A novel shaped-electrode coplanar array capacitive sensor with intrinsically rotationally invariant sensitivity field for defect imaging

Coplanar capacitive sensors (CCSs) are an emerging non-destructive testing (NDT) technique. For large-scale components, scanning measurements enhance data density and resolution. This paper reveals that during rotational scanning, the sensitivity field of typical rectangular electrode layouts generates direction-dependent distortion distributions. To address this, a coplanar electrode geometry with intrinsic rotational consistency is proposed. A sensor geometric model based on the topological layout of electrodes is first established, and the distribution characteristics of the sensitivity matrix are analysed to reveal the intrinsic influence of electrode geometric topology on the spatial uniformity of the sensitivity field. Furthermore, an evaluation framework based on singular value decomposition (SVD) of the sensitivity field matrix is developed to quantify the intrinsic correlation between electrode geometry and imaging information. Experimental results demonstrate that the proposed design outperforms the conventional rectangular electrode structure. The developed 12-electrode sector capacitive sensor (12-ESCS) achieves a 25.14% reduction in root mean square error (RMSE) and a 4.63% improvement in Pearson correlation coefficient (PCC) compared with the traditional coplanar array sensor, effectively suppressing rotation-induced geometric distortions and enhancing structural consistency in imaging. These findings provide a practical sensor design approach for rotational scanning NDT applications.