JP2013217986A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To control charge potential to be a target value with higher accuracy at lower costs compared to the case where a potential sensor is attached to an image carrier.SOLUTION: An image forming apparatus includes: a Zener diode 17 that is connected with a conductive path for grounding an intermediate transfer belt 13a that has constant potential over the entire periphery by application of voltage; an ammeter 18 that measures the size of a current flowing through the Zener diode 17; and a control device 2 that derives, when image formation is performed, charge potential of an image carrier 8 by using the size of a non-charge period current D6 and a charge period current D7, and controls a charging power source 6 to make the derived charge potential close to a target potential that is set in advance.

Description

  The present invention relates to an image forming apparatus such as a copying machine or a printer having a function of forming an image on a recording material such as a sheet.

  In recent years, image forming apparatuses are required to have high image quality. As a factor for reducing the image quality, there is a change in image density caused by a change in the charging potential of the image carrier. In the charging process during image formation, a constant voltage is applied to the charging roller that is in contact with the image carrier. As a result, a discharge is generated between the charging roller and the image carrier to charge the image carrier to a desired charging potential. However, when printing is repeatedly performed, the resistance of the charging roller may fluctuate due to deterioration of energization or contamination. For this reason, there is a concern that the discharge state between the charging roller and the image carrier changes, and the charged potential of the image carrier changes. In the exposure step, an electrostatic latent image is formed on the image carrier by irradiating the charged image carrier with a laser. When the charging potential of the image carrier is fluctuating, the exposed portion potential of the electrostatic latent image also fluctuates. In the developing step, the toner is developed on the image carrier in accordance with the potential difference between the exposed portion potential and the developing roller potential. When the exposed portion potential fluctuates, the potential difference between the exposed portion potential and the developing roller potential fluctuates, and the amount of toner to be developed changes. As a result, the image density varies.

In order to reduce the fluctuation of the charging potential, the following method has been proposed.
(1) A system using a potential sensor for an image carrier (Patent Document 1)
In this method, a surface potential sensor for measuring the surface potential of the image carrier after the charging step is arranged on the image carrier. Using this surface potential sensor, the charged potential of the image carrier is measured. The control device controls the charging power supply so that the charging potential measured by the surface potential sensor approaches the target value.
Further, the following method has been proposed in order to reduce fluctuations in the charging potential without using a surface potential sensor.
(2) Method of calculating the potential of the image carrier from the resistance value of the primary transfer system and the current value flowing therethrough (Patent Document 2)
In this method, first, a current is measured by applying a voltage to the primary transfer roller while the image carrier is not charged. From the result, the resistance value of the primary transfer system is calculated. Next, the current value flowing through the primary transfer system when the primary transfer system is grounded with the image carrier charged is measured. The charged potential of the image carrier is calculated by multiplying the resistance value thus obtained and the current value. Then, the control device controls the charging power source to control the calculated charging potential so as to approach the target value.

Japanese Patent Laid-Open No. 06-274011 JP 2006-0665113 A

However, in the above-described conventional technology, there is a concern that the following problems occur.
That is, in the method using the potential sensor for the image carrier described in the above (1), the surface potential sensor must be disposed close to each image carrier. For this reason, the same number of surface potential sensors as the number of image carriers are required, so that the cost of the image forming apparatus is increased.
In the method of calculating the potential of the image carrier from the resistance value of the primary transfer system and the current value flowing therethrough described in (2) above, the resistance value of the primary transfer system having a resistance component and a capacitance component is measured. Must. When measuring the resistance value of a system having a resistance component and a capacitance component, the resistance value changes depending on the measurement conditions. For this reason, the measurement accuracy of the resistance value is low. As a result, the accuracy of the charging potential obtained as a result of multiplication of the resistance value and the current value is also lowered.

  The present invention has been made in view of the circumstances as described above, and is capable of controlling the charging potential to a target value with higher accuracy while reducing the cost as compared with the case where a potential sensor is attached to the image carrier. Objective.

In order to achieve the above object, the present invention provides:
An image carrier on which an electrostatic latent image is formed;
A charging member for uniformly charging the image carrier;
Charging voltage applying means for applying a voltage to the charging member;
Developing means for developing the electrostatic latent image into a developer image;
An endless rotatable intermediate transfer body onto which a developer image on the image carrier developed by the developing means is transferred at a primary transfer portion;
A transfer member disposed so as to be in contact with the intermediate transfer member and forming a secondary transfer portion with the intermediate transfer member, and is primarily transferred to the intermediate transfer member by applying a voltage. A transfer member for secondary transfer of the toner image to the recording material at the secondary transfer portion;
Transfer voltage applying means for applying a voltage to the transfer member;
An image forming apparatus for forming an image on a recording material,
A constant voltage element that is connected to a conductive path for grounding the intermediate transfer member and that conducts when a voltage having a magnitude equal to or greater than a preset value is applied;
Current measuring means for measuring the magnitude of the current flowing through the constant voltage element;
When image formation is performed and a voltage is applied to the transfer member by the transfer voltage application unit,
The magnitude of the non-charging current flowing through the constant voltage element measured by the current measuring means when no voltage is applied to the charging member by the charging voltage applying means; and
When the voltage is applied to the charging member by the charging voltage applying means, the charging potential of the image carrier is determined using the magnitude of the charging current flowing through the constant voltage element measured by the current measuring means. Derived,
Control means for controlling the charging voltage application means so that the derived charging potential approaches a preset target value;
It is characterized by providing.

