Note: Descriptions are shown in the official language in which they were submitted.
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Background o~ the Invention
In the art of electrostatic copying in which
the surface of a photoconductor carried by a conduc-tor
su~port first is charged, then exposed to a light image
and then subjected to the action of a developer, organic
photoconductors have recently come into relatively wide use.
While pho~oconductors of this type have many advantages
over inorganic photoconductors, they have one significant
disadvantage. Upon exposure to light the charge on the
photoconductor does not leak off as rapidly as is desirable.
Thus, in any co~ying apparatus whlch is to operate at a
reasonable rate of speed, an organic photoconductor
retains a significant charge in background or non-image
areas after normal exposure to the copy to be reproduced.
This background level may be in the range of from about
100 to about 200 voltsO
~any attempts have been made in the prior art
to overcome the problem of deposit of developer upon
background areas owing to the residual potential -thereon.
For example, it has been suggested that the developer
station be provided with a biasing electrode to which
a potential is applied to counteract the effect of the
residual potential in background areas. One problem
in using a fixed biasing potential is that the background
potential varies over a relatively wide range so that
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either deve:Lopment of background areas takes place :if the
biasin~ po-tential is not large enough or toner is deposited
on the biasing el~ctrode if -too large a biasing potential
is employed. ~-t will be apprecla-ted further that a
biasing po-ten-tial should be applied to the electrode only
durlng the period oE time durinq which the latent image
is passing through the developer system. If the biasing
potential is not switched off~ relatively great amounts of
toner will be deposited on the biasing electrode when
uncharged areas of the drum pass through the deve].oper stationO
Attempts have been made in the prior art to
provide systems which vary the biasing potential in response
to variations in the potential of bac~ground areas. For
example, Coriale Patent 3,611,982 shows an arrangement
in which a capacitive probe, located outside and just
before t:he developer unit, is exposed by a shutter to a
charged and fulIy exposed strip at the edge of the photo-
conductor drum. The sensed potential is amplified and
is used to control a variable power source which provides
the biasing potential for the electrode located in the
developer system. Another example of a bias voltage
con-trol system is shown in Coriale Patent 3,788,739,
in which a capacitive probe, located outside and just
ahead of the developer system, senses the potential of a
part of an oversize exposed area outside the image area
to control the bias potential applied to an electrode in
the de~elo~er unit. A further example of the use of a
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capacitance probe to regulate the bias applied by a
source to a biasing electrode is shown in Smith Patent
3,782,818. The probe of Smith, like those of Coriale,
is located just ahead of and outside of the developer
applicator unit in which the biasing electrode is disposed.
Parmigiani Patent 3,575,505 shows an arrangement in which
the developer system bias voltage is changed in response
; to the number of`copies made in an attempt to compensate
~or changes in the characteristics of the photoconductor
over a period of time.
The systems of the prior art discussed herein~
above sense photoconductor voltage by the use of delicate
and sensitive instruments such as elec-trometers for measuring
the charge in residual areas of the photoconductor.
Such instruments are not only expenslve, but also involve
critical factors such as the particular geometry of the
probe and the critical distance of the probe from the
surface carrying the potential to be sensed. The arrange-
ments of the prior art, moreover, employ switching arrange~
: 20 ments for rendering the bias
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effec-tive only for the period o:E time during which the image
passes through the developer system. In addi-tionl owing to the
deposition of toner particles on the biasing electrode, unless
some means is provided for cleaning this electrode, it will
rapidly become so contaminated as to render the system
inoperative.
We have invented an automatic development ele~trode . .
bias control system for inhibiting deposit of toner on back-
ground areas which overcomes the defects of systems o the
1~ prior art. The parameters of our system are non-criticalO
Our assembly is relatively inexpensive to construct. Our
construction is such as to insure that the bias will at all
ti.mes be sufficient to prevent deposition of toner on back-
ground areas. We provide our system with automatic means for
removing toner deposited on the biasing electrode without the
use of mechanical cleaning means.
SUMMARY OF THE INVENTION
An object of the invention is to provide an automatic
development electrode bias control system.
In one particular aspect of the present invention
there is provided a method of electrically biasing a
developing electrode disposed closely adjacent to a photo-
conductive member of an electrophotographic device after the
photoconductive member has been charged and exposed to a light
image. The electrophotographic aevice is of the wet-type
having a developer unit utilizing a developing solution.
The method comprisiny the s-teps of a) automatically sensing
through the developing solution the potential remaining on the
photoconductive member by means of electrostatic induction and
the electrical conductivitv of the developing solu-tion; b)
computing biasing voltage in accordance with a predetermined
value of the sensed potential; and c) automatically applying
the biasing voltage to the developing electrode.
