JPH06324541A - Color image forming device - Google Patents

Abstract

PURPOSE:To provide a color image forming device that enables effective operation of units by enabling color image formation to continue within an allowable range even when a driving system for either of plural image forming units has failed. CONSTITUTION:A color image forming device has a driving state detection means 40 for detecting the driving states of image forming units 1K, 1C, 1M, 1Y, a judgment means 42 which, when a detection signal from the driving state detection means indicates abnormality, determines whether or not the image forming unit that has yielded the abnormality signal can form images alone and judges the image formation modes of colors that can be produced by combination of the other sound image forming units, a selection means 43 for selecting one of the image formation modes judged by the judgment means 42, and a control means 44 for driving and controlling each of the image forming units 1K, 1C, 1M, 1Y according to the image formation mode selected by the selection means 43.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a color image forming apparatus such as a tandem type color electrophotographic copying machine or a color printer having a plurality of image forming units, and more particularly to a driving system for any one of the plurality of image forming units. The present invention relates to a color image forming apparatus capable of continuing a color image forming operation within a possible range even when a failure occurs.

[0002]

2. Description of the Related Art Conventionally, as a color image forming apparatus such as the above-mentioned tandem type color electrophotographic copying machine or color printer, for example, there is the following one. As shown in FIG. 17, this color image forming apparatus includes four color image forming units 100Y, 1 for yellow, magenta, cyan, and black.
00M, 100C, 100K are arranged in parallel at a predetermined interval, and each of these image forming units 100Y,
The toner images of yellow, magenta, cyan, and black are formed by 100M, 100C, and 100K. The image forming units 100Y, 100M, 10
The toner images of yellow, magenta, cyan, and black formed by 0C and 100K are conveyed to the conveyor belt 101.
The color image is formed by sequentially transferring the toner images transferred onto the transfer sheet 102 conveyed by the transfer sheet 102 and then fixing the toner image transferred onto the transfer sheet 102. The conveyor belt 101 is
Each image forming unit 100Y, 100M, 100C, 1
It is arranged below 00K so as to form a linear transport path over the entire image forming units.

By the way, in the above image forming apparatus,
Each image forming unit 100Y, 100M, 100C, 1
The toner images of the respective colors formed by 00K are sequentially transferred onto the transfer paper 102 conveyed by the conveyor belt 101 to form a color image. Therefore, in order to obtain a high-quality color image, the transfer paper is used. It is necessary to accurately superimpose the toner images of the respective colors transferred onto the toner 102.
Therefore, in the image forming apparatus, it is necessary to rotate the photosensitive drums 103Y, 103M, 103C, 103K of the image forming units 100Y, 100M, 100C, 100K with high accuracy.

Therefore, as a technique relating to the drive control device of the photosensitive drum, for example, Japanese Patent Laid-Open No. 63-7575.
Some of them are disclosed in Japanese Patent No. This image carrier drive control device, in an image forming apparatus that forms an image on an image carrier that moves endlessly, moves the image carrier as a mechanism for moving the image carrier via a reduction gear train having an integer number of teeth. A stepping motor to be driven, a memory for storing a pulse generation pattern for canceling one rotation rotation variation of the final gear of the gear train for one final rotation, and a means for detecting the home position of the final gear When the image carrier is moved, a pulse is generated based on the pulse generation pattern to drive the stepping motor.

However, in the case of the drive control device for the image carrier according to the above proposal, the pulse generation pattern for canceling the rotational fluctuation of the image carrier is stored in advance in the memory as a fixed pattern, and is stored in this memory. A pulse is generated on the basis of the fixed pulse generation pattern thus generated, and the stepping motor is driven by this pulse to prevent the rotation fluctuation of the image carrier. Therefore, if the state of the rotational fluctuation of the reduction gear train that rotationally drives the image carrier changes due to environmental changes such as temperature changes and changes over time due to long-term use, etc., the pulse generation pattern that cancels this rotational fluctuation is Since it is a fixed pattern stored in, it is not possible to cope with an unexpected rotation fluctuation due to environmental changes and the like. As a result, it is not possible to sufficiently suppress the rotational fluctuation of the image carrier caused by the environmental change or the temporal change, and the rotational fluctuation remains on the image carrier, so that there is a problem that a color shift or the like occurs in the formed image. there were.

In order to solve the above problems, the present applicant has already proposed a rotation control method and device in a multiple transfer device disclosed in Japanese Patent Application Laid-Open No. 2-43574. The rotation control device according to this proposal is a multi-transfer device in which a plurality of images are multi-transferred to a common transfer roll, and the transfer roll when a drive motor for driving the transfer roll is rotated at a constant angular velocity in advance. The information about the change in the angular velocity is stored in the storage unit, the information about the change in the angular velocity is read from the storage unit at the time of transfer, and the angular velocity of the drive motor is changed based on the information.

The rotation control device in the multiple transfer device according to this proposal has a transfer roll which is stored in advance in the storage means even when a new rotation change occurs in the image carrier due to environmental changes or changes over time. When the angular velocity change information of is read out and the angular velocity of the drive motor is changed based on this information, it can be corrected as a change in the angular velocity, so that it is possible to cope with environmental changes and changes over time. Has become.

However, in the case of the device according to the above proposal,
It has the following problems. That is, in the case of the rotation control method in the multiple transfer device proposed by the applicant, the information on the change in the angular velocity of the transfer roll is stored in the storage unit as it is, and the information on the change in the angular velocity is read from the storage unit. Then, the angular velocity of the drive motor is directly changed based on the information. Therefore, the change information of the angular velocity of the transfer roll to be stored in the storage means,
When the precision of the rotation control of the transfer roll is improved by greatly increasing the number of divisions of the angular velocity, the amount of change due to correction gradually increases, and the rotation axis of the transfer roll changes from the drive motor to the rotation shaft of the transfer roll. There is a problem in that it becomes a source of vibration for the system up to the point where it may oscillate or the amplitude of the natural frequency of the system may increase. Further, in this case, in order to improve the accuracy of the rotation control of the transfer roll, it is necessary to use a high-precision encoder as the encoder for detecting the change information of the angular velocity of the transfer roll, which increases the cost. There was also the problem of inviting them.

Therefore, the applicant of the present invention has solved this problem, and even when the speed control of the rotating body is performed with high accuracy, the rotating body does not oscillate or increase the cost. A drive control device has already been proposed (Japanese Patent Application No. 4-80279).

