JP2007079233A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus capable of immediately outputting a stable image by always suitably controlling toner concentration even if developer is left for a long time between jobs. <P>SOLUTION: The image forming apparatus includes an image carrier, an optical writing means, a developing means, a toner concentration detecting means, a time measuring means, a storage means for storing data detected by the toner concentration detecting means, and a control means for controlling the operation of each section. In the image forming apparatus, control means causes the toner concentration detecting means to detect the concentration of toner before or after each of all job for image formation, causes the storage means to store detected data, compares data captured at the start of each job and data captured after the previous job, thereby calculates the degree of change in quantity of charged developer, and controls the operation of each section at the start of each job according to the calculated degree of change in quantity of charged developer. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an image forming apparatus that forms an image by an electrophotographic method, and more particularly to an image forming apparatus having a developing device that performs development using a two-component developer.

  In an image forming apparatus equipped with a developing unit that performs development using a two-component developer composed of toner and carrier, the toner concentration is detected by measuring the magnetic permeability of the developer to control the toner concentration of the developer. It is widely done. Toner density detection using changes in magnetic permeability corresponding to changes in developer toner density makes use of light and mass that are difficult due to the inherent functions of toner, such as opacity and small specific gravity. It is used instead of detection.

  The toner density detection using the magnetic permeability of the developer (hereinafter referred to as a magnetic permeability sensor) is a detection method that uses the change in density of the magnetic material in the developer when the toner density of the developer changes. It is known that the density of the magnetic material in the developer changes due to causes other than the change in toner density.

  The developer is agitated in the development process, and as a result of the reduction in the bulk density of the developer by this agitation, the density of the magnetic substance in the developer is lowered. On the other hand, if the developer is left for a long time without being driven by the developing device, the bulk density of the developer increases and the density of the magnetic material also increases. Such a phenomenon causes a problem that the value detected by the magnetic permeability sensor changes even if the toner density of the developer does not change.

  As a measure for solving the problem that occurs when the developer is left for a long time, a technique is disclosed in which the toner supply is stopped for a certain time when the leaving time exceeds a predetermined time (for example, Patent Documents). 1). However, the technique described in Patent Document 1 does not deal with the influence of environmental conditions such as temperature and humidity, and does not deal with the influence of the toner consumption per screen that changes depending on the image density of the output image. It can be said that it is difficult to keep the image density constant only by the time factor.

A technique for controlling the toner replenishment amount according to the difference in the output value of the toner density sensor before and after the developer is left for a long time is also disclosed (for example, see Patent Document 2). Measures for controlling the toner concentration are performed only during warm-up. When the image forming apparatus is idling for a long time, the toner density sensor output changes due to the decrease in the charge amount of the developer. The problem of replenishing toner beyond the change cannot be dealt with.
Japanese Patent Laid-Open No. 9-6120 JP-A-10-186833

  The present invention solves the above-described problems, and even when the developer is left unattended for a long time after turning on the power, the toner density is always properly controlled to stably output a stable image. An object of the present invention is to provide an image forming apparatus.

  The object of the present invention can be achieved by the following constitution.

(1) an image carrier to which a uniform potential is applied to the surface;
A developing device that mounts a two-component developer on a position facing the image carrier and rotates the developer carrier, and visualizes the electrostatic latent image formed on the image carrier; ,
Toner density detecting means disposed in the developing device;
Storage means for storing data detected by the toner density detection means;
In an image forming apparatus having
For one or a plurality of jobs for image formation, the toner density is detected by the toner density detection unit before and after each job, and the detected data is stored in the storage unit. Control that compares the data at the end of the previous job to calculate the change amount of the developer charge amount, and controls the toner concentration by the toner concentration control means at the start of the job according to the calculated change amount of the developer charge amount An image forming apparatus comprising: means.

  (2) The image forming apparatus according to (1), wherein the toner density control means is a toner density detection sensor, and the toner density is controlled by controlling an output value of the toner density detection sensor.

  (3) The image forming apparatus according to (1) or (2), wherein the toner concentration detection sensor is a magnetic permeability sensor that measures the magnetic permeability of the developer.

  (4) The image forming apparatus according to any one of (1) to (3), wherein the storage unit includes a nonvolatile memory.

  (5) The control unit includes a time measuring unit that measures a continuous driving time of the developing device, and causes the time measuring unit to measure the continuous driving time of the developing device at the end of each job, and the time 5. The image formation according to any one of (1) to (4), wherein a control voltage value of the toner density detection sensor is corrected according to a continuous driving time of the developing device measured by a measuring unit. apparatus.

