Image forming apparatus method and storage device storing a program for controlling image forming operation of primarily transferring an image onto an intermediate transfer member

An image forming apparatus which is capable of reducing a color misalignment in a color overlapping process, and a color misalignment due to variation of the circumferential length of an intermediate transfer member due to an environmental change over time during a successive copy operation. The image forming apparatus carries out image formation by primarily transferring an image electrophotographically formed on an image carrier onto the rotatably driven intermediate transfer member, and then secondarily transferring the images on the intermediate transfer member onto a recording medium. An image forming operation of primarily transferring the image onto the intermediate transfer member is controlled according to the length of the intermediate transfer member in a circumferentially moving direction thereof and a variation of a predetermined parameter relating to the intermediate transfer member.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image forming apparatus, a control method therefor and a program for implementing the control method, and more particularly relates to an image forming apparatus that electrophotographically forms an image on a recording medium, by primarily transferring a toner image formed on a photosensitive member onto an intermediate transfer member, and then secondarily transferring the toner image on the intermediate transfer member onto the recording medium, such as a copying machine, a multifunction apparatus, and a printer, as well as a control method for the image forming apparatus and a program for implementing the control method.

2. Description of the Related Art

There has been known an image forming apparatus that electrophotographically forms an image, such as a copying machine, a multifunction apparatus, and a printer, in which a toner image formed on a photosensitive member is once primarily transferred onto an intermediate transfer member, the toner image is then secondarily transferred onto a recording medium such as a recording sheet or an OHP sheet, and the toner image on the recording medium is fixed, thereby forming an image. As the intermediate transfer member used in the above described transfer, a drum-shaped intermediate transfer member and a belt-shaped intermediate transfer member are actually used. The intermediate transfer belt method using the belt-shaped intermediate transfer member is currently attracting attention due to it being advantageous in saving installation space in an image forming apparatus since the miniaturization of image forming apparatuses has been desired these days.

When a full-color image is formed by an image forming apparatus that carries out the transfer using the intermediate transfer belt, since it is difficult to form overlapped toner images on the photosensitive member, toner images of three colors of yellow, cyan, and magenta or those of four colors including black in addition to these three colors are sequentially primarily transferred from the photosensitive member onto the intermediate transfer belt, and the toner images of the full color overlapped on the intermediate transfer belt are secondarily transferred onto a recording medium at once, thereby forming a full-color image.

To achieve a good image quality of the full-color image obtained by the above described process, it is necessary to accurately align the multi-color toner images to be overlapped on the intermediate transfer belt. Specifically, if the toner images in three colors or four colors are slightly displaced from the position in which they are to be overlapped, the resulting image has a color completely different from that of the original image formed on a medium such as an original, which necessitates carrying out the accurate alignment.

Conventionally, to accurately align multi-color toner images on the intermediate transfer belt, a reference mark serving as a reference of the image formation timing is provided at a predetermined position on the intermediate transfer belt, the reference mark is detected by an optical sensor or the like provided at a predetermined position on a conveying path for the intermediate transfer belt, and the image forming process is started in predetermined timing after the detection of the reference mark so that the multi-color toner images are primarily transferred and overlapped at a given position on the intermediate transfer belt. In addition, other improved techniques have been proposed for more accurate alignment of multi-color toner images (for example, Japanese Laid-Open Patent Publications (Kokai) No. H7-92763 and No. H7-281536).

However, if the image formation is carried out successively using these conventional methods, a defect may occur in the image due to degradation of the intermediate transfer belt. Specifically, according to these methods, since the toner images are always overlapped at a certain area on the intermediate transfer belt, there occurs such a phenomenon that an aging change of the state of a conducting agent inside the intermediate transfer belt causes a decrease in the resistance value of that area on the intermediate transfer belt. Such decrease in the resistance value of the specific area on the intermediate transfer belt causes a difference in primary and second transferability between the area having the decreased resistance and the other areas, and an image defect such as a void becomes remarkable when a large halftone image is formed across the area having the decreased resistance value and another area.

To solve this problem, there has been proposed a technique that a plurality of reference marks are provided on the intermediate transfer belt, any one of these reference marks is detected by a photo sensor, the timing of exposure on the photosensitive members is controlled to predetermined timing so as to accurately align multi-color toner images formed, and at the same time, primarily transfer the toner images at different positions on the intermediate transfer belt (for example, Japanese Laid-Open Patent Publication (Kokai) No. H8-146698).

When the timing of the image forming process is controlled based on the plurality of reference marks provided on the intermediate transfer belt as above, an identification mark should be added to each reference mark for identification, and the control should be carried out while the identification mark is identified using a sensor. Specifically, for example, if a yellow toner image is transferred onto the intermediate transfer belt with reference to a reference mark “a” provided at a predetermined position on the intermediate transfer belt, the reference mark “a” must be also used as a reference when the next toner image such as a cyan toner image is transferred onto the intermediate transfer belt to overlap the next toner image on the yellow toner image. If another reference mark “b” is used as a reference, a color misalignment occurs.

However, there is such a case where the sensor cannot identify the identification mark added to the reference mark on the intermediate transfer belt which rotates in synchronism with the speed of image formation on the recording medium. Particularly, recently, high speed image formation has been required, so that it is difficult for the sensor to accurately read the identification marks on the intermediate transfer belt which rotates at a high speed for such high speed image formation. Although this problem can be solved by using a high performance sensor which can accurately read the identification marks even if the intermediate transfer belt is rotating at a high speed, such a sensor is disadvantageous in terms of cost. Apart from this problem, there is a problem that the identification marks disappear when the surface of the intermediate transfer belt is cleaned using a cleaning blade, and consequently the sensor cannot read the identification marks on the intermediate transfer belt. In these cases, the proper timing control cannot be carried out, and as a result, a color misalignment may occur.

Further, if the timing of the image forming process is controlled based on the plurality of reference marks provided on the intermediate transfer belt as described above, after preparation for (toner) image formation for a first color has been completed, a first reference mark is detected and then image formation is started. As a result, at least a wait time period from the completion of preparation for the image formation to the detection of the first reference mark is added to a FCOT (first copy out time) for the full-color image formation.

Therefore, a method for actively reducing the above described wait time has recently been studied. According to this method, the circumferential length in the circumferential direction (rotational direction) of the intermediate transfer member is detected and stored in a RAM or the like in advance. After the preparation for image formation is completed, image formation start signals are generated in arbitrary timing according to a program. Specifically, an image formation start signal for a first color is generated in arbitrary timing, and then a next image formation start signal for a next color is generated upon the lapse of a one-turn time period required for the intermediate transfer member to make one turn, which is calculated from the stored circumferential length and the rotational speed of the intermediate transfer member. As a result, the wait time until the detection of the first reference mark can be eliminated, providing an advantage of reduction of the FCOT for the full-color image formation compared with the method of starting the image formation based on the reference marks (for example, Japanese Laid-Open Patent Publication (Kokai) No. H10-20614).

Further, in the case where the image formation start signal is generated using the one-turn time period calculated in advance as described above, when a plurality of full-color images are successively output, there has been the problem that various mechanical shocks or mechanical load fluctuations occur due to contacting and separation of the cleaning blade with and from the intermediate transfer member, that is, a mechanical shock caused by the separation of the cleaning blade from the intermediate transfer member when a toner image is formed on the intermediate transfer member for a first color of a first recording sheet; a mechanical shock caused by contacting of a secondary transfer roller with the recording sheet when a color toner image is secondarily transferred on a recording sheet after a tone image of a fourth color is overlapped on the intermediate transfer member; a mechanical shock caused by contacting of the cleaning blade with the intermediate transfer member for cleaning the same; and other mechanical load fluctuations caused by contacting and separation of the cleaning blade with and from the intermediate transfer member. These mechanical load fluctuations cause variations in the rotational speed of the intermediate transfer member such that the one-turn time period varies between the respective colors. This results in a color misalignment between the first color and second and subsequent colors in the color overlapping process.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an image forming apparatus, a control method therefor, and a program for implementing the control method, which are capable of reducing a color misalignment in a color overlapping process, and a color misalignment due to variation of the circumferential length of an intermediate transfer member due to an environmental change over time during a successive copy operation.