  According to the present invention, it is possible to control the charged potential to the target value with higher accuracy while reducing the cost as compared with the case where the potential sensor is attached to the image carrier.

Sectional drawing which shows schematic structure of the image forming apparatus of Example 1. FIG. FIG. 6 is a diagram for explaining a method of measuring the resistance of the intermediate transfer belt according to the first embodiment. The figure which shows the flowchart showing the printing operation of Example 1. FIG. The figure which shows the flowchart showing the charging potential adjustment operation | movement of Example 1. FIG. The figure which shows the data structure of the memory provided in the control apparatus of Example 1. FIG. 6 is a diagram for explaining the principle of calculating the charging potential of the image carrier of Example 1. The figure which shows the measurement result of the surface potential and the electric current when changing the charging potential of the image carrier

DETAILED DESCRIPTION Exemplary embodiments for carrying out the present invention will be described in detail below with reference to the drawings. However, the dimensions, materials, shapes, and relative arrangements of the components described in this embodiment should be appropriately changed according to the configuration of the apparatus to which the invention is applied and various conditions. It is not intended to limit the scope to the following embodiments.
The present invention relates to an image forming apparatus such as a copying machine, a printer, and a facsimile machine that forms an image by electrophotography, and more particularly to an image forming apparatus having a mechanism that stabilizes a charged potential of an image carrier.

Example 1 will be described below.
First, the schematic configuration of the electrophotographic image forming apparatus of this embodiment will be described.
(Electrophotographic image forming apparatus)
FIG. 1 is a cross-sectional view showing a schematic configuration of an image forming apparatus 1 of the present embodiment. In this embodiment, the image forming apparatus 1 is a quadruple tandem image forming apparatus. The image forming apparatus 1 includes first, second, third, and fourth image forming units (PY, PM, PC, and PK) that respectively form yellow, magenta, cyan, and black images as a plurality of image forming units. Have

In the image forming apparatus 1 of this embodiment, a host device such as a personal computer is connected to be communicable. Therefore, in accordance with image information from the host device, four color full-color images of yellow (Y), magenta (M), cyan (C), and black (K) are recorded on a recording material (recording paper, plastic) using an electrophotographic method. Sheet, cloth, etc.).
In this embodiment, the configuration of each image forming unit (PY, PM, PC, PK) is substantially the same except that the development colors are different. Accordingly, in the following, when there is no particular need for distinction, the subscripts Y, M, C, and K for indicating an element belonging to any one of the image forming units will be generally described without adding any reference numerals.
More specifically, as shown in FIG. 1, the image forming apparatus 1 includes a control device 2. The control device 2 includes a central processing unit 3 as a calculation unit, a memory 4, and various input / output (I / O). ). Input / output (I / O) is connected to the computer 5, the image forming apparatus 1, the charging power source 6, the secondary transfer power source 7, and the ammeter 18. Here, the control device 2 corresponds to control means. The charging power source 6 corresponds to charging voltage applying means for applying a voltage to a charging roller 9 as a charging member. The secondary transfer power source 7 corresponds to a transfer voltage applying unit.

When receiving a print start signal from the computer 5, the control device 2 starts image formation. The image carrier 8 is formed by coating a surface of an aluminum cylinder having a diameter of 24 mm with a 20 μm photosensitive layer comprising a hole blocking layer, a charge generation layer, and a charge transfer layer. In this embodiment, the image carrier 8 is configured as a photosensitive drum. Has been.
The aluminum cylinder is grounded. The image carrier 8 is rotated clockwise in FIG. 1 at a surface speed of 240 mm / sec.

A charging roller 9 is disposed in the charging portion so as to be in contact with the image carrier 8. The charging roller 9 has a metal shaft having a diameter of 5.5 mm, a conductive elastic body having a thickness of 1.5 mm and a volume resistivity of about 1 × 10 ⁴ Ωcm, and a volume resistivity of 1 × 0.05 μm. A surface layer of about 10 ¹⁰ Ωcm is formed.
When a voltage of −1115 V is applied to the charging roller 9 and the image carrier 8 rotates, the charging roller 9 circulates and rotates at a substantially equal speed with the image carrier 8, and discharge between the charging roller 9 and the image carrier 8 occurs. The image carrier 8 is uniformly charged. When the surface potential of the image carrier 8 was measured with a surface potential meter Model 344 manufactured by Trek, it was about -500V.

The charged image carrier 8 is irradiated with a laser emitted from a scanning exposure type exposure apparatus 10 including a laser, a polygon mirror, and a lens. The laser scans and exposes the surface of the image carrier 8 in accordance with the rotation of the image carrier 8 by a rotating polygon mirror. At this time, an electrostatic latent image is formed on the image carrier 8 by controlling the amount of laser light based on the image data. The surface potential of the portion that has received the maximum light amount decreases to about -100V.
The portion of the surface of the image carrier 8 on which the electrostatic latent image is formed then contacts the developer carrier 11a of the developing unit 11 as a developing unit.
The developer 12 is a non-magnetic toner produced by suspension polymerization and has an average particle size of about 6.5 μm. In order to improve the surface property, silicon oxide particles of about 20 nm are uniformly attached to the surface in an amount of about 1.5% of the toner weight. Here, the average particle diameter of the toner is a volume average particle diameter measured with a laser diffraction particle size distribution analyzer LS-230 manufactured by Beckman Coulter, Inc.

