U.S. patent number 3,815,988 [Application Number 05/361,112] was granted by the patent office on 1974-06-11 for image density control apparatus.
This patent grant is currently assigned to Xerox Corporation. Invention is credited to James H. McVeigh, George N. Tsilibes.
United States Patent |
3,815,988 |
McVeigh , et al. |
June 11, 1974 |
IMAGE DENSITY CONTROL APPARATUS
Abstract
An apparatus in which the potential of a sample electrostatic
latent image recorded on a photoconductive surface is detected for
controlling the density of toner particles deposited on a single
color electrostatic latent image recorded thereon.
Inventors: |
McVeigh; James H. (Rochester,
NY), Tsilibes; George N. (Rochester, NY) |
Assignee: |
Xerox Corporation (Stamford,
CT)
|
Family
ID: |
23420693 |
Appl.
No.: |
05/361,112 |
Filed: |
May 17, 1973 |
Current U.S.
Class: |
399/39; 399/56;
118/691; 430/43.1 |
Current CPC
Class: |
G03G
15/0121 (20130101); G03G 15/065 (20130101) |
Current International
Class: |
G03G
15/01 (20060101); G03G 15/06 (20060101); G03g
015/00 () |
Field of
Search: |
;355/3,4 ;118/637
;117/17.5 ;96/1.2 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Greiner; Robert P.
Attorney, Agent or Firm: Fleischer; H. Ralabate; James J.
Green; C. A.
Claims
What is claimed is:
1. An apparatus for controlling the cut-off density of toner
particles deposited on a single color electrostatic 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
photoconductive 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 depositing means to a
potential corresponding to the sample electrostatic 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.
2. An apparatus as recited in claim 1, wherein said sensing and
biasing means includes;
a probe positioned adjacent to the photoconductive surface and
arranged to detect the potential of the sample electrostatic latent
image recorded thereon prior to toner particles being deposited on
the photoconductive surface;
a voltage source for electrically biasing said toner particle
depositing means; and
means, in electrical communication with said probe and said voltage
source, for generating an electrical output signal indicative of
the sample electrostatic latent image potential detected by said
probe to regulate the output voltage of said voltage source.
3. An apparatus as recited in claim 2, wherein successive
distinguishable single color electrostatic latent images are
recorded on the charged photoconductive surface, including an
indexable support member having a plurality of discrete neutral
density samples disposed thereon, said support member being mounted
in a light-receiving relationship with said illuminating means,
each of said neutral density samples having a pre-selected density
substantially about the cut-off density of the respective single
color electrostatic latent image corresponding thereto.
4. An apparatus as recited in claim 3, wherein the neutral density
samples disposed on said support member include:
a first neutral density sample for a green electrostatic latent
image;
a second neutral density sample spaced from said first neutral
density sample for a blue electrostatic latent image; and
a third neutral density sample spaced from said first and second
neutral density samples for a red electrostatic latent image.
5. An apparatus as recited in claim 2, wherein said electrical
signal generating means includes:
means for periodically sampling the sample electrostatic latent
image potential detected by said probe; and
circuit means for analyzing the periodically detected sample
electrostatic latent image potential and forming a continuous
electrical output signal indicative thereof.
6. 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 preselected density
corresponding to substantially about the predetermined cut-off
density of the single color electrostatic latent image;
means for exposing the charged photoconductive 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 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
photoconductive 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 depositing means to a
potential corresponding to the sample electrostatic 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.
7. A printing machine as recited in claim 6, wherein said sensing
and biasing means includes:
a probe positioned adjacent to the photoconductive surface and
arranged to detect the potential of the sample electrostatic latent
image recorded thereon prior to toner particles being deposited on
the photoconductive surface;
a voltage source for electrically biasing said toner particle
depositing means; and
means, in electrical communication with said probe and said voltage
source, for generating an electrical output signal indicative of
the sample electrostatic latent image potential detected by said
probe to regulate the output voltage of said voltage source.
8. A printing machine as recited in claim 7, wherein successive
distinguishable single color electrostatic latent images are
recorded on the charged photoconductive surface, including an
indexable support member having a plurality of discrete neutral
density samples disposed thereon, said support member being mounted
on the printing machine in a light-receiving relationship with said
exposing means, each of said neutral density samples having a
pre-selected density substantially about the cut-off density of the
respective single color electrostatic latent image corresponding
thereto.
