JPH05196671A - Calibration method for surface potential detector in image forming apparatus - Google Patents

Abstract

(57) [Abstract] [Purpose] Without increasing costs or complicating the device,
Further, even if the residual potential is generated on the photoconductor, the surface potential of the photoconductor is accurately detected by calibrating the surface potential detector so that stable image quality can be obtained. [Construction] After leaving the photoconductor belt 5 for a time sufficient to recover its fatigue, the relay switch 30 is closed normally.
Is opened and the normally open contact a is closed, and a reference voltage is applied from the developing bias power source 32 to the Al substrate 5a of the photoconductor belt 5, and the potential of the surface of the photoconductor belt 5 (organic photoconductor 5b) is used as a reference. The surface potential sensor (surface potential detector) 21 is calibrated using this value.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for calibrating a surface potential detector in an image forming apparatus such as an optical printer such as a laser printer, a copying machine or a facsimile machine.

[0002]

2. Description of the Related Art In an electrophotographic image forming apparatus such as a copying machine, a drum-shaped or belt-shaped photosensitive member is uniformly charged to a predetermined potential by a charger, and its charged surface is electrostatically charged by light from an exposure device. After forming the electrostatic latent image, toner is applied by the developing unit to make it visible, and the toner image is transferred to the paper from the paper feeding unit.

In such an image forming apparatus, for example, JP-A-56-95255 and JP-A-63-8374 are used.
As disclosed in Japanese Patent No. 3 or the like, there is one in which a surface potential detector (surface potential sensor) is arranged at a position facing the photoconductor, and the surface potential detector measures the surface potential of the photoconductor, and the measurement is performed. According to the value, the voltage applied to the charger, the developing roller, etc. is adjusted and set to an optimum value to stabilize the image quality.

[0004]

However, a calibration volume is provided in the generally used surface potential detector as described above, and the surface potential detector can be used as the main body of the image forming apparatus or the calibration volume. It is necessary to calibrate the surface potential detector with high accuracy according to the frequency of use such as the degree of deterioration of the surface potential detector, which is a very troublesome work. Moreover, since the calibration also varies depending on the service person, the surface potential detector is inferior in accuracy as a result.

Therefore, it is conceivable to use a highly accurate and highly reliable one, but if this is done, the cost will increase and the problem that it cannot be mounted on a low-priced machine arises.

Further, as described in JP-A-56-95255, a reference plate (calibration electrode plate) is installed so as to face the measurement surface of the surface potential detector, and the reference plate is used. There is also a system in which the surface potential detector is automatically calibrated, but since an electrode plate for special calibration is required as a reference plate, the cost will increase accordingly.

Moreover, even if the surface potential detector can accurately measure the surface of the photoconductor, a residual potential unnecessary for image formation is generated on the photoconductor due to the passage of time or the environment. Since it has to be canceled, the device and control become complicated.

Further, if a relatively inexpensive surface potential detector having a distance dependency is used, the cost will be reduced, but the distance between the surface potential detector and the calibration electrode plate will be the distance between the surface potential detector and the photosensitive electrode. It is necessary to make the distance equal to or similar to the distance to the body, which complicates the image forming apparatus as a whole.

The present invention has been made in view of the above points and does not increase the cost and complicate the apparatus, and even if the residual potential is generated on the photoconductor, the surface potential detector is simply calibrated. An object of the present invention is to accurately detect the surface potential of the photoconductor so that stable image quality can be obtained.

[0010]

SUMMARY OF THE INVENTION To achieve the above object, the present invention provides an image forming apparatus equipped with a surface potential detector for measuring the surface potential of a photoconductor, in which the fatigue of the photoconductor is sufficiently recovered. There is provided a method for calibrating a surface potential detector, which comprises applying a reference voltage to a substrate of a photoconductor after leaving it for a period of time, and calibrating the surface potential detector using the potential of the surface of the photoconductor as a reference value.

It is desirable to keep the temperature of the photoconductor at the time of being left above the temperature at the time of image formation or to keep the photoconductor in a stopped state at the time of calibrating the surface potential detector. Further, the substrate of the photoconductor is usually connected to ground, and at the time of calibration of the surface potential detector, it may be connected to a developing bias power source.
Furthermore, the fatigue recovery timing of the photoconductor may be determined by the continuous stop time of the photoconductor or by the temperature of the fixing device after the power supply to the image forming apparatus is turned on. For example, it is determined that the fatigue of the photoconductor has recovered when the temperature of the fixing device is a predetermined temperature lower than the temperature at the time of image formation.

