Note: Descriptions are shown in the official language in which they were submitted.
This invention relates generally to an
electrophotographic imaging apparatus, and more
particularly provides a method and apparatus for
establishing a prede-termined apparent surface voltage
charge on the photoconduc-tive surface of an elec-tro
photographic member at the star-t of toning and providing
exposure con-trol thereof.
An electrophotographic member having an
outwardly facing photoconductive sur-Eace is secured to
a pla-ten mounted on a linearly translata~le carriage
to bring said photoconductive surface past plural
functional stations including a charging sta-tion, an
exposure of imaging station, a toning or developing
station and an optional image transfer station. A
corona generating device at the charging sta-tion
applies a surface voltage charge to the photoconduc-tive
surface, same then being moved to the exposure or
imaging station. Light is projected in a pattern to the
charged surface forming a,latent electrostatic image
on said photoconduc-tive surface comprising exposed and
unexposed areas. The laten-t image is developed (toned).
At the start of toning it is desirable to have a
predetermined apparent surface voltage charge on the
unexposed areas of the photoconductive surface.
At the start of the toning function the
apparent surface voltage charge of the respective
exposed and unexposed areas of the,photoconduc-tive surface
generally a,re determined by a) the level to which the
photoconductive surface was initially charged by -the
corona means~ b) the exposure or imaging light in~ensity
and duration; c) the elapsed time between the ini-tial
charging and the start of the tonin~ function, and,
d) the characteristics of the :individual electrophotographic
- 2 - ~
membex employed. Among the characteristics of the
pho-toconductive surface are the individual dar~ decay
slope and aging. Other characteristics may be consider~d.
A prime factor for assuring acceptance of
the electropho~ographic processes is provision oE
consistent and repeatable imaging. If one is to gain
the maximum benefit available through utilization of
a medium such as disclosed in U.S. Patents 4,025,339
and 4,269,919, one must reduc~ the variable in the
process, to obtain consistent and repeatable results
independent of any particular electrophotographie
medium selected or t~e particular electrophotographic
machine employed, to reduce operator error and to
reduce costs of manufacture and operation.
It is desirable to provide apparatus which
can determine the best eharging and exposure conditions
for any one of a wide range of photoconductor members.
The achievement of the dynamie selection of the best
charge and exposure conditions enables the machine to
be adaptable to a wide range of photoconductor
performance eharacteristics thereby redueing the cost
of selection for the photoconductor member while
yielding more repeatable and consistent image results,
Accordingly, -there is provided a method
for controlling the electrostatic field charge on a
photoconduc-tive surface of an electrophotographic
reeording member applied thereto by a corona
generator, comprising the steps; of applying an
electrostatic surface eharge on the photoconductive
surface, at least partially discharging regions of said
charged surface by exposing same to radiation to provide
$~
exposed regi.ons and bloc]cing other regions of said
surface to provide unexposed regions, generating
signa~ representative of the comparison of said
exposed and unexposed regions on the same
photoconductive surface and controlling said corona
generator in response to said comparison signals.
Further there is,provided apparatus for -the
practicing of the above method comprising a
charging device for charging the photoconductive
surface in a succession of levels extending from at
least a lesser to a greater level, an illuminating
device for partially discharging by illumina-ting a
region for successive charge levels Gf ~he surface to
produce an exposed region and an unexposed region, a
sensor for detecting the eIe~trostatic field charge
in each of the successive charge levels of the exposed
and unexposed regions of the photoconductive surface,
a signal generator producing a predetermined signal,
a comparator for comparing at least said det.ected
electrostatic field charge signal to a predetermined
signal and providing said compared signals for control
tasks in.accordance with said detected signal and
said predetermined signal~
The invention further provides for use
in practicing the method above stated, an electrometer
for detecting the electrostatic charge on a moving .
photoconductive surface comprisiny, a'housing having
an aperture therein ancl disposed adjacent the
photoconductive surface, a sensor head enclosed in
said housing and mounted to coincide with said
aperture,,a rotatable member having a plurality
of e~uispaced apertures therein and mounted on said
housing such that said member apertures coincide
with said housing aperture and disposed between said
sensor head a~d the photoconductive surface, a
drive mechanism coupled to said rotatable member for
repeti.tively interrupting and coupling said sensor
head at a requency related to the rotation speed
of said member and the number of equispaced apertures
therein, and an amplifier circuit coupled to said
sensor head.