To attain the above objects, in a first aspect of the present invention, there is provided an image forming apparatus that carries out image formation by primarily transferring an image electrophotographically formed on an image carrier onto a rotatably driven intermediate transfer member, and then secondarily transferring the images on the intermediate transfer member onto a recording medium, comprising a controller that controls an image forming operation of primarily transferring the image onto the intermediate transfer member, according to a length of the intermediate transfer member in a circumferentially moving direction thereof and a variation of a predetermined parameter relating to the intermediate transfer member.

Preferably, the image forming apparatus comprises a circumferential length detecting device that detects a circumferential length as the length of the intermediate transfer member in the circumferentially moving direction thereof, a signal generating device that generates an image formation start signal for a plurality of respective colors, a target value setting device that sets a target value of image formation timing to be input to the signal generating device based on the circumferential length detected by the circumferential length detecting device, and an offset value adding device that adds an offset value determined according to an expected load variation to the target value set by the target value setting device.

More preferably, the circumferential length detecting device comprises a reference member detecting device that detects a reference member attached to the intermediate transfer member, and a measuring device that measures a time period elapsed from generation of a first detection signal acquired from the reference member detecting device to generation of a second detection signal acquired from the reference member detecting device as a result of circumferential movement of the intermediate transfer member.

More preferably, the signal generating device comprises four signal generating devices provided respectively for yellow, magenta, cyan, and black, and the target value setting device sets target values of image formation timing for respective ones of the four signal generating devices.

More preferably, the signal generating device comprises at least two signal generating devices provided respectively at least for a face A corresponding to recording mediums at odd number-th positions attached to the intermediate transfer member, and a face B corresponding to recording mediums attached to the intermediate transfer member at even number-th positions, and the target value setting device sets target values of image formation timing for respective ones of the two signal generating devices for the face A and the face B.

More preferably, the offset value added by the offset value adding device is for correcting values of mechanical shocks different between respective colors, generated during the image forming operation of primarily transferring the image onto the intermediate transfer member.

More preferably, the offset value added by the offset value adding device is for correcting a change in the circumferential length of the intermediate transfer member due to an environmental change over time during a successive output operation of successively forming images.

Still more preferably, the image forming apparatus comprises an environmental change detecting device that detects a change in temperature and humidity as the environmental change.

Preferably, the intermediate transfer member comprises one selected from the group consisting of a belt type and a drum type.

Preferably, the image forming apparatus comprises one selected from the group consisting of a printer, a copying machine, and a multifunction apparatus.

To attain the above objects, in a second aspect of the present invention, there is provided an image formation control method for an image forming apparatus that carries out image formation by primarily transferring an image electrophotographically formed on an image carrier onto a rotatably driven intermediate transfer member, and then secondarily transferring the images on the intermediate transfer member onto a recording medium, comprising a control step of controlling an image forming operation of primarily transferring the image onto the intermediate transfer member, according to a length of the intermediate transfer member in a circumferentially moving direction thereof and a variation of a predetermined parameter relating to the intermediate transfer member.

Preferably, the image formation control method comprises a circumferential length detecting step of detecting a circumferential length as the length of the intermediate transfer member in the circumferentially moving direction thereof, a signal generating step of generating an image formation start signal for a plurality of respective colors, a target value setting step of setting a target value of image formation timing to be input to the signal generating step based on the circumferential length detected in the circumferential lengths detecting step, and an offset value addition step of adding an offset value determined according to an expected load variation to the target value set in the target value setting step.

More preferably, the circumferential length detecting step comprises a reference member detecting step of detecting a reference member attached to the intermediate transfer member, and a measurement step of measuring a time period from generation of a first detection signal acquired in the reference member detecting step to generation of a second detection signal acquired in the reference member detecting step as a result of circumferential movement of the intermediate transfer member.

More preferably, the signal generating step comprises four signal generating steps provided respectively for yellow, magenta, cyan, and black, and the target value setting step comprises setting target values of image formation timing for respective ones of the four signal generating steps.

More preferably, the signal generating step comprises at least two signal generating steps provided respectively at least for a face A corresponding to recording mediums at odd number-th positions attached to the intermediate transfer member, and a face B corresponding to recording mediums attached to the intermediate transfer member at even number-th positions, and the target value setting step comprises setting target values of image formation timing for respective ones of the two signal generating steps for the face A and the face B.

More preferably, the offset value added in the offset value addition step is for correcting values of mechanical shocks different between respective colors, generated during the image forming operation of primarily transferring the image onto the intermediate transfer member.

More preferably, the offset value added in the offset addition step is for correcting a change in the circumferential length of the intermediate transfer member due to an environmental change over time during a successive output operation of successively forming images.

Still more preferably, the image formation control method comprises an environmental change detecting step of detecting a change in temperature and humidity as the environmental change.

Preferably, the intermediate transfer member comprises one selected from the group consisting of a belt type and a drum type.

Preferably, the image formation control method is applied to an image forming apparatus selected from the group consisting of a printer, a copying machine, and a multifunction apparatus.

To attain the above objects, in a third aspect of the present invention, there is provided a program for causing a computer to execute an image formation control method for an image forming apparatus that carries out image formation by primarily transferring an image electrophotographically formed on an image carrier onto a rotatably driven intermediate transfer member, and then secondarily transferring the images on the intermediate transfer member onto a recording medium; comprising a control module for controlling an image forming operation of primarily transferring the image onto the intermediate transfer member, according to a length of the intermediate transfer member in a circumferentially moving direction thereof and a variation of a predetermined parameter relating to the intermediate transfer member.

According to the present invention, in the image forming apparatus that carries out image formation by primarily transferring an image electrophotographically formed on an image carrier onto the rotatably driven intermediate transfer member, and then secondarily transferring the images on the intermediate transfer member onto a recording medium, the image forming operation of primarily transferring the image onto the intermediate transfer member is controlled according to the length of the intermediate transfer member in the circumferentially moving direction thereof and a variation of the predetermined parameter relating to the intermediate transfer member. As a result, it is possible to reduce a color misalignment in the color overlapping process and a color misalignment due to a change in the circumferential length of the intermediate transfer member due to an environmental change over time during a successive copy operation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will now be described later in detail with reference to the accompanying drawings showing preferred embodiments thereof. In the drawings, elements and parts which are identical throughout the views are designated by identical reference numeral, and duplicate description thereof is omitted.

FIG. 1is a schematic cross sectional view showing the construction of an image forming apparatus according to a first embodiment of the present invention. The image forming apparatus100according to the present invention is implemented by a copying machine, for example. The image forming apparatus100is comprised of a scanner unit1including a laser unit (hereinafter simply referred to as “the laser”)6, a polygon mirror7, a scanner motor8, and a beam detection signal (BD signal) generating circuit200, a photosensitive drum3, an intermediate transfer belt4, a circumferential length detecting sensor5, a developing rotary10including developer units10ato10dof respective colors, a secondary transfer roller11, an environment sensor13, cleaning blades14and15, a fixing device16, recording mediums17such as recording sheets, a sheet feed cassette18, a manual feed cassette19, and a sheet discharge opening20. In the present and following second embodiments, a description will be given mainly of control relating to color alignment in a sub scanning direction of respective colors: yellow (Y), magenta (M), cyan (C), and black (Bk) in the image forming apparatus100, and illustration and description of an original reading mechanism which reads an image from an original to be copied are omitted.

