JPH04309977A - Image forming device - Google Patents

Abstract

PURPOSE:To always obtain an excellent image by accurately detecting the change of sensitivity characteristic caused by using a photosensitive body and feeding back the detected value to the control of forming the image in a toner image transfer type image forming device. CONSTITUTION:The surface of the photosensitive drum which is electrostatically charged to be specified potential Vo by an electrostatic charger is exposed with light quantities of E1 and E2 by an exposing lamp. The exposed part is developed to form two test toner images. The density of the test toner image is detected by a photosensor and the sensitivity characteristic alpha is calculated in accordance with the detected values Mo1 and Mo2', and further the quantity of exposing light Eopt' at the time of copying processing is calculated.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a toner image transfer type image forming apparatus using electrophotographic copying, and more particularly to an automatic image density control mechanism thereof.

0002

2. Description of the Related Art In recent years, organic photoreceptors have been mainly used in toner image transfer type electrophotographic copying machines. Some types of organic photoreceptors have a characteristic that their sensitivity to light deteriorates as the number of times they are used increases. The deterioration of the sensitivity characteristics results in an increase in the potential of the background portion of the image, resulting in frequent occurrence of so-called toner fog and a decrease in the contrast of the image.

0003

[Object, structure, and operation of the invention] Therefore, the object of the present invention is to
In a toner image transfer type image forming apparatus, by accurately detecting changes in sensitivity characteristics due to the use of a photoreceptor and feeding back the detected value to control during image formation, it is possible to always obtain copied images of good image quality. It is in. In order to achieve the above object, an image forming apparatus according to the present invention includes a charging means for charging the surface of a photoreceptor to a predetermined potential, and a surface of the photoreceptor charged to a predetermined potential by the charging means. At least two test toner images having different densities are formed on the surface of the photoreceptor exposed by the exposure means while applying a developing bias of a predetermined voltage to an exposure means for exposing with at least two different amounts of light and a developing electrode. The image forming apparatus includes a developing means, a density detecting means for detecting the density of the test toner image, and a light amount controlling means for controlling the amount of exposure light during image formation according to the detected density value of the toner image.

In the above configuration, the photoreceptor is charged to a predetermined potential Vo by the charging means and exposed to light by the exposure means. This exposure is predetermined at least E1,
This is performed using two types of light amounts E2, and the potential of the photoreceptor surface is lowered to a level corresponding to the light amounts E1 and E2. Toner is adhered to this potential portion while applying a developing bias of a predetermined voltage Vb by a developing device. In this way, at least two test toner images having different densities are formed, and their densities are detected by a detection means (for example, a photosensor). The density of the test toner image is a constant value depending on the sensitivity characteristics of the photoreceptor, and the sensitivity characteristics are calculated based on the detected density value. Based on this calculation result, the amount of exposure light during image formation is controlled. That is, the amount of exposure light is increased in accordance with the deterioration of the sensitivity characteristics of the photoreceptor, thereby preventing toner fogging on the background portion of the image and deterioration of the contrast of the image.

[0005]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of an image forming apparatus according to the present invention will be described below with reference to the accompanying drawings. In this embodiment, the present invention is applied to a toner image transfer type electrophotographic copying machine. (Schematic configuration and operation of copying machine) As shown in FIG. The image is exposed to the exposure portion X by the optical system 20, and an electrostatic latent image corresponding to the original image is formed on the photoreceptor drum 5.

The optical system 20 includes an exposure lamp 21 and a mirror 22.
It is composed of a to 22d and a projection lens 23. During image exposure, the exposure lamp 21 and mirror 22a move in the direction of arrow b at a speed of v/m (m: copying magnification), and the mirrors 22b and 22c can move at a speed of v/2m. Around the photosensitive drum 5, there is a charger 6 that uniformly charges the surface to a predetermined potential, an interimage eraser 10 that erases interimage charges in areas other than the latent image forming area, and static electricity formed by an optical system 20. A developing device 7 attaches toner to the electrostatic latent image and visualizes it, and the toner image is transferred to the registration roller 3.
A transfer charger 28 that transfers the image onto the copy paper P fed from 0, a separation charger 29 that separates the copy paper P from the photoreceptor drum 5, a cleaner device 9 that removes residual toner adhering to the photoreceptor drum 5, and a photoreceptor drum. A main eraser 8 for erasing residual charges on the 5 is provided.

