Abstract:
An image forming device and method improves image quality by detecting the density of a pattern and adjusting the voltage applied to a developer supply member if necessary in response to the detected density. The pattern can be a medium duty tone pattern, which is relatively sensitive to environmental changes that can affect image density. The density detection pattern can be produced and detected after every predetermined number of sheets are printed, to maintain an acceptable image density during large print jobs. The pattern is formed on the surface of a transferring unit, which transfers the sheets.

Description:
CROSS REFERENCE 
     The present application is related to, claims priority from and incorporates by reference Japanese patent application number 2009-147368, filed on Jun. 22, 2009. 
     TECHNICAL FIELD 
     The present application relates to an image forming device and method, and more particularly to an image forming device and method for adjusting the density of images. 
     BACKGROUND 
     Conventionally, an image forming device, such as a printer, a copier, a facsimile machine and a multifunction machine, for example a color printer, has image forming units of colors of black, yellow, magenta and cyan. A photoreceptor drum is charged by a charge roller at each of the image forming units, a latent image is formed by a light emitting diode (LED) head exposing the photoreceptor drum, and a toner image of each color is formed by electrostatically adhering toner that is a developer on a developing roller. The toner is formed in a thin-layered manner on the developing roller for the latent image. 
     Then, each toner image is transferred onto a carried sheet in order by a transferring roller in accordance with a transferring belt running along with each image forming unit, and each of the color toner images are formed. Subsequently, the sheet is sent to a fuser, the color toner images are fused on the sheet at the fuser. The color image is formed. 
     With this kind of printer, it is necessary to detect the density of the toner image of each color in order to adjust the color image. In order to do so, the predetermined shaped image is formed as a density detecting pattern on the transferring belt by the toner of each color. Subsequently, the density of image part of each color that structures the density detecting pattern is detected, and the voltage that is applied to the developing roller, in other words, the developing voltage is corrected based on the detected density. Accordingly, density of each toner image can be corrected and the color image can be adjusted. (for example, see Japanese laid-open patent application number 2004-341100) 
     However, with the conventional printer, for example, when the toner is overly charged due to the temperature and moisture around the printer or the like, the density of the toner image of each color becomes high beyond the density range. As a result, the density of each toner image cannot be corrected by only correcting the developing voltage. The color image cannot be adjusted sufficiently; therefore, the image quality decreases. 
     The present application shows an image forming device that can solve the problems of the conventional printers, and can improve image quality even when the developer is overly charged due to the environmental changes. 
     SUMMARY 
     For the purpose, an image forming device of the present application includes an image carrier; an exposure device that forms a latent image on a surface of the image carrier; a developer carrier that develops the latent image and forms a developer image by attaching developer on the image carrier; a developer supplying member that supplies the developer to the developer carrier; a transferring member that transfers the developer image on a medium; a pattern forming processing part that forms a predetermined density detecting pattern on a pattern forming medium; a density detection processing part that detects a density of the density detecting pattern on the pattern forming medium; and a supply voltage changing processing part that changes a supply voltage that is applied to the developer supplying member. 
     In another view of the embodiment, an image forming method of the present application includes forming a latent image on a surface of an image carrier; developing the latent image and forming a developed image by attaching developer on the image carrier; supplying the developer to a developer carrier with a developer supplying member; transferring the developer image to a sheet, which is carried by a transferring unit; forming a predetermined density detecting pattern on a surface of the transferring unit; detecting a density of the density detecting pattern; and changing a supply voltage that is applied to the developer supplying member if the detected density is outside of a predetermined range. 
     With such a structure, a density detecting pattern formed with the developer is formed on the predetermined pattern forming medium, the density of the density detecting pattern is detected, and the supply voltage is changed based on the detected density. Accordingly, even when the developer is overly charged due to the environmental changes, the high image quality is maintained. In other words, the image quality is improved compared with the quality by the conventional devices. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a control block diagram of a printer of a first embodiment. 
