JP2008216737A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus which is capable of preventing continuation of wasteful printing with an insufficient position precision by precisely detecting positional deviation between images on front and rear sides when forming double-side images. <P>SOLUTION: A reading means for reading positions of images formed on a front side and a rear side of a sheet is provided on a paper ejection path, and positions of detection patterns for detecting the positions of images formed on the sheet are read by the reading means to calculate the amount of positional deviation between images formed on the front side and the rear side of the sheet, and an error is determined in the case that the calculated amount of positional deviation between images exceeds a prescribed value. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an image forming apparatus that forms images on both sides.

  In recent years, an electrophotographic image forming apparatus has been used in the field of light printing such as POD (Print On Demand), and a high quality is required for image quality and image position accuracy.

  Particularly in the field of light printing, in double-sided printing in which image formation is performed on both sides of a sheet, strict standards are set for the alignment of the front and back images. In such a case, since the printed material that does not satisfy the standard for the positional accuracy of the front and back surfaces needs to be defective, it is necessary to check the quality of the output printed material. The printed matter that has become a defective product needs to be discarded, and it is necessary to perform printing again. Accordingly, there is a problem that resources and labor are wasted.

  As a means for detecting a printed matter that is a defective product, in Patent Document 1, a sheet misalignment amount is detected by a reading means in which a plurality of reading pixels are arranged, and the detected sheet misalignment amount is larger than a predetermined threshold value. Is disclosed as an error sheet.

Further, as a means for aligning the front and back images, in the image forming apparatus disclosed in Patent Document 2, a reading means for reading the image on the first surface is arranged in the recording material conveyance path, and the reading means reads the image. The image formation position at the time of image formation on the second surface (back surface) is corrected based on the image information.
JP 2003-327345 A JP 2000-305324 A

  However, the image forming apparatus disclosed in Patent Document 1 detects the positional deviation of the paper by detecting the edge of the paper, and does not detect the position of the image formed on the paper. Further, because of such a configuration, the detection accuracy depends on the variation in the edge shape of the paper. Further, since it is determined whether or not an error sheet is generated by detecting the positional deviation of the paper for each side, it is necessary to determine whether the front and back images are aligned with high accuracy during double-sided image formation. Is not enough.

  Further, in the image forming apparatus disclosed in Patent Document 2, a positional deviation at the time of forming the second surface image is predicted and the prospective correction is performed based on the information obtained by reading the position of the image formed on the first surface of the recording material. Therefore, it is necessary to consider the case where the prospect correction is not properly performed. Further, paper expansion / contraction caused by thermal fixing or the like that occurs after image formation on the first surface is not taken into consideration.

  SUMMARY OF THE INVENTION In view of the above problems, the present invention provides an image forming apparatus capable of accurately detecting a positional deviation between front and back images during double-sided image formation and preventing continuous useless printing with insufficient positional accuracy. It is intended.

  The above object is achieved by the invention described below.

(1) An image forming apparatus having image forming means for forming images on both sides of a sheet based on image data,
A reading unit that reads the positions of detection patterns on the front and back surfaces formed on the sheet by the image forming unit, and a first reading unit that reads the front side of the sheet and a second reading unit that reads the back side of the sheet Reading means comprising:
Control for calculating the position deviation amount of the front and back surfaces of the image formed on the sheet by reading the position of the detection pattern with the reading means, and determining that an error occurs when the calculated image position deviation amount exceeds a predetermined value Means,
An image forming apparatus comprising:

  (2) The control unit calculates a positional deviation amount between the front surface and the back surface of the image formed on the sheet by reading the position of the sheet end portion and the detection pattern formed on the sheet by the reading unit. The image forming apparatus according to (1).

  (3) The image forming apparatus according to (1) or (2), wherein the reading unit is arranged in a sheet discharge path for discharging sheets formed with images on both sides to the outside of the apparatus.

