JP3452289B2 - Image forming device - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus such as a copying machine, a facsimile and a printer.

[0002]

2. Description of the Related Art In a printing field, an image forming apparatus used as a proofreading output machine for performing a trial printing before producing a final printing plate uses a sublimable ink sheet and a laser optical system. Some of them use an inkjet writing device, but since the device main body and consumables are expensive, a more inexpensive device is desired.

Conventionally, as an image forming apparatus using a latent image transfer method, an electrostatic latent image is formed by charging an image carrier by a charging means and exposing it by an exposing means, and an electrostatic latent image on the image carrier is formed. It is known that a latent image is electrostatically transferred onto a recording medium on a recording medium conveying means, and the electrostatic latent image on the recording medium is developed by a developing means to be visualized. In this image forming apparatus, a photoreceptor such as OPC used in a normal electrophotographic apparatus is used as an image carrier, and a corona discharge device such as a scorotron is used as a charging unit to charge the image carrier. It is charged to about -600 to 800 V by the means.

In the latent image forming step of forming an electrostatic latent image on the image bearing member, the surface of the image bearing member is charged by an external power source for the convenience of charging the image bearing member by the charging means and removing the charge by exposure by the exposing means. Since a predetermined voltage is applied, it is common to ground the image carrier. The electrostatic latent image on the image carrier is electrostatically transferred onto the recording medium on the recording medium conveying means by the electrostatic transfer method. In this electrostatic transfer, the charges on the image carrier are efficiently transferred onto the recording medium. For transfer, the image carrier and the recording medium are brought into close contact with each other, and a transfer bias voltage is applied between the image carrier and the recording medium by an external power source.

The electrostatic latent image thus formed on the recording medium is about 100 V at most, and a good image cannot be obtained even if it is developed by a dry developing device using powder toner, so that it is usually used. A wet developing device is used as a developing means that can sufficiently develop an electrostatic latent image having a low potential. Further, in the image forming apparatus, as a means for keeping the quality of the output image stable, a means for detecting the developer concentration in the wet developing device is provided for the purpose of controlling the image density on the recording medium. Has proposed a number of methods for keeping the developer concentration within a predetermined range. Further, in the image forming apparatus, a method has been proposed in which the image density on the recording medium is detected and the developer concentration in the wet type developing apparatus is controlled.

Further, in a general electrophotographic apparatus,
An electrostatic latent image is formed by charging the photoconductor to a predetermined potential by the charging means and exposing by the exposing means, and the electrostatic latent image is developed by the developing means and transferred to the recording paper by the transferring means. . Then, the surface potential of the photoconductor and the image density are generally detected, and the charging potential of the charging means and the toner density of the developer are controlled based on the results.

[0007]

In the above-mentioned image forming apparatus, the means for keeping the quality of the output image stable is to control the concentration of the developing solution, and the electrostatic transfer to the recording medium is performed. No means for controlling the potential of the latent image is provided, and the charging potential of the image carrier by the charging means and the transfer bias voltage applied between the image carrier and the recording medium by the external power source are positively set. No control was done. Therefore, when the characteristics of the image carrier or the charging unit are changed due to deterioration over time and environmental changes, the correction is not performed, and an image with stable quality cannot be obtained.

Further, in the method of detecting the image density on the recording medium and controlling the developer density in the wet type developing device, deterioration with time and environmental change of the image carrier and the charging means are not taken into consideration. This method is not sufficient when there is a limit in itself and high-quality image output is required. Further, in a general electrophotographic apparatus, not limited to the one using the latent image transfer method, the surface potential of the photoconductor and the image density are detected, and the charging potential of the charging unit and the toner density of the developer are controlled based on the results. However, when the toner on the photoconductor is transferred onto the recording paper, the toner on the photoconductor is accurately transferred onto the recording paper due to the influence of the paper type and environment of the recording paper. Since it is not always transferred, it is hard to say that proper control is performed, and it was unreliable to use it as a calibration output machine. SUMMARY OF THE INVENTION It is an object of the present invention to provide an image forming apparatus capable of maintaining high-quality image output without being affected by aging of an image carrier or environmental changes.

[0009]

In order to achieve the above object, the invention according to claim 1 has a recording medium conveying means for holding and conveying a recording medium, an image carrier, and a predetermined size of the image carrier. Charging means for charging to an electric potential; exposure means for exposing the image carrier after charging by the charging means to form an electrostatic latent image; and the image carrier and a recording medium on the recording medium conveying means. And a voltage application unit that applies a predetermined voltage to transfer the electrostatic latent image on the image carrier to the recording medium on the recording medium conveying unit, and develops the electrostatic latent image on the recording medium. In the image forming apparatus having a developing unit for forming a visible image, an image carrier potential detecting unit that detects a potential on the image carrier, a recording medium potential detecting unit that detects a potential on the recording medium, and Image density to detect optical density of visible image on recording medium It is obtained by a known means.

According to a second aspect of the present invention, in the image forming apparatus according to the first aspect, the charging potential of the charging means and the applied voltage of the voltage applying means are based on the detection result of the image carrier potential detecting means. A control means for controlling the light output of the exposure means is provided.

According to a third aspect of the present invention, in the image forming apparatus according to the first aspect, the detection result of the image carrier potential detection means and the latent image transfer characteristic obtained from the detection result of the recording medium potential detection means are used. A charging potential of the charging means,
A control means for controlling the applied voltage of the voltage application means and the light output of the exposure means is provided.

The invention according to claim 4 is the invention according to claim 2 or 3.
In the image forming apparatus described above, the image carrier potential detection means detects a potential on the image carrier immediately before transfer to the recording medium.

According to a fifth aspect of the present invention, in the image forming apparatus according to the third aspect, the recording medium potential detecting means detects a potential immediately after transfer from the image carrier on the recording medium.

According to a sixth aspect of the present invention, in the image forming apparatus according to the third aspect, the detection result of the recording medium potential detecting means and the charging potential vs. image density characteristic obtained from the detection result of the image density detecting means are used. A means for judging the developing ability of the developing means is provided.

According to a seventh aspect of the invention, in the image forming apparatus according to the sixth aspect, the recording medium potential detecting means detects the potential on the recording medium immediately before development by the developing means.

According to an eighth aspect of the invention, in the image forming apparatus according to the first aspect, the detection result of the potential of the recording medium potential detecting means immediately after the transfer from the image carrier on the recording medium, A means for detecting an electrostatic defect of the recording medium is provided based on the detection result of the potential on the recording medium immediately before the development by the developing means.

