding roller method in most cases. In such a situation, the reference of the transfer material conveyance direction is different between the front and back surfaces, which is very large in achieving image printing accuracy, particularly the leading edge margin deviation in the sub-scanning direction (transfer material conveyance direction). It will be harmful. This is because there is a small variation in the dimensions of the transfer material cut in advance. That is, as described above, if the toner image on the intermediate transfer belt 506 and the timing of the transfer material are simply matched, the leading edge margin is also equal to the variation in the size of the transfer material as long as the transfer is performed from the opposite direction on the front and back sides. Shift. As a result, in the cutting or folding process, an image may be lost, or a blank space may be introduced.

  With respect to these problems, in the prior art, various proposals have been made to recognize the same reference when transferring the front and back images. For example, there is one in which a dot pattern that is not conspicuous is added to a transfer material, and the amount of deviation is detected and corrected by counting this (for example, see Patent Document 1). However, since an originally unnecessary pattern is formed on the transfer material, unnecessary toner consumption is caused, and in some cases, the pattern itself may become a claim.

  Therefore, the method of detecting an edge, which is the reference of the transfer material itself, has become the mainstream. For example, the transfer material is provided with a detecting means for detecting the trailing edge of the transfer material (the leading edge that was the reference at the time of surface transfer) in the process of refeeding the leading and trailing edges, and based on the detection signal, There is one that calculates the tip position and image formation timing (for example, see Patent Document 2).

  Also, a detecting means for detecting the leading and trailing edges of the transfer material is provided, and the trailing edge margin is calculated from the relationship between the trailing edge of the transfer material detected during the surface transfer and the image position information. As a result, when the front end of the transfer material (the rear end detected at the front side) is detected during the back side transfer, the back image position is set based on the value of the rear end margin (see, for example, a patent). Reference 3).

  However, in practice, it is difficult to satisfy a sufficient quality as a printed product only by matching the leading margin between the front and back images. This is because, as in the image forming apparatus 50 described with reference to FIG. 9, the transfer material toward the transfer of the back image has the toner image already transferred to the front surface fixed thereon by the fixing device 58. This is because it contracts in the sub-scanning direction. The main scanning direction is the width direction of the transfer material orthogonal to the transfer material conveyance direction, and the sub-scanning direction is the transfer material conveyance direction.

Such shrinkage after passing through the fixing device 58 varies depending on the gap direction of the fibers of the transfer material and the evaporation rate of the contained water amount. Therefore, in order to precisely match the image position accuracy between the front and back sides, it is necessary to provide means for matching the leading edge margins and matching the magnifications of the front and back images. As described above, there is a technique that pays attention to the change in magnification of the front and back images and matches the both (see, for example, Patent Document 4 and Patent Document 5).
JP-A-10-190975 JP 2003-35974 A JP-A-11-237768 JP 2002-338084 A JP2003-241610A

  In general, when the image position accuracy of the printed product is strictly required as in the printing market, the deviation between the front and back images in the sub-scanning direction needs to be about ± 0.5 to 1.0 mm. Factors that cause these deviations are mainly mechanical tolerances and variations caused by the transfer material. The former can be suppressed to a certain extent by devising the number and accuracy of the intervening mechanical parts, but the latter is difficult to suppress directly. In other words, the superiority or inferiority of the printing accuracy capability of the image forming apparatus depends on how to suppress variations due to the transfer material.

  Therefore, various proposals for predicting the length of a transfer material by using a detection unit that detects the leading and trailing ends of the transfer material as in the conventional art have been made. However, in view of achieving the required accuracy of about ± 0.5 to 1.0 mm as described above, the detection / prediction accuracy of such a transfer material length must be at least ± 0.3 mm or less. Judgment is difficult to put into practical use. ± 0.3 mm is an approximate value of the expansion / contraction variation of the transfer material under the same conditions (type, image, environment, etc.). Therefore, if the detection / prediction accuracy of the transfer material length is below this, it is rather more troublesome for the operator to measure the image misalignment between the front and back sides of the output sample and select a method for entering a uniform correction value. However, excellent image position accuracy can be obtained.

  From this point of view, all of the above-described conventional techniques have many error factors in the detection and prediction of the transfer material length, which is insufficient to achieve strict image position accuracy that can withstand the printing market. is there. This is because, when viewed strictly, the transfer material to be conveyed is skewed (transfer material is conveyed in an oblique posture) and skew between the conveyance rollers (transfer due to the difference in peripheral speed between the left and right conveyance rollers). The material is slightly tilted). In addition, since the conveyance rollers and the like have an initial outer diameter variation and endurance change / variation due to wear, there is a difference in conveyance speed among a plurality of rollers conveying the transfer material. As described above, the signal of the detection means includes a large error, and the expected transfer material length is greatly deviated from the actual length.

  In addition, the detection means in the conventional technique can detect the passage timing of the front and rear ends, but cannot detect the influence of skew, skew, conveyance speed difference, and the like. In addition, although it is described that the transfer material length and the shift amount of the image formation timing are calculated based on the detection signal, the transfer material transport speed is a method calculated on the assumption that it is an ideal speed. Even in this step, an error that may greatly affect the prediction accuracy is included.

  Therefore, in the present invention, focusing on the point that high-precision image position accuracy cannot be obtained simply by providing detection means for detecting the leading and trailing edges of the transfer material, in order to solve the above-mentioned problems, the arrangement configuration of the detection means and the error factors The purpose is to propose how to cancel.

  The present invention is an image forming apparatus for forming an image on a transfer material by an image forming unit, the first transport unit and the second transport unit sequentially disposed along the transport direction of the transfer material, and the first transport unit. Two first A sensors and a first B sensor arranged along a width direction perpendicular to the transfer material conveyance direction at a first detection position set between the first detection sensor and the second conveyance means, and the first detection At the second detection position set downstream of the position, two second A sensors and second B sensors arranged along the width direction orthogonal to the transfer material conveyance direction, and the transfer detected by the four sensors An arithmetic processing unit that calculates the length of the transfer material by correcting the length detection error in the transfer material conveyance direction caused by the posture of the transferred transfer material and the change in posture based on the detection signal of the leading and trailing edges of the material And by the arithmetic processing unit And control means for adjusting the image forming position to the transfer material in the image forming unit based on the length information in the conveying direction of the resulting transfer material, characterized in that it comprises a.

  According to the present invention, the following effects can be obtained.

