Developing device and image forming apparatus provided with same

A developing device includes a development housing, a developer carrier, a toner carrier, a bias applying unit, a leakage detecting unit, a bias control unit and a leakage detection control unit. The developer carrier carries a developer layer. The toner carrier receives the toner from the developer layer and supplies the toner to an image carrier. The bias applying unit includes one transformer and applies direct-current voltages and alternating-current voltages to the developer carrier and the toner carrier. The leakage detecting unit detects leakage occurring between the image carrier and the toner carrier or between the toner carrier and the developer carrier. The leakage detection control unit detects a value of an inter-peak voltage, at which the leakage occurs, by applying the same direct-current voltage to the toner carrier and the developer carrier and changing the inter-peak voltages of the alternating-current voltages.

INCORPORATION BY REFERENCE

This application is based on Japanese Patent Application No. 2014-052913 filed with the Japan Patent Office on Mar. 17, 2014, the contents of which are hereby incorporated by reference.

BACKGROUND

The present disclosure relates to a developing device and an image forming apparatus provided with the same.

An image forming apparatus adopting an electrophotographic method such as a copier, a printer or a facsimile machine forms a toner image on an image carrier (e.g. photoconductive drum or transfer belt) by supplying toner to an electrostatic latent image formed on the image carrier to develop the electrostatic latent image. A touch-down development method using a two-component developer containing nonmagnetic toner and magnetic carrier is known as one of methods for performing the above development. In this case, a two-component developer layer (so-called magnetic brush layer) is carried on a magnetic roller, the toner is transferred from the two-component developer layer onto a developing roller and a toner layer is carried on the developing roller. Further, the electrostatic latent image is visualized by the supply of the toner from the toner layer to the image carrier. Conventionally, there has been known a technology on a leakage detecting operation for detecting a leakage voltage, at which leakage occurs, by changing inter-peak voltage of alternating-current voltages in a developing device adopting the touch-down development method.

SUMMARY

A developing device according to one aspect of the present disclosure includes a development housing, a developer carrier, a toner carrier, a bias applying unit, a leakage detecting unit, a bias control unit and a leakage detection control unit. The development housing stores a developer containing toner to be charged to a predetermined polarity and carrier. The developer carrier receives the developer in the development housing and carries a developer layer by being rotated. The toner carrier receives the toner from the developer layer, carries a toner layer and supplies the toner to an image carrier having an electrostatic latent image formed on a surface and carrying a toner image to be developed by the toner by being rotated in a state in contact with the developer layer. The bias applying unit includes one transformer and applies direct-current voltages and alternating-current voltages having the same frequency and phases opposite to each other to the developer carrier and the toner carrier. The leakage detecting unit detects leakage occurring between the image carrier and the toner carrier or leakage occurring between the toner carrier and the developer carrier. The bias control unit provides a predetermined potential difference of the direct-current voltages between the toner carrier and the developer carrier and applies the alternating-current voltages so that the toner is transferred from the developer carrier to the toner carrier by controlling the bias applying unit during a developing operation in which the toner is supplied from the toner carrier to the image carrier. The leakage detection control unit detects a value of an inter-peak voltage, at which the leakage occurs, by applying the same direct-current voltage to the toner carrier and the developer carrier and changing the inter-peak voltages in a state where a ratio of the inter-peak voltages of the alternating-current voltages applied to the toner carrier and the developer carrier is kept constant during a leakage detecting operation different from the developing operation.

An image forming apparatus according to another aspect of the present disclosure includes the above developing device and the image carrier configured to carry the electrostatic latent image and the toner image.

DETAILED DESCRIPTION

Hereinafter, an embodiment of the present disclosure is described in detail based on the drawings. Note that the present disclosure can be applied to an image forming apparatus adopting an electrophotographic method such as a copier, a printer, a facsimile or a complex machine provided with these functions.

FIG. 1is a front view in section showing the structure of an image forming apparatus1according to one embodiment of the present disclosure. The image forming apparatus1includes an image forming station12, a fixing device13, a sheet feeding unit14, a sheet discharging unit15, a document reading unit16and the like in an apparatus main body11.

The apparatus main body11includes a lower main body111, an upper main body112arranged to face the lower main body111from above and a coupling portion113interposed between these upper and lower main bodies112,111. The coupling portion113is a structure for coupling the lower and upper main bodies111,112to each other in a state where the sheet discharging unit15is formed between the both, and stands on a left part and a rear part of the lower main body111to be L-shaped in a plan view. The upper main body112is supported on an upper end part of the coupling portion113.

The image forming station12, the fixing device13and the sheet feeding unit14are housed in the lower main body111and the document reading unit16is housed in the upper main body112.