         FIG. 2  illustrates a schematic diagram of the printer of the first embodiment. 
         FIG. 3  illustrates a flow diagram showing print operation of the printer of the first embodiment. 
         FIG. 4  illustrates a density detecting pattern on a transferring belt of the first embodiment. 
         FIG. 5  illustrates a control block diagram of the printer of a second embodiment. 
         FIG. 6  illustrates a first flow diagram showing print operation of the printer of the second embodiment. 
         FIG. 7  illustrates a second flow diagram showing the print operation of the printer of the second embodiment. 
         FIG. 8  illustrates a density detecting pattern on a transferring belt of the second embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Hereafter, the embodiments of the present application are explained in detail referring to the drawings. In this case, a color printer as an image forming device is explained. 
       FIG. 2  illustrates a schematic diagram of a printer of the first embodiment. 
     As shown in the figures, image forming parts Bk, Y, M and C for each of four colors of toners  14  (black, yellow, magenta and cyan) serve as a developer in the printer. LED heads  23 , serve as exposing devices that correspond to each of the image forming part Bk, Y, M and C. A transferring unit  21  is located below the image forming parts Bk, Y, M and C. Since the structures of each of the image forming parts Bk, Y, M and C are basically the same, only the image forming part Bk is explained. 
     The image forming part Bk includes an image forming unit  15 Bk for forming black toner images. The image forming unit  15 Bk includes a photoreceptor drum  11 , or image carrier, which uses an organic photoreceptor on its surface. Around the photoreceptor drum  11 Bk are a charge roller  12 Bk, or charge device, a developing roller  16 Bk, or developer carrier, and a cleaning blade  19 Bk, or cleaning member, which are arranged to contact the photoreceptor drum  11 Bk. On the image forming unit  15 Bk are a toner supplying roller  18 Bk and a developing blade  17 Bk, which are arranged to contact the developing roller  16 Bk. The toner supplying roller  18 Bk charges the toner  14 Bk that is supplied from the toner cartridge  13 Bk, or developer cartridge, and supplies the toner  14 Bk to the developing roller  16 Bk, or developer supplying member. The developing blade  17 Bk, which serves as a developer layer restricting member, forms a thin layer of the toner  14 Bk (namely a toner layer) that is supplied from the toner supplying roller  18 Bk, is arranged to contact the developing roller  16 . The developing blade  17  is arranged to press and contact the developing roller  16 Bk at the downstream side of the toner supplying roller  18 Bk considering the rotating direction of the developing roller  16 Bk. In the present embodiment, negative conductivity toner is used for the toner  14 Bk, and the average grain diameter of the pulverized shape is 8 μm by using a pulverization method. The toner  14 Bk is formed from polyester resin, coloring agent, charge controlling agent and release agent, and an additive agent (hydrophobic silica) is included. 
     The transferring unit  21  provides drive rollers  25   a  and  25   b , which serve as first and second carrying parts, a transferring belt  24 , which is movably provided in a tensioned state between the drive rollers  25   a  and  25   b  and is arranged to contact each of the photoreceptor drums  11 , and a transferring roller  22 , which serves as a transferring member that is opposed to each of the photoreceptor drums  11  through the transferring belt  24 . 
     Then, a cleaning blade  26 , or cleaning member, is arranged to scrape the toner  14  that is attached on the transferring belt  24 , and the toner  14  that is scraped by the cleaning blade  26  is collected as waste toner in a waste toner box  27 . A density sensor  28 , or density detecting part, which is configured with a light emitting part and a light receiving part, is; located downstream of a drive roller  25   a  in a running direction of the transferring belt  24 . 
     Then, at the downstream side of the image forming unit C in the carrying direction of sheet P, or medium, a fuser  35 , or fusing device, which includes first and second fusing rollers R 1  and R 2 . 
     With the printer discussed above, at the image forming units  15 Bk,  15 Y,  15 M and  15 C (hereinafter referred to as image forming units  15 ), photoreceptor drums  11 Bk,  11 Y,  11 M, and  11 C (photoreceptor drums  11 ) are charged by the charge rollers  12 Bk,  12 Y,  12 M, and  12 C (charge rollers  12 ). Electrostatic latent images are formed by the LED heads  23 Bk,  23 Y,  23 M, and  23 C (LED heads  23 ). The thin layered toners  14 Bk,  14 Y,  14 M, and  14 C (toner  14 ) on the developing rollers  16 Bk,  16 Y,  16 M, and  16 C (developing rollers  16 ) are statically adhered to the electrostatic latent image so that toner images of different colors are formed. 
