Abstract:
Sheetmaking processes such as papermaking making systems employ water weight sensors underneath the moving water permeable wire that supports the wet stock (pulp slurry). A dynamically compensated calibration equation that equates the water weight plus fiber weight plus wire weight (total weight) to the resistance measured by the water weight sensor is developed for controlling the continuous process. Dynamic compensation accounts for changing papermaking machine conditions or states that affect the intrinsic conductivity of the wet stock being measured. The amount of correction to apply is determined by the conductance measured by a reference sensor.

Description:
FIELD OF THE INVENTION 
     The present invention generally relates to controlling continuous sheetmaking and, more specifically, to dynamically calibrating water weight sensors used to measure the water weight of paper stock 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 are well known in the art and are described, for example, in  Handbook for Pulp  &amp;  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. No. 5,539,634 to He, U.S. Pat. No. 5,022,966 to Hu, U.S. Pat. No. 4,982,334 to Balakrishnan, U.S. Pat. No. 4,786,817 to Boissevain et al., and U.S. Pat. No. 4,767,935 to Anderson et al. 
     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 papermaking 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. 
     U.S. Pat. No. 5,891,306 to Chase et al. describes a sensor that measures water weight on the wire of a papermaking machine. The sensor detects changes in resistance of the wet stock between the electrodes in an electrode array. The resistance of the wet stock between the electrodes is dependent on the amount of water above the electrodes, i.e., the water weight, and on the conductivity of the water. Since the conductivity of the water changes from time to time, the resistance measurement does not uniquely determine the amount of water unless some correction for the conductivity is provided. Consequently, the sensor also includes a separate reference cell which is designed to cancel out all affects that change the resistance between the electrodes other than the water weight. For instance, the resistance measurement is affected by changes in conductivity due to changes in the wet stock temperature or chemical composition. 
     SUMMARY OF THE INVENTION 
     The present invention is based in part on the development of a dynamically compensated calibration equation that equates the water weight plus fiber weight plus wire weight (total weight) to the resistance measured by the above described water weight sensor. Dynamic compensation is required to account for changing papermaking machine conditions or states that affect the intrinsic conductivity of the wet stock, i.e., pulp slurry, being measured. The amount of correction to apply is determined by the conductance measured by the reference sensor. 
     In one aspect, the invention is directed to a method of monitoring the formation of a sheet of wet stock comprising fibers wherein the wet stock is formed on a water permeable movable wire of a de-watering machine that has a headbox with a plurality of apertures through which wet stock is introduced onto the wire at a controlled flow rate, said method includes the steps of: 
     (a) positioning three or more water weight sensors (measurement sensors) underneath and adjacent to the wire and upstream from a dry line which develops during operation of the machine wherein all the measurement sensors have substantially the same configuration; 
     (b) positioning a reference sensor so that it will measure the wet stock under saturated conditions; 
     (c) calibrating the measurement sensors to equate conductance measurements made by the three or more water weight sensors to water weight (or value) above the three or more water weight sensors to develop a calibration equation, and, following step (c); 
     (d) measuring the conductance of the wet stock with one or more of the measurement sensors and using the calibration equation to provide the absolute water weight(s) in substantially real time. 
     In another aspect, the invention is directed to a system of controlling the formation of wet stock which comprises fibers on a moving water permeable wire of a de-watering machine that includes: 
     wet-dry devices that comprises (i) means for supplying an amount of pulp from at least one source, (ii) means for adding an amount of non-fibrous additives to the wet stock, (iii) a refiner that subjects the fibers to mechanical action, said refiner having a motor load controller, and (iv) 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, and 
