Patent Publication Number: US-9835989-B2

Title: Image forming apparatus

Description:
INCORPORATION BY REFERENCE 
     The present application claims priority under 35 U.S.C. §119 to Japanese Patent Application No. 2016-007081, filed on Jan. 18, 2016. The contents of this application are incorporated herein by reference in their entirety. 
     BACKGROUND 
     The present disclosure relates to an image forming apparatus. 
     It is known that a value of a current flowing through a transfer roller in an image forming apparatus is measured. 
     For example, a generally known image forming apparatus determines a resistance value of a transfer roller from a value of a current detected by a detection circuit. This image forming apparatus includes the detection circuit, a constant voltage circuit and a controller. The detection circuit detects a value of a current flowing through the transfer roller. The constant voltage circuit performs constant voltage control of a voltage that is applied to the transfer roller. Further, the resistance value of the transfer roller is determined from the value of the current detected by the detection circuit when the constant voltage control is performed. The controller controls a transfer voltage based on the determined resistance value. 
     Through the above image forming apparatus, an appropriate transfer voltage can be applied. 
     SUMMARY 
     An image forming apparatus of the present disclosure forms an image on a recording medium. The image forming apparatus includes a specified number of developing devices, the specified number of primary transfer rollers, an intermediate transfer belt, voltage applicators, a current detector, and a voltage controller. The specified number is two or more. The specified number of developing devices each form a toner image on a corresponding one of the specified number of photosensitive drums. The specified number of primary transfer rollers are each located opposite to a corresponding one of the specified number of photosensitive drums. The intermediate transfer belt is held between the specified number of photosensitive drums and the specified number of primary transfer rollers. The voltage applicators each apply a voltage to a corresponding one of the specified number of primary transfer rollers. The current detector detects a total current value that is a sum of values of currents flowing through the specified number of primary transfer rollers. The voltage controller controls voltage values of voltages that the voltage applicators apply to the specified number of primary transfer rollers. The voltage controller causes a detection voltage to be applied to one primary transfer roller of the specified number of primary transfer rollers while causing a voltage having the same polarity as the detection voltage to be applied to all other of the primary transfer rollers. The detection voltage is a voltage that is applied to detect a resistance value of the one primary transfer roller and that has a predetermined voltage value. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view illustrating a configuration of an image forming apparatus according to an embodiment of the present disclosure. 
         FIG. 2  is a side view illustrating configurations of an image forming unit and a transfer section illustrated in  FIG. 1 . 
         FIG. 3  is a side view illustrating a configuration of a power supply section illustrated in  FIG. 1 . 
         FIG. 4  is a diagram illustrating a configuration of a controller illustrated in  FIG. 1 . 
         FIGS. 5A to 5D  are graphs showing an example of operation of a voltage controller when the voltage value of a second voltage VL 2  is the same as the voltage value of a first voltage VL 1 .  FIG. 5A  is a graph showing variation of an applied voltage applied to a primary transfer roller for Y color.  FIG. 5B  is a graph showing variation of an applied voltage applied to a primary transfer roller for C color.  FIG. 5C  is a graph showing variation of an applied voltage applied to a primary transfer roller for M color.  FIG. 5D  is a graph showing variation of an applied voltage applied to a primary transfer roller for K color. 
         FIG. 6  is a graph showing a relationship between voltage values of a voltage applied by a voltage applicator illustrated in  FIG. 3  and current values detected by a current detector. 
         FIG. 7  is a flowchart illustrating operation of the controller illustrated in  FIG. 4 . 
         FIG. 8  is a flowchart illustrating the operation of the controller illustrated in  FIG. 4 . 
         FIGS. 9A to 9D  are graphs showing an example of operation of the voltage controller when the voltage value of the second voltage VL 2  is greater than the voltage value of the first voltage VL 1 .  FIG. 9A  is a graph showing variation of an applied voltage applied to the primary transfer roller for Y color.  FIG. 9B  is a graph showing variation of an applied voltage applied to the primary transfer roller for C color.  FIG. 9C  is a graph showing variation of an applied voltage applied to the primary transfer roller for M color.  FIG. 9D  is a graph showing variation of an applied voltage applied to the primary transfer roller for K color. 
     
    
    
     DETAILED DESCRIPTION 
     The following describes an embodiment of the present disclosure with reference to the drawings ( FIGS. 1 to 9D ). Elements that are the same or substantially equivalent are indicated by the same reference signs in the drawings and explanation thereof is not repeated. 
     First, an image forming apparatus  1  according to the present embodiment will be described with reference to  FIG. 1 .  FIG. 1  is a diagram illustrating a configuration of the image forming apparatus  1  according to the present embodiment. The image forming apparatus  1  according to the present embodiment is a color copier. 
     The image forming apparatus  1  forms an image on paper P. The image forming apparatus  1  includes a housing  10 , a paper feed section  2 , a conveyance section L, a toner replenishment unit  3 , an image forming unit  4 , a transfer section  5 , a power supply section  6 , a fixing section  7 , an ejection section  8 , and a controller  9 . The paper P corresponds to an example of what is referred to as “a recording medium”. 
     The paper feed section  2  is disposed in a lower location of the housing  10  and feeds the paper P to the conveyance section L. The paper feed section  2  can accommodate a plurality of sheets of the paper P. The paper feed section  2  feeds the paper P to the conveyance section L one uppermost sheet of the paper P at a time. 
     The conveyance section L conveys the paper P fed by the paper feed section  2  to the ejection section  8  through the transfer section  5  and the fixing section  7 . 
     The toner replenishment unit  3  includes four toner cartridges  3   y ,  3   c ,  3   m , and  3   k  which are containers for supplying toners to the image forming unit  4 . The toner cartridge  3   y  contains a yellow toner. The toner cartridge  3   c  contains a cyan toner. The toner cartridge  3   m  contains a magenta toner. The toner cartridge  3   k  contains a black toner. 
