JP2002072605A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve the problems that jitter in a subscanning direction and the color slippage of a color overlapped image are caused when there is the rotational unevenness of an intermediate transfer body. SOLUTION: This image forming device is provided with plural marks formed at uniform intervals in a rotating direction in an image non-forming area on the intermediate transfer body 1, a detecting means 14 to detect the plural marks and a PLL circuit to which the output signal of the detecting means 14 is inputted, and the output signal of the PLL circuit is set as the write-in start reference signals of write-in means 6 and 11.

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 such as a printer, a copying machine, a facsimile, etc., having an intermediate transfer member and a plurality of image forming means arranged to face the moving surface of the intermediate transfer member. .

[0002]

2. Description of the Related Art In an image forming apparatus such as a printer, a copying machine or a facsimile, a toner image of each color is formed on an image carrier such as a photosensitive drum or a photosensitive belt by an image forming means. There is a type in which the toner images of each color are superimposed and transferred on an intermediate transfer body such as an intermediate transfer belt, and the superimposed toner image on the intermediate transfer body is transferred onto a sheet-like recording medium such as transfer paper. In this image forming apparatus, the image forming means forms an electrostatic latent image by writing on the image carrier with the image signal synchronized with the synchronization signal by the writing means, and forms the electrostatic latent image on the image carrier. Is developed by a developing means to form a toner image. The synchronizing signal is generated at a constant cycle, a mark provided on an intermediate transfer member such as an intermediate transfer belt is detected by a detecting means, and the synchronizing signal is initialized by a mark detecting signal from the detecting means. .

Japanese Patent Application Laid-Open No. 9-68845 discloses that an exposure beam reflected by a rotary polygon mirror is irradiated on a photosensitive member to form an electrostatic latent image corresponding to an image forming signal of a plurality of colors. The formed latent image is visualized, a plurality of visual images on the photoreceptor are sequentially superimposed and transferred to an intermediate transfer member to form a multicolor visual image, and the visual image transferred by being superimposed and transferred on the intermediate transfer member is A color image forming apparatus for transferring images onto another transfer body, comprising: a detection unit that detects a reference position on the intermediate transfer body; and a control unit that starts image formation at a detection timing by the detection unit. Detects the detection timing of the first color and the detection timing of the second and subsequent colors, and the control means controls the rotational phase of the rotary polygon mirror of the second and subsequent colors based on the detected detection timing signal. Mosquito characterized by Over the image forming apparatus is described.

Japanese Patent Application Laid-Open No. 10-104970 discloses an image carrier which carries a toner image and rotates, a toner image forming means for forming a toner image on the image carrier,
An intermediate transfer member that is placed in contact with the image carrier and rotates and that transfers and holds the toner image on the image carrier, and a transfer bias for temporarily transferring the toner image on the image carrier onto the intermediate transfer member can be switched on and off Primary transfer means applied to
A secondary transfer unit to which a transfer bias for secondary transfer of the toner image on the intermediate transfer body to the recording medium is applied so as to be switchable on and off, and a timing for detecting a timing of forming a toner image on an image carrier provided on the intermediate transfer body Reference mark, mark detection means for detecting the reference mark, image formation timing control means for controlling a toner image formation timing of the toner image formation means based on a detection signal from the mark detection means, The transfer bias of the primary transfer unit and the secondary transfer unit is switched on at least in accordance with the primary transfer and the secondary transfer timing of the toner image based on the detection signal from the unit, and the reference mark on the intermediate transfer body is changed to the primary transfer unit. Transfer bias control means for turning off the transfer bias of the primary transfer means and the secondary transfer means while passing through the secondary transfer means, respectively. It describes a color image forming apparatus characterized by a.

