Series capacitor ink sensor for monitoring liquid developer material

An apparatus for determining charge density and mobility in a liquid solution having electrically charged particles therein. The apparatus includes an electrode, and a dielectric member having a first surface situated opposite the electrode for providing a volume therebetween in which a sample of the liquid may be placed. A fixed bias voltage applied to the electrode to produce an electrical current flow through the liquid solution and the dielectric member. A device coupled to the dielectric member, measures the electrical current as a function of time to provide a measure of voltage decay across the liquid solution. The voltage decay corresponds to the charge density of the liquid solution.

This invention relates generally to an electrostatographic printing 
machine, and more particularly concerns an apparatus for monitoring total 
density of toner charge and the mobility thereof in a liquid developer 
material in a liquid developing material-based copying or printing 
machine. 
Generally, the process of electrostatographic copying is initiated by 
exposing a light image of an original document to a substantially 
uniformly charged photoreceptive member. Exposing the charged 
photoreceptive member to a light image discharges the photoconductive 
surface thereof in areas corresponding to non-image areas in the original 
input document while maintaining the charge in image areas, resulting in 
the creation of an electrostatic latent image of the original document on 
the photoreceptive member. This latent image is subsequently developed 
into a visible image by a process in which developer material is deposited 
onto the surface of the photoreceptive member. Typically, this developer 
material comprises carrier granules having toner particles adhering 
triboelectrically thereto, wherein the toner particles are 
electrostatically attracted from the carrier granules to the latent image 
for forming a powder toner image on the photoreceptive member. 
Alternatively, liquid developer materials comprising a liquid carrier 
material having toner particles dispersed therein have been utilized, 
wherein the liquid developer material is applied to the latent image with 
the toner particles being attracted toward the image areas to form a 
liquid image. Regardless of the type of developer material employed, the 
toner particles of the developed image are subsequently transferred from 
the photoreceptive member to a copy sheet, either directly or by way of an 
intermediate transfer member. Once on the copy sheet, the image may be 
permanently affixed to provide a "hard copy" reproduction of the original 
document or file. In a final step, the photoreceptive member is cleaned to 
remove any charge and/or residual developing material from the 
photoconductive surface in preparation for subsequent imaging cycles. 
The above described electrostatographic reproduction process is well known 
and is useful for light lens copying from an original, as well as for 
printing applications involving electronically generated or stored 
originals. Analogous processes also exist in other printing applications 
such as, for example, digital laser printing where a latent image is 
formed on the photoconductive surface via a modulated laser beam, or 
ionographic printing and reproduction where charge is deposited on a 
charge retentive surface in response to electronically generated or stored 
images. In addition, variant processes are also known, wherein the 
electrostatic latent image is formed directly in a toner layer, with image 
areas and non-image areas subsequently separate to produce a developed 
image. Some of these printing processes develop toner on the discharged 
area, known a DAD or "write black" systems, in contradistinction to the 
light lens generated image systems which develop toner on the charged 
areas, known as CAD, or "write white" systems. The subject invention 
applies to both such systems. 
The use of liquid developer materials in imaging processes is well known. 
Likewise, the art of developing electrostatographic latent images with 
liquid developer materials is also well known. Indeed, various types of 
liquid developing material development systems have heretofore been 
disclosed. 
Liquid developers have many advantages. For example, images developed with 
liquid developers can be made to adhere to paper without a fixing or 
fusing step, thereby eliminating a requirement to include a resin in the 
liquid developer for fusing purposes. In addition, the toner particles can 
be made to be very small without resulting in problems often associated 
with small particle toners, such as airborne contamination which can 
adversely affect machine reliability and can create potential health 
hazards. The use of very fine toner particles enable the production of 
higher quality images than those generally formed with dry toners. In full 
color imaging processes, production of a texturally attractive output 
document is enabled through the use of liquid developers due to minimal 
multilayer toner height build-up (whereas full color images developed with 
dry toners often exhibit substantial height build-up of the image in 
regions where color areas overlap). In addition, full color imaging with 
liquid developers is economically attractive, particularly if surplus 
liquid carrier containing the toner particles can be economically 
recovered without cross contamination of colorants. Further, full color 
prints made with liquid developers can be processed to a substantially 
uniform finish, whereas uniformity of finish is difficult to achieve with 
powder toners due to variations in the toner pile height as well as a need 
for thermal fusion, among other factors. 
Although specific liquid development systems may vary, one well known type 
of system includes a roll member adapted to transport liquid developer 
material into a position proximate to a surface to be coated. In such 
systems, is the roll member is typically partly submerged in a sump of 
liquid developer material with the roll member being rotated at a 
sufficiently high velocity so as to transport the liquid developer to the 
surface in the form of a relatively thin toner layer formed along the 
surface of the roll member. In addition, an electrical bias may be applied 
to the roll member for generating an electrical field across a gap between 
the roll member and the surface to maintain a toning meniscus across the 
gap so as to provide a desired density of toner particles entrained in the 
liquid developer and to reduce undesirable background staining of the 
photoreceptor as it passes the developer apparatus. 
Generally, in the field of electrostatographic printing and copying, 
development of a latent image takes place at high speeds, which requires 
that a large amount of uniformly characteristic liquid developer material 
be supplied to the photoconductive surface as uniformly as possible to 
produce a high quality image without any variations in the development 
thereof. Accordingly, it has been found, that it is advantageous to 
monitor the process of applying the liquid developer material to detect 
any property variations that are relevant to the development process. The 
present invention provides an ink sensor for separately monitoring the 
total density of toner charge and its mobility in the developer material. 