Further, image formation by the image forming apparatus is prohibited between the time when the image forming apparatus is turned on and the calibration of the surface potential detector is completed, or the developing roller is calibrated prior to the calibration of the surface potential detector. Should be separated from the photoconductor, or the calibration of the surface potential detector should be prohibited at least until the time when the surface potential detector can be operated stably after the power is turned on. Further, it is desirable to calibrate the surface potential detector so that the measured value obtained from the surface potential detector with respect to the reference voltage becomes constant.

[0013]

In the method of calibrating the surface potential detector of the present invention, the photoconductor itself is used in place of the reference plate that is usually used when calibrating the surface potential detector. After leaving it for a time sufficient for recovery, a reference voltage was applied to the substrate, and the surface potential detector was calibrated using the potential of the surface of the photoconductor as a reference value.
The surface potential of the photoconductor can be accurately detected without increasing the cost, and stable image quality can be obtained.

If the calibration methods according to the second to eleventh aspects are used, the surface potential detector can be calibrated with high accuracy, so that the surface potential detection accuracy of the photoconductor is further enhanced.

[0015]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The embodiments of the present invention will be specifically described below with reference to the accompanying drawings. Before that, the points of the present invention will be briefly described here.

The photoreceptor used in the image forming apparatus is
As shown in FIG. 2, since the charging potential decreases with time and the residual potential Vr rises, it cannot be used in place of the reference plate when calibrating the surface potential detector unless this is eliminated.

FIG. 3 shows the relationship between the residual time of the photoconductor and the residual potential, although it depends on the degree of fatigue of the photoconductor. As can be seen from this figure, if the residual potential is high, it takes a long time to recover the photoconductor, and during that time, the photoconductor cannot be used as a reference plate. Further, the higher the temperature of the photoconductor is, the faster the recovery is. If the temperature is above a certain temperature, the recovery time does not change regardless of the level of the residual potential.

This certain temperature is related to the temperature at which the photoconductor becomes fatigued by being subjected to hazards such as charging and light irradiation during image formation. The degree of fatigue is higher as the temperature at the time of hazard is lower. It was found that the longer the time was (the time passed), the higher the value became.

Although the reason is not clear, the time required for recovery of the photoconductor is always almost the same regardless of the degree of fatigue by controlling the temperature of the photoconductor when left to stand to be the maximum temperature that is received during image formation. I found out.

FIG. 4 is an overall block diagram of a copying machine showing an embodiment of the present invention, and FIG. 5 is an enlarged view of the essential parts thereof. This copier includes a copier body 1 and a circulation type document feeder (hereinafter referred to as "RD").
(Abbreviated as "H") 40.

The copying operation of this copying machine is started by setting the necessary copying conditions from the operation panel provided in the copying machine main body 1 and pressing the copy start key. Here, the RDH 40 is provided with a document tray 41,
The original placed on the original tray 41 facing downward is fed by the original feeding belt 42 and conveyed onto the contact glass 2 through the original conveying path 43.

When the original is conveyed onto the contact glass 2, the exposure lamp 3 emits light and is reflected by the irradiation mirror 3a, and the entire image surface of the original is irradiated for a predetermined time.
The light reflected from the document is reflected by the first mirror 4 that constitutes the optical system.
The surface of the photoconductor belt 5 is exposed through a, the through lens 4b, and the second mirror 4c.

A charging charger 6 is attached to the photosensitive belt 5.
Are uniformly charged in advance by an electric charge, and an electrostatic latent image is formed on the surface thereof by exposure, and the eraser 7 removes electric charges in unnecessary portions, and then the toner is attached by the developing roller 8a in the developing unit 8. After development by development, the image is transferred to a transfer sheet by the transfer charger 9 at the transfer section.

The transfer paper is the paper feed trays 10a, 10
Paper is fed from either b or 10c, passes through the conveyance path 11,
The registration roller 12 sends the toner image to the transfer portion in time. The transfer paper after transfer is the conveyor belt 1.
3 is sent to the fixing device 14 and is fixed to the fixing roller 14a.
Is fixed by heat, and then discharged to the discharge tray 15 or the double-sided tray 1
Stacked at 0d.

When the transfer sheets copied on one side are stacked on the double-sided tray 10d, the transfer sheets are re-fed at a predetermined timing and are conveyed again to the transfer section with the front and back surfaces reversed. The above-mentioned copying process processing is also applied to the other surface.

