JP5196966B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus having correction means for correcting image forming conditions in an image forming apparatus using an electrophotographic system.

  Conventionally, a charging member for charging the surface of the photosensitive member is provided, and an electrostatic latent image is formed by irradiating the charged surface of the photosensitive member with a laser beam, and the bright portion potential VL of the electrostatic latent image and the developing DC bias Vdc. Many image forming apparatuses that supply toner to an electrostatic latent image in accordance with the value of the potential difference Vcont are known.

  In such an image forming apparatus, since the density of the obtained image greatly depends on the amount of toner supplied to the electrostatic latent image, the potential difference Vcont between the bright portion potential VL of the electrostatic latent image and the development DC bias Vdc is set to a desired value. It is important to keep the potential difference. In the following description, the potential difference Vcont between the bright portion potential VL of the electrostatic latent image and the developing bias Vdc is referred to as the developing contrast Vcont.

  In order to maintain the development contrast Vcont at a desired value, in recent years, a method of charging a photoconductor by a contact AC charging method has been widespread (Patent Document 1). In the contact AC charging method, for example, a charging roller as a charging member is brought into contact with the surface of a photosensitive drum used as a photosensitive member, and an AC voltage is superimposed on the charging roller with an AC component superimposed on the charging roller. This is a method of applying a voltage that changes automatically).

  In contrast to the contact AC charging method, the contact DC charging method is also known in the past, but the contact AC charging method is more stable than the contact DC charging method in which the resistance value of the charging roller is likely to fluctuate due to environmental fluctuations. Thus, the surface of the photosensitive drum can be charged to a desired voltage.

  In recent years, organic photoconductors (OPC) are often used as materials for photoreceptors used in image forming apparatuses. As such an OPC photoreceptor, for example, a stacked photoreceptor in which functions such as a charge injection blocking layer, a charge generation layer, and a charge transport layer are further separated on a metal substrate is often used.

  According to this configuration, the charge generation layer generates positive and negative charge pairs upon exposure, and the charge transport layer transports only positive charges generated in the charge generation layer to the surface of the photoreceptor. Therefore, when the surface of the photoreceptor negatively charged by the contact AC charging is exposed, the negative charge is attenuated by the positive charge transported through the charge transport layer, and thereby the surface of the photoreceptor is electrostatically charged. A latent image is formed.

  However, the OPC photoconductor has a property that the sensitivity of the photoconductor is likely to change due to variations in use environment, accumulation of light exposure, and manufacturing of the photoconductor. Further, the OPC photoreceptor has a property that the surface of the photoreceptor is easily scraped. If the surface is scraped off, the electrostatic capacity of the photoreceptor will change greatly.

  That is, even if the surface of the photosensitive member is stably charged by the contact AC charging method, the bright portion potential VL of the electrostatic latent image obtained by irradiating the laser beam after charging is equal to the sensitivity of the photosensitive member described above. It tends to fluctuate due to changes and changes in capacitance, and it is not easy to keep it constant. When the bright portion potential VL fluctuates, it becomes difficult to keep the development contrast Vcont constant, and it becomes difficult to stabilize the image density.

  On the other hand, in order to stabilize the image density, a method of arranging a sensor or the like for detecting fluctuations in the bright portion potential VL in the image forming apparatus is also conceivable. However, if such a sensor is separately provided, the image forming apparatus is reduced in size and cost.

Therefore, in Patent Document 2, contact AC charging is performed with respect to the light portion potential VL of the photosensitive member, and the charging DC current Idc flowing at that time is measured (hereinafter, this process is referred to as “current detection process”). Discloses a method for detecting the light portion potential VL.
JP-A 63-149669 Japanese Patent No. 3239441

  However, when the conventional image forming apparatus performs continuous printing particularly in a low temperature and low humidity environment, there arises a problem that the image density varies between pages. This is because the surface of the photoconductor is exposed a plurality of times when continuous printing is performed, and as a result, the sensitivity of the surface of the photoconductor changes.

  For example, in the multilayer photoconductor described above, when exposure is repeated in a low temperature and low humidity environment, the resistance of the layer between the charge generation layer and the metal substrate increases. As a result, it is considered that the positive and negative charge pairs generated by exposure are easily trapped inside the photoconductor, the negative charge on the surface of the photoconductor is not attenuated, and the bright portion potential is increased.

  FIG. 10 is a conceptual diagram showing changes in the surface potential of the photoreceptor when continuous printing is performed. The horizontal axis represents the image formation time, and the vertical axis represents the bright portion potential. A solid line a indicates a tendency in a low temperature and low humidity environment, and a broken line b indicates a tendency in a normal temperature and normal humidity environment.

  Under a low-temperature and low-humidity environment, the initial bright portion potential is VL1, from which a potential increase (minus direction) is observed in a stepwise manner, and is saturated at the potential VL2. This tendency is called VL up. As a result, the development contrast Vcont becomes smaller at the last Vcont2 than the initial Vcont1 of continuous printing, the amount of toner adhering to the electrostatic latent image is reduced, and the image density is reduced.

  On the other hand, in the normal temperature and normal humidity environment, the potential increase tendency is not observed, and the initial potential VL1 is maintained, and a stable image density is obtained.

  That is, the above-described conventional image forming apparatus is configured to detect the bright portion potential VL in advance at the time of the pre-rotation of printing, but corrects the variation in VL caused by the exposure of the photosensitive member a plurality of times during the continuous printing process. No information is disclosed about the configuration. Therefore, in the conventional image forming apparatus, it is difficult to feed back fluctuations in the bright portion potential VL during continuous printing and to correct the image forming conditions accordingly.