     Subsequently, the toner image of each color is transferred to a sheet P in order by the transferring rollers  22 . The sheet P is carried along with the running transferring belt  24  so that a four color toner image is formed. The sheet P is sent to the fuser  35 , and at the fuser  35 , the color toner image is fused on the sheet P to complete the color image. 
     Next, a controller of the afore-mentioned printer is explained. 
       FIG. 1  illustrates a control block diagram of a printer of a first embodiment. 
     In the figure, 10 designates a printer, and 40 designates a print controlling part, or a first controller. The print controlling part  40  entirely controls the printer  10 . In the print controlling part  40 , an interface part  41 , which receives print data from a host computer  52 , or host device, a memory  42 , or memory device, a CPU  43 , which performs a density correction calculation and serves as a second controller, and various kinds of sensors  44 , such as a sensor, or medium detecting part, that detects the sheet P ( FIG. 2 ) are connected. The memory  42  includes a ROM  42   a , or first memory part, a RAM  42   b , or second memory part, and a pattern recording part  42   c , or third memory part, in which a later-mentioned density detecting pattern is recorded. 
     In the print controlling part  40 , a process controlling part  45 , or third controller, a developing voltage controlling part  46 , or fourth controlling part, a supply voltage controlling part  47 , or fifth controller, a layer forming voltage controlling part  48 , or sixth controller, a motor controlling part  49 , or seventh controlling part, and a density sensor controlling part  51  are connected. The process controlling part  45  controls a charge voltage that is applied to the charge roller  12 , a head drive voltage applied to the LED heads  23 , a transfer voltage applied to the transferring rollers  22 , and the other voltages. The developing voltage controlling part  46  controls the developing voltage that is applied to the developing rollers  16 . The supply voltage controlling part  47  controls the supply voltage that is applied to toner supplying rollers  18 Bk,  18 Y,  18 M, and  18 C (toner supplying rollers  18 ). The layer forming voltage controlling part  48  controls the layer forming voltage that is applied to developing blades  17 Bk,  17 Y,  17 M, and  17 C (as developing blades  17 ). Moreover, a developing unit  20  is configured with the developing rollers  16 , the developing blades  17  and toner supplying rollers  18 . 
     Each of the motors  50 , or driving parts, is driven by the motor controlling part  49  to rotate the drive rollers  25   a ,  25   b  or the like of the developing rollers  16 , toner supplying rollers  18 , photoreceptor drums  11 , the charge rollers  12 , transferring rollers  22 Bk,  22 Y,  22 M, and  22 C (transferring rollers  22 ) and the transferring belt  24 . The sensor output of the density sensor  28  is read by the density sensor controlling part  51 . 
     Next, the print operation of the printer  10  of the aforementioned structure is explained. 
       FIG. 3  illustrates a flow diagram showing print operation of a printer of the first embodiment and  FIG. 4  illustrates a density detecting pattern on the transferring belt  24  of the first embodiment. 
     First, when the power of the printer  10  is turned on initially, a drive processing part of the print controlling part  40 , which is not shown in the figures, performs drive processing, and drives each of motors  50  by the motor controlling part  49 . A voltage application processing part of the print controlling part  40 , which is not shown in the figures, performs voltage application processing. In the processing, the voltage application processing part reads the setting values (bias setting) of the developing voltage, the supply voltage, the layer forming voltage or the like from ROM  42   a . The developing voltage controlling part  46  applies a developing voltage of −200[V] to the developing rollers  16 . The supply voltage controlling part  47  applies a supply voltage of −300[V] to the toner supplying rollers  18 . The layer forming voltage controlling part  48  applies a layer forming voltage of −300[V] to the developing blades  17 . The process controlling part  45  applies the charge voltage of −1000[V] to the charge rollers  12 . Accordingly, the surface of the photoreceptor drum  11  is uniformly charged, and the transfer voltage of +4000 [V] is applied to the transferring rollers  22 . 
     Incidentally, with the printer  10  of the aforementioned structure, it is necessary to detect the density of toner images of each color in order to adjust the color image. Therefore, images of predetermined shapes are formed as a density detecting pattern by each color of the toners  14  ( 14 Bk,  14 Y,  14 M, and  14 C) on the transferring belt  24 . Then, the density of each color pattern that forms the density detecting pattern is detected, and the developing voltage is corrected based on the detected density. Accordingly, the density of each toner image can be corrected and color image can be adjusted. 