     The image forming unit  4  includes four image forming sections  4   y ,  4   c ,  4   m , and  4   k . The yellow toner is supplied from the toner cartridge  3   y  to the image forming section  4   y . The cyan toner is supplied from the toner cartridge  3   c  to the image forming section  4   c . The magenta toner is supplied from the toner cartridge  3   m  to the image forming section  4   m . The black toner is supplied from the toner cartridge  3   k  to the image forming section  4   k . A configuration of the image forming unit  4  will be described further below with reference to  FIG. 2 . 
     The transfer section  5  includes an intermediate transfer belt  54 . The image forming unit  4  forms toner images on the intermediate transfer belt  54 , and the transfer section  5  transfers the toner images onto the paper P. A configuration of the transfer section  5  will be described further below with reference to  FIG. 2 . 
     The power supply section  6  applies transfer voltages to the transfer section  5 . The power supply section  6  also detects values of transfer currents flowing through the transfer section  5 . A configuration of the power supply section  6  will be described further below with reference to  FIG. 3 . 
     The fixing section  7  includes a heating roller  71  and a pressure roller  72  as a pair of rollers for fixing the toner images transferred onto the paper P by the transfer section  5 . The heating roller  71  and the pressure roller  72  apply heat and pressure respectively to the paper P. Through the above, the fixing section  7  fixes the unfixed toner images transferred onto the paper P by the transfer section  5 . The ejection section  8  ejects the paper P having the toner images fixed thereon out of the apparatus. 
     The controller  9  controls operation of the image forming apparatus  1 . A configuration of the controller  9  will be described further below with reference to  FIG. 4 . 
     Next, the configurations of the image forming unit  4  and the transfer section  5  will be described with reference to  FIG. 2 .  FIG. 2  is a side view illustrating the configurations of the image forming unit  4  and the transfer section  5 . As illustrated in  FIG. 2 , the image forming unit  4  includes the four image forming sections  4   y ,  4   c ,  4   m , and  4   k . A “specified number” is four in the present embodiment. 
     The image forming sections  4   y ,  4   c ,  4   m , and  4   k  each include a light exposure device  41 , a photosensitive drum  42 , a developing device  43 , a charging roller  44 , and a cleaning blade  45 . The four image forming sections  4   y ,  4   c ,  4   m , and  4   k  have substantially the same configuration except the colors of the toners to be supplied thereto. The present specification therefore describes the configuration of the image forming section  4   y  to which the yellow toner is supplied, and omits description of the configurations of the image forming sections other than the image forming section  4   y , that is, image forming sections  4   c ,  4   m , and  4   k.    
     The image forming section  4   y  has a light exposure section  41   y  ( 41 ), a photosensitive drum  42   y  ( 42 ), a developing device  43   y  ( 43 ), a charging roller  44   y  ( 44 ), and a cleaning blade  45   y  ( 45 ). 
     The charging roller  44   y  charges the photosensitive drum  42   y  to a specific potential. The light exposure section  41   y  irradiates the photosensitive drum  42   y  with laser light to form an electrostatic latent image on the photosensitive drum  42   y . The developing device  43   y  includes a development roller  431   y . The development roller  431   y  supplies the yellow toner to the photosensitive drum  42   y  and develops the electrostatic latent image to form a toner image. As a result, the toner image in yellow is formed on a circumferential surface of the photosensitive drum  42   y.    
     An edge of the cleaning blade  45   y  (the top edge in  FIG. 2 ) is in sliding contact with the circumferential surface of the photosensitive drum  42   y . The edge of the cleaning blade  45   y  in sliding contact with the circumferential surface of the photosensitive drum  42   y  removes the yellow toner remaining on the circumferential surface of the photosensitive drum  42   y.    
     The transfer section  5  transfers toner images onto the paper P. The transfer section  5  includes four primary transfer rollers  51  ( 51   y ,  51   c ,  51   m , and  51   k ), a secondary transfer roller  52 , a drive roller  53 , the intermediate transfer belt  54 , a driven roller  55 , and a blade  56 . 
     The transfer section  5  transfers onto the intermediate transfer belt  54  toner images respectively formed on the photosensitive drums  42  ( 42   y ,  42   c ,  42   m , and  42   k ) of the image forming sections  4   y ,  4   c ,  4   m , and  4   k  such that the toner images are superimposed on one another. The transfer section  5  further transfers the superimposed toner images from the intermediate transfer belt  54  to the paper P. 
     The primary transfer roller  51   y  is disposed opposite to the photosensitive drum  42   y  with the intermediate transfer belt  54  therebetween. The primary transfer roller  51   y  can come in or out of press contact with the photosensitive drum  42   y  with the intermediate transfer belt  54  therebetween through driving by a driving mechanism, not illustrated. The primary transfer roller  51   y  is normally in press contact with the photosensitive drum  42   y  with the intermediate transfer belt  54  therebetween. Like the primary transfer roller  51   y , the other primary transfer rollers  51   c ,  51   m , and  51   k  are each in press contact with a corresponding one of the photosensitive drums  42  ( 42   c ,  42   m , or  42   k ), with the intermediate transfer belt  54  therebetween. 
     The drive roller  53  is disposed opposite to the secondary transfer roller  52  and drives the intermediate transfer belt  54 . 
     The intermediate transfer belt  54  is an endless belt that is stretched around the four primary transfer rollers  51 , the drive roller  53 , and the driven roller  55 . The intermediate transfer belt  54  is driven by the drive roller  53  to circulate in a counterclockwise direction as indicated by arrows F 1  and F 2  in  FIG. 2 . An outer surface of the intermediate transfer belt  54  is in contact with circumferential surfaces of the respective photosensitive drums  42  ( 42   y ,  42   c ,  42   m , and  42   k ). Toner images are transferred by the primary transfer rollers  51  ( 51   y ,  51   c ,  51   m , and  51   k ) from the photosensitive drums  42  ( 42   y ,  42   c ,  42   m , and  42   k ) to the outer surface of the intermediate transfer belt  54 . 
     The driven roller  55  is driven to rotate by circulation of the intermediate transfer belt  54 . The blade  56  is disposed opposite to the driven roller  55  with the intermediate transfer belt  54  therebetween. The blade  56  removes toners remaining on the outer surface of the intermediate transfer belt  54 . 
