JP2002072634A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming apparatus with which the life of an image carrier is prolonged and defective electrification is prevented. SOLUTION: An AC voltage for electrifying the surface of the image carrier 1 is applied to an electrifying member 12 by using a high voltage power supply 18, and also, when the AC voltage is applied, the current flowing into the electrifying member 12 is detected by a current detecting means 64. Besides, a repeat waveform having the same frequency as that of the current detected by the current detecting means 64 or an integer multiple of the frequency is generated by a repeat waveform generating means 63, and also the difference between the current detected by the current detecting means 64 and the repeat waveform from the repeat waveform generating means 63 or the sum of them is calculated by a calculating means 65. Moreover, the calculation result by the calculating means 65 is compared with a reference value, and also the output of the high voltage power supply 18 is controlled by an output control means 66 in accordance with the comparison result and then the discharge current value between the image carrier 1 and the electrifying member 12 is kept constant.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to an image forming apparatus, and more particularly to control of a voltage applied to a charging member for charging a surface of an image carrier.

[0002]

2. Description of the Related Art Conventionally, in an image forming apparatus such as a copying machine or a printer using an electrophotographic system or an electrostatic recording system, a charging means for an image carrier such as an electrophotographic photosensitive member and an electrostatic recording dielectric is used. Corona chargers have been used as
In recent years, it has advantages such as low ozone and low electric power. Therefore, as a means for charging the image bearing member, a contact charging device, that is, a charging member in which a voltage is applied to the image bearing member to be charged is brought into contact with the charged member. 2. Description of the Related Art Apparatus of a system for charging a body has been put to practical use.

[0003] In such a contact charging device, a roller charging type device using a conductive roller (hereinafter, referred to as a charging roller) as a charging member is preferably used from the viewpoint of stabilizing charging. Here, in this roller charging type contact charging device, the charging roller is brought into pressure contact with the member to be charged, and a voltage is applied thereto to charge the member to be charged.

FIG. 13 is a diagram schematically showing such a roller charging type contact charging device. Reference numeral 11 denotes an image bearing member as a member to be charged, for example, a rotating drum type electrophotographic photosensitive member (hereinafter, referred to as a photosensitive member). The photosensitive drum 11 is driven to rotate clockwise as indicated by an arrow. Reference numeral 12 denotes a charging roller as a charging member.
Is pressed against the surface of the photosensitive drum with a predetermined pressing force, and is rotated following the rotation of the photosensitive drum 11.

Reference numeral 18 denotes a voltage application power supply for applying a voltage to the charging roller 12. The voltage application power supply 18 applies a predetermined voltage to the charging roller 12 so that the photosensitive drum 11 Surface potential. In addition to the charging roller 12, necessary image forming process devices (not shown) are arranged around the photosensitive drum 11, thereby constituting an image forming mechanism.

In the contact charging device having such a structure, contact charging is performed by discharging from the charging roller 12 to the photosensitive drum 11, and a voltage higher than a certain threshold voltage is applied to the charging roller 12. As a result, charging of the photosensitive drum 11 is started.

For example, when a charging process is performed by pressing the charging roller 12 against a photosensitive drum (charged member) 11 having an OPC photosensitive layer having a thickness of 25 μm, the charging roller 12 When a voltage of about 600 V is applied from the voltage applying power supply 18, the surface potential of the photoconductor drum 11 starts to increase, and thereafter, the surface potential of the photoconductor drum increases linearly with a constant gradient with respect to the applied voltage.

Thereafter, this threshold voltage, that is, the DC voltage (DC voltage) is applied to the charging roller (charging member), and the threshold voltage is applied to the charging roller 11 when the charging of the photosensitive drum (charging member) 11 is started. The voltage value is defined as a charging (discharging) start voltage value Vth of the photosensitive drum (charged body) 11.

