Note: Descriptions are shown in the official language in which they were submitted.
1281763
Title of the Invention
LIQUID DEVELOPER CHARGE DIRECTOR CONTROL
Background of the Invention
Liquid developers generally include a liquid phase,
comprising an insulating carrier liquid such as an isoparaf-
finic hydrocarbon, and a solid phase, comprising toner
particles composed of a pigment and a binder. The solid phase
toner is dispersed or suspended in the liquid phase carrier.
Liquid developers further include a minor amount of charge
director which insures that the toner particles are uniformly
charged with the same polarity, which may be either positive orl
negative depending upon the particular application. The liquid,
eveloper is used to develop a latent image formed on a photo-
onductive imaging surface. Usually the photoconductive
urface is charged with one polarity; and the toner particles
re charged with an opposite polarity. If the liquid developer
ontains exces~ive charge director, then the images developed
ill tend to be somewhat faint because of loss of image charge
ue to leakage in the higher conductivity liquid developer.
n the other hand if the liquid developer contains insufficient
harge director, then the images developed will also tend to be
omewhat faint since toner particles having reduced charge move
ith reduced velocity through the developer liquid to the
maging surface;
A more serious problem with liquid developers having
nsufficient charge director is that the toner tends to drop
ut of suspension, forming sludge deposits which continually
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grow until operation o~ the electrostatic copier must be
interrupted for cleaning. It is the maintenance of the
' charge on the toner particles by the charg~ director which
Il causes the toner particles to repel one another, maintaining
~! them dispersed, and preventing them from agglomerating and
! forming sludge deposits
li Summary of the Invention
Il In general our invention contemplates a liquid
il developer control system wherein a liquid carrier, such as
10 1, isodecane, from a first container is added to a diluted
working developer suspension to maintain the volume of such
I, working developer constant. The optical transmissivity
;~ of the working developer is measured; and toner concentrate
I frorn a second container is added to the working developer
to maintain its optical transmissivity at a predetermined
! value. The conductivity of the working developer is also
'I measured and charge director concentrate from a third
container is added to the working developer to maintain
I, its conductivity at a predetermined value.
20 ! In the making of a copy with a liquid developer, a
constant amount of carrier liquid is deposited over the
entire surface of the copy sheet. This carrier liguid from
! the working suspen~ion contains an associated amount of
l liquid phase charge director. There is further deposited
25 ¦ upon the copy sheet an amount of toner proportional to the
¦ extent of printing on the copy sheet. This toner includes
¦ an associated amount of solid phase charge director.
¦ Accordingly, during the making of a copy, there is lost
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from the working developer a first constant quantity of
charge director associated with the carrier liquid and a
second variable quantity of charge director associated with
the toner solids. The toner concentrate in the second
container is preferably provided with a total amount of
charge director not appreciably exceeding that associated
with the solid phase toner. This prevents detectable
sludging of the toner concentrate and insures that the
toner concentrate can not cause the conductivity of the
working developer to exceed its desired value. The charge
director concentrate from the third container effectively
supplies the charge director lost by transfer of carrier
liquid to each copy sheet.
One object of our invention is to provide a liquid
developer control system wherein a charge director concentrate
is supplied to the working developer suspension in response
to a conductivity measurement thereof.
Another object of our invention is to provide a
liquid developer control system wherein a toner concentrate
is added to the working suspension in response to a
transmissivity measurement thereof.
Still another object of our invention is to provide
a liquid developer control system wherein the toner
concentrate contains a total amount of charge director not
appreciably exceeding that associated with the solid phase
toner.
A further object of our invention is to disable
an electrophotocopier if the temperature of the liquid
developer becomes excessive in order to prevent sludging.
~8~7~3
A further ob~ect of our invention is to ~isable
an electrophotocopier if the conduotivity of the liquid
developer drops appreciably below its proper value in order
to prevent sludging.
Other and further objects of our inventlon will
appear from the following description.
The Prior Art
Gardiner et al 3,981,267 (Re. 30,477) uses a
conductivity measurement of the liquid developer to control
a constant current source providing biasing current to the
development ele~trode.