  According to the present invention, a four-sensor posture of a transfer material represented by skew and a transfer material conveyance direction length detection error due to a change in posture (so-called skew) is canceled. The length can be determined. As a result, it is possible to accurately form an image on the transfer material on the basis of highly accurate transfer material length detection information.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

(Embodiment 1)
FIG. 1 shows a cross-sectional view of an image forming apparatus according to Embodiment 1 of the present invention. 1 is the same in basic configuration and operation as the image forming apparatus in FIG. 9 already described, and common portions will be described using the same reference numerals. The image forming apparatus 1 in FIG. 1 is an intermediate transfer tandem color image forming apparatus in which four sets of Y, M, C, and Bk image forming units 513 are arranged on an intermediate transfer belt 506. The image forming apparatus 1 is an image forming apparatus capable of forming images on both the front and back surfaces of the transfer material S.

  The image forming apparatus 1 includes a secondary transfer unit (secondary transfer inner part) that connects a leading end of a toner image formed by sequentially superimposing four colors on the intermediate transfer belt 506 and a leading end of a transfer material S conveyed by a sheet feeding unit 53. A means for matching at the transfer nip between the roller 503 and the secondary transfer outer roller 56 is provided. Specifically, the pattern detection unit 2 is provided at a position facing the intermediate transfer belt 506, and the image leading end pattern formed on the intermediate transfer belt 506 is read by the pattern detection unit 2. The image leading edge pattern is a mark image provided at the leading edge of the toner image that is actually transferred, and serves as a reference when matching the leading edge of the transfer material S. Accordingly, it is determined how long the toner image on the intermediate transfer belt 506 will reach the secondary transfer portion.

  On the other hand, the transfer material S is transported from the precursor paper feeding means 53 to the resist device 55 via the transport unit 54, and the secondary transfer portion is transferred by the sensor 8 included in the resist device 55 in the remaining time. Is reached. As described above, by controlling the image formation timing or the change in the conveyance speed of the registration roller 7 based on the determination results of both, it is possible to match the image and the leading edge of the transfer material S at a desired position. In the image forming apparatus 1 shown in FIG. 1, it is assumed that the leading edge is aligned by a method of controlling the conveyance speed of the registration roller 7. Note that the error of the sensor 8 is reduced when the skew of the transfer material is corrected and the position is closer to the secondary transfer position (for example, downstream of the registration roller 7 shown in FIG. 1).

  The registration device 55 can be of various methods. In the image forming apparatus shown in FIG. 1, a method of performing skew correction by using a skew feeding roller and an abutting reference member is taken as an example. 3A to 3D are top views of the oblique feeding type registration apparatus. The registration device 55 mainly includes a movable guide 30, a fixed guide 33, and a registration roller 7. The movable guide 30 is movable in the main scanning direction (the width direction of the transfer material orthogonal to the transfer material conveyance direction) according to the size of the transfer material S, and includes the abutting reference member 31 and a plurality of skew feeding rollers. 32. The skew feeding roller 32 is inclined by an angle α with respect to the sub-scanning direction (transfer material transport direction), and is set so as to obtain an abutting conveyance component with respect to the abutting reference member 31.

  The fixed guide does not move regardless of the size of the transfer material S, and functions as a conveyance guide for the transfer material S. As shown in FIG. 3A, it is assumed that the transfer material S has entered the registration device 55 with the skew angle β. The transfer material S sent to the skew feeding roller 32 by the transport roller 34 is transported obliquely toward the abutting reference member 31 as shown in FIG. Note that when the transfer material S starts to be conveyed by the skew feeding roller 32, the nip of the conveying roller 34 is released.

  Thereafter, as shown in FIG. 3C, the transfer roller S is conveyed to the downstream registration roller 7 while urging the end surface of the transfer material S against the abutting reference member 31. When conveyance is started by the registration roller 7, the skew feeding roller 32 releases the nip, and the registration roller 7 moves in the main scanning direction while nipping and conveying the transfer material S as shown in FIG. The center position of the image on the transfer material S and the intermediate transfer belt is adjusted.

  After that, the registration roller 7 that has transferred the transfer material S to the secondary transfer roller releases the nip and moves again in the main scanning direction to return to the next job standby state. Here, in FIG. 3D, the registration roller 7 performs a reciprocating operation in the main scanning direction. This is because the abutting reference member 31 is offset so that the transfer material S does not collide with the abutting reference member 31 in consideration of variations in the position of the transferred transfer material S in the main scanning direction. This is because the position is set.

  By using the skew feeding type registration device 55 described above, the transfer material S reversed (hereinafter referred to as switchback reversal) by the switchback method is the same side of the transfer material S in both single-sided and double-sided image formation. The end face can be abutted against the abutting reference member 31. Therefore, it is possible to achieve high accuracy with respect to the front and back image position accuracy in the main scanning direction. On the other hand, regarding the front and back image position accuracy in the sub-scanning direction, the leading and trailing ends of the transfer material S are switched by the switchback reversal of the reversing and conveying device 501, so that the reference in the conveying direction of the transfer material changes, which is disadvantageous in accuracy. Become. Note that the first surface of the transfer material on which the first image is formed is expressed as the front surface, the second surface on the back of the first surface is expressed as the back surface, and the image position accuracy on the front and back is the first image and the second image. Is the degree of accuracy with which the position is formed at the same position with respect to the transfer material.

  For this purpose, the image forming apparatus 1 shown in FIG. 1 is configured such that the transfer material S is placed in a section between the conveyance roller (first conveyance unit) 5 and the conveyance roller (second conveyance unit) 6 provided in the double-side conveyance path 502. A first detection position 3 and a second detection position 4 are provided which are sensors for detecting the position. Thereby, the actual length of the transfer material S conveyed at the time of double-sided refeeding can be detected with higher accuracy.

  If there is information on the actual length of the transfer material S, it is theoretically possible to achieve high front / back image position accuracy, and therefore the actual length detection accuracy must be increased. However, in order to increase the accuracy of detection of the actual length, it becomes impossible to ignore the influence of slight skew, skew, transport speed variation, etc. that occur during transport. Note that the skew means that the transfer material is conveyed in an oblique posture, and the skew means that the posture of the transfer material is changed obliquely due to a difference in peripheral speed between the left and right conveying rollers. Therefore, in the first embodiment, a sensor arrangement as shown in FIG. 2 is adopted in order to consider and cancel these influences, and the length detection error in the transfer material conveyance direction caused by the posture of the transfer material and the posture change is corrected. Thus, high-precision detection of the actual length of the transfer material is realized.

