Patent Publication Number: US-8983317-B2

Title: Method for detecting surface potential of image bearing member and image forming apparatus

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image forming apparatus that detects the surface potential of a photosensitive drum as an image bearing member and controls operations thereof based on a detection result. 
     2. Description of the Related Art 
     As an image forming apparatus that forms an image on a recording material, the configuration and general operation of an electrophotographic printer will be described with reference to  FIG. 14 . The printer illustrated in  FIG. 14  includes a photosensitive drum  101  as an image bearing member, a semiconductor laser  102  as a light source, a rotational polygon mirror (also referred to as a polygonal mirror)  103  that is rotated by a scanner motor  104 , and a laser beam  105  that is irradiated from the semiconductor laser  102  and scans the surface of the photosensitive drum  101 . 
     A charging roller  106  acts as a charging member for uniformly charging the photosensitive drum  101 . A development unit  107  is for developing an electrostatic latent image formed on the photosensitive drum  101  with toner. A transfer roller  108  acts as a transfer member for transferring a toner image developed on the photosensitive drum  101  by the development unit  107  onto a recording material. A fixing roller  109  acts as a fixing member that heats the toner image transferred onto the recording material to fuse the toner image on the recording material. 
     A feeding roller  110  acts as a feeding member that rotates to feed a recording material from a cassette in which the recording material is stacked onto a conveyance path. The cassette has a function of identifying the size of the recording material. A manual feeding roller  111  feeds a recording material from a manual feed port, which is a separate feed port to the cassette. Conveyance rollers  114  and  115  convey the fed recording material. 
     A recording material detection sensor  116  is for detecting a leading edge and a trailing edge of the fed recording material. A pre-transfer conveyance roller  117  feeds the conveyed recording material to a transfer unit configured of the photosensitive drum  101  and the transfer roller  108 . A synchronization sensor  118  is for synchronizing the writing of the electrostatic latent image (image) on the photosensitive drum  101  and the recording material to be conveyed with the fed paper. Further, the synchronization sensor  118  also measures the length in the conveyance direction of the fed recording material. A discharge detection sensor  119  is for detecting the presence of a fixed recording material. A discharge roller  120  is for discharging a fixed recording material out of the apparatus. 
     A flapper  121  switches the conveyance destination (discharge out of the apparatus, or convey to a two-sided unit) of the recording material on which an image has been formed. A conveyance roller  122  is for conveying a recording material conveyed to a two-sided unit to a reversing unit. A reversal detection sensor  123  detects the leading edge and the trailing edge of the paper conveyed to the reversing unit. A reversing roller  124  reverses the recording material and conveys the recording material to a re-feeding unit by sequentially switching between forward direction rotation and reverse direction rotation. 
     A re-feeding sensor  125  detects the presence of a recording material at the re-feeding unit. A re-feeding roller  126  re-feeds the recording material at the re-feeding unit into a conveyance path for conveyance toward the transfer unit. 
     Next, a block diagram illustrating the configuration of a control circuit for controlling operations of the above-described printer will be described with reference to  FIG. 15 . In  FIG. 15 , a printer controller  201  rasterizes image data sent from a (not illustrated) external device, such as a host computer, into the bit data necessary for printing by the printer, reads information in the printer, and controls operations based on that information. 
     A printer engine control unit  202  controls operation of each unit in the printer engine based on instructions from the printer controller  201 , and sends information in the printer engine to the printer controller  201 . A paper conveyance control unit  203  drives and stops the motors (conveyance roller etc.) for feeding and conveying the recording material based on instructions from the printer engine control unit  202 . 
     A high-voltage control unit  204  controls the output of high voltages in the various steps such as charging, development, and transfer in the electrophotographic process based on instructions from the printer engine control unit  202 . An optical system control unit  205  controls the driving and stopping of the scanner motor  104 , or the turning on of a laser beam based on instructions from the engine control unit  202 . 
     A fixing device temperature regulation control unit  207  is for regulating the temperature of the fixing device to a temperature specified by the printer engine control unit  202 . A two-sided unit control unit  208  controls operation of a two-sided unit that can be attached/detached from the printer main body. The two-sided unit control unit  208  performs a paper reversal operation and a re-feeding operation based on instructions from the printer engine control unit  202 , and simultaneously notifies the printer engine control unit  202  of those operation states. 
     Next, a schematic configuration of a typical charging voltage application circuit will be described with reference to  FIG. 16 . This charging voltage application circuit is a high-voltage circuit for applying a high voltage to the charging roller  106 . In  FIG. 16 , a circuit  401  generates a direct current (DC) voltage (also referred to as DC bias) applied to the charging roller. A voltage setting circuit unit  402  is a circuit whose setting value is changed when a pulse-width modulation (PWM) signal is received. The charging voltage application circuit illustrated in  FIG. 16  also includes a transformer drive circuit unit  403  and a high-voltage transformer  404 . 
