Rotating-body driving device and image forming apparatus

A rotating-body driving device that includes a rotating body; a driving source; a reduction gear that includes an output shaft and a gear rotating at a non-integer ratio of rotation period to a rotation period of the output shaft, and the reduction gear reducing rotation speed of the driving source; a pulse-signal generating unit; a pulse-count storage unit; a speed-fluctuation storage unit that stores therein a rotation speed fluctuation of the output shaft; and a driving-source control unit. The driving-source control unit detects the speed fluctuation information associated with the accumulated number of pulse signals from the speed-fluctuation storage unit, and performs for the driving source a feedforward control to set off the rotation speed fluctuation.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority to and incorporates by reference the entire contents of Japanese Patent Application No. 2012-183938 filed in Japan on Aug. 23, 2012.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a rotating-body driving device for driving a rotating body and an image forming apparatus, such as a copier, a printer, or a facsimile machine, equipped with the rotating-body driving device.

2. Description of the Related Art

In general, as a color image forming apparatus, there is known, for example, a direct transfer method of tandem-type image forming apparatus that forms a color image on a recording medium by transferring solid black (Bk), yellow (Y), magenta (M), and cyan (C) images formed on photosensitive drums onto the recording medium being carried/conveyed on a recording-medium conveyance belt in a superimposed manner.

In such a tandem-type image forming apparatus, generally, a motor provided as a driving source of a photosensitive drum and a reduction gear unit are installed in the main body side of the image forming apparatus, and the reduction gear unit is connected to the photosensitive drum so as to transmit a driving force to the photosensitive drum.

Furthermore, in the tandem-type image forming apparatus, the quality of an image is greatly affected by the accuracy of surface moving speed of each photosensitive drum. Respective transfer positions of solid color images on the photosensitive drums are relatively shifted by fluctuation in surface moving speed which periodically occurs in the individual photosensitive drums. This causes a color shift of a color image formed on a recording medium or a so-called “banding phenomenon”, density unevenness that periodically appears like strips, in a range of the color image formed.

Such fluctuation in surface moving speed is caused by an error in transmission of a drive transmission system installed on a shaft of a photosensitive drum (a transmission error due to gear eccentricity or cumulative tooth pitch deviation, and the like) and a transmission error due to a coupling provided to removably attach the photosensitive drum to the drive transmission system (axial tilt, shaft misalignment, and the like).

The periodic fluctuation in surface moving speed occurs with a rotation period of the shaft, a rotation period of gears, and a rotation period of a higher-order component, and constantly occurs in a driving state. Furthermore, the magnitude of the periodic fluctuation in surface moving speed varies according to the progression of gear wear with time or changes in installation conditions, such as a hygrothermal environment, of the image forming apparatus. Therefore, to correct the color shift, it is necessary to suppress the periodic fluctuation in surface moving speed of the photosensitive drums that varies with time and environment.

For example, Japanese Patent No. 2754582, Japanese Patent No. 3259440, and Japanese Patent Application Laid-open No. 2008-099490 have disclosed a technology of detecting an angular velocity of a shaft of a photosensitive drum when a drive motor is rotated at a predetermined constant angular velocity with a rotary encoder, storing information on fluctuation in angular velocity of the photosensitive drum during one revolution, and changing the angular velocity of the drive motor on the basis of the stored fluctuation information at the timing of a home position signal output with each rotation of the rotary encoder, i.e., executing so-called feedforward control. This technology can eliminate an oscillation phenomenon such as an increase in rotation speed fluctuation which is a concern in feedback control and achieve stable drive control, and therefore can suppress periodic fluctuation in surface moving speed of the photosensitive drum.

Furthermore, Japanese Patent Application Laid-open No. 2010-008924 has disclosed a technology of counting the number of pulses output from a rotary encoder and outputting a timing signal when it comes to a pulse count corresponding to one revolution of the rotary encoder, thereby detecting a home position signal.

In this manner, to perform feedforward control on the basis of fluctuation information detected in advance based on a preset home position on a rotating shaft is effective as a method to suppress periodic fluctuation in surface moving speed of a photosensitive drum.

However, in such a conventional image forming apparatus using feedforward control based on a home position in each rotation of a rotary encoder (in each rotation of a photosensitive drum), a period of fluctuation to be corrected is limited to only an integral period with respect to a rotation period of the photosensitive drum.

Therefore, if a reduction gear with a non-integral reduction gear ratio, for example, a planetary gear mechanism is adopted, there is a problem that there exist gears with a non-integral ratio or non-terminating decimal ratio of rotation period, and fluctuation cannot be corrected on the basis of the home position in each rotation of the photosensitive drum.

As a means for feedforward control of gears with a non-integral ratio or non-terminating decimal ratio of rotation period, for example, a home position could be set on each gear.

However, this configuration has a problem that a component for detecting a home position of each gear has to be installed, which results in an increase in the number of parts and an increase in cost.

There have been needs to solve these problems and to provide a rotating-body driving device capable of performing feedforward control enabling, even when a reduction gear having gears that each rotate with a non-integral ratio of rotation period to a rotation period of a shaft of a photosensitive drum is used in a drive transmission system of the photosensitive drum, to suppress periodic fluctuation generated with the respective rotation periods of the gears.

SUMMARY OF THE INVENTION

According to an aspect of the invention, a rotating-body driving device is provided. The rotating-body driving device includes: a rotating body; a driving source that generates a driving force for rotating the rotating body; a reduction gear that includes an output shaft connected to the rotating body and a gear rotating at a non-integer ratio of rotation period to a rotation period of the output shaft, and the reduction gear reducing rotation speed of the driving source with the gear and transmitting the driving force to the rotating body via the output shaft; a pulse-signal generating unit that generates a pulse signal associated with the number of revolutions of the output shaft; a pulse-count storage unit that accumulates and stores therein the number of pulse signals generated by the pulse-signal generating unit; a speed-fluctuation storage unit that stores therein a rotation speed fluctuation of the output shaft occurring every rotation period of the gear as speed fluctuation information associated with the number of pulse signals; and a driving-source control unit that controls the driving source, wherein the driving-source control unit detects the speed fluctuation information associated with the accumulated number of pulse signals from the speed-fluctuation storage unit on the basis of the accumulated number of pulse signals stored in the pulse-count storage unit, and performs for the driving source a feedforward control using the speed fluctuation information to set off the rotation speed fluctuation.

According to another aspect of the invention, an image forming apparatus is provided. The image forming apparatus includes: the rotating-body driving device; and an image forming unit that transfers the image carried on the photosensitive drum onto a recording medium, thereby forming the image on the recording medium.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Exemplary embodiments of the present invention will be explained below with reference to accompanying drawings.

FIGS. 1 to 11are diagrams illustrating an embodiment of a rotating-body driving device and an image forming apparatus according to the present invention, and shows a case where the rotating-body driving device according to the present invention is applied to an electrophotographic color multifunction peripheral (hereinafter, referred to as an “MFP”) as an example of the image forming apparatus.

First, a configuration of the MFP is explained.

FIG. 1is a diagram illustrating a configuration of an MFP1according to the present embodiment. The MFP1is a so-called tandem type of image forming apparatus, and adopts a dry two-component developing method using dry two-component developer. InFIG. 1, the MFP1includes an MFP main body2, a sheet feeder3, a scanner4, and an automatic document feeder5as image forming means.

In the MFP1, the MFP main body2is set up on top of the sheet feeder3, and the scanner4is installed on top of the MFP main body2, and the automatic document feeder5is installed on top of the scanner4.

The MFP1receives image data which is information on a scanned image from the scanner4or receives print data from an external device, such as a personal computer, and performs an image forming process.

The MFP main body2includes four photosensitive drums6Y,6M,6C, and6Bk (for yellow (Y), magenta (M), cyan (C), and black (Bk) color images) provided as rotating bodies. The photosensitive drums6Y,6M,6C, and6Bk are driven bodies and cylindrical latent-image carriers.

