Thickness variation detector of photoconductor, image formation unit, image formation apparatus and method for thickness variation of photoconductor

A thickness variation detector of a photoconductor includes: a current detection unit that detects a value of current being used for charging a surface of the photoconductor in a state in which a charging unit is in contact with a surface of the photoconductor; and a thickness variation detection unit that detects a thickness variation along a rotation direction of the photoconductor based on the value of current.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based on and claims priority under 35 USC §119 from Japanese Patent Application No. 2006-318287 filed Nov. 27, 2006.

BACKGROUND

(i) Technical Field

This invention relates to a thickness variation detector of a photoconductor, an image formation unit using the thickness variation detector, an image formation apparatus, a method for detecting a thickness variation of a photoconductor.

(ii) Related Art

Hitherto, an image formation apparatus of a printer, a copier, a facsimile, etc., adopting electrophotography has been configured as follows: After the surface of a photoconductor drum is charged to a potential by a charging roll, image light exposure is applied to the surface of the photoconductor drum to form an electrostatic latent image responsive to image information, the electrostatic latent image formed on the surface of the photoconductor drum is visualized by a developing unit to form a toner image, and the toner image formed on the photoconductor drum is transferred directly onto a record paper and then is fixed, thereby forming an image or is transferred onto the record paper through an intermediate transfer body and then is fixed, thereby forming an image.

In such an image formation apparatus, when a photosensitive layer is formed on the surface of the photoconductor drum shaped like a drum or a belt, it is practically difficult to uniformly form a layer over the full face of the photoconductor and when an image is formed, it is difficult to provide a uniform toner image along the circumferential direction of the photoconductor.

SUMMARY

According to an aspect of the invention, there is provided a thickness variation detector of a photoconductor including:

a current detection unit that detects a value of current being used for charging a surface of the photoconductor in a state in which a charging unit is in contact with a surface of the photoconductor; and

a thickness variation detection unit that detects a thickness variation along a rotation direction of the photoconductor based on the value of current.

Wherein13Y,13M,13C,13K each represents an image formation unit,14represents a ROS,15represents a photoconductor drum,16represents a charging roll,17represents a developing unit,60represents a current detection circuit,61represents a thickness variation detection section,67represents a gradation correction section, and110represents a density correction section.

DETAILED DESCRIPTION

Referring now to the accompanying drawings, there are shown exemplary embodiments of the invention.

First Exemplary Embodiment

FIG. 2shows a tandem full-color printer as an image formation apparatus incorporating a thickness variation detection unit of a photoconductor and an image formation unit according to a first exemplary embodiment of the invention. The tandem color printer includes an image reader and also functions as a full-color copier. The full-color printer may include no image reader, of course.

InFIG. 2, numeral1denotes the main body of the tandem full-color printer. An image reader (IIT: Image Input Terminal)3for reading an image of an original document2is disposed in the upper part of one end side (in the figure, the left) of the full-color printer main body1. The image reader3uses a light source6to illuminate an original document2placed on platen glass5in a pressed state by a platen cover4by an automatic original transporter (ADF: Automatic Document Feeder) not shown for automatically transporting an original document or the like, scans a reflected light image from the original document2over an image read element11of CCD, etc., through a reduction optical system made up of a full rate mirror7, half rate mirrors8and9, and an image formation lens10, and reads the image of the original document2by the image read element11at a dot density (for example, 400 dpi or 600 dpi).

The image of the original document2read by the image reader3is sent to an image processing apparatus12(IPS: Image Processing System) for performing image processing for image data of three colors of red (R), green (G), and blue (B), for example, and the image processing apparatus12performs image processing of shading correction, position shift correction, lightness/color space conversion, gamma correction, frame removal, color/move edit etc., for the image data of the original document2.

The image data subjected to the image processing by the image processing apparatus12as described above is converted into image data of four colors of yellow (Y), magenta (M), cyan (C), and black (K) (each eight bits) by the image processing apparatus12and the image data is sent to an exposing apparatus ROS (Raster Output Scanners)14, including14Y,14M,14C, and14K, which are image exposing units of color image formation units13Y,13M,13C, and13K of yellow (Y), magenta (M), cyan (C), and black (K). The ROSs14Y,14M,14C, and14K execute image exposure using a laser beam LB in response to the image data of each color as described below.

By the way, the four image formation units13Y,13M,13C, and13K of yellow (Y), magenta (M), cyan (C), and black (K) are placed in series with a given spacing in the horizontal direction in the tandem full-color printer main body1.

The four image formation units13Y,13M,13C, and13K are configured in a similar manner except the color of the image to be formed. Each is roughly made up of a photoconductor drum15as a photoconductor driven at a rotation speed along the arrow A direction, a charging roll16as a contact charging unit for uniformly charging the surface of the photoconductor drum15, the above-mentioned ROS14for exposing an image corresponding to each color to form a latent image on the surface of the photoconductor drum15, a developing unit17for developing the latent image formed on the photoconductor drum15in toner of the corresponding color, and a cleaning unit18. The image formation units13Y,13M,13C, and13K can be separately attached to and detached from the full-color printer main body1except the ROS14.

