Patent Publication Number: US-7907854-B2

Title: Image forming apparatus and image forming method

Description:
BACKGROUND 
     (1) Technical Field 
     The present invention relates to an image forming apparatus having a mechanism that homogeneously charges a photoconductor by applying an AC component and a DC component thereto according to a contact or proximity charging method on a charge-by-discharge basis, and more specifically to a technique of measuring the film thickness of a photoconductor. 
     (2) Related Art 
     Various components (such as a charging roller, a development brush, a transfer roller, a cleaning brush, and a cleaning blade) are provided on a surface of a photoconductor provided in an image forming apparatus in physical contact with the surface. A photosensitive layer formed on the surface of the photoconductor has its surface gradually worn by repetitive physical contact with such components for each step of image forming processing. Frictional force by the cleaning brush and the cleaning blade is particularly significant and plays a large part in the wearing away of the photosensitive layer. 
     When the photosensitive layer has its thickness reduced to a predetermined degree or more by the wear, the photosensitivity may significantly be reduced or the charge characteristic degrades, the surface cannot be charged homogeneously to a predetermined potential, and a clear image can no longer be formed. The thickness of the photosensitive layer of the photoconductor should be measured, and the useful life of the photoconductor should be notified. 
     SUMMARY 
     According to an aspect of the invention, there is provided an image forming apparatus including a photoconductor driven to rotate and having a photosensitive thin film formed on its surface, a charging member that charges the photosensitive thin film of the photoconductor, a voltage applying unit that applies at least one of voltage of a DC component and voltage of an AC component to the charging member, a capacitance unit connected to a superposition point for the DC component and the AC component, a DC current measuring unit that measures the value of DC current passed from the charging member to the photoconductor when the voltage applying unit applies voltage to the charging member, a capacitance measuring unit that measures the electrostatic charge amount of current coming into the capacitance unit when the voltage applying unit applies the voltage to the charging member, and a control unit that integrates the DC current value measured by the DC current measuring unit with time for which the voltage is applied to the photoconductor and calculates a charge amount corresponding to the thickness of the photosensitive thin film by subtracting the electrostatic charge amount measured by the capacitance measuring unit from the result of integration. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the invention will be described in detail in conjunction with the following figures, wherein: 
         FIG. 1  is a schematic configurational diagram of the hardware of an image forming apparatus; 
         FIG. 2  is a block diagram of the configuration of the image forming apparatus; 
         FIG. 3  is a diagram of the configuration of a power supply device; 
         FIG. 4  is a flowchart for use in illustrating film thickness determination processing; 
         FIGS. 5A and 5B  are characteristic graphs representing the relation of a charge amount and a resistance value to thickness reduction in a photosensitive thin film; 
         FIGS. 6A and 6B  are charts showing measurement current in a film thickness measuring mode with respect to the time base; 
         FIG. 7  is a schematic configurational diagram of the hardware of an image forming apparatus (according to a second exemplary embodiment); 
         FIG. 8  is a flowchart for use in illustrating film thickness determination processing (according to the second exemplary embodiment); 
         FIG. 9  is a diagram of a power supply system for a charging roller (according to a third exemplary embodiment); 
         FIG. 10  is a flowchart for use in illustrating film thickness determination processing (according to the third exemplary embodiment); 
         FIG. 11  is a diagram of a power supply system for a charging roller (according to a fourth exemplary embodiment); 
         FIG. 12  is a flowchart for use in illustrating film thickness determination processing (according to the fourth exemplary embodiment); and 
         FIG. 13  is a diagram for use in illustrating related art. 
     
    
    
     DETAILED DESCRIPTION 
     First Exemplary Embodiment 
       FIG. 1  is a schematic configurational diagram of the hardware of an image forming apparatus  1  according to an exemplary embodiment of the invention. A charging roller  3 , an ROS  4 , a developer  5 , a transfer roller  6 , a cleaning blade  7 , a static eliminating lamp  8  and other elements are provided around a photoconductor drum  2  provided in the image forming apparatus  1 . 
     The photoconductor drum  2  includes a conductive drum base  2 A and a photosensitive thin film  2 B of an OPC (Organic Photoconductor) formed the surface of the drum base  2 A. The photoconductor drum  2  is driven to rotate at a predetermined process speed (peripheral velocity) in the clockwise direction as indicated by the arrow around the central axial line. 
