JP2009251127A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus able to set an inter-peak voltage for an AC voltage during image non-formation period without increasing the capacity of a power source due to a small amount of AC current flowing when the AC voltage which is a high inter-peak voltage is applied. <P>SOLUTION: The inter-peak voltage Vpp of an AC voltage Vac applied to a charging roller 2a in a charging step of a printing process is reset during a pre-rotation at the start of the image forming apparatus or each time the cumulative number of sheets subjected to image formation reaches 200. When image formation is not carried out, an inter-peak voltage Vpp for the AC voltage Vac is set using a constant voltage obtained by interpolating a discharge current value measured using a plurality of levels of AC voltage at a frequency lower than that used for regular image formation. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an image forming apparatus for charging an image carrier by applying a charging voltage including a constant alternating voltage to a charging member, and more specifically, a control for setting a constant voltage of an alternating voltage used during image formation during non-image formation. About.

  2. Description of the Related Art Image forming apparatuses that apply a charging voltage in which an AC voltage is superimposed on a DC voltage to a charging member such as a charging roller to uniformly charge the surface of the image carrier are widely used. The AC voltage forms a bidirectional discharge state between the charging member and the image carrier, eliminates the potential difference between the surface of the image carrier and the charging member, and charges the image carrier to a potential equal to the DC voltage. Let

  Patent Document 1 discloses an image forming apparatus in which a charging voltage obtained by superimposing a constant voltage controlled AC voltage on a DC voltage is applied to a charging roller to charge the surface of a photosensitive drum. Here, during non-image formation, an AC voltage with different peak-to-peak voltages is applied to the charging roller to detect the AC current at each stage, and the AC voltage used during image formation is determined based on the detection results. The voltage (voltage between peaks) is set.

JP 2001-201921 A

  In the control of Patent Document 1, an alternating current is measured using a high peak voltage and a low peak voltage prepared in advance so as to sandwich a range of peak voltages used during image formation. This is because the setting accuracy of the peak-to-peak voltage can be increased by setting the peak-to-peak voltage at the time of image formation inside the two or more peak-to-peak voltages in which the alternating current is detected (within the interpolation range).

  However, if an AC voltage with a peak-to-peak voltage that is higher than the range of peak-to-peak voltages used during image formation is applied to the charging member, an excessive AC current that does not normally flow may flow and damage the power supply. is there.

  In particular, when a charging member whose resistance value increases as image formation accumulates is used, a peak-to-peak voltage much higher than the range of peak-to-peak voltages that are always used is used in anticipation of a future increase in resistance value. It is done. For this reason, in the stage where the initial resistance value with little accumulation of image formation is low, a considerable overcurrent flows, and it is necessary to provide a considerably large capacity power source in order to avoid damage.

  The present invention provides an image forming apparatus capable of setting a peak-to-peak voltage of an AC voltage during non-image formation without increasing a power supply capacity because an alternating current flowing when an alternating voltage having a high peak-to-peak voltage is applied is small. The purpose is that.

  An image forming apparatus according to the present invention includes a charging member that charges an image carrier by applying a charging voltage including a constant alternating voltage, and a power source that can output alternating voltages having different peak-to-peak voltages. is there. The image forming apparatus includes a control unit that applies an AC voltage having a frequency lower than that at the time of image formation to the charging member during non-image formation and sets the constant voltage used at the time of image formation.

  Even if the frequency of the AC voltage is lowered, the discharge start voltage and the discharge current that change according to the facing condition between the charging member to which the bias is applied and the image carrier do not change, but the series circuit of the charging member and the image carrier is not changed. The flowing alternating current decreases. In other words, even when the AC voltage is lowered by reducing the frequency of the AC voltage of the test bias, the discharge state at a high frequency is properly reproduced, and the charging bias (DC voltage) is image-bearing via the charging member. The peak-to-peak voltage that can be transferred to the body without excess or deficiency can be set. An appropriate constant voltage in the image forming mode can be set based on the detection result with the power supply load kept small.

  Accordingly, since an alternating current that flows when an alternating voltage having a higher peak voltage than that always used is applied is small, the peak voltage of the alternating voltage can be set during non-image formation without increasing the power supply capacity.

  Hereinafter, some embodiments of the present invention will be described in detail with reference to the drawings.

  In this example, as long as the frequency of the AC voltage applied to the charging member at the time of non-image formation is lower than that at the time of image formation, a part or all of the configuration of each example is replaced with the alternative configuration. The present invention can also be implemented.

  Therefore, the present invention is not limited to the intermediate transfer belt method, and a method of sequentially transferring and superimposing toner images on a recording material carried on a recording material conveying belt, or a method of directly transferring from a photosensitive member to a recording material can also be implemented. Not only a tandem type in which a plurality of photosensitive drums are arranged along the belt member but also a single drum type in which a plurality of developing devices are arranged on one photosensitive drum.

  In this embodiment, only main parts related to toner image formation / transfer will be described. However, in the present invention, in addition to necessary equipment, equipment, and a housing structure, a printer, various printing machines, a copier, a FAX, a composite, It can be implemented in various applications such as a machine.

  Further, the charging member is not necessarily in contact with the image carrier. If a dischargeable region determined by the voltage between the gap and the modified Paschen curve (discharge condition equation) is ensured between the charging member and the image carrier, for example, there is a 10 μm air gap and no contact is made. It may be arranged in close proximity. In the following, such a case of proximity charging is also referred to as contact charging.

  In addition, about the general matter regarding the image formation apparatus shown by patent document 1, a charging device, and the setting control of the peak-to-peak voltage, illustration is abbreviate | omitted and the overlapping description is abbreviate | omitted. In the description, the reference symbols in parentheses attached to the component names used in the claims are examples for helping understanding of the invention, and the configuration is limited to the corresponding members in the examples. Is not.

