Patent Publication Number: US-9885972-B2

Title: Image forming apparatus that ensures setting surface potential of photoreceptor drum with simple constitution

Description:
INCORPORATION BY REFERENCE 
     This application is based upon, and claims the benefit of priority from, corresponding Japanese Patent Application No. 2015-193569 filed in the Japan Patent Office on Sep. 30, 2015, the entire contents of which are incorporated herein by reference. 
     BACKGROUND 
     Unless otherwise indicated herein, the description in this section is not prior art to the claims in this application and is not admitted to be prior art by inclusion in this section. 
     As a typical image forming apparatus employing an electrophotographic method such as a printer and a copier, there has been known an image forming apparatus that includes a photoreceptor drum, a charging apparatus, an exposure apparatus, a developing device, and a transfer apparatus. The charging apparatus uniformly charges a circumference surface of the photoreceptor drum. The exposure apparatus irradiates the photoreceptor drum with exposure light according to image information to form an electrostatic latent image. The developing device supplies the photoreceptor drum with toner to develop the electrostatic latent image into a toner image. The transfer apparatus transfers the toner image from the photoreceptor drum to a sheet. 
     To obtain good images, it is necessary that a surface potential of the photoreceptor drum in the image forming apparatus be set to be a desired electric potential. Especially, when the charging apparatus includes a charging roller that rotates while contacting a surface of the photoreceptor drum, even if a voltage applied to the charging roller is identical, the surface potential of the photoreceptor drum is likely to vary depending on an environmental variation or a similar factor. With the charging roller to which an ion conducting agent is combined, since a resistance value of the roller is likely to vary depending on the environment or a similar factor, a variation in displacement of the photoreceptor drum is likely to be especially remarkable. 
     There has been disclosed a typical image forming apparatus that includes a surface electrometer opposed to a circumference surface of a photoreceptor drum. Feeding back a measurement result of an electric potential by the surface electrometer to a voltage applied to a charging apparatus sets a surface potential of the photoreceptor drum to be a desired electric potential. 
     SUMMARY 
     An image forming apparatus according to one aspect of the disclosure includes an apparatus main body, a photoreceptor drum, a charging apparatus, a developing device, a transfer apparatus, a charging bias applying unit, a developing bias applying unit, a bias adjusting unit, and a print density measurement unit. The photoreceptor drum has a circumference surface on which an electrostatic latent image including a background portion and an image portion is formed. The photoreceptor drum is rotationally driven in a predetermined rotation direction. The charging apparatus is arranged in contact with or close to the circumference surface of the photoreceptor drum. The charging apparatus charges the circumference surface at a predetermined electric potential. The developing device includes a developing roller disposed opposed to the photoreceptor drum. The developing device supplies the photoreceptor drum with toner to develop the electrostatic latent image into a toner image. The transfer apparatus transfers the toner image from the photoreceptor drum to a sheet or an intermediate transfer belt. The charging bias applying unit applies a predetermined charging bias to the charging apparatus. The developing bias applying unit applies a predetermined developing bias to the developing roller. The bias adjusting unit performs a charging bias adjusting operation. The charging bias adjusting operation adjusts an electric potential at the background portion in the electrostatic latent image on the photoreceptor drum to a predetermined target electric potential. The print density measurement unit measures a print density of the toner image. The bias adjusting unit, in the charging bias adjusting operation, controls the charging bias applying unit to form a non-charged area, to which the charging bias is not applied, on the circumference surface of the photoreceptor drum, and controls the developing bias applying unit to apply the developing bias constituted of a first electric potential to the developing roller, so as to form a first toner image by an electric potential difference between the non-charged area and the developing roller. In the charging bias adjusting operation, the charging bias applying unit controls the charging bias applying unit to apply the charging bias found by subtracting the first electric potential from a first tentative charging bias preset corresponding to the target electric potential to form a predetermined electric potential area on the circumference surface of the photoreceptor drum, and controls the developing bias applying unit to apply the target electric potential to the developing roller, so as to form a second toner image by an electric potential difference between the electric potential area and the developing roller. The charging bias applying unit decides a value of the charging bias corresponding to the target electric potential from measurement results of print densities of the first toner image and the second toner image measured by the print density measurement unit. 
     These as well as other aspects, advantages, and alternatives will become apparent to those of ordinary skill in the art by reading the following detailed description with reference where appropriate to the accompanying drawings. Further, it should be understood that the description provided in this summary section and elsewhere in this document is intended to illustrate the claimed subject matter by way of example and not by way of limitation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a cross section of an internal structure of an image forming apparatus according to one embodiment of the disclosure; 
         FIG. 2  illustrates an electrical block diagram of a control unit of the image forming apparatus according to the embodiment of the disclosure; 
         FIG. 3  illustrates a charging bias adjusting operation according to a first embodiment of the disclosure; 
         FIG. 4  schematically illustrates an electric potential relationship in the charging bias adjusting operation according to the first embodiment; 
         FIG. 5  schematically describes a timing of the charging bias adjusting operation according to the first embodiment; 
         FIG. 6  illustrates the timing of the charging bias adjusting operation according to the first embodiment; 
         FIG. 7  schematically illustrates an electric potential relationship of the charging bias adjusting operation according to a second embodiment of the disclosure; and 
         FIG. 8  illustrates a calibration operation according to the one embodiment of the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Example apparatuses are described herein. Other example embodiments or features may further be utilized, and other changes may be made, without departing from the spirit or scope of the subject matter presented herein. In the following detailed description, reference is made to the accompanying drawings, which form a part thereof. 
     The example embodiments described herein are not meant to be limiting. It will be readily understood that the aspects of the present disclosure, as generally described herein, and illustrated in the drawings, can be arranged, substituted, combined, separated, and designed in a wide variety of different configurations, all of which are explicitly contemplated herein. 
     The following describes an image forming apparatus  10  according to embodiments of the disclosure in detail based on the accompanying drawings. This embodiment exemplifies a tandem type color printer as an exemplary image forming apparatus. The image forming apparatus may be devices such as a copier, a facsimile device, and a multi-functional peripheral of these devices. 
       FIG. 1  illustrates a cross section of an internal structure of the image forming apparatus  10 . This image forming apparatus  10  includes an apparatus main body  11  with a box-shaped chassis structure. This apparatus main body  11  internally includes a paper sheet feeder  12 , which feeds a sheet P, an image forming unit  13 , which forms a toner image to be transferred to the sheet P fed from the paper sheet feeder  12 , an intermediate transfer unit  14  on which the toner image is primarily transferred, a secondary transfer roller  145 , a toner replenishment unit  15 , which replenishes the image forming unit  13  with toner, and a fixing unit  16 , which fixes an unfixed toner image formed on the sheet P to the sheet P. Furthermore, at the upper portion of the apparatus main body  11 , a paper sheet discharge unit  17  to which the sheet P fixed by the fixing unit  16  is discharged is provided. 
     At an appropriate position on the top surface of the apparatus main body  11 , an operation panel (not illustrated) for an input operation for an output condition to the sheet P or a similar operation is located. This operation panel includes a power key, a touch panel to input the output condition, and various operation keys. Additionally, the apparatus main body  11  internally includes a sheet conveyance path  111 , which extends in a vertical direction, at a position right side of the image forming unit  13 . The sheet conveyance path  111  includes a conveyance roller pair  112  to feed the sheet at an appropriate position. A registration roller pair  113  is arranged upstream with respect to a nip portion in the sheet conveyance path  111 . The registration roller pair  113  performs skew correction on the sheet and sends out the sheet to the nip portion for a secondary transfer, which will be described later, at a predetermined timing. The sheet conveyance path  111  is a conveyance path that feeds the sheet P from the paper sheet feeder  12  to the paper sheet discharge unit  17  via the image forming unit  13  (the secondary transfer nip portion) and the fixing unit  16 . 
