Patent Publication Number: US-9897947-B2

Title: Image forming apparatus executing charge removal for photoconductor thereof and control method for same

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This patent application is based on and claims priority pursuant to 35 U.S.C. § 119 to Japanese Patent Application No. 2015-181594, filed on Sep. 15, 2015, in the Japan Patent Office, the entire disclosure of which is hereby incorporated by reference herein. 
     BACKGROUND 
     Technical Field 
     Exemplary aspects of the present disclosure relate to an image forming apparatus and a control method for the image forming apparatus. 
     Related Art 
     In recent years, digitization of information tends to be promoted, and image processing apparatuses such as printers and facsimile machines used for output of digitized information and scanners used for digitization of documents become indispensable. In most cases, such image processing apparatuses have an image capturing function, an image forming function, and a communication function to serve as a multi-function peripheral capable of being used as a printer, a facsimile machine, a scanner, and a copier. 
     Among such image processing apparatuses, an electrophotographic image forming apparatus that is one example of image forming apparatuses used for output of digitized documents is widely used. The electrophotographic image forming apparatus irradiates a photoconductor thereof with light to form an electrostatic latent image on the photoconductor, develops the electrostatic latent image with developer such as toner to form a toner image on the photoconductor, transfers the toner image to a sheet using a transfer device, and outputs the sheet with the transferred image. 
     After transferring the toner image developed on the photoconductor, the electrophotographic image forming apparatus removes residual electric charge from the photoconductor. The electric charge remaining on the photoconductor can be removed by irradiating a surface of the photoconductor with light (hereinafter called “charge removal irradiation”) or discharging the surface of the photoconductor (hereinafter called “charge removal discharge”). 
     SUMMARY 
     In at least one embodiment of this disclosure, there is provided an improved image forming apparatus that includes a photoconductor on which an electrostatic latent image is formed by irradiation of the photoconductor with light, charger, a developing device, a transfer device, a charge removal execution determiner, and a power supply controller. The charger receives a superimposed voltage of a direct current voltage and an alternating current voltage to charge the photoconductor. The developing device develops the electrostatic latent image formed on the photoconductor into a toner image. The transfer device transfers the toner image developed by the developing device to a recording medium. The charge removal execution determiner issues a charge removal command when a flow of electric charge from the transfer device into the photoconductor has occurred in an image forming outputting operation. The power supply controller applies only the alternating current voltage to the charger for a predetermined period in a state in which the photoconductor is rotated, when the charge removal execution determiner issues the charge removal command. 
     In at least one embodiment of this disclosure, there is provided an improved method for controlling an image forming apparatus. The control method includes charging a photoconductor disposed in the image forming apparatus and on which an electrostatic latent image is formed by irradiation of the photoconductor with light using a charger that receives superimposed voltage of a direct current voltage and an alternating current voltage, developing the electrostatic latent image formed on the photoconductor into a toner image using a developing device, transferring the toner image developed by the developing device to a recording medium using a transfer device, issuing an charge removal command when a flow of electric charge from the transfer device into the photoconductor has occurred in an image forming outputting operation, applying only the alternating current voltage to the charger for a predetermined period in a state in which the photoconductor is rotated when the charge removal execution determiner issues the charge removal command. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The aforementioned and other aspects, features, and advantages of the present disclosure would be better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
         FIG. 1  is a block diagram illustrating hardware of an image forming apparatus according to an exemplary embodiment; 
         FIG. 2  is a block diagram illustrating a functional configuration of the image forming apparatus according to the exemplary embodiment; 
         FIG. 3  is a schematic diagram illustrating the image forming apparatus according to the exemplary embodiment; 
         FIG. 4  is a diagram illustrating an image forming unit disposed in the image forming apparatus according to the exemplary embodiment; 
         FIG. 5  is a diagram illustrating electric charge flowing to a photoconductor drum disposed in the image forming apparatus according to the exemplary embodiment; 
         FIG. 6  is a diagram illustrating the electric charge flowing to the photoconductor drum; 
         FIG. 7  is a diagram illustrating a charge power supply device disposed in the image forming apparatus according to the exemplary embodiment; 
         FIG. 8  is a diagram illustrating a transfer power supply device disposed in the image forming apparatus according to the exemplary embodiment; 
         FIG. 9  is a diagram illustrating a control configuration of the image forming apparatus according to the exemplary embodiment; 
         FIG. 10  is a flowchart illustrating a recovery operation performed by the image forming apparatus according to the exemplary embodiment; 
         FIG. 11  is a diagram illustrating a relation between a charge potential and environment of an image forming apparatus according to another exemplary embodiment; and 
         FIG. 12  is a diagram illustrating a relation between a travel distance and a layer thickness of a photoconductor drum of an image forming apparatus according to another exemplary embodiment. 
     
    
    
     The accompanying drawings are intended to depict exemplary embodiments of the present disclosure and should not be interpreted to limit the scope thereof. The accompanying drawings are not to be considered as drawn to scale unless explicitly noted. 
     DETAILED DESCRIPTION 
     In describing embodiments illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the disclosure of this patent specification is not intended to be limited to the specific terminology so selected and it is to be understood that each specific element includes all technical equivalents that have the same function, operate in a similar manner, and achieve similar results. 