  FIG. 2 is a top view of a part of the double-sided conveyance path, in which the transfer material S is re-feeded and conveyed on both sides in the direction of the arrow. A first detection position 3 and a second detection position 4 are between the adjacent conveyance rollers 5 and 6. The first detection position 3 and the second detection position are substantially symmetrical with respect to the conveyance center reference (reference with the conveyance reference position of the transfer material as the center) at an interval N (first A sensor) SN1 and (first B sensor) SN2. , (Second A sensor) SN3 and (second B sensor) SN4.

The first detection position 3 is disposed at a distance a downstream from the transport roller 5, and the second detection position 4 is disposed at a distance b upstream from the transport roller 6. The interval between the detection position 3 and the second detection position 4 is m. In this way, four sensors SN1 to SN4 are arranged, and through signals obtained from these sensors,
(1) Detection error cancellation due to conveyance speed variation (2) Detection error cancellation due to skew (3) By canceling detection error due to skew feeding, the actual length of the transfer material can be detected with high accuracy.

  A specific canceling method will be described below.

(1) Detection error cancellation due to variation in transport speed Since the distance between the first detection position 3 and the second detection position 4 is known as m, the time when the leading edge and the trailing edge of the transfer material S pass through the distance m, respectively. If it can be detected, the actual conveyance speed of the transfer material S can be calculated in real time. Without acceleration / deceleration control, the theoretical transport speed should be known, but in reality the transport speed varies due to various factors such as the tolerance of the transport roller diameter and the difference in wear over time. Here, the conveyance roller and conveyance guide for conveying the transfer material S are also arranged upstream and downstream of the interval m, and the conveyance roller and conveyance involved in conveyance when passing the interval m. Strictly speaking, the guide differs between the front end and the rear end. In particular, in the case where there is a conveyance guide having a curvature upstream or downstream of the distance m, the conditions such as resistance that the transfer material S receives during conveyance are different. In addition, since the front and rear ends of the transfer material S are easily affected by an error due to curling or the like, the front and rear ends are not necessarily the same even when passing through the same interval m. Accordingly, the actual length L of the transfer material S is calculated at an actually measured speed obtained from time 1 when either the leading edge or the trailing edge passes through the interval m. In many cases, it contains many errors. In contrast, in this embodiment, the time when each of the leading end and the trailing end passes through the interval m is obtained, and the error is canceled by using the average value of these times.

  For example, from the signal that the tip of the transfer material S reaches the sensor SN1 to SN3,

Thus, the conveyance speed (in this case, the conveyance speed on the back side) is obtained. Here, t ₁ and t ₃ are SN1 and SN3 ON signals.

  Similarly, from the signal that the rear end of the transfer material S reaches the sensors SN1 to SN3,

Thus, the conveyance speed (in this case, the conveyance speed on the back side) is obtained. Here, t ′ ₁ and t ′ ₃ are SN1 and SN3 OFF signals.

  And it is the average value of these two transport speeds,

3 as the transport speed (in this case, the back side) when passing through the interval m, the accuracy of predicting the actual length of the transfer material S is greatly improved.

(2) Detection error cancellation due to skew error A slight difference in conveyance speed occurs between the front side and the rear side in the main scanning direction due to the balance of the applied pressure not only between different conveyance rollers but also between the same conveyance rollers. Accordingly, when the actual length of the transfer material S is calculated from only the sensor signal on either the front side or the back side, the transfer material S is calculated to be excessive or small due to the influence of the skew generated by the difference in the conveyance speed. There is a fear. On the other hand, in the configuration in which sensors are provided on the near side (SN2 and SN4) and the far side (SN1 and SN3) substantially symmetrically with respect to the transport center reference as shown in FIG. 2, transport is performed from the near side and the far side state. By obtaining the state at the position of the center reference, the influence of the skew can be averaged.

The transport speeds V _F1 , V _F2 , and V _FAvg on the near side can be calculated in the same manner as the transport speeds V _R1 , V _R2 , and V _RAvg on the far side already described in (1), and the transport center is obtained by averaging each. A conveyance speed component V _{CAvg at the} reference position is obtained.

  That is,

It becomes.

In order to obtain the actual length L of the transfer material S, a passing signal from the _front end to the rear end of the transfer material S may be _{known in} addition to the V _CAvg . In the case of FIG. 2, a passing signal at the first detection position 3 and the second detection position 4 can be considered. Again, as already described in (1), strictly speaking, the transport roller and the transport guide involved in transport when passing through the first detection position 3 and the second detection position 4 are strictly described. Different. In particular, when there is a conveyance guide or the like having a curvature upstream or downstream of the first detection position 3 and the second detection position 4, conditions such as resistance that the transfer material S receives during conveyance are different. The time required to pass from the end to the rear end is not necessarily the same. Therefore, by averaging the passing signals at the first detection position 3 and the second detection position 4, errors and variations caused by the difference in conditions as described above are canceled as much as possible. Further, in order to cancel the influence of the skew which is the original purpose of (2), the back side (the sensors SN1 and SN3) and the near side (the sensors SN2 and SN4) are averaged as in the case of the conveyance speed. ing. That is, the transit time T at the transport center reference position is

It is predicted. Here, t ₁ , t ₂ , t ₃ and t ₄ are ON signals of the sensors SN1, SN2, SN3 and SN4, and t ′ ₁ , t ′ ₂ , t ′ ₃ and t ′ ₄ are the sensors SN1, It is an OFF signal of SN2, SN3, SN4. From t ₂ or more, the detection length L ′ at the transport center reference position is
L ′ = V _CAvg T
As obtained.

(3) Detection Error Cancellation Caused by Skew While the detection length L ′ at the transport center reference position of the transfer material S has been described above, the actual transfer material S has a skew angle θ as shown in FIG. It is being conveyed by. Therefore, strictly speaking, the detection length L ′ obtained in (2) is a length detected obliquely with respect to the length of the transfer material S in the sub-scanning direction, and includes an error for skew. ing. On the other hand, in the configuration in which sensors are arranged on the near side and the far side substantially symmetrically with respect to the transport center reference as shown in FIG. 2, the skew angle θ can be calculated from the difference between the detection times of the two.

