Patent Publication Number: US-9841707-B2

Title: Image forming apparatus, control method, and control program in which a magnitude of a current to flow in a paper sheet at the time of toner image transfer can be set

Description:
The entire disclosure of Japanese Patent Application No. 2015-248686 filed on Dec. 21, 2015 including description, claims, drawings, and abstract are incorporated herein by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present disclosure relates to control of an image forming apparatus, and more particularly, relates to control of electrophotographic image forming apparatus. 
     Description of the Related Art 
     Electrophotographic image forming apparatuses are widely used today. An electrophotographic image forming apparatus performs a printing process that includes a process of forming a toner image on an image carrier such as a photosensitive member or an intermediate transfer member, a process of transferring the toner image from the image carrier onto a paper sheet, and a process of fixing the toner image to the paper sheet. 
     The transfer of a toner image from an image carrier onto a paper sheet is performed by a transfer member. The transfer member is installed in contact with the image carrier. A voltage of the opposite polarity of that of the toner image (this voltage will be hereinafter also referred to as “transfer voltage”) is applied to the transfer member. With this transfer voltage, the toner image is attracted to the transfer member from the image carrier, and is then transferred onto the paper sheet being conveyed between the image carrier and the transfer member. 
     The resistance value of the transfer member varies with the surrounding environment or the like. Furthermore, the resistance value of a paper sheet varies with the type of the paper sheet. With such changes, the transfer voltage to be applied to the transfer member might also change, and cause a decrease in print quality. Particularly, unlike the resistance value of plain paper, the resistance value of a paper sheet to be used to achieve special image quality or a special visual or tactile effect (such a paper sheet will be hereinafter also referred to as “special paper”) noticeably reflects a decrease in print quality. To increase the print quality of special paper, the user of the image forming apparatus manually sets an allowable range of change in the transfer voltage for the special paper. In relation to a technology for reducing the load of this setting process, JP 2010-145955 A discloses an image forming apparatus for simplifying the operation to set the transfer voltage for special paper. 
     When an image carrier and a transfer member are left in an environment where ozone is readily generated or at an ultralow temperature for a long period of time, the resistance values of the image carrier and the transfer member might become higher than expected. As a result, current does not smoothly flow in paper sheets. Furthermore, in a case where printing is performed on a paper sheet with an unexpectedly high resistance value (such as a paper sheet with poor quality), current does not smoothly flow in the paper sheet. In such a case, even if the transfer voltage is finely adjusted, the current flowing in a paper sheet does not greatly change, and therefore, the print quality of the paper sheet does not greatly change, either. In view of this, in the image forming apparatus disclosed in JP 2010-145955 A, the user or maintenance staff needs to set a wide allowable range of transfer voltage change, and conduct test printing repeatedly to achieve desired print quality. In such circumstances, there is a demand for an image forming apparatus in which the magnitude of the current to flow in the paper sheet at the time of printing can be set, instead of the transfer voltage that hardly affects print quality. 
     SUMMARY OF THE INVENTION 
     The present disclosure has been made to solve the above problems, and an object thereof is to provide an image forming apparatus in which the magnitude of the current to flow in the paper sheet at the time of toner image transfer can be set. Another object of the present disclosure is to provide a method of controlling an image forming apparatus in which the magnitude of the current to flow in the paper sheet at the time of toner image transfer can be set. Yet another object of the present disclosure is to provide a program for controlling an image forming apparatus in which the magnitude of the current to flow in the paper sheet at the time of toner image transfer can be set. 
     To achieve at least one of the abovementioned objects, according to an aspect, an image forming apparatus reflecting one aspect of the present invention comprises: an image carrier configured to carry a toner image; a transfer member configured to transfer the toner image from the image carrier onto a transfer target member by applying a transfer voltage of the opposite polarity of the polarity of the toner image to the transfer target member passing through a portion in contact with the image carrier, the transfer member being in contact with the image carrier; an accepting unit configured to accept a setting for changing a preset current range to a new current range, the preset current range being defined by at least one of a lower limit of a current flowing in the transfer target member passing through the contact portion between the image carrier and the transfer member and an upper limit of the current; a sensing unit configured to sense a magnitude of the current flowing in the transfer target member passing through the contact portion between the image carrier and the transfer member; and a control device configured to control the transfer voltage so that the magnitude of the current sensed by the sensing unit falls within the new current range. 
     The image forming apparatus preferably has a setting mode as an operation mode, and the accepting unit preferably accepts a change of the preset current range while the operation mode is the setting mode. 
     The accepting unit preferably accepts a change of the preset current range by accepting at least one of a new lower limit of the current and a new upper limit of the current. 
     The control device preferably gradually changes the transfer voltage from a predetermined initial voltage so that the magnitude of the current flowing between the transfer member and the image carrier falls within the new current range, and a magnitude of the initial voltage preferably varies with a change of the preset current range. 
     The image forming apparatus preferably further comprises a storage device storing printing information, the printing information being voltage adjustment amounts associated with a plurality of sets of printing conditions, the sets being different from one another, and the control device preferably obtains, from the printing information, the voltage adjustment amount associated with the set of the printing conditions for the transfer target member to be subjected to printing, and changes the transfer voltage by the voltage adjustment amount each time so that the magnitude of the current flowing between the transfer member and the image carrier falls within the new current range. 