  The present invention has been made in view of the above-described situation, and it is possible to obtain a stable image density by predicting the potential fluctuation of the surface of the photoreceptor with a simple configuration and correcting the image forming conditions according to the prediction result. An object is to provide a possible image forming apparatus.

In order to achieve the above object, according to the present invention, a photoconductor that is rotationally driven, a charging member that charges the surface of the photoconductor, an exposure device that emits exposure light to the surface of the photoconductor, and the charging An image forming apparatus comprising: a current detection unit configured to detect a charging current flowing through the member; and charging the surface of the photosensitive member by the charging member, the exposure device exposing the charged surface of the photosensitive member, and the exposure A series of steps in which the charging member recharges the exposed portion by the apparatus, the current detecting means detects the charging current flowing through the charging member at that time, and the potential of the exposed portion is calculated based on the detection result. The image forming apparatus includes a correcting unit that performs a plurality of times in succession and corrects the image forming conditions based on the calculated potentials of the plurality of exposed portions.

  According to the present invention, there is provided an image forming apparatus capable of obtaining a stable image density by predicting a potential change on the surface of a photoreceptor with a simple configuration and correcting an image forming condition according to the prediction result. It becomes possible.

  The best mode for carrying out the present invention will be exemplarily described in detail below based on the embodiments with reference to the drawings. However, the dimensions, materials, shapes, relative arrangements, and the like of the components described in this embodiment are not intended to limit the scope of the present invention only to those unless otherwise specified. Absent.

[First embodiment]
An image forming apparatus according to a first embodiment of the present invention will be described. The image forming apparatus according to the present embodiment is an electrophotographic color image forming apparatus that performs contact AC charging.

(Overall configuration of image forming apparatus)
FIG. 11 shows a schematic configuration of the image forming apparatus according to the present embodiment. As shown in FIG. 11, the image forming apparatus according to the present embodiment includes an image forming unit 20 that forms an image on a sheet material, and a controller 10 that controls driving of the image forming unit 20.

  The image forming unit 20 includes stations corresponding to toners of four colors of yellow (Y), magenta (M), cyan (C), and black (K), and each station has a toner image on the intermediate transfer belt 27. Is transferred in a superimposed manner to obtain a desired image. The process speed of the image forming apparatus according to the present embodiment is 100 mm / s.

  First, the image forming unit 20 exposes the surfaces of the photosensitive drums (photosensitive members) 22Y, 22M, 22C, and 22K with laser light (exposure light) irradiated based on image information, thereby forming an electrostatic latent image. The photosensitive drum 22 is configured to be rotatable. The laser light is emitted from the scanner units (exposure devices) 24Y, 24M, 24C, and 24K based on the image information input from the controller 10.

  Further, the photosensitive drum 22 before being irradiated with the laser light is charged in advance by chargers 23Y, 23M, 23C, and 23K as primary charging members. Each charger 23 is provided with charging rollers 23YS, 23MS, 23CS, and 23KS having an outer diameter of Φ14 mm for charging by contacting the surface of the corresponding photosensitive drum 22. In this embodiment, the surface of the photosensitive drum 22 is charged by a contact AC charging method.

  Developers 26Y, 26M, 26C, and 26K that are configured to be attachable to and detachable from the apparatus main body supply toner to the electrostatic latent images formed on the surfaces of the respective photosensitive drums 22; The electrostatic latent image is developed as a single color toner image. The toner of each color is accommodated in toner cartridges 25Y, 25M, 25C, and 25K, and the developing sleeves 26YS, 26MS, 26CS, and 26KS provided in the developing units 26 contact the surface of the photosensitive drum 22 to supply the toner. To do.

  When supplying toner to the surface of the photosensitive drum 22, a developing DC bias Vdc is applied to the developing device 26, and a potential difference (development contrast Vcont) with the bright portion potential VL of the electrostatic latent image is used. In this configuration, toner is electrostatically supplied to the surface of the photosensitive drum 22.

  The monochromatic toner images developed on the respective photosensitive drums 22 are superimposed and transferred onto the intermediate transfer belt 27, and a desired multicolor toner image is developed on the intermediate transfer belt 27. The intermediate transfer belt 27 is provided in contact with each photosensitive drum 22 and rotates clockwise in FIG.

  The desired multicolor toner image formed on the intermediate transfer belt 27 is conveyed to the nip portion between the intermediate transfer belt 27 and the transfer roller 28 along with the movement of the intermediate transfer belt 27, where the multicolor toner image is formed on the sheet material 11. Transcribed. The transfer roller 28 contacts the sheet material 11 at the position 28a during transfer, and is separated to the position 28b when transfer is not performed (when non-image formation is performed).

  A plurality of sheet materials 11 are stacked on the feeding unit, and the image forming apparatus according to the present embodiment includes a feeding unit 21a and a manual feeding unit 21b. The toner remaining without being transferred onto the sheet material from the intermediate transfer belt 27 is scraped off by the cleaning unit 29.

  The sheet material 11 to which the multicolor toner image has been transferred is conveyed to the fixing unit 30 on the downstream side of the conveyance path of the sheet material 11, and the multicolor toner image is fixed on the sheet material 11 in the fixing unit 30. The fixing unit 30 includes a fixing roller 31 and a pressure roller 32, and heaters 33 and 34 are provided therein. The sheet material 11 is conveyed to the nip portion between the fixing roller 31 and the pressure roller 32, and is fixed by being pressurized and heated at the nip portion.

