BACKGROUND

[0001] An image forming apparatus may include a charging device disposed so as not to contact a photoconductor. In such an image forming apparatus, a surface of the photoconductor is charged by applying a voltage obtained by superimposing an alternating voltage component on a direct voltage component to the charging device.

BRIEF DESCRIPTION OF DRAWINGS

[0002] FIG. 1 is a schematic diagram of an example image forming apparatus.

[0003] FIG. 2 is a schematic diagram of an example photoconductor charging system.

[0004] FIG. 3 is a graph of a wear rate of an example photoconductor relative to a frequency of an alternating voltage applied to an example noncontact charging device that charges a surface of the photoconductor.

[0005] FIG. 4 is a schematic diagram showing example print data.

[0006] FIG. 5 is a schematic diagram showing an example of a relationship between a toner image to be formed on the surface of the photoconductor and the frequency of the alternating voltage applied to the charging device.

[0007] FIG. 6 is a schematic diagram showing an example of a relationship between a flow of a print job and the frequency of the alternating voltage applied to the charging device.

[0008] FIG. 7 is a graph of background density measurements relative to the frequency of the alternating voltage applied to the charging device that charges the surface of the photoconductor.

[0009] FIG. 8 is a graph showing the frequency of the alternating voltage applied to the charging device, at which the background density reaches a threshold density T, for various gap sizes between the charging device and the photoconductor.

[0010] FIG. 9 is a flowchart showing an example of a frequency switching process.

[0011] FIG. 10 is a flowchart showing an example of a second frequency setting process.

[0012] FIG. 11 is a table showing test results of a life extension test.

[0013] FIG. 12 is a table showing test results of a life extension test.

[0014] FIG. 13 is a schematic diagram of another example photoconductor charging system.

DETAILED DESCRIPTION

[0015] An example image forming apparatus includes: a photoconductor having a surface to form a toner image, a charging device to charge the surface of the photoconductor based on an alternating voltage, a conveyance path to convey the toner image, a sensor to measure a toner density on the conveyance path, and a control unit (or controller) to change a frequency of the alternating voltage applied to the charging device based on the measured toner density. Since the control unit changes the frequency of the alternating voltage applied to the charging device based on the toner density measured on the conveyance path (e.g., on a conveyance surface forming the conveyance path), it is possible to decrease the frequency of the alternating voltage applied to the charging device, for example, when the toner density on the conveyance path is low. Accordingly, it is possible to suppress the wear of the photoconductor compared to a case in which the frequency of the alternating voltage applied to the charging device is constantly high.

[0016] According to another example, a photoconductor charging system of an image forming apparatus includes a non-contact charging device disposed to be separated from a photoconductor and charging a surface of the photoconductor based on an alternating voltage, and a control unit (or controller) to change a frequency of the alternating voltage applied to the charging device based on an electrostatic latent image to be formed on the surface of the photoconductor. Since the control unit changes the frequency of the alternating voltage applied to the charging device based on the electrostatic latent image to be formed on the surface of the photoconductor, it is possible to decrease the frequency of the alternating voltage applied to the charging device, for example, when an image such as a picture is not included in the electrostatic latent image to be formed on the surface of the photoconductor. Accordingly, it is possible to suppress the wear of the photoconductor compared to a case in which the frequency of the alternating voltage applied to the charging device is constantly high.

[0017] In the following description, with reference to the drawings, the same reference numbers are assigned to the same components or to similar components having the same function, and overlapping description is omitted.

[0018] With reference to FIG. 1 , the example image forming apparatus 1 forms a color image using toners of four colors, namely magenta, yellow, cyan, and black. The image forming apparatus 1 includes a conveying device 10, a plurality of photoconductors 20, a plurality of developing devices 30, a transfer device 40, a fixing device 50, a discharge device 60, and a control unit (or controller) 70.

[0019] The conveying device 10 conveys a sheet (e.g., a sheet of paper) M that is a recording medium on which an image is to be formed. The plurality of photoconductors 20 include photoconductors 20M, 20Y, 20C, and 20K. The photoconductors 20M, 20Y, 20C, and 20K respectively form electrostatic latent images to respectively forming magenta, yellow, cyan, and black toner images. Since the photoconductors 20M, 20Y, 20C, and 20K have similar configurations, they may be collectively described herein as the photoconductors 20, and a single one of the photoconductors 20 may be referred to as the photoconductor 20. Each of the photoconductors 20 has a surface 21 which is a surface (peripheral surface) on which the associated electrostatic latent image is to be formed.