[0005] Japanese Patent Application Laid-Open No. 10-104970 discloses 2
In an image forming apparatus for forming an image on each of the front and back surfaces of a recording material by one image forming unit, an image position adjusting unit is provided in which front and back image positions formed on each of the front and back surfaces of the recording material by each image forming unit are matched with each other, The image forming unit includes a first image carrier on which the first image is carried, a first intermediate transfer member arranged to face the first image carrier, and a first intermediate image on the first image carrier. A first intermediate primary transfer unit that performs primary transfer on a transfer body, a second image carrier that carries a second image, a second intermediate transfer body that is arranged to face the second image carrier, and a second image carrier A second intermediate primary transfer unit for primary-transferring a second image on the body to a second intermediate transfer body, and an intermediate secondary transfer unit for secondary-transferring the primary transfer image on each intermediate transfer body to both sides of the recording material. And the image position adjusting means is provided on the first and second intermediate transfer members. A double-sided image forming apparatus, wherein a reference mark is provided, and front and back image positions formed on the front and back surfaces of the recording material by respective image forming units are matched based on a detection result of the reference mark by the detection means. Is described.

[0006]

A toner image is formed on an image carrier by the above-described image forming means, and the toner image is transferred onto an intermediate transfer member such as an intermediate transfer belt. In an image forming apparatus for transferring a mark onto a sheet-like recording medium such as transfer paper, a mark on the intermediate transfer body is detected by a detection unit, a synchronization signal of the writing unit is initialized by an output signal of the detection unit, and a writing signal is initialized by the writing unit. Even when writing is performed on the image carrier, the above-mentioned synchronization signal is generated at a constant period. Therefore, if there is uneven rotation of the intermediate transfer body, jitter (banding) in the sub-scanning direction occurs, or a color superimposed image is generated. Color shift occurs.

SUMMARY OF THE INVENTION It is an object of the present invention to provide an image forming apparatus capable of preventing a jitter in a sub-scanning direction and a color shift of a superimposed image.

[0008]

According to a first aspect of the present invention, there is provided an image forming apparatus comprising: an intermediate transfer member; and a plurality of image forming units disposed to face a moving surface of the intermediate transfer member. The image forming unit includes one image carrier, one writing unit, and at least two developing units for developing the electrostatic latent image formed on the image carrier by the writing unit. A switching unit for selectively selecting at least two developing units and driving the plurality of developing units, wherein a plurality of marks formed at equal intervals in a rotation direction in an image non-forming region on the intermediate transfer body. When,
There is provided a detecting means for detecting the mark, and a PLL circuit to which an output signal of the detecting means is inputted, and the output signal of the PLL circuit is used as a writing start reference signal of the writing means.

According to a second aspect of the present invention, in the image forming apparatus of the first aspect, the intermediate transfer is performed when the PLL circuit does not lock the phase of the output signal of the detection means even after a predetermined time has elapsed. The writing operation by the writing means is stopped assuming that the body is abnormal.

According to a third aspect of the present invention, in the image forming apparatus according to the first aspect, a frequency dividing ratio setting means for setting a frequency dividing ratio of the PLL circuit, and smoothing of a phase comparator output voltage of the PLL circuit. A time constant setting means for setting the time constant of the filter to be set, and a variable amplifier for amplifying the output of the filter, wherein the time constant of the filter is changed according to the frequency division ratio by changing the frequency division ratio of the PLL circuit. And by setting the amplification factor of the variable amplifier to a previously determined time constant and amplification factor,
The writing density is variable.

According to a fourth aspect of the present invention, in the image forming apparatus of the first, second or third aspect, a distance between the respective positions when the exposure position of each of the writing means is replaced with a position on the intermediate transfer member. Is the same as the mark interval on the intermediate transfer member or a distance that is an integral multiple of the mark interval.

[0012]

FIG. 1 schematically shows a first embodiment of the present invention. The first embodiment is an embodiment of the first aspect of the present invention. In the first embodiment, image forming stations 2 and 3 serving as two image forming units are provided on a lower moving surface of the intermediate transfer belt 1 below the intermediate transfer belt 1 as an intermediate transfer body that is driven to rotate by a driving unit. Are arranged at predetermined intervals in the direction of movement. The image forming station 2 includes a photoconductor 4 as an image carrier.
A charging device 5 as a charging device, a writing device 6 as a writing device, and developing devices 7 and 8 as two developing devices for respectively developing magenta and cyan colors;
And a cleaning device (not shown).