     A feedback circuit unit  405  detects the value of the voltage applied to the charging roller  106  using a resistor R 71 , and transmits the detected voltage value to the voltage setting circuit unit as an analog value. Then, based on this analog value, a constant voltage is applied to the charging member. 
     Based on such a configuration, by performing a series of controls, a constant voltage can be applied to the charging roller acting as a charging member. Japanese Patent Application Laid-Open No. 6-3932 discusses such a technology, in which a constant voltage is applied to a charging roller. 
     The voltage at which discharge starts for the photosensitive drum acting as an image bearing member by applying a high voltage to the charging roller is known to change based on, for example, the temperature and humidity of the environment in which the printer is set, and the film thickness of the photosensitive drum. 
     The fact that the characteristics of the discharge start voltage to the photosensitive drum are different based on the environment (temperature and humidity) and the film thickness will now be described with reference  FIG. 17 . In  FIG. 17 , the horizontal axis represents the voltage applied to the photosensitive drum, and the vertical axis represents the current flowing to the photosensitive drum. The point at which the current starts to flow is the voltage at which discharge started. It can be seen from  FIG. 17  that since the discharge voltage varies, the potential (Vd) of the photosensitive drum surface is not constant even if a constant voltage is applied to the photosensitive drum. 
     Further, since the sensitivity of the photosensitive drum surface to the laser beam also varies based on the environment (temperature and humidity) and the film thickness of the photosensitive drum (thickness: large (thick)&gt;medium (standard)&gt;small (thin)), the surface potential of the photosensitive drum also varies after laser irradiation even if a constant laser light amount is irradiated on the photosensitive drum. 
       FIG. 18  illustrates the fact that the potential (VL) of the photosensitive drum after irradiation by the laser beam exhibits different characteristics based on differences in the film thickness of the photosensitive drum. In  FIG. 18 , the horizontal axis represents the light amount of the laser beam, and the vertical axis represents the potential of the photosensitive drum after irradiation with the laser beam (expressed as VL). Based on this data, it can be seen that that the potential (VL) of the photosensitive drum after irradiation with the laser beam is not constant even if a constant laser light amount is irradiated on the photosensitive drum. 
     Further, as a photosensitive drum characteristic, fluctuation (also referred to as drum memory) in the surface potential of the photosensitive drum irradiated with light, such as by irradiation with a laser beam, also occurs. Normally, although the surface potential of the photosensitive drum is ideally 0 V after charge on the photosensitive drum surface has been removed, since the potential is negative due to the influence of this potential fluctuation, variation in the surface potential of the photosensitive drum after irradiation with the laser beam occurs. 
     Conventionally, to correct this variation, for example, a storage element (a non-volatile memory) has been provided in the cartridge as a replaceable part in the photosensitive drum for storing information indicating the sensitivity of the photosensitive drum, and application voltage values based on the usage amount of the photosensitive drum. Based on the information in the storage device, the high voltages (charging voltage and development voltage) are variably controlled to match the sensitivity and the usage amount. 
     Further, the light amount of the laser beam has been also variably controlled. However, the increases in conveyance speed and drive speed during printing and the increases in the capacity of the cartridges containing the toner made to improve the productivity of the printer have made it more difficult to sufficiently correct this variation with conventional technology that performs control based on information about the storage element. 
     The reason why it is difficult to correct this variation will be described referring to  FIG. 19 . In  FIG. 19 , if the potential after a photosensitive drum has been charged by a charging roller is Vd, the potential after exposure by a laser beam is VL, and the development potential when developing with a development unit is Vdc, the potential difference Vdc−VL during a normal period and the potential difference Vdc−VL when the sensitivity of the photosensitive drum has deteriorated are different. Since it is difficult to correct this potential difference, density unevenness occurs in the image. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to an image forming apparatus capable of controlling the potential of a photosensitive drum appropriately to form an image that is free from density unevenness, regardless of changes in environment or differences in the film thickness of the photosensitive drum. 
     According to an aspect of the present invention, an image forming apparatus includes an image bearing member on which an image is formed, a charging unit configured to charge the image bearing member, a transfer unit configured to transfer the image formed on the image bearing member onto a transfer member, a voltage application unit configured to apply a voltage to the charging unit and the transfer unit, and a current detection unit configured to detect a current flowing to the image bearing member via the transfer unit when a voltage is applied to the transfer unit, wherein in a state where a voltage is applied to the charging unit, a surface potential of the image bearing member is determined using a first voltage applied from the voltage application unit when a current value obtained by, after applying a predetermined voltage to the transfer unit, detecting the current value with the current detection unit while changing the applied voltage to a positive direction, reaches a discharge current value, and a second voltage applied from the voltage application unit when a current value obtained by, after applying the predetermined voltage to the transfer unit, detecting the current value with the current detection unit while changing the applied voltage to a negative direction, reaches the discharge current value. 
