Image forming device

Abstract

To provide an image forming apparatus that can prevent the reduction in the accuracy of detecting toner concentration after the rotation speed of a conveying member conveying a developer is switched from one to the other.SOLUTION: An image forming apparatus comprises a CPU 50 that controls conveying screws 28, 29 and a permeability sensor 53, the CPU 50 inputting control voltage to the permeability sensor 53 and receiving input of detection voltage according to the magnetic permeability of the developer and the control voltage from the permeability sensor 53. When switching a mode between a normal mode and a low-speed mode different from each other in the rotation speed of the conveying screws 28, 29, the CPU 50 changes the control voltage in the mode after the switching according to the execution time of the mode before the switching and the execution time of the mode after the switching.SELECTED DRAWING: Figure 8

Description

[Technical field]

[0001] The present invention relates to an image forming apparatus that forms an image on a sheet by using an electrophotographic method. [Background technology]

[0002] When forming an image with an electrophotographic image forming apparatus, a developer is first applied to an electrostatic latent image formed on the surface of a photoreceptor by a developing device to form a developer image, and then the developer image is transferred to a sheet to form an image.

[0003] A two-component developer containing a non-magnetic toner and a magnetic carrier is widely used as a developer in an image forming apparatus. When using a two-component developer, it is required to optimize the toner concentration, which is the ratio of toner to carrier in the developing device. This is because if the toner concentration is too high, toner scattering is likely to occur, and if the toner concentration is too low, the image density becomes low. Therefore, it is common to detect the toner concentration from the magnetic permeability of the developer detected by a magnetic permeability sensor, and control the amount of developer replenished to the developing device based on the detected toner concentration, thereby optimizing the toner concentration of the developer in the developing device.

[0004] In addition, the image forming apparatus changes the image forming speed according to the type of sheet on which the image is formed and the setting of the image resolution. For example, when forming an image on thick paper, the image forming apparatus forms the image at a slower image forming speed than when forming an image on thin paper in order to ensure the fixation of the toner image in the fixing unit. When changing the image forming speed in this way, the rotation speed of the photoconductor and the rotation speed of the developing sleeve change, and accordingly, the rotation speed of the transport member that transports the developer in the developing device also changes.

[0005] When the rotation speed of the transport member that transports the developer changes, the bulk density of the developer in the developing device changes. Specifically, when the rotation speed of the transport member is fast, the bulk density of the developer is lower than when the rotation speed of the transport member is slow. When the bulk density of the developer changes in this way, the detection result of the magnetic permeability of the developer by the magnetic permeability sensor may change due to the change in the bulk density of the developer around the magnetic permeability sensor, even though the actual toner concentration in the developing device has not changed. In this case, the detection accuracy of the toner concentration of the developer detected based on the detection result of the magnetic permeability of the developer by the magnetic permeability sensor may decrease.

[0006] In response to this, Patent Document 1 describes a configuration in which a control voltage input to a magnetic permeability sensor is adjusted so that an output voltage corresponding to the magnetic permeability of a developer output from the magnetic permeability sensor immediately after switching the rotation speed of a transport member is set to the output voltage last output from the magnetic permeability sensor before switching the rotation speed of the transport member. With this configuration, even if the rotation speed of the transport member is switched and the bulk density of the developer around the magnetic permeability sensor changes, it is possible to suppress a decrease in the detection accuracy of the toner concentration. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] JP 2011-232421 A Summary of the Invention [Problem to be solved by the invention]

[0008] The state of the developer after the rotation speed of the transport member is switched depends on the driving time of the transport member before the rotation speed of the transport member is switched and the driving time of the transport member after the rotation speed of the transport member is switched. For example, consider the case where the rotation speed of the transport member is switched from low to high. When the rotation speed of the transport member is slow, the force for stirring the developer is weak, so the developer is not sufficiently stirred and the bulk density of the developer increases. In addition, there is a gap between the transport member and the inner wall of the developing device for rotating the transport member, and the developer with high bulk density that has accumulated in this gap is pressed against the inner wall as the transport member rotates and aggregates. The longer the driving time of the transport member at low speed, the higher the degree of aggregation of the developer that aggregates in this gap.

[0009] When the transport member is switched from a low speed to a high speed, the developer is gradually agitated in response to the drive of the transport member, and the aggregated developer is broken down. If the drive time at low speed before the transport member is switched to high speed is long, the time until the aggregated developer is broken down after the transport member is switched to high speed is longer than that when the drive time at low speed before the transport member is switched to high speed is short. In this way, the state of the developer after the rotation speed of the transport member is switched changes depending on the drive time of the transport member before the rotation speed of the transport member is switched and the drive time of the transport member after the rotation speed of the transport member is switched.

[0010] Here, in the configuration of Patent Document 1, the control input value input to the magnetic permeability sensor after the rotation speed of the transport member is switched is not changed according to the driving time of the transport member before and after the switching of the rotation speed of the transport member. Therefore, the state of the developer is not fully reflected in the detection result of the magnetic permeability of the developer by the magnetic permeability sensor, and there is a risk of a decrease in the detection accuracy of the toner concentration of the developer detected based on this detection result.

[0011] SUMMARY OF THE PRESENTLY PREFERRED EMBODIMENTS An object of the present invention is to provide an image forming apparatus capable of suppressing a decrease in the detection accuracy of the toner concentration after the rotation speed of a transport member that transports developer is switched. [Means for solving the problem]

[0012] A representative configuration of an image forming apparatus according to the present invention for achieving the above object is a developing unit that develops an electrostatic latent image formed on the surface of the photoconductor with a developer containing non-magnetic toner and a magnetic carrier, the developing unit having a developer container that contains the developer, a transport member that rotates to transport the developer contained in the developer container, and a magnetic permeability sensor that outputs a detection voltage according to the magnetic permeability of the developer contained in the developer container, a drive unit that drives the transport member to rotate, and a control unit that controls the drive unit and the magnetic permeability sensor, and a control unit to which a control voltage is input to a magnetic permeability sensor and the detection voltage corresponding to the magnetic permeability and the control voltage is input from the magnetic permeability sensor, wherein the control unit has a first mode in which the drive unit rotates the conveying member at a first rotation speed, and a second mode in which the drive unit rotates the conveying member at a second rotation speed slower than the first rotation speed, and when switching between the first mode and the second mode, the control voltage in the mode after switching is changed depending on the execution time of the mode before switching and the execution time of the mode after switching. Effect of the Invention

[0013] According to the present invention, in an image forming apparatus, it is possible to suppress a decrease in the detection accuracy of the toner concentration after the rotation speed of a transport member that transports developer is switched. [Brief description of the drawings]

[0014] [Figure 1] FIG. 1 is a schematic cross-sectional view of an image forming apparatus. [Diagram 2] FIG. [Diagram 3] FIG. [Figure 4] FIG. 2 is a perspective view of a magnetic permeability sensor. [Diagram 5] 1 is a block diagram showing a system configuration of an image forming apparatus. [Figure 6] 4 is a graph showing the relationship between a control voltage Vin and a detection voltage Vout. [Figure 7] 11 is a graph showing the results of measuring the value of a detection voltage Vout before and after the image formation mode is switched from a low-speed mode to a normal mode. [Figure 8] 13 is a flowchart of a replenishment sequence. [Figure 9] 1 is a graph showing the relationship between a control voltage Vin(adj) and a detection voltage Vout. [Figure 10] 1 is a graph showing the relationship between a control voltage Vin(adj) and a detection voltage Vout. [Figure 11] 13 is a flowchart of a replenishment sequence. [Figure 12] 13 is a flowchart of a replenishment sequence. [Figure 13] 13 is a flowchart of a replenishment sequence. [Figure 14] 11 is a graph showing data relating the number of images formed in the image forming mode before switching and the idling time of the conveying screw. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] (First embodiment) <Image forming device> The overall configuration of the image forming apparatus according to the first embodiment of the present invention will be described below together with the operation during image formation with reference to the drawings. Note that the dimensions, materials, shapes, and relative positions of the components described below are not intended to limit the scope of the present invention unless otherwise specified.

