JP5413296B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an electrophotographic image forming apparatus.

  In a known image forming apparatus in the electrophotographic system, the image forming process includes a process of transferring a toner image formed on a photosensitive drum onto a sheet. Specifically, the toner image carried on the surface of the photosensitive drum is transferred to the sheet while the sheet passes between the photosensitive drum and a transfer roller disposed to face the photosensitive drum.

  In such an image forming apparatus, the transfer roller is usually applied with a transfer bias in order to transfer the toner image onto a sheet. The transfer bias has an optimum value depending on the area of the toner image printed on the paper. Therefore, a method is known in which the printing area of a toner image is calculated by counting the number of dots of image data to be printed, and the transfer bias is controlled according to the value of the printing area (for example, Patent Document 1). .

Japanese Patent Laid-Open No. 11-125978

  By the way, it is known that the electrical resistance value of the paper is lowered by the moisture absorption of the paper in a high humidity environment where the humidity inside the machine is high. Therefore, in a high humidity environment, a large amount of transfer current applied to the toner image via the paper tends to flow to a white paper portion where the toner image is not transferred, and a sufficient transfer bias is not supplied to the toner image. Therefore, in a high humidity environment, the transfer current applied to the toner image with many dots adjacent to the white paper portion is larger than the transfer current applied to a solid image or the like where the dots are adjacent and few dots are adjacent to the white paper portion. It needs to be a large value.

  However, the method of controlling the transfer current by counting the number of dots as in the prior art cannot distinguish between image pattern differences of image data having the same number of dots. That is, it is impossible to determine whether the image data is an image pattern with many white dots adjacent to each other or an image pattern with many dots not adjacent to each other. Therefore, an appropriate transfer current cannot be applied to the change in the resistance value of the paper, causing a transfer failure such as blurring or ghosting.

  The present invention provides an image forming apparatus capable of selecting a transfer current in accordance with a change in electrical resistance value of a sheet due to a change in humidity environment in the apparatus and an image pattern and performing good transfer. Objective.

The image forming apparatus according to claim 1, wherein a photoconductor that forms a toner image based on input image data and a transfer member that transfers a toner image on the photoconductor to a recording medium when a transfer bias is applied. A humidity environment detection unit that detects a parameter related to the humidity environment in the apparatus, and a plurality of pixels based on the input image data are arranged on a two-dimensional grid and developed from among the pixels of the image From the image composed of a plurality of pixels arranged on a two-dimensional grid, and the printing pixel and toner at the time of development, the printing pixel counting unit that counts the number of printing pixels that are pixels to which toner is sometimes attached An edge count unit that counts the number of edges that are boundaries on the grid line with a blank pixel that should not be a pixel, the humidity environment detection unit, the print pixel count unit, Have a control unit for controlling the transfer bias applied to the transfer member in accordance with a detection result obtained by the edge count unit, wherein the control unit from said obtained from humidity detector parameters, high in the apparatus When it is determined that there is a wet environment and the ratio of the number of edges obtained by the edge count unit to the number of print pixels obtained from the print pixel count unit is smaller than a predetermined value, the parameter and the It is characterized in that the transfer bias having the smallest value is selected from transfer biases determined in advance according to the ratio of the number of edges to the number of print pixels .

The image forming apparatus according to claim 2 , wherein a photoconductor on which a toner image is formed based on input image data and a transfer member that applies a transfer bias to transfer the toner image on the photoconductor to a recording medium. A humidity environment detection unit that detects a parameter related to the humidity environment in the apparatus, and a plurality of pixels based on the input image data are arranged on a two-dimensional grid and developed from among the pixels of the image From the image composed of a plurality of pixels arranged on a two-dimensional grid, and the printing pixel and toner at the time of development, the printing pixel counting unit that counts the number of printing pixels that are pixels to which toner is sometimes attached An edge count unit that counts the number of edges that are boundaries on the grid line with a blank pixel that should not be a pixel, the humidity environment detection unit, the print pixel count unit, A control unit for controlling the transfer bias applied to the transfer member in accordance with a detection result obtained by the edge count unit, wherein the control unit from said obtained from humidity detector parameters, high in the apparatus If the in-humidity environment Ru is determined, the control unit, based on the ratio of the number of edges for the number of printing pixels, and selecting one of a plurality of control functions prepared in advance, selected the control It is characterized by controlling the transfer current using a curve .

The image forming apparatus according to claim 3 is the image forming apparatus according to claim 1 or 2, wherein the control unit, the transfer bias based on a ratio of the number of edges for the number of printing pixels in the image data of one page It is characterized by controlling.

According to a fourth aspect of the present invention , in the image forming apparatus according to the first or second aspect , the control unit is configured to divide the image forming area on the recording medium into a plurality of areas divided in the conveyance direction of the recording medium. The transfer bias is controlled for each region based on the ratio of the number of edges to the number of print pixels included.

The image forming apparatus according to claim 5 is the image forming apparatus according to claim 4 , wherein the image forming area on the recording medium is divided into a plurality of areas in the recording medium conveyance direction. The width of the photosensitive member and the transfer member are equal to each other.

The image forming apparatus according to claim 6 is the image forming apparatus according to any one of claims 1 to 5 , wherein the environment detection unit includes a recording medium between the photoconductor and the transfer member. In an existing state, an electrical resistance value in a section including a space between the photoconductor and the transfer member is detected.