The image forming station 3 includes a photosensitive member 9 as an image carrier, a charger 10, a writing device 11 as a writing unit, and a developing unit as two developing units for developing black and yellow, respectively. Vessel 12,
13 and a cleaning device (not shown). Although the photoconductor drums are used as the photoconductors 4 and 9, an image carrier such as a photoconductor belt may be used. A switching unit (not shown) selectively drives the developing units 7 and 8 to drive them.
3 is selectively switched to drive.

When a full-color image is formed, the photosensitive drums 4 and 9 are driven to rotate in synchronization with the rotation of the intermediate transfer belt 1 by a drive unit (not shown), and the peripheral speed of the photosensitive drums 4 and 9 is set at the peripheral speed of the intermediate transfer belt 1. Match the speed. Normally, the photosensitive drums 4 and 9 are slightly separated from the intermediate transfer belt 1, and when the toner images on the photosensitive drums 4 and 9 are transferred to the intermediate transfer belt 1, the intermediate transfer belt 1 is Contact

As shown in FIG. 2, a plurality of marks M are arranged at equal intervals in the rotation direction of the intermediate transfer belt 1 in an image non-forming area on one end side in the width direction orthogonal to the rotation direction on the upper surface of the intermediate transfer belt 1. This mark M is detected by the optical detection means 14 at a fixed position.

When it is determined by the control means (not shown) that the speed of the intermediate transfer belt 1 has reached a constant speed after the rotation of the intermediate transfer belt 1 has started, the mark M is detected from the intermediate transfer belt 1 whose running is stable. The image forming station 2 starts the image forming operation of the first sheet by being detected by. In the image forming station 2, the photosensitive drum 4 is uniformly charged by the charger 5 and then exposed by the writing device 6 to write the first magenta image information to form an electrostatic latent image. The electrostatic latent image on the photosensitive drum 4 is developed by the developing device 7 to become a magenta toner image. The magenta toner image on the photosensitive drum 4 is transferred to an image forming area on the intermediate transfer belt 1 by a transfer unit, and the photosensitive drum 4 is cleaned by a cleaning device.

When the writing operation of the image forming operation for forming a magenta toner image on the photosensitive drum 4 is completed and the mark M is detected from the intermediate transfer belt 1 by the detecting means 14, the image forming station 2 is newly set up. The image forming operation for the first sheet is started. In the image forming station 2,
The photosensitive drum 4 is uniformly charged by the charger 5 and is then exposed by the writing device 6 to write the second magenta image information to form an electrostatic latent image. The electrostatic latent image is developed by the developing device 7 to become a magenta toner image. The magenta toner image on the photosensitive drum 4 is transferred to an image forming area on the intermediate transfer belt 1 by a transfer unit, and the photosensitive drum 4 is cleaned by a cleaning device.

On the other hand, in the image forming station 3, while the first magenta toner image on the intermediate transfer belt 1 approaches as the intermediate transfer belt 1 travels, the photosensitive drum 9 is uniformly charged by the charger 10. The electrostatic latent image is formed by being charged and exposed by the writing device 11 to write the first yellow image information, and the electrostatic latent image on the photosensitive drum 9 is developed by the developing device 13 to form a yellow image. It becomes a toner image. The yellow toner image on the photosensitive drum 9 is transferred onto the intermediate transfer belt 1 by the transfer means so as to be superimposed on the first magenta toner image, and the photosensitive drum 9 is cleaned by the cleaning device.

In the image forming station 3, the formation of the first yellow toner image is completed, and the second magenta toner image on the intermediate transfer belt 1 approaches as the intermediate transfer belt 1 travels. The photosensitive drum 9 is a charging device 10
After being uniformly charged, the writing device 11 exposes the image to write the second yellow image information, thereby forming an electrostatic latent image. The electrostatic latent image on the photosensitive drum 9 is developed by the developing device 13. To form a yellow toner image. The yellow toner image on the photosensitive drum 9 is transferred onto the intermediate transfer belt 1 by the transfer means so as to be superimposed on the second magenta toner image, and the photosensitive drum 9 is cleaned by the cleaning device.