The contact charging method includes a “DC charging method” in which only a DC voltage is applied to a charging member to charge a member to be charged, and a voltage having an AC voltage component and a DC voltage component (oscillation voltage: along with time). "AC charging method" in which a charged object is charged by applying a voltage whose voltage value changes periodically)
There is.

Here, in the DC charging method, in order to charge the member to be charged to a desired surface potential VD, the surface voltage VD and the charging start voltage value Vth of the member to be charged are added to VD + V.
The DC voltage of th is applied to the charging member. However, this charging method has insufficient charging uniformity and has no potential convergence with respect to a potential overcharged above the surface potential VD. Therefore, it is necessary to perform the pre-exposure of the photosensitive drum as the member to be charged.

On the other hand, the AC charging system is disclosed in
As disclosed in JP-A-149669 or the like, an oscillating voltage in which an AC voltage having a peak-to-peak voltage Vpp of 2 × Vth or more is superimposed on an offset DC voltage corresponding to a desired surface potential VD of a member to be charged is applied to a charging member. The applied member is charged by applying the voltage.

Here, this type is intended for the effect of leveling the potential by the AC voltage component, and by having such a configuration, as shown in FIG. 14, a photosensitive drum as a member to be charged is used. The surface potential converges to the middle of the peak voltage according to the separation between the charging member and the photosensitive drum, and is excellent in charging uniformity.

Further, in the image forming apparatus, the charging device charges the image carrier to a constant potential in order to form an electrostatic latent image, and after the image has been formed, erases the potential history on the image carrier. In the AC charging method, the DC voltage corresponding to a desired dark portion potential of the image carrier is used as the offset voltage when the image carrier is charged, and 0 V is used as the offset voltage when the image carrier is removed. Since the carrier surface potential can be uniformly converged to VD or 0 V, it is advantageous as compared with other charging devices such as a DC charging system and a corona charger.

As a method of controlling the AC voltage applied to the charging roller 12, a constant voltage control and a constant current control are generally used. Here, the constant voltage control is to keep the AC voltage applied to the charging roller 12 at a constant value, which is simple and has a cost merit.
AC current changes due to environmental fluctuation of impedance of
Leakage, poor charging, etc. may occur.

On the other hand, in the constant current control, the AC current value applied to the charging roller 12 is maintained at a constant value. By controlling the AC current value at a constant value in this manner, the impedance of the charging roller 12 is controlled. Environmental fluctuations can be automatically corrected. In addition, as a constant current control method,
Generally, there is a control method for setting a peak value, an average value, an effective value, and the like of an alternating current to a constant value.

[0016]

However, in a conventional image forming apparatus which performs such charging control of the AC charging system, the charging control alternates between positive and negative voltages as applied voltages and repeats discharge / reverse discharge. Due to such discharge, the surface of the photosensitive drum, which is a member to be charged, is greatly deteriorated, and the deteriorated surface of the photosensitive drum is scraped off by friction with a contact member such as a cleaning blade. Body drum scraping occurs.

When such abrasion of the photosensitive drum occurs, the photosensitive layer of the photosensitive drum gradually becomes thinner. Or the charge retention ability of the surface is reduced, which results in poor charging. This allows
The life of a process cartridge having an image forming apparatus and a photosensitive drum is defined by the number of prints used until the photosensitive drum wears to a limit film thickness.

On the other hand, in recent years, due to environmental problems and an increase in the burden on the printer due to networking of computers, the demand for high durability of the process cartridge is increasing, and accordingly, the life of the photosensitive drum is required to be prolonged. ing. Here, as a method of extending the life of the photosensitive drum, a method of increasing the initial film thickness of the photosensitive drum or a method of reducing the deterioration of the photosensitive drum by reducing the discharge amount of the charging roller can be considered.

However, when the thickness of the photosensitive drum is simply increased, the holding power of the surface charge of the photosensitive drum is reduced, and the electrostatic image may be blurred. Also, it has been found that when the amount of discharge is excessively reduced, the discharge tends to be unstable, causing uneven charging and image defects such as poor charging.