     To achieve at least one of the abovementioned objects, according to an aspect, there is provided a method of controlling an image forming apparatus, the image forming apparatus including: an image carrier configured to carry a toner image; and a transfer member configured to transfer the toner image from the image carrier onto a transfer target member by applying a transfer voltage of the opposite polarity of the polarity of the toner image to the transfer target member passing through a portion in contact with the image carrier, the transfer member being in contact with the image carrier, and the method reflecting one aspect of the present invention comprises: a step of accepting a setting for changing a preset current range to a new current range, the preset current range being defined by at least one of a lower limit of a current flowing in the transfer target member passing through the contact portion between the image carrier and the transfer member and an upper limit of the current; a step of sensing a magnitude of the current flowing in the transfer target member passing through the contact portion between the image carrier and the transfer member; and a step of controlling the transfer voltage so that the magnitude of the current sensed by the sensing unit falls within the new current range. 
     The image forming apparatus preferably has a setting mode as an operation mode, and the accepting step preferably includes accepting a change of the preset current range while the operation mode is the setting mode. 
     The accepting step preferably includes accepting a change of the preset current range by accepting at least one of a new lower limit of the current and a new upper limit of the current. 
     The controlling step preferably includes gradually changing the transfer voltage from a predetermined initial voltage so that the magnitude of the current flowing between the transfer member and the image carrier falls within the new current range, and a magnitude of the initial voltage preferably varies with a change of the preset current range. 
     The image forming apparatus preferably further includes a storage device storing printing information, the printing information being voltage adjustment amounts associated with a plurality of sets of printing conditions, the sets being different from one another, and the controlling step preferably includes obtaining, from the printing information, the voltage adjustment amount associated with the set of the printing conditions for the transfer target member to be subjected to printing, and changing the transfer voltage by the voltage adjustment amount each time so that the magnitude of the current flowing between the transfer member and the image carrier falls within the new current range. 
     To achieve at least one of the abovementioned objects, according to an aspect, there is provided a non-transitory recording medium storing a computer readable program for controlling an image forming apparatus, the image forming apparatus including: an image carrier configured to carry a toner image; and a transfer member configured to transfer the toner image from the image carrier onto a transfer target member by applying a transfer voltage of the opposite polarity of the polarity of the toner image to the transfer target member passing through a portion in contact with the image carrier, the transfer member being in contact with the image carrier, and the control program reflecting one aspect of the present invention causes the image forming apparatus to carry out: a step of accepting a setting for changing a preset current range to a new current range, the preset current range being defined by at least one of a lower limit of a current flowing in the transfer target member passing through the contact portion between the image carrier and the transfer member and an upper limit of the current; a step of sensing a magnitude of the current flowing in the transfer target member passing through the contact portion between the image carrier and the transfer member; and a step of controlling the transfer voltage so that the magnitude of the current sensed by the sensing unit falls within the new current range. 
     The image forming apparatus preferably has a setting mode as an operation mode, and the accepting step preferably includes accepting a change of the preset current range while the operation mode is the setting mode. 
     The accepting step preferably includes accepting a change of the preset current range by accepting at least one of a new lower limit of the current and a new upper limit of the current. 
     The controlling step preferably includes gradually changing the transfer voltage from a predetermined initial voltage so that the magnitude of the current flowing between the transfer member and the image carrier falls within the new current range, and a magnitude of the initial voltage preferably varies with a change of the preset current range. 
     The image forming apparatus preferably further includes a storage device storing printing information, the printing information being voltage adjustment amounts associated with a plurality of sets of printing conditions, the sets being different from one another, and the controlling step preferably includes obtaining, from the printing information, the voltage adjustment amount associated with the set of the printing conditions for the transfer target member to be subjected to printing, and changing the transfer voltage by the voltage adjustment amount each time so that the magnitude of the current flowing between the transfer member and the image carrier falls within the new current range. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, advantages and features of the present invention will become more fully understood from the detailed description given hereinbelow and the appended drawings which are given by way of illustration only, and thus are not intended as a definition of the limits of the present invention, and wherein: 
         FIG. 1  is a diagram showing a configuration for performing secondary transfer of a toner image from an intermediate transfer belt onto a paper sheet; 
         FIG. 2  is a diagram showing the inner structure of an image forming apparatus; 
         FIG. 3  is a diagram showing a setting screen that is an example of an accepting unit; 
         FIG. 4  shows graphs indicating temporal changes in transfer voltage and transfer current; 
         FIG. 5  shows a graph indicating the correlations between the transfer voltage and the transfer current; 
         FIG. 6  is a graph showing the initial voltage that varies with the setting of the lower limit of a current range; 
         FIG. 7  is a graph showing the initial voltage that varies with the setting of the upper limit of a current range; 
         FIG. 8  is a table showing the contents of printing information; 
         FIG. 9  is a flowchart showing part of a process to be performed by the image forming apparatus; and 
         FIG. 10  is a block diagram showing the principal hardware configuration of the image forming apparatus. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Hereinafter, an embodiment of the present invention will be described with reference to the drawings. However, the scope of the invention is not limited to the illustrated examples. In the description below, like components and constituent elements are denoted by like reference numerals. Like components and constituent elements also have like names and functions. Therefore, detailed explanation of them will not be unnecessarily repeated. It should be noted that the embodiments and the modifications described below may be selectively combined as appropriate. 
     The above and other objects, features, aspects, and advantages of the present invention will become apparent from the detailed description given below in relation to the present invention to be understood in conjunction with the accompanying drawings. 
     [Secondary Transfer Process] 
     An electrophotographic image forming apparatus  100  performs a printing process that includes a process of forming a toner image on a photosensitive member, a process of transferring the toner image from the photosensitive member onto an intermediate transfer belt as a primary transfer process, and a process of transferring the toner image from the intermediate transfer belt onto a paper sheet as a secondary transfer process. 