For example, the case of using a detection signal in the first detection position 3, the tip passes the time position of the tip passage time t ₁ and the conveying central reference of the sensor SN1

Time difference and main scanning direction distance

From the ratio of

Is obtained.

Now, as shown in FIG. 2, between the actual length L of the transfer material S and the detected length L ′ detected in (2),
L = L'cosθ
Thus, by substituting tan θ or θ already obtained, the actual length L of the transfer material S in which the skew error has been corrected can be obtained with high accuracy.

  Here, only the front skew angle θ of the transfer material S at the first detection position 3 has been described. However, since the rear end skew angle at the first detection position 3 and the front end and rear end skew angles at the second detection position 4 are obtained in the configuration of FIG. Correction processing using an average value or a weighted average value is also possible. Alternatively, it may be a method having a table for correcting a skew error determined in advance from a difference between sensor passing times on the near side and the far side.

  By processing the calculations described in (1), (2), and (3) above by the arithmetic processing unit 9 included in the image forming apparatus 1, the actual length L of the transfer material S with high detection accuracy in which various errors are canceled. Is obtained.

  As shown in FIG. 4, when the front end of the transfer material S toward the transfer of the back surface image is detected by the sensor 8, if the actual length L of the transfer material S is known in advance, the rear end position (that is, the front surface) The standard for image transfer) can be determined. If the rear end position can be determined, the front end margin w of the front surface image and the image length G when transferred from the intermediate transfer belt 506 are known values. The leading edge margin (image position) to be controlled at the time of transfer is known. In this way, since the actual length L of the transfer material S to be re-fed on both sides can be detected in the transport process to the registration device 55, the variable timing of the transport speed of the registration roller 7 can be determined based on the information. It can be determined and controlled according to the toner image on the intermediate transfer belt. As a result, high front and back image position accuracy can be realized not only in the main scanning direction but also in the sub scanning direction. Note that control such as variable timing of the conveyance speed of the registration rollers 7 is performed by the control means C.

Further, in FIG. 2, the distance m between the first detection position 3 and the second detection position 4 with respect to the roller diameter d of the conveyance roller 5 and the conveyance roller 6,
m = Nπd (N is an integer)
By doing so, the detection accuracy can be further improved. In other words, the first detection position 3 and the second detection position 4 can match the phase of the speed fluctuation caused by the eccentricity or shake of the conveyance roller 5 and the conveyance roller 6. Therefore, an effect of preventing a range error within the speed fluctuation period from being included in the actual length L of the transfer material S can be obtained.

  That is, even when the outer diameters of the transport roller 5 and the transport roller 6 are decentered or shaken, the speed fluctuations when the leading and trailing ends of the transfer material reach the first detection position and the second detection position are synchronized. Can be made. As a result, it is possible to cancel the speed fluctuation error caused by the eccentricity or the shake and improve the transfer material length detection accuracy.

  Further, as shown in FIG. 2, by providing a restraining roller 20 between the first detection position 3 and the second detection position 4, it is possible to prevent the transfer material S from violating in the gap of the conveyance guide, and the sensor. Detection of SN1 to SN4 can be stabilized. In FIG. 2, the distances a and b are small, and the effect of suppressing the nip between the transport roller 5 and the transport roller 6 can be sufficiently obtained. Therefore, the roller is suppressed only between the first detection position 3 and the second detection position 4. 20 was provided. However, when the distances a and b are relatively large, it is more effective to provide them immediately before and after the sensors SN1 to SN4. In FIG. 2, the holding roller 20 is used. However, the shape is not particularly limited as long as it is a pressing member such as a guide that is brought into contact with the transfer material S to prevent the rampage. By providing the pressing member in this way, it is possible to obtain an effect of preventing the detection accuracy of the sensors SN1 to SN4 from being affected even if curling or undulation occurs.

  Further, the process described so far does not include the correction of expansion / contraction of the transfer material S accompanying the passage of the fixing device 58, but of course the front / back image position accuracy is greatly improved by considering the rate of change of the transfer material size accompanying expansion / contraction. To do. For example, if the table value of the size change rate corresponding to the type of transfer material, environment, type of image, etc. is given to the image forming apparatus 1 in advance, the magnification correction is performed based on information input and set by the operator from the operation unit or the like. The quantity is automatically referenced and determined.

  The magnification correction amount obtained in this way is considered not only for the size of the back surface image but also for the values of the leading edge margin w and the image length G of the front surface image described in FIG. It is possible to transfer a back image having an appropriate size. Therefore, it is possible to cope with a change in size due to the expansion and contraction of the transfer material S, and it is possible to provide an image forming apparatus with excellent front and back image position accuracy.

(Embodiment 2)
FIG. 5 shows a cross-sectional view of an image forming apparatus according to Embodiment 2 of the present invention. FIG. 5 is a cross-sectional view of a monochrome image forming apparatus. Although the image forming process is slightly different, the basic configuration and operation are the same as those of the color image forming apparatus already described in FIG. 1 or FIG. Hereinafter, common parts will be described using the same reference numerals.

  The image forming apparatus 60 develops the electrostatic latent image on the photoconductor 508 formed by the exposure device 511 and the diffraction unit 512 by the developing device 510 and then transfers the image onto the transfer material S by the transfer device 61. As already described with reference to FIG. 5, the transfer material S is subjected to skew correction by the registration device 55 from the paper feeding device 51 through the conveyance path 54 a included in the conveyance unit 54, and then is transferred to the transfer device 61 by the transfer device 61. The timing coincides with the toner image on the photoconductor 508. In the first embodiment, the transfer material S and the image are aligned by controlling the conveyance speed of the registration roller 7. However, since the image forming apparatus 60 in FIG. 5 has a shorter distance from exposure to transfer than the image forming apparatus 1 in FIG. 1, sub-scanning can be performed by using a signal from the sensor 62 or the like in the transport path as an image generation timing. It is possible to control the image position in the direction. After the toner image is transferred onto the transfer material S by the transfer device 61, in the case of double-sided printing, it enters the reverse conveying device 501 through the fixing device 58, and the front and rear ends are switched by the switchback reverse operation to replace the double-sided conveying device. It is conveyed to 502.