The photoconductor drum15has a photoconductor layer52of an OPC (organic photoconductor), etc., formed in a thickness d on a surface of a conductive cylinder51made of metal, for example, as shown inFIG. 3and is driven at a rotation speed along the arrow A direction (seeFIG. 2) by a drive source (not shown). The photoconductor drum15has the photoconductor layer52of an OPC (organic photoconductor), etc., formed in the thickness d on the surface of the conductive cylinder51made of metal as described above; however, it is difficult to uniformly form the photoconductor layer52over the full surface of the conductive cylinder51and the photoconductor layer52is non-uniformly worn by the cleaning unit18, etc., with time and thus partial thickness d variation (thickness unevenness) of the photoconductor layer52inevitably occurs along the rotation direction of the photoconductor drum15(subscanning direction). The photoconductor layer52may be formed of one layer or may be made up of a plurality of functionally separated photoconductor layers. The photoconductor may be shaped not only like a drum, but also like a belt, of course.

The ROS14modulates a semiconductor laser101in response to image data by a laser driver109and emits a laser beam LB from the semiconductor laser101in response to image data output from the image processing apparatus (IPS)12as shown inFIG. 4. The laser beam LB emitted from the semiconductor laser101is reflected by a reflecting face104aof a rotating polygon mirror104rotating in the arrow C direction through a collimator lens102and a cylindrical lens103and is deflected and scanned along the arrow D direction and is scanned and exposed as a long line image along the main scanning direction (arrow D direction) on the photoconductor drum15as a photoconductor through a reflecting mirror106in a state in which the focal length is adjusted in response to the scan direction through an f-θ lens105. A reflecting mirror107is disposed at any position other than the image formation area at the start end of the laser beam LB in the scanning direction thereof, and the laser beam LB reflected on the reflecting mirror107is made incident on an SOS (Start of Scan) sensor108. Whenever the laser beam LB scans over the surface of the photoconductor drum15, the first laser beam LB of each scan line is made incident on the SOS sensor108. The SOS sensor108detects the application timing for each scan line over the surface of the photoconductor drum15and generates a signal indicating the application start timing (SOS signal).

The laser driver109for outputting a laser drive signal responsive to the image data output from the image processing apparatus (IPS)12at a timing is connected to the semiconductor laser101. The laser driver109demodulates the semiconductor laser101to perform ON/OFF control in response to the image data from the image processing apparatus12. Accordingly, the laser beam LB corresponding to the image data is output from the semiconductor laser101. The laser driver109is also connected to the SOS sensor108and the SOS signal generated by the SOS sensor108is input to the laser driver109. The laser driver109sets the start timing of output of a laser drive signal for the semiconductor laser101in response to the SOS signal from the SOS sensor108.

Further, a density correction section110is connected to the laser driver109. The density correction section110generates a light amount setting signal to suppress density unevenness in the subscanning direction caused by partial thickness variation of the photoconductor drum15and outputs the light amount setting signal to the laser driver109. The laser driver109adjusts the light amount of the laser beam LB output from the semiconductor laser101in response to the light amount setting signal from the density correction section110. The light amount of the laser beam LB is adjusted by the time the surface of the photoconductor drum15is actually scanned and exposed after a thickness variation detection section61detects thickness variation of the photoconductor layer52of the photoconductor drum15as described later. The density correction section110may be disposed in the image processing apparatus (IPS)12.

Thus, color image data is output in sequence from the image processing apparatus (IPS)12to the ROSs14Y,14M,14C, and14K of the image formation sections13Y,13M,13C, and13K of colors of yellow (Y), magenta (M), cyan (C), and black (K), and the laser beams LB emitted in response to image data from the ROSs14Y,14M,14C, and14K are scanned over the surfaces of photoconductor drums15Y,15M,15C, and15K to form electrostatic latent images thereon. The electrostatic latent images formed on the photoconductor drums15Y,15M,15C, and15K are developed by developing units17Y,17M,17C, and17K as color toner images of yellow (Y), magenta (M), cyan (C), and black (K).

The color toner images of yellow (Y), magenta (M), cyan (C), and black (K) formed in sequence on the photoconductor drums15Y,15M,15C, and15K of the image formation sections13Y,13M,13C, and13K are transferred onto an intermediate transfer belt25as an interface transfer body placed below the image formation sections13Y,13M,13C, and13K in a state in which the toner images are superposed on each other by primary transfer rolls26Y,26M,26C, and26K, as shown inFIG. 2. The intermediate transfer belt25is placed under a given tension on a drive roll27, a tension roll28, a steering roll29, an idler roll30, a backup roll32against which a feeding roll31is abutted, and an idler roll33, and is circulated at a speed roughly equal to that of the photoconductor drum15Y,15M,15C,15K along the arrow B direction by the drive roll27rotated by a dedicated drive motor excellent in constant speed property (not shown).

The intermediate transfer belt25is formed of a film-like endless belt with a proper amount of an antistatic agent of carbon black, etc., contained in a resin of polyimide, polyamide, or the like. It is formed so that volume resistivity becomes 106to 1014Ωcm, for example, and the thickness is set to about 0.1 mm, for example.

