Efforts Towards the Design and Development of an Electromagnetic Induction Sensor Optimized for Detection and Discrimination of Unexploded Ordnance

Abstract

This thesis provides a detailed description of efforts towards the development of a time domain electromagnetic induction (EMI) system optimized for detection and discrimination of Unexploded Ordnance (UXO). A time domain (or pulsed) EMI system with good discrimination capability must be able to accurately capture a UXO target’s late-time response. Here, late-time response is defined as the time period starting a few hundred microseconds after the transmitter current turn off and extending to approximately 20 milliseconds or to just before the beginning of the next transmitter current pulse. A pulsed EMI system must have sufficient bandwidth, especially at the lower end, to accurately measure a UXO target’s late time response. EMI system components include transmitter and receiver coils and corresponding transmitter and receiver coil amplifiers as well as the data acquisition system (analogue-to-digital converter). This thesis addresses EMI system component design and tradeoffs necessary to achieve the bandwidth and sensitivity for good UXO detection and discrimination performance. Field measurements were taken at the Blossom Point Test Site in Maryland in order to evaluate the detection and discrimination performance of the pulsed EMI sensor. Although overall performance was good, several deficiencies were noted. In particular, the low 3dB frequency of the receiver coil amplifier was inadvertently set too high (above 1 kHz) which resulted in a prematurely truncated late-time response. In short, we did not observe the expected long exponential “tails” indicative of larger ferrous test targets. This problem has recently been corrected using a current-to-voltage converter as the first amplification stage following the receiver coil. The current-to-voltage converter has very low input impedance so that the system lower 3 dB frequency is fixed solely by the resistance-inductance ratio of the receiver coil (currently set to 80 Hz). Testing also indicated a deficiency in system sensitivity. This problem has been attributed to insufficient receiver coil moment; specifically, the area of each half of the currently employed bucking receiver coil is too small. Future plans are to replace each 1 ft2 half of the bucking receiver coils with a vertically stacked arrangement of two 1 m x 1 m coils, one above and the other below the transmitter coil. The moment of the new coils will increase by a factor of 10 over that of the old and thus overall sensitivity should increase by the same factor. Additionally, future plans include purchasing a 1 kW transmitter coil amplifier capable of delivering over 40 amps at rated voltage (22.5 V rms). The transmitter coil amplifier used in the Blossom Point tests delivered a peak current of less than 20 amps so the new amplifier, all other system parameters held constant, should improve overall sensitivity by more than 3 dB. Of course sensitivity can be improved by either increasing the received signal level or by reducing noise. A “boot strapped” differential amplifier has been tested that has significantly improved noise performance compared to the previously used single ended amplifier. Furthermore, testing is currently underway to evaluate a three operational amplifier configuration that should even further reduce noise yet still have very low input impedance and thus very good low frequency performance. Lastly, during the Blossom Point tests some problems were encountered with the test cart wheel bearings. At the very least we plan to replace the “home made” PVC wheel bearings with manufactured bearings. We learned about the source of the manufactured bearings from a colleague at a recent UXO workshop. In addition to the description of efforts towards the development of an EMI sensor optimized for detection and discrimination of UXO, a brief description of the efforts towards the development of a Microelectromechanical Systems (MEMS) Capa