Patent Publication Number: US-9904200-B2

Title: Image forming apparatus

Description:
This application is a Continuation of U.S. application Ser. No. 14/987,993, filed Jan. 5, 2016, which claims priority from Japanese Patent Application No. 2015-002615 filed Jan. 8, 2015, which are hereby incorporated by reference herein in their entireties. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to an image forming apparatus using an electrophotographic process. 
     Description of the Related Art 
     Image forming apparatuses using an electrophotographic or electrostatic recording method have used a corona charger as a unit for charging an image-bearing member such as an electrophotographic photosensitive member or an electrostatic recording dielectric. 
     In recent years, a contact charger has been put into practice as the charge processing unit of an image-bearing member because of its advantages of low ozone production and low power consumption. The contact charger charges a charge target member in such a manner that a charging member to which a voltage is applied is brought into contact with the image-bearing member. 
     The charge method used for such a contact charger includes a “direct-current (DC) charge method” by which only a DC voltage is applied to a charging member to charge a charge target member. The charge method also includes an “alternating-current (AC) charge method” by which a charge target member is charged by applying an oscillation voltage that has an AC voltage component and a DC voltage component and whose voltage value periodically changes with time. In recent years, the “AC charge method” having good charge uniformity has been used widely. 
     When an image forming apparatus using such an AC charge method performs charge control, the image forming apparatus alternately applies positive and negative voltages and repeats discharge and back discharge. Accordingly, the discharge increases deterioration of a photoconductor drum that is a charged member on its surface. The deteriorated surface portion of the photoconductor drum is shaved due to friction with an abutting member such as a cleaning blade, and the life of the photo conductor is thus decreased. 
     Hence, many methods for controlling and minimizing a discharge current amount in the AC charge method have been proposed (for example, Japanese Patent Laid-Open No. 2010-231188). 
     An image forming apparatus proposed in Japanese Patent Laid-Open No. 2010-231188 uses a high-pass filter to extract a discharge current component from a current that flows between the photo conductor and the charger when an AC voltage is applied to the charger. Based on the extracted discharge current component, a peak-to-peak voltage value of the AC voltage is controlled. 
     SUMMARY OF THE INVENTION 
     As noted above, Japanese Patent Laid-Open No. 2010-231188 discloses controlling a peak-to-peak voltage value of AC voltage applied to a charger. However, studies by the inventors have proved that, in a case where an environmental factor such as the temperature or the humidity is changed, a discharge start voltage is changed, and the frequency of the discharge current component is thus changed. In a case where the frequency of the discharge current component is changed as described above, the method in which the frequency band of a filter for extracting the discharge current component as described in Japanese Patent Laid-Open No. 2010-231188 does not enable highly accurate detection of a discharge current amount. 
     The present invention provides an image forming apparatus that enables an AC voltage applied to a charging member to be controlled with high accuracy even in a case where an environmental factor is changed. 
     According to an aspect of the present invention, an image forming apparatus includes an image-bearing member, a charging member configured to charge the image-bearing member by receiving a charge voltage in which a direct-current voltage and an alternating-current voltage are superposed on each other, a power source configured to apply a voltage to the charging member, a toner-image forming unit configured to form a toner image on the image-bearing member charged by the charging member, a detector configured to detect a current flowing through the charging member when the power source applies the charge voltage to the charging member, an extraction unit configured to extract, from the current detected by the detector when the charge voltage is applied to the charging member, a current in a frequency band including a discharge current component, an adjustment unit configured to adjust the alternating-current voltage based on the current extracted by the extraction unit, an acquisition unit configured to acquire environmental information, and a setting unit configured to set, based on the environmental information acquired by the acquisition unit, a frequency band for extraction performed by the extraction unit. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments (with reference to the attached drawings). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating a schematic configuration of an image forming apparatus according to an embodiment of the invention. 
         FIG. 2  is a diagram illustrating a schematic configuration of a control circuit that controls a discharge current amount of the image forming apparatus. 
         FIG. 3  is a graph illustrating waveforms of currents and a voltage of a charge voltage applied to a charge roller. 
         FIG. 4  is a graph illustrating a relationship between an AC voltage amplitude and an output current amount. 
         FIG. 5  is a graph illustrating a relationship between a peak current amount Ip of a total output current applied to the charge roller and a discharge current amount. 
         FIG. 6  is a graph illustrating a relationship between a discharge current amount and the total number of output print pages. 
         FIG. 7  is a flowchart illustrating steps of a discharge current control process performed by a controller. 
         FIG. 8  illustrates an example of frequency band control tables for setting the frequency band of an extraction unit. 
         FIG. 9  illustrates an example of frequency band control tables for setting the frequency band of the extraction unit. 
         FIG. 10  illustrates an example of frequency band control tables for setting the frequency band of the extraction unit. 