A description will now be given of the constructions of the respective sections of the image forming apparatus100. In the scanner unit1, the laser6emits laser light modulated based on an image signal output from an image forming section27shown inFIG. 4described later. The polygon mirror7is a rotary polygon mirror which scans the surface of the photosensitive drum3by deflecting the laser light emitted from the laser6, and forms an electrostatic latent image on the photosensitive drum3. The scanner motor8rotatably drives the polygon mirror7. The beam detection signal (BD signal) generating circuit200detects the laser light deflected by the polygon mirror7in the main scanning direction. The developing rotary10develops the electrostatic latent image formed on the photosensitive drum3using developer units10a,10b,10c, and10dof the respective colors: yellow (Y), magenta (M), cyan (C), and black (Bk). The photosensitive drum3primarily transfers the developer on the photosensitive drum3developed by the developing rotary10onto the intermediate transfer belt4. The secondary transfer roller11is disposed in contact with the intermediate transfer belt4, and secondarily transfers the developers on the intermediate transfer belt4onto the recording medium such as a recording sheet fed from the sheet feed cassette18or the manual feed tray19. The circumferential length detecting sensor5detects a circumferential length, which is the length of the intermediate transfer belt4in the circumferential direction (rotational direction), and is disposed in the measuring circuit300provided inside a unit of the intermediate transfer belt4. An optical reflection type sensor is used as the circumferential length detecting sensor5in the present embodiment.

The intermediate transfer belt4is stretched across the outer peripheries of a plurality of rollers as shown inFIG. 1, and is driven for rotation by the respective rollers. A reference mark12is provided on the rear surface of the intermediate transfer belt4. In the present embodiment, the reference mark12is comprised of a seal made of a material having a high reflectivity. Specifically, a light source such as an LED, not shown, irradiates light on the reference mark12provided on the rear surface of the intermediate transfer belt4, and the circumferential length detecting sensor5detects the reflected light from the reference mark12. It should be noted that inFIG. 1, the photosensitive drum3is rotatively driven in the clockwise direction, and the intermediate transfer belt4is rotatively driven in the counterclockwise direction which is reverse to the rotational direction of the photosensitive drum3, by a drive mechanism, not shown, both at the same constant speed. The environment sensor13detects the temperature and the humidity, and the amount of the moisture around the intermediate transfer belt4is calculated based on the detection result of the environment sensor13. The details of control using the environment sensor13will be described with reference to a second embodiment of the invention, described later.

The cleaning blade14is always disposed in contact with the photosensitive drum3, and cleans the photosensitive drum3by scraping off residual toner on the surface. The cleaning blade15is configured and disposed such that it can be separated from and brought in contact with the intermediate transfer belt4, and cleans the intermediate transfer belt4by scraping off residual toner on the surface when it is in contact with the belt4. The fixing device16carries out a fixing operation by heating and pressing toner images which have been transferred onto the recording sheet17. The sheet feed cassette18stores a plurality of recording sheets17, and a recording sheet17fed out from the sheet feed cassette18is fed to a secondary transfer position on the intermediate transfer belt4. The manual feed tray19is used for manually feeding a recording sheet17, and a recording sheet17inserted into the manual feed tray19is fed to the secondary transfer position on the intermediate transfer belt4. The sheet discharge opening20discharges the recording sheet17on which the image formation (copy) has completed.

A description will now be given of the operations of the respective sections of the image forming apparatus.100. First, the image formation is carried out for yellow (Y) data. Specifically, upon receiving a start instruction for an image forming job by an user via an operating section, not shown, of the image forming apparatus100, initialization is carried out for image forming preparation, and then top signal (TOP*) generation counters, not shown, which are provided inside the top signal generating section22shown inFIG. 4, described later, and have set target values for the respective colors, are started by a trigger of an electrical START signal generated according to a program. A top signal for yellow (Y), the first color, is generated when the value of a top signal generating counter for yellow (Y) reaches the target value, the write timing of the laser6inside the scanner unit1is set according to the top signal, thereby causing the laser6to emit laser light, whereby a latent image according to the data of yellow (Y) is formed on the photosensitive drum3.

Then, the photosensitive drum3is rotated by the drive mechanism, not shown, and the latent image on the photosensitive drum3is visualized by the developer of yellow (Y) at a position where the photosensitive drum3comes in contact with the developer unit of yellow (Y)10ain the developing rotary10. The photosensitive drum3is further rotated by the drive mechanism, and the developer of yellow (Y) on the photosensitive drum3is primarily transferred onto the intermediate transfer belt4at a position where the photosensitive drum3comes in contact with the intermediate transfer belt4. Then, the developing rotary10rotates by approximately 90 degrees in preparation for the development of the next color, magenta (M).

Then, the image formation for magenta (M) is carried out. Specifically, top signal generating counters, not shown, which are provided inside the top signal generating section22shown inFIG. 4, and have set target values for the respective colors, are started as described above by a trigger of the top signal generated when the yellow (Y) data was generated. A top signal for magenta (M), the second color, is generated when the value of the top signal generating counter for magenta (M) reaches the target value, the write timing of the laser6inside the scanner unit1is set according to the top signal, thereby causing the laser6to emit laser light. A latent image according to the data of magenta (M) is formed on the photosensitive drum3by the emission of the laser light from the laser6when the intermediate transfer belt4is at the same rotation position as the formation of the latent image of yellow (Y).

Then, the photosensitive drum3is rotated by the drive mechanism, and the latent image on the photosensitive drum3is visualized by the developer of magenta (M) when the intermediate transfer belt4is at the same rotation position as the visualization of the latent image of yellow (Y). The photosensitive drum3is further rotated by the drive mechanism, and the developer of magenta (M) on the photosensitive drum3is primarily transferred onto the intermediate transfer belt4when the intermediate transfer belt4is at the same rotation position as the primary transfer of the developer of yellow (Y).

Thereafter, similar control is carried out for cyan (C) and black (Bk) in the image forming process described above. When the developers of the four colors: yellow (Y), magenta (M), cyan (C), and black (Bk) have been overlapped on the intermediate transfer belt4, a recording sheet17is fed from the sheet feed cassette18or the manual feed tray19, and the secondary transfer roller11is brought in contact with the intermediate transfer belt4. Consequently, the secondary transfer roller11secondarily transfers the developers on the intermediate transfer belt4onto the recording sheet17. Then, the secondary transfer roller11, which has been in contact with the intermediate transfer belt4, is separated after the entire developers have been transferred onto the recording sheet17. Then, the developers on the recording sheet17are fixed by the fixing device16, and the recording sheet17on which the image has been formed is discharged into the discharge opening20.

A description will now be given of the cleaning operation of the intermediate transfer belt4using the cleaning blade15, described later. As preprocessing for the above described image formation of the four colors, the cleaning blade15is brought in contact with the intermediate transfer belt4to clean the intermediate transfer belt4before the development of yellow (Y), which is the first color, is carried out. The cleaning blade15, which has been in the contact state, is separated from the intermediate transfer belt4before the leading end of the developer of yellow (Y), which is the first color primarily transferred onto the intermediate transfer belt4, reaches the cleaning blade15, and the preprocessing of cleaning is completed. Further, when the developers of the four colors have been overlapped and secondarily transferred onto the recording sheet17as described above, the cleaning blade15is again brought in contact with the intermediate transfer belt4to scrape off the remaining developers on the intermediate transfer belt4. When the developers have been completely scraped off, the blade15is separated from the intermediate transfer belt4, and the preprocessing of cleaning is completed.

It should be noted that the above described target values set for the respective colors: yellow (Y), magenta (M), cyan (C), and black (Bk) are determined based on the detection result of the circumferential length of the intermediate transfer belt4by the circumferential length detecting sensor5provided inside the unit of the intermediate transfer belt4.

A description will now be given of how to detect the circumferential length.

FIG. 2is a block diagram showing the construction of the measuring circuit300of the image forming apparatus100inFIG. 1.

As shown inFIG. 2, the measuring circuit300is comprised of an oscillator301, a frequency divider302, a CPU306, a circumferential length detecting counter307including a counter section303and a circumferential length register section304, and the circumferential length detecting sensor5appearing inFIG. 1, and measures the circumferential length of the intermediate transfer belt4.

The oscillator301generates primary clock signal (base clock). The frequency divider302generates a reference clock for the circumferential length detecting counter307based on the primary clock input from the oscillator301. The CPU306is connected to the circumferential length detecting counter307, and controls the respective sections inFIG. 2. The counter section303carries out a count operation, described later. The circumferential length register section304stores a count value counted by the counter section303.