A primary transfer section is implemented as the primary transfer roll26placed facing the photoconductor drum15with the intermediate transfer belt25between. The primary transfer roll26has a shaft and a sponge layer as an elastic layer fixedly secured to the surrounding of the shaft. The shaft is a cylindrical rod made of metal of iron, stainless steel, etc. The sponge layer is formed of blend rubber of NBR, SBR, and EPDM mixed with a conductive agent of carbon black, etc., and is a sponge-like cylindrical roll whose volume resistivity is 107.5to 108.5Ωcm, for example.

The color toner images of yellow (Y), magenta (M), cyan (C), and black (K) multiplely transferred onto the intermediate transfer belt25are secondarily transferred onto record paper35as a record medium by a pressing force and an electrostatic force by a secondary transfer roll34for coming in press contact with the backup roll32through the intermediate transfer belt25, and the record paper35onto which the color toner images are transferred is transported to a fuser38with a transport belt36and a transport guide37. The record paper35onto which the color toner images are transferred is subjected to fixing treatment by heat and pressure by a heating roll39and a pressurization roll40of the fuser38and is ejected by an ejection roll41onto an ejection tray42provided outside the printer main body1.

A secondary transfer section is made up of the secondary transfer roll34placed on the toner image support face side of the intermediate transfer belt25and the backup roll32. The backup roll32is made up of a tube of blend rubber of EPDM and NBR with carbon dispersed on a surface and EPDM rubber inside the backup roll. It is formed so that surface resistivity becomes 107to 1010Ω/□, for example, and the hardness is set to 70° (ASKER C stiffness), for example.

The secondary transfer roll34is made up of a shaft and a sponge layer as an elastic layer fixedly secured to the surrounding of the shaft. The shaft is a cylindrical rod made of metal of iron, stainless steel, etc. The sponge layer is formed of blend rubber of NBR, SBR, and EPDM mixed with a conductive agent of carbon black, etc., and is a sponge-like cylindrical roll whose volume resistivity is 107.5to 108.5Ωcm, for example.

The record paper35of a size and material is once transported from a paper tray43disposed on the bottom of the printer main body1via a paper transport passage48made up of a paper feed roller44and roller pairs45,46, and47for transporting paper to a registration roll49and then is stopped as shown inFIG. 2. The record paper35supplied from the paper tray43is sent to the secondary transfer position of the intermediate transfer belt25by the registration roll49driven at a timing.

The transfer toner remaining on the intermediate transfer belt25is removed by a cleaning unit50for the intermediate transfer belt, placed at a position opposed to the idler roll33.

InFIG. 2, numeral51denotes a density sensor for detecting a toner patch for process control and registration control, formed on the intermediate transfer belt25.

By the way, in the embodiment, the image formation apparatus includes a photoconductor being rotated; a contact charging unit for charging the surface of the photoconductor in a state in which the charging unit is in contact with the surface of the photoconductor; a bias voltage application unit for applying a DC bias voltage on which an AC voltage is superposed to the charging unit; a current detection unit for detecting DC current flowing into the charging unit; a thickness variation detection unit for detecting a thickness variation along the rotation direction of the photoconductor in response to the DC current value detected by the current detection unit; and a controller for controlling an image formation condition in response to the detection result of the thickness variation detection unit.

That is, in the embodiment, the surface of the photoconductor drum15is charged to a potential by the contact charging roll16for charging the surface of the photoconductor drum15in a state in which the charging roll16comes in contact with the surface of the photoconductor drum15as shown inFIG. 1. The charging roll16is made up of a cylindrical core16amade of metal of iron, stainless steel, etc., and a conductive elastic layer16bput on the outer periphery of the cylindrical core16a. As a bias power supply70for charging, an AC power supply71for applying a high voltage of AC and a DC power supply72for applying a given DC high voltage are connected in series to the cylindrical core16aof the charging roll16as shown inFIG. 5. The DC power supply72applies a negative DC voltage to the cylindrical core16aof the charging roll16.

The contact charging roll charges the surface of the photoconductor drum15to the potential as DC voltage on which AC voltage is superposed is applied to the cylindrical core16aof the charging roll16from the AC power supply and the DC power supply72and minute gap discharge, etc., is produced between the conductive elastic layer16bof the charging roll16and the surface of the photoconductor drum15.

DC voltage on which AC voltage is superposed is applied to a developing roll17aof the developing unit17from an AC power supply74for applying high voltage of AC and a DC power supply75for applying high voltage of DC as a developing bias power supply73. Further, a transfer bias of positive polarity is applied to the primary transfer roll26from a DC power supply77for applying high voltage of DC as a transfer bias power supply76.

As shown inFIG. 6, the photoconductor drum15has the photoconductor layer52made up of a charge generation layer, a charge transport layer, etc., of an OPC (organic photoconductor), etc., formed in the thickness d on the surface of the conductive cylinder51; it functions as a capacitor having the conductive cylinder51as one electrode and the photoconductor layer52as a dielectric layer from the electric viewpoint. The surface of the photoconductor drum15is charged by the charging roll16, whereby negative-polarity charges are held on the surface of the photoconductor drum15as shown inFIG. 6.

