Eddy current thickness gauge for conductive foils - uses phase-sensitive control circuit to reduce amplitude errors

摘要

The eddy-current measuring system for determining thicknesses of layers and electrically conducting foils comprises a measuring probe with coil driven from a reference HF generator. A differential amplifier has its inputs fed from the coil and from the HF signal via a network. The output of the amplifier controls the phase-shift of a shift network whose output passes to a phase-discriminator fed from the reference source. When the inputs to the discriminator have a 90 deg. separation, no output signal is produced. Any variation from this produces an output signal which is filtered and amplified in the control loop amplifier whose output is fed back to the phase-shifter. The system is insensitive to h.f interference and harmonics.