The image forming station12performs an image forming operation of forming a toner image on a sheet P fed from the sheet feeding unit14. The image forming station12includes a magenta unit12M using magenta toner, a cyan unit12C using cyan toner, a yellow unit12Y using yellow toner and a black unit12Bk using black toner successively arranged from an upstream side toward a downstream side in a horizontal direction, an intermediate transfer belt125and a secondary transfer roller196held in contact with the outer peripheral surface of the intermediate transfer belt125.

The unit of each color of the image forming station12integrally includes a photoconductive drum121, a developing device122, a toner cartridge (not shown) containing the toner, a charging device123and a drum cleaning device127. Further, an exposure device124for exposing each photoconductive drum121to light is horizontally arranged below the adjacent developing devices122.

The photoconductive drum121has an electrostatic latent image formed on the circumferential surface thereof and carries a toner image obtained by developing the electrostatic latent image by the toner. The developing device122supplies the toner to an electrostatic latent image on the circumferential surface of the photoconductive drum121rotating in a direction of an arrow to form a toner image corresponding to image data on the circumferential surface of the photoconductive drum121. The toner is appropriately supplied to each developing device122from the toner carrier. The charging device123uniformly charges the circumferential surface of the photoconductive drum121. The exposure device124irradiates the charged circumferential surface of the photoconductive drum121with laser light corresponding to each color based on image data input from a computer or the like or image data obtained by the document reading unit16, thereby forming an electrostatic latent image on the circumferential surface of each photoconductive drum121. Note that the exposure device124irradiates the laser light according to an exposure light amount set in advance in order to form a predetermined latent image potential on the photoconductive drum121. The drum cleaning device127cleans the circumferential surface of the photoconductive drum121by removing the residual toner.

The intermediate transfer belt125is an endless, electrically conductive and soft belt. The intermediate transfer belt125is mounted on a plurality of tension rollers arranged substantially in the horizontal direction. The tension rollers include a drive roller125A arranged near the fixing device13to rotationally drive the intermediate transfer belt125and a driven roller125E arranged at a predetermined distance from the drive roller125A in the horizontal direction and configured to rotate, following the rotation of the intermediate transfer belt125. The intermediate transfer belt125is driven to rotate in a clockwise direction inFIG. 1.

A secondary transfer bias applying unit (not shown) is electrically connected to the secondary transfer roller196. A toner image formed on the intermediate transfer belt125is transferred to a sheet P conveyed from a pair of conveyor rollers192located below by a transfer bias applied between the secondary transfer roller196and the drive roller125A.

The fixing device13includes a heating roller132integrally provided with a heating source and a pressure roller134arranged to face the heating roller132. The fixing device13applies a fixing process to a toner image on a sheet P transferred in the image forming station12. The color-printed sheet P completed with the fixing process is discharged toward a sheet discharge tray151provided on the top of the apparatus main body11through a sheet discharge conveyance path194extending from an upper part of the fixing device13.

The sheet feeding unit14includes a manual feed tray141and a sheet cassette142. The sheet cassette142stores a sheet stack P1formed by stacking a plurality of sheets P. A pickup roller143is provided above the sheet cassette142and feeds the uppermost sheet P of the sheet stack P1stored in the sheet cassette142to a sheet conveyance path190. The manual feed tray141is a tray for manually feeding sheets P one by one toward the image forming station12.

The vertically extending sheet conveyance path190is formed to the left of the image forming station12. The pair of conveyor rollers192are provided at a suitable position in the sheet conveyance path190and conveys a sheet P fed from the sheet feeding unit14toward a secondary transfer nip portion formed by the secondary transfer roller196. The sheet discharging unit15is formed between the lower and upper main bodies111,112. The sheet discharging unit15includes the sheet discharge tray151formed on the upper surface of the lower main body111.

The document reading unit16includes a contact glass161which is mounted in an upper surface opening of the upper main body112and on which a document is to be placed, a document pressing cover162which is free to open and close and presses a document placed on this contact glass161and a scanning mechanism163which scans and reads an image of a document placed on the contact glass161. The scanning mechanism163optically reads an image of a document using an image sensor and generates image data. Further, the apparatus main body11includes an image processing unit (not shown) for generating an image from this image data.

<Configuration of the Developing Device>

Next, the developing device122is described in detail.FIG. 2is a vertical and lateral sectional view schematically showing an internal structure of the developing device122, andFIG. 3is a plan view showing the internal structure of the developing device122. The developing device122includes a development housing80defining an internal space of the developing device122. This development housing80includes a developer storage81for storing a developer containing nonmagnetic toner to be charged to a predetermined polarity and magnetic carrier. As an example, an average particle diameter of the toner is 6.8 μm. Further, a magnetic roller82(developer carrier) arranged above the developer storage81, a developing roller83(toner carrier) arranged to face the magnetic roller82at a position obliquely above the magnetic roller82and a developer regulation blade84arranged to face the magnetic roller82are arranged in the development housing80.