     Although the exemplary embodiments are described with technical limitations with reference to the attached drawings, such description is not intended to limit the scope of the disclosure and all of the components or elements described in the exemplary embodiments of this disclosure are not necessarily indispensable. 
     Referring now to the drawings, exemplary embodiments of the present disclosure are described below. In the drawings for explaining the following exemplary embodiments, the same reference codes are allocated to elements (members or components) having the same function or shape and redundant descriptions thereof are omitted below. 
     Hereinafter, a multifunctional peripheral (MFP) is described as one example of an image forming apparatus of an exemplary embodiment. 
       FIG. 1  is a block diagram illustrating hardware of an image forming apparatus  1  according to the exemplary embodiment. As illustrated in  FIG. 1 , a configuration of the image forming apparatus  1  is similar to that of a general personal computer (PC) or an information processing apparatus such as a server. That is, the image forming apparatus  1  according to the exemplary embodiment includes a central processing unit (CPU)  11 , a random access memory (RAM)  12 , a read only memory (ROM)  13 , a hard disk drive (HDD)  14 , and an interface (I/F)  15  that are connected via a bus  19 . Moreover, the image forming apparatus  1  includes a liquid crystal display (LCD)  16 , a control panel  17 , and a dedicated device  18  that are connected to the I/F  15 . 
     The CPU  11  as an operation unit comprehensively controls operations of the image forming apparatus  1 . The RAM  12  is a volatile storage medium, and information can be read from and written in the RAM  12  at high speed. The RAM  12  is used as a working area when the CPU  11  processes information. The ROM  13  is a non-volatile read only storage medium in which programs such as firmware are stored. The HDD  14  is a non-volatile storage medium, and information can be read from and written in the HDD  14 . For example, the HDD  14  stores an operating system (OS), various control programs, and application programs. 
     The I/F  15  connects the bus  19  to various hardware or a network, and controls such connection. The LCD  16  as a visual user interface is used when a user checks a state of the image forming apparatus  1 . The control panel  17  as a user interface is used when the user inputs information to the image forming apparatus  1 . In the exemplary embodiment, the control panel  17  includes a touch panel or hard keys. 
     The dedicated device  18  of hardware operates so that the image forming apparatus  1  provides a specific function. The dedicated device  18  is, for example, a print engine for forming an image on a sheet, and a scanner unit for reading an image on a sheet. The image forming apparatus  1  of the exemplary embodiment is characterized by the print engine. 
     Moreover, a temperature humidity sensor for measuring temperature and humidity inside the image forming apparatus  1  may be disposed as the dedicated device  18 . In such a case, the temperature humidity sensor includes a thermistor having a low heat capacity or a temperature sensor such as a silicon-type integrated circuit (IC) sensor, and a humidity sensor such as a polymer-film variable resistance sensor. 
     With such a hardware configuration, the CPU  11  performs computation according to a program stored in the ROM  13  or a program read from the HDD  14  or a recording medium such as an optical disk to the RAM  12  to provide a software controller. A combination of the software controller and the hardware provides a functional block by which each function of the image forming apparatus  1  is executed. 
     Next, a functional configuration of the image forming apparatus  1  according to the exemplary embodiment is described. 
       FIG. 2  is a block diagram illustrating the functional configuration of the image forming apparatus  1 . As illustrated in  FIG. 2 , the image forming apparatus  1  includes a controller  100 , an automatic document feeder (ADF)  101 , a scanner unit  102 , a sheet ejection tray  103 , a display panel  104 , a sheet feeding table  105 , a print engine  106 , a sheet ejection tray  107 , and a network I/F  108 . 
     The controller  100  includes a main controller  110 , an engine controller  120 , an image processing unit  130 , an operation display controller  140 , and an input output controller  150 . As illustrated in  FIG. 2 , the image forming apparatus  1  according to the exemplary embodiment is configured as a multifunctional peripheral including the scanner unit  102  and the print engine  106 . In  FIG. 2 , a solid-line arrow indicates an electrical connection, whereas a broken-line arrow indicates a flow of a sheet. 
     The display panel  104  serves as not only an output interface for visually displaying a state of the image forming apparatus  1 , but also an input interface. The display panel  104  of the input interface is used as a touch panel when the user directly operates the image forming apparatus  1  or inputs information with respect to the image forming apparatus  1 . That is, the display panel  104  has a function of displaying an image to receive an operation from the user. The display panel  104  functions with the LCD  16  and the control panel  17  illustrated in  FIG. 1 . 
     The network I/F  108  enables the image forming apparatus  1  to communicate with other devices via a network. The network I/F  108  includes an Ethernet (registered trademark) interface or a universal serial bus (USB) interface. The network I/F  108  can perform communication using a transmission control protocol/Internet protocol (TCP/IP). Moreover, the network I/F  108  can function as an interface for transmitting a facsimile when the image forming apparatus  1  functions as a facsimile machine. Thus, the network I/F  108  is also connected to a telephone line. The network I/F  108  functions with the I/F  15  illustrated in  FIG. 1 . 
     The controller  100  includes a combination of software and hardware. In particular, the controller  100  includes the software controller and hardware such as an integrated circuit. The software controller is provided by performing computation by the CPU  11  according to a program loaded to a volatile memory (hereinafter called a memory) such as the RAM  12  from the ROM  13  or a non-volatile memory and to a program loaded to the memory from the HDD  14  or a non-volatile storage medium such as an optical disk. The controller  100  functions to comprehensively control the image forming apparatus  1 . 
