JP6403613B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus using an electrophotographic system.

  In an image forming apparatus such as an electrophotographic apparatus or an electrostatic recording apparatus, a voltage is applied to a charging roller (charging member) to discharge the photosensitive drum (image carrier), thereby charging the drum surface to a predetermined potential. Let The discharge start voltage Vth at which charging of the photosensitive drum starts varies depending on the temperature and humidity environment, the film thickness of the photosensitive layer of the drum, and the like.

  FIG. 5 shows the relationship between the charging roller applied voltage and the photosensitive drum potential when the temperature and humidity environment and the thickness of the photosensitive layer are different. The horizontal axis represents the voltage applied to the charging roller, and the vertical axis represents the charging potential (dark portion potential Vd) of the photosensitive drum. The applied voltage at which the potential of the photosensitive drum starts to rise is the discharge start voltage Vth. Thus, when the temperature / humidity environment and the thickness of the photosensitive layer are different, the discharge start voltage Vth is different. In the “DC charging method”, when the discharge start voltage Vth is different, the dark portion potential Vd does not become constant even when a constant voltage is applied to the photosensitive drum.

  The sensitivity of the photosensitive drum varies depending on the temperature / humidity environment and the thickness of the photosensitive layer. Here, if the potential when the surface of the photosensitive drum is irradiated with a constant amount of laser light is the bright portion potential VL, the bright portion potential VL changes depending on the temperature / humidity environment and the difference in the film thickness of the photosensitive layer. FIG. 6 shows the relationship between the film thickness of the photoreceptor layer and the light portion potential VL. The horizontal axis represents the laser light amount, and the vertical axis represents the bright portion potential VL. FIG. 6 shows that the bright portion potential VL changes depending on the film thickness of the photoreceptor layer.

  Here, the DC voltage applied to the developing roller (developer carrier) is defined as Vdc_dev, and defined as development contrast Vcont = | VL−Vdc_dev | and back contrast Vback = | Vdc_dev−Vd |. In the electrophotographic apparatus, the development amount of toner (developer) from the developing roller to the photosensitive drum exposure unit is set to a desired value by controlling the development contrast Vcont. Further, by controlling the back contrast Vback, toner adhesion (fogging) from the developing roller to the non-exposed portion of the photosensitive drum is prevented. Therefore, when the temperature / humidity environment and the film thickness of the photosensitive layer change, and the dark portion potential Vd and the bright portion potential VL change, the development contrast Vcont and the back contrast Vback change, and the print image density and fog change.

  In order to prevent such changes in the development contrast Vcont and the back contrast Vback, it is necessary to detect the potential of the photosensitive drum and correct the dark portion potential Vd and the bright portion potential VL to desired values. Patent Document 1 proposes an image forming apparatus in which a surface potential meter is installed in the vicinity of a photosensitive drum to detect the potential of the photosensitive drum. Patent Document 2 proposes an image forming apparatus that obtains a discharge start voltage from a current flowing through a photosensitive drum when a voltage is applied to a transfer roller (transfer means), and detects the potential of the photosensitive drum from the discharge start voltage. Yes.

JP 2004-29092 A JP 2013-125097 A

In the invention of Patent Document 1, it is necessary to newly install a means for detecting a potential, and the configuration becomes complicated. Further, when the diameter of the photosensitive drum is reduced, a sufficient space for arranging the surface electrometer in the vicinity of the surface of the photosensitive drum cannot be taken. On the other hand, in the invention of Patent Document 2, the drum potential can be detected without adding a member for detecting the drum potential. However, the invention of Patent Document 2 has the following problems when applied to an image forming apparatus having post-transfer exposure means.

  In an electrophotographic apparatus, when there is no means for removing static electricity before charging a photosensitive drum, the drum potential before charging changes depending on the presence or absence of image exposure. The drum potential after charging varies depending on the drum potential before charging. That is, there is a difference in the drum potential after charging between the drum portion where the image is exposed to form an electrostatic latent image and the drum portion where the image is not exposed. The difference in drum potential after charging does not disappear even when an image is exposed to form an electrostatic latent image, resulting in a difference in image density. Hereinafter, this image density difference is referred to as “ghost”. “Ghost” can be prevented by removing the charge after transfer and before charging. The reason is that the drum potential before charging can be made uniform regardless of the presence or absence of image exposure. An exposure unit can be used as the charge eliminating unit. In many cases, the exposure means is arranged after transfer because of easy arrangement.

  When the post-transfer exposure unit is provided, when the photosensitive drum is exposed by the post-transfer exposure unit, the exposure is performed up to the vicinity of the nip where the transfer roller and the photosensitive drum are in contact with each other. As a result, the drum potential immediately after passing through the transfer nip changes. In Patent Document 2, the surface potential of the photosensitive drum is obtained from two applied voltages (first voltage and second voltage), which are positive and negative, at which the discharge current starts to flow. When the photosensitive drum is exposed by the post-transfer exposure means after passing through the transfer nip portion, the drum potential changes and the discharge current changes. In this case, an error occurs in the detection of the first voltage and the second voltage at which the discharge current starts to flow, and the drum potential cannot be detected with high accuracy.

  On the other hand, when the post-transfer exposure means is not always exposed, the surface potential of the photosensitive drum can be detected with high accuracy. However, during image formation, the photosensitive drum is always exposed by the post-transfer exposure means. Therefore, when the drum potential is detected without always exposing the post-transfer exposure means, the dark portion potential Vd and the bright portion potential VL at the time of image formation cannot be detected. In order to accurately detect the dark portion potential Vd and the bright portion potential VL during image formation, “the drum portion for detecting the surface potential is exposed by the post-transfer exposure means before one round of detection” and “surface When the potential is detected, it is necessary to turn off the post-transfer exposure means.

  The simplest method for satisfying the above conditions and performing detection in a short time is to turn on the post-transfer exposure means and not detect the surface potential every time the photosensitive drum rotates once. This is a method of alternately switching off and turning on and detecting the surface potential. In this case, if the post-transfer exposure means lighting time is t2, and the surface potential detection time is t3, t2 = t3. That is, the time required for the surface potential detection takes at least twice the surface potential detection time t3.

  An object of the present invention is to provide a technique capable of accurately detecting the potential of a photosensitive drum in a short time with a simple configuration.