The sensor is applicable to undiluted inks or their component charge 
director solutions alike. By combining a simple and rugged cell design 
with standard pulse height electronics, both of low cost, the sensor has 
broad applicability and value for image quality control in liquid ink 
development based products. The following publication may be particularly 
relevant; "Space-Charge-Perturbed Electrophoresis in Nonpolar Colloidal 
Dispersions", J. Appl. Physics, Vol. 80, No. 12, Dec. 15, 1996. The 
foregoing article is specifically incorporated into the present disclosure 
by reference. 
In accordance with one aspect of the present invention, there is provided 
an apparatus for determining charge density and mobility in a liquid 
solution having electrically charged particles therein. The apparatus 
includes an electrode. A dielectric member having a first surface situated 
opposite the electrode provides for a volume therebetween in which a 
sample of the liquid may be placed. A fixed bias voltage is applied to the 
electrode to produce an electrical current flow through the liquid 
solution and the dielectric member. Means coupled to the dielectric member 
measure the electrical current as a function of time to provide a measure 
of voltage decay across the liquid solution. The voltage decay corresponds 
to the charge density of the liquid solution. 
In accordance with another aspect of the present invention, there is 
provided a sensor for determining charge density and mobility in a liquid 
developer comprising liquid carrier and electrically charged toner 
particles therein. The sensor includes an electrode. A dielectric member 
having a first surface situated opposite the electrode provides for a 
volume therebetween in which a sample of the liquid may be placed. A fixed 
bias voltage is applied to the electrode to produce an electrical current 
flow through the liquid solution and the dielectric member. Means coupled 
to the dielectric member measure the electrical current as a function of 
time to provide a measure of voltage decay across the liquid solution. The 
voltage decay corresponds to the charge density of the liquid solution. 
In accordance with yet another aspect of the present invention, there is 
provided an electrostatographic printing machine of the type having a 
liquid developer comprising liquid carrier and electrically charged toner 
particles therein. The improvement includes an electrode. A dielectric 
member having a first surface situated opposite the electrode provides for 
a volume therebetween in which a sample of the liquid may be placed. A 
fixed bias voltage is applied to the electrode to produce an electrical 
current flow through the liquid solution and the dielectric member. Means 
coupled to the dielectric member measure the electrical current as a 
function of time to provide a measure of voltage decay across the liquid 
solution. The voltage decay corresponds to the charge density of the 
liquid solution.

For a general understanding of the features of the present invention, 
reference is made to the drawings, wherein like reference numerals have 
been used throughout to designate identical elements. FIG. 7 is a 
schematic elevational view illustrating an exemplary full-color, 
single-pass, image-on-image, liquid developing material based 
electrostatographic printing machine incorporating the features of the 
present invention. It will become apparent from the following discussion 
that the apparatus of the present invention may be equally well-suited for 
use in a wide variety of printing processes and machine architectures such 
that the present invention is not necessarily limited in its application 
to the particular electrostatographic process or system described herein. 
Thus, although the present invention will be described in connection with 
a preferred system environment and embodiment thereof, it will be 
understood that the description of the invention is not intended to limit 
the invention to this preferred environment and/or embodiment. Indeed, the 
description 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. 
Turning now to FIG. 7, inasmuch as the art of electrostatographic printing 
is well known, the various processing stations will be described briefly 
with reference thereto. The liquid developing material based multicolor 
electrostatographic printing machine employs a photoreceptor 18 in the 
form of a continuous multilayered belt member, generally comprising a 
photoconductive surface deposited on an electrically grounded conductive 
substrate. The photoreceptor 18 is entrained about a plurality of rollers, 
at least one of which is rotatably driven by a drive mechanism (not shown) 
for advancing the belt along a curvilinear path in the direction of arrow 
16, such that successive portions of the photoreceptive belt 18 can be 
transported through the various processing stations disposed about the 
path of movement thereof. 
The electrostatographic printing process is initiated by applying a 
substantially uniform charge to the photoreceptive surface of 
photoreceptor 18. As such, an initial processing station is shown as a 
charging station, including a corona generating device 20. The corona 
generating device 20 is capable of applying a relatively high and 
substantially uniform charge potential to the surface of the photoreceptor 
belt 18. 
After the substantially uniform charge is placed on the surface of 
photoreceptor belt 18, the electrostatographic printing process proceeds 
by either imaging an input document placed on the surface of a transparent 
imaging platen (not shown), or by providing a computer generated image 
signal, for selectively discharging the photoconductive surface in 
accordance with the image to be generated. For multicolor printing and 
copying, the imaging process involves separating the imaging information 
into the three primary colors plus black to provide a series of 
subtractive imaging signals, with each subtractive imaging signal being 
proportional to the intensity of the incident light of each of the primary 
colors or black. These imaging signals are then transmitted to a series of 
individual raster output scanners (ROSs), shown schematically by reference 
numerals 22, 32, 42 and 52, for generating complementary color-separated 
latent images on the charged photoreceptive belt 18. Typically, each ROS 
22, 32, 42 and 52 writes the latent image information in a pixel by pixel 
manner. 