Mochizuki et al 4,310,238 shows a liquid developer
transmissivity control system, wherein the desired
transmis6ivity value is modified by a measurement of the
onductivity of the working suspension. More particularly,
as the conductivity of the working suspension gradually
increases during the making of many copies, the resulting
"fatigue" of the developer is compensated for by adding
more toner concentrate to decrease the transmissivity of
the working developer.
¦ Brief Description of the Drawing
¦ The accompanying drawing, which forms part of the
instant specification and is to be read in conjunction
¦therewith, is a schematic view of a preferred embodiment
¦of our liquid developer charge director control.
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Description of Preferred Embodiment
Referring now to the drawing, the working suspension
is contained within a tank 10. A first container 12 having
a neck 24 is provided with a supply of carrier liquid such
as Isopar-G (a trademark of Exxon Corporation) which is a
mixture of isoparaffinic hydrocarbons, lar~ely isodecane,
having a extremely low bulk conductivity. Container 12 is
held inverted; and carrier liquid issues from neck 24 until
the height of the working suspension in tank lO covers the
lower end of neck 24. The working developer is relatively
dilute and typically might be 1~ toner solids and 99~ carrier
liquid by weight.
Within a second container 14 we provided a supply
of toner concentrate. Typically the concentrate might be
10~ toner solids by weight and 90~ carrier liquid such as
Isopar. The rate of sludging depends upon the temperature
and the amount of charge director. Sludging is increased
by temperature and is decreased by charge director. The
sludging of the toner concentrate in container 14 is
2Q negligible compared with that of the working suspension in
tank 10 because the concentrate is substantially at or only
slightly above ambient or room temperature while the working
suspension in tank 10 may approach temperatures as high as
100F. Any local hot spots within tan~ 10 form sludging
sites.
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To further reduce any minimal sludging of the toner
concentrate in the second container 14, we further provide
the toner concentrate with a relatively large amount of
charge director which, however, does not appreciabily
exceed that associated with the solid phase. Bo,th toner
solids and charge director associated therewith are removed
from the system by transfer to copy paper and by sludging
as well. Carrier liquid may be removed from the system both
by transfer to copy paper and by evaporation. Both carrier
liquid and charge director associated therewith are removed
from the system by transfer to copy paper. However,
evaporation of carrier liquid produces no loss in charge
director. If the copier is maintained at operating
temperature for an extended period of time and the sludging
and evaporation rates are made up solely by toner
concentrate from the second container 14, the amount of charge
director within tank 10 will continually increase if the
toner concentrate contains any charge director in excess of
that a6sociated with the toner solids. We ensure that the
charge director control system will not lose control due
to excessive charge director in tank 10 by ensuring that the
charge director in the toner concentrate does not appreciably
exceed thatassociated with the toner solids.
Within a third container 16 we provide a charge
director concentrate which may contain ~5 % by weight of
charge director and 95~ carrier liquid such as Isopar.
lZ1~176~3
i Electrophotocopiers employ a photoconductive sur~ace which
may be charged either positively or negatively. The charge
¦director imparts a charge to the toner particles which is
¦ordinarily opposite to that with which the photoconductive
¦surface is charged. For example if the surface is char~ed
¦positivcly, then a negative charge director would be employed
¦both in the toner concentrate container 14 and in the charge
director concentrate container 16. Such negative charge
directors include metal salts of fatty acids such as calcium
1(~ palmitate and metal salts of naphthenic acid such as barium
petronate. If the photoconductive surface is negatively
oharged, then a positive charge director would be employed.
Such positive charge directors include transition metal
salts of fatty acids such as aluminum stearate and transition
¦metal salts of naphthenic acid such as cobalt naphthanate.
¦Furthcr charge directors known to the prior art include
¦sodium dioctyl sulfosuccinate, lecithin and calcium petronate,
¦sometimes known as "mahogany soap". Generally we prefer
¦charge directors which are soluble in the carrier liquid.
