Image forming apparatus including a developing device having first conveyance chamber and a second conveyance chamber disposed above the first conveyance chamber

An image forming apparatus includes a developing device, a toner reservoir, a toner replenishing device, a development driving device, a developer amount detecting sensor, and a controller. The developing device includes a developing container, a first stirring-conveyance member, a second stirring-conveyance member, and a developer carrier. The developing container includes a first conveyance chamber and a second conveyance chamber disposed above the first conveyance chamber. The developer amount detecting sensor detects an amount of developer that falls from the second conveyance chamber to the vicinity of a toner replenishing port in the first conveyance chamber. The controller controls the driving of the toner replenishing device or the driving of the development driving device based on a result of detection performed by the developer amount detecting sensor, and thereby keeps a replenished-toner concentration in the developer substantially constant in the vicinity of the toner replenishing port.

INCORPORATION BY REFERENCE

This application is based upon and claims the benefit of priority from the corresponding Japanese Patent Application No. 2017-92351 filed on May 8, 2017, the entire contents of which are incorporated herein by reference.

BACKGROUND

The present disclosure relates to image forming apparatuses such as copiers, printers, facsimile machines, and so on, and in particular, relates to image forming apparatuses including a developing device having a first conveyance chamber and a second conveyance chamber disposed above the first conveyance chamber.

In an image forming apparatus, an electrostatic latent image formed on an image carrier, such as a photosensitive member, is developed by a developing device and visualized as a toner image. A known example of such a developing device is one that includes a developing container which holds therein a developer, first and second stirring-conveyance members which convey the developer, while stirring the developer, and a developing roller (a developer carrier) which carries thereon the developer supplied thereto from the second stirring-conveyance member. The first stirring-conveyance member conveys the developer to one side in the axial direction of the developing roller, and the second stirring-conveyance member supplies the developer to the developing roller while conveying the developer to the other side (the side opposite from the one side).

In recent years, there has been an increasing demand for miniaturization of image forming apparatuses, and in particular, in color image forming apparatuses, in which a plurality of developing devices are disposed, there has been a demand for miniaturization of developing devices. As an example of developing devices meeting such a demand, there is known one that includes a first conveyance chamber inside which a first stirring-conveyance member is disposed and a second conveyance chamber which is disposed above the first conveyance chamber and inside which a second stirring-conveyance member is disposed. By arranging the first conveyance chamber and the second conveyance chamber one above the other in this developing device, it is possible to make it compact in the horizontal direction. Accordingly, it is possible to reduce space for installing the developing device, and thus, it is possible to achieve the miniaturization of image forming apparatuses.

However, in the above-described developing device, there is an area where the developer is conveyed against gravity, from the first conveyance chamber to the second conveyance chamber. With this structure, when change in flowability of the developer results from factors such as durable printing and environmental variation, the circulation balance of the developer is likely to change, and thus uneven distribution of the developer is likely to occur inside the developing device. When, in such a condition, toner is replenished to a portion where only a small amount of developer exists, it will create a portion where the concentration of the replenished toner is locally high. The replenished toner is not sufficiently mixed with a carrier in the developer and thus is low in charge amount, and accordingly, when the portion with a high replenished-toner concentration is used for development, it will result in problems such as fogged images and uneven image density.

To prevent such problems, there have been proposed various methods for fully mixing developer inside a developing device with replenished toner. For example, there has been known a developing device which has a communication path through which the developer is delivered from a developing chamber to a stirring chamber by conveying the developer to fall from a downstream side of the developing chamber to an upstream side of the stirring chamber in the developer conveyance direction, and in which replenished developer replenished through a developer replenishing port above the stirring chamber falls onto an area in the stirring chamber where the communication path joins the stirring chamber.

Furthermore, there has been known a developing device where, for the purpose of maintaining the circulation balance, after executing a low speed mode in which a developer carrier and a conveyance member are driven at a speed lower than usual, an idle driving mode is executed in which the conveyance member is idly driven at a high driving speed for a predetermined time.

