JP2022082352A - Image forming device, control method of image forming device, and program - Google Patents

Abstract

Kind Code: A1 In a conventional method for detecting discharge start voltage and film thickness of a photoreceptor in a DC charging method using Paschen discharge, a large error occurs according to changes in the pre-charging potential of the photoreceptor drum due to environmental changes and endurance fluctuations. There was a problem. In the present invention, in a Vth detection sequence, by acquiring two or more sets of applied voltage Vc(n) during charging and discharging current Id(n), discharge start voltage Vth and dielectric layer of photosensitive drum are detected. Calculate the thickness d. As a result, it is possible to provide an image forming apparatus capable of obtaining a more appropriate voltage applied to the photosensitive drum and a more accurate life of the photosensitive member. [Selection drawing] Fig. 5

Description

The present invention relates to an image forming apparatus such as a copier, a printer, and a facsimile apparatus using an electrophotographic method or an electrostatic recording method, a control method of the image forming apparatus, and a program.

Conventionally, in an image forming apparatus using, for example, an electrophotographic method, as a method of charging a photoconductor as an image carrier, the photoconductor is in contact with the photoconductor or in a non-contact proximity with a very narrow gap (several tens to several hundreds of μm or less). There is a Paschen discharge method in which a voltage is applied to the charged member. As the charging member, a roller-shaped charging roller is often used. The charging roller has, for example, a configuration in which a conductive elastic body layer is provided on the outer periphery of the conductive support, and the surface of the conductive elastic body layer is covered with a conductive surface layer.

In the Paschen discharge method, the surface of the photoconductor is charged by a discharge (Paschen discharge) generated in a minute gap between the photoconductor and the charging member. The pasture discharge method includes an "AC charging method" in which a voltage obtained by superimposing a DC voltage and an AC voltage on a charging member is applied, and a "DC charging method" in which only a DC voltage is applied to the charging member. Since the DC charging method does not require an AC power supply, it is advantageous in terms of miniaturization, simplification of configuration, and cost reduction. However, unlike AC charging, the DC charging method does not have the potential convergence effect of the photosensitive drum, so if the same voltage is continuously applied to the charging member, the problem is that the drum surface potential gradually increases as the film thickness of the photoconductor decreases. There is. It is necessary to control the DC voltage applied to the charging member according to the number of prints in order to prevent the occurrence of problems such as fog due to the fluctuation of the drum surface potential.

As described above, in the DC charging method, since the change in the drum potential depends on the film thickness of the photoconductor, various means for detecting or predicting the film thickness of the photoconductor have been proposed. For example, Patent Document 1 proposes a method capable of detecting the film thickness of the photoconductor layer (dielectric layer) of the photosensitive drum as a method peculiar to the Paschen discharge method. Paschen discharge has the characteristic that the voltage applied to the charging member exceeds a certain threshold value (hereinafter referred to as "discharge start voltage"), the discharge starts, and a current flows. Since this discharge start voltage depends on the shared voltage of the capacitance component of the photoconductor, if the voltage applied to the charging member and the current value flowing through the photoconductor can be detected, the discharge start voltage and the photoconductor can be detected. It is explained that the film thickness can be obtained.
To summarize Patent Document 1, when the voltage applied to the charging member during the film thickness detection sequence during non-image formation is Vc1 and Vc2, and the DC current flowing from the charging member to the photoconductor is Id1 and Id2, the film of the photoconductor layer is formed. It can be expressed as thickness d = α · (Vc2-Vc1) / (Id2-Id1). Here, since α is a known fixed value determined by the permittivity of the photoconductor, the charge width of the photoconductor, the rotation speed of the photoconductor (so-called process speed), etc., it is possible to derive the film thickness d of the photoconductor. It is explained that.

Japanese Unexamined Patent Publication No. 5-223513

However, as a result of the study by the present inventor in accordance with Patent Document 1, it has been found that an error occurs in the value of the film thickness detection result of the photoconductor depending on the residual potential before charging (pre-charging potential) on the photosensitive drum. According to the study of the present inventor, since the precharge potential changes depending on various factors such as the temperature and humidity environment of the image forming apparatus and the exposure history of the photoconductor, it is difficult to completely predict the precharge potential. It was found that no fluctuation was included as the calculated value error of the film thickness of the photoconductor. It was also found that the value of the drum surface potential calculated from the film thickness of the photoconductor also contained an error.

Therefore, an object of the present invention is to obtain the discharge start voltage of the photoconductor with higher accuracy in a DC charging type image forming apparatus using Paschen discharge, without depending on the precharge potential of the photoconductor. ..

The present invention comprises a photosensitive member, a charging member that performs a charging process on the photosensitive member by a passage discharge, a voltage applying means for applying a DC voltage to the charging member, and a current flowing from the photosensitive member to the charging member. A current detecting means for measuring a DC current, an exposure means for exposing the photosensitive member to form a latent image, and a developing means for forming a toner image on the latent image formed on the photosensitive member. In the image forming apparatus having the transfer means for transferring the toner image to the transfer material, the voltage application means is the voltage at which the discharge from the charging member to the photoconductor starts in the first step. A first charging voltage and a second charging voltage exceeding the voltage are applied to the photoconductor in order to charge the photoconductor, and the voltage applying means further applies the discharge start voltage in the second step. A lower static elimination voltage is applied to the photoconductor in order to eliminate static electricity from the photoconductor, and the current detecting means is used in the second step after the first charging voltage is applied in the first step. The first DC current flowing from the photoconductor to the charging member in the portion to which the static elimination voltage is applied, and the static elimination in the second step after the second charging voltage is applied in the first step. The second DC current flowing from the photoconductor to the charging member in the portion to which the voltage is applied is measured, and the first set of the first charging voltage and the first DC current, and the second. It is characterized in that the discharge start voltage is calculated again based on the second set of the charging voltage and the second DC current.

According to the present invention, it is possible to provide an image forming apparatus capable of acquiring the discharge start voltage of the photoconductor with higher accuracy. This makes it possible to calculate a more appropriate voltage applied to the photoconductor and a more accurate life of the photoconductor.

Sectional view of image forming apparatus Cross-sectional view of the image forming part Control circuit that controls the image forming device Main flowchart of image formation operation Flowchart of Vth detection sequence (Example 1) Time transition of applied voltage Vc and drum surface potential Vd (Example 1) Time transition of DC current Id flowing from the photosensitive drum (Example 1) The figure explaining the method of obtaining the discharge start voltage Vth1. The figure explaining the error of the drum surface potential Vd / DC current Id Flowchart of Vth detection sequence (Example 2) Time transition of applied voltage Vc and drum surface potential Vd (Example 2) Time transition of DC current Id flowing from the photosensitive drum (Example 2) Flowchart of Vth detection sequence (Example 3) Time transition of applied voltage Vc and drum surface potential Vd (Example 3) Time transition of DC current Id flowing from the photosensitive drum (Example 3) Partially enlarged view of FIG.

Hereinafter, examples for carrying out the present invention will be described with reference to the drawings.
However, the examples described below are merely examples, and are not intended to limit the scope of the present invention to them. Also, not all combinations of features described in the following examples are essential to the solution of the present invention.