(4) An image forming apparatus having image forming means for forming images on both sides of a sheet based on image data,
A reversing path for reversing the front and back of the sheet imaged on both sides;
A reading means for reading the positions of the detection patterns on the front and back surfaces formed on the sheet by the image forming means, wherein the other sheet surface is read after one sheet surface is read and the sheet is reversed on the reverse path. Reading means having one reading unit for reading
When the position of the detection pattern and the edge of the sheet are read by the reading unit to calculate the amount of positional deviation of the front and back surfaces of the image formed on the sheet, and the calculated amount of positional deviation of the image exceeds a predetermined value, Control means for determining an error;
An image forming apparatus comprising:

(5) The reading unit includes an exposure unit that exposes the sheet from the same side as the reading unit, and a back exposure unit that exposes the sheet from the side opposite to the reading unit,
(4) The control unit may switch between exposure by the exposure unit and exposure by the back exposure unit based on the amount information of the sheet when reading the detection pattern. Image forming apparatus.

  (6) The image forming apparatus according to any one of (1) to (5), wherein the reading unit is a contact image sensor.

(7) has warning means,
The image forming apparatus according to any one of (1) to (6), wherein the control unit performs warning by the warning unit or cancellation of image formation when it is determined as an error.

(8) The image forming apparatus includes a plurality of paper discharge trays for discharging the image-formed sheet,
(1) to (7) wherein the control unit discharges the error sheet determined to have exceeded a predetermined value and the sheet not determined to be in error to different discharge trays. ).

  (9) In any one of (1) to (8), when the control unit determines that an error has occurred, the control unit forms an image again based on the image data formed on the error sheet. Image forming apparatus.

  According to the present invention, it is possible to obtain an image forming apparatus capable of accurately detecting the positional deviation of the front and back images during double-sided image formation and preventing continuing unnecessary printing with insufficient positional accuracy. It becomes.

  Although the present invention will be described based on an embodiment, the present invention is not limited to the embodiment.

  FIG. 1 is a central cross-sectional view of the image forming apparatus according to the present embodiment. The image forming apparatus A is called a tandem color image forming apparatus, and includes an image forming unit A1, a scanner unit 1, an operation display unit 2, and an automatic document feeder D.

  The image forming unit A1 includes an image writing unit 3, a plurality of sets of image creating units 4Y (yellow), 4M (magenta), 4C (cyan), 4K (black), a belt-like intermediate transfer member 42, a paper feed cassette 5, A sheet feeding unit 6, a sheet discharge unit 7, a duplex conveyance path 9, and a fixing device 10 are included.

  The image creating unit 4 (4Y, 4M, 4C, 4K) has a developing unit, and each has a small particle size toner of yellow (Y), magenta (M), cyan (C), and black (K). And a two-component developer composed of a carrier.

  An automatic document feeder D is mounted on the upper part of the image forming apparatus A. The document placed on the document table of the automatic document feeder D is conveyed in the direction of the arrow, the image of one or both sides of the document is read by the optical system of the scanner unit 1, and is read by the CCD image sensor 1A.

  The analog signal photoelectrically converted by the CCD image sensor 1A is subjected to analog processing, A / D conversion, shading correction, image compression processing, and the like in the memory control unit, and then sent to the image writing unit 3.

  In the image writing unit 3, the output light from the semiconductor laser is irradiated onto the photosensitive drum 41 (reference numerals are omitted for M, C, and K) of the image creating unit 4 to form a latent image. In the image creating unit 4, processes such as charging, exposure, development, transfer, separation, and cleaning are performed. The toner images of the respective colors formed by the image creating unit 4 are sequentially transferred by the primary transfer unit onto the rotating intermediate transfer body 42 to form a combined color image.

  The toner image on the intermediate transfer body 42 is transferred by the secondary transfer unit 43 from the paper feed cassette 5 to the sheet (paper) S transported by the paper feed transport unit 6. The sheet S carrying the toner image is heated and pressure-fixed by the fixing device 10, discharged from the discharge unit 7 to the outside of the apparatus, and placed on the discharge tray 15.