  Then, the sheet material 11 on which the multicolor toner image is fixed is discharged onto a discharge tray (not shown). Hereinafter, the configurations of the contact AC charging method, the controller 10, and the photosensitive drum 22 described above will be described in more detail.

(Contact AC charging method)
The contact AC charging method in this embodiment will be described. In general, the charging method for charging the surface of the photosensitive drum mainly includes a contact DC charging method and a contact AC charging method.

  In both charging methods, the photosensitive drum is charged by bringing a conductive charging roller into pressure contact with the photosensitive drum and applying a high voltage thereto. Specifically, since charging is performed by discharging from the charging roller to the photosensitive drum, charging is started by applying a voltage equal to or higher than a threshold voltage on the charging roller. This voltage is referred to as a charging start voltage Vth.

  For example, when the charging roller is brought into pressure contact with a photosensitive drum (layer) having a thickness of 25 μm, the surface potential of the photosensitive drum starts to rise when a voltage of about 640 V or more is applied, and thereafter, the applied voltage is applied. On the other hand, the potential of the photosensitive drum surface increases linearly with an inclination of 1.

  From the above, in order to obtain a desired photosensitive drum surface potential VD required for the image forming process, a voltage of VD + Vth may be applied to the charging roller. Here, a charging method in which a DC voltage is applied to the charging roller is referred to as a contact DC charging method.

However, in the contact DC charging method, if the resistance value of the charging roller fluctuates due to a change in the surrounding environment, the value of the charging start voltage Vth fluctuates, so it is difficult to set the surface potential of the photosensitive drum to a desired value VD. .

  Therefore, in this embodiment, the surface of the photosensitive drum is charged by the contact AC charging method. In the contact AC charging method, an oscillating voltage obtained by superimposing an AC component having a peak-to-peak voltage of 2 × Vth or more on a DC voltage corresponding to a desired photosensitive drum surface potential VD (a voltage whose voltage value periodically changes with time), This is a charging method applied to the charging roller.

  This is for the purpose of smoothing the potential due to the AC component. The potential on the surface of the photosensitive drum converges to VD, which is the center of the peak of the AC potential, and may be affected by disturbances such as the surrounding environment. Is low.

(Configuration of controller)
The controller 10 includes an arithmetic unit (arithmetic unit) 12 including a CPU, a ROM 13 in which a program of the arithmetic unit 12 is stored, and a RAM 14 in which work areas and various tables used during control execution are defined. The controller 10 controls the tone correction unit such as a look-up table based on the output value from each sensor to provide a desired color on the sheet material by feedback. It also controls the operation of the image forming process.

(Configuration of photosensitive drum)
The layer structure of the photosensitive drums 22Y, 22M, 22C, and 22K in the present embodiment is shown in FIG. The photosensitive drum 22 in the present embodiment is configured by applying an organic optical transmission layer to the outer periphery of an aluminum cylinder, and rotates when a driving force of a driving motor (not shown) is transmitted. The drive motor rotates the photosensitive drums 22Y, 22M, 22C, and 22K in the counterclockwise direction according to the image forming operation. Further, the outer diameter of the photosensitive drum 22 of the present embodiment is φ30 mm.

  The substrate of the photosensitive layer is not particularly limited as long as it has conductivity, and the Al substrate 22a is used in the present embodiment. An undercoat layer 22b having a barrier function and an adhesive function is provided on the Al base 22a.

  Polyvinyl alcohol, polyethylene oxide, nitrocellulose, or ethylcellulose is used as the material for the undercoat layer 22b used in the present embodiment. Alternatively, methyl cellulose, ethylene-acrylic acid copolymer, alcohol-soluble amide, polyamide, polyurethane, casein, glue, gelatin, or the like may be used. The undercoat layer 22b is formed by applying a solution obtained by dissolving these materials in an appropriate solvent on the Al base 22a and drying it.

  On the undercoat layer 22b, a medium-resistance positive charge injection preventing layer 22c that serves to prevent the positive charge injected from the aluminum substrate 22a from canceling the negative charge charged on the surface of the photosensitive drum 22 is provided. Provided.

  A charge generation layer 22d containing a charge generation material is provided on the positive charge injection prevention layer 22c. Examples of the charge generation material used for the charge generation layer 22d include azo pigments such as monoazo, disazo, and trisazo, and phthalocyanine pigments such as metal phthalocyanine and nonmetal phthalocyanine.

  Indigo pigments such as indigo and thioindigo, perylene pigments such as perylene anhydride and perylene imide, polycyclic quinone pigments such as anthraquinone and pyrenequinone, squarylium dyes, pyrylium salts and thiapyrylium salts may also be used. .

  Furthermore, triphenylmethane dyes, inorganic substances such as selenium, selenium-tellurium, amorphous silicon, quinacridone pigments, azulenium salt pigments, cyanine dyes, xanthene dyes, quinoneimine dyes, styryl dyes, cadmium sulfide, zinc oxide, etc. Good. Among these, metal phthalocyanines such as oxytitanium phthalocyanine, hydroxygallium phthalocyanine, and chlorogallium phthalocyanine are particularly preferable.

  The charge generation layer 22d can be formed by applying a charge generation layer coating solution obtained by dispersing a charge generation material together with a binder resin and a solvent and drying the coating solution. Examples of the dispersion method include a method using a homogenizer, an ultrasonic wave, a ball mill, a sand mill, an attritor, a roll mill and the like. The ratio between the charge generating material and the binder resin is preferably in the range of 10: 1 to 1:10 (mass ratio), and more preferably in the range of 3: 1 to 1: 1 (mass ratio).