(Explanation of each part of the image forming apparatus)
<Example 1>
1-1. Overall Configuration and Operation of the Image Forming Device FIG. 1 is a schematic cross-sectional view of the image forming apparatus 100 of the present embodiment. The image forming apparatus 100 is a multifunction device having the functions of a tandem type (in-line method) copying machine, a printer, and a facsimile machine that employ an intermediate transfer method and can form a full-color image using an electrophotographic method. .. The image forming apparatus 100 employs a contact charging method, particularly a DC charging method, and can form an image on a transfer material P having a maximum size of A3.
The image forming apparatus 100 forms a plurality of first, second, third, and fourth image forming units that form yellow (Y), magenta (M), cyan (C), and black (K) images, respectively. It has SY, SM, SC, and SK. In the following, the reference numerals (Y, M, C,) indicating that the elements having the same or corresponding functions or configurations in each image forming unit SY, SM, SC, SK are elements for any of the colors. K) may be omitted for general explanation.

FIG. 2 is a schematic cross-sectional view showing one image forming unit S. In this embodiment, the image forming unit S is composed of a photosensitive drum 1, a charging roller 2, an exposure device 3, a developing device 4, a primary transfer roller 5, a drum cleaning device 6, and the like, which will be described later.
The image forming apparatus 100 has a photosensitive drum 1 which is a rotatable drum-shaped (cylindrical) photoconductor as an image carrier. The photosensitive drum 1 is rotationally driven at a predetermined peripheral speed (process speed) in the direction of arrow R1 in the figure by a drive motor (not shown) as a drive means. In this embodiment, the photosensitive drum 1 is a negatively charged drum-shaped organic photosensitive member, and a photosensitive layer (OPC layer) is formed on a substrate made of a conductive material such as aluminum.
The surface of the rotating photosensitive drum 1 is uniformly charged to a predetermined potential having a predetermined polarity (negative electrode property in this embodiment) by a charging roller 2, which is a roller-type charging member as a charging means. During the charging step, a DC voltage (also referred to as charging bias or charging voltage) Vc consisting of only a DC component is applied to the charging roller 2 from the high voltage power supply E1 which is a voltage applying means. In the charging treatment on the surface of the photosensitive drum 1, at least one of the photosensitive drum 1 and the charging roller 2 on the upstream side or the downstream side in the rotation direction of the photosensitive drum 1 with respect to the contact portion N between the photosensitive drum 1 and the charging roller 2. It is carried out by the Paschen discharge generated in the minute gap between them. The surface of the charged photosensitive drum 1 is scanned and exposed by an exposure device 3 as an exposure means (electrostatic image forming means), and an electrostatic image (electrostatic latent image) is formed on the photosensitive drum 1. In this embodiment, the exposure apparatus 3 is a laser beam scanner using a semiconductor laser.

The electrostatic image formed on the photosensitive drum 1 is developed (visualized) by a developing device 4 as a developing means using a developing agent, and a toner image is formed on the photosensitive drum 1. In this embodiment, the charging polarity of the photosensitive drum 1 (negative electrode property in this embodiment) is applied to the exposed portion on the photosensitive drum 1 in which the absolute value of the potential is lowered by being exposed after being uniformly charged. Toner charged with the same polarity adheres. That is, in this embodiment, the normal charging polarity of the toner, which is the charging polarity of the toner at the time of development, is the negative electrode property.
In this embodiment, the developing apparatus 4 uses a two-component developer having a toner (non-magnetic toner particles) and a carrier (magnetic carrier particles) as a developing agent. The developing apparatus 4 is a developing container 4a for accommodating the developing agent 4e, and a non-magnetic hollow cylindrical member rotatably provided in the developing container 4a so that a part of the developing container 4a is exposed to the outside from the opening of the developing container 4a. It has a developed sleeve 4b formed. Inside the developing sleeve 4b (hollow portion), a magnet roller 4c is fixedly arranged with respect to the developing container 4a. Further, the developing container 4a is provided with a regulating blade 4d so as to face the developing sleeve 4b. Further, two stirring members (stirring screws) 4f are provided in the developing container 4a. Toner is appropriately replenished from the toner hopper 4g to the developing container 4a.

The amount of the developer 4e supported on the developing sleeve 4b by the magnetic force of the magnet roller 4c is regulated by the regulating blade 4d as the developing sleeve 4b rotates, and then on the facing portion (developing portion) with the photosensitive drum 1. Be transported. The developer 4e on the developing sleeve 4b conveyed to the developing unit stands up by the magnetic force of the magnet roller 4c to form a magnetic brush (magnetic spike), and is brought into contact with or close to the surface of the photosensitive drum 1. Further, during the development process, the development sleeve 4b receives a vibration voltage in which a DC voltage (DC component) and an AC voltage (AC component) are superimposed as a development voltage (development bias) from the development power supply (high voltage power supply circuit) E2. Applied. As a result, the toner moves from the magnetic brush on the developing sleeve 4b onto the photosensitive drum 1 according to the electrostatic image on the photosensitive drum 1, and the toner image is formed on the photosensitive drum 1.

In this embodiment, the surface potential (dark area potential) of the photosensitive drum 1 formed by being charged by the charging roller 2 is −700 V, and the surface potential (bright area potential) of the photosensitive drum 1 formed by being exposed by the exposure apparatus 3 is −700 V. ) Is -300V, and the charging amount and the exposure amount are adjusted. Further, in this embodiment, the DC component of the developing voltage is set to −550V. Further, in this embodiment, the process speed of the photosensitive drum 1 is 120 mm / sec, and the charge processing width of the photosensitive drum 1 on the photosensitive drum 1 in the rotation axis direction is 320 mm. Further, in this embodiment, the charge amount of the toner is set to about −40 μC / g, and the amount of toner on the photosensitive drum 1 of the solid image portion is set to about 0.4 mg / cm ² .

An intermediate transfer belt 7 composed of an endless belt as an intermediate transfer body is arranged so as to face each of the photosensitive drums 1Y, 1M, 1C, and 1K. The intermediate transfer belt 7 is hung on a drive roller 71, a tension roller 72, and a secondary transfer facing roller 73 as a plurality of tension rollers, and is tensioned with a predetermined tension. The intermediate transfer belt 7 is rotationally driven by the drive roller 71 to rotate (rotate) in the direction of arrow R2 in the figure at a peripheral speed substantially equal to the peripheral speed of the photosensitive drum 1.
On the inner peripheral surface side of the intermediate transfer belt 7, the primary transfer rollers 5Y, 5M, 5C, and 5K, which are roller-type primary transfer members as the primary transfer means, correspond to the photosensitive drums 1Y, 1M, 1C, and 1K. Is placed. The primary transfer roller 5 is pressed toward the photosensitive drum 1 via the intermediate transfer belt 7 to form a primary transfer portion (primary transfer nip) T1 in which the photosensitive drum 1 and the intermediate transfer belt 7 come into contact with each other.

The toner image formed on the photosensitive drum 1 as described above is primarily transferred onto the intermediate transfer belt 7 by the action of the primary transfer roller 5 in the primary transfer unit T1. During the primary transfer step, a primary transfer voltage (primary transfer bias), which is a DC voltage opposite to the normal charging polarity of the toner, is applied to the primary transfer roller 5 from the primary transfer power supply (high voltage power supply circuit) E3. .. For example, at the time of forming a full-color image, the toner images of each color of yellow, magenta, cyan, and black formed on each photosensitive drum 1 are sequentially transferred so as to be superimposed on the intermediate transfer belt 7.