Therefore, in order to achieve both high image quality and long life of the image forming apparatus and the process cartridge,
Use a photosensitive drum with a thickness of the photosensitive layer that can maintain the sharpness of the latent image, and a charging roller with an appropriate discharge amount that prevents charging failure due to insufficient discharge and reduces deterioration of the photosensitive drum. It is necessary to use the contact charging member.

As for the method of controlling the AC voltage for a contact charging member such as a charging roller, the above-described constant current control is generally used. It controls the amount of current flowing through the drum to be constant, and does not directly control the discharge current that affects the life of the photosensitive drum.

Therefore, for example, when the AC current value is set to a discharge current amount that does not cause charging failure in the initial state, the initial charged state cannot be maintained throughout the durability. This is because charging characteristics change due to toner contamination of the charging roller and a decrease in the thickness of the photosensitive drum. For this reason, in the constant current control in which the charging state is initially set to be appropriate, the discharge current amount increases as compared with the initial state, and the amount of scraping of the photosensitive drum increases in accordance with the durability, and the life of the photosensitive drum increases. Is shortened.

Accordingly, the present invention has been made in view of such circumstances, and an image forming apparatus capable of prolonging the life of a photosensitive drum (image carrier) and preventing defective charging is provided. It is intended to provide.

[0024]

According to the present invention, there is provided an image carrier,
A high-voltage power supply for applying, to the charging member, an AC voltage for charging the surface of the image carrier, the charging device being provided with a charging member for charging the surface of the image carrier by pressing against the image carrier. Current detecting means for detecting a current flowing through the charging member when an AC voltage is applied from the high-voltage power supply; and generating a repetitive waveform having the same frequency or an integral multiple of the current detected by the current detecting means. A repetitive waveform generating means, a calculating means for calculating a difference or a sum between a current detected by the current detecting means and the repetitive waveform from the repetitive waveform generating means, and a calculation result and a reference value by the calculating means. And an output control means for controlling the output of the high-voltage power supply in accordance with the comparison result.

Further, in the present invention, when the phase difference between the current detected by the current detecting means and the repetitive waveform is 0, the calculating means calculates the difference between the detected current and the repetitive waveform, Is 180 °, the sum of the detected current and the repetitive waveform is calculated.

Further, in the present invention, the repetitive waveform and the peak value of the detection current are the same.

The present invention is also characterized in that the alternating voltage applied to the charging member is a sine wave, and the repetitive waveform is a sine wave or a half sine wave.

According to the present invention, the repetitive waveform is generated from a clock pulse.

Further, the present invention is characterized in that the repetitive waveform is generated based on an AC voltage applied to the charging member.

Further, the present invention is characterized in that the output control means controls the output of the high voltage power supply so as to keep the discharge current value constant based on a peak value of a calculation result of the calculation means. is there.

Further, the present invention is characterized in that the output control means controls an output of the high voltage power supply so as to keep the discharge current value constant based on an integrated value of a calculation result of the calculation means. is there.

Further, the present invention is characterized in that the output control means controls the output of the high-voltage power supply so as to keep the discharge current value constant based on the average value of the calculation result of the calculation means. is there.

Further, the present invention is characterized in that the output control means controls the output of the high voltage power supply so as to keep the discharge current value constant based on the effective value of the calculation result of the calculation means. is there.

Further, as in the present invention, an AC voltage for charging the surface of the image carrier by a high voltage power supply is applied to a charging member which is in pressure contact with the image carrier, and when the AC voltage is applied, a current detecting means is provided. To detect the current flowing through the charging member. The repetitive waveform generating means generates a repetitive waveform having the same frequency as the current detected by the current detecting means or an integer multiple of the frequency, and the calculating means calculates the detected current from the current detecting means and the repetitive waveform from the repetitive waveform generating means. The difference or the sum is calculated. The output control means compares the calculation result of the calculation means with the reference value, and controls the output of the high-voltage power supply according to the comparison result, thereby irrespective of the contamination of the charging roller and the change in the thickness of the photosensitive drum. The discharge current value between the image carrier and the charging member can be kept substantially constant, and the amount of scraping of the photosensitive drum can be reduced.

