JP6065739B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus including a toner density sensor that detects the density of toner on an image carrier.

  In an electrophotographic image forming apparatus, a patch image having a predetermined image pattern is formed, and an image forming condition is adjusted according to the output of the toner density sensor so that a desired image density can be stably obtained. ing.

  By the way, ripple noise due to an AC bias is generated in the output of the toner concentration sensor due to unnecessary radiation from an AC component generated by an AC component generation source such as a discharge electrode existing in the vicinity of the toner concentration sensor.

  As a means for removing noise, for example, when data that may contain noise is detected in the output of the toner density sensor, a noise component is calculated from adjacent data, and the noise component is offset to obtain an accurate output. An image forming apparatus to be obtained is known (see Patent Document 1). Specifically, an accurate sensor output is obtained by replacing data having a value more than a certain distance from an average value of sampling data obtained by reading a certain area of the patch image with an average value of adjacent data.

Japanese Patent Laid-Open No. 2004-361871

  However, with the technique described in Patent Document 1, although specific noise can be removed, ripple noise cannot be removed. Hereinafter, the reason will be described with reference to FIGS.

FIG. 9 is an explanatory diagram showing a conventional noise removal processing method. FIG. 10 is an explanatory diagram when a conventional noise removal processing method is performed when ripple noise occurs. 9 and 10, the horizontal axis indicates the position of the sampling point, and the vertical axis indicates the detection value (sampling value) detected by the toner density sensor.
As shown in FIG. 9, the sampling value 202 of the patch image Ip attached along the driving direction of the intermediate transfer belt 6 is far away from the value (average value) obtained by averaging the sampling values of other sampling points. And contains noise. When the sampling value 202 including noise is detected, the sampling value at the sampling point is replaced with the average value 202 ′ of the sampling values of the two adjacent sampling points 201 and 203.

  However, as shown in FIG. 10, due to the influence of the ripple noise 210 having an AC waveform, depending on the sampling position, the sampling value is an average value (broken line) offset by an arrow from a normal value (solid line) when there is no ripple noise 210. ) Is output. That is, when a conventional noise removal processing method is performed when ripple noise occurs, ripple noise is not detected, and image forming conditions cannot be adjusted appropriately.

  From the above situation, a technique for removing the influence of ripple noise from the detection value of the toner density sensor has been desired.

An image forming apparatus according to one aspect of the present invention is an image forming apparatus that changes image forming conditions in accordance with an output of a toner density sensor that detects a correction toner image carried on an image carrier.
Analyzing the frequency of ripple noise generated by an AC component of a dark current detection unit that detects a dark current value of the toner concentration sensor at a predetermined sampling period and one or more AC component generation sources existing in the vicinity of the toner concentration sensor; Based on the analysis result, the ripple noise period calculation unit that calculates the period of the ripple noise, and the sampling period of the dark current detection unit and the toner density sensor based on the ripple noise period calculated by the ripple noise period calculation unit. A sampling cycle control unit to be set, and a correction unit that corrects the detection value of the correction toner image output from the toner density sensor in accordance with the sampling cycle set by the sampling cycle control unit.
In the above configuration, the dark current detection unit detects a dark current value of the toner concentration sensor in a state where an AC component is generated from the AC component generation source, based on the sampling cycle set by the sampling cycle control unit.
The correction unit calculates a difference value between a dark current value in a state where an AC component is generated from the AC component generation source and a dark current value in a state where no AC component is generated from the AC component generation source.
Then, the toner density sensor detects the correction toner image carried on the image carrier detected by the conditions of the same period and the same phase as the sampling period set by the sampling period control unit, and calculates the difference value calculated by the correction unit. Use to correct.

  In the above configuration, the period of the ripple noise is set from the frequency of the ripple noise caused by the AC component of the AC component generation source. The dark current value when the AC component is generated is detected by the set sampling cycle. A difference value between the dark current value when the alternating current component detected in the set sampling period is generated and the dark current value when the alternating current component is not generated is calculated. Then, the correction toner image is sampled at the same period and the same phase as the set sampling period. As described above, the sampling point when the AC component is generated when the difference value is calculated is matched with the cycle and phase of the sampling point when the AC component is generated when the correction toner image is detected. . Then, the detected value of the toner density sensor is appropriately corrected using the dark current difference value.

  According to the present invention, it is possible to remove the influence of ripple noise from the detection value of the toner density sensor and appropriately correct the image.

