Patent Publication Number: US-8543021-B2

Title: Image forming apparatus

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image forming apparatus including a charge bias application circuit for charging an image bearing member. 
     2. Description of the Related Art 
     Description is given below by taking a printer as an example of the image forming apparatus. Conventionally, the printer has a configuration as illustrated in  FIG. 10A . A rotating polygon mirror  103  is rotated by a scanner motor  104 . A laser beam  205  is emitted from a laser light source  207 , and scans a photosensitive drum  201  serving as an image bearing member. A charge roller  202  uniformly charges the photosensitive drum  201 . A developing roller (also referred to as “developing sleeve”)  203  develops an electrostatic latent image formed on the photosensitive drum  201  with toner. A transfer roller  204  transfers a toner image developed by the developing sleeve  203  onto fed paper. Fixing rollers  109  fuse and fix the toner image transferred onto the paper with heat. A cassette paper feeding roller  110  feeds the paper from a cassette to send out the paper to a conveyance path. Pairs of conveyance rollers  114  and  115  convey the paper fed from the cassette to a transfer position formed between the photosensitive drum  201  and the transfer roller  204 . 
       FIG. 10B  is a block diagram illustrating a circuit configuration of a control system for controlling the above-mentioned mechanical parts. Referring to  FIG. 10B , a printer controller  501  loads image code data sent from an external device (not shown), such as a host computer, as bit data necessary for printing to be performed in the printer, and at the same time, reads and displays printer internal information. An engine control part  502  controls each part of the printer in response to an instruction from the printer controller  501 , and at the same time, notifies the printer controller  501  of the printer internal information. A charge bias application circuit  206  controls, in response to an instruction from the engine control part  502 , an output of a charge bias in a charge step among charge, development, and transfer steps. A laser driving circuit  505  controls ON/OFF of the laser light source  207  in response to an instruction from the engine control part  502 . 
       FIG. 11  illustrates a schematic configuration of a charge bias application circuit part  601  for applying the charge bias to the charge roller  202  serving as a charge material for charging the photosensitive drum  201  serving as the image bearing member. The charge bias application circuit part  601  is an example of the above-mentioned charge bias application circuit  206 . A voltage setting circuit part  602  is capable of changing a setting value according to a PWM signal. The PWM signal is input according to a target value of the charge bias to be output. A transformer drive circuit part  603  and a high voltage transformer part  604  are further provided. A feedback circuit part  605  detects a voltage value applied to the charge member/charge material (load) through a resistor R 81 , and transmits the voltage value to the voltage setting circuit part  602 . In the subsequent control, a PWM signal (target value) is obtained so that the detected value is input, and a constant voltage is applied to the charge member/charge material (load). Through the control with such a configuration, a constant voltage can be applied to the charge member/charge material (load). For example, Japanese Patent Application Laid-Open No. H06-003932 discloses a high voltage power source device that employs such a technology of charge bias application. 
     However, a voltage for starting charging between the charge material (charge roller  202 ) and the charge member (photosensitive drum  201 ) changes depending on ambient temperature, a drum layer thickness, or the like. Hence, variations in voltage of the photosensitive drum  201  occur when the predetermined voltage is merely applied ( FIG. 12A ).  FIG. 12A  is a graph showing a relationship between an application voltage (V) applied to the photosensitive drum  201  and a drum voltage (V) of the photosensitive drum  201 . In  FIG. 12A , a circumstance H/H, a circumstance N/N, and a circumstance L/L represent that the state of the circumstance is high temperature and high humidity, normal temperature and normal humidity, and low temperature and low humidity, respectively. When an application voltage (Vout) is set constant, it is found from  FIG. 12A  that variations in voltage of the photosensitive drum  201  occur due to the difference in drum layer thickness or the difference in circumstance. From the fact that the sensitivity of the photosensitive drum  201  also differs due to the circumstance or the drum layer thickness, in a case where a laser beam with a constant light amount is emitted to the photosensitive drum  201 , there also occur variations in voltage of the electrostatic latent image on the photosensitive drum after the laser illumination ( FIG. 12B ).  FIG. 12B  is a graph showing a relationship between a laser illumination light amount and a voltage (VL) of the photosensitive drum after the laser illumination. When the laser illumination light amount is set constant (for example, vertical chain line of  FIG. 12B ), it is found from  FIG. 12B  that variations in voltage (VL) of the photosensitive drum  201  after the laser illumination occur due to the drum layer thickness (in  FIG. 12B , for example, −128 V in a case of thicker drum layer and −197 V in a case of thinner drum layer). 