In the developing unit 11, the developer 12 in the developer container 11b is conveyed downward by a stirring member 11c that rotates clockwise in FIG. The developer supply member 11d is for supplying the developer 12 to the developer carrier 11a. The developer 12 carried on the developer carrying member 11a is charged while the developer layer thickness is regulated by the developer layer thickness regulating member 11e. At this time, the developer amount per unit area was about 0.4 mg / cm ² , and the developer charge amount per unit mass was about −30 μC / g.
Here, the charge amount of the developer 12 per unit mass and the developer amount per unit area were determined as follows. That is, the developer 12 on the developer carrier 11a is sucked and collected by a suction type Faraday gauge having a filter inside, the charge measured by the Faraday gauge, the collection area of the developer 12, and the mass of the filter. Obtained from the increase. That is, the charge amount of the developer 12 per unit mass is obtained by dividing the charge measured by the Faraday gauge by the increase in the filter mass, and the developer amount per unit area is obtained by dividing the increase in the filter mass by the collection area. And asked.

  A voltage of −300 V is applied to the developer carrier 11a. The surface potential of the portion of the electrostatic latent image that has received the maximum amount of laser light is −100 V, and an electric field is generated in the direction in which the developer 12 is attracted to the image carrier 8. On the other hand, the surface potential of the portion of the electrostatic latent image that has not been exposed to the laser is −500 V, and an electric field is generated in the direction in which the developer 12 repels the image carrier 8. By the electrostatic force, the developer 12 adheres only to the exposed portion of the electrostatic latent image on the image carrier 8, whereby the electrostatic latent image is developed and becomes a developer image (toner image).

Next, the developer image on the image carrier is brought into contact with the intermediate transfer belt 13 a constituting the intermediate transfer body 13. The intermediate transfer belt 13a is provided in an endless manner so as to be rotatable, and constitutes a primary transfer portion (hereinafter referred to as a primary transfer nip N1) by contacting the image carrier 8. As described above, the intermediate transfer belt 13a forms the primary transfer nip N1 of the intermediate transfer body 13 on which the toner image is primarily transferred from the image carrier 8.
The intermediate transfer belt 13a is configured to have a constant potential over the entire circumference when a voltage is applied. Here, the secondary transfer power source 7 for applying a voltage to the secondary transfer roller 16 as a transfer member applies an intermediate voltage in contact with the secondary transfer roller 16 by applying a voltage to the secondary transfer roller 16. A voltage is applied to the transfer belt 13a.
In this embodiment, the intermediate transfer belt 13a having a low resistance is used as will be described later, so that the current applied from the secondary transfer power source 7 passes through the secondary transfer roller 16 and the intermediate transfer belt 13a to the image carrier 8 ( It is configured to be supplied to the primary transfer nip N1). A necessary primary transfer electric field is formed by this current.
In this way, by adopting a configuration in which the secondary transfer power supply also serves as the primary transfer power supply, the cost of the image forming apparatus can be reduced.
The secondary transfer roller 16 is disposed so as to contact the intermediate transfer belt 13a and forms a secondary transfer portion (hereinafter referred to as a secondary transfer nip N2) with the intermediate transfer belt 13a. The secondary transfer roller 16 is applied with a voltage by the secondary transfer power source 7 to secondarily transfer the toner image primarily transferred to the intermediate transfer belt 13a onto the recording material at the secondary transfer nip N2.

The intermediate transfer belt 13a is an annular sheet in which carbon black is dispersed in polyphenylene sulfide (PPS) resin, and has a thickness of 100 μm and a resistance of 110 kΩ. On the surface (surface in contact with the image carrier), a surface layer of acrylic resin having a thickness of 0.5 to 3 μm and a high resistance is provided.
The resistance of the intermediate transfer belt 13a was measured by connecting a Hiresta UP dedicated probe URS probe to a resistivity meter Hiresta UP manufactured by Mitsubishi Chemical Analytech Co., Ltd., but the resistance was low and out of range. It was. Therefore, measurement was performed as shown in FIG. 2A using the image forming apparatus 1 used in this example. FIG. 2 is a diagram for explaining a method of measuring the resistance of the intermediate transfer belt 13a.

As shown in FIG. 2A, a metal secondary transfer roller 16M having the same shape as the secondary transfer roller 16 is disposed instead of the secondary transfer roller 16, and a charge is used instead of the image carrier 8 of the image forming unit PK. An aluminum cylinder 8M not coated with a moving layer or the like was disposed. Then, the charging roller 9 and the developing unit 11 of the image forming unit PK were removed, and the entire image forming unit was removed from the image forming units PY, PM, and PC.
The secondary transfer power source 7 was connected to the metal secondary transfer roller 16M, and the aluminum cylinder 8M was grounded via the ammeter 23. Then, the current flowing when 100 V was applied from the secondary transfer power source 7 was measured while driving the intermediate transfer belt 13a at a surface speed of 240 mm / sec.
A value obtained by dividing the applied voltage 100 V by the measured current was used as the resistance of the intermediate transfer belt 13a. The distance from the primary transfer nip N1 to the secondary transfer nip N2 of the aluminum cylinder 8M with respect to the movement direction (rotation direction) of the surface of the intermediate transfer belt 13a was 370 mm, and the opposite side was 420 mm. The resistance of the intermediate transfer belt 13a measured by this method is 1.1 × 10 ⁵ Ω.

  Further, when the resistance of the intermediate transfer belt 13a was measured by changing the position where the aluminum cylinder 8M was installed to the image forming portion PY as shown in FIG. 2B, the resistance value was almost the same. The distance from the primary transfer nip N1 to the secondary transfer nip N2 of the aluminum cylinder 8M with respect to the moving direction of the surface of the intermediate transfer belt 13a was 710 mm, and the opposite side was 80 mm.

