Development of embedded modulated scatterer technique: single- and dual-loaded scatterers

Health monitoring of infrastructure is an important ongoing issue. Therefore, it is important that a cost-effective and practical method for evaluating complex composite structures be developed. A promising microwave-based embedded sensor technology is developed based on the Modulated Scatterer Technique (MST). MST is based on illuminating a probe, commonly a dipole antenna loaded with a PIN diode (also referred to as a single-loaded scatterer, or SLS), with an electromagnetic wave. This impinging wave induces a current along the scatterer length, which causes a scattered field to be reradiated. Modulating the PIN diode also modulates the signal scattered by the probe, resulting in two different states of the probe. By measuring this scattered field, information about the material in the vicinity of the probe may be determined. Using the ratio of both states of the probe removes the dependency of MST on several measurement parameters. In order to separate the scattered signal from reflections from other targets present in the total detected signal, a swept-frequency measurement process and subsequent Fourier Transform (time-gate method) was incorporated into MST. Additionally, a full electromagnetic study of the SLS, as applied to MST, was also conducted. The increased measurement complexity and data processing resulting from the time-gate method prompted the development of a novel dual-loaded scatterer (DLS) probe design, with four possible modulation states. By taking a differential ratio, the reflections from other targets can be effectively removed, while preserving the measurement parameter independence of the SLS ratio. A full electromagnetic derivation and analysis of the capabilities of the DLS as applied to MST is included in this investigation, as well as representative measurements using the DLS probe --Abstract, page iii

Two-Dimensional In-Plane Strain Fss-Based Sensor

Frequency selective surface (FSS) based sensors have found application as sensors in the last decade. In this paper, a new sensor design is proposed for two-dimensional in-plane strain sensing. The unit cell of the FSS-based sensor includes two slot dipoles, oriented normal to one another and each with different dimensions, to create two unique resonant frequencies when interrogated with an incident electric field normal to the direction of measured strain. In this way, 2D strain can be characterized concurrently and independently. The error due to strain orthogonal to the direction of interest, along with the error due to the presence of shear strain, has also been characterized. The sensor has a maximum of 12% error for an applied strain due to 4% strain orthogonal to the measurement direction, and no more than 8% error for a maximum of 4% of shear strain

Practical Fss-Based Sensor Sensitivity

Frequency selective surfaces are a periodic array of unit cells that, when illuminated externally, have a specific frequency response that depends on element geometry, spacing, and substrate properties. Theoretically, FSS is assumed as an infinite array of unit cells with a plane wave excitation. However, in practice, an FSS is finite and hence, due to edge effects, the limited number of unit cells, and non-uniform illumination, the response will deviate from the theoretical. As it relates to FSS-based sensing in particular, a localized illumination is often used in order to improve the sensing resolution. However, due to the aforementioned factors, the sensitivity of the sensor may suffer as a result. Hence, the effect of these factors is studied on the FSS sensor response. Then, taking strain measurement as an example, the degradation in the sensor sensitivity to strain is evaluated in comparison with that of a theoretical FSS. The simulation results show that a finite FSS with non-uniform illumination has reduced sensitivity to strain. This degradation in sensitivity of reduces as the number of illuminated unit cells increases. However, the sensitivity of a finite FSS with uniform illumination is nearly constant with respect to the number of illuminated unit cells

Thermal Diffusivity Materials Characterization Via Active Microwave Thermography

Active microwave thermography (AMT) is a relatively new nondestructive evaluation method which is proposed in this work for thermal materials characterization. Specifically, AMT is investigated as a single-sided measurement option for out-of-plane thermal diffusivity (a parameter traditionally measured using a two-sided technique). Simulation and measurement results support the use of AMT for such a characterization for materials backed by an electromagnetically absorptive material. Both lossless and lossy materials may be measured, with better accuracy for lossless materials. The effect of heating time was also considered. The results indicate that for the 50 W system used here, 100 seconds of electromagnetic illumination is necessary to achieve less than 10% error in measured out-of-plane thermal diffusivity for lossless and lossy materials

Improved Quantification Of Defect Cross-Section For Active Microwave Thermography