  Reference numeral 9 denotes a double-sided conveyance path. When images are formed on both sides, a sheet on which an image is formed on the first surface (front surface) and fixed by the fixing device 10 is conveyed to the double-sided conveyance path 9 and is switched to the switchback path sb. Are reversed and conveyed to the image forming unit 4 again to form an image on the second surface (back surface), and then discharged from the apparatus by the paper discharge unit 7 and placed on the paper discharge tray 15.

[Reading means]
A reading unit 8 reads the positions of the detection patterns on the front and back surfaces of the sheet. The reading unit includes a first reading unit 81 that reads the first surface (front surface) of the sheet, and a second reading unit 82 that reads the second surface (back surface). A path from the fixing device 10 to the paper discharge unit 7 is referred to as a paper discharge path, and the reading unit 8 is disposed in the paper discharge path.

  FIG. 2 is an enlarged view around the reading unit 8. 2A is a cross-sectional view, and FIG. 2B is a plan view. The two reading units 81 and 82 are arranged with the sheet S interposed therebetween. M is a cross-shaped detection pattern (hereinafter referred to as a registration mark) formed by K toner on the sheet. The detection pattern M is formed on both the first surface and the second surface of the sheet.

  The reading units 81 and 82 are “contact image sensors (CIS)”, and include a line-shaped image pickup device using CMOS and a line-shaped LED exposure unit (810, which will be described later), and a paper discharge unit 7. Are arranged so that the direction y perpendicular to the conveyance direction of the sheet S is the longitudinal direction on the upstream side in the conveyance direction. By using the contact image sensor, there are advantages that the position of the detection pattern can be detected with high accuracy and that the reading unit can be made small.

  The reading unit 81 and the reading unit 82 have the same shape, and are arranged so as to be targeted around the sheet conveyance path (paper discharge path). In the figure, the reading unit 81 and the reading unit 82 are arranged so as to be shifted by Lx so that the conveyance direction x does not face each other. This is to prevent exposure (not shown) for one reading unit from interfering with the other reading unit. Of course, interference may be prevented by time-sharing control of both exposures as a relative positional relationship.

  In addition, when the reading units 81 and 82 are attached to the image forming apparatus A, the positional relationship of each image sensor with respect to the sheet conveyance reference position (for example, the sheet conveyance center) of the image forming apparatus A is the memory of the main body (described later). Is remembered. As a result, the relative positional relationship of each image sensor between the reading unit 81 and the reading unit 82 can be understood. Further, since the reading unit 8 is arranged on the downstream side in the conveyance direction of the fixing device 10, the image is read after both sides are fixed, in other words, after the thermal expansion and contraction of the sheet due to the heating pressure fixing of the fixing device 10 occurs. ing. By doing so, it is possible to read an image in a state closer to the final output.

  FIG. 3 is a block diagram of a control system in the image forming apparatus. In the figure, the periphery of the portion necessary for the explanation of the operation of the present embodiment is mainly described, and other portions known as terminal devices are omitted.

  The image forming apparatus includes an image forming unit A1, a control unit (control unit) A2, a reading unit 8, an operation display unit 2, and a paper feeding / conveying unit 6.

  A CPU (Central Processing Unit) 101 of the control unit A 2 controls the entire operation of the image forming apparatus A, and constitutes a control unit together with a ROM (Read Only Memory) 102 and a RAM (Random Access Memory) 103. The CPU 101 reads out various control programs stored in the ROM 102 and develops them in the RAM 103 to control the operation of each unit. Further, the CPU 101 executes various processes in accordance with the program expanded in the RAM 103, stores the processing results in the RAM 103 and displays them on the operation display unit 2. Then, the processing result stored in the RAM 103 is stored in a predetermined storage destination.

  The ROM 102 stores programs, data, and the like in advance, and this recording medium is composed of a magnetic or optical recording medium or a semiconductor memory. The RAM 103 forms a work area that temporarily stores data processed by various control programs executed by the CPU 101.

  The reading unit 8 includes a reading drive circuit 80, a first reading unit 81, and a second reading unit 82. In the reading units 81 and 82, electric charges are accumulated according to the amount of light received by a plurality of image sensors (pixels). In the reading drive circuit 80, the accumulated charge of each pixel is read and converted into a digital signal. At this time, a horizontal synchronization signal is inputted to read out timing, and further, an image having two-dimensional position information by combining this timing information (x direction) and position information of each image sensor (y direction). It can be handled as data.