     dry-end devices that dry a sheet of material from the wire, which system includes: 
     (a) at least one water weight measurement sensor that is positioned adjacent to the wire and upstream from a dry line which develops during operation of the machine; 
     (b) a reference sensor that measures the water weight of the wet stock under saturated conditions, wherein the measurement sensor has been calibrated in accordance with procedures (a), (b), and (c) of the inventive method; and 
     (c) means for adjusting at least one of the wet-end or dry-end devices in response to water weight measurements. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  shows a sheetmaking system implementing the technique of the present invention; 
         FIG. 1B  shows the relationship of the slices in the headbox and wire and their proximity to the reference water weight sensor; 
         FIGS. 2 and 3  show the measurement apparatus and its electrical representation, respectively; 
         FIG. 4A  shows the top plan view of an electrode configuration having reference cells built into the measurement electrode configuration; 
         FIG. 4B  shows the top view of a sensor that is embedded in a foil; 
         FIG. 5  is a graph of the uncorrected conductance measurements vs. total water weight; 
         FIG. 6  is a schematic graph of the uncorrected conductance measurements vs. total water weight indicating how the correction for state is obtained; 
         FIG. 7  is a graph of the corrected conductance measurements vs. total water weight; 
         FIG. 8  is a schematic graph of the reference sensor conductance difference from reference state conductance vs. calibration curve difference from reference state calibration curve and illustrates how the parameter used in dynamically calculating the correction factor is obtained; 
         FIG. 9  is a graph of the conductivity vs. 1/(saturated reference resistance); 
         FIG. 10  is a graph of conductivity vs. 1/(signal resistance); 
         FIG. 11  is a graph of water weight height vs. slope and of water weight height vs. intercept; and 
         FIG. 12  is schematic of a process control system. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present invention generally relates to devices for detecting properties of an aqueous mixture, e.g., wet stock, on a de-watering machine wherein the devices exhibit enhanced precision and sensitivity over a wide range of mixture concentrations. Measurements from the devices for instance can be employed for process control in papermaking machines. 
     The invention is based in part on the development of calibration techniques that can be employed with any water weight sensing device that measures the amount of a liquid product by detecting its conductance/resistance. Particularly preferred sensing devices have sensors or electrodes that are positioned below the liquid product to be measured with the electrodes facing the liquid product. The sensing device has an effective range whereby conductance/resistance measurements correlate to the amount of liquid product present, resting on the electrodes. 
     Papermaking Machine 
       FIG. 1A  shows a system for producing continuous sheet material that comprises a headbox  10 , a calendaring stack  21 , and reel a  22 . The papermaking raw material is metered, diluted, mixed with any necessary additives, and finally screened and cleaned as it is introduced into the headbox  10  from a source  50  by a fan or feeding pump  52 . This pump mixes stock with the white water and delivers the blend to the headbox  10 . 
     The process of preparing the wet stock includes the step of subjecting the fibers to mechanical action in the refiner  56  which includes a variable motor load controller  54 . 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 characteristics of the raw materials, e.g., fiber morphology, equipment characteristics, and the pH. 
     The actuators  23  in the headbox  10  discharge raw material through a plurality of slices onto supporting web or wire  13  which rotates between the rollers  14  and  15  which are driven by motors  44  and  45 , respectively. Controller  54  regulates the speed of the motors. Foils and vacuum boxes (not shown) drain liquid, 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 the 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. No. 5,094,535 to Dalquist et al., U.S. Pat. No. 4,879,471 to Dalquist, U.S. Pat. No. 5,315,124 to Goss et al., and U.S. Pat. No. 5,432,353 to Goss et al., 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. 
