Image forming apparatus and method of controlling the same

An image forming apparatus includes a photoconductor, a charging member, a first power supply circuit which supplies electric power to the charging member, a detector configured to detect a current value of an alternating current which flows to the charging member, and a controller configured to control an operation of the first power supply circuit. The controller is configured to lower a frequency of alternating-current power supplied to the charging member by the first power supply circuit when the current value detected by the detector in application of a voltage of a prescribed value to the charging member by the first power supply circuit is equal to or smaller than a predetermined value.

Japanese Patent Application No. 2016-244274 filed on Dec. 16, 2016 including description, claims, drawings, and abstract the entire disclosure is incorporated herein by reference in its entirety.

BACKGROUND

Technological Field

The present disclosure relates to an image forming apparatus and a method of controlling the same, and particularly to an image forming apparatus using alternating-current power and a method of controlling the same.

Description of the Related Art

An image forming apparatus which forms an image with electrophotography or electrostatic recording has conventionally been used. In such an image forming apparatus, recently, adoption of contact charging for uniformly charging a surface of a photoconductor by arranging a roller type charging member in contact with or in proximity to the surface of the photoconductor and applying an oscillating voltage as a direct-current voltage and an alternating-current voltage being superimposed on each other to the charging member has become mainstream from a point of view of a low-voltage process, a small amount of ozone generation, and low costs.

In contact charging, a peak-to-peak voltage Vpp of a charging voltage is determined, for example, as follows. A first approximation function and a second approximation function between a peak-to-peak value of a voltage and an alternating current value are derived, and a differential function indicating a differential value between these two functions is derived. Such a peak-to-peak voltage value that a rate of increase in current differential value per unit peak-to-peak voltage is a prescribed value K is specified as a peak-to-peak voltage Vpp used in control.

Japanese Laid-Open Patent Publication No. 2014-38259 discloses a technique to change peak-to-peak voltage Vpp in accordance with an environment where an image forming apparatus is located. More specifically, the apparatus increases a value for peak-to-peak voltage Vpp for compensating for defective charging of a charging member due to lowering in temperature when a temperature at a location where the apparatus is located lowers.

With increase in peak-to-peak voltage Vpp, however, abrasion of a film of a photoconductor tends to proceed in an image forming apparatus. Therefore, running costs of the image forming apparatus may increase. Furthermore, when significant increase in peak-to-peak voltage Vpp is allowed in the image forming apparatus, a circuit which is capable of providing a high output should be adopted as a circuit to supply electric power to a charging member. Therefore, cost for manufacturing the image forming apparatus may increase. Reduction in cost for the image forming apparatus is demanded.

SUMMARY

To achieve at least one of the above-mentioned objects, according to an aspect of the present disclosure, an image forming apparatus reflecting one aspect of the present disclosure is provided. The image forming apparatus includes a photoconductor, a charging member provided in proximity to the photoconductor, a first power supply circuit configured to supply alternating-current power to the charging member, a detector configured to detect a current value of an alternating current which flows to the charging member, and a controller configured to control an operation of the first power supply circuit. The controller is configured to lower a frequency of alternating-current power supplied to the charging member by the first power supply circuit when the current value detected by the detector in application of a voltage of a prescribed value to the charging member by the first power supply circuit is equal to or smaller than a predetermined value.

To achieve another of the above-mentioned objects, according to an aspect of the present disclosure, a method of controlling an image forming apparatus reflecting one aspect of the present disclosure is provided, the image forming apparatus including a photoconductor and a charging member provided in proximity to the photoconductor and supplied with electric power containing an alternating-current component. The method includes obtaining a value of a current which flows to the charging member when a voltage of a prescribed value is applied to the charging member and lowering a frequency of alternating-current power supplied to the charging member when the obtained value of the current is equal to or smaller than a predetermined value.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1is a diagram for illustrating a technical concept realized by an image forming apparatus according to the present disclosure. In the image forming apparatus according to the present disclosure, a frequency of alternating-current power supplied to a charging roller (one example of a charging member) is lowered in response to the fact that a value of a current which flows to the charging roller in application of a voltage of a prescribed value (for example, 2000 V) to the charging roller is equal to or smaller than a predetermined value. The charging member in the present disclosure may be in a shape other than a cylindrical shape, such as a prismatic shape.

