Image forming apparatus

An image forming apparatus includes a microcomputer. The number of image forming operation of a photosensitive drum is counted by a counter as a copy quantity, and data of the copy quantity is applied to the microcomputer. By referring to a copy quantity-voltage table, voltage setting data according to the copy quantity is applied by the microcomputer to a bias voltage source so that the bias voltage source applies a required bias voltage according to the voltage setting data to a conductive substrate of the photosensitive drum.

BACKGROUND OF THE INVENTION 
1. Field of the invention 
The present invention relates to an image forming apparatus. More 
specifically, the present invention relates to an image forming apparatus 
such as a copying machine, printer, facsimile and etc. in which a 
photosensitive member is exposed with a light image to form an 
electrostatic latent image which is then developed by a toner. 
2. Description of the Prior Art 
In a kind of an electrophotographic apparatus, electrophotographic 
processes such as charging, exposure, developing, transferring, 
separating, cleaning, erasing, and etc. are repeated around a 
photosensitive drum. Not only does the sensitivity of the photosensitive 
drum become low but also a residual voltage (Vr) increases due to (1) 
fatigue of the photosensitive drum by an exposure light and an erasing 
light, (2) deterioration of a photosensitive layer by ozone which is 
generated in charging, transferring and separating, (3) decrease of the 
thickness of the photosensitive layer by friction with a cleaning blade, 
and etc. Resultingly, a voltage of a portion corresponding to a white 
ground of an original, that is, an exposure voltage (VL) which was 
approximately equal to zero in an initial state increases. Therefore, a 
so-called fogging phenomenon occurs, and accordingly, the life of the 
photosensitive member decreases. 
Therefore, in the past, it was known to use an electrophotographic 
apparatus in which a developing bias voltage (Vb) is increased for 
prolonging the life of the photosensitive member (Japanese Patent 
Application Laying-Open No. 50-46334), and an electrophotographic 
apparatus in which the amount of light from the exposure lamp is increased 
for prolonging the life of the photosensitive member. 
In the former apparatus, there was a problem that the image density is 
lowered because a voltage difference between a dark voltage (Vd) 
corresponding to a black portion of the original and the developing bias 
voltage (Vb) becomes small. On the other hand, in the latter apparatus, it 
was impossible to deal with the increase of the exposure voltage (VL) 
according to the increase of the residual voltage (Vr), and therefore, the 
latter apparatus had its limit. 
SUMMARY OF THE INVENTION 
Therefore, a principal object of the present invention is to provide a 
novel image forming apparatus. 
Another object of the present invention is to provide an image forming 
apparatus in which it is possible to prolong the life of a photosensitive 
member without the problem that the image density becomes thin. 
An image forming apparatus according to the present invention comprises a 
photosensitive member including a conductive substrate; exposing means for 
exposing the photosensitive member; detecting means for detecting the 
number of use times, that is, the image forming operations, of the 
photosensitive member; and changing means for changing an electric 
potential of the conductive substrate in response to an output of the 
detecting means. 
By utilizing the fact that an exposure voltage is increased with increase 
of a copy quantity, for example, a relationship between the copy quantity 
and an electric potential of the conductive substrate is stored in advance 
in the ROM of a microcomputer as a copy quantity-voltage table. The number 
of image forming operations of the photosensitive member, that is, the 
copy quantity is detected by the detecting means such as a counter, and 
voltage data corresponding to a detected copy quantity is read-out from 
the table, and in accordance with the voltage data, a bias voltage source 
is controlled by a microcomputer, for example. Therefore, the electric 
potential of the conductive substrate is changed according to the copy 
quantity, that is, the number of use times of the photosensitive member. 
Accordingly, the exposure voltage is suppressed from increasing. 
In accordance with the present invention, since the exposure voltage is 
prevented from increasing by changing the electric potential of the 
conductive substrate, it is possible to prolong the life of the 
photosensitive member without a problem that the image density is lowered. 