Each of these color-separated electrostatic latent images are serially 
developed into visible images on the photoreceptive belt 18 via coating 
flow applicators identified by reference numerals 24, 34, 44 and 54. Each 
coating flow applicator operates as an apparatus for transporting liquid 
developing material and for applying a thin coating layer of liquid 
developing material to the surface of belt 18. Adjacent to each of the 
flow applicators is a series-capacitor sensor of the present invention. 
Each of these sensors, identified by reference numerals 62, 64, 66, and 
68, separately monitor the total charge density and mobility thereof in 
the developer material applied to the belt surface. The sensor will be 
described in grater detail following the instant description of the 
electrostatographic printing system of FIG. 7. 
Each coating flow applicator transports a different color developer 
material into contact with a different electrostatic latent image on the 
photoreceptor surface for developing the latent image with pigmented toner 
particles, and creating a visible image. By way of example, coating flow 
applicator 24 transports cyan colored liquid developer material, coating 
flow applicator 34 transports magenta colored liquid developer material, 
coating flow applicator 44 transports yellow colored liquid developer 
material, and coating flow applicator 54 transports black colored liquid 
developer material. Each different color liquid developing material 
comprises pigmented toner particles immersed in a liquid carrier medium, 
wherein the toner particles are charged to a polarity opposite in to the 
latent image on the photoconductive surface of belt 18 such that the toner 
particles are attracted to the electrostatic latent image to create a 
visible developed image thereof. 
Generally, in a liquid developing material-based system, the liquid carrier 
medium makes up a large amount of the liquid developer composition. 
Specifically, the liquid medium is usually present in a range of from 
about 80 to about 98 percent by weight in the developing material, 
although this amount may vary outside of the stated range. By way of 
example, the liquid carrier medium may be selected from a wide variety of 
materials, including, but not limited to, any of several hydrocarbon 
liquids, such as high purity alkanes having from about 6 to about 14 
carbon atoms, exemplified by such commercial products as Norpar.RTM. 12; 
Norpar.RTM. 13 and Norpar.RTM. 15, as well as isoparaffinic hydrocarbons 
such as Isopar.RTM. G, H, L, and M, available from Exxon Corporation. 
Other examples of materials suitable for use as a liquid carrier include 
Amsco.RTM. 460 Solvent, and Amsco.RTM. OMS, available from American 
Mineral Spirits Company, Soltrol.RTM., available from Phillips Petroleum 
Company, Pagasol.RTM., available from Mobil Oil Corporation, 
Shellsol.RTM., available from Shell Oil Company. Isoparaffinic 
hydrocarbons may provide a preferred liquid media since they are 
colorless, environmentally safe, and possess a sufficiently high vapor 
pressure so a thin film of the liquid evaporates from the contacting 
surface within seconds at ambient temperatures. 
The toner particles utilized in liquid developer compositions can be any 
pigmented particle compatible with the liquid carrier medium, such as, for 
example those disclosed in U.S. Pat. Nos. 3,729,419; 3,841,893; 3,968,044; 
4,476,210; 4,707,429; 4,762,764; 4,794,651; 5,066,559 and 5,451,483, among 
various other patents and disclosures known to one of skill in the art. 
Preferably, the toner particles have an average particle diameter from 
about 0.2 to about 10 microns, and more preferably in the range from about 
0.5 to about 2 microns. In addition, the toner particles may be present in 
amounts of from about 1 to about 10 percent by weight, and preferably from 
about 1 to about 4 percent by weight of the developer composition. The 
toner particles can consist solely of pigment particles, or may comprise a 
resin and a pigment; a resin and a dye; or a resin, a pigment, and a dye. 
Suitable resins include poly(ethyl acrylate-co-vinyl pyrrolidone), 
poly(N-vinyl-2-pyrrolidone), and the like. Suitable dyes include Orasol 
Blue 2GLN, Red G, Yellow 2GLN, Blue GN, Blue BLN, Black CN, Brown CR, all 
available from Ciba-Geigy, Inc., Mississauga, Ontario, Morfast Blue 100, 
Red 101, Red 104, Yellow 102, Black 101 Black 108, all available from 
Morton Chemical Company, Ajax, Ontario. Dyes generally are present in an 
amount of from about 5 to about 30 percent by weight of the toner 
particle, although other amounts may be present provided that the 
objectives of the present invention are achieved. Suitable pigment 
materials include carbon blacks such as Microlith.RTM. CT, available from 
BASF, Printex.RTM. 140 V, available from Degussa, Raven.RTM. 5250 and 
Raven.RTM. 5720, available from Colombian Chemicals Company. Pigment 
materials may be colored, and may include magenta pigments such as 
Hostaperm Pink E (American Hoechst Corporation) and Lithol Scarlet (BASF), 
yellow pigments such as Diarylide Yellow (Dominion Color Company), cyan 
pigments such as Sudan Blue OS (BASF), and the like. Generally, any 
pigment material is suitable provided that it consists of small particles 
that combine well with any polymeric material also included in the 
developer composition. Pigment particles are generally present in amounts 
of from about 10 to about 40 percent by weight of the toner particles, and 
preferably from about 10 to about 30 percent by weight. 