In the graph shown inFIG. 1, the ordinate represents a frequency of alternating-current power supplied to the charging roller of the image forming apparatus (hereinafter also referred to as a “charging frequency”) and the abscissa represents a value of a current which flows to the charging roller in application of a voltage of a prescribed value to the charging roller (hereinafter also referred to as a “criterion current value”). In the example shown inFIG. 1, variation in charging frequency is shown with a “frequency FA” and a “frequency FB.” A first frequency shown as “frequency FA” is higher than a second frequency shown as “frequency FB.”

In the example inFIG. 1, the charging frequency is controlled to “frequency FA” until the criterion current value attains to a predetermined value (“900 μA” in the example inFIG. 1). When the criterion current value is lowered to the predetermined value due to an internal temperature of an image forming apparatus200, the charging frequency is controlled to “frequency FB.” Thereafter, when the criterion current value increases, the charging frequency is returned to “frequency FA.” More detailed description will be given below.

The graph inFIG. 1shows four states (a state (1) to a state (4)) of the image forming apparatus.

As shown as the state (1) in the example inFIG. 1, when the criterion current value is greater than the predetermined value (for example, “900 μA”), “frequency FA” is set as the charging frequency.

Thereafter, as shown as the state (2), when an instruction to print is received while the criterion current value is equal to or smaller than the predetermined value (900 μA), the image forming apparatus sets “frequency FB” as the charging frequency and performs printing.

Thereafter, as shown as the state (3), when the criterion current value increases to a prescribed value (for example, “1100 μA”) or greater, the image forming apparatus returns the charging frequency to “frequency FA” (the state (4)).

In the example inFIG. 1, in the image forming apparatus, the criterion current value (900 μA) defining a condition for change in charging frequency from “frequency FA” to “frequency FB” and the criterion current value (1100 μA) defining a condition for change in charging frequency from “frequency FB” to “frequency FA” are different from each other. The two criterion current values may be set to the same value.

[Configuration of Image Forming Apparatus]

FIG. 2is a diagram illustrating a configuration example of image forming apparatus200according to one embodiment. In one embodiment, image forming apparatus200is an electrophotographic image forming apparatus such as a laser printer or a light emitting diode (LED) printer. As shown inFIG. 2, image forming apparatus200includes an intermediate transfer roller1as a belt member substantially in a central portion of the inside. Four imaging units2Y,2M,2C, and2K corresponding to colors of yellow (Y), magenta (M), cyan (C), and black (K), respectively, are arranged as being aligned along intermediate transfer roller1under a lower horizontal portion of intermediate transfer roller1. Imaging units2Y,2M,2C, and2K have photoconductors3Y,3M,3C, and3K configured to be able to carry toner images, respectively.

Charging rollers4Y,4M,4C, and4K for charging corresponding photoconductors, print head portions5Y,5M,5C, and5K, development rollers6Y,6M,6C, and6K, and primary transfer rollers7Y,7M,7C, and7K opposed to photoconductors3Y,3M,3C, and3K with intermediate transfer roller1being interposed are arranged sequentially around photoconductors3Y,3M,3C, and3K along a direction of rotation thereof, respectively. In the present disclosure, the development roller represents one example of a development member. The development member may be in a shape other than a columnar shape, such as a prismatic shape.

A secondary transfer roller9is brought in pressure contact with a portion of intermediate transfer roller1supported by an intermediate transfer belt drive roller8and secondary transfer is performed in that region. A fixing and heating portion20including a fixing roller10and a pressurization roller11is arranged at a downstream position in a transportation path R1subsequently to a secondary transfer region.

A paper feed cassette30is arranged in a lower portion of image forming apparatus200. Paper feed cassette30is attachable to and removable from a main body of image forming apparatus200. Paper P loaded and accommodated in paper feed cassette30is sent one by one from a sheet of paper located at the top to transportation path R1as a paper feed roller31rotates.

An operation panel80is arranged in an upper portion of image forming apparatus200. Operation panel80is constituted of a touch panel in which a touch sensor and a display are layered on each other and a physical button by way of example.

In one aspect, intermediate transfer roller1, charging rollers4Y,4M,4C, and4K, primary transfer rollers7Y,7M,7C, and7K, and secondary transfer roller9may function as an ion conductive member. By way of example, such a conductive member may contain ion conductive rubber in which hydrin rubber, acrylonitrile butadiene rubber, or epichlorohydrin rubber is blended. Each conductive member may contain an appropriate ion conductive material depending on a required characteristic.

