Image forming apparatus and image forming method

An image forming apparatus includes a photoconductor, a charging member, 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, a capacitance measuring unit that measures the electrostatic charge amount of current coming into the capacitance unit, 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.

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.

DETAILED DESCRIPTION

First Exemplary Embodiment

FIG. 1is a schematic configurational diagram of the hardware of an image forming apparatus1according to an exemplary embodiment of the invention. A charging roller3, an ROS4, a developer5, a transfer roller6, a cleaning blade7, a static eliminating lamp8and other elements are provided around a photoconductor drum2provided in the image forming apparatus1.

The photoconductor drum2includes a conductive drum base2A and a photosensitive thin film2B of an OPC (Organic Photoconductor) formed the surface of the drum base2A. The photoconductor drum2is 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)3is a charging member in contact with the photoconductor drum2. The charging roller3rotates following the rotation of the photoconductor drum2and homogeneously charges a surface of the photoconductor drum2(negatively charged in the exemplary embodiment) to a predetermined potential in a predetermined polarity in response to high voltage supplied from a power supply device10that will be described.

The ROS (Raster Optical Scanner: image writing unit)4directs an image modulated laser beam to a surface of the photoconductor drum2to be charged (scanning exposure). The potential at the exposed part is attenuated and an electrostatic latent image forms at the photosensitive thin film2B of the photoconductive drum2. When the photoconductor drum2rotates and the electrostatic latent image comes to a developing position A opposing the developer5, an amount of negatively charged toner is supplied from the developer5and a toner image is formed by reversal development.

The transfer roller6is positioned on the downstream side of the developer5when viewed in the rotation direction of the photoconductor drum2and provided in contact with the photoconductor drum2under pressure. The position of the nip portion between the transfer roller6and the photoconductor drum2is a transfer position B.

When the toner image formed on the surface of the photoconductor drum2reaches the transfer position B as the photoconductor drum2rotates, a paper sheet is supplied to the transfer position B in this timing, and predetermined voltage is applied to the transfer roller6at the same time, so that the toner image is transferred from the surface of the photoconductor drum2to 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 drum2after the transfer is scraped off with the cleaning blade7, and the photoconductor drum2has its surface cleaned and readied for the next image forming operation. The electrostatic latent image on the photoconductor drum2is eliminated by the static eliminating lamp8.

Now, a power supply system to the charging roller3will be described.

The power supply system includes a power supply device10including an AC power source unit11that supplies the charging roller3with high voltage, a DC power source unit16, and a current measuring unit20, and a control unit30that controls the operation of the power supply device10.

The power supply device10includes the AC power source unit11that generates AC voltage as shown in the block diagram inFIG. 2and the DC power source unit16that generates DC voltage. The configurations of the power source units11and16and the current measuring unit20will later be described. The current measuring unit20measures a measurement current Iref corresponding to a film thickness in a film thickness measuring mode.

The control unit30includes a controller31, an input/output controller32, and a memory33and these components each include a CPU (Central Processing Unit) or a RAM (Random Access Memory). The input/output controller32has its input and output sides connected with the AC power source unit11and the DC power source unit16of the power supply device10and its output side connected with a display41. The control unit30outputs a command signal Aon to the AC power source unit11and a command signal Don to the DC power source unit16.

The controller31carries out image forming processing, film thickness determination processing and the like that will be described according to a control program stored in the memory33. Among these kinds of processing, the turning on/off and variation of a constant current output in the AC power source unit11and the turning on/off and variation of a constant voltage output in the DC power source unit16are carried out to keep the photosensitive thin film2B of the photoconductor drum2homogeneously 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 device10will briefly be described with reference to the circuit diagram inFIG. 3.

In the AC power source unit11, an AC power drive circuit12operates in response to a command signal Aon received from the control unit30, a boosted AC component is produced through a transformer13, and one end of the secondary side of the transformer13is connected to the charging roller3. The other end of the secondary side of the transformer13is connected with an output from the DC power source unit16and a detection diode15through a DC regulating capacitor14. The detection diode15feeds back the AC component of current passed through a circuit including the charging roller3, the photoconductor drum2, a ground, and a detection circuit as a half-wave rectified monitor signal IAC to a control section in the power supply device10.

