JP2017111355A - Developing device, image forming apparatus, control method of developing device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make it possible to prevent a deterioration in developability and generation of a noise image due to a toner on a developing roller that has not been transferred to an image carrier accumulating on the developing roller.SOLUTION: A DC bias adjustment part 82 adjusts a second reference voltage of a second bias voltage to be applied to a magnetic roller through predetermined calibration processing. An AC bias adjustment part 83, when the difference between a first reference voltage of a first bias voltage V1 to be applied to a developing roller and the second reference voltage after adjustment performed by the DC bias adjustment part 82 exceeds a predetermined tolerance, executes correction to reduce a duty ratio of an AC component of the first bias voltage.SELECTED DRAWING: Figure 6

Description

  The present invention relates to a developing device, an image forming apparatus, and a developing device control method.

  In general, in an electrophotographic image forming apparatus, a developing device may include a magnetic roller and a developing roller, and develop using an interactive touchdown method. Note that the interactive touchdown method may be referred to as a hybrid development method.

  In the interactive touch-down developing device, the magnetic roller rotates while carrying a two-component developer including toner and a carrier. The developing roller carries and rotates the toner supplied from the magnetic roller, and supplies the toner to an image carrier on which an electrostatic latent image is formed. Usually, the toner remaining on the developing roller without moving to the image carrier is collected in the developing device by the magnetic roller.

  In the developing device, a layer thickness of the toner on the developing roller is detected by a sensor, and a duty ratio of an AC component in a bias voltage applied to the developing roller is in accordance with a detection result of the toner layer thickness. Is known to be adjusted (see, for example, Patent Document 1).

JP 2008-20640 A

  By the way, when the charge amount of the toner is increased in a low temperature and low humidity environment, the toner on the developing roller that has not been transferred to the image carrier is less likely to be collected by the magnetic roller. In particular, the fine powder toner, which is the toner having a small particle diameter, tends to stay on the developing roller. Then, the toner staying on the developing roller causes so-called charge-up, and developability deteriorates. In addition, a noise image due to charging aggregation of the toner tends to occur.

  SUMMARY OF THE INVENTION An object of the present invention is to provide a developing device capable of preventing deterioration of developability and generation of noise images due to the toner on the developing roller that has not transferred to the image bearing member staying in the developing roller, and an image forming apparatus having the developing device It is an object to provide a method for controlling a developing device.

  A developing device according to one aspect of the present invention includes a developing roller, a magnetic roller, a first bias applying unit, a second bias applying unit, a DC bias adjusting unit, and an AC bias adjusting unit. The developing roller carries toner and rotates, and supplies the toner to the image carrier to develop the electrostatic latent image on the surface of the image carrier into a toner image. The magnetic roller carries a developer including the toner and a carrier and rotates to supply the toner to the developing roller. The first bias application unit applies a first bias voltage including a first AC component that changes with a duty ratio set in advance to the plus side and the minus side with respect to the first reference voltage to the developing roller. The second bias applying unit changes to a plus side and a minus side at a level corresponding to a level of the first AC component of the first bias voltage with reference to a second reference voltage, and the first AC component A second bias voltage including a second AC component that changes in an opposite phase is applied to the magnetic roller. The direct current bias adjustment unit adjusts the second reference voltage by a predetermined calibration process. The AC bias adjusting unit is configured to detect the first bias when a reference potential difference that is a difference between the first reference voltage and the second reference voltage adjusted by the DC bias adjusting unit exceeds a predetermined tolerance. Correction for reducing the duty ratio of the first bias voltage in the application unit is executed. The first bias applying unit maintains the maximum voltage of the first bias voltage constant when the duty ratio of the first alternating current component is changed, and the first bias application unit uses the first reference voltage as a reference. The absolute value of the product of the negative peak voltage of one AC component and the residual ratio representing the proportion of the negative AC voltage generation period in the first AC component is the first AC voltage based on the first reference voltage. The minimum voltage of the first bias voltage is adjusted in a direction that matches the absolute value of the product of the positive peak voltage of the component and the duty ratio.

  An image forming apparatus according to another aspect of the present invention includes an image carrier, the developing device, and a transfer device. An electrostatic latent image is formed on the surface of the image carrier. The developing device develops the electrostatic latent image on the surface of the image carrier into a toner image. The transfer device transfers the toner image of the image carrier to a sheet.