     However, for example, when the toners  14  are overly charged by environmental changes such as the temperature and humidity around the printer  10 , and the density of the toner image of each color becomes high beyond the predicted density range, the density of each toner image cannot be corrected sufficiently by only correcting the developing voltage, the color image cannot be adjusted sufficiently, and the image quality deteriorates. 
     Therefore, as for the present application, the supply voltage and the developing voltage are corrected based on the density of the pattern of each color. 
     Because of this, the pattern forming processing part  40 , which is not shown in the figures, performs pattern forming processing, drives each of the LED heads  23  by the process controlling part  45 , reads the density detecting pattern from the pattern recording part  42   c , exposes the photoreceptor drum  11  by irradiating light that corresponds to the image data that forms the density detecting pattern on each of the photoreceptor drums  11 , and forms the electrostatic latent image of the pattern in each color. Accordingly, the electrostatic latent image is developed by each of the developing rollers  16 , the toner images of patterns in each color are formed, and the patterns for detecting density are formed at predetermined places on the running transferring belt  24  by each of the transferring rollers  22  as shown in  FIG. 4 . The transferring belt  24  functions as a pattern forming medium that forms the density detecting pattern. 
     Then, a black pattern pBK, a yellow pattern pY, a magenta pattern pM and a cyan pattern pC are formed as the patterns in each color. In this case, the density detecting pattern is formed in two duty types, a high tone duty and middle tone duty. In the present application, the high tone duty (or 100% duty), or first pattern, is formed from each color to form patterns pBK 100 , pY 100 , pM 100  and pC 100 . The middle tone duty (or 70% duty), or second pattern, is formed from each color to form patterns pBK 70 , pY 70 , pM 70  and pC 70 . Herein, the density detecting pattern includes image data for forming the developer with a predetermined area ratio on a two dimensional plan region under a standard print environment. The term “duty” refers to a developer formed area ratio where the developer is formed on the plan region using the image data of the density detecting pattern. A 100% duty of the density detecting pattern means that the developer is formed with 100% area ratio of developer (or covers all the plan region) and with 0% area ratio of non-developer on the plan region of the density detecting pattern. A 70% duty of the density detecting pattern means that the developer is formed with 70% area ratio of developer and with 30% area ratio of non-developer. 
     Next, the density detecting processing part of the print controlling part  40  (not illustrated) performs density detection processing, reads the sensor output of the density sensor  28 , detects the densities pBk 100   a , pY 100   a , pM 100   a , pC 100   a , pBk 70   a , pY 70   a , pM 70   a  and pC 70   a  of the patterns pBk 100 , pY 100 , pM 100 , pC 100 , pBk 70 , pY 70 , pM 70  and pC 70 , respectively, and records an optical density value (hereafter O.D. value) of the densities pBk 100   a , pY 100   a , pM 100   a , pC 100   a , pBk 70   a , pY 70   a , pM 70   a  and pC 70   a  into the RAM  42   b.    
     However, when the toner  14  is overly charged, and the potential vt of the toner layer (toner layer potential) on the development roller  16  becomes negatively greater, a dot becomes large, and the densities pBk 70   a , pY 70   a , pM 70   a  and pC 70   a  also become high. In this case, the densities pBk 70   a , pY 70   a , pM 70   a  and pC 70   a  become unstable, and their tones become low, causing the image quality to deteriorate. 
     Therefore, in the present application, when the densities pBk 70   a , pY 70   a , pM 70   a  and pC 70   a  become high, the supply voltage is changed so that the toner  14  is adequately charged. 
     For such a purpose, a correct voltage calculation processing part of the CPU  43  (not illustrated) performs correct voltage calculation processing, reads the O.D. value of the densities pBk 70   a , pY 70   a , pM 70   a  and pC 70   a , and calculates the correct voltage, in other words, the supply voltage correction value, in order to change the supply voltage. The correct voltage is preset in corresponding to each of the O.D. values. 
     Since the patterns pBk 70 , pY 70 , pM 70  and pC 70  that are formed with 70% duty are relatively more sensitive to the environmental changes, the densities pBk 70   a , pY 70   a , pM 70   a  and pC 70   a  easily change when environment conditions change. This makes it easier to judge whether or not the density is stable. Therefore, in the present application, the supply voltage correction value is calculated based on the densities pBk 70   a , pY 70   a , pM 70   a  and pC 70   a.    
     For the middle tone duty, the patterns pBk 70 , pY 70 , pM 70  and pC 70  are formed with 70% duty; however, it is also practical to form the patterns with a duty of equal to or more than 30% and with a duty of equal to or less than 80% duty for the middle tone duty. 
     Table 1 shows the relationship between the O.D. value, which represents the densities pBk 70   a , pY 70   a , pM 70   a  and pC 70   a , and the supply voltage correction value. 
     