The operation display unit 2 includes a display unit including a liquid crystal panel and an operation unit including a touch panel in which a touch screen is disposed on the liquid crystal panel. In the operation display section, a warning such as an error display can be displayed to the user, or the paper feed cassette 5 for feeding sheets can be selected. Further, it is possible to input the amount (g / m ² ) and other information of the sheets stored in the sheet cassette from the operation display unit 2. The control unit A2 controls various conditions such as the secondary transfer 43, the fixing device 10, and the reading unit 8 based on the input sheet information.

  The paper feeding / conveying unit 6 includes a feeding motor 61, a registration clutch 62, a fixing paper discharge sensor 63, and a path switching gate 64. The feeding motor 61 drives a conveyance roller that conveys the sheet of the paper feeding conveyance unit 6. In the registration clutch 620, transmission of driving by the feeding motor 61 to the registration roller 62 is turned on and off. The registration roller 62 temporarily stops the conveyed sheet S and then conveys the sheet S in synchronization with the image formation, thereby matching the timing of image formation on the sheet. The fixing paper discharge sensor 63 is installed near the exit of the fixing device 10 and detects the conveyance timing of the sheet that has passed through the fixing device 10. The path switching gate solenoid 64 controls the path for transporting the sheet by switching the opening position of the switching gate at the branch point on the transport path.

[Detection pattern (register mark) placement]
FIG. 4 shows an example in which a register mark is written on a sheet. The register mark M has a cross shape as shown in the figure, and is formed with K toner by the image creating unit 4K of the image forming unit 4. The detection pattern is stored in advance in the ROM 102, and the control unit A2 controls the image forming unit A1 to form the detection pattern.

  D in the figure is an image area and is arranged at the center of the sheet S. For example, if the size of the image area is A3 size (297 mm × 420 mm), the size of the sheet S is an A3 nobi size obtained by adding a registration mark forming area to the A3 size. In general, the registration mark M is given for a reference mark when performing post-processing such as cutting on an image-formed sheet. However, in the present embodiment, the registration mark M is applied to an image area D formed on the sheet. It is also used to detect the amount of misalignment between the front and back sides.

  Since the registration mark positions are arranged so as to have a predetermined positional relationship with respect to the image area D, the position of the image area (image position) can be determined by detecting the registration mark position. In the example shown in FIG. 4, four register marks (M1 to M4) are arranged at the four corners of the sheet. However, the present invention is not limited to this, and two register marks or more than four register marks are provided. It may be arranged. Furthermore, in the present embodiment, an example in which a cross-shaped registration mark pattern is formed as a detection pattern has been described. However, the present invention is not limited to this, and may be a simple square shape or a circle.

[Detection algorithm for detection pattern (dragonfly)]
In the following, an algorithm for detecting a crossing point of a register mark (hereinafter referred to as a register mark position) will be described with reference to FIGS. 5, 6, and 7.

  FIG. 5 is a schematic diagram showing a register mark M1 formed on a sheet and a reading result thereof. Py in the figure shows a signal profile obtained by reading the register mark M1 by the reading means 8. (1) to (3) in Py show the results of reading one line at the reading position (scanning timing) (1) to (3) on the sheet S.

  In Py, when the light exposed from the reading unit 8 is reflected on the white background (non-image forming unit) of the sheet, the amount of light received by the image sensor of the reading unit 81 (or 82, the same applies hereinafter) is large, and therefore H output When there is no sheet or when it is reflected by a register mark (image forming unit) of the sheet, the amount of light received by the image sensor of the reading unit 81 is small, and therefore L output is indicated.

  In Py (1), since there is no register mark M1 at the reading position (1), all the corresponding positions of the sheet are H output, and the outside of the sheet end portion y0 indicates L output. In Py (2), a line portion extending in the x direction is detected in the register mark M1, and therefore an L output is shown at the corresponding position y2. In Py (3), since the line portion extending in the y direction of the register mark M1 is detected, L output is indicated between the corresponding positions y1 to y3.

