Background of the Invention In the conventional fourdrinier papermaking process, a water slurry, or suspension, of cellulosic fibers (known as the paper"stock") is fed onto the top of the upper run of an endless belt of woven wire and/or synthetic material that travels between two or more rollers. The belt, often referred to as a"forming fabric ", provides a papermaking surface on the upper surface of its upper run which operates as a filter to separate the cellulosic fibers of the paper stock from the aqueous medium, thereby forming a wet paper web. The aqueous medium drains through mesh openings of the forming fabric, known as drainage holes, by gravity alone or with assistance from one or more suction boxes located on the lower surface (i. e., the"machine side ") of the upper run of the fabric.

After leaving the forming section, the paper web is transferred to a press section of the paper machine, in which it is passed through the nips of one or more pairs of pressure rollers covered with another fabric, typically referred to as a "press felt."Pressure from the rollers removes additional moisture from the web ;

the moisture removal is often enhanced by the presence of a"batt"layer on the press felt. The paper is then conveyed to a drier section for further moisture removal. After drying, the paper is ready for secondary processing and packaging.

Notably, each of the aforementioned steps in the fourdrinier process involves the removal of moisture from the paper web. Predictably, the rate at which moisture is removed from the web can significantly impact the properties (such as thickness, strength, gloss, color, smoothness and the like) of the paper being made. For example, if moisture is removed at different rates at various positions across the width of the web, the properties of the paper at these positions can also vary substantially. Also, inconsistent removal at a particular location over time can cause the properties of the paper to vary depending on when it was processed. These problems can be exacerbated for different lots of paper manufactured on different days. As such, it is important to monitor the moisture in the web during processing to assess the rate of removal.

The determination of web moisture can be rather difficult, as the web travels on forming fabrics and press felts at very high speeds (as fast as 7, 000 feet per minute). Interrupting of web travel to take moisture readings can adversely impact production and can also provide inaccurate readings, so typically moisture levels are determined as the web is moving. Also, moisture readings should not contact the web itself in a manner that marks or damages the web.

There have been several techniques proposed to measure moisture in a moving paper web without contacting the web itself. One technique relies on exposure of the paper web to infrared radiation (IR). As exemplified in U. S.

Patent No. 4, 006, 358 to Howarth, IR is passed through the moving paper web, then is separated by wavelength and detected. The ratio of components having 1. 7 and 1. 94 micron wavelengths is used to calculate the moisture content of the web. Other IR-based techniques are discussed in U. S. Patent Nos. 3, 851, 175 to Dahlin et al, 3, 551, 678 to Mitchell, 4, 840, 706 to Campbell, 5, 124, 552 to Anderson, and 5, 276, 327 to Bossen. Unfortunately, these methods have generally turned out to be unsatisfactory, because the IR devices tend to be very

costly to install and maintain ; also, if maintenance is not performed rigorously, results tend to be inaccurate.

Another relatively common technique for measuring moisture is discussed in U. S. No. 4, 569, 069 to LaMarche et al.. With this technique, a device containing a radioactive source (known in the industry as a"gamma gauge") is directed to the moving web. The backscatter from the web is detected and correlated to the mass of the moist web. Once the mass of the web is determined, the moisture content can be calculated. Although the gamma gauge is relatively widely used, it carries the obvious disadvantage of employing radioactive materials. As such, measuring devices are stringently regulated, as are plants employing the devices. Also, any leakage from the device may require the plant to discontinue operations for clean-up. As a result, this technique is unpopular with users.

Other techniques for detecting moisture in a moving web include the use of laser range fmders (U. S. Patent No. 5, 492, 601 to Ostermayer et al), vibrating members that draw air through the web to create an equilibrium condition that is monitored with a humidity sensor (U. S. Patent No. 3, 595, 070 to Smith), probes that measure the electrical resistance of the web (U. S. Patent No.