In order to achieve the above object, an image forming apparatus of the present invention includes:
A rotating photosensitive drum carrying a toner image for forming an image on a recording material;
A charging unit for charging the photosensitive drum;
A first exposure unit that exposes the circumferential surface at a downstream side in a rotation direction of the photosensitive drum from a charging position charged by the charging unit on the circumferential surface of the photosensitive drum;
A voltage applying member that contacts the peripheral surface and applies a voltage to the photosensitive drum at a downstream side of the rotation direction from an exposure position exposed by the first exposure unit on the peripheral surface;
A second exposure unit that exposes the circumferential surface at a position downstream of the voltage application position in contact with the voltage application member on the circumferential surface and upstream of the charging position with respect to the charging direction;
A current detection unit for detecting a current value flowing through the photosensitive drum;
On the peripheral surface, an applied voltage value in the voltage application obtained by applying a voltage from the voltage application member to a post-static charge region exposed by the second exposure unit and charged by the charging unit; A potential detection unit for determining a surface potential of the photosensitive drum based on a relationship with a detected current value detected by the current detection unit by applying the voltage;
With
In the image forming apparatus in which an electrostatic latent image for forming the toner image is formed on the surface of the photosensitive drum based on the surface potential obtained by the potential detection unit.
When the surface potential is obtained by the potential detection unit, the voltage application member applies voltage to the charged region after the charge removal while the exposure by the second exposure unit is stopped while the photosensitive drum rotates once. And a static elimination period for stopping the voltage application and performing exposure by the second exposure unit are alternately repeated a plurality of times.

  According to the present invention, the potential of the photosensitive drum can be accurately detected with a simple configuration and in a short time.

1 is a schematic configuration diagram of an image forming apparatus according to an embodiment of the present invention. Relationship diagram of photosensitive drum surface potential, positive discharge start voltage, and negative discharge start voltage Relationship between the voltage applied to the transfer roller and the current value flowing through the photosensitive drum Diagram showing that the current value varies depending on the resistance value of the transfer roller Relationship between charging roller applied voltage and photosensitive drum potential Relationship between the thickness of the photoreceptor layer and the light portion potential VL The figure which shows the exposure width of the exposure equipment after transfer Timing chart to shorten the time required to detect the photosensitive drum potential Flowchart of sequence for detecting photosensitive drum potential Flowchart of sequence for detecting photosensitive drum potential Operation process diagram of image forming apparatus

  DESCRIPTION OF EMBODIMENTS Hereinafter, embodiments for carrying out the present invention will be exemplarily described in detail 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. That is, it is not intended to limit the scope of the present invention to the following embodiments.

(Example)
(1) Configuration of Printer and Image Forming Operation FIG. 1 is a schematic configuration diagram illustrating an image forming apparatus according to the present embodiment. This image forming apparatus is an electrophotographic laser beam printer. When image information is input from a host device 21 such as a personal computer to a control circuit unit (CPU) 16 that is a control means, an image can be formed on a transfer material (recording material) P and output. The control circuit unit 16 transmits and receives electrical information to and from the host device 21 and the printer operation unit 22, and also controls the image forming operation of the image forming apparatus stored in a memory (not shown) and a predetermined control program or reference. Control according to the table. The image forming sequence control and other various controls described below are executed by the control circuit unit 16, and the control circuit unit 16 corresponds to the potential detection unit (calculation unit) in the present invention. In the following description, the calculated values obtained by the respective arithmetic expressions based on various detected values may be actually calculated in real time, or a table in which the detected values are associated with the calculated values in advance is prepared. In addition, it may be acquired by referring to the table. The printer operation unit 22 includes various printing condition setting keys (not shown) and a display 22a.

  The image forming apparatus includes a photosensitive drum 1 as a rotatable image carrier that carries a toner image. Around the photosensitive drum 1, a charging roller 2, a developing device 4, a transfer roller 5, a post-transfer exposure device 14, and a cleaning device 6 are disposed as process means related to the photosensitive drum 1. Is provided with an exposure device 3. Further, a transfer guide 7 is disposed in the transfer material conveyance path upstream of the transfer nip portion N between the photosensitive drum 1 and the transfer roller 5 (contact portion) in the transfer material conveyance direction. A paper feed cassette (paper feed unit) 17 is disposed upstream of the transfer guide 7 in the transfer material conveyance direction. Further, on the transfer material conveyance path downstream of the transfer nip portion N in the transfer material conveyance direction, a static elimination needle 8, a post-transfer exposure device 14, a conveyance guide 9, and a fixing device 10 are arranged.

  The photosensitive drum 1 is a negatively charged organic photosensitive member in this embodiment, and has a photosensitive layer 1b on the outer peripheral surface of an aluminum drum base 1a. The peripheral speed is 94 mm / s by a driving means (not shown). Is rotated in the clockwise direction of the arrow. The outer diameter of the photosensitive drum is 30 mm. The surface of the photosensitive drum 1 is uniformly charged to a predetermined polarity and potential by a charging roller 2 as a contact charging unit during the rotation process. The charging roller 2 is a conductive roller, and is disposed in contact with the photosensitive drum 1 at a charged portion at a predetermined pressing force. The charging roller 2 is rotated by the rotation of the photosensitive drum 1. By applying a predetermined charging bias to the charging roller 2 using the charging bias power source 11, the photosensitive drum 1 is uniformly charged to a predetermined negative potential. The configuration relating to the charging of the photosensitive drum 1 corresponds to the charging unit of the present invention.

  The exposure apparatus 3 is a laser scanner provided with a laser driver (not shown), a laser diode, a polygon mirror, and the like. Laser light modulated in accordance with the image signal input from the control circuit unit 16 is output from the laser diode. The output laser beam is scanned with a polygon mirror that rotates at high speed, and the exposed portion of the photosensitive drum 1 is subjected to image exposure L via the reflection mirror 3a. Thereby, an electrostatic latent image corresponding to the image information is formed on the surface of the photosensitive drum 1. The configuration relating to the exposure of the circumferential surface of the photosensitive drum 1 on the downstream side in the rotation direction from the charging position charged by the charging roller 2 and on the upstream side in the rotation direction from the development position by the developing device 4 is the first aspect of the present invention. One exposure part corresponds.

  The developing device 4 includes a rotatable developing sleeve 4a that substantially contacts the surface of the photosensitive drum 1 at the developing portion (developing position) of the photosensitive drum 1 downstream of the exposure position. By applying a predetermined developing bias to the developing sleeve 4a using the developing bias power source 12, in the developing unit, the negatively charged toner is attached to the electrostatic latent image on the photosensitive drum 1 to form a toner image. Visualize.

  The transfer roller (transfer member) 5 is a rotatable conductive roller provided with a conductive elastic layer on a core metal, and corresponds to the voltage application member of the present invention. The transfer roller 5 is arranged in contact with the photosensitive drum 1 with a predetermined pressing force at a transfer site (transfer position) on the downstream side of the developing unit, and the photosensitive drum 1 and the transfer nip N are connected to each other. Forming. The transfer material P is introduced into the transfer nip portion N from the paper feed cassette 17 side at a predetermined control timing and is nipped and conveyed.

A sheet (cut sheet, sheet) as a transfer material (recording material) P is stacked and stored in the paper feed cassette 17. The uppermost recording material P of the stacked transfer materials P is separated by a paper feed roller 18 whose rotation is controlled at a predetermined paper feed timing and fed out from the paper feed cassette 17 to be transferred to a transfer material transport path. 19 is conveyed toward the transfer nip N. The transfer material P is temporarily received by the registration roller pair 20 and is subjected to skew correction during conveyance. Then, it is conveyed at a predetermined control timing by the subsequent rotation of the registration roller pair 20 and introduced into the transfer nip portion N via the transfer guide 7.