A description will now be given of the operation of the above construction. The primary clock generated by the oscillator301is input to the frequency divider302, which in turn generates the reference clock for the circumferential length detecting counter307. The circumferential length detecting counter307is connected to the CPU306. The CPU306can always read the count value of the counter section303loaded in the circumferential length register section304of the circumferential length detecting counter307, and generates an enable signal for the counter section303of the circumferential length detecting counter307.

The counter section303of the circumferential length detecting counter307starts counting the reference clock in response to a trigger composed of the enable signal from the CPU306and the detection signal from the circumferential length detecting sensor5. When the next detection signal is input from the circumferential length detecting sensor5, the counter section303loads the count value at this point into the circumferential length register section304, and then the counter section303is cleared, and repeats the count. Namely, the counter section303measures a time period from a first detection signal acquired from the circumferential length detecting sensor5to a second detection signal acquired from the same as a result of the rotation (circumferential movement) of the intermediate transfer-belt4.

A description will now be given of a setting sequence of the actual target values set for the respective colors: yellow (Y), magenta (M), cyan (C), and black (Bk) with the above described construction of the image forming apparatus100. First, in timing when a mechanical shock applied to the intermediate transfer belt4, which occurs during image formation, for example, during the initialization upon turning-on of the power supply of the image forming apparatus100(such as shocks caused by contacting/separation of the cleaning blade15and the secondary transfer roller11with/from the intermediate transfer belt4), a circumferential length detection sequence is carried out for detection of the circumferential length of the intermediate transfer belt4using the circumferential length detecting sensor5and the circumferential length detecting counter307.

FIG. 3is a view useful in explaining the operation of the circumferential length detecting counter307inFIG. 2. First, the circumferential length detecting sensor5detects the reference mark12on the rear surface of the intermediate transfer belt4as the intermediate transfer belts4rotates, and the counter section303of the circumferential length detecting counter307receives the detection signal (HP signal) from the circumferential length detecting sensor5. The counter section303starts counting the reference clock supplied to the circumferential length detecting counter307upon rise of the detection signal. When the intermediate transfer belt4further rotates, the circumferential length detecting sensor5again detects the reference mark12. At this point, the counter section303of the circumferential length detecting counter307stores the number of the reference clock inputs supplied until immediately before the input of the detection signal (HP signal) generated by the second detection by the sensor5, and loads the count value into the circumferential length register section304inside the circumferential length detecting counter307.

In this way, the circumferential length of the intermediate transfer belt4can be measured with the resolution of the reference clock supplied to the circumferential length detecting counter307based on the count value acquired as described above, and the one-turn time period of the intermediate transfer belt4can be managed based on the circumferential length of the intermediate transfer belt4and the rotational speed (speed of rotating operation) of the intermediate transfer belt4during the image formation. However, the actual one-turn time period of the intermediate transfer belt4for each color has a certain offset to the one-turn time period calculated as described above due to mechanical shocks applied to the intermediate transfer belt4(such as shocks caused by contacting/separation of the cleaning blade15and the secondary transfer roller11with/from the intermediate transfer belt4) during the image formation, as described later. Thus, the target values of the respective colors input to the top signal generating counters (signal generating sections) for the respective colors during the image formation are set by adding the respective offset values thereto.

The method of calculating the offset values includes, for example, a method in which the cleaning blade15and the secondary transfer roller11are intentionally brought into contact and separated for each one turn of the intermediate transfer belt4, and the difference Δ in one-turn time period from the case where the cleaning blade15and the secondary transfer roller11are not brought into contact and separated is calculated and stored as the offset value before the delivery of the image forming apparatus from the factory, and a method in which a predetermined value is initially set as the offset value, and during the image formation, the CPU306causes the circumferential length detecting counter307to start operation in timing when the cleaning blade15and the secondary transfer roller11are not brought into contact and separated, the circumferential length of the intermediate transfer belt4is measured, then further, the CPU306causes the circumferential length detecting counter307to start operation in timing when the cleaning blade15and the secondary transfer roller11are brought into contact and separated, the circumferential length of the intermediate transfer belt4is measured, and the resulting difference Δ in one-turn time period is calculated as the offset value, to correct the initially set value using the calculated offset value and store the correct value.

Further, the target values of the top signal generating counters (signal generating sections) can be set independently for the respective four colors: yellow (Y), magenta (M), cyan (C), and black (Bk). Further, the target values can also be set independently for a surface A corresponding to odd number-th recording sheets attached to the intermediate transfer belt4, and for a surface B corresponding to even number-th recording sheets attached to the intermediate transfer belt4.

On the other hand, even when the top positions (image leading end positions as the leading end of the image formation timing) for the respective colors: yellow (Y), magenta (M), cyan (C), and black (Bk) are accurately synchronized with each other, if the top signal (TOP*) indicating a start position of writing in the sub scanning direction for each of the respective colors acquired by the rotation of the intermediate transfer belt4, and the beam detection signal (BD) indicating a start position of writing in the main scanning direction for the color acquired by the rotation of the scanner motor8are not synchronized with each other, the start position of writing for the color in the sub scanning direction can be displaced by an amount corresponding to the difference between the phase of the top signal and that of the BD signal, namely by an amount corresponding to one line in the sub scanning direction at the maximum. This problem might be solved if the one-turn time period of the intermediate transfer belt4were exactly an integer multiple of the period of the BD signal. However, in actuality, it is difficult to exactly set the one-turn time period of the intermediate transfer belt4to an integer multiple of the period of the BD signal, since such setting restricts the design of the image forming apparatus100.

To solve this problem, the present embodiment employs a known prior technique using a simple method in which a target signal as a reference corresponding to the position of the polygon mirror7provided on the scanner motor8is generated every time the intermediate transfer belt4makes one rotation, and the rotation of the scanner motor8is controlled by phase control based on the target signal. With this prior technique, the image forming apparatus100can be completely free from color misalignment between the respective colors: yellow (Y), magenta (M), cyan (C), and black (Bk), as a multi-color (full color) image forming apparatus.

FIG. 4is a block diagram showing the construction of a scanner motor control system of the image forming apparatus100. The image forming apparatus100is comprised of the laser6, the polygon mirror7, the scanner motor8including a scanner motor driving circuit8-1and a scanner motor main body (SM)8-2, a CPU21, the top signal generating section22, a timer23, a ROM24, an oscillator25, a laser controller26, the image forming section (image formation control circuit)27, a drum motor controller28, a scanner motor control circuit29, an oscillator30, and the beam detection signal (BD signal) generating circuit200. Parts and elements inFIG. 4corresponding to those inFIG. 1are designated by identical reference numerals.

The CPU21controls the entire image forming apparatus100based on a program stored in the ROM24, and carries out processes shown in respective flowcharts, described later, by controlling the CPU306, the circumferential length detecting counter307, the environment sensor13, and others. The CPU21has a memory (work area for the CPU21), not shown, therein or at another location. The ROM24stores various control programs executed by the CPU21. The drum motor controller28rotates and stops the intermediate transfer belt4and the photosensitive drum3. The top signal generating section22starts the timer23based on a predetermined step number for one turn of the intermediate transfer belt4and the one-turn time period determined in advance as described above, thereby electrically generating the top signals (TOP*) for the respective colors during the actual image formation.

The oscillator25generates a clock signal serving as a reference time of the operation of the CPU21. The timer23divides the output frequency of the oscillator25, to provided a divided frequency clock as a reference of time period measurement or the like. At least part of the construction ofFIG. 4may be implemented by a one-chip CPU in general, which makes it possible to accommodate the CPU21, the top signal generating section22, the timer23, the ROM24, and the drum motor controller28in the one chip, and thus further reduce the size and cost of the image forming apparatus100.