1 is an overall configuration diagram of an image forming system including 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. FIG. 2 is an explanatory diagram illustrating an internal configuration of a main part and a ripple noise removing unit of an image forming apparatus according to an embodiment of the present invention. 5 is a flowchart showing ripple noise removal processing by the image forming apparatus according to the embodiment of the present invention. It is explanatory drawing which shows the determination method of the presence or absence of the ripple noise by the dark current of the toner density sensor which concerns on one embodiment of this invention. 6 is a graph illustrating an example of sampling data output from a toner concentration sensor when there is one AC component generation source according to an embodiment of the present invention. It is explanatory drawing which shows an example of the alternating current waveform output from a removal electrode and a separation pole. 6 is a graph showing an example of sampling data output from a toner concentration sensor when there are two AC component generation sources according to an embodiment of the present invention. It is explanatory drawing which shows the conventional noise removal processing method. It is explanatory drawing at the time of implementing the conventional noise removal processing method at the time of ripple noise generation | occurrence | production.

  Hereinafter, an example of an embodiment for carrying out the present invention will be described with reference to the accompanying drawings. In the drawings, common components are denoted by the same reference numerals, and redundant description is omitted.

[Example of overall configuration of image forming system]
FIG. 1 is an overall configuration diagram of an image forming system including an image forming apparatus according to an embodiment of the present invention. In FIG. 1, the portion considered necessary for the description of the present invention in the configuration of the image forming system is mainly described.

  As shown in FIG. 1, the image forming system 1 includes a paper feeding device 20, an image forming device 10, and a fixing device 30.

  The paper feeding device 20 is provided with a plurality of paper storage portions 20a according to the paper size and type. In the paper feeding device 20, the corresponding paper storage unit 20 a is selected based on an instruction from the image forming device 10, the paper S is taken out by a paper feeding unit (not shown), and is sent to the conveyance path C of the image forming device 10. .

  The image forming apparatus 10 employs an electrophotographic system that forms an image using static electricity. For example, toner images of four colors of yellow (Y), magenta (M), cyan (C), and black (K) are used. Is a tandem type color image forming apparatus. The image forming apparatus 10 includes an operation display unit 105, an image forming unit 5, an intermediate transfer belt 6 (image carrier), and a secondary transfer unit 7.

  The image forming unit 5 includes four image forming units 5Y, 5M, 5C, and 5K in order to form toner images of colors of yellow, magenta, cyan, and black. Further, the image forming apparatus 10 includes a plurality of rollers (conveying rollers) 11, 12, and 13 for conveying paper. The rollers 11, 12, and 13 are usually constituted by a pair of rollers. Each roller described below has the same configuration. The operation display unit 105 has a function as an operation unit that instructs the start of a job such as image forming processing.

  In the image forming mode, the image forming apparatus 10 charges the photosensitive member 51 included in the image forming units 5Y, 5M, 5C, and 5K, erases the electric charge according to the original image, and exposes the photosensitive member 51 to the electrostatic latent image. Form an image. Then, toner is attached to the electrostatic latent images of the photoreceptors 51 of yellow, magenta, cyan, and black using a developing unit to form toner images of each color. Next, the toner images formed on the yellow, magenta, cyan, and black photoconductors 51 are sequentially primary-transferred onto the surface of the intermediate transfer belt 6 that rotates in the direction of the arrow.

  Next, a sheet of toner image of each color primarily transferred onto the intermediate transfer belt 6 by the secondary transfer unit 7 (secondary transfer roller) is supplied from the sheet feeding device 20 and is conveyed by the rollers 11 and 12. Secondary transfer to S. The toner images of the respective colors on the intermediate transfer belt 6 are secondarily transferred to the paper S, whereby a color image is formed. The image forming apparatus 10 discharges the sheet S on which the color toner image is formed to the fixing device 30 by the roller 13.

  A toner density sensor 15 is disposed downstream of the primary transfer nip portion formed by the intermediate transfer belt 6 and the photosensitive member 51 of the image forming unit and upstream of the secondary transfer portion 7 in the sheet conveyance direction. Yes. The toner density sensor 15 detects the toner density of each color toner image transferred to the intermediate transfer belt 6 before being secondarily transferred by the secondary transfer unit 7 and outputs the detection result to the control unit 100 described later.

  As the toner density sensor 15, for example, a transmissive photosensor in which a light emitting member and a light receiving member are arranged to face each other, or a reflective photosensor that detects reflected light from an emitted light object is used. Alternatively, a line sensor in which a light emitting member and a plurality of photoelectric conversion elements are arranged linearly over a part or the entire area in the paper width direction, or an image sensor in which photoelectric conversion elements are arranged in a matrix form is used. As the line sensor and the image sensor, a CCD type image sensor or a CMOS type (including MOS type) image sensor can be used.

