JP2016156888A - Image forming apparatus and image forming method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To maintain the optimal density of an image on an image carrier, and in turn, on a sheet, even when a developer is used for a long period and a carrier is largely deteriorated.SOLUTION: An image forming method includes the steps of: forming a toner patch image for controlling the density of an image on an image carrier when a potential difference ΔVd between an exposure potential and a developing bias potential is a predetermined potential difference (steps S11 and S12); reading the density of the toner patch image with an image density sensor to acquire an output value Dsig of the image density sensor (step S13); determining that adhesion of a carrier has occurred, that is, a carrier phenomenon has occurred when an error to a reference value of the output value Dsig of the image density sensor is equal to or more than a predetermined value (for example, 0.5 V) (step S14); and correcting the output value Dsig of the image density sensor to control an image forming condition (step S15).SELECTED DRAWING: Figure 9

Description

  The present invention relates to an image forming apparatus and an image forming method, and more particularly to an electrophotographic image forming apparatus and an image forming method.

  In an electrophotographic image forming apparatus, due to the characteristics of forming an image using static electricity, environmental conditions such as temperature and humidity in the environment in which the apparatus is used, and deterioration of the developer over time, that is, durability The density and the line width of the image fluctuate due to the change in. As a result, a stable image cannot be formed.

  Therefore, in an electrophotographic image forming apparatus, in order to realize stable image formation, control is performed to detect the density of a toner patch image for image density control with an image density sensor and feed back to the image forming conditions. (For example, refer to Patent Document 1). The toner patch image is a patch-like toner image formed exclusively for controlling (adjusting) the image density.

JP 2005-165091 A

  Incidentally, in an image forming apparatus using a two-component developer, the carrier deteriorates as the durability of the developer advances. Specifically, as the durability of the developer progresses, the coat layer of the carrier is depleted and the resistance value of the carrier decreases. As a result, carrier adhesion (hereinafter referred to as “carrier development”) occurs on the image carrier. On the other hand, the diameter of the toner is about 5 [μm], whereas the diameter of the carrier is about 60 [μm], which is about 10 times the size of the toner. Therefore, when the carrier is developed in the toner patch image for image density control, the image density sensor erroneously detects the density of the toner patch image. As a result, the image density on the image carrier and consequently on the paper cannot be optimally maintained.

  Therefore, the present invention provides an image forming apparatus capable of optimally maintaining the image density on the image carrier, and thus on the paper, even when the durability of the developer advances and the deterioration of the carrier increases. An object is to provide an image forming method.

In order to achieve the above object, an image forming apparatus of the present invention includes:
An image forming unit that forms an image density control pattern image on the image carrier when the potential difference between the exposure potential and the developing bias potential is a predetermined potential difference;
A density detector for detecting the density of the image density control pattern image;
The presence or absence of carrier adhesion on the image carrier is determined based on the toner adhesion amount obtained from the output value based on the density detected by the density detection unit. A control unit for controlling
It is characterized by providing.

The image forming method of the present invention includes
Forming an image density control pattern image on the image carrier when the potential difference between the exposure potential and the development bias potential is a predetermined potential difference;
Detecting the density of the image density control pattern image;
The presence or absence of carrier adhesion on the image carrier is determined based on the toner adhesion amount obtained from the output value based on the density detected by the density detection unit. Controlling the process,
The process of each process is performed.

  In the image forming apparatus or the image forming method configured as described above, by detecting the density of the image density control pattern image formed on the image carrier when the potential difference between the exposure potential and the development bias potential is a predetermined potential difference, The toner adhesion amount on the image carrier can be obtained from the detected value. Further, it is possible to determine the presence or absence of carrier adhesion on the image carrier from the toner adhesion amount on the image carrier. When it is determined that there is carrier adhesion, the image forming conditions of the image forming unit are controlled.

  According to the present invention, the presence or absence of carrier adhesion is determined, and the image forming conditions are controlled when carrier adhesion is present. The image density on the carrier and thus on the paper can be maintained optimally.

1 is an overall configuration diagram illustrating an outline of a system configuration of an image forming apparatus according to an embodiment of the present invention. 2 is a block diagram illustrating a hardware configuration of each unit of the image forming apparatus according to the embodiment of the present disclosure. FIG. It is a figure which shows the change of the number of carriers with respect to potential difference (DELTA) Vd of exposure potential and developing bias potential. FIG. 5 is a model diagram for explaining erroneous detection of an image density sensor when a carrier adheres to a toner patch image for image density control. It is a figure which shows the change of the output value Dsig of an image density sensor with respect to potential difference (DELTA) Vd of exposure potential and developing bias potential. FIG. 6 is a diagram illustrating a change in the amount of toner adhesion on an image carrier with respect to a potential difference ΔVd between an exposure potential and a development bias potential. FIG. 10 is a diagram illustrating a specific example for obtaining a slope of a potential difference ΔVd−toner adhesion amount curve when carrier development occurs. It is a figure showing the ratio of the inclination with respect to the inclination of each electric potential difference range in case with generation | occurrence | production of carrier development, and a reference | standard. 3 is a flowchart illustrating a processing procedure of a method for controlling an image forming condition according to the first exemplary embodiment. 10 is a flowchart illustrating a processing procedure of an image forming condition control method according to the second embodiment. 10 is a flowchart illustrating a processing procedure of an image forming condition control method according to a third embodiment.

  Hereinafter, modes for carrying out the present invention (hereinafter referred to as “embodiments”) will be described in detail with reference to the drawings. The present invention is not limited to the embodiment, and various numerical values in the embodiment are examples. In the following description and each drawing, the same reference numerals are used for the same elements or elements having the same functions, and duplicate descriptions are omitted.

[Configuration example of image forming apparatus]
FIG. 1 is an overall configuration diagram showing an outline of a system configuration of an image forming apparatus according to an embodiment of the present invention. In this embodiment, a case where the present invention is applied to a copying machine is taken as an example.

