Position setting method of photosensitive body and image forming apparatus Number:7,395,012 from the United States Patent and Trademark Office (PTO) owispatent

Title: Position setting method of photosensitive body and image forming apparatus

Abstract: A drum/belt drive starting procedure simultaneous starts the rotation of photosensitive drums and the conveyance of a belt. A drum/belt driving time calculating procedure sets a rotational drive stop time of each photosensitive drum and a conveyance drive stop time of the conveying belt on the basis of a detection result of a reference mark detecting unit. A drum/belt drive stopping procedure stops the rotation of each photosensitive drum and the conveyance of the conveying belt on the basis of a calculation result of the drum/belt driving time calculating procedure. Rotational phase differences of every plural photosensitive drums are set to a predetermined value in the state where rubbing with the belt is small.

Patent Number: 7,395,012 Issued on 07/01/2008 to Kojima

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Inventors:	Kojima; Tomoyuki (Tokyo, JP)
Assignee:	Oki Data Corporation (Tokyo, JP)
Appl. No.:	11/144,894
Filed:	June 3, 2005

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Foreign Application Priority Data

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Jun 04, 2004 [JP]			2004-167018

Current U.S. Class:	399/167 ; 399/300; 399/302; 399/308
Current International Class:	G03G 15/00 (20060101)
Field of Search:	399/167,298,299,300,302,303,308

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References Cited [Referenced By]

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U.S. Patent Documents

6070041	May 2000	Nakayasu et al.
6360070	March 2002	Taka et al.

Foreign Patent Documents

	2000-187428		Jul., 2000		JP

Primary Examiner: Gray; David M
Assistant Examiner: Walsh; Ryan D.
Attorney, Agent or Firm: Panitch Schwarze Belisario & Nadel LLP

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Claims

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What is claimed is:

1. An image forming apparatus comprising: a first photosensitive body and a second photosensitive body; a first driving unit which gives a driving force to said first photosensitive body and a second driving unit which gives a driving force to said second photosensitive body; a belt which is arranged between said first photosensitive body and said second photosensitive body; a first mark whose position changes when said first photosensitive body rotates and a second mark whose position changes when said second photosensitive body rotates; a first detecting unit which detects said first mark and a second detecting unit which detects said second mark; and a control unit which obtains a time difference between timing when said first mark passes through said first detecting unit and timing when said second mark passes through said second detecting unit, determines a time that is required until said second driving unit is stopped after the stop of said first driving unit, and set stop timing of said belt driving unit to timing between the stop of said first driving unit and the stop of said second driving unit.

2. The apparatus according to claim 1, wherein said first photosensitive body has a first photosensitive body gear which receives the driving force from said first driving unit, said second photosensitive body has a second photosensitive body gear which receives the driving force from said second driving unit, a position of said first mark is rotated synchronously with rotation of said first photosensitive body gear, and a position of said second mark is rotated synchronously with rotation of said second photosensitive body gear.

3. The apparatus according to claim 1, wherein said control unit stops said belt driving unit together with the stop of said first driving unit and, thereafter, stops said second driving unit.

4. The apparatus according to claim 1, further comprising: a first photosensitive body driving gear which has said first mark, receives the driving force from said first driving unit, and rotates said first photosensitive body; and a second photosensitive body driving gear which has said second mark, receives the driving force from said second driving unit, and rotates said second photosensitive body.

5. The apparatus according to claim 4, wherein said control unit stops said belt driving unit together with the stop of said first driving unit and, thereafter, stops said second driving unit.

6. An image forming apparatus comprising: a plurality of photosensitive bodies and a plurality of driving units which rotate said photosensitive bodies; a belt driving unit which drives a belt that is come into contact with said plurality of photosensitive bodies; a plurality of detection sections which are rotated in association with the rotation of said photosensitive bodies; a plurality of detecting units each of which is arranged so as to face each of said rotated detection sections at a same fixing position and outputs a detection signal when each of said rotated detection sections is detected at the facing position; a drive control unit which simultaneously drives said plurality of driving units and drives said belt driving unit so as to simultaneously rotate said plurality of photosensitive bodies; a stop control unit which discriminates rotational phase differences of said photosensitive bodies on the basis of a reception time difference each time a detection signal is received from each of said detecting units and sequentially stops the driving units of said photosensitive bodies in correspondence to said rotational phase differences; and a belt drive stopping unit which stops said belt driving unit before the driving unit whose stopping order is the last is stopped after said driving unit whose stopping order is the first was stopped.

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Description

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BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a position setting method of a photosensitive body for suppressing friction between a belt and the photosensitive body as an image holding body and to an image forming apparatus.

2. Related Background Art

Ordinarily, in a multicolor image forming apparatus such as a color electrophotographic printer or the like, for example, image holding bodies (hereinbelow, referred to as photosensitive drums) for forming toner images of K (black), Y (yellow), M (magenta), and C (cyan) are arranged in a line on a belt at predetermined intervals. In the following description, for example, it is assumed that the photosensitive drums are arranged from an upstream in the conveying direction of a sheet toward its downstream in order of K, Y, M, and C.

In order to simultaneously start the rotation of those four photosensitive drums, simultaneously stop the rotation of them, and laminate the toner images of four colors of K, Y, M, and C to the same position on the sheet conveyed on the belt, assuming that time when an image is irradiated onto the photosensitive drum K is set to reference time Tk, time Ty when an image is irradiated onto the photosensitive drum Y has to be equal to time after the elapse of time Lky/V obtained by dividing a distance Lky from a contact point of the photosensitive drum K and the belt to a contact point of the photosensitive drum Y and the belt by a conveying speed V of the belt.

Time Tm when an image is irradiated onto the photosensitive drum M has to be equal to time after the elapse of time Lkm/V obtained by dividing a distance Lkm from the contact point of the photosensitive drum K and the belt to a contact point of the photosensitive drum M and the belt by the conveying speed V of the belt. Similarly, time Tc when an image is irradiated onto the photosensitive drum C has to be equal to time after the elapse of time Lkc/V obtained by dividing a distance Lkc from the contact point of the photosensitive drum K and the belt to a contact point of the photosensitive drum C and the belt by the conveying speed V of the belt.

However, even if the irradiating time is accurately set as mentioned above, there is a case where a positional deviation (hereinafter, referred to as a color drift) occurs among the toner images of four colors of K, Y, M, and C due to an eccentricity or the like of each of the photosensitive drums. A technique for making the occurrence of the color drift inconspicuous by periodically making the generation timing of the color drift coincident has been disclosed (for example, refer to JP-A-2000-187428).