9. A printing machine as recited in claim 8, wherein the neutral
density samples disposed on said support member include:
a first neutral density sample for a green electrostatic latent
image;
a second neutral density sample spaced from said first neutral
density sample for a blue electrostatic latent image;
a third neutral density sample spaced from said first and second
neutral density samples for a red electrostatic latent image.
10. A printing machine as recited in claim 7, wherein said
electrical signal generating means includes
means for periodically sampling the sample electrostatic image
potential detected by said probe; and
circuit means for analyzing the periodically detected sample
electrostatic latent image potential and forming a continuous
electrically output signal indicative thereof.
Description
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
electrostatically, 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. Pat. No.
2,297,691 issued to Carlson in 1942.
Multi-color electrophotographic printing is similar 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.
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 to 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 electrophotographic 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 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 a primary object of the present invention to
improve the development system utilized in a multi-color
electrophotographic printing machine by sensing changes in the
electrical characteristics of the photoconductive surface and
varying the potential of the development system in response
thereto.
SUMMARY OF THE INVENTION
Briefly stated, and in accordance with the present invention, there
is 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 the present instance, 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 latent 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 electrostatic latent image has a potential
intermediate that of the 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 the present invention, the
sensing and electrical biasing means electrically bias the toner
particle depositing means to a potential corresponding 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.
BRIEF DESCRIPTION OF THE DRAWINGS
Other objects and advantages of the present invention will become
apparent upon reading the following detailed description and upon
reference to the drawings, in which:
FIG. 1 is a schematic perspective view of a multi-color
electrophotographic printing machine having the present invention
incorporated therein;
FIG. 2 is a schematic illustration of the light source and disc of
neutral density samples utilized in the FIG. 1 printing
machine;
FIG. 3 is a partial elevational view of the development system and
the probe utilized therein to sense the potential of the sample
electrostatic latent image recorded on the photoconductive surface;
and
FIG. 4 is 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, FIG. 1 schematically
illustrates a multi-color electrophotographic 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 FIG. 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 electrophotographic printing
machines, and is not necessarily limited to the particular
embodiment shown therein.
As shown in FIG. 1, the printing machine employs a drum 10 having a
photoconductive surface 12 secured thereto and entrained about the
exterior circumferential 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. Pat. 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. Pat. 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. A suitable moving lens system is disclosed
in U.S. Pat. No. 3,062,108 issued to Mayo in 1962, and a suitable
color filter mechanism is described in co-pending application Ser.
No. 830,282 filed in 1969. 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
positioned in filter 20 forming a sample electrostatic latent image
on photoconductive surface 12 which is a strip discharged to the
desired potential. The potential of the sample electrostatic latent
image recorded on photoconductive surface 12 is detected by probe
28, i.e. a suitable electrometer 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 sample 36 is a neutral 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 appropriate 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 predetermined 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
photoconductive 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 is less thin than that of the development
system. Each of the neutral density 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 corresponds 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 FIG. 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
electromagnetic wave spectrum, heretofore referred to as a single
color electrostatic latent image.
After exposure, drum 10 rotates the single 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 development system employing a plurality of
developer units is disclosed in co-pending application Ser. No.
255,259, filed in 1970. 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
developer roll 86, 88 and 90 (FIG. 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 substantially 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 respective 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 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. Pat.
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 velocity
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 FIG. 1, the sheet feeding path for
advancing support material 44 to transfer roll 46 will be briefly
described hereinafter. A stack 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. Next, 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. Pat. No.
3,498,592 issued to Moser et al. in 1970. After the multi-layered
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 photoconductive surface 12 as it passes through cleaning
station F. At cleaning station F, residual toner particles are
initially brought under the influence 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. Pat. No.
3,590,412 issued to Gerbasi in 1971. Brush 76 is positioned at
cleaning station F and maintained 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 FIG. 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 25 disposed 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 FIG. 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 electrostatic latent
image corresponding in density to the neutral density sample is
recorded on photoconductive surface 12.
Referring now to FIG. 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
electrostatic 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 operatively 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 photoconductive 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 carriage 78 when light source 26 is in the initial
position. This enables disc 22 to index such that a neutral density
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 FIG. 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 hold circuit by normally
open contact 102.
Referring once again to FIG. 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 to power supply 84 (FIG.
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
electrostatic 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 prescribed set of conditions and having
specified densities to achieve desired copy characteristics.
Furthermore, 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. an 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
predetermined 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 substantially the same potential. Thus, toner particles are
attracted to those regions of the photoconductive surface 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 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 in
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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