Non-destructive control device using eddy currents with switching over additive-substractive flux

Abstract From the Canadian patent

Eddy current probe for non-destructive monitoring with switching between additive and subtractive fields including: a generator providing a sine-wave signal; two power amplifiers whose gains have the same absolute value but opposite signs; a reverse current relay; two impedances; two coils creating a Wheatstone bridge along the two impedances; an amplifier that can be controlled by a binary signal in order to have either a subtractive or additive effect; two synchronous detectors to provide signals representing the real and the imaginary part of the difference between the complex impedances of the coils; and a control device to create two additive or subtractive fields in the coils and …