  The transfer timing of the transfer material P by the registration roller pair 20 is the timing at which the leading edge of the toner image formed on the photosensitive drum 1 reaches the transfer nip N, and the leading edge of the transfer material P reaches the transfer nip N. It is time to do. While the transfer material P is nipped and conveyed through the transfer nip portion N, a transfer bias having a predetermined potential opposite to the charging polarity of the toner is applied to the transfer roller 5 from the transfer bias power source 13. As a result, the toner images on the photosensitive drum 1 are sequentially transferred to the transfer material P by the electrostatic force generated between the photosensitive drum 1 and the transfer roller 5.

  The transfer material P that has exited the transfer nip N is neutralized by the static elimination needle 8, separated from the photosensitive drum 1, and introduced into the fixing device 10 via the conveyance guide 9. The fixing device 10 includes a rotatable fixing roller 10a and a pressure roller 10b. While the transfer material P is nipped and conveyed at the fixing nip between the fixing roller 10a and the pressure roller 10b, the toner image transferred to the surface of the transfer material P is heated and pressed to be thermally fixed. The transfer material P exiting the fixing device 10 is discharged to the outside of the image forming apparatus as an image formed product.

  The post-transfer exposure device 14 includes a post-transfer exposure device light-emitting unit configured by arranging a plurality of light-emitting diodes along the longitudinal direction of the photosensitive drum 1. By emitting light from the light emitting diode, the photosensitive drum 1 is exposed, and the surface potential of the photosensitive drum 1 (photosensitive layer 1b) is neutralized. The circuit control unit 16 controls lighting and extinguishing of the light emitting diode. The configuration relating to the exposure of the circumferential surface of the photosensitive drum 1 on the downstream side in the rotation direction from the transfer position (voltage application position) by the transfer roller 5 and the upstream side in the rotation direction from the charging position by the charging roller 2 is the present invention. This corresponds to the second exposure part.

  The cleaning device 6 has an elastic cleaning blade (cleaning member) 6 a that rubs the surface of the photosensitive drum 1 at the cleaning site. On the surface of the photosensitive drum 1 after passing through the transfer portion, the toner that could not be transferred to the transfer material P at the transfer nip portion N remains. This transfer residual toner is removed and collected by rubbing with the cleaning blade 6a. The photosensitive drum 1 cleaned by the cleaning blade 6a is repeatedly used for image formation.

  FIG. 10 shows an operation process diagram of the image forming apparatus executed by the control circuit unit 16.

1) Pre-multi-rotation step: a start (start) operation period (warming period) of the image forming apparatus. When a main power switch (not shown) of the apparatus is turned on, a driving means (not shown) of the apparatus is activated to execute a preparation operation for a required process device.

2) Standby: After the predetermined start operation period, the driving unit is stopped and the apparatus is kept in a standby (standby) state until an image formation start signal (print job start signal) is input.

3) Pre-rotation step: This is a period in which the drive unit is re-driven based on the input of the image formation start signal and the pre-image formation operation of the required process equipment is executed. More practically, a: the image forming apparatus receives the image formation start signal, b: the image is developed by the controller (the development time varies depending on the image data amount and the formatter processing speed), c: the pre-rotation process starts, It becomes the order. If an image formation start signal is input during the pre-multi-rotation process of 1), the process proceeds to the pre-rotation process without standby of 2) after the pre-multi-rotation process.

4) Image forming operation: When a predetermined pre-rotation process is completed, the image forming process is subsequently executed, and an image-formed recording material is output. In the case of a continuous image forming job, the above-described image forming process is repeated, and a predetermined number of image-formed transfer materials are sequentially output.

5) Inter-sheet process: In the case of a continuous image forming job, this is an interval process between the trailing edge of one transfer material P and the leading edge of the next transfer material P. is there.

6) Post-rotation process: after the image-formed recording material is output in the case of only one image-forming job, or in the case of a continuous image-forming job, the last image-formed recording of the continuous image-forming job Even after the material is output, the driving means is continuously driven for a predetermined time. This is a period during which the post-image forming job operation of the required process device is executed.

7) Standby: After completion of the predetermined post-rotation process, the driving unit stops driving, and the apparatus is kept in a standby state until the next image formation start signal is input.

  In the above, 4) image forming execution time is an image forming operation, 1) pre-multi-rotation process, 3) pre-rotation process, 5) paper-to-paper process, and 6) post-rotation process. In addition, an arbitrary interruption period of the image forming operation is during non-image formation. Also, during non-image formation, at least one of the above-mentioned pre-multi-rotation process, pre-rotation process, paper-to-paper process, post-rotation process, interrupted process, and within that process time At least the predetermined time.

(2) Description of Photosensitive Drum Potential Detection Method (2-A) Photosensitive Drum Potential Detection Principle The transfer bias power supply 13 has a transfer voltage application circuit 13 a that applies a DC voltage to the transfer roller 5. The transfer voltage application circuit 13 a includes a current detection circuit 13 b (current detection unit) that detects a current value that flows through the photosensitive drum 1 through the transfer roller 5 when a DC voltage is applied to the transfer roller 5. In the present invention, during non-image formation, a current value is detected when a DC voltage having a different value is applied to the transfer roller 5 that is in contact with the charged area after static elimination or the exposed area after charging on the circumferential surface of the photosensitive drum. Then, the discharge start voltage value for the photosensitive drum 1 is obtained. There is a correlation between the discharge start voltage and the surface potential. Below, a correlation is demonstrated.

FIG. 2 shows the relationship between the surface potential of the photosensitive drum 1 (photoreceptor layer 1b), the positive discharge start voltage, and the negative discharge start voltage. The horizontal axis in FIG. 2 is the applied voltage value, and the vertical axis is the current value (detected current value). Assuming that the surface potential of the photosensitive drum 1 is V, the first voltage which is a positive discharge start voltage is V1, and the second voltage which is a negative discharge start voltage is V2, FIG.
V = (V1 + V2) / 2
It shows that it satisfies.
That is, an applied voltage value intermediate between the two discharge start voltage values V 1 and V 2 can be regarded as the surface potential V of the photosensitive drum 1. This principle is maintained even if the environment (temperature and humidity) and the film thickness of the photosensitive layer 1b of the photosensitive drum 1 change. The present invention uses this principle. Specifically, the surface potential of the photosensitive drum 1 is detected by obtaining a positive discharge start voltage V1 and a negative discharge start voltage V2.

  FIG. 3 shows the relationship between the voltage applied to the transfer roller 5 and the value of the current flowing through the photosensitive drum 1. As shown by the straight line 1 in FIG. 3, the relationship between the applied voltage and the current value is a straight line until the discharge starts. As shown by the curve 2 in FIG. 3, the relationship between the applied voltage and the current value becomes a curve away from the straight line 1 when the discharge starts. Therefore, the discharge current value is a Δ value obtained by subtracting the straight line 1 from the curve 2. A point in time when the Δ value reaches a desired current value It (target discharge current value) is defined as a discharge start voltage.

