JPH04131876A - Image forming device - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

Detailed Description of the Invention [Industrial Application Field 1] This invention involves arranging a plurality of image bearing members side by side and sequentially transferring images formed on each image bearing member to a recording medium conveyed by a transfer belt. The present invention relates to an image forming apparatus that can form images in multiple colors.

[Prior Art] Conventionally, a plurality of image carriers are arranged side by side, and images formed on each image carrier are sequentially transferred to a recording medium conveyed by a transfer belt to form a multi-color image. A forming device has been put into practical use.

FIG. 8 is a cross-sectional configuration diagram illustrating the configuration of this type of image forming apparatus, which is composed of a printer section P, a league section R, etc., and the printer section P prints a full color image based on the color document information read by the league section R. It is configured to be able to form an image. As shown in the figure, the image forming station is composed of four image forming stations, and corresponds to an image forming apparatus that sequentially overlays and transfers yellow, magenta, cyan, and black developers to obtain a full-color image.

In the league section R, the reading section 14 moves in the direction of the arrow and reads the document O on the document table glass 13. The reading unit 14 is
Illumination lamp 1431 reflecting mirror 142, optical element 141. C
It is composed of a light receiving element 144 such as a CD, etc., and is driven by a timing belt 18 . The document 0 is scanned along the document 0 via the pulley 17.

The yellow image forming station of the printer section P is provided with a photosensitive drum 1Y, a charger 2Y, a laser scanning optical system 3Y, a developing device 4Y, and a cleaner 5Y, and a yellow toner image is formed by a well-known electrophotographic process. The transfer material S is directed by the transfer belt 6a, conveyed in the direction of arrow A, transferred by the transfer charger 6bY, and then conveyed to the next magenta image forming station.

The magenta image forming station of the printer section P is provided with a photosensitive drum IM, a charger 2M, a laser scanning optical system 3M, a developing device 4M, and a cleaner 5M, and a magenta toner image is formed by a well-known electrophotographic process. Transfer material S
is directed to the transfer belt 6a and conveyed in the direction of the arrow,
After being transferred by the transfer charger 6bM, it is transported to the next cyan image forming station.

The cyan image forming station of the printer section P includes a photosensitive drum IC, a charger 2C, a laser scanning optical system 3C, and a developer 4.
C and a cleaner 5C are provided, and a cyan toner image is formed by a well-known electrophotographic process. The transfer material S is directed by the transfer belt 6a, conveyed in the direction of arrow A, transferred by the transfer charger 6bC, and then conveyed to the next black image forming station.

The black image forming station of the printer section P includes a photosensitive drum 1BK, a charger 2BK, and a laser scanning optical system 3BK.
, a developing device 4BK, and a cleaner 5BK are provided, and a black toner image is formed by a well-known electrophotographic process.

The transfer material S is directed by a transfer bell) 6a, is conveyed in the direction of arrow A, is transferred by a transfer charger 6bBK, is fixed by a fixing device 8, and is discharged onto a tray 9.

In an image forming apparatus in which a plurality of image forming stations are arranged side by side in this way, since images are transferred sequentially onto the same surface of the same transfer material S, if the transferred image position at each image forming station deviates from the ideal position, a difference in color tone may occur. In other words, color shift occurs and image quality is degraded. In addition, 10 is a detector, 2
7Y, 27M, 27C, 27BK, 28Y, 28M, 2
8C and 28BK are linear actuators, and L is a laser beam.

FIG. 9 is a schematic diagram showing the causes of color misregistration in the image forming apparatus shown in FIG.
This is the laser scanning direction with respect to IBK.

For example, as shown in Figure (a), if a positional shift occurs in the direction of the arrow (indicated by the broken line in the figure) from the normal writing start position (indicated by the solid line in the figure), the top margin shift will result in a mismatch in the top margin. arise.

In addition, as shown in Figure (b), when a positional shift occurs in the direction of arrow B (indicated by a broken line in the figure) from the normal writing start position (indicated by a solid line in the figure), the left margin misalignment causes a mismatch in the left margin. occurs.

Furthermore, as shown in Figure (c), if the writing direction is tilted from the normal writing direction (shown by the solid line in the figure) to the diagonal direction shown by the broken line in the figure,
Tilt deviation occurs.

Further, as shown in FIG. 2D, a magnification error may occur as shown by the broken line in the figure from the normal writing direction (indicated by the solid line in the figure).