- 4a -
4~2
The preferred embodiments of this invention
now will be described, by way of example, with
reference to the drawings accompanying this specification
in which:
Figure 1 is a perspective view of a charge
poten-tial level sensing apparatus accordinc~ to the
invention herein;
Figure 2 is a top plan view of the apparatus
of Figure 1, a panel being removed and portions broken
away to show interior details;
Figure 3 is a schematic representa-tion of
the amplifier and motor circuit of the apparatus of
Figure l;
Figure 4 is a diagrammatic representation
illustrating the method of the invention; - ,
Figure 5 is a diagrammatic block diagram
of the control logic circuitry according to the
invention;
Figure 6 is a timing diagram illustrating
the operati.on of the apparatus according to the
inven-tion; and
~ Figure 7 is an enlarged diagramma-tic detail
of the photoconductive surface illustrated in Figure 4.
- 5
9z
~ riefly according to the invention, in an
elec-trophotographic imaging apparatusj a method and
apparatus are provided for establishing a
predetermined apparent surace charge on the exposed
and unexposed areas of a photoconductive surface at
the start of toning and providing exposure control
thereof. A full range of optimum charge levels thereby
can be provided at the instant of toning or develo~ing
a latent electrostatic image formed on the
photoconductive surface of said electrophotographlc
member. The electrophotographic member is mounted on
a platen which is secured to a linearly translatable
carriage. ~he carriage is mounted for travel alon~
a path sequentially from a home position through the
respective functional stations for charging, imaging,
toning, and, optionally, transfer and cleaning. A
~alibration technique provides an optimum corona level
and a best level of light e~posure whereby during
electrophotographic imaging operation, the charging
and imaging functions are controlled in accordance
with the charge behavior characteristics oE the
photoconductive surface of the particular
electrophoto~raphic member employed.
Referring to Figures 1 and 2, an
electrostatic field detector apparatus 10 is
illustrated having housing 12. A disk 14 is disposed
over one face of detector 10 by a shaft 16 coupled to
a drive motor 18. The disk 14 has a plurality
-- 6 --
of apertures 24 formed therein and disposed concentric
with the axis of the disc 14, equispaced inwardly of
the periphery thereof. As illustrated in Figure 1,
fifteen equally spaced apertures. It was preferable
that a finite relationship be maintained between the
number of apertures and the power line frequency for
reducing the deleterious effect of a.c. hum. The
op-timum arrangement of the apertures can be defined byo
N~ = 60 Fc
where NF F
Fc = P ~ P
where
Fc = chop frequency
Fp = power line (hum) frequency
NA = number of apertures
NFp = an "N" multiple of the power
line frequency, an integer.
S = disc rotating speed in R.P.M.
Therefore, for 60 Hz power, 600 rpm speed~ Fc = 150 Hz
sltuated midway between the a.c. power second harmonic
(120) Hz) and third harmonic ~180 Hz). The sensox
electrode 20 is enclosed in the housing 12 and disposed
adjacent an aperture 26 that is formed in housing 12.
The sensor electrically is connected to an amplifier
circuit shown as 22 in Figure 2. The apertures 24
formed in the disk 14 are coincident with aperture 26 in
~ 7 ~
4G92
the enclosure 12. As the clisk 14 is rotated by motor 18,
the electrode 20 alternately is blocked and exposed.