[0016] Image forming apparatus A is an intermediate tandem type image forming apparatus that transfers four colors of toner, yellow Y, magenta M, cyan C, and black K, as developers onto an intermediate transfer belt, and then transfers the image onto a sheet to form an image. Note that in the following description, although the components that use the above-mentioned toners of each color are given the suffixes Y, M, C, and K, the configurations and operations of the components are essentially the same except for the color of the toner used, and therefore the suffixes will be omitted as appropriate unless a distinction is required.

[0017] FIG. 1 is a schematic cross-sectional view of an image forming apparatus A. As shown in FIG. 1, the image forming apparatus A includes an image forming section 100 that forms a toner image on a sheet S. The image forming section 100 includes a photosensitive drum 1 (1Y, 1M, 1C, 1K), a charging roller 30 (30Y, 30M, 30C, 30K), and a developing device 4 (4Y, 4M, 4C, 4K) having a developing sleeve 23 (23Y, 23M, 23C, 23K). The image forming section 100 also includes a primary transfer roller 5 (5Y, 5M, 5C, 5K), a cleaning blade 9 (9Y, 9M, 9C, 9K), a laser scanner unit 3 (3Y, 3M, 3C, 3K), an intermediate transfer belt 6, a secondary transfer roller 7, and a secondary transfer opposing roller 8.

[0018] Furthermore, image forming apparatus A includes toner bottle 55 (developer replenishing unit) that contains developer and supplies the developer to developing device 4. Toner bottle 55 is configured to be detachable from image forming apparatus A, and when the contained developer runs out, it is replaced with a new toner bottle 55 by a user or a maintenance worker.

[0019] The image forming apparatus A also includes an operation panel 56 (detection unit) that allows the user to operate to perform various settings related to image formation. The user can specify the number of images to be formed, the size of the sheets on which the images are to be formed, and the like by operating the operation panel 56. The user can also input the basis weight of the sheets S stored in the sheet cassette 10 (for example, cardboard, plain paper, sheet brand, etc.) by operating the operation panel 56.

[0020] The operation panel 56 is electrically connected to the CPU 50 (FIG. 5). The CPU 50 controls the rotation speed of various rollers that transport the sheet S according to the basis weight of the sheet S detected from the operation panel 56, and sets the transport speed of the sheet S during image formation. In this embodiment, the CPU 50 executes the normal mode when forming an image on a sheet S having a basis weight less than a predetermined value, such as plain paper or thin paper, and executes the low-speed mode in which the transport speed of the sheet S is slower than that in the normal mode when forming an image on a sheet S having a basis weight of a predetermined value or more, such as thick paper. This extends the time for which the toner image carried on the sheet S with a large basis weight is heated by the fixing device 16, thereby applying more heat to the toner image and the sheet S, and stably fixing the toner image on the sheet S with a large heat capacity. In this embodiment, the image forming apparatus A forms 30 images per minute in the normal mode, and forms 15 images per minute in the low-speed mode.

[0021] The timing at which the CPU 50 switches between the low-speed mode and the normal mode is not limited to the above timing. For example, the CPU 50 may set the low-speed mode when forming a high-quality image, and set the resolution in the sub-scanning direction, which is the rotation direction of the photosensitive drum 1, to be higher than that in the normal mode.

[0022] Next, a description will be given of the image forming operation of the image forming apparatus A. First, when an image formation job signal is input to the CPU 50 (FIG. 5), a sheet S stored in a sheet cassette 10 is sent to a registration roller 15 by a pickup roller 11, a feeding roller 12, and conveying rollers 13 and 14. Thereafter, the registration roller 15 conveys the sheet S to a secondary transfer section formed by a secondary transfer roller 7 and a secondary transfer opposing roller 8 at a predetermined timing.

[0023] Meanwhile, in the image forming unit 100, a charging bias is first applied to the charging roller 30Y, thereby charging the surface of the photosensitive drum 1Y (photosensitive member) in contact with the charging roller 30Y. Next, an electrostatic latent image is formed on the surface of the photosensitive drum 1Y by a laser scanner unit 3Y irradiating the surface of the photosensitive drum 1Y with laser light in accordance with image data transmitted from an image reading unit (not shown) or an external device (not shown).

[0024] Next, a developing bias is applied to a developing sleeve 23Y of the developing device 4Y (developing unit), so that yellow toner adheres to the electrostatic latent image formed on the surface of the photosensitive drum 1Y, and a yellow toner image is formed on the surface of the photosensitive drum 1Y. A primary transfer bias is applied to a primary transfer roller 5Y, so that the toner image formed on the surface of the photosensitive drum 1Y is primarily transferred to the intermediate transfer belt 6. Toner remaining on the photosensitive drum 1Y after the primary transfer is scraped off by a cleaning blade 9Y.

[0025] In a similar process, photosensitive drums 1M, 1C, and 1K are also irradiated with laser light from laser scanner units 3M, 3C, and 3K according to image data for each color, forming electrostatic latent images. Then, magenta, cyan, and black toner images are formed by developing devices 4M, 4C, and 4K. After that, a primary transfer bias is applied to primary transfer rollers 5M, 5C, and 5K, so that these toner images are transferred and superimposed on the yellow toner image on the intermediate transfer belt 6. As a result, a full-color toner image according to the image data is formed on the surface of the intermediate transfer belt 6.

[0026] Thereafter, the intermediate transfer belt 6 receives a driving force from the secondary transfer opposing roller 8 and moves in a circular motion, thereby sending the full-color toner image to the secondary transfer section. Then, in the secondary transfer section, a secondary transfer bias is applied to the secondary transfer roller 7, so that the full-color toner image on the intermediate transfer belt 6 is transferred to the sheet S.

[0027] The sheet S onto which the toner image has been transferred is conveyed to the fixing device 16, where it is subjected to a heating and pressure treatment, thereby fixing the toner image on the sheet S to the sheet S. Thereafter, the sheet S onto which the toner image has been fixed is discharged onto a discharge tray 18 by a discharge roller 17.

[0028] <Developing device> Next, the configuration of the developing device 4 will be described.

[0029] Fig. 2 is a schematic cross-sectional view of the developing device 4 cut in a direction perpendicular to the rotation axis direction of the developing sleeve 23. Fig. 3 is a schematic cross-sectional view of the developing device 4 cut along the rotation axis direction of the developing sleeve 23. As shown in Figs. 2 and 3, the developing device 4 includes a developing container 20 as a housing. The developing container 20 has a developing chamber 21 as a developer containing section that contains developer, and a stirring chamber 22.

[0030] The developing chamber 21 and the stirring chamber 22 are partitioned by a partition wall 25 provided inside the developing container 20. In the direction of the rotation axis of the developing sleeve 23, both ends of the partition wall 25 do not reach the inner wall of the developing container 20, thereby forming a communication portion 25a (FIG. 3) that communicates between the developing chamber 21 and the stirring chamber 22. In this embodiment, the developer contained in the developing chamber 21 and the stirring chamber 22 is a two-component developer in which non-magnetic toner and a magnetic carrier are mixed.

[0031] The developing chamber 21 and the stirring chamber 22 are provided with transport screws 28 and 29 (transport members) that stir and transport the developer, respectively. The transport screw 28 is disposed at the bottom of the developing chamber 21 so that its shaft portion 28a is aligned with the rotation axis direction of the developing sleeve 23. The transport screw 28 has a screw structure in which a spiral blade 28b is provided on the outer periphery of the shaft portion 28a, which serves as the rotation axis. When the shaft portion 28a of the transport screw 28 rotates, the blade 28b transports the developer in the developing chamber 21 along the rotation axis direction of the developing sleeve 23.