According to the image forming apparatus of the first aspect, the transfer bias is determined according to the detection results of the humidity environment detection unit, the print pixel count unit, and the edge count unit. By counting the number of edges of the print pixel, it is possible to determine whether a blank paper portion or a print pixel is adjacent to the print pixel. That is, good transfer can be performed in accordance with the change in the electrical resistance value of the paper and the image pattern due to the humidity environment in the apparatus.
In addition, by determining whether the ratio of the number of edges to the number of print pixels is smaller than a predetermined value, the image data includes many print pixels that are not adjacent to the blank page portion, or prints that are adjacent to the blank page portion. It can be determined whether or not there are many pixels. Therefore, when the electrical resistance value of the recording medium is reduced in a high humidity state in the apparatus, the transfer bias is applied between an image including many print pixels where the blank page portion is not adjacent and an image including many print pixels where the blank page portion is adjacent. Transfer defects such as blurring and ghosting can be suppressed.

According to the image forming apparatus of the second aspect , the transfer bias is determined according to the detection results of the humidity environment detection unit, the print pixel count unit, and the edge count unit. By counting the number of edges of the print pixel, it is possible to determine whether a blank paper portion or a print pixel is adjacent to the print pixel. That is, good transfer can be performed in accordance with the change in the electrical resistance value of the paper and the image pattern due to the humidity environment in the apparatus.
Further, when the electrical resistance value obtained from the resistance value detection unit is smaller than a predetermined value, an optimal control function is selected from a plurality of control functions prepared in advance based on the ratio of the number of edges to the number of print pixels. Select and control the transfer bias. Therefore, it is possible to control the transfer bias in detail according to the image pattern in a high humidity environment.

According to the image forming apparatus of the third aspect , the optimum transfer bias is determined from the ratio of the number of edges to the number of print pixels in one page of print data. Accordingly, since the image data can be determined with a small amount of calculation, the calculation process is simplified.

According to the image forming apparatus of the fourth aspect , the image data on the recording medium is divided into a plurality of areas in the recording medium conveyance direction. Then, the transfer bias is controlled for each area according to the ratio of the number of edges to the print pixels included in the image data corresponding to the plurality of divided areas. Therefore, since the transfer bias can be controlled for each divided area, it is possible to perform the optimum transfer bias control for each area in the page, rather than controlling the transfer bias in one page of image data.

According to the image forming apparatus of the fifth aspect , the width of the divided area according to the fourth aspect in the recording medium conveyance direction is a width in which the photosensitive member and the transfer member are in contact with each other in the recording medium conveyance direction. equal. Therefore, since the transfer bias is controlled in accordance with actual image formation, good transfer can be performed.

According to the image forming apparatus of claim 6 , in a state where the recording medium exists between the photoconductor and the transfer member, the resistance value of a section including between the photoconductor and the transfer member is set. To detect. Since the humidity environment in the apparatus is determined based on the resistance value, a humidity sensor or the like is not used, so that the configuration of the image forming apparatus can be simplified.

It is a longitudinal cross-sectional view showing schematic structure of a laser printer. It is a block diagram of a control part. It is a figure which shows the edge and oblique end part of a printing pixel. 3 is a flowchart according to the first embodiment. 10 is a table showing a correspondence relationship between a detected resistance value, a ratio of the number of edges to a print pixel, and an applied transfer bias. FIG. 6 is a diagram illustrating an example of an image in which print pixels are arranged on a two-dimensional lattice. FIG. 6 is a diagram illustrating an example of an image in which print pixels are arranged on a two-dimensional lattice. It is a flowchart concerning a 2nd embodiment. (A) It is a figure showing the correspondence of a control function and the ratio of the number of edges with respect to the number of printing pixels. (B) It is a figure showing an actual control function. FIG. 6 is a diagram illustrating an example of an image in which print pixels are arranged on a two-dimensional lattice. FIG. 6 is a diagram illustrating an image forming area divided into predetermined ranges and a correspondence relationship between the divided area and pixel data.

<Overall configuration of image forming apparatus>
<First Embodiment>
Next, embodiments of the present invention will be described in detail with reference to the drawings as appropriate.

  FIG. 1 is a side sectional view of an image forming apparatus according to an embodiment of the present invention.

  As shown in FIG. 1, a laser printer 1 as an example of an image forming apparatus has a feeder unit 4 for feeding a sheet 3 into a main body casing 2 and a sheet 3 as an example of a fed recording medium. An image forming unit 5 for forming an image is provided.

  A front cover 11 that can be opened and closed is provided on the front side of the main casing 2 (in the following description, the right in FIG. 1 is the front and the left is the rear), and an opening that is formed when the front cover 11 is opened. Thus, a process cartridge 30 described later is detachable.

<Configuration of feeder section>
The feeder unit 4 includes a paper feed tray 12 that is detachably attached to the bottom of the main body casing 2, and a paper pressing plate 13 provided in the paper feed tray 12. The feeder unit 4 includes a paper feed roller 14 and a paper feed pad 15 provided above one end of the paper feed tray 12, and a paper provided downstream of the paper feed roller 14 in the conveyance direction of the paper 3. Powder removing rollers 16 and 17 are provided. Further, the feeder unit 4 includes a registration roller 18 provided on the downstream side with respect to the paper dust removing rollers 16 and 17.

  In the feeder unit 4 configured as described above, the sheet 3 in the sheet feeding tray 12 is brought close to the sheet feeding roller 14 by the sheet pressing plate 13 and is sent out by the sheet feeding roller 14 and the sheet feeding pad 15. Then, after passing through the various rollers 1618, they are conveyed one by one to the image forming unit 5.