SUMMARY

According to one aspect of the present disclosure, an image forming apparatus includes a developing device, a toner reservoir, a toner replenishing device, a development driving device, a developer amount detecting sensor, and a controller. The developing device includes a developing container, a first stirring-conveyance member, a second stirring-conveyance member, and a developer carrier. The developing container includes a first conveyance chamber, a second conveyance chamber disposed above the first conveyance chamber, a first communication portion which allows the first conveyance chamber and the second conveyance chamber to communicate with each other at end portions thereof on a downstream side in a first direction, a second communication portion which allows the first conveyance chamber and the second conveyance chamber to communicate with each other at end portions thereof on a downstream side in a second direction, and a toner replenishing port which is disposed at an end portion of the first conveyance chamber on the downstream side in the second direction for toner replenishment to the developing container. The developing container holds therein a two-component developer including a carrier and a toner. The first stirring-conveyance member is rotatably supported inside the first conveyance chamber, and conveys the developer inside the first conveyance chamber in the first direction, while stirring the developer. The second stirring-conveyance member is rotatably supported inside the second conveyance chamber, and conveys the developer inside the second conveyance chamber in the second direction, which is a direction opposite to the first direction, while stirring the developer. The developer carrier is rotatably supported in the developing container, and carries on a surface thereof the developer in the second conveyance chamber. The toner reservoir holds therein the toner to be replenished to the developing device. The toner replenishing device replenishes the toner from the toner reservoir to the developing device. The development driving device drives the first stirring-conveyance member and the second stirring-conveyance member. The controller controls driving of the toner replenishing device and driving of the development driving device. The developer amount detecting sensor detects an amount of developer conveyed from the second conveyance chamber, through the second communication portion, to fall to the vicinity of the toner replenishing port in the first conveyance chamber. The controller controls driving of the toner replenishing device or driving of the development driving device based on a result of detection performed by the developer amount detecting sensor, and thereby keeps a replenished-toner concentration in the developer substantially constant in the vicinity of the toner replenishing port.

Further features and advantages of the present disclosure will become apparent from the description of embodiments given below.

DETAILED DESCRIPTION

Hereinafter, embodiments of the present disclosure will be described with reference to the accompanying drawings.FIG. 1is a schematic sectional view of a color printer100embodying the present disclosure, and the color printer100illustrated here is a tandem-type color printer. Inside a main body of the color printer100, four image formers Pa, Pb, Pc, and Pd are arrange in this order from a downstream side in a transport direction (the right side inFIG. 1). The image formers Pa to Pd are provided corresponding to images of four different colors (cyan, magenta, yellow, and black), and sequentially form images of cyan, magenta, yellow, and black through charging, exposure, developing, and transfer processes.

The image formers Pa to Pd are each provided with a corresponding one of photosensitive drums1a,1b,1c, and1d, which each carry a visible image (a toner image) of a corresponding color, and further, an intermediate transfer belt8, which rotates in a clockwise direction inFIG. 1, is provided adjacent to the image formers Pa to Pd.

When image data is fed from a host device such as a personal computer, first, charging devices2ato2duniformly charge surfaces of the photosensitive drums1ato1d. Then, an exposure device5irradiates the photosensitive drums1ato1dwith light in accordance with the image data to form an electrostatic latent image on each of the photosensitive drums1ato1din accordance with the image data. Developing devices3ato3dare each filled, by toner containers4ato4d, with a predetermined amount of two-component developer (which hereinafter may be referred to simply as developer) including a toner of a corresponding one of the four colors of cyan, magenta, yellow and black, and the toner included in the developer is supplied by a corresponding one of the developing devices3ato3donto a corresponding one of the photosensitive drums1ato1dto electrostatically adhere thereto. Thereby, a toner image is formed in accordance with the electrostatic latent image, which has been formed by the exposure to the light emitted from the exposure device5.

Then, by primary transfer rollers6ato6d, an electric field is applied at a predetermined transfer voltage between the primary transfer rollers6ato6dand the photosensitive drums1ato1d, and the toner images of cyan, magenta, yellow, and black on the photosensitive drums1ato1dare primarily transferred onto the intermediate transfer belt8. After the primary transfer, residual toner and the like left on the surfaces of the photosensitive drums1ato1dare removed by cleaning devices7ato7d.

