| Electrical time domain reflectometry (ETDR) distributed sensors have great potential for the application of online health monitoring of large engineering structures like aircrafts, bridges, etc. They have the capability of capturing deflection profiles of the entire structure and detecting crack damages in the early stages. In order to study the defects in the structure a high frequency step pulse is sent through the sensing medium, and based on the reflections from the discontinuity, the severity of the damage can be determined. As of today, only primary reflections have been considered in the algorithm to calibrate ETDR signal waveforms. The effects of secondary reflections were considered insignificant and neglected in the calibration algorithm. In this thesis, a new calibration algorithm taking into account the effects of secondary reflections has been developed. Moreover, an experiment measuring the ETDR signal waveform for a sensing line composed of a number of segments with substantially mismatched impedances has been conducted. The measured signal was then processed using the algorithm developed in this study. The results were then compared with those obtained using the algorithm considering only the primary reflections. It was found that the effects of the secondary reflections appear to be significant when a large discontinuity is present on the sensing line. The new algorithm was shown to be an improved calibration scheme, which yields a more accurate interpretation of ETDR signal waveforms for distributed reflection coefficients and characteristic impedance profile along the sensing line. |