<Configuration of image forming unit>
The image forming unit 5 includes a scanner unit 20, a process cartridge 30, a fixing device 40, and the like.

<Configuration of scanner unit>
The scanner unit 20 is provided in the upper part of the main casing 2 and includes a laser light emitting unit (not shown), a polygon mirror 21 that is driven to rotate, lenses 22 and 23, reflecting mirrors 24, 25, and 26. . As indicated by the chain line, the laser beam based on the image data emitted from the laser light-emitting unit passes or reflects in the order of the polygon mirror 21, the lens 22, the reflecting mirrors 24 and 25, the lens 23, and the reflecting mirror 26, and the process cartridge. The surface of the 30 photosensitive drums 32 is irradiated by high-speed scanning.

<Configuration of process cartridge>
The process cartridge 30 is disposed below the scanner unit 20 and is configured to be detachably attached to the main body casing 2. The process cartridge 30 includes a photosensitive cartridge 31 and a developing cartridge 33 as an example of a developing device.

<Configuration of developer cartridge>
The developing cartridge 33 is detachably attached to a hollow photosensitive frame 31A that constitutes the outer frame of the photosensitive cartridge 31, and includes a developing roller 36, a layer thickness regulating blade 37, a supply roller 38, and a toner hopper 39. ing. The developing roller 36 and the supply roller 38 are rotatably supported by a hollow developing frame 33A that constitutes an outer frame of the developing cartridge 33. The toner as an example of the developer in the toner hopper 39 is supplied to the developing roller 36 by the rotation of the supply roller 38 in the arrow direction (counterclockwise direction). At this time, the supply roller 38 and the developing roller 36 Positively frictionally charged. The toner supplied onto the developing roller 36 enters between the layer thickness regulating blade 37 and the developing roller 36 as the developing roller 36 rotates in the arrow direction (counterclockwise direction), and is thin with a certain thickness. It is carried on the developing roller 36 as a layer.

<Configuration of photoconductor cartridge>
The photosensitive cartridge 31 is mainly provided with a photosensitive drum 32, a scorotron charger 34, and a transfer roller 35. The photoconductor drum 32 is supported by the photoconductor frame 31A so as to be rotatable in the arrow direction (clockwise). The photosensitive drum 32 has a drum body grounded and a surface portion formed of a positively chargeable photosensitive layer.

  The scorotron charger 34 is disposed above the photosensitive drum 32 so as to face the photosensitive drum 32 with a predetermined interval therebetween. The scorotron charger 34 is a positively charged scorotron charger that generates corona discharge from a charging wire such as tungsten, and is configured to uniformly charge the surface of the photosensitive drum 32 to a positive polarity. Has been.

  The transfer roller 35 is disposed below the photoconductive drum 32 so as to face and contact the photoconductive drum 32, and is supported by the photoconductive frame 31A so as to be rotatable in an arrow direction (counterclockwise). . The transfer roller 35 is configured by covering a metal roller shaft with a conductive rubber material. A transfer current having a polarity opposite to that of the toner is applied to the transfer roller 35 at the time of transfer.

  The surface of the photosensitive drum 32 is uniformly positively charged by the scorotron charger 34 and then exposed in the image forming area by high-speed scanning of the laser beam from the scanner unit 20.

  Thereby, the potential of the exposed part is lowered, and an electrostatic latent image based on the image data is formed. Here, the “electrostatic latent image” refers to an exposed portion of the surface of the photosensitive drum 32 that is uniformly positively charged and exposed to a laser beam to have its potential lowered. Next, the toner carried on the developing roller 36 is supplied to the electrostatic latent image formed on the surface of the photosensitive drum 32 when the toner carried on the developing roller 36 contacts the photosensitive drum 32 by the rotation of the developing roller 36. Is done. The toner is made visible by being selectively carried on the surface of the photosensitive drum 32, whereby a toner image is formed by reversal development.

  Thereafter, the photosensitive drum 32 and the transfer roller 35 are rotationally driven so as to sandwich and convey the sheet 3 therebetween, and the sheet 3 is conveyed between the photosensitive drum 32 and the transfer roller 35. Then, the toner image carried on the surface of the photosensitive drum 32 is transferred onto the paper 3.

<Configuration of fixing device>
The fixing device 40 is disposed on the downstream side of the process cartridge 30, and is provided with a heating roller 41, a pressure roller 42 that is disposed opposite to the heating roller 41 and sandwiches the sheet 3 between the heating roller 41, and these heating rollers 41. And a pair of transport rollers 43 provided on the downstream side of the pressure roller 42. In the fixing device 40 configured as described above, the toner transferred onto the sheet 3 is thermally fixed while the sheet 3 passes between the heating roller 41 and the pressure roller 42, and then the sheet is transferred. 3 is transported to the paper discharge path 44 by the transport roller 43 and the flapper 49. The paper 3 sent to the paper discharge path 44 is discharged onto a paper discharge tray 46 by a paper discharge roller 45.

<Configuration of control unit>
The operation of each device described above is controlled by the control unit 100. Next, control by the control unit 100 will be described. FIG. 2 is a block diagram of the control unit.

  As shown in FIG. 2, the function of the control unit 100 will be described within the scope of the present invention. The control unit 100 mainly includes a CPU and the like, and includes an image detection unit 110, an image determination unit 120, a storage unit 130, and a resistance value detection unit 150 as an example of a humidity environment detection unit. The control unit 100 receives a print job that is data (print data) including a print instruction from a device such as a personal computer (PC) 200, and performs printing according to the print job. Yes.