In addition, if an exposure light amount is also changed, an increasing 
range of a bias voltage which is applied to the conductive substrate can 
be made small, and therefore, it is possible to further prolong the life 
of the photosensitive member. 
The above described objects and other objects, features, aspects and 
advantages of the present invention will become more apparent from the 
following detailed description of the present invention when taken in 
conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, an electrophotographic apparatus 10 of this embodiment 
includes a negative polarity photosensitive drum 16 having a conductive 
substrate 12 made of a metal, for example, and a photosensitive layer 14 
which is made of polyvinyl carbazole, for example, and formed on a surface 
of the conductive substrate 12. However, instead of the photosensitive 
drum 16, another type of photosensitive member such as a photosensitive 
belt, photosensitive plate and etc. may be utilized. An output terminal 
(not shown) of a bias voltage source 20 which is controlled by a 
microcomputer 18 is connected to the conductive substrate 12. The 
microcomputer 18 receives as an input the output of a counter 22 which 
counts a copy quantity, that is, the number of use times of the 
photosensitive drum 16. The microcomputer 18 also controls an AVR 
(Automatic Voltage Regulator) 24 which functions as an electric power 
source for driving a light source 28 which exposes an original (not shown) 
put on an original table 26. A relationship between the copy quantity and 
a required bias voltage is stored in a ROM (not shown) of the 
microcomputer 18 as a copy quantity-voltage table. In addition, a 
relationship between the copy quantity and a required exposure light 
amount is also stored in the ROM as a copy quantity-light amount table. 
Arranged around the photosensitive drum 16, is a charger 30, an exposure 
unit (not shown), a developer 32, a transferer 34, a separating claw 36, a 
cleaning blade 38, an erasing lamp 40, and other necessary components. 
The charger 30 includes a mesh-like grid electrode 42 which is connected to 
ground through a varistor 44 which functions as a bi-directional constant 
voltage diode. The varistor voltage is set as -400 volts, and therefore, 
the surface voltage of the photosensitive drum 16 is controlled at -400 
volts. 
When an original image is irradiated onto the photosensitive drum 16 which 
has been charged by the charger 30 by the exposure unit (not shown) 
including a short focal distance lens array or the like, an electrostatic 
latent image is formed on the photosensitive drum 16 in accordance with a 
photosensitive characteristic of the photosensitive drum 16. As well 
known, the developer 32 includes a toner box, a magnetic brush, and etc. 
By agitating a carrier having a negative charging characteristic and a 
toner in the toner box, the toner is charged with a positive polarity. The 
toner is adhered to the electrostatic latent image which is formed with a 
negative polarity charge, and therefore, the electrostatic latent image is 
toner-developed. In addition, the magnetic brush (not shown) of the 
developer 32 is connected to ground such that the developing bias voltage 
(Vb) can be set as 0 volts. 
The transferer 34 transfers a toner image formed on the photosensitive drum 
16 to a paper sheet 46 which is fed from a left side of FIG. 1. More 
specifically, when a direct-current corona discharging of a negative 
polarity is applied to the paper 46 from a back surface thereof by the 
transferer 34, the toner which has formed the toner image on the 
photosensitive drum 16 is attracted by an electric field of the transferer 
34 so as to be transferred onto the paper 46. The paper 46 onto which the 
toner image has been transferred is separated from the photosensitive drum 
16 by the separating claw 36. The paper 46 is then fed toward a fixing 
unit (not shown), and the toner image is fixed to the paper 46 which is 
then discharged. 
After the toner image is transferred from the photosensitive drum 16 to the 
paper 46, the cleaning blade 38 removes a toner and paper powder remaining 
on the photosensitive drum, and the erasing lamp 40 eliminates a residual 
charge of the photosensitive drum 16. 
A single copy is thus completed by charging, exposure, developing, 
transferring, separating, cleaning and charge erasing. Each completed copy 
is counted by the counter 22. The output of the counter 22, that is, the 
copy quantity, is applied to the microcomputer 18. 
