JPH04163565A - Charge control unit for image forming device - Google Patents

Abstract

PURPOSE:To keep the extent of charged potential constant even if thickness of a sensitizer is varied, by changing the grid voltage of a scolotron charger according to a compensation value operated and controlling a charged current. CONSTITUTION:A charged current detecting means 31 detects a charged current flowing in space between a scolotron charger 9 and a sensitizer 7, while a comparison operational means 32 seeks a charged pressure variation value of the charged current, comparing it with the reference value, whereby a variation in the thickness of the sensitizer 7 is detected, operating the compensation value conformed to the thickness. Since a charged current control means 34 changes a grid voltage of the scolotron charger 9 and controls it so as to make the charged current conformed to the compensation value flow thereinto, the charged potential of the sensitizer 7 is kept constant. With this constitution, even if thickness of the sensitizer 7 is varied, the charged potential can be kept constant.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a charge control device for an image forming apparatus using an electrostatic latent image method, which is equipped with a negatively charged photoreceptor.

[Prior Art] An electrostatic latent image obtained by exposing a charged photoreceptor to light is visualized by development, and an image is formed by transferring the visualized image onto a transfer object, such as plain paper. Electrostatic latent image type image forming device, such as PPC copier or laser printer, L, ED (light emitting diode) printer, L
There are optical printers such as CDA (liquid crystal array) printers.

These photoreceptors include those based on inorganic semiconductors and those based on organic semiconductors (hereinafter referred to as "rOPC"), but recently, compared to inorganic semiconductors that contain heavy metals such as selenium, which may cause pollution, O20-based photoreceptors are used because they are non-polluting and have excellent manufacturing costs.

While most inorganic semiconductors are positively charged, O
Since most of the 20 series are negatively charged, they are charged by negative corona discharge, but as is well known, negative corona discharge is more unstable and causes more uneven discharge than positive corona discharge.

Therefore, as a charger for negatively charging a photoreceptor, a scorotron charger is used, which has a simple structure consisting of a discharge electrode and a shield case, and adds a control grid to stabilize the discharge.

Regarding charging control of a photoreceptor using a corotron charger, for example, as shown in Japanese Patent Laid-Open No. 54-5444, a light-dependent resistor is provided between a constant voltage high-voltage power source and the charger, and the photoreceptor drum ( A proposal to control the charging current by changing the resistance value by changing the amount of light irradiating the light-dependent resistor according to the charging current flowing through the plate, or as shown in Japanese Patent Application Laid-Open No. 57-40364,
There have been proposals to provide a current control means for detecting and controlling the charging current, and all of these methods control the charging current flowing through the photoreceptor to a constant level.

On the other hand, a scorotron charger has a self-current control function that, in principle, equalizes the (surface) charging potential of a photoconductor placed in close proximity to the bias voltage applied to the grid, so the discharge It has the advantage of being not only stable but also easy to control the charging potential.

Therefore, charging control using a scorotron charger involves controlling the bias voltage applied to the grid to be constant.

[Problems to be Solved by the Invention] However, PC photoreceptors are softer than inorganic semiconductor-based photoreceptors, and have the disadvantage that their thickness changes due to wear as they are used.

If the inductivity of vacuum is ευ, and the constants of the photoreceptor are dielectric constant ε, thickness d4 area S, capacitance C9 charge Q, charging voltage V, charging current i, and charging time t, then C=ευ×εX S/d Q=CXV= i X t , “, V=ixt/C = (ixtxd)/ (ε υ × ε × S
) Among these, the dielectric constant ευ and the ε9 area S are constant,
In general, the charging time determined by the photoconductor passing speed and the width of the charger is constant, so if the coefficient is k (constant), then V = kXiXd, and the charging voltage V is proportional to the charging current l and the thickness d. do.

Therefore, when the charging current 1 is controlled to be constant, the charging voltage decreases as the thickness d decreases over time.

FIG. 4 is a diagram showing an example of a change in the charging potential with respect to the number of sheets used by the copying machine, and it can be seen that the charging potential begins to decrease when the number of sheets exceeds 30,000.

FIG. 5 is a diagram showing an example of a change in charging potential depending on the thickness d of a photoreceptor when charging is performed by constant current control for a corotron charger and a scorotron charger.

In this way, compared to a corotron charger, although a scorotron charger with a self-current control function has a smaller change in charging potential due to the thickness d, this does not mean that there is no change in actuality, so it is shown in Figure 4. This causes a decrease in the charging potential, resulting in problems such as a decrease in image density.

