Patent Publication Number: US-7899352-B2

Title: Image forming apparatus

Description:
This application is based on applications No. 2007-055484 and No. 2007-055485 filed in Japan, the contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image forming apparatus into which a charging control device is incorporated. The charging control device includes a high-voltage generation circuit which applies an oscillating voltage to a charging member which is disposed while brought into contact with or close to an image bearing body, a direct-current voltage and an alternating-current voltage being superimposed to form the oscillating voltage; and a voltage control unit which controls a peak-to-peak voltage value Vpp of the alternating-current voltage to a target voltage. 
     2. Description of the Related Art 
     Recently, a charging control device in which a contact charging method is adopted is becoming the mainstream from the viewpoints of low-voltage process of lowering a charging control voltage applied to an image bearing body, reduction of ozone generated in charging control, and cost reduction. In the contact charging method, a roller type or a blade type charging member is disposed while brought into contact or close to a surface of the image bearing body, and an oscillating voltage in which a direct-current voltage and a alternating-current voltage are superimposed is applied to the charging member to evenly charge the surface of the image bearing body. At this point, the oscillating voltage is not limited to a sine wave, but any periodically-changing oscillation waveform such as a rectangular wave, triangular wave, and a pulsating wave can be used. 
     Japanese Laid-Open Patent Publication No. 63-149668 discloses a technique in which the following charging characteristics are exerted when the above-described contact charging method is adopted. 
     That is, when a peak-to-peak voltage value of the alternating-current voltage in the oscillating voltage is boosted, a charging voltage of the image bearing body is increased in proportion to the increase in peak-to-peak voltage value. A charging potential is saturated when the peak-to-peak voltage value reaches about double a charging start voltage of the direct-current voltage, and the charging potential is not changed even if the peak-to-peak voltage value is further boosted. In order to ensure evenness of the charging, it is necessary to apply the oscillating voltage having the peak-to-peak voltage not lower than double the charging start voltage in applying the direct-current voltage determined by various characteristics of the image bearing body. The charging voltage obtained at that time depends on a direct-current component of the applied voltage. 
     Japanese Laid-Open Patent Publication No. 2001-201921 discloses a charging control method, wherein the image bearing body is evenly charged by adjusting a discharge amount from the charging member to the image bearing body such that problems such as deterioration of the image bearing body, toner adhesiveness, and image deletion due to the discharge are not generated even if a resistance value of the charging member fluctuates due to an influence of an environment. 
     Specifically, the control is performed as follows. During a non-image formation period, an alternating current value passed from the charging member to the image bearing body is detected, when at least one alternating-current voltage whose peak-to-peak voltage is lower than double a direct-current threshold voltage Vth is applied to the charging member. The direct-current threshold voltage Vth is one at which the discharge is started from the charging member to the image bearing body when the direct-current voltage is applied to the charging member. 
     Then, alternating current value passed from the charging member to the image bearing body are detected, when at least two alternating-current voltages whose peak-to-peak voltages are not lower than double the threshold voltage Vth are applied to the charging member. 
     The peak-to-peak voltage of the alternating-current voltage which should be applied in forming the image is determined based on the plural alternating current values detected in each step, and whereby the alternating-current voltage is controlled such that the peak-to-peak voltage is maintained in forming the image. 
     More specifically, a peak-to-peak voltage-alternating current function F 1  and a peak-to-peak voltage-alternating current function F 2  are determined on a two-dimensional coordinate in which a horizontal axis is set to a peak-to-peak voltage while a vertical axis is set to an alternating current. The peak-to-peak voltage-alternating current function F 1  expresses a line segment connecting an origin (0 point) and an alternating current value which is detected when the peak-to-peak voltage lower than double the threshold voltage Vth is applied to the charging member. The peak-to-peak voltage-alternating current function F 2  expresses a line segment including at least two alternating current values which are detected when the peak-to-peak voltage not lower than double the threshold voltage Vth is applied to the charging member. A peak-to-peak voltage value which becomes an intersecting point of the line segments expressed by the functions F 1  and F 2  is determined as the peak-to-peak voltage of the alternating-current voltage which should be applied in forming the image. 
     However, in an epichlorohydrin-rubber charging roller used as the charging member, characteristics fluctuate largely depending on an environment such as temperature and humidity. The adoption of the conventional charging control method in the epichlorohydrin-rubber charging roller causes the following problems. At this point, the image bearing body having the diameter of 30 mm is formed by depositing an amorphous silicon photoconductive layer having a thickness of 20 μm. The charging roller is disposed in contact with the image bearing body with a pressing force of 1 Kgf. 
     That is, as shown in  FIG. 2 , in a low-temperature environment (low-temperature environment  1  of  FIG. 2 ), an electric resistance value of the epichlorohydrin rubber is increased to slow down motion of conductive ions in the rubber, which decreases the charging potential. 
     Accordingly, in order to adjust the charging potential of the image bearing body to a stable target potential, it is necessary that the peak-to-peak voltage value Vpp  1  of the alternating-current voltage be maintained at a peak-to-peak voltage value Vpp 2  larger than a peak-to-peak voltage value Vpp  1  of ambient temperature environment. 
     However, in an extremely-low-temperature environment (low-temperature environment  2  of  FIG. 2 ) such as 0° C., the peak-to-peak voltage value cannot be adjusted to a predetermined target potential even if the peak-to-peak voltage value is increased. In such low-temperature environments, because the charging potential at the image bearing body does not reach the target potential, problems such as fog (phenomenon in which toner adheres slightly to a background except for the image) and uneven density (phenomenon in which unevenness of charging state is generated to cause a fluctuation in density) are generated in the image formed in the image bearing body. 