  As shown in FIG. 1, the image forming apparatus 1 according to the present embodiment employs an electrophotographic system that forms an image using static electricity, and includes yellow (Y), magenta (M), and cyan (C). And a tandem color image forming apparatus that superimposes four color toners of black (K). The image forming apparatus 1 includes a document conveying unit 10, a paper storage unit 20, an image reading unit 30, an image forming unit 40, an intermediate transfer belt 50, a secondary transfer unit 60, a fixing unit 80, a control unit, and a control unit. And a substrate 90.

  The document transport unit 10 includes a document feed table 11 on which a document is set, a plurality of rollers 12, a transport drum 13, a transport guide 14, a document discharge roller 15, and a document discharge tray 16. The documents G set on the document feeder 11 are conveyed one by one to the reading position of the image reading unit 30 by the plurality of rollers 12 and the conveying drum 13. The conveyance guide 14 and the document discharge roller 15 discharge the document G conveyed by the plurality of rollers 12 and the conveyance drum 13 to the document discharge tray 16.

  The image reading unit 30 reads an image of the document G transported by the document transport unit 10 or the document placed on the document table 31 and generates image data. Specifically, the image of the original G is irradiated by the lamp L. The reflected light from the document G based on the light emitted from the lamp L is guided in the order of the first mirror unit 32, the second mirror unit 33, and the lens unit 34, and forms an image on the light receiving surface of the image sensor 35. The image sensor 35 photoelectrically converts incident light and outputs a predetermined image signal. The output image signal is created as image data by A / D conversion.

  The image reading unit 30 has an image reading control unit 36. The image reading control unit 36 performs well-known image processing such as shading correction, dither processing, and compression on the image data created by A / D conversion, and stores the image data in a RAM (not shown) mounted on the control board 90. To do. Note that the image data is not limited to data output from the image reading unit 30, and may be data received from an external device such as a personal computer connected to the image forming apparatus 1 or another image forming apparatus.

  The paper storage unit 20 is disposed in the lower part of the apparatus main body, and a plurality of paper storage units 20 are provided according to the size and type of the paper S. The sheet S is fed by the sheet feeding unit 21 and sent to the transport unit 23, and is transported by the transport unit 23 to the secondary transfer unit 60 that is a transfer position. Further, a manual feed portion 22 is provided in the vicinity of the paper storage portion 20. From the manual feed section 22, special paper such as paper of a size not set in the paper storage section 20, tag paper having a tag, or OHP sheet set by the user is sent to the transfer position.

  Between the image reading unit 30 and the paper storage unit 20, an image forming unit 40 and an intermediate transfer belt 50 as an image carrier are arranged. The image forming unit 40 includes four image forming units 40Y, 40M, 40C, and 40K in order to form toner images of each color of yellow (Y), magenta (M), cyan (C), and black (K). .

  The first image forming unit 40Y forms a yellow toner image, and the second image forming unit 40M forms a magenta toner image. The third image forming unit 40C forms a cyan toner image, and the fourth image forming unit 40K forms a black toner image. These four image forming units 40Y, 40M, 40C, and 40K have the same configuration. Therefore, here, the first image forming unit 40Y will be described.

  The first image forming unit 40Y includes a drum-shaped photoconductor 41 as an image carrier, a charging unit 42 disposed around the photoconductor 41, an exposure unit 43, a developing unit 44, and a cleaning unit 45. have. The photoconductor 41 rotates under driving by a drive motor (not shown). The charging unit 42 uniformly charges the surface of the photoconductor 41 by applying a charge to the photoconductor 41. The exposure unit 43 forms an electrostatic latent image on the photoconductor 41 by exposing the surface of the photoconductor 41 based on image data read from the document G or image data transmitted from an external device. To do.

  The developing unit 44 develops the electrostatic latent image formed on the photoconductor 41 using a two-component developer composed of toner and carrier. The toner is particles that form an image. The carrier has a function of imparting an appropriate charge to the toner by frictional charging in mixing with the toner in the developing unit 44, a function of transporting the toner to the developing area facing the photoconductor 41, and an electrostatic on the photoconductor 41. It has a function of forming a developing electric field so that the toner can be faithfully developed on the latent image. The developing unit 44 attaches yellow toner to the electrostatic latent image formed on the photoreceptor 41. As a result, a yellow toner image is formed on the surface of the photoreceptor 41.

  The developing unit 44 of the second image forming unit 40M attaches magenta toner to the photoconductor 41, and the developing unit 44 of the third image forming unit 40C attaches cyan toner to the photoconductor 41. Then, the developing unit 44 of the fourth image forming unit 40K adheres black toner to the photoconductor 41.

  The cleaning unit 45 removes the toner remaining on the surface of the photoreceptor 41.

  The toner attached on the photoreceptor 41 is transferred to an intermediate transfer belt 50 that is an example of an intermediate transfer member. The intermediate transfer belt 50 is formed in an endless shape and is stretched around a plurality of rollers. The intermediate transfer belt 50 rotates in a direction opposite to the rotation (movement) direction of the photoconductor 41 under the drive of a drive motor (not shown).

  A primary transfer portion 51 is provided at a position on the intermediate transfer belt 50 that faces the photoreceptor 41 of each of the image forming units 40Y, 40M, 40C, and 40K. The primary transfer unit 51 transfers the toner attached on the photoreceptor 41 to the intermediate transfer belt 50 by applying a voltage having a polarity opposite to that of the toner to the intermediate transfer belt 50.

  As the intermediate transfer belt 50 rotates, the toner images formed by the four image forming units 40Y, 40M, 40C, and 40K are sequentially transferred onto the surface of the intermediate transfer belt 50. Thus, a color image is formed on the intermediate transfer belt 50 by overlapping toner images of yellow, magenta, cyan, and black.

  A belt cleaning device 53 faces the intermediate transfer belt 50. The belt cleaning device 53 cleans the surface of the intermediate transfer belt 50 that has finished transferring the toner image onto the paper S.