         FIG. 11  is a diagram illustrating a schematic configuration of a control circuit that controls a discharge current amount of the image forming apparatus. 
         FIG. 12  illustrates an example of frequency band control tables for setting the frequency band of the extraction unit. 
         FIG. 13  is a diagram illustrating a schematic configuration of a control circuit that controls a discharge current amount of the image forming apparatus. 
         FIG. 14  is a diagram illustrating a schematic configuration of a control circuit that controls a discharge current amount of the image forming apparatus. 
         FIG. 15  illustrates an example of frequency band control tables for selecting the frequency band of the extraction unit. 
         FIG. 16  is a diagram illustrating a schematic configuration of a control circuit that controls a discharge current amount of the image forming apparatus. 
         FIGS. 17A and 17B  are graphs each illustrating a detected current waveform having undergone Fourier transformation. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Hereinafter, embodiments of the invention will be described in detail with reference to the drawings. Components denoted by the same reference numerals in the respective drawings have the same configuration or act in the same manner, and repeated description thereof is omitted as appropriate. Note that the dimensions, the material, the shape, the relative position, and the like of each component are not intended to limit the applicable scope of the technical idea to only these unless otherwise particularly stated. 
     First Embodiment 
       FIG. 1  is a diagram illustrating a schematic configuration of an image forming apparatus  200  according to a first embodiment of the invention. 
     In  FIG. 1 , a photoconductor drum  1  is an image-bearing member that is a charge target member, and the photoconductor drum  1  has an electro-conductive support member  1   a  and a photoconductive layer  1   b  formed on the electro-conductive support member  1   a . The image forming apparatus  200  further includes a charge roller  12  that is a charging member, a developing device  14  that is a developing unit, a transfer roller  15  that is a transfer unit, a cleaner  16  that is a cleaning unit, and the like that are disposed around the photoconductor drum  1  and listed in a direction of an arrow A in which the photoconductor drum  1  rotates. In addition, a scanner unit  13  is disposed above the photoconductor drum  1 . The charge roller  12  is brought into pressure contact with the photoconductor drum  1  by using a spring (not illustrated), and the photoconductor drum  1  is thereby charged. 
     A charge power source  18  corresponds to an application unit that applies a charge voltage for charging the photoconductor drum  1  to the charge roller  12 . The charge power source  18  applies an AC voltage as the charge voltage on which a DC voltage is superposed to the charge roller  12 . A development power source  19  supplies a development bias to a developing sleeve  14   a  of the developing device  14 . A transfer power source  20  supplies a transfer bias to the transfer roller  15 . The image forming apparatus  200  is also provided with a discharging needle  24 , transportation guides  21  and  22 , and a fixing device  17  serving as a fixing unit. 
     An image forming operation of the image forming apparatus  200  will next be described. 
     When the image forming operation is started, the photoconductor drum  1  is first driven by a drive unit (not illustrated) to rotate in the direction of the arrow A at a process speed of 200 mm/sec. Discharge occurs toward the photoconductor drum  1  from the charge roller  12  to which a charge voltage is applied, and the photoconductor drum  1  is thereby charged evenly at a predetermined polarity and a predetermined electric potential. 
     In the photoconductor drum  1  having the surface charged by the charge roller  12 , the surface is subsequently exposed to laser L emitted from the scanner unit  13  serving as an exposure unit, the laser L being emitted in accordance with image information such as characters and figures transmitted from an external information apparatus such as a personal computer. A portion, of a photo conductor, irradiated with the laser L, undergoes charge removal and has a bright-section potential (VL) having a small electric potential. As the result, an electrostatic latent image is formed on the surface of the photoconductor drum  1 . 
     The electrostatic latent image undergoes toner development performed by the developing device  14  serving as the developing unit, and a toner image is formed on the surface of the photoconductor drum  1 . A superposed voltage (development voltage) of the AC voltage and the DC voltage is supplied from the development power source  19  to the developing sleeve  14   a  of the developing device  14  and causes a potential difference between the developing sleeve  14   a  and the electrostatic latent image on the photoconductor drum  1 . The potential difference causes the toner to be transferred to the electrostatic latent image and thereby causes the toner image to be formed on the photoconductor drum  1 . The scanner unit  13  and the developing device  14  correspond to a toner image forming unit. 
     Meanwhile, recording sheets S that are recording media have been stored in a paper cassette (not illustrated). In parallel with the toner image forming operation, one of the recording sheets S is transported to a nip between the photoconductor drum  1  and the transfer roller  15  at a predetermined timing. A transfer bias applied to the transfer roller  15  causes the toner image on the photoconductor drum  1  to be transferred onto the recording sheet S at a predetermined position. 
     The recording sheet S bearing a transferred unfixed toner image on its surface is separated from the photoconductor drum  1  by using the grounded discharging needle  24  and is introduced into the fixing device  17  serving as a fixing unit by using the transportation guide  22 . The transfer medium S subsequently undergoes pressure heating in the fixing device  17 , the unfixed toner image thereby becomes a permanent image, and the recording sheet S having the toner image permanently fixed thereon is ejected to the outside. 
