JPH11184348A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming device effective to accurately detect the deviation of transfer timing. SOLUTION: A latent image is formed on a photoreceptor belt 5 by compositing a mark outputted from a mark signal output part 2 with an image signal, the mark is detected by a mark detection sensor 8 disposed just before a transfer part, and the detected result is outputted to an image deviation prediction part 12. Simultaneously, the head of paper P is detected by a paper detection sensor 9 disposed just before the transfer part, and the detected result is outputted to the prediction part 12. The prediction part 12 measures an interval between the time when the mark is detected by the sensor 8 and the time when the head of the paper P is detected by the sensor 9 and judges whether the interval is normal or not.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus effective for accurately detecting a transfer timing shift.

[0002]

2. Description of the Related Art Generally, in an image forming apparatus such as a copying machine or a printer, a process of continuously supplying sheets of a predetermined size into the apparatus and sequentially forming an image on the supplied sheets is performed.

For example, in an electrophotographic high-speed laser printer, a paper feeding tray and a stacker are connected to an image forming apparatus, and an image is formed on paper continuously supplied from the paper feeding tray. The latter sheet is discharged to a stacker and stacked.

Here, an example of an image forming process by the image forming apparatus will be described below.

First, an image forming apparatus receives an image signal generated by an external device or a reading device such as a scanner. Then, the image signal is modulated to generate a laser beam, and the laser beam is applied to the photoconductor. As a result, a latent image corresponding to the image signal is formed on the photoconductor. Thereafter, the toner is attracted to the latent image, and the sheet is supplied at a predetermined timing, so that the toner is transferred to the sheet. Then, the paper on which the toner has been transferred is pressed by a heating roller to fix the image on the paper.

The image formation by the image forming apparatus is performed as described above. In order to prevent the image formed on the sheet from being shifted, the image forming apparatus has a means for monitoring the transfer timing in addition to the above configuration. Some are provided with:

FIG. 10 is a conceptual diagram showing the structure of a transfer section of a conventional image forming apparatus. The transfer unit shown in FIG. 1 exposes the photoreceptor belt 5 corresponding to the photoreceptor described above, a photoreceptor belt drive roller 6 for driving the photoreceptor belt 5, and the photoreceptor belt 5 based on an image signal. Exposure device 80
A developing unit 11 for adsorbing toner on the latent image formed by the exposure device 80; and a charging roller 7 for charging the supplied sheet P and transferring the toner adsorbed on the photoreceptor belt 5 to the sheet P. And a cleaner 10 for removing excess toner remaining on the photoreceptor belt 5 after the transfer of the toner, a paper detection sensor 9 for detecting the leading end of the supplied paper P, and an image output from the exposure device 80. A comparison unit 81 that compares the timing with the detection timing of the paper detection sensor 9 is provided, and operates as follows.

First, the photoreceptor belt driving roller 6 rotates the photoreceptor belt 5 in the direction of the arrow shown in the figure, ie, counterclockwise. The exposure device 80 sequentially exposes the surface of the rotating photoreceptor belt 5 to form an image corresponding to the sheet P as a latent image on the surface. The latent image formed by the exposure device 80 is sent to the developing device 11 with the rotation of the photoreceptor belt, and toner is supplied. Then, the contact portion between the photosensitive belt driving roller 6 and the charging roller 7 as it is,
That is, it is sent to the transcription site.

On the other hand, the paper P is supplied to the transfer section at a timing when the portion on which the toner is formed passes the transfer portion.

Then, the toner and the paper P are pressed against each other by the photosensitive belt driving roller 6 and the charging roller 7, and the toner is transferred to the paper P.

During the above series of processing, the paper detection sensor 9 outputs a detection signal to the comparison unit 81 while the paper P passes over the paper detection sensor 9, and the comparison unit 81
The detection signal output from the paper detection sensor 9 and the image output signal output from the exposure device 80 are compared.

FIG. 11 is a time chart for explaining the comparison process executed by the sheet detection sensor shown in FIG. As shown in the figure, the comparing unit 81 measures an interval “T” between the rise of the detection signal output from the paper detection sensor 9 and the rise of the image output signal output from the exposure device 80. The interval “T” means an interval between the time when the head of the image is formed on the photoreceptor belt 5 and the time when the head of the paper P passes through the paper detection sensor 9 as shown in FIG.