  FIG. 1 is an explanatory diagram of a configuration of an image forming apparatus according to the first exemplary embodiment, FIG. 2 is an explanatory diagram of a configuration of an image forming unit, and FIG. 3 is an explanatory diagram of a layer configuration of a photosensitive drum.

  As shown in FIG. 1, the image forming apparatus 100 is a tandem full-color copying machine in which yellow, magenta, cyan, and black image forming portions Pa, Pb, Pc, and Pd are arranged along an intermediate transfer belt 11.

  In the image forming portion Pa, a yellow toner image is formed on the photosensitive drum 1 a and is primarily transferred to the intermediate transfer belt 11. In the image forming portion Pb, a magenta toner image is formed on the photosensitive drum 1 b and is primarily transferred to the yellow toner image on the intermediate transfer belt 11. In the image forming portions Pc and Pd, a cyan toner image and a black toner image are formed on the photosensitive drums 1c and 1d, respectively, and similarly, the toner images on the intermediate transfer belt 11 are overlapped with each other and sequentially primary transferred.

  The four-color toner images primarily transferred to the intermediate transfer belt 11 are pulled out one by one from the recording material cassette 14 and are collectively secondary transferred to the recording material P fed to the secondary transfer portion T2. The recording material P on which the toner image is secondarily transferred by the secondary transfer portion T2 is heated and pressed by the fixing device 9 to fix the toner image on the surface, and then is discharged to the discharge tray.

  The intermediate transfer belt 11 is supported around a tension roller 15, a drive roller 16, and a backup roller 12, and rotates in a direction indicated by an arrow R2 at a predetermined process speed.

  The belt cleaning device 13 is disposed with the cleaning blade always in contact with the intermediate transfer belt 11, and cleans transfer residual toner and fog toner attached to the intermediate transfer belt 11 that has passed through the secondary transfer portion T2.

  The image forming portions Pa, Pb, Pc, and Pd are substantially the same except that the color of toner used in each developing device is different from yellow, magenta, cyan, and black. Hereinafter, the image forming unit Pa will be described with reference to FIG. 2, 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”. Shall.

  As shown in FIG. 2, in the image forming unit Pa, a charging roller 2a, an exposure device 3a, a developing device 4a, a primary transfer roller 5a, and a cleaning device 6a are arranged around the photosensitive drum 1a. However, as shown in FIG. 1, a corona charger is used as a charging device for the image forming unit Pd at the final position in the image feeding direction.

  The photosensitive drum 1a is formed with an organic photoconductor layer (OPC) having a negative polarity on the outer peripheral surface of a conductive drum base, and finished to an outer diameter of 30 mm. In the direction of arrow R1 at a process speed of 300 mm / sec. Rotate.

  The charging roller 2a is driven to rotate while being pressed against the photosensitive drum 1a, and charges the surface of the photosensitive drum 1a to a uniform negative potential by a contact charging method. The power source D3 outputs a charging voltage obtained by superimposing an AC voltage on a DC voltage to the charging roller 2a. The conductive drum base of the photosensitive drum 1a is connected to the ground potential.

  The exposure device 3a scans, with a rotating mirror, a laser beam obtained by ON-OFF modulation of scanning line image data obtained by developing a yellow color separation image, and writes an electrostatic image of the yellow image on the surface of the charged photosensitive drum 1a.

  The developing device 4a agitates a two-component developer in which nonmagnetic toner is mixed with a magnetic carrier, and charges the nonmagnetic toner to negative polarity and the magnetic carrier to positive polarity. The charged two-component developer is carried on the developing sleeve 41a rotating in the counter direction around the fixed magnetic pole in a stand-up state and rubs against the photosensitive drum 1a.

  The power source D4 applies an oscillating voltage obtained by superimposing an AC voltage on a negative DC voltage to the developing sleeve 41a, and applies a non-magnetic toner to the electrostatic image on the photosensitive drum 1a that has a relatively positive polarity relative to the developing sleeve 41a. The electrostatic image is reversed and developed.

  Specifically, 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 of −550 V is applied to the developing sleeve 41a.

  The toner bottle 7a replenishes the nonmagnetic toner taken out from the developing device 4a with the development of the electrostatic image, and maintains the toner / carrier ratio in the developing device 4a constant.

  The primary transfer roller 5a is pressed against the photosensitive drum 1a with a predetermined pressing force so as to sandwich the intermediate transfer belt 11 to form a primary transfer portion Ta.

  The power source D1 applies a positive DC voltage to the primary transfer roller 5a, and negatively charges the toner image carried on the photosensitive drum 1a to the intermediate transfer belt 11 passing through the primary transfer portion Ta. Let

  The cleaning device 6a slides the cleaning blade on the photosensitive drum 1a to remove the transfer residual toner attached to the surface of the photosensitive drum 1a that has passed through the primary transfer portion Ta.

  As shown in FIG. 3, the photosensitive drum 1 is composed of four layers. Three layers of an undercoat layer 1β, an optical charge generation layer 1γ, and a charge transport layer 1ω, which suppress the interference of light and improve the adhesion of the upper layer, are sequentially formed on the surface of the aluminum cylinder 1α disposed on the innermost side. It has a layered structure.

<Charging device>
FIG. 4 is an explanatory diagram of a configuration of a charging roller as a charging member, and FIG. 5 is a block diagram of a control configuration of a charging voltage applied to the charging roller.

  The charging roller 2a (2b, 2c) has an outer diameter of 16 mm and a length in the longitudinal direction (axial direction of the photosensitive drum 1a) of 330 mm.

  As shown in FIG. 4, the layer structure of the charging roller 2a is a two-layer structure in which a base layer 2β and a surface layer 2γ are sequentially laminated from the inner side on the outer peripheral surface of a core metal 2α using a stainless steel rod having a diameter of 6 mm. .

The base layer 2β is a conductive layer for obtaining a uniform resistance as a whole of the charging roller 2a, and urethane rubber having a layer thickness of 5.0 mm, in which ion conductive particles are dispersed and the resistance value is adjusted to 1.6 × 10 ⁵ Ω. Consists of.