     The cleaner  16  removes, from the photoconductor drum  1  from which the toner image is transferred, the toner remaining on the surface without being transferred to the recording sheet S, and the photoconductor drum  1  is ready for the next image forming. Repeating the operations described above enables the image forming one after another. 
       FIG. 2  is a diagram illustrating a schematic configuration of a control circuit  300  that controls a discharge current amount of the image forming apparatus  200  in  FIG. 1 . 
     In  FIG. 2 , a high-voltage transformer drive circuit  61  generates a sinusoidal wave on the basis of a frequency setting signal and a voltage setting signal that are input by a controller  100  including a central processing unit (CPU)  99  and a read-only memory (ROM)  98  that has control data stored therein. The frequency of the sinusoidal wave in the embodiment is 2000 Hz. In the control data in the ROM  98 , use amount information of the photoconductor drum  1  and environmental information are stored. The use amount information includes the total number of pages (the total number of print pages) acquired by a use-history detector  97  serving as a use-amount acquisition unit (use-history detector), the pages having undergone image forming performed by the image forming apparatus  200 . The use amount information also includes a rotation time of the photoconductor drum  1 , a voltage application time in which the charge power source  18  applies a voltage to the charge roller  12 , and other information. The environmental information includes temperature, relative humidity, absolute humidity, and the like that are detected by an environment sensor  96  serving as an acquisition unit (environment detector). The voltage of the sinusoidal wave generated by the high-voltage transformer drive circuit  61  is increased by a high-voltage transformer  60 . 
     A DC high-voltage generation circuit  62  generates a DC high voltage. The generated DC voltage and the AC high voltage that has been increased by the high-voltage transformer  60  are superposed on each other and are applied to the charge roller  12 . 
     A current detection circuit  64  corresponds to a detector (current detector) that detects a current flowing through the charge roller  12  to which the AC voltage is applied by the charge power source  18 . The current detection circuit  64  detects, by using a full-wave rectification, the current caused to flow through the charge roller  12  by the voltage applied from the high-voltage transformer drive circuit  61  and the DC high-voltage generation circuit  62 . A bandpass filter  101  corresponds to an extraction unit that extracts a current component in a predetermined frequency band including a discharge current component of a current waveform detected by the current detector. The bandpass filter  101  may be an analog signal circuit or a digital signal circuit. In the embodiment, analog-to-digital (A/D) conversion is performed by using a sampling frequency of 44,100 Hz on the current waveform detected by the current detection circuit  64 , and a discharge current component is thereafter extracted in digital signal processing. A digital signal processing circuit for removing current components other than the discharge current component is configured by using an application specific integrated circuit (ASIC). Note that a field programmable gate array (FPGA) may also be used, and a high-general-purpose digital signal possessor (DSP) may be operated in accordance with a program. 
     The setting of the frequency band for the bandpass filter  101  performed by the controller  100  serving as a setting unit is determined by using one of frequency band control tables stored in the ROM  98 , on the basis of the use amount information received from the use-history detector  97  and the environmental information received from the environment sensor  96 . Specifically, a median value of the frequency band is set. In the embodiment, control is performed by using the setting based on information regarding the total number of output print pages and temperature information as the use amount information and the environmental information, respectively. An output signal in the predetermined frequency band extracted by the bandpass filter  101  is input to the controller  100 . 
       FIG. 3  is a graph illustrating waveforms of currents and an AC voltage applied to the charge roller  12  by the charge power source  18  in  FIG. 1 . In  FIG. 3 , the vertical axis represents voltage and current, and the horizontal axis is a temporal axis. 
     When an AC voltage (Vo) illustrated in  FIG. 3  is applied to the charge roller  12 , a resistive load current (Izr) having the same phase as that of the AC voltage (Vo) flows to a resistive load between the charge roller  12  and the photoconductor drum  1 . 
     In addition, a capacitive load current (Izc) having a phase advanced by 90 degrees from the AC voltage (Vo) flows to a capacitive load between the charge roller  12  and the photoconductor drum  1 . Further, while the amplitude of the AC voltage is being equal to or higher than that of a discharge start voltage (Vth) in the configuration described above, discharge occurs between the charge roller  12  and the photoconductor drum  1  and causes a flow of a pulse discharge current (Is). The discharge occurs in a gap portion adjacent to a nip portion between the charge roller  12  and the photoconductor drum  1 . Accordingly, in a case where the discharge start voltage (Vth) varies depending on the use amount information of an apparatus or the installation environment, the frequency of the discharge current (Is) also varies. 
     As a total current of the resistive load current (Izr), the capacitive load current (Izc), and the discharge current (Is), a current Io flows. A detected current waveform Im represents a waveform observed when an alternating current drawn from a charge roller to a high-voltage power source is detected. 