If we shorten the charging time or significantly increase the charging current 1, we may be able to obtain charging potential characteristics that are equal to the original grid voltage of the scorotron, but high speed is required and the size of the charger is also limited. In some image forming apparatuses, the charging time t cannot be extended very much.

Furthermore, if the charging current 1 is increased, the discharge current also increases, and an increase in ozone associated with corona discharge is unavoidable, causing unpleasant ozone odor and health problems for workers.

There would be no problem if the charging potential itself could be detected and the voltage and current could be feedback-controlled, but even if it is possible to detect electrostatic potentials such as charging potential in experiments, it is not possible to detect the charging potential itself, and it is not possible to detect electrostatic potentials such as the charging potential itself. It is almost impossible to detect the charged potential in an image forming apparatus in which developing units and the like are disposed closely together.

The present invention has been made in view of the above points, and an object of the present invention is to provide a charge control device that maintains a constant charging potential even if the thickness of a photoreceptor changes.

[Means for Solving the Problems] In order to achieve the above object, the present invention develops an electrostatic latent image obtained by exposing a negatively charged photoreceptor, and develops the developed image. A charging control device for an image forming apparatus that forms an image by transferring an image onto a transfer target includes a scorotron charger that negatively charges a photoreceptor and a charging current flowing to the photoreceptor by the scorotron charger. a charging current detection means for detecting a charging current, a comparison calculation means for calculating a correction amount by comparing a voltage variation value of the charging current detected by the charging current detection means with a reference value set in advance; A charging current control means for controlling the charging current by changing the grid voltage of the scorotron charger according to the calculated correction amount is provided to keep the charging potential of the photoreceptor constant.

[Function] In the charging control device configured as described above, the charging current detection means detects the charging current flowing between the scorotron charger and the photoreceptor, and the comparison calculation means calculates the variation value of the charging current with respect to voltage. A change in the thickness of the photoreceptor is detected by determining and comparing it with the reference value, and a correction amount corresponding to the change in thickness is calculated.

The charging current control means controls the charging current by changing the grid voltage of the scorotron charger so that a charging current according to the correction amount flows, so that the charging potential of the photoreceptor is kept constant.

[Example] Hereinafter, an example of the present invention will be specifically described with reference to the drawings.

FIG. 2 is a schematic cross-sectional view showing an example of the mechanism of a copying machine which is an image forming apparatus equipped with a charge control device according to the present invention.

This copying machine 1 includes a copying optical system for copying a document 2, an image forming system for developing an electrostatic latent image formed by exposure, and transferring it onto paper (plain paper) as a transfer target. It is composed of a paper transport system that transports the paper, fixes the image transferred thereon, and then discharges the paper.

The copying optical system includes a scanning illumination system 3 that is integrally constructed with a lamp 3a that illuminates the original 2 and a first mirror 3b, and a second mirror 4a and a third mirror '4b whose optical axes are orthogonal to each other. A scanning optical system 4 configured as shown in FIG. Become.

The image forming system is ○PC, which is a photoreceptor whose surface is negatively charged.
A ○PC drum 8 covered by a ○PC drum 8 on which an electrostatic latent image is formed by exposure, and a scorotron charger 9 disposed around the ○PC drum 8 in a clockwise direction. Developing unit 1
0. Transfer charger 11, separation charger 12. It consists of a cleaning unit 13 and a static elimination lamp 14.

The paper conveyance system includes a paper feed tray 18 on which paper 17 is placed,
Paper feed roller 19. Conveyance path 20. Registration roller pair 2
1. Conveyor belt 22. It is composed of a conveyance mechanism including a paper ejection roller 23, a fixing unit 24 that fixes an image on the paper, and a paper ejection tray 25 from which the paper 17 that has been fixed is ejected.

When the copying operation starts, the PC drum 8 rotates clockwise, the Scorotron charger 9 starts discharging, and the lamp 3a lights up.

The negatively charged part of the surface of the OPC drum 8 is the exposure position 8.
When reaching point a, the scanning illumination system 3 moves to the right at the same speed as the circumferential speed of the ○Pc drum 8, and the scanning optical system 4 moves to the right at half the speed to scan the surface of the document 2, and the lens 5, the image of the document is exposed on the PC drum 8 to form an electrostatic latent image, and then toner is attached to the electrostatic latent image by the developing unit 10, and the electrostatic latent image is converted into a visible toner image.