  A secondary transfer unit 60 is disposed in the vicinity of the intermediate transfer belt 50 and downstream of the transport unit 23 in the paper transport direction. The secondary transfer unit 60 transfers the toner image formed on the outer peripheral surface of the intermediate transfer belt 50 to the sheet S by bringing the sheet S conveyed by the conveyance unit 23 into contact with the intermediate transfer belt 50.

  The secondary transfer unit 60 has a secondary transfer roller 61. The secondary transfer roller 61 is in pressure contact with the counter roller 52. A portion where the secondary transfer roller 61 and the intermediate transfer belt 50 come into contact becomes a secondary transfer nip portion 62. The position of the secondary transfer nip portion 62 is a transfer position at which the toner image formed on the outer peripheral surface of the intermediate transfer belt 50 is transferred to the paper S.

  A fixing unit 80 is provided on the discharge side of the sheet S in the secondary transfer unit 60. The fixing unit 80 pressurizes and heats the paper S to fix the transferred toner image on the paper S. The fixing unit 80 includes, for example, a fixing upper roller 81 and a fixing lower roller 82 which are a pair of fixing members. The upper fixing roller 81 and the lower fixing roller 82 are arranged in pressure contact with each other, and a fixing nip portion is formed as a pressing portion between the upper fixing roller 81 and the lower fixing roller 82.

  A heating unit is provided inside the fixing upper roller 81. The roller portion of the fixing upper roller 81 is warmed by the radiant heat from the heating portion. Then, the heat of the roller portion of the fixing upper roller 81 is transmitted to the sheet S, whereby the toner image on the sheet S is fixed.

  The sheet S is conveyed so that the surface on which the toner image is transferred by the secondary transfer unit 60 (the surface to be fixed) faces the fixing upper roller 81, and passes through the fixing nip portion. Accordingly, the sheet S passing through the fixing nip is subjected to pressure by the fixing upper roller 81 and the fixing lower roller 82 and heating by the heat of the roller portion of the fixing upper roller 81.

  A switching gate 24 is disposed downstream of the fixing unit 80 in the conveyance direction of the sheet S. The switching gate 24 switches the transport path of the paper S that has passed through the fixing unit 80. That is, the switching gate 24 moves the paper S straight when performing face-up paper discharge in image formation on one side of the paper S. As a result, the paper S is discharged by the pair of paper discharge rollers 25. The switching gate 24 guides the paper S downward when performing face-down paper discharge in image formation on one side of the paper S and when performing image formation on both sides of the paper S.

  When face-down paper discharge is performed, the paper S is guided downward by the switching gate 24, and then the front and back of the paper S is reversed and transported upward by the paper reversal transport unit 26. As a result, the paper S whose front and back are reversed is discharged by the pair of paper discharge rollers 25. When image formation is performed on both sides of the paper S, the paper S is guided downward by the switching gate 24, and then the front and back of the paper S are reversed by the paper reversing conveyance unit 26. Then, the sheet S with the front and back reversed is sent again to the transfer position by the refeed path 27.

  A post-processing device that folds the paper S or performs stapling processing or the like on the paper S may be disposed on the downstream side of the pair of paper discharge rollers 25.

  Further, an image density sensor 91 that is an example of a density detection unit that detects the density of an image is disposed between the belt cleaning device 53 and the secondary transfer unit 60 in the intermediate transfer belt 50. That is, the image density sensor 91 is disposed on the downstream side of the secondary transfer unit 60 in the rotation direction of the intermediate transfer belt 50. The image density sensor 91 includes a light emitting unit that emits light toward the intermediate transfer belt 50 and a light receiving unit that receives reflected light from the intermediate transfer belt 50 based on the irradiated light. The image density sensor 91 is provided so that the light emitting portion and the light receiving portion face the surface of the intermediate transfer belt 50.

[Hardware Configuration of Each Part of Image Forming Apparatus]
Next, a hardware configuration of each unit of the image forming apparatus 1 will be described with reference to FIG. FIG. 2 is a block diagram illustrating a hardware configuration of each unit of the image forming apparatus 1.

  As illustrated in FIG. 2, the image forming apparatus 1 includes a control unit 100. The control unit 100 is configured on the control board 90 (see FIG. 1) described above.

  The control unit 100 includes, for example, a CPU (Central Processing Unit) 101, a ROM (Read Only Memory) 102 for storing programs executed by the CPU 101, and a RAM (Random Access Memory) used as a work area of the CPU 101. 103. As the ROM 102, for example, a programmable ROM that can be normally electrically erased can be used.

  The CPU 101 controls the entire image forming apparatus 1. The CPU 101 is connected to the HDD 104, the operation display unit 105, and the communication unit 108 via the system bus 107, respectively. Further, the CPU 101 is connected to the image reading unit 30, the image processing unit 106, the image forming unit 40, the paper feeding unit 21, the secondary transfer unit 60, the fixing unit 80, and the image density sensor 91 via the system bus 107. .

  The HDD 104 stores image data of an original image obtained by reading by the image reading unit 30, or stores output image data and the like. The operation display unit 105 is a touch panel including a display such as a liquid crystal display (LCD) or an organic ELD (Electro Luminescence Display). The operation display unit 105 displays an instruction menu for the user, information about the acquired image data, and the like. Further, the operation display unit 105 includes a plurality of keys, receives input of various instructions, data such as characters and numbers, and outputs an input signal to the control unit 100.

  The communication unit 108 receives job information transmitted from an external information processing apparatus PC (personal computer) 120 via a communication line. The received job information is sent to the control unit 100 via the system bus 107. The job information includes image data of an image to be formed and information such as the type and number of sheets to be used associated with the image data.

  In this embodiment, an example in which a personal computer is applied as an external device has been described. However, the present invention is not limited to this, and various other devices such as a facsimile device can be applied as the external device.