     Further, as a characteristic of the photosensitive drum  201 , drum memory adversely occurs through the laser illumination. The drum memory is a phenomenon that, though the drum voltage of the photosensitive drum  201  is supposed to be 0 V after a voltage remaining on the surface thereof is eliminated, the drum voltage becomes negative, resulting in variations in drum voltage after the laser illumination. In order to reduce the variations, the following measure has been taken. That is, a memory is provided to a process cartridge including the photosensitive drum  201 , and, for example, a bias value according to the sensitivity and usage of the photosensitive drum  201  is stored in the memory. Then, based on the information, the charge bias, the developing bias, and the laser light amount corresponding to the sensitivity and the usage are corrected, to thereby reduce the variations in voltage. However, the control based on the information of the cartridge memory is predictive control. Therefore, as the printing speed or the cartridge toner amount is increased, the system using the predictive control based on the information of the cartridge memory has a limitation in the correction of the variations in voltages between Vd−Vdc and between Vdc−VL as shown in  FIGS. 13A and 13B . In  FIGS. 13A and 13B , Vd represents a drum voltage after the charging by the charge roller, Vdc represents a developing bias, and VL represents a drum voltage after the laser illumination. 
     SUMMARY OF THE INVENTION 
     The purpose of the present invention is to provide an image forming apparatus capable of forming a high-quality image irrespective of a change in circumstance or drum layer thickness. 
     Another purpose of the present invention is to provide an image forming apparatus, including an image bearing member; a first voltage application section for applying a first DC voltage to a charge section for charging the image bearing member, a second voltage application section for applying a second DC voltage, which has a polarity reverse to a polarity of the first DC voltage, to the charge section for charging the image bearing member, and a calculation section for calculating a surface voltage of the image bearing member based on a first charge start voltage between the charge section and the image bearing member, which is obtained when the first voltage application section applies the first DC voltage to the charge section, and a second charge start voltage between the charge section and the image bearing member, which is obtained when the second voltage application section applies the second DC voltage to the charge section. 
     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 schematic diagram illustrating an image forming part of an image forming apparatus according to a first embodiment of the present invention. 
         FIG. 2A  is a graph showing a drum characteristic according to the first embodiment. 
         FIGS. 2B and 2C  are graphs showing results of the drum characteristic. 
         FIG. 3  is a diagram illustrating a charge bias application circuit part according to the first embodiment. 
         FIG. 4  is a schematic graph showing a V-I characteristic at the time of charge bias application according to the first embodiment. 
         FIG. 5  is a configuration diagram illustrating a laser driving circuit according to the first embodiment. 
         FIG. 6  is comprised of  FIGS. 6A and 6B  showing flowcharts illustrating charge bias control according to the first embodiment. 
         FIGS. 7A ,  7 B,  7 C, and  7 D are graphs showing voltages of a photosensitive drum obtained as a result of the charge bias control according to the first embodiment. 
         FIG. 8  is comprised of  FIGS. 8A and 8B  showing flowcharts illustrating charge bias control according to a second embodiment of the present invention. 
         FIGS. 9A ,  9 B, and  9 C are graphs showing voltages of the photosensitive drum obtained as a result of the charge bias control according to the second embodiment. 
         FIG. 10A  is a configuration diagram illustrating an image forming apparatus according to the embodiments of the present invention and a conventional example. 
