Measuring and testing – Sheet – woven fabric or fiber
Reexamination Certificate
1998-04-06
2002-01-29
Williams, Hezron (Department: 2856)
Measuring and testing
Sheet, woven fabric or fiber
C073S073000
Reexamination Certificate
active
06341522
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to systems for controlling continuous sheetmaking systems and, more specifically, to sensors for measuring the fiber weight of wetstock in a papermaking machine.
2. State of the Art
In the art of modern high-speed papermaking, it is well known to continuously measure certain properties of the paper material in order to monitor the quality of the finished product. These on-line measurements often include basis weight, moisture content, and sheet caliper (i.e., thickness). The measurements can be used for controlling process variables with the goal of maintaining output quality and minimizing the quantity of product that must be rejected due to upsets in the manufacturing process.
The on-line sheet property measurements are often accomplished by scanning sensors that periodically traverse the sheet material from edge to edge. For example, a high-speed scanning sensor may complete a scan in a period as short as twenty seconds, with measurements being read from the sensor at about 50 milliseconds intervals. It is also know that a series of stationary sensors can be used to make similar on-line measurements.
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 suction through the fabric. The web is then transferred to the pressing section where more water is removed by pressure and vacuum. The web next enters the dryer section where steam heated dryers complete the drying process. The paper machine is, in essence, a de-watering, i.e., water removal, system. A typical forming section of a papermaking machine includes an endless traveling papermaking mesh fabric or wire which travels over a series of water removal elements such as table rolls, foils, vacuum foils, and suction boxes. As the material travels on the mesh fabric over the series of water removal elements, there is a distinct line of demarcation showing a change in the state of the stock from an extremely wet state to a relatively dryer state. This visible line of demarcation (referred to as the dry line) is characterized in that one side of the dry line has a glossy appearance (i.e., wet state) and the other side of the line has a non-glossy appearance (i.e., relatively dry state). The stock is carried on the top surface of the papermaking fabric and is de-watered as the stock travels over the successive de-watering elements to form a sheet of paper. Finally, the wet sheet is transferred to the press section of the papermaking machine where enough water is removed to form a sheet of paper. Other, papermaking devices well known in the art are described for example in U.S. Pat. No. 5,400,258.
Many factors influence the rate at which water is removed which ultimately affects the quality of the paper produced. As is apparent, it would be advantageous to monitor the dynamic process so as to, among other things, predict and control the dry stock weight of the paper that is produced.
It is conventional to measure the moisture content on leaving the main dryer section or at the take up reel employing scanning sensors. Such measurement may be used to adjust the machine operation toward achieving desired parameters. One technique for measuring moisture content is to utilize the absorption spectrum of water in the infra-red. Monitoring or gauge apparatus for this purpose is commonly in use. Such apparatus conventionally uses either a fixed gauge or a gauge mounted on a scanning head which is repetitively scanned transversely across the web at the exit from the dryer section and/or upon entry to the take up reel, as required by the individual machines. The gauges typically use a broad-band infra-red source and one or more detectors with the wavelength of interest being selected by a narrow-band filter, for example, an interference type filter. The gauges used fall into two main types: the transmissive type in which the source and detector are on opposite sides of the web and, in a scanning gauge, are scanned in synchronism across it, and the scatter type (sometimes called “reflective” type) in which the source and detector are in a single head on one side of the web, the detector responding to the amount of source radiation scattered from the web.
SUMMARY OF THE INVENTION
The present invention, in general, is a sensor imbedded roller which, in one embodiment, is used in a measurement apparatus in a sheetmaking machine. In a preferred embodiment, the measurement apparatus includes a fixed impedance element coupled in series with a detection cell in the sensor which is coupled between an input signal and a reference potential (e.g., ground) and which has a variable impedance. The fixed impedance element and the detection cell form a voltage divider network such that changes in impedance of the detection cell results in changes in voltage on the output of the measurement system. The impedance of the detection cell represents the impedance of the physical configuration of electrodes within the sensor and the material residing between and in close proximity to the electrodes. The impedance relates to the property of the material being measured.
In one embodiment, the measurement apparatus is used to measure the conductivity of an aqueous mixture (referred to as wetstock) in a sheetmaking system. The conductivity of the wetstock is directly proportional to the total water weight within the wetstock, consequently providing information which can be used to monitor and control the quality of the paper sheet produced by the papermaking system. In this embodiment the sensor imbedded roller is positioned in the wet end of the sheetmaking system so as to detect conductivity changes.
In another embodiment, the measurement apparatus is used to measure the weight of dry paper sheet in a sheetmaking machine. In this application, the conductivity is negligible and the capacitive impedance is inversely proportional to the dielectric constant and the amount of paper between the electrodes of the measurement apparatus. In this embodiment the sensor embedded roller is positioned in the dry end of the sheetmaking system so as to detect capacitive impedance changes.
The fixed impedance element can be embodied as a resistor, capacitor, inductor or a combination of the three and the input signal is an analog signal. In the embodiment in which the impedance element is an inductor, the impedance of the inductor can be selected to be a particular magnitude by setting the frequency of the input signal so that the impedance of the fixed impedance element can be set to the same range as the impedance of the sensor for optimum sensor sensitivity. Hence, in the case in which the impedance of the sensor varies due to fluctuations in operating conditions of the system or the material being sensed, the impedance of the inductor can be customized to match the sensor impedance without any hardware changes.
The sensor comprises an array of electrodes having a particular configuration which forms measurement cells, each cell being independently coupled to an input signal provided by a signal generator through the impedance element. In a preferred embodiment used to detect the conductivity of an aqueous fibrous mixture, the impedance elements are implemented as resistive elements. Each cell forms a voltage divider network made-up of the resistive element coupled between the signal generator and portions or segments of electrodes within a given cell and of a resistance resulting from the effective water resistance between the electrode portions and segments. The output of each cell is taken from an electrode segment or portion, i.e., the point between the resistive element and the cell. As the conductance of the aqueous mixture changes so does the output voltage of the cell. The output voltage of each cell is coupled to a detector which, in one embodiment, includes circuitry for enhancing the signal such as an amplifier for amplifyin
Chase Lee
Goss John D.
Burns Doane Swecker & Mathis
Cygan Michael
Measurex Corporation
Miologos Anthony
Williams Hezron
LandOfFree
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