  FIG. 7 shows a top view of the registration device 55 in the image forming apparatus 60. The registration device 55 shown in FIG. 7 employs a system in which the skew is removed by abutting the transfer material S against the nip portion of the registration roller 7 stopped by the conveyance roller 81 and forming a loop. Here, the registration device 55 has a line sensor 80 in the main scanning direction, and can detect not only the passing timing of the transfer material S but also the positional deviation in the main scanning direction. Therefore, if the image forming timing in the main scanning direction is controlled based on the detection result of the line sensor 80, the image position can be adjusted with high accuracy in both the main scanning and the sub scanning.

  It should be noted that the conveyance roller portions 82F and 82R of the conveyance roller 81 are independently controlled by separate drive motors (not shown) or the like according to the front / back difference of the transfer material S passing timing detected by the line sensor 80. Thus, an active method for correcting the skew of the transfer material S may be used. In this case, it is not necessary to stop the transfer material S once against the registration roller 7, so that productivity can be improved. Of course, there is no problem with the skew feeding roller type registration apparatus described in the first embodiment, and in this case, a larger amount of skew feeding can be dealt with.

  Image position adjustment and skew correction can be realized by the image generation timing and the registration device as described above, but strictly speaking, considering that the leading and trailing ends (reference) of the transfer material S are switched by switchback inversion. Otherwise, the accuracy is disadvantageous. In order to compensate for this, it is necessary to further correct the image writing in the sub-scanning direction based on the sensor 62 for image generation timing. Therefore, in the image forming apparatus 60 shown in FIG. 5, the first detection position 3 and the second detection made up of sensors for detecting the position of the transfer material S in the section of the arbitrary conveyance roller 5 and the conveyance roller 6 constituting the double-side conveyance path 502. Position 4 is provided. Thereby, the actual length L of the transfer material S conveyed at the time of double-sided refeeding can be accurately detected.

Hereinafter, a method for detecting the actual length L of the transfer material with high accuracy will be described with reference to FIG. The configuration in FIG. 6 is basically the same as the configuration already described with reference to FIG. In addition, the same code | symbol shall be used regarding the same structure. FIG. 6 is a top view of a part of the double-sided conveyance path, and the transfer material S is re-fed and conveyed on both sides in the direction of the arrow in the drawing. The first detection position 3 and the first detection position 3 are arranged at substantially the same position in the conveyance direction of the transfer material as the substantially nip position of the conveyance roller (first conveyance unit) 5 and the conveyance roller (second conveyance unit) 6 sequentially arranged in the conveyance direction. 2 detection positions 4 are arranged. Each of the first detection position 3 and the second detection position is substantially symmetrical with respect to the conveyance center reference at intervals N of the sensors SN1 (first A sensor) and SN2 (first B sensor), SN3 (second A sensor) and SN4 (first 2B sensor). With these configurations, as in the first embodiment,
(1) Detection error cancellation due to conveyance speed variation (2) Detection error cancellation due to skew (3) Detection error due to skew feeding can be canceled, and high front and back image position accuracy can be realized in the sub-scanning direction. In the present embodiment, a cancellation method different from that in the first embodiment will be specifically described.

(1) Detection error cancellation due to variation in conveyance speed Generally, as shown in FIG. 2, the distance (a + m + b) between the conveyance roller 5 and the conveyance roller 6 is set smaller than the minimum size in the specifications of the image forming apparatus 1. Has been. In the transfer process of the transfer material S, the time when the transfer roller 5 is mainly transferred, the time when the transfer roller 6 is mainly transferred, and the transfer roller 5 and the transfer roller 6 are both nipped. Then, there are three times of carrying. Without acceleration / deceleration control, the conveyance speeds in these three times should theoretically be equal, but in reality, there are variations in the conveyance speed due to various factors such as tolerances of the diameter of the conveyance rollers and differences in wear over time. Therefore, it goes without saying that the ideal speed is used as a substitute, and if the length of the transfer material S is calculated only by the actual speed obtained from a single conveying roller, many errors are included. On the other hand, in the configuration in which two detection positions are provided in the sub-scanning direction as shown in FIG. 2, the above-described three time components can be extracted.

  For example, from the signal that the tip of the transfer material S reaches the sensor SN1 to SN3,

As described above, the conveyance speed of the conveyance roller 5 (in this case, the conveyance speed on the back side) is obtained using only the time during which the conveyance roller 5 is mainly used. Here, t ₁ and t ₃ are SN1 and SN3 ON signals.

  Similarly, from the signal that the rear end of the transfer material S reaches the sensors SN1 to SN3,

As described above, the conveyance speed of the conveyance roller 6 (in this case, the conveyance speed on the back side) can be obtained using only the time when the conveyance roller 6 is mainly used. Here, t ′ ₁ and t ′ ₃ are SN1 and SN3 OFF signals.

Further, from the signal that the leading edge of the transfer material S has reached the sensor SN3 and the signal that the trailing edge has reached the sensor SN1, the speed V _{R1 + 2} (in this case, the back side) Transport speed) is

Is obtained by substituting the values of V _R1 and V _R2 described above into the relationship. In this equation, the conveyance speed from the time when the leading edge of the transfer material S reaches the sensor SN3 to the time when the trailing edge reaches the sensor SN1 is decomposed into V _R1 , V _R2 , and VR 1 _{+ 2} components. 6. The weighted average is performed at a ratio determined by each distance between the first detection position 3 and the second detection position 4. Note that L _ideal is the ideal size of the transfer material S (420 mm for A3), and is used because it is necessary to calculate the ratio when the actual length L is unknown.

As described above, the general arrangement has been described based on FIG. 2, but the case of FIG. 6 is also considered as a derivative system of FIG. 2, and since a = b = 0, the above V _{R1 + 2} is

It becomes.

  As described above, by obtaining the three velocity components, the conveyance state of the transfer material S can be traced in detail, so that the prediction accuracy is greatly improved.

(2) Detection error cancellation due to skew error A slight difference in conveyance speed occurs between the front side and the rear side in the main scanning direction due to the balance of the applied pressure not only between different conveyance rollers but also between the same conveyance rollers. Accordingly, when the actual length L of the transfer material S is calculated from only the sensor signal on either the front side or the back side, it is detected as being excessively or too small due to the influence of the skew generated by the difference in the conveyance speed. End up. On the other hand, in the configuration in which sensors are provided on the near side (SN2 and SN4) and the far side (SN1 and SN3) substantially symmetrically with respect to the reference that is being conveyed as shown in FIG. By obtaining the state at the position of the transport center reference, the influence of skew can be averaged.