If the photoconductor drum15is viewed as a capacity as described above, as shown inFIG. 6(b), the thickness d of the photoconductor layer52made of a dielectric body varies partially due to application variations, age biased wear, etc., and as the thickness d of the photoconductor layer52becomes thin, electrostatic capacity C of the photoconductor layer52increases. Letting the dielectric constant of the photoconductor layer52be ∈ and the thickness of the photoconductor layer52be d, the electrostatic capacity C per unit area of the photoconductor layer52can be represented as C=∈/d. The thickness d of the photoconductor layer52decreases due to application variations and biased wear and in the change portion from the thickness d to thickness (d−Δd), the electrostatic capacity C of the photoconductor layer52increases from C1=∈/d to C2=∈/(d−Δd) (C1<C2). Then, when the surface of the photoconductor drum15is charged by the charging roll16, the surface of the photoconductor drum15is negatively charged to a uniform potential VH regardless of the thickness unevenness of the photoconductor layer52. However, when the thickness of the photoconductor layer52becomes thin, the electrostatic capacity C of the photoconductor layer52increases as described above. Thus, many negative charges are supplied to the portion where the thickness of the photoconductor layer52is thin, and the current flowing into the charging roll16increases in the portion.

Thus, when image exposure is applied to the surface of the photoconductor drum15to form an electrostatic latent image, the negative charge amount increases in the portion where the thickness of the photoconductor layer52is thin and therefore to lower the potential of the portion where the thickness is thin, more positive charges need to be produced by image exposure. However, since the image exposure amount is constant on the photoconductor drum15, exposure part potential VL of the portion where the thickness of the photoconductor layer52is thin becomes high as compared with other portions, namely, potential unevenness occurs.

Thus, if there are variations in the exposure part potential VL of the photoconductor drum15caused by biased wear of the thickness of the photoconductor layer52of the photoconductor drum15or the like, as shown inFIG. 7, when image exposure is applied to the surface of the photoconductor drum15using the laser beam LB to form an electrostatic latent image53and the electrostatic latent image53is reversally developed by the developing roll17aof the developing unit17, if the exposure part potential VL of the photoconductor drum15varies, the developing electric field fluctuates and the density of the developed toner image varies. That is, the density of the developed toner image, when the exposure part potential VL of the photoconductor drum15is high, becomes thin as compared with the case where the exposure part potential VL of the photoconductor drum15is low.

Consequently, if the thickness d of the photoconductor layer52of the photoconductor drum15varies, as shown inFIG. 8, the exposure part potential VL of the photoconductor drum15changes in response to the variation of the thickness d of the photoconductor layer52and when an image at a uniform density is formed on the surface of the photoconductor drum15, etc., partial inconsistencies in density (banding) appear along the rotation direction of the photoconductor drum15.

Then, in the embodiment, when the surface of the photoconductor drum15is charged by the charging roll16, a current detection circuit60detects DC current IDCflowing into the charging roll16and the thickness variation detection section61as a thickness variation detection unit detects the biased wear amount of the partial variation of the thickness d of the photoconductor layer52of the photoconductor drum15in response to the DC current IDCdetected by the current detection circuit60as shown inFIG. 1. The thickness variation detection section61is implemented as a CPU, etc., as a controller of the printer, but is not limited to it and may be implemented as an independent circuit, of course.

The DC current IDCflowing into the charging roll16is a value resulting from dividing charge amount Q given to the photoconductor layer52by charging time T when the photoconductor layer52of the photoconductor drum15functioning as a capacity is charged to a potential, and is given according to the following expression:
IDC=Q/T

Since surface potential V of the photoconductor drum15is given as V=Q/C, the electrostatic capacity C per unit area of the photoconductor layer52of the photoconductor drum15becomes C=Q/V and is represented as C=∈/d using the dielectric constant ∈ and the thickness d of the photoconductor layer52and therefore the thickness d of the photoconductor layer52is found as d=∈V/(IDC·T) relative to the DC current IDCflowing into the charging roll16as shown inFIG. 9.

Therefore, when the surface of the photoconductor drum15is charged by the charging roll16, the current detection circuit60detects the DC current IDCflowing into the charging roll16, whereby the thickness variation detection section61detects the partial thickness d of the photoconductor layer52of the photoconductor drum15in response to the DC current IDC.

In the embodiment, a number-of-photoconductor-use-cycles count section62cumulatively counts the number of use cycles (the number of revolutions) of each photoconductor drum15in the image formation sections13Y,13M,13C, and13K of colors of yellow (Y), magenta (M), cyan (C), and black (K) as shown inFIG. 1. Further, the fact that the thickness d of the photoconductor layer52of the photoconductor drum15decreases gradually in response to the number of use cycles of the photoconductor drum15is previously found by experiment as shown inFIG. 10.

Then, the number-of-photoconductor-use-cycles count section62previously retains data of the thickness d of the photoconductor layer52as shown inFIG. 10in response to the number of use cycles (the number of revolutions) of the photoconductor drum15cumulatively counted. The photoconductor layer52shown inFIG. 10has the initial thickness d set to 24 μm. The data of the thickness d of the photoconductor layer52is an average value in one round of the photoconductor drum15used with the printer. The number-of-photoconductor-use-cycles count section62may count the number of revolutions of the photoconductor drum15as the number of use cycles of the photoconductor drum15. However, since the effect on wear of the photoconductor layer varies depending on whether or not a toner image is formed on the photoconductor drum15, the number-of-photoconductor-use-cycles count section62may count the number of image formation cycles, namely, the number of revolutions in the image formation operation rather than the number of revolutions of the photoconductor drum15or may count the number of print sheets.