The developer storage81includes two developer storage chambers81a,81bextending in a longitudinal direction of the developing device122. The developer storage chambers81a,81bare partitioned by a partition plate801that is integrally formed to the development housing80and extending in the longitudinal direction, but communicate with each other through communication paths803,804at opposite end parts in the longitudinal direction as shown inFIG. 3. Screw feeders85,86for agitating and conveying the developer by rotating about their axes are housed in the respective developer storage chambers81a,81b. The screw feeders85,86are rotationally driven by an unillustrated driving mechanism, and rotating directions thereof are set to be opposite to each other. In this way, the developer is conveyed in a circulating manner between the developer storage chambers81a,81bwhile being agitated as shown by an arrow inFIG. 3. By this agitation, the toner and the carrier are mixed and the toner is positively charged in this embodiment.

The magnetic roller82is arranged along the longitudinal direction of the developing device122and rotationally driven in a clockwise direction inFIG. 2. A fixed so-called magnet roller (not shown) is arranged in the magnetic roller82. The magnet roll includes a plurality of poles, in this embodiment, a draw-up pole821, a regulating pole822and a main pole823. The draw-up pole821faces the developer storage81, the regulating pole822faces the developer regulation blade84and the main pole823faces the developing roller83. Further, the magnetic roller82is rotated in a direction opposite to the developing roller83(counter direction) at a facing position at a circumferential speed which is 1.5 times as fast as that of the developing roller83.

The magnetic roller82magnetically draws up (receives) the developer onto a circumferential surface82A thereof from the developer storage81by a magnetic force of the draw-up pole821. The magnetic roller82magnetically carries the drawn-up developer as a developer layer (magnetic brush layer) on the circumferential surface82A. With the rotation of the magnetic roller82, the developer is conveyed toward the developer regulation blade84.

The developer regulation blade84is arranged upstream of the developing roller83when viewed in a rotating direction of the magnetic roller82and regulates a layer thickness of the developer layer magnetically adhering to the circumferential surface82A of the magnetic roller82. The developer regulation blade84is a plate member made of a magnetic material and extending along a longitudinal direction of the magnetic roller82and supported by a predetermined supporting member841fixed at a suitable position of the development housing80. Further, the developer regulation blade84has a regulation surface842(i.e. tip surface of the developer regulation blade84) for forming a regulation gap G of a predetermined dimension between the regulation surface842and the circumferential surface82A of the magnetic roller82.

The developer regulation blade84formed of the magnetic material is magnetized by the regulating pole822of the magnetic roller82. In this way, a magnetic path is formed between the regulation surface842of the developer regulation blade84and the regulating pole822, i.e. in the regulation gap G. When the developer layer adhering to the circumferential surface82A of the magnetic roller82is conveyed into the regulation gap G by the draw-up pole821with the rotation of the magnetic roller82, the layer thickness of the developer layer is regulated in the regulation gap G. In this way, the uniform developer layer having a predetermined thickness is formed on the circumferential surface82A.

The developing roller83is arranged to extend along the longitudinal direction of the developing device122and in parallel to the magnetic roller82and rotationally driven in a clockwise direction inFIG. 2. The developing roller83has a circumferential surface83A for carrying a toner layer by receiving the toner from the developer layer while rotating in a state in contact with the developer layer held on the circumferential surface82A of the magnetic roller82. At the time of development during which an developing operation is performed, the developing roller83supplies the toner of the toner layer to the circumferential surface of the photoconductive drum121. In this embodiment, the developing roller83is a roller formed by applying resin coating (urethane coating) to an alumite surface. Further, the developing roller83is rotated in the same direction as the photoconductive drum121(with rotation) at a facing position at a circumferential speed which is 1.3 times as fast as that of the photoconductive drum121.

The developing roller83and the magnetic roller82are rotationally driven by a driving unit962to be described later. A clearance S of a predetermined dimension is formed between the circumferential surface83A of the developing roller83and the circumferential surface82A of the magnetic roller82. The clearance S is, for example, set at 0.3 mm. The developing roller83is arranged to face the photoconductive drum121through an opening formed on the development housing80and a clearance of a predetermined dimension is also formed between the circumferential surface83A and the circumferential surface of the photoconductive drum121. In this embodiment, this clearance is set at 0.12 mm.

Next, a main electrical configuration of the image forming apparatus1is described. The image forming apparatus1(developing device122) includes a control unit90for comprehensively controlling the operation of each component of the image forming apparatus1.FIG. 4is a functional block diagram of the control unit90.FIG. 5is a diagram showing the developing operation of the developing device122according to this embodiment. The control unit90is composed of a CPU (Central Processing Unit), a ROM (Read Only Memory) storing a control program, a RAM (Random Access Memory) used as a work area of the CPU and the like. Further, a development bias applying unit88(bias applying unit), a leakage detecting unit89, the driving unit962, an image memory963, an I/F964and the like are electrically connected to the control unit90in addition to each member of the developing device122.