  The fixing device 30 is a device that performs a fixing process on the paper S on which a color toner image is formed, which is supplied from the image forming apparatus 10. The fixing device 30 includes, for example, a fixing unit 31, a roller 32 near the carry-in port, and a roller 33 near the discharge port.

  The fixing unit 31 provided in the fixing device 30 pressurizes and heats the conveyed paper S, and fixes the transferred toner image on the paper S. The fixing unit 31 includes, for example, a fixing upper roller and a fixing lower roller which are a pair of fixing members. The upper fixing roller and the lower fixing roller are arranged in pressure contact with each other, and a fixing nip portion is formed as a pressing portion between the upper fixing roller and the lower fixing roller.

  A heating unit is provided inside the fixing upper roller. The roller part at the outer peripheral part of the fixing upper roller is heated by the radiant heat from the heating part. Then, the heat of the roller portion of the fixing upper roller is transmitted to the sheet, whereby the toner image on the sheet is thermally fixed.

  The sheet S is conveyed so that the surface on which the toner image is transferred by the secondary transfer unit 7 (surface to be fixed) faces the fixing upper roller, and passes through the fixing nip portion. Accordingly, the sheet passing through the fixing nip is subjected to pressure by the upper fixing roller and the lower fixing roller and heating by the heat of the upper fixing roller. The sheet S on which the fixing process has been performed by the fixing unit 31 is discharged to the discharge tray 50 by the roller 33.

[Configuration of control system of image forming system]
Next, a control system of the image forming apparatus 10 provided in the image forming system 1 will be described.
FIG. 2 is a block diagram illustrating a hardware configuration of each unit of the image forming apparatus 10.

  As shown in FIG. 2, the image forming apparatus 10 includes a control unit 100. The control unit 100 includes, for example, a CPU (Central Processing Unit) 101, a ROM (Read Only Memory) 102 for storing programs and data executed by the CPU 101, and a RAM (Random Access) used as a work area of the CPU 101. Memory) 103. The ROM 102 stores, for example, control programs for various jobs, setting value data used for ripple noise removal processing, and the like. As the ROM 102, for example, an electrically erasable programmable ROM is used. The control unit 100 controls each block, that is, controls the entire apparatus.

  The control unit 100 is connected to an HDD (Hard disk drive) 104 and an operation display unit 105 via a system bus 107, respectively. The control unit 100 is connected to the image reading unit 4, the image processing unit 106, the image forming unit 5, and the secondary transfer unit 7 via the system bus 107. Further, the control unit 100 is connected to the toner density sensor 15 and the ripple noise removal unit 111 via the system bus 107, respectively.

  The HDD 104 is a large-capacity storage device that stores image data of an original image obtained by reading by the image reading unit 4 and 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) and a touch sensor (an example of an operation unit). The operation display unit 105 displays an instruction menu for the user, information about the acquired image data, and the like. Furthermore, the operation display unit 105 includes a plurality of keys, receives input of various instructions, data such as characters and numbers by user key operations, generates an input signal, and outputs the input signal to the control unit 100.

  Image data generated by the image reading unit 4 and image data transmitted from a PC (personal computer) 120 which is an example of an external device connected to the image forming apparatus 10 are sent to the image processing unit 106 for image processing. Is done. The image processing unit 106 performs processes such as analog processing, A / D conversion, shading correction, and image compression on the received image data.

  The communication unit 108 receives job information transmitted from the PC 120 which is an external information processing apparatus via a communication line. The received job information is sent to the control unit 100 via the system bus 107.

  The control unit 100 controls driving of the image forming unit 5 and the secondary transfer unit 7 based on the job information. The control unit 100 may be configured to control the fixing process of the fixing unit 31 disposed in the fixing device 30.

  Under the control of the control unit 100, the toner density sensor 15 lights the light emitting member with a preset reference light emission setting value, receives the reflected light from the intermediate transfer belt 6 on which the toner image has been primarily transferred, The detection value (detection signal) is output to the control unit 100 at a sampling period of. The control unit 100 calculates the toner adhesion amount (toner concentration) of the toner image transferred to the intermediate transfer belt 6 based on the toner adhesion amount characteristic information that associates the detection value of the toner density sensor 15 with the toner adhesion amount. . Then, the control unit 100 applies the calculated toner density to a determination unit such as a determination table or a conversion formula, and controls the toner amount of the toner image transferred to the intermediate transfer belt 6 in the image forming unit 5. The toner adhesion amount characteristic information and the data relating to the determination unit are stored in the ROM 102 or the HDD 104 in advance.