[0017]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 14 schematically shows an embodiment of the present invention. This embodiment is an embodiment of the invention described in claims 1 to 8 and is an example of an image forming apparatus including a printer. An electrostatic recording paper (hereinafter simply referred to as a recording paper) 11 as a recording medium is wound around a core 12 in a roll shape, and is mounted as a recording paper roll in this embodiment by a paper tube holder (not shown). The recording paper 11 is fed by a feeding roller 13, cut into a predetermined length by a cutter 14, conveyed by a feed roller 15, held by a clamp drum 16 as a recording medium conveying means, and conveyed.

A flat surface portion 16a is formed on a part of the outer peripheral surface of the clamp drum 16, and the recording paper 1 is formed on the flat surface portion 16a.
Clamp claw 17 for grasping the tip of 1 and clamp drum 16
An eject pin 18 for ejecting the upper recording paper is provided. The clamp drum 16 grips the leading edge of the recording paper conveyed by the feed roller 15 with the clamp claw 17,
The recording paper is wound around the outer peripheral surface of the clamp drum 16 as the clamp drum 16 rotates, and is held and conveyed around the clamp drum 16.

Around the clamp drum 16, an image carrier 19 composed of a photosensitive drum having a photosensitive member provided on a conductive drum, an exposure device 20 composed of a laser optical system as an exposing means, and a wet developing as a developing means. The device 21 and the like are arranged. The clamp drum 16 and the photoconductor drum 19 are rotationally driven at the same peripheral speed by the main motor, the photoconductor drum 19 is uniformly charged to a predetermined potential by the charging scorotron 22 as a charging means, and a plurality of colors are exposed by the exposure device 20. For example, yellow, magenta, cyan, and black electrostatic latent images are sequentially formed at predetermined time intervals (time intervals at which the clamp drum 16 makes one rotation).

The exposure apparatus 20 including an optical system unit is
A light source composed of a semiconductor laser (LD) is driven by an LD driving substrate with image signals of respective colors to emit laser light, and the laser light is deflected by a polygon mirror 20a and is applied to the photosensitive drum 19. The polygon mirror 20a is rotationally driven by a polygon motor 20b and deflects the laser light from the semiconductor laser to deflect it in the main scanning direction (photosensitive drum 19).
Scanning repeatedly in the width direction.

The electrostatic latent images of the four colors are clamped by applying a transfer bias between the photosensitive drum 19 and the recording paper 11 on the clamp drum 16 from a transfer bias power source as transfer means (not shown). Each time the clamp drum 16 makes one rotation, the recording paper 11 on the drum 16 is sequentially superposed and electrostatically transferred. The electrostatic latent image of each color on the recording paper 11 has a mirror image relationship with the electrostatic latent image on the photosensitive drum 19, and the developing device 21 corresponds the electrostatic latent image of each color on the recording paper 11 to that color. Development is performed with a developer as a wet two-component developer containing color toners.

The developing device 21 includes a plurality of slit developing heads (hereinafter simply referred to as developing heads) 23a and 2a for developing electrostatic latent images of respective colors on the recording paper 11 into images of those colors.
3b, 23c, and 23d. The developing head 23c for developing a cyan electrostatic latent image on the recording paper 11 into a cyan image is supplied with a developing solution containing cyan toner by a suction pump 24c, a developing tank 25c and a developing solution supply pipe 26c. Development tank 2 via liquid recovery pipe 27c
Recovered in 5c. That is, the developing solution containing cyan toner in the developing tank 25c is supplied to the developing head 23c through the developing solution supply pipe 26c by the suction pump 24c, and the developing solution discharged from the developing head 23c passes through the developing solution recovery pipe 27c to the developing tank 25c. Will be collected. Further, the developer in the developing tank 25c is supplied to the toner replenishing device 28.
Cyan toner is replenished.

Developing heads 23a, 23 for developing the yellow, magenta, and black electrostatic latent images on the recording paper 11 into yellow, magenta, and black images, respectively.
Similarly, in b and 23d, the developing solutions containing the yellow, magenta and black toners are respectively supplied from the developing tanks by the suction pump and the developing solution supply pipe and collected in the developing tanks by the developing solution collecting pipes. The developer is replenished with yellow, magenta, and black toners by a toner replenishing device.

Developing heads 23a, 23b, 23c, 23
The d is arranged radially around the clamp drum 16 and is shaped so as to be in close contact with the surface of the clamp drum 16. At the time of non-development, the developer is not supplied and is separated from the surface of the clamp drum 16 by a predetermined distance. It will be in a state of being.
Further, the developing heads 23a, 23b, 23c, 23d are
At the time of development for developing the electrostatic latent image of each color on the recording paper 11, the electrostatic latent image of each color on the recording paper 11 is moved to the developing position so as to be in close contact with the recording paper on the clamp drum 16. Develop with a developer containing toner.

As described above, on the recording paper on the clamp drum 16, the electrostatic latent images of the respective colors are sequentially superposed and transferred from the photosensitive drum 19, and the developing heads 23a, 23b, 23.
A full-color image is formed by being developed with a developing solution containing toner of those colors by c and 23d, and
Each time the clamp drum 16 rotates, the charge is removed by the charge removing device 30, and then the eject pin 18 discharges the charge from the clamp drum 16 and discharges it to the outside.

FIG. 15 shows the control system of this embodiment. The control system as the control means of the present embodiment is roughly divided into a sequence control unit 31, an LD control unit 32, a process control unit 33, and an operation unit 34. The sequence control unit 31 detects the interface unit inserted between the external device 35 and the control signal from the operation unit 34 and each sensor of the present embodiment in order to control the operation of the present embodiment. An input unit for inputting a signal, a drive system for driving the motors and pumps of the present embodiment, an output unit for controlling a high voltage power supply of each charger of the present embodiment, a transfer bias power supply, and the like are provided. ing.

At the input of the sequence controller 31, the clamp drum 16, the developing heads 23a to 23d, the recording paper 1 are provided.
The detection signals of the position sensors for detecting the position such as 1 are input. Further, the operator can operate the operation unit 34 to directly instruct the operation of the present embodiment. Further, the sequence control unit 31 is connected to the external device 35 by a serial line through an interface unit with the external device 35, decodes a command from the external device 35, and performs a predetermined operation on each unit of this embodiment. To perform. The sequence control unit 31 turns on from the output unit to the main motor for rotationally driving the clamp drum 16, the paper feed / discharge motor, the suction pump and electromagnetic valve for each color, the high voltage power supply for each charger, and the high voltage power supply for static elimination. / Off signal is output.