  FIG. 6 is a histogram display of the reading result of FIG. In FIG. 5, Hy represents the signal profile in the y direction shown in FIG. 5 as a histogram obtained by integrating the reading results of a predetermined range, for example, the peripheral area of the register mark M1, with the horizontal axis as the y position (coordinate). . Hx is a histogram obtained by accumulating the reading results up to a predetermined range in the reading unit 81, for example, the peripheral area of the register mark M1, with the horizontal axis as the x position (scanning timing).

  The x position (xm) of the vertical line (line extending in the y direction) of the registration mark M1 is calculated from Hx shown in the figure, and the y position (ym) of the horizontal line of the registration mark M1 is calculated from Hy. As a calculation method, in the histograms Hx and Hy shown in FIG. 6, the center positions of the regions where the frequency is peak (in the region of the sheet) are xm and ym. At this time, the edge portion whose frequency changes in the vicinity of the end portion of the sheet may be determined as the sheet end portion (x0, y0), and the distances Lxm and Lym to the sheet end portion may be obtained.

  In this way, the x coordinate is calculated from the vertical line of the two lines of the registration mark M, and the y coordinate is calculated from the horizontal line, thereby detecting the intersection coordinate of the cross of the registration mark. Next, the cooperation of the two reading units 81 and 82 will be described.

  FIG. 7 is a timing chart of the reading unit 8. In the figure, the control unit A2 operates the reading units 81 and 82 via the reading drive circuit 80. In the figure, operation signals (b) and (e) are for energizing the reading units 81 and 82, respectively, and when the operation signal is turned on, an exposure unit (not shown) of the reading units 81 and 82 emits light. . When the horizontal synchronization signals (c) and (f) are input to the reading units 81 and 82, the charge accumulated in the image sensor of each reading unit is read (one line). An example of the read signal is the image signal (d) (g). The horizontal synchronization signal ts is transmitted at a constant cycle (sampling cycle), and this one cycle (ts) is a readout cycle for one line.

  In FIG. 7, when the signal of the fixing paper discharge sensor 63 (a) is turned on (time t11), the operation signals (b) and (e) of the reading units 81 and 82 are turned on (time t12). Immediately after time t12, a horizontal synchronization signal is transmitted to the first reading unit 81 at time t13, and reading of the image signal is started. In the reading unit 82, when a predetermined time t (Lx) has elapsed from t13, a horizontal synchronization signal is transmitted and reading of an image signal is started (synchronization of reading timing).

  Here, the predetermined time t (Lx) corresponds to the length Lx in FIG. 2, and is a time for which the sheet moves by an amount corresponding to the length of Lx. For example, if the sheet moving speed (conveying speed) is 300 mm / sec and Lx is 15 mm, t (Lx) is 0.05 sec. This is because the reading unit 82 is shifted from the reading unit 81 by the length Lx in the x direction to the downstream side in the conveyance direction. This is because the image signal is detected with a shift of (Lx). That is, in the comparison between the reading unit 81 and the reading unit 82, it is determined whether or not the formation positions of the registration marks M in the x direction match by comparing image signals having the same elapsed time from the start of the horizontal synchronization signal. Can do.

  In the registration mark position detection algorithm, an example in which the registration error between the front and back registration marks is detected without detecting the sheet edge (edge) is described, but the present invention is not limited to this. The position of the registration mark relative to the sheet edge (for example, Lxm, Lym in FIG. 6) may be detected, and the position of the image area on the front and back sides may be detected by comparing the positions of the registration marks. In this case, the registration mark position and the image position (image area D) with respect to the sheet are detected.

  In the present embodiment, since it is necessary to determine a positional deviation amount of about 0.1 to 0.2 mm, the resolution of the reading unit is set to a resolution several times higher than that. For example, the line-shaped image sensors (pixels) of the reading unit 81 are arranged at a density of 600 dpi (about 0.042 mm pitch) in the y direction. Further, the reading cycle ts is set so that the x-direction resolution on the sheet is the same as that in the y-direction. For example, if the sheet conveyance speed is 300 mm / sec, the reading sampling period (horizontal synchronization signal) ts is set to 7.1 kHz (= 300 / 0.042) in order to set the same resolution as in the y direction. Yes.

