JP5744153B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus that charges a photosensitive member by applying an oscillating voltage obtained by superimposing an alternating voltage on a direct current voltage to a charging member, and more particularly to control for reducing the burden of the alternating voltage on a photosensitive layer on the surface of the photosensitive member.

  2. Description of the Related Art An image forming apparatus that applies an oscillating voltage obtained by superimposing an AC voltage on a DC voltage to a charging member to uniformly charge the surface of a photoreceptor having a photosensitive layer on the surface is widely used (Patent Document 1). . When an oscillating voltage is used, the charge necessary for charging is transferred from the charging member to the photosensitive member with a bidirectional discharge due to an AC voltage, so that a charging potential equal to the DC voltage applied to the charging member is the surface of the photosensitive member. It is copied to. For this reason, even if there is some contact failure between the photoconductor and the charging member, the surface of the photoconductor can be charged to a uniform potential.

  However, when an oscillating voltage is used, a large alternating current with a high voltage is forced to flow, so that deterioration of the photosensitive layer composed of a semiconductor thin film is accelerated compared to the case where only the direct current voltage is applied to the charging member. . Further, since the surface roughness accompanying the discharge increases the wear rate of the photosensitive layer, the life of the photosensitive layer due to wear is also a problem.

  For this reason, it is desirable that the alternating voltage used for the oscillating voltage be as low as possible. However, if the alternating voltage is too low, the discharge current is insufficient and the photosensitive member cannot be charged to the required charging potential (Patent Document 2).

  Therefore, there has been proposed a control for setting an optimum constant voltage of an alternating voltage that can ensure a necessary discharge current within a range in which an excessive discharge current does not flow during non-image formation (Patent Document 3).

  In Patent Document 3, as shown in FIG. 6, during a print preparation rotation operation prior to image formation, an alternating current that flows through the photosensitive drum by applying three stages of alternating current test voltages to the charging roller in each of the discharge area and the non-discharge area. Is detected. Then, the relational expression between the AC voltage and the discharge current (Formula 2) and the relational expression between the AC test voltage and the alternating current in the undischarged area (Formula 3) Therefore, the constant voltage of the peak-to-peak voltage of the AC voltage that can obtain an appropriate discharge current is set.

JP-A 63-149669 Japanese Patent Publication No. 06-093150 JP 2001-201920 A

  According to the control of Patent Literature 3, the AC voltage in the oscillating voltage is variably set so that an appropriate discharge current corresponding to the resistance value of the charging member at that time can be obtained. However, when the AC voltage is set by the control of Patent Document 3, as the photosensitive layer of the photosensitive drum wears, the load of the photosensitive layer when the test AC voltage is applied increases, and the life of the photosensitive layer is shortened. It turned out to be damaged.

  When the photosensitive layer of the photosensitive drum becomes worn and thin, the electrostatic capacity of the photosensitive layer increases. Therefore, even when the same test AC voltage is applied, the discharge of each AC voltage pulse between the photosensitive drum and the charging member becomes longer. Subsequently, the discharge current increases. As a result, when an AC test voltage for setting an appropriate discharge current is applied, deterioration or wear of the photosensitive layer due to excessive discharge current proceeds.

  In view of this, there has been proposed a control in which the abrasion state of the photosensitive layer of the photosensitive drum is estimated based on the cumulative usage time of the photosensitive drum, and the AC test voltage is gradually decreased as the cumulative usage time increases. However, the wear state of the photosensitive layer of the photosensitive drum stops or accelerates regardless of the cumulative use time. For example, when the image formation is continued with a high image ratio (a lot of fog toner on the photosensitive drum), the wear of the photosensitive layer does not progress much. However, when an image is formed with a recording material containing a large amount of hard additive, the wear of the photosensitive layer proceeds at a stretch. Even due to troubles such as jamming of the recording material and carrier adhesion in the developing device, the wear rate of the photosensitive layer may increase in subsequent image formation.

  In such a case, a gap occurs between the wear state of the photosensitive layer estimated from the cumulative usage time of the photosensitive drum and the actual wear state, and the discharge current when an AC test voltage is applied becomes excessive. There is a case.

  The present invention directly determines the film thickness of the photosensitive layer, sets the AC test voltage more appropriately than when indirectly estimated by the cumulative number of images, etc., and deteriorates the photosensitive layer due to the application of the AC test voltage. An object of the present invention is to provide an image forming apparatus capable of suppressing wear and wear.

An image forming apparatus according to the present invention includes a photosensitive member having a photosensitive layer, a charging member disposed in contact with the surface of the photosensitive member and charging the photosensitive member, and a direct current voltage to an alternating voltage to charge the photosensitive member. A power source that applies a charging bias superimposed on the charging member, an image forming unit that forms a toner image on the photosensitive member charged by the charging member, and a first detection that detects an alternating current flowing through the charging member A second detection unit for detecting a direct current flowing through the charging member, and a plurality of AC peak test voltages having different peak-to-peak voltages in the discharge region and the non-discharge region, respectively. first mode and an AC voltage having a peak-to-peak voltage of the discharge area for setting a peak voltage of the AC voltage in the charging bias based on the alternating current detected respectively by the first detecting portion Which comprises an execution unit for executing a second mode in which the detection of the DC current flowing through the charging member by the second detection unit when an oscillating voltage obtained by superimposing a DC voltage is applied to the charging member It is. In the first mode, the execution unit detects the peak-to-peak voltage of the AC test voltage having the largest peak-to-peak voltage among the plurality of AC test voltages in the undischarged region in the second mode. If the direct current is the first current, the first direct current is set to the first voltage, and the absolute value of the direct current detected in the second mode is the second current larger than the first current. In some cases, the second voltage is set lower than the first voltage.

In the image forming apparatus of the present invention, the peak-to-peak voltage of the AC test voltage having the largest peak-to-peak voltage among the plurality of AC test voltages in the undischarged region is determined based on the detection result of the DC current flowing through the charging member. An appropriate value can be set according to the film thickness. For this reason, the peak-to-peak voltage of the AC test voltage can be set more appropriately than the case where the film thickness of the photosensitive layer is indirectly estimated by the cumulative number of images and the like to set the peak-to-peak voltage of the AC test voltage.

1 is an explanatory diagram of a configuration of an image forming apparatus. It is explanatory drawing of a structure of an image formation part. It is explanatory drawing of the film thickness of the photosensitive layer of a photosensitive drum. It is a block diagram of a control system of a voltage applied to the charging roller. It is explanatory drawing of the relationship between the peak voltage of the alternating voltage applied to a charging roller, and alternating current. It is explanatory drawing of the control which sets the peak voltage of an alternating voltage. It is explanatory drawing of the film thickness measurement of the photosensitive layer based on the direct current amount which flows into a photosensitive drum. It is explanatory drawing of the change of the relationship between a peak-to-peak voltage and an alternating current. It is explanatory drawing of the increase in the discharge current accompanying abrasion of a photosensitive layer. It is explanatory drawing of the setting of the alternating current test voltage according to the measurement result of the film thickness of a photosensitive layer. 3 is a flowchart of an AC voltage setting mode according to the first embodiment. 6 is an explanatory diagram of a configuration for measuring a film thickness of a photosensitive layer in Example 3. FIG. It is explanatory drawing of the relationship between an alternating current and the film thickness of a photosensitive layer. It is explanatory drawing of the alternating voltage superimposed on a direct-current voltage.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the present invention, as long as the voltage applied to the charging roller is reduced when the AC voltage is set according to the wear of the film thickness of the photosensitive layer, a part or all of the configuration of the embodiment is replaced with the alternative configuration. Other embodiments can also be implemented.

  Therefore, the present invention can be equally applied to an image forming apparatus using a charging member to which an oscillating voltage is applied, regardless of the type using an intermediate transfer member, the method using a recording material conveyance member, the tandem type, the one drum type, the full color, and the monochrome. In the present embodiment, only main parts related to toner image formation / transfer will be described. However, the present invention includes a printer, various printing machines, a copier, a fax machine, a composite machine, in addition to necessary equipment, equipment, and a housing structure. It can be implemented in various applications such as a machine.

  In addition, about the general matter of the image forming apparatus shown by patent documents 1-3, illustration is abbreviate | omitted and the overlapping description is abbreviate | omitted.

<Image forming apparatus>
FIG. 1 is an explanatory diagram of the configuration of the image forming apparatus. FIG. 2 is an explanatory diagram of the configuration of the image forming unit.

  As shown in FIG. 1, the image forming apparatus 100 is a tandem intermediate transfer type full color printer in which image forming portions Pa, Pb, Pc, and Pd are arranged along an intermediate transfer belt 31.