With reference toFIG. 5, the development bias applying unit88is composed of a direct-current power supply and an alternating-current power supply and applies development biases, in which an alternating-current voltage is superimposed on a direct-current voltage, to the magnetic roller82and the developing roller83in the developing device122based on a control signal from a bias control unit92or a leakage detection control unit93to be described later. In this embodiment, the development bias applying unit88is composed of one transformer. In other words, development biases are applied to the magnetic roller82and the developing roller83from the common development bias applying unit88and a specific bias applying unit (transformer) is not arranged for each of the magnetic roller82and the developing roller83. Thus, the developing device122is inexpensively configured. The development bias applying unit88applies direct-current voltages and alternating-current voltages having the same frequency and phases opposite to each other to the magnetic roller82and the developing roller83.

With reference toFIG. 5, the development bias applying unit88includes an alternating current applying unit88A, a first direct current applying unit88B and a second direct current applying unit88C. Two terminals from which development biases are output are arranged in the development bias applying unit88. One terminal is a first terminal K1and the other is a second terminal K2. The development bias is applied to the magnetic roller82via the first terminal K1and applied to the developing roller83via the second terminal K2.

The leakage detecting unit89(FIG. 5) is electrically connected to the development bias applying unit88. The leakage detecting unit89detects leakage occurring between the photoconductive drum121and the developing roller83or between the developing roller83and the magnetic roller82. At this time, the leakage detecting unit89detects leakage based on a variation of the value of a current (overcurrent) flowing in the developing roller83.

The driving unit962(FIG. 4) is composed of a motor and a gear mechanism for transmitting a torque of the motor and rotationally drives the developing roller83, the magnetic roller82and the screw feeders85,86in the developing device122in addition to the photoconductive drum121during a developing operation and a leakage detecting operation in accordance with a control signal from the control unit90. In this embodiment, the developing roller83, the magnetic roller82and the screw feeders85,86are rotationally driven in synchronization by the driving unit962.

The image memory963temporarily stores image data to be printed given from an external apparatus such as a personal computer when this image forming apparatus1functions as a printer. Further, the image memory963temporarily stores image data optically read by an ADF (Auto Document Feeder) when the image forming apparatus1functions as a copier.

The I/F964is an interface circuit for realizing data communication with external apparatuses and, for example, generates a communication signal conforming to a communication protocol of a network connecting the image forming apparatus1and the external apparatuses and converts a communication signal from a network side into data of a format processable by the image forming apparatus1. A print instruction signal transmitted from a personal computer or the like is given to the control unit90via the I/F964and image data is stored in the image memory963via the I/F964.

The control unit90functions to include the drive control unit91, the bias control unit92and the leakage detection control unit93by the CPU executing the control program stored in the ROM.

The drive control unit91rotationally drives the developing roller83, the magnetic roller82and the screw feeders85,86by controlling the driving unit962. Further, the drive control unit91rotationally drives the photoconductive drum121by controlling an unillustrated drive mechanism. In this embodiment, the drive control unit91rotationally drives each of the above members in a developing operation during an image forming operation and a leakage detecting operation.

The bias control unit92provides a potential difference of a direct-current voltage between the magnetic roller82and the developing roller83by controlling the development bias applying unit88during the developing operation in which the toner is supplied from the magnetic roller82to the developing roller83and further from the developing roller83to the photoconductive drum121. The toner is transferred from the magnetic roller82to the developing roller83by the above potential difference. Further, the bias control unit92applies alternating-current voltages having the same frequency and phases opposite to each other to the magnetic roller82and the developing roller83during the developing operation. Note that duty ratios of the alternating-current voltages are fixed. The transfer of the toner from the magnetic roller82to the developing roller83is promoted by the alternating-current voltages. Further, the toner is transferred from the developing roller83to the photoconductive drum121by the above development bias applied to the developing roller83. The development biases during the developing operation are described in detail later.

The leakage detection control unit93applies direct-current voltages and alternating-current voltages having opposite phases to the magnetic roller82and the developing roller83by controlling the development bias applying unit88during the leakage detecting operation. In the leakage detecting operation, an inter-peak voltage of the alternating-current voltage that leaks between the photoconductive drum121and the developing roller83or between the magnetic roller82and the developing roller83is detected out of the development bias applied to the developing roller83. At this time, the leakage detection control unit93causes leakage to occur between the photoconductive drum121and the developing roller83or between the magnetic roller82and the developing roller83while increasing the inter-peak voltages of the alternating-current voltages of the development biases. The leakage detecting operation is performed prior to the developing operation and the inter-peak voltage (leakage causing voltage) at which leakage occurs is detected. Then, during the developing operation, the inter-peak voltages of the alternating-current voltages are set in a range not reaching the leakage causing voltage and the occurrence of leakage is prevented. Note that the development biases during the leakage detecting operation are described in detail later.