On the outer peripheral surface side of the intermediate transfer belt 7, a secondary transfer roller 8 which is a roller type secondary transfer member as a secondary transfer means is arranged at a position facing the secondary transfer facing roller 73. The secondary transfer roller 8 is pressed toward the secondary transfer facing roller 73 via the intermediate transfer belt 7, and the secondary transfer portion (secondary transfer nip) in which the intermediate transfer belt 7 and the secondary transfer roller 8 come into contact with each other. Form T2.
The toner image formed on the intermediate transfer belt 7 as described above is sandwiched and conveyed between the intermediate transfer belt 7 and the secondary transfer roller 8 by the action of the secondary transfer roller 8 in the secondary transfer unit T2. It is secondarily transferred to a transfer material (sheet, recording material) P such as recording paper. During the secondary transfer step, the secondary transfer roller 8 receives a secondary transfer voltage (secondary transfer bias) from the secondary transfer power supply (high voltage power supply circuit) E4, which is a DC voltage opposite to the normal charging polarity of the toner. ) Is applied.
The transfer material P is sent out one by one by a feeding device (not shown) and conveyed to the resist roller pair 9, and the resist roller pair 9 matches the timing with the toner image on the intermediate transfer belt 7 to the secondary transfer unit T2. Is supplied. Further, the transfer material P to which the toner image is transferred is conveyed to the fixing device 10 as a fixing means, and the toner image is fixed (melted and fixed) by being heated and pressed by the fixing device 10. After that, the transfer material P is output to the outside of the image forming apparatus 100.

On the other hand, the toner remaining on the photosensitive drum 1 during the primary transfer (primary transfer residual toner) is removed from the photosensitive drum 1 by the drum cleaning device 6 as a photoconductor cleaning means and recovered. The drum cleaning device 6 has a cleaning blade 6a as a cleaning member and a cleaning container 6b. The drum cleaning device 6 rubs the surface of the rotating photosensitive drum 1 with a cleaning blade 6a arranged in contact with the photosensitive drum 1. As a result, the primary transfer residual toner on the photosensitive drum 1 is scraped off from the photosensitive drum 1 and stored in the cleaning container 6b.
Further, on the outer peripheral surface side of the intermediate transfer belt 7, a belt cleaning device 74 as an intermediate transfer body cleaning means is arranged at a position facing the drive roller 71. The toner remaining on the intermediate transfer belt 7 during the secondary transfer step (secondary transfer residual toner) is removed from the intermediate transfer belt 7 by the belt cleaning device 74 and recovered.
In this embodiment, in each image forming unit SY, SM, SC, SK, the photosensitive drum 1, the charging roller 2, and the drum cleaning device 6 are integrally detachable from the image forming device 100. (Drum CRG) 110 constitutes.

Here, with reference to FIG. 3, an overall view of a circuit for controlling the image forming apparatus 100, including a current detecting circuit 103 which is a means for detecting a current flowing from the charging roller 2 to the photosensitive drum 1, will be described.
The CPU 101 controls the entire image forming apparatus 100, and is electrically connected to the internal memory 102, the high-voltage power supply E1, the current detection circuit 103, and the external memory 104. The external memory 104 is a non-volatile memory held in the drum CRG 110, and the drum CRG 110 itself has a function of holding information about the photosensitive drum 1 and the charging roller 2 independently of the image forming apparatus 100.
The high-voltage power supply E1 and the charging roller 2 are electrically connected by a feeding contact (not shown). The current detection circuit 103 is composed of a reference resistor and an operational amplifier (not shown), detects a direct current (DC current) flowing from the high voltage power supply E1 to the charging rollers 2Y, 2M, 2C, and 2K of each image forming unit, and determines the value. It is converted into an electric signal and transferred to the CPU 101.

1-2. Outline of Problems and Solutions Next, problems in the prior art that employs the DC charging method will be described.
Since the life of the drum cartridge often depends on the film thickness of the photoconductor, various means for detecting or predicting the film thickness of the photoconductor have been proposed. Among these, as a method peculiar to the Paschen discharge method, a method capable of detecting the film thickness of the photoconductor is known as disclosed in Patent Document 1. In Paschen discharge, there is a characteristic that the discharge starts after the voltage applied to the charging member exceeds a certain threshold value (the above-mentioned "discharge start voltage"), and a current flows. Since this discharge start voltage depends on the shared voltage of the capacitance component of the photoconductor, if the voltage applied to the charging member and the current value flowing through the photoconductor can be detected, the capacitance of the photoconductor can be determined. It is explained that the film thickness of the photoconductor can be finally obtained.

More specifically, when the voltage applied to the charging roller 2 is Vc, the surface potential of the photosensitive drum 1 (drum surface potential) is Vd, and the discharge start voltage is Vth, the following relational expression is obtained.

Next, the thickness of the air layer between the photosensitive drum 1 and the charging roller 2 is z [μm], the thickness of the dielectric layer of the photosensitive drum 1 is d [μm], the relative permittivity of the photosensitive drum 1 is k, and the photosensitive drum 1 is used. Assuming that the shared voltage applied to the air layer between the charging roller 2 and the charging roller 2 is Vgap [V], the following relational expression is obtained.

Further, from the empirical formula of Townsend, the spark starting voltage Vs in the atmosphere of one atmosphere is as shown in the following formula (3).

Here, when Vgap = Vs, discharge starts from the charging roller 2 to the photosensitive drum 1, and the applied voltage Vc at that time is equal to the discharge start voltage Vth, so that equations (2) and (3) are used. Therefore, the discharge start voltage Vth is as shown in the following equation (4).

Solving equation (4) for z yields equation (5) below.

Since the thickness z of the air layer in the equation (5) has only one solution at the start of discharge, the following equation (6) is derived from the multiple solution condition b ² -4ac = 0 in the quadratic equation ax ² + bx + c = 0. )become that way.

Solving the equation (6) with respect to the discharge start voltage Vth gives the following equation (7).

Here, it was found that the relative permittivity k of the dielectric layer of the photosensitive drum 1 is a value determined by the material, and the value of k = 3 is appropriate for the organic photosensitive layer of the photosensitive drum 1 used in this embodiment. There is. That is, if the discharge start voltage Vth is known, the film thickness d of the dielectric layer of the photosensitive drum 1 can be obtained by using the equation (7).
Further, as shown in the equation (1), the discharge start voltage Vth can be obtained from the applied voltage Vc and the drum surface potential Vd, but as in this embodiment, the image forming apparatus does not have a drum surface electrometer. In, the drum surface potential Vd cannot be directly measured and obtained.

Therefore, the DC current flowing from the charging roller 2 to the photosensitive drum 1 is Id [μA], the process speed of the photosensitive drum 1 is Vp [mm / s], the charging width W [mm], and the relative permittivity of the dielectric layer is k. When the dielectric constant of the vacuum is ε0, the following equation (8) can be obtained by modifying the capacitance equation Q = CV.

If two or more sets of the combination of the voltage Vc applied to the charging roller 2 and the DC current Id at that time can be obtained from the equations (1) and (8), the film thickness d of the photosensitive drum 1 which is two unknowns can be obtained. It is explained that it is theoretically possible to obtain the discharge start voltage Vth.

However, as examined by the present inventor, even when according to Patent Document 1, the value of the film thickness detection result of the drum may differ depending on the residual potential before charging on the photoconductor in the DC charging method. found.
That is, when the potential before charging of the photosensitive drum 1 is V0 and the potential after charging is Vd, the correct relational expression of the equation (8) is as follows.