     The problems are generated in not only a monochrome image forming apparatus in which the black toner is used, but also a tandem-type full-color image forming apparatus in which image bearing bodies are arranged in series along a sheet conveyance belt or an indirect transfer belt according to the yellow (Y), magenta M), cyan (C), and black (K) colors. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing, an object of the invention is to provide an image forming apparatus which can properly control the charging potential of the image bearing body such that the fog and uneven density are not generated even in the low-temperature environment. 
     In accordance with an aspect of the invention, an image forming apparatus having a charging member which is disposed while brought into contact with or close to an image bearing body, the image forming apparatus includes a high-voltage generation circuit which applies an oscillating voltage to the charging member, a direct-current voltage and an alternating-current voltage being superimposed to form the oscillating voltage; a current detection unit which detects a direct current passed from the charging member to the image bearing body; a voltage control unit which controls a peak-to-peak voltage value of the alternating-current voltage applied from the high-voltage generation circuit to the charging member such that the direct current detected by the current detection unit falls within a target current range; and an aging control unit which rotates and drives the image bearing body while retaining the alternating-current voltage and the direct-current voltage at previously-set predetermined voltages. 
     Other aspects of the present invention will become apparent with reference to the following embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram showing a charging control device of an image forming apparatus according to an embodiment of the invention; 
         FIG. 2  is a graph showing a relationship between a peak-to-peak voltage value and a charging potential; 
         FIG. 3  is a graph showing a relationship between a direct current and a charging potential; 
         FIG. 4  shows an appearance of a digital copying machine according to an embodiment of the invention; 
         FIG. 5  is an explanatory view showing the digital copying machine of the embodiment; 
         FIG. 6  is a block diagram showing a control unit of the digital copying machine; 
         FIG. 7  is an explanatory view of a temperature table; 
         FIG. 8  is a flowchart for explaining an aging operation; 
         FIG. 9  is a block diagram showing a charging control device of the embodiment; 
         FIG. 10A  is an explanatory view showing a color digital copying machine according to an embodiment of the invention; 
         FIG. 10B  is an explanatory view showing an image forming unit; 
         FIG. 11  is a block diagram showing a high-voltage generation circuit; 
         FIG. 12  is a circuit diagram showing a direct-current transformer; 
         FIG. 13  is a circuit diagram showing a shunt regulator; 
         FIG. 14  is a circuit diagram showing a current detection circuit; 
         FIG. 15  is an explanatory view showing a temperature table; 
         FIG. 16  is a flowchart showing an aging operation; and 
         FIG. 17  is a flowchart showing the aging operation. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A monochrome digital copying machine will be described below. The monochrome digital copying machine is an example of an image forming apparatus according to an embodiment of the invention into which a charging control device is incorporated. 
     As shown in  FIGS. 4 and 5 , a digital copying machine  1  includes functional blocks such as a document placing unit  2 , an image scanning unit  3 , an image forming portion  4 , a fixing unit  5 , plural sheet feed cassettes  7  ( 7   a  to  7   d ), a conveyance unit  6 , and a manipulation unit  8 . A document is placed on the document placing unit  2 . The image scanning unit  3  scans a document image to convert the document image into electronic data. The image forming portion  4  forms a toner image on a sheet based on the image data which is converted into the electronic data by the image scanning unit  3 . The fixing unit  5  heats and fixes the toner image formed on the sheet. The plural sheet feed cassettes  7  ( 7   a  to  7   d ) accommodate different sizes or types of the sheets respectively. The conveyance unit  6  conveys the sheet accommodated in the sheet feed cassettes  7  ( 7   a  to  7   d ) to the image forming portion  4 . Plural menu setting keys are provided in the manipulation unit  8  to set various copy menus. 
     As shown in  FIG. 5 , an image bearing body  41  is provided in the image forming portion  4 . A charging member  42 , a printhead  43 , a development unit  44 , a transfer unit  46 , a cleaner unit  47 , and an antistatic lamp  48  are disposed around the image bearing body  41  along a rotating direction of the image bearing body  41 . 
     The image bearing body  41  is formed by a photosensitive drum, and the photosensitive drum is rotated and driven about a shaft of the photosensitive drum by a driving device. In the photosensitive drum, a photosensitive layer is provided on a surface of an aluminum cylinder having a diameter of 30 mm, and the photosensitive layer is formed so as to have a thickness of 20 μm by depositing amorphous silicon which is of a positively-charged photoconductive material. 
     The charging member  42  is formed by a charging roller in which a cored bar  421  is coated with an epichlorohydrin-rubber layer  422 , and the charging roller is brought into contact with the photosensitive drum with a pressing force of 1 Kgf (see  FIG. 1 ). The epichlorohydrin-rubber layer  422  is an elastic material having electrical conductivity. 
     A toner cartridge  45  which is of an exchange unit is provided in the development unit  44 , and a black toner is stably supplied into the development unit  44 . 
     An image forming process will be described below. The charging member  42  evenly charges the image bearing body  41 , and the image bearing body  41  is exposed to form an electrostatic latent image by the printhead  43  driven based on the image data. The development unit  44  visualizes the electrostatic latent image to form a toner image on the image bearing body  41  using the electrostatically adhering toner. 
     After the transfer unit  46  transfers the toner image formed on the image bearing body  41  to the sheet, the cleaner unit  47  recovers the residual toner, and the antistatic lamp  48  erases a residual potential of the image bearing body  41 . A series of image forming processes from the charging to the erase corresponds to the process of printing the image on the one sheet, and the continuous printing process is realized by the series of image forming processes. 
