U.S. patent number 3,892,481 [Application Number 05/479,659] was granted by the patent office on 1975-07-01 for automatic development electrode bias control system.
This patent grant is currently assigned to Savin Business Machines Corporation. Invention is credited to Kenneth W. Gardiner, Louis F. Schaefer.
United States Patent |
3,892,481 |
Schaefer , et al. |
July 1, 1975 |
Automatic development electrode bias control system
Abstract
An automatic control system for the bias on a development
electrode in which a plurality of ground-insulated, narrow floating
electrodes are spaced along a line adjacent the entrance of a
liquid developer applicator unit. The floating electrodes
relatively scan image areas of the surface of an organic
photoconductor carried by a conductive support moving through the
developer unit. Owing to conduction of a charge from the
photoconductive surface through the developer liquid to the
floating electrodes, they assume potentials each of which is a
function of the average potential of the image area subtended by
the floating electrode. The potential of each floating electrode is
sensed by a high input impedance measuring circuit which selects
the potential of the lowest value, amplifies the selected potential
and applies the amplified voltage to the biasing electrode or
electrodes of the developer system. A fully charged and unexposed
area of the surface following the image area produces a reverse
bias which cleans the biasing electrodes as the fully charged area
passes through the developer system.
Inventors: |
Schaefer; Louis F. (Palo Alto,
CA), Gardiner; Kenneth W. (Menlo Park, CA) |
Assignee: |
Savin Business Machines
Corporation (Valhalla, NY)
|
Family
ID: |
23904891 |
Appl.
No.: |
05/479,659 |
Filed: |
June 17, 1974 |
Current U.S.
Class: |
399/56; 399/239;
118/665; 399/245; 118/664; 324/72 |
Current CPC
Class: |
G03G
15/065 (20130101); G03G 15/10 (20130101) |
Current International
Class: |
G03G
15/06 (20060101); G03G 15/10 (20060101); G03g
009/04 () |
Field of
Search: |
;355/10,14,3R ;117/37LE
;96/1LY ;118/DIG.23,7,4 ;324/72 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Moses; Richard L.
Attorney, Agent or Firm: Shenier & O'Connor
Claims
Having thus described our invention, what we claim is:
1. A system for automatically controlling the biasing potential on
a developer electrode in an electrostatic copying machine including
in combination, a support, a surface layer of photoconductive
material on said support adapted to receive a latent electrostatic
image to be developed, a developer unit for contacting developer
fluid with said image to develop the same, a biasing electrode in
said developer unit, a sensing electrode insulated from ground,
means mounting said sensing electrode in said developer unit at a
location at which developer fluid is positioned between and in
contact with both said photoconductive surface and said sensing
electrode to enable said sensing electrode to assume substantially
the potential on said surface by conduction, and means responsive
to the potential of said sensing electrode for applying a biasing
potential to said biasing electrode.
2. A system as in claim 1 in which said sensing electrode mounting
means mounts said sensing electrode at a location at which it
registers with a portion of said image in the course of a
developing operation.
3. A system as in claim 2 including means mounting said support and
said developer unit for relative movement, and in which said
sensing electrode mounting means positions said electrode adjacent
to the point at which said image enters said developer unit.
4. A system as in claim 1 in which said developer is a liquid
developer made up of charged particles suspended in a liquid having
a high electrical resistance.
5. A system as in claim 4 in which said developer between said
surface and said sensing electrode and said photoconductor has a
certain high resistance and in which said means responsive to the
potential sensed by said sensing electrode includes a measuring
circuit having an input resistance which is orders of magnitude
greater than said resistance.
6. A system as in claim 1 in which said biasing potential is
greater than the potential in background areas of said image.
7. A system as in claim 1 including means for producing a reverse
bias between said surface and said biasing electrode in the course
of operation of said machine.
8. A system as in claim 7 in which said reverse bias producing
means comprises means for producing a charged unexposed area on
said surface outside the area of said image.
9. A system as in claim 8 including means for moving said support
and said developer unit relative to each other and in which said
charged unexposed area trails said image area in the direction of
said relative movement.
10. A system as in claim 1 in which said photoconductor is an
organic photoconductor.
11. A system as in claim 1 in which said sensing electrode is an
electrically floating electrode.
12. Apparatus for developing a latent electrostatic image carried
by the surface of a photoconductor including in combination, a
developer applicator unit for applying developer to said surface,
said applicator unit having an entry and an exit, means for moving
said surface and said developer applicator unit relative to each
other to carry said image through said unit in a direction from
said entry toward said exit, a biasing electrode in said developer
unit, an electrically floating sensing electrode of conductive
material, means mounting said sensing electrode in said developer
unit at a location at which developer passes between said sensing
electrode and said surface to cause said electrode to sense the
potential of said surface by virtue of conduction through said
developer, said developer between said sensing electrode and said
surface having a certain resistance, and means including a
measuring circuit having an input impedance orders of magnitude
greater than said resistance and responsive to said potential
sensed by said sensing electrode for applying a biasing potential
to said biasing electrode.
