Image forming apparatus for setting an electrification voltage

An image forming apparatus includes photoconductors. Electrifiers uniformly electrify surfaces of the photoconductors. A power source applies an electrifying voltage to the electrifiers. A current measurer measures alternating current caused to flow by application of AC voltage by the power source. A controller calculates discharge starting voltage. Environment detectors detect an environment inside of the apparatus. The controller operates the current measurer at each predetermined timing to acquire the discharge starting voltage. When acquiring the discharge starting voltage, the controller changes peak-to-peak voltage at pre-discharge voltage and at post-discharge voltage. The current measurer measures alternating current at measurement points of each of the pre-discharge and post-discharge voltages. The controller calculates a voltage value at an intersection of a first line and a second line. After acquiring the discharge starting voltage, the controller calculates environment-correction discharge starting voltage, and sets electrification voltage based on the calculated voltage.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority under 35 U.S.C. §119 to Japanese Patent Application No. 2014-028558, filed Feb. 18, 2014. The contents of this application are incorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

2. Discussion of the Background

Conventionally, an image forming apparatus of electrophotography has included an electrifier to electrify the surface of each photoconductor. As this electrifier, there have been known contact electrifiers of, for example, a roller type and a blade type. Moreover, among such contact electrifiers, there have been known electrifiers to which an electrifying voltage having an alternating-current (AC) voltage superposed on a direct-current (DC) voltage is applied. It should be noted that in the following description, not only an electrifier in direct contact with a photoconductor but also an electrifier not in contact but closely adjacent will be referred to as a contact electrifier.

When the AC voltage is applied, a contact electrifier causes discharge between the electrifier and a photoconductor to appropriately electrify the surface of the photoconductor. Excessive discharge caused by the electrifier may damage the photoconductor. In view of this, the magnitude of AC component of the electrifying voltage applied to the electrifier is controlled to maintain an amount of discharge within a suitable range (see Japanese Unexamined Patent Application Publication No. 2001-201920 and Japanese Unexamined Patent Application Publication No. 2007-199094). Furthermore, image forming apparatuses recited in Japanese Unexamined Patent Application Publication No. 2001-201920 and Japanese Unexamined Patent Application Publication No. 2007-199094 include environment sensors to detect environmental changes inside of the apparatuses such as temperature and humidity. In accordance with the environmental changes inside of the apparatuses detected by such environment sensors, AC component of the electrifying voltage applied to the electrifier is controlled.

The contents of Japanese Unexamined Patent Application Publication No. 2001-201920 and Japanese Unexamined Patent Application Publication No. 2007-199094 are incorporated herein by reference in their entirety.

Recently, there has been a demand for increasing the thickness of a photosensitive layer to prolong the service life of a photoconductor. Therefore, as the frequency of use of the photoconductor increases, the photosensitive layer becomes thinner than an initial state. Consequently, application of the electrifying voltage having AC component set in the initial state may unfortunately cause excessive discharge with respect to the photoconductor.

In this respect, in the image forming apparatus disclosed in Japanese Unexamined Patent Application Publication No. 2001-201920, the AC component of the electrifying voltage is set based on a plurality of measurement points in the initial stage. However, the AC component of the electrifying voltage is then set based on a value measured in the printing step and a setting log. This decreases setting accuracy. Also, in the image forming apparatus disclosed in Japanese Unexamined Patent Application Publication No. 2007-199094, there is only one measurement point to cause discharge with respect to the photoconductor. Similarly to the image forming apparatus disclosed in Japanese Unexamined Patent Application Publication No. 2001-201920, setting accuracy of the AC component of the electrifying voltage is not high. Therefore, when the image forming apparatuses disclosed in Japanese Unexamined Patent Application Publication No. 2001-201920 and Japanese Unexamined Patent Application Publication No. 2007-199094 include the photoconductor having a thick photosensitive layer in the initial state, it is difficult to set the optimum electrifying voltage depending on states of use.

In view of the above-described problems, it is an object of the present invention to provide an image forming apparatus to set the optimum electrifying voltage even though a photoconductor having a thick photosensitive layer is used.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, an image forming apparatus includes photoconductors, electrifiers, a power source, a current measurer, a controller, and environment detectors. The photoconductors are configured to carry electrostatic latent images. The electrifiers are disposed in contact with or adjacent to the respective photoconductors and configured to uniformly electrify surfaces of the photoconductors. The power source is configured to apply an electrifying voltage to the electrifiers. The electrifying voltage has an AC voltage superposed on a DC voltage. The current measurer is configured to measure an alternating current caused to flow by application of an AC voltage by the power source. The controller is configured to calculate a discharge starting voltage, which is a peak-to-peak voltage of the AC voltage at which discharge between the photoconductor and the electrifier is started. The environment detectors are configured to detect an environment inside of the apparatus. The controller is configured to operate the current measurer at each predetermined timing to acquire the discharge starting voltage. The controller is configured to, when acquiring the discharge starting voltage, change the peak-to-peak voltage of the AC voltage applied by the power source in at least two stages at pre-discharge voltage lower than the discharge starting voltage and at post-discharge voltage higher than the discharge starting voltage. The current measurer is configured to measure alternating current at two or more measurement points of each of the pre-discharge voltage and the post-discharge voltage. The controller is configured to calculate a voltage value at an intersection of a first line and a second line. The first line is acquired from a relationship between a peak-to-peak voltage of an AC voltage and an alternating current at two or more measurement points of the pre-discharge voltage. The second line is acquired from a relationship between a peak-to-peak voltage of an AC voltage and an alternating current at two or more measurement points of the post-discharge voltage. The controller is configured to, after acquiring the discharge starting voltage, calculate an environment-correction discharge starting voltage by correcting the discharge starting voltage based on the environment inside of the apparatus detected by the environment detectors. The controller is configured to set an electrification voltage based on the environment-correction discharge starting voltage. The electrification voltage is a peak-to-peak voltage of the AC voltage applied by the power source in image formation.

According to the embodiment of the present invention, alternating current is measured at two or more measurement points of each of the pre-discharge voltage and the post-discharge voltage. Based on a measurement result, the electrification voltage (AC component of the electrifying voltage) is set. Consequently, in accordance with an amount of change in the thickness of the photosensitive layer depending on the frequency of use of the photoconductor, the optimum electrification voltage is set. In order to prolong the service life of the photoconductor, the thickness of the photosensitive layer is increased. Even in the case of the photoconductor having such a thick photosensitive layer, an electrification state is constantly maintained appropriately. At the same time, excessive discharge is suppressed to prevent damage to the photoconductor.

According to the embodiment of the present invention, in the second and subsequent measurement, the number of measurement points is smaller than the number of measurement points in the first measurement. This shortens the time for the second and subsequent measurement and reduces the power consumption required for the measurement. Moreover, according to the embodiment of the present invention, the thickness deviation of the photosensitive layer is predicted to correct the electrification voltage based on the thickness deviation. This suppresses random variation in electrification states due to the thickness deviation, and enables image formation of high definition with less image irregularity. Furthermore, according to the embodiment of the present invention, the DC voltage applied for the measurement is set to be smaller than the absolute value of the DC voltage applied for image formation. Therefore, in the measurement at the post-discharge voltage, leak current is prevented from flowing to the photoconductor owing to excessive discharge. This suppresses damage to the photoconductor.

DESCRIPTION OF THE EMBODIMENTS

Embodiments of the present invention will be described below with reference to the accompanying drawings. In the following description, terms to represent specific directions and positions (such as “left and right” and “above and below”) are used as necessary. In such an occasion, a view as seen in a direction perpendicular to the surface of the sheet ofFIG. 2is a front view. This direction is regarded as a reference. Such terms are intended only for convenience's sake of description, and will not limit the technical scope of the present invention.

<Configuration of Image Forming Apparatus>

First, the general arrangement of an image forming apparatus according to an embodiment of the present invention will be described below with reference to the drawings.FIG. 1is an external perspective view of the image forming apparatus according to the embodiment.FIG. 2is a schematic diagram illustrating an internal configuration of the image forming apparatus.