Furthermore, the MFP main body2includes, as members for an electrophotographic process, charging units8Y,8M,8C, and8Bk, developing units9Y,9M,9C, and9Bk, cleaning units10Y,10M,10C, and10Bk, and neutralization lamps11Y,11M,11C, and11Bk around the photosensitive drums6Y,6M,6C, and6Bk in the order of the process.

The MFP main body2includes an optical writing device12above the photosensitive drums6Y,6M,6C, and6Bk. Furthermore, the MFP main body2includes primary transfer rollers13Y,13M,13C, and13Bk, which are primary transfer means, in positions opposed to the photosensitive drums6Y,6M,6C, and6Bk across an intermediate transfer belt7.

The photosensitive drums6Y,6M,6C, and6Bk have contact with the intermediate transfer belt7which is an endless belt supported by multiple rotatable rollers including a drive roller, and are arranged side-by-side along a moving direction of the intermediate transfer belt7.

The intermediate transfer belt7is supported by support rollers14and15, a drive roller16, and a tension roller17, and is driven to rotate by rotation of the drive roller16driven to rotate by a driving source (not shown).

A belt cleaning unit18is installed in a position opposed to the support roller15across the intermediate transfer belt7, and removes residual toner remaining on the intermediate transfer belt7after secondary transfer.

The support roller14is an opposed secondary transfer roller opposed to a secondary transfer roller19which is a secondary transfer means. A secondary transfer nip portion is formed between the support roller14and the secondary transfer roller19across the intermediate transfer belt7.

A transfer-sheet conveyance belt21is supported by support rollers20aand20b, and is installed on the downstream side of the secondary transfer nip portion in a transfer-sheet conveying direction. The transfer-sheet conveyance belt21conveys a transfer sheet onto which a toner image has been secondary-transferred to a fixing unit22.

The fixing unit22includes fixing rollers23aand23b, and fixes an unfixed toner image on a transfer sheet by applying heat and pressure to the transfer sheet at a fixing nip portion formed by abutting contact between the fixing rollers23aand23b.

Subsequently, a copy operation of the MFP is explained.

When a user forms a full-color image with use of the MFP1according to the present embodiment, first, the user sets an original on an original table24of the automatic document feeder5. Or, the user opens the automatic document feeder5and sets an original on a platen glass25of the scanner4, and closes the automatic document feeder5to cover it.

After that, if the user pushes a START switch (not shown), the original is conveyed onto the platen glass25in the case where the original has been set in the automatic document feeder5. When the original has been on the platen glass25, first and second traveling bodies26and27of the scanner4start traveling.

A light from the first traveling body26is reflected by the original on the platen glass25, and the reflected light is further reflected by a mirror of the second traveling body27and is guided into a read sensor29through an imaging lens28, thereby the scanner4scans image information of the original.

Furthermore, when the user has pushed the START switch, the MFP main body2drives a motor (not shown), thereby driving the drive roller16to rotate, so that the intermediate transfer belt7is driven to move by the rotation of the drive roller16.

Furthermore, at the same time that the intermediate transfer belt7is driven to move, a photoreceptor driving device30Y (not shown) as a rotating-body driving device to be described later drives the photosensitive drum6Y to rotate in a direction of arrow, and the rotating photosensitive drum6Y is uniformly charged by the charging unit8Y.

After that, the optical writing device12emits an optical beam31Y to the photosensitive drum6Y, and a Y electrostatic latent image is formed on the photosensitive drum6Y. The Y electrostatic latent image is developed into a Y-toner image by transfer of Y toner contained in developer applied by the developing unit9Y.

At the developing, a predetermined developing bias is applied to between a developing roller and the photosensitive drum6Y, and Y toner on the developing roller is electrostatically-transferred onto the Y electrostatic latent image on the photosensitive drum6Y.

In accordance with the rotation of the photosensitive drum6Y, the Y-toner image formed on the photosensitive drum6Y is conveyed to a primary transfer position at which the photosensitive drum6Y has contact with the intermediate transfer belt7. At the primary transfer position, the primary transfer roller13Y applies a predetermined bias voltage to the back side of the intermediate transfer belt7.

Then, by a primary-transfer electric field generated by the application of the bias voltage, the Y-toner image is attracted to the side of the intermediate transfer belt7, and is primary-transferred onto the intermediate transfer belt7.

Likewise, an M-toner image, a C-toner image, and a Bk-toner image are sequentially primary-transferred onto the intermediate transfer belt7so as to be superimposed on the Y-toner image. Incidentally, residual toner remaining on the intermediate transfer belt7after secondary transfer is removed by the belt cleaning unit18.

Moreover, when the user has pushed the START switch, the sheet feeder3rotates a sheet feed roller40corresponding to transfer paper that the user has selected, and sends transfer sheets out from one of sheet cassettes33.

The sent transfer sheets are separated one by one by separation rollers34aand34band sequentially fed into a sheet feed path35, and conveyed to a sheet feed path37in the MFP main body2by a conveyance roller36. The conveyed transfer sheet is stopped by bumping against a registration roller38.

When transfer sheets which have not set in any of the sheet cassettes33are used, the MFP main body2sends transfer sheets set in a manual feed tray39by means of the sheet feed roller32. The sent transfer sheets are separated one by one by a separation roller41, and sequentially conveyed to the registration roller38through a manual feed path42.

The superimposed four-color toner image on the intermediate transfer belt7is conveyed to a secondary transfer position opposite to the secondary transfer roller19in accordance with the movement of the intermediate transfer belt7. Furthermore, the registration roller38starts rotating in keeping with the timing at which the compound toner image formed on the intermediate transfer belt7is conveyed to the secondary transfer position, and conveys the transfer sheet to the secondary transfer position.

Then, at the secondary transfer position, the secondary transfer roller19applies a predetermined bias voltage to the back side of the transfer sheet. By a secondary-transfer electric field generated by the application of the bias voltage and a contact pressure at the secondary transfer position, the toner image on the intermediate transfer belt7is secondary-transferred onto the transfer sheet.

The transfer sheet onto which the toner image has been secondary-transferred is conveyed to the fixing unit22by the transfer-sheet conveyance belt21. Here, the fixing unit22performs a process of fixing the toner image on the transfer sheet by means of the fixing rollers23aand23bincluded in the fixing unit22.

After the fixing process, the transfer sheet is discharged and stacked on a copy receiving tray44installed outside of the MFP1by sheet discharge rollers43aand43b.

Subsequently, a configuration of a rotating-body driving device including a reduction gear is explained. Incidentally, the photosensitive drums6Y,6M,6C, and6Bk, which are driven bodies, are driven to rotate by photoreceptor driving devices30Y,30M,30C, and30Bk having the same configuration; therefore, in the explanation described below, alphabetic color codes Y, M, C, and Bk are omitted.

As a reduction gear with a non-integral reduction gear ratio, a reduction gear using a gear train with a non-integral ratio of the number of gear teeth is commonly used.

By adopting a reduction gear using a gear train with a non-integral ratio of the number of gear teeth, the range of options for reduction gear ratios is expanded, and an optimum reduction gear, ratio can be set according to output characteristics, such as the number of revolutions and efficiency of the motor.

Furthermore, by adopting a reduction gear using a gear train with a non-integral ratio of the number of gear teeth, engagement of teeth between gears varies with each rotation, and therefore it is possible to prevent uneven wear.

In the photoreceptor driving device30according to the present embodiment, as a reduction gear using a gear train with a non-integral ratio of the number of gear teeth, a planetary reduction, gear capable of enhancing the durability, the miniaturization, and the high precision in addition to the above effects is used.

FIGS. 2 to 5are diagrams showing a configuration of the photoreceptor driving device30. The photoreceptor driving device30includes a motor45as a driving source, a planetary reduction gear46as a reduction gear and a planetary gear mechanism, a joint47, and a drum shaft48.