     The paper sheet feeder  12  includes a sheet feed tray  121 , a pickup roller  122 , and a feed roller pair  123 . The sheet feed tray  121  is insertably/removably mounted to a lower position of the apparatus main body  11  to accumulate a sheet bundle P 1 , which is a plurality of the stacked sheets P. The pickup roller  122  feeds out the sheet P on the uppermost surface of the sheet bundle P 1  accumulated at the sheet feed tray  121  one by one. The feed roller pair  123  sends out the sheet P fed out by the pickup roller  122  to the sheet conveyance path  111 . The paper sheet feeder  12  includes a manual paper feed tray, which is mounted to a left side surface of the apparatus main body  11  illustrated in  FIG. 1 . The manual paper feed tray includes a bypass tray  124 , a pickup roller  125 , and a feed roller pair  126 . The bypass tray  124  is a tray on which the sheet P is manually placed. When the sheet P is manually fed, as illustrated in  FIG. 1 , the bypass tray  124  is opened from the side surface of the apparatus main body  11 . The pickup roller  125  feeds out the sheet P placed on the bypass tray  124 . The feed roller pair  126  sends out the sheet P fed out by the pickup roller  125  to the sheet conveyance path  111 . 
     The image forming unit  13  forms a toner image to be transferred to the sheet P. The image forming unit  13  includes a plurality of image forming units, which form toner images of different colors. As this image forming unit, this embodiment includes a magenta unit  13 M, which uses a magenta (M) color developer, a cyan unit  13 C, which uses a cyan (C) color developer, a yellow unit  13 Y, which uses a yellow (Y) color developer, and a black unit  13 Bk, which uses a black (Bk) color developer, sequentially from upstream to downstream in a rotation direction of an intermediate transfer belt  141  (from the left side to the right side shown in  FIG. 1 ). The image forming units  13 M,  13 C,  13 Y, and  13 Bk each include a photoreceptor drum  20  (an image carrier), a charging apparatus  21 , which is arranged at a peripheral area of the photoreceptor drum  20 , a developing device  23 , and a cleaning apparatus  25 . An exposure apparatus  22  shared by the respective image forming units  13 M,  13 C,  13 Y, and  13 Bk is arranged below these image forming units. 
     The photoreceptor drum  20  is rotatably driven in a direction of the arrow in  FIG. 1  (a predetermined rotation direction) around the axis, and an electrostatic latent image and a toner image are formed on a circumference surface of the photoreceptor drum  20 . The electrostatic latent image formed on the photoreceptor drum  20  includes a background portion and an image portion according to image information. A rotation shaft of the photoreceptor drum  20  extends in a front-rear direction (a direction perpendicular to the paper surface of  FIG. 1 ). As this photoreceptor drum  20 , a photoreceptor drum using an organic photo conductor (OPC) based material is applicable. As illustrated in  FIG. 1 , a plurality of the photoreceptor drums  20  corresponding to the respective colors are arranged at predetermined intervals in a lateral direction (a horizontal direction). 
     The charging apparatus  21  uniformly charges the circumference surface of the photoreceptor drum  20  at a predetermined electric potential. As the charging apparatus  21 , a charging apparatus with a contact electrification method can be employed. The charging apparatus  21  includes a charging roller  21 A, which contacts the circumference surface of the photoreceptor drum  20  and is arranged and rotationally driven, and a charging cleaning brush  21 B to remove toner attached to the charging roller  21 A. In another embodiment, the charging roller  21 A may close the circumference surface of the photoreceptor drum  20 . The exposure apparatus  22  includes various optical system devices such as a light source, a polygon mirror, a reflection mirror, and a deflecting mirror. The exposure apparatus  22  irradiates the uniformly charged circumference surface of the photoreceptor drum  20  with light modulated based on image data to form the above-described electrostatic latent image. The cleaning apparatus  25  cleans the circumference surface of the photoreceptor drum  20  after toner image transfer. 
     The developing device  23  supplies the circumference surface of the photoreceptor drum  20  with toner to develop the electrostatic latent image formed on the photoreceptor drum  20 . The developing device  23  is for two-component developer constituted of toner and a carrier. The developing device  23  supplies the toner to the circumference surface of the photoreceptor drum  20  to develop the electrostatic latent image. The developing device  23  includes a developing roller  23 C opposed to the photoreceptor drum  20 , a magnetic roller  23 B, and a pair of screws  23 A. As the developing device  23 , another constitution including the developing roller  23 C may be applied. In this embodiment, the toner has a property that charges to a positive polarity. 
     The intermediate transfer unit  14  is located at the space between the image forming unit  13  and the toner replenishment unit  15 . The intermediate transfer unit  14  includes the intermediate transfer belt  141 , a drive roller  142 , a tension roller  143 , a plurality of primary transfer rollers  24  (transfer rollers), and a belt cleaning apparatus  144 . 
     The intermediate transfer belt  141  is an endless belt-shaped rotator and is suspended across the drive roller  142  and the tension roller  143  such that its circumference surface side is brought into contact with the circumference surfaces of the respective photoreceptor drums  20 . The intermediate transfer belt  141  is circularly driven in one direction along the lateral direction, and carries the toner image transferred from a plurality of the photoreceptor drums  20  on its surface. The intermediate transfer belt  141  is a conductive soft belt with a laminated structure formed of a base layer, an elastic layer, and a coat layer. 
     The drive roller  142  stretches the intermediate transfer belt  141  at a right end side of the intermediate transfer unit  14 , and causes the intermediate transfer belt  141  to circularly drive. The drive roller  142  is constituted of a metal roller. The tension roller  143  passively rotates at a left end side of the intermediate transfer unit  14 . The tension roller  143  stretches the intermediate transfer belt  141 . The tension roller  143  provides the intermediate transfer belt  141  with a tensile strength. The belt cleaning apparatus  144  (see  FIG. 1 ), which is located at the proximity of the tension roller  143 , removes a remnant toner on the circumference surface of the intermediate transfer belt  141 . 
     The primary transfer roller  24  is located across the intermediate transfer belt  141 , and opposed to the photoreceptor drum  20 . This forms primary transfer nip portions between the primary transfer rollers  24  and the photoreceptor drums  20  to primarily transfer the toner image, which is on the photoreceptor drum  20 , on the intermediate transfer belt  141 . As illustrated in  FIG. 1 , respective primary transfer rollers  24  are opposed to the photoreceptor drums  20  for the respective colors. The primary transfer roller  24  is a roller extending in the front-rear direction, and rotationally driven along with the intermediate transfer belt  141 . 
     The secondary transfer roller  145  is opposed to the drive roller  142  across the intermediate transfer belt  141 . The secondary transfer roller  145  is pressed and contacts the circumference surface of the intermediate transfer belt  141  to form a secondary transfer nip portion. The toner image primarily transferred on the intermediate transfer belt  141  is secondarily transferred on the sheet P supplied from the paper sheet feeder  12  at the secondary transfer nip portion. In this embodiment, the intermediate transfer unit  14  and the secondary transfer roller  145  constitute a transfer apparatus. The transfer apparatus transfers the toner image from the photoreceptor drum  20  to the sheet P. 
     The toner replenishment unit  15  retains toners used for an image formation. The toner replenishment unit  15  according to the embodiment includes a magenta toner container  15 M, a cyan toner container  15 C, a yellow toner container  15 Y, and a black toner container  15 Bk. These toner containers  15 M,  15 C,  15 Y, and  15 Bk retain respective replenishment toners for the respective colors M, C, Y, and Bk. The toner containers  15 M,  15 C,  15 Y, and  15 Bk replenish the toners for the respective colors to the developing devices  23  for the image forming units  13 M,  13 C,  13 Y, and  13 Bk, which correspond to the respective colors M, C, Y, and Bk, from toner discharge ports  15 H, which are formed on the bottom surfaces of the containers, via a toner conveying unit (not illustrated). 
     The fixing unit  16  includes a heating roller  161 , which internally includes a heat source, a fixing roller  162 , which is located opposed to the heating roller  161 , a fixing belt  163 , which is stretched between the fixing roller  162  and heating roller  161 , and a pressure roller  164 , which is opposed to the fixing roller  162  via the fixing belt  163  and forms a fixing nip portion. The sheet P supplied to the fixing unit  16  passes through the fixing nip portion to be heated and pressurized. This fixes the toner image, which has been transferred to the sheet P at the secondary transfer nip portion, to the sheet P. 
     The paper sheet discharge unit  17  is formed by depressing the top of the apparatus main body  11 . The bottom portion of this concave portion forms a sheet discharge tray  171  that receives the discharged sheet P. The sheet P on which the fixing process has been performed is discharged to a sheet discharge tray  171  via the sheet conveyance path  111  running from the upper portion of the fixing unit  16 . 
