Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF THE INVENTION
This invention relates generally to a multi-color
electrophotographic printing machine, and more particularly
concerns an apparatus which controls the concentration of
toner particles in a developer mix employed therein.
In the process of electrophotographic printing, a
charged photoconductive surface is irradiated to create an
electrostatic latent image corresponding to an original docu-
ment. The electrostatic latent image is developed with toner
particles from a developer mix. Thereafter, the toner particles
are transferred to a sheet of support material and permanently
affixed thereto in image configuration. In the foregoing
manner, a copy of the original document is created.
This process is not limited to black and white copies
but may be employed with color copies in a similar manner. In
color reproduction, a plurality of successive single color
toner powder images are transferred to the sheet of support
material in superimposed registration with one another. Each
image corresponds to a filtered light image of the original
document. Thus, the multi-layered toner powder image, when per-
manently affixed to the sheet of support material, corresponds
in color to the original document. In order to insure that
the toner powder image is satisfactory, developability must
be controlled. Developability is related to the concentration
of toner particles in the developer mix, i.e. the percentage
of toner particles relative to carrier granules in the developer
mix.
Various prior art systems have been devised to
regulate the concentration of toner particles in the developer
mix. One such ~ystem is described in U.S. Patent No. 3,399,652
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issued to Gawron in 1968~ This patent discloses a rotating
reflective disc positioned in the developer mix. The disc is
electrically-biased to attract toner particles from the
developer mix. A light beam is reflected from the surface
of the disc and transmitted to a photoelectric unit. The
intensity of the light rays received by the photoelectric unit
is indicative of the density of toner particles adhering to
the disc surface. An electrical output signal from the photo-
electric unit initiates dispensing of toner particles to the
developer mix to replenish the supply thereof.
Another patent of interest is U.S. Patent No. 3,754,821
issued to Whited in 1973~ The apparatus disclosed therein is
a transparent electrode mounted on a photoconductive drum. As
the electrostatic latent image on the photoconductive drum is
developed, the transparent electrode is biased electrically
to attract toner particles thereto. The transparent electrode,
with the toner particles adhering thereto, is illuminated and
the light rays transmitted therethrough are detected by a photo-
sensor mounted in a thermal chamber. The light rays are trans-
mitted to the photosensor by fiber optics disposed between the
photosensor and the transparent electrode. Utilization of a
thermal chamber and the fiber optics in the foregoing apparatus
substantially increases the cost and difficulties of manufacture.
However, this arrangement is required in order to compensate
for the thermal sensitivity of the photosensor. For example,
a phototransistor has a sensitivity of 1.5%/C, and a photo-
darlington has a sensitivity of 3~5%/C~ The foregoing tem-
perature sensitivity exceeds the allowable system tolerance,
Temperature sensitivities of this magnitude would result in
the ~y~tem detecting variations in temperature rather than
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variations in toner particle density. One type of photosensor
which has a sufficiently low thermal sensitivity is a photo-
diode. A photodiode has a sensitivity of 0.2%/C. However,
at the light levels involved, the current generated by a
photodiode would be extremely low.
Accordingly, it is an object of an aspect of the
present invention to improve the apparatus utilized to regul-
ate the concentration of toner particles within a developer
mix by providing a substantially thermally insensitive photo-
sensor cooperating electrically with a stable amplificationcircuit.
SUMM~RY OF THE INVENTION
In accordance with one aspect of the present
invention there is provided an apparatus for controlling the
concentration of particles in a mix, including: electrode
means biased electrically to attract particles thereto as
said electrode means passes through the mix; means for
illuminating said electrode means having the particles
deposited thereon with light rays; a thermally insensitive
photodiode for detecting the intensity of the light rays
transmitted from said electrode means, said photodiode being
adapted to generate a substantially thermally insensitive
electrical output signal indicative of the density of
particles deposited on said electrode means; and amplifying
circuit means, in electrical communication with said photo-
diode, for stably amplifying the thermally insensitive
electrical output signal from said photodiode to at least a
preselected level.
In accordance with another aspect of the present
invention there is provided an electrophotographic printing
machine of the type having a photoconductive member and a
development system employing a developer mix of carrier
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granules and toner particles to form a toner powder image on
an electrostatic latent image recorded on the photoconductive
member, including: electrode means mounted on the photo-
conductive member and biased electrically to attract toner
particles thereto as said electrode means passes through
the developer mix; means for illuminating said electrode
means having the toner particles deposited thereon with light
rays; a thermally insensitive photodiode for detecting the
intensity of the light rays transmitted from said electrode
means, said photodiode being adapted to generate a substan- - - -
tially thermally insensitive electrical output signal indica-
tive of the density of toner particles deposited on said :
electrode means; and amplifying circuit means, in electrical .-
communication with said photodiode, for stably amplifying
the thermally insensitive electrical output signal from said
photodiode to at least a preselected level.