Further, the thickness variation detection section61includes an average value calculation section63for averaging the DC current IDCflowing into the charging roll16, detected by the current detection circuit60and a thickness unevenness calculation section64for calculating partial thickness d of the photoconductor layer52in response to the DC current values IDCfrom the number-of-photoconductor-use-cycles count section62, the average value calculation section63, and the current detection circuit60, as shown inFIG. 1. The DC current IDCflowing into the charging roll16varies depending on the thickness d of the photoconductor layer52as shown inFIG. 10; the thickness d of the photoconductor layer52differs partially in response to the position along the rotation direction of the photoconductor drum15depending on the DC current IDCon an enlarged scale shown inFIG. 12.

In the embodiment, the average value calculation section63for averaging the DC current IDCflowing into the charging roll16, detected by the current detection circuit60as described above as shown inFIG. 1is included. The average value calculation section63calculates the average value (in one round of the photoconductor drum15) of the DC current IDCpartially varying in response to the position along the rotation direction of the photoconductor drum15as shown inFIG. 12, and the thickness unevenness calculation section64references a graph as shown inFIG. 10in response to the average value of the DC current IDCand the number of use cycles of the photoconductor drum15counted by the number-of-photoconductor-use-cycles count section62described above and finds the thickness d of the photoconductor layer52. Here, the found thickness d of the photoconductor layer52is the thickness d averaged per revolution of the photoconductor drum15.

Further, in addition to the average value of the thickness d of the photoconductor layer52found as described above, the thickness unevenness calculation section64references the relationship between the DC current value IDCand the thickness d of the photoconductor layer52as shown inFIG. 9based on the DC current value IDCinput from the current detection circuit60, and calculates the partial thickness d of the photoconductor layer52in one round of the photoconductor drum15, namely, thickness unevenness d of the photoconductor layer52.

That is, although the average thickness d of the photoconductor layer52responsive to the number of use cycles of the photoconductor drum15is found in response to the average value of the DC current IDCcalculated by the average value calculation section63, the thickness d of the photoconductor layer52is calculated in response to each DC current value IDCand the thickness d partially varies from one position to another along the rotation direction of the photoconductor drum15.

Then, the thickness unevenness calculation section64calculates the partial thickness d of the photoconductor layer52in one round of the photoconductor drum15, namely, thickness unevenness of the photoconductor layer52as d1, d2, . . . in addition to the average value of the thickness d of the photoconductor layer52. In the thickness unevenness calculation section64, to remove the effect of noise on the DC current value IDCinput from the current detection circuit60, a filter of a low-pass filter, a band-pass filter, etc., for a current detection signal may be added in the current detection circuit60and a gradation curve computation section66described later.

In the embodiment, a density correction section110is also included as shown inFIG. 1. This density correction section110is made up of a gradation curve computation section66and a gradation correction section67. The gradation curve computation section66computes a gradation curve to correct the effect of biased wear of partial thickness unevenness of the photoconductor layer52of the photoconductor drum15in response to unevenness of the partial thickness d of the photoconductor layer52calculated by the thickness unevenness calculation section64.

Further, in the embodiment, as shown inFIG. 1, if the density correction section110determines that the thickness d of the photoconductor layer52calculated by the thickness unevenness calculation section64falls below the lower limit value even partially, the density correction section110prohibits print operation and displays the fact that the photoconductor drum15reaches its life on a user interface (not shown). At this time, since it is feared that the thickness d of the photoconductor layer52may be determined to fall below the lower limit value because of erroneous detection in the thickness unevenness calculation section64, print operation may be prohibited and the fact that the photoconductor drum15reaches its life may be displayed on a user interface (not shown) only if it has been determined successive times that the thickness d of the photoconductor layer52falls below the lower limit value.

The gradation curve computation section66previously stores data as to how the image density output relative to the input image data changes in response to variation of the thickness d of the photoconductor layer52if the thickness d of the photoconductor layer52of the photoconductor drum15varies as shown inFIG. 14. In the figure, C1indicates the case where the photoconductor drum15is in the initial state; the thickness d of the photoconductor layer52is 24 μm, for example. C2indicates a state in which the thickness d of the photoconductor layer52decreases 5 μm, C3indicates a state in which the thickness d of the photoconductor layer52decreases 10 μm, and C4indicates a state in which the thickness d of the photoconductor layer52decreases 15 μm.

Then, in the thickness unevenness calculation section64, when the fact that the thickness d of the photoconductor layer52averaged in response to the number of use cycles of the photoconductor drum15cumulatively counted by the number-of-photoconductor-use-cycles count section62as shown inFIG. 10decreases 5 μm, for example, is input, it is recognized that the gradation curve changes as indicated by C2inFIG. 14.

Likewise, in the thickness unevenness calculation section64, if the DC current detection value IDCpartially varies along the rotation direction of the photoconductor drum15in response to the DC current detection value IDCdetected by the current detection circuit60as shown inFIGS. 11 and 12, it is recognized that the thickness d of the photoconductor layer52varies along the rotation direction of the photoconductor drum15.

