Note: Descriptions are shown in the official language in which they were submitted.
~Q37~00
This invention relates to electrostatography and
more particularly to improvements in the control of develop-
ment in electrostatographic apparatus.
In one well known form of automatic xerographic re-
production 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 la~er thus form an
electrostatic charge pattern in conformity with the configur-
ation 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
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` 10371(~0
SyStelll S~lCh as :is described in U.S. Patent Specificat:ion
No. 308~1043.
. It often occurs that the electrostatlc 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 llquid development system) this is
~ 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.~ith
the copy material then the toner deposited in these zones
and left behind on.the plate surface after tlle transIer
operation must be removed or cleaned from the plate sur-
face 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 over-
load 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.
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Variolls what rnay conveniently be termed (~ e Lad~-o-lt
systems have been proposed for the purpose of avoiding
or reducing wastage of developer material and of elimina-
ting or reducing solid marginal edge zones on copies.
Thus in U.S. Specification No. 3687538, 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 Specification
No. 1230526, which is incorporated in a machine employing
flash exposure, a lamp e~tending transversely across the
path of a photoreceptor belt is activa-ted by a suitable
logic system a short~predetermined time after de-energi-
sation of the flash exposure lamps. In U.S. Specification
No. 3685894, a photoelectrostatic copying machine is
described whi~h has reduction mode copying facility and,
in reduction mode, portions of the copy sheet, which in
this case lS 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 otherside occur in reduc-
tion mode of operation a pair of lamps are pro~ided over
the edge zones which are switched in only when reduced mode
is selected so as automa~ically to be~energised in this mode
and scan the predetermined edge zones.
lnlilst all of the machines described above~have the
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desira~le eE~ct o~ re~lucin-r ~i~stage of toner ~nd/o~
of avoi~ing 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. Specification No.
3685894.
,
,
From one aspect, the present invention prorides 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,
. Id) 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.
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From another aspect, the inrention prorides, in or
for an electrostatographic apparatus including a photo-
; conductive plate, means for applying a uniform electro-
static charge to the plate, exposure means for exposing the
charged plate to a pattern of light and shadow to produce
an eleFtrostatic latent image on the plate, and~developer
.
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mearls for developing charged areas of the plate; Ineans
for de-tecting continuous zones of the plate which do not
receive sufficient ligllt 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 cla1ms, 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. And the discharge means may comprise
one or more discharge lamps extending transversely across
the plate.
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In a preferred form of the invention for use in amachine 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
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achieved by determil~:lng -the intensity of light required
to reduce the platc charge to a value such that developer
material will not be attracted thereto and provid:ing 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 e~ample a discharge device in the form of a
9ingle 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 ligh~.
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 i~age
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,
eaoh operated by one, or preferably a transv~rsely extend-
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in~ serics ol', detectors. The lengths o:t` such sections
wou'Ld be chosen according to known parameter~ such as
paper size, document size or reduc-tion 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 arrangements will resul'c 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.'
Whilst in some instances, the present invention will
perform the same functions as are achieved by the known
devices described above, it provides the slgniflcant
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
1 .
oi 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
.
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drawings, in wllich~ 37~Q0
Figure 1 is a schematic view of one form of document
.
- scanning sys-tem for an electrostatographic machlne~
Figure 2 is a schematic view of one emboaiment of
development control system according to the invention,
Figure 3 is a view illustrating the embodiment of
Figure 2 taken along the line A-A of Figure 1,
Figure 4 is a vlew of a unitary mounting arrangement
for the embodiment of Figure 2,
Figure 5 is an illustration of a suitable eleotronic
oontrol circuit,
Figure 6 is a representation of a document to be
copied for the purpose of explaining modes of operation
.
of development oontrol systems of this inventlon,
.
Figure 7 is a view like that of Figure 3 of a second
embodiment oI the invention, and
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Figure 8 is a vie~ lilce that of Figure 3 of a third
embodiment of the invention.
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. 1037~00
To f~lcilit~te appr~ciation of this invention~ one
form of document scanning system Eor a xerographic machine
in ~Yhich the xerographic plate is exposed to light re~
flected from succes~ive portions of a document to be
copied through a slit will first be described with
reference to the schematic representation of Figure 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 dls-
tance constant between the document 11 and a lens 1~.
A tubular lamp 15 extending across the platen 10 parallel
to the mirror 12 moves with the mirror 12 in fixed re-
lation thereto. The lamp 15 is provided as illumination
means to illuminate the document 11 through the platen 10
during scanning.
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- 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 Figure 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.
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The conLiguration or shape of the slit 19 i5, as
shown, narrower at its mid-point than at its e~tremities.
