U.S. patent number 3,872,824 [Application Number 05/381,035] was granted by the patent office on 1975-03-25 for xerographic toner concentration control apparatus.
This patent grant is currently assigned to Van Dyk Research Corporation. Invention is credited to Daniel Richard Erny, Leon Albert Tysko.
United States Patent |
3,872,824 |
Erny , et al. |
March 25, 1975 |
Xerographic toner concentration control apparatus
Abstract
An arrangement for measuring the concentration of toner powder
in a xerographic developer mixture by causing a sample of the
developer to impinge upon a transparent plate. By providing the
developer sample in the form of a turbulent stream, inclining the
plate at the proper angle, and arranging the geometry of the
measuring apparatus so that the stream impinges upon the
transparent plate with the proper kinetic energy, a layer of toner
powder is formed on the surface of a portion of the plate, the
thickness of the toner layer increasing and decreasing with
corresponding changes in the concentration of toner in the
developer mixture. A photoelectric system is employed to monitor
the light transmission of the toner-coated plate to provide a
measure of toner concentration.
Inventors: |
Erny; Daniel Richard (Boonton,
NJ), Tysko; Leon Albert (Rockaway, NJ) |
Assignee: |
Van Dyk Research Corporation
(Whippany, NJ)
|
Family
ID: |
26921921 |
Appl.
No.: |
05/381,035 |
Filed: |
July 20, 1973 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
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227965 |
Feb 22, 1972 |
3791744 |
|
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Current U.S.
Class: |
399/65; 427/10;
430/30 |
Current CPC
Class: |
G03G
15/0855 (20130101) |
Current International
Class: |
G03G
15/08 (20060101); G03g 013/00 () |
Field of
Search: |
;118/4,7,9,10,637
;117/17.5 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Stein; Mervin
Assistant Examiner: Millstein; Leo
Attorney, Agent or Firm: Lessler; Arthur L.
Parent Case Text
This is a division of copending application Ser. No. 227,965, filed
Feb. 22, 1972 and assigned to the assignee of the instant
application.
Claims
We claim:
1. Apparatus for controlling the toner concentration of the
developer in the developer tank of a xerographic copying apparatus,
comprising:
a developer sample storage hopper;
a supply conduit for transporting a portion of the developer in
said tank to said hopper;
a lower orifice in said hopper for permitting developer stored in
said hopper to flow through said orifice in a stream at a rate
limited by the size of the orifice;
first and second inclined light transmissive toner collecting
plates;
a developer stream deflecting element situated below said orifice
for (i) introducing a substantial amount of turbulence into said
stream by disrupting the flow of developer in said stream, for (ii)
dividing the stream into at least two relatively turbulent
streamlets, and for (iii) directing said turbulent streamlets onto
respective ones of said collecting plates
with kinetic energy sufficient to dislodge toner particles from the
developer carrier beads to which the particles are
electroscopically adherent, said dislodged toner particles being
deposited upon said collecting plate portions, the kinetic energy
and turbulence of said developer streamlets being sufficient to
remove toner particles from said collecting plate portions when the
concentration of toner in said streamlets decreases, said kinetic
energy being sufficiently low so that the developer is not damaged
by impact with said collecting plates;
at least one light source for illuminating said collecting plate
portions;
a photosensitive element for generating a control signal responsive
to the light from said source which is transmitted through said
collecting plate portions;
means coupled to said element for varying the concentration of
toner in the developer within said tank in accordance with said
control signal; and
conduit means for returning the developer in said streamlets to
said tank after the streamlets have impinged on said collecting
plates.
2. Apparatus according to claim 1, further including a bypass
conduit for returning a portion of any developer in said sample
storage hopper to said developer tank without permitting said
developer portion to impinge on said collecting plates, said
developer portion being returned at a sufficient rate to insure
that the concentration of toner in the developer within said hopper
is at all times substantially equal to the concentration of toner
in the developer within said tank.
3. Apparatus according to claim 2, wherein said light source and
said photosensitive element are situated in dust-free
compartments.
4. Apparatus according to claim 2, wherein the carrier beads of
said developer comprise steel pellets having a coating thereon of a
material removed from the material of said toner in the
triboelectric series.
5. Apparatus according to claim 2, wherein said deflecting element
is in the form of an inverted V.
6. Apparatus according to claim 5, wherein said deflecting element
is electrically conductive.
7. Apparatus according to claim 6, wherein said deflecting element
is grounded.
8. Apparatus according to claim 6, wherein said deflecting element
comprises steel or aluminum.
9. Apparatus according to claim 5, wherein the angle formed by the
V of the deflecting element is on the order of 60 degrees.
10. Apparatus according to claim 2, wherein said collecting plates
are disposed below said deflecting element and the normals to said
plates are inclined at an angle of 30 to 75.degree. with respect to
the vertical.
11. Apparatus according to claim 10, wherein said angles are on the
order of 60.degree..
12. Apparatus according to claim 10, wherein the vertical distance
between said orifice and the adjacent portion of said deflecting
element directly beneath said orifice is on the order of
three-sixteenth inch.
13. Apparatus according to claim 12, wherein the vertical distances
between said collecting plate portions and the parts of said
deflecting element above said plate portions are on the order of
0.25 inch.
Description
This invention relates to an apparatus for measuring the toner
concentration in the developer of a xerographic copying
apparatus.
In the practice of xerography, an electrostatic image of a desired
pattern is formed on an insulating surface. This is usually
accomplished by providing a photoconductive insulating material
affixed to a conductive backing, uniformly electrostatically
charging the photoconductive surface (typically by a corona
charging technique), and subsequently exposing the charged
photoconductive surface to an electromagnetic radiation pattern
(usually a visible light pattern) of the image to be reproduced.
The electromagnetic radiation pattern discharges the
photoconductive surface in the areas where the surface is
irradiated, thus forming an electrostatic charge pattern on the
photoconductive surface corresponding to the pattern of the desired
image.
In order to render the image defined by the electrostatic charge
pattern visible and permanent, the photoconductive surface is
contacted with microscopic particles which may be in the form of a
fine powder, the particles having been provided by some means with
an electrostatic charge opposite in sign to the charge remaining on
those portions of the photoconductor which have not been discharged
(or which have only been partially discharged) by the incident
electromagnetic radiation. As a result, these microscopic particles
(commonly known as "toner") adhere to the photoconductor only in
those areas which retain an electrostatic charge, i.e. those areas
which have not been irradiated.
The pattern of toner particles, which corresponds to the pattern of
the desired image, is subsequently either (i) fused to the
photoconductive surface by application of heat or a suitable
solvent to form a permanent image, or (ii) transferred to another
surface, which may comprise ordinary paper, and subsequently fused
thereto by suitable application of heat or solvent.
In order to carry the toner particles to the surface of the
photoconductor which has been selectively discharged in accordance
with the desired pattern, and to provide toner particles with the
desired electrostatic charge, i.e. opposite to the charge remaining
on those areas of the photoconductor which have not been discharged
by irradiation in accordance with the desired pattern, a granular
carrier material, usually in the form of small beads of glass, sand
or steel, is provided. The toner, which is usually a pigmented or
dyed resin based powder, is mixed with the carrier particles,
pellets or beads, which are coated with a material removed in the
triboelectric series from the toner material, so that a
triboelectric charge is acquired by the toner and the granular
carrier upon mutual interaction therebetween. The resulting
triboelectric charging effect causes the relatively small
(typically on the order of 0.1 to 20 microns) toner particles to
adhere to the relatively large (typically on the order of 350 to
500 microns) carrier granules.