On the other hand, the paper 17 on the paper feed tray 18 is taken out by the paper feed roller 19, passes through the conveyance path 2°, comes into contact with a pair of registration rollers 21, and is temporarily stopped. It is sent out in sync with the

When the paper 17 overlaps the toner image on the PC drum 8, the transfer charger 1 consisting of a corotron charger is activated.
1. The toner image is transferred onto the paper 17 by the separation charger 12, and the paper 17 separated from the PC drum 8 is transported to the fixing unit 24 by the transport belt 22 with the toner image placed thereon.

Paper 17 on which the toner image is fixed by the fixing unit 24
is discharged onto the paper discharge tray 25 by the paper discharge roller 23.

A small amount of toner remaining on the PC drum 8 after the transfer is collected by the cleaning unit 13,
The charge is extinguished by the irradiation of the discharge lamp 14, and a cycle begins with the next charge.

FIG. 1 is a block diagram showing an embodiment of a charging control device according to the present invention.

This charging control device 30 includes a discharge electrode 9a, a grid 9b
, a scorotron charger 9 consisting of a case 9c, a resistor 31 which is a charging current detection means, and a microcomputer (hereinafter referred to as rCPU) which is a comparison calculation means and has a built-in comparison calculation unit (not shown), an A/D converter, a memory, etc.
32, a corona power source 33 consisting of a constant current high voltage power source, and a grid power source 34 which is a charging current control means and consisting of a variable voltage high voltage power source.

The CPU 32 also performs sequence control of each section by outputting timing signals and control signals as needed.

Corona power supply 33. The grid power sources 34 each have DC2
It consists of a DC-DC converter that converts DC power from a 4V drive power source (not shown) into negative high-voltage power and outputs it, and the corona power source 33
outputs constant current high voltage power to the discharge electrode 9a of the scorotron charger 9, and the grid power supply 34 outputs high voltage power at a voltage corresponding to the correction amount input from the CPU 32 to the grid 9b and case 9c, respectively.

The resistor 31 is installed between the PC drum 8 and the ground,
The charging current that flows between the scorotron charger 9 and the ground due to corona discharge through the PC drum 8 and the OPC 7 on its surface is converted into an analog voltage signal and sent to the CPU 3.
Output to 2.

The CPU 32 outputs a correction amount for controlling the grid voltage to the grid power supply 34 based on the charging current value inputted as an analog voltage signal, but this control does not need to be performed every time, for example, after replacing the OPC drum 8 with a new one. It is sufficient to record the number of copies and perform this in the initial setting routine when the power is turned on next time the number of copies exceeds 500.

The oPC drum 8 during execution includes not only the scorotron charger 9 but also a transfer charger 119 and a separation charger 12.
Since the current flows in a superimposed manner, the grid voltage control rotates the OPC drum 8, turns on the static elimination lamp 14, and
This is done with other chargers etc. stopped.

In Figure 3, the right side of the vertical axis showing the charging current (μA) is the grid voltage VG (V) and the charging current, and the left side is the charging current and (O
These are diagrams showing an example of the relationship characteristics with the charging potential SV (V) of the PC, and the diagonal line A indicates the O
PC (A), diagonal line B is OP whose thickness has decreased due to considerable use
C (B) are shown respectively.

Since it is a negative corona discharge, the current flows from the ground toward the scorotron charger 9, and the detected charging current, voltage, and potential are also negative, but there are also difference and comparison calculations below, so for easy understanding, Both FIG. 3 and FIG. 3 will be explained using absolute values.

Grid voltage control is performed as follows.

At the time of factory adjustment, set the grid voltage VG in advance so that the charging potential Sv of the new OPC (A) is 800V, set it as Va (e.g. 800V), and set the charging current at that time as AO (e.g. 60μA). do.

Next, set the grid voltage VC to a voltage vb (for example, 500 V) lower than Va, and set the charging current at that time to BO (
For example, 40 μA).

Calculate the current difference (AO-BO) and Do (20μA
), and with this value Do as the reference value from then on, the voltage V
It is stored in non-volatile memory along with a and Vb.

Grid voltage control after installation on the user side first detects the charging currents Al and Bl when the grid voltage VG is set to Va, V)), calculates the difference D1, and calculates the calculated value D]. It is compared with the stored base value Do.