This rotary body drive control device is a rotary body drive control device for controlling the drive of a rotary body used in an image forming apparatus. A rotation detecting unit, a highly accurate second rotation detecting unit that is used only when the image forming apparatus is manufactured and detects the rotation speed of the rotating body, and a driving unit that rotationally drives the rotating body are driven at a constant speed. At this time, storage means for storing the rotation speed information detected by the first rotation detection means and the second rotation detection means for each predetermined divided section, and the first rotation detection means at the time of image formation. The rotation speed of the rotating body is detected, the rotation speed information of the first rotation detection means and the second rotation detection means stored in the storage means is read, and the rotation speed detected by these first rotation detection means is read. Drive means for averaging the rotation speed information of the first rotation detecting means and the second rotation detecting means stored in the information and storage means, and for rotating the rotating body based on the averaged rotation speed information. And a control means for controlling.

[0011]

However, the above-mentioned prior art has the following problems. That is, in the case of the drive control device for a rotating body according to the proposal of the present applicant, needless to say, it is possible to cope with environmental changes, changes over time, and the like, and speed control of the rotating body is performed with high accuracy. Even in such a case, oscillation does not occur in the rotating body and the cost is not increased.

However, when such a drive control device for a rotating body is applied to a tandem type color image forming apparatus, each image forming unit 100Y, 100M, 100.
C, 100K photosensitive drums 103Y, 103M, 10
While 3C and 103K can be rotationally driven with high accuracy, each of the photoconductor drums 103Y, 103M and 103 can be driven.
A feedback control system for controlling the driving means based on the change information of the rotation speed, the slit used as the first rotation detection means for detecting the rotation speeds of C and 103K, the slit clogging, the disconnection of the optical sensor, etc. If a failure occurs in the image forming unit 100, the failure occurs in the image forming unit 100.
Y, 100M, 100C, 100K photoconductor drum 10
3Y, 103M, 103C, 103K cannot be rotationally driven with high precision. At that time, the photosensitive drums 103Y, 103M, 103C and 103K of the normal image forming units 100Y, 100M, 100C and 100K are
Since the rotational state is controlled with high accuracy, even if the rotational fluctuations of the photosensitive drums 103Y, 103M, 103C, and 103K in which the failure has occurred are small, the rotational fluctuations appear as remarkable.

Therefore, in the above image forming apparatus,
Each image forming unit 100Y, 100M, 100C, 1
It becomes impossible to record a predetermined color image by superimposing the toner images of each color formed by 00K with high accuracy, and the user notices the formed color image and notices the device failure, and calls the service engineer. However, there is a problem that the image forming apparatus cannot be used at all until the repair of the apparatus is completed. Therefore, even if there is another image forming unit that can be driven normally, the image forming operation cannot be continued and the image forming apparatus cannot be effectively used.

Therefore, as a technique capable of partially solving such a problem, a technique disclosed in Japanese Patent Laid-Open No. 4-116571 has already been proposed. A color copying apparatus according to this proposal includes an instruction unit for instructing an operation for reading and recording an original image, an input unit for reading the original image according to an instruction from the instruction unit, and an image signal for inputting the image signal. In a color copying machine having a forming means for forming a visible image based on an image signal inputted by the input means in accordance with the instruction of 1., a detecting means for detecting an internal abnormality and a content of the internal abnormality detected by the detecting means. The determining means determines the effective instruction range by the instructing means, and the specifying means that clearly indicates the instruction range determined by the determining means.

However, in the case of the color copying apparatus according to this proposal, when an internal abnormality such as a lack of toner occurs,
This is detected by the detecting means, and the image forming operation can be performed by the forming means other than the image forming means for which the toner is absent. When an abnormality occurs in the drive system, the image forming operation cannot be continued and the use of the apparatus must be interrupted.

Therefore, the present invention has been made in order to solve the above-mentioned problems of the prior art, and its purpose is to provide a drive system failure in any one of a plurality of image forming units. An object of the present invention is to provide an image forming apparatus capable of continuing the color image forming operation within a possible range and effectively operating the apparatus.

[0017]

According to the present invention, a plurality of image forming units are arranged side by side, and an image formed by each of the image forming units is conveyed to a transfer position of each of the image forming units by a transfer medium and sequentially transferred. In a color image forming apparatus for forming a color image by means of the drive state detection means for detecting the drive state of each of the image forming units, and when the detection signal from the drive state detection means is abnormal, the abnormality signal is generated. Determination means for determining whether or not the image forming unit can form an image independently and an image forming mode of a color that can be formed by combining other normal image forming units, and an image forming mode determined by the determining means. Selection means for selecting one from among the selection means, and drive control of each of the image forming units according to the image creation mode selected by the selection means. It is configured to include a Cormorant control means.

As the drive state detecting means, for example,
The one that measures the drive state by measuring the interval of the output pulse output from the rotary encoder that detects the rotation speed of the photosensitive drum of each image forming unit is used.

As the drive state detecting means, when the frequency of the drive pulse supplied to the driver for driving the drive motor for rotationally driving the photosensitive drum of each image forming unit is significantly deviated from the standard frequency, For example, one that detects that a failure has occurred in the feedback control system that changes the angular velocity of the drive motor is used.

However, as the drive state detecting means,
Of course, other materials may be used.

As the driving means for driving the image forming unit, for example, a stepping motor is used, but other than this, a DC servo motor or a direct drive motor can be used for control.

[0022]

According to the present invention, the drive state detecting means detects the drive state of each image forming unit, and when the detection signal from the drive state detecting means is abnormal, the image forming unit which has generated the abnormal signal is detected. Whether or not an image can be formed independently and an image forming mode of a color that can be formed by combining other normal image forming units are judged by the judging means, and one of the image forming modes judged by the judging means is selected. When the user selects by the selecting means, the control means controls the drive of each of the image forming units according to the image forming mode selected by the selecting means, so that the drive system of any one of the plurality of image forming units is controlled. Even if a failure occurs, the color image forming operation can be continued within a possible range.

[0023]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to illustrated embodiments.

FIG. 2 shows an embodiment of the color image forming apparatus according to the present invention.