¦ A shaded pole induction motor 18 drives a stirrer
19 within tank 10 to prevent settling of toner in the working
suspension. A further shaded pole induction motor 20 drives
a stirrer 21 which extends through a hole 26 in the top of
the second container 14 to prevent settling of the toner
~s in the conc trate, The char irect~r conc ntrate con~ainer
1~8~763
, ~ ~
,~, 16 is provided with an air hole 28 in the top thereo~. The
l second container 14 is provided with a short outlet pipe 30
1, which is coupled by flexible tubing 34 to the inlet of a
~, positive displacement pump 38 having an eccentric shaft 39 1i
5~l which is driven by a direct current motor 40. The outlet of
,l pump 38 is connected by flexible tubing 46 to a short inlet
pipe 50 ~f tank 10. The third container lÇ is provided with
,1 a short outlet pipe 32 which is coupled by flexible tubing 36
Il to the inlet of a positive displacement pump 42 having an
10¦¦ eccentric shaft 43 which is driven by DC motor 44. The
¦ outlet of pump 42 is coupled by flexlble tubing 48 to a short
inlet pipe 52 o~ tank 10. Positive displacement pumps 38 and
42 may be of the flexible vane type; and both the rotors
thereof and motors 40 and 44 rotate counterclockwise in the
direction of the arrows. Excitation of motor 40 drives pump
38, supplying toner concentrate from container 14 to tank 10.
Excitation of motor 44 supplies charge director concentrate
~rom container 16 to tank 10.
One wall o~ tank 10 is provided with a tran~par~nt
plate 62, which permits light ~rom an incandescent lamp 60 to
pase therethrough to a photosensitive device 66 mounted
within tank 10. Device 66 may comprise a photoconductor such
a~ cadmium sulfide. The toner control may be substantially
as shown in U.S. Patent 4,579,253, issued April 1, 1986 to
Shenier for Toner Control System. Light rom lamp 60 is also
directed through a variable or adjustable aperture 64 to a
reference photosensitive device 68 which is similar to device
66. The primary winding 56 of a transformer indicated
generally by the reference numeral 54 is excited
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¦ by an alternating current source of, for example, 115 volts.
Transformer 54 is provided with a secondary winding 58
¦ having a center tap which is grounded. Terminals 57 and 59
¦ of winding 58 respectively pro~-ide +10 and -10 volts AC.
¦ Lamp 60 is connected between terminals 57 and 59. Terminal
¦ 57 is connected through device 68 to one terminal of a
potentiometer 67, the other terminal of which is connected
through devlce 66 to terminal 59. The slider of potentiometer
67 is connected through an AC amplifier 80 to the input of a
phase sensitive detector 84 which receives a reference input
from terminal 59. Positive outputs of detector 84 actuate a
trigger circuit 86 which enables a free running power
multivibrator 88. Multivibrator 88 is coupled to one
terminal of motor 40, the other terminal of which is grounded.¦
Terminal 57 is connected to one electrode 70 of a
pair of spaced electrodes 70 and 71 disposed within tank lO.
Electrode 71 is connected through a 50 megohm resistor 74
to the slider of a 200 kilohm potentiometer 73 connected
botween terminal 59 and ground. Terminal 59 is connected
through an adjustable capacitor 72 having a nominal value
of 35 picofarads to electrode 71. Terminal 59 is further
connected serially through a rheostat 76 having a nominal
value of 33 kilohms and a .07 microfarad capacitor 77 to
ground. Electrode 71 is connected to the gate of an insulated-
gate field-effect transistor amplifier 81, which may comprise
a source follower having a voltage amplification of
substantially unity with an extremely high input impedance
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and a low output impedance. The source output of field
effect transistor amplifier 81 is connected through an
AC amplifier 82 to a phase sensitive detector 83, the
reference input of which is supplied by a conduct~r 78
connected to the ~unction of rheostat 76 and capacitor 77.
Detector 83 drives the positive input of a unity gain
Il differential amplifier 85. Positive o~tputs of amplifier
¦1 85 actuate a trigger circuit 87 which enables a free running
¦ power multivibrator 90. Multivibrator 90 is connected to
¦ one terminal of motor 44, the other terminal of which is
¦ grounded.