       FIG. 2  illustrates an electrical block diagram of a control unit  50  of the image forming apparatus  10  according to the embodiment. The image forming apparatus  10  includes the control unit  50 , which integrally controls the respective operations of this image forming apparatus  10 . The control unit  50  is constituted of a Central Processing Unit (CPU), a Read Only Memory (ROM), which stores control programs, a Random Access Memory (RAM), which is used as a work area for the CPU, and a similar member. In addition to the above-described photoreceptor drum  20 , charging apparatus  21 , exposure apparatus  22 , developing device  23 , and primary transfer roller  24  of the image forming unit  13  and a similar member, to the control unit  50 , a driving unit  61 , a charging bias applying unit  62 , a developing bias applying unit  63 , an environmental sensor  64  (an environment detector), a print density sensor  65  (a print density measurement unit), and a similar member are electrically connected. 
     The driving unit  61  is formed of a gear mechanism that transmits a motor and a torque of the motor. The driving unit  61  rotates the respective members such as the image forming unit  13  and the secondary transfer roller  145  according to a control signal from a drive control unit  51 , which will be described later. 
     The charging bias applying unit  62  is constituted of a DC power supply. Based on a control signal from a bias control unit  52 , which will be described later, the charging bias applying unit  62  applies a predetermined charging bias to the charging roller  21 A of the charging apparatus  21 . 
     The developing bias applying unit  63  is constituted of a DC power supply and an AC power supply. Based on the control signal from the bias control unit  52 , the developing bias applying unit  63  applies a predetermined developing bias to the developing roller  23 C and the magnetic roller  23 B of the developing device  23 . 
     The environmental sensor  64  (see  FIG. 1 ) is provided with the apparatus main body  11 . The environmental sensor  64  detects temperature and humidity inside the apparatus main body  11 . In another embodiment, the environmental sensor  64  may detect the temperature and humidity around the apparatus main body  11 . 
     The print density sensor  65  (see  FIG. 1 ) detects an image density of the toner image formed on the intermediate transfer belt  141  and converts the image density into an electric signal. The print density sensor  65  includes a light-emitting element, which emits light on a belt surface of the rotatably driven intermediate transfer belt  141 , and a light receiving portion (not illustrated), which receives a reflected light from this belt surface. An image condition adjusting unit  53 , which will be described later, refers to information on the image density output from the print density sensor  65 , and the information is reflected to a charging bias adjusting operation, which will be described later. 
     An execution of the control program stored in the ROM by the CPU causes the control unit  50  to function as the drive control unit  51 , the bias control unit  52 , the image condition adjusting unit  53 , a storage unit  54 , and a count unit  55 . 
     The drive control unit  51  controls the driving unit  61  according to an image forming operation by the image forming apparatus  10  and the charging bias adjusting operation, which will be described later. The drive control unit  51  controls a driving mechanism (not illustrated) as well as the driving unit  61  to drive other driving members in the image forming apparatus  10 . 
     Similarly, the bias control unit  52  controls the charging bias applying unit  62  and the developing bias applying unit  63  according to the image forming operation by the image forming apparatus  10 , the charging bias adjusting operation, and a calibration operation. The bias control unit  52  controls a bias applying unit (not illustrated) as well as the charging bias applying unit  62  and the developing bias applying unit  63  to apply a predetermined bias to other members inside the image forming apparatus  10 . As one example, the bias control unit  52  applies a primary transfer bias and a secondary transfer bias to the primary transfer roller  24  and the secondary transfer roller  145 , respectively. 
     The image condition adjusting unit  53  executes various image condition adjusting operations in the image forming apparatus  10 . This image condition adjusting operation includes the charging bias adjusting operation. In the charging bias adjusting operation, the image condition adjusting unit  53  adjusts an electric potential at the background portion in the electrostatic latent image on the photoreceptor drum  20  to a predetermined target electric potential. 
     The storage unit  54  stores various pieces of reference information referred to by the drive control unit  51 , the bias control unit  52 , and the image condition adjusting unit  53 . As one example, the storage unit  54  stores electric potential information referred to in the charging bias adjusting operation. 
     The count unit  55  counts various pieces of accumulated information in the image forming operation by the image forming apparatus  10  and the image condition adjusting operation. As one example, the count unit  55  counts the number of printed sheets to which the toner image is transferred, a printing interval period of the sheets (a period during which the image forming apparatus  10  is left), the number of accumulated rotations of the photoreceptor drum  20 , and an accumulated application period of the charging bias by the charging apparatus  21 . 
     Charging Bias Adjusting Operation 
     The following describes the charging bias adjusting operation according to a first embodiment of the disclosure.  FIG. 3  illustrates the charging bias adjusting operation according to the embodiment.  FIG. 4  schematically illustrates an electric potential relationship between the photoreceptor drum  20  and the developing roller  23 C in the charging bias adjusting operation according to the embodiment.  FIG. 4  indicates a surface potential of the photoreceptor drum  20  as Vdr and an electric potential of a DC bias of the developing roller  23 C as Vdc. As described above, this embodiment includes the charging roller  21 A, which contacts with the circumference surface of the photoreceptor drum  20  and rotates. Especially in this embodiment, an ion conducting agent is combined in the charging roller  21 A. Since a resistance value of such ion-conductive charging roller  21 A has a property that is likely to change depending on an environmental condition such as a temperature and a humidity, it is difficult to hold the surface potential of the photoreceptor drum  20  constant. In such case, arranging a well-known surface electrometer opposed to the circumference surface of the photoreceptor drum  20  ensures performing a feedback control on the charging bias applied to the charging roller  21 A based on a measurement result by the surface electrometer. However, this causes a problem of cost increase in the image forming apparatus  10 . To solve such problem, this embodiment does not include an electrometer, which measures the surface potential of the photoreceptor drum  20 , but the image condition adjusting unit  53  performs the charging bias adjusting operation to accurately set the surface potential of the photoreceptor drum  20  to a target electric potential. This embodiment performs the charging bias adjusting operation in order on the photoreceptor drums  20  for the respective colors. In another embodiment, the charging bias adjusting operation may be concurrently performed on the photoreceptor drums  20  for a plurality of colors. 
     With reference to  FIG. 3 , the charging bias adjusting operation includes seven steps, a formation of a band latent image  1  (Step S 1 ), a development of the band latent image  1  (Step S 2 ), a measurement of a print density of a band toner image  1   a  (Step S 3 ), a formation of a band latent image  2  (Step S 4 ), a development of the band latent image  2  (Step S 5 ), a measurement of a print density of the band toner image  2   a  (Step S 6 ), and a decision of a charging bias (Step S 7 ). Roughly dividing the charging bias adjusting operation, the charging bias adjusting operation is classified into a first phase until the measurement of the print density of the band toner image  1   a  (Step S 3 ), a second phase until the measurement of the print density of the band toner image  2   a  (Step S 6 ), and a third phase of the decision of the charging bias (Step S 7 ). A timing that the charging bias adjusting operation is performed will be described later in detail. 
     The execution of the charging bias adjusting operation forms the band latent image  1  in  FIG. 4  by the image condition adjusting unit  53  (Step S 1 ). To form a good image by the image forming apparatus  10 , a preset target electric potential at the background portion of the photoreceptor drum  20  is defined as V 0  (V). As described above, this embodiment does not directly measure the surface potential of the photoreceptor drum  20  by, for example, the electrometer. On the other hand, by controlling an input signal input from the bias control unit  52  to the charging bias applying unit  62 , it is possible to control a value of the charging bias applied to the charging roller  21 A by the charging bias applying unit  62  within a predetermined error range. In view of this, the charging bias adjusting operation leads the value of the charging bias such that the surface potential of the photoreceptor drum  20  becomes V 0  (V). The storage unit  54  (see  FIG. 2 ) preliminary stores a value of a charging bias Vref. The charging bias Vref is a value preliminary derived experimentally such that the surface potential of the photoreceptor drum  20  becomes V 0  (V). Even if this charging bias Vref is applied to the charging roller  21 A of the charging apparatus  21 , the surface potential of the photoreceptor drum  20  is not always set to V 0  (V). Therefore, the above-described charging bias adjusting operation is required. 