13. Apparatus as in claim 12 in which said sensing electrode is
positioned at a location at which a portion of the image area of
said surface registers with said sensing electrode as said image
area moves through said unit.
14. Apparatus as in claim 12 including means for producing a
reverse bias between said biasing electrode and said surface
following a developing operation.
15. Apparatus as in claim 14 in which said means for producing said
reverse bias comprises means for producing a reverse field between
the development electrode and an area on said surface outside the
area of said image.
16. Apparatus as in claim 15 in which said photoconductor is an
organic photoconductor and in which said developer is made up of
charged particles suspended in a liquid having a high
resistance.
17. Apparatus as in claim 12 in which said sensing electrode is
adjacent to the entry of said developer unit and in which said
measuring circuit comprises a storage capacitor, means responsive
to movement of an initial portion of an image into said developer
unit for enabling bidirectional charging of said capacitor in
response to a sensed potential, means responsive to movement of an
intermediate portion of said image into said developer unit for
enabling only unidirectional charging of said capacitor in response
to sensed potential and means responsive to entry of the remainder
of said image into said delivery unit for disabling charging of
said capacitor.
18. Apparatus as in claim 12 in which said means including said
measuring circuit comprises a source of power for said circuit and
means for disconnecting said source from said circuit as non-image
areas of said surface pass through said developer unit.
19. Apparatus for developing a latent electrostatic image carried
by the surface of a photoconductor including in combination, a
developer applicator unit for applying developer to said surface,
said applicator unit having an entry and an exit, means for moving
said surface and said developer applicator unit relative to each
other to carry said image through said unit in a direction from
said entry toward said exit, a developer electrode in said unit, a
plurality of electrically floating sensing electrodes of conductive
material, means mounting said sensing electrodes along a line
extending generally across the direction of relative movement of
said surface and said unit and at a location at which developer
passes between said sensing electrodes and said surface to cause
said sensing electrodes to sense the potential of said surface in
areas thereof adjacent to said electrodes by virtue of conduction
through the developer and means responsive to the potential sensed
by said sensing electrodes for applying a potential to said
development electrode.
20. Apparatus as in claim 19 in which said plurality of sensing
electrodes includes a central electrode adapted to scan the portion
of the image corresponding to the central normally printed area of
an original and an edge electrode adapted to scan the portion of
the image corresponding to a normally unprinted border area of an
original.
21. Apparatus as in claim 19 in which said means responsive to the
potential sensed by said sensing electrodes comprises means for
selecting the potential of the lowest magnitude sensed by said
sensing electrodes.
22. Apparatus as in claim 19 in which said plurality of sensing
electrodes includes a central electrode adapted to scan the portion
of the image corresponding to the normally printed area of an
original and an edge electrode adapted to scan a portion of the
image corresponding to a normally unprinted border area of an
original, and in which said means responsive to the potential
sensed by said sensing electrode comprises means for selecting the
sensed potential of the lowest magnitude.
23. Apparatus as in claim 19 including a plurality of development
electrodes spaced in the direction of relative movement of said
surface and said unit, and means for applying said biasing
potential to said development electrodes in decreasing steps from
the first development electrode to the last development electrode
in said direction of relative movement.
24. Apparatus as in claim 19 in which said line along which said
electrodes are disposed is adjacent to the entry of said developer
unit.
25. Apparatus as in claim 24 in which said means responsive to said
potential sensed by said sensing electrodes comprises a storage
capacitor, means responsive to movement of an initial portion of an
image into said developer unit for enabling bidirectional charging
of said capacitor in response to a sensed potential, means
responsive to movement of an intermediate portion of said image
into said developer unit for enabling only unidirectional charging
of said capacitor in response to sensed potential and means
responsive to entry of the remainder of said image into said
developer unit for disabling charging of said capacitor.
26. Apparatus as in claim 19 in which said means responsive to said
potential sensed by said sensing electrodes comprises a sensing
circuit, a source of power for said sensing circuit and means for
disconnecting said source from said circuit as non-image areas of
said surface pass through said developer unit.
Description
BACKGROUND OF THE INVENTION
In the art of electrostatic copying in which the surface of a
photoconductor carried by a conductor support 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 photoconductors 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 copying apparatus which 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 volts.