When the formation of the second yellow toner image is completed and the mark M on the intermediate transfer belt 1 is detected again by the detecting means 14, the image forming station 2 starts an image forming operation. In the image forming station 2, the photosensitive drum 4 is uniformly charged by the charger 5 and is then exposed by the writing device 6 to write the first sheet of cyan image information to form an electrostatic latent image. The electrostatic latent image on the photosensitive drum 4 is developed by the developing device 8 to become a cyan toner image. The cyan toner image on the photosensitive drum 4 is transferred onto the intermediate transfer belt 1 by the transfer means so as to overlap the first magenta toner image and the yellow toner image, and the photosensitive drum 4 is cleaned by the cleaning device.

When the writing operation of the image forming operation for forming the first sheet of cyan toner image is completed and the mark M on the intermediate transfer belt 1 is detected by the detecting means 14, the image forming station 2 newly starts image forming. Start operation. In the image forming station 2, the photosensitive drum 4 is uniformly charged by the charger 5 and then exposed by the writing device 6 to write the second cyan image information to form an electrostatic latent image. The electrostatic latent image on the photosensitive drum 4 is developed by the developing device 8 to become a cyan toner image.
The cyan toner image on the photosensitive drum 4 is transferred onto the intermediate transfer belt 1 by the transfer means so as to overlap the second magenta toner image and the yellow toner image, and the photosensitive drum 4 is cleaned by the cleaning device.

On the other hand, in the image forming station 3, while the first toner image on the intermediate transfer belt 1 approaches as the intermediate transfer belt 1 travels, the photosensitive drum 9 is charged by the charger 1.
After being uniformly charged to 0, the writing device 11 exposes the image to write the first sheet of black image information, thereby forming an electrostatic latent image. 12 to form a black toner image. The black toner image on the photosensitive drum 9 is transferred onto the intermediate transfer belt 1 by the transfer means so as to be superimposed on the first magenta toner image, the yellow toner image, and the cyan toner image. The photosensitive drum 9 is formed and is cleaned by the cleaning device.

In the image forming station 3, while the formation of the first black toner image is completed and the second toner image on the intermediate transfer belt 1 approaches as the intermediate transfer belt 1 travels, After the photosensitive drum 9 is uniformly charged by the charger 10, the photosensitive drum 9 is exposed by the writing device 11 and the second black image information is written to form an electrostatic latent image. The electrostatic latent image is developed by the developing device 12 to become a black toner image. The black toner image on the photosensitive drum 9 is transferred onto the intermediate transfer belt 1 by the transfer means so as to be superimposed on the second magenta toner image, yellow toner image, and cyan toner image. Formed, the photosensitive drum 9
Is cleaned by the cleaning device.

The first full-color image and the second full-color image formed on the intermediate transfer belt 1 are respectively transferred to a sheet-like recording medium such as transfer paper by a transfer unit (not shown). The full-color image is fixed and discharged by a fixing device that does not perform the fixing operation.

The image forming stations 3 and 4 use three or four developing units for developing different colors, instead of the developing units 7, 8, 12, and 13, respectively. While selecting and driving one of the developing units,
By writing image information of three or four colors on the photosensitive drums 4 and 9 by the writing means 6 and 11, toner images of three or four colors are formed and transferred onto the intermediate transfer belt 1 in a superimposed manner. To form a full-color image.

In the image forming apparatus according to the first embodiment, when a print instruction signal (print start instruction signal) is received from a host (personal computer), the intermediate transfer belt 1 and the photosensitive drums 4 and 9 rotate to rotate. The image forming operation as described above is started. As shown in FIG. 2, a plurality of marks M are provided at equal intervals in the rotation direction of the intermediate transfer belt 1 in an image non-forming area on one end side in the width direction orthogonal to the rotation direction on the upper surface of the intermediate transfer belt 1. Intermediate transfer belt 1
Is rotated, the mark M on the intermediate transfer belt 1 is detected by the detection means 14 at the fixed position. FIG. 3 shows the operation timing of the first embodiment, and a mark (belt mark) detection signal as shown in FIG.