     The main controller  110  has a function of controlling each unit of the controller  100 , and issues a command to each of the units of the controller  100 . The engine controller  120  functions as a drive unit for controlling or driving the print engine  106  and the scanner unit  102 , for example. The image processing unit  130 , according to the control by the main controller  110 , generates rendering information based on image information to be printed. The term “rendering information” represents information that is used to render an image to be formed by the print engine  106  including image forming units  30 Y,  30 M,  30 C, and  30 K in an image forming operation. 
     Moreover, the image processing unit  130  processes captured-image data that is input from the scanner unit  102  to generate image data. The term “image data” represents information to be stored as a scanner operation result in a storage area of the image forming apparatus  1 , or information to be transmitted to another information processing terminal or storage device via the network I/F  108 . 
     The operation display controller  140  displays information on the display panel  104 , or notifies the main controller  110  of information that is input via the display panel  104 . The input output controller  150  inputs information that is input via the network I/F  108  to the main controller  110 . Moreover, the main controller  110  controls the input output controller  150  to access other devices connected to a network via the network I/F  108  and the network. 
     When the image forming apparatus  1  operates as a printer, the input output controller  150  first receives a print job via the network I/F  108 . The input output controller  150  transfers the received print job to the main controller  110 . Upon receipt of the print job, the main controller  110  controls the image processing unit  130  to generate rendering information based on document information or image information included in the print job. 
     In the exemplary embodiment, the print job includes information of a parameter that is set for image formation in addition to image information in which information of an output target image is described in a format analyzable by the image processing unit  130  of the image forming apparatus  1 . The parameter information is, for example, information of a two-sided print setting, an aggregate print setting, and a color/monochrome setting. 
     When the rendering information is generated by the image processing unit  130 , the engine controller  120  controls the print engine  106 , based on the generated rendering information, to form an image on a sheet conveyed from the sheet feeding table  105 . That is, the image processing unit  130 , the engine controller  120 , and the print engine  106  function as an image forming outputting unit. In particular, an electrophotographic image forming system is used as the print engine  106  in the exemplary embodiment. A document with the image formed by the print engine  106  is ejected to the sheet ejection tray  107 . 
     When the image forming apparatus  1  operates as a scanner, the operation display controller  140  or the input output controller  150  transfers a scan execution signal to the main controller  110  according to an operation of the display panel  104  by the user or an scan execution instruction input by another device via the network I/F  108 . The main controller  110  controls the engine controller  120  based the received scan execution signal. 
     The engine controller  120  drives the ADF  101  to convey an image capturing target document placed on the ADF  101  to the scanner unit  102 . Moreover, the engine controller  120  drives the scanner unit  102  to capture an image of the document conveyed from the ADF  101 . If the document is directly placed on the scanner unit  102  instead of the ADF  101 , the scanner unit  102  captures an image of the document according to the control by the engine controller  120 . That is, the scanner unit  102  operates as an image capturing unit, and the engine controller  120  function as a reading controller. 
     In the image capturing operation, an image capturing device such as a contact image sensor (CIS) or a charge-coupled device (CCD) disposed in the scanner unit  102  optically scans the document to generate captured-image information based on the optical information. The engine controller  120  transfers the captured-image information generated by the scanner unit  102  to the image processing unit  130 . Subsequently, the image processing unit  130  generates image information based on the captured-image information received from the engine controller  120  according to the control by the main controller  110 . 
     The main controller  110  acquires the image information generated by the image processing unit  130 , and stores the image information in a storage medium such as the HDD  14  attached to the image forming apparatus  1 . That is, the scanner unit  102 , the engine controller  120 , and the image processing unit  130  operate in response to one another to function as an image input unit. The image information generated by the image processing unit  130  is stored as is in the storage medium such as the HDD  14  according to an instruction from the user, or transmitted to an external device via the input output controller  150  and the network I/F  108 . 
     Moreover, when the image forming apparatus  1  operates as a copier, the image processing unit  130  generates rendering information based on captured-image information received by the engine controller  120  from the scanner unit  102  or image information generated by the image processing unit  130 . Similar to the operation performed when the image forming apparatus  1  operates as the printer, the engine controller  120  drives the print engine  106  based on the rendering information. 
     Next, the print engine  106  of the image forming apparatus  1  according to the exemplary embodiment is described with reference to  FIG. 3 . The print engine  106  of a tandem type includes the image forming units  30 Y,  30 M,  30 C, and  30 K arranged along a conveyance belt  301  of an endless moving member. Moreover, the print engine  106  includes transfer rollers  35 Y,  35 M,  35 C, and  35 K. A sheet P (one example of the recording media) from a sheet feeding tray  302  is fed by a sheet feeding roller  303 , and then conveyed along the conveyance belt  301  as an intermediate transfer belt on which an intermediate transfer image to be transferred to the sheet P is formed. The plurality of image forming units (electrophotographic processing units)  30 Y,  30 M,  30 C, and  30 K are arranged in order from an upstream side in the direction of movement of the conveyance belt  301 . In the following description, the image forming units  30 Y,  30 M,  30 C, and  30 K may be collectively called the image forming units  30  as necessary. 
