JPH09230711A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To always correctly maintain synchronism between a sheet leading end and an exposure starting position, at the time of transferring a toner image formed on a photoreceptor onto the sheet, in an electrophotographic copying machine or a printer. SOLUTION: This image forming device is composed so as to read out an original image, while to make expose-scanning on the photoreceptor drum 21, by means of the image scanning optical system 10, and to transfer the toner image formed on the photoreceptor to the sheet transported by a transporting roller 35. On detecting the sheet leading end by a sensor SE1, the optical system 10 is allowed to start the moving, so that the timing required for transporting the leading end of the sheet up to the position F matches with the timing required for the optical system 10 to move from the standby position D' to the exposure starting position D.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus which develops an electrostatic latent image formed on a photosensitive member with toner and transfers the toner image onto a sheet.

[0002]

2. Description of the Related Art Generally, in an electrophotographic copying machine, it is necessary to accurately synchronize the leading edge of the image forming area on the photoconductor (that is, the image exposure start position) and the leading edge of the sheet at the transfer position. Explaining with reference to FIG. 5, the photosensitive drum 50 rotates at a peripheral speed of V ₁ , and a is an image exposure position and b
Is the transfer position. The time T ₁₂ during which the photosensitive drum 50 moves the distance L ₁₁ from the exposure position a to the transfer position b is L ₁₁ /
It is V ₁ . A registration roller 51 is installed upstream of the transfer position b in the sheet conveying direction (see arrow h).
The registration roller 51 is composed of a lower roller 51a and an upper roller 51b, and its nip portion is c. Normally, as shown in FIG. 6, the lower roller 51 a is connected to a drive source (main motor) via an electromagnetic clutch 52, and the upper roller 5 a
1b is rotatable. The registration roller 51 temporarily stops the leading end of the fed sheet at the nip portion c, and the electromagnetic clutch 52 is turned on based on the generation of the synchronization signal to restart the conveyance of the sheet (in the direction of arrow h).
The sheet conveying speed by the registration rollers 51 is V ₂ , and the conveying distance is L ₁₃ .

Incidentally, it takes time T ₁₄ as shown in FIG. 7 for the speed of the registration roller 51 to rise to a constant speed after the electromagnetic clutch 52 is turned on. Therefore, the time T ₁₃ when the sheet is conveyed by the distance L ₁₃ is T ₁₃ = T ₁₄ + {L ₁₃ − (V ₂ × T _14/2 )} / V ₂ .

The time T ₁₃ is set longer than the time T ₁₁ , and the synchronizing signal is issued when the exposure position a moves to a '. It is necessary to set the distance L ₁₂ that the synchronization signal generating position a ′ moves to the transfer position b to be equal to V ₂ × T ₁₃ . Thus, time T ₁₅ from the image exposure is started until the synchronization signal is generated is a _{T 15 = (L 11 -L 12} ) / V 1.

[0005]

However, in the seat registration mechanism having the above structure, the drive characteristics of the electromagnetic clutch 52 change due to variations in load torque, drive transmission torque, etc. It fluctuates as shown by the alternate long and short dash line. On the other hand, since the generation timing of the synchronization signal is always controlled to be constant, the synchronization between the leading edge of the sheet at the transfer position b and the exposure start position on the photosensitive member is deviated.

SUMMARY OF THE INVENTION An object of the present invention is to provide an image forming apparatus capable of always maintaining the synchronization between the leading edge of a sheet and the exposure start position on the photosensitive member at the image transfer position.

[0007]

In order to achieve the above object, the image forming apparatus according to the present invention conveys a fed sheet to a transfer position without temporarily stopping it and, at the same time, conveys the leading edge of the sheet. Is detected, and the image forming operation on the photoconductor is started based on this detection signal.

Further, in the image forming apparatus according to the present invention, an image scanning optical system for scanning and exposing an image on a photoconductor, a conveying means for conveying a sheet to a transfer position of a toner image, and a conveying means for conveying the sheet. A detection means for detecting the sheet at a predetermined position; a storage means for storing the control values of the conveyance means and the image scanning optical system; and a control means for controlling the conveyance means and the image scanning optical system based on the control values, The control means generates control timing of the image scanning optical system from the sheet front end detection signal from the detection means and the control value so that the front end of the sheet and the exposure start position on the photoconductor coincide at the transfer position. To do.