[Control flow]
FIG. 8 is a diagram illustrating a control flow of the image forming apparatus. This control flow is executed by the control unit A2 of the image forming apparatus A. First, in step S11, a registration mark (detection pattern) is written on the first surface (front surface) of the sheet. In step S <b> 12, registration marks are formed in the same arrangement as the first surface on the second surface (back surface) of the sheet conveyed to the image forming unit 4 again through the double-sided conveyance path 9.

  In step S13, the register marks formed on the first and second surfaces of the sheet are read by the reading means 8, and the positions of all the register marks are detected by the algorithm shown in FIGS. In step S14, the positions of the corresponding register marks respectively formed on the first surface and the second surface are compared with each other, and the positional deviation amount d is calculated. As the positional deviation amount d, for example, a method of calculating the difference dx between the x positions of the register marks corresponding to the front and back surfaces and the difference dy between the y positions and setting the square sum square of the two as the positional deviation amount d can be considered. Of course, dy and dx may be used as the positional deviation amount d.

  In step S15, it is determined whether or not the positional deviation amount d calculated in step S14 is larger than a predetermined value. For example, when it is determined that the positional deviation amount d is equal to or less than a predetermined value (step S15: No) with 0.2 mm as a predetermined value (step S15: No), it is determined that the front and back image areas satisfy a predetermined standard. To finish. On the other hand, when it is determined that the positional deviation amount exceeds the predetermined value (step S15: Yes), an error determination (determination as an error) is performed.

  Step S21 and subsequent steps are flows for executing various processes in accordance with the error determination in step S16. In step S21, if an image is currently being formed, new image formation thereafter is stopped, and new conveyance of the sheet from the sheet feeding cassette 5 is also stopped. If there is a sheet being conveyed in the image forming apparatus A, a process of discharging the image forming apparatus A out of the apparatus is performed.

  Subsequently, in step S22, a warning message for urging the user is displayed on the operation display unit 2 that also functions as a warning unit.

  In step S23, confirmation is made when the image is formed again on another sheet based on the image data formed on the sheet (error sheet) on which the error is determined (determined that the image displacement amount exceeds the predetermined value). Step to perform. This is an effective process when, for example, the user who has viewed the warning display in step S22 wants to resume image formation when analyzing the cause and determining that the cause has been removed. This step is performed by displaying a sentence confirming resumption on the operation display unit 2 and inputting consent on the touch panel of the operation display unit 2.

  In the case of restarting (step S23: Yes), image formation is performed again based on the canceled image data (step S24), and the process ends.

  The image forming apparatus is provided with a plurality of paper discharge trays 15 for discharging the sheet on which the image is formed, and the error sheet determined that the image positional deviation exceeds a predetermined value is different from the sheet not determined as an error. The paper may be discharged to a paper discharge tray. This eliminates the need to sort error sheets from normal sheet bundles, improving work efficiency.

  In this way, the detection pattern is written on the front and back surfaces of the sheet, the amount of positional deviation between the front and back surfaces of the image formed on the sheet is calculated from the written detection pattern, and the calculated amount of positional deviation of the image exceeds a predetermined value. In such a case, by using an image forming apparatus having a control unit that determines that an error has occurred, it is possible to accurately detect misalignment of the front and back images during double-sided image formation and continue to perform unnecessary printing with insufficient positional accuracy. Can be prevented.

[Other Embodiments]
FIG. 9 is a central sectional view of the vicinity of the fixing device 10 of the image forming apparatus A according to another embodiment. In the figure, reference numeral 95 denotes a reverse conveyance path, and the reading means 8 has one reading unit 81. In the present embodiment, other configurations are the same as those in the embodiment of FIGS.