  The solvent used in the coating solution for the charge generation layer is selected from the solubility and dispersion stability of the binder resin and charge generation material used, and the organic solvents include alcohols, sulfoxides, ketones, ethers, esters, aliphatic halogens. Hydrocarbons and aromatic compounds.

  When applying the coating solution for the charge generation layer, for example, a coating method such as a dip coating method (a dip coating method), a spray coating method, a spinner coating method, a roller coating method, a Meyer bar coating method, or a blade coating method is used. be able to.

  A charge transport layer 22e containing a charge generation material is provided on the charge generation layer 22d. The charge transport layer 22e is formed of a suitable charge transport material, and examples thereof include triarylamine compounds, hydrazone compounds, styryl compounds, stilbene compounds, pyrazoline compounds, oxazole compounds, thiazole compounds, and triallylmethane compounds.

  As the binder resin for the charge transport layer 22e, acrylic resin, styrene resin, polyester resin, polycarbonate resin, or polyarylate resin is used. Further, a polysulfone resin, a polyphenylene oxide resin, an epoxy resin, a polyurethane resin, an alkyd resin, an unsaturated resin, or the like may be used. In particular, polymethyl methacrylate resin, polystyrene resin, styrene-acrylonitrile copolymer resin, polycarbonate resin, polyarylate resin, diallyl phthalate resin and the like are preferable. These can be used singly or in combination of two or more as a mixture or copolymer.

  The charge transport layer 22e can be formed by applying a charge transport layer coating solution obtained by dissolving a charge transport material and a binder resin in a solvent, and drying it. The ratio between the charge transport material and the binder resin is preferably in the range of 2: 1 to 1: 2 (mass ratio).

  As the solvent for the coating solution for the charge transport layer, ketones such as acetone and methyl ethyl ketone, esters such as methyl acetate and ethyl acetate, ethers such as dimethoxymethane and dimethoxyethane are used. Alternatively, aromatic hydrocarbons such as toluene and xylene, hydrocarbons substituted with halogen atoms such as chlorobenzene, chloroform, or carbon tetrachloride may be used.

  When applying the coating solution for the charge transport layer, for example, a coating method such as a dip coating method (dip coating method), a spray coating method, a spinner coating method, a roller coating method, a Meyer bar coating method, a blade coating method, or the like is used. be able to.

A surface protective layer 22f is provided as a surface layer on the charge transport layer 22e. The surface protective layer 22f is formed by applying a coating solution obtained by dissolving or diluting a curable phenolic resin with a solvent or the like onto the photosensitive layer, thereby forming a polymerization reaction after coating. Is.

(Outline of image density control method)
An outline of an image density control method for stably maintaining an image density at a desired density in consideration of a variation in the bright portion potential VL that occurs during continuous printing will be described.

  Part of the image density control is a control that keeps the maximum density of each color constant (hereinafter referred to as Dmax control) and a control that keeps the halftone gradation characteristics linear with respect to the image signal (hereinafter referred to as Dhalf control). Is going.

  Dmax control takes into account that the maximum density of each color is affected by the film thickness of the photosensitive drum and the atmospheric environment, and the charging bias and development bias are obtained from the result of the environment detection and the CRG tag information so as to obtain a desired maximum density. The image forming conditions such as are set.

  On the other hand, Dhalf control cancels the γ characteristic in order to prevent the output density from deviating from the input image signal due to the nonlinear input / output characteristic (γ characteristic) peculiar to electrophotography. In this control, image processing is performed to keep the input / output characteristics linear.

  Specifically, a plurality of toner patches having different input image signals are detected by an optical sensor to obtain a relationship between the input image signal and density. From this relationship, the control configuration converts the image signal input to the image forming apparatus so that a desired density is obtained with respect to the input image signal. This Dhalf control is performed after image forming conditions such as a charging bias and a developing bias are determined by Dmax control.

  When the density of the output image changes with the image formation time due to fluctuations in the bright portion potential VL generated in the process of continuous printing, the Dmax control and the Dhalf control are frequently performed, for example, for every five printed sheets, It is possible to suppress taste variation.

  However, frequently performing the Dmax control and the Dhalf control is not realistic because the printing speed is greatly reduced and the productivity of the image forming apparatus is significantly reduced. For this reason, in the present embodiment, the Dmax control and the Dhalf control are performed only once for every 1000 printed sheets.

  The Dmax control and the Dhalf control in the present embodiment are set to the timing of once per 1000 printed sheets. However, the timing is not limited to this, and other timings may be used. The structure which does not perform may be sufficient. Further, the timing for performing Dmax control and Dhalf control may be determined based on toner consumption and the like instead of the number of prints.

  In the present embodiment, the Dmax control and the Dhalf control are performed only once for every 1000 printed sheets. Therefore, in a low humidity environment, the light portion potential VL varies greatly during that time. Therefore, when image density control is performed only with Dmax control and Dhalf control, a stable image density cannot be obtained.

  Therefore, in this embodiment, an image density control method other than Dmax control and Dhalf control is performed. That is, a control method is adopted in which the development DC bias Vdc (image forming conditions) determined by the Dmax control is sequentially corrected so that the development contrast Vcont becomes constant by predicting VL fluctuations that occur during the continuous printing process. To do.

In the present embodiment, a method of correcting the development DC bias Vdc will be described as an example of an image forming condition to be corrected. Specifically, the “current detection step” is performed to determine the bright portion potential VL by measuring the charging current Idc flowing in the bright portion potential, and the VL fluctuation is predicted by performing this for a predetermined number of times. In this configuration, the development DC bias Vdc is corrected based on the fluctuation.