BRIEF DESCRIPTION OF THE DRAWINGS
Other objects and advantages of the present inven-
tion will become apparent upon reading the following detailed
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description and upon reference to the drawing, in which:
Figure 1 is a schematic perspective view of a multi-
color electrophotographic printing machine embodying the
teachings of the present invention therein;
Figure 2 is a sectional elevational view of the
photoconductive drum employed in the Figure 1 printing machine;
Figure 3 is an electrical diagram illustrating the
amplifying circuit of the present invention;
Figure 4 is an electrical diagram depicting the
comparator circuit of the present invention;
Figure 5 is an electrical diagram showing the hold
circuit of the present invention; and
Figure 6 is an electrical diagram depicting the
circuitry of Figures 3, 4 and 5 electrically connected to
one another.
While the present invention will be described in
connection with a preferred embodiment thereof, it will be
understood that it is not intended to limit the invention to
that embodiment. On the contrary, it is intended to cover all
alternatives, modifications and equivalents as may be included
within the spirit and scope of the invention as defined by
the appended claims.
DETAILED DESCRIPTIO~ OF THE I~VENTION
With continued reference to the drawings wherein like
reference numerals have been used throughout to designate
like elements, Figure 1 schematically illustrates an electro-
photographic printing machine arranged to produce multi-color
copies from a colored original document. Although the control
apparatus of the present invention is particularly well adapted
for use in the electrophotographic printing machine depicted
in Figure 1, it should become evident from the following dis-
cussion that it is equally well suited for use in a wide variety
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of electrostatographic printing machines and is not necessarily
limited to the particular embodiment shown herein.
The printing machine depicted in Figure 1 employs a
drum 10 having a photoconductive surface 12 mounted thereon
and entrained about the exterior circumferential surface thereof.
Photoconductive surface 12, preferably, is manufactured from a
material having a relatively panchromatic response to white
light. For example, one type of suitable photoconductive
material is disclosed in U.S. Patent No. 3,655,377 issued to
Sechak in 1972. Drum 10 is mounted rotatably within the machine
frame and adapted to rotate in the direction of arrow 14. A
series of processing stations are positioned such that as drum
10 rotates in the direction of arrow 14, photoconductive surface
12 passes sequentially therethrough. Drum 10 is driven at a
predetermined speed by a drive motor (not shown) relative to
the various machine operating mechanisms. The timing disc
mounted in the region of one end portion of the shaft on which
drum 10 is disposed cooperates with the machine logic to coor-
dinate each operation at the respective stations producing the
proper sequence of events thereat.
Initially, drum 10 advances photoconductive surface 12
through charging station A. Charging station A has positioned
thereat a corona generating device, indicated generally by the
reference numeral 16. Corona generating device 16 extends in
a generally transverse direction across photoconductive surface
12. In this manner, corona generating device 16 is readily
able to charge photoconductive surface 12 to a relatively high
~ubstantially uniform potential. A suitable corona generating
device is described in U.S. Patent ~o. 2,778,946 issued to Mayo
in 1957.
After photoconductive surface 12 is charged to a
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substantially uniform level, drum 10 rotates to exposure station
B. At exposure station B, a moving lens system, generally
designated by the reference numeral 18, and a color filter
mechanism, shown generally at 20, create a color filtered light
image which irradiates charged photoconductive surface 12. An
original document 22 is stationarily supported face down upon
transparent viewing platen 24. Original document 22 may be
a sheet of paper, book or the like which is disposed upon platen
24. Lamp assembly 26, filter mechanism 20 and lens 18 move in
a timed relation with drum 10 to scan successive incremental
areas of original document 22 disposed upon platen 24. During
exposure, filter mechanism 20 interposes selected color filters
into the optical path of lens 18. A selected color filter
operates on the light rays passing through lens 18 to create a
single color light image. The single color light image irradi-
ates charged photoconductive surface 12 to thereby record an
electrostatic latent image thereon corresponding to a presel-
ected spectral region of the electromagnetic wave spectrum,
hereinafter referred to as a single color electrostatic latent
image.