  The reverse conveyance path 95 is a reverse path that reverses the front and back sides of the paper on which images are formed on both sides, and is disposed in the conveyance path between the fixing device 10 and the paper discharge unit 7. An image on one surface (first surface or front surface) is read by the reading unit 81 of the reading unit 8 with respect to the sheet on which images are formed on both sides, and the subsequently conveyed sheet is reversed and conveyed by the path switching solenoid 64. It is introduced into the path 95. The sheet that has passed through the reverse conveyance path 91 is conveyed forward and backward in the lower switchback path sb, and the image on the other surface (second surface or back surface) is read again by the reading unit 81.

  In the present embodiment, since the reading of both sides is performed one by one by only one reading unit 81, the position of the detection pattern is detected from the end of the read sheet (Lxm, FIG. 6). Lym) is stored, and the positional deviation between the front and back image areas is calculated by comparing the positions.

  By providing the reverse conveyance path 95 in this manner, both sides of the sheet can be read by one reading unit 81, so that the cost can be reduced.

  FIG. 10 is a schematic diagram for explaining another embodiment. FIG. 10A is an enlarged view of the periphery of the reading unit 8, and FIG. 10B is a diagram for explaining a positional deviation detection method using transmitted light. Other parts are the same as those in the embodiment of FIG.

  In FIG. 10A, the reading unit 8 has one reading unit 81. In addition to the exposure unit 810 in the reading unit 81, a back exposure unit 811 is provided. The exposure unit 810 exposes the sheet from the same side as the reading unit 81, and the back exposure unit 811 exposes the sheet from the side opposite to the reading unit 81. The detection pattern formed on the sheet is read by the reflected light from the sheet by the exposure of the exposure unit 810 and the transmitted light from the sheet by the exposure of the back exposure unit 811.

FIG. 11 is a diagram showing a control flow in the embodiment of FIG. Prior to the control flow shown in the drawing, information on the amount (g / m ² ) of the sheets stored in the sheet cassette is input from the operation display unit 2. In addition, a sheet thickness detection sensor may be provided, and the amount information may be automatically detected by the sheet thickness detection sensor and input may be omitted. In the control unit A2, the amount information associated with the paper cassette 5 can be acquired by determining the paper cassette 5 to be used.

  The control flow in FIG. 11 corresponds to step S13 in FIG. 8, and the other control flow is the same as the control flow in FIG.

  In step S131, it is determined from the amount information associated with the sheet feeding cassette 5 that is transporting the sheet whether the amount is equal to or less than a predetermined amount. Here, the light amount information is used by determining the light transmittance of the sheet, and when the back exposure unit 811 performs exposure from the back side of the sheet, the sheet transmitted light reaching the reading unit 81 is a sufficient amount of light. This is to determine whether or not there is.

If the soot amount is a predetermined amount, for example, 80 g / m ² or less (step 131: Yes), the reading unit 81 determines that a sufficient amount of transmitted light can be obtained, and the back exposure unit 811 performs exposure (step 132). In this case, the reading unit 81 determines whether the registration mark positions on both sides coincide with each other by detecting the line width (thickness) of the registration mark M or the like. Based on this determination, it is determined whether or not the image areas of the first surface and the second surface coincide with each other.

  As shown in (2) of FIG. 10B, when the line position of the register mark M on the first surface is shifted from the line position of the register mark M on the second surface, the line width w2 detected by the transmitted light. Becomes thicker (up to twice) as the amount of deviation increases. On the other hand, the line width becomes narrower as the positions of both register marks M coincide with each other, and when they coincide completely, the register marks M overlap as shown in (1) of FIG. Is the width of one.

  In this embodiment, since the misalignment of the registration marks M on both sides is detected by the overlapping of the registration marks on both sides, the positional relationship between the registration marks M and the image area is determined when the registration marks M on both sides overlap. The positional relationship is such that the image areas on both sides match.

  On the other hand, if the soot amount is not less than or equal to the predetermined amount (step S131: No), the exposure unit 810 inside the reading unit 81 is exposed (similar to FIG. 2). In this case, since the registration mark M needs to be detected on each side of the sheet by the reading unit 81, after the detection of the registration mark M on the first surface, the sheet is reversed on the reverse conveyance path 95 and then the second surface. The registration mark M is detected (steps S135 to S137).

