Aiming at the thickness measurement of non-metallic coatings on metal substrates

a detection method based on digital demodulation and eddy current technology is proposed.For the inductive sensor probe that picks up the eddy current signal

the relationship between the size of the eddy current inductance and the thickness of the coating is theoretically analyzed. A symmetrical low-noise differential circuit integrating excitation and amplifying processing is designed

and STM32 micro-programmed control unit（MCU）is used to collect and process the differential amplified signal.The signal fitting algorithm based on the principle of digital demodulation is applied to automatically solve the characteristic value of the collected signal.An experimental platform is built by using lift-off distance measurement instead of coating thickness measurement.The displacement platform is used to move the lift-off amount of 10～500 μm in a certain step length

and the moving displacement of the platform is measured in real time by the grating. The lift-off amount is calculated inversely according to the measurement signal.The experimental results show that the proposed detection method achieves a relative precision of ±2 % within the measurement range

and has high repeatability.