     The secondary transfer roller  52  is pressed against the drive roller  53 . As a result, the secondary transfer roller  52  and the drive roller  53  form a nip N therebetween. The secondary transfer roller  52  and the drive roller  53  transfer the toner images from the intermediate transfer belt  54  to the paper P while the paper P is passing through the nip N. 
     Next, the power supply section  6  will be described with reference to  FIG. 3 .  FIG. 3  is a side view illustrating the configuration of the power supply section  6 . The power supply section  6  includes voltage applicators  61  and a current detector  62 . The voltage applicators  61  apply voltages to the primary transfer rollers  51 . 
     The voltage applicators  61  include four voltage applicators  61   y ,  61   c ,  61   m , and  61   k . The four voltage applicators  61   y ,  61   c ,  61   m , and  61   k  apply voltages to the primary transfer rollers  51   y ,  51   c ,  51   m , and  51   k , respectively. For example, the voltage applicator  61   y  applies to the primary transfer roller  51   y  a voltage of opposite polarity (a negative voltage in the present embodiment) to charging polarity of the toner. The photosensitive drums  42  ( 42   y ,  42   c ,  42   m , and  42   k ) are grounded. As a result, the voltage applicator  61   y  applies a voltage between the primary transfer roller  51   y  and the photosensitive drum  42   y.    
     The current detector  62  detects a total current value JS that is a sum of values of currents flowing through the respective four primary transfer rollers  51   y ,  51   c ,  51   m , and  51   k.    
     Next, the configuration of the controller  9  will be described with reference to  FIG. 4 .  FIG. 4  is a diagram illustrating the configuration of the controller  9 . The controller  9  includes a central processing unit (CPU) and a memory. A control program is stored in the memory. The CPU implements various functional sections through execution of the control program. Also, the CPU causes the memory to implement various functional sections through execution of the control program. As a result, the various functional sections implemented by the controller  9  control operation of the image forming apparatus  1 . The controller  9  includes a voltage controller  911 , a current acquiring section  912 , a resistance calculator  913 , an adjuster  914 , and a voltage and current storage section  92 . 
     The voltage and current storage section  92  stores therein voltage values VT of voltages that the voltage applicators  61  apply to the primary transfer rollers  51  in association with total current values JS detected by the current detector  62 . The voltage values VT of the voltages and the total current values JS stored in the voltage and current storage section  92  are read by the resistance calculator  913  and the adjuster  914 . 
     The voltage controller  911  controls the voltages that the voltage applicators  61  apply to the primary transfer rollers  51   y ,  51   c ,  51   m , and  51   k . More specifically, the voltage controller  911  causes a detection voltage VS to be applied to one primary transfer roller  51  of the four primary transfer rollers  51  while causing a first voltage VL 1  having the same polarity as the detection voltage VS to be applied to all the other primary transfer rollers  51 . The one primary transfer roller  51  is for example the primary transfer roller  51   y , and all the other primary transfer rollers  51  are for example the primary transfer rollers  51   c ,  51   m , and  51   k . The detection voltage VS is a voltage that is applied to detect a resistance value R between the one primary transfer roller  51  and a corresponding one of the photosensitive drums  42 . The detection voltage VS has a predetermined voltage value (for example, 1000 V). The first voltage VL 1  has a voltage value of at least one-200th and no greater than one-tenth (for example, 100 V) of the voltage value of the detection voltage VS. 
     The voltage controller  911  also causes a second voltage VL 2  to be applied to all the four primary transfer rollers  51   y ,  51   c ,  51   m , and  51   k . A specific method for controlling voltages by the voltage controller  911  will be described further below with reference to  FIG. 5 . 
     The voltage controller  911  also causes the detection voltage VS that is varied to have different voltage values to be applied to one primary transfer roller  51  (for example, the primary transfer roller  51   y ) while causing the first voltage VL 1  to be applied to all the other primary transfer rollers (for example, the primary transfer rollers  51   c ,  51   m , and  51   k ). A relationship between the voltage values of the detection voltage VS and the total current values JS will be described further below with reference to  FIG. 6 . 
     The current acquiring section  912  acquires the total current values JS detected by the current detector  62 . The current acquiring section  912  also stores to the voltage and current storage section  92  the total current values JS in association with voltage values VT of voltages that the voltage applicators  61  apply to the four primary transfer rollers  51   y ,  51   c ,  51   m , and  51   k.    
     The resistance calculator  913  determines a resistance value R of each of the primary transfer rollers  51 . Specifically, the resistance calculator  913  determines the resistance value R of each of the primary transfer rollers  51  by dividing the voltage value of the detection voltage VS by the total current value JS. More specifically, the voltage controller  911  initially causes the detection voltage VS to be applied to the primary transfer roller  51   y  and the first voltage VL 1  to be applied to the other three primary transfer rollers  51   c ,  51   m , and  51   k . During the voltage application, the current acquiring section  912  acquires a first total current value JSy. The resistance calculator  913  determines a value of resistance Ry between the primary transfer roller  51   y  and the photosensitive drum  42   y  by dividing the voltage value of the detection voltage VS by the first total current value JSy. 
     Next, the voltage controller  911  causes the detection voltage VS to be applied to the primary transfer roller  51   c  and the first voltage VL 1  to be applied to the other three primary transfer rollers  51   y ,  51   m , and  51   k . During the voltage application, the current acquiring section  912  acquires a first total current value JSc. The resistance calculator  913  determines a value of resistance Rc between the primary transfer roller  51   c  and the photosensitive drum  42   c  by dividing the voltage value of the detection voltage VS by the first total current value JSc. 
     Then, the voltage controller  911  causes the detection voltage VS to be applied to the primary transfer roller  51   m  and the first voltage VL 1  to be applied to the other three primary transfer rollers  51   y ,  51   c , and  51   k . During the voltage application, the current acquiring section  912  acquires a first total current value JSm. The resistance calculator  913  determines a value of resistance Rm between the primary transfer roller  51   m  and the photosensitive drum  42   m  by dividing the voltage value of the detection voltage VS by the first total current value JSm. 
