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A Rotationally Invariant Capacitive Probe for Materials Evaluation

Motivated by potential applications, such as anomaly detection in aircraft radome sandwich structures, a rotationally invariant capacitive probe with concentric coplanar electrodes was designed based on a previously published theoretical model. Two versions of the probe, with different target sensing penetration depths, were constructed in such a way that removal of parasitic capacitances could be achieved. Capacitance measurements for the probes in surface contact with laminar structures showed agreement to within 10% of theoretical predictions. Important factors governing the penetration depth of concentric capacitive sensors were investigated numerically, followed by experimental investigation on stepped samples, where results were found to be in accordance with numerical predictions. The effect of probe liftoff on inferred permittivity of dielectric slabs was also investigated. As liftoff increased, uncertainty in inferred permittivity increased dramatically. On the other hand, the capacitive probes show good sensitivity to low-contrast inhomogeneities embedded in laminar structures; 1 mL of a low contrast liquid injected into the core of a glass fiber-honeycomb-glass fiber sandwich structure was successfully detected.

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