The disk 14 can be rotated, for example, at a speed of
600 r.p.m., thereby providing a chopping fxequency oE
150 Hz that is removed from a harmonic frequency of 60
hertz. The electrode 20 conveniently can be provided
as a flat screw, preferably plating the head 21 thereof
with gold or silver. The size of the head 21 of the
electrode 20 approximately is equal to the size o
apertures 24 in the disk.
The rotation of the disk 14 alternately
couples and interrupts the capacitive coupling between
the electrode 20 and the electrostatic field of the
photoconductive surface 28, thereby inducing an A.C.
signal on said measurement electrode 20. The electrode
20 is disposed adjacent the photoconductive surface 28. The
A.C. signal induced on electrode 20 is c~upled to
amplifier 2~.
An example of one useful amplifier 22 is
illustrated in Figure 3 wherein the amplifier 22
primarily comprises two operational amplifiers 32,34.
The operational amplifiers 32,34 are coupled throu~h
current-limitiny resistors 36,38 to a positi~e fifteen
volt power supply 40 and through current-limitin~
resistors 42,44 to a negative fifteen volt power
supply 46. The operational amplifiers 32,34 may have
a field-effect transistor, FET input, such as RCA
type CA3140E. The biasing resistors J and the by-pass
and coupling capacitors are provided as -follows:
-- 8 --
gz
Resistors 36, 38, 42, 44, 48 100 ohms
Resistors 60 1.5 Kilohm
Resistor 62 22 megohm
Resistors ~4, 66 22 Kilohm
Resistor 68 270 Kilohm
Resistor 70 3.9 megohm
Capacitors 50 r 52, 58 10 micro-farad
Capacitors 54, 56 470 nanofarad
Variable resistor 72 100 Kilohm
~ Many variations could be made from the above example with
the same results achieved.
The motor 18 is coupled through a switch 74
to an A.C. power supply 76.
The low level A.C. measured si~nal provided by
probe 20 is coupled to the input of operational ~mplifier
32. The output of amplifier 3~ is connected to variable
resistor 72 so that a portion of the output is coupl~d to
the input of operational amplifier 34, for further
amplification of the measured signal. The output 80 of
operational amplifier 34 is an employable electrostatic
field signal for coupling to a control logic unit.
Referring now to Figure 4, the process
according to the invention is illustrated diagrammatically.
; Figure 6 graphically illustrates the timing of
the events involved.
Step 1 of Figure 4 illustrates the platen
82 ~in a home~position. A corona genera-ting device 84 is
shown positioned relative to the photoconductive surface
28 of an electrophotographic member secured to platen 82.
~The corona generating device 84 applies a surface voltage
_ g
, ~
charge to the photoconductive surface 2~ when translated
thereacross. Measuring electrode 20 and amplifier 22
are shown positioned adjacent the photoconductive
surface 28. The output 80 of amplifier 22 is a signal
proportional to the apparent surface voltage electrostatic
field charge level of the photoconductive surface 28.
A section of the photoconductive surface 28 is
shielded by baffle 88 from illumination provided by an
exposure lamp 86.
The platen 82 is moved at a constant speed
from left to right direction as viewed in Figure 4.
In step 2 of the corona generating device 84 is shown
the process o~ applying a charge to the photoconductive
surface 28 dur1ng movement thereof from left to right~
The corona level output is varied in a sequence of
levels sy~chronously with the movement of surface 28.
A staircase pattern of corona level outputs is
illustrated in Figure 6 s-tarting at a minimum level at
time T0 and increasing in equal steps to a maximum level
at time T5 and decreasing in steps from ti~e T7 to time
Tll. Step 2 of Figure 4 is represented in the chart
of Figure 6 from time T~ to time T12. At the time T~,
the corona generating device 84 acts upon the leading
edge of the moving photoconductive su.rface 28. At
time T6, the corona generating device 84 ac-ts upon the
middle portion of the surface 28. At time T12 the
corona generating device ac~s upon the trailing edge
of the moving photoconduc~ive surface 28
-- 10 --
g~
and is translated past corona device 84.