As shown inFIGS. 1 and 2, the image forming apparatus1includes an image reader3, sheet feed trays4, a transfer unit5, a fixing unit6, a sheet discharge tray7, and an operation panel9. The image reader3reads an image from a document P1. The sheet feed trays4contain recording sheets P2on which images are to be formed. The transfer unit5transfers a toner image to each recording sheet P2fed from the sheet feed tray4. The fixing unit6fixes the toner image, which has been transferred by the transfer unit5, onto the recording sheet P2. The recording sheet P2on which the image is fixed and formed at the fixing unit6is discharged to the sheet discharge tray7. The operation panel9receives operation commands to the image forming apparatus1. In the image forming apparatus1, the image reader3is disposed on an upper portion of an apparatus main body2. The transfer unit5is disposed below the image reader3.

The sheet discharge tray7is disposed above the transfer unit5in the apparatus main body2so as to receive the recording sheet P2discharged after the image is recorded at the transfer unit5and the fixing unit6. The sheet feed trays4are detachably inserted below the transfer unit5in the apparatus main body2. With this configuration, as will be described later, a recording sheet P2contained in the sheet feed tray4is fed into the apparatus main body2and conveyed upwardly. An image is transferred onto the recording sheet P2in the transfer unit5above the sheet feed tray4and fixed in the fixing unit6. Then, the recording sheet P2is discharged to the sheet discharge tray7disposed in a space (recessed space) between the image reader3and the transfer unit5.

The image reader3on the upper portion of the apparatus main body2includes a scanner31and an automatic document feeder (ADF)32. The scanner31reads an image from a document P1. The ADF32is disposed on an upper portion of the scanner31and feeds documents P1to the scanner31one by one. The operation panel9is disposed on the front side of the apparatus main body2. The user operates the keys while checking, for example, a monitor of the operation panel9. Thus, the user performs setting of a function selected from various kinds of functions of the image forming apparatus1, and instructs the image forming apparatus1to execute work.

Next, referring toFIG. 2, the internal configuration of the apparatus main body2will be described. The scanner31of the image reader3on the upper portion of the apparatus main body2includes a document table33, a light source34, an image sensor35, an image formation lens36, and a mirror group37. The document table33includes platen glass (not shown) on an upper surface thereof. The light source34irradiates a document P1with light. The image sensor35performs photoelectric conversion of reflected light from the document P1into image data. The image formation lens36forms an image of the reflected light on the image sensor35. The mirror group37reflects the reflected light from the document P1successively to make the reflected light incident on the image formation lens36. The light source34, the image sensor35, the image formation lens36, and the mirror group37are disposed inside of the document table33. The light source34and the mirror group37are arranged to be laterally movable with respect to the document table33.

On the upper side of the scanner31, the ADF32is disposed to be openable from the document table33in a cantilever manner. The ADF32extends over the document P1on the platen glass (not shown) of the document table33, thus also serving to bring the document P1in close contact with the platen glass (not shown). The ADF32includes a document mounting tray38and a document discharge tray39.

When the image reader3of the above-described configuration reads a document P1on the platen glass (not shown) of the document table33, the light source34moving in the right direction (subscanning direction) irradiates the document P1with light. The light reflected from the document P1is successively reflected by the mirror group37moving in the right direction similarly to the light source34. The reflected light is made incident on the image formation lens36, and an image of the reflected light is formed on the image sensor35. In accordance with the intensity of the incident light, the image sensor35executes photoelectric conversion of each picture element and generates image signals (RGB signals) corresponding to the image of the document P1.

In reading a document P1on the document mounting tray38, the document P1is conveyed to a reading position by a document conveyance mechanism40including components such as a plurality of rollers. At this time, the light source34and the mirror group37of the scanner31are fixed at predetermined positions inside of the document table33. Therefore, a portion of the document P1at the reading position is irradiated with the light from the light source34. Through the mirror group37and the image formation lens36of the scanner31, an image of the reflected light is formed on the image sensor35. Then, the image sensor35converts the formed image into image signals (RGB signals) corresponding to the image of the document P1, and the document P1is discharged to a document discharge tray39.

The transfer unit5to transfer a toner image to a recording sheet P2includes image formation portions51, an exposure portion52, an intermediate transfer belt53, primary transfer rollers54, a drive roller55, a driven roller56, a secondary transfer roller57, and a cleaner58. The image formation portions51respectively generate toner images of colors yellow (Y), magenta (M), cyan (C), and black (K). The exposure portion52is disposed below the image formation portions51. The intermediate transfer belt53is in contact with the image formation portions51of the colors disposed horizontally. The toner images of the colors are transferred from the image formation portions51to the intermediate transfer belt53. The primary transfer rollers54are respectively disposed above and opposite to the image formation portions51of the colors in such a manner that the primary transfer rollers54and the image formation portions51clamp the intermediate transfer belt53. The drive roller55rotates the intermediate transfer belt53. Rotation of the drive roller55is transmitted to the driven roller56through the intermediate transfer belt53to rotate the driven roller56. The secondary transfer roller57is disposed opposite to the drive roller55with the intermediate transfer belt53interposed therebetween. The cleaner58is disposed opposite to the driven roller56with the intermediate transfer belt53interposed therebetween.

Each of the image formation portions51includes a photoconductive drum61, an electrifier62, a developer63, and a cleaner64. The photoconductive drum61is in contact with an outer peripheral surface of the intermediate transfer belt53. The electrifier62electrifies an outer peripheral surface of the photoconductive drum61. After stirring and electrifying toner, the developer63applies the toner to the outer peripheral surface of the photoconductive drum61. After the toner image is transferred to the intermediate transfer belt53, the cleaner64removes residual toner on the outer peripheral surface of the photoconductive drum61. At this time, the photoconductive drum61is disposed opposite to the primary transfer roller54with the intermediate transfer belt63interposed therebetween. Also, the photoconductive drum61rotates clockwise, as seen inFIG. 2. Around the photoconductive drum61, the primary transfer roller54, the cleaner64, the electrifier62, and the developer63are disposed in sequence in the rotation direction of the photoconductive drum61.

The intermediate transfer belt53is made of, for example, an endless belt member having electric conductivity, and wound around the drive roller55and the driven roller56without slackness. Thus, in accordance with rotation of the drive roller55, the intermediate transfer belt53rotates counterclockwise, as seen inFIG. 2. Around the intermediate transfer belt53, the secondary transfer roller57, the cleaner58, and the image formation portions51of the colors Y, M, C, and K are disposed in sequence in the rotation direction of the intermediate transfer belt53.

In order to fix the toner image transferred to the recording sheet P2, the fixing unit6includes a heating roller59and a pressurizing roller60. The heating roller59includes a heat source such as a halogen lamp to heat and fix the toner image on the recording sheet P2. The pressurizing roller60clamps the recording sheet P2with the heating roller59and pressurizes the recording sheet P2. It should be noted that the heating roller59may produce eddy current on the surface by electromagnetic induction to heat the surface of the heating roller59.

A sheet feed unit8including a plurality of sheet feed trays4is provided with draw rollers81. Each of the draw rollers81draws out recording sheets P2contained in the sheet feed tray4from an uppermost sheet to a sheet feed path R1. A main conveyance path R0is a route in which the recording sheet P2mainly passes in the steps of image formation (printing). The sheet feed path R1is provided for each of the sheet feed trays4and communicates with the main conveyance path R0. The recording sheets P2in the sheet feed tray4are drawn out one by one from an uppermost sheet to the sheet feed path R1by rotation of the corresponding draw roller81. Then, the recording sheet P2is sent to the main conveyance path R0.

A manual bypass tray93is disposed on a lateral side portion (right side portion in this embodiment) of the apparatus main body2. With the manual bypass tray93, recording sheets P2of a predetermined size are fed from the outside. The manual bypass tray93is an auxiliary tray in addition to the normal sheet feed trays4inside of the apparatus main body2. The manual bypass tray93is attached to the lateral side portion of the apparatus main body2rotatably to be open from and closed to the apparatus main body2. By rotation of a draw roller and such components, the recording sheets P2on the manual bypass tray93are drawn out one by one from an uppermost sheet and sent through a bypass sheet feed path R2toward the main conveyance path R0. Further, a sheet discharge roller pair91to discharge the printed recording sheet P2are disposed on the most downstream end of the main conveyance path R0. The printed recording sheet P2is discharged to the sheet discharge tray7by rotation of the sheet discharge roller pair91.

Next, description will be made on printing operation by the image forming apparatus1. When receiving a command through the operation panel9or an external terminal to start the printing operation, the image forming apparatus1starts control operation for the printing operation. First, the sheet feed unit8drives the draw roller81to draw out an uppermost recording sheet P2from the sheet feed tray4and feed the recording sheet P2to the sheet feed path R1. The recording sheet P2, which has been fed from the sheet feed tray4to the sheet feed path R1, is sent from the sheet feed path R1to the vertical main conveyance path R0through a vertical conveyance roller pair84.