In FIG. 2, 1Y ', 1M', 1C ',
Reference numeral 1K ′ denotes an image forming unit that forms a toner image of each color of yellow, magenta, cyan, and black. The image forming units 1Y ′, 1M ′, 1C ′,
1K ′ is a photoconductor drum 1Y, 1Y having a toner image of each color of yellow, magenta, cyan, and black formed on its surface.
M, 1C and 1K are provided respectively, and these photoconductor drums 1Y, 1M, 1C and 1K are arranged in parallel at predetermined intervals. Each photoconductor drum 1
The surfaces of Y, 1M, 1C, and 1K have primary chargers 2Y and 2Y.
After being uniformly charged by M, 2C, and 2K, exposure optical systems 3Y and 3Y including a semiconductor laser and a polygon mirror are provided.
The images are sequentially exposed by M, 3C, and 3K to form an electrostatic latent image. These photosensitive drums 1Y, 1M, 1
The electrostatic latent image formed on the surface of C, 1K is the developing device 4Y,
4M, 4C, and 4K develop toners of yellow, magenta, cyan, and black, respectively, to form visible toner images. These visible toner images are discharged by pre-transfer charge eliminators 5Y, 5M, 5C, and 5K, Transfer sheet 10 is charged by transfer chargers 6Y, 6M, 6C, and 6K.
Transferred on top.

Photosensitive drums 1Y, 1M, 1C, 1K
The transfer paper 10 to which the toner images are sequentially transferred from is supplied from a paper feed cassette (not shown), and is conveyed while being electrostatically held on the transfer body conveyance belt 12, and the photosensitive drums 1Y, 1M ,. It is sequentially conveyed to the transfer position located below 1C and 1K. Then, each photosensitive drum 1Y,
The transfer paper 10 on which the toner images of the respective colors are sequentially transferred from 1M, 1C, and 1K is separated from the transfer body transfer belt 12 and is transferred to the fixing device unit 13, and the fixing device unit 13 transfers the respective colors on the transfer paper 10. The toner image of 1 is superposed and the color image is fixed.

On the other hand, the surface of each of the photosensitive drums 1Y, 1M, 1C and 1K on which the transfer of the toner image has been completed is charged by the post-transfer chargers 7Y, 7M, 7C and 7K to be discharged and cleaned. After the residual toner and the like are removed by the containers 8Y, 8M, 8C and 8K, the erase lamp 9Y,
Prepared for the next image formation by receiving static elimination by 9M, 9C and 9K.

FIG. 3 shows a mechanism for rotationally driving the photosensitive drum.

In the figure, reference numerals 1Y, 1M, 1C and 1K indicate the photoconductor drums, and the photoconductor drums 1Y, 1M, 1C and 1K are drive motors 15 each including a stepping motor, and The first gear 16 fixed to the drive shaft of the drive motor 15, the second gear 17 meshing with the first gear 16, the third gear 18 fixed to the same shaft as the second gear 17, It is adapted to be rotationally driven by a third gear 18 and a photoconductor gear 19 fixed to the rotary shafts of the photoconductor drums 1Y, 1M, 1C, and 1K meshing with each other. Moreover, the photosensitive drums 1Y, 1M, 1
Flywheels 30 as inertial bodies are attached to the rotation shafts of C and 1K, respectively.

Further, the photosensitive drums 1Y, 1M, 1
A rotary encoder 21 and a calibration rotary encoder 22 are attached to the rotary shafts of C and 1K, and the rotary encoder 21 and the calibration rotary encoder 22 include the photosensitive drums 1Y, 1M, and
It is for detecting the angular velocities of rotations of 1C and 1K. The rotary encoder 21 and the calibration rotary encoder 22 are connected to the control unit 20, and the drive motor 15 is connected to the control unit 20. As the rotary encoder 21, a relatively low precision one is used, for example, one that outputs 180 or 360 pulses per rotation is used. On the other hand, the rotary encoder 2 for calibration
As 2 is used a highly accurate one, for example, one that outputs 10,000 pulses per rotation is used.
The calibration rotary encoder 22 is attached to the rotary shafts of the photoconductor drums 1Y, 1M, 1C, and 1K only when adjusting the color image forming apparatus before shipment in the factory, and is shipped from the factory. It is designed to be removed from the device when it is opened.

FIG. 4 is a block diagram showing a circuit configuration of the control unit.

In the figure, reference numeral 23 is a photosensitive drum 1Y, 1
A CPU that controls the driving operation of M, 1C, and 1K, 24 is a PROM that stores programs executed by the CPU 23, predetermined data, and the like, and 25 is the rotary encoder 2
1 or RA for storing the time interval for each divided section, which is the data read from the calibration rotary encoder 22.
M and 21 are photosensitive drums 1Y and 1Y at the time of image recording.
A rotary encoder for detecting the rotational speeds of M, 1C, and 1K, 22 is a calibration rotary encoder for calibrating the rotary encoder 21 at the time of adjustment before shipment of the apparatus, and 26 is a pulse output from the calibration rotary encoder 22. A pulse counter 27 for dividing the pulse signal output from the pulse oscillator 28 to output a drive pulse having a predetermined command frequency. Reference numeral 29 denotes a drive pulse output from the interval counter 27. Is a drive motor driver that rotationally drives the drive motor 15.

Before the drive control of this embodiment is performed, the rotary drive mechanism for the photosensitive drums includes a drive motor 15 for rotationally driving the photosensitive drums 1Y, 1M, 1C and 1K.
Even when the command frequency to the photosensitive drum 1 is fixed,
Eccentricity of Y, 1M, 1C and 1K rotary shafts, and drive gear 1
As shown in FIG. 5, when the photoconductor drums 1Y, 1M, 1C, and 1K rotate due to the meshing error of 6, 17, 18, and 19 and the like, a positional error ( Represents the deviation from the ideal position) appears.

The relationship between the variation of the rotational angular velocity and the position error when the photosensitive drum rotates is generally expressed by the following equation (1). The integration of the position error X (t) is performed from 0 to t.

[0035] (number 1) ω (t) = ω 0 + Δω 1 · cosω 1 t + ... + Δω i · cosω i t + ... X (t) = ∫ω (t) dt = ω 0 · t + (Δω 1 / ω 1 _{) · sinω 1 t + ... ...} + (Δω i / ω i) · sinω i t + ... where, omega _0; average angular velocity [Delta] [omega _i of the photosensitive body shaft portion; angular amplitude in the vibration frequency f _i (0~pe
ak) f _i; oscillation frequency X (t); rotation angle

The result of the fast Fourier transform (FFT) analysis of the angular velocity of the photoconductor drum is shown in FIG.

