Patent Publication Number: US-2011064429-A1

Title: Image forming apparatus and image forming method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of U.S. Provisional Application No. 61/242,992, filed on Sep. 16, 2009; the entire contents of which are incorporated herein by reference. 
    
    
     FIELD 
     Embodiments described herein relate generally to an image forming apparatus and an image forming method. 
     BACKGROUND 
     Various developing apparatuses are used in an image forming apparatus such as a copier or a printer. For example, there is used a developing apparatus that performs the development using a two component developer. Generally, a developing apparatus, which uses the two component developer of a toner and a carrier, supplies the toner consumed by the development operation. However, although the toner is supplied, when the performance of the carrier declines, the performance for electrifying the toner deteriorates. 
     JP-A-6-348134 discloses a trickle developing method for suppressing the deterioration of the electrification performance of the toner due to the performance deterioration of the carrier. In the trickle developing method, a new carrier is supplied to the toner within a developing container, and the excessive developer is discharged from a discharge port. As a result, the new carrier is gradually replaced with the deteriorated carrier. 
     However, in the above-mentioned developing apparatus, the supply amount of the carrier into the developing container is decided depending on the toner amount to be consumed. For this reason, there was a case where the carrier of the required amount was not supplied, even though the deterioration of the carrier was noticeable. If the supply amount of the carrier is insufficient, the electrification shortage of the toner is generated. If the electrification shortage of the toner is generated, there is a high possibility in which the image quality of the developed toner image deteriorates or disadvantage such as a ground fog is generated. On the contrary, if the carrier is excessively supplied, the carrier with a low degree of deterioration is discharged, whereby the carrier is excessively consumed. 
     Thus, if the carrier can be suitably supplied into the developing container depending on the deterioration of the carrier, the clear developing image can be obtained without excessively consuming the carrier and the disadvantage such as the ground fog can be prevented to promote an improvement in image quality. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an exemplary schematic configuration diagram showing a color printer which is an image forming apparatus according to an embodiment. 
         FIG. 2  is an exemplary detailed configuration diagram showing respective process units according to an embodiment. 
         FIG. 3  is an exemplary perspective view showing a developing apparatus according to an embodiment. 
         FIG. 4  is an exemplary longitudinal sectional view showing a developing apparatus according to an embodiment. 
         FIG. 5  is an exemplary cross sectional view showing a developing apparatus according to an embodiment. 
         FIG. 6  is an exemplary longitudinal sectional view showing a developer supply unit and a developing apparatus according to an embodiment. 
         FIG. 7  is an exemplary block diagram showing a control system for controlling a new carrier supply to the respective developing apparatuses according to an embodiment. 
         FIG. 8  is a schematic exemplary functional configuration diagram showing a control system in an image forming apparatus according to an embodiment. 
         FIG. 9  is an exemplary diagram showing a relationship of a developing contrast voltage and a toner electrification amount according to an embodiment. 
         FIG. 10  is an exemplary diagram showing a relationship of a developing contrast voltage and a correction coefficient according to an embodiment. 
         FIG. 11  is an exemplary flow chart showing a schematic image quality maintenance control process sequence according to an embodiment. 
         FIG. 12  is an exemplary diagram showing a relationship of a grid bias voltage, a non light exposure portion potential, a light exposure potential, and a developing bias voltage according to an embodiment. 
         FIG. 13  is an exemplary diagram showing a relationship of a pattern area on a photoconductive drum and a toner attachment amount measuring portion  44  according to an embodiment. 
         FIG. 14  is a diagram showing an exemplary carrier supply time correction coefficient database managed in a correction coefficient database according to an embodiment. 
         FIG. 15  is an exemplary flow chart showing an operation sequence of a carrier supply time control portion according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In general, according to one embodiment, an image forming apparatus includes a developing unit configured to store a developer comprising a toner and a carrier and to form a developer image; a toner attachment amount detection portion configured to detect an attachment amount of a toner formed on a photoconductor or an intermediate transfer body; a developing bias voltage change portion configured to change a plurality of developing bias voltages including a developing contrast voltage in a manner that the detected attachment amount of the toner becomes a value in a predetermined range; a carrier supply time correction portion configured to correct a carrier supply time in response to the value of the changed developing contrast voltage; and a carrier supply portion configured to supply a carrier in a corrected carrier supply time at a predetermined timing. 
     Hereinafter, an embodiment of the present invention will be described. 
       FIG. 1  is an exemplary schematic configuration diagram of a color printer  1  which is an image forming apparatus according to an embodiment. The color printer  1  is of a four-drum tandem type. A process velocity is 150 mm/S. 
     The color printer  1  includes a paper discharge portion  3  at an upper part. The color printer  1  has an image forming unit  11  at a lower side of a intermediate transfer belt  10 . The image forming unit  11  includes four process units  11 Y,  11 M,  11 C and  11 K disposed in parallel along the intermediate transfer belt  10 . The process units  11 Y,  11 M,  11 C and  11 K form toner images of yellow (Y), magenta (M), cyan (C), and black (K), respectively. 
       FIG. 2  shows an exemplary detailed configuration diagram of the respective process units  11 Y,  11 M,  11 C and  11 K according to an embodiment. The respective process units  11 Y,  11 M,  11 C and  11 K have photoconductor drums  12 Y,  12 M,  12 C and  12 K that are image carriers, respectively. The respective photoconductor drums  12 Y,  12 M,  12 C and  12 K can rotate in an arrow m direction. Around the respective photoconductor drums  12 Y,  12 M,  12 C and  12 K, electrification chargers  13 Y,  13 M,  13 C and  13 K, developing apparatuses  14 Y,  14 M,  14 C and  14 K, and photoconductor cleaners  16 Y,  16 M,  16 C, and  16 K are disposed along a rotational direction. In the respective electrification chargers  13 Y,  13 M,  13 C and  13 K, the respective drums  12 Y,  12 M,  12 C, and  12 K are equally electrified to negative (−). 