     Moreover, conveyance of the sheet P fed from the sheet feeding tray  302  is temporality stopped by a registration roller  304 . The registration roller  304  times the conveyance of the sheet P with image formation in the image forming units  30 Y,  30 M,  30 C, and  30 K to feed the sheet P to an image transfer position from which the image is transferred from the conveyance belt  301 . 
     Each of the image forming units  30 Y,  30 M,  30 C, and  30 K is substantially similar to every other except for the color of a toner image to be formed. The image forming units  30 Y,  30 M,  30 C, and  30 K respectively form images of yellow, magenta, cyan, and black. Accordingly, a description is hereinafter given of configurations of only the image forming unit  30 Y as a representative of the image forming units  30 Y,  30 M,  30 C, and  30 K. Since each component illustrated with a reference numeral with color abbreviation Y of the image forming unit  30 Y is similar to each component of the image forming units  30 M,  30 C, and  30 K except for the color of toner, descriptions of the image forming units  30 M,  30 C, and  30 K are omitted. 
     The conveyance belt  301  as an endless belt looped around a drive roller  305  and a driven roller  306 . The drive roller  305  is rotated by a drive motor. The drive motor, the drive roller  305 , and the driven roller  306  function as a drive unit for moving the conveyance belt  301  of the endless moving member. In  FIG. 3 , although an optical writing device  310  is configured to irradiate each of the photoconductor drums  31 Y,  31 M,  31 C, and  31 K with light, optical writing devices  310 Y,  310 M,  310 C, and  310 K are also illustrated for the sake of the following description. 
     When an image is formed, the first image forming unit  30 Y transfers a yellow toner image to the conveyance belt  301  being rotated. Hereinafter, the image forming unit  30 Y of the image forming apparatus  1  according to the exemplary embodiment is described with reference to a sectional view illustrated  FIG. 4 . The image forming unit  30 Y includes a photoconductor drum  31 Y as a photoconductor, a charging roller  32 Y as a charger disposed opposite the photoconductor drum  31 Y, the optical writing device  310 Y, a developing device  33 Y, a photoconductor cleaner  34 Y, and a toner supply unit  36 Y. In  FIG. 4 , the optical writing device  310 Y irradiates the photoconductor drum  31 Y. 
     The photoconductor drum  31 Y includes an organic photoconductive layer and a surface layer that are sequentially laminated around a drum-shaped conductive supporting member. The organic photoconductive layer includes a charge generation layer and a charge transport layer. The charge transport layer has a thickness that can be selected from a range of 10 μm to 40 μm according to a characteristic of the photoconductor drum  31 Y. Moreover, a subbing layer can be formed between the conductive supporting member and the organic conductive layer as necessary. 
     The charging roller  32 Y includes a cored bar to which a charging bias is applied by a direct current (DC) power supply or an alternating current (AC) power supply. Electrical discharge occurs in an air gap between the charging roller  32 Y and the photoconductor drum  31 Y, so that the photoconductor drum  31 Y is uniformly charged via a charge gap. A cleaning brush roller  322 Y is disposed to contact the charging roller  32 Y to remove toner adhering to the charging roller  32 Y. 
     The optical writing device  310 Y irradiates the uniformly charged photoconductor drum  31 Y with light based on the rendering information to form an electrostatic latent image on the photoconductor drum  31 Y. The optical writing device  310 Y employs an optical writing method such as a polygon scanning method and a light emitting diode (LED) array method. 
     The developing device  33 Y develops the electrostatic latent image formed by the optical writing device  310 Y by rendering toner adhere to the photoconductor drum  31 Y. This forms a yellow toner image on the photoconductor drum  31 Y. Herein, the toner supply unit  36 Y supplies the toner to the developing device  33 Y. 
     In a position (a transfer position) in which the photoconductor drum  31 Y and the conveyance belt  301  contact each other or are closest to each other, the toner image is transferred to the conveyance belt  301  by a transfer roller  35 Y as a transfer device. Hence, the yellow toner image is formed on the conveyance belt  301 . After the toner image is transferred from the photoconductor drum  31 Y, a photoconductor cleaner  34 Y removes an unnecessary toner remaining on a circumferential surface of the photoconductor drum  31 Y. Subsequently, the optical writing device  310 Y irradiates the photoconductor drum  31 Y with light again, thereby removing charge from the photoconductor drum  31 Y. When the charge is removed by the light, the photoconductor drum  31 Y is on standby for next image formation. 
     The image forming unit  30 Y performs such operations, so that a series of electrophotographic processes in the image forming apparatus  1  according to the exemplary embodiment is completed. In the series of the electrophotographic processes, an emergency stop may be made partway through the processes due to an inadequate amount of toner or a conveyance failure of a sheet P. In such a case, the image forming apparatus  1  cannot form or output an image. As illustrated in  FIG. 5 , if the image forming unit  30 Y makes an emergency stop, electric charge flows into the photoconductor drum  31 Y from the transfer roller  35 Y for transferring the toner image. Consequently, the photoconductor drum  31 Y is charged with excessive electric charge. 