  (6) The control means corrects the control voltage value of the toner density detection sensor based on the environmental condition measured by the environmental condition measurement means. Any one of (1) to (5) The image forming apparatus described in 1.

  According to the present invention, before and after each job for forming an image, the toner density detection unit is used to detect the toner density, the detected toner density data is stored in the storage unit, and data before the job is started. And the data after the previous job start is calculated to calculate the change amount of the developer charge amount. According to the calculated change amount of the developer charge amount, the toner concentration control means controls the toner concentration before starting the job. As a result, a good image can be output even if a long time elapses between jobs after power-on. Further, by detecting the toner density before and after each job, stable toner density control can always be performed without being affected even if there is a variation in image density between jobs.

  The image forming apparatus of the present invention will be described below with reference to the drawings. Note that the present invention is not limited thereby.

  FIG. 1 is a configuration diagram of an image forming apparatus according to an embodiment of the present invention.

  The image forming apparatus A is called a tandem type color image forming apparatus, and includes a plurality of sets of image forming units 10Y, 10M, 10C, and 10K, a belt-shaped intermediate transfer member 6, a sheet feeding device 20, and a fixing device 30. It is composed of

  Above the image forming apparatus A, an image reading device SC is installed. The document placed on the document table is scanned and exposed by the optical system of the document image scanning exposure device of the image reading device SC and read by the line image sensor. The analog signal photoelectrically converted by the line image sensor is subjected to analog processing, A / D conversion, shading correction, image compression processing, etc. in the image processing unit, and then input to the optical writing means 3Y, 3M, 3C, 3K. Is done.

  An image forming unit 10Y that forms a yellow (Y) image includes a charging unit 2Y, an optical writing unit 3Y, a developing device 4Y, and a cleaning unit 5Y disposed around a photosensitive drum 1Y as an image carrier. . An image forming unit 10M that forms a magenta (M) color image includes a photosensitive drum 1M as an image carrier, a charging unit 2M, an optical writing unit 3M, a developing device 4M, and a cleaning unit 5M. An image forming unit 10C for forming a cyan (C) color image includes a photosensitive drum 1C as an image carrier, a charging unit 2C, an optical writing unit 3C, a developing device 4C, and a cleaning unit 5C. The image forming unit 10K that forms a black (K) image includes a photosensitive drum 1K as an image carrier, a charging unit 2K, an optical writing unit 3K, a developing device 4K, and a cleaning unit 5K.

  The charging unit 2Y and the optical writing unit 3Y, the charging unit 2M and the optical writing unit 3M, the charging unit 2C and the optical writing unit 3C, and the charging unit 2K and the optical writing unit 3K constitute a latent image forming unit.

  Reference numerals 4Y, 4M, 4C, and 4K denote developing devices that contain a two-component developer composed of a small particle size toner of yellow (Y), magenta (M), cyan (C), and black (K) and a carrier.

  The intermediate transfer body 6 is wound around a plurality of rollers and is rotatably supported.

  Each color image formed by the image forming units 10Y, 10M, 10C, and 10K is sequentially transferred onto the rotating intermediate transfer body 6 by the primary transfer units 7Y, 7M, 7C, and 7K, and a combined color image is obtained. It is formed.

  A recording medium (hereinafter referred to as a recording sheet) P accommodated in a sheet feeding cassette 21 of the sheet feeding device 20 is fed by a sheet feeding means (first sheet feeding unit) 22 and fed by sheet feeding rollers 23, 24, 25, 26, registration rollers (second paper feed unit) 27, etc., are conveyed to the secondary transfer means 9, and the color image is transferred onto the recording paper P.

  Note that the three-stage sheet feeding cassettes 21 arranged vertically in the vertical direction below the image forming apparatus A have substantially the same configuration. The three-stage sheet feeding means 22 has almost the same configuration. The sheet feeding cassette 21 and the sheet feeding means 22 are collectively referred to as a sheet feeding device 20.

  The recording paper P onto which the color image has been transferred is sandwiched by the fixing device 30 and the color toner image (or toner image) on the recording paper P is fixed by applying heat and pressure to the recording paper P. The sheet is fixed on the upper side, sandwiched between the sheet discharge rollers 28 and placed on a sheet discharge tray 29 outside the apparatus.

  On the other hand, after the color image is transferred to the recording paper P by the secondary transfer means 9, the residual toner is removed by the cleaning means 8 from the intermediate transfer body 6 from which the recording paper P is separated by curvature.