  A developing device control method according to another aspect of the present invention is a method for controlling a developing device including the developing roller, the magnetic roller, the first bias applying unit, and the second bias applying unit. . The method includes the following steps. One of the plurality of steps is a step of adjusting the second reference voltage by a predetermined calibration process. Another one of the plurality of steps is when a reference potential difference that is a difference between the first reference voltage and the second reference voltage after adjustment by the DC bias adjustment unit exceeds a predetermined tolerance, It is a step of executing a correction for reducing the duty ratio of the first bias voltage in the first bias applying unit. The first bias applying unit maintains the maximum voltage of the first bias voltage constant when the duty ratio of the first alternating current component is changed, and the first bias application unit uses the first reference voltage as a reference. The absolute value of the product of the negative peak voltage of one AC component and the residual ratio representing the proportion of the negative AC voltage generation period in the first AC component is the first AC voltage based on the first reference voltage. The minimum voltage of the first bias voltage is adjusted in a direction that matches the absolute value of the product of the positive peak voltage of the component and the duty ratio.

  According to the present invention, a developing device that can prevent deterioration in developability and generation of a noise image due to the toner on the developing roller that has not transferred to the image carrier staying in the developing roller, an image forming apparatus including the developing device, and It becomes possible to provide a control method of the developing device.

FIG. 1 is a configuration diagram of an image forming apparatus including a developing device according to the embodiment. FIG. 2 is a configuration diagram of the developing device according to the embodiment. FIG. 3 is a time chart of the first bias voltage and the second bias voltage in the developing device according to the embodiment. FIG. 4 is a diagram illustrating a change of the first bias voltage and the second bias voltage for one cycle in the developing device according to the embodiment. FIG. 5 is a diagram illustrating a specific example of the duty ratio of the AC component of the first bias voltage and the related voltage in the developing device according to the embodiment. FIG. 6 is a block diagram of a control-related unit in an image forming apparatus including the developing device according to the embodiment. FIG. 7 is a flowchart illustrating an example of a procedure of bias adjustment processing in the developing device according to the embodiment. FIG. 8 is a graph showing the relationship between the number of sheets and the number of noise pixels when continuous image forming processing is performed under conditions in which the duty ratio of the bias voltage of the developing roller is different.

  Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. In addition, the following embodiment is an example which actualized this invention, Comprising: It does not have the character which limits the technical scope of this invention.

  First, the configuration of the image forming apparatus 10 including the developing device 43 according to the embodiment will be described with reference to FIG. The image forming apparatus 10 is an apparatus that forms an image on a sheet by an electrophotographic method. The sheet is a sheet-like image forming medium such as paper and an envelope.

  The image forming apparatus 10 includes a sheet supply unit 2, a sheet conveyance unit 3, an image forming unit 40, an optical scanning unit 46, a fixing device 49, a plurality of toner supply units 400, a control unit 8, and the like in the main body unit 1.

  The image forming unit 40 is a general term for devices related to image formation by the developer 9 on the sheet, and includes the developing unit 4. Developer 9 includes toner 9a and carrier 9b.

  The carrier 9b is a granular material having magnetism. For example, it is conceivable that the carrier 9b is a granular body including a granular magnetic body and a synthetic resin film such as an epoxy resin coated on the surface of the magnetic body.

  An image forming apparatus 10 shown in FIG. 1 is a tandem image forming apparatus and a color printer. Therefore, the image forming apparatus 10 includes a plurality of developing units 4 corresponding to each color of cyan, magenta, yellow, and black.

  Each developing unit 4 includes a photoreceptor 41, a charging device 42, a developing device 43, a primary transfer unit 44, a primary cleaning device 45, and the like.

  The tandem image forming apparatus 10 further includes an intermediate transfer belt 47, a secondary transfer unit 48, and a secondary cleaning device 470.

  In the sheet supply unit 2, the sheet delivery unit 22 sends out the sheets one by one from the sheet cassette 21 that stores a plurality of the sheets to the conveyance path 30.

  The sheet conveyance unit 3 conveys the sheet along the conveyance path 30 by a plurality of conveyance rollers 31. Further, a conveyance roller 31 disposed at the exit of the conveyance path 30 discharges the sheet on which the image is formed from the conveyance path 30 onto the discharge tray 101.

  The intermediate transfer belt 47 is a belt-like member formed in an annular shape. The intermediate transfer belt 47 rotates while being stretched between two rollers.

  In each developing unit 4, the drum-shaped photoconductor 41 rotates, and the charging device 42 uniformly charges the surface of the photoconductor 41. The optical scanning unit 46 writes an electrostatic latent image on the surface of the photoconductor 41 by scanning with laser light.