As described above, the frequency f _i and the resonance frequency f _{n of the} system are controlled before the drive control of the photosensitive drum is performed.
Also, a peak with a high amplitude level appears in the vicinity of a high frequency due to the drive gear, etc., and due to the rotational fluctuation of the angular velocity of these photoconductor drums, color misregistration or color unevenness is sequentially generated in the color images transferred onto the transfer paper 10. Appears.

The rotational fluctuation of the angular velocity of the photosensitive drum is
Low-frequency fluctuations or the eccentricity component is generated by a rotation of the photosensitive drum as one period, variation of the intermediate frequency corresponding to the resonant frequency f _n of the system, or the like variations in the high-frequency component due to the drive gear.

By the way, in this embodiment, at the time of adjustment before shipment at the factory, the respective photosensitive drums 1Y, 1M, 1C,
A rotary encoder 21 for calibration is attached to the rotary shaft of 1K in addition to the rotary encoder 21, and a correction table is created as described below. here,
Although the work of creating the correction table for the photoconductor drum 1Y will be described, the same work is performed for the other photoconductor drums 1M, 1C, and 1K.

First, the correction table for the photosensitive drum 1Y is set.
In order to create, the photoconductor drum 1Y rotates at a constant angular velocity.
It is rotationally driven. That is, the photosensitive drum 1Y is rotated.
As shown in FIG. 4, the drive motor 15 for driving the drive motor 15
Output pulse S of oscillator 28 _OSCIs an interval count
Drive pulse S of a predetermined frequency after being divided by_DPWhen
Will be supplied. This drive pulse S_DPFrequency is P
Standard frequency f in ROM 24_SPre-stored as
It The CPU 23 reads the standard frequency f from the PROM 24._Sof
Read the data and use this as preset data S_PSAs
The interval counter 27 is loaded. interval
The counter 27 outputs the pulse output from the pulse oscillator 28.
Count and the count value of this pulse is preset
Data S_PSDrive pulse S every time_DPIs output. So
Then, the photosensitive drum 1Y is driven by the drive motor driver 2
Predetermined frequency f supplied to the drive motor 15 via 9_S
Drive pulse S_DPIt is driven to rotate by.

Then, as the photoconductor drum 1Y rotates, the rotary encoder 21 and the calibration rotary encoder 22 mounted on the rotary shaft of the photoconductor drum 1Y will be shown in FIGS. d) As shown in (e), output pulses S1 _RE , S2 _RE and zero phase pulse S
1 ₀ is output to the CPU 23. At that time, the output pulse S2 _RE of the calibration rotary encoder 22 is output to the CPU 23 via the pulse counter 26. The output pulses S1 _RE and S2 _RE are output by the rotary encoder 2
1 is a signal output each time the rotary encoder 22 for calibration rotates by a predetermined angle. Further, the zero phase pulse S1 ₀ is for the rotary encoder 21 is output at the reference position upon rotation, both encoders 2
The reference positions of 1 and 22 are set to the same position.
The pulse counter 26 frequency-divides the output pulse S2 _RE output from the calibration rotary encoder 22 as shown in FIG. 7C, and outputs C as an interrupt signal S _INT.
It is designed to be output to the PU 23.

When the correction table is created, the rotational fluctuation of the photosensitive drum 1Y is changed by the rotary encoder 2.
1 and the rotary encoder 22 for calibration. Pulses S1 _RE , S2 output from the rotary encoder 21 and the rotary encoder 22 for calibration
_RE is output at almost constant intervals unless the rotation of the photosensitive drum 1Y fluctuates, and the number of pulses output in a predetermined divided section is always constant. However, if the rotation of the photosensitive drum 1Y changes, the output intervals of the pulses S1 _RE and S2 _RE output from the rotary encoder 21 and the calibration rotary encoder 22 change, and FIG.
As shown in, the time interval output in a predetermined divided section differs depending on each divided section.

Therefore, the CPU 23 uses the calibration rotary
Output pulse S2 of encoder 22 _REPulse counter
Interrupt signal S divided by 26_INTIs input
And the interval driven by the output of the pulse oscillator 28.
The count value of the counter 27b is read and shown in FIG.
As described above, the data is stored in the RAM 25. That is, the CPU 23
Is the zero phase pulse S1 from the rotary encoder 21.₀
Is input, the first interrupt signal S_INTInterrupted by
When there is a count of the interval counter 27b
Interval T corresponding to the first section by reading the value
₁Is stored in the RAM 25. Then, the next section
Corresponding interrupt signal S_INTIs also read when is input
And the cow of the interval counter 27b read before
The difference between the interval value and the interval value of the interval T₂But
It is measured and stored in the RAM 25. This work is for calibration
This is repeated for one cycle of the rotary encoder 22.

At this time, the zero phase pulse S1 ₀ of the rotary encoder 21 is supplied to the CPU 23, and the initial value of the address is C based on the zero phase pulse S1 _0.
The interval T _(N) of each divided section is set by the PU 23, and thereafter, by adding and designating the address by a constant value for each section, that is, every time the interrupt signal S _INT is input, the interval T _(N) of each divided section is determined by the RAM 25. Stored in.

The CPU 23 also drives the interval counter 27 driven by the output of the pulse oscillator 28 each time the output pulse S1 _RE of the rotary encoder 21 is input.
The count value of b is read, and as shown in FIG.
25. That is, the CPU 23, like the rotary encoder 22 for calibration, uses the rotary encoder 2
When the next output pulse S1 _RE is input after the zero phase pulse S1 ₀ is input from 1, the interval counter 2
The count value of 7b is read and stored in the RAM 25 as the interval T _L1 corresponding to the first section. Then, when the output pulse S1 _RE corresponding to the next section is also read in the same manner, the difference from the previously read count value of the interval counter 27b is calculated, the interval T _{L2 of} that section is measured, and the RAM 25 Stored in. This work is also repeated for one cycle of the rotary encoder 21.

At this time, the calibration rotary encoder 22
The number of interrupt signals S _INT that are output during one rotation of is set equal to the number of output pulses S1 _RE that are output during one rotation of the rotary encoder 21. That is,
The division intervals for counting the time intervals of the pulses S2 _RE and S1 _RE output from the calibration rotary encoder 22 and the rotary encoder 21 are both set to the same number.

The interval counter 27b is used.
Is driven by the output S _OSC having a constant frequency from the pulse oscillator 28, the count value of the interval counter 27b indicates the elapsed time.