The LD controller 32 controls the light output of the semiconductor laser in the exposure apparatus (optical system unit) 20 and drives the polygon motor 20b. In this case, the LD control unit 32 controls the L from the LD drive substrate in the exposure apparatus 20.
The D power monitor signal, the detection signal of the synchronization detection sensor in the exposure apparatus 20, the lock signal of the polygon motor 20b, etc. are input from the input unit. Further, the LD control unit 32 outputs an image signal, a power control signal of the semiconductor laser, and a drive signal of the polygon motor 20b to the exposure apparatus 20 from the output unit.

The process control unit 33 controls the charging potential by the charging scorotron 22, the transfer bias voltage,
The semiconductor laser power control and the developer concentration control in the exposure device 20 are performed. In this case, the process control unit 33
The analog signals such as the output monitor signals of the potential sensors, the image density sensor, the developing solution density sensor, and the high voltage power source of the present embodiment are input from the input unit. The developer concentration sensor includes a toner concentration sensor 29c that detects the concentration of the developer in the developing tank 25c and three toner concentration sensors that detect the concentrations of the developer in the other three developing tanks. Further, the process control unit 33 controls the output of the high voltage power supply, the transfer bias power supply, the high voltage power supply for static elimination of each charger, the LD power setting signal to the LD control unit 32, and the control of the toner replenishing device. Output a signal.

FIG. 1 shows an outline of components related to the process control of the process control unit 33. At the input of the process control unit 33, a photoconductor potential sensor 41 as an image carrier potential detection unit, a recording paper potential sensor 42 as a recording medium potential detection unit, and an optical density of a visible image on the recording medium 11 are detected. The detection signals from the four types of detecting means, namely, the image density sensor 43 as the image density detecting means, the toner density sensor 29c ...

The process control section 33 performs calculation based on input signals from the photoconductor potential sensor 41, recording paper potential sensor 42, image density sensor 43, toner density sensor 29c, ... Control each unit connected to. The unit connected to the output unit of the process control unit 33 is a high-voltage power supply (charging PP) that applies a voltage to the charge wire of the charging scorotron 22.
44, a grid bias power supply (grid PP) 45 for applying a predetermined potential to the grid of the charging scorotron 22,
There are a transfer bias power supply 46 for applying a transfer bias between the photosensitive drum 19 and the recording paper on the clamp drum 16, and a toner replenishing device 28c. Further, the process control unit 33 outputs a semiconductor laser power setting signal from the output unit to the LD control unit 32 in order to control the optical output of the semiconductor laser in the exposure apparatus 20.

The photoconductor potential sensor 41 and the recording paper potential sensor 42 are the same, and FIG. 2 shows an example of the photoconductor potential sensor 41. The photoconductor potential sensor 41 includes a potential detection unit (hereinafter referred to as a probe) 41a and a control board 41b.
The probe 41a is installed at a distance of 1 to 3 mm from the surface of the photosensitive drum 19 as the object to be measured, and detects the surface potential of the photosensitive drum 19.
The detection result of the probe 41a is output from the output section of the control board 41b to the process control section 33 as an analog signal.

The photoconductor potential sensor 41 is the photoconductor drum 1
It is arranged between the exposure position on the optical disc 9 and the position in contact with the clamp drum 16. This is the photoconductor potential sensor 41.
The electrostatic latent image on the photosensitive drum 19 is the clamp drum 16.
Photosensitive drum 19 immediately before being transferred to the upper recording paper
This is because the purpose is to measure the upper potential.
As a result, it is possible to minimize variations in the potential detection result due to dark decay of the photosensitive drum 19.

The recording paper potential sensor 42 is disposed between the developing device 21 and a position where the recording paper on the clamp drum 16 and the photosensitive drum 19 are in contact with each other.
The potential of the electrostatic latent image on the recording paper on 6 is detected. This aims to detect the potential of the electrostatic latent image on the recording paper immediately after transfer and the potential of the electrostatic latent image immediately before development. Normally,
The electric potential of the electrostatic latent image immediately before the development is detected by the recording paper electric potential sensor 42, and the control is performed as described below according to the result, but the electric potential of the electrostatic latent image just after the transfer is detected by the recording paper electric potential sensor 42. There are the following reasons.

That is, in this embodiment, the main purpose is to improve the image quality of the output image, and when a high print speed is not required, the exposure device 20 exposes the photosensitive drum 19 to write an image at the time of development. The exposure process and the development process are performed separately to avoid the influence of load fluctuations. Therefore, when attention is paid to a certain point on the recording paper, the clamp drum 16 makes one rotation from the transfer of the electrostatic latent image to the development, and the electric potential on the recording paper is slightly attenuated during that time. This attenuation of the electric potential is usually not a problem, but when the recording paper has a local defect, the electric potential on the recording paper may be greatly attenuated.

Therefore, as will be described later in detail, when a desired transfer characteristic is obtained, the detection result of the electrostatic latent image potential immediately after the transfer is used, and when a desired developing characteristic is obtained, the electrostatic latent image immediately before the development is obtained. By using the detection result of the electric potential, it is possible to minimize the variation in the detection result of the recording paper electric potential sensor 42 due to the electrostatic latent image on the recording paper being attenuated over time.

Further, by comparing the electric potentials on the recording paper immediately after the transfer and immediately before the development, it is possible to detect a decrease in the electrostatic latent image electric potential due to a local defect of the recording paper, so that the image quality is deteriorated. By preventing the image output in advance, waste of image output time can be eliminated. The mesh-shaped grid arranged at the opening of the charging scorotron 22 is connected to the grid bias power supply 45, and when the charging scorotron 22 is turned on, the surface of the photoconductor drum 19 is connected to the grid of the charging scorotron 22 by the grid bias power supply 4.
It is charged to substantially the same potential as that applied from 5. Therefore, the charging potential of the photosensitive drum 19 can be controlled by adjusting the output value of the grid bias power supply 45.

A voltage of about -300 to -380 V is applied by the transfer bias power supply 46 between the photosensitive drum 19 and the conductive layer provided on the recording paper on the clamp drum 16. This voltage is set to a voltage value such that the potential of the exposed portion of the photosensitive drum 19 is not transferred to the recording paper, but a slight width is provided due to the variation of the residual potential of the photosensitive drum 19 after exposure. .