  As control factors for the amount of toner adhered to the intermediate transfer belt 6, for example, the toner density in the developing unit of the image forming unit 5, the developing bias, the amount of exposure to the photosensitive member by the exposing unit, and the charging of the photosensitive unit by the charging unit The voltage, the content of the image data, etc. are mentioned. The ROM 102 holds a table for associating the toner adhesion amount with its control factor, and the control unit 100 controls the toner adhesion amount as an image forming condition with reference to the table.

  The ripple noise removing unit 111 appropriately changes the sampling period of the toner density sensor 15 under the control of the control unit 100 and removes the influence of ripple noise from the detection value of the toner density sensor 15. The sampling period of the toner concentration sensor 15 is determined based on the frequency of the AC component output from the AC component generation source existing in the vicinity of the toner concentration sensor 15. The control unit 100 functions as a correction unit that averages a plurality of sampling values output from the toner density sensor 15 in accordance with a set sampling cycle and corrects a toner image formed by the image forming unit 5. The averaging process may be performed by, for example, addition averaging.

  In this embodiment, an example in which the personal computer 120 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. it can.

[Internal configuration of ripple noise elimination unit]
Hereinafter, the ripple noise removing unit 111 will be described in detail.
FIG. 3 is an explanatory diagram illustrating an internal configuration of a main part of the image forming apparatus 10 and the ripple noise removing unit 111.
Each of the image forming units 5Y, 5M, 5C, and 5K includes photoreceptors 51Y, 51M, 51C, and 51K. On the downstream side of the nip portion between the intermediate transfer belt 6 and each photoconductor 51, the electrode removal electrode 8 is disposed. Here, the photoconductor 51K will be described as a representative. The removal electrode 8 erases the charge remaining on the photoconductor 51 under the control of the control unit 100. A separation pole 9 is disposed on the downstream side of the secondary transfer unit 7. The separation pole 9 erases the charge remaining on the sheet S after the secondary transfer under the control of the control unit 100 and separates the sheet S from the intermediate transfer belt 6. The removal electrode 8 and the separation electrode 9 are an example of an AC component generation source located near the toner concentration sensor 15.
In the example of FIG. 3, the toner density sensor 15 is disposed on the downstream side of the removal electrode 8 and on the upstream side of the secondary transfer unit 7.

  As described above, since the toner concentration sensor 15 is composed of a semiconductor device such as a photo sensor, a line sensor, or an image sensor, an electric charge (dark current) is generated in a state where light is blocked. When an AC component generation source exists in the vicinity of the toner concentration sensor 15, this dark current varies depending on the AC component output from the AC component generation source. Therefore, the dark current of the toner density sensor 15 is analyzed, and the influence of ripple noise included in the output of the toner density sensor 15 is removed.

  The ripple noise removal unit 111 includes a dark current detection unit 112, a dark current value storage unit 113, a ripple noise determination unit 114, a ripple noise cycle calculation unit 115, a noise cycle least common multiple storage unit 116, and a sampling cycle control unit. 117.

  The dark current detection unit 112 detects dark current based on a sampling value output by the toner concentration sensor 15 during a predetermined sampling period when light is blocked, and the detection result is a ripple noise determination unit 114 and a dark current value storage unit 113. Output to.

  The dark current value storage unit 113 is a storage unit that stores a dark current detection result of the toner density sensor 15 output from the dark current detection unit 112. As the dark current value storage unit 113, the RAM 103 or the HDD 104 may be used.

  The ripple noise determination unit 114 determines whether or not the ripple noise that needs to remove the influence on the dark current is included based on the detection result of the dark current input from the dark current detection unit 112, and determines the determination result. Output to the ripple noise period calculation unit 115. More specifically, the dark current value (amplitude) of the toner concentration sensor 15 detected by the dark current detection unit 112 when the AC component generation source (for example, the removal electrode 8 and the separation electrode 9) generates an AC component. Is within the set threshold range. When the dark current value is outside the set threshold range, it is determined that ripple noise that needs to be removed has occurred.

  The ripple noise period calculation unit 115 analyzes the frequency of the ripple noise included in the dark current of the toner density sensor 15 detected by the dark current detection unit 112, and based on the analysis result (AC frequency information), the period of the ripple noise. Is calculated. For example, a fast Fourier transform (FFT) apparatus can be used for the frequency analysis. As a result of analyzing the frequency of the ripple noise, the ripple noise cycle calculation unit 115 calculates the least common multiple of the cycles of the plurality of ripple noises when the ripple noise of the plurality of frequencies is detected, and the noise cycle least common multiple storage unit 116. Output to. Further, when the frequency of the ripple noise is one, the ripple noise period calculation unit 115 outputs the period of the ripple noise as the period of the least common multiple.

