Noncontacting proximity gage utilizing induced eddy currents,having improved dynamic response and interference discrimination

Claim 2

An electrical proximity measuring instrument responsive to induced eddy currents comprising in combination an oscillator, a primary transmission line, a differential sensing probe having a primary coil and two secondary coils, two secondary transmission lines, two tuned carrier-level detectors, means bypassing and filtering radio frequency, and means balancing and algebraically adding two opposing signals; said oscillator producing a carrier current at a frequency sufficient for good envelope resolution when said carrier current is under very rapid modulation, said primary transmission line coupling an output of said oscillator to said primary coil of said sensing probe, said two secondary coils closely coupling by inductance to said primary coil, all of said coils having low internal capacitance, said two secondary coils connecting individually to said two secondary transmission lines, all of said coils having impedances of the same order as all of said transmission lines and functioning together as a relatively low impedance differential coupler along said transmission lines in response to said induced eddy currents, said secondary transmission lines functioning as dual extensions of said primary transmission line by means of said primary and secondary coils and carrying differentiaL carrier currents resulting from said eddy currents, each of said secondary transmission lines individually terminating at an input of each of said carrier-level detectors, said carrier-level detectors being tuned to the frequency of said oscillator, said carrierlevel detector''s inputs being optimized for best transmission line termination and least characteristic resonance effect from parametric variations of said secondary coils, said means bypassing and filtering radio frequency being attached to the outputs of said tuned detectors, said tuned detectors producing opposing DC voltages proportional to the instantaneous carrier current and during the same half cycle of said carrier current, and said opposing DC voltages being balanced and algebraically added by said means balancing and algebraically adding to produce a final output signal proportional to the effect of said eddy currents at said sensing probe and unresponsive to common-mode signals and carrier frequencies outside the selected passband.

Claim 3

An electrical proximity measuring instrument responsive to induced eddy currents comprising in combination an oscillator, a primary transmission line, a differential proximity sensing probe having at least one primary coil and and at least two secondary coils, dual short secondary transmission lines, a network for balancing and cancelling common-mode carrier currents, a main secondary transmission line, a logarithmic amplifier, a carrier-level detector, a radiofrequency filter, and an output; said oscillator producing a carrier current of sufficient frequency for good envelope resolution when said carrier current is modulated at very high rates, said primary transmission line connecting an output of said oscillator to said at least one primary coil, said at least two secondary coils individually connecting to said dual short secondary transmission lines, said dual short secondary transmission lines being no longer than a few wavelengths of said carrier current, all of said coils having relatively low impedances of the same order as all of said transmission lines, all of said coils having relatively low internal capacitance, said at least two secondary coils closely coupling by inductance to said at least one primary coil and functioning therewith as a differential coupler along said transmission lines in response to said induced eddy currents, said dual short secondary transmission lines functioning as dual extensions of said primary transmission line by means of said differential coupler, each of said dual short transmission lines individually terminating at individual inputs of said network for balancing and cancelling common-mode carrier currents, said network having means for adjusting the phase angle and amplitudes of incoming carrier currents, said network having an output delivering a carrier current proportional to a differential of said carrier currents arriving at said individual inputs but effectively terminating the carrier current energy arriving in the common mode, said main secondary transmission line having any desired practical length and connecting said output of said network to an input of said logarithmic amplifier, said logarithmic amplifier being tuned to the frequency of said oscillator and having a frequency-bandwidth sufficient for said carrier current and modulation sidebands, said carrier level detector receiving an output of said logarithmic amplifier and producing therefrom a DC signal proportional to the instantaneous envelope amplitude of said logarithmic amplifier output, adjustable means feeding back a sample of said instantaneous envelope amplitude to a gain-sensitive area of said logarithmic amplifier, said radiofrequency filter removing any remaining carrier current ripple from said DC signal, and said output delivering a final output signal proportional to the instantaneous distance between said proximity sensing probe and an object under observation.

Claim 4

An electrical proximity measuring instrument responsive to induced eddy currents comprising in combination an oscillator, a primary transmission line, a differential proximity sensing probe having at least one primary coil and and at least two secondary coils, dual short secondary transmission lines, a network for balancing and cancelling common-mode carrier currents, a main secondary transmission line, a logarithmic amplifier, a carrier-level detector, a radiofrequency filter, and an output; said oscillator producing a carrier current of sufficient frequency for good envelope resolution when said carrier current is modulated at very high rates, said primary transmission line connecting an output of said oscillator to said at least one primary coil, said at least two secondary coils individually connecting to said dual short secondary transmission lines, said dual short secondary transmission lines being no longer than a few wavelengths of said carrier current, all of said coils having relatively low impedances of the same order as all of said transmission lines, all of said coils having relatively low internal capacitance, said at least two secondary coils closely coupling by inductance to said at least one primary coil and functioning therewith as a differential coupler along said transmission lines in response to said induced eddy currents, said dual short secondary transmission lines functioning as dual extensions of said primary transmission line by means of said differential coupler, each of said dual short transmission lines individually terminating at individual inputs of said network for balancing and cancelling common-mode carrier currents, said network having means for adjusting the phase angle and amplitudes of incoming carrier currents, said network having an output delivering a carrier current proportional to a differential of said carrier currents arriving at said individual inputs but effectively terminating the carrier current energy arriving in the common mode, said main secondary transmission line having any desired practical length and connecting said output of said network to an input of said logarithmic amplifier, said logarithmic amplifier being tuned to the frequency of said oscillator and having a frequency-bandwidth sufficient for said carrier current and modulation sidebands, said carrier level detector receiving an output of said logarithmic amplifier and producing therefrom a DC signal proportional to the instantaneous envelope amplitude of said logarithmic amplifier output, adjustable means feeding back a sample of said instantaneous envelope amplitude to a gain-sensitive area of said logarithmic amplifier, said radiofrequency filter removing any remaining carrier current ripple from said DC signal, and said output delivering a final output signal proportional to the instantaneous distance between said proximity sensing probe and an object under observation.