  The control unit 50 includes a control board and a motor drive board (not shown) for controlling the operation of the image forming units Pa, Pb, Pc, Pd, and other mechanisms in the unit. The environmental sensor 51 is arranged so that the temperature and humidity around the apparatus can be accurately measured without being affected by the fixing device 40 that is a heat source in the apparatus. The control unit 50 performs various controls based on the output of the environment sensor 51.

  In the image forming portion Pa, a yellow toner image is formed on the photosensitive drum 11 a and is primarily transferred to the intermediate transfer belt 31. In the image forming unit Pb, a magenta toner image is formed on the photosensitive drum 11 b and is primarily transferred to the yellow toner image on the intermediate transfer belt 31. In the image forming portions Pc and Pd, a cyan toner image and a black toner image are formed on the photosensitive drums 11c and 11d, respectively, and similarly, are sequentially transferred to the intermediate transfer belt 31 in order.

  The four-color toner images primarily transferred to the intermediate transfer belt 31 are conveyed to the secondary transfer portion T2 and collectively transferred to the recording material P. The recording material P onto which the toner image has been transferred is separated from the intermediate transfer belt 31 and conveyed to the fixing device 40, where the toner image is fixed.

  The intermediate transfer belt 31 is supported around a tension roller 33, a drive roller 32, and a counter roller 34, and is driven by the drive roller 32 to rotate in the direction of arrow R2 at a process speed of 320 mm / sec. For example, PET [polyethylene terephthalate] or PVdF [polyvinylidene fluoride] is used as the material of the intermediate transfer belt 31. The intermediate transfer belt 31 uses polyimide having a thickness of 100 μm, and the width in the thrust direction of the primary transfer portions Ta, Tb, Tc, and Td is 330 mm.

  The driving roller 32 is coated with rubber (urethane or chloroprene) having a thickness of several millimeters on the surface of the metal roller to prevent slipping, and is driven to rotate by a pulse motor (not shown). The tension roller 33 applies a predetermined tension to the intermediate transfer belt 31. The secondary transfer roller 36 is in pressure contact with the opposing roller 34 with the intermediate transfer belt 31 interposed therebetween, thereby forming a secondary transfer portion T <b> 2 between the intermediate transfer belt 31 and the secondary transfer roller 36.

  The separation roller 23 separates the recording material P drawn from the recording material cassette 21 by the pickup roller 22 one by one and sends it to the registration roller 25. The registration roller 25 receives and waits for the recording material P in the stopped state, and sends the recording material P to the secondary transfer portion T2 in synchronization with the toner image on the intermediate transfer belt 31.

  The recording material P conveyed to the secondary transfer portion T2 is conveyed while being sandwiched between the intermediate transfer belt 31 carrying the toner image and the secondary transfer roller. Meanwhile, the toner image on the intermediate transfer belt 31 is secondarily transferred to the recording material P by the power source D2 applying a positive DC voltage having a polarity opposite to the charging polarity of the toner to the secondary transfer roller 36.

  The belt cleaning device 38 brings a cleaning blade into contact with the intermediate transfer belt 31 and collects transfer residual toner remaining on the surface of the intermediate transfer belt 31 that has passed through the secondary transfer portion T2.

  The fixing device 40 presses a pressure roller 42 against a fixing roller 41 having a heater at the center to form a heating nip. The recording material P is heated and pressurized in the process of being nipped and conveyed by the heating nip, melts the toner image, fixes the full color image on the surface, and then is discharged to the discharge tray 46 through the discharge roller 45.

  The image forming portions Pa, Pb, Pc, and Pd are substantially the same except that the toner colors used in the developing devices 14a, 14b, 14c, and 14d are different from yellow, magenta, cyan, and black. Therefore, hereinafter, the image forming unit Pa will be described, and the other image forming units Pb, Pc, and Pd will be described by replacing “a” at the end of the reference numerals with “b”, “c”, and “d”. .

  As shown in FIG. 2, in the image forming unit Pa, a charging roller 12a, an exposure device 13a, a developing device 14a, a primary transfer roller 35a, and a cleaning device 15a are arranged around the photosensitive drum 11a.

  The photosensitive drum 11a is a rotating drum type electrophotographic photosensitive member, more specifically, an organic photoconductor (OPC) thin film having a negative polarity on the outer peripheral surface of an aluminum cylinder. The entire photosensitive drum 11a has an outer diameter of 30 mm, and is rotationally driven in the direction of arrow R1 with a process speed (peripheral speed) of 320 mm / sec.

  As will be described later, the charging roller 12a is applied with an oscillating voltage obtained by superimposing an AC voltage on a DC voltage, and charges the surface of the photosensitive drum 11a to a uniform potential.

  The exposure device 13a scans the scanning line image data obtained by developing the yellow separation color image with a rotating mirror, and writes an electrostatic image of the image on the surface of the charged photosensitive drum 11a.

  In the developing device 14a, a developing container 14e is filled with a two-component developer obtained by mixing a yellow nonmagnetic toner and a magnetic carrier. As a characteristic of the color toner, a weight average particle diameter of 5 to 8 μm is preferable for forming a good image. If the weight average particle diameter is within this range, it has sufficient resolution, can form clear and high-quality images, and has less adhesive force and cohesive force than electrostatic force, reducing various troubles. Because it does. The weight average particle diameter of the nonmagnetic toner particles can be measured by various methods such as a sieving method, a sedimentation method, and a photon correlation method.

  The toner particles can be produced by a pulverization method in which the constituent materials are made uniform by heating and melting, and then cooled, solidified, and pulverized to produce toner particles. However, since the toner particles obtained by the pulverization method are generally indeterminate, it is necessary to perform mechanical, thermal, or some special treatment in order to obtain a substantially spherical shape. In addition, it is necessary to classify the toner particles after the spheroidizing treatment in order to obtain the weight average particle diameter in the above-mentioned range. For this reason, it is preferable to employ a polymerization method as a toner particle production method rather than a pulverization method.

  A stirring screw (not shown) disposed in the developing container 14e circulates the two-component developer while stirring to charge the nonmagnetic toner to negative polarity and the magnetic carrier to positive polarity. The developing sleeve 14h rotates around the fixed magnet roller 14i, carries the charged two-component developer on the surface by the magnetic force of the magnet roller 14i, and rubs the photosensitive drum 11a with the two-component developer in a standing state. Let The power source D4 applies an oscillating voltage obtained by superimposing the AC voltage Vpp on the negative DC voltage Vdc to the developing sleeve 14h. More specifically, it is an oscillating voltage obtained by superimposing a rectangular wave AC voltage having a frequency of 9.2 kHz and a peak-to-peak voltage Vpp of 1.8 kV on a DC voltage Vdc of −550V. As a result, the charged toner is transferred from the developing sleeve 14h to the exposed portion of the photosensitive drum 11a having a relatively positive polarity, and the electrostatic image is reversely developed.

  The primary transfer roller 35a is in pressure contact with the photosensitive drum 11a via the intermediate transfer belt 31, and forms a primary transfer portion T1 between the intermediate transfer belt 31 and the photosensitive drum 11a. While the toner image on the photosensitive drum 11a passes through the primary transfer portion T1, the power source D1 applies a positive DC voltage having a polarity opposite to the charging polarity of the toner to the primary transfer roller 35a. The toner image is primarily transferred to the intermediate transfer belt 31. The charge holding amount of the toner on the photosensitive drum 11a is 30 μC / g, and the constant voltage output from the power source D1 is set so that a current of 40 μA flows through the core bar 35e of the primary transfer roller 35a.

The primary transfer roller 35 is 4mm thick polyurethane foam on the outside of the metal core 35e with a diameter φ8mm a urethane sponge roller having an outer diameter of φ16mm forming the elastic layer 35f, the 5 × 10 ⁷ Ω voltage application of 1kV It has a resistance value.

  The cleaning device 15a brings the cleaning blade 15e into contact with the photosensitive drum 11a, and scrapes off and collects the transfer residual toner attached to the surface of the photosensitive drum 11a that has passed through the primary transfer portion T1. The cleaning device 15a uses a counter blade method, and the cleaning blade 15e has a free length of 8 mm. The cleaning blade 15e is an elastic blade mainly composed of urethane, and is brought into contact with the photosensitive drum 11a by pressing with a linear pressure of about 35 g / cm.

<Photosensitive drum>
FIG. 3 is an explanatory diagram of the film thickness of the photosensitive layer of the photosensitive drum. As shown in FIG. 3, in the photosensitive drum 11a, an undercoat layer 11β, a charge generation layer 11γ, and a charge transport layer 1ω are sequentially applied and stacked on the surface of an aluminum cylinder support (conductive drum base) 11α. Combined four-layer structure.