At Step 3 of Figure 4 the platen 82 reverses
and moves from right to left. Step 3 Figure 4 is
represented in Figure 6 from time T12 to the time T24.
The corona ou*put level is varied in the same sequence
of levels during movement of the photoconductive surface
28 represented by Step 2 of Fi~ure 4. The '7double pass"
charging acts to apply a relatively constant and uniform
series of charge levels in staircase-like s-teps, or a
ramp format, on the surface 28.
Step 4 of Figure 4 illustrates the platen 82
moved back to its home position. The platen 82 then is
moved over the baffle 88 to shield a predetermined
portion of the photoconductive surface 2g from light,
such as one-half thereof as shown in Step 5 of Figure 4.
The baffle 88 acts to shield or block the illumination
of the exposure lamp 86 from th~ section of the photo-
conductive surface 28 extending to the right of the
baffle 88. Step 5 of Figure 4 is shown on the chart of
Figure 6 from the time T25 to the time T26. At time T25
the exposure lamp 86 is energized to achieve a predetermined
intensity for a predetermined time duration. The
exposure lamp 86 is deenergized at the time T26. The
effective exposure period from the time T25 to the time
T26 typically is provided as a few seconds~ The start
of the exposure period at the time T25 typically is
provided in the range of a few seconds to about twenty
seconds after the completion of the charging function
a-t the time T24.
-- 11 --
0~2
From the time T25 to the time T26, the exposure
lamp 86 emits illumination having a constant intensi.ty,
thereby uniformly discharging the exposed section of the
photoconductive surface 28 during this time peri.od. The
exposed section of the photoconductive surface is designated
as region KA and the unexposed section of the photoconductive
surface 28 is designated as region KB.
The platen 82 is moved from left to right (in
Figure 4) to a position on the right side of the electro-
static field electrode 20 as represented in step 6. Atime delay is efEected that is equal to the time between
the completion of the charging function and the start of
the toning function in the normal operation of the
electrophotographic imaging apparatus. The time delay
between time T24 and time T27 (when the electrostatic
field detector apparatus lO is activated), is prov.ided
generally in the range of thirty to fifty seconds.
The chart of Figure 6 shows the electrostatic
field detector apparatus or electrometer 10 activated at
the time T27 through the time T28 as the photoconductive
surface 28 moves across the electrode 20. A platen position
encoder llO synchronously defines each position of the
moving platen 82 with the amplified measured signal 80
and is coupled to a control logic unit lO0. The
measured signal output 80 from electrode 20 is illustrated
for the partially exposed region KA and unexposed
region KB. The resulting measured signal 80 has a
triangular ramp-like staircase shape ha~ing a lesser
leading staircase ramp due to the light exposure în
the region KA and a greater trailing staircase ramp in
the region KB representing the exposed and the unexposed
apparent surface voltage charge levels.
Referring to Figure 7, the electrode
signal 80 is illustrated relative to the photoconductive
surface 28. The exposed regi.on KA and u~exposed regi.on
KB are shown as having ~ands comprising increments o:E
charge variation according to the seguence corona level
outputs as the photoconductive surface 28 moved
thereacross. The shaded band regions 29 are extended
~elow the photoconductive surface 2g to illustrate the
stepwise change in the electrode signal 80 with the
charge bands of the photoconductive surface 28. In
practice, these charge bands appear more l.ike a smooth
transition than the discrete steps shown.
The coincidence unexposed line of the chart
of Figure 6 shows a co.incidence level between the
measured signal 80 and a predetermined apparent surface
voltage charge level stored in memory for the unexposed
region K~. The exposed line represents the correlated
measured signal 80 for the exposed region KB at the
corona ou-tput ievel corresponding to the above coincidence
level.
In the normal operation of the electro-
photographic imaging machine the corona generating unit8~ is controlled to provide a corona level output
corresponding to the coincidence level. The correlated
measuxed signal in the exposed region ~A isused to
control the exposure lamp 86 in accordance with the
exposure lamps characteristics to provide the
predetermined apparent surface voltage charge in the
exposed areas of the photoconductive surface 28.