Claim 5

The instrument defined in claim 2 where said network for balancing anD cancelling said common-mode carrier current contains a delay line for phase inversion and a potentiometer serving as a null selector and common-mode terminating resistor.

Claim 6

The instrument defined in claim 2 where said network for balancing and cancelling said common-mode carrier contains an inductive coupler having at least two primary windings and at least one secondary winding, said windings having adjustable coupling by inductance to provide cancellation of common-mode signals and transmission of differential-mode signals.

Claim 7

The instrument defined in claim 2 where the impedance of said primary winding is made lower than that of said primary transmission line by the use of a tuned stub.

Claim 8

An electrical proximity-measuring instrument responsive to induced eddy currents comprising in combination an oscillator, a primary transmission line, a differential sensing probe having at least one primary coil and at least two secondary coils, a secondary transmission line, a logarithmic amplifier, a carrier level detector, a radiofrequency filter, and an output; said oscillator producing a carrier current of sufficient frequency for good envelope resolution when said carrier current is under very rapid modulation, said primary transmission line connecting an output of said oscillator to said at least one primary coil, said at least two secondary coils being connected in series opposition and having an output connected to said secondary transmission line, said secondary coils tightly coupling to said at least one primary coil by inductance, said secondary coils producing an output proportional to any unbalance in said coupling to said at least one primary coil, all of said coils being of low impedance and absorbing carrier current energy and serving as termination for said primary transmission line with the exception that differential energy is coupled out to said secondary transmission line, said differential sensing probe having means to adjust said coupling of said secondary coils, said secondary transmission line being of any practical length and terminating at an input of said logarithmic amplifier, said logarithmic amplifier being tuned to the frequency of said oscillator and having a frequency-bandwidth sufficient for said carrier current and modulation sidebands, said carrier level detector receiving an output of said logarithmic amplifier and producing therefrom a DC signal proportional to the instantaneous envelope amplitude of said logarithmic amplifier output, adjustable means feeding back a sample of said instantaneous envelope amplitude to a gain-sensitive area of said logarithmic amplifier, said radiofrequency filter removing remaining carrier current from said DC signal, and said output delivering a final output signal proportional to the instantaneous distance between said proximity probe and an object under observation.

Claim 9

An electrical proximity-measuring instrument responsive to induced eddy currents comprising in combination an oscillator, a primary transmission line, a differential sensing probe having at least one primary coil and at least two secondary coils, a secondary transmission line, a logarithmic amplifier, a carrier level detector, a radiofrequency filter, and an output; said oscillator producing a carrier current of sufficient frequency for good envelope resolution when said carrier current is under very rapid modulation, said primary transmission line connecting an output of said oscillator to said at least one primary coil, said at least two secondary coils being connected in series opposition and having an output connected to said secondary transmission line, said secondary coils tightly coupling to said at least one primary coil by inductance, said secondary coils producing an output proportional to any unbalance in said coupling to said at least one primary coil, all of said coils being of low impedance and absorbing carrier current energy and serving as termination for said primary transmission line with the exception that differential energy is coupled out to said secondary transmission line, said differential sensing probe having means to adjust said coupling of said secondary coils, said secondary transmission line being of any practical length and terminating at an input of said logarithmic amplifier, said logarithmic amplifier being tuned to the frequency of said oscillator and having a frequency-bandwidth sufficient for said carrier current and modulation sidebands, said carrier level detector receiving an output of said logarithmic amplifier and producing therefrom a DC signal proportional to the instantaneous envelope amplitude of said logarithmic amplifier output, adjustable means feeding back a sample of said instantaneous envelope amplitude to a gain-sensitive area of said logarithmic amplifier, said radiofrequency filter removing remaining carrier current from said DC signal, and said output delivering a final output signal proportional to the instantaneous distance between said proximity probe and an object under observation.

Claim 10

The instrument defined in claim 6 where said means to adjust said coupling of said secondary coils is an adjustable shorting ring.

Claim 11

The instrument defined in claim 6 where the carrier current termination capability of said at least one primary coil is improved by placing said coil at one end of a tuned stub.