Imaging of absolute electrical properties using electroquasistatic and magnetoquasistatic sensor arrays

Abstract

This research focuses on the enhancement of electroquasistatic and magnetoquasistatic nondestructive evaluation techniques. The terminals of the sensors involved are connected to conductors which are traditionally located on a single plane and have a spatially-periodic structure. The sensor operates as a two-port device with one conductor used to excite the sensor and a second conductor used to sense the response to test materials. Existing measurement capabilities are extended: 1. Multiple sensing elements are incorporated into electroquasistatic and magneto-quasistatic sensors such that the response can be accurately modeled. Single sensing element sensors which remain stationary on a test material's surface cannot provide information about variations in material properties along the surface. Scanning of a single element sensor requires many passes in order to provide high resolution property mapping of the surface. By introducing an array of sensing elements it is possible to provide stationary resolution and increase the rate at which a test material's surface can be mapped. Multiple sensing elements can also provide the ability to independently measure material properties that may otherwise be inseparable.

(cont.) The sensors developed allow semi-analytic models to accurately predict their response to layered-media. The sensor is then able to measure absolute material properties using only an air calibration. 2. Existing sensor modeling methods are extended to address new sensor structures. Traditional formulations for models of spatially-periodic sensors were limited to simple conductor patterns on a single plane. These models have been reformulated to address more complex conductor patterns and allow placement on multiple sensor planes. In addition, the models have been used to approximate the sensor response of sensors with aperiodic conductor patterns. 3. Instrumentation for characterizing the terminal response of a many-element sensor is developed. The two-port nature of a single element sensor allows for its characterization by commonly available impedance measurement instruments. The complete realization of the capabilities of a multiple element sensor requires that its terminal response can be rapidly and accurately characterized. An impedance instrument compatible with sensors having up to 39 elements was developed along with methods for accurate calibration.

(cont.) 4. A perturbation method is presented for rapidly predicting the response of a magnetoquasistatic sensor to a notch in a conducting material. Since magnetoquasistatic sensors are often used in the detection of cracks, the ability to model the sensor response to a simplified notch representation is desired. Due to the computational efficiency offered by the spatially-periodic layered-media models, a method of utilizing the computed material current density in the absence of the notch is sought. An approximate response is determined by introducing the notch in a way that perturbs the original current distribution. The extended capabilities are demonstrated through measurements on a variety of material configurations. In some cases the measurements can be represented as images of absolute material properties. In addition to the application of this research to quasistatic measurement methods, other disciplines can benefit from this work. Modeling techniques presented are valuable for microstrip and strip-line transmission lines, microcircuits, and can possibly be applied in fields such as geology and geological exploration. Methodologies applied to these sensor arrays may generically be applied to other array types such as: acoustic, optical, thermal, pressure, antenna, and chemical.