     In operation of the papermaking machine, one or more water weight sensors can be positioned underneath the wire  13 . CD and/or MD arrays of such sensors can also be employed. Signals from the sensors can be used for process control as further described herein. In the embodiment as shown in  FIG. 1A , an MD array, that includes five measurement water weight sensors  42 A– 42 E, is positioned underneath the web  13 . By this meant that each sensor is positioned below a portion of the web that supports the wet stock. The sensors are positioned upstream from the dry line  43 . 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 machine direction (MD) at the points where it passes each sensor. 
     Production mode of operation of the papermaking machine should be distinguished from calibration mode of operation. The latter requires an MD array with a minimum of three sensors as described herein; preferably four to six 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 individual sensors are about 30 to 60 cm apart. For practical purposes, the same MD arrangement with 3 or more sensors can be used for both calibration and production operations. 
     The term “water weight” refers to the mass or weight of water per unit area of the wet or paper stock that 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 wire. 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  43  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. 
       FIG. 1B  illustrates a headbox  10  having a plurality of slices or apertures  50  which discharge wet stock  55  onto wire  13 . For a headbox that is 300 inches (7.62 m) 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 a corresponding actuator which, for example, regulates the diameter of the nozzle. The function of the headbox is to take the stock that is 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. A reference water weight sensor  42 F is positioned underneath web  13  and adjacent to aperture  50 . The reference sensor preferably has substantially the same structure and configuration as that of the measurement water weight sensors. 
     Dynamic Calibration 
     Embodiments of the present invention provide methods of calibrating water weight sensors that are employed in a papermaking machine. The methods yield mathematical relationships that correlate conductance measurements of the wet stock generated by the sensors positioned underneath the wire to corresponding water weights. Essentially real time data from the sensors are measured and manipulated accordingly. 
     For calibration, three or more and preferably four to six measurement sensors arranged in an MD array are employed. Preferably, the measurement sensors all have substantially the same configuration. The reference sensor may have substantially the same configuration as the measurement sensors. 
     Two related calibration techniques are presented; both techniques rely on conductance measurements from a papermaking machine operating in the calibration mode while making different grades of paper to generate data for deriving the mathematical relationship. While the techniques are applicable to any de-watering machine that employs a moving, porous conveyer on which an aqueous mixture is transported, the techniques have been demonstrated on papermaking machines similar to that shown in  FIGS. 1A and 1B . 
     The first technique is based on the discovery that, within certain operating parameters, there exists a linear relationship between the total water weight height on the wire for the paper stock and the inverse of the measured sensor signal resistance. Furthermore, it was observed that this relationship remains linear regardless of the grade of wet stock. Indeed, a series of substantially parallel calibration lines were obtained from data generated when the papermaking machine was operated in the calibration mode using different grades of paper stocks. (These different grades of paper stocks exhibit different conductances and are used to make different grades of paper in the production mode.) The second technique is an improvement of the first and in essence is an extension of the characterization of the sensor behavior over a larger range of paper stock conductances and a wider range of water weights. 
     The calibration equations that are derived provide absolute water weight measurements based on conductance measurements from water weight sensors. An absolute measurement of water weight is used to accurately monitor any changes in drainage on the wire which can affect the quality of paper produced. 
     Method 1. The first on-line calibration technique has been demonstrated to be particularly suited for measuring paper stock with water weights that range from about 2250 g/m 2  to 8500 g/m 2 . In this realm, the dynamically compensated calibration is based on the following calibration equation (Equation 1):
 