In order to eliminate these deviations, the scan timing of the light beam is electrically adjusted for the top margin and left margin, and the tilt deviation and magnification error deviation in the diagonal direction are eliminated by moving and adjusting the mirror in the scanning optical system. Proposals have already been made to do so.

That is, laser scanning optical systems 3Y, 3M, 3C, 3BK
A reflector 24 that reflects the laser beam L emitted from the
By moving Y, 24M, 24C, and 24BK in the a direction by linear actuators 27Y, 27M, 27C, and 27BK, the magnification error is corrected, and the reflector 24
Two linear actuators 2BY, 28M, and 28C placed in front of Y, 24M, 24C, and 24BK.
, 28BK correspond to reflectors 24Y, 24M, 24
By shifting C and 24BK in the b direction, the tilt deviation in the diagonal direction is corrected.

In addition, in the above four types of misalignment detection processing, the registration correction mark written on the transfer belt Sa is detected by the detector 1.
o, and the reflector 24 is shifted based on it.

In this way, even after correcting image shift, color unevenness 1
In order to suppress color shift, four photosensitive drums IY and IM are required.
, IC, and 18, and measures such as keeping the moving speed of the transfer material constant are required.

For this reason, conventionally, the photosensitive drums 1Y and IM.

An encoder is directly connected to the IC, IBK, and the transfer roller drive motor, and the transfer roller drive motor is controlled to maintain a constant rotation using PLL control, and the four photosensitive drums IY
, 1M, IC, and IBK rotation speeds are matched,
The moving speed of the transfer material S is kept constant.

[Problem to be Solved by the Invention 1] However, speed fluctuations occur in driven objects such as photosensitive drums due to eccentric components of the necessary reduction ratio gear train from the drive motor or meshing errors of the teeth of the gear train. .

This speed fluctuation becomes a speed fluctuation on the surface of the photosensitive drum, and the laser scanning optical system 3Y, 3M, 3C, 3B serving as an exposure means
When forming an image using K, the image writing position is misaligned. This positional shift is caused by the drive system of the four photosensitive drums, all of which have different aspects, and it is very difficult to adjust the rotational speed of all four photosensitive drums to a constant speed using conventional methods. It is difficult to prevent color shift.

Further, as in monochromatic image forming apparatuses using one photosensitive drum, there is a problem in that the image expands and contracts due to similar positional deviations due to eccentricity and meshing errors of the gear train for driving the photosensitive drum.

However, speed fluctuations due to eccentricity and gear meshing errors, which are causes of image misalignment (color shift) and expansion/contraction, must be avoided by using high-precision gears that minimize eccentricity and meshing errors, and by ensuring precision installation. Although it is possible to reduce the size by increasing the size, this is a disadvantage in terms of cost.

We also attempted to reduce the influence of the gear train by installing a low-resolution encoder on the drum shaft and performing PLL control. Not only does this require a fast control circuit, leading to increased costs, but it is also difficult to suppress speed fluctuations of the photosensitive drum due to delays in the transmission system from the motor.

Therefore, as shown in FIG. 10, each photosensitive drum 1Y, IM
, by adding a predetermined pattern FAT to the end of the IC, IBK, and detecting this pattern FAT optically (consisting of optical elements 51 to 54, etc.) to individually detect pitch fluctuations, the eccentric phase can be determined. The angle is detected and drive control is performed for each photosensitive drum IY, IM, IC, and IBK of each image station so that each image writing position is at the same angle with respect to the eccentric phase angle to prevent positional deviation. Proposals have been made to do this, but there were problems such as difficulty in capturing accurate speed fluctuations including the deceleration mechanism.

This invention was made in order to solve the above problems, and it reads and analyzes images for registration correction formed by each image carrier, and calculates the results due to speed fluctuation components of each image carrier. By individually analyzing the amount of positional deviation and controlling the image writing start position of each image carrier so that the relative positional deviation characteristics match, the speed fluctuation phase difference of the driving means of each image carrier can be eliminated. It is an object of the present invention to provide an image forming apparatus capable of sequentially overlapping and transferring clear images without color shift onto a recording medium.