[0020] One of the developing devices 30, a charging device 22, an exposure device 23, and a cleaning unit 24 are provided adjacent the associated photoconductor 20. The charging device 22 charges the surface of the photoconductor 20 to a predetermined potential, and is provided so as not to contact the photoconductor 20. The exposure device 23 exposes the surface 21 of the photoconductor 20 having been previously charged by the charging device 22, to a light in response to an image to be formed on the sheet M. Accordingly, an electrostatic latent image corresponding to the image to be formed on the sheet M is formed on the surface 21 of the photoconductor 20. The cleaning unit 24 collects excess toner remaining on the photoconductor 20.

[0021] The plurality of developing devices 30 include developing devices 30M, 30Y, 30C, and 30K. The developing devices 30M, 30Y, 30C, and 30K are respectively arranged adjacent the photoconductors 20M, 20Y, 20C, and 20K to develop the electrostatic latent images formed on the respective surfaces of the photoconductors 20M, 20Y, 20C, and 20K to form a toner image. Since the developing devices 30M, 30Y, 30C, and 30K have similar configurations, they may be collectively described herein as the developing devices 30, and a single one of the developing devices 30 may be referred to as the developing device 30. [0022] Each of the developing devices 30 includes a developing roller 31 which supplies a toner to the surface 21 of the adjacent photoconductor 20. The developing roller 31 supplies a toner to the surface 21 of the photoconductor 20, to develop the electrostatic latent image formed on the surface 21 of the photoconductor 20. As a result, a toner image corresponding to the electrostatic latent image, namely, a toner image corresponding to an image to be formed on the sheet M is formed on the surface 21 of the photoconductor 20.

[0023] The transfer device 40 conveys the toner images having been developed by the developing devices 30, and transfers the toner images onto the sheet M. The transfer device 40 includes an intermediate transfer belt 41 to which the toner images formed on the surface 21 of the photoconductor 20 are primarily transferred and layered into a single composite toner image, and which secondarily transfers the composite toner image to the sheet M. When the toner images are primarily transferred from the surface 21 of the photoconductor 20 to the intermediate transfer belt 41 , the composite toner image is formed on the intermediate transfer belt 41 . Therefore, both the photoconductor 20 and the intermediate transfer belt 41 form a conveyance path which conveys the toner image. Namely, the conveyance path may be formed by the surface 21 of the photoconductor 20 and a conveyance surface 41a of the intermediate transfer belt 41.

[0024] The fixing device 50 fixes the toner images to the sheet M by heating and pressing the sheet M with the toner image. The discharge device 60 discharges the sheet M with the composite toner images fixed thereto to the outside of the apparatus.

[0025] The control unit 70 is an electronic control device or controller which includes a CPU (Central Processing Unit), a ROM (Read Only Memory), a RAM (Random Access Memory), and the like. The control unit 70 executes various controls by loading a program stored in the ROM into the RAM and executing the program in the CPU. The control unit 70 may include a plurality of electronic control devices or a single electric control device. The control unit 70 executes various controls to operate the image forming apparatus 1 .

[0026] With reference to FIGS. 1 and 2 an example photoconductor charging system 80 will be described. The example photoconductor charging system 80 charges the surface 21 of the photoconductor 20. The photoconductor charging system 80 includes the charging device 22, a power supply 81 , a toner density sensor 82, and the control unit (or controller) 70.

[0027] The charging device 22 charges the surface 21 of the photoconductor 20 to a predetermined potential. The charging device 22 is a non-contact charging device. The charging device 22 is spaced apart (or separated) from the photoconductor 20 to form a gap G between the charging device and the photoconductor 20. The photoconductor 20 and the charging device 22 extend substantially in parallel so that a separation distance between the surface 21 of the photoconductor 20 and the outer peripheral surface of the charging device 22 (namely, a size of the gap G between the photoconductor 20 and the charging device 22 in the charging direction) is set to be substantially constant along the axial direction of the photoconductor 20. In order to ensure the uniformity of charging of the photoconductor 20, the size of the gap G between the photoconductor 20 and the charging device 22 in the charging direction may be of 10 pm to 100 pm. In this way, since the charging device 22 is provided so as not to contact the photoconductor 20, the wear of the photoconductor 20 due to the friction is prevented.