In the present invention, unlike the conventional case, the sheet conveying roller installed immediately before the transfer position is not used as a registration roller which is rotated / stopped by turning on and off the electromagnetic clutch, but is continuously rotated and kept at a constant speed. Transport the sheet.
This sheet conveyance control value is constant, and the control value for operating the image scanning optical system is also constant corresponding to the copy mode. For example, the time required for the analog image scanning optical system to move from the standby position to the image exposure start position and the time to move from the exposure position on the photoconductor to the transfer position are constant. Further, the sheet detection position by the detection means is also constant. Therefore, when the leading end of the sheets that are continuously conveyed without being temporarily stopped is detected by the detection unit, the time for the leading end of the sheet to reach the transfer position can be determined. Therefore, the control timing of the image scanning optical system is calculated backward by using the sheet leading edge detection signal generation timing as a reference.

When a digital image scanning optical system is used, a sheet leading edge detection signal is generated as in the case of the analog type if the memory through type is used in which the original image is read and the photoconductor is directly exposed without memory. The control timing of the image scanning optical system may be calculated backward by using the timing as a reference.

On the other hand, even in the digital type image scanning optical system, when the image information is once stored in the memory or the image information transferred from the host computer is output as a laser beam, the sheet leading edge detection signal is output. Based on the above, the vertical synchronizing signal to the image scanning optical system may be generated so that the leading edge of the sheet and the exposure start position on the photoconductor coincide with each other at the transfer position.

According to the present invention as described above, since the sheet is continuously conveyed without being stopped by the conveying rollers, there is no possibility of the synchronization deviation due to the fluctuation of the starting characteristic of the electromagnetic clutch as in the conventional case, and the leading edge of the sheet is prevented. The exposure start position on the photoconductor can always be accurately synchronized with the transfer position.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of an image forming apparatus according to the present invention will be described below with reference to the accompanying drawings. FIG.
The embodiment shown in FIG. 4 is one in which the present invention is applied to an analog type electrophotographic copying machine.

In FIG. 1, the electrophotographic copying machine is provided with an image scanning optical system 10 in the upper stage, an image forming unit 20 in the middle stage, and a sheet feeding unit 30 in the lower stage. The image forming unit 20 has an arrow a.
Drum 2 that can be driven to rotate at system speed V in any direction
1, a charger 22, an inter-image / image-end eraser 23, a developing device 24, a transfer charger 25, a sheet separating charger 26, a residual toner cleaner 27, and a residual charge eraser 28 are installed around the periphery of 1. The configurations and operations of these elements are well known, and description thereof will be omitted.

The image scanning optical system 10 includes an exposure lamp 11,
It is composed of movable mirrors 12, 13, 14, an imaging lens 15, and a fixed mirror 16. Lamp 11, mirror 1
2, 13 and 14 stand by at the home position shown by the dotted line in FIG. 2, and the lamp 11 and the mirror 12 are moved in the direction of arrow b by V / m (V is the system speed, m Is the copy magnification), and the mirrors 13 and 1
4 moves in the direction of arrow b at a speed of V / 2 m. A pulse motor M is used as a drive source, and the motor M rotates in synchronization with the frequency of the pulse signal sent from the driver 43 (see FIG. 3). The image scanning optical system 10 moves based on the rotation of the motor M, and scans and exposes the image of the document set on the platen glass 19 onto the photosensitive drum 21. The exposure start point on the platen glass 19 is point D, and the exposure position on the photoconductor drum 21 is point A.

The paper feed unit 30 feeds the sheets contained in the cassette 31 upward one by one based on the rotation of the paper feed roller 32. The fed sheet is conveyed by the conveyance roller 33,
It is conveyed to the transfer position B through 34 and 35 without stopping halfway. At the transfer position B, the toner image formed on the photosensitive drum 21 is transferred to the sheet, and then the sheet is sent from the air suction belt 36 to the fixing device 37 where the toner is heated / fixed, and is transferred onto the tray 38. Is discharged.