[Means for Solving the Problems 1] An image forming apparatus according to the present invention includes a pattern forming means for developing a predetermined test pattern formed on each image carrier and transferring it to a transfer belt, and a pattern forming means using the pattern forming means. An image reading means that reads each test pattern transferred to the transfer belt, a calculation means that analyzes the image information read by the image reading means and calculates the amount of image position shift, and an image read by the image reading means. A calculation means that analyzes the information and calculates the amount of image writing position deviation of each image carrier, and a calculation means that analyzes the phase difference of the image writing position deviation amount of each image carrier calculated by this calculation means and calculates each image. The apparatus is further provided with determining means for determining the starting position of image writing on the carrier.

Further, the pattern forming means is configured to transfer a predetermined test pattern onto the recording medium conveyed by the transfer belt.

Further, the calculating means is configured to calculate the amount of image position shift by analyzing the test pattern on the recording medium read by the document reading means.

Further, the test pattern is composed of line patterns arranged at equal pitches in the rotational direction of the image carrier.

[Operation] In this invention, when the predetermined test pattern developed on each image carrier by the pattern forming means is transferred to the transfer belt, the image reading means reads each test pattern transferred to the transfer belt, The calculating means calculates the amount of image position deviation by analyzing the read image information, and based on the calculated image position deviation amount, the determining means calculates the calculated phase difference of the image writing position deviation amount of each image carrier. is analyzed to determine the image writing start position of each image carrier, and it is possible to drive all the image carriers by matching the phase of image position shift fluctuation per rotation of each image carrier.

Further, the pattern forming means transfers a predetermined test pattern onto the recording medium conveyed by the transfer belt, thereby making it possible to read the test pattern externally.

Furthermore, the calculation means analyzes the test pattern on the recording medium read by the document reading means, and makes it possible to calculate the amount of image position deviation caused by the deceleration mechanism from the result of observation from the outside.

Further, a test pattern composed of line patterns arranged at equal pitches in the rotational direction of the image carrier is transferred onto a transfer belt or a recording medium, and is made to function as a pattern for detecting image positional deviation caused by a deceleration mechanism or the like.

[Embodiment] FIG. 1 is a block diagram showing the main structure of an image forming apparatus according to an embodiment of the present invention, and the same parts as in FIG. 9 are given the same reference numerals.

In the figure, PCON is the printer controller and the 9th
To comprehensively control the printer section P shown in the figure (7)
CPU100. RAM101. It is equipped with a ROM 102 and the like.

In the figure, 22Y, 22M, 22C, and 22BK are stepping motors (STM) that drive the photosensitive drum IY via a reduction gear train 20 as described later.

It is configured to transmit rotational force to IM, 1C, and IBK, and this reduction gear train 20 becomes a factor of speed fluctuation, causing the above-mentioned positional deviation. 33Y, 33M, 33C, 3
3BK is a stepping motor drive circuit that drives the stepping motors 22Y, 22M, 22C, and 22B based on the drive pulse train output from the printer controller PCON.
Drive K.

103 is a laser driver, and the laser scanning optical system 3Y, 3
Controls the driving of M, 3C, and 3BK semiconductor lasers.

In the image forming apparatus configured as described above, each image carrier (in this embodiment, the photosensitive drums 1Y, IM, IC, IBK) is
) The predetermined test pattern developed on the transfer belt 6
a, the image reading means (in this embodiment, the detector 10) reads each test pattern transferred to the transfer belt 6a, analyzes the read image information, and calculates the image information by the calculation means (in this embodiment, the CPU 100). ) calculates the amount of image position deviation, and based on the calculated amount of image position deviation, the determining means (in this embodiment, CPtJ 100 also serves) calculates the calculated phase difference of the amount of image writing position deviation of each image carrier. It becomes possible to drive all the image carriers by analyzing the image writing start position of each image carrier and matching the phase of the image position shift fluctuation per rotation of each image carrier. .

Further, the pattern forming means transfers a predetermined test pattern onto the recording medium conveyed by the transfer belt 6a, and enables external reading.

Furthermore, the calculation means analyzes the test pattern on the recording medium read by the document reading means, and makes it possible to calculate the amount of image position shift from the external reading result.

In addition, a test pattern consisting of line patterns arranged at equal pitches in the rotational direction of the image carrier is transferred to the transfer belt 6.
a or a recording medium, and it is possible to accurately measure the amount of image position shift caused by a deceleration mechanism or the like.

FIG. 2 is a block diagram showing the drive mechanism of the stepping motor shown in Section 1, and the same parts as in FIG. 1 are given the same reference numerals. For example, the case of magenta station is shown. Note that the other image forming stations have similar configurations, so the configuration and operation of the magenta station will be explained.