  The image density sensor 91 is driven and controlled by the control unit 100 to irradiate the intermediate transfer belt 50 with light and to receive light reflected from the intermediate transfer belt 50. The image density sensor 91 detects the density of the image density control pattern image on the intermediate transfer belt 50 by the irradiation and reception of the light, and transmits the detection result to the control unit 100. Based on the information received from the image density sensor 91, the control unit 100 adjusts image forming conditions when the image forming unit 40 forms an image. Details of the adjustment of the image forming conditions will be described later.

  The image reading unit 30 optically reads a document image and converts it into an electrical signal. For example, when reading a color document, image data having luminance information of gradation of 10 bits for each of RGB per pixel is generated. Image data generated by the image reading unit 30 and image data transmitted from the PC 120 which is an example of an external device connected to the image forming apparatus 1 are sent to the image processing unit 106 and subjected to image processing. The image processing unit 106 performs processes such as analog processing, A / D conversion, shading correction, and image compression on the received image data.

  For example, when a color image is formed by the image forming apparatus 1, R, G, B image data generated by the image reading unit 30 or the like is input to a color conversion LUT (Look Up Table) in the image processing unit 106. Then, the image processing unit 106 performs color conversion of the R, G, and B data into Y, M, C, and K image data. Then, correction of gradation reproduction characteristics, screen processing such as halftone dots with reference to the density correction LUT, or edge processing for emphasizing thin lines is performed on the color-converted image data.

  The control unit 100 drives and controls the image forming unit 40 to form a toner image for image density control or a toner image for image formation, and performs primary transfer onto the intermediate transfer belt 50. In addition, the control unit 100 drives and controls the secondary transfer unit 60 to secondary-transfer the toner image carried by the intermediate transfer belt 50 onto the paper S. Further, the control unit 100 drives and controls the fixing unit 80 to pressurize and heat the paper S, thereby fixing the toner image on the paper S.

[Image stabilization control]
In the electrophotographic image forming apparatus 1 described above, image stabilization control for adjusting image forming conditions of the image forming unit 40 is performed so that the density of an image to be formed (that is, an output image) becomes a target density. Examples of the image forming conditions include development bias voltage, development θ, toner density, and the like. The development bias voltage is set to be larger by ΔVd than the exposure potential, that is, the surface potential of the photoreceptor 41 after the exposure. As a result, an electric field is generated between the photosensitive member 41 and the developer carrying member due to the potential difference ΔVd. Further, the development θ is a ratio of the developer conveyance speed to the process speed.

  In the image stabilization control, the image density sensor 91 detects the density of the image density control pattern image formed on the image carrier such as the intermediate transfer belt 50, and the detection result (output value) is the image of the image forming unit 40. This is performed by feeding back to the forming conditions and reflecting them in the image forming conditions. Here, the “image density control pattern image” is a patch-like toner image formed to control (adjust) the image density, for example, called a toner patch image. By performing this image stabilization control, an image can be stably formed even if there is a factor that makes the image formation unstable.

[About carrier development]
In the electrophotographic image forming apparatus 1, when the potential difference ΔVd between the exposure potential and the developing bias potential increases, the number of carriers attached to the toner portion (hereinafter referred to as “carrier number”) increases. This is carrier development. FIG. 3 shows a change in the number of carriers [piece / unit area] with respect to the potential difference ΔVd [V]. In FIG. 3, a change in the case of the developer at the end of the durability is indicated by a solid line, and a change in the case of the end of the durability of the developer is indicated by a broken line.

  In the early stage of the durability of the developer, the number of carriers attached to the toner portion is extremely small until the potential difference ΔVd is about 700 [V], and slightly increases when the potential difference ΔVd exceeds 700 [V]. On the other hand, at the end of the durability of the developer, the number of carriers adhering to the toner portion is small until the potential difference ΔVd is about 400 [V], but starts increasing when the potential difference ΔVd exceeds 400 [V], and is 50 at 600 [V]. It increases to about [piece / unit area], about 800 [V], about 120 [piece / unit area], and about 900 [V] to about 200 [piece / unit area].

  Further, as the durability of the developer progresses and the coat layer of the carrier is depleted, the carrier resistance value is lowered and carrier development is likely to occur. Specifically, when the resistance value of the carrier decreases, negative charges are injected into the carrier when a negative high voltage is applied with the developing bias, and the same polarity as the toner is obtained. As a result, the carrier having a lowered resistance value is subjected to a force by an electric field from the surface layer of the developer carrying member (developing sleeve) of the developing unit 44 toward the photosensitive member 41 and is developed in the same manner as normal toner. That is, carrier development occurs.

[Error detection of image density sensor by carrier]
Here, since the carrier is about 10 times larger than the toner, when the carrier has been developed in the toner patch image, which is the image density control pattern image, the density of the toner patch image is set to the image. The density sensor 91 will falsely detect. The erroneous detection of the image density sensor 91 by this carrier will be described more specifically with reference to FIG.

  FIG. 4 is a model diagram illustrating erroneous detection of the image density sensor 91 when a carrier adheres to the toner patch image for image density control. FIG. 4A shows a model when no carrier development occurs, and FIG. 4B shows a model when carrier development occurs. The image density sensor 91 includes a light emitting unit 911 made of a light emitting diode or the like, and a light receiving unit 912 made of a phototransistor or the like.

  When no carrier development occurs (FIG. 4A), the light irradiated from the light emitting unit 911 to the toner patch image on the image carrier is reflected by the surface of the toner, and the reflected light enters the light receiving unit 912. As a result, the image density sensor 91 outputs a density detection signal at a level corresponding to the amount of reflected light. The image density sensor 91 outputs a large level density detection signal because the amount of reflected light from the image carrier increases when the amount of toner is small. Conversely, when the amount of toner is large, the image density sensor 91 reflects from the image carrier. Since the amount of light decreases, a low level density detection signal is output.