Based on image data of the colors Y, M, C, and K, light emitting diodes (not shown) inside of the exposure portion52are driven to form electrostatic latent images on the photoconductive drums61of the respective colors Y, M, C, and K. Specifically, in each of the image formation portions51of the colors Y, M, C, and K, the photoconductive drum61is electrified by the electrifier62, and the surface of the photoconductive drum61is irradiated with a laser beam from the exposure portion52. Thus, an electrostatic latent image corresponding to an image of each of the colors Y, M, C, and K is formed.

Toner electrified by the developer63is transferred to the surface of the photoconductive drum61on which the electrostatic latent image is formed, and a toner image is formed on the photoconductive drum61serving as a first image carrier (development). When the toner image carried on the surface of the photoconductive drum61and rendered manifest is brought into contact with the intermediate transfer belt53, the toner image is transferred to the intermediate transfer belt53by transfer current or transfer voltage applied to the primary transfer roller54. Consequently, the toner images of the colors Y, M, C, and K superposed on each other are formed on the surface of the intermediate transfer belt53serving as a second image carrier (primary transfer). After the toner image is transferred to the intermediate transfer belt53, the toner, which has not been transferred but remained on the photoconductive drum61, is scraped by the cleaner64and removed from the surface of the photoconductive drum61.

The recording sheet P2conveyed to the main conveyance path R0reaches a timing roller pair87. At the timing when the toner image is transferred to the intermediate transfer belt53, the timing roller pair87are operated to convey the recording sheet P2to the transfer unit5. When the intermediate transfer belt53is rotated by the drive roller55and the driven roller56, the toner image transferred to the intermediate transfer belt53moves to a transfer nip area in contact with the secondary transfer roller57and is transferred to the recording sheet P2conveyed to the transfer nip area on the main conveyance path R0(secondary transfer). After the toner image is transferred to the recording sheet P2, the toner, which has not been transferred but remained on the intermediate transfer belt53, is scraped by the cleaner58and removed from the surface of the intermediate transfer belt53.

After the toner image is transferred to the recording sheet P2at the position in contact with the secondary transfer roller57, the recording sheet P2is conveyed to the fixing unit6made up of the heating roller59and the pressurizing roller60. When the heating roller59and the pressurizing roller60are rotated, the heating roller59heats the recording sheet P2at the same time. Thus, the recording sheet P2on one side of which the unfixed toner image is carried passes a fixing nip portion of the fixing unit6. Then, the recording sheet P2is heated and pressurized by the heating roller59and the pressurizing roller60to fix the unfixed toner image on the recording sheet P2. After the toner image is fixed (after single-side printing), the recording sheet P2is conveyed to the sheet discharge roller pair91and discharged to the sheet discharge tray7by the sheet discharge roller pair91.

<Configuration of Image Formation Portion>

Detailed configurations of components of the image formation portion51will be described below. As shown inFIG. 3, the electrifier62includes an electrification roller621and a cleaning roller622. The cleaning roller622is in contact with the electrification roller621at a position on a side opposite to the photoconductive drum61side. The electrifier62, the photoconductive drum61, and the cleaner64are housed in a drum housing611and constitute a photoconductor unit601. The photoconductor unit601is detachably attached to the apparatus main body2(apparatus frame). Needless to say, a specific configuration may be selected as desired. For example, the electrifier62and the cleaner64may constitute a single detachable unit.

The electrification roller621includes a shaft on which a conductive rubber elastic layer is formed. A nip is formed in a portion of the electrification roller621that is in contact with the photoconductive drum61. A rough surface layer is formed on the surface of the conductive rubber elastic layer of the electrification roller621. The conductive rubber elastic layer of the electrification roller621is made of an elastic material, for example, epichlorohydrin rubber (such as ECO and CO), nitrile rubber (NBR), ethylene-propylene-diene rubber (EPDM), silicone rubber, urethane rubber, styrene-butadiene rubber (SBR), isoprene rubber (IR), chloroprene rubber (CR), and natural rubber (NR). In particular, ethylene-propylene-diene rubber (EPDM), epichlorohydrin rubber, and nitrile rubber are preferably adopted.

As a conductive material to be mixed in an elastic material constituting the conductive rubber elastic layer, there are adopted carbon black such as Ketjen black and acetylene black, graphite, metal powder, conductive metallic oxide, various ionic conductive materials such as quaternary ammonium salt such as tetramethylammonium perchlorate, trimethyloctadecylammonium perchlorate, and benzyltrimethylammonium chloride. In order to roughen the surface layer formed on the surface of the conductive rubber elastic layer, the surface of the conductive rubber elastic layer is coated with coating resin to which roughening particles are added. The roughening particles are organic particles or inorganic particles having an average diameter of several μm to several ten μm. The roughness of the surface layer is regulated by changing the size and addition amount of the particles and the coating thickness.

The cleaning roller622includes a metal shaft on which a conductive elastic material is wound. The cleaning roller622is in contact with the electrification roller621under a predetermined pressure. Consequently, the nip is formed in the contact portion of the cleaning roller622with the electrification roller621. The cleaning roller622is disposed on the side of the axis of the electrification roller621that is opposite to the photoconductive drum61side. In other words, the cleaning roller622is in contact with the outer peripheral surface of the electrification roller621at the farthest portion from the photoconductive drum61.

The developer63includes a developer housing631, a development roller632, a supply roller633, a stirring roller634, and a development chamber635. The development chamber635contains a carrier and a toner as a developing solution. A development bias having an AC voltage superposed on a DC voltage is applied to the development roller632. An electrostatic latent image formed on the surface of the photoconductive drum61is developed by the toner under the effect of the development bias. Thus, a toner image is formed on the surface of the photoconductive drum61. It should be noted that the toner includes a coloring agent in a binder resin to which an external additive is added and processed. Desirably, the toner has a particle diameter of 3 to 15 μm although this should not be construed in a limiting sense. As necessary, the binder resin contains a charge control agent and a release agent.

The toner in the developing solution is produced by a conventional method in general use such as pulverization, emulsion polymerization, and suspension polymerization. Examples of the binder resin for the toner include styrene resin (homopolymer or copolymer containing styrene or styrene substitution product), polyester resin, epoxy resin, vinyl chloride resin, phenol resin, polyethylene resin, polypropylene resin, polyurethane resin, and silicone resin. Preferably, the binder resin, which is a simple one of these resins or a complex of these resins, has a softening temperature of 80° C. to 160° C. or a glass transition point of 50° C. to 75° C.

As the coloring agent, conventional coloring agents in general use are adopted. Examples include carbon black, aniline black, active carbon, magnetite, benzine yellow, permanent yellow, naphthol yellow, phthalocyanine blue, fast sky blue, ultramarine blue, rose bengal, and lake red. Preferably, the coloring agent is used to be 2 to 20 weight % with respect to 100 weight % of the above-described binder resin.

As the charge control agent contained in the binder resin, in the case of a positively electrifiable toner, nigrosine dye, quaternary ammonium salt compound, triphenylmethane compound, imidazole compound, and polyamine resin are used. In the case of the charge control agent for a negatively electrifiable toner, azo dye containing metal such as chromium, cobalt, aluminum, and iron, salicylic acid metal compound, alkyl salicylic acid metal compound, and calixarene compound are used. Preferably, the charge control agent is used to be 0.1 to 10 weight % with respect to 100 weight % of the binder resin. As the release agent contained in the binder resin, polyethylene, polypropylene, carnauba wax, and Sasolwax are singly used or a combination of two or more of these release agents is used. Preferably, the release agent is used to be 0.1 to 10 weight % with respect to 100 weight % of the binder resin.

Particles (external additive) are externally added to the toner to improve fluidity. For example, silica, titanium oxide, and aluminum oxide are used. In particular, these particles are preferably made water-repellant by silane coupler, titanium coupler, and silicone oil. Preferably, the fluidizer serving as the external additive is used to be 0.1 to 5 weight % with respect to 100 weight % of the toner. Also, preferably, the external additive has an average primary particle diameter of 10 to 100 nm.

As the carrier, for example, binder carrier and coat carrier are used. Preferably, the carrier has a particle diameter of 15 to 100 μm although this should not be construed in a limiting sense. The toner and the carrier are mixed at a ratio controlled to acquire a predetermined amount of toner electrification. Preferably, the toner ratio to the sum of the toner and the carrier is 3 to 30 weight %. Further preferably, the toner ratio is 4 to 20 weight %.