The toner replenishing device 28c ... Is provided as a means for replenishing the developing solution in the developing tank 25c ... With new toner to control the toner concentration of the developing solution. The toner replenishing device 28c ... Is configured to replenish a predetermined amount of toner to the developing solution in the developing tank 25c. The toner replenishing operation is repeated until the detection signal level reaches a predetermined output value.

When the power supply of the present embodiment is turned on and the initialization is performed, the LD control unit 32 once sets the emission power of the semiconductor laser in the exposure apparatus 20 to the default value, but the process control unit 33 to the LD control unit 32. On the other hand, when the semiconductor laser power setting signal is output, the LD control unit 32 automatically sets the emission power of the semiconductor laser in the exposure apparatus 20 according to the semiconductor laser power setting signal from the process control unit 33.

Next, the process control by the process control unit 33 will be described. In the embodiment, in the image forming process, the non-exposed portion, that is, the portion where the electrostatic latent image exists (image portion) is developed, that is, so-called positive / negative.
Although it is a positive process, it goes without saying that the same effect can be obtained even when the image forming process is a negative / positive process.

FIG. 3 shows the characteristics of the image forming process in this embodiment. In FIG. 3, Vst represents the potential difference between the photosensitive drum 19 and the recording paper on the clamp drum 16, and the charging potential on the photosensitive drum 19 immediately before transfer is V.
When the transfer bias voltage applied between the photosensitive drum 19 and the recording paper on the clamp drum 16 by the transfer bias power source 46 is Vt, there is a relationship of Vst = Vs + Vt.

Further, Vp represents the electric potential of the recording paper after the electric potential of the photosensitive drum 19 is transferred to the recording paper, and Vst and Vst
p has a substantially direct proportional relationship. A characteristic curve showing the relationship between Vst and Vp is obtained from the output value of the photoconductor potential sensor 41 and the output value of the recording paper potential sensor 42. In FIG. 3, the point A indicates the transfer start voltage, and the photosensitive drum 1
When the potential difference Vst between the sheet 9 and the recording paper on the clamp drum 16 becomes equal to or higher than this transfer start voltage, it means that the electrostatic latent image on the photosensitive drum 19 is transferred to the recording paper on the clamp drum 16. .

Further, ID represents the image density on the recording paper, and the relationship between Vp and ID is a substantially exponential function.
A characteristic curve showing the relationship between Vp and ID (developing characteristic of the developing device 21) is obtained from the output value of the recording paper potential sensor 42 and the output value of the image density sensor 43. Process control unit 3
3 is a characteristic curve showing the relationship between Vp and ID in FIG.
The developing ability of the developing device 21 is determined based on the detection result of the toner concentration sensor 29c ... And based on this characteristic curve in the state where the toner concentration of the developer of each color is controlled within a predetermined range. Depending on the developing capacity of the developing device 21, the charging bias scorotron 22 from the grid bias power source 45
The image density ID is always controlled to the same value by controlling the grid voltage to or to the transfer bias voltage output from the transfer bias power source 46 to keep the value of Vst constant.

However, when the photoconductor drum 19 is deteriorated with time or light is fatigued, the dark decay property of the photoconductor drum 19 is deteriorated, and the photosensitivity is lowered.
Since the above-mentioned characteristic curve changes, it is necessary to correct it. The following four factors are mainly mentioned as the factors of the change of the characteristic curve. I. Changes in charging characteristics of the photoconductor drum 19 (including deterioration of dark decay characteristics) b. Deterioration of photosensitivity characteristics of the photosensitive drum 19 c. Variation in transfer characteristics d. Variations in transfer start voltage These variations will be described in detail below.

As for the fluctuation of the dark attenuation characteristic of the photosensitive drum 19, as shown in FIG. 4, the photosensitive drum 19 whose dark attenuation characteristic is deteriorated has a certain time immediately after charging as compared with a normal photosensitive member. The potential drop becomes large as time passes. The point indicated by A in FIG. 4 indicates the time from immediately after the charging of the photosensitive drum 19 until the contact between the photosensitive drum 19 and the recording paper, and when the dark attenuation characteristic deteriorates, the grid of the charging scorotron 22. Even if the voltage applied to the same is set to the same value, there arises a problem that the electric potential at the time of transfer of the photosensitive drum 19 becomes lower than that at the normal time. This will be described with reference to FIG. 5. The light emission power (LD power) of the semiconductor laser and the surface potential Vs of the photosensitive drum 19 in the exposure apparatus 20.
The characteristic curve showing the relationship with V is slid in the direction shown by the arrow in FIG. 5 due to the deterioration of the dark attenuation characteristic, and as a result, the surface potential Vs of the photosensitive drum 19 is lowered and Vst is ΔV.
It will be decreased by st.

Therefore, for example, as described above, the photoconductor potential sensor 4 is placed at a position corresponding to point B in FIG. 4 (position immediately before transfer).
1 is installed and the surface potential Vs of the photoconductor drum 19 is detected by the photoconductor potential sensor 41, and it is determined how much the current dark decay characteristic of the photoconductor drum 19 is deteriorated, and the photoconductor drum 19 is charged. By correcting the electric potential Vs, it is necessary to bring the electric potential of the photosensitive drum 19 at the time of transfer close to the normal value.

Next, regarding the fluctuation of the photosensitivity characteristic of the photoconductor drum 19, the potential of the photoconductor drum 19 after exposure by the exposure device 20 depends on the LD power of the exposure device 20 as shown in FIG.
It changes as shown in. It is generally known that the photosensitivity characteristic of the photosensitive drum 19 deteriorates due to fatigue over time, and when the photosensitive drum 19 is exposed with the same LD power, the deteriorated photosensitive drum is a normal photosensitive drum. The residual potential becomes higher.

When the residual potential of the photosensitive drum 19 becomes high, electric charges are transferred to the background portion at the time of transfer, so that there is a problem that background contamination occurs. As shown in FIG. 5, in order to make the post-exposure potential of the photoconductor drum 19 that is exhausted the same as the post-exposure potential of the normal photoconductor drum 19, the LD power of the exposure device 20 is increased from A to B. You know what you have to do.