     As shown in  FIG. 6 , plural control units are provided in the digital copying machine  1  in order to control the functional blocks. Specifically, an image scanning control unit  100 , an image output control unit  200 , and a manipulation control unit  300  are provided in the digital copying machine  1 . The image scanning control unit  100  controls the document scanning operation performed by the image scanning unit  3 . The image output control unit  200  collectively controls the system of the digital copying machine  1 , and the image output control unit  200  also controls the image forming portion  4 , the fixing unit  5 , the conveyance unit  6 , and the sheet feed cassettes  7 . The manipulation control unit  300  controls input and output signals of the manipulation unit  8 . 
     Each of the control units  100 ,  200 , and  300  is formed by one or more control boards. One or more CPUs, a ROM in which a control program executed by the CPU is stored, a RAM in which control data is stored, and an input and output interface circuit are provided on the control board. The input and output interface circuit outputs a signal to various loads which is of a control target, and detection values are inputted from various sensors to the input and output interface circuit. 
     The CPUs are connected to one another through a serial communication line  400  to construct a distributed control system. In the digital copying machine  1 , the functional blocks are operated in conjunction with one another by the control program executed by each CPU and related hardware, which realizes a predetermined image forming operation. 
     An output line of the charging control device  9  according to an embodiment of the invention is connected to the charging member  42 , and a high voltage is applied to the charging member  42  in order to control a charging voltage to the image bearing body  41 . 
     As shown in  FIG. 1 , the charging control device  9  includes a high-voltage generation circuit  91 , a current detection unit  92 , a voltage control unit  96 , and an aging control unit  95 . The high-voltage generation circuit  91  applies an oscillating voltage to the charging member  42 . A direct-current voltage and an alternating-current voltage are superimposed with each other in the oscillating voltage. The current detection unit  92  detects a direct current between the image bearing body  41  and the charging member  42 . The voltage control unit  96  controls an output voltage of the high-voltage generation circuit  91 . An environmental sensor  10  is disposed near the charging member  42  to detect a temperature and humidity, and detection signals of the environmental sensor  10  are inputted to the voltage control unit  96  through the aging control unit  95 . 
     The high-voltage generation circuit  91  includes a direct-current voltage power supply  911  and an alternating-current voltage power supply  912 . The direct-current voltage power supply  911  converts an alternating-current high voltage boosted by a pulse transformer into a direct-current voltage and outputs the direct-current voltage. The alternating-current voltage power supply  912  outputs an alternating-current high voltage boosted by the pulse transformer, and the alternating-current high voltage is formed by a sine wave having a predetermined frequency (1.6 kHz in the embodiment, but not limited to). 
     The current detection unit  92  detects the direct current passed between the charging member  42  and the image bearing body  41  using the oscillating voltage applied to the charging member  42  from the high-voltage generation circuit  91 . 
     The voltage control unit  96  and the aging control unit  95  are implemented by a CPU incorporated into the image output control unit  200 , a peripheral circuit, and a control program. The voltage control unit  96  includes a direct-current voltage control unit  93  which controls an output level of the direct-current voltage power supply  911  and an alternating-current voltage control unit  94  which controls an output level of the alternating-current voltage power supply  912 . 
     The direct-current voltage outputted from the direct-current voltage power supply  911  is set to a threshold voltage Vth at which a discharge is started from the charging member  42  to the image bearing body  41 , and a peak-to-peak voltage of the alternating-current voltage outputted from the alternating-current voltage power supply  912  is gradually increased. Therefore, the oscillating voltage applied to the charging member  42  causes the current detection unit  92  to detect a direct current value Idc. 
     As shown in  FIG. 3 , through various experiments, the inventors confirm that a proportional relationship exists between the detected direct current value Idc and a charging potential Vo and the proportional relationship is not largely changed by environmental variations such as the temperature and humidity and aging of the image bearing body or charging member. 
     As described above, in the oscillating voltage applied to the charging member  42 , when the peak-to-peak voltage value of the alternating-current voltage is boosted, the charging voltage of the image bearing body  41  is increased in proportion to the peak-to-peak voltage value. When the peak-to-peak voltage value reaches about double the threshold voltage Vth, the charging potential is saturated, and the charging potential is not change any more even if the peak-to-peak voltage value is further boosted. 
     On the basis of these facts, the peak-to-peak voltage value is adjusted while the direct current value Idc is monitored, which allows the charging potential of the image bearing body  41  to be set to a target potential. The current detection unit  92  is a circuit which detects a direct-current component in the discharge current passed between the charging member  42  and the image bearing body  41 , and the current detection unit  92  can be configured simply and at low cost compared with the conventional circuit for detecting an alternating-current component. 
     The image bearing body  41  exhibits variations in threshold voltage Vth and direct current which should be controlled at proper charging potentials. Accordingly, a ROM in which a proper target voltage (direct-current voltage at the beginning of the discharge) and a target direct current value control range are previously stored is incorporated in each image bearing body  41 . A predetermined current range centering on a target current value Idc necessary to adjust the image bearing body  41  to a predetermined surface potential is set as the target direct current value control range. 
     The voltage control unit  96  adjusts the oscillating voltage, i.e., value of the direct-current voltage and the alternating-current voltage based on the target voltage (direct-current voltage at the beginning of the discharge) and direct current control range read from the ROM. 
     The direct-current voltage control unit  93  reads the target voltage from the ROM to perform the control such that the output voltage of the direct-current voltage power supply  911  becomes the target voltage. In the embodiment, the target voltage is set to about 400V, but is not limited to. 
     The alternating-current voltage control unit  94  reads the direct-current voltage value and the target current range from the ROM to maintain the output voltage of the alternating-current voltage power supply  912  at the peak-to-peak voltage value double the direct-current voltage value (about 800V in the embodiment). 