     Based on these relationships and by using the amplitude of the AC bias voltage (Vo), the frequency of the AC bias voltage (Vo), and the discharge start voltage (Vth) in the configuration described above, setting of the frequency of the bandpass filter  101  using a discharge current (Is) section as a passband is in advance calculated based on experiments and is stored in the frequency band control tables in the ROM  98 . 
       FIG. 4  is a graph illustrating a relationship between the amplitude of the AC voltage applied to the charge roller  12  and an amount of an output current that is an alternating current flowing to the charge roller  12  at that time. In  FIG. 4 , the vertical axis represents output current amount, and the horizontal axis represents AC voltage amplitude. 
     In  FIG. 4 , while being equal to or lower than the amplitude of the discharge start voltage (Vth) at which discharge is started between the charge roller  12  and the photoconductor drum  1 , the AC voltage amplitude is almost proportional to the output current amount. While the AC voltage amplitude is equal to or lower than the discharge start voltage (Vth), the resistive load current (Izr) and the capacitive load current (Izc) are proportional to the AC voltage amplitude, and the AC voltage amplitude is small. Accordingly, discharge does not occur, and the discharge current (Is) does not flow. 
     In contrast, when the AC voltage amplitude is gradually increased, the discharge is started at the predetermined amplitude of the AC voltage (Vth). Accordingly, the AC voltage amplitude is not proportional to the total output current (Io), and the output current flow is increased by an amount of the discharge current (Is). 
       FIG. 5  is a graph illustrating a relationship between a peak current amount Ip of a total output current to be applied to the charge roller  12  and a discharge current amount. In  FIG. 5 , the vertical axis represents discharge current amount, and the horizontal axis represents peak current amount. 
     In  FIG. 5 , when a characteristic at the early stage of using the charge roller  12  is compared with a characteristic after a predetermined period of use, the charge roller  12  after the predetermined period of use has a low value of a discharge start current that starts flowing at the time of discharge. This is because impedance is changed due to a build-up of toner, a change in film thickness of the photoconductor drum  1 , and the like. In addition, the discharge current amount in a peak current amount (Ip) is increased from Is 0  to Is 1 . 
       FIG. 6  is a graph illustrating a relationship between a discharge current amount and the total number of output print pages. In  FIG. 6 , the vertical axis represents discharge current amount and photoconductor-drum shave amount per 1000 pages, and the horizontal axis represents the total number of output print pages. 
     When control is performed to keep the peak current amount (Ip) constant, an increase in the total number of output print pages leads to an increase in the discharge current amount from the discharge current amount Is 0  at the early stage of use to the discharge current amount Is 1 , as illustrated in  FIG. 6 . 
     The amount of shaving of the surface of the photoconductor drum  1  that leads to deterioration of the photoconductor drum  1  is increased in proportion to the discharge current amount. 
     Accordingly, in the constant current control as in the related art, as the total number of output print pages is increased, the photoconductor drum  1  is shaved at an accelerated pace, and the life of the photoconductor drum  1  is thus decreased. Hence, a discharge current component is directly controlled in the embodiment in order to control the amount of shaving of the photoconductor drum  1 . 
       FIG. 7  is a flowchart illustrating steps of a discharge current control process performed by the controller  100  in  FIG. 2 . 
     In  FIG. 7 , an image forming operation and an adjusting operation are started (step S 200 ). When the use-history detector  97  and the environment sensor  96  serving as the acquisition unit (environment detector) detect the use amount information and the environmental information (steps S 201  and S 202 ), the controller  100  acquires a target discharge-current amount associated with the environmental information from an environment table stored in the ROM  98  (step S 203 ). The environment table has target discharge-current amounts each for achieving appropriate charge in accordance with the state of the image forming apparatus  200 . Also in the embodiment, the information regarding the total number of output print pages and the temperature information are used as the use amount information and the environmental information, respectively. Also in the embodiment, the information regarding the total number of output print pages is acquired by a counter serving as the use-amount acquisition unit. The controller  100  sets the frequency band of a bandpass filter by using one of the frequency band control tables in  FIG. 8  that are stored in the ROM  98  on the basis of the discharge start voltage (Vth) and the amplitude (Vot) of the applied voltage waveform, the discharge start voltage (Vth) varying with the use amount information, the amplitude (Vot) being available for obtaining a target discharge-current amount (step S 204 ). The discharge start voltage (Vth) and the amplitude (Vot) have been calculated in advance based on experiments. 
     If output of the AC voltage as the charge voltage is started (YES in step S 205 ), the controller  100  outputs, to the high-voltage transformer drive circuit  61 , a frequency setting signal (clock) for setting the frequency of the AC voltage (step S 206 ). 
     Further, the controller  100  outputs a voltage setting signal (initial value) for setting the amplitude of the AC voltage (step S 207 ). The voltage setting signal (initial value) used here has been stored in advance. 