     At Step S 1 , the image condition adjusting unit  53  refers to an intermediate charging bias Vm preliminary stored in the storage unit  54  (see  FIG. 2 ) and controls the charging bias applying unit  62  to apply this intermediate charging bias Vm. The intermediate charging bias Vm is a bias value whose absolute value is smaller than the charging bias Vref. Consequently, the surface of the photoreceptor drum  20  is charged to the intermediate electric potential (Vm). This intermediate electric potential can be set providing a certain degree of freedom. When a two-component development method is used as a development method, an excessively high intermediate electric potential is likely to generate a carrier development due to an electric potential difference between the surface potential Vdr of the photoreceptor drum  20  and the electric potential Vdc of the developing roller  23 C. Accordingly, the intermediate electric potential of the photoreceptor drum  20  is preferably a value before and after 50% of the target electric potential V 0 . When the two-component development method is not used as the development method, similar to Step S 4 , which will be described later, the photoreceptor drum  20  may be charged with the charging bias Vref. 
     If the surface potential Vdr at the background portion of the photoreceptor drum  20  is lower than the developing bias Vdc, a background portion fog occurs. This is likely to generate an error in the print density measurement at Step S 3 , which will be described later. In view of this, the surface potential Vdr at the background portion of the photoreceptor drum  20  at Step S 1  is preferably higher than the developing bias Vdc. Next, the image condition adjusting unit  53  controls the charging bias applying unit  62  to form a charging bias 0 V (charging bias: OFF) area by a predetermined period. Consequently, as illustrated in  FIG. 4 , on the circumference surface of the photoreceptor drum  20 , a non-charged area where the surface potential is approximately 0 V is formed. As will be described later, adjusting the values of the developing bias applied to the developing roller  23 C and the transfer bias applied to the primary transfer roller  24  at a good timing ensures further making the surface potential of the photoreceptor drum  20  in the non-charged area close to 0 V. 
     At Step S 2 , the band latent image  1  is developed. The image condition adjusting unit  53  sets the developing bias Vdc applied to the developing roller  23 C to a preset electric potential a (V) to develop the latent image (the band latent image  1 , the non-charged area), which is formed at Step S 1 . Consequently, the electric potential difference between the developing roller  23 C to which the developing bias Vdc at a (V) has been applied and the non-charged area forms the band toner image  1   a  (see I 1  in  FIG. 4 ) on the circumference surface of the photoreceptor drum  20 . This embodiment sets a=100 V, and this a value is also preliminary stored in the storage unit  54 . The a value is preferably in a range of 50 to 200 V and 100 to 150 V is more preferable. 
     At Step S 3 , the print density of the band toner image  1   a  formed at Step S 2  is measured. The toner image on the photoreceptor drum  20  is transferred to the intermediate transfer belt  141  at a predetermined primary transfer bias applied to the primary transfer roller  24 . The toner image carried on the intermediate transfer belt  141  passes through immediately above the print density sensor  65  in  FIG. 1 . In this respect, the print density sensor  65  measures the print density of the toner image. The storage unit  54  (see  FIG. 2 ) stores the print density result of the respective toner images measured by the print density sensor  65 . 
     At Step S 4 , the band latent image  2  is formed. Here, the image condition adjusting unit  53  controls the charging bias applying unit  62  to apply the charging bias Vref to the charging roller  21 A. At this phase, the surface potential of the photoreceptor drum  20  is possibly set to a value shifted from the target electric potential V 0 . Further, the image condition adjusting unit  53  controls the charging bias applying unit  62  and causes the charging bias applying unit  62  to apply a value found by subtracting the above-described a (V) from the charging bias Vref by the predetermined period. Consequently, as illustrated in  FIG. 4 , the band latent image  2  (the electric potential area) is formed on the circumference surface of the photoreceptor drum  20 . 
     At Step S 5 , the band latent image  2  is developed. The image condition adjusting unit  53  sets the developing bias Vdc applied to the developing roller  23 C to the target electric potential V 0  (V) of the photoreceptor drum  20  and develops the latent image (the band latent image  2 ) formed at Step S 4 . Consequently, the electric potential difference between the developing roller  23 C to which the developing bias Vdc at V 0  (V) has been applied and the band latent image  2  forms the band toner image  2   a  (see  12  in  FIG. 4 ) on the circumference surface of the photoreceptor drum  20 . 
     At Step S 6 , the image condition adjusting unit  53  controls the print density sensor  65  to measure the print density of the band toner image  2   a  formed at Step S 5 . 
     At Step S 7 , a print density D 1  of the band toner image  1   a  measured at Step S 3  is compared with a print density D 2  of the band toner image  2   a  measured at Step S 6  and corrects the charging bias Vref as necessary. As described above, when the electric potential at the non-charged area, which is formed at Step S 1 , is 0 (V), the print density D 1  of the band toner image  1   a  is formed through movement of the toner caused by the electric potential difference a (V) between the photoreceptor drum  20  and the developing roller  23 C. At Step S 4 , assuming that, the application of the charging bias Vref sets the surface potential Vdr of the photoreceptor drum  20  to the target electric potential V 0  (V), the surface potential Vdr at the background portion of the photoreceptor drum  20  becomes identical to the electric potential of the developing roller  23 C. In view of this, since the movement of the toner caused by the electric potential difference a (v) forms the print density D 2  of the band toner image  2   a , print density D 1 =print density D 2 . 
     On the other hand, the print density D 2  measured at Step S 6  is larger than the print density D 1 , the surface potential Vdr at the background portion of the photoreceptor drum  20  at Step S 4  is smaller than the target electric potential V 0 . Accordingly, in this case, the image condition adjusting unit  53  decides a value larger than the charging bias Vref as a charging bias with respect to the target electric potential V 0  (V). In details, the application of the charging bias found by adding a preset increment value m (V) to the charging bias Vref to the photoreceptor drum  20  executes Steps S 4  to S 6  again. Thus, while correcting the value of the charging bias applied to the charging roller  21 A, the image condition adjusting unit  53  extracts the charging bias so as to meet the print density D 1 =print density D 2 . When the print density D 2  measured at Step S 6  is smaller than the print density D 1 , the image condition adjusting unit  53  decides a value smaller than the charging bias Vref as a charging bias with respect to the target electric potential V 0  (V). Thus, the value of the charging bias corresponding to the target electric potential V 0  for the photoreceptor drum  20  is decided. 
     As described above, in this embodiment, the image condition adjusting unit  53  controls the charging bias applying unit  62  in the charging bias adjusting operation to form the non-charged area to which the charging bias is not applied on the circumference surface of the photoreceptor drum  20 . The image condition adjusting unit  53  controls the developing bias applying unit  63  to apply the developing bias Vdc constituted of a (V) (a first electric potential) to the developing roller  23 C, thus forming the band toner image  1   a  (a first toner image) by the electric potential difference between the non-charged area and the developing roller  23 C. Further, the image condition adjusting unit  53  controls the charging bias applying unit  62  to apply a charging bias found by subtracting a (V) from the charging bias Vref (a first tentative charging bias), which is preset corresponding to the target electric potential V 0  (V), on the circumference surface of the photoreceptor drum  20  to form the band latent image  2  (a predetermined electric potential area). The image condition adjusting unit  53  controls the developing bias applying unit  63  to apply the target electric potential V 0  (V) to the developing roller  23 C, thus forming the band toner image  2   a  (a second toner image) by the electric potential difference between the band latent image  2  and developing roller  23 C. The image condition adjusting unit  53  decides the value of the charging bias corresponding to the target electric potential V 0  from the results of print density measurements of the band toner image  1   a  and the band toner image  2   a  measured by the print density sensor  65  (D 1  and D 2 ). Accordingly, based on a relationship between the charging bias at the band toner image  1   a  and the print density of the toner image, an amount of discrepancy of the charging bias Vref with respect to the target electric potential V 0  is decidable. This ensures setting the surface potential of the photoreceptor drum  20  to the target electric potential with a simple configuration without providing a surface electrometer opposed to the photoreceptor drum  20 . 