     Referring now to  FIG. 1 , the secondary transfer process to be performed for a toner image  32  by the image forming apparatus  100  is described.  FIG. 1  is a diagram showing a configuration for performing secondary transfer of the toner image  32  from an intermediate transfer belt  30  onto a paper sheet S. The primary transfer process will be described later. 
     As shown in  FIG. 1 , the image forming apparatus  100  includes the intermediate transfer belt  30 , a secondary transfer member  33 , an accepting unit  34 , a voltage source  35 , a sensing unit  36 , a driving roller  39 , and a control device  101 . 
     The intermediate transfer belt  30  is an image carrier for carrying the toner image  32 . The intermediate transfer belt  30  is stretched around the later described following roller  38  (see  FIG. 2 ) and the driving roller  39 . Receiving a drive force from the driving roller  39 , the intermediate transfer belt  30  rotates to convey the toner image  32  to the secondary transfer member  33 . Although  FIG. 1  shows the intermediate transfer belt  30  as an example of the image carrier, the image carrier may be a later described photosensitive member  10  (see  FIG. 2 ). 
     The secondary transfer member  33  (the transfer member) is installed in contact with the intermediate transfer belt  30 . The secondary transfer member  33  applies a transfer voltage of the opposite polarity of that of the toner image  32  to the paper sheet S (a transfer target member) passing through the portion in contact with the intermediate transfer belt  30 , so that the toner image  32  on the intermediate transfer belt  30  is transferred onto the paper sheet S. With the application of the transfer voltage, the toner image  32  is attracted to the secondary transfer member  33  from the intermediate transfer belt  30 , and is then transferred onto the paper sheet S being conveyed between the image carrier and the transfer member. Although  FIG. 1  shows the secondary transfer member  33  as an example of the transfer member, the transfer member may be a later described primary transfer member  31  (see  FIG. 2 ). Although  FIG. 1  shows the paper sheet S as an example of a transfer target member onto which a toner image is to be transferred, the transfer target member may be some other kind of sheet. 
     The accepting unit  34  accepts a setting to change a preset current range to a new current range. The preset current range is defined by a lower limit M 1  of the current flowing in the paper sheet S passing through the contact portion between the intermediate transfer belt  30  and the secondary transfer member  33  (this current will be hereinafter also referred to as “transfer current”), and/or an upper limit M 2  of the transfer current. The accepting unit  34  will be described later with reference to  FIG. 3 . 
     The voltage source  35  applies a voltage to the intermediate transfer belt  30  and the secondary transfer member  33 . The voltage source  35  is a variable voltage source, and outputs a different voltage in accordance with a voltage control instruction from the control device  101 . The voltage source  35  is electrically connected to a node N 2  of the secondary transfer member  33  and ground, and is located between the node N 2  and the ground. 
     The intermediate transfer belt  30 , the secondary transfer member  33 , and the driving roller  39  are in contact with one another between nodes N 1  and N 2 , and the nodes N 1  and N 2  are connected to the ground. Accordingly, a transfer voltage corresponding to a resistance R between the nodes N 1  and N 2  is generated in the paper sheet S. As a result, a transfer current flows between the intermediate transfer belt  30  and the secondary transfer member  33 . The sensing unit  36  senses the magnitude of the transfer current flowing in the paper sheet S passing through the contact portion between the intermediate transfer belt  30  and the secondary transfer member  33 . The sensing unit  36  is a current sensor, for example. The magnitude of the transfer current is represented by a current value, for example. 
     The control device  101  controls the transfer voltage so that the magnitude of the transfer current to be sensed by the sensing unit  36  falls within the current range that is set as described above. More specifically, when the magnitude of the transfer current is smaller than the lower limit M 1 , the control device  101  issues a voltage control instruction for increasing the transfer voltage at this point of time, to the voltage source  35 . When the magnitude of the transfer current is greater than the upper limit M 2 , the control device  101  issues a voltage control instruction for lowering the transfer voltage to the voltage source  35 . The control device  101  repeats the sensing of the transfer current and the control of the transfer voltage until the transfer current falls within the current range. 
     In the above manner, the image forming apparatus  100  accepts a setting of a current range. In a case where printing is to be performed on a paper sheet with an unexpectedly high resistance value (a paper sheet with poor quality, for example), print quality can be more efficiently controlled with a change in the transfer current than with a change in the transfer voltage. Accordingly, the user of the image forming apparatus  100  and the maintenance staff for the image forming apparatus  100  can greatly change print quality by setting a current range. In this manner, the number of times test printing needs to be performed to achieve desired print quality can be reduced. Thus, the workload can be reduced. 
     [Inner Structure of the Image Forming Apparatus  100 ] 
     Referring now to  FIG. 2 , the image forming apparatus  100  is described.  FIG. 2  is a diagram showing the inner structure of the image forming apparatus  100 . 
     The image forming apparatus  100  as a color printer is shown in  FIG. 2 . Although the image forming apparatus  100  as a color printer will be described below, the image forming apparatus  100  is not necessarily a color printer. For example, the image forming apparatus  100  may be a monochrome printer, a facsimile machine, or multifunctional peripherals (MFP) that function as a monochrome printer, a color printer, and a facsimile machine. 
     The image forming apparatus  100  includes image forming units  1 Y,  1 M,  1 C, and  1 K, the intermediate transfer belt  30 , the primary transfer member  31 , the secondary transfer member  33 , a cassette  37 , the following roller  38 , the driving roller  39 , timing rollers  40 , a fixing device  50 , a cleaning blade  42 , and the control device  101 . 
