Captive Sensor for Cylindrical Structures



Figure 7: Curved patch capacitive sensors for quantitative evaluation of wire insulation.

Motivated by the need to detect and characterize degradation in aircraft wiring insulation, curved patch capacitive sensors have been developed, Figure 7. The sensor consists of two curved electrodes that are exterior to and coaxial with a wire structure. The relationship between the complex permittivity of the wire insulation and the sensor capacitance has been established in a numerical model [2].



Figure 8: Prototype capacitive probe with conforming electrodes, for inspection of wire type MIL-W- 81381/12. Subfigure: capacitive probe clipping a wire sample under test.

Figure 8 shows a prototype capacitive probe for insulation inspection of wire type MIL-W- 81381/12. The probe consists of two 2 cm by 4 cm acrylic plates and an acrylic rod that holds the two plates. The lower plate is attached to the acrylic rod using a plastic screw, whereas the upper plate can glide up and down by adjusting the other plastic screw perpendicular to the two plates. The curved sensor electrodes are formed by brushing a layer of silver paint onto the symmetric grooves in the two plates. The two electrodes are connected to two pins, which are then connected to an LCR meter for capacitance measurements.


Figure 9: Experimental arrangement for thermal degradation and hydrolytic exposure of wire type MIL-W-81381/12. Clockwise from left: muffle furnace used for thermal exposure; wire samples after heat exposure (brown) and a control wire (yellow); a wire sample used in hydrolytic exposure experiment – both ends of the sample are sealed with wax prior to immersion.

In order to demonstrate the feasibility of utilizing the capacitive probe for evaluation of wiring insulation status, groups of wire samples have been thermally and hydrolytically exposed under different conditions, to induce dielectric property changes in the wire insulation. Figure 9 shows the experimental arrangement for thermal degradation and hydrolytic exposure. Groups of wire samples were isothermally heated in the furnace at 400, 425, 450, and 475 oC, for 1 to 5 hours. Groups of wax-sealed wires were immersed in water at room temperature for 0.5, 1, 2, 3, and 4 days, respectively.




Figure 10: Inferred real permittivity of thermally exposed MIL-W-81381/12 wires compared with that of Polyimide HN films which have undergone a similar aging process.

Figure 10 shows the real permittivity ε’ of the thermally exposed wires, along with the permittivity of thermally-exposed polyimide HN films [3], measured independently.



Figure 11: Inferred imaginary permittivity of thermally exposed MIL-W-81381/12 wires.

Figure 11 shows the imaginary permittivity ε’’ of the thermally exposed wires. These permittivity values are inferred from the measured probe capacitance by application of a physical (numerical) model of the system. Comparisons made between the complex permittivity of the damaged wires and the control wires (exposure time equals 0) show that,both the real and imaginary parts of the insulation permittivity of the damaged wires increase as the thermal exposure temperature and exposure time increase, and are higher than those of the control wires. Especially, changes in the imaginary permittivity are more significant than those in the real part. For example, the imaginary permittivity was observed to increase by up to 39% while the real part by up to 17%, for exposures at temperatures between 400 and 475oC for various times up to 5 hours.



Figure 12: Inferred complex permittivity for hydrolytically exposed wires in comparison with that of polyimide HN films. Left: real permittivity. Right: imaginary permittivity.

Figure 12 shows the real and imaginary permittivity of the hydrolytically exposed wires, along with that of hydrolytically-exposed polyimide HN films [3], measured independently. It is clearly seen that both the real and imaginary parts of the wire insulation increase as immersion time increases up to three days.  The observed change of insulation complex permittivity as a function of hydrolytic exposure agrees well with independent measurement results on polyimide HN films.


Handheld capacitive probes have been developed for materials evaluation and damage detection for both planar and cylindrical test-pieces. For the planar sensor configuration, experimental results have demonstrated the excellent capability of the probe in detecting low-contrast defects in sandwich structures, e.g., the probe successfully detected 1 cc of oil injected into the core of a glassfiber-honeycomb-glassfiber structure. For the cylindrical sensor configuration, the probe has successfully detected and quantified change in insulation permittivity for wires of type MIL-W- 81381/12.


  • Dielectric property evaluation for  planar and cylindrical low-conductivity structures.
  • Anomaly detection in multilayered planar and cylindrical dielectric materials.


[1] T. Chen and N. Bowler, “Analysis of a concentric coplanar capacitive sensor for nondestructive evaluation of multi-layered dielectric structures”, IEEE Transactions on Dielectrics and Electrical Insulation, Vol. 17, pp. 1307-1318, Aug. 2010.

[2] T. Chen and N. Bowler, “Analysis of a capacitive sensor for the evaluation of circular cylinders with a conductive core”, to be submitted to Measurement Science and Technology.

[3] L. Li, N. Bowler, P. R. Hondred, M. R. Kessler, “Influence of thermal degradation and saline exposure on dielectric permittivity of polyimide”, Journal of Physics and Chemistry of Solids, Vol. 72, pp. 875-881, Apr. 2011.