Next, the CPU 23 causes the difference C _(N) = _{TL (N)} between the interval T of the divided section of the rotary encoder 22 for calibration stored in the RAM 25 and the interval T _L of the divided section of the rotary encoder 21. −T _(N) is calculated for each divided section, and the calculation result C _(N) is stored in the RAM 25 for each divided section, as shown in FIG.

This completes the work of creating the correction table. As described above, the work of creating this correction table is as follows.
This is performed when adjusting the color image forming apparatus in the factory.

When the user uses the color image forming apparatus, the CPU 23 uses the rotary encoder 21 based on the following correction formula during the image forming operation.
Is corrected and the photosensitive drum is driven to rotate.

(Equation 2) f _n = f _s [1 + α {FILS / T _ID -1} + β {T _ID / (FILD-ΣDIFF) -1}]

The correction formula is not limited to the above formula (2), and a correction formula as shown in the following formula (3) may be used.

(Equation 3) f _n = f _s [1 + α {FILS / T _ID −1}] × [1 + β {T _ID / (FILD−ΣDIFF) −1}]

Here, f _n ; corrected section frequency f _s ; standard frequency FILS stored in advance in PROM 24; interval value T _(N) of calibration rotary encoder 22 stored in RAM 25 as a correction table, Interval value after averaging from the section (N) to the previous section of only m, that is, FILS = 1 / m (T _{(N-m + 1)} + T _{(N-m + 2)} + T
_{(N-m + 3)} + ... + T _(N-1) + T _(N) ) FILD; Interval of the rotary encoder 22 read in real time and the ideal value ΣT _ID of one section ahead in the correction operation, which is stored in the PROM 24 in advance. The value after averaging the calculated value (ΣT _ID −ΣT ′ _(N) ) of the difference with the value ΣT ′ _{(N )} over the interval m from the interval (N), that is, FILD = 1 / M {(ΣT _ID −ΣT ′ _{(N-m + 1)} ) + (Σ
T _ID -ΣT ' _{(N-m + 2)} ) + (ΣT _ID -ΣT' _{(N -m + 3)} )
+ ... + (ΣT _ID −ΣT ' _(N-1) ) + (ΣT _ID −ΣT'
_(N) )} ΣDIFF; The interval values _{TL (N)} and T _(N) of the rotary encoder 21 and the calibration rotary encoder 22 are set to the interval m from the interval (N) to the previous m, like FILS. After averaging over, the difference data (FILS-FILS) of these values is integrated from the zero-phase pulse, that is, ΣDIFF = Σ (FILS-FILS) 0 to N = Σ {1 / m ( _{TL (N-m + 1)} ＋ _{TL (N-m + 2)} ＋ _{TL (N-m + 3)} ＋
… + _{TL (N-1)} + _{TL (N)} )-1 / m (T _{(N-m + 1)} + T
_{(N-m + 2)} + T _{(N-m + 3)} + ... + T _(N-1) + T _(N) )} (Σ
Is from 0 to N) T _ID ; ideal calculation value for one section pre-stored in the PROM 24, that is, an interval value for one section in an ideal state in which there is no rotational fluctuation of the rotation axis of the photosensitive drum α; PROM24 Is a constant β of the feed-forward unit prestored in P; a constant of the feedback unit pre-stored in the PROM 24. The above equations employ averaging, but depending on the system, weighting can be used to change the coefficient of each section.

It should be noted that, based on the experimental data and the logical analysis using only the feedforward control, since there is no response delay when α = 1, the previously known value can be corrected by that amount, so the correction effect is the best. Therefore, when α = 1 is substituted into the equation (1) (α is set to a value close to 1 depending on the system), the following equation 4 is obtained.

(Equation 4) f _n = f _s [1 + α {FILS / T _ID -1} + β {T _ID / (FILD-ΣDIFF) -1}] = f _s × (FILS / T _ID ) + f _s × β _{{T ID / (FILD-ΣDIFF} ) -1} = f s × (FILS / T ID) + β × {f s · T ID / (FILD-ΣDIFF) -f s}

In this embodiment, the equation (4) is used as the correction equation.

By the way, in the image forming apparatus according to this embodiment, the drive control of the photosensitive drum is performed as follows. That is, in order to form a color image in the color image forming apparatus, as shown in FIG. 2, the photoconductor drums 1Y, 1M, 1C and 1K are rotationally driven, and the photoconductor drums 1Y, 1M, 1C, and Yellow, magenta, cyan, and black toner images are formed on the surface of 1K, respectively. At that time, the photosensitive drums 1Y, 1M, 1C, 1
The rotation state of K is controlled as follows.

When starting the formation of a color image, the CP
As shown in FIG. 4, U23 drives the drive motor 15 at the standard frequency f _s , and the photoconductor drums 1Y, 1M, 1
Rotate C and 1K. And the rotor encoder 2
After detecting the zero phase pulse S1 ₀ of 1, the correction and the interval value T table stored _(N), based on the output from the rotary encoder 22 measured interval value T _(N) in each section , The correction frequency f _n of the next section (N + 1) is calculated according to the equation (4), and is output to the drive motor 15.

The correction of the drive frequency based on the equation 4 is
Although it is always performed, it may be performed at a predetermined time if necessary.

Then, the CPU 23 outputs the data of the corrected driving frequency f _n to the interval counter 27, and the interval counter 27 outputs the pulse oscillator 28.
The pulses output from are counted, and the drive pulse S _DP is output every time the count value of this pulse reaches a value corresponding to the corrected drive frequency f _n . As a result, the photosensitive drum 1Y is rotationally driven by the drive pulse S _DP having the corrected drive frequency f _n supplied to the drive motor 15 via the drive motor driver 29.

Now, if there is no rotational fluctuation of the photosensitive drum 1Y, FILS, FILD and Σ
For each value of DIFF, FILS and FILD are T _ID ,
Since ΣDIFF becomes 0, the equation 4 becomes f _n = f _s , and as a matter of course, the corrected drive frequency f _n becomes equal to the standard frequency f _s .

Next, when the interval value of the correction table in the Kth section to be corrected is long, that is, when the drive motor 15 is rotated at a constant angular speed, the angular speed of the photosensitive drum 1Y is the Kth section. If it is late at, the interval values T _(K) and T ' _{(K) of the section} are T
_Since it becomes larger than _ID , the correction frequency f _n becomes higher as follows. The value of T ' _(K) is strictly unknown because it is read in real time. That is, in the correction equation number 4, FILS, FILD and ΣDIF
Each value of F is as follows.