  The noise cycle least common multiple storage unit 116 is a storage unit that stores the period (least common multiple) of the ripple noise output from the ripple noise cycle calculation unit 115. For the noise cycle least common multiple storage unit 116, a rewritable storage unit such as the RAM 103 or the HDD 104 may be used.

  The sampling cycle control unit 117 is a control unit that sets (varies) the sampling cycle of the toner density sensor 15 based on the ripple noise cycle calculated by the ripple noise cycle calculation unit 115.

Since the image quality deterioration level changes depending on the charge amount of the toner, the optimum output value of the deelectrode 8 is varied. Further, since the separation property between the intermediate transfer belt 6 and the paper changes depending on the paper type, the output value of the separation pole 9 after the secondary transfer is variable. For example, the output value of the removal electrode 8 and the output value of the separation electrode 9 can be varied within the following range as an example. These output values change, for example, when the user operates the operation display unit 105 to set the image quality of the toner image.
Output of the electrode removal electrode AC: 1 to 10 kV, DC: −5 to 1 kV, frequency: 500 Hz to 3 kHz
-Output of separation pole 9 AC: 1 to 10 kV, DC: -5 to 0 kV, frequency: 100 Hz to 1 kHz

  Since the frequency of the AC component generated by the AC component generating source such as the electrode removal electrode 8 or the separation electrode 9 is preset, the set value is stored in the storage means. As the storage means, for example, a rewritable storage means such as the RAM 103 or the HDD 104 is used.

  The ripple noise removing unit 111 described above can be configured using hardware or software. When configured using software, the CPU 101 in the control unit 100 reads and executes a ripple noise removal program stored in the ROM 102, thereby realizing its function.

[Ripple noise elimination processing]
FIG. 4 is a flowchart showing ripple noise removal processing by the ripple noise removal unit 111 of the image forming apparatus 10.
First, the control unit 100 of the image forming apparatus 10 detects the start of a job based on an operation signal input from the operation display unit 105 or job information input via the communication unit 108. When the control unit 100 detects the start of a job, it starts ripple noise removal processing.

  Then, under the control of the control unit 100, the dark current detection unit 112 of the ripple noise removal unit 111 performs toner concentration in a state where the AC component generation source does not generate an AC component (hereinafter referred to as “AC output off”). The dark current of the sensor 15 is detected at an arbitrary sampling period (step S1). The dark current detection unit 112 averages the detected sampling value of the dark current when the AC output is off, and stores it in the dark current value storage unit 113 (step S2).

  Next, the dark current detection unit 112 detects the dark current of the toner concentration sensor 15 at an arbitrary sampling period in a state where the AC component generation source generates an AC component (hereinafter referred to as “AC output ON”). . Then, the detected sampling value of the dark current when the AC output is turned on is averaged (step S3). When the AC output is on, the control unit 100 causes the AC electrode generation source, for example, the electrode removal 8 and the separation electrode 9 to generate an AC component based on the set value as described above. In other words, since the AC output is normally off except during image formation, control is performed to turn on the AC output when dark current is detected.

  Next, the ripple noise determination unit 114 compares the dark current value of the toner density sensor 15 when the AC output is on and the dark current value of the toner density sensor 15 when the AC output is off, and compares the dark current value of the toner density sensor 15 when the AC output is off. It is determined whether or not the dark current value is within a predetermined range (−a ≦ x ≦ a x: toner density sensor output value) (step S4).

  Here, FIG. 5 is an explanatory diagram showing a method for determining the presence or absence of ripple noise due to dark current. In the figure, the horizontal axis indicates the position of the sampling point, and the vertical axis indicates the detection value (dark current value) detected by the toner density sensor 15. The dark current is measured, for example, at a time T other than the detection time of the toner density sensor 15 used for correcting the toner image, that is, when the toner density sensor 15 is in an off state.

As shown in FIG. 5, when the dark current value of the toner density sensor 15 is within the range of the ripple noise occurrence threshold (−a ≦ x ≦ a) (YES in step S4), the ripple noise determination unit 114 It is determined that no ripple noise has occurred, and the ripple noise removal process is terminated.
On the other hand, when the dark current value of the toner density sensor 15 is outside the range of the ripple noise occurrence threshold (−a ≦ x ≦ a) (NO in step S4), the ripple noise determination unit 114 indicates that the ripple noise 121 to be removed is It is determined that the error has occurred, and the determination result is output to the ripple noise cycle calculation unit 115.