[0028] The power supply 81 applies a voltage to charge the photoconductor 20 to the charging device 22. The voltage applied from the power supply 81 to the charging device 22 is a voltage obtained by superimposing a direct current voltage (DC voltage) and an alternating current voltage (AC voltage). When a voltage is applied to the charging device 22, a discharge occurs between the charging device 22 and the photoconductor 20. This discharge causes an alternating current to flow from the charging device 22 to the photoconductor 20 so as to charge a portion facing the charging device 22 in the surface 21 of the photoconductor 20. In accordance with the rotation of the photoconductor 20, the surface 21 of the photoconductor 20 is charged over the entire circumference.

[0029] The power supply 81 includes a frequency converter and has a function of converting the frequency of the alternating voltage applied to the charging device 22. The frequency of the alternating voltage is controlled by the control unit 70. Further, the DC component of the voltage applied from the power supply 81 to the charging device 22 has, for example, a negative bias voltage of -900 V to -300 V. On the other hand, the alternating component of the voltage applied from the power supply 81 to the charging device 22 has, for example, an inter-peak voltage of 1500 V to 3000 V. When such a voltage is applied to the charging device 22, a discharge occurs between the charging device 22 and the photoconductor 20 so as to charge a target region 21 a, namely a portion of the surface 21 of the photoconductor 20 that faces the charging device 22 so as to be charged by the charging device 22.

[0030] The surface 21 of the photoconductor 20 is charged by the charging device 22 while the photoconductor 20 rotates in the rotating direction D. The target region 21 a having been charged by the charging device 22 moves with the rotation of the photoconductor 20, so as to be subsequently exposed to a light output from the exposure device 23. The exposure device 23 includes, for example, a laser device that emits a laser beam to the target region 21 a of the photoconductor 20 in accordance with the image to be formed on the sheet M, namely in response to print data (e.g., image data) stored in memory (e.g., a print memory). Accordingly, a potential of the portion of the surface 21 having been exposed by the exposure device 42 changes so as to form an electrostatic latent image on the surface 21 of the photoconductor that corresponds to the print data. Then, a toner supplied by the developing roller 31 of the developing device 30 is transferred to the electrostatic latent image formed on the surface 21 of the photoconductor 20. Accordingly, a toner image corresponding to the print data is formed on the surface 21 of the photoconductor 20. Subsequently, the toner image is primarily transferred from the photoconductor 20 to the intermediate transfer belt 41 , and secondarily transferred from intermediate transfer belt 41 to the sheet M, as previously described.

[0031] The control unit (or controller) 70 controls the application of the voltage to the charging device 22 via the power supply 81. Additionally, the control unit 70 controls the frequency of the alternating voltage applied to the charging device 22 by the power supply 81 . Namely, the control unit 70 changes the frequency of the alternating voltage applied to the charging device 22 by the power supply 81 .

[0032] FIG. 3 is a graph showing an example of a relationship between the frequency of the alternating voltage and the wear rate of the photoconductor 20 when the surface 21 of the photoconductor 20 is charged by the non-contact charging device 22. With reference to FIG. 3, the wear rate of the photoconductor increases as the frequency of the alternating voltage applied to the charging device increases, and the wear rate of the photoconductor decreases as the frequency of the alternating voltage applied to the charging device decreases. This is because the alternating current flowing from the charging device to the photoconductor increases when the frequency of the alternating voltage applied to the charging device increases, so as to accelerate the wear of the photoconductor, and because the alternating current flowing from the charging device to the photoconductor decreases when the frequency of the alternating voltage applied to the charging device decreases, so as to suppress the wear of the photoconductor.