  The control means 60 is disposed inside the image forming apparatus A and controls the operation of each part of the apparatus. A memory 61 that is a storage unit is built in the control unit 60.

  The time measuring unit 70 is a device that measures the driving time and operation stop time of the developing device using a known technique, and is included in the image forming apparatus A together with the environmental condition measuring unit 71 that measures the temperature and humidity in the image forming apparatus. Arranged.

  In the description of the image forming apparatus A, the color image formation has been described. However, the present invention includes a case of forming a monochrome image.

  Hereinafter, the photosensitive drums 1Y, 1M, 1C, and 1K are referred to as an image carrier (hereinafter referred to as a photosensitive drum) 1, the charging units 2Y, 2M, 2C, and 2K are referred to as a charging unit 2 and the optical writing unit 3Y. 3M, 3C, and 3K are referred to as an optical writing unit 3, and the developing devices 4Y, 4M, 4C, and 4K are referred to as a developing device 4.

  FIG. 2 is a cross-sectional view showing an embodiment of the developing device.

  The housing of the developing device 4 has a two-part configuration including a lower lower casing 40 and an upper upper casing 50, and can be opened and closed.

  In the lower casing 40 of the developing device 4, a developing roller 41 that is a developer carrying member, a first developer conveying and agitating member 43 that is first and second developer conveying and agitating means, and a second developer. A conveying stirring member 44, a developer supply means 45, a developer supply roller 45, a developer guide member 46, and the like are arranged.

  The lower casing 40 includes a developer transport supply chamber 401 that houses the first developer transport stirring member 43 and a developer stirring chamber 402 that houses the second developer transport stirring member 44. The developer conveyance supply chamber 401 and the developer agitation chamber 402 are formed on both sides with a partition wall 403 standing upright from the bottom of the lower casing 40.

  The developing roller 41 includes a rotatable developing sleeve 41A and a fixed magnet roll (magnetic field generating means) 41B, and is driven and rotated by a developing driving motor M (not shown).

  At the opposing proximity point between the developing sleeve 41A and the first developer conveying and agitating member 43, the developing sleeve 41A rotates from below to above, and the first developer conveying and agitating member 43 rotates from above to below.

  In the upper casing 50 covering the upper part of the lower casing 40, a developer layer thickness regulating member 42 as developer layer thickness regulating means, a layer thickness regulating member mounting plate 51 for supporting the developer layer thickness regulating member 42, a developer A developer upper surface guide plate 52 and the like for guiding the developer conveyed by the supply roller 45 are disposed. The developer upper surface guide plate 52 is provided with a toner concentration detection sensor 48 as toner concentration detection means.

  A layer thickness regulating member mounting plate 51 that holds the developer layer thickness regulating member 42 and a developer upper surface guide plate 52 that guides the developer to be conveyed are fixed to the ceiling portion inside the upper casing 50. From the surface of the developer supply roller 45 to the surface of the developing sleeve 41A, an upper surface of the developer guide member 46, a layer thickness regulating member mounting plate 51 and a developer upper surface guide plate 52 fixed to the ceiling portion inside the upper casing 50 A developer supply chamber 49 having a wide space is formed between them, and a large amount of developer can be accommodated.

  By disposing the developer supply roller 45 above the first developer conveying and agitating member 43, it is possible to form a developer layer without unevenness in density or pitch on the developing roller.

  Further, in the developer supply chamber 49 formed by installing the developer upper surface guide plate 52 and the developer guide member 46, the developer can be stably conveyed and supplied in a uniform bulk density state. It was.

  Since the toner density detection sensor is disposed in the developer supply chamber 49 separated from the developer delivery path V4, the toner density can be normally detected without being affected by the drop of the developer.

  The developing roller 41 is disposed to face the photosensitive drum 1 carrying the electrostatic latent image, and is driven to rotate by a driving source (not shown). On the developing sleeve 41A, an AC voltage from the AC power source E1 and a DC voltage from the DC power source E2 are superimposed as a developing bias.

  The magnet roll 41B is disposed inward of the developing sleeve 41A and has seven magnetic poles N1, N2, N3, S1, S2, S3, and S4. The magnetic pole N1 is a main magnetic pole, the magnetic pole S1 is a stripped magnetic pole, and the magnetic pole S2 is a raised magnetic pole. The developer layer thickness regulating member 42 is disposed in the vicinity of the magnetic pole N2 of the magnet roll 41B.