である。すなわち、式（１）に示されるように、ドラム表面電位Ｖｄは帯電前電位Ｖ０によらず印加電圧Ｖｃと放電開始電圧Ｖｔｈによって決定されるのに対し、その時に流れる電流Ｉｄは帯電前電位Ｖ０に依存するため、Ｉ－Ｖ特性にズレが生じるのである。
そこで、式（１）と式（９）を感光体の膜厚ｄに対して解くと、下記の式（１０）のようになる。

Is. That is, as shown in the equation (1), the drum surface potential Vd is determined by the applied voltage Vc and the discharge start voltage Vth regardless of the precharge potential V0, whereas the current Id flowing at that time is the precharge potential V0. Because it depends on the voltage, the IV characteristics are deviated.
Therefore, when the equations (1) and (9) are solved with respect to the film thickness d of the photoconductor, the following equation (10) is obtained.

FIG. 9 is a graph showing this in an easy-to-understand manner. In FIG. 9, the vertical axis is the value obtained by dividing the drum surface potential Vd by the DC current Id flowing through the charging roller 2 when the drum surface potential Vd = −700 V, and the horizontal axis is the dielectric layer of the photosensitive drum 1. Shows the thickness of. The process speed of the photoconductor is 120 mm / sec, the width of the charger is 320 mm, and the relative permittivity of the dielectric of the photoconductor is 3. The result when the pre-charging potential V0 is 0V is shown by a solid line, and the result when the pre-charging potential V0 is −100V is shown by a broken line.
When the precharge potential V0 fluctuates between 0V and −100V due to environmental fluctuations and durability fluctuations, the film thickness d also fluctuates between the solid line and the broken line in FIG. In the example of FIG. 9, it can be seen that the detection error is about 14%. According to the results of the study by the present inventor, the precharge potential V0 changes depending on various factors such as the temperature and humidity environment of the image forming apparatus and the exposure history of the photoconductor, so that it is difficult to completely predict it, and the fluctuation of the precharge potential V0 It was found that the film thickness d contained an error by the amount. Further, since the discharge start voltage Vth also includes an error, it was found that the value of the drum surface potential Vd calculated from the error also includes an error.

Therefore, in this embodiment, in the DC charging method using Paschen discharge, the applied voltage required to obtain a desired drum surface potential without depending on the fluctuation of the residual potential (pre-charging potential) V0 of the photoconductor. , It is an object of the present invention to provide a method capable of obtaining a more accurate film thickness of a photoconductor.
Specifically, at the time of non-image formation, a voltage Vc (more specifically, Vc> 2Vth) exceeding the discharge start voltage Vth is applied to the charging roller 2 to exceed the discharge start voltage Vth on the photosensitive drum 1. A charging process is performed to form the drum surface potential Vd. Next, the drum surface potential Vd formed on the photosensitive drum 1 is statically eliminated by the charging roller 2, and the DC current Id flowing from the photosensitive drum 1 to the charging roller 2 is detected at that time. In this way, a sequence for obtaining a plurality of combinations of the applied voltage Vc at the time of charging and the DC current Id at the time of static elimination (hereinafter, referred to as “Vth detection sequence”) is performed. As a result, the discharge start voltage Vth and the film thickness d of the photosensitive drum 1 can be obtained independently of the change in the residual potential (pre-charging potential) of the photosensitive drum 1. This principle will be explained below using several equations.

The Vth detection sequence is performed at the time of non-image formation, and is composed of a charging process by the charging roller 2 and a static elimination process as one set. Then, in the charging step, different high voltages are applied up to N times. Here, assuming that the voltage applied to the charging roller 2 in the nth charging step is Vc (n), the following equation (11) is obtained from the equation (1).

However, Vc (n) is set to a value more than twice the temporary discharge start voltage Vth0.
Here, as for the temporary discharge start voltage Vth0, since the true discharge start voltage Vth1 is unknown in a state where the Vth detection sequence has never been performed, the tolerance fluctuation of the film thickness of the initial photosensitive drum 1 at the time of mass production is large. It is desirable to use the numerical value calculated by substituting the maximum value into the equation (7). After performing the Vth detection sequence even once, the true discharge start voltage Vth1 obtained in the previous Vth detection sequence is used as the temporary discharge start voltage Vth0.

In this embodiment, since the maximum value of the film thickness tolerance of the mass-produced product of the photosensitive drum 1 is 23 μm, the temporary discharge start voltage Vth0 is set to about 620V as a result of calculation by the equation (7). Further, since the DC current Id at the time of static elimination increases as the drum surface potential increases, it is preferable that the drum surface potential is high within the range of high voltage output for stable measurement, and Vc (n) is at least Vth0. 2.1 times or more is preferable. Therefore, in this embodiment, Vc (1) = -1300V and Vc (2) = -1400V are held in the memory. The data of Vc (n) may be held in the internal memory 102 of the image forming apparatus 100, or may be held in the external memory 104 provided in the drum CRG 110.

If you want to increase the number of measurement points to further improve the accuracy, for example, Vc (1) = -1300V, Vc (2) = -1400V, Vc (3) = -1500V, and so on. The voltage data of is stored in the memory. Further, in addition to the method of directly holding Vc (n) in the memory, a means such as calculating Vc (n) each time using the true discharge start voltage Vth1 obtained after the first Vth detection sequence is used. You can also take it. In this case, for example, the temporary discharge start voltage Vth0 is held in the external memory 104, and N coefficients β (2.1, 2.2, ..., Etc.) exceeding 2 are held in the internal memory 102. Then, it may be calculated by Vc (n) = β × Vth0 at the next Vth detection sequence. The advantage of this means is that the capacity of the external memory 104 can be mainly reduced.

In this embodiment, the voltage applied to the charging roller 2 after the charging bias of Vc (1) is applied to the charging roller 2 and then the photosensitive drum 1 makes at least one rotation (after the charging step of the Vth detection sequence is completed). Is set to Vdis (n), which is a value lower than the discharge start voltage. As a result, the drum surface potential Vd is statically eliminated by the charging roller 2 and converges to Vth + Vdis (n) (the Vth detection sequence ends).
Here, assuming that the DC current (static elimination current) flowing from the photosensitive drum 1 to the charging roller 2 during the static elimination step of the Vth sequence is Id (n), it can be expressed as the following equation (12).

This means that the drum surface potential Vd of the photosensitive drum 1 converges to the discharge start voltage Vth + Vdis (n) by static elimination, and the statically eliminated drum surface potential Vd is converted into a current. Further, since the charging high voltage has a negative polarity, a negative current flows as a DC current Id during the charging process, but a positive current apparently flows from the photosensitive drum 1 to the charging roller 2 during the static elimination process. It flows.

In principle, the potential Vdis (n) (also referred to as static elimination voltage) applied to the charging roller 2 in the static elimination step can take either a positive or negative value. However, in order to apply the opposite (same, plus side) voltage of the normal potential of the photosensitive drum 1 (in the case of this embodiment, the minus side), it is necessary to make the high voltage power supply of the charging roller 2 compatible with both positive and negative poles. Therefore, the cost increases, and a history called a drum memory may be generated in the photosensitive drum 1. Therefore, it is basically not preferable to apply a reverse voltage to the photosensitive drum 1. Further, when a negative voltage is applied, it becomes difficult to eliminate the potential of the photosensitive drum 1, which is also not very preferable. Therefore, in the present embodiment, the static elimination voltage applied to the charging roller 2 will be 0V for static elimination of the photosensitive drum 1, and Vdis (n) = 0V will be described.

When the equation (11) and the equation (12) are combined and Vdis (n) = 0 is substituted and arranged, the discharge start voltage Vth can be expressed as follows.