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In a further particulax aspect the presen-t
invention provides a development apparatus for an electrostatic
copier including in combination, a drum having an arcuate
surface, a photoconductor comprising at least a portion of the
arcuate surface of said drum, means for electrastatically
charging said photoconductor, means for exposing a portion of
the charged photoconductor to a pattern o~ light and shade to
produce an electrostatic image, said image occupying a region
of the arcuate surface of said drum of less than 360 angular
extent, the arcuate surface of said drum including a non-image
area of appreciable angular extent, development means for
applying to said image a developer liquid containing dispersed
toner particles~ means for rotating the drum to carr-y
sequentially said image and said non-lmage area past the
development means, the development means comprising an
electrode and means for electrically bia~ing said electrode
to provide adjacent said electrode a first electric field
which attracts toner particles to said electrode, and means
operable during passage of a non-image area past said
development means for providing adjacent said electrode a
second electric field which repels toner particles from said
electrode.
BRIEF DES~RIPTION OF THE DRAWINGS
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In the accompanying drawings to which reference
- is made in the accompanying specification and in which like
reference characters indicate like parts in the various views:
~ igure 1 is a partially schematic end elevation of an
electrostatic copying machine which may be provided with our
automatic development electrode bias control system.
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Figure 2 ls a perspeetive view with parts removed,
with other parts broken away, and with parts shown ln ~ection,
illustrating our automatic development
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electrode bias control s~stem.
FIGUI~E 3 is a schematic view of one form
of an electrical circuit which may.be employed in our
automatic development electrode bias control s~stem.
Description of the Preferred Embodiment
Referring now to FIGURES 1 and 2, a machine
indicated generally by the reference character 10,
with which our system ma~ be used, includes a drum
indicated generally by the reference character 12 made
up of a conducti1ve cylinder 14, the outer surface of
whi.ch carries a layer 16 of organic photoconductive material
well known to the art. Drum 12 includes respective end
plates 18 and 20 carrying stub shafts 22 and 24 by means
of which the drum is mounted for rotary movement in a
manner known to the artu
A corona discharge unit 26 is adapted to be
connected to a suitable source of power 28 ~hrough a
switch 30 to provide a corona discharge for applying a
uniform electrostatic charge to -the photoconductor 16 as
the drum 12 rotates. After having been charged, the
photoconductor surEace moves past an exposure unit 32 of
any type known to the art, adapted to be connected to
a control unit 34 upon the closure of a switch 36.
After having been exposed to an original of the
image to be copied, the photoconductive surface moves ir.to
: cooperative relationship with a developer unit indicated
generally by the reference character 38. Developer unit
38 may, for example, be of -the t~pe which includes an
applicator tank 40 di.sposed within a return tray 42. As is
known in the art, developer made up of charged toner
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particles disPosed in a carrier li~uid having a ~e]ativelY
high volume resistivity is ~ed into the tan~ 40 through a
pipe 44. The tank 40 fills -to a point at which ~he liquid
developer comes into contact with the surface of the drum
12 and then overflows into the tray 42, from whence it is
re-turned to the supply ~not shown) through a pipe 46.
It will readily be a~preciated that any means may
be employed to control the operation of the various units
of the machine 10. For reasons which will be explained
more fully hereinbelow, we wish to provide a region on the
photoconductive surface 16 following the image area, ~hich
region is fully charged but not exposed. By way of example,
in order to achieve this result we may mount a cam 48 on
shaft 22 for rotation therewith, so as to actuate a
follower 50 to close switch 30 so that a predetermined
region around the drum is fully charged. A second cam 54 ~-
on shaft 22 is adapted to operate a follower 56 to close
swItch 36 to place the ex~osure unit 32 into operation.