Therefore, it is necessary to detect the post-exposure potential of the photoconductor drum 19 with the photoconductor potential sensor 41 and set the appropriate LD power of the exposure device 20 from the value.
However, the LD power of the exposure apparatus 20 has an upper limit, and even if the LD power of the exposure apparatus 20 is increased to the maximum value, the potential of the photoconductor drum 19 cannot be corrected,
Since the photosensitive drum 19 is severely deteriorated, the photosensitive drum 1
It is judged that 9 is unusable.

Next, when the transfer characteristic of the photoconductor is changed, Vst and V as shown in FIG. 6 are obtained from the output signals of the photoconductor potential sensor 41 and the recording paper potential sensor 42.
A characteristic curve showing the relationship with p is obtained. From FIG. 6, even if the potential of the photosensitive drum 19 is transferred to the recording sheet with the same Vst, the same recording sheet potential Vp cannot be obtained and the recording sheet potential Vp is ΔV.
When it decreases by p, it is considered that the transfer efficiency has decreased.

The cause of this decrease in transfer efficiency is considered to be a change in the photoconductor drum 19 and the recording paper lot. When the photoconductor drum 19 is replaced, or the recording paper roll 11 is changed.
Can occur if is set. In this case, the decrease in transfer efficiency can be corrected by increasing the charging potential Vs of the photosensitive drum 19 until the same recording paper potential Vp is obtained.

When the transfer start voltage at the time of transfer for transferring the potential of the photosensitive drum 19 to the recording paper changes, the characteristic curve showing the relationship between Vst and Vp as shown in FIG. It is almost a parallel movement, and it is thought to occur mainly when environmental conditions such as atmospheric pressure change significantly. In this case, the transfer bias voltage is set to Vst and V
It is possible to correct the change in the transfer start voltage by changing the amount of parallel movement of the characteristic curve showing the relationship with p.

The process when the process controller 33 actually performs the correction for each variation described above will be described with reference to the flowcharts shown in FIGS. The process control unit 33 turns on / off the Vs correction flag under a predetermined condition and repeatedly executes the correction for each variation. However, as shown in FIG. 7, it is determined in step 1 whether the Vs correction flag is turned on. If the Vs correction flag is not turned on, the process jumps to step 6.

When the Vs correction flag is turned on, the process control section 33 checks the output signal of the photoconductor potential sensor 41 in step 2 to check the surface potential Vs of the photoconductor drum 19 detected by the photoconductor potential sensor 41. Between the target value Vs-norm and the preset target value Vs-norm of the solid portion, and the photosensitive drum 19 detected by the photosensitive potential sensor 41.
Difference between the post-exposure potential Vexp and a preset target value Vmin of the photoconductor drum 19 after the exposure ΔVex
p is determined, and it is determined in step 3 whether the absolute value | ΔVs | of ΔVs is larger than the allowable fluctuation range Vs-dif of the surface potential of the photosensitive drum 19.

In the process controller 33, | ΔVs | is Vs
If larger than -dif, execute the correction 1 routine,
The correction process is once ended. In the correction 1 routine, the process control unit 33 resets the grid voltage Vg applied to the grid of the charging scorotron 22 to the grid bias power source 45 by ΔVs as shown in FIG. The charging potential Vs of 19 is corrected. Next, the process control unit 33 causes the reset Vg
Is judged whether or not it exceeds the maximum allowable value Vg-max of the grid voltage setting value, and if Vg does not exceed Vg-max, the correction 1 routine is ended, and if Vg exceeds Vg-max. Error processing is performed and the correction 1 routine ends.

Further, the process control unit 33 uses | ΔVs |
Is not larger than Vs-dif, Δ in step 4
It is determined whether or not the absolute value | ΔVexp | of Vexp is larger than the allowable fluctuation range Vexp-dif of the potential of the photosensitive drum 19 after exposure, and | ΔVexp | is Vexp-dif.
If it is larger than the above, the correction 2 routine is executed. In the correction 2 routine, the process control unit 33 uses the semiconductor laser power setting signal LD-set to increase or decrease the LD power of the exposure apparatus 20 by a value according to ΔVexp as shown in FIG. Only the value LD (Vexp) corresponding to ΔVexp is subtracted and LD-set is reset.

Next, the process control unit 33 determines whether or not the reset LD-set exceeds the maximum allowable value LD-max of the LD power setting value of the exposure apparatus 20.
If et does not exceed LD-max, the correction 2 routine is ended, and if LD-set exceeds LD-max, error processing is executed and then the correction 2 routine is ended.

The process control unit 33 determines that | ΔVexp | is not larger than Vexp-dif, that is, | ΔVexp.
When Vs | converges to Vs-dif or less and | ΔVexp | converges to Vexp-dif or less, the Vs correction flag is turned off in step 5 and the process proceeds to step 6. In step 6, the process control unit 33 determines that the recording paper potential sensor 42
Of the recording paper potential Vp detected by the recording paper potential sensor 42 and the target value Vp− of the recording paper potential
The difference from the norm ΔVp = Vp−Vp−norm is obtained, and in step 7, it is determined whether or not the absolute value | ΔVp | of ΔVp is larger than the fluctuation allowable width Vp-dif of the recording paper potential. In the process control unit 33, | ΔVp | is Vp-dif
If it is larger, ΔVp in step 8 and the recording paper potential Vp− of the background portion detected by the recording paper potential sensor 42.
When ΔVp is almost equal to Vp-exp compared with exp, the correction 3 routine is executed, and ΔVp is Vp-exp.
If the value is not substantially equal to, the correction 4 routine is executed.

In the correction 3 routine, the process control section 33 resets the transfer bias voltage Vt so that the transfer bias voltage Vt is increased or decreased by ΔVt with respect to the transfer bias power source 46 as shown in FIG. That is, the process control unit 33 uses the Vst (Vs) and Vst as shown in FIG.
The slope of the characteristic curve showing the relationship with p dVst / dVp (=
dVs / dVp) is calculated from the output value of the photoconductor potential sensor 41 and the output value of the recording paper potential sensor 42, and the conversion formula ΔVt
= ΔVt is obtained from dVs / dVp × ΔVp, and the transfer bias voltage setting value Vt− for the transfer bias power supply 46 is obtained.
The transfer bias voltage is reset so that the set is increased or decreased by ΔVt.

Next, the process control section 33 judges whether or not the reset transfer bias voltage setting value Vt-set exceeds the maximum allowable transfer bias voltage setting value Vt-max, and Vt-set If it does not exceed Vt-max, the correction 3 routine is ended and Vt-set becomes Vt-max.
If it exceeds, error processing is performed and the correction 3 routine is ended.