<Concerning the Developing Operation>

Next, a development mechanism of an electrostatic latent image on the photoconductive drum121in the developing operation is described with reference toFIGS. 5 and 6.FIG. 6is a diagram showing the waveforms of development biases applied to the magnetic roller82and the developing roller83during the developing operation of the developing device122according to this embodiment. A section (A) ofFIG. 6shows the waveform of one cycle of the alternating-current voltage of the development bias applied to the developing roller83and a section (B) ofFIG. 6shows the waveform of one cycle of the alternating-current voltage of the development bias applied to the magnetic roller82. Note that the sections (A) and (B) ofFIG. 6show positions adjusted in the vertical direction (bias magnitude indicating direction) to relatively compare a magnitude relationship of direct-current biases. The image forming apparatus1according to this embodiment has a print speed of 25 pages/min. A circumferential speed of the photoconductive drum121is set at 120 mm/sec. Further, in this embodiment, coating ferrite carrier having a volume specific resistance of 1010Ω·m, a saturation magnetization of 65 emu/g and an average particle diameter of 35 μm is used as the carrier in the developer. As described above, the bias control unit92controls the development bias applying unit88to apply development biases in the case of performing the developing operation of the developing device122in the image forming operation of the image forming apparatus1.

With reference toFIG. 5, the magnetic brush layer on the circumferential surface82A of the magnetic roller82is conveyed toward the developing roller83with the rotation of the magnetic roller82after a layer thickness thereof is uniformly regulated by the developer regulation blade84(FIG. 2). Thereafter, a multitude of magnetic bristles DB in the magnetic brush layer come into contact with the circumferential surface83A of the developing roller83in rotation in an area where the magnetic roller82and the developing roller83face each other.

At this time, the bias control unit92applies development biases, each composed of a direct-current voltage and an alternating-current voltage as described above, to the magnetic roller82and the developing roller83by controlling the development bias applying unit88. This causes a predetermined potential difference (development potential difference ΔV, difference between Vsldcof the section (A) ofFIG. 6and Vmgdcof the section (B) ofFIG. 6) between the circumferential surface82A of the magnetic roller82and the circumferential surface83A of the developing roller83. The development potential difference ΔV is set in a range of 100 V to 350 V depending on an environment and the like. The toner layer on the developing roller83is thick if ΔV is large, and the toner layer on the developing roller83is thin if ΔV is small. Due to this potential difference, only toner particles T are transferred from the magnetic bristles DB to the circumferential surface83A at the facing position of the circumferential surfaces82A and83A (facing position of the main pole823(FIG. 2) and the circumferential surface83A) and the carrier particles C and the remaining toner particles of the magnetic bristles DB remain on the circumferential surface82A. In this way, a toner layer TL having a predetermined thickness is carried on the circumferential surface83A of the developing roller83.

The toner layer TL on the circumferential surface83A is conveyed toward the circumferential surface of the photoconductive drum121with the rotation of the developing roller83. A superimposed voltage of a direct-current voltage and an alternating-current voltage is applied to the developing roller83. Thus, a predetermined potential difference is generated between the circumferential surface of the photoconductive drum121having a potential on the surface according to the electrostatic latent image and the circumferential surface83A of the developing roller83. Due to this potential difference, the toner particles T of the toner layer TL are transferred to the circumferential surface of the photoconductive drum121. In this way, the electrostatic latent image on the circumferential surface of the photoconductive drum121is developed to form a toner image.

Note that examples of the development biases applied to the magnetic roller82and the developing roller83by controlling the development bias applying unit88during the developing operation by the bias control unit92are as follows.

Duty ratio (Duty 1) of the alternating-current voltage of the developing roller83; 27%

Duty ratio (Duty 2) of the alternating-current voltage of the magnetic roller82; 73%

Image part potential VL of the photoconductive drum121: +100 V

Background part potential Vo of the photoconductive drum121; +430 V

On the other hand,FIG. 8shows potential conditions of the magnetic roller82, the developing roller83and the photoconductive drum121when the above development biases and potentials on the photoconductive drum121are set.

A potential relationship during the developing operation is further described in detail with reference toFIGS. 8 and 6. As shown inFIG. 6, the alternating-current voltages of the development biases applied to the magnetic roller82and the developing roller83are set to have opposite phases during the developing roller. Thus, a cyclic potential difference based on the alternating-current voltages is set between the magnetic roller82and the developing roller83in addition to the aforementioned development potential difference ΔV composed of a direct-current voltage. With reference to the section (A) ofFIG. 6, a direct-current bias Vsldcof 250 V and an alternating-current bias Vslac of 1000 V including an inter-peak voltage are applied to the developing roller83. At this time, since a duty ratio (Duty 1) on a positive side of the alternating-current bias is 27%, a peak voltage Vslpp1on the positive side of the alternating-current bias of the developing roller83is 730 V. As a result, a maximum value Vmaxsl of the alternating-current voltage is 250+730=980 V (FIG. 8). Similarly, a peak voltage Vslpp2on a negative side of the alternating-current bias of the developing roller83is 270 V. As a result, a minimum value Vminsl of the alternating-current voltage is 250−270=−20 V (FIG. 8).