  The surface layer 2γ is a protective layer that is specially coated with fluorine or the like so that the external additive slipping through the cleaning device (6a: FIG. 2) is difficult to adhere, and has a surface roughness Rz≈10 μm.

  As shown in FIG. 2, the charging roller cleaning member 22a formed by the charging roller 2a and the woolen fabric is rotatably held at both ends of each cored bar by a bearing member 23a with a constant center distance. . The bearing member 23a is urged toward the photosensitive drum 1a by the pressing spring 21a, and presses the charging roller 2a against the surface of the photosensitive drum 1a with a predetermined pressing force.

  The charging roller cleaning member 22a rotates following the rotation of the charging roller 2a, and cleans the toner, external additive, paper powder, and the like that passes through the cleaning device 6a and adheres to the surface of the charging roller 2a by adsorbing or sweeping. To do.

  The charging roller 2a rotates following the photosensitive drum 1a, and a vibration in which a DC voltage having a predetermined polarity and potential and an AC voltage having a predetermined peak-to-peak voltage are superimposed on a cored bar of the charging roller 2a by a power source D3. A voltage is biased. As a result, the surface of the rotating photosensitive drum 1a is contact-charged to a predetermined polarity / potential.

  Specifically, during normal image formation, an oscillating voltage obtained by superimposing a sine AC voltage having a frequency of 2.0 kHz and a peak-to-peak voltage Vpp of 1.5 kV on a DC voltage of −700 V is used. Due to this oscillating voltage, the surface of the photosensitive drum 1a is uniformly charged to the dark portion potential Vd of −700 V, which is the same as the DC voltage applied to the charging roller 2a.

  As shown in FIG. 5, the power source D3 generates a vibration voltage (Vdc + Vac) obtained by superimposing the AC voltage Vac of the frequency f1 output from the AC power source 612 on the DC voltage Vdc output from the DC power source 611. Apply to the core 2α.

  The DC power supply 611 and the AC power supply 612 are controlled by a control circuit 613 which is an example of a control unit. The control circuit 613 has a function of individually turning on / off the DC power supply 611 and the AC power supply 612 and applying a superposed voltage of the DC voltage Vdc and the AC voltage Vac to the charging roller 2a.

  The control circuit 613 sets the value of the peak voltage of the AC voltage Vac applied from the AC power source 612 to the charging roller 2a during non-image formation, and then sets the value of the DC voltage Vdc applied from the DC power source 611 to the charging roller 2a. Has the function to set.

  The AC current measurement circuit 614 measures the AC current (or peak-to-peak voltage) when the AC voltage Vac is applied from the AC power source 612 to the charging roller 2a, and inputs AC current value information (measurement result) to the control circuit 613. To do.

  The environmental sensor 615 measures the temperature and humidity of the place where the image forming apparatus (100: FIG. 1) is installed, and inputs environmental information (measurement result) to the control circuit 613.

  The control circuit 613 obtains alternating current value information and environmental information during non-image formation, and calculates / sets an appropriate peak-to-peak voltage Vpp of the alternating current Vac output from the power source D3 in the charging step of the printing step. Run the decision program.

  The contact charging type charging member includes a roller-shaped charging roller and a blade-shaped charging blade. In particular, the charging roller can stably charge the image carrier over a long period of time. Then, by applying an oscillating voltage in which an alternating voltage is superimposed on a direct current voltage to the charging member and setting the alternating voltage so as to alternately repeat the discharge between the charging member and the image carrier, the surface of the image carrier is Can be charged uniformly.

  The oscillating voltage has the effect of leveling the charged potential on the surface of the image carrier. The waveform of the oscillating voltage is not limited to a sine wave, and may be a rectangular wave, a triangular wave, or a pulse wave. The oscillating voltage is the same as the voltage of a rectangular wave formed by periodically turning the DC voltage OFF / ON or the superimposed voltage of the AC voltage and the DC voltage by periodically changing the value of the DC voltage. Includes output.

  In order to alternately repeat the discharge between the charging member and the image carrier, an AC voltage having a peak-to-peak voltage equal to or higher than the discharge start threshold voltage (charging start voltage) of the image carrier when a DC voltage is applied is used. There is a need.

  However, in the AC charging method in which a high AC voltage Vac is applied to the charging member, there is a problem in that the amount of discharge to the image carrier increases, and deterioration of the image carrier such as roughening or abrasion is promoted. There is also a problem that abnormal images such as image flow occur in a high-temperature and high-humidity (H / H) environment due to discharge products.

  Therefore, in the image forming apparatus 100 of the first embodiment, the discharge for setting an appropriate discharge current by resetting the peak-to-peak voltage Vpp of the AC voltage Vac at the time of non-image formation (pre-rotation and 200-sheet image formation). Perform current control. Thereby, an appropriate constant voltage (inter-peak voltage Vpp) of AC voltage Vac that is neither excessive nor insufficient to charge the image carrier to a predetermined potential is set, and image formation is started.

  By the way, if an unnecessarily high frequency is used when setting the peak-to-peak voltage Vpp of the alternating voltage Vac, a large amount of alternating current is output as compared with the case of a low frequency. This means that power supply costs increase and abnormal images such as image flows are likely to occur in response to the recent trend of increasing image forming speed in image forming apparatuses. In order to solve these problems, it is preferable to minimize the discharge current generated alternately between the charging member and the image carrier using the minimum necessary peak-to-peak voltage Vpp.

  However, the relationship between the peak-to-peak voltage Vpp and the discharge current is not always constant, and changes depending on the film thickness of the photosensitive layer and dielectric layer of the image carrier, the usage history of the charging member, and environmental information. It is difficult to sufficiently suppress the increase or decrease in the amount of discharge current due to variations in charging member manufacturing and resistance values due to contamination, fluctuations in electrostatic capacity of the image carrier due to accumulation of usage history, variations in high-voltage power supply devices, etc. is there.