The charger 6 uses a scorotron system equipped with a mesh-like grid, has stable discharge characteristics according to the grid voltage, and uniformly charges the surface of the photoreceptor drum 5 to a predetermined potential. . Further, the exposure lamp 21 of the optical system 20 can adjust the amount of exposure light by controlling the applied voltage, and the adjustment method will be described later.

(Configuration and operation of developing device) The developing device 7 is
Development is carried out by so-called regular development, in which toner is attached to the charged portions of the latent image passing through the development section Y using a well-known magnetic brush method using a developer made of a mixture of a magnetic carrier and an insulating toner. . Inside the developing tank 70, a developing sleeve 71 containing a magnetic roller 72 and a height regulating plate 73 are provided.
, a bucket roller 74, and a screw roller 75 are installed. The developer is attracted to the outer circumferential surface of the developing sleeve 71 by the magnetic force of the magnetic roller 72 based on the rotation of the bucket roller 74 in the direction of the arrow c, and the developer is attracted to the outer peripheral surface of the developing sleeve 71 by the magnetic force of the magnetic roller 72 based on the rotation of the developing sleeve 71 in the direction of the arrow d or the direction of the arrow d' of the magnetic roller 72. It is transported to the developing section Y based on the rotation. Developing sleeve 7
Reference numeral 1 functions as a developing electrode, to which a developing bias of a predetermined voltage having a polarity opposite to the charged polarity of the toner is applied to prevent the toner from covering the background portion of the image.

On the other hand, a toner replenishment roller 77 that is rotationally driven by a replenishment motor 78 is installed at the bottom of a toner tank 76 provided above the developer tank 70 . The toner in the toner tank 76 is replenished onto the screw roller 75 based on the rotation of the replenishment roller 77 , and is agitated and mixed with the already existing developer by the rotation of the roller 75 .
Sent to 4. During stirring and mixing, the toner is triboelectrically charged with the magnetic carrier to a predetermined amount of charge. The toner concentration in the developer is detected by a magnetic sensor (not shown) provided in the developer tank 70, and the replenishment motor 78 is controlled to maintain a constant toner concentration at all times.

(Deterioration of sensitivity characteristics of photoreceptor, detection thereof, and control of exposure light amount) The photoreceptor used in this copying machine is an organic photoreceptor, and one of its characteristics is that the sensitivity characteristics deteriorate with use. deteriorates. FIG. 5 shows the surface potential V and toner adhesion amount M of the photoreceptor with respect to the exposure light amount E, where the solid line A shows the initial sensitivity characteristic and the dotted line B shows the sensitivity characteristic when the photoreceptor has been used for about half its life. Further, the solid line C indicates the development characteristic indicating the toner adhesion amount M with respect to the surface potential V when the sensitivity characteristic is A, and the dotted line D indicates the development characteristic when the sensitivity characteristic is B. Figure 5
As is clear from the figure, the residual potential is approximately constant at Vr both at the initial stage and after use, but the sensitivity characteristics change from the solid line A to the dotted line B with use, and at the same time the development characteristics change from the solid line C.
to dotted line D.

That is, in the initial stage, when exposed with a relatively large amount of light E2 (close to the background portion of the image), the photoreceptor surface potential is Vs2, and the amount of toner adhering to the potential Vs2 portion is Mo2. Further, when exposed with a relatively small amount of light E1 (corresponding to the image area), the photoreceptor surface potential becomes Vs1, and the amount of toner adhering to the potential Vs1 portion is Mo1. On the other hand, after use, the toner adhesion amount increases to Mo2' under the exposure light amount E2, and decreases to Mo1' under the exposure light amount E1. In other words, toner fogging occurs on the background portion of the image, resulting in a reduction in the contrast of the copied image. Therefore, after use, it is necessary to increase the amount of light from the exposure lamp 21 to eliminate such problems.