Many 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 either development of background areas takes
place if the biasing potential is not large enough or toner is
deposited on the biasing electrode if too large a biasing potential
is employed. It will be appreciated further that a biasing
potential should be applied to the electrode only during the period
of time during 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
developer station.
Attempts have been made in the prior art to provide systems which
vary the biasing potential in response to variations in the
potential of background areas. For example, Coriale U.S. Pat. No.
3,611,982 shows an arrangement in which a capacitive probe, located
outside and just before the developer unit, is exposed by a shutter
to a charged and fully exposed strip at the edge of the
photoconductor drum. The sensed potential is amplified and is used
to control a variable power source which provides the biasing
potential of the electrode located in the developer system. Another
example of a bias voltage control system is shown in Coriale U.S.
Pat. No. 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 developer unit. A
further example of the use of a capacitive probe to regulate the
bias applied by a source to a biasing electrode is shown in Smith
U.S. Pat. No. 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 U.S.
Pat. No. 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 for changes in the characteristics
of the photoconductor over a period of time.
The systems of the prior art discussed hereinabove sense
photoconductor voltage by the use of delicate and sensitive
instruments such as electrometers for measuring the charge in
residual areas of the photoconductor. Such instruments are not only
expensive, 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 arrangements
of the prior art, moreover, employ switching arrangements for
rendering the bias effective only for the period of time during
which the image passes through the developer system. In addition,
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 electrode bias control
system for inhibiting deposit of toner on background areas which
overcomes the defects of systems of the prior art. The parameters
of our system are non-critical. Our assembly is relatively
inexpensive to construct. Our construction is such as to insure
that the bias will at all times be sufficient to prevent deposition
of toner on background 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
One object of our invention is to provide an automatic development
electrode bias control system.
Another object of our invention is to provide a system for
overcoming the effect of background potential which avoids the
defects of systems of the prior art intended to achieve this
purpose.
Another object of our invention is to provide an automatic
development electrode bias control system the parameters of which
are not critical.
Another object of our invention is to provide for automatic
exposure control by the automatic bias control to permit high
quality copies to be made from both white and colored background
originals without requiring operator adjustment.
A still further object of our invention is to provide an automatic
development electrode bias control system which is relatively
inexpensive to manufacture.
A still further object of our invention is to provide an automatic
development electrode bias control system having means for removing
toner particles from the biasing electrode without the use of
mechanical cleaning means.
Other and further objects of our invention will appear from the
following description.
In general our invention contemplates the provision of an automatic
development electrode biasing control system for an electrostatic
copying machine using a liquid developer in which a plurality of
narrow sensing electrodes are spaced along a line in the developer
unit adjacent to the entrance thereof. These electrodes afford a
measure of the average potential along the image areas subtended by
the electrodes owing to conduction of a small portion of the charge
on the photoconductive surface through the developer liquid
disposed between and in contact with both the surface and with the
electrode. The voltages thus sensed are measured by a high input
impedance measuring circuit, which selects the potential of lowest
value, and amplifies it to provide the biasing voltage for
application to the biasing electrodes. We provide the
photoconductive surface with a fully charged and unexposed region
following the image area to provide a reverse bias which draws
toner particles which have been deposited on the biasing electrode
in the course of a developing operation from the biasing
electrode.
BRIEF DESCRIPTION OF THE DRAWINGS
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:
FIG. 1 is a partially schematic end elevation of an electrostatic
copying machine which may be provided with our automatic
development electrode bias control system.
FIG. 2 is a perspective view with parts removed, with other parts
broken away, and with parts shown in section, illustrating our
automatic developmen electrode bias control system.
FIG. 3 is a schematic view of one form of an electrical circuit
which may be employed in our automatic development electrode bias
control system.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to FIGS. 1 and 2, a machine indicated generally by
the reference character 10, with which our system may be used,
includes a drum indicated generally by the reference character 12
made up of a conductive cylinder 14, the outer surface of which
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 art.
A corona discharge unit 26 is adapted to be connected to a suitable
source of power 28 through 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 photoconductive surface 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 into cooperative relationship
with a developer unit indicated generally by the reference
character 38. Developer unit 38 may, for example, be of the type
which includes an applicator tank 40 disposed within a return tray
42. As is known in the art, developer made up of charged toner
particles disposed in a carrier liquid having a relatively high
volume resistivity is fed into the tank 40 through a pipe 44. The
tank 40 fills to a point at which the liquid developer comes into
contact with the surface of the drum 12 and then overflows into the
tray 42, from whence it is returned to the supply (not shown)
through a pipe 46.