The binder carrier includes the binder resin in which magnetic particles are dispersed. Also, positively or negatively electrifiable particles are fixed to the surface of the carrier, or a surface coating layer is formed on the surface of the carrier. Electrification properties of the binder carrier is controlled by a material of the binder resin, the electrifiable particles, and a kind of the surface coating layer. As the binder resin, thermoplastic resin such as vinyl resin represented by polystyrene resin, polyester resin, nylon resin, and polyolefin resin, and thermosetting resin such as phenol resin are used.

As the magnetic particles dispersed in the binder carrier, for example, there are used spinel ferrite such as magnetite and γ iron oxide, spinel ferrite containing one or more of metals other than iron (such as manganese, nickel, magnesium, and copper), magnetoplumbite ferrite such as barium ferrite, and particles of iron or alloy covered with iron oxide. When high magnetization is required, iron ferromagnetic particles are preferably used. When chemical stability is considered, ferromagnetic particles of spinel ferrite or magnetoplumbite ferrite are preferably used. A kind and content of the ferromagnetic particles are suitably selected to obtain a carrier having a predetermined magnetization. The magnetic particles may have a particulate or spherical or pin shape. Preferably, 50 to 90 weight % magnetic particles are added to the carrier.

In the case of the binder carrier on which electrifiable or conductive particles are fixed, the particles are uniformly mixed in magnetic resin carrier and attached to the surface of the carrier. Then, exertion of mechanical or thermal impact causes the particles to be hit and fixed into the magnetic resin carrier on the surface of the carrier. At this time, the particles are not completely embedded in the magnetic resin carrier but part of the particles are fixed to protrude from the surface of the magnetic resin carrier.

When electrifiable particles are used as such particles, an organic or inorganic insulating material is used. Specifically, for example, organic insulating particles of polystyrene, styrene copolymer, acryl resin, various acryl copolymers, nylon, polyethylene, polypropylene, fluororesin, and cross-linked products of these substances are used. The material, polymerization catalyst, and surface processing of the organic insulating particles are appropriately selected to set an electrification level and polarity of the carrier as desired. As inorganic particles, negatively electrifiable inorganic particles such as silica and titanium bioxide, or positively electrifiable inorganic particles such as strontium titanate and alumina are used.

In the case of a binder carrier including a surface coating layer, silicone resin, acryl resin, epoxy resin, and fluororesin are used as a material to form the surface coating layer. Thus, the surface of the binder carrier is coated with the resin material and cured to form the surface coating layer so as to improve electrifiability.

The coat carrier includes carrier core particles of magnetic material that are coated with coat resin. In the case of the coat carrier, similarly to the binder carrier, positively or negatively electrifiable particles are fixed on the surface of the carrier. Electrification properties of the coat carrier such as the polarity are controlled by the kind of the surface coating layer and the kind of the electrifiable particles. The coat carrier is made of a material similar to the material of the binder carrier. Also, the carrier core particles are coated with a resin similar to the binder resin of the binder carrier.

As shown in a partial cross-sectional view ofFIG. 4, the photoconductive drum61includes an intermediate layer614and a photosensitive layer615that are laminated in sequence on an outer peripheral surface of a conductive support613. The intermediate layer614has adhesiveness. An electrostatic latent image is formed on the photosensitive layer615. The conductive support613is made of a conductive material. Examples include: metal such as aluminum, copper, chromium, nickel, zinc, and stainless steel that is molded in a drum or sheet shape; metal foil such as aluminum and copper that is laminated on a plastic film; aluminum, indium oxide, and tin oxide that is evaporated on a plastic film; and conductive matter singly or with binder resin applied to form a conductive layer.

The intermediate layer614has a barrier function in addition to the adhesion function to adhere the photosensitive layer615to the conductive support613. The intermediate layer614is formed, for example, by dissolving a binder resin in a solvent and immersing the conductive support613in the solution. Examples of the binder resin include casein, polyvinyl alcohol, nitrocellulose, ethylene acrylate copolymer, polyamide, polyurethane, and gelatin. Among such binder resins, alcohol-soluble polyamide resin is preferable. As the solvent used for forming the intermediate layer614, preferably, inorganic particles such as the above-described conductive particles and metal oxide particles are dispersed, and binder resin represented by polyamide resin is dissolved. Specifically, alcohol having carbon number of 2 to 4 such as ethanol, n-propyl alcohol, isopropyl alcohol, n-butanol, t-butanol, and sec-butanol is preferable. Such alcohol implements favorable solubility and coating performance with respect to polyamide resin. In order to improve preservability and dispersiveness of inorganic particles, co-solvent may be also used with the solvent. Examples of this co-solvent include methanol, benzyl alcohol, toluene, cyclohexanone, and tetrahydrofuran.

The density of the binder resin at the time of forming the coating solution is suitably selected in accordance with the thickness of the intermediate layer614and the coating method. When inorganic particles are dispersed in the binder resin, the mixing ratio of inorganic particles to the binder resin is preferably 20 to 400 weight % with respect to 100 weight % of the binder resin, and more preferably, 50 to 200 weight %. Examples of dispersing means of the inorganic particles include an ultrasonic disperser, a ball mill, a sand grinder, and a homomixer. After the binder resin is coated on the outer peripheral surface of the conductive support613and subjected to a drying step suitably selected from various drying methods such as heat drying, the intermediate layer614is formed. Preferably, the thickness of the intermediate layer614is 0.1 to 15 μm, and more preferably, 0.3 to 10 μm.

The photosensitive layer615on the surface of the photoconductive drum61includes a charge generation layer (CGL)615A and a charge transport layer (CTL)615B. The charge generation layer615A has a charge generation function, and the charge transport layer615B has a charge transport function. These layers are laminated to provide the photosensitive layer615with a layer configuration of separate functions. For this reason, an increase in residual potential owing to continuous use is controlled and suppressed to a low level. In addition, this facilitates control of various kinds of electrophotography properties in accordance of an object of use. When the photoconductive drum61has a negative electrification property, the charge generation layer615A is laminated on the intermediate layer614, and the charge transport layer615B is further laminated on the charge generation layer615A, as shown inFIG. 3. When the photoconductive drum61has a positive electrification property, the charge transport layer615B is laminated on the intermediate layer614, and the charge generation layer615A is further laminated on the charge transport layer615B. Preferably, the photosensitive layer615is a negative electrification photoconductor having the function separation configuration. However, the photosensitive layer615may have a single layer configuration including one layer of the charge generation function and the charge transport function.

The charge generation layer615aof the photosensitive layer615contains a charge generation material and binder resin. Examples of the charge generation material include azo dye such as Sudan Red and diane blue, quinone pigment such as pyrene quinone and Anthanthrone, quinocyanine pigment, perylene pigment, indigo pigment such as indigo and thioindigo, and phthalocyanine pigment. Examples of the binder resin include polystyrene resin, polyethylene resin, polypropylene resin, acryl resin, methacryl resin, vinyl chloride resin, vinyl acetate resin, polyvinyl butyral resin, epoxy resin, polyurethane resin, phenol resin, polyester resin, alkyd resin, polycarbonate resin, silicone resin, melamine resin, copolymer resin containing two or more of these resins (such as vinyl chloride-vinyl acetate copolymer resin, vinyl chloride-vinyl acetate-maleic anhydride copolymer resin), and polyvinylcarbazole resin.

In order to form the charge generation layer615a, binder resin is dissolved in solvent, and the charge generation material is dispersed in the solution by a disperser to prepare coating solution. After coating a surface with the coating solution to have a uniform thickness by a coater, a coating film is dried to form the charge generation layer615aas part of the photosensitive layer615. As the solvent to form the charge generation layer615a, examples include toluene, xylene, methyl ethyl ketone, cyclohexane, ethyl acetate, butyl acetate, methanol, ethanol, propanol, butanol, methyl cellosolve, ethyl cellosolve, tetrahydrofuran, 1-dioxane, 1,3-dioxolane, pyridine, and diethylamine.