     Especially, with the print density of the band toner image  1   a  higher than the print density of the band toner image  2   a  in the charging bias adjusting operation, the image condition adjusting unit  53  decides a value smaller than the charging bias Vref as the charging bias corresponding to the target electric potential V 0 . With the print density of the band toner image  1   a  lower than the print density of the band toner image  2   a , the image condition adjusting unit  53  decides a value larger than the charging bias Vref as the charging bias corresponding to the target electric potential V 0 . In view of this, from the comparison result of print density between the band toner image  1   a  and the band toner image  2   a , the charging bias corresponding to the target electric potential V 0  can be easily decided. 
     In this embodiment, the image condition adjusting unit  53  controls the charging bias applying unit  62  in the charging bias adjusting operation to apply a predetermined intermediate charging bias Vm, thus setting a background portion electric potential before and after the rotation direction of the non-charged area (0 V). The intermediate charging bias Vm is set smaller than the target electric potential V 0 . In view of this, even if the developing device uses a two-component developer, this restrains the movement of a large amount of carrier from the developing roller  23 C to the photoreceptor drum  20  during the charging bias adjusting operation. 
     Next, the following describes a timing control of the charging bias adjusting operation according to the embodiment.  FIG. 5  is a schematic view describing the timing of the charging bias adjusting operation according to the embodiment.  FIG. 6  illustrates the timing of the charging bias adjusting operation according to the embodiment. 
       FIG. 5  schematically illustrates a disposition of an eraser (a static eliminator), which is not illustrated in  FIG. 1 , at the peripheral area of the photoreceptor drum  20  in addition to the charging apparatus  21 , the exposure apparatus  22 , the developing device  23 , and the primary transfer roller  24 . In  FIG. 5 , the photoreceptor drum  20  is rotationally driven in an arrow D 1  direction. The intermediate transfer belt  141 , which is sandwiched between the photoreceptor drum  20  and the primary transfer roller  24 , circles around in an arrow D 2  direction. With reference to  FIG. 5 , a distance on the circumference surface of the photoreceptor drum  20  from a position at which the primary transfer roller  24  is opposed to the photoreceptor drum  20  (a position at which a straight line connecting a shaft center of the primary transfer roller  24  and a shaft center of the photoreceptor drum  20  intersects with the circumference surface of the photoreceptor drum  20 ) to a position at which the charging apparatus  21  is opposed to the photoreceptor drum  20  (a position at which a straight line connecting a shaft center of the charging roller  21 A and the shaft center of the photoreceptor drum  20  intersects with the circumference surface of the photoreceptor drum  20 ) is defined as L. Similarly, a distance on the circumference surface of the photoreceptor drum  20  from a position at which the charging apparatus  21  is opposed to the photoreceptor drum  20  to a position at which the developing device  23  is opposed to the photoreceptor drum  20  (a position at which a straight line connecting a shaft center of the developing roller  23 C and the shaft center of the photoreceptor drum  20  intersects with the circumference surface of the photoreceptor drum  20 ) is defined as M. Further, a distance on the circumference surface of the photoreceptor drum  20  from a position at which the developing device  23  is opposed to the photoreceptor drum  20  to a position at which the primary transfer roller  24  is opposed to the photoreceptor drum  20  is defined as N. As one example, this embodiment sets the distance L=28.4 mm, the distance M=15 mm, and the distance N=32 mm. 
     In  FIG. 6  the charging bias adjusting operation according to the embodiment is performed from a time T 1  to a time T 14 .  FIG. 6  illustrates ON/OFF timings of the charging bias, which is applied to the charging roller  21 A, an AC bias and a DC bias of the developing bias, which are applied to the developing roller  23 C, and the primary transfer bias (the transfer bias), which is applied to the primary transfer roller  24 . With reference to the timings of the charging bias in  FIG. 6 , the time T 1  to a time T 7  correspond to Step S 1  in  FIG. 3 , and the time T 7  to a time T 11  correspond to Step S 4  in  FIG. 3 . 
     At Step S 1  in  FIG. 3 , to set the surface potential of the photoreceptor drum  20  at the non-charged area closer to 0 (V), this embodiment preferably controls the application timing of the AC bias of the developing bias applied to the developing roller  23 C and the application timing of the primary transfer bias applied to the primary transfer roller  24 . That is, by turning off the charging bias from a time T 5  to a time T 6  forms the above-described non-charged area. Prior to this, the AC bias of the developing bias is not preliminarily applied to the circumference surface of the photoreceptor drum  20  corresponding to the non-charged area (from the time T 1  to a time T 2 ). In this respect, a phase of the application timing of the developing bias is controlled so as to be shifted by the above-described distance N+L. Various waveforms are applicable to an AC waveform of the developing bias; however, a sine waveform or a rectangular waveform is preferable. To restrain a leakage between the photoreceptor drum  20  and the developing roller  23 C and a variation in print density, an amplitude of an AC waveform (a voltage between peaks) Vpp is preferably 500 to 1500 (V). Developing the band toner image  1   a  and the band toner image  2   a  while the AC bias of the developing bias is kept to be applied secures stable print density of the toner image (from the time T 2  to the time T 14 ). 
     Similarly, with reference to  FIG. 6 , the primary transfer bias is not preliminarily applied to the circumference surface of the photoreceptor drum  20  corresponding to the non-charged area (from a time T 3  to a time T 4 ). In this respect, the phase of the application timing of the primary transfer bias is controlled so as to be shifted by the above-described distance L. 
     An influence that the DC bias of the developing bias gives the electric potential (0 V) at the non-charged area is little; however, as illustrated in  FIG. 6 , this embodiment controls the DC bias and the AC bias of the developing bias to synchronously turn ON at the time T 2 . In  FIG. 6 , from a time T 8  to a time T 12 , the target electric potential V 0  of the photoreceptor drum  20  is applied to the developing roller  23 C. The ON/OFF state of the charging bias and the developing bias are controlled to shift the phases by the time T 2 −the time T 1 , accommodating the distance between the charging apparatus  21  and the developing device  23 . 
     Thus, in this embodiment, the image forming apparatus  10  includes a transfer bias applying unit (not illustrated), which applies a predetermined transfer bias to the primary transfer roller  24 . In the charging bias adjusting operation, the image condition adjusting unit  53  stops the primary transfer bias before an area corresponding to the non-charged area in the circumference surface of the photoreceptor drum  20  passing through the charging apparatus  21  and when passing through the primary transfer roller  24 . This ensures setting the non-charged area closer to 0 V, thereby ensuring accurately deciding the charging bias corresponding to the target electric potential V 0 . 
     The developing bias applying unit  63  applies the developing bias configured by superimposing the AC bias to the DC bias to the developing roller  23 C. In the charging bias adjusting operation, the image condition adjusting unit  53  at least stops the AC bias of the developing bias before the area corresponding to the non-charged area in the circumference surface of the photoreceptor drum  20  passing through the charging apparatus  21  and when passing through the developing device  23 . This ensures setting the non-charged area closer to 0 V, thereby ensuring accurately deciding the charging bias corresponding to the target electric potential V 0 . In another embodiment, only one of the developing bias and the primary transfer bias may be stopped. Further, in another embodiment, the non-charged area may be formed only the application of the charging bias is stopped by the charging apparatus  21 . That is, corresponding to the non-charged area, the AC bias of the developing bias and the primary transfer bias may not necessarily to be stopped. 
     The following describes the charging bias adjusting operation according to a second embodiment of the disclosure.  FIG. 7  is a schematic view illustrating an electric potential relationship between the photoreceptor drum  20  and the developing roller  23 C in the charging bias adjusting operation according to the embodiment. Compared with the above-described first embodiment, this embodiment partially differs in formation of the band latent image  2  at Step S 4  and development of the band latent image  2  at Step S 5 . Therefore, the following describes only these differences and omits descriptions on other common control aspects. 
     With reference to  FIG. 7 , this embodiment features the surface potential Vdr of the photoreceptor drum  20  during the formation of the band latent image  2  and a value of the developing bias Vdc during the development of the band latent image  2  among the charging bias adjusting operation. The values of the surface potential Vdr of the photoreceptor drum  20  when the band latent image  1  is formed and the developing bias Vdc when the band latent image  1  is developed are similar to those of the first embodiment. 
