Guarded capacitance probe and related measurement circuit
First Claim
1. A capacitance probe for continuously sensing a fluid'"'"'s level within a container over a predetermined height range, comprising:
- first and second spaced-apart insulated electrodes located within the container, each electrode having a capacitive area of a predetermined length and width such that the length of each electrode extends over the predetermined height range, to allow continuous sensing of the fluid'"'"'s level; and
an insulated guard conductor symmetrically located between the capacitive areas of the first and second electrodes, for modifying an electric field surrounding the first and second electrodes.
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Accused Products
Abstract
A continuous liquid level measuring system having a guarded capacitance probe and related drive circuit for providing accurate level measurements, even when measuring the level of ionic and highly viscous fluids. The capacitance probe has two parallel electrodes, each constructed of a half-tube section, that are separated from each other by an insulated guard wire. The probe is excited by a 1 MHz excitation signal applied across the electrodes. The current through the electrodes is a function of the capacitance between the electrodes, which is affected by the relative permitivity of the surrounding liquid. The effects of highly viscous ionic liquids that adhere to the probe are reduced by exciting the guard wire with a guard signal that is 180° out of phase from the probe'"'"'s excitation signal. The guard signal changes the electric field existing in the space adjacent the junctions of the two electrodes to reduce any liquid coating effects on the level measurement. The capacitance probe is connected to a capacitance measurement circuit through at least one 1:1 ratio pulse transformer for isolation, effectively eliminating the effects of noise currents on the probe'"'"'s capacitance. Preferably, a second pulse transformer is provided to isolate the guard wire.
73 Citations
20 Claims
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1. A capacitance probe for continuously sensing a fluid'"'"'s level within a container over a predetermined height range, comprising:
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first and second spaced-apart insulated electrodes located within the container, each electrode having a capacitive area of a predetermined length and width such that the length of each electrode extends over the predetermined height range, to allow continuous sensing of the fluid'"'"'s level; and an insulated guard conductor symmetrically located between the capacitive areas of the first and second electrodes, for modifying an electric field surrounding the first and second electrodes. - View Dependent Claims (2)
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3. A capacitance probe for continuously sensing a fluid'"'"'s level within a container over a predetermined height range, comprising:
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first and second spaced-apart insulated electrodes located within the container, wherein the first and second electrodes are first and second half-tube sections configured to form an elongated cylinder, and each electrode has a capacitive area of a predetermined length and width such that the length of each electrode extends over the predetermined height range, to allow continuous sensing of the fluid'"'"'s level; and an insulated guard element located between the first and second electrodes, for modifying an electric field surrounding the first and second electrodes, wherein the guard element is an insulated wire that extends along the length of one side of the elongated cylinder, between a first set of adjacent edges of the two half-tube sections, and that returns along an opposite side of the cylinder, between a second set of adjacent edges of the two half-tube sections. - View Dependent Claims (4)
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5. A capacitance probe for continuously sensing a fluid'"'"'s level within a container over a predetermined height range, comprising;
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first and second spaced-apart insulated electrodes located within the container, each electrode having a capacitive area of a predetermined length and width such that the length of each electrode extends over the predetermined height range, to allow continuous sensing of the fluid'"'"'s level; an insulated guard element located between the first and second electrodes, for modifying an electric field surrounding the first and second electrodes; and a capacitance measurement circuit having a first circuit terminal for providing an excitation signal with respect to ground, a second circuit terminal for providing a guard signal with respect to ground, and a third circuit terminal for providing an output signal with respect to ground, wherein the first circuit terminal is coupled to the first and second electrodes such that the excitation signal is applied across the first and second electrodes to induce an electric field between and surrounding the first and second electrodes, the second circuit terminal is coupled to the guard element such that the guard signal is applied to the guard element, and the output signal is based on the fluid'"'"'s effect on the electric field. - View Dependent Claims (6, 7, 8, 9)
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10. A circuit for measuring the capacitance of a capacitance probe, comprising:
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an excitation circuit having a drive terminal and an output terminal, wherein the excitation circuit provides an excitation signal with respect to ground at the drive terminal for exciting the capacitance probe, and further provides an output signal at the output terminal based on the capacitance probe'"'"'s effect on the excitation signal; and a first isolation transformer having isolated primary and secondary windings, wherein the primary winding is coupled between the drive terminal and a ground terminal and the secondary winding is coupled across the capacitance probe, such that the capacitance probe is dc isolated from the drive terminal and the ground terminal.
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11. A circuit for measuring the capacitance of a capacitance probe, comprising:
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an excitation circuit having a drive terminal, an output terminal, an operational amplifier having an inverting input terminal, a noninverting input terminal, and an output terminal, wherein the inverting input terminal is coupled to the drive terminal and the operational amplifier'"'"'s output terminal is coupled to the excitation circuit'"'"'s output terminal, an oscillator connected to the noninverting input of the operational amplifier, and a feedback capacitor connected between the operational amplifier'"'"'s inverting input terminal and the excitation circuit'"'"'s output terminal; wherein the excitation circuit provides an excitation signal with respect to ground at the drive terminal for exciting the capacitance probe, and further provides an output signal at the output terminal based on the capacitance probe'"'"'s effect on the excitation signal; and a first isolation transformer having isolated primary and secondary windings, wherein the primary winding is coupled between the drive terminal and a ground terminal and the secondary winding is coupled across the capacitance probe. - View Dependent Claims (12, 13, 14, 15, 16, 17, 18, 19)
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20. A fluid level measurement system for continuously sensing a fluid'"'"'s level over a predetermined height range, comprising:
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first and second spaced-apart insulated half-tube section electrodes spaced apart and configured to form an elongated cylindrical capacitance probe, each electrode having a capacitive area of a predetermined length and width such that the length of each electrode extends over the predetermined range to allow continuous sensing of the fluid'"'"'s level; an insulated guard element located between the electrodes such that the guard element extends along the length of one side of the probe between a first set of adjacent edges of the two half-tube section electrodes, and that returns on an opposite side of the probe between a second set of adjacent edges of the two half-tube sections, to modify an electric field surrounding the first and second electrodes; a capacitance measurement circuit having a drive terminal for providing an excitation signal with respect to ground, a guard terminal for providing a guard signal with respect to ground, and a output terminal for providing an output signal with respect to ground based on the probe'"'"'s response to the excitation signal; and a first and second isolation transformers, each transformer having isolated primary and secondary windings and each winding having first and second winding terminals, wherein the primary winding of the first isolation transformer is coupled between the drive terminal and a ground terminal, the secondary winding of the first isolation transformer has its first and second winding terminals coupled to the first and second electrodes, respectively, the primary winding of the second isolation transformer is connected between the guard terminal and the ground terminal, and the secondary winding of the second isolation transformer has its first winding terminal coupled to the guard element and its second winding terminal connected to the second secondary winding terminal of the first isolation transformer such that the excitation signal and the guard signal are 180°
out of phase with each other.
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Specification