The comparing section 81 considers the time from when the photosensitive belt 5 is exposed to when it reaches the transfer portion and the time when the top of the sheet P detected by the sheet detection sensor 9 reaches the transfer portion. Then, it is determined whether or not the “T” is a normal interval.

Thus, the conventional image forming apparatus is
The timing of transferring the image to the paper is monitored.

[0015]

However, in the above-described monitoring means, the paper P is moved from the position where the photosensitive belt 5 is exposed.
Is determined based on the rotation of the photosensitive belt drive roller 6 and the slip generated between the photosensitive belt drive roller 6 and the photosensitive belt 5. Derangement can occur. as a result,
Actually, even if an image shift occurs, a problem arises that the image is handled as if the image formation was performed normally.

In particular, in a large-sized image forming apparatus, the distance from the exposure position to the transfer position is long, and in a high-speed image forming apparatus, detection accuracy is required.

For example, in an apparatus for forming an image on 150 sheets of paper per minute, the photosensitive belt 5 is 550 mm / sec.
Transported at a speed of When detecting an image shift within 1 mm with this apparatus, it is necessary to detect a time difference within 1.8 ms from "T". 1.8ms like this
The degree of the time difference can be sufficiently affected by the rotation fluctuation of the photosensitive belt driving roller 6 described above.

Further, in a large-sized image forming apparatus, 0.1
Since it is necessary to detect a time difference of about ms to 1.0 ms, it is less susceptible to rotation fluctuation and the like.

As a technique for solving such a problem,
The technique disclosed in Japanese Patent Application Laid-Open No. H4-261887 is also conceivable. However, in the technique disclosed in the publication, a mark for detection is printed on a sheet at a fixed period, and the printed mark is detected. Since it is determined whether or not image formation is performed normally, an unnecessary mark is formed on the sheet.

In addition, this technique has a drawback in that it is not possible to determine an image shift before forming an image, and wasteful printing is performed on paper.

As described above, the conventional technique still has a problem to be solved.

Accordingly, it is an object of the present invention to provide an image forming apparatus which is effective for accurately detecting a transfer timing shift.

[0023]

According to a first aspect of the present invention, there is provided an image signal output means for outputting an image to be formed on a sheet as an image signal, and a photosensitive member based on the image signal. A latent image forming means for forming a latent image on the photoreceptor, a developing means for developing the latent image formed on the photoreceptor by the latent image forming means, and transferring the image developed by the developing means to a sheet at a predetermined transfer timing Transfer means for supplying paper to the transfer means in accordance with the transfer timing, and image shift prediction means for estimating a shift between the paper supply timing by the paper supply means and the transfer timing. In the image forming apparatus that forms an image at a predetermined position on the sheet, the latent image forming unit may mark a mark outside the area where the latent image is formed on the photoconductor in synchronization with the formation of the latent image. form Mark forming means for detecting the mark formed by the mark forming means, a sheet detecting means for detecting a sheet supplied by the sheet supplying means, The image processing apparatus further includes a prediction unit that predicts the image shift based on a detection interval between the detection of the mark by the mark detection unit and the detection of the sheet by the sheet detection unit.

According to a second aspect of the present invention, in the first aspect, the sheet detecting means detects a first side end of the sheet supplied by the sheet supplying means. Detection means, and second side edge detection means for detecting a second side edge of the paper supplied by the paper supply means, wherein the mark forming means is provided on a side corresponding to the first side edge. First mark forming means for forming a first mark, and second mark forming means for forming a second mark on a side corresponding to the second side end, wherein the mark detecting means comprises: , A first mark detecting means for detecting the first mark, and a second mark detecting means for detecting the second mark, wherein the predicting means is provided by the first side edge detecting means. The first side edge is detected and the first mark is detected by the first mark detecting means. A first detection interval measuring unit for measuring a detection interval between the first mark detection, the second side edge detection by the second side edge detection unit, and a second detection by the second mark detection unit. The second detection interval measuring means for measuring the detection interval between the detection of the second mark and the measurement results of the first detection interval measuring means and the second detection interval measuring means are compared, and the skew of the image is calculated. Skew deviation predicting means for predicting the skew.

According to a third aspect of the present invention, in the first or the second aspect of the present invention, the mark forming means is arranged so as to correspond to a leading end position of a latent image formed by the latent image forming means. A mark is formed, and the sheet detection unit detects a leading end position of the sheet supplied by the sheet supply unit.