FILS is 1 / m (T_{(N-m + 1)}+ T
_{(N-m + 2)}+ T_{(N-m + 3)}+ ... + T_(N-1)T_(N-1)+
T_(N)), So T_IDWill be a larger value. Well
FILD is 1 / m {(ΣT_ID-ΣT '_{(N-m + 1)})
+ (ΣT_ID-ΣT '_{(N-m + 2)}) + (ΣT _ID-ΣT '
_{(N-m + 3)}) + ... + (ΣT_ID-ΣT '_(N-1)) + (ΣT_ID
-ΣT '_(N))}, So T_IDClose to, but T_IDThan
Is also a small value. Furthermore, ΣDIFF is Σ {1 / m
(T_{L (N-m + 1)}+ T_{L (N-m + 2)}+ T_{L (N-m + 3)}+ ... + T_{L (N-1)}
+ T_{L ( N)}) -1 / m (T_{(N-m + 1)}+ T_{(N-m + 2)}+ T
_{(N-m + 3)}+ ... + T_(N-1)+ T_(N))}
The interval values of the rotary encoders 21 and 22 are equal.
If it is 0, the calibration rotary encoder 22 interface
If the global value is larger, it becomes negative, and the rotary
If the interval value of the coder 21 is larger, it becomes positive
The value is a small value close to 0.

Therefore, the number of correction equations 4
4'in the equation, (FILS / T _ID) Is close to 1
Value is greater than 1, and T_ID/ (FILD-ΣD
The value of (IFF) is close to 1, but is larger than 1.

The value of β in the correction equation (2) is set appropriately.

As a result, the value of f _{n in} the correction equation 4 is f
_The value becomes larger than _s , and the correction frequency f _n becomes higher. As a result, the photosensitive drum 1 driven by the drive motor 15
The angular velocity of rotation of Y is controlled to be constant, and the peripheral speed of the photosensitive drum 1Y is constant.

Further, due to changes over time, changes in temperature, etc., the expansion and contraction of the shape and dimensions affect the runout of the gear tooth grooves or the total pitch engagement error, which changes the eccentric component. When the angular velocity of the Kth section is increased due to the change over time, the temperature change, or the like, that is, when the interval value of the correction table of the Kth section to be corrected is short, the interval value T of the section is _{Since L (K)} and T _(K) are smaller than T _ID , the correction frequency f _n
Becomes lower as follows. That is, in the correction equation 4, the respective values of FILS, FILD and ΣDIFF are as follows.

FILS is 1 / m (T _{(N-m + 1)} + T
_{(N-m + 2)} + T _{(N-m + 3)} + ... + T _(N-1) + T _(N-1) + T
_(N) ), the value is smaller than T _ID . Also,
FILD is 1 / m {(ΣT _ID −ΣT ′ _{(N-m + 1)} ) +
(ΣT _ID −ΣT ' _{(N-m + 2)} ) + (ΣT _ID −ΣT'
_{(N-m + 3)} ) + ... + (ΣT _ID −ΣT ' _(N-1) ) + (ΣT _ID
Because it is -ΣT _'(N))}, is close to T _ID value larger than T _ID. Furthermore, ΣDIFF is Σ {1 / m
( _{TL (N-m + 1)} + _{TL (N-m + 2)} + _{TL (N-m + 3)} + ... + _{TL (N-1)}
+ T _{L ( N)} −1 / m (T _{(N-m + 1)} + T _{(N-m + 2)} + T
_{(N-m + 3)} + ... + T _(N-1) + T _(N) )}, the interval values of both rotary encoders 21 and 22 are equal to 0, and the interval value of the calibration rotary encoder 22 is the same. Is negative when the value is large, and positive when the interval value of the rotary encoder 21 is larger, and the value is a small value close to zero.

Therefore, the number of correction equations 4
4'in the equation, (FILS / T _ID) Is close to 1
However, the value becomes smaller than 1, and T_ID/ (FILD-ΣD
The value of (IFF) is close to 1 but smaller than 1.

As a result, the value of f _{n in} the correction equation 4 is f
_The value becomes smaller than _s , and the correction frequency f _n becomes low. As a result, the photosensitive drum 1 driven by the drive motor 15
The angular velocity of rotation of Y is controlled to be constant, and the peripheral speed of the photosensitive drum 1Y is constant.

As described above, according to this embodiment, not only the change in the angular velocity of the photosensitive drum unique to each image forming apparatus can be corrected, but also the dynamic angular velocity caused by the change with time, the change in temperature, etc. Changes can also be corrected.

Moreover, the rotary encoders 21 and 22
The interval value output from is used for correction after being averaged by the CPU 23.
Even when the angular velocity division information stored in the RAM 25 is greatly increased, the interval values output from the rotary encoders 21 and 22 are averaged and the variation due to the correction is increased. As a result, it serves as a vibration source for the system from the drive motor 15 through the gear to the rotation axis of the photosensitive drum, and it is possible to prevent the possibility of oscillation and increase in the amplitude of the natural frequency of the system.

The calibration rotary encoder 22 is used only at the time of adjustment in the factory, and is removed from the color image forming apparatus at the time of shipment from the factory, so that the cost of the color image forming apparatus can be increased without increasing the cost. The accuracy can be controlled.

As a result, the four photosensitive drums 1Y, 1
The velocities of the M, 1C, and 1K transfer portions are constant, and the positional deviation between the transfer portions can be reduced. For example, the corrected position error Δx becomes extremely small as shown in FIG.

The position error is measured by a measuring device based on the output of the rotary encoder 21 by the method described below.

That is, the output of the rotary encoder 21 is F / V converted, further A / D converted, and the digital value is stored in a proper sampling cycle. Then, the stored digital values are averaged, and the difference between each digital value and the average value is obtained. Since this difference is a difference in speed, this is integrated over time to obtain the position error.

Further, in this embodiment, the flywheels 30 as inertial members are attached to the rotary shafts of the photoconductor drums 1Y, 1M, 1C and 1K, respectively, so that the gear teeth are high. It is possible to prevent the frequency component from fluctuating, and it is possible to prevent the occurrence of color unevenness or the like in the image.