The intermediate transfer belt 13a is stretched by three rollers of a driving roller 13b, a tension roller 13c, and a secondary transfer counter roller 13d (hereinafter, the three may be collectively referred to as “stretching roller”). Here, the driving roller 13b, the tension roller 13c, and the secondary transfer counter roller 13d correspond to a stretching member. The secondary transfer counter roller 13d corresponds to a counter member.
On the surface layer of the driving roller 13b, a conductive rubber layer having a high friction and a volume resistance of 10 ⁵ Ωcm or less is provided. The drive roller 13b is driven by a drive motor (not shown) to rotate the intermediate transfer belt 13a counterclockwise in FIG. 1 at the same surface speed as the image carrier 8 at 240 mm / sec.

The tension roller 13c is made of a metal roller, applies a tension of a total pressure of about 60 N to the intermediate transfer belt 13a, and rotates following the intermediate transfer belt 13a. The secondary transfer counter roller 13d is provided with a rubber layer on the surface in order to form the secondary transfer nip N2. As the surface rubber layer, a conductive material having a volume resistance of 10 ⁵ Ωcm or less is used. The secondary transfer counter roller 13d rotates following the intermediate transfer belt 13a. The driving roller 13b, the tension roller 13c, and the secondary transfer counter roller 13d are all grounded through a Zener diode 17 and an ammeter 18 having a breakdown voltage of 200 V connected in series. The zener diode 17 is connected with the anode on the intermediate transfer member 13 side and the cathode on the ammeter 18 side. Here, the Zener diode 17 is connected to a conductive path for grounding the intermediate transfer belt 13a, and is a constant voltage element that is turned on when a voltage having a magnitude greater than or equal to a breakdown voltage (greater than a preset value) is applied. It corresponds to. The constant voltage element is for holding the intermediate transfer belt 13a at a preset potential. The ammeter 18 corresponds to current measuring means for measuring the magnitude of the current flowing through the Zener diode 17. A varistor may be used for the constant voltage element.

A primary transfer roller 14 in which a metal shaft having a diameter of 6 mm is coated with a conductive foam having a thickness of 4 mm is disposed so as to sandwich the intermediate transfer belt 13 a together with the image carrier 8. The rubber hardness measured by a weight of 1 kg weight (9.8 N) with an Asker rubber hardness meter C type manufactured by Kobunshi Keiki Co., Ltd. is 30 degrees, and the volume resistance is 10 ⁵ to 10 ⁹ Ωcm.
The primary transfer roller 14 presses the intermediate transfer belt 13a toward the image carrier 8 with a total pressure of about 9.8 N, thereby forming a primary transfer nip N1. The primary transfer roller 14 rotates following the rotation of the intermediate transfer belt 13a. The primary transfer roller 14 is electrically floating.

  A circuit that is grounded through the intermediate transfer belt 13a, the stretching rollers 13b, 13c, and 13d, the zener diode 17, and the ammeter 18 through the contact from the secondary transfer roller 16 to the secondary transfer nip N2 is formed. When a voltage of 3000 V is applied from the secondary transfer power source 7 to the secondary transfer roller 16, the surface potential of the intermediate transfer belt 13a increases. When the surface potential of the intermediate transfer belt 13a was measured with a surface potential meter Model 344 manufactured by Trek, the surface potential was 200 V equal to the breakdown voltage of the Zener diode 17 at any image carrier position.

In the primary transfer nip N1, an electric field is formed by the difference between the surface potential of the image carrier 8 (-500 V for the non-exposed portion and -100 V for the exposed portion) and the surface potential (200 V) of the intermediate transfer belt 13a. The direction of the electric field is a direction in which the negatively charged developer 12 is transferred from the image carrier 8 to the intermediate transfer member 13 regardless of the exposed portion and the non-exposed portion of the image carrier 8. By this electric field, the developer 12 on the image carrier 8 moves onto the intermediate transfer belt 13 a of the intermediate transfer member 13.
The developer 12 remaining on the image carrier 8 without being primarily transferred is removed by an image carrier cleaning means 15 attached to the image carrier 8. The image carrier cleaning means 15 is provided so that the rubber blade 15 a contacts the image carrier 8. The developer 12 is dammed up at the contact portion between the rubber blade 15a and the image carrier 8, so that the developer 12 is removed from the image carrier 8 and collected in the developer collection container 15b.

As described above, the intermediate transfer belt 13a is rotated counterclockwise in FIG. 1 by the driving roller 13b at a surface speed of 240 mm / sec, and the above steps are sequentially performed by the four image forming portions PY, PM, PC, and PK. . Thus, the yellow, magenta, cyan, and black color developers 12 are sequentially transferred so as to overlap the intermediate transfer belt 13a.
As a result, a color developer image is formed on the intermediate transfer belt 13 a of the intermediate transfer member 13.