When the developer mixture, consisting of carrier granules having
electroscopic toner particles adhering thereto by mutual
triboelectric interaction, is cascaded over, poured on, brushed
against or otherwise brought into contact with the photoconductive
surface having the desired electrostatic image thereon, the toner
particles are pulled away from the carrier granules by the stronger
attraction of the latent electrostatic image.
These attracted toner particles are subsequently fused to the
photoconductor or another surface to form the desired permanent
image. The carrier granules are not attracted to the latent
electrostatic image, and are eventually returned to the developer
reservoir.
As the number of copies produced by the xerographic apparatus
increases, toner powder is depleted from the developer mixture,
while the carrier granules are not consumed. As a result, the
relative concentration of toner in the developer mixture decreases
as more and more copies are produced. This effect causes the
density of print, i.e. of toner deposited upon the portions of the
photoconductor surface which retain charge in accordance with the
latent electrostatic image pattern, to decrease, producing copies
which are undesirably light and of non-uniform quality.
Since the amount of toner consumed by the xerographic apparatus is
dependent not only upon the number of copies produced, but also
upon the amount of print produced on each copy, it is virtually
impossible to estimate the periods when toner should be added, and
the proper quantity of toner to be added at such periods.
Attempts to manually monitor the toner concentration by viewing the
density of print on the copies produced by the xerographic
apparatus have proven to be impractical, especially in the case of
high speed copier-duplicator machines, where a large number of
defective copies may be produced before the need for toner
replenishment is discovered.
An additional problem encountered with attempts to manually control
the toner concentration in a xerographic apparatus is that
defective copies and impairment of machine performance can result
from addition of an excessive concentration of toner to the
developer mixture. On the other hand, an insufficient concentration
of toner can result in mechanical damage to the triboelectric
coating on the developer beads.
An excessive concentration of toner in the developer mixture
results in an undesirable deposit of toner in the image areas,
commonly known as "background." Such "background" causes copies to
exhibit poor contrast with blotchy images or poor resolution.
Excessive toner concentration also increases the rate of
accumulation of toner powder on critical machine components,
requiring more frequent replacement of filters and cleaning of such
components.
An additional problem resides in the difficulty of removing excess
toner from the developer mixture, so that once an excessive amount
of toner has been added, it is usually necessary to produce a
relatively large number of defective copies (which must be
discarded) in order to reduce the toner concentration to an
acceptable value.
Accordingly, those skilled in the xerographic art have sought to
provide an arrangement for automatically monitoring the
concentration of toner in a xerographic developer mixture, and
utilizing such an arrangement to automatically control the toner
concentration by feeding toner to the developer mixture from a
suitable motor operated dispenser when required.
One approach which has been suggested for the measurement of toner
concentration involves the provision of a photoelectric sensing
arrangement to measure the light transmission through the cloud of
toner which is generated within the developer tank due to the
disturbances which occur when the developer is transported from the
tank to be brought into contact with the photoconductive surface.
The density of the toner cloud, and therefore the light
transmission through the cloud, varies as a function of the
concentration of toner in the developer mixture within the
developer tank.
It has been found, however, that the required photoelectric light
sources and sensors, which must necessarily be in optical
communication with the toner cloud, become permanently coated with
toner, resulting in a false indication of high toner
concentration.
Another approach to the measurement of toner concentration has been
to provide sample areas of photoconductive material on the
photoconductive surface but outside the area on which the desired
image is to be produced. The photoconductive sample area or areas
are charged in accordance with a predetermined pattern when the
machine is in operation, and toner is deposited on the charged
sample areas by bringing a portion of the developer mixture into
contact therewith. The density of the toner so deposited on the
sample areas is then measured optically, or by any other suitable
technique. The measured density of the toner so deposited is
presumably a function of the concentration of toner in the
developer within the developer tank. Such a sample deposition
system has not proven satisfactory, however, in part because (i)
the amount of toner deposited on the sample pattern is affected by
variations in various machine operating parameters, and (ii) the
continuous exposure of the sample area results in fatigue of the
photoconductive material therein, characterized by a decrease in
photoconductivity, a lower rate of toner deposition for a given
toner concentration, and an indication of a lower toner
concentration than is actually present in the developer
mixture.
An improved version of the technique of "developing" a sample area
which has been electrostatically charged involves the provision of
an "artificial" electrostatic charge pattern formed by application
of a voltage between adjacent conductive electrodes, thus providing
a DC electric field therebetween. See, e.g., U.S. Pat. No.
3,430,606. When a portion of the developer mixture is passed over
the electrodes, or over an insulating surface adjacent the
electrodes, toner particles are caused to deposit on that electrode
which has a charge opposite to that of the toner particles. By
periodically reversing the polarity of the voltage applied between
the electrodes, the toner particles are caused to periodically be
repelled by one electrode and attracted by the other. By employing
transparent electrodes adjacent a transparent insulator such as
glass, and measuring the variation in light transmission through
the deposited toner layer, an indication of toner concentration may
be provided. Such an arrangement, while workable, requires
conductive electrodes, a voltage source of periodically variable
polarity and relatively elaborate electronic sensing circuitry.
Still another toner concentration measuring technique is described
in U.S. Pat. No. 3,610,205. However, this technique is limited to
arrangements wherein the toner and carrier particles have different
optical characteristics, and is of limited versatility.
As herein described, there is provided apparatus for measuring the
toner concentration in the developer of a xerographic copying
apparatus. The measuring apparatus includes a toner collecting
plate, and supply means for providing a stream of the developer
containing the toner whose concentration is to be measured. The
developer stream is directed onto a surface of the collecting plate
to cause toner particles to be dislodged from the carrier beads,
pellets or granules to which the toner particles are
electroscopically adherent to deposit a layer of toner on at least
a portion of the collecting plate. The developer stream removes
toner from the collecting plate when the concentration of toner in
the stream decreases. Means is provided for sensing the amount of
toner deposited in the form of the aforementioned layer.
In the drawing:
FIG. 1 shows a conventional xerographic copying apparatus with
which the present invention may be employed.
FIG. 2 illustrates, in schematic form, a toner concentration
measuring technique according to the prior art.
FIG. 3 is a schematic representation which illustrates the
principle of an embodiment of the present invention.
FIG. 4 is a schematic representation which illustrates the
principle of an alternative embodiment of the invention.
FIGS. 5 and 6 show graphs useful in explaining the operation of the
invention.
FIg. 7 shows graphs useful in explaining the operation of the
preferred embodiment of the invention.
FIG. 8 is an elevation view of the developer tank employed in the
apparatus of FIG. 1, with an toner concentration analyzing device
according to the present invention affixed thereto.
FIG. 9 is a side elevation view, in partial crosssection, of the
developer tank arrangement shown in FIG. 8.
FIG. 10 shows a front cross-sectional elevation view of a toner
concentration analyzer according to an embodiment of the invention,
taken along the cutting plane X--X' as shown in FIG. 8.
FIG. 11 shows a front cross-sectional elevation view of a toner
concentration analyzer according to a preferred embodiment of the
invention, taken along the cutting plane X-X' as shown in FIG.
8.
FIG. 12 shows a side elevation cross-sectional view of a portion of
the toner concentration analyzer shown in FIG. 11, taken along the
cutting plane Y-Y'.