     In a case in which the operation is resumed in such a state, the photoconductor drum  31 Y is rotated while a surface thereof is being charged with the electric charge as illustrated in  FIG. 6 , a diagram illustrating the electric charge flowing to the photoconductor drum. If the rotation of the photoconductor drum  31 Y continues as is, the excessive electric charge on the photoconductor drum  31 Y flows into the charging roller  32 Y. 
     Herein, the charging roller  32 Y receives superimposition of DC power supply and AC power supply. Generally, the AC power supply takes longer from the beginning of operation to activation than the DC power supply. The flow of the electric charge into the charging roller  32 Y during such a time may cause a failure in a power supply device that supplies DC power to the charging roller  32 Y. In a case in which the photoconductor drum  31 Y is charged with the excessive electric charge, the image forming apparatus  1  according to the exemplary embodiment can reduce the electric charge flowing from the photoconductor drum  31 Y into the charging roller  32 Y. 
     In the exemplary embodiment, the transfer roller  35 Y receives power supply from a DC power supply device, whereas the charging roller  32 Y receives power supply from a power supply device in which DC power supply and AC power supply are superimposed.  FIG. 7  is a diagram illustrating a power supply device (a charge power supply device  321 ) connected to the charging roller  32 Y, and  FIG. 8  is a diagram illustrating a power supply device (a transfer power supply device  351 ) connected to the transfer roller  35 Y. Hereinafter, the charge power supply device  321  and the transfer power supply device  351  are respectively described with reference to  FIGS. 7 and 8 . Similar to the above description, the image forming unit  30 Y is used as a representative of the image forming units  30 Y,  30 M,  30 C, and  30 K in the following description. 
     As illustrated in  FIG. 7 , the charge power supply device  321  includes a DC power supply  710  and an AC power supply  720 . The charge power supply device  321  supplies power by superimposing power supply from the AC power supply  720  on power supply from the DC power supply  710 . Thus, the charging roller  32 Y as the charger and the charge power supply device  321  cooperate with each other. Accordingly, in an electric circuit of the charge power supply device  321  for supplying power by superimposing the power supply from the AC power supply  720  on the power supply from the DC power supply  710 , electric connectors  716  and  726  are electrically connected via a harness  717 . Moreover, a DC voltage transformer  713  outputs a DC voltage to the AC power supply  720  via the harness  717 . A description is given of configurations of the DC power supply  710  and the AC power supply  720  of the charge power supply device  321 . 
     The DC power supply  710  includes a DC output controller  711 , a DC drive unit  712 , the DC voltage transformer  713 , a DC output detector  714 , an output malfunction detector  715 , and the electric connector  716 . A power supply controller  700  includes hardware such as the CPU  11  and the RAM  12  having a computation function, and controls the DC power supply  710 . 
     The DC output controller  711  receives a DC_PWM signal from the power supply controller  700 . The DC_PWM signal is used to control a DC voltage output. Moreover, the DC output controller  711  receives an output value of the DC voltage transformer  713  from the DC output detector  714 , the output value being detected by the DC output detector  714 . The DC output controller  711  controls the DC voltage transformer  713  based on a duty ratio of the received DC_PWM signal and the received output value of the DC voltage transformer  713 . In particular, the DC output controller  711  controls the driving of the DC voltage transformer  713  via the DC drive unit  712  such that an output value of the DC voltage transformer  713  is an output value designated by the DC_PWM signal. 
     The DC drive unit  712  drives the DC voltage transformer  713  according to the control by the DC output controller  711 . The DC voltage transformer  713  is driven by the DC drive unit  712  to output a high DC voltage having a negative polarity. Similar to the charging roller  32 Y, in a device that is driven by receiving power supply by superimposing an AC voltage on a DC voltage from the DC power supply  710 , the electric connectors  716  and  726  are electrically connected via the harness  717 . Therefore, the DC voltage transformer  713  outputs a DC voltage to an AC voltage transformer  724  via the harness  717 . 
     The DC output detector  714  detects an output value of the high DC voltage of the DC voltage transformer  713 , and outputs the detected output value to the DC output controller  711 . Moreover, the DC output detector  714  outputs the detected output value to the power supply controller  700  as an FB_DC signal (a feedback signal). The FB_DC signal is output, so that the power supply controller  700  controls duty of the DC_PWM signal to prevent degradation in transferability due to environment or load. 
     The output malfunction detector  715  is disposed on an output line of the DC power supply  710  to output a service channel (SC) signal indicating an output malfunction such as a leakage to the power supply controller  700 . Upon receipt of the SC signal, the power supply controller  700  executes a control operation to stop the high-voltage output from the DC power supply  710 . Such a control operation can stop the high-voltage output from the DC power supply  710  to the charging roller  32 Y when a power supply leakage occurs. 
     Next, the AC power supply  720  is described. The AC power supply  720  includes an AC output controller  722  to which an AC_PWM signal is input from the power supply controller  700 . The AC_PWM signal is used to control an AC voltage output. Moreover, the AC output controller  722  receives an output value of the AC voltage transformer  724  from an AC output detector  721 , the output value being detected by the AC output detector  721 . The AC output controller  722  controls the AC voltage transformer  724  based on a duty ratio of the received AC_PWM signal and the received output value of the AC voltage transformer  724 . In particular, the AC output controller  722  controls the driving of the AC voltage transformer  724  via an AC drive unit  723  such that an output value of the AC voltage transformer  724  is an output value designated by the AC_PWM signal. 