  The control unit 100 executes control of each unit according to various programs stored in the storage unit 130.

  The storage unit 130 mainly includes a ROM, a RAM, and the like. The ROM stores various programs for controlling transfer current and image forming processing, and a resistance threshold value, a pixel edge threshold value, and the like, which will be described later. Further, the RAM temporarily stores an electrical resistance value obtained from a resistance value detection unit 150 (to be described later), the number of print pixels and the number of edges obtained by the print pixel count unit 111 and the edge count unit 112, and the like.

  As shown in FIG. 2, the transfer current application power source 140 as an example of the transfer current application unit is connected to the control unit 100 and the roller shaft of the transfer roller 35, and is controlled by the control unit 100 under constant current control. The transfer roller 35 is configured to apply a predetermined transfer current.

  The voltage detector 160 is connected in the middle of a circuit connected from the transfer current application power source 140 to the transfer roller 35. An example of the voltage detection unit 160 is a voltmeter.

  In a state where the sheet 3 is sandwiched between the photosensitive drum 32 and the transfer roller 35, the resistance value detection unit 150 applies a preset transfer current from the transfer current application power source 140 to the transfer roller 35, and determines the voltage value at this time. Read. Then, the resistance value detection unit 150 measures the electrical resistance value between the transfer roller 35 and the photosensitive drum 32 in a state where the sheet 3 is sandwiched between the photosensitive drum 32 and the transfer roller 35 from the voltage value. This electrical resistance value is used as a parameter when determining the humidity environment in the laser printer 1.

  The image detection unit 110 includes a print pixel count unit 111 and an edge count unit 112. The print pixel count unit 111 and the edge count unit 112 count the number of print pixels and the number of edges of one page of image data input from the PC 200 or the like to the control unit 100.

  Specifically, image data input from the PC 200 or the like is developed into pixel data (raster data) binarized by the control unit 100 in accordance with the resolution of the laser printer 1. Pixel data is represented as a set of points in which input image data is arranged in a two-dimensional grid. FIG. 3 shows an example of pixel data developed by the control unit 100. FIG. 3 defines the main scanning direction and the sub-scanning direction of the pixel data. In FIG. 3, the thick line portion of the outer frame represents the boundary between the image forming area and the non-image forming area when the toner image is transferred onto the paper. The image forming area is an area where an image is formed on the paper 3 and usually indicates an area excluding the peripheral edge of the paper 3. The filled area in the grid represents a printing pixel that is a pixel to which toner should adhere during development. A grid represented in white represents a blank pixel that is a pixel to which toner should not adhere during development.

  The print pixel counting unit 111 counts pixels to which toner should adhere during development. In the image shown in FIG. 3, the print pixel counting unit 111 counts the number of print pixels as 7, and temporarily stores the value in the storage unit 130.

The edge count unit 112 detects edges using a known technique such as an edge detection filter.

  In the present embodiment, the edge is an adjacent side of a print pixel and a blank pixel adjacent to the print pixel when a pixel arranged on a two-dimensional grid is regarded as a quadrilateral. The edge count unit 112 counts the adjacent side as an edge. In FIG. 3, when the print pixel has a boundary side between the image forming area and the non-image forming area, the edge counting unit 112 counts the boundary side as an edge. For example, in the case of the image shown in FIG. 3, the side of the quadrilateral indicated by A represents the adjacent side of the print pixel and the blank pixel. A quadrilateral side indicated by B is a boundary side between the image forming area and the non-image forming area of the print pixel.

  In the pixel data of FIG. 3, the edge count unit 112 counts the number of edges as 4 for the pixel 300, counts 4 for the pixel 301, counts 8 for the pixel set 303 in which the four pixels are aggregated, Count four. Therefore, the pixel data shown in FIG. 3 has 20 edges.

  The print pixel count and the edge count values counted by the print pixel count unit 111 and the edge count unit 112 are temporarily stored in the storage unit 130.

  The image determination unit 120 includes a resistance value determination unit 121 and a pixel edge determination unit 122. The resistance value determination unit 121 compares a predetermined value (resistance threshold value) stored in advance in the storage unit 130 with the resistance value obtained by the resistance value detection unit 150, and the detection result of the resistance value detection unit 150 is smaller than the resistance threshold value. It is determined whether or not. The resistance value determination unit 121 compares the resistance value obtained from the resistance value detection unit 150 with the resistance threshold value to determine whether the apparatus is in a high humidity environment and the resistance value of the sheet 3 is reduced. . In this embodiment, the high humidity environment refers to a time when the relative humidity is 70% or more.

  The pixel edge determination unit 122 is configured so that the ratio of the number of edges obtained by the edge counting unit 112 to the number of printing pixels obtained by the printing pixel counting unit 111 and a predetermined value (pixel edge threshold) stored in advance in the storage unit 130 are larger or smaller. Compare Then, the pixel edge determination unit 122 determines whether the ratio of the number of edges to the number of print pixels included in the image data is smaller than the pixel edge threshold value.

  The control unit 100 determines a transfer current based on the determination results of the resistance value determination unit 121 and the pixel edge determination unit 122, and applies the transfer current to the transfer roller 35 by the transfer current application power supply 140.