     Finally, the voltage controller  911  causes the detection voltage VS to be applied to the primary transfer roller  51   k  and the first voltage VL 1  to be applied to the other three primary transfer rollers  51   y ,  51   c , and  51   m . During the voltage application, the current acquiring section  912  acquires a first total current value JSk. The resistance calculator  913  determines a value of resistance Rk between the primary transfer roller  51   k  and the photosensitive drum  42   k  by dividing the voltage value of the detection voltage VS by the first total current value JSk. In the description given below, the values of resistance Ry, Rc, Rm, and Rk may be respectively referred to as the resistance value Ry of the primary transfer roller  51   y , the resistance value Rc of the primary transfer roller  51   c , the resistance value Rm of the primary transfer roller  51   m , and the resistance value Rk of the primary transfer roller  51   k  for convenience. As described above, the resistance calculator  913  determines the resistance values Ry, Rc, Rm, and Rk of the respective four primary transfer rollers  51   y ,  51   c ,  51   m , and  51   k.    
     The adjuster  914  adjusts the resistance values R (Ry, Rc, Rm, and Rk). Specifically, the adjuster  914  adjusts the resistance values R based on a second total current value JSL obtained when the voltage controller  911  causes the second voltage VL 2  to be applied to all the four primary transfer rollers  51   y ,  51   c ,  51   m , and  51   k . More specifically, the resistance values R (Ry, Rc, Rm, and Rk) are adjusted using Formulas (1) to (4) given below.
 
Resistance value  Ry =(voltage value of detection voltage  VS )/{(first total current value  JSy )−(second total current value  JSL )×3/4}  (1)
 
Resistance value  Rc =(voltage value of detection voltage  VS )/{(first total current value  JSc )−(second total current value  JSL )×3/4}  (2)
 
Resistance value  Rm =(voltage value of detection voltage  VS )/{(first total current value  JSm )−(second total current value  JSL )×3/4}  (3)
 
Resistance value  Rk =(voltage value of detection voltage  VS )/{(first total current value  JSk )−(second total current value  JSL )×3/4}  (4)
 
     That is, for example the first total current value JSy is a sum of values of currents flowing through the four primary transfer rollers  51   y ,  51   c ,  51   m , and  51   k  when the detection voltage VS is applied to the primary transfer roller  51   y  and the first voltage VL 1  is applied to the other three primary transfer rollers  51   c ,  51   m , and  51   k . Accordingly, the first total current value JSy includes a sum of values of currents flowing through the primary transfer rollers  51   c ,  51   m , and  51   k  through application of the first voltage VL 1  thereto in addition to a value of a current flowing through the primary transfer roller  51   y . Therefore, the sum of the values of the currents flowing through the primary transfer rollers  51   c ,  51   m , and  51   k  is obtained as “(second total current value JSL)×3/4”. Then, the value of the current flowing through the primary transfer roller  51   y  is determined by subtracting “(second total current value JSL)×3/4” from the first total current value JSy. The adjuster  914  divides the voltage value of the detection voltage VS by the current value determined as above to adjust the resistance value Ry determined by the resistance calculator  913 . 
     The following describes an example of operation of the voltage controller  911  with reference to  FIGS. 5A to 5D .  FIGS. 5A to 5D  are graphs showing an example of operation of the voltage controller  911  when the voltage value of the second voltage VL 2  is the same as the voltage value of the first voltage VL 1 . In explanation of  FIGS. 5A to 5D , the voltage value of the detection voltage VS may be referred to as the detection voltage VS and the voltage value of the first voltage VL 1  may be referred to as the first voltage VL 1  for convenience.  FIG. 5A  is a graph G 11  showing variation of a voltage VY applied to the primary transfer roller  51   y  for a Y (yellow) color.  FIG. 5B  is a graph G 21  showing variation of a voltage VC applied to the primary transfer roller  51   c  for a C (cyan) color.  FIG. 5C  is a graph G 31  showing variation of a voltage VM applied to the primary transfer roller  51   m  for a M (magenta) color.  FIG. 5D  is a graph G 41  showing variation of a voltage VK applied to the primary transfer roller  51   k  for a K (black) color. The horizontal axis in each of the graphs represents time T and the vertical axis represents the voltage VY, VC, VM, or VK. The direction of the arrow on the vertical axis in each of the graphs indicates that the voltages VY, VC, VM, and VK are negative. In explanation of  FIGS. 5A to 5D , the first voltage VL 1  is for example 100 V. 
     First, variation of the voltage VY will be described with reference to  FIG. 5A . At time point T 11 , the voltage controller  911  causes the first voltage VL 1  to be applied to the primary transfer roller  51   y . Then at time point T 12 , the voltage controller  911  changes the voltage VY applied to the primary transfer roller  51   y  from the first voltage VL 1  to the detection voltage VS. Next at time point T 13 , the voltage controller  911  changes the voltage VY applied to the primary transfer roller  51   y  from the detection voltage VS to the first voltage VL 1 . Further at time point T 14 , the voltage controller  911  changes the voltage VY applied to the primary transfer roller  51   y  from the first voltage VL 1  to zero. 
     Next, variation of the voltage VC will be described with reference to  FIG. 5B . At time point T 11 , the voltage controller  911  causes the first voltage VL 1  to be applied to the primary transfer roller  51   c . Then at time point T 13 , the voltage controller  911  changes the voltage VC applied to the primary transfer roller  51   c  from the first voltage VL 1  to the detection voltage VS. Next at time point T 21 , the voltage controller  911  changes the voltage VC applied to the primary transfer roller  51   c  from the detection voltage VS to the first voltage VL 1 . Further at time point T 14 , the voltage controller  911  changes the voltage VC applied to the primary transfer roller  51   c  from the first voltage VL 1  to zero. 
     Next, variation of the voltage VM will be described with reference to  FIG. 5C . At time point T 11 , the voltage controller  911  causes the first voltage VL 1  to be applied to the primary transfer roller  51   m . Then at time point T 21 , the voltage controller  911  changes the voltage VM applied to the primary transfer roller  51   m  from the first voltage VL 1  to the detection voltage VS. Next at time point T 31 , the voltage controller  911  changes the voltage VM applied to the primary transfer roller  51   m  from the detection voltage VS to the first voltage VL 1 . Further at time point T 14 , the voltage controller  911  changes the voltage VM applied to the primary transfer roller  51   m  from the first voltage VL 1  to zero. 