     According to another aspect of the present invention, a method for detecting a surface potential of an image bearing member on which an image is formed, includes applying a voltage to a charging unit configured to charge the image bearing member, in a state where a voltage is applied to the transfer unit, applying a predetermined voltage to a transfer unit configured to transfer the image on the image bearing member onto a transfer member, and detecting a first current value flowing to the transfer member while changing the applied voltage to a positive direction, after applying the predetermined voltage to the transfer unit, detecting a second current value flowing to the transfer member while changing the applied voltage to a negative direction, and determining a surface potential of the image bearing member using a first voltage applied to the transfer unit when the detected first current value reaches a discharge current value and a second voltage applied from a voltage application unit when the detected second current value reaches the discharge current value. 
     Further features and aspects of the present invention will become apparent from the following detailed description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate exemplary embodiments, features, and aspects of the invention and, together with the description, serve to explain the principles of the invention. 
         FIG. 1  illustrates a characteristic of a photosensitive drum. 
         FIGS. 2A ,  2 B, and  2 C are graphs illustrating measurement results of a photosensitive drum characteristic. 
         FIG. 3  is a schematic diagram of an image forming apparatus according to an exemplary embodiment of the present invention. 
         FIG. 4  illustrates a transfer voltage application circuit diagram according to a first exemplary embodiment. 
         FIG. 5  is a graph illustrating a V-I characteristic during transfer voltage application. 
         FIG. 6  is a graph illustrating a current characteristic during transfer negative bias application. 
         FIG. 7  is a laser drive circuit configuration diagram according to the first exemplary embodiment. 
         FIG. 8  ( 8 A and  8 B) is a flowchart according to the first exemplary embodiment. 
         FIG. 9  is a timing chart according to the first exemplary embodiment. 
         FIGS. 10A ,  10 B,  10 C, and  10 D illustrate changes in the potential of a photosensitive drum according to the first exemplary embodiment. 
         FIG. 11  ( 11 A and  11 B) is a flowchart according to a second exemplary embodiment. 
         FIG. 12  is a timing chart according to the second exemplary embodiment. 
         FIGS. 13A ,  13 B,  13 C, and  13 D illustrate changes in the potential of a photosensitive drum according to the second exemplary embodiment. 
         FIG. 14  is a configuration schematic diagram of an image recording apparatus main body. 
         FIG. 15  is a schematic block diagram of a control unit in an image recording apparatus. 
         FIG. 16  illustrates a conventional charging voltage application circuit. 
         FIG. 17  is a graph illustrating that variation is produced in the potential Vd of a photosensitive drum. 
         FIG. 18  is a graph illustrating that variation is produced in the potential VL of a photosensitive drum after laser irradiation. 
         FIG. 19  illustrates that variation is produced in the surface potential of a photosensitive drum. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Various exemplary embodiments, features, and aspects of the invention will be described in detail below with reference to the drawings. 
     The present exemplary embodiment is based on the assumption of a circuit configuration that includes a transfer voltage application circuit that applies a transfer voltage, which is a direct current (DC) voltage generated by a constant voltage power source, to a transfer roller acting as a transfer member in the above-described image forming apparatus, and a detection circuit for detecting the value of the current flowing to a photosensitive drum acting as a an image bearing member via a transfer roller during output of the DC voltage from the constant voltage power source. 
     Such a configuration enables the value of the current flowing to the photosensitive drum to be detected based on a simple circuit configuration using a transfer voltage application circuit, without having to provide a dedicated circuit for applying a DC voltage for current detection. 
     In the present exemplary embodiment, each discharge start voltage for the photosensitive drum is determined based on each current value detected by a current detection circuit when DC voltages with different negative values are respectively applied to a transfer roller during a period over which an image is not formed (non-image forming period). Further, the present exemplary embodiment is characterized by calculating the potential difference needed for the photosensitive drum to discharge and the surface potential of the photosensitive drum using the determination results. 
       FIG. 1  illustrates the symmetry of discharge start voltages, which forms the basis of the present exemplary embodiment.  FIG. 1  illustrates that a discharge voltage V 1 , which is a negative first voltage, and a discharge voltage V 2 , which is a negative second voltage, are symmetrical. 
     As an example of a photosensitive drum discharge characteristic, as described above, the voltage value at which discharge starts changes based on the environment (temperature and humidity) and the film thickness of the photosensitive drum. However, even if the environment where the photosensitive drum is located or the film thickness is different, a characteristic of photosensitive drums is that the potential difference necessary for starting discharging with respect to a predetermined potential of the photosensitive drum is the same. This characteristic is similar to the discharge characteristic within a gap (between flat faces) when applying a high voltage. 