On the other hand, the exposed contact glass 2
The upper original is sent out by the original conveying belt 44 and returned to the original tray 41 by the original discharge roller 45. Further, the charges and toner remaining on the photoconductor belt 5 after the transfer are removed by the cleaning unit 16, and the photoconductor belt 5 is charged again by the charger 6 to prepare for the next exposure.

As the photoreceptor belt 5, as shown in FIG. 1, a laminate type organic photoconductor 5b is coated on an Al substrate 5a made of aluminum is used.
Reference numeral 21 denotes a surface potential sensor which is a surface potential detector, the detection surface of which is fixedly arranged facing the photoconductor belt 5, and the surface potential detection output is supplied to the charger 6 and the developing roller 8a in the developing unit 8. It is used to adjust and set the applied voltage to the optimum value.

As the surface potential sensor 21, a low-cost surface potential sensor of the type having a distance dependency in which the output changes depending on the distance (gap) from the surface of the photosensitive belt 5 is used. The gap is set to about 4 mm in consideration of the ease of attaching and detaching the photosensitive belt 5.

Reference numeral 22 denotes a density sensor (P sensor) composed of a photo sensor and the like, which is also fixedly arranged at a position facing the photoconductor belt 5 and has a toner image and a photoconductor surface formed outside the image area on the surface thereof. Each density of the background portion of (1) is detected (light is emitted and the amount of reflected light is received), and the detection signal is output to the control unit of the copying machine. The control unit corrects the developing bias according to the ratio of the respective densities, and maintains the image densities appropriately.

Reference numeral 23 denotes a contact brush, which is arranged in contact with the Al substrate 5a of the photosensitive belt 5 as shown in FIG. 1, and grounds the Al substrate 5a through a changeover switch which will be described later or the surface of the Al substrate 5a. A reference voltage for calibrating the potential sensor 21 is applied.

Reference numeral 24 is a pre-transfer charge eliminating lamp (PTL), which applies light from the back side of the photoreceptor belt 5 to eliminate the surface potential of the photoreceptor belt 5. Reference numeral 25 denotes a heater, which warms the photoreceptor belt 5 from the back side while the power supply to the copying machine is cut off so that the fatigue can be recovered in a short time.

FIG. 1 is a circuit diagram showing only the portion of the control system of this copying machine relating to the present invention. The Al substrate 5a of the photoconductor belt 5 is normally grounded (device casing) via a relay switch (changeover switch) 30 of a relay (not shown).
It is connected to the.

31 is a microprocessor, ROM, RA
A microcomputer (hereinafter abbreviated as “CPU”) including M, a timer, a counter, I / O, etc., performs the processing described later in the copy mode to obtain the actual surface potential of the photoconductor belt 5, and develops it. Bias power supply 32
(Drive power supply for the surface potential sensor 21 and the surface potential sensor 2
1 also serves as a reference power source for calibration), and an optimum developing bias voltage is applied to the developing roller 8a (developing sleeve) in the developing unit 8.

Although not shown, the voltage applied to the charger 6 in FIG. 2 is also optimally controlled according to the result of measuring the surface potential. Further, the CPU 31 always measures the temperature of the photoconductor belt 5 by the thermistor 33 arranged close to the photoconductor belt 5, and stores the maximum temperature in the internal RAM. The contents are stored and retained by another power source (backup power source) even when the power to the copying machine is turned off by the main switch (not shown).

After that, at the same time when the power supply to the copying machine is turned off, the heater 25 is turned on and the timer for measuring the leaving time of the photosensitive belt 5 is activated. This timer is used to determine whether or not the photoconductor belt 5 has been left until the time when the fatigue of the photoconductor belt 5 is recovered and the surface potential sensor 21 can be calibrated.

The photoconductor belt 5 has about 6
If left for a long time, it can be sufficiently used as a reference plate for calibration of the surface potential sensor 21, but it is set to 10 hours (about overnight) according to the fluctuation of the temperature control of the photosensitive belt 5 and the actual frequency of use. ing. Therefore, when the power supply to the copying machine is turned off and the heater 25 is turned on, the temperature control is performed using the thermistor 33 so that the temperature of the photoconductor belt 5 is kept at the maximum temperature level until then.

When the power to the copying machine is turned on, it is determined whether 10 hours or more have passed since the power to the copying machine was turned off. If 10 hours or more have elapsed, the sensor calibration mode is entered, and after 4 minutes including the stabilization time of the developing bias power source 32, the potential level output from the surface potential sensor 21 is set to the initial value (the set value at the time of factory shipment). Then, calibrate to 0 V here).