In addition to the liquid carrier vehicle and toner particles which 
typically make up the liquid developer materials, a charge control 
additive, sometimes referred to as a charge director, is also included for 
facilitating and maintaining a uniform charge on toner particles by 
imparting an electrical charge of selected polarity (positive or negative) 
to the toner particles. Examples of suitable charge control agents include 
lecithin, available from Fisher Inc.; OLOA 1200, a polyisobutylene 
succinimide, available from Chemical Company; basic barium petronate, 
available from Witco Inc.; zirconium octoate, available from Nuodex; as 
well as various forms of aluminum stearate; salts of calcium, manganese, 
magnesium and zinc; heptanoic acid; salts of barium, aluminum, cobalt, 
manganese, zinc, cerium, and zirconium octoates and the like. The charge 
control additive may be present in an amount of from about 0.01 to about 3 
percent by weight, and preferably from about 0.02 to about 0.05 percent by 
weight of the developer composition. 
Returning to a description of the process carried out by the system of FIG. 
7, the amount of liquid developing material, and in particular the liquid 
carrier portion of the liquid developing material that is deposited on the 
surface of the photoreceptor belt 18 is preferably reduced by an incipient 
amount during or after image development. To this end, metering rollers 
26, 36, 46 and 56 are positioned slightly downstream of, and adjacent to, 
respective developing material coating flow applicators 24, 34, 44 and 54, 
in the direction of movement of the photoreceptor 18. Preferably, the 
peripheral surface of each metering roller is situated in close proximity 
to the surface of the photoreceptor 18 and may or may not contact the 
surface of the photoreceptor 18 and/or the liquid layer thereon. In 
addition, the peripheral surface of the metering roller 26 is preferably 
rotated in a direction opposite the path of movement of the photoreceptor 
in order to create a substantial shear force against the thin layer of 
liquid developing material present between it and the photoreceptor 18. 
This shear force removes a predetermined amount of excess developing 
material, in particular carrier liquid, from the surface of the 
photoreceptor and transports this excess developing material in the 
direction of the developing material flow applicator 24, with the excess 
developing material eventually falling away from the rotating metering 
roll 26 for collection in a sump (not shown) or other liquid developer 
collection and reclaim system. 
As shown, the metering rolls 26, 36, 46 and 56 are electrically biased by 
supplying DC voltage thereto for repelling or attracting toner particles 
present in the liquid developing material on the photoreceptor belt 18. It 
will be recognized that, by providing a predetermined electrical bias at 
the metering roll of the same charge polarity as the charge on the 
developed image, removal of deposited toner particles from the surface of 
the photoreceptor due to the shear forces created by the movement of the 
metering roll can be inhibited. Conversely, by providing a predetermined 
electrical bias to the metering roll which is opposite in polarity to the 
charge of the developed image, excess toner material or background image 
removal can be induced, if desired. 
After the above-described metering process is completed, the developed 
liquid image on the photoconductor may preferably be further processed or 
"conditioned" to pack or condense the image onto the surface of the 
photoreceptor and to further remove some of the liquid carrier therefrom. 
This basic concept is shown, for example, by previously cited U.S. Pat. 
No. 4,286,039, as well as U.S. Pat. Nos 4,974,027 and 5,028,964, among 
various other patents. Thus, an image conditioning system may be utilized 
for conditioning a developed liquid image on a photoreceptor surface or on 
any surface which is used to transport a developed image (e.g. an 
intermediate transfer belt), or for conditioning any liquid developer 
layer on the photoreceptor surface or other surface, whereby the liquid 
developer is first subjected to a large electric field for 
electrostatically driving the colorant containing toner particles of the 
liquid developer toward the surface, followed by removal of excess liquid 
from the liquid developer layer on the belt surface. In either method of 
use, an ink conditioning system is shown at reference numerals 28, 38, 48, 
and 58, wherein a biased roller is urged against the photoreceptor 18 to 
electrostatically compress the liquid developer on the photoreceptor belt 
18 while further removing excess liquid therefrom. It will be recognized 
that various methods for forming high electric fields, such as a corona 
generating device, an electrically biased non-contact blade member, a 
charging "shoe", or a non-contact biased roller, can also be used in 
combination with a non-biased contact roller as an alternative to the 
described conditioning apparatus. 
Following developer conditioning, belt 18 continues to advance in the 
direction of arrow 16. The photoreceptor belt 18 is first optionally 
exposed to a flood lamp 29 for erasing any residual charge therefrom, and 
then moved to a subsequent recharge station where another corona 
generating device 30 is utilized to recharge the photoconductor belt 18, 
having a first developed color separation thereon, to establish a new 
substantially uniform potential thereon. The belt then continues to travel 
to the next exposure station, where ROS 32 selectively dissipates the 
charge laid down by corotron 30 to record another color separated 
electrostatic latent image corresponding to regions to be developed with a 
magenta developer material. This color separated electrostatic latent 
image may be totally or partially superimposed on the image previously 
developed on the surface. Thereafter, the electrostatic latent image is 
advanced to the next successive coating flow applicator 34 which deposits 
magenta toner thereon. 
After the electrostatic latent image has been developed with magenta toner, 
the photoconductive surface of belt 18 continues to be advanced to the 
next metering roll 36, to the next image conditioning station 38 and 
onward to flood lamp 39 and corona generating device 40, which, once 
again, recharges the photoconductive surface to a substantially uniform 
potential. Thereafter, ROS 42 selectively discharges this new charge 
potential on the photoconductive surface to record yet another color 
separated electrostatic latent image, which may be partially or totally 
superimposed on the prior cyan and magenta developed images, for 
development with yellow toner. In this manner, a yellow toner image is 
formed on the photoconductive surface of belt 18 in superimposed 
registration with the previously developed cyan and magenta images. It 
will be understood that the color of the toner particles at each 
development station may be provided in an arrangement and sequence that is 
different than described herein. 
