Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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BACKGROUND OF THE INVENTION
This invention relates generally to a multi-color
electrophotographic printing machine, and more particularly
concerns an apparatus for controlling the density of toner
particles deposited on a single color electrostatic latent
image recorded on a charged photoconductive surface.
In the process of electrophotographic printing, a
photoconductive surface is uniformly charged and exposed
to a light image of an original document. Exposure of the
photoconductive surface records thereon an electrostatic
latent image corresponding to the original document. The
electrostatic latent image is then rendered visible by
depositing thereon toner particles which adhere electro-
statically, in image configuration, thereto. Thereafter,
the toner powder image may be transferred to a sheet of
support material. The toner powder image is, then,
permanently affixed to the support material to provide
a copy of the original document. The foregoing process
was originally disclosed in U. S. Patent No. 2,297,691
issued to Carlson in 1942.
Multi-color electrophotographic printing is slmilar
to the heretofore discussed process with the following
exceptions. Rather than forming a total light image of
the original document, the light image is filtered producing
a single color light image which is a partial light image
of the original. The foregoing single color light image
exposes the charged photoconductive surface to record thereon
a single color electrostatic latent image. This single color
electrostatic latent image is developed with toner particles
of a color complementary to the single color light image.
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Subsequently, the single color toner powder image is
transferred to the sheet of support material. The
foregoing process is repeated a plurality of cycles
with differently colored light images and the respective
complementary colored toner particles. Each single color
toner powder image is transferred ~ the support material
superimposed in registration with the prior toner powder
image to form a composite multi-layer powder image thereon.
This multi-layered toner powder image is coalesced and
permanently affixed to the support material forming a
composite image corresponding in color to the original
document.
It is apparent that in multi-color electro-
photographic printing machines, the characteristics of
the photoconductive surface are critical. Preferably,
the electrical characteristics of the photoconductive
surface should remain substantially constant. However,
it has been found that the electrical characteristics
of the photoconductive surfaces will vary with temperature
changes or with continuous usage thereof. Hence, it is
extremely difficult to maintain substantially the same
potential on the photoconductive surface for light images
projected thereon having substantially identical intensities.
Moreover, electrophotographic printing machines frequently
utilize magnetic brushes to produce viewable toner powder
images on the electrostatic latent image recorded on the
photoconductive surface. Toner particles are attracted
from the magnetic brush to the charged photoconductive
surface.
In multi-color electrophotographic printing, the
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imaged areas are developed with the toner particles whereas -~
the non-image areas remain substantially devoid of toner
particles. However, it is evident that some toner particles -
will be attracted to the non-image areas inasmuch as a residual
charge remains thereon. Hence, it is desirable to electrically ~ ~ :
bias the magnetic brush to a potential intermediate that of
the non-image areas respective single color electrostatic
latent image.
Accordingly, it is an object of an aspect of the
present invention to improve the development system utilized in
a multi-color electrophotographic printing maching by sensing
changes in the electrical characteristics of the photocon-
ductive surface and varying the potential of the development
system in response thereto.
SUMMARY OF THE INVENTION
Briefly stated, and in accordance with one aspect
of the present invention, there i5 provided an apparatus for -
controlling the density of toner particles deposited on a
single color electrostatic latent image recorded on a charged
photoconductive surface.
In an embodiment, the apparatus includes at least
one neutral density sample, illuminating means, toner particle
depositing means, and sensing and electrical biasing means.
Preferably, the neutral density sample has a pre-selected
density corresponding to substantially about a predetermined
cut-off density for the single color electrostatic la~ent
image recorded on the photoconductive surface. The illuminating
means irradiates the neutral density sample and projects the
light image formed thereof onto the charged photoconductive
surface. In this way, a sample electrostatic latent image is
recorded on the photoconductive surface. The sample electro-
static latent image has a potential intermediate that of the ~ -
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single color electrostatic latent image and the non-image
regions of the charged photoconductive surface. The potential
of the sample electrostatic latent image recorded on the
charged photoconductive surface is detected by the sensing and
electrical biasing means. Pursuant to an aspect of the present
invention, the sensing and electrical biasing means electrically
bias the toner particle depositing means to a potential corres-
ponding to that of the sample electrostatic latent image
recorded on the charged photoconductive surface. Hence, toner
particles are deposited on regions of the photoconductive
surface having a potential greater than the potential of the
sample electrostatic latent image.