Next, a relay (not shown) is activated to open the relay switch 30 of the normally closed contact b and the normally open contact a.
To close the Al substrate 5 of the photoreceptor belt 5
The reference voltage is applied to the Al substrate 5a by connecting a to the output terminal of the developing bias power source 32. This voltage may be determined as necessary depending on the accuracy and stability of the surface potential sensor 21.

At this time, the voltage level output from the surface potential sensor 21 is calibrated to the same value as the reference voltage (here, -1000 V). Here, the calibration of the surface potential sensor 21 means that the output of the surface potential sensor 21 is set to 0 V and stored in the internal RAM, and the content is stored and held until the next calibration time.

After the calibration is completed, the relay switch 30 is returned to the state where the normally closed contact b is closed, the Al substrate 5a of the photosensitive belt 5 is connected to the ground again, and the sensor calibration mode is exited. After that, the heater 25 is turned off to clear the maximum temperature of the photoconductor that has been memorized so far, and at the same time, the exposure time measurement of the photoconductor by the timer is turned off (timer clear).
Details of the process relating to the calibration of the surface potential sensor 21 in this embodiment are shown in the flowchart of FIG. In the figure, N is the count value of the internal counter.

As described above, in this embodiment, after leaving the photoconductor belt 5 for a time sufficient to recover its fatigue,
Since the reference voltage is applied to the substrate and the surface potential sensor 21 is calibrated by using the surface potential of the photoconductor belt 5 as a reference value, the surface potential of the photoconductor can be detected with high accuracy without increasing the cost. And stable image quality can be obtained.

Further, since the change in the residual potential Vr at the time of copying can be measured, it becomes possible to control by applying the developing bias corresponding to the residual potential Vr, and the image can be stabilized more easily. Further, since the temperature of the photoconductor belt 5 when left standing is kept at the maximum temperature during copying to keep the fatigue recovery time of the photoconductor belt 5 constant, the time setting is fixed and the processing becomes simple.

FIG. 7 is a circuit diagram showing only a portion of the control system according to another embodiment of the present invention related to the present invention, and the same or corresponding portions as those in FIG. 1 are designated by the same reference numerals.
In this embodiment, the temperature of the heater 25 is controlled by a thermostat. That is, the heater power (100 V AC) is applied even while the main switch is off, and the normally-closed contact 35 of the relay (not shown) is closed. Therefore, if the heater temperature is low, the thermostat thermoswitch 34 is closed and the heater 25 is closed. Energize. Then, when the temperature of the heater 25 becomes higher than a predetermined temperature (the maximum temperature of the photosensitive belt 5 at the time of image formation), the thermostat operates to open the thermo switch 34 to turn off the heater 25, and when it becomes lower than the predetermined temperature, The thermo switch 34 is closed and the heater 25 is turned on.

FIG. 8 shows a processing routine relating to the calibration of the surface potential sensor 21 by the CPU 31 in this embodiment. This routine starts when it is called from a main routine (not shown) when the power is turned on, first starts the internal timer, and then determines whether the temperature of the fixing device 14 (fixing temperature) is 100 ° C. or lower. That is,
The surface temperature of the fixing roller 14a provided in the fixing device 14 is measured using a temperature sensor (thermistor) not shown, and the detected temperature is a predetermined temperature lower than the proper temperature of 196 ° C., about 50% in this example. Low temperature of 100 ℃
It is determined whether or not

FIG. 9 shows the relationship between the time for which the photoconductor is left and the temperature of the fixing device (fixing temperature). From this relationship, the temperature of the fixing device 14 drops from the proper temperature to 100 ° C. or less. If so, it can be determined that the fatigue time of the photoconductor belt 5 has been recovered since the leaving time has passed for 6 hours or more.

If the temperature of the fixing device 14 is not lower than 100 ° C., the image forming mode is entered. If the temperature is lower than 100 ° C., the sensor calibration mode is entered, and after the rotation of the photosensitive belt 5 and the image forming are prohibited. If 4 minutes (time required for the surface potential sensor 21 to operate stably) have elapsed since the internal timer started, the calibration operation of the surface potential sensor 21 is executed by the sensor calibration command.