  In the case of occurrence of carrier development (FIG. 4B), light is emitted from the light emitting portion 911 to the toner patch image on the image carrier, but the carrier (with a diameter of about 5 [μm]) is much larger than the toner (diameter is about 5 μm). Part of the light reflected from the surface of the toner cannot enter the light receiving portion 912. As a result, the level (output value) of the density detection signal of the light receiving unit 912 becomes lower than when no carrier development occurs, so the image density sensor 91 erroneously detects that the image density is high.

[Relationship of sensor output value Dsig to potential difference ΔVd]
FIG. 5 shows a change in the output value (density detection signal level) Dsig [V] of the image density sensor 91 with respect to the potential difference ΔVd [V] between the exposure potential and the development bias potential. In FIG. 5, the change when no carrier development occurs is indicated by a solid line, and the change when carrier development occurs is indicated by a broken line. FIG. 5 shows changes in the output value Dsig of the image density sensor 91 when no carrier development occurs when the potential difference ΔVd is changed every 100 [V] to create a toner patch image for image density control. It shows the change in the output value Dsig of the image density sensor 91 when carrier development occurs.

  Carrier development occurs as the durability of the developer progresses. Therefore, regarding the change in the output value Dsig of the image density sensor 91 when no carrier development occurs, the potential difference ΔVd is changed every 100 [V] in the initial durability of the developer, and the toner patch image for image density control is used. Can be obtained by detecting the density of the image by the image density sensor 91. On the other hand, the change in the output value Dsig of the image density sensor 91 when carrier development occurs is caused by changing the potential difference ΔVd every 100 [V] during the endurance of the developer and the toner patch for image density control. The measured value of the image density sensor 91 changes when the image is created.

  As is apparent from FIG. 5, when no carrier development occurs, the output value Dsig of the image density sensor 91 gradually decreases as the potential difference ΔVd between the exposure potential and the development bias potential increases. On the other hand, when carrier development occurs, the output value Dsig of the image density sensor 91 greatly decreases in a region where the potential difference ΔVd between the exposure potential and the development bias potential is high.

  According to the experiment by the present inventor, it has been confirmed that white spots due to carriers occur on the image under the condition that the potential difference ΔVd between the exposure potential and the development bias potential is ΔVd = 600 [V]. In the vicinity of ΔVd = 600 [V], the error of the output value Dsig of the image density sensor 91 between the case where no carrier development occurs and the case where carrier development occurs is about 0.5 [V].

  The comparison between the developer at the initial stage of durability and the developer having advanced durability is performed by aligning the toner charge amount Q / M (Q is the charge amount, and M is the mass). When the developer is in the early stage of durability, a plurality of levels are set by changing the toner density Tc [%] in the developer, and the potential difference ΔVd and output value of the image density sensor 91 at a plurality of different charge amounts Q / M. A relational expression with Dsig is obtained and stored.

  From the above-described relationship between the output value Dsig of the image density sensor 91 and the potential difference ΔVd between the exposure potential and the development bias potential, the output value Dsig of the image density sensor 91 is about 0.5 [ When the error (difference) in V] occurs, it can be determined that carrier development has occurred. Specifically, the output value Dsig of the image density sensor 91 when no carrier development occurs is used as a reference value for determining whether carrier development has occurred. Then, when the difference between the actually measured value (output value Dsig) of the image density sensor 91 with respect to the reference value is equal to or larger than a predetermined value (for example, 0.5 [V]), it is determined that the carrier is attached, that is, the carrier development occurs. .

[Relationship of toner adhesion amount on image carrier to potential difference ΔVd]
FIG. 6 shows a change in the toner adhesion amount [g / m ² ] on the image carrier with respect to the potential difference ΔVd [V] between the exposure potential and the development bias potential. In FIG. 6, the change when no carrier development occurs is indicated by a solid line, and the change when carrier development occurs is indicated by a broken line. FIG. 6 shows a change in toner adhesion amount when no carrier development occurs and the occurrence of carrier development when a toner patch image for image density control is created by changing the potential difference ΔVd every 100 [V]. The change in the toner adhesion amount in the case of.

  When no carrier development occurs, the toner adhesion amount on the image carrier increases linearly with respect to the potential difference ΔVd between the exposure potential and the development bias potential. On the other hand, when carrier development occurs, the relationship between the toner adhesion amount and the potential difference ΔVd between the exposure potential and the development bias potential is not a linear relationship but changes so as to have an inflection point. Therefore, it is possible to determine the presence or absence of carrier adhesion, that is, the occurrence of carrier development, based on whether or not there is an inflection point in the change in toner adhesion amount with respect to the potential difference ΔVd between the exposure potential and the development bias potential.

  Whether there is an inflection point in the change in the toner adhesion amount with respect to the potential difference ΔVd can be determined as follows. In the example shown in FIG. 6, since there is almost no carrier development in a region where the potential difference ΔVd between the exposure potential and the development bias potential is small, specifically, in the region of 100 [V] to 300 [V], in this region. The slope of the linear straight line is used as a reference. An inflection point is obtained when the slope of the curve in the region of 300 [V] or more is a predetermined ratio or more with respect to the slope of the reference with respect to the slope in the region of 100 [V] -300 [V]. It is determined that carrier development has occurred.

  Here, a specific example of obtaining the slope of the curve of potential difference ΔVd−toner adhesion amount when carrier development occurs will be described with reference to FIG. In the example shown in FIG. 7, 100 [V] -300 [V], 300 [V] -500 [V], 400 [V] -600 [V], 500 [V] -700 [V], 700 [V] ] In the potential difference range of −900 [V], for example, the slope of the straight line in each potential difference range is calculated from the data of each three points every 100 [V]. For example, in the potential difference range of 300 [V] -500 [V], the slope is obtained from three points of data of 300 [V], 400 [V], 500 [V] indicated by □, and 500 [V] -700. In the potential difference range of [V], the slope is obtained from three points of data of 500 [V], 600 [V] and 700 [V] indicated by Δ.