     The image forming unit  1 Y receives a supply of toner from a toner bottle  15 Y, and forms a yellow (Y) toner image. The image forming unit  1 M receives a supply of toner from a toner bottle  15 M, and forms a magenta (M) toner image. The image forming unit  1 C receives a supply of toner from a toner bottle  15 C, and forms a cyan (C) toner image. The image forming unit  1 K receives a supply of toner from a toner bottle  15 K, and forms a black (BK) toner image. 
     The image forming units  1 Y,  1 M,  1 C, and  1 K are arranged along the intermediate transfer belt  30  in the direction of rotation of the intermediate transfer belt  30 . The image forming units  1 Y,  1 M,  1 C, and  1 K each include a photosensitive member  10 , a charging unit  11 , an exposing unit  12 , a developing unit  13 , and a cleaning blade  17 . 
     The charging unit  11  uniformly charges the surface of the photosensitive member  10 . The exposing unit  12  emits laser light onto the photosensitive member  10  in accordance with a control signal from the control device  101 , and exposes the surface of the photosensitive member  10  in accordance with an image pattern that has been input. As a result, an electrostatic latent image corresponding to the input image is formed on the photosensitive member  10 . 
     The developing unit  13  applies a developing bias to a developing roller  14  while rotating the developing roller  14 , so that toner adheres to the surface of the developing roller  14 . The toner is then transferred from the developing roller  14  onto the photosensitive member  10 , and a toner image corresponding to the electrostatic latent image is developed on the surface of the photosensitive member  10 . 
     The photosensitive member  10  and the intermediate transfer belt  30  are in contact with each other at the portion where the primary transfer member  31  is provided. The primary transfer member  31  is in the form of a roller, and is designed to rotate. As a transfer voltage of the opposite polarity of that of the toner image is applied to the primary transfer member  31 , the toner image is transferred from the photosensitive member  10  onto the intermediate transfer belt  30 . The yellow (Y) toner image, the magenta (M) toner image, the cyan (C) toner image, and the black (BK) toner image are sequentially transferred from the photosensitive member  10  onto the intermediate transfer belt  30  in an overlapping manner. As a result, a color toner image is formed on the intermediate transfer belt  30 . 
     The intermediate transfer belt  30  is stretched around the following roller  38  and the driving roller  39 . The driving roller  39  is connected to a motor (not shown). The motor is controlled by the control device  101 , for example. The method of controlling the motor may be pulse width modulation (PWM) control, for example. As the control device  101  controls the motor, the driving roller  39  rotates. The intermediate transfer belt  30  and the following roller  38  rotate with the driving roller  39 . As a result, the toner image on the intermediate transfer belt  30  is conveyed to the secondary transfer member  33 . 
     The cleaning blade  17  is pressed against the photosensitive member  10 . The cleaning blade  17  collects toner remaining on the surface of the photosensitive member  10  after the transfer of the toner image from the photosensitive member  10  onto the intermediate transfer belt  30 . 
     Paper sheets S are stored in the cassette  37 . The paper sheets S are sent one by one from the cassette  37  to the secondary transfer member  33  through a conveyance path  41  by the timing rollers  40 . In time with the sending of each paper sheet S, the control device  101  controls the transfer voltage to be applied to the secondary transfer member  33 . 
     The secondary transfer member  33  is in the form of a roller, and is designed to rotate. The secondary transfer member  33  applies a transfer voltage of the opposite polarity of that of the toner image to the paper sheet S being conveyed. As a result, the toner image is attracted to the secondary transfer member  33  from the intermediate transfer belt  30 . Thus, the toner image on the intermediate transfer belt  30  is transferred. The timing of conveyance of the paper sheet S to the secondary transfer member  33  is controlled by the timing rollers  40  in accordance with the position of the toner image on the intermediate transfer belt  30 . As a result, the toner image on the intermediate transfer belt  30  is transferred to an appropriate position on the paper sheet S. 
     The fixing device  50  includes a heating roller  51  and a pressure roller  52 . The fixing device  50  causes the paper sheet S to pass through the portion between the heating roller  51  and the pressure roller  52 , and applies pressure and heat to the paper sheet S. As a result, the toner image transferred onto the paper sheet S is fixed to the paper sheet S. After that, the paper sheet S is discharged onto a tray  48 . 
     The cleaning blade  42  is pressed against the intermediate transfer belt  30 . The cleaning blade  42  collects toner remaining on the surface of the intermediate transfer belt  30  after the transfer of the toner image from the intermediate transfer belt  30  onto the paper sheet S. The collected toner is conveyed by a conveyance screw (not shown), and is stored into a toner waste container (not shown). 
     [Current Range Setting Screen] 
     Referring now to  FIG. 3 , an example of the accepting unit  34  (see  FIG. 1 ) that accepts a setting of a current range is described.  FIG. 3  is a diagram showing a setting screen  70  that is an example of the accepting unit  34 . 
     The setting screen  70  accepts a setting for changing a preset current range to a new current range. As shown in  FIG. 3 , the setting screen  70  includes a graph  71  and an input region  75 . The setting screen  70  is displayed on a display unit of a later described operation panel  107  (see  FIG. 10 ), for example. 