The scanner motor8has attached thereto the polygon mirror7appearing inFIG. 1, includes the scanner motor driving circuit8-1and the scanner motor main body (SM)8-2, and rotates and stops under the control of the scanner motor control circuit29according to instructions from the CPU21. The beam detection signal (BD signal) generating circuit200generates the beam detection signal (BD signal) serving as a start reference signal (synchronizing signal in the main scanning direction) in the main scanning direction by detecting laser light deflected by the polygon mirror7as the polygon mirror7rotates. If the polygon mirror7has six surfaces, the beam detection signal (BD signal) is generated six times during one rotation of the scanner motor8.

The oscillator30generates a reference clock for the operation of the image forming section (image formation control circuit)27. The image forming section27is comprised of a sub scanning control circuit and a main scanning control circuit, generates timing for video data generation through communication with a controller, not shown, synchronizes the sub scanning and the main scanning with each other based on the top signal (TOP*) and the beam detection signal (BD signal), and generates a laser light emission signal corresponding to a video signal. The laser controller26synchronizes the sub scanning of the respective colors according to a print instruction from the CPU21and the top signal (TOP*) from the top signal generating section22, to thereby control the driving of the laser6. The laser6receives a signal from the laser controller26, and forms a latent image on the photosensitive drum3using the laser light. The scanner motor control circuit29has a control circuit operating to eliminate the phase difference from the actual BD signal by generating a target BD signal serving as a reference immediately after the generation of the electrical top signal (TOP*).

FIG. 5is a block diagram showing the detailed construction of the scanner motor control circuit29inFIG. 4. The scanner motor control circuit29is comprised of a counter31, a phase comparison circuit34, and a charge pump circuit35. Reference numeral22designates the top signal generating section;2, the BD signal inside the scanner motor control circuit29; and33, the target BD signal inside the scanner motor control circuit29. Parts and elements inFIG. 5corresponding to those inFIG. 4are designated by identical reference numerals.

The counter31of the scanner motor control circuit29generates the target BD signal33as the reference. The scanner motor control circuit29is configured so as to reset the counter31to newly generate the target BD signal immediately after the detection of the output (TOP*) from the top signal generating section22. The phase comparison circuit34compares the phase of the target BD signal33generated by the counter31and the phase of the actual BD signal2detected by the beam detection signal (BD signal) generating circuit200with each other, and outputs a LAG signal and a LEAD signal, described later. The charge pump circuit35receives the output signals from the phase comparison circuit34, and converts the phase difference between the two signals into a control voltage. Specifically, the time period corresponding to the phase difference is directly used as a control variable for use in proportional operation, and the charge pump circuit35generates control voltage which is constant in absolute value but has a positive value or negative value depending upon whether the phase difference indicates “lead” or “lag”.

FIG. 6is a block diagram showing the detailed construction of the scanner motor control/driving circuit of the scanner motor8inFIG. 4. The scanner motor8is comprised of the scanner motor driving circuit8-1, the scanner motor main body (SM)8-2, the frequency divider41, a speed discriminator42, a resistor43, an integrator44, an integrating filter45, a control amplifier46, and a resistor48. InFIG. 6, reference numeral25designates the oscillator appearing inFIG. 4. Parts and elements inFIG. 6corresponding to those inFIG. 4are designated by identical reference numerals.

The scanner motor control/driving circuit constructed as above is a control circuit that drivingly controls the scanner motor main body (SM)8-2using the control signal from the scanner motor control circuit29appearing inFIG. 4. The frequency divider41divides the frequency of the reference clock generated by the oscillator25with a predetermined division ratio, thereby generating a frequency serving as a reference speed of the scanner motor main body8-1. The speed discriminator42compares the BD signal2used for the detection of the rotational speed of the polygon mirror7(seeFIG. 1) attached to the scanner motor8, and the output signal from the frequency divider41which generates the frequency serving as the reference speed of the polygon mirror7, and discriminates the speed of the polygon mirror7based on the comparison result.

The integrator44receives the control signal output from the scanner motor control circuit29via the resistor48, and a control signal output from the speed discriminator42via the resistor43, and operates as an integrator having predetermined gain and frequency characteristics determined by the integrating filter45comprised of a resistor and capacitors, and the resistor43. The control amplifier46receives a signal output from the integrator44and amplifies the signal to a predetermined gain so as to drive the scanner motor main body8-2. The scanner motor driving circuit8-1is composed of transistors and other devices and parts, and drives the scanner motor main body8-2.

A description will now be given of the operation of controlling the scanner motor8. When the rotation control of the scanner motor8by the scanner motor control/driving circuit constructed as above is carried out, the speed discriminator42carries out the rotation control through a feedback control loop in which it is determined whether the scanner motor8is operating at a predetermined rotational speed or not by monitoring the BD signal2, and then an output signal is generated such that if the rotational speed of the scanner motor8has not reached the predetermined rotational speed, the rotational speed is increased, or if the rotational speed has exceeded the predetermined rotational speed, the rotational speed is decreased. It should be noted that since this feedback control loop does not include control based on the phase difference between the BD signal and the output signal from the frequency divider41whose frequency serves as the reference rotational speed, the scanner motor8is controlled to a rotational speed slightly deviated from the predetermined rotational speed due to an offset voltage of the integrator44.

To accurately control the rotational speed of the scanner motor8to the predetermined reference rotational speed, an output indicative of the phase difference between the target BD signal33and the actual BD signal2obtained from the scanner motor control circuit29is input to the integrator44via the resistor48in parallel with the input via the resistor43, thereby carrying out PLL (Phase Locked Loop) speed control. The gain of the PLL control loop can be considerably smaller than the gain of the speed discriminator42, and thus the resistance value of the resistor48may be set to ten times or more of the resistance value of the resistor43. This is because if the gain of the PLL control is high, the follow-up to the reference phase is improved, but the ability to lock-in of the PLL degrades. As a result of the additional provision of the PLL control of the phase difference between the target BD signal33and the actual BD signal2, it is possible to control the rotational speed of the scanner motor8to the rotational speed at which the actual BD signal2is generated with the period of the target BD signal33.

A detailed description will now be given of the operation of the PLL control operation of the image forming apparatus100with reference to a timing chart inFIG. 7.

FIG. 7is a timing chart showing the PLL control operation of the scanner motor8by the scanner motor control circuit29inFIG. 4.

InFIG. 7, symbol “ENABLE *” designates a signal indicating a print area/a non-print area (an area where latent image is not formed in the sub scanning direction on the photosensitive drum3). “High” areas filled in black in the chart indicate print areas, and the other areas indicate non-print areas. Symbol “TOP *N” designates a TOP signal, which is generated by the top signal generating section22as a synchronizing signal for the start of the print in the sub scanning direction. Symbol “REFBD*” designates the target BD signal, which is generated by the counter31of the scanner motor control circuit29. Symbol “BD*” designates the actual BD signal, which is generated by the beam detection signal (BD signal) generating circuit200as a synchronizing signal for the start of the print in the main scanning direction. Symbol “LAG*”n designates a LAG signal, which represents the phase lag of the actual BD signal (BD*) from the target BD signal (REFBD*), and is output from the phase comparison circuit34of the scanner motor control circuit29.

Symbol “LEAD*” designates a LEAD signal, which represents the phase lead of the actual BD signal (BD*) from the target BD signal (REFBD*), and is output from the phase comparison circuit34of the scanner motor control circuit29. It should be noted that the LAG signal (LAG*) goes “low” only when the phase of the actual BD signal (BD*) lags behind that of the target BD signal (REFBD*), and the LEAD signal (LEAD*) goes “low” only when the phase of the actual BD signal (BD*) leads that of the target BD signal (REFBD*). Symbol “CPUMP” designates a synthesized signal of the LAG signal (LAG*) and the LEAD signal (LEAD*) output from the phase comparison circuit34of the scanner motor control circuit29, which is generated by the charge pump circuit35of the scanner motor control circuit29. Symbol “Is” designates a current which is actually output to the scanner motor main body8-2.

With reference toFIG. 7, a description will now be given of the PLL control operation by the scanner motor control/driving circuit (frequency divider41through resistor48) inside the scanner motor8shown inFIG. 6.