  The support 11α arranged on the innermost side can be used without particular limitation as long as it is a material exhibiting conductivity and is in a range that does not affect the measurement of hardness. For example, a metal or alloy such as aluminum, copper, chromium, nickel, zinc and stainless steel formed into a photosensitive drum shape can be used.

  The undercoat layer 11β improves the adhesion of the upper layer by suppressing light interference. Also formed to improve the adhesion of the photosensitive layer, improve coating properties, protect the support, cover defects on the support, improve charge injection from the support, or protect against electrical breakdown of the photosensitive layer Is done.

  As a material for the undercoat layer 11β, polyvinyl alcohol, poly-N-vinylimidazole, polyethylene oxide, ethyl cellulose, ethylene-acrylic acid copolymer, and casein can be used. Polyamide, N-methoxymethylated 6 nylon, copolymer nylon, glue and gelatin can also be used. These are dissolved in a suitable solvent and coated on the support 11α. The thickness of the undercoat layer 11β is preferably 0.1 to 2 μm.

  The photosensitive layer of the photosensitive drum 11a is a laminated photosensitive layer having a thickness of 30 μm obtained by functionally separating an organic semiconductor (OPC) into a charge generating layer 11γ having a thickness of 1 μm and a charge transporting layer 11ω having a thickness of 29 μm. is there.

  The charge generation layer 11γ absorbs the energy of exposure light (laser beam) incident from the surface and generates charge carrier pairs. Examples of the charge generation material used for the charge generation layer 11γ include selenium-tellurium, pyrylium, thiapyrylium dyes, various central metals, and crystal systems. Specific examples include phthalocyanine compounds having crystal types such as α, β, γ, ε, and X, anthanthrone pigments, dibenzpyrenequinone pigments, pyranthrone pigments, trisazo pigments, disazo pigments, and the like. And monoazo pigments, indigo pigments, quinacridone pigments, asymmetric quinocyanine pigments, quinocyanines, and amorphous silicon described in JP-A No. 54-143645. In this example, the charge generation layer 11γ using a phthalocyanine compound capable of increasing sensitivity in order to realize high image quality was used. The charge generation layer 11γ uses a disazo pigment as a CGL layer (carrier generation layer).

  The charge generation material is dispersed together with a binder resin and a solvent in an amount of 0.3 to 4 times using a method such as a homogenizer, ultrasonic dispersion, ball mill, vibration ball mill, sand mill, attritor, and roll mill. After the dispersion, the charge generation layer 11γ is formed by applying the dispersion onto the undercoat layer and drying it. Alternatively, the charge generation layer 11γ is formed by depositing a film made of a single composition of the charge generation material on the undercoat layer B by using an evaporation method or the like. The charge generation layer 11γ preferably has a thickness of 5 μm or less, particularly preferably in the range of 0.1 to 2 μm.

  As the binder resin, polymers and copolymers of vinyl compounds such as styrene, vinyl acetate, vinyl chloride, acrylic acid ester, methacrylic acid ester, vinylidene fluoride, and trifluoroethylene can be used. Polyvinyl alcohol, polyvinyl acetal, polyvinyl butyral, polycarbonate, polyester, polysulfone, polyphenylene oxide, and polyurethane can also be used. Cellulose resin, phenol resin, melamine resin, silicon resin, epoxy resin and the like can also be used.

  The charge transport layer 11ω carries the charge transferred from the charge carriers generated in the charge generation layer 11γ to the surface. The charge transport layer 11ω is formed by dispersing / dissolving a suitable charge transport material in a solvent together with a suitable binder resin, applying the solution onto the charge generation layer 11γ using the method described above, and drying the solution.

  As the charge transport material, a heterocyclic compound such as poly-N-vinylcarbazole or polystyrylanthracene or a polymer compound having a condensed polycyclic aromatic, or a heterocyclic compound such as pyrazoline, imidazole, oxazole, triazole or carbazole can be used. . Low molecular compounds such as triarylalkane derivatives such as triphenylmethane, triarylamine derivatives such as triphenylamine, phenylenediamine derivatives, N-phenylcarbazole derivatives, stilbene derivatives, and hydrazone derivatives can also be used. As the binder resin, a resin similar to the binder resin of the charge generation layer 11γ can be used.

  The ratio of the charge transport material to the binder resin is preferably 20 to 100, more preferably 30 to 100, when the total weight of both is 100. If the amount of the charge transporting material is less than that, the charge transporting ability is lowered, and problems such as a decrease in sensitivity and an increase in residual potential occur.

  The film thickness of the charge transport layer 11ω in the multilayer photoreceptor is preferably 1 to 50 μm, more preferably 3 to 30 μm. In this embodiment, the charge transport layer 11ω is formed with a thickness of 29 μm as a CTL layer (carrier transport layer) obtained by mixing hydrazone and resin.

<Charging device>
As a charging device for the photosensitive drum, a contact charging system in which a charging member such as a roller type or a blade type is brought into contact with the surface of the photosensitive drum is widely employed. In particular, the charging roller can stably charge over a long period of time. it can.

  In order to obtain charging uniformity, the charging roller has an AC voltage having a peak-to-peak voltage Vpp that is equal to or more than twice the discharge start voltage Vth when the DC voltage is applied to the DC voltage Vdc corresponding to the desired dark portion potential VD. An oscillating voltage superimposed is applied. This is because the use of the oscillating voltage eliminates local potential unevenness on the photosensitive drum, and the charged dark portion potential VD converges uniformly to the DC voltage value Vdc.

The charging roller 12a is a contact type charging roller that makes contact with the photosensitive drum 11a without making a gap by using the elasticity of the elastic layer 12f. A metal shaft core 12e is used as a conductive support for applying an oscillating voltage. The core 12e is composed of a bearing, a bearing also serving as a voltage application unit, and a coating having an outer diameter of φ14 mm. Are integrally formed. On the peripheral surface of the covering portion, a resistance adjustment layer having a volume resistance value of 10 ⁵ to 10 ⁷ Ωcm of an ABS resin, which is a thermoplastic resin containing an ion conductive polymer compound such as polyether ester amide, is formed by injection molding. The coating is formed to a thickness of 0.5 to 1 mm. A protective layer made of a thermoplastic resin composition in which conductive fine particles such as tin oxide are dispersed is formed on the surface of the resistance adjustment layer.

The resistance adjustment layer is made of conductive rubber having a thickness of 1 to 2 mm in which a conductive agent such as carbon black is dispersed and mixed, and the resistance value is 10 ⁵ to 10 in order to prevent charging unevenness during image formation. ^It may be adjusted to ⁷ Ωcm.

<Detection means, control means>
FIG. 4 is a block diagram of a control system for the voltage applied to the charging roller. FIG. 5 is an explanatory diagram of the relationship between the peak-to-peak voltage of the alternating voltage applied to the charging roller and the alternating current. FIG. 6 is an explanatory diagram of control for setting the peak-to-peak voltage of the AC voltage.

  When an oscillating voltage is applied to the charging member, the discharge current to the photosensitive member is increased as compared with a case where only a direct current component is applied, so that the surface deterioration tends to be promoted such that the surface of the photosensitive member is rough and worn. Therefore, it is necessary to suppress the peak-to-peak voltage Vpp of the AC voltage as much as possible to prevent the charging member from being excessively discharged with respect to the photosensitive member.

  In addition, when the discharge current increases in a high temperature and high humidity environment, the discharge product adheres to the surface of the photoreceptor, and the attached discharge product attracts moisture in the air and forms uneven resistance on the surface. Such unevenness of resistance makes the electrostatic image formed on the surface of the photoreceptor non-uniform, and causes the so-called image blur such as the dots of the electrostatic image being stretched, thickened and scattered. Therefore, it is necessary to suppress the peak-to-peak voltage Vpp of the AC voltage as much as possible to prevent the charging member from being excessively discharged with respect to the photosensitive member.

  The relationship between the peak voltage Vpp of the AC voltage and the discharge current varies depending on the environmental temperature and humidity, the material of the charging member, the durability state, and the like. For example, even when the same peak-to-peak voltage is applied to the charging member, the resistance value of the charging member increases in a low temperature and low humidity environment, so that the discharge current decreases. In the high temperature and high humidity environment, the resistance value decreases, and the discharge current increases.

  In particular, since a recent charging roller as a charging member aims at a long life, an ionic conductive agent having high charge responsiveness inside the charging roller is used. The charging roller using an ionic conductive agent is more durable than the conventional charging roller using an electronic conductive agent, but has a large amplitude of resistance due to temperature and humidity fluctuations. Variations in the current relationship become significant.