That is, the fluctuations of these high frequency components can be suppressed by optimally selecting the flywheel 30 as an inertial body.

It has been found that when the size of the flywheel 30 (inertia moment J _L ) is within the following range, the high frequency component can be reduced and the correction control also works effectively. J _L ≦ 0.5 Kg · cm · s ²

FIG. 13 shows photosensitive drums 1Y, 1M, 1C, 1
FIG. 14 shows the case where the flywheel 30 of J _L = 0.25 kg · cm · s ² is attached to the rotary shaft of K, and the flywheel 30 is attached to the rotary shafts of the photosensitive drums 1Y, 1M, 1C, and 1K in FIG. The case is not shown. As is clear from these figures, the photosensitive drums 1Y, 1M, 1C, 1
By attaching the flywheel 30 to the rotary shaft of K, the high frequency component near 100 Hz can be suppressed small.

By combining the above methods, the position error and the FFT analysis result due to the fluctuation of the rotational angular velocity at the shaft of the photosensitive drum are as shown in FIGS.
It can be seen that the rotational fluctuation can be suppressed to a very small level.

Further, in the above embodiment, it is possible to perform only the feedback control, and in this case, the adjustment process such as the mounting of the high precision encoder for creating the fixed pattern is unnecessary, and the memory for the fixed pattern is also used. The cost is reduced because it is unnecessary.

By the way, as described above, the photosensitive drum 1
Even when the rotations of Y, 1M, 1C, and 1K are controlled with high accuracy, the rotary encoder 21 that detects the rotation speed of each of the photoconductor drums 1Y, 1M, 1C, and 1K is not clogged with slits or the optical sensor is disconnected. If a failure occurs in the feedback control system that changes the angular velocity of the drive motor based on the rotation speed information, the photosensitive drums 1Y, 1M, 1C, 1K in which the failure has occurred are driven to rotate with high accuracy. Can not be. Therefore, in the above-described image forming apparatus, it is not possible to accurately superimpose the toner images of the respective colors formed by the photosensitive drums 1Y, 1M, 1C, and 1K to record a predetermined color image.

In view of this, in this embodiment, when the drive state detecting means for detecting the drive state of each image forming unit and the detection signal from the drive state detecting means are abnormal, the image forming which has generated the abnormal signal is generated. One of the image forming mode judged by the judging unit, which judges whether the unit can form an image independently and an image forming mode of a color which can be formed by combining other normal image forming units. It is configured to include a selection unit that selects one of the image forming units and a control unit that controls the drive of each of the image forming units in accordance with the image creation mode selected by the selection unit.

FIG. 1 is a block diagram showing a control system for controlling the operation of the color image forming apparatus when a failure occurs in the drive system of the color image forming apparatus according to this embodiment.

In FIG. 1, reference numerals 40 and 41 denote drive state detecting means for detecting the drive state of each image forming unit. As shown in FIG. 4, the drive state detecting means 40 obtains the interval of the output pulse S1 _RE output from the rotary encoder 21 by the count value of the interval counter 27b, and the rotary encoder 21
When the interval of the output pulse S1 _RE output from is outside the predetermined range, it is detected as an abnormality of the rotary encoder or the optical sensor 21. Further, the other drive state detecting means 41 drives when the frequency of the drive pulse S _DP supplied to the driver 29 which drives the drive motor 15 deviates significantly from the standard frequency f _s , as shown in FIG. It detects that a failure has occurred in the feedback control system that changes the angular velocity of the motor 15. These drive state detecting means 40 and 41 are configured, for example, so that the CPU 23 shown in FIG. 4 also has the same function, but may be configured by their own drive state detecting means. In addition, each image forming unit 1Y ′, 1M ′, 1
When the intervals of the output pulses S1 _RE output from the C ′ and 1K ′ rotary encoders 21 are compared with each other and fall outside a predetermined range, it is detected as an abnormality of the rotary encoder or the optical sensor 21. Further, the drive pulse S supplied to the driver 29 that drives the drive motors 15 of the image forming units 1Y ′, 1M ′, 1C ′, and 1K ′.
_It may be possible to detect that a failure has occurred in the feedback control system in which the frequency of _DP changes the angular velocity of the drive motor 15.

Reference numeral 42 denotes each image forming unit 1Y ', 1
The determination means 4 is a determination means to which signals from the drive state detection means 40 and 41 of M ′, 1C ′, and 1K ′ are input.
2 is each image forming unit 1Y ', 1M', 1C ', 1
Which image forming unit 1 based on the drive state detection signals output from the drive state detection means 40 and 41 of K ′.
It is to determine whether a failure has occurred in Y ', 1M', 1C ', 1K'. Reference numeral 43 is a selection unit that selects which image forming mode the image forming operation is to be executed on the basis of the determination signal from the determination unit 42. Reference numeral 44 denotes each image forming unit 1Y ′, 1M ′, based on the signal from the selecting means.
The control unit 1C ', 1K' is controlled by the control unit 44. For example, the CPU 23 shown in FIG. 4 is also used as the control unit 44.

With the above-described structure, in the color image forming apparatus according to the present embodiment, the slits are formed in the rotary encoder 21 for reading the change information of the rotational states of the photosensitive drums 1Y, 1M, 1C and 1K as follows. Continuation of color image forming operation in possible image forming mode even if clogging, disconnection of optical sensor, etc. occur, or failure of feedback control system that changes angular velocity of drive motor based on change information of rotation state You can do it.

That is, in the color image forming apparatus, as shown in FIG.
Driving of M ', 1C', and 1K 'is started (step 1). Next, as shown in FIG. 1, this color image forming apparatus detects the driving state of each image forming unit 1Y ', 1M', 1C ', 1K' by the driving state detecting means 40, 41 (step 2). . Then, when the image forming units 1Y ', 1M', 1C ', and 1K' are normally driven, the normal image forming operation is executed (step 3).

On the other hand, the image forming units 1Y ', 1
When an abnormality is detected in the driving states of M ′, 1C ′, and 1K ′, whether or not the image forming unit in which the abnormality is detected can form an image is detected by the detection signals of the driving state detecting units 40 and 41. It is determined (step 4). In this case, for example, even when an abnormality is detected by the drive state detection unit 40 that detects the abnormality of the rotary encoder 21 based on the interval of the output pulse S1 _RE output from the rotary encoder 21, the driver that drives the drive motor 15 When the frequency of the drive pulse S _DP supplied to 29 is not largely deviated from the standard frequency f _s , it is determined that image formation is possible.