[0035]

Embodiments of the present invention will be described below in detail with reference to the drawings.

FIG. 1 is a view showing a schematic configuration of a laser printer which is an example of an image forming apparatus according to a first embodiment of the present invention. In FIG. 1, reference numeral 1 denotes an image carrier which is a member to be charged. The photosensitive drum 1 has a conductive layer 1b formed on a conductive support 1a.

Further, around the photosensitive drum 1,
A charging roller 12 as a charging member, a developing device 14, a transfer roller 15, a cleaner 16, and the like are arranged along the rotation direction (the direction of arrow A). A scanner unit 13 is arranged above the photosensitive drum 1.

In the figure, reference numeral 18 denotes the charging roller 1
2, a charging power supply which is a high-voltage power supply for applying an AC voltage obtained by superimposing a DC voltage on a developing roller 14;
And a transfer power supply 20 for supplying a transfer bias to the transfer roller 15. Also,
Reference numeral 24 denotes a static elimination needle, reference numerals 21 and 22 denote transport guides, and reference numeral 17 denotes a fixing device.

Next, an image forming operation in the laser printer configured as described above will be described.

When the image forming operation is started, first, the photosensitive drum 1 rotated and driven in the direction of arrow A by a driving unit (not shown) is uniformly charged to a predetermined polarity and a predetermined potential by the charging roller 12. You. The photosensitive drum 1 whose surface has been charged in this way is irradiated with a laser beam emitted from the scanner unit 13 in accordance with image information such as characters and figures sent from an external information device such as a personal computer. Exposure is performed by L, and as a result, the charge in the exposed portion is removed to form an electrostatic latent image.

Next, the electrostatic latent image is developed with toner by the developing device 14 to form a toner image on the photosensitive drum. In the developing device 14, the developing power supply 19
Bias and DC supplied to the developing roller 14a from the
A potential difference is formed between the developing roller 14a and the electrostatic latent image on the photosensitive drum 1 by the bias superimposed voltage, and the toner is transferred onto the electrostatic latent image by the potential difference.
A toner image is formed on the photosensitive drum.

On the other hand, in parallel with the toner image forming operation, the recording paper S stored in a paper cassette (not shown)
Is transferred to the nip between the photosensitive drum 1 and the transfer roller 15 at a predetermined timing, and the toner image on the photosensitive drum is transferred to a predetermined position on the recording paper by the transfer bias applied to the transfer roller 15. You.

Next, the recording paper S carrying the unfixed toner image on the surface by the transfer is transferred to the grounding needle 2 which is grounded.
4 and separated from the photosensitive drum 1 by the transport guide 22.
It moves up and is introduced into the fixing device 17. Then, the transfer material S is pressurized and heated by the fixing device 17, whereby the unfixed toner image becomes a permanent image, and thereafter, the recording paper S on which the toner image is permanently fixed is discharged outside the apparatus.

The photosensitive drum 1 after the transfer of the toner image
The toner remaining on the surface without being transferred to the recording paper S is removed by the cleaner 16 to prepare for the next image formation. Image formation can be performed one after another by repeating the above operation.

FIG. 2 is a diagram showing the waveforms of the AC bias voltage and current applied to the charging roller 12 by the charging power supply 18, and the AC bias voltage (Vo) shown in FIG. When the voltage is applied to the charging roller 12, a resistive load between the charging roller 12 and the photosensitive drum 1 is applied to a resistive load current (Izr) having the same phase as the AC bias voltage (Vo) as shown in FIG. Flows and charging roller 1
A capacitive load current (Izc) whose phase is advanced by 90 ° from the AC bias voltage (Vo) flows through the capacitive load between the photosensitive drum 1 and the photosensitive drum 1. Further, the AC bias voltage (V
At the peak of the voltage amplitude of o), a pulsed discharge current (Is) flows between the charging roller 12 and the photosensitive drum 1.

