Paper stock shear and formation control

System and method for producing paper are provided. The system controls formation of wet stock comprising fibers on a moving water permeable wire of a de-watering machine that has a refiner that is subject to a variable load and a headbox having at least one slice, wherein each slice has an aperture through which wet stock is discharged at a certain stock jet speed onto the wire that is moving at a certain wire speed. The system includes: a) at least two water weight sensors that are positioned adjacent to the wire wherein the at least two sensors are positioned at different locations in the direction of movement of the wire and upstream from a dry line which develops during operation of the machine and the sensors generate signals indicative of a water weight profile made up of a multiplicity of water weight measurements; and b) means for adjusting at least one of the stock jet speed, wire speed, or to cause the water weight profile to match a preselected or optimal water weight profile.

FIELD OF THE INVENTION 
The present invention generally relates to controlling continuous 
sheetmaking and, more specifically, to controlling formation and fiber 
shear on the fourdriner wire of a papermaking machine. 
BACKGROUND OF THE INVENTION 
In the art of making paper with modern high-speed machines, sheet 
properties must be continually monitored and controlled to assure sheet 
quality and to minimize the amount of finished product that is rejected 
when there is an upset in the manufacturing process. The sheet variables 
that are most often measured include basis weight, moisture content, and 
caliper (i.e., thickness) of the sheets at various stages in the 
manufacturing process. These process variables are typically controlled 
by, for example, adjusting the feedstock supply rate at the beginning of 
the process, regulating the amount of steam applied to the paper near the 
middle of the process, or varying the nip pressure between calendaring 
rollers at the end of the process. Papermaking devices well known in the 
art are described, for example, in "Handbook for Pulp & Paper 
Technologists" 2nd ed., G. A. Smook, 1992, Angus Wilde Publications, Inc., 
and "Pulp and Paper Manufacture" Vol III (Papermaking and Paperboard 
Making), R. MacDonald, ed. 1970, McGraw Hill. Sheetmaking systems are 
further described, for example, in U.S. Pat. Nos. 5,539,634, 5,022,966 
4,982,334, 4,786,817, and 4,767,935. 
In the manufacture of paper on continuous papermaking machines, a web of 
paper is formed from an aqueous suspension of fibers (stock) on a 
traveling mesh papermaking fabric and water drains by gravity and vacuum 
suction through the fabric. The web is then transferred to the pressing 
section where more water is removed by dry felt and pressure. The web next 
enters the dryer section where steam heated dryers and hot air completes 
the drying process. The paper machine is essentially a de-watering system. 
In the sheetmaking art, the term machine direction (MD) refers to the 
direction that the sheet material travels during the manufacturing 
process, while the term cross direction (CD) refers to the direction 
across the width of the sheet which is perpendicular to the machine 
direction. 
In the papermaking process, the major factors at the wire that influence 
the formation and strength of the paper include: (1) the stock jet speed 
to wire speed (jet/wire) ratio; (2) the angle that the stock jet lands on 
the wire; and (3) the rate of water drainage from the web. The speed 
differential between the stock jet and the wire speed determines the 
average orientation of the pulp fibers throughout the paper web between 
the cross, machine, and Z (wet stock height) directions. The average 
orientation of the fibers within the sheet is critical to both paper 
formation and sheet strength. 
Current machine start-up procedures require optimization of the papermaking 
machine at different jet/wire ratios and to perform laboratory tests to 
identify the jet/wire ratio that produces the requisite formation and 
strength characteristics of the paper. The test results may take several 
hours and require several trial-and-error changes to the jet/wire ratio 
before acceptable results are obtained. 