1, 623, 436 to Peschl), a high frequency resonant electrical circuit (U. S. Patent No. 3, 713, 966 to Lippke), and microwave radiation (U. S. Patent No. 5, 349, 845 to Blom). However, each of these techniques has shortcomings that has limited its use.

Summary of the Invention In view of the foregoing, it is an object of the present invention to provide a method of measuring the moisture in a moving paper web that does not require radioactive materials.

It is also an object of the present invention to provide a method of measuring the moisture in a moving paper web that is accurate, relatively inexpensive and easily used.

These and other objects are satisfied by the present invention, which

is directed to a method for measuring the moisture content of a web of paper stock utilizing the principles of Time Domain Reflectometry (TDR). The method comprises the steps of positioning a first transmission probe having an open end adjacent the web of paper stock ; generating a plurality of electrical pulses with a pulse-generating source ; sequentially transmitting the electrical pulses from the pulse-generating source to the open end of the transmission probe, wherein each of the pulses reflects from the open end to the pulse-generating source ; detecting the duration of each of the pulses after reflection of the pulse from the open end of the probe ; and correlating the duration of each pulse to the moisture content of the web of paper stock. With these steps, the moisture in the paper web can be quickly, easily, and inexpensively determined.

It is preferred that the present method include the step of increasing the duration of the pulses proportionately prior to correlation of the duration data.

Doing so facilitates the manipulation of the data. It is also preferred that the pulse duration data be correlated to moisture content with a quadratic equation relating these parameters.

In one embodiment, the method can be carried out with an apparatus that includes : a pair of open-ended transmission probes ; a pulse generator connected to the open ended transmission probes ; a pulse duration detector ; a scaling circuit connected to the pulse duration detector to increase the duration of pulses detected by the pulse duration detector proportionately ; a converter connected to the scaling circuit for converting the increased duration pulses to numerical values ; and a correlator connected to the converter to correlate the numerical values produced by the converter to moisture content values. These moisture content values can then be displayed on an output device, such as a handheld LCD panel or a personal computer monitor. The apparatus can be a manually operated device, or can be permanently mounted below the paper web as a subcomponent of another paper machine component, such as a foil blade, or can be permanently mounted as a separate component of the paper machine.

Brief Description of the Drawings Figure 1 is a perspective view of a moisture detector of the present

invention.

Figure 2 is a top view of the moisture detector of Figure 1.

Figure 3 is a side view of the moisture detector of Figure 1.

Figure 4 is a perspective view of a moisture detector of the present invention positioned for operation beneath a paper web.

Figure 5 is a schematic diagram illustrating the components of the time domain reflectometry sensor and processor of the moisture detector of Figure 1.

Figure 6 is a chart showing the relationship between pulse duration and moisture content defined by a specific form of Formula I below.

Figure 7 is a schematic diagram illustrating the operative steps employed in using the moisture detector of Figure 1.

Figure 8 is a perspective view of an another embodiment of a moisture detector of the present invention located within a paper machine foil blade.

Figure 9 is an end view of a set of moisture detectors of Figure 8 positioned beneath a paper web.

Detailed Description of the Preferred Embodiment The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein ; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art.

Referring now to the Figures, a moisture detector of the present invention, designated broadly at 10, is illustrated in Figures 1 through 3. The moisture detector 10 comprises a shaft 12, a sensor head 20, a processor 36, and an output device 40. These components are described in greater detail hereinbelow.

The shaft 12 is elongate, cylindrical and hollow ; as illustrated, the shaft 12 includes multiple sections that are connected end to end with couplings 13.

A hand grip 14 is attached to the shaft 12 at one end to facilitate manual manipulation of the moisture detector 10. A head bracket 16 is fixedly mounted to the opposite end of the shaft 12. Preferably, the shaft 12 is between about 48 and 96 inches in length, as such a length can generally be easily handled manually by most operators but has sufficient length to enable an operator to obtain moisture readings at any transverse position on a moving paper web. Those skilled in this art will appreciate that other shaft configurations are also suitable for use with the present invention. For example, the shaft may have a different length or cross section, may telescope to adjust in length, may be nonlinear rather than straight, or may lack a hand grip.

