A Study of Electromagnetic Induction Systems for the Detection of Unexploded Ordnance
Type of DegreeThesis
Electrical and Computer Engineering
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This thesis presents a study of a time domain electromagnetic induction (EMI) system used for detection and discrimination of unexploded ordnance. In general, EMI system components include transmitter and receiver coils, corresponding transmitter and receiver coil amplifiers and a data acquisition system. This thesis explains differences in electronic circuitry of EMI sensors that measure the current or voltage response of a target. In the case of a current measurement it is essential to use a current-to-voltage converter as the first stage before any amplification is done so that the lower 3 dB point of the receiver coil is sufficiently low to capture all important target characteristics. Also using a compensation circuit to lower the 3 dB point makes it possible to acquire the undistorted late time response of targets. This capability is essential for discrimination purposes. In voltage measurement methods, however, only a voltage amplifier is used on the receiver side and there is no need for compensation circuitry as the 3 dB point of the receiver coil is high enough to capture an undistorted version of the target response. Both voltage and current measurement methods were used to record the target response and it was observed that in general, the voltage measurement method is better than the current measurement method. Comparison of the response collected from a commercial EMI sensor and a constructed EMI sensor (voltage method) is made. Observations are made for possible reasons for superior response from the commercial EMI sensor In addition to the above topics, a software tool is developed for evaluating electromagnetic transmitter coil designs and configurations that are used in EMI sensor arrays. This software tool generates static sensitivity maps in any desired plane when transmitter and receiver coil space coordinates are given as inputs. These maps give a good idea of how effective an EMI sensor layout can be in terms of its sensitivity to a target’s presence at different points in space. The advantage of using static sensitivity maps is that they are independent of target characteristics and thus can be used as an effective tool to measure the performance of a proposed coil layout. Maps have been presented for different coil configurations and important observations and conclusions are made. To verify the fidelity of the software tool output field measurements are taken using a commercial sensor and compared to the same from the software model. In general, good qualitative agreement was obtained though more careful measurements should be done in the future. In addition to sensitivity maps, streamline plots can also be generated from a similar software tool with minor changes in software coding. Streamline plots are helpful in visualizing the direction of the magnetic field due to transmitter or receiver coils in any orientation.