According to such a technique, a reference point is added to each of the four photosensitive drums and phase differences among the reference points of the four photosensitive drums are always held constant, thereby periodically making the generation timing of the color drift coincident. To always hold the phase differences among the reference points of the four photosensitive drums constant, a process for setting the phase differences among the reference points is executed in accordance with a predetermined procedure at a print start time point.

According to the process for setting the phase differences among the reference points, the belt is conveyed and while simultaneously rotating the four photosensitive drums, the reference points of the four photosensitive drums are detected by using reference mark detecting units. Subsequently, while the belt is conveyed, the photosensitive drums are individually stopped on the basis of each reference point detecting time (the photosensitive drum and the belt enter a rubbing state).

When the last photosensitive drum is stopped, the belt is also simultaneously stopped and the process for setting the phase differences among the reference points is completed. The stop time of each photosensitive drum is calculated from a relation between the reference point detecting time obtained by the reference point detecting units and rotational speeds of the four photosensitive drums. By setting the stopping timing as mentioned above, each time a printing step is executed, the phase differences among the four photosensitive drums are held constant, so that the color drift becomes inconspicuous.

A problem to be solved is that in the process for setting the phase differences among the reference points of the above conventional technique, while the belt is conveyed, the photosensitive drums are individually stopped on the basis of each reference point detecting time and simultaneously with the stop of the last photosensitive drum, the belt is also stopped. That is, according to such a process, the rubbing state between the photosensitive drum and the belt increases to a level larger than it is needed. Not only an abrasion between the photosensitive drum and the belt but also deflection occurs in the belt, so that the color drift is further liable to occur.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a position setting method of a photosensitive body for suppressing a friction between a belt and the photosensitive body as an image holding body and an image forming apparatus.

According to the present invention, there is provided a position setting method of a photosensitive body in which a reference position of each of a plurality of photosensitive bodies arranged in a conveying unit is detected by a reference position detecting unit during rotation of the photosensitive bodies, the rotation of the plurality of photosensitive bodies and conveyance of the conveying unit are stopped at predetermined time on the basis of a result of the detection, and rotational phase differences of every the plurality of photosensitive bodies are set to a predetermined value, comprising:

a drum/belt driving time calculating step which sets rotational drive stop time of each of the plurality of photosensitive bodies and conveyance drive stop time of the conveying unit on the basis of the detection result of the reference position detecting unit; and

a drive stopping step which stops the conveyance of the conveying unit in correspondence to drive stop timing of one of the photosensitive bodies on the basis of a result of the calculation of the drum/belt driving time calculating step.

Moreover, In the drum/belt driving time calculating step, the photosensitive body whose reference position has been detected first by the reference position detecting unit is set at reference rotational drive stop time and the rotational drive stop time is set in order of the photosensitive body whose reference position has subsequently been detected.

Moreover, in the drum/belt driving time calculating step, the photosensitive body into which the reference rotational drive stop time is set is changed every setting of the phase difference and the rotational drive stop time is set.

Moreover, in the drum/belt driving time calculating step, among the plurality of photosensitive bodies, the photosensitive body in which the accumulated number of rotating times is the smallest is selected and the reference rotational drive stop time is set.

Moreover, in the drum/belt driving time calculating step, among the rotational drive stop time excluding the earliest rotational drive stop time and the latest rotational drive stop time from the plurality of the rotational drive stop time of the plurality of photosensitive bodies, the stop time of the conveying unit is set to timing between the earliest rotational drive stop time and the latest rotational drive stop time.

Moreover, the method may further comprise at least either a drum speed changing step which calculates rotation amounts (rotational speeds.times.rotating time) of the plurality of photosensitive bodies and changes the rotational speeds while maintaining each of the rotation amounts every the plurality of photosensitive bodies, or a belt speed changing step which calculates a movement amount (moving speed.times.moving time) of the conveying unit and changes the moving speed while maintaining the movement amount, on the basis of the calculation result of the drum/belt driving time calculating step.

According to the present invention, there is also provided an image forming apparatus comprising:

a first photosensitive body and a second photosensitive body;

a first driving unit which gives a driving force to the first photosensitive body and a second driving unit which gives a driving force to the second photosensitive body;

a belt which is arranged between the first photosensitive body and the second photosensitive body;

a first mark whose position changes when the first photosensitive body rotates and a second mark whose position changes when the second photosensitive body rotates;

a first detecting unit which detects the first mark and a second detecting unit which detects the second mark; and

a control unit which obtains a time difference between timing when the first mark passes through the first detecting unit and timing when the second mark passes through the second detecting unit, determines a time that is required until the second driving unit is stopped after the stop of the first driving unit, and set stop timing of the belt driving unit to timing between the stop of the first driving unit and the stop of the second driving unit.

In the apparatus, the first photosensitive body has a first photosensitive body gear which receives the driving force from the first driving unit, the second photosensitive body has a second photosensitive body gear which receives the driving force from the second driving unit, a position of the first mark is rotated synchronously with rotation of the first photosensitive body gear, and a position of the second mark is rotated synchronously with rotation of the second photosensitive body gear.

Moreover, in the apparatus, the control unit stops the belt driving unit together with the stop of the first driving unit and, thereafter, stops the second driving unit.

Moreover, The apparatus may further comprise a first photosensitive body driving gear which has the first mark, receives the driving force from the first driving unit, and rotates the first photosensitive body; and a second photosensitive body driving gear which has the second mark, receives the driving force from the second driving unit, and rotates the second photosensitive body.

Moreover, in the apparatus, the control unit stops the belt driving unit together with the stop of the first driving unit and, thereafter, stops the second driving unit.

According to the present invention, there is also provided an image forming apparatus comprising:

a plurality of photosensitive bodies and a plurality of driving units which rotate the photosensitive bodies;

a belt driving unit which drives a belt that is come into contact with the plurality of photosensitive bodies;

a plurality of detection sections which are rotated in association with the rotation of the photosensitive bodies;

a plurality of detecting units each of which is arranged so as to face each of the rotated detection sections at a same fixing position and outputs a detection signal when each of the rotated detection sections is detected at the facing position;

a drive control unit which simultaneously drives the plurality of driving units and drives the belt driving unit so as to simultaneously rotate the plurality of photosensitive bodies;

a stop control unit which discriminates rotational phase differences of the photosensitive bodies on the basis of a reception time difference each time a detection signal is received from each of the detecting units and sequentially stops the driving units of the photosensitive bodies in correspondence to the rotational phase differences; and

a belt drive stopping unit which stops the belt driving unit before the driving unit whose stopping order is the last is stopped after the driving unit whose stopping order is the first was stopped.