Transfer sheets P onto one of which the toner images are to be transferred are stacked in a sheet cassette16disposed in a lower part inside the color printer100, and a transfer sheet P is conveyed at a predetermined timing via a sheet feeding roller12aand a registration roller pair12bto a nip portion (secondary transfer nip portion) between the intermediate transfer belt8and a secondary transfer roller9provided adjacent to the intermediate transfer belt8. The transfer sheet P, after having the toner images transferred thereon, is conveyed to a fixer13.

To the transfer sheet P, which has been transported to the fixer13, heat and pressure is applied by a fixing roller pair13a, and thereby the toner images are fixed to a surface of the transfer sheet P, and thereby a predetermined full-color image is formed. The transfer sheet P, on which the full-color image has been formed, is discharged onto a discharge tray17by a discharge roller pair15as it is (or after being directed by a branching unit14into a reverse transport path18and having an image formed on the other side, too).

FIG. 2is a side sectional view illustrating a structure of the developing device3aincorporated in the color printer100of the first embodiment of the present disclosure. Here, in the following description, only the developing device3adisposed in the image former Pa inFIG. 1will be dealt with as a representative example, and the developing devices3bto3darranged in the image formers Pb to Pd will not be described. This is because the developing devices3bto3dall have basically the same structure as the developing device3a. As illustrated inFIG. 2, the developing device3aincludes a developing roller (developer carrier)20, a stirring-conveyance member42, and a developing container22.

The developing container22forms a housing of the developing device3a, and is divided, by a partition22b, into a first conveyance chamber22cand a second conveyance chamber22d. The first conveyance chamber22cand the second conveyance chamber22dhold therein a two-component developer including a toner and a carrier. The developing container22rotatably holds the stirring-conveyance member42and the developing roller20. Furthermore, in the developing container22, an opening22ais formed through which the developing roller20is exposed toward the photosensitive drum1a.

The stirring-conveyance member42is composed of two spirals, namely, a first spiral (a first stirring-conveyance member)43and a second spiral (a second stirring-conveyance member)44. The first spiral43is disposed below the second spiral44, inside the first conveyance chamber22c. The second spiral44is disposed in the second conveyance chamber22d, which is disposed above the first conveyance chamber22c.

The first and second spirals43and44stir the developer to charge the toner in the developer to a predetermined level. This enables the toner to be held on the carrier. Furthermore, at both end portions of the partition22bin its longitudinal direction (a direction perpendicular to the sheet on whichFIG. 2is drawn), the partition22bdividing the first conveyance chamber22cand the second conveyance chamber22dfrom each other, communication portions (a first communication portion22eand a second communication portion22fwhich will be described later) are disposed. When the first spiral43rotates, the charged developer is conveyed through one of the communication portions, disposed in the partition22b, to the second spiral44, and thereby the developer circulates inside the first conveyance chamber22cand the second conveyance chamber22d. Then, the developer is supplied from the second spiral44to the developing roller20to form a magnetic brush on the developing roller20.

The developing roller20includes a fixed shaft (not shown) and a developing sleeve20aformed in a cylindrical shape. The developing sleeve20ais rotatably held on the fixed shaft. Near the developing sleeve20a, a regulation blade21is disposed at a predetermined distance from the developing sleeve20a. The regulation blade21regulates the layer thickness of the magnetic brush formed on the surface of the developing sleeve20a. The developing sleeve20ais caused to rotate in an arrow direction inFIG. 2(a clockwise direction) by a driving mechanism including a motor and a gear, of which none is illustrated. Furthermore, to the developing sleeve20a, a developing bias is applied which is obtained by superposing an alternating current voltage on a direct current voltage.

When the developing sleeve20a, to which the developing bias is applied, rotates in the clockwise direction inFIG. 2, a potential difference between the developing bias and the exposed portion of the photosensitive drum1acauses the toner to be supplied from the magnetic brush carried on the surface of the developing sleeve20ato the photosensitive drum1a. The toner sequentially adheres to the exposed portion on the photosensitive drum1arotating in a counter-clockwise direction, and an electrostatic latent image on the photosensitive drum1ais developed with the toner.

On a side surface of the second conveyance chamber22d, a developer amount detecting sensor50is disposed which detects an amount of developer conveyed inside the second conveyance chamber22d. Used here as the developer amount detecting sensor50is a piezoelectric sensor which generates an electric signal when pressure is applied to a detection surface. The higher the level (bulk) of the developer conveyed inside the second conveyance chamber22dbecomes, the larger the pressure applied to the detection surface of the developer amount detecting sensor50becomes, and thus, an amount of developer is detectable based on a detection signal transmitted from the developer amount detecting sensor50.