  Therefore, in the image forming apparatus 100, during non-image formation, a plurality of stages of AC test voltages are applied to measure AC current, and a constant voltage of AC voltage is set so that a predetermined discharge current can be obtained based on the measurement result. doing. The AC current Iac when the peak voltage Vpp in the discharge region is applied to the charging roller 12a and the AC current Iac when the peak voltage Vpp in the non-discharge region is applied to the charging roller 12a are measured. Then, the measurement data is interpolated to obtain a relational expression between the discharge current ΔIac and the peak-to-peak voltage Vpp, and by substituting a predetermined discharge current D (for example, 50 μA) into this calculation formula, the constant voltage of the peak-to-peak voltage Vpp is obtained. Set.

  In this way, since the discharge current ΔIac is controlled more directly, even if the relationship between the peak-to-peak voltage Vpp and the discharge current varies with changes in temperature and humidity, it is highly accurate so as to keep the discharge current constant. AC voltage can be controlled.

  As shown in FIG. 4, an oscillation voltage obtained by superimposing an AC voltage Vac having a frequency f on a DC voltage Vdc is applied from the power source D3 to the charging roller 12a through the cored bar 12e, so that the peripheral surface of the photosensitive drum 11a is predetermined. Is charged to a potential of. The power source D3 includes a DC power source 101 and an AC power source 102.

  The control unit 50 has a function of applying on / off control of the DC power supply 101 and the AC power supply 102 to apply either a DC voltage or an AC voltage or a superimposed voltage of both to the charging roller 12a. The control unit 50 also has a function of controlling a DC voltage value applied from the DC power supply 101 to the charging roller 12a and a peak-to-peak voltage value of the AC voltage applied from the AC power supply 102 to the charging roller 12a.

  The alternating current measuring circuit 104 measures the alternating current flowing through the charging roller 12a via the photosensitive drum 11a. The measured alternating current information is input from the alternating current measuring circuit 104 to the control unit 50. The environment sensor 51 detects the temperature and humidity of the environment where the image forming apparatus 100 is installed. Temperature and humidity information detected by the environmental sensor 51 is input to the control unit 50.

  The controller 50 determines an appropriate peak-to-peak voltage in the alternating voltage of the oscillating voltage applied to the charging roller 12a from the alternating current value information input from the alternating current measurement circuit 104 and the temperature / humidity information input from the environmental sensor 105. Is calculated and determined.

Conventionally, the various studies, the following amounts of digitized discharge current delta Iac actual AC discharge surrogate manner indicate by definition, scraping of the photosensitive drum 11a, the image flow, charging uniformity and strongly correlated It has been found.

As shown in FIG. 5, the alternating current Iac flowing through the charging roller 12a has a linear relationship in the undischarged region of the peak-to-peak voltage Vpp less than the discharge start voltage Vth × 2 (V). In the discharge region where the peak-to-peak voltage Vpp is equal to or higher than the discharge start voltage Vth × 2 (V), the alternating current Iac gradually shifts in the increasing direction. However, when a similar experiment was performed in a vacuum in which no discharge occurred, the linear relationship was maintained in the undischarged region even when the discharge start voltage Vth × 2 (V) or higher. it is considered to be incremental delta Iac of current are involved in. When the ratio of the current Iac to the peak-to-peak voltage Vpp less than the discharge start voltage Vth × 2 (V) is α, the AC current (nip current) flowing to the contact portion other than the current due to discharge is α · Vpp. . Then, define the alternating current Iac measured during discharge start voltage Vth × 2 (V) or more of the peak-to-peak voltage Vpp is applied, the amount of discharge difference delta Iac nip current alpha · Vpp and the discharge current showing surrogate .
Δ Iac = Iac-α · Vpp ··· Formula 1

Discharge current delta Iac, when performing charging with the oscillating voltage of the constant peak voltage Vpp, varies as the accumulated usage time of the temperature and humidity and the image forming apparatus environment. This is because the relationship between the discharge current delta Iac between the peak-to-peak voltage Vpp is varied by these factors.

  Therefore, in the AC voltage setting mode, in order to always obtain a desired discharge current, the AC voltage setting mode is controlled in the following manner to obtain a constant voltage of the AC voltage. The controller 50 executes the AC voltage setting mode program during the print preparation rotation operation, and sets the constant voltage of the peak-to-peak voltage Vpp of the AC voltage so that a desired discharge current D can be obtained in the charging process during the printing process. decide. The controller 50 calculates and determines an appropriate peak-to-peak voltage value Vpp of the alternating voltage Vac of the oscillating voltage applied to the charging roller 12a.

  The controller 50 controls the AC power supply 102 during the print preparation rotation operation, and as shown in FIG. 6, the peak voltage (Vpp) in the discharge region is three points, the peak-to-peak voltage in the non-discharge region is three points, Sequentially applied to the charging roller 12a. Then, the alternating current value Iac flowing through the charging roller 12a through the photosensitive drum 11a at that time is measured by the alternating current measuring circuit 104 and input to the control unit 50.

The control unit 50 linearly approximates the relationship between the peak-to-peak voltage Vpp in the discharged and undischarged regions and the AC current Iac from the measured AC currents Iac for each of the three points using the least square method. Equation 3 is calculated.
Approximate straight line of discharge region: Yα = αXα + A Equation 2
Approximate straight line in the undischarged area: Yβ = βXβ + B Equation 3

The difference between the approximate straight line of the discharge region of Equation 2 and the approximate straight line of the undischarged region of Equation 3 is the discharge current ΔIac. Therefore, the peak-to-peak voltage Vpp1 at which this discharge current ΔIac becomes a predetermined discharge current amount D is determined by Equation 4.
First peak-to-peak voltage: Vpp1 = (D−A + B) / (α−β) Equation 4

  Then, the peak-to-peak voltage Vpp to be applied to the charging roller 12a is switched to Vpp1 obtained by Expression 4, and the process proceeds to a printing process using an oscillating voltage in which the AC voltage is constant-voltage controlled with the peak-to-peak voltage Vpp1.

  In the printing process, an oscillating voltage including an AC voltage whose constant voltage is controlled to the peak-to-peak voltage Vpp1 is applied to the charging roller 12a, and the AC current Iach flowing through the charging roller 12a in this state is measured by the AC current measuring circuit 104.

  In addition, an oscillating voltage including an AC voltage having a peak-to-peak voltage Vppm in an undischarged region is applied to the charging roller 2 between images (between sheets) between images in the printing process, and the AC current Iacm value flowing at that time is applied. Is measured by the alternating current measuring circuit 104.

  Then, the newly measured relationship between the peak-to-peak voltage Vpp1 and the alternating current Iach and the relationship between the peak-to-peak voltage Vppm and the alternating current Iacm are added to the relationship between the peak-to-peak voltage Vpp and the alternating current Iac measured during the print preparation rotation operation. To perform statistical processing. Thereby, the following formulas 5 and 6 are calculated.

The measurement point (peak-to-peak voltage Vpp1 / AC current Iach) during the new printing process and the measurement point between images (peak-to-peak voltage Vppm / AC current Iacm) are added to the measurement points obtained during the print preparation rotation operation, Increase the number of measurement points and recalculate using the least squares method.
Approximate straight line of discharge region: Yα = α′Xα + A ′ Expression 5
Approximate straight line of undischarged area: Yβ = β′Xβ + B (6)

The difference between the approximate straight line of the discharge region of Equation 5 and the approximate straight line of the undischarged region of Equation 6 is the discharge current ΔIac corrected by a new measurement. For this reason, the peak voltage Vpp2 for the next time corrected so that the discharge current ΔIac becomes a predetermined discharge current amount D is determined by Expression 7.
The peak-to-peak voltage for the next time: Vpp2 = (DA′−B) / (α′−β ′) Equation 7

  Then, the control unit 50 switches the peak-to-peak voltage Vpp1 applied to the charging roller 12a in the first image formation to the peak-to-peak voltage Vpp2 obtained by Expression 7, and performs the next image formation. In the next image formation, the control unit 50 similarly measures the alternating current Iac during the printing process and between the papers, and performs statistical processing with newly obtained measurement data added thereto. 5. Correct Equation 6. The relational expression (Equation 7) between the peak-to-peak voltage and the discharge current is corrected for each sheet, and the peak-to-peak voltage of the AC voltage applied to the charging roller 12a during the printing process is continuously corrected for each sheet.

  In this way, the peak-to-peak voltage Vpp1 necessary for obtaining the predetermined discharge current amount D in the first printing process is calculated during the printing preparation rotation operation. During the printing process, an image is formed using the obtained peak-to-peak voltage Vpp1, and a peak-to-peak voltage Vpp2 required to obtain a predetermined discharge current amount D by adding newly obtained measurement data. Correct. In the continuous printing mode, the alternating current Iach during the printing process and the alternating current Iacm when the peak-to-peak voltage Vppm of the undischarged region is applied to the charging roller 12a between the paper are measured and applied during the next printing process. The peak-to-peak voltage Vpp2 is corrected.