     At Step S 4  in  FIG. 3 , the image condition adjusting unit  53  controls the charging bias applying unit  62  to apply a value found by subtracting b (V) (the second electric potential) from the preset charging bias Vref (Vref−b) to the charging roller  21 A, thus setting the background portion electric potential. Additionally, to form the band latent image  2 , the image condition adjusting unit  53  applies a value found by subtracting b (V) and a (V) (the first electric potential) from the preset charging bias Vref (Vref−b−a) to the charging roller  21 A. Consequently, the band latent image  2  is formed on the photoreceptor drum  20 . The storage unit  54  preliminary stores the threshold b (V). Additionally, at Step S 5  in  FIG. 3 , the image condition adjusting unit  53  controls the developing bias applying unit  63  to apply a value found by subtracting b (V) from the target electric potential V 0  for the photoreceptor drum  20  to the developing roller  23 C. This forms the band toner image  2   a  by the electric potential difference (V 0 −Vref−a) between the developing roller  23 C and the band latent image  2  (see  12  in  FIG. 7 ). Accordingly, similar to the first embodiment, the image condition adjusting unit  53  is configured to decide the charging bias corresponding to the target electric potential V 0  through the comparison between the print density D 1  and the print density D 2 . 
     Additionally, in this embodiment, compared with the first embodiment, the developing bias that the developing bias applying unit  63  applies is reduced by b (V) during the charging bias adjustment. This restrains a cost increase of the developing bias applying unit  63 , which is constituted of a high-voltage power supply. 
     Execution Timing of Charging Bias Adjusting Operation 
     The following describes the execution timing of the charging bias adjusting operation according to the above-described first and second embodiments (hereinafter referred to as the embodiments). In the image forming apparatus  10 , when the surface potential of the photoreceptor drum  20  varies, an image defect such as a print density variation occurs. Accordingly, it is preferable to perform the charging bias adjusting operation under a condition where the surface potential of the photoreceptor drum  20  is likely to vary from the target electric potential V 0 . The following describes the preferable conditions. 
     First, it is preferable that the charging bias adjusting operation is performed when the image forming apparatus  10  is left for a long time after a termination of the previous image forming operation. In this case, temperature and humidity environments inside and outside the image forming apparatus  10  or a similar factor may vary or the property of the charging roller  21 A of the charging apparatus  21  may change. In this embodiment, the image forming apparatus  10  includes the count unit  55  (see  FIG. 2 ). The count unit  55  operates a difference between an end time of the previous image forming operation and a request time of the next image forming operation. In other words, the count unit  55  counts a printing interval period between sheets. When the printing interval period by the count unit  55  exceeds a preset threshold stored in the storage unit  54 , it is only necessary for the image condition adjusting unit  53  to perform the charging bias adjusting operation prior to the next image forming operation. This prevents the image defect in association with the variation of the surface potential of the photoreceptor drum  20  even if the unused image forming apparatus  10  is left over a long period of time. 
     Secondary, if the temperature and humidity inside and outside the machine of the image forming apparatus  10  largely change, the charging bias adjusting operation is preferably performed. In this case, due to the variation of the temperature and humidity environments, the property of the charging roller  21 A of the charging apparatus  21  may change. In this embodiment, the image forming apparatus  10  includes the environmental sensor  64  (see  FIG. 2 ). Accordingly, if the temperature or the humidity detected by the environmental sensor  64  exceeds the preset threshold, which is stored in the storage unit  54 , it is only necessary for the image condition adjusting unit  53  to perform the charging bias adjusting operation prior to the next image forming operation. This prevents the image defect in association with the variation of the surface potential of the photoreceptor drum  20  even if the temperature and humidity inside and outside the machine of the image forming apparatus  10  largely change. A detection timing of the temperature and humidity by the environmental sensor  64  may be performed at constant time intervals. If the temperature and humidity when the previous charging bias adjusting operation has been performed are stored in the storage unit  54  and amounts of variation from these stored temperature and humidity are large, whether to perform the charging bias adjusting operation or not may be determined. 
     Thirdly, if the number of printed sheets printed within a predetermined period exceeds the preset threshold stored in the storage unit  54 , the image condition adjusting unit  53  may perform the charging bias adjusting operation. Continuous executions of the image forming operation over a long time are likely to vary the surface potential of the photoreceptor drum  20  due to a temperature rise of the photoreceptor drum  20 , the property change of the charging roller  21 A, or a similar cause. Accordingly, with the large number of printed sheets within the predetermined time, accurately adjusting the surface potential V 0  of the photoreceptor drum  20  prevents the image defect. 
     The above-described execution timing of the charging bias adjusting operation may be almost identical to a timing of the calibration operation (adjustments of developability, an amount of exposure, and color shift correction) performed by the image forming apparatus  10 . In view of this, the image condition adjusting unit  53  may perform the charging bias adjusting operation simultaneous with the execution of the calibration operation.  FIG. 8  illustrates a flowchart of the calibration operation according to the embodiment. As one example, when the unused image forming apparatus  10  is left since the night on the previous day and a power supply of the image forming apparatus  10  is turned on in the morning of the next day, the image condition adjusting unit  53  performs the calibration operation in  FIG. 8 . The image condition adjusting unit  53  first performs a developing bias calibration (Step S 11 ). This calibration adjusts the value of the DC bias of the developing bias, the waveform of the AC bias, and a similar factor according to a detection result of the temperature and humidity by the environmental sensor  64 . Next, the image condition adjusting unit  53  performs the charging bias adjusting operation (the correction of charging bias) according to the embodiment (Step S 12 ). Afterwards, the image condition adjusting unit  53  performs a light amount calibration of the exposure apparatus  22  (Step S 13 ). Here, an amount of laser light of the exposure apparatus  22  is adjusted to obtain an appropriate print density for a halftone image. Afterwards, the image condition adjusting unit  53  performs a tone table correction (a print density tone adjustment calibration) (Step S 14 ). Here, continuous tone print densities from a low print density area to a high print density area are adjusted. Afterwards, the image condition adjusting unit  53  performs a registration correction (Step S 15 ). This adjusts a color shift correction of a full-color image or a similar defect. 
     Thus, in this embodiment, the image condition adjusting unit  53  performs the charging bias adjusting operation (Step S 12 ), and then the calibration operation (Step S 13 ), which adjusts the print density tone of the toner image, is performed. Accordingly, the print density tone of the toner image is adjusted with the surface potential V 0  of the photoreceptor drum  20  stably held. This ensures obtaining a stable image quality in the subsequent image forming operation. 
     Correction of Charging Bias Vref 
     The following describes a third embodiment of the disclosure. Compared with the above-described first and second embodiments, this embodiment differs in predictive control of the charging bias Vref performed in advance. Therefore, the following describes only this difference and omits descriptions on other common control aspects. Vref, which is used in the charging bias adjusting operation, is preferably a value that can accurately reproduce the target surface potential V 0  for the photoreceptor drum  20 . However, the charging bias Vref required to reproduce the identical target electric potential V 0  is likely to largely change due to the environment (the temperature and humidity), a period of using the photoreceptor drum  20  (a degree of deterioration of a surface layer of the photoreceptor drum  20 ), or a similar factor. In view of this, in this embodiment, the image condition adjusting unit  53  corrects the value of the charging bias Vref (the first tentative charging bias) according to a predetermined correction condition prior to the charging bias adjusting operation (see  FIG. 3 ). 
     Table 1 shows an amount of correction of the charging bias Vref corrected by the image condition adjusting unit  53  when the temperature and the humidity detected by the environmental sensor  64  change. The storage unit  54  preliminary stores this amount of correction. As one example, with the detected temperature and humidity at 18 degrees and 30% RH, a value found by adding 76 V to a predetermined reference value is set as the charging bias Vref, and the charging bias adjusting operation is started. With this correction, even if the properties of the photoreceptor drum  20  and the charging apparatus  21  change according to the temperature and humidity, the adjusting operation is performed in the electric potential area close to the actual target electric potential V 0 . Therefore, the charging bias adjusting operation is quickly and accurately achieved. 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                   
                   
