JPS6247657A - Image forming device - Google Patents

Abstract

PURPOSE:To form an image of good quality by moving a photodetecting element at right angles to the movement of a form and detecting the quantity of position shifting, and moving a lens block for magnification variation according to said quantity and correcting the formation position of the image. CONSTITUTION:An optical detection unit 130 which has a light emitting element 132 and a photodetecting element 133 is fixed on a timing belt 135 and moved by a pulse motor 138 along a guide shaft 134. The element 133 moves to inside the maximum shifting quantity of a form firstly and moves to outside by being driven by a motor 138 when the head end of the form fed by a register roller 19, thereby detecting the side edge of the form. Its movement distance is calculated from the number of pulses of the motor 138 and the lens block for variable magnification moves on the basis of the position shifting quantity to correct the formation position of the image. Consequently, the position shifting quantity of the form is detected and corrected to obtain an image of good quality.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to an image forming apparatus applied to, for example, an electronic copying machine.

(Technical background of the invention and its problems) As is well known, in conventional copying machines, misalignment of paper supplied to the transfer unit (misalignment relative to the direction of paper movement, or misalignment perpendicular to the direction of movement) There was no means to detect or correct the positional deviation (direction).

Generally, the paper fed from the paper cassette has a length of ±3 m.
A positional shift of about 1 liter may occur. However, in conventional copying machines, even if there is a misalignment in the supplied paper, it cannot be detected or corrected. There were times when the quality deteriorated.

[Purpose of the invention]

This invention was made based on the above circumstances,
The purpose of this is to be able to detect the amount of positional deviation of the transfer material with high precision, and also to be able to highly accurately correct the image formation position based on the detected positional deviation, thereby improving quality. An object of the present invention is to provide an image forming apparatus that can form high-quality images.

(Summary of the Invention) The present invention provides a light receiving element movable in a direction perpendicular to the moving direction of paper, for example, in front of a register roller.
This light-receiving element is moved to detect the edge of the supplied paper, and the displacement of the paper is detected based on the moving distance or time of this light-receiving element, and according to this detection output signal, the variable magnification lens block is moved in a direction perpendicular to the moving direction of the first carriage to correct the image forming position on the paper.

[Embodiments of the invention]

An embodiment of the present invention will be described below with reference to the drawings.

FIGS. 4 and 5 schematically show an image forming apparatus of the present invention, such as a copying machine. That is, 1 is a main body of a copying machine, and a document table (transparent glass) 2 is fixed to the upper surface of the main body 1 to support a document. The document table 2 is provided with a fixed scale 21 that serves as a reference for setting the document, and furthermore, a document cover 11 and a work table 12, which can be opened and closed, are provided near the document table 2. The original placed on the document table 2 is processed by the optical system consisting of the exposure lamp 4 and mirrors 5, 6, and 7 reciprocating along the lower surface of the document table 2 in the direction of arrow a. Exposure scanning is performed during the reciprocation. In this case, mirror 6.7
moves at half the speed of the mirror 5 so as to maintain the optical exposure length. The light reflected from the original by the scanning of the above optical system,
That is, the light reflected from the original by the light irradiation of the exposure lamp 4 is reflected by the mirror 5, 6, 7, passes through the variable magnification lens block 8, is further reflected by the mirror 9, and is guided to the photoreceptor drum 10. An image of the original is formed on the surface of the photosensitive drum 10.

The photosensitive drum 10 is rotated in the direction of arrow C in the figure, and first, the surface thereof is charged by the charging device 11. Thereafter, the image is exposed to slit light to form an electrostatic latent image on the surface. This electrostatic latent image is formed by toner being deposited by the developing device 12. ff will be converted to e.

- 10,000, the paper (transfer material) P is taken out one by one from the selected upper paper feed cassette 13 or lower paper feed cassette 14 by the delivery roller 15 or 16, and this taken out paper is taken out from the paper guide path 17. Alternatively, it is guided through 18 to a pair of registration rollers 19, and guided to a transfer section by this pair of registration rollers 19. Here, the paper feed cassettes 13 and 14 are detachably provided at the lower right end of the main body 1, and one of them can be selected on the operation panel described in vi. The cassette size of each of the paper feed cassettes 13 and 14 is detected by cassette size detection switches 601 and 602, respectively. The cassette size detection switches 601 and 602 are composed of a plurality of microswitches that are turned on and off according to the insertion of cassettes of different sizes.