The sensitivity characteristics and development characteristics of the photoreceptor have a certain correlation as shown in FIG. Therefore, in this example,
For image stabilization, the surface of the photosensitive drum 5, which has been charged to a predetermined potential Vo by the charger 6, is connected to an optical system 20.
Exposure is performed with light quantities E1 and E2 to form test latent images with two different potentials. Then, a developing bias of a predetermined voltage Vb is applied to this test latent image to the developing sleeve 71 to cause toner to adhere to the test latent image, thereby forming two test toner images. Furthermore, the density of this test toner image is optically detected by a reflective photosensor 19 (see FIG. 1).

The photoreceptor surface potential V has a relationship between the exposure light amount E and the charging potential Vo by the charger 6 as shown in equation (1). V=-αE+Vo
...(1)
α: Photoreceptor sensitivity characteristic Further, the toner adhesion amount M has a relationship between the post-exposure surface potential Vs and the developing bias voltage Vb as shown in equation (2).

M=(Vs−Vb)γ
...(2) On the other hand, the surface potentials Vs1, V with respect to the exposure light amounts E1, E2
s2 has the relationship shown in equations (3) and (4). Vs1=-αE1+Vo
...(3)
Vs2=-αE2+Vo
...(4) Also, for the surface potentials Vs1, Vs2, the toner adhesion amounts Mo1, M
o2 has the relationship shown in equations (5) and (6).

Mo1=(Vs1−Vb)γ
…(5) M
o2=(Vs2-Vb)γ
...(6) Formulas (3) and (4) above
When the sensitivity characteristic α and the development characteristic γ are determined from equations (5) and (6), the following equations (7) and (8) hold true. α=(Vo-Vb)(Mo1-Mo2)/(M
o1・E2−Mo2・E1)

…(7)
γ=(Mo1・E2−Mo2・E1)/(Vo−Vb)
(E2-E1)

(8) As a result of the above, when the toner adhesion amounts Mo1 and Mo2 are detected, the initial sensitivity characteristic α and development characteristic γ are calculated using equations (7) and (8). Similarly, by detecting the toner adhesion amounts Mo1' and Mo2', the current sensitivity characteristic α' and development characteristic γ' can be calculated.

α'=(Vo-Vb)(Mo1'-Mo2')
/(Mo1'・E2-Mo2'・
E1)
...(7') Therefore, the sensitivity characteristic is α
In a state exhibiting the characteristic ', the amount of light Eopt' to be exposed during copying processing is calculated using equation (9), where Eopt is the initial standard exposure light amount.

Eopt'=α'・Eopt/α
...(9) Furthermore, the formula (
9), a constant k is introduced as necessary. Incidentally, Eopt and α are values set for each product, and are stored in advance in a storage means such as RAM of the main body control section at the time of factory shipment. In particular, since the photoreceptor sensitivity characteristic α differs depending on the type of photoreceptor used or manufacturing lot, it is necessary to measure it in advance by a known method.

If the exposure light amount Eopt' calculated here is used as the exposure light amount of the exposure lamp 21 in the subsequent copying process, a copied image with good contrast and no toner fog in the image background can be obtained. In addition, when performing such image stabilization processing, ■Charging potential Vo
(2) The surface potential Vs after exposure has a constant relationship with the exposure light amount E, (2) The residual potential Vr of the photoreceptor remains approximately constant over time, (2) The toner adhesion amount M is Vs-
It is preferable that it has a certain relationship with Vb. In this embodiment, the condition (1) is stabilized by using a scorotron charger as the charger 6, and the conditions (2) and (2) can be satisfied in view of the characteristics of the photoreceptor. Furthermore, the condition (2) is such that an ordinary copying machine has sufficiently satisfactory performance. Development characteristics are affected by variations in toner charge amount and toner density, but by forming test toner images with at least two different densities and detecting the densities (adhesion amounts), accurate photoconductor sensitivity characteristics can be determined. can be found.