Active microwave thermography (AMT) is an integrated nondestructive testing and evaluation (NDT&E) technique that features a microwave-based excitation and subsequent thermographic inspection via an infrared camera. AMT has been successfully employed in several industries including aerospace and civil for NDT&E inspections. Since the excitation is microwave-based, an antenna is used to irradiate the sample under test and hence the heating pattern will vary spatially (following the antenna pattern). This nonuniform thermal excitation may limit the ability of AMT to quantify defect cross-sections. Therefore, this work seeks to expand the capabilities of AMT by incorporating a post-processing technique to improve defect cross-section quantification. Specifically, an approach based on the temperature gradient is considered, with results compared to other well-established approaches. The effect of noise is also considered. The results, from both simulation and measurement, indicate that the temperature gradient approach provides the least amount of error in defect cross-section quantification

A Meander Line-Based Frequency Selective Surface for Strain Sensing

Frequency selective surfaces (FSSs) are periodic arrays of conductive elements that have distinct reflection and transmission responses. In this work, an FSS sensor designed to operate in Ka-band to measure a wide range of uni-directional strain using a meander line-based unit cell is presented. Specifically, the proposed unit cell of the sensor consists of a convoluted meander line geometry designed on a thin dielectric substrate. Strain sensing is achieved by monitoring the change in the resonant frequency of the FSS when under strain that is parallel to an interrogating signal linearly polarized and aligned with the convoluted dimension of the meander line element. Simulation results of strain measurement over two ranges, small-(0%-0.5%) and large-scale (0%-5%), are presented. The simulated results indicate that the sensitivity of the sensor to small-scale strain is 21 MHz/0.1% strain and 230 MHz/1.0% strain for large-scale

PCB based Modulated Scatter with Enhanced Modulation Depth

The Modulated Scatterer Technique (MST) Has Shown Promise for Applications in Microwave Imaging, Electric Field Mapping, and Materials Characterization. Traditionally, MST Scatterers Are Dipoles Centrally Loaded with an Element Capable of Modulation (E.g., a PIN Diode). by Modulating the Load Element State, the Scattered Fields Are Also Modulated. However, Due to the Small Size of Such Scatterers, It Can Be Difficult to Reliably Detect the Response. Increasing the Modulation Depth (MD) of the Scattered Signal May Improve Detectability. This Paper Presents Simulations and Measurements of PCB-Based MST Elements that, through Reactive Loading, Are Designed to Be Electrically Invisible during the Reverse Bias State of the Modulated Element (A PIN Diode in This Case) While Producing Detectable Scattering during the Forward Bias State. the Results Show a Significant (\u3e 90%) Improvement in the MD of the Scattered Signal When Compared to a Traditional MST Scatterer

High Permittivity Anisotropic 3d Printed Material

In this work, the diagonal elements of the permittivity matrix for dielectric samples, 3D printed with a carbon fiber-loaded material (XT-CF20), are measured for frequencies within the range of 1 MHz to 18 GHz. These permittivity measurements demonstrated a strong anisotropy, indicating that the electromagnetic properties of the CF20 material depend on the infill method used to print. The importance of understanding the anisotropy for microwave device design is demonstrated via a dielectric-loaded cavity resonator application

Pulsed-Active Microwave Thermography

Active microwave thermography (AMT) is a thermographic nondestructive testing and evaluation technique that utilizes an electromagnetic-based excitation with a subsequent infrared measurement of the surface thermal profile of the material or structure of interest. AMT has been successfully applied to several aerospace and civil infrastructure applications. This work seeks to expand the performance of AMT by incorporating a signal processing technique common to traditional (flash-lamp) thermography, referred to as pulsed thermography (PT). PT operates on the premise of a pulsed excitation, as opposed to a constant or step excitation (ST) over a given time-period that is typical to traditional active thermography. This work applies the pulsed approach to AMT, herein referred to as P-AMT, and compares the thermal contrast (TC) and signal-to-noise ratio (SNR) of traditional and pulsed AMT inspections as applied to a moisture ingress detection need. The results suggest that the optimal heating time (indicated through SNR) for P-AMT is less than that of traditional AMT with a reduced overall (absolute) temperature. This is important as it relates to any inspection with concerns for thermal damage as well an overall reduction in required inspection time