     The charging roller (BCR: Bias Charging Roller)  3  is a charging member in contact with the photoconductor drum  2 . The charging roller  3  rotates following the rotation of the photoconductor drum  2  and homogeneously charges a surface of the photoconductor drum  2  (negatively charged in the exemplary embodiment) to a predetermined potential in a predetermined polarity in response to high voltage supplied from a power supply device  10  that will be described. 
     The ROS (Raster Optical Scanner: image writing unit)  4  directs an image modulated laser beam to a surface of the photoconductor drum  2  to be charged (scanning exposure). The potential at the exposed part is attenuated and an electrostatic latent image forms at the photosensitive thin film  2 B of the photoconductive drum  2 . When the photoconductor drum  2  rotates and the electrostatic latent image comes to a developing position A opposing the developer  5 , an amount of negatively charged toner is supplied from the developer  5  and a toner image is formed by reversal development. 
     The transfer roller  6  is positioned on the downstream side of the developer  5  when viewed in the rotation direction of the photoconductor drum  2  and provided in contact with the photoconductor drum  2  under pressure. The position of the nip portion between the transfer roller  6  and the photoconductor drum  2  is a transfer position B. 
     When the toner image formed on the surface of the photoconductor drum  2  reaches the transfer position B as the photoconductor drum  2  rotates, a paper sheet is supplied to the transfer position B in this timing, and predetermined voltage is applied to the transfer roller  6  at the same time, so that the toner image is transferred from the surface of the photoconductor drum  2  to the paper sheet. The paper sheet transferred with the toner image at the transfer position B is transported to a fixing unit, has its toner image fixed and is then discharged to the outside of the apparatus. 
     Meanwhile, the toner remaining on the surface of the photoconductor drum  2  after the transfer is scraped off with the cleaning blade  7 , and the photoconductor drum  2  has its surface cleaned and readied for the next image forming operation. The electrostatic latent image on the photoconductor drum  2  is eliminated by the static eliminating lamp  8 . 
     Now, a power supply system to the charging roller  3  will be described. 
     The power supply system includes a power supply device  10  including an AC power source unit  11  that supplies the charging roller  3  with high voltage, a DC power source unit  16 , and a current measuring unit  20 , and a control unit  30  that controls the operation of the power supply device  10 . 
     The power supply device  10  includes the AC power source unit  11  that generates AC voltage as shown in the block diagram in  FIG. 2  and the DC power source unit  16  that generates DC voltage. The configurations of the power source units  11  and  16  and the current measuring unit  20  will later be described. The current measuring unit  20  measures a measurement current Iref corresponding to a film thickness in a film thickness measuring mode. 
     The control unit  30  includes a controller  31 , an input/output controller  32 , and a memory  33  and these components each include a CPU (Central Processing Unit) or a RAM (Random Access Memory). The input/output controller  32  has its input and output sides connected with the AC power source unit  11  and the DC power source unit  16  of the power supply device  10  and its output side connected with a display  41 . The control unit  30  outputs a command signal Aon to the AC power source unit  11  and a command signal Don to the DC power source unit  16 . 
     The controller  31  carries out image forming processing, film thickness determination processing and the like that will be described according to a control program stored in the memory  33 . Among these kinds of processing, the turning on/off and variation of a constant current output in the AC power source unit  11  and the turning on/off and variation of a constant voltage output in the DC power source unit  16  are carried out to keep the photosensitive thin film  2 B of the photoconductor drum  2  homogeneously charged in the image forming processing. The film thickness determination processing is carried out separately from the image forming processing. The film thickness determination processing is carried out in a measuring mode in a preset condition (for example after printing a predetermined number of sheets, after elapse of a predetermined time period, or in response to a user command). 
     Now, the configuration of the power supply device  10  will briefly be described with reference to the circuit diagram in  FIG. 3 . 
     In the AC power source unit  11 , an AC power drive circuit  12  operates in response to a command signal Aon received from the control unit  30 , a boosted AC component is produced through a transformer  13 , and one end of the secondary side of the transformer  13  is connected to the charging roller  3 . The other end of the secondary side of the transformer  13  is connected with an output from the DC power source unit  16  and a detection diode  15  through a DC regulating capacitor  14 . The detection diode  15  feeds back the AC component of current passed through a circuit including the charging roller  3 , the photoconductor drum  2 , a ground, and a detection circuit as a half-wave rectified monitor signal IAC to a control section in the power supply device  10 . 