SUMMARY OF THE INVENTION 
The present invention is based in part on the development of an underwire 
water weight sensor (referred to herein as the "UW.sup.3 " sensor) which 
is sensitive to three properties of materials: the conductivity or 
resistance, the dielectric constant, and the proximity of the material to 
the UW.sup.3 sensor. Depending on the material, one or more of these 
properties will dominate. The UW.sup.3 sensors are positioned in a 
papermaking machine in the MD direction, and are used to measure the 
conductivity of an aqueous mixture (referred to as wet stock) in a 
papermaking system. In this case, the conductivity of the wet stock is 
high and dominates the measurement of the UW.sup.3 sensor. The proximity 
is held constant by contacting the support web in the papermaking system 
under the wet stock. The conductivity of the wet stock is directly 
proportional to the total water weight within the wet stock; consequently, 
the sensors provide information which can be used to monitor and control 
the quality of the paper sheet produced by the papermaking system. With 
the present invention, an array of UW.sup.3 sensors is employed to measure 
the water weight in the MD on the web of a fourdriner paper machine and 
generate water weight or drainage profiles. These sensors have a very fast 
response time (1 msec) and are capable of providing an accurate value of 
the water weight, which relates to the basis weight of the paper. Indeed, 
the water weight measurements can be computed from the under the wire 
weight sensor 600 times a second. By monitoring the MD trend of each of 
the MD sensors in the array, it is possible to correlate the variation of 
the water weight down the table between each of these sensors. The offset, 
in terms of time, that is required to overlay these trends to provide the 
desired correlation is the time that it takes for the unsupported stock 
slurry to travel from one sensor to the next. From this time, the control 
system can calculate the speed of the stock down the wire with relation to 
the wire speed. Since this unsupported stock slurry speed relates to the 
original stock jet speed, the control system can then monitor and control 
the jet-to-wire speed ratio and optimize this ratio to give the optimal 
sheet formation and strength. 
The method for tuning the operation of a fourdriner machine to produce a 
specific paper grade comprises a three-step procedure. The first step 
comprises tuning process parameters of the fourdriner machine to obtain an 
optimized configuration which produces acceptable quality paper as 
determined by direct measurement. The drainage profile corresponding to 
this optimized configuration is then measured with water weight sensors 
distributed along the machine direction, and recorded. 
This optimal drainage profile may then be fitted to various parameterized 
functions (such as an exponential) using standard curve fitting 
techniques. This curve fitting procedure has the effect of smoothing out 
the effects of noise on the profile, and interpolating between measured 
points. 
During subsequent production runs of the fourdriner machine, the objective 
is to reproduce the previously determined optimal drainage profile. If the 
measured moisture content at a given position is either above or below the 
optimal value for that position, the machine parameters, such as the stock 
jet speed to wire speed ratio, are adjusted as necessary to bring that 
measurement closer toward the optimal value. 
In one aspect, the invention is directed to a system of controlling that 
formation of wet stock which comprises fibers on a moving water permeable 
wire of a de-watering machine that comprises a refiner that subjects the 
fibers to mechanical action, said refiner having a motor load controller, 
and a headbox having at least one slice, wherein each slice has an 
aperture through which wet stock is discharged at a certain stock jet 
speed onto the wire that is moving at a certain wire speed, which system 
includes: 
a) at least two water weight sensors that are positioned adjacent to the 
wire wherein the at least two sensors are positioned at different 
locations in the direction of movement of the wire and upstream from a dry 
line which develops during operation of the machine and the sensors 
generate signals indicative of a water weight profile made up of a 
multiplicity of water weight measurements; and 
b) means for adjusting at least one of the stock jet speed, wire speed, or 
motor load controller to cause the water weight profile to match a 
preselected water weight profile. 
The invention will, among other things, increase productivity as the 
papermaker can now quickly determine the proper jet-to-wire ratio for a 
particular grade of paper. The paper produced will have optimum fiber 
orientation that is reflected in the sheet formation and strength.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
The present invention employs a system that includes a plurality of sensors 
that measure water weight in the MD along the web or wire at the wet end 
of a papermaking machine, e.g., fourdrinier. These UW.sup.3 sensors have a 
very fast response time (1 msec) so that an essentially instantaneous MD 
profile of water weight can be obtained. Although the invention will be 
described as part of a fourdrinier papermaking machine, it is understood 
that the invention is applicable to other papermaking machines including, 
for example, twin wire and multiple headbox machines and to paper board 
formers such as cylinder machines or Kobayshi Formers. Some conventional 
elements of a papermaking machine are omitted in the following disclosure 
in order not to obscure the description of the elements of the present 
invention. 
FIG. 1A shows a system for producing continuous sheet material that 
comprises headbox 10, a calendaring stack 21, and reel 22. Actuators 23 in 
headbox 10 discharge raw material through a plurality of slices onto 
supporting web or wire 13 which rotates between rollers 14 and 15 which 
are driven by motors 150 and 152, respectively. Controller 54 regulates 
the speed of the motors. Foils and vacuum boxes (not shown) remove water, 
commonly known as "white water", from the wet stock on the wire into the 
wire pit 8 for recycle. Sheet material exiting the wire passes through a 
dryer 24. A scanning sensor 30, which is supported on supporting frame 31, 
continuously traverses the sheet and measures properties of the finished 
sheet in the cross-direction. Multiple stationary sensors could also be 
used. Scanning sensors are known in the art and are described, for 
example, in U.S. Pat. Nos. 5,094,535, 4,879,471, 5,315,124, and 5,432,353, 
which are incorporated herein. The finished sheet product 18 is then 
collected on reel 22. As used herein, the "wet end" portion of the system 
depicted in FIG. 1A includes the headbox, the web, and those sections just 
before the dryer, and the "dry end" comprises the sections that are 
downstream from the dryer. 