FIG. 13 is a block diagram of a toner concentration control system
employing the toner concentration analyzer illustrated in FIGS. 11
and 12.
The xerographic copying apparatus 10 shown in FIG. 1, which
apparatus is typical of the prior art, comprises a number of
operating stations situated about the periphery of a rotatable drum
11. The drum 11 is rotatably mounted on an axle 12, and is
continually rotated by a suitable drive motor (not shown) while
copies are to be made. The outer surface 13 of the drum 11 is
coated with a relatively hard photoconductive material, such as
vitreous selenium.
A document 14 bearing an image to be copied is placed (face down)
on a curved transparent support plate 15. The image to be copied is
then illuminated by a suitable light source (not shown) through the
transparent support plate 15, and scanned by a rotatable mirror 16,
which reflects light from the document 14 through a lens 17 and a
fixed mirror 18 onto the photoconductive surface 13 of the rotating
drum 11, through the exposure slot 19.
The rotation of the scanning mirror 16 is accurately synchronized
with the rotation of the drum 11, so that the linear velocity at
which the surface of the document 14 is scanned is equal to the
linear velocity of the photoconductive layer 13 disposed on the
outer periphery of the drum 11.
Prior to exposure to the image information bearing light beam
through the aperture 19, the photoconductive surface 13 of the drum
11 is passed under a corona emitter or corotron 20, which charges
the photoconductive surface 13 to establish a uniform electrostatic
charge density thereon.
Upon passing beneath the masking aperture 19, the charged
photoconductive surface 13 is selectively discharged in accordance
with the pattern of the image on the document 14 to be copied,
resulting in the formation of a latent electrostatic image on at
least a portion of the photoconductive surface 13 (the size of the
portion of the photoconductive surface which contains the latent
electrostatic image is dependent upon the size of the document 14
to be copied).
As the drum 11 continues to rotate, the latent electrostatic image
on the photoconductive surface 13 enters the developer tank 21.
Situated in the lower portion or sump region of the developer tank
21 is a granular developer mixture 22, which consists of resin
coated steel beads and toner powder. The material of the resin
coating is removed from the toner powder material in the
triboelectric series.
The developer mixture is continually carried from the sump region
of the developer tank to a hopper 23 at or near the top of the
developer tank by means of a conveyor belt 24 having a plurality of
carrier buckets 25 affixed thereto.
As the drum 11 continues to rotate, the developer mixture in the
hopper 23 continually flows out the orifice at the bottom of the
hopper, and cascades over the photoconductive surface 13 of the
drum. As the developer mixture cascades over the photoconductive
surface, toner particles are attracted away from the moving carrier
beads and adhere to the charged portions of the photoconductive
surface, thus converting the latent electrostatic image thereon to
a corresponding visible pattern of pigmented or dyed toner
particles. The developer mixture (less any toner particles which
have adhered to the latent electrostatic image on the
photoconductive surface 13 of the drum 11) falls back into the sump
of the developer tank 21 after the developer has cascaded over the
photoconductive surface 13.
Upon further rotation of the drum 11, the pattern of toner
particles (the toner particles being electroscopically adherent to
the charged areas of the latent electrostatic image on the
photoconductive surface 13) is brought into juxtaposition with a
moving paper 26, which is caused to progress at a velocity equal to
the peripheral velocity of the drum 11, so that there is
substantially no relative motion between the paper 26 and the
adjacent portion of the periphery of the drum 11. As the toner
pattern (corresponding to the latent electrostatic image which in
turn corresponds to the image of the document 14) passes the moving
paper 26 in contact therewith, an image transfer corona emitter or
corotron 27 attracts the toner particles away from the
photoconductive surface 13 and onto the adjacent portion of the
moving paper 26. The toner pattern is thus transferred onto the
moving paper 26, and is then passed under a radiant heater or fuser
28 which fuses the toner particle pattern to the paper 26 to form a
permanent copy of the image on the document 14.
The paper 26 is moved along beneath the fuser 28 by a moving belt
29.
After the toner pattern has been transferred from the
photoconductive surface 13 of the drum 11 onto the paper 26, some
residual toner particles remain on the drum surface. In order to
remove these residual toner particles, the photoconductive surface
13 is subsequently exposed to a further corona emitting device or
corotron 30 which neutralizes any residual charge remaining on the
photoconductive surface, thus reducing or cancelling the
electrostatic attraction between the residual toner particles and
the adjacent photoconductor surface.
A rotating brush 31 situated in a dust-tight compartment 32
mechanically removes any remaining toner particles from the
photoconductive surface 13 of the drum 11, the toner particles so
removed being drawn out through the conduit 33 by air pressure as a
result of the application of a suitable vacuum source (not shown)
to the conduit 33. Before the toner-laden air is returned to the
atmosphere, it is filtered by suitable means (not shown) to remove
the toner particles therefrom.
After the residual toner particles have been removed from the
photoconductive surface 13 by the action of the brush 31 and the
vacuum source associated therewith, the photoconductive surface 13
is irradiated by light from a suitable light source 34, to insure
substantially complete discharge of the photoconductive
surface.
Thereafter the corona emitter or corotron 20 recharges the
photoconductive surface 13 in a uniform manner in preparation for
the next cycle of machine operation.
The developer 22 employed in the apparatus 10 may be of any
suitable conventional type. For example, the following composition
has been found to provide excellent results:
Carrier: steel beads having an average diameter on on the order of
350 to 500 microns, coated with an acrylic or styrene copolymer
mixed with a suitable triboelectric activating dye such as Hansa
yellow, the weight of the resin coating being on the order of 0.3
percent of the total carrier bead weight.
Toner: a thermoplastic styrene-based resin or copolymer mixed with
a suitable pigment such as carbon black.
Toner Concentration: Ratio of toner to developer (toner plus
carrier) - 0.5 percent by weight.
In order to maintain uniform quality of print produced on the paper
26, the concentration of toner within the developer 22 must
maintain within a particular range. As previously mentioned, a
number of systems have been proposed for measuring the
concentration of toner in the developer mixture 22, and
automatically dispensing additional toner into the sump of the
developer tank 21 to mix with the developer 22 and maintain the
toner concentration therein within the desired range. One such
system is illustrated in FIG. 2.
As illustrated schematically in FIG. 2, a glass plate 35 is
provided having suitably contoured transparent electrodes 36 and 37
disposed thereon. A variable polarity DC voltage is applied between
the electrodes 36 and 37 by means of a suitable voltage source 38,
the polarity of the voltage produced by the source 38 being
reversed at regular intervals.
A sample of developer from the developer tank 21 is removed
therefrom through a suitable conduit and caused to cascade over the
glass plate 35 and the electrodes 36 and 37 by a hopper of proper
configuration, illustrated schematically as 39. The electric field
produced between the electrodes 36 and 37 by the voltage source 38
simulates that of a latent electrostatic image such as is formed on
the photoconductive surface 13 of drum 11 during normal operation
of the apparatus 10. As developer leaves the hopper 39 and cascades
over the electrodes 36 and 37, toner particles are attracted away
from the carrier granules to which they are electroscopically
adherent, and toward the electrode which at that time has a charge
opposite in polarity to that of the toner particles. Presumably,
the density of the "image" deposited upon the aforementioned
electrode varies in accordance with the concentration of toner in
the developer mixture. The resulting variation in light
transmission through the transparent plate 35 and the conductive
electrodes may then be measured by illumination thereof with a
suitable light source 40, while monitoring the transmission
therethrough by means of a suitable photosensitive element 41. The
output signals developed by the photosensitive element 41, which
presumably vary in accordance with the toner concentration in the
developer mixture, may then be utilized to control the operation of
a toner dispenser to vary the rate at which toner is added to the
developer 22 within the developer tank 21.
