U.S. patent number 3,912,387 [Application Number 05/450,269] was granted by the patent office on 1975-10-14 for electrostatography.
This patent grant is currently assigned to Xerox Corporation. Invention is credited to Edric Raymond Brooke.
United States Patent |
3,912,387 |
Brooke |
October 14, 1975 |
**Please see images for:
( Certificate of Correction ) ** |
Electrostatography
Abstract
An electrostatographic apparatus including means for charging,
exposing and developing a photosensitive member and transferring
the developed image to suitable support material including means
for detecting continuous zones on the photosensitive member which
are not discharged by the exposure means and discharge means for
discharging the photosensitive member in said zones.
Inventors: |
Brooke; Edric Raymond (Welwyn
Garden City, EN) |
Assignee: |
Xerox Corporation (Stamford,
CT)
|
Family
ID: |
10061375 |
Appl.
No.: |
05/450,269 |
Filed: |
March 11, 1974 |
Foreign Application Priority Data
|
|
|
|
|
Mar 30, 1973 [GB] |
|
|
15564/73 |
|
Current U.S.
Class: |
399/190 |
Current CPC
Class: |
G03G
15/047 (20130101); G03G 2215/0443 (20130101); G03G
2215/0446 (20130101); G03G 2215/0473 (20130101); G03G
2215/0448 (20130101); G03G 2215/0439 (20130101) |
Current International
Class: |
G03G
15/045 (20060101); G03G 15/047 (20060101); G03G
015/00 () |
Field of
Search: |
;355/14,3 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Horan; John M.
Claims
What I claim is:
1. In an electrostatographic apparatus including a photoconductive
plate, means for applying a uniform electrostatic charge to the
plate, exposure means for exposing the charged plate to a pattern
of light and shadow to produce an electrostatic latent image on the
plate, and developer means for developing charged areas of the
plate;
detecting means comprising one or more light sensitive devices
arranged in the optical path of the exposure means for detecting
continuous zones of the plate which do not receive sufficient light
from said exposure means during exposure of the plate to discharge
the plate; and,
discharge means controlled by said detecting means for discharging
the plate in said zones.
2. The invention as claimed in claim 1, wherein said discharge
means comprises one or more discharge lamps extending transversely
across the plate.
3. The invention as claimed in claim 2, wherein said discharge
means is adapted to operate only to discharge predetermined zones
of a photoconductive plate.
4. In an electrostatographic apparatus including an
electrostatographic plate, means for applying a uniform
electrostatic charge to the plate, and means for discharging
selected portions of the charged plate to produce an electrostatic
latent image on the plate;
sensing means adapted for detecting continuous zones of the plate
which are not discharged by said means for discharging selected
portions of the plate; and,
discharging means controlled by said sensing means for discharging
the plate in said continuous zones.
5. The invention as claimed in claim 4 wherein said discharge means
extends transversely across the plate, said detecting means
comprising a plurality of light-sensitive detectors, whereby
operation of said discharge means is effected only when all
detectors receive less than a predetermined intensity of light.
6. The invention as claimed in claim 5, wherein said discharge
means comprises a single tubular lamp.
7. The invention as claimed in claim 5, wherein at least two
transversely extending discharge means arranged end-to-end is each
associated with one or more light-sensitive detectors.
8. The invention as claimed in claim 2, wherein a row of lamps
extends transversely across the plate and a light-sensitive
detector is associated with each lamp.
9. The invention as claimed in claim 8, including means for
manually switching out the discharge means.
10. Electrostatographic apparatus according to claim 8 in which, in
operation, successive portions of a document to be reproduced are
exposed through a slit on to a relatively moving photoconductive
plate.
11. An electrostatographic process including the steps of
a. applying a uniform electrostatic charge to a photoconductive
plate,
b. exposing the charged plate to a pattern of light and shadow to
produce an electrostatic latent image on the plate,
c. developing charged areas of the plate,
d. detecting continuous zones of the plate which do not receive
sufficient light during exposure step (b) to discharge the plate,
and
e. discharging the plate in said zones.
Description
This invention relates to electrostatography and more particularly
to improvements in the control of development in
electrostatographic apparatus.