An array of five UW.sup.3 sensors 42A-42E is positioned underneath web 13. 
By this meant that each sensor is positioned below a portion of the web 
which supports the wet stock. As further described herein, each sensor is 
configured to measure the water weight of the sheet material as it passes 
over the sensor. The sensor provides continuous measurement of the sheet 
material along the MD direction at the points where it passes each sensor. 
The sensors are positioned upstream from the dry line 43. A water weight 
profile made up of a multiplicity of water weight measurements at 
different locations in the MD is developed. An MD array with a minimum of 
two sensors is required, preferably 4 to 6 sensors are employed and 
preferably the sensors are positioned in tandem in the MD about 1 meter 
from the edge of the wire. Preferably, the sensors are about 30 to 60 cm 
apart. 
In another embodiment, each sensor in the MD array can be replaced with a 
CD array of the UW.sup.3 sensors, that is, each of the five sensors 
42A-42E comprises a CD array. Each CD array provides a continuous 
measurement of the entire sheet material along the CD direction at the 
point where it passes the array. A profile made up of a multiplicity of 
water weight measurements at different locations in the CD is developed. 
An average of these multiple measurements is obtained for each of the five 
CD arrays can be obtained and an MD profile based on the five average 
values generated. 
The term "water weight" refers to the mass or weight of water per unit area 
of the wet paper stock which is on the web. Typically, the water weight 
sensors are calibrated to provide engineering units of grams per square 
meter (gsm). As an approximation, a reading of 10,000 gsm corresponds to 
paper stock having a thickness of 1 cm on the fabric. The term "basis 
weight" refers to the total weight of the material per unit area. The term 
"dry weight" or "dry stock weight" refers to the weight of a material 
(excluding any weight due to water) per unit area. 
It has been demonstrated that fast variations of water weight on the wire 
correlate well to fast variations in dry basis weight of the sheet 
material produced when the water weight is measured upstream from dry line 
on the wire. The reason is that essentially all of the water on the wire 
is being held by the paper fibers. Since more fibers hold more water, the 
measured water weight correlates well to the fiber weight. 
The papermaking raw material is metered, diluted, mixed with any necessary 
additives, and finally screened and cleaned as it is introduced into 
headbox 10 from source 130 by fan or feeding pump 131. This pump mixes 
tock with the white water and deliver the blend to the headbox 10. 
The process of preparing the wet stock includes the step of subjecting the 
fibers to mechanical action in refiner 135 which includes a variable motor 
load controller 136. By regulating the refiner one can, among other 
things, regulate strength development and stock drainability and sheet 
formation. Many variables affect the refining process and these generally 
include, for example, the raw materials (e.g., fiber morphology), 
equipment characteristics, and process variables (e.g., pH). With respect 
to fiber morphology, it is known that the source of the wood pulp fibers 
will influence the properties of the paper. Two important characteristics 
are fiber length and cell wall thickness. A minimum length is required for 
interfiber bonding, and length is proportional to tear strength. The ratio 
of pulp fiber length to cell wall thickness which is as an index of 
relative fiber flexibility and the fiber coarseness value, which is the 
weight of fiber wall material in a specified fiber length, are two 
indications of fiber behavior. Generally, pulp characteristics of softwood 
species differ from those of hardwood species and the paper stock can 
comprise different blends of softwood and hardwood. This stock ratio of 
softwood and hardwood can be regulated to affect changes in, for example, 
the drainability of the wet stock on the wire. 
FIG. 2B illustrates headbox 10 having slices 50 which discharge wet stock 
55 onto wire 13. In actual papermaking systems, the number of slices in 
the headbox will be higher. For a headbox that is 300 inches in length, 
there can be 100 or more slices. The rate at which wet stock is discharged 
through the nozzle 52 of the slice can be controlled by corresponding 
actuator which, for example regulates the diameter of the nozzle. The 
function of the headbox is to take the stock delivered by the fan pump and 
transform a pipeline flow into an even, rectangular discharge equal in 
width to the paper machine and at uniform velocity in the machine 
direction. 