A desired current value It (target discharge current value) is set according to the resistance value of the transfer roller 5.
A dark current flows through the transfer roller 5 until discharge starts. This dark current is determined by the resistance value of the transfer roller 5. FIG. 4 shows that the current value varies depending on the resistance value of the transfer roller 5. The horizontal axis in FIG. 4 is the applied voltage, and the vertical axis is the current value.

  As shown in FIG. 4, the dark current value varies depending on the resistance value of the transfer roller 5. Therefore, the resistance value of the transfer roller 5 is calculated by detecting the current value that flows when a predetermined voltage is applied. In FIG. 4, the resistance value can be calculated from the current value detected when a DC voltage of −1200 V is applied. From the resistance value, a correction current value at the time when discharge starts is obtained. From this corrected current value, a desired current value It (target discharge current value) is set. The correction current value for the resistance value is stored as a table in a nonvolatile memory in the control unit of the image forming apparatus. It is also possible to calculate using an arithmetic expression instead of a table.

(2-B) Outline of Photosensitive Drum Potential Detection Method By applying a DC voltage to the charging roller 2, the photosensitive drum 1 is charged to a negative potential (negative potential). Using the transfer voltage application circuit 13a, a different voltage is applied to the expected potential of the photosensitive drum 1 by changing it in the positive direction or the negative direction. Then, a positive discharge start voltage V1 and a negative discharge start voltage V2 are detected with respect to the potential of the photosensitive drum 1. The dark portion potential Vd of the photosensitive drum 1 is calculated from the discharge start voltages V1 and V2. 1/2 of the difference between the absolute values of the discharge start voltages V1 and V2 is set as a voltage difference ΔV necessary for the photosensitive drum 1 to start discharge.

  Further, after exposing the photosensitive drum 1 to laser using the exposure device 3, the transfer voltage applying circuit 13 a is used to change the potential of the photosensitive drum 1 in the negative direction and apply a different voltage. The discharge start voltage V3 is detected from the current value detected at that time. From the discharge start voltage V3 and the voltage difference ΔV necessary for the photosensitive drum 1 to start discharging, the bright portion potential VL = V3 + ΔV on the photosensitive drum 1 after the laser exposure using the exposure device 3 is calculated.

(3) Photosensitive drum potential detection timing (3-A) Method for shortening the time taken to detect the photosensitive drum potential FIG. 7 shows the exposure width of the post-transfer exposure device 14 (static discharge exposure on the circumferential surface of the photosensitive drum). Width (d) along the circumferential direction of the irradiated region. As shown in FIG. 7, the exposure width d of the post-transfer exposure device 14 is the circumference of the surface of the photosensitive drum 1 from the contact nip outlet P <b> 1 between the transfer roller 5 and the photosensitive drum 1 to the most upstream position P <b> 2 of the cleaning device 6. The length of the direction.

  FIG. 8 is a timing chart for shortening the time taken to detect the photosensitive drum potential. The horizontal axis is time, and the vertical axis is the position around the photosensitive drum. The vertical axis indicates a position P2 where the post-transfer exposure ends and a position P1 where the post-transfer exposure starts. The on / off of the post-transfer exposure device 14 is controlled in accordance with the position P2. Further, the execution and pause of the photosensitive drum potential detection are controlled in accordance with the position P1. Here, the lighting cycle of the post-transfer exposure device 14 is t1, the lighting time of the post-transfer exposure device 14 is t2, the photosensitive drum potential detection time is t3, the circumferential length of the photosensitive drum 1 is L1, and the circumferential speed of the photosensitive drum 1 is V. And

In order to accurately detect the photosensitive drum potential during image formation, the following two conditions must be satisfied.
Condition 1: The circumferential portion of the photosensitive drum for detecting the potential is exposed by the post-transfer exposure apparatus and charged before the potential detection is completed.
Condition 2: When the photosensitive drum potential is detected, the post-transfer exposure device is turned off.

In order to satisfy the above two conditions and detect the photosensitive drum potential in a short time, the following expression must be satisfied.
t1 = t2 + t3 (Formula 1)
t3 = t2 + d / V (Formula 2)
n * t1 = L1 / V + t3-d / V (n is an integer of 2 or more) (Formula 3)

Expression 1 indicates that “post-transfer exposure device is turned on and photosensitive drum potential detection is stopped” and “post-transfer exposure device is turned off and photosensitive drum potential detection is executed” are alternately switched.
Expression 2 indicates that the circumferential portion (t2 × V + d) exposed by the post-transfer exposure device 14 and the photosensitive drum potential detection portion (t3 × V) are matched.
Equation 3 indicates that there is a timing at which the lighting start of the post-transfer exposure device coincides with the end of the photosensitive drum potential detection while the photosensitive drum 1 rotates once.

  By satisfying Equation 1, Equation 2, and Equation 3, the potentials of all the circumferential portions of the photosensitive drum 1 that satisfy the conditions 1 and 2 and are exposed by the post-transfer exposure device 14 can be detected, and are constant no matter how many times the drum rotates. The photosensitive drum potential detection can be executed in the cycle of. As a result, the time required to detect the photosensitive drum potential can be shortened.

  FIG. 8 shows the time taken to detect the photosensitive drum potential when Expression 1, Expression 2, and Expression 3 are satisfied and the post-transfer exposure device 14 is turned on twice while the photosensitive drum 1 rotates once. It is a timing chart which shortens. Here, in the timing chart of FIG. 8, the number of times the post-transfer exposure device 14 switches from lighting to off or the number of times from switching off to lighting is 2 or more for any one rotation of the photosensitive drum 1. is there.

From Equation 1, Equation 2, and Equation 3,
2 * t1 = L1 / V + t3-d / V
= L1 / V + (1/2) * t1- (1/2) * (d / V)
That is,
t1 = (1/3) × (2 × L1-d) / V (Formula 4-1)
t2 = (1/3) × (L1-2 × d) / V (Formula 4-2)
Meet.

Similarly, when Expression 1, Expression 2, and Expression 3 are satisfied, and the post-transfer exposure device 14 is turned on three times while the photosensitive drum 1 rotates once, the following expression is satisfied.
t1 = (1/5) × (2 × L1-d) / V (Formula 5-1)
t2 = (1/5) × (L1−3 × d) / V (Formula 5-2)

Similarly, when Expression 1, Expression 2, and Expression 3 are satisfied and the post-transfer exposure device 14 is turned on n times while the photosensitive drum 1 rotates once, the following expression is satisfied.
t1 = (1 / (2n-1)) * (2 * L1-d) / V (Formula 6-1)
t2 = (1 / (2n-1)) * (L1-n * d) / V (Formula 6-2)
Here, n is an integer. Also, since t2 is positive, from Equation 6-2,
2 ≦ n <L1 / d (Formula 6-3)

Table 1 shows an embodiment in which the time taken to detect the photosensitive drum potential is shortened by changing n and d in the formula using Formula 6-1, Formula 6-2, and Formula 6-3. . Here, the potential detection time ratio was t1 / t3.
(Table 1)

  In Comparative Example 1, every time the photosensitive drum 1 rotates, “post-transfer exposure device is turned on and photosensitive drum potential detection is stopped” and “post-transfer exposure device is turned off and photosensitive drum potential detection is executed” are alternately switched. Is the method. In this case, the potential detection time ratio = 2.