  By controlling the AC voltage setting mode in this way, the discharge current D is minimized by absorbing fluctuations in resistance values due to manufacturing variations of the charging roller 12a and environmental variations in the material and high-voltage variations in the main unit. The charging step can be performed at In the continuous printing mode, even if the resistance value fluctuation of the charging roller 12a is corrected for each sheet, the desired discharge current D can be reliably controlled.

  By the way, the influence of the film thickness of the photosensitive layer of the photosensitive drum exists as a factor that greatly changes the peak-to-peak voltage Vpp and the alternating current Iac. When an oscillating voltage is applied to the charging roller in contact with the photosensitive drum to charge the photosensitive drum, the amount of abrasion of the photosensitive drum increases due to discharge, which hinders the extension of the life of the photosensitive drum. The amount of abrasion of the photosensitive drum is particularly remarkable when an OPC photosensitive drum is used, and the charge transport layer (11ω: FIG. 3) serving as the surface layer is mainly scraped. The shaving of the photosensitive drum means that the surface of the photosensitive drum is shaved and the film thickness of the charge transport layer (11ω: FIG. 3) is reduced.

The relationship of Formula 8 is established between the surface potential V0 of the photosensitive drum 11a given by the charging roller 12a and the film thickness d of the photosensitive layer of the photosensitive drum 11a. The film thickness d is the distance from the surface of the photosensitive layer to the surface of the conductive substrate (11d: FIG. 3). Q is the amount of charge per unit area given to the photosensitive layer, C is the capacitance per unit area of the photosensitive layer, ε0 is the dielectric constant in vacuum, and εr is the relative dielectric constant of the photosensitive layer.
Q = CV0 = ε0 · εr · 1 / d · V0 Equation 8

  As can be seen from Equation 8, when the photosensitive drum 11a is worn and the photosensitive layer thickness d decreases, more charge Q is required to apply the same surface potential V0 to the photosensitive drum 11a. That is, Q required to secure the same surface potential V0 increases. Therefore, when the setting of the alternating voltage for charging is controlled with a constant voltage, the value of the alternating current decreases the film thickness of the photosensitive drum 11a. As it grows. Correspondingly, the discharge current that deteriorates the surface of the photosensitive drum 11a also increases.

  Therefore, even if the thickness of the photosensitive layer is reduced, if the same AC test voltage is continuously applied as when it is thick, an excessive discharge current is generated along with the application of the AC test voltage, and the photosensitive layer of the photosensitive drum 11a is deteriorated. Will progress. An excessive discharge current is generated in an AC voltage setting mode for setting a discharge current that can suppress deterioration of the photosensitive layer of the photosensitive drum 11a.

  Therefore, the amount of photosensitive layer scraping due to the accumulation of image formation is estimated based on the accumulated number of image formations, the accumulated rotation time of the photosensitive drum, and the like, and the AC test voltage used in the AC voltage setting mode is reduced according to the estimation result. Control was proposed. However, the film thickness of the photosensitive layer of the photosensitive drum 11a is not predictable only by the number of sheets used or the rotation time of the photosensitive drum, but depends on the temperature and humidity of the environment in which the photosensitive drum 11a is used and the copy volume that is flowed daily. . In addition, the photosensitive drum 11a may be rapidly consumed after sudden troubles such as developer carrier adhesion to the photosensitive drum 11a and jamming of the recording material. Therefore, in order to accurately know the film thickness of the photosensitive layer of the photosensitive drum 11a, it is effective to detect the film thickness of the photosensitive layer of the photosensitive drum 11a in real time.

  Therefore, in the following embodiments, the film thickness of the photosensitive layer of the photosensitive drum 11a is measured using the existing configuration in the AC voltage setting mode, and an AC test voltage in an appropriate range according to the film thickness is real-time. It is set. This prevents an excessive discharge current from being generated by the AC test voltage in the AC voltage setting mode, and sets the AC test voltage in a range close to the appropriate discharge current to increase the setting accuracy of the discharge current.

<Example 1>
FIG. 7 is an explanatory diagram for measuring the thickness of the photosensitive layer based on the amount of direct current flowing through the photosensitive drum. FIG. 8 is an explanatory diagram of changes in the relationship between the peak-to-peak voltage and the alternating current according to the film thickness of the photosensitive layer and the environment. FIG. 9 is an explanatory diagram of an increase in discharge current accompanying wear of the photosensitive layer. FIG. 10 is an explanatory diagram of the setting of the AC test voltage according to the measurement result of the film thickness of the photosensitive layer. FIG. 11 is a flowchart of the AC voltage setting mode according to the first embodiment.

  In Example 1, the AC test voltages AC1, AC2, AC3, AC1 ′, AC2 ′, and AC3 ′ used during the print preparation rotating operation and the AC test voltage ACm (peak-to-peak voltage Vppm) used between the images are used. Set according to.

  As shown in FIG. 4, a direct current measuring circuit 106 is provided between the photosensitive drum 11a and the ground potential as a life detecting means for detecting the film thickness of the photosensitive layer of the photosensitive drum 11a. The DC current measuring circuit 106 has a resistance R for measuring the DC current flowing from the charging roller 12a to the photosensitive drum 11a by the DC voltage of the oscillating voltage, and a capacitor C for bypassing the AC current flowing to the photosensitive drum 11a by the AC voltage. Including.

  The DC current measuring circuit 106 is an existing configuration for monitoring the DC voltage applied to the charging roller 12a by the control unit 50, and this existing configuration also serves as a film thickness measurement of the photosensitive layer of the photosensitive drum 11a. Yes.

  The controller 50 measures the voltage across the resistor R and calculates the current film thickness of the photosensitive layer of the photosensitive drum 11a based on the measured value. The controller 50 determines the current film thickness of the photosensitive drum 11a so that the AC test voltage does not become excessive.

  As shown in FIG. 9, when the charging potential (VD) of the photosensitive drum is constant and the film thickness of the photosensitive drum 11a changes, the direct current flowing from the photosensitive drum 11a to the charging roller 12a changes.

  In the first embodiment, an oscillating voltage including a DC voltage of −600 V is applied from the power source D3 to the charging roller 12a at the timing of rotation before start-up when the main power source is turned on at the beginning of the day, and the DC current measuring circuit 106 Thus, the direct current component at that time is measured.

  Since the amount of shaving of the photosensitive drum 11a does not progress so much during the day, it is sufficient to perform the film thickness measurement about once a day. The rotation before start-up when the main power supply is turned on at the beginning of the day is preferable because the surface potential, temperature, and the like of the photosensitive drum 11a are stable. On the other hand, during image formation, the surface potential of the photosensitive drum 11a is not guaranteed by laser exposure or the like, which is not preferable.

  If the frequency of film thickness measurement is increased, it is also possible to execute film thickness measurement at the timing of the previous rotation when returning from sleep, or at the timing between images during continuous image formation by widening the image interval. . However, since the reduction in the thickness of the photosensitive layer progresses through the accumulation of image formation, it is sufficient only when the main power supply is turned on even if the number of images formed by a heavy user per day is assumed.

  The DC voltage used for measuring the film thickness of the photosensitive layer is preferably -400V to -800V in the combination of the photosensitive drum 11a and the charging roller 12a of the embodiment. In particular, a stable measurement result with the highest reproducibility is obtained at −600V.

  When measuring the film thickness of the photosensitive layer, it is preferable to apply an oscillating voltage in which an AC voltage is superimposed on a DC voltage. The DC voltage used at the time of film thickness measurement may actually be any voltage as long as the DC current can be accurately detected. However, the AC voltage superimposed on the DC voltage is a voltage that can secure a sufficient discharge current between the photosensitive drum 11a and the charging roller 12a and can form a charging potential (VD) equal to the DC voltage on the photosensitive drum 11a. It is preferable. FIG. 14 is an explanatory diagram of an AC voltage superimposed on a DC voltage.

  As shown in FIG. 14A, if the AC voltage is not sufficient, the DC voltage applied to the charging roller 12a cannot be sufficiently reflected on the charging potential (drum potential) of the photosensitive drum 11a.

  As shown in FIG. 14B, if the AC voltage is not sufficient, the relationship between the current value measured through the charging roller 12a and the DC voltage becomes unstable.

  In short, the DC voltage at the time of film thickness measurement only needs to converge the charging potential (drum potential) to the DC voltage applied to the charging roller 12a, and the measured DC current is in a stable region. Therefore, what is important at the time of measuring the film thickness is that the AC voltage is not lower than the DC start voltage but is not less than the discharge start range.