For easy understanding, assuming that the photoconductor drum15is in an unused state, the gradation curve of the photoconductor drum15matches the curve of C1previously found by experiment as shown inFIG. 15. However, it is assumed that the thickness d of the photoconductor layer52of the photoconductor drum15varies partially along the rotation direction and that the thickness unevenness calculation section64calculates that the variation of the thickness d of the photoconductor layer52is 5 μm in response to the DC current detection value IDCdetected by the current detection circuit60.

Then, in the gradation curve computation section66, since the thickness d of the photoconductor layer52decreases 5 μm on the surface of the photoconductor drum15in the part as shown inFIG. 16, to form an image at a density of Cin=40%, for example, the image density increases by D in the portion where the thickness d of the photoconductor layer52decreases 5 μm.

Then, to suppress the image density variation of D in the portion where the thickness d of the photoconductor layer52decreases 5 μm, the gradation curve computation section66computes the gradation curve so as to correct the gradation curve by correcting the image data output to the ROS14in response to the input image data so that the density image in Cin=40% becomes equal to or becomes roughly equal to the image density of the photoconductor in the unused state.

Specifically, the gradation curve computation section66computes the gradation curve by finding output image data by referencing graph or table in response to curve D2, D3, D4as shown inFIG. 17as input image data so that the image density like the case where the photoconductor drum15is in an unused state is obtained even in the portion where the thickness d of the photoconductor layer52decreases 5 μm, for example. The curve D2corresponds to the curve C2inFIG. 14, the curve D3corresponds to the curve C3, and the curve D4corresponds to the curve C4.

For intermediate values of the curves C1, C2, C3, C4shown inFIG. 14, the values of the curves C1, C2, C3, C4may be interpolated for use or the value corresponding to change in the thickness d of the photoconductor layer52every μm may be included aside fromFIG. 14.

The gradation correction section67as a controller makes a correction to the input image data in response to the gradation curve calculated by the gradation curve computation section66, outputs image data to the ROS14as an exposing device, and corrects the on time and the light amount of the semiconductor laser101.

The gradation correction section67corrects the on time and the light amount of the semiconductor laser101in succession in synchronization when the surface of the photoconductor drum15where the DC current IDCflowing into the charging roll16is detected by the current detection circuit60moves to the position of the ROS14.

In the gradation correction section67, when the image data output to the ROS14is corrected, if the density is corrected according to area coverage modulation, the on time of the semiconductor laser101rather than the light amount is controlled.

In the described configuration, the full-color printer according to the embodiment makes it possible to suppress inconsistencies in density of the image caused by thickness variation of the photoconductor, etc., without using a unit for directly detecting the toner amount of a toner image, the thickness or sensitivity unevenness of the photoconductor, or the physical amount of the surface potential of the photoconductor, etc., as follows:

In the full-color printer according to the embodiment, as shown inFIG. 2, to print a full-color image, the surfaces of the photoconductor drums15are charged to a potential by the charging rolls16in the image formation sections13Y,13M,13C, and13K of colors of yellow (Y), magenta (M), cyan (C), and black (K) and then image exposure corresponding to the color image data is executed on the surfaces of the photoconductor drums15by the ROSs14Y,14M,14C, and14K to form electrostatic latent images and the electrostatic latent images formed on the photoconductor drums15are visualized in corresponding color toners by the developing units17to form toner images. The toner images of colors of yellow (Y), magenta (M), cyan (C), and black (K) formed on the photoconductor drums15are multiplely transferred onto the intermediate transfer belt25and then are secondarily transferred from the intermediate transfer belt25in batch onto record paper35by the secondary transfer roll34and are fixed by the fuser38to form a full-color image.

At the time, when the surface of each photoconductor drum15is charged to the potential by the charging roll16, the current value IDCof the DC current flowing into the charging roll16is detected immediately by the current detection circuit60as shown inFIG. 1. The DC current value IDCdetected immediately by the current detection circuit60is input to the average value calculation section63, which then calculates average current amount Ave IDC.

The average current amount Ave IDCcalculated by the average value calculation section63is input to the thickness unevenness calculation section64. The DC current value IDCdetected by the current detection circuit60is also input to the thickness unevenness calculation section64. Further, the thickness data of the photoconductor layer52responsive to the number of use cycles of the photoconductor drum15cumulatively counted by the number-of-photoconductor-use-cycles count section62is also stored in the thickness unevenness calculation section64.

The thickness unevenness calculation section64calculates averaged thickness unevenness of the photoconductor layer52in response to the number of use cycles of the photoconductor drum15cumulatively counted by the number-of-photoconductor-use-cycles count section62.

The thickness unevenness calculation section64also calculates the difference between the DC current value IDCdetected by the current detection circuit60and the average current amount Ave IDCcalculated by the average value calculation section63, and calculates the partial thickness unevenness value of the photoconductor layer52in response to difference ΔIDCbetween the DC current value IDCdetected by the current detection circuit60and the average current amount Ave IDCcalculated by the average value calculation section63.

The partial thickness unevenness value of the photoconductor layer52is input to the gradation curve computation section66, which then computes the correction value to the gradation curve shown inFIG. 17in response to the relationship shown inFIG. 14, for example, as described above. As the correction to the gradation curve, if the correction value of the thickness data of the photoconductor layer52based on the difference ΔIDCbetween the DC current value and the average current amount is found as 5 μm, for example, relative to the gradation curve based on the thickness data of the photoconductor layer52retained in the number-of-photoconductor-use-cycles count section62as shown inFIG. 10, image data output by performing correction computation of the input image data is found in response to the curve D2of the gradation curve with the thickness 5 μm in the gradation curve as shown inFIG. 17.