Thereafter, when it is judged by the judging means 42 shown in FIG. 1 that image formation is possible (step 5),
The image forming possible mode is selected by the selecting means 43 (step 6). Here, for example, when an abnormality occurs in the black image forming unit 1K ', the other yellow, magenta, and cyan image forming units 1Y', 1M 'are used without using the black image forming unit 1K'. ,
By using 1C ', it is possible to form a normal color image.
A full-color image forming mode by color is selected. Further, when an abnormality occurs in the yellow image forming unit 1Y ', full-color image formation becomes impossible. Therefore, image forming modes other than the color image formation of the color in which the yellow color is superimposed are formed. Is selected.

In this case, the image formable mode selected by the selecting means 43 is blinking although the "yellow mode has failed on the display unit 50 of the operation panel as shown in FIG. The color can be copied ”message is displayed. In addition, in FIG.
Reference numeral 1 denotes a keyboard in which a large number of keys 52 for designating colors capable of forming an image are arranged. In addition to this display, a message prompting the repair of the image forming unit having the failure is also displayed on the operation panel as usual.

When the user specifies a color capable of forming an image on the operation panel, the image forming unit in which the abnormality has occurred does not perform the feedback control, and the oscillator 28
The pulse output from is driven to open only by the drive pulse S _DP via the interval counter 27 (step 7), and the image of the color formed by the image forming unit in which the abnormality has occurred is independently formed. .
This is because even if the image forming unit has an abnormality,
This is because if the image can be formed independently, the image can be formed in a state where the image quality is not hindered. Further, the other normal image forming units execute a color image forming mode of colors that can be formed by combining these (step 8) and form a color image (step 9).

On the other hand, when the determination means 42 shown in FIG. 1 determines that the image formation is impossible (step 10), the selection means 43 stops the driving of the image forming unit having the abnormality (step 11). The color of the abnormal unit is displayed on the operation panel (step 12). Here, for example, when an abnormality occurs in the black image forming unit 1K ′ and it is determined that black image formation is impossible,
The black image forming unit 1K 'is stopped. But,
Since a full-color image can be formed without using the black image forming unit 1K ', in this case, the full-color image forming mode of three colors of yellow, magenta and cyan is selected (step 13). Further, when an abnormality occurs in the yellow image forming unit 1Y ', full-color image formation becomes impossible. Therefore, image forming modes other than the color image formation of the color in which the yellow color is superimposed are formed. Is selected (Step 1
4). Then, an image forming operation of a single color or a plurality of colors is executed (step 15), and the operation ends.

As described above, in the color image forming apparatus, the driving state detecting means 40 and 41 detect the driving states of the respective image forming units 1Y ', 1M', 1C 'and 1K', and the driving state detecting means are used. When the detection signals from 40 and 41 are abnormal, whether or not the image forming units 1Y ′, 1M ′, 1C ′, and 1K ′ that have generated the abnormal signal can form an image independently and other normal image formation. Unit 1
The determination unit 42 determines the image forming mode of the color that can be formed by combining Y ′, 1M ′, 1C ′, and 1K ′.
When the user selects one of the image forming modes determined by the determining unit 42 by the selecting unit 43, the image forming units 1Y ′, 1M are selected according to the image forming mode selected by the selecting unit 43. ',
The drive control of 1C 'and 1K' is performed by the control means 44.

Therefore, the plurality of image forming units 1
Even if a drive system failure occurs in any one of Y ', 1M', 1C ', and 1K', the image forming operation can be continued in the possible image forming mode. Even before the end, it is possible to effectively use the color image forming apparatus to execute the formation of the color image.

[0098]

According to the present invention, which has the above-described structure and operation, the color image forming operation is continued within a possible range even when a drive system failure occurs in any of the plurality of image forming units. Therefore, it is an object of the present invention to provide an image forming apparatus capable of effectively operating the apparatus.

[Brief description of drawings]

FIG. 1 is a block diagram showing a control unit of an embodiment of a color image forming apparatus according to the present invention.

FIG. 2 is a block diagram showing an embodiment of a color image forming apparatus according to the present invention.

FIG. 3 is a configuration diagram showing a drive system of the image forming unit.

FIG. 4 is a block diagram showing a drive control system of the image forming unit.

FIG. 5 is a graph showing fluctuations in rotation of the photosensitive drum.

FIG. 6 is a graph showing fluctuations in rotation of the photosensitive drum.

7A to 7E are waveform charts showing control signals, respectively.

FIG. 8 is an explanatory diagram showing a memory table.

FIG. 9 is an explanatory diagram showing a memory table.

FIG. 10 is an explanatory diagram showing a memory table.

FIG. 11 is a graph showing fluctuations in rotation of the photosensitive drum.

FIG. 12 is a graph showing fluctuations in rotation of the photosensitive drum.

FIG. 13 is a graph showing fluctuations in rotation of the photosensitive drum.

FIG. 14 is a graph showing fluctuations in rotation of the photosensitive drum.

FIG. 15 is a flowchart showing the operation of an embodiment of the color image forming apparatus according to the present invention.

FIG. 16 is an explanatory diagram showing a display panel.

FIG. 17 is a block diagram showing a conventional color image forming apparatus.

[Explanation of symbols]

1Y ', 1M', 1C ', 1K' image forming unit,
40, 41 drive state detection means, 42 determination means, 43
Selection means, 44 control means.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Minoru Kasama 2274 Hongo, Ebina City, Kanagawa Prefecture Fuji Xerox Co., Ltd. (72) Inventor Toshio Ansai 2274, Hongo Ebina City, Kanagawa Prefecture Fuji Xerox Co., Ltd.

Claims (1)

[Claims] 1. A color image forming a color image by arranging a plurality of image forming units side by side and transferring the images formed by the respective image forming units to a transfer position of each of the image forming units and sequentially transferring the transfer medium. In the forming apparatus, when the drive state detection unit that detects the drive state of each of the image forming units and the detection signal from the drive state detection unit are abnormal, the image forming unit that has generated the abnormal signal independently displays an image. Judgment means for judging whether or not formation is possible and an image formation mode of a color that can be formed by combining other normal image formation units, and selection for selecting one from the image formation modes judged by the judgment means Means, and a control means for controlling the drive of each of the image forming units according to the image forming mode selected by the selecting means. Color image forming apparatus according to claim.