It will be seen from FIGURE 1 that the angular extent of
the cam 48 is greater than that of cam 54, so that a
greater region of the surface layer 16 is charged than is
exposed. Moreover, -the arrangement is such that exposure
starts at the beginning of the charged region, so that the
fully charged and unexposed region 60 follows the image in
the directlon of movement of the drum. It will further be
appreciated by those skilled in t'ne art that such an
arrangement could, if desired, readily be adapted to a
system in which the controls are so set as to permit of the
making of co~ies of different lengths
In our automatic development electrode bias
control system, we dispose a small centrally located
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electrode 62, and edge electrodes 64 and 66 of conductive
material in the developer tank ~0 adjacent to the entrance
thereof. We so locate the electrodes 62, 64 and 66 as to
insure that the image area on -the drum passes over the
electrodes as the image area moves through the developer
uni-t 38. Moreover, the electrodes 62, 64 and 66 are so
located that developer liquld flows between the electrodes
and the drum and contacts the surfaces of both the electrodes
and the drum. Our electrodes 62, 64 and 66 are completely
insulated from ground or "floating" so that they are
permitted to assume their own potentials. When developer
is disposed between and contacting both surfaces of the
electrodes and of the drum, charged toner particles are
attracted to the surface of the photoconducbor resulting
in charges on the electrodes 62, 64 and 66 such that ea~h
electrode assumes a potential which is a measure of that
of an -area on the surface of layer 16. The resistance of
the toner is high but not a comPlete insulatorO In the
particular orientation shown, each electrode 62, 64 and 66
will assume a potential which is a measure oE the average
potential over that portion of the image area which
registers with the electrode. The potential the electrode
assumes is nearly independent of the elec-trode-to-
photoconductor spacing owing to conductive interconnection
by the toner liquid. It is also reasonably independent of
the electrode capacity-to-ground and resistive capcity-to-
ground, providing that the capacities are small and that
the resistances are fairly high. It will thus be seen that
our sensing electrodes 62, 64 and 66 operate on the
principle of conduction, rather than capacitance.
In order to utilize the potentials sensed by
electrodes 62, 64 and 66, we connect the electrodes to a
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high input imp~dancc- measurin~ circui-t 68 which selec-ts as
its ou-tput the lowest potential sensed. An amPliEier 70,
which recelves ; ts input from the measuring circuit 68
applies a biasing potential -to biasing electrodes 72, 7~,
76 and 78 in a manner to be described. The average voltage
of each electrode 62, 64 and 66 over the image area being
sensed thereby will be equal -to -the residual or background
potential in clear areas with no printing and greater than
the residual potential in areas with printing.
~s indicated in Figure 3, each of the development
electrodes 72, 74, 76 and 78 extends across substantially
the entire width W of a copy to be ~roduced. Moreover,
dimensioning of the sensing electrodes 62, 64 and 66 and
~the positioning thereof across the width of the copy to be
produced are so selected that the electrode 62 scans the
central portion oi~ the image which normally corresponds to
that part of the original, such as a typewritten page,
which contains printing, while the electrodes 64 and 66
scan areas corresponding to margin or border areas of the
original which normally are devoid of printing. By virtue
of this arrangement of multiple electrodes, one on each
edge and one in the middle of the image area, we are able
to, and our circuit 78 does, select the biasing voltage
from the sensing electrode having the lowest reading.
Since, as is pointed out hereinabove, mos-t originals include
one or more clear border areas, our arrangement ensures
that a minimum bias is provided for most copies. Our
circuit 68 also permits of the insertion o E a small
additional bias to the development electrode to provide an
overall bias which is slightly greater than the potential
value sensed in a clear area, thus ensuring that no develop-
ment will take place in the background areas. In the course
of our investigation, we discovered that the resistance of
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the liquid developer between a scnsing electrode and the
drum is of the order oE 109 ohms. Our high impedance
measuring circuit 68 has an in~ut impedance of more than
1012 ohms, or at least three orders of magnitude greater
than the resistance be-tween the e:lectrode and the drum
surEace. In this way we are ab]e to obtain a good reading
of the average potential along the region of the image area
in registry with the electrodes 62.
When the fully charged and unexposed area 60 of
the drum 12 arrives at the developer unit at a location in
registry with the biasing electrodes, the high potential o~
this area produces a reverse bias. It will readily be
appreciated that, even with the amplifier 70 putting out
its deliberately limited maximum value, the potential of
the development electrodes will be well below that of the
unexposed area 60. Consequently, toner particles which may
have been deposited on the biasing electrodes in the course
of the developing o~eration, are drawn toward the surface
of the drum. In the course of that operation, many of the
developer particles return to suspension in the carrier
liquid. It is, of course, true that the area 60 will be to
some extent developed by the toner Particles. This does not
present a serious problem in most commerical app]ications,
however, since such units are provided with mechanical means
for cleaning the surface of the photoconductor 16 in the
course of each operation of the machine.
Alternatively to providing the fully charged and
unexposed region for cleaning the biasing electrodes, we
may provide a section of the drum with a thin plastic
coating rather than a conductor, or we may switch a reverse
polarity voltage onto the development electrodes during
passage of non-image-areas of the drum through the
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developer system.