     The graph  71  shows correlations  72  through  74 , the lower limit M 1  of the current range, the upper limit M 2  of the current range, and an initial voltage V 0 . The initial voltage V 0  will be described later in detail. The correlation  72  indicates the relationship between the transfer voltage and the transfer current in the case of low-resistance paper. The correlation  73  indicates the relationship between the transfer voltage and the transfer current in the case of normal-resistance paper. The correlation  74  indicates the relationship between the transfer voltage and the transfer current in the case of high-resistance paper. As shown in the graph  71 , as the resistance of the paper sheet becomes higher, the increase in the transfer current relative to an increase in the transfer voltage becomes smaller. 
     The user can input the lower limit M 1  of the transfer current and the upper limit M 2  of the transfer current to the input region  75 . Buttons  76  through  79  are displayed in the input region  75 . When the button  76  is pressed, the lower limit M 1  becomes higher. When the button  77  is pressed, the lower limit M 1  becomes lower. When the button  78  is pressed, the upper limit M 2  becomes higher. When the button  79  is pressed, the upper limit M 2  becomes lower. The user can also input the lower limit M 1  directly to a text box  80 . The user can input the upper limit M 2  directly to a text box  81 . The display of the lower limit M 1  and the upper limit M 2  in the graph  71  preferably changes with values that are input to the input region  75 . 
     The lower limit M 1  and the upper limit M 2  shown in the graph  71  are proportional to values that are input to the input region  75 . In the example shown in  FIG. 3 , “−5” is input as an input value of the lower limit M 1  in the input region  75 . With this input, the lower limit M 1  in the graph  71  decreases from 60 μA to 50 μA. That is, every time the input value in the input region  75  becomes lower by “1”, the lower limit M 1  shown in the graph  71  becomes lower by “2 μA”. 
     The image forming apparatus  100  preferably has a setting mode as an operation mode. While the operation mode of the image forming apparatus  100  is the setting mode, the setting screen  70  accepts a change of the preset current range. That is, when the user selects the setting mode as the operation mode, the image forming apparatus  100  displays the setting screen  70 . 
     When the user presses a save button  83 , the image forming apparatus  100  saves the lower limit M 1  and the upper limit M 2  input to the input region  75  as the current range. It is not necessary to set both the lower limit M 1  and the upper limit M 2 , and it is possible to set only either the lower limit M 1  or the upper limit M 2 . The setting screen  70  accepts a change of the preset current range by newly accepting a lower limit M 1  of the transfer current and/or an upper limit M 2  of the transfer current. 
     As the current range is set in the above manner, the user can conduct printing with desired image quality. Noise that adversely affects image quality may be roughness or white dots, for example. However, some users allow such noise. For example, some users prefer to reduce roughness but allow white dots. As the current range is set in the above manner, users can achieve any desired image quality. 
     Although the setting screen  70  has been described above as an example of the accepting unit  34 , the accepting unit  34  is not necessarily the setting screen  70 . For example, the accepting unit  34  may be a settings file in which the lower limit M 1  of the transfer current and the upper limit M 2  of the transfer current are specified. In this case, the user sets the lower limit M 1  and the upper limit M 2  of the transfer current in the settings file. The settings file is stored in a later described storage device  120  (see  FIG. 10 ), for example. 
     [Transfer Voltage Control Process] 
     As described above, the control device  101  (see  FIG. 1 ) controls the transfer voltage so that the transfer current falls within the current range. Referring now to  FIGS. 4 and 5 , a transfer voltage control process to be performed by the control device  101  is described.  FIG. 4  shows graphs indicating temporal changes in the transfer voltage and the transfer current.  FIG. 5  shows a graph indicating the correlations between the transfer voltage and the transfer current. 
     As shown in  FIG. 4 , the control device  101  gradually changes the transfer voltage from the preset initial voltage V 0 , so that the magnitude of the transfer current falls within a newly set current range. The initial voltage V 0  is determined beforehand by auto transfer voltage control (ATVC), for example. ATVC is a control method for automatically determining the initial value of a transfer voltage. More specifically, the image forming apparatus  100  that uses ATVC causes a constant current to flow between the intermediate transfer belt  30  (see  FIG. 1 ) and the secondary transfer member  33  (see  FIG. 1 ), and calculates the resistance value between the intermediate transfer belt  30  and the secondary transfer member  33  from the voltage generated at the time. After that, in accordance with a preset table in which the correlations between resistance values and transfer voltages are specified, the image forming apparatus  100  identifies the transfer voltage corresponding to the calculated resistance value, and determines the identified transfer voltage to be the initial voltage V 0 . 
     At time T 0 , the top edge of a paper sheet reaches the contact portion between the intermediate transfer belt  30  and the secondary transfer member  33 . Because of this, the control device  101  applies the initial voltage V 0  to the paper sheet. As a result, a transfer current I 0  flows in the paper sheet. 
     At time T 1 , the control device  101  compares a current range  85  with the transfer current I 0 . Since the magnitude of the transfer current I 0  is smaller than the lower limit M 1  of the current range  85 , the control device  101  increases the transfer voltage by a constant value ΔV. Accordingly, the transfer voltage increases from the initial voltage V 0  to a voltage V 1 . As a result, a transfer current I 1  flows in the paper sheet. 
     At time T 2 , the control device  101  compares the current range  85  with the transfer current I 1 . Since the magnitude of the transfer current I 1  is smaller than the lower limit M 1  of the current range  85 , the control device  101  increases the transfer voltage by the constant value ΔV. Accordingly, the transfer voltage increases from the voltage V 1  to a voltage V 2 . As a result, a transfer current I 2  flows in the paper sheet. 