     When the oscillating voltage is applied to the charging member  42 , a discharge current is passed between the charging member  42  and image bearing body  41 , and the current detection unit  92  detects a direct-current component of the discharge current. 
     The alternating-current voltage control unit  94  performs feedback control of the alternating-current voltage power supply  912  such that the direct current detected by the current detection unit  92  falls within the target current range. 
     Specifically, the alternating-current voltage control unit  94  maintains the peak-to-peak voltage value outputted from the alternating-current voltage power supply  912  when the direct current detected by the current detection unit  92  falls within the target current range, the alternating-current voltage control unit  94  performs control so as to increase the peak-to-peak voltage value when the direct current value is shifted lower than the target current range, and the alternating-current voltage control unit  94  performs control so as to decrease the peak-to-peak voltage value when the direct current value is shifted higher than the target current range. 
     The voltage control unit  96  rotates and drives the image bearing body  41  in power-on of the apparatus, in transition of the apparatus from a power saving mode to a normal mode, or in start-up of an image forming operation. The voltage control unit  96  turns on and drives the antistatic lamp  48  to adjust the peak-to-peak voltage value such that the image bearing body  41  is kept at a predetermined charging potential based on the direct current value detected by the current detection unit  92 . The adjusted peak-to-peak voltage value is stored in the RAM, and the peak-to-peak voltage value is adjusted while the stored peak-to-peak voltage value is used as an initial value in the following image forming operation. 
     The peak-to-peak voltage value adjusting procedure performed in power-on of the apparatus, or the like is not limited to the above-described procedure. Alternatively, for example, after the output voltage of the direct-current voltage power supply  911  is adjusted to the target voltage, the peak-to-peak voltage value Vpp outputted from the alternating-current voltage power supply  912  is gradually increased, and the peak-to-peak voltage value Vpp may be set to the initial value when the direct current Idc detected by the current detection unit  92  is saturated. 
     However, as described above, in the low-temperature environment, because the electric resistance value of the epichlorohydrin rubber which is of the charging member  42  becomes increased, the direct-current value cannot be adjusted within the target current range even if the output voltage of the alternating-current voltage power supply  912  is increased by the alternating-current voltage control unit  94 . Accordingly, the image bearing body  41  cannot be adjusted to the predetermined target potential. 
     Therefore, in the power-on of the apparatus, in the transition of the apparatus from the power saving mode to the normal mode, or in the start-up of the image forming operation, when the voltage control unit  96  cannot control the direct-current value Idc within the target current range, or when the temperature detected by the environmental sensor  10  is lower than a predetermined temperature, the aging control unit  95  is started up to perform running-in (hereinafter sometimes referred to as “aging operation”) of the charging member  42 . 
     The aging control unit  95  retains the direct-current voltage and alternating-current voltage, outputted from the high-voltage generation circuit  91 , at previously-set predetermined voltages through the direct-current voltage control unit  93  and alternating-current voltage control unit  94 . The aging control unit  95  also turns on and drives the antistatic lamp  48  to rotate and drive the image bearing body  41 . 
     The conductive ions in the epichlorohydrin-rubber layer are oscillated to lower the electric resistance value by performing the running-in. In the embodiment, the direct-current voltage is controlled at 400V, and the alternating-current voltage is controlled in a voltage higher than by 1.5 kV than the peak-to-peak voltage Vpp at which the charging can stably performed in the ambient temperature environment. The value is the maximum value which can be outputted from the alternating-current voltage power supply  912 , the invention is not limited to the value. 
     For example, in the power-on of the apparatus or in the transition of the apparatus from the power saving mode to the normal mode, when the temperature detected by the environmental sensor  10  is lower than the predetermined temperature, the aging control unit  95  performs the running-in according to a maximum aging time T defined in an aging table stored in the ROM of the image output control unit  200 . 
     As shown in  FIG. 7 , at ambient temperature lower than 15° C., in the aging table, each aging time T is defined according to a temperature range divided into plural portions. For example, the aging time T is 800 seconds at ambient temperature lower than 3° C., and the aging time T is 200 seconds at ambient temperature not lower than 7° C. to lower than 10° C. 
     It is not necessary to perform the running-in in a temperature range not lower than 15° C. where the voltage control unit  96  can control the direct-current value Idc within the target current range. However, even in the ambient temperature not lower than 15° C., the running-in may be performed when the voltage control unit  96  cannot control the direct-current value Idc within the target current range. 
     The aging control unit  95  performs the running-in according to the aging time T defined in the aging table, and the aging control unit  95  monitors the direct current value Idc detected by the current detection unit  92  at predetermined intervals during the running-in. 
     The aging control unit  95  ends the running-in when the direct current value Idc detected by the current detection unit  92  reaches the target current range. When the aging time T defined in the aging table elapses, the aging control unit  95  ends the running-in even if the direct current value Idc detected by the current detection unit  92  does not reach the target current range. 
     Then, the digital copying machine  1  makes a transition to a normal start-up operation in the power-on or in recovering from the power saving mode. 
     The operation of the aging control unit  95  will be described with reference to a flowchart of  FIG. 8 . 
     When the digital copying machine  1  is powered on (SA 1 ), the aging control unit  95  determines the maximum aging time T based on the detection value of the environmental sensor  10  (SA 2 ). 
     The aging control unit  95  starts the aging operation (SA 5 ), when the ambient temperature detected by the environmental sensor  10  is lower than 15° C. (YES in SA 3 ), and when the voltage control unit  96  cannot control the direct current value Idc within the target current range (NO in SA 4 ). 
     On the other hand, the aging operation is not performed, when the ambient temperature is not lower than 15° C. (NO in SA 3 ), or when the voltage control unit  96  can control the direct current value Idc within the target current range (YES SA 4 ). Then, the voltage control unit  96  performs the peak-to-peak voltage value adjusting process in step SA 12 . 