As illustrated in later-describedFIG. 3, the developer amount detecting sensor50is disposed in the second conveyance chamber22d, at a position immediately adjacent to an upstream side of the second communication portion22fwith respect to a developer conveyance direction in the second conveyance chamber22d(an A2 direction indicated by arrow A2inFIG. 3). Here, instead of the piezoelectric sensor, a magnetic permeability sensor, which detects magnetic permeability of the developer and outputs a voltage value corresponding to the detected magnetic permeability, may be used as the developer amount detecting sensor50.

On a bottom of the first conveyance chamber22c, a toner concentration detecting sensor51is disposed. The toner concentration detecting sensor51detects a ratio of toner to carrier (T/C) in the developer conveyed inside the developing container22, and used as the toner concentration detecting sensor51is, for example, a magnetic permeability sensor which detects magnetic permeability of the developer inside the developing container22. As illustrated in later-describedFIG. 3, the toner concentration detecting sensor51is disposed in the first conveyance chamber22c, at a position immediately adjacent to an upstream side of the first communication portion22ewith respect to a developer conveyance direction in the first conveyance chamber22e(an A1 direction indicated by arrow A1inFIG. 3).

FIG. 3is a vertical sectional view (taken along line YY′ ofFIG. 2) illustrating a structure of a stirring unit of the developing device3a. As illustrated inFIG. 3, the developing container22includes the partition22b, the first conveyance chamber22c, the second conveyance chamber22d, the first communication portion22e, and the second communication portion22f.

The first communication portion22eand the second communication portion22fare respectively formed at one end and the other end (an A1-direction side end and an A2-direction side end) of the partition22bin its longitudinal direction. The first communication portion22eallows the first conveyance chamber22cand the second conveyance chamber22dto communicate with each other at their A1-direction (first-direction) end portions. The second communication portion22fallows the first conveyance chamber22cand the second conveyance chamber22dto communicate with each other at their A2-direction (second-direction) end portions. Here, the second communication portion22fis formed large enough to prevent the developer conveyed by the second spiral44from stagnating. And the developer circulates inside the developing container22by passing through the first conveyance chamber22c, the first communication portion22e, the second conveyance chamber22d, and the second communication portion22f.

The first spiral43, which is disposed inside the first conveyance chamber22c, has a rotary shaft43band a first spiral blade43adisposed integrally with the rotary shaft43band formed in a spiral shape with a predetermined pitch in the axial direction of the rotary shaft43b. The rotary shaft43bis rotatably supported in the developing container22. The first spiral blade43arotates in the counter-clockwise direction inFIG. 2, and thereby conveys the developer inside the first conveyance chamber22cin the A1 direction (to one side in the axial direction of the developing roller20), while stirring the developer.

Furthermore, in an end surface of the first conveyance chamber22cin the A2-direction, there is provided a toner replenishing port23through which toner is replenished into the developing container22. The toner replenishing port23has connected thereto a toner replenishing path24, which leads to the toner container4a(seeFIG. 1). The rotary shaft43bextends, passing through the toner replenishing port23, into a toner replenishing path24. The portion of the rotary shaft43bthat is disposed inside the toner replenishing path24has integrally formed thereon a replenishing blade43c, which is formed in a shape of a spiral with a constant pitch along the axial direction of the rotary shaft43b. The replenishing blade43cis a spiral blade wound in the same direction (the same phase) as the first spiral blade43a, and is formed with a smaller pitch and a smaller diameter as compared with the first spiral blade43a.

The second spiral44disposed inside the second conveyance chamber22dhas a rotary shaft44band a second spiral blade44awhich is integrally formed with the rotary shaft44band which is formed in a shape of a spiral wound in the same direction (the same phase) as the first spiral blade43a. The rotary shaft44bis disposed parallel to the rotary shaft43b, and rotatably supported in the developing container22. The second spiral blade44arotates in a clockwise direction inFIG. 2, and thereby conveys the developer existing in the second conveyance chamber22din the A2 direction (a direction opposite to the A1 direction), while stirring the developer, to supply the developer to the developing roller20(seeFIG. 2).