  When there is ripple noise, the ripple noise period calculation unit 115 analyzes the frequency of the ripple noise, calculates the period of the ripple noise, and stores it in the noise cycle least common multiple storage unit 116 (step S5). Here, the ripple noise period calculation unit 115 obtains the least common multiple of the periods of the plurality of ripple noises when the ripple noises of the plurality of frequencies are detected. When the frequency of the ripple noise is one, the period of the ripple noise is processed as the least common multiple period.

  Next, the sampling cycle control unit 117 performs a change to replace the sampling cycle of the toner density sensor 15 with the cycle of the ripple noise stored in the noise cycle least common multiple storage unit 116 (step S6).

  The dark current detection unit 112 detects the dark current of the toner density sensor 15 when the AC output is on in the changed sampling cycle, averages the detected dark current sampling value, and the dark current value storage unit 113. (Step S7).

  And the control part 100 which is a correction | amendment part is a difference value (offset) of the dark current value detected by arbitrary sampling periods when the alternating current output is off, and the dark current value detected by the changed sampling period when the alternating current output is on. Amount) is calculated (step S8). The processing from step S1 to step S8 described above is preparation before the toner density sensor 15 detects a patch image (correction toner image). Hereinafter, steps S9 and S10 are processes executed when a patch image is detected.

  Next, the control unit 100 drives the image forming unit 5 to form a patch image and transfers it to the intermediate transfer belt 6. The toner density sensor 15 detects the patch image transferred to the intermediate transfer belt 6 in the same cycle and the same phase as the changed sampling cycle set by the sampling cycle control unit 117 (step S9).

  Next, the control unit 100 corrects by adding the difference value to the detection value (sampling value) of the patch image detected in the changed sampling cycle (step S10).

  The control unit 100 changes the image forming condition using the corrected detection value of the patch image, and corrects the toner image to be formed.

  This ripple noise removal processing may be performed at the start of the job or before the start of the job as described above, or may be performed during execution of image formation. For example, when the set value of the AC component changes for some reason during image formation based on a certain job, ripple noise removal processing is performed between sheets, and the sampling period of the toner density sensor 15 is changed. Good.

According to the above-described embodiment, first, the difference value (offset amount) between the dark current value when the AC output is off and the dark current value when the AC output is on is calculated. When the AC output is on, the dark current of the toner density sensor 15 is detected by the sampling period changed to the least common multiple of the ripple noise period. The toner density sensor 15 detects the patch image at the same cycle and phase as the changed sampling cycle, and corrects the detection value of the patch image using the difference value.
Here, since the sampling cycle and phase of the dark current offset amount and the sampling cycle and phase of the detection value (sampling value) of the patch image are the same, an accurate offset amount is added to the detection value of the patch image. be able to. Therefore, the detection value of the patch image output from the toner density sensor 15 can be accurately corrected by the offset amount 124. Thereby, a normal value obtained by removing the influence of ripple noise from the detection value of the toner density sensor 15 can be obtained. As a result, the image forming conditions can be changed based on the normal value of the toner density sensor 15, and the formed toner image can be corrected appropriately.

  In the example of FIG. 4 described above, when the ripple noise determination unit 114 determines whether or not the ripple noise is generated, the dark current when the AC output is off is detected. An accurate dark current value when the AC output is off can be obtained. However, a dark current value determined for each device of the toner density sensor 15 may be used. The known dark current value of the toner density sensor 15 is stored in the ROM 102 or the like.

[When there is only one AC component source: electrode removal only]
As described above, since the ripple noise frequency changes according to the user setting, the sampling period must be changed each time. Hereinafter, an example of the ripple noise removal process in the case where there is only one AC component generation source in the vicinity of the toner density sensor 15 will be described. In this example, an example of changing an appropriate sampling period with respect to the set value of the electrode removal electrode 8 as an AC component generation source is shown.