That is, a relay (not shown) is turned on and the relay switch 30 is switched to close the normally open contact a, the Al substrate 5a of the photosensitive belt 5 is connected to the output end of the developing bias power source 32, and the Al substrate is opened. A reference voltage is applied to 5a, and the voltage output from the surface potential sensor 21 at this time is stored as a reference voltage in the internal RAM. Then, the relay switch 30 is returned to the original state in which the normally closed contact b is closed, and the photosensitive belt 5 is removed. The Al substrate 5a is again connected to the ground.
Here, the same voltage as the target potential at the time of image formation is applied as a reference voltage to improve the potential accuracy at the time of image formation.

For example, the surface potential sensor 21 is set to −50 to −.
When calibrating in the range of 1050V, first, the developing bias voltage is set to -50V and applied to the Al substrate 5a of the photoconductor belt 5, and then the voltage output from the surface potential sensor 21 is set to -50V in the internal RAM. After memory
Next, set the developing bias voltage to -1050V and set it to A.
The voltage output from the surface potential sensor 21 at that time is set to −1050V and stored in the internal RAM. As a result, the photoconductor potential becomes -50 to -1050.
The surface potential sensor 21 is calibrated in the range of V. The stored contents are retained until the power of this copying machine is turned off.

When the calibration operation is completed in this way, the normally-closed contact 35 is opened by the operation of a relay (not shown) to turn off the heater 25, and the timer is cleared to clear the photosensitive belt 5.
After the prohibition of rotation and the prohibition of image formation are released, the process returns to the main routine and returns to the sequence of the main body.

Therefore, in this embodiment, in addition to the same effects as described above, the following effects can be obtained. That is, if the photosensitive belt 5 is rotated during the calibration of the surface potential sensor 21, its surface will be charged due to friction. (There is also a method of using a charge eliminating charger and light irradiation to prevent this. Therefore, the true output cannot be obtained from the calibrated surface potential sensor 21. However, in this embodiment, the surface potential sensor 2 cannot be obtained.
Since the rotation of the photosensitive belt 5 is prohibited during the calibration of No. 1, it is possible to eliminate the potential fluctuation (reference voltage fluctuation) due to frictional charging.

In addition to this, image formation is prohibited between the time when the power of the copying machine is turned on and the time when the calibration of the photosensitive belt 5 at the time of image formation is completed, or at least the surface potential sensor 21 is turned on. Since the calibration of the surface potential sensor 21 is prohibited until the time when the surface potential sensor 21 can be stably operated, the surface potential sensor 21 can be calibrated more accurately, thereby forming an image. Since the potential of the photosensitive belt 5 at that time can be accurately measured, it becomes possible to easily realize long-term stabilization of the image.

Further, in the above-described embodiment, in order to determine the fatigue recovery time of the photoconductor belt 5, the time for which the photoconductor is left after the power supply to the copying machine is turned off is measured by a timer. Although an internal power source (backup power source) such as a battery had to be used, in this embodiment, since the relationship between the fatigue recovery time of the photoconductor and the temperature decrease time of the fixing device was found, Since the heater 25 for control is directly driven by a thermostat, it is not necessary to use a special internal power source for the copying machine. Therefore, the entire copying machine is simple and low in cost, and the power source is not required to be replaced, which is excellent in maintainability.

Although not shown here, it is determined whether copying is possible in the main body sequence,
If possible, the copy operation ON signal is issued. Further, in the image forming mode of this copying machine, the photoconductor belt 5 is always set to stop at the same position with respect to the surface potential sensor 21 after the completion of copying. A portion facing the surface potential sensor 21 is a non-image forming area.

Therefore, even if the photoconductor belt 5 deteriorates due to the image forming mode, the potential detection position by the sensor 21 during calibration of the surface potential sensor 21 should be in the same hazard as in the initial stage or in a region with less deterioration over time. Therefore, the calibration accuracy of the surface potential sensor 21 is further enhanced.

FIG. 10 is an overall constitutional view of a copying machine showing still another embodiment of the present invention, and FIG. 11 is an enlarged view of the main part thereof, which is provided with the heater shown in FIGS. 4 and 5 and a heater 25, respectively. Since it is the same except that it is not present, a description thereof will be omitted. FIG. 12 is a circuit diagram showing only a portion of the control system of this copying machine relating to the present invention, and the same portions as those in FIG. 1 are designated by the same reference numerals.

