U.S. patent number 3,611,074 [Application Number 04/879,054] was granted by the patent office on 1971-10-05 for corona discharge device.
This patent grant is currently assigned to International Business Machines Corporation. Invention is credited to Heinz H. Weichardt.
United States Patent |
3,611,074 |
Weichardt |
October 5, 1971 |
CORONA DISCHARGE DEVICE
Abstract
A corona generator for providing a uniform and highly efficient
corona discharge for use in such apparatus as electrophotographic
printers.
Inventors: |
Weichardt; Heinz H. (Los Gatos,
CA) |
Assignee: |
International Business Machines
Corporation (Armonk, NY)
|
Family
ID: |
25373341 |
Appl.
No.: |
04/879,054 |
Filed: |
November 24, 1969 |
Current U.S.
Class: |
361/229; 250/326;
361/235 |
Current CPC
Class: |
G03G
15/0291 (20130101) |
Current International
Class: |
G03G
15/02 (20060101); G03g 015/02 () |
Field of
Search: |
;250/49.5ZC
;317/262A,2 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Shoop, Jr.; William M.
Assistant Examiner: Moose, Jr.; Harry E.
Claims
What is claimed is:
1. A charging apparatus for use in such apparatus as
electrophotographic mechanisms for depositing increments of charge
on a charge-bearing surface, said apparatus comprising
a corona discharge wire held in spaced relationship to said
surface;
a pair of bias rods each positioned in substantially the same
spaced relationship to said surface as said corona discharge wire;
means for holding one of said bias rods in fixed position on each
side of said wire; and
means to impress an electrical voltage potential on said wire and
rods whereby said rods effect a virtual ground plane extending to
the surface to enhance the charge flow from said wire to said
surface.
2. The charging apparatus according to claim 1 wherein said bias
rods are positioned parallel to said corona discharge wire.
3. The charging apparatus according to claim 2 wherein said bias
rods and said corona discharge wire are included in a plane
substantially parallel to said charge-bearing surface.
4. The charging apparatus according to claim 1 wherein said
electrical voltage potential on said corona discharge wire
comprises a potential in the range of 6 to 8 kilovolts and the
electric voltage potential on said bias rods comprises a range from
substantially ground potential to one quarter of the corona wire
potential.
5. The charging apparatus according to claim 1 wherein said
electrical voltage potential on said corona wire comprises a
potential of 7 kilovolts and the electrical voltage potential on
said bias rods comprises substantially ground potential.
6. A charging apparatus for use in such apparatus as
electrophotographic mechanisms for depositing increments of charge
on a charge-bearing surface, said apparatus comprising:
a corona discharge wire held in spaced relationship to said
surface;
a pair of bias rods positioned in substantially the same spaced
relationship to said surface as said corona discharge wire;
means for holding one of said bias rods in fixed position on each
side of said wire;
a suppressor screen positioned in spaced relationship intermediate
said wire and said charge-bearing surface; and
means to impress a first relatively high electrical voltage
potential on said wire, a second relatively lower electrical
voltage potential on said rods and a third electric voltage
potential relatively lower than said second potential on said
suppressor screen, whereby the charge flow from said wire to said
surface is enhanced.
7. The charging apparatus according to claim 6 wherein said bias
rods are positioned parallel to said corona discharge wire.
8. The charging apparatus according to claim 7 wherein said bias
rods and said corona discharge wire are included in a first plane
substantially parallel to said charge-bearing surface and spaced a
first distance therefrom; and said suppressor screen is included in
a second plane parallel to said first plane and said charge-bearing
surface, said second plane being located a closer distance to said
charge-bearing surface than said first distance.
9. A charging apparatus for use in such apparatus as
electrophotographic mechanisms for depositing increments of charge
on a charge-bearing surface, said apparatus comprising:
a plurality of corona discharge wires held in spaced relationship
to said surface;
a plurality of bias rods each positioned in substantially the same
spaced relationship to said surface as said corona discharge wires;
means for holding one of said bias rods in flanking relation on
each side of each of said wires; and
means to impress an electrical voltage potential on said wires and
rods whereby said rods effect a virtual ground plane extending to
the surface to enhance the charge flow from said wires to said
surface.