Though image forming apparatus200adopts a tandem intermediate transfer scheme inFIG. 2, limitation thereto is not intended. Specifically, image forming apparatus200may be an image forming apparatus adopting a cycle scheme or an image forming apparatus adopting a direct transfer scheme in which toner is directly transferred from a development apparatus to a printing medium.

Image forming apparatus200includes a control box700containing a control unit (a “controller70” which will be described later with reference toFIG. 3) which controls an operation of image forming apparatus200. A temperature sensor51is attached to control box700. A position where temperature sensor51is located is not limited to the position shown inFIG. 2so long as temperature sensor51can measure an internal temperature of image forming apparatus200. The internal temperature refers, for example, to a temperature of the inside of a cover which covers an outer shell of image forming apparatus200.

When an image signal is input to the control unit of image forming apparatus200from an external apparatus (such as a personal computer), the control unit generates digital image signals obtained by conversion of this image signal into signals of colors of yellow, cyan, magenta, and black and has print head portions5Y,5M,5C, and5K of respective imaging units2Y,2M,2C, and2K emit light based on the input digital signals for exposure. Electrostatic latent images formed on respective photoconductors3Y,3M,3C, and3K are thus developed by respective development rollers6Y,6M,6C, and6K to become toner images of respective colors. The toner images of these colors are primarily transferred onto intermediate transfer roller1which moves in a direction shown with an arrow A inFIG. 2as being successively superimposed on one another as a result of functions of primary transfer rollers7Y,7M,7C, and7K. The toner image thus formed on intermediate transfer roller1is secondarily collectively transferred onto paper P as a result of a function of secondary transfer roller9.

The toner image secondarily transferred to paper P reaches fixing and heating portion20. The toner image is fixed to paper P as a result of functions of heated fixing roller10and pressurization roller11. Paper P to which the toner image has been fixed is ejected to a paper ejection tray60through a paper ejection roller50.

[Configuration in Vicinity of Charging Roller]

FIG. 3is a diagram schematically showing a configuration in the vicinity of charging rollers4Y,4M,4C, and4K inFIG. 2.FIG. 3shows imaging units2Y,2M,2C, and2K as “imaging unit2” for illustrating a configuration common to four imaging units2Y,2M,2C, and2K. Photoconductors3Y,3M,3C, and3K are shown as “photoconductor3” for illustrating a configuration common to four photoconductors3Y,3M,3C, and3K. Charging rollers4Y,4M,4C, and4K are shown as “charging roller4” for illustrating a configuration common to four charging rollers4Y,4M,4C, and4K. Development rollers6Y,6M,6C, and6K are shown as “development roller6” for illustrating a configuration common to four development rollers6Y,6M,6C, and6K. Primary transfer rollers7Y,7M,7C, and7K are shown as “primary transfer roller7” for illustrating a configuration common to four primary transfer rollers7Y,7M,7C, and7K.

Referring toFIG. 3, in image forming apparatus200, a charging voltage supply portion44supplies electric power to charging roller4. Supplied electric power contains an alternating-current component. Supplied electric power may contain a direct-current component. Charging voltage supply portion44is implemented, for example, by a power supply circuit. Image forming apparatus200includes a current detector43for detecting a current value of an alternating-current component of electric power supplied to charging roller4.

Image forming apparatus200includes controller70. Controller70is accommodated, for example, in control box700(FIG. 2). Controller70includes a central processing unit (CPU)511representing one example of a processor which executes a program and a memory512which stores data such as a program. Controller70obtains a detection output from temperature sensor51. Controller70controls an operation of charging voltage supply portion44.

Image forming apparatus200includes a development voltage supply portion54which supplies electric power to development roller6and a transfer voltage supply portion55which supplies electric power to primary transfer roller7. Electric power supplied to development roller6contains an alternating-current component. A frequency of electric power supplied to development roller6may be referred to as a “development frequency” in the description below. Each of development voltage supply portion54and transfer voltage supply portion55is implemented, for example, by a power supply circuit. Controller70controls an operation of development voltage supply portion54and transfer voltage supply portion55.

In imaging unit2, charging roller4abuts on photoconductor3and charging voltage supply portion44applies a voltage required for formation of an image to the charging roller. Charging voltage supply portion44supplies, for example, a voltage as a direct-current (DC) voltage and an alternating-current (AC) voltage being superimposed on each other to charging roller4. As the voltage is applied from charging voltage supply portion44to charging roller4, a potential difference is produced between a surface of charging roller4and photoconductor3.