After the yellow toner image has been formed on the photoconductive surface 
of belt 18, the belt 18 continues to advance to the next metering roller 
46, image conditioning station 48, and onward to flood lamp 49 and 
recharge station 50 and corresponding ROS 52 for selectively discharging 
those portions of belt 18 which are to be developed with black toner via a 
process known as black undercolor removal process wherein the developed 
image is located only on those portions of the photoconductive surface 
adapted to have black in the printed page and may not be superimposed over 
the prior cyan, magenta, and yellow developed images. This final developed 
image is once again metered and image conditioned at an image conditioning 
station 58 to compact the image and subsequently remove excess liquid from 
the image. Using the process described hereinabove, a composite multicolor 
toner image is formed on the photoconductive surface of belt 18. It will 
be recognized that the present description is directed toward a Recharge, 
Expose, and Develop (REaD) process, wherein the charged photoconductive 
surface of photoreceptive belt 18 is serially exposed to record a series 
of latent images thereon corresponding to the subtractive color of one of 
the colors of the appropriately colored toner particles at a corresponding 
development station. Thus, the photoconductive surface is continuously 
recharged and re-exposed to record latent images thereon corresponding to 
the subtractive primary of another color of the original. This latent 
image is therefore serially developed with appropriately colored toner 
particles until all the different color toner layers are deposited in 
superimposed registration with one another on the photoconductive surface. 
It should be noted that either discharged area development (DAD), wherein 
discharged portions are developed, or charged area development (CAD), 
wherein charged areas are developed can be employed. 
After the composite multicolor image is formed on the photoreceptor, the 
multilayer developed image may be further conditioned with corona and/or 
light and then advanced to a transfer station, whereat a sheet of support 
material 100, typically a sheet of paper or some similar sheet like 
substrate, is guided into contact with the photoreceptor 18. At the 
transfer station, a corona generating device 108 directs ions onto the 
back side of the support material 100 for attracting the composite 
multicolor developed image on belt 18 to the support material 100. While 
direct transfer of the composite multicolor developed image to a sheet of 
paper has been described, one skilled in the art will appreciate that the 
developed image may be transferred to an intermediate member, such as a 
belt or drum, and then, subsequently, transferred and fused to the sheet 
of paper, as is well known in the art. 
After the image has been transferred to the support substrate, a conveyor 
belt 110 moves the sheet of paper in the direction of arrow 112 to a 
drying or fusing station (not shown). The fusing station may include a 
heated roll in combination with a back-up or pressure roll, the rolls 
being resiliently urged into engagement with one another to form a nip 
through which the sheet of paper passes. The fusing station operates to 
affix the toner particles to the copy substrate so as to bond the 
multicolor image thereto. After fusing, the finished sheet is discharged 
and further transported for removal by the machine operator. Often, after 
the developed image is transferred from belt 18, residual developer 
material tends to remain, undesirably, on the surface thereof. In order to 
remove this residual toner from the surface of the belt 18, a cleaning 
roller 60, typically formed of an appropriate synthetic resin, is driven 
in a direction opposite to the direction of movement of belt 18 for 
contacting and cleaning the surface thereof. It will be understood that a 
number of photoconductor cleaning means exist in the art any of which 
would be suitable for use with the present invention. 
The foregoing discussion provides a general description of the operation of 
a liquid developing material based electrostatographic printing machine 
incorporating a plurality of sensors in accordance with the present 
invention therein. The detailed structure of the series-capacitor sensors 
of the present invention will be described hereinafter with reference to 
FIGS. 1-6. It will be understood that the series-capacitor sensor of the 
present invention may be utilized in any liquid developing material-based 
printing machine, including multicolor or monocolor systems as well as 
systems wherein a latent image is created in a layer of liquid developing 
material with the image and background area subsequently separated to 
create a developed image. The developed image may be transferred directly 
to the copy sheet, as described, or to an intermediate member prior to 
transfer to the copy sheet. Multicolor printing machines may use this type 
of sensor where successive latent images are developed to form a composite 
multicolor toner image which is subsequently transferred to a copy sheet 
or, in lieu thereof, single color liquid images may be transferred in 
superimposed registration with one another directly to the copy sheet or 
to an intermediate transfer member in like manner. It will be understood 
that each series-capacitor sensor 62, 64, 66, and 68, shown in the 
apparatus of FIG. 7 is substantially identical. 
The most convenient way of characterizing the electrical properties of 
liquid developers is by conductivity measurement facilitated by an induced 
low AC field (less than 10 volts/centimeter). While conductivity is an 
important parameter, it is the product of two components, namely, the 
charge density and mobility, that individually determine the efficiency of 
the development process. The traditional method of charge density and 
mobility measurements uses a sweep-out cell with a pair of parallel 
electrodes held at a constant voltage. However, neither the low AC field, 
or the constant voltage represents the actual development environment, 
where the field could be as high as 10.sup.5 volts/centimeter, and the 
voltage across the ink layer decreases in the millisecond time domain as 
the toner deposition progresses. Similarly, techniques based on transient 
optical phenomena have been implemented. The use of these techniques are 
usually limited to toner concentrations much lower than those in actual 
liquid developing material. Since liquid developing material properties 
are field and concentration dependent, such characterization methods do 
not reveal the true behavior of the materials in the actual development 
process. 