       FIGS. 2A ,  2 B, and  2 C illustrate measurement results of an actual photosensitive drum discharge characteristic.  FIG. 2A  illustrates the characteristic for an ordinary temperature and a low temperature, respectively, and  FIG. 2B  illustrates the characteristic for a case when the film thickness is thin and thick, respectively. The horizontal axis in the graph represents application voltage (V), and the vertical axis represents current (μA). The graph is drawn by plotting actual discharge voltages V 1  and V 2 , and a center (V 1 +V 2 )/2 value. 
     In  FIG. 2A , in an ordinary temperature environment, +602 V and −659 V are discharge voltages V 1  and V 2 , respectively, with a middle of 3.5 V. In a low-temperature environment, +652 V and −621 V are discharge voltages V 1  and V 2 , respectively, with a middle of 9.5 V. 
     Further,  FIG. 2B  illustrates that the discharge voltages when the film thickness of a photosensitive drum  201  is thin and when thick are symmetrical, with a middle of about 0 V. 
     Based on the above data, it can be confirmed that the discharge voltages V 1  and V 2  at each of which discharge starts are symmetrical with respect to the application voltage even if temperature varies or the film thickness changes. This data is for a case in which the potential of the photosensitive drum is roughly 0 V, and is a measurement result when both positive and negative DC voltages were applied. 
     This symmetry exhibits the same characteristic even when the potential of the photosensitive drum surface is not 0 V, for example, when the potential of the photosensitive drum surface is a negative value. An example of this is illustrated in  FIG. 2C , which illustrates measurement data for a case in which the photosensitive drum surface has a negative potential.  FIG. 2C  shows that the discharge voltages V 1  and V 2  are symmetrical, with a middle of −1,150 V. 
     The present exemplary embodiment, focusing on this symmetry characteristic, is characterized by determining the potential difference necessary for the photosensitive drum to discharge and the surface potential of the photosensitive drum, and based on these detection results, setting the value of the voltage to be applied to the charging roller, and setting the light amount of the laser beam. 
       FIG. 3  is a schematic diagram illustrating members and high-voltage application circuits acting on the photosensitive drum according to the present exemplary embodiment. The image forming apparatus illustrated in  FIG. 3  includes a photosensitive drum  201 , a charging roller  202  acting as a charging member that charges the photosensitive drum  201 , a development roller  203  as a development member that develops an electrostatic latent image formed on the photosensitive drum with toner, a transfer roller  204  as a transfer member that transfers a toner image developed on the photosensitive drum onto a recording material, a charging voltage application circuit  205  that applies a high voltage to the charging roller  202 , a transfer voltage application circuit  206  that applies a DC voltage to the transfer roller  204 , and a light source  207  as an exposure unit. 
     Once residual potential on the photosensitive drum  201  has been removed by applying an Alternating Current (AC) voltage to the charging roller  202  from the charging voltage application circuit, a voltage application operation by the transfer voltage application circuit  206  and an operation to detect the potential difference necessary for photosensitive drum discharge and the surface potential are started. 
       FIG. 4  illustrates a schematic configuration of a transfer voltage application circuit  301  according to the present exemplary embodiment. Broadly speaking, this circuit includes two circuits, a positive voltage application circuit unit  301   a  that applies a positive polarity voltage to the transfer roller  204  (photosensitive drum  201 ), which has a negative charge, and a negative voltage application circuit unit  301   b  that applies a negative polarity voltage (negative voltage). In the present exemplary embodiment, since the operation is performed based on application of a negative voltage, a description of the circuit applying a positive voltage will be omitted. 
     In the negative voltage application circuit unit  301   b  illustrated in  FIG. 4 , a voltage setting circuit unit  302  can control the value of the output voltage based on an input PWM signal. The negative voltage application circuit unit  301   b  also includes a high-voltage transformer  304  and a drive circuit unit  303  for driving the high-voltage transformer  304 . 
     A feedback circuit unit  306  is a circuit that detects a voltage output from the high-voltage transformer  304  via the resistor R 61  in order to control a drive operation of the drive circuit unit  303  so that the voltage value is based on the PWM signal setting. A current detection circuit unit  305  is a circuit that detects with a resistor R 63  a current value I 63  obtained by adding a current value I 62  flowing to the photosensitive drum acting as a carrier member and a current value I 61  flowing from the feedback circuit unit  306 , and transmits from a terminal J 501  the detected current value I 63  to the engine control unit  202  as an analog value. 
     Until discharge starts between the photosensitive drum  201  and the transfer roller  204 , the section between the output device  210  and the transfer roller  204  is insulated. Consequently, until discharge is started, the current flowing to a detection resistor R 63  is only the current I 61  that is flowing from the feedback circuit unit  306 . The current I 61  is determined by the following formula based on the voltage value Vpwm set by the PWM signal, a reference voltage Vref, R 64 , and R 65 .