     At time T 3 , the control device  101  compares the current range  85  with the transfer current I 2 . Since the magnitude of the transfer current I 2  is smaller than the lower limit M 1  of the current range  85 , the control device  101  increases the transfer voltage by the constant value ΔV. Accordingly, the transfer voltage increases from the voltage V 2  to a voltage V 3 . As a result, a transfer current I 3  flows in the paper sheet. 
     At time T 4 , the control device  101  compares the current range  85  with the transfer current I 3 . Since the magnitude of the transfer current I 3  is smaller than the lower limit M 1  of the current range  85 , the control device  101  increases the transfer voltage by the constant value ΔV. Accordingly, the transfer voltage increases from the voltage V 3  to a voltage V 4 . As a result, a transfer current I 4  flows in the paper sheet. 
     At time T 5 , the control device  101  compares the current range  85  with the transfer current I 4 . Since the magnitude of the transfer current I 4  is greater than the lower limit M 1  of the current range  85 , the control device  101  maintains the transfer voltage at this point of time. In this manner, the control device  101  increases the transfer voltage from the initial voltage V 0  by the constant value ΔV each time, so that the transfer current falls within the current range  85 . 
     The control device  101  preferably controls the transfer voltage after determining whether the paper sheet to be subjected to printing is high-resistance paper. More specifically, when the transfer current at the time of application of the initial voltage V 0  is smaller than the lower limit M 1  of the current range  85 , the control device  101  determines that the paper sheet to be subjected to printing is high-resistance paper, as shown in  FIG. 5 . After determining that the paper sheet to be subjected to printing is high-resistance paper, the control device  101  repeatedly increases the transfer voltage until the transfer current becomes greater than the lower limit M 1  of the current range  85 . 
     In the example shown in  FIG. 5 , the transfer voltage is repeatedly increased until the transfer current exceeds 60 μA. As a result, the transfer voltage reaches 2000 V. After the transfer current exceeds 60 μA, the control device  101  maintains the transfer voltage at 2000 V. If the transfer current again becomes lower than 60 μA before the printing is completed, the control device  101  cancels the maintenance of the transfer voltage, and again repeats the sensing of the transfer current and the adjustment of the transfer voltage until the transfer current becomes 60 μA or greater. 
     More preferably, the control device  101  controls the transfer voltage after determining whether the paper sheet to be subjected to printing is low-resistance paper. When the transfer current at the time of application of the initial voltage V 0  is greater than the upper limit M 2  of the current range  85 , the control device  101  determines that the paper sheet to be subjected to printing is low-resistance paper. After determining that the paper sheet to be subjected to printing is low-resistance paper, the control device  101  repeatedly lowers the transfer voltage until the transfer current becomes smaller than the upper limit M 2  of the current range  85 . 
     In the example shown in  FIG. 5 , the transfer voltage is repeatedly lowered until the transfer current becomes lower than 150 μA. As a result, the transfer voltage is lowered to 1300 V. After the transfer current becomes lower than 150 μA, the control device  101  maintains the transfer voltage at 1300 V. If the transfer current again becomes higher than 150 μA before the printing is completed, the control device  101  cancels the maintenance of the transfer voltage, and again repeats the sensing of the transfer current and the adjustment of the transfer voltage until the transfer current becomes 150 μA or smaller. 
     When the transfer current at the time of application of the initial voltage V 0  is not smaller than the lower limit M 1  of the current range  85  and not greater than the upper limit M 2  of the current range  85 , the control device  101  determines that the paper sheet to be subjected to printing is normal-resistance paper. After determining that the paper sheet to be subjected to printing is normal-resistance paper, the control device  101  does not change the transfer voltage at this point of time, and maintains the transfer current at this point of time. 
     [Variation of the Initial Voltage V 0 ] 
     Referring now to  FIGS. 6 and 7 , the initial voltage V 0  is further described.  FIG. 6  is a graph showing the initial voltage V 0  that varies with the setting of the lower limit M 1  of a current range.  FIG. 7  is a graph showing the initial voltage V 0  that varies with the setting of the upper limit M 2  of a current range. 
     As described above, the control device  101  gradually changes the transfer voltage from the preset initial voltage V 0 , so that the magnitude of the current flowing between the intermediate transfer belt  30  and the secondary transfer member  33  falls within a newly set current range. The magnitude of the initial voltage V 0  varies with change that is made to the preset current range. 
     More specifically, the user may change the lower limit M 1  of the transfer current to a lower limit M 1 ′, for example, as shown in  FIG. 6 . In this case, the control device  101  changes the initial voltage V 0  to an initial voltage V 0 ′ in accordance with the change from the lower limit M 1  to the lower limit M 1 ′. In the example shown in  FIG. 6 , the user has changed the lower limit M 1  of the transfer current from 60 μA to 50 μA, and accordingly, the initial voltage V 0  has been increased from 1500 V to 1700 V. 
     The control device  101  starts adjusting the transfer voltage at the changed initial voltage V 0 ′, and repeats the adjustment of the transfer voltage until the transfer current exceeds the lower limit M 1 ′. The transfer current is adjusted in real time when printing is performed on the paper sheet. Because of this, the transfer current is also adjusted while the paper sheet passes through the contact portion between the intermediate transfer belt  30  (see  FIG. 1 ) and the secondary transfer member  33  (see  FIG. 1 ). As a result, a time lag is generated between the time when the top edge of the paper sheet reaches the contact portion and the time when the transfer current is adjusted to an optimum transfer current. The time required for the transfer current to exceed the lower limit M 1 ′ is shorter in a case where the transfer voltage adjustment is started at the initial voltage V 0 ′ than in a case where the transfer voltage adjustment is started at the initial voltage V 0  (as indicated by arrows  87  and  88 ). With this, the image forming apparatus  100  can also increase image quality at the top edge of the paper sheet. 