As shown inFIGS. 2 and 3, an output shaft50of the planetary reduction gear46is connected and fixed to the drum shaft48by the joint47. Furthermore, in the photoreceptor driving device30, a bearing49is press-fitted in the drum shaft48.

A near-tip portion of the drum shaft48of the photoreceptor driving device30is fitted in a bearing53installed on a front side plate52, which is a front-side main body side plate fixed to a housing of the MFP main body2. Furthermore, the photoreceptor driving device30is installed on a back side plate51, which is a back-side main body side plate fixed to the housing of the MFP main body2, via the bearing49.

Namely, the photoreceptor driving device30is supported and positioned by being installed on the front and back side plates52and51, which are part of the housing of the MFP main body2, via the bearings53and49of the drum shaft48.

In the planetary reduction gear46according to the present embodiment, a 2K-H type two-stage planetary gear mechanism is used. In general, a planetary reduction gear is composed of four parts: a sun gear, planetary gears, a planetary carrier which supports the revolution of the planetary gears, and an outer gear. In the 2K-H type planetary gear mechanism, a shaft of the sun gear, a shaft of the planetary carrier, and a shaft of the outer gear are basic shafts.

The 2K-H type planetary gear mechanism has three elements: rotation of the sun gear, rotation of the planetary gears (rotation of the planetary carrier), and rotation of the outer gear, and any one of the three elements is fixed, another one is connected to input, and the remaining one is connected to output.

The 2K-H type planetary gear mechanism can switch a reduction gear ratio and a rotation direction by which of fixed, input, and output are the three elements assigned to, respectively; therefore, switching of multiple reduction gear ratios and rotation directions can be achieved by one unit.

A reduction gear ratio of the 2K-H type two-stage planetary gear mechanism is calculated by the following equation (1), where Za denotes the number of teeth of the sun gear, Zb denotes the number of teeth of the planetary gear, and Zc denotes the number of teeth of the outer gear. Incidentally, suffixes 1 and 2 in the equation (1) denote the first stage and the second stage, respectively.

The 2K-H type two-stage planetary gear mechanism according to the present embodiment is categorized as a compound planetary gear mechanism (of two or more 2K-H type planetary gear mechanisms), and the shaft of the sun gear is set as an input shaft, the shaft of the outer gear is set as a fixed shaft, and the shaft of the planetary carrier is set as an output shaft.

InFIG. 3, a first-stage planetary gear mechanism of the planetary reduction gear46includes a first sun gear55, an outer gear57, first planetary gears58, a first carrier59, and a first carrier pin60. The outer gear57is integrally formed with an outer gear of a second-stage planetary gear mechanism.

To reduce the number of components, the first sun gear55is formed by directly cutting a portion of a motor output shaft54which is a drive shaft of the motor45.

The first planetary gears58engage with the first sun gear55and the outer gear57fixed to a bracket56, and revolve along an outer periphery of the first sun gear55while being supported by the first carrier59.

For the sake of rotational balance and torque sharing, the first planetary gears58are arranged in equally-spaced positions where the first carrier59is concentrically divided into three equal parts in a circumferential direction. The first planetary gears58each rotate while being supported by the first carrier pin60installed on the first carrier59.

By engagement with the first sun gear55and the outer gear57, the first planetary gears58rotate and revolve. This reduces the rotation of the first carrier59supporting the first planetary gears58to lower speed than the first sun gear55, and thus a first-stage reduction gear ratio is acquired.

Incidentally, the first carrier59has no rotation support part, and is configured to rotate in a floating state.

The second-stage planetary gear mechanism of the planetary reduction gear46includes a second sun gear61, the outer gear57, second planetary gears62, a second carrier63, a second carrier pin64, and the output shaft50.

To reduce the number of components, the second sun gear61is integrally formed with the first carrier59in the rotation center of the first carrier59, and the second sun gear61is input of the second-stage planetary gear mechanism.

The second planetary gears62engage with the second sun gear61and the outer gear57, and revolve along an outer periphery of the second sun gear61while being supported by the second carrier63.

The second planetary gears62are arranged in equally-spaced positions where the second carrier63is concentrically divided into three equal parts in a circumferential direction. The second planetary gears62each rotate while being supported by the second carrier pin64installed on the second carrier63.

The second carrier63rotates in accordance with the rotation and revolution of the second planetary gears62driven by engagement with the second sun gear61and the outer gear57. An outer-gear cap65is installed in the end of the outer gear57on the side of the photosensitive drum6so as to cover the carriers and the planetary gears, and a bearing65ais press-fitted in the inside of the outer-gear cap65.

The output shaft50is installed in the rotation center of the second carrier63corresponding to the final stage, and is connected to the drum shaft48having the same diameter via the hollow cylindrical joint47. The output shaft50is positioned by the outer gear57, and is supported by the bearing65apress-fitted in the outer-gear cap65.

The outer-gear cap65is positioned by being fitted in a groove that has been formed on an inner periphery of the outer gear57and has about the same diameter as the outer diameter of the outer-gear cap65. Consequently, the planetary reduction gear46can minimize coaxiality between the output shaft50and the central axis of the outer gear57.

In the photoreceptor driving device30, the drum shaft48and the output shaft50are coaxially connected and integrated by the joint47which is a connecting member. Here, the joint47is configured as shown inFIG. 4.

As shown inFIG. 4, the joint47has a hollow cylindrical shape, and a portion of the joint47on the side of the drum shaft48is press-fitted in the drum shaft48. Furthermore, a portion of the joint47on the side of the output shaft50is loosely fitted in the output shaft50, and is connected and fixed to the output shaft50with a shoulder screw66.

Alternatively, the joint47can be configured as shown inFIG. 5. The joint47shown inFIG. 5has a slit47ain the central part of the hollow cylindrical shape. The output shaft50is connected and fixed to the joint47by a force due to friction with the joint47bent by a screw67.

In any of the configurations shown inFIGS. 4 and 5, the joint47in the present embodiment is preferably configured to minimize misalignment between the central axes of the drum shaft48and the output shaft50in the joint part and be able to transmit a driving source.

Incidentally,FIG. 5Ais a diagram illustrating a method of connecting and fixing the output shaft50and the drum shaft48by the joint47;FIG. 5Bis a front view of the joint47viewed from a direction of the central axis of the output shaft50.

As shown inFIG. 3, the motor45is supported by the bracket56. Furthermore, the outer gear57is fixed to the bracket56with screws68. In this manner, the bracket56fixes and holds the motor45and the outer gear57.

The bracket56is fixed to a drive side plate69with screws. Furthermore, the drive side plate69is supported and positioned by studs70swaged into the back side plate51.

A hollow cylindrical boss is formed on the central axis of the outer gear57on the side of the motor45, and the motor45is positioned by being fitted in the inner periphery of the boss having about the same diameter as the outer diameter of a bearing installed on the motor45side.

The outer gear57is configured to be positioned by fitting the outer periphery of the boss in a hole that has been formed on the bracket56and has about the same diameter as the outer diameter of the boss.

By such a configuration, the planetary reduction gear46can arrange the motor output shaft54, the bracket56, and the output shaft50so that their central axes are coaxially arranged with reference to the outer gear57. Furthermore, by the present configuration, the planetary reduction gear46can minimize coaxiality due to dimensional variations in parts of the motor output shaft54, the bracket56, and the output shaft50.

Incidentally, in the photoreceptor driving device30according to the present embodiment, the first sun gear55, the first planetary gears58, the second sun gear61, and the second planetary gears62compose a gear. Incidentally, in the present embodiment, the outer gear57is fixed; however, in a case where another rotational element is fixed, the outer gear57composes a gear.