 
 I 61=( V ref− Vpwm )/ R 64 −Vpwm/R 65  (Formula 1)
 
     Further, the output voltage can also be determined by formula 2 by flowing the current value I 61  through the resistor R 61  in the feedback circuit unit  306 .
 
 V out= I 61 ×R 61+ Vpwm□I 61 ×R 61  (Formula 2)
 
       FIG. 5  illustrates a relationship between the application voltage to the transfer roller  204  (photosensitive drum  201 ) as a negative charge and the value of the current flowing to the photosensitive drum  201 . As illustrated by the straight line  1  in  FIG. 5 , until discharge is started, because the only current flowing to the resistor R 63  in the current detection circuit unit  305  is the I 61  based on the PWM signal, the relationship between the application voltage and the current is a straight line. 
     However, when discharge between the photosensitive drum  201  and the transfer roller  204  starts, the current value I 62  flowing to the photosensitive drum  201  flows via a resistor R 71  in the circuit to which a positive voltage is applied. 
     Thus, the current flowing here is I 63 , which is obtained by adding the current value I 62  and the current value I 61  flowing from the feedback circuit unit  306 . Specifically, as illustrated in  FIG. 5 , the relationship between the application voltage and the current is represented by curve  2  that has a branch point at the point where discharge starts. 
     Therefore, the current flowing between the photosensitive drum  201  and the transfer roller  204  can be calculated based on a Δ value obtained by subtracting the value of straight line  1  from curve  2 . The point at which the Δ value is the desired current value (target discharge current value) I is determined as the voltage at which discharge has started. 
     The desired current value (target discharge current value) I needs to be set based on a resistance value of the transfer roller  204 . Although slight, a dark current flows through the transfer roller  204  until discharge is started. 
     This dark current is determined based on the resistance value of the transfer roller  204 .  FIG. 6  illustrates the difference in the flowing current value based on the difference in the resistance value of the transfer roller  204 . As illustrated in  FIG. 6 , the value of the dark current is different based on the difference in the resistance value of the transfer roller  204 . This difference can be understood as having an effect on the current detection accuracy. 
     The resistance value of the transfer roller  204  can be determined based on a difference calculated by applying a pre-set constant voltage and detecting the flowing current value at that point from the relationship illustrated in  FIG. 6 . In  FIG. 6 , for example, the resistance value can be determined based on the current value detected when a voltage of −1,200 V is applied. 
     If the resistance value can be determined, a correction current value at the point where discharge started can be obtained based on the resistance value. The desired current value I (target discharge current value) is set in consideration of this correction current value. Correction current values according to the resistance value are stored as a table in a non-volatile memory in the image forming apparatus control unit. However, these values may also be calculated using a calculation formula rather than a table. 
     After the potential of the photosensitive drum  201  is charged to a predetermined minus potential (negative potential) by applying to the charging roller  202  a predetermined voltage composed of a DC voltage and an alternating current (AC) voltage, different voltages are applied from the transfer voltage application circuit by either changing the voltage in the positive direction (decreasing the absolute value of the voltage) or changing the voltage in the negative direction (increasing the absolute value of the voltage) with respect to that minus potential. 
     Two discharge start voltages are detected, the discharge start voltage V 1  having a small absolute value and the discharge start voltage V 2  having a large absolute value. One-half of the difference in the absolute values of the discharge start voltages V 1  and V 2  is set as the voltage difference ΔV necessary for the photosensitive drum  201  to start discharge (refer to  FIG. 1 ). 
     Further, after the laser beam is irradiated from the light source  207  on the photosensitive drum  201 , a voltage with the greater absolute value is again applied from the transfer voltage application circuit. The discharge start voltage obtained based on the current detected at that point is set as V 3 . The potential VL of the photosensitive drum after irradiation with a laser beam from the light source  207  can be calculated using this discharge start voltage V 3  and the voltage value ΔV obtained as described above. In addition, the light amount value of the irradiated laser beam is set (corrected) so as to match the calculated value of the potential VL. 
     By controlling in this manner, the potential (after laser beam irradiation) VL of the photosensitive drum−development voltage Vdc can be stabilized even if there are changes in the environment (temperature and humidity) or differences in the film thickness of the photosensitive drum. 
       FIG. 7  illustrates a schematic configuration of a laser drive circuit according to the present exemplary embodiment. In  FIG. 7 , while monitoring the amount of light emitted from the laser diode with a PD sensor  316 , a laser driver  314  performs control so that the light amount is constant. 
     A light amount change signal (also referred to as a PWM signal)  313  is input between a control circuit unit  311  and the laser driver  314 , which enables the amount of light emitted from the laser beam to be varied based on this light amount change signal (PWM signal). 