<Control method>
A specific transfer current control method by the control unit 100 will be described. In the drawings to be referred to, FIG. 4 is a flowchart regarding control of a transfer current in the first embodiment.

  A specific example of processing will be described. The control unit 100 receives the print job and obtains the image data to be printed (S10). The acquired image data is temporarily stored in the storage unit 130.

  As described above, the image data is developed into pixel data by the control unit 100 on the storage unit 130.

  Here, control of the transfer current when the developed pixel data for one page is data as shown in FIG. 5 will be described. In FIG. 5, a colored grid represents a print pixel.

  The print pixel count unit 111 and the edge count unit 112 count the number of print pixels and the number of edges included in the pixel data (S11). In this case, the print pixel counting unit 111 counts 45 print pixels. Further, as described above, the edge count unit 112 counts the adjacent sides of the print pixel and the blank pixel as edges, so the number of edges is 100. The obtained print pixel number and edge number are temporarily stored in the storage unit 130.

  Then, the control unit 100 obtains the ratio of the number of edges to the number of print pixels based on the obtained values of the number of print pixels and the number of edges, and stores the ratio in the storage unit 130 (S12). In the case of the image data shown in FIG. 5, since the number of print pixels is 45 and the number of edges is 100, the ratio of the number of edges to the number of print pixels is 2.22. This value is temporarily stored in the storage unit 130.

  In step S13, a transfer current is applied to the transfer roller 35 while the sheet 3 is sandwiched between the photosensitive drum 32 and the transfer roller 35. Based on the value of the voltage detection unit 160 at this time, the resistance value detection unit 150 determines that the transfer roller 35 and the photosensitive drum 32 in a state where the sheet 3 is sandwiched between the photosensitive drum 32 and the transfer roller 35 from the voltage value. Measure the electrical resistance value between.

  And the resistance value determination part 121 determines whether the obtained resistance value is smaller than a resistance threshold value (S14). If the inside of the apparatus (laser printer 1) is in a high humidity environment and the electrical resistance value of the paper 3 decreases, a large amount of current flows through the paper 3 easily. Therefore, it is necessary to change the value of the transfer current depending on whether or not the print pixel of the image pattern to be printed is adjacent to the blank pixel. Accordingly, the electrical resistance value obtained from the resistance value detection unit 150 is compared with the resistance threshold value, and the state of the electrical resistance value of the paper 3 is determined.

  In S14, the resistance value determination unit 121 determines that the electrical resistance value obtained in S13 is equal to or greater than the resistance threshold value (S14: N). In this case, the control unit 100 controls the transfer current in a normal mode, which will be described later, and applies a transfer current to the transfer roller 35 by the transfer current application power source 140 (S15).

  In S14, when the resistance value determination unit 121 determines that the electrical resistance value obtained in S13 is smaller than the resistance threshold value, the process proceeds to S16 (S14: Y).

  In S16, the pixel edge determination unit 122 determines whether the ratio of the number of edges to the number of print pixels obtained in S12 is smaller than the pixel edge threshold (S16). In the first embodiment, the value of the pixel edge threshold is 1. In the pixel data shown in FIG. 5, since the ratio of the number of edges to the number of print pixels is 2.22, the value is equal to or greater than the pixel edge threshold. Therefore, the pixel edge determination unit 122 determines that the ratio of the number of edges to the number of print pixels is equal to or greater than the pixel edge threshold (S16: N).

  In step S15, the control unit 100 controls the transfer current in the normal mode.

  Next, transfer current control when the pixel data is data as shown in FIG. 6 will be described.

  As described above, after receiving the print job (S10), the image data is developed into pixel data on the storage unit 130. Based on this pixel data, the print pixel count unit 111 and the edge count unit 112 count the number of print pixels and the number of edges (S11).

In the pixel data shown in FIG. 6, the number of print pixels is 45 and the number of edges is 28. From this value, the control unit 100 calculates the ratio of the number of edges to the number of print pixels as 0.62, and the storage unit 130
(S12).

  In step S13, a transfer current is applied to the transfer roller 35 while the sheet 3 is sandwiched between the photosensitive drum 32 and the transfer roller 35. Based on the value of the voltage detection unit 160 at this time, the resistance value detection unit 150 determines that the transfer roller 35 and the photosensitive drum 32 in a state where the sheet 3 is sandwiched between the photosensitive drum 32 and the transfer roller 35 from the voltage value. Measure the electrical resistance value between.

  The resistance value determination unit 121 determines whether or not the obtained electrical resistance value is smaller than a resistance threshold value (S14).

  In S14, the resistance value determination unit 121 determines that the resistance value obtained in S13 is equal to or greater than the resistance threshold value (S14: N). In this case, the control unit 100 controls the transfer current in the normal mode, and applies the transfer current to the transfer roller 35 from the transfer current application power source 140 (S15).

  In S14, when the resistance value determination unit 121 determines that the electrical resistance value obtained in S13 is smaller than the resistance threshold value, the process proceeds to S16 (S14: Y).

  In S16, the pixel edge determination unit 122 determines whether the ratio of the number of edges to the number of print pixels obtained in S12 is smaller than the pixel edge threshold (S16). In the pixel data shown in FIG. 5, since the ratio of the number of edges to the number of print pixels is 0.62, the value is smaller than the pixel edge threshold. Therefore, the pixel edge determination unit 122 determines that the ratio of the number of edges to the number of print pixels is smaller than the pixel edge threshold (S16: Y).

