Single coil conductance measuring apparatus
First Claim
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1. A method of quantitative measurement of the resistance of a remote substance using a single coil comprising the steps of:
- (a) exciting the coil with an electrical signal to produce an electromagnetic field enveloping the remote substance;
(b) allowing the frequency of the electric signal to be at the resonant frequency of the coil as affected by the remote substance;
(c) adjusting a control signal controlling the amplitude of the electrical signal so that the electric signal at the coil substantially equals a predetermined standard level; and
(d) deriving a resistance value of the remote substance based on the adjusted control signal independent of any deformation of the coil, wherein the resistance value of the remote substance is only substantially indicative of a real portion of an impedance of the remote substance and is not substantially indicative of any reactance or inductance of the remote substance or the coil.
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Abstract
A conductance sensor uses a high impedance feedback oscillator to drive a single coil at the natural resonant frequency of the coil coupled to a measured material. The amplitude of the driving signal is also feedback controlled. The error signal of the amplitude control yields a measurement of resistance of the measured material. The combination of frequency adjustment and amplitude control eliminates non-resistance impedance effects and amplitude-dependent effects allowing high precision, high sensitivity measurement.
40 Citations
12 Claims
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1. A method of quantitative measurement of the resistance of a remote substance using a single coil comprising the steps of:
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(a) exciting the coil with an electrical signal to produce an electromagnetic field enveloping the remote substance;
(b) allowing the frequency of the electric signal to be at the resonant frequency of the coil as affected by the remote substance;
(c) adjusting a control signal controlling the amplitude of the electrical signal so that the electric signal at the coil substantially equals a predetermined standard level; and
(d) deriving a resistance value of the remote substance based on the adjusted control signal independent of any deformation of the coil, wherein the resistance value of the remote substance is only substantially indicative of a real portion of an impedance of the remote substance and is not substantially indicative of any reactance or inductance of the remote substance or the coil. - View Dependent Claims (2, 3, 4, 5)
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6. A sensor for remote measurement of resistance of a sample material comprising:
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a conductive coil;
an oscillator connected to the conductive coil to;
(1) drive the conductive coil with an oscillating current;
(2) to receive feedback from the conductive coil to allow the frequency of the oscillating current to be at a varying resonant frequency of the conductive coil as coupled to the sample; and
an amplitude control circuit connected to the oscillator to adjust the oscillating current; and
an output providing an error signal indicating the adjustment of the amplitude control circuit, the error signal providing a measure of the effective resistance of the sample, wherein the measure of the effective resistance of the sample can be determined independent of any deformation of the conductive coil, and wherein the effective resistance of the sample is only substantially indicative of a real portion of an impedance of the sample and is not substantially indicative of any reactance or inductance of the sample or the conductive coil. - View Dependent Claims (7, 8, 9, 10, 11, 12)
wherein the error signal is the gain control input.
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9. The sensor of claim 8 wherein the gain control input is provided by a rectifier, rectifying the oscillating current synchronously with the oscillating current.
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10. The sensor of claim 9 wherein the rectifier is a multiplier multiplying the oscillating current by itself.
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11. The sensor of claim 6 including a guide for positioning the measured material in a predetermined relationship with respect to the coil.
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12. The sensor of claim 6 wherein the guide is a non-conductive tube through which a liquid may pass.
Specification