     When the aging operation is started (SA 5 ), the aging control unit  95  controls the direct-current voltage power supply  911  and alternating-current voltage power supply  912  through the direct-current voltage control unit  93  and alternating-current voltage control unit  94  to apply the oscillating voltage to the charging member  42  (SA 6 ). 
     Then, the current detection unit  92  detects the direct current value Idc passed from the charging member  42  to the image bearing body  41  (SA 7 ), the aging control unit  95  determines whether or not the direct-current value Idc reaches the target current range in each predetermined time (SA 8 ), and the aging operation is ended (SA 10 ) when the direct current value Idc reaches the target current range (YES in SA 8 ). 
     On the other hand, when the direct current value Idc does not reach the target current range (NO in SA 8 ), the processes from step SA 7  to step SA 9  are repeated. When the maximum aging time T elapses (YES in SA 9 ), the aging operation is ended (SA 10 , and the application of the oscillating voltage to the charging member  42  is stopped (SA 11 ). 
     Then, the voltage control unit  96  performs the alternating-current adjusting process (SA 12 ). That is, the direct-current voltage is controlled at 400V, and the peak-to-peak voltage of the alternating-current voltage is adjusted such that the direct-current value Idc falls within the target current range. 
     A tandem-type digital copying machine which is of another example of the image forming apparatus according to an embodiment of the invention into which a charging control device is incorporated will be described below. In the following description, the substantially same component as the monochrome digital copying machine is designated by the same numeral, and the description of the overlapping portion is not given. 
     As shown in  FIG. 10A , image forming units  4   a  to  4   d  corresponding to yellow (Y), magenta AM), cyan (C), and black (K) colors are arranged along a sheet conveyance belt in the image forming portion  4 . 
     As shown in  FIG. 10B , the image bearing body  41  is provided in each of the image forming units  4   a  to  4   d . The charging member  42 , the printhead  43 , the development unit  44 , the transfer unit  46 , the cleaner unit  47 , and the antistatic lamp  48  are disposed around the image bearing body  41  along the rotating direction of the image bearing body  41 . 
     The image bearing body  41  is formed by the photosensitive drum, and the photosensitive drum is rotated and driven about a shaft of the photosensitive drum by the driving device. In the photosensitive drum, the photosensitive layer is provided on the surface of the aluminum cylinder, and the photosensitive layer is formed by depositing amorphous silicon which is of the positively-charged photoconductive material. 
     Similarly to  FIG. 1 , the charging member  42  is formed by the charging roller in which the cored bar  421  is coated with the epichlorohydrin-rubber layer  422 , and the charging roller is disposed in contact with the photosensitive drum. The epichlorohydrin-rubber layer  422  is the elastic material having electrical conductivity. 
     The toner cartridges  45  which are of the exchange unit are provided in the development unit  44 , and color toners are stably supplied into the development unit  44 . 
     An image forming process will be described below. 
     In each of the image forming units  4   a  to  4   d , the charging member  42  evenly charges the image bearing body  41 , and the image bearing body  41  is exposed to form an electrostatic latent image by the printhead  43  driven based on the image data. The development unit  44  visualizes the electrostatic latent image to form a toner image on the image bearing body  41  using each of the electrostatically adhering yellow (Y), magenta (M), cyan (C), and black (K) toners. 
     After the transfer unit  46  transfers the toner image formed on the image bearing body  41  to the sheet transferred by the sheet conveyance belt, the cleaner unit  47  recovers the residual toner, and the antistatic lamp  48  erases a residual potential of the image bearing body  41 . A series of image forming processes from the charging to the erase corresponds to the process of printing the image on the one sheet, and the continuous full-color printing process is realized by repeating the series of image forming processes in the image forming units  4   a  to  4   d.    
     The output line of the charging control device  9  of the embodiment is connected to each charging member  42 , and a high voltage is applied to control the charging voltage to the image bearing body  41 . 
     As shown in  FIG. 9 , the charging control device  9  includes the high-voltage generation circuit  91 , the current detection unit  914 , the voltage control unit  96 , and the aging control unit  95 . The high-voltage generation circuit  91  applies the oscillating voltage to the charging member  42  ( 42   a  to  42   d ) incorporated into each of the image forming units  4   a  to  4   d . The direct-current voltage and the alternating-current voltage are superimposed with each other in the oscillating voltage. The current detection unit  914  detects a direct current between the image bearing body  41  ( 41   a  to  41   d ) and the charging member  42  ( 42   a  to  42   d ). The voltage control unit  96  controls the output voltage of the high-voltage generation circuit  91 . 
     The environmental sensor  10  is disposed near the charging member  42  to detect the temperature and humidity, and the detection signals of the environmental sensor  10  are inputted to the voltage control unit  96  through the aging control unit  95 . 
     The voltage control unit  96  and the aging control unit  95  are implemented by the CPU incorporated into the image output control unit  200 , the peripheral circuit, and the control program. 
     As shown in  FIG. 11 , the high-voltage generation circuit  91  includes a direct-current voltage power supply and an alternating-current voltage power supply. The direct-current voltage power supply converts an alternating-current high voltage boosted by a pulse transformer into a direct-current voltage and outputs the direct-current voltage. The alternating-current voltage power supply outputs an alternating-current high voltage boosted by the pulse transformer, and the alternating-current high voltage is formed by a sine wave having a predetermined frequency (1.6 kHz in the embodiment, but is not limited to). 
     The direct-current voltage power supply includes single direct-current transformer  911  and four linear direct-current regulators  912  connected in parallel on a secondary side of the direct-current transformer  911 . 