The developer image on the intermediate transfer belt 13a is carried by the rotation of the intermediate transfer belt 13a and moves to the secondary transfer nip N2. The secondary transfer roller 16 faces the secondary transfer counter roller 13d and abuts against the intermediate transfer belt 13a with a total pressure of about 39.2N to form a secondary transfer nip N2. The secondary transfer roller 16 is configured by covering a metal shaft having a diameter of 8 mm with an elastic layer made of an EPDM foam having a thickness of 4 mm. The rubber hardness is 30 degrees and the volume resistivity is 10 ⁷ to 10 ⁹ Ωcm, measured by a weight of 1 kg weight (9.8 N) using an Asker rubber hardness meter C type manufactured by Kobunshi Keiki Co., Ltd.
At the secondary transfer nip N2, a negative developer image is transferred from the intermediate transfer belt 13a to the secondary transfer nip N2 due to the difference between the surface potential (200V) of the intermediate transfer belt 13a and the voltage (3000V) applied to the secondary transfer roller 16. An electric field directed toward the transfer roller 16 is formed. The recording material S stored in the recording material storage cassette 19 passes through the secondary transfer nip N2 at a timing when the printing position matches the developer image on the intermediate transfer belt 13a. When the recording material S passes through the secondary transfer nip N2, the developer image on the intermediate transfer belt 13a moves (transfers) to the recording material S by the electric field formed in the secondary transfer nip N2. Here, in this example, high-quality paper (basis weight 75 g / m ² ) was used as the recording material S.

The developer remaining on the intermediate transfer belt 13a without being subjected to the secondary transfer is removed by an intermediate transfer member cleaning unit 20 attached to the intermediate transfer belt 13a. The intermediate transfer member cleaning means 20 is provided so that the elastic blade 20a contacts the intermediate transfer belt 13a. The developer remaining on the intermediate transfer belt 13a is removed from the intermediate transfer belt 13a by being dammed at the contact portion between the elastic blade 20a and the intermediate transfer belt 13a, and collected in the developer collection container 20b.
The developer image on the recording material S is heated and pressed by a fixing device 21 having a fixing roller 21a and a pressure roller 21b and fixed on the recording material S. The recording material S on which the image is formed is discharged onto the discharge tray 22 and the image forming process is completed. The image forming apparatus 1 according to the present embodiment has a printable width of 200 mm and can print A4 size recording material at 40 sheets per minute.

(Charge potential adjustment operation)
FIG. 3 is a flowchart illustrating the printing operation according to the first embodiment.
When the printing operation (image forming operation) is started, a “printing routine” shown in FIG. 3 is started. In this printing routine, a “charging potential adjustment routine” shown in FIG. 4 is called. FIG. 4 is a flowchart illustrating the charging potential adjustment operation according to the first embodiment. FIG. 5 is a diagram showing a data structure of the memory 4 provided in the control device 2, and this data structure is composed of constants, variables, and a reference table (LUT).

In the present embodiment, various constants hold predetermined values.
In FIG. 5, the following constants are specific values of the image forming apparatus. The dielectric constant (C1) of the photosensitive layer of the image carrier 8, the contact width (C2) between the image carrier 8 and the intermediate transfer belt 13a, the discharge threshold (C3) between the image carrier 8 and the intermediate transfer belt 13a, This is the breakdown voltage (C4) of the Zener diode 17. Further, they are the surface speed (C5) of the image carrier 8, the number of image forming portions (C6), and the waiting time (C7). The adjustment period (C8), the target potential (C9), and the tolerance (C10) are values determined by setting conditions of the image forming apparatus. In addition, the number in the said parenthesis has shown the reference number in FIG.

Various variables store values obtained during operation. In FIG. 5, the cumulative number of printed sheets (cumulative number, D1), the adjustment deadline number (D2), and the charging power source 6 output voltage (D3) are one for each of the image forming portions PY, PM, PC, and PK. It has 4 storage areas. These are non-volatile memories that continue to store values even when the image forming apparatus is turned off. The cumulative number of printed sheets (D1) and the adjustment deadline number (D2) are reset to 0 when the image forming unit is replaced. The photosensitive layer thickness (D4), the adjustment target index (D5), the non-charging current (D6), the charging current (D7), the current potential (D8), and the deviation (D9) are volatilized values that are temporarily stored. Memory.
Cumulative number of printed sheets / photosensitive layer film thickness LUT (TA) is an array of two rows, the cumulative number of printed sheets is held in the first column, and the photosensitive layer of the image carrier corresponding to the cumulative number of printed sheets in the second column. Maintain film thickness.
In the image forming apparatus 1 of the present embodiment shown in FIG. 1, the control device 2 has the central processing unit 3, the memory 4, and various inputs / outputs (I / O) as described above. Input / output (I / O) is connected to the computer 5, the image forming apparatus 1, the charging power source 6, the secondary transfer power source 7, and the ammeter 18.

  When receiving a print start command from the computer 5, the control device 2 starts a print routine shown in FIG. In the charging potential adjustment, the four image forming portions PY, PM, PC, and PK are adjusted in order. Therefore, an index is associated with each image forming unit (PY = 0, PM = 1, PC = 2, PK = 3).

First, the adjustment target index D5 is set to 0 (corresponding to the image forming unit PY) (S1-1). Then, driving of the image carrier 8 and the intermediate transfer member 13 is started. At the same time, the output of the secondary transfer power supply 7 is started (S1-2). Control is transferred to the image forming unit PY corresponding to the adjustment target index D5 (S1-3).
Next, it is inspected whether the cumulative number of printed sheets D1 corresponding to the image forming unit PY is smaller than the adjustment deadline number D2 (S1-4). When the cumulative number of printed sheets D1 is equal to or greater than the adjustment deadline number D2, the charging potential adjustment routine is performed (S2-1 to S2-11), and the adjustment period C8 is added to the adjustment deadline number D2 to determine the cumulative number of printed sheets for the next adjustment. Obtain (S1-5). If the cumulative number of printed sheets D1 is smaller than the adjustment deadline number D2, the potential adjustment routine is passed. Thereafter, 1 is added to the adjustment target index D5 (S1-6). While the adjustment target index D5 is smaller than the number C6 of image forming units, the procedure from S1-3 to S1-6 is repeated (S1-7). When the adjustment target index D5 becomes equal to or greater than the number C6 of image forming units, the adjustment of the charging potential is completed in all the image forming units PY, PM, PC, and PK. In this state, image formation is performed (S1-8).
When the image formation is completed, the driving of the image carrier 8 and the intermediate transfer member 13 is stopped and the output of the secondary transfer power source 7 is stopped (S1-9). The number of images formed is added to the cumulative number of printed sheets D1 (S1-10), and the printing routine is terminated.
By the above routine, the charging potential of the image carrier 8 can be controlled so as to approach the target potential (target value).