1M is a photosensitive drum serving as an image carrier, and 2o is a reduction gear train that reduces the rotation speed of the stepping motor 21M and transmits the rotation to the photosensitive drum 1M. And these gears 20a to 20d
The number of teeth Za, Zb, Zc.

The ratio of Zd is an integer ratio of 8:4:2:1, and the gear 2a at the final stage is directly connected to the photosensitive drum 1M through the photosensitive drum shaft, and when the gear 20a rotates once, the photosensitive drum 1M
also rotates once. This is configured such that each gear rotates an integral number of times while the photosensitive drum 1M rotates once. In this embodiment, the gear 20b rotates twice, and the gear 20c rotates twice.
rotates 4 times, and the gear 20d rotates 8 times.

33M is a stepping motor drive circuit that outputs an excitation pulse signal to the stepping motor 21M based on the output of the stepping motor control circuit 32. Reference numeral 34 denotes a photosensitive drum home position signal generating section, which outputs a home position signal obtained by detecting the slitted disc 22 fixed to the drum shaft 1a with the photo ink club 23 to the stepping motor control circuit 32.

With this configuration, when a pulse train at constant intervals (constant frequency) is applied to the stepping motor 21M to drive the motor at a constant speed, the rotational speed variation ΔV of the drum shaft 1a of the photosensitive drum 1M is caused by the eccentricity of the gears 20a to 20d. Depending on the components, etc., it changes periodically as shown in equation (1) below.

Δ■: 3. CO3ω+az CO8((ωt/2)+
φ, ) 10 as CO8 ((ωt/4)+φ2)+a,
CO3 ((ωt/8) ψ,)+F (t)
...fl) Here, ω is the rotational angular velocity of the photosensitive drum.

Furthermore, F (t) is the rotational speed fluctuation component due to the gear meshing error, and if the number of teeth in the gear train is an integer ratio, the same teeth will always mesh with each other, so one rotation of the photosensitive drum is equal to 1 rotation.
It becomes a periodic function with a period. In other words, Δ■ is a periodic function whose period corresponds to one rotation of the photosensitive drum. When the product R*Δ■ of the speed variation ΔV and the radius R of the photosensitive drum is integrated with respect to time, this becomes the amount of positional deviation Δ° C. of the photosensitive drum surface from its normal position when rotated at a constant speed.

When the stepping motor 21M is driven at a constant speed with this configuration, the characteristic of the positional deviation amount Δe as shown in FIG. 3 is obtained.

FIG. 3 is a diagram showing the characteristics of the positional deviation amount Δ°C of the photosensitive drum 1M shown in FIG. 1, where the vertical axis shows the positional deviation amount Δ°C, and the horizontal axis shows time.

As can be seen from this figure, the characteristic of the positional deviation amount Δ°C of the photosensitive drum 1M (the same applies to the other photosensitive drums 1Y, 1C, and 1BK) is limited by one rotation as one period.

Now, in order to detect the above positional deviation amount Δ℃, ROM10
The pattern data (constant frequency data) for detecting the amount of positional deviation stored in 2 or stored in the fixed pattern storage memory is read out and laser scanned onto the photosensitive drum 1M by the laser scanning optical system 3M of each image forming station. A pattern image developed with a developer (not shown) is transferred to a transfer belt 6a.

FIG. 4 is a plan view of a main part showing an example of a test pattern image in an image forming apparatus according to the present invention.
The pattern is arranged at mm pitch intervals.

For example, in the case of a printer with a process speed of 100 mm/sec and a scanner that rotates a 10-sided polygon mirror at 12,000 rpm, lines can be written at intervals of 0.05 m5 ec in the sub-scanning direction, and a laser will be emitted every two rotations of the polygon mirror. If controlled so as to do so, it is possible to output a test pattern with a pitch of 1 mm in the sub-scanning direction as shown in FIG.

Such a test pattern image is generated on the photosensitive drums 1Y, IM, 1C, and 1C of each image forming station after the adjustment of the top margin of each image forming station is completed.
A transfer belt 6 formed of IBK and conveyed at a constant speed
transferred onto a. The pattern images of each color thus transferred onto the transfer belt 6a are sequentially read by a detector 10 (disposed downstream of the black image forming station in the paper transport direction).

The process speed of the transfer belt 6a is 100 mm/sec.
Therefore, if there is no displacement of the line image, a line image is input to the detector 10 every 0.05 m5ec.

