Final Report - Strategic Environmental Research and Development ...

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Compact Low-Cost Ultra-Wideband EMI SensorSERDP MR-2105with the current electronics, and the system is able to record early-time signal starting at 20 s(see Figure 7). The sensor sensitivity was reduced to 16% of maximum during EMI experimentsso that very early-time measurements could be made without saturating the electronics. A sensorwith improved electronics that re-aligns the permalloy magnetic domains more quickly couldbegin recording at 10 s. This bench-top setup was also used to measure the passive magneticresponse to a target.Table 2. Transmitter moments of common EMI instruments.Sensor Type Dipole Moment Peak Current Turn-Off Time/EarliestTime GateEM61 200 A-m 2 6-8 A 216 s time gateEM63 1024 A-m 2 ? 180 s time gateZonge DNT 25 A-m 2 3 A Depends on loop sizesALLTEM 693 A-m 2 11 A 200 μs minimumresponse time.MetalMapper 120 A-m 2 6 A Transmitter turn-off to1% is on the order of 10us. Receiver useable100 μs maximum afterinitiating transmitterturn-off.MPV Undertermined 2.75 A Receiver useable 100 μsmaximum afterinitiating transmitterturn-off.AMR sensor prototype2 A-m 2 (design existsfor ~40 A-m 2 )0.5 A (designexists for ~20 A)< 10 s turn-off, 20 stime gateFigure 6. Photograph of a bench-top TD EMI system with the transmitter coil visible and an 81 mm mortar (left).Photograph of the AMR sensor board and the transmitter board inside the transmitter coil form (right).9
Compact Low-Cost Ultra-Wideband EMI SensorSERDP MR-2105Figure 7. Photograph of pickup coil used to monitor the transmitted signal from the bench-top time-domain EMIsystem (left). Oscilloscope screen capture of the early-time AMR sensor response (right). The transmitted signalreceived by the monitoring coil is shown in yellow, and the raw received signal from the AMR sensor is shown in green.The prototype was designed to use a configurable magnetic offset to compensate for theambient magnetic field, bridge offset voltage, thermal drift, and the unwanted EMI response ofconductors in the sensor. Although this approach works, it is impractical. It produces significantpower dissipation in the AMR chip due to the resistance of the offset coil, and power dissipationin the coil driver. It also causes the sensor to consume more power which is a negative attributefor battery powered instruments. Power dissipation in the AMR chip exacerbates thermal drift.To alleviate these difficulties, the prototype was modified to enable an electronic offsetadjustment. At this time, the electronic offset adjustment is static, meaning that its operatingpoint is set at the beginning of an experiment and not changed during the experiment. This setupaddresses the ambient magnetic field and bridge offset compensation, but is not able tocompensate for thermal drift and the unwanted EMI response of the instrument’s conductors.A frequency-domain EMI system was not built because a separate bucking coil would beneeded at each receiver for each transmitter. This is impractical when more than one transmitterpolarization is used. For this reason, FD EMI sensors have not been used in ‘advanced’ EMIsensors that have many polarization combinations of transmitters and receivers.5 Results and DiscussionThis section presents the experimental results and discusses implications of these results.Before conducting response tests of the AMR sensor, the noise of the measurement system wasevaluated. The voltage output of the AMR bridge was shorted and the noise spectral density wasmeasured as described in the Materials and Methods section. The left panel in Figure 8 showsthe noise density of the system with no modulation. The right panel is the results afterelectronically modulating the noise out of measurement band (1 kHz). Both graphs indicateincreasing fluctuation noise (i.e. 1/f noise) for frequencies less than 200 Hz. The modulation10
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