A wall plug 92 provides a source of 115 volts AC
at 60 Hertz. Conductor 93 of plu~ 92 i5 connected to one
Il terminal of primary winding 56 and to the armature of a
!¦ normally closed relay switch 105 which may be opened by
¦¦ energization of a DC relay winding 104. Conductor 93 is
jl further connected to one terminal of each of stirring motors
¦l 18 and 20. Conductor 91 of plug 92 is connected to the
~' armature of a normally open relay switch 99 which may be
¦ closed by energization of an AC relay winding 98. The fixed
¦ contact of relay switch 99 is connected to the other terminal
of each of stirring motors 18 and 20, to the other terminal
of primary winding 56, and through a resistor 102 to one
terminal of relay winding 98. The other terminal of winding
98 is connected to the fixed contact of relay switch 105.
Conductor 91 is connected to one contact of a spring-loaded
normally-open "on" switch 94. The other contact of switch
94 is connected through a resistor 96 to the first terminal
of winding 98. The fixed contact of relay switch 99 also
30 1 provides power to the copier as is well-known to the art.
' ~ 10-
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Ter~inal 59 is c~nnected to one contact of a
¦ spring-loaded, normally-open "off" switch 108. The other
contact of switch 108 is connected through a resistor 110
I and forwardly through a diode 112 to a first terminal of
¦ relay winding 104 the other terminal of which is grounded.
¦ The output of amplifier 8S is connected through a normally-
¦ closed, manually-operable switch 114 and forwardly through
¦ a diode 115 to the input of a trig~er circ~lit 116. The
¦ output of trigger circuit 116 is connected to the first
¦ terminal of relay winding 104. Upon the making of each
j copy, an output is provided by circuit 118which is coupled
to the resetting input of a one minute timer 120. The output
of timer is connected to the first terminal of relay winding
1lo4.
¦ The temperature of the working suspension is measured
by a sensor 122 providing an electrical output scaled, for
example, in degrees Pahrenheit. The output of temperature
sensor 122 is applied to the negative input of a unity gain
differential amplifier 128 and to the positive input of
a unity gaindifferential amplifier 130. A source of a fixed
voltage 124 scaled to represent ambient or room temperature
of 70F. is applied to the positive input of differential
amplifier 128, A source of a fixed voltage 126 scaled to
¦ represent a temperature of 100F. is applied to the negative
¦ input of differential amplifier 130. The output of
differential amplifier 128 is applied to the negative input
¦ of differential amplifier 85. The output of differential
¦ amplifier 13~ is appl~ed through an amplifier 132 to a trigger
¦ circuit 134, the output of which is connected to the first
terminal of relay winding 104.
1 ~281763
In operation of our invention, the "on" switch 94
! is momentarily closed which, through resistor 96, energizes
¦ relay winding 98, the circuit being completed through switch
j 105. Relay switch 99 closes, applying power from conductor
1¦ 91 through resistor~102 to winding 98, maintaining it energ-
~¦ zed. The closing of switch 99 also excites stirring motors
¦¦ 18 and 20, energizes primary winding 56, and provides power
to the copier through other connections not shown.
I In operation of the toner control system, adjustable
¦ aperture 64 causes the amount of light falling on device
68 ~o be equal to that passing through window 62 and falling
upon device 66 when the working suspension has the desired
transmissivity. In such event, equal voltage drops occur
11 across devices66 and 68; and the input of amplifier 80 is
ll at ground potential. Potentiometer 67 is adjusted to
compensate or slight mismatch in devices 66 and 68. As
the transmissivity o~ the working developer increases due
to depletion of toner, more light falls upon device
66, reducing its resistance. An AC voltage appears at the
~ input of amplifier 80 which is in phase with that at terminal
59. Phase sensitive detector 84 now provides a positive
output. When such output exceeds +0.5 volt, for example,
trigger circuit 86 is actuated, causing multivibrator 88
1 to excite motor 40 with a duty cycle of, for example, one
second "on" and four seconds "off". During each one second
¦ interval that motor 44 is excited, it rotates counterclockwise
correspondingly driving pump 38 and supplying toner
concentrate from container 14 to tank 10. The four second
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1 - .
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¦ interval between successive excitations of motor 40 permits
j stirrer 19 to circulate the toner concentrate throughout the
~ working suspension. As the transmissivity of the working
¦ suspension decreases due to increase in amount of toner,
¦ the output of detector 84 decreases; and at zero volts,
¦ for example, trigger circuit 84 is turned "off", disabling
¦ multivibrator 88 and motor 40.