                 Temperature (T° C.) 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                   
                 0 
                 5 
                 12 
                 14 
                 16 
                 18 
                 20 
                 22 
                 23 
                 24 
                 26 
                 28 
                 30 
                 32 
                 40 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Humidity 
                 15% 
                 346 
                 274 
                 180 
                 161 
                 139 
                 118 
                 114 
                 111 
                 109 
                 100 
                 80 
                 61 
                 48 
                 31 
                 −24 
               
               
                 (H %) 
                 20% 
                 337 
                 265 
                 173 
                 150 
                 127 
                 104 
                 98 
                 93 
                 90 
                 81 
                 62 
                 44 
                 31 
                 16 
                 −31 
               
               
                   
                 25% 
                 328 
                 257 
                 166 
                 139 
                 115 
                 90 
                 82 
                 74 
                 71 
                 62 
                 44 
                 27 
                 15 
                 1 
                 −37 
               
               
                   
                 30% 
                 319 
                 248 
                 159 
                 129 
                 102 
                 76 
                 66 
                 56 
                 51 
                 43 
                 26 
                 10 
                 −2 
                 −14 
                 −43 
               
               
                   
                 35% 
                 313 
                 242 
                 152 
                 122 
                 94 
                 66 
                 55 
                 44 
                 39 
                 31 
                 15 
                 0 
                 −10 
                 −20 
                 −44 
               
               
                   
                 40% 
                 307 
                 236 
                 146 
                 115 
                 86 
                 57 
                 44 
                 32 
                 26 
                 18 
                 4 
                 −10 
                 −18 
                 −26 
                 −45 
               
               
                   
                 45% 
                 301 
                 230 
                 139 
                 108 
                 77 
                 47 
                 34 
                 20 
                 13 
                 6 
                 −7 
                 −20 
                 −26 
                 −32 
                 −46 
               
               
                   
                 50% 
                 295 
                 224 
                 132 
                 100 
                 69 
                 38 
                 23 
                 8 
                 0 
                 −6 
                 −18 
                 −31 
                 −35 
                 −39 
                 −47 
               
               
                   
                 55% 
                 291 
                 220 
                 128 
                 96 
                 64 
                 32 
                 18 
                 4 
                 −3 
                 −9 
                 −20 
                 −31 
                 −35 
                 −39 
                 −47 
               
               
                   
                 60% 
                 287 
                 216 
                 124 
                 91 
                 58 
                 26 
                 13 
                 0 
                 −6 
                 −11 
                 −21 
                 −32 
                 −35 
                 −39 
                 −47 
               
               
                   
                 65% 
                 283 
                 212 
                 120 
                 86 
                 53 
                 19 
                 8 
                 −3 
                 −9 
                 −14 
                 −23 
                 −32 
                 −35 
                 −40 
                 −47 
               
               
                   
                 70% 
                 279 
                 208 
                 116 
                 82 
                 47 
                 13 
                 3 
                 −7 
                 −12 
                 −16 
                 −24 
                 −33 
                 −36 
                 −40 
                 −47 
               
               
                   
                 75% 
                 275 
                 204 
                 112 
                 77 
                 42 
                 7 
                 −2 
                 −10 
                 −15 
                 −19 
                 −26 
                 −33 
                 −36 
                 −41 
                 −47 
               
               
                   
                 80% 
                 271 
                 200 
                 108 
                 72 
                 37 
                 1 
                 −7 
                 −14 
                 −18 
                 −21 
                 −28 
                 −34 
                 −36 
                 −41 
                 −47 
               
               
                   
               
            
           
         
       
     
     Table 2 shows an amount of correction of the charging bias Vref corrected by the image condition adjusting unit  53  according to a driving period of the photoreceptor drum  20  detected by the count unit  55 . The storage unit  54  preliminary stores this amount of correction. As one example, with the detected driving period of the photoreceptor drum  20  of 50 hours, a value found by adding 50 V to a predetermined reference value is set as the charging bias Vref and the charging bias adjusting operation is started. In this case, even if the charging characteristic of the photoreceptor drum  20  changes according to the driving period of the photoreceptor drum  20 , the charging bias adjusting operation is quickly and accurately achieved. In another modified embodiment, the count unit  55  may count an accumulated application period of the charging bias by the charging apparatus  21 . It is only necessary that the storage unit  54  preliminary stores correction values shown in Table 2 according to the accumulated application period of the charging bias. In this case as well, even if the charging characteristic of the charging roller  21 A changes according to the accumulated application period of the charging bias, the charging bias adjusting operation is quickly and accurately achieved. With the above-described respective amounts of correction in combination with one another, the charging bias Vref may be adjusted by the temperature and humidity inside and outside of the machine of the image forming apparatus  10 , the driving period of the photoreceptor drum  20 , and a similar factor. The charging bias Vref may be adjusted according to other correction conditions. The above-described respective correction values may be stored not as a table but as a predetermined correction formula. After the above-described charging bias Vref is corrected, the charging bias adjusting operation similar to the above-described first or second embodiment is performed. 
     