Then, these resistance load currents (Izr),
When the capacitance load current (Izc) and the discharge current (Is) are totaled, a current Io having a waveform as shown in FIG. FIG. 3D shows a detected current waveform Im when an AC current drawn from the charging roller to the high-voltage power supply is detected.

FIG. 3 is a graph showing the relationship between the AC voltage amplitude and the output current amount. As is apparent from FIG. 3, the voltage amplitude and the output current are almost proportional to each other when the voltage amplitude is equal to or less than the predetermined voltage amplitude. This is because the resistive load current (Izr) and the capacitive load current (I
zc) is proportional to the voltage amplitude, and since the voltage amplitude is small, the discharge phenomenon does not occur, and the discharge current (Is) does not flow.

On the other hand, when the output voltage amplitude is increased, a discharge phenomenon starts at a predetermined voltage amplitude (Vs), whereby the total output current (Io) also deviates from the proportional relation and flows more by the discharge current (Is). Become like

FIG. 4 shows the relationship between the peak current amount (Ip in FIG. 2) of the total output current applied to the charging roller 12 and the discharge current amount. As shown in FIG. Comparing the characteristics at the beginning of use and the characteristics after use for a certain period, the charging roller 12 after the use for a certain period
In this case, the discharge start current value becomes small due to a change in impedance due to toner contamination, a change in the film thickness of the photosensitive drum 1, and the like. For this reason, when the peak current amount (Ip) is controlled to be constant by the constant current control as described above, the discharge current amount increases from the initial Is0 to Is1 as the integrated output print number increases, as shown in FIG. Would.

Here, the amount of abrasion on the surface of the photosensitive drum, which causes deterioration of the photosensitive drum 1, increases in proportion to the amount of discharge current. Photoconductor drum 1 as it increases
The speed at which the surface of the photosensitive drum 1 is scraped is accelerated and the life of the photosensitive drum 1 is shortened. Therefore, in the present embodiment, the discharge current component is directly controlled in order to control the scraping of the photosensitive drum 1.

FIG. 6 is a diagram showing a control circuit for controlling such a discharge current component. In FIG.
The high voltage transformer drive circuit generates a sine wave based on the clock pulse input by U72. The sine wave generated by the high voltage transformer drive circuit 61 is boosted by the high voltage transformer 60.

Reference numeral 62 denotes a DC high-voltage generating circuit. The DC high voltage generated by the DC high-voltage generating circuit 42 and the AC high voltage boosted by the high-voltage transformer 40 are applied to the charging roller 12. .

On the other hand, 64 is a high-voltage transformer drive circuit 6
1 and a current detecting circuit which is a current detecting means for detecting a current flowing through the charging roller 12 by a voltage applied from the charging power supply 18 having the DC high voltage generating circuit 62, and 63 is a repetitive waveform generating means for generating a repetitive waveform. The repetitive waveform generating circuit 65 includes the current shown in FIG. 7A detected by the current detecting circuit 64 and the repetitive waveform generating circuit 6.
7 is an arithmetic circuit as arithmetic means for calculating the repetitive waveform signal shown in FIG.

In the present embodiment, a difference circuit is used as the arithmetic circuit 65, and the repetitive waveform generation circuit 63 synchronizes with the phase detection signal of the current detected by the current detection circuit 64 and generates a peak. A repetitive waveform having the same value is generated.

A comparison circuit 66 compares the operation result of the operation circuit 65 shown in FIG. 7C with a predetermined set current from the set current circuit 67, which is a reference value. As described above, when the phase difference between the current detected by the current detection circuit 64 and the repetitive waveform is 0, the discharge current amount, which is the difference between the phase difference and the predetermined set current from the set current circuit 67, is compared. A predetermined value, for example, the initial setting I
The control signal is fed back to the high voltage transformer drive circuit 61 so as to be s0.