Further, in the correction 4 routine, the process control section 33 uses the grid bias power source 4 as shown in FIG.
5, the grid voltage setting value Vg-set is reset so that the grid voltage Vg of the charging scorotron 22 is increased or decreased by a value converted from ΔVp, and the charging potential of the photosensitive drum 19 is corrected.

That is, the process control unit 33 determines the output value of the photoconductor potential sensor 41 based on the slope dVst / dVp (= dVs / dVp) of the characteristic curve showing the relationship between Vst (Vs) and Vp as shown in FIG. And the output value of the recording paper potential sensor 42, the conversion formula ΔVs = dVs / dVp × ΔV
The ΔVs is obtained from p, and the grid voltage setting value Vg-set is reset so that the grid bias power supply 45 increases or decreases Vg by ΔVs to correct the charging potential of the photosensitive drum 19.

Then, the process control unit 33 determines that the reset Vg-set is the maximum allowable value Vg of the grid voltage set value.
Vg-set is Vg by judging whether or not it exceeds -max.
If -max is not exceeded, the correction 4 routine is terminated,
If Vg-set exceeds Vg-max, error processing is performed and the correction 4 routine is ended.

Further, the process control unit 33 determines that | ΔVp
If | is not larger than Vp-dif, the output signal of the recording paper potential sensor 42 is checked in step 9 and the recording paper potential Vp-exp of the background portion is larger than 0 (Vp-e
xp was detected) and Vp-exp is 0.
If it is not larger, the process for each fluctuation is temporarily terminated, and if Vp-exp is larger than 0, the correction 5 routine is executed. In the correction 5 routine, the process control unit 33 causes the LD control unit 32 to increase the LD power set value LD- so as to increase the LD power of the exposure apparatus 20 by a value converted from Vp-exp as shown in FIG. reset the set.

In this case, the process control unit 33 controls the LD power and the potential on the recording paper obtained from the relationship between the LD power of the exposure device 20 and the potential of the photosensitive drum 19 and the relationship between Vst (Vs) and Vp described above. Relationship with ΔLD = LD
Based on (Vp-exp), the LD power variation ΔLD is obtained from Vp-exp, and L is set so that the LD power of the exposure apparatus 20 is increased by ΔLD with respect to the LD control unit 32.
LD by subtracting ΔLD from the D power setting value LD-set
Reset the power setting value LD-set.

Next, the process control unit 33 determines that the reset LD-set is the maximum allowable value LD-m of the LD power set value.
LD-set is LD-m by judging whether or not ax is exceeded.
If it does not exceed ax, the correction 5 routine is ended, and LD
If -set exceeds LD-max, it is determined that the photoconductor drum 19 is unusable due to severe deterioration of the photoconductor drum 19 and error processing is performed, and then the correction 5 routine is ended.

Now, other variable factors that cannot be completely corrected by the above-mentioned correction means will be described. As described above, when a local electrostatic defect is generated on the recording paper 11, the potential on the recording paper at that portion is greatly attenuated, and as a result, an abnormal image is generated. However, since the above-mentioned correction means detects the recording paper potential immediately after transfer, there is a problem in that it cannot detect more than this. On the other hand, by comparing the recording paper potential immediately after transfer with the recording paper potential immediately before development, the output of an abnormal image can be prevented.

When the developing ability of the toner deteriorates with time, the relationship between Vp and ID shown in FIG. 3 is lost, and the toner concentration of the developing device 21 is controlled to be constant by the toner concentration sensor 29c. However, there is a problem that a predetermined image density cannot be obtained. On the other hand, from the ideal image density calculated from the recording paper potential immediately before development and the actual image density obtained by the image density sensor 43, the toner developing ability which cannot be detected by the toner density sensor 29c is detected. By doing so, the abnormality processing can be performed.

The processing for actually performing the above control will be described below with reference to the flowchart shown in FIG. In step 3, the process controller 33 determines the recording paper potential Vp (1) immediately after the transfer and the recording paper potential V immediately before the development.
The difference of p (2) is calculated, and in step 4, the difference ΔVp and the maximum allowable potential drop ΔVp-ma of the recording paper potential.
When .DELTA.Vp is larger than .DELTA.Vp-max, the process jumps to step 5 and it is determined that there is an electrostatic abnormality on the recording paper, and error processing such as printing stop is performed.

The process control unit 33 determines ΔV in step 4.
If p is not larger than ΔVp-max, in step 6, the ideal image density ID-p is calculated from the value of the recording paper potential Vp (2) immediately before transfer as ID-p = f (Vp
(2)). This formula ID-p = f (Vp
(2) is a calculation formula obtained from the relationship between Vp and ID in FIG.

Next, the process control unit 33 causes the step 7
At, the actual image density ID-s is obtained from the detection result of the image density sensor 43, and the difference ΔID between the two is obtained at step 8. If the ΔID does not exceed the allowable value ΔID-max, the process control unit 33 determines that the process is normal and terminates the process.
The toner concentration sensor 29c detects the developer concentration. If the detection result is normal, the process control unit 33 determines that the developing ability of the toner is abnormal, jumps to step 12, and performs error processing. If the detection result is not normal, the process control unit 33 jumps to step 13, replenishes the developing solution, and repeats similar control.

As described above, the process control unit 33 determines the grid voltage applied to the grid of the charging scorotron 22 from the grid bias power supply 45 based on the detection result of the photoconductor potential sensor 41 and the detection result of the recording paper potential sensor 42, and the transfer. By controlling the transfer bias voltage applied between the photoconductor drum 19 and the recording paper on the clamp drum 16 from the bias power source 46 and the LD power of the exposure device 20,
Even if the transfer efficiency or the transfer start voltage changes, the potential of the electrostatic latent image transferred to the recording paper can be kept constant. Further, when the process control unit 33 compares the potential Vp on the recording paper immediately after the transfer with the potential Vp immediately before the development and the difference is abnormally large, the recording paper has some electrostatic defect and is recorded. It can be determined that the charges on the paper have leaked, the output of the abnormal image can be prevented, and the output time can be saved.