The transport speeds V _F1 , V _F2 , and V _{F1 + 2} on the near side can be calculated in the same manner as the transport speeds V _R1 , V _R2 , and V _{R1 + 2} on the back side already described in (1). The conveyance speed components V _C1 , V _C2 , and V _{C1 + 2 at the} reference position are obtained.

  That is,

It becomes. In order to obtain the actual length L of the transfer material S with high accuracy, it is only necessary to know the passing signals at the front and rear ends of the transfer material S at the first detection position 3 or the second detection position 4. Here, a case where a passing signal at the first detection position 3 is used is considered. At this time, considering the ratio of each conveyance roller involved in conveyance of the transfer material S, in general, the average conveyance speed V _{C at the} conveyance center reference position is as shown in FIG.

It is predicted.

  In the case of FIG. 6, it can be considered as a derivative system of FIG. 2, and a = b = 0 is substituted.

Is expected.

Further, from the average value of the detection signals t ′ ₁ -t ₁ and t ′ ₂ -t ₂ by the sensors SN1 and SN2, the passing time T at the transport center reference position is:

It is predicted.

From the above, the detection length L ′ ₁ at the transport center reference position is
L ′ ₁ = V _C T
As obtained.

In addition, according to the same theory, the detection length L ′ ₂ when the passing signal at the second detection position 4 is used is also obtained.
L ′ = (L ′ ₁ + L ′ ₂ ) / 2
As a result, the detection length L ′ with higher accuracy can be obtained.

(3) Detection Error Cancellation Caused by Skew While the detection length L ′ at the transport center reference position of the transfer material S has been described above, the actual transfer material S has a skew angle θ as shown in FIG. It is being conveyed by. Therefore, strictly speaking, the detection length L ′ obtained in (2) is a length detected obliquely with respect to the length of the transfer material S in the sub-scanning direction, and includes an error for skew. ing. On the other hand, in the configuration in which sensors are arranged on the near side and the far side substantially symmetrically with respect to the transport center reference as shown in FIG. 6, the skew angle θ can be calculated from the difference between the detection times of the two.

For example, the case of using a detection signal in the first detection position 3, the tip passes the time position of the tip passage time t ₁ and the conveying central reference of the sensor SN1

Time difference and main scanning direction distance

From the ratio of

Is obtained.

Now, as shown in FIG. 2, between the actual length L of the transfer material S and the detected length L ′ detected in (2),
L = L'cosθ
Thus, by substituting tan θ or θ already obtained, the actual length L of the transfer material S in which the error in the skew feeding is corrected can be obtained.

  Here, only the front skew angle θ of the transfer material S at the first detection position 3 has been described. However, in the configuration of FIG. 6, the rear end skew angle at the first detection position 3 and the front end and rear end skew angles at the second detection position 4 are obtained, so that an average value of the respective skew angles is obtained as necessary. Alternatively, correction processing using a weighted average value is also possible. Alternatively, it may be a method having a table for correcting a skew error determined in advance from a difference between sensor passing times on the near side and the far side.

  The above-described calculations (1), (2), and (3) are processed by the arithmetic processing unit 9 included in the image forming apparatus 60, so that the actual length L of the transfer material S with high detection accuracy in which various errors are canceled. Is obtained. As shown in FIG. 4, when the front end of the transfer material S toward the transfer of the back surface image is detected by the sensor 62, if the actual length L of the transfer material S is known in advance, the rear end position (that is, the front surface) The standard for image transfer) can be determined.

  If the trailing edge position can be determined, the leading edge margin w and the image length G of the front surface image are known values. Therefore, the trailing edge margin w ′ of the front surface image, that is, the leading edge margin (image to be controlled) when transferring the back surface image. Position). As described above, if the actual length L of the transfer material S to be re-fed on both sides can be detected before the back side image is written, the image writing timing by the exposure device 511 is matched with the margin w ′ based on the information. Can make decisions and control. As a result, high front and back image position accuracy can be realized not only in the main scanning direction but also in the sub scanning direction.

  Further, in FIG. 6, the first detection position 3 and the second detection position 4 are set to be substantially nip positions of the conveyance roller 5 and the conveyance roller 6, respectively, so that the transfer material S is reliably detected at the detection portions of the sensors SN1 to SN4. Can be held between. As a result, the detection posture is stabilized without being affected by flapping or curling of the transfer material S in the gap of the conveyance guide. Further, since it is not necessary to separately provide the pressing roller 20 as described in the first embodiment, advantages such as simplification of the configuration and cost reduction can be obtained.

Further, in FIG. 6, the distance m between the first detection position 3 and the second detection position 4 with respect to the roller diameter d of the conveyance roller 5 and the conveyance roller 6 (that is, the distance between the conveyance roller 5 and the conveyance roller 6). The
m = Nπd (N is an integer)
By doing so, the detection accuracy can be further improved. That is, the phase of the speed fluctuation caused by the eccentricity or the shake of the transport roller 5 and the transport roller 6 can be made to coincide at the first detection position 3 and the second detection position 4, and the range within the speed fluctuation cycle can be obtained. An effect of preventing the error from being included in the actual length L of the transfer material S can be obtained.

  Further, the process described so far does not include the correction of expansion / contraction of the transfer material S accompanying the passage of the fixing device 58, but of course the front / back image position accuracy is greatly improved by considering the rate of change of the transfer material size accompanying expansion / contraction. To do. For example, if the table value of the size change rate corresponding to the type of transfer material, environment, type of image, etc. is given to the image forming apparatus 1 in advance, the magnification correction is performed based on information input and set by the operator from the operation unit or the like. The quantity is automatically referenced and determined.

  The magnification correction amount obtained in this way is considered not only for the size of the back surface image but also for the values of the leading edge margin w and the image length G of the front surface image described in FIG. It is possible to transfer a back image having an appropriate size. Therefore, it is possible to cope with a change in size due to the expansion and contraction of the transfer material S, and it is possible to provide an image forming apparatus with excellent front and back image position accuracy.

(Embodiment 3)
FIG. 8 shows a cross-sectional view of an image forming apparatus according to Embodiment 3 of the present invention. FIG. 8 has the same basic configuration and operation as the image forming apparatus in FIGS. 1 and 5 already described, and common portions will be described using the same reference numerals. An image forming apparatus 90 in FIG. 8 is an intermediate transfer tandem type color image forming apparatus in which four sets of Y, M, C, and Bk image forming portions 513 are arranged on an intermediate transfer belt 506.

