Radio frequency coupled differential sensor coil for improved muzzle velocity measurements

Claim 1

A radio frequency coupled differential sensor assembly for improved measurement of projectile velocity at the muzzle end of a gun, which comprises; clamping means for fixedly holding said sensor assembly to said muzzle end of said gun; oscillator means for generating a radio frequency signal; first sensor means electrically coupled to said oscillator means for resonating at approximately the same frequency of said oscillator means, and for generating a modulated signal in response to passage of a projectile therethrough and in response to extraneous sources; second sensor means operatively disposed in the same plane and coaxial with said first sensor means and electrically coupled to said oscillator means, for resonating at the same frequency of said oscillator, and for generating a modulated signal in response to said extraneous sources but not in response to the passage of said projectile through said first and second sensor means; shield means fixedly positioned by said clamping means in front of said first and second sensor means for electromagnetically shielding said first and second sensor means and for confining the radiating fields from said first and second sensor means; seal means operatively disposed intermediate said first and second sensor means for preventing high pressure gases from penetrating between the muzzle end of said gun and first and second sensor means; first detector means electrically coupled to said first sensor means for generating a detected and filtered signal; second detector means electrically coupled to said second sensor mean for providing a filtered extraneous signal; differential amplifier means, having a non-inverting input terminal electrically coupled to the output of said first detector means, and an inverting input terminal electrically coupled to the output of said second detector means, for canceling the components of said detected and filtered signal and said filtered extraneous signal which are equal, and for generating an output pulse of the components of said detected and filtered signal and said filtered extraneous signals which are not common to each other; and amplifier means electrically coupled to the output of said differential amplifier means for generating an output signal proportional to the length of said projectile.