Examples of the disperser of the charge generation material in the binder resin include an ultrasonic disperser, a ball mill, a sand grinder, and a homomixer. As for the mixing ratio of the charge generation material to the binder resin, preferably, 1 to 600 weight % of the charge generation material with respect to 100 weight % of the binder resin, and more preferably, 50 to 500 weight %. Preferably, the thickness of the charge generation layer615ais 0.01 to 5 μm, and more preferably, 0.05 to 3 μm. It should be noted that foreign matter and agglomerates are filtered from the coating solution for the charge generation layer615aprior to coating so as to prevent occurrence of image defects. The charge generation layer615ais formed also by vacuum evaporation of pigment as the charge generation material.

Examples of the binder resin for the charge transport layer615binclude polycarbonate resin, polyacrylate resin, polyester resin, polystyrene resin, styrene-acrylonitrile copolymer resin, polymethacrylic acid-ester resin, and styrene-methacrylic acid ester copolymer resin. Of these resin materials, polycarbonate resin is preferable. In consideration of crack resistance, abrasion resistance, and electrification properties, polycarbonate resin such as bisphenol A (BPA), bisphenol Z (BPZ), dimethyl BPA, BPA-dimethyl BPA copolymer is more preferable.

Similarly to the charge generation layer615a, the charge transport layer615bis formed by the coating method with the solvent described above. Concerning the mixing ratio of the binder resin and the charge transport material, preferably, the charge transport material is 10 to 500 weight % with respect to 100 weight % of the binder resin, and more preferably, 20 to 100 weight %. The thickness of the charge transport layer615bis preferably 5 to 60 μm, and more preferably, 10 to 40 μm. Antioxidant may be added to the charge transport layer615b. For example, antioxidant disclosed in Japanese Unexamined Patent Application Publication No. 2000-305291 may be used.

As described above, the intermediate layer614, the charge generation layer615a, and the charge transport layer615b, which constitute the photoconductive drum61, are respectively formed on the outer peripheral surface of the conductive support613by a conventional coating method. Specifically, examples of the conventional coating method include dip coating, spray coating, spinner coating, bead coating, blade coating, beam coating, and circular amount-restriction coating. The coating method for each of the layers of the photoconductive drum61will not be limited to one kind. A plurality of coating methods may be combined or coating may be performed a plurality of times.

In the image formation portion51having the above-described configuration, the electrifier62electrifies the surface of the photoconductive drum61uniformly. For this purpose, as shown inFIG. 5, a voltage having an AC voltage superposed on a DC voltage is applied to the electrification roller621by a power source unit100. The power source unit100includes a DC power source101, an AC power source102, and a current measurer103. The DC power source101applies a DC voltage Vg serving as an electrifying voltage to electrify the photoconductive drum61. The AC power source102superposes the AC voltage on the DC voltage Vg of the DC power source101. The current measurer103measures a value of current passing the electrification roller621.

A controller110controls each component of the apparatus main body2. In order to set application voltage to the electrifier62, the controller110gives control signals to the power source unit100. The controller110sets the DC voltage Vg by the DC power source101and a peak-to-peak voltage Vpp of the AC voltage by the AC power source102. Thus, the application voltage to the electrifier62is set. The controller110detects the minimum value Vth of the peak-to-peak voltage Vpp discharged between the photoconductive drum61and the electrification roller621at a predetermined timing (hereinafter referred to as “discharge starting voltage”). The controller110sets a peak-to-peak voltage of the AC voltage applied to the electrifier62by the AC power source102(hereinafter referred to as “electrification voltage”).

In detection of the discharge starting voltage, the controller110sets application voltage for measuring the discharge starting voltage (hereinafter referred to as “measurement voltage”) based on values of measurement by a temperature sensor112and a humidity sensor113(environment detectors) to measure temperature and humidity environment inside of the apparatus main body2. Then, the controller110refers to data tables stored in a memory111, and changes the peak-to-peak voltage of the AC voltage by the AC power source102in stages from low voltage to high voltage. Also, the controller110receives a current value measured by the current measurer103, and detects a value of alternating current passing the photoconductive drum51and the electrification roller621.

When the AC voltage from the AC power source102is lower than the discharge starting voltage, the controller110detects a current value of nip current based on contact resistance between the electrification roller621and the photoconductive drum61. When the AC voltage from the AC power source102is higher than the discharge starting voltage, the controller110detects a current value by adding discharge current between the photoconductive drum61and the electrification roller621to the nip current between the photoconductive drum61and the electrification roller621. The controller110changes the AC voltage from the AC power source, and measures the current value in the above-described manner. Based on the measured current value, the controller110calculates and store a discharge starting voltage Vth in the memory111.

When performing printing operation of the above-described image forming apparatus1, the controller110sets an electrification voltage Vac from the AC power source102based on the discharge starting voltage Vth stored in the memory111and the temperature and humidity environment inside of the apparatus main body2measured by the temperature sensor112and the humidity sensor113. Therefore, the controller110gives control signals to the power source unit100to output, from the AC power source102, an AC voltage (AC voltage having an amplitude Vac/2) from the set electrification voltage Vac and to output a DC voltage Vg from the DC power source101at the same time. Thus, the power source unit100outputs an AC voltage having an amplitude Vac/2 (AC voltage of Vg±Vac/2) with DC voltage Vg from the DC power source101as central voltage, and applies the AC voltage to the electrification roller621.

Concerning the electrifying voltage to be applied to the electrification roller621corresponding to each of the colors Y, M, C, and K, the controller110may execute the above-described operation of setting the electrification voltage. Thus, with respect to the electrification rollers621of the colors Y, M, C, and K, the electrifying voltage is set in accordance with states of the corresponding photoconductive drums61. The following embodiments have the configuration and operation described above in common, and are characterized in detection operation of the discharge starting voltage. Therefore, in the following embodiments, the detection operation of the discharge starting voltage by the controller110will be mainly described.

First Embodiment

An image forming apparatus according to a first embodiment of the present invention will be described below with reference to the drawings.FIG. 6is a diagram illustrating a configuration of tables stored in a memory in the image forming apparatus according to the first embodiment.FIGS. 7 and 8are timing charts illustrating transition timings of measurement voltage in current value measurement for calculating discharge starting voltage.FIG. 9is a graph illustrating a relationship between measurement voltage and measured current values and is used for describing a method for calculation of discharge starting voltage.

In the image forming apparatus1according to the first embodiment, as shown inFIG. 6, the memory111stores a measurement voltage setting table (first setting table) DT1, a discharge starting voltage correction table (first correction table) DT2, a measurement voltage correction table (second correction table) DT3, and a measurement voltage setting table (second setting table) DT4. The first setting table DT1stores measurement voltages Vpp corresponding to environment values of the apparatus main body2(temperature and humidity inside of the apparatus). The first correction table DT2stores discharge starting voltage correction values (first correction values) Vx for correcting discharge starting voltage Vth calculated by the controller110. The second correction table DT3stores reference voltage correction values (second correction values) Vy for setting reference values Vpp0of measurement voltage Vpp of the second and subsequent measurement. The second setting table DT4is used for setting the measurement voltage Vpp of the second and subsequent measurement.

In addition to a table storage area storing the above-described tables DT1to DT4, the memory111includes a setting value storage area and a calculation area. The setting value storage area stores the discharge starting voltage Vth and the electrification voltage Vac acquired by the controller110. The calculation area is for calculating the discharge starting voltage Vth and the electrification voltage Vac in the controller110. It should be noted that the memory111may include all of the table storage area, the setting value storage area, and the calculation area, and also, individual memories may be respectively provided for the corresponding areas.

The image forming apparatus1provided with the memory111starts measurement operation of discharge starting voltage Vth by the controller110at predetermined timings. The predetermined timings include when the power of the apparatus main body2is switched on, when printing exceeds the predetermined number of sheets (for example, when 500 or more sheets are printed continuously), and when a change amount of the environment value of the apparatus main body2exceeds a threshold. When the controller110confirms that measurement operation is performed for the first time, the controller110receives environment values (temperature and humidity inside of the apparatus) respectively measured by the temperature sensor112and the humidity sensor113. Also, the controller110retrieves measurement voltages Vpp1to Vpp8corresponding to the environment values from the first setting table DT1. Specifically, the measurement voltages Vpp1to Vpp8are set to be, with respect to Vpp1corresponding to an environment value Sn, Vpp2=Vpp1+ΔV1, Vpp3=Vpp2+ΔV1, Vpp4=Vpp3+ΔV1, Vpp5=Vpp4+ΔV2(ΔV2>ΔV1), Vpp6=Vpp5+ΔV1, Vpp7=Vpp6+ΔV1, and Vpp8=Vpp7+ΔV1.