[0032] The transport screw 29 disposed in the stirring chamber 22 is disposed at the bottom of the stirring chamber 22 so that its shaft portion 29a is aligned with the direction of the rotation axis of the developing sleeve 23. The transport screw 29 has a screw structure in which a spiral blade 29b is provided on the outer periphery of the shaft portion 29a, which serves as the rotation shaft. Here, the blade 29b of the transport screw 29 is provided in the opposite direction to the blade 28b of the transport screw 28. As a result, the shaft portion 29a of the transport screw 29 rotates in the same direction as the shaft portion 28a of the transport screw 28, and the blade 29b transports the developer in the stirring chamber 22 in the opposite direction to the direction in which the developer is transported by the transport screw 28.

[0033] In this manner, the developer is stirred and transported in the developing chamber 21 and the stirring chamber 22, and the developer circulates between the developing chamber 21 and the stirring chamber 22 through the communication portion 25a. The developing sleeve 23 and the transport screws 28, 29 are connected and driven by the driving force of a developing motor 52 (FIG. 5) serving as a drive portion being transmitted via a gear train (not shown).

[0034] The developing chamber 21 has an opening formed at a position facing the photosensitive drum 1, and a developing sleeve 23 is provided in the opening so as to be partially exposed on the photosensitive drum 1 side. The developing sleeve 23 is made of a non-magnetic material. The developing sleeve 23 also contains a magnet roll 24 in a non-rotating state. The magnet roll 24 has a total of five magnetic poles, namely, poles S1, S2, S3, N1, and N2.

[0035] In addition, a developing blade 26 is provided in the developing chamber 21 at a position facing the developing sleeve 23. The developing blade 26 comes into contact with the toner carried on the developing sleeve 23 and regulates the amount of toner carried on the developing sleeve 23.

[0036] During image formation, first, the developer is attracted to the surface of the developing sleeve 23 by the magnetic force of the S2 pole of the magnet roll 24. Next, when the developing sleeve 23 rotates in the direction of the arrow shown in Fig. 2, the developer made to stand up by the N1 pole on the surface of the developing sleeve 23 receives a shearing force from the developing blade 26, and the amount of developer is regulated. As a result, a developer layer of a predetermined thickness is formed on the developing sleeve 23.

[0037] Next, the developer layer on the developing sleeve 23 is transported to a developing area facing the photosensitive drum 1 by the rotation of the developing sleeve 23. The toner in the developer, with magnetic chains formed by the S1 pole, adheres to the electrostatic latent image formed on the surface of the photosensitive drum 1. This forms a toner image on the surface of the photosensitive drum 1. Thereafter, the developer remaining on the surface of the developing sleeve 23 moves while being supported on the surface of the developing sleeve 23 by the magnetic force of the N2 pole as the developing sleeve 23 rotates. Thereafter, the developer is peeled off from the developing sleeve 23 by the magnetic force of the S3 pole and returned to the developing chamber 21.

[0038] Further, one end of the mixing chamber 22 is provided with a supply port 43 for receiving toner discharged from a toner bottle 55. The toner bottle 55 rotates by receiving a driving force from a supply motor 54 (FIG. 5), and as a result, the toner is discharged from the toner bottle 55. The toner supplied from the supply port 43 is stirred and transported by the transport screw 29 in the mixing chamber 22, and then sent to the developing chamber 21 via the communication part 25a.

[0039] Further, a magnetic permeability sensor 53 for detecting the magnetic permeability of the developer is provided near the conveying screw 29 on the side surface of the stirring chamber 22. The magnetic permeability of the developer detected by the magnetic permeability sensor 53 is used to detect the toner concentration. The toner concentration is the ratio of the toner weight to the total weight of the toner and carrier. Note that although the magnetic permeability sensor 53 is disposed in the stirring chamber 22 in this embodiment, the magnetic permeability sensor 53 may be disposed in another position as long as there is enough developer near the magnetic permeability sensor 53 to detect the toner concentration, and the developer flows when the conveying screws 28, 29 rotate.

[0040] Fig. 4 is a perspective view of the magnetic permeability sensor 53. As shown in Fig. 4, the magnetic permeability sensor 53 has a base portion 53a, a cylindrical detection head 53b provided on the top surface of the base portion 53a, and a signal line 53c that electrically connects the magnetic permeability sensor 53 to a CPU 50 (Fig. 5) that controls the magnetic permeability sensor 53.

[0041] A transformer (not shown) is embedded inside the detection head 53b. The transformer is composed of a total of three windings: one primary winding and two secondary windings, a reference winding and a detection winding. The detection winding is disposed on the top surface side of the detection head 53b, and the reference winding is disposed on the back surface side of the detection head 53b, sandwiching the primary winding therebetween.

[0042] When a control voltage Vin is input from the CPU 50 shown in FIG. 5 to the magnetic permeability sensor 53, and a current having a fixed waveform signal is input from an oscillator (not shown) provided on the base portion 53a to the primary winding, a current having a certain waveform signal also flows through the secondary winding due to electromagnetic induction. The magnetic permeability sensor 53 compares the fixed waveform signal from the oscillator at this time with a certain waveform signal of a current flowing from the detection winding due to electromagnetic induction in a comparison circuit in the base portion 53a, and outputs a detection voltage Vout according to the magnetic permeability of the developer to the CPU 50. For example, when toner is consumed during an image forming operation, the ratio of toner in the developer near the detection head 53b decreases, and the detection voltage Vout output from the magnetic permeability sensor 53 increases. In this way, the magnetic permeability sensor 53 detects the density of the magnetic material on the top surface side of the detection head 53b, i.e., the amount of carrier.

[0043] <Control Unit> Next, the configuration of the control section of the image forming apparatus A will be described, focusing on the control system that controls the developing device 4.

[0044] Fig. 5 is a block diagram showing the system configuration of the image forming apparatus A. As shown in Fig. 5, the image forming apparatus A includes a CPU 50 (control unit) and a memory 51 (storage unit). To the CPU 50, a developing motor 52 that drives the developing sleeve 23 and the conveying screws 28 and 29, a supply motor 54 that drives a toner bottle 55, a photosensitive motor 57 that drives the photosensitive drum 1, a magnetic permeability sensor 53, and an operation panel 56 are connected.

[0045] The CPU 50 controls the developing motor 52 and the photosensitive motor 57 in accordance with the image forming mode set by the user via the operation panel 56, and controls the rotation speeds of the photosensitive drum 1 and the developing sleeve 23. Specifically, in order to control the conveying speed of the sheet S, when the low-speed mode is executed, the CPU 50 sets the rotation speeds of the photosensitive drum 1 and the developing sleeve 23 to be slower than when the normal mode is executed.

[0046] As described above, the drive of the developing sleeve 23 is linked to the drive of the conveying screws 28, 29. Therefore, the rotation speed of the conveying screws 28, 29 also changes with the change in the rotation speed of the developing sleeve 23 due to switching of the image formation mode. That is, the conveying screws 28, 29 rotate at a first rotation speed in the normal mode (first mode), and rotate at a second rotation speed slower than the first rotation speed in the low-speed mode (second mode). In this embodiment, the conveying screws 28, 29 rotate at 300 rpm in the normal mode, and at 150 rpm in the low-speed mode.

[0047] In this embodiment, a configuration in which the developing sleeve 23 and the transport screws 28, 29 are coupled to each other is described, but each may be driven by a different motor. Even when this configuration is adopted, the CPU 50 increases or decreases the rotation speed of the transport screws 28, 29 in accordance with an increase or decrease in the rotation speed of the developing sleeve 23. This keeps the circulation ratio of the developer supplied from the developing sleeve 23 to the photosensitive drum 1 and the developer circulating in the developing device 4 constant.