Step 7 of Figure 4 illustrates the exposure
lamp 86 illuminating the photoconductive surface 28 in
order to fully discharge the surface 28. Steps 8 and 9
of Figure 4 illustrate the normal operation of the
electrophotographic imaging apparatus. In step 3 the
platen 82 is moving across the corona generating
device 84 fxom left to right. Step 9 sho~s the platen 82
positioned to the left of the corona generating de~ice
84 after moving thereacross from right to left,
completing charging in a double pass. The sensing
device 10 in the form of an electrome~er measures the
apparent surface voltage charge on the photoconductive
surface 28. This initial charge measurement is
relatively meaningless in relation to the charge level
at the start of the toning function: however, the initial
charge measurement can be utilized to determine when
the useful capability o~ the photoconductive surface 28
has been exhausted.
Attention i5 now invited to Fi~ure 5 whîch
diagrammatically illustrates the control logic unit 100
according to the invention. The amplified elec-trode
signal B0 is coupled to an analog-to-digital (A/D)
converter 102. The A/D converter 102 produces a
digital detected charge signal 103 in the form o-f a
binary word, usually on the order of six bits. The
binary word signal 103 is coupled to the date input of
a random access memory ~RAM) 104. Control signals KA,
KB corresponding to the exposed and unexposed regions
of the photoconductive surface, as shown in Step 5 of
Figure 4 r are coupled -to a mode control 106. The
mode control unit is coupled to the most significant
bit (MSB) input of the random access memory 104.
The travel of platen 82 is encoded by position
detector 110, such as a tachometer or like device.
The platen position encoder 110 is coupled to the input
of a platen travel pulse generator 112. The platen
travel pulse generator 112 produces a pulse train
corresponding to the travel of platen 28. For example,
each pulse produced by the pulse generator 112 may
represent one tenth of an inch of travel of the platen 82.
The output of the platen travel pulse generator 112
is coupled to the clock input of a memory address
counter 108. The reset line of the memory address
counter 108 is connected to the mode control function
106. A reset pulse having a brief, spike-like
confiyuration is produced at the onset of either
region KA, KB and resets the counter 108 effectively
to zero. The output of the mode control 106 that is
coupled to the MSB input of R~M 104 can be provided,
for example, as a binary LOW for the exposed region KA
and as a binary HIGH for -the unexposed region XB of
the photoconductive surface 28. The input from the
mode control 107 to the MSB input of the RAM 104
effects the addressing of two different files in the
RAM 104. The memory address counter 108 is coupled to
the address input of the RAM 104 and scans the same
remaining address lines of RAM 104.
The memory address counter 108 produces a
most significant bit minus one (MSB - 1) signal that
- 15 -
is coupled -to a complementor 114. The complementor 114,
when the active state of the significant bit of the
address word which i5 equivalent to one bit less than
the MSB occurs,will invert the relative sense of the
binary words passing therethrough.
The complementor 114 is coupled to a
staircase ramp generator 116. With the complemen-tor
114 addressing the staircase ramp generator 116, the
generator 116 will count up, count down, count up and
count down, corresponding to the corona level output
illustrated in Figure 6 for regions KA, ~B for both
directions of the travel of platen 82~ During the
calibration function, the staircase ramp generator 116
is coupled through switch 132 to the corona level
control unit 130 while the predeterminèd seguence
of corona output levels are produced by the corona
generating device 84.
The output 105 of RAM 104 is coupled to the
DA input of a coincidence detector 118. The
coincidence detector 118 conveniently may be a binary
comparator. The output of mode control 106 is coupled
through the invexter 119 to the EN input of the
coincidence detector 118. The DB input of the
coincidence detector 118 is coupled to a predetermined
binary word 120 equal to the desired apparent surface
voltage charge at the start of the toning Eunction.