By further adding the step of stopping the belt while rotating the photosensitive drum, a period of time during which the photosensitive drum and the belt enters the rubbing state can be further shortened. Therefore, a decrease in life of both of the belt and the photosensitive drum due to the rubbing between them is reduced and, further, the occurrence of the deflection in the belt can be prevented. Thus, such an effect that a preferable output image with a small color drift can be obtained.

The above and other objects and features of the present invention will become apparent from the following detailed description and the appended claims with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a construction of an embodiment 1;

FIG. 2 is a schematic cross sectional view of a print mechanism unit of an image forming apparatus to which the invention is applied;

FIG. 3 is a block diagram of a control system of the invention;

FIG. 4 is an internal constructional diagram of a mechanism control unit;

FIG. 5 is an explanatory diagram of marks and detecting units of the invention;

FIGS. 6A and 6B are explanatory diagrams (part 1) of a phase difference setting principle of the embodiment 1;

FIGS. 7A and 7B are explanatory diagrams (part 2) of the phase difference setting principle of the embodiment 1;

FIGS. 8A and 8B are explanatory diagrams of a phase difference setting principle of an embodiment 2;

FIG. 9 is a block diagram of a construction of an embodiment 3;

FIG. 10 is a block diagram of a construction of an embodiment 4;

FIGS. 11A and 11B are explanatory diagrams of a phase difference setting principle of the embodiment 4;

FIGS. 12A and 12B are explanatory diagrams of a main section of an embodiment 5;

FIG. 13 is a diagram showing a photodetecting mechanism of the embodiment 5; and

FIG. 14 is an explanatory diagram of a phase difference setting principle of the embodiment 5.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A process for setting phase differences among reference points according to the invention is realized only by changing an executing program of a control procedure in the process for setting the phase differences among the reference points of the conventional technique.

Embodiment 1

FIG. 1 is a block diagram of a construction of an embodiment 1.

As shown in the diagram, an image forming apparatus to execute a position setting method of a photosensitive drum according to the embodiment 1 is constructed by: a belt roller 1; a belt 2; photosensitive drums 3 (K, Y, M, C); marks 4 (K, Y, M, C); reference mark detecting units 5 (K, Y, M, C); a belt driving unit 6; a belt driving gear 7; photosensitive drum driving units 8 (K, Y, M, C); photosensitive drum driving gears 9 (K, Y, M, C); and a control unit 10.

An outline of a print mechanism unit to which the invention is applied will now be described.

FIG. 2 is a cross sectional view of a main section of a print mechanism unit.

As shown in the diagram, four image forming units 21K, 21Y, 21M, and 21C are arranged in the print mechanism unit along a conveying path directing from an inserting side of a recording medium to its ejecting side. It is now assumed that K, Y, M, and C denote colors of black, yellow, magenta, and cyan (hereinafter, this is true of the following explanation). The photosensitive drums 3K, 3Y, 3M, and 3C whose surfaces are uniformly charged by charging rollers 22K, 22Y, 22M, and 22C are included in the image forming units 21K, 21Y, 21M, and 21C, respectively. Electrostatic latent images according to image data are formed on the surfaces of the photosensitive drums 3K, 3Y, 3M, and 3C by LED heads 23K, 23Y, 23M, and 23C.

Toner of predetermined colors is supplied to the electrostatic latent images from toner cartridges 27K, 27Y, 27M, and 27C by developing rollers 24K, 24Y, 24M, and 24C, developing blades 25K, 25Y, 25M, and 25C, sponge rollers 26K, 26Y, 26M, and 26C, and the like and developed.

By transfer rollers 28K, 28Y, 28M, and 28C, the toner which has developed the electrostatic latent images is transferred onto the recording medium conveyed on the conveying belt 2 in the direction from the image forming unit 21K to the image forming unit 21C.

A color drift detecting sensor 16 to detect a color drift, a cleaning blade 33, and a drain toner tank 34 are arranged around the conveying belt 2. The color drift detecting sensor 16 is a sensor for irradiating a predetermined light beam to a positional deviation detection pattern formed on the conveying belt 2, receiving its reflection light, and detecting a print positional deviation value. The cleaning blade 33 is a section for removing the positional deviation detection pattern formed on the conveying belt 2. The drain toner tank 34 is a section for enclosing the removed drain toner.

Upon printing, when the recording medium is picked up from a sheet enclosing cassette 35 by a hopping roller 36, it is guided by a guide 37 and reaches a resist roller 38. An oblique feeding or the like of the recording medium is corrected by the resist roller 38 and a pinch roller 39 which faces it. The recording medium is conveyed to a position between an adsorbing roller 40 and the conveying belt 2 by the resist roller 38. The adsorbing roller 40 allows the recording medium to be come into pressure contact with the belt roller 1, thereby charging the recording medium and allowing it to be electrostatically adsorbed onto the conveying belt 2.

The recording medium on which the toner images have been transferred by the transfer rollers 28K, 28Y, 28M, and 28C is sent to a heat roller 41 and a pressing roller 42. In this position, the toner images are heated and fixed onto the recording medium. A temperature of the heat roller 41 is detected by a thermistor 14.

The recording medium after the fixing passes through a guide 43 and is enclosed onto a stacker 44 and a printing process is finished.

To detect the position of the recording medium during the processing steps described above, position sensors 13-1, 13-2, 13-3, and 13-4 are arranged in predetermined positions.

Explanation regarding an outline of the print mechanism unit is finished in this manner. A control system of the invention will now be described in detail with reference to FIG. 3.

A host interface (I/F) unit 81 is a section for playing a role of an interface with an external apparatus, that is, a host computer.

A command/image processing unit 82 is a section for analyzing a print job received from the external apparatus, editing and developing it, and outputting bit map data and various control instructions.

An LED head interface (I/F) unit 53 is a section for sending the bit map data of one line outputted from the command/image processing unit 82 to the LED heads 23K, 23Y, 23M, and 23C.

As mentioned above, the LED heads 23K, 23Y, 23M, and 23C are sections for receiving the bit map data of one line from an LED head interface (I/F) unit 53, turning on the LEDs corresponding to the bit map data, and forming electrostatic latent images onto the surfaces of the photosensitive drums 3K, 3Y, 3M, and 3C.

A high voltage control unit 54 is a section for controlling generation of a charging voltage (CH), a developing bias voltage (DB), and a transfer voltage (TR) under the control of a mechanism control unit 18.

