A rapid-acquisition electrical time-domain reflectometer for analysis of time-variant impedance discontinuities

A distributed crack sensor has been developed for the measurement of cracks in concrete structures. The sensor is measured using a distributed measurement technique known as electrical time-domain reflectometry (ETDR). ETDR has traditionally been used to measure time-invariant (i.e. unchanging with time) impedance discontinuities, however applications of the sensor in structural failure analysis require measurement of time-variant (i.e. changing with time) impedance discontinuities at rates as high as 10 k measurements per second. ETDR is a suitable measurement technique for these applications since a time-domain reflectometer (TDR) acquisition can be performed in less than 100 µs. Employment of ETDR in these applications, however, requires a TDR that supports measurement rates as high as 10 k measurements per second. Commercial TDRs are not suitable for these applications since their measurement rates are typically less than 10 measurements per second. In order to satisfy the high measurement rates required for these applications, a rapid-acquisition TDR was developed that supports measurement rates as high as 10.1725 k measurements per second. The acquisition rate of the TDR was evaluated by modulating the voltage reflected from a short termination with a voltage variable attenuator. The TDR was able to monitor the reflected voltage at modulation frequencies as high as 1 kHz. The TDR was applied in the monitor of a crack sensor embedded in a bridge column during a shake-table experiment. The TDR was able to monitor the evolution of a crack which formed in the column during the experiment. The operation, design, evaluation, and application of the TDR are discussed herein --Abstract, page iii