       
         
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
             
             
               
                 Potential of Toner  
                 −60 ≦ vt 
                 −90 &lt; vt &lt; −60 
                 Vt ≦ −90 
               
               
                 Layer [V] 
                   
                   
                   
               
               
                 O.D. Value 
                 O.D. ≦ 1.0 
                 1.0 &lt; O.D. &lt; 1.3 
                 1.3 ≦ O.D. 
               
               
                 Supply Voltage 
                 0 
                 20 
                 40 
               
               
                 Correction Value [V] 
               
               
                   
               
             
          
         
       
     
     In this case, each O.D. value is shown corresponding to the potential vt of the toner layer on the developing roller  16 . The supply voltage correction value is the value to adequately charge the toner  14  and is the correction value to correct each O.D. value. 
     For example, when the potential vt of the toner layer is −60[V]≦vt, the toner  14  is adequately charged, the O.D. value becomes O.D.≦1.0 and is stabilized, and the supply voltage correction value is set as +0 [V]. 
     When the potential vt of the toner layer is −90[V]&lt;vt&lt;−60 [V], the toner  14  is overly charged, the O.D. value becomes 1.0&lt;O.D.&lt;1.3 and becomes unstable, and the supply voltage correction value is set as +20 [V]. 
     Then, when the potential vt is vt≦−90[V], the toner  14  is further overly charged, the O.D. value becomes 1.3≦O.D. and becomes more unstable, and the supply voltage correction value is set as +40 [V]. 
     Therefore, the O.D. value of the densities pBk 70   a , pY 70   a , pM 70   a  and pC 70   a  and the supply voltage correction value are recorded in the ROM  42   a  in correspondence. The correct voltage calculation processing part judges whether or not the O.D. value is equal to or less than 1.0 when reading the O.D. value. When the O.D. value is equal to or less than 1.0, the supply voltage correction value that corresponds to the O.D. value is calculated by reading from the ROM 42   a , and recorded in the RAM  42   b . In this case, the voltage applied to the photoreceptor drums  11  is constant, and the layer forming voltage is made to be equal to the supply voltage. 
     Subsequently, the supply voltage change processing part (not illustrated) in the CPU43 performs the supply voltage change processing, reads the supply voltage correction value from the RAM  42   b , changes the supply voltage based on the supply voltage correction value, and records the changed supply voltage into the RAM  42   b . In the present application, the supply voltage correction value is added to a standard supply voltage. For example, when the supply voltage correction value is +20 [V], since the standard voltage is −300 [V], the supply voltage after the change is −280 [V],
 
−300 [V]+20 [V]=−280 [V].
 
     Then, the density detecting pattern is formed according to the supply voltage after the change, and the calculation of the supply voltage by the supply voltage correction value is repeated until the O.D. value becomes a normal value of equal to or less than 1.0. In the present application, the calculation is repeated until the O.D. value becomes 1.0. When the O.D. value becomes 1.0, the developing voltage corresponds to the supply voltage. 
     In this case, when the O.D. value of the densities pBk 70   a , pY 70   a , pM 70   a  and pC 70   a  is approximately 1.0, the tone can be stabilized (stable). The O.D. value of the densities pBk 70   a , pY 70   a , pM 70   a  and pC 70   a  is defined as densities at the time of when the patterns pBk 70 , pY 70 , pM 70  and pC 70  are formed with 70% of duty 
     Therefore, the developing voltage change processing part of the voltage apply processing part performs developing voltage change processing, calculates a optimum value of the developing voltage based on the O.D. value of the densities pBk 100   a , pY 100   a , pM 100   a  and pC 100   a , changes the developing voltage to the optimum value, and records it to the RAM  42   b.    
     In this case, it is defined that the control target value is Tod (Tod=1.5), the optimum value of the developing voltage is Vdk, and a coefficient that is a change ratio of the developing voltage at the time of the supply voltage changed is k (k=0.003). Herein, the control target value is a target of the O.D. value of the densities pBk 100   a , pY 100   a , pM 100   a  and pC 100   a . Since the standard developing voltage is −200 [V], the optimum value Vdk is calculated by performing linear interpolation: 
     
       
         
           
             
               
                 
                   
                     
                       Vdk 
                       = 
                       
                         
                           - 
                           200 
                         
                         + 
                         
                           
                             ( 
                             
                               O 
                               . 
                               D 
                               . 
                               