The paper P sent to the transfer section comes into close contact with the surface of the photoreceptor drum 10 at the transfer charger 20, so that the toner image on the photoreceptor drum 1o is transferred by the action of the charger 20. . This transferred paper P is transferred to the peeling charger 2
1, it is electrostatically peeled off from the photoreceptor drum 10, conveyed by the conveyor belt 22, and sent to a fixing roller 23 as a fixing device provided at the end thereof. The transferred image is fixed by passing through this, and the paper P after fixing is
is discharged onto a tray 25 outside the main body 1 by a pair of paper discharge rollers 24. After the photosensitive drum 10 has been transferred, the charge is removed by the charger 26, residual toner on the surface is removed by the cleaner 27, and afterimages are roughly erased by the charge removal lamp 28, so that the photoreceptor drum 10 returns to its initial state. He is set to return. Note that 29 is a cooling fan for preventing the temperature inside the main body 1 from rising.

FIG. 6 shows the operation panel 30 provided on the main body 1. 301 is a copy key for instructing the start of copying; 302
is a numeric keypad for setting the number of copies, etc., 303 is a display unit that displays the operating status of each part and paper jams, etc., 304 is a cassette selection key for selecting the upper and lower paper feed cassettes 13 and 14, and 30s is the selected cassette. 306 is a magnification setting key for setting the copy enlargement/reduction magnification in a predetermined relationship, 307 is a zoom key for steplessly setting the enlargement/reduction magnification, and 30a is a display for displaying the set magnification. A section 309 is a density setting section for setting copy density.

FIG. 7 shows an example of the configuration of the drive source for each drive section of the copying machine configured as described above, and is composed of the following motors. That is, 31 is a lens motor, which is a motor for moving the position of the lens block 8 for changing the magnification.

32 is a mirror motor, which is a motor for changing the distance (optical path length) between the mirror 5 and the mirror 6.7 for changing the magnification.

A scanning motor 33 is a motor for moving the exposure lamp 4, the mirror 5, and the mirror 6.7 for scanning the original. A shutter motor 34 is a motor for moving a shutter (not shown) for adjusting the charging width of the photosensitive drum 10 by the charger 11 during zooming. A developing motor 35 is a motor for driving the developing roller of the developing device 12 and the like. 36 is a drum motor, which is a motor for driving the photosensitive drum 10. Reference numeral 37 denotes a fixing motor, which is a motor for driving the paper conveyance path 22, the pair of fixing rollers 23, and the pair of paper discharge rollers 24.

38 is a paper feed motor, and the feed roller 15, 16
This is a motor for driving. A paper feeding motor 39 is a motor for driving the registration roller pair 19. A fan motor 40 is a motor for driving the cooling fan 29.

FIG. 8 shows a drive 17Ii for reciprocating the optical system.
This shows the structure. That is, the mirror 5 and the exposure lamp 4 are supported by a first carriage 411, and the mirror 6.7 is supported by a second carriage 422, and these carriages 411.412 are guided by guide rails 421.422 in the direction of arrow a. It can be moved in parallel. That is,
The four-phase pulse motor 33 drives a pulley 43. Seven endless belts 45 are passed between this pulley 43 and idle pulley 44, and this belt j.
A first carriage 41 that supports the mirror 5 in the middle of 45
! One end is fixed. On the other hand, the guide portion 46 of the second carriage 422 supporting the mirror 6.7 has a rail 4
Two pulleys 47.47 are rotatably provided apart from each other in the axial direction of 22, and a wire 48 is stretched between these pulleys 47.47. One end of this wire 48 is fixed to a fixed part 49, and the other end is fixed to the fixed part 49 via a coil spring 50. Further, one end of a first carriage 411 is fixed to a midway portion of the wire 48. Therefore, when the pulse motor 33 is rotated, the belt 45 is rotated and the first carriage 4 is rotated.
11 is moved, and along with this, the second carriage 42
2 is also moved. At this time, since the pulleys 47 and 47 act as movable pulleys, the second
Carriage 422 is moved in the same direction at 1/2 speed. Note that the moving directions of the first and second carriages 411 and 412 are controlled by switching the rotation direction of the pulse motor 33.

Further, on the document table 2, a copyable range corresponding to the specified paper is displayed. That is, the cassette selection key 304
If the paper size specified by is (PX, F'y> binding, and the copy magnification specified by the magnification setting keys 30s and 307 is K, then the copyable range (X, Y) is ``x-
Px/KJ.