[0033] Accordingly, the control unit 70 executes a frequency switching process in order to suppress the wear of the photoconductor 20. The frequency switching process changes the frequency of the alternating voltage applied to the charging device 22 between a first frequency FH and a second frequency FL. The second frequency FL is a frequency that is lower than the first frequency FH. The charging device 22 progressively charges the surface 21 of the photoconductor 20, as the photoconductor rotates in a rotating direction D that corresponds to the progressive transfer of the subsequently formed toner image onto the sheet M while the sheet M is conveyed in the conveying direction (or printing direction). As previously described, the toner image that is subsequently formed on the photoconductor 20 is primarily transferred to the intermediate transfer belt 41 , and secondarily transferred from the intermediate transfer belt 41 to the sheet M. In the frequency switching process, the control unit 70 sets the frequency of the alternating voltage applied to the charging device 22 to the first frequency FH when the target region 21 a of the surface 21 of the photoconductor 20 is set to subsequently form an electrostatic latent image that generates a toner image including a picture. Additionally, the control unit 70 sets the frequency of the alternating voltage applied to the charging device 22 to the second frequency FL when the target region 21 a is not set to form any toner image of a picture. The target region 21 a that is not set to form any toner image of a picture may include, for example, a portion of the surface 21 where the toner image of a text is to be formed or a portion in which no toner image is to be formed. [0034] The determination as to whether the target region 21a is set to form a toner image including a picture may be made based on the print data stored in the print memory.

[0035] For example, the print data stored in the print memory, representing the content to be printed onto the sheet M, may be divided according to a plurality of divided areas representing subdivisions of a printing area of the sheet M arranged along the printing direction, so as to obtain the distribution of the image density of the pixels in each divided area. Each divided area represents a subdivision extending over the entire printing area of the sheet M in a transverse direction that is orthogonal to the printing direction. A divided area in which the image density distribution is represented by the binary values of black and white is determined as an area that does not include any picture print data. Namely, a divided area corresponding to print data having no more than two pixel values is determined as an area of the print data that does not include any picture print data. On the other hand, a divided area in which the image density distribution is ternary or more is determined as an area having picture print data. Namely, a divided area corresponding to print data having three or more pixel values is determined as an area of the print data having picture print data. Accordingly, a target region 21 a corresponding to an area including picture print data is set to form a toner image (or electrostatic latent image) including a picture, and a target region 21 a corresponding to an area not including any picture print data is set so as not to form any toner image (or electrostatic latent image) of a picture. Additionally, it is possible to more accurately determine whether or not to set the target region 21 a to form a toner image (or electrostatic latent image) of a picture by increasing the amount of the print data or increasing the divided areas so as to correspond to narrower subdivisions of the sheet M in the printing direction.

[0036] Additionally, in some examples, halftone dot or halftone data may be extracted from the print data stored in the print memory. A portion of the print data having halftone dot or halftone data is determined as an area having picture print data, and a portion of the print data having no halftone dot or halftone data is determined as an area having no picture print data. Accordingly, when the target region 21 a of the photoconductor 20 corresponds to an area including the picture print data, the target region 21 a is determined as being set to form a toner image (and electrostatic latent image) of a picture, and when the target region 21 a corresponds to an area having no picture print data, then the target region 21 a is determined as being set not to form a toner image (and electrostatic latent image) of a picture.

[0037] FIG. 4 is a diagram showing example print data 100. The print data 100 shown in FIG. 4 corresponds to print data for a first page. In the print data 100, a first non-picture containing portion 102 not including any picture 101 , a picture-containing portion 103 including the picture 101 , and a second non-picture containing portion 104 not including the picture 101 , are arranged in this order from the head to the foot of the page. The first non-picture containing portion 102 may include a header margin, text, and the like. The picture-containing portion 103 may include text or the like in addition to the picture 101. The second nonpicture containing portion 104 may include a footer margin, text, and the like.

[0038] FIG. 5 is a schematic diagram illustrating the frequency of the alternating voltage to be applied to the charging device 22 in relation to the toner image to be formed on the surface 21 of the photoconductor 20. In FIG. 5, the surface 21 of the photoconductor 20 is developed to be illustrated along a plane. Further, FIG. 5 represents a case of printing the print data 100 shown in FIG. 4. With reference to FIGS. 4 and 5, when the print data 100 is printed, an electrostatic latent image L corresponding to the print data 100 is formed on the surface 21 of the photoconductor 20. The electrostatic latent image L to be formed on the surface 21 of the photoconductor 20 includes a first non-picture- containing print area L1 corresponding to the first non-picture containing portion 102 of the print data 100, a picture-containing print area L2 corresponding to the picture-containing portion 103 of the print data 100, and a second non-picture- containing print area L3 corresponding to the second non-picture containing portion 104 of the print data 100 in the rotating direction D of the photoconductor 20. Further, in the surface 21 of the photoconductor 20, a pre-processing area L0 is allocated on the upstream side of the electrostatic latent image L in the rotating direction D of the photoconductor 20, in which no electrostatic latent image L is to be formed, and a post-processing area L4 is allocated on the downstream side of the electrostatic latent image L in the rotating direction D of the photoconductor 20, in which no electrostatic latent image is to be formed. Additionally, the pre-processing area L0 and the post-processing area L4 are charged by the charging device 22.

