Capacitive Sensors for Measuring Complex Permittivity of Planar and Cylindrical Test-pieces

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1 Capacitive Sensors for Measuring Complex Permittivity of Planar and Cylindrical Test-pieces Nicola Bowler and Tianming Chen Center for NDE Iowa State University

2 Online Graduate Certificate in NDE 13 or 14 credit hours of graduate coursework teaches the fundamentals of three major NDE techniques; eddy-current, ultrasonic and penetrating radiation methods aims to train professionals working in numerous industrial/government sectors Further information: Visit to receive updates Nicola Bowler (Director of Certificate Studies):

3 Principle of Capacitive NDE (Dielectrometry) D = ε 0 E + P D = εe ε is the material permittivity and may be complex due to resonance or relaxation. Detect inhomogeneities based on changes in measured capacitance. Figure: Kasap, McGraw Hill, 19

4 Capacitive Sensors Prior Work Capacitive sensor arrays developed by Auld et al: Figure source: Characterization of capacitive arrays for NDE applications, Research in Nondestructive Evaluation, vol. 2, Staring mode: all array elements are simultaneously excited. This mode controls the shape and extent of the interrogating field generated by the array. Scanning mode: individual elements or groups of elements in the array are sequentially excited. This mode electronically controls the spatial position of the interrogating field.

5 Capacitive Sensors Prior Work Interdigital sensors developed at MIT and Jentek Sensors Inc. Figure source: Interdigital sensors and transducers, Proceedings of the IEEE, vol. 92, Figure source: Cylindrical geometry electroquasistatic dielectrometry sensors, IEEE Transaction on Dielectrics and Electrical Insulation, vol. 12, 2005.

6 Capacitive Sensors Prior Work Capacitance tomography developed at University of Manchester, UK. object sensor image sensing electronics measured data control signals Figure source: Image reconstruction algorithms for electrical capacitance tomography, Measurement Science and Technology, vol. 14, 2003.

7 Motivation of Current Research Planar capacitive sensor: Cylindrical capacitive sensor: Figure source: Furse et al, Down to the Wire, IEEE Spectrum, 2001.

8 Modeling Scheme - Planar Approximate the radome as a multilayered structure Establish a quantitative relationship between sensor capacitance and test-piece permittivity Green s function analysis and method of moments [Chen & Bowler, IEEE Trans. Dielectr. Electr. Insul., 2010] Detect inhomogeneities based on contrasts in measured capacitance.

9 Modeling Scheme - Cylindrical Approximate the wire as a dielectric-coated conductor Find quantitative relationship between wire insulation permittivity and sensor capacitance Green s function analysis and method of moments [Chen, Bowler & Bowler, IEEE Trans. Instr. Meas., 2011] Assess wire insulation status based on measured sensor

10 Measurement Procedures Agilent probe test fixture 16095A Agilent LCR meter E4980A

11 Experimental Verification: Planar Sensor Structure Benchmark experiments of planar sensors in surface contact with multi-layered test-pieces show good agreement with numerical predictions (to within 4%). Test-piece: 2.4-mm-thick Acrylic plate.

12 Detection of Contrast Zones in a Delrin Plate Two rows of holes of different diameters, 2.5, 5.0, 7.5, and 10 mm, were drilled in a 3.17-mm-thick Delrin plate with rpermittivity Wax filled holes, One row of holes was left r / h 2 empty while the other was filled with Paraffin wax r 2.1 ( ) to form zones with different permittivity contrasts. Air filled holes, r / h 4 13

13 Detection of Contrast Zones in a Delrin Plate (Contd.) Hole diameter (mm) Air filled holes ( ε 4) Measure d C Relative diff. (%) Wax filled holes ( ε 2) Measure d C Relative diff. (%) The relative difference is compared to the intact area capacitance value, 1.93 pf. 14

14 Detection of Inhomogeneities in Sandwich Structures Different amounts (1, 3, and 5 cc) of water and olive oil r ( 78 and at 1 MHz and room temperature, respectively) were injected into the honeycomb core of a glassfiberhoneycomb-glassfiber structure. r 3 1 cc of injected liquid corresponds to 4 honeycomb cells with total surface area of 88 mm 2, compared to the surface area of sensors A and B 2 Parameter Core thickness Skin thickness Cell volume Surface area of cell Panel length and width Value 7.62 mm mm 0.25 cc 22 mm mm 15

15 Detection of Inhomogeneities in Sandwich Structures (Contd.) Capacitance measured as hand-held probes scan over glassfiber panels containing injected water. ε contrast

16 Detection of Inhomogeneities in Sandwich Structures (Contd.) Capacitance measured as hand-held probes scan over glassfiber panels containing injected olive oil. ε contrast 3. 17

17 Experimental Verification: Cylindrical Sensor Structure Large-scale experimental verifications have been performed on different dielectriccoated Acrylic-coated conductor structures. copper: Electrode length l (cm) Calculated C (pf) Measured C (pf) Relative difference (%) Absolute difference (pf) ± ± Experimental results agree with calculated results to within an average of 7%. 19

18 Experimental Verification (Contd.) Acrylic-coated copper: comparison of dissipation factor D Electrode length l (cm) Calculated D Measured D ± ± Major sources of uncertainty in the experimental verification: Variations in the test-piece outer diameters. Variations in the dielectric tube thickness. Air gaps existing between the electrodes and the samples. 20

19 Capacitive Probe Based on the Cylindrical Sensor Configuration Groups of wire samples were heated in a muffle furnace at 400, 425, 450 and 475 o C, for 0 to 5 hours. 21

20 Thermal Exposure of Wire Samples (Contd.) The real permittivity ε is inferred from measured probe capacitance. 22

21 Thermal Exposure of Wire Samples (Contd.) 23

22 Thermal Exposure of Wire Samples (Contd.) The imaginary permittivity ε is inferred from measured probe dissipation factor. 24

23 Hydrolytic Exposure of Wire Samples Groups of wire samples were immersed in water for 0 to 4 days. 25

24 Hydrolytic Exposure of Wire Samples (Contd.) 26

25 Present Developments Interdigital spiral sensors Concentric interdigital sensors Interdigital sensors that conform to the curvature of wires 29

26 Acknowledgment This work was supported by the American Society of Nondestructive Testing Graduate Fellowship Award, by the Air Force Research Laboratory under contract FA C-5228 and by NASA under cooperative agreement NNX07AU54A at Iowa State University s Center for NDE.