  In each potential difference range, the inclination of two straight lines is obtained from data of two points out of the data of each three points, and the average value of the inclinations of the two straight lines is set as the inclination of the straight line of each potential difference range. Here, the slope of the straight line in each potential difference range is calculated from the data of each three points every 100 [V], but the slope may be calculated from the data of two points, or every 50 [V]. Thus, the inclination may be calculated from data of four or more points. The greater the number of data, the higher the accuracy of calculating the slope.

  As an example, in the case of the example shown in FIG. 7, the equation of the straight line in the potential difference range of 100 [V] −300 [V] is y = 0.0097x + 0.0133, and the inclination is 0.0097. . The equation of the straight line in the potential difference range of 300 [V] -500 [V] is y = 0.0109x-0.2967, and the equation of the straight line in the potential difference range of 400 [V] -600 [V] is y = 0.012x-0.7667. Further, the equation of the straight line in the potential difference range of 500 [V] -700 [V] is y = 0.0145x-21.667, and the equation of the straight line in the potential difference range of 700 [V] -900 [V] is , Y = 0.02x-6. Incidentally, the expression of the linear line when no carrier development occurs is y = 0.0089x + 0.3375.

  Then, the ratio of the slope of each potential difference range is calculated based on the slope of the potential difference range of 100 [V] -300 [V]. FIG. 8 shows the slope of each potential difference range and the ratio of the slope to the reference when carrier development occurs. When the slope in the potential difference range of 100 [V] -300 [V] is used as a reference, the ratio of the slope with respect to this reference is 1.12 times and 400 [V in the potential difference range of 300 [V] -500 [V]. ] 1.24 times in the potential difference range of -600 [V], 1.49 times in the potential difference range of 500 [V] -700 [V], 2.06 times in the potential difference range of 700 [V] -900 [V] It becomes.

  As described above, according to the experiments of the present inventors, white spots due to carriers may occur on the image under the condition that the potential difference ΔVd between the exposure potential and the developing bias potential is ΔVd = 600 [V]. It has been confirmed. Therefore, the slope in the potential difference range of 100 [V] −300 [V] is used as a reference from the relationship of the slope with respect to the slope of the reference linear line in each potential difference range. Then, it is determined that there is an inflection point in the change in the toner adhesion amount with respect to the potential difference ΔVd in the potential difference range where the ratio of the slope is about 1.3 times or more with respect to the reference slope, that is, carrier development has occurred. can do.

  As described above, the change in the toner adhesion amount on the image carrier with respect to the potential difference ΔVd is obtained from the output values Dsig of the image density sensor 91 regarding the plurality of toner patch images corresponding to the plurality of potential differences ΔVd. Based on this, it is possible to determine the presence or absence of carrier adhesion, that is, the presence or absence of occurrence of carrier development.

  That is, the presence or absence of occurrence of carrier development is determined based on the difference (error) of the actually measured value (output value Dsig) of the image density sensor 91 with respect to the reference value acquired from the output value Dsig of the image density sensor 91 for a plurality of toner patch images. Judgment can be made. Specifically, in the case of the example shown in FIG. 5, the output value Dsig of the image density sensor 91 when no carrier development occurs is used as a reference value, and the difference between the measured values of the image density sensor 91 with respect to the reference value is When the value is equal to or higher than a predetermined value (for example, 0.5 [V]), it can be determined that carrier development has occurred.

  The presence or absence of carrier adhesion can also be determined based on whether or not there is an inflection point in the change in the toner adhesion amount on the image carrier relative to the potential difference ΔVd between the exposure potential and the development bias potential. Specifically, in the case of the example shown in FIG. 7, the potential difference in which the ratio of the slope is 1.3 times or more with respect to the reference slope in the potential difference range of 100 [V] −300 [V]. In the range, it can be determined that there is an inflection point in the change in the toner adhesion amount with respect to the potential difference ΔVd, and that carrier development has occurred.

Alternatively, “change in toner adhesion amount on the image carrier with respect to potential difference ΔVd between the exposure potential and the development bias potential” in consideration of changes in the toner charge amount Q / M in durability and the toner concentration Tc [%] in the developer. Is measured in advance. The difference between the potential difference ΔVd and the toner adhesion amount on the image carrier obtained from the output value Dsig of the image density sensor 91 is, for example, 1 [ g / m ² ] or more, it can be determined that carrier development has occurred.

[Control of image forming conditions when carrier development occurs]
In one embodiment of the present invention, the presence or absence of carrier development is determined based on the toner adhesion amount on the image carrier obtained from the output value Dsig of the image density sensor 91, and carrier development has occurred, that is, carrier adhesion has occurred. In this case, the image forming conditions of the image forming unit 40 are controlled. This control is executed under the control of the control unit 100 in FIG.

  That is, the control unit 100 obtains the toner adhesion amount on the image carrier obtained from the output value Dsig of the image density sensor 91, determines whether carrier development has occurred based on the toner adhesion amount, and generates carrier development. When there is, the image forming conditions are controlled. By this control, even when the durability of the developer advances and the deterioration of the carrier becomes large, the image density on the image carrier and consequently on the paper can be optimally maintained.

  A specific example of a method for controlling image forming conditions (image forming method) executed under the control of the control unit 100 when carrier development occurs will be described below. Since the carrier development occurs when the durability of the developer progresses, it is preferable to execute this control from the stage where the usage period of the developer has passed a predetermined period. Further, the control by the control unit 100 is executed at a certain period, for example, at a time interval of about 20 to 30 hours after a predetermined period of use of the developer.

Example 1
FIG. 9 is a flowchart illustrating the processing procedure of the image forming condition control method according to the first embodiment.

  First, the control unit 100 executes a process of setting a potential difference ΔVd between the exposure potential and the development bias potential (step S11). The potential difference ΔVd can be changed by changing the developing bias voltage or the exposure energy of the image forming unit. The same applies to the embodiments described later. In the process of step S11, first, the potential difference ΔVd is set to 100 [V] (see FIG. 5).