A CH generating unit 55 is a section for supplying the charging voltage (CH) to the image forming units 21K, 21Y, 21M, and 21C under the control of the high voltage control unit 54. The charging voltage (CH) is applied to the charging rollers 22K, 22Y, 22M, and 22C (FIG. 2).

A DB generating unit 56 is a section for supplying the developing bias voltage (DB) to the image forming units 21K, 21Y, 21M, and 21C under the control of the high voltage control unit 54. The developing bias voltage (DB) is applied to the developing rollers 24K, 24Y, 24M, and 24C (FIG. 2).

A TR generating unit 57 is a section for supplying the transfer voltage (TR) to the image forming units 21K, 21Y, 21M, and 21C under the control of the high voltage control unit 54. The transfer voltage (TR) is applied to the transfer rollers 28K, 28Y, 28M, and 28C (FIG. 2).

A hopping motor 58 is a motor for driving the hopping roller 36 (FIG. 2) under the control of the mechanism control unit 18.

A resist motor 59 is a motor for driving the resist roller 38 (FIG. 2) under the control of the mechanism control unit 18.

A heater motor 61 is a motor for driving the heat roller 41 (FIG. 2) under the control of the mechanism control unit 18.

Drum driving motors 62 (K, Y, M, C) are motors for driving the photosensitive drums 3K, 3Y, 3M, and 3C (FIG. 2) under the control of the mechanism control unit 18.

The sensors 13-1, 13-2, 13-3, and 13-4 are position sensors for detecting the position of the print medium during the printing step. Position detection signals of those sensors are sent to the mechanism control unit 18.

The thermistor 14 is a temperature sensor to detect a fixing temperature of the heat roller 41 (FIG. 2). The detection temperature is sent to the mechanism control unit 18.

An environment temperature sensor 15 is a sensor to detect a temperature in the apparatus.

A heater 17 is a heater to heat the heat roller 41.

The mechanism control unit 18 is a section for controlling the whole control system described above and has the following internal construction.

FIG. 4 is an internal constructional diagram of the mechanism control unit.

As shown in the diagram, the mechanism control unit 18 has: a CPU (Central Processing Unit) 18-1; a program ROM 18-2 in which a control program for executing control means has been stored; a memory 18-3 for storing data and the like necessary to execute the control means; and a custom LSI 18-4 for dividing the control system into four systems of K, Y, M, and C.

Further, the control system is divided every four colors from the custom LSI 18-4 and the drum driving motors 62K, 62Y, 62M, and 62C and motor driving ICs 18-6K, 18-6Y, 18-6M, and 18-6C for outputting driving pulses to the drum driving motors are connected to the divided control systems. Further, pulse counters 18-5K, 18-5Y, 18-5M, and 18-5C each for counting the number of driving pulses outputted from the motor driving ICs are arranged.

FIG. 5 is an explanatory diagram of drum gear/idle gear separating means.

Only the image forming unit 21K (FIG. 2) will be described here.

As shown in FIG. 5, a rotating force to drive the photosensitive drum 3K is transferred from the drum driving motor 62K to the photosensitive drum 3K through a drum motor gear 62Ka which is axially supported to a rotary shaft of the drum driving motor 62K, an idle gear 51K which is come into engagement with the drum motor gear 62Ka, the driving gear 9K (FIG. 1) which is axially supported to a rotary shaft of the idle gear 51K, and a photosensitive drum gear 23Ka that is axially supported to the photosensitive drum 3K. Since each of the image forming unit 21Y (FIG. 2), the image forming unit 21M (FIG. 2), and the image forming unit 21C (FIG. 2) is substantially similar to the image forming unit 21K (FIG. 2), their description is omitted here.

Drum gear/idle gear separating means 52 for releasing the engagement of the driving gear 9K and the photosensitive drum gear 23Ka is further provided. Thus, rotational phases of the photosensitive drum and the driving gear can be independently set. Since each of the image forming unit 21Y (FIG. 2), the image forming unit 21M (FIG. 2), and the image forming unit 21C (FIG. 2) is almost similar to that of the image forming unit 21K (FIG. 2), their explanation is omitted here.

Returning to FIG. 1, the belt roller 1 is a section for receiving a driving force of the belt driving unit 6 through the belt driving gear 7 and conveying the belt under the control of the control unit 10.

The belt 2 is a section which is conveyed in the state where it sandwiches a recording medium 100 together with the photosensitive drums 3 (K, Y, M, C).

While the photosensitive drums 3 (K, Y, M, C) are conveying the recording medium 100 in the state where they sandwich it together with the belt 2, the surfaces of the drums 3 are charged to a negative electric potential by a charging roller. A predetermined image is irradiated to the charged section by the LED head of each color, so that an electrostatic latent image is formed there. The electrostatic latent image is developed to a toner image by a developing unit. The toner image is transferred onto the recording medium by a transfer unit.

Ordinarily, in a multicolor image forming apparatus such as a color electrophotographic printer or the like, the photosensitive drums 3 (K, Y, M, C) for forming the toner images of K (black), Y (yellow), M (magenta), and C (cyan) are arranged in a line on the belt 1 at predetermined intervals. For example, as shown in the diagram, the photosensitive drums are arranged from the upstream in the sheet conveying direction toward its downstream in order of K, Y, M, C.

As already described in the paragraphs of "Related Background Art", in order to simultaneously start the rotation of those four photosensitive drums, simultaneously stop the rotation of them, and laminate the toner images of four colors of K, Y, M, and C to the same position on the sheet conveyed on the belt 2, the time when the light from the LED to form the image is irradiated onto the photosensitive drum 3K, photosensitive drum 3Y, photosensitive drum 3M, and photosensitive drum 3C has to be accurately set.

However, even if the irradiating time is accurately set as mentioned above, there is a case where a color drift occurs among the toner images of four colors of K, Y, M, and C due to the eccentricity or the like of the photosensitive drums. Since the rotational driving forces have been applied to the photosensitive drums 3 (K, Y, M, C) from the photosensitive drum driving units 8 (K, Y, M, C) through the photosensitive drum driving gears 9 (K, Y, M, C), the eccentricities of all of the photosensitive drums 3 (K, Y, M, C), the photosensitive drum driving gears 9 (K, Y, M, C), and the photosensitive drum driving units 8 (K, Y, M, C) can become factors of the color drift.