  On the other hand, by performing Example 1-1 to Example 1-5, the potential detection time ratio can be made smaller than 2, and the photosensitive drum potential can be accurately detected in a short time. That is, the potential detection time ratio can be made smaller than 2 when the number of times the post-transfer exposure device 14 is switched from lighting to extinguishing or the number of times switching from extinguishing to lighting is two or more during one rotation of the photosensitive drum 1. In a short time, the photosensitive drum potential can be accurately detected.

Further, a period t1 at which the post-transfer exposure apparatus switches from turning off to turning on,
t1 = (1 / (2n-1)) * (2 * L1-d) / V
Thus, the photosensitive drum potential can be detected with high accuracy in a shorter time.
In particular, as in Example 1-3 and Example 1-5, the photosensitive drum can be formed in a shorter time by increasing the exposure width d and increasing the number of times of lighting n within a range that satisfies Expression 6-3. The potential can be detected with high accuracy.

(3-B) Photosensitive Drum Potential Detection Sequence FIGS. 9A and 9B show a flowchart of a sequence for detecting the photosensitive drum potential. Hereinafter, a description will be given based on a flowchart. The operation based on the flowcharts of FIGS. 9A and 9B is controlled by the control circuit unit 16. Here, the number n of times that the post-transfer exposure device 14 is turned on during one rotation of the photosensitive drum 1 is set to six. The exposure width d was set to 13.1 mm.

Step 1: After the printer is turned on or a print command is received, a pre-multi rotation (after power-on) or a pre-rotation (after reception of the print command), which is an initialization operation, is executed.
Step 2: The rotation of the photosensitive drum 1 is started during the pre-multi rotation or the pre-rotation.
Step 3: A DC voltage is applied to the charging roller 2, the post-transfer exposure device 14 is turned on, and the photosensitive drum 1 is charged to a negative potential (forms a charged area after static elimination). The timer of the control circuit unit 16 is set to t = 0, and the timer count is started.
Step 4: It is determined whether t ≧ (L1 + L2) / V is satisfied. Here, the circumferential length of the photosensitive drum 1 is L1, and the circumferential length of the photosensitive drum 1 between the charging position and the transfer position (the distance when the photosensitive drum 1 travels from the charging position to the transfer position in the rotation direction) is L2. Let V be the peripheral speed of the photosensitive drum 1. If t ≧ (L1 + L2) / V is satisfied, the process proceeds to Step 5. If not, return to Step 4.
Step 5: The post-transfer exposure device 14 is turned off, the timer of the control circuit unit 16 is set to t = t2, and the timer count is restarted.
Step 6: A predetermined negative voltage is applied to the transfer roller 5.
Step 7: The resistance value of the transfer roller 5 is calculated, and the target current value It is set.

A discharge start voltage V1 in the positive direction is obtained with respect to the surface potential of the photosensitive drum 1.
Step 8: Determine whether t <t1 is satisfied. If so, go to Step 9. If not, the process proceeds to the subroutine “Detection of detection timing”.

The subroutine “detection timing determination” will be described.
Sub1: The post-transfer exposure device 14 is turned on, the timer t of the control circuit unit 16 is reset to t−t1, and the timer count is restarted.
Sub2: It is determined whether t ≧ t2 is satisfied. If so, go to Sub3. If not, return to Sub2.
Sub3: The post-transfer exposure device 14 is turned off, and the subroutine “detection timing determination” ends. If the discharge start voltage V1 is not detected, the process returns to Step 8.

  If the discharge start voltage V1 has been detected and the discharge start voltage V2 has not been detected, the process returns to Step 18. If the discharge start voltage V1 has been detected, the discharge start voltage V2 has been detected, and the discharge start voltage V3 has not been detected, the process returns to Step 31.

Return to the main flow.
Step 9: A positive voltage Vp (x) is applied to the transfer roller 5 with respect to the surface potential of the photosensitive drum 1. The initial value is a predetermined voltage V5 that is sufficiently positive with respect to the surface potential of the photosensitive drum 1.
Step 10: Detects the current value flowing through the photosensitive drum 1 via the transfer roller 5.
Step 11: The discharge current value Is is detected from the current value by the method described above.
Step 12: It is determined whether Is> It + ΔIs is satisfied. Here, It is a target current value, and ΔIs is a tolerance of the target current value It. When Is> It + ΔIs is satisfied, the process proceeds to Step 13. When not satisfy | filling, it progresses to Step14.
Step 13: Change the transfer voltage in the negative direction. Thereby, the discharge current value Is can be reduced. Then, it progresses to Step16.
Step 14: It is determined whether or not Is <It−ΔIs is satisfied. If Is <It−ΔIs is satisfied, the process proceeds to Step 15. If not, go to Step 17.
Step 15: Change the transfer voltage in the positive direction. Thereby, the discharge current value Is can be increased. Then, it progresses to Step16.
Step 16: The voltage Vp (x) applied to the transfer roller 5 changed in Step 13 or Step 15 is held in the memory of the control circuit unit 16.
Step 17: It is determined that the discharge current value Is has converged between It−ΔIs and It + ΔIs. The transfer voltage at this time is a discharge start voltage V1 in the positive direction with respect to the surface potential of the photosensitive drum 1. Then, it progresses to Step18.

A discharge start voltage V2 in the negative direction is obtained with respect to the surface potential of the photosensitive drum 1.
Step 18: Determine whether t <t1 is satisfied. If so, go to Step 19. If not, the process proceeds to the subroutine “Detection of detection timing”.
Step 19: A negative voltage Vn (x) is applied to the transfer roller 5 with respect to the surface potential of the photosensitive drum 1. The initial value is a predetermined voltage V6 that is sufficiently large and negative with respect to the surface potential of the photosensitive drum 1.
Step 20: Detects the current value flowing through the photosensitive drum 1 via the transfer roller 5.
Step 21: The discharge current value Is is detected from the current value by the method described above.
Step 22: Determine whether Is> It + ΔIs is satisfied. Here, It is a target current value, and ΔIs is a tolerance of the target current value It. When Is> It + ΔIs is satisfied, the process proceeds to Step 23. When not satisfy | filling, it progresses to Step24.
Step 23: Change the transfer voltage in the positive direction. Thereby, the discharge current value Is can be reduced. Then, it progresses to Step26.
Step 24: It is determined whether or not Is <It−ΔIs is satisfied. When Is <It−ΔIs is satisfied, the process proceeds to Step 25. If not, go to Step 27.
Step 25: Change the transfer voltage in the negative direction. Thereby, the discharge current value Is can be increased. Then, it progresses to Step26.
Step 26: The voltage Vn (x) applied to the transfer roller 5 changed in Step 23 or Step 25 is held in the memory of the control circuit unit 16.
Step 27: It is determined that the discharge current value Is has converged between It−ΔIs and It + ΔIs. The transfer voltage at this time is a discharge start voltage V2 in the negative direction with respect to the surface potential of the photosensitive drum 1.