In accordance with another aspect of this invention
there is provided an apparatus for controlling the cut-off
density of toner particles deposited on a single color electro-
static latent image recorded on a charged photoconductive
surface, including: at least one neutral density sample having
a pre-selected density corresponding to substantially about the
predetermined cut-off density of the single color electrostatic
latent image; means for illuminating said neutral density
sample and projecting the light image formed thereof onto the
charged photoconductive surface to record thereon a sample
electrostatic latent image having a potential intermediate the
single color electrostatic latent image and the non-image
regions of the charged photoconductive surface; means for
depositing toner particles, complementary in color to the
single color electrostatic latent image, on the charged photo-
conductive surface; and means for sensing the potential of the
sample electrostatic latent image recorded on the charged
photoconductive surface and electrically biasing said toner
particle depositinq means to a potential correspondinq to the
sample electrostatic latent image potential so that toner
particles are deposited on regions of the photoconductive surface
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having a potential substantially greater than the potential , -
of the sample electrostatic latent image.
In accordance with another aspect of this invention
there is provided an electrophotographic printing machine of
the type having a photoconductive surface, including: means
for charging the photoconductive surface to a substantially
uniform potential; at least one neutral density sample having ~'
a pre-selected density corresponding to substantially about ~:
the predetermined cut-off density of the single color electro- '~
static,latent image; means for,exposing the charged photo-
conductive surface to a single color light image of an original '
document to record thereon a single color electrostatic latent '
image, said exposing means being arranged to illuminate said
neutral density sample and project a light image thereof onto . ::
the charged photoconductive surface to record thereon a sample
eleetrostatic latent image having a potential intermediate
the single color electrostatic latent image and the non- ~.,
image regions of the charged photoconduetive surface; means '
for depositing toner particles, complementary in color to the ~ :
20 'single color electrostatic latent image, on the charged ~'
photoconductive surface; and means for sensing the potential ..
of the sample eleetrostatic latent image recorded on the
charged photoconduetive surfaee and eleetrieally biasing said
toner particle depositing means to a potential corresponding
to the sample electrostatie latent image potential so that
toner particles are deposited on regions of the photoconductive
surface having a potential substantially greater than the
potential of the sample electrostatic latent image.
In accordance with another aspect of this invention
there is provided apparatus for controlling the cut-off density
of toner particles deposited by a developer means on a single
color latent electrostatic image recorded on a charged photo-
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conductive surface, said developer means including potential
means for biasing the toner particles, the bias on said devel-
oper means being controlled in response to variations in the
electrical characteristics of the photoconductive surfaces
which the single color electrostatic latent image is formed,
at least one neutral density sample having a pre-selected
density corresponding substantially to the predetermined
cut-off density of the single color electrostatic latent image,
means to illuminate the sample and pro]ect the light image.
thereof onto a charged photoconductive surface, a sensor
located in proximity of the charged photoconductive surface
for detecting the potential level of the sample image on the
charged photoconductive surface prior to the deposition of
toner particles on the surface, the sensor coupled to said
developer potential means to bias the toner particles to a
potential corresponding to the potential of the sample image.
BRIEF DESCRIPTION OF THE DRAWINGS
Other objects and advantages of the present inven-
tion will become apparent upon reading the following detailed
description and upon reference to the drawings, in which
Figure 1 is a schematic perspective view of a
multi-color electrophotographic printing machine having the
present invention incorporated therein;
Figure 2 is a schematic illustration of the light
source and disc of neutral density samples utilized in the
Figure 1 printing machine.