     Between from the electrification chargers  13 Y,  13 M,  13 C and  13 K to the developing apparatuses  14 Y,  14 M,  14 C and  14 K around the respective photoconductor drums  12 Y,  12 M,  12 C and  12 K, the respective exposure lights are emitted by a laser exposure apparatus  17 . By the emission of the exposure light, electrostatic latent image is formed on the respective photoconductor drums  12 Y,  12 M,  12 C and  12 K. The respective electrification chargers  13 Y,  13 M,  13 C and  13 K and the laser exposure apparatus  17  constitute a latent image forming portion. 
     The respective developing apparatuses  14 Y,  14 M,  14 C and  14 K develop the electrostatic latent image on the photoconductor drums  12 Y,  12 M,  12 C and  12 K. The respective developing apparatuses  14 Y,  14 M,  14 C and  14 K perform the development by the use of the two component developer having the respective toners and carriers of yellow (Y), magenta (M), cyan (C), and black (K) that are the developers. 
     The intermediate transfer belt  10  is spanned by a backup roller  21 , a driven roller  20  and first to third tension rollers  22  to  24 , and rotates in an arrow s direction. 
     The intermediate transfer belt  10  faces and comes into contact with the photoconductor drums  12 Y,  12 M,  12 C and  12 K. At positions of the intermediate transfer belt  10  facing the photoconductor drums  12 Y,  12 M,  12 C and  12 K, primary transfer rollers  18 Y,  18 M,  18 C and  18 K are provided. The respective primary transfer rollers  18 Y,  18 M,  18 C and  18 K primarily transfer the toner images formed on the respective photoconductor drums  12 Y,  12 M,  12 C and  12 K to the intermediate transfer belt  10 . 
     The respective photoconductor cleaners  16 Y,  16 M,  16 C and  16 K remove the surface potential remaining on the respective photoconductor drums  12 Y,  12 M,  12 C and  12 K after the primary transfer. The respective photoconductor cleaners  16 Y,  16 M,  16 C and  16 K remove and collect the residual toners on the respective photoconductor drums  12 Y,  12 M,  12 C and  12 K. 
     A secondary transfer roller  27  is disposed at a secondary transfer portion which is a transfer position supported by the backup roller  21  of the intermediate transfer belt  10 . In the secondary transfer portion, a predetermined secondary transfer bias is applied to the backup roller  21 . When the sheet paper passes between the intermediate transfer belt  10  and the secondary transfer roller  27 , the toner image on the middle transfer roller belt  10  is secondarily transferred onto the sheet paper. The sheet paper P is fed from a paper feeding cassette  4  or a manual feed mechanism  31 . After the secondary transfer is finished, the middle transfer roller belt  10  is cleaned by a belt cleaner  10   a . At a position facing the tension roller  22 , a toner attachment amount meter  39  for measuring the toner attachment amount on the intermediate transfer belt is provided. 
     Between from the paper feeding cassette  4  to the secondary transfer roller  27 , a pickup roller  4   a , a separation roller  28   a , a transport roller  28   b , and a resist roller pair  36  are provided. Between from a manual feed tray  31   a  of the manual feed mechanism  31  to the resist roller pair  36 , a manual feed pickup roller  31   b  and a manual feed separation roller  31   c  are provided. In addition, a fixing apparatus  30  is provided at a downstream of the secondary transfer portion along a direction of a longitudinal transport path  34 . The fixing apparatus  30  fixes the toner image, which is transferred onto the sheet paper P by the secondary transfer portion, to the sheet paper P. At the downstream of the fixing apparatus  30 , a gate  33 , which divides the sheet paper into a paper discharge roller  41  direction or a re-transport unit  32  direction, is provided. The sheet paper guided to the paper discharge roller  41  is discharged to the paper discharge portion  3 . The sheet paper guided to the re-transport unit  32  is guided in the secondary transfer roller  27  direction again. 
     Next, the developing apparatuses  14 Y,  14 M,  14 C and  14 K will be described with reference to  FIGS. 2 to 7 . Since the developing apparatuses  14 Y,  14 M,  14 C and  14 K have the same configuration, the description will be made using the common reference numerals. The respective developing apparatuses  14 Y,  14 M,  14 C and  14 K have a case  50  that is a developing container, a developing roller  58 , a first screw  56  and a second screw  57  that are transport portions, a restriction blade  60 , and a toner concentration sensor  61 . 
       FIG. 7  is an exemplary block diagram of a control system for controlling a new carrier supply to the respective developing apparatuses according to an embodiment. To an input side of a CPU  80 , which is a control portion that controls the overall color printer  1  and controls the supply amount of the carrier, there are connected a control panel  8 , a toner concentration sensor  61 , a toner empty sensor  68  for detecting that the toner cartridge  63  is empty, a photo coupler  77  for detecting the revolution of the developing roller  58 , a page counter  81  for cumulatively counting the printing number (printing sheet number) of the color printer  1 , a pixel counter  82  for detecting the printing rate of the image, a timer  83 , and an environmental sensor  84 . The printing rate is defined by a percentage of an area to be printed and a printed area. 
     First to third motor drivers  86  to  88  are connected to the output side of the CPU  80 . The first motor driver  86  drives the developing roller  58 , the first screw  56  and the second screw  57 . The second motor driver  87  drives a toner supply auger  66 . The third motor driver  88  drives a carrier supply auger  67 . 