The conveying roller 35 installed immediately before the transfer position B is conventionally called a registration roller.
In order to match the exposure start position on the photoconductor drum 21 with the leading edge of the sheet at the transfer position B, the sheet conveyance is temporarily stopped and then turned on and off to be restarted.
However, in the present embodiment, the main motor (not shown) continuously rotates. Further, a sensor SE1 for detecting a sheet is installed immediately before the transport roller 35. The sheet detection signal from the sensor SE1 is shown in FIG.
Is input to the microcomputer (CPU) 40 shown in FIG. Based on the input sheet leading edge detection signal, the CPU 40 generates a timing signal in accordance with the copy mode which is the target of control at that time, starts the movement of the image scanning optical system 10, and causes the leading edge of the sheet and the photosensitive drum 21. The upper exposure start position is matched at the transfer position B.

Here, the generation of the timing signal will be described. FIG. 3 shows the main part of the control circuit of the copying machine. The CPU 40, which is the center of control, is a read only memory (RO
M) 41 and random access memory (RAM) 42. In the ROM 41, parameters for executing control according to the copy mode including the copy magnification are stored in advance as a function or table data. Sensor S
The sheet detection signal emitted from E1 is direct or I /
It is input to the CPU 40 via the O gate. At this time,
The CPU 40 is already controlling the copying machine in the copying mode such as the copying magnification set by the operator. Therefore, CP
When the U40 receives the detection signal from the sensor SE1, it can determine the movement pattern of the image scanning optical system 10 controlled by itself.

By the way, as shown in FIG.
The sheet conveyance distance from the detection point E of 1 to the transfer position B is L
₁ , the transport distance until the timing signal is generated is L ₂ ,
The transport distance after the timing signal is generated is L ₃ . The rotation peripheral speed of the photosensitive drum 21 is the system speed V, and the sheet conveyance speed by the conveyance rollers 35 is set equal to the system speed V. Therefore, the distance that the exposure position A on the photosensitive drum 21 moves to the transfer position B is equal to the sheet conveying distance L ₃ . On the other hand, the distance that the image scanning optical system 10 moves from the standby position D ′ to the exposure start position D is L ₄ .

Under the above conditions, for example, when the movement pattern of the image scanning optical system 10 is controlled as shown in FIG. 4, the timing when the image scanning optical system 10 reaches the exposure start position D and the leading edge of the sheet Distance L ₃ from transfer position B
It is only necessary that the timings of reaching the position F on the upstream side in the sheet conveying direction (see the arrow h) match. The acceleration pattern of the image scanning optical system 10 by the pulse motor M is not necessarily a straight line as shown in FIG. 4, and is set to an optimum pattern for accelerating the optical system 10, but it is illustrated as a linear pattern for convenience. .

In the case of 1: 1 copying, the time T ₄ from the standby position D'to the exposure start position D of the scanning optical system 10 is L ₄
/ (V / 2). When this time T ₄ elapses, the leading edge of the sheet may reach the position F on the upstream side from the transfer position B by the distance L ₃ . The time T ₂ during which the sheet is conveyed from the detection point E of the sensor SE1 to the position F is L ₂ / V. The time T ₂ is set to be larger than the time T ₄ , and the CPU 40 calculates the time difference Ts = T ₂ −T ₄ ,
When the sheet leading edge detection signal is input from the sensor SE1,
When the time difference Ts has elapsed from the input timing, the pulse motor M is activated to move the image scanning optical system 10 to the standby position D ′.
To move in the acceleration pattern shown in FIG. With this,
At the transfer position B, the leading edge of the sheet and the photosensitive drum 21
The upper exposure start position will be correctly matched.

When performing variable magnification copying in this type of copying machine, as described above, the moving speed of the image scanning optical system 10 changes corresponding to the set magnification m. In this case, it is necessary to correct the time difference Ts based on the difference in acceleration speed or acceleration distance of the image scanning optical system 10.

On the other hand, in the copying machine in which the set value of the copy magnification is not continuous but can be selected stepwise, as shown in FIG. 2, the copy magnification is set to a position corresponding to a specific magnification in the sheet conveying path. Other sensors SE1 'may be provided. If the CPU 40 receives the sheet leading edge detection signal from the sensor SE1 'and generates a timing signal to the image scanning optical system 10 at the time of copying at that magnification, the time difference T
The process of correcting s is unnecessary.

Further, the sensor SE1 'has the time difference Ts.
Is used for control as it is, it can be used in the copy mode in which a margin for sheet binding is formed at the edge of the sheet.

