Metal Detectors Metal Detectors By: Benjamin Arkin By: Benjamin - - PowerPoint PPT Presentation

Metal Detectors Metal Detectors

By: Benjamin Arkin By: Benjamin Arkin Irma Bocova Irma Bocova Arthur Connors Arthur Connors

Types/Applications Types/Applications

► ► Type:

Type:

• Very Low Frequency

Very Low Frequency

• Pulse Induction

Pulse Induction

• Beat Frequency Oscillator

Beat Frequency Oscillator

► ► Applications:

Applications:

• VLF (below 30kHz): is the most

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 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

• 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 = - Np dΦ/dt

►Magnetic Field at center of field coil is

B = Nf μ0 I(t)/(2Rf)

►This creates a flux in the pick-up coil:

Φ = ( Rp2) Nf μ0 I(t)/(2Rf )

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

• f 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 audible change in “ “beats beats” ”

► ► BFO Metal Detectors are cheap to

BFO Metal Detectors are cheap to make, but aren make, but aren’ ’t as accurate or t as accurate or 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