The aforementioned toner concentration measuring arrangement, as
described with reference to FIG. 2, is disclosed in more detail in
U.S. Pat. No. 3,430,606.
A disadvantage of the arrangement illustrated in FIG. 2 is the need
for providing transparent conductive electrodes, and for biasing
these electrodes with a DC voltage of periodically varying
polarity. Unless the polarity of the applied voltage is varied at
regular intervals, toner continues to accumulate on one of the
electrodes, giving a false indication of high toner
concentration.
Another disadvantage of the arrangement illustrated in FIG. 2 is
the need for relatively elaborate circuitry to process the signal
obtained from the photosensitive element 41, which has a generally
sawtooth waveform as a result of periodic reversal of the polarity
of the voltage generated by the source 38.
In the aforementioned arrangement, as illustrated in FIG. 2 and
described in U.S. Pat. No. 3,430,606, developer is cascaded over
the plate 35 in relatively turbulence-free fashion. If the
electrodes 36 and 37 and the voltage source 38 were not provided, a
layer of toner would accumulate on the entire exposed surface of
plate 35, the thickness of the toner layer increasing until nearly
all the light transmitted through the plate 35 by the light source
40 was blocked. Thus the device shown in FIG. 2 would be useless
for providing a measurement of toner concentration without the
electrodes 36 and 37 and the source 38.
Another arrangement, disclosed in U.S. Pat. No. 3,610,205,
describes a system for determining the toner concentration in the
developer of a xerographic copying apparatus in the particular case
where the toner and carrier particles have different optical
characteristics. As shown in FIGS. 1 and 6 of this reference, the
developer mixture is caused to smoothly slide over the interior
surface 36 of a transparent window 35. Light from a source 37 is
transmitted through the window 35 and reflected by the developer
mixture. The reflected light is measured by an arrangement
comprising the photosensor 40, amplifier 56 and meter 58.
In the aforementioned arrangement, since the toner and carrier
particles have different optical qualities, the total amount of
light reflected by the developer mixture depends upon the relative
proportions of such particles, i.e. on the toner concentration.
In order for the arrangement shown in U.S. Pat. No. 3,610,205 to
operate effectively, it is essential that the developer mixture
slide across the interior surface 36 of the transparent window 35
in such a manner that deposition of toner on the surface 36 is
minimized. The structure according to the present invention, on the
other hand, is capable of measuring the toner concentration of
developer mixtures in which the toner and carrier particles have
the same optical qualities. In the arrangement described in the
instant application, the developer mixture is formed into a stream
which is caused to impinge upon the transparent collecting plate in
order to deposit a toner layer thereon, the optical transmission of
the deposited toner layer providing an accurate measure of toner
concentration in the developer mixture. According to the instant
invention, the developer stream is directed onto the collecting
plate with sufficient kinetic energy to dislodge toner particles
from the carrier beads to which they are electroscopically
adherent. In contradistinction, the dislodging of toner particles
from their associated carrier beads is deliberately avoided in the
structure of U.S. Pat. No. 3,610,205.
In U.S. Pat. No. 3,610,205, toner concentration is measured by, in
effect, determining the relative amounts of light reflected by the
carrier and toner particles respectively. On the other hand, the
structure herein described determines toner concentration by
measuring the optical transmission of a deposited toner layer, the
carrier beads flowing across the optical path having an
insignificant effect upon this measurement.
Thus, while the arrangement of U.S. Pat. No. 3,610,205 seeks to
avoid the deposition of a toner layer on the transparent surface to
be optically illuminated, the structure herein described seeks to
dislodge toner particles from the associated carrier beads to form
a toner layer on the transparent collecting plate, such that the
density of the toner layer increases and decreases with
corresponding variations in the toner concentration within the
developer mixture.
The operation of the toner concentration measuring arrangement
according to the present invention is based in part upon the
discovery that, by causing the developer mixture to impinge upon a
collecting plate with a proper value of kinetic energy and in a
substantially turbulent manner, a layer of toner is deposited on a
portion of the collecting plate, which layer increases and
decreases in thickness or density as the concentration of toner in
the developer mixture varies correspondingly.
The principle of operation of the present invention will be more
clearly understood by reference to FIGS. 3 and 4, which illustrate
a feature of the toner concentration measurement arrangement
according to an embodiment of the present invention.
As illustrated in FIG. 3, a developer sample storage hopper 42
contains a sample of developer mixture 43, which has the same toner
concentration as the developer mixture 22 within the developer tank
21 (see FIG. 1). An orifice 44 at the bottom of the hopper 42
permits the developer sample 43 to flow from the hopper at a rate
limited by the size of the orifice. Situated below the orifice 44
is a transparent collecting plate 45, upon surface 46 of which a
stream of developer emanating from the orifice 44 of the hopper 42
is caused to impinge.
A developer stream deflecting member 48, preferably in the form of
a suitable shaped conductive plate, is situated in the path of the
developer stream 47. The purpose of the deflector plate 48 is to
introduce a substantial amount of turbulence into the developer
stream 47, and to direct the stream 47 onto the surface 46 of the
transparent collecting plate 45.
By properly selecting the size and shape of the orifice 44, the
configuration of the deflecting plate 48, the angle of inclination
of the collecting plate 45, the relative spacings between the
orifice 44 and the portion of the deflecting plate 48 immediately
beneath the orifice, the inclination of the deflecting plate 48,
and the vertical distance between the point at which the stream 47
impinges upon the collecting plate 45 and the portion of the
deflecting plate 48 above said point, the developer stream 47 is
caused to impinge upon the surface 46 of the collecting plate 45
with kinetic energy sufficient to dislodge toner particles from the
developer carrier beads to which the particles are
electroscopically adherent, so that the dislodged toner particles
are deposited on at least a portion of the collecting plate 45. At
the same time, the kinetic energy and turbulence of the developer
stream 47 are sufficient to remove toner particles from the
aforementioned collecting plate portion when the concentration of
toner in the developer stream 47 decreases.
If the kinetic energy and turbulence of the developer stream 47 are
too low, the aforementioned effects will not occur and toner will
build up on the collecting plate 45 to give a false indication of
high toner concentration. If the kinetic energy of the stream 47
impinging on the collecting plate 45 is too great, damage will
result to the triboelectric resin coating on the carrier granules,
and the uniformity of the deposited toner layer will be disturbed
by splattering effects.
It has not proven possible to specify the required range of kinetic
energy and turbulence of the developer stream in terms of specific
parameters, since it is difficult if not impossible to measure
these quantities. In practice, the apparatus shown in FIG. 3 in
schematic form is assembled, and the inclination of the deflector
plate 48 is varied until the toner layer deposited on a portion of
the surface 46 of the collecting plate 45 exhibits the desired
increase and decrease of thickness or density with increase or
decrease of toner concentration in the developer sample 43. In
order to measure the amount of toner deposited in the form of the
aforementioned layer on a portion of the collecting plate 45, a
photoelectric system comprising a light source 49 and a
photosensitive element 50 is aligned so that light transmitted from
the source 49 to the element 50 passes through the portion of the
collecting plate 45 upon which is deposited a layer of toner which
increases and decreases in thickness or density as the
concentration of toner in the developer mixture increases or
decreases. The light transmitted through this portion of the
collecting plate, as sensed by the photosensitive element 50, is
converted to an electrical output signal which is utilized to
control a toner dispenser by controlling the operation of a motor
which releases toner from the dispenser into the sump of the
developer tank 21 when the toner concentration in the developer is
low.