At this time, the image part voltage VL of the photoconductive drum121is set at +100 V and the background part potential VL is set at +430V as described above. Thus, a potential difference of the direct-current bias between the developing roller83and the photoconductive drum121(interval DS) is Vsldc−VL=150 V. Further, since the alternating-current bias is applied to the developing roller83, a potential difference between an image part of the photoconductive drum121and the developing roller83is Vmaxsl−VL=980−100=880 V (FIG. 8). Further, a potential difference between a background part of the photoconductive drum121and the developing roller83is Vo−Vminsl=430−(−20)=450 V (FIG. 8).

With reference to the section (B) ofFIG. 6, a direct-current bias Vmgdc of 550 V and an alternating-current bias Vmgac of 600 V including an inter-peak voltage are applied to the magnetic roller82. At this time, since a duty ratio (Duty 2) on a positive side of the alternating-current bias is 73%, a peak voltage Vmgpp1on the positive side of the alternating-current bias of the magnetic roller82is 600×0.27=162 V. As a result, a maximum value Vmaxmg of the alternating-current voltage is 550+162=712 V (FIG. 8). Similarly, a peak voltage Vmgpp2on a negative side of the alternating-current bias of the magnetic roller82is 438 V. As a result, a minimum value Vminmg of the alternating-current voltage is 550−438=112 V (FIG. 8).

As described above, the potentials shown in the section (A) ofFIG. 6are set for the developing roller83. Thus, a potential difference of the direct-current bias between the developing roller83and the magnetic roller82(interval MS) is Vmgdc−Vsldc=550−250=300 V. Further, since the alternating-current biases are applied to the developing roller83and the magnetic roller82, a potential difference on a return side for collecting the toner from the developing roller83to the magnetic roller82is Vmaxsl−Vminmg=980−112=868 V (FIG. 8). Further, a potential difference on a feed side for supplying the toner from the magnetic roller82to the developing roller83is Vmaxmg−Vminsl=712−(−20)=732 V (FIG. 8).

By setting the potential differences as described above, the transfer of the toner from the magnetic roller82to the developing roller83and from the developing roller83to the photoconductive drum121is promoted. Thus, the development biases can be stably applied to the magnetic roller82and the developing roller83by the development bias applying unit88including a single transformer.

On the other hand, if specific bias applying units (transformers) are provided for the magnetic roller82and the developing roller83unlike the developing device122according to this embodiment, specific development biases can be applied to the magnetic roller82and the developing roller83in performing the developing operation. Further, specific development biases can be applied to the magnetic roller82and the developing roller83also in detecting a leakage causing voltage at which leakage occurs between the photoconductive drum121and the developing roller83and between the developing roller83and the magnetic roller82. Thus, it becomes possible to suppress the transfer of the toner from the magnetic roller82to the developing roller83during the leakage detecting operation and perform the leakage detecting operation in a state where the surface of the developing roller83is maximally exposed. Particularly, in the case of including the specific transformers, the transfer of the toner from the magnetic roller82to the developing roller83can be prevented by reversing the magnitude relationship of the direct-current biases applied to the magnetic roller82and the developing roller83during the developing operation. On the other hand, the cost of the developing device122is largely increased in the case of including the specific bias applying unit (transformer) for each of the magnetic roller82and the developing roller83in this way.

In this embodiment, the leakage detecting operation of the developing device122can be stably performed utilizing the development bias applying unit88composed of one transformer as described above.FIG. 7is a diagram showing the waveforms of development biases applied to the magnetic roller82and the developing roller83during the leakage detecting operation of the developing device122according to this embodiment. A section (A) ofFIG. 7shows the waveform of one cycle of an alternating-current voltage of the development bias applied to the developing roller83and a section (B) ofFIG. 7shows the waveform of one cycle of an alternating-current voltage of the development bias applied to the magnetic roller82. Note that the sections (A) and (B) ofFIG. 7show positions adjusted in the vertical direction (bias magnitude indicating direction) to relatively compare a magnitude relationship of direct-current biases.

The leakage detection control unit93(FIG. 4) performs the leakage detecting operation at a timing different from that during the imaging forming operation (during the developing operation), i.e. when the image forming apparatus1is shipped, when the developing device122or the photoconductive drum121is exchanged, when an environment (temperature, humidity) around the image forming apparatus1is changed or when a predetermined number of printing operations have been performed. In the leakage detecting operation, the leakage detection control unit93rotationally drives the photoconductive drum121and each member of the developing device122by controlling the drive control unit91. Further, the leakage detection control unit93forms an electrostatic latent image on the photoconductive drum121(potential VL on the photoconductive drum121) by controlling the charging device123and the exposure device124. Then, the leakage detection control unit93detects an inter-peak voltage, at which leakage occurs, by detecting an overcurrent by the leakage detecting unit89while increasing (changing) the inter-peak voltages of the alternating-current voltages applied to the developing roller83and the magnetic roller82.