Next, the charging potential adjustment routine shown in FIG. 4 will be described.
With reference to the cumulative number of printed sheets / photosensitive layer film thickness LUT (TA), for the maximum row where the current cumulative number of printed sheets D1 does not exceed the value in the first column of the cumulative number of printed sheets / photosensitive layer film thickness LUT (TA). Then, the value in the second column is taken out and set to the photosensitive layer thickness D4 of the image carrier 8 (S2-1). In order to initialize the deviation D9, the deviation D9 is set to 0 (S2-2). Since the current is already supplied from the secondary transfer power supply 7, the current flows through the Zener diode 17. The current is sampled by the ammeter 18 and stored in the non-charging current D6 (S2-3). Here, the accuracy of current measurement is increased by sampling the ammeter 18 a plurality of times, and adding it to the non-charging current D6 and dividing by the number of samplings. The non-charging current D6 is the magnitude of the current flowing through the Zener diode 17 measured by the ammeter 18 when no voltage is applied to the charging roller 9 by the charging power source 6.

Next, a deviation D9 is added to the charging power source 6 output voltage D3 (S2-4), and output of the charging power source 6 of the image forming unit to be adjusted is started (S2-5). Here, when S2-4 is repeatedly executed in S2-10, which will be described later, in S2-4, a deviation D9 is added to the previous charging power source 6 output voltage D3.
Wait until the standby time C7 elapses after the output of the charging power source 6 is started (S2-6). When the charged portion of the image carrier 8 reaches the primary transfer nip N1, electric current flows due to discharge between the image carrier 8 and the intermediate transfer belt 13a. When such a current flows, the current flowing through the Zener diode 17 decreases. This current is sampled by the ammeter 18 and stored in the charging current D7 (S2-7). Here, the accuracy of current measurement is improved by sampling the ammeter 18 a plurality of times, sequentially adding to the charging current D7 and dividing by the number of samplings. Here, the charging current D7 is the magnitude of the current flowing through the Zener diode 17 measured by the ammeter 18 when a voltage is applied to the charging roller 9 by the charging power source 6.
Subsequently, using the non-charging current D6 and the charging current D7, a charging potential is calculated (derived) from the following Equation 1, and the result is set to the current potential D8 (S2-8).

・・・（式１）

... (Formula 1)

  Subsequently, the deviation D9 is set to a value obtained by subtracting the target potential C9 from the current potential D8 (S2-9). These procedures from S2-4 to S2-9 are repeated while the deviation D9 is equal to or greater than the tolerance C10 (S2-10). When the deviation D9 becomes smaller than the tolerance C10, the output of the charging power source 6 is stopped (S2-11), and the charging potential adjustment routine is completed. Thus, in the charging potential adjustment routine, the charging power source 6 is controlled so that the charging potential of the image carrier 8 approaches the target potential C9.

(Principle of measurement of charged potential of image carrier)
Next, the principle by which the charged potential of the image carrier 8 can be calculated by measuring the current flowing through the Zener diode 17 will be described with reference to FIG. FIG. 6 is a diagram for explaining the principle of calculating the charging potential of the image carrier 8 in the present embodiment.
When the intermediate transfer member 13 and the image carrier 8 are driven and the output of the secondary transfer power supply 7 is started, a current flows through the low-resistance intermediate transfer belt 13a. The current (indicated by 25a in FIG. 6) output from the secondary transfer power source 7 passes through the intermediate transfer belt 13a and the stretching rollers 13b, 13c, and 13d and all flows to the Zener diode 17 (this current flow is indicated by 25b in FIG. 6). ). The non-charging current D6, which is the value of the ammeter 18 in this state, is equal to the output current of the secondary transfer power source 7.

Next, the output of the charging power source 6 is started and a voltage is applied to the charging roller 9. A discharge occurs between the charging roller 9 and the image carrier 8, and the image carrier 8 is charged. The charged portion of the image carrier 8 approaches the primary transfer nip N1 as it rotates. When the potential difference between the surface potential of the image carrier 8 and the intermediate transfer belt 13a is equal to or greater than the discharge threshold, the image carrier is configured such that discharge occurs and the potential difference between the image carrier 8 and the intermediate transfer belt 13a becomes the discharge threshold. The surface potential of the body 8 is changed.
The surface potential of the image carrier 8 before passing through the primary transfer nip includes the surface potential of the intermediate transfer belt 13a, the discharge threshold value of the image carrier 8 and the intermediate transfer belt 13a, and the image carrier before and after passing through the primary transfer nip N1. Is the sum of the amount of change in surface potential (Equation 2).

・・・（式２）

... (Formula 2)

  The amount of change in the surface potential of the image carrier 8 at this time is determined by the electrostatic capacity per unit area of the image carrier 8 and the surface area of the image carrier 8 passing through the primary transfer nip N1 per second. The divided value (Equation 3).