     Note that the DC regulating capacitor  14  prevents the current of the AC component supplied from the AC current power source unit  11  from being passed to the ground side of the DC power source unit  16 . Therefore, a capacitor with a capacitance C 0  (such as 2200 pF) whose impedance is about ten times as large as that of the load capacitance is used. It is only necessary to increase the capacitance C 0  of the DC regulating capacitor  14  in order to completely prevent the DC component current from being passed to the ground side, but if the capacitance is increased too much, the time constant when the AC component current is supplied becomes too large, which causes delayed response. 
     Therefore, in practice, the capacitance C 0  is set in expectation of a small current flow to the ground side of the DC power source unit  16  through the DC regulating capacitor  14 . 
     Upon receiving a command signal Don from the control unit  30 , the DC power source unit  16  turns on a switching transistor  17  to apply DC specified voltage Vdd (for example 24 V) to the primary side of a transformer  18 , and boosted DC voltage (for example −750 V) is produced through the transformer  18 . One end of the secondary side of the transformer  18  is connected to the other end of the secondary side (low potential side) of the transformer  13  at the AC power source unit  11  and the DC component is superposed to the AC component. A voltage dividing resistor  19  and the current measuring unit  20  are connected in series to the output of the DC power source unit  16 , a monitor signal VDC produced from a signal picked up from the midway of the voltage dividing resistor  19  is fed back to the control section in the power supply device  10 . 
     The current measuring unit  20  is connected to the low potential side of the DC power source unit  16  and forms a differential circuit including OP amplifiers  21  and  22  activated in response to the specified voltage Vdd as basic components. The ground of the current measuring unit  20  is used in common as the ground of the photoconductor drum  2 , and therefore current passed through the photosensitive thin film  2 B of the photoconductor drum  2  through the charging roller  3  comes into the current measuring unit  20 . Then, current corresponding to the circuit constant (impedance) of the current measuring unit  20  is measured as a measurement current Iref. The measurement current Iref measured at the current measuring unit  20  is output to the control unit  30 . 
     The AC component of the voltage supplied to the charging roller  3  and the photoconductor drum  2  forms a closed circuit with the AC power source unit  11  through the ground of the photoconductor drum  2 , and the DC component forms a closed circuit with the DC power source unit  16  and the AC power source unit  11  through the ground of the photoconductor drum  2  and the current measuring unit  20 . 
     Now, with reference to the flowchart in  FIG. 4 , the film thickness determination processing according to the exemplary embodiment will be described. 
     The control unit  30  determines whether or not a film thickness measuring mode is attained (step S 10 ). If the film thickness measuring mode is attained (YES in step S 10 ), it is then determined whether or not an electrostatic charge amount measuring mode is attained (step S 20 ). In the electrostatic charge amount measuring mode, the amount of charge possessed by the DC regulating capacitor  14  is measured. 
     If the electrostatic charge amount measuring mode is attained (YES in step S 20 ), the control unit  30  outputs a command signal Don to the DC power source unit  16  that makes a command for applying voltage in a level insufficient to charge the photosensitive thin film  2 B (for example −400 V) (step S 30 ). Upon receiving the command signal Don, the DC power source unit  16  supplies DC component current to the charging roller  3 . In this way, the DC component current is supplied to the charging roller  3 , but the charge is not supplied from the charging roller  3  to the photosensitive thin film  2 B, and the charge comes into the current measuring unit  20 . 
     The control unit  30  reads the measurement current Iref for the current coming into the current measuring unit  20  (step S 40 ). The control unit  30  then calculates an electrostatic charge amount Q 2  by integrating the read measurement current Iref with the time for which the DC component current is supplied (step S 50 ) and stores the electrostatic charge amount Q 2  in the memory  33  (step S 60 ). 
     Then, the control unit  30  outputs a command signal Aon to the AC power source unit  11  (step S 70 ) and then outputs a command signal Don to the DC power source unit  16  that makes a command for applying voltage about in a level sufficient to charge the photosensitive thin film  2 B (for example −750 V) (step S 80 ). In this way, current produced by superposing the DC component to the AC component is sequentially supplied to the charging roller  3  and charges the photosensitive thin film  2 B, and then the current comes into the current measuring unit  20 . The current produced by superposing the DC component to the AC component is used because a material having a dielectric constant close to that of an insulator is charged. 