First, inFIG. 7, before the top signal generating section22generates the top signal (TOP*), the rotational speed of the scanner motor8is controlled by the speed discriminator control and the PLL control such that the phase of the target BD signal (REFBD*) and that of the actual BD signal (BD*) coincide with each other.

Then, when the top signal (TOP*) is generated, the counter31of the scanner motor control circuit29that is generating the target BD signal (REFBD*) is immediately cleared at the falling edge of the top signal (TOP*), whereupon the counter31restarts the count operation, so that the target BD signal (REFBD*) is newly generated. Since the speed of the scanner motor8cannot be changed rapidly, the actual BD signal (BD*) continues to be output with the same period. The phase comparison circuit34of the scanner motor control circuit29outputs the LAG signal (LAG*) at “low” level only when the phase of the actual BD signal (BD*) lags behind the phase of the target BD signal (REFBD*), and outputs the LEAD signal (LEAD*) at “low” level only when the phase of the actual BD signal (BD*) leads the phase of the target BD signal (REFBD*).

Namely, the phase comparison circuit34of the scanner motor control circuit29outputs the LAG signal (LAG*) at “low” while the LEAD signal (LEAD*) remains “high” when the phase of the actual BD signal (BD*) lags behind the phase the target BD signal (REFBD*), and outputs the LEAD signal (LEAD*)at “low” while the LAG signal (LAG*) remains “high” when the phase of the actual BD signal (BD*) leads the phase of the target BD signal (REFBD*).

The charge pump circuit35of the scanner motor control circuit29synthesizes the LAG signal (LAG*) indicating the phase lag and the LEAD signal (LEAD*) indicating the phase lead into the CPUMP signal. The charge pump circuit35of the scanner motor control circuit29is configured such that a positive (“+”) voltage for accelerating the scanner motor8is generated if the phase lags, and output a negative (“−”) voltage for decelerating the scanner motor8is generated if the phase leads.

When this control signal is input as a signal relating to the PLL control to the scanner motor control/driving circuit of the scanner motor8inFIG. 6, the scanner motor8is controlled to have its speed slightly increased so that the phase lag gradually decreases, and the scanner motor8is controlled continuously so as to be maintained at the equilibrium. Specifically, the actual BD signal (BD*) comes in phase with the target BD signal (REFBD*), with the speed difference being zero, and the phase difference cancels or eliminates the speed deviation in the speed discriminator42of the scanner motor8, whereby the equilibrium is maintained.

If printing is started at a time when the actual BD signal (BD*) comes in phase with the target BD signal (REFBD*), the printing positions (printing start positions in the sub scanning direction) for the respective colors can be accurately aligned with each other. Further, even during the printing operation the scanner motor control circuit29operates to keep the actual BD signal (BD*) in phase with the target BD signal (REFBD*), so that the scanner motor8can be controlled such that the actual BD signal (BD*) and the target BD signal (REFBD*) are synchronized until the end of the printing operation.

In this way, even in the image forming apparatus100where the one-turn time period of the intermediate transfer belt4is not set to an integer multiple of the BD period, it is possible to bring the main scanning synchronizing signal and the sub scanning synchronizing signal (top signal) into phase with each other.

A detailed description will now be given of operations and effects specific to the image forming apparatus100according to the present embodiment constructed as described above.

FIG. 8is a sequence diagram showing generation of the TOP signal (TOP*) in a color print by the image forming apparatus100inFIG. 1. The intermediate transfer belt4used in the present embodiment allows two-sheet attachment of recording sheets in A4 size, for example, on the one-turn circumferential length (i.e. allows forming images corresponding to two recording sheets on the intermediate transfer belt4at the same time), andFIG. 8shows a sequence of color image formation for the two-sheet attachment for small-sized recording sheets such as A4. It should be noted that counters for the respective colors such as a yellow face-A (YA) counter and a yellow face-B (YB) counter, described later, are provided inside the top signal generating section22.

InFIG. 8, first, the electrical START signal is generated according to the program as a trigger to cause the yellow face-A (YA) counter and the yellow face-B (YB) counter to start counting at the same time. Here, the face A (the face of a recording sheet at an odd number-th position in a sequence of the recording sheets) corresponds to the first half of the one turn of the intermediate transfer belt4, and the face B the (face of a recording sheet at an even number-th position) corresponds to the latter half of the same. As shown inFIG. 8, a VYA* signal and a VYB* signal as TOP signals (TOP*) corresponding respectively to the face A and the face B of yellow (Y) are generated when respective predetermined count time periods (TYA and TYB) elapse. These signals are received as the write timing of the laser6by the scanner unit1, thereby causing the emission of laser light from the laser6. In this way, latent images of the data of yellow (Y) are formed on the photosensitive drum3.

Then, a VMA* signal and a VMB* signal as top signals (TOP*) corresponding respectively to the face A and the face B of magenta (M), are generated when start timing of respective predetermined count time periods (TMA and TMB) approximately corresponding to the one-turn time period of the intermediate transfer belt is reached after the generation of the VYA* and VYB* signals of yellow (Y) as triggers. These signals are received as the write timing of the laser6in the scanner unit1, thereby causing emission of laser light from the laser6. In this way, latent images of the data of magenta (M) are formed on the photosensitive drum3.

Then, similar control is also carried out for cyan (C) and black (Bk), so that latent images according to the data of cyan (C) and black (Bk) are formed on the photosensitive drum3. After the developers of the four colors are thus overlapped on the intermediate transfer belt4, respective registration-on signals (RA and RB) are sequentially generated based on registration-on counters which started respective counting operations with reference to the respective VKA* and VKB* signals as the top signals (TOP*) of black (Bk), to thereby cause recording sheets17to be fed from the sheet feed cassette18or the manual feed cassette19and then bring them into contact with the secondary transfer roller11, so that the developers of the four colors on the intermediate transfer belt4are secondarily transferred onto the recording sheets17.

FIG. 9is a diagram showing the circuit configuration of video data request signal generation counters corresponding to the respective colors (yellow, magenta, cyan, and black) of the image forming apparatus100according to the first embodiment. InFIG. 9, the sequence of the first embodiment is enabled by a cascade construction where the START signal described above is input to the face-A and face-B counters of the first color, yellow (Y), and the top signals generated by the counters of previous colors trigger counters of the respective following colors.

FIG. 10shows a sequence of image top timing in an actual color print by the image forming apparatus, in which mechanical shocks generated during actual image formation (such as a mechanical shock caused by the separation of the cleaning blade15during the formation of toner images on the intermediate transfer belt4) based on the construction of the image forming apparatus100shown inFIGS. 1,2,4,5, and6, and the top signal generation sequence in a color print shown inFIG. 8.

The sequence diagram ofFIG. 10shows the sequence ofFIG. 8and further shows timing of mechanical shocks applied to the intermediate transfer belt4and corresponding actual image top timing. As shown inFIG. 10, in an actual image formation by the image forming apparatus100, the cleaning blade15which has been in contact with the intermediate transfer belt4for cleaning the intermediate transfer belt4as the preprocessing of the image formation for the four colors, is separated from the intermediate transfer belt4at a point in the latter half of the yellow (Y) face-B image formation, and is brought into contact with the intermediate transfer belt4at a point in the latter half of the black (Bk) face-B image formation as the post processing for cleaning. Also, the second transfer roller11comes into contact with the intermediate transfer belt4in timing in which the developers of the four colors overlapped on the intermediate transfer belt4are transferred onto the recording sheet (at a point in the latter half of the Black (Bk) face-A image formation inFIG. 10), as described earlier.

In actuality, the separation of the cleaning blade15from the intermediate transfer belt4from the contact state acts to reduce the load toque applied to the intermediate transfer belt4, and consequently the intermediate transfer belt4rotates (moves in the circumferential direction thereof) faster momentarily. Conversely, the contacting of the cleaning blade15with the intermediate transfer belt4from the separate state acts to increase the load torque applied to the intermediate transfer belt4, and consequently the intermediate transfer belt4rotates slower momentarily. Also when the secondary transfer roller11comes into contact with the intermediate transfer belt4, this contacting motion acts to increase the load torque applied to the intermediate transfer belt4, and consequently the intermediate transfer belt4rotates slower momentarily.