Examples of the development biases applied to the magnetic roller82and the developing roller83by controlling the development bias applying unit88during the leakage detecting operation by the leakage detection control unit93are as follows.

(where Vmgacand Vslacare respectively made variable with a ratio thereof fixed at a ratio of voltage values during the developing operation, i.e. 600:1000)

Duty ratio (Duty 1) of the alternating-current voltage of the developing roller83; 27%

Duty ratio (Duty 2) of the alternating-current voltage of the magnetic roller82; 73%

Image part potential VL of the photoconductive drum121: +100 V

Background part potential Vo of the photoconductive drum121; +430 V

Note that the leakage detecting operation is performed at the image part potential VL on the photoconductive drum121. The background part potential Vo of the photoconductive drum121is a potential as a prerequisite for setting the image part potential VL by the exposure device124.FIG. 9shows potential conditions of the magnetic roller82, the developing roller83and the photoconductive drum121when the above development biases during the leakage detecting operation and the potentials on the photoconductive drum121are set. Note that calculation methods for the respective numerical values are omitted since they are similar to those during the previous developing operation.

As shown inFIG. 7, in this embodiment, the direct-current voltage Vmgdcof the magnetic roller82and the direct-current voltage Vsldcof the developing roller83are set at the same value in the leakage detecting operation. Particularly, as compared with the developing operation, the direct-current voltage Vsldcof the developing roller83is set to have the same value as the direct-current voltage Vmgdcof the magnetic roller82. Characteristics of the direct-current voltage Vmgdcof the magnetic roller82and the direct-current voltage Vsldcof the developing roller83during this leakage detecting operation are further described. With reference toFIGS. 6 and 8, leakage that occurs in the interval DS (between the photoconductive drum121and the developing roller83) during the developing operation is mainly in the image part. Specifically, leakage occurs when a potential difference VRd(DS) of the section (A) ofFIG. 6is large. Further, leakage that occurs in the interval MS (between the magnetic roller82and the developing roller83) during the developing operation is mainly on a return side. Specifically, leakage occurs when a potential difference VRd(MS) of the sections (A), (B) ofFIG. 6is large. As described above, since a single transformer is used as the development bias applying unit88in this embodiment, a ratio of the inter-peak voltages of the alternating-current biases applied to the magnetic roller82and the developing roller83is constant. This ratio is determined by a ratio of numbers of turns of predetermined coils in the development bias applying unit88.

Accordingly, the inter-peak voltages are increased in a state where a ratio of the inter-peak voltages of the alternating-current biases applied to the magnetic roller82and the developing roller83is kept constant also during the leakage detecting operation as during the developing operation. On the other hand, it is desirable to remove the toner adhering on the developing roller83in the leakage detecting operation as described above. This is because the toner becomes resistance to cause an error in the leakage causing voltage if a large amount of toner adheres on the developing roller83. It is thought to reverse the magnitude relationship of Vsldcand Vmgdcin the sections (A), (B) ofFIG. 6in performing the leakage detecting operation in order to prevent the above toner adhesion. However, in this case, a balance between VRd(DS) and VRd(MS) largely varies as the direct-current biases are shifted. If Vmgdcin the section (B) ofFIG. 6is set to be lower than Vsldcin the section (A) ofFIG. 6by 100 V as an example, the transfer of the toner from the magnetic roller82to the developing roller83is suppressed. However, in this case, a value of the VRd(DS) does not change, but a value of VRd(MS) becomes larger by Vmgdc−Vsldc+100 V. As just described, if the leakage detecting operation is performed in a state where a balance of VRd(DS) and VRd(MS) is largely varied, leakage first occurs in the interval MS although it is supposed to first occur in the interval DS. Thus, it becomes difficult to perform a highly accurate leakage detecting operation assuming the developing operation.

The discloser of the present disclosure newly found out a control of performing a stable leakage detecting operation in a state where a balance of VRd(DS) and VRd(MS) is kept in a predetermined range while toner adhesion to the developing roller83during the leakage detecting operation is prevented. Specifically, in this embodiment, the direct-current voltage Vsldcof the developing roller83is set to have the same value as the direct-current voltage Vmgdcof the magnetic roller82during the leakage detecting operation as compared with during the developing operation as described above (FIG. 7). As shown inFIG. 9, if an alternating-current bias of an inter-peak voltage of 1000 V is applied to the developing roller83and an alternating-current bias of an inter-peak voltage of 600 V is applied to the magnetic roller82during the leakage detecting operation, a potential difference in the image part in the interval DS (between the photoconductive drum121and the developing roller83) (VRe(DS) in the section (A) ofFIG. 7) is 1180 V. Similarly, a potential on the return side in the interval MS (between the magnetic roller82and the developing roller83) (VRe(MS) in the sections (A), (B) ofFIG. 7) is 1168 V. If these potential differences are compared with VRd(DS) and VRd(MS) during the developing operation with reference toFIGS. 8 and 6, VRe(DS)−VRd(DS)=300V, VRe(MS)−VRd(MS)=300V. Specifically, a balance of VRe(DS) and VRe(MS) during the leakage detecting operation can maintain a relationship similar to a balance of VRd(DS) and VRd(MS) during the developing operation. Note that, in order to maintain a balance of VRd(DS) and VRd(MS) during the developing operation in this way, the direct-current voltage Vsldcof the developing roller83and the direct-current voltage Vmgdcof the magnetic roller82during the leakage detecting operation are desirably set at the same value as the direct-current voltage Vmgdcof the magnetic roller82during the developing operation.