The series-capacitor ink sensor of the present invention, unlike the 
methods described hereinabove, simulates the temporal and spatial 
variations of the electrical field in the development zone. This enables 
the variations of to properties more relevant to the development process 
to be detected. 
Referring now to FIG. 1, there is shown a schematic configuration for a 
series-capacitor discharge of liquid developing material samples according 
to the present invention. In FIG. 1, a layer of liquid developing material 
70 to be monitored is flanked by a grounded dielectric 72, which is a 
perfect capacitor acting as the surrogate photoreceptor in the dark, and a 
counter-electrode 74 held at a constant bias voltage V.sub.b. It can be 
shown that the external current J.sub.T is a measure of the decay of 
voltage across the developing material layer 70 due to charge transport 
therein. While this phenomenon is essentially a dielectric relaxation, 
distinct features associated with space-charge-perturbed condition can 
arise from the low mobility and high charge density of the developing 
materials and the decaying voltage across the developing material layer. 
Although the present invention is directed to determining charge density 
and mobility in a liquid solution having electrically charged species 
therein, one skilled in the art will appreciate that the liquid solution 
includes collidal dispersions. Moreover, the electrically charged species 
include particles, counter-ions and co-ions. 
FIG. 2 is an electrical equivalent circuit of dielectric relaxation in the 
series-capacitor configuration shown in FIG. 1. The transport properties 
of the system are represented by a sample resistance 76 (R) and 
capacitance's 78 and 80 of the sample (C.sub.s) and the dielectric layers 
(C.sub.d), respectively. The current density J.sub.RC as shown in FIG. 2 
is given by, 
EQU J.sub.RC =(V.sub.b /R)C.sub.d /(C.sub.s +C.sub.d)!.sup.2 exp(-t/.tau.)(1) 
where the relaxation time .tau. is, 
EQU .tau.=(C.sub.s +C.sub.d)R. (2) 
The resistance 76 is defined in terms of the charge density at equilibrium. 
In cases where charge transport becomes non-Ohmic, either due to a limited 
supply of charge species or the field being perturbed by the space-charge, 
the charge density could become non-uniform and differ significantly from 
the equilibrium value such that the concept of a well-defined "resistance" 
is no longer valid. In the following, the applicability of the above 
formulae is examined by deriving the current expression from the first 
principle charge transport theory. 
In general, the total current density J.sub.T at a time t, for the system 
shown in FIGS. 1 and 2, can be written as the sum of conduction and 
displacement current densities as, 
EQU J.sub.T (t)=(.mu..sub.p .rho..sub.p +.mu..sub.n 
.rho..sub.n)E(x,t)+.di-elect cons.(.differential.E/.differential.t)(3) 
where E(x,t) is the electric field, .di-elect cons. is the permitivity, 
.rho. is the volume density and .mu. is the mobilities of charge species. 
The subscripts p and n refer to the positive and the negative species, 
respectively. 
Noting that there is no conduction current in a perfect capacitor (the 
"dielectric"), the separate integrations of Eq.(3) over the dielectric of 
thickness L.sub.d and the leaky capacitor (the "sample") of thickness 
L.sub.s yield, 
##EQU1## 
where V.sub.d and V.sub.s denote the voltages across the dielectric and 
the sample, respectively, and .di-elect cons..sub.d and .di-elect 
cons..sub.s are the permittivities of the dielectric and the sample, 
respectively. The time derivatives of V's can be eliminated from the 
condition that the sum of V.sub.d and V.sub.s equals to the constant bias 
voltage V.sub.b, i.e., 
EQU dV.sub.d /dt+dV.sub.s /dt=dV.sub.b /dt=0. (5) 
Then, the total current density can be expressed as, 
##EQU2## 
and the decay rates of voltages are given by, 
EQU dV.sub.s /dt=-dV.sub.d /dt=-J.sub.T /C.sub.d, (7) 
where C.sub.s =.di-elect cons..sub.s /L.sub.s and C.sub.d =.di-elect 
cons..sub.d /L.sub.d are the capacitances of the sample and the 
dielectric, respectively. These results allow the calculations of the 
total current density J.sub.T and the layer voltages, V.sub.s and V.sub.d, 
as functions of time using the instantaneous local values of charge 
densities p(x,t)'s and fields E(x,t). The latter quantities can be 
obtained by solving the coupled continuity and Poisson's equations, which 
are given in one-dimensional geometry as, 
EQU .differential..rho..sub.p 
/.differential.t=-(.differential./.differential..times.)(.mu..sub.p 
.rho..sub.p E)+g, (8) 
EQU .differential..rho..sub.n 
/.differential.t=(.differential./.differential..times.)(.mu..sub.n 
.rho..sub.n E)-g, (9) 
EQU .differential.E/.differential..times.=(.rho..sub.p +.rho..sub.n)/.di-elect 
cons., (10) 
where g represents the rate of charge generation, which can be a function 
of position and time through its dependence on the local field E(x,t). 
The boundary conditions for these equations specify the interaction between 
the charge species in the sample layer and the electrodes. For example, in 
the case of inks one may assume that there is no charge exchange between 
the ink and the electrodes, hence, the densities of charge species are 
conserved in the ink layer. This is represented by the vanishing of 
outgoing conduction currents: for the positively charged species, 
EQU J.sub.p =.mu..sub.p .rho..sub.p E&gt;=0, at x=0, 
and 
EQU J.sub.p =.mu..sub.p .rho..sub.p E&lt;=0, at x=L.sub.s (11) 
and for the negatively charged species, 
EQU J.sub.n =.mu..sub.n .rho..sub.n E&lt;=0, at x=0, 
and 
EQU J.sub.n=.mu..sub.n .rho..sub.n E&gt;=0, at x=L.sub.s 12) 
Alternatively, the electrode at x=L.sub.s can be assumed to inject charge 
of the same polarity as the bias voltage at a rate which may depend on the 
field at x=L.sub.s. 