The sensor head 20 comprises a head panel 21 and a TDR sensor 30. The head panel 21 is pivotally mounted to the head bracket 16 at a pivot 20a such that the head panel 21 is free to pivot about an axis normal to the longitudinal axis of the shaft 12. Such pivotal movement can improve sensor placement, but the head panel 21 may be fixed to the shaft 12 as desired. The head panel 21 is illustratively and preferably formed of a polymeric material, with ultra high molecular weight polyethylene being a particularly preferred material ; other nonconducting, low dielectric materials may also be suitable for use with the present invention.

The TDR sensor 30 is mounted generally on the lower surface of the head panel 21, with the exception of a pair of open-ended transmission probes 22a, 22b that are mounted on the upper surface of the head panel 21. The transmission probes 22a, 22b are embedded in and reside on the upper surface of the head panel 21 and are substantially parallel to each other and to the longitudinal axis of the shaft 12, with their free or"open"ends extending away from the hand grip 14. As used herein, an"open-ended"probe is one that forms a single electrical pathway ; it contains no circuits or"loops"therein such that an electrical signal can follow multiple pathways. Instead, an electrical signal travels the length of the probe to reach the"open"end, then returns, or"reflects,"from the open end along the same

pathway. Those skilled in this art will recognize that, although the probes 22a, 22b are linear, other configurations in which the probes are parallel to each other, such as serpentine, arcuate, and"zig-zag"layouts, may also be used with the present invention.

The probes 22a, 22b are formed of an electrically conductive material (such as a metal), with stainless steel being a preferred material. It is also preferred that the probes 22a, 22b be between about 6 and 12 inches in length, between about 0. 125 and 0. 5 inches in thickness, and be separated from each other by a gap of between about 0. 5 and 1. 0 inches.

Referring now to Figure 5, the TDR sensor 30 includes a pulse generator 32 and a scaling circuit 34. These components enable the TDR sensor 30 to utilize TDR in the detection of moisture. The TDR technique is discussed generally in Bilskie, Using Dielectric Properties to Measure Water Content, Sensors 26 (July 1997) and in U. S. Patent No. 5, 136, 249 to White, the disclosure of which is hereby incorporated herein by reference. TDR is based on the reducing effect that dielectric materials have on the velocity of electromagnetic energy propagating on a waveguide element (such as the probes 22a, 22b). By exposing such electromagnetic energy to a highly dielectric material (like water) and detecting the changes in the velocity of the electromagnetic energy, a correlation can be drawn between the velocity change and the quantity of water present in the environment around the waveguide.

The pulse generator 32, which is mounted to the underside of the head panel 21, is connected to the probes 22a, 22b as they extend through apertures 23a, 23b in the head panel 21. The pulse generator 32 generates electrical pulses that travel from the pulse generator 32 along the paths defined by the probes 22a, 22b to the open ends of the probes 22a, 22b. The pulses then return, or"reflect", along the same paths. The pulse generator 32 is oscillatory in that it is configured to (a) generate a pulse that travels to the open end of the probe 22a and reflects therefrom back to the pulse generator 32, then (b) reverse polarity and transmit a pulse that travels to the open end of the pulse 22b and reflects therefrom. The pulse generator 32 preferably generates pulses that are between

about 1. 3 and 4 nanoseconds in length at a frequency of between about 250 and 750 MHz, as these properties have proven to be particularly effective in the detection of moisture.