     The control unit  30  reads the measurement current Iref for the current coming into the current measuring unit  20  (step S 90 ). The control unit  30  then integrates the read measurement current Iref with the time for which the current of superposed components is supplied to produce an integrated charge amount Q 1  (step S 100 ). 
     The control unit  30  reads out the electrostatic charge amount Q 2  stored in the memory  33  in step S 60  (step S 110 ), and the electrostatic charge amount Q 2  is subtracted from the integrated charge amount Q 1  obtained in step S 100  to produce a charge amount Q 3  (step S 120 ). 
     The control unit  30  determines whether or not the charge amount Q 3  exceeds the threshold charge amount Q 0  (step S 130 ) If Q 3 &gt;Q 0  holds (YES in step S 130 ) in the determination processing, the photosensitive thin film  2 B reaches a limit value for film reduction (limit film thickness), and therefore a command for requesting “replacement of the photoconductor drum” is indicated at the display  41  (step S 140 ). 
     The control unit  30  then stops outputting the command signal Don to the DC power source unit  16  (step S 150 ) and stops outputting the command signal Aon to the AC power source unit  11  (step S 160 ), and the film thickness determination processing ends. 
     The film thickness determination processing will be described further in detail with reference to  FIGS. 5A ,  5 B,  6 A and  6 B. 
       FIG. 5A  shows the characteristic of the charge amount Q of the photosensitive thin film  2 B according to reduction in the thickness of the photosensitive thin film  2 B.  FIG. 5B  shows the characteristic of the resistance value R of the photosensitive thin film  2 B according to reduction in the thickness of the photosensitive thin film  2 B.  FIGS. 6A and 6B  show the measurement current Iref in the film thickness measuring mode with respect to the time base, and each interval on the scale of the abscissa represents time for the photoconductor drum  2  to make one rotation. Note that electricity is described in terms of current for the ease of description. 
     As can be seen from  FIG. 5A , at the photoconductor drum  2 , the charge amount Q increases as a function of increase in the reduction in the thickness of the photosensitive thin film  2 B (i.e., as the film thickness decreases), and the charge limit is reached when the wear limit for the photosensitive thin film  2 B is reached. The characteristic of the resistance value R shown in  FIG. 5B  is inversely proportional to the charge amount Q and therefore the resistance value R decreases as the film thickness decreases. 
     As described above, the DC regulating capacitor  14  prevents the DC component current from coming into the ground side. When however the DC component current is supplied, a potential difference is generated at the DC regulating capacitor  14  and current is transiently passed, which causes overshoot in the measurement current Iref. The overshoot causes the actually measured values to follow the characteristic lines as denoted by the dotted lines in  FIGS. 6A and 6B . 
     In contrast, in the film thickness determination processing in step S 30 , the command signal Don that makes a command for applying voltage in a level insufficient to charge the photosensitive thin film  2 B (for example −400 V) is output to the DC power source unit  16 , and DC component current is supplied from the DC power source unit  16  to the charging roller  3  for the time for which the photoconductor drum  2  makes two rotations. When the measurement current Iref for the current coming into the current measuring unit  20  is read (step S 40 ), and the measurement current Iref is integrated with the time corresponding to three rotations of the drum for which the DC component current is supplied (step S 50 ), an electrostatic charge amount Q 2  substantially equal to the overshoot by the DC regulating capacitor  14  can be obtained as indicated by the measurement current Iref 1  in  FIG. 6 . 
     Then in step S 70 , a command signal Aon is output to the AC power source unit  11  and AC component current is supplied from the AC power source unit  11  to the charging roller  3  for the time in which the photoconductor drum  2  makes two rotations. In this state, a command signal Don that makes a command for applying voltage in a level sufficient to charge the photosensitive thin film  2 B (for example −750 V) is output to the DC power source unit  16  in step S 80 , and DC component current is supplied from the DC power source unit  16  to the charging roller  3  for the period in which the photoconductor drum makes three rotations. The measurement current Iref for the current coming into the current measurement portion  20  is read (step S 90 ) and the measurement current Iref is integrated with the time corresponding to three rotations of the drum for which the DC component current is supplied (S 100 ). Then, as indicated by the measurement current Iref 2  in  FIG. 6 , an electrostatic charge amount Q 1  substantially equal to the sum of the charge amount of the photosensitive thin film  2 B and the overshoot by the DC regulating capacitor  14  can be obtained. 
