Investigation of Signal Conditioning for Tx-Rx PEC Probe at High Lift-Off Using a Modified Maxwell’s Bridge

Abstract

Pulsed eddy current testing is widely used because of its richness in spectral components. However, the degrading sensitivity as lift-off increases poses a challenge to its application in thick insulation or buried structures and weld areas. In the transmitter-receiver probe, self-impedance (offset) of receiver coil dominates the output signal, reducing the signal-to-noise ratio, especially at high lift-off. This limits the subsequent amplification of the output signal by the signal conditioning circuit. In practice, bridge-based measurements are used to mitigate this challenge but result in small linear range input-output characteristics. Electromagnetic interference and stray capacitance effects also cause measurement errors. This paper investigates a modified Maxwell's inductance bridge based on operational amplifier configuration for high lift-off testing. The receiver coil self-impedance is removed to improve signal-to-noise ratio and to make output signal proportional to impedance change only. Experiments are performed to evaluate the performance of the circuit in crack detection. The results show improved signal-to-noise ratio with maximum linearity deviation of 0.30%, and a higher crack detection sensitivity at 30mm high lift-off, in comparison to the conventional bridge circuit with maximum linearity deviation of 0.53%.