     Next, variation of the voltage VK will be described with reference to  FIG. 5D . At time point T 11 , the voltage controller  911  causes the first voltage VL 1  to be applied to the primary transfer roller  51   k . Then at time point T 31 , the voltage controller  911  changes the voltage VK applied to the primary transfer roller  51   k  from the first voltage VL 1  to the detection voltage VS. Next at time point T 14 , the voltage controller  911  changes the voltage VK applied to the primary transfer roller  51   k  from the detection voltage VS to zero. 
     As described above with reference to  FIGS. 5A to 5D , during the period between time points T 11  and T 12 , the voltage controller  911  causes the first voltage VL 1  to be applied to all the four primary transfer rollers  51   y ,  51   c ,  51   m , and  51   k . A second total current value JSL that the current acquiring section  912  acquires during the above period is used by the adjuster  914  to adjust the resistance values R. 
     During the period between time points T 12  and T 13 , the voltage controller  911  causes the detection voltage VS to be applied to the primary transfer roller  51   y  and the first voltage VL 1  to be applied to the other three primary transfer rollers  51   c ,  51   m , and  51   k . A first total current value JSy that the current acquiring section  912  acquires during the above period is used by the resistance calculator  913  to determine the resistance value Ry of the primary transfer roller  51   y.    
     During the period between time points T 13  and T 21 , the voltage controller  911  causes the detection voltage VS to be applied to the primary transfer roller  51   c  and the first voltage VL 1  to be applied to the other three primary transfer rollers  51   y ,  51   m , and  51   k . A first total current value JSc that the current acquiring section  912  acquires during the above period is used by the resistance calculator  913  to determine the resistance value Rc of the primary transfer roller  51   c.    
     During the period between time points T 21  and T 31 , the voltage controller  911  causes the detection voltage VS to be applied to the primary transfer roller  51   m  and the first voltage VL 1  to be applied to the other three primary transfer rollers  51   y ,  51   c , and  51   k . A first total current value JSm that the current acquiring section  912  acquires during the above period is used by the resistance calculator  913  to determine the resistance value Rm of the primary transfer roller  51   m.    
     During the period between time points T 31  and T 14 , the voltage controller  911  causes the detection voltage VS to be applied to the primary transfer roller  51   k  and the first voltage VL 1  to be applied to the other three primary transfer rollers  51   y ,  51   c , and  51   m . A first total current value JSk that the current acquiring section  912  acquires during the above period is used by the resistance calculator  913  to determine the resistance value Rk of the primary transfer roller  51   k.    
     Next, the following describes operation of the controller  9  with reference to  FIG. 6 .  FIG. 6  is a graph G 5  showing a relationship between voltage values VT of the detection voltage VS applied by one of the voltage applicators  61  and first total current values JS detected by the current detector  62 . In the graph G 5 , the horizontal axis represents the voltage values VT and the vertical axis represents the first total current values JS. Square marks indicate measurement points PT. The resistance calculator  913  may determine the resistance value R (Ry, Rc, Rm, or Rk) of each of the primary transfer rollers  51  based on the slope of the straight line in the graph G 5 . 
     Specifically, the voltage controller  911  initially selects a plurality of (for example, 12) voltage values VT 1  to VT 12  from a predetermined range (for example, from 50 V to 1050 V). Then, the voltage controller  911  controls the detection voltage VS explained above with reference to  FIGS. 5A to 5D  to the voltage values VT 1  to VT 12 . The current acquiring section  912  acquires the first total current values JS respectively corresponding to the voltage values VT 1  to VT 12  from the current detector  62 . The resistance calculator  913  then determines a straight line from coordinates of the 12 measurement points in accordance with the least square method, for example. The resistance calculator  913  determines the resistance value R of each of the primary transfer rollers  51  by determining the inverse of the slope of the straight line. 
     When the resistance calculator  913  determines the resistance value R of each of the primary transfer rollers  51  according to the procedure explained above with reference to  FIG. 6 , the adjuster  914  need not adjust the resistance value R determined by the resistance calculator  913 . This is because the resistance value R is determined using Formula (5) given below based on the slope of the straight line that represents an amount of change ΔJS of the first total current value JS relative to an amount of change ΔVT of the voltage value VT of the detection voltage VS as illustrated in  FIG. 6 .
 
Resistance value  R =(amount of change Δ VT  of voltage value  VT )/(amount of change Δ JS  of first total current value  JS )  (5)
 
     The following describes operation of the controller  9  with reference to  FIGS. 5A to 5D, 7, and 8 .  FIGS. 7 and 8  are flowcharts illustrating the operation of the controller  9 . First, as illustrated between time points T 11  and T 12  in  FIGS. 5A to 5D , the voltage controller  911  causes the second voltage VL 2  to be applied to all the four primary transfer rollers  51   y ,  51   c ,  51   m , and  51   k  (step S 101 ). Then, the current acquiring section  912  acquires the second total current value JSL from the current detector  62  (step S 103 ). 
     Next, as illustrated between time points T 12  and T 13  in  FIGS. 5A to 5D , the voltage controller  911  causes the detection voltage VS to be applied to the primary transfer roller  51   y  and the first voltage VL 1  to be applied to the other three primary transfer rollers  51   c ,  51   m , and  51   k  (step S 105 ). Then, the current acquiring section  912  acquires the first total current value JSy from the current detector  62  (step S 107 ). Next, the resistance calculator  913  determines the resistance value Ry of the primary transfer roller  51   y  by dividing the detection voltage VS by the first total current value JSy (step S 109 ). 
     Next, as illustrated between time points T 13  and T 21  in  FIGS. 5A to 5D , the voltage controller  911  causes the detection voltage VS to be applied to the primary transfer roller  51   c  and the first voltage VL 1  to be applied to the other three primary transfer rollers  51   y ,  51   m , and  51   k  (step S 111 ). Then, the current acquiring section  912  acquires the first total current value JSc from the current detector  62  (step S 113 ). Next, the resistance calculator  913  determines the resistance value Rc of the primary transfer roller  51   c  by dividing the detection voltage VS by the first total current value JSc (step S 115 ). 