  In this case, the control unit 100 controls the transfer current in the low current mode in which a transfer current having a smaller value than that in the normal mode is applied to the transfer roller 35 (S16). When the electrical resistance value of the paper 3 is reduced, a large amount of transfer current easily flows through a white paper portion where the toner image is not transferred. Therefore, the transfer current appropriate for an image including many print pixels adjacent to blank pixels needs to have a larger absolute value than the transfer current appropriate to an image including many print pixels not adjacent to blank pixels. . Therefore, when the electrical resistance value of the paper is lowered, it is necessary to change the transfer current applied to the toner image according to the image data to be printed. By determining the image pattern based on the ratio of the number of edges to the number of print pixels, the control unit 100 can apply an optimum transfer current to the transfer roller 35 even for image patterns having the same number of print pixels.

  Then, the control unit 100 applies a transfer current to the transfer roller 35 by the transfer current application power source 140 in the low current mode (S17).

  FIG. 7 shows a table showing an example of the correspondence relationship between the detected electrical resistance value, the ratio of the number of edges to the print pixel, and the applied transfer current in the first embodiment. In FIG. 7, T represents the electrical resistance value obtained from the resistance value detection unit 150, and C represents the ratio of the number of edges to the number of print pixels. When the electrical resistance value between the transfer roller 35 including the paper 3 and the photosensitive drum 32 is 50 MΩ, the temperature in the apparatus is 32.5 ° C. and the relative humidity is 70%. Therefore, in this embodiment, the resistance threshold is set to 50 MΩ.

  For example, as shown in FIG. 7, the transfer current applied to the transfer roller 35 is determined according to the detection value of the resistance value detection unit 150 and the ratio of the number of edges to the number of print pixels.

Specifically, the normal mode is a transfer current control mode selected when T ≧ 50 MΩ or C ≧ 1 and T <50 MΩ. When T ≧ 50 MΩ when the normal mode is selected, the transfer current applied to the transfer roller 35 is −15 μA, and C ≧ 1 and T
When <50 MΩ, the transfer current applied to the transfer roller 35 is −20 μA. On the other hand, in the low current mode, that is, when 0 <C <1 and T <50 MΩ are satisfied, −10 μA is applied to the transfer roller 35.

  As described above, the control unit 100 applies a transfer current having a smaller absolute value in the low current mode than in the normal mode. That is, when the detection result of the resistance value detection unit 150 is smaller than 50 MΩ and the ratio of the number of edges to the number of print pixels is smaller than 1, the control unit 100 generates a transfer current (−10 μA) having the smallest absolute value. Apply to the transfer roller 35.

  As described above, even if the image data has the same number of print pixels, by counting the number of edges, the image data to be printed has many print pixels adjacent to blank pixels, or no blank pixels are adjacent to each other. It is possible to identify whether there are many print pixels. Therefore, even when the resistance value of the paper 3 is reduced in a high humidity environment, a transfer current suitable for the image pattern can be applied, so that transfer defects such as blurring and ghosting can be suppressed.

<Second Embodiment>
Next, a second embodiment will be described. FIG. 8 is a flowchart regarding control of the transfer current according to the second embodiment.

  In the second embodiment, the storage unit 130 stores a control function f corresponding to a ratio of the number of edges to a predetermined number of print pixels.

  In the second embodiment, as shown in FIG. 9A, one control function f is stored in the storage unit 130 so as to correspond to a predetermined range of the ratio of the number of edges to the number of print pixels. ing. Specifically, the ratio (C) of the number of edges to the number of print pixels is divided into two ranges of 0 <C ≦ 0.5 and 0.5 <C ≦ 1, and one control function f is provided for each range. It corresponds.

  FIG. 9B shows a schematic diagram of a control function used in the second embodiment. The control function is a curve indicating a correspondence relationship between the electrical resistance value detected by the resistance value detection unit 150 that varies depending on the humidity environment in the apparatus and the transfer current value corresponding to each electrical resistance value.

  FIG. 9B shows a control function F in the normal mode, and a control function f1 and a control function f2 corresponding to a ratio of the number of edges to a predetermined number of print pixels. As for the control function F used in the normal mode, the inventor experimentally obtained the transfer current corresponding to the electric resistance value of the photosensitive drum 32, the transfer roller 35, and the paper 3 sandwiched between them, and approximated the curve (control function). ). Then, it is proposed to control the transfer current based on this function (see JP 2009-157231 A).

  In the second embodiment, furthermore, in a high humidity environment, that is, in a range where the detected electric resistance value is smaller than the resistance threshold (50 MΩ), the control unit 100 transfers a value having a smaller absolute value than in the normal mode. A control function f is predetermined so as to apply a current. Similarly to the control function F, the control function f can be obtained by experimentally obtaining a transfer current corresponding to the electric resistance value and approximating the experimental value.

  In the second embodiment, the normal mode is a control function F that is selected when the detected electrical resistance value is equal to or greater than a predetermined value (resistance threshold) or the ratio of the number of edges to the number of print pixels is greater than 1. Based on this, a case where a transfer current is applied to the transfer roller 35 is shown.

  Next, transfer current control according to the second embodiment will be described with reference to FIG.

  The control unit 100 acquires a print job (S20), and develops the input image data into pixel data.

  Next, a transfer current is applied to the transfer roller 35 while the sheet 3 is held between the photosensitive drum 32 and the transfer roller 35. Based on the value of the voltage detection unit 160 at this time, the resistance value detection unit 150 determines that the transfer roller 35 and the photosensitive drum 32 in a state where the sheet 3 is sandwiched between the photosensitive drum 32 and the transfer roller 35 from the voltage value. The electrical resistance value between them is measured (S21).