     Meanwhile, a charging operation has been started when the charge voltage is applied to the charge roller  12  on the basis of the voltage setting signal (initial value), and a detected current waveform has been obtained by the current detection circuit  64  in the charge power source  18 . 
     The waveform signal undergoes the A/D conversion performed using the sampling frequency of 44,100 Hz and is input to the controller  100  through the bandpass filter  101  in the determined frequency band. 
     The CPU  99  acquires an output value from the bandpass filter  101  (step S 208 ). 
     The CPU  99  computes a measured discharge-current amount H (measured amount) on the basis of the acquired output value (step S 209 ). 
     The measured discharge-current amount H is subsequently compared with the target discharge-current amount, and a voltage-correction set amount that is an amount of correction to the voltage setting signal is computed so that the difference between the measured discharge-current amount H and the target discharge-current amount can be decreased (step S 210 ). The voltage setting signal (correction value) having undergone the correction is output to the high-voltage transformer drive circuit  61  (step S 211 ). Step S 211  corresponds to an operation of an adjustment unit that performs adjustment on an AC voltage applied by the charge power source  18 , the adjustment being performed using the measured amount determined from the output value of the bandpass filter and a reference amount predetermined to control an amount of the discharge current flowing from the charge roller  12  to the photoconductor drum  1 . 
     The successive correction made to the voltage setting signal is continued at fixed sampling intervals until application of the AC voltage as the charge voltage is terminated (YES in step S 212 ), and the AC voltage output is terminated (step S 213 ). 
     With the processing described above, more accurate discharge-current-amount control can be performed any time and in real time in the embodiment. 
     In the embodiment described above, the frequency band of a bandpass filter is changed depending on the apparatus-use amount information and the installation environment, and the discharge current section in the detected current waveform can thereby be extracted more accurately. A discharge current component can thus be directly estimated with high accuracy, while preventing extraction of a high-frequency component such as noise of the high-voltage power source. Accordingly, even in a case where the environment factor is changed, highly accurate detection of the discharge current component enables an AC voltage applied to a charging member to be controlled with higher accuracy. Also in the embodiment, control based on the use amount information of the photo conductor is performed. Even in a case where the film thickness of the photo conductor is changed, the AC voltage applied to the charging member can be controlled with higher accuracy. In addition, the discharge-current-amount control can be performed in real time. Accordingly, a uniform charge state can be maintained in continuous image formation, and print in high quality and high image quality can be output stably for a long period. 
     Second Embodiment 
     In the first embodiment, the example in which the temperature information acquired from the environment sensor and the information regarding the total number of output print pages are used as the environmental information and the use amount information, respectively, has been described. In a second embodiment, an example in which information regarding a total rotation time of the photoconductor drum  1  and relative humidity information are used as the use amount information and the environmental information, respectively, will be described. 
     Note that the same components as those in the first embodiment are denoted by the same reference numerals, and repeated explanation is omitted as appropriate. 
       FIG. 9  illustrates frequency band control tables used to set the frequency band of the bandpass filter  101  serving as the extraction unit on the basis of the photoconductor rotation time information and the relative humidity information. Relationships between a total rotation time of the photoconductor drum  1  obtained in advance based on experiments and the frequency band of the bandpass filter  101  are stored, in the ROM  98 , as tables respectively provided for relative humidity values. Each table illustrated in  FIG. 9  also contains the discharge start voltage (Vth) and the amplitude (Vot) of the AC voltage applied to the charge roller  12  and available for obtaining a target discharge-current amount. The frequency band of the bandpass filter  101  is set by performing computation on the basis of the information. The frequency band of the bandpass filter  101  is set by the controller  100 . 
     The embodiment is applicable to such cases where the photoconductor drum  1  has been relatively largely shaved and rotating operations of the photoconductor drum  1  themselves accelerates the shaving and where a value of resistance of the charge roller  12  varies with the relative humidity. 
     In addition, performing the control in the same manner as in the foregoing first embodiment provides the same effects as in the first embodiment. 
     In the embodiment as described above, the discharge current amount that changes depending on the total rotation time of the photoconductor drum and the relative humidity is detected with higher accuracy, and the AC voltage applied to the charging member can thereby be controlled with higher accuracy. 
     Third Embodiment 
     In the second embodiment, the example in which the relative humidity information acquired from the environment sensor and the photo-conductor rotation time information are used as the environmental information and the use amount information, respectively, has been described. In a third embodiment, absolute humidity information acquired from the environment sensor and information regarding a total application time in which the AC voltage is applied from the charge power source  18  to the charge roller  12  are used as the environmental information and the use amount information, respectively. 
     Note that the same components as those in the first embodiment are denoted by the same reference numerals, and repeated explanation is omitted as appropriate. 