As described above, the present embodiment is an embodiment of the invention described in claim 1, and is the recording medium conveying means 16 comprising the clamp drum for holding and conveying the recording paper 11 as the recording medium. , An image carrier 1 including a photosensitive drum
9, a charging unit 22 composed of a charging scorotron for charging the image carrier 19 to a predetermined potential, and an exposure unit for exposing the image carrier 19 after charging by the charging unit 22 to form an electrostatic latent image. Exposure apparatus 20 and image carrier 1
9 and a recording medium 11 on the recording medium conveying means 16 are applied with a predetermined voltage to transfer the electrostatic latent image on the image carrier 19 to the recording medium 11 on the recording medium conveying means 16. In the image forming apparatus having the voltage applying unit 46 including the above and the developing device 21 as a developing unit that develops the electrostatic latent image on the recording medium 11 into a visible image, the image carrier 1
9 is an image carrier potential detecting unit 41 including a photoconductor potential sensor for detecting the potential on the recording medium 9, and a recording medium potential detecting unit 42 including a recording paper potential sensor for detecting the potential on the recording medium 11.
And the image density sensor 43 as an image density detecting means for detecting the optical density of the visible image on the recording medium 11, the charging means in the image forming process using the latent image transfer method, and the image carrier The characteristics of each of the transfer means and the developing means for transferring the electrostatic latent image of to the recording medium can be individually grasped, and the high-level process control can be performed by the appropriate feedback control by the process control section 33. It is possible to maintain a high-quality image output that can be endured as a calibration output device without being affected by changes with time or environmental changes of the image carrier or the like.

Further, this embodiment is an embodiment of the invention described in claim 2, and in the image forming apparatus described in claim 1, the charging means 22 is based on the detection result of the image carrier potential detection means 41. Since the process control unit 33 is provided as a control unit for controlling the charging potential, the voltage applied by the voltage application unit 46, and the light output of the exposure unit 20, the dark decay characteristics of the image carrier due to light fatigue or the like over time. It is possible to prevent changes in the charging potential of the image carrier during transfer due to deterioration and deterioration of photosensitivity.

Further, the present embodiment is an embodiment of the invention described in claim 3, and in the image forming apparatus according to claim 1, the detection result of the image carrier potential detection means 41 and the recording medium potential. According to the latent image transfer characteristics obtained from the detection result of the detection unit 42, the process control unit 33 as a control unit that controls the charging potential of the charging unit 22, the applied voltage of the voltage application unit 46, and the light output of the exposure unit 20 is used. Since it is provided, it is possible to prevent a change in the electrostatic latent image potential of the recording medium at the time of transfer due to a change in electrostatic transfer characteristic caused by a difference in electrostatic characteristic due to variations in the image carrier and the recording medium.

Further, this embodiment is an embodiment of the invention described in claim 4, and in the image forming apparatus according to claim 2 or 3, the image carrier potential detecting means 41 is on the image carrier 19. Since the potential of the image carrier immediately before being transferred to the recording medium 11 is detected, the detection result of the image carrier potential detecting unit immediately before the transfer by the transfer unit is used to minimize the variation in the image carrier potential detection value due to the dark decay of the image carrier. You can keep it to the limit.

This embodiment is an embodiment of the invention described in claim 5, and in the image forming apparatus according to claim 3, the recording medium potential detecting means 42 is an image carrier on the recording medium 11. Since the potential immediately after transfer from 19 is detected,
By using the detection result of the recording medium potential detecting means, it is possible to minimize the variation of the recording medium potential detection value due to the attenuation of the charge of the recording medium.

Further, the present embodiment is an embodiment of the invention described in claim 6, and in the image forming apparatus according to claim 3, the detection result of the recording medium potential detection means 42 and the image density detection means. Since the process control section 33 as a means for judging the developing ability of the developing means 21 is provided based on the charging potential vs. image density characteristic obtained from the detection result of 43, a dedicated detecting means for detecting the developing ability of the developing means is provided. It is possible to determine the life of the developer without the need.

This embodiment is an embodiment of the invention described in claim 7, and in the image forming apparatus according to claim 6, the recording medium potential detecting means 42 is the developing means 21 on the recording medium 11. Because it detects the potential just before development by
By using the detection result of the recording medium potential detecting means, it is possible to minimize the variation of the recording medium potential detection value due to the attenuation of the charge of the recording medium.

Further, the present embodiment is an embodiment of the invention described in claim 8, and in the image forming apparatus according to claim 1, the recording medium 11 of the recording medium potential detecting means 42.
The detection result of the potential immediately after the transfer from the upper image carrier 19, and
Since the process control unit 33 is provided as means for detecting an electrostatic defect of the recording medium 11 based on the detection result of the potential immediately before the development by the developing unit 21 on the recording medium 11, the latent image potential of the recording medium is It is possible to detect an abnormal decrease, and to prevent waste of output time by illegally outputting an image having poor image quality. The present invention is not limited to the above-described embodiment, but can be similarly applied to an image forming apparatus such as a copying machine or a facsimile.

[0082]

As described above, according to the first aspect of the invention, recording medium conveying means for holding and conveying a recording medium,
An image carrier, charging means for charging the image carrier to a predetermined potential, exposing means for exposing the image carrier by the charging means to form an electrostatic latent image, and the image carrier Voltage applying means for applying a predetermined voltage between the body and the recording medium on the recording medium conveying means to transfer the electrostatic latent image on the image carrier to the recording medium on the recording medium conveying means, In an image forming apparatus having a developing unit for developing an electrostatic latent image on the recording medium to form a visible image, an image carrier potential detecting unit for detecting a potential on the image carrier, and an image carrier on the recording medium. Since the recording medium potential detecting means for detecting the potential and the image density detecting means for detecting the optical density of the visible image on the recording medium are provided, the charging means, the image in the image forming step using the latent image transfer method, Transfer hand that transfers the electrostatic latent image on the carrier to the recording medium The characteristics of each developing means can be individually grasped, and it is possible to perform advanced process control by appropriate feedback control, without being affected by changes with time of the image carrier or environmental changes. It is possible to maintain high-quality image output.

According to a second aspect of the present invention, in the image forming apparatus according to the first aspect, the charging potential of the charging means and the applied voltage of the voltage applying means are based on the detection result of the image carrier potential detecting means. And the control means for controlling the light output of the exposure means, the deterioration of dark decay characteristics due to light fatigue and the like of the image carrier and the change of the charging potential of the image carrier at the time of transfer due to the deterioration of photosensitivity. Can be prevented.