The gradation correction section67makes a gradation correction to the image data in response to the correction gradation curve computed by the gradation curve computation section66and when the surface of the photoconductor drum15charged by the charging roll16moves to the position of the ROS14as an exposing device, image exposure is executed in a state in which the light amount is corrected on the photoconductor drum15by the ROS14as an exposing device.

Thus, if the thickness of the photoconductor layer52of the photoconductor drum15contains partial variation along the rotation direction, a gradation correction is made to the image data in response to the gradation curve and the exposure amount of the image is corrected, so that inconsistencies in density of the image caused by thickness variation of the photoconductor layer52are suppressed.

Second Embodiment

FIG. 18shows a second embodiment of the invention. Parts identical with those of the first embodiment described above are denoted by the same reference numerals in the second embodiment. In the second embodiment, a phase detection unit for detecting the rotation phase of a photoconductor is included and variation of DC current flowing into a charging roll is detected in response to the rotation phase of the photoconductor detected by the phase detection unit, thickness variation of a photoconductor layer in the rotation phase of the photoconductor is previously found, and a gradation correction is executed in response to gradation correction data based on the found thickness.

That is, in the second embodiment, an image exposing device14is operated in response to a light amount setting signal set by a density correction section110, thereby suppressing inconsistencies in density along the rotation direction of a photoconductor drum15as shown inFIG. 18. The density correction section110acquires data of partial thickness of a photoconductor layer52along the rotation direction of the photoconductor drum15at an appropriate timing from a thickness variation detection section61, and sets gradation correction data to generate a light amount setting signal in response to the result.

A phase mark M1is formed on the surface of the photoconductor drum15as shown inFIG. 18. The phase mark M1is formed outside an image formation area (area where electrostatic latent image and toner image can be formed) in the photoconductor drum15, for example, as shown inFIG. 2. A phase detection sensor71for detecting the phase mark M1is placed at a position opposed to the surface of the photoconductor drum15. The phase detection sensor71detects the phase mark M1each time the photoconductor drum15makes one revolution. Whenever the phase detection sensor71detects the phase mark M1, it outputs a phase signal PS1to the thickness variation detection section61. The phase detection sensor71also outputs the phase signal PS1to the density correction section110.

The phase mark M1is formed by filling a part of the surface of the photoconductor drum15, for example, as shown inFIG. 2, but the invention is not limited to the mode. Specifically, for example, the surface state of a part of the photoconductor drum15(for example, surface roughness) may be changed or a notch may be made in a part on an end side. The rotation period of the photoconductor drum15can also be acquired, for example, by providing a sensor for detecting drive torque of the photoconductor drum15or counting the number of pulse signals of the motor for driving the photoconductor drum15instead of the sensor for reading the mark.

FIG. 19is a block diagram to show the configuration of the thickness variation detection section61shown inFIG. 18.

The thickness variation detection section61includes a current value data storage section (memory)81, a synchronization processing section82, a data extraction section83, a period data storage section84, and an averaging processing section85.

The current value data storage section81stores data of DC current value IDCdetected by a current detection circuit60as current value data (digital data) arranged in the rotation direction of the photoconductor drum15. The synchronization processing section82synchronizes the current value data IDCread from the current value data storage section81and the phase signal PS1input from the phase detection sensor71with each other in synchronization with each other. That is, it determines which position of the current value data corresponds to the formation part of the mark M1in the photoconductor drum15as shown inFIG. 20.

The period data storage section84stores the period of one revolution of the photoconductor drum15(which will be hereinafter referred to as first period T1). The first period T1is predetermined in response to the outer diameter and the rotation speed of the photoconductor drum15. The data extraction section83reads the first period T1stored in the period data storage section84. The data extraction section83extracts current value data in a plurality of rounds (a plurality of first periods T1) of the photoconductor drum15for each first period T1from the current value data already subjected to the synchronization processing input from the synchronization processing section82.

The averaging processing section85averages a plurality of pieces of current value data input from the data extraction section83for each identical part on the photoconductor drum15. A current value-thickness conversion section86converts the current value data averaged by the averaging processing section85into thickness data as shown inFIG. 21. The current value data averaged by the averaging processing section85is data provided by averaging the current value data at each position on the photoconductor drum15over a plurality of periods of the photoconductor drum15rather than averaged data in one round of the photoconductor drum15.

A thickness unevenness storage section87stores thickness unevenness corresponding to one period along the rotation direction of the photoconductor drum15in response to the thickness data provided by the current value-thickness conversion section86as shown inFIG. 21(b).

A gradation correction data computation section88computes gradation correction data in response to the data of the thickness unevenness of the photoconductor layer52input from the thickness unevenness storage section87of the thickness variation detection section61, stores the result as gradation correction data LC1, and outputs the data at a timing. The gradation correction data LC1stored in the gradation correction data computation section88is output upon reception of a request from the gradation correction section67.