  The relationship between the thickness of the photosensitive layer and the direct current also varies depending on the material of the photosensitive drum 11a, the process speed, and the temperature and humidity. This is because the resistance value of the photosensitive layer which is a semiconductor thin film varies depending on the temperature. However, if these parameters are equal, the direct current flowing through the photosensitive drum 11a increases as the thickness of the photosensitive layer decreases. Therefore, a film thickness conversion table divided according to the material of the photosensitive drum 11a, the process speed, and the temperature and humidity is prepared, and is referred to by the DC current measurement result using the DC current measuring circuit 106. The film thickness of the layer can be determined.

  In general, the film thickness detection means (106) of the photosensitive drum 11a is used to calculate the lifetime of the photosensitive drum 11a. That is, by measuring the change in the film thickness of the photosensitive drum 11a, the limit to the image defect of the cartridge including the photosensitive drum 11a is predicted.

  As shown in FIG. 10A, the relationship between the peak-to-peak voltage Vpp applied to the charging roller 12a and the alternating current Iac greatly depends on the film thickness of the photosensitive drum.

  As shown in FIG. 10 (b), the relationship between the peak-to-peak voltage Vpp applied to the charging roller 12a and the alternating current Iac is greatly influenced by fluctuations in the temperature and humidity (absolute humidity) of the environment in which it is used. In consideration of this, as a conventional means, there has been a proposal to change the AC test voltages AC1, AC2, AC3, AC1 ′, AC2 ′, and AC3 ′ based on the detection result of the environmental sensor 51. Yes. That is, it is possible to predict the environment in which the image forming apparatus 100 is used, and to correct the change in the relationship between the peak-to-peak voltage and the alternating current applied to the charging roller due to environmental fluctuations.

  However, the present situation is that no correction is made with respect to the change in the relationship between the peak-to-peak voltage applied to the charging roller and the alternating current that occurs with the change in the film thickness of the photosensitive drum 11a.

  As shown in FIG. 9, when the film thickness of the photosensitive layer of the photosensitive drum is reduced from 29 μm to 15 μm, if the same AC test voltages AC1, AC2, AC3 as when the film thickness is 29 μm are applied, an excessive discharge current is generated. And a large alternating current flows. As the film thickness decreases, the relationship between the peak-to-peak voltage Vpp and the discharge current ΔIac changes. For this reason, the peak-to-peak voltage Vpp1 in Equation 4 that provides a predetermined discharge current D is set to a value far from the AC test voltages AC1, AC2, and AC3 used when the film thickness of the photosensitive layer is 29 μm. become.

  That is, when the film thickness of the photosensitive layer is 15 μm, the AC test voltages AC1, AC2, and AC3 used when the film thickness is 29 μm are unsuitable for obtaining the peak-to-peak voltage Vpp1 and have an excessively large error between the peaks. It has become. As the film thickness changes, the relationship between the peak-to-peak voltage Vpp applied to the charging roller 12a and the AC current Iac changes greatly, so that the calculation error of the above-described equations 2 and 3 increases, and the reliability of the AC voltage setting mode is increased. Sex is reduced.

  Further, when the film thickness of the photosensitive layer is 29 μm, the AC test voltage AC1 ′ in the undischarged area applied to the charging roller 11a between the papers in the continuous printing process becomes a peak-to-peak voltage in the discharge area when the film thickness is 15 μm. It has become. In this case, the alternating current / peak voltage data (AC1 ') in the discharge area is used as the alternating current / peak voltage in the undischarged area, and correction of the peak-to-peak voltage between sheets cannot be performed properly.

  For this reason, in the first embodiment, in the AC voltage setting mode, the AC test voltages in the undischarged area and the discharged area used for calculating the above-described Expressions 2 and 3 are set according to the environmental variation and the film thickness of the photosensitive drum 11a. change. Further, the AC test voltage in the undischarged area applied to the charging roller 11a between the sheets in the continuous printing process is also changed according to the environmental fluctuation and the film thickness of the photosensitive drum.

  As shown in FIG. 6, in the AC voltage setting mode, a three-stage AC test voltage set in the discharge region and a three-step AC test voltage set in the non-discharge region are used.

  As shown in FIG. 9, the relationship between the peak-to-peak voltage applied to the charging roller 12a and the discharge start voltage (Vth × 2) varies depending on the change in the thickness of the photosensitive layer of the photosensitive drum 11a and the environmental fluctuation. When the film thickness is 29 μm, the three-stage AC test voltages AC1 ′, AC2 ′, and AC3 ′ are all selected to be undischarged region values lower than the discharge start voltage. However, when the film thickness is 15 μm, the AC test voltage AC1 ′ crosses the discharge start voltage and moves to the discharge region. For this reason, when the relational expression (formula 2) of the undischarged region is obtained using the data of the alternating current Iac / peak-to-peak voltage Vpp of the alternating test voltages AC1 ', AC2', and AC3 ', an error increases.

  Further, the relationship between the peak-to-peak voltage Vpp and the alternating current Iac in the discharge region exceeding the discharge start voltage is not actually a linear proportional relationship as shown in FIG. 6, but a quadratic function as shown in FIG. It becomes a relationship. The relationship between the peak-to-peak voltage Vpp applied to the charging roller 12a and the discharge current ΔIac does not change linearly in proportion. Therefore, when the peak-to-peak voltage Vpp is controlled so as to set a predetermined discharge current D, the AC test voltages AC1, AC2, and AC3 that are higher than the discharge start voltage are as close as possible to the actually used peak-to-peak voltage Vpp1. Must be set. And it is necessary to calculate the discharge current D (Formula 4) in a range close to the peak-to-peak voltage Vpp1 that is actually used.

  For this reason, in Example 1, the AC test voltages AC1, AC2, AC3 in three stages in the discharge region and the AC test voltages AC1 ′, AC2 ′, AC3 ′ in the non-discharge region are applied to the photosensitive layer of the photosensitive drum 11a. It is set according to the film thickness. It is set according to the environmental variation shown in Table 1 and the film thickness of the photosensitive layer shown in Table 2.

  As shown in FIG. 4, in Example 1, the thickness of the photosensitive layer of the photosensitive drum 11a is accurately measured by referring to the table in Table 2 with the DC current [μA] measured by the DC current measuring circuit 106. . Then, according to the measured film thickness, as shown in Table 3, three stages of AC test voltages AC1, AC2, and AC3 in the discharge region and three stages of AC test voltages AC1 ′, AC2 ′, and AC3 ′ in the undischarged region. Set.

  As shown in Table 3, the AC test voltages AC1, AC2, AC3, AC1 ′, AC2 ′, and AC3 ′ are set lower as the photosensitive layer thickness of the photosensitive drum 11a measured by the DC current measuring circuit 106 becomes smaller. . Even if the film thickness of the photosensitive layer changes, the discharge current D is set constant.

  As shown in Table 1, the AC test voltages AC 1, AC 2, AC 3, AC 1 ′, AC 2 ′, and AC 3 ′ are set lower as the moisture content (absolute humidity) in the air measured by the environment sensor 105 is larger. The discharge current D is set lower as the moisture content (absolute humidity) in the air increases.

  As shown in FIG. 11 with reference to FIG. 4, the control unit 50 performs continuous image formation while calculating Vpp1 from Equation 4 as needed. The control for calculating Vpp1 (S13 to 15) was performed once every time the number of copies was counted and the cumulative number of sheets from the previous setting counted 200 (YES in S12). As described with reference to FIG. 6, the alternating current due to the alternating current test voltage was measured in the discharge region (S13) and the non-discharge region (S14), and the peak-to-peak voltage Vpp1 corresponding to the predetermined discharge current D was obtained. (S15).

  Further, regarding the film thickness detection control (S20 to S22) of the photosensitive drum 11a, every time the number of copies is counted and the cumulative number from the previous AC test voltage adjustment counts 1000 (YES in S19), 1 I went twice. The film thickness of the photosensitive drum 11a was detected (S21), and the AC test voltage was adjusted (S22).

  Further, the alternating current due to the peak-to-peak voltage in the discharge region is measured in each printing step (S16), the alternating current due to the peak-to-peak voltage in the undischarged region is measured between the papers (S17), and the peak-to-peak voltage Vpp is determined from the paper. Adjustments were made at intervals (S18).

  Of course, the control frequency for calculating Vpp1 may be changed according to the configuration of the main body, the characteristics of the photosensitive drum 11a, the characteristics of the charging roller 12a, and the like. In general, the control frequency is often once per 100 to 1000 sheets, and is preferably once per 200 to 500 sheets in consideration of productivity and control accuracy.

  Further, the control frequency may be changed with respect to the film thickness detection control of the photosensitive drum 11a, and it is desirable to keep a single pace while the photosensitive drum 11a is scraped by 1 μm in consideration of the scraping amount of the photosensitive drum 11a. In this case, it is set to once per 1000 sheets. However, when using a photosensitive drum with a small amount of scraping recently, it is possible to perform control such as performing only at the time of warm-up immediately after the main body is turned on.