[0026]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of an image forming apparatus according to the present invention will be described below in detail with reference to the accompanying drawings.

First, the outline of the present invention will be described with reference to FIG. FIG. 1 is a conceptual diagram illustrating a structure of a transfer unit included in the image forming apparatus according to the present invention.

According to the present invention, as shown in the figure, a mark output from the mark signal output unit 2 is combined with an image signal to form a latent image on the photoreceptor belt 5, and is disposed immediately before a transfer portion. The mark is detected by the mark detection sensor 8,
The detection result is output to the image shift prediction unit 12. At the same time, the head of the sheet P is detected by the sheet detection sensor 9 disposed immediately before the transfer portion, and the detection result is output to the image shift prediction unit 12. The image shift prediction unit 12 measures an interval between a time when the mark is detected by the mark detection sensor 8 and a time when the head of the sheet P is detected by the sheet detection sensor 9 and determines whether the interval is normal. By making a judgment, the above-described problem is solved.

Hereinafter, the contents of the present invention will be described in more detail.

The transfer section shown in FIG. 1 is provided in an image forming apparatus such as a copying machine or a printer, and transfers a toner image to a sheet supplied from a paper feed tray and outputs the toner image to a fixing section such as a heating roller. The detailed configuration of the image forming device and paper feed tray
Since the information is disclosed in a conventionally known copying machine, printer, or the like, the description thereof is omitted, and the description will focus on the transfer unit.

In the transfer section shown in FIG. 1, an image signal serving as a basis of an image formed on a sheet is output from an image signal output section 1. The image signal output unit 1 may be an external terminal such as a personal computer or an image reading device such as a scanner.

The mark formed on the photosensitive belt 5 is output from the mark signal output unit 2 as a mark signal. The mark signal output unit 2 stores a mark having a predetermined shape such as a linear shape or a rectangular shape.

The synchronizing signal generating unit 13 includes an image signal output unit 1 and a mark signal output unit 2 according to the printing timing.
To output an exposure synchronization signal. The image signal output unit 1 and the mark signal output unit 2 receiving the exposure synchronization signal output the image signal and the mark signal to the synthesizing unit 3, respectively.

FIG. 2 shows a synchronizing signal generator 13 shown in FIG.
5 is a time chart illustrating timings of signals output by an image signal output unit 1 and a mark signal output unit 2; As shown in the figure, the exposure synchronizing signal is an enable signal output for each number of sheets, and the image signal is output while the exposure synchronizing signal is enabled. The mark signal is output simultaneously with the image signal, and the rising edge of the mark signal corresponds to the head of the image on each sheet.

The synthesizing unit 3 synthesizes the received image signal and mark signal and outputs the synthesized image signal and mark signal to the exposing unit 4. The exposure unit 4 modulates the received signal into a laser drive signal and irradiates the photosensitive belt 5 with laser light. As a result, a latent image of an image and a mark is formed on the surface of the photoreceptor belt 5.

FIG. 3 is a top view showing a state where an image and a latent image of a mark are formed on the surface of the photosensitive belt 5 shown in FIG. As shown in the figure, the image and the latent image of the mark are formed in an image forming area 50 and a mark forming area 51 provided on the surface of the photoreceptor belt 5, respectively.

Here, the image forming area 50 is provided so as to correspond to the maximum width of the sheet P to be supplied.
1 is provided outside thereof. That is, the mark forming area 5
The latent image formed in 1 does not contribute to the transfer to the paper P.

The photoreceptor belt 5 shown in FIG. 3 is moving in the direction indicated by the arrow in the figure, and the left end mark A21a is provided on the surface of the photoreceptor belt 5 at both outer sides of the image A20a.
The right end mark A22a is formed, and the photoreceptor belt 5 shown in FIG. 3 is moving in the direction shown by the arrow in the figure. Mark A21a and right end mark A2
2a are formed, and a left end mark B21b and a right end mark B22b are formed outside both ends of the image B20b. Here, the image A20a is a latent image corresponding to an image formed on the first sheet, and the image B20b is a latent image corresponding to an image formed on the second sheet.

The left end mark A21a, the right end mark A22a, the left end mark B21b, and the right end mark B22b correspond to an image A20a and an image B20b, respectively.
Is formed in accordance with the leading position in the traveling direction of.