  Two magnetic poles S1 and S2 adjacent to each other among the plurality of magnetic poles of the magnet roll 41B are arranged in the same polarity to form a repulsive magnetic field. The stripping magnetic pole S1 for stripping off the developer strips off and disperses the developer on the developing sleeve 41A. The developer peeled off from the developing sleeve 41 </ b> A is transferred to the first developer transport stirring member 43 and stirred while being transported in a direction parallel to the rotation axis of the developing roller 41. The developer receiving upper magnetic pole S2 draws up the developer supplied by the developer supply roller 45 and deposits it on the developing sleeve 41A.

  The first developer conveying and stirring member 43 agitates and conveys the developer conveyed from the second developer conveying and agitating member 44 and supplies it uniformly to the developer supply roller 45. Both the first developer conveying and stirring member 43 and the second developer conveying and stirring member 44 are formed of a helical screw member.

  The second developer conveyance stirring member 44 is arranged in parallel with the first developer conveyance stirring member 43, and mixes and stirs the new toner replenished from the toner replenishing means 47 and the developer refluxed from the developing sleeve 41A. To the upstream portion of the first developer transport stirring member 43.

  The first developer conveyance stirring member 43 conveys the developer in the direction of the rotation axis and releases the developer in a direction substantially perpendicular to the rotation axis.

  The developing roller 41 of the developing device 4 shown in FIG. 2 includes a magnet roll 41B having a 7-pole fixed magnet. The developer layer thickness regulating member 42 is made of a magnetic material, and regulates the layer thickness of the developer on the developing sleeve 41A. The developer supply roller 45 includes a magnet roll having five-pole fixed magnets N1, N2, N3, S1, and S2. The developer supply roller 45 holds the developer conveyed from the first developer conveyance agitating member 43 and holds it on the developing sleeve 41A. Supply.

  In the vicinity of the opposing proximity point between the developing sleeve 41A and the first developer conveying and agitating member 43, the lower developer peeled off from the developing sleeve 41A and conveyed in the direction of the white arrow shown in FIG. 2, and the developing sleeve A developer guide member 46 that separates the upper developer supplied to 41A and deposits the developer conveyed by the developer supply roller 45 and guides it to the developing roller 41 is disposed in an inclined manner.

  The toner replenishing means 47 is disposed above the developing device 4 and is provided in the developer stirring chamber from an opening (not shown) of the upper casing 50 formed on the upstream side in the developer transport direction of the second developer transport stirring member 44. Toner is supplied into 402.

  FIG. 3 is a graph showing how the toner density sensor output value at the time of image formation of the developer housed in the developing device varies depending on the number of prints depending on the developer leaving time. The graph shown in FIG. 3 shows the relationship between the vertical axis representing the developer density and the horizontal axis representing the number of prints. In FIG. 3, solid lines a1, a2, and a3 represent the output values of the toner density sensor at the image forming time Tg1 of job 1, the image forming time Tg2 of job 2, and the image forming time Tg3 of job 3, respectively. Further, the leaving time when the developer leaving time between job 1 and job 2 is small is represented by Th1, and the leaving time when the developer leaving time between job 2 and job 3 is long is represented by Th2. .

  As can be seen from FIG. 3, the difference between the final output value Vs11 for job 1 and the first output value Vs20 for job 2 is small before and after Th1 when the developer leaving time is small, but the developer leaving time is long. Before and after Th2, the difference between the final output value Vs21 for job 2 and the first output value Vs30 for job 3 increases. That is, as the time for which the toner is left is increased, the bulk density of the developer increases, and the toner concentration detection sensor outputs a toner shortage detection signal even though the toner concentration does not actually change when image formation is resumed. The toner density is replenished and the toner density value (output value) increases.

  To solve this problem, in the image forming apparatus according to the present invention, the toner density is measured by the toner density detection sensor before and after all the jobs for image formation, and the calculated toner density data (output value) is used for each job. The amount of change in developer charge is calculated by comparing before and after, and the toner concentration in the developing device is corrected by controlling the output value of the toner concentration detection sensor according to the calculated amount of change in charge. Yes.

  FIG. 4 is a control block diagram relating to toner density control of the image forming apparatus according to the embodiment of the present invention.

  In the image forming apparatus A, a toner density detection control program is input to the memory 61 in advance, and the toner density detection control program repeats toner density detection within a short time (for example, 1 second). It consists of a plurality of tasks for density control processing.