  For this reason, in use, the image forming apparatus 100 performs high-frequency discharge current amount control at a rate of at least once every several hundred sheets.

  However, in the discharge current amount control, in order to find a target peak-to-peak voltage, a case where a larger amount of current flows than the amount of current necessary for image formation, an increase in capacity of the high-voltage power supply device and an increase in cost are caused.

  According to experimental facts, when an equal AC voltage is applied, the frequency of the AC voltage and the output AC current value (total current) are proportional to each other, but the discharge start voltage and the amount of discharge current depend on the frequency. I made sure I didn't rely on it. That is, it was found that if the discharge current control is performed at a low frequency, the control sequence can be completed with a small total current amount.

  Therefore, in discharging current amount control, when outputting a sampling voltage (AC voltage) as a test bias, a frequency lower than the frequency used during image formation is used. This makes it possible to perform discharge current control without exceeding the total current required for image formation, reducing the capacity of the high-voltage power supply that was necessary only for control, and making it compact and inexpensive. I was able to do it with a high-voltage power supply.

  FIG. 6 is a flowchart of control according to the first embodiment.

  The peak-to-peak voltage Vpp of the AC voltage Vac applied to the charging roller 2a in the charging process of the printing process is 200 at the time of pre-rotation when the image forming apparatus (100: FIG. 1) starts up or the cumulative number of image formations. It will be reset every time. The peak-to-peak voltage Vpp of the AC voltage Vac is set using an AC voltage having a lower frequency than that used during normal image formation during non-image formation.

  The pre-rotation process is a preparatory rotation operation period before image formation from when the print signal is turned on until the actual image formation (printing) process operation is performed. When the print signal is input during the initial rotation operation, the initial rotation process is performed. It is executed following the operation. When the print signal is not input, the drive of the main motor is temporarily stopped after the initial rotation operation is finished, and the rotation drive of the photosensitive drum 1 is stopped. The image forming apparatus 100 is in standby f (standby) until the print signal is input. ) Is kept in a state. When the print signal is input, the print preparation rotation operation is executed.

  In the first embodiment, an appropriate peak-to-peak voltage value (or alternating current value) calculation / determination program of the alternating voltage Vac applied in the charging step of the printing step is executed during the printing preparation rotation operation period.

  As shown in FIG. 6 with reference to FIG. 5, when the cumulative number of image formed sheets from the previous rotation or the previous setting reaches 200 sheets (YES in S11), the peak-to-peak voltage Vpp of the AC voltage Vac is set. It is started (S12 to S19).

  The control circuit 613 acquires environment information from the environment sensor 615, and acquires image formation history information from the internal memory (S12).

  The control circuit 613 switches the frequency f of the AC voltage Vac to a frequency f2 lower than the frequency f1 at the time of image formation (S13), and turns on the AC power supply 612 with the DC power supply 611 turned off (S14).

  The control circuit 613 outputs AC voltages AC1 ′, AC2 ′, and AC3 ′ having three stages of test bias set in the undischarged region from the AC power source 612, and outputs three stages of AC current Iac1 ′ from the AC current measuring circuit 614. , Iac2 ′, Iac3 ′ are acquired (S15). The outputs of the AC voltages AC1 ', AC2', and AC3 'are preferably output in descending order. The AC voltages AC1 ', AC2', and AC3 'use effective values, and are appropriately set according to the environmental classification of temperature and humidity (high temperature and high humidity to low temperature and low humidity).

  The control circuit 613 outputs the three-stage AC voltages AC1, AC2, and AC3 set in the discharge region from the AC power supply 612, and acquires the three-stage AC currents Iac1, Iac2, and Iac3 from the AC current measurement circuit 614 ( S16). The outputs of the AC voltages AC1, AC2, and AC3 are preferably output in descending order. The AC voltages AC1, AC2, and AC3 use effective values, and are appropriately set according to the environmental classification of temperature and humidity (high temperature and high humidity to low temperature and low humidity).

  The control circuit 613 obtains the relational expression of the approximate curve in the undischarged area from the three data of AC voltage-AC current acquired in the undischarged area using the least square method (S17).

  In the equation, Xβ is the average value of the effective values of the alternating voltage applied in the undischarged region, Yβ is the average value of the effective value of the alternating current output in the undischarged region, β1 is the slope of the approximate line, and B1 is , Each represents an intercept of the approximate line.

  The control circuit 613 obtains a relational expression of an approximate curve in the discharge region from the three data of AC voltage-AC current acquired in the discharge region using the least square method (S17).

  In the formula, Xα is an average value of effective values of alternating current applied in the discharge region, Yα, an average value of effective values of alternating current output in the discharge region, α is an inclination of an approximate line, and B1 is an approximate line. Represents each of the sections.

  The control circuit 613 obtains a relational expression of AC voltage-discharge current (VI characteristic) from the approximate curve of the discharge region and the non-discharge region (S17).

  The control circuit 613 substitutes the discharge current amount D set according to the environment and the image formation history into the relational expression of the discharge current to obtain the AC voltage V1 used at the time of image formation (S18).

  The control circuit 613 sets the obtained peak-to-peak voltage Vpp in the AC power source 612 (S19).

  In Example 1, alternating current measurement is performed at three points in both the discharge region and the non-discharge region. Then, from the difference between the relational expression for the discharge area and the relational expression for the undischarged area, a relational expression between the peak voltage Vpp of the AC voltage Vac and the discharge current D in the current environmental conditions and image formation history is derived. The peak-to-peak voltage Vpp is calculated by applying the discharge current amount D to be controlled to this relational expression. In other words, the peak-to-peak voltage Vpp at which the difference between the approximate straight lines of the discharge area and the undischarge area matches the target discharge current amount D determined by the environmental conditions is used during image formation.