     A case  50  contains the developer  51  having the toner and the carrier. The respective developers  51  of the developing apparatuses  14 Y,  14 M,  14 C and  14 K differ in colors, respectively. A developer supply port  52  is formed on an upper part of a front side of the case  50 . A developer supply unit  62  is provided at the front side of the case  50 . The developer supply unit  62  integrally has a toner cartridge  63  and a carrier cartridge  64 . The toner cartridge  63  contains a new toner for supply that is a toner supply portion. The carrier cartridge  64  contains a new carrier for supply that is a carrier supply portion. On a bottom part of the toner cartridge  63 , a toner supply auger  66 , which supplies a new toner to the developer supply port  52 , is provided. On a bottom portion of the carrier cartridge  64 , a carrier supply auger  67 , which supplies a new carrier to the developer supply port  52 , is provided. 
     The toner supply auger  66  rotates so as to supply a predetermined amount of toner by the detection result of the toner concentration sensor  61 . Although the details will be described later, deterioration property of the developer  51  in the case  50  is detected in the present embodiment. The carrier supply auger  67  rotates so as to supply a predetermined amount of carrier depending on the characteristic variation of the developer  51  in the case  50 . The restriction blade  60  almost regularly controls the height of the two component developer. 
     The toner concentration sensor  61  is disposed at the lower part of a rear side of the first screw  56 . It is desirable that the toner concentration sensor  61  be arranged separately from the developer supply port  52  in the case  50 . By the arrangement, the toner concentration sensor  61  improves accuracy of the measurement of the toner concentration in the developer  51 . The toner concentration sensor  61  uses, for example, a magnetic permeability, sensor or the like. The detection result which is an output of the toner concentration sensor  61  is displayed as the voltage value. When the toner concentration of the developer  51  in the case  50  fluctuates, the output value of the toner concentration sensor  61  fluctuates. Furthermore, if the electrification amount of the toner of the developer  51  is fluctuated, the output value of the toner concentration sensor  61  is fluctuated. 
     When the toner concentration of the developer  51  in the case  50  declines, the toner concentration sensor  61  inputs the detection result into the CPU  80 . The CPU  80  drives the toner supply auger  66  depending on the detection result, thereby supplying a new toner in the toner cartridge  63 . As a result, toner concentration of the developer  51  in the case  50  is regularly maintained. 
     A developer discharge port  53 , which is the discharge portion, is formed at a side portion of the front side of the case  50 . The new toner and carrier are supplied, whereby, as the volume in the case  50  increases, the excessive developer is discharged from the developer discharge port  53  and is collected. As a result, in the case  50 , the amount of the developer  51  is regularly maintained. Concurrently, in the developer  51  in the case  50 , the deteriorated old carrier is gradually replaced with the new carrier. 
     The developing roller  58  is rotatably provided in the case  50 . The developing roller  58  supplies the toner to the electrostatic latent image, which is formed on the respective photoconductor drums  12 Y,  12 M,  12 C, and  12 K, thereby forming the toner image. The inner part of the case  50  is partitioned by a partition plate  70  along an axial direction of the respective photoconductor drums  12 Y,  12 M,  12 C, and  12 K. The inner part of the case  50  is partitioned into a stirring transport chamber  71  and a stirring supply chamber  72  by the partition plate  70 . In the stirring transport chamber  71 , the new toner and the new carrier supplied from the developer supply port  52  and the developer  51  in the case  50  are stirred by the first screw  56  and are transported in an arrow x direction. As a result, the toner of the developer  51  is electrified. 
     The developer  51 , which is stirred and transported by the first screw  56 , passes through a first passage portion  73  of a rear side of the partition plate  70  and is supplied to the stirring supply chamber  72 . In the stirring supply chamber  72 , the developer  51  is stirred and transported by the second screw  57  in an arrow y direction, thereby being supplied to the developing roller  58 . 
     A discharge screw  76  is formed at a front side of the second screw  57 . As shown in  FIG. 4 , the discharge screw  76  reduces the diameter of the screw and narrows the pitch of the screw to reduce the flow velocity of the developer  51 . Thus, as shown by solid line y, the surface of the developer  51  transported in the arrow y direction bulges in the form of a mountain. If the volume of the developer  51  in the case  50  is equal to or less than a predetermined amount, even though the developer  51  bulges by the discharge screw  76 , the developer  51  does not reach up to the height of the developer discharge port  53 . If the carrier is supplied from the carrier cartridge  64  in this state, the volume of the developer  51  increases. In addition, the discharge screw  76  causes the bulged developer  51  to reach up to the height of the developer discharge port  53 . The developer  51  reached the developer discharge port  53  is discharged from the developer discharge port  53 . The developer discharge port  53  is disposed so that the peak of the mountain shape of the developer  51 , which bulges by the discharge screw  76 , coincides with an approximately middle portion of a longitudinal direction of the developer discharge port  53 . Thus, the developer, which is excessive by supplying the carrier, is discharged from the developer discharge port  53 . The developer  51 , which has passed through the discharge screw  76 , passes through the second passage portion  74  of the front side of the partition plate  70  and is cyclically transported to the stirring transport chamber  71 . 
     In the color printer  1  of the present embodiment, when performing the image forming, depending on the detection result of the toner concentration sensor  61 , in the respective developing apparatuses  14 Y,  14 M,  14 C, and  14 K, the new toner is supplied from the toner cartridge  63  to the case  50 . Furthermore, depending on the detection result of the deterioration of the carrier of the developer  51 , in the respective developing apparatuses  14 Y,  14 M,  14 C, and  14 K, the new carrier is supplied from the carrier cartridge  64  to the case  50 . 
     Next, a method of detecting of the deterioration of the carrier to supply the new carrier into the case  50  will be described in detail. In the image forming apparatus, by the use of a method that is called an image quality maintenance control, a developing bias voltage VD or a grid bias voltage VG, which is used at the time of an actual image forming, is determined. In the present embodiment, the deterioration state of the developer is presumed by the use of a developing contrast voltage VC determined in the image quality maintenance control. In addition, the supply amount of the new carrier is corrected based on the presumption value. 