In one well known form of automatic xerographic reproduction
machine, a moving photoconductive plate, generally in the form of
an endless surface, such as a drum or the like, is first uniformly
charged and the surface then exposed to a light pattern of the
image sought to be reproduced thereby to discharge the charge in
the areas where light strikes the plate. The undischarged areas of
the layeer thus form an electrostatic charge pattern in conformity
with the configuration of the original image pattern.
The electrostatic latent image may then be developed into visible
form by applying a developer material, e.g. a powder, generally
referred to as toner, to the plate using any one of a number of
development means generally known and used in the art. Subsequent
to the development operation, the now visible image is transferred
from the plate to a sheet of final support material such as paper
or the like and suitably affixed to it thereby forming a permanent
print.
Instead of being developed by means of a powder, the latent image
may be developed using a liquid development system such as is
described in U.S. Pat. No. 3,084,043.
It often occurs that the electrostatic latent image area formed is
smaller than the area which has been charged so that after exposure
to the light image edge zones of the plate bounding the latent
image remain unexposed to light and therefore retain the charge. In
machines having solid area development capability (which is a
feature of the above-mentioned liquid development system) this is a
particular problem since when the photoconductive layer is then
developed, toner is deposited in these edge zones and consequently
produces solid areas of toner in these zones. If these regions do
not come into contact with the copy material then the toner
deposited in these zones and left behind on the plate surface after
the transfer operation must be removed or cleaned from the plate
surface in some manner before a new imaging cycle can be initiated.
Removal of this toner can prove to be a problem in the automatic
xerographic process not only because it is dirt producing, but also
because where large volumes of residual toner are involved, these
may act rapidly to overload the machine's cleaning system and
result in a waste of material to the user. On the other hand if
these edge zones come into contact with the copy material then the
latter will have solid coloured marginal zone along one or more
edges, which is unsightly.
Various what may conveniently be termed edge fade-out systems have
been proposed for the purpose of avoiding or reducing wastage of
developer material and of eliminating or reducing solid marginal
edge zones on copies. Thus in U.S. Pat. No. 3,687,538, for example,
illuminating means disposed in a rectangular frame surrounding the
image area is energised to discharge the photoreceptor in the areas
surrounding the image area. In the system described in British
Patent Specification No. 1,230,526, which is incorporated in a
machine employing flash exposure, a lamp extending transversely
across the path of a photreceptor belt is activated by a suitable
logic system a short predetermined time after de-energisation of
the flash exposure lamps. In U.S. Pat. No. 3,685,894, a
photoelectrostatic copying machine is described which has reduction
mode copying facility and, in reduction mode, portions of the copy
sheet, which in this case is itself adapted to receive the
electrostatic latent image and so forms the xerographic plate, are
not fully exposed. In order to avoid the production of dark or
"dirty" edge zones which would otherwise occur in reduction mode of
operation a pair of lamps are provided over the edge zones which
are switched in only when reduced mode is selected so as
automatically to be energised in this mode and scan the
predetermined edge zones.
While all of the machines described above have the desirable effect
of reducing wastage of toner and/or of avoiding or reducing solid
marginal areas on copies, a need still exists to be able to predict
the production of an unwanted solid development area other than
under specific predetermined conditions such as operation in
reduction mode in the case of U.S. Pat. No. 3,685,894.
From one aspect, the present invention provides an
electrostatographic process including the steps of
a. applying a uniform electrostatic charge to a photoconductive
plate,
b. exposing the charged plate to a pattern of light and shadow to
produce an electrostatic latent image on the plate,
c. developing charged areas of the plate,
d. detecting continuous zones of the plate which do not receive
sufficient light during exposure step (b) to discharge the plate,
and
e. discharging the plate in said zones.
From another aspect, the invention provides, in or for an
electrostatographic apparatus including a photoconductive plate,
means for applying a uniform electrostatic charge to the plate,
exposure means for exposing the charged plate to a pattern of light
and shadow to produce an electrostatic latent image on the plate,
and developer means for developing charged areas of the plate;
means for detecting continuous zones of the plate which do not
receive sufficient light from said exposure means during exposure
of the plate to discharge the plate and discharge means controlled
by said detecting means for discharging the plate in said
zones.
It is to be understood that by the references to discharging zones
of the photoconductive plate made herein and in the claims, is
meant that the charge on the plate will be reduced to a value such
that developer material will not be attracted thereto.
