Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Reexamination Certificate
1999-05-17
2001-08-21
Le, Que T. (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
C363S021030
Reexamination Certificate
active
06278103
ABSTRACT:
FIELD OF THE INVENTION AND RELATED ART
The present invention relates to a charging apparatus, the charging member of which is placed in contact with, or placed microscopically close to, an object to be charged, for example, a photosensitive member or the like, in order to charge the object. In particular, it relates to a charging apparatus which applies to its charging member a voltage comprising an oscillating component.
In the past, a corona discharging apparatus (corona type charging device) has been widely used as a means for charging an object such as a photosensitive member in an image forming apparatus (copying machine, laser printer, and the like) which employed, for example, an electrophotographic process
A corona discharging apparatus is a noncontact type charging apparatus. It comprises a charging electrode constituted of a piece of wire or the like, and a shield electrode which surrounds the discharging electrode. The shield electrode is provided with a corona discharging opening directed toward the surface of an object to be charged, not in contact with the object. In operation, a high voltage is applied to the discharging electrode and the shield electrode to generate discharge current (corona shower) to which the surface of the object is exposed to be charged to predetermined polarity and potential level.
In recent years, however, a contact type charging apparatus has been put to practical use. In the case of contact type charging apparatus, the electrically conductive charging member of a contact type charging apparatus, to which a voltage is being applied, is placed in contact with an object to be charged, in order to charge the surface of the object to predetermined polarity and potential level. In charging an object with the use of a contact type charging apparatus, two distinctive charging systems (mechanisms or principles) play mixed roles. One is an electrical discharge based charging system, or mechanism. According to an electrical discharge based charging system, the surface of an object to be charged is charged by an electrical discharge which occurs through microscopic gaps between a charging member and the object to be charged. The other is a charge injection based system, or mechanism, according to which electrical charge is directly injected from a charging member into an object to be charged to charge the surface of the object. Thus, the characteristic of a contact type charging apparatus varies depending on which of the two systems is dominant in charging an object.
An electrically conductive charging member may vary in shape and material. For example, it may be of a roller type, a blade type, a fur brush type, or a magnetic brush type.
There are two systems for applying voltage to a charging member. There are a DC application system which applies only a DC voltage to a charging maser, and an AC application system which applies an oscillating voltage (voltage, value of which periodically changes) to a charging member. Regarding an AC application system, applying to an object to be charged, a compound oscillating voltage composed of an AC voltage Vac with a peak-to-peak voltage, the level of which is twice or more the level of the threshold voltage required to charge an object by applying DC voltage to the object, is effective to uniformly charge the object; the object can be more uniformly charged by this type of AC application system than a DC application system (Japanese Laid-Open Application No. 149668/1988).
It is not mandatory that a charging member be placed in contact with the surface of an object to be charged, with the application of a certain amount of contact pressure, as long as it is assured that a region in which electrical discharge is allowed to occur is provided between the charging member and the object to be charged. In other words, a charging member and an object to be charged do not need to be placed in contact with each other as long as they are disposed extremely close to each other, that is, close enough to trigger electrical discharge between them. In the case of the present invention, a charging system based on such an arrangement between a charging member and an object to be charged is included in the category of a contact type charging apparatus. Whether or not electrical discharge occurs is determined by the voltage across the gap between the charging member and the object to be charged, and Paschen curve.
The latter charging system, the contact type charging system, has many advantages over the former charging system, a corona discharging apparatus. For example, (1) it is unnecessary to apply bias constituted of high voltage, (2) charging efficiency in high, (3) the amount of the products such as ozone for which the discharging of corona is responsible is extremely small, and (4) there is no problem that the wire is soiled.
However, a contact type charging apparatus suffers from its own problems. That is, the electrical resistance or electrostatic capacity of the resistive layer of a charging member fluctuates in response to ambient factors, in particular, temperature and humidity, and as a result, leak or charge failure occurs.
Generally, in order to solve these problems peculiar to a contact type charging apparatus of an AC application type, compensation is automatically made for the fluctuation of the electrical resistance of the resistive layer of a charging member, which is caused by the ambient factors, by controlling the AC voltage, that is, the oscillating component of the oscillating voltage applied to the charging member, so that the current through the charging member remains stable.
Here, such a method will be briefly described. Referring to
FIG. 12
, a referential character
1
designates an object to be charged. In this example of an electrophotographic image forming apparatus, the object to be charged is an electrophotographic photosensitive member in the form of a rotative drum (hereinafter, “photosensitive drum”). A referential character
2
designates a contact type charging member, which is disposed in contact with the photosensitive drum
1
. In this example, the contact type charging member is constituted of an electrically conductive member of a roller type (hereinafter, “charge roller”). The charge roller
2
consists of an electrically conductive metallic core
2
a
, an electrically conductive elastic layer
2
b
concentrically formed on the peripheral surface of the metallic core
2
a
, and an electrically resistive layer
2
c
coated on the peripheral surface of the conductive elastic layer
2
b
. A referential character S designates a high voltage power source (bias application power source) for the charge roller
2
. From this power source S, a compound oscillating voltage compound of a DC voltage Vdc and an AC voltage Vac is applied to the charge roller
2
through the metallic core
2
a
. As a result, the peripheral surface of the photosensitive drum
1
is uniformly charged to predetermined polarity and potential level.
FIG. 13
depicts an example of an electrical circuit for controlling the AC voltage, or the oscillating component, of the oscillating voltage applied to the charge roller
2
, so that the current through the charge roller remains stable. A referential character
11
designates an oscillating deice (oscillating circuit), from which a voltage with a rectangular wave-form having the same frequency as that of the AC voltage applied to the charge roller
2
is outputted. The outputted voltage with the rectangular wave-form is amplified by an amplifying device
12
, converted into a voltage with a sinusoidal wave-form as it is passed through a low-pass filter
13
, and inputted into a transformer driving circuit
14
. The transformer driving circuit
14
applies the voltage with the sinusoidal wave-form to the primary coil of a step-up transformer
15
. As a result, a high voltage with the sinusoidal wave-form is generated at one end (coil end
3
) of the secondary coil, and is outputted through resistors
16
and
17
, and a condenser
18
; the AC voltage Vac is applied to
Canon Kabushiki Kaisha
Fitzpatrick ,Cella, Harper & Scinto
Le Que T.
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