Electrophotography – Control of electrophotography process – Control of charging
Reexamination Certificate
2001-04-27
2003-05-13
Braun, Fred L (Department: 2852)
Electrophotography
Control of electrophotography process
Control of charging
C361S235000, C399S176000
Reexamination Certificate
active
06564023
ABSTRACT:
FIELD OF THE INVENTION AND RELATED ART
The present invention relates to an image forming apparatus such as a copying machine, printer, facsimile machine or the like of an electrophotographic type, more particularly to an apparatus wherein a charge member contacted to the image bearing member to electrically charge the image bearing member.
The image formation process in an electrophotographic apparatus includes a uniform charging step of electrically charging an electrophotographic photosensitive member (photosensitive drum) to a predetermined uniform potential, as is well known. With an example of charging means for this purpose, a charge member in the form of a roller (charging roller) is contacted to the surface of the photosensitive drum, and the charging roller is supplied with a charging bias (a voltage in the form of superimposing DC high voltage and sine wave AC high voltage) It is empirically known that discharge current is preferably not lower than a predetermined level in order to provide a stabilized charging.
When the output voltage (sine wave AC voltage (Vo)) as shown in
FIG. 19
, for example, is applied to the charging roller from the high voltage source, a current having the same phase as the AC voltage (Vo), that is, a resistance load current through a resistance load between the charging roller and the photosensitive drum, a current having the phase which is advanced by 90° beyond the AC voltage (Vo), that is a capacity load current through a capacity load between the charging roller and the photosensitive drum, a pulse current flowing at the peak of the amplitude of the AC voltage (Vo), that is, a discharge current between the charging roller and the photosensitive drum. In total, the waveform of the outputing current is as indicated by Io. Designated by Im is a detected current waveform of the AC current attracted to the high voltage source from the charging roller.
FIG. 20
shows a relation between the amplitude of the AC voltage (the output voltage) and the outputing current (Io). When the amplitude of the AC voltage is gradually increased, the amplitude of the AC voltage and the outputing current are substantially proportional to each other as long as the voltage amplitude is lower than a predetermined level. As shown in
FIG. 19
, this is because a resistance load current (Izr) and a capacity load current (Izc) are proportional to the voltage amplitude, and discharge phenomenon does not occur because the voltage amplitude is small, which means that no discharge current (Is) flows. When the amplitude of the AC voltage (output voltage) is further increased, the discharge phenomenon occurs at the predetermined voltage amplitude (Vs), and the total outputing current (Io) does not satisfy the proportional relationship, and the discharge current (Is) alone increases.
Therefore, in the prior art, the peak value (Ip in
FIG. 19
) of the total outputing current is controlled at a predetermined level by a control system which will be described hereinafter, by which the discharge current (Is) is intended to be substantially constant.
FIG. 21
shows a charging bias control circuit for applying the charging bias voltage to the charging roller. As shown in this Figure, the charging roller
2
contacted to the photosensitive drum
1
is connected with a high voltage source
3
and a control device
4
for controlling the high voltage source
3
. When the high voltage source
3
receives a clock pulse of a CPU
5
of the control device
4
, a transistor
8
switches through a pull-up resistor
6
and a base resistor
7
to produce a clock pulse having an amplitude corresponding to an output of an operational amplifier
11
connected with a pull-up resistor
9
through a diode
10
.
When the amplitude of the clock pulse is large, the driving voltage amplitude of the sine wave inputted to the high voltage transformer
12
is also large, and as a result, the amplitude of the AC voltage outputted to the charging roller
2
is also large, the clock pulse is inputted to the filter circuit
32
, which in turn produces a sine wave output having the central value of +12V. The output is inputted to a primary coil of the high voltage transformer
12
through a high voltage transformer drive, and a sine wave AC high voltage is produced at the secondary coil. One side of the secondary coil is connected with a DC high voltage generating circuit
46
through a resistor
45
, and a charging bias voltage in the form of a superimposed DC high voltage and AC high voltage is supplied to the charging roller
2
through an output protection resistor
47
.
The filter circuit
32
is constituted by fourth butterworth filter including resistors
13
,
14
,
15
,
16
,
17
,
18
,
19
,
20
,
21
,
22
,
23
, capacitors
24
,
25
,
26
,
27
,
28
,
29
and operational amplifiers
30
,
31
and a primary high path filter. The high voltage transformer drive circuit
44
is constituted by resistors
33
,
34
,
35
,
36
,
37
,
38
, a capacitor
39
, transistors
40
,
41
,
42
and a Zenorun-diode
43
.
The current flowing into the high voltage source
3
from the charging roller
2
is detected by a high voltage capacitor
49
for separating the DC current of the peak current detection circuit
48
from the high voltage source
3
and a current monitoring resistor
50
. More particularly, the peak voltage of the detected voltage is held by the diode
51
and the capacitor
52
so that peak current is detected.
The resistor
53
is a discharge resistor for the capacitor
52
, and the diode
54
is for current discharge protection.
In order to control the current attracted from the charging roller
2
at a predetermined level, the output of the peak current detection circuit
48
is inputted to a “−” (negative) terminal or contact of the operational amplifier
11
, and a reference voltage provided by the resisters
55
and
56
is inputted to a “+” (positive) terminal or contact, and the output terminal or contact of the operational amplifier
11
is connected to an emitter of the transistor
8
through the diode
10
, so that amplitude of the clock pulse inputted to the circuit
32
is controlled.
In the above-described conventional example of the charging bias control, as shown in
FIG. 22
, a discharge start current I
1
in an initial property e (initial stage of use) is not kept constant but reduces to a discharge start current I
2
as shown in property f after use in the certain term, because of contamination of the charging roller
2
with toner or the like. The discharge current of the peak value Ip increases from Is
0
to Is
1
.
Therefore, if the peak current is controlled to be constant, the discharge current g increases from Is
0
to Is
1
with the increase of the integrated number of output prints (number of the image formations, as shown In FIG.
23
. With further increase of the number of output prints, it exceeds Is
1
.
On the other hand, as shown in
FIG. 23
, an amount of scrape of a photosensitive layer at the surface of the photosensitive drum
1
(deterioration of the photosensitive drum
1
) increases proportionally to the discharge current, and as a result, the speed of the scrape acceleratedly increases. This has shortened the service life of the photosensitive drum
1
.
SUMMARY OF THE INVENTION
Accordingly, it is a principal object of the present invention to provide an image forming apparatus in which deterioration of an image bearing member attributable to a discharge current is prevented while avoiding improper charging. According to an aspect of the present invention, there is provided an image forming apparatus comprising an image bearing member;
a charge member for electrically charging said image bearing member while contacting to said image bearing member; voltage applying means for applying an oscillating voltage including a component of AC voltage to said charge member; first detecting means for detecting an average of the AC current applied to said charge member from said voltage applying means; second detecting means for detection a value of
Nakamori Tomohiro
Saito Seiji
Shimura Masaru
Takami Hiroshi
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