Therefore, the image input to the detector 1o is the original 0,05
By calculating the difference time Δt, which indicates how much time varies with respect to each m5ec, correspondence can be established with the amount of positional shift Δ°C of the image of each hue line.

FIG. 5 is a characteristic diagram showing the relative difference time between the pattern detection timing and the ideal detection timing by the detector 10 shown in FIG. 1, where the horizontal axis shows the line number of the test pattern line image and the vertical axis The relative difference time Δt is shown.

As can be seen from this figure, when the number of teeth in the gear train is an integer ratio, the same teeth always mesh with each other, so the relative difference time Δt becomes a periodic function with one rotation of the photosensitive drum as one period. Depending on the series, the following equations (2) to (5)
It can be approximated by Eq.

△t(θ) =ao” at CO8θ+a2 cos
2θ ten... ten an C03nθ+bo+ bl co
sθ+bz C082θ+...' + t)n C08
nθ − (21Δt(θ) = a, +c+ cos
(θ+δ1)+C2cos (2θ+δ2) +Cn CO5(nθ+δ.)...(3),
Each constant a is determined by the least squares method as the time shift Δt8 of the k-th line pattern. ,a,,,.

If bm4cm is calculated, then comparing the above equations (2) and (3), the constant Cm is:
5).

In addition, in equation (3), the governing term with period 2π is C+
Since CO3 (θ+61), tan δ, = a I
/ b + , and the phase angle δ1 can be calculated.

As a result, the phase angle δIY of each image forming station,
δ, 2. δlc+δ, K is determined.

FIG. 6 is a characteristic diagram in which the fluctuation of each relative time difference of the photosensitive drum shown in FIG. 1 is approximated by a Fourier series, and the vertical axis shows the difference time Δt.

In the figure, ΔtY indicates the differential time variation of the Y image produced at the first image forming station, and Δt,A indicates the differential time variation of the M image produced at the second image forming station.

As can be seen from this figure, in order to eliminate the color shift between the two, it is sufficient to form an image with the phase angle δlY+61M matched. Note that each phase angle δ1 is the photosensitive drum IY, IM of each image forming station.

The angle from the reference point of IC and IBK is converted into the amount of driving pulses of the stepping motor and stored. Then, as shown in FIG. 2, the slit disk 22 is fixed to the drum shaft 1a of the photosensitive drum 1M,
By detecting its home position by
The phase angle δ1 is determined using this home position as a reference point.

FIG. 7 is a flowchart showing an example of a phase angle calculation procedure for each image forming station in the image forming apparatus according to the present invention. Note that (1) to (4) indicate each step.

First, the test pattern image read from the ROM 102 of the printer controller PCON is output onto the transfer belt 6a (1).Then, the test pattern image transferred to the transfer belt 6a is read by the detector 10 (2), and each color is The amount of positional deviation Δ°C of the test pattern output image (actually, the detector 10
Relative difference time Δt of the test pattern image input to
(3) Next, the phase angle of each image forming station is calculated (4) and stored in the RAM (not shown) of the stepping motor control circuit 25 of each station.

When an image forming start signal (not shown) is input, the steering/sobbing motor 2 of each image forming station is
2Y, 22M, 22C, and 228 start driving at a constant driving frequency. When the home position signal of each image forming station is output from the phytointerrupter, the stepping motors 22M, 22C, and 22BK of the second to fourth image forming stations stop, and only the stepping motor 22Y continues to rotate. The drive is controlled so that the image writing start angle at the first image forming station becomes a phase angle δ, Y with reference to the home position signal.

At each station after the second image forming station, at which pulse after the home position signal of the first image forming station is manually input, the stepping motors 22M, 22C, and 22BK are started to be driven. The image writing angle is δ1. , δlc+
δ1. , and calculate the respective timings as T slJ, T ! As ic+TSB, driving of the stepping motors 22M, 22C, and 22BK is started, and the phase angles of the photosensitive drums 1M, 1C, and IBK can be made to match the phase angle δ1Y of the photosensitive drum 1Y. Therefore, even if the stepping motors 22Y, 22M, 22C, and 22BK of each image forming station are driven at a constant driving frequency, the photosensitive drums IY, IM, and IC.

IBK image position shifts occur in substantially the same phase, making it possible to form a clear image without color shift.