       FIG. 8  is a schematic exemplary functional configuration diagram of a control system in an image forming apparatus according to an embodiment. As shown in  FIG. 8 , an input portion  42 , a toner attachment amount measuring portion  44 , and an environmental detection portion  45  are connected to the control apparatus  41 . 
     In addition, the control apparatus  41  has a main control portion  51 , a printing-related data acquiring portion  52 , a memory portion  53 , an image quality maintenance control portion  54 , a carrier supply time control portion  55 , and an input-output interface  57 . The input-output interface  57  connects the main control portion  51 , the printing-related data acquiring portion  52 , the memory portion  53 , the image quality maintenance control portion  54 , and the carrier supply time control portion  55  to the control apparatus  41 . 
     The main control portion  51  includes a CPU (Central Processing Unit) or a MPU (Micro Processing Unit), a ROM (Read Only Memory), a RAM (Random Access Memory). The main control portion  51  generally controls the image forming apparatus  1  by creating and supplying various control signals. 
     The printing-related data acquiring portion  52  acquires printing-related data from the input portion  42 , or from an external device (not shown) via an electric cable or the like, by the operation of the display panel, button or the like by an user. In addition, the printing-related data acquiring portion  52  provides the acquired printing-related data to the data memory portion  58  of the memory portion  53 . 
     The memory portion  53  includes a data memory portion  58  and a correction coefficient database  59 . The data memory portion  58  acquired the printing-related data provided from the printing-related data acquiring portion  52  and stores the acquired printing-related data. In addition, the data memory portion  58  properly provides various data stored in the respective portions of the image forming apparatus  1  according to instructions of the main control portion  51 . In the correction coefficient database  59 , an environmental condition such as a relative humidity, an operation condition such as a voltage, and a correction coefficient of the carrier supply time are correlated and registered in advance. 
     The image quality maintenance control portion  54  includes a calculation unit  60 , a comparison determination unit  61 , and a developing voltage change portion  62 . 
     The calculation unit  60  calculates the developing contrast voltage VC and a background voltage VBG serving as the standard based on the coefficients K 1  to K 4  which are stored in the data memory portion  58  as a known data in advance. Herein, the coefficients K 1  to K 4  are constants when a light exposure portion potential VL and a non light exposure portion potential VO are indicated by the grid bias voltage VG. The calculation portion  60  calculates the developing contrast voltage VC and the background voltage VBG using the light exposure portion potential VL and the non light exposure portion potential VO. In addition, the calculation portion  60  calculates the grid bias voltage VG and the developing bias voltage VD corresponding to the calculated contrast voltage VC and background voltage VBG. 
     The comparison determination portion  61  compares and determines the measurement data of the toner attachment amount provided from the toner attachment amount measuring portion  44  and data relating to the standard value of the toner attachment amount stored in the data memory portion  58 , thereby providing the comparison determination results to the calculation portion  60 . 
     Furthermore, the calculation portion  60  calculates the deviation based on the comparison determination results provided from the comparison determination portion  61  and calculates the developing contrast voltage VC, the standard background voltage VBG, the grid bias voltage VG, and the developing bias voltage VD based on the calculated deviation. 
     The developing voltage change portion  62  changes the developing contrast voltage VC, the background voltage VBG, the grid bias voltage VG, and the developing bias voltage VD based on the calculation results in the calculation portion  60 . The developing voltage change portion  62  provides the data memory portion  58  with data relating to the developing contrast voltage VC, the background voltage VBG, the grid bias voltage VG, and the developing bias voltage VD to be actually applied. 
     The toner attachment amount measuring portion  44  includes, for example, the toner attachment amount meter  39  of  FIG. 1 , measures the toner attachment amount attached to the photoconductor drum  12  or the intermediate transfer belt  10  according to the instructions of the main control portion  51 , and provides the image quality maintenance control portion  54  with the measurement data of the toner attachment amount. 
     The environmental detection portion  45  includes, for example, the environmental sensor  38  of  FIG. 1 , detects the temperature or the relative humidity in the image forming apparatus  1  according to the instructions of the main control portion  51 , and creates the environmental detection signal based on the detected temperature or relative humidity. The environmental detection portion  45  provides the respective portions of the control apparatus  41  with the environmental detection signal. The environmental detection signal includes environmental data such as the temperature or the relative humidity in the image forming apparatus  1 . 
     The carrier supply time control portion  55  includes the carrier supply time calculation portion  64 , the carrier supply time correction coefficient setting portion  65 , and the carrier supply time change portion  66 . The carrier supply time control portion  55  changes the supply time of the carrier based on the operation results in the image quality maintenance control portion  54 . The details of the operation of the carrier supply time control portion  55  will be described later. 
       FIG. 9  is an exemplary diagram showing a relationship of the developing contrast voltage and the toner electrification amount according to an embodiment. As shown by a solid line a in  FIG. 9 , between the developing contrast voltage VC and the toner electrification amount, a correlation, which has a predetermined width and is linear, is recognized. That is, when the toner electrification amount is small, a low developing contrast voltage VC is sufficient. On the contrary, when the toner electrification amount is large, a high developing contrast voltage VC is necessary. 
     However, in the image quality maintenance control process, the developing contrast voltage VC for obtaining a predetermined toner attachment amount is required. In that case, by the use of the correlation of the developing contrast voltage VC and the toner electrification amount shown in  FIG. 9 , it is possible to predict that, when the developing contrast voltage VC calculated by the image quality maintenance control process is low, the toner electrification amount becomes smaller, and when the developing contrast voltage VC is high, the toner electrification amount becomes larger. Thus, by grasping the developing contrast voltage VC, the toner electrification amount, that is, deterioration degree of the toner can be grasped. Accordingly, when the developing contrast voltage VC calculated by the image quality maintenance control process is largely deviated from the predetermined standard value, it is determined that the toner electrification amount is also largely deviated from the range of the predetermined standard value, and the carrier supply time can be corrected based on the determination results. 