As described above, in the exposure process in the image forming apparatus, the photosensitive belt 5 is continuously exposed according to the number of sheets supplied.

The latent image formed as described above is sent to the developing unit 11 by the photosensitive belt driving roller 6. The developing device 11 supplies and attracts toner to the sent latent image. As a result, a toner image is formed on the photosensitive belt 5.

The toner image is sent to a transfer portion as the photosensitive belt 5 moves. This transfer site and the developing device 11
Between them, a mark detection sensor 8 is provided, and the mark detection sensor 8 detects a mark formed on the photoreceptor belt 5. The mark detection sensor 8 is configured by irradiating the photoreceptor belt 5 with light and using a photosensor or the like for detecting reflected light.

On the other hand, the paper P on which an image is to be formed is supplied to a transfer portion at a transfer timing. Immediately before the transfer site, a paper detection sensor 9 is provided, and this paper detection sensor 9 detects the head of the supplied paper.

By providing the mark detection sensor 8 and the paper detection sensor 9 as close as possible to the transfer area, errors due to fluctuations in the rotation of the photosensitive belt driving roller 6 can be reduced.

The mark detection sensor 8 and the paper detection sensor 9 are designed to take the moving speed of the photoreceptor belt 5 and the transport speed of the paper P into consideration so that the mark and the paper reach the transfer portion at the same time. It is preferable to dispose the image in this manner, and by disposing in this way, it is possible to simplify an image shift prediction process described later. Since the moving speed of the photoreceptor belt 5 and the conveying speed of the paper P are generally the same, it is preferable that the mark detection sensor 8 and the paper detection sensor are arranged at the same distance from the transfer portion.

FIG. 4 is a perspective view showing the structure near the transfer site shown in FIG. As shown in the figure, the mark detection sensor is composed of a mark detection sensor A8a and a mark detection sensor B8b provided corresponding to the mark formation areas 51 shown in FIG. The mark detection sensor A8a detects the left end mark A21a and the left end mark B21b shown in FIG.
Detects the right end mark A22a and the right end mark B22b shown in FIG.

The paper detection sensor is composed of a paper detection sensor A9a and a paper detection sensor B9b which are respectively provided corresponding to the width of the supplied paper. These sheet detection sensors detect both ends of the sheet P, respectively.

As described above, the marks are formed on both sides of the photoreceptor belt 5, and the means for independently detecting the marks and the means for independently detecting the both sides of the sheet are provided, so that the image is skewed. The deviation can be predicted.

FIG. 5 is a time chart showing an example of detection signals output by the mark detection sensor and the paper detection sensor shown in FIG. As shown in the drawing, the paper detection sensor A9a outputs a detection signal while the left end of the paper P passes, and the mark detection sensor B8b outputs the left end mark A21a or the left end mark B21b shown in FIG. Outputs the detection signal while passing. Similarly, the paper detection sensor B
9b and the mark detection sensor B8b also output the detection signals shown in FIG. The respective detection results are output to the image shift prediction unit 12, where a determination process described later is performed.

The paper P supplied to the transfer portion is pressed against the photoreceptor belt 5 on which the toner image is formed by the photoreceptor belt driving roller 6 and the charging roller 7, and the toner image is transferred.

After the transfer of the toner image, the toner image remaining on the photosensitive belt 5 is sent to the cleaner 10 and removed as the photosensitive belt 5 moves.

FIG. 6 is a block diagram showing the internal configuration of the image shift prediction section shown in FIG. As shown in the figure, the image shift prediction unit 12 includes a counter A6 that counts a detection interval between the sheet detection sensor A9a and the mark detection sensor A8a.
1 and a register A6 for storing the value of the counter A61.
2, sheet detection sensor B9b and mark detection sensor B8b
B63 for counting the detection interval of the counter B63, a register B64 for storing the value of the counter B63, and a ROM
65 are connected to the CPU 60.

The ROM 65 is provided with an allowable time difference storage area 70 for storing an allowable time difference and an allowable skew amount storage area 71 for storing an allowable skew amount.

Here, the permissible time difference is the permissible value of the detection interval between the mark detection by the mark detection sensor 8 and the paper detection by the paper detection sensor.
Is determined in consideration of the distance from the position where is disposed to the transfer part and the distance from the position where the paper detection sensor 9 is disposed to the transfer part.