Headboxes are typically categorized, depending on the required speed of 
stock delivery, as open or pressurized types. Pressurized headboxes can be 
further divided into air-cushioned and hydraulic designs. In the hydraulic 
design, the discharge velocity from the slice depends directly on the 
feeding pump pressure. In the air-cushioned type the discharge energy is 
also derived from the feeding pump pressure, but a pond level is 
maintained and the discharge head is attenuated by air pressure in the 
space above the pond. 
The total head (pressure) within the box determines the slice jet speed. 
According to Bernoulli's equation: v=(2 gh).sup.1/2 where v=jet velocity 
or speed (m/s); h=head of liquid (m); and g=acceleration due to gravity 
(9.81 m/s.sup.2). The jet of stock emerging from a typical headbox slice 
contracts in thickness and deflects downward as a result of slice 
geometry. The jet thickness, together with the jet velocity, determines 
the volumetric discharge rate from the headbox. The headbox slice is 
typically a full-width orifice or nozzle with a completely adjustable 
opening to give the desired rate of flow. The slice geometry and opening 
determine the thickness of the slice jet, while the headbox pressure 
determines the velocity. 
The main operating variables for the headbox are typically stock 
consistency and temperature and jet-to-wire speed ratio. Typically, the 
consistency is set low enough to achieve good sheet formation, without 
compromising first-pass retention or exceeding the drainage capability of 
the forming section. Since higher temperature improves stock drainage, 
temperature and consistency are interrelated variables. Consistency is 
varied by raising or lowering the slice opening. Since the stock addition 
rate is typically controlled only by the basis weight valve (not shown), a 
change in slice opening will mainly affect the amount of white water 
circulated from the wire pit under the wire. 
The ratio of jet velocity to wire velocity is usually adjusted near unity 
to achieve best sheet formation. If the jet velocity lags the wire, the 
sheet is said to be "dragged"; if the jet velocity exceeds the wire speed, 
the sheet is said to be "rushed". Sometimes, it is necessary to rush or 
drag the sheet slightly to improve drainage or change fiber orientation. 
The jet speed is not actually measured, but is inferred from the headbox 
pressure. Typically, the papermaking machine is operated so that the ratio 
is not equal to 1, rather the ratio preferably ranges from about 0.95 to 
0.99 or 1.01 to 1.05. 
Practice of the invention relies in part on the development of one or more 
water weight profiles created during operation of the papermaking machine. 
The term "water weight profile" refers to a set of water weight 
measurements as measured by the MD array of sensors. Alternatively, the 
water weight profile can comprise a curve that is developed by standard 
curve fitting techniques from this set of measurements. In operation, 
water weight profiles are created for different grades of paper that are 
made under different operating conditions including different ambient 
conditions (e.g., temperature and humidity). For instance, when the 
machine of FIG. 1A is operating and making a specific grade of paper that 
has the desired physically properties as determined by laboratory analysis 
and/or measurement by the scanning sensor, measurements are taken with the 
UW.sup.3 sensors. The measurements will be employed to create a base or 
optimal water weight profile for that specific grade of paper and under 
the specific conditions. A database containing base water weight profiles 
(or base profiles) for different grades of paper manufactured under 
various operating conditions can be developed. It should be noted that 
besides developing and maintaining a database of the base water weight 
profiles, the stock jet speed to wire speed ratio for each profile will 
also be recorded. Furthermore, this ratio will be close to but not equal 
to 1. In this fashion, when the base profile from the database is employed 
to operate the papermaking machine, initially the machine will begin 
operation at the recorded jet/wire ratio. Thereafter, the ratio is 
manipulated in order to reproduce the base profile. 
During start-up of the papermaking machine, the operator will select the 
proper base profile from the database. The array of UW.sup.3 continuously 
develops measured water weight profiles which are compared to the base 
water weight profile. The stock jet speed to wire speed ratio is adjusted 
until the measured profile matches the base profile. Continual monitoring 
of the measured water weight profile allows the operator to adjust the jet 
speed to wire speed ratio should the measured profile deviated beyond a 
preset range from base profile. Only the wet end of the machine needs to 
operate during this initial start-up stage. Materials are recycled during 
this period. 