[0039] The control unit 70 sets the frequency of the alternating voltage applied to the charging device 22 to the first frequency FH to charge the picturecontaining print area L2 in the surface 21 of the photoconductor 20 and sets the frequency of the alternating voltage applied to the charging device 22 to the second frequency FL to charge an area other than the picture-containing print area L2. The area other than the picture-containing print area L2 includes the pre-processing area L0, the first non-picture-containing print area L1 , the second non-picture-containing print area L3, and the post-processing area L4. [0040] FIG. 6 is a schematic diagram illustrating the frequency of the alternating voltage applied to the charging device in relation to the flow of an example print job. The example print job illustrated in FIG. 6 prints the print data of the second page on the sheet M. According to this print job, a pre-processing J1 , a first-page printing J2, an inter-page processing J3, a second-page printing J4, and a post-processing J5 are performed in this order. The pre-processing J1 is a preparatory process prior to executing the print job and is executed during a period before the print job is started after the control unit 70 receives the print data. The pre-processing J1 is also called a pre-rotation before the start of the job. The inter-page processing J3 is a process of waiting for the sheet M of the next page to be conveyed between the pages. The post-processing J5 is an end process for stopping the image forming apparatus 1 and is executed during a period before the image forming apparatus 1 is stopped after the print job ends. The post-processing J5 is also called a post-rotation after the end of the job. In the pre-processing J1 , the inter-page processing J3, and the post-processing J5, according to examples, the electrostatic latent image and the toner image are not formed on the surface of the photoconductor 20, the photoconductor 20 is idled, and the surface 21 of the photoconductor 20 is charged by the charging device 22.

[0041] The print data of the second page printed by the print job shown in FIG. 6 includes a non-picture containing portion 111 not including any picture and a picture-containing portion 112 including a picture. The non-picture containing portion 111 may include a header margin, text, and/or the like. The picturecontaining portion 112 may include text or the like in addition to the picture. The control unit 70 sets the frequency of the alternating voltage applied to the charging device 22 to the first frequency FH when an area corresponding to the picturecontaining portion 112 of the first-page printing J2 and the picture-containing portion 112 of the second-page printing J4 in the surface 21 of the photoconductor 20 is charged. Additionally, the control unit 70 sets the frequency of the alternating voltage applied to the charging device 22 to the second frequency FL to charge an area not corresponding to the picture-containing portions 112 of the first-page printing J2 and of the second-page printing J4, on the surface 21 of the photoconductor 20. The area not corresponding to the picture-containing portions 112 includes an area corresponding to the pre-processing J1 , the non-picture containing portions 111 of the first-page printing J2, the inter-page processing J3, the non-picture containing portions 111 of the second-page printing J4, and the post-processing J5.

[0042] Referring back to FIG. 3, since the wear of the photoconductor 20 is suppressed as the frequency of the alternating voltage applied to the charging device decreases, it is conceivable that the second frequency FL may set to a value as low as possible. However, if the frequency of the alternating voltage applied to the charging device is too low, there is a possibility that background contamination occurs. The background contamination may cause non-printing portion (background) of the sheet M to be contaminated unnecessarily by toner. The background contamination may refer to a phenomenon, for example, in which the entire sheet M or a part of the sheet M is grayed out by an unnecessary toner. [0043] Therefore, a relationship between the frequency of the alternating voltage applied to the charging device 22 and the background density when the surface 21 of the photoconductor 20 was charged by the charging device 22 was examined. The background density is one of the evaluation indexes of background contamination, and represents the toner density in the area of the intermediate transfer belt 41 where no toner image is formed. In this examination, non-exposure development, described further below, was executed while changing the frequency of the alternating voltage applied to the charging device 22 in stages and the toner density of the intermediate transfer belt 41 on the downstream side of the photoconductor 20 was measured by the toner density sensor 82 (cf. FIGS. 1 and 2) as the background density. The toner density sensor 82 is a sensor which is disposed in the vicinity of the intermediate transfer belt 41 on the downstream side of the photoconductor 20 to measure the toner density of the intermediate transfer belt 41 on the downstream side of the photoconductor 20. The non-exposure development is a process in which the surface 21 of the photoconductor 20 is not exposed by the exposure device 23 although the surface 21 of the photoconductor 20 is charged by the charging device 22 and a toner is supplied from the developing roller 31 of the developing device 30 to the surface 21 of the photoconductor 20. In the non-exposure development, since the electrostatic latent image is not formed on the surface 21 of the photoconductor 20, no toner image is not formed on the surface 21 of the photoconductor 20. Therefore, the toner density sensor 82 measures the toner density in the area of the intermediate transfer belt 41 that is free of any toner image.