(Specific Example of Image Stabilization Processing) Formation of a test toner image, detection of its density, and control of exposure light amount will now be explained in detail. As shown in FIG. 2, when exposed with a relatively small amount of light E1, a relatively dark test toner image T1 is formed, and when exposed with a relatively large amount of light E2, a relatively thin test toner image T2 is formed. These concentrations are detected by a reflective photosensor 19. The photosensor 19 includes a light emitting element 19a and a light receiving element 19b, and is installed at a position facing the surface of the photosensitive drum 5 from the transfer section Z to the cleaner device 9 and corresponding to the test toner images T1 and T2. ing. Therefore, this photosensor 19 can detect the density of the test toner images T1 and T2. If it is dark, the amount of reflected light is small, and the output voltage Vs from the light receiving element 19b becomes low. The thinner it is, the more reflected light it is.
The output voltage Vs from the light receiving element 19b becomes higher.

As shown in FIG. 3, the light emitting element 19a is grounded via the switch SW11 and the variable resistor 19c, and the output voltage Vs of the light receiving element 19b is grounded via the microcomputer 2.
The signal is input to the A/D port of 01 (hereinafter referred to as CPU). The input sensor output voltage Vs is converted into a digital signal by an A/D converter, stored in a battery-backed random access memory 202 (hereinafter referred to as RAM) via a data bus 203, and taken out as appropriate for data processing. be done.

An arithmetic circuit 210 for calculating the amount of exposure light and a dimming circuit 211 for adjusting the amount of light from the exposure lamp 21 are connected to the output port of the CPU 201. The light control circuit 211 uses the calculated exposure light amount Eo during copy processing.
pt', and during image stabilization processing, the exposure lamp 21 is caused to emit light so that predetermined exposure light amounts E1 and E2 are achieved. The dimming circuit 211 includes switches SW101 and SW102. When the switches SW101 and SW102 are turned on at the same time, the voltage applied to the exposure lamp 21 is controlled so that the amount of exposure light becomes E1, and a test toner image T1 is formed. When the switch SW101 is turned on alone, the voltage applied to the exposure lamp 21 is controlled so that the amount of exposure light becomes E2, and a test toner image T2 is formed.

Next, FIG. 4 shows the process of image stabilization processing.
This will be explained with reference to the time chart. The image stabilization process is first executed when the main switch is turned on and the copying machine is powered on. It is also executed every time a predetermined period of time elapses. When the main switch is turned on, warm-up is performed to start up the fixing device (not shown) for a predetermined time T0. Thereafter, the main motor is turned on and the photoreceptor drum 5 starts rotating, and the charger 6 is turned on, so that the surface of the photoreceptor drum 5 is charged to a predetermined surface potential Vo. Just before the charged portion reaches the exposure section X, switches SW101 and SW102 are turned on to expose the charged portion to light with an amount of light E1. Then switch SW10
2 is turned off, and the charged portion is exposed to light with a light amount E2. This test latent image is developed at a developing section Y under a predetermined developing bias voltage Vb, and test toner images T1 and T2 are formed.

Switch S for operating the photosensor 19
W11 indicates that the test toner images T1 and T2 are the photosensor 1.
It is turned on just before passing the detection point No. 9 to prepare for measuring the amount of reflected light. When no toner is attached, the light receiving element 19
The output voltage Vs of b is 12V in this embodiment. When the test toner images T1 and T2 pass through the detection point, the output voltage Vs of the light receiving element 19b is increased by 5 at a period of time t.
They are read twice and averaged to obtain the adhesion amounts Mo1' and Mo2'. Based on this detected value, the sensitivity characteristic α' is calculated using the above equation (7'), and furthermore, the exposure light amount Eopt' used during normal copying processing is calculated using the above equation (9).