Furthermore, in the photoreceptor driving device30according to the present embodiment, the first sun gear55and the second sun gear61compose a sun gear, the first planetary gears58and the second planetary gears62compose planetary gears, and the first carrier59and the second carrier63compose a carrier.

The photosensitive drum6is composed of a cylindrical drum71and drum flanges72aand72b. The drum71is configured to be positioned by the drum shaft48via the drum flanges72aand72binstalled on both ends of the drum71.

A hole having about the same diameter as the drum shaft48is formed on each of the drum flanges72aand72bat the position of the central axis of the drum71, and the drum71is attached to the drum shaft48by inserting the drum shaft48into the holes, thereby being positioned. Accordingly, the photosensitive drum6, which is a driven body, is supported and positioned by the housing of the MFP main body2via the drum shaft48.

To transmit a driving force to the drum71, a joint73is press-fitted in the drum shaft48. The drum71is configured to be driven via the drum flange72aconnected to the joint73by rotation of the joint73in accordance with rotation of the drum shaft48.

A rotary encoder74as a pulse-signal generating unit is installed on the output shaft50. The rotary encoder74is a rotation-speed detecting means including an encoder circular plate74aand two sensors74b.

The encoder circular plate74ais attached to the output shaft50so as to be mounted coaxially with the central axis of the outer gear57, the motor output shaft54, the bracket56, and the output shaft50. Furthermore, the encoder circular plate74ais placed on the upstream side of the joint47of the output shaft50in a driving-force transmitting direction.

Slits are formed on the encoder circular plate74ain a circumferential direction at even intervals, and the sensors74beach optically detect a slit of the encoder circular plate74aand output a detection signal to a controller75to be described later.

The two sensors74bdetect a slit of the encoder circular plate74aat positions having a phase difference of 180 degrees, respectively; even if the encoder circular plate74ais installed eccentrically to the output shaft50, the controller75averages data detected by the two sensors74b, so that a rotation angular velocity of the output shaft50can be detected with high accuracy.

Incidentally, instead of an optical encoder, a magnetic encoder which detects a magnetic mark put on the concentric circle of a disk composed of a magnetic body with a magnetic head can be adopted as the rotary encoder74. Or, a well-known tacho generator can be used.

As described above, the photoreceptor driving device30uses the planetary reduction gear46, and therefore can suppress rotation fluctuation of the photosensitive drum6without installing a large-diameter gear or installing a direct drive motor as a driving source.

Furthermore, the photoreceptor driving device30according to the present embodiment can arrange the motor output shaft54and integrally-formed first sun gear55, the outer gear57, the first carrier59and integrally-formed second sun gear61, the second carrier63and integrally-formed output shaft50, the drum shaft48, the central axis of the drum71composing the photosensitive drum6, and the encoder circular plate74aall on the same axis.

Consequently, the photoreceptor driving device30can minimize coaxiality due to dimensional variations in parts.

Moreover, the photoreceptor driving device30is supported in a state where the first carrier59floats with respect to the outer gear57.

Consequently, a concentric error between the first carrier59and the outer gear57is suppressed by the action of alignment by supporting the photoreceptor driving device30in the state where the first carrier59floats, and therefore the photoreceptor driving device30can further suppress the rotation fluctuation of the photosensitive drum6.

Furthermore, the photoreceptor driving device30includes the rotary encoder74; therefore, by performing feedback control (hereinafter, referred to as “FB control”) of the motor45, the photoreceptor driving device30can further suppress rotation fluctuation of the photosensitive drum6resulting from a concentric error caused by an installation error or the like.

Consequently, it is possible to provide the photoreceptor driving device30capable of driving the high-accuracy rotation of the photosensitive drum6of which the rotation fluctuation is further suppressed.

The first sun gear55, the first carrier pin60, the second carrier63, and the second carrier pin64composing the planetary reduction gear46are made of metallic material, such as stainless steel or carbon steel.

Furthermore, the first planetary gears58, the first carrier59, the second sun gear61integrally formed with the first carrier59, the second planetary gears62, and the outer gear57integrally formed with the housing case are moldings made of resin material, such as polyacetal.

The planetary reduction gear46is made of a hybrid of metal and resin as described above; therefore, the metallic output shaft50can be integrally provided with the second carrier63.

In this manner, the output shaft50and the second carrier63are made of metal; therefore, the output shaft50and the second carrier63can withstand a high load of the photosensitive drum6as compared with a planetary gearbox of which the major components are all made of resin.

Therefore, the planetary reduction gear46according to the present embodiment can respond to weight saving and resource saving, and can further withstand high load of the photosensitive drum6than a planetary gearbox of which the major components are all made of resin.

In the photoreceptor driving device30, the drum shaft48is rotatably supported by the back side plate51via the bearing49in a state where a position of the drum shaft48in a radial direction is fixed by the back side plate51via the bearing49. Furthermore, in the photoreceptor driving device30, the outer gear57of the planetary reduction gear46is also fixed to the back side plate51via the bracket56and the studs70.

Therefore, when the photoreceptor driving device30is installed in the MFP main body2, if there is shaft misalignment between the drum shaft48and the output shaft50of the planetary reduction gear46, rotation fluctuation resulting from the shaft misalignment may arise.

Consequently, in the planetary reduction gear46, the outer gear57, the first planetary gears58, the second planetary gears62, the first carrier59, and the second sun gear61integrally formed with the second carrier63are made of resin and are configured to be elastically deformable in a radial direction.

Furthermore, by configuring the photoreceptor driving device30to be elastically deformable, even in the event of shaft misalignment between the drum shaft48and the output shaft50, the photoreceptor driving device30can align the drum shaft48and the output shaft50by elastic deformation of components configured to be elastically deformable. Consequently, the photoreceptor driving device30can drive the photosensitive drum6to rotate with high accuracy.

Moreover, the components of the photoreceptor driving device30which are configured to be elastically deformable can distribute an elastic deformation amount in the alignment, and therefore can improve the durability of the photoreceptor driving device30.

Furthermore, by installing the metallic output shaft50, the photoreceptor driving device30can use the joint47capable of minimizing misalignment between the central axes of the drum shaft48and the output shaft50and transmitting a driving force in connection between the drum shaft48and the output shaft50.

Generally, in a photoreceptor driving device of which the major components are all made of resin, for example, a spline joint with backlash is used in connection between a shaft of a driven body and an output unit of a planetary gear mechanism.

However, in the photoreceptor driving device30according to the present embodiment, the joint47is used to connect and unite the drum shaft48and the output shaft50; therefore, it is possible to eliminate rotation fluctuation caused by backlash.

Furthermore, in the photoreceptor driving device30, unevenness of rotation between the drum shaft48and the output shaft50does not occur by installation of the joint47; therefore, an installation position of the rotary encoder74is not limited to the downstream side of the joint47in the driving-force transmitting direction.

Therefore, the rotary encoder74can be placed on the upstream side of the joint47of the output shaft50in the driving-force transmitting direction, i.e., in the planetary reduction gear46.

In this manner, by placing the rotary encoder74on the side of the planetary reduction gear46, mounting of the rotary encoder74in the photoreceptor driving device30can be achieved without deteriorating assemblability of the photoreceptor driving device30.

When the rotary encoder74is mounted in the photoreceptor driving device30, for example, the encoder circular plate74aof the rotary encoder74is attached to the output shaft50of the planetary reduction gear46fixed to the bracket56together with the motor45.

Then, the sensors74bof the rotary encoder74are attached to the housing case integrated with the outer gear57, and positions of the encoder circular plate74aand the sensors74bare adjusted and fixed.

Next, the output shaft50is integrally connected to the drum shaft48by the joint47. Then, the photoreceptor driving device30is implemented in such a manner that the drum shaft48is inserted into a hole formed on the back side plate51, and the planetary reduction gear46is inserted into a hole formed on the drive side plate69, thereby positions of the drum shaft48and the planetary reduction gear46are adjusted and fixed.