Note that in the above embodiment, the detector 10 provided in the printer section P
The case where the detection pattern shown in Fig. 4 is read has been explained, but the detection pattern is outputted to a recording medium, the output paper is read by the league section R shown in Fig. 8, and the amount of positional deviation Δ°C is calculated. It can also be a configuration. Further, in the above embodiment, when calculating the phase angle δ, attention was paid to the differential time fluctuation Δt, but any parameter may be used as long as it is a periodic function whose period corresponds to one revolution of the drum.

[Effects of the Invention] As explained above, the present invention includes a pattern forming means that develops a predetermined test pattern formed on each image carrier and transfers it to a transfer belt, and a pattern forming means that develops a predetermined test pattern formed on each image carrier and transfers it to the transfer belt. an image reading means for reading each test pattern, a calculation means for analyzing the image information read by the image reading means and calculating the amount of image writing position deviation of each image carrier; Since a determining means is provided for determining the image writing start position of each image bearing member by analyzing the phase difference of the image writing position shift amount of each image bearing member, the image position per one rotation of each image bearing member is determined. The phases of shift fluctuations can be matched, color shifts caused by phase differences can be prevented, and four-color images can form synchronized, high-quality images without color shift.

Further, since the pattern forming means is configured to transfer a predetermined test pattern onto the recording medium conveyed by the transfer belt, the test pattern can be read by an external document reading means.

Furthermore, since the test pattern is composed of line patterns arranged at equal pitches in the rotational direction of the image carriers, it is possible to accurately grasp the amount of positional deviation caused by fluctuations in the rotational speed of each image carrier.

Therefore, it is possible to match the phase of the positional shift caused by the phase difference in the rotational speed fluctuation of each image carrier, which cannot be corrected by conventional registration correction processing, so that the color images on each image carrier are successively superimposed with high precision. This makes it possible to transfer a four-color image, which has the advantage of always being able to provide a high-quality image that is synchronized and free from color shift.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing the main structure of an image forming apparatus according to an embodiment of the present invention, FIG. 2 is a block diagram showing the driving mechanism of the stepping motor shown in FIG. 1, and FIG. A diagram showing the characteristics of the amount of positional deviation of the photosensitive drum shown in FIG. 1,
FIG. 4 is a plan view of a main part showing an example of a test pattern image in the image forming apparatus according to the present invention, and FIG. 5 is a relative difference time between the pattern detection timing and the ideal detection timing by the detector shown in FIG. FIG. 6 is a characteristic diagram showing the fluctuation characteristics of each relative time difference of the photosensitive drum shown in FIG. 1, and FIG. 7 is a phase angle calculation procedure for each image forming station in the image forming apparatus according to the present invention. Flowchart showing an example FIG. 8 is a cross-sectional configuration diagram explaining the configuration of this type of image forming apparatus, FIG. 9 is a schematic diagram showing color shift factors in the image forming apparatus shown in FIG. 8, and FIG. 10 is a flowchart showing an example. FIG. 2 is a perspective view showing an eccentric phase difference detection mechanism in an image carrier of this type of image forming apparatus. In the figure, IY, 1M, IC, and IBK are photosensitive drums, 6
a Transfer belt, 22Y, 22M, 22C. 22BK is stepping motor, 100 is CPU, 10
1 is a RAM, and 102 is a ROM. O Figure Figure Figure (a) A (c) Figure (b) (d) Figure

Claims (3)

[Claims] (1) Multiple image carriers driven by stepping motors are arranged side by side, and images formed on each image carrier are sequentially transferred to a recording medium conveyed by a transfer belt to form a multi-color image. An image forming apparatus includes a pattern forming means for developing a predetermined test pattern formed on each image carrier and transferring it to the transfer belt, and a pattern forming means for developing a predetermined test pattern formed on each image carrier and transferring the test pattern to the transfer belt. An image reading means for reading, a calculation means for calculating the amount of image writing position deviation of each image carrier by analyzing the image information read by the image reading means, and each image carrier calculated by the calculation means. 1. An image forming apparatus comprising: determining means for determining an image writing start position for each image carrier by analyzing a phase difference in an image writing position shift amount. (2) The image forming apparatus according to claim 1, wherein the pattern forming means transfers a predetermined test pattern onto the recording medium conveyed by the transfer belt. (3) The image forming apparatus according to claim 2, wherein the calculating means calculates the image position shift amount by analyzing a test pattern on the recording medium read by the document reading means. (4) The image forming apparatus according to claim (1), wherein the test pattern is constituted by a line pattern arranged at equal pitches in the rotational direction of the image carrier.