  Here, how to select the sampling voltage (AC voltage AC1, AC2, AC3, AC1 ', AC2', AC3 ') will be described.

  The smaller the number of sampling voltages used in the undischarged region, the better. Since it is considered that it already has a zero point of zero voltage and zero current, it is sufficient to take at least one other point, but if there is only one point, there is a possibility that the inclination of the approximate straight line may be greatly mistaken due to an error. At least two points are desirable. The sampling voltage interval is preferably 10% or more of the peak-to-peak voltage Vpp at the time of image formation because the AC power supply 612 has an error range.

  On the other hand, for the discharge region, it is desirable to use at least three sampling voltages in order to perform linear approximation. Similarly, the sampling voltage interval is 10% or more of the peak-to-peak voltage Vpp during image formation. desirable.

  However, if the sampling voltage interval is too large, a wide range of AC voltage-discharge current (VI characteristics) that is inherently non-linear is linearly approximated, and errors in the control result increase. For this reason, it is desirable to narrow the sampling voltage interval as long as the error range of the AC power supply 612 allows.

  The linear approximation used in Example 1 does not correctly describe an AC voltage-discharge current (VI characteristic) that is inherently nonlinear. However, there is no relational expression that correctly describes the discharge area and undischarged illustrator under any environmental conditions and usage conditions, and even if polynomial approximation etc. are performed, accurate results are not necessarily obtained, Linear approximation was adopted.

  As shown in FIG. 5, when an AC voltage is applied between the cylindrical charging roller 2a having a resistance value of several MΩ and the photosensitive drum 1a, the distance between the charging roller 2a and the photosensitive drum 1a is very close. The region acts as a capacitor. For this reason, the capacitive reactance decreases as the frequency of the AC voltage increases, and as a result, the overall resistance between the charging roller 2a and the photosensitive drum 1a decreases and the AC current tends to increase. Here, simply considering a direct current circuit having a resistance R (Ω) and a capacitance C (F), and assuming that the voltage applied to the resistance R and the capacitance C is VR and VC, respectively, the reactance Z (Ω) of the entire circuit. Is represented by the following equation.

  That is, when a low frequency is used, the alternating current decreases because the capacitive reactance is high. However, the amount of charge necessary for converging the unit length of the circumferential length of the photosensitive drum 1a to a constant potential is constant regardless of the frequency, and the effective value of the discharge current amount in the discharge region is also increased or decreased. Regardless, it is constant.

In the first embodiment, the resistance values of the charging roller 2a and the photosensitive drum 1a are 1.5 × 10 ⁶ Ω and 1.0 × 10 ⁶ Ω, respectively. The electrostatic capacity between the charging roller 2a and the photosensitive drum 1a varies depending on the temperature and humidity, but from a normal temperature and low humidity condition of a temperature of 23 degrees C and a humidity of 20% to a high temperature and high humidity condition of a temperature of 30 degrees C and a humidity of 80%. In the range of 1.0 to 2.2 × 10 ⁻¹⁰ F.

  In the first embodiment, the AC power source 612 can output an AC voltage Vac having a frequency of 2.3 kHz to 0.7 kHz. However, from the viewpoint of preventing moire images, it is not preferable to use a very low frequency during normal image formation. The moire image also depends on the basic pattern of the image (hereinafter referred to as a screen image) used at the time of image formation. In the first embodiment, an alternating voltage having a frequency of 2.3 kHz is used based on the moire formula at the time of image formation. Vac is used.

  Here, a moire image generation mechanism that occurs due to interference between the screen image and the frequency of the AC voltage applied to the charging roller 2a will be described.

  Assuming that the frequency f of the AC voltage is f (Hz) and the peripheral speed Vp (mm / sec) of the photosensitive drum 1a, charging in a constant cycle represented by ν1 = (Vp / f) mm in the peripheral length direction of the photosensitive drum 1a. Unevenness occurs. The charging unevenness is unevenness of the dark portion potential Vd, and when the uneven dark portion potential Vd surface is uniformly exposed, unevenness of the bright portion potential VL is formed and unevenness occurs in the development contrast voltage Vcont. Accordingly, the charging unevenness due to the frequency f appears as unevenness of the development contrast Vcont, that is, image density unevenness during image formation.

  However, in general, the frequency f is set to a sufficiently high frequency of several kHz or higher compared to the peripheral speed of the photosensitive drum 1a. In Example 1, since the frequency f = 2.3 kHz with respect to the peripheral speed Vp = 300 mm / sec, the charging uneven period ν1 = 0.14 mm. Generally, what is captured by human visual characteristics is a pitch having a length of sub-millimeter order or more, so density unevenness with a period ν1 = 0.14 mm is not generally at a level where human visual characteristics can be captured.

  On the other hand, the spatial frequency determined by the drawing interval of the screen image, which is a basic pattern for image formation, is ν2. The spatial frequency ν2 is also designed on the order of several μm to several tens of μm, which is a level that generally does not capture human visual characteristics.

  The density unevenness expressed by beat F = | (1 / ν1) − (1 / ν2) | formed by interference between the density unevenness of the period ν1 and the spatial frequency ν2 does not necessarily depend on human visual characteristics. It is not always within the range that cannot be captured. Such density unevenness caused by the beat F is an image called pitch unevenness or moire caused by interference with the frequency f of the AC voltage Vac.

  In Example 1, when the frequency f of the AC voltage Vac is 2.3 kHz and a screen image of 1 line, 5 spaces, and 1200 dpi is taken as a reference, the moire interval determined by the beat F is 0.443 mm. This moire interval is 0.5 mm or less, which is a pitch period that is generally felt uncomfortable to the human eye.

<Effect of Example 1>
FIG. 7 is an explanatory diagram of the relationship between the peak-to-peak voltage Vpp and the alternating current in Example 1, FIG. 8 is an explanatory diagram of the relationship between the peak-to-peak voltage and the discharge current, and FIG. 9 is an explanatory diagram of the influence of excessive discharge current.