For example, when the mark detection sensor 8 and the paper detection sensor 9 are arranged so that the mark and the paper arrive at the transfer portion from the respective sensors at the same time, the allowable time difference is different from the transport speed of the paper. It is a value obtained by converting the allowable range of the image shift into time based on the time.

The allowable skew amount is an allowable amount that defines a shift in an oblique direction of an image formed on the sheet P when the sheet P is supplied in a state of being inclined obliquely. Is defined as the difference between the detection times of the sensor at the right end and the sensor at the right end.

Hereinafter, an image shift prediction process by the image shift predicting unit 12 configured as described above will be described with reference to flowcharts shown in FIGS.

FIG. 7 is a flowchart showing an execution procedure of an image shift prediction process executed by the image shift predicting section 12 shown in FIG. 6 based on the sheet detection sensor A9a and the mark detection sensor A8a. The process shown in the figure is performed for each sheet.

In the process shown in the figure, first, the CPU 60 resets the value of the counter A61 (step 100).
It waits until a sheet is detected by the sheet detection sensor A9a or a mark is detected by the mark detection sensor A8a (No in step 101). When either the sheet or the mark is detected (step 101)
Yes), the counter A61 starts incrementing (step 102).

Here, if a sheet is detected in step 101, a mark is detected in step 103, and if a mark is detected in step 101, the sheet is detected in step 103.

Thereafter, the control waits for a detection signal from the sheet detection sensor A9a or the mark detection sensor A8a again. If there is a detection signal (Yes in step 103), the value of the counter A61 is stored in the register B64 (step 107). ). Thereafter, the process proceeds to the skew feeding prediction process shown in FIG.

On the other hand, if there is no detection signal (No in step 103), it is determined whether or not the value of the counter A61 exceeds the allowable time difference stored in the ROM 65 (step 104). If the difference does not exceed the allowable time difference (No in Step 104), the process returns to Step 102 again, and the increment of the counter A61 is continued.

In step 104, the counter A61
If the value exceeds the allowable time difference, the occurrence of an image shift is notified (step 105), and the image forming process is interrupted (step 106).

FIG. 8 is a flowchart showing an execution procedure of an image shift prediction process executed by the image shift predicting section 12 shown in FIG. 6 based on the sheet detection sensor B9b and the mark detection sensor B8b. The processing shown in the figure is performed in the same manner as the processing shown in FIG.

FIG. 9 is a flowchart showing an execution procedure of the skew deviation prediction processing executed by the image deviation prediction section 12 shown in FIG. In the process shown in FIG.
Are a sheet detection sensor A9a and a sheet detection sensor B9b
Wait for the detection signals output from both to fall,
That is, the process waits until the trailing edge of the sheet is detected by each of the sensors (step 300).

When the trailing edge of the sheet is detected (step 300), the CPU 60 sets the register A62 and the register B
The value stored in the ROM 64 is read (step 301), the absolute value of the difference between the values is obtained (step 302), and whether the absolute value is within the range of the allowable skew amount stored in the ROM 65 is determined. Is determined (step 303).

If the absolute value obtained in step 302 is within the range of the allowable skew amount (Yes in step 303), the process is terminated. If the absolute value is not within the range of the allowable skew amount (step 303). No), the skew of the image is notified (step 304), and the image forming process is interrupted (step 30).
5).

[0068]

As described above, according to the present invention,
It is possible to provide an image forming apparatus that is effective for accurately detecting a transfer timing shift.

Further, marks are formed on both side edges of the photoreceptor belt 5 and means for independently detecting the respective marks and means for independently detecting both side edges of the paper are provided. Prediction becomes possible.

[Brief description of the drawings]

FIG. 1 is a conceptual diagram illustrating a structure of a transfer unit included in an image forming apparatus according to the present invention.

FIG. 2 is a time chart showing timings of signals respectively output by a synchronization signal generation unit 13, an image signal output unit 1, and a mark signal output unit 2 shown in FIG.

FIG. 3 is a top view showing a state in which an image and a latent image of a mark are formed on the surface of the photoreceptor belt 5 shown in FIG. 1;

FIG. 4 is a perspective view showing a structure near a transfer site shown in FIG. 1;

FIG. 5 is a time chart showing an example of detection signals output by the mark detection sensor and the paper detection sensor shown in FIG.