Here, k, ε0, Vp, and W in the equation (13) are known values, and Id (n) can be obtained by performing a Vth detection sequence using a predetermined set value Vc (n). Therefore, there are two complete unknowns, Vth and d. Therefore, if at least two sets of Vc (n) and Id (n) are obtained, the remaining two unknowns, the discharge start voltage Vth and the film thickness d of the photosensitive drum 1, can be obtained. When there are three or more sets of Vc (n) and Id (n), the discharge start voltage Vth and the film thickness d of the photosensitive drum 1 with less measurement error can be obtained by the least squares method described later. ..

1-3. Vth detection sequence of the image forming apparatus Next, the Vth detection sequence of this embodiment will be described with reference to FIGS. 3 to 7.
First, the main flowchart of the image forming operation shown in FIG. 4 will be described. The process shown in the flowchart of FIG. 4 is realized by the CPU 101 executing a program stored in the internal memory 102.

First, in S101, the CPU 101 executes a normal image forming operation.
At the time of non-image formation after the normal image formation operation is completed, in S102, the CPU 101 determines whether or not to enter the Vth detection sequence. The criterion for determining whether or not to enter the Vth detection sequence can be set by using a combination of various conditions such as an integrated count value of the number of prints and the installation environment of the image forming apparatus. This is because the film thickness of the photosensitive drum 1 fluctuates little by little during the printing operation over a long period of time, so by performing the Vth detection sequence too frequently, the waiting time does not increase and usability is not impaired. be. In this embodiment, the Vth detection sequence is entered at the end of the first print job immediately after the power supply operation or at the end of the print job for every 1,000 A4 prints.
If it is determined that the Vth detection sequence is entered, the process proceeds to S103. If it is determined that the Vth detection sequence is not entered, the process proceeds to S107.

In S103, the CPU 101 ends the primary transfer operation by the primary transfer unit T1 of the image forming apparatus 100 and turns off the primary transfer bias before entering the Vth detection sequence. Further, the exposure apparatus 3 which is a static elimination means for the photosensitive drum 1 is also turned off before entering the Vth detection sequence.
The static elimination process for the photosensitive drum 1 other than the charging roller 2 in the Vth detection sequence causes an error in the Vth detection sequence. Therefore, it is desirable to set the timing so that the portion of the photosensitive drum 1 that has undergone static elimination finally enters the Vth detection sequence after passing through the charging roller 2. Further, even if the primary transfer bias is turned off, the primary transfer unit T1 may be configured to eliminate static electricity from the photosensitive drum 1. In this case, a means for separating the primary transfer unit from the photosensitive drum 1 or a range in which the primary transfer unit does not discharge the photosensitive drum 1 (the voltage difference between the drum surface potential of the photosensitive drum 1 and the primary transfer bias is less than Vth). ), It is desirable to take measures such as applying a voltage having the same polarity as the charging roller 2.

Next, the Vth detection sequence is entered in S104. The details of the Vth detection sequence will be described with reference to the flowchart of FIG. Further, FIG. 6 shows the time transition of the voltage Vc applied to the charging roller 2 and the surface potential (drum surface potential) Vd of the photosensitive drum 1 in the Vth detection sequence of this embodiment. Further, FIG. 7 shows the time transition of the DC current Id flowing from the photosensitive drum 1 to the charging roller 2.
Moving to the flowchart of FIG. 5, in S111, the CPU 101 reads the maximum number of measurements N from the memory. In this embodiment, N = 2 is set, but N = 3 or more may be set in order to obtain more accuracy. Further, the memory described in this embodiment may be the internal memory 102 of the image forming apparatus connected to the CPU 101, or the external memory 104 attached to the drum CRG 110.

Next, in S112, the CPU 101 sets the current number of measurements n to 1. Hereinafter, in this embodiment, one set of the charging step and the static elimination step in the Vth detection sequence is referred to as one step. Hereinafter, the charging step in the n steps is referred to as n-1, and the static elimination step in the n steps is referred to as n-2.

Next, in S113, the CPU 101 starts the charging step of step n-1. Since the first step is n = 1, it is the charging step of step 1-1. When n = 2, the charging step is step 2-1.
At n = 1, the CPU 101 first reads Vc (1) = -1300V, which is a value more than twice the temporary discharge start voltage Vth0 = −620V, from the memory (internal memory 102 or external memory 104).

Next, in S114, the CPU 101 applies the charging bias Vc (1) = -1300V to the charging roller 2 for one rotation of the photosensitive drum 1. As a result, the drum surface potential of the photosensitive drum 1 converges to around Vd = −720V. (However, the image forming apparatus 100 of this embodiment cannot detect the drum surface potential Vd itself.)
The photosensitive drum 1 to be the target of the Vth detection sequence may be all photosensitive drums of YMCK, or may be only the photosensitive drum 1K in the Bk mode. The operating time of step n-1 is one rotation of the photosensitive drum 1 (0.785 sec in this embodiment), but it is photosensitive depending on the situation of the image forming apparatus 100, for example, to stabilize the potential. The drum 1 may be rotated a plurality of times.

After one rotation of the photosensitive drum 1 by the charging step of S114, the CPU 101 starts the static elimination step of step n-2 in S115. Since the first step is n = 1, it is step 1-2, and the voltage applied to the charging roller 2 is changed to Vdis (1) = 0V (in this embodiment, Vdis (n) = always. Set to 0V).
The drum surface potential Vd of the photosensitive drum 1 charged to -720V by the applied voltage Vc (1) = -1300V in the step 1-1 is statically eliminated after passing through the charging roller 2 which is 0V. It converged to around 570 to 580V. (However, as described above, the image forming apparatus 100 of this embodiment cannot detect the converged drum surface potential Vd itself.)

During the operation of the static elimination step of S115 (in this embodiment, during 0.785 sec in which the photosensitive drum 1 rotates once), in S116, the CPU 101 causes the DC current flowing from the photosensitive drum 1 to the charging roller 2 by the current detection circuit 103. Id (n) is measured. Then, the measured DC current Id (n) is averaged and stored in the memory.
In step 1-2 of this embodiment, the average value of the DC current detected by the current detection circuit 103 was Id (1) = 7.2 μA.

After one rotation of the photosensitive drum 1 by the static elimination step of S116, in S117, the CPU 101 compares the current number of measurements n with the maximum number of measurements N.
When n ≧ N, the process proceeds to S118.
If n <N, the process proceeds to S119 and 1 is added to n. After that, the process proceeds to S113 again, and the same sequence is performed (for example, when n = 2, the charging step of step 2-1 and the static elimination step of step 2-2 are performed).

For example, in the charging step of step 2-1 the CPU 101 reads Vc (2) = -1400V from the memory and charges the photosensitive drum 1. Further, in the static elimination step of step 2-2, the CPU 101 sets the voltage applied to the charging roller 2 to Vdis (2) = 0V, performs static elimination processing on the photosensitive drum 1, and is averaged by the current detection circuit 103. The DC current Id (2) is detected. In this example, the DC current Id (2) was 12.2 μA.
In this embodiment, since the maximum number of measurements N is set to 2, the loop from S113 to S117 ends here.

In S118, the CPU 101 again calculates the true discharge start voltage Vth1 and the film thickness d of the photosensitive drum 1 from Vc (1) and Id (1), and Vc (2) and Id (2) as follows. Ask for.
From equation (13), the true discharge start voltage Vth1 is the value of the y-intercept at x = 0 from two points on the xy coordinates with Id as the x-axis and Vc as the y-axis. It becomes as shown in 14).

