Capacitive sensors for measuring complex permittivity of planar and cylindrical structures

With the increasing use of low-conductivity structural and functional materials, there has been a greater need for the efficient and reliable nondestructive evaluation (NDE) of these materials. One approach to evaluate low-conductivity structural and functional materials is to characterize the material dielectric property. In this thesis, capacitive sensors are developed for measuring complex permittivity of planar and cylindrical materials. For each sensor configuration, models are developed to allow for inverse determination of material permittivity from measured capacitance, therefore realizing quantitative characterization of material dielectric properties. In the first half of the thesis, coplanar concentric capacitive sensors are developed to meet the need of detecting water or excessive inhomogeneities caused by repairs in aircraft radome structures. Another important motivation is the absolute dielectric property characterization of laminar structures. Three coplanar sensor configurations are designed: the simple two-electrode concentric configuration, the interdigital spiral and the interdigital concentric configurations. Corresponding numerical models are developed to predict the sensor capacitance for given test-piece structures. The validity of the models is verified by comparing numerical predictions and measurement results. The advantage and disadvantage of each sensor configuration is discussed. For the two-electrode concentric configuration, a prototype handheld probe is also fabricated, and has detected successfully 1 cc of low contrast liquid in a simulated radome structure. Curved patch capacitive sensors, presented in the second half of the thesis, are developed with the motivation of accurate and convenient permittivity measurement of cylindrical structures. It is demonstrated that the permittivity of homogeneous dielectric rods is inferred easily from measured sensor capacitance, based on analytical and numerical models developed here. Another practical application of the curved patch capacitive sensors is the quantitative evaluation of aircraft wiring insulation condition. In this work, wires are modeled as cylindrical dielectrics with a conductive core. A numerical relationship between the complex permittivity of the insulation and the sensor capacitance and dissipation factor is established. A prototype probe, developed based on this model, has distinguished successfully degraded wires from the control ones. The feasibility of utilizing the presented capacitive approach for quantitative evaluation of aircraft wiring insulation condition is demonstrated. Although the development of the capacitive sensors in this thesis is motivated by aerospace engineering related applications, results presented in this work have the potential to be applied to other engineering fields. Potential sensor applications and recommended future research are suggested at the end of the thesis

Design of interdigital spiral and concentric capacitive sensors for materials evaluation

This paper describes the design of two circular coplanar interdigital sensors with i) a spiral interdigital configuration and ii) a concentric interdigital configuration for the nondestructive evaluation of multilayered dielectric structures. A numerical model accounting for sensor geometry, test-piece geometry and real permittivity, and metal electrode thickness has been developed to calculate the capacitance of the sensors when in contact with a planar test-piece comprising up to four layers. Compared with a disk-and-ring coplanar capacitive sensor developed previously, the interdigital configurations are predicted to have higher signal-to-noise ratio and better accuracy in materials characterization. The disk-and-ring configuration, on the other hand, possesses advantages such as deeper penetration depth and better immunity to lift-off variations

Concentric Coplanar Capacitive Sensor System with Quantitative Model

A concentric coplanar capacitive sensor includes a charged central disc forming a first electrode, an outer annular ring coplanar with and outer to the charged central disc, the outer annular ring forming a second electrode, and a gap between the charged central disc and the outer annular ring. The first electrode and the second electrode may be attached to an insulative film. A method provides for determining transcapacitance between the first electrode and the second electrode and using the transcapacitance in a model that accounts for a dielectric test piece to determine inversely the properties of the dielectric test piece

Analytical solution for capacitance calculation of a curved patch capacitor that conforms to the curvature of a homogeneous cylindrical dielectric rod

This Letter presents an analytical expression for the capacitance of a curved patch capacitor whose electrodes conform to the curvature of a long, homogeneous, cylindrical dielectric rod. The capacitor is composed of two infinitely long curved electrodes, symmetrically placed about a diameter of the cylinder cross-section. The resulting capacitance per unit length depends on both the dielectric properties of the material under test and the capacitor configuration. A practical capacitance measurement is also presented, with appropriately guarded finite electrodes. Very good agreement between measured and theoretically predicted capacitances were observed, to within 2.4 percent. The analytical result presented in this Letter can be applied for extremely rapid evaluation of rod permittivity from measured capacitance

Electromagnetic Nondestructive Evaluation of Wire Insulation and Models of Insulation Material Properties

Polymers have been widely used as wiring electrical insulation materials in space/air-craft. The dielectric properties of insulation polymers can change over time, however, due to various aging processes such as exposure to heat, humidity and mechanical stress. Therefore, the study of polymers used in electrical insulation of wiring is important to the aerospace industry due to potential loss of life and aircraft in the event of an electrical fire caused by breakdown of wiring insulation. Part of this research is focused on studying the mechanisms of various environmental aging process of the polymers used in electrical wiring insulation and the ways in which their dielectric properties change as the material is subject to the aging processes. The other part of the project is to determine the feasibility of a new capacitive nondestructive testing method to indicate degradation in the wiring insulation, by measuring its permittivity

Spatiotemporal evolution and multi-scenario prediction of habitat quality in the Yellow River Basin

IntroductionThe Yellow River Basin (YRB) is not only a vital area for maintaining ecological security but also a key area for China’s economic and social development. Understanding its land-use change trends and habitat quality change patterns is essential for regional ecological conservation and effective resource allocation.MethodsThis study used the patch-generating land-use simulation (PLUS) and Integrated Valuation of Ecosystem Services and Trade-offs (InVEST) models to analyze and predict the spatial and temporal trends of habitat quality in the YRB from 2000 to 2030 under natural development (ND) and ecological conservation and high-quality development (ECD) scenarios. The PLUS model was used to predict land-use change in 2030 under different scenarios, after which the InVEST model was used to obtain the habitat quality distribution characteristics from the 2000–2030 period.Results(1) The mean values of habitat quality in the YRB in 2000, 2010, and 2020 were 0.6849, 0.6992, and 0.7001, respectively. The mean habitat quality values were moderately high. Spatial distribution characteristics were high in the west and low in the east and along the water. In 2030, habitat quality (0.6993) started to decline under ND, whereas under ECD, there was an indication of substantial improvement in habitat quality (0.7186). (2) The mean habitat degradation values in 2000, 2010, and 2020 were 0.0223, 0.0219, and 0.0231, respectively. The level of habitat degradation showed a decreasing trend, followed by an increasing trend with a stable spatial distribution pattern. The mean level of habitat degradation in 2030 (0.0241) continued to increase under ND, while a substantial decrease in the level of habitat degradation occurred under ECD (0.0214), suggesting that the level of habitat degradation could be effectively contained under the ECD scenario. (3) During the study period, the conversion of building land—both negative and positive—had the most pronounced impact on habitat quality per unit area. Further, the conversion of grassland was shown to be a key land transformation that may either lead to the deterioration or improvement of the ecological environment. The results provide scientifific theoretical support and a decision basis for ecological conservation and the high-quality development of the YRB