     In this configuration, since the laser beam light amount that is irradiated on the photosensitive drum  201  can be controlled, after the potential of the photosensitive drum after laser irradiation (VL) is detected, if that value is different from the desired value, the VL value can be corrected by varying the laser beam light amount. By performing such a correction, the drum potential (after laser beam irradiation)−development voltage (Vdc) can be obtained. 
     Next, the controls performed in the present exemplary embodiment will be described with reference to the flowchart of  FIG. 8 , the timing chart of  FIG. 9 , and the potential diagrams of  FIGS. 10A ,  10 B,  10 C, and  10 D. The operations performed in the flowchart of  FIG. 8  are controlled by the engine control unit  202  (refer to  FIG. 14 ). 
     In  FIG. 8 , first, in step S 300 , the power of the image forming apparatus is turned on or a print command is received. Then, in step S 301 , pre-rotation (after the power is turned on) or pre-rotation (after a print command is received), which are an initialization operation, is executed. In step S 302 , during the period that the photosensitive drum  201 , which is an image bearing member, is rotating (non-image period during which an image is not formed on the photosensitive drum), residual charge on the photosensitive drum  201  is removed by applying an AC voltage to the charging roller  202 . 
     Then, in step S 303 , the photosensitive drum  201  is charged to a negative potential by applying a desired AC voltage to the charging roller  202  using a charging voltage application circuit (refer to  FIG. 16 ). In step S 304 , a predetermined voltage (negative voltage) is applied to the transfer roller  204 . In step S 305 , the desired current value I is determined as described above by calculating the voltage value applied at that point and the resistance value of the transfer roller based on the detected current value. 
     In step S 306 , a negative voltage is applied to the transfer roller with respect to the charging voltage value when the photosensitive drum  201  was charged by applying the desired AC voltage. First, the absolute value of the negative voltage gradually decreases. Then, in step S 307 , the current I 63  obtained by adding the current I 62  flowing from the transfer roller  204  and the current I 61  flowing from the feedback circuit is detected as an analog value input from the terminal J 501 . 
     In step S 308 , based on that detection value, the discharge current is calculated based on the method described above. Then, in step S 309 , the calculated discharge current value and the desired current value (target discharge current value) I are compared to determine whether that current value I is within a tolerance. 
     Specifically, if the calculated discharge current value is greater than the desired current value I+tolerance (“GREATER THAN” in step S 309 ), it is determined that the discharge start voltage is set to a lower voltage, so the processing proceeds to step S 310 . In step S 310 , the voltage value is increased by taking the PWM signal value up a step. 
     However, if the calculated discharge current value is smaller than the desired current value I−tolerance (“LESS THAN” in step S 309 ), it is determined that the discharge start voltage is set to a higher voltage, so that the processing proceeds to step S 311 . In step S 311 , the voltage value is decreased by taking the PWM signal value down a step. 
     If the PWM signal has been controlled so that the calculated discharge current value and the desired current value are within the tolerance, then in step S 312 , the voltage value at that point is set as the discharge start voltage V 1  for the side with the low absolute value. 
     Then, once again, in step S 313 , a negative voltage is applied to the transfer roller  204  with respect to the charging voltage value when the photosensitive drum  201  was charged by applying the desired AC voltage. However, this time the absolute value of the negative voltage gradually increases. Then, in step S 314 , the current I 63  obtained by adding the current I 62  flowing from the transfer roller  204  and the current I 61  flowing from the feedback circuit is detected as an analog value input from the terminal J 501 . In step S 315 , based on that detection value, the discharge current is calculated based on the method described above. 
     Then, in step S 316 , the calculated discharge current value and the desired current value I are compared to determine whether the desired current value I is within a tolerance. Specifically, if the calculated discharge current value is greater than the desired current value I+tolerance (“GREATER THAN” in step S 316 ), it is determined that the discharge start voltage is set to a lower voltage, so that the processing proceeds to step S 317 . In step S 317 , the voltage value is increased by taking the PWM signal value up a step. 
     However, if the calculated discharge current value is smaller than the desired current value I−tolerance (“LESS THAN” in step S 316 ), it is determined that the discharge start voltage is set to a higher voltage, so that the processing proceeds to step S 318 . In step S 318 , the voltage value is decreased by taking the PWM signal value down a step. 
     If the PWM signal has been controlled so that the calculated discharge current value and the desired current value are within the tolerance, then in step S 319 , the voltage value at that point (PWM signal value B) is set as the discharge start voltage V 2  for the side with the high absolute value. Then, in step S 320 , ½ of the difference in the absolute values of the discharge start voltages V 1  and V 2  is calculated, and based on the calculated value, the voltage difference ΔV necessary for the photosensitive drum  201  to start discharge and the surface potential Vdram of the photosensitive drum  201  are calculated. 