If the thickness of 0PC7 does not change, its capacitance C also does not change, so Dl=DO or DI-DO.

Di) If it is Do, it can be determined that the OPC thickness d has decreased, but in the case of 2, the amount of decrease is not a problem and SVI
Since the purpose is to return the charging potential SV that has fallen to the initial SVO, calculate I c = A 1 + (D I - Do) and adjust the grid so that the charging current becomes Ic. The voltage VG is controlled, the voltage Vc at that time is stored, and the subsequent grid voltage VG is set to Vc.

That is, in the example shown in FIG. 3, the initial charging potential SV
Va=800■ so that 5V1=800■ and lower Vb=500V are set, and the charging current at that time is l\0-60μA, respectively.

Since B○=4071A, the reference value DO=20μA
It was set to .

When controlling the grid voltage by OPC (B) whose thickness has decreased during use, the grid voltage VG is set to ~.

The charging current when set to a and Vb is A1 = 75, respectively.
μA, Bl- = 50 μA were detected, so D1 = AI
-81=25μA, which is the standard value.

(20 μA).

Therefore, the charging current is I c=AI+ (Dl −D
O) = 75μA +5μA = 80/, by controlling the grid voltage VG so that LA becomes VC = 860V,
The grid voltage VG thereafter is set to 860V.

When the grid voltage VG is set to 800V (Va) and a charging current of 75 μA (AI) flows, the charging potential Sv is:
(Although it cannot be detected except at the time of shipping adjustment when a measuring device can be used) It had fallen to about SV1#740V.

Therefore, by controlling the grid voltage VG to 860V (Vc) so that a charging current of 80 μA (Ic) flows, the charging potential SV is set to 800V (SVO) even though the thickness d of 0PC7 has already decreased. I was able to get him back.

As explained above, by detecting the charging current and controlling the grid voltage of the scorotron charger according to the correction amount obtained by performing comparison calculations, it is possible to prevent the charging potential from decreasing due to a decrease in the thickness of the OPC. As a result, the effective service life of the ○PC drum can be extended until the grid voltage becomes too high or the OPC thickness decreases too much, resulting in image unevenness.

In addition, even if the charging current increases by controlling the grid voltage, the corona discharge current does not increase because the corona power source of the discharge electrode is a constant current and the potential of the case is set equal to the grid voltage. Therefore, the amount of ozone generated does not change, so it is possible to prevent unpleasant ozone odor and health problems for workers, and it also prevents excessive ozone from adsorbing to the 020 surface and causing image blurring due to its hygroscopic effect. It can be prevented.

[Effects of the Invention] As explained above, the charge control device according to the present invention has the following effects:
Even if the thickness of the photoreceptor changes, the charging potential can be kept constant.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an embodiment of the charging control device according to the present invention, FIG. 2 is a schematic sectional view showing an example of the mechanism of the copying machine, and FIG. 3 is the grid voltage, charging current, and charging Figure 4 is a diagram showing an example of potential characteristics. Figure 4 is a diagram showing an example of the number of sheets used and charging potential change characteristics using a conventional charge control device. Figure 5 is a diagram showing photoreceptor thickness and charging due to constant current control. FIG. 3 is a diagram showing an example of the relationship with electric potential. 1... Copying machine (image forming device) 7... OPC (organic semiconductor, negatively charged photoreceptor) 8
...OPC drum 9...Scorotron charger 9a...Discharge electrode 9b...Grid 30...Charging control device 31...Resistor (charging current detection means) 32...CPU (microcomputer, comparison calculation Means) 33...Corona power supply 34...Grid power supply (charging current control means) Applicant
Ricoh Co., Ltd. - Agent Patent Attorney Takashi Inuzawa. Figure 1 Figure 2

Claims (1)

[Scope of Claims] 1. An electrostatic latent image obtained by exposing a negatively charged photoreceptor to light is visualized by development, and the visualized image is transferred onto a transfer target to create an image. A charging control device for an image forming apparatus includes: a scorotron charger for negatively charging the photoreceptor; a charging current detection means for detecting a charging current flowing to the photoreceptor by the scorotron charger; a comparison calculation means for calculating a correction amount by comparing the voltage variation value of the charging current detected by the detection means with a reference value set in advance; A charging control device for an image forming apparatus, characterized in that a charging current control means for controlling the charging current by changing the grid voltage of a rotron charger is provided, and the charging potential of the photoreceptor is kept constant. .