Preferably, the deflecting plate 48 is electrically conductive and
is grounded to avoid undesirable electrostatic interaction with the
developer mixture. Suitable materials for the plate 48 are steel or
aluminum.
While the collecting plate 45 may comprise a suitable transparent
insulating material such as glass, it is not necessary that the
collecting plate 45 be transparent, provided that a suitable
technique (such as, e.g. a capacitance measuring arrangement) is
employed to measure the amount of toner deposited in the form of
the aforementioned layer on a portion of the collecting plate 45.
No adverse effects have been detected resulting from the use of a
conductive toner collecting surface to receive the toner stream 47,
rather than an electrically insulating surface.
In order to provide an accurate measurement of toner concentration,
as determined by the light attenuation introduced by the deposited
toner layer, it is highly desirable that the light source 49 be of
constant intensity, and that the photoelectric system (comprising
light source 49 and photosensitive element 50) be arranged in such
a manner that toner does not accumulate on the surfaces of the
light source and photosensitive element, since such toner
accumulation would give a false indication of high toner
concentration.
With an arrangement of the general type schematically illustrated
in FIG. 3, a dynamic range of light intensity on the order of 90:1
has been obtained. That is, the output signal derived from the
photosensitive element 50 has been observed to vary over a 90:1
range as the concentration of toner within the developer mixture 43
was varied from a relatively low (0.4 percent by weight) to a
relatively high (0.75 percent by weight) value. With such a dynamic
range, variations in the intensity of the light generated by the
source 49 on the order of .+-.10 percent can be tolerated without
substantially adversely affecting the performance of the
concentration measurement system.
Where very great toner concentration measurement accuracy is
desired, or where substantial variations in power line voltages are
to be expected, the intensity of the light generated by the source
49 may be maintained constant by (i) driving the light source from
a regulated power supply, or (ii) employing an additional
photosensitive element in conjunction with a feedback control
system to continuously monitor the intensity of the light generated
by the source 49 and feed back an error signal to maintain the
intensity at a desired value.
Alternatively, where it is desired to minimize the effect of
variation of the intensity of the light generated by the source 49,
the arrangement shown in FIG. 4 may be employed.
The arrangement illustrated schematically in FIG. 4 operates in a
generally similar manner to that of FIG. 3, the parts of FIG. 4
which correspond to those of FIG. 3 being identified by the same
reference numerals.
In the arrangement of FIG. 4, as in the system illustrated
schematically in FIG. 3, a layer of toner is deposited on the
surface 46 of the transparent collecting plate 45, which layer
varies in thickness or density in accordance with variations in the
concentration of toner within the developer mixture 43. The
arrangement for sensing the light attenuation of the deposited
toner layer differs, however, from that of FIG. 3 in that
additional elements are provided, viz. a light splitter 51, a
second photosensitive element 52, and a comparator circuit 53. The
light splitter 51, which may be in the form of a half-silvered
mirror or other conventional optical element, divides the light
beam emanating from the source 49 into two portions, one portion
being directed through the transparent plate 45 and the other
portion being directed to the photosensitive element 52.
Preferably, the light from the source 49 is divided into unequal
portions by the splitter 51, the portion of greatest intensity
being directed toward the transparent collecting plate 45.
The output signals generated by the photosensitive elements 50 and
52 on lines 54 and 55 respectively, are combined within the
comparator 53, which may take the form of a bridge of the general
type illustrated in FIG. 6 of the aforementioned U.S. Pat. No.
3,430,606, or alternatively may comprise a differential amplifier
for providing an output signal proportional to the difference
between the signals on lines 54 and 55. The comparator 53 also
includes circuitry for comparing the derived signal, indicative of
toner concentration, with a reference signal indicative of a
desired toner concentration, and providing a control signal on line
56 to dispense toner to the developer tank 21 at the proper rate,
by controlling a suitable toner feed motor coupled to a toner
dispenser which is coupled to the developer tank 21.
By use of the arrangement shown in FIG. 4, variations in the
intensity of light generated by the source 49 affect the signals on
both lines 54 and 55, such effects being substantially cancelled
out by the comparator 53.
With the arrangement shown in FIGS. 3 and 4, it has been observed
that when a collecting plate 45 which is initially clean (free of
toner) is subjected to a developer stream 47, the rate of
deposition of a toner layer on the surface 46 of the collecting
plate 45 is initially quite rapid, the rate of deposition
thereafter tapering off and finally stabilizing, resulting in a
substantially constant light attenuation introduced by the
deposited toner layer (this light attenuation value remaining
substantially constant even though the developer stream 47
continues to flow across the surface 46 of the collecting plate
45), the stabilized light attenuation value corresponding to the
concentration of toner within the developer mixture 43 and
developer stream 47.
This phenomenon is more clearly illustrated in FIG. 5, which shows
the variation in light attenuation introduced by the toner layer
deposited on the portion of the collecting plate 45 in the path of
the light beam transmitted from the source 49 to the photosensitive
element 50, as sensed by the output signal produced by the
photosensitive element. In FIG. 5, the variation in light
attenuation with time is shown for the case where the developer
stream 47 is caused to impinge upon an initially clean collecting
plate 45. The curves A, B, and C correspond to results obtained
with developer streams having relatively high, moderate and low
respective toner concentrations.
From FIG. 5, it is seen that the rate of increase of light
attenuation is high for an initial time T.sub.1 (typically on the
order of 3 to 10 seconds for a developer flow rate on the order of
2 to 7 grams per second), and thereafter tapers off, eventually
approaching a stable equilibrium value at time T.sub.s, which is
typically on the order of 1 to 3 minutes for the aforementioned
developer flow rate. The value of light attenuation (and thus of
thickness or density of the deposited toner layer) reached at time
T.sub.1 is typically on the order of one-half the final value at
which the light attenuation stabilizes at time T.sub.s.
FIG. 6 shows curves similar to those of FIG. 5, for the case in
which the developer stream 47 is caused to impinge upon a
collecting plate 45 which is initially quite "dirty" i.e. heavily
coated with toner. The measured light attenuation (and thus the
thickness or density of the deposited toner layer) rapidly
decreases for an initial time period T.sub.1 ' , the rate of
decrease of light attenuation then tapering off, with the light
attenuation finally stabilizing at a time T.sub.s ' and at values
corresponding substantially to those shown in FIG. 5 for
corresponding concentrations of toner in the developer stream 47.
The curves A, B, and C of FIG. 6 correspond to toner concentration
within the developer stream 47 which are identical to the toner
concentrations represented by the corresponding curves A, B, and C
of FIG. 5.
Thus the curves shown in FIGS. 5 and 6 indicate that, for a given
concentration of toner within the developer stream 47, the light
attenuation introduced by the deposited toner layer, as measured by
the photosensitive element 50, stabilizes at a substantially
constant value which is relatively independent of the amount of
toner initially present on the collector plate 45 before the
developer stream 47 is caused to impinge thereon. Generally
speaking, it has been found that the value at which the measured
light attenuation stabilizes is in all cases an accurate measure of
toner concentration.