     Next, the processing proceeds to a sequence for detecting the potential VL of after the photosensitive drum  201  is irradiated with a laser beam. In step S 321 , the photosensitive drum  201  is charged by applying to the charging roller  202  a charging voltage based on the potential difference ΔV and the surface potential Vdram. Then, in step S 322 , the surface of the photosensitive drum  201  is set to a potential VL state by irradiating the laser beam on the photosensitive drum  201 . 
     Next, in step S 323 , a predetermined negative voltage based on the voltage difference ΔV is applied to the transfer roller  204 . Then, in that state, in step S 324 , the current I 63  obtained by adding the current I 62  flowing from the transfer roller  204  and the current I 61  flowing from the feedback circuit is detected as an analog value input from the terminal J 501 . 
     In step S 325 , based on that detection value, the discharge start current value is calculated based on the method described above. Then, in step S 326 , the calculated discharge current value and the desired current value I are compared to determine whether the current value I is within a tolerance. In step S 327 , if the calculated discharge current value is greater than the desired current value I+tolerance (“GREATER THAN” in step S 326 ), it is determined that the potential VL of the photosensitive drum  201  surface is set low, so that the processing proceeds to step S 327 . In step S 327 , the laser beam light amount is decreased by taking the laser light amount setting value down a step. 
     However, if the calculated discharge current value is less than the desired current value I−tolerance (“LESS THAN” in step S 326 ), it is determined that the potential VL of the photosensitive drum  201  surface is set high, so that the processing proceeds to step S 328 . In step S 328 , the laser beam light amount is increased by taking the laser light amount setting value up a step. If the current value I is within the tolerance based on the above-described control (“within tolerance” in step S 326 ), then in step S 329 , the setting value of the laser beam light amount at that point is confirmed as the desired laser beam light amount. 
     By executing the above-described sequence, the VL−Vdc potential difference is controlled to a predetermined value. In step S 330 , after these settings have been completed, the image forming operation is started. 
     Next, the voltage application to the charging roller, the voltage application to the transfer roller, the timing of laser beam irradiation from the light source, and the state of the corresponding photosensitive drum potential at each step of the control described in  FIG. 8  will be described with reference to  FIG. 9  and  FIGS. 10A ,  10 B,  10 C, and  10 D. 
     In  FIG. 9 , an AC voltage and a DC voltage (a voltage in which an AC voltage and a DC voltage are superimposed) are applied to the charging roller at a timing corresponding to steps S 302  and S 303  in  FIG. 8 . Then, the resistance value of the transfer roller is calculated by applying a negative voltage to the transfer roller  204  at a timing corresponding to steps S 302  and S 303  in  FIG. 8 , and the desired current value I is set. 
     Then, at a timing corresponding to steps S 306  to S 319 , the discharge start voltages V 1  and V 2  are detected, and at a timing corresponding to step S 320 , the drum surface potential Vdram and the potential difference ΔV are calculated. Next, while applying current and voltage to the charging roller based on ΔV and Vdram at a timing corresponding to step S 321 , the laser beam is irradiated on the photosensitive drum at a timing corresponding to step S 322 . 
     At a timing corresponding to steps S 323  to  326 , the photosensitive drum surface potential VL is detected, and at a timing corresponding to steps S 327  to  331 , the photosensitive drum potential is controlled to VL by varying the light amount of the laser beam. 
       FIGS. 10A ,  10 B,  10 C, and  10 D each illustrate a state of the photosensitive drum surface potential at the respective steps.  FIG. 10A  illustrates a state of the photosensitive drum surface potential at a timing corresponding to step S 303  of  FIG. 8 .  FIG. 10B  illustrates a state of the photosensitive drum surface potential at a timing corresponding to steps S 306  to S 319  of  FIG. 8 . 
       FIG. 10C  illustrates a state of the photosensitive drum surface potential at a timing corresponding to steps S 320  to S 323  of  FIG. 8 .  FIG. 10D  illustrates a state of the photosensitive drum surface potential at a timing corresponding to step S 329  of  FIG. 8 . Based on the above control, the potential difference between VL (exposure potential) and Vdc (development voltage) can be stabilized at a desired potential difference. 
     Thus, according to the present exemplary embodiment, a high-quality image with less density unevenness can be formed by appropriately controlling the potential of a photosensitive drum, regardless of changes in environment or differences in the film thickness of the photosensitive drum. 
     A second exemplary embodiment will now be described. The present exemplary embodiment is based on an assumption of the same configuration as the first exemplary embodiment. The difference with the first exemplary embodiment is that in the second exemplary embodiment, the potential difference necessary for the photosensitive drum to discharge and the surface potential of the photosensitive drum are detected, and based on those detection results, the voltage applied to the development roller is set. 
     The configuration in the present exemplary embodiment does not include a function of varying the laser beam light amount like in the first exemplary embodiment. Since a function of varying the laser beam light amount is not included, the configuration is cheaper. Further, since the configuration and the operations for detecting the potential difference and the surface potential are the same as in the first exemplary embodiment, a description thereof will be omitted here. 