Another boundary condition is the continuity of displacements at the 
interface, x=0, 
EQU .di-elect cons..sub.s E(0)-.di-elect cons..sub.d E.sub.d =Q.sub.s,(13) 
where E.sub.d =-V.sub.d /L.sub.d is the uniform electric field in the 
(space-charge free) dielectric. In the electrographic applications, 
Q.sub.s represents the latent image charge which is the driving force for 
electrophoresis. The field needed to drive the charge species is provided 
by the bias voltage V.sub.b, and hence, the case for Q.sub.s =0is 
considered. 
The usual initial condition is that the sample is charge neutral with 
uniform distributions of positive and negative charge species, 
EQU .rho..sub.p (X,0)=-.rho..sub.n (X,0)=.rho..sub.o at all X and t=0,(14) 
hence, the initial field distribution is also uniform, 
EQU E(X,0)=-V.sub.b /L.sub.s (1+C.sub.s /C.sub.d) at all X and t=0.(15) 
The coupled equations, Eqs.(8, 9, 10) are solved numerically by the finite 
difference method. The numerical results are presented in a system of 
units with the sample thickness L.sub.s, the permittivity .di-elect 
cons..sub.s, the mobility of negative species .mu..sub.n, and the charge 
density .rho..sub.o as the basic units. Other units can be derived from 
these four basic units as shown in Table I, together with the typical 
value of each unit. The derived units have important physical meanings. 
For example, the unit voltage V.sub.o corresponds to the voltage at which 
the total charge per unit area of sample .rho..sub.o L.sub.s is equal to 
"one-CV's worth". The time unit t.sub.o, is the transit time of negative 
species at the unit voltage and is also equal to the (intrinsic) 
dielectric relaxation time of the sample. 
TABLE I 
______________________________________ 
System of Units and Typical Values 
______________________________________ 
Thickness: L.sub.s (of sample) 
5 .times. 10.sup.-3 cm 
Permittivity: .epsilon..sub.s (of sample) 
5 .times. 10.sup.-13 F/cm 
Charge mobility: .mu..sub.n (of negative species) 
10.sup.-4 cm.sup.2 /Vsec 
Volume charge density: .rho..sub.o (in sample) 
10.sup.-5 Coul/cm.sup.3 
Area charge density: Q.sub.o .ident. .rho..sub.o L.sub.s 
5 .times. 10.sup.-8 Coul./cm.sup.2 
Voltage: V.sub.o .ident. Q.sub.o L.sub.s /.epsilon..sub.3 = .rho..sub.o 
L.sub.s.sup.2 /.epsilon..sub.s 
500 V 
Time: t.sub.o .ident. L.sub.s.sup.2 /.mu..sub.n V.sub.o = .epsilon..sub.s 
/.mu..sub.n .rho..sub.o 
5 .times. 10.sup.-4 sec 
Current density: J.sub.o .ident. Q.sub.o /t.sub.o = .mu..sub.n .rho..sub.o 
.sup.2 L.sub.s /.epsilon..sub.s 
10.sup.-4 amp/cm.sup.2 
______________________________________ 
Turning now to FIG. 3, there is schematically shown a laboratory apparatus 
10 designed to simulate the electrophoresis that occurs in liquid 
electrographic development. The apparatus 10 illustrated in FIG. 3, 
consists of two capacitors representing sample 70 and dielectric 72 
connected in series. A constant bias voltage from a DC power supply 88 is 
applied to capacitor 70 by way of a fast acting relay switch 90. A 
coupling capacitor 82 connects capacitor 72 to an operational amplifier 
84. The output of the amplifier 84 is connected to a visual output device 
86, such as a cathode ray tube (CRT). Capacitor 72 is a perfect 
dielectric, which in electrographic applications, would be the imaging 
member bearing electrographic charges (i.e. the latent electrostatic 
images). The other capacitor 70 comprises the liquid ink sample system and 
consists of a film containing, for example, surfactant molecules in a 
hydrocarbon liquid with or without a dispersion of charged macroscopic 
particles of about 1 .mu.m diameter. Although a power supply 88 provides a 
constant bias voltage applied to apparatus 10, the colloid-layer voltage 
in capacitor 70 decays as charged species move within the liquid sample. 
Thus, apparatus 10 allows for the observation of voltage decay, and the 
dielectric relaxation of a liquid developing material sample, by the 
measurement of external currents. Since this is a purely electrical 
measurement that does not involve optical detection, the technique is 
equally applicable to dispersions containing only the surfactant molecule 
and its aggregates, such as inverse micelles. Apparatus 10 provides a 
means for a comparative study of the electrical properties of dispersions 
with and without the presence of colloidal particles. However, because of 
the spatially uniform distribution of bipolar species, the technique is 
sensitive to the motion of all charges and cannot easily distinguish 
between the contributions of positive and negative species. 