After the electrostatic latent image is recorded on
photoconductive surface 12, drum 10 rotates to development
station C. Development station C includes three individual
developer units generally designated by the reference numerals
28, 30 and 32, respectivel~y. A suitable development station
employing a plurality of developer units is disclosed in
U.S. Patent No. 3,854,449 issued December 17, 1974. The
development units are all of a type generally referred to as
magnetic brush developer units. A typical magnetic brush
developer unit employs a magnetizable developer mix comprising
carrier granules and toner particles. The developer mix is
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continually brought through a direction flux field to form a
brush thereof. The developer mix is continually moving to
provide the brush consistently with fresh material. Develop-
ment is achieved by bringing the brush into contact with
photoconductive surface 12. Each of the developer units 28,
30, 32 apply toner particles corresponding to the complement
of the specific single color electrostatic latent image recorded
on photoconductive surface 12. The toner particles are adapted
to absorb light within a preselected spectral region of the
electromagnetic wave spectrum corresponding to the wave length
of light transmitted through the filter. For example, a green
filtered electrostatic image is rendered visible by depositing
green absorbing magenta toner particles thereon. Similarly,
blue and red latent images are developed with yellow and cyan
toner particles, respectively.
The control apparatus of the present invention includes
a transparent electrode assembly 46 mounted on photoconductive
surface 12 of drum 10. Illuminating means, such as light source
48, in cooperation with fiber optics 50, transmit light rays
through transparent electrode assembly 46. During development,
toner particles are deposited on transparent electrode 46 and
the intensity of the light rays passing therethrough is indica-
tive of the density thereof. Sensing means, such as photo-
diode 52, is adapted to receive the light rays transmitted
through transparent electrode assembly 46. Photodiode 52
develops an electrical output signal corresponding to the
intensity of the light rays received thereon. Electrical
circuitry 54 processes the electrical output signal from photo-
diode 52. It should be noted that photodiode 52 is substantially
thermally insensitive having a coefficient of thermal sensi-
tivity of about 0. 2%/C in the mode presently being used.
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Electrical circuitry 54 lS adapted to amplify the electrical
output signal from photodiode 52 which is at a low level for the
intensity of light rays transmitted thereto. In addition,
electrical circuitry 54 compares the amplified electrical output
signal to a preselected reference and develops an error signal
corresponding to the deviation therebetween. Finally, electrical
circuitry 54 includes a hold circuit adapted to hold the error
signal until it is interrogated by the logic associated with one
of the respective toner dispensers 34, 36 or 38. Thus, the
electrical output signal from electrical circuitry 54 is adapted
to energize the corresponding toner dispenser to adjust the con-
centration of toner particles within the associate developer mix
to a preselected level optimizing development thereby. The
detailed structural configuration of the circuitry employed in
electrical circuit 54 will be discussed hereinafter with refer-
ence to Figures 2 through 6, inclusive. The foregoing control
apparatus is disclosed in greater detail in U.S. Patent No.
3,754,821 issued to Whited in 1973.
After development, drum 10 is rotated to transfer
station D. At transfer station D, the toner powder, adhering
electrostatically to photoconductive surface 12, is transferred
to a sheet of support material 56. Support material 56 may be
plain paper or a sheet of thermoplastic material, amongst others.
Transfer station D includes thereat an electrically biased
transfer roll shown generally at 58. The surface of transfer
roll 58 is biased electrically to a potential having a magni-
tude and polarity sufficient to electrostatically attract toner
particles from photoconductive surface 12 to support material 56. ;
A single sheet of support material 56 is supported on transfer
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roll 58 which is arranged to move in synchronism with photo-
conductive surface 12 (at substantially the same angular velocity
therewith) so that each of the developed images is placed in
superimposed registration upon support material 56.
Prior to proceeding with the remainder of the processes
in the electrophotographic printing machine, the sheet feeding
arrangement will be briefly discussed. Support material 56 is
advanced from a stack thereof. A feed roll, in operative communi-
cation with a retard roll, advances and separates the uppermost
sheet from the stack. The advancing sheet moves into a chute
which directs it into the nip between a pair of register rolls.
Thereafter, gripper finger, mounted on transfer roll 58, secure
releasably thereon support material 56 for movement in a
recirculating path therewith.
After a plurality of toner powder images have been
transferred to support material 56, the gripper fingers on
transfer roll 58 release support material 56 and space it
therefrom. A stripper bar is then interposed therebetween to
separate support material 56 from transfer roll 58. Subse-
quently, an endless belt conveyor advances support material 56
to fixing station E.
At fixing station E, a suitable fuser permanently
affixes the transferred multi-layered toner powder image to
support material 56. One type of suitable fuser is described
in U.S. Patent No. 3,498,592 issued to Moser et al. in 1970.
After the fixing process, support material 56 is advanced by
a plurality of endless belt conveyors to a catch tray for sub-
sequent removal from the machine by an operator.