Charge director in the working suspension is depleted
l not only by transfer of carrier liquid to a copy sheet
¦ but also by loss of toner due either to transfer thereof to a
copy sheet or to sludging thereof withln tank 10. Electrodes
70 and 71 may be formed as parallel plates or as concentric
cylinders having respective diameters of .55 inch and .45 inch
l with len~ths of approximately 2.5 inches. The spacing
', between the cylinders may thus be approximately .05 inch.
The area bctween spaced electrodes 70 and 71 is approximately i
¦ 3.9 square inches. The working suspension may have optimum
¦ performance with a bulk conductivity of 50 picohos per
I centimeter, for example. This produces a resistance between
¦ electrodes 70 and 71 of approximately 100 megohms. The input
to amplifier 81 will be at ground potential with the slider
¦ of potentiometer 73 positioned substantially at its midpoint
and providing -5 VAC. Potentiometer 73 acts as a variable
potential source having a maximum internal resistance, when
~ ¦ positioned at its midpoint, of 50 kilohms which is one-
thousandth the 50 megohms resistance of fixed resistor 74.
¦ The conductance betweer, the parallel plates or
concentric cylinders 70 and 71 is proportional to the surface
~.28'1763
area and inversely proportional to the spacing therebetween.
The same relationship also prevails for the capacitance
between the plates or cylinders 70 and 71. Isopar-G has
a dielectric constant of 2.0; and the capacitance between
plates 70 and 71 is approximately 35 picofarads. At a
frequency of 60 Hertz, the capacitive reactance bet~een
plates 70 and 71 is 7S megohms. Capacitor 72, having a
nominal value of 35 picofarads, is adjusted to equality
l with the capacitance of plates 70 and 71, so that the input
¦ to amplifier 81 can have zero quadrature comDonent. The
eqùivalent capacitance of plates 70 and 71 and capacitor
¦72 is 70 picofarads which at a frequency of 60 Hertz has
i a capacitive reactance of 37.5 megohms. This is shunted
¦by the 33.3 megohms equivalent resistance of resistor 74
1 and plates 70-71.
lZ81763
As the conductivity of the w~rking suspension in
tank 10 decreases due to the depletion of charge director,
¦ the re~istance between plates 70 and 71 increases. In
¦ the absence of capacitive effects, this would produce an
¦ AC voltage at the input of amplifier 81 which is in phase
with that of the -10 VAC potential at terminal 59.
¦ Because of the capacitive effect of plates 70~71 and
¦ neutralizing capacitor 72, the voltage at the input of
¦ amplifier 81 lags that at terminal 59 by arctan (33.3/37.5)
¦ = tan 1,89 - 41,6. Capacitor 77 has a value which is
approximately a thousand times the 70 picofarad equivalent
capacitance of plates 70-71 and capacitor 72; and resistor
76 has a nominal resistance which is one-thousandth that of
the 33.3 megohm equivalent resistance of plates 70-71 and
resistor 74, Resistor 76 is adjusted so that the reference
voltage on conductor 78 lags that at terminal S9 by 41.6.
Small errors in the adjustment of capacitor 72 produce
at the input of amplifier 81 only voltage components in
quadrature with the reference signal on conductor 78
provided to detector 83, Accordingly, detector 83 will
produce no voltage output. However, increase in the
resistance between plates 70 and 71, due to decrease in
the conductivity of the working suspension, produces at
the input of amplifier 81 a voltage component in phase
with the reference signal provided to detector B3 on
conductor 78. Phase sensitive detector 83 now produces
positive output. We prefer AC conductance measurement,
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lZ81763
despite capacitive effects, to prevent any net deposition of
toner on electrodes 70 and 71, as would occur if DC potentials ¦
ere applied thereto. I
The working suspension has a positive temperature
coefficient of bulk conductivity which may, for example, be
+O,S picomho per centimeter per degree Fahrenheit. At a
temperature of 100F. the same optimum performance working
',suspension m~y thus have an increased bulk conductivity of
1150 ~ 0.5 (100-70) = 65 picomhos per centimeter. As the
~Itemperature of the working suspension increases above 70F.,
jlthe output of differential amplifier 128 becomes negative.