Description
BACKGROUND OF THE INVENTION
Field of the Invention
This invention relates specifically to corona discharge devices for
generating an ion flow particularly adapted to charge an
electrophotographic plate used in electrophotography
Xerography.
In electrophotographic copiers and other such printing devices, it
is common to use corona charging devices to charge the
electrophotographic plate surface. The charge is effected by
imposing a high-voltage potential on a conductor spaced from the
surface to impart a high-field intensity on the surrounding air.
The voltage potential causes the air to break down resulting in a
migration of ions to the wire with ions of opposite potential
migrating to the electrophotographic surface. The migration of ions
must be sufficient to deposit a predetermined charge on the
electrophotographic plate for satisfactory operation of the
electrophotography process. Additionally, the charge must be
uniformly distributed across the plate.
In the past, the primary method of equalizing the charge
distribution across the surface has been to position a ground
shield on the opposite side of the charged conductors from the
plate. It has also been common practice to drive the emitting
conductors at a very high potential because, if not so driven, the
phenomena occurs which is referred to as beading. The effect of
such beading is a streaking of the charge pattern deposited on the
electrophotographic surface. The shield acts to raise the total ion
flow along the corona wire length and thus at least partially
prevents the beading.
While solving some of the more immediate problems, the ground
shield also presents disadvantages. One disadvantage is that the
shield is not transparent and therefore cannot be used where the
charging and exposure of the electrophotographic plate must be
concurrent. Additionally, the ground shield serves as a conductor
for bleeding off ions which strike it and thereby limits the ion
flow to the plate and decreases the efficiency of the corona unit.
In the past, less than 10 percent of the ions actually reach the
plate and more than 90 percent migrate to the ground shield thereby
making the corona unit operate at a very low efficiency.
SUMMARY OF THE INVENTION
It is the primary object of the subject invention to provide a
corona charging unit which operates at a higher efficiency than
prior art devices and distributes a more uniform charge on the
surface being charged.
An additional object of the present invention is to provide a
simple and relatively inexpensive corona charging unit.
In accordance with these and other objects, the invention is
embodied in a charging apparatus for use in electrophotographic
devices to deposit increments of charge on a charge-bearing surface
and comprises a corona discharge wire held in spaced relationship
to the surface to be charged, a bias rod positioned in spaced
relationship on each side of said wire, and means to impress an
electrical voltage potential on said wire and rods whereby said
rods effect a virtual ground plane extending to the surface to be
charged on either side of the wire to enhance the charge flow from
the wire to the surface.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates a diagrammatic schematic view of an
electrophotographic process in which the subject invention can be
used;
FIG. 2 illustrates the corona charging unit embodying the subject
invention with the primary components disassembled;
FIG. 3 is an enlarged cross-sectional view of the corona charging
unit embodying the subject invention in assembled form;
FIG. 4 is an enlarged schematic view of a corona wire plus bias
rods showing the approximate path of the ions;
FIG. 5 illustrates an alternate embodiment of the corona charging
unit embodying the subject invention with primary components
disassembled;
FIG. 6 is an enlarged cross-sectional view of the corona charging
unit of FIG. 5 embodying the subject invention in assembled
form.
DESCRIPTION OF THE PREFERRED EMBODIMENT
One example of a particular apparatus in which the subject
invention is adapted for use is the electrophotographic apparatus
shown in FIG. 1. In this apparatus, a rotatable drum 1 carries
around its periphery an electrophotographic photoconductive member
2 on which is directed an image which it is desired to reproduce on
the paper sheets 3 stored in a hopper 4. A document having the
image to be copied is placed upon a transparent plate 5 mounted
within a table 6 and the light sources 7 are energized which direct
light onto the image for reflection back through a lens 8 onto the
surface of the photoconductor 2. The document is moved at a speed
synchronized with the surface speed of drum 1 in a manner known in
the art. The surface of the photoconductor has been
electrostatically charged in a manner to be described later and
where illuminated by the image, is discharged leaving a charged
pattern in a form of the image to be copied.
Thereafter, a drum passes a development station 10 at which a
toner-carrier mixture 11 is cascaded across the electrostatic image
on the surface of the photoconductor 2. The toner, having a charge
opposite from the polarity of the electrostatic image charge, is
attracted to the drum surface to render the image visible. An
endless conveyor belt 12 carries the toner-carrier mixture to a
position for gravity feeding across the surface of the
photoconductive member.