  The developing device 43 uses the two-component developer 9 to develop the electrostatic latent image on the photoreceptor 41 as an image of the toner 9a. As a result, an image of the toner 9 a is formed on the surface of the photoreceptor 41.

  The toner replenishing unit 400 is provided for each color of the toner 9a. Each toner replenishing unit 400 replenishes toner 9 a to each developing device 43.

  The primary transfer unit 44 transfers the image of the toner 9 a on the surface of the photoreceptor 41 to the intermediate transfer belt 47. On the intermediate transfer belt 47, an image of the toner 9a is transferred from each of the plurality of photoreceptors 41. As a result, a color image including a plurality of color toner images 9 a is formed on the intermediate transfer belt 47. The primary cleaning device 45 removes the toner 9 a remaining on the surface of the photoreceptor 41.

  The secondary transfer unit 48 transfers the color image formed on the intermediate transfer belt 47 to the sheet in the conveyance path 30. The secondary cleaning device 470 removes the toner 9a remaining on the intermediate transfer belt 47.

  The fixing device 49 heats the color image transferred to the sheet in the conveyance path 30 to fix the color image on the sheet.

  As described above, the electrostatic latent image is formed on the surface of the photoreceptor 41, and the developing device 43 develops the electrostatic latent image on the surface of the photoreceptor 41 into a toner image. Further, the primary transfer unit 44, the intermediate transfer belt 47, and the secondary transfer unit 48 transfer the toner image on the photoreceptor 41 to the sheet.

  The photoconductor 41 is an example of an image carrier, and the primary transfer unit 44, the intermediate transfer belt 47, and the secondary transfer unit 48 are examples of a transfer device.

  The image forming apparatus 10 further includes a density sensor 800. The density sensor 800 is a sensor that detects the density of the pattern image transferred to the intermediate transfer belt 47. The pattern image is the toner image having a predetermined pattern. The pattern increase is formed on the intermediate transfer belt 47 by the developing unit 4 when a calibration process for adjusting image forming parameters is performed.

  For example, the density sensor 800 is a reflective photosensor including a light emitting unit and a light receiving unit. In this case, the light emitting unit irradiates light on a portion of the intermediate transfer belt 47 where the pattern image is formed, and the light receiving unit receives light irregularly reflected by the pattern image.

  In the density sensor 800, the amount of reflected light from the intermediate transfer belt 47 detected by the light receiving unit represents the density of the pattern image.

  The imaging parameter is adjusted so that the density of the pattern image detected by the density sensor 800 approaches a preset target density. As will be described later, the image forming parameter includes a bias voltage in the developing device 43.

  The control unit 8 controls the developing unit 4 including the developing device 43 and other devices. A part of the control unit 8 constitutes a part of the developing device 43.

  For example, it is conceivable that the control unit 8 includes a micro processor unit (MPU) and a random access memory (RAM) that execute a program stored in a non-illustrated nonvolatile computer-readable storage unit. It is also conceivable that the control unit 8 is configured by one or a combination of the MPU, ASIC (Application Specific Integrated Circuit), and DSP (Digital Signal Processor).

[Configuration of Developing Device 43]
As illustrated in FIG. 2, the developing device 43 includes a developing container 4300, a developing roller 430, a magnetic roller 431, a magnet 432, a blade 434, a stirring mechanism 435, and the like. The magnetic roller 431 is a drum-shaped nonmagnetic material that encloses the magnet 432.

  The developing device 43 including the magnetic roller 431 and the developing roller 430 is a so-called interactive touch-down developing device.

  The developing roller 430, the magnetic roller 431, and the stirring mechanism 435 rotate in the developing container 4300 that stores the developer 9. The stirring mechanism 435 stirs the developer 9, and the toner 9a is charged by being stirred.

  The magnetic roller 431 rotates while carrying the two-component developer 9. The magnetic roller 431 supports the carrier 9b by the magnetic force of a plurality of magnets 432 included therein. The magnetic roller 431 supplies the toner 9 a of the carried developer 9 to the developing roller 430.

  The developing roller 430 rotates while carrying the toner 9a supplied from the magnetic roller 431 on the outer peripheral surface. Further, the developing roller 430 supplies the toner 9 a to the portion of the electrostatic latent image on the surface of the photoreceptor 41. As a result, the developing roller 430 develops the electrostatic latent image as the toner image.

  The plurality of carriers 9 b on the magnetic roller 431 form a magnetic brush by the magnetic field of the magnet 432. The magnetic brush contacts the outer peripheral surface of the developing roller 430. At that time, the toner 9 a adhering to the carrier 9 b moves from the magnetic roller 431 to the developing roller 430.