When a potential difference between the surface of charging roller4and photoconductor3is equal to or greater than a predetermined potential difference determined under the Paschen's law, discharging occurs and hence photoconductor3is charged. As charges move between charging roller4and charged photoconductor3, a current flows. Current detector43detects a value of a current which flows between charging roller4and photoconductor3. A value of the current which flows between charging roller4and photoconductor3in application of a voltage of a predetermined value to charging roller4may vary depending on image forming apparatus200(for example, a temperature, a humidity, or a barometric pressure) and a thickness of a film of photoconductor3.

[Control (1) Based on Criterion Current Value]

In image forming apparatus200, operation setting is made in stabilization control. The operation setting includes setting of peak-to-peak voltage Vpp of a voltage (charging voltage) applied to each of charging rollers4Y,4M,4C, and4K. Peak-to-peak voltage Vpp of the charging voltage is determined, for example, as follows. A first approximation function and a second approximation function between a peak-to-peak value of the voltage and an AC current value are derived, and then a differential function indicating a differential value between these two functions is derived. In deriving the first and/or second approximation function(s), a plurality of predetermined peak-to-peak voltage values (detection voltage values) are used for measurement of an AC current value.

Image forming apparatus200lowers a charging frequency when an AC current value obtained by using one voltage value (for example, 2000 V) of the detection voltage values is equal to or smaller than a predetermined value (for example, “900 μA” inFIG. 1). Such control is carried out, for example, by execution of a given program by CPU511(FIG. 3).FIG. 4is a flowchart of processing performed by CPU511. In the description below, a detection voltage value used for detection of a criterion AC current value among the detection voltage values is referred to as a “specific voltage value.”

As shown inFIG. 4, whether or not timing of stabilization control in image forming apparatus200has come is determined in step S110. The process remains in step S110until CPU511determines that the timing has come. When CPU511determines that the timing has come, it carries out stabilization control including setting of a charging voltage, and thereafter the process proceeds to step S120.

CPU511determines in step S120whether or not a criterion current value Iac is equal to or smaller than a predetermined value (for example, “900 μA”). Criterion current value Iac is a value of a current which flows to charging roller4in application of the specific voltage value to charging roller4and detected by current detector43(FIG. 3). When CPU511determines that criterion current value Iac has exceeded 900 μA, the process returns to step S110, and when the CPU determines that criterion current value Iac is equal to or smaller than 900 μA, the process proceeds to step S130.

In step S130, CPU511instructs charging voltage supply portion44to lower the charging frequency. The frequency of an AC component of electric power supplied from charging voltage supply portion44to charging rollers4Y,4M,4C, and4K is thus lowered from frequency FA to frequency FB. Thereafter, the process proceeds to step S140.

In step S140, CPU511instructs development voltage supply portion54to lower a development frequency. The frequency of an AC component of electric power supplied from development voltage supply portion54to development rollers6Y,6M,6C, and6K is thus lowered. Thereafter, the process proceeds to step S150.

The development frequency may correspond to the charging frequency. For example, when the charging frequency is set to frequency FA, the development frequency is set to a frequency FX, and when the charging frequency is set to frequency FB, the development frequency is set to a frequency FY. Frequency FX has a value which is an integral multiple of frequency FA. Frequency FY has a value which is an integral multiple of frequency FB. As the development frequency is changed with change in charging frequency in image forming apparatus200, such relation that the development frequency is an integral multiple of the charging frequency is maintained. To whichever of frequency FA and frequency FB the charging frequency may be set, production of interference fringes in electric power supplied to each of charging rollers4Y,4M,4C, and4K and electric power supplied to each of development rollers6Y,6M,6C, and6K is more reliably avoided and hence generation of noise in a developed image can more reliably be avoided. There may also be a case that only the charging frequency is changed with change in criterion current value Iac and the development frequency is not changed (step S140and step S190which will be described later are not performed).

In step S150, CPU511changes a set value for a system speed. The system speed refers, for example, to a speed of transportation of paper P in image forming apparatus200. In step S150, for example, the system speed is lowered. The speed of transportation of paper P is thus lowered. Thereafter, the process proceeds to step S160.

CPU511determines in step S160whether or not timing of new stabilization control has come. The process remains in step S160until CPU511determines that the timing has come, and when the CPU determines that the timing has come, the process proceeds to step S170.

CPU511determines in step S170whether or not criterion current value Iac at that time point is equal to or greater than a value (for example, “1100 μA”) equal to or greater than a value defined as a threshold value in step S120. When CPU511determines that criterion current value Iac is smaller than 1100 μA, the process returns to step S160, and when the CPU determines that criterion current value Iac is equal to or greater than 1100 μA, the process proceeds to step S180.