water weight= S   ref *( G+ΔG   ref )+ I   ref   (1)
 
where: S ref  is the slope of the reference state calibration curve;
 
     I ref  is the y-intercept (value of the water weight at zero conductance) of the reference state calibration curve; 
     G=1/R is the reciprocal of the water weight sensor resistance reading, i.e., the conductance reading; and 
     ΔG ref =1/R ref −1/R, is the correction that refers the measured G values at any state back to the reference state. 
     ΔG ref  is the difference between G that is measured at any particular state and G that is measured at the reference state (G ref −G in  FIG. 6 ). This accounts for the conductance contribution from changes in the state of the paper stock (Paper stock is also referred as the “solution” herein). 
     The values of the parameters of the calibration equation are derived by the following steps 1–4: 
     (1) Referring o the papermaking machine shown in  FIG. 1A , an array MD sensors under the wire, e.g.,  42 A to  42 E, are employed to obtain uncompensated conductance readings during calibration mode operation of the papermaking machine under different operating conditions. As the uncompensated readings are made, the actual total water weights over the measurement water weight sensors, which comprise the weight of the wet stock and wire, are also measured with a gauge that measures the height of the paper stock above each sensor. In principle, two sets of measurements taken under two different operation conditions of the papermaking machine will suffice. In practice measurements from more than two are desirable for increased accuracy. 
       FIG. 5  is a graph of the conductance versus total weight that shows the results of  22  measurement water weight sensor readings under the 7 sets of operating conditions. Each of the 22 reading is from one of the measurement MD water weight sensors. The conductance readings are uncompensated for the state of the paper stock and are divided into 7 groups, i.e., operating conditions, each with its own curve, i.e., line that is generated by conventional curve fitting techniques. These curves cover a sufficiently narrow range of operating conditions and therefore the slopes of the curves are essentially the same. 
     (2) While the papermaking machine is operating under each of the 7 different operating conditions in step (1), the corresponding saturated water weight conductance is measured with the reference sensor  42 F that is positioned near the headbox as illustrated in  FIG. 1B . 
     (3) Setting the first calibration group (curve A of  FIG. 5 ) as a reference state calibration curve, the other 6 calibration curves are shifted to the left and converged into the reference state curve by applying a correction factor. This shifting process is illustrated in  FIG. 6  which is graph of conductance vs. total weight measured under three operating conditions. One curve is selected as the reference state and two correction factors, i.e., G ref −G 1  and G ref −G 2 , are shown. 
     Any of the 7 curves in  FIG. 5  could have been selected as the reference state calibration curve since the selection of the reference state is arbitrary. As an example, to shift the individual readings of another curve (referred to as curve B) into curve A, the correction factor applied is proportional to the difference in the saturation water weight conductances as measured for curves A and B (G A −G B ). Each shift for the remaining 5 other curves requires a different correction factor. 
     Once all the readings have been shifted, a reference state calibration curve is created by curve fitting all 22 readings as shown in  FIG. 7 . As is apparent, the calibration curve of  FIG. 7  is similar but not necessarily identical to curve A of  FIG. 5 . This calibration curve will be employed to continuously calculate water weight measurements during operations based on measurement readings and the corresponding reference sensor readings. Specifically, the reference state calibration curve provides the values for (i) S ref  which is the slope of the curve, (ii) I ref  which is the y-intercept (value of water weight at zero conductance) of the calibration. The only remaining variable for the calibration equation (1) is the correction factor ΔG ref . 
     (4) The value of the correction factor (ΔG ref ) is calculated from the conductance measurement of any saturated reference sensor, i.e., any water weight sensor with a saturated signal. Saturated measurement conditions are achieved when any water weight sensor reading is unaffected by any further increases in water weight. Under these saturated conditions, this water weight sensor effectively measures changes in intrinsic bulk conductivity only. The correction factor is calculated using the deviation from the reference state conductance of the reference sensor. The relationship of the correction factor is defined in the following Equation 2:
 
Δ G   ref =( G   Sref   −G   S )/ B   (2)
 
where:
 