The control signal from the comparison circuit 66 as the output control means is transmitted to the high-voltage transformer drive circuit 6.
By feeding back to 1, the AC high voltage applied to the charging roller 12 can be controlled, whereby the discharge current amount can be kept constant.

FIG. 8 is a diagram showing the relationship between the cumulative number of output prints and the increase / decrease of the discharge current amount and the relationship between the amount of scraping of the photosensitive drum 1 in this embodiment. In contrast, in the case of the constant current control in which the peak current of the alternating current shown in FIG. 5 is controlled to a constant value as described above, the discharge current amount increases, whereas in the configuration of the present embodiment, the discharge current is reduced to the initial value. It can be kept at the setting Is0.

As described above, the discharge current component in the output current is cut out as the calculation result of the output current and the repetitive waveform, and is directly controlled, so that the shaving amount of the photosensitive drum 1 proportional to the discharge current amount is kept constant. It is possible to keep.
Further, it is possible to suppress an increase in the amount of discharge current due to contamination of the charging roller 12, a change in the thickness of the photosensitive drum 1, and the like.

As a result, not only can the speed at which the photosensitive drum 1 is abraded be reduced even if the total number of output prints increases, but also the photosensitive drum 1 and the charging roller 12 can be charged according to the environment and the like. Even if the impedance changes and the ratio of the resistance load current, the capacitance load current and the discharge current changes, only the discharge current contributing to charging can be controlled to be constant, thereby preventing charging failure due to environment and durability. And stability of charging can be obtained.

Further, since the discharge current can be kept constant, the amount of scraping of the photosensitive drum 1 can be accurately predicted, and the life of the photosensitive drum 1 can be detected with higher accuracy. In the present embodiment, the cut-out discharge current amount is controlled to a constant value, but it goes without saying that the discharge current amount can be controlled by changing the set current value.

Next, an image forming apparatus according to a second embodiment of the present invention will be described.

FIG. 9 is a diagram showing a control circuit for controlling a discharge current component provided in the image forming apparatus according to the present embodiment. 6, the same reference numerals as those in FIG. 6 indicate the same or corresponding parts. Further, in the image forming apparatus according to the present embodiment, 1000
A sine wave AC voltage of Hz and a DC voltage of -650 V are superimposed.

In the figure, reference numeral 63A denotes a repetitive waveform generating circuit. The repetitive waveform generating circuit 63A receives a clock pulse from the CPU 72 in the printer control unit 70 and receives a detected current signal from the current detecting circuit 64. The phase of the output current output to the charging roller 12 is detected, and a repetitive waveform is generated by the phase detection signal and the clock pulse.

The current detected by the current detection circuit 64 is
As shown in FIG. 10A, this is a half-wave of the output current, and the repetitive waveform generating circuit 63A generates a phase detection signal shown in FIG. Further, as shown in (c), a repetitive waveform synchronized with the phase detection signal is generated. Then, the repetitive waveform is input to the arithmetic circuit 65.

Here, in the present embodiment, the arithmetic circuit 65 calculates the difference between the detection current of the current detection circuit 64 and the repetition waveform from the repetition waveform generation circuit 63A. The discharge current can be obtained as shown in (d) of FIG.

Then, the peak value of the discharge current thus obtained is obtained by the peak detection circuit 92, and this output is compared with the set current value from the set current circuit 67 by the comparison circuit 66. By feeding back the control signal to the high-voltage transformer drive circuit 61 so that the value becomes, for example, Is0, which is the initial setting, the discharge current can be held at a constant value.

As described above, by discharging the discharge current from the output current and maintaining the discharge current at a constant value, the endurance and the discharge current caused by the impedance change of the charging roller 12 and the photosensitive drum 1 due to environmental change are changed. It is possible to suppress the increase, and to suppress the speed at which the photosensitive drum abrasion proceeds regardless of the use condition of the cartridge.

