Note: Descriptions are shown in the official language in which they were submitted.
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ELECTROSTATIC IMAGING APPARATUS AND METHOD
PROVIDING STABLE REFERENCE POTENTIAL
BACKGROUND OF THE INVENTION
Field of the Invention
. .
The present invention relates to electrostatic
imaging and more particularly to improved structural and
functional approaches for use in electrostatic imaging
apparatus to simply yet stably maintain the potential of
the imaging member's conductive layer near a desired
reference potential level, e.g. ground.
Description of the Prior Art
In electrostatic imaging, e.g. electrophotog-
raphy or dielectric (s~ylus) recording, an electrostatic
charge pattern is formed on the surface of an insulator
layer (in electrophotography the surface of the photo-
conductive insulator layer) of an image member and
developed with marking particles which are electro-
statically attracted thereto. Adjacent the charge-
retentive insulator layer, such imaging members
generally include an electrically conductive layer which
is adap~ed to be electrically coupled to a source of
reference potential, e.g. ground. If the potential on
the conductive layer is maintained closely proximate a
nominal reference potential level, the relative magni-
tudes of the different portions of the electrostaticcharge pattern will accurately represent the imaged
pattern to be reproduced and during development the
marking particles will be attracted in accurate propor-
tion according to the charge pattern magnitudes.
However, if the reference potential of the conductive
layer varies, the variation will be reflected in the
formation and development of the latent electrostatic
image (e.g. in a variation in the magnitude of attracted
charge and/or attracted marking particles). In many
applications the image layer and conductive layer are
disposed on an insulative film support. Because the
conductive layer is thus sandwiched between two
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insulators, the couplin~ of the conductive layer to the
reference potential source is more complicated.
A wide variety of approaches have been
developed for electrically connecting such sandwiched
conductive layers to a reference potential source, which
is commonly termed "grounding" the imaging member. A
non-imaged portion of the imaging member can be
specially fabricated to allow the electrical coupling of
a reference potential with the conductive layer. For
example, a non-image portion of the imaging member can
be perforated or bared, a non-image portion of a sheet
or strip of the imaging member can be imbibed with a
conductive coating material which penetrated to the
conductive layer, or the edge of a sheet, strip or roll
of the imaging member can be imbibed with a conductive
coatin~ material.
Electrical coupling can be effected in various
ways, e.g. by physical contact between the conductive
layer and a conductive grounding member or by directing
a corona discharge into the conductive layer. Physical
contact has been the most commonly used technique but it
requires special fabrication of the imaging member.
Corona grounding via the edge of the image member has
been suggested as an approach to obviate the need for
such special fabrications (see U.S. 3,650,622). How-
ever, past attempts to use corona grounding have
confronted difficulties in maintaining the reference
potential accurately when variations occur in system
parameters such as atmospheric conditions, operating
velocities, operating voltage levels, imaging member
composition, etc. In view of these difficulties, it has
been suggested to provide, as grounding means, a corona
discharge device energized by a programmable D.C. power
supply and means for adjusting such energization in
response to a feedback signal ~rom an electrometer
positioned to sense conductive layer portions moving
therepast.
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SUMMARY OF THE INVENTION
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A significant purpose of the present invention
is to provide functional and structural corona grounding
approaches that improve upon the D.C. energization-
feedback control system mentioned above. Morespecifically, the present invention provides, in corona
grounding, the advantages of rapid response (to varia-
tion of the conductive layer from the desired reference
potential) and accurate stabilization of the conductive
layer potential (to levels closely proximate the
reference potential level). The present invention also
affords advantages in flexibility as to operability with
different (e.g. bipolar) imaging operation modes. A
further significant advantage of the present invention
is that it is subject to sîmple and inexpensive
structural implementations.
The above and other objectives and advantages
are accomplished in accordance with the present inven-
tion by providing for imaging apparatus of the kind
adapted to form and develop an electrostatic image on an
imaging member having a charge-retentive layer and an
electrically conductive layer, improved means and method
for controlling the potential of such conductive layer.