It becomes "y-Py/K". This copyable range (X, y)
In the X direction, a pointer 51 provided on the back side of the document table 2
．． 52, and the X direction is displayed by a scale 53 provided on the upper surface of the front second first carriage 411.

As shown in FIG.
55 via a spring 56. The pulley 55 is rotated by a motor 58, and by driving the motor 58 according to the determined copyable range in the X direction, the distance between the needles 51 and 52 is changed. It is now possible to

Further, the first carriage 411 is moved to a predetermined position by driving the motor 33 according to the paper size and magnification.
II (home position according to the magnification). Then, when the copy key 30r is pressed,
゛The first carriage 411 first moves to the second carriage 41.
It is moved in two directions, then the lamp 4 is turned on and it is moved in a direction away from the second carriage 412. When scanning of the original is completed, the lamp 4 is turned off 1, and the first carriage 411 is returned to the home position.

FIG. 10 shows a drive mechanism for the conversion lens block 8. As shown in FIG. The motor 31 is connected to a first carriage 411
A lead screw 61 disposed along the moving direction (X direction) is rotated. Bushes 631 and 632 provided at one end of the board 62 are screwed into this lead screw 61.
1 is rotated, the substrate 62 is moved in the X direction.

A guide member 621 is provided at the other end of the substrate 62, and the guide member 621 is slidably engaged with the guide rail 64. Further, the substrate 62 includes a substrate 62 .
A movable body 65 is provided, which is movable in a direction (X direction) orthogonal to the above, and in which the variable magnification lens block 8 is disposed. That is, supports 651. are provided at both ends of the moving body 65.
652 are provided, and these supports 651, 652 are guided and held by guide members 661, 662 provided on the base plate 62. Further, a rack 653 is provided on the side surface in the longitudinal direction of the support body 651, and a pinion 68 rotated by a pulse motor 67 provided on the base plate 62 is engaged with this rack 653. Therefore, the variable power lens block 8 is moved in the X direction by driving the motor 67. In addition, the micro switches 691 and 692 are connected to the substrate 62 and the moving body 6, respectively.
This is to detect the initial position of No. 5.

Next, the relationship between the operation of the variable magnification lens block 8 and the image formed will be explained. In FIG. 11(a), the focal length of the variable power lens block 8 is f, the optical path length from the document table 2 to the variable power lens block 8 is ya, and the optical path length from the variable power lens block 8 to the photosensitive drum 10 is Optical path length is y
b. Let yc be the total optical path length from the document table 2 to the photosensitive drum 10, then the optical formula is expressed as follows.

1/f=1/ya+1/yb Also, the magnification is expressed as -yb/ya. Since the focal length f of the variable power lens block 8 is constant, it is understood that in order to focus during variable power, it is necessary to change not only the total optical path length yc but also ya or yb. These ya and yb can be changed by moving the variable power lens block 8 in the X direction. Further, the total optical path length yc can be changed by moving the second carriage 412 and changing the position of the mirror 6.7.

On the other hand, as shown in FIG. 11(b), the distances between the document table 2, the lens block 8 for variable magnification, and the photosensitive drum 10 are kept constant, and the lens block 8 for variable magnification is moved by the motor 67.
When the image on the photosensitive drum 10 is moved by, for example, a distance x 1 in the direction, the image on the photosensitive drum 10 is moved at a distance i! given by the following equation. It is moved by 1lXz.

×2 Swamp X1 xyb/ya Also, in the case of a same-size copy, it becomes ×2...2X1. In this way, by moving the variable magnification lens block 8 in the X direction, the center of the copied image can be moved.

FIG. 12 shows the overall control circuit, in which the main processor group 71 and the first . Second sub-processor group 72
．． It is mainly composed of 73. The main processor group 71 detects inputs from the operation panel 30 and input devices 75 such as various switches and sensors, such as the cassette size detection switches 601 and 602, and a high-voltage transformer 76 that drives the various chargers, and the static elimination lamp. 28, controlling the blade solenoid 27'a of the cleaner 27, the heater 23a of the fixing roller pair 23, the exposure lamp 4, and each of the motors 31 to 40, 58, 67, etc. to perform the above-described copying operation; , detects paper misalignment using the light emitting element 132, light receiving element 133, and pulse motor 138 of the optical detection unit 130,
Depending on the state, an operation is performed to adjust the image forming position on the paper. Note that details of the optical detection unit 130 and the like will be described later.