  The resistance value determination unit 121 determines whether or not the resistance value obtained by the resistance value detection unit is smaller than a resistance threshold (50 MΩ) (S22).

  The resistance value determination unit 121 determines that the resistance value obtained by the resistance value detection unit 150 is equal to or greater than the resistance threshold value (50 MΩ) (S22: N). In this case, the process proceeds to S23, and the control unit 100 controls the transfer current in the normal mode (S23). Then, the control unit 100 applies a transfer current to the transfer roller 35 by the transfer current application power source 140. The storage unit 100 stores in advance a control function F that is selected by the control unit 100 when the normal mode is selected, and controls the transfer current based on the control function F. The control function F is determined so that a transfer current larger than the control function f2 is applied to the transfer roller 35.

  Next, transfer current control when the resistance value determination unit 121 determines that the value obtained from the resistance value detection unit 150 is smaller than a predetermined value (50 MΩ) (S22: Y) will be described.

  The resistance value determination unit 121 determines that the electrical resistance value obtained from the resistance value detection unit 150 is smaller than the resistance threshold value (S22: Y). In that case, the process proceeds to S24, and the print pixel count unit 111 and the edge count unit 112 count the number of print pixels and the number of edges of the image data input from the PC 200 (S24).

  Then, the control unit 100 calculates the ratio of the number of edges to the number of print pixels based on the obtained number of print pixels and the number of edges, and temporarily stores it in the storage unit 130 (S25).

  For example, when the image data is input from the PC 200 and the developed pixel data is the diagram shown in FIG. 5, the ratio of the number of edges to the number of print pixels is 2.22 as described above.

  Proceeding to the next step, it is determined whether the ratio of the number of edges to the number of print pixels obtained is 1 or less (S26).

  In the case of the pixel data shown in FIG. 5, the ratio of the number of edges to the number of print pixels is 2.22. Therefore, the pixel edge determination unit 122 determines that the ratio of the number of edges to the obtained number of print pixels is greater than 1 (S26: N).

  In this case, the process proceeds to S23, where the control unit 100 selects the control function F and controls the transfer current in the normal mode (S23). Then, the control unit 100 applies a transfer current to the transfer roller 35 by the transfer current application power source 140.

  Further, the case where the pixel data developed from the image data acquired by the control unit 100 is the data shown in FIG. 6 will be described. As described above, the ratio of the number of edges to the number of print pixels is 0.62.

Therefore, in S26, the pixel edge determination unit 122 determines that the ratio of the number of edges to the print pixel is 1 or less (S26: Y).

  Proceeding to the next step, the control unit 100 selects a predetermined control function in accordance with the ratio of the number of edges to the obtained number of print pixels. Accordingly, the control unit 100 selects the control function f2 from the table shown in FIG. 9A (S27).

  Then, the control unit 100 controls the transfer current based on the selected control function f2, and applies the transfer current to the transfer roller 35 by the transfer current application power source 140 (S28).

  Further, the case where the pixel data developed from the image data acquired by the control unit 100 is the data shown in FIG. 10 will be described. In the case of the pixel data shown in FIG. 10, since the number of print pixels is 90 and the number of edges is 38, the ratio of the number of edges to the number of print pixels is 0.42.

  Therefore, in S26, the pixel edge determination unit 122 determines that the ratio of the number of edges to the print pixel is 1 or less (S26: Y).

  Proceeding to the next step, the control unit 100 selects a predetermined control function f in accordance with the ratio of the number of edges to the obtained number of print pixels. Therefore, the control unit 100 selects the control function f1 from the table shown in FIG. 9A (S27).

  Then, the control unit 100 controls the transfer current based on the selected control function f1, and applies the transfer current to the transfer roller 35 by the transfer current application power source 140 (S28).

  Thus, a plurality of control functions are stored in the storage unit 130 in advance. Then, when the electrical resistance value obtained from the resistance value detection unit 150 is smaller than the resistance threshold value and the ratio of the number of edges to the number of print pixels is 1 or less, the control unit 100 determines the number of edges relative to the number of print pixels. The control function is selected such that the smaller the ratio, that is, the closer the printed image data is to a solid black image, the smaller the value of the transfer current. Thereby, when the electric resistance value of the sheet 3 is lowered, a transfer current corresponding to the image pattern can be applied in more detail, and transfer defects can be suppressed.

  In the second embodiment, in order to reduce the memory consumption of the storage unit 130, the range (C) of the number of edges with respect to the number of print pixels is divided into two areas. By providing such a control function, more detailed control of the transfer current may be performed.

  In the above-described embodiment, the detection of the electric resistance value of the paper 3 may be detected a plurality of times every predetermined period, and the transfer current may be controlled according to the electric resistance value of the paper 3 each time. . Thereby, the electric resistance value of the paper 3 can be detected finely, and a good toner image can be transferred.

  In the above embodiment, the transfer current is controlled by determining the ratio of the number of edges to the number of print pixels in the entire image data of one page. However, the image data is divided into a plurality of areas, and the divided areas are divided. The transfer current may be controlled every time.