FIG. 6 is a graph showing an example of sampling data output from the toner concentration sensor 15 when there is one AC component generation source. The horizontal axis indicates the position (msec) of the sampling point (white circle), and the vertical axis indicates the detection value (sampling value) detected by the toner density sensor 15. The patch image is formed on the surface of the intermediate transfer belt 6 along the driving direction of the intermediate transfer belt 6. The scanning direction of the patch image is opposite to the driving direction of the intermediate transfer belt 6. As an example, the output value of the electrode removal 8 is set as follows. These can be set by the user operating the operation display unit 105.
-Output value of the electrode removal electrode AC: 4 kV, DC: -2 kV, frequency f: 1 kHz

  Since the period of the ripple noise 121 is 1 msec (= 1 / f) when the frequency f of the output value of the discharge electrode 8 is 1 kHz, the sampling period control unit 117 changes the sampling period to 1 msec. The toner concentration sensor 15 detects the dark current average value 123 when the AC output is on, based on the changed sampling cycle. Then, the control unit 100 calculates the offset amount 124 from the dark current average value 122 when the AC output is off and the dark current average value 123 when the AC output is on.

  Even when a patch image is detected, the toner density sensor 15 samples the patch image under the same cycle and phase conditions as the changed sampling cycle, and obtains a detection value 125 (sampling point) of the patch image. As a result, the period and phase of the sampling point when the AC output is ON when calculating the offset amount and the sampling point when the AC output is ON when detecting the patch image are the same. Therefore, the detection value (sampling value) of the patch image output from the toner density sensor 15 can be accurately corrected by taking the offset amount 124 into consideration.

[When there are two AC component generation sources: electrode removal and separation electrode]
FIG. 7 is an explanatory diagram illustrating an example of an AC waveform output from the electrode removal electrode 8 and the separation electrode 9. FIG. 8 is a graph showing an example of sampling data output from the toner concentration sensor 15 when there are two AC component generation sources. As an example, output values and correction conditions of the electrode removal 8 and the separation electrode 9 that are AC component generation sources are set as follows. These can be set by the user operating the operation display unit 105.
-Output value of electrode removal electrode AC: 4 kV, DC: -2 kV, frequency f: 500 Hz
-Output value of separation pole 9 AC: 2 kV, DC: -1 kV, frequency f: 200 Hz

As shown in FIG. 7, when the frequency f of the output value of the discharge electrode 8 is 500 Hz, the period of the ripple noise 12 6 a by the discharge electrode 8 is 2 msec (= 1 / f). Similarly, the period N of the ripple noise 12 6 b due to the separation pole 9 is 5 msec. Therefore, the period of the combined waveform 12 6 (see FIG. 8) of the ripple noise 12 6 a and the ripple noise 12 6 b output from the removal electrode 8 and the separation electrode 9 is 5 msec. Further, the least common multiple of the period of the output values of the electrode removal 8 and the separation electrode 9 is 10 msec.

Since the least common multiple of the period of the output values of the deelectrode 8 and the separation electrode 9 is 10 msec, the sampling period control unit 117 changes the sampling period to 10 msec. The toner density sensor 15 by the sampling period after the change, to detect the dark current average value 12 8 when the AC output on. Then, the control unit 100, by any sampling period, and calculates the offset amount 12 9 from the dark current average value 12 8 when the dark current average value 12 7 and the AC output on when the AC output off.

As in the example of FIG. 6, even when a patch image is detected, the toner density sensor 15 samples the patch image at the same cycle and phase as the changed sampling cycle. Thus, the sampling value of the patch image output from the toner density sensor 15 can be accurately corrected by considering the offset amount 12 9.

  Thus, in this example, the least common multiple of a plurality of frequencies of AC components output from a plurality of AC component generation sources (in this example, the removal electrode 8 and the separation electrode 9) is set as the sampling period. By sampling the dark current and the patch image at the least common multiple sampling period, ripple noise including a plurality of AC components can be sampled in the same phase, so that the sampling value is stable. Thus, even when there are two or more AC component generation sources, a normal value obtained by removing the influence of ripple noise from the detection value of the toner density sensor 15 can be obtained with high accuracy.

  As described with reference to FIGS. 6 to 9, the period of the difference value between the dark current value of the toner density sensor 15 when the AC output is off and the dark current value of the toner density sensor 15 when the AC output is on and The phase and the cycle and phase of the sampling point when detecting the patch image are matched. Therefore, the sampling value of the patch image output from the toner density sensor 15 can be accurately corrected using the difference value. Thereby, a normal value obtained by removing the influence of ripple noise from the detection value of the toner density sensor 15 can be obtained. As a result, the image forming conditions can be changed based on the normal value of the toner density sensor 15, and the formed toner image can be corrected appropriately.