Periodicity occurs in the color drift since the photosensitive drums 3 (K, Y, M, C), the photosensitive drum driving gears 9 (K, Y, M, C), and the photosensitive drum driving units 8 (K, Y, M, C) are rotary members. To make the color drift most inconspicuous, it is necessary not only to accurately set the time when the images are irradiated onto the photosensitive drums 3K, 3Y, 3M, and 3C but also to accurately control the rotational phase differences among the photosensitive drums 3 (K, Y, M, C) (refer to JP-A-2000-187428). To satisfy such a requirement, the marks 4 (K, Y, M, C) are added to the photosensitive drums 3 (K, Y, M, C), respectively, and the reference mark detecting units 5 (K, Y, M, C) are arranged.

As shown in FIGS. 1 and 5, the marks 4 (K, Y, M, C) are constructed in such a manner that, for example, a metal film, a glass plate, or the like to reflect the light is adhered in one position on a circumference of a non-transfer section of each of the photosensitive drums 3 (K, Y, M, C). For example, the reference mark detecting units 5 (K, Y, M, C) are photodetecting sensors and arranged in non-rotating sections around the photosensitive drums 3 (K, Y, M, C) so as to irradiate the light to the marks 4 (K, Y, M, C) and receive the reflection light.

The control unit 10 is a CPU (Central Processing Unit) for controlling the whole apparatus. In the embodiment, particularly, the control unit 10 is a section for executing drum/belt drive starting procedure 10-1 (K, Y, M, C, V), drum/belt driving time calculating procedure 10-2 (K, Y, M, C, V), and drum/belt drive stopping procedure 10-3 (K, Y, M, C, V) in this order. In other words, the control unit 10 includes drum/belt drive starting portion, drum/belt driving time calculating portion and drum/belt drive stopping portion.

The drum/belt drive starting procedure 10-1 (K, Y, M, C, V) is procedure for driving the photosensitive drum driving units 8 (K, Y, M, C), simultaneously starting the rotation of the photosensitive drums 3 (K, Y, M, C), further, simultaneously driving the belt driving unit 6, and starting the conveyance of the belt 2. These procedures are control procedures of the control unit 10 which are formed when the CPU executes computer-readable programs which have previously been stored in a storing unit (not shown) in the apparatus.

The drum/belt driving time calculating procedure 10-2 (K, Y, M, C, V) is a procedure for setting the rotational drive stop time of a plurality of photosensitive drums (K, Y, M, C) and the conveyance drive stop time of the conveying belt 2 on the basis of detection results of the reference mark detecting units 5 (K, Y, M, C). These procedures are control procedures of the control unit 10 which are formed when the CPU executes the computer-readable programs which have previously been stored in the storing unit (not shown) in the apparatus.

The drum/belt drive stopping procedure 10-3 (K, Y, M, C, V) is a procedure for stopping the rotational drive of a plurality of photosensitive drums (K, Y, M, C) and the conveyance drive of the conveying belt 2 on the basis of calculation results of the drum/belt driving time calculating procedure 10-2 (K, Y, M, C, V). These procedures are control procedures of the control unit 10 which are formed when the CPU executes the computer-readable programs which have previously been stored in the storing unit (not shown) in the apparatus.

The position setting operation of the photosensitive drums in the embodiment 1 will now be described.

FIGS. 6A and 6B are explanatory diagrams (part 1) of a phase difference setting principle of the embodiment 1.

FIG. 6A is a diagram showing an example of phase differences of the photosensitive drums 3 (K, Y, M, C) (FIG. 1). In the diagram, a large circle of a solid line is a diagram showing the state where the phase differences of the four photosensitive drums 3 (K, Y, M, C) (FIG. 1) are shown on one circle in a lump. A point S indicates positions where the reference mark detecting units 5 (K, Y, M, C) (FIG. 1) are set. Y, K, M, and C on the circumference indicate phase lags of the marks 4 (Y, K, M, C) (FIG. 1) of the photosensitive drums 3 (Y, K, M, C) from the point S, respectively.

The diagrams show the case where the mark 4Y of the photosensitive drum 3Y is delayed from the point S by .pi./4, the mark 4K of the photosensitive drum 3K is delayed from the point S by 3.pi./4, the mark 4M of the photosensitive drum 3M is delayed from the point S by .pi., and the mark 4C of the photosensitive drum 3C is delayed from the point S by 3.pi./2, respectively.

FIG. 6B is a diagram showing a step of making the phase differences of the marks 4 (K, Y, M, C) coincident from the state of FIG. 6A. In the diagram, an axis of ordinate indicates rotational peripheral velocities (=conveying speeds of the belt) of the photosensitive drums 3 (Y, K, M, C) and an axis of abscissa indicates an elapsed time. In the diagram, in order from the top, S denotes detecting time of the marks 4 (Y, K, M, C) at the point S; 3Y a relation between the rotational peripheral velocity of the photosensitive drum 3Y and the elapsed time; 3K a relation between the rotational peripheral velocity of the photosensitive drum 3K and the elapsed time; 3M a relation between the rotational peripheral velocity of the photosensitive drum 3M and the elapsed time; and 3C a relation between the rotational peripheral velocity of the photosensitive drum 3C and the elapsed time. A belt (1) shows a relation between the conveying speed in the case where the belt has been stopped at the shortest elapsed time and the elapsed time. A belt (2) shows a relation between the conveying speed in the case where the belt has been stopped at the longest elapsed time and the elapsed time. The time shown at the bottom stage shows a change point of each diagram by the common time.

The operations of the photosensitive drums 3 (K, Y, M, C) and the belt (1) will now be described in accordance with the time shown at the bottom stage of FIG. 6B. After that, the operations of the photosensitive drums 3 (K, Y, M, C) and the belt (2) will be described. It is assumed that acceleration at the time of accelerating the photosensitive drums 3 (K, Y, M, C) and the belt (1, 2) and that at the time of decelerating them are equal.

Time T1:

By the drum/belt drive starting procedure 10-1 (K, Y, M, C, V), the rotation of the four photosensitive drums 3 (K, Y, M, C) is simultaneously started and the conveyance of the belt (1) is also simultaneously started.

Time T2:

After the rotational speeds of the four photosensitive drums 3 (K, Y, M, C) and the conveying speed of the belt (1) reach predetermined speeds, the rotation or the conveyance of the predetermined speed is continued.

Time T3:

The reference mark detecting unit 5Y (FIG. 1) detects the mark 4Y (FIG. 1). In FIG. 6A, this time is the time when the photosensitive drum 3Y finishes the rotation of .pi./4.

Time T4:

The reference mark detecting unit 5K detects the mark 4K (FIG. 1). In FIG. 6A, this time is the time when the photosensitive drum 3K finishes the rotation of 3/.pi./4. A time interval from time T3 to time T4 is assumed to be t1.