Step 28: ΔV and Vd are calculated. The voltage difference ΔV necessary for the photosensitive drum 1 to start discharging is ½ of the difference between the absolute values of the discharge starting voltages V1 and V2. The dark portion potential Vd of the photosensitive drum 1 is ½ of the sum of the discharge start voltages V1 and V2.
Step 29: The charging voltage is changed according to the detection results of ΔV and Vd, and a DC voltage is applied to the charging roller 2.
Step 30: Using the exposure device 3, the photosensitive drum 1 is irradiated with laser (forms an exposure area after charging), and the potential of the photosensitive drum 1 is set to the bright portion potential VL.

A discharge start voltage V3 in the negative direction is obtained with respect to the light portion potential VL of the photosensitive drum 1.
Step 31: It is determined whether t <t1 is satisfied. If so, go to Step 32. If not, the process proceeds to the subroutine “Detection of detection timing”.
Step 32: A negative voltage Vn (x) is applied to the transfer roller 5 with respect to the surface potential of the photosensitive drum 1. The initial value is a predetermined voltage V7 that is sufficiently negative with respect to the light portion potential VL of the photosensitive drum 1.
Step 33: A value of a current flowing through the photosensitive drum 1 via the transfer roller 5 is detected.
Step 34: The discharge current value Is is detected from the current value by the method described above.
Step 35: It is determined whether Is> It + ΔIs is satisfied. Here, It is a target current value, and ΔIs is a tolerance of the target current value It. When Is> It + ΔIs is satisfied, the process proceeds to Step 36. If not, go to Step 37.
Step 36: Change the transfer voltage in the positive direction. Thereby, the discharge current value Is can be reduced. Then, it progresses to Step39.
Step 37: It is determined whether or not Is <It−ΔIs is satisfied. If Is <It−ΔIs is satisfied, the process proceeds to Step 38. When not satisfy | filling, it progresses to Step40.
Step 38: Change the transfer voltage in the negative direction. Thereby, the discharge current value Is can be increased. Then, it progresses to Step39.
Step 39: The voltage Vn (x) applied to the transfer roller 5 changed in Step 36 or Step 38 is held in the memory of the control circuit unit 16.
Step 40: It is determined that the discharge current value Is has converged between It−ΔIs and It + ΔIs. The transfer voltage at this time is a discharge start voltage V3 in the negative direction with respect to the light portion potential VL of the photosensitive drum 1.

Step 41: VL is calculated. The voltage difference ΔV necessary for the photosensitive drum 1 to start discharging is ½ of the difference between the absolute values of the discharge starting voltages V1 and V2. Accordingly, the light portion potential VL (second surface potential) of the photosensitive drum 1 is a value obtained by adding ΔV in the positive direction to the discharge start voltage V3 in the negative direction.
Step 42: The photosensitive drum potential detection sequence is terminated.

  By executing the above sequence, the potential detection time ratio t1 / t3 can be set to 1.10. When a current is detected by applying a voltage to the transfer roller 5 a plurality of times, according to this embodiment, it is possible to shorten the time required to detect the photosensitive drum potential.

<Features of this embodiment>
The drum surface potential detection method according to this embodiment is characterized in that a plurality of detection times are provided during one rotation of the photosensitive drum, and an area where detection is performed on the circumferential surface of the photosensitive drum for each detection. It is to neutralize (region passing through the voltage application unit). The neutralization starts immediately after the end of the one-time detection, and is performed until the entire region where the detection is performed is neutralized. That is, the circumferential length of the region passing through the voltage application position on the drum circumferential surface during one detection period is equal to the circumferential length of the region subjected to the static elimination exposure on the drum circumferential surface during one static elimination period. It is comprised so that it may become. When the static elimination is completed, the next detection is started, and when the detection is completed, the static elimination is performed again. Thereafter, detection and static elimination are alternately repeated a plurality of times until a desired detection result is obtained. The detection period and the charge removal period are provided so as not to overlap each other in order to suppress the influence of the charge removal exposure on the voltage application in the configuration in which the voltage application position is included in the irradiation range of the charge removal exposure. That is, the voltage application in the detection period is performed after the discharge exposure in the immediately previous discharge period is stopped.

  The above detection and neutralization are repeated as many times as necessary, and do not synchronize with the number of rotations of the photosensitive drum. When the repetition of detection and static elimination is synchronized with the number of rotations of the photosensitive drum as in Comparative Example 1 (t1 = 2 × L1), the circumference of the detection area and the circumference of the photosensitive drum (an integer thereof) required for detection Double), a useless rotation distance is generated. That is, even if a desired detection result is obtained before one rotation of the photosensitive drum is completed, the rotation of the photosensitive drum is continued until one rotation is completed. Therefore, as in this embodiment, the detection after the desired detection result is obtained by repeating detection and static elimination a plurality of times regardless of the number of rotations of the photosensitive drum and during one rotation of the photosensitive drum. It is possible to reduce the useless rotation distance of the drum. By repeating this detection and static elimination at a shorter cycle, it is possible to further reduce the useless rotation distance of the photosensitive drum (for example, Example 1-3 with respect to Examples 1-1 and 1-2). .

  Here, on the outer peripheral surface of the photosensitive drum, the portion where the voltage is applied is substantially one point on the peripheral surface, whereas the portion where the static elimination is performed (exposure irradiation range) is in the circumferential direction on the peripheral surface. This is a region having a predetermined width (d) and wider in the circumferential direction than the voltage application location. That is, voltage application and static elimination exposure are different from each other in the range processed at one time. Therefore, the time required for processing differs depending on whether a voltage is applied to the region of the same distance on the drum peripheral surface or the case where the charge is removed. That is, in the case of static elimination (t2 × V), the distance can be increased by the width (d) of the static elimination region (irradiation range) than in the case of voltage application (t3 × V), and the time required for processing is increased. (It becomes t2 = t3-d / V from Formula 2). As the width (d) of the charge removal region increases, the time (t3) that can be used for detection increases when the charge removal time (t2) is constant. Therefore, it is possible to shorten the time until a desired detection result is obtained, and it is possible to improve detection efficiency (for example, Example 1-5 with respect to Examples 1-2 and 1-4).

(4) Operation after Photosensitive Drum Potential Detection In the electrophotographic apparatus, the toner development amount from the developing sleeve 4a to the exposed portion of the photosensitive drum 1 is set to a desired value by controlling the development contrast Vcont. Further, by controlling the back contrast Vback, toner adhesion (fogging) from the developing sleeve 4a to the non-exposed portion of the photosensitive drum 1 is prevented. Here, the DC voltage applied to the developing sleeve 4a is defined as Vdc_dev, developing contrast Vcont = | VL−Vdc_dev |, and back contrast Vback = | Vdc_dev−Vd |.