While the reasons for the phenomena discussed above in connection
with FIGS. 3-6 are not thoroughly understood, it is believed that
the developer stream 47 continually deposits toner particles on the
collector plate 45 and "cleans" toner particles from the collecting
plate, thus establishing a dynamic equilibrium at a toner layer
thickness or density corresponding to the toner concentration
within the developer stream 47. It has been found that, when the
developer stream 47 is caused to cascade over the surface 46 of the
collecting plate 45 in a substantially non-turbulent fashion (as in
the case of the structure illustrated in FIG. 3 of the
aforementioned U.S. Pat. No. 3,430,606) curves generally similar to
those of FIG. 5 (but differing somewhat in shape) are obtained, but
the "cleaning" phenomenon illustrated by the curves of FIG. 6 does
not occur. However, when the deflecting member 48 of proper
configuration is provided, and the distances between the various
elements shown in FIG. 3 as well as the orientations of said
elements are properly selected, as previously described, so as to
impart sufficient kinetic energy and turbulence to the developer
stream 47, the curves illustrated in FIGS. 5 and 6 are obtained.
With such an arrangement, it has been found that the stabilized
value of light attenuation, as measured by the photosensitive
element 50, accurately "tracks" increases and decreases in toner
concentration within the developer stream 47.
One of the practical difficulties encountered in attempting to
obtain a high level of performance from the arrangement shown
schematically in FIGS. 3 and 4, and a difficulty also inherent in
the arrangement shown in U.S. Pat. No. 3,430,606, results from the
fact that the developer stream, which contains opaque carrier beads
as well as toner particles, is at all times flowing past the
photoelectric sensing system, thus introducing undesirable
background noise.
An experiment was undertaken to periodically interrupt the flow of
the developer stream 47, and measure the light attenuation
introduced by the deposited toner layer only during those periods
when developer was not flowing across the surface 46 of the
collecting plate 45. When this was done, a rather unexpected and
surprising result was obtained, this result best being illustrated
by the curves shown in FIG. 7.
The various curves shown in FIGS. 7a-7d are drawn to a common time
base, and are vertically aligned.
FIG. 7a illustrates the manner in which the developer stream 47 is
turned on and off on a periodic basis. It has been found
advantageous to periodically turn the developer stream on for a
time t.sub.on on the order of one to three seconds, with the
developer flow being turned off for time periods t.sub.off on the
order of three to seven seconds, the period t.sub.rec between
recurrences of developer flow being on the order of four to ten
seconds.
FIG. 7b illustrates the manner in which the measured light
attenuation (caused by the deposited toner layer on the collecting
plate 45) would ordinarily be expected to vary with time, when flow
of the developer stream 47 is periodically interrupted in the
manner shown in FIG. 7a. The curves shown in FIG. 7b correspond to
those shown in FIG. 5 for the case where flow of the developer
stream 47 is continuous, the developer stream flow rates being
essentially the same.
As shown in FIG. 7b, one would ordinarily expect the light
attenuation to increase by small increments during successive
periods of developer flow, and to remain constant during the
periods between, when developer is not impinging on the surface 46
of the collecting plate 45. One would also expect the light
attenuation to ultimately stabilize at a value substantially equal
to that shown in FIG. 5.
Rather surprisingly, the actual light attenuation curves obtained,
as shown in FIG. 7c, differ quite markedly from the theoretical
curves shown in FIG. 7b. The solid curves of FIG. 7c correspond to
the case where a developer stream 47 is caused to periodically
impinge upon an initially "clean" collecting plate 45, while the
dashed curves correspond to the case where the developer stream 47
is caused to periodically impinge upon an initially "dirty"
collecting plate 45. The curves, A, B, and C of FIG. 7c correspond
to developer streams having toner concentrations identical to those
represented by the corresponding curves of FIGS. 5, 6 and 7b.
As seen in FIG. 7c, the measured light attenuation, instead of
gradually ascending to the value represented by the corresponding
curve of FIG. 5 or FIG. 6 for the case of a continuous developer
stream, rapidly rises to, and stabilizes at a light attenuation
level somewhat below the stabilized level reached when a continuous
developer stream is employed. While for the sake of simplicity the
curves shown in FIG. 7c illustrate stabilization of the measured
light attenuation in a time T.sub.a which corresponds to a single
developer flow interval (see FIG. 7a), in practice it has been
found that, for developer flow intervals on the order of 1 to 3
seconds, at a flow rate on the order of 2 to 7 grams per second (3
grams per second being typical) with the period between recurrences
being on the order of 4 to 10 seconds, several developer flow
intervals are required for stabilization of the measured light
attenuation, the stabilization time T.sub.a being on the order of
15 to 20 seconds for these parameters.
Thus it has been found that, with intermittent developer flow as
described above in connection with FIGS. 7a to 7c, a stabilized
light attenuation reading which provides an accurate indication of
the toner concentration within the developer stream can be obtained
(starting with a very clean or very dirty collecting plate 45) in
much less time (15 to 20 seconds) than is required for the case
where a continuous developer stream is employed (1 to 3 minutes).
Actually, the performance of the intermittent developer flow
arrangement is even better than the aforementioned Figures would
indicate, since in practice the concentration of toner in the
developer stream varies at a relatively slow rate, so that the
intermittent developer flow measurement arrangement described above
provides very rapid response in normal machine operation.
Another advantage of the intermittent flow arrangement is that such
an arrangement permits the light attenuation to be measured, if
desired, only during those periods when developer is not flowing
onto the collecting plate, thus eliminating the spurious background
noise effects caused by interruption of the light beam by the
developer carrier beads.
FIG. 7d illustrates the manner in which light attenuation is
measured, the periods of measurement occurring between the periods
of developer flow (see FIG. 7a which is vertically aligned with
FIG. 7d) and being indicated by the numerals 57. It has been found
that initial transients in developer flow due to start-up of the
apparatus 10, and to initial disturbances within the developer tank
21, result in spurious light attenuation measurements during an
initial period of a few seconds after the apparatus 10 is turned
on, and the drum 11 and developer carrier belt 24 begin to move.
Accordingly, as shown in FIG. 7d, measurement of light attenuation
is inhibited for an initial period D which may typically be on the
order of 3 seconds.
FIG. 8 illustrates a practical system, utilizing the principles
previously described, for measuring and controlling the
concentration of toner in the developer mixture situated within the
sump of the developer tank 21 (see FIG. 1).
Affixed to the developer tank 21, as shown in FIG. 8, is a toner
dispenser 58. Situated within the toner dispenser 58 is a rotatable
shaft 59 which may be rotated by a toner dispenser motor 60.
Secured to the shaft 59 and disposed within the housing of the
toner dispenser 58 are a plurality of fingers for transferring
toner from the dispenser 58 to the sump of the developer tank 21,
and a cam which drives an agitating member to allow toner to be
transferred from the dispenser 58 to the sump of the developer tank
21 whenever the toner feed motor 60 rotates the shaft 59. The
aforementioned structural elements of the toner dispenser 58 are
not shown in detail in FIG. 8.
Secured to a side wall of the developer tank 21 by means of a
bracket 61 is a toner concentration analyzing device 62, which
incorporates the elements indicated schematically in FIGS. 3 and
4.