The last processing station in the direction of
drum rotation, as indicated by arrow 14, is cleaning station F.
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Although a preponderance of the toner particles are transferred
to support material 56, residual toner particles remain on
photoconductive surface 12 after transfer of the powder image
therefrom. These residual toner particles are removed from
photoconductive surface 12 as it passes through cleaning station F.
Here, the residual toner particles are initially brought under
the influence of a cleaning corona generating device (not shown)
adapted to neutralize the electrostatic charge remaining on
photoconductive surface 12 and the residual toner particles
adhering thereto. The neutralized toner particles are then
cleaned from photoconductive surface 12 by a rotatably mounted
fibrous brush 60 in contact therewith. A suitable brush cleaning
device is described in U.S. Patent No. 3,590,412 issued to
Gerbasi in 1971. In this manner, residual toner particles re-
maining on photoconductive surface 12 are readily removed therefrom.
Referring now to Figure 2, drum 10 is illustrated
therein with transparent electrode assembly 46 mounted thereon.
Electrode assembly 46 is located in a non-image portion of
photoconductive surface 12. As electrode assembly 46 passes
through the development zone, the conductive surface thereof
is biased to an electrical potential simulating the electro-
static latent image on photoconductive surface 12 of drum lOo
Preferably, the electrode is biased to about 200 volts above
developer bias, normal developer bias being about 500 volts.
However, the electrode may be biased from about 100 volts to
about 600 volts above the developer bias. The density of toner
particles on transparent electrode 46 is sensed by photodiode 52.
The output signal from photodiode 52 is processed by electrical
circuitry 54 and, depending upon the density of toner particles
remaining on electrode 46, additional toner particles may or may
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not be furnished to the developer mix. Photodiode 52 is mounted
exterior to and spaced from photoconductive surface 12 of drum
10 to sense the intensity of light rays passing through trans- -
parent electrode 46 just prior to the cleaning of photoconductive
surface 12, i.e. before drum 10 is rotated to cleaning station E
since electrode 46 undergoes the normal photoconductive drum
cleaning process. Light source 48 may be inside drum 10, or as
shown in Figure 2, external to drum 10 with the light rays
conducted therein by means of fiber optics 62. Shaft 64, which
supports drum 10, is a tubular member and permits fiber optics 62
to pass through the hollow central core thereof and out therefrom
to photoconductive surface 12. This enables light rays from
light source 48 to be directed to transparent electrode assembly
46.
In order to apply the appropriate voltage corresponding
to the electrostatic latent image recorded on photoconductive
surface 12 of drum 10, transparent electrode assembly 46 is
biased to a suitable voltage level. Preferably, this is
achieved by mounting a commutator assembly, indicated generally
at 66, in the region of the end bell of drum 10. A suitable
slip ring assembly may be used in lieu of commutator assembly 66.
The timing of the application of the bias voltage to an electrode
assembly 46 may be controlled by suitable electronic switching
or by the use of a split commutator ring, i.e. the electrode
being biased over one portion of the commutator and not over the
remaining portion. The bias voltage is removed from transparent
electrode assembly 46 during the cleaning process. In lieu
of applying a bias voltage to transparent electrode assembly 46,
a suitable bias may be applied thereto by electrical charging.
3 Turning now to Figure 3, there is shown the ampli-
fying circuit means 68 of electrical circuitry 54. Amplifying
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circuit means 68 is electrically connected to photodiode 52.
Operational amplifier 70 thereof is electrically connected to
photodiode 52. A 100,000 ohm resistor 72 and a 0.01 microfarad
capacitor 74 are connected in parallel with operational amplifier
70. In this manner, operational amplifier 70 operates with
resistive feedback in an inverting mode to maintain a low input
impedence, thereby operating photodiode 52 in a short circuit
condition. As shown in Figure 3, the input stage thereof pre-
ferably has a 10 to 1 gain variation control. Gain variation
is achieved by adjusting the resistance of a suitable resistor
76 connected to the output of operational amplifier 70. By way
of example, photodiode 52 may be a silicon wafer cell Model No.
SS-22 manufactured by Solar Systems, Inc.