  This control may be performed at any station of yellow (Y), magenta (M), cyan (C), and black (K). However, for the sake of simplicity, correction is performed at the black station (K). Take it. Hereinafter, the image density control method according to the present embodiment is divided into (a method for predicting the fluctuation of the bright portion potential VL), (a method for detecting the Idc flowing through the charging roller), and (a method for correcting the development DC bias Vdc). Do.

(Prediction method of fluctuation of bright part potential VL)
First, a method for predicting fluctuations in the bright portion potential VL during continuous printing will be described. In the present embodiment, the bright portion potential VL is obtained a plurality of times during the pre-rotation operation before the image forming process is performed, and the fluctuation of the VL is predicted from the obtained values of the plurality of VL. When obtaining the bright portion potential VL, the “current detection step” is performed to detect the charging current Idc flowing through the charging roller, and the measured value of Idc is substituted into the calculation formula for obtaining the bright portion potential VL. In this configuration, the partial potential VL is calculated.

  FIG. 1 shows a timing chart of the “current detection step” performed during the pre-rotation operation before the image forming process. FIG. 1 shows the main motor, charging bias, exposure, development bias ON / OFF timing, photosensitive drum potential (upper side is minus side), and charging DC current value in order from the top, with the horizontal axis representing time. .

  First, when the image forming apparatus receives a print signal, a motor for driving the photosensitive drum starts driving, and a high voltage start-up operation in the order of AC bias (charging AC) and DC bias (charging DC) connected to the charging roller. To do.

  As a result, the photosensitive drum surface is charged to a desired dark portion potential VD. Subsequently, after the charged portion on the surface of the photosensitive drum passes through the exposure position, scanner exposure is performed to partially change the photosensitive drum potential to a bright portion potential (exposed portion). The charging current Idc that flows when charging is detected (Ibc1, Ibc2).

  The process of detecting the charging current Idc flowing through the charging roller in this way is referred to as a “current detection process”, and in this embodiment, the “current detection process” is performed a plurality of times, thereby predicting fluctuations in the bright part potential VL. To do.

  In one “current detection step”, the bright portion potential VL can be obtained from the following equation. That is, the bright portion potential VL is obtained by VL = Idc · d / K + VD (1). However, K = ε · ε0 · L · Vp.

In this formula,
Idc: charging current flowing through the charging roller measured by the current detection step d: film thickness of the photosensitive drum VD: (desired dark portion potential described above)
ε: dielectric constant ε0: dielectric constant of vacuum L: effective charging width of charging roller Vp: process speed (100 mm / sec in this embodiment)
It is.

The film thickness d of the photosensitive drum may be obtained by measuring the charging current Idc that flows through the charging roller when the potential is changed from 0 V to VD in Equation (1), or based on the rotation information of the photosensitive drum. In addition, the film thickness d may be obtained by prediction from the relationship between the rotational speed of the photosensitive drum and the film thickness.

(Detection method of Idc flowing through charging roller)
According to Expression (1), the bright portion potential VL can be obtained by charging the bright portion potential again during the pre-rotation operation and substituting the measured value of the current Idc flowing through the charging roller. Here, a method for detecting the charging current Idc flowing through the charging roller will be described.

  The charging current Idc is detected by a current detection circuit shown in FIG. In the current detection circuit shown in FIG. 4, a vibration bias in which an AC bias is superimposed on a DC bias is applied to the core of the charging roller 23KS that is in contact with the photosensitive drum 22K by a high voltage power source 23a.

  In the high voltage power source 23a, a DC bias power source 23c and an AC bias power source 23b are connected in series via a resistor 23e that blocks the AC component of the bias. According to this configuration, it is possible to detect the charging current Idc flowing through the charging roller by amplifying and reading the current flowing through the detection resistor 23h arranged on the upstream side of the DC bias power supply 23c with the amplifier 23g.

  On the other hand, the AC component of the AC bias flows to the ammeter 23f via the capacitor 23d, and is controlled by the controller 10 so that the AC current becomes a desired value. In the controller 10, an arithmetic unit 12 having a CPU, a ROM 13 in which a program is stored, and a RAM 14, which is a storage means, are arranged. The flowing charging current Idc is detected.

  In addition, the arithmetic unit 12 controls the motor driving of the image forming apparatus, the high-voltage circuit driving, the scanner lighting, and the like by a built-in CPU based on a program held in the ROM 13.

(Method for correcting development DC bias Vdc)
A method for correcting the development DC bias Vdc so as to stabilize the image density while keeping the development contrast Vcont constant will be described based on the above-described method for predicting the fluctuation of the bright portion potential VL. Here, the description will be made based on the sequence shown in FIG.

  First, when a print signal is received (S101), the photosensitive drum starts rotating, and the photosensitive drum is charged to the dark portion potential VD (S102). After the charging portion has completely passed the exposure position, the laser is exposed for one charging roller (corresponding to approximately half the circumference of the photosensitive drum in this embodiment) to form a bright portion potential VL1 ( S103).

  Next, the controller 10 measures the charging current Idc that flows when the bright portion potential is charged again to the dark portion potential VD by the charging roller (S104). The value of the charging current Idc is obtained as an average value of the current that flows when the section where exposure is performed is charged again.

  From the average value of the charging current Idc obtained by measurement, the film thickness d of the photosensitive member, and the charging DC bias (substantially equal to the dark portion potential VD) determined by the immediately preceding Dmax control, the bright portion potential VL is expressed by the formula ( Obtained by 1). The obtained data is stored in the RAM 14 in the controller 10 as the n-th light portion potential VL (n) (S105).