[0044] Further, the separation distance (or size of the gap G) between the charging device 22 and the photoconductor 20 increases due to the wear of the photoconductor 20. Therefore, the background density was measured for three examples in which the separation distance between the charging device 22 and the photoconductor 20 was 21 pm, 50 pm, and 98 pm. The example in which the separation distance between the charging device 22 and the photoconductor 20 is 21 pm is an example of modeling a state in which the photoconductor 20 is not worn, the example in which the separation distance is 50 pm is an example of modeling a state in which the photoconductor 20 is moderately worn, and the example in which the separation distance is 98 pm is an example of modeling a state in which the photoconductor 20 is considerably worn. The measurement results are shown in FIGS. 7 and 8. FIG. 7 is a graph showing a background density in relation to the frequency of the alternating voltage applied to the charging device 22 to charge the surface 21 of the photoconductor 20. FIG. 8 illustrates the frequency of the alternating voltage at which the background density corresponds substantially to a threshold density T (cf. FIG. 7), for various sizes of the gap G (e.g., separation distances) between the charging device 22 and the photoconductor 20. Accordingly, FIG. 8 shows an example relationship between the size of the gap G and the frequency of the alternating voltage applied to the charging device 22 to charge the surface 21 of the photoconductor 20 when the background density corresponds substantially to the threshold density T. The threshold density T is set to suitably determine whether or not the background contamination is considered to be present. FIG. 7 shows two examples in which the separation distance between the charging device and the photoconductor is 21 pm and 98 pm for ease of understanding of the graph.

[0045] With reference to FIG. 7, the background density increases as the frequency of the alternating voltage applied to the charging device 22 decreases, and the background density exceeds the threshold density T when the frequency of the alternating voltage applied to the charging device 22 is too low. Further, as shown in FIGS. 7 and 8, the frequency of the alternating voltage applied to the charging device 22 increases when the background density exceeds the threshold density T, as the separation distance (size of the gap G) between the charging device 22 and the photoconductor 20 increases.

[0046] Accordingly, the control unit 70 may change the frequency of the alternating voltage applied to the charging device 22 based on the background density measured by the toner density sensor 82. In this example, the control unit 70 changes the second frequency FL based on the background density measured by the toner density sensor 82.

[0047] The control unit 70 executes a second frequency setting process to change the second frequency FL based on the background density measured by the toner density sensor 82. In the second frequency setting process, the control unit 70 sets the second frequency FL within a frequency range that causes the toner density of an area of the intermediate transfer belt 41 without any toner image to be equal to or less than the threshold density T. For example, the control unit 70 may set the second frequency FL to the minimum frequency (lowest frequency) in the frequency range.

[0048] The control unit 70 acquires the background density measured by the toner density sensor 82 at the time of executing the non-exposure development as the toner density of the area of the intermediate transfer belt 41 without any toner image. Then, the control unit 70 sets the second frequency FL within the frequency range that causes the background density measured by the toner density sensor 82 to be equal to or less than the threshold density T. In this case, the control unit 70 may set the second frequency FL to the minimum frequency of the frequency range by which the background density measured by the toner density sensor 82 is equal to or less than the threshold density T. In some examples, the control unit 70 may execute the non-exposure development while changing the frequency of the alternating voltage applied to the charging device 22 in stages and may measure the background density via the toner density sensor 82. The control unit 70 may set the second frequency FL to any frequency within a frequency range corresponding to frequencies selected among the plurality of frequencies changed in stages that cause the background density measured by the toner density sensor 82 to be equal to or less than the threshold density T. In this case, the control unit 70 may set the second frequency FL to the minimum frequency of the frequency range.