FIG. 6 is a block diagram showing an internal configuration of an image shift prediction unit shown in FIG. 1;

7 is a flowchart showing an execution procedure of an image shift prediction process executed by the image shift predicting unit 12 shown in FIG. 6 based on the sheet detection sensor A9a and the mark detection sensor A8a.

FIG. 8 is a flowchart showing an execution procedure of an image shift prediction process executed by the image shift prediction unit 12 shown in FIG. 6 based on the sheet detection sensor B9b and the mark detection sensor B8b.

9 is a flowchart showing an execution procedure of a skew deviation prediction process executed by the image deviation prediction unit 12 shown in FIG.

FIG. 10 is a conceptual diagram illustrating a structure of a transfer unit of a conventional image forming apparatus.

FIG. 11 is a time chart illustrating a comparison process performed by the sheet detection sensor illustrated in FIG. 10;

[Description of Signs] 1 ... image signal output section, 2 ... mark signal output section, 3 ... compositing section, 4 ... exposure section, 5 ... photosensitive belt, 6 ... photosensitive belt drive roller, 7 ... charge roller, 8 ... Mark detection sensor,
8a: mark detection sensor A, 8b: mark detection sensor B, 9: paper detection sensor, 9a: paper detection sensor A, 9
b: paper detection sensor B, 10: cleaner, 11: developing device, 12: image shift prediction unit, 13: synchronization signal generation unit, 2
0a image A, 20b image B, 21a left end mark A, 21b left end mark B, 22a right end mark A, 22b right end mark B, 50 image formation area, 5
DESCRIPTION OF SYMBOLS 1 ... Mark formation area, 60 ... CPU, 61 ... Counter A, 62 ... Register A, 63 ... Counter B, 64 ... Register B, 65 ... ROM, 70 ... Permissible time difference storage area, 7
1: Allowable skew amount storage area, 80: Exposure device, 81: Comparison unit, P: Paper.

Claims (3)

[Claims] An image signal output unit for outputting an image to be formed on a sheet as an image signal; a latent image forming unit for forming a latent image on a photosensitive member based on the image signal; Developing means for developing the latent image formed on the body, transfer means for transferring the image developed by the developing means to a sheet at a predetermined transfer timing, and feeding the sheet to the transfer means in accordance with the transfer timing An image forming apparatus for forming an image at a predetermined position on the sheet, comprising: a sheet supply unit; and an image shift prediction unit configured to estimate a shift between a sheet supply timing by the sheet supply unit and the transfer timing. The image forming unit includes a mark forming unit that forms a mark in synchronization with the formation of the latent image, outside a region where the latent image is formed on the photoconductor, and the image shift prediction unit includes: Mark detecting means for detecting a mark formed by the mark forming means; sheet detecting means for detecting the sheet supplied by the sheet supplying means; detection of the mark by the mark detecting means; and An image forming apparatus comprising: a prediction unit configured to predict the image shift based on a detection interval between sheet detection and the sheet detection. 2. The sheet detecting device according to claim 1, wherein the sheet detecting unit detects a first side edge of the sheet supplied by the sheet supplying unit, and a second side detecting unit detects a second side of the sheet supplied by the sheet supplying unit. A second mark detecting means for detecting a side edge of the first mark, the mark forming means, a first mark forming means for forming a first mark on a side corresponding to the first side end, A second mark forming means for forming a second mark on a side corresponding to the second side end, wherein the mark detecting means comprises: a first mark detecting means for detecting the first mark; And a second mark detecting means for detecting the second mark, wherein the predicting means detects the first side edge by the first side edge detecting means and the first mark detecting means. Measuring a detection interval between detection of the first mark by the first method Output interval measuring means; and second detection for measuring a detection interval between the detection of the second side edge by the second side edge detecting means and the detection of the second mark by the second mark detecting means. An interval measuring unit; and a skew deviation predicting unit that compares measurement results obtained by the first detection interval measuring unit and the second detection interval measuring unit and predicts a skew deviation of an image. The image forming apparatus according to claim 1. 3. The mark forming means forms the mark corresponding to a leading end position of a latent image formed by the latent image forming means, and the paper detecting means comprises a sheet supplied by the sheet supplying means. The image forming apparatus according to claim 1, wherein a position of a tip of the image forming apparatus is detected.