Time T5:

The reference mark detecting unit 5M detects the mark 4M (FIG. 1). In FIG. 6A, this time is the time when the photosensitive drum 3M finishes the rotation of .pi.. A time interval from time T3 to time T5 is assumed to be t2.

Time T6:

The reference mark detecting unit 5C detects the mark 4C (FIG. 1). In FIG. 6A, this time is the time when the photosensitive drum 3C finishes the rotation of 3.pi./2. At the same time, the drum/belt drive stopping procedure 10-3Y (FIG. 1) starts the deceleration of the photosensitive drum 3Y. A time interval from time T3 to time T6 is assumed to be t3.

Time T7:

The photosensitive drum 3Y is stopped.

Time T8:

The drum/belt drive stopping procedure 10-3K (FIG. 1) starts the deceleration of the photosensitive drum 3K. A time interval from time T6 to time T8 is set to be equal to t1.

Time T9:

The drum/belt drive stopping procedure 10-3M (FIG. 1) starts the deceleration of the photosensitive drum 3M. A time interval from time T6 to time T9 is set to be equal to t2. The photosensitive drum 3K is stopped. The stop of the photosensitive drum 3K is not concerned with t2.

Time T10:

The photosensitive drum 3M is stopped.

Time T11:

The drum/belt drive stopping procedure 10-3C (FIG. 1) starts the deceleration of the photosensitive drum 3C. At the same time, the drum/belt drive stopping procedure 10-3V (FIG. 1) starts the deceleration of the belt (1). A time interval from time T6 to time T11 is set to be equal to t3.

Time T12:

The photosensitive drum 3C and the belt (1) are stopped.

By executing the procedures described above, the marks 4 (K, Y, M, C) are set to the same phase.

Rubbing between the photosensitive drums 3 (K, Y, M, C) and the belt (1) will now be described. (1) The photosensitive drum 3Y and the belt (1) are in the rubbing state for the period of time from time T6 to time T12. A rubbing amount corresponds to an area S(Y) of a hatched region where oblique lines drop from the right side to the left side. (2) The photosensitive drum 3K and the belt (1) are in the rubbing state for the period of time from time T8 to time T12. A rubbing amount corresponds to an area S(K) of a hatched region where oblique lines drop from the right side to the left side. (3) The photosensitive drum 3M and the belt (1) are in the rubbing state for the period of time from time T9 to time T12. A rubbing amount corresponds to an area S(M) of a hatched region where oblique lines drop from the right side to the left side. (4) The rubbing of the photosensitive drum 3C is equal to 0.

The operations of the photosensitive drums 3 (K, Y, M, C) and the belt (2) will now be described.

For the period of time from time T1 to time T6, since the operations are substantially similar to those of the photosensitive drums 3 (K, Y, M, C) and the belt (1), their description is omitted and the operations after time T6 will be explained.

Time T6:

The reference mark detecting unit 5C detects the mark 4C (FIG. 1). The drum/belt drive stopping procedure 10-3Y (FIG. 1) starts the deceleration of the photosensitive drum 3Y and the belt (2).

Time T7:

The photosensitive drum 3Y and the belt (2) are stopped.

Time T8:

The drum/belt drive stopping procedure 10-3K (FIG. 1) starts the deceleration of the photosensitive drum 3K. A time interval from time T6 to time T8 is set to be equal to t1.

Time T9:

The drum/belt drive stopping procedure 10-3M (FIG. 1) starts the deceleration of the photosensitive drum 3M. A time interval from time T6 to time T9 is set to be equal to t2. The photosensitive drum 3K is stopped. The stop of the photosensitive drum 3K is not concerned with t2.

Time T10:

The photosensitive drum 3M is stopped.

Time T11:

The drum/belt drive stopping procedure 10-3C (FIG. 1) starts the deceleration of the photosensitive drum 3C. A time interval from time T6 to time T11 is set to be equal to t3.

Time T12:

The photosensitive drum 3C is stopped.

By executing the procedures described above, the marks 4 (K, Y, M, C) are set to the same phase.

Rubbing between the photosensitive drums 3 (K, Y, M, C) and the belt (2) will now be described. (5) The rubbing of the photosensitive drum 3Y is equal to 0. (6) The photosensitive drum 3K and the belt (2) are in the rubbing state for the period of time from time T6 to time T9. A rubbing amount corresponds to an area s(K) of a hatched region where oblique lines drop from the left side to the right side. (7) The photosensitive drum 3M and the belt (2) are in the rubbing state for the period of time from time T6 to time T10. A rubbing amount corresponds to an area s(M) of a hatched region where oblique lines drop from the left side to the right side. (8) The photosensitive drum 3C and the belt (2) are in the rubbing state for the period of time from time T6 to time T12. A rubbing amount corresponds to an area s(C) of a hatched region where oblique lines drop from the left side to the right side.

A position setting method whereby, in the initial state, the photosensitive drums 3 (K, Y, M, C) having the phase differences shown in FIG. 6A are set to the same phase at the lowest limit rubbing amount will now be described.

FIGS. 7A and 7B are explanatory diagrams (part 2) of the phase difference setting principle of the embodiment 1.

FIG. 7A is a diagram showing an example of the phase differences of the photosensitive drums 3 (K, Y, M, C) (FIG. 1). In the diagram, a large circle of a solid line is a diagram showing the state where the phase differences of the four photosensitive drums 3 (K, Y, M, C) (FIG. 1) are shown on one circle in a lump. The point S indicates the positions where the reference mark detecting units 5 (K, Y, M, C) (FIG. 1) are set. Y, K, M, and C on the circumference indicate phase lags of the marks 4 (Y, K, M, C) (FIG. 1) of the photosensitive drums 3 (Y, K, M, C) from the point S, respectively. This state is set to the same state shown in FIGS. 6A and 6B.

The diagrams show the case where the mark 4Y of the photosensitive drum 3Y is delayed from the point S by .pi./4, the mark 4K of the photosensitive drum 3K is delayed from the point S by 3.pi./4, the mark 4M of the photosensitive drum 3M is delayed from the point S by .pi., and the mark 4C of the photosensitive drum 3C is delayed from the point S by 3.pi./2, respectively.