In this circuit, 51 is a temperature sensor (thermistor), which is an organic photoconductor 5b of the photosensitive drum 5.
It is fixedly installed at a position opposed to, detects the temperature of its surface, and outputs the detection signal to the CPU 31. Reference numeral 52 denotes a sensor output conversion unit, which is configured as shown in FIG. 13, for example, in which the operational amplifier 53 inputs the output of the surface potential sensor 21 via the resistor 54 and outputs it to the electronic volume 55,
It is corrected by each resistance value of 56 and output to the CPU 31.

FIG. 14 shows the CPU 31 in this embodiment.
7 is a flowchart showing a process related to calibration of the surface potential sensor 21 by the method. This routine starts when it is called from a main routine (not shown) by turning on the power source, first starts the internal timer, and then the fixing unit 14
It is determined whether the temperature (fixing temperature) is 100 ° C. or less.

If the temperature of the fixing device 14 is not lower than 100 ° C., the normally closed contact b of the relay switch 30 is opened after the measurement mode for measuring the surface potential for the calibration of the surface potential sensor 21 is prohibited. If the Al substrate 5a of the photoconductor belt 5 is connected to the ground in the closed state, the Al substrate 5a of the photoconductor belt 5 is opened in the closed state and the development bias power source 32 If it is connected to the output terminal, the relay is turned off, the normally closed contact b is closed, the Al substrate 5a is connected to the ground, the internal timer is cleared, and the process returns to the main routine.

When the fixing temperature is 100 ° C. or lower, the continuous stop time of the photosensitive belt 5, the temperature, the operating time (proportional to the number of copies after the replacement of the photosensitive member), and the stop of the photosensitive belt 5. The present residual potential Vr is estimated by calculating each factor of the residual potential Vr at that time. At that time, normal PI
D control may be used, or fuzzy control or a neural network which has been actively used in recent years may be used.

If the estimated residual potential Vr is not Vr≤20V capable of calibrating the surface potential sensor 21, each process such as the above-described measurement mode inhibition process is performed and the process returns to the main routine, but Vr≤20V. If so, the measuring mode is entered to prohibit the image forming mode (copy mode), and after 4 minutes from which the surface potential sensor 21 operates stably, the developing roller 8a in FIG. The developer is scraped off and separated from the photosensitive belt 5 to bring it into a non-contact state.

Then, the relay is turned on to close the normally open contact a of the relay switch 30, and the Al of the photosensitive belt 5 is closed.
Connect the substrate 5a to the output terminal of the developing bias power source 32, and
After the lapse of sec, the developing bias power source 32 is turned on to enter the sensor calibration mode, and then the sensor calibration processing of FIGS. 15 and 16 described later is performed. When the sensor calibration process is completed, the developing bias power source 32 is turned off to prevent the contact mark, and then the relay is turned off to return the normally closed contact b of the relay switch 30 to the closed state. With the above, the measurement mode ends, and after 100 msec has elapsed, the internal timer is cleared, the prohibition of the image forming mode is canceled and the measurement mode is prohibited, and then the process returns to the main routine.

FIGS. 15 and 16 are flowcharts showing the subroutine of the sensor calibration process of FIG. 14. First, the internal counter is set to "1" (N ← 1), and then the developing bias power source 32 is used as a reference for the Al substrate 5a. As the voltage (calibration voltage), the lower limit value of 100 V of the usage area is applied, and after the elapse of 50 msec to ensure that the surface potential sensor 21
The surface potential of the photoconductor belt 5 is measured using the sensor output converter 52 (FIG. 12). That is, a voltage (hereinafter referred to as “potential measurement value”) obtained by correcting the output voltage of the surface potential sensor 21 by the sensor output conversion unit 52 is input. The output of the surface potential sensor 21 is assumed to be about 1/200 of the input (surface potential of the photosensitive belt 5).

Next, the count value N of the counter is "1".
It is judged whether or not it is, but since it is set to "1" in the first step, the potential measurement value with respect to the applied voltage of 100 V is V
After being stored as 1 in the internal memory A and clearing the counter to “0” (N ← 0), the developing bias power source 32 outputs Al
The upper limit value 800V of the use area is applied as a reference voltage (calibration voltage) to the substrate 5a, and after 50 msec, the surface potential sensor 21
Also, the surface potential of the photosensitive belt 5 is measured using the sensor output converter 52.

Next, the count value N of the counter is "1".
It is determined whether or not it is "0" this time. Therefore, after the potential measurement value for the applied voltage of 800 V is stored in the internal memory B as V2, Vcal1 = V2-V1 is calculated. Here, assuming that the allowable range of the surface potential of the photosensitive belt 5 obtained by the potential measurement value when the voltage is applied to the Al substrate 5a is ± 10 V of the actual surface potential,
The normal range of Vcal 1 is 3.5 ± 0.05V.