The scaling circuit 34 is connected with the pulse generator 32 and receives each pulse as it reflects from the open end of its respective probe 22a, 22b. The scaling circuit 34 (also known as a"flip-flop"circuit) is configured to reduce the effective frequency (i. e., increase the effective duration, or"pulse width ") of the pulses to facilitate manipulation thereof by the processor 36. One exemplary configuration operates by counting a predetermined number of pulses (preferably all positive pulses from the probe 22a or all negative pulses from the probe 22b) with a counter, then summing the durations of those pulses and providing the sum as an output signal ; however, those skilled in this art will recognize that other scaling circuit configurations that proportionately increase the pulse duration may also be employed with the present invention. It is preferred that the scaling circuit 34 reduce the pulses to a frequency of between about 800 and 1, 300 Hz to facilitate data manipulation ; this corresponds to an increased pulse duration of between about 384 and 625 microseconds (assuming that one positive pulse is one-half the square wave period). It is also preferred that the scaling circuit 34 provide as output increased duration pulses in the form of a square wave.

It will also be understood that other devices capable of detecting and manipulating pulse duration may also be suitable for use with the present invention, and that some systems may elect to employ a duration detector of a different configuration and omit the scaling function performed by the scaling circuit entirely.

An exemplary TDR sensor 30 is Model No. CS615, manufactured by Campbell Scientific, Inc. (Logan, Utah), although other pulse generators known to those skilled in this art can also be employed with the present invention.

The scaling circuit 34 is connected to a head cable 24 that extends from the TDR sensor 30 at head end of the shaft 12 through the lumen of the shaft 12 to emerge from the hand grip 14 as a tail cable 26. An actuation switch 28 is attached to the shaft 12 adjacent the hand grip 14 and is electrically connected to

the tail cable 26. After emerging from the hand grip 14, the tail cable 26 is connected to the processor 36.

The processor 36 includes a converter 37, a memory 38 and a correlator 39. The converter 37 is configured to measure the duration of each increased duration pulse and convert it into a numerical value. In the illustrated embodiment, the converter 37 is a microprocessor-based timer/counter, although those skilled in this art will recognize that other devices capable of detecting pulse duration and converting it to a numerical form, such as discrete digital timer/counter combinations, can also be used with the present invention.

Duration information about each increased duration pulse is transferred from the converter 37 and stored numerically in the memory 38 until a predetermined number of pulses (for example, 256 pulses) is received and stored.

The memory 38 is configured to erase the pulse data after the predetermined number of pulse durations are stored and transferred to correlator 39. In addition, the memory 38 may be omitted entirely, and the increased duration pulse data may be transferred directly to the correlator 39 for conversion.

The correlator 39 is configured to convert the numerical data from the memory 38 from a pulse duration quantity to a moisture content quantity. To do so, the correlator 39 is configured to average the numerical values stored in the memory 38 that correspond to increased pulse durations, then to apply a mathematical equation to the average increased pulse duration in order to calculate the moisture content of the moving web. An exemplary quadratic equation relating pulse duration to moisture content is set forth below as Formula I : y = Ax'+ Bx (I) wherein"y"is moisture content in g/m,"x"is pulse duration (of the increased duration pulses produced by the scaling circuit 34) in microseconds minus the duration in microseconds when no moisture is present, and A and B are numerical constants. A graph of this relationship between pulse duration and moisture content is shown in Figure 6, in which A is 0. 2158 and B is 15. 58. Alternatively,

the correlator 39 may comprise a two dimensional array of duration values and corresponding moisture values. Those skilled in this art will recognize that other converter configurations may also be suitable for use with the present invention.

The correlator 39 is connected to the output device 40, which displays or otherwise provides moisture content data to an operator. The output device 40 can be a personal computer monitor, an LCD panel, a printer, a plotter, or any other output device known by those skilled in this art to be suitable for providing output data. The output device 40 may be directly connected to the correlator 39 or may be separate therefrom, in which case the moisture content data may be transferred to the output device by conventional means, such as by the downloading of a computer disk.

As illustrated in Figure 1, the converter 36 and output device 40 can be contained within a single integral unit, and preferably a single unit that can be carried in a pocket or on the belt of an operator. Alternatively, the converter 36 and the output device 40 can be contained in separate units.