The developer in the first conveyance chamber22cis conveyed in the A1 direction, while being stirred, by the first spiral43, and gradually accumulates on one side (first communication portion-22eside) of the first conveyance chamber22c. The developer already existing on the one side of the first conveyance chamber22cis pushed by another portion of the developer newly coming to the one side, and is forced up into the second conveyance chamber22dvia the first communication portion22e.

Then, the developer is conveyed in the A2 direction, while being stirred, by the second spiral44, to be supplied to the developing roller20. The developer remaining on the developing roller20without being used in development falls from the developing roller20, and is collected by the second conveyance chamber22d. Then, the collected developer is conveyed by the second spiral44to the other side (second communication portion-22fside) of the second conveyance chamber22d, and falls into the first conveyance chamber22cvia the second communication portion22f.

In the developing device3a, the second conveyance chamber22dis disposed above the first conveyance chamber22c. That is, the first conveyance chamber22cand the second conveyance chamber22dare arranged one over the other. With this arrangement, it is possible to make the developing device3amore compact in the horizontal direction, and thus it is possible to make the color printer100more compact. Here, in the color printer100, which is a color image forming apparatus, four developing devices3ato3dare arranged in the horizontal direction, and thus, making the developing devices3ato3dcompact is particularly effective.

FIG. 4is a block diagram showing an example of a control route used in the color printer100of the first embodiment. In use of the color printer100, various controls of the portions thereof are performed, and thus the control route in the entire color printer100is complex. Here, for convenience of description, the following description will focus on portions of the control route that are necessary for the embodiment of the present disclosure.

A toner replenishing motor60replenishes the toner stored in each of the toner containers4ato4dto a corresponding one of the developing devices3ato3dat a predetermined speed. In the present embodiment, the magnetic permeability of the toner is detected by the toner concentration detecting sensor51, and a voltage value corresponding to the detection result is fed to a controller80, which will be described later, and the controller80determines the toner concentration inside each of the developing devices3ato3dbased on the value fed from the toner concentration detecting sensor51. The controller80transmits a control signal to the toner replenishing motor60in accordance with the thus determined toner concentration, and a predetermined amount of toner is replenished from each of the toner containers4ato4dvia the toner replenishing port23(seeFIG. 3) to a corresponding one of the developing devices3ato3d.

A development driving motor70is coupled via a gear train (not shown) to the first spiral43and the second spiral44disposed inside each of the developing devices3ato3d, and drives the first spiral43and the second spiral44to rotate at a predetermined speed based on a control signal from the controller80. Note that, by coupling the development driving motor70, via the gear train, also to the developing roller20, the development driving motor70functions also as a driving source of the developing roller20.

FIG. 5is a flowchart illustrating an example of drive control of the color printer100of the first embodiment. Referring toFIG. 1toFIG. 4as necessary, a description will be given of the procedure of replenishing the toner to each of the developing devices3ato3d, along the steps inFIG. 5.

When a printing instruction is fed from a host device such as a computer, a control signal is transmitted from the controller80to the development driving motor70, and the first spiral43and the second spiral44inside each of the developing devices3ato3dstart to be driven to rotate. The controller80detects an amount of developer based on a detection signal transmitted from the developer amount detecting sensor50(step S1). The developer amount detecting sensor50is disposed at a position immediately adjacent to the upstream side of the second communication portion22f, and thus it detects an amount of developer that is conveyed from the second conveyance chamber22d, passing through the second communication portion22f, to fall into the first conveyance chamber22c.

Next, the controller80changes a toner replenishing speed at which the toner is replenished to each of the developing devices3ato3d, in accordance with output level of the developer amount detecting sensor50. Specifically, a table for correcting the toner replenishing speed, the table being used to determine the toner replenishing speed based on the output level of the developer amount detecting sensor50, is stored in a storage (a ROM or a RAM) within the controller80, and the toner replenishing speed is determined using the output level of the detection signal from the developer amount detecting sensor50and the table for correcting the toner replenishing speed. Table 1 shows an example of the table for correcting the toner replenishing speed.