Alternatively, as the drive source of the image scanning optical system 10, in addition to the pulse motor M, a control value (pulse cycle)
Alternatively, an AC synchronous motor or the like may be used as long as it is a motor that is rotationally driven in synchronization. Further, an electromagnetic clutch may be attached to the transport roller 35, similarly to the conventional registration roller, so that the stop and start can be controlled. This is because it is used for avoidance in the event of an abnormality.

On the other hand, the present invention can be applied to a digital type electronic copying machine. Although the digital copying machine is not shown, the image scanning optical system 10 of the copying machine shown in FIG. 1 is replaced with a laser beam scanning optical system, and an image of an original set on the platen glass 19 is displayed. It is equipped with an image reader for reading. In the case of the data-through method, the image information read by the image reader is directly input to the drive circuit of the laser beam scanning optical system without being digitized and stored in the drive circuit of the laser beam scanning optical system. The image is written line by line. Therefore, also in the case of the data through method, similarly to the control of the image scanning optical system 10,
The time difference Ts may be calculated based on the starting characteristic of the image reader, and the image reader may be started when the time difference Ts has elapsed from the generation of the sheet leading edge detection signal.

Further, even in the case of the digital type, when the image information read by the image reader is once stored in the memory or the image information transferred from the host computer is output as a laser beam, the sheet leading edge detection signal is detected. Based on the above, the laser beam scanning optical system may be controlled so that the leading end of the sheet and the exposure start position on the photoconductor coincide with each other at the transfer position. That is, the laser beam scanning optical system is controlled based on a vertical synchronizing signal that determines the leading end position of the image and a horizontal synchronizing signal that determines the writing start position for each line, as is well known. For such a laser beam scanning optical system, the CPU 40 determines the time T _{2 for} transporting the leading edge of the sheet to the position F (see FIG. 2), that is, L ₂ / V, based on the sheet leading edge detection signal from the sensor SE1. A vertical synchronizing signal for the laser beam scanning optical system is output when the time T ₂ has elapsed from the generation of the sheet leading edge detection signal. By such control, at the transfer position B, the leading edge of the sheet and the exposure start position on the photosensitive drum 21 are correctly aligned. Of course, the sensor SE1
May be placed at position F, in which case it is not necessary to calculate time T ₂ .

Further, in the digital copying machine or printer, in addition to the sensor SE1 as described above, another sensor S
It is also possible to provide E1 '.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram showing a copying machine according to an embodiment of the present invention.

FIG. 2 is an explanatory diagram showing a positional relationship of each component of the copying machine shown in FIG.

3 is a block diagram showing a main part of a control circuit of the copying machine shown in FIG.

4 is a graph showing a control pattern of the image scanning optical system of the copying machine shown in FIG.

FIG. 5 is an explanatory diagram showing a registration operation in a conventional copying machine.

FIG. 6 is a perspective view showing a registration roller in a conventional copying machine.

FIG. 7 is a graph showing the rotation speed of a registration roller in a conventional copying machine.

[Explanation of Codes] 10 ... Image scanning optical system 21 ... Photosensitive drums 34, 35 ... Conveying rollers 40 ... CPU 41 ... ROM SE1 ... Sheet detection sensor A ... Exposure position B ... Transfer position D ... Exposure start position D '... Standby Position E ... Sheet edge detection position

Claims (3)

[Claims] 1. An image forming apparatus that develops an electrostatic latent image formed on a photoconductor with toner and transfers the toner image onto a sheet. The image is exposed while the original image is read by moving in one direction. An image scanning optical system for scanning and exposing on a body, a conveying unit for conveying a sheet to a transfer position of a toner image, a detecting unit for detecting a sheet conveyed by the conveying unit at a predetermined position, a conveying unit and image scanning A storage unit that stores a control value of the optical system, controls the conveying unit and the image scanning optical system based on the control value, and detects the sheet front end from the sheet front end detection signal and the control value from the detection unit. An image forming apparatus comprising: a control unit that generates a control timing of an image scanning optical system so that an exposure start position on a photoconductor coincides with a transfer position. 2. The image forming apparatus according to claim 1, wherein the control unit generates a movement start timing signal of the image scanning optical system based on a sheet leading edge detection signal. 3. An image forming apparatus that develops an electrostatic latent image formed on a photoconductor with toner and transfers the toner image onto a sheet, and conveys the fed sheet to a transfer position without temporarily stopping it. At the same time, the leading edge of the sheet is detected during conveyance, and the image forming operation on the photoconductor is started based on the detection signal.