                                 - 
                                 Tod 
                               
                             
                             ) 
                           
                           / 
                           k 
                         
                       
                     
                     ) 
                   
                   ⁡ 
                   
                     [ 
                     V 
                     ] 
                   
                 
               
             
             
               
                 
                   
                     
                       = 
                       
                         
                           - 
                           200 
                         
                         + 
                         
                           
                             ( 
                             
                               O 
                               . 
                               D 
                               . 
                               
                                 - 
                                 1.5 
                               
                             
                             ) 
                           
                           / 
                           0.003 
                         
                       
                     
                     ) 
                   
                   ⁡ 
                   
                     [ 
                     V 
                     ] 
                   
                 
               
             
           
         
       
     
     For example, when the O.D. value is 1.6, the optimum value Vdk is −167 [V] (Vdk=−167 [V]). 
     The supply voltage change processing part calculates the optimum value Vsk and records it on the RAM  42   b . When the supply voltage correction value is +20 [V], the difference between the supply voltage and the developing voltage is:
 
−200 [V]−(−300 [V]+20 [V])=+80 [V]).
 
Therefore, it becomes −247 [V] (Vsk=Vdk−80 [V]=−247 [V]).
 
     Accordingly, when the O.D. value of the densities pBk 70   a , pY 70   a , pM 70   a  and pC 70   a  is 1.0, and the O.D value of the densities pBk 100   a , pY 100   a , pM 100   a  and pC 100   a  is 1.5, the supply voltage can be the optimum value Vsk, and the developing voltage can be the optimum value Vdk. 
     Then, the print processing part of the print controlling part  40  (not shown) applies the developing voltage of the optimum value Vdk to the developing roller  16  and applies the supply voltage of the optimum value Vsk to the supply roller  18  by performing print processing. 
     Accordingly, even when the toner  14  is overly charged due to the environmental changes, the tone can be high, and image quality can be improved. 
     Since the O.D. value is read based on the density detecting pattern, and the supply voltage becomes the optimum value Vsk based on the O.D. value, the densities of pBk 100   a , pY 100   a , pM 100   a , pC 100   a , and the densities of pBk 70   a , pY 70   a , pM 70   a  and pC 70   a  can be stabilized. As a result, the tone is higher, and the image quality is improved. 
     Next, flow diagrams are explained. At S 1 , a power is tuned on. At S 2 , the density detecting pattern is formed. At S 3 , the density sensor  28  detects the densities of pBk 100   a , pY 100   a , pM 100   a , pC 100   a , pBk 70   a , pY 70   a , pM 70   a  and pC 70   a . At S 4 , the densities pBk 100   a , pY 100   a , pM 100   a , pC 100   a , pBk 70   a , pY 70   a , pM 70   a  and pC 70   a  are recorded in the RAM  42   b . At S 5 , the processing judges whether or not the O.D. value is equal to or less than 1.0. When the O.D. value is equal to or less than 1.0 (Yes), the processing proceeds to S 8 . When the O.D. value is greater than 1.0 (No), the processing proceeds to S 6 . At S 6 , the supply voltage correction value is calculated. At S 7 , the supply voltage is changed, and returns to S 2 . At S 8 , the optimum value of the developing voltage is calculated. At S 9 , the developing voltage is changed. At S 10 , performs the print processing and ends the processing. 
     Subsequently, the second embodiment of the present application is explained. The same numbers are used for parts that have the same structure as the first embodiment. The effects of the second embodiment that derive from the structure of the first embodiment are the same as those of the first embodiment. 
       FIG. 5  illustrates a control block diagram of a printer of the second embodiment. 
     In the figure,  53  is a print counter that counts the number of print sheets (or the amount of print sheets), and the print counter  53  counts the number of print sheet as print index that shows cumulative printing amount, and sends the count value to the print controlling part  40  as the first controller. The print counter  53  increments the count number every time that the print operation is performed per sheet of the sheet P of a lateral feeding of an A4 size sheet (see  FIG. 2 ). 