[0049] Further, if the wear of the photoconductor 20 progresses and the separation distance (size of the gap G) between the charging device 22 and the photoconductor 20 increases even after the second frequency FL is once set, the toner density measured by the toner density sensor 82 may exceed the threshold density T. Therefore, the control unit 70 executes the second frequency setting process at an initial setting timing of the image forming apparatus 1 , at a periodic timing, or at a selected timing. Then, when the background density measured by the toner density sensor 82 exceeds the threshold density T, the control unit 70 increases the second frequency FL so that the background density measured by the toner density sensor 82 is equal to or less than the threshold density T.

[0050] FIG. 9 is a flowchart showing an example of the frequency switching process carried out by the control unit 70. For example, the frequency switching process shown in FIG. 9 may be repeated until the printing of the print data ends after the control unit 70 receives the print data. With reference to FIG. 9, at operation S1 , when the control unit 70 receives the print data, and determines whether or not the target region 21 a to be charged, is set to form a toner image (electrostatic latent image) of a picture. The control unit 70 determines whether or not the target region 21 a is set to form a toner image (electrostatic latent image) of a picture, for example, by analyzing the print data stored in the print memory. At operation S2, when it is determined that the target region 21 a to be charged is set to forms a toner image (electrostatic latent image) of a picture, the control unit 70 applies a voltage to the charging device 22 by setting the frequency of the alternating voltage to the first frequency FH. At operation S3, when it is determined that the target region 21 a to be charged is set to form a toner image (electrostatic latent image) of a picture, the control unit 70 applies a voltage to the charging device 22 by setting the frequency of the alternating voltage applied to the charging device 22 to the second frequency FL which is lower than the first frequency FH.

[0051] FIG. 10 is a flowchart showing an example of the second frequency setting process carried out by the control unit 70. The second frequency setting process shown in FIG. 10 is executed at the initial setting timing of the image forming apparatus 1 , at a periodic timing, or at a selected timing. With reference to FIG. 10, at operation S11 , the control unit 70 executes the non-exposure development while changing the frequency of the alternating voltage applied to the charging device 22 in stages and receives the background density measured by the toner density sensor 82. At operation S12, the control unit 70 determines a frequency range frequencies among the plurality of frequencies applied at operation S11 , that cause the background density measured by the toner density sensor 82 to be equal to or less than the threshold density T. At operation S13, the control unit 70 sets the second frequency FL to the minimum frequency of the frequency range determined.

[0052] According to examples of the image forming apparatus 1 , the control unit 70 changes at least the second frequency FL in the frequency of the alternating voltage applied to the charging device 22 based on the toner density on the intermediate transfer belt 41 measured by the toner density sensor 82. Therefore, at least the second frequency FL in the frequency of the alternating voltage applied to the charging device can be decreased when the toner density on the intermediate transfer belt 41 is low. Accordingly, the wear of the photoconductor 20 can be suppressed compared to a case in which the frequency of the alternating voltage applied to the charging device 22 is constantly high.

[0053] Further, examples of the image forming apparatus 1 includes the non-contact charging device 22 which is disposed to be spaced apart from the photoconductor 20 to charge the surface 21 of the photoconductor 20 based on the alternating voltage, and the control unit 70 changes the frequency of the alternating voltage applied to the charging device 22 based on the electrostatic latent image to be formed on the surface 21 of the photoconductor 20. Therefore, the frequency of the alternating voltage applied to the charging device 22 can be decreased when the electrostatic latent image to be formed on the surface 21 of the photoconductor 20 does not corresponds to any image including a picture, such as a photograph for example. Accordingly, the wear of the photoconductor 20 can be suppressed compared to a case in which the frequency of the alternating voltage applied to the charging device 22 is constantly high.

[0054] Further, according to examples of the image forming apparatus 1 , the control unit 70 changes the second frequency FL based on the background density measured by the toner density sensor 82 in the second frequency setting process. Namely, the control unit 70 sets the second frequency FL within the frequency range that causes the background density to be equal to or less than the threshold density T via the second frequency setting process. Accordingly, the wear of the photoconductor 20 can be better suppressed while suppressing the occurrence of background contamination due to the wear of the photoconductor 20 or the like.