Suitably said detecting means comprises one or more light sensitive
detectors arranged in the optical path of the exposure means. The
discharge means may comprise one or more discharge lamps extending
transversely across the plate.
In a preferred form of the invention for use in a machine in which,
in operation, successive portions of a document to be reproduced
are exposed through a slit onto a moving electrostatographic plate,
detecting means sensitive to light is provided in the light path
defined by the exposure means and one or more discharge devices,
suitably lamps, are operated as a function of the intensity of
light received by said detecting means. This may be achieved by
determining the intensity of light required to reduce the plate
charge to a value such that developer material will not be
attracted thereto and providing that the detecting means will cause
the discharge device(s) to be activated only when the detecting
means receives light of less than such intensity (including no
light).
The discharge system of this invention may operate only to
discharge predetermined zones of a photoconductive plate. For
example a discharge device in the form of a single lamp, or a row
of lamps connected to operate in unison, may be arranged
transversely across the plate and a plurality of light-sensitive
detectors may be provided transversely across the plate, operation
of the discharge device only being effected where all detectors
receive less than a predetermined intensity of light. Such an
arrangement would permit the discharge of solid or continuous
charge zones at opposite ends, in the direction of movement of the
plate, of the desired image area whilst enabling solid area
development to be achieved within the desired image area.
With the above arrangement, undesired solid or continuous charge
zones may still be produced at the sides of the desired image area
and in order to avoid this, the discharge device could be split
into three or more sections, each operated by one, or preferably a
transversely extending series of, detectors. The lengths of such
sections would be chosen according to known parameters such as
paper size, document size or reduction mode.
In a further form of the invention a row of lamps extends
transversely across the plate and a detector is associated with
each lamp. Whilst in some circumstances the above arrangement will
result in the elimination of desired solid or continuous charge
areas within the image area, manual means could be provided for
switching out the discharge means in these circumstances.
While in some instances, the present invention will perform the
same functions as are achieved by the known devices described
above, it provides the significant advantage of enabling the
elimination of solid development areas which cannot or have not
been predetermined in terms of machine function. Thus, for example,
in the form described above, in which a lamp means is controlled by
a plurality of detectors spaced across the plate, the elimination
of continuous charge zones at opposite ends of the image area is
effected regardless of their length in the direction of plate
movement and without control by any other machine function.
In order that the invention may be more readily understood,
reference will now be made to the accompanying drawings, in
which:
FIG. 1 is a schematic view of one form of document scanning system
for an electrostatographic machine,
FIG. 2 is a schematic view of one embodiment of development control
system according to the invention,
FIG. 3 is a view illustrating the embodiment of FIG. 2 taken along
the line A--A of FIG. 1,
FIG. 4 is a view of a unitary mounting arrangement for the
embodiment of FIG. 2,
FIG. 5 is an illustration of a suitable electronic control
circuit,
FIG. 6 is a representation of a document to be copied for the
purpose of explaining modes of operation of development control
systems of this invention,
FIG. 7 is a view like that of FIG. 3 of a second embodiment of the
invention, and
FIG. 8 is a view like that of FIG. 3 of a third embodiment of the
invention.
To facilitate appreciation of this invention, one form of document
scanning system for a xerographic machine in which the xerographic
plate is exposed to light reflected from successive portions of a
document to be copied through a slit will first be described with
reference to the schematic representation of FIG. 1.
A platen 10 is provided to support the document 11 to be copied. A
scanning mirror system includes two moveable mirrors 12 and 13
shown in their extreme left and right positions in full and dotted
outline respectively. The mirror 13 is arranged to move at half the
speed of the mirror 11 during scanning to maintain the optical
distance constant between the document 11 and a lens 14. A tubular
lamp 15 extending across the platen 10 parallel to the mirror 12
moves with the mirror 12 in fixed relation thereto. The lamp 15 is
provided as illumination means to illuminate the document 11
through the platen 10 during scanning.
An optical or light path extending from the platen 10 to the lens
14 continues beyond the lens to be reflected in sequence by mirrors
16 and 17 towards a photoreceptor drum 18. An optical slit 19,
better seen in FIG. 3, is provided in a cover plate 20. A platen
cover 22 is provided over the platen so that it will lie on the
document 11.