Referring further to FIG. 3, sheet 73 of 25 .mu.m Mylar.RTM., a registered 
trademark for a polyvinyl film material manufactured by E. I. DuPont de 
Nemours and Co. of Wilmington, Del., is placed on a polished aluminum 
block electrode 75 to form a "perfect" capacitor 72. A known amount of the 
liquid solution (approximately 0.13 cc) is placed on sheet 73 such that, 
upon covering it with another aluminum plate 77 (5.times.5 cm), the 
solution flows to fill the entire area of 25 cm.sup.2. Knowing the carrier 
fluid density and weight of the solution, the thickness of the liquid 
layers formed is calculated to be approximately 60 .mu.m. Some 
measurements were made using a cell giving liquid heights of approximately 
125 .mu.m. Although the total experimental time is typically much less 
than a minute, any possible complications due to settling out of colloidal 
particles on the sheet 73 of apparatus 10 out in the course of operation 
is avoided by using only fresh layers of solution for each measurement 
run. 
The DC voltage is applied to capacitor 70 and relay 90 is activated by an 
external trigger on CRT 86, which is a Nicolet, Model Pro 10, 
oscilloscope. With the arrangement shown in FIG. 3, the change in the 
voltage across capacitor 70, and the related charge in the ink sample is 
measured and electronically differentiated by coupling capacitor 82 and 
amplifier 84 to yield the total current. With a variety of available 
sampling times (the shortest being 1 .mu.sec) and employing different 
total numbers of points (up to 50 thousand), it is possible to measure the 
time-resolved current transients and integrated collected charge over a 
time range from 10.sup.-5 to 10.sup.-2 seconds. 
FIG. 4 shows examples of experimental current versus time curves, obtained 
with a typical liquid developing material containing about 2% toner 
particles and about 0.1% charge directors in hydrocarbon liquid, at 
various bias voltages Vb. A comparison with the results of charge director 
solutions (containing no toners) identifies the first current peaks (at 
the shortest time) as due to the transit of toner particles. The toner 
mobility, together with the applied bias voltage, the liquid layer 
thickness and the properties of the dielectric, determines the transit 
time or the time when the current peaks appear. 
The time integral of current from the curves of FIG. 4 provide a measure of 
the toner charge that can be collected within a period of time. Thus, 
changes in toner concentration or toner charge/mass ratio are reflected in 
the current integral versus time relations shown in FIG. 5. A decrease in 
the toner concentration without significant decrease in the charge/mass 
ratio, (and hence, the mobility), causes the saturation level of the 
integral to decrease but without much delay in the saturation time. On the 
other hand, a change in the charge/mass ratio, (and hence, the mobility), 
results in a decreased saturation value and delayed saturation time. Thus, 
a decrease in the current integral value measured at a time comparable to 
the development time is a signal of the potential for insufficient DMA 
(development mass per area) because of changes in liquid developing 
material performance and the need for adjustments to developing material 
composition or development process parameters. 
Turning now to FIG. 6, there is shown schematically a series-capacitor 
sensor 12 based upon the laboratory apparatus 10 discussed hereinabove 
with reference to FIG. 3. Sensor 12 is intended for use in the printing 
machine discussed with reference to FIG. 7. The sensor 12 comprises a pair 
of concentric cylinders 94 and 96. A dielectric layer 98 is coated on the 
inner surface of cylinder 94 to form the "perfect" capacitor. A sample of 
liquid developing material flows into sensor 12. An electrode 93 coaxial 
with cylinders 94 and 96 connects to a DC power supply 88 through a fast 
acting relay 90. Electrode 93 conveys a voltage to the ink sample. The 
outer surface of cylinder 94 is connected to one plate of coupling 
capacitor 82. The other plate of capacitor 82 connects to an inverting 
input 81 of operational amplifier 84. In this manner, the voltage across 
the liquid developing material sample is conveyed to amplifier 84 for 
measurement thereof. A noninverting input 83 of operational amplifier 84 
is grounded. A feedback resistor 89, connected between input 81 and output 
terminal 85, returns a portion of an output voltage to input 81 that is 
out of phase with the input voltage. Feedback resistor 89, coupling 
capacitor 82, and operational amplifier 84 form a differentiating circuit 
such that the output signal thereof is proportional to the rate-of-change 
of the input signal. An indicator 87 is connected between output terminal 
85 and ground, and triggered by power supply 88 through a conductor 91. 
The present invention comprises a series-capacitor sensor that monitors 
the voltage decay and relaxation of a liquid developing material sample by 
measuring external currents. When there is a change in the liquid 
developing material performance, the sensor activates external devices to 
adjust the liquid developing material composition or the development 
process. 
In review, the present invention provides an apparatus for determining 
charge density and mobility in a liquid solution having electrically 
charged particles therein. The apparatus includes, an electrode. A 
dielectric member having a first surface situated opposite the electrode 
provides for a volume therebetween in which a sample of the liquid may be 
placed. A fixed bias voltage is applied to the electrode to produce an 
electrical current flow through the liquid solution and the dielectric 
member. Means coupled to the dielectric member measure the electrical 
current as a function of time to provide a measure of voltage decay across 
the liquid solution. The voltage decay corresponds to the charge density 
of the liquid solution. 
It is, therefore, apparent that there has been provided, in accordance with 
the present invention, series-capacitor in sensor for liquid developers. 
This apparatus fully satisfies the aspects of the invention hereinbefore 
set forth. 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 such alternatives, 
modifications, and variations as fall within the spirit and broad scope of 
the appended claims.