  The blade 434 limits the layer thickness of the developer 9 carried by the magnetic roller 431. The blade 434 is provided with a gap with respect to the magnetic roller 431 at a position upstream of the position of the magnetic roller 431 facing the developing roller 430 in the rotation direction of the magnetic roller 431. Thereby, the height of the magnetic brush is made substantially constant.

  The toner 9 a moves from the developing roller 430 to the photosensitive member 41 in a part of the layer of the toner 9 a carried on the developing roller 430 facing the electrostatic latent image of the photosensitive member 41. The toner 9 a remaining on the developing roller 430 without moving to the photoreceptor 41 is collected in the developing container 4300 by the magnetic roller 431.

  As shown in FIG. 2, the developing device 43 includes a bias applying unit 7 that applies a bias voltage to each of the developing roller 430 and the magnetic roller 431. The bias application unit 7 includes a first bias application unit 71 that applies a first bias voltage V1 to the developing roller 430, and a second bias application unit 72 that applies a second bias voltage V2 to the magnetic roller 431.

  The first bias voltage V1 is a voltage in which the first AC voltage Va1 is superimposed on the DC first reference voltage Vd1. That is, the first AC voltage Va1 is an AC component of the first bias voltage V1 that changes in a constant cycle from the first reference voltage Vd1 to the plus side and the minus side (see FIG. 3). Hereinafter, this AC component is referred to as a first AC component.

  The second bias voltage V2 is a voltage obtained by superposing the second AC voltage Va2 on the DC second reference voltage Vd2. In other words, the second AC voltage Va2 is an AC component of the first bias voltage V1 that changes in a constant cycle from the second reference voltage Vd2 to the plus side and the minus side. Hereinafter, this AC component is referred to as a second AC component.

  As shown in FIG. 3, the first AC voltage Va1, which is the first AC component, changes with a duty ratio Dt10 set in advance to the plus side and the minus side with respect to the first reference voltage Vd1. The duty ratio Dt10 is a ratio representing the ratio of the positive voltage generation period in the first AC component.

  In the following description, the ratio representing the ratio of the negative voltage generation period in the first AC component is referred to as a residual ratio Dt11.

  When the duty ratio Dt10 is expressed as a percentage, the sum of the duty ratio Dt10 and the residual ratio Dt11 is 100%. That is, the remainder obtained by subtracting the duty ratio Dt10 from 100% is the remaining ratio Dt11.

  As shown in FIG. 4, the positive side peak voltage of the first AC component with respect to the first reference voltage Vd1, that is, the positive side of the first AC voltage Va1 that changes on the basis of the first reference voltage Vd1. The amplitude is referred to as a first plus side peak voltage Vp11. Similarly, the negative peak voltage of the first AC component with reference to the first reference voltage Vd1 is referred to as a first negative peak voltage Vp12.

  Similarly, the positive-side peak voltage of the second AC component with respect to the second reference voltage Vd2, that is, the positive-side amplitude of the second AC voltage Va2 that changes with the second reference voltage Vd2 as a reference. This is referred to as 2 plus side peak voltage Vp21. Similarly, the negative peak voltage of the second AC component with reference to the second reference voltage Vd2 is referred to as a second negative peak voltage Vp22.

  The first bias applying unit 71 maintains the maximum voltage V1max of the first bias voltage V1 at a predetermined constant level. Further, in the first bias applying unit 71, the absolute value of the product of the first plus-side peak voltage Vp11 and the duty ratio Dt10 matches the absolute value of the product of the first minus-side peak voltage Vp12 and the residual ratio Dt11. Thus, the minimum voltage V1min of the first bias voltage V1 is adjusted.

  Accordingly, the first bias applying unit 71 maintains the maximum voltage V1max constant when the duty ratio Dt10 of the first AC component is changed, and the product of the first negative peak voltage Vp12 and the residual ratio Dt11. The minimum voltage V1min is adjusted so that the absolute value of coincides with the absolute value of the product of the first plus-side peak voltage Vp11 and the duty ratio Dt10.

  On the other hand, the second bias applying unit 72 generates a second bias voltage V 2 as shown in FIGS. 3 and 4 and applies it to the magnetic roller 431.

  As shown in FIG. 3, the second AC voltage Va2 that is the second AC component changes with an opposite phase to the first AC component with respect to the second reference voltage Vd2. Accordingly, the duty ratio Dt20 of the second AC component is equal to the residual ratio Dt11 of the first AC component. Similarly, the residual ratio Dt21 of the second AC component is equal to the duty ratio Dt10 of the first AC component.