・・・（式３）

... (Formula 3)

Here, the capacitance per unit area of the image carrier 8 is a value obtained by dividing the dielectric constant C1 of the photosensitive layer of the image carrier 8 by the photosensitive layer thickness D4. The surface area of the image carrier 8 passing through the primary transfer nip N1 per second is the product of the contact width between the image carrier 8 and the intermediate transfer belt 13a and the surface speed C5 of the image carrier 8.
This discharge current is supplied from the secondary transfer power source 7 via the intermediate transfer belt 13a (the flow of this current is indicated by 25c in FIG. 6). Therefore, the current (25b) flowing through the Zener diode 17 is reduced by this discharge current when no discharge is generated between the image carrier 8 and the intermediate transfer belt 13a. The charging current D7, which is the value of the ammeter 18 in this state, is equal to the value obtained by subtracting the discharge current from the output current of the secondary transfer power supply 7 (Equation 4).

・・・（式４）

... (Formula 4)

Substituting Equations 3-4 into Equation 2 yields Equation 1. By using Equation 1, the charging potential of the image carrier 8 can be calculated from the current flowing through the Zener diode 17.
In order to confirm this principle, the current flowing through the Zener diode 17 was measured by changing the surface potential of the image carrier 8.
In order to obtain the actual surface potential of the image carrier 8, the potential sensor 26a that measures the surface potential of the image carrier 8 before passing through the primary transfer nip N1, and the image carrier 8 after passing through the primary transfer nip N1. Measurement was performed by attaching a potential sensor 26b for measuring the surface potential. A potential sensor 26c was attached to measure the surface potential of the intermediate transfer belt 13a. For these potential sensors, a surface potential meter Model 344 manufactured by Trek was used. Further, an ammeter 24 was attached to measure the output current of the secondary transfer power source 7.
The result is shown in FIG.

The potential of the intermediate transfer belt 13a is equal to 200V, which is the breakdown voltage of the Zener diode 17, since the secondary transfer power supply 7 has an output voltage of 3000V and supplies a sufficient current.
In the region where the surface potential of the image carrier 8 before passing through the primary transfer nip N1 is on the positive side (0V side) from −415V, the potential of the image carrier 8 before and after passing through the primary transfer nip N1 is equal, and the zener diode 17 The flowing current (25b) is constant and equal to the output current (25a) of the secondary transfer power source 7. This indicates that no discharge is generated between the image carrier 8 and the intermediate transfer belt 13a, and all the current (25a) output from the secondary transfer power source 7 flows to the Zener diode 17 (25b). ing.
In a region where the surface potential of the image carrier 8 before passing through the primary transfer nip N1 is a negative side (a region larger on the negative side) than −415 V, regardless of the potential of the image carrier 8 before passing through the primary transfer nip N1, The surface potential of the image carrier 8 after passing through the primary transfer nip N1 becomes constant at −415V. In addition, the current (25b) flowing through the Zener diode 17 decreases and becomes smaller than the current (25a) output from the secondary transfer power supply 7. The reduction amount of this current and the surface potential difference of the image carrier 8 before and after passing through the primary transfer nip N1 satisfy Expression 2. This indicates that a part of the current output from the secondary transfer power supply 7 that did not flow to the Zener diode 17 was used for static elimination of the image carrier 8 via the intermediate transfer belt 13a. ing.

  As described above, the surface potential of the image carrier 8 can be calculated from the current flowing through the Zener diode 17 using Equation 1. Note that the method for calculating the surface potential of the image carrier 8 uses the discharge between the image carrier 8 and the intermediate transfer belt 13a, so that the surface potential of the image carrier 8 and the intermediate transfer belt 13a can be calculated. It cannot be used in a region where the potential difference is smaller than the discharge threshold.

(Accuracy of charge potential control of image carrier)
The accuracy of charge potential control of the image carrier 8 depends on the measurement accuracy of the charge potential of the image carrier. In the configuration of this embodiment, the charging potential can be directly measured from the measurement result of the current flowing through the Zener diode 17. Therefore, the measurement accuracy of the charged potential in this embodiment depends on the accuracy of current measurement. On the other hand, when the charging potential is calculated from the product of the resistance and current of the primary transfer system as in the prior art, the charging potential is calculated by the product of current and resistance. For this reason, the measurement accuracy of the charged potential depends on the product of variations in both current measurement and resistance measurement.
Therefore, the measurement result of the charged potential of the configuration of the present embodiment can be controlled with higher accuracy than the conventional case because it is not affected by the measurement result of the resistance.

(Calculation of constants necessary for charging potential adjustment)
Next, a method for calculating a constant necessary for adjusting the charging potential will be described.
The dielectric constant of the photosensitive layer of the image carrier 8 was determined as follows. A test piece having a 1 cm ² aluminum plate coated with the same photosensitive layer as the image carrier 8 and platinum deposited on the surface was prepared. The capacitance was measured with an LCR meter AG-4304 manufactured by Ando Electric Co., Ltd. under conditions of a frequency of 100 Hz and an applied voltage of 1V. The dielectric constant was determined by dividing the capacitance by the area of the test piece and multiplying by the thickness of the photosensitive layer. The dielectric constant of the photosensitive layer of the image carrier 8 of this example was 2.0 × 10 ⁻¹¹ .
The discharge threshold value C3 between the image carrier 8 and the intermediate transfer belt 13a is before passing through the primary transfer nip N1 when the current flowing through the Zener diode 17 described in the principle of measuring the charged potential of the image carrier starts to decrease. The surface potential of the image carrier 8 and the potential difference of the intermediate transfer belt 13a were used. In this example, it was set to 615V.