In the image forming apparatus according to the first embodiment,
A series of mark detection signals (mark detection signals from the detection means 14) generated when the intermediate transfer belt 1 is driven are converted to a PLL.
A signal input to the circuit and generated by the PLL circuit is used as a main scanning synchronization signal (writing start reference signal) of the writing means 6 and 11. The writing means 6 and 11 are connected to the PLL
Writing is performed on the photosensitive drums 4 and 9 using image information synchronized with the output signal of the circuit to form an electrostatic latent image. In the first embodiment, the LED array head is used as the writing means 6, 11.

It is determined by control means (not shown) that the speed of the intermediate transfer belt 1 has reached a constant speed after the rotation of the intermediate transfer belt 1 has started. This control means obtains a constant speed reaching time t0 from when the speed of the intermediate transfer belt 1 starts to rotate to when the speed of the intermediate transfer belt 1 reaches the constant speed, and measures the driving time of the intermediate transfer belt 1 in advance. It is determined whether or not the measurement time has reached the constant speed arrival time t0.
It can also be determined from the lock state of the circuit. When the control means determines that the speed of the intermediate transfer belt 1 has reached a constant speed after the rotation of the intermediate transfer belt 1 has started, it causes the image forming station 2 to start the image forming operation of the first sheet.

FIG. 4 shows an example of the PLL circuit. The PLL circuit includes a phase comparator (PD) 15, a low-pass filter (LPF) 16, an amplifier (Amp) 17, a voltage controlled oscillator (VCO) 18, and a frequency divider (Div) 19. The phase comparator 15 outputs a mark detection signal Pm from the detection unit 14 and a signal Pf obtained by dividing the output of the voltage controlled oscillator 18 by 1 / n by the frequency divider 19 so as to have the same cycle as the mark detection signal Pm. And outputs a signal corresponding to the phase difference.

The low-pass filter 16 includes a phase comparator 15
, And the amplifier 17 amplifies the output signal of the low-pass filter 16 at a preset amplification factor and outputs the amplified signal to the voltage-controlled oscillator 18. The voltage controlled oscillator 18
The oscillation output (oscillation frequency) Fo is controlled by the voltage input from the amplifier 17. The frequency divider 19 divides the frequency of the output signal Fo of the voltage controlled oscillator 18 so that the mark detection signal Pm from the detection means 14 and the output signal Pf maintain the phase difference at the time of phase lock. The output signal Pf of the frequency divider 19 is fed back to the phase comparator 15.

FIG. 5 shows the mark detection signal Pm from the detection means 14, the output signal Fo of the voltage controlled oscillator 18, and the frequency divider 1
9 shows the relationship of the output signal Pf. Assuming that the period of the mark detection signal Pm from the detector 14 is T, the period of the oscillation output Fo of the voltage controlled oscillator 18 is t, and the frequency division ratio of the frequency divider 19 is n, T = n × t.

For example, marks M are formed on the intermediate transfer belt 1 at intervals of 1 inch in the direction of movement, and the dividing ratio n is set to 60.
If it is set to 0, the oscillation output Fo of the voltage controlled oscillator 18 is generated for 600 cycles while the intermediate transfer belt 1 moves by one inch. If the oscillation output Fo of the voltage controlled oscillator 18 is used as a main scanning synchronization signal for the writing means 6 and 11, the writing density is 600 dpi.

Even if the peripheral speed (rotational speed) of the intermediate transfer belt 1 changes, the PLL circuit always outputs the same number of signals in the same phase with respect to one mark detection signal Pm if the lock is maintained. Is generated and output. Since the mark detection signal Pm is a position signal indicating the position on the intermediate transfer belt 1, the oscillation output Fo of the voltage controlled oscillator 18 is always generated at the same position on the intermediate transfer belt 1.