  The duty ratio Dt20 is a ratio that represents the ratio of the positive voltage generation period in the second AC component. Further, the residual ratio Dt21 is a ratio representing the ratio of the negative voltage generation period in the second AC component.

  Further, the second bias applying unit 72 converts the second AC voltage Va2 that is the level of the second AC component in the second bias voltage V2 to the first AC that is the level of the first AC component of the first bias voltage V1. Proportional to voltage Va1.

  Since the first AC component and the second AC component are 180 ° out of phase, the first AC voltage Va1 and the second AC voltage Va2 are opposite in polarity. Therefore, when the proportionality coefficient from the first AC voltage Va1 to the second AC voltage Va2 is K0, Va2 = −K0 × Va1.

  The proportional coefficient K0 is a coefficient determined by the ratio of the number of turns of the pair of coils in the transformer included in the second bias applying unit 72.

  Therefore, the absolute value of the product of the second plus-side peak voltage Vp21 and the duty ratio Dt20 matches the absolute value of the product of the second minus-side peak voltage Vp22 and the residual ratio Dt21.

  In the following description, a potential difference obtained by subtracting the first reference voltage Vd1 from the second reference voltage Vd2 is referred to as a reference potential difference ΔVd. When the development is performed, the bias applying unit 7 sets the first reference voltage Vd1 and the second reference voltage Vd2 so that the reference potential difference ΔVd has the same polarity as the charging polarity of the toner 9a.

  The carrier 9b is charged with the opposite polarity to the charging polarity of the toner 9a by frictional charging. The second reference voltage Vd2 based on the ground potential is a voltage having a polarity opposite to the charging polarity of the carrier 9b.

  For example, when the charging polarity of the toner 9a is positive, the reference potential difference ΔVd is positive. Further, when the charging polarity of the toner 9a is positive, the charging polarity of the carrier 9b is negative. Therefore, the polarity of the second reference voltage Vd2 with respect to the ground potential is positive.

  For example, in the bias application unit 7 in which the proportional coefficient K0 is 0.7053, the first reference voltage Vd1 is 70 (V), the first positive side peak voltage Vp11 is 900 (V), and the second reference voltage Vd2 is 360 (V ) Is set.

  In the above case, the reference potential difference ΔVd is 290 (V). The first plus-side peak voltage Vp11 is 630 (V), and the maximum voltage V1max of the first bias voltage V1 is 970 (V). Also,

  In the above case, the bias applying unit 7 generates voltages related to the first bias voltage V1 and the second bias voltage V2, as shown in FIG. 5, according to the duty ratio Dt10 of the first AC component set separately. Adjust.

  For example, when the duty ratio Dt10 is set to 40%, the first bias applying unit 71 sets the first negative peak voltage Vp12 to − so that the absolute value of Dt10 × Vp12 matches the absolute value of Dt10 × Vp11. Adjust to 600 (V). Accordingly, the minimum voltage V1min of the first bias voltage V1 is adjusted to −530 (V). In this case, the residual ratio Dt11 is 60%.

  Further, the second bias application unit 72 adjusts the second negative peak voltage Vp22 to −635 (V) so that Vp22 = −K0 × Vp11, and the second bias application unit 72 adjusts the second negative peak voltage Vp22 to −635 (V). The 2 plus side peak voltage Vp21 is adjusted to 423 (V). As a result, the minimum voltage V2min of the second bias voltage V2 is adjusted to -530 (V).

  Therefore, the minimum voltage V2min of the second bias voltage V2 is −275 (V), and the maximum voltage V2max of the second bias voltage V2 is 783 (V).

  Further, the first peak potential difference ΔVp1 which is the difference between the maximum voltage V1max of the first bias voltage V1 and the minimum voltage V2min of the second bias voltage V2 becomes 1245 (V) (see FIGS. 4 and 5). Further, the second peak potential difference ΔVp2 that is the difference between the maximum voltage V2max of the second bias voltage V2 and the minimum voltage V1min of the first bias voltage V1 is 1313 (V).

  When the duty ratio Dt10 of the first AC component is set to 36% and 32%, respectively, the first negative peak voltage Vp12, the second positive peak voltage Vp21 and the second negative side are the same as described above. The peak voltage Vp22 is adjusted to a voltage as shown in FIG.

  By the way, when the charge amount of the toner 9a increases in a low-temperature and low-humidity environment, the toner 9a on the developing roller 430 that has not moved to the photoconductor 41 is difficult to be collected by the magnetic roller 431. In particular, the fine powder toner, which is the toner 9 a having a small particle diameter, tends to stay on the developing roller 430. Then, the toner 9a staying on the developing roller 430 causes so-called charge-up, and developability deteriorates. In addition, a noise image due to charge aggregation of the toner 9a is likely to occur.