Specifically, as shown in FIG. 11, the image forming area is divided into ranges indicated by arrows R in the conveyance direction of the sheet 3. Then, the transfer current is controlled for each area based on the ratio of the number of edges to the print pixels included in the area. As a result, an appropriate transfer current can be determined for each of the divided areas. Therefore, the transfer current is controlled more optimally for each area than when the transfer current is controlled based on the ratio of the number of edges to the print pixels in one page of print data. Transfer can be performed.

  Further, the range of the arrow R is the contact width between the photosensitive drum 32 and the transfer roller 35 in the conveyance direction of the paper 3, and the transfer current is controlled based on the ratio of the number of edges to the number of print pixels included in the pixel data area. I do. By doing so, the transfer current can be controlled in accordance with the actual image formation, so that good transfer can be performed.

  In the above embodiment, the resistance detection unit 150 obtains the electrical resistance value from the voltage value detected when a predetermined current is applied to the transfer roller 35, but applies the predetermined voltage to the transfer roller 35. The electric resistance value may be obtained from the detected current value.

  In the first embodiment, the resistance detection unit 150 detects the electrical resistance value after counting the print pixels and the number of edges. However, the resistance value is detected before the print pixels and the number of edges are counted. By doing so, the transfer current may be controlled. In the second embodiment, after the electrical resistance value is detected by the resistance value detection unit 150, the number of print pixels and the number of edges included in the pixel data are counted. However, the number of print pixels and the number of edges are counted. After the measurement, the electric resistance value may be detected.

  In the above embodiment, when the sheet 3 exists between the photosensitive drum 32 and the transfer roller 35, the electrical resistance value between the transfer roller 35 and the photosensitive drum 32 is measured, and the electrical resistance value is compared with the resistance threshold value. As a result, the humidity environment in the apparatus was judged. However, the method for determining the humidity environment in the apparatus is not limited to this, and the humidity environment in the apparatus may be determined using the relative humidity as a parameter by detecting the relative humidity using a humidity sensor or the like in the apparatus.

  In the above embodiment, the case where the transfer current is controlled has been described. However, even when the transfer voltage is controlled, good transfer can be performed by the same control as when the transfer current is controlled. .

3 Paper 32 Photosensitive drum 35 Transfer roller 100 Control unit 111 Print pixel count unit 112 Edge count unit 121 Resistance value determination unit 122 Pixel edge determination unit 130 Storage unit 140 Transfer current application power supply 150 Resistance value detection unit 160 Voltage detection unit

Claims (6)

A photoreceptor on which a toner image based on input image data is formed;
A transfer member to which a transfer bias is applied to transfer the toner image on the photoconductor to a recording medium;
A humidity environment detector for detecting parameters related to the humidity environment in the device ;
A print pixel that counts the number of print pixels to which toner is to be attached during development from among the pixels of an image configured by arranging a plurality of pixels based on the input image data on a two-dimensional grid A counting section;
Counts the number of edges, which are the boundary on the grid line, between the print pixel and the blank pixel that should not be attached with toner during development from an image composed of a plurality of pixels arranged on a two-dimensional grid. An edge count unit to perform,
It possesses said humidity detecting unit, a control unit for controlling the transfer bias applied to the transfer member in accordance with the print pixel counting unit and the edge count portion detection result obtained by,
The controller is
The ratio of the number of edges obtained by the edge count unit to the number of print pixels obtained from the print pixel count unit when it is determined from the parameters obtained from the humidity environment detection unit that the inside of the apparatus is in a high humidity environment Is smaller than a predetermined value, the transfer bias having the smallest value is selected from the transfer bias determined in advance according to the ratio of the parameter and the number of edges to the number of print pixels. Image forming apparatus. A photoreceptor on which a toner image based on input image data is formed;
A transfer member to which a transfer bias is applied to transfer the toner image on the photoconductor to a recording medium;
A humidity environment detector for detecting parameters related to the humidity environment in the device;
A print pixel that counts the number of print pixels to which toner is to be attached during development from among the pixels of an image configured by arranging a plurality of pixels based on the input image data on a two-dimensional grid A counting section;
Counts the number of edges, which are the boundary on the grid line, between the print pixel and the blank pixel that should not be attached with toner during development from an image composed of a plurality of pixels arranged on a two-dimensional grid. An edge count unit to perform,
A control unit for controlling a transfer bias applied to the transfer member in accordance with the detection results obtained by the humidity environment detection unit, the print pixel count unit and the edge count unit;
The controller is
When it is determined from the parameters obtained from the humidity environment detection unit that the inside of the apparatus is in a high humidity environment, based on the ratio of the number of edges to the number of print pixels, a plurality of control functions prepared in advance choose one, images forming device you control means controls the transfer bias with said selected control curve. Wherein the control unit, the image forming apparatus according to claim 1 or 2, characterized in that to control the transfer bias based on a percentage of the number of edges for the number of printing pixels in the image data of one page. The control unit sets the image forming area on the recording medium for each area based on the ratio of the number of edges to the number of print pixels included in the image data corresponding to the area divided into a plurality of areas in the conveyance direction of the recording medium. the image forming apparatus according to claim 1 or 2, characterized in that to control the transfer bias to. An area obtained by dividing the image forming area on the recording medium into a plurality of parts in the recording medium conveyance direction is equal to a width in which the photosensitive member and the transfer member contact in the recording medium conveyance direction. The image forming apparatus according to claim 4 . The environment detection unit, the recording medium is in a state that exists between the transfer member and the photosensitive member, and characterized by detecting the resistance value of the interval including between the transfer member and the photosensitive member The image forming apparatus according to any one of claims 1 to 5 .