(Control Procedure) Next, the control procedure performed by the CPU 201 regarding the image stabilization processing described above will be explained with reference to the flowcharts shown in FIGS. 6 and 7. Figure 6 shows CPU2
01 main routine is shown. The power is turned on and the CPU
When the device 201 is reset, the program starts, and in step S1 clears the RAM 202, initializes various registers, and performs initial settings to set each device in an initial mode. Subsequently, warm-up processing is performed in step S2, and image stabilization processing is performed in step S3. Details of the subroutine here will be explained with reference to FIG.

Next, in step S4, a timer T is set, and in step S5, input signals from various keys on an operation panel (not shown) of the copying machine are processed. In step S6, it is determined whether the print switch has been turned on, and if NO, the process returns to step S5. If YES, step S7
The copying operation is processed in step S8, and other processes such as temperature control of the fixing device are performed in step S8. Subsequently, in step S9, it is determined whether or not the timer T has reached a predetermined time T1, and N
If YES, the process returns to step S5, and if YES, the process returns to step S3 to perform image stabilization processing.

FIG. 7 shows a subroutine for image stabilization processing executed in step S3. First, step S1
1, test toner images T1 and T2 are formed in the above-mentioned process,
In step S12, the photosensor 19 detects toner concentration, that is, toner adhesion amounts Mo1' and Mo2'. Subsequently, in step S13, the sensitivity characteristic α' is calculated based on the above equation (7'), and in step S14, the exposure light amount Eopt' corresponding to the sensitivity characteristic α' is calculated based on the above equation (9). Then, in step S15, the exposure light amount used for subsequent copying processing is set to the calculated light amount Eopt'.

(Other Embodiments) The image forming apparatus according to the present invention is not limited to the embodiments described above, and can be modified in various ways within the scope of the gist. For example, in the embodiment described above, the image stabilization process is executed not only immediately after the power is turned on but also every time a predetermined time T1 elapses.
The process may be executed every time a predetermined number of copies are made or every time the photosensitive drum 5 rotates a predetermined number of times.

[0028]

As is clear from the above description, according to the present invention, as an image stabilization process, the charged portion of the photoreceptor is exposed to at least two different amounts of light, and this portion is developed. Since two test toner images are formed and the detected density values are fed back to the exposure light amount during image formation, there is no toner fog in the background of the image, regardless of the deterioration of sensitivity characteristics due to the use of the photoreceptor. Images with good contrast can be obtained.

[Brief explanation of the drawing]

The drawing shows an electrophotographic copying machine which is an embodiment of the present invention.

FIG. 1 is a schematic configuration diagram of an electrophotographic copying machine.

FIG. 2 is a chart showing the relationship between the amount of toner adhering to a test toner image and the sensor output voltage.

FIG. 3 is a block diagram of a control circuit.

FIG. 4 is a time chart diagram showing an image stabilization process.

FIG. 5 is a graph showing the surface potential of a photoreceptor and the amount of toner adhesion with respect to the amount of exposure light.

FIG. 6 is a flowchart showing the main routine of the CPU.

FIG. 7 is a flowchart showing a subroutine of image stabilization processing.

[Explanation of symbols]

5...Photoreceptor drum 6...Electrical charger 7...Developing device 19...Photo sensor 21...Exposure lamp 71...Developing sleeve 201...CPU 210... Arithmetic circuit 211...Dimmer circuit

Claims (1)

[Claims] 1. An electrostatic latent image is formed by an image exposure means on a photoreceptor that is uniformly charged to a predetermined potential by a charging means, and this electrostatic latent image is transferred as a toner image onto copy paper by a developing means. In the image forming apparatus, a charging means for charging a surface of a photoreceptor to a predetermined potential; an exposure means for exposing the surface of the photoreceptor charged to a predetermined potential by the charging means to at least two different amounts of light; a developing means for forming at least two test toner images with different densities on the surface of the photoreceptor exposed by the exposing means while applying a developing bias of a predetermined voltage to a developing electrode; and detecting the density of the test toner images. An image forming apparatus comprising: a density detecting means for detecting the density of the toner image; and a light amount controlling means for controlling the amount of exposure light during image formation according to the detected density value of the toner image.