     When the carrier supply time is corrected based on the determination result relating to the bulk of the toner electrification amount, specifically, the following correction is performed. That is, when a regular voltage is applied, if the electrification amount of the toner becomes larger, the attachment amount of the toner generally becomes smaller. For this reason, to maintain the toner attachment amount in the regular range, it is necessary to control the voltage depending on the bulk of the electrification amount of the toner. 
     As shown in  FIG. 10 , for example, as the lower limit value and the upper limit value of the developing contrast voltage VC in which the toner electrification amount can be regarded to be in the range of the predetermined value, 200 (V) and 400 (V) are set in advance. In addition, the control section is divided into three sections (an optimal electrification area, a low electrification area, and a high electrification area) by the value of the developing contrast voltage VC. 
     The optimal electrification area is an area in which the electrification amount of the toner can be regarded to be in the range of the predetermined value, and sets the range from 200 (V) to 400 (V) as the range (a-b section) of the developing contrast voltage VC. The low electrification area is an area in which the electrification amount of the toner is lower than the range of the predetermined value, and sets the range of less than 200 (V) as the range (A section) of the developing contrast voltage VC. The high electrification area is an area in which the electrification amount of the toner is larger than the range of the predetermined value, and sets the range larger than 400 (V) as the range (B section) of the developing contrast voltage VC. 
     In regard to the deterioration of the carrier, there are various types, for example, a case where a coat agent of the carrier surface is peeled off, a case where an external additive with a low resistance such as titanium dioxide included in the toner is attached, so that the electrification of the toner drops, a case where resin or an external additive with a high resistance included in the toner is attached, so that the electrification amount of the toner rises or the like. 
     In the following embodiments, a case where the electrification amount drops by the deterioration of the carrier will be described. 
     In the case of an environmental condition of  FIG. 10 , it is considered that, in the a-b section which is the optimal electrification area and the B section which is the high electrification area, the carrier does not deteriorate. Therefore, the carrier supply time correction coefficient is set to 1.0. The supply of the carrier is performed at the supply time which is obtained by multiplying the normal carrier supply time by the carrier supply time correction coefficient 1.0. Thus, in this case, the carrier amount supplied at the time of supplying is not changed. 
     In the A section that is the low electrification area, it is considered that the carrier deteriorates. Therefore, the carrier supply time correction coefficient is set to a value larger than 1.0, e.g., 1.5. The carrier is supplied only at the supply time which is obtained by multiplying the normal carrier supply time by the carrier supply time correction coefficient 1.5. Thus, in this case, the carrier amount supplied at the time of supplying increases 1.5 times the ordinary amount. 
     When the carrier deteriorates, since the new carriers are supplied in a greater amount than the ordinary time, the degree of the deterioration is improved, which makes it possible to maintain a satisfactory developer performance. The carrier supply time correction coefficient is a value that fluctuates by the environmental condition such as the temperature or the relative humidity. 
     Hereinafter, the carrier supply time control process using the correlation of the developing contrast voltage VC and the toner electrification amount will be described. 
       FIG. 11  is a flow chart showing a schematic image quality maintenance control process sequence. The image quality maintenance control process is carried out when the image forming apparatus  1  is in a predetermined timing. The predetermined timing is the timing in which, when the image forming apparatus  1  is activated, so that the fixing device completes a warm-up process up to a predetermined temperature, or after, the image forming apparatus  1  performs the last image quality maintenance control process, a predetermined number of copies, for example, one thousand copies are printed. 
     In Act S 1 , the main control portion  51  exposes the laser exposure apparatus  17  in two gradation patterns of high concentration and low concentration for the toner attachment amount measurement on the photoconductor drum  12 .  FIG. 12  is an exemplary diagram showing a relationship of a grid bias voltage VG, a non exposure portion potential VO, an exposure portion potential VL, and a developing bias voltage VD according to an embodiment. The grid bias voltage VG is output from a grid electrode of the electrification charger  13 . The non exposure portion potential VO is a surface potential of the photoconductor drum  12 . The exposure portion potential VL is a surface potential of the photoconductor drum  12  which is attenuated by being entirely exposed by a regular light amount via the laser exposure apparatus  17 . In the case of the example of  FIG. 12 , the polarity of the voltage is negative due to the reverse development. 
     As shown in  FIG. 12 , when the grid bias voltage VG increases (changes in a left direction of the drawing), absolute values of the non exposure portion potential VO and the exposure portion potential VL decrease, respectively (changes in an upper direction of the drawing). The exposure portion potential VL and the non exposure portion potential VO relative to the grid bias voltage VG can be indicated by formulas (1) and (2) by a linear approximation. 
         VO ( VG )= K 1 ×VG+K 2  formula (1)
 
         VL ( VG )= K 3 ×VG+K 4  formula (2)
 
     Herein, symbols K 1  to K 4  are coefficients. VO, VL, and VG are absolute values. VO (VG) and VL (VG) show that VO and VL are indicated by variable VG. 
     Generally, the toner attachment amount (developing concentration) is changed by the relationship of three values of the developing bias voltage VD, the exposure portion potential VL, and the non exposure portion potential VO. The developing contrast voltage VC and the background voltage VBG are indicated by formulas (3) and (4) 
     
       
         
           
             
               
                 
                   
                     
                       
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                               VG 
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     Herein, VD (VG) shows the bulk of the developing bias voltage VD relative to an arbitrary grid bias voltage VG. 