         FIG. 10B  is a block diagram illustrating a circuit configuration of a control system. 
         FIG. 11  is a diagram illustrating a charge bias application circuit part of the image forming apparatus according to the conventional example. 
         FIG. 12A  is a graph showing a relationship between an application voltage and a drum voltage in a photosensitive drum according to the conventional example. 
         FIG. 12B  is a graph showing a relationship between a laser illumination light amount and the drum voltage. 
         FIGS. 13A and 13B  are graphs showing drum voltages of the photosensitive drum after laser illumination according to the conventional example. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Hereinbelow, configurations and operations according to the present invention are described. Note that, embodiments described below are merely exemplary, and hence the technical scope of the present invention is not limited to the embodiments. Hereinbelow, referring to the attached drawings, modes for carrying out the present invention are described in detail by way of the embodiments. 
     First, a first embodiment of the present invention is described. 
     Configuration of Image Forming Apparatus 
       FIG. 1  is a schematic diagram illustrating an image forming part of an image forming apparatus according to this embodiment. The image forming apparatus includes a photosensitive drum  201 , a charge roller  202  for uniformly charging the photosensitive drum  201 , a developing sleeve (developing material)  203  for developing an electrostatic latent image, a transfer roller  204 , a charge bias application circuit  206  serving as a voltage application circuit, and a laser light source  207 . The charge bias application circuit  206  applies an alternative current bias voltage (hereinafter, referred to as “AC bias”) to eliminate the voltage remaining on the photosensitive drum  201 , and then a series of control is started. Note that, the image forming apparatus of this embodiment includes the same control system described above with reference to  FIG. 10B . 
     As a charge characteristic of the photosensitive drum  201 , a voltage difference necessary for the charging differs due to a difference in circumstance or a difference in drum layer thickness. However, as shown in  FIG. 2A , there is such a characteristic that, under a certain condition of the photosensitive drum  201 , the voltage difference necessary to start the charging has a symmetric relationship between the positive voltage and the negative voltage (hereinafter, referred to as “positive-negative symmetry”) with respect to a surface voltage (zero drum voltage) of the photosensitive drum  201 . This characteristic is the same as the charge characteristic in a gap (plane to plane).  FIGS. 2B and 2C  show results of the characteristic of the photosensitive drum  201  obtained through actual measurement.  FIG. 2B  shows a characteristic based on the difference in circumstance, while  FIG. 2C  shows a characteristic based on the difference in drum layer thickness. The two pieces of data each indicate the positive-negative symmetry. Focusing on this characteristic, the image forming apparatus of this embodiment has a feature of detecting the surface voltage of the photosensitive drum  201  and the voltage difference necessary for the charging by the photosensitive drum  201 , and setting high voltages (charge bias and developing bias) and a laser illumination light amount based on the detection results. 
     Configuration of Charge Bias Application Circuit 
       FIG. 3  illustrates, in the upper part thereof, a schematic configuration of a charge bias application circuit  301  for a negative bias according to this embodiment. Note that, the charge bias application circuit  301  and a charge bias application circuit  401  described later constitute the above-mentioned charge bias application circuit  206 . A voltage setting circuit part  302  is capable of changing a bias value to be output according to a PWM signal. The charge bias application circuit  301 A further includes a transformer drive circuit part  303  and a high voltage transformer part  304 . A feedback circuit part  306  is a circuit for monitoring an output voltage through a resistor R 61 , the feedback circuit part  306  being provided so that an output voltage value is obtained according to the setting of the PWM signal. A current detection circuit part  305  detects, through a resistor R 63 , a current I 63  obtained by adding a current I 62  flowing through a charge member/charge material and a current I 61  flowing from the feedback circuit part  306 , and transmits the current I 63  as an analog value from J 301  to an engine control part  502  (see  FIG. 10B ). 