This process is repeated for two rotations of the photosensitive drum (S106). Here, in order to measure a change in sensitivity when the photosensitive drum is repeatedly exposed, it is necessary that the exposure on the second round of the photosensitive drum has almost the same timing as the exposure position on the first round of the photosensitive drum. There is.

  Subsequently, as the development DC bias applied during the image forming operation, a value obtained by correcting the value of the development DC bias Vdc determined by the previous Dmax control by the variation ΔVL (= VL1−VL2) of the bright portion potential is used. (S107). The corrected development DC bias is applied in accordance with the image writing timing to form a toner image on the surface of the photosensitive drum (S108). These corrections are performed by the controller 10 (correction means).

  Next, it is determined whether the requested number of prints has been completed (S109). If not, image formation is performed with the adjusted development DC bias until the number of continuous prints is 10 (predetermined number) (S110). ).

  When the number of continuous prints exceeds 11, the development DC bias is corrected again (S111), and image formation is performed with the same development DC bias up to 20 prints. Further, when the number of continuous prints exceeds 21, the third development DC bias is corrected (S112), and thereafter image formation is repeated without correcting the development DC bias up to the requested print number (S112). S113, S114).

  As described above, the correction of the development DC bias is completed when the number of continuous prints is up to 20, thereby preventing the development contrast Vcont from becoming too large and conversely increasing the image density. Finally, when a predetermined number of prints are completed, the development DC bias set value is returned to the value of the development DC bias determined by the Dmax control, and the process ends (S115).

  FIG. 5 shows changes in the setting of the development DC bias Vdc during continuous printing in the 15 ° C./10% RH environment (water content 1.1 g / m 3), with the horizontal axis representing the number of prints and the vertical axis representing the potential. It is shown. It can be seen that by changing the setting of the development DC bias Vdc stepwise, the change in development contrast can be suppressed as compared with the conventional case.

  FIG. 13A shows a case where the density difference between the first print and the 20th print when 20 sheets are continuously printed in two environments with different moisture amounts is not corrected in the present embodiment and the conventional case. It is the result compared with. Here, the printing pattern was black halftone (printing rate 50%), and the density measurement was performed by measuring the color of the image sample after fixing using Gretag Spectrolino (manufactured by Gretag Macbeth).

  As can be seen from FIG. 13A, according to the correction method of the present embodiment, the density fluctuation is smaller than in the conventional case in any environment, and the present embodiment is effective against the density fluctuation. I understand that.

  In this embodiment, the time for exposing the laser during the pre-rotation operation is set to one turn of the charging roller. However, the laser exposure time during the pre-rotation operation does not have to be that, and the exposure current Idc that flows when each of the exposure portions is formed as a short time and the photosensitive drum makes one turn and is charged again is changed. On average, the charging current Idc for the first round of the photosensitive drum may be used. If the image forming apparatus includes a pre-exposure process, the exposure unit may be formed by pre-exposure.

  As described above, in the present embodiment, the “current detection step” is performed twice during the pre-rotation operation without providing a special sensor for measuring the surface potential of the photosensitive drum, whereby the bright portion potential VL at the time of image formation. Is fed back to the correction of the development DC bias Vdc. Therefore, even when used in a low temperature and low humidity environment, the development contrast Vcont can be stabilized, and a desired image density can be stably obtained.

[Second Embodiment]
In the first embodiment, regardless of the environment in which the image forming apparatus is used, the “current detection step” is always performed twice during the pre-rotation operation to predict the fluctuation amount of the bright portion potential VL, and the development DC bias Vdc. A method of correcting the above has been described.

  However, it is known that the fluctuation amount of the light portion potential VL of the photosensitive drum varies depending on the atmospheric environment. That is, depending on the atmospheric environment, the bright portion potential VL does not vary so much during the continuous printing process, and it may not be necessary to correct the development DC bias Vdc described above.

  In the second embodiment, the temperature / humidity sensor (temperature / humidity detecting means) 42 is provided in the image forming apparatus. Then, based on the absolute moisture content obtained from the detection result (temperature, humidity) of the temperature / humidity sensor 42, a configuration for selecting whether or not to correct the development DC bias Vdc described in the first embodiment is selected. Was adopted.

  FIG. 12 shows a schematic configuration diagram of the image forming apparatus according to the present embodiment. As shown in FIG. 12, the temperature and humidity detected by the temperature / humidity sensor 42 are output to the arithmetic unit 12 of the controller 10.

The arithmetic unit 12 calculates the absolute moisture content of the ambient environment from the data input from the greenhouse humidity sensor 42, and stores the temperature and absolute moisture content of the ambient environment in the RAM 14 in units of 0.1 ° C. and 0.1 g / m ^3. To do. The place where the temperature / humidity sensor 42 is provided is not limited to this, and it may be provided around the photosensitive drum 22 or may be another place.

  As shown in FIG. 12, the image forming apparatus according to the present embodiment is substantially the same as the configuration of the image forming apparatus according to the first embodiment except for the presence / absence of the temperature / humidity sensor 42. Therefore, description of the overall configuration of the image forming apparatus is omitted here.

  FIG. 6 is a result of measuring the variation ΔVL of the bright portion potential VL on the first page and the second page when the solid image is continuously printed in the image forming apparatus according to the present embodiment while changing the use environment. is there.

  The horizontal axis represents the absolute water content obtained from the detection result of the temperature / humidity sensor 42, and the vertical axis represents the fluctuation amount ΔVL of the bright portion potential VL on the first and second pages.