Among the motors 31 to 40,58,67,138, the motor 35, 37, 40 and the toner motor 77 that supplies toner to the developing device 12 are controlled by the main processor group 71 via the motor driver 78. motor 31
- 34.67.138 are controlled by the first sub-processor group 72 via a pulse motor driver 79, - Motor 36° 39.38.58 is controlled by a pulse motor driver 80
It is controlled by the second sub-processor group 73 via.

Further, the exposure lamp 4 is controlled by the main processor group 71 via the lamp regulator 81, and the heater 23a is controlled by the main processor group 71 via the heater control section 82. Then, from the main processor group 71, the first. Commands to drive and stop each motor are sent to the second sub-processor group 72, 73, and the first sub-processor group 72. Second sub-processor group 72.
73 to the main processor group 71 for driving each motor,
A status signal indicating a stopped state is sent. Also, the first
The sub-processor group 72 includes motors 31 to 34, 67 .
Position information from a position sensor 83 that detects each initial position of 138 is input.

FIG. 13 shows an example of the configuration of the main processor group 71. That is, numeral 91 is a one-chip microcomputer (hereinafter simply referred to as microcomputer), which, via an input/output board 92, detects human input of keys on an operation panel (not shown) and controls various displays. Further, the microcomputer 91 is expanded by input/output ports 93-96. The input/output boat 93 is connected to a high voltage transformer 76, a motor driver 78, a lamp regulator 81, and other outputs, and the input/output boat 94 is connected to a size switch for detecting paper size and its individual inputs. -1 to 95 are connected to copy condition setting switches and their respective inputs.

Note that the input/output port 96 is an option.

FIG. 14 shows an example of the configuration of the first sub-processor group 72. That is, 101 is a microcomputer, which is connected to the main processor group 71.

102 is a programmable interval timer for controlling the phase switching interval time of the pulse motor;
By setting a set value from 1, a count is performed based on the set value, and when the count is out, an end pulse is output to the interrupt line of the microcomputer 101. Timer 10 above
2, a reference clock pulse is input. Further, the microcomputer 101 receives position information from the position sensor 83 and is connected to input/output ports 103 and 104. The input/output boat 104 is connected to the motors 31 to 31 through the pulse motor driver 79.
34.67.138 is connected. The input/output board 103 is used, for example, to output status signals of each pulse motor to the main processor group 71.

FIG. 15 shows the configuration of the second sub-processor group 73. That is, 111 is a microcomputer, which is connected to the main processor group 71.

112 is a programmable interval timer for controlling the phase switching interval time of the pulse motor;
By setting a set value from 1, counting is performed based on the set value, and when the count reaches 1, a termination pulse is output. This termination pulse is latched by the latch circuit 113, and its output is supplied to the interrupt line and input/output board input line of the microcomputer 111. Also, microcomputer 111
An input/output boat 114 is connected to the input/output boat 114, and motors 36, 38, 39, and 58 are connected to the input/output boat 114 via the pulse motor driver 80.

Figure 16 shows the control circuit of the pulse motor, and shows the input/output boat 121 (input/output boat 1 in Figures 14 and 15).
04.114) has a pulse motor driver 122
(Equivalent to pulse motor driver 79.80 in Figure 12)
A pulse motor 123 (the pulse motors 31 to 34, 36, 3) is connected to this pulse motor driver 122.
8.39.58.67.13B) windings A and B
, A, and B are connected.

FIG. 17 shows a method for controlling the speed of a pulse motor. FIG. 3(a) shows the speed curve of the pulse motor, and FIG. 1(b) shows the phase switching interval.

As is clear from this figure, the phase switching interval is long at the beginning;
They gradually shorten, then become evenly spaced, then gradually lengthen again, and then stop. In other words, this indicates the through-up and through-down of the pulse motor, starting from the self-starting region, being used in the high-speed region, and finally falling down. Note that tl, t2, . . . tx indicate the time of the phase switching interval.