Then, by referring to the stored copy quantity-voltage table (not shown), 
the voltage setting data according to the copy quantity is read-out by the 
microcomputer 18 which applies the data to the bias voltage source 20 so 
that a bias voltage from the bias voltage source 20 is controlled by the 
voltage setting data. Thus, the bias voltage (Vpc) according to the 
voltage setting data is applied to the conductive substrate 12. 
Furthermore, with referring to the stored copy quantity-light amount table 
(not shown), the light amount setting data according to the copy quantity 
is read-out by the microcomputer 18 which then applies the data to the AVR 
24 so that the driving voltage for the light source 28, that is, the light 
amount is adjusted in accordance with the light amount setting data. 
FIG. 2 is a graph showing a change of the surface voltage of the 
photosensitive drum 16 in a case where the exposure light amount is kept 
constant while the bias voltage (Vpc) is changed in accordance with the 
copy quantity. As shown in the graph, by changing the bias voltage (Vpc) 
according to the copy quantity, it is possible to keep the exposure 
voltage (VL) approximately constant, and therefore, a good image without 
fogging can be obtained for a long time. Therefore, the life of the 
photosensitive drum 16 can be prolonged. 
As shown in a graph of FIG. 3, if the exposure light amount as well as the 
bias voltage (Vpc) are simultaneously changed, the increase range of the 
required bias voltage (Vpc) can be reduced, and therefore, it is possible 
to further prolong the life of the photosensitive drum 16. In a case where 
the bias voltage source having an output voltage with an upper limit of 
the 600 volts, for example, is used, if the light amount is constant at 
100%, it is seen that the fogging phenomenon occurs at the copy quantity 
of approximately 30,000; however, by increasing the light amount by 120%, 
it is possible to delay the time that the fogging phenomenon occurs, and 
therefore, the life of the photosensitive drum 16 can be prolonged. 
FIG. 4 is a graph showing a model of the surface voltage of the 
photosensitive drum 16 in a case where a bias voltage (Vpc) of 200 volts 
is applied to the conductive substrate 12 in the above described 
embodiment. 
In addition, in the above described embodiment, although the grid electrode 
42 of the charger 30 is connected to ground through the varistor 44, it is 
possible to consider that the grid electrode 42 is directly connected to 
ground and each of the developing bias voltage (Vb) and the bias voltage 
(Vpc) is increased by 400 volts. In this case, a model of the surface 
voltage of the photosensitive drum 16 becomes the same as the model of the 
surface voltage of the above described embodiment (FIG. 4), as shown in 
FIG. 5. 
In an electrophotographic apparatus 50 according to another embodiment, as 
shown in FIG. 6, a relay 52 is connected to the conductive substrate 12 of 
the photosensitive drum 16. Switching the relay 52 selects a state where 
the conductive substrate 12 is directly connected to the ground or a state 
where the same is connected to the ground through the varistor 54 having 
the varistor voltage of 50 volts. The relay 52 is driven by a driving 
power source 56 which is controlled by a microcomputer 58. The 
microcomputer 58 receives an output of a counter 60 for counting the copy 
quantity. The developing bias voltage (Vb) of approximately -250 volts is 
applied to the magnetic brush (not shown) of the developer 32 by a 
developing bias voltage source 62. In addition, the charger 46 of this 
embodiment may be the same as the charger 30 of FIG. 1 or the charger 46 
may be one having no grid electrode. 
In operation, the number of use times of the photosensitive drum 16 is 
counted by the counter 60 as the copy quantity. The output of the counter 
60, that is, the copy quantity, is applied to the microcomputer 58. The 
microcomputer 58 determines whether or not the copy quantity is less than 
27,500, for example, and in response to such determination, the relay 52 
is switched by the microcomputer 58. More specifically, if it is 
determined that the copy quantity is less than 27,500, the conductive 
substrate 12 is connected to ground through the varistor 54 by the relay 
52. On the other hand, when it is determined that the copy quantity is 
more than 27,500, the conductive substrate 12 is directly connected to the 
ground by the relay 52. Therefore, if the copy quantity is less than 
27,500, the exposure voltage (VL) of the photosensitive drum 16 is changed 
as shown by a dotted line in FIG. 7. The exposure voltage (VL) of this 
case is higher than that of the case where the conductive substrate 12 is 
directly connected to ground (a solid line in FIG. 7) by approximately 50 
volts. If the relay 52 is not switched to a state where the conductive 
substrate 12 is directly connected to the ground (the solid line in FIG. 