Conventionally, as means for removing ripple noise, (1) shield the toner density sensor, (2) remove noise by circuit processing of the toner density sensor, and (3) turn off the AC bias output during the detection operation of the toner density sensor. And so on. However, the removal means (1) increases the installation space and costs. Further, the removing means (2) causes an increase in cost and an increase in circuit area. Furthermore, since the removal means (3) stops the AC bias output that is originally required for image formation, the productivity and the image quality are degraded.
On the other hand, according to the configuration of the present embodiment, the accurate output of the toner density sensor 15 is obtained by uniformly varying the sampling period of dark current measurement and patch image measurement in consideration of the ripple noise period. It is possible to obtain high-quality image quality without causing a decrease in productivity, an increase in cost, a reduction in correction accuracy, and the like.

[Modification]
The embodiment to which the invention made by the present inventor is applied has been described above. However, the present invention is not limited by the description and drawings which form part of the disclosure of the invention according to the above-described embodiments, and various modifications may be made without departing from the spirit of the invention described in the claims. Is possible.

  For example, in the above-described embodiment, the example in which the image forming unit 5 is provided with the four image forming units 5Y, 5M, 5C, and 5K to form a color image has been described. However, the single color in which only one image forming unit is provided. You may apply to the image forming apparatus which forms an image.

  Further, in the above-described embodiment, an example in which the toner image formed on the photoconductor 51 (photosensitive drum) is transferred to the intermediate transfer belt 6 and the toner image is secondarily transferred from the intermediate transfer belt 6 to a sheet is described. However, the present invention may be applied to an image forming apparatus that directly transfers a toner image from the photosensitive member 51 to a sheet as an image carrier.

  Further, in the above-described embodiment, the configuration in which the ripple noise removing unit 111 executes the ripple noise removing process has been described. However, the control unit 100 may execute part or all of the configuration.

  Further, in the above-described embodiment, when there are a plurality of ripple noise frequencies, the ripple noise frequency outside the threshold range may be specified, and only the ripple noise having the specified frequency may be removed.

DESCRIPTION OF SYMBOLS 5 ... Image formation part, 6 ... Intermediate transfer belt, 7 ... Secondary transfer part, 10, 10A ... Image formation apparatus, 15 ... Toner density sensor, 111 ... Ripple noise removal part, 112 ... Dark current detection part, 113 ... Dark Current value storage unit 114... Ripple noise determination unit 115. Ripple noise cycle calculation unit 116. Noise cycle least common multiple storage unit 117. Sampling cycle control unit 122. 12 7. Dark current average value 123. 12 8 ... dark current average value (sampling cycle after change), 125 ... detection value, 126 ... composite waveform, 124 , 12 9 ... offset amount

Claims (2)

In an image forming apparatus that changes image forming conditions in accordance with an output of a toner density sensor that detects a correction toner image carried on an image carrier.
A dark current detector that detects a dark current value of the toner density sensor at a predetermined sampling period;
The frequency of ripple noise generated by the AC component of one or more AC component generation sources existing in the vicinity of the toner density sensor is analyzed, and when the frequency of the ripple noise included in the dark current is one, the ripple A ripple noise period calculation unit that calculates a cycle of noise and calculates a least common multiple of the period of the plurality of ripple noises as a period of the ripple noise when the frequency of the ripple noise included in the dark current is plural ,
When the frequency of the ripple noise included in the dark current is one, the ripple noise cycle calculated by the ripple noise cycle calculation unit is set as a sampling cycle of the dark current detection unit and the toner density sensor. When the frequency of the ripple noise included in the dark current is plural, the least common multiple of the period of the plurality of ripple noises calculated by the ripple noise period calculation unit is used as the dark current detection unit and the toner. A sampling period control unit that is set as the sampling period of the density sensor ;
A correction unit that corrects the detected value of the correction toner image output from the toner density sensor in accordance with the sampling cycle set by the sampling cycle control unit,
The dark current detection unit detects a dark current value of the toner density sensor in a state where an AC component is generated from the AC component generation source according to the sampling cycle set by the sampling cycle control unit,
The correction unit calculates a difference value between a dark current value in a state where an AC component is generated from the AC component generation source and a dark current value in a state where no AC component is generated from the AC component generation source. ,
The correction unit calculates the correction toner image carried on the image carrier detected by the toner density sensor under the same cycle and phase conditions as the sampling cycle set by the sampling cycle control unit. An image forming apparatus that corrects using a difference value. It is determined whether or not the dark current value of the toner density sensor in a state where an alternating current component is generated from the alternating current component generation source is within a set threshold range, and the dark current value is outside the threshold range. The image forming apparatus according to claim 1, further comprising: a ripple noise determination unit that determines that ripple noise is generated.

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