     Next, the print operation of the printer  10  of the aforementioned structure is explained. 
       FIG. 6  illustrates a first flow diagram showing print operation of the printer of the second embodiment.  FIG. 7  illustrates a second flow diagram showing print operation of the printer of the second embodiment.  FIG. 8  illustrates a density detecting pattern on a transferring belt of the second embodiment. 
     When the power of the printer  10  is turned on initially, as in the first embodiment, the pattern forming processing part of the print controlling part  40  forms the density detecting pattern as shown in  FIG. 8 . 
     The density detecting patterns are formed from two types: the first pattern of each color (pBK 100 , pY 100 , pM 100  and pC 100 ) is formed with 100% duty, and the second pattern of each color (pBK 70 , pY 70 , pM 70  and pC 70 ) is formed with 70% of duty. 
     First, the density detection processing part detects the densities pBk 100   a , pY 100   a , pM 100   a , pC 100   a , pBk 70   a , pY 70   a , pM 70   a  and pC 70   a  of the patterns pBk 100 , pY 100 , pM 100 , pC 100 , pBk 70 , pY 70 , pM 70  and pC 70 , and memorizes the O.D. value of the densities pBk 100   a , pY 100   a , pM 100   a , pC 100   a , pBk 70   a , pY 70   a , pM 70   a  and pC 70   a  in the RAM  42   b.    
     Subsequently, the correct voltage calculation processing part of the CPU  43 , which serves as a calculation device and as the second controlling part, reads each of the O.D. values, and calculates the correct voltage, in other words, the supply voltage correction value, in order to change the supply voltage that is preset in correspondence to each of the O.D. values. 
     Next, the correct voltage calculation processing part of the CPU  43  reads the supply voltage correction value from the RAM  42   b , changes the supply voltage based on the supply voltage correction value, and records the supply voltage in the RAM  42   b.    
     Then, the density detecting pattern is formed again by the newly corrected supply voltage. The calculation of the supply voltage by the supply voltage correction value is repeated until the O.D. value becomes a normal value, which is equal to or less than 1.0. In the present application, the calculation is repeated until the O.D. value becomes 1.0. When the O.D. value is 1.0, the developing voltage is changed in corresponding to the supply voltage. 
     Therefore, the developing voltage change processing part of the voltage apply processing part calculates the optimum value Vdk based on the O.D. value of the densities of pBk 100   a , pY 100   a , pM 100   a  and pC 100   a , changes the developing voltage to the optimum value Vdk, and records it in the RAM  42   b.    
     The voltage change processing part calculates the optimum value Vsk of the supply voltage, changes the supply voltage to the optimum voltage Vsk, and records it in the RAM  42   b.    
     Accordingly, when the O.D. value of the densities pBk 70   a , pY 70   a , pM 70   a  and Pc 70   a  become 1.0, and when the densities pBk 100   a , pY 100   a , pM 100   a  and pC 100   a  become 1.5, the supply voltage can be the optimum value Vsk, and the developing voltage can be the optimum value Vdk. 
     Subsequently, the print processing part of the print controlling part  40  performs the printing operation by applying the optimum value Vdk of the developing voltage to the developing rollers  16  as the developer carrier, and by applying the optimum value Vsk of the supply voltage to the toner supplying rollers  18  as the developer supplying member. 
     Accordingly, while the printing operation is being repeated, the print counter  53  counts the print sheets, sends the count value to the print controlling part  40 , and the print controlling part  40  records the count value in the RAM  42   b.    
     Consequently, the print amount judging processing part reads the count value and judges whether the count value exceeds 500 that is a threshold by performing print amount judging processing. 
     Then, when the count value exceeds 500 that is the threshold value of the count value, the pattern forming processing part of the print controlling part  40 , as shown in  FIG. 8 , forms the density detecting pattern between sheets P on the transferring belt  24  as a belt member. In other words, the density detecting pattern is formed on an area of the transferring belt that does not correspond to an area occupied by a sheet, or medium, on which images are being formed. 
     In this case, the density detecting pattern is configured with patterns of each color pBk 71 , Py 71 , pM 1  and pC 71  that are formed with 70% duty. 