     Therefore, the charge amount obtained by subtracting the electrostatic charge amount Q 1  from the electrostatic charge amount Q 2  can be interpreted as the charge amount of the photosensitive thin film  2 B itself. 
     According to the exemplary embodiment described above, the overshoot in the measurement current Iref generated when the DC component current is supplied to the photoconductor drum  2  through the charging roller  3  is measured, and then an electrostatic charge amount obtained based on the measurement result is removed by the processing in the control unit  30 . In this way, the electrostatic charge amount Q 3  removed of the overshoot is calculated. The calculated electrostatic charge amount Q 3  is represented by the solid line in  FIG. 5A , and therefore an accurate value is indicated as the amount of thickness reduction corresponding to the charge amount of the photosensitive thin film  2 B itself. 
     Consequently, erroneous determination as would be encountered in the case of using the charge amount Q including the overshoot such as erroneously determining replacement timing for the photoconductor drum  2  though the drum has not yet reached the limit of its usefulness can be prevented, and the reliability of the image forming apparatus  1  can be improved. 
     Furthermore, the charge amount is calculated based on the actual measurement value in the electrostatic charge amount measuring mode, and therefore if the capacitance C 0  of the DC regulating capacitor  14  changes for each film thickness determination processing, the charge amount Q 3  with a reduced error can be calculated by accurately calculating the electrostatic charge amount Q 2 . 
     Second Exemplary Embodiment 
     A second exemplary embodiment of the invention will be described. 
       FIG. 7  is a schematic configurational diagram of the hardware of an image forming apparatus  1  according to the exemplary embodiment. As shown in  FIG. 7 , the image forming apparatus  1  is different from the first exemplary embodiment in that the device includes a retract driving part  91  that separates the charging roller  3  from the photoconductor drum  2  at such a distance that the photosensitive thin film  2 B of the photoconductor drum  2  is not charged. 
       FIG. 8  is a flowchart for use in illustrating film thickness determination processing according to the exemplary embodiment. In  FIG. 8 , the processing in the electrostatic charge amount measuring mode from steps S 30  to S 60  shown in  FIG. 4  is replaced by processing from steps S 21  to S 62 . The processing in the series of steps will be described. When the electrostatic charge amount measuring mode is attained (YES in step S 20 ), the control unit  30  separates the charging roller  3  from the photoconductor drum  2  (step S 21 ). The control unit  30  then outputs a command signal Don that makes a command for applying voltage to the DC power source unit  16  (step S 31 ). Upon receiving the command signal Don, the DC power source unit  16  supplies DC component current to the charging roller  3  through the other end of the secondary side of the transformer  13  in the AC power source unit  11 . However, since the charging roller  3  is separated from the photoconductor drum  2  by the retract driving part  91 , only current leaked to the DC regulating capacitor  14  is allowed to come into the current measuring unit  20 . 
     The control unit  30  reads the measurement current Iref for the current coming into the current measuring unit  20  (step S 41 ). The control unit  30  calculates an electrostatic charge amount Q 2  by integrating the read measurement current Iref with the time for which the DC component current is supplied (step S 51 ), stores the electrostatic charge mount Q 2  in the memory  33  (step S 61 ), and then cancels the separated state of the charging roller  3  and the photoconductor drum  2  (step S 62 ). 
     Thereafter, the same processing as that in and after step S 70  in  FIG. 4  is carried out. 
     Third Exemplary Embodiment 
     A third exemplary embodiment of the invention will be described. 
       FIG. 9  is a circuit diagram of a power supply device  10  in an image forming apparatus  1  according to the exemplary embodiment. As shown in  FIG. 9 , the power supply device  10  includes a switch  92  on a wire from the other end of the secondary side of the transformer  13  in the AC power source unit  11  serving as a superposing position for the AC and DC components to the charging roller  3 . The switch  92  opens/closes in response to an output-load ON/OFF signal from the control unit  30 . 