FIG. 6 shows the relationship between the mark detection signal Pm and the oscillation output Fo of the voltage controlled oscillator 18 when the rotation of the intermediate transfer belt 1 has unevenness. Even if the period of the mark detection signal Pm changes, the period of the oscillation output Fo of the voltage controlled oscillator 18 used for the synchronization signal of the writing means 6 and 11 also changes to t1 and t2.
(T1 ≠ t2), so that jitter (banding) in the sub-scanning direction does not occur. In addition, even when the colors are superimposed (the toner images of the respective colors are superimposed), the color shift does not increase.

As described above, according to the first embodiment, a plurality of marks M formed at equal intervals in the rotation direction in the non-image forming area on the intermediate transfer belt 1 as an intermediate transfer member, and the marks M Detecting means 14 for detecting the
And a PLL circuit to which the output signal of P.4 is input.
Since the output signal of the LL circuit is used as the writing start reference signal of the writing means 6 and 11, jitter in the sub-scanning direction and color shift of the superimposed image can be prevented.

Next, a second embodiment of the present invention will be described. This second embodiment is an embodiment of the invention according to claim 2. In the second embodiment, as shown in FIG. 7, in the first embodiment, the PLL circuit is provided with a phase lock detecting means (also referred to as a phase lock detecting means) 2.
0, the phase synchronization lock detecting means 20 detects an abnormal rotation of the intermediate transfer belt 1 and a mark detection error from the output signal Vo of the amplifier 17, and the control means such as the CPU uses the detected signal to control the engine of the present embodiment. And the image forming operation is stopped.

FIG. 8 shows an example of an input signal of the voltage controlled oscillator 18 when the mark detection signal Pm is input to the phase comparator 15. After a predetermined time to elapses after the generation of the mark detection signal Pm, the PLL circuit
Locks to m and stabilizes. Since the PLL circuit has a steady phase error and maintains lock, the output signal V
o changes from the voltage Vf during the free run by Vs.

When the mark detection signal Pm is input to the phase comparator 15 or when the phase difference between the mark detection signal Pm and the output signal Pf of the frequency divider 19 is out of the locked state,
As shown in FIG. 8, the output signal Vo of the amplifier 17 fluctuates greatly as compared with the case where the output signal is stable (at the time of locking). The phase lock detection means 20 detects the oscillation (fluctuation) of the voltage from the output voltage of the amplifier 17 at the time of stability, and the control means judges this from the output signal of the phase lock detection means 20 as an error and writes the error. The writing of the means 6 and 11 is stopped.

As shown in FIG. 9, the phase locked lock detecting means 20 comprises two comparators 21 and 22 and an OR circuit 2.
3 and a counter 24. Here, the reference voltages Ref1, R2 of the comparators 21, 22 are
ef2 is set to a value within which the input voltage of the voltage controlled oscillator 18 when the PLL circuit is in a stable state as shown in FIG. 8 (when the input voltage of the voltage controlled oscillator 18 when the PLL circuit is in a stable state) It is set so as to fall between the reference voltages Ref1 and Ref2). The signal En is a signal notifying the progress of the above to, and is applied to the counter 24 from a circuit (not shown).

The output signal Vo of the amplifier 17 is compared with reference voltages Ref1 and Ref2 by comparators 21 and 22, and the comparator 21 compares the output signal Vo of the amplifier 17 with the reference voltage Rf.
A pulse signal is output when the signal becomes higher than ef1. The comparator 21 determines that the output signal Vo of the amplifier 17 is equal to the reference voltage R.
When it becomes lower than ef2, a pulse signal is output and amplifier 1
7 is out of the range between the reference voltages Ref1 and Ref2, the OR circuit 2
3, a pulse signal is input.

The counter 24 enters a countable state when a predetermined time To elapses from the start of synchronization pull-in to the lock of the PLL circuit and the signal En goes high, and the counter 24 enters a countable state. By counting, the output signal Vo of the amplifier 17 becomes equal to the reference voltage Ref.
1, the number of times out of the range between Ref2 is counted, and when the count value reaches a predetermined number, a signal Co is generated as an error.
Is output.