     The AC drive unit  723  receives an AC_CLK signal for controlling a frequency of the AC voltage output. The AC drive unit  723  drives the AC voltage transformer  724  based on the control by the AC output controller  722  and the AC_CLK signal. The AC drive unit  723  controls the driving of the AC voltage transformer  724  via the AC drive unit  723  based on the AC_CLK signal such that an output value of the AC voltage transformer  724  is a value designated by the AC_CLK signal. 
     The AC voltage transformer  724  is driven by the AC drive unit  723  to generate an AC voltage, and superimposes the generated AC voltage on a high DC voltage output from the DC voltage transformer  713  to generate a superimposed voltage. Then, the AC voltage transformer  724  outputs the superimposed voltage to the charging roller  32 Y via an electric connector  727  and a harness  728 . If an AC voltage is not generated, the AC voltage transformer  724  outputs the high DC voltage output from the DC voltage transformer  713  to the charging roller  32 Y via the electric connector  727  and the harness  728 . 
     The AC output detector  721  detects an output value of the AC voltage of the AC voltage transformer  724 , and outputs the detected output value to the AC output controller  722 . Moreover, the AC output detector  721  outputs the detected output value to the power supply controller  700  as an FB_AC signal (a feedback signal). The FB_AC signal is output, so that the power supply controller  700  controls duty of the AC_PWM signal to prevent degradation in transferability due to environment or load. 
     Moreover, the AC power supply  720  includes an output malfunction detector  725 . The output malfunction detector  725  is disposed on an output line of the AC power supply  720  to output a service channel (SC) signal indicating an output malfunction such as a leakage to the power supply controller  700 . 
     In the exemplary embodiment, the AC power supply  720  performs the constant voltage control operation. However, the AC power supply  720  may perform a constant current control operation. Moreover, the AC voltage generated by the AC voltage transformer  724  (the AC power supply  720 ) may be any of a sine wave and a rectangular wave. 
     As illustrated in  FIG. 8 , the transfer power supply device  351  supplies power using a DC power supply. A functional configuration of the transfer power supply device  351  is common to that of the DC power supply  710  illustrated in  FIG. 7 . Hereinafter, the transfer power supply device  351  is described by referring to the differences between the transfer power supply device  351  illustrated in  FIG. 8  and the DC power supply  710  illustrated in  FIG. 7 . 
     As illustrated in  FIG. 8 , in an electric circuit of the transfer power supply device  351  for supplying power using the DC voltage output from the DC power supply  710 , the transfer roller  35 Y and the electric connector  716  are electrically connected via a harness  718 . Accordingly, the DC voltage transformer  713  outputs the DC voltage to the transfer roller  35 Y via the harness  718 . Unlike the charge power supply device  321  illustrated in  FIG. 7  in which the power supply from the AC power supply  720  is superimposed, the DC power supply  710  of the transfer power supply device  351  illustrated in  FIG. 8  supplies power to the transfer roller  35 Y without superimposition. Hence, the DC voltage output from the DC power supply  710  is applied to the transfer roller  35 Y via the harness  718 . 
     Therefore, the transfer power supply device  351  and the charge power supply device  321  respectively control the power supply to the transfer roller  35 Y and the charging roller  32 Y according to the exemplary embodiment. 
     Next, a control configuration of the image forming apparatus  1  is described with reference to  FIG. 9 . 
     The image forming apparatus  1  includes an engine controller  121 , a charge removal execution determiner  122 , a roller power controller  123 , and an optical writing controller  124 . The engine controller  121  receives a command from a higher-level controller, and inputs a command to form an electrostatic latent image corresponding to an output target image. The image forming unit  30 Y executes an electrophotographic process according to the command output from the engine controller  121 . Moreover, the engine controller  121  determines whether a charge removal operation is necessary in the control by the image forming apparatus  1  to control the charge removal operation. 
     The charge removal execution determiner  122  includes a current value detector  125 , a temperature humidity detector  126 , and a photoconductor layer thickness detector  127 . The charge removal execution determiner  122  controls the charging roller  32 Y or the optical writing device  310 Y based on the command input by the engine controller  121 , so that charge is removed from the photoconductor drum  31 Y. If the image forming apparatus  1  transfers a toner image without an emergency stop in the course of electrophotographic process, the charge removal execution determiner  122  outputs a command to the optical writing controller  124  to remove the charge from the photoconductor drum  31 Y using the optical writing device  310 Y. If there is an emergency stop in the course of the electrophotographic process, the charge removal execution determiner  122  outputs a command (an charge removal command) to the roller power controller  123  to remove the charge from the photoconductor drum  31 Y by applying an AC voltage to the charging roller  32 Y. 
     The roller power controller  123  receives the charge removal command from the charge removal execution determiner  122  to remove the charge from the photoconductor drum  31 Y, and renders the AC power supply  720  to supply power to the charging roller  32 Y to remove the charge from the photoconductor drum  31 Y (to execute AC charge removal discharge). 