Figure 3 is a partial elevational view of the
development system and the probe utiliæed therein to sense `.
the potential of the sample electrostatic latent :.:
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image recorded on the photoconductive surface; and
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Figure 4nis a schematic circuit diagram for
periodically sampling the sensed sample electrostatic
latent image potential.
While the present invention will be described
in connection with a preferred embodiment, it will be
understood that it is not intended to limit the invention
to that embodiment. On the contrary, it is intended to
cover all alternatives, modifications, and equivalents
as may be included within the spirit and scope of the
invention as defined by the appended claims.
DETAILED DESCRIPTION OF THE INVENTION
With continued reference to the drawings,
Figure 1 schematically illustrates a multi-color electro-
photographic printing machine employing the present
invention. In the drawings, like reference numerals
have been used throughout to designate like elements.
The multi-color electrophotographic printing machine
shown schematically in Figure 1, illustrates the various
components used to produce multi-color copies from a
colored original. Although the apparatus of the present
invention is particularly well adapted for use in an
electrophotographic printing machine, it will become
evident from the following description that it is equally
well suited for use in a wide variety of electrophoto-
graphic printing machines, and is not necessarily limited
to the particular embodiment shown therein.
As shown in Figure 1, the printing machine
employs a drum 10 having a photoconductive surface 12
secured thereto and entrained about the exterior circum-
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ferential surface thereof. Drum 10 is mounted rotatably '
within the machine frame (not shown). A series of
processing stations are positioned such that as drum 10
rotates in the direction of arrow 14, photoconductive
surface 12 passes sequentially therethrough. Drum 10 is
driven at a predetermined speed relative to the other
machine operating mechanisms by a common drive motor
(not shown). One type of suitable photoconductive
material is disclosed in U. S. Patent No. 3,655,377,
issued to Sechak in 1972. A timing wheel is mounted
in the region of one end of drum 10 and adapted to
trigger the logic circuitry of the printing machine.
This coordinates the various machine operations with one
another to produce the proper sequence of the events at
the appropriate processing stations.
Initially, drum 10 moves photoconductive surface
12 through charging station A. Charging station A has
positioned thereat a corona generating device, indicated
generally at 16. Corona generating device 16 extends in
a generally longitudinal direction transversely across
photoconductive surface 12. This readily enables corona
generating device 16 to charge photoconductive surface
12 to a relatively high substantially uniform potential.
Preferably, corona generating device 16 is of the type
described in U. S. Patent No. 2,778,946 issued to Mayo in
1957.
Drum 10, thereafter, is rotated to exposure
station B. Exposure station B includes thereat a moving
lens system, generally designated by the reference number
18, and a color filter mechanism shown generally at 20.
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A suitable moving lens system is disclosed in U.5. Patent
No. 3,062,108 issued to Mayo in 1962, and a suitable color
filter mechanism is described in Canadian patent ~15,490
issued November 28, 1972. Disc 22 has a plurality of neutral
density samples (in this case 3) disposed thereon. Disc 22
is mounted rotatably in the printing machine and is disposed
beneath transparent platen 24 within the half angle of the
optical system. Before the light source lamps indicated
generally by the reference numeral 26, begin to scan, they
will be actuated to illuminate one of the neutral density
samples. In this way, a sample electrostatic latent image is
recorded, on photoconductive surface 12 as drum 10 rotates.
Lamps 26 are stationary and the appropriate filter is posi-
tioned in filter 20 forming a sample electrostatic latent
image on photoconductive surface 12 which is a strip dis-
charged to the desired potential. The potential of the sam-
ple electrostatic latent image recorded on photoconductive
surface 12 is detected by probe 28, i.e. a suitable elec-
trometer disposed adjacent to photoconductive surface 12
intermediate exposure station B and development station C.