Note that the DC regulating capacitor14prevents the current of the AC component supplied from the AC current power source unit11from being passed to the ground side of the DC power source unit16. Therefore, a capacitor with a capacitance C0(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 C0of the DC regulating capacitor14in 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 C0is set in expectation of a small current flow to the ground side of the DC power source unit16through the DC regulating capacitor14.

Upon receiving a command signal Don from the control unit30, the DC power source unit16turns on a switching transistor17to apply DC specified voltage Vdd (for example 24 V) to the primary side of a transformer18, and boosted DC voltage (for example −750 V) is produced through the transformer18. One end of the secondary side of the transformer18is connected to the other end of the secondary side (low potential side) of the transformer13at the AC power source unit11and the DC component is superposed to the AC component. A voltage dividing resistor19and the current measuring unit20are connected in series to the output of the DC power source unit16, a monitor signal VDC produced from a signal picked up from the midway of the voltage dividing resistor19is fed back to the control section in the power supply device10.

The current measuring unit20is connected to the low potential side of the DC power source unit16and forms a differential circuit including OP amplifiers21and22activated in response to the specified voltage Vdd as basic components. The ground of the current measuring unit20is used in common as the ground of the photoconductor drum2, and therefore current passed through the photosensitive thin film2B of the photoconductor drum2through the charging roller3comes into the current measuring unit20. Then, current corresponding to the circuit constant (impedance) of the current measuring unit20is measured as a measurement current Iref. The measurement current Iref measured at the current measuring unit20is output to the control unit30.

The AC component of the voltage supplied to the charging roller3and the photoconductor drum2forms a closed circuit with the AC power source unit11through the ground of the photoconductor drum2, and the DC component forms a closed circuit with the DC power source unit16and the AC power source unit11through the ground of the photoconductor drum2and the current measuring unit20.

Now, with reference to the flowchart inFIG. 4, the film thickness determination processing according to the exemplary embodiment will be described.

The control unit30determines whether or not a film thickness measuring mode is attained (step S10). If the film thickness measuring mode is attained (YES in step S10), it is then determined whether or not an electrostatic charge amount measuring mode is attained (step S20). In the electrostatic charge amount measuring mode, the amount of charge possessed by the DC regulating capacitor14is measured.

If the electrostatic charge amount measuring mode is attained (YES in step S20), the control unit30outputs a command signal Don to the DC power source unit16that makes a command for applying voltage in a level insufficient to charge the photosensitive thin film2B (for example −400 V) (step S30). Upon receiving the command signal Don, the DC power source unit16supplies DC component current to the charging roller3. In this way, the DC component current is supplied to the charging roller3, but the charge is not supplied from the charging roller3to the photosensitive thin film2B, and the charge comes into the current measuring unit20.

The control unit30reads the measurement current Iref for the current coming into the current measuring unit20(step S40). The control unit30then calculates an electrostatic charge amount Q2by integrating the read measurement current Iref with the time for which the DC component current is supplied (step S50) and stores the electrostatic charge amount Q2in the memory33(step S60).

Then, the control unit30outputs a command signal Aon to the AC power source unit11(step S70) and then outputs a command signal Don to the DC power source unit16that makes a command for applying voltage about in a level sufficient to charge the photosensitive thin film2B (for example −750 V) (step S80). In this way, current produced by superposing the DC component to the AC component is sequentially supplied to the charging roller3and charges the photosensitive thin film2B, and then the current comes into the current measuring unit20. 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 unit30reads the measurement current Iref for the current coming into the current measuring unit20(step S90). The control unit30then integrates the read measurement current Iref with the time for which the current of superposed components is supplied to produce an integrated charge amount Q1(step S100).

The control unit30reads out the electrostatic charge amount Q2stored in the memory33in step S60(step S110), and the electrostatic charge amount Q2is subtracted from the integrated charge amount Q1obtained in step S100to produce a charge amount Q3(step S120).

The control unit30determines whether or not the charge amount Q3exceeds the threshold charge amount Q0(step S130) If Q3>Q0holds (YES in step S130) in the determination processing, the photosensitive thin film2B 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 display41(step S140).

The control unit30then stops outputting the command signal Don to the DC power source unit16(step S150) and stops outputting the command signal Aon to the AC power source unit11(step S160), and the film thickness determination processing ends.

The film thickness determination processing will be described further in detail with reference toFIGS. 5A,5B,6A and6B.