     Next, the controls performed in the present exemplary embodiment will be described with reference to the flowchart of  FIG. 11  ( 11 A and  11 B), the timing chart of  FIG. 12 , and the potential diagrams of  FIGS. 13A ,  13 B,  13 C, and  13 D. 
     The operations performed in the flowchart of  FIG. 11  are controlled by the engine control unit  202  (refer to  FIG. 14 ). Further, since steps S 300  to S 325  in the flowchart of  FIG. 11  are the same as the control performed in  FIG. 8  according to the first exemplary embodiment, a description of those steps will be omitted here. The controls performed in steps S 426  to  431  regarding setting of the development voltage according to the present exemplary embodiment will now be described. 
     In step S 426 , the engine control unit  202  determines whether the calculated discharge start voltage (step S 325 ) is greater than the desired current value I+tolerance (“GREATER THAN” in step S 426 ) or whether the discharged discharge start voltage is less than the desired current value I−tolerance (“LESS THAN” in step S 426 ). 
     Based on that detection value, the discharge current value is calculated based on the same method as in the first exemplary embodiment. That calculated value and the desired current value I are then compared to determine whether the current value is within a tolerance for the I value. If the calculated discharge current value is greater than the desired current value I+tolerance (“GREATER THAN” in step S 426 ), it is determined that the discharge start voltage is a low setting, so that the processing proceeds to step S 427 . In step S 427 , the transfer voltage is increased by taking the PWM signal value (transfer voltage applied to the transfer roller) up a step. 
     However, if the calculated discharge current value is less than the desired current value I−tolerance (“LESS THAN” in step S 426 ), it is determined that the discharge start voltage is a high setting, so that the processing proceeds to step S 428 . In step S 428 , the transfer voltage is decreased by taking the PWM signal value (transfer voltage) down a step. 
     If the current value I is within the tolerance for the desired current value I based on the above-described control (“WITH IN TOLERANCE”), then in step S 429 , the value (transfer voltage) of the PWM signal at that point is set as the discharge start voltage V 3  for the potential VL after laser beam irradiation. 
     In step S 430 , the potential VL after laser beam irradiation is calculated by determining the difference between the potential difference ΔV necessary for photosensitive drum  201  discharge to start obtained above and the discharge start voltage V 3  for the potential VL after laser beam irradiation. The calculated value is VL=|V 3 −ΔV|, which is an absolute value. 
     In step S 431 , based on the calculated VL value, the value of the development voltage applied to the development roller is set. By controlling in this manner, the VL−Vdc voltage is controlled to a predetermined value. In step S 432 , after these settings have been completed, the image forming operation is started. 
     Next, the voltage application to the charging roller, the voltage application to the transfer roller, the timing of laser beam irradiation from the light source, and the state of the corresponding photosensitive drum potential at each step of the control described in  FIG. 11  will be described with reference to  FIG. 12  and  FIGS. 13A ,  13 B,  13 C, and  13 D. 
     In  FIG. 12 , since the on/off state corresponding to steps S 302 , S 305 , S 306  to  320 , and S 322  of  FIG. 9  is the same, a description thereof will be omitted here. In the present exemplary embodiment, the application of the transfer voltage to the transfer roller and the voltage correction in steps S 426  to S 428 , and calculation of the exposure potential VL and the setting (adjustment) of the development voltage in steps S 429  to  431  are different. 
     A description of the states in  FIG. 13A to 13D  that are the same as in  FIG. 10A ,  10 B, and  10 C ( FIG. 10A : timing corresponding to step S 302 ,  FIG. 10B : timing corresponding to steps S 306  to S 319 , and  FIG. 10C : timing corresponding to steps S 323  to S 331 ) will be omitted here. In the present exemplary embodiment, the timing of step S 431 , which is illustrated in  FIG. 13D , is different from the first exemplary embodiment. In this step, potential difference of the VL (exposure potential)−Vdc (development potential) is stabilized at the desired potential difference by setting the laser beam light amount to a constant level and correcting the value of the development voltage. 
     Thus, according to the present exemplary embodiment, a high-quality image with less density unevenness can be formed based on a simple configuration by appropriately controlling the potential of a photosensitive drum, regardless of changes in environment or differences in the film thickness of the photosensitive drum. 
     Although the configuration has been described above that transfers an image on a photosensitive drum acting as an image bearing member onto a recording material, the present invention is not limited to this. For example, the configurations described in the first and second exemplary embodiments may also be applied in an apparatus that transfers an image on a photosensitive drum onto a transfer member (intermediate transfer belt, intermediate transfer drum etc.) other than a recording material. 
     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 modifications, equivalent structures, and functions. 
     This application claims priority from Japanese Patent Application No. 2011-272760 filed Dec. 13, 2011, which is hereby incorporated by reference herein in its entirety.