Continuing the clockwise rotation of the drum 1, a copy paper 3 is
fed into contact with the developed electrostatic image by the
rollers 14. Preferably a corona unit 15 is disposed beneath the
paper at the area of contact with a polarity opposite that of the
toner thereby attracting the toner to the copy paper. After this
so-called corona transfer, the paper 3 is separated from the drum
and fed past a heating element 16 which serves to fuse and
permanently fix the toner to the paper. The drum continues to
rotate past a cleaning brush 17 which wipes the surface of the
photoconductor and removes any excess toner which drops into a
receptacle 18. With the exception of the charging station, this
describes the complete cycling of the drum for reproducing the
image desired.
In accordance with the present invention, the corona unit 20
comprises at least one corona discharge wire held in spaced
relationship to the photoconductor surface with a bias rod
positioned to either side and approximately an equal distance from
the photoconductor surface such that by impressing a high voltage
on the wire and a lower voltage on the rod, the rods form a virtual
ground plane extending normal to the photoconductor surface and
through the rods so as to guide most of the resulting ions from the
wire to the photoconductor surface for the charging of the
photoconductor.
Accordingly, in FIGS. 2 and 3, is shown the wire support unit
comprising a plurality of small diameter corona wires 21 mounted on
a pair of insulator blocks 22 and 24 having extending fingers 25
contacting the wires. The wires are supported in tension between a
pair of spring fingers 26 made of a conductive metal and fixed to
the end of a mounting block 27 by screws 28.
Similarly, the rod support unit holds the bias rods 30 mounted to
extend between a pair of insulator blocks 31 and 32 held in
parallel spaced relationship by a pair of support rods 34 at each
end. The inner side of each of the mounting blocks includes a
concave inset portion 35 sized to permit the entry of the wire
support unit so that the corona wires 21 fit between and extend
parallel to the bias rods 30. Terminals 36 and 37 connect through
the conductor plate to the bias rods for supplying a reference
potential V.sub.R to the rods 30. A terminal 38 extends through a
slot 39 in the insulator block 31 when the support units are
assembled for supplying a high-voltage potential V.sub.H to the
corona wires through the spring fingers 26.
The voltage to the corona unit 20 is provided by power supply 33.
The voltage is usually referenced to the support member for
photoconductor 2 which is generally maintained at ground potential.
The voltage V.sub.R applied to bias rods 30 may be selected at a
value from ground potential to about 25 percent V.sub.H and provide
a substantial increase in efficiency over the type of corona unit
which utilizes a ground shield. However, the preferred potential
for V.sub.R is ground potential. The magnitude of the potential
V.sub.H selected for the corona wires may vary over a fairly wide
range dependent upon many design factors known to those in the art.
However, the preferred range is a potential of 6 to 8 kilovolts and
a polarity the same as that desired for the charge to be produced
on the surface of photoconductor 2.
As shown in FIG. 3, the support units are held together by a
plurality of spring biased buttons 40 which snap between the
support rods 34. This construction facilitates the servicing of the
unit since the parts can be easily disassembled by snapping the
supports apart. This construction also gives access to the
individual parts for cleaning, adjusting or replacing any of the
individual parts of the corona unit.
The corona unit is suitable for those applications in which
charging and exposure of the photoconductor surface must be
concurrent since the area where the wires run is substantially free
of interfering material as shown in FIG. 1. The embodiment shown in
FIGS. 2 and 3 also possesses this advantage since support 27 can be
constructed of a transparent material or alternatively support 27
can be constructed as a frame-shaped member which is open in the
area where the wires run.
The schematic view of FIG. 4 shows the approximate path of the ions
from the corona wires 21 to the photoconductive member 2. It can be
shown mathematically that the effect of the bias rods 30 on the
electrostatic field is similar to that of two grounded shields
whose planes are perpendicular to the surface of the
photoconductive member 2 and parallel to rods 30, and this effect
is essential to establish a high-corona current. The ions tend to
migrate to the ground planes but, inasmuch as the region from the
rods 30 toward the photoconductor is essentially at ground
potential and no conductor is present there, a large percentage of
the ions continue on to the photoconductor surface thereby
resulting in an efficient transfer of the high-corona current to
charge on the photoconductor surface.