In the example shown inFIG. 6, ΔV1=100 V and ΔV2=300 V. With respect to environment values S1to S4, Vpp1is respectively set to be 1300V, 1200V, 1100V, and 1000V. When the temperature and the humidity inside of the apparatus are the lowest, the measurement voltages Vpp1to Vpp8are set to be values corresponding to the environment values S1. When the temperature and the humidity inside of the apparatus are the highest, the measurement voltages Vpp1to Vpp8are set to be values corresponding to the environment values S4. When the temperature and the humidity inside of the apparatus are in a normal range, the measurement voltages Vpp1to Vpp8are set to be values corresponding to the environment values S3. Of the environment values S1to S4, an environment value denoted by a small number represents an environment inside of the apparatus in which the resistance of the electrification roller621is high, and an environment value denoted by a large number represents an environment inside of the apparatus in which the resistance of the electrification roller621is low.

When the controller110sets the measurement voltages Vpp1to Vpp8in this manner, the controller110sends control signals to the power source unit100to change peak-to-peak voltage of the AC voltage supplied from the AC power source102in stages from the measurement voltage Vpp1at the minimum to the measurement voltage Vpp8at the maximum. Then, the controller110superposes the AC voltage on DC voltage Vg from the DC power source101. Specifically, as shown inFIG. 7, when the controller110starts measurement operation, the AC voltage from the AC power source102is set as a measurement voltage Vpp1. When a predetermined period of time T1(for example, 100 msec) elapses after the AC voltage from the AC power source102is set as the measurement voltage Vpp1, the controller110acquires a current value measured by the current measurer103. When the controller110starts acquisition of the measured current value, as shown inFIGS. 7 and 8, the controller110receives measured current values from the current measurer103N times (for example, 120 times) continuously at intervals of a predetermined period of time T2(for example, 5 msec).

Acquiring the measured current values of N times at the measurement voltage Vpp1, the controller110calculates an average value Iac1of the acquired measured current values. At the same time, as shown inFIG. 7, the controller110changes the peak-to-peak voltage of the AC voltage supplied from the AC power source102to a measurement voltage Vpp2. When the predetermined period of time T1elapses after the change to the measurement voltage Vpp2, as shown inFIGS. 7 and 8, the controller110receives measured current values from the current measurer103N times continuously at intervals of the predetermined period of time T2. Then, the controller110calculates an average value Iac2of the acquired measured current values of N times, and at the same time, the controller110changes the peak-to-peak voltage of the AC voltage supplied from the AC power source102to a measurement voltage Vpp3.

At intervals of a period of time T1+T2×N, the controller110changes the peak-to-peak voltage of the AC voltage supplied from the AC power source102in stages from the measurement voltage Vpp3to a measurement voltage Vpp8. The controller110respectively calculates average values Iac3to Iac8of the measured current values of N times at the measurement voltages Vpp3to Vpp8. It should be noted that the interval T2of acquisition of the measured current value is set based on resolution of the measured current value. The number N of acquisitions of the measured current values is set at such a value that the electrification roller621rotates one turn or more in a period of time T2×N.

As described above, the controller110respectively calculates the average values Iac1to Iac8of the measured current values at the measurement voltages Vpp1to Vpp8. Based on a relationship between the measurement voltages Vpp1to Vpp8and the average measured current values Iac1to Iac8, as shown inFIG. 9, the controller110calculates a discharge starting voltage Vth. Specifically, referring to the measurement voltages Vpp1to Vpp4as pre-discharge voltages, and based on a relationship between the pre-discharge voltages and the average measured current values Iac1to Iac4, the controller110acquires a line L1representing a relationship between electrifying voltage and nip current by the least squares method. Also, referring to the measurement voltages Vpp5to Vpp8as post-discharge voltages, and based on a relationship between the post-discharge voltages and the average measured current values Iac5to Iac8, the controller110acquires a line L2representing a relationship of electrifying voltage, nip current, and discharge current by the least squares method.

As described above, based on the measurement voltages Vpp1to Vpp8and the average measured current values Iac1to Iac4, the controller110acquires the lines L1and L2in the graph ofFIG. 9. Then, the controller110calculates an electrifying voltage at an intersection X1of the acquired lines L1and L2, and assumes the calculated electrifying voltage at the intersection X1as a discharge starting voltage Vth. After calculating the discharge starting voltage Vth, the controller110refers to the first correction table DT2and retrieves a first correction value Vx based on the environment value Sn. The discharge starting voltage Vth is corrected by the first correction value Vx. The resultant value Vth+Vx is assumed as an environment-correction discharge starting voltage Vth1[1] and stored in the memory111. In the first correction table DT2in the example ofFIG. 6, the first correction value Vx with respect to the environment value S1is −200 V, the first correction value Vx with respect to the environment value S2is −100 V, and the first correction value Vx with respect to the environment values S3and S4is 0 V.

Based on the calculated environment-correction discharge starting voltage Vth1[1], the controller110sets a peak-to-peak voltage of the AC voltage from the AC power source102as an electrification voltage Vac. This electrification voltage Vac is a voltage value to cause discharge between the photoconductive drum61and the electrification roller621. The electrification voltage Vac may be a voltage value Vth1[1]+ΔV, which is the sum of the environment-correction discharge starting voltage Vth1[1] and a predetermined voltage ΔV. Also, the electrification voltage Vac may be a voltage value K×Vth1[1], which is the product of the environment-correction discharge starting voltage Vth1[1] and a predetermined coefficient K (K>1). The controller110stores the set electrification voltage Vac in the memory111, and also controls the AC power source102to apply the AC voltage having the set electrification voltage Vac as the peak-to-peak voltage to the electrification roller621.

As described above, in the first measurement operation, the controller110refers to the first setting table DT1and the first correction table DT2to calculate the environment-correction discharge starting voltage Vth1[1] in accordance with the environment value Sn and to set the electrification voltage Vac. In the second and subsequent measurement operation, the controller110refers to the second correction table DT3and the second setting table DT4and uses the environment-correction discharge starting voltage Vth1[n−1], which has been acquired in the previous measurement operation, and the environment value Sn. Thus, the controller110calculates the environment-correction discharge starting voltage Vth1[n] and sets the electrification voltage Vac.

In the second and subsequent measurement operation, the controller110retrieves the previous environment-correction discharge starting voltage Vth1[n−1] stored in the memory111, which is assumed as a previous measurement voltage Vth2[n]. Then, the controller110receives the environment values Sn respectively measured by the temperature sensor112and the humidity sensor113. Referring to the second correction table DT3of the memory111, the controller110retrieves second correction values Vy corresponding to the environment values Sn and adds the second correction values Vy to the previous measurement voltage Vth2[n]. Thus, a reference value Vpp0(=Vth2[n]+Vy) of the measurement voltage Vpp is calculated. In the second correction table DT3in the example ofFIG. 6, the second correction value Vy with respect to the environment value S1is +200 V, the second correction value Vy with respect to the environment value S2is +100 V, and the second correction value Vy with respect to the environment values S3and S4is 0 V.

After calculating the measurement voltage reference value Vpp0, the controller110refers to the second setting table DT4and acquires measurement voltages Vpp1ato Vpp4ahaving a relationship Vpp1a<Vpp2a<Vpp0<Vpp3a<Vpp4a. The measurement voltage Vpp1ais set to be Vpp0−ΔV1aby subtracting a voltage ΔV1afrom the reference value Vpp0. The measurement voltage Vpp2ais set to be Vpp0−ΔV2a(ΔV1a>ΔV2a) by subtracting a voltage ΔV2afrom the reference value Vpp0. The measurement voltage Vpp3ais set to be Vpp0+ΔV3aby adding a voltage ΔV3ato the reference value Vpp0. The measurement voltage Vpp4ais set to be Vpp0+ΔV4a(ΔV4a>ΔV3a) by adding a voltage ΔV4ato the reference value Vpp0. In the example ofFIG. 6, with the measurement voltage reference value Vpp0being a central value, ΔV1a=ΔV4a=200 V, and ΔV2a=ΔV3a=100 V.

The controller110sets the measurement values Vpp1aand Vpp2aas two pre-discharge voltages and the measurement values Vpp3aand Vpp4aas two post-discharge voltages. Then, as shown inFIG. 10, in sequence from the measurement value Vpp1a, the peak-to-peak voltage of the AC voltage supplied from the AC power source102is changed. Each time the controller110changes the measurement value, the controller110executes measurement operation similar to the first measurement operation. That is, when the predetermined period of time T1elapses immediately after the change of the measurement value, the controller110acquires measured current values by the current measurer103N times continuously at intervals of the period of time T2. Also, similarly to the first measurement operation, the controller110calculates average values Iac1ato Iac4aof the acquired measured current values of N times with respect to the respective measurement voltages Vpp1ato Vpp4a.

