Modern frequency-domain electromagnetic induction (EMI) sensors for detection of unexploded ordnance (UXO) use wideband transmitter circuitry design. The inductor is driven using a sinusoidal signal from a time-varying voltage source. This driving voltage produces sinusoidal current in the transmitter coil at the desired frequency or frequencies. Producing low frequency sinusoidal transmit waveforms requires that the power amplifiers internally dissipate a large amount of heat energy because of the inefficiency of generating a sinusoidal waveform. This requirement effectively precludes the operation of commercial systems at frequencies below 30 hertz.
The objective of this project is to design a frequency-domain EMI UXO sensor using a direct current (DC)-driven transmitter coil. The transmitter coil mechanically rotates (using slip rings) about one of its axes. As the coil rotates, a distant object sees a sinusoidal time-varying magnetic inducing field at the rotation frequency. A pair of identically shaped receiver coils spaced equidistant above and below the transmitter coil will be used as a gradient detector.
This project will make extensive use of high-level EMI modeling software to predict the performance of various transmitter and receiver mechanical and electronic designs. The very low frequencies present a challenge to the ability to receive and amplify signals in the receiver coils with a flat response system. The secondary induced voltages at these frequencies are very small. This could be countered by increasing the effective loop areas of the receiver coils. Ultimately, system sensitivity will be controlled by the ability to reduce the resistance of the receiver coil, cabling, and amplifier input. Unconventional design approaches will be used to develop a new receiver circuit.
The benefit of this approach is predicated upon the importance of new information to be acquired in the extremely low frequency (ELF) region pertaining to a metallic object's shape, composition, and wall thickness. This information may be critical in differentiating intact UXO from ordnance and explosives (OE) scrap. Substantially reducing the number of false alarm excavations will provide significant savings in UXO clearances.