  Thus, when the amount of sheet wrinkles is small, the registration marks on the front and back sides can be detected simultaneously by the transmitted light by one reading unit 81. In this case, since it is not necessary to pass through the reverse conveyance path 95, it is possible to detect an image position shift without reducing productivity.

1 is a central sectional view of an image forming apparatus according to an embodiment. It is an enlarged view of the periphery of the reading means. 2 is a block diagram of a control system in the image forming apparatus. FIG. This is an example in which a register mark is written on a sheet. It is a schematic diagram showing a register mark M1 formed on the sheet and the reading result. FIG. 6 is a histogram display of the reading result of FIG. 6 is a timing chart of the reading unit 8. FIG. 3 is a diagram illustrating a control flow of the image forming apparatus. FIG. 6 is a central sectional view of an image forming apparatus according to another embodiment. It is an enlarged view of the periphery of reading means according to another embodiment. It is a figure which shows the control flow in embodiment of FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS A Image forming apparatus A1 Image forming part A2 Control part 2 Operation display part 7 Paper discharge part 8 Reading means 81 1st reading part 82 2nd reading part 810 Exposure part 811 Back surface exposure part 9 Double-sided conveyance path 95 Reverse conveyance path ( Reverse path)
10 Fixing device

Claims (9)

An image forming apparatus having image forming means for forming images on both sides of a sheet based on image data,
A reading unit that reads the positions of detection patterns on the front and back surfaces formed on the sheet by the image forming unit, and a first reading unit that reads the front side of the sheet and a second reading unit that reads the back side of the sheet Reading means comprising:
Control for calculating the position deviation amount of the front and back surfaces of the image formed on the sheet by reading the position of the detection pattern with the reading means, and determining that an error occurs when the calculated image position deviation amount exceeds a predetermined value Means,
An image forming apparatus comprising: 2. The control unit according to claim 1, wherein the reading unit calculates a positional deviation amount between the front surface and the back surface of the image formed on the sheet by reading the position of the sheet end portion and the detection pattern formed on the sheet. The image forming apparatus described in 1. 3. The image forming apparatus according to claim 1, wherein the reading unit is disposed in a paper discharge path for discharging the sheet on which the image is formed on both sides to the outside of the apparatus. An image forming apparatus having image forming means for forming images on both sides of a sheet based on image data,
A reversing path for reversing the front and back of the sheet imaged on both sides;
A reading means for reading the positions of the detection patterns on the front and back surfaces formed on the sheet by the image forming means, wherein the other sheet surface is read after one sheet surface is read and the sheet is reversed on the reverse path. Reading means having one reading unit for reading
When the position of the detection pattern and the edge of the sheet are read by the reading unit to calculate the amount of positional deviation of the front and back surfaces of the image formed on the sheet, and the calculated amount of positional deviation of the image exceeds a predetermined value, Control means for determining an error;
An image forming apparatus comprising: The reading unit has an exposure unit that exposes the sheet from the same side as the reading unit, and a back exposure unit that exposes the sheet from the side opposite to the reading unit,
5. The control unit according to claim 4, wherein when the detection pattern is read, the control unit switches between exposure by the exposure unit and exposure by the back exposure unit on the basis of sheet amount information. Image forming apparatus. 6. The image forming apparatus according to claim 1, wherein the reading unit is a contact image sensor. Have warning means,
The image forming apparatus according to claim 1, wherein the control unit performs warning by the warning unit or cancellation of image formation when it is determined that an error has occurred. The image forming apparatus has a plurality of paper discharge trays for discharging a sheet on which an image is formed,
8. The control unit according to claim 1, wherein the control unit discharges an error sheet determined to have exceeded a predetermined value and a sheet not determined to be in error to different discharge trays. The image forming apparatus according to claim 1. 9. The image formation according to claim 1, wherein when the control unit determines that an error has occurred, the image formation is performed again based on the image data formed on the error sheet. apparatus.

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