     The developing contrast voltage VC mainly relates to the concentration of the solid color portion, and the background voltage VBG mainly relates to the concentration of the low concentration portion of a multi gradation method using a pulse width modulation. Thus, it is possible to change the toner attachment amount by the developing contrast voltage VC and the background voltage VBG. 
     Herein, when both sides of formulas (3) and (4) are added to obtain VG, formula (5) can be obtained. In addition, when VD is obtained from the formula (4), formula (6) can be obtained. 
         VG −( VC+VBG−K 2+ K 4)/( K 1 −K 3)  formula (5)
 
         VD=K 1 ×VG+K 2− VBG   formula (6)
 
     By formulas (5) and (6), the grid bias voltage VG and the developing bias voltage VD are indicated by the use of the developing contrast voltage VC and the background voltage VBG. 
     In this manner, if the coefficients K 1  to K 4 , which indicate the relationship of the exposure portion potential VL and the non exposure portion potential VO relative to the grid bias voltage VG, are already known, by determining the developing contrast voltage VC and the background voltage VBG, it is possible to primarily calculate the grid bias voltage VG and the developing bias voltage VD by the use of formulas (5) and (6) accordingly. 
     Therefore, the developing contrast voltage VC and the background voltage VBG are determined on the basis of the coefficients K 1  to K 4  which show the relationship of the exposure portion potential VL and the non exposure portion potential VO relative to the grid bias voltage VG stored in the data memory portion  58  as known data in advance. 
     In Act S 2  of  FIG. 11 , the calculation portion  60  of the image maintenance control portion  54  reads the coefficients K 1  to K 4  stored in the data memory portion  58  as the known data in advance. In Act S 3 , the calculation portion  60  calculates the developing contrast voltage VC and the background voltage VBG serving as the standard based on the read coefficients K 1  to K 4 , and calculates the grid bias voltage VG and the developing bias voltage VD corresponding to the developing contrast voltage VC and the background voltage VBG serving as the calculated standard. 
     The main control portion  51  controls the respective portions of the image forming apparatus  1 , can carries out the developing treatment based on the developing contrast voltage VC and the background voltage VBG serving as the calculated standard, and the grid bias voltage VG and the developing bias voltage VD corresponding to these. That is, the main control portion  51  forms a high concentration pattern area (a high concentration patch) corresponding to the gradation data of the high concentration pattern and a low concentration pattern area (a low concentration patch) corresponding to the gradation pattern of the low concentration with a lower density than the high concentration pattern, on the photoconductor drum  12 . 
       FIG. 13  is a diagram showing a relationship of a pattern area and a toner attachment amount measurement portion  44  on the photoconductor drum. 
     In Act S 4  of  FIG. 11 , after the gradation patterns of the high concentration and the low concentration irradiated on the photoconductor drum  12  are developed by the developing apparatus  14 , the toner attachment amount measurement portion  44  measures the toner attachment amount on the photoconductor drum  12  in synchronous with the movement of the gradation pattern to a measurable position, and provides the comparison determination portion  61  with the measurement data of the toner attachment amount. 
     In Act S 5 , the comparison determination portion  61  acquires the measurement data of the toner attachment amount provided from the toner attachment amount measurement portion  44 , and reads a predetermined standard value of the toner attachment amount stored in the data memory portion  58  in advance. The comparison determination portion  61  refers to a predetermined standard value of the read toner attachment amount, compares the standard value based on the measurement data of the acquired toner attachment amount, and determined whether or not the toner permissible amount is in an allowable range. For example, it is determined whether or not the measurement values of the toner attachment amount of the high concentration pattern and the low concentration pattern of the toner are in a predetermined range. When it is determined that the measurement value of the toner attachment amount is not in the allowable range, based on the measurement data of the acquired toner attachment amount, the comparison determination portion  61  provides the calculation portion  60  with the comparison determination results. 
     In Act S 6 , the calculation portion  60  calculates the deviation based on the comparison determination results provided from the comparison determination portion  61 . In Act S 7 , the calculation portion  60  calculates a correction developing contrast voltage ΔVC and a correction background voltage ΔVBG based on the calculated deviation. The calculated correction developing contrast voltage ΔVC and correction background voltage ΔVBG are, for example, shown in  FIG. 12 . In Act S 8 , the calculation portion  60  calculates the developing contrast voltage VC and the background voltage VBG to be applied based on the developing contrast voltage VC becoming the standard, the background voltage VBG becoming the standard, and the calculated correction developing contrast voltage ΔVC and correction background voltage ΔVBG, and calculates the grid bias voltage VG and the developing bias voltage VD corresponding to these. Herein, setting the combination of the developing contrast voltage to be applied and the background voltage to (VC, VBG) or to (VC*, VBG*) differs according to the value of the deviation. 
     Thereafter, returning to Act S 4 , the treatments after Act S 4  are repeatedly carried out. That is, the main control portion  51  controls the respective portions of the image forming apparatus  1 , performs the developing treatment based on the calculated developing contrast voltage VC and the background voltage VBG, and the grid bias voltage VG and developing bias voltage VD corresponding to these, and forms the high concentration pattern area (the high concentration patch) and the low concentration pattern area (the low concentration patch) on the photoconductor drum  12 . The toner attachment amount is measured by the toner attachment amount measurement portion  44 , and the same treatment is repeated until it is determined that the toner attachment amount is in the allowable range compared to the predetermined standard value. As a result, it is possible to calculate the optimal developing contrast voltage VC and background voltage VBG, and grid bias voltage VG and developing bias voltage VD corresponding to these. 
     In the case of Yes in Act S 5 , that is; in the case where it is determined that the toner attachment amount is in the allowable range based on the measurement data of the acquired toner attachment amount, the comparison determination portion  61  provides the calculation portion  60  with the comparison determination results. The calculation portion  60  recognizes that the toner attachment amount is in the allowable range compared to the predetermined standard value based on the comparison determination results provided from the comparison determination portion  61 , and provides the developing voltage change portion  63  with the current developing contrast voltage VC and background voltage VBG, and the grid bias voltage VG and the developing bias voltage VD corresponding to these. 