Therefore, this time, it is judged whether or not the value of Vcal 1 is within the normal range of 3.5 ± 0.05 V, and if it is not within the normal range, the electronic volume of FIG. 13 is set so that it falls within the normal range. The resistance value R2 of 55 is variably set. That is,
If Vcal 1 <3.45V, the resistance value R2 of the electronic volume 55 is increased by a preset value, and Vcal 1> 3.
If it is 55V, it is decreased by a preset value. Then, return to the first step and repeat the above process to
When cal 1 is within 3.5 ± 0.05V, ie 80
The resistance value R2 of the electronic volume 55 is stored in the internal memory C when the slope of the straight line passing through the potential measurement value when 0V is applied and the potential measurement value when 100V is applied has a predetermined slope.

Next, proceeding to the processing of FIG. 16, the Al substrate 5
100 V is again applied to a, and after the lapse of 50 msec, the surface potential of the photosensitive belt 5 is measured using the surface potential sensor 21 and the sensor output converter 52. Then, when the potential measurement value is set to Vcal 2, it is determined whether or not Vcal 2 is within the normal range of 0.5 ± 0.02 V, and if it is not within the normal range, it is set within the normal range. The resistance value R3 of the electronic volume 56 of 13 is variably set.

That is, if Vcal 2 <0.48V, the resistance value R3 of the electronic volume 56 is increased by a preset value, and if Vcal 2> 0.52V, it is decreased by a preset value. After that, the surface potential is measured again and this process is repeated. When Vcal 2 becomes 0.5 ± 0.02 V, the resistance value R3 of the electronic volume 56 is stored in the internal memory D, and the process returns to the routine of FIG. ..

The reason why 100V is selected as the voltage to be applied to the Al substrate 5a when adjusting the electronic volume 56 is that the accuracy of the low voltage is required in the usage area. Of (800-10
It is better to select 0) / 2 = 350V.

Therefore, according to this embodiment, substantially the same effects as the above-mentioned embodiments can be obtained, and the following effects can also be obtained. That is, prior to the calibration of the surface potential sensor 21, the developing roller 8a is rotated in the reverse direction to scrape the toner on the circumference thereof and separate it from the photoconductor belt 5, so that the voltage leak can be prevented and the photoconductor belt 5 can be prevented from leaking. Al substrate 5a
It is possible to apply an accurate reference voltage.

Further, even if the gap between the photoconductor belt and the surface potential sensor differs between machines or the sensitivity of the surface potential sensor varies, it is applied to the Al substrate 5a of the photoconductor belt 5 when the surface potential sensor is calibrated. Since the potential measurement value with respect to the reference voltage can be held constant, the surface potential sensor can be calibrated with higher accuracy. Further, since the degree of fatigue recovery of the photoconductor belt is estimated more finely based on each factor of the continuous stop time of the photoconductor belt, the temperature, the use time, and the residual potential when the photoconductor belt is stopped, the surface potential sensor The calibration time can be determined more accurately.

The embodiments in which the present invention is applied to a full-exposure electrophotographic copying machine using a belt-shaped photosensitive member as a photosensitive member have been described above, but the present invention is not limited to this.
The present invention can be applied not only to a scanning exposure type electrophotographic copying machine using a drum-shaped photoconductor but also to an image forming apparatus such as an optical printer such as a laser printer or a facsimile machine.

[0072]

As described above, according to the present invention, the photoconductor is used as a reference plate while the mounting position of the surface potential detector is fixed, and the potential of the photoconductor surface during non-image formation is set. Since the surface potential detector is calibrated by using it as the reference value, the cost is reduced by not using the reference plate, and no special work is required at the time of calibration.
Moreover, since the relative surface potential of the photoconductor can be detected with high accuracy, stable image quality can be obtained.

Further, even a low-cost surface potential sensor (having a distance dependency) can be stably used for a long period of time, so that it can be mounted on a low-priced image forming apparatus. Further, according to the inventions of claims 2 to 11, the calibration accuracy of the surface potential detector can be further enhanced.

[Brief description of drawings]

1 is a circuit diagram showing only a portion related to the present invention of a control system of the copying machine shown in FIG.

FIG. 2 is a diagram showing a relationship between an exposure amount and a photoreceptor surface potential.

FIG. 3 is a diagram showing a relationship between a photoconductor leaving time and a residual potential.