  As can be seen from FIG. 6, when the value of the absolute water content decreases, the fluctuation amount ΔVL of the bright portion potential VL on the first page and the second page increases. That is, it can be seen that the light portion potential fluctuation between pages depends on the absolute moisture content of the atmospheric environment. Therefore, the threshold value for determining whether to correct the development DC bias Vdc is set to a value at which the potential fluctuation between pages starts to increase (in this embodiment, the water amount is 2.5 g / m 3).

  FIG. 7 shows an operation flow of a control sequence for correcting the developing bias Vdc in the present embodiment. First, when printing is started, the absolute moisture content is obtained from the output of the temperature / humidity sensor 42 (S116).

Here, it is determined whether the absolute water content is larger or smaller than 2.5 g / m 3 (S117). When the number is large, a normal image forming operation without adjusting the development DC bias Vdc is performed. When the number is small, a control sequence for correcting the development DC bias Vdc is performed. Since this control sequence is the same as the control sequence shown in the first embodiment, description thereof is omitted. As a result, the photosensitive drum is not rotated unnecessarily in an environment other than the low humidity environment.

  That is, in this embodiment, the absolute moisture content of the atmosphere environment of the image forming apparatus is calculated, and the development DC bias Vdc is corrected based on the calculated absolute moisture content. Thereby, even when used in a low temperature and low humidity environment, the development contrast Vcont can be stabilized, and a desired image density can be stably obtained.

[Third embodiment]
An image forming apparatus according to a third embodiment of the present invention will be described. The configuration of the image forming apparatus according to the present embodiment is the same as that of the image forming apparatus according to the first embodiment or the second embodiment. Therefore, a description of the overall configuration of the image forming apparatus is omitted here.

  In the first embodiment, the development DC bias Vdc is corrected according to the number of printed pages in consideration of the difference in the bright portion potential VL predicted by performing the “current detection step” twice, and the image density is stabilized. The method of making it explained.

  In the present embodiment, when there is a margin in the time for performing the pre-rotation (for example, when it takes time to make the fixing device print ready), the number of times of performing the “current detection step” is increased. A method for more accurately grasping the fluctuation of the partial potential VL will be described.

  FIG. 8 shows an operation flow of a control sequence for correcting the development DC bias Vdc in the present embodiment.

  First, when a print signal is received (S201), the absolute moisture content is obtained from the output of the temperature / humidity sensor (S202), and it is determined whether the moisture content is greater than or less than 2.5 g / m3 (S203).

  If the amount of water is greater than 2.5 g / m 3, a normal image forming operation without adjusting the development DC bias Vdc is executed (S214). On the contrary, when the number is small, the “current detection step” is executed a plurality of times.

  That is, the photosensitive drum is charged to the dark portion potential VD (S204), and after the charged portion has completely passed through the exposure position, the laser is exposed for one rotation of the charging roller (approximately half of the photosensitive drum) to be the bright portion potential. VL1 is formed (S205).

  Next, the charging current Idc that flows when this bright portion potential is charged again by the charging roller is measured by the controller, and an average value is obtained (S206). From the average value of the charging current Idc obtained by measurement, the film thickness d of the photosensitive drum, and the charging DC bias (substantially equal to the dark portion potential VD) determined by the immediately preceding Dmax control, the bright portion potential VL is calculated as described above. It calculates | requires by Formula (1) demonstrated. Then, data is stored in the memory in the controller 10 as the n-th rotation light portion potential VL (n) (S207).

  Next, it is determined whether there is time to rotate the photosensitive drum before starting the image writing (S208). If there is time to rotate the photosensitive drum, the operations from S204 to S207 are repeated, and data is stored in the memory in the controller 10 as the bright portion potential VL (n) of the nth rotation.

When there is no time to rotate the photosensitive drum, the data VL (n) stored in the memory
ΔVL (= VL (n−1) −VL (n)) is obtained from the above, and coefficients a and b in equation (2) shown below are calculated by fitting (S209). That is, a and b are calculated by ΔVL = −a · Log _e (N) + b (2). Here, N is the number of times the “current detection step” has been performed.

FIG. 13B shows the result of performing the “current detection step” three times in a 10 ° C./5% RH environment (water content 0.38 g / m 3).

  As shown in FIG. 13B, VL (1) is calculated at the first rotation of the photosensitive drum, and VL (2) and ΔVL are obtained at the second rotation of the photosensitive drum. Further, in the third rotation of the photosensitive drum, the coefficients a and b in the equation (2) are obtained to determine the equation (2), and the fluctuation of the bright portion potential VL during image formation is predicted.

  Thereafter, the development DC bias is corrected by predicting the fluctuation of the bright portion potential VL using the relationship of the expression (2) in which the values of a and b are determined (S210), and the development DC bias is adjusted in accordance with the image formation timing. Vdc is applied (S211).

  The above process is repeated for a predetermined number of continuous prints (S212). Finally, the development DC bias setting value is returned to the value of the development DC bias Vdc determined by the Dmax control, and the process ends (S213).

  FIG. 9 shows changes in the setting of the developing bias Vdc during continuous printing by the above sequence operation, with the horizontal axis representing the number of prints and the vertical axis representing the potential. The development contrast Vcont can be kept more constant than when the monotonous change is made by ΔVL every predetermined number of continuous prints as in the first embodiment.

  Further, FIG. 13C compares the density difference between the first and 20th printed sheets when 20 sheets are continuously printed in two environments with different moisture contents, as compared with the first embodiment and the conventional example. It is a result.

  As shown in FIG. 13C, in the present embodiment, the concentration variation can be further suppressed in the low humidity environment where the potential variation of the bright portion potential VL is large compared to the first embodiment.