  As shown in FIG. 7, the alternating current flowing to the photosensitive drum 1a through the charging roller 2a changes according to the selected frequency f. The alternating current is the sum of the alternating current flowing through the reactance of the charging roller 2a and the photosensitive drum 1a depending on the frequency f and the discharge current generated at the opposite portion of the charging roller 2a and the photosensitive drum 1a almost independently of the frequency f. It is.

  A discharge current is required to make the DC voltage Vdc applied to the charging roller 2a equal to the charging potential of the photosensitive drum 1a.

  In Example 1, when the peak-to-peak voltage Vpp was 1800 V under an environmental condition of 1 atm 23 ° C. and 20% relative humidity, the alternating current was 2.805 mA with respect to the normal frequency 2.3 kHz. In contrast, at a sampling voltage frequency of 1.43 kHz, an alternating current of 1.757 mA is sufficient when the peak-to-peak voltage Vpp = 1800V.

  As a result, the AC current in the control for setting the peak-to-peak voltage Vpp using the sampling voltage was reduced by about 1.05 mA, the reduction ratio was about 60%, and the ratio was almost the same as the frequency ratio. Since the reduction ratio of the alternating current is the same even under conditions of high temperature and high humidity up to 30 degrees C and relative humidity of 80%, the power consumption can be reduced at the same ratio as the frequency ratio of the alternating voltage.

  However, the relationship between the peak-to-peak voltage and the discharge current (VD property) derived from the relationship between the peak-to-peak voltage and the alternating current (VI characteristic) in FIG. 7 is as shown in FIG.

  As shown in FIG. 8, regardless of whether the sampling voltage frequency f is 2.3 kHz or 1.43 kHz, if the same peak-to-peak voltage Vpp is used, the discharge currents generated at the opposing portions of the charging roller 2a and the photosensitive drum 1a are substantially equal. It is.

  As shown in FIG. 7, the relationship between the peak-to-peak voltage Vpp and the alternating current (VI characteristic curve) is not necessarily linear, particularly in the discharge region. For this reason, in order to control the discharge current with high accuracy in the control of the first embodiment in which the VI characteristic curve of the undischarged region is linearly approximated, a plurality of sampling voltages in the discharge region particularly include the final set value within the range. desirable.

  For this reason, the sampling voltage used during control has a sampling peak-to-peak voltage Vpp higher than the peak-to-peak voltage Vpp of the final set value used during image formation. However, in the first embodiment, since the high sampling peak voltage Vpp is output at a low frequency, it is possible to reduce the alternating current and reduce the capacity of the AC power supply 612. That is, a compact and inexpensive power supply configuration is possible.

  As shown in FIG. 9, a method of lowering the frequency at the time of setting control of the peak-to-peak voltage Vpp was compared with a method of using the same frequency as that at the time of image formation as before. The amount of discharge current determined by the control for setting the peak-to-peak voltage Vpp of the AC voltage Vac has a strong correlation with the shaving of the photosensitive drum 1, image flow, and charging uniformity. Further, the amount of alternating current detected at this time also has a strong correlation with the deterioration of the surface of the photosensitive drum 1, that is, the potential drop.

  When the deterioration of the photosensitive drum 1 (potential drop due to dark decay) is compared in a low-humidity environment where unevenness in current tends to occur, the potential drop of about 10 V is achieved in the method of decreasing the frequency of Example 1 at the time of the cumulative number of 40,000 sheets. Was able to be suppressed. As described above, the life of the photosensitive drum 1 can be extended by using the control of the first embodiment.

  A sampling voltage having a frequency lower than that used during image formation is used during discharge current control for determining the peak-to-peak voltage when constant voltage control is performed on the AC voltage used during image formation. Thereby, the peak-to-peak voltage of the AC voltage used at the time of image formation can be determined without exceeding the amount of current required at the time of image formation when performing the discharge current control. It is possible to minimize drum scraping and toner fusion in the discharge current control that is frequently performed, and to reduce the capacity of the high-voltage power source that is necessary only for the control.

  This example has the same configuration as that of the first embodiment except for the configuration described later, and the detailed description is omitted by adding the same reference numerals to such a similar configuration.

  In the first embodiment, in the control for setting the peak-to-peak voltage Vpp of the AC voltage Vac, the three sampling voltages set for the discharge region and the non-discharge region are changed and set according to the temperature / humidity environmental information. .

  In the second embodiment, in the control for setting the peak-to-peak voltage Vpp of the AC voltage Vac, three sampling voltages set in the discharge area and the non-discharge area are used as the usage history (cumulative image formation) of the charging roller 2a and the photosensitive drum 1a. The number was changed according to the number of images.

  In the control for setting the peak-to-peak voltage Vpp of the AC voltage Vac, the AC voltage used for adjusting the discharge current is set according to the temperature and humidity of the surrounding environment, the usage history of the charging roller and the photosensitive drum, etc. Can be improved.

Specifically, it is set based on the amount of water contained in the ambient atmosphere from the temperature and humidity. For example, in a low-humidity environment where the amount of water is 2.0 [g / m ³ ] or less, the sampling voltage in the discharge area is Vpp1 = 1.78 kV, Vpp2 = 1.70 kV, Vpp3 = 1 in the initial stage of use of the photosensitive drum. Set to 64 kV. On the other hand, in a high humidity environment where the water content is 18.0 [g / m ³ ] or more, the sampling voltage in the discharge region is set to Vpp1 = 1.42 kV, Vpp2 = 1.36 kV, and Vpp3 = 1.31 kV.

  As for the sampling voltage in the non-discharge region, a voltage of about 1.0 kV or less is used regardless of the amount of moisture.

  When the water content is between the above values, a value obtained by supplementing the sampling voltage approximately linearly according to the water content is used.