In one general aspect this improvement comprises
directing an alternating corona current to the conduc-
tive layer of an imaging member as it moves along the
operative path of such apparatus, detecting the conduc-
tive layer potential of such moving imaging member and
varying the net corona current in accordance with the
detected potential so as to maintain the conductive
layer potential closely proximate a nominal reference
potential level. In another general aspect such
improvement comprises A.C. corona discharge means for
directing alternating corona current to the conductive
layer of such imaging member; adjustable means for
varying such corona current; detector means for sensing,
and providing a signal representative of~ the potential
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level of successive portions of such imaging member's
conductive layer; and control means for receiving such
signal and adjusting said current-varying means to main-
tain the conductive layer potential closely proximate a
nominal reference potential level.
BRIEF ~ESCRIPTION OF THE DRAWINGS
The subsequent description of preferred embodi-
ments of the present invention is made with reference to
the attached drawings wherein:
Figure 1 is a schematic illustration of one
embodiment of electrostatic imaging apparatus in
accordance with the present invention;
Figure 2 is a schematic illus~ration showing
details of one preferred corona device and adjustment
15 circuit in accordance with the present invention;
Figure 3 is a circuit d;agram illustrating one
preferred a~plifier/filter and phase sensitive detector
embodiment in accordance with the present invention;
Figure 4 is a circuit diagram illustrating one
preferred signal processing and control embodiment in
accordance with the present invention; and
Fi~ure 5 is a graph illustrating the variation
in net alternating corona current effected, for example,
by the Fig. 2 embodiment.
DETAIL.ED DESC~IPTION OF THE PREFERRED EMBODIMENTS
The subsequent description pertains to a
particular electrophotographic embodiment of the present
invention; however, it will be appreciated by those
skilled in the art that the advantages afforded by tne
present invention also can be implemented in many other
electrophotographic embodiments, as well as in other
electrostatic imaging systems that utilize a reference
potential on their image member's conductive layer.
The electrophotographic imaging apparatus 10
shown in Fig. 1 is one particularly adapted to form
images on a strip imaging member 11, e.g. a 16m~ film.
Referring briefly to Fig. 2, it can be seen that the
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film 11 comprises a photoconductive insulator layer 12
adjacent an electrically conductive layer 13 on a film
support 14. As shown in Fig. 1, the imaging film 11 is
moved along an operative path of the apparatus 10 from a
supply roll 15 past a pr;mary charging station 16, where
a uniform electrostatic charge is applied upon the
exterior surface of photoconductor layer 12. Downstream
along the operative path from charging station 16 are an
image exposure station 17 and a liquid development
station 18 which function in the usual manner to form
and develop latent electrostatic images. After the
development station 18, a hot air fixing station 19 is
provided to dry and fuse the toner image on the film.
The film is then fed to a storage or utilization means
(not shown), e.g. a take-up reel. Rollers 20 and 21
support and transport the film along the operative
process path ~f the apparatus. As stated earlier, in
order to obtain uniform primary-charging and development
of the film 11, it is necessary that its conductive
layer 13 be coupled effectively to a source of reference
potential. This is accomplished by corona discharge
device 25 (see Fig. 2) which can be of a conventional
construction and is disposed along an edge of the film
path.
In accordance with the present invention, the
corona discharge device 25 is energized with alternating
current continuously controlled in accordance with a
signal provided from detector means for sensing the
potential of the film conductive layer 13. Such detec-
tor means can comprise one of various kinds of
commercial electrometers and is represented in Fig. 1 by
probe and housing 30, and cooperating timing driver 31,
amplifier/filter 32 and phase sensitive detector 33.
The signal provided by such detector means is applied to
signal processing and control circuit 35 and corona
adjustment circuit 26 which, in cooperation~ provide
proper ener~ization of the corona discharge electrode 25
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to effect a net corona current to the conductive layer
13 that will maintain its potential closely proximate
the desired nominal reference potential.