FIG. 4 is an overall configuration diagram of a copying machine that is an embodiment of the present invention.

5 is an enlarged view of a main part of FIG.

FIG. 6 is a flowchart showing a process relating to calibration of a surface potential sensor by a CPU 31 of FIG.

FIG. 7 is a circuit diagram showing only a portion related to the present invention of a control system in another embodiment of the present invention.

FIG. 8 is a flowchart showing a process relating to calibration of the surface potential sensor by the CPU 31 of FIG.

FIG. 9 is a diagram showing a relationship between a photoconductor leaving time and a fixing temperature.

FIG. 10 is an overall configuration diagram of a copying machine that is still another embodiment of the present invention.

11 is an enlarged view of a main part of FIG.

12 is a circuit diagram showing only a portion of the control system of the copying machine shown in FIG. 10 related to the present invention.

13 is a block diagram illustrating a configuration example of a sensor output conversion unit 52 in FIG.

FIG. 14 is a flowchart showing a process relating to calibration of the surface potential sensor by the CPU 31 of FIG.

15 is a flowchart showing a subroutine of the sensor calibration process of FIG.

FIG. 16 is a flowchart of the same continuation.

[Explanation of symbols]

1 Copying Machine Main Body 5 Photosensitive Belt 5a Al Substrate 5b Organic Photoconductor 8 Developing Unit 8a Developing Roller 14 Fixing Device 14a Fixing Roller 21 Surface Potential Sensor 23 Contact Brush 25 Heater 30 Relay Switch 31 Microcomputer 32 Development Bias Power Supply 33, 51 Temperature sensor 34 Thermo switch 35 Normally closed contact 52 Sensor output converter

Claims (11)

[Claims] 1. An image forming apparatus equipped with a surface potential detector for measuring the surface potential of a photoconductor, wherein the photoconductor is left for a time sufficient to recover its fatigue, and then a reference voltage is applied to a substrate of the photoconductor. Is applied, and the surface potential detector is calibrated by using the potential of the surface of the photoconductor as a reference value. 2. The method for calibrating a surface potential detector in an image forming apparatus according to claim 1, wherein the temperature of the photoconductor when left as it is is maintained at a temperature equal to or higher than a temperature when forming an image. Calibration method. 3. The method for calibrating a surface potential detector in an image forming apparatus according to claim 1, wherein the photoconductor is held in a stopped state when the surface potential detector is calibrated. Calibration method. 4. The calibration method for a surface potential detector in an image forming apparatus according to claim 1, wherein the substrate of the photoconductor is normally connected to ground, and the surface potential detector A calibration method for a surface potential detector, characterized in that it is connected to a developing bias power source during calibration. 5. The method for calibrating a surface potential detector in an image forming apparatus according to claim 1, wherein the fatigue recovery timing of the photoconductor is determined by a continuous stop time of the photoconductor. A method for calibrating a surface potential detector characterized by: 6. The method for calibrating a surface potential detector in an image forming apparatus according to claim 1, wherein the fixing recovery time after the power of the image forming apparatus is recovered from the fatigue recovery timing of the photoconductor. A method for calibrating a surface potential detector, characterized in that it is judged by the temperature of. 7. The method for calibrating a surface potential detector in an image forming apparatus according to claim 6, wherein when the temperature of the fixing device is a predetermined temperature lower than the temperature at the time of image formation, it is determined that the fatigue of the photoconductor is recovered. A method for calibrating a surface potential detector, which is characterized in that 8. The calibration method for a surface potential detector in an image forming apparatus according to claim 1, wherein the calibration of the surface potential detector is completed after the image forming apparatus is powered on. Until then, a method for calibrating the surface potential detector is characterized in that image formation by the image forming apparatus is prohibited. 9. The method of calibrating a surface potential detector in an image forming apparatus according to claim 1, wherein the developing roller is separated from the photoconductor prior to the calibration of the surface potential detector. A method for calibrating a surface potential detector, which is characterized in that 10. The method for calibrating a surface potential detector in an image forming apparatus according to claim 1, wherein at least the surface potential detector is operated stably after being powered on. A method for calibrating a surface potential detector, characterized in that the calibration of the surface potential detector is prohibited until a time when it is possible to reach the limit. 11. The method of calibrating a surface potential detector in an image forming apparatus according to claim 1, wherein a measured value obtained from the surface potential detector is constant with respect to the reference voltage. A method for calibrating a surface potential detector, characterized in that the surface potential detector is calibrated as follows.