In order to detect the level at which toner particles
are to be dispensed into the developer mix, comparator circuit
69 employs operational amplifier 78 wired as a level detector
with a slight amount of positive feedback to provide hysteresis
about the switch point. Resistor 80 is connected in parallel
with operational amplifier 78, and preferably, has a resistance
of 50,000 ohms. The output from operational amplifier 70 is
connected to the input of operational amplifier 78. A suitable
voltage source (not shown) develops a -15 volt reference voltage
to operational amplifier 78 through resistor 82, which is pre-
ferably 6,500 ohms. The positive terminal of operational ampli-
fier 78 is connected to ground by resistor 84 which is, preferably,
1000 ohms. In this circuit, a voltage slightly more negative
than -2.24 volts will cause the output of operational amplifier 78
to switch negative. The circuit holds that state until the input
voltage is slightly less negative than -1.76 volts. The hysteresis
cycle which is preferably about 2 volts + .24 volts is useful to
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prevent high frequency oscillation at the switch point due to
circuit instability and noise on the input signal. Temperature
stability of this circuit is preferably about 0.5 micro volts/ C
over the total range of 20C. Thus, the total error will be
about 10 microvolts over the total 20C range. Hence, it is
evident that the major temperature sensitivity in comparator
circuit 69 will be contributed by the thermal variations of the
resistors. To reduce these to a minimum, metal film resistors
are used which have a 0.01% per degree sensitivity.
Referring now to Figure 5, hold electrical circuit 86
is depicted therein. Hold circuit 86 includes a voltage source
preferably of 15 volts (not shown) electrically connected in
series to a suitable relay 88. Relay 88 is connected ~o a
silicon controlled switch 90. The output from comparator circuit
69 is also connected to silicon controlled switch 90. This
circuit detects the occurrence of a switch point and holds it
until the logic is ready to interrogate it. The logic connec-
tion is shown at terminals 92. A detailed description of the
logic associated with the toner dispensers is to be found in -
U.S. Patent No. 3,873,002 issued March 25, 1975. Hold circuit
86 also converts the low level signal from the operational
amplifier circuitry to the level required to drive the toner
dispenser motors. The toner dispenser motors are connected
through logic lines 94. The reset of hold circuit 86 is
accomplished by a signal from the logic circuitry to open relay
contact 91.
Referring now to Figure 6, there is shown a preferred
circuit configuration for electrical circuitry 54 embodying
the various elements depicted in Figures 3 through 5, inclusive.
As shown in Figure 6, photodiode 52 is oonnected to operational
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amplifier 70. Resistor 72 and compacitor 74 are connected in
parallel with operational amplifier 70. A 500,000 ohm resistor
94 is connected in series with the foregoing parallel arrangement
and the output of operational amplifier 70. The output terminal
of operational amplifier 70 is connected to a parallel circuit
having three 5000 ohm resistors 96 in series with three 50,000
ohm variable resistors 98 connected to one another through
switches 100. This circuit arrangement is connected to operational
amplifier 78 of comparator 69. Operational amplifier 78 is
connected to operational amplifier 70 and a 15 volt reference is
applied thereto through resistor 82. The positive terminal of
operational amplifier 78 is electrically grounded through resistor 84.
Resistor 80 is connected in parallel with operational amplifier
78 as hereinbefore described. The output from operational
amplifier 78 passes through resistor 102, which is preferably
2000 ohms. Resistor 102 is connected to ground through a 470
ohm resistor 104. Resistor 102 is also connected to silicon
controlled switch 90. The anode of silicon controlled switch
90 is connected to relay 88 which, in turn, is connected to a
15 volt reference source. In addition, 100,000 ohm resistor 106
is connected from the anode gate of silicon controlled switch
90 to the +15 volt voltage sourceO As previously indicated,
the output along terminals 94 is directed to drive the toner
dispenser oscillator motors. Interrogation of the hold circuit
is initiated by a signal from the toner dispenser along logic
lines 92.
In recapitulation, it is evident that the control
apparatus of the present invention achieves the requisite
regulation through the utilization of a thermally insensitive
photosen~or having a low level signal output therefrom which is
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amplified by a stable amplification circuit. The amplified
electrical output signal is then compared to a reference by
suitable comparator circuitry and then held in a hold circuit
until interrogated by the toner dispenser logic circuitry.
When the interrogated error signal is developed, it actuates
the toner dispenser oscillator motor to dispense toner particles
therefrom into the corresponding developer mix, thereby adjusting
the concentration thereof to substantially about the preselected
level. In this manner, the control apparatus insures that the
image density and color balance of the multi-color copies being
reproduced in the printing machine are repeatedly of a high
quality.
It is, therefore, apparent that there has been pro-
vided in accordance with the present invention an apparatus
for controlling the concentration of toner particles in a
developer mix that fully satisfies the objects, aims and
advantages set forth above. While this invention has been
described in conjunction with specific embodiments thereof, it
is evident that many alternatives, modifications and variations
will be apparent to those skilled in the art. Accordingly, it
is intended to embrace all alternatives, modifications and
variations that fall within the spirit and broad scope of the
appended claims~