     The optical writing controller  124  receives the charge removal command from the charge removal execution determiner  122 , and renders the optical writing device  310 Y to remove the charge from the photoconductor drum  31 Y. In the image forming apparatus  1  according to the exemplary embodiment, the optical writing device  310 Y irradiates the photoconductor drum  31 Y with light in normal image formation. The charge of the photoconductor drum  31 Y is removed by irradiation of the photoconductor drum  31 Y with light by the optical writing device  310 Y. 
     In the normal electrophotographic process, the optical writing device  310 Y optically removes the charge from the photoconductor drum  31 Y after the toner image corresponding to the electrostatic latent image is transferred. However, in a case in which the electrophotographic process stops partway, a positive electric charge flows into the photoconductor drum  31 Y by the DC voltage applied to the transfer roller  35 Y. This causes the photoconductor drum  31 Y to be charged with excessive positive electric charge. 
     In a case in which the image forming apparatus  1  performs a recovery operation in a state in which the photoconductor drum  31 Y remains charged with the excessive positive electric charge, the positive electric charge flows into the charging roller  32 Y due to a potential difference between the photoconductor drum  31 Y and the charging roller  32 Y. In a case in which the flow of the positive electric charge into the charging roller  32 Y occurs in a state in which the DC power supply is applied to the charge power supply device  321 , the charge power supply device  321  malfunctions and the image forming apparatus  1  stops working. 
     Such an event needs to be prevented. Accordingly, if there is an emergency stop partway through the electrophotographic process, the image forming apparatus  1  according to the exemplary embodiment performs AC charge removal discharge at activation of the charge power supply device  321  to remove the charge from the photoconductor drum  31 Y by rotating the photoconductor drum  31 Y. Alternatively, the image forming apparatus  1  can perform AC charge removal discharge using the charging roller  32 Y after rotation of the photoconductor drum  31 Y is resumed. The image forming apparatus  1  may start the AC charge removal discharge, and then execute DC charging at a time when the photoconductor drum  31 Y has made one rotation. In such a case, an image forming operation can be executed again without necessity of a long time period even if the image forming apparatus  1  makes an emergency stop. 
       FIG. 10  is a flowchart illustrating a procedure performed by the image forming apparatus  1  according to the exemplary embodiment. In step S 1001 , the charge removal execution determiner  122 , based on a command from the engine controller  121 , detects that the image forming apparatus  1  has made an emergency stop partway through the electrophotographic process. Upon such detection, the charge removal execution determiner  122  outputs a command to the roller power controller  123  to execute AC charge removal discharge by applying an AC voltage to the charging roller  32 Y. 
     Upon receipt of the command to execute the AC charge removal discharge from the charge removal execution determiner  122 , the roller power controller  123  transmits such a command to the power supply controller  700  of the charge power supply device  321  which supplies power to the charging roller  32 Y. In step S 1002 , the power supply controller  700  receives the command from the roller power controller  123 , and controls the charge power supply device  321  to execute the AC charge removal discharge according to the command. 
     When the AC charge removal discharge in the charging roller  32 Y is completed, the process proceeds to step S 1003  in which the image forming unit  30 Y forms an image by image forming outputting operation. Subsequently, in step S 1004 , the optical writing controller  124  controls the optical writing device  310 Y, so that the photoconductor drum  31 Y is irradiated with light to delete electrostatic latent image history (to remove charge). 
     The procedure illustrated in  FIG. 10  has been described using an example case in which the image forming apparatus  1  performs the recovery operation for recovering from the emergency stop, and a series of electrophotographic processes ends without a malfunction. In a case in which any malfunction occurs in a series of the processes illustrated in  FIG. 10 , the process may return to step S 1001  to execute the series of the processes illustrated in  FIG. 10  again. 
     In the exemplary embodiment, therefore, the image forming apparatus  1  removes charge from the photoconductor drum  31 Y by AC charging at activation of the charge power supply device  321  such that a potential difference between the photoconductor drum  31 Y and the charging roller  32 Y is reduced. Accordingly, such reduction in the potential difference between the photoconductor drum  31 Y and the charging roller  32 Y reduces the positive electric charge flowing from the photoconductor drum  31 Y into the charging roller  32 Y, thereby reducing a malfunction of the charging device. 
     Another Exemplary Embodiment 
     The image forming apparatus  1  according to the above exemplary embodiment executes emergency stop control if the main controller  110  detects a malfunction such as toner exhaustion and a sheet jam in any of the image forming units  30 Y,  30 M,  30 C, and  30 K. Hereinafter, an image forming apparatus according to another exemplary embodiment is described. Components and configurations that are similar to the above exemplary embodiment are given the same reference numerals as above and description thereof will be omitted. Similar to the above exemplary embodiment, each of image forming units  30 Y,  30 M,  30 C, and  30 K is substantially similar to every other except for the color of a toner image to be formed, the image forming unit  30 Y is described as a representative the image forming units  30 Y,  30 M,  30 C, and  30 K. An image forming apparatus  1  can allow a current value detector  125 , a temperature humidity detector  126 , and a photoconductor layer thickness detector  127  in the charge removal execution determiner  122  to determine whether to render a charging roller  32 Y to execute AC charge removal discharge. 