  In the image forming apparatus 90 in FIG. 8, the transfer material S fed from the paper feeding device 51 joins in the middle of the double-sided transport path 502 from the joining path 91 and is transported to the registration device 55 via the transport unit 54. Here, as in the first embodiment (see FIG. 3), the registration device 55 adopts a system in which skew feeding correction is performed by the skew feeding roller 32 and the abutting reference member 31. Further, the image forming apparatus 90 can match the transfer material S and the leading end of the image at a desired position by the same method as in the first embodiment (see FIG. 1). Since fixing, reverse conveyance (switchback reversal) and double-sided conveyance after secondary transfer have already been described with reference to FIGS. 1 and 5, they are omitted here.

  As described above, by including the skew feeding type registration device 55, the transfer material S that has been switched back by the reversing conveyance device 501 has the same end surface of the transfer material S in both one-side and both-side image formation. It can abut against the abutting reference member 31. Therefore, high accuracy can be realized with respect to the front and back image position accuracy in the main scanning direction. On the other hand, the front and back image position accuracy in the sub-scanning direction is disadvantageous in terms of accuracy because the front and rear ends (reference) of the transfer material S are switched by the switchback reversal of the reversing conveyance device 501. In order to compensate for this, the image forming apparatus 90 shown in FIG. 8 has means for detecting the actual length of the transfer material S on the double-sided conveyance path 502 with the same configuration as in FIG. 2 or FIG. Then, the actual length of the transfer material S is detected by any of the arithmetic processing methods described in the first embodiment or the second embodiment.

  In the image forming apparatus 90 shown in FIG. 8, the position where the merge path 91 merges with the double-sided conveyance path 502 is set on the upstream side of the conveyance roller 5. As a result, the actual length L can be detected not only for the transfer material S toward double-sided refeeding but also for the transfer material S fed from the paper feeding device 51. As a result, in the first and second embodiments, it is possible to automatically perform correction (magnification correction) of the size change rate of the transfer material S that has been uniformly corrected based on information input by the operator from the operation unit or the like. It becomes. As an effect obtained by this, not only the operator's work load is reduced, but also the expansion / contraction correction for each sheet can cancel the expansion / contraction variation, which was impossible with the uniform correction. The expansion / contraction correction will be described in detail below.

First, the transfer material S toward the transfer of the paper feed surface image from the sheet feeding device 51 passes through the first detection position 3 and the second detection position 4, the original real length L ₁ is detected Is done. With said actual length L ₁ information is stored in the memory unit, by means for counting the order in which they are conveyed to the paper feeding device 51 wherein the order two-sided conveyance path 502 to be fed from the actual length L It is possible to specify which transfer material the information of ₁ is.

As a result, possible relative comparison between the actual length L ₂ to be detected when passing through the switchback the transfer material S toward the transfer of the post-inversion on the back image again first detection position 3 and the second detection position 4 It becomes. Generally, it said actual length L ₂ by water content variation or the like accompanying the passage of the fixing device 58, changes from the original real length L _1. By inputting the expansion ratio and the actual length L ₂ information of the obtained transfer material S in advance in the image forming unit 513 and the resist 55 from here, to match the front and back image position, including the magnification change Can do.

Specifically, when the leading edge of the transfer material S toward the transfer of the back side image is detected by the sensor 8, the rear end position is indexed from the actual length L ₂ information as shown in FIG. Then, from the value obtained by adding the above-mentioned magnification change to the value of the leading edge margin w and the image length G of the front surface image, the trailing edge margin w ′ of the front surface image, that is, the leading edge margin (image to be controlled) when transferring the back surface image. Position). Therefore, the variable timing of the conveyance speed of the registration roller 7 is determined.

  On the other hand, the back image itself is exposed and developed with a size that takes into account the value of the magnification change inputted in advance, and if it is secondarily transferred at the position of the margin w ', the front and back image positions can be printed appropriately. According to the configuration of the present embodiment, since the above image alignment can be applied to each of the transfer materials passing through the first detection position 3 and the second detection position 4, the front and back image position accuracy It is possible to provide an excellent image forming apparatus.

  Although the intermediate transfer tandem color image forming apparatus is used in the third embodiment, the present invention is not limited to this. For example, also in the monochrome image forming apparatus shown in FIG. 5 of the second embodiment, the joining path 91 from the sheet feeding device is similarly set in the middle of the duplex conveying path 502 and upstream of the conveying roller 5. A monochrome image forming apparatus with high front / back image position accuracy including magnification correction can be obtained. In this case, the configuration required for detecting the actual length of the transfer material S may be the already described FIG. 2 or FIG. 6, and the registration apparatus may be the method of FIG. 3 or FIG.

  In the third embodiment, the merge path 91 merges in the middle of the double-sided conveyance path. However, the present invention is not limited to this, and both the transfer material S toward the transfer of the front surface image and the back surface image are present. The detection means of the present invention may be provided in the transport path that passes.

  The present invention is not limited to the registration means described in the above embodiment. For example, the transfer material is temporarily stopped by a registration roller, and the registration roller is adjusted in accordance with the position of the image on the image carrier. May be used to feed the transfer material.

Sectional drawing of the image forming apparatus explaining Embodiment 1 of this invention The top view explaining the sensor arrangement configuration in Embodiment 1 of the present invention The top view explaining the registration apparatus in Embodiment 1 of this invention The figure explaining the image alignment of this invention Sectional drawing of the image forming apparatus explaining Embodiment 2 of this invention Top view for explaining the sensor arrangement in the second embodiment of the present invention Top view for explaining a resist device according to Embodiment 2 of the present invention. Sectional drawing of the image forming apparatus explaining Embodiment 3 of this invention Sectional drawing explaining the conventional image forming apparatus

Explanation of symbols

2 Pattern detection means 3 1st detection position 4 2nd detection position 5 and 6 Conveyance roller 7 Registration roller 9 Arithmetic processing part 20 Stopping roller a Distance between conveyance roller 5-1st detection position b Between 2nd detection position-conveyance roller 6 Distance d Outer diameter of transport roller m Distance between first detection position and second detection position N Distance between sensors in main scanning direction SN1, SN2, SN3, SN4 Sensor 30 Movable guide 31 Abutting reference member 32 Inclined feed roller 33 Fixed type Guide 51 Paper feeding device 54 Transport device 55 Registration device 56 Secondary transfer outer roller 58 Fixing device 501 Reverse transport device 502 Double-sided transport device 506 Intermediate transfer belt 508 Photoconductor 511 Exposure device 513 Image forming unit 80 Line sensors 82F and 82R Part 91 Junction pass