The waiting time C7 is a value obtained by dividing the distance along the rotation direction of the image carrier 8 from the contact position of the charging roller 9 on the surface of the image carrier 8 to the primary transfer nip N1 by the surface speed of the image carrier 8. Using. In the configuration of this embodiment, the distance from the contact position of the charging roller 9 on the surface of the image carrier 8 to the primary transfer nip N1 is 45.6 mm, and the rotational speed of the image carrier 8 is 240 mm / sec. The standby time C7 was set to 0.19 sec.
The adjustment cycle C8 was set to 100 sheets, and the target potential C9 was set to 500V. The tolerance C10 was set to 10V because the difference in image density was recognized visually when there was a potential fluctuation of 10V.

  The cumulative number of printed sheets / photosensitive layer film thickness LUT (TA) is the instantaneous multi-photometry system MCPD- manufactured by Otsuka Electronics Co., Ltd. Measured at 2000. This was measured until 20000 sheets, and the relationship between the cumulative number of printed sheets and the photosensitive layer thickness was linearly fitted. Using the fitting result, the photosensitive layer thickness was obtained for every 2500 sheets, and the cumulative number of printed sheets / photosensitive layer thickness LUT (TA) as shown in Table 1 was prepared.

In order to confirm the result of the charging potential adjustment, the charging potential when the charging potential was changed by attaching the developer 12 to the charging roller 9 was measured. The charging potential of the image carrier 8 was measured by a potential sensor 26a. The charging potential of the image carrier 8 when the output of the charging power source was set to −1115 V without performing the charging potential control of this embodiment was −500 V. After the measurement, 0.1 mg / cm ² of developer 12 was adhered to the entire charging roller 9. The charging potential when the output of the charging power source was set to −1115 V without performing the charging potential control of this example was lowered to −450 V. On the other hand, when the charging potential control of the present embodiment was performed, the charging potential of the image carrier 8 could be controlled within ± 10V with respect to −500V.
As described above, according to the present embodiment, the charged potential of the image carrier 8 can be increased with higher accuracy while realizing low cost as compared with the case where the potential sensor is attached to the image carrier as in the prior art. It becomes possible to control to the target value.

  DESCRIPTION OF SYMBOLS 1 ... Image forming apparatus, 2 ... Control apparatus, 6 ... Charging power supply, 7 ... Secondary transfer power supply, 8 ... Image carrier, 9 ... Charging roller, 11 ... Developing part, 13 ... Intermediate transfer body, 16 ... Secondary transfer Roller, 17 ... Zener diode, 18 ... Ammeter

Claims (8)

An image carrier on which an electrostatic latent image is formed;
A charging member for uniformly charging the image carrier;
Charging voltage applying means for applying a voltage to the charging member;
Developing means for developing the electrostatic latent image into a developer image;
An endless rotatable intermediate transfer body onto which a developer image on the image carrier developed by the developing means is transferred at a primary transfer portion;
A transfer member disposed so as to be in contact with the intermediate transfer member and forming a secondary transfer portion with the intermediate transfer member, and is primarily transferred to the intermediate transfer member by applying a voltage. A transfer member for secondary transfer of the toner image to the recording material at the secondary transfer portion;
Transfer voltage applying means for applying a voltage to the transfer member;
An image forming apparatus for forming an image on a recording material,
A constant voltage element that is connected to a conductive path for grounding the intermediate transfer member and that conducts when a voltage having a magnitude equal to or greater than a preset value is applied;
Current measuring means for measuring the magnitude of the current flowing through the constant voltage element;
When image formation is performed and a voltage is applied to the transfer member by the transfer voltage application unit,
The magnitude of the non-charging current flowing through the constant voltage element measured by the current measuring means when no voltage is applied to the charging member by the charging voltage applying means; and
When the voltage is applied to the charging member by the charging voltage applying means, the charging potential of the image carrier is determined using the magnitude of the charging current flowing through the constant voltage element measured by the current measuring means. Derived,
Control means for controlling the charging voltage application means so that the derived charging potential approaches a preset target value;
An image forming apparatus comprising: The control means includes
The difference between the derived charging potential of the image carrier and the target value is added to the previous charging current applied to the charging member by the charging voltage applying unit, and a new current is charged. A step of deriving a charged potential of the image carrier as a current when
2. The image forming apparatus according to claim 1, wherein the image forming apparatus is operated until a difference between the derived charged potential of the image carrier and the target value becomes smaller than a preset value. The control means includes
Deriving the film thickness of the photosensitive layer that forms the surface of the image carrier from the relationship with the accumulated number of recording materials on which image formation has been performed in advance,
3. The image formation according to claim 1, wherein a charging potential of the image carrier is derived from a relationship between the non-charging current, the charging current, and the film thickness that is stored in advance. apparatus. A counter member facing the transfer member via the intermediate transfer member;
The image forming apparatus according to claim 1, wherein the constant voltage element is connected to the facing member. A plurality of stretching members that stretch an intermediate transfer belt that forms a primary transfer portion of the intermediate transfer body to which a toner image is primarily transferred from the image carrier;
The image forming apparatus according to claim 1, wherein the constant voltage element is connected to the stretching member. When the transfer voltage applying unit applies a voltage to the transfer member, a current is supplied from the transfer voltage applying unit to the image carrier via the transfer member and the intermediate transfer member. The image forming apparatus according to claim 1.   The image forming apparatus according to claim 1, wherein the constant voltage element is a Zener diode.   The image forming apparatus according to claim 1, wherein the constant voltage element is a varistor.

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