[0048] Also, the CPU 50 drives the supply motor 54 to execute a toner supply operation according to the toner concentration of the developer in the developing device 4 detected based on the detection voltage Vout of the permeability sensor 53. Specifically, the CPU 50 compares the target voltage Vtg corresponding to the target toner concentration stored in the memory 51 with the detection voltage Vout output from the permeability sensor 53. When Vtg < Vout, it means that the toner concentration is low. In this case, the CPU 50 sets the driving time of the supply motor 54 based on a table stored in the memory 51 in which the detection voltage Vout and the supply time of the developer are associated, and drives the supply motor 54 to execute a toner supply operation. In the present embodiment, the CPU 50 monitors the detection voltage Vout output from the permeability sensor 53 every time an image is formed by the image forming apparatus A, and executes a toner supply operation according to the detection voltage Vout.

[0049] In the present embodiment, the target toner concentration is 8% (by weight) common to the developers of all colors, and the target voltage Vtg corresponding to the toner concentration of 8% is 2.5 V. The setting of the target voltage Vtg is performed as described below. That is, when a new developing device 4 is attached to the image forming apparatus A, the CPU 50 drives the developing motor 52 for a certain period of time to spread the developer evenly throughout the developing container 20. Next, the CPU 50 drives the developing motor 52 for another 1 second without stopping its driving, and detects the detection voltage Vout of the permeability sensor 53 20 times at intervals of 50 msec during this 1 second. Then, the CPU 50 stores the average value of the 20 detection voltages Vout as the target voltage Vtg in the memory 51. The developing device 4 includes a fuse (not shown) that contacts an electrical contact (not shown) of the image forming apparatus A. When the power of the image forming apparatus A is turned on, if a new developing device 4 is attached, current flows through the fuse and is cut off, and the CPU 50 determines that the developing device 4 is new. When a non-new developing device 4 is attached, the fuse has already been cut off and no current flows, so the CPU 50 determines that the developing device 4 is a durable product.

[0050] Here, when the rotation speed of the transport screws 28, 29 is fast, the detection voltage Vout of the magnetic permeability sensor 53 tends to be smaller than when the rotation speed is slow. This is because when the rotation speed of the transport screws 28, 29 is fast, the transport screws 28, 29 quickly stir the developer and wind it up, reducing the bulk density of the developer and decreasing the carrier density in the vicinity of the detection head 53b. This means that after the image formation mode is switched, even if the toner concentration of the developer in the developing container 20 is the same, the detection voltage Vout of the magnetic permeability sensor 53 will have a different value.

[0051] Fig. 6 is a graph showing the relationship between the control voltage Vin output from the CPU 50 and the detection voltage Vout output from the magnetic permeability sensor 53 when the toner concentration of the developer is 8%. As shown in Fig. 6, the control voltage Vin and the detection voltage Vout are in a proportional relationship, and as the control voltage Vin increases, the detection voltage Vout also increases. For example, when the control voltage Vin is 12.5V, the detection voltage Vout is 2.5V, and when the control voltage Vin is 12.6V, the detection voltage Vout is 2.75V.

[0052] The CPU 50 changes the control voltage Vin according to the image forming mode so that the relationship between the toner concentration and the detection voltage Vout is the same in the normal mode and the low-speed mode, and corrects the detection voltage Vout. That is, in this embodiment, the CPU 50 sets the control voltage Vin to 12.5V in the normal mode and 12.3V in the low-speed mode so that the detection voltage Vout is 2.5V when the toner concentration is 8%. These values ​​are stored in the memory 51. As a result, even if the bulk density of the developer changes depending on the image forming mode, the same detection voltage Vout can be obtained when the toner concentration is the same. Note that these values ​​are reference values ​​of the control voltage input from the CPU 50 to the magnetic permeability sensor 53, and are corrected as described later according to the number of images formed in each image forming mode.

[0053] <Transient phenomenon> Next, a transient phenomenon that occurs from when the image forming mode is switched until the state of the developer becomes stable will be described.

[0054] FIG. 7 is a graph showing the results of measuring the value of the detection voltage Vout before and after the image forming mode is switched from the low-speed mode to the normal mode. In this measurement, when switching from the low-speed mode to the normal mode, the control voltage Vin input from the CPU 50 to the magnetic permeability sensor 53 is immediately switched from 12.3V to 12.5V. FIG. 7(a) is a graph when switching to the normal mode after forming 20 images in the low-speed mode. FIG. 7(b) is a graph when switching to the normal mode after forming 50 images in the low-speed mode. FIG. 7(c) is a graph when switching to the normal mode after forming 100 images in the low-speed mode. In FIG. 7, the vertical axis indicates the value of the detection voltage Vout, and the horizontal axis indicates the number of images formed. The image related to the image formation is a solid white image that does not consume toner.

[0055] As shown in FIG. 7, immediately after the image forming mode is switched, the value of the detection voltage Vout changes drastically even though the toner concentration of the developer does not change. This is due to the following reason. That is, the bulk density of the developer immediately after the image forming mode is switched does not change from the bulk density of the developer before the switching, and the bulk density of the developer does not become the expected bulk density after the switching unless the conveying screws 28 and 29 rotate to a certain extent after the switching. Despite this, the CPU 50 switches the value of the control voltage Vin sent to the magnetic permeability sensor 53 from 12.3V to 12.5V immediately after switching the image forming mode from the low speed mode to the normal mode. Then, even though the bulk density of the developer is high, the control voltage Vin becomes high, and the detection voltage Vout becomes extremely high.

[0056] In addition, the time until the developer reaches the expected bulk density after the image formation mode is switched depends on the driving time of the conveying screws 28, 29 before the image formation mode is switched. Since the conveying screws 28, 29 are mainly driven during image formation, it can also be said that the time until the developer reaches the expected bulk density after the image formation mode is switched depends on the number of images formed before the image formation mode is switched. Specifically, the more images formed in the low-speed mode, the longer the time until the developer reaches the expected bulk density in the normal mode, and the more images formed in the normal mode, the longer the time until the developer reaches the expected bulk density in the low-speed mode.

[0057] For example, as shown in Fig. 7, when the number of images formed in the low-speed mode is 20, the number of images formed after switching to the normal mode until the developer bulk density reaches that expected in the normal mode is about 10. When the number of images formed in the low-speed mode is 50, the number of images formed after switching to the normal mode until the developer bulk density reaches that expected in the normal mode is about 30. When the number of images formed in the low-speed mode is 100, the number of images formed after switching to the normal mode until the developer bulk density reaches that expected in the normal mode is about 85.

[0058] This is due to the following reason. That is, in the low-speed mode, the rotation speed of the transport screws 28, 29 is slow, and the force for stirring the developer is weak, so the developer is not sufficiently stirred and the bulk density of the developer becomes high. In addition, between the transport screws 28, 29 and the inner wall of the developing chamber 21 or the inner wall of the stirring chamber 22, there is a gap for rotating the transport screws 28, 29. The developer with high bulk density that accumulates in this gap is pressed against the inner wall as the transport screws 28, 29 rotate and coagulates. The longer the driving time of the transport screws 28, 29 at low speed, the higher the degree of coagulation of the developer that coagulates in this gap.

[0059] When the image formation mode is switched to the normal mode and the conveying screws 28, 29 are switched to a high speed, the developer is gradually agitated and the aggregated developer is broken down in response to the driving of the conveying screws 28, 29. Here, if the driving time at low speed before the conveying screws 28, 29 are switched to a high speed is long, the time until the aggregated developer is broken down after the conveying screws 28, 29 are switched to a high speed is long compared to when the driving time is short.