Here, FIG. 8 shows a graph showing in an easy-to-understand manner how to obtain the discharge start voltage Vth1 from Vc (1) and Id (1), and Vc (2) and Id (2). In the graph of FIG. 8, the vertical axis represents the applied voltage Vc (n) and the horizontal axis represents the DC current Id. The process speed of the photosensitive drum 1 is 120 mm / sec, and the charging width is 320 mm.
As described above, when the drum surface potential Vd of the photosensitive drum 1 charged with Vc (1) = -1300V in the first charging step is statically eliminated by the 0V charging roller 2 in the first static elimination step. The detected current was Id (1) = 7.2 μA. Further, the detection current when the drum surface potential Vd of the photosensitive drum 1 charged with Vc (2) = -1400V in the second charging step is statically eliminated by the 0V charging roller 2 in the second static elimination step is Id (). 2) = 12.2 μA.

From the equation (14), since the true discharge start voltage Vth1 is equal to the DC current Id of 0 μA, that is, one half of the voltage when the static elimination is stopped, the value of the y-intercept of the broken line in FIG. 8 is set to 2. It can be seen that the divided value, that is, 578V, is the true discharge starting voltage Vth1 of this photoconductor. Since the surface potential of the photosensitive drum 1 does not intervene in FIG. 8 and the formula (14), it is not necessary to measure the surface potential Vd of the photosensitive drum 1. Further, since the pre-charging potential V0 does not intervene, it can be seen that the true discharge start voltage Vth1 of the photosensitive drum 1 can be derived without including the error of the detection current due to the pre-charging potential V0.

Further, when the equation (7) is solved with respect to d, the following equation (15) is obtained.

By substituting the discharge start voltage Vth = -578V and k = 3, which is the relative permittivity of the dielectric of the photoconductor, into this equation (15), the film thickness d = 19.8 [μm] of the photosensitive drum 1. I was able to get.
Since the actual thickness of the photosensitive drum 1 used in the experiment in this example was 20.1 μm on average in length, it was proved that the film thickness of the photosensitive drum 1 could be obtained with an error of about 1.5%. .. This is more accurate because it is an order of magnitude smaller than the film thickness error of about 14% when the precharge potential V0 fluctuates from 0 to 100 V as shown in FIG. It was confirmed that a film thickness detection method can be provided.

Further, the current detection error in this embodiment is mainly the current noise of the current detection circuit 103. Therefore, regarding the method of obtaining the discharge start voltage Vth when the maximum number of measurements N of the Vth detection sequence exceeds 2, the minimum approximation method is used in order to reduce the error in the electric circuit such as the current detection noise. Can be done. Specifically, for n data (x1, y1), (x2, y2), ... (Xn, yn), xn = Id (n), yn = Vc by the minimum approximation method y = ax + b. Substituting (n) to obtain b as in equation (16).

Then, by obtaining the discharge start voltage Vth with Vth = b / 2, the detection error can be further reduced.
When the true discharge start voltage Vth1 and the film thickness d of the photosensitive drum 1 are calculated in S118, the Vth detection sequence is terminated and the process returns to the main sequence of FIG.

Returning to the flowchart of FIG. 4, in S105, the CPU 101 stores the calculated true discharge start voltage Vth1 in the memory (internal memory 102 or external memory 104), and high voltage is applied to the charging roller 2 at the time of the next normal image formation. Feedback to control. This makes it possible to form an image of the photosensitive drum 1 at a more appropriate potential from the time of the next normal image formation. Further, by storing the true discharge start voltage Vth1 in the external memory 104 of the drum CRG110, the correct charging bias can be applied even if the drum CRG110 is attached or detached.

Next, in S106, the CPU 101 reflects the film thickness d calculated in the Vth detection sequence on the display of the drum life of the image forming apparatus 100. In this embodiment, when the film thickness d of the photosensitive drum 1 is reduced by 10 μm from the initial value, the life is displayed as 0% on the display (not shown) provided in the image forming apparatus 100. Therefore, it is displayed as 100% in the initial state, and the numerical value of the drum life is reduced and displayed in 1% units for each change of −0.1 μm. As a result, the operator or user of the image forming apparatus 100 can know more accurately when to replace the drum CRG 110.
After displaying the drum life in S106, in S107, the CPU 101 executes the remaining post-rotation operation (density adjustment, etc.) of the image forming apparatus 100. This ends the main sequence of FIG.

As described above, in the first embodiment, in the Vth detection sequence, the charging step and the static elimination step of the photosensitive drum are repeatedly performed a plurality of times, and the applied voltage Vc (n) at the time of charging and the current Id (n) at the time of static elimination are obtained. Get multiple sets of combinations. As a result, a more accurate discharge start voltage Vth can be obtained, and correspondingly, the applied voltage at the time of image formation can be set more appropriately. Further, it is possible to obtain a more accurate thickness d of the dielectric layer of the photosensitive drum, and to provide an image forming apparatus capable of displaying a more accurate drum life and a drum CRG replacement time.

<Example 2>
2-1. Overall configuration and operation of the image forming apparatus Since the overall configuration and operation of the image forming apparatus 100 is the same as that of the first embodiment, the description thereof will be omitted.
2-2. Outline of Problem and Solution Means Since the problem and outline are the same as in Example 1, the Vth detection sequence, which is a difference from Example 1, will be mainly described below.

In the second embodiment, in the charging step, the voltage applied to the photosensitive drum 1 during one rotation of the photosensitive drum 1 is changed from the applied voltage Vc (1) to Vc (n), which is more than twice the temporary discharge start voltage Vth0. The charging process is performed by gradually changing the voltage. In Example 2, Vth is obtained by two-step steps Vc (1) and Vc (2), but as in Example 1, if it is desired to increase the number of measurement points and further improve the accuracy, Vc (3). , Vc (4), ... may be increased.

Next, in the static elimination step, the photosensitive drum 1 is statically eliminated by the charging roller 2 with an applied voltage Vdis (n) having a temporary discharge start voltage of Vth0 or less while the photosensitive drum 1 makes the next rotation. Then, while the photosensitive drum 1 makes one rotation, the static elimination current Id (1) flowing from the portion to which the Vc (1) of the photosensitive drum 1 is applied to the charging roller 2 and the Vc (2) of the photosensitive drum 1 are applied. The static elimination current Id (2) flowing from the charged portion to the charging roller 2 is obtained. (Note that the voltage Vdis (n) applied to the charging roller 2 in the static elimination step is 0 V, as in the first embodiment.)

As a result, regardless of the number of steps n in which the high voltage is applied to the photosensitive drum 1, the rotation speed of the photosensitive drum 1 in the Vth detection sequence is two rotations, one rotation in the charging process and another rotation in the static elimination process. It ends with. Therefore, it is possible to minimize the increase in downtime due to the Vth detection sequence and the decrease in film thickness due to the increase in rotation of the photosensitive drum 1.
Regarding the method of calculating the discharge start voltage Vth and the film thickness d of the photosensitive drum 1 using the Id (1), Id (2) and the applied voltages Vc (1) and Vc (2) detected by the above Vth detection sequence. , Since it is the same as Example 1, the explanation is omitted.

2-3. Vth detection sequence of the image forming apparatus The Vth detection sequence of the second embodiment will be described with reference to the flowchart of FIG. Further, FIG. 11 shows the time transition of the voltage Vc applied to the charging roller 2 and the drum surface potential Vd in the Vth detection sequence of the second embodiment. Further, FIG. 12 shows the time transition of the current value Id flowing from the photosensitive drum 1 to the charging roller 2. Since the main flowchart is the same as FIG. 4 described in the first embodiment, the description thereof will be omitted.