When the temperature and humidity environment and the thickness of the photosensitive layer of the photosensitive drum are different, the discharge start voltage Vth is different, and the dark portion potential Vd and the bright portion potential VL change. Accordingly, the development contrast Vcont and the back contrast Vback also change. Therefore, in this embodiment, the development contrast Vcont and the back contrast Vback are based on the detection results of the dark part potential Vd and the bright part potential VL.
The image forming conditions such as are changed. The control circuit unit 16 corrects the dark part potential Vd and the bright part potential VL to desired values based on the detection result of the photosensitive drum potential. A method for correcting the dark part potential Vd and the bright part potential VL will be exemplified below.

(Correction method 1)
When the absolute value of the light portion potential VL is lower than a desired value, the development contrast Vcont increases. In this case, the amount of laser irradiation is reduced. Conversely, when the bright portion potential VL is higher than a desired value, the development contrast Vcont becomes small. In this case, the amount of laser irradiation is increased.
(Correction method 2)
When the development contrast Vcont is lower than a desired value, the absolute value of the DC voltage Vdc_dev applied to the developing sleeve 4a is increased. Conversely, when the development contrast Vcont is higher than a desired value, the absolute value of the DC voltage Vdc_dev applied to the developing sleeve 4a is decreased.
(Correction method 3)
A method of setting the back contrast Vback and the development contrast Vcont to desired values will be described. Vback + Vcont = | Vd−VL |. Therefore, first, Vback + Vcont is set to a desired value. That is, when Vback + Vcont is lower than a desired value, the absolute value of the DC voltage Vdc_c applied to the charging roller 2 is increased. On the contrary, when Vback + Vcont is higher than a desired value, the absolute value of the DC voltage Vdc_c applied to the charging roller 2 is decreased. After setting Vback + Vcont to a desired value, the DC voltage Vdc_dev applied to the developing sleeve 4a is adjusted to set Vback and Vcont to desired values.

  As described above, by correcting the dark part potential Vd and the bright part potential VL after detecting the photosensitive drum potential, even if the temperature / humidity environment or the film thickness of the photosensitive layer changes, the change in the development contrast Vcont and the back contrast Vback. Can be prevented. Thereby, the image quality such as the print image density and the fog can be maintained.

(5) Supplement (5-A) In this embodiment,
t1 = (1 / (2n-1)) * (2 * L1-d) / V (Formula 6-1)
t2 = (1 / (2n-1)) * (L1-n * d) / V (Formula 6-2)
t3 = (1 / (2n−1)) × (L1 + (n−1) × d) / V
2 ≦ n <L1 / d (Formula 6-3)
However, this is not necessarily the case. Here, the lighting cycle of the post-transfer exposure apparatus is t1, the lighting time of the post-transfer exposure apparatus is t2, and the photosensitive drum potential detection time is t3.

An example different from the present embodiment will be shown.
A time obtained by integrating the photosensitive drum potential detection time t3 until the photosensitive drum potential is detected is defined as t5. here,
α = (1 / (2n−1)) × (L1 + (n−1) × d) / V
age,
t5 = m × α + β, m is an integer, β is 0 ≦ β <α
And

In this case, the lighting cycle of the post-transfer exposure apparatus is t1 ′ = (1 / (2n−1)) × (2 × L1-d) / V−β / m (formula 6-1 ′).
It is good.

In this case, Condition 1, Condition 2, and Expression 3 are substantially satisfied, so that it is possible to shorten the time taken to detect the photosensitive drum potential. Here, the condition 1 is “the circumferential position of the photosensitive drum for detecting the potential is exposed by the post-transfer exposure device and charged before one cycle of potential detection”. Condition 2 is “When the photosensitive drum potential is detected, the post-transfer exposure apparatus is turned off”. Formula 3 is [n * t1 = L1 / V + t3-d / V (n is an integer of 2 or more)].
As described above, the period t1 for switching the post-transfer exposure unit from turning off to turning on may be approximately (1 / (2n-1)) × (2 × L1-d) / V.

(5-B) In this embodiment, t1 = t2 + t3 (Formula 1) is used, but this is not necessarily the case. For example, t1 = t2 + t3 + t4 (formula 1 ') may be used. Here, the lighting cycle of the post-transfer exposure apparatus is t1, the lighting time of the post-transfer exposure apparatus is t2, the photosensitive drum potential detection time is t3, and the "post-transfer exposure apparatus is extinguished and the photosensitive drum potential detection pause" time is t4. To do. The time t4 may be a time for switching off and on the post-transfer exposure apparatus or a time for switching between execution and pause of the photosensitive drum potential detection. However, it is assumed that the above conditions 1 and 2 and expression 3 are satisfied.

(5-C) In this embodiment, t3 = t2 + d / V (Formula 2), but this is not necessarily the case. For example, t3 <t2 + d / V (formula 2 ') may be used. Here, d is the exposure width of the post-transfer exposure apparatus, and V is the peripheral speed of the photosensitive drum. (Equation 2 ') is satisfied when the potential of the photosensitive drum portion where the exposure amount of the post-transfer exposure apparatus is small is not detected and only the photosensitive drum portion similar to the post-transfer exposure history during image formation is detected. However, it is assumed that the above conditions 1 and 2 and expression 3 are satisfied.

(5-D) In this embodiment, when the photosensitive drum potential detection sequence is executed, the exposure width d is set to a width that is actually exposed by the post-transfer exposure apparatus. You may set smaller than the width | variety exposed with an apparatus. For example, depending on the exposure position of the photosensitive drum, the distance and angle with the post-transfer exposure apparatus are different, and the exposure amount is different. Further, the exposure time by the post-transfer exposure apparatus is also different. Therefore, the width of the photosensitive drum portion similar to the post-transfer exposure history during image formation may be set as the exposure width d.

(5-E) In this embodiment, when the photosensitive drum potential detection sequence is executed, the change width for changing the transfer voltage is not changed, but this is not necessarily the case. For example, coarse tone and fine tone may be combined. An example is shown below.
At the time of rough adjustment, the change width of the DC voltage applied to the transfer roller 5 is set sufficiently larger than the tolerance ΔIs. If the discharge current value Is does not converge between It−ΔIs and It + ΔIs only by rough adjustment, the operation shifts to fine adjustment.
At the time of fine adjustment, the change width of the DC voltage is set to a tolerance ΔIs or less. Thereby, the discharge current value Is converges between It−ΔIs and It + ΔIs.
The photosensitive drum potential detection sequence can be completed in a shorter time by detecting the photosensitive drum potential by combining coarse adjustment and fine adjustment as described above than detecting the photosensitive drum potential only by fine adjustment.

(5-F) In this embodiment, the photosensitive drum potential detection is performed during multiple pre-rotations or pre-rotations, but this is not necessarily limited thereto. For example, it may be executed at the time of non-image formation such as when a plurality of images are being output. Noise generated from other high-voltage power sources such as for development affects an electric circuit for obtaining a current flowing through the transfer roller. In order to avoid this noise, it is desirable to detect the surface potential of the photosensitive drum by detecting the current flowing through the transfer roller when other high-voltage power supply for development or the like is not operating.