       FIG. 10  is a flowchart for use in illustrating film thickness determination processing according to the exemplary embodiment. In  FIG. 10 , the processing in the electrostatic charge amount measuring mode from steps S 30  to S 60  in  FIG. 4  is replaced by the processing from steps S 24  to S 65 . The processing in the series of steps will be described. When the electrostatic charge amount measuring mode is attained (YES in step S 20 ), the control unit  30  supplies an output-load ON/OFF signal and thus opens the output end of the power supply, in other words, opens the switch  92  between the other end of the secondary side of the transformer  13  in the AC power source unit  11  and the charging roller  3  (step S 24 ). The control unit then outputs a command signal Don that makes a command for applying voltage to the DC power source unit  16  (Step S 34 ). Upon receiving the command signal Don, the DC power source unit  16  supplies DC component current to the other end of the secondary side of the transformer  13  in the AC power source unit  11 . However, the switch on the wire from the other end of the secondary side of the transformer  13  in the AC power source unit  11  to the charging roller  3  is opened, and therefore only the current leaked to the DC regulating capacitor  14  is allowed to come into the current measuring unit  20 . 
     The control unit  30  reads the measurement current Iref for the current coming into the current measuring unit  20  (step S 44 ). The control unit  30  calculates the electrostatic charge amount Q 2  by integrating the read measurement current Iref with the time for which the DC component current is supplied (step S 54 ), and the electrostatic charge amount Q 2  is stored in the storage  33  (step S 64 ). 
     The control unit  30  connects the switch between the other end of the secondary side of the transformer  13  in the AC power source unit  11  and the charging roller  3  (step S 65 ), and thereafter the same processing as that in and after step S 70  shown in  FIG. 4  is carried out. 
     Fourth Exemplary Embodiment 
     A fourth exemplary embodiment of the invention will be described. 
       FIG. 11  is a circuit diagram of a power supply device  10  in an image forming apparatus  1  according to the exemplary embodiment. As shown in  FIG. 11 , the power supply device  10  includes a switch  93  between the other end of the secondary side of a transformer  13  of an AC power source unit  11  as a superposition point between AC and DC components and a DC regulating capacitor  14 . The switch  93  opens/closes in response to a capacitance ON/OFF signal from the control unit  30 . 
       FIG. 12  is a flowchart for use in illustrating film thickness determination processing according to the exemplary embodiment. 
     The control unit  30  determines whether or not a film thickness measuring mode is attained (step S 10 ). If a film thickness measuring mode is attained (YES in step S 10 ), a capacitance ON/OFF signal is supplied. In this way, the switch  93  between the output end of the power supply, in other words, the other end of the secondary side of the transformer  13  in the AC power source unit  11  and the DC regulating capacitor  14  is opened (step S 26 ), and then the control unit outputs a command signal Don that makes a command for applying voltage in a level sufficient to charge the photosensitive thin film  2 B (for example −1500 V) to the DC power source unit  16  (step S 36 ). Upon receiving the command signal Don, the DC power source unit  16  supplies DC component current to the other end of the secondary side of the transformer  13  in the AC power source unit  11 . Then, DC component current sufficient to charge the photosensitive thin film  2 B is sequentially supplied to the charging roller  3 , charges the photosensitive thin film  2 B, and then is allowed to come into the current measuring unit  20 . 
     The control unit  30  reads the measurement current Iref for the current coming into the current measuring unit  20  (step S 46 ). The control unit  30  then calculates an integrated charge amount Q 1  by integrating the read measurement current Iref with the time for which the current for superposed components is supplied (step S 106 ), and the integrated charge amount Q 1  is determined as a charge amount Q 3  (step S 126 ). 
     The control unit  30  then determines whether or not the charge amount Q 3  obtained in step S 126  exceeds the threshold charge amount Q 0  (step S 136 ). If Q 3 &gt;Q 1  holds (YES in step S 136 ) in the determination processing, the reduction in the photosensitive thin film  2 B has reached the limit value (limit film thickness), and therefore a command for requesting “replacement of photoconductor drum  2 ” is indicated at the display  41  (step S 146 ). 
     The control unit  30  also stops outputting the command signal Don to the DC power source unit  16  (step S 156 ), supplies a capacitance ON/OFF signal to connect the switch  93  between the other end of the secondary side of the transformer  13  in the AC power source unit  11  and the DC regulating capacitor  14  (step S 166 ) and then ends the film thickness determination processing. 
     The foregoing description of the exemplary embodiments of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in the art. The exemplary embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, thereby enabling others skilled in the art to understand the invention for various embodiments and with the various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.