In S201, the CPU 101 reads the number of detection steps N from the memory. In this embodiment, N = 2 and a two-step charging high voltage setting is used.
Next, in S202, the CPU 101 starts the charging process. First, Vc (1) = -1300V and Vc (2) = -1400V, which are values more than twice the temporary discharge start voltage Vth0 = −620V, are read from the memory.
Next, in S203, the CPU 101 steps the high-voltage power supply E1 so as to apply the charging bias Vc (1) = -1300V and Vc (2) = -1400V to the charging roller 2 while the photosensitive drum 1 makes one rotation. Control.
Here, the photosensitive drum 1 to be the target of the Vth detection sequence may be all photosensitive drums of YMCK, or may be only the photosensitive drum 1K in the Bk mode. In this embodiment, the time for one rotation of the photosensitive drum 1 is 0.785 sec.

Next, in S204, the CPU 101 starts the static elimination step after the charging process for one rotation of the photosensitive drum 1 is completed. First, the voltage applied to the charging roller 2 is changed to Vdis (1) = 0 for the next rotation of the photosensitive drum 1.
During the operation of the static elimination step of S204, in S205, the CPU 101 measures the DC current Id (n) flowing from the photosensitive drum 1 to the charging roller 2 by the current detection circuit 103. Here, the static elimination current Id (1) is measured after one rotation of the photosensitive drum 1 that has been charged with the applied voltage Vc (1) (after 0.785 sec in this embodiment). Similarly, the measurement of Id (2) is performed after one rotation (also 0.785 sec) of the photosensitive drum 1 that has been charged with the applied voltage Vc (2).
The measured DC currents Id (1) and Id (2) are averaged and stored in a memory.
In this example, the DC current was Id (1) = 7.2 μA and Id (2) = 12.2 μA.

Next, in S206, the CPU 101 calculates the true discharge start voltage Vth1 and the film thickness d of the photosensitive drum 1 from Vc (1) and Id (1), and Vc (2) and Id (2). These calculation methods are the same as those described in S118 in Example 1.

As described above, in the second embodiment, the charging treatment is performed with the applied voltage Vc (n) in a plurality of stages during one rotation of the photosensitive drum 1, and after one rotation, the charging roller 2 is applied to each of the charging treatment surfaces. Detects the static elimination current Id (n) flowing through. Thereby, the thickness d of the dielectric layer of the photosensitive drum 1 and the discharge start voltage Vth can be obtained in a shorter operation time.

<Example 3>
3-1. Overall configuration and operation of the image forming apparatus Since the overall configuration and operation of the image forming apparatus are the same as those of the first and second embodiments, the description thereof will be omitted.
3-2. Outline of Problem and Solution Since the problem and outline are the same as those in Example 1 and Example 2, the Vth detection sequence, which is a difference from these Examples, will be mainly described.

The Vth detection sequence in Example 3 will be described.
Let Vc (t) be the voltage applied to the charging member at a predetermined timing t. First, as a charging step, while the photosensitive drum 1 makes one rotation, an applied voltage Vc (T1) that is more than twice the temporary discharge start voltage Vth0 is continuously applied to an applied voltage Vc (T2) that is less than twice Vth0. The photosensitive drum 1 is charged by changing it to a (slope shape).
Next, as a static elimination step, the photosensitive drum 1 is subjected to static elimination processing with an applied voltage Vdis (t) having a temporary discharge start voltage of Vth0 or less while the photosensitive drum 1 makes one rotation. Then, during the period of the static elimination step, the time T3 at which the DC current Id (t) flowing through the charging roller 2 becomes 0 (zero cross) is obtained (Id (T3) = 0). That is, for the current Id (t), which is a function of the time t, the time T3 at which the current Id (t) = 0 is obtained.

Here, the time required for one rotation of the photosensitive drum 1 is Tdr. From the equations (11) and (12), the true discharge start voltage Vth1 is the applied voltage Vc (T3-Tdr) when the surface of the photosensitive drum 1 is charged at the time (T3-Tdr) one rotation before the drum from the time T3. It is expressed by the following equation (17) using T3-Tdr).

In this embodiment, since the bias applied to the charging roller 2 during the static elimination step is always 0V (Vdis (t) = 0V), the true discharge start voltage Vth1 can be obtained by the following equation (18). ..

3-3. Vth detection sequence of the image forming apparatus The Vth detection sequence of the third embodiment will be described with reference to the flowchart of FIG. Further, FIG. 14 shows the time transition of the voltage Vc applied to the charging roller 2 and the drum surface potential Vd in the Vth detection sequence of the third embodiment. Further, FIG. 15 shows the time transition of the current value Id flowing from the photosensitive drum 1 to the charging roller 2, and FIG. 16 shows a partially enlarged view of FIG. Since the main flowchart is the same as FIG. 4 described in the first embodiment, the description thereof will be omitted.

In S301, the CPU 101 starts the charging process. First, the temporary discharge start voltage Vth0 = -600V from the memory, and the start value Vc (T1) = -1800V (= 3 × Vth0) and the end value Vc (T2) of the charge bias Vc applied by changing in a slope shape. = -900V (= 1.5 x Vth0) is read out.

Next, in S302, the CPU 101 controls the high-voltage power supply E1 so as to apply the charging bias Vc (t) = -1800V to -900V to the charging roller 2 while changing it in a slope shape during one rotation of the photosensitive drum 1. do.
The drum surface potential Vd of the photosensitive drum 1 was formed so as to change from -1200V to -300V as shown in FIG. 14 during one rotation of the photosensitive drum 1. (However, as described above, the image forming apparatus 100 of this embodiment cannot detect the drum surface potential Vd itself.) The photosensitive drum 1 targeted for the Vth detection sequence is all photosensitive with YMCK. It may be a drum, or in the case of Bk mode, only the photosensitive drum 1K may be used. The operating time of the charging process is one rotation of the photosensitive drum 1 (0.785 sec in this embodiment), but may be one rotation or less of the photosensitive drum 1.

Next, in S303, the CPU 101 starts the static elimination step. First, the voltage applied to the charging roller 2 is changed to Vdis = 0V. As a result, after passing through the charging roller 2, the drum surface potential Vd of the photosensitive drum 1 converges to about 570V to 580V in the portion charged to about 570V or higher, and the potential of the portion charged to less than that is almost the same. Met.
At the same time as the static elimination step, in S304, the CPU 101 applies the DC current Id (t) flowing from the photosensitive drum 1 to the charging roller 2 during one rotation of the photosensitive drum 1 (0.785 sec in this embodiment). It is detected by using the detection circuit 103 and stored in the internal memory 102.

Next, in S305, the CPU 101 obtains the true discharge start voltage Vth1 by obtaining the time T3 at which the DC current Id (t) crosses zero (see FIG. 15). As a means for that, the CPU 101 calculates an approximate straight line of Id (t) by the method of least squares (see FIG. 16). As the data range for linear approximation, a range in which Id (t) at the time of static elimination has a significant value is extracted. In this embodiment, from the time Ta'where the DC applied voltage Vc (t) becomes 2.9 times the temporary discharge start voltage Vth0 to the time Tb' where the DC applied voltage Vc (t) becomes 2.1 times, the time Tdr for one rotation of the photosensitive drum 1. The range from time Ta to Tb, which is the sum of (= drum outer circumference length ÷ process speed), was set as the analysis range. (That is, Vc (Ta') = 2.9 x Vth0, Vc (Tb') = 2.1 x Vth0).