     Next, the density detecting processing part of the print controlling part  40  detects the densities pBk 71   a , pY 71   a , pM 71   a  and pC 71   a  of the patterns pBk 71 , pY 71 , pM 1  and pC 71 , and records the O.D. value of the densities pBk 71   a , pY 71   a , pM 71   a  and pC 71   a  in the RAM  42   b.    
     Subsequently, the correct voltage calculation processing part of the CPU  43  reads each of the O.D. values, calculates the supply voltage correction value that is predetermined as in the first embodiment in correspondence with each of the O.D. values and records it in the RAM  42   b.    
     Next, the supply voltage change processing part of the CPU  43  reads the supply voltage correction value from the RAM  42   a , changes the supply voltage based on the supply voltage correction value, and records it in the RAM  42   b.    
     Then, the density detecting pattern is formed by the supply voltage that has been changed, and the calculation of the supply voltage by the supply voltage correction value is repeated until the O.D. value becomes normal value, which is equal to or less than 1.0. In the present application, the calculation is repeated until the O.D. value becomes 1.0. When the O.D. value is 1.0, the supply voltage change processing part clears the count value, and the print processing part of the print controlling part  40  prints the printing operation by applying the changed supply voltage to the toner supplying roller  18 . 
     In the present embodiment, every time the count value exceeds 500 and the number of the print sheets exceeds 500, the density detecting pattern is formed between sheets P, and the densities pBk 71   a , pY 71   a , pM 71   a  and pC 71   a  of the patterns pBk 71 , pY 71 , pM 1  and pC 71  are detected. Consequently, the supply voltage can be changed (adjusted) based on the densities pBk 71   a , pY 71   a , pM 71   a  and pC 71   a  when the toner  14 , as a developer, is overly charged while the printing operation continues to be performed for a long period. 
     Accordingly, the tone is higher, and the image quality is improved. 
     Since the O.D. value is read based on the density detecting pattern, and the supply voltage is made the optimum value based on the O.D. value, the densities pBk 71   a , pY 71   a , pM 71   a  and pC 71   a  can be stabilized. As a result, the tone is higher and the image quality is improved. 
     Next, the flow diagrams are explained referring to  FIGS. 6 and 7 . At S 21 , power is tuned on. At S 22 , the density detecting pattern is formed. At S 23 , the density sensor  28  detects the densities pBk 100   a , pY 100   a , pM 100   a , pC 100   a , pBk 70   a , pY 70   a , pM 70   a  and pC 70   a . At S 24 , the densities pBk 100   a , pY 100   a , pM 100   a , pC 100   a , pBk 70   a , pY 70   a , pM 70   a  and pC 70   a  are recorded in the RAM  42   b . At S 25 , the processing judges whether or not the O.D. value is equal to or less than 1.0. When the O.D. value is equal to or less than 1.0 (Yes), the processing proceeds to S 29 . When the O.D. value is greater than 1.0 (No), the processing proceeds to S 26 . At S 26 , the supply voltage correction value is calculated. At S 27 , the supply voltage is changed, and returns to S 22 . At S 28 , the optimum value of the developing voltage is calculated. At S 29 , the developing voltage is changed. At S 30 , the processing judges whether or not the count value exceeds 500. When the count value exceeds 500 (Yes), the processing proceeds to S 31 , and when the count value is equal to or less than 500 (No), the processing proceeds to S 38 . At S 31 , the density detecting pattern is formed. At S 32 , the density sensor  28  detects the densities pBk 71   a , pY 71   a , pM 71   a  and pC 71   a . At S 33 , the densities pBk 71   a , pY 71   a , pM 71   a  and pC 71   a  are recorded in RAM  42   b . At S 34 , the processing judges whether or not the O.D. value is equal to or less than 0.1. When the O.D. value is equal to or less than 1.0 (Yes), the processing proceeds to S 37 , and when the O.D. value is greater than 0.1 (No), the processing proceeds to S 35 . At S 35 , the optimum value of the supply voltage is calculated. At S 36 , the supply voltage is changed. At S 37 , the count value is cleared. At S 38 , the print processing is performed, and then the processing proceeds to the end. 
     According to each of the embodiments, the tandem type printer that directly transfers a toner image on the sheet P is employed. However, the present application can be applied to an intermediate transferring method printer. 
     According to each of the embodiments, the color printer is explained as the embodiments. However, the present application can be applied to a copy machine, facsimile machine, multifunction machine or the like. 
     Moreover, the present application is not limited to each of the aforementioned embodiments, and can be modified, and should not limit the scope of the present application.