According to the invention of claim 3, in the image forming apparatus of claim 1, the latent image transfer obtained from the detection result of the image carrier potential detecting means and the detection result of the recording medium potential detecting means. According to the characteristics, the charging means has a charging potential, the voltage applied by the voltage applying means, and the control means for controlling the light output of the exposing means. It is possible to prevent the electrostatic latent image potential of the recording medium from changing at the time of transfer due to the change in the electrostatic transfer characteristic caused by the difference.

According to a fourth aspect of the invention, in the image forming apparatus according to the second or third aspect, the image carrier potential detecting means detects a potential on the image carrier immediately before transfer to the recording medium. Therefore, it is possible to minimize the variation in the detected value of the image carrier potential due to the dark decay of the image carrier by using the detection result of the image carrier potential detection unit immediately before the transfer by the transfer unit.

According to the fifth aspect of the invention, in the image forming apparatus according to the third aspect, the recording medium potential detecting means detects the potential immediately after the transfer from the image carrier on the recording medium. By using the detection result of the recording medium potential detecting means, it is possible to minimize the variation of the recording medium potential detection value due to the attenuation of the charge of the recording medium.

According to a sixth aspect of the invention, in the image forming apparatus according to the third aspect, the charging potential versus the image density obtained from the detection result of the recording medium potential detecting means and the detection result of the image density detecting means. Since the means for determining the developing ability of the developing means is provided according to the characteristics, the life of the developing solution can be determined without providing a dedicated detecting means for detecting the developing ability of the developing means.

According to a seventh aspect of the invention, in the image forming apparatus according to the sixth aspect, the recording medium potential detecting means detects the potential on the recording medium immediately before the development by the developing means. By using the detection result of the potential detecting means, it is possible to minimize the variation of the recording medium potential detection value due to the attenuation of the charge of the recording medium.

According to the eighth aspect of the invention, in the image forming apparatus according to the first aspect, the detection result of the potential of the recording medium potential detecting means immediately after the transfer from the image carrier on the recording medium is detected. Since the means for detecting the electrostatic defect of the recording medium from the detection of the potential of the recording medium immediately before the development by the developing means is provided, the abnormal decrease of the latent image potential of the recording medium is detected. Therefore, it is possible to prevent the image output with poor image quality from being fraudulent and reduce the waste of output time.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing components related to process control of a process control unit in an embodiment of the present invention.

FIG. 2 is a schematic view showing a photoconductor potential sensor according to the embodiment.

FIG. 3 is a characteristic curve diagram showing characteristics of an image forming process in the embodiment.

FIG. 4 is a characteristic curve diagram showing a dark attenuation characteristic of the photosensitive drum in the embodiment.

FIG. 5 is a characteristic curve diagram showing the relationship between the semiconductor laser power and the photosensitive drum surface potential in the exposure apparatus of the embodiment.

FIG. 6 is a characteristic curve diagram showing a relationship between Vst and Vp in the example of the embodiment.

FIG. 7 is a first flowchart showing a correction process for each variation of the process control unit in the embodiment.

FIG. 8 is a flowchart showing in detail the correction 1 routine of FIG.

9 is a flow chart showing the correction 2 routine of FIG. 7 in detail.

FIG. 10 is a flowchart showing in detail the correction 3 routine of FIG.

FIG. 11 is a flowchart showing in detail the correction 4 routine of FIG.

FIG. 12 is a flowchart showing in detail the correction 5 routine of FIG.

FIG. 13 is a second flowchart showing a correction process for each variation of the process control unit in the embodiment.

FIG. 14 is a sectional view showing the outline of the embodiment.

FIG. 15 is a block diagram showing a control system of the embodiment.

[Explanation of symbols] 11 Recording paper 16 clamp drum 19 photoconductor drum 20 Exposure equipment 21 Development device 22 Charged Scorotron 33 Process control unit 32 LD control unit 41 Photoconductor potential sensor 42 Recording paper potential sensor 43 Image density sensor 45 grid bias power supply 46 Transfer bias power supply

Front page continued (58) Fields surveyed (Int.Cl. ⁷ , DB name) G03G 15/18 G03G 15/00 303 G03G 15/22 G03G 15/24

Claims (8)

(57) [Claims] 1. A recording medium carrying means for holding and carrying a recording medium, an image carrier, a charging means for charging the image carrier to a predetermined potential, and a charging means for the image carrier. An electrostatic latent image on the image carrier is formed by applying a predetermined voltage between the exposure unit that forms an electrostatic latent image by exposing after charging and the image carrier and the recording medium on the recording medium conveying unit. An image forming apparatus comprising: a voltage applying unit that transfers an image onto a recording medium on the recording medium conveying unit; and a developing unit that develops an electrostatic latent image on the recording medium into a visible image. An image carrier potential detecting means for detecting an upper potential, a recording medium potential detecting means for detecting a potential on the recording medium, and an image density detecting means for detecting an optical density of a visible image on the recording medium. An image forming apparatus characterized by being provided. 2. The image forming apparatus according to claim 1, wherein the charging potential of the charging means, the applied voltage of the voltage applying means, and the light output of the exposing means are determined based on the detection result of the image carrier potential detecting means. An image forming apparatus comprising a control means for controlling the image forming apparatus. 3. The image forming apparatus according to claim 1, wherein the charging of the charging means is carried out based on a detection result of the image carrier potential detection means and a latent image transfer characteristic obtained from the detection result of the recording medium potential detection means. An image forming apparatus comprising: a control unit that controls a potential, a voltage applied by the voltage applying unit, and a light output of the exposing unit. 4. The image forming apparatus according to claim 2, wherein the image carrier potential detecting means detects a potential on the image carrier immediately before transfer to the recording medium. apparatus. 5. The image forming apparatus according to claim 3, wherein the recording medium potential detecting means detects a potential immediately after transfer from the image carrier on the recording medium. 6. An image forming apparatus according to claim 3, wherein the developing means develops by the detection result of the recording medium potential detecting means and the charging potential vs. image density characteristic obtained from the detection result of the image density detecting means. An image forming apparatus comprising means for determining the ability. 7. The image forming apparatus according to claim 6, wherein the recording medium potential detecting means detects a potential on the recording medium immediately before development by the developing means. 8. The image forming apparatus according to claim 1, wherein the detection result of the potential of the recording medium potential detecting means immediately after the transfer from the image carrier on the recording medium, and the development on the recording medium. An image forming apparatus comprising means for detecting an electrostatic defect of the recording medium based on a detection result of the potential immediately before the development by the means.