The configuration or shape of the slit 19 is, as shown, narrower at
its mid-point than at its extremities. This shape, as is already
known in the art, is to compensate for the non-uniformity of light
output from the lamp across its length.
The document 11 is scanned by the sweep of the mirrors from left to
right producing a latent image of the information on the document
on the photoreceptor 18 which rotates in synchronism with the
movement of the mirrors 12 and 13 in the direction of arrow 23.
Where the underside of the platen cover has a reflective surface,
light striking the platen cover beyond the edges of the document
will be reflected along the optical path.
As illustrated in FIG. 1 a xerographic machine incorporating such a
scanning system is also provided with charging, development and
transfer stations C, D and T arranged in the relative locations
indicated. In this embodiment, a liquid development system is
employed, being represented schematically by an applicator roll of
such a system, but it should be appreciated that in the case of a
powder or particulate development system a fusing station should be
provided following the transfer station for securing the toner
particles to the copy material S in a permanent form.
The general operation of the machine illustrated is as follows.
Prior to exposure, the drum surface, which comprises a layer of
photoconductive insulating material which may, for example, be
vitreous selenium supported on a conductive backing such as
aluminium, is sensitised by means of a corona generating device at
the charging station C, which is energised from a suitable high
potential source (not shown). Such generating device may be an
adaption of the type disclosed in U.S. Pat. No. 2,965,756.
Exposure of the drum to the light image discharges the
photoconductive layer in the areas struck by light, whereby there
remains on the drum a latent electrostatic image in image
configuration corresponding to the light image projected from the
document. As the drum surface continues its movement the
electrostatic latent image passes through the developer station D
at which, in this machine configuration, a liquid developer
material is applied to the drum to produce a visible image thereon.
Following development the visible image passes through the image
transfer station T at which the visible image is transferred to a
sheet of copy paper, which is fed into contact with the rotating
drum in coordinated registration with the arrival of the developed
image at the transfer station. Following transfer, a suitable
pick-off mechanism (not illustrated) separates the copy sheet from
the drum surface whereafter the copy sheet is directed into a
suitable copy-holder for removal by an operator.
After transfer, the xerographic drum surface passes through a
cleaning station (not illustrated) at which the surface is cleaned
suitably by a doctor blade arranged in contact with the drum for
doctoring the developer material remaining on the drum surface.
One embodiment of development control system according to this
invention will now be described with particular reference to FIGS.
2 and 3. In the system illustrated the exposure slit 19 is narrower
than the optical path so that the rays having imaged on the drum at
X pass on to a row of photo diodes 31 extending transversely across
the drum. Each photo diode is associated with a lamp 32 controlled
as a function of the intensity of light impinging on the photodiode
by a suitable electronic logic system L in the following manner.
When light of less than a predetermined intensity (sufficient to
reduce the charge on the plate to a value such that developer
material will not be attracted thereto) is impinging on a
photodiode the corresponding lamp is energised thus discharging the
drum area passing beneath it; when light of greater than the
predetermined intensity impinges upon the photodiode the lamp is
not energised. The lamp should extinguish or light, as the case may
be, only after a predetermined time delay dependent upon the
circumferential distance between the photodiode and the lamp and
the rotational speed of the drum following detection of a change in
intensity by the corresponding photodiode. This is necessary since
the lamp is arranged in a trailing position (in the direction of
drum rotation) relative to the photodiode. Filament lamps will have
their own delay characteristic which may provide the required time
delay.
Suitable choice of the extent by which the photodiodes extend into
the optical path and of the threshold value of light intensity to
which the photodiodes respond, will permit lines on the document of
desired width to be reproduced on the copy. The arrangement
above-described with its separately operable multiplicity of lamps
each acting on only a short transverse section of the drum enables
close control of development to be achieved. In order to prevent,
or at least reduce to a minimum, overlapping effects from adjacent
lamps, baffles or shields 33 are provided between the lamps to
restrict the area of effect of each lamp, and for the additional
purpose of providing a sharp contrast between those zones of the
drum surface which are discharged by the lamps and those which are
not.
If desired, instead of being switched off completely when
inoperative, the lamps may be maintained lit at a low level of
intensity insufficient to discharge the drum area therebeneath in
the manner described in British patent specification No.