According to the second embodiment, the PLL
If the circuit does not lock the phase of the output signal Pm of the detecting means 14 even after a predetermined time has passed, it is considered that the intermediate transfer belt 1 as an intermediate transfer member is abnormal, and the writing operation by the writing means 6 and 11 is performed. Since the image is stopped, an image having deteriorated quality is not formed.

Next, a third embodiment of the present invention will be described. This third embodiment is an embodiment of the invention according to claim 3. In the third embodiment, as shown in FIG. 10, a variable amplifier 17a is used as the amplifier 17 in the first embodiment, and the frequency division ratio of the frequency divider 19 is changed according to the write density of the present embodiment. A frequency division ratio setting means for setting and a time constant setting means for setting a time constant of the low-pass filter 16 are provided. In other words, the frequency divider 19 is a frequency divider with frequency division ratio setting means capable of changing the frequency division ratio.
As the low-pass filter 16, a low-pass filter having a time constant setting means capable of changing a time constant is used.

The frequency divider 19 is loaded with preset data as a signal corresponding to the write density from the write density means for setting the write density of the present embodiment, so that the frequency division ratio is determined by the frequency division ratio setting means. It can be changed according to the writing density of the embodiment. The amplification factor of the variable amplifier 17a is set to a gain determined in advance according to the writing density of the present embodiment (according to the frequency division ratio of the frequency divider 19) by a signal corresponding to the writing density from the writing density means. In the low-pass filter 16, the time constant is set by the time constant setting means according to the signal corresponding to the writing density from the writing density means according to the writing density of the present embodiment (according to the division ratio of the frequency divider 19). The time constant is set to a previously obtained time constant.

In order to increase the writing density, preset data is set so that the frequency division ratio of the frequency divider 19 is increased.
To lower the writing density, preset data is set so that the frequency division ratio of the frequency divider 19 becomes smaller. Further, by optimizing the gain of the variable amplifier 17a according to the writing density, a more stable and responsive PLL circuit can be configured.

According to the third embodiment, the division ratio setting means for setting the division ratio of the PLL circuit and the time constant of the filter 16 for smoothing the output voltage of the phase comparator 15 of the PLL circuit are set. It has a time constant setting means and a variable amplifier 17a for amplifying the output of the filter 16, and changes the frequency division ratio of the PLL circuit to change the time constant of the filter 16 and the amplification factor of the variable amplifier 17a according to the frequency division ratio. Since the writing density is variable by setting the time constant and the amplification factor obtained in advance, even when the writing density is changed, the deterioration of the image quality due to the uneven rotation of the intermediate transfer belt 1 as the intermediate transfer member is prevented. Can be prevented.

Next, other embodiments of the present invention will be described. These embodiments are embodiments of the invention according to claim 4. In these embodiments, in the above-described first to third embodiments, the distance m of the mark M on the intermediate transfer belt 1 in the moving direction of the intermediate transfer belt 1 is set to the distance m between the two image forming stations 2 and 3, respectively. The distance d is made equal to the distance d in the moving direction of the intermediate transfer belt 1, whereby the second mark is detected based on the mark detected by the detecting unit 14 next to the mark used as the reference for starting image formation by the first image forming station 2. Image formation by the image forming station 3 can be started.

The distance m of the mark M on the intermediate transfer belt 1 in the moving direction of the intermediate transfer belt 1 is multiplied by k (k: integer), and the exposure position of each of the writing means 6 and 11 is set on the intermediate transfer belt 1. Is equal to the distance between the respective positions when the position is replaced by the above-mentioned position, whereby the division ratio of the frequency divider 19 can be reduced when the writing density is the same, and the configuration of the frequency divider 19 is simplified. In addition, the time constant of the filter 16 can be reduced,
Responsiveness (following) is improved. Further, the writing timing of the second image forming station 3 can be easily obtained by providing a counter for counting the output signal of the detecting means 14 from the start of the writing of the first image forming station 2.