     Next, as illustrated between time points T 21  and T 31  in  FIGS. 5A to 5D , the voltage controller  911  causes the detection voltage VS to be applied to the primary transfer roller  51   m  and the first voltage VL 1  to be applied to the other three primary transfer rollers  51   y ,  51   c , and  51   k  (step S 117  in  FIG. 8 ). Then, the current acquiring section  912  acquires the first total current value JSm from the current detector  62  (step S 119 ). Next, the resistance calculator  913  determines the resistance value Rm of the primary transfer roller  51   m  by dividing the detection voltage VS by the first total current value JSm (step S 121 ). 
     Next, as illustrated between time points T 31  and T 14  in  FIGS. 5A to 5D , the voltage controller  911  causes the detection voltage VS to be applied to the primary transfer roller  51   k  and the first voltage VL 1  to be applied to the other three primary transfer rollers  51   y ,  51   c , and  51   m  (step S 123 ). Then, the current acquiring section  912  acquires the first total current value JSk from the current detector  62  (step S 125 ). Next, the resistance calculator  913  determines the resistance value Rk of the primary transfer roller  51   k  by dividing the detection voltage VS by the first total current value JSk (step S 127 ). 
     Then, the adjuster  914  adjusts the resistance values Ry, Rc, Rm, and Rk using the second total current value JSL acquired in step S 103  (step S 129 ), whereby the processing ends. More specifically, the adjuster  914  adjusts the resistance values Ry, Rc, Rm, and Rk using Formulas (1) to (4) explained above with reference to  FIG. 4 . 
     The following describes another example of operation of the voltage controller  911  with reference to  FIGS. 9A to 9D .  FIGS. 9A to 9D  are graphs showing an example of operation of the voltage controller  911  when the voltage value of the second voltage VL 2  is greater than the voltage value of the first voltage VL 1 .  FIG. 9A  is a graph G 12  showing variation of the voltage VY applied to the primary transfer roller  51   y  for the Y (yellow) color.  FIG. 9B  is a graph G 22  showing variation of the voltage VC applied to the primary transfer roller  51   c  for the C (cyan) color.  FIG. 9C  is a graph G 32  showing variation of the voltage VM applied to the primary transfer roller  51   m  for the M (magenta) color.  FIG. 9D  is a graph G 42  showing variation of the voltage VK applied to the primary transfer roller  51   k  for the K (black) color. In each of the graphs, the horizontal axis represents time T and the vertical axis represents the voltage VY, VC, VM, or VK. Further, the direction of the arrow on the vertical axis in each of the graphs indicates that the voltages VY, VC, VM, and VK are negative. In explanation of  FIGS. 9A to 9D , the first voltage VL 1  is for example 50 V and the second voltage VL 2  is for example 70 V. 
     First, variation of the voltage VY will be described with reference to  FIG. 9A . At time point T 11 , the voltage controller  911  causes the second voltage VL 2  to be applied to the primary transfer roller  51   y . Then at time point T 12 , the voltage controller  911  changes the voltage VY applied to the primary transfer roller  51   y  from the second voltage VL 2  to the detection voltage VS. Next at time point T 13 , the voltage controller  911  changes the voltage VY applied to the primary transfer roller  51   y  from the detection voltage VS to the first voltage VL 1 . Further at time point T 14 , the voltage controller  911  changes the voltage VY applied to the primary transfer roller  51   y  from the first voltage VL 1  to zero. 
     Next, variation of the voltage VC will be described with reference to  FIG. 9B . At time point T 11 , the voltage controller  911  causes the second voltage VL 2  to be applied to the primary transfer roller  51   c . Then, at time point T 12 , the voltage controller  911  changes the voltage VC applied to the primary transfer roller  51   c  from the second voltage VL 2  to the first voltage VL 1 . Then at time point T 13 , the voltage controller  911  changes the voltage VC applied to the primary transfer roller  51   c  from the first voltage VL 1  to the detection voltage VS. Next at time point T 21 , the voltage controller  911  changes the voltage VC applied to the primary transfer roller  51   c  from the detection voltage VS to the first voltage VL 1 . Further at time point T 14 , the voltage controller  911  changes the voltage VC applied to the primary transfer roller  51   c  from the first voltage VL 1  to zero. 
     Next, variation of the voltage VM will be described with reference to  FIG. 9C . At time point T 11 , the voltage controller  911  causes the second voltage VL 2  to be applied to the primary transfer roller  51   m . Then at time point T 12 , the voltage controller  911  changes the voltage VM applied to the primary transfer roller  51   m  from the second voltage VL 2  to the first voltage VL 1 . Then at time point T 21 , the voltage controller  911  changes the voltage VM applied to the primary transfer roller  51   m  from the first voltage VL 1  to the detection voltage VS. Next at time point T 31 , the voltage controller  911  changes the voltage VM applied to the primary transfer roller  51   m  from the detection voltage VS to the first voltage VL 1 . Further at time point T 14 , the voltage controller  911  changes the voltage VM applied to the primary transfer roller  51   m  from the first voltage VL 1  to zero. 
     Next, variation of the voltage VK will be described with reference to  FIG. 9D . At time point T 11 , the voltage controller  911  causes the second voltage VL 2  to be applied to the primary transfer roller  51   k . Then at time point T 12 , the voltage controller  911  changes the voltage VK applied to the primary transfer roller  51   k  from the second voltage VL 2  to the first voltage VL 1 . Then at time point T 31 , the voltage controller  911  changes the voltage VK applied to the primary transfer roller  51   k  from the first voltage VL 1  to the detection voltage VS. Next at time point T 14 , the voltage controller  911  changes the voltage VK applied to the primary transfer roller  51   k  from the detection voltage VS to zero. 