The developer whose toner concentration is to be analyzed is
transferred from the hopper of the developer tank 21 to the toner
concentration analyzer 62 by means of a supply conduit 63, this
developer sample being returned to the sump of the developer tank
21 by means of the return conduit 64. The toner concentration
analyzer 62, and the supply and return conduits 63 and 64, are
situated in such a manner that developer flows through these
elements by gravity, no additional active elements being required
to provide the desired developer flow through the analyzer 62.
As shown in FIG. 9, the supply tube 63 has an open upper end 65
positioned within the hopper 23 below the normal level of developer
in said hopper. The size of the supply tube 63 is chosen to be
sufficiently large to provide reliable nonclogging flow of
developer therethrough, while being sufficiently small so that the
normal operation of the apparatus 10 is not disturbed thereby.
The toner concentration analyzer 62 shown in FIGS. 8 and 9 may be
of the continuous flow type shown in FIG. 10, comprising a metallic
housing 66, preferably of aluminum, having an upper aperture 103
therein through which developer is transferred from the supply
conduit 63 to provide a supply sample 43 in the developer supply
sample storage hopper 42a. A lower aperture 74 permits developer to
leave the analyzer 62 to be returned to the sump of the developer
tank 21 by the return conduit 64. The hopper 42a is preferably
constructed of aluminum, is of generally pyramidal or conical
cross-section, and is secured at its upper end to the interior wall
of the housing 66.
An orifice 44 at the bottom of the hopper 42a permits the developer
sample 43 to flow therethrough, and to be deflected by the
developer stream deflecting member 48a, which is preferably
electrically conductive and constructed of a suitable metal such as
steel or aluminum.
The orifice 44 is preferably of rectangular crosssection, measuring
one-eighth inch .times. one-fourth inch. The deflecting member 48a
is preferably shaped in the form of an inverted V, with the apex
thereof situated directly below the orifice 44 and spaced therefrom
by a vertical distance an equal to three-sixteenth inch. The angle
d which each outer surface of the deflecting member 48 a makes with
the vertical is preferably on the order of 30 degrees.
The cross-sectional area of the orifice 44 is preferably small
compared to that of the supply conduit 63, so that a developer
supply sample accumulates in the hopper 42a and the rate of flow of
developer from the hopper 42a is limited by the size of the orifice
44.
Situated below and adjacent to the deflecting member 48a (which is
secured at its opposite ends to the housing 66 and electrically
grounded) are a pair of baffles 67 and 68, each baffle having an
inclined upper portion and a vertically oriented lower portion.
Preferably, the upper inclined portions of the baffles 67 and 68
are arranged so that the normals to the inclined surfaces are
oriented at an angle of 30 to 75.degree. with respect to the
vertical, an angle of 60.degree. being preferred. This angle is the
same as the angle c which the inclined upper portion of each baffle
makes with the horizontal.
The vertically oriented lower portions of the baffles 67 and 68 are
spaced in close proximity to one another, to form a channel
therebetween which communicates with the return conduit 64. An
additional baffle member, not shown in FIG. 10, is situated between
the vertically oriented lower portions of the baffles 67 and 68 and
inclined in a direction perpendicular to the drawing, this
additional baffle cooperating with the vertically oriented lower
portions of the baffles 67 and 68 to form a funnel for guiding
developer emanating from the orifice 44 into the return conduit
64.
Apertures 69 and 70 are formed in the inclined upper portions of
the baffles 67 and 68 respectively, adjacent the vertically
oriented lower portions thereof. Transparent collecting plates 45a
and 45b, preferably of glass, are secured to the upper inclined
portions of the baffles 67 and 68 adjacent the apertures 69 and 70
respectively.
The baffles 67 and 68 are secured to the housing 66 in a
substantially dust-tight manner, the collecting plates 45a and 45b
being similarly secured to the corresponding baffles, so that the
baffle 67 and collecting plate 45a cooperate with the adjacent
portion of the housing 66 to form a dust-tight enclosure 71, while
the baffle 68 and collecting plate 45b cooperate with the adjacent
portion of the housing 66 to form a second dust-tight enclosure
72.
A light source 49a, which may for example be an incandescent lamp
or a light emitting semiconductor diode, is situated within the
enclosure 71 and oriented to project a beam of light through the
aperture 69, collecting plates 45a and 45b, and the aperture 70 to
illuminate the photosensitive element 50a with light which varies
in intensity in accordance with the amount of toner deposited in
the form of a layer on those portions of the collecting plates 45a
and 45b which are in the path of the light beam.
The photosensitive element 50a, which may for example comprise a
suitable phototransistor, is secured to a block of insulating
material 73, which in turn is secured to the upper inclined portion
of the baffle 68 adjacent the aperture 70 therein. The insulating
block 73 is provided with a hole therethrough which permits light
from the source 49a to reach the phototransistor or other
photosensitive element 50a.
In operation, whenever there is developer in the hopper 23 of the
developer tank 21, a sample of such developer is transported to the
hopper 42a by the supply conduit 63. The developer sample 43 flows
out the bottom of the hopper 42a through the orifice 44 therein,
the rate of flow of developer being determined primarily by the
orifice size.
The stream of developer flowing through the orifice 44 strikes the
deflecting member 48a, which introduces a substantial amount of
turbulence into the developer stream and divides the stream into
two streamlets. Each streamlet formed by the deflecting member 48a
impinges upon a corresponding one of the collecting plates 45a and
45b. Preferably, the vertical distance the point at which each
streamlet impinges upon one of the collecting plates and the point
on the deflecting member 48a above the point of impingement is on
the order of 0.25 inch.
As the developer streamlets impinge upon and flow across the
collecting plates 45a and 45b, the amount of deposited toner
increases or decreases in accordance with variations in the
concentration of toner in the streamlets, which concentration is
substantially equal to the concentration of toner in the developer
sample 43, in the developer within the hopper 23, and in the
developer 22 within the sump of the developer tank 21.
Thus the signal derived at the output of the phototransistor or
other photosensitive element 54 varies in accordance with
variations in the concentration of toner in sump of the developer
tank 21.
Since the toner concentration analyzer 62 provides a measure of the
concentration of toner within the developer stream emanating from
the orifice 44, a substantial time lag may result, when the hopper
42a is full or nearly full, before the analyzer 62 provides an
indication of change in the concentration of toner entering the
hopper 42a through the supply conduit 63. In order to reduce this
time lag, a suitable bypass conduit (not shown in FIG. 10) may be
provided to continually remove a portion of the developer 43 from
the lower portion of the hopper 42a, and to return the developer so
removed directly to the sump of the developer tank 21 without
causing such developer to flow across or impinge upon the
collecting plates 45a and 45b.
An alternative form of toner concentration analyzer, which operates
on the periodically interrupted flow basis previously discussed in
connection with FIG. 7, is shown in FIGS. 11 and 12.
As shown in FIG. 11, the toner concentration analyzer 62a
incorporates a toner sample supply valve 75 in the upper portion of
the body thereof. The toner sample supply valve 75 includes a valve
body 76 which is secured to the housing 66a of the concentration
analyzer 62a (the valve body 76 forming a part of the analyzer
housing), a rotatable vane 77 mounted for rotation on a shaft 78, a
valve drive motor 79 for continually rotating the shaft 78 (the
vane 77 being affixed to the shaft 78), and a valve position switch
80 including a switching element 81 and a cam 82 affixed to the
shaft 78 for providing a gating signal responsive to the angular
position of the vane 77, said gating signal indicating the times at
which the position of the vane 77 is such that developer is not
permitted to flow through the orifices 44a.