The increased bulk conductivity of the working suspension
produces a negative output from detector 83. The gradient in
Ivolts per F. of sensor 122 and the gain of amplifier 82
should be such that the negative outputs of both amplifier
128 and detector 83 are substantially equal, so that no
output is produced from differential amplifier 85 as long as
thè composition oP the working suspension does not change,
lirrespective of changes in its temperature. The conductivity
ilmeasurement is thus modified by correcting for changes in
¦¦conductivity caused by variations in developer temperature
llfrom 70F. As the amount of charge director in the working
,¦developer composition gradually decreases, the output of
¦ amplifier 85 gradually increases.
When the output of amplifier 85 exceeds ~0.5 volt,
for example, trigger circuit 87 is actuated, causing
multivibrator 90 to excite motor 44 with a duty cycle of, for
example, one-quarter second "on" and five seconds "off".
; -16-
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Pump 42 may have one-fifth the volume or displacement of
pump 38. ~he amount of charge director concentrate to be
added is extremely small. The five seconds "off" interval
permits stirrer 19 to thoroughly mix the added charge director
with the working suspension. During each one-quarter second
interval that motor 44 is excited, it drives pump 42
counterclockwise, supplying charge director from the third
container 16 to tank 10. As the conductivity of the working
developer increases, the output of amplifier 85 decreases;
I and at zero volts, for example, trigger circuit 87 is turned
"off", disabling multivibrator 90 and motor 44.
~he following examples of working developer and
toner concentrate are given by way of illustration and not
by way of limitation. A 1% w~rking developer suspens on
¦ may comprise 990 grams of Isopar G and 10 grams of toner
solids including pigment and binder. Optimum performance
Il may be at a conductivity of 50picomhos per centimeter
¦¦ at 70F. which may require 238 millig.ams of a given charge
~I director. The loss of each gram of toner solids, whether
~ by transfer to a copy sheet or by sludging within tank 10,
1~ e~v~ gr~ cb~9 ~r~
P 6t~
~ -17-
~ lZ81763
Each gram of carrier liquid transferred to a copy sheet
may remove 0.2 milligram of charge director. The 10 grams
of toner solids requires 4 x 10 = 40 milligrams of charge
1 director; the Isopar carrier requires 990 x 0.2 - 198
¦ milligrams of charge director; and the total amount of
charge director is 40 ~ 198 = 238 milligrams. A 10% toner
¦ concentrate may contain 100 grams of toner solids, including
¦ pigment and binder, and 900 grams of Isopar-G. The toner
¦ concentrate preferably further includes 4 milligrams of
¦ charge director associated with each gram of toner solids,
¦ or 4 x 100 - 400 milligrams of charge director, to prevent
sludging. However, the toner concentrate preferably
¦ contains little or no further charge director associated
with the 900 grams of Isopar. The toner concentrate is thus
deficient by 900 x 0.2 ~ 180 milligrams of charge director
to ~nsure that the system cannot lose control. The carrier
liquid in the first container 12 is also deficient in charge
director since preferably little or none is provided
therein. All further charge director needs are supplied
from the charge director concentrate in the third container
16.
To turn the copier off, the spring-loaded, manually-
operable "off" switch 108 is momentarily depressed, supplying
DC current through resistor 110 and rectifier 112 to winding
104. Engerization of winding 104 opens switch 105, disabling
the circuit for winding 98. Switch 99 thus opens, removing
power from the copier. The copier also turns itself off
automati 11y if no copies are made for a period of one minut~.
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¦ As long as copies are being made more frequently than once
¦ each minute, the copy sensor 118 will reset the one
minute timer 120 before it provides an output. ~owever, if
no copy is made for a period-exceeding one minute, then the
copy sensor 118 will not reset timer 120 before its timing
interval has expired. When the one minute timing interval of
timer 120 expires, an output signal is produced which
energizes winding 104, opening switch 105 and disabling the
circuit for winding 98. Switch 99 now opens, removing power
from the copier.
