# High-Resolution Spatiotemporal-Coded Differential Eddy-Current Array Probe for Defect Detection in Metal Substrates

## TL;DR

A new high-resolution eddy-current probe is developed to detect and quantify near-surface defects in metal substrates with improved accuracy and spatial resolution.

## Contribution

A novel spatiotemporal-coded eddy-current array probe is introduced for enhanced detection of low-contrast near-surface metal defects.

## Key findings

- The M-DECA probe achieves a balance between high common-mode rejection and high-density spatial sampling.
- A power-law relationship between defect area and differential eddy-current response amplitude is established with R2=0.9034.
- Quantitative characterization of defect sizes shows relative errors below 6.35% across different scales.

## Abstract

To address the problems of weak geometric features, low signal response amplitude, and insufficient spatial resolvability of near-surface defects in metal substrates, a high-resolution spatiotemporal-coded eddy-current array probe is proposed. The probe adopts an array topology with time-multiplexed excitation and adjacent differential reception, achieving a balance between high common-mode rejection ratio and high-density spatial sampling. First, a theoretical electromagnetic coupling model between the probe and the metal substrate is established, and finite-element simulations are conducted to investigate the evolution of the skin effect, eddy-current density distribution, and differential impedance response over an excitation frequency range of 1–10 MHz. Subsequently, a 64-channel M-DECA probe and an experimental testing platform are developed, and frequency-sweeping experiments are carried out under different excitation conditions. Experimental results indicate that, under a 50 kHz excitation frequency, the array eddy-current response achieves an optimal trade-off between signal amplitude and spatial geometric consistency. Furthermore, based on the pixel-to-physical coordinate mapping relationship, the lateral equivalent diameters of near-surface defects with different characteristic scales are quantitatively characterized, with relative errors of 6.35%, 4.29%, 3.98%, 3.50%, and 5.80%, respectively. Regression-based quantitative analysis reveals a power-law relationship between defect area and the amplitude of the differential eddy-current array response, with a coefficient of determination R2=0.9034 for the bipolar peak-to-peak feature. The proposed M-DECA probe enables high-resolution imaging and quantitative characterization of near-surface defects in metal substrates, providing an effective solution for electromagnetic detection of near-surface, low-contrast defects.