Next, main parts of the present invention will be explained. In this practical example, it is possible to detect the positional deviation of the paper and correct it. That is, as shown in FIG. 1, the optical detection unit 130 is provided between the pair of registration rollers 19 described above and the photosensitive drum 10 (not shown), which detects a lateral positional deviation of the paper P. . The optical detection unit 130 includes a case 131 having a substantially U-shape, a light emitting element 132 disposed inside the case 131 at a predetermined distance in a direction perpendicular to the front and back sides of the paper P, and It is composed of a light receiving element 133. The case 131 is movably provided on a guide shaft 134 disposed along the registration roller pair 17, and the case 131 is fixed to a timing belt 135 disposed along the guide shaft 134. . This timing belt 135 is stretched between pulleys 136 and 137, of which the pulley 137 is connected to the pulse motor 13.
8. therefore,
When the motor 138 is driven, the optical detection unit 130 is moved along the guide shaft 134. Furthermore, a microswitch 139 as a position sensor for detecting the initial position of the optical detection unit 130 is provided at one end of the guide shaft 134. Although the registration roller pair 19 and the guide shaft 134 are shown separated from each other in FIG. 1, the distance 111 type between them is extremely narrow.

In the above-mentioned prompting, a paper misalignment detection operation using the optical detection unit 130 will be described.

When the cassette selection key 304 of the operation panel 30 is operated, the light receiving element 133 of the optical detection unit 130 is moved to a position inside the set maximum positional deviation amount of the paper, as shown in 21(a). , the standard is filxg. In this case, 1x(1>maximum positional deviation amount of paper).

In this state, first, the paper P is moved to the registration roller pair 1.
When the leading edge of the paper P is detected by the light receiving element 133, the motor 138 is driven under the control of the main processor group 71, and the optical detection unit 13
0 is moved in the direction of the force shown by the arrow xd. And the same figure (1
)) In the state shown in FIG.
The side edges of are detected. Paper P may be stopped when its leading edge is detected, but it is best to continue feeding it or to
As shown by yl in b), it is more convenient to advance the paper a little for detecting the side edges of the paper. Paper P detected in this way
The positional deviation llx in the direction perpendicular to the moving direction of is 1x-Ixa-1xa, where lxa is the distance from the reference position @lXo to the side edge of the paper. The moving distance of the light receiving element 133 can be determined by the number of pulses supplied to the pulse motor 138 that drives the optical detection unit 130.

Further, when the paper size is changed from Pa to pb as shown in FIG. 2(C), the reference position of the light receiving element 133 is moved from IXG to I×1. The movement eis at this time is S=(xa-xb)/2, where the width of the paper Pa is Xa and the width of the paper Pb is xb, and assuming that these papers are fed centering on the center CT. At this position, the positional deviation of the paper in the X direction (direction perpendicular to the movement direction) is detected by the same operation as described above.

Furthermore, the positional deviation of the paper in the y direction (movement direction) can be detected by detecting the timing from the start of paper feeding until the output signal is detected by the optical sensor 133. This detection method involves counting the number of pulses supplied to the motor 39 that drives the pair of registration rollers 19 until the optical sensor 133 detects the paper, and comparing this counted value with a preset reference value. By doing so, the amount of positional deviation in the y direction can be detected.

In the above configuration, a paper misalignment detection operation using the optical detection unit 130 will be described.

When the copy key 301 of the operation panel 30 is operated,
The main processor group 71 performs operations as shown in FIG. That is, first, in step STI, the upper paper feed cassette 13 and the lower paper feed cassette 14 are selected by operating the cassette selection key 304 on the operation panel 3o.
It is determined which paper feed cassette is selected. As a result, if it is determined that the upper paper feed cassette 13 is being operated, the control moves to step ST2, and if it is determined that the lower paper feed cassette 14 is being operated, Control is transferred to step ST3. In these steps ST2 and ST3, the optical detection unit 130 is moved to the reference position corresponding to the size of the selected paper, as described above.

Note that the paper P is fed so that its center in the width direction is aligned with the center in the longitudinal direction of the pair of registration rollers 19.

As described above, in a state where the optical detection unit 130 is moved to the reference position of the selected paper, step ST
In step ST5, paper is fed from the selected paper feed cassette, and in step ST5, the positional deviation of the fed paper in the X direction and the y direction is j! It is detected using the optical detection unit 130 as shown above.