  The developing device 43 has a function of preventing deterioration in developability and generation of a noise image due to the toner 9a on the developing roller 430 that has not moved to the photoreceptor 41 staying in the developing roller 430. Hereinafter, control of the first bias voltage V1 and the second bias voltage V2 for realizing the function will be described.

[Configuration of Control Unit 8]
As shown in FIG. 6, the control unit 8 includes an image formation control unit 81, a DC bias adjustment unit 82, an AC bias adjustment unit 83, and the like. The image forming control unit 81 controls the image forming unit 40.

  For example, the image forming control unit 81 causes the developing unit 4 to execute a process for forming the pattern image on the intermediate transfer belt 47 when the calibration process is performed. As described above, the density sensor 800 detects the density of the pattern image.

  The DC bias adjusting unit 82 controls the second bias applying unit 72 based on the detected density of the pattern image in a bias adjusting process described later. As a result, the DC bias adjusting unit 82 adjusts the second reference voltage Vd2.

  The AC bias adjusting unit 83 controls the first bias applying unit 71 when a predetermined condition is satisfied in a bias adjusting process described later. Accordingly, the AC bias adjusting unit 83 adjusts the parameter of the first AC component of the first bias voltage V1.

[Bias adjustment processing]
Next, an example of the procedure of the bias adjustment process executed by the control unit 8 will be described with reference to the flowchart shown in FIG.

  The bias adjustment process is executed when the image forming apparatus 10 is started up and when the image forming process is completed. In the following description, S1, S2,... Represent identification codes of processes executed by the control unit 8.

<Process S1>
In the bias adjustment process, first, the calibration process is executed. In the calibration process, the image forming control unit 81 causes the developing unit 4 to execute a process for forming the predetermined pattern image on the intermediate transfer belt 47.

  Further, in the calibration process, the DC bias adjusting unit 82 adjusts the second reference voltage Vd2 of the second bias applying unit 72 according to the detected density of the pattern image by the density sensor 800.

  In the calibration process, the DC bias adjusting unit 82 adjusts at least the second reference voltage Vd2. It is also conceivable that the DC bias adjusting unit 82 adjusts both the first reference voltage Vd1 and the second reference voltage Vd2.

  For example, in the calibration process, the DC bias adjusting unit 82 is different from the first bias applying unit 71 and the second bias applying unit 72 in a plurality of sets of the first reference voltage Vd1 and the second reference voltage, each having a different reference potential difference ΔVd. Vd2 is set sequentially.

  In addition, every time the first reference voltage Vd1 and the second reference voltage Vd2 are set, the image forming control unit 81 causes the developing unit 4 to execute the process of forming the same pattern image. As a result, the density sensor 800 obtains the detected densities of the plurality of pattern images corresponding to the set combinations of the first reference voltage Vd1 and the second reference voltage Vd2.

  Then, the DC bias adjusting unit 82 is configured to output the first reference voltage Vd1 and the second reference voltage Vd1 when the detected density closest to the preset target density is obtained among the plurality of detected densities obtained by the density sensor 800. The reference voltage Vd2 is set in the first bias application unit 71 and the second bias application unit 72.

<Process S2>
Next, whether the AC bias adjusting unit 83 has a reference potential difference ΔVd that is a difference between the first reference voltage Vd1 and the second reference voltage Vd2 adjusted by the DC bias adjusting unit 82 exceeds a predetermined tolerance ΔV0. Determine whether or not.

  When the first reference voltage Vd1 is adjusted by the DC bias adjusting unit 82, the reference potential difference ΔVd compared with the tolerance ΔV0 is a difference between the adjusted first reference voltage Vd1 and second reference voltage Vd2.

<Process S3>
When the reference potential difference ΔVd exceeds the allowable difference ΔV0, the AC bias adjusting unit 83 performs correction for reducing the duty ratio Dt10 of the first bias voltage V1 with respect to the first bias applying unit 71.

  For example, the AC bias adjusting unit 83 performs correction for reducing the duty ratio Dt10 by about 10% to 20%. In this case, if the duty ratio Dt10 before correction is 40%, the duty ratio Dt10 is corrected to a range of about 36% to 32%.