[0060] In this way, when the output of the magnetic permeability sensor 53 is controlled using only the control voltage Vin, erroneous detection of the toner concentration occurs until the state of the developer stabilizes after the image formation mode is switched. Similarly, when the image formation mode is switched from the normal mode to the low-speed mode, erroneous detection of the toner concentration of the developer occurs until the state of the developer stabilizes, although the degree of the transient phenomenon is smaller than when the image formation mode is switched from the low-speed mode to the normal mode.

[0061] <Resupply sequence> In order to suppress erroneous detection of toner concentration due to the above-mentioned transient phenomenon, the CPU 50 executes a supply sequence described below, corrects the control voltage Vin, transmits it to the magnetic permeability sensor 53, and executes a developer supply operation. The supply sequence will be described below with reference to the flowchart shown in Fig. 8. The CPU 50 executes the supply sequence every time one image is formed.

[0062] 8, first, the CPU 50 detects the number of images formed in the image formation mode before switching to the currently set image formation mode, i.e., the previous image formation mode (S1). Next, the CPU 50 obtains the amount of erroneous detection Va of the detection voltage Vout when the control voltage Vin corresponding to the image formation mode stored in the memory 51 is input to the magnetic permeability sensor 53 (S2). The control voltage Vin corresponding to the image formation mode stored in the memory 51 here is 12.5V when the current image formation mode is the normal mode, and 12.3V when the current image formation mode is the low-speed mode.

[0063] Here, in order to obtain the amount of erroneous detection Va, the following definitions are made. The number of sheets on which images are formed in the current image formation mode is N, the number of sheets on which images are formed in the previous image formation mode is M, and the last detection voltage detected by the magnetic permeability sensor 53 in the previous image formation mode is Vlast. At this time, the CPU 50 calculates the amount of erroneous detection Va from the following formula 1.

[0064] (Formula 1) Va=Vlast×K1×exp^(-(N-1) / (K2×M 2 +K3×M) )

[0065] Here, K1, K2, and K3 in Equation 1 are constants, and are unique values ​​that differ for each image forming apparatus A, and are obtained as follows. That is, as shown in FIG. 7, before shipping the image forming apparatus A, the conditions of the driving time (defined here as the number of images formed) of the conveying screws 28 and 29 before switching the image forming mode are changed in an experiment to obtain the detection voltage Vout after switching the image forming mode. Since the attenuation curve of the detection voltage Vout after switching the image forming mode differs depending on the driving time of the conveying screws 28 and 29 before switching the image forming mode, K1, K2, and K3 are determined so that the attenuation curve obtained experimentally is the one obtained by curve fitting. In this embodiment, K1 is 0.08, K2 is 0.0032, and K3 is 0.0581.

[0066] Next, the CPU 50 calculates (S3) an offset value Voff, which is a correction amount for the control voltage Vin corresponding to the image formation mode stored in the memory 51, based on the calculated amount of erroneous detection Va. The offset value Voff is a correction amount for the control voltage Vin that can shift the detection voltage Vout by the amount of erroneous detection Va, and is calculated from the following formula 2.

[0067] (Formula 2) Voff = Va × K4

[0068] Here, K4 in Equation 2 is a constant, which is an inherent value that differs for each image forming apparatus A, and is the amount of change in the control voltage Vin relative to the amount of erroneous detection Va. In this embodiment, K4 is 0.4 based on the relationship between the control voltage Vin and the detection voltage Vout shown in FIG.

[0069] Next, the CPU 50 calculates the corrected control voltage Vin(adj) from the following equation 3 based on the calculated offset value Voff and the control voltage Vin corresponding to the image formation mode stored in the memory 51 (S4).

[0070] (Formula 3) Vin(adj)=Vin-Voff

[0071] Next, the CPU 50 transmits the control voltage Vin(adj) to the magnetic permeability sensor 53 and receives the detected voltage Vout from the magnetic permeability sensor 53 (S5, S6). If the detected voltage Vout received from the magnetic permeability sensor 53 is less than the target voltage Vtg stored in the memory 51, the CPU 50 drives the supply motor 54 based on the table stored in the memory 51 to perform a supply operation of the developer (S7, S8, S9, S10).

[0072] As described above, the developer state has a transient characteristic in which the developer state gradually adapts from the state before switching the image formation mode to the state after switching. Also, the longer the number of images formed in the previous image formation mode, the longer the number of images formed until the developer state adapts to the state assumed in the current image formation mode. Therefore, the erroneous detection amount Va and the offset value Voff are also values ​​that are gradually attenuated according to the number of images formed after switching the image formation mode, and they need to be values ​​that reduce the amount of attenuation according to the number of images formed in the current image formation mode as the number of images formed in the previous image formation mode increases.

[0073] In this regard, according to formulas 1 and 2, as N, which is the number of sheets of image formation in the current image formation mode, increases, the amount of erroneous detection Va and the offset value Voff decrease, and finally converge to a value infinitesimally close to zero. Also, the value of N until the amount of erroneous detection Va and the offset value Voff converge to a value infinitesimally close to zero increases as the value of M, which is the number of sheets of image formation in the previous image formation mode, increases.

[0074] According to formula 3, as N, which is the number of images formed in the current image forming mode, increases, the corrected control voltage Vin(adj) approaches the control voltage Vin before correction, and finally converges to a value that is as close as possible to the control voltage Vin. The value of N until the control voltage Vin(adj) converges to a value that is as close as possible to the control voltage Vin increases as M, which is the number of images formed in the previous image forming mode, increases.

[0075] Fig. 9 is a graph showing the relationship between the control voltage Vin(adj) and the detection voltage Vout according to the number of images formed when the normal mode is executed after 20 consecutive images are formed in the low-speed mode. Fig. 10 is a graph showing the relationship between the control voltage Vin(adj) and the detection voltage Vout according to the number of images formed when the normal mode is executed after 150 consecutive images are formed in the low-speed mode.

[0076] 9 and 10, as the number of images formed in the normal mode increases, the control voltage Vin(adj) approaches the pre-correction control voltage Vin of 12.5 V, and finally converges to a value that is extremely close to the control voltage Vin. In addition, the number of images formed until the post-correction control voltage Vin(adj) converges to a value that is extremely close to the pre-correction control voltage Vin of 12.5 V is greater when the number of images formed in the low-speed mode, which is the previous image formation mode, is 150 sheets than when it is 20 sheets.

[0077] As described above, according to the configuration of this embodiment, the CPU 50 predicts the state of the developer according to the drive time of the conveying screws 28, 29 before and after switching the rotation speed of the conveying screws 28, 29, and changes the control voltage to be input to the magnetic permeability sensor 53 after switching the rotation speed according to this prediction. This makes it possible to reflect the state of the developer that changes according to the drive time of the conveying screws 28, 29 before and after switching the rotation speed of the conveying screws 28, 29 in the detection result of the magnetic permeability of the developer by the magnetic permeability sensor 53. Therefore, it is possible to suppress a decrease in the detection accuracy of the toner concentration of the developer detected based on the detection result of the magnetic permeability of the developer by the magnetic permeability sensor 53.

[0078] The state of the developer changes depending on the drive time of the conveying screws 28, 29 before and after switching the rotation speed of the conveying screws 28, 29, and the conveying screws 28, 29 are driven not only during image formation but also at other times such as pre-rotation and post-rotation. Therefore, in the above formula 1, N may be the drive time before switching the rotation speed of the conveying screws 28, 29, and M may be the drive time after switching the rotation speed of the conveying screws 28, 29. This makes it possible to accurately reflect the state of the developer, which changes depending on the drive time of the conveying screws 28, 29 before and after switching the rotation speed of the conveying screws 28, 29, in the detection result of the magnetic permeability of the developer by the magnetic permeability sensor 53.