       
         
           
               
               
               
               
               
               
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
             
            
               
                 Photoreceptor  
                 0 
                 10 
                 20 
                 30 
                 40 
                 50 
                 60 
                 500 
                 1000 
               
               
                 driving time 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 [Time] 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 Amount of  
                 0 
                 10 
                 20 
                 30 
                 40 
                 50 
                 60 
                 60 
                 50 
               
               
                 Vref correction 
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                 [V] 
               
               
                   
               
            
           
         
       
     
     The image forming apparatus  10  according to the embodiments of the disclosure is described above in detail; however, the disclosure is not limited to this. The disclosure can employ, for example, the following modified embodiments. 
     (1) The above-described respective embodiments describe the aspect that the toner is charged to a positive polarity; however, the disclosure is not limited to this. When the toner is charged to a negative polarity, the similar charging bias adjustment control is executable with polarities of the above-described respective biases inverted. 
     (2) The above-described embodiments describe the aspect that the image forming apparatus  10  is the full-color image forming apparatus; however, the disclosure is not limited to this. The image forming apparatus  10  may be a monochrome printer or a similar printer that forms a single color image. 
     (3) The above-described first embodiment describes the aspect that the one band latent image  2  (the band toner image  2   a ) is formed from Step S 4  to Step S 6  in  FIG. 3 ; however, the disclosure is not limited to this. In another embodiment, while the a value in  FIG. 4  is changed, a plurality of the band latent images may be formed. In this case, the image condition adjusting unit  53  can accurately detect the difference between Vref and the target electric potential V 0  from a print density measurement result of the plurality of band toner images  2   a.    
     (4) The above-described first embodiment describes the aspect that the value a (V) of the developing bias applied to the developing roller  23 C when the band latent image  1  is formed is identical to a value of the electric potential difference a (V) subtracted from the charging bias Vref when the band latent image  2  is formed; however, the disclosure is not limited to this. Both values may not be identical values. When different values are applied, it is only necessary that the value of the charging bias derived by the difference between both is corrected at Step S 7  in  FIG. 3 . 
     WORKING EXAMPLES 
     The following further describes the embodiments of the disclosure in detail with the working examples; however, the disclosure is not limited to only the following working examples. 
     Working Example 1 
     With the above-described image forming apparatus  10  according to the first embodiment, under conditions that the distance on the circumference surface of the photoreceptor drum  20  from the developing device  23  to the charging apparatus  21  in  FIG. 5  is 60 mm (a rotation period of the photoreceptor drum  20 : 0.4 sec) and the distance on the circumference surface of the photoreceptor drum  20  from the primary transfer roller  24  to the charging apparatus  21  is 30 mm (a rotation period of the photoreceptor drum  20 : 0.2 sec), a peripheral velocity of the photoreceptor drum  20  is set to 150 mm/sec. Similar to the first embodiment, corresponding to the non-charged area, the primary transfer bias and the AC bias of the developing bias are preliminary turned off. The measurement of the surface potential at the non-charged area on the photoreceptor drum  20  by a surface electrometer for experiment turned out to be 0 V. 
     In this Working Example 1, at Step S 2  in  FIG. 3 , the band latent image  1  was developed at a developing bias Vdc=a=150 V and the band toner image  1   a  was formed. This resulted in print density D 1 =0.52. Next, at Step S 4 , to obtain the target electric potential V 0 , the charging bias Vref=1350 V was tentatively applied, and Vref−a (V)=1200 V was applied to form the band latent image  2 . Afterwards, at Step S 5 , the developing bias Vdc=target electric potential V 0 =450 (V) was set to form the band toner image  2   a . This resulted in print density D 2 =0.42. From this result, from the relationship of print density D 1 &gt; print density D 2 , a value found by subtracting 10 (V) from the above-described charging bias Vref was set as a charging bias Vref after correction=1340 (V). While the image condition adjusting unit  53  controlled the charging bias applying unit  62  to apply the charging bias Vref=1340 (V), the surface potential of the photoreceptor drum  20  was actually measured. This resulted in V 0 =460 (V). 
     On the other hand, at Step S 4 , to obtain the target electric potential V 0 , the charging bias Vref=1330 V was tentatively applied, and Vref−a (V)=1180 V was applied to form the band latent image  2 . Afterwards, at Step S 5 , the developing bias Vdc=target electric potential V 0 =450 (V) was set to form the band toner image  2   a . This resulted in print density D 2 =0.52. In this case, since print density D 1 =print density D 2  was met, the charging bias Vref=1330 V was decided as the charging bias corresponding to the target electric potential V 0 =450 (V). 
     Working Example 2 
     Compared with Working Example 1, Working Example 2 turns on the AC bias of the developing bias corresponding to the non-charged area. The measurement of the surface potential at the non-charged area on the photoreceptor drum  20  by the surface electrometer for experiment turned out to be −5 V. The measurement result of the print density in Working Example 2 is omitted. 
     Working Example 3 
     Compared with Working Example 1, Working Example 3 turns on the primary transfer bias and the AC bias of the developing bias corresponding to the non-charged area. The measurement of the surface potential at the non-charged area on the photoreceptor drum  20  by the surface electrometer for experiment turned out to be −20 V. 
     In this Working Example 3, at Step S 2  in  FIG. 3 , the band latent image  1  was developed at a developing bias Vdc=a=150 V and the band toner image  1   a  was formed. This resulted in print density D 1 =0.62. Next, at Step S 4 , to obtain the target electric potential V 0 , the charging bias Vref=1350 V was tentatively applied, and Vref−a (V)=1200 V was applied to form the band latent image  2 . Afterwards, at Step S 5 , developing bias Vdc=target electric potential V 0 =450 (V) was set to form the band toner image  2   a . This resulted in print density D 2 =0.42. From this result, from the relationship of print density D 1 &gt; print density D 2 , a value found by subtracting 10 (V) from the above-described charging bias Vref was set as a charging bias Vref after correction=1340 (V). While the image condition adjusting unit  53  controlled the charging bias applying unit  62  to apply the charging bias Vref=1340 (V), the surface potential of the photoreceptor drum  20  was actually measured. This resulted in V 0 =460 (V). 
     On the other hand, at Step S 4 , to obtain the target electric potential V 0 , the charging bias Vref=1330 V was tentatively applied, and Vref−a (V)=1180 V was applied to form the band latent image  2 . Afterwards, at Step S 5 , developing bias Vdc=target electric potential V 0 =450 (V) was set to form the band toner image  2   a . This resulted in print density D 2 =0.52. In this case, since print density D 1 &gt; print density D 2  was met, a value found by further subtracting 10 (V) from the above-described charging bias Vref was set as a charging bias Vref after correction=1320 (V). While the image condition adjusting unit  53  controlled the charging bias applying unit  62  to apply the charging bias Vref=1320 (V), the surface potential of the photoreceptor drum  20  was actually measured. This resulted in V 0 =440 (V). Consequently, the charging bias Vref=1320 V was decided as the charging bias corresponding to the target electric potential V 0 . 
     The above-described all working examples decide the charging bias corresponding to the target electric potential V 0  for the photoreceptor drum  20  with simple configuration. Further, like Working Example 1, preliminary turning off the primary transfer bias and the AC bias of the developing bias corresponding to the non-charged area ensures execution of the charging bias adjusting operation with the less number of steps. 
     While various aspects and embodiments have been disclosed herein, other aspects and embodiments will be apparent to those skilled in the art. The various aspects and embodiments disclosed herein are for purposes of illustration and are not intended to be limiting, with the true scope and spirit being indicated by the following claims.