FIG. 5Ashows the characteristic of the charge amount Q of the photosensitive thin film2B according to reduction in the thickness of the photosensitive thin film2B.FIG. 5Bshows the characteristic of the resistance value R of the photosensitive thin film2B according to reduction in the thickness of the photosensitive thin film2B.FIGS. 6A and 6Bshow 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 drum2to make one rotation. Note that electricity is described in terms of current for the ease of description.

As can be seen fromFIG. 5A, at the photoconductor drum2, the charge amount Q increases as a function of increase in the reduction in the thickness of the photosensitive thin film2B (i.e., as the film thickness decreases), and the charge limit is reached when the wear limit for the photosensitive thin film2B is reached. The characteristic of the resistance value R shown inFIG. 5Bis 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 capacitor14prevents 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 capacitor14and 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 inFIGS. 6A and 6B.

In contrast, in the film thickness determination processing in step S30, the command signal Don that makes a command for applying voltage in a level insufficient to charge the photosensitive thin film2B (for example −400 V) is output to the DC power source unit16, and DC component current is supplied from the DC power source unit16to the charging roller3for the time for which the photoconductor drum2makes two rotations. When the measurement current Iref for the current coming into the current measuring unit20is read (step S40), 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 S50), an electrostatic charge amount Q2substantially equal to the overshoot by the DC regulating capacitor14can be obtained as indicated by the measurement current Iref1inFIG. 6.

Then in step S70, a command signal Aon is output to the AC power source unit11and AC component current is supplied from the AC power source unit11to the charging roller3for the time in which the photoconductor drum2makes 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 film2B (for example −750 V) is output to the DC power source unit16in step S80, and DC component current is supplied from the DC power source unit16to the charging roller3for the period in which the photoconductor drum makes three rotations. The measurement current Iref for the current coming into the current measurement portion20is read (step S90) 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 (S100). Then, as indicated by the measurement current Iref2inFIG. 6, an electrostatic charge amount Q1substantially equal to the sum of the charge amount of the photosensitive thin film2B and the overshoot by the DC regulating capacitor14can be obtained.

Therefore, the charge amount obtained by subtracting the electrostatic charge amount Q1from the electrostatic charge amount Q2can be interpreted as the charge amount of the photosensitive thin film2B 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 drum2through the charging roller3is measured, and then an electrostatic charge amount obtained based on the measurement result is removed by the processing in the control unit30. In this way, the electrostatic charge amount Q3removed of the overshoot is calculated. The calculated electrostatic charge amount Q3is represented by the solid line inFIG. 5A, and therefore an accurate value is indicated as the amount of thickness reduction corresponding to the charge amount of the photosensitive thin film2B 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 drum2though the drum has not yet reached the limit of its usefulness can be prevented, and the reliability of the image forming apparatus1can 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 C0of the DC regulating capacitor14changes for each film thickness determination processing, the charge amount Q3with a reduced error can be calculated by accurately calculating the electrostatic charge amount Q2.

Second Exemplary Embodiment

A second exemplary embodiment of the invention will be described.

FIG. 7is a schematic configurational diagram of the hardware of an image forming apparatus1according to the exemplary embodiment. As shown inFIG. 7, the image forming apparatus1is different from the first exemplary embodiment in that the device includes a retract driving part91that separates the charging roller3from the photoconductor drum2at such a distance that the photosensitive thin film2B of the photoconductor drum2is not charged.

FIG. 8is a flowchart for use in illustrating film thickness determination processing according to the exemplary embodiment. InFIG. 8, the processing in the electrostatic charge amount measuring mode from steps S30to S60shown inFIG. 4is replaced by processing from steps S21to S62. The processing in the series of steps will be described. When the electrostatic charge amount measuring mode is attained (YES in step S20), the control unit30separates the charging roller3from the photoconductor drum2(step S21). The control unit30then outputs a command signal Don that makes a command for applying voltage to the DC power source unit16(step S31). Upon receiving the command signal Don, the DC power source unit16supplies DC component current to the charging roller3through the other end of the secondary side of the transformer13in the AC power source unit11. However, since the charging roller3is separated from the photoconductor drum2by the retract driving part91, only current leaked to the DC regulating capacitor14is allowed to come into the current measuring unit20.

The control unit30reads the measurement current Iref for the current coming into the current measuring unit20(step S41). The control unit30calculates an electrostatic charge amount Q2by integrating the read measurement current Iref with the time for which the DC component current is supplied (step S51), stores the electrostatic charge mount Q2in the memory33(step S61), and then cancels the separated state of the charging roller3and the photoconductor drum2(step S62).