As noted, the electrometer can be of various
conventional constructions. In one preferred configura-
tion, the probe and housing 30 comprises an electro-
magnet enclosed in a metallic housing which shields its
electrical noise. Below the electromagnet, a thin piece
of electrical shielding is provided. This piece
preferably is non-ferromagnetic and of relatiYely low
electrical conductivity so as to provide shielding from
electrical noise but not attenuate the magnetic field
generated by the electromagnet. A thin reed, which is
made of magnetic material (e.g. springsteel or other
magnetic alloys), is sandwiched between two pieces of
electrical insulator and fastened below the electro-
magnet. The operative film pa~h is configured so that
film 11 passes below such reed with the photoconductor
side facing the reed. This minimi~es the possibility of
detection error due to static charges on the film
support.
The electromagnet is driven by timing driver
circuit 31, preferably at the resonant frequency of the
magnetic reed; and the magnetic field generated by the
electromagnet forces the magnetic reed to vibrate.
Since the vibrating reed is capacitively coupled to the
conducting layer of the photoconductive film below it,
the electrostatic potential of the layer can be detected
from the vibrating reed. Thus, the amplitude of the
signal detected from the vibrating reed depends on the
magnitude of the electrical potential of the conducting
layer, the displacement of the vibrating reed and the
film-to-reed distance. The displacement of the
vibrating plate is defined by the strength of the
magnetic field and also the frequency of the timing
driver. Therefore the amplitude of the signal detected
in the vibrating reed represents the magnitude of the
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electrical potential of film 11, and the phase of the
signal detected with respect to the timing driver gives
the sign of the voltage of the film. That is, the phase
of the signal detected with positive voltage on the film
is 180 out of phase with the signal detected with
negative voltage on the film.
Referring to Fig. 3, the signal from the
vibrating reed is amplified and filtered in circuit 32
of the electrometer, wherein an FET operational ampli-
fier 70 provides high input impedance for the signal anda band-pass filter 71 has a frequency centered at the
frequency of timing driver 31. The amplified and
filtered signal is fed to the input of an analog FET 72
in the phase sensitive detector 33, which, in coopera-
tion with a capacitor 73 at its output end, acts as asample and hold circuit. The timing position of sample
and hold is provided by a retriggerable multivibrator 74
which is controlled by timing driver 31. The detected
signal from the band-pass filter 71 is converted into a
D.C. voltage at the output of the analog FET 72, e.g. a
positive D.C. voltage will show at the output of the
analog FET 72 if the voltage on the film is positive and
the reed is vibrating at its resonant frequency. This
D.C. voltage which is proportional to the potential of
the film's conductive layer is fed to an output control
amplifier 75 in the phase sensitive detector circuit
33. A bias voltage 76 can also be provided on the out-
put control amplifier if it is desired that the refer-
ence potential on the conductive layer be other than
ground.
The signal from phase sensitive detector 33 is
input to signal processing and control circuit 35, one
preferred embodiment of which is shown in more detail in
Fig. 4. This embodiment CQmpriSeS an inverting ampli-
fier circuit 36, a differentiator circuit 37 and anintegrator circuit 38 which are coupled in parallel
between the input from phase sensitive detector 33 and
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an output inverting amplifier 39. The output o~
amplifier 39 is applied to a voltage to current
converter circuit 40 which energizes a light emissive
element 41, e.g. an LED, located in cooperative relation
with a photoresistor 42 in corona adjustment circu~t 26.
Details of one preferred corona adjustment
circuit are shown in Fig. 2. Thus high voltage trans-
former 50 provides an output voltage of amplitude
sufficient to excite the corona, e.g. in the order of
7 K.V. peak. The net corona current (current averaged
over a full cycle) delivered by the charger is varied by
controlling the amplitude of the ne~ative half cycle of
voltage applied to the corona wire via the parallel high
voltage diode 51 and variable resistance 42 circuit
shown in Fig. 2. The simplicity of this control circuit
depends on the physical phenomenon that a symmetric
(unrectified) voltage applied to the corona wire pro-
vides a strongly negative net corona current. Thus,
when the resistance 42 in parallel with the diode 51 is
at a low value, the corona delivers relatively high
net-negative current. The diode 51 is forward biased
during the positive half cycle of the energizing
voltage, and as resistance 42 is increased, the negative
half cycle of voltage applied to the corona wire is
attenuated so that the net current becomes less nega-
tive, going through zero, and becoming positive. The
fixed resistor, 44, in parallel with the diode 51 and
variable resistance 42 is provided to limit the maximum
resistance of this branch so as to insure a path to dis-
charge the capacitance of the charger in a time less~han about a half cycle. Resistance 44 is much greater
than ~he resistance which will bias the corona to zero
net current.