  Next, the control unit 100 creates a toner patch image for image density control on the image carrier under the set potential difference ΔVd (step S12), and then acquires the output value Dsig of the image density sensor 91 ( Step S13). From the output value Dsig of the image density sensor 91, the density of the toner patch image, more specifically, the toner adhesion amount on the image carrier can be obtained.

  Next, when the output value Dsig when there is no occurrence of carrier development is used as a reference value, the control unit 100 determines that the error of the actually measured value (output value Dsig) of the image density sensor 91 with respect to the reference value is, for example, about 0. It is determined whether it is 5 [V] or more (step S14). If the error amount of the output value Dsig of the image density sensor 91 is about 0.5 [V] or more as a result of this determination processing, carrier adhesion is present on the image carrier, that is, the carrier, as described with reference to FIG. It can be determined that development has occurred.

  If the control unit 100 determines in step S14 that the error of the output value Dsig of the image density sensor 91 is less than about 0.5 [V] (NO), the control unit 100 returns to step S11 and goes from step S11 to step S14. Repeat the process. As described with reference to FIG. 5, in the vicinity of ΔVd = 600 [V], the error of the output value Dsig of the image density sensor 91 between the case where no carrier development occurs and the case where carrier development occurs is about 0.5 [V]. Therefore, in the case of this example, the processing from step S11 to step S14 is repeated until the potential difference ΔVd is set to 600 [V] in step S11.

  In step S14, the control unit 100 determines (YES) that the error amount of the output value Dsig of the image density sensor 91 that is an actual measurement value is about 0.5 [V] or more, that is, the occurrence of carrier development. In this case, the image forming condition is changed by correcting the output value Dsig of the image density sensor 91 (step S15). In the process of correcting the output value Dsig of the image density sensor 91, the control unit 100 performs a correction process of adding an error (0.5 [V]) to the output value Dsig (detected value) of the image density sensor 91.

  As described above, according to the control method according to the first embodiment, when it is determined that carrier development has occurred, the output value Dsig (detected value) of the image density sensor 91 has an error (0.5 [V]). The control which adds is performed. As a result, even when the durability of the developer progresses and the deterioration of the carrier becomes large, the image density on the image carrier and consequently on the paper can be optimally maintained.

(Example 2)
FIG. 10 is a flowchart illustrating the processing procedure of the image forming condition control method according to the second embodiment.

  The control unit 100 first executes a process of setting a potential difference ΔVd between the exposure potential and the developing bias potential (step S21), and then the toner for controlling the image density on the image carrier under the set potential difference ΔVd. A patch image is created (step S22), and then the output value Dsig of the image density sensor 91 is acquired (step S23).

  Next, when the control unit 100 uses the output value Dsig when no carrier development occurs as a reference, whether the error of the output value Dsig of the image density sensor 91 with respect to the reference is about 0.5 [V] or more, for example. It is determined whether or not (step S24). In the case of this example, the control unit 100 repeats the processing from step S21 to step S24 until the potential difference ΔVd is set to 600 [V] in step S21.

  If the controller 100 determines in step S24 that the error of the output value Dsig of the image density sensor 91 is about 0.5 [V] or more (YES), that is, if it is determined that carrier development has occurred, The image forming conditions are changed by correcting the target value for image density control (step S25).

  The control threshold value of the output value Dsig when the image density is controlled to be constant based on the output value Dsig of the image density sensor 91 varies depending on the configuration of the image density sensor 91 and the image forming apparatus 1 being used. Here, as an example, the control threshold value of the output value Dsig is set to 2.0 [V], and normally Dsig = 2.0 [V] is set as a target value for image density control. When carrier development has occurred, the control unit 100 determines that an error (−0...) Is included in the normal target value (2.0 [V]), that is, the control threshold value of the image density sensor 91 in the process of step S25. Correction processing is performed with 1.5 [V] plus 5 [V]) as a target value.

  As described above, according to the control method according to the second embodiment, when it is determined that carrier development has occurred, control for correcting the target value for image density control is performed. As a result, even when the durability of the developer progresses and the deterioration of the carrier becomes large, the image density on the image carrier and consequently on the paper can be optimally maintained.

  When the degree of carrier adhesion on the image carrier is low (the amount of adhesion is small), the output value Dsig of the image density sensor 91 is corrected because the influence of image carrier adhesion is small (or no effect is seen). This control can be performed by control (Example 1) for performing image correction and control (Example 2) for correcting a target value for image density control. However, when the degree of carrier adhesion on the image carrier is high (the amount of adhesion is large), image defects such as white spots occur, and thus it is necessary to limit the potential difference ΔVd to be used. In view of this point, Example 3 described below is made.

(Example 3)
FIG. 11 is a flowchart illustrating the processing procedure of the image forming condition control method according to the third embodiment.

  The control unit 100 first executes a process of setting a potential difference ΔVd between the exposure potential and the developing bias potential (step S31), and then the toner for controlling the image density on the image carrier under the set potential difference ΔVd. A patch image is created (step S32), and then the output value Dsig of the image density sensor 91 is acquired (step S33).

  Next, when the control unit 100 uses the output value Dsig when no carrier development occurs as a reference, whether the error of the output value Dsig of the image density sensor 91 with respect to the reference is about 0.5 [V] or more, for example. It is determined whether or not (step S34). In the case of this example, the control unit 100 repeats the processing from step S31 to step S34 until the potential difference ΔVd is set to 600 [V] in step S31.

  If the controller 100 determines in step S34 that the error of the output value Dsig of the image density sensor 91 is about 0.5 [V] or more (the occurrence of carrier development) (YES), the exposure potential and the development bias are determined. The potential difference ΔVd from the potential is limited to a region where carrier development does not occur (step S35). In this example, the region where carrier development does not occur is a potential difference region of less than 600 [V], and the potential difference ΔVd can be limited to a region where carrier development does not occur by changing the development bias voltage.