     In Act S 9 , the developing voltage change portion  62  changes the developing contrast voltage VC, the background voltage VBG, the grid bias voltage VG, and the developing bias voltage VD, based on the calculation results provided from the calculation portion  60 . The developing voltage change portion  62  provides the data memory portion  58  with data relating to the changed developing contrast voltage VC, background voltage VBG, grid bias voltage VG, and developing bias voltage VD. 
       FIG. 14  is an exemplary carrier supply time correction coefficient database managed in a correction coefficient database  59  according to an embodiment. In a first row to fifth row of the carrier supply time correction coefficient database of  FIG. 14 , “relative humidity (%)”, “lower limit value (V)”, “upper limit value (V)”, “α”, and “β” are described. “Relative humidity (%)” is a value of the relative humidity in the image forming apparatus  1 . “Lower limit value (V)” is a lower limit value of the developing contrast voltage VC in which the electrification amount of the toner can be regarded to be in the range of the predetermined value. “Upper limit value (V)” is an upper limit value of the developing contrast voltage VC in which the electrification amount of the toner can be regarded to be in the range of the predetermined value. “α” is a correction coefficient of the carrier supply time in the low electrification area. “β” is a correction coefficient of the carrier supply time in the high electrification area. 
     In the case of a first line of the database of  FIG. 14 , “relative humidity (%)” is “up to 29.9(%)” and shows that the value of the relative humidity in the image forming apparatus  1  is “up to 29.9(%)”. “Lower limit value (V)” is “200 (V)” and shows that the lower limit value of developing contrast voltage VC in which the electrification amount of the toner can be regarded to be in the range of the predetermined value is “200 (V)”. “Upper limit value (V)” is “400 (V)” and shows that the upper limit value of developing contrast voltage VC in which the electrification amount of the toner can be regarded to be in the range of the predetermined value is “400 (V)”. “α” is “1.2” and shows that the correction coefficient of the carrier supply time in the low electrification area is “1.2”. “β” is “1.0” and shows that the correction coefficient of the carrier supply time in the high electrification area is “1.0”. 
     In the case of a second line of the database of  FIG. 14 , “relative humidity (%)” is “30.0 to 49.9(%)” and shows that the value of the relative humidity in the image forming apparatus  1  is “30.0 to 49.9(%)”. “Lower limit value (V)” is “180 (V)” and shows that the lower limit value of developing contrast voltage VC in which the electrification amount of the toner can be regarded to be in the range of the predetermined value is “180 (V)”. “Upper limit value (V)” is “380 (V)” and shows that the upper limit value of developing contrast voltage VC in which the electrification amount of the toner can be regarded to be in the range of the predetermined value is “380 (V)”. “α” is “1.5” and shows that the correction coefficient of the carrier supply time in the low electrification area is “1.5”. “β” is “1.0” and shows that the correction coefficient of the carrier supply time in the high electrification area is “1.0”. 
     In the case of a third line of the database of  FIG. 14 , “relative humidity (%)” is “45.0 to 59.9(%)” and shows that the value of the relative humidity in the image forming apparatus  1  is “45.0 to 59.9(%)”. “Lower limit value (V)” is “160 (V)” and shows that the lower limit value of developing contrast voltage VC in which the electrification amount of the toner can be regarded to be in the range of the predetermined value is “160 (V)”. “Upper limit value (V)” is “360 (V)” and shows that the upper limit value of developing contrast voltage VC in which the electrification amount of the toner can be regarded to be in the range of the predetermined value is “360 (V)”. “α” is “1.5” and shows that the correction coefficient of the carrier supply time in the low electrification area is “1.5”. “β” is “1.0” and shows that the correction coefficient of the carrier supply time in the high electrification area is “1.0”. 
     In the case of a fourth line of the database of  FIG. 14 , “relative humidity (%)” is “60.0 to 74.9(%)” and shows that the value of the relative humidity in the image forming apparatus  1  is “60.0 to 74.9(%)”. “Lower limit value (V)” is “140 (V)” and shows that the lower limit value of developing contrast voltage VC in which the electrification amount of the toner can be regarded to be in the range of the predetermined value is “140 (V)”. “Upper limit value (V)” is “340 (V)” and shows that the upper limit value of developing contrast voltage VC in which the electrification amount of the toner can be regarded to be in the range of the predetermined value is “340 (V)”. “α” is “1.7” and shows that the correction coefficient of the carrier supply time in the low electrification area is “1.7”. “β” is “1.0” and shows that the correction coefficient of the carrier supply time in the high electrification area is “1.0”. 
     In the case of a fifth line of the database of  FIG. 14 , “relative humidity (%)” is “75.0(%) or more” and shows that the value of the relative humidity in the image forming apparatus  1  is “75.0(%) or more”. “Lower limit value (V)” is “120 (V)” and shows that the lower limit value of developing contrast voltage VC in which the electrification amount of the toner can be regarded to be in the range of the predetermined value is “120 (V)”. “Upper limit value (V)” is “320 (V)” and shows that the upper limit value of developing contrast voltage VC in which the electrification amount of the toner can be regarded to be in the range of the predetermined value is “320 (V)”. “α” is “2.0” and shows that the correction coefficient of the carrier supply time in the low electrification area is “2.0”. “β” is “1.0” and shows that the correction coefficient of the carrier supply time in the high electrification area is “1.0”. 
       FIG. 15  is an exemplary flow chart showing an operation sequence of a carrier supply time control portion  55  according to an embodiment. 