When the conductivity control system is operating
normally, the output from amplifier 85 will gradually
increase from zero volts to approximately 0.5 volt,
whereupon charge director concentrate is added until the
output of amplifier 85 drops back to zero volts again. If
the third container 16 i8 empty of charge director
concentrate, or if there is a failurein trigger circuit 87,
multivibrator 90, motor 44, or pump 42, then the output from
I amplifier 85 will gradually increase beyond 0.5 volt. Diode
115 may be formed of silicon and requires approximately 0.5
¦ volt of forward bias for conduction. The input of trigger
¦ circuit 116 now begins to rise from ground potential. When
the output of amplifier 85 exceeds t 1.0 volt, the input of
trigger circuit 116 exceeds l0.5 volt, thereby actuating it
¦ and energizing winding 104. Relay switch 105 opens,
disabling the circuit for winding 98. This opens switch 99,,
removing power from the copier. While the copier may be
momentarily turned "on" by depressing switch 94, it will
immediatëly thereafter be turned "off" by amplifier 85
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through switch 114, diode 115, trigger circuit 116, and
winding 104, which opens switch 105. It is desired that the
copier not be permitted to operate with significantly less
charge director in the working developer than that required
to bring the system to optimum performance. Significantly
reduced amounts of charge director would lead to more rapid
sludging of toner in tank 10, resulting not only in loss of
t~er material but also more frequent maintenance.
~ Generally operation of the copier can be restored
10 ¦ by providing a fresh supply of charge director concentrate
¦ in container 16. However, the output of amplifier 85 still
¦ exceeds ~1.0 volt. To prevent this output of amplifier 85
from turning "off" the copier, switch 11~ is opened. The
input of trigger circuit 116 drops to ground potential,
turning "off" trigger 116, and disabling relay winding 104.
Motor 44 now causes charge director concentrate to be pumped
¦ from container 16 into tank 10. When the output of amplifier
¦ ~5 drops below +1.0 volt, switch 114 may be closed. Trigger
¦ circuit 116 will not be turned "onN, since its input i8 less
¦ than +0.5 volt. Charge director concentrate will continue to
¦ be pumped until the output of amplifier 85 drops to ground
¦ potential, turning noff" trigger circuit 87.
¦ The copier normally operates with developer
¦ temperatures Iess than 100F., where the sludging rate
is acceptably small. If, for example, a cooling fan fails,
then the developer temperature will rise above 100F. The
output of differenti~l amplifier 130 will become positive
and~through amplifier 132, actuate trigger circuit 134. This
energizes winding 104, turning "off" the copier and preventing
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¦ any further rise in developer temperature or increase in
¦ sludging rate.
¦ It will be seen that we have accomplished the objects
¦ of our invention. We have provided a system for independently
1 controiling the quantity of carrier liquid, the quantity of
¦ of toner solids and the quantity of charge director in a
¦ workin~ liquid developer. The height or volume of carrier
, liquid is controlled by the neck 24 of container 12. The
¦ quantity of toner concentrate is governed by a balanced
1 transmissivity measuring system wherein equal amounts of
¦ light fall upon matched photosensitive devices. The
¦ guantity of charge director concentrate is governed by an
¦ alternating current conductivity measurement wherein the
!¦ inherent capacitance between the conductivity measuring
I electrodes is compensated for by a balanced circuit employing
¦ a phase sensitive detector. To prevent either sludging of
the toner concentrate or loss of control of the charge
director control system, the toner concentrate contains a
total amount of charge director which does not appreciably
exceed that associated with the toner solids and contains
little or no charge director associated with the carrier
li~uid component. Our system exhibits none of the so called
"fatigue" effects of prior art systems which supply excessive
charge director with the toner concentrate. In such systems,
the conductivity of the working developer gradually increases,
causing an excessive discharge rate of the photoconductive
surface and the latent image thereon. To prevent excessive
sludging of the working suspension when the charge director
l concentrate is exhausted or the control system fails, the
¦ copier is automatically turned off; and further operation is
inhibited. The copier ls further disabled if the temperature
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of the working suspension rises above a point where the
sludging rate is still acceptably small. The conductivity
¦measurement is modified by correcting for changes in
¦developer conductivity caused by changes in its temperature.
¦ It will be understood that certain features and
Isubcombinations 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.
Having thus described our invention, what we
claim iss
i!
,1
!¦ .
,
I -22-