Because the stock jet velocity is generally easier to controlled than the 
wire speed, a preferred method of adjusting the jet/wire ratio is to 
maintain a substantially constant wire speed and adjust the pressure in 
the headbox to regulate the stock jet velocity. It is understood that the 
invention is applicable where the ratio is adjusted by controlling of the 
wire speed while maintaining a constant stock jet velocity or by 
controlling both the jet velocity and wire speed. 
In operation of the system as illustrated in FIG. 2, wet stock is pumped by 
feed pump 72 from source 70 to headbox 74. The wet stock is partially 
dewatered in the wet end process 76 that yields a partially dewatered 
product. During this initial start-up stage the partially dewatered 
product 90 can be collected for recycle. After this initial process has 
been completed, the partially dewatered product 92 will enter the dry end 
process 78 which yields finished paper that is collected at the reel 80. A 
scanning sensor 82 measures the dry end basis weight to confirm that the 
process parameters (e.g., jet/wire ratio) have been correctly selected. 
During the initial stage, an MD array of sensors 84 measures the water 
weight at the wet end and transmit signals to computer 86 which 
continuously develops water weight profiles of the wet end process. These 
measured water weight profiles are compared to the base or optimal water 
weight profile that has been selected for the particular grade of paper 
being made from a database. FIG. 8 is a graph of water weight versus wire 
position illustrating implementation of the process. As shown, curve A 
represents a base or optimal profile that has been preselected from the 
database for the grade of paper that is being made. During the start-up 
phase, water weight measurements at the wire are made by the MD array of 
sensors and from measurements curve B is created using standard curve 
fitting methods. 
As is apparent, in this case the measured water weight values are higher 
than those of the base profile. As a result, the computer will transmit 
appropriate signals to controller 94 that will regulate feed pump 72. This 
curve comparison procedure continues until the measured water weight 
profile matches the preselected optimized profile. In practice, 100% 
matching will not be necessary or practical and the level of deviation can 
be set by the operator. Therefore, it is understood that the term "match" 
or "matching" implies that the measured water weight profile has the same 
or approximately the same values as that of the preselected water base 
weight profile. Referring to FIG. 8, a preferred method of comparing the 
measured water weight values with those of the base profile entails 
comparing the three measurements at positions x, y, and z for each profile 
rather than the two curves. Furthermore, depending on the grade of paper, 
it may be that measurements closer to the dry line at position z may be 
more significant that those near the headbox at position x. In this case, 
the operator may require a higher degree of agreement at position z than 
at position x. After the proper jet/wire ratio is reached, i.e., when the 
measured profile matches the base profile, the dry end process goes on 
line and finished product is made. 
As indicated above, the system is preferably operated within certain 
jet/wire ranges. To assure that the machine is operating within this 
parameter, the system preferable includes computer 100 which receives 
signals from wire speed measuring device (e.g., tachometer) 102 and 
headbox pressure gauge 104. The computer calculates the stock jet speed to 
wire speed ratio. If the ratio is outside the ratio range (e.g., 1.01 to 
1.05) that is set by the operator, the stock jet velocity and/or wire 
speed can be adjusted accordingly. For example, signal 106 can be 
transmitted to the controller 110 which increases or decreases the speed 
of the pump 72. This in turn increases or decreases the stock jet 
velocity. The computer can also transmit appropriate signals to 108 to 
controller 112 which regulate the speed of the motors that drive the wire. 
In addition, the controller can transmit signal 114 to controller 94 which 
temporarily overrides operation of controller 94 until the jet/wire speed 
returns to the preset ratio range. 
As is apparent, while it is preferred to maintain the jet/wire ratio within 
a preset range, in the case where either the stock jet velocity or the 
wire speed is kept constant, it is not necessary to calculate the jet/wire 
ratio in order to implement the profile matching procedure. The only 
critical requirement is that the measured water weight profile matches the 
base profile. 
FIG. 2 also illustrates a method of controlling the motor load of refiner 
180 in response to wet end process signals. Specifically, when as in the 
case above, the measured water weight values are higher than those of the 
base profile, computer 86 will transmit appropriate signals to controller 
185 that will regulate the load (e.g, energy to variable motor) of refiner 
180. Furthermore, the jet speed to wire speed ratio is outside the ratio 
range that is set by the operator, signal 191 is transmitted by computer 
100 to controller 193 to increase or decrease the motor load. The computer 
can also transmit appropriate signals 197 to controller 185 temporarily 
overrides operation of controller 185 until the jet/wire speed returns to 
the preset ratio range. 