     G Sref  is the conductance of the reference sensor at the reference state, i.e., for the selected reference state calibration curve; 
     G S  is the conductance of the saturated water weight sensor at the measured state; and 
     B is a factor that relates the shift in the calibration curve to the corresponding shift in the saturated reference sensor conductance due to a change in the solution state. 
     The “B” factor is calculated by correlating the shift that was applied to the 6 state dependent calibration curves as each was converged onto the reference state calibration curve to the corresponding shift in the reference sensor conductance from its reference state value. For example, referring to  FIG. 6 , each data point of the state 1 curve was shifted by a value of G ref −G 1  and each data point of the state 2 curve was shifted by a value of G ref −G 2  to the left to converged into the reference state calibration curve. The corresponding difference between the reference water weight sensor conductances for the reference state and the state 1 and state 2 curves were also calculated. 
       FIG. 8  is a schematic graph plotting the shift (amount of correction) for data points of a curve versus the corresponding difference between the reference sensor conductances. The slope of this graph is equal to the “B” factor in Equation 2. As is apparent, the “B” factor is a constant that relates the difference between a particular measured conductance and the reference conductance at the reference sensor to the shift of the calibration curve. The “B” factor is determined empirically for each papermaking machine calibration under different operation conditions, since it will vary with machine condition, reference and measurement sensor locations, and reference sensor geometry. 
     In practice, after the calibration curve is established, the “B” factor is calculated and the calibration equation can be employed thereafter to provide continuous corrected water weight readings from the measurement water weight sensors. The water weight can be determined from the total weight measured by subtracting the contribution of the wire that is fairly constant. 
     Method 2. The following improved dynamically compensated calibration can be applied for measuring paper stock(s) having a total water weight ranging from 0 to 20000 g/m 2  and higher. Method 2 is based in part on the discovery that for a wide range of conductances and water weights, calibrating curves can also be consolidated into a single calibration curve using multiple adjustment parameters in a calibration equation. It has been observed that the relationship between the paper stock or solution conductance (SC) and the reference sensor resistance (or equivalently, the conductance) is linear.  FIG. 9  is a graph of independent measurements of conductance vs. the inverse of saturated reference resistance for a paper stock. 
     The linear relationship can be expressed as Equation 3:
 
 SC =(Δ SC/ΔG   ref )* G   ref   +SC   Gref0   (3)
 
where:
         ΔSC/ΔG ref  is the change in solution conductance with change in the reference sensor conductance;       

     G ref  is the reference sensor conductance reading; and 
     SC Gre0  is the projected solution conductance at G ref =0. 
     Similarly, for the measurement sensors, the relationship is linear for a particular water weight and can be expressed as Equation 4:
 
 SC =(Δ SC/ΔG )* G+SC   G0   (4)
 
where: ΔSC/ΔG and G refer to similar readings for the measurement sensor; and
         SC G0 (is the projected solution conductance at G=0.       

     The relationship between conductance and the inverse of measurement sensor signal resistance is plotted in  FIG. 10  for a large range of paper stock water weights. 
     ΔSC/ΔG and SC G0  can be plotted against water weight height as shown in  FIG. 11 . ΔSC/ΔG is linear versus (1/water weight height) for any given water weight and can be expressed as Equation 5:
 
Δ SC/ΔG ={Slope(Δ SC/ΔG )/( WWH+WWH   offset )}+(Δ SC/ΔG ) 0   (5)
 
where: Slope(ΔSC/ΔG) is the slope of the line {i.e. change in ΔSC/ΔG with (WWH+WWH offset )};
         WWH is the water weight height;   WWH offset  is the water weight height offset; and   (ΔSC/ΔG) 0  is the projected ΔSC/ΔG at 1/(WWH+WWH offset )=0       

     The projected conductance at any given water weight, SC G0  can be expressed as Equation 6:
 
 SC   G0 ={( A *exp(&#39; WWH /height R0 )}+Offset  (6)
 
where: the pre-exponential A, and the “Offset” term are fitting parameters; and
         height R0  is a normalizing constant.       

     ΔSC/ΔG from Equation 4 can be substituted in Equation 5 to yield Equation 7:
 
 WWH =Slope(Δ SC/ΔG )/{( SC−SC   C0 )/ G −(Δ SC/ΔG ) 0   }−WWH   offset   (7)
 
     With these relationships, the water weight can be determined from conductance measurements from a reference sensor and a measurement sensor using the following iterative method:
         1. Calculate SC from the reference sensor conductance, G ref , using Equation 3.   2. Applying Equation 5 to the data in  FIG. 11  yields:   a. Slope(ΔSC/ΔG)=1952   b. (ΔSC/ΔG) 0 =451 ohms   c. WWH offset =0.17 mm   3. Assuming SC G0 =0 for the first iteration, substituting the measured G, SC into Equation 7 yields an initial WWH   4. Use the initial WWH in Equation 6 to calculate SC G0 .   5. Use the calculated SC G0  to calculate a final WWH with Equation 7.
 