     In Act S 16 , the carrier supply time correction coefficient setting portion  65  reads data relating to the developing contrast voltage VC stored in the data memory portion  58 . In Act S 17 , the environmental detection portion  45  detects the environment (temperature, relative humidity, etc.) in the image forming apparatus  1  according to the instructions of the main control portion  51 , creates the environmental detection signal, and provides the carrier supply time correction coefficient setting portion  65  with the signal. The environmental detection signal includes data relating to the environment in the image forming apparatus  1 . 
     In Act S 18 , the carrier supply time correction coefficient setting portion  65  refers to the carrier supply time correction coefficient database managed in the read correction coefficient database  59 , and sets the carrier supply time correction coefficient based on data relating to the read developing contrast voltage VC and the environmental detection signal provided from the environmental detection portion  45 . 
     Specifically, when the relative humidity is 35(%) and the developing contrast voltage VC is 160 (V), the carrier supply time correction coefficient corresponds to the second line in the database of  FIG. 14 . In addition, since the developing contrast voltage VC is in the low electrification area, the carrier supply time correction coefficient is “1.5”. As a result, it is possible to set the correction coefficient of the carrier supply time according to the toner electrification amount and the environment. 
     The carrier supply time correction coefficient setting portion  65  provides the carrier supply time calculation portion  64  with data of the set carrier supply time correction coefficient. 
     In Act S 19 , the carrier supply time calculation portion  64  acquires the carrier supply time correction coefficient data provided from the carrier supply time correction coefficient setting portion  65 , and calculates the carrier supply time after the correction according to the toner electrification amount based on the acquired carrier supply time correction coefficient data and the carrier supply time. That is, the carrier supply time calculation portion  64  calculates the value which is obtained by multiplying the carrier supply time by the carrier supply time correction coefficient, and provides the carrier supply time change portion  66  with the calculation results. In Act S 20 , the carrier supply time change portion  66  changes the carrier supply time based on the calculation results provided from the carrier supply time calculation portion  64 . 
     Furthermore, although the case where the electrification amount declines due to the deterioration of the carrier is described in the above-mentioned case, the present invention can also be applied to a case where the electrification amount increases due to the deterioration of the carrier. In this case, the control is performed so as to change the carrier supply time by the carrier supply time correction coefficient corresponding to the high electrification amount area. 
     In addition, although the carrier supply time correction coefficient corresponding to the low electrification amount area is set to values different from 1.0 in the above-mentioned case, the carrier supply time correction coefficient corresponding to the low electrification amount area and the high electrification amount area may be set to the values different from 1.0. 
     In addition, the toner attachment amount meter may measure the toner attachment amount on the intermediate transfer belt  10  as shown in  FIG. 1 , and may measure the toner attachment amount on the photoconductor drum  12 . 
     Furthermore, when the value of the developing contrast voltage VC deviates from the predetermined limit values (the upper limit value and the lower limit value), information showing that the maintenance is necessary may be output to stop the image forming apparatus. 
     In addition, although multiplication calculation is performed using the correction coefficient to correct the carrier supply time in the above-mentioned case, an adjustment calculation may be performed using the correction value. 
     In the image forming apparatus  1  shown in the present embodiment, with reference to carrier supply time correction coefficient database managed in the correction coefficient database  59 , on the basis of data relating to the developing contrast voltage VC changed by the image quality maintenance control treatment, and environmental data (data relating to the temperature or the relative humidity) included in the environmental detection signal provided from the environmental detection portion  45 , the carrier supply time correction coefficient is set. As a result, when the toner electrification amount greatly deviates from the range of the predetermined standard value, it is possible to correct the carrier supply time based on the set carrier supply time correction coefficient. Thus, it is possible to maintain the very satisfactory developer performance. 
     In addition, although, in the image forming apparatus  1  shown in the present embodiment, the area is divided into three sections (the optimal electrification area, the low electrification area, and the high electrification area) by the value of the developing contrast voltage VC, so that the correction coefficient is set to the different values for each sections, the present invention is not limited to the case, the area may be divided into two or four sections or more, and the appropriate correction coefficient may be calculated and set according to the value of the developing contrast voltage VC. In this case, at least one section is set to the appropriate electrification area. 
     In the present embodiment, when the image forming is performed by the color printer  1 , the page counter  81  cumulatively counts the image forming sheets and inputs the number of the sheets to the CPU  80 . When the detection result of the page counter  81  reaches a predetermined number of image forming sheets, the CPU  80  controls the third motor driver  88 . The third motor driver  88  drives the carrier supply auger  67  by a predetermined amount, thereby supplying the carrier in the carrier cartridge  64  to the case  50  by the predetermined amount. In the present embodiment, the capacity of the developer in the case  50  is 400 g, it is necessary to normally supply the carrier at the rate of 4 g whenever the number of print sheets reaches one thousand, and the time necessary for the supply is about 8 seconds. With respect to the supply time, the above-mentioned correction is performed and the carrier is supplied at the supply time after the correction. 
     As the carrier is supplied and the volume increases, the excessive developer is discharged from the developer discharge port  53 . As a result, in the developer  51  in the case  50 , the carrier of about 4 g is newly replaced. Thus, in the respective developing apparatuses  14 Y,  14 M,  14 C and  14 K, the deterioration of the carrier in the case  50  is suppressed. As a result, the carrier can always maintain the satisfactory property and can sufficiently electrify the toner. 
     In addition, the respective functions described in the above-mentioned embodiments may be constituted by the use of hardware and may be realized by reading a program, in which the respective functions are described, into a computer using software. Furthermore, the respective functions may be constituted by appropriately selecting any one of the software and hardware. 
     In addition, the respective functions can be realized by reading the program stored in a recording medium (not shown) into a computer. Herein, if the recording medium in the present embodiment is a recording medium that can record the program and can be read by the computer, the recording type may be any form. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.