From the minimum approximation method of the linear function (y = ax + b) to n data groups (t1, Id1), (t2, Id2), ... (Tn, Idn) ... In this range [Ta, Tb].
a = (nΣxy-Σx · Σy) / ( ^{nΣx2- (Σx) 2 )}
b = (Σx2 ・ Σy－Σxy ・ Σx) / ( ^{nΣx2- (Σx) 2 )}
Therefore, a and b can be obtained by substituting Xn = tun and Yn = Idn, and the time T3 at which the DC current value Id crosses zero can be obtained with 0 = at + b.

Next, in S306, the CPU 101 obtains the time T4 one rotation before the drum at the time T3 at which the zero crosses. In this embodiment, as shown in FIG. 16, T3 = 2.925s was determined, and since Tdr = 0.785, T4 = T3-Tdr = 2.13s was determined.

Next, in S307, the CPU 101 reads out the applied voltage Vc (T4) applied to the charging roller 2 at the time T4 from the memory. From the equation (13), the true discharge start voltage Vth1 is equal to 0 μA of the DC current Id, that is, one half of the voltage when the static elimination is stopped. From this, it can be seen that the applied voltage Vc (T4) at time T4 divided by 2, that is, in FIG. 14, -1156V / 2 = -578V is the true discharge start voltage Vth1 of this photoconductor. .. Since the processing after the true discharge start voltage Vth1 is obtained is the same as that in the first embodiment, the description thereof will be omitted.

After the end of the Vth detection sequence, the CPU 101 stores the true discharge start voltage Vth1 in the memory and feeds it back to the high voltage control to the charging roller 2 at the time of the next normal image formation. Further, the temporary discharge start voltage Vth0 stored in the memory is used for calculating the start value and end value of the applied voltage Vc at the Vth detection sequence and the Id, but at the time of the next and subsequent Vth detection sequences. The newly obtained Vth1 may be used instead of Vth0.

As described above, in the third embodiment, the photosensitive drum 1 is charged to a potential that changes in a slope shape, and then static elimination treatment is performed, and the combination of the applied voltage Vc (t) and the static elimination current Id (t) at the time of charging is performed. To get. As a result, the thickness d of the dielectric layer of the photosensitive drum 1 and the discharge start voltage Vth can be obtained with high accuracy in a shorter time.

<Other embodiments>
The present invention supplies a program that realizes one or more functions of the above-described embodiment to a system or device via a network or storage medium, and one or more processors in the computer of the system or device reads and executes the program. It can also be realized by the processing to be performed. It can also be realized by a circuit (for example, ASIC) that realizes one or more functions.
Further, the present invention may be applied to a system composed of a plurality of devices or a device composed of one device.
The present invention is not limited to the above-described embodiment, and various modifications can be made based on the gist of the present invention, and these are not excluded from the scope of the present invention. That is, all the configurations in which the above-mentioned configuration example and the modification thereof are combined are included in the present invention.

1 Photosensitive drum 2 Charging roller 3 Exposure device 100 Image forming device 101 CPU
103 Current detection circuit E1 High voltage power supply

Claims (14)

Photoreceptor and
A charging member that charges the photoconductor by Paschen discharge, and
A voltage applying means for applying a DC voltage to the charging member, and
A current detecting means for measuring a direct current flowing from the photoconductor to the charged member, and
An exposure means that exposes the photoconductor to form a latent image,
A developing means for forming a toner image with respect to a latent image formed on the photoconductor, and
A transfer means for transferring the toner image to the transfer material,
In an image forming apparatus having
In the first step, the voltage applying means applies a first charging voltage and a second charging voltage that exceed the discharge starting voltage, which is the voltage at which the charging member starts discharging to the photoconductor, to the photoconductor. Apply to the photoconductor to charge it,
Further, in the second step, the voltage applying means applies a static elimination voltage having a lower discharge start voltage to the photosensitive member in order to eliminate static electricity from the photosensitive member.
The current detecting means has a first flow from the photoconductor to the charging member in the portion where the static elimination voltage is applied in the second step after the first charging voltage is applied in the first step. A direct current and a second direct current flowing from the photoconductor to the charging member in the portion where the static elimination voltage is applied in the second step after the second charging voltage is applied in the first step. Measure and
The discharge start voltage is based on a first set of the first charge voltage and the first DC current and a second set of the second charge voltage and the second DC current. An image forming apparatus characterized in that the voltage is calculated again. The image forming apparatus according to claim 1, wherein the current detecting means detects the first direct current and the second direct current in the second step. The image forming apparatus according to claim 1 or 2, wherein the first step and the second step are performed at the time of non-image forming. The present invention according to any one of claims 1 to 3, wherein the DC voltage applied to the charging member from the voltage applying means at the time of image formation is set based on the newly calculated discharge start voltage. The image forming apparatus according to the description. The image forming apparatus according to any one of claims 1 to 4, wherein the film thickness of the dielectric layer in the photoconductor is calculated based on the newly calculated discharge start voltage. The image forming apparatus according to claim 5, wherein the life of the photoconductor is acquired based on the calculated film thickness of the dielectric layer. The image forming apparatus according to claim 6, further comprising a display means for displaying the life of the photoconductor. The first charging voltage and the second charging voltage are each applied during one rotation of the photoconductor.
The image according to any one of claims 1 to 7, wherein the first direct current and the second direct current are each applied while the photoconductor makes another rotation. Forming device. The first charging voltage and the second charging voltage are applied while the photoconductor makes one rotation.
The image forming apparatus according to any one of claims 1 to 7, wherein the first direct current and the second direct current are applied while the photoconductor makes another rotation. .. The image according to claim 9, wherein the first charging voltage and the second charging voltage are applied by gradually changing the DC voltage during one rotation of the photoconductor. Forming device. The image according to claim 9, wherein the first charging voltage and the second charging voltage are applied by changing the DC voltage in a slope shape while the photoconductor makes one rotation. Forming device. It is characterized in that the discharge start voltage is calculated again based on the DC voltage applied to the charging member in the time before the photoconductor makes one rotation with respect to the time when the DC current becomes 0. The image forming apparatus according to claim 11. Photoreceptor and
A charging member that charges the photoconductor by Paschen discharge, and
A voltage applying means for applying a DC voltage to the charging member, and
A current detecting means for measuring a direct current flowing from the photoconductor to the charging member, and
An exposure means that exposes the photoconductor to form a latent image,
A developing means for forming a toner image with respect to a latent image formed on the photoconductor, and
A transfer means for transferring the toner image to the transfer material,
In the control method of the image forming apparatus having
A first charging voltage and a second charging voltage exceeding the discharge starting voltage, which is the voltage at which the discharge from the charging member to the photoconductor starts, are applied to the photoconductor in order to charge the photoconductor. Process and
A second step of applying a static elimination voltage having a lower discharge start voltage to the photosensitive member in order to eliminate static electricity from the photosensitive member.
The first direct current flowing from the photoconductor to the charging member in the portion where the static elimination voltage is applied in the second step after the first charging voltage is applied in the first step, and the first DC current. A step of measuring the second DC current flowing from the photoconductor to the charging member in the portion where the static elimination voltage is applied in the second step after the second charging voltage is applied in the first step.
The discharge start voltage is based on a first set of the first charge voltage and the first DC current and a second set of the second charge voltage and the second DC current. A control method for an image forming apparatus, which comprises a process of recalculating a current. A program for executing the control method of the image forming apparatus according to claim 13 by a computer.

Images