       FIG. 10  illustrates frequency band control tables used to set the frequency band of the bandpass filter  101  serving as the extraction unit on the basis of the information regarding the total application time in which the AC voltage is applied and the absolute humidity information. Relationships between the total application time in which the AC voltage is applied and the frequency band of the bandpass filter  101  are stored, in the ROM  98 , as tables respectively provided for absolute humidity values. The total application time has obtained in advance based on experiments. Each table illustrated in  FIG. 10  also contains the discharge start voltage (Vth) and the amplitude (Vot) of the AC voltage applied to the charge roller  12  and available for obtaining a target discharge-current amount. The frequency band of the bandpass filter  101  is set by performing computation on the basis of the information. The frequency band of the bandpass filter  101  is set by the controller  100 . 
     The embodiment is applicable to such cases where the discharge exerted from the charge roller to the photoconductor drum notably accelerates an increase in the amount of shaving of the photoconductor drum  1  and where a value of resistance of the charge roller  12  varies with the absolute humidity. 
     In addition, performing the control in the same manner as in the foregoing first embodiment provides the same effects as in the first embodiment. 
     In the embodiment as described above, the discharge current amount that changes depending on the absolute humidity and the total application time in which the AC voltage is applied from the charge power source to the charge roller is detected with higher accuracy, and the AC voltage applied to the charging member can thereby be controlled with higher accuracy. 
     Fourth Embodiment 
     In the third embodiment, the example has been described in which the absolute humidity information acquired from the environment sensor and the information regarding the total application time in which the AC voltage is applied from the charge power source  18  to the charge roller  12  are used as the environmental information and the use amount information, respectively. In a fourth embodiment, the absolute humidity information acquired from the environment sensor is used as the environmental information. In addition, information regarding combined impedance of the photoconductor drum  1  and the charge roller  12  is used as the use amount information. 
     Note that the same components as those in the first embodiment are denoted by the same reference numerals, and repeated explanation is omitted as appropriate. 
       FIG. 11  is a diagram illustrating a schematic configuration of a control circuit  300  that controls a discharge current amount of the image forming apparatus  200  in the embodiment. 
     In the configuration, the use-history detector  97  serving as the use-amount acquisition unit corresponds to an impedance calculation unit that calculates combined impedance of the photoconductor drum  1  and the charge roller  12 . The impedance calculation unit calculates the impedance on the basis of a charge voltage applied by the charge power source  18  and a value of a current detected by the current detection circuit  64 , the current flowing to the charge roller  12  at the time of application of the charge voltage. 
     When the controller  100  sets the frequency band of the bandpass filter  101 , control is performed by using the impedance information and the absolute humidity information as the use amount information and the environmental information, respectively. In other words, the use-history detector  97  serving as the use-amount acquisition unit acquires a result of detection performed by the current detection circuit  64  serving as a detector, and the controller  100  sets the frequency band of the bandpass filter on the basis of the acquired detection result and the environmental information. 
     In the embodiment, the voltage applied by the charge power source  18  at the time of calculating the impedance is an AC voltage having a peak-to-peak voltage of 1800V. 
       FIG. 12  illustrates frequency band control tables used to set the frequency band of the bandpass filter  101  serving as the extraction unit on the basis of an effective value of the alternating current detected by the current detection circuit  64  and the absolute humidity information. Relationships between the effective value obtained in advance based on experiments and the frequency band of the bandpass filter  101  are stored, in the ROM  98 , as tables respectively provided for absolute humidity values. Each table illustrated in  FIG. 12  also contains the impedance calculated based on the applied voltage and the effective value of the alternating current detected by the current detection circuit  64 , the discharge start voltage (Vth), and the amplitude (Vot) of the AC voltage applied to the charge roller  12  and available for obtaining a target discharge-current amount. The frequency band of the bandpass filter  101  is set by performing computation on the basis of the information. The frequency band of the bandpass filter  101  is set by the controller  100 . 
     In the embodiment, the amount of shaving of the photoconductor drum  1  having a largest influence on the discharge start voltage and the resistance of the charge roller can be estimated directly from the impedance information, and the frequency band of the bandpass filter can thus be set with higher accuracy. 
     In addition, performing the control in the same manner as in the foregoing first embodiment provides the same effects as in the first embodiment. 
     In the embodiment as described above, the discharge current amount that changes depending on the absolute humidity and the information regarding the combined impedance of the photoconductor drum and the charge roller is detected with higher accuracy, and the AC voltage applied to the charging member can thereby be controlled with higher accuracy. 
     Fifth Embodiment 
     Hereinafter, another embodiment of the invention will be described with reference to the drawings. 
     Note that the same components as those in the first embodiment are denoted by the same reference numerals, and repeated explanation is omitted as appropriate. 
       FIG. 13  is a diagram illustrating a schematic configuration of a control circuit  300  that controls a discharge current amount of the image forming apparatus  200  in  FIG. 1 . 