     The photosensitive drum  201  serving as an image bearing member is isolated from the charge roller  202  serving as the charge material until the charging starts between the photosensitive drum  201  and the charge roller  202 . Accordingly, the current flowing through the resistor R 63  is only the current I 61  flowing from the feedback circuit part  306  until the charging starts. The current I 61  is determined from Vpwm, which is set based on the PWM signal, Vref, R 64 , and R 65 , and has the following relationship.
 
 I 61=( V ref− V pwm)/ R 64 −V pwm/ R 65
 
     Further, when the current I 61  flows through the resistor R 61 , an output voltage Vout is set as follows.
 
 V out= I 61 ×R 61 +V pwm≈ I 61× R 61
 
       FIG. 4  is a schematic graph showing transition of a current value (μA) with respect to the application voltage. As indicated by a linear line I, only the current I 61  according to the PWM signal flows through the resistor R 63  until the charging starts. However, when the charging starts between the photosensitive drum  201  and the charge roller  202 , the current I 63  obtained by adding the current I 62  flowing through the photosensitive drum  201  and the current I 61  flowing from the feedback circuit flows through the resistor R 63 . In other words, as indicated by a curved line II of  FIG. 4 , there is obtained a curved line having a branch point around the time when the charging starts. Thus, a charge current flowing through the photosensitive drum  201  can be calculated from a Δ value obtained by subtracting the linear line I from the curved line II. Then, a point at which the Δ value becomes a predetermined current value is determined as the application voltage at the time when the charging starts. 
       FIG. 3  further illustrates, in the lower part thereof, a schematic configuration of the charge bias application circuit  401  for a positive bias according to this embodiment. A voltage setting circuit part  402  is capable of changing a bias value according to a PWM signal. A transformer drive circuit part  403  and a high voltage transformer part  404  are further provided. A feedback circuit part  406  is a circuit for monitoring an output voltage through a resistor R 71 , the feedback circuit part  406  being provided so that an output voltage value is obtained according to the setting of the PWM signal. A current detection circuit part  405  detects, through a resistor R 73 , a current I 73  obtained by adding a current I 72  flowing through the charge member/charge material and a current I 71  flowing through the feedback circuit part  406 , and transmits the current I 73  as an analog value from J 401  to the engine control part  502 . The method of calculating the voltage at the time when the charging starts is the same as that in the case of the charge bias application circuit  301  for the negative bias, and description thereof is therefore omitted herein. 
     A relay circuit part  511  switches between the above-mentioned positive and negative bias application circuits. Under the condition in which such two circuits are provided respectively for the positive bias and the negative bias, biases of a positive polarity and a negative polarity are applied with respect to the voltage of the photosensitive drum  201 , and charge start voltages of both the polarities (detection voltage of the positive bias: V 1  and detection voltage of the negative bias: V 2 ) are detected. Then, a value obtained by halving a difference between the voltage value V 1  and the voltage value V 2  is set as a voltage difference ΔV that is necessary to start the charging by the photosensitive drum  201 , and a central value between V 1  and V 2  is set as a zero drum voltage (Vdram) of the photosensitive drum  201 . In the subsequent control, a bias to be applied to the photosensitive drum  201  serving as the charge member, and a bias to be applied to the developing sleeve  203  are set according to the setting values. Through the control described above, a predetermined relationship, that is, (voltage of the photosensitive drum  201 )−(developing bias) (Vd−Vdc), can be obtained irrespective of the fluctuation in drum layer thickness, circumstance, or the like. 
     Further,  FIG. 5  illustrates a schematic configuration of a laser driving circuit  505  according to this embodiment. A laser driver  354  monitors an exposure amount of the laser light source  207  by using a PD sensor  356  to control an emission amount to be constant. A light amount variable signal (PWM signal)  353  is input from a control circuit part  351  to the laser driver  354 , with the result that the light amount is variably set according to the light amount variable signal (PWM signal)  353 . With this configuration, the light amount for illuminating the photosensitive drum  201  is variably set, and hence, when a drum voltage (VL) after the laser illumination is detected and its value differs from a predetermined value, the value of VL can be corrected by changing the laser light amount. Through the correction described above, a predetermined relationship, that is, (voltage of the photosensitive drum  201  after the laser illumination)−(developing bias) (VL−Vdc), can be obtained. 