FIG. 7B is a diagram showing a step of making the phase differences of the marks 4 (K, Y, M, C) coincident from the state of FIG. 7A. In the diagram, an axis of ordinate indicates the rotational peripheral velocities (=conveying speeds of the belt) of the photosensitive drums 3 (Y, K, M, C) and an axis of abscissa indicates the elapsed time. In the diagram, in order from the top, S denotes the detecting time of the marks 4 (Y, K, M, C) at the point S; 3Y the relation between the rotational peripheral velocity of the photosensitive drum 3Y and the elapsed time; 3K the relation between the rotational peripheral velocity of the photosensitive drum 3K and the elapsed time; 3M the relation between the rotational peripheral velocity of the photosensitive drum 3M and the elapsed time; and 3C the relation between the rotational peripheral velocity of the photosensitive drum 3C and the elapsed time. The belt (1) shows the relation between the conveying speed of the belt in the case where the belt has been stopped at the shortest elapsed time and the elapsed time. The belt (2) shows the relation between the conveying speed of the belt in the case where the belt has been stopped at the longest elapsed time and the elapsed time. A belt (3) shows a relation between the conveying speed in the case where the deceleration of the belt is started between the photosensitive drum whose deceleration has been started for the first time (assumed to be the photosensitive drum 3Y here) and the photosensitive drum whose deceleration has been started lastly (assumed to be the photosensitive drum 3C here) and the elapsed time. Among the remaining photosensitive drums excluding the photosensitive drum whose deceleration has been started for the first time (assumed to be the photosensitive drum 3Y here) and the photosensitive drum whose deceleration has been started lastly (assumed to be the photosensitive drum 3C here), a belt (4) shows a relation between the conveying speed in the case where the deceleration of the belt is started between the photosensitive drum whose deceleration has been started for the first time (assumed to be the photosensitive drum 3K here) and the photosensitive drum whose deceleration has been started lastly (assumed to be the photosensitive drum 3M here) and the elapsed time. The time shown at the bottom stage shows a change point of each diagram by the common time.

The relations of the photosensitive drums 3 (K, Y, M, C) and the belts (1) and (2) are substantially similar to those in FIGS. 6A and 6B already described above, their description is omitted. Only the operations of the belts (3) and (4) will now be described.

As shown in the diagrams, the belt (3) has started the deceleration at time Tp (one example) between the photosensitive drum whose deceleration has been started for the first time (assumed to be the photosensitive drum 3Y here) and the photosensitive drum whose deceleration has been started lastly (assumed to be the photosensitive drum 3C here). At this time, the rubbing amount between the belt (3) and the photosensitive drum 3Y corresponds to an area S(p) of a hatched region where oblique lines drop from the right side to the left side. The rubbing amount between the belt (3) and the photosensitive drum 3C corresponds to an area s(p) of a hatched region where oblique lines drop from the left side to the right side. Therefore, it will be understood that the sum of the area S(p) and the area s(p) is constant so long as time Tp exists between deceleration time T6 of the photosensitive drum 3Y whose deceleration has been started for the first time and deceleration time T11 of the photosensitive drum 3C whose deceleration has been started lastly. If time Tp is out of the range from deceleration time T6 and deceleration time T11, the sum of the area S(p) and the area s(p) increases.

The belt (4) has started the deceleration at time Tq (one example) between the photosensitive drum whose deceleration has been started for the first time (assumed to be the photosensitive drum 3K here) and the photosensitive drum whose deceleration has been started lastly (assumed to be the photosensitive drum 3M here) among the remaining photosensitive drums excluding the photosensitive drum whose deceleration has been started for the first time (assumed to be the photosensitive drum 3Y here) and the photosensitive drum whose deceleration has been started lastly (assumed to be the photosensitive drum 3C here). The rubbing amount between the belt (4) and the photosensitive drum 3K corresponds to an area S(q) of a hatched region where oblique lines drop from the right side to the left side. The rubbing amount between the belt (4) and the photosensitive drum 3M corresponds to an area s(q) of a hatched region where oblique lines drop from the left side to the right side. Therefore, it will be understood that the sum of the area S(q) and the area s(q) is constant so long as time Tq exists between deceleration time T8 of the photosensitive drum 3K whose deceleration has been started for the first time and deceleration time T9 of the photosensitive drum 3M whose deceleration has been started lastly in the state where the photosensitive drums 3Y and 3C are excluded. If time Tq is out of the range from deceleration time T8 and deceleration time T9, the sum of the area S(q) and the area s(q) increases.

From the results described above, it will be understood that by making the deceleration time of the belt 2 (FIG. 1) coincide with that of the belt (3) in the above description, the rubbing amount is minimized and the phase differences among a plurality of rotary drums can be set.

Although the embodiment has been described above by limiting to the case where the number of photosensitive drums is equal to 4, the invention is not limited to such an example.

That is, if the number of photosensitive drums is larger than 4, in the state where the photosensitive drum whose deceleration time is the earliest and the photosensitive drum whose deceleration time is the latest are excluded, the photosensitive drum of the second earliest deceleration time and the photosensitive drum of the second latest deceleration time are selected. Further, in the state where the selected two photosensitive drums are excluded, the next photosensitive drums are successively selected in a manner similar to the above. It is sufficient that the deceleration time of the belt is set to timing between the deceleration time of the last two remaining photosensitive drums (the case where three photosensitive drums remain is also included).

As described above, by further adding the step of stopping the belt while the photosensitive drums are rotated, the time interval during which the photosensitive drum and the belt enter the rubbing state can be further reduced. Therefore, the decrease in life of both of the belt and the photosensitive drum due to the rubbing between them can be reduced and, further, the occurrence of the deflection in the belt can be prevented. Thus, such an effect that an output image with the small color drift can be obtained.

To reduce the costs of the belt driving unit, a torque which is needed by a motor of the belt driving unit can be decreased. However, in the case where each photosensitive drum is stopped with the time difference as in the embodiment, if the motor of the belt driving unit is stopped in the state where the number of stopped photosensitive drum is small, particularly, in accordance with the stop of the driving of the photosensitive drum which is stopped first, a frictional load occurring since the belt is conveyed in the state where the photosensitive drum is stopped and come into contact with the belt can be distributed to the drum motor of each photosensitive drum driving unit. The state where only the motor of the belt driving unit is set to the torque which is fairly higher than the torque necessary in the normal print mode can be lightened.

In this case, it is possible to make the torque distribution in consideration of the frictional load distributed to the drum motor of each photosensitive drum. In the case where the medium serving as a load is conveyed to a position between the belt and the photosensitive drum by the printing operation, print data can be printed up to the medium of a high load as compared with the case where only the motor of the belt driving unit can endure the high load.

Embodiment 2

In the embodiment 2, averaging of friction of the photosensitive drums is realized by sequentially changing the photosensitive drum serving as a reference of the phase difference setting. Its details will be described hereinbelow.