When the amount of positional deviation in the X direction and the y direction is detected in this way, in step ST6, the variable magnification lens block 8 is moved in accordance with the detected positional deviation in the X direction,
At step ST7, the operation timings of the first carriage 411 and the exposure lamp 4 are controlled according to the amount of positional deviation in the y direction, and the above-described copying operation is performed. Therefore, the position at which an image is formed on the paper is changed depending on the amount of positional deviation of the paper, and the original image is formed at a predetermined position on the paper. In this way, the paper on which the original image has been copied is ejected to the paper ejection tray 25 in step ST8.
All processing is finished.

According to the above embodiment, the optical detection unit is moved to detect the edge of the fed paper, and the moving direction and the moving direction of the fed paper are determined based on the moving distance or the moving time of the optical detection unit. Detects positional misalignment in the direction orthogonal to the direction, and moves the variable magnification lens block 8 in accordance with the detected amount of misalignment, and also controls the mist timing of the original and the paper feeding timing to perform the copying operation. I try to do this. Therefore, even if a misalignment occurs in the fed paper, an image can be formed at a predetermined position on the paper, and the quality of the copied image can be improved. Furthermore, since the optical detection unit 130 and the variable magnification lens block 8 are each driven by a pulse motor, it is possible to detect and correct positional deviations with high precision.

Additionally, since a single light-receiving element can detect misalignment in the direction of paper movement and in the direction orthogonal to the direction of movement, it is possible to reduce the size of the device and reduce the price. It is.

Note that this invention is not limited to the above embodiments,
Of course, various modifications can be made without departing from the gist of the invention.

[Effects of the Invention] As detailed above, according to the present invention, it is possible to detect displacement of the transferred material with high precision, and
It is possible to provide an image forming apparatus that can correct the image forming position with high precision based on the detected positional deviation, and can form a high quality image.

[Brief explanation of the drawing]

FIG. 1 shows an embodiment of an image forming apparatus according to the present invention, and shows a perspective view of the main parts showing an optical detection unit;
FIG. 2 is a plan view of the main parts shown to explain the sheet misalignment detection operation, and FIG. 3 is a diagram shown to explain the operation.
4 and 5 show the configuration of the image forming apparatus, respectively. FIG. 4 is an external perspective view, FIG. 5 is a side sectional view, and FIG. 6 is a plan view showing the configuration of the operation panel. FIG. 7 is a perspective view showing the structure of the drive section, FIG. 8 is a perspective view schematically showing the drive mechanism of the optical system, FIG. 9 is a perspective view schematically showing the pointer drive mechanism, and FIG. 10 is a perspective view schematically showing the drive mechanism of the pointer. FIG. 11 is a perspective view of the main parts showing the configuration of the variable power lens block, FIG. 11 is a diagram shown to explain the operation of the variable power lens block, FIG. 12 is a configuration diagram showing the overall control circuit, and FIG. Figure 14 is a configuration diagram showing the main processor group, Figure 14 is a configuration diagram showing the first sub-processor group, Figure 15 is a configuration diagram showing the second sub-processor group,
Figure 6 is a schematic configuration diagram showing the control circuit of the pulse motor.
FIG. 7 is a diagram for explaining the speed control method of the pulse motor. DESCRIPTION OF SYMBOLS 1... Main body, 2... Original table, 8... Lens block for variable magnification, 10... Photosensitive drum, 19... Registration roller pair, 30... Operation panel, 71... Main processor group, 130... optical detection unit, 132
... Light emitting element, 133 ... Light receiving element, 67.138
...Pulse motor. Applicant's agent Patent attorney Takehiko Suzue Figure 3 Figure 4 Figure 7 Figure 9 58

Claims (3)

[Claims] (1) A scanning means for optically scanning a document placed on a document table; a transfer means for transferring an image corresponding to the document image guided by the scanning means onto a transfer material; In an image forming apparatus comprising a moving means for changing an image forming position, the apparatus is provided near the transfer means and is movable in a direction perpendicular to the moving direction of the transferred material, and detects a positional shift of the supplied transferred material. The detection means is set with the inside of the transfer material as a reference, the detection means is moved to the outside of the transfer material, and the positional shift of the transfer material is detected by the moving distance or moving time of the detection means. An image forming apparatus characterized by comprising: a control means for detecting and correcting an image forming position on a transfer material by operating the moving means in accordance with the detection output signal. (2) The image forming apparatus according to claim 1, wherein the detection means comprises a light receiving element. (3) The image forming apparatus according to claim 1, wherein the control means moves the detection means to a reference position corresponding to the transfer material when the width of the transfer material changes. .