Papermaking Machine Process Control
       

     Water weight sensors (or an array arranged in the MD and/or CD underneath the wire) can be employed to optimize papermaking machines. Process control techniques for papermaking machines are further described, for instance, in U.S. Pat. No. 6,149,770 to Hu et al., U.S. Pat. No. 6,092,003 to Hagart-Alexander et. al, U.S. Pat. No. 6,080,278 to Heaven et al., U.S. Pat. No. 6,059,931 to Hu et al., U.S. Pat. No. 6,853,543 to Hu et al., and U.S. Pat. No. 5,892,679 to He, which are all incorporated herein by reference. 
     As is apparent, a number of parameters of the wet end and dry end of the papermaking machine as illustrated in  FIGS. 1A and 1B  can be regulated.  FIG. 12  depicts a papermaking machine  110  having a wet end  112  and a dry end  116 . Control unit  114  that includes a computer receives readings from measurement and reference water weight sensors from machine  110 . The conductance readings are converted to water weight measurements with the mathematical relationships developed using the calibration techniques. In the case where an MD array of sensors is employed, a continuous water weight profile of the paper stock on the web can be generated and compared to an “ideal” profile for making a particular grade of paper. Depending on the degree of deviation from ideal, wet end and/or dry end parameters can be adjusted accordingly. See, for example, U.S. Pat. No. 6,092,003 to Hagart-Alexander which is incorporated herein. While dry end parameters, e.g., temperature of heating devices, can be controlled to achieve the desired final product, typically the wet end parameters are more important and will be further described herein. 
     A wide range of chemicals is utilized in the papermaking stock furnish to impart or enhance specific sheet properties or to serve other necessary purposes. Such additives as alum, sizing agents, mineral fillers, starches and dyes are commonly used. Chemicals for control purposes such as drainage aids, defoamers, retention aids, pitch dispersants, slimicides, and corrosion inhibitors are added as required. Fabrication of quality paper required addition of the proper amount of these chemicals. 
     Wet end chemistry deals with all the interactions between furnish materials and the chemical/physical processes occurring at the wet end of the papermaking machine. The major interactions at the molecular and colloidal level are surface charge, flocculation, coagulation, hydrolysis, time-dependent chemical reactions and microbiological activity. These interactions are fundamental to the papermaking process. For example, to achieve effective retention, drainage, sheet formation, and sheet properties, it is necessary that the filler particles, fiber fines, size and starch be flocculated and/or adsorbed onto the large fibers with minimal flocculation between the large fibers themselves. 
     Control of wet-end chemistry is vital to ensure that a uniform paper product is manufactured. The wet end of a papermaking machine is also critical in determining the long-term stability of the machine and ultimately the quality of the resulting product. Wet end control is further described in U.S. Pat. No. 6,1166,839 to Heaven et al. and U.S. Pat. No. 6,086,716 to Watson et al., which are both incorporated herein. 
     Typically, the papermaking furnish or raw material is metered, diluted, mixed with any necessary additives, and finally screened and cleaned as it is introduced into headbox from a fan pump. Any of these unit operations can be regulated. For example, paper stock is supplied to a machine chest from a refiner which includes adjustable mechanical elements, e.g., motorized disk elements or plates to grind the paper fiber surfaces. Generally, the refiner is part of the stock preparation system which prepares, conditions, and/or treats the pulp or stock in such a manner that a satisfactory sheet of paper can be produced. Adjusting the load will increase or decrease the degree of mechanical action on the pulp by the mechanical elements in the refiner. The refiner is connected to sources of thick stock and water. For high quality paper typically more than one source of pulp is used. Vigorously grinding the paper stock in the refiner reduces the rate at which water will drain through the wire mesh. Thus, it is common to refer to a rapidly draining stock as being “free”, or having high freeness, whereas more highly grinded stock is referred to as being slow, or having low freeness. In addition, wet end control also includes means for adding non-fibrous additives to the papermaking stock described above. Chemical additives are added at different steps in the process. 