     As shown in  FIG. 12 , an output of a pulse signal generation unit  911   a  is connected to a primary winding of the direct-current transformer  911 , a high-voltage alternating-current voltage outputted from a secondary winding is smoothed by a diode D 10  and a capacitor C 10 , and a high-voltage direct-current voltage is outputted from output terminals t 1  and t 2 . 
     The pulse signal generation unit  911   a  outputs a constant-level pulse signal to drive the direct-current transformer  911  based on a remote signal inputted from the voltage control unit  96 . Accordingly, the direct-current voltage outputted from the direct-current transformer  911  is kept constant. 
     Each linear direct-current regulator  912  is formed by a shunt regulator  912 . As shown in  FIG. 13 , the linear direct-current regulator  912  includes an operational amplifier OP 20  which acts as a differential amplifier, a pass transistor Q 20  which is driven by an output current of the operational amplifier OP 20 , a Zener diode ZD 20  (breakdown voltage of 250V) which is connected to a collector of the pass transistor Q 20 . The shunt regulator is described only by way of example, and another type of linear direct-current regulator may be used. 
     A divided-voltage into which the output voltage of the shunt regulator  912  is divided by resistors R 21  and R 20  is inputted to a noninverting input terminal of the operational amplifier OP 20 , and a reference voltage is inputted to the noninverting input terminal. 
     Accordingly, a base current is supplied from the operational amplifier OP 20  to the pass transistor Q 20  such that the reference voltage and the divided-voltage are equal to each other, and whereby the direct-current voltage Vdc is adjusted by the current passed through the resistor R 23  and Zener diode ZD 20 . 
     The reference voltage is adjusted in a variable manner by a comparative voltage Vref (fixed value) and control voltages Vcnt controlled by the voltage control unit  96 , and the control voltages Vcnt are separately adjusted. Therefore, a direct-current voltage Vdc applied to each charging member  42  is adjusted variably and stably in the range of 250V to 750V. 
     As shown in  FIG. 11 , the alternating-current voltage power supply includes alternating-current transformers  913  according to the number of image bearing bodies  41 . 
     The output of the pulse signal generation unit is connected to a primary winding of the alternating-current transformer  913 , and a high-voltage alternating-current voltage is outputted from a secondary winding. The pulse signal generation unit outputs a variable-level pulse signal to drive the alternating-current transformer  913  based on the remote signal and voltage control signal inputted from the voltage control unit  96 . Accordingly, the alternating-current voltage outputted from the alternating-current transformer  913  is controlled in a variable manner. 
     An output terminal of the shunt regulator  912  is connected to a secondary-side terminal of each alternating-current transformer  913  through a capacitor C bypassing the alternating-current voltage, and the oscillating voltage in which the direct-current voltage of the shunt regulator  912  and the alternating-current voltage of the alternating-current transformer  913  are superimposed is applied to each charging member  42 . 
     Additionally, a single current detection unit  914  is provided in the high-voltage generation circuit  91  to detect the direct-current component in the discharge current passed from each charging member  42  to each image bearing body  41 . 
     As shown in  FIG. 14 , the current detection circuit  914  includes a current-voltage converting operational amplifier OP 41  and an amplifying operational amplifier OP 40 . 
     The low-voltage terminal t 2  of the secondary winding of the direct-current transformer  911  is connected to a noninverting input terminal of the operational amplifier OP 41 , and the reference voltage is inputted to the noninverting input terminal. The reference voltage is a divided-voltage into which the comparative voltage Vref is divided by resistors R 43  and R 44 . 
     The current value passed through the feedback resistor R 42  is converted into the voltage such that the reference voltage is equal to the voltage between the secondary low-voltage side terminals t 2 , and the voltage is amplified by the operational amplifier OP 40  and inputted to the voltage control unit  96 . 
     The detection of the direct current Idc will be described in detail. In the operational amplifier OP 41 , the direct current Idc between the charging member  42  and the image bearing body  41  is passed from the image bearing body  41  to the ground and passed to the output terminal from the ground-side terminal of the control voltage applied to the operational amplifier OP 41 . The direct current Idc is passed to the low-voltage side of the direct-current transformer  911  through the resistor R 42  to form a loop of the direct-current component of the high-voltage generation circuit  91 . 
     A voltage-current conversion table with which the voltage value detected by the current detection unit  914  is converted into the current value is previously stored in the ROM, and the voltage control unit  96  determines the direct-current value Idc from the output voltage of the operational amplifier OP 40  based on the voltage-current conversion table. 
     The current detection unit  914  detects the total value of the direct currents passed from the charging members  42  to all the image bearing bodies  41  of the image forming units  4   a  to  4   d.    
     Accordingly, when the current detection unit  914  separately measures the direct currents Idc passed between the charging members  42  and the image bearing bodies  41  of the image forming units  4   a  to  4   d , the outputs of the shunt regulators  912  and alternating-current transformers  913  are adjusted lower than the discharge start voltage except for the shunt regulator  912  and alternating-current transformer  913  corresponding to the charging member  42  which becomes the measurement target of the voltage control unit  96 . 
     Specifically, the control voltages Vcnt of the three shunt regulators  912  except for the shunt regulator  912  which becomes the measurement target of the voltage control unit  96  are adjusted so as to become about 250V lower than the discharge start voltage, and the values of the current detection unit  914  are read after the three alternating-current transformers  913  except for the alternating-current transformer  913  which becomes the measurement target are turned off. 
     The voltage control unit  96  performs the adjustment such that the output of each shunt regulator  912  and output of each alternating-current transformer  913  are maintained at predetermined target values based on each direct-current value Idc passed from the charging member  42  to the image bearing body  41 . 