     As shown in  FIG. 7 , the user may change the upper limit M 2  of the transfer current to an upper limit M 2 ′, for example. In this case, the control device  101  changes the initial voltage V 0  to an initial voltage V 0 ′ in accordance with the change from the upper limit M 2  to the upper limit M 2 ′. In the example shown in  FIG. 7 , the user has changed the upper limit M 2  of the transfer current from 150 μA to 160 μA, and accordingly, the initial voltage V 0  has been lowered from 1500 V to 1300 V. 
     The control device  101  starts adjusting the transfer voltage at the initial voltage V 0 ′, and repeats the adjustment of the transfer voltage until the transfer current becomes lower than the upper limit M 2 ′. The time required for the transfer current to become lower than the upper limit M 2 ′ is shorter in a case where the transfer voltage adjustment is started at the initial voltage V 0 ′ than in a case where the transfer voltage adjustment is started at the initial voltage V 0  (as indicated by arrows  90  and  91 ). With this, the image forming apparatus  100  can also increase image quality at the top edge of the paper sheet. 
     In the above manner, the control device  101  causes the initial voltage to vary with the setting of the current range. When the lower limit M 1  of the current range is changed, the control device  101  preferably increases the initial voltage. When the upper limit M 2  of the current range is changed, the control device  101  preferably lowers the initial voltage. 
     [Adjustment Amounts of Transfer Voltage and Transfer Current] 
     As the magnitude of the transfer current changes with printing conditions, the degree of change of the transfer current also changes with printing conditions. In view of this, to further increase print precision, the transfer current and the transfer voltage are preferably adjusted in accordance with printing conditions. 
     Referring now to  FIG. 8 , a method of determining adjustment amounts of the transfer current and the transfer voltage in accordance with printing conditions is described.  FIG. 8  is a table showing the contents of printing information  124 . 
     In the printing information  124 , current adjustment amounts and voltage adjustment amounts are associated with each set of printing conditions. The printing conditions for a paper sheet include the sheet conveyance speed, the type of the paper sheet, the printing side, the sheet width, the coverage, and the environment at the time of printing. The coverage indicates the proportion of the area of the toner image in the area of the paper sheet. The environment at the time of printing is indicated by the temperature and the humidity of the inside of the image forming apparatus  100 , for example. 
     Each current adjustment amount specified in the printing information  124  indicates an amount of change in transfer current with respect to a value input to the current range setting screen  70  (see  FIG. 3 ). Each current adjustment amount is specified with respect to both the lower limit M 1  and the upper limit M 2  of a current range. In a case where the current adjustment amount at the lower limit M 1  of a current range is 5 μA, for example, the lower limit M 1  of the current range changes by 5 μA every time the value input to the setting screen  70  changes by “1”. In a case where the current adjustment amount at the upper limit M 2  of a current range is 30 μA, the upper limit M 2  of the current range changes by 30 μA every time the value input to the setting screen  70  changes by “1”. 
     Each voltage adjustment amount specified in the printing information  124  is equivalent to the constant value ΔV shown in  FIG. 4 . As described above, the control device  101  changes the transfer voltage by the constant value ΔV each time when adjusting the transfer current. From the printing information  124 , the image forming apparatus  100  obtains the voltage adjustment amount associated with the printing conditions at the time of printing on a paper sheet. When adjusting the transfer current, the image forming apparatus  100  changes the transfer voltage by the obtained voltage adjustment amount each time. 
     As described above, voltage adjustment amounts associated with the respective sets of printing conditions that differ from one another are stored as the printing information  124  in the image forming apparatus  100 . The printing information  124  is stored in the later described storage device  120  (see  FIG. 10 ), for example. From the printing information  124 , the control device  101  obtains the voltage adjustment amount associated with the printing conditions for the paper sheet to be subjected to printing. As the control device  101  changes the transfer voltage by the obtained voltage adjustment amount each time, the magnitude of the transfer current flowing between the intermediate transfer belt  30  and the secondary transfer member  33  falls within the newly set current range. With this, the image forming apparatus  100  can change the transfer voltage in accordance with printing conditions, and further increase print quality. 
     The sheet conveyance speed, the type of the paper sheet, the printing side, and the sheet width are obtained from the print settings that are set at the time of printing. The environment in the image forming apparatus  100  is determined in accordance with a temperature sensor (not shown), a humidity sensor (not shown), and the like installed in the image forming apparatus  100 . The coverage of a paper sheet is calculated in accordance with an image obtained by taking an image of the toner image. 
     Although the printing information  124  is shown as a table in  FIG. 8 , the printing information  124  is not necessarily expressed as a table. For example, the printing information  124  may be shown as relational expressions that indicate the printing conditions as explanatory variables, and current adjustment amounts or voltage adjustment amounts as objective variables. 
     [Control Structure of the Image Forming Apparatus  100 ] 
     Referring now to  FIG. 9 , the control structure of the image forming apparatus  100  is described.  FIG. 9  is a flowchart showing part of a process to be performed by the image forming apparatus  100 . The process shown in  FIG. 9  is performed when the control device  101  executes a program. In other embodiments, part of or all of the process may be performed by a circuit element or some other hardware. 