In Table 1, output values of the developer amount detecting sensor50are ranked into Levels 1 to 6, and an ON-time/OFF-time DUTY (ratio) of the toner replenishing motor60corresponding to each of Levels 1 to 6 is stored. Here, output values that are 2.5 V or higher but lower than 3V are ranked in Level 5 (the reference value), and the ON/OFF DUTY of the toner replenishing motor60at that time is set to 5:5. And the DUTY is changed such that the ratio of the ON time of the toner replenishing motor60decreases (that is, the toner replenishing speed becomes lower) with the output level of the developer amount detecting sensor50.

Back toFIG. 5, the controller80makes a judgment on whether or not the output level of the developer amount detecting sensor50is Level 5 (the reference value) or Level 6 (step S2). When the output level is Level 5 or Level 6 (Yes at step S2), the controller80keeps the ON time/OFF time DUTY of the toner replenishing motor60at 5:5 (step S3), and the flow returns to step S1, and the detection of the amount of developer is continued.

When the output level is not Level 5 or Level 6 (No at step S2), then, a judgment is made on whether or not the output level is Level 4 (step S4). When the output level is Level 4 (Yes at step S4), the controller80changes the ON-time/OFF-time DUTY of the toner replenishing motor60to 4:6 (step S5). Then, the flow returns to step S1, and the detection of the amount of developer is continued.

Likewise, the controller80makes judgments on whether or not the output level of the developer amount detecting sensor50is Level 3, Level 2, and Level 1 (steps S6, S8, and S10). Then, when the output level is Level 3, the controller80changes the ON-time/OFF-time DUTY of the toner replenishing motor60to3:7(step S7), when the output level is Level 2, the controller80changes the ON-time/OFF-time DUTY of the toner replenishing motor60to2:8(step S9), and when the output level is Level 1, the controller80changes the ON-time/OFF-time DUTY of the toner replenishing motor60to 1:9 (step S11). Thereafter, in whichever case, the flow returns to step S1, and the detection of the amount of developer is continued.

According to the control described above, when the amount of developer that is conveyed from the second conveyance chamber22dto fall to the vicinity of the toner replenishing port23in the first conveyance chamber22cis small, it is possible to lower the toner replenishing speed at which the toner is replenished from each of the toner containers4ato4dto a corresponding one of the developing devices3ato3d. As a result, even when the amount of developer that joins replenished toner is small, the ratio of the replenished toner with respect to the developer (the concentration of the replenished toner in the developer) is kept constant. Accordingly, it is possible to effectively reduce occurrence of problems which are caused by locally high concentration of the replenished toner, such as fogging of images and uneven image density.

Note that the amount of toner to be replenished to each of the developing devices3ato3dis determined based on the detection signal from the toner concentration detecting sensor51, and thus has no relation to the output level of the developer amount detecting sensor50. For example, in a case where the output level of the developer amount detecting sensor50is low (the ON time ratio of the toner replenishing motor60is small) and the amount of toner to be replenished determined based on the detection signal from the toner concentration detecting sensor51is large, a predetermined amount of toner is replenished to each of the developing devices3ato3dby prolonging the driving time of the toner replenishing motor60.

FIG. 6is a flowchart illustrating an example of drive control of the a color printer100according to a second embodiment of the present disclosure. The structure of the developing devices3ato3dand the control route in the color printer100of the second embodiment are similar to those in the first embodiment, but in the second embodiment, in the developing devices3ato3d, it is possible to change the rotation rate (the number of rotations per unit time) of the first spiral43and that of the second spiral44independently of each other by means of the development driving motor70. Referring toFIG. 1toFIG. 4as necessary, a description will be given of the procedure of controlling the developing device3a, along the steps inFIG. 6.

A control signal is transmitted from the controller80to the development driving motor70, and the first spiral43and the second spiral44in each of the developing devices3ato3dstart to be driven to rotate. The controller80starts the rotation of the second spiral44with Rn (the default value of which is R0) as the number of rotations of the second spiral44per unit time (step S1). Further, the controller80detects the amount of developer based on a detection signal transmitted from the developer amount detecting sensor50(step S2).