Under Wire Water Weight (UW.sup.3) Sensor 
In its broadest sense, the sensor can be represented as a block diagram as 
shown in FIG. 3A, which includes a fixed impedance element (Zfixed) 
coupled in series with a variable impedance block (Zsensor) between an 
input signal (Vin) and ground. The fixed impedance element may be embodied 
as a resistor, an inductor, a capacitor, or a combination of these 
elements. The fixed impedance element and the impedance, Zsensor, form a 
voltage divider network such that changes in impedance, Zsensor, results 
in changes in voltage on Vout. The impedance block, Zsensor, shown in FIG. 
3A is representative of two electrodes and the material residing between 
the electrodes. The impedance block, Zsensor, can also be represented by 
the equivalent circuit shown in FIG. 3B, where Rm is the resistance of the 
material between the electrodes and Cm is the capacitance of the material 
between the electrodes. The sensor is further described in U.S. patent 
application Ser. No. 08/766,864 filed on Dec. 13, 1996, which is 
incorporated herein. 
As described above, wet end BW measurements can be obtained with one or 
more UW.sup.3 sensors. Moreover, when more than one is employed, 
preferably the sensors are configured in an array of sensor cells. 
However, in some cases when an array does not physically fit in a location 
in the sheetmaking machine, a single sensor cell may be employed. 
The sensor is sensitive to three physical properties of the material being 
detected: the conductivity or resistance, the dielectric constant, and the 
proximity of the material to the sensor. Depending on the material, one or 
more of these properties will dominate. The material capacitance depends 
on the geometry of the electrodes, the dielectric constant of the 
material, and its proximity to the sensor. For a pure dielectric material, 
the resistance of the material is infinite (i.e. Rm=.infin.) between the 
electrodes and the sensor measures the dielectric constant of the 
material. In the case of highly conductive material, the resistance of the 
material is much less than the capacitive impedance (i.e. Rm Z.sub.cm), 
and the sensor measures the conductivity of the material. 
To implement the sensor, a signal Vin is coupled to the voltage divider 
network shown in FIG. 3A and changes in the variable impedance block 
(Zsensor) is measured on Vout. In this configuration the sensor impedance, 
Zsensor, is: Zsensor=Zfixed*Vout/(Vin-Vout) (Eq. 1). The changes in 
impedance of Zsensor relates physical characteristics of the material such 
as material weight, temperature, and chemical composition. It should be 
noted that optimal sensor sensitivity is obtained when Zsensor is 
approximately the same as or in the range of Zfixed. 
Cell Array 
FIG. 4 illustrates a block diagram of one implementation of the sensor 
apparatus including cell array 24, signal generator 25, detector 26, and 
optional feedback circuit 27. Cell array 24 includes two elongated 
grounded electrodes 24A and 24B and center electrode 24C spaced apart and 
centered between electrodes 24A and 24B and made up of sub-electrodes 
24D(1)-24D(n). A cell within array 24 is defined as including one of 
sub-electrodes 24D situated between a portion of each of the grounded 
electrodes 24A and 24B. For example, cell 2 includes sub-electrode 24D(2) 
and grounded electrode portions 24A(2) and 24B(2). For use in the system 
as shown in FIGS. 1 and 2, cell array 24 resides beneath and in contact 
with supporting web 12 and can be positioned either parallel to the 
machine direction (MD) or to the cross-direction (CD) depending on the 
type of information that is desired. In order to use the sensor apparatus 
to determine the weight of fiber in a wetstock mixture by measuring its 
conductivity, the wetstock must be in a state such that all or most of the 
water is held by the fiber. In this state, the water weight of the 
wetstock relates directly to the fiber weight and the conductivity of the 
water weight can be measured and used to determine the weight of the fiber 
in the wetstock. 
Each cell is independently coupled to an input voltage (Vin) from signal 
generator 25 through an impedance element Zfixed and each provides an 
output voltage to voltage detector 26 on bus Vout. Signal generator 25 
provides Vin. In one embodiment Vin is an analog waveform signal, however 
other signal types may be used such as a DC signal. In the embodiment in 
which signal generator 25 provides a waveform signal it may be implemented 
in a variety of ways and typically includes a crystal oscillator for 
generating a sine wave signal and a phase lock loop for signal stability. 
One advantage to using an AC signal as opposed to a DC signal is that it 
may be AC coupled to eliminate DC off-set. 