[0079] Second embodiment Next, a second embodiment of the image forming apparatus according to the present invention will be described with reference to the drawings. The same reference numerals are used to designate parts that are the same as those in the first embodiment, and the description thereof will be omitted.

[0080] In the first embodiment, a configuration was described in which, when the image formation mode is switched, the control voltage Vin is corrected according to the number of sheets of image formation in the previous image formation mode. In this embodiment, when the image formation mode is switched, the target voltage Vtg is corrected according to the number of sheets of image formation in the previous image formation mode. The configuration of the image forming apparatus A according to this embodiment differs from the configuration of the first embodiment only in the content of the supply sequence, and the other configurations are the same as the configuration of the first embodiment.

[0081] Fig. 11 is a flowchart of a supply sequence according to this embodiment. In Fig. 11, steps that perform the same processes as those described in the first embodiment with reference to Fig. 8 are given the same reference numerals and their description will be simplified.

[0082] 11, first, the CPU 50 detects the number of images formed in the image forming mode before switching to the currently set image forming mode, i.e., the previous image forming mode (S1). Next, the CPU 50 obtains the amount of erroneous detection Va of the detection voltage Vout when the control voltage Vin corresponding to the image forming mode stored in the memory 51 is input to the magnetic permeability sensor 53 by using the formula 1 (S2).

[0083] Next, the CPU 50 calculates the corrected target voltage Vtg(adj) using the following equation 4 based on the calculated amount of erroneous detection Va and the target voltage Vtg stored in the memory 51 (S21).

[0084] (Formula 4) Vtg(adj)=Vtg+Va

[0085] Next, the CPU 50 transmits the control voltage Vin corresponding to the image formation mode stored in the memory 51 to the magnetic permeability sensor 53, and receives the detection voltage Vout from the magnetic permeability sensor 53 (S22, S6). The control voltage Vin corresponding to the image formation mode stored in the memory 51 here is 12.5 V when the current image formation mode is the normal mode, and is 12.3 V when the current image formation mode is the low-speed mode.

[0086] Next, the CPU 50 compares the detected voltage Vout received from the magnetic permeability sensor 53 with the corrected target voltage Vtg(adj). If the detected voltage Vout is less than the corrected target voltage Vtg(adj), the CPU 50 drives the supply motor 54 based on the table stored in the memory 51 to perform a developer supply operation (S23, S8, S9, S10).

[0087] As described above, the developer state has a transient characteristic in which the developer state gradually adapts from the state before switching the image formation mode to the state after switching. Also, the longer the number of images formed in the previous image formation mode, the longer the number of images formed until the developer state adapts to the state assumed in the current image formation mode. Therefore, the amount of erroneous detection Va is a value that is gradually attenuated according to the number of images formed after switching the image formation mode, and it is necessary to set the amount of attenuation according to the number of images formed in the current image formation mode to a value that is smaller according to the number of images formed in the previous image formation mode.

[0088] In this regard, according to formula 1, as N, which is the number of sheets of image formation in the current image formation mode, increases, the amount of erroneous detection Va decreases, and finally converges to a value infinitesimally close to zero. Also, the value of N until the amount of erroneous detection Va converges to a value infinitesimally close to zero increases as the value of M, which is the number of sheets of image formation in the previous image formation mode, increases.

[0089] According to formula 4, as N, which is the number of images formed in the current image forming mode, increases, the corrected target voltage Vtg(adj) approaches the pre-correction target voltage Vtg, and finally converges to a value that is as close as possible to the target voltage Vtg. The value of N until the corrected target voltage Vtg(adj) converges to a value that is as close as possible to the pre-correction target voltage Vtg, also increases with the value of M, which is the number of images formed in the previous image forming mode.

[0090] Thus, according to the configuration of this embodiment, the CPU 50 predicts the state of the developer according to the driving time of the conveying screws 28, 29 before and after switching the rotation speed of the conveying screws 28, 29, and changes the target voltage after switching the rotation speed according to this prediction. This makes it possible to reflect the state of the developer that changes according to the driving time of the conveying screws 28, 29 before and after switching the rotation speed of the conveying screws 28, 29 in the detection result of the toner concentration of the developer. Therefore, even when the image formation mode is switched, it is possible to suppress a decrease in the detection accuracy of the toner concentration of the developer.

[0091] The state of the developer changes depending on the drive time of the conveying screws 28, 29 before and after switching the rotation speed of the conveying screws 28, 29, and the conveying screws 28, 29 are driven not only during image formation but also at other times such as pre-rotation and post-rotation. Therefore, in the above formula 1, N may be the drive time before switching the rotation speed of the conveying screws 28, 29, and M may be the drive time after switching the rotation speed of the conveying screws 28, 29. This allows the state of the developer, which changes depending on the drive time of the conveying screws 28, 29 before and after switching the rotation speed of the conveying screws 28, 29, to be more accurately reflected in the detection result of the toner concentration.

[0092] Third embodiment Next, a third embodiment of the image forming apparatus according to the present invention will be described with reference to the drawings. The same reference numerals are used to designate parts that are the same as those in the first embodiment, and the description thereof will be omitted.

[0093] In the first embodiment, a configuration was described in which, when the image formation mode is switched, the control voltage Vin is corrected according to the number of sheets on which images were formed in the previous image formation mode. In this embodiment, when the image formation mode is switched, the detection voltage Vout output from the magnetic permeability sensor 53 is corrected according to the number of sheets on which images were formed in the previous image formation mode. The configuration of the image forming apparatus A according to this embodiment differs from the configuration of the first embodiment only in the content of the replenishment sequence, and the other configurations are similar to the configuration of the first embodiment.

[0094] Fig. 12 is a flowchart of a supply sequence according to this embodiment. In Fig. 12, steps that perform the same processes as those described in the first embodiment with reference to Fig. 8 are given the same reference numerals, and the description thereof will be simplified.

[0095] 12, first, the CPU 50 detects the number of images formed in the image formation mode before switching to the currently set image formation mode, i.e., the previous image formation mode (S1). Next, the CPU 50 obtains the amount of erroneous detection Va of the detection voltage Vout when the control voltage Vin corresponding to the image formation mode stored in the memory 51 is input to the magnetic permeability sensor 53 by using the formula 1 (S2).

[0096] Next, the CPU 50 transmits the control voltage Vin corresponding to the image formation mode stored in the memory 51 to the magnetic permeability sensor 53, and receives the detection voltage Vout from the magnetic permeability sensor 53 (S31, S6). The control voltage Vin corresponding to the image formation mode stored in the memory 51 here is 12.5 V when the current image formation mode is the normal mode, and is 12.3 V when the current image formation mode is the low-speed mode.

[0097] Next, the CPU 50 calculates the corrected detection voltage Vout(adj) using the following equation 5 based on the calculated amount of erroneous detection Va and the detection voltage Vout received from the magnetic permeability sensor 53 (S32).

[0098] (Formula 5) Vout(adj)=Vout-Va

[0099] Next, the CPU 50 compares the corrected detected voltage Vout(adj) with the target voltage Vtg stored in the memory 51. If the corrected detected voltage Vout(adj) is less than the target voltage Vtg, the supply motor 54 is driven based on the table stored in the memory 51 to perform a developer supply operation (S33, S8, S9, S10). Note that in the table of this embodiment, the corrected detected voltage Vout(adj) is associated with the developer supply time.

[0100] As described above, the developer state has a transient characteristic in which the developer state gradually adapts from the state before switching the image formation mode to the state after switching. Also, the longer the number of images formed in the previous image formation mode, the longer the number of images formed until the developer state adapts to the state assumed in the current image formation mode. Therefore, the amount of erroneous detection Va is a value that is gradually attenuated according to the number of images formed after switching the image formation mode, and it is necessary to set the amount of attenuation according to the number of images formed in the current image formation mode to a value that is smaller according to the number of images formed in the previous image formation mode.