Referring now to Figure 7, in operation the moisture detector 10 is first positioned beneath a moving web W of paper stock with the probes 22a, 22b adjacent the lower surface of the papermaker's forming fabric F that carries the web W (see Figure 4 for positioning of the moisture detector 10). Alternatively, the moisture detector 10 may be used with other papermaking fabrics, such as press felts, or may even be placed directly on the web W as desired. Preferably, the probes 22a, 22b are positioned to contact the underside of the fabric F.

To initiate operation, the actuation switch 28 is activated. Pulses are generated by the pulse generator 32 that travel to the open ends of, alternately, the probes 22a, 22b. As the pulses travel, they travel adjacent the moving web W and experience dielectric losses due to polarization and electrical conduction of the water present in and draining from the web W. The extent of these dielectric losses (which are dependent on the amount of moisture present in the web W) reduces the velocity of the pulses. As the pulses return, they are processed by the scaling circuit 34, which produces increased duration pulses that travel through the head and tail cables 24,26 to the processor 36.

As the increased duration pulses reach the processor 36, their durations are converted to numerical values in the converter 37 and stored in the memory 38. Once 256 reduced frequency pulse durations have been stored in the memory 38, the durations are transferred to the correlator 39, where they are averaged. The moisture content is calculated from the average duration value using Formula I or another suitable formula that correlates pulse duration to moisture content. This value is then passed to the output device 40 for display.

Those skilled in this art will recognize that, although the operative steps described above are preferred, other techniques for carrying out the present invention may also be employed. For example, a moisture content value can be produced with more or fewer pulses (actually as few as one pulse). Of course, a different correlation technique, such as one in which the gathered data is compared to pre-stored data in the correlator to arrivât a moisture content, may also be used.

In addition, although the use of two substantially parallel open ended probes is preferred, the present method can be performed with a single open ended probe.

An additional embodiment of the present invention is illustrated in Figures 8 and 9, which show a number of moisture detectors 60 having open- ended probes 62 mounted in foil blade segments 64 that are fixed relative to and in contact with the papermaker's fabric F that supports the web W. Each of the foil blade segments 64 includes a deflecting edge 66 that extends from the body 65 of the foil blade segment 64. The foil blade segment 64 encourages the removal of water from the moving paper web W ; the deflecting edge 66 acts as a"squeegee" to force water to drain downwardly from the web W. The foil blade segment should be formed of the types of materials described hereinabove for the head panel 21 of the moisture detector 10.

As illustrated in Figure 9, the moisture detectors 60 are included as segments in a multi-piece foil blade 70 mounted on a frame 74. As illustrated, four moisture detectors 60 are included across the width of the web W ; these are separated by non-detecting foil blade segments 72. Each of the moisture detectors 60 is connected to a controller 76 and is configured and operated in a manner similar to that of the moisture detector 10 illustrated in Figures 1 through 7.

However, this embodiment enables the operator to obtain moisture content readings at multiple locations across the width of the web W simultaneously, and also ensures that the position of the probes 62 relative to the web W is constant to minimize differences in moisture measurement.

The foregoing discussion demonstrates that the present invention can be used to accurately detect moisture content of a moving paper web. The detector can be positioned manually or fixed into place, and can be provided as a separate device, operated within an existing component like a foil blade, or mounted in place without the foil blade. Measurements can be taken and displayed quickly and easily, and have proven to be accurate. Finally, and importantly, this method lacks the safety and regulatory concerns associated with the radioactive materials used in a gamma gauge.

The foregoing is illustrative'ofthe present invention and is not to be construed as limiting thereof. Although exemplary embodiments of this invention have been described, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of this invention. Accordingly, all such modifications are intended to be included within the scope of this invention as defined in the claims. The invention is defined by the following claims, with equivalents of the claims to be included therein.