FIG. 22is a diagram to describe the configuration of the gradation correction section67in the density correction section110shown inFIG. 18. The gradation correction section67as a controller includes a first counter91and a first correction section92.

The first counter91counts the number of SOS signals input from an SOS sensor108(seeFIG. 2). The phase signal PS1is input from the phase detection sensor71(seeFIG. 18) to the first counter91. Whenever the phase signal PS1is input to the first counter91, the count of the first counter91(called first count value) is reset. The first correction section92references the gradation correction data LC1read from the gradation correction data computation section88(seeFIG. 18) and outputs the first correction value responsive to the first count value input from the first counter91.

In the described configuration, the full-color printer according to the second embodiment operates as follows:

In the second embodiment, the thickness detection operation of the photoconductor drum15is performed at a timing before the print operation of an image as shown inFIG. 18. The thickness detection operation is executed at a timing, for example, when power of the printer is turned on, after completion of the print operation of the number of sheets, etc.

The thickness detection operation is performed by rotating the photoconductor drum15, charging the surface of the photoconductor drum15to a potential by a charging roll16, and detecting the DC current IDCflowing into the charging roll16is detected immediately by the current detection circuit60as shown inFIG. 18. At this time, image exposure and developing process are not performed on the surface of the photoconductor drum15.

The DC current value IDCflowing into the charging roll16detected by the current detection circuit60is input to the thickness variation detection section61as shown inFIG. 18and is stored in the current value data storage section81of the thickness variation detection section61as digital data. Next, the synchronization processing section82synchronizes the current value data stored in the current value data storage section81in response to the phase signal PS1input from the phase detection sensor71(seeFIG. 18) and the data extraction section83extracts the current value data stored in the current value data storage section81as shown inFIG. 20every period of the photoconductor drum15and then the averaging processing section85averages the current value for each position along the circumferential direction of the photoconductor drum15as shown inFIG. 21(a). The current value for each position averaged by the averaging processing section85is converted into thickness data of the photoconductor layer52by the current value-thickness conversion section86and the thickness data is stored in the thickness unevenness storage section87. The averaging processing section85averages the current value for each position along the circumferential direction of the photoconductor drum15, whereby the detection accuracy of the thickness based on the current value improves.

The gradation correction data computation section88computes gradation correction data in response to the data of thickness unevenness of the photoconductor layer52input from the thickness unevenness storage section87of the thickness variation detection section61and stores the result as gradation correction data LC1.

Then, in the full-color printer, as shown inFIG. 2, to print a full-color image, etc., the surfaces of the photoconductor drums15are charged to a potential by the charging rolls16in image formation sections13Y,13M,13C, and13K of colors of yellow (Y), magenta (M), cyan (C), and black (K) and then image exposure corresponding to the color image data is executed on the surfaces of the photoconductor drums15by ROSs14Y,14M,14C, and14K to form electrostatic latent images and the electrostatic latent images formed on the photoconductor drums15are visualized in corresponding color toners by developing units17to form toner images. The toner images of colors of yellow (Y), magenta (M), cyan (C), and black (K) formed on the photoconductor drums15are multiplely transferred onto an intermediate transfer belt25and then are secondarily transferred from the intermediate transfer belt25in batch onto record paper35by a secondary transfer roll34and are fixed by a fuser38to form a full-color image.

At the time, in the full-color printer, the phase detection sensor71detects the phase of the photoconductor drum15as shown inFIG. 18and outputs a phase signal PS1. The phase signal PS1output from the phase detection sensor71is input to the first counter91as shown inFIG. 22.

The first counter91counts the number of SOS signals input from the SOS sensor108(seeFIG. 2). The phase signal PS1is input to the first counter91from the phase detection sensor71(seeFIG. 18). Whenever the phase signal PS1is input to the first counter91, the count of the first counter91(called first count value) is reset. The first correction section92references the gradation correction data LC1read from the gradation correction data computation section88(seeFIG. 18) and outputs the first correction value responsive to the first count value input from the first counter91. The ROS14as an exposure unit executes image exposure for the surface of the photoconductor drum15in a state in which the gradation data is corrected in response to the correction with the first correction value.

Consequently, if the thickness of the photoconductor layer52of the photoconductor drum15contains thickness unevenness along the rotation direction of the photoconductor drum15, the exposure amount of the image exposure is controlled at a position along the rotation direction of the photoconductor drum15in response to the thickness unevenness of the photoconductor layer52, whereby inconsistencies in density caused by thickness unevenness of the photoconductor layer52are suppressed.

Other components and functions are similar to those of the first embodiment and will not be discussed again.

In the description of the embodiment, partial thickness unevenness of the photoconductor layer52in the photoconductor drum15appears as surface potential unevenness of the photoconductor drum15caused by the charging rolls16. In addition, if inconsistencies in density along the rotation direction of the photoconductor drum15are caused to occur as partial thickness unevenness of the photoconductor layer52in the photoconductor drum15affects the developing electric field in the developing unit17or the transfer electric field in primary transfer roll26, the thickness variation detection section61may detect thickness unevenness corresponding to the rotation period of the developing roll or the primary transfer roll26and image exposure, etc., may be corrected in response to the detected thickness unevenness.

The invention can be applied not only to the full-color image formation apparatus, but also to a monochrome image formation apparatus, of course.