Frequency-modulated eddy-current proximity gage

Claim 1

A system for making dimensional measurements by means of induced eddy-currents, comprising in combination a transducer comprising a sensing coil, at least one capacitor connected with said sensing coil forming a resonant circuit, at least one active component connected with said resonant circuit forming a variable oscillator whose frequency is determined by said resonant circuit and varied in response to the proximity of said sensing coil to an electrically conducting object; said transducer producing a frequency-varying output; a first reference oscillator producing a first reference frequency approximately equal to the frequency of said frequency-varying output when said sensing coil is at its closest proximity to an electrically conducting object; a first frequency subtracting means receiving said frequencyvarying output and said first reference frequency, and producing therefrom a first difference frequency; a second reference oscillator producing a second reference frequency approximately equal to the frequency of said frequency-varying output when said sensing coil is remote from any electrically conducting object; a second frequency subtracting means receiving said frequencyvarying output and said second reference frequency and producing therefrom a second difference frequency; a scaler receiving said second difference frequency and producing therefrom an output frequency proportional thereto; and a digital counting means receiving the output frequency of said scaler and gating on in proportion to the period thereof; said digital counting means also receiving said first difference frequency and counting directly in proportion thereto.

Claim 2

A system for making dimensional measurements by means of induced eddy-currents, comprising in combination a transducer comprising a sensing coil, at least one capacitor connected with said sensing coil forming a resonant circuit, at least one active component connected with said resonant circuit forming a variable oscillator whose frequency is determined by said resonant circuit and varied in response to the proximity of said sensing coil to an electrically conducting object; said transducer producing a frequency-varying output; a first reference oscillator producing a first reference frequency approximately equal to the frequency of said frequency-varying output when said sensing coil is at its closest proximity to an electrically conducting object; a first frequency subtracting means receiving said frequencyvarying output and said first reference frequency, and producing therefrom a first difference frequency; a second reference oscillator producing a second reference frequency approximately equal to the frequency of said frequency-varying output when said sensing coil is remote from any electrically conducting object; a second frequency subtracting means receiving said frequencyvarying output and said second reference frequency and producing therefrom a second difference frequency; a scaler receiving said second difference frequency and producing therefrom an output frequency proportional thereto; and a digital counting means receiving the output frequency of said scaler and gating on in proportion to the period thereof; said digital counting means also receiving said first difference frequency and counting directly in proportion thereto.

Claim 4

A system for making dimensional measurements by means of induced eddy-currents, comprising in combination a transducer comprising a sensing coil, at least one capacitor connected with said sensing coil forming a resonant circuit, at least one active component connected with said resonant circuit forming a variable oscillator whose frequency is determined by said resonant circuit and varied in response to the proximity of said sensing coil to an electrically conducting object; said transducer producing a frequency-varying output; a limiting amplifier receiving said frequency-varying output and producing therefrom a similar frequency-varying signal of fixed amplitude; a plurality of coupling means, each receiving said frequency-varying signal; a plurality of frequency-selective filters, each connected to one of said coupling means and receiving a portion of said frequency-varying signal; a plurality of amplitude detectors, each connected to one of said frequency selective filters, and a common bus connecting all of said amplitude detectors to a common output.

Claim 5

A system for making dimensional measurements by means of induced eddy-currents, comprising in combination a transducer comprising a sensing coil, at least one capacitor connected with said sensing coil forming a resonant circuit, at least one active component connected with said resonant circuit forming a variable oscillator whose frequency is determined by said resonant circuit and varied in response to the proximity of said sensing coil to an electrically conducting object; said transducer producing a frequency-varying output; a limiting amplifier receiving said frequency-varying output and producing therefrom a similar frequency-varying signal of fixed amplitude; a plurality of coupling means, each receiving said frequency-varying signal; a plurality of frequency-selective filters, each connected to one of said coupling means and receiving a portion of said frequency-varying signal; a plurality of amplitude detectors, each connected to one of said frequency selective filters, and a common bus connecting all of said amplitude detectors to a common output.