     The water drainage profile on a fourdrinier wire is a complicated function principally dependent on the arrangement and performance of drainage elements, characteristics of the wire, tension on the wire, stock characteristics (for example freeness, pH and additives), stock thickness, stock temperature, stock consistency wire speed and refiner load or power. By controlling one or more operating parameters of the system the quality of the paper fabricated can be regulated. Although one may adjust the concentration of additives to regulate the final product, and/or regulate the flow of pulp into the refiner when more than one source is employed, generally for a particular grade of paper, it is preferred to maintain the concentration of the additives and pulp flow rates once the optimum levels are set. 
     In one embodiment of the control system, one or more of the other process parameters while keeping the flow of additives and pulp within certain set points. One such parameter is the refiner power. This can be accomplished by using a refiner that has a refiner plate position control system. By subjecting fibers to different levels of mechanical action, the paper stock flowing onto the wire will exhibit different properties, e.g., drainage characteristics. 
     Finally, the ratio of jet velocity of the paper stock through the slice of headbox to wire velocity is usually adjusted near unity to achieve best sheet formation. Typically, this ratio is maintained between 0.95 to 1.05 but usually it is not maintained at exactly 1. 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. The jet-to-wire ratio can be changed by adjusting the wire speed or the jet speed. The wire speed is typically adjusted by changing the speed of the large rolls at the beginning and end of the wire which the wire travels on. Often times the couch roll, i.e., the end roll, controls the speed of the wire. The jet speed is adjusted by the headbox pressure. 
     Water Weight Sensors 
     Suitable sensing devices for use in the present invention include water weight sensors which are available under the trade name SPECTRAFOIL from Honeywell, Inc. and which are described in U.S. Pat. No. 5,954,923 to Chase et al., which is incorporated herein. These sensors have a very fast response time (1 msec) so that an essentially instantaneous water weight can be obtained. The SPECTRAFOIL brand sensors are positioned underneath the wire of a papermaking machine, e.g., fourdrinier. The invention will be described with the water weight sensors having the construction illustrated herein but it is understood that other water weight sensors having similar characteristics can be employed. 
       FIG. 2  shows the basic configuration of a water weight sensor that includes a sensor array with two elongated grounded electrodes  24 A and  24 B and a segmented electrode  24 C. Measurement cells (cell  1 , cell  2 , . . . cell n) each includes a segment of electrode  24 C and a corresponding portion of the grounded electrodes ( 24 A and  24 B) opposite the segment. Each cell detects a resistance of the wet stock and specifically the water portion of the stock residing in the space between the segment and its corresponding opposing portions of grounded electrode. Each cell is independently coupled to an input measurement 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. Device  26  includes circuitry for detecting variations in voltage from each of the segments in electrodes  24 C and any conversion circuitry for converting the voltage variations into useful information relating to the physical characteristics of the aqueous mixture. 
       FIG. 3  illustrates an electrical representation of the measuring apparatus shown in  FIG. 2  including cells  1 –n of sensor array  24  for measuring conductivity of an aqueous mixture. 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 center segment  24 D(n) and portions  24 A(n) and  24 B(n) (opposite segment  24 D(n)) are coupled to ground. Also shown in  FIG. 3  are resistors Rs 1  and Rs 2  which represent the resistance of the aqueous mixture between the segments and the grounded portions. Resistors Ro, Rs 1 , and Rs 2  form a voltage divider network between Vin and ground. It should be understood that the apparatus shown in  FIGS. 2 and 3  can be implemented with a single grounded electrode which is adjacent and positioned opposite to a single segmented electrode. 
     Resistances Rs 1  and Rs 2  are dependent on changes in the water depth and the bulk conductivity of the aqueous mixture. The bulk conductivity of the mixture in turn is influenced by a number of factors, including, for example, mixture temperature, chemical additions, the concentration of fiber. When using the measurement apparatus to measure only water weight, it is necessary to cancel out the affects of the bulk conductivity seen in the detected resistance between the electrodes. This is done with a feedback apparatus  27 , as shown in  FIGS. 2 and 3 , which generates a feedback signal to adjust Vin to compensate for changes in bulk conductivity. 
     The 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 weight is desired to be sensed then the weight is kept constant so that any voltage changes generated by the reference cell are due to physical characteristics other than 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  converts 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. 