     Charge Bias Control 
     Next, referring to flowcharts of  FIGS. 6A and 6B  and voltage graphs of  FIGS. 7A to 7D , the control of this embodiment is described. First, after the engine control part  502  is powered on or receives a print command, the engine control part  502  executes a print preparation operation while rotating the photosensitive drum  201  for a predetermined period of time (also referred to as “multiple initial rotation” or “initial rotation”) (Step (hereinafter, referred to as “S”)  301 ). Then, the charge bias application circuit  206  applies the AC bias to the photosensitive drum  201  to eliminate the remaining voltage (S 302 ). After that, the charge bias application circuit  401  applies a predetermined positive bias (PWM( 1 )) (S 303 ). Then, the engine control part  502  detects, by using the current detection circuit part  405 , the current I 73  obtained by summing the current I 72  flowing through the photosensitive drum  201  and the current I 71  flowing through the feedback circuit part  406 , to thereby detect the analog value of J 401  (S 304 ). The engine control part  502  calculates the charge current from the detection value (S 305 ), and compares the calculation value and the Δ value to determine whether or not the calculation value falls within a tolerance of the Δ value (S 306 ). Specifically, the engine control part  502  determines whether or not the calculation value falls within a range between a lower limit of the Δ value and an upper limit of the Δ value. When the determination result shows that the calculation value is larger than the upper limit of the Δ value, the engine control part  502  determines that the charge start voltage is set to a lower value, and hence causes the charge bias application circuit  401  to step up the bias value (PWM( 1 )) (S 307 ). On the other hand, when the determination result shows that the calculation value is smaller than the lower limit of the Δ value, the engine control part  502  determines that the charge start voltage is set to a higher value, and hence causes the charge bias application circuit  401  to step down the bias value (PWM( 1 )) (S 308 ). Through this operation, the engine control part  502  determines that the positive side voltage of  FIG. 2A  can be detected when the calculation value falls within the tolerance of the Δ value, and sets the bias value (PWM( 1 )) at this time as the charge start voltage V 1  of the positive bias (S 309 ). 
     Subsequently, the engine control part  502  switches the relay by using the relay circuit part  511 , to thereby switch from the positive bias application to the negative bias application (S 310 ). After that, the charge bias application circuit  206  applies the AC bias to the photosensitive drum  201  to eliminate the remaining voltage (S 311 ). Then, the charge bias application circuit  301  applies a predetermined negative bias (PWM( 2 )) (S 312 ). Subsequently, the engine control part  502  detects, by using the current detection circuit part  305 , the current I 63  obtained by summing the current I 62  flowing from the photosensitive drum  201  and the current I 61  flowing from the feedback circuit part  306 , to thereby detect the analog value of J 301  (S 313 ). The engine control part  502  calculates the charge current from the detection value (S 314 ). Then, the engine control part  502  compares the calculation value and the Δ value to determine whether or not the calculation value falls within the tolerance of the Δ value (S 315 ). When it is determined that the calculation value is larger than the upper limit of the Δ value, the engine control part  502  determines that the charge start voltage is set to a lower value, and hence causes the charge bias application circuit  301  to step up the bias value (PWM( 2 )) (S 316 ). On the other hand, when it is determined that the calculation value is smaller than the lower limit of the Δ value, the engine control part  502  determines that the charge start voltage is set to a higher value, and hence causes the charge bias application circuit  301  to step down the bias value (PWM( 2 )) (S 317 ). Through this operation, the engine control part  502  determines that the negative side voltage of  FIG. 2A  can be detected when the calculation value falls within the