In Example 1, as a measurement point of control for calculating the charging pressure conditions Vpp1 during imaging, AC1, AC2, AC3, and AC1', AC2', using six points AC3'. However, of course, the present invention is not limited to this, and it is sufficient that at least Equation 2 in the discharge region and Equation 3 in the undischarged region can be calculated. If there is enough. Conversely, if it is desired to increase the accuracy of equations 2 and 3, the AC test voltage level can be increased to 6 or more.

  As shown in Table 1, the relationship between the peak-to-peak voltage Vpp applied to the charging roller 12a and the AC current Iac was calculated, and control was performed to determine the peak-to-peak voltage Vpp1 of the AC voltage during image formation. The change in the film thickness of the photosensitive drum 11a is predicted by the film thickness detection control of the photosensitive layer, and the test AC voltages AC1, AC2, AC3, AC1 ′, AC2 ′, and AC3 ′ are changed according to the change in the film thickness. .

  As a result, the discharge current D, which is the basis for the deterioration of the photosensitive drum 11a, can be accurately controlled throughout the life of the photosensitive drum 11a. Then, abnormal discharge that has been a problem when the film thickness of the photosensitive drum 11a has been reduced has been prevented. As a result, image blur due to deterioration of the photosensitive drum 11a and image unevenness due to uneven shading of the photosensitive drum 11a do not occur at all, and image evaluation after passing through 50,000 sheets of the photosensitive drum 11a of the image forming apparatus 100 is performed. There were no image defects.

  In the first embodiment, it is possible to appropriately maintain a charging discharge current that greatly affects the deterioration of the surface of the photosensitive drum 11a. The relationship between the peak voltage of the alternating voltage and the alternating current can be measured, and the discharge current amount can be calculated from the relational expression. Control of the amount of discharge current at the end of life, which has been difficult in the past, is made possible by accurately grasping the film thickness of the photosensitive drum 11a and changing the AC test voltage according to the change in the film thickness. It became possible to control the current accurately. As a result, the discharge current discharged from the charging roller 12a to the photosensitive drum 11a, which causes deterioration of the photosensitive drum 11a, can be minimized. A reduction in the reliability of the AC voltage setting mode when the film thickness of the photosensitive drum 11a changes is prevented by changing the AC test voltage used for control based on the detection result of the film thickness detecting means of the photosensitive drum 11a. To do. Therefore, it is possible to remarkably suppress image blurring that is likely to occur and defective images due to uneven shading of the photosensitive drum 11a according to the usage state of the photosensitive drum 11a, and the image quality of the image forming apparatus 100 is remarkably high. Improved.

  In the first embodiment, as an example of detecting the film thickness of the photosensitive drum 11a, an embodiment is described in which a DC current detection circuit 106 is provided as a life detection means between the photosensitive drum 11a and the ground potential. However, the present invention is not limited to this, and the DC current measurement circuit 106 according to the first embodiment may be disposed between the charging roller 12a and the DC power supply 101, or may be disposed between the DC power supply 101 and the ground potential. . Similarly, the AC voltage measurement circuit 104 may be disposed between the charging roller 12a and the AC power source 102, or may be disposed between the AC power source 102 and the ground potential.

<Example 2>
In Example 1, as shown in Table 1 above, the AC test voltages AC1, AC2, AC3, AC1 ′, AC2 ′, and AC3 ′ are applied in increments for each combination of the absolute humidity (g / kg Air) and the film thickness of the photosensitive layer. Set. On the other hand, in Example 2, the AC test voltages AC1, AC2, AC3, AC1 ′, AC2 ′ at the initial film thickness of the photosensitive layer (the new state of the photosensitive drum 11a) at every increment of absolute humidity (g / kg Air). , AC3 ′ was set. Then, after the use of the photosensitive drum 11a from the new state is started, the current flowing through the photosensitive layer when a predetermined DC voltage is applied is measured in the same manner as in Example 1 to calculate the thickness of the photosensitive layer, and the initial film Find the amount of wear from the thickness. And AC test voltage AC1, AC2, AC3, AC1 ', AC2', AC3 'was set for every step of the amount of wear. As shown in FIG. 10, every time the amount of wear reached 3 μm, the AC test voltages AC1, AC2, AC3, AC1 ′, AC2 ′, and AC3 ′ were uniformly reduced by 20V.

  In Table 4, the AC test voltage is not linear as in the first embodiment, but is linearly changed according to the decrease in the thickness of the photosensitive layer of the photosensitive drum 11a calculated based on the DC voltage measurement result by the DC current measuring circuit 106. Decreased. A table of AC test voltages was set by providing regularity so that the peak-to-peak voltage Vpp is reduced by 20 V with respect to the wear amount of 3 μm on the photosensitive drum 11a.

  In Table 5, the step of decreasing the film thickness of the photosensitive layer is made larger than in Table 2, and the AC test is performed with regularity so that the peak-to-peak voltage Vpp decreases by 40 V with respect to the wear amount of 5 μm on the photosensitive drum 11a. A voltage table was set up.

  Using the table in Table 4, experiments for continuous image formation were performed in the same manner as in Example 1. An alternating current that determines the peak-to-peak voltage Vpp1 of the alternating voltage during image formation by calculating the relationship between the peak-to-peak voltage applied to the charging roller 12a and the alternating current at a predetermined timing during the pre-preparation rotating operation and during continuous image formation. The voltage setting mode was performed. The change in the film thickness of the photosensitive layer of the photosensitive drum 11a is electrically measured, and the AC test voltages AC1, AC2, AC3, AC1 ′, AC2 ′, and AC3 ′ are uniformly changed in accordance with the film thickness change. It was.

  As a result, the discharge current D, which is the basis for the deterioration of the photosensitive drum 11a, can be accurately controlled throughout the life of the photosensitive drum 11a. Image blur due to deterioration of the photosensitive drum 11a due to abnormal discharge and image unevenness due to uneven shading of the photosensitive drum 11a no longer occur. In the image evaluation after continuous 50,000 copies were made by the image forming apparatus 100, there were no image defects.

<Example 3>
FIG. 12 is an explanatory diagram of a configuration for measuring the film thickness of the photosensitive layer in Example 3. FIG. 13 is an explanatory diagram of the relationship between the alternating current and the film thickness of the photosensitive layer.

  In Examples 1 and 2, the film thickness of the photosensitive drum was evaluated according to the current flowing through the photosensitive layer of the photosensitive drum when a predetermined DC voltage was applied to the charging roller 12a, that is, the DC resistance of the photosensitive layer. On the other hand, in Example 3, the film thickness of the photosensitive drum is evaluated according to the AC current that flows when a predetermined AC voltage is applied to the charging roller 12a, that is, the electrostatic capacitance of the photosensitive layer.

  As shown in FIG. 12, in the third embodiment, an alternating current measuring circuit 107 is provided between the photosensitive drum 11a and the ground potential as a lifetime detecting unit for detecting the film thickness of the photosensitive drum 11a. The control unit 50 detects the voltage between the terminals of the AC current measuring circuit 107, measures the current film thickness of the photosensitive layer of the photosensitive drum 11a, and sets an AC test voltage corresponding to the current film thickness of the photosensitive layer.

  The AC current measuring circuit 107 is a resistor R for measuring the AC current flowing through the charging roller 12a applied with a DC bias and an AC bias superimposed voltage as a charging bias in contact with the photosensitive drum 11a, and a capacitor for bypassing the DC current. C.

  FIG. 13 is a graph showing changes in the alternating current flowing through the photosensitive drum 11a when the film thickness of the photosensitive drum 11a changes with the charging potential kept constant. As can be seen from FIG. 13, as in the first and second embodiments, not only the direct current flowing through the photosensitive drum 11a but also the alternating current is applied to the photosensitive drum 11a as the photosensitive layer thickness decreases. The amount of current flowing is increasing.

  This is because, as described with respect to Equation 8, as the film thickness d of the photosensitive layer decreases, the capacitance C of the photosensitive layer increases and the alternating current Iac increases when the equal alternating voltage Vac is applied. Therefore, the means for detecting the film thickness of the photosensitive drum as in the first embodiment can be performed not only by the direct current measuring circuit (106: FIG. 4) but also by the alternating current measuring circuit 107.

  In the configuration in which the setting condition of the AC voltage Vac during image formation is determined by the relationship between the peak-to-peak voltage Vpp applied to the charging roller 12a and the AC current Iac, an AC current Iac detection circuit is originally required. . Since the image forming apparatus 100 is premised on the configuration in which the alternating current measuring circuit 107 exists, when the film thickness detection of the photosensitive layer of the photosensitive drum 12a is performed with the alternating current, the setting condition of the alternating voltage is originally determined. The AC current measurement circuit used for control can be used as it is. Therefore, there is an advantage that it is not necessary to provide the direct current measuring circuit 106 as in the first embodiment.