7), the voltage difference between the exposure voltage (VL) and the 
developing bias voltage (Vb) becomes too large in a period that the copy 
quantity is small, and therefore, there is the possibility that a 
so-called carrier attracting phenomenon where the carrier within the 
developer 32 is adhered to the photosensitive drum 16, and thus, the 
photosensitive drum 16 is damaged. 
However, in accordance with the embodiment shown in FIG. 6, since the 
developing bias voltage (Vb) applied to the magnetic brush of the 
developer 32 is set to be approximately "-250 volts", even if the exposure 
voltage (VL) is increased by -200 volts, no fogging phenomenon occurs. 
In addition, in the embodiment shown in FIG. 6, only a single varistor 54 
is provided for defining the voltage of the conductive substrate 12; 
however, a plurality of varistors having different varistor voltages may 
be provided and the varistors may be switched by a relay. Furthermore, 
instead of the varistor(s), a resistor(s) may be utilized. In this case, a 
voltage is developed on the resistor in accordance with a resistance value 
thereof by a current flowing the photosensitive layer 14, and therefore, 
an advantage similar to that of the varistor can be obtained. 
Furthermore, in the embodiments shown in FIG. 1 and FIG. 6, the bias 
voltage (Vpc) is set for the photosensitive drum 16 on the basis of the 
output of the counter 22 (or 60) for counting the copy quantity. However, 
in such a kind of image forming apparatus, papers having different sizes 
as such a letter size and a legal size are usually utilized. In the image 
forming apparatus capable of using papers of different sizes, on the basis 
of a paper having a specific size and a feeding direction of the paper, 
the copy quantity-voltage table, the copy quantity-exposure light amount 
table, the value of the copy quantity at that the relay is switched, and 
so on may be stored in the ROM. Then, if an exposure length to the 
photosensitive drum according to a relationship of a used paper and a 
feeding direction thereof becomes different from the above described 
standard stored in the ROM. By providing a correcting means for correcting 
a counted value of the counter 22 (or 60), it becomes possible to surely 
set the bias voltage (Vpc). For example, if the above described standard 
is stored in a state where the paper is the letter size and the feeding 
direction thereof is a longitudinal direction, when a paper of a legal 
size is fed in a longitudinal direction, the correcting means corrects the 
copy quantity, that is, the counted value of the counter by 1.3 times that 
is equal to a ratio of the length. The counted value of 1.3 times is added 
to the total copy quantity which is used as the number of use times of the 
photosensitive drum 16. However, if the standard is stored in a state 
where a paper is letter size and the feeding direction thereof is in a 
lateral direction, even if a paper of legal size is used in a feeding 
direction of a lateral direction thereof, the exposure length to the 
photosensitive drum 16 becomes equal to that of the standard, it is not 
necessary to correct the counted value, that is, the copy quantity. That 
is, the correcting means corrects the copy quantity only when the exposure 
length that is relevant to the life of the photosensitive member is 
different from the exposure length of the standard stored in the ROM. 
In addition, in the above described embodiments, the electrophotographic 
apparatus 10 or 50 is used as a copying machine; however, an image forming 
apparatus to which the present invention can be applied may be a printer, 
a facsimile, and etc. In such a case, an LED array, laser beam or the like 
can be used as the exposing means. 
Although the present invention has been described and illustrated in 
detail, it is clearly understood that the same is by way of illustration 
and example only and is not to be taken by way of limitation, the spirit 
and scope of the present invention being limited only by the terms of the 
appended claims.