     In the embodiment as illustrated in  FIG. 13 , there is provided a replaceable cartridge  300  including the photoconductor drum  1 , the charge roller  12 , and a contact tag  95  that are integrated into the cartridge  300 . The tag  95  is provided for storing the use amount information received from the use-history detector  97 . When the cartridge  300  is attached to the image forming apparatus  200 , the controller  100  stores, in the control data in the ROM  98 , the total number of output print pages of the image forming apparatus  200 , the rotation time of the photoconductor drum  1 , and the voltage application time in which the charge power source  18  applies a voltage to the charge roller  12  that are read from the tag  95 . The controller  100  also stores the temperature, the relative humidity, and the absolute humidity that are detected by the environment sensor  96 . 
     In the embodiment, the setting of the frequency band for the bandpass filter  101  performed by the controller  100  is determined by using one of the frequency band control tables in  FIG. 10  stored in the ROM  98 , on the basis of the information regarding the total application time in which the AC voltage is applied from the charge power source  18  to the charge roller  12  and the absolute humidity information received from the environment sensor  96 . 
     In the embodiment as described above, in the image forming apparatus having the configuration using the cartridge, the use conditions of the cartridge are stored in the tag. Even before or after replacement of the cartridge, the discharge current amount is detected with higher accuracy, and the AC voltage applied to the charging member can thereby be controlled with higher accuracy. 
     Sixth Embodiment 
     Hereinafter, another embodiment of the invention will be described in detail with reference to the drawings. 
     Note that the same components as those in the first embodiment are denoted by the same reference numerals, and repeated explanation is omitted as appropriate. 
       FIG. 14  is a diagram illustrating a schematic configuration of a control circuit  300  that controls a discharge current amount of the image forming apparatus  200  in  FIG. 1 . 
     The bandpass filter  101  in a sixth embodiment corresponds to the extraction unit that extracts a current component in a predetermined frequency band of a current waveform detected by the current detector and extracts the discharge current component in such a manner as to select one of a plurality of analog signal circuits set to respectively different frequency bands. 
     The controller  100  selects one of bandpass filters  101   a ,  101   b , and  101   c  set to respectively different frequency bands, by using frequency band control tables stored in the ROM  98 , on the basis of use amount information acquired from the use-history detector  97  serving as a use-amount acquisition unit and detector and the environmental information acquired from the environment sensor  96 . The controller  100  selects one of the bandpass filters  101   a ,  101   b , and  101   c  that has a frequency band associated with the use amount information and the environmental information that are closest to the acquired information. In the embodiment, control is performed by using the information regarding the total number of output print pages and the temperature information as the use amount information and the environmental information, respectively. An output signal in the predetermined frequency band extracted by the selected bandpass filter  101  is input to the controller  100 . 
       FIG. 15  illustrates the frequency band control tables from which the frequency band of the bandpass filter  101  is selected on the basis of the discharge start voltage (Vth) and the amplitude (Vot) of the applied voltage waveform, the discharge start voltage (Vth) varying depending on the total number of output print pages of the image forming apparatus, the total number being calculated in advance based on experiments, the amplitude (Vot) being available for obtaining the target discharge-current amount determined based on the temperature information. The frequency band control tables are stored in the ROM  98 , and the controller  100  selects one of the bandpass filters  101   a ,  101   b , and  101   c.    
     The embodiment is applicable to a case where changing the frequency band of the bandpass filter is difficult, such as a case where an analog signal circuit is used as the extraction unit that extracts a current component in a predetermined frequency band. 
     In addition, performing the control in the same manner as in the foregoing first embodiment provides the same effects as in the first embodiment. 
     Seventh Embodiment 
     In each of the first to sixth embodiments, the example in which one bandpass filter  101  extracts the discharge current component has been described. 
     In a seventh embodiment, an example in which an AC voltage applied to the charging member is controlled based on a plurality of discharge current components extracted from the plurality of bandpass filters in respective different frequency bands will be described. 
     Note that the same components as those in the first embodiment are denoted by the same reference numerals, and repeated explanation is omitted as appropriate. 
       FIG. 16  is a diagram illustrating a schematic configuration of a control circuit  300  that controls a discharge current amount of the image forming apparatus  200 . 
     In  FIG. 16 , four bandpass filters  101   d ,  101   e ,  101   f , and  101   g  are provided. The bandpass filters  101   d  to  101   g  correspond to extraction units that extract current components in respective different predetermined frequency bands of the current waveform detected by the current detector. The bandpass filters  101   d  to  101   g  may each be an analog signal circuit or a digital signal circuit. In the embodiment, the A/D conversion is performed by using the sampling frequency of 44,100 Hz on the current waveform detected by the current detection circuit  64 , and thereafter discharge current components are extracted in the digital signal processing. A digital signal processing circuit for removing current components other than the discharge current component is configured by using an ASIC. Note that a FPGA may also be used, and a high-general-purpose DSP may be operated in accordance with a program. 