Next, the controller80changes the number of rotations of the second spiral44per unit time (hereinafter, the number of rotations per unit time will be referred to simply as rotation rate) based on the output level (Levels 1 to 6 in Table 1) of the developer amount detecting sensor50. Specifically, first, a judgement is made on whether or not the output level of the developer amount detecting sensor50is Level 5 (the reference value) (step S3). When the output level is Level 5 (Yes at step S3), the rotation rate of the second spiral44is kept at R0, and the flow returns to step S1.

When the output level is not Level 5 (No at step S3), then a judgment is made on whether or not the output level is Level 4 or lower (step S4). When the output level is Level 4 or lower (Yes at step S4), the controller80changes the rotation rate of the second spiral44to Rn+1=Rn×1.1 (step S5).

When the output level is not Level 4 or lower (No at step S4), the output level is Level 6, and thus the controller80changes the rotation rate of the second spiral44to Rn+1=Rn×0.9 (step S6). Thereafter, Rn+1 is replaced with Rn (step S7), and then the flow returns to step S1, and the detection of the amount of developer and changing of the rotation rate of the second spiral44are repeated until the output level becomes Level 5.

According to the control described above, when the amount of developer that is conveyed from the second conveyance chamber22dto fall to the vicinity of the toner replenishing port23in the first conveyance chamber22cis small, the rotation rate (rotation speed) of the second spiral44is increased to thereby increase the amount of developer that is conveyed from the second conveyance chamber22d, passing through the second communication portion22f, into the first conveyance chamber22c. On the other hand, when the amount of developer that is conveyed from the second conveyance chamber22dto fall to the vicinity of the toner replenishing port23in the first conveyance chamber22cis large, the rotation rate (rotation speed) of the second spiral44is reduced to thereby reduce the amount of developer that is conveyed from the second conveyance chamber22d, passing through the second communication portion22f, into the first conveyance chamber22c.

Thereby, the amount of developer that is conveyed from the second conveyance chamber22d, passing through the second communication portion22f, to fall to the vicinity of the toner replenishing port23in the first conveyance chamber22cis stabilized, and thus, as in the first embodiment, the ratio of the replenished toner with respect to the developer (the concentration of the replenished toner in the developer/per unit volume of the developer) is kept constant. Accordingly, it is possible to effectively reduce occurrence of problems which are caused by locally high concentration of the replenished toner, such as fogging of images and uneven image density. Moreover, the circulation balance of the developer inside the developing container22also is adjusted, and this contributes to stable supply of the developer from the second conveyance chamber22dto the developing roller20.

The embodiments described above are in no way meant to limit the present disclosure, which thus allows for many modifications and variations within the spirit of the present disclosure. For example, the above embodiments have dealt with examples where the developer is supplied to a developing roller from a stirring-conveyance member, but they are by no means meant to limit the scope of the present disclosure. A developer carrier such as a magnetic roller or a like may further be disposed between the stirring-conveyance member and the developing roller, such that developer is supplied from the stirring-conveyance member to the magnetic roller or the like and then the developer is supplied from the magnetic roller or the like to the developing roller. Furthermore, the above embodiments have dealt with examples of the developing devices3ato3din each of which the second conveyance chamber22dis disposed vertically above the first conveyance chamber22c, but the second conveyance chamber22dand the first conveyance chamber22cmay be displaced from each other in the horizontal direction.

Further, the above embodiments have dealt with examples where a developer amount detecting sensor50is disposed in a second conveyance chamber22d, at a position immediately adjacent to an upstream side of a second communication portion22fwith respect to a developer conveyance direction (the A2 direction), but instead, for example, as illustrated inFIG. 7, the developer amount detecting sensor50may be disposed on a side surface of a second communication portion22f.

Further, the present disclosure is applicable not only to the tandem type color printer100illustrated inFIG. 1, but also to various image forming apparatuses including both digital and analog types of monochrome copiers, color copiers, facsimile machines, and so on which each incorporate a developing device including a first conveyance chamber and a second conveyance chamber disposed above the first conveyance chamber.

The present disclosure is applicable to a developing device having a first conveyance chamber and a second conveyance chamber disposed above the first conveyance chamber. Use of the present disclosure makes it possible to provide a developing device which prevents occurrence of a portion with locally high concentration of replenished toner, even when the circulation balance of developer is changed, and thus is capable of reducing occurrence of problems such as fogging of images and uneven image density, and an image forming apparatus incorporating such a developing device.