tolerance of the Δ value, and sets the bias value (PWM( 2 )) at this time as the charge start voltage V 2  of the negative bias (S 318 ). After that, the engine control part  502  calculates the value obtained by halving the difference between V 1  and V 2  as the voltage difference ΔV of  FIG. 2A  that is necessary to start the charging by the photosensitive drum  201 , and calculates the central value between V 1  (V of  FIG. 2A ) and V 2  (−V of  FIG. 2A ) as the zero drum voltage (Vdram) of the drum (S 319 ). The engine control part  502  adds a bias value (ΔPWM) corresponding to the drum voltage into the PWM value according to the calculated voltage difference ΔV and zero drum voltage (Vdram), to thereby set a charge bias (PWM( 3 )) to be output from the charge bias application circuit  206  (S 320 ). The setting value is ΔV+Vdram+Vd, provided that Vd represents a voltage to be superposed onto the photosensitive drum  201 . Through the setting described above, the voltage Vd becomes constant as shown in  FIG. 7A . Subsequently, the engine control part  502  sets a developing bias (PWM( 4 )) according to the set bias (PWM( 3 )) of the charge bias application circuit  206  (S 321 ). Through this sequence, the voltage between Vd−Vdc is controlled to be a predetermined value as shown in  FIG. 7B . 
     Subsequently, the process proceeds to a sequence of detecting the voltage VL after the laser illumination. First, the charge bias application circuit  206  applies the AC bias to the photosensitive drum  201  to eliminate the remaining voltage (S 322 ). After that, the charge bias application circuit  206  applies the charge bias (PWM( 3 )) determined in S 320  to the photosensitive drum  201  (S 323 ), and emits laser of a laser light amount value PWM( 6 ) onto the photosensitive drum  201  to set the voltage on the photosensitive drum  201  to VL (S 324 ). Subsequently, the charge bias application circuit  301  applies a DC negative bias (PWM( 5 )), which is a predetermined DC voltage, to the photosensitive drum  201  (S 325 ). Then, the engine control part  502  detects, by using the current detection circuit part  305 , the current I 63  obtained by summing the current I 62  flowing from the photosensitive drum  201  and the current I 61  flowing from the feedback circuit part  306 , to thereby detect the analog value of J 301  (S 326 ). The engine control part  502  calculates the charge current from the detection value (S 327 ). Then, the engine control part  502  compares the calculation value and the Δ value to determine whether or not the calculation value falls within the tolerance of the Δ value (S 328 ). When it is determined that the calculation value is larger than the upper limit of the Δ value, the engine control part  502  determines that the VL value is set to a lower value, and hence causes the control circuit part  351  of the laser driving circuit  505  to step down the laser light amount value (PWM( 6 )), to thereby decrease the light amount (S 329 ). On the other hand, when it is determined that the calculation value is smaller than the lower limit of the Δ value, the engine control part  502  determines that the VL value is set to a higher value, and hence causes the control circuit part  351  to step up the laser light amount setting value (PWM( 6 )), to thereby increase the light amount (S 330 ). Through this control, the engine control part  502  determines that, when the calculation value falls within the tolerance of the Δ value, the laser light amount value (PWM( 6 )) at this time is the predetermined laser light amount, and causes the control circuit part  351  to set the laser light amount value (PWM( 6 )) (S 331 ). Through this sequence, the voltage between VL−Vdc is controlled to be a predetermined value as shown in  FIG. 7C . After those settings are completed, the printing is started. Through the control described above, a stabilized voltage as shown in  FIG. 7D  is obtained irrespective of the condition of the circumstance or the drum layer thickness, with the result that a high-quality image can be realized. 
     With the image forming apparatus of this embodiment, a high-quality image can be obtained irrespective of a change in circumstance or drum layer thickness. 
     Next, a second embodiment of the present invention is described. 