The electrical output signal from probe 28 is processed by
circuit elements 30 which regulate voltage source or power
supply 84 adjusting the bias voltage of the respective
developer unit having toner particles complementary in color ~-
to the filter of filter mechanism 20. Preferably, disc 22
includes three equally spaced neutral density samples
located about the periphery thereof. Sample 32 is a neutral
density sample for green separation, sample 34 is a neutral
density sample for red separation and
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sample 36 is a neutral density sample for blue separation.
Preferably the green separation sample has a density of
0.32, the blue separation sample a density of 0.35, and
the red separation sample a density of 0.15. The appro-
priate neutral density sample is illuminated by light
source 26 to produce a sample electrostatic image
corresponding to a predetermined development density
for the filter being used, i.e. a green filter will have
neutral density sample 32 illuminated forming a sample
electrostatic latent image corresponding to the predeter-
mined development density for the green separation.
In multi-color electrophotographic printing, a
single color light image exposes the charged photoconductive
surface. The potential on the charged photoconductive
surface in the area irradiated by the single color light
image is reduced. The potential of the charged photo-
conductive surface in the non-irradiated areas remain
substantially unchanged. During development, toner
particles, complementary in color to the single color
light image, are deposited on the photoconductive surface.
The irradiated areas remain substantially devoid of toner
particles. The development system is biased such that the
potential thereof is intermediate the irradiated and non-
irradiated areas. In this way, toner particles are
attracted to the non-irradiated areas from the development
system since the potential of the non-irradiated areas is
greater than the potential of the development system,
whereas toner particles are not attracted to the irradiated
areas inasmuch as the charge thereof if less thin than that
of the development system. Each of the neutral density
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samples form a sample electrostatic latent image. The
charge of the sample electrostatic latent image is greater
than that of the irradiated areas and less than that of the
non-irradiated areas. The developer unit is adjusted to
the potential of the sample electrostatic latent image.
Thus, toner particles are attracted to all regions of the
charged photoconductive surface having a potential greater
than that of the sample electrostatic latent image. The
potential of the sample electrostatic latent image corre-
sponds to the washout density of the single color toner
powder image, i.e. the potential beneath which development
of the single color electrostatic latent image does not
occur. However, if the potential of the single color
electrostatic latent image is greater than that of the
sample electrostatic latent image development will occur.
With continued reference to the Figure 1, after
the sample electrostatic latent image is formed on the
charged photoconductive surface, an original document 25,
such as a book, sheet of paper, or the like, disposed
upon transparent viewing platen 24 is scanned. Lamps 26
and lens 18 move in a timed relation with drum 10 to scan
successive incremental areas of original document 25
disposed upon platen 24. This creates a flowing light
image of original document 25 which is projected onto
charged photoconductive surface 12. Filter mechanism 20
is adapted to interpose selected color filters into the
optical light path. The appropriate color filter operates
on the light rays passing through lens 18 to record an
electrostatic latent image on photoconductive surface 12
corresponding to a pre-selected spectral region of the
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electromagnetic wave spectrum, heretofore referred to as a
single color electrostatic latent image.
After exposure, drum 10 rotates the sing~e color
electrostatic latent image recorded on photoconductive surface
12 to development station C. Development station C includes
thereat three individual developer units, generally indicated
by the reference numerals 38, 40, and 42. A suitable develop-
ment system employing a plurality of developer units is dis-
closed in Canadian patent 982,886 issued February 3, 1976.
Preferably, the developer units are all of a type generally
referred to as magnetic brush developer units. A typical
magnetic brush developer unit utilizes a magnetizable developer
mix having carrier granules and toner particles. The developer ;
mix is continually brought through a directional flux field to
form a brush thereof. Each developer unit includes a develop-
er roll ~6, 88 and 90 (Figure 3) electrically biased to the
appropriate potential such that toner particles are attracted
to the image areas (non-irradiated areas) rather than the non-
image areas (irradiated areas) of the photoconductive surface
12. The potential applied to the developer roll is substan-
tially equal to that of the sample electrostatic latent image
recorded on photoconductive surface 12 and detected by probe
28. The single color electrostatic latent image recorded on
photoconductive surface 12 is developed by bringing the brush
of developer mix into contact therewith. Each of the respec-
tive developer units contain discretely colored toner particles
corresponding to the complement of the spectral region of the
wavelength of light transmitted through filter 21, e.g. a
green filtered
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electrostatic latent image is rendered visible by depositing
green absorbing magenta toner particles therein, blue and
red latent images are developed with yellow and cyan toner
particles, respectively.