     The current value detector  125  determines that AC charge removal discharge is to be executed if an electric current exceeding an electric current value determined from a discharge start voltage and a resistance value of the transfer roller  35 Y flows to the transfer roller  35 Y. Herein, the discharge start voltage can be determined by a function of atmospheric pressure and an air gap width (a nip width) between the photoconductor drum  31 Y and the transfer roller  35 Y. Since the electrophotographic image forming apparatus  1  is used under the atmospheric pressure, the discharge start voltage is determined by a function that depends on only the air gap width between the photoconductor drum  31 Y and the transfer roller  35 Y. 
     The image forming apparatus  1  includes a temperature humidity sensor including a thermistor having a low heat capacity or a temperature sensor such as a silicon-type IC sensor, and a humidity sensor such as a polymer-film variable resistance sensor.  FIG. 11  is a graph illustrating a charge potential Vd of a photoconductor interface with respect to temperature and humidity. As illustrated in  FIG. 11 , the higher the absolute humidity and the relative humidity, the lower the charge potential Vd of the photoconductor interface. An increase in the absolute humidity and the relative humidity facilitates diffusion of static electricity. This increases electric inductivity on the photoconductor interface, and electric charge leakage speed is increased. Hence, the graph illustrated in  FIG. 11  is obtained. 
     The temperature humidity detector  126  determines whether execution of AC charge removal discharge is needed based on measurements of temperature and humidity inside the image forming apparatus  1 , the measurements being obtained by a temperature humidity sensor. Herein, the temperature humidity detector  126  defines a threshold value based on fluctuations in electric inductivity that is unique to a material used as a base material of the photoconductor. If the temperature and humidity exceeds the threshold value, the temperature humidity detector  126  determines to execute the AC charge removal discharge. 
     The photoconductor drum  31 Y deteriorates over time due to abrasion of a surface layer thereof. When the cumulative number of rotations of the photoconductor drum  31 Y increases, the surface layer of the photoconductor drum  31 Y is abraded, and thus a circumference of the photoconductor drum  31 Y is reduced.  FIG. 12  is a diagram illustrating a relation between a travel distance and a layer thickness of the photoconductor drum  31 Y based on the cumulative number of rotations of the photoconductor drum  31 Y. As illustrated in  FIG. 12 , the layer thickness of the photoconductor drum  31 Y decreases as the travel distance of the photoconductor drum  31 Y increases. 
     The photoconductor layer thickness detector  127  counts the cumulative number of rotations of the photoconductor drum  31 Y, and calculates a travel distance of the photoconductor drum  31 Y based on the counted number to determine whether to render the charging roller  32 Y to execute AC charge removal discharge based on the calculated result. Herein, the information indicating the relation between the travel distance and the layer thickness of the photoconductor drum  31 Y illustrated in  FIG. 12  is stored beforehand in a storage area such as an HDD  14  disposed in the image forming apparatus  1 . As illustrated in  FIG. 12 , the greater the travel distance of the photoconductor drum  31 Y, the smaller the layer thickness of the photoconductor drum  31 Y. Consequently, the smaller the layer thickness, the less likely the photoconductor drum  31 Y is to be charged with a positive electric charge. 
     The photoconductor layer thickness detector  127  calculates a layer thickness of the photoconductor drum  31 Y from a travel distance of the photoconductor drum  31 Y. Herein, if the layer is abraded to a thickness where a malfunction no longer occurs in the charge power supply device  321  by movement of electric charge between the photoconductor drum  31 Y and the charging roller  32 Y, the photoconductor layer thickness detector  127  determines that the travel distance of the photoconductor drum  31 Y exceeds a predetermined travel distance. Moreover, if the travel distance of the photoconductor drum  31 Y exceeds the predetermined travel distance, the photoconductor layer thickness detector  127  determines to advance an application time of a DC voltage to perform AC charge removal discharge. The DC voltage is applied after the AC voltage is applied to an area corresponding to one circumference of the photoconductor drum  31 Y, the one circumference being calculated based on the layer thickness acquired when the travel distance exceeds the predetermined travel distance. Thus, when the photoconductor layer thickness detector  127  determines to execute the AC charge removal discharge, an application time of the DC voltage to the charging roller  32 Y is advanced. Accordingly, when an application time of the DC voltage is advanced, the image forming apparatus  1  can perform a recovery operation promptly. 
     Moreover, a photoconductor cleaner  34 Y may include a lubricant. In such a case, when AC charge removal discharge is to be executed, superimposition of a DC voltage can be advanced. When the photoconductor cleaner  34 Y includes the lubricant, the photoconductor drum  31 Y is coated with the lubricant. This suppresses the flow of a positive electric change into the photoconductor drum  31 Y. Herein, the DC voltage is applied after an AC voltage is discharged to the photoconductor drum  31 Y in an area at least from an air gap between a transfer roller  35 Y and the photoconductor drum  31 Y to an air gap between the charging roller  32 Y and the photoconductor drum  31 Y. The lubricant used herein can be a natural wax such as carnauba wax and a fatty acid metal salt, such as zinc stearate, or fluororesin, such as polytetrafluoroethylene. 
     The present disclosure has been described above with reference to specific exemplary embodiments but is not limited thereto. Various modifications and enhancements are possible without departing from the scope of the disclosure. It is therefore to be understood that the present disclosure may be practiced otherwise than as specifically described herein. For example, elements and/or features of different illustrative exemplary embodiments may be combined with each other and/or substituted for each other within the scope of the present disclosure.