     As described above with reference to  FIGS. 9A to 9D , during the period between time points T 11  and T 12 , the voltage controller  911  causes the second voltage VL 2  to be applied to all the four primary transfer rollers  51   y ,  51   c ,  51   m , and  51   k . A second total current value JSL that the current acquiring section  912  acquires during the above period is used by the adjuster  914  to adjust the resistance values R. 
     During the period between time points T 12  and T 13 , the voltage controller  911  causes the detection voltage VS to be applied to the primary transfer roller  51   y  and the first voltage VL 1  to be applied to the other three primary transfer rollers  51   c ,  51   m , and  51   k . A first total current value JSy that the current acquiring section  912  acquires during the above period is used by the resistance calculator  913  to determine the resistance value Ry of the primary transfer roller  51   y.    
     During the period between time points T 13  and T 21 , the voltage controller  911  causes the detection voltage VS to be applied to the primary transfer roller  51   c  and the first voltage VL 1  to be applied to the other three primary transfer rollers  51   y ,  51   m , and  51   k . A first total current value JSc that the current acquiring section  912  acquires during the above period is used by the resistance calculator  913  to determine the resistance value Rc of the primary transfer roller  51   c.    
     During the period between time points T 21  and T 31 , the voltage controller  911  causes the detection voltage VS to be applied to the primary transfer roller  51   m  and the first voltage VL 1  to be applied to the other three primary transfer rollers  51   y ,  51   c , and  51   k . A first total current value JSm that the current acquiring section  912  acquires during the above period is used by the resistance calculator  913  to determine the resistance value Rm of the primary transfer roller  51   m.    
     During the period between time points T 31  and T 14 , the voltage controller  911  causes the detection voltage VS to be applied to the primary transfer roller  51   k  and the first voltage VL 1  to be applied to the other three primary transfer rollers  51   y ,  51   c , and  51   m . A first total current value JSk that the current acquiring section  912  acquires during the above period is used by the resistance calculator  913  to determine the resistance value Rk of the primary transfer roller  51   k.    
     As described above with reference to  FIGS. 3 to 9D , the voltage applicators  61  respectively apply voltages to a specified number of (for example, four) primary transfer rollers  51  ( 51   y ,  51   c ,  51   m , and  51   k ). Further, the current detector  62  detects the first total current values JS (JSy, JSc, JSm, and JSk), each of which is a sum of values of currents flowing through the respective four primary transfer rollers  51 . 
     Thus, the resistance values R (Ry, Rc, Rm, and Rk) of the four primary transfer rollers  51  can be determined only with one current detector  62 . More specifically, for example, the voltage applicators  61  apply the detection voltage VS to the primary transfer roller  51   y , which is one of the four primary transfer rollers  51 , and apply no voltage to the other primary transfer rollers  51   c ,  51   m , and  51   k  to detect a first total current value JS. The resistance value Ry of the primary transfer roller  51   y  to which the detection voltage VS has been applied can be determined by dividing the voltage value of the detection voltage VS by the first total current value JS. The resistance values R of the four primary transfer rollers  51  can be determined through the voltage applicators  61  applying the detection voltage VS to the four primary transfer rollers  51  in order. Thus, the number of current detectors  62  for detecting the values of the currents flowing through the primary transfer rollers  51  can be reduced. As a result, the manufacturing cost of the image forming apparatus  1  can be reduced. 
     Furthermore, when the voltage controller  911  causes the detection voltage VS to be applied to the primary transfer roller  51   y , which is one of the four primary transfer rollers  51 , a voltage having the same polarity as the detection voltage VS is applied to the other primary transfer rollers  51   c ,  51   m , and  51   k . In this case, the detection voltage VS is a voltage that is applied for detecting the resistance value Ry of the primary transfer roller  51   y . The value of the detection voltage VS is a predetermined voltage value (for example, 1000 V). By applying the voltage having the same polarity as the detection voltage VS to the other primary transfer rollers  51   c ,  51   m , and  51   k , it is possible to reduce leakage of current from the one primary transfer roller  51   y  to the other primary transfer rollers  51   c ,  51   m , and  51   k . Thus, the resistance value Ry of the primary transfer roller  51   y  can be detected accurately. Likewise, the resistance values Rc, Rm, and Rk can be detected accurately. Through the above, transfer voltages each having an appropriate magnitude can be applied to the four primary transfer rollers  51 . 
     Furthermore, when the voltage controller  911  causes the detection voltage VS to be applied to the primary transfer roller  51   y , which is one of the four primary transfer rollers  51 , the first voltage VL 1  having a voltage value of no greater than one-tenth of the voltage value of the detection voltage VS is applied to the other primary transfer rollers  51   c ,  51   m , and  51   k . Through application of the first voltage VL 1 , currents flowing into the other primary transfer rollers  51   c ,  51   m , and  51   k  can be reduced. Furthermore, the first voltage VL 1  having a voltage value of at least one-200th of the voltage value of the detection voltage VS is applied to the other primary transfer rollers  51   c ,  51   m , and  51   k . Therefore, leakage of current from the one primary transfer roller  51   y  to the other primary transfer rollers  51   c ,  51   m , and  51   k  can be prevented. As a result, the resistance value Ry of the primary transfer roller  51   y  can be detected more accurately. Likewise, the resistance values Rc, Rm, and Rk can be detected more accurately. Consequently, transfer voltages each having a more appropriate magnitude can be applied to the four primary transfer rollers  51 . 
     Furthermore, the voltage controller  911  causes the second voltage VL 2  having the same polarity as the detection voltage VS to be applied to the four primary transfer rollers  51  and the current detector  62  detects the second total current value JSL. Then, the adjuster  914  adjusts the resistance values R based on the second total current value JSL. Meanwhile, when the detection voltage VS is applied to the one primary transfer roller  51   y  and the first voltage VL 1  having the same polarity as the detection voltage VS is applied to the other primary transfer rollers  51   c ,  51   m , and  51   k , currents flow through the other primary transfer rollers  51   c ,  51   m , and  51   k . Therefore, the adjuster  914  adjusts the resistance value Ry of the primary transfer roller  51   y  based on the second total current value JSL. Through the above, it is possible to adjust the resistance value Ry by compensating for influence of the currents flowing through the primary transfer rollers  51   c ,  51   m , and  51   k  to which the detection voltage VS has not been applied. Thus, the resistance values R of the four primary transfer rollers  51  can be detected more accurately. Consequently, transfer voltages each having a more appropriate magnitude can be applied to the four primary transfer rollers  51 . 