  The above correction means that the first bias applying unit 71 is adjusted to increase the minimum voltage V1min without changing the maximum voltage V1max of the first bias voltage V1. As a result, the second bias applying unit 72 adjusts the second bias voltage V1 in the direction of decreasing the maximum voltage V2min without changing the minimum voltage V2min.

<Step S4>
On the other hand, when the reference potential difference ΔVd does not exceed the tolerance ΔV0, the AC bias adjusting unit 83 cancels the correction of the duty ratio Dt10 for the first bias applying unit 71.

  After the process of step S3 or step S4 is performed, the bias adjustment process ends.

  When the calibration process is performed in a state where the charge amount of the toner 9a is high, the reference potential difference ΔVd increases so that a necessary amount of toner 9a is secured on the developing roller 430.

  Therefore, when the image density is optimized by the calibration process, a large reference potential difference ΔVd indicates that the charge amount of the toner 9a is high. Under such circumstances, the fine powder toner stays on the developing roller 430, and developability deteriorates due to the charge-up, and a noise image is easily generated due to charge aggregation of the toner 9a.

  On the other hand, the smaller the second peak potential difference ΔVp2 is, the more the toner 9a is peeled off from the developing roller 430 by the magnetic roller 431. As described above, the second peak potential difference ΔVp2 is the difference between the maximum voltage V2max of the second bias voltage V2 and the minimum voltage V1min of the first bias voltage V1 (see FIG. 4).

  Accordingly, when the first bias voltage V1 and the second bias voltage V2 are adjusted so that the second peak potential difference ΔVp2 is reduced under a situation where the charge amount of the toner 9a is high, the toner 9a on the developing roller 430 is charged up. Is suppressed. As a result, it is possible to avoid deterioration in developability and generation of noise images.

  On the other hand, when the first peak potential difference ΔVp <b> 1 is small, the toner 9 a on the magnetic roller 431 is not easily transferred to the developing roller 430. Therefore, the first peak potential difference ΔVp1 needs to be maintained at an appropriate level.

  As shown in FIGS. 4 and 5, when the duty ratio Dt10 of the first AC component of the first bias voltage V1 is lowered, the minimum voltage V1min of the first bias voltage V1 is increased, and the maximum of the second bias voltage V2 is increased. The voltage V2max is lowered. As a result, the second peak potential difference ΔVp2 is reduced.

  Therefore, when the reference potential difference ΔVd is large, the AC bias adjusting unit 83 performs correction to reduce the duty ratio Dt10 of the first AC component of the first bias voltage V1 (S3). As a result, the charge-up of the toner 9a on the developing roller 430 is suppressed, and deterioration of developability and generation of a noise image can be avoided.

  Further, as shown in FIG. 5, even when the duty ratio Dt10 decreases, the first peak potential difference ΔVp1 is maintained. Therefore, the transferability of the toner 9a from the magnetic roller 431 to the developing roller 430 is appropriately maintained.

  FIG. 8 is a test of a situation in which noise pixels appear on each of the sheets when continuous image forming processing is performed on five sheets under the conditions of the three types of duty ratios Dt10 shown in FIG. Results are shown.

As shown in FIG. 8, when the reference potential difference ΔVd is 290 (V) and the duty ratio Dt10 is 40%, more noise pixels are added to the sheet as the number of sheets for continuous image formation increases. appear.

  On the other hand, even if the reference potential difference ΔVd is 290 (V), if the duty ratio Dt10 is lowered to 36% or 32%, the number of noise pixels appearing on the sheet is greatly reduced to the extent that the image quality is not deteriorated. Decrease.

  Further, even when the duty ratio Dt10 was lowered by about 10% to 20% from the state after the calibration process was performed, there was no noticeable adverse effect on the image quality.

[Application example]
It is conceivable that the developing device 43 shown above is applied to a monochrome image forming apparatus. In this case, in the calibration process, the density sensor 800 detects the density of the pattern image formed on the surface of the photoreceptor 41, for example.

  The developing device, the image forming apparatus, and the developing device control method according to the present invention can be freely combined or implemented within the scope of the invention described in each claim. It is also possible to configure by modifying the form and application examples as appropriate or omitting some of them.

