Compensating Low-Frequency Signal With Indirect Current Feedback in Rectangular Wave Eddy Current Testing Using Tunnel Magnetoresistive Sensor

Rectangular wave eddy current testing (RECT) identifies flaws in conductive materials by acquiring signals at multiple frequencies simultaneously. In eddy current testing (ECT), magnetoresistive (MR) sensors are used for low-frequency ECT due to the high sensitivity of low-frequency inspection for detecting deep flaws. Therefore, a tunnel MR (TMR) sensor, which is less affected by temperature, was used as the detection element in this study. However, because the output signal of an MR sensor is a differential voltage signal, it is susceptible to the offset voltage of the MR element. To address this, we developed a compensation method using an indirect current feedback (ICF) instrumentation amplifier with a digital-to-analog converter (DAC) to eliminate the effect of offset voltage. A 12 mm thick aluminum plate with an artificial flaw measuring 3 mm in diameter and 2, 4, and 6 mm in height was used as a specimen. The measurement results show that the offset was successfully eliminated using this compensation method, allowing smaller flaws on the backside to be identified with low-frequency excitation (10 Hz).