  The sampling voltage is changed according to the usage history of the photosensitive drum. The value determined by the humidity environment described above is defined as a REF value R, and a coefficient Pd determined from the photosensitive drum usage history Nd (unit: k sheets). At this time, the coefficient Pd is rounded down every 20k of the usage history Nd. For example, Pd = 0 is calculated when Nd = 19k, and Pd = 20 when Nd = 21k.

The sampling voltage V to be used is expressed by the following equation using a coefficient D1 determined by the life of the photosensitive drum. In this embodiment, D1 = 150.
V = R × (1 + Pd / D1)

  FIG. 10 is a flowchart of control according to the third embodiment. In the third embodiment, the image forming speed (process speed), which is the rotational speed of the image forming process equipment such as the photosensitive drum 1a, is set in three stages of a reference speed, a 1/2 speed, and a 1/3 speed according to the type of recording material. Can be set in stages.

  In this example, the frequency of the charging bias applied from the AC power source 612 to the charging roller 2a during image formation is switched according to the switching of the image forming speed. Specifically, when the image forming speed is switched from the reference speed to 1/2 speed or 1/3 speed, the charging bias frequency applied to the charging roller 2a is changed from the reference frequency = 2.3 kHz to 1/2 frequency. = 1.15 kHz and 1/3 frequency = 770 kHz, respectively.

  Note that, regardless of whether the image forming speed is the reference speed, 1/2 speed, or 1/3 speed, the setting value of the peak-to-peak voltage of the charging bias applied at the time of image formation is the same.

  That is, in this example, control for setting the peak-to-peak voltage of the charging bias to be applied at the time of image formation is performed.

  As shown in FIG. 1, the image forming apparatus 100 has an image at 1/2 speed and 1/3 speed so that the fixing device 9 does not run out of heating capacity when a special recording material P such as cardboard is used. Run formation mode. Thereby, the generation condition of the moire image (particularly, the interference moire image with the screen image) in the image forming mode of the 1/2 speed and the 1/3 speed is aligned with the reference speed mode.

  The frequency of the charging bias used in the AC charging method is preferably changed in proportion to the image forming speed. This is because, when the frequency of the alternating voltage of the charging voltage is insufficient with respect to the process speed, there is a possibility that the charging potential unevenness due to the frequency, and consequently the density unevenness on the image may occur. When the charging potential unevenness due to the frequency exceeds a certain value, it is recognized as a so-called moiré image that is unsightly to the human eye, especially when an image is output as a uniform screen image such as a halftone. End up.

  Therefore, in Example 3, the frequency of the sampling voltage when performing the control for setting the peak-to-peak voltage Vpp of the AC voltage Vac is set to 1/3 frequency = 770 kHz, which is the lowest frequency among the frequencies used during image formation. Set. In other words, the frequency of the sampling voltage when performing the control for setting the peak-to-peak voltage Vpp of the AC voltage Vac is set to the frequency of the charging bias used in the image forming mode with the slowest image forming speed.

  Therefore, even when the reference speed or the 1/2 speed is selected as the image forming speed, the frequency of the sampling voltage when performing the control for setting the peak-to-peak voltage Vpp of the AC voltage Vac is 1/3 frequency. = 770 kHz.

  Next, a sequence for executing control for setting the peak-to-peak voltage Vpp of the AC voltage Vac will be described using the flowchart of FIG.

  As shown in FIG. 10, the image forming apparatus has different image forming speeds depending on the type of recording material used. Specifically, if the weight per basis weight of the recording material is less than 105 g, the reference speed (YES in S21), 1/2 speed (YES in S25) if 105 g or more and less than 230 g, and 1/3 if 230 g or more. The speed is determined (NO in S25) and image formation is performed (S31).

  The determination of the image forming speed may be either automatic detection or manual setting in the service mode. When the image forming operation is performed as described above, as described in the first embodiment, the discharge current that sets the peak-to-peak voltage Vpp of the AC voltage Vac at the time of the pre-rotation or every time the cumulative number of image formation reaches 200 sheets. Control enters.

  At this time, for each image forming speed, the charging bias frequency used for the discharge current control is determined as follows.

(1) When the image forming speed is the reference speed (YES in S21)
When the timing of the discharge current control comes (YES in S22), the discharge current control is executed using the charging frequency used at the time of 1/3 speed image formation (S23). After the control is completed, the charging frequency is returned to the reference speed (S24), and the image forming operation is continued (S31).

(2) When the image forming speed is 1/2 speed (YES in S25)
When the timing of the discharge current control comes (YES in S26), the discharge current control is executed using the charging frequency used at the time of image formation at 1/3 speed (S27). After the end of the control, the charging frequency is returned to the half speed (S28), and the image forming operation is continued (S31).

(3) When the image forming speed is 1/3 (NO in S25)
When the timing of the discharge current control comes (YES in S29), the discharge current control is executed using the charging frequency used at the time of image formation at 1/3 speed (S30). After completion of the control, the image forming operation is continued with the same charging frequency (S31).

  As a result, it is possible to reduce channels in the frequency band that is prepared and used in the AC power supply 612. Since the frequency used for the discharge current control is selected from them, it is possible to realize a configuration in which the frequency use channel is reduced and the frequency is switched to set the peak-to-peak voltage Vpp at low cost.

  FIG. 11 is a flowchart of control according to the fourth embodiment. In the fourth embodiment, the discharge current control is always performed at the fastest peripheral speed (reference speed) in the image forming mode.

  Since the fourth embodiment has the same configuration as the first to third embodiments except for the following points, the description overlapping with these is omitted. Specifically, also in this example, as in the third embodiment, the image forming speed that is the rotational speed of the photosensitive drum 1a and the like can be set stepwise in three stages. However, in this example, unlike Example 3, the rotational speed of the photosensitive drum and the charging roller when performing control for setting the peak-to-peak voltage Vpp of the AC voltage Vac is set to the reference speed.