The binary word 120 can be provided from a manually
operated switch or a databus of a separate digital
system. The A = B output of the coincidence detector 118
- 16 -
is coupled to the clock input of a latch 122. When the
DA input equals the DB input to the coincidence
detector 118, the A = B output of the detector 118
clocks the latch 122, the latch 118 will latch onto the
instantaneous count state of the word that is addressing
the RAM 104.
The output of the latch 122 is coupled to
the input of a digital-to-analog (D/A) converter 124
and the DB input of a coincidence detector 126. The
D/A converter 124 produces an analog signal 128
corresponding to the digital coincidence word~ The
analog signal 128 is coupled to a corona level control
unit 130 and is the control signal thereto when the
switch 132 is provided in the operate position, a~ter
the completion of the calibration function according to
the invention.
The me~sured signal gO that is sequentially
stored in a second file position in the memory 104
corresponding to the exposed region KA is compared in
the coincidence de-tector 126 to the output of the l.atch
122 that is coupled to the DB input of detector 1260
The output of mode control 106 is coupled to the EN input
of the coincidence detector 126. The coincidence output
A =- B of detector 126 corresponds to the measured
charge signal 80 in the exposed region KA for the
corona level output as determined by the value stored
in the latch 122. The A = B output of the coincid2nce
detector 126 is coupled to the clock input of a latch
134. The latched output of the latch 134 is coupled to
- 17 -
~4q3~32
a least significan-t bit (LSB) input of a memory 136.
The memory 136 ca.n be a programmable read only memory
~PROM).
The latched, discharged signal of the
latch 134 generally is higher than the predetermined or
desired apparent surface voltage charge for the exposed
area~ The memory 136 couples to a digital lamp control
unit 138 and a timed switch control 140~ The memory
136 functions as a loo~-up table of predetermined values
~Ihich are used to determine a control word for coupling
to the digital lamp control 138 and the timed swikch
control 140. An operato.r adjust light level unit 142
is coupled to the most significant bit (MSB~ input of
the memory 136, thereby allowing for manual adjustment
of the light level.
The memory 136 stores the non-linear
characteristics of the exposure lamp 86 relative to the
changes in power applied thereto by the digital lamp
control unit 138. The memory 136 can include
compensation data for such important factors as the
shift in exposure lamp color temperature relative to the
pho-toconductor sensitivity and non-linear lamp
illumination output relative to changes in voltage
applied to the exposure lamp 86. The profiling of
the characteristics of the exposure lamp 86 provides
for proper control of both the intensity of the exposure
lamp 86 and the time duration that the lamp 86 is
energized.
- 18 -
The control logic unit 100 can include the
following:
A/D converter 102 AD571JD (Analog
Devicesl Inc.,
Norwood, Mass.)
Memory 104 93422 (Fairchild Inc.)
Concidence CD 4063 (RCA, etc.)
detector 118,126
Latch 122, 134 CD4508BE (RCAJ etc.)
D/A Converter 110,
124, staircase ramp AD 7523JN ~Analog
generator 116 Devices, Inc.)
Memory 136 HM 7502 (H~rris
Semiconductor)
Memory address counter
108 CD 4040BE (RCA, etc.)
10 Complementor 114 CD 4070 BE tRCA, etc.)
Many variations could be made from the above
example and the same results achieved, without departing
frvm the invention.
In the practice of the invention a fully
exposed, maxial-clear separation film can be provided in
the optical path during the calibrate expose cycle, thereby
compensating or the residual density of -the separa-tion
film substrate. The photoconductive surface 28 acts as
the light measuring aevice.
In conclusion, the method and apparatus
according to the invention provide control for the charging
and imaging ~ucntions thereby providing the de~ired
charge levels on the photoconductive surface 28 for
the exposed regions KA and unexposed regions KB according
to the image pattern at the s-tart of the toning function
in the normal operation of an electrophotographic imaging
machine.
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