Claims (12)

An image forming apparatus for forming an image on a transfer material by an image forming unit,
A first transport unit and a second transport unit sequentially disposed along the transport direction of the transfer material;
Two first A sensors and a first B sensor disposed along a direction orthogonal to the transfer direction of the transfer material at a first detection position set between the first transfer unit and the second transfer unit;
Two second A sensors and a second B sensor arranged along a direction perpendicular to the transfer material transport direction at a second detection position set downstream of the first detection position;
Based on the detection signals of the leading and trailing edges of the transfer material detected by the four sensors, the transfer material is corrected by correcting the length detection error in the transfer material conveyance direction caused by the posture of the transferred transfer material and the posture change. An arithmetic processing unit for calculating the length of
Control means for adjusting the image forming position on the transfer material in the image forming unit based on the length information in the conveyance direction of the transfer material obtained by the arithmetic processing unit;
An image forming apparatus comprising:   The arithmetic processing unit uses a transfer material conveyance speed used for calculating the length of the transfer material, a conveyance speed for conveying the transfer material by the first conveyance unit, and a conveyance for conveying the transfer material by the second conveyance unit. The first transport unit, the first detection position, and the second detection position are respectively decomposed into speeds and transport speeds for simultaneously transporting the transfer material by the first transport unit and the second transport unit. 2. The image forming apparatus according to claim 1, wherein calculation is performed by weighted averaging based on a ratio determined by each interval in the conveyance direction of the transfer material of the second conveyance unit.   The first A sensor and the second A sensor are arranged on both sides with the transfer center reference of the transfer material as a boundary, and the first B sensor and the second B sensor are arranged on both sides with the transfer center reference of the transfer material as a boundary, The arithmetic processing unit is based on a detection signal at the leading and trailing ends of the transfer material detected by the first A sensor and the second A sensor, and is one of the width directions orthogonal to the transfer material transport direction with the transfer material transport center reference as a boundary. The transfer speed of the transfer material on the other side is calculated, and from the detection signal of the front and rear ends of the transfer material detected by the sensor 1B sensor and the second B sensor, Calculate the transport speed, calculate the average transport speed of the transfer material at the transport center reference position by averaging the two transport speeds, and calculate the length of the transfer material based on the average transport speed The feature The image forming apparatus according to claim 1 or 2.   The first transport unit and the second transport unit each have transport rollers having the same diameter, and an interval along the transfer material transport direction between the first detection position and the second detection position is a circumferential length of the transport roller. 4. The image forming apparatus according to claim 1, wherein the image forming apparatus is substantially an integral multiple of.   5. A pressing member for pressing the transfer material when the transfer material is conveyed is provided between the first detection position and the second detection position. The image forming apparatus described.   The position of the first detection position in the sheet conveyance direction substantially coincides with the position of the first conveyance unit, and the position of the second detection position in the sheet conveyance direction substantially coincides with the position of the second conveyance unit, respectively. The image forming apparatus according to claim 1, wherein the image forming apparatus is arranged.   The image forming unit includes a double-sided conveying unit for reversing the transfer material on which the image is formed on the first surface, replacing the leading and trailing ends, feeding the image to the image forming unit again, and forming an image on the second surface, A transfer material in which the first and second transport units and the four sensors are arranged in a duplex transport path of a duplex transport unit, and the arithmetic processing unit passes through the duplex transport unit based on detection information from each sensor. The image forming apparatus according to claim 1, wherein the length of the image forming apparatus is calculated.   A detection sensor for detecting the passage of the transfer material downstream from the second detection position, and a detection signal for the transfer material re-feeded by the double-sided conveyance unit for image formation on the second surface by the detection sensor. And the control means performs transfer so that the image forming positions of the image on the first surface and the image formed on the second surface coincide with each other based on the length information in the conveyance direction of the transfer material calculated by the arithmetic processing unit. The image forming apparatus according to claim 7, wherein an image is formed on the material.   An image carrier that carries a toner image transferred to a transfer material by the image forming unit; and a pattern detection unit that detects an image pattern on the image carrier; a registration roller on the upstream side of the image forming unit; A registration sensor that detects the passage of the transfer material, image position information on the image carrier obtained from the pattern detection means, a passage signal of the transfer material obtained from the registration sensor, and the arithmetic processing unit. Based on the obtained length information in the conveyance direction of the transfer material, the control means conveys the registration roller so that the image formation position formed on the second surface matches the image on the first surface formed on the transfer material. The image forming apparatus according to claim 7, wherein the speed is controlled.   An image carrier that carries a toner image to be transferred onto the transfer material by the image forming unit, a fixing device that fixes the toner image transferred onto the transfer material by the image carrier, and the fixing device Setting means for setting the rate of change of the size of the subsequent transfer material for each type of transfer material, and the control means uses the length information of the transfer material conveyed by the double-sided conveyance means and the setting means. 10. The image forming operation in the image forming unit is controlled based on the set change rate so that the magnifications of the first image and the second image coincide with each other. The image forming apparatus according to claim 1.   A sheet feeding device for supplying a transfer material to the image forming unit and a transfer material on which an image is formed on the first surface by the image forming unit are reversed and the leading and trailing ends are switched, and the image forming unit is again fed to the image forming unit. A double-sided conveyance unit for forming an image on the second side, and the double-sided conveyance path of the double-sided conveyance unit is merged with the conveyance path between the sheet feeding device and the image forming unit, The first and second transport units and the four sensors are arranged in a transport path between the image forming unit and the arithmetic processing unit is sent out from the paper feeding device based on detection information from each sensor. 11. The image forming apparatus according to claim 1, wherein a length in a conveyance direction of the transfer material to be conveyed and the transfer material conveyed through the double-sided conveyance path is calculated.   The arithmetic processing unit calculates the length in the conveyance direction of the transfer material sent out from the paper feeding device, and further, the transfer material has an image formed by the image forming unit and is conveyed by the duplex conveying unit. The control means detects the change in magnification from the length of the transfer material calculated before and after the image formation, and based on the change in magnification, The image forming apparatus according to claim 11, wherein an image forming operation in the image forming unit is controlled so that magnifications of images on the second side are matched.

Images