Detector 26 includes circuitry for detecting variations in voltage from 
each of the sub-electrodes 24D and any conversion circuitry for converting 
the voltage variations into useful information relating to the physical 
characteristics of the aqueous mixture. Optional feedback circuit 27 
includes a reference cell also having three electrodes similarly 
configured as a single cell within the sensor array. The reference cell 
functions to respond to unwanted physical characteristic changes in the 
aqueous mixture other than the physical characteristic of the aqueous 
mixture that is desired to be measured by the array. For instance, if the 
sensor is detecting voltage changes due to changes in water weight, the 
reference cell is configured so that it measures a constant water weight. 
Consequently, any voltage/conductivity changes exhibited by the reference 
cell are due to aqueous mixture physical characteristics other than weight 
changes (such as temperature and chemical composition). The feedback 
circuit uses the voltage changes generated by the reference cell to 
generate a feedback signal (Vfeedback) to compensate and adjust Vin for 
these unwanted aqueous mixture property changes (to be described in 
further detail below). The non-weight related aqueous mixture conductivity 
information provided by the reference cell may also provide useful data in 
the sheetmaking process. 
Individual cells within sensor 24 can be readily employed in the system of 
FIGS. 1 and 2 so that each of the individual cells (1 to n) corresponds to 
each of the individual UW.sup.3 sensors in the machine or cross direction. 
The length of each sub-electrode (24D (n)) determines the resolution of 
each cell. Typically, its length ranges from 1 in. to 6 in. 
The sensor cells are positioned underneath the web, preferably upstream of 
the dry line, which on a fourdrinier, typically is a visible line of 
demarcation corresponding to the point where a glossy layer of water is no 
longer present on the top of the stock. 
A method of constructing the array is to use a hydrofoil or foil from a 
hydrofoil assembly as a support for the components of the array. In a 
preferred embodiment, the grounded electrodes and center electrodes each 
has a surface that is flushed with the surface of the foil. 
FIG. 5A shows an electrical representation of sensor cell array 24 
(including cells 1-n) and the manner in which it functions to sense 
changes in conductivity of an aqueous mixture (i.e., wetstock). As shown, 
each cell is coupled to Vin from signal generator 25 through an impedance 
element which, in this embodiment, is resistive element Ro. Referring to 
cell n, resistor Ro is coupled to the center sub-electrode 24D(n). The 
outside electrode portions 24A(n) and 24B(n) are both coupled to ground. 
Also shown in FIG. 5A are resistors Rs1 and Rs2 which represent the 
conductance of the aqueous mixture between each of the outside electrodes 
and the center electrode. The outside electrodes are designed to be 
essentially equidistant from the center electrode and consequently the 
conductance between each and the center electrode is essentially equal 
(Rs1=Rs2=Rs). As a result, Rs1 and Rs2 form a parallel resistive branch 
having an effective conductance of half of Rs (i.e. Rs/2). It can also be 
seen that resistors Ro, Rs1, and Rs2 form a voltage divider network 
between Vin and ground. FIG. 5B also shows the cross-section of one 
implementation of a cell electrode configuration with respect to a 
sheetmaking machine in which electrodes 24A(n), 24B(n), and 24D(n) reside 
directly under the web 12 immersed within the aqueous mixture. 
The sensor apparatus is based on the concept that the resistance Rs of the 
aqueous mixture and the weight/amount of an aqueous mixture are inversely 
proportional. Consequently, as the weight increases/decreases, Rs 
decreases/increases. Changes in Rs cause corresponding fluctuations in the 
voltage Vout as dictated by the voltage divider network including Ro, Rs1, 
and Rs2. 
The voltage Vout from each cell is coupled to detector 26. Hence, 
variations in voltage directly proportional to variations in resistivity 
of the aqueous mixture are detected by detector 26 thereby providing 
information relating to the weight and amount of aqueous mixture in the 
general proximity above each cell. Detector 26 may include means for 
amplifying the output signals from each cell and in the case of an analog 
signal will include a means for rectifying the signal to convert the 
analog signal into a DC signal. In one implementation well adapted for 
electrically noisy environments, the rectifier is a switched rectifier 
including a phase lock-loop controlled by Vin. As a result, the rectifier 
rejects any signal components other than those having the same frequency 
as the input signal and thus provides an extremely well filtered DC 
signal. Detector 26 also typically includes other circuitry for converting 
the output signals from the cell into information representing particular 
characteristics of the aqueous mixture such as weight. 