Drum 10 is, next, rotated to transfer station D
where the toner powder image adhering electrostatically to
photoconductive surface 12 is transferred to a sheet of final
support material 44. Support material 44 may be plain paper,
or a sheet of transparent, thermoplastic material. A
transfer roll, shown generally at 46, rotates support
material 44 in the direction of arrow 48. Transfer roll
46 is electrically biased to a potential of sufficient
magnitude and polarity to electrostatically attract toner
particles from photoconductive surface 12 to support
material 44. U. S. Patent No. 3,612,677, issued to
Langdon et al. in 1972, discloses a suitable electrically
biased transfer roll. Transfer roll 46 is arranged to
rotate in synchronism with drum 10, i.e. transfer roll
46 and drum 10 rotate at substantially the same angular
velooity and have substantially the same outer diameter.
Inasmuch as support material 44 is secured to transfer
roll 46 for movement therewith in a recirculating path,
successive toner powder images may be transferred from
photoconductive surface 12 to support material 44, in
superimposed registration with one another. Hence, a
multi-color toner powder image corresponding in color to
the original document is formed on support material 44.
With continued reference to Figure 1, the sheet
feeding path for advancing support material 44 to transfer
roll 46 will be briefly described hereinafter. A stack
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50 of support material 44 is supported on tray 52. Feed
roll 54, operatively associated with retard roll 56, separates
and advances the uppermost sheet from stack 50. The advancing
sheet moves into chute 58 and is directed into the nip of
register rolls 60. ~ext, gripper fingers 62, mounted on
transfer roll 46, releasably secure thereto support material
44 for movement therewith in a recirculating path.
After all of the discretely colored toner powder
images have been transferred to support material 44, gripper
fingers 62 space support material 44 from transfer roll 46.
This enables stripper bar 64 to be interposed between support
material 44 and transfer roll 46 separating support material
44 therefrom. After support material 44 is stripped from
transfer roll 46, it is moved on endless belt conveyor 66 to
fixing station E.
At station E, a suitable fuser, indicated
generally at 68, coalesces and permanently affixes the
toner powder image to support material 44. A typical
fuser is described in U. S. Patent ~o. 3,498,592 issued
to Moser et al. in 1970. After the multi-layer toner
powder image is fixed to support material 44, endless
belt conveyors 68 and 70 advance support material 44 to
catch tray 72. Catch tray 72 is readily accessible so
that an operator may remove the final multi-color copy
from the printing machine.
Invariably, residual toner particles remain on
photoconductive surface 12 after the transfer of the
toner powder image therefrom to support material 44.
These residual toner particles are removed from photo-
conductive surface 12 as it passes through cleaning
station F. At cleaning station F, residual toner
particles are initially brought under the influence
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of a cleaning corona generating device (not shown) adapted
to neutralize the electrostatic charge remaining on the
residual toner particles and photoconductive surface.
The neutralized toner particles are then removed from
photoconductive surface 12 by rotatably mounted brush
76. A suitable brush cleaning device is described in
U. S. Patent No. 3,590,412 issued to Gerbasi in 1971.
Brush 76 is positioned at cleaning station F and main-
tained in contact with photoconductive surface 12. Thus,
the residual toner particles remaining on photoconductive
surface 12, after each successive transfer operation, are
readily removed therefrom.