According to these embodiments, when the exposure positions of the writing means 6 and 11 are replaced with the positions on the intermediate transfer belt 1 as the intermediate transfer member, the distance between the positions is set on the intermediate transfer belt 1. Since the distance between the marks M is set to be the same as the distance of the mark M or an integer multiple of the distance between the marks M, even when a plurality of image forming units are used to form a superimposed image, the writing timing control of the writing unit in each image forming unit becomes easy.

[0050]

As described above, according to the first aspect of the present invention, it is possible to prevent the jitter in the sub-scanning direction and the color shift of the superimposed image. According to the second aspect of the present invention, it is not necessary to form an image having deteriorated quality. According to the third aspect of the present invention, it is possible to prevent deterioration of image quality due to uneven rotation of the intermediate transfer member even when the writing density is changed. According to the fourth aspect of the invention, even when a plurality of image forming units are used to form a superimposed image, it is easy to control the writing timing of the writing unit in each image forming unit.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view schematically showing a first embodiment of the present invention.

FIG. 2 is a perspective view showing a part of the first embodiment.

FIG. 3 is a timing chart showing the operation timing of the first embodiment.

FIG. 4 is a block diagram illustrating an example of a PLL circuit used in the first embodiment.

FIG. 5 is a timing chart showing a relationship among a mark detection signal Pm from a detection unit, an output signal Fo of a voltage controlled oscillator, and an output signal Pf of a frequency divider in the first embodiment.

FIG. 6 is a timing chart showing an easy-to-understand relationship between the mark detection signal Pm and the oscillation output Fo of the voltage-controlled oscillator when there is uneven rotation in the intermediate transfer belt in the first embodiment.

FIG. 7 is a block diagram showing a part of a second embodiment of the present invention.

FIG. 8 is a waveform chart showing an example of an input signal of a voltage controlled oscillator when a mark detection signal Pm is input to a phase comparator in the second embodiment.

FIG. 9 is a block diagram illustrating a configuration of a phase synchronization lock detecting unit according to the second embodiment.

FIG. 10 is a block diagram showing a part of a third embodiment of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Intermediate transfer belt 2, 3 Image forming means 4, 9, Photoreceptor drum 5, 10 Charger 6, 11, Writing means 7, 8, 12, 13 Developing device 14 Detecting means 15 Phase comparator 16 Low-pass filter 17 Amplifier 17a Variable amplifier 18 Voltage controlled oscillator 19 Divider M mark 20 Phase locked lock detecting means 21, 22 Comparator 23 OR circuit 24 Counter

Claims (4)

[Claims] 1. An image forming apparatus comprising: an intermediate transfer member; and a plurality of image forming means arranged to face a moving surface of the intermediate transfer member. Writing means, at least two developing means for developing the electrostatic latent image formed on the image carrier by the writing means, and switching means for selectively selecting and driving the at least two developing means And a plurality of marks formed at equal intervals in the rotation direction in an image non-formation area on the intermediate transfer body, detection means for detecting the marks, and an output signal of the detection means. And an output signal of the PLL circuit is used as a write start reference signal of the writing means. 2. An image forming apparatus according to claim 1, wherein said PLL circuit does not lock the phase of the output signal of said detecting means even after a predetermined time has elapsed, and considers that said intermediate transfer member is abnormal. An image forming apparatus configured to stop a writing operation by the writing unit. 3. The image forming apparatus according to claim 1, wherein a frequency dividing ratio setting means for setting a frequency dividing ratio of said PLL circuit, and a time constant of a filter for smoothing a phase comparator output voltage of said PLL circuit. A time constant setting means for setting, and a variable amplifier for amplifying the output of the filter, wherein the frequency constant of the filter and the amplification of the variable amplifier are changed according to the frequency division ratio by changing the frequency division ratio of the PLL circuit. An image forming apparatus characterized in that a writing density is variable by setting a rate to a time constant and an amplification rate obtained in advance. 4. The image forming apparatus according to claim 1, wherein the distance between the respective positions when the exposure position of each of the writing means is replaced with a position on the intermediate transfer member is set on the intermediate transfer member. An image forming apparatus, wherein the distance is equal to the mark interval or an integral multiple of the mark interval.