Referriny now to FIGURE 3, we have shown one
example of a high input impedance measuring circuit
indicated generally by the reference character 68, including
a sample-and-hold portion to be described hereinbelow
and an amplifier indicated generally by the reference
character 70, which w~e may employ in our automatic
developer electrode bias control s~stem. In the arrange-
ment shown, we provide respective shields 80, 82 and 84
for the conductors leading from the sensing electrodes 66,
62 and 64. Res~ective resistors 86, 88 and 90 connect the
sensing electrodes 66, 62 and 64 to insulated gate field
effect transistors 92, 9~ and 96 having a common drain
line 98 and a common source line 100 connected by a
resistor 102 to the terminal ln4 of a source of potential
having a value of, for example.-600 volts. ~he high
input impedance of the measuring circuit 68 is provided
by the transistors 92, 94 and 96. These transistors, in
- res~onse to the sensed voltages, serve to shunt current
away from the base emitter junction of a transitor 106.
The common source line 100, which is connected to -the base
of transistor 106, supplies the base current for the
transistor through the resistor 102. A transistor.1~8
forms a current source for providing the emitter current
for transistor 106. Owing to this arrangement, the emitter
of transistor 1~6 normally is a few volts more positive
; than the input to the field effect transistors 92, 94 and
96, assuming that all of these transistors were fed from
the same source. As a matter of fact, however, as is
indicated in FIGURE 3, the field effect transistors 92,
94 and 96 are fed with in~ut voltages from the respective
sensing electrodes 66, 62 and 64O In the arrangement
shown, the circuit responds to the least negative of the
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sensed voltages ignoring the o-ther sensed voltages. It
will readily be aPparent that -the least negative voltage
is produced on the probe which is sensing the most
discharged area of the photoconductor which normally would
be in the margin of the original. A parallel RC circui-t,
indicated generally by the reference character 109,
couples the emitter of transistor 106 to the shields 80,
82 and 84, so that the capacitance between the input
conductor and the shield does not load the sensing electrode.
The negative voltage source of the sensing circuit is a
Zener diode 110 connected to the source of -600 volts by
a resistor 112.
: .Our measuring circuit 68 includes a sample-and-
hold circu1t which is respons~ve to the potential at the
common terminal of diode 110 and resistor 112. This
signal is applied to the base of a transistor 114 which
. base is connected to the emitter by means of.a diode llÇo
The collector of transistor 114 is connected to a source
:: of, for example, -300 volts. The transistox 114 forms a
low impedance c1ri~er which is adapted to appl~ a potential .
to a storage capacitor 124. The sample-and-hold circuit
~: includes back-to-back diodes 118 and 120, the common
terminal of which is connected to ground and to one
terminal of the storage capacitor 124 by a resistor 122.
A pair of microswitches 126 and 130 are adapted to be
closed to control the charging of the capacitor 124. A
- resistor 128 connects one terminal of switch 126 to the
common terminal of diodes 116 and 118. We connect the
common terminal of the two switches 126 and 130 to the
diode 120. The other terminal of switch 130 is connected
to capacitor 124. From the circuit it can be seen that
with switch 126 closed transistor 114 is ~ermitted to
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charge the storage capacl-tor 124 very rapidly in either
direction. Operation of microswi-tch 130 with switch 126
open permits the capacitor to charge only in the positive
direction.
We so arrange our circuit that swi-tch 126 is
closed during the first two or -three centime-ters of the
copy image and switch 130 is closed for about the first
twelve centimeters of the copy image. In order to achieve
this result, we may, for example, mount a first cam 132
on shaft 22 for rotation therewith. A follower 134,
positioned at a location around shaft 22 corresponding
to that at which the latent image is entering the
developer system 38, is adapted to be actuated by the
cam 132 to close switch 126 and to hold the switch closed
for approximately two to three centimeters of the copy.
Another cam 136 on shaft 22 is adapted to actuate a
follower 138 located at a position corresponding to that o~
fo~ower 134 to close switch 130 for approximately the
first twelve centimeters of the copy ~ength. Thus, during
the first -two to three centimeters of the image, transistor
114 is permitted to charge capacitor 124 rapidly in either
direction. During the next portion of the copy image up to
approxlmately twelve centimeters, transistor 114 can charge
capacitor 124 only in the positive direction and at a
` controlled charging rate which is a compromise among a
number of factors.