Turning now to Figure 2, there is shown lamp
carriage 78 supporting a pair of light sources or lamps
26 thereon. Lamp carriage 78 is arranged to traverse
platen 24 illuminating incremental widths of original
document 25disposed therein. A suitable belt drive
system advances lamp carriage 78 in the direction of
arrow 80 to scan successive incremental areas of the
original document 25 and returns lamp carriage 78 in the
direction of arrow 82 to the initial position. Disc 22
is mounted rotatably on the printing machine frame and
is interposed between lamp carriage 78 and platen 24.
Thus, when lamp carriage 78 is in the initial position,
prior to the initiation of the scan cycle, disc 22 is
indexed so that light source 26 illuminates one of the
neutral density samples disposed thereon. For example,
in Figure 1, neutral density sample 32 is shown in
position to be illuminated. Light source 26 remains
stationary as drum 10 rotates so that a sample electro-
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static latent lmage corresponding in density to the neutral
density sample is recorded on photoconductive surface 12.
Referring now to Figure 3, there is shown
developer units 38, 40 and 42, probe 28 and drum 10.
Probe 28 is secured in the machine frame and positioned
between exposure station B and development station C.
Probe 28 is seated within the machine's support housing
and arranged to detect the sample electrostatic latent
image recorded on photoconductive surface 12. A light
image of the neutral density sample is projected onto
the charged photoconductive surface recording a sample
electrostatic latent image thereon. The sample electro-
static latent image is detected by probe 28. The machine
logic is arranged to generate a signal during each print
cycle initiating the formation of the sample electrostatic
latent image. In practice, the signal is generated when
light source 26 is in the initial position prior to scanning
of original document 25. A voltage indicative of the sample
electrostatic latent image is sensed by probe 28 and processed
by electrical circuitry 30 to produce an electrical output
signal regulating voltage source or variable power supply
84. Power supply 84 is operati.vely connected to developer
rolls 86, 88 and 90, respectively, of the corresponding
developer units 38, 40, and 42. Power supply 84 regulates
the electrical potential applied to the respective developer
rolls 86, 88 and 90. In this way, each of the developer
rolls is selectively biased to a potential substantially
identical to that of the appropriate sample electrostatic
latent image potential recorded on photoconductive surface :
12. Thus, the developer roll potential is intermediate
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the potential of the irradiated and non-irradiated areas
in photoconductive surface 12. The signal generated by
the machine logic has a pulse of sufficient duration to
de-energize the drive of lamp carrlage 78 when light
source 26 is in the initial position. This enables
disc 22 to index such that a neutral densi~ty sample is
illuminated by light source 26. The resulting light
image thereof is projected onto the moving photoconductive
surface forming the sample electrostatic latent image
thereon. After the sample electrostatic latent image is
formed, a second pulse of sufficient duration is generated -
by the machine logic actuating the drive system of lamp
carriage 78 so that light source 26 illuminates incremental
portions of original document 25 as it moves thereacross.
This creates a single color electrostatic latent image on
photoconductive surface 12 after the corresponding sample
electrostatic latent image is recorded thereon. As drum
10 rotates the sample electrostatic latent image recorded
thereon, it passes adjacent to probe 28. Probe 28 senses
the potential of the sample electrostatic latent image
and develops a voltage signal indicative thereof.
As shown in Figure 4, the voltage signal from
probe 28 is processed by unity gain amplifier 92. A
suitable amplifier having a high impedance can be utilized
in conjunction with the probe of the present invention.
The electrical output from amplifier 92, is transmitted
through two successive amplifier stages 94 and 96, and
then applied to a hold circuit including a high impedance
unity gain amplifier 98 and a capacitor 100. However,
the signal is initially prevented from passing to the
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hold circuit by normally open contact 102.
Referring once again to Figure 4, probe 28
includes a sensing element 104 surrounded by an insulator
106. Insulator 106 is preferably fabricated from a material
which is electrically insensitive to humidity changes and
functions to maintain a high probe-to-ground resistance.