In this manner, the rotary encoder74is mounted in the photoreceptor driving device30, thereby the photoreceptor driving device30can drive the photosensitive drum6to rotate with high accuracy by FB control using the rotary encoder74.

Furthermore, by the above-described configuration, the photoreceptor driving device30can achieve both resource saving resulting in weight saving and highly-accurate rotary drive of the photosensitive drum6.

In the MFP main body2according to the present embodiment, when the diameter of the photosensitive drum6is 60 mm, surface moving speed of the photosensitive drum6and conveying speed of the intermediate transfer belt7are 350 mm/s, and therefore the number of revolutions of the photosensitive drum6is 112 rpm. Incidentally, the diameter of the photosensitive drum6is not limited to this.

The photosensitive drum6is required to have highly-accurate constant-speed rotational performance; therefore, a motor capable of controlling the rotation speed, such as a DC servomotor or a stepping motor, is adopted as the motor45. The motor45according to the present embodiment is composed of an outer rotor type DC brushless motor with stable rotation characteristics and low power consumption.

To efficiently rotate the outer rotor type DC brushless motor that outputs about 20 to 30 W to drive the photosensitive drum6or a transfer belt, it is preferable that the outer rotor type DC brushless motor is driven to rotate at about 2400 to 3600 rpm.

Therefore, the planetary reduction gear46is required to reduce the number of revolutions of the output shaft50to one twentieth to thirtieth of the number of revolutions of the motor output shaft54.

Furthermore, in the layout of the photoreceptor driving device30, space conservation can be achieved by eliminating constraints of interference with a development driving device and a toner supply unit around the photosensitive drum6and placing a driving device near the side plate of the photosensitive drum6.

Therefore, when a reduction gear using a large-diameter gear, for example, a gear having the substantially larger diameter than that of the photosensitive drum6is adopted, the reduction gear has to be installed by displacing either the large-diameter gear or a development driving device in an axial direction of the photosensitive drum6for avoiding interference with the development driving device.

Furthermore, there exists a large unutilized region (dead space) around the large-diameter gear. This leads to increases in size and cost of the entire device.

Therefore, in the photoreceptor driving device30according to the present embodiment, the 2K-H type two-stage planetary reduction gear46is adopted as a reduction gear to achieve the requirements of a reduction gear ratio of 20 to 30 and the outer diameter of 60 mm.

FIG. 6is a diagram showing gear specifications of the planetary reduction gear46according to the present embodiment. In the planetary reduction gear46, a first-stage reduction gear unit on the side of the motor45, which is a driving source, is the input side, and a second-stage reduction gear unit on the side of the photosensitive drum6, which is an object to be driven, is the output side.

A reduction gear ratio of the input side is 7.08, and a reduction gear ratio of the output side is 4.16, and the total reduction gear ratio of the planetary reduction gear46is 29.4. Furthermore, the planetary reduction gear46is configured so that a root circle diameter of the outer gear57is about 33.3 mm and an outer diameter is not more than 50 mm.

Generally, in a planetary gear mechanism, if both have the same outer diameter, one having a smaller reduction gear ratio than the other is lower in load torque acting on a gear engagement part. Therefore, reduction gear ratios of the input side and the output side are preferably set so that the reduction gear ratio of the output side on which load torque largely acts is smaller than that of the input side to improve the durability of the planetary gear mechanism.

In the planetary reduction gear46, an integrally-molded gear shared by the input side and the output side is adopted as the outer gear57to reduce the cost. Therefore, in the planetary reduction gear46, to increase the reduction gear ratio of the input side, the number of teeth of the first sun gear55is 13 which is fewer than 25 teeth of the output-side second sun gear61.

Generally, in a planetary reduction gear, two or more planetary gears are arranged at equal spaces. The planetary reduction gear46includes three first planetary gears58and three second planetary gears62.

Furthermore, to improve the rotation accuracy, the number of teeth of a sun gear is preferably the non-integral multiple of the number of teeth of a planetary gear.

Therefore, in the planetary reduction gear46according to the present embodiment, the number of teeth of the first sun gear55is set to 13, and the number of teeth of the second sun gear61is set to 25 so that the number of teeth of the first and second sun gears55and61are the non-integral multiple of the number of (three) teeth of the first and second planetary gears58and62, respectively.

Consequently, the timing for each of the three first planetary gears58and the three second planetary gears62to engage with the first sun gear55and the second sun gear61is out of synchronization, so engagement vibration generated due to a difference in tooth pitch between engagement parts causes a phase difference between the first planetary gears58and the second planetary gears62, and this reduces the vibration.

Furthermore, the first and second planetary gears58and62according to the present embodiment have an odd number of teeth (the first planetary gear58has 13 teeth, and the second planetary gear62has 25 teeth).

Consequently, the first planetary gears58generate a phase difference between engagement vibration generated due to the tooth pitch of an engagement part engaged with the first sun gear55and engagement vibration generated due to the tooth pitch of an engagement part engaged with the outer gear57, and therefore reduces the vibration.

Therefore, the rotation accuracy of the first planetary gears58is improved. The second planetary gears62can also achieve the same effect, and the rotation accuracy of the second planetary gears62is improved.

Incidentally, to further reduce engagement vibration generated due to a difference in tooth pitch between engagement parts of gears, helical gears are adopted as gears of the planetary reduction gear46, and the face width and helix angle are set so that a tooth contact ratio is 3 or higher.

Consequently, respective reduction gear ratios of the input and output sides of the planetary reduction gear46determined by the number of teeth of gears shown inFIG. 6are both a non-integral and non-terminating decimal reduction gear ratio.

Incidentally, generally, there are many design examples of planetary gear mechanisms where even if the number of gear teeth as described above is not selected, and a reduction gear ratio is an integral ratio, a rotation period of a planetary gear shows a non-integral ratio to a rotation period of a carrier which is an output shaft.

FIG. 7is a diagram showing generated frequencies of major fluctuation components periodically generated due to rotation-speed fluctuation factors which are components of the planetary reduction gear46when the photosensitive drum6is driven at 1 Hz.

Incidentally, in the following explanation, parts composing the planetary reduction gear46are rotation fluctuation factors, and major fluctuation components periodically generated due to the rotation fluctuation factors are rotation fluctuation components.

The rotation fluctuation factors of the planetary reduction gear46include one tooth contacts of the first sun gear55, the second sun gear61, the first planetary gears58, the second planetary gears62, the first carrier59, and the second carrier63, and exist in both the first and second stages.

A rotation fluctuation component generated with a rotation period of the first sun gear55in the first stage is generated due to rotation fluctuation of the motor45and gear accuracy of the first sun gear55formed by cutting a portion of the motor output shaft54.

A rotation fluctuation component generated with a rotation period of the first planetary gears58in the first stage is generated due to gear accuracy of the first planetary gears58.

A rotation fluctuation component generated with a rotation period of the first carrier59which is the first-stage output is generated due to part accuracy of the first carrier59and gear accuracy of the second sun gear61integrally molded with the first carrier59.

A rotation fluctuation component generated with a rotation period of the second sun gear61in the second stage is generated due to part accuracy of the first carrier59and gear accuracy of the second sun gear61integrally molded with the first carrier59.

A rotation fluctuation component generated with a rotation period of the second planetary gears62in the second stage is generated due to gear accuracy of the second planetary gears62.

A rotation fluctuation component generated with a rotation period of the second carrier63which is the second-stage output is generated due to part accuracy of the second carrier63.

Furthermore, a rotation fluctuation component generated with a period of one tooth contact in each stage is generated due to tooth form accuracy of each gear.

In an image forming apparatus, a high degree of rotation accuracy is required; therefore, it is necessary to take a measure to suppress the fluctuation in all of these rotation fluctuation factors.