  For example, when ΔVd = 600 [V] is originally used and ΔVd = 400 [V] is set due to a restriction to an area where carrier development does not occur, the potential difference ΔVd is set lower by 200 [V]. The amount of carrier adhesion decreases. For this reason, the image (density) stabilization processing ensures the density by setting the development θ, which is the ratio of the developer conveyance speed to the process speed, higher than usual. That is, by limiting the potential difference ΔVd to an area where carrier development does not occur, the image density can be secured by another parameter (development θ in this example).

  As a hardware restriction, there is a setting range of usable development θ. Therefore, by limiting the potential difference ΔVd to an area where carrier development does not occur, there is a possibility that the development θ exceeds the set range. Even in such a case, if the toner concentration Tc [%] in the developer is increased, the image density can be secured without any trouble.

  As described above, according to the control method according to the third embodiment, when it is determined that carrier development has occurred, control is performed to limit the potential difference ΔVd to a region where carrier development does not occur. As a result, even when the durability of the developer progresses and the deterioration of the carrier becomes large, the image density on the image carrier and consequently on the paper can be optimally maintained.

  In the first to third embodiments described above, the output value Dsig of the image density sensor 91 when no carrier development occurs is used as a reference value, and the actually measured value (output value Dsig) of the image density sensor 91 with respect to the reference value. Although it is determined whether or not carrier development has occurred based on the difference, the present invention is not limited to this. Specifically, for example, the presence / absence of carrier development can be determined based on whether there is an inflection point in the change in the toner adhesion amount with respect to the potential difference ΔVd between the exposure potential and the development bias potential. Alternatively, “change in toner adhesion amount on the image carrier with respect to potential difference ΔVd between the exposure potential and the development bias potential” in consideration of changes in the toner charge amount Q / M in durability and the toner concentration Tc [%] in the developer. Can be measured in advance, and the presence or absence of occurrence of carrier development can be determined based on this measurement.

[Modification]
As mentioned above, although this invention was demonstrated using embodiment, this invention is not limited to the range as described in the said embodiment. That is, various changes or improvements can be added to the above-described embodiment without departing from the gist of the present invention, and embodiments to which such changes or improvements are added are also included in the technical scope of the present invention.

  For example, in the above-described embodiment, a copying machine has been described as an example of the image forming apparatus 1 according to an embodiment of the present invention, but the present invention is not limited to this application example. That is, the present invention can be applied to all electrophotographic image forming apparatuses using a two-component developer such as a printer, a facsimile machine, a printing machine, and a multifunction machine in addition to a copying machine.

  In the above embodiment, a color image forming apparatus is exemplified as the image forming apparatus 1 according to an embodiment of the present invention, and a toner patch image for image density control on the intermediate transfer belt 50 as an image carrier is displayed. Although the density is detected by the image density sensor 91, the present invention is not limited to this application example. That is, the present invention can be applied to a monochrome image forming apparatus in addition to a color image forming apparatus.

  In the case of application to a monochrome image forming apparatus, the image density sensor 91 detects the density of the toner patch image for image density control on the photoconductor 41 that is an image carrier. Further, in order to eliminate the influence of the shake of the photoconductor 41 in obtaining the slope of the potential difference ΔVd−toner adhesion amount curve in the case where carrier development has occurred as described with reference to FIG. The toner patch images having the same conditions may be created every 90 ° and 180 ° in the rotation angle, and the average may be taken.

  DESCRIPTION OF SYMBOLS 1 ... Image forming apparatus, 10 ... Document conveyance part, 20 ... Paper storage part, 30 ... Image reading part, 40 ... Image forming part, 50 ... Intermediate transfer belt, 51 ... Primary transfer part, 60 ... Secondary transfer part, 80: fixing unit 91: image density sensor 100: control unit

Claims (9)

An image forming unit that forms an image density control pattern image on the image carrier when the potential difference between the exposure potential and the developing bias potential is a predetermined potential difference;
A density detector for detecting the density of the image density control pattern image;
Based on the toner adhesion amount obtained from the output value based on the density detected by the density detector, the presence / absence of carrier adhesion on the image carrier is determined. A control unit for controlling the image forming conditions of
An image forming apparatus comprising: The image forming section forms a plurality of image density control pattern images corresponding to the plurality of potential differences when a plurality of potential differences are set as the predetermined potential differences. Image forming apparatus. The control unit is configured to determine a toner adhesion amount on the image carrier with respect to a potential difference between an exposure potential and a development bias potential based on output values of the density detection unit regarding a plurality of image density control pattern images formed by the image forming unit. The image forming apparatus according to claim 2, wherein a change is obtained and the presence or absence of carrier adhesion on the image carrier is determined based on the change. The control unit includes a carrier in the image carrier based on a difference between an actual measurement value detected by the density detection unit and a reference value acquired based on an output value of the density detection unit regarding the plurality of image density control pattern images. The image forming apparatus according to claim 3, wherein presence or absence of adhesion is determined. The control unit determines whether or not there is carrier adhesion on the image carrier based on whether or not there is an inflection point based on a change in toner adhesion amount with respect to a potential difference between an exposure potential and a development bias potential. Item 4. The image forming apparatus according to Item 3. The image forming apparatus according to claim 1, wherein the control unit controls the image forming condition by correcting an output value of the density detection unit. The image forming apparatus according to claim 1, wherein the control unit controls the image forming condition by correcting a control threshold included in the density detection unit. The image forming apparatus according to claim 1, wherein the control unit limits a potential difference between an exposure potential and a development bias potential to a region where carrier adhesion does not occur on the image carrier. Forming an image density control pattern image on the image carrier when the potential difference between the exposure potential and the development bias potential is a predetermined potential difference;
Detecting the density of the image density control pattern image;
Determining the presence or absence of carrier adhesion on the image carrier based on the amount of toner adhesion on the image carrier obtained from the output value based on the detected density, and controlling image forming conditions when carrier adhesion is present And having
An image forming method, wherein the processing of each step is executed.

Images