In the present embodiment, the cut-out discharge current amount is controlled to a constant value. However, the discharge current amount can be controlled by changing the set current value. The current detected by the current detection circuit 64 is a half wave of the output current, but is not particularly limited. Further, the current detection circuit 6
It is needless to say that the same effect can be obtained by synchronizing the detection current at 4 with the clock pulse from the CPU 72 and calculating the waveform repeatedly.

Next, an image forming apparatus according to a third embodiment of the present invention will be described.

FIG. 11 is a diagram showing a control circuit for controlling a discharge current component provided in the image forming apparatus according to the present embodiment. 6, the same reference numerals as those in FIG. 6 indicate the same or corresponding parts. In the image forming apparatus according to the present embodiment, a sine wave AC voltage of 1000 Hz and a DC voltage of −650 V are superimposed on charging roller 12.

In the figure, reference numeral 63B denotes a repetitive waveform generating circuit. The repetitive waveform generating 63B is boosted by the high-voltage transformer 40, and is supplied to the current detecting circuit 64 based on the high AC voltage applied to the charging roller 12. A repetitive waveform having the same peak value and phase as the detected current is formed.

The current detected by the current detection circuit 64 is
As shown in FIG. 12A, the output current is a half-wave of the output current, and the phase detection signal is based on the AC high voltage applied to the charging roller 12. Therefore, in the repetitive waveform generation 63B,
As shown in (b), the waveform is generated in response to the rise of the detection current, whereby a repetitive waveform synchronized with the phase detection signal is generated as shown in (c) and input to the arithmetic circuit 65. It has become.

Here, in this embodiment, the output value of the arithmetic circuit 65 is the difference between the detection current of the current detection circuit 64 and the repetitive waveform, and the output value from the arithmetic circuit 65 is integrated by the integration circuit 100. After that, the comparison signal is compared with the set current value from the set current circuit 67 in the comparison circuit 66, and then the control signal is fed back to the high voltage transformer drive circuit 61 so that the discharge current amount becomes a predetermined value, for example, Is0 which is the initial setting. Like that. By feeding back the control signal to the high-voltage transformer drive circuit 61, the discharge current can be kept at a constant value.

As described above, by cutting out the discharge current from the output current and maintaining the discharge current at a constant value, the increase in the discharge current due to the change in the impedance of the charging roller 12 due to the durability and environmental fluctuation is suppressed. Regardless of the use condition of the cartridge, the scraping speed of the photosensitive drum 1 can be suppressed.

In this embodiment, after the output value of the arithmetic circuit 65 is integrated, the output value is compared with the set current value from the set current circuit 67 in the comparison circuit 66. However, the average value, peak value, effective value Needless to say, the same effect can be obtained by controlling based on the above.

In the above description, when the phase difference between the current detected by the current detecting means and the repetitive waveform is 0, the case where the difference is taken to control the charging power supply 18 has been described. The present invention is not limited to this, and when the phase difference is 180 °, the sum of them is calculated and the charging power supply 18 is calculated.
The same effect can be obtained even if the output is controlled.

[0077]

As described above, according to the present invention, when an AC voltage is applied to the charging member, the discharge current value between the image carrier and the charging member is kept constant. In addition, it is possible to suppress the scraping speed of the image carrier and prevent charging failure due to an excessive decrease in the discharge current, thereby making it possible to extend the life of the image carrier and prevent charging failure. be able to.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating a schematic configuration of a laser printer as an example of an image forming apparatus according to a first embodiment of the present invention.

FIG. 2 is a diagram showing waveforms of an AC bias voltage and a current applied to a charging roller of the laser printer.

FIG. 3 is a diagram illustrating a relationship between an output current amount flowing through a charging roller and an AC bias amplitude when the AC bias is applied.

FIG. 4 is a diagram showing a relationship between a discharge current amount and a peak current when the AC bias is applied.