[0055] The life extension of the photoconductor 20 was examined at the time of executing the frequency switching process of changing the frequency of the alternating voltage applied to the charging device 22 between the first frequency FH and the second frequency FL and the second frequency setting process of changing the second frequency FL. In this examination, the life of the photoconductor 20 in Test Examples 1 to 3 was measured by using three types of print data in which the ratio between the picture and the text is as follows: picture : text = 0 : 100 in Test Example 1 , picture : text = 10 : 90 in Test Example 2, and picture : text = 20 : 80 in Test Example 3. FIG. 11 is a table showing the conditions of Test Examples 1 to 3. As shown in FIG. 11 , in Example 1 , neither of the frequency switching process and the second frequency setting process was executed and the frequency of the alternating voltage applied to the charging device 22 was kept constant at the first frequency FH. In Test Example 2, the frequency switching process was executed, but the second frequency setting process was not executed. In Test Example 3, both the frequency switching process and the second frequency setting process were executed. Then, the life (lifespan) of the photoconductor 20 in Example 1 was set as the reference life (reference lifespan), and the life extension ratio of the life of the photoconductor 20 in Test Examples 2 and 3 were calculated with respect to the reference life (reference lifespan). The calculation result is shown in FIG. 12.

[0056] As shown in FIGS. 11 and 12, in Test Examples 2 and 3 in which the frequency switching process was executed, the life of the photoconductor 20 was significantly extended compared to Test Example 1 in which the frequency switching process was not executed. Further, in Test Examples 2 and 3, the life of the photoconductor 20 was significantly extended as the ratio of the text with respect to the picture increased. Further, in Test Example 3 in which the frequency switching process was executed, the life of the photoconductor 20 was significantly extended compared to Test Example 3 in which the frequency switching process was not executed.

[0057] It should be understood that not all aspects, advantages, and features described herein are necessarily achieved or included by any one particular example. In fact, various examples have been described and shown herein, but it should be clear that other examples can be modified in their arrangement and details.

[0058] For example, in the above-described examples, a case has been described in which the control unit 70 changes the second frequency FL based on the background density measured by the toner density sensor 82. However, in addition to changing the second frequency FL, the control unit 70 may also change the first frequency FH based on the background density measured by the toner density sensor 82.

[0059] In addition, in the above-described examples, a case has been described in which the background density measured by the toner density sensor 82 at the time of executing the non-exposure development corresponds to the toner density of the area without the toner image on the intermediate transfer belt 41 . However, if the area without the toner image is identified even in the case other than the non-exposure development, the toner density in the corresponding area measured by the toner density sensor 82 may be used as the toner density of the area without the toner image in the intermediate transfer belt 41 .

[0060] In addition, in the above-described examples, a case has been described in which the charging device 22 is of a non-contact type separated from the photoconductor 20. However, the charging device may be of a contact type that contacts the photoconductor 20 similarly to a charging device 22A of another example photoconductor charging system 80A shown in FIG. 13.

[0061] In addition, in the above-described examples, a case has been described in which the toner density sensor 82 measures the toner density on the intermediate transfer belt 41 . However, as described above, the photoconductor 20 in addition to the intermediate transfer belt 41 may constitute the conveyance path conveying the toner image. Therefore, the toner density sensor 82 may measure the toner density on the photoconductor 20 instead of or in addition to the toner density on the intermediate transfer belt 41 in order to determine the toner density on the conveyance path that conveys the toner image.

[0062] In addition, in the above-described examples, the image forming apparatus has been described as an apparatus that forms a color image by using four color toners of magenta, yellow, cyan, and black. However, the image forming apparatus may be an apparatus for forming a monochromatic image using a monochromatic toner. In such example, the image forming apparatus for forming a monochromatic image may not include any intermediate transfer belt, for example, so as to directly transfer the toner image from the photoconductor to the sheet. However, as described above, the photoconductor also constitutes the conveyance path conveying the toner image. Therefore, in the example apparatus excluding an intermediate transfer belt such as in the image forming apparatus to form a monochromatic image, the toner density sensor may measure the toner density on the photoconductor as the toner density on the conveyance path conveying the toner image.

[0063] It is to be understood that not all aspects, advantages and features described herein may necessarily be achieved by, or included in, any one particular example. Indeed, having described and illustrated various examples herein, it should be apparent that other examples may be modified in arrangement and detail is omitted.