A resistor 140 ap~lies the stored voltage to
the amplifi~r 70, which is made up of a pair of transistors
142 and 144, to provide the development electrode biasing
voltage on a conductor 146. We apply the voltage on line
146 to the various development electrodes 72, 74, 76 and
78 by means of a string of diodes 148, 150, 152 and a
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resistor 154, all connected in series between the line 146
and ground. In the arrangement shown, the electrode 72,
which is the first electrode adjacent to which the copy
passes as it moves through the developer system, receives
the full biasing potential. The second electrode 74
receives the potential at the common -terminal of diodes 148
and 150. Electrode 76 receives the potential at the
common terminal of diodes 150 and 152, while the last
development electrode 78 receives the potential at the
common terminal of diode 152 and resistor 154.
It is desirable that no voltage be applied to
the development electrodes during times when no development ,r"
- is to take place, in order to prevent excessive
deposit of toner on the development electrodes. This
result may be accomplished in any convenient manner. For
; example, as we have indlcated schematically in FIGURE 3,
the power supply 156, which supplies the -600 volt potential
and the -300 volt potential to various points in the
circuit, may be disconnected from the sensing ci~uit by any
; 20 convenient means. By way of example, we have indicated a
.
switch 158 in the output line of supply 156. A cam
follower 160 is adapted to be operated to close switch 158
to apply po~er to the sensing cirucit. Follower 160 may
be operated in any convenient manner. For example, we may
position the follower 160 in line with followers 134 and
138 and at a position at which it is actuated by the
exposure ca~ 54 which will cause switch 158 to be closed
; all during the period of time when the latent image is
passing -through the developer system~ It will readily be
appreciated that an~ other suitable means might be employed
to control the application of power to the sensing circuit~
In operation of our automatic develop~en-k
electrode bias control system, when the machine 10 is set
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in operation, drum 12 rotates in the direction ~f -the arro~s
shown in FIGURES 1 and 2. Cam ~8 actuates follower 50 to
apply power from the source 28 to the corona 26 so that the
surface of layer 16 receives a uniform charge over the
period o~ time for which the cam ~8 actuates the follower 50.
After the drum has rotated to a point at which the leading
edge of the charged area is adjacent to the optical system
32, cam 54 actuates follower 56 to close switch 36 to
connect the control arrangement 34 to the optical system 32
to begin the exposure step. This exposure step lasts for
the extent of cam 54 so that, as can be seen from FIGURE l,
there is a fully charged but unexposed area 60 following
the image area. ~s the image area enters the develo~er
system 38, cam 54 closes switch 158 to apply power to the
sensing circuit 68. As the image passes electrodes 62, 64
- and 66, the electrodes sense the potentials of areas of
- the image covered thereby. The sensing circuit selects
the least negative of the potentials which is sampled and
held. The resultant signal is amplified and applied to
~o the development electrodes 72, 7g, 76 and 78. It will
readily be appreciated that this potential will be equal
to or somewhat greater than the actual residual potential
in background areas of the image so that we ensure that no
development of these background areas takes place.
It will further be appreciated, as is pointed
- out hereinabove, that in the course of this development
operation some toner particles will collect on the
biasing electrodes. However, as the area 60 moves over
the development electrodes, there is produced a reverse
bias owing to the fact that the fully charged but
unexposed area 60 is at a much greater potential than
the maximum biasing potential provided by the circuit
including amplifier 70. This reverse bias causes toner
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to migra~e from the development electrodes 6~ and 66
toward the surface of the drum. In the course of this
operation some of the toner particles coming off the
electrodes will go back into suspension in the developer
carrier liquid. It is true that, in the course of this
operation, the area 60 will be developed at least to some
extent. As is further pointed out hereinabove, however,
this presents no great problem in a commerical machine,
since some means already is provided for cleaning the
surface of the drum 12 on each operation of the machine.
It will be seen that we have accomplished the
ob]ects of our invention. We have provided an automatic
development electrode biaslng control system. Our biasing
system overcomes the defects of systems of the prior art
intended to inhibit background development. Our s~stem
provides a variable bias which produces the effect of
automatic e~posure control. The parameters of our system
are not critical~ We provide our svstem with means for
cleaning the biasing electrodes without the necessity of
employing mechanical cleaners. Our system is appreciably
less expensive than are systems of the prior art employing
instruments such as electrometersO
It will be understood that certain features
and subcombinations are of utility and may be employed
without reference to other features and subcombinations.
This is contemplated by and is within the scope o~ our
claims. It is further obvious that various changes may
be made in details within the scope of our claims
without depar-ting from the spirit of our inventionO
It is, therefore~ to be understood that our invention is
not to be limited to the specific details shown and described.
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