Thereafter, the same processing as that in and after step S70inFIG. 4is carried out.

Third Exemplary Embodiment

A third exemplary embodiment of the invention will be described.

FIG. 9is a circuit diagram of a power supply device10in an image forming apparatus1according to the exemplary embodiment. As shown inFIG. 9, the power supply device10includes a switch92on a wire from the other end of the secondary side of the transformer13in the AC power source unit11serving as a superposing position for the AC and DC components to the charging roller3. The switch92opens/closes in response to an output-load ON/OFF signal from the control unit30.

FIG. 10is a flowchart for use in illustrating film thickness determination processing according to the exemplary embodiment. InFIG. 10, the processing in the electrostatic charge amount measuring mode from steps S30to S60inFIG. 4is replaced by the processing from steps S24to S65. The processing in the series of steps will be described. When the electrostatic charge amount measuring mode is attained (YES in step S20), the control unit30supplies an output-load ON/OFF signal and thus opens the output end of the power supply, in other words, opens the switch92between the other end of the secondary side of the transformer13in the AC power source unit11and the charging roller3(step S24). The control unit then outputs a command signal Don that makes a command for applying voltage to the DC power source unit16(Step S34). Upon receiving the command signal Don, the DC power source unit16supplies DC component current to the other end of the secondary side of the transformer13in the AC power source unit11. However, the switch on the wire from the other end of the secondary side of the transformer13in the AC power source unit11to the charging roller3is opened, and therefore only the current leaked to the DC regulating capacitor14is allowed to come into the current measuring unit20.

The control unit30reads the measurement current Iref for the current coming into the current measuring unit20(step S44). The control unit30calculates the electrostatic charge amount Q2by integrating the read measurement current Iref with the time for which the DC component current is supplied (step S54), and the electrostatic charge amount Q2is stored in the storage33(step S64).

The control unit30connects the switch between the other end of the secondary side of the transformer13in the AC power source unit11and the charging roller3(step S65), and thereafter the same processing as that in and after step S70shown inFIG. 4is carried out.

Fourth Exemplary Embodiment

A fourth exemplary embodiment of the invention will be described.

FIG. 11is a circuit diagram of a power supply device10in an image forming apparatus1according to the exemplary embodiment. As shown inFIG. 11, the power supply device10includes a switch93between the other end of the secondary side of a transformer13of an AC power source unit11as a superposition point between AC and DC components and a DC regulating capacitor14. The switch93opens/closes in response to a capacitance ON/OFF signal from the control unit30.

FIG. 12is a flowchart for use in illustrating film thickness determination processing according to the exemplary embodiment.

The control unit30determines whether or not a film thickness measuring mode is attained (step S10). If a film thickness measuring mode is attained (YES in step S10), a capacitance ON/OFF signal is supplied. In this way, the switch93between the output end of the power supply, in other words, the other end of the secondary side of the transformer13in the AC power source unit11and the DC regulating capacitor14is opened (step S26), 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 film2B (for example −1500 V) to the DC power source unit16(step S36). Upon receiving the command signal Don, the DC power source unit16supplies DC component current to the other end of the secondary side of the transformer13in the AC power source unit11. Then, DC component current sufficient to charge the photosensitive thin film2B is sequentially supplied to the charging roller3, charges the photosensitive thin film2B, and then is allowed to come into the current measuring unit20.

The control unit30reads the measurement current Iref for the current coming into the current measuring unit20(step S46). The control unit30then calculates an integrated charge amount Q1by integrating the read measurement current Iref with the time for which the current for superposed components is supplied (step S106), and the integrated charge amount Q1is determined as a charge amount Q3(step S126).

The control unit30then determines whether or not the charge amount Q3obtained in step S126exceeds the threshold charge amount Q0(step S136). If Q3>Q1holds (YES in step S136) in the determination processing, the reduction in the photosensitive thin film2B has reached the limit value (limit film thickness), and therefore a command for requesting “replacement of photoconductor drum2” is indicated at the display41(step S146).

The control unit30also stops outputting the command signal Don to the DC power source unit16(step S156), supplies a capacitance ON/OFF signal to connect the switch93between the other end of the secondary side of the transformer13in the AC power source unit11and the DC regulating capacitor14(step S166) and then ends the film thickness determination processing.