[0101] In this regard, according to formula 1, as N, which is the number of sheets of image formation in the current image formation mode, increases, the amount of erroneous detection Va decreases, and finally converges to a value infinitesimally close to zero. Also, the value of N until the amount of erroneous detection Va converges to a value infinitesimally close to zero increases as the value of M, which is the number of sheets of image formation in the previous image formation mode, increases.

[0102] According to formula 5, as N, which is the number of images formed in the current image forming mode, increases, the corrected detection voltage Vout(adj) approaches the pre-correction detection voltage Vout, and finally converges to a value that is as close as possible to the detection voltage Vout. The value of N until the corrected detection voltage Vout(adj) converges to a value that is as close as possible to the pre-correction detection voltage Vout, increases as M, which is the number of images formed in the previous image forming mode, increases.

[0103] As described above, according to the configuration of this embodiment, the CPU 50 predicts the state of the developer according to the driving time of the conveying screws 28, 29 before and after switching the rotation speed of the conveying screws 28, 29, and corrects the detection voltage Vout output from the magnetic permeability sensor 53 after switching the rotation speed according to this prediction. This makes it possible to reflect the state of the developer that changes according to the driving time of the conveying screws 28, 29 before and after switching the rotation speed of the conveying screws 28, 29 in the detection result of the magnetic permeability of the developer by the magnetic permeability sensor 53. Therefore, it is possible to suppress a decrease in the detection accuracy of the toner concentration of the developer detected based on the detection result of the magnetic permeability of the developer by the magnetic permeability sensor 53.

[0104] The state of the developer changes depending on the drive time of the conveying screws 28, 29 before and after switching the rotation speed of the conveying screws 28, 29, and the conveying screws 28, 29 are driven not only during image formation but also at other times such as pre-rotation and post-rotation. Therefore, in the above formula 1, N may be the drive time before switching the rotation speed of the conveying screws 28, 29, and M may be the drive time after switching the rotation speed of the conveying screws 28, 29. This makes it possible to accurately reflect the state of the developer, which changes depending on the drive time of the conveying screws 28, 29 before and after switching the rotation speed of the conveying screws 28, 29, in the detection result of the magnetic permeability of the developer by the magnetic permeability sensor 53.

[0105] (Fourth embodiment) Next, a fourth embodiment of the image forming apparatus according to the present invention will be described with reference to the drawings. The same reference numerals are used to designate parts that are the same as those in the first embodiment, and the description thereof will be omitted.

[0106] In the first embodiment, a configuration has been described in which, when the image formation mode is switched, the control voltage Vin is corrected according to the number of images formed in the previous image formation mode. In this embodiment, when the image formation mode is switched, before supplying developer in the image formation mode after the switch, the conveying screws 28 and 29 are rotated idly until the developer reaches the bulk density expected in the image formation mode after the switch. The configuration of the image forming apparatus A according to this embodiment differs from the configuration of the first embodiment only in the content of the supply sequence, and the other configurations are the same as the configuration of the first embodiment.

[0107] Fig. 13 is a flow chart of the supply sequence according to this embodiment. As shown in Fig. 13, first, the CPU 50 judges whether the image forming mode has been switched based on the received image forming job signal (S41, S42). If the CPU 50 judges that the image forming mode has been switched, it detects the number of images formed in the image forming mode before the switching (S43).

[0108] Next, the CPU 50 sets the idling time of the conveying screws 28, 29 according to the number of images formed in the image formation mode before the switching (S44). The CPU 50 sets the idling time of the conveying screws 28, 29 as follows. That is, the memory 51 stores data that associates the number of images formed in the image formation mode before the switching with the idling time of the conveying screws 28, 29. In this embodiment, the data shown in FIG. 14 is stored in the memory 51. The CPU 50 sets the idling time of the conveying screws 28, 29 based on this data.

[0109] The data is generated as follows. That is, as shown in FIG. 7, when the number of images formed in the low-speed mode is 20, the number of images formed until the developer reaches the bulk density assumed in the normal mode after switching to the normal mode is about 10. When the number of images formed in the low-speed mode is 50, the number of images formed until the developer reaches the bulk density assumed in the normal mode after switching to the normal mode is about 30. When the number of images formed in the low-speed mode is 100, the number of images formed until the developer reaches the bulk density assumed in the normal mode after switching to the normal mode is about 85. Such correlation data between the number of images formed before switching the image mode and the number of images formed until the developer reaches the bulk density assumed in the image formation mode after switching is obtained by experiment before shipping the image forming apparatus A, and is stored in the memory 51.

[0110] Next, the CPU 50 rotates the conveying screws 28 and 29 idly for the idling time set in step S44 before switching the image forming mode (S45). This causes the state of the developer to have a bulk density of the developer expected in the image forming mode after switching.

[0111] Next, the CPU 50 transmits the control voltage Vin corresponding to the image formation mode stored in the memory 51 to the magnetic permeability sensor 53, and receives the detection voltage Vout from the magnetic permeability sensor 53 (S46, S47). The control voltage Vin corresponding to the image formation mode stored in the memory 51 here is 12.5 V when the current image formation mode is the normal mode, and is 12.3 V when the current image formation mode is the low-speed mode.

[0112] Next, the CPU 50 compares the detected voltage Vout received from the magnetic permeability sensor 53 with the target voltage Vtg stored in the memory 51. If the detected voltage Vout is less than the target voltage Vtg, the CPU 50 drives the supply motor 54 based on the table stored in the memory 51 to perform a developer supply operation (S48, S49, S50, S51).

[0113] Thus, according to the configuration of this embodiment, when the image mode is switched, the CPU 50 rotates the conveying screws 28 and 29 idly until the developer reaches a bulk density expected for the image forming mode after the switch. This makes it possible to suppress a decrease in the detection accuracy of the toner concentration even when the image forming mode is switched.

[0114] The state of the developer changes depending on the driving time of the conveying screws 28, 29 before and after the rotation speed of the conveying screws 28, 29 is switched, and the conveying screws 28, 29 are driven not only during image formation but also at other times such as pre-rotation and post-rotation. Therefore, data correlating the driving time before switching the rotation speed of the conveying screws 28, 29 with the idling time after switching the rotation speed is stored in the memory 51. Then, in step S43, the driving time before switching the rotation speed of the conveying screws 28, 29 is detected, and in step S44, the idling time is set according to the detected driving time. This makes it possible to further suppress a decrease in the detection accuracy of the toner concentration.

[0115] In addition, although the present embodiment has been described with respect to a configuration for varying the idling time after switching the image formation mode, the present invention is not limited to this. That is, the same effect as above can be obtained by a configuration for executing idling at the end of the image formation operation in anticipation of a change in the image formation mode, or by a configuration for providing an idling time under the conditions of the image formation mode after switching by interrupting the image formation before switching the image formation mode.

[0116] In the first to fourth embodiments, the conveying screws 28, 29 are driven at two rotation speeds, a normal mode and a low speed mode, but the present invention is not limited to this. That is, the conveying screws 28, 29 may be driven at three or more speeds, and the same effect as above can be obtained by performing the above-mentioned control in response to the fluctuation in the rotation speed of the conveying screws 28, 29. [Explanation of symbols]

[0117] 1...Photosensitive drum (photoconductor) 4...Developing device (developing unit) 21...Developing chamber (developer storage section) 22...Mixing chamber (developer storage section) 28...Conveying screw (conveying member) 29...Conveying screw (conveying member) 50…CPU (control unit) 51...Memory (storage unit) 53...Magnetic permeability sensor 55...Toner bottle (developer supply unit) 56...Operation panel (detection section) A: Image forming device

Claims

1. A photoconductor; a developing unit which develops an electrostatic latent image formed on the surface of the photoconductor with a developer containing a non-magnetic toner and a magnetic carrier, the developing unit having a developer container which contains the developer, a transport member which rotates to transport the developer contained in the developer container, and a magnetic permeability sensor which outputs a detection voltage according to the magnetic permeability of the developer contained in the developer container; A drive unit for rotating the conveying member; a control unit that controls the drive unit and the magnetic permeability sensor, the control voltage being input to the magnetic permeability sensor, and the detection voltage corresponding to the magnetic permeability and the control voltage being input from the magnetic permeability sensor; Equipped with The control unit is a first mode in which the drive unit rotates the transport member at a first rotation speed, and a second mode in which the drive unit rotates the transport member at a second rotation speed slower than the first rotation speed, An image forming apparatus characterized in that, when switching between the first mode and the second mode, the control voltage in the mode after the switching is changed depending on the execution time of the mode before the switching and the execution time of the mode after the switching.