In a particular embodiment the bias rods 30 were constructed of
metal rods having a diameter of 0.050 of an inch. The corona wires
were polished tungsten wires 0.002 of an inch in diameter. The
adjacent bias rods 30 as shown in FIG. 4 were approximately 1.5
centimeters apart. The bias rods and the corona wires were in a
plane substantially parallel to the photoconductive member and
spaced approximately 0.65 centimeter from the photoconductor member
in one operation. This structure was energized with a potential
V.sub.H of -7 kilovolts applied to the corona wire and a reference
potential V.sub.R coupled to bias rods 30 of ground potential. The
voltage of the photoconductor support member was also ground
potential. This configuration provided a charge current
approximately 10 times as high as conventional corona units having
ground shields and supplying the same total current. In another
embodiment the plane including the bias rods and the corona wires
was spaced from the photoconductive member by 0.5 centimeter, and
this embodiment showed a further four-fold increase in the
efficiency of transferring charge to the photoconductive surface as
compared to a conventional corona unit having ground shields.
Thus, the disclosed construction provides greatly increased corona
current as well as a higher efficiency in transfer of charge to the
photoconductive surface. This advantage results in a decreased
high-voltage power supply capacity requirement for a given corona
requirement, thereby resulting in a reduced cost for the system.
This construction has the added advantage of being an easily
iterated structure. Additional individual units of corona wires and
adjacent bias rods can be added without appreciably influencing the
operation of adjacent units. Thus, the disclosed construction has
the advantage of not only increased corona current and a higher
efficiency in transfer of charge to the photoconductive member, but
also the construction facilitates the servicing of the unit since
the individual parts are easily accessible.
In some cases the high-current density provided by the corona
charging device described above may cause damage to the
photoconductor in places in which small pinholes or depressions
exist in the photoconductive coating. The irregularities in the
photoconductive coating give rise to strong local fields in the
area of the photoconductive member surrounding the pinhole. These
strong local fields cause the onset of a glow discharge which
eventually results in electrical breakdown and arc-over to the
corona wire. To significantly reduce the change of electrical
breakdown in the corona charging unit, a suppressor screen is added
between the corona wire and the photoconductive member and
energized with a relatively low potential. The addition of the
suppressor screen not only reduces the chance of arc-over, but also
leads to a charge limiting effect so that the charge on the
photoconductor is uniformly placed.
With the addition of the suppressor screen a somewhat different
mode of operation can be used for the corona charging unit. By
applying to the bias rods an intermediate level voltage, the corona
wires can be operated at a higher voltage thereby operating at a
greater efficiency due to the field gradient being greater between
the corona wires and the photoconductive member.
The embodiment of the invention which includes the suppressor
screen is shown in FIGS. 5 and 6. In these drawings components
which are substantially the same as the embodiment shown in FIGS. 2
and 3 are assigned the same reference number followed by a prime
('). Suppressor screen 42 is added in a plane parallel to the
photoconductive member between the plane which includes corona
wires 21' and the bias rods 30'. The suppressor screen comprises a
plurality of spaced wires attached at the ends to mounting plates
43. Plates 43 are attached to insulator blocks 31' by screws 44,
for example, to maintain screen 42 in a plane. Suppressor screen 42
is energized by a suitable voltage V.sub.1 from power supply 45. In
a particular embodiment that works well the photoconductive member
2' is maintained at a reference voltage V.sub.R of ground potential
and the suppressor screen is maintained at a voltage V.sub.1 of -1
kilovolts. In this case bias rods 30' are energized with a
potential V.sub.2 of -2 kilovolts and corona wires 21' are
energized with a potential V.sub.3 of -7 kilovolts. This corona
device produces a uniform charge on the photoconductive member
without damaging the photoconductor surface and also produces an
efficient utilization of current to produce charge on the
photoconductive surface. This has the practical result of requiring
a smaller size corona device for a specified charging current and a
smaller capacity power supply to produce the required charge
density.
While the invention has been particularly shown and described with
reference to preferred embodiments thereof, it will be understood
by those skilled in the art that the foregoing and other changes in
form and details may be made therein without departing from the
spirit and scope of the invention.
* * * * *