1,230,526.
FIG. 4 shows a convenient mounting arrangment for the development
control system described above in which the detector/lamp sub-units
are mounted in an elongate unit extending across the width of the
drum and which also includes the necessary control circuitry, a
simplified form of which for a single sub-unit is illustrated in
FIG. 5. The operation is as follows. When less than the
predetermined threshold intensity of light is falling on the
photocell 31 it has an effective high resistance and the voltage
across resistor R1 is applied to the emitter of transistor 34 and
current flows through the lamp 32. When the photocell receives
light above the threshold intensity its resistance drops causing
the voltage across R1 to drop sufficiently to cause the lamp
current to drop and extinguish the lamp.
As shown in FIG. 4, the circuits associated with each lamp are all
connected to bus-bars 35 leading to a common supply. In the unit
illustrated, each sub-unit is separated by a light shield or baffle
33 and the complete unit may be incapsulated in a transparent
resin.
This embodiment has particular application to the elimination of
unwanted solid area development of marginal edges zones of a copy
sheet which would otherwise occur where the document to be copied
is small compared with the glass platen size and either the platen
cover is raised or the document has a substantial thickness (e.g.
it may be a book) so preventing the platen cover from coming close
to the platen. In this event, light from the lamp 15 which passes
through the platen in the areas surrounding the document 11 (see
FIG. 6) will not be reflected down the optical path and the
corresponding areas of the drum surface will not be discharged
during exposure. The development control system described above
will operate as follows. Until the image of the document edge
X.sub.1 X.sub.2 reaches the photodiodes 31, all lamps 32 will be
lit and the drum will be discharged. As edge X.sub.1 X.sub.2
reflects light the lamps will be extinguished across the portion of
the drum between X.sub.1 and X.sub.2 and an electrostatic latent
image of the document information will be produced on the drum
surface. After the Y.sub.1 Y.sub.2 edge passes the exposure slit 19
no further light will reach the photodiodes 31 and all the lamps 32
will again be lit, discharging the remaining area of the drum. The
pitch of the photocells 31 and the lamps 32 will determine the
sharpness of the X.sub.1 Y.sub.1 and X.sub.2 Y.sub.2 edges. In the
above description of operation it is presupposed that the document
contains only line information and no substantial solid dark
areas.
Further embodiments of the invention are schematically illustrated
in FIGS. 7 and 8. In the embodiment of FIG. 7 a single tubular lamp
32 is arranged transversely across the drum surface and a row of
photocells 31 extend in a row across the drum. The photocells 31
are so connected to the lamp 32 that the lamp will only be lit when
all the photocells receive less than the pre-set threshold
intensity of light. With this arrangement, in the circumstances
described above with reference to FIG. 6, only those areas or zones
of the drum beyond the document edges X.sub.1 X.sub.2 and Y.sub.1
Y.sub.2 will be discharged, but solid areas within the image area
on the durm will be reproduced. The embodiment of FIG. 8 permits
the elimination of the undischarged marginal zones along the image
edges corresponding to X.sub.1 Y.sub.1 and X.sub.2 Y.sub.2 which
will remain with the embodiment of FIG. 7, without eliminating
solid area development within the image area, where the width of
these marginal zones can be predetermined, e.g. as a function of
copy size or reduction mode of operation of the machine. Here the
lamp 32 is split into three sections 32a, 32b and 32c, each
associated with its own series of photocells 31. Beyond the edges
corresponding to X.sub.1 X.sub.2 and Y.sub.1 Y.sub.2 of FIG. 6, all
three lamps will be lit but between X.sub.1 X.sub.2 and Y.sub.1
Y.sub.2 only the outer lamps 32a and 32c which are of a length
corresponding to predetermined marginal zones of the drum, will be
lit.
While particular embodiments have been described above, it will be
appreciated that various modifications may be made to the specific
details referred to herein without departing from the scope of the
invention. For example, instead of being arranged as shown, the
photocells could be arranged across the drum at position A
indicated in FIG. 2 so as to receive advance information of the
intensity of light about to fall on the drum.
Further, it is contemplated that embodiments of this invention
could be used in conjunction with known development control
systems, as comprehended for example by the prior art described
above.
* * * * *