This shape, as is already ~nown in the art, is to compensate
for the non-uni~ormity 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 in-
formation 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 2~. Where the under-
side of the platen cover has a reflective surface, light
striking the platen cover beyond the edges of the
document will be re~lected along the optical path.
',
As illustrated in Figure 2 a ~erographic 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 em-
ployed, being represented schematically by an applicator
roll of such a system, but it should be appreciated that
in the case of a po~der or particulate development system
a fusing station should be provided follo~ing the transfer
station for securing the toner particles to the copy
material S in a permanent form.
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The gene.ral operation ol` the machine illustrated is
- as Iollows. Prior to exposure, the drum surface, which
comprises a layer of photoconductive insulatlng 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. Patent No. 2965756.
''
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
im~ge in image configuration corresponding to the light
image projected from the document. As the drum surface
cont nues 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 registra-
tion 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
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. 1037100
~ tO a suil;able copy~holde~ for removal by cm operator~
.
After transfer, the ~erographic drum surface passes
through a cleaning station (not illustrated) at which the
sur-face 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 Figures 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 pho-to diodes 31 extending trans-
versely across the drum. Each photo diode is associated
with a lamp ~2 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 intenslty (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 discharing the~ drum area passing beneath
it; when light o-f 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 dependant upon the
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circulllferential dislallce l~e-tweell the r~hotod-iod~ an(l the
lamp and the rotational speed Or the drum follo-ring
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 photodlode. Filament lamps wlll 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 multipliclty 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 discha~rged 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
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". ~0371'00
low level of intensity insu~ficient to discharge the drum
area therebeneath in the manner described in British
Patènt Specification No. 1230526.
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Figure 4 shows a convenient mounting arrangement for
the development control system described above in which
the detector/lamp sub-units are mounted ln an elongate
unit extending aoross the width of the drum and which
also includes the necessary control circuitry, a simpli~ied
~orm of which ior a single sub-unit is illustrated in
Fi~ure 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 su~ficiently to cause the
~ lamp current to drop and extinguish the lamp.
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As shown in Figure 4, the circuits associated with
each laap are all conneoted to bus-bars 35 leading to a
-: common supply. In the unit illustrated, each sub-unit
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is separated by a light~shield or baffle 33 and the
complete unit may be encapsulated in a transparent resin.
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This embodiment has particular application to the
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1037100
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 glas~
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 thi~ event, light from the lamp lS,which passes through the
platen in the areas surrounding the document 11 (see Figure 6)
will not be reflected down the optical path and the corres-
ponding areas of the drum surface will not be discharged dur-
ing exposure. The development control system described above
will operate as follows. Until the image of the document
edge XlX2 reaches the photodiodes 31, all lamps 32 will be lit
and the drum will be discharged. Ao edge XlX2 reflects light
the lamps will be extinguished across the portion of the drum
between Xl and X2 and an electrostatic latent image of the
document information will be produced on the drum surface.
After the YlY2 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 XlYl and X2Y2 edges. In the above
description of operation it is presupposed that the document
contains only line information and no substantial solid dark
areas.
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Further embodiments of the invention are schematically
illustrated in ~igures 7 and 8. In the embodiment of
Figure 7 a single tubular lamp 32 is arranged transversely
acros~ the drum surface and a row of photocells 31 extend
ln a row across the drum. The photocells 31 are so con-
nected to the lamp 32 that the lamp will only be lit when
all the photocells recei.ve less than the pre-set threshold
intensity of light. With this arrangement, in the cir-
cumstances described above with reference to Figure 6,
only those areas or zones of the drum beyond the document
edgcs XIX2 and YlY2 will be discharged,~Out solid~areas
within the image area on the drum will be~reproduoed. The
embodiment of Figure 8 permits the elimination of the
undischarged marglnal zones along the image cdges corres
ponding to XlYl and X2Y2 which will remain with the
embodiment of Figure 7, without eliminating solid area
development within the lmage area, where the width of
~, . . , . :
these marginal zones can be prcdetermined, c.g. as a
function of copy s1ze or reduction mode of operation of
the mac,hine. Here the lamp 32 is split into three
sections 32a, 32b and 32c, each associated with lts own
series of photocells 31. Beyond the edges corresponding
to XlX2 and YlY2 of Figure 6~ all threé lamps will be lit
but between XlX2 and YlY2 only the outer lamps 32a and
32c which are of a length corresponding to predetermincd
marginal zones of the drum, will be lit.
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~ nlilst ~articular eln~olliments have bcen 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
e~ample, instead of being arranged as shown, the photocells
could be arranged across the drum at position A indicated
in Figure 2 so as to receive adyance information of the
intensity of light about to fall on the drum.
,
Further, it is contemplated that embodimënts of this
invention could be used in conjunction with known develop-
ment control systemsS as comprehended for example by the
prior art described~above. ~ ~ ~
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