In step S180, CPU511has charging voltage supply portion44(FIG. 3) change the frequency of electric power (charging frequency) supplied to charging rollers4Y,4M,4C, and4K from frequency FB to frequency FA. Thereafter, the process proceeds to step S190.

In step S190, CPU511has development voltage supply portion54(FIG. 3) return the frequency of electric power (development frequency) supplied to development rollers6Y,6M,6C, and6K from the frequency after change in step S140to the frequency before change in step S140. Thereafter, the process proceeds to step S200.

In step S200, CPU511returns the set value for the system speed to the state before change in step S150. For example, the speed of transportation of paper P is increased and returns to the state before lowering in step S150. Thereafter, the process returns to step S110.

According to the process inFIG. 4described above, whether or not the criterion current value is equal to or smaller than a first threshold value is determined at the timing of stabilization control (step S120), and when the criterion current value is equal to or smaller than the first threshold value, the charging frequency is lowered.

After the charging frequency is lowered, whether or not the criterion current value is equal to or greater than a second threshold value is determined (step S170), and when the criterion current value is equal to or greater than the second threshold value, the charging frequency is returned to the original frequency.

In the example inFIG. 4, the first threshold value is set to 900 μA and the second threshold value is set to 1100 μA. The second threshold value should only be equal to or greater than the first threshold value. Namely, the second threshold value may be equal to the first threshold value.

In image forming apparatus200, setting of the charging frequency in steps S130and S180, setting of the development frequency in steps S140and S190, and setting of the system speed in steps S150and S200may be made as a part of stabilization control.

A voltage value used for detecting criterion current value Iac in the process described with reference toFIG. 4does not necessarily have to be included in the voltage values used for determining a charging voltage in stabilization control.

Setting of the charging frequency based on the criterion current value as described above may be made at timing other than stabilization control. Such an example will be described below.

[Control (2) Based on Criterion Current Value]

FIG. 5is a diagram for illustrating a modification of the process inFIG. 4. As compared with the process inFIG. 4, steps S112, S114, and S162are added in a process inFIG. 5. As a premise of the process inFIG. 5, CPU511is configured to store information for specifying an accumulated value of durations of application of a voltage to charging rollers4C,4K,4M, and4Y in memory512.

In the process inFIG. 5, after CPU511determines in step S110that timing of stabilization control has not yet come (NO in step S110), the process proceeds to step S112.

CPU511determines in step S112whether or not an accumulated value (an accumulated time period Ta) of durations of application of a voltage to charging rollers4C,4K,4M, and4Y has reached a predetermined threshold value TS1. When CPU511determines that accumulated time period Ta has not reached threshold value TS1, the process proceeds to step S114, and when the CPU determines that accumulated time period Ta has reached threshold value TS1, the process proceeds to step S120.

CPU511determines in step S114whether or not an accumulated value (an accumulated time period Tb) of durations of application of a voltage to charging rollers4C,4K,4M, and4Y after previous stabilization control has reached a predetermined threshold value TS2. When CPU511determines that accumulated time period Tb has not reached threshold value TS2, the process returns to step S110, and when the CPU determines that accumulated time period Tb has reached threshold value TS2, the process proceeds to step S120.

In step S120to step S160, the process as described with reference toFIG. 4is performed.

When CPU511determines in step S160that the timing of new stabilization control has not yet come, the process proceeds to step S162.

CPU511determines in step S162whether or not an accumulated value (accumulated time period Tb) of durations of application of a voltage to charging rollers4C,4K,4M, and4Y after previous stabilization control has reached predetermined threshold value TS2. When CPU511determines that accumulated time period Tb has not reached threshold value TS2, the process returns to step S160, and when the CPU determines that accumulated time period Tb has reached threshold value TS2, the process proceeds to step S170.

In step S170to step S200, the process as described with reference toFIG. 4is performed.

In the process inFIG. 5, the charging frequency may be changed based on the criterion current value when a time period (accumulated time period Ta) since start of application of a voltage to charging rollers4C,4K,4M, and4Y reaches threshold value TS1or when a time period (accumulated time period Tb) since start of application of a voltage to charging rollers4C,4K,4M, and4Y since execution of stabilization control reaches threshold value TS2, in addition to the timing of stabilization control.

Although embodiments of the present invention have been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and not limitation, the scope of the present invention should be interpreted by terms of the appended claims.