FIG. 5A also shows feedback circuit 27 including reference cell 28 and 
feedback signal generator 29. The concept of the feedback circuit 27 is to 
isolate a reference cell such that it is affected by aqueous mixture 
physical characteristic changes other than the physical characteristic 
that is desired to be sensed by the system. For instance, if water weight 
is desired to be sensed then the water weight is kept constant so that any 
voltage changes generated by the reference cell are due to physical 
characteristics other than water weight changes. In one embodiment, 
reference cell 28 is immersed in an aqueous mixture of recycled water 
which has the same chemical and temperature characteristics of the water 
in which cell array 24 is immersed in. Hence, any chemical or temperature 
changes affecting conductivity experienced by array 24 is also sensed by 
reference cell 28. Furthermore, reference cell 28 is configured such that 
the weight of the water is held constant. As a result voltage changes 
Vout(ref. cell) generated by the reference cell 28 are due to changes in 
the conductivity of the aqueous mixture, not the weight. Feedback signal 
generator 29 convert the undesirable voltage changes produced from the 
reference cell into a feedback signal that either increases or decreases 
Vin and thereby cancels out the affect of erroneous voltage changes on the 
sensing system. For instance, if the conductivity of the aqueous mixture 
in the array increases due to a temperature increase, then Vout(ref. cell) 
will decrease causing a corresponding increase in the feedback signal. 
Increasing Vfeedback increases Vin which, in turn, compensates for the 
initial increase in conductivity of the aqueous mixture due to the 
temperature change. As a result, Vout from the cells only change when the 
weight of the aqueous mixture changes. 
One reason for configuring the cell array as shown in FIG. 5A, with the 
center electrode placed between two grounded electrodes, is to 
electrically isolate the center electrode and to prevent any outside 
interaction between the center electrode and other elements within the 
system. However, it should also be understood that the cell array can be 
configured with only two electrodes. FIG. 6A shows a second embodiment of 
the cell array for use in the sensor. In this embodiment, the sensor 
includes a first grounded elongated electrode 30 and a second partitioned 
electrode 31 including sub-electrodes 32. A single well is defined as 
including one of the sub-electrodes 32 and the portion of the grounded 
electrode 30 which is adjacent to the corresponding sub-electrode. FIG. 6A 
shows cells 1-n each including a sub-electrode 32 and an adjacent portion 
of electrode 30. FIG. 6B shows a single cell n, wherein the sub-electrode 
32 is coupled to Vin from the signal generator 25 through a fixed 
impedance element Zfixed and an output signal Vout is detected from the 
sub-electrode 32. It should be apparent that the voltage detected from 
each cell is now dependent on the voltage divider network, the variable 
impedance provided from each cell and the fixed impedance element coupled 
to each sub-electrode 32. Hence, changes in conductance of each cell is 
now dependent on changes in conductance of Rs1. The remainder of the 
sensor functions in the same manner as with the embodiment shown in FIG. 
6A. Specifically, the signal generator provides a signal to each cell and 
feedback circuit 27 compensates Vin for variations in conductance that are 
not due to the characteristic being measured. 
In still another embodiment of the cell array shown in FIGS. 7A and 7B, the 
cell array includes first and second elongated spaced apart partitioned 
electrodes 33 and 34, each including first and second sets of 
sub-electrodes 36 and 35, (respectively). A single cell (FIG. 7B) includes 
pairs of adjacent sub-electrodes 35 and 36, wherein sub-electrode 35 in a 
given cell is independently coupled to the signal generator and 
sub-electrode 36 in the given cell provides Vout to a high impedance 
detector amplifier which provides Zfixed. This embodiment is useful when 
the material residing between the electrodes functions as a dielectric 
making the sensor impedance high. Changes in voltage Vout is then 
dependent on the dielectric constant of the material. This embodiment is 
conducive to being implemented at the dry end of a sheetmaking machine 
(and particularly beneath and in contact with the dry sheet since dry 
paper has high resistance and its dielectric properties are easier to 
measure. 
The foregoing has described the principles, preferred embodiments and modes 
of operation of the present invention. However, the invention should not 
be construed as being limited to the particular embodiments discussed. 
Thus, the above-described embodiments should be regarded as illustrative 
rather than restrictive, and it should be appreciated that variations may 
be made in those embodiments by workers skilled in the art without 
departing from the scope of the present invention as defined by the 
following claims.