     The voltage value of the second voltage VL 2  is most preferably the same as the voltage value of the first voltage VL 1  (the configuration illustrated in  FIGS. 5A to 5D ). In this case, the adjuster  914  can accurately adjust the resistance value Ry by compensating for the influence of the currents flowing through the primary transfer rollers  51   c ,  51   m , and  51   k  to which the detection voltage VS has not been applied. Thus, the resistance values R of the four primary transfer rollers  51  can be detected more accurately. Consequently, transfer voltages each having a more appropriate magnitude can be applied to the four primary transfer rollers  51 . 
     Alternatively, the voltage value of the second voltage VL 2  may be greater than the voltage value of the first voltage VL 1  (the configuration illustrated in  FIGS. 9A to 9D ). In other words, the voltage value of the first voltage VL 1  may be smaller than the voltage value of the second voltage VL 2 . In the above configuration, the currents flowing through the primary transfer rollers  51   c ,  51   m , and  51   k  to which the detection voltage VS has not been applied can be reduced. Therefore, the adjuster  914  can accurately adjust the resistance value Ry by compensating for the influence of the currents flowing through the primary transfer rollers  51   c ,  51   m , and  51   k  to which the detection voltage VS has not been applied. Thus, the resistance values R of the four primary transfer rollers  51  can be detected accurately. Furthermore, transfer voltages each having a more appropriate magnitude can be applied to the four primary transfer rollers  51 . 
     Furthermore, after application of the second voltage VL 2  to the four primary transfer rollers  51 , the voltage controller  911  causes the detection voltage VS to be applied to the primary transfer roller  51   y , which is one of the four primary transfer rollers  51 , and the first voltage VL 1  to be applied to the other primary transfer rollers  51   c ,  51   m , and  51   k . Accordingly, the voltage applied to the primary transfer roller  51   y  changes from the first voltage VL 1  to the detection voltage VS. Therefore, a time necessary to change the voltage applied to the primary transfer roller  51   y  to the detection voltage VS can be reduced. Consequently, a time necessary to detect the resistance values R of the four primary transfer rollers  51  can be reduced. 
     Further, the resistance calculator  913  determines the resistance value Ry of the primary transfer roller  51   y  based on the plurality of (for example, 12) voltage values VT 1  to VT 12  of the detection voltage VS that is applied to the one primary transfer roller  51   y  and the first total current values JS. Likewise, the resistance values Rc, Rm, and Rk of the other primary transfer rollers  51   c ,  51   m , and  51   k  are determined. Thus, the resistance values R of the four primary transfer rollers  51  can be determined accurately. More specifically, the resistance value R of each of the four primary transfer rollers  51  can be determined more accurately by for example determining a straight line representing a relationship between the voltage values of the detection voltage VS and the first total current values JS, and determining the inverse of the slope of the straight line. Through the above, transfer voltages each having an appropriate magnitude can be applied to the four primary transfer rollers  51 . 
     Through the above, an embodiment of the present disclosure has been described with reference to the drawings. However, the present disclosure is not limited to the above embodiment and may be implemented in various different forms that do not deviate from the essence of the present disclosure (for example, as described below in sections (1) to (5)). The drawings schematically illustrate elements of configuration in order to facilitate understanding and properties of the elements of configuration illustrated in the drawings, such as thickness, length, and number thereof, may differ from actual properties thereof in order to facilitate preparation of the drawings. Furthermore, properties of elements of configuration described in the above embodiment, such as shapes and dimensions, are merely examples and are not intended as specific limitations. Various alterations may be made so long as there is no substantial deviation from the configuration of the present disclosure. 
     (1) The present disclosure is described with reference to  FIG. 1  for a configuration in which the image forming apparatus  1  includes four primary transfer rollers  51   c ,  51   m ,  51   y ,  51   k  and four photosensitive drums  42   c ,  42   m ,  42   y , and  42   k . However, the present disclosure is not limited to such a configuration. In other words, the “specified number” is not limited to four. It is only required that the image forming apparatus  1  includes a specified number of primary transfer rollers and the specified number of photosensitive drums. The specified number may for example be two, three, five or more. 
     (2) The present disclosure is described with reference to  FIG. 4  for a configuration in which the voltage value of the first voltage VL 1  is at least one-200th and no greater than one-tenth of the voltage value of the detection voltage VS. However, the present disclosure is not limited to such a configuration. No particular limitations are placed on the first voltage VL 1  so long as the first voltage VL 1  has the same polarity as the detection voltage VS. 
     (3) The present disclosure is described with reference to  FIGS. 5A to 5D  for a configuration in which the voltage controller  911  causes the detection voltage VS to be applied to the four primary transfer rollers  51   y ,  51   c ,  51   m , and  51   k  in the noted order. However, the present disclosure is not limited to such a configuration. In other words, the voltage controller  911  may cause the detection voltage VS to be applied to the primary transfer rollers  51  in any order. In a configuration, for example, the detection voltage VS may be applied to the four primary transfer rollers  51   k ,  51   m ,  51   c , and  51   y  in the noted order. 
     (4) The present disclosure is described with reference to  FIG. 6  for a configuration in which the resistance calculator  913  determines the inverse of the slope of the straight line. However, the present disclosure is not limited to such a configuration. In a configuration, the resistance calculator  913  may obtain a curve that approximates coordinates of the measurement points PT instead of the straight line. In such a configuration, the resistance value R is determined as the inverse of the slope of the curve. 
     (5) The present disclosure is described with reference to  FIGS. 9A to 9D  for a configuration in which the voltage value of the second voltage VL 2  is greater than the voltage value of the first voltage VL 1 . However, the present disclosure is not limited to such a configuration. In a configuration, the voltage value of the second voltage VL 2  may be no greater than the voltage value of the first voltage VL 1 .