     Specifically, a direct current value control range having a proper target voltage (direct-current voltage at the beginning of the discharge) is previously stored in the ROM, and the ROM is incorporated in each image bearing body  41 . 
     The voltage control unit  96  controls each shunt regulator  912  such that the target voltage value (about 500V in the embodiment) read from the ROM of each image bearing body  41  is outputted. 
     The voltage control unit  96  also reads the target voltage value and target current range from the ROM of each image bearing body  41  to control the alternating-current transformer  913  such that the alternating-current voltage (about 1000V in the embodiment) whose peak-to-peak voltage Vpp becomes double the target voltage value is outputted. 
     In the power-on of the apparatus, in the transition of the apparatus from the power saving mode to the normal mode, or in the start-up of the image forming operation, the voltage control unit  96  turns on and drives the antistatic lamp  48  while rotating and driving the image bearing body  41  in each of the image forming units  4   a  to  4   d . On the basis of the direct current value detected by the current detection unit  914 , the voltage control unit  96  performs the feedback control of each peak-to-peak voltage such that the image bearing body  41  is maintained at a predetermined charging potential. Each of the adjusted peak-to-peak voltage value is stored in the RAM, and the peak-to-peak voltage value is adjusted while the stored peak-to-peak voltage value is used as the initial value in the following image forming operation. 
     The peak-to-peak voltage adjusting procedure performed in power-on of the apparatus, or the like is not limited to the above-described procedure. Alternatively, for example, after the output voltage of the shunt regulator  912  is adjusted to the target voltage, the peak-to-peak voltage value Vpp outputted from the alternating-current transformer  913  is gradually increased, and the peak-to-peak voltage value Vpp may be set to the initial value when the direct current Idc detected by the current detection unit  914  is saturated. 
     Additionally, the antistatic lamps  48  are turned on and driven while all the image bearing bodies  41  of the image forming units  4   a  to  4   d  are rotated and driven, and the peak-to-peak voltage value Vpp outputted from each alternating-current transformer  913  may be adjusted. In this case, when each direct current Idc is separately detected, the oscillating voltage applied to the charging member  42  which is not the measurement target is turned off. 
     However, similarly to the previous embodiment, in the low-temperature environment, because the electric resistance value of the epichlorohydrin rubber which is of the charging member  42  becomes increased, the direct-current value Idc cannot be adjusted within the target current range even if the output voltage of the alternating-current transformer  913  is increased to the maximum peak-to-peak voltage. Accordingly, the image bearing body  41  cannot be adjusted to the predetermined target potential. 
     Therefore, in the power-on of the apparatus, in the transition of the apparatus from the power saving mode to the normal mode, or in the start-up of the image forming operation, when the voltage control unit  96  cannot control the direct-current value Idc within the target current range, or when the temperature detected by the environmental sensor  10  is lower than a predetermined temperature, the aging control unit  95  is started up to perform running-in of the charging member  42 . 
     The aging control unit  95  retains the direct-current voltage and alternating-current voltage, outputted from the high-voltage generation circuit  91 , at previously-set predetermined voltages. The aging control unit  95  also turns on and drives the antistatic lamp  48  to rotate and drive the image bearing body  41 . 
     The conductive ions in the epichlorohydrin-rubber layer are oscillated to lower the electric resistance value by performing the running-in. In the embodiment, the direct-current voltage is controlled at 500V, and the alternating-current voltage is controlled in a voltage higher than by 1.5 kV the peak-to-peak voltage Vpp at which the charging can stably performed in the ambient temperature environment. The value is the maximum value which can be outputted from the alternating-current transformer  913 , but the invention is not limited to the value. 
     For example, in the power-on of the apparatus or in the transition of the apparatus from the power saving mode to the normal mode, when the temperature detected by the environmental sensor  10  is lower than the predetermined temperature, the aging control unit  95  performs the running-in according to the maximum number of aging times T defined in an aging table stored in the ROM of the image output control unit  200 . 
     As shown in  FIG. 15 , at ambient temperature lower than 15° C., in the aging table, the number of aging times N is defined according to a temperature range divided into plural portions. For example, the number of aging times N is 30 times at ambient temperature lower than 3° C., and the number of aging times N is 6 times at ambient temperature not lower than 7° C. to lower than 10° C. In this case, although the reference time per aging is set to 30 seconds, the invention is not limited to the 30 seconds. 
     It is not necessary to perform the running-in in a temperature range not lower than 15° C. where the voltage control unit  96  can control the direct current value Idc within the target current range. However, even in the ambient temperature not lower than 15° C., the running-in may be performed when the voltage control unit  96  cannot control the direct current value Idc within the target current range. 
     The aging control unit  95  turns on and drives the antistatic lamp  48  while rotating and driving all the image bearing bodies  41  of the image forming unit  4   a  to  4   d , and the aging control unit  95  adjusts the output voltages of the shunt regulators  912  to the target voltages. Then, the aging control unit  95  adjusts the peak-to-peak voltage Vpp outputted from the alternating-current transformer  913  to the maximum value of 1.5 kV and performs the running-in for 30 seconds. 
     When the 30 seconds elapsed, the aging control unit  95  turns off the outputs of the shunt regulators  912  and alternating-current transformers  913  corresponding to the three image forming units  4   b  to  4   d  except for the image forming unit  4   a , or the aging control unit  95  decreases the outputs of the shunt regulators  912  and alternating-current transformers  913  corresponding to the three image forming units  4   b  to  4   d  to levels lower than the discharge start voltage. Then, the aging control unit  95  monitors the direct current Idc corresponding to the specific image forming unit  4   a  for one second using the current detection unit  914 . 