As described above, the controller110respectively calculates the average values Iac1ato Iac4aof the measured current values at the measurement voltages Vpp1ato Vpp4a. Then, based on the relationship shown inFIG. 11, the controller110calculates a discharge starting voltage Vth. Specifically, based on a relationship between the measurement voltages Vpp1aand Vpp2aas the pre-discharge voltages and the average measured current values Iac1aand Iac2a, the controller110acquires a line L1arepresenting a relationship between electrifying voltage and nip current by the least squares method. Also, based on a relationship between the measurement voltages Vpp3aand Vpp4aas the post-discharge voltages and the average measured current values Iac3aand Iac4a, the controller110acquires a line L2arepresenting a relationship of electrifying voltage, nip current, and discharge current by the least squares method.

Then, the controller110calculates an electrifying voltage at an intersection X1aof the lines L1aand L2ain the graph ofFIG. 11, and assumes the electrifying voltage as a discharge starting voltage Vth. After calculating the discharge starting voltage Vth, the controller110refers to the first correction table DT2and retrieves a first correction value Vx based on the environment value Sn. The discharge starting voltage Vth is corrected by the first correction value Vx, and the resultant value Vth+Vx is assumed as an environment-correction discharge starting value Vth1[n] and stored in the memory111. Further, based on the calculated environment-correction discharge starting value Vth1[n], the controller110sets an electrification voltage Vac that is a peak-to-peak voltage of the AC voltage from the AC power source102and controls application operation by the power source unit100.

Thus, in the second and subsequent measurement operation, two measurement points are set for each of the pre-discharge voltage and the post-discharge voltage. Consequently, in a period of time shorter than the first measurement operation, the electrification voltage Vac that is a peak-to-peak voltage of the AC voltage from the AC power source102is set. It should be noted that in the second and subsequent measurement operation, the number of measurement points at the pre-discharge voltage and the post-discharge voltage should be smaller than the number of the measurement points in the first measurement operation. For example, when the number of measurement points in the first measurement operation is Y1, the number of measurement points in the second and subsequent measurement operation should be two or more and (Y1−1) or less.

In the first embodiment, in the second and subsequent measurement operation, the second correction value Vy corresponding to the environment value Sn is retrieved and added to the previous measurement voltage Vth2[n] (=Vth1[n−1]) so as to calculate the measurement voltage reverence value Vpp0. However, in the third and subsequent measurement operation, not only the previous measurement voltage Vth2[n] but also the second previous measurement voltage Vth3[n] (=Vth1[n−2]) may be used for calculation. Specifically, in the third and subsequent measurement operation, for example, the second correction value Vy is added to the previous measurement voltage Vth2[n] to calculate a first reference value Vpp0a(=Vth2[n]+Vy). The second correction value Vy is added to the second previous measurement voltage Vth3[n] to calculate a second reference value Vpp0b(=Vth3[n]+Vy). Then, an average value of the first and second reference values Vpp0aand Vpp0bmay be assumed as a measurement voltage reference value Vpp0. A weighted average value of the first and second reference values Vpp0aand Vpp0bmay be assumed as a measurement voltage reference value Vpp0.

Moreover, as described above, in the third and subsequent measurement operation, the previous two environment-correction discharge starting voltages are used to calculate the measurement voltage reference value Vpp0. In this manner, in each measurement operation, a plurality of environment-correction discharge starting voltages may be stored as a history, and the stored history may be used to calculate the measurement voltage reference value Vpp0. In order to calculate the measurement voltage reference value Vpp0, all the history of the environment-correction discharge starting voltages stored in the memory111may be retrieved. Also, the predetermined number of environment-correction discharge starting voltages, for example, previous three, may be retrieved.

Second Embodiment

An image forming apparatus according to a second embodiment of the present invention will be described below with reference to the drawings.FIG. 12is a diagram illustrating a configuration of tables stored in a memory in the image forming apparatus according to the second embodiment. In the second embodiment, the same components and operations as in the first embodiment will be denoted by the same reference numerals and will not be elaborated here.

In the image forming apparatus1according to the second embodiment, as shown inFIG. 12, similarly to the first embodiment (seeFIG. 6), the memory111stores a measurement voltage setting table (first setting table) DT1, a discharge starting voltage correction table (first correction table) DT2, a measurement voltage correction table (second correction table) DT3, and a measurement voltage setting table (second setting table) DT4. The memory111further stores an electrification voltage correction table (third correction table) DT5storing electrification voltage correction values (third correction values) Vz for correcting an electrification voltage Vac in accordance with the number of rotations of the photoconductive drum61.

In the image forming apparatus1according to the second embodiment, similarly to the first embodiment, the controller110changes a peak-to-peak voltage of the AC voltage superposed on a DC voltage Vg at each predetermined timing to execute measurement operation of a discharge starting voltage Vth. In the first measurement operation, the controller110refers to the first setting table DT1, and based on a measurement result in operating the power source unit100, the controller110calculates the discharge starting voltage Vth (seeFIG. 9). In the second and subsequent measurement operation, the controller110refers to the second correction table DT3and the second setting table DT4, and based on a measurement result in operating the power source unit100, the controller110calculates the discharge starting voltage Vth (seeFIG. 11).

Then, similarly to the first embodiment, referring to the first correction table DT2, the controller110corrects the acquired discharge starting voltage Vth in accordance with the environment value Sn and calculates an environment-correction discharge starting voltage Vth1[n]. The controller110stores the acquired environment-correction discharge starting voltage Vth1[n] in the memory111. Also, based on the environment-correction discharge starting voltage Vth1[n], the controller110sets an electrification voltage Vac that is a peak-to-peak voltage of the AC voltage from the AC power source102.

Of the photoconductive drum61, as indicated by the solid line in the graph ofFIG. 13, the thickness of the photosensitive layer615in an initial state is M1μm and approximately uniform in an axial direction of the photoconductive drum61. However, when the photoconductive drum61rotates in an operation of the image forming apparatus1such as printing, the surface of the photoconductive drum61is abraded. Consequently, as the number of rotations of the photoconductive drum61increases, the thickness of the photosensitive layer615is reduced. At positions on the surface of the photoconductive drum61, amounts of accumulated toner are different in accordance with an image to be formed. For such a reason, as indicated by the dot-dash line in the graph ofFIG. 13, when the average thickness of the photosensitive layer615is reduced to a thickness M2(M2<M1) μm, the thickness of the photosensitive layer615lacks uniformity in the axial direction of the photoconductive drum61.

In other words, as the number of rotations of the photoconductive drum61increases, the thickness of the photosensitive layer615decreases, and at the same time, the thickness of the photosensitive layer615becomes uneven in the axial direction of the photoconductive drum61. When the electrification voltage Vac set as described above is applied to the electrification roller621at the time of image formation (printing processing), unevenness (deviation) of the thickness of the photosensitive layer615on the photoconductive drum61causes defective electrification at a portion of the photosensitive layer615increased in thickness.

In the second embodiment, at the time of image formation (printing processing), the controller110predicts the thickness deviation of the photosensitive layer615from the number of rotations of the photoconductive drum61, and corrects the electrification voltage Vac at the time of image formation (printing processing) in accordance with the maximum thickness of the photosensitive layer615on the photoconductive drum61. Consequently, in the image formation, the controller110notifies the power source unit100of the electrification voltage Vac1corrected in accordance with the thickness deviation of the photoconductive drum61. Thus, the AC voltage applied to the electrification roller621by the power source unit100has a dischargeable amplitude Vac1/2even at a portion of the photosensitive layer615on the photoconductive drum61that has the maximum thickness.

The correction processing of the electrification voltage Vac in the image formation will now be described. When the printing processing (image formation) starts, the controller110confirms the number of rotations of the photoconductive drum61. At this time, for example, the controller110measures operation time of a motor (not shown) to give torque to the photoconductive drum61and the rotation rate of the motor. The operation time and the rotation rate of the motor, and the drum diameter of the photoconductive drum61are used for calculation to acquire the number of rotations of the photoconductive drum61. This number of rotations of the photoconductive drum61may be stored in the memory111each time the calculation is executed by the controller110.