1: Main unit 2: Sheet supply unit 3: Sheet transport unit 4: Development unit 7: Bias application unit 8: Control unit 9: Developer 9a: Toner 9b: Carrier 10: Image forming apparatus 21: Sheet cassette 22: Sheet delivery Part 30: Conveying path 31: Conveying roller 40: Image forming part 41: Photoconductor (image carrier)
42: charging device 43: developing device 44: primary transfer unit 45: primary cleaning device 46: optical scanning unit 47: intermediate transfer belt 48: secondary transfer unit 49: fixing device 71: first bias applying unit 72: second bias Application unit 81: Image formation control unit 82: DC bias adjustment unit 83: AC bias adjustment unit 101: Discharge tray 400: Toner replenishment unit 430: Development roller 431: Magnetic roller 432: Magnet 434: Blade 435: Stirring mechanism 470: Two Next cleaning device 800: Density sensor 4300: Developer container Dt10: Duty ratio Dt11: Residual ratio Dt20: Duty ratio Dt21: Residual ratio K0: Proportional coefficient V1: First bias voltage V1max: Maximum voltage V1min of first bias voltage: First Minimum bias voltage V2: second bias Voltage V2max: Maximum voltage V2min of the second bias voltage: Minimum voltage Va1 of the second bias voltage: First AC voltage Va2: Second AC voltage Vd1: First reference voltage Vd2: Second reference voltage Vp11: First positive peak Voltage Vp12: First negative peak voltage Vp21: Second positive peak voltage Vp22: Second negative peak voltage ΔV0: Tolerance ΔVd: Reference potential difference ΔVp1: First peak potential difference ΔVp2: Second peak potential difference

Claims (4)

A developing roller for carrying and rotating the toner, and developing the electrostatic latent image on the surface of the image carrier to a toner image by supplying the toner to the image carrier;
A magnetic roller that carries and rotates the developer including the toner and carrier, and supplies the toner to the developing roller;
A first bias applying unit that applies a first bias voltage including a first AC component that changes at a preset duty ratio to the plus side and the minus side with respect to the first reference voltage to the developing roller;
The second reference voltage is changed to a plus side and a minus side at a level corresponding to the level of the first AC component of the first bias voltage with reference to the second reference voltage, and the phase is changed in an opposite phase with respect to the first AC component. A second bias applying unit that applies a second bias voltage including two AC components to the magnetic roller;
A DC bias adjusting unit that adjusts the second reference voltage by a predetermined calibration process;
When the reference potential difference, which is the difference between the first reference voltage and the second reference voltage adjusted by the DC bias adjusting unit, exceeds a predetermined tolerance, the first bias in the first bias applying unit An AC bias adjustment unit that performs correction to reduce the duty ratio of the voltage,
The first bias applying unit maintains the maximum voltage of the first bias voltage constant when the duty ratio of the first alternating current component is changed, and the first bias application unit uses the first reference voltage as a reference. The absolute value of the product of the negative peak voltage of one AC component and the residual ratio representing the proportion of the negative AC voltage generation period in the first AC component is the first AC voltage based on the first reference voltage. A developing device that adjusts the minimum voltage of the first bias voltage in a direction that coincides with an absolute value of a product of a plus-side peak voltage of a component and the duty ratio.   2. The developing device according to claim 1, wherein the second bias applying unit causes the level of the second AC component in the second bias voltage to be proportional to the level of the first AC component of the first bias voltage. An image carrier on which an electrostatic latent image is formed on the surface;
The developing device according to claim 1 or 2, wherein the electrostatic latent image on the surface of the image carrier is developed into a toner image.
A transfer device that transfers the toner image of the image carrier to a sheet. A developing roller that carries and rotates toner and supplies the toner to the image carrier;
A magnetic roller that carries and rotates the developer including the toner and carrier, and supplies the toner to the developing roller;
A first bias applying unit that applies a first bias voltage including a first AC component that changes in a constant cycle to the plus side and the minus side with respect to the first reference voltage to the developing roller;
A second bias that includes a second AC component that changes in accordance with a change in the first AC component of the first bias voltage with reference to a second reference voltage and that changes in an opposite phase to the first AC component. A second bias applying unit that applies a voltage to the magnetic roller, and a control method of the developing device,
Adjusting the second reference voltage by a predetermined calibration process;
When the reference potential difference, which is the difference between the first reference voltage and the second reference voltage adjusted by the DC bias adjusting unit, exceeds a predetermined tolerance, the first bias in the first bias applying unit Performing a correction to reduce the duty ratio of the voltage,
The first bias applying unit maintains the maximum voltage of the first bias voltage constant when the duty ratio of the first alternating current component is changed, and the first bias application unit uses the first reference voltage as a reference. The absolute value of the product of the negative peak voltage of one AC component and the residual ratio representing the proportion of the negative AC voltage generation period in the first AC component is the first AC voltage based on the first reference voltage. A developing device control method for adjusting a minimum voltage of the first bias voltage in a direction that coincides with an absolute value of a product of a plus-side peak voltage of a component and the duty ratio.

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