It will readily be appreciated 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, which 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 exposure unit 32 into
operation. It will be seen from FIG. 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 direction of movement of the drum. It will
further be appreciated by those skilled in the 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 copies
of different lengths.
In our automatic development electrode bias control system, we
dispose a small centrally located electrode 62, and edge electrodes
64 and 66 of conductive material in the developer tank 40 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 unit 38.
Moreover, the electrodes 62, 64 and 66 are so located that
developer liquid 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 photoconductor resulting in
charges on the electrodes 62, 64 and 66 such that each 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 insulator. In the particular orientation shown, each
electrode 62, 64 and 66 will assume a potential which is a measure
of 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 electrode-to-photoconductor spacing
owing to conductive interconnection by the toner liquid. It is also
reasonably independent of the electrode capacity-to-ground and
resistive capacity-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 high input impedance measuring
circuit 68 which selects as its output the lowest potential sensed.
An amplifier 70, which receives its input from the measuring
circuit 68 applies a biasing potential to biasing electrodes 72,
74, 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.
As indicated in FIG. 3, each of the development electrodes 72, 74,
76 and 78 extends across substantially the entire width W of a copy
to be produced. 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 of 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 herinabove, most 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 of 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
development will take place in the background areas. In the course
of our investigation, we discovered that the resistance of the
liquid developer between a sensing electrode and the drum is of the
order of 10.sup.9 ohms. Our high impedance measuring circuit 68 has
an input impedance of more than 10.sup.12 ohms, or at least three
orders of magnitude greater than the resistance between the
electrode and the drum surface. In this way we are able 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 of 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 operation, 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
commercial applications, 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 developer system.
Referring now to FIG. 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 we may employ in our automatic developer
electrode bias control system. In the arrangement shown, we provide
respective shields 80, 82 and 84 for the conductors leading from
the sensing electrodes 66, 62 and 64. Respective resistors 86, 88
and 90 connect the sensing electrodes 66, 62 and 64 to insulated
gate field effect transistors 92, 94 and 96 having a common drain
line 98 and a common source line 100 connected by a resistor 102 to
the terminal 104 of a source of potential having a value of, for
example, -600 volts. The high input impedance of the measuring
circuit 68 is provided by the transistors 92, 94 and 96. These
transistors, in response to the sensed voltages, serve to shunt
current away from the base emitter junction of a transistor 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 108 forms a current source
for providing the emitter current for transistor 106. Owing to this
arrangement, the emitter of transistor 106 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 FIG.
3, the field effect transistors 92, 94 and 96 are fed with input
voltages from the respective sensing electrodes 66, 62 and 64. In
the arrangement shown, the circuit responds to the least negative
of the sensed voltages ignoring the other 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 circuit, 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 circuit which
is responsive 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 116. The collector of transistor 114 is connected to a source
of, for example, -300 volts. The transistor 114 forms a low
impedance driver which is adapted to apply 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 adpated 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 permitted to charge
the storage capacitor 124 very rapidly in either direction.
Operation of microswitch 130 with switch 126 open permits the
capacitor to charge only in the positive direction.
We so arrange our circuit that switch 126 is closed during the
first 2 or 3 centimeters 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 2 to 3 centimeters of the copy. Another
cam 136 on shaft 22 is adapted to actuate a follower 138 located at
a position corresponding to that of follower 134 to close switch
130 for approximately the first twelve centimeters of the copy
length. Thus, during the first 2 to 3 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
approximately 12 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 applies the stored voltage to the amplifier 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 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 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 indicated schematically in FIG. 3,
the power supply 156, which supplies the -600 volt potential and
the -300 bolt potential to various points in the circuit, may be
disconnected from the sensing circuit by any 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 power to the sensing circuit. 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 cam 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 any other suitable means might be
employed to control the application of power to the sensing
circuit.
In operation of our automatic development electrode bias control
system, when the machine 10 is set in operation, drum 12 rotates in
the direction of the arrows shown in FIGS. 1 and 2. Cam 48 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 of time for which the cam 48 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 FIG. 1, there is a fully charged but unexposed area 60
following the image area. As the image area enters the developer
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 cuircuit selects the least negative of the
potentials which is sampled and held. The resultant signal is
amplified and applied to the development electrodes 72, 74, 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 to migrate from the development electrodes 64 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 commercial 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 objects of our
invention. We have provided an automatic development electrode
biasing control system. Our biasing system overcomes the defects of
systems of the prior art intended to inhibit background
development. Our system provides a variable bias which produces the
effect of automatic exposure control. The parameters of our system
are not critical. We provide our system 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 electrometers.
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 of our claims. It is further obvious that various changes may
be made in details within the scope of our claims without departing
from the spirit of our invention. 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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