Therefore, in the photoreceptor driving device30according to the present embodiment, rotation fluctuation components of the gears and carrier in each stage that are fluctuations in a low-frequency band of 50 Hz or less indicated by bold font inFIG. 7are controlled by feedforward control (hereinafter, referred to as “FF control”) which is rotation control of the motor45.

Furthermore, in the photoreceptor driving device30according to the present embodiment, a fluctuation component generated with a period of one tooth contact that is a fluctuation in a high-frequency band of 50 Hz or more indicated by regular font inFIG. 7is reduced by choice of the number of gear teeth.

The fluctuation component generated with the period of one tooth contact, which is difficult to suppress by the control rotation control of the motor45, is suppressed by setting the number of teeth of the first and second sun gears55and61to be the non-integral multiple of the number of teeth of the first and second planetary gears58and62, respectively, and setting the number of teeth of the planetary gears to be odd numbers as shown inFIG. 6.

As a result, the fluctuation component generated with the period of one tooth contact is reduced; however, the fluctuations of the rotation fluctuation factors subject to the FF control are generated at a non-integral and non-terminating decimal ratio to a rotation period of the drum shaft48(a rotation period of the second carrier63).

Therefore, the photoreceptor driving device30cannot set a home position on the drum shaft48and therefore cannot perform the FF control according to an amount of previously-detected fluctuation.

Consequently, the photoreceptor driving device30according to the present embodiment performs the FF control based on accumulated pulse count of the rotary encoder74without using a home position, thereby suppressing the fluctuations of the rotation fluctuation factors.

FIG. 8shows an outline of a control system of the photoreceptor driving device30according to the present embodiment. InFIG. 8, configurations of the motor45, the planetary reduction gear46, and the rotary encoder74are as described above.

The rotary encoder74transmits a pulse signal according to a rotation amount of the output shaft of the planetary reduction gear46to the controller75serving as a pulse-number storage unit and a speed-fluctuation storage unit.

The controller75measures a time interval between pulse signals transmitted from the rotary encoder74, and calculates the current rotation speed of the output shaft50, and performs FB control for controlling the rotation speed of the motor45so that the rotation speed of the output shaft50becomes a target value.

Furthermore, the controller75previously detects a rotation fluctuation component for which the time interval between pulse signals periodically fluctuates, and performs the FF control at predetermining timing.

Then, a motor driver76drives the motor45in accordance with a motor-speed command value transmitted from the controller75.

FIG. 9is a block diagram for explaining a control system of the controller75.

The present control system includes an FB control system designed for a controlled object77including the motor driver76and the rotary encoder74.

Furthermore, the present control system further includes an FF control system in addition to the FB control system. The FF control system adds an FF control value to an output unit of the FB control system (a motor-speed command value). The FF control value here means a motor-speed command value output from the FF control system.

The FB control system performs control for suppressing rotation fluctuation caused by a non-periodic change in load on the drum shaft48by means of various parts having abutting contact with the photosensitive drum6.

In the FB control system, the controller75obtains speed information from a signal output from the rotary encoder74. Then, the controller75causes a comparator79to calculate speed deviation information, which is a difference between speed information and target speed information, on the basis of the speed information and target speed information obtained from a target-speed commanding unit78.

Then, the controller75causes a PID calculating unit89to calculate a motor-speed command value from the speed deviation information calculated by the comparator79.

Then, the controller75causes a filtering unit81to filter the motor-speed command value calculated by the PID calculating unit89. This filtering is performed to stabilize the FB control system while maintaining a control area of the FB control system.

The photoreceptor driving device30according to the present embodiment is a two-inertia system in which the motor45and the photosensitive drum6are inertia fields, and therefore, vibration is likely to be generated at a resonance point.

Consequently, to prevent excitation of resonant vibration due to driving of the motor of the FB control system, the filtering unit81adopts a quaternary Butterworth filter as a low-pass filter.

The controller75executes the FB control system with a control period of 1 msec, thereby suppressing various disturbance fluctuations and controlling the photosensitive drum6to rotate at the target speed constantly.

InFIG. 9, the FF control system is composed of a fluctuation-component detecting unit83, a switch84, and an FF-control-value calculating unit85, and is configured to add a result of calculation by the FF control system to the FB control system.

The FB control system is executed with the control period of 1 msec, whereas in the FF control system, operations of the fluctuation-component detecting unit83and the switch84are executed with a period of a few seconds. Then, the FF-control-value calculating unit85calculates a numerical value by performing multi-sampling with a period two to three times longer than the control period of the FB control system.

Subsequently, operation of the controller75in the FF control system is explained.

First, in a state where the switch84is OFF, the controller75causes the fluctuation-component detecting unit83to detect fluctuation information as speed fluctuation information included in rotation fluctuation components of the first planetary gears58, the second planetary gears62, the first carrier59, and the second carrier63from pulse signals transmitted from the rotary encoder74. The fluctuation information here means data on the amplitude and phase of a rotation fluctuation component.

Then, the controller75turns the switch840N, and transfers the fluctuation information detected by the fluctuation-component detecting unit83to the FF-control-value calculating unit85. Then, after the transfer of the fluctuation information, the controller75turns the switch84OFF, and causes the FF-control-value calculating unit85to calculate an FF control value, which offsets the fluctuation, on the basis of the fluctuation information and a current drum rotation phase.

Then, in a state where the switch84is OFF, the controller75causes an adder82to add the FF control value calculated by the FF-control-value calculating unit85to control output of the FB control system.

Consequently, the controller75can compensate only a disturbance caused by an objective periodic fluctuation in the form of FF control without disturbing closed-loop characteristics of the FB control system.

The planetary reduction gear46according to the present embodiment is a gear reducer, and therefore a relationship between the rotation periods of the gears is unchanged. A fluctuation of a rotation fluctuation factor to be suppressed occurs with a fixed period as shown inFIG. 7with respect to the rotation of the output shaft50to which the rotary encoder is attached.

Therefore, on the assumption that this periodic fluctuation is sinusoidal, a disturbance estimation observer is installed, and the fluctuation-component detecting unit83periodically detects a disturbance estimate, i.e., respective fluctuation information of rotation fluctuation components by using this observer.

By turning the switch84OFF, fluctuation information used by the FF-control-value calculating unit85is updated during the estimation of a disturbance by the fluctuation-component detecting unit83, thereby preventing the disturbance being estimated from being changed.

Consequently, the fluctuation information of the FF-control-value calculating unit85is updated during the estimation of a disturbance by the fluctuation-component detecting unit83, and therefore the controller75can avoid a change in the estimate disturbance estimated by the fluctuation-component detecting unit83and significant reduction of the FF control accuracy.

The FF-control-value calculating unit85calculates an FF control value for fluctuation offset with a period close to the control period of the FB control system in consideration of a function of transfer from control input to signal output of the encoder (a function of sensitivity to an input disturbance).

Incidentally, in the photoreceptor driving device30according to the present embodiment, the fluctuation-component detecting unit83, the switch84, and the FF-control-value calculating unit85compose an updated-speed-fluctuation storage unit.

FIG. 10is a flowchart of major steps of the operation of the FF control system shown in the block diagram ofFIG. 9.

InFIG. 10, Steps S11to S14are executed by the fluctuation-component detecting unit83, Step S15is executed by the FF-control-value calculating unit85when the switch84has been turned ON, and Step S16is subsequently executed by the FF-control-value calculating unit85.

To perform FF control, learning action of detecting a rotation fluctuation and management of a phase of the photosensitive drum6are required. By using these, the FF-control-value calculating unit85can calculate a sinusoidal FF control value that is opposite in amplitude value but the same in phase with respect to the detected periodic fluctuation and execute the FF control.

Consequently, to manage the phase of the photosensitive drum6, the controller75includes a phase-management pulse counter that accumulatively counts the number of pulses transmitted from the rotary encoder74.

The pulse counter starts counting at the time of startup of the motor45.