  In addition, as a method of predicting the fluctuation of the bright part potential VL with higher accuracy, there is a method in which the current detection step is performed during the pre-multi-rotation to obtain Equation (2) in advance. Here, the pre-multi-rotation is a period from when the power switch of the image forming apparatus is turned on to when the photosensitive drum is rotated and the apparatus is in a print ready state. It performs operations such as detection, density adjustment, and fixing device startup.

  Since the time required for this pre-multi rotation is generally sufficiently longer than that for the pre-rotation, the change in the bright portion potential VL during continuous printing is obtained by performing the current detection step a plurality of times during the pre-multi rotation. It is possible to predict with higher accuracy using.

  As a result, the current detection process only needs to be performed once during the pre-rotation operation, and the image density can be stabilized without extending the time required to output the sheet material from the image forming apparatus after receiving the print signal. It becomes possible to make it.

(Other embodiments)
The present invention can also be applied to an image forming apparatus that performs “contact DC charging” as a charging method. As described above, the first to third embodiments are configured to perform contact AC charging, but the charging method of the present invention is not limited to this.

  In the case of contact DC charging, if the resistance value of the charging roller varies, the value of the charging start voltage Vth varies, so the surface potential of the photosensitive drum may deviate from the desired dark portion potential VD. This is particularly noticeable in a low-temperature and low-humidity environment, and is several tens of volts lower (plus side) than the desired potential.

Therefore, in the case of an image forming apparatus that performs “contact DC charging”, a value of α in the following equation (3) corresponding to temperature / humidity information is tabulated and stored in the ROM 13 in advance.
VD = (applied voltage) −Vth + α (3)

  That is, α is data obtained by converting the shift amount of the dark portion potential in the low temperature and low humidity environment obtained in advance by experiments, and the dark portion potential can be corrected from the measurement result by the temperature / humidity sensor provided in the apparatus main body.

  Therefore, even in an image forming apparatus that performs “contact DC charging”, if the image forming conditions are corrected by the method described in the first embodiment or the second embodiment, the same as in the case of “contact AC charging”. In addition, the image density can be stabilized.

  In the first to third embodiments, the development DC bias is corrected in accordance with the fluctuation of the bright portion potential VL. However, the image forming condition to be corrected is not limited to the development DC bias. .

  The image forming condition to be corrected according to the fluctuation of the bright portion potential VL may be the light amount (exposure light amount) of the laser light emitted from the scanner unit 24. That is, when the surface of the photosensitive drum 22 is exposed during image formation, it is possible to keep the developing bias Vcont constant by correcting the amount of exposure light in accordance with the predicted fluctuation in VL.

Timing chart of current detection process in the first embodiment Sequence for correcting development bias in the first embodiment The figure showing the layer structure of the photosensitive drum in 1st Embodiment Schematic configuration diagram of a current detection circuit in the first embodiment The figure showing the change of the development bias setting value in a 1st embodiment Relationship between absolute water content and amount of fluctuation ΔVL Sequence for correcting development bias in the second embodiment Sequence for correcting development bias in the third embodiment The figure showing the change of the development bias setting value in 3rd Embodiment Temporal change of bright part potential VL during continuous printing in low temperature and low humidity environment (a) and normal temperature and normal humidity environment (b) 1 is a schematic configuration diagram of an image forming apparatus according to a first embodiment. Schematic configuration diagram of an image forming apparatus according to a second embodiment Comparison results with conventional examples

Explanation of symbols

10 Controller 12 Arithmetic unit (CPU)
13 ROM
14 RAM
DESCRIPTION OF SYMBOLS 20 Image formation part 22 Photosensitive drum 23 Charging device 27 Intermediate transfer belt 28 Transfer roller 30 Fixing device

Claims (7)

A rotationally driven photoreceptor;
A charging member for charging the surface of the photoreceptor;
An exposure apparatus that emits exposure light to the surface of the photoreceptor;
Current detection means for detecting a charging current flowing through the charging member;
In an image forming apparatus comprising:
Charging the surface of the photoreceptor with the charging member;
The exposure device exposes the surface of the charged photoreceptor,
The charging member again charges the exposed portion by the exposure apparatus,
At that time, the current detection means detects the charging current flowing through the charging member,
A series of steps for calculating the potential of the exposed portion based on the detection result is continuously performed a plurality of times,
An image forming apparatus comprising correction means for correcting an image forming condition based on the calculated potentials of the plurality of exposed portions. The process includes
The image forming apparatus according to claim 1, wherein the image forming apparatus is performed during a pre-rotation operation of rotating the photosensitive member before the apparatus main body performs image formation. The correction means includes
3. The image forming apparatus according to claim 1, wherein the image forming condition is corrected each time the apparatus main body forms an image on a predetermined number of sheet materials. Based on the difference in potential of the exposed portion obtained by performing the step twice,
The image forming apparatus according to claim 1, wherein the correcting unit corrects the image forming unit. The device body is
With temperature and humidity detection means to detect the temperature and humidity of the atmosphere environment,
5. The process according to claim 1, wherein the step is not performed when the absolute moisture value obtained based on the detection result of the temperature and humidity detection means is higher than a predetermined value. The image forming apparatus described. The image forming conditions are
In the electrostatic latent image obtained by exposing the surface of the photosensitive member charged to the charging member,
The image forming apparatus according to claim 1, wherein the image forming apparatus is a developing bias that forms a potential difference with a bright portion potential of the electrostatic latent image. The image forming conditions are
6. The image forming apparatus according to claim 1, wherein the exposure light amount is an exposure light amount when the exposure apparatus exposes the surface of the photoconductor during image formation.

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