Since a construction of the embodiment 2 is substantially similar to that of the embodiment 1, its explanation is omitted and the phase difference setting operation of the photosensitive drums in the embodiment 2 will be described.

FIGS. 8A and 8B are explanatory diagrams of a phase difference setting principle of the embodiment 2.

FIG. 8A is a diagram showing an example of the phase differences of the photosensitive drums 3 (K, Y, M, C) (FIG. 1). In the diagram, a large circle of a solid line is a diagram showing the state where the phase differences of the four photosensitive drums 3 (K, Y, M, C) (FIG. 1) are shown on one circle in a lump. The point S indicates the positions where the reference mark detecting units 5 (K, Y, M, C) (FIG. 1) are set. Y, K, M, and C on the circumference indicate phase lags of the marks 4 (Y, K, M, C) (FIG. 1) of the photosensitive drums 3 (Y, K, M, C) from the point S, respectively.

The diagrams show the case where the mark 4Y of the photosensitive drum 3Y is delayed from the point S by .pi./4, the mark 4K of the photosensitive drum 3K is delayed from the point S by 3.pi./4, the mark 4M of the photosensitive drum 3M is delayed from the point S by .pi., and the mark 4C of the photosensitive drum 3C is delayed from the point S by 3.pi./2, respectively. This state is set to the same state in FIGS. 6A and 6B.

FIG. 8B is a diagram showing the step of making the phase differences of the marks 4 (K, Y, M, C) coincident from the state of FIG. 8A. In the diagram, an axis of ordinate indicates the rotational peripheral velocities (=conveying speeds of the belt) of the photosensitive drums 3 (Y, K, M, C) and an axis of abscissa indicates the elapsed time. In the diagram, in order from the top, S denotes the detecting time of the marks 4 (Y, K, M, C) at the point S; 3K the relation between the rotational peripheral velocity of the photosensitive drum 3K and the elapsed time; 3M the relation between the rotational peripheral velocity of the photosensitive drum 3M and the elapsed time; 3C the relation between the rotational peripheral velocity of the photosensitive drum 3C and the elapsed time; and 3Y the relation between the rotational peripheral velocity of the photosensitive drum 3Y and the elapsed time. The belt (1) shows the relation between the conveying speed in the case where the belt has been stopped at the shortest elapsed time and the elapsed time. The belt (2) shows the relation between the conveying speed in the case where the belt has been stopped at the longest elapsed time and the elapsed time. The time shown at the bottom stage shows the change point of each diagram by the common time.

The operations of the photosensitive drums 3 (K, Y, M, C) and the belt (1) will now be described in accordance with the time shown at the bottom stage of FIG. 8B. It is assumed that acceleration at the time of accelerating the photosensitive drums 3 (K, Y, M, C) and the belt (1, 2) and that at the time of decelerating them are equal.

Time T21:

By the drum/belt drive starting procedure 10-1 (K, Y, M, C, V), the rotation of the four photosensitive drums 3 (K, Y, M, C) is simultaneously started and the conveyance of the belt (1) is also simultaneously started.

Time T22:

After the rotational speeds of the four photosensitive drums 3 (K, Y, M, C) and the conveying speed of the belt (1) reach predetermined speeds, the rotation or the conveyance of the predetermined speed is continued.

Time T23:

The reference mark detecting unit 5Y (FIG. 1) detects the mark 4Y (FIG. 1). In FIG. 8A, this time is the time when the photosensitive drum 3Y finishes the rotation of .pi./4.

Time T24:

The reference mark detecting unit 5K detects the mark 4K (FIG. 1). In FIG. 8A, this time is the time when the photosensitive drum 3K finishes the rotation of 3.pi./4.

Time T25:

The reference mark detecting unit 5M detects the mark 4M (FIG. 1). In FIG. 8A, this time is the time when the photosensitive drum 3M finishes the rotation of .pi.. A time interval from time T24 to time T25 is assumed to be t11.

Time T26:

The reference mark detecting unit 5C detects the mark 4C (FIG. 1). In FIG. 8A, this time is the time when the photosensitive drum 3C finishes the rotation of 3.pi./2. A time interval from time T24 to time T26 is assumed to be t12.

Time T27:

The reference mark detecting unit 5C detects the mark 4Y (FIG. 1) again. In FIG. 8A, this time is the time when the photosensitive drum 3Y further finishes the rotation of 2.pi. from time T23. At the same time, the drum/belt drive stopping procedure 10-3K (FIG. 1) starts the deceleration of the photosensitive drum 3K. A time interval from time T24 to time T27 is set to be equal to t13.

Time T28:

The drum/belt drive stopping procedure 10-3M (FIG. 1) starts the deceleration of the photosensitive drum 3M. A time interval from time T27 to time T28 is set to be equal to t11. The photosensitive drum 3K is stopped here. The stop of the photosensitive drum 3K is not concerned with t11.

Time T29:

The photosensitive drum 3M is stopped.

Time T30:

The drum/belt drive stopping procedure 10-3C (FIG. 1) starts the deceleration of the photosensitive drum 3C. A time interval from time T27 to time T30 is set to be equal to t12.

Time T31:

The photosensitive drum 3C is stopped.

Time T32:

The drum/belt drive stopping procedure 10-3Y (FIG. 1) starts the deceleration of the photosensitive drum 3Y. At the same time, the drum/belt drive stopping procedure 10-3V (FIG. 1) starts the deceleration of the belt (1). A time interval from time T27 to time T32 is set to be equal to t13.

Time T33:

The photosensitive drum 3Y and the belt (1) are stopped.

By executing the procedures described above, the marks 4 (K, Y, M, C) are set to the same phase.

Rubbing between the photosensitive drums 3 (K, Y, M, C) and the belt (1) will now be described. (11) The photosensitive drum 3K and the belt (1) are in the rubbing state for the period of time from time T27 to time T33. A rubbing amount corresponds to an area S(k) of a hatched region where oblique lines drop from the right side to the left side. (12) The photosensitive drum 3M and the belt (1) are in the rubbing state for the period of time from time T28 to time T33. A rubbing amount corresponds to an area S(m) of a hatched region where oblique lines drop from the right side to the left side. (13) The photosensitive drum 3C and the belt (1) are in the rubbing state for the period of time from time T30 to time T33. A rubbing amount corresponds to an area S(c) of a hatched region where oblique lines drop from the right side to the left side. (14) The rubbing of the photosensitive drum 3Y is equal to 0.

The operations of the photosensitive drums 3 (K, Y, M, C) and the belt (2) will now be described.

For the period of time from time T21 to time T27, since the operations are substantially similar to those of the