Since the magnetic roller82and the developing roller83are set at the same potential by the direct-current biases, the transfer of the toner from the magnetic roller82to the developing roller83is prevented. Thus, the toner hardly adheres to the surface of the developing roller83and leakage between the photoconductive drum121and the developing roller83or between the developing roller83and the magnetic roller82can be detected in a state where the surface of the developing roller83is exposed. Further, the developing operation and the leakage detecting operation are realized by one transformer (development bias applying unit88). As a result, a cost reduction of the developing device122and the image forming apparatus1and space saving are realized and a complicated control circuit is not required.

While maintaining the potential relationship illustrated inFIGS. 7 and 9, the leakage detection control unit93increases the inter-peak voltages of the alternating-current voltages applied to the developing roller83and the magnetic roller82and detects the inter-peak voltage at which leakage occurs. As an example, it is assumed that leakage occurs in the image part in the interval DS at the potential value shown inFIG. 9, i.e. at a potential difference of 1180 V. In this case, the alternating-current bias of the inter-peak voltage of 1000 V is applied to the developing roller83. The inter-peak voltage at this time is assumed to be Va(FIG. 9). Similarly, an alternating-current bias of an inter-peak voltage of 600 V is set for the magnetic roller82. As described above, the direct-current bias Vsldcof the developing roller82is shifted during the leakage detecting operation. Thus, the leakage detection control unit93derives a value of an alternating-current bias (assumed to be Vb) at which a potential difference of 1180 V is generated in the interval DS in the relationship of Vsldcand Vmgdcduring the developing operation using the following equation.
Vb={Vmgdc−Vsldc}+Va×(100−Duty1)/100}/{100−Duty1}/100}  (1)
In Equation (1), Vmgdcand Vsldcare values of the direct-current biases respectively applied during the developing operation.FIG. 10shows Vbwhen Va=1000 V and Duty 1=27% ofFIG. 9are applied to Equation (1) and potential conditions of the magnetic roller82and the developing roller83corresponding to the value of Vb.

As shown inFIG. 10, if Vbderived in Equation (1) is applied to the developing roller83, the same voltage of 1180 V as inFIG. 9is applied to the image part in the interval DS. Thus, it is possible to derive the value of the alternating-current bias (Vb) at which leakage actually occurs during the developing operation after the influence of the direct-current bias Vsldcshifted in the leakage detecting operation is eliminated.

Further, the leakage detection control unit93sets Vcobtained by subtracting a predetermined margin voltage Vt(offset voltage) from the derived Vb=1411 (V) as a development bias associated with the next image forming operation. In this embodiment, the margin voltage is set at 100 V in advance in consideration of a safety rate. Specifically, an inter-peak voltage Vc=Vb−Vt=1411−100=1311 (V) is applied to the developing roller83during the developing operation. Further, an inter-peak voltage of 1311×600/1000=787 (V) is applied to the magnetic roller82. As a result, the occurrence of leakage between the photoconductive drum121and the developing roller83and between the magnetic roller82and the developing roller83is prevented with high accuracy and a stable image forming operation is realized.

Although the developing device122and the image forming apparatus1provided with the same according to the embodiment of the present disclosure have been described above, the present disclosure is not limited to this. For example, the following modifications may be adopted.

(1) In the above embodiment, a mode is described in which the leakage detecting unit89detects leakage based on a variation of the value of the current (overcurrent) flowing in the developing roller83. The present disclosure is not limited to this. The leakage detecting unit89may adopt another mode such as the one in which leakage is detected by detecting the number of times the above current value exceeds a threshold value set in advance.

(2) Further, although a mode is described in which the toner is charged to have a positive polarity in the above embodiment, the present disclosure is not limited to this. Even if the toner is charged to have a negative polarity, it is possible to apply development biases to the developing roller83and the magnetic roller82from a single transformer and perform the leakage detecting operation by executing a control similar to the above. In this case, the surface potential of the photoconductive drum121and the polarities of the development biases applied to the magnetic roller82and the developing roller83may be adjusted according to the polarity of the toner.