     Then, the aging control unit  95  turns off the outputs of the shunt regulators  912  and alternating-current transformers  913  corresponding to the three image forming units  4   a ,  4   c , and  4   d  except for the image forming unit  4   b , or the aging control unit  95  decreases the outputs of the shunt regulators  912  and alternating-current transformers  913  corresponding to the three image forming units  4   a ,  4   c , and  4   d  to levels lower than the discharge start voltage. Then, the aging control unit  95  monitors the direct current Idc corresponding to the specific image forming unit  4   b  for one second using the current detection unit  914 . The cycle of the above-described operations is repeated by the number of aging times N set in the aging table. 
     The aging control unit  95  ends the running-in, when all the direct currents Idc of the image forming units  4   a  to  4   d  reach the target current ranges, or when the number of cycles reaches the number of aging times N. 
     Then, the digital copying machine  1  makes a transition to a normal start-up operation in the power-on or in recovering from the power saving mode. 
     The operation of the aging control unit  95  will be described with reference to flowcharts of  FIGS. 16 and 17 . 
     When the color digital copying machine  1  is powered on (SB 1 ), the aging control unit  95  determines the maximum number of iterations N based on the detection value of the environmental sensor  10  (SB 2 ). 
     The aging control unit  95  starts the aging operation to all the image bearing bodies  41   a  to  41   d  and all the charging members  42   a  to  42   d  (SB 5 ), when the ambient temperature detected by the environmental sensor  10  is lower than 15° C. (YES in SB 3 ), and when the voltage control unit  96  cannot controls the direct current value Idc corresponding to one of the charging members  42  within the target current range (NO in SB 4 ). 
     On the other hand, the aging operation is not performed, when the ambient temperature is not lower than 15° C. (NO in SB 3 ), or when the voltage control unit  96  can controls the direct current values Idc corresponding to all the charging members  42  within the target current range (YES in SB 4 ). Then, the voltage control unit  96  performs the peak-to-peak voltage value adjusting process in step SB 15 . 
     When the aging operation is started (SB 5 ), the aging control unit  95  applies the oscillating voltages for 30 seconds to the charging members  42   a  to  42   d  from the high-voltage generation circuit  91  through the voltage control unit  96  (SB 6 ). 
     Then, the aging control unit  95  turns off the oscillating voltages applied to the charging members  42   b  to  42   d  except for the specific charging member  42   a , or the aging control unit  95  adjusts the oscillating voltages to a voltage lower than the discharge start voltage. In this state of things, the direct current value Idc (direct current value Idc passed between charging member  42   a  and the image bearing body  41   a ) detected by the current detection unit  914  is monitored for one second (SB 7 ). 
     Similarly to step SB 7 , the aging control unit  95  turns off the oscillating voltages applied to the charging members  42  except for the specific charging member  42   b , or the aging control unit  95  adjusts the oscillating voltages to a voltage lower than the discharge start voltage. In this state of things, the direct current value Idc (direct current value Idc passed between charging member  42   b  and the image bearing body  41   b ) detected by the current detection unit  914  is monitored for one second (SB 8 ). The similar process is performed to the charging members  42   c  and  42   d  (SB 9  and SB 10 ). 
     The aging control unit  95  ends the aging operation (SB 13 ), when all the direct current values Idc corresponding to the monitored charging member  42  reach the target current ranges (YES in SB 11 ). 
     On the other hand, when one of the direct current values Idc does not reach the target current range (NO in SB 11 ), the aging control unit  95  repeats the one-cycle aging operation from step SB 6  to step SB 11  by the number of aging times N set in the aging table (SB 12 ). 
     After the number of cycles reaches the number of aging times N, the aging control unit  95  ends the aging operation (SB 13 ), and the aging control unit  95  stops the application of the oscillating voltage to charging member  42  (SB 14 ). 
     Then, the voltage control unit  96  performs the alternating-current adjusting process (SB 15 ). That is, the direct-current voltage is controlled at 500V, and the peak-to-peak voltage of the alternating-current voltage is adjusted such that the direct current value Idc falls within the target current range. 
     Other embodiments will be described below. In the above-described embodiments, the voltage control unit  96  adjusts the alternating-current voltage such that the direct current value Idc is maintained in the target current range. Furthermore, the direct-current voltage may be controlled such that the direct current value Idc is maintained in the target current range as well as the alternating-current voltage adjustment. 
     In the above-described embodiments, the relationship between the ambient temperature and the aging time or the number of aging times is defined by the aging table. The aging table may be configured while the environmental humidity is added. 
     In the above-described embodiments, the photosensitive drum in which the photosensitive layer is made of amorphous silicon is used as the image bearing body  41 . The invention can be applied to the image forming apparatus including the photosensitive body made of a material except for the amorphous silicon photosensitive material. For example, the invention can be applied to an image forming apparatus made of an organic photosensitive material or a selenium photosensitive material. Particularly, the invention can effectively be applied to the amorphous silicon photosensitive material having the hard surface layer. 
     In the above-described embodiments, the charging member  42  is formed by the charging roller in which the cored bar  421  is coated with the epichlorohydrin-rubber layer  422 . Alternatively, the charging member  42  may be formed by the charging roller with which the epichlorohydrin-rubber layer  422  is coated. 
     When the aging control unit  95  cannot control the direct current value Idc so as to fall within the target current range, the voltage control unit  96  may control the direct current value Idc to the maximum peak-to-peak voltage, or a message that the charging potential is not normally set may be displayed on the manipulation unit  8 . 
     The charging member  42  is not always disposed in contact with the image bearing body  41 , but the charging member  42  may be brought close to the image bearing body  41  with a small gap. 
     The waveform of the alternating-current voltage superimposed with the direct-current voltage in the form of the oscillating voltage is not limited to the sine wave, but any waveform such as rectangular wave, a triangular wave, and a pulsating wave may be used.