     Specifically, a predetermined frequency band F calculated using any method described in the first to fifth embodiments is set for the bandpass filter  101   f.    
     In addition, a frequency f applied to the high-voltage transformer is set as a passband in the bandpass filter  101   d  in the embodiment, and a frequency of 2/3 and a frequency of 4/3 of the frequency band F set for the bandpass filter  101   f  are set as the passbands for the bandpass filters  101   e  and  101   g , respectively. 
     As described above, the plurality of bandpass filters are provided and have respective different frequencies serving as the passbands of the bandpass filters. 
     Smoothing circuits  102   a ,  102   b ,  102   c , and  102   d  are peak hold circuits, and output signals are input to the controller  100  through digital-to-analog (D/A) conversion ports (not illustrated). 
     Also in the embodiment, the current detection circuit  64  provides the detected current waveform as illustrated in  FIG. 3 . 
     The waveform signal is input to the D/A conversion ports of the controller  100  through the bandpass filters ( 101   d  to  101   g ) that are set for the predetermined frequency bands and the smoothing circuit  102  that are the peak hold circuits. 
     The CPU  99  acquires output values from the smoothing circuits  102   a  to  102   d.    
     The CPU  99  subsequently computes a measured discharge-current amount H (measured amount) (S 209  in  FIG. 7 ). In the embodiment, the measured discharge-current amount H takes on a value computed in accordance with Formula (1) below.
 
 H=K   1   ×V   1   +K   2   ×V   2   +K   3   ×V   3   +K   4   ×V   4   +C   (1)
 
V 1 : Output from bandpass filter  101   d  
     V 2 : Output from bandpass filter  101   e      V 3 : Output from bandpass filter  101   f      V 4 : Output from bandpass filter  101   g      K 1 , K 2 , K 3 , K 4 , and C: Predetermined coefficients obtained based on experiments   

     As described above, the measured discharge-current amount H is a linear sum of the output values of the bandpass filters  101   d  to  101   g  and represents an amount of a discharge current from the charge roller  12  to the photoconductor drum  1 . 
     As described above, the measured amount corresponding to the discharge current amount is obtained as the linear sum of the output values of the bandpass filters. Even in a case where an amount of only positive current is detected due to a limited budget for inexpensive circuits or where the original waveform is distorted, K 1 , K 2 , K 3 , K 4 , and C based on the measured amount that well match the discharge current amount can be obtained from experiments. Control can be performed by setting K 1 , K 2 , K 3 , K 4 , and C to −0.1, 2.3, 0.3, −0.2, and −7.1, respectively. 
     In the embodiment, the measured amount is computed based on the linear sum of the output values of the bandpass filters. However, as long as such K 1 , K 2 , K 3 , K 4 , and C that well match the discharge current amount are set, the method for computing a measured amount is not limited to the method using the linear sum. 
     The measured discharge-current amount H is subsequently compared with the target discharge-current amount, and a voltage-correction set amount applied to the voltage setting signal is computed so that the difference between the measured discharge-current amount H and the target discharge-current amount can be decreased (S 210  in  FIG. 7 ). The voltage setting signal (correction value) having undergone the correction is output to the high-voltage transformer drive circuit  61  (S 211  in  FIG. 7 ). The voltage setting signal correction is continued at fixed sampling intervals until the AC charge is terminated. 
       FIG. 17A  illustrates a pseudo current waveform (without discharge current) having undergone Fourier transformation, and  FIG. 17B  illustrates a detected current waveform (with discharge current) having undergone Fourier transformation. In  FIGS. 17A and 17B , the vertical axis represents frequency component, and the horizontal axis represents frequency. 
     In  FIGS. 17A and 17B , a difference between the two waveforms is mainly observed at a peak subsequent to the frequency of 2/3 of the frequency band F set for the bandpass filter  101   f . The difference corresponds to the waveform of the discharge current. 
     Accordingly, if a predetermined frequency component of the detected current waveform is observed in real time, the component of the discharge current amount can be extracted. By utilizing this and in consideration for distortion or the like at the time of current detection, Formula (1) uses the linear sum of the output values of the bandpass filters to calculate the discharge current amount H. The coefficients in Formula (1) may be obtained from experiments depending on the image forming apparatus. 
     In the configuration as described above, the plurality of frequency bands of the bandpass filters are set as in the first to fifth embodiments. The discharge current amount can thereby be obtained with further higher accuracy, and the AC voltage applied to the charging member can be controlled with higher accuracy. 
     Other Embodiments 
     In each of the first to seventh embodiments, the example in which the frequency band of the bandpass filter is set based on the use amount information and the environmental information has been described. However, the frequency band of the bandpass filter may be set based on only one of the use amount information and the environmental information. 
     According to the invention, even in a case where the environment factor is changed, the AC voltage applied to the charging member can be controlled with high accuracy. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.