Conductive shield 108 is disposed around insulator 106
and the output from amplifier 92 is fed back to shield
108. This maintains shield 108 at the same potential
as amplifier 92 reducing current leakage from sensing
element 104 to the surrounding electrical ground. The
machine logic, preferably, includes suitable circuitry
adapted to close contact 100 at the appropriate time.
Thus, the sample voltage is applied across the high impedance
unity gain amplifier 98. Closing contact 102 causes two
discrete conditions to occur. Initially, the sensed
sample electrostatic latent image potential is applied
across the high impedance amplifier 98 and secondarily,
capacitor 100, in the hold circuitry, is charged to the
sample electrostatic latent image potential. Termination
of the signal from the machine logic after the sample
electrostatic latent image has passed probe 28 permits
contact 102 to return to its normally open position.
However, the sample electrostatic latent image potential
is stored on capacitor 100 and continues to be impressed
across amplifier 98. Because of the high impedance of
amplifier 98, a relatively constant output is maintained
during the hold periods until the subsequent reclosing
of contacts 102 provides a new sample electrostatic
latent image potential. This output voltage is applied
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to power supply 84 ( Figure 3 ) holding the output voltage
therefrom substantially constant until the next sample
signal is received thereby. If the potential level of the
next sample electrostatic latent image differs from that
of the first sample electrostatic latent image, capacitor
100 is allowed to recharge to the new potential through
contact 102 and through the circuitry of amplifier 96.
The new sample electrostatic latent image potential is
impressed across the high-impedance hold amplifier 98 and
capacitor 100 is recharged to this new voltage. The output
voltage is supplied to power supply high-voltage operational
amplifier 110 which holds the voltage output from power
supply 84 substantially constant until the next signal is
received. At the end of the sample period, contact 102 is
again open and the hold circuit waits for the next sample.
It is evident, therefore, that this type of arrangement
permits the present apparatus to detect both increases
and decreases in the potential of the sample electro-
static latent image recorded on photoconductive surface
12 while, substantially simultaneously therewith, generating
a continuous control signal for regulating the potential
applied to developer rolls 86, 88 and 90 of developer units
38, 40 and 42, respectively.
While the present invention has been described
in connection with a single set of three neutral density
samples, one skilled in the art will appreciate that the
invention is not necessarily so limited and that a plurality
of such sets may be utilized, each set corresponding to a
prescrihed set of conditions and having specified densities
to achieve desired copy characteristics. Furthermore,
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while the present invention has been described as utilizing
a disc, it will be apparent to one skilled in the art that
the neutral density samples may be mounted on any suitable
support arranged to be appropriately indexed, e.g. and
endless conveyor belt.
In recapitulation, it is apparent that the apparatus
of the present invention controls the cut-off density of
toner particles deposited on a single color electrostatic
latent image recorded on a charged photoconductive surface.
This is achieved by exposing the charged photoconductive
surface to a neutral density sample having a pre-selected
density corresponding to substantially about the predeter-
mined cut-off density of the single color electrostatic
latent image. In this way, a sample electrostatic latent
image is recorded on the photoconductive surface. The
potential of the sample electrostatic latent image is
employed to electrically bias the developer roll of the
corresponding magnetic brush developer unit to substan-
tially the same potential. Thus, toner particles are
attracted to those regions of the photoconductive sur-
face having a potential greater than that of the sample
electrostatic latent image. Inasmuch as the potential
of the non-image region is substantially less than that
of the image region, toner particles are not attracted
thereto and the image region of photoconductive surface
12 has toner particles deposited thereon.
It is, therefore, evident that there has been
provided, in accordance with the present invention, an
apparatus for controlling the cut-off density of toner
particles deposited on a single color electrostatic latent
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image recorded on a photoconductive surface that fully
satisfies the objects, aims, and advantages set forth
above. While this invention has been described in
conjunction with specific embodiments thereof, it is
evident that many alternatives, modifications and
variations will be apparent to those skilled iIl the
art. Accordingly, it is intended to embrace all
alternatives, modifications, and variations that
fall within the spirit and broad scope of the appended
claims.
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