  In the first task after the start of the job, the control means 60 uses the toner concentration detection data of the developer by the toner concentration detection sensor 48 in the final task at the time of image formation of the previous job, and the toner concentration in the first task after the job starts. The detection data is compared, and the process conditions, that is, the exposure amount of the optical writing means 3, the rotation speed of the development drive motor M, or the bias voltages of the development bias power supplies E1 and E2 are controlled according to the difference. Since the memory 61 is composed of a non-volatile memory, data is stored even when the power is turned off.

  In the second and subsequent tasks after the job starts, the control unit 60 measures the developer stirring time and controls the process conditions. Since the memory 61 is composed of a non-volatile memory, data is stored even when the power is turned off.

  Further, the controller 60 detects the toner concentration detection sensor according to the continuous driving time data of the developing device 4 sent via the time measuring means 70 and according to the temperature / humidity data sent from the environmental condition measuring means 71. The control voltage value of 48 is corrected.

  FIG. 5 is a graph based on experimental data showing the relationship between the developer leaving time in which the developer is left in the state where the developing device 4 is not driven and the output change amount of the toner density detection sensor 48. FIG. An experiment showing the relationship between the continuous stirring drive time when the apparatus 4 is continuously stirred and the leaving time correction amount obtained by converting the output value of the toner density detection sensor 48 recovered by the continuous stirring into the correction amount of the developer leaving time. It is the graph created based on data. The amount of change in the output value of the toner density detection sensor 48 can be calculated from the developer leaving time in which the developer is left in a state where the developing device 4 is not driven, using the graph of FIG. The correction value of the developer leaving time can be calculated from the time during which the developing device 4 is continuously driven. Further, by programming the data created based on the graphs of FIGS. 5 and 6 and storing it in the memory 61, the amount of change in the output value of the toner density detection sensor 48 can be calculated from the developer leaving time. The correction amount of the developer leaving time can be easily calculated from the continuous driving time of the developing device 4. Further, by combining the two graphs, the correction amount of the output value of the toner density detection sensor 48 can be calculated from the continuous drive time of the developing device 4.

  FIG. 7 is a flowchart regarding toner density control of the image forming apparatus according to the embodiment of the present invention.

  First, a start signal for the current job is input by the operator (step S21). The control means 60 calls the toner density detection control program from the memory 61 to start toner density control. First, the toner density detection task for the first time after the print job for toner density control processing of the current job is started. If it is the first toner concentration detection task (Yes at Step S22), the process proceeds to Step S23, and if it is the second or later toner concentration detection task (No at Step S22), Step S27 is performed. Proceed to

  When it is the first toner density detection task after the start of the print job (Yes at Step S22), the process proceeds to Step 23, where the control means 60 controls the control voltage value (reference value) and sensor output value of the last task in the previous job. Are called from the memory 61 (step S23). Next, with respect to the sensor control voltage value (reference value) of the first task in the current job, the number of image formation correction, temperature / humidity correction, and developer agitation time correction are calculated and executed to determine the output of the control voltage value. The value is determined, and the sensor output value is detected by the toner density sensor 48 (step S24).

  Next, the control means 60 compares and calculates the output value detected this time and the output value of the previous job, and calculates the change amount of the developer charge amount (step S25).

  Specifically, the previous toner density sensor control voltage value stored in the memory 61 is represented by Vc1, the toner density sensor output value is represented by Vs1, the current toner density sensor control voltage value is represented by Vc2, and the toner density sensor output value. Is represented by Vs2, the change amount of the developer charge amount of the toner density sensor control voltage value (expressed by the difference between the toner density sensor control voltage value Vc2 and the toner density sensor output value Vs2, also referred to as a toner density false detection amount). .) ΔTr (Vc), the change amount ΔTr (Vs) of the developer charge amount of the toner density sensor output value, and the change amount ΔTr of the developer charge amount are expressed by the following equations, so that stable toner density control can be performed. It has become possible.

ΔTr (Vc) = {(Vc2−Vc1) / 20} × 100
ΔTr (Vs) = {(Vs2−Vs1) / 35} × 100
ΔTr = ΔTr (Vs) −ΔTr (Vc)
Next, the control means 60 corrects the sensor output value from the change amount of the developer charge amount of the toner density sensor control voltage value by using a program inputted in advance to the memory 61 (step 26), and proceeds to step S31. .