     In step S 10 , the control device  101  determines whether the setting mode is selected as the operation mode of the image forming apparatus  100 . The operation mode of the image forming apparatus  100  may be the setting mode, a print mode, a scan mode, or the like. If the control device  101  determines that the setting mode is selected as the operation mode of the image forming apparatus  100  (YES in step S 10 ), the control process is switched to step S 12 . If the control device  101  determines that the setting mode is not selected (NO in step S 10 ), the control process is switched to step S 20 . 
     In step S 12 , the control device  101  accepts a setting to change a preset current range to a new current range. The preset current range is defined by the lower limit M 1  of the transfer current flowing in the paper sheet passing through the contact portion between the intermediate transfer belt  30  (see  FIG. 1 ) and the secondary transfer member  33  (see  FIG. 1 ), and/or the upper limit M 2  of the transfer current. 
     In step S 20 , the control device  101  determines whether printing has been started. If the control device  101  determines that printing has been started (YES in step S 20 ), the control process is switched to step S 22 . If the control device  101  determines that printing has not been started (NO in step S 20 ), the control process returns to step S 10 . 
     In step S 22 , the control device  101  senses the magnitude of the transfer current flowing in the transfer target member passing through the contact portion between the intermediate transfer belt  30  (see  FIG. 1 ) and the secondary transfer member  33  (see  FIG. 1 ). 
     In step S 24 , the control device  101  controls the transfer voltage so that the magnitude of the transfer current sensed in step S 22  falls within the current range. More specifically, if the transfer current is smaller than the lower limit M 1  of the current range, the control device  101  increases the transfer voltage. If the transfer current is greater than the upper limit M 2  of the current range, the control device  101  lowers the transfer voltage. 
     In step S 30 , the control device  101  determines whether printing has been completed on all the paper sheets. If the control device  101  determines that printing has been completed on all the paper sheets (YES in step S 30 ), the transfer voltage control process comes to an end. If the control device  101  determines that printing has not been completed on all the paper sheets (NO in step S 30 ), the control process returns to step S 22 . 
     By virtue of the procedure in step S 30 , the steps S 22  and S 24  are repeated until printing is completed. In this manner, the sensing of the transfer current and the control of the transfer voltage are repeated until the transfer current falls within the current range. 
     [Hardware Configuration of the Image Forming Apparatus  100 ] 
     Referring now to  FIG. 10 , an example of the hardware configuration of the image forming apparatus  100  is described.  FIG. 10  is a block diagram showing the principal hardware configuration of the image forming apparatus  100 . 
     As shown in  FIG. 10 , the image forming apparatus  100  includes the control device  101 , a read only memory (ROM)  102 , a random access memory (RAM)  103 , a network interface  104 , the operation panel  107 , and the storage device  120 . 
     The control device  101  is formed with at least one integrated circuit, for example. An integrated circuit is formed with at least one central processing unit (CPU), at least one application specific integrated circuit (ASIC), at least one field programmable gate array (FPGA), or a combination of these circuits. 
     The control device  101  controls operation of the image forming apparatus  100  by executing various programs, such as a control program  122  according to this embodiment. Upon receipt of an instruction to execute the control program  122 , the control device  101  reads the control program  122  from the storage device  120  into the ROM  102 . The RAM  103  functions as a working memory, and temporarily stores various kinds of data necessary for executing the control program  122 . 
     An antenna (not shown) or the like is connected to the network interface  104 . The image forming apparatus  100  exchanges data with external communication devices via the antenna. Examples of such external communication devices include mobile communication terminals, such as smartphones, and servers. The image forming apparatus  100  may be designed to download the control program  122  from a server via the antenna. 
     The operation panel  107  is formed with a display unit and a touch panel. The display unit and the touch panel are overlapped on each other, and the operation panel  107  accepts a touch operation performed on the display unit. The operation panel  107  accepts a print operation, a scan operation, and the like for the image forming apparatus  100 . The display unit displays the current range setting screen  70  (see  FIG. 3 ) and the like. 
     The storage device  120  is a storage medium, such as a hard disk or an external storage device. The storage device  120  stores the control program  122  according to this embodiment, the printing information  124  (see  FIG. 10 ), and the like. The location of storage of the printing information  124  is not necessarily the storage device  120 . The printing information  124  may be stored in a storage area (such as a cache) in the control device  101 , the ROM  102 , the RAM  103 , an external device (such as a server), or the like. 
     The control program  122  may not be provided as a single program, but may be incorporated into any appropriate program. In that case, the control process according to this embodiment is performed in cooperation with any appropriate program. Even such a program that does not include some module does not depart from the scope of the control program  122  according to this embodiment. Further, some function(s) or all of the functions to be provided by the control program  122  may be provided by special-purpose hardware. Alternatively, the image forming apparatus  100  may be in the form a cloud service, and at least one server performs part of the process according to the control program  122 . 
     SUMMARY 
     In the above described manner, the image forming apparatus  100  accepts a setting of a current range that indicates the variation range of the transfer current. In a case where printing is to be performed on a paper sheet with an unexpectedly high resistance value (a paper sheet with poor quality, for example), print quality can be more efficiently controlled with a change in the transfer current than with a change in the transfer voltage. Accordingly, the user of the image forming apparatus  100  and the maintenance staff for the image forming apparatus  100  can greatly change print quality by setting a current range. In this manner, the number of times test printing needs to be performed to achieve desired print quality can be reduced. Thus, the workload can be reduced. 
     Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustrated and example only and is not to be taken byway of limitation, the scope of the present invention being interpreted by terms of the appended claims. It should be understood that equivalents of the claimed inventions and all modifications thereof are incorporated herein.