The valve body 76 is provided with a cylindrical chamber 83 within
which the vane 77 rotates. Developer from the supply conduit 63
enters the chamber 83 through an aperture 103 in the valve body 76
communicating therewith.
During those periods when the vane 77 is oriented so that developer
may flow through the orifices 44a, the developer streams leaving
the orifices 44a are deflected by the inclined developer stream
deflecting plate 48b (which is preferably electrically conductive
and grounded, steel and aluminum being preferred for the plate
material) onto the exposed surfaces of the glass collecting plates
45a and 45b.
The arrangement and operation of the remaining elements of the
toner concentration analyzer 62a is similar to that of the
corresponding elements of the analyzer 62 shown in FIG. 10,
corresponding parts being identified by the same reference
numerals.
The shaft 78, to which the vane 77 and cam 82 are affixed, extends
into the valve chamber 83 through an aperture therein, in which a
shaft support bearing 84 is provided. An end thrust bearing 85 is
provided to support the end of the shaft 78 which is remote from
the valve drive motor 79.
The orifices 44a communicate with the valve chamber 83 through
holes 86 (see FIG. 12) in the valve body 76.
A bypass tube 87 communicates with the valve chamber 83 through a
hole 88 in the valve body, the lower end of the bypass tube 87
being disposed near the top of and closely adjacent to the channel
defined by the lower vertically oriented portions of the baffles 67
and 68. The inner diameter of the bypass tube 87, and the size of
the hole 88 are selected so as to continually remove a portion of
the developer situated within the cavity 83, to insure that the
developer streams emanating from the orifices 44a and deflected by
the deflecting member 48b to impinge upon the collecting plates 45a
and 45b are at all times an up-to-date sample of the developer 22
within the sump of the developer tank 21, i.e. to insure that the
concentration of toner in the streams impinging upon the collecting
plates is substantially the same as the concentration of toner in
the developer situated within the sump of the developer tank
21.
As seen in FIG. 12, the inclined developer stream deflecting member
48b is secured to the valve body 76 at the upper end of the
deflecting member by means of screws 89. The deflecting member 48b
may be bent to change its angle of inclination and to thereby vary
the manner in which the developer streams deflected thereby are
perturbed, so as to provide the proper variation of thickness or
density of the deposited toner layer on the collecting plates 45a
and 45b with increase or decrease of toner concentration in the
developer streams.
As shown in FIG. 12, the rotating valve element comprises the vane
77, which is preferably made of steel, and a mass of polyurethane
foam 90 bonded thereto. The width of the vane 77 is sufficiently
less than the diameter of the chamber 83 so that developer carrier
beads do not bind between the edges of the vane 77 and the walls of
the chamber 83. The mass of polyurethane foam 90 is in low friction
contact with the chamber walls, and serves to preclude leakage of
developer or toner through the clearance space between the edges of
the vane 77 and the walls of the chamber 83.
As the valve element (consisting of the vane 77 and the
polyurethane mass 90) rotates, it alternately (i) permits developer
from the supply conduit 63 to enter the valve chamber 83, (ii)
permits a portion of the developer within the chamber 83 to exit
via the hole 88 and bypass tube 87, and (iii) permits developer
within the chamber 83 to exit through the holes 86 in the form of
streams which are deflected by the deflecting member 48b onto the
collecting plates 45a and 45b.
Suitable circuitry for utilizing the toner concentration analyzer
62a, as shown in FIGS. 11 and 12, to control the toner
concentration of the developer 22 within the developer tank 21, is
illustrated in FIG. 13.
The dashed lines in FIG. 13 indicate mechanical connections and
developer flow, whereas the solid lines indicate electrical
connections.
As previously described, upon application of electric power
thereto, the toner feed motor 60, which is mechanically connected
to the toner feeder or dispenser 58, commences to rotate, causing
the dispenser 58 to release toner into the sump of the developer
tank 21. A sample of the developer within the hopper 23 of the
developer tank 21 is caused to periodically flow across the toner
collector plates 45a and 45b, developer flow across the collector
plates being periodically interrupted by the toner sampling valve
75. Developer removed from the hopper 23 by the supply conduit 63
is returned to the sump of the developer tank 21 by the sample
bypass tube 87 and the developer sample return conduit 64.
The rotary vane 77 of the valve 75 (and the urethane mass 90 bonded
to the vane 77) is continually rotated by the toner sampling valve
drive motor 79, to which is mechanically coupled a valve position
switch 80. The electrical output signal of the valve position
switch 80, which indicates when the vane position is such that
developer is not permitted to flow through the orifices 44a and
across the collecting plates 45a and 45b, is connected to a logical
AND circuit 91.
A machine "on" switch 92, which provides a signal when the
apparatus 10 is in operation, is coupled to the logical AND gate 91
through a delay circuit 93, which may typically introduce a delay D
on the order of 3 seconds.
The resultant signal output of the logical AND circuit 91 on line
94 indicates when a reliable indication of toner concentration is
provided by measurement of the light attenuation introduced by the
toner layers on the portions of the collecting plates 45a and 45b
which are in the path of the light beam transmitted from the light
source 49a to the phototransistor or other photosensitive element
50a. The sampling signal on line 94 has the waveform shown in FIG.
7d.
A toner concentration signal determined by the photocurrent through
the phototransistor 50a is provided to differential amplifier 95 on
line 96.
A reference voltage indicative of a desired toner concentration is
provided to an oppositely poled input terminal of differential
amplifier 95 by the toner ratio selector 97, the output voltage of
differential amplifier 95 on line 98 having a relatively high or
low value, depending upon whether the measured toner concentration,
as indicated by the toner concentration signal on line 96, is above
or below the desired toner concentration as set by the toner ratio
selector switch 97. Since the internal gain of the differential
amplifier 95 is quite high, the toner concentration correction
signal appearing on line 98 is essentially two-valued, i.e. high or
low depending upon the polarity of the voltage difference between
the signal on line 96 and the signal provided to the differential
amplifier 95 by the toner ratio selector switch 97. If desired, the
response sensitivity of the differential amplifier 95 may be
further enhanced by providing positive feedback from the output
line 98 to vary the signal provided to the input of the
differential amplifer 95 on line 99 by the toner ratio selector
switch 97.
The sampling signal on line 94 (see FIG. 7d) controls the gate 100
to couple the concentration correction signal on line 98 to a
bistable memory 101, which stores the concentration correction
signal during the period between reliable measurements thereof, the
concentration correction signal on line 98 being permitted to
update the memory 101 whenever the sampling signal on line 94
indicates that the concentration correction signal is a reliable
measure of the concentration of toner within the developer tank
21.
The output of bistable memory 101, which may be a setreset bistable
multivibrator or other conventional circuit, is employed to conrol
a power distribution gate 102, which provides electric power to the
toner feed motor 60 whenever the content of the bistable memory
indicates that the concentration of toner within the developer tank
21 is lower than the desired value. When the content of bistable
memory 101 indicates that the concentration of toner within the
developer tank 21 is greater than the desired value, the gate 102
is disabled, disconnecting power from the toner feed motor 60, and
allowing toner to be depleted from the developer tank 21 as copies
are produced by the apparatus 10, until such time as the content of
bistable memory 101 again indicates that the concentration of toner
in the developer within the developer tank 21 has dropped below the
desired value. In this manner, the toner concentration in the
developer 22 situated within the sump of the developer tank 21 is
continuously and automatically controlled within close limits.
* * * * *