  In step S22, when the print job is a toner density detection task for the second and subsequent times after start (No in step S22), the process proceeds to step S27, and the control unit 60 calculates the sensor control voltage value of the current toner density detection task. The output determination value is determined, and the toner concentration detection sensor 48 is made to detect the sensor output value (step S27). Next, the control means 60 stores the calculated sensor control voltage value and the detected sensor output value in the memory 61 (step S28).

  Next, the control means 60 determines whether or not toner replenishment is necessary from the amount of change in the developer charge amount (Step S29). If necessary (Yes in Step S29), the process proceeds to Step S30, and if not necessary (Step S29). (No in S29) Proceed to step S31.

  In step S30, the control means 60 instructs the toner replenishing means 47 to replenish toner in an amount corresponding to the sensor output judgment value of the toner density detection sensor 48 (step S30).

  In step S31, the control means 60 determines whether or not the current job is finished. When the current job is not completed (No at Step S31), the process returns to Step S7, and the operations after Step S7 are repeated.

  When the job is finished (Yes at Step S31), the process proceeds to Step S32, and the control means 60 issues a command to the image forming means 10, stores the sensor control voltage value calculated last in the memory 61, and finishes the job. (Step S32).

  By correcting the control voltage value of the toner density detection sensor 48 in accordance with the continuous driving time of the developing device 4 measured by the time measuring means 70 according to the present invention, the sensor output value that is changed while the control voltage value is neglected. As a result, the toner density control can be quickly returned to the normal state.

  Further, by correcting the control voltage value of the toner concentration detection sensor 48 in accordance with the continuous driving time of the developing device 4 measured by the environmental condition measuring means 71 according to the present invention, the control voltage value varies with temperature and humidity. It is possible to quickly return the toner density control to the normal state by approaching the sensor output value that changes with the charge amount of the developer.

1 is a configuration diagram of an image forming apparatus according to an embodiment of the present invention. It is sectional drawing which shows embodiment of the developing device which concerns on this invention. 6 is a graph showing a change in toner density of a developer stored in a developing device when an image is formed and when the image is left standing over time. FIG. 3 is a control block diagram relating to toner density control of the image forming apparatus according to the embodiment of the present invention. 6 is a graph based on experimental data showing a relationship between a developer leaving time in which a developer is left in a state where the developing device 4 is not driven and an output change amount of a toner density detection sensor 48; 7 is a graph showing a relationship between a continuous stirring driving time of the developing device 4 and a leaving time correction amount obtained by converting an output value of the toner density detection sensor 48 into a developer leaving time correction amount. 6 is a flowchart relating to toner density control of the image forming apparatus according to the embodiment of the present invention.

Explanation of symbols

1, 1Y, 1M, 1C, 1K Photosensitive drum (image carrier)
4, 4Y, 4M, 4C, 4K Developing device 10, 10Y, 10M, 10C, 10K Image forming means 47 Toner replenishing means 48 Toner density detecting sensor (toner density detecting means)
60 Control means 61 Memory (storage means)
70 Time measuring means 71 Environmental condition measuring means A Image forming apparatus

Claims (6)

An image carrier to which a uniform potential is applied to the surface;
A developing device that mounts a two-component developer on a position facing the image carrier and rotates the developer carrier, and visualizes the electrostatic latent image formed on the image carrier; ,
Toner density detecting means disposed in the developing device;
Storage means for storing data detected by the toner density detection means;
In an image forming apparatus having
For one or a plurality of jobs for image formation, the toner density is detected by the toner density detection unit before and after each job, and the detected data is stored in the storage unit. Control that compares the data at the end of the previous job to calculate the change amount of the developer charge amount, and controls the toner concentration by the toner concentration control means at the start of the job according to the calculated change amount of the developer charge amount An image forming apparatus comprising: means. The image forming apparatus according to claim 1, wherein the toner density control unit is a toner density detection sensor, and the toner density is controlled by controlling an output value of the toner density detection sensor. The image forming apparatus according to claim 1, wherein the toner concentration detection sensor is a magnetic permeability sensor that measures the magnetic permeability of the developer. The image forming apparatus according to claim 1, wherein the storage unit includes a nonvolatile memory. The control unit includes a time measuring unit that measures a continuous driving time of the developing device, causes the continuous driving time of the developing device at the end of each job to be measured by the time measuring unit, and the time measuring unit 5. The image forming apparatus according to claim 1, wherein the control voltage value of the toner density detection sensor is corrected in accordance with the measured continuous drive time of the developing device. The image forming apparatus according to claim 1, wherein the control unit corrects a control voltage value of the toner density detection sensor based on an environmental condition measured by the environmental condition measuring unit. apparatus.

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