One device that can be used to provide the
remotely controllable variable resistance is a
"Photomod"~ Model CLM-9000 made by Clairex Corporation.
It contains a light emitting diode ~e.g. such as 41 in
,
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Fig. 2) and a cadmium-sulfide photoresistor (e.g. such
as 42 in Fig. 2) in a package that provides high voltage
isolation between the diode and the photoresistor. The
function of this device could also be performed by an
LED and a photoresistor fabricated as separate compo-
nents. The net current delivered by the corona charger
as a function of current in light emitting diode 41 is
shown in Fig. 5.
Now consider the overall operation of these
circuits. If it is desired to maintain the conductive
layer 13 at ground potential, the bias of the output
control amplifier of phase sensitive circuit 33 is
adjusted so that its output is a predetermined voltage
which maintains the collector current in the transistor
output of the voltage-to-current converter 40 at a given
level (when the conducting layer is at zero volts).
This given current level, operating through LED 41,
adjusts the photoresistance of element 42 so that the
corona output is balanced between positive and nega-
tive. This balanced corona current is injected via anair gap to the conductive layer in the photoconductor
maintaining its potential at ground. If the conductive
layer in the photoconductor starts to have a positive
potential, for example because of the charges of the
toners or main corona charger, the detecting means will
sense this change and, operating through signal
processing and control circuit 35, provide a signal to
the corona adjustment circuit 26 which turns photo-
resistor 40 more "on", i.e. decreasing its resistance.
This causes injection of a predominantly negative corona
to the conductive layer, driving the conductive layer
back to the preset potential, zero in this case.
Similarly, if the film conductive layer 13 goes nega-
tive, the detector means senses this condition and
similarly turns the photoresistor less "on", i.e.
increasing its resistance. This creates a predominantly
positive corona which drives the conductive layer to the
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preset potential, zero in this case. If the conductive
layer is to be maintained at a potential other than
zero, the bias of the output control ampli~ier can be
appropriately adjusted. Thus it can be seen that after
selecting a desired nominal potential for the conductive
layer of the film, the present invention will actively
maintain the conductive layer closely proximate such
nominal potential. This operation is independent, for
example, of the polarity of the main corona charger
used, the polarity of the toner, the film velocity and
film width.
One other structural aspect of the Fig. 1
embodiment must be noted. It is highly desirable that
rollers 20 located along the operative path in position
to contact the surface of the photoconductive layer 12
be conductive and coupled to ground. This provides a
capacitance between ground and conductive layer 13 that
is much larger than the corona wire and the conducting
layer. This is important to prevent the corona from
inducing a large A.C. voltage on the conducting layer.
Considering the foregoing, it will be apprecia-
ted by one skilled in the art that the present invention
provides a simple, yet extremely flexible and accurate
means for maintaining the conductive layer potential
closely proximate a desired nominal reference potential
level. The provision of an A.C. corona current to
control the voltage of the conductive layer enables
rapid response to a wide variety of operative system
requirements, and the signal processing and control
circuit modulates the signal provided by the charge
detector means to create high stability for the servo-
loop between the conductive layer and the corona.
Examples of the utility of such flexible feedback A.C.
grounding arrangements include modes of operation where
it is desired to develop (or charge and develop) differ-
ent segments of the film strip with different polarity
or reference levels. Also, when operating in a run-out
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condition (i.e. where the primary charger and exposure
operations are terminated but it is desired to develop
existing latent images), the requirements for the
grounding corona will change rapidly and in large magni-
tude. The present invention is uniquely adapted tohandle these special modes, as well as more common
variations in operating parameters caused by changes in
film velocity, humidity, etc.
The invention has been described in detail with
particular reference to certain preferred embodiments
thereof, but it will be understood that variations and
modifications can be effected within the spirit and
scope of the invention.