     Similarly to the first embodiment, the second embodiment utilizes the characteristic that the voltage difference necessary to start the charging is symmetric between the positive voltage and the negative voltage with respect to the zero drum voltage (positive-negative symmetry). However, the image forming apparatus of this embodiment is different from that of the first embodiment in that the laser light amount variable function is not provided. Accordingly, the image forming apparatus of this embodiment can be made more inexpensive than that of the first embodiment. 
     Charge Bias Control 
     The configurations of the image forming apparatus and the charge bias application circuit according to this embodiment are the same as those of the first embodiment, and description thereof is therefore omitted herein. Next, referring to flowcharts of  FIGS. 8A and 8B  and voltage graphs of  FIGS. 9A to 9C , the control of this embodiment is described. The process of from S 5401  to S 420  of  FIGS. 8A and 8B  is the same as the process of from S 301  to S 320  of  FIGS. 6A and 6B  according to the first embodiment, and description thereof is therefore omitted herein. 
     The setting value of the charge bias (PWM( 3 )) to be output from the charge bias application circuit  206  is ΔV+Vdram+Vd, provided that Vd represents a voltage to be superposed onto the photosensitive drum  201 . With this set voltage, the voltage Vd becomes constant as shown in  FIG. 9A . 
     Subsequently, the process proceeds to a sequence of detecting the voltage VL after the laser illumination. First, the charge bias application circuit  206  applies the AC bias to the photosensitive drum  201  to eliminate the remaining voltage on the photosensitive drum  201  (S 421 ). After that, the charge bias application circuit  206  applies the charge bias (PWM( 3 )) determined in S 420  to the photosensitive drum  201  (S 422 ), and emits laser onto the photosensitive drum  201  to set the voltage on the photosensitive drum  201  to VL after the laser illumination (S 423 ). Subsequently, the charge bias application circuit  301  applies a predetermined DC negative bias (PWM( 5 )) (S 424 ). Then, the engine control part  502  detects, by using the current detection circuit part  305 , the current I 63  obtained by summing the current I 62  flowing from the charge member and the current I 61  flowing from the feedback circuit part  306 , to thereby detect the analog value of J 301  (S 425 ). The engine control part  502  calculates the charge current from the detection value (S 426 ). Then, the engine control part  502  compares the calculation value and the Δ value to determine whether or not the calculation value falls within the tolerance of the Δ value (S 427 ). When it is determined that the calculation value is larger than the upper limit of the Δ value, the engine control part  502  determines that the charge start voltage is set to a lower value, and hence steps up the bias value (PWM( 5 )) (S 428 ). On the other hand, when the determination result shows that the calculation value is smaller than the lower limit of the Δ value, the engine control part  502  determines that the charge start voltage is set to a higher value, and hence so as to step down the bias value (PWM( 5 )) (S 429 ). Through this operation, when the calculation value falls within the tolerance of the Δ value, the engine control part  502  sets the bias value (PWM( 5 )) at this time as a charge start voltage V 3  of the negative bias (S 430 ). From the charge start voltage V 3  at VL and the voltage difference ΔV necessary to start the charging obtained through the above-mentioned sequence, the engine control part  502  calculates VL by an expression of V 3 −ΔV=VL (S 431 ). In this manner, the voltage between VL−Vd can be detected as shown in  FIG. 9B . 
     Then, according to the values of Vd and VL that are set and calculated through the above-mentioned sequence, the engine control part  502  sets the developing bias (PWM( 4 )) (S 432 ). When setting the developing bias (PWM( 4 )), it is considered that the value of the voltage between VL−Vdc, which may affect the contrast, falls within the predetermined range. Through the control described above, a predetermined voltage as shown in  FIG. 9C  is obtained irrespective of the condition of the circumstance or the drum layer thickness, with the result that a high-quality image can be realized. 
     With the image forming apparatus of this embodiment, a high-quality image can be obtained irrespective of a change in circumstance or drum layer thickness. 
     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. 
     This application claims the benefit of Japanese Patent Application No. 2010-149375, filed Jun. 30, 2010, which is hereby incorporated by reference herein in its entirety.