After completion of the startup of the motor45, the fluctuation-component detecting unit83starts sampling of encoder speed data (hereinafter, referred to as “ENC speed data sampling”) as the first operation of the FF control system (Step S11). At the start of the ENC speed data sampling, the controller75stores therein a phase-management pulse count value C1.

Then, the fluctuation-component detecting unit83performs a moving average process on the sampled data stored at Step S11, thereby removing a noise component in a higher frequency band than a component of a periodic fluctuation to be detected (Step S12). The moving average process here is a process of summing all the stored sampled data and dividing the sum by the number of the sampled data.

Then, the fluctuation-component detecting unit83performs downsampling of data to be stored in a memory as frequently as every 10 outputs at Step S12(Step S13). Through Step S13, the fluctuation-component detecting unit83can reduce a calculation load of a rotation-fluctuation-component estimating process to be subsequently performed.

After completion of the storage of the downsampled data, the fluctuation-component detecting unit83performs a process of estimating rotation fluctuation components of the first and second sun gears55and61and rotation fluctuation components of the first and second planetary gears58and62that are included in the downsampled data (Step S14). The estimating process here is to calculate estimated fluctuation data from each rotation fluctuation component.

Rotation fluctuation components subject to the estimating process include the rotation fluctuation components generated in the first sun gear55, the first planetary gears58, the second sun gear61integrally formed with the first carrier59, and the second planetary gears62. Generation of these fluctuation components has already been predicted in assessment of a prototype, and therefore, these fluctuation components have been set as objects of the estimating process.

Incidentally, a rotation fluctuation component of the second carrier63is expected to get a sufficient effect of control by the FB control system and therefore is excluded from an object of the estimating process performed by the FF control system.

In the rotation-fluctuation-component estimating process, the fluctuation-component detecting unit83performs a matrix operation on each of the rotation fluctuation components by using an estimated fluctuation coefficient matrix set in advance, and calculates an in-phase component (I component) and quadrature component (Q component) of each of the rotation fluctuation components.

After completion of the rotation-fluctuation-component estimating process, the fluctuation-component detecting unit83calculates fluctuation information from estimated fluctuation data created through the rotation-fluctuation-component estimating process. Then, the switch84is turned ON, the fluctuation information is transmitted to the FF-control-value calculating unit85. The FF-control-value calculating unit85updates the fluctuation information saved therein with the phase component and quadrature component of the received fluctuation information (Step S15).

Then, the FF-control-value calculating unit85calculates a phase value of each of the rotation fluctuation components from the updated fluctuation information and the pulse count value C1at the start of the ENC speed data sampling and a current pulse count value of the phase-management pulse counter. Then, the FF-control-value calculating unit85starts calculation of an FF control value (Step S16).

Incidentally, the pulse counter used for phase management is an accumulation counter, so there is a concern about overflow. As the timing to reset the accumulation counter in order to avoid overflow, the start of the ENC speed data sampling is preferably adopted so that all it takes is changing only the phase of the FF-control-value calculating unit85.

To improve the FF control accuracy, and respond to temporal changes of components of the planetary reduction gear46, and correct a phase management error caused by miscounting of the pulse counter, the controller75repeatedly executes the control mode shown inFIG. 10. This repeatedly-performed control mode is hereinafter referred to as a “constant learning type”.

In the constant learning type, the fluctuation information is constantly updated to the latest fluctuation information, and the FF-control-value calculating unit85calculates an FF control value on the basis of the updated fluctuation information.

FIG. 11is a diagram showing details of the processes performed by the FF-control-value calculating unit85at Steps S15and S16inFIG. 10after the switch84is turned ON in the constant learning type.

Each time the ON-OFF operation of the switch84is repeated by the execution of FF control action by the controller75, the FF-control-value calculating unit85acquires fluctuation information as a learning value. Here, the following value is acquired as a learning value.

For example, a first learning value acquired through the first FF control action is fluctuation information acquired when the motor45is driven at constant speed.

Furthermore, a second learning value acquired through the second FF control action is fluctuation information acquired when the motor45is driven by an FF control value calculated from the first learning value.

The FF-control-value calculating unit85acquires a learning value in accordance with the ON operation of the switch84each time the controller75executes the FF control action. Namely, an Nth learning value acquired through the Nth FF control action is fluctuation information acquired when the motor45is driven by an FF control value calculated from an (N−1)th learning value. Therefore, fluctuation information is updated on the basis of Nth acquired fluctuation information and (N−1)th updated fluctuation information.

At Step S21inFIG. 11, the timing at which an Nth learning value was acquired (a phase of each rotation fluctuation component) is different from the timing at which an (N−1)th learning value was acquired, so the FF-control-value calculating unit85performs a phase correction.

The phase correction here is a process of correcting the phase component and quadrature component of each rotation fluctuation component calculated at Step S14to an appropriate value to be handled at Step S22.

This converts the (N−1)th fluctuation information into the same phase as the Nth learning timing on the basis of a difference value between the (N−1)th and Nth pulse count values.

Then, the FF-control-value calculating unit85updates the fluctuation information (Step S22). An update-value calculation formula is expressed by the following equation (2).
Nth fluctuation information (update value)=(N−1)th fluctuation information (previous update value)−Nth fluctuation information (Nth learning value)  (2)

By subtracting the Nth fluctuation information from the (N−1)th fluctuation information, a reversed-phase component that the Nth detected rotation fluctuation component is inverted is added to the (N−1)th fluctuation information as an FF-control-value calculating parameter.

Then, the FF-control-value calculating unit85corrects attenuation (smoothing) of the amplitude and delay in phase of each rotation fluctuation component due to the moving average process at Step S12(Step S23). Then, the FF-control-value calculating unit85converts the Nth fluctuation information (update value) on the basis of an attenuation rate and an amount of phase delay of each rotation fluctuation component.

Then, the FF-control-value calculating unit85calculates an FF control value from fluctuation information derived by the update of the Nth fluctuation information. The FF-control-value calculating unit85calculates an FF control value by calculating a current phase from a current pulse count value based on the pulse count value C1of the phase-management pulse counter at the start of the ENC speed data sampling (Step S24).

In this manner, by adopting the constant learning type, the controller75can use characteristics of FF control and FB control. Therefore, if the process shown inFIGS. 10 and 11, which is performed with the constant learning type control period, is performed with a shorter period, the controller75generates mutual interference between the constant learning type and the existing FB control.

Consequently, the constant learning type control period of the FF control has to be a sufficiently long period with respect to the control period of the FB control, and is preferably more than 100 times longer than the control period of FB control. In the controller75according to the present embodiment, the FB control is performed with the control period of 1 msec, whereas the constant learning type control period of the FF control, i.e., an update period of fluctuation information is a period of 0.5 to 3 msec.

FIG. 12is a diagram showing results of Verification of the suppressing effects on rotation fluctuation due to the FF control and FB control according to the present embodiment obtained by analyzing a rate of rotation speed fluctuation of the photosensitive drum6using a fast Fourier transform (FFT) method.

InFIG. 12, the photosensitive drum6is driven at 1.8 Hz, and fluctuation components shown are all primary components.

FIG. 12Ais a diagram showing a rotation speed fluctuation rate due to each rotation fluctuation factor when the motor45is controlled to be driven at constant speed by output (a motor FG signal) from a rotation detector mounted on the motor output shaft54.

As can be seen fromFIG. 12A, in the photoreceptor driving device30, the output shaft50(“output shaft primary” inFIG. 12) and the first planetary gears58(“first planetary primary” inFIG. 12) have fluctuation in rotation speed with their respective rotation periods. Furthermore, in the photoreceptor driving device30, the second planetary gears62(“second planetary primary” inFIG. 12) and the first sun gear55(“first sun primary” inFIG. 12) have fluctuation in rotation speed with their respective rotation periods.

FIG. 12Bis a diagram showing a rotation speed fluctuation rate due to each rotation fluctuation factor when driving of the motor45is FB-controlled by the FB control system using output from the rotary encoder74installed on the output shaft50.

