Metal Detectors Metal Detectors By: Benjamin Arkin By: Benjamin Arkin Irma Bocova Irma Bocova Arthur Connors Arthur Connors
Types/Applications Types/Applications ► Type: Type: ► Applications: Applications: ► ► VLF (below 30kHz): is the most � Very Low Frequency Very Low Frequency VLF (below 30kHz): is the most � � � popular detection technology at popular detection technology at the present time the present time Limited in urban areas as PI is � Limited in urban areas as PI is � � Pulse Induction Pulse Induction � unable to reject ferrous (iron) trash unable to reject ferrous (iron) trash Ideal under extreme conditions Ideal under extreme conditions � � such as salt water beaches and such as salt water beaches and highly mineralized ground highly mineralized ground � Beat Frequency Oscillator Beat Frequency Oscillator � Good choice for beginning Good choice for beginning � � hobbyists or children as they hobbyists or children as they are cheap and easy to make are cheap and easy to make
Theory Theory ► According to Faraday's law, the emf induced in the pick-up coil is ε = - N p d Φ /dt ► Magnetic Field at center of field coil is B = N f μ 0 I(t)/(2R f ) ► This creates a flux in the pick-up coil: Φ = ( R p2 ) N f μ 0 I(t)/(2R f )
Very Low Frequency (VLF) Very Low Frequency (VLF) ► Uses two coils: Uses two coils: ► -Transmitter coil (search head, antenna) Transmitter coil (search head, antenna) - � � -Receiver coil Receiver coil - � � ► There is a delay in the received signal There is a delay in the received signal ► compared to the transmitted signal compared to the transmitted signal called phase shift called phase shift -Resistive materials (small phase shift) - Resistive materials (small phase shift) � � -Inductive materials (large phase shift) - Inductive materials (large phase shift) � � -Ferromagnetic materials (little phase shift) - Ferromagnetic materials (little phase shift) � � ► The signal is sent to the control box where it is The signal is sent to the control box where it is ► analyzed by sensors analyzed by sensors
Pulse Induction (PI) Pulse Induction (PI) ► Simple when compared to VLF detectors Simple when compared to VLF detectors ► ► Advantages/Disadvantages Advantages/Disadvantages ► ► Generates Generates “ “Pulses Pulses” ” of Current Through Search Coil of Current Through Search Coil ► Magnetic Field Generated: Magnetic Field Generated: � � ► Pulse reaches its peak and reverses direction, Pulse reaches its peak and reverses direction, ► collapsing very suddenly, thus creating a sharp collapsing very suddenly, thus creating a sharp electrical spike. electrical spike. ► Pulses continue to be generated from a mere dozen to Pulses continue to be generated from a mere dozen to ► about a few thousand pulses per second. about a few thousand pulses per second. Detection of Desired Targets: Detection of Desired Targets: � � ► Reflected portion of signal becomes delayed Reflected portion of signal becomes delayed ► which is sampled, amplified, and sent to which is sampled, amplified, and sent to audio circuit to notify user. audio circuit to notify user. ► Manual vs. Automatic Tuning Manual vs. Automatic Tuning ►
Beat Frequency Oscillator (BFO) Beat Frequency Oscillator (BFO) ► Uses two coils and sends a current Uses two coils and sends a current ► through each through each ► Coils use slightly different Coils use slightly different ► frequencies, which produces audible frequencies, which produces audible “beats “ beats” ” ► When passed over metal object, When passed over metal object, ► induces a current in object induces a current in object ► Current causes a magnetic field, Current causes a magnetic field, ► which interferes with field produced which interferes with field produced by detector by detector ► Interference changes frequency of Interference changes frequency of ► current in large coil, and thus an current in large coil, and thus an audible change in “ “beats beats” ” audible change in ► BFO Metal Detectors are cheap to BFO Metal Detectors are cheap to ► make, but aren’ ’t as accurate or t as accurate or make, but aren dependable as more expensive types dependable as more expensive types
References References http://www.mines.edu/~jamcneil/TPT_Meta ► http://www.mines.edu/~jamcneil/TPT_Meta ► lDetector.pdf lDetector.pdf http://www.gi.alaska.edu/~jesse/treasure/ ► http://www.gi.alaska.edu/~jesse/treasure/ ► misc/howdetector.html misc/howdetector.html http://electronics.howstuffworks.com/metal- - ► http://electronics.howstuffworks.com/metal ► detector.htm detector.htm
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