  That is, even when the 1/2 speed or 1/3 speed is selected as the image forming mode, the control for setting the peak-to-peak voltage Vpp of the AC voltage Vac is performed by adjusting the rotational speed of the photosensitive drum or the charging roller. It is characterized by executing after switching to the reference speed. In other words, the rotational speed of the photosensitive drum and the charging roller when detecting the sampling voltage is set to the reference speed regardless of (regardless of) the selected image forming mode.

  This is due to the following reason. When the discharge current control is performed at the reference speed, sampling is performed in a short time as much as possible in the circumferential direction over a distance of a little over one turn with respect to the photosensitive drum and the charging roller. On the other hand, if the control is performed at the same sampling interval as the reference speed at 1/2 speed and 1/3 speed, the control position is unevenly distributed with respect to the photosensitive drum and the charging roller, and the discharge current amount is excessive or insufficient. This is because there is a risk that the robustness against the resulting image defect may be lost. In addition, if the photosensitive drum or the charging roller is controlled at a speed of 1/2 or more at a distance of more than one turn, the control time is extended and the productivity is lowered.

  In this example, when the control for setting the peak-to-peak voltage Vpp of the AC voltage Vac is performed, the frequency of the sampling voltage is not the reference frequency = 2.3 kHz but the 1/2 frequency = 1.15 kHz or 1 / Three frequencies = 770 kHz are used. In this example, an example using 1/2 frequency = 1.15 kHz will be described in detail.

  Next, a sequence for executing control for setting the peak-to-peak voltage Vpp of the AC voltage Vac will be described using the flowchart of FIG.

  As shown in FIG. 11, the image forming apparatus has different image forming speeds depending on the type of recording material to be used, and the specific judgment is as described in the third embodiment.

  When the image forming operation is performed as described above, the discharge current control is performed at the time of the previous rotation or every time the cumulative number of image formation reaches 200 sheets. At this time, control is performed as follows for each image forming speed.

(1) When the image forming speed is the reference speed (YES in S41)
The discharge current control (YES in S42) is performed by using the charging frequency used at the time of image formation at 1/2 speed at the same image forming speed (S43). After the control is completed, the charging frequency is returned to the reference speed (S44), and the image forming operation is continued (S53).

(2) When the image forming speed is 1/2 speed (YES in S45)
The discharge current control (YES in S46) is performed using the charging frequency that is used at the time of image formation at 1/2 speed with the image forming speed once returned to the reference speed (S47). After the control is completed, the image forming speed is returned to 1/2 speed (S48), and the image forming operation is continued (S53).

(3) When the image forming speed is 1/3 speed (NO in S45)
The discharge current control (YES in S49) is performed by using the charging frequency used at the time of image formation at 1/2 speed with the image forming speed once returned to the reference speed (S50). After completion of the control, the image forming speed (S51) and the charging frequency (S52) are returned to the specifications at the 1/3 speed, and the image forming operation is continued (S53).

1 is an explanatory diagram of a configuration of an image forming apparatus according to Embodiment 1. FIG. It is explanatory drawing of a structure of an image formation part. It is explanatory drawing of the layer structure of a photosensitive drum. It is explanatory drawing of a structure of the charging roller as a charging member. It is a block diagram of the control structure of the charging voltage applied to a charging roller. 3 is a flowchart of control according to the first embodiment. It is explanatory drawing of the relationship between the peak voltage Vpp and alternating current in Example 1. FIG. It is explanatory drawing of the relationship between the peak-to-peak voltage and discharge current. It is explanatory drawing of the influence of an excessive discharge current. 10 is a flowchart of control according to the third embodiment. 10 is a flowchart of control according to the fourth embodiment.

Explanation of symbols

1a Image carrier (photosensitive drum)
2a Charging member (charging roller)
3a Exposure device 4a Developing device 5a Primary transfer roller 6a Cleaning device 9 Fixing device 11 Intermediate transfer belt 100 Image forming device 611 DC power source 612 AC power source 613 Control means (control circuit)
614 Detection means (AC current measurement circuit)
D3 Power supply Pa, Pb, Pc, Pd Image forming unit

Claims (6)

An image bearing member; a charging member that contacts and charges the image bearing member; a bias applying unit that applies a charging bias in which a DC voltage and an AC voltage are superimposed on the charging member; and charging by the bias applying unit. In an image forming apparatus comprising: a detection unit that detects an alternating current when a test bias is applied to a member; and a control unit that controls a peak-to-peak voltage of the charging bias based on an output of the detection unit.
An image forming apparatus comprising setting means for setting a test bias frequency to a frequency lower than that during image formation.   2. The image forming apparatus according to claim 1, wherein the control unit controls the peak-to-peak voltage of the charging bias based on the detection result of the alternating current when a plurality of test biases having different peak-to-peak voltages are applied. An image bearing member; a charging member that contacts and charges the image bearing member; a bias applying unit that applies a charging bias in which a DC voltage and an AC voltage are superimposed on the charging member; and charging by the bias applying unit. In an image forming apparatus comprising: a detection unit that detects an alternating current when a test bias is applied to a member; and a control unit that controls a peak-to-peak voltage of the charging bias based on an output of the detection unit.
Switching means for switching the charging bias frequency in accordance with switching of image forming modes having different image forming speeds, and setting means for setting the test bias frequency to the charging bias frequency in the image forming mode having a low image forming speed. An image forming apparatus comprising:   4. The image forming apparatus according to claim 3, wherein the control unit controls the peak-to-peak voltage of the charging bias based on the detection result of the alternating current when a plurality of test biases having different peak-to-peak voltages are applied.   5. The image forming apparatus according to claim 3, wherein the setting means sets the frequency of the test bias to the frequency of the charging bias in the image forming mode in which the image forming speed is the slowest.   6. The image according to claim 3, wherein a peripheral speed of the image carrier when the alternating current is detected by the detecting means is set to a peripheral speed in an image forming mode in which the image forming speed is the fastest. Forming equipment.

Images