     Instead of using an external reference cell and feedback circuit, the electrode configuration can include a built-in reference cell within the measurement electrode configuration.  FIG. 4A  shows a measurement electrode configuration having a first center elongated grounded electrode or rail  60  with second and third segmented measurement electrodes  62  on either side of grounded electrode  60 . As with the measurement apparatus shown in  FIGS. 2 and 3 , each measurement electrode segment is coupled to an impedance element (not shown) which, in turn, is coupled to a measurement input signal. For instance, each measurement electrode segment  60  is coupled to a resistor Ro (as shown in  FIG. 3 ) which is coupled to Vin. An output voltage signal Vout is taken from each electrode segment which corresponds to a detected measurement electrode resistance (R measured ) of the mixture between each electrode segment and ground. 
     The electrode configuration further includes a plurality of interspaced reference electrodes  64  built into grounded electrode  60 . A circular layer of dielectric insulates each reference electrode from the elongated grounded center electrode. The reference electrodes form an array of reference cells each including a reference electrode and the portion of the grounded electrode surrounding the reference electrode. As is with the measurement electrodes, each reference electrode is coupled to an impedance element and a measurement input signal Vin in order to measure the reference electrode resistance (R ref ) of the aqueous mixture between the reference electrode and ground formed by the circle of dielectric material encircling the reference electrode. In another embodiment, more than one reference electrode can be associated with a single measurement electrode segment. In still another embodiment, a single segmented electrode can be used instead of two on either side of the ground electrode, wherein the measurement electrode configuration only includes one elongated, segmented electrode and an elongated, grounded electrode. 
       FIG. 4B  illustrates an alternative embodiment of a sensor  80  that is embedded in a ceramic foil or support  92  that can be positioned under the web of a papermaking machine. The sensor  80  includes a center reference electrode  90 , outer ground electrode  86 , and measurement electrode  84  which are insulated by dielectric materials  82  and  88 . 
     The measurement and reference electrodes can be constructed so that they exhibit different sensitivities to a first property but exhibit relatively the same sensitivity to a second property. For instance, both the reference and measurement electrodes can have the same sensitivity to changes in bulk conductivity on the wet stock but have different sensitivities to changes in water depth. In particular, if the bulk conductivity of the wet stock changes, each of the reference and measurement electrodes detects a similar change in resistance, when the water depth is kept constant. However, the reference and measurement electrodes have different sensitivities to changes in water depth. As a result, for the same depth of the aqueous mixture, each of the reference and measurement electrodes will detect a different resistance. 
     The sensitivity of either a reference or measurement electrode cell to the depth of water depends on the spacing between the grounded electrode and the electrode opposite the grounded electrode which is coupled to the impedance element. For instance, the spacing between one of the measurement electrode segments and the grounded elongated electrode determines the sensitivity of that measurement cell. Similarly, the spacing of the dielectric which encircles the reference electrode between one of the reference electrodes and the grounded elongated electrode determines the sensitivity of the reference cell to water depth. 
     When the sensitivity of the reference electrodes to changes in water depth is made sufficiently low, then its output will be dominated by changes in the intrinsic bulk conductivity of the liquid. Its output then can be utilized to compensate for the effects of changes in the intrinsic bulk conductivity of the liquid on the measurement electrode output. The resistance, or its reciprocal the conductance, measured by the sensor is the sum of the contribution due to the intrinsic bulk conductivity of the liquid and of the contribution due to the water weight or depth. This behavior can thus be described in a single equation. It is not necessary that both the measurement and reference electrodes have a different sensitivity to a first property but have relatively the same sensitivity to a second property. Alternatively, the measurement and reference electrodes are constructed so that they have different sensitivities to both the first and second properties. 
     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.