(5-G) In this embodiment, an example of a rotatable transfer roller in which a conductive elastic layer is provided on a cored bar is given as the shape and resistance configuration of the transfer member, but is not limited thereto. For example, a multi-layered conductive blade / brush may be used as the transfer member.

(5-H) In the present embodiment, as a conductive member that applies a voltage in order to obtain a discharge start voltage,
Although a transfer member (transfer roller) is used, this is not necessarily the case. For example, the conductive member R may be disposed downstream of the transfer member and upstream of the charging member, and a voltage may be applied to the conductive member R to detect the photosensitive drum potential. When the conductive member R and the photosensitive drum are in contact with or close to each other to form a nip, and the photosensitive drum is discharged by exposing the nip upstream or downstream of the nip, the present invention can be implemented in a shorter time to achieve the photosensitive drum potential. Can be detected accurately. However, it is preferable to use the transfer member as the conductive member as in this embodiment because it can also be used as a circuit for applying a voltage to the conductive member, so that the image forming apparatus can be reduced in size and cost.

(5-I) In this embodiment, the DC voltage is applied to the charging member both when the photosensitive drum potential is detected and during the image forming operation. For example, a charging bias in which an AC voltage is superimposed on a DC voltage may be applied to the charging member when the photosensitive drum potential is detected and when an image is formed.

(5-J) The image forming apparatus is not limited to an electrophotographic apparatus. An electrostatic recording apparatus using an electrostatic recording dielectric as an image carrier on which an electrostatic latent image is formed may be used.

  As described above, by implementing the present invention, the potential of the photosensitive drum can be detected with a simple configuration and in a short time. Further, since it is not necessary to add a new member near the surface of the photosensitive drum, the potential of the photosensitive drum can be detected even when the diameter of the photosensitive drum is reduced.

  DESCRIPTION OF SYMBOLS 1 ... Photosensitive drum, 2 ... Charging roller (charging part), 3 ... Exposure apparatus (1st exposure part), 4 ... Developing apparatus, 5 ... Transfer roller (voltage application member), 13b ... Current detection circuit (current detection part) , 14 ... Post-transfer exposure apparatus (second exposure unit), 16 ... Control circuit unit (potential detection unit)

Claims (12)

A rotatable photosensitive drum carrying a toner image for forming an image on a recording material;
A charging unit for charging the photosensitive drum;
A first exposure unit that exposes the circumferential surface at a downstream side in a rotation direction of the photosensitive drum from a charging position charged by the charging unit on the circumferential surface of the photosensitive drum;
A voltage applying member that contacts the peripheral surface and applies a voltage to the photosensitive drum at a downstream side of the rotation direction from an exposure position exposed by the first exposure unit on the peripheral surface;
A second exposure unit that exposes the circumferential surface at a position downstream of the voltage application position in contact with the voltage application member on the circumferential surface and upstream of the charging position with respect to the charging direction;
A current detection unit for detecting a current value flowing through the photosensitive drum;
On the peripheral surface, an applied voltage value in the voltage application obtained by applying a voltage from the voltage application member to a post-static charge region exposed by the second exposure unit and charged by the charging unit; A potential detection unit for determining a surface potential of the photosensitive drum based on a relationship with a detected current value detected by the current detection unit by applying the voltage;
With
In the image forming apparatus in which an electrostatic latent image for forming the toner image is formed on the surface of the photosensitive drum based on the surface potential obtained by the potential detection unit.
When the surface potential is obtained by the potential detection unit, the voltage application member applies voltage to the charged region after the charge removal while the exposure by the second exposure unit is stopped while the photosensitive drum rotates once. An image forming apparatus characterized in that a detection period of 2 and a static elimination period for stopping the voltage application and performing exposure by the second exposure unit are alternately repeated a plurality of times.   The image forming apparatus according to claim 1, wherein the voltage application position is included in an irradiation range of exposure by the second exposure unit.   The image forming apparatus according to claim 1, wherein an irradiation range of exposure by the second exposure unit on the peripheral surface has a predetermined width in a circumferential direction.   The irradiation range of the exposure by the second exposure unit on the peripheral surface is wider in the circumferential direction than the region where the voltage application member contacts on the peripheral surface. 2. The image forming apparatus according to item 1.   5. The image forming apparatus according to claim 1, wherein one static elimination period is shorter than one detection period. 6.   The circumferential length of a region passing through the voltage application position on the circumferential surface during one detection period and the circumferential direction of the region exposed by the second exposure unit on the circumferential surface during one static elimination period The image forming apparatus according to claim 1, wherein the length of the image forming apparatus is equal to the length of the image forming apparatus. The cycle in which the static elimination period is repeated is t1,
The length of one charge removal period is t2,
The length of one detection period is t3,
And when
t1 = t2 + t3
The image forming apparatus according to claim 1, wherein: The length of one charge removal period is t2,
The length of one detection period is t3,
The circumferential length of the irradiation range of the exposure by the second exposure unit on the peripheral surface is d,
The peripheral speed of the photosensitive drum is V,
And when
t3 = t2 + d / V
The image forming apparatus according to claim 1, wherein: The cycle in which the static elimination period is repeated is t1,
The circumferential length of the photosensitive drum is L1,
The circumferential length of the irradiation range of the exposure by the second exposure unit on the peripheral surface is d,
The peripheral speed of the photosensitive drum is V,
And when
t1 = (1 / (2n-1)) * (2 * L1-d) / V
(N is an integer satisfying 2 ≦ n <L1 / d)
The image forming apparatus according to claim 1, wherein: The potential detection unit includes:
Obtaining a discharge start voltage value which is a positive and negative applied voltage value at which discharge starts to occur between the photosensitive drum and the voltage application member;
The image forming apparatus according to claim 1, wherein an applied voltage value intermediate between the two discharge start voltage values is the surface potential. The potential detection unit is further configured to expose the voltage applying member to a post-charge exposure area exposed by the second exposure unit, charged by the charging unit, and further exposed by the first exposure unit on the peripheral surface. The second surface potential of the photosensitive drum is obtained by applying a voltage to the photosensitive drum based on a relationship between an applied voltage value in the voltage application and a detected current value detected by the current detector by the voltage application. Seeking
When the second surface potential is obtained by the potential detection unit, exposure by the second exposure unit is stopped and the voltage application member applies voltage to the post-charge exposure region while the photosensitive drum rotates once. 11. The detection period for performing the above and the neutralization period for performing the exposure by the second exposure unit after stopping the voltage application are alternately repeated a plurality of times, respectively. The image forming apparatus according to claim 1.   The voltage applying member is a transfer member that applies a transfer voltage for transferring a toner image formed on a peripheral surface of the photosensitive drum to a recording material, to the photosensitive drum. The image forming apparatus according to any one of the above.

Images