2. a storage unit that stores a reference value of the control voltage, the reference value being different between the first mode and the second mode; The image forming apparatus according to claim 1 , wherein the control unit changes the control voltage in the post-switching mode so that the control voltage approaches the reference value of the post-switching mode as the execution time of the post-switching mode becomes longer.

3. When the reference value of the mode after the switching is Vin, the detection voltage last output from the magnetic permeability sensor in the mode before the switching is Vlast, the execution time of the mode before the switching is M, the execution time of the mode after the switching is N, the control voltage of the mode after the switching is Vin(adj), and K1, K2, K3, and K4 are constants, Va=Vlast×K1×exp^-(N-1) / K2×M ２ (3×) Voff=Va×K4 Vin(adj)=Vin-Voff 3. The image forming apparatus according to claim 2, wherein the control voltage in the mode after the switching is determined based on the value obtained by the above formula (2).

4. a developer supply unit that supplies developer to the developer storage unit, 4. The image forming apparatus according to claim 1, wherein the control unit compares the detected voltage with a target voltage, and when the detected voltage is lower than the target voltage, causes the developer supply unit to supply developer.

5. a detection unit for detecting a basis weight of a sheet on which an image is to be formed, 5. The image forming apparatus according to claim 1, wherein the control unit executes the first mode when the basis weight of the sheet is less than a predetermined value, and executes the second mode when the basis weight of the sheet is equal to or greater than the predetermined value.

6. A photoconductor; a developing unit which develops an electrostatic latent image formed on the surface of the photoconductor with a developer containing a non-magnetic toner and a magnetic carrier, the developing unit having a developer container which contains the developer, a transport member which rotates to transport the developer contained in the developer container, and a magnetic permeability sensor which outputs a detection voltage according to the magnetic permeability of the developer contained in the developer container; A drive unit for rotating the conveying member; a developer supply section that supplies developer to the developer storage section; a control unit that controls the drive unit and the developer supply unit, the control unit comparing the detected voltage with a target voltage, and causing the developer supply unit to supply developer when the detected voltage is lower than the target voltage; Equipped with The control unit is a first mode in which the drive unit rotates the transport member at a first rotation speed, and a second mode in which the drive unit rotates the transport member at a second rotation speed slower than the first rotation speed, An image forming apparatus characterized in that, when switching between the first mode and the second mode, the target voltage in the mode after the switching is changed depending on the execution time of the mode before the switching and the execution time of the mode after the switching.

7. a storage unit that stores a reference value of the target voltage; The image forming apparatus according to claim 6 , wherein the control unit changes the target voltage in the post-switching mode such that the target voltage approaches the reference value as an execution time of the post-switching mode becomes longer.

8. When the reference value is Vtg, the detection voltage last output from the magnetic permeability sensor in the mode before the switching is Vlast, the execution time of the mode before the switching is M, the execution time of the mode after the switching is N, the target voltage of the mode after the switching is Vtg(adj), and K1, K2, K3, and K4 are constants, the control unit: Va=Vlast×K1×exp^-(N-1) / K2×M ２ (3×) Vtg(adj)=Vtg+Va 8. The image forming apparatus according to claim 7, wherein the value obtained by the above formula (1) is set as the target voltage value for the mode after the switching.

9. a detection unit for detecting a basis weight of a sheet on which an image is to be formed, 9. The image forming apparatus according to claim 6, wherein the control unit executes the first mode when the basis weight of the sheet is less than a predetermined value, and executes the second mode when the basis weight of the sheet is equal to or greater than the predetermined value.

10. A photoconductor; a developing unit which develops an electrostatic latent image formed on the surface of the photoconductor with a developer containing a non-magnetic toner and a magnetic carrier, the developing unit having a developer container which contains the developer, a transport member which rotates to transport the developer contained in the developer container, and a magnetic permeability sensor which outputs a detection voltage according to the magnetic permeability of the developer contained in the developer container; A drive unit for rotating the conveying member; a developer supply section that supplies developer to the developer storage section; a control unit that controls the drive unit and the developer supply unit, the control unit comparing the detected voltage with a target voltage, and causing the developer supply unit to supply developer when the detected voltage is lower than the target voltage; Equipped with The control unit is a first mode in which the drive unit rotates the transport member at a first rotation speed, and a second mode in which the drive unit rotates the transport member at a second rotation speed slower than the first rotation speed, an image forming apparatus comprising: an image forming unit that, when switching between the first mode and the second mode, corrects the detection voltage output from the magnetic permeability sensor in the mode after the switching depending on the execution time of the mode before the switching and the execution time of the mode after the switching.

11. When the detection voltage last output from the magnetic permeability sensor in the mode before the switching is Vlast, the execution time of the mode before the switching is M, the execution time of the mode after the switching is N, the detection voltage output from the magnetic permeability sensor in the mode after the switching is Vout, the detection voltage corrected in the mode after the switching is Vout(adj), and K1, K2, K3, and K4 are constants, Va=Vlast×K1×exp^-(N-1) / K2×M ２ (3×) Vout(adj)=Vout−Va 11. The image forming apparatus according to claim 10, wherein the value obtained by the above formula (1) is used as the detected voltage value corrected in the mode after the switching.

12. a detection unit for detecting a basis weight of a sheet on which an image is to be formed, 12. The image forming apparatus according to claim 10, wherein the control unit executes the first mode when the basis weight of the sheet is less than a predetermined value, and executes the second mode when the basis weight of the sheet is equal to or greater than the predetermined value.

13. A photoconductor; a developing unit which develops an electrostatic latent image formed on the surface of the photoconductor with a developer containing a non-magnetic toner and a magnetic carrier, the developing unit having a developer container which contains the developer, a transport member which rotates to transport the developer contained in the developer container, and a magnetic permeability sensor which outputs a detection voltage according to the magnetic permeability of the developer contained in the developer container; A drive unit for rotating the conveying member; a developer supply section that supplies developer to the developer storage section; a control unit that controls the transport member and the developer supply unit, the control unit comparing the detected voltage with a target voltage, and causing the developer supply unit to supply developer when the detected voltage is lower than the target voltage; Equipped with The control unit is a first mode in which the driving unit rotates the transport member at a first rotation speed to form an image, and a second mode in which the driving unit rotates the transport member at a second rotation speed slower than the first rotation speed to form an image, When switching between the first mode and the second mode, the conveying member is rotated for a predetermined time in the switched mode before receiving the detection voltage from the magnetic permeability sensor in the switched mode; The image forming apparatus according to claim 1, wherein the predetermined time is set to be longer when the execution time of the mode before the switching is long than when the execution time of the mode before the switching is short.

14. a detection unit for detecting a basis weight of a sheet on which an image is to be formed, 14. The image forming apparatus according to claim 13, wherein the control unit executes the first mode when the basis weight of the sheet is less than a predetermined value, and executes the second mode when the basis weight of the sheet is equal to or greater than the predetermined value.

Images