FIG. 5 shows a discharge current amount in a conventional laser printer;
FIG. 5 is a diagram illustrating a relationship between an integrated image formation number and a shaving amount per unit number of photosensitive drums.

FIG. 6 is a block diagram of a control circuit for controlling a discharge current component of the laser printer.

FIG. 7 is a diagram showing a detection current, a repetitive waveform, and an output of an arithmetic circuit in the control circuit.

FIG. 8 is a diagram showing a relationship among a discharge current amount, an integrated image forming number, and a shaving amount per unit number of photosensitive drums in the laser printer.

FIG. 9 is a block diagram of a control circuit for controlling a discharge current component provided in an image forming apparatus according to a second embodiment of the present invention.

FIG. 10 is a diagram showing a detection current, a phase detection signal, a repetitive waveform, and an output of an arithmetic circuit in the control circuit.

FIG. 11 is a block diagram of a control circuit for controlling a discharge current component provided in an image forming apparatus according to a third embodiment of the present invention.

FIG. 12 is a diagram showing a detection current, a phase detection signal, a repetitive waveform, and an output of an arithmetic circuit in the control circuit.

FIG. 13 is a diagram schematically illustrating a roller charging type contact charging device used in a conventional image forming apparatus.

FIG. 14 is a diagram showing a relationship between an applied voltage and a photosensitive drum surface potential of a conventional roller charging type contact charging device.

[Explanation of symbols]

 Reference Signs List 1 photosensitive drum 12 charging roller 21 high voltage power supply 61 high voltage transformer drive circuit 62 DC high voltage generation circuit 63 repetitive waveform generation circuit 64 current detection circuit 65 arithmetic circuit 66 comparison circuit 67 setting current circuit 72 CPU

Claims (10)

[Claims] 1. An image forming apparatus comprising: an image carrier; and a charging member that presses against the image carrier and charges the surface of the image carrier. A high-voltage power supply that applies a voltage to the charging member; a current detection unit that detects a current flowing through the charging member when an AC voltage is applied from the high-voltage power supply; and a same frequency as the current detected by the current detection unit. Or a repetitive waveform generating means for generating a repetitive waveform having an integral multiple of frequency; a calculating means for calculating a difference or a sum between a current detected by the current detecting means and a repetitive waveform from the repetitive waveform generating means; Output control means for comparing an operation result of the operation means with a reference value, and controlling an output of the high-voltage power supply in accordance with the comparison result. Equipment. 2. The method according to claim 1, wherein the calculating unit calculates a difference between the detected current and the repetitive waveform when a phase difference between the current detected by the current detecting unit and the repetitive waveform is 0;
2. The image forming apparatus according to claim 1, wherein when the phase difference is 180 [deg.], A sum of the detected current and a repetitive waveform is calculated. 3. The image forming apparatus according to claim 1, wherein the repetitive waveform has the same peak value as the detected current. 4. The method according to claim 1, wherein the alternating voltage applied to the charging member is a sine wave, and the repetitive waveform is a sine wave or a half wave of the sine wave. The image forming apparatus as described in the above. 5. The image forming apparatus according to claim 1, wherein the repetitive waveform is generated from a clock pulse. 6. The image forming apparatus according to claim 1, wherein the repetitive waveform is generated based on an AC voltage applied to the charging member. 7. The output control means controls the output of the high voltage power supply so as to keep the discharge current value constant based on a peak value of a calculation result of the calculation means. The image forming apparatus according to any one of the above. 8. The output control means controls the output of the high-voltage power supply so as to maintain the discharge current value constant based on an integrated value of a calculation result of the calculation means. The image forming apparatus according to any one of the above. 9. The output control means controls the output of the high voltage power supply so as to keep the discharge current value constant based on an average value of the calculation results of the calculation means. The image forming apparatus according to any one of the above. 10. The output control means controls an output of the high-voltage power supply based on an effective value of a calculation result of the calculation means so as to keep the discharge current value constant. The image forming apparatus according to any one of the above.