In this way, the rotation or circumferential motion of the intermediate transfer belt4varies due to the above-mentioned mechanical loads (the cleaning blade15and the secondary transfer roller11) being applied to the intermediate transfer belt4, and consequently the actual image top timing changes i.e. advances or retards as shown inFIG. 10. In the present sequence, the actual image top timing of the respective colors depends upon the top signals (TOP* in the present embodiment) of the respective colors generated by the top signal (TOP*) generation counters of the respective colors, irrespective of the above described load variations. Therefore, a displacement of ΔL occurs in the actual image top timing as shown inFIG. 10, and an accumulation of such displacements for the respective colors in the image formation of the four colors results in color misalignment in the full-color image formation by the image forming apparatus100. Specifically, as shown inFIG. 10, the one-turn time period for both the face A and face B in the area from yellow (Y) to magenta (M) on the intermediate transfer belt4decreases by ΔLy-c due to the separation of the cleaning blade15from the intermediate transfer belt4. Also, the one-turn time period in the area from cyan (C) to black (Bk) on the intermediate transfer belt4increases by ΔLc-k due to the contacting of the secondary transfer roller11with the intermediate transfer belt4. The actual color misalignment due to these variations of the one-turn time period is approximately 50 μm to 100 μm (description and illustration of the contacting of the cleaning blade15and the separation of the secondary transfer roller11are omitted since these actions have negligibly small influences in the present embodiment).

However, the generation timing of the above described shocks due to the separation of the cleaning blade15from the intermediate transfer belt4and due to the contacting of the secondary transfer roller11with the intermediate transfer belt4is fixed in the image forming sequence, and hence the actual variations of the rotation of the intermediate transfer belt4due to these shocks have a certain periodicity.

FIGS. 11A,11B,12A, and12B are flowcharts the procedure of setting the top signal generating counters.FIG. 11Ashows the setting of the top signal generating counters for yellow;FIG. 11B, magenta;FIG. 12A, cyan; andFIG. 12B, black.

First, as shown inFIG. 11A, if the setting of the yellow (Y) counters is to be carried out (“YES” to a step S100), since the time period from the generation of the START signal to that of the image top signal for yellow (Y) is constant irrespective of the circumferential length of the intermediate transfer belt4, the counter values TYA for the face A and TYB for the face B are respectively set to predetermined values (step S101).

Then, as shown inFIG. 11B, if the setting of the magenta (M) counters is to be carried out (“YES” to a step S111), and if the present time is after a circumferential length detecting mode where the circumferential length of the intermediate transfer belt4is detected (namely, the circumferential length of the intermediate transfer belt4has been measured, and the actual circumferential length value has been stored in the circumferential length register section304in the circumferential length detecting counter307) (“YES” to a step S112), the circumferential length of the intermediate transfer belt4, which has been measured by the circumferential length detecting sensor5and stored in the circumferential length register section304in the circumferential length detecting counter307, is stored in a RAM, not shown, in the CPU306(step S113). Then, the counter values TMA and TMB corresponding to the one-turn time period of the intermediate transfer belt4are calculated based on the circumferential length of the intermediate transfer belt4stored in the RAM of the CPU306, and a predetermined image forming speed (step S114). Then, an offset value Mc1-off of the time period corresponding to the rotation variation of the intermediate transfer belt4due to the mechanical shock generated by the separation of the cleaning blade15from the intermediate transfer belt4is added to the calculated counter values TMA and TMB, to thereby set target values for the magenta (M) counters, TMA′ and TMB′, respectively for the surface A and the surface B (step S115).

Then, as shown inFIG. 12A, if the setting of the cyan (C) counters is to be carried out (“YES” to a step S121), and if the present time is after the circumferential length detecting mode where the circumferential length of the intermediate transfer belt4is detected (“YES” to a step S122), the circumferential length of the intermediate transfer belt4, which has been measured by the circumferential length detecting sensor5and stored in the circumferential length register section304in the circumferential length detecting counter307, is stored in the RAM, not shown, in the CPU306(step S123). Then, the counter values TCA and TCB corresponding to the one-turn time period of the intermediate transfer belt4are calculated based on the circumferential length of the intermediate transfer belt4stored in the RAM of the CPU306, and the predetermined image forming speed (step S124). Since there is no expected mechanical shock in the image forming process corresponding to the time period from magenta (M) to cyan (C), target values TCA and TCB of the cyan (C) counters are respectively set for the face A and the face B.

Finally, as shown inFIG. 12B, if the setting of the black (Bk) counters is to be carried out (“YES” to a step S131), if the present time is after the circumferential length detecting mode where the circumferential length of the intermediate transfer belt4is detected (“YES” to a step S132), the circumferential length of the intermediate transfer belt4, which has been measured by the circumferential length detecting sensor5and stored in the circumferential length register section304in the circumferential length detecting counter307, is stored in the RAM, not shown, in the CPU306(step S133). Then, the counter values TKA and TKB corresponding to the one-turn time period of the intermediate transfer belt4are calculated based on the circumferential length of the intermediate transfer belt4stored in the RAM of the CPU306, and the predetermined image forming speed (step S134). Since there is no mechanical shock on the intermediate transfer belt4during the time period corresponding to the face A, the target value TKA of the black face A (BA) counter is set for the face A (step S135). On the other hand, as for the face B, added to the target value TKB is an offset value Kc1-on of the time period corresponding to the circulation variation of the intermediate transfer belt4due to the mechanical shock generated by the contacting of the secondary transfer roller11to the intermediate transfer belt4, thereby setting the target value for the black (Bk) counter, TKB′, for the surface B (step S136).

By setting the target values as described above with reference toFIGS. 11A,11B,12A, and12B, it is possible to generate the top signals (TOP*) for the respective colors approximately in synchronism with the actual image top timing even when the separation of the cleaning blade15from the intermediate transfer belt4, and the contacting of the secondary transfer roller11with the intermediate transfer belt4occur in the image forming sequence as shown inFIG. 10. As a result, a proper image can be output without a large color misalignment by the image forming apparatus100.

As described above, according to the first embodiment, it is possible to prevent color misalignment which occurs between first and subsequent colors during the color overlapping process due to variations of the one-turn time period of the intermediate transfer belt4between the respective colors caused by mechanical load variations causing differences in the rotational speed of the intermediate transfer belt4, which are generated by the contacting of the respective loads (such as the cleaning blade15and the secondary transfer roller11) with the intermediate transfer belt4, and the separation of them from the intermediate transfer belt4for the primary transfer in the image forming process.

A description will now be given of a second embodiment of the present invention. An image forming apparatus, a circumferential length detecting counter, a scanner motor control system, a scanner motor control circuit and a scanner motor control/driving circuit according to the present embodiment are identical with those of the above described first embodiment (FIGS. 1 and 2, andFIGS. 4 to 6), and hence detailed description thereof is omitted.

The present embodiment is characterized in that the environment sensor13is provided in the periphery of the intermediate transfer belt4(on the outer peripheral side thereof, for example) as shown inFIG. 1, to monitor the humidity and temperature and calculate the amount of moisture in the periphery of the intermediate transfer belt4. Specifically, the environment sensor13detects the temperature and humidity, and based on the detection result of the environment sensor13, the amount of moisture around the intermediate transfer belt4is calculated by the CPU301(FIG. 2).

FIG. 13is a sequence diagram showing the generation of TOP signals (TOP*) generation for the color print by the image forming apparatus100according to the second embodiment. In the present embodiment, the intermediate transfer belt4allows the two-sheet attachment of recording sheets in A4 size, for example, on the one-turn circumferential length as is the same with the first embodiment, andFIG. 13shows the sequence of color image formation for the two-sheet attachment for small size recording sheets such as A4.

InFIG. 13, electrical START signals generated respectively for the face A and face B as triggers according to a program cause the yellow face-A (YA) counter and the yellow face-B (YB) counter to start counting. As shown inFIG. 13, the VYA* signal and the VYB* signal corresponding respectively to the face A and the face B of yellow (Y) are generated when the respective predetermined count time periods (TYA and TYB) elapse. These signals are received as the write timing of the laser6in the scanner unit1, thereby causing the emission of laser light from the laser6. In this way, latent images of the data yellow (Y) are formed on the photosensitive drum3.

Then, the VMA* signal and the VMB* signal as top signals (TOP*) corresponding respectively to the face A and the face B of magenta (M) are generated when the respective predetermined count time periods (TMA and TMB) approximately corresponding to the one-turn time period of the intermediate transfer belt4elapse from the VYA* and VYB* signals of yellow (Y) as triggers. These signals are received as the write timing of the laser6in the scanner unit1, thereby causing the emission of laser light from the laser6. In this way, latent images of the data of magenta (M) are formed on the photosensitive drum3.

Then, similar control is also carried out for cyan (C) and black (Bk), so that latent images according to the data of cyan (C) and black (Bk) are formed on the photosensitive drum3. After the developers of the four colors are overlapped on the intermediate transfer belt4, the respective registration-on signals (RA and RB) are sequentially generated based on the registration-on counters which started respective counting operations with reference to the respective VKA* and VKB* signals as the top signals (TOP*) for black (Bk), to thereby cause recording sheets17to be fed from the sheet feed cassette18or the manual feed cassette19and then bring them into contact with the secondary transfer roller11, so that the developers of the four colors on the intermediate transfer belt4are secondarily transferred onto the recording sheets17.

FIG. 14is a diagram showing the circuit configuration of the video data request signal generation counters corresponding to the respective colors (yellow, magenta, cyan, and black) of the image forming apparatus100according to the second embodiment. The sequence of the second embodiment is enabled by a cascade construction where gates, ENABLE_A and ENABLE_B, are provided respectively on prior stages of the face-A and face-B counters of the first color of yellow (Y) as compared with the circuit configuration (FIG. 9) of the first embodiment, and the START signal is input for the face A and the face B by toggling the ON/OFF of the respective gates, and video data request signals generated by the counters of previous colors trigger the counters of the respective following colors.

In the present embodiment, in addition to the correction of the circumferential length variation caused by mechanical shocks, the circumferential length value of the intermediate transfer belt4measured in the circumferential length detecting mode where the circumferential length of the intermediate transfer belt4is detected can be changed when the level of the moisture quantity calculated using the environment sensor13exceeds a predetermined level, to thereby correct a circumferential length variation of the intermediate transfer belt4generated by an environment change which occurs when image formation on a large number of recording sheets and output thereof are carried out. By reflecting the changed circumferential length value upon the target values of the top signal (TOP*) generation counters for the respective colors, it is possible to cope with an aging change in the circumferential length of the intermediate transfer belt4due to an environmental change, namely a change in the moisture quantity around the intermediate transfer belt4during execution of an image formation job of forming images on recording sheets.

In actuality, the temperature inside the image forming apparatus100increases by 30° C. or so over long-term execution of an image formation job which is started at a room temperature, and the humidity changes accordingly. The circumferential length of the intermediate transfer belt4(made of a polyimide material in the present embodiment) actually changes by a few micrometers.

FIGS. 15A,15B,16A, and16B are flowcharts the procedure of setting the top signal generating counters during a successive copy operation.FIG. 15Ashows the setting of the top signal generating counter for yellow;FIG. 15B, magenta;FIG. 16A, cyan; andFIG. 16B, black during the successive copy operation.

First, as shown inFIG. 15A, if the setting of the yellow (Y) counters is to be carried out (“YES” to a step S141), since the time period from the generation of the START signal to that of the top signal for yellow (Y) is constant irrespective of the circumferential length of the intermediate transfer belt4, the counter values TYA for the face A and TYB for the face B are respectively set to predetermined values (step S141).

Then, as shown inFIG. 15B, if the setting of the magenta (M) counters is to be carried out (“YES” to a step S151), whenever a predetermined number of sheets have been subjected to image formation after the start of the successive copy operation (“YES” to a step S152), the moisture quantity around the intermediate transfer belt4is calculated based on the temperature and humidity detected by the environment sensor13(step S153). Further, the calculated moisture quantity around the intermediate transfer belt4and the moisture quantity acquired at the time of the detection of the circumferential length of the intermediate transfer belt4are compared (“YES” in step S154). If the difference between the moisture quantities is more than a predetermined quantity, a counter offset value Thum according to the environmental change calculated based on an offset value Lhum of the intermediate transfer belt4according the moisture difference is added to the magenta counter values TMA and TMB which have already been set, to thereby newly set environmentally-corrected target values TMA′ and TMB′ for the face A and face B (step S155).

Thereafter, the counter offset value Thum is added respectively to the counter values of cyan (C) and black (Bk) for the face A and the face B in a similar manner as the counter target values of magenta (M), as shown inFIGS. 16A and 16B, (steps S161through S165inFIG. 16Aand S171through S175inFIG. 16B).

In this way, according to the present embodiment, deviation of the image top timing due to a circumferential length change of the intermediate transfer belt4caused by-an environmental change over time during a successive copy operation can be corrected in addition to the correction for mechanical shocks applied to the intermediate transfer belt4described with reference to the first embodiment. As a result, the top signals (TOP*) of the respective colors in more accurate timing according to the actual image top timing than in the first embodiment, to thereby enable the image forming apparatus100to output a proper image without a large color misalignment.

As described above, according to the second embodiment, it is possible to prevent color misalignment which occurs between first and subsequent colors during the color overlapping process due to variations of the one-turn time period of the intermediate transfer belt4between the respective colors caused by mechanical load variations causing differences in the rotational speed of the intermediate transfer belt4, which are generated by the contacting of the respective loads (such as the cleaning blade15and the secondary transfer roller11) with the intermediate transfer belt4, and the separation of them from the intermediate transfer belt4for the primary transfer in the image forming process. In addition, according to the second embodiment, it is possible to reduce a color misalignment due to a change circumferential length of the intermediate transfer belt4caused by an environmental change over time during a successive copy operation.

It should be understood that the present invention is not limited to the first and second embodiments described above, but various variations of the above described embodiments may be possible without departing from the spirit of the present invention.

Although in the first and second embodiments, the intermediate transfer belt4is used as the intermediate transfer member provided in the image forming apparatus100, the present invention is not limited to this, and may be applied to a case where an intermediate transfer drum is used as the intermediate transfer member.

Although in the first and second embodiments, the two-sheet attachment of recording sheets in A4 size along the one-turn circumferential length of the intermediate transfer belt4of the image forming apparatus100is employed, the present invention is not limited to this, and it is possible to arbitrarily set the size of recording sheets and the number of images corresponding to the recording sheets, attached or formed on the intermediate transfer belt4within the spirit of the present invention.

Although in the above described embodiments, a copying machine is employed as the image forming apparatus100, the present invention is not limited to this, and may be also applied to a printer and a multifunction apparatus.

It goes without saying that the object of the present invention may also be accomplished by supplying a system or an apparatus with a storage medium (or a recording medium) in which a program code of software, which realizes the functions of either of the above described embodiments is stored, and causing a computer (or CPU or MPU) of the system or apparatus to read out and execute the program code stored in the storage medium.

In this case, the program code itself read from the storage medium realizes the novel functions of either of the above described embodiments, and hence the program code and a storage medium on which the program code is stored constitute the present invention.

Examples of the storage medium for supplying the program code include a floppy (registered trademark) disk, a hard disk, an optical disk, a magnetic-optical disk, a CD-ROM, a CD-R, a CD-RW, a DVD-ROM, a DVD-RAM, a DVD-RW, DVD+RW, a magnetic tape, a nonvolatile memory card, a ROM, and an EEPROM. Alternatively, the program is supplied by downloading via a network or the like.