As can be seen fromFIG. 12B, in the photoreceptor driving device30, rotation speed fluctuation generated with rotation periods of the output shaft50and the second planetary gears62, which belong to a low-frequency band of 10 Hz or less, is suppressed by performing the FB control.

FIG. 12Cis a diagram showing a rotation speed fluctuation rate due to each rotation fluctuation factor when control of the FF control as well as the FB control, which is a control form of the photoreceptor driving device30according to the present embodiment, is executed.

As can be seen fromFIG. 12C, in the photoreceptor driving device30, rotation speed fluctuation generated with rotation periods of the first planetary gears58and the first sun gear55, which belong to a high-frequency band of 10 Hz or more, is suppressed by performing the FF control.

As described above, the photoreceptor driving device30according to the present embodiment includes the photosensitive drum6, the motor45that generates a driving force for driving the photosensitive drum6to rotate, the output shaft50connected to the photosensitive drum6, and the planetary reduction gear46that includes the first sun gear55, the first planetary gears58, the second sun gear61, and the second planetary gears62, which each rotate with a non-integral ratio of rotation period to a rotation period of the output shaft50, and reduces the rotation speed of the motor45and transmits the driving force to the photosensitive drum6via the output shaft50.

The photoreceptor driving device30according to the present embodiment further includes the rotary encoder74, which generates a pulse signal according to the number of revolutions of the output shaft50, and the controller75, which accumulates and stores the number of pulse signals generated by the rotary encoder74, and stores rotation speed fluctuation of the output shaft50generated with each of the rotations periods of the first sun gear55, the first planetary gears58, the second sun gear61, and the second planetary gears62as fluctuation information corresponding to the number of pulse signals, and controls the rotation speed of the motor45.

The controller75is configured to detect fluctuation information corresponding to the number of accumulated pulse signals and execute FF control of driving the motor45so as to offset the rotation speed fluctuation of the output shaft50by using the fluctuation information.

Therefore, the photoreceptor driving device30can suppress the periodic rotation fluctuation of the output shaft50caused by the rotation of the gears of the planetary reduction gear46by the FF control without using a home position sensor in each gear.

Furthermore, the photoreceptor driving device30according to the present embodiment is configured so that the number of teeth of the first and second sun gears55and61are the non-integral multiple of the number of (three) teeth of the first and second planetary gears58and62, respectively.

Consequently, the photoreceptor driving device30can stagger the engagement timing between the three first planetary gears58engaged with the first sun gear55and the three second planetary gears62engaged with the second sun gear61. Therefore, engagement vibration generated due to a difference in tooth pitch between engagement parts causes a phase difference between the planetary gears, so that the vibration of the photoreceptor driving device30can be reduced.

Moreover, in the photoreceptor driving device30according to the present embodiment, the number of teeth of the first planetary gears58and the number of teeth of the second planetary gears62are set to odd numbers, respectively.

Consequently, the first planetary gears58can generate a phase difference between engagement vibration generated due to the tooth pitch of an engagement part engaged with the first sun gear55and engagement vibration generated due to the tooth pitch of an engagement part engaged with the outer gear57, and therefore can reduce the vibration.

Therefore, the rotation accuracy of the first planetary gears58can be improved. Furthermore, the second planetary gears62can also achieve the same effect, and the rotation accuracy of the second planetary gears62can be improved.

Furthermore, in the photoreceptor driving device30according to the present embodiment, the controller75detects rotation speed fluctuation of the output shaft50generated with each of the rotations periods of the first sun gear55, the first planetary gears58, the second sun gear61, and the second planetary gears62on the basis of detected pulse signals from the rotary encoder74during the execution of the FF control, and stores the rotation speed fluctuation in the fluctuation-component detecting unit83, the switch84, and the FF-control-value calculating unit85as fluctuation information.

Then, the controller75updates the fluctuation information stored in the fluctuation-component detecting unit83, the switch84, and the FF-control-value calculating unit85on the basis of the fluctuation information stored in the fluctuation-component detecting unit83, the switch84, and the FF-control-value calculating unit85during the execution of the previous FF control and the fluctuation information stored in the fluctuation-component detecting unit83, the switch84, and the FF-control-value calculating unit85during the execution of the current FF control, and performs the FF control of driving the motor45so as to offset the rotation speed fluctuation by using the fluctuation information stored in the fluctuation-component detecting unit83, the switch84, and the FF-control-value calculating unit85.

Consequently, the controller75can repeatedly improve the FF control accuracy, and respond to temporal changes of components of the planetary reduction gear46, and correct a phase management error caused by miscounting of the pulse counter.

Moreover, in the photoreceptor driving device30according to the present embodiment, the controller75is configured to perform the FB control for controlling the rotation speed of the motor45with the control period of 1 msec on the basis of a pulse signal transmitted from the rotary encoder74.

Consequently, the controller75can suppress rotation fluctuation caused by a non-periodic change in load on the drum shaft48by means of various parts having abutting contact with the photosensitive drum6.

Furthermore, in the photoreceptor driving device30according to the present embodiment, the controller75again detects rotation speed fluctuation of the output shaft50after the update of the fluctuation information stored in the fluctuation-component detecting unit83, the switch84, and the FF-control-value calculating unit85.

Consequently, the controller75can update the fluctuation information of the FF-control-value calculating unit85while the fluctuation-component detecting unit83is estimating a disturbance, and therefore can prevent an estimate disturbance estimated by the fluctuation-component detecting unit83from being changed, thereby resulting in significant reduction of the FF control accuracy.

Moreover, in the photoreceptor driving device30according to the present embodiment, the controller75updates the fluctuation information stored in the fluctuation-component detecting unit83, the switch84, and the FF-control-value calculating unit85with a period more than 100 times longer than the control period of 1 msec.

Consequently, the controller75can prevent mutual interference between the constant learning type having characteristics of FB control and the existing FB control.

Incidentally, in the present embodiment, the photoreceptor driving device30is applied to a drive shaft of the photosensitive drum6; however, the present invention is not limited to this, and the photoreceptor driving device30can be used as a roller driving device of the drive roller16and a rotating-body driving device of each drive roller in a secondary transfer drive unit and a fixing drive unit, etc.

Furthermore, in the present embodiment, the number of the first planetary gears58and the number of the second planetary gears62are both three; however, the number of the planetary gears is not limited to three, and can be any number as long as there are two or more planetary gears.

In a planetary gear mechanism, both have the same outer diameter and reduction gear ratio, one having more planetary gears than the other is lower in load torque acting on a gear engagement part. Therefore, to further improve the durability while curbing an increase in cost, the number of planetary gears can be, for example, two on the input side and four on the output side.

Furthermore, in the present embodiment, the photosensitive drum6and the planetary reduction gear46are separate components; however, the present invention is not limited to this configuration, and part or all of the planetary reduction gear46can be housed in the photosensitive drum6.

Moreover, in the present embodiment, the first sun gear55, the first carrier pin60, the second carrier63, and the second carrier pin64are made of metal, and the other components of the planetary reduction gear46are made of resin; however, the present invention is not limited to this.

For example, the outer gear57can be made of resin, and the first planetary gears58, the second planetary gears62, the first carrier pin60, and the second sun gear61integrally formed with the first carrier59can be made of metal as needed.

Even in this case, the planetary reduction gear46can be made lighter than that of which the major components are all made of metal, and can further withstand high load of the photosensitive drum6than that of which the major components are all made of resin.

According to the present invention, it is possible to provide a rotating-body driving device capable of performing feedforward control enabling, even when a reduction gear having gears that each rotate with a non-integral ratio of rotation period to a rotation period of a shaft of a photosensitive drum is used in a drive transmission system of the photosensitive drum, to suppress periodic fluctuation generated with the respective rotation period of the gears.