The controller110refers to the third correction table DT5in the memory111, and based on the acquired number of rotations of the photoconductive drum61, the controller110acquires a third correction value Vz, and retrieves the electrification voltage Vac stored in the memory111. In the third correction table DT5in the example ofFIG. 12, when the number of rotations of the photoconductive drum61is less than 400,000 rotations (400 krot), the third correction value Vz is 0 V. When the number of rotations of the photoconductive drum61is equal to or more than 400,000 rotations, the third correction value Vz is 15 V. Each time the number of rotations of the photoconductive drum61increases by 100,000 rotations, the third correction value Vz increases by 5 V. When the number of rotations of the photoconductive drum61is equal to or more than 800,000 rotations, the third correction value Vz is 35 V.

The controller110corrects the electrification voltage Vac by adding the third correction value Vz, and notifies the power source unit100of the resultant value Vac+Vz as a thickness-correction electrification voltage Vac1. Therefore, the AC power source102outputs an AC voltage peak-to-peak voltage of which is the thickness-correction electrification voltage Vac1. That is, the power source unit100outputs an AC voltage having an amplitude of Vac1/2(AC voltage of Vg±Vac1/2) with a DC voltage Vg from the DC power source101being a central voltage. The AC voltage is applied to the electrification roller621.

In the second embodiment, the controller110predicts the thickness deviation of the photosensitive layer615from the number of rotations of the photoconductive drum61, and the memory111stores the third correction table DT5shown inFIG. 12. However, based on the calculated discharge starting voltage Vth, the thickness deviation of the photosensitive layer615may be predicted. Specifically, as the thickness of the photosensitive layer615decreases, the discharge starting voltage Vth decreases. Therefore, it is predicted that when the discharge starting voltage Vth is low, the thickness deviation of the photosensitive layer615will be large.

At this time, for example, as shown inFIG. 14, the memory111stores a third correction table DT5ain place of the above-described third correction table DT5. Then, the controller110assumes the discharge starting voltage Vth0in the first measurement as a reference. Also, when acquiring the discharge starting voltage Vth acquired in the second and subsequent measurement, the controller110refers to the third correction table DT5a. Thus, based on a decrease amount of the discharge starting voltage Vth from the reference voltage Vth0, the controller110may acquire the third correction value Vz.

In the example shown inFIG. 14, when the decrease amount of the discharge starting voltage Vth from the reference voltage Vth0is less than 150 V, the third correction value Vz is 0 V. When the decrease amount of the discharge starting voltage Vth from the reference voltage Vth0is equal to or more than 150 V, the third correction value Vz is 15 V. Further, each time the decrease amount of the discharge starting voltage Vth from the reference voltage Vth0increases by 50 V, the third correction value Vz increases by 5 V. When the decrease amount of the discharge starting voltage Vth from the reference voltage Vth0is equal to or more than 400 V, the third correction value Vz is 35 V. As in a third correction table DT5bshown inFIG. 15, the reference voltage Vth0of the discharge starting voltage Vth may be set at a fixed value (1800 V in the example ofFIG. 15).

Third Embodiment

An image forming apparatus according to a third embodiment of the present invention will be described below with reference to the drawings.FIG. 16is a diagram illustrating a configuration of tables stored in a memory in the image forming apparatus according to the third embodiment. In the third embodiment, the same components and operations as in the first embodiment will be denoted by the same reference numerals and will not be elaborated here.

In the image forming apparatus1according to the third embodiment, as shown inFIG. 16, similarly to the first embodiment (seeFIG. 6), the memory111stores a measurement voltage setting table (first setting table) DT1, a discharge starting voltage correction table (first correction table) DT2, a measurement voltage correction table (second correction table) DT3, and a measurement voltage setting table (second setting table) DT4. The memory111further stores a measurement voltage setting table (third setting table) DT6for setting a DC voltage Vg1in the measurement in accordance with the number of rotations of the photoconductive drum61.

The third embodiment is different from the first and second embodiments in that in measurement operation of a discharge starting voltage Vth, the DC voltage from the DC power source101is changed based on the thickness of the photosensitive layer615on the photoconductive drum61. Specifically, in the measurement operation of the discharge starting voltage Vth at each predetermined timing, the controller110confirms the number of rotations of the photoconductive drum61, and refers to the third setting table DT6to set an absolute value |Vg1| of the DC voltage (DC voltage for measurement, hereinafter referred to as measurement DC voltage) from the DC power source101. This measurement DC voltage (absolute value) |Vg1| is set with an absolute value |Vg| of the DC voltage (DC voltage for printing, hereinafter referred to as printing DC voltage) as a reference value. The absolute value |Vg| of the printing DC voltage is constant at the time of image formation (printing processing). As the number of rotations of the photoconductive drum61increases, the absolute value |Vg1| decreases.

In the third setting table DT6in the example ofFIG. 16, when the number of rotations of the photoconductive drum61is less than 400,000 rotations (400 krot), the measurement DC voltage (absolute value) |Vg1| is equal to the printing DC voltage (absolute value) |Vg|. When the number of rotations of the photoconductive drum61is equal to or more than 400,000 rotations, the measurement DC voltage (absolute value) |Vg1| is a voltage value (|Vg|−50) V. Each time the number of rotations of the photoconductive drum61increases by 100,000 rotations, the measurement DC voltage (absolute value) |Vg1| decreases by 50 V. When the number of rotations of the photoconductive drum61is equal to or more than 800,000, the measurement DC voltage (absolute value) |Vg1| is a voltage value (|Vg|−250) V.

In the third embodiment, in the measurement operation of the discharge starting voltage Vth, the controller110sets the measurement DC voltage (absolute value) |Vg1| to decrease as the number of rotations of the photoconductive drum61increases. In the measurement operation of the discharge starting voltage Vth when the thickness of the photosensitive layer615is small, an AC voltage having peak voltage higher than the electrification voltage Vac is applied from the AC power source102. Even in this case, a potential difference between the photoconductive drum61and the electrification roller621is decreased. Therefore, even if the thickness of the photosensitive layer615is small in the application of the AC voltage having peak voltage higher than the electrification voltage Vac from the AC power source102at the time of the measurement, generation of leak current with respect to the photoconductive drum61is suppressed to prevent damage to the photoconductive drum61.

In the third embodiment, the controller110predicts the thickness of the photosensitive layer615from the number of rotations of the photoconductive drum61, and the memory111stores the third setting table DT6shown inFIG. 16. However, prediction of the thickness of the photosensitive layer615may be executed based on the calculated discharge starting voltage Vth. In this case, as shown inFIG. 17, the memory111stores a third setting table DT6ain place of the above-described third setting table DT6.

In the example ofFIG. 17, when a decrease amount of the discharge starting voltage Vth from the reference voltage Vth0is less than 150 V, the measurement DC voltage Vg1is −500 V. When the decrease amount of the discharge starting voltage Vth from the reference voltage Vth0is equal to or more than 150 V, the measurement DC voltage Vg1is −450 V. Each time the decrease amount of the discharge starting voltage Vth from the reference voltage Vth0increases by 50 V, the measurement DC voltage Vg1increases by −50 V. When the decrease amount of the discharge starting voltage Vth from the reference voltage Vth0is equal to or more than 400 V, the measurement DC voltage Vg1is −250 V.

In the third embodiment, based on the thickness of the photosensitive layer615on the photoconductive drum61, the measurement DC voltage is changed in stages. However, irrespective of the thickness of the photosensitive layer615, the absolute value of the measurement DC voltage Vg1may be set to be lower than the printing DC voltage Vg by a constant value. For example, the measurement DC voltage (absolute value) |Vg1| is set to be lower than the printing DC voltage (absolute value) |Vg| constantly by approximately 200 V.

Moreover, in the third embodiment, the memory111may store the electrification voltage correction table (third correction table) DT5similarly to the second embodiment. At the time of image formation (printing processing), based on the predicted thickness deviation of the photosensitive layer615, the electrification voltage Vac may be corrected. Thus, the AC voltage applied to the electrification roller621by the power source unit100has a dischargeable amplitude Vac1/2at a portion of the photosensitive layer615on the photoconductive drum61that has the maximum thickness.

The image forming apparatus according to the embodiment of the present invention may be a multifunction peripheral (MFP) having a copy function, a scanner function, a printer function, and a fax function. Also, the image forming apparatus may be a printer or a copying machine or a facsimile.