  Further, in the method of measuring the film thickness of the photosensitive drum 12a according to the alternating current Iac, the alternating test voltages AC1, AC2, AC3, AC1 ′, AC2 ′, and AC3 ′ serving as the basis for calculating Expressions 1 and 2 are used. It is also possible to detect the film thickness of the photosensitive drum 12a during the measurement.

  For example, the AC test voltage AC4 ′ is added to the measurement point, the measurement conditions of the AC test voltage AC4 ′ are kept the same every time, and the film thickness of the photosensitive drum 11a is detected from the detection result of the AC current while the AC test voltage AC4 ′ is being applied. Can be detected.

  Originally, the measurement point of the AC test voltage AC3 ′ used for the control of the AC voltage setting mode is always fixed to a constant AC voltage, and the photosensitive drum 11a is detected from the detection result of the AC current while the AC test voltage AC3 ′ is being applied. The film thickness may be measured.

  In any case, if the film thickness detecting means for the photosensitive layer of the photosensitive drum 12a and the discharge current control means for determining the setting condition of the AC voltage can be used in combination, the control time can be remarkably shortened and the productivity can be improved. Become.

  In Example 3, the film thickness of the photosensitive layer of the photosensitive drum 12a is detected with an alternating current. Based on the detection result, the AC test voltages AC1, AC2, AC3, AC1 ′, AC2 ′, and AC3 ′ are shown in Table 3. Has been changed arbitrarily.

  The alternating current measured using the table in Table 6 is converted into the film thickness of the photosensitive layer. The setting of the AC test voltage according to the film thickness is the same as in Example 1.

  As shown in Table 7, in Example 3, the AC test voltages AC1, AC2, AC3, AC1 ′, AC2 ′, and AC3 ′ are also changed due to environmental changes. In Example 3, the target of the discharge current D was changed according to the environment, and the AC voltage setting mode was controlled while calculating the peak-to-peak voltage Vpp1 of the AC current as needed using Equation 4. The control for calculating the peak-to-peak voltage Vpp1 was performed once every time the number of copies was counted and the integrated value counted 200 sheets. Further, regarding the film thickness detection control of the photosensitive drum 12a, the number of copies was counted, and it was performed once every time the integrated value counted 1000 sheets to detect the film thickness of the photosensitive drum 12a.

<Example 4>
By the way, the constant current system which controls the alternating current component of an oscillating voltage with a constant current is proposed. In the constant current method, the alternating current Iac flowing through the photosensitive drum is detected, and the applied voltage is controlled so as to be constant. When the constant current control method is used, the AC peak voltage can be freely changed with respect to the impedance change due to the environmental change, so that the AC current Iac can be kept almost constant regardless of the environmental change.

However, the constant-current control method of the AC voltage, and controls the total current flowing from the charging member to the photosensitive member (the sum of the nip current alpha · Vpp discharge current delta Iac) to a constant value. The constant current control, the discharge current delta Iac but also for controlling the nip current alpha · Vpp be included at a constant value, a current required to charge actually photoreceptor discharge current delta Iac constant value I have not been able to control.

Therefore, in the constant current control, even if controlled by the same current value, discharge current delta Iac The more nip current alpha · Vpp by a change in the resistance of the material of the charging member is reduced, nip current alpha · Vpp is discharge current delta Iac if Hele would increasing.

  Therefore, in the constant current control method disclosed in Patent Document 2, it is impossible to completely suppress the increase / decrease in the amount of discharge current. When aiming for a long life, both the shaving of the photosensitive drum 11a and the charging uniformity can be achieved. It was difficult to realize. For this reason, in the constant current method for controlling the AC current Iac flowing through the photosensitive drum to a constant level, a phenomenon such as photosensitive drum scraping or image blur due to overdischarge is caused, or a poorly charged image such as a sand image due to insufficient discharge is generated. It was generated.

Alternatively, the discharge current delta Iac, when subjected to charging with a constant current control to maintain a constant alternating current, varies as the accumulated usage time of the temperature and humidity and the image forming apparatus environment. This relationship of the alternating current Iac and the discharge current delta Iac is because varies by these factors.

  Therefore, in the AC voltage setting mode of the fourth embodiment, during non-image formation, the AC voltage is measured by applying an AC test current in which a constant current is set in a plurality of stages in each of the discharge region and the non-discharge region to the charging roller 12a. To do. Then, a relational expression between the alternating current and the discharge current is obtained from a relational expression between the alternating current test voltage and the alternating voltage in the discharge region and a relational expression between the alternating current test voltage and the alternating voltage in the non-discharge region. Then, a predetermined discharge current (for example, 50 μA) is substituted into the “relational expression between alternating current and discharge current” to set a constant current value of the alternating current used during image formation.

  However, in this case, when the thickness of the photosensitive layer of the photosensitive drum 11a is reduced, the AC test current becomes excessive, which may cause deterioration of the photosensitive drum due to abnormal discharge. Therefore, in Example 4, the film thickness of the photosensitive layer is electrically measured as in Examples 1 to 3, and the AC test current is gradually reduced according to the wear of the film thickness. That is, in Example 4, the control means executes the AC voltage setting mode, and sets the constant current of the AC voltage according to the AC voltage detected by applying the AC test current to the charging member. Then, a detection means for electrically detecting the film thickness of the photosensitive layer is provided. In the AC voltage setting mode, the AC test current is set lower as the film thickness of the photosensitive layer becomes smaller based on the detection result of the detection means.

11a, 11b, 11c, 11d Photosensitive member (photosensitive drum)
12a, 12b, 12c, 12d Charging member (charging roller)
13a, 13b, 13c, 13d Exposure devices 14a, 14b, 14c, 14d Development devices 35a, 35b, 35c, 35d Primary transfer roller 31 Intermediate transfer belt, 34 Counter roller, 36 Secondary transfer roller 50 Control unit, 51 Environmental sensor, 52 Operation Panel 101 DC Power Supply, 102 AC Power Supply 106 DC Current Measurement Circuit, 107 AC Current Measurement Circuit D1, D2, D3, D4 Power Supply Pa, Pb, Pc, Pd Image Forming Unit

Claims (2)

A photoreceptor having a photosensitive layer;
A charging member arranged in contact with the surface of the photoconductor to charge the photoconductor;
A power source that applies a charging bias in which an AC voltage is superimposed on a DC voltage to charge the photosensitive member to the charging member;
An image forming unit that forms a toner image on the photosensitive member charged by the charging member;
A first detector for detecting an alternating current flowing through the charging member;
A second detection unit for detecting a direct current flowing through the charging member;
AC voltage at the charging bias based on the AC current detected by the first detector when a plurality of AC test voltages having different peak-to-peak voltages are applied to the charging member in the discharge region and the non-discharge region, respectively. The charging voltage is applied by the second detection unit when an oscillating voltage in which an AC voltage having a peak-to-peak voltage in the discharge region and a DC voltage are superimposed is applied to the charging member. An execution unit for executing a second mode for detecting a direct current flowing through the member,
In the first mode, the execution unit detects the peak-to-peak voltage of the AC test voltage having the highest peak-to-peak voltage among the plurality of AC test voltages in the undischarged region in the second mode. When the direct current is the first current, the first voltage is set, and the absolute value of the direct current detected in the second mode is the second current larger than the first current. The image forming apparatus is characterized in that the second voltage smaller than the first voltage is set. A photoreceptor having a photosensitive layer;
A charging member arranged in contact with the surface of the photoconductor to charge the photoconductor;
A power source that applies a charging bias in which an AC voltage is superimposed on a DC voltage to charge the photosensitive member to the charging member;
An image forming unit that forms a toner image on the photosensitive member charged by the charging member;
A first detector for detecting a peak-to-peak voltage of an alternating current applied to the charging member;
A second detection unit for detecting a direct current flowing through the charging member;
The AC at the charging bias based on the peak-to-peak voltage detected by the first detection unit when a plurality of AC test currents having different current values are applied to the charging member in the discharge region and the non-discharge region, respectively. The charging is performed by the second detection unit when a first mode for setting a current value of the voltage and an oscillating voltage in which an AC voltage having a peak-to-peak voltage in a discharge region and a DC voltage are applied to the charging member. An execution unit for executing a second mode for detecting a direct current flowing through the member,
In the first mode, the execution unit detects a current value of an AC test current having the largest current value among a plurality of the AC test currents in at least the undischarged region in the second mode. When the current is the